Urea production apparatus, method for improving urea production apparatus, and urea production method

Abstract

A urea production apparatus (10) includes a first evaporator (EV1) obtaining a first off-gas and a concentrated urea solution from a urea solution, a condenser (C1) cooling the first off-gas to obtain condensate water, a second evaporator (EV2) obtaining a second off-gas and a highly concentrated urea solution from the concentrated urea solution, a granulator (F) obtaining a solid urea product from the highly concentrated urea solution, a scrubber (SCR) contacting the second off-gas and a third off-gas discharged from the granulator (F) with a cleaning solution to obtain a cleaning-processed recovered solution containing a clean gas, ammonium salt, and urea, and a recovery pipe configured to supply the cleaning-processed recovered solution to the second evaporator (EV2). The second evaporator (EV2) obtains the highly concentrated urea solution and the second off-gas from a mixture of the concentrated urea solution and the cleaning-processed recovered solution.

Description

DESCRIPTION TITLE OF INVENTION: UREA PRODUCTION APPARATUS, METHOD FOR IMPROVING UREA PRODUCTION APPARATUS, AND UREA PRODUCTION METHOD TECHNICAL FIELD

[0001] The present disclosure relates to a urea production apparatus, a method for modifying a urea production apparatus, and a method for producing urea. BACKGROUND ART

[0002] A known urea production apparatus produces a solid urea product using a urea solution containing urea obtained by synthesizing carbon dioxide and ammonia as raw materials. Patent Literature 1 discloses a urea production apparatus including a first evaporator and a second evaporator that concentrate a urea solution before the urea solution is supplied to a finishing section. The urea production apparatus includes a first condenser and a second condenser. The first condenser condenses a first vapor obtained in the first evaporator to obtain first condensate. The first condensate is supplied to a wastewater treatment section. Patent Literature 1 describes that urea is hydrolyzed in the wastewater treatment section so that carbon dioxide and ammonia are recycled to urea synthesis. The second condenser condenses a second vapor obtained in the second evaporator to obtain second condensate. The second condensate is supplied to a scrubber.

[0003] Patent Literature 2 discloses a urea production apparatus including a first evaporator and a second evaporator that concentrate a urea solution before the urea solution is supplied to a finishing section. The urea production apparatus includes a first compartment and a second compartment. First condensate is obtained by a first compartment and is supplied to a wastewater treatment section. Patent Literature 2 discloses that the wastewater treatment section includes a hydrolyser. Second condensate is obtained by a second compartment and is supplied to the scrubber. CITATION LIST Patent Literature

[0004] Patent Literature 1: JP2023-503177A Patent Literature 2: US2022 / 0089527A SUMMARY OF INVENTION Technical Problem

[0005] The urea production apparatuses disclosed in Patent Literature 1 and Patent Literature 2 each perform a wastewater treatment process that processes a condensate. In the waste water treatment process, urea is hydrolyzed to be recovered to carbon dioxide and ammonia. Since the urea is obtained by synthesis and again decomposed, energy loss during the process is large. Solution to Problem

[0006] Each aspect of a urea production apparatus according to the present disclosure will be described. An example of a urea production apparatus includes a first evaporator, a condenser, a second evaporator, a granulator, a scrubber, and a recovery pipe. The first evaporator is configured to selectively separate a first off-gas containing water and ammonia from a urea solution to obtain the first off-gas and a concentrated urea solution. The concentrated urea solution is a liquid having a higher urea concentration than the urea solution. The condenser is configured to cool the first off-gas to obtain condensate water. The second evaporator is configured to selectively separate a second off-gas containing water and ammonia from the concentrated urea solution to obtain the second off-gas and a highly concentrated urea solution. The highly concentrated urea solution is a liquid having a higher urea concentration than the concentrated urea solution. The granulator is configured to obtain a solid urea product from the highly concentrated urea solution. The scrubber is configured to contact the second off-gas, which is supplied as a gas from the second evaporator, and a third off-gas discharged from the granulator with an acidic cleaning solution containing an acid to obtain a clean gas and a cleaning-processed recovered solution containing an ammonium salt and urea. The recovery pipe is configured to supply the cleaning-processed recovered solution to the second evaporator. The second evaporator is configured to obtain the highly concentrated urea solution and the second off-gas from a mixture of the concentrated urea solution and the cleaning-processed recovered solution.

[0007] An example of a urea production apparatus includes a first evaporator, a condenser, a second evaporator, a granulator, a scrubber, a stripping tower, a recovery pipe, and a supply pipe. The first evaporator is configured to selectively separate a first off-gas containing water and ammonia from a urea solution to obtain the first off-gas and a concentrated urea solution. The concentrated urea solution is a liquid having a higher urea concentration than the urea solution. The condenser is configured to cool the first off-gas to obtain condensate water. The second evaporator is configured to selectively separate a second off-gas containing water and ammonia from the concentrated urea solution to obtain the second off- gas and a highly concentrated urea solution. The highly concentrated urea solution is a liquid having a higher urea concentration than the concentrated urea solution. The granulator is configured to obtain a solid urea product from the highly concentrated urea solution. The scrubber is configured to contact the second off-gas, which is supplied as a gas from the second evaporator, and a third off-gas discharged from the granulator with an acidic cleaning solution containing an acid to obtain a clean gas and a cleaning-processed recovered solution containing an ammonium salt and urea. The stripping tower is configured to strip the condensate water with steam to obtain a gas containing ammonia and processed condensate water. The recovery pipe is configured to supply the cleaning-processed recovered solution to the second evaporator. The supply pipe is configured to supply the processed condensate water to the scrubber. The second evaporator is configured to obtain the highly concentrated urea solution and the second off-gas from a mixture of the concentrated urea solution and the cleaning-processed recovered solution. The scrubber uses the processed condensate water as a make-up water supplied to the cleaning solution.

[0008] Each aspect of a method for modifying a urea production apparatus according to the present disclosure will be described. An example of a method is for modifying an existing urea production apparatus that includes a granulator configured to obtain a solid urea product from a liquid containing urea, an evaporator configured to selectively separate a first off-gas, which is a gas containing water and ammonia, from a urea solution to obtain the first off-gas and a liquid having a higher urea concentration than the urea solution and being supplied to the granulator, a condenser configured to cool the first off-gas to obtain condensate water, a waste water treatment processing unit including a urea hydrolyzer and a stripping tower and configured to process the condensate water to obtain treated waste water, a scrubber configured to contact an off-gas discharged from the granulator with a cleaning solution to obtain a clean gas and a cleaning-processed recovered solution, and a supply pipe configured to supply the treated waste water, which is obtained in the waste water treatment processing unit, to the scrubber. The method includes adding an additional evaporator between the evaporator and the granulator, adding a discharge pipe configured to supply an additional off-gas including water and ammonia from the additional evaporator to the scrubber, adding an acid supply pipe configured to supply an acid to the scrubber so that an off-gas discharged from the granulator and the additional off-gas contact an acidic cleaning solution containing an acid to obtain a clean gas and a cleaning-processed recovered solution, adding a recovery pipe configured to supply the cleaning-processed recovered solution to the additional evaporator, and configuring the waste water treatment processing unit so that the condensate water is processed by the stripping tower to obtain waste water, and the waste water is supplied to the scrubber through the supply pipe without operating the urea hydrolyzer.

[0009] An example of a method is for modifying an existing urea production apparatus that includes a granulator configured to obtain a solid urea product from a liquid containing urea, an evaporator configured to selectively separate a first off-gas, which is a gas containing water and ammonia, from a urea solution to obtain the first off-gas and a liquid having a higher urea concentration than the urea solution and being supplied to the granulator, a condenser configured to cool the first off-gas to obtain condensate water, a waste water treatment processing unit including a urea hydrolyzer and a stripping tower and configured to process the condensate water to obtain processed waste water, a scrubber configured to contact an off-gas discharged from the granulator with a cleaning solution to obtain a clean gas and a cleaning-processed recovered solution, and a supply pipe configured to supply the treated waste water, which is obtained in the waste water treatment unit, to the scrubber. The method includes adding an additional evaporator between the evaporator and the granulator, adding a discharge pipe configured to supply an additional off-gas including water and ammonia from the additional evaporator to the scrubber, adding an acid supply pipe configured to supply an acid to the scrubber so that an off-gas discharged from the granulator and the additional off-gas contact an acidic cleaning solution containing an acid to obtain a clean gas and a cleaning-processed recovered solution, adding a recovery pipe configured to supply the cleaning-processed recovered solution to the additional evaporator, and configuring the waste water treatment unit to supply the condensate water to the scrubber without operating the urea hydrolyzer and the stripping tower.

[0010] An example of a method is for modifying an existing urea production apparatus that includes a granulator configured to obtain a solid urea product from a liquid containing urea, an evaporator configured to selectively separate a first off-gas, which is a gas containing water and ammonia, from a urea solution to obtain the first off-gas and a liquid having a higher urea concentration than the urea solution and being supplied to the granulator, a condenser configured to cool the first off-gas to obtain condensate water, a waste water treatment unit including a urea hydrolyzer and a stripping tower and configured to process the condensate water to obtain treated waste water, a scrubber configured to contact an off-gas discharged from the granulator with an acidic cleaning solution containing an acid to obtain a clean gas and a cleaning-processed recovered solution, and a supply pipe configured to supply the treated waste water, which is obtained in the waste water treatment unit, to the scrubber. The method includes adding an additional evaporator between the evaporator and the granulator, adding a discharge pipe configured to supply an additional off-gas including water and ammonia from the additional evaporator to the scrubber, adding a recovery pipe configured to supply the cleaning-processed recovered solution to the additional evaporator, and configuring the waste water treatment unit so that the condensate water is processed by the stripping tower to obtain treated waste water, and the treated waste water is supplied to the scrubber through the supply pipe without operating the urea hydrolyzer.

[0011] An example of a method is for modifying an existing urea production apparatus that includes a granulator configured to obtain a solid urea product from a liquid containing urea, an evaporator configured to selectively separate a first off-gas, which is a gas containing water and ammonia, from a urea solution to obtain the first off-gas and a liquid having a higher urea concentration than the urea solution and being supplied to the granulator, a condenser configured to cool the first off-gas to obtain condensate water, a waste water treatment unit including a urea hydrolyzer and a stripping tower and configured to process the condensate water to obtain treated waste water, a scrubber configured to contact an off-gas discharged from the granulator with an acidic cleaning solution containing an acid to obtain a clean gas and a cleaning-processed recovered solution, and a supply pipe configured to supply the treated waste water, which is obtained in the waste water treatment unit, to the scrubber. The method includes adding an additional evaporator between the evaporator and the granulator, adding a discharge pipe configured to supply an additional off-gas including water and ammonia from the additional evaporator to the scrubber, adding a recovery pipe configured to supply the cleaning-processed recovered solution to the additional evaporator, and configuring the waste water treatment unit to supply the condensate water to the scrubber without operating the urea hydrolyzer and the stripping tower.

[0012] An example of a method is for modifying an existing urea production apparatus that includes a granulator configured to obtain a solid urea product from a liquid containing urea, a first-stage evaporator configured to selectively separate a first off-gas containing water and ammonia from a urea solution to obtain the first off-gas and a concentrated urea solution, the concentrated urea solution being a liquid having a higher urea concentration than the urea solution, a first condenser configured to cool a gas discharged from the first- stage evaporator to obtain first condensate water, a second-stage evaporator configured to selectively separate a gas containing water and ammonia from a liquid supplied from the first-stage evaporator to obtain the separated gas and a liquid having a higher urea concentration than the liquid supplied from the first evaporator and being configured to be supplied to the granulator, a second condenser configured to cool a gas discharged from the second-stage evaporator to obtain second condensate water, a waste water treatment unit including a urea hydrolyzer and a stripping tower and configured to process the first condensate water and the second condensate water to obtain treated waste water, a scrubber configured to contact an off-gas discharged from the granulator with a cleaning solution to obtain a clean gas and a cleaning-processed recovered solution, and a supply pipe configured to supply the treated waste water, which is obtained in the waste water treatment unit, to the scrubber. The method includes adding an additional evaporator between the second-stage evaporator and the granulator, adding a discharge pipe configured to supply an additional off-gas including water and ammonia from the additional evaporator to the scrubber, adding an acid supply pipe configured to supply an acid to the scrubber so that an off-gas discharged from the granulator and the additional off-gas contact an acidic cleaning solution containing an acid to obtain a clean gas and a cleaning-processed recovered solution, adding a recovery pipe configured to supply the cleaning-processed recovered solution to the additional evaporator, and configuring the waste water treatment unit so that the first condensate water and the second condensate water are processed by the stripping tower to obtain treated waste water, and the treated waste water is supplied to the scrubber through the supply pipe without operating the urea hydrolyzer.

[0013] An example of a method is for modifying an existing urea production apparatus that includes a granulator configured to obtain a solid urea product from a liquid containing urea, a first-stage evaporator configured to selectively separate a first off-gas containing water and ammonia from a urea solution to obtain the first off-gas and a concentrated urea solution, the concentrated urea solution being a liquid having a higher urea concentration than the urea solution, a first condenser configured to cool a gas discharged from the first- stage evaporator to obtain first condensate water, a second-stage evaporator configured to selectively separate a gas containing water and ammonia from a liquid supplied from the first-stage evaporator to obtain the separated gas and a liquid having a higher urea concentration than the liquid supplied from the first evaporator and being configured to be supplied to the granulator, a second condenser configured to cool a gas discharged from the second-stage evaporator to obtain second condensate water, a waste water treatment unit including a urea hydrolyzer and a stripping tower and configured to process the first condensate water and the second condensate water to obtain treated waste water, a scrubber configured to contact an off-gas discharged from the granulator with a cleaning solution to obtain a clean gas and a cleaning-processed recovered solution, and a supply pipe configured to supply the treated waste water, which is obtained in the waste water treatment unit, to the scrubber. The method includes adding an additional evaporator between the second-stage evaporator and the granulator, adding a discharge pipe configured to supply an additional off-gas including water and ammonia from the additional evaporator to the scrubber, adding an acid supply pipe configured to supply an acid to the scrubber so that an off-gas discharged from the granulator and the additional off-gas contact an acidic cleaning solution containing an acid to obtain a clean gas and a cleaning-processed recovered solution, adding a recovery pipe configured to supply the cleaning-processed recovered solution to the additional evaporator, and configuring the waste water treatment unit to supply the first condensate water and the second condensate water to the scrubber without operating the urea hydrolyzer and the stripping tower.

[0014] An example of a method is for modifying an existing urea production apparatus that includes a granulator configured to obtain a solid urea product from a liquid containing urea, a first-stage evaporator configured to selectively separate a first off-gas containing water and ammonia from a urea solution to obtain the first off-gas and a concentrated urea solution, the concentrated urea solution being a liquid having a higher urea concentration than the urea solution, a first condenser configured to cool a gas discharged from the first- stage evaporator to obtain first condensate water, a second-stage evaporator configured to selectively separate a gas containing water and ammonia from a liquid supplied from the first-stage evaporator to obtain the separated gas and a liquid having a higher urea concentration than the liquid supplied from the first evaporator and being configured to be supplied to the granulator, a second condenser configured to cool a gas discharged from the second-stage evaporator to obtain second condensate water, a waste water treatment unit including a urea hydrolyzer and a stripping tower and configured to process the first condensate water and the second condensate water to obtain treated waste water, a scrubber configured to contact an off-gas discharged from the granulator with an acidic cleaning solution containing an acid to obtain a clean gas and a cleaning-processed recovered solution, and a supply pipe configured to supply the treated waste water, which is obtained in the waste water treatment unit, to the scrubber. The method includes adding an additional evaporator between the second-stage evaporator and the granulator, adding a discharge pipe configured to supply an additional off-gas including water and ammonia from the additional evaporator to the scrubber, adding a recovery pipe configured to supply the cleaning- processed recovered solution to the additional evaporator, and configuring the waste water treatment unit so that the first condensate water and the second condensate water are processed by the stripping tower to obtain treated waste water, and the treated waste water is supplied to the scrubber through the supply pipe without operating the urea hydrolyzer.

[0015] An example of a method is for modifying an existing urea production apparatus that includes a granulator configured to obtain a solid urea product from a liquid containing urea, a first-stage evaporator configured to selectively separate a first off-gas containing water and ammonia from a urea solution to obtain the first off-gas and a concentrated urea solution, the concentrated urea solution being a liquid having a higher urea concentration than the urea solution, a first condenser configured to cool a gas discharged from the first- stage evaporator to obtain first condensate water, a second-stage evaporator configured to selectively separate a gas containing water and ammonia from a liquid supplied from the first-stage evaporator to obtain the separated gas and a liquid having a higher urea concentration than the liquid supplied from the first evaporator and being configured to be supplied to the granulator, a second condenser configured to cool a gas discharged from the second-stage evaporator to obtain second condensate water, a waste water treatment unit including a urea hydrolyzer and a stripping tower and configured to process the first condensate water and the second condensate water to obtain treated waste water, a scrubber configured to contact an off-gas discharged from the granulator with an acidic cleaning solution containing an acid to obtain a clean gas and a cleaning-processed recovered solution, and a supply pipe configured to supply the treated waste water, which is obtained in the waste water treatment unit, to the scrubber. The method includes adding an additional evaporator between the second-stage evaporator and the granulator, adding a discharge pipe configured to supply an additional off-gas including water and ammonia from the additional evaporator to the scrubber, adding a recovery pipe configured to supply the cleaning- processed recovered solution to the additional evaporator, and configuring the waste water treatment unit to supply the first condensate water and the second condensate water to the scrubber without operating the urea hydrolyzer and the stripping tower.

[0016] Each aspect of a method for producing urea according to the present disclosure will be described. An example of a method for producing urea includes a first evaporation step, a condensation step, a second evaporation step, a granulation step, and a cleaning step. The first evaporation step selectively separates a first off-gas containing water and ammonia from a urea solution to obtain the first off-gas and a concentrated urea solution. The concentrated urea solution is a liquid having a higher urea concentration than the urea solution. The condensation step cools the first off-gas to obtain condensate water. The second evaporation step selectively separates a second off-gas containing water and ammonia from the concentrated urea solution to obtain the second off-gas and a highly concentrated urea solution. The highly concentrated urea solution is a liquid having a higher urea concentration than the concentrated urea solution. The granulation step obtains a solid urea product from the highly concentrated urea solution. The cleaning step contacts the second off-gas, which is obtained by the second evaporation step, and a third off-gas discharged by the granulation step with an acidic cleaning solution containing an acid to obtain a clean gas and a cleaning- processed recovered solution containing an ammonium salt and urea. The second evaporation step includes obtaining the highly concentrated urea solution and the second off- gas from a mixture of the concentrated urea solution and the cleaning-processed recovered solution.

[0017] An example of a method for producing urea includes a first evaporation step, a condensation step, a second evaporation step, a granulation step, a cleaning step, and a stripping step. The first evaporation step selectively separates a first off-gas containing water and ammonia from a urea solution to obtain the first off-gas and a concentrated urea solution. The concentrated urea solution is a liquid having a higher urea concentration than the urea solution. The condensation step cools the first off-gas to obtain condensate water. The second evaporation step selectively separates a second off-gas containing water and ammonia from the concentrated urea solution to obtain the second off-gas and a highly concentrated urea solution. The highly concentrated urea solution is a liquid having a higher urea concentration than the concentrated urea solution. The granulation step obtains a solid urea product from the highly concentrated urea solution. The cleaning step contacts the second off-gas, which is obtained by the second evaporation step, and a third off-gas discharged by the granulation step with an acidic cleaning solution containing an acid to obtain a clean gas and a cleaning-processed recovered solution containing an ammonium salt and urea. The stripping step obtains a gas containing ammonia and processed condensate water by stripping the condensate water with steam. The second evaporation step includes obtaining the highly concentrated urea solution and the second off-gas from a mixture of the concentrated urea solution and the cleaning-processed recovered solution. The cleaning step uses the processed condensate water as a make-up water supplied to the cleaning solution. BRIEF DESCRIPTION OF DRAWINGS

[0018] [Fig. 1] Fig. 1 is a schematic diagram showing a first embodiment of a urea production apparatus. [Fig. 2] Fig. 2 is a schematic diagram showing a second embodiment of a urea production apparatus. [Fig. 3] Fig. 3 is a schematic diagram showing a third embodiment of a urea production apparatus. [Fig. 4] Fig. 4 is a schematic diagram showing a first existing urea production apparatus. [Fig. 5] Fig. 5 is a schematic diagram showing a fourth embodiment of a urea production apparatus. [Fig. 6] Fig. 6 is a schematic diagram showing a fifth embodiment of a urea production apparatus. [Fig. 7] Fig. 7 is a schematic diagram showing a sixth embodiment of a urea production apparatus. [Fig. 8] Fig. 8 is a schematic diagram showing a seventh embodiment of a urea production apparatus. [Fig. 9] Fig. 9 is a schematic diagram showing a second existing urea production apparatus. [Fig. 10] Fig. 10 is a schematic diagram showing an eighth embodiment of a urea production apparatus. [Fig. 11] Fig. 11 is a schematic diagram showing a ninth embodiment of a urea production apparatus. [Fig. 12] Fig. 12 is a schematic diagram showing a third existing urea production apparatus. [Fig. 13] Fig. 13 is a schematic diagram showing a tenth embodiment of a urea production apparatus. [Fig. 14] Fig. 14 is a schematic diagram showing an eleventh embodiment of a urea production apparatus. [Fig. 15] Fig. 15 is a schematic diagram showing a fourth existing urea production apparatus. [Fig. 16] Fig. 16 is a schematic diagram showing a twelfth embodiment of a urea production apparatus. [Fig. 17] Fig. 17 is a schematic diagram showing a thirteenth embodiment of a urea production apparatus. [Fig. 18] Fig. 18 is a schematic diagram showing a fourteenth embodiment of a urea production apparatus. [Fig. 19] Fig. 19 is a schematic diagram showing a modified example of a urea production apparatus. [Fig. 20] Fig. 20 is a schematic diagram showing another modified example of a urea production apparatus. DESCRIPTION OF EMBODIMENTS

[0019] [First Embodiment] A first embodiment of a urea production apparatus 10 will now be described with reference to Fig. 1. The urea production apparatus 10 is configured to obtain a solid urea product from a urea solution. The urea solution is produced by, for example, a urea synthesis step. The urea synthesis step is a step of obtaining a urea solution from carbon dioxide (CO2) and ammonia (NH3), which are raw materials. The urea synthesis step is not particularly limited and may be performed by a known process. The urea solution obtained by the urea synthesis step contains, for example, urea, ammonia, water, biuret, and the like. Although not particularly limited, the urea content of the urea solution is, for example, 70 wt% or less.

[0020] Fig. 1 is a diagram showing a urea synthesis unit U0 performing the urea synthesis step. The urea synthesis unit U0 is not particularly limited and may have a known configuration. In an example, the urea synthesis unit U0 includes a synthesis device U1, a purification device U2, and a recovery device U3. The synthesis device U1 synthesizes urea from carbon dioxide and ammonia. The purification device U2 separates carbon dioxide and ammonia from a synthesis solution obtained by the synthesis device U1 to obtain a purified urea solution. The recovery device U3 obtains a recovered solution by absorbing carbon dioxide and ammonia, obtained by the purification device U2, into water. The recovered solution is supplied to the synthesis device U1 and is used for synthesizing urea.

[0021] In each drawing, a solid line arrow indicates that a liquid is discharged from the tail side of the arrow. The solid line arrow also indicates that the liquid is supplied to the head side of the arrow. A dashed line arrow indicates that gas is discharged from the tail side of the arrow. The dashed line arrow also indicates that the gas is supplied to the head side of the arrow.

[0022] As shown in Fig. 1, the urea production apparatus 10 includes a first evaporator EV1, a second evaporator EV2, a granulator F, a condenser C1, and a scrubber SCR. The urea production apparatus 10 includes pipes connected to each component of the urea production apparatus 10. In Fig. 1, each arrow with "L#" indicates line L#. Each line L# corresponds to a pipe of the urea production apparatus 10. The sign # includes at least one of a Roman letter, a numeral, and a symbol.

[0023] In this specification, the phrase "at least one of" as used in this disclosure means "one or more" of a desired choice. As one example, the phrase "at least one of" as used in this disclosure means "only one choice" or "both of two choices" in a case where the number of choices is two. In another example, in this specification, the phrase "at least one of" as used in this disclosure means "only one single choice" or "any combination of two or more choices" if the number of its choices is greater than or equal to three.

[0024] <First Evaporator EV1 and Condenser C1> The first evaporator EV1 selectively separates a first off-gas containing water and ammonia from a urea solution. The first evaporator EV1 obtains a concentrated urea solution. The concentrated urea solution is a liquid having a higher urea concentration than the urea solution. The first off-gas may contain urea. Although not particularly limited, the urea content of the concentrated urea solution is, for example, 80 wt% or greater.

[0025] Although not particularly limited, the first evaporator EV1 may be, for example, a known heat exchanger such as a shell and tube heat exchanger. Operation conditions of the first evaporator EV1 are such that, for example, the pressure is preferably greater than or equal to 10 kPa and less than or equal to 80 kPa in terms of absolute pressure. The temperature is preferably, for example, greater than or equal to 90°C and less than or equal to 140°C.

[0026] The condenser C1 cools the first off-gas to obtain condensate water. The condenser C1 includes an ejector Ej that reduces the pressure of the first evaporator EV1 and the pressure of the condenser C1. An example of the ejector Ej uses steam as a driving source. Fig. 1 shows line LS1 supplying steam for driving the ejector Ej. When the first off-gas contains urea, the condensate water also contains urea.

[0027] The urea solution is supplied from line L1 to the first evaporator EV1. The concentrated urea solution is discharged from the first evaporator EV1 to line L2. The first off-gas is discharged from the first evaporator EV1 to line L8.

[0028] The condensate water is supplied to the scrubber SCR. The condensate water supplied to the scrubber SCR may be a portion of or the entirety of the condensate water obtained by the condenser C1. Fig. 1 shows line L10 supplying the condensate water from the condenser C1 to the scrubber SCR. The line L10 corresponds to a supply pipe supplying at least a portion of the condensate water to the scrubber SCR.

[0029] In this specification, the expression "at least a portion" includes a portion and the entirety. The condensate water may be supplied to the urea synthesis unit U0. In an example, the condensate water may be supplied to the recovery device U3 through line L9.

[0030] In the present embodiment, the condensate water is discharged from the condenser C1 through the two lines L9 and L10. Alternatively, one of the lines discharging the condensate water from the condenser C1 may be divided into two branches so that one of the branches is connected to the scrubber SCR while the other branch is connected to the urea synthesis unit U0.

[0031] <Second Evaporator EV2> The second evaporator EV2 selectively separates a second off-gas containing water and ammonia from the concentrated urea solution to obtain a highly concentrated urea solution having a higher urea concentration than the concentrated urea solution. The second evaporator EV2 is supplied with a cleaning-processed recovered solution, which will be described later. The second evaporator EV2 is configured to obtain the highly concentrated urea solution and the second off-gas from a mixture of the concentrated urea solution and the cleaning-processed recovered solution. The urea content of the highly concentrated urea solution is, for example, greater than or equal to 95 wt% and less than or equal to 99.8 wt%. Preferably, the urea content of the highly concentrated urea solution is greater than or equal to 97 wt% and less than or equal to 99.8 wt%.

[0032] Fig. 1 shows line L16 to which the cleaning-processed recovered solution is supplied. In an example, as shown in Fig. 1, the lines L2 and L16 are joined so that the mixture of the concentrated urea solution and the cleaning-processed recovered solution is supplied to the second evaporator EV2 through line L3. Alternatively, the line L2 and the line L16 may each be connected to the second evaporator EV2.

[0033] Preferably, the second evaporator EV2 is a liquid falling film evaporator. In an example, the second evaporator EV2 is configured to bring the concentrated urea solution into countercurrent contact with heated air at atmospheric pressure.

[0034] Preferably, the second evaporator EV2 is operated at atmospheric pressure. However, there is no limit to such a configuration. An operation condition of the second evaporator EV2 is that the pressure is close to atmospheric pressure and may be higher than or lower than atmospheric pressure. Preferably, the operation condition is, for example, greater than or equal to 90 kPa and less than or equal to 110 kPa in terms of absolute pressure. Another operation condition of the second evaporator EV2 is that the heating temperature is preferably, for example, greater than or equal to 130°C and less than or equal to 140°C.

[0035] The second evaporator EV2 is not limited to the configuration described above. The second evaporator EV2 may be configured in any manner as long as the second off-gas, which is selectively separated from the concentrated urea solution and contains water and ammonia, is supplied to the scrubber SCR. In an example, the second evaporator EV2 may be operated at pressure lower than atmospheric pressure, and gas may be supplied from the second evaporator EV2 to the scrubber using an ejector or a blower. Gas and liquid may flow through a pipe in a concurrent manner.

[0036] The highly concentrated urea solution is discharged from the second evaporator EV2 to line L4. The heated air is supplied from line LA1 to the second evaporator EV2. The second off-gas is supplied from the second evaporator EV2 to the scrubber SCR through the line L11. The second off-gas contains heated air that has passed through the second evaporator EV2. More specifically, the heated air and the gas separated from the concentrated urea solution are supplied from the line L11 to the scrubber SCR.

[0037] <Scrubber SCR> The scrubber SCR contacts the second off-gas, which is supplied as a gas from the second evaporator EV2, and a third off-gas discharged from the granulator F with an acidic cleaning solution containing an acid to obtain a clean gas and a cleaning-processed recovered solution containing an ammonium salt and urea.

[0038] The scrubber SCR is a device that limits ammonia discharge from the system of the urea production apparatus 10. The scrubber SCR discharges the clean gas from line L15 to the outside of the system of the urea production apparatus 10. The clean gas is a processed gas having a low ammonia concentration.

[0039] The cleaning-processed recovered solution is discharged from the scrubber SCR to the line L16. The line L16 corresponds to a recovery pipe supplying the cleaning-processed recovered solution to the second evaporator EV2. The acid contained in the acidic cleaning solution is, for example, sulfuric acid, nitric acid, or phosphoric acid, and is preferably sulfuric acid. When the cleaning solution contains sulfuric acid, ammonium sulfate is produced as the ammonium salt contained in the cleaning-processed recovered solution. The acid is supplied from the line L14 to the scrubber SCR.

[0040] Water is supplied to the scrubber SCR to adjust the concentration of the solution in the scrubber SCR and compensate for evaporation loss of water. The scrubber SCR uses at least a portion of the condensate water as a make-up water supplied to the cleaning solution. As described above, the condensate water is supplied from the condenser C1. Water may be supplied from the line L13 to the scrubber SCR. An example of the cleaning solution is a liquid in which the above-described acid, water, and the condensate water are mixed. The cleaning solution may be a liquid in which the above- described acid and the condensate water are mixed.

[0041] <Granulator F> The granulator F obtains a solid urea product from the highly concentrated urea solution. Fig. 1 shows line L6 to which the highly concentrated urea solution is supplied. Fig. 1 shows line L7 discharging the solid urea product from the granulator F.

[0042] Although not particularly limited, the granulator F is preferably a device using the heated air as fluidizing air to form a fluidized bed. In the description hereafter, the device may be referred to as a fluidized bed granulator. Japanese Laid-Open Patent Publication No. S62-74443 describes an example of a fluidized bed granulator. The granulator F may be a prilling tower that obtains urea prills. The solid urea product obtained using the prilling tower has a smaller particle size than the solid urea product obtained using the fluidized bed granulator. Thus, the size of particles of the solid urea product is not particularly limited.

[0043] Urea dust is produced in the granulator F. The urea dust is supplied as the third off- gas to the scrubber SCR. Fig. 1 shows line L12 discharging the third off-gas from the granulator F.

[0044] When the fluidized bed granulator is used as the granulator F, the urea production apparatus 10 may include an air supply device FAN configured to supply the heated air to the granulator F. Fig. 1 shows line LA2 supplying the heated air from the air supply device FAN to the granulator F. When the urea production apparatus 10 includes the air supply device FAN, the air supply device FAN may supply heated air that comes into countercurrent contact with the concentrated urea solution in the second evaporator EV2. More specifically, the heated air supplied from the air supply device FAN may be divided and supplied to the second evaporator EV2 and the granulator F. Alternatively, the urea production apparatus 10 may include two air supply devices. More specifically, the urea production apparatus 10 may include an air supply device configured to supply heated air to the second evaporator EV2 and an air supply device configured to supply heated air to the granulator F.

[0045] <Additive> The urea production apparatus 10 may be configured to supply an additive to the liquid between the first evaporator EV1 and the granulator F. More specifically, the additive may be configured to be supplied to the liquid downstream of the second evaporator EV2 and upstream of the granulator F. Alternatively, the additive may be configured to be supplied to the liquid downstream of the first evaporator EV1 and upstream of the second evaporator EV2. Alternatively, the additive may be supplied to the liquid downstream of the second evaporator EV2 and upstream of the granulator F and the liquid downstream of the first evaporator EV1 and upstream of the second evaporator EV2.

[0046] Fig. 1 shows line L5 corresponding to an additive pipe supplying the additive and the ammonium salt for adjusting an ammonium salt concentration of a product to the liquid downstream of the second evaporator EV2 and upstream of the granulator F. In an example, as shown in Fig. 1, the lines L4 and L5 are joined so that the highly concentrated urea solution and the additive are supplied to the granulator F through the line L6.

[0047] The additive is a component supplied from outside the system of the urea production apparatus 10. The additive includes, for example, formaldehyde, a urea-formaldehyde solution, calcium nitrate, potassium nitrate, or a mixture of polyvinyl alcohol and calcium sulfate.

[0048] The ammonium salt for adjusting an ammonium salt concentration of a product is a component supplied from the outside of the system of the urea production apparatus 10. The ammonium salt includes, for example, ammonium nitrate or ammonium sulfate.

[0049] The additive and the ammonium salt for adjusting an ammonium salt concentration of a product may be one of the components described above or a combination of two or more of the components. When the additive and the ammonium salt for adjusting an ammonium salt concentration of a product are both used, the urea production apparatus 10 may include one additive pipe. Alternatively, the urea production apparatus 10 may include a first additive pipe supplying the additive and a second additive pipe supplying ammonium salt.

[0050] The ammonium salt for adjusting an ammonium salt concentration of a product may be an ammonium salt supplied from outside the system or may be an ammonium salt formed by synthesizing a salt supplied to the cleaning solution of the scrubber and ammonia (NH3), which is a raw material used in the urea synthesis step. In an example, the cleaning- processed recovered solution containing the ammonium salt is supplied through the line L16.

[0051] <Method for Producing Urea> An example of a urea production method for producing a solid urea product from a urea solution will now be described. The urea production method of the present embodiment includes, for example, a first evaporation step, a condensation step, a second evaporation step, a granulation step, and a cleaning step.

[0052] The first evaporation step selectively separates a first off-gas containing water and ammonia from a urea solution to obtain a concentrated urea solution. The concentrated urea solution is a liquid having a higher urea concentration than the urea solution.

[0053] The condensation step cools the first off-gas to obtain the condensate water. The second evaporation step selectively separates a second off-gas containing water and ammonia from the concentrated urea solution to obtain a highly concentrated urea solution having a higher urea concentration than the concentrated urea solution. The second evaporation step obtains the highly concentrated urea solution and the second off-gas from a mixture of the concentrated urea solution and the cleaning-processed recovered solution, which will be described later.

[0054] The granulation step obtains a solid urea product from the highly concentrated urea solution. The cleaning step contacts the second off-gas, which is obtained by the second evaporation step, and the third off-gas discharged by the granulation step with an acidic cleaning solution containing an acid to obtain a clean gas and a cleaning-processed recovered solution containing an ammonium salt and urea.

[0055] The cleaning step may use at least a portion of the condensate water as a make-up water supplied to the cleaning solution. The second evaporation step may use a liquid falling film evaporator configured to bring the concentrated urea solution into countercurrent contact with the heated air at atmospheric pressure. In this case, the second off-gas contains the heated air that has passed through the evaporator.

[0056] The urea production method may further include a step of supplying an additive and an ammonium salt for adjusting an ammonium salt concentration of a product to the liquid between the first evaporation step and the granulation step. <Operation and Advantages> The operation of the present embodiment will now be described.

[0057] In the present embodiment, the scrubber SCR processes the third off-gas containing urea dust discharged from the granulator F. The cleaning-processed recovered solution obtained by the scrubber SCR is supplied to the second evaporator EV2. In the second evaporator EV2, the highly concentrated urea solution and the second off-gas are obtained from a mixture of the concentrated urea solution obtained by the first evaporator EV1 and the cleaning-processed recovered solution. Thus, the cleaning-processed recovered solution, from which urea contained in the third off-gas is recovered, is again supplied to the second evaporator EV2.

[0058] In the present embodiment, in addition to the off-gas from the granulator F, the second off-gas is supplied to the scrubber SCR. Thus, the scrubber SCR uses a relatively large quantity of water for processing the off-gases. In this regard, in the present embodiment, the condensate water is obtained as a result of condensation of the first off-gas and supplied to the scrubber SCR. This replenishes the water of the scrubber SCR.

[0059] The first off-gas and the condensate water obtained by condensing the first off-gas may include urea. In the present embodiment, the condensate water does not undergo a waste water treatment process and is processed in the scrubber SCR and then recovered as the cleaning-processed recovered solution. In this configuration, when the condensate water contains urea, the urea in the condensate water is supplied to the second evaporator EV2 without being decomposed. Thus, while replenishing the water of the scrubber SCR, urea is recovered from the condensate water.

[0060] Moreover, the second off-gas may contain urea. When the second off-gas contains urea, the cleaning-processed recovered solution, from which urea contained in the second off-gas is recovered, is again supplied to the second evaporator EV2.

[0061] The advantage of the present embodiment will now be described. (1-1) While the second off-gas and the third off-gas are processed by the scrubber SCR, urea is efficiently recovered. This increases the efficiency of producing urea.

[0062] (1-2) Urea contained in the condensate water is recovered without being hydrolyzed. This increases the efficiency of producing urea. (1-3) The second evaporator EV2 is operated at atmospheric pressure so that the second off-gas obtained by the second evaporator EV2 is directly supplied to the scrubber SCR.

[0063] (1-4) When the heated air is supplied to the second evaporator EV2, the second off- gas supplied to the scrubber SCR contains the heated air. This facilitates evaporation of water in scrubber SCR. Since the amount of water evaporated in the scrubber SCR is increased, even when a large amount of condensed water is supplied to the scrubber SCR, the condensate water is completely processed by the scrubber SCR. Therefore, excess condensate water is less likely to be produced. This eliminates the need for separately arranging a waste water treatment unit that processes the condensate water.

[0064] (1-5) The ammonium salt is produced by the scrubber SCR and is mixed with the solid urea product. Thus, properties derived from the ammonium salt are added to the solid urea product. In an example, when ammonium sulfate is mixed into the solid urea product, the solid urea product may be used as an effective fertilizer for alkaline soils.

[0065] (1-6) Urea is recovered without being decomposed while the discharging of urea outside the system is limited. Thus, the solid urea product is produced efficiently. (1-7) The ammonium salt is recovered without being discharged outside the system. Thus, the solid urea product containing the ammonium salt is produced efficiently. This eliminates the need for facilities for processing the ammonium salt outside the system such as a urea mixing facility or a concentration solidification facility. Thus, the number of facilities is less likely to be increased as compared to a configuration that processes the ammonium salt outside the system.

[0066] (1-8) The second evaporator EV2 and the granulator F share the air supply device FAN configured to supply heated air. Thus, the number of facilitates is less likely to be increased as compared to a configuration that includes two air supply devices.

[0067] (1-9) Addition of the additive and the ammonium salt for adjusting an ammonium salt concentration of a product improves the quality of the solid urea product. When the additive is added, the solid urea product has additional properties. In an example, when supplying an ammonium salt corresponding to an acid supplied to the scrubber SCR, the concentration of ammonium salt contained in the solid urea product is increased.

[0068] (1-10) In the urea production process, the use of steam increases energy consumption. In this regard, in the present embodiment, less steam is used in the process of producing the solid urea product from the urea solution. For example, in the present embodiment, the second off-gas is not condensed. This eliminates the need for a condenser and an ejector. Thus, less steam is used in the process of producing the solid urea product.

[0069] (1-11) In the present embodiment, the urea production apparatus 10 does not need the waste water treatment unit. Since there is no need for steam for operating the waste water treatment unit, less steam is used in the process of producing the solid urea product as compared to a configuration that operates the waste water treatment unit.

[0070] (1-12) In the present embodiment, the urea production apparatus 10 reduces the number of facilities as compared to a configuration in which each of the evaporators includes a condenser. (1-13) In the present embodiment, the urea production apparatus 10 does not need the waste water treatment unit. Thus, the urea production apparatus 10 reduces the number of facilities as compared to a configuration that includes the waste water treatment unit.

[0071] [Second Embodiment] A second embodiment of a urea production apparatus 20 will now be described with reference to Fig. 2. The urea production apparatus 20 differs from the first embodiment in that a stripping tower ST is arranged between the condenser C1 and the scrubber SCR. In the urea production apparatus 20, same reference characters are given to those components that are the same as the corresponding components of the first embodiment. The description of such components may be omitted.

[0072] <Stripping Tower ST> The stripping tower ST strips the condensate water with steam to obtain a gas containing ammonia and processed condensate water. Fig. 2 shows line LSS supplying the steam to the stripping tower ST. The processed condensate water is a liquid having a lower ammonia content than the condensate water. The processed condensate water is, for example, a liquid containing urea.

[0073] The condensate water discharged from the condenser C1 is supplied to the stripping tower ST through line L10'. The processed condensate water is discharged from the stripping tower ST to the line L17. The processed condensate water is supplied from the line L17 to the scrubber SCR. The line L17 corresponds to a supply pipe supplying at least a portion of the processed condensate water to the scrubber SCR. The scrubber SCR of the urea production apparatus 20 may have the same configuration as the scrubber SCR of the first embodiment except that the processed condensate water is used instead of the condensate water.

[0074] The gas containing ammonia is discharged from the stripping tower ST to the line L18. The gas containing ammonia may be supplied to the urea synthesis unit U0. In an example, the gas containing ammonia may be supplied to the purification device U2 through the line L18.

[0075] <Method for Producing Urea> An example of a urea production method for producing a solid urea product from a urea solution will now be described. The method of the present embodiment for producing urea differs from that of the first embodiment in the following point but otherwise is the same as that of the first embodiment.

[0076] The method of the present embodiment for producing urea further includes a stripping step. The stripping step strips the condensate water with steam to obtain a gas containing ammonia and processed condensate water. In the method of the present embodiment for producing urea, the cleaning step uses the processed condensate water as a make-up water supplied to the cleaning solution.

[0077] <Operation and Advantages> The present embodiment has the following advantage in addition to the advantages of the first embodiment.

[0078] (2-1) In the present embodiment, ammonia is removed from the condensate water by the stripping tower ST. The processed condensate water processed by the stripping tower ST is supplied to the scrubber SCR. This allows a reduction in the amount of acid used in the scrubber SCR as compared to when the condensate water is not processed by the stripping tower ST.

[0079] [Third Embodiment] A third embodiment of a urea production apparatus 30 will now be described with reference to Fig. 3. In the urea production apparatus 30, same reference characters are given to those components that are the same as the corresponding components in the urea production apparatus 10 of the first embodiment. The description of such components may be omitted.

[0800] The urea production apparatus 30 includes a first-stage evaporator EVa, a second- stage evaporator EVb, and a third-stage evaporator EVc. The urea production apparatus 30 includes a first condenser C1 and a second condenser C2.

[0081] The urea production apparatus 30 includes a first-stage scrubber SCR1 and a second-stage scrubber SCR2. The urea production apparatus 30 includes a waste water treatment unit WWT.

[0082] <Evaporators EVa, EVb, and Evc> The first-stage evaporator EVa, the second-stage evaporator EVb, and the third- stage evaporator EVc are arranged in series in the order of the first-stage evaporator EVa, the second-stage evaporator EVb, and the third-stage evaporator EVc. The urea solution is supplied from the line L1 to the first-stage evaporator EVa. The third-stage evaporator EVc discharges a highly concentrated urea solution to line L30.

[0083] The first-stage evaporator EVa may be, for example, an evaporator similar to the first evaporator EV1. The second-stage evaporator EVb may be, for example, a known heat exchanger such as a shell and tube heat exchanger. Operation conditions of the second-stage evaporator EVb are such that, for example, the pressure is preferably greater than or equal to 10 kPa and less than or equal to 40 kPa in terms of absolute pressure. The temperature is preferably, for example, greater than or equal to 120°C and less than or equal to 140°C.

[0084] The third-stage evaporator EVc may be an evaporator similar to the second evaporator EV2. Heated air is supplied from the line LA1 to the third-stage evaporator EVc. Preferably, the heated air is supplied from the air supply device FAN configured to supply heated air to the granulator F.

[0085] In Fig. 3, lines L2', L4", and L6" are shown. The first-stage evaporator EVa increases the urea concentration of a liquid from that of the urea solution. The liquid is discharged from the first-stage evaporator EVa to the line L2'. The second-stage evaporator EVb obtains a concentrated urea solution from the liquid, which is supplied from the line L2' and has a higher urea concentration than the urea solution. The concentrated urea solution is discharged from the second-stage evaporator EVb to the line L4". The concentrated urea solution is supplied from the line L4" to the third- stage evaporator EVc. The lines L30 and L5 are joined so that the highly concentrated urea solution and an additive are supplied to the granulator F through the line L6".

[0086] In Fig. 3, lines L8, L11', and L31 are shown. The line L8 supplies a first off-gas discharged from the first-stage evaporator EVa to the first condenser C1. The line L11' supplies an off-gas discharged from the second-stage evaporator EVb to the second condenser C2. The line L31 supplies an off-gas discharged from the third-stage evaporator EVc to the first-stage scrubber SCR1.

[0087] <Condensers C1 and C2> The first condenser C1 and the second condenser C2 may have the same configuration as the condenser C1 of the urea production apparatus 10 in the first embodiment. In the following description, the condensate water that is obtained by the first condenser C1 may be referred to as first condensate water. The condensate water that is obtained by the second condenser C2 may be referred to as second condensate water.

[8800] In Fig. 3, lines LS1, LS2, L21, and L22 are shown. The line LS1 supplies steam to the ejector Ej configured to reduce the pressure of the first-stage evaporator EVa and the pressure of the first condenser C1. The line LS2 supplies steam to the ejector Ej configured to reduce the pressure of the second-stage evaporator EVb and the pressure of the second condenser C2.

[0089] The line L21 supplies the condensate water from the first condenser C1 to the waste water treatment unit WWT. The line L22 supplies the condensate water from the second condenser C2 to the waste water treatment unit WWT.

[0090] <Waste Water Treatment Unit WWT> The waste water treatment unit WWT is configured to process condensate water. The waste water treatment unit WWT includes the stripping tower ST.

[0091] The stripping tower ST strips the condensate water with steam to obtain a gas containing ammonia and a liquid containing urea. The liquid obtained from the waste water treatment unit WWT may be referred to as treated waste water.

[0092] In Fig. 3, lines L23, L24, L26, and L27 are shown. The line L23 supplies the treated waste water from the waste water treatment unit WWT to the first-stage scrubber SCR1. The line L24 supplies the treated waste water from the waste water treatment unit WWT to the second-stage scrubber SCR2. The lines L23 and L24 each correspond to a supply pipe supplying the processed condensate water, which is the condensate water that has been processed by the stripping tower ST, to the scrubber. The line L26 supplies the gas containing ammonia from the waste water treatment unit WWT to the urea synthesis unit U0, for example, to the purification device U2. The line L27 supplies the waste water from the waste water treatment unit WWT to the urea synthesis unit U0, for example, to the recovery device U3.

[0093] In Fig. 3, the components indicated by two-dot chain lines are not components of the urea production apparatus 30. For example, the urea production apparatus 30 does not include line L25. In another example, the urea production apparatus 30 is connected to a pipe corresponding to the line L25. However, the urea production apparatus 30 is configured not to discharge the treated waste water from the pipe.

[0094] <Scrubbers SCR1 and SCR2> The first-stage scrubber SCR1 and the second-stage scrubber SCR2 form a dual- stage scrubber.

[0095] The first-stage scrubber SCR1 performs water cleaning. The first-stage scrubber SCR1 contacts a gas containing urea dust discharged from the granulator F and an off-gas discharged from the third-stage evaporator EVc with the treated waste water supplied from the waste water treatment unit WWT to obtain a first-stage cleaning-processed recovered solution and a first-stage cleaning gas. The first-stage cleaning-processed recovered solution contains urea.

[0096] A portion of the first-stage cleaning-processed recovered solution is supplied to the urea synthesis unit U0. For example, the first-stage cleaning-processed recovered solution is supplied to the purification device U2 through the line L28. The second-stage scrubber SCR2 performs acid cleaning. The second-stage scrubber SCR2 obtains a clean gas and a second-stage cleaning-processed recovered solution containing an ammonium salt from the first-stage cleaning gas, the treated waste water supplied from the waste water treatment unit WWT, and an acid supplied from the line L14.

[0097] The urea production apparatus 30 is configured to supply the second-stage cleaning-processed recovered solution to the third-stage evaporator EVc. Fig. 3 shows line L29' supplying the cleaning-processed recovered solution from the second-stage scrubber SCR2 to the third-stage evaporator EVc. The lines L4" and L29' may be joined to supply the second-stage cleaning-processed recovered solution to the third-stage evaporator EVc. Alternatively, the second-stage cleaning-processed recovered solution may be directly supplied to the third-stage evaporator EVc.

[0098] [Correspondence] The correspondence between the description in the embodiment and the description in the section titled "Solution to Problem" is as follows.

[0099] At least one of the first-stage evaporator EVa and the second-stage evaporator EVb corresponds to "first evaporator." In other words, an evaporator unit including the first-stage evaporator EVa and the second-stage evaporator EVb that are connected in series corresponds to the first evaporator. The third-stage evaporator EVc corresponds to "second evaporator."

[0100] <Method for Modifying Urea Production Apparatus> The urea production apparatus 30 may be manufactured as a new apparatus or may be manufactured by modifying an existing urea production apparatus. The method for modifying a urea production apparatus will now be described.

[0101] [Example of First Existing Urea Production Apparatus] An example of a first existing urea production apparatus will now be described. Fig. 4 shows a first existing urea production apparatus 100. In the first existing urea production apparatus 100, the description of components that are the same as the components of the urea production apparatus 30may be omitted.

[0102] The first existing urea production apparatus 100 includes the first-stage evaporator EVa and the second-stage evaporator EVb. The first-stage evaporator EVa and the second- stage evaporator EVb are connected in series. The highly concentrated urea solution is discharged from the second-stage evaporator EVb to the line L4'. The lines L4' and L5 are joined so that the highly concentrated urea solution, an additive, and an ammonium salt for adjusting an ammonium salt concentration of a product are supplied to the granulator F through line L6'.

[0103] In the urea production apparatus 100, the waste water treatment unit WWT includes a urea hydrolyzer UH and the stripping tower ST. The stripping tower ST strips the condensate water with steam to obtain a gas containing ammonia and a liquid containing urea.

[0104] The urea hydrolyzer UH hydrolyzes urea to obtain carbon dioxide and ammonia. In the first existing urea production apparatus 100, the lines L23 and L24 each correspond to a supply pipe supplying treated waste water obtained in the waste water treatment unit to a scrubber.

[0105] In the urea production apparatus 100, a portion of waste water treated by the waste water treatment unit WWT may be discharged outside the system of the urea production apparatus 100. Fig. 4 shows the line L25 discharging the treated waste water outside the system of the waste water treatment unit WWT. The line L25 corresponds to a discharge pipe discharging the treated waste water, which is obtained in the waste water treatment unit, outside the system of the urea production apparatus.

[0106] The urea production apparatus 100 is configured to discharge the second-stage cleaning-processed recovered solution, which has been processed by the second-stage scrubber SCR2, outside the system of the urea production apparatus 100. Fig. 4 shows line L29 performing the discharging to the outside of the system of the second-stage scrubber SCR2.

[0107] [Modifying Method] A method for modifying the first existing urea production apparatus 100 shown in Fig. 4 is as follows.

[0108] As shown in Figs. 3 and 4, in the method for modifying a urea production apparatus, the third-stage evaporator EVc is added between the first-stage evaporator EVa and the granulator F. More specifically, the third-stage evaporator EVc is added between the second-stage evaporator EVb and the granulator F. The third-stage evaporator EVc corresponds to an additional evaporator. The additional evaporator further concentrates, for example, a liquid before being supplied to the granulator F to obtain a highly concentrated urea solution having a higher urea concentration than the liquid and an additional off-gas, which contains water and ammonia. The second-stage cleaning solution is mixed with a liquid discharged from an evaporator arranged upstream of the additional evaporator to obtain a liquid having a lower concentration, and the liquid is supplied to the additional evaporator. Hence, the liquid discharged from the evaporator arranged upstream of the additional evaporator may have the same urea concentration as the liquid discharged from the additional evaporator.

[0109] As shown in Figs. 3 and 4, in the method for modifying the urea production apparatus, the line L31 is added to supply the off-gas discharged from the third-stage evaporator EVc to the first-stage scrubber SCR1. The line L31 corresponds to a discharge pipe supplying an additional off-gas to the scrubber.

[0110] As shown in Figs. 3 and 4, in the method for modifying the urea production apparatus, the line L29' is added to supply the second-stage cleaning-processed recovered solution discharged from the second-stage scrubber SCR2 to the third-stage evaporator EVc. The line L29' corresponds to a recovery pipe supplying the second-stage cleaning-processed recovered solution to the additional evaporator. The line L29' may be obtained by connecting the line L29 of the first existing urea production apparatus 100 to the third-stage evaporator EVc. Alternatively, the line L29' may be arranged as a pipe differing from the line L29 of the first existing urea production apparatus 100. In this case, the line L29 may be removed. Alternatively, the line L29 may be configured not to discharge the second-stage cleaning- processed recovered solution.

[0111] In the method for modifying the urea production apparatus, operation of the urea hydrolyzer UH is stopped in the waste water treatment unit WWT. When the urea hydrolyzer UH is not operated, condensate water is processed by the stripping tower ST to obtain treated waste water. The treated waste water is supplied to the scrubbers through the lines L23 and L24.

[0112] When the first existing urea production apparatus 100 includes the line L25 configured to discharge the treated waste water, which is obtained in the waste water treatment unit WWT, to the outside of the system of the urea production apparatus 100, the method for modifying the urea production apparatus may be configured not to discharge treated waste water from the line L25. In an example, the urea production apparatus 100 may be configured not to discharge treated waste water from the line L25 by stopping the use of the line L25. In another example, in the process of modifying the urea production apparatus, the line L25 may be removed.

[0113] <Operation and Advantages> The present embodiment obtains the same advantages as <semantics>(1−1)<annotation encoding="application / x-tex">(1-1)< / annotation>< / semantics> to <semantics>(1−10)<annotation encoding="application / x-tex">(1-10)< / annotation>< / semantics> and <semantics>(1−12)<annotation encoding="application / x-tex">(1-12)< / annotation>< / semantics> of the first embodiment and (2-1) of the second embodiment. Steam is used to operate the urea hydrolyzer UH. In the present embodiment, the urea hydrolyzer UH is not operated. Thus, steam consumption is reduced.

[0114] [Fourth Embodiment] A fourth embodiment of a urea production apparatus 40 will now be described with reference to Fig. 5. In the urea production apparatus 40, same reference characters are given to those components that are the same as the corresponding components in the urea production apparatus 30 of the third embodiment. The description of such components may be omitted.

[0115] The urea production apparatus 40 is configured to supply condensate water discharged from the first condenser C1 and the second condenser C2 to the first-stage scrubber SCR1 and the second-stage scrubber SCR2.

[0116] The urea production apparatus 40 is configured to supply the condensate water discharged from the first condenser C1 and the second condenser C2 to the urea synthesis unit U0. An example of the urea production apparatus 40 will now be described.

[0117] In Fig. 5, lines L33, L23', and L24' are shown. The line L23' supplies the condensate water to the first-stage scrubber SCR1. The line L24' supplies the condensate water to the second-stage scrubber SCR2. The condensate water discharged from the first condenser C1 and the condensate water discharged from the second condenser C2 are joined. Then, a portion of the joined condensate water is supplied to the lines L23' and L24' through the line L33.

[0118] In Fig. 5, lines L32 and L27' are shown. The line L27' supplies the condensate water to the recovery device U3. The condensate water discharged from the first condenser C1 and the condensate water discharged from the second condenser C2 are joined. Then, another portion of the joined condensate water is supplied to the line L27' through the line L32.

[0119] Fig. 5 shows an example of configuration in which the condensate water discharged from the first condenser C1 and the condensate water discharged from the second condenser C2 are joined, and then the joined condensate water is supplied to the scrubbers SCR1 and SCR2 and the urea synthesis unit U0. Alternatively, the condensate water discharged from the first condenser C1 and the condensate water discharged from the second condenser C2 may be configured to be separately supplied to the scrubbers SCR1 and SCR2 and the urea synthesis unit U0.

[0120] In Fig. 5, the components indicated by two-dot chain lines are not components of the urea production apparatus 40. For example, the urea production apparatus 40 does not include the waste water treatment unit WWT. In another example, in the urea production apparatus 40, the line supplying the condensate water to the waste water treatment unit WWT is stopped.

[0121] [Correspondence] The correspondence between the description in the embodiment and the description in the section titled "Solution to Problem" is as follows.

[0122] At least one of the first-stage evaporator EVa and the second-stage evaporator EVb corresponds to "first evaporator." In other words, an evaporator unit including the first-stage evaporator EVa and the second-stage evaporator EVb that are connected in series corresponds to the first evaporator. The third-stage evaporator EVc corresponds to "second evaporator."

[0123] <Method for Modifying Urea Production Apparatus> The urea production apparatus 40 may be manufactured as a new apparatus or may be manufactured by modifying the first existing urea production apparatus 100. The method for modifying a urea production apparatus will now be described.

[0124] [Modifying Method] A method for modifying the first existing urea production apparatus 100 shown in Fig. 4 is as follows.

[0125] As shown in Figs. 4 and 5, in the method for modifying a urea production apparatus, the third-stage evaporator EVc is added between the first-stage evaporator EVa and the granulator F. More specifically, the third-stage evaporator EVc is added between the second-stage evaporator EVb and the granulator F. The third-stage evaporator EVc corresponds to an additional evaporator. The additional evaporator further concentrates, for example, a liquid before being supplied to the granulator F to obtain a highly concentrated urea solution having a higher urea concentration than the liquid and an additional off-gas, which contains water and ammonia.

[0126] As shown in Figs. 4 and 5, in the method for modifying the urea production apparatus, the line L31 is added to supply the off-gas discharged from the third-stage evaporator EVc to the first-stage scrubber SCR1. The line L31 corresponds to a discharge pipe supplying an additional off-gas to the scrubber.

[0127] As shown in Figs. 4 and 5, in the method for modifying the urea production apparatus, the line L29' is added to supply the second-stage cleaning-processed recovered solution discharged from the second-stage scrubber SCR2 to the third-stage evaporator EVc. The line L29' corresponds to a recovery pipe supplying the second-stage cleaning-processed recovered solution to the additional evaporator.

[0128] In the method for modifying the urea production apparatus, operations of the urea hydrolyzer UH and the stripping tower ST are stopped in the waste water treatment unit WWT. In the method for modifying the urea production apparatus, the line L33 is added to supply condensate water to the scrubbers. In the method for modifying the urea production apparatus, the line L32 may be added to supply the condensate water to the urea synthesis unit U0.

[0129] <Operation and Advantages> The present embodiment obtains the same advantages as <semantics>(1−1)<annotation encoding="application / x-tex">(1-1)< / annotation>< / semantics> to <semantics>(1−9)<annotation encoding="application / x-tex">(1-9)< / annotation>< / semantics>, <semantics>(1−11)<annotation encoding="application / x-tex">(1-11)< / annotation>< / semantics>, and (1-13) of the first embodiment.

[0130] [Fifth Embodiment] A fifth embodiment of a urea production apparatus 50 will now be described with reference to Fig. 6. In the urea production apparatus 50, same reference characters are given to those components that are the same as the corresponding components in the urea production apparatus 30 of the third embodiment. The description of such components may be omitted.

[0131] In the urea production apparatus 50, the first-stage evaporator EVa and the third- stage evaporator EVc are directly connected to each other in series. In Fig. 6, lines L2" and L3' are shown.

[0132] The concentrated urea solution is discharged from the first-stage evaporator EVa to the line L2". The lines L2" and L29' are joined so that the mixture of the concentrated urea solution and the second-stage cleaning-processed recovered solution is supplied to the third- stage evaporator EVc through the line L3'.

[0133] In Fig. 6, the components indicated by two-dot chained lines are not components of the urea production apparatus 50. For example, the urea production apparatus 50 does not include the second-stage evaporator EVb. In another example, in the urea production apparatus 50, the line connecting the first-stage evaporator EVa and the second-stage evaporator EVb is stopped.

[0134] In an example, the urea production apparatus 50 does not include the second condenser C2. In another example, in the urea production apparatus 50, the second condenser C2 is stopped. In an example, the urea production apparatus 50 does not include the line L25. In another example, the urea production apparatus 50 is connected to a pipe corresponding to the line L25. However, the urea production apparatus 50 is configured not to discharge the waste water from the pipe.

[0135] [Correspondence] The correspondence between the description in the embodiment and the description in the section titled "Solution to Problem" is as follows.

[0136] The first-stage evaporator EVa corresponds to "first evaporator." The third-stage evaporator EVc corresponds to "second evaporator." <Method for Modifying Urea Production Apparatus> The urea production apparatus 50 may be manufactured as a new apparatus or may be manufactured by modifying the first existing urea production apparatus 100. The method for modifying a urea production apparatus will now be described.

[0137] [Modifying Method] A method for modifying the first existing urea production apparatus 100 shown in Fig. 4 is as follows.

[0138] As shown in Figs. 4 and 6, in the method for modifying a urea production apparatus, the third-stage evaporator EVc is added between the first-stage evaporator EVa and the granulator F. More specifically, the second-stage evaporator EVb is replaced with the third-stage evaporator EVc. In addition, the second condenser C2 is removed. The third- stage evaporator EVc corresponds to an additional evaporator further concentrating a liquid before being supplied to the granulator F to obtain a highly concentrated urea solution having a higher urea concentration than the liquid and an additional off-gas, which contains water and ammonia.

[0139] Instead of replacing the second-stage evaporator EVb with the third-stage evaporator EVc, the urea production apparatus 100 may be modified as follows. The third- stage evaporator EVc is added between the first-stage evaporator EVa and the granulator F so as to be connected in series to each of the first-stage evaporator EVa and the granulator F. Then, the second-stage evaporator EVb and the second condenser C2 are stopped.

[0140] As shown in Figs. 4 and 6, in the method for modifying the urea production apparatus, the line L31 is added to supply the off-gas discharged from the third-stage evaporator EVc to the first-stage scrubber SCR1. The line L31 corresponds to a discharge pipe supplying an additional off-gas to the scrubber.

[0141] As shown in Figs. 4 and 6, in the method for modifying the urea production apparatus, the line L29' is added to supply the second-stage cleaning-processed recovered solution discharged from the second-stage scrubber SCR2 to the third-stage evaporator EVc. The line L29' corresponds to a recovery pipe supplying the second-stage cleaning-processed recovered solution to the additional evaporator. The line L29' may be obtained by connecting the line L29 of the first existing urea production apparatus 100 to the third-stage evaporator EVc. Alternatively, the line L29' may be arranged as a pipe differing from the line L29 of the first existing urea production apparatus 100. In this case, the line L29 may be removed. Alternatively, the line L29 may be configured not to discharge the second-stage cleaning- processed recovered solution.

[0142] In the method for modifying the urea production apparatus, operation of the urea hydrolyzer UH is stopped in the waste water treatment unit WWT. When the urea hydrolyzer UH is not operated, condensate water is processed by the stripping tower ST to obtain treated waste water. The treated waste water is supplied to the scrubbers through the lines L23 and L24.

[0143] When the first existing urea production apparatus 100 includes the line L25 configured to discharge the treated waste water, which is obtained in the waste water treatment unit WWT, outside the system of the urea production apparatus 100, the method for modifying the urea production apparatus may be configured not to discharge from the line L25. In an example, it may be configured not to discharge from the line L25 by stopping the use of the line L25. In another example, in the process of modifying the urea production apparatus, the line L25 may be removed.

[0144] <Operation and Advantages> The present embodiment obtains the same advantages as <semantics>(1−1)<annotation encoding="application / x-tex">(1-1)< / annotation>< / semantics> to <semantics>(1−10)<annotation encoding="application / x-tex">(1-10)< / annotation>< / semantics> and <semantics>(1−12)<annotation encoding="application / x-tex">(1-12)< / annotation>< / semantics> of the first embodiment and (2-1) of the second embodiment. Steam is used to operate the urea hydrolyzer UH. In the present embodiment, the urea hydrolyzer UH is not operated. Thus, steam consumption is reduced.

[0145] [Sixth Embodiment] A sixth embodiment of a urea production apparatus 60 will now be described with reference to Fig. 7. In the urea production apparatus 60, same reference characters are given to those components that are the same as the corresponding components in the urea production apparatus 50 of the fifth embodiment. The description of such components may be omitted.

[0146] The urea production apparatus 60 is configured to supply condensate water discharged from the first condenser C1 to the first-stage scrubber SCR1 and the second-stage scrubber SCR2. Also, the urea production apparatus 60 is configured to supply the condensate water discharged from the first condenser C1 to the urea synthesis unit U0.

[0147] An example of the urea production apparatus 60 will now be described. In Fig. 7 lines L33', L23', and L24' are shown. The line L23' supplies the condensate water to the first-stage scrubber SCR1. The line L24' supplies the condensate water to the second-stage scrubber SCR2. A portion of the condensate water discharged from the first condenser C1 is supplied to the lines L23' and L24' through the line L33'.

[0148] In Fig. 7, lines L32' and L27' are shown. The line L27' supplies the condensate water to the recovery device U3. Another portion of the condensate water discharged from the first condenser C1 is supplied to the line L27' through the line L32'.

[0149] In Fig. 7, the components indicated by two-dot chained lines are not components of the urea production apparatus 60. For example, the urea production apparatus 60 does not include the waste water treatment unit WWT. In another example, in the urea production apparatus 60, the line supplying the condensate water to the waste water treatment unit WWT is stopped.

[0150] In addition, for example, the urea production apparatus 60 does not include the second-stage evaporator EVb. In another example, in the urea production apparatus 60, the line connecting the first-stage evaporator EVa and the second-stage evaporator EVb is stopped.

[0151] For example, the urea production apparatus 60 does not include the second condenser C2. In another example, in the urea production apparatus 60, the second condenser C2 is stopped. [Correspondence] The correspondence between the description in the embodiment and the description in the section titled "Solution to Problem" is as follows.

[0152] The first-stage evaporator EVa corresponds to "first evaporator." The third-stage evaporator EVc corresponds to "second evaporator." <Method for Modifying Urea Production Apparatus> The urea production apparatus 60 may be manufactured as a new apparatus or may be manufactured by modifying the first existing urea production apparatus 100. The method for modifying a urea production apparatus will now be described.

[0153] [Modifying Method] A method for modifying the first existing urea production apparatus 100 shown in Fig. 4 is as follows.

[0154] As shown in Figs. 4 and 7, in the method for modifying a urea production apparatus, the third-stage evaporator EVc is added between the first-stage evaporator EVa and the granulator F. More specifically, the second-stage evaporator EVb is replaced with the third-stage evaporator EVc. In addition, the second condenser C2 is removed. The third- stage evaporator EVc corresponds to an additional evaporator further concentrating a liquid before being supplied to the granulator F to obtain a highly concentrated urea solution having a higher urea concentration than the liquid and an additional off-gas, which contains water and ammonia.

[0155] Instead of replacing the second-stage evaporator EVb with the third-stage evaporator EVc, the urea production apparatus 100 may be modified as follows. The third- stage evaporator EVc is added between the first-stage evaporator EVa and the granulator F so as to be connected in series to each of the first-stage evaporator EVa and the granulator F. Then, the second-stage evaporator EVb and the second condenser C2 are stopped.

[0156] As shown in Figs. 4 and 7, in the method for modifying the urea production apparatus, the line L31 is added to supply the off-gas discharged from the third-stage evaporator EVc to the first-stage scrubber SCR1. The line L31 corresponds to a discharge pipe supplying an additional off-gas to the scrubber.

[0157] As shown in Figs. 4 and 7, in the method for modifying the urea production apparatus, the line L29' is added to supply the second-stage cleaning-processed recovered solution discharged from the second-stage scrubber SCR2 to the third-stage evaporator EVc. The line L29' corresponds to a recovery pipe supplying the second-stage cleaning-processed recovered solution to the additional evaporator. The line L29' may be obtained by connecting the line L29 of the first existing urea production apparatus 100 to the third-stage evaporator EVc. Alternatively, the line L29' may be arranged as a pipe differing from the line L29 of the first existing urea production apparatus 100. In this case, the line L29 may be removed. Alternatively, the line L29 may be configured not to discharge the second-stage cleaning- processed recovered solution.

[0158] In the method for modifying the urea production apparatus, operations of the urea hydrolyzer UH and the stripping tower ST are stopped in the waste water treatment unit WWT. In the method for modifying the urea production apparatus, the line L33' is added to supply condensate water to the scrubbers. In the method for modifying the urea production apparatus, the line L32' may be added to supply the condensate water to the urea synthesis unit U0.

[0159] <Operation and Advantages> The present embodiment obtains the same advantages as (1-1) to (1-13) of the first embodiment.

[0160] [Seventh Embodiment] A seventh embodiment of a urea production apparatus 207 will now be described with reference to Fig. 8. In the urea production apparatus 207, differences from the urea production apparatus 50 of the fifth embodiment will be described. In the urea production apparatus 207, same reference characters are given to those components that are the same as the corresponding components in the urea production apparatus 50 of the fifth embodiment. The description of such components may be omitted.

[0161] The urea production apparatus 207 includes a scrubber SCR. The scrubber SCR of the urea production apparatus 207 may have the same configuration as the scrubber SCR of the urea production apparatus 10 in the first embodiment. More specifically, the scrubber SCR of the urea production apparatus 207 is a single-stage scrubber that uses an acid cleaning solution containing acid.

[0162] The urea production apparatus 207 connects a first evaporator EVa and a second evaporator EVc in series. In Fig. 8 lines L2*, L3*, and L16* are shown.

[0163] The concentrated urea solution is discharged from the first evaporator EVa to the line L2*. The lines L2* and L16* are joined so that the mixture of the concentrated urea solution and the cleaning-processed recovered solution is supplied to the second evaporator EVc through the line L3*.

[0164] In Fig. 8, the components indicated by two-dot chain lines are not components of the urea production apparatus 207. For example, the urea production apparatus 207 does not include line L28* supplying the cleaning-processed recovered solution obtained from the scrubber SCR to the urea synthesis unit U0. In another example, the urea production apparatus 207 is connected to a pipe corresponding to the line L28*. However, the urea production apparatus 207 is configured not to supply the cleaning-processed recovered solution from the pipe to the urea synthesis unit U0.

[0165] In an example, the urea production apparatus 207 does not include the line L25. In another example, the urea production apparatus 207 is connected to a pipe corresponding to the line L25. However, the urea production apparatus 207 is configured not to discharge the waste water from the pipe.

[0166] [Correspondence] The correspondence between the description in the embodiment and the description in the section titled "Solution to Problem" is as follows.

[0167] The first evaporator EVa corresponds to "first evaporator." The second evaporator EVc corresponds to "second evaporator." <Method for Modifying Urea Production Apparatus> The urea production apparatus 207 may be manufactured as a new apparatus or may be manufactured by modifying an existing urea production apparatus. The method for modifying a urea production apparatus will now be described.

[0168] [Example of Second Existing Urea Production Apparatus] An example of a second existing urea production apparatus will now be described. Fig. 9 shows a second existing urea production apparatus 110. In the second existing urea production apparatus 110, the description of components that are the same as the components of the first existing urea production apparatus 100 shown in Fig. 4 may be omitted.

[0169] The second existing urea production apparatus 110 includes a first evaporator EVa configured to concentrate a urea solution supplied from the line L1. The first evaporator EVa is configured to supply, to the granulator F, a liquid having a higher urea concentration than the urea solution supplied from the line L1.

[0170] In the second existing urea production apparatus 110, line L23* corresponds to a supply pipe supplying treated waste water obtained in the waste water treatment unit to a scrubber. The second existing urea production apparatus 110 includes a water cleaning scrubber SCR*. The water cleaning scrubber SCR* is configured to contact an off-gas discharged from the granulator F with the cleaning solution to obtain a clean gas and a cleaning-processed recovered solution. The water cleaning scrubber SCR* differs from the scrubber SCR in that an acid is not supplied to the cleaning solution. The clean gas discharged from the water cleaning scrubber SCR* through the line L15* has a higher ammonia concentration than the clean gas discharged from the scrubber SCR through the line L15. The cleaning-processed recovered solution obtained from the water cleaning scrubber SCR* has a lower ammonia concentration than the cleaning-processed recovered solution obtained from the scrubber SCR.

[0171] [Modifying Method] A method for modifying the second existing urea production apparatus 110 shown in Fig. 9 is as follows.

[0172] As shown in Figs. 8 and 9, in the method for modifying a urea production apparatus, the second evaporator EVc is added between the first evaporator EVa and the granulator F. The second evaporator EVc corresponds to an additional evaporator further concentrating a liquid before being supplied to the granulator F to obtain the highly concentrated urea solution having a higher urea concentration than the liquid and an additional off-gas, which contains water and ammonia.

[0173] As shown in Figs. 8 and 9, in the method for modifying a urea production apparatus, the water cleaning scrubber SCR* is replaced with the scrubber SCR. Alternatively, an acid supply pipe L14 supplying an acid to a scrubber is added so that the water cleaning scrubber SCR* serves as the scrubber SCR. The scrubber SCR may be configured to receive water from outside the system.

[0174] As shown in Figs. 8 and 9, in the method for modifying the urea production apparatus, the line L31 is added to supply an off-gas discharged from the second evaporator EVc to the scrubber SCR. The line L31 corresponds to a discharge pipe supplying an additional off-gas to the scrubber.

[0175] As shown in Figs. 8 and 9, in the method for modifying the urea production apparatus, the supply of the cleaning-processed recovered solution discharged from the scrubber SCR to the urea synthesis unit U0 is stopped. In an example, the line L28* may be removed. Alternatively, the line L28* may be configured not to discharge the cleaning- processed recovered solution.

[0176] As shown in Figs. 8 and 9, in the method for modifying the urea production apparatus, the line L16* is added to supply the cleaning-processed recovered solution discharged from the scrubber SCR to the second evaporator EVc. The line L16* corresponds to a recovery pipe supplying the cleaning-processed recovered solution to the additional evaporator. The line L16* may be obtained by connecting the line L28* of the second existing urea production apparatus 110 to the second evaporator EVc. The line L16* may be arranged as a pipe differing from the line L28* of the second existing urea production apparatus 110.

[0177] In the method for modifying the urea production apparatus, operation of the urea hydrolyzer UH is stopped in the waste water treatment unit WWT. When the urea hydrolyzer UH is not operated, condensate water is processed by the stripping tower ST to obtain treated waste water. The treated waste water is supplied to the scrubber through the line L23.

[0178] When the second existing urea production apparatus 110 includes the line L25 configured to discharge the treated waste water, which is obtained in the waste water treatment unit WWT, outside the system of the urea production apparatus 110, the method for modifying the urea production apparatus may be configured not to discharge from the line L25. In an example, it may be configured not to discharge from the line L25 by stopping the use of the line L25. In another example, in the process of modifying the urea production apparatus, the line L25 may be removed.

[0179] <Operation and Advantages> The present embodiment obtains the same advantages as <semantics>(1−1)<annotation encoding="application / x-tex">(1-1)< / annotation>< / semantics> to <semantics>(1−10)<annotation encoding="application / x-tex">(1-10)< / annotation>< / semantics> and <semantics>(1−12)<annotation encoding="application / x-tex">(1-12)< / annotation>< / semantics> of the first embodiment and (2-1) of the second embodiment.

[0180] In the modifying method, the water cleaning scrubber SCR* is replaced with the scrubber SCR. Thus, the concentration of ammonia in the clean gas is decreased. When the water cleaning scrubber SCR* is replaced with the scrubber SCR, the concentration of ammonia in the cleaning-processed recovered solution is increased. This increases the efficiency of recovering ammonia while limiting the discharging of ammonia outside the system.

[0181] [Eighth Embodiment] An eighth embodiment of a urea production apparatus 208 will now be described with reference to Fig. 10. In the urea production apparatus 208, differences from the urea production apparatus 60 of the sixth embodiment will be described. In the urea production apparatus 208, same reference characters are given to those components that are the same as the corresponding components in the urea production apparatus 60 of the sixth embodiment. The description of such components may be omitted.

[0182] The urea production apparatus 208 includes a scrubber SCR. The scrubber SCR is a single-stage scrubber that uses an acid cleaning solution containing acid. The urea production apparatus 208 connects the first evaporator EVa and the second evaporator EVc in series.

[0183] In Fig. 10, the lines L2*, L3*, and L16* are shown. The concentrated urea solution is discharged from the first evaporator EVa to the line L2*. The lines L2* and L16* are joined so that the mixture of the concentrated urea solution and the cleaning-processed recovered solution is supplied to the second evaporator EVc through the line L3*.

[0184] In Fig. 10, the components indicated by two-dot chain lines are not components of the urea production apparatus 208. For example, the urea production apparatus 208 does not include line L28* supplying the cleaning-processed recovered solution obtained from the scrubber SCR to the urea synthesis unit U0. In another example, the urea production apparatus 208 is connected to a pipe corresponding to the line L28*. However, the urea production apparatus 208 is configured not to supply the cleaning-processed recovered solution from the pipe to the urea synthesis unit U0.

[0185] For example, the urea production apparatus 208 does not include the waste water treatment unit WWT. In another example, in the urea production apparatus 208, the line supplying the condensate water to the waste water treatment unit WWT is stopped.

[0186] [Correspondence] The correspondence between the description in the embodiment and the description in the section titled "Solution to Problem" is as follows.

[0187] The first evaporator EVa corresponds to "first evaporator." The second evaporator EVc corresponds to "second evaporator." <Method for Modifying Urea Production Apparatus> The urea production apparatus 208 may be manufactured as a new apparatus or may be manufactured by modifying an existing urea production apparatus. The method for modifying a urea production apparatus will now be described.

[0188] [Modifying Method] A method for modifying the second existing urea production apparatus 110 shown in Fig. 9 is as follows.

[0189] As shown in Figs. 9 and 10, in the method for modifying a urea production apparatus, the second evaporator EVc is added between the first evaporator EVa and the granulator F. The second evaporator EVc corresponds to an additional evaporator further concentrating a liquid before being supplied to the granulator F to obtain the highly concentrated urea solution having a higher urea concentration than the liquid and an additional off-gas, which contains water and ammonia.

[0190] As shown in Figs. 9 and 10, in the method for modifying a urea production apparatus, the water cleaning scrubber SCR* is replaced with the scrubber SCR. Alternatively, an acid supply pipe L14 supplying an acid to a scrubber is added so that the water cleaning scrubber SCR* serves as the scrubber SCR.

[0191] As shown in Figs. 9 and 10, in the method for modifying the urea production apparatus, the line L31 is added to supply an off-gas discharged from the second evaporator EVc to the scrubber SCR. The line L31 corresponds to a discharge pipe supplying an additional off-gas to the scrubber.

[0192] As shown in Figs. 9 and 10, in the method for modifying the urea production apparatus, the supply of the cleaning-processed recovered solution discharged from the scrubber SCR to the urea synthesis unit U0 is stopped. In an example, the line L28* may be removed. Alternatively, the line L28* may be configured not to discharge the cleaning- processed recovered solution.

[0193] As shown in Figs. 9 and 10, in the method for modifying the urea production apparatus, the line L16* is added to supply the cleaning-processed recovered solution discharged from the scrubber SCR to the second evaporator EVc. The line L16* corresponds to a recovery pipe supplying the cleaning-processed recovered solution to the additional evaporator. The line L16* may be obtained by connecting the line L28* of the second existing urea production apparatus 110 to the second evaporator EVc. The line L16* may be arranged as a pipe differing from the line L28* of the second existing urea production apparatus 110.

[0194] In the method for modifying the urea production apparatus, operations of the urea hydrolyzer UH and the stripping tower ST are stopped in the waste water treatment unit WWT. In the method for modifying the urea production apparatus, the line L33' is added to supply condensate water to the scrubbers. In the method for modifying the urea production apparatus, the line L32' may be added to supply the condensate water to the urea synthesis unit U0.

[0195] [Ninth Embodiment] A ninth embodiment of a urea production apparatus 209 will now be described with reference to Fig. 11. In the urea production apparatus 209, differences from the urea production apparatus 50 of the fifth embodiment will be described. In the urea production apparatus 209, same reference characters are given to those components that are the same as the corresponding components in the urea production apparatus 50 of the fifth embodiment. The description of such components may be omitted.

[0196] The urea production apparatus 209 connects the first evaporator EVa and the second evaporator EVc in series. In Fig. 11, the lines L2* and L3* are shown.

[0197] The concentrated urea solution is discharged from the first evaporator EVa to the line L2*. The lines L2* and L29' are joined so that the mixture of the concentrated urea solution and the cleaning-processed recovered solution is supplied to the second evaporator EVc through the line L3*.

[0198] In Fig. 11, the components indicated by two-dot chain lines are not components of the urea production apparatus 209. For example, the urea production apparatus 209 does not include the line L25. In another example, the urea production apparatus 209 is connected to a pipe corresponding to the line L25. However, the urea production apparatus 209 is configured not to discharge the waste water from the pipe.

[0199] [Correspondence] The correspondence between the description in the embodiment and the description in the section titled "Solution to Problem" is as follows.

[0200] The first evaporator EVa corresponds to "first evaporator." The second evaporator EVc corresponds to "second evaporator." <Method for Modifying Urea Production Apparatus> The urea production apparatus 209 may be manufactured as a new apparatus or may be manufactured by modifying an existing urea production apparatus. The method for modifying a urea production apparatus will now be described.

[0201] [Example of Third Existing Urea Production Apparatus] An example of a third existing urea production apparatus will now be described. Fig. 12 shows a third existing urea production apparatus 120. In the third existing urea production apparatus 120, the description of components that are the same as the components of the first existing urea production apparatus 100 shown in Fig. 4may be omitted.

[0202] The third existing urea production apparatus 120 includes the first evaporator EVa configured to concentrates a urea solution supplied from the line L1. The first evaporator EVa is configured to supply, to the granulator F, a liquid having a higher urea concentration than the urea solution supplied from the line L1.

[0203] [Modifying Method] A method for modifying the third existing urea production apparatus 120 shown in Fig. 12 is as follows.

[0204] As shown in Figs. 11 and 12, in the method for modifying a urea production apparatus, the second evaporator EVc is added between the first evaporator EVa and the granulator F. The second evaporator EVc corresponds to an additional evaporator further concentrating a liquid before being supplied to the granulator F to obtain the highly concentrated urea solution having a higher urea concentration than the liquid and an additional off-gas, which contains water and ammonia.

[0205] As shown in Figs. 11 and 12, in the method for modifying the urea production apparatus, the line L31 is added to supply an off-gas discharged from the second evaporator EVc to the first-stage scrubber SCR1. The line L31 corresponds to a discharge pipe supplying an additional off-gas to the scrubber.

[0206] As shown in Figs. 11 and 12, in the method for modifying the urea production apparatus, the line L29' is added to supply the second-stage cleaning-processed recovered solution discharged from the second-stage scrubber SCR2 to the second-stage evaporator EVc. The line L29' corresponds to a recovery pipe supplying the second-stage cleaning- processed recovered solution to the additional evaporator. The line L29' may be obtained by connecting the line L29 of the third existing urea production apparatus 120 to the second evaporator EVc. The line L29' may be arranged as a pipe differing from the line L29 of the third existing urea production apparatus 120. In this case, the line L29 may be removed. Alternatively, the line L29 may be configured not to discharge the second-stage cleaning- processed recovered solution.

[0207] In the method for modifying the urea production apparatus, operation of the urea hydrolyzer UH is stopped in the waste water treatment unit WWT. When the urea hydrolyzer UH is not operated, condensate water is processed by the stripping tower ST to obtain treated waste water. The treated waste water is supplied to a scrubber through the lines L23 and L24.

[0208] When the third existing urea production apparatus 120 includes the line L25 configured to discharge the treated waste water, which is obtained in the waste water treatment unit WWT, outside the system of the urea production apparatus 120, the method for modifying the urea production apparatus may be configured not to discharge from the line L25. In an example, it may be configured not to discharge from the line L25 by stopping the use of the line L25. In another example, in the process of modifying the urea production apparatus, the line L25 may be removed.

[0209] [Tenth Embodiment] A tenth embodiment of a urea production apparatus 210 will now be described with reference to Fig. 13. In the urea production apparatus 210, differences from the urea production apparatus 60 of the sixth embodiment will be described. In the urea production apparatus 210, same reference characters are given to those components that are the same as the corresponding components in the urea production apparatus 60 of the sixth embodiment. The description of such components may be omitted.

[0210] The urea production apparatus 210 connects the first evaporator EVa and the second evaporator EVc in series. In Fig. 13, the lines L2* and L3* are shown.

[0211] The concentrated urea solution is discharged from the first evaporator EVa to the line L2*. The lines L2* and L29' are joined so that the mixture of the concentrated urea solution and the cleaning-processed recovered solution is supplied to the second evaporator EVc through the line L3*.

[0212] In Fig. 13, the components indicated by two-dot chain lines are not components of the urea production apparatus 210. For example, the urea production apparatus 210 does not include the waste water treatment unit WWT. In another example, in the urea production apparatus 210, the line supplying the condensate water to the waste water treatment unit WWT is stopped.

[0213] [Correspondence] The correspondence between the description in the embodiment and the description in the section titled "Solution to Problem" is as follows.

[0214] The first evaporator EVa corresponds to "first evaporator." The second evaporator EVc corresponds to "second evaporator." <Method for Modifying Urea Production Apparatus> The urea production apparatus 210 may be manufactured as a new apparatus or may be manufactured by modifying an existing urea production apparatus. The method for modifying a urea production apparatus will now be described.

[0215] [Modifying Method] A method for modifying the third existing urea production apparatus 120 shown in Fig. 12 is as follows.

[0216] As shown in Figs. 12 and 13, in the method for modifying a urea production apparatus, the second evaporator EVc is added between the first evaporator EVa and the granulator F. The second evaporator EVc corresponds to an additional evaporator further concentrating a liquid before being supplied to the granulator F to obtain the highly concentrated urea solution having a higher urea concentration than the liquid and an additional off-gas, which contains water and ammonia.

[0217] As shown in Figs. 12 and 13, in the method for modifying the urea production apparatus, the line L31 is added to supply an off-gas discharged from the second evaporator EVc to the first-stage scrubber SCR1. The line L31 corresponds to a discharge pipe supplying an additional off-gas to the scrubber.

[0218] As shown in Figs. 12 and 13, in the method for modifying the urea production apparatus, the line L29' is added to supply the second-stage cleaning-processed recovered solution discharged from the second-stage scrubber SCR2 to the second-stage evaporator EVc. The line L29' corresponds to a recovery pipe supplying the second-stage cleaning- processed recovered solution to the additional evaporator. The line L29' may be obtained by connecting the line L29 of the third existing urea production apparatus 120 to the second evaporator EVc. The line L29' may be arranged as a pipe differing from the line L29 of the third existing urea production apparatus 120. In this case, the line L29 may be removed. Alternatively, the line L29 may be configured not to discharge the second-stage cleaning- processed recovered solution.

[0219] In the method for modifying the urea production apparatus, operations of the urea hydrolyzer UH and the stripping tower ST are stopped in the waste water treatment unit WWT. In the method for modifying the urea production apparatus, the line L33' is added to supply condensate water to the scrubbers. In the method for modifying the urea production apparatus, the line L32' may be added to supply the condensate water to the urea synthesis unit U0.

[0220] [Eleventh Embodiment] An eleventh embodiment of a urea production apparatus 211 will now be described with reference to Fig. 14. In the urea production apparatus 211, differences from the urea production apparatus 30 of the third embodiment will be described. In the urea production apparatus 211, same reference characters are given to those components that are the same as the corresponding components in the urea production apparatus 30 of the third embodiment. The description of such components may be omitted.

[0221] The urea production apparatus 211 includes a scrubber SCR. More specifically, the scrubber SCR of the urea production apparatus 211 is a single-stage scrubber that uses an acid cleaning solution containing acid.

[0222] The urea production apparatus 211 includes the line L16* supplying the cleaning- processed recovered solution discharged from the scrubber SCR to the third-stage evaporator EVc. In Fig. 14, the components indicated by two-dot chain lines are not components of the urea production apparatus 211.

[0223] For example, the urea production apparatus 211 does not include line L28 supplying the cleaning-processed recovered solution obtained from the scrubber SCR to the urea synthesis unit U0. In another example, the urea production apparatus 211 is connected to a pipe corresponding to the line L28. However, the urea production apparatus 211 is configured not to supply the cleaning-processed recovered solution from the pipe to the urea synthesis unit U0.

[0224] In an example, the urea production apparatus 211 does not include the line L25. In another example, the urea production apparatus 211 is connected to a pipe corresponding to the line L25. However, the urea production apparatus 211 is configured not to discharge the waste water from the pipe.

[0225] <Method for Modifying Urea Production Apparatus> The urea production apparatus 211 may be manufactured as a new apparatus or may be manufactured by modifying an existing urea production apparatus. The method for modifying a urea production apparatus will now be described.

[0226] [Example of Fourth Existing Urea Production Apparatus] An example of a fourth existing urea production apparatus will now be described. Fig. 15 shows a fourth existing urea production apparatus 130. In the fourth existing urea production apparatus 130, the description of components that are the same as the components of the first existing urea production apparatus 100 shown in Fig. 4 may be omitted.

[0227] The fourth existing urea production apparatus 130 includes a water cleaning scrubber SCR*. The water cleaning scrubber SCR* is configured to contact an off-gas discharged from the granulator F with the cleaning solution to obtain a clean gas and a cleaning-processed recovered solution.

[0228] [Modifying Method] A method for modifying the fourth existing urea production apparatus 130 shown in Fig. 15 is as follows.

[0229] As shown in Figs. 14 and 15, in the method for modifying a urea production apparatus, the third-stage evaporator EVc is added between the first-stage evaporator EVa and the granulator F. More specifically, the third-stage evaporator EVc is added between the second-stage evaporator EVb and the granulator F. The third-stage evaporator EVc corresponds to an additional evaporator. The additional evaporator further concentrates, for example, a liquid before being supplied to the granulator F to obtain a highly concentrated urea solution having a higher urea concentration than the liquid and an additional off-gas, which contains water and ammonia.

[0230] As shown in Figs. 14 and 15, in the method for modifying a urea production apparatus, the water cleaning scrubber SCR* is replaced with the scrubber SCR. Alternatively, an acid supply pipe L14 supplying an acid to a scrubber is added so that the water cleaning scrubber SCR* serves as the scrubber SCR.

[0231] As shown in Figs. 14 and 15, in the method for modifying the urea production apparatus, the line L31 is added to supply an off-gas discharged from the third-stage evaporator EVc to the scrubber SCR. The line L31 corresponds to a discharge pipe supplying an additional off-gas to the scrubber.

[0232] As shown in Figs. 14 and 15, in the method for modifying the urea production apparatus, the supply of the cleaning-processed recovered solution discharged from the scrubber SCR to the urea synthesis unit U0 is stopped. In an example, the line L28* may be removed. Alternatively, the line L28* may be configured not to discharge the cleaning- processed recovered solution.

[0233] As shown in Figs. 14 and 15, in the method for modifying the urea production apparatus, the line L16* is added to supply the cleaning-processed recovered solution discharged from the scrubber SCR to the third-stage evaporator EVc. The line L16* corresponds to a recovery pipe supplying the cleaning-processed recovered solution to the additional evaporator. The line L16* may be obtained by connecting the line L28* of the fourth existing urea production apparatus 130 to the third-stage evaporator EVc. The line L16* may be arranged as a pipe differing from the line L28* of the fourth existing urea production apparatus 130.

[0234] In the method for modifying the urea production apparatus, operation of the urea hydrolyzer UH is stopped in the waste water treatment unit WWT. When the urea hydrolyzer UH is not operated, condensate water is processed by the stripping tower ST to obtain treated waste water. The treated waste water is supplied to the scrubber through the line L23.

[0235] When the fourth existing urea production apparatus 130 includes the line L25 configured to discharge the treated waste water, which is obtained in the waste water treatment unit WWT, outside the system of the urea production apparatus 130, the method for modifying the urea production apparatus may be configured not to discharge from the line L25. In an example, it may be configured not to discharge from the line L25 by stopping the use of the line L25. In the process of modifying the urea production apparatus, for example, the line L25 may be removed.

[0236] [Twelfth Embodiment] A twelfth embodiment of a urea production apparatus 212 will now be described with reference to Fig. 16. In the urea production apparatus 212, differences from the urea production apparatus 40 of the fourth embodiment will be described. In the urea production apparatus 212, same reference characters are given to those components that are the same as the corresponding components in the urea production apparatus 40 of the fourth embodiment. The description of such components may be omitted.

[0237] The urea production apparatus 212 includes a scrubber SCR. More specifically, the scrubber SCR of the urea production apparatus 212 is a single-stage scrubber that uses an acid cleaning solution containing acid.

[0238] The urea production apparatus 212 includes the line L16* supplying the cleaning- processed recovered solution discharged from the scrubber SCR to the third-stage evaporator EVc. In Fig. 16, the components indicated by two-dot chain lines are not components of the urea production apparatus 212.

[0239] <Method for Modifying Urea Production Apparatus> The urea production apparatus 212 may be manufactured as a new apparatus or may be manufactured by modifying an existing urea production apparatus. The method for modifying a urea production apparatus will now be described.

[0240] [Modifying Method] A method for modifying the fourth existing urea production apparatus 130 shown in Fig. 15 is as follows.

[0241] As shown in Figs. 15 and 16, in the method for modifying a urea production apparatus, the third-stage evaporator EVc is added between the first-stage evaporator EVa and the granulator F. More specifically, the third-stage evaporator EVc is added between the second-stage evaporator EVb and the granulator F. The third-stage evaporator EVc corresponds to an additional evaporator. The additional evaporator further concentrates, for example, a liquid before being supplied to the granulator F to obtain a highly concentrated urea solution having a higher urea concentration than the liquid and an additional off-gas, which contains water and ammonia.

[0242] As shown in Figs. 15 and 16, in the method for modifying a urea production apparatus, the water cleaning scrubber SCR* is replaced with the scrubber SCR. Alternatively, an acid supply pipe L14 supplying an acid to a scrubber is added so that the water cleaning scrubber SCR* serves as the scrubber SCR.

[0243] As shown in Figs. 15 and 16, in the method for modifying the urea production apparatus, the line L31 is added to supply an off-gas discharged from the second evaporator EVc to the scrubber SCR. The line L31 corresponds to a discharge pipe supplying an additional off-gas to the scrubber.

[0244] As shown in Figs. 15 and 16, in the method for modifying the urea production apparatus, the supply of the cleaning-processed recovered solution discharged from the scrubber SCR to the urea synthesis unit U0 is stopped. In an example, the line L28* may be removed. Alternatively, the line L28* may be configured not to discharge the cleaning- processed recovered solution.

[0245] As shown in Figs. 15 and 16, in the method for modifying the urea production apparatus, the line L16* is added to supply the cleaning-processed recovered solution discharged from the scrubber SCR to the third-stage evaporator EVc. The line L16* corresponds to a recovery pipe supplying the cleaning-processed recovered solution to the additional evaporator. The line L16* may be obtained by connecting the line L28* of the fourth existing urea production apparatus 130 to the third-stage evaporator EVc. The line L16* may be arranged as a pipe differing from the line L28* of the fourth existing urea production apparatus 130.

[0246] In the method for modifying the urea production apparatus, operations of the urea hydrolyzer UH and the stripping tower ST are stopped in the waste water treatment unit WWT. In the method for modifying the urea production apparatus, the line L33 is added to supply condensate water to the scrubbers. In the method for modifying the urea production apparatus, the line L32 may be added to supply the condensate water to the urea synthesis unit U0.

[0247] [Thirteenth Embodiment] A thirteenth embodiment of a urea production apparatus 213 will now be described with reference to Fig. 17. In the urea production apparatus 213, differences from the urea production apparatus 50 of the fifth embodiment will be described. In the urea production apparatus 213, same reference characters are given to those components that are the same as the corresponding components in the urea production apparatus 50 of the fifth embodiment. The description of such components may be omitted.

[0248] The urea production apparatus 213 includes a scrubber SCR. More specifically, the scrubber SCR of the urea production apparatus 213 is a single-stage scrubber that uses an acid cleaning solution containing acid.

[0249] The urea production apparatus 213 includes the line L16* supplying the cleaning- processed recovered solution discharged from the scrubber SCR to the third-stage evaporator EVc. In Fig. 17, the components indicated by two-dot chain lines are not components of the urea production apparatus 213.

[0250] <Method for Modifying Urea Production Apparatus> The urea production apparatus 213 may be manufactured as a new apparatus or may be manufactured by modifying an existing urea production apparatus. The method for modifying a urea production apparatus will now be described.

[0251] [Modifying Method] A method for modifying the fourth existing urea production apparatus 130 shown in Fig. 15 is as follows.

[0252] As shown in Figs. 15 and 17, in the method for modifying a urea production apparatus, the third-stage evaporator EVc is added between the first-stage evaporator EVa and the granulator F. More specifically, the second-stage evaporator EVb is replaced with the third-stage evaporator EVc. In addition, the second condenser C2 is removed. The third- stage evaporator EVc corresponds to an additional evaporator further concentrating a liquid before being supplied to the granulator F to obtain a highly concentrated urea solution having a higher urea concentration than the liquid and an additional off-gas, which contains water and ammonia.

[0253] Instead of replacing the second-stage evaporator EVb with the third-stage evaporator EVc, the urea production apparatus 130 may be modified as follows. The third- stage evaporator EVc is added between the first-stage evaporator EVa and the granulator F so as to be connected in series to each of the first-stage evaporator EVa and the granulator F. Then, the second-stage evaporator EVb and the second condenser C2 are stopped.

[0254] As shown in Figs. 15 and 17, in the method for modifying a urea production apparatus, the water cleaning scrubber SCR* is replaced with the scrubber SCR. Alternatively, an acid supply pipe L14 supplying an acid to a scrubber is added so that the water cleaning scrubber SCR* serves as the scrubber SCR.

[0255] As shown in Figs. 15 and 17, in the method for modifying the urea production apparatus, the line L31 is added to supply an off-gas discharged from the third-stage evaporator EVc to the scrubber SCR. The line L31 corresponds to a discharge pipe supplying an additional off-gas to the scrubber.

[0256] As shown in Figs. 15 and 17, in the method for modifying the urea production apparatus, the supply of the cleaning-processed recovered solution discharged from the scrubber SCR to the urea synthesis unit U0 is stopped. In an example, the line L28* may be removed. Alternatively, the line L28* may be configured not to discharge the cleaning- processed recovered solution.

[0257] As shown in Figs. 15 and 17, in the method for modifying the urea production apparatus, the line L16* is added to supply the cleaning-processed recovered solution discharged from the scrubber SCR to the third-stage evaporator EVc. The line L16* corresponds to a recovery pipe supplying the cleaning-processed recovered solution to the additional evaporator. The line L16* may be obtained by connecting the line L28* of the fourth existing urea production apparatus 130 to the third-stage evaporator EVc. The line L16* may be arranged as a pipe differing from the line L28* of the fourth existing urea production apparatus 130.

[0258] In the method for modifying the urea production apparatus, operation of the urea hydrolyzer UH is stopped in the waste water treatment unit WWT. When the urea hydrolyzer UH is not operated, condensate water is processed by the stripping tower ST to obtain treated waste water. The treated waste water is supplied to the scrubber through the line L23.

[0259] When the fourth existing urea production apparatus 130 includes the line L25 configured to discharge the treated waste water, which is obtained in the waste water treatment unit WWT, outside the system of the urea production apparatus 130, the method for modifying the urea production apparatus may be configured not to discharge from the line L25. In an example, it may be configured not to discharge from the line L25 by stopping the use of the line L25. In the process of modifying the urea production apparatus, for example, the line L25 may be removed.

[0260] [Fourteenth Embodiment] A fourteenth embodiment of a urea production apparatus 214 will now be described with reference to Fig. 18. In the urea production apparatus 214, differences from the urea production apparatus 60 of the sixth embodiment will be described. In the urea production apparatus 214, same reference characters are given to those components that are the same as the corresponding components in the urea production apparatus 60 of the sixth embodiment. The description of such components may be omitted.

[0261] The urea production apparatus 214 includes a scrubber SCR. More specifically, the scrubber SCR of the urea production apparatus 214 is a single-stage scrubber that uses an acid cleaning solution containing acid.

[0262] The urea production apparatus 214 includes the line L16* supplying the cleaning- processed recovered solution discharged from the scrubber SCR to the third-stage evaporator EVc. In Fig. 18, the components indicated by two-dot chain lines are not components of the urea production apparatus 214.

[0263] <Method for Modifying Urea Production Apparatus> The urea production apparatus 214 may be manufactured as a new apparatus or may be manufactured by modifying an existing urea production apparatus. The method for modifying a urea production apparatus will now be described.

[0264] [Modifying Method] A method for modifying the fourth existing urea production apparatus 130 shown in Fig. 15 is as follows.

[0265] As shown in Figs. 15 and 18, in the method for modifying a urea production apparatus, the third-stage evaporator EVc is added between the first-stage evaporator EVa and the granulator F. More specifically, the second-stage evaporator EVb is replaced with the third-stage evaporator EVc. In addition, the second condenser C2 is removed. The third- stage evaporator EVc corresponds to an additional evaporator further concentrating a liquid before being supplied to the granulator F to obtain a highly concentrated urea solution having a higher urea concentration than the liquid and an additional off-gas, which contains water and ammonia.

[0266] Instead of replacing the second-stage evaporator EVb with the third-stage evaporator EVc, the urea production apparatus 130 may be modified as follows. The third- stage evaporator EVc is added between the first-stage evaporator EVa and the granulator F so as to be connected in series to each of the first-stage evaporator EVa and the granulator F. Then, the second-stage evaporator EVb and the second condenser C2 are stopped.

[0267] As shown in Figs. 15 and 18, in the method for modifying a urea production apparatus, the water cleaning scrubber SCR* is replaced with the scrubber SCR. Alternatively, an acid supply pipe L14 supplying an acid to a scrubber is added so that the water cleaning scrubber SCR* serves as the scrubber SCR.

[0268] As shown in Figs. 15 and 18, in the method for modifying the urea production apparatus, the line L31 is added to supply an off-gas discharged from the third-stage evaporator EVc to the scrubber SCR. The line L31 corresponds to a discharge pipe supplying an additional off-gas to the scrubber.

[0269] As shown in Figs. 15 and 18, in the method for modifying the urea production apparatus, the supply of the cleaning-processed recovered solution discharged from the scrubber SCR to the urea synthesis unit U0 is stopped. In an example, the line L28* may be removed. Alternatively, the line L28* may be configured not to discharge the cleaning- processed recovered solution.

[0270] As shown in Figs. 15 and 18, in the method for modifying the urea production apparatus, the line L16* is added to supply the cleaning-processed recovered solution discharged from the scrubber SCR to the third-stage evaporator EVc. The line L16* corresponds to a recovery pipe supplying the cleaning-processed recovered solution to the additional evaporator. The line L16* may be obtained by connecting the line L28* of the fourth existing urea production apparatus 130 to the third-stage evaporator EVc. The line L16* may be arranged as a pipe differing from the line L28* of the fourth existing urea production apparatus 130.

[0271] In the method for modifying the urea production apparatus, operations of the urea hydrolyzer UH and the stripping tower ST are stopped in the waste water treatment unit WWT. In the method for modifying the urea production apparatus, the line L33' is added to supply condensate water to the scrubbers. In the method for modifying the urea production apparatus, the line L32' may be added to supply the condensate water to the urea synthesis unit U0.

[0272] (Modified Examples) The embodiments described above may be modified as follows. The above embodiments and the following modified examples may be implemented in combination with each other as long as technical contradiction does not occur.

[0273] -In the third embodiment, the operation of the urea hydrolyzer UH is stopped in the waste water treatment unit WWT. Alternatively, the operations of the urea hydrolyzer UH and the stripping tower ST may be stopped in the waste water treatment unit WWT. Further, the waste water treatment unit WWT in which the operations of the urea hydrolyzer UH and the stripping tower ST are stopped may be configured to supply condensate water from the condensers C1 and C2 to the scrubbers SCR1 and SCR2. With this configuration, an off-gas is not discharged from the waste water treatment unit WWT to the line L26.

[0274] -In the first and second embodiments, a dual-stage scrubber may be used as described in the third embodiment. With this configuration, the discharging of urea outside the system is further limited. The first-stage cleaning-processed recovered solution is supplied from the first-stage scrubber to the purification device U2. This reduces the load on the second evaporator EV2. The reduction in the load on the second evaporator EV2 allows for miniaturization of the second evaporator EV2. The miniaturization of the second evaporator EV2 reduces the amount of air supplied to the second evaporator EV2, thereby allowing for miniaturization of the scrubber. In the same manner as this modified example, a dual-stage scrubber may be used in the seventh, eighth, eleventh, twelfth, thirteenth, and fourteenth embodiments.

[0275] -In the third to sixth, ninth, and tenth embodiments, a single-stage scrubber may be used as described in the first embodiment. With this configuration, the scrubber equipment is simplified.

[0276] -In the seventh, eighth, eleventh, and twelfth embodiments, the modifying method includes replacing the water cleaning scrubber SCR* with the single-stage scrubber SCR. Alternatively, the modifying method may include replacing the water cleaning scrubber SCR* with a dual-stage scrubber.

[0277] -In the third to sixth embodiments, the method for modifying the first existing urea production apparatus 100 is described as an example of the method for modifying an existing urea production apparatus. However, the existing urea production apparatus is not limited to the first existing urea production apparatus 100. For example, the urea production apparatus 40 of the fourth embodiment may be obtained by modifying the urea production apparatus 30 of the third embodiment. Similarly, in the modifying methods described in the seventh to twelfth embodiments, the existing urea production apparatus subject to modification is not limited to that of each embodiment.

[0278] -In the second embodiment, the urea production apparatus 20 may be obtained by modifying an existing urea production apparatus. The method for modifying an existing urea production apparatus to manufacture the urea production apparatus 20 includes, for example, the following modifying methods [1] to [6].

[0279] [1] (A) A modification is performed on the second existing urea production apparatus 110 through the same method for manufacturing the urea production apparatus 207 described in the seventh embodiment. (B) A line is added to supply condensate water from the condenser C1 to the urea synthesis unit U0. (C) The processed condensate water discharged from the stripping tower ST is configured to be entirely supplied to the scrubber. In an example, the line L27 is stopped. Alternatively, the line L27 may be removed.

[0280] [2] (A) A modification is performed on the third existing urea production apparatus 120 through the same method for manufacturing the urea production apparatus 209 described in the ninth embodiment. (B) The same modification as (B) in [1] is performed. (C) A dual-stage scrubber is replaced with a single-stage scrubber SCR. (D) The line L28* is stopped or removed.

[0281] [3] (A) A modification is performed on the fourth existing urea production apparatus 130 through the same method for manufacturing the urea production apparatus 211 described in the eleventh embodiment. (B) The first-stage evaporator EVa is connected to the third-stage evaporator EVc in series. The second-stage evaporator EVb and the second condenser C2 are stopped. Alternatively, the second-stage evaporator EVb and the second condenser C2 are removed. (C) The same modification as (B) in [1] is performed.

[0282] [4] (A) A modification is performed on the first existing urea production apparatus 100 through the same method for manufacturing the urea production apparatus 213 described in the thirteenth embodiment. (B) The same modification as (B) in [1] is performed.

[0283] [5] (A) A modification is performed on the first existing urea production apparatus 100 through the same method for manufacturing the urea production apparatus 50 described in the fifth embodiment. (B) The same modification as (B) in [1] is performed. (C) A dual- stage scrubber is replaced with a single-stage scrubber SCR. (D) The line L28 is stopped or removed.

[0284] [6] (A) A modification is performed on the first existing urea production apparatus 100 through the same method for manufacturing the urea production apparatus 30 described in the third embodiment. (B) The first-stage evaporator EVa and the third-stage evaporator EVc are connected in series. The second-stage evaporator EVb and the second condenser C2 are stopped. Alternatively, the second-stage evaporator EVb and the second condenser C2 are removed. (C) The same modification as (B) in [1] is performed. (D) A dual-stage scrubber is replaced with the single-stage scrubber SCR. (E) The line L28 is stopped or removed.

[0285] The waste water treatment unit WWT that includes only the stripping tower ST is substantially the same as the stripping tower ST of the urea production apparatus 20 in the second embodiment. Therefore, the urea production apparatus modified by any one of the modifying methods [1] to [6] has the same configuration as the urea production apparatus 20.

[0286] -In the first embodiment, the urea production apparatus 10 may be obtained by modifying an existing urea production apparatus. The urea production apparatus 10 of the first embodiment may be obtained by stopping the stripping tower ST in addition to any one of the modifying methods [1] to [6] for modifying an existing urea production apparatus to manufacture the urea production apparatus 20.

[0287] The method for modifying an existing urea production apparatus to manufacture the urea production apparatus 10 includes, for example, the following modifying methods [7] to

[12] . [7] (A) A modification is performed on the second existing urea production apparatus 110 through the same method for manufacturing the urea production apparatus 208 described in the eighth embodiment. That is, the urea production apparatus 208 has the same configuration as the urea production apparatus 10.

[0288] [8] (A) A modification is performed on the third existing urea production apparatus 120 through the same method for manufacturing the urea production apparatus 210 of the tenth embodiment. (B) The dual-stage scrubber is replaced with the single-stage scrubber SCR. (C) The line L28* is stopped or removed.

[0289] [9] (A) A modification is performed on the fourth existing urea production apparatus 130 through the same method for manufacturing the urea production apparatus 212 of the twelfth embodiment. (B) The first-stage evaporator EVa is connected to the third- stage evaporator EVc in series. The second-stage evaporator EVb and the second condenser C2 are stopped. Alternatively, the second-stage evaporator EVb and the second condenser C2 are removed.

[0290]

[10] (A) A modification is performed on the first existing urea production apparatus 100 through the same method for manufacturing the urea production apparatus 214 of the fourteenth embodiment. That is, the urea production apparatus 214 has the same configuration as the urea production apparatus 10.

[0291]

[11] (A) A modification is performed on the first existing urea production apparatus 100 through the same method for manufacturing the urea production apparatus 60 of the sixth embodiment. (B) The dual-stage scrubber is replaced with the single-stage scrubber SCR. (C) The line L28 is stopped or removed.

[0292]

[12] (A) A modification is performed on the first existing urea production apparatus 100 through the same method for manufacturing the urea production apparatus 40 of the fourth embodiment. (B) The first-stage evaporator EVa is connected to the third-stage evaporator EVc in series. The second-stage evaporator EVb and the second condenser C2 are stopped. Alternatively, the second-stage evaporator EVb and the second condenser C2 are removed. (C) A dual-stage scrubber is replaced with a single-stage scrubber SCR. (D) The line L28 is stopped or removed.

[0293] -In each of the embodiments and the modified examples, the ammonium salt produced in the scrubber SCR or the second-stage scrubber SCR2 is recovered as the mixture of the solid urea product without discharging the ammonium salt outside the system. Alternatively, the urea production apparatus may be configured to discharge the ammonium salt outside the system. In the same manner, the method for producing urea may include a step of discharging an ammonium salt outside the system.

[0294] An example of a urea production apparatus configured to discharge an ammonium salt outside the system will now be described with reference to Figs. 19 and 20. Fig. 19 shows a urea production apparatus 215.

[0295] The urea production apparatus 215 differs from the urea production apparatus 10 of the first embodiment in that a portion of the cleaning-processed recovered solution discharged from the scrubber SCR is discharged outside the system, and the remaining portion of the cleaning-processed recovered solution is supplied to the second evaporator EV2. In the urea production apparatus 215, the description of components that are the same as the components of the urea production apparatus 10 of the first embodiment may be omitted.

[0296] More specifically, the urea production apparatus 215 includes a line L16a supplying the cleaning-processed recovered solution to the second evaporator EV2 and a line L16b diverging from the line L16a and discharging the cleaning-processed recovered solution outside the system. The cleaning-processed recovered solution discharged through the line L16b is transferred to a processing facility such as a urea mixing facility or a concentration solidification facility for processing the ammonium salt outside the system. The line L16a corresponds to the recovery pipe. The line L16b corresponds to the recovered solution discharge pipe.

[0297] In Fig. 19, the lines L16a and L16b diverge at the outside of the scrubber SCR. Instead, two lines may be arranged to discharge the cleaning-processed recovered solution from the scrubber SCR.

[0298] Fig. 20 shows a urea production apparatus 216. The urea production apparatus 216 differs from the urea production apparatus 30 of the third embodiment in that a portion of the second-stage cleaning-processed recovered solution discharged from the second-stage scrubber SCR2 is discharged outside the system. In the urea production apparatus 216, the description of components that are the same as the components of the urea production apparatus 30 of the third embodiment may be omitted.

[0299] More specifically, the urea production apparatus 216 includes a line L29a supplying the second-stage cleaning-processed recovered solution to the third-stage evaporator EVc and a line L29b diverging from the line L29a and discharging the second- stage cleaning-processed recovered solution outside the system. The second-stage cleaning- processed recovered solution discharged through the line L29b is transferred to a processing facility such as a urea mixing facility or a concentration solidification facility for processing the ammonium salt outside the system. The line L29a corresponds to the recovery pipe. The line L29b corresponds to the recovered solution discharge pipe.

[0300] In Fig. 20, the lines L29a and L29b diverge at the outside of the second-stage scrubber SCR2. Alternatively, two lines may be arranged to discharge the second-stage cleaning-processed recovered solution to the second-stage scrubber SCR2.

[0301] With the urea production apparatuses 215 and 216 having the configuration described above, while urea is efficiently recovered without being hydrolyzed, the ammonium salt is discharged outside the system.

[0302] -In the embodiments and modified examples that include the scrubber SCR, a portion of the cleaning-processed recovered solution discharged from the scrubber SCR may be configured to be discharged outside the system as in the urea production apparatus 215 shown in Fig. 19.

[0303] -In the embodiments and modified examples that include the second-stage scrubber SCR2, a portion of the second-stage cleaning-processed recovered solution discharged from the second-stage scrubber SCR2 may be configured to be discharged outside the system as in the urea production apparatus 216 shown in Fig. 20.

[0304] -The urea production apparatus 216 shown in Fig. 20 is configured to discharge a portion of the second-stage cleaning-processed recovered solution outside the system. Alternatively, the entirety of the second-stage cleaning-processed recovered solution may be discharged outside the system. That is, at least a portion of the second-stage cleaning- processed recovered solution is discharged outside the system. Even when the entirety of the second-stage cleaning-processed recovered solution is discharged outside the system, urea is recovered effectively without being hydrolyzed. This is because in the urea production apparatus including a dual-stage scrubber, the first-stage cleaning-processed recovered solution is supplied to the urea synthesis unit U0.

[0305] -A urea production apparatus configured to discharge the cleaning-processed recovered solution or the second-stage cleaning-processed recovered solution outside the system may be manufactured by modifying an existing urea production apparatus. In an example, in addition to the method for modifying an existing urea production apparatus to manufacture a urea production apparatus described in the embodiments and the modified examples, a step of adding a recovered solution discharge pipe may be performed to discharge a portion of the cleaning-processed recovered solution or a portion of the second- stage cleaning-processed recovered solution outside the system. The recovered solution discharge pipe may diverge from the recovery pipe supplying the cleaning-processed recovered solution to an additional evaporator or may be added as a pipe differing from the recovery pipe. When the entirety of the second-stage cleaning-processed recovered solution is discharged outside the system, a step of adding the recovered solution discharge pipe may be performed instead of the step of adding the recovery pipe.

[0306] -The method for producing urea may include, in addition to those of the embodiments, a releasing step of discharging a portion of the cleaning-processed recovered solution obtained by the cleaning step outside the system. In this case, the second evaporation step includes obtaining the highly concentrated urea solution and the second off- gas from a mixture of the concentrated urea solution and a portion of the cleaning-processed recovered solution excluding the portion discharged outside the system. That is, when a portion of the cleaning-processed recovered solution is discharged outside the system, the cleaning-processed recovered solution is divided into a liquid discharged outside the system and a liquid mixed with the concentrated urea solution. [Examples]

[0307] The present disclosure will be described in further detail based on the examples described below. The present disclosure is not limited to the configurations described in the examples. (Example 1) A solid urea product is produced based on the urea production apparatus 10 shown in Fig. 1. The detail is as follows.

[0308] The urea solution, which is the raw material, was supplied from the line L1. Formaldehyde was added through the line L5 as an addition agent. A sulphuric acid was added from the line L14 as an acid supplied to the scrubber SCR. The solid urea product was obtained from the line L7. Tables 1 to 3 show the components contained in each line per hour.

[0309] The first evaporator EV1 was operated at 47 kPa and 104°C. The second evaporator EV2 was operated at atmospheric pressure and 140°C. The fluidized bed granulator was used as the granulator F.

[0310] [Table 1] [Image disponible dans le document PDF, Image available in the PDF document]

[0311] [Table 2] [Image disponible dans le document PDF, Image available in the PDF document] [Image disponible dans le document PDF, Image available in the PDF document]

[0313] (Example 2) The solid urea product was produced based on the urea production apparatus 20 shown in Fig. 2 while the other configuration is the same as that of Example 1. Tables 4 to 6 show the components contained in each line per hour.

[0314] [Table 4] [Image disponible dans le document PDF, Image available in the PDF document]

[0315] [Table 5] [Image disponible dans le document PDF, Image available in the PDF document]

[0316] [Table 6] [Image disponible dans le document PDF, Image available in the PDF document]

[0317] <Evaluations> In Example 1, as shown in Table 3, the amount of urea in the line L16 is equal to a value obtained by subtracting the amount of urea that is present in the line L9 and recycled to the urea synthesis unit and the amount of urea that is contained in the line L15 and discharged from the scrubber SCR to the atmosphere from the amount of urea that is contained in the gas in the evaporator and the granulator, that is, urea contained in the lines L8, L11, and L12. In other words, the amount of urea in L16 = the amount of urea in (L8 + <semantics>L11+L12<annotation encoding="application / x-tex">L11 + L12< / annotation>< / semantics>) - the amount of urea in (L9 + L15). This shows that urea contained in the gas in the evaporator and the granulator is recovered without being decomposed. The waste water treatment unit does not include a hydrolyzer and a stripping tower. Thus, water is not discharged outside the system. In Example 2, as shown in Table 6, the amount of urea in L16 = the amount of urea in <semantics>(L8+L11+L12)<annotation encoding="application / x-tex">(L8 + L11 + L12)< / annotation>< / semantics> - the amount of urea in <semantics>(L9+L15)<annotation encoding="application / x-tex">(L9 + L15)< / annotation>< / semantics>. This shows that urea contained in the gas in the evaporator and the granulator is recovered without being decomposed. Water is not discharged outside the system. The urea hydrolyzer is not used. As described above, urea is recovered without being hydrolyzed. This reduces the amount of urea that is decomposed into ammonia and carbon dioxide and resynthesized from the ammonia and carbon dioxide, thereby increasing the efficiency of producing the solid urea product.

Claims

1. A urea production apparatus, comprising: a first evaporator configured to selectively separate a first off-gas containing water and ammonia from a urea solution to obtain the first off-gas and a concentrated urea solution, the concentrated urea solution being a liquid having a higher urea concentration than the urea solution; a condenser configured to cool the first off-gas to obtain condensate water; a second evaporator configured to selectively separate a second off-gas containing water and ammonia from the concentrated urea solution to obtain the second off-gas and a highly concentrated urea solution, the highly concentrated urea solution being a liquid having a higher urea concentration than the concentrated urea solution; a granulator configured to obtain a solid urea product from the highly concentrated urea solution; a scrubber configured to contact the second off-gas, which is supplied as a gas from the second evaporator, and a third off-gas discharged from the granulator with an acidic cleaning solution containing an acid to obtain a clean gas and a cleaning-processed recovered solution containing an ammonium salt and urea; and a recovery pipe configured to supply the cleaning-processed recovered solution to the second evaporator, wherein the second evaporator is configured to obtain the highly concentrated urea solution and the second off-gas from a mixture of the concentrated urea solution and the cleaning-processed recovered solution.

2. The urea production apparatus according to claim 1, further comprising: a supply pipe configured to supply at least a portion of the condensate water to the scrubber, wherein the scrubber uses at least a portion of the condensate water as a make-up water supplied to the cleaning solution.

3. A urea production apparatus, comprising: a first evaporator configured to selectively separate a first off-gas containing water and ammonia from a urea solution to obtain the first off-gas and a concentrated urea solution, the concentrated urea solution being a liquid having a higher urea concentration than the urea solution; a condenser configured to cool the first off-gas to obtain condensate water; a second evaporator configured to selectively separate a second off-gas containing water and ammonia from the concentrated urea solution to obtain the second off-gas and a highly concentrated urea solution, the highly concentrated urea solution being a liquid having a higher urea concentration than the concentrated urea solution; a granulator configured to obtain a solid urea product from the highly concentrated urea solution; a scrubber configured to contact the second off-gas, which is supplied as a gas from the second evaporator, and a third off-gas discharged from the granulator with an acidic cleaning solution containing an acid to obtain a clean gas and a cleaning-processed recovered solution containing an ammonium salt and urea; a stripping tower configured to strip the condensate water with steam to obtain a gas containing ammonia and processed condensate water; a recovery pipe configured to supply the cleaning-processed recovered solution to the second evaporator; and a supply pipe configured to supply the processed condensate water to the scrubber, wherein the second evaporator is configured to obtain the highly concentrated urea solution and the second off-gas from a mixture of the concentrated urea solution and the cleaning- processed recovered solution, and the scrubber uses the processed condensate water as a make-up water supplied to the cleaning solution.

4. The urea production apparatus according to any one of claims 1 to 3, wherein the second evaporator includes a liquid falling film evaporator and is configured to bring the concentrated urea solution into countercurrent contact with heated air at atmospheric pressure, and the second off-gas contains heated air that has passed through the second evaporator.

5. The urea production apparatus according to claim 4, further comprising: an air supply device configured to supply heated air to the granulator, wherein the granulator is configured to use the heated air as fluidizing air to form a fluidized bed, and the heated air brought into countercurrent contact with the concentrated urea solution in the second evaporator is supplied from the air supply device.

6. The urea production apparatus according to claim 4, further comprising: an additive pipe configured to supply an additive to a liquid downstream of the first evaporator and upstream of the second evaporator or a liquid downstream of the second evaporator and upstream of the granulator.

7. The urea production apparatus according to claim 6, wherein the additive is a component supplied from outside a system of the urea production apparatus, and is at least one selected from a group consisting of formaldehyde, a urea- formaldehyde solution, calcium nitrate, potassium nitrate, and a mixture of polyvinyl alcohol and calcium sulfate.

8. The urea production apparatus according to claim 4, further comprising: an additive pipe configured to supply an ammonium salt for adjusting an ammonium salt concentration of a product to a liquid downstream of the first evaporator and upstream of the second evaporator or a liquid downstream of the second evaporator and upstream of the granulator.

9. The urea production apparatus according to claim 8, wherein the ammonium salt for adjusting the ammonium salt concentration of the product is at least one selected from a group consisting of ammonium nitrate and ammonium sulfate.

10. The urea production apparatus according to claim 9, wherein the ammonium salt for adjusting the ammonium salt concentration of the product is formed from an acid and ammonia supplied to the scrubber.

11. The urea production apparatus according to claim 4, further comprising: a recovered solution discharge pipe configured to discharge a portion of the cleaning-processed recovered solution discharged from the scrubber outside a system of the urea production apparatus, wherein the recovery pipe is configured to supply the cleaning-processed recovered solution excluding the portion discharged outside the system to the second evaporator.

12. A method for modifying an existing urea production apparatus, wherein the existing urea production apparatus includes: a granulator configured to obtain a solid urea product from a liquid containing urea; an evaporator configured to selectively separate a first off-gas, which is a gas containing water and ammonia, from a urea solution to obtain the first off-gas and a liquid having a higher urea concentration than the urea solution and being supplied to the granulator; a condenser configured to cool the first off-gas to obtain condensate water; a waste water treatment unit including a urea hydrolyzer and a stripping tower and configured to process the condensate water to obtain treated waste water; a scrubber configured to contact an off-gas discharged from the granulator with a cleaning solution to obtain a clean gas and a cleaning-processed recovered solution; and a supply pipe configured to supply the treated waste water, which is obtained in the waste water treatment unit, to the scrubber, the method comprising: adding an additional evaporator between the evaporator and the granulator; adding a discharge pipe configured to supply an additional off-gas including water and ammonia from the additional evaporator to the scrubber; adding an acid supply pipe configured to supply an acid to the scrubber so that an off-gas discharged from the granulator and the additional off-gas contact an acidic cleaning solution containing an acid to obtain a clean gas and a cleaning-processed recovered solution; adding a recovery pipe configured to supply the cleaning-processed recovered solution to the additional evaporator; and configuring the waste water treatment unit so that the condensate water is processed by the stripping tower to obtain treated waste water, and the treated waste water is supplied to the scrubber through the supply pipe without operating the urea hydrolyzer.

13. A method for modifying an existing urea production apparatus, wherein the existing urea production apparatus includes: a granulator configured to obtain a solid urea product from a liquid containing urea; an evaporator configured to selectively separate a first off-gas, which is a gas containing water and ammonia, from a urea solution to obtain the first off-gas and a liquid having a higher urea concentration than the urea solution and being supplied to the granulator; a condenser configured to cool the first off-gas to obtain condensate water; a waste water treatment unit including a urea hydrolyzer and a stripping tower and configured to process the condensate water to obtain treated waste water; a scrubber configured to contact an off-gas discharged from the granulator with a cleaning solution to obtain a clean gas and a cleaning-processed recovered solution; and a supply pipe configured to supply the treated waste water, which is obtained in the waste water treatment unit, to the scrubber, the method comprising: adding an additional evaporator between the evaporator and the granulator, adding a discharge pipe configured to supply an additional off-gas including water and ammonia from the additional evaporator to the scrubber, adding an acid supply pipe configured to supply an acid to the scrubber so that an off-gas discharged from the granulator and the additional off-gas contact an acidic cleaning solution containing an acid to obtain a clean gas and a cleaning-processed recovered solution; adding a recovery pipe configured to supply the cleaning-processed recovered solution to the additional evaporator; and configuring the waste water treatment unit to supply the condensate water to the scrubber without operating the urea hydrolyzer and the stripping tower.

14. A method for modifying an existing urea production apparatus, wherein the existing urea production apparatus includes: a granulator configured to obtain a solid urea product from a liquid containing urea; an evaporator configured to selectively separate a first off-gas, which is a gas containing water and ammonia, from a urea solution to obtain the first off-gas and a liquid having a higher urea concentration than the urea solution and being supplied to the granulator; a condenser configured to cool the first off-gas to obtain condensate water; a waste water treatment unit including a urea hydrolyzer and a stripping tower and configured to process the condensate water to obtain treated waste water; a scrubber configured to contact an off-gas discharged from the granulator with an acidic cleaning solution containing an acid to obtain a clean gas and a cleaning-processed recovered solution; and a supply pipe configured to supply the treated waste water, which is obtained in the waste water treatment unit, to the scrubber, the method comprising: adding an additional evaporator between the evaporator and the granulator; adding a discharge pipe configured to supply an additional off-gas including water and ammonia from the additional evaporator to the scrubber; adding a recovery pipe configured to supply the cleaning-processed recovered solution to the additional evaporator; and configuring the waste water treatment unit so that the condensate water is processed by the stripping tower to obtain treated waste water, and the treated waste water is supplied to the scrubber through the supply pipe without operating the urea hydrolyzer.

15. A method for modifying an existing urea production apparatus, wherein the existing urea production apparatus includes: a granulator configured to obtain a solid urea product from a liquid containing urea; an evaporator configured to selectively separate a first off-gas, which is a gas containing water and ammonia, from a urea solution to obtain the first off-gas and a liquid having a higher urea concentration than the urea solution and being supplied to the granulator; a condenser configured to cool the first off-gas to obtain condensate water; a waste water treatment unit including a urea hydrolyzer and a stripping tower and configured to process the condensate water to obtain treated waste water; a scrubber configured to contact an off-gas discharged from the granulator with an acidic cleaning solution containing an acid to obtain a clean gas and a cleaning-processed recovered solution; and a supply pipe configured to supply the treated waste water, which is obtained in the waste water treatment unit, to the scrubber, the method comprising: adding an additional evaporator between the evaporator and the granulator; adding a discharge pipe configured to supply an additional off-gas including water and ammonia from the additional evaporator to the scrubber; adding a recovery pipe configured to supply the cleaning-processed recovered solution to the additional evaporator; and configuring the waste water treatment unit to supply the condensate water to the scrubber without operating the urea hydrolyzer and the stripping tower.

16. The method according to claim 12 or 13, further comprising: adding a recovered solution discharge pipe configured to discharge a portion of the cleaning-processed recovered solution outside a system of the urea production apparatus, wherein the recovery pipe is configured to supply the cleaning-processed recovered solution excluding the portion discharged outside the system to the additional evaporator.

17. The method according to claim 14 or 15, wherein the existing urea production apparatus further includes a recovered solution discharge pipe configured to discharge a portion of the cleaning-processed recovered solution, which is discharged from the scrubber, outside a system of the urea production apparatus, and the recovery pipe is configured to supply the cleaning-processed recovered solution excluding the portion discharged outside the system to the additional evaporator.

18. The method according to any one of claims 12 to 15, further comprising: configuring the additional evaporator to further concentrate a liquid before being supplied to the granulator and the cleaning-processed recovered solution in the additional evaporator to obtain a highly concentrated urea solution that is a liquid having a higher urea concentration than a liquid supplied to the granulator.

19. The method according to any one of claims 12 to 15, wherein the additional evaporator includes a liquid falling film evaporator and is configured to bring a liquid before being supplied to the granulator into countercurrent contact with heated air at atmospheric pressure, and the additional off-gas contains heated air that has passed through the additional evaporator.

20. The method according to any one of claims 12 to 15, wherein the existing urea production apparatus further includes a discharge pipe configured to discharge the treated waste water, which is obtained in the waste water treatment unit, outside a system of the urea production apparatus, the method, further comprising: configuring the discharge pipe so that discharging from the discharge pipe is not performed.

21. The method according to claim 19, wherein the evaporator in the existing urea production apparatus is a first-stage evaporator, the condenser in the existing urea production apparatus is a first condenser, the existing urea production apparatus includes: a second-stage evaporator configured to selectively separate a gas containing water and ammonia from a liquid supplied from the first-stage evaporator to obtain the separated gas and a liquid having a higher urea concentration than the liquid supplied from the first evaporator and being configured to be supplied to the granulator; and a second condenser configured to cool the gas discharged from the second-stage evaporator to obtain condensate water, the method, further comprising: replacing the second-stage evaporator with the additional evaporator, thereby adding the additional evaporator; and removing the second condenser.

22. A method for modifying an existing urea production apparatus, wherein the existing urea production apparatus includes: a granulator configured to obtain a solid urea product from a liquid containing urea; a first-stage evaporator configured to selectively separate a first off-gas containing water and ammonia from a urea solution to obtain the first off-gas and a concentrated urea solution, the concentrated urea solution being a liquid having a higher urea concentration than the urea solution; a first condenser configured to cool a gas discharged from the first-stage evaporator to obtain first condensate water; a second-stage evaporator configured to selectively separate a gas containing water and ammonia from a liquid supplied from the first-stage evaporator to obtain the separated gas and a liquid having a higher urea concentration than the liquid supplied from the first evaporator and being configured to be supplied to the granulator; a second condenser configured to cool a gas discharged from the second-stage evaporator to obtain second condensate water; a waste water treatment unit including a urea hydrolyzer and a stripping tower and configured to process the first condensate water and the second condensate water to obtain treated waste water; a scrubber configured to contact an off-gas discharged from the granulator with a cleaning solution to obtain a clean gas and a cleaning-processed recovered solution; and a supply pipe configured to supply the treated waste water, which is obtained in the waste water treatment unit, to the scrubber, the method comprising: adding an additional evaporator between the second-stage evaporator and the granulator; adding a discharge pipe configured to supply an additional off-gas including water and ammonia from the additional evaporator to the scrubber; adding an acid supply pipe configured to supply an acid to the scrubber so that an off-gas discharged from the granulator and the additional off-gas contact an acidic cleaning solution containing an acid to obtain a clean gas and a cleaning-processed recovered solution; adding a recovery pipe configured to supply the cleaning-processed recovered solution to the additional evaporator; and configuring the waste water treatment unit so that the first condensate water and the second condensate water are processed by the stripping tower to obtain treated waste water, and the treated waste water is supplied to the scrubber through the supply pipe without operating the urea hydrolyzer.

23. A method for modifying an existing urea production apparatus, wherein the existing urea production apparatus includes: a granulator configured to obtain a solid urea product from a liquid containing urea; a first-stage evaporator configured to selectively separate a first off-gas containing water and ammonia from a urea solution to obtain the first off-gas and a concentrated urea solution, the concentrated urea solution being a liquid having a higher urea concentration than the urea solution; a first condenser configured to cool a gas discharged from the first-stage evaporator to obtain first condensate water; a second-stage evaporator configured to selectively separate a gas containing water and ammonia from a liquid supplied from the first-stage evaporator to obtain the separated gas and a liquid having a higher urea concentration than the liquid supplied from the first evaporator and being configured to be supplied to the granulator; a second condenser configured to cool a gas discharged from the second-stage evaporator to obtain second condensate water; a waste water treatment unit including a urea hydrolyzer and a stripping tower and configured to process the first condensate water and the second condensate water to obtain treated waste water; a scrubber configured to contact an off-gas discharged from the granulator with a cleaning solution to obtain a clean gas and a cleaning-processed recovered solution; and a supply pipe configured to supply the treated waste water, which is obtained in the waste water treatment unit, to the scrubber, the method comprising: adding an additional evaporator between the second-stage evaporator and the granulator; adding a discharge pipe configured to supply an additional off-gas including water and ammonia from the additional evaporator to the scrubber; adding an acid supply pipe configured to supply an acid to the scrubber so that an off-gas discharged from the granulator and the additional off-gas contact an acidic cleaning solution containing an acid to obtain a clean gas and a cleaning-processed recovered solution; adding a recovery pipe configured to supply the cleaning-processed recovered solution to the additional evaporator; and configuring the waste water treatment unit to supply the first condensate water and the second condensate water to the scrubber without operating the urea hydrolyzer and the stripping tower.

24. A method for modifying an existing urea production apparatus, wherein the existing urea production apparatus includes: a granulator configured to obtain a solid urea product from a liquid containing urea; a first-stage evaporator configured to selectively separate a first off-gas containing water and ammonia from a urea solution to obtain the first off-gas and a concentrated urea solution, the concentrated urea solution being a liquid having a higher urea concentration than the urea solution; a first condenser configured to cool a gas discharged from the first-stage evaporator to obtain first condensate water; a second-stage evaporator configured to selectively separate a gas containing water and ammonia from a liquid supplied from the first-stage evaporator to obtain the separated gas and a liquid having a higher urea concentration than the liquid supplied from the first evaporator and being configured to be supplied to the granulator; a second condenser configured to cool a gas discharged from the second-stage evaporator to obtain second condensate water; a waste water treatment unit including a urea hydrolyzer and a stripping tower and configured to process the first condensate water and the second condensate water to obtain treated waste water; a scrubber configured to contact an off-gas discharged from the granulator with an acidic cleaning solution containing an acid to obtain a clean gas and a cleaning-processed recovered solution; and a supply pipe configured to supply the treated waste water, which is obtained in the waste water treatment unit, to the scrubber, the method comprising: adding an additional evaporator between the second-stage evaporator and the granulator; adding a discharge pipe configured to supply an additional off-gas including water and ammonia from the additional evaporator to the scrubber; adding a recovery pipe configured to supply the cleaning-processed recovered solution to the additional evaporator; and configuring the waste water treatment unit so that the first condensate water and the second condensate water are processed by the stripping tower to obtain treated waste water, and the treated waste water is supplied to the scrubber through the supply pipe without operating the urea hydrolyzer.

25. A method for modifying an existing urea production apparatus, wherein the existing urea production apparatus includes: a granulator configured to obtain a solid urea product from a liquid containing urea; a first-stage evaporator configured to selectively separate a first off-gas containing water and ammonia from a urea solution to obtain the first off-gas and a concentrated urea solution, the concentrated urea solution being a liquid having a higher urea concentration than the urea solution; a first condenser configured to cool a gas discharged from the first-stage evaporator to obtain first condensate water; a second-stage evaporator configured to selectively separate a gas containing water and ammonia from a liquid supplied from the first-stage evaporator to obtain the separated gas and a liquid having a higher urea concentration than the liquid supplied from the first evaporator and being configured to be supplied to the granulator; a second condenser configured to cool a gas discharged from the second-stage evaporator to obtain second condensate water; a waste water treatment unit including a urea hydrolyzer and a stripping tower and configured to process the first condensate water and the second condensate water to obtain treated waste water; a scrubber configured to contact an off-gas discharged from the granulator with an acidic cleaning solution containing an acid to obtain a clean gas and a cleaning-processed recovered solution; and a supply pipe configured to supply the treated waste water, which is obtained in the waste water treatment unit, to the scrubber, the method comprising: adding an additional evaporator between the second-stage evaporator and the granulator; adding a discharge pipe configured to supply an additional off-gas including water and ammonia from the additional evaporator to the scrubber; adding a recovery pipe configured to supply the cleaning-processed recovered solution to the additional evaporator; and configuring the waste water treatment unit to supply the first condensate water and the second condensate water to the scrubber without operating the urea hydrolyzer and the stripping tower.

26. The method according to any one of claims 22 to 25, further comprising: configuring the additional evaporator to further concentrate a liquid before being supplied to the granulator and the cleaning-processed recovered solution in the additional evaporator to obtain a highly concentrated urea solution that is a liquid having a higher urea concentration than a liquid supplied to the granulator.

27. The method according to claim 22 or 23, further comprising: adding a recovered solution discharge pipe configured to discharge a portion of the cleaning-processed recovered solution outside a system of the urea production apparatus, wherein the recovery pipe is configured to supply the cleaning-processed recovered solution excluding the portion discharged outside the system to the additional evaporator.

28. The method according to claim 24 or 25, wherein the existing urea production apparatus further includes a recovered solution discharge pipe configured to discharge a portion of the cleaning-processed recovered solution, which is discharged from the scrubber, outside a system of the urea production apparatus, and the recovery pipe is configured to supply the cleaning-processed recovered solution excluding the portion discharged outside the system to the additional evaporator.

29. The method according to any one of claims 22 to 25, wherein the additional evaporator includes a liquid falling film evaporator and is configured to bring a liquid before being supplied to the granulator into countercurrent contact with heated air at atmospheric pressure, and the additional off-gas contains heated air that has passed through the additional evaporator.

30. The method according to any one of claims 22 to 25, wherein the existing urea production apparatus further includes a discharge pipe configured to discharge the treated waste water, which is obtained in the waste water treatment unit, outside a system of the urea production apparatus, the method, further comprising: configuring the discharge pipe so that discharging from the discharge pipe is not performed.

31. A method for producing urea, the method, comprising: a first evaporation step of selectively separating a first off-gas containing water and ammonia from a urea solution to obtain the first off-gas and a concentrated urea solution, the concentrated urea solution being a liquid having a higher urea concentration than the urea solution; a condensation step of cooling the first off-gas to obtain condensate water; a second evaporation step of selectively separating a second off-gas containing water and ammonia from the concentrated urea solution to obtain the second off-gas and a highly concentrated urea solution, the highly concentrated urea solution being a liquid having a higher urea concentration than the concentrated urea solution; a granulation step of obtaining a solid urea product from the highly concentrated urea solution; and a cleaning step of contacting the second off-gas, which is obtained by the second evaporation step, and a third off-gas discharged by the granulation step with an acidic cleaning solution containing an acid to obtain a clean gas and a cleaning-processed recovered solution containing an ammonium salt and urea, wherein the second evaporation step includes obtaining the highly concentrated urea solution and the second off-gas from a mixture of the concentrated urea solution and the cleaning-processed recovered solution.

32. The method according to claim 31, wherein the cleaning step uses at least a portion of the condensate water as a make-up water supplied to the cleaning solution.

33. A method for producing urea, the method, comprising: a first evaporation step of selectively separating a first off-gas containing water and ammonia from a urea solution to obtain the first off-gas and a concentrated urea solution, the concentrated urea solution being a liquid having a higher urea concentration than the urea solution; a condensation step of cooling the first off-gas to obtain condensate water; a second evaporation step of selectively separating a second off-gas containing water and ammonia from the concentrated urea solution to obtain the second off-gas and a highly concentrated urea solution, the highly concentrated urea solution being a liquid having a higher urea concentration than the concentrated urea solution; a granulation step of obtaining a solid urea product from the highly concentrated urea solution; a cleaning step of contacting the second off-gas, which is obtained by the second evaporation step, and a third off-gas discharged by the granulation step with an acidic cleaning solution containing an acid to obtain a clean gas and a cleaning-processed recovered solution containing an ammonium salt and urea; a stripping step of obtaining a gas containing ammonia and processed condensate water by stripping the condensate water with steam, wherein the second evaporation step includes obtaining the highly concentrated urea solution and the second off-gas from a mixture of the concentrated urea solution and the cleaning-processed recovered solution, and the cleaning step uses the processed condensate water as a make-up water supplied to the cleaning solution.

34. The method according to any one of claims 31 to 33, wherein the second evaporation step uses a liquid falling film evaporator configured to bring the concentrated urea solution into countercurrent contact with heated air at atmospheric pressure, and the second off-gas contains heated air that has passed through the evaporator.

35. The method according to claim 34, further comprising: a releasing step of discharging a portion of the cleaning-processed recovered solution obtained by the cleaning step, wherein the second evaporation step includes obtaining the highly concentrated urea solution and the second off-gas from a mixture of the concentrated urea solution and the cleaning-processed recovered solution excluding the portion discharged.