UREA PRODUCTION PLANT, METHOD FOR IMPROVING A UREA PRODUCTION PLANT, AND METHOD FOR PRODUCING UREA

Abstract

A urea production apparatus (10) comprises: a first evaporator (EV1) for obtaining a first exhaust gas and a concentrated urea aqueous solution from a urea aqueous solution; a condenser (C1) for cooling the first exhaust gas to thereby obtain condensed water; a second evaporator (EV2) for obtaining a second exhaust gas and a high-concentration urea liquid from the concentrated urea aqueous solution; a granulation device (F) for obtaining a solid urea product from the high-concentration urea liquid; a scrubber (SCR) for bringing the second exhaust gas and a third exhaust gas discharged from the granulation device (F) into contact with a cleaning liquid to thereby obtain a clean gas and a post-cleaning recovery liquid containing an ammonium salt and urea; and a recovery pipe configured so as to supply the post-cleaning recovery liquid to the second evaporator (EV2). The second evaporator (EV2) obtains the high-concentration urea liquid and the second exhaust gas from a mixture liquid of the concentrated urea aqueous solution and the post-cleaning recovery liquid.

Description

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

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

[0002] A urea production apparatus is known that produces a solid urea product using a urea solution containing urea synthesized using carbon dioxide and ammonia as raw materials. The urea production apparatus disclosed in Patent Document 1 includes a first evaporator and a second evaporator that concentrate the urea solution before supplying it to a finishing section. The urea production apparatus also includes a first condenser and a second condenser. The first condenser condenses a first steam obtained in the first evaporator to obtain a first condensate. The first condensate is supplied to a wastewater treatment unit. Patent Document 1 also describes that urea is hydrolyzed in the wastewater treatment unit, thereby recycling carbon dioxide and ammonia to the urea synthesis. The second condenser condenses a second steam obtained in the second evaporator to obtain a second condensate. The second condensate is supplied to a scrubber.

[0003] The urea production apparatus disclosed in Patent Document 2 includes a first evaporator and a second evaporator that concentrate the urea aqueous solution before supplying it to the finishing section. The urea production apparatus also includes a first compartment and a second compartment. The first condensate obtained from the first compartment is supplied to the wastewater treatment section. Patent Document 2 describes that the wastewater treatment section includes a hydrolyser. The second condensate obtained from the second compartment is supplied to a scrubber.

[0004] JP 2023-503177 A U.S. Patent Application Publication No. 2022 / 0089527

[0005] The urea production apparatuses disclosed in Patent Documents 1 and 2 both perform wastewater treatment to treat the condensate. In the wastewater treatment, urea is hydrolyzed and returned to carbon dioxide and ammonia. However, since the urea obtained by synthesis is decomposed again, there is a problem of a large energy loss.

[0006] The following describes various aspects of the urea production apparatus of the present disclosure. One example of the urea production apparatus includes a first evaporator configured to selectively separate a first exhaust gas, which is a gas containing water and ammonia, from a urea aqueous solution to obtain a concentrated urea aqueous solution, which is a liquid having a higher urea concentration than the urea aqueous solution, and the first exhaust gas; a condenser configured to cool the first exhaust gas to obtain condensed water; a second evaporator configured to selectively separate a second exhaust gas, which is a gas containing water and ammonia, from the concentrated urea aqueous solution to obtain a high-concentration urea solution, which is a liquid having a higher urea concentration than the concentrated urea aqueous solution, and the second exhaust gas; the second evaporator is configured to obtain the high-concentration urea solution and the second exhaust gas from a mixture of the concentrated aqueous urea solution and the post-washing recovered liquid; a scrubber configured to obtain a clean gas and a post-washing recovered liquid containing an ammonium salt and urea by bringing the second exhaust gas supplied as a gas from the second evaporator and the third exhaust gas discharged from the granulation apparatus into contact with an acidic washing liquid containing an acid; and a recovery pipe configured to supply the post-washing recovered liquid to the second evaporator, wherein the second evaporator is configured to obtain the high-concentration urea solution and the second exhaust gas from a mixture of the concentrated aqueous urea solution and the post-washing recovered liquid.

[0007] An example of a urea production apparatus includes a first evaporator configured to selectively separate a first exhaust gas that is a gas containing water and ammonia from a urea aqueous solution to obtain a concentrated urea aqueous solution that is a liquid having a higher urea concentration than the urea aqueous solution, and the first exhaust gas; a condenser configured to cool the first exhaust gas to obtain condensed water; a second evaporator configured to selectively separate a second exhaust gas that is a gas containing water and ammonia from the concentrated urea aqueous solution to obtain a high-concentration urea liquid that is a liquid having a higher urea concentration than the concentrated urea aqueous solution, and the second exhaust gas; a granulator configured to obtain a solid urea product from the high-concentration urea liquid; and a condenser configured to cool the first exhaust gas to obtain condensed water from the concentrated urea aqueous solution. the second evaporator is configured to obtain the high-concentration urea solution and the second exhaust gas from a mixed solution of the concentrated aqueous urea solution and the post-cleaning recovered solution, and the scrubber is configured to obtain a clean gas and a post-cleaning recovered solution containing an ammonium salt and urea by bringing the third exhaust gas discharged from the second evaporator into contact with an acidic cleaning solution containing an acid; the stripping tower is configured to obtain a gas containing ammonia and post-cleaning condensed water by stripping the condensed water with steam; a recovery pipe configured to supply the post-cleaning recovered solution to the second evaporator; and a make-up pipe configured to supply the post-cleaning condensed water to the scrubber, wherein the second evaporator is configured to obtain the high-concentration urea solution and the second exhaust gas from a mixed solution of the concentrated aqueous urea solution and the post-cleaning recovered solution, and the scrubber uses the post-cleaning condensed water as make-up water to replenish the cleaning solution.

[0008] The present disclosure relates to a method for improving a urea production apparatus, and the method includes: a granulator configured to obtain a solid urea product from a liquid containing urea; an evaporator configured to selectively separate a first exhaust gas, which is a gas containing water and ammonia, from an aqueous urea solution to obtain a liquid having a higher urea concentration than the aqueous urea solution and to supply the liquid to the granulator; a condenser configured to cool the first exhaust gas to obtain condensed water; a wastewater treatment unit including a urea hydrolyzer and a stripper tower and configured to treat the condensed water to obtain treated wastewater; a scrubber configured to contact the exhaust gas discharged from the granulator with a washing liquid to obtain a clean gas and a washed recovered liquid; and a supply pipe configured to supply the treated wastewater obtained in the wastewater treatment unit to the scrubber. The improved method includes adding an additional evaporator between the evaporator and the granulation apparatus, adding an exhaust pipe configured to supply additional exhaust gas, which is a gas containing water and ammonia, from the additional evaporator to the scrubber, adding an acid supply pipe configured to supply acid to the scrubber so as to bring the exhaust gas discharged from the granulation apparatus and the additional exhaust gas into contact with an acidic washing liquid containing an acid, thereby obtaining a clean gas and a post-wash recovery liquid, adding a recovery pipe configured to supply the post-wash recovery liquid to the additional evaporator, and configuring the wastewater treatment unit so as to supply wastewater, after the condensed water has been treated by the stripper tower, to the scrubber via the supply pipe, without operating the urea hydrolyzer.

[0009] One example of the method for improving a urea production apparatus is directed to an existing urea production apparatus, and the existing urea production apparatus includes: a granulation apparatus configured to obtain a solid urea product from a liquid containing urea; an evaporator configured to selectively separate, from an aqueous urea solution, a first exhaust gas that is a gas containing water and ammonia, to obtain, from the aqueous urea solution, a liquid having a higher urea concentration than the aqueous urea solution and to be supplied to the granulation apparatus, and the first exhaust gas; a condenser configured to cool the first exhaust gas to obtain condensed water; a wastewater treatment unit including a urea hydrolyzer and a stripper tower, and configured to treat the condensed water to obtain treated wastewater; a scrubber configured to obtain a clean gas and a post-wash recovery liquid by bringing the exhaust gas discharged from the granulation apparatus into contact with a washing liquid; and a supply pipe configured to supply the treated wastewater obtained in the wastewater treatment unit to the scrubber. The improved method includes adding an additional evaporator between the evaporator and the granulation apparatus, adding an exhaust pipe configured to supply additional exhaust gas, which is a gas containing water and ammonia, from the additional evaporator to the scrubber, adding an acid supply pipe configured to supply acid to the scrubber so that a cleaned gas and a post-wash recovery liquid can be obtained by contacting the exhaust gas discharged from the granulation apparatus and the additional exhaust gas with an acidic washing liquid containing an acid, adding a recovery pipe configured to supply the post-wash recovery liquid to the additional evaporator, and configuring the wastewater treatment unit to supply the condensed water to the scrubber without operating the urea hydrolyzer and the stripper tower.

[0010] One example of the method for improving a urea production apparatus is directed to an existing urea production apparatus, and the existing urea production apparatus comprises: a granulation apparatus configured to obtain a solid urea product from a liquid containing urea; an evaporator configured to selectively separate, from an aqueous urea solution, a first exhaust gas that is a gas containing water and ammonia, to obtain a liquid having a higher urea concentration than the aqueous urea solution and to be supplied to the granulation apparatus, and the first exhaust gas; a condenser configured to cool the first exhaust gas to obtain condensed water; a wastewater treatment unit including a urea hydrolyzer and a stripper tower, and configured to treat the condensed water to obtain treated wastewater; a scrubber configured to obtain a clean gas and a post-wash recovery liquid by bringing the exhaust gas discharged from the granulation apparatus into contact with an acidic washing liquid containing an acid; and a supply pipe configured to supply the treated wastewater obtained in the wastewater treatment unit to the scrubber. The improved method includes adding an additional evaporator between the evaporator and the granulation apparatus, adding an exhaust pipe configured to supply additional exhaust gas, which is a gas containing water and ammonia, from the additional evaporator to the scrubber, adding a recovery pipe configured to supply the post-wash recovery liquid to the additional evaporator, and configuring the wastewater treatment unit so that, without operating the urea hydrolyzer, wastewater from which the condensed water has been treated by the stripper tower is supplied to the scrubber via the supply pipe.

[0011] One example of the method for improving a urea production apparatus is directed to an existing urea production apparatus, and the existing urea production apparatus comprises: a granulation apparatus configured to obtain a solid urea product from a liquid containing urea; an evaporator configured to selectively separate, from an aqueous urea solution, a first exhaust gas that is a gas containing water and ammonia, to obtain a liquid having a higher urea concentration than the aqueous urea solution and to be supplied to the granulation apparatus, and the first exhaust gas; a condenser configured to cool the first exhaust gas to obtain condensed water; a wastewater treatment unit including a urea hydrolyzer and a stripper tower, and configured to treat the condensed water to obtain treated wastewater; a scrubber configured to obtain a clean gas and a post-wash recovery liquid by bringing the exhaust gas discharged from the granulation apparatus into contact with an acidic washing liquid containing an acid; and a supply pipe configured to supply the treated wastewater obtained in the wastewater treatment unit to the scrubber. The improved method includes adding an additional evaporator between the evaporator and the granulation apparatus, adding an exhaust pipe configured to supply additional exhaust gas, which is a gas containing water and ammonia, from the additional evaporator to the scrubber, adding a recovery pipe configured to supply the post-wash recovery liquid to the additional evaporator, and configuring the wastewater treatment unit so that the urea hydrolyzer and the stripper tower are not operated and the condensed water is supplied to the scrubber.

[0012] One example of the method for improving a urea production apparatus is directed to an existing urea production apparatus, and 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 exhaust gas, which is a gas containing water and ammonia, from a urea aqueous solution to obtain a liquid having a higher urea concentration than the urea aqueous solution and the first exhaust gas; a first condenser configured to cool the gas discharged from the first-stage evaporator to obtain first condensed water; and a first condenser configured to selectively separate a gas containing water and ammonia from a liquid supplied from the first-stage evaporator to obtain a liquid having a higher urea concentration than the urea aqueous solution and the first exhaust gas. a second-stage evaporator configured to obtain a liquid having a high urea concentration to be supplied to the granulation apparatus and a separated gas; a second condenser configured to cool the gas discharged from the second-stage evaporator to obtain second condensed water; a wastewater treatment unit including a urea hydrolyzer and a stripper tower configured to treat the first condensed water and the second condensed water to obtain treated wastewater; a scrubber configured to obtain clean gas and a post-wash recovery liquid by bringing the exhaust gas discharged from the granulation apparatus into contact with a washing liquid; and a supply pipe configured to supply the treated wastewater obtained in the wastewater treatment unit to the scrubber. The improved method includes adding an additional evaporator between the second-stage evaporator and the granulation apparatus, adding an exhaust pipe configured to supply additional exhaust gas, which is a gas containing water and ammonia, from the additional evaporator to the scrubber, adding an acid supply pipe configured to supply acid to the scrubber so that a clean gas and a post-wash recovered liquid can be obtained by bringing the exhaust gas discharged from the granulation apparatus and the additional exhaust gas into contact with an acidic washing liquid containing an acid, adding a recovery pipe configured to supply the post-wash recovered liquid to the additional evaporator, and configuring the wastewater treatment unit so that, without operating the urea hydrolyzer, wastewater obtained after the first condensed water and the second condensed water have been treated in the stripper tower is supplied to the scrubber via the supply pipe.

[0013] One example of the method for improving a urea production apparatus is directed to an existing urea production apparatus, and 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 exhaust gas, which is a gas containing water and ammonia, from a urea aqueous solution to obtain a liquid having a higher urea concentration than the urea aqueous solution and the first exhaust gas; a first condenser configured to cool the gas discharged from the first-stage evaporator to obtain first condensed water; and a first condenser configured to selectively separate a gas containing water and ammonia from a liquid supplied from the first-stage evaporator to obtain a liquid having a higher urea concentration than the urea aqueous solution and the first exhaust gas. a second-stage evaporator configured to obtain a liquid having a high urea concentration to be supplied to the granulation apparatus and a separated gas; a second condenser configured to cool the gas discharged from the second-stage evaporator to obtain second condensed water; a wastewater treatment unit including a urea hydrolyzer and a stripper tower configured to treat the first condensed water and the second condensed water to obtain treated wastewater; a scrubber configured to obtain clean gas and a post-wash recovery liquid by bringing the exhaust gas discharged from the granulation apparatus into contact with a washing liquid; and a supply pipe configured to supply the treated wastewater obtained in the wastewater treatment unit to the scrubber. The improved method includes adding an additional evaporator between the second-stage evaporator and the granulation apparatus, adding an exhaust pipe configured to supply additional exhaust gas, which is a gas containing water and ammonia, from the additional evaporator to the scrubber, adding an acid supply pipe configured to supply acid to the scrubber so that a clean gas and a post-wash recovery liquid can be obtained by contacting the exhaust gas discharged from the granulation apparatus and the additional exhaust gas with an acidic washing liquid containing an acid, adding a recovery pipe configured to supply the post-wash recovery liquid to the additional evaporator, and configuring the wastewater treatment unit to supply the first condensed water and the second condensed water to the scrubber without operating the urea hydrolyzer and the stripper tower.

[0014] One example of the method for improving a urea production apparatus is directed to an existing urea production apparatus, and 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 exhaust gas, which is a gas containing water and ammonia, from a urea aqueous solution to obtain a liquid having a higher urea concentration than the urea aqueous solution and the first exhaust gas; a first condenser configured to cool the gas discharged from the first-stage evaporator to obtain first condensed water; and a first condenser configured to selectively separate a gas containing water and ammonia from a liquid supplied from the first-stage evaporator to obtain a liquid having a higher urea concentration than the liquid. a second condenser configured to cool the gas discharged from the second-stage evaporator to obtain second condensed water; a wastewater treatment unit including a urea hydrolyzer and a stripper tower, configured to treat the first condensed water and the second condensed water to obtain treated wastewater; a scrubber configured to contact the exhaust gas discharged from the granulation apparatus with an acidic washing liquid containing an acid to obtain clean gas and a post-wash recovery liquid; and a supply pipe configured to supply the treated wastewater obtained in the wastewater treatment unit to the scrubber. The improved method includes adding an additional evaporator between the second-stage evaporator and the granulation apparatus, adding an exhaust pipe configured to supply additional exhaust gas, which is a gas containing water and ammonia, from the additional evaporator to the scrubber, adding a recovery pipe configured to supply the post-wash recovery liquid to the additional evaporator, and configuring the wastewater treatment unit so that, without operating the urea hydrolyzer, the wastewater after the first condensed water and the second condensed water have been treated in the stripper tower is supplied to the scrubber via the supply pipe.

[0015] One example of the method for improving a urea production apparatus is directed to an existing urea production apparatus, and 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 exhaust gas, which is a gas containing water and ammonia, from a urea aqueous solution to obtain a liquid having a higher urea concentration than the urea aqueous solution and the first exhaust gas; a first condenser configured to cool the gas discharged from the first-stage evaporator to obtain first condensed water; and a first condenser configured to selectively separate a gas containing water and ammonia from a liquid supplied from the first-stage evaporator to obtain a liquid having a higher urea concentration than the liquid. a second condenser configured to cool the gas discharged from the second-stage evaporator to obtain second condensed water; a wastewater treatment unit including a urea hydrolyzer and a stripper tower, configured to treat the first condensed water and the second condensed water to obtain treated wastewater; a scrubber configured to contact the exhaust gas discharged from the granulation apparatus with an acidic washing liquid containing an acid to obtain clean gas and a post-wash recovery liquid; and a supply pipe configured to supply the treated wastewater obtained in the wastewater treatment unit to the scrubber. The improved method includes adding an additional evaporator between the second-stage evaporator and the granulation apparatus, adding an exhaust pipe configured to supply additional exhaust gas, which is a gas containing water and ammonia, from the additional evaporator to the scrubber, adding a recovery pipe configured to supply the post-wash recovery liquid to the additional evaporator, and configuring the wastewater treatment unit to supply the first condensed water and the second condensed water to the scrubber without operating the urea hydrolyzer and the stripper tower.

[0016] Each aspect of the disclosed urea production process will now be described. An example of a method for producing urea includes: a first evaporation step of selectively separating a first exhaust gas, which is a gas containing water and ammonia, from a urea aqueous solution to obtain a concentrated urea aqueous solution, which is a liquid having a higher urea concentration than the concentrated urea aqueous solution, and the first exhaust gas; a condensation step of cooling the first exhaust gas to obtain condensed water; a second evaporation step of selectively separating a second exhaust gas, which is a gas containing water and ammonia, from the concentrated urea aqueous solution to obtain a high-concentration urea solution, which is a liquid having a higher urea concentration than the concentrated urea aqueous solution, and the second exhaust gas; a granulation step of obtaining a solid urea product from the high-concentration urea solution; and a washing step of contacting the second exhaust gas obtained by the second evaporation step and a third exhaust gas discharged by the granulation step with an acidic washing liquid containing an acid to obtain a clean gas and a post-wash recovery liquid containing an ammonium salt and urea, wherein the second evaporation step is a step of obtaining the high-concentration urea solution and the second exhaust gas from a mixed liquid of the concentrated urea aqueous solution and the post-wash recovery liquid.

[0017] One example of a method for producing urea includes a first evaporation step of selectively separating a first exhaust gas, which is a gas containing water and ammonia, from a urea aqueous solution to obtain a concentrated urea aqueous solution, which is a liquid having a higher urea concentration than the urea aqueous solution, and the first exhaust gas; a condensation step of cooling the first exhaust gas to obtain condensed water; a second evaporation step of selectively separating a second exhaust gas, which is a gas containing water and ammonia, from the concentrated urea aqueous solution to obtain a high-concentration urea solution, which is a liquid having a higher urea concentration than the concentrated urea aqueous solution, and the second exhaust gas; a granulation step of obtaining a solid urea product from the high-concentration urea solution; and and a stripping step of stripping the condensed water with steam to obtain a gas containing ammonia and post-treatment condensed water, wherein the second evaporation step is a step of obtaining the high-concentration urea solution and the second exhaust gas from a mixed solution of the concentrated aqueous urea solution and the post-treatment condensed water, and the cleaning step uses the post-treatment condensed water as makeup water to replenish the cleaning solution.

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

[0019] First Embodiment A urea production apparatus 10 according to a first embodiment will be described with reference to FIG. 1. The urea production apparatus 10 is an apparatus for obtaining a solid urea product from an aqueous urea solution. The aqueous urea solution can be produced, for example, by a urea synthesis process. The urea synthesis process uses carbon dioxide (CO ２ ) and ammonia (NH ３ ) as a raw material to obtain a urea aqueous solution. The urea synthesis step is not particularly limited, and a known method can be adopted. The urea aqueous solution obtained by the urea synthesis step contains, for example, urea, ammonia, water, biuret, etc. The urea content of the urea aqueous solution is not particularly limited, but is, for example, 70 wt % or less.

[0020] FIG. 1 illustrates a urea synthesis unit U0 that performs a urea synthesis step. The urea synthesis unit U0 is not particularly limited and can have a known configuration. For example, the urea synthesis unit U0 includes a synthesis apparatus U1, a purification apparatus U2, and a recovery apparatus U3. The synthesis apparatus U1 is an apparatus that synthesizes urea from carbon dioxide and ammonia. The purification apparatus U2 is an apparatus that obtains a purified urea aqueous solution by separating carbon dioxide and ammonia from the synthesis solution obtained by the synthesis apparatus U1. The recovery apparatus U3 is an apparatus that obtains a recovered solution by absorbing the carbon dioxide and ammonia obtained by the purification apparatus U2 into water. The recovered solution can be supplied to the synthesis apparatus U1 and used for urea synthesis.

[0021] In each figure, a solid arrow indicates that liquid is being discharged from the base end side, a solid arrow indicates that liquid is being supplied to the tip end side, a dashed arrow indicates that gas is being discharged from the base end side, and a dashed arrow indicates that gas is being supplied to the tip end side.

[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 the respective components of the urea production apparatus 10. In Fig. 1, each arrow marked with "L#" indicates a line L#. Each line L# corresponds to a pipe included in the urea production apparatus 10. The # is at least one of an alphanumeric character and a symbol.

[0023] The phrase "at least one" as used herein means "one or more" of the desired options. As an example, the phrase "at least one" as used herein means "only one option" or "both of two options" if the number of options is two. As another example, the phrase "at least one" as used herein means "only one option" or "any combination of two or more options" if the number of options is three or more.

[0024] <First Evaporator EV1 and Condenser C1> The first evaporator EV1 can selectively separate the first exhaust gas, which is a gas containing water and ammonia, from the urea aqueous solution. The first evaporator EV1 obtains a concentrated urea aqueous solution, which is a liquid having a higher urea concentration than the urea aqueous solution. The first exhaust gas may contain urea. The urea content of the concentrated urea aqueous solution is not particularly limited, but is, for example, 80 wt % or more.

[0025] The first evaporator EV1 is not particularly limited, and may be, for example, a known heat exchanger such as a shell-and-tube heat exchanger. Regarding the operating conditions of the first evaporator EV1, the pressure is preferably, for example, 10 kPa or more and 80 kPa or less in absolute pressure, and the temperature is preferably, for example, 90°C or more and 140°C or less.

[0026] The condenser C1 cools the first exhaust gas to obtain condensed water. The condenser C1 includes a first evaporator EV1 and an ejector Ej for reducing the pressure inside the condenser C1. One example of the ejector Ej is driven by steam. FIG. 1 illustrates a line LS1 for supplying steam to drive the ejector Ej. If the first exhaust gas contains urea, the condensed water also contains urea.

[0027] The urea aqueous solution is supplied to the first evaporator EV1 through a line L1. The concentrated urea aqueous solution is discharged from the first evaporator EV1 through a line L2. The first exhaust gas is discharged from the first evaporator EV1 through a line L8.

[0028] The condensed water is supplied to the scrubber SCR. The condensed water supplied to the scrubber SCR may be a part or all of the condensed water obtained by the condenser C1. Fig. 1 illustrates a line L10 for supplying the condensed water from the condenser C1 to the scrubber SCR. The line L10 corresponds to a supply pipe for supplying at least a part of the condensed water to the scrubber SCR.

[0029] The expression "at least a portion" as used herein means a portion or all of the condensed water. The condensed water may be supplied to the urea synthesis unit U0. For example, the condensed water may be supplied to the recovery device U3 via line L9.

[0030] In this embodiment, two lines L9 and L10 for discharging condensed water from the condenser C1 are illustrated as an example. Alternatively, one line for discharging condensed water from the condenser C1 may be branched, one of which is connected to the scrubber SCR and the other of which is connected to the urea synthesis unit U0.

[0031] <Second Evaporator EV2> The second evaporator EV2 selectively separates the second exhaust gas, which is a gas containing water and ammonia, from the concentrated urea aqueous solution to obtain a high-concentration urea solution, which is a liquid having a higher urea concentration than the concentrated urea aqueous solution. The second evaporator EV2 is supplied with the post-cleaning recovery liquid described below. The second evaporator EV2 obtains the high-concentration urea solution and the second exhaust gas from a mixture of the concentrated urea aqueous solution and the post-cleaning recovery liquid. The urea content of the high-concentration urea solution is, for example, 95 wt % or more and 99.8 wt % or less. The urea content of the high-concentration urea solution is preferably 97 wt % or more and 99.8 wt % or less.

[0032] 1 illustrates a line L16 through which the post-cleaning recovery liquid is supplied. For example, as shown in FIG. 1, by joining the line L2 and the line L16, a mixture of the concentrated urea aqueous solution and the post-cleaning recovery liquid can be supplied to the second evaporator EV2 via the line L3. Alternatively, each of the line L2 and the line L16 may be connected to the second evaporator EV2.

[0033] The second evaporator EV2 is preferably a falling film type evaporator. For example, the second evaporator EV2 is configured to allow countercurrent contact between the concentrated aqueous urea solution and heated air under atmospheric pressure.

[0034] The second evaporator EV2 is preferably operated at atmospheric pressure, but is not limited to this. The operating conditions of the second evaporator EV2 may be a pressure close to atmospheric pressure, and may be a pressure higher or lower than atmospheric pressure. The operating conditions are preferably, for example, an absolute pressure of 90 kPa or higher and 110 kPa or lower. Regarding the operating conditions of the second evaporator EV2, the heating temperature is preferably, for example, 130°C or higher and 140°C or lower.

[0035] The second evaporator EV2 is not limited to the above configuration. The second evaporator EV2 may be configured to supply the second exhaust gas, which is a gas containing water and ammonia selectively separated from the concentrated urea aqueous solution, to the scrubber SCR. For example, the second evaporator EV2 may be configured to supply the exhaust gas from the second evaporator EV2, which operates at a pressure lower than atmospheric pressure, to the scrubber using an ejector or a blower. A type in which gas and liquid flow in parallel inside the pipe may also be used.

[0036] The high-concentration urea solution is discharged from the second evaporator EV2 to a line L4. Heated air is supplied to the second evaporator EV2 from a line LA1. The second exhaust air is supplied from the second evaporator EV2 to a scrubber SCR via a line L11. The second exhaust air contains the heated air that has passed through the second evaporator EV2. That is, the gas separated from the concentrated urea aqueous solution and the heated air are supplied to the scrubber SCR via a line L11.

[0037] <Scrubber SCR> The scrubber SCR brings the second exhaust gas supplied as a gas from the second evaporator EV2 and the third exhaust gas discharged from the granulation device F into contact with an acidic washing liquid containing an acid, thereby obtaining a clean gas and a post-wash recovery liquid containing an ammonium salt and urea.

[0038] The scrubber SCR is a device that suppresses ammonia discharge to the outside of the system of the urea production plant 10. The scrubber SCR can discharge clean gas from a line L15 to the outside of the system of the urea production plant 10. The clean gas is a treated gas with a low ammonia concentration.

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

[0040] Water is supplied to the scrubber SCR to adjust the concentration of the liquid in the scrubber and to compensate for water evaporation loss. The scrubber SCR can use at least a portion of the condensed water as makeup water to replenish the cleaning liquid. The condensed water is supplied from the condenser C1 as described above. Water may also be supplied to the scrubber SCR from the line L13. An example of the cleaning liquid is a liquid mixture of the above-mentioned acid, water, and condensed water. The cleaning liquid may also be a liquid mixture of the above-mentioned acid and condensed water.

[0041] <Granulation Apparatus F> The granulation apparatus F obtains a solid urea product from a high-concentration urea solution. Fig. 1 illustrates a line L6 through which the high-concentration urea solution is supplied. Fig. 1 also illustrates a line L7 through which the solid urea product is discharged from the granulation apparatus F.

[0042] The granulation apparatus F is not particularly limited, but is preferably an apparatus that forms a fluidized bed by using heated air as fluidizing air. Hereinafter, this apparatus may also be referred to as a fluidized bed granulator. An example of a fluidized bed granulator is the apparatus described in JP-A-62-74443. The granulation apparatus F may be a granulation tower that produces small prilled urea. The particle size of the solid urea product obtained when a granulation tower is used is smaller than the particle size of the solid urea product obtained when a fluidized bed granulator is used. In other words, the particle size of the solid urea product is not particularly limited.

[0043] Urea dust is generated in the granulation apparatus F. The urea dust is supplied to the scrubber SCR as third exhaust gas. Figure 1 illustrates a line L12 for discharging the third exhaust gas from the granulation apparatus F.

[0044] When a fluidized bed granulator is used as the granulation apparatus F, the urea production apparatus 10 may be provided with an air supply device FAN configured to supply heated air to the granulation apparatus F. FIG. 1 illustrates a line LA2 for supplying heated air from the air supply device FAN to the granulation apparatus F. When the urea production apparatus 10 is provided with the air supply device FAN, the heated air that is brought into countercurrent contact with the concentrated urea aqueous solution in the second evaporator EV2 may be supplied from the air supply device FAN. That is, the heated air supplied from the air supply device FAN may be branched and supplied to the second evaporator EV2 and the granulation apparatus F. Alternatively, the urea production apparatus 10 may be provided with two air supply devices. That is, the urea production apparatus 10 may be provided with an air supply device that supplies heated air to the second evaporator EV2 and an air supply device that supplies heated air to the granulation apparatus F.

[0045] <Additives> The urea production apparatus 10 may be configured to supply additives to the liquid between the first evaporator EV1 and the granulation device F. Specifically, the urea production apparatus 10 may be configured to supply additives to the liquid downstream of the second evaporator EV2 and upstream of the granulation device F. Alternatively, the urea production apparatus 10 may be configured to supply additives to the liquid downstream of the first evaporator EV1 and upstream of the second evaporator EV2. Alternatively, the additives may be supplied to both the liquid downstream of the second evaporator EV2 and upstream of the granulation device F and the liquid downstream of the first evaporator EV1 and upstream of the second evaporator EV2.

[0046] 1 corresponds to an addition pipe that supplies additives and ammonium salts for adjusting the ammonium salt concentration in the product to the liquid downstream of the second evaporator EV2 and upstream of the granulation apparatus F. For example, as shown in FIG. 1, by joining the lines L4 and L5, the high-concentration urea solution and additives can be supplied to the granulation apparatus F via the line L6.

[0047] The additives are components supplied from outside the system of the urea production apparatus 10. Examples of the additives include formaldehyde, a urea formaldehyde solution, calcium nitrate, potassium nitrate, and a mixture of polyvinyl alcohol and calcium sulfate.

[0048] The ammonium salt for adjusting the ammonium salt concentration of the product is a component supplied from outside the system of the urea manufacturing apparatus 10. Examples of the ammonium salt include ammonium nitrate and ammonium sulfate.

[0049] The ammonium salt for adjusting the ammonium salt concentration of the additive or the product may be any of the above components, or two or more components may be used in appropriate combination. When both the additive and the ammonium salt for adjusting the ammonium salt concentration of the product are used, the urea production apparatus 10 may be equipped with one addition pipe, or may be equipped with a first addition pipe for supplying the additive and a second addition pipe for supplying the ammonium salt.

[0050] The ammonium salt for adjusting the ammonium salt concentration of the product may be supplied from outside the system, or may be the acid supplied to the cleaning liquid of the scrubber and ammonia (NH ３ For example, a post-wash recovery liquid containing an ammonium salt can be supplied via line L16.

[0051] <Urea Production Method> An example of a urea production method for obtaining a solid urea product from an aqueous urea solution will be described. The example of the urea production method in this embodiment includes a first evaporation step, a condensation step, a second evaporation step, a granulation step, and a washing step.

[0052] The first evaporation step is a step of selectively separating the first exhaust gas, which is a gas containing water and ammonia, from the urea aqueous solution to obtain a concentrated urea aqueous solution, which is a liquid having a higher urea concentration than the urea aqueous solution.

[0053] The condensation step is a step of cooling the first exhaust gas to obtain condensed water. The second evaporation step is a step of selectively separating the second exhaust gas, which is a gas containing water and ammonia, from the concentrated urea aqueous solution to obtain a high-concentration urea solution, which is a liquid having a higher urea concentration than the concentrated urea aqueous solution. Here, the second evaporation step is a step of obtaining the high-concentration urea solution and the second exhaust gas from a mixed liquid of the concentrated urea aqueous solution and a post-cleaning recovery liquid described below.

[0054] The granulation step is a step of obtaining a solid urea product from the high-concentration urea solution, and the washing step is a step of obtaining a clean gas and a post-wash recovery solution containing ammonium salt and urea by contacting the second exhaust gas obtained in the second evaporation step and the third exhaust gas discharged in the granulation step with an acidic washing solution containing an acid.

[0055] In the cleaning step, at least a part of the condensed water may be used as makeup water to replenish the cleaning liquid. In the second evaporation step, a falling film evaporator configured to allow countercurrent contact between the concentrated aqueous urea solution and heated air at atmospheric pressure may be used. In this case, the second exhaust gas contains the heated air after passing through the evaporator.

[0056] The method for producing urea may further include a step of supplying an additive or an ammonium salt for adjusting the ammonium salt concentration in the product to the liquid between the first evaporation step and the granulation step. <Functions and Effects> The functions of this embodiment will be described.

[0057] In this embodiment, the third exhaust gas containing urea dust discharged from the granulation device F is treated in the scrubber SCR. The post-washing recovered liquid obtained by the scrubber SCR is supplied to the second evaporator EV2. In the second evaporator EV2, a high-concentration urea solution and the second exhaust gas are obtained from a mixture of the concentrated urea aqueous solution obtained by the first evaporator EV1 and the post-washing recovered liquid. In this way, the post-washing recovered liquid from which the urea contained in the third exhaust gas has been recovered can be supplied again to the second evaporator EV2.

[0058] In this embodiment, the second exhaust gas is supplied to the scrubber SCR in addition to the exhaust gas from the granulation device F. Therefore, the scrubber SCR requires a large amount of water to treat the exhaust gas. Therefore, in this embodiment, condensed water obtained by condensing the first exhaust gas is supplied to the scrubber SCR, thereby replenishing the water for the scrubber SCR.

[0059] Incidentally, the first exhaust gas and the condensed water obtained by condensing the first exhaust gas may contain urea. In this embodiment, condensed water that does not go through wastewater treatment is treated in the scrubber SCR and then recovered as post-cleaning recovered liquid. Therefore, if the condensed water contains urea, the urea in the condensed water is supplied to the second evaporator EV2 without being decomposed. This makes it possible to recover the urea from the condensed water while replenishing water to the scrubber SCR.

[0060] In addition, the second exhaust gas may contain urea. When the second exhaust gas contains urea, the post-cleaning recovery liquid from which the urea contained in the second exhaust gas has been recovered can be supplied again to the second evaporator EV2.

[0061] The effects of this embodiment will be described below. (1-1) Urea can be efficiently recovered while the second exhaust gas and the third exhaust gas are treated by the scrubber SCR. This improves the efficiency of urea production.

[0062] (1-2) Urea contained in the condensed water can be recovered without hydrolysis, thereby improving the efficiency of urea production. (1-3) By operating the second evaporator EV2 at atmospheric pressure, the second exhaust gas obtained by the second evaporator EV2 can be supplied directly to the scrubber SCR.

[0063] (1-4) When heated air is supplied to the second evaporator EV2, the second exhaust gas supplied to the scrubber SCR contains heated air. This makes it easier for water to evaporate in the scrubber SCR. By increasing the amount of water evaporated in the scrubber SCR, even when a large amount of condensed water is supplied to the scrubber SCR, all of the condensed water can be treated by the scrubber SCR. Because excess condensed water is less likely to be generated, there is no need to provide a separate wastewater treatment unit or the like to treat the condensed water.

[0064] (1-5) Ammonium salts produced by the scrubber SCR process can be mixed with the solid urea product, thereby adding characteristics derived from the ammonium salts to the solid urea product. For example, by mixing ammonium sulfate with the solid urea product, the solid urea product can be used as an effective fertilizer for alkaline soil.

[0065] (1-6) Urea can be recovered without decomposing it while suppressing its discharge outside the system. This allows for efficient production of a solid urea product. (1-7) Ammonium salt can be recovered without being discharged outside the system. This allows for efficient production of a solid urea product containing ammonium salt. Since there is no need to install treatment equipment such as urea mixing equipment or concentration and solidification equipment for treating ammonium salt outside the system, the increase in equipment can be suppressed compared to a configuration in which ammonium salt is treated outside the system.

[0066] (1-8) By sharing the air supply device FAN that supplies heated air between the second evaporator EV2 and the granulation device F, the increase in equipment can be suppressed compared to a configuration with two air supply devices.

[0067] (1-9) The quality of the solid urea product can be improved by adding an additive or an ammonium salt for adjusting the ammonium salt concentration of the product. By adding an additive, characteristics can be added to the solid urea product. For example, when an ammonium salt corresponding to the acid supplied to the scrubber SCR is supplied, the concentration of the ammonium salt contained in the solid urea product can be increased.

[0068] (1-10) In the urea production process, the use of steam consumes a large amount of energy. In this regard, in the present embodiment, it is possible to reduce the amount of steam used in the process of producing a solid urea product from a urea aqueous solution. For example, in the present embodiment, the second exhaust gas is not condensed. Therefore, a condenser and an ejector are not required. This makes it possible to reduce the amount of steam used in the process of producing a solid urea product.

[0069] (1-11) The urea production apparatus 10 of this embodiment does not require a wastewater treatment unit. Because steam for operating a wastewater treatment unit is not required, the amount of steam used in the process of producing a solid urea product can be reduced compared to when a wastewater treatment unit is operated.

[0070] (1-12) The urea production apparatus 10 of this embodiment can reduce the amount of equipment compared to a configuration in which multiple evaporators each have a condenser. (1-13) The urea production apparatus 10 of this embodiment does not require a wastewater treatment unit. Therefore, the urea production apparatus 10 can reduce the amount of equipment compared to a configuration in which a wastewater treatment unit is provided.

[0071] Second Embodiment A urea production apparatus 20 according to a second embodiment will be described with reference to Fig. 2. The urea production apparatus 20 differs from the first embodiment in that a stripper tower ST is provided between the condenser C1 and the scrubber SCR. In the urea production apparatus 20, components common to the first embodiment are denoted by the same reference numerals as in the first embodiment. Description of components common to the first embodiment will be omitted where appropriate.

[0072] <Stripping tower ST> The stripping tower ST strips the condensed water with steam to obtain a gas containing ammonia and treated condensed water. Figure 2 shows an example of a line LSS that supplies steam to the stripping tower ST. The treated condensed water is a liquid that contains less ammonia than the condensed water. The treated condensed water is a liquid that contains, for example, urea.

[0073] The condensed water discharged from the condenser C1 is supplied to the stripper tower ST via a line L10'. The treated condensed water is discharged from the stripper tower ST to a line L17. The treated condensed water is supplied to the scrubber SCR from the line L17. The line L17 corresponds to a supply pipe that supplies the treated condensed water to the scrubber SCR. The scrubber SCR in the urea production system 20 can have the same configuration as the scrubber SCR in the first embodiment, except that the treated condensed water is used instead of condensed water.

[0074] The ammonia-containing gas is discharged from the stripper ST to a line L18. The ammonia-containing gas may be supplied to the urea synthesis unit U0. For example, the ammonia-containing gas may be supplied to the purification device U2 via the line L18.

[0075] <Urea Production Method> An example of a urea production method for obtaining a solid urea product from an aqueous urea solution will be described. The urea production method of this embodiment differs from the urea production method of the first embodiment in the following respects. The other steps are common to the urea production method of the first embodiment.

[0076] The urea production method of the present embodiment further includes a stripping step in which condensed water is stripped with steam to obtain a gas containing ammonia and treated condensed water. In the urea production method of the present embodiment, the cleaning step uses the treated condensed water as makeup water to replenish the cleaning liquid.

[0077] <Functions and Effects> According to this embodiment, in addition to the effects obtained by the first embodiment, the following effects can be obtained.

[0078] (2-1) According to this embodiment, ammonia can be removed from the condensed water by the stripper tower ST. The condensed water treated by the stripper tower ST is supplied to the scrubber SCR. Therefore, the amount of acid used in the scrubber SCR can be reduced compared to when the condensed water is not treated by the stripper tower ST.

[0079] Third Embodiment A urea production apparatus 30 according to a third embodiment will be described with reference to Fig. 3. In the urea production apparatus 30, components common to the urea production apparatus 10 according to the first embodiment will be denoted by the same reference numerals, and descriptions thereof will be omitted as appropriate.

[0080] The urea production apparatus 30 includes a first stage evaporator EVa, a second stage evaporator EVb, and a third stage evaporator EVc, and also includes a first condenser C1 and a second condenser C2.

[0081] The urea production plant 30 includes a first stage scrubber SCR1 and a second stage scrubber SCR2. The urea production plant 30 includes a wastewater 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 this order. Aqueous urea solution is supplied to the first-stage evaporator EVa through line L1. The third-stage evaporator EVc discharges high-concentration urea solution into 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. Regarding the operating conditions of the second-stage evaporator EVb, the pressure is preferably, for example, 10 kPa or more and 40 kPa or less in absolute pressure, and the temperature is preferably, for example, 120°C or more and 140°C or less.

[0084] The third-stage evaporator EVc may be the same as the second evaporator EV2. Heated air may be supplied to the third-stage evaporator EVc from a line LA1. The heated air is preferably supplied from the air supply device FAN that supplies heated air to the granulation device F.

[0085] FIG. 3 shows lines L2', L4", and L6". A liquid having a higher urea concentration than the urea aqueous solution by the first-stage evaporator EVa is discharged from the first-stage evaporator EVa to line L2'. The second-stage evaporator EVb obtains a concentrated urea aqueous solution from a liquid having a higher urea concentration than the urea aqueous solution supplied from line L2'. The concentrated urea aqueous solution is discharged from the second-stage evaporator EVb to line L4". The concentrated urea aqueous solution is supplied to the third-stage evaporator EVc from line L4". By merging line L30 and line L5, the high-concentration urea solution and additives can be supplied to the granulation device F via line L6".

[0086] 3 illustrates lines L8, L11', and L31. Line L8 supplies the first exhaust gas discharged from the first-stage evaporator EVa to the first condenser C1. Line L11' supplies the exhaust gas discharged from the second-stage evaporator EVb to the second condenser C2. Line L31 supplies the exhaust gas discharged from the third-stage evaporator EVc to the first-stage scrubber SCR1.

[0087] <Condensers C1, C2> The first condenser C1 and the second condenser C2 may have the same configuration as the condenser C1 in the urea production apparatus 10 of the first embodiment. Hereinafter, the condensed water obtained by the first condenser C1 may also be referred to as first condensed water. The condensed water obtained by the second condenser C2 may also be referred to as second condensed water.

[0088] 3 illustrates lines LS1, LS2, L21, and L22. The line LS1 supplies steam to an ejector Ej for depressurizing the first-stage evaporator EVa and the first condenser C1. The line LS2 supplies steam to an ejector Ej for depressurizing the second-stage evaporator EVb and the second condenser C2.

[0089] A line L21 supplies condensed water from the first condenser C1 to the wastewater treatment unit WWT, and a line L22 supplies condensed water from the second condenser C2 to the wastewater treatment unit WWT.

[0090] <Wastewater Treatment Unit WWT> The wastewater treatment unit WWT can treat condensed water and is composed of a stripping tower ST.

[0091] The stripping tower ST strips the condensed water with steam to obtain a gas containing ammonia and a liquid containing urea. Hereinafter, the liquid obtained by the wastewater treatment unit WWT may also be referred to as treated wastewater.

[0092] 3 illustrates lines L23, L24, L26, and L27. Line L23 supplies treated wastewater from the wastewater treatment unit WWT to the first-stage scrubber SCR1. Line L24 supplies treated wastewater from the wastewater treatment unit WWT to the second-stage scrubber SCR2. Lines L23 and L24 correspond to supply pipes that supply treated condensed water to the scrubbers after the condensed water has been treated in the stripper tower ST. Line L26 supplies ammonia-containing gas from the wastewater treatment unit WWT to a urea synthesis unit U0, for example, a purification unit U2. Line L27 supplies wastewater from the wastewater treatment unit WWT to a urea synthesis unit U0, for example, a recovery unit U3.

[0093] 3 are not components of the urea production apparatus 30. As an example, the urea production apparatus 30 does not include a line L25. As another example, although a pipe corresponding to the line L25 is connected to the urea production apparatus 30, the urea production apparatus 30 is not configured to discharge treated wastewater from the pipe.

[0094] <Scrubbers SCR1, SCR2> A two-stage scrubber is composed of a first-stage scrubber SCR1 and a second-stage scrubber SCR2.

[0095] The first-stage scrubber SCR1 performs water washing. The first-stage scrubber SCR1 brings the exhaust gas containing urea dust discharged from the granulator F and the exhaust gas discharged from the third-stage evaporator EVc into contact with treated wastewater supplied from the wastewater treatment unit WWT, thereby obtaining a first-stage post-wash recovery liquid and a first-stage wash gas. The first-stage post-wash recovery liquid contains urea.

[0096] A portion of the first-stage post-scrubbing recovery liquid is supplied to the urea synthesis unit U0. For example, the first-stage post-scrubbing recovery liquid is supplied to the purification device U2 via line L28. The second-stage scrubber SCR2 performs acid washing. The second-stage scrubber SCR2 obtains clean gas and second-stage post-scrubbing recovery liquid containing ammonium salts from the first-stage scrubbing gas, treated wastewater supplied from the wastewater treatment unit WWT, and acid supplied from line L14.

[0097] The urea production apparatus 30 is configured to supply the second-stage post-cleaning recovered liquid to the third-stage evaporator EVc. Fig. 3 illustrates a line L29' for supplying the post-cleaning recovered liquid from the second-stage scrubber SCR2 to the third-stage evaporator EVc. The second-stage post-cleaning recovered liquid may be supplied to the third-stage evaporator EVc by merging the line L4'' and the line L29', or may be supplied directly to the third-stage evaporator EVc.

[0098] [Correspondence] The correspondence between the matters in this embodiment and the matters described in the "Means for solving the problems" section above is as follows.

[0099] At least one of the first-stage evaporator EVa and the second-stage evaporator EVb corresponds to the "first evaporator." It can also be said that an evaporator unit including the first-stage evaporator EVa and the second-stage evaporator EVb connected in series corresponds to the first evaporator. The third-stage evaporator EVc corresponds to the "second evaporator."

[0100] <Method of Improving Urea Production Apparatus> The urea production apparatus 30 may be manufactured as a new apparatus or may be manufactured by improving an existing urea production apparatus. Hereinafter, a method of improving a urea production apparatus will be described.

[0101] [Example of an existing first urea production apparatus] An example of an existing first urea production apparatus will be described. Fig. 4 shows an existing first urea production apparatus 100. Regarding the existing first urea production apparatus 100, the description of the components common to the urea production apparatus 30 will be omitted as appropriate.

[0102] The existing first urea production apparatus 100 includes a first-stage evaporator EVa and a second-stage evaporator EVb. The first-stage evaporator EVa and the second-stage evaporator EVb are arranged in series. A high-concentration urea solution is discharged from the second-stage evaporator EVb to a line L4'. By merging the lines L4' and L5, the high-concentration urea solution, additives, and ammonium salts for adjusting the ammonium salt concentration of the product can be supplied to the granulation apparatus F via a line L6'.

[0103] The wastewater treatment unit WWT in the urea production apparatus 100 is composed of a urea hydrolyzer UH and a stripper tower ST. The stripper tower ST strips the condensed 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 existing first urea production apparatus 100, the lines L23 and L24 correspond to the supply pipes that supply treated wastewater obtained in the wastewater treatment unit to the scrubber.

[0105] The urea production apparatus 100 may be configured so that a portion of the wastewater after treatment by the wastewater treatment unit WWT is discharged to the outside of the system of the urea production apparatus 100. Fig. 4 shows an example of a line L25 for discharging the treated wastewater from the wastewater treatment unit WWT to the outside of the system. The line L25 corresponds to a drain pipe for discharging the treated wastewater obtained in the wastewater treatment unit to the outside of the system of the urea production apparatus.

[0106] The urea production plant 100 is configured so that the second-stage post-cleaning recovered liquid after treatment by the second-stage scrubber SCR2 is discharged to the outside of the system of the urea production plant 100. Fig. 4 illustrates an example of a line L29 for discharging the liquid from the second-stage scrubber SCR2 to the outside of the system.

[0107] [Improvement Method] The method for improving the existing first urea production apparatus 100 shown in FIG. 4 is as follows.

[0108] As shown in Figures 3 and 4, in the method for improving the urea production apparatus, a third-stage evaporator EVc is added between the first-stage evaporator EVa and the granulation apparatus F. More specifically, the third-stage evaporator EVc is added between the second-stage evaporator EVb and the granulation apparatus F. The third-stage evaporator EVc corresponds to an additional evaporator. The additional evaporator, for example, further concentrates the liquid before it is supplied to the granulation apparatus F to obtain a high-concentration urea liquid, which is a liquid with a higher urea concentration than the liquid, and additional exhaust gas, which is a gas containing water and ammonia. Note that, because the liquid discharged from the evaporator upstream of the additional evaporator is mixed with the second-stage cleaning liquid and the liquid with a reduced concentration is supplied to the additional evaporator, the urea concentration of the liquid discharged from the evaporator upstream of the additional evaporator may not change.

[0109] 3 and 4, in the method for improving the urea production plant, a line L31 is added to supply the exhaust gas discharged from the third-stage evaporator EVc to the first-stage scrubber SCR1. The line L31 corresponds to an exhaust pipe that supplies the additional exhaust gas to the scrubber.

[0110] As shown in FIGS. 3 and 4 , in the method for improving a urea production apparatus, a line L29′ is added to supply the post-second-stage cleaning recovered liquid discharged from the second-stage scrubber SCR2 to the third-stage evaporator EVc. The line L29′ corresponds to a recovery pipe that supplies the post-second-stage cleaning recovered liquid to the additional evaporator. The line L29′ may be the line L29 provided in the existing first urea production apparatus 100, connected to the third-stage evaporator EVc. The line L29′ may be provided as a pipe separate from the line L29 provided in the existing first urea production apparatus 100. In this case, the line L29 may be removed, or the line L29 may be configured so that the post-second-stage cleaning recovered liquid is not discharged from the line L29.

[0111] In the method for improving a urea production plant, the operation of the urea hydrolyzer UH in the wastewater treatment unit WWT is stopped. By not operating the urea hydrolyzer UH, treated wastewater after condensed water has been treated in the stripper tower ST is supplied to a scrubber via lines L23 and L24.

[0112] If the existing first urea production apparatus 100 is equipped with a line L25 for discharging treated wastewater obtained in the wastewater treatment unit WWT to the outside of the urea production apparatus 100, the method for improving the urea production apparatus may be configured to not discharge wastewater from the line L25. For example, the method for improving the urea production apparatus may be configured to not discharge wastewater from the line L25 by stopping use of the line L25. Furthermore, the method for improving the urea production apparatus may be configured to remove the line L25, for example.

[0113] <Actions and Effects> This embodiment can achieve the same effects (1-1) to (1-10) and (1-12) as those achieved by the first embodiment, and the same effect (2-1) as that achieved by the second embodiment. Furthermore, although steam is required to operate the urea hydrolyzer UH, this embodiment can reduce the amount of steam consumed by not operating the urea hydrolyzer UH.

[0114] [Fourth embodiment] A urea production apparatus 40 according to a fourth embodiment will be described with reference to Fig. 5. In the urea production apparatus 40, components common to the urea production apparatus 30 according to the third embodiment will be denoted by the same reference numerals, and descriptions thereof will be omitted as appropriate.

[0115] The urea production plant 40 is configured so that condensed water discharged from the first condenser C1 and the second condenser C2 is supplied to the first stage scrubber SCR1 and the second stage scrubber SCR2.

[0116] The urea production apparatus 40 is configured to supply condensed 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 be described.

[0117] 5 illustrates lines L33, L23', and L24'. Line L23' supplies condensed water to the first-stage scrubber SCR1. Line L24' supplies condensed water to the second-stage scrubber SCR2. The condensed water discharged from the first condenser C1 and the condensed water discharged from the second condenser C2 are joined together, and a portion of the joined condensed water is supplied to lines L23' and L24' via line L33.

[0118] 5 illustrates lines L32 and L27'. The line L27' supplies condensed water to the recovery device U3. The condensed water discharged from the first condenser C1 and the condensed water discharged from the second condenser C2 are combined, and another portion of the combined condensed water is supplied to the line L27' via the line L32.

[0119] 5 shows an example in which the condensed water discharged from the first condenser C1 and the condensed water discharged from the second condenser C2 are joined together and then supplied to the scrubbers SCR1 and SCR2 and the urea synthesis unit U0. Alternatively, the condensed water discharged from the first condenser C1 and the condensed water discharged from the second condenser C2 may be supplied to the scrubbers SCR1 and SCR2 and the urea synthesis unit U0, respectively.

[0120] 5 are not components of the urea production apparatus 40. As an example, the urea production apparatus 40 does not include a wastewater treatment unit WWT. As another example, in the urea production apparatus 40, the line through which condensed water is supplied to the wastewater treatment unit WWT is stopped.

[0121] [Correspondence] The correspondence between the matters in this embodiment and the matters described in the "Means for solving the problems" section above is as follows.

[0122] At least one of the first-stage evaporator EVa and the second-stage evaporator EVb corresponds to the "first evaporator." It can also be said that an evaporator unit including the first-stage evaporator EVa and the second-stage evaporator EVb connected in series corresponds to the first evaporator. The third-stage evaporator EVc corresponds to the "second evaporator."

[0123] <Method of Improving Urea Production Apparatus> The urea production apparatus 40 may be manufactured as a new apparatus, or may be manufactured by improving the existing first urea production apparatus 100. Hereinafter, a method of improving the urea production apparatus will be described.

[0124] [Improvement Method] The method for improving the existing first urea production apparatus 100 shown in FIG. 4 is as follows.

[0125] 4 and 5, in the method for improving the urea production apparatus, a third-stage evaporator EVc is added between the first-stage evaporator EVa and the granulation apparatus F. More specifically, the third-stage evaporator EVc is added between the second-stage evaporator EVb and the granulation apparatus F. The third-stage evaporator EVc corresponds to an additional evaporator. The additional evaporator, for example, further concentrates the liquid before it is supplied to the granulation apparatus F, thereby obtaining a high-concentration urea solution, which is a liquid having a higher urea concentration than the liquid, and additional exhaust gas, which is a gas containing water and ammonia.

[0126] 4 and 5, in the method for improving the urea production plant, a line L31 is added to supply the exhaust gas discharged from the third-stage evaporator EVc to the first-stage scrubber SCR1. The line L31 corresponds to an exhaust pipe that supplies the additional exhaust gas to the scrubber.

[0127] 4 and 5, in the method for improving a urea production plant, a line L29' is added to supply the second-stage scrubbed recovered liquid discharged from the second-stage scrubber SCR2 to the third-stage evaporator EVc. The line L29' corresponds to a recovery pipe that supplies the second-stage scrubbed recovered liquid to the additional evaporator.

[0128] In the method for improving a urea production apparatus, the operation of the urea hydrolyzer UH and the stripper tower ST in the wastewater treatment unit WWT is stopped. Also, in the method for improving a urea production apparatus, a line L33 is added to supply condensed water to a scrubber. In the method for improving a urea production apparatus, a line L32 may be added to supply condensed water to the urea synthesis unit U0.

[0129] <Operations and Effects> According to this embodiment, the same effects as those (1-1) to (1-9), (1-11) and (1-13) obtained by the first embodiment can be obtained.

[0130] Fifth Embodiment A urea production apparatus 50 according to a fifth embodiment will be described with reference to Fig. 6. In the urea production apparatus 50, components common to the urea production apparatus 30 according to the third embodiment will be denoted by the same reference numerals, and descriptions thereof will be omitted as appropriate.

[0131] In the urea production plant 50, the first-stage evaporator EVa and the third-stage evaporator EVc are directly connected in series. In Fig. 6, lines L2'' and L3' are shown as examples.

[0132] The concentrated urea aqueous solution is discharged from the first-stage evaporator EVa to a line L2''. By joining the line L2'' and a line L29', a mixture of the concentrated urea aqueous solution and the second-stage post-wash recovery liquid is supplied to the third-stage evaporator EVc via a line L3'.

[0133] 6 are not components of the urea production apparatus 50. As an example, the urea production apparatus 50 does not include a second-stage evaporator EVb. As 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] As an example, the urea production apparatus 50 does not include a second condenser C2. As another example, the second condenser C2 is stopped in the urea production apparatus 50. As an example, the urea production apparatus 50 does not include a line L25. As another example, although a pipe corresponding to the line L25 is connected to the urea production apparatus 50, the urea production apparatus 50 is not configured to discharge wastewater from the pipe.

[0135] [Correspondence] The correspondence between the matters in this embodiment and the matters described in the "Means for solving the problems" section above is as follows.

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

[0137] [Improvement Method] The method for improving the existing first urea production apparatus 100 shown in FIG. 4 is as follows.

[0138] As shown in Figures 4 and 6, in the method for improving the urea production apparatus, a third-stage evaporator EVc is added between the first-stage evaporator EVa and the granulation apparatus F. More specifically, the second-stage evaporator EVb is replaced with the third-stage evaporator EVc. The second condenser C2 is then removed. The third-stage evaporator EVc corresponds to an additional evaporator that further concentrates the liquid before it is supplied to the granulation apparatus F to obtain a high-concentration urea solution, which is a liquid with a higher urea concentration than the liquid, and additional exhaust gas, which is a gas containing 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 improved as follows: A third-stage evaporator EVc is added between the first-stage evaporator EVa and the granulation apparatus F, and is connected in series with both the first-stage evaporator EVa and the granulation apparatus F. Then, the second-stage evaporator EVb and the second condenser C2 are stopped.

[0140] 4 and 6, in the method for improving the urea production plant, a line L31 is added to supply the exhaust gas discharged from the third-stage evaporator EVc to the first-stage scrubber SCR1. The line L31 corresponds to an exhaust pipe that supplies the additional exhaust gas to the scrubber.

[0141] As shown in FIGS. 4 and 6 , in the method for improving a urea production apparatus, a line L29′ is added to supply the post-second-stage cleaning recovered liquid discharged from the second-stage scrubber SCR2 to the third-stage evaporator EVc. The line L29′ corresponds to a recovery pipe that supplies the post-second-stage cleaning recovered liquid to the additional evaporator. The line L29′ may be the line L29 included in the existing first urea production apparatus 100, connected to the third-stage evaporator EVc. The line L29′ may be provided as a pipe separate from the line L29 included in the existing first urea production apparatus 100. In this case, the line L29 may be removed, or the line L29 may be configured so that the post-second-stage cleaning recovered liquid is not discharged from the line L29.

[0142] In the method for improving a urea production plant, the operation of the urea hydrolyzer UH in the wastewater treatment unit WWT is stopped. By not operating the urea hydrolyzer UH, treated wastewater after condensed water has been treated in the stripper tower ST is supplied to a scrubber via lines L23 and L24.

[0143] If the existing first urea production apparatus 100 is equipped with a line L25 for discharging treated wastewater obtained in the wastewater treatment unit WWT to the outside of the urea production apparatus 100, the method for improving the urea production apparatus may be configured to not discharge wastewater from the line L25. For example, the method for improving the urea production apparatus may be configured to not discharge wastewater from the line L25 by stopping use of the line L25. Furthermore, the method for improving the urea production apparatus may be configured to remove the line L25, for example.

[0144] <Actions and Effects> This embodiment can achieve the same effects (1-1) to (1-10) and (1-12) as those achieved by the first embodiment, and the same effect (2-1) as that achieved by the second embodiment. Furthermore, although steam is required to operate the urea hydrolyzer UH, this embodiment can reduce the amount of steam consumed by not operating the urea hydrolyzer UH.

[0145] Sixth Embodiment A urea production apparatus 60 according to a sixth embodiment will be described with reference to Fig. 7. In the urea production apparatus 60, components common to the urea production apparatus 50 according to the fifth embodiment will be denoted by the same reference numerals, and descriptions thereof will be omitted as appropriate.

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

[0147] An example of a urea production apparatus 60 will be described. Fig. 7 illustrates lines L33', L23', and L24'. Line L23' supplies condensed water to the first-stage scrubber SCR1. Line L24' supplies condensed water to the second-stage scrubber SCR2. A portion of the condensed water discharged from the first condenser C1 is supplied to lines L23' and L24' via line L33'.

[0148] 7 illustrates lines L32' and L27'. The line L27' supplies condensed water to the recovery device U3. Another portion of the condensed water discharged from the first condenser C1 is supplied to the line L27' via the line L32'.

[0149] 7 are not components of the urea production apparatus 60. As an example, the urea production apparatus 60 does not include a wastewater treatment unit WWT. As another example, in the urea production apparatus 60, the line through which condensed water is supplied to the wastewater treatment unit WWT is stopped.

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

[0151] Also, as an example, the urea production apparatus 60 does not include the second condenser C2. As another example, the second condenser C2 is stopped in the urea production apparatus 60. [Correspondence] The correspondence between the matters in this embodiment and the matters described in the above section "Means for solving the problem" is as follows.

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

[0153] [Improvement Method] The method for improving the existing first urea production apparatus 100 shown in FIG. 4 is as follows.

[0154] As shown in Figures 4 and 7, in the method for improving the urea production apparatus, a third-stage evaporator EVc is added between the first-stage evaporator EVa and the granulation apparatus F. More specifically, the second-stage evaporator EVb is replaced with the third-stage evaporator EVc. The second condenser C2 is then removed. The third-stage evaporator EVc corresponds to an additional evaporator that further concentrates the liquid before it is supplied to the granulation apparatus F to obtain a high-concentration urea solution, which is a liquid with a higher urea concentration than the liquid, and additional exhaust gas, which is a gas containing 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 improved as follows: A third-stage evaporator EVc is added between the first-stage evaporator EVa and the granulation apparatus F, and is connected in series with both the first-stage evaporator EVa and the granulation apparatus F. Then, the second-stage evaporator EVb and the second condenser C2 are stopped.

[0156] 4 and 7, in the method for improving a urea production plant, a line L31 is added to supply exhaust gas discharged from the third-stage evaporator EVc to the first-stage scrubber SCR1. The line L31 corresponds to an exhaust pipe that supplies the additional exhaust gas to the scrubber.

[0157] As shown in FIGS. 4 and 7 , in the method for improving a urea production apparatus, a line L29′ is added to supply the post-second-stage cleaning recovered liquid discharged from the second-stage scrubber SCR2 to the third-stage evaporator EVc. The line L29′ corresponds to a recovery pipe that supplies the post-second-stage cleaning recovered liquid to the additional evaporator. The line L29′ may be the line L29 included in the existing first urea production apparatus 100, connected to the third-stage evaporator EVc. The line L29′ may be provided as a pipe separate from the line L29 included in the existing first urea production apparatus 100. In this case, the line L29 may be removed, or the line L29 may be configured so that the post-second-stage cleaning recovered liquid is not discharged from the line L29.

[0158] In the method for improving the urea production apparatus, the operation of the urea hydrolyzer UH and the stripper tower ST in the wastewater treatment unit WWT is stopped. Also, in the method for improving the urea production apparatus, a line L33' is added to supply condensed water to a scrubber. In the method for improving the urea production apparatus, a line L32' may be added to supply condensed water to the urea synthesis unit U0.

[0159] <Operations and Effects> According to this embodiment, the same effects as those (1-1) to (1-13) obtained by the first embodiment can be obtained.

[0160] [Seventh embodiment] A urea production apparatus 207 according to the seventh embodiment will be described with reference to Fig. 8. Differences between the urea production apparatus 207 and the urea production apparatus 50 according to the fifth embodiment will be described. In the urea production apparatus 207, components common to the urea production apparatus 50 according to the fifth embodiment will be denoted by the same reference numerals, and descriptions thereof will be omitted as appropriate.

[0161] The urea production plant 207 is equipped with a scrubber SCR. The scrubber SCR equipped in the urea production plant 207 can have the same configuration as the scrubber SCR equipped in the urea production plant 10 in the first embodiment. That is, the scrubber SCR equipped in the urea production plant 207 is a single-stage scrubber that uses an acidic cleaning solution containing an acid.

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

[0163] The concentrated urea aqueous solution is discharged from the first evaporator EVa to a line L2*. The line L2* and the line L16* are joined together, and a mixture of the concentrated urea aqueous solution and the post-cleaning recovered liquid is supplied to the second evaporator EVc via a line L3*.

[0164] 8 are not components of the urea production apparatus 207. As an example, the urea production apparatus 207 does not include a line L28* that supplies the post-cleaning recovered liquid obtained from the scrubber SCR to the urea synthesis unit U0. As another example, although a pipe corresponding to the line L28* is connected to the urea production apparatus 207, the urea production apparatus 207 is not configured to supply the post-cleaning recovered liquid from the pipe to the urea synthesis unit U0.

[0165] Also, as an example, the urea production apparatus 207 does not include the line L25. As another example, although a pipe corresponding to the line L25 is connected to the urea production apparatus 207, the urea production apparatus 207 is not configured to discharge wastewater from the pipe.

[0166] [Correspondence] The correspondence between the matters in this embodiment and the matters described in the "Means for solving the problems" section above is as follows.

[0167] The first evaporator EVa corresponds to the "first evaporator." The second evaporator EVc corresponds to the "second evaporator." <Method for improving the urea production apparatus> The urea production apparatus 207 may be manufactured as a new apparatus, or may be manufactured by improving an existing urea production apparatus. The method for improving the urea production apparatus will be described below.

[0168] [Example of an existing second urea production apparatus] An example of an existing second urea production apparatus will be described. Fig. 9 shows an existing second urea production apparatus 110. Regarding the existing second urea production apparatus 110, the description of the components common to the existing first urea production apparatus 100 shown in Fig. 4 will be omitted as appropriate.

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

[0170] In the existing second urea production apparatus 110, the line L23* corresponds to a supply pipe that supplies treated wastewater obtained in the wastewater treatment unit to the scrubber. The existing second urea production apparatus 110 is equipped with a water wash scrubber SCR*. ​​The water wash scrubber SCR* obtains clean gas and a post-wash recovery liquid by contacting exhaust gas discharged from the granulation apparatus F with a wash liquid. Unlike the scrubber SCR, the water wash scrubber SCR* does not supply acid to the wash liquid. Note that the clean gas discharged from the water wash scrubber SCR* via line L15* has a higher ammonia concentration than the clean gas discharged from the scrubber SCR via line L15. Furthermore, the post-wash recovery liquid obtained from the water wash scrubber SCR* has a lower ammonia concentration than the post-wash recovery liquid obtained from the scrubber SCR.

[0171] [Improvement Method] The method for improving the existing second urea production unit 110 shown in FIG. 9 is as follows.

[0172] 8 and 9, in the method for improving the urea production apparatus, a second evaporator EVc is added between the first evaporator EVa and the granulation apparatus F. The second evaporator EVc corresponds to an additional evaporator that further concentrates the liquid before it is supplied to the granulation apparatus F to obtain a high-concentration urea liquid, which is a liquid having a higher urea concentration than the liquid, and additional exhaust gas, which is a gas containing water and ammonia.

[0173] As shown in Figures 8 and 9, in the method for improving a urea production plant, the water-washing scrubber SCR* is replaced with a scrubber SCR. Alternatively, an acid supply pipe L14 for supplying acid to the scrubber is added, so that the water-washing scrubber SCR* functions as a scrubber SCR. The scrubber SCR may be configured to supply water from outside the system.

[0174] 8 and 9, in the method for improving the urea production plant, a line L31 is added to supply the exhaust gas discharged from the second evaporator EVc to the scrubber SCR. The line L31 corresponds to an exhaust pipe that supplies the additional exhaust gas to the scrubber.

[0175] 8 and 9, in the method for improving the urea production apparatus, the supply of the post-cleaning recovered liquid discharged from the scrubber SCR to the urea synthesis unit U0 is stopped. For example, the line L28* may be removed, or the line L28* may be configured not to discharge the post-cleaning recovered liquid.

[0176] As shown in Figures 8 and 9, in the method for improving the urea production apparatus, a line L16* is added to supply the post-cleaning recovered liquid discharged from the scrubber SCR to the second evaporator EVc. The line L16* corresponds to a recovery pipe that supplies the post-cleaning recovered liquid to the additional evaporator. The line L16* may be obtained by connecting the line L28* provided in the existing second urea production apparatus 110 to the second evaporator EVc. The line L16* may be provided as a pipe different from the line L28* provided in the existing second urea production apparatus 110.

[0177] In the improvement method of the urea production plant, the operation of the urea hydrolyzer UH in the wastewater treatment unit WWT is stopped. By not operating the urea hydrolyzer UH, the treated wastewater after the condensed water has been treated in the stripper tower ST is supplied to a scrubber via a line L23.

[0178] If the existing second urea production apparatus 110 is equipped with a line L25 for discharging treated wastewater obtained in the wastewater treatment unit WWT to the outside of the urea production apparatus 110, the method for improving the urea production apparatus may be configured to not discharge wastewater from the line L25. For example, the method for improving the urea production apparatus may be configured to not discharge wastewater from the line L25 by stopping use of the line L25. Furthermore, the method for improving the urea production apparatus may be configured to remove the line L25, for example.

[0179] <Functions and Effects> According to this embodiment, the same effects as those (1-1) to (1-10) and (1-12) obtained by the first embodiment, and the same effect as that (2-1) obtained by the second embodiment can be obtained.

[0180] Furthermore, according to the above-described improved method, by replacing the water wash scrubber SCR* with a scrubber SCR, the ammonia concentration in the purified gas can be reduced. By replacing the water wash scrubber SCR* with a scrubber SCR, the ammonia concentration in the post-scrubbing recovery liquid can be increased. This makes it possible to suppress ammonia from being discharged outside the system and improve the efficiency of ammonia recovery.

[0181] Eighth Embodiment A urea production apparatus 208 according to an eighth embodiment will be described with reference to Fig. 10. Differences between the urea production apparatus 208 and the urea production apparatus 60 according to the sixth embodiment will be described. In the urea production apparatus 208, components common to the urea production apparatus 60 according to the sixth embodiment will be denoted by the same reference numerals, and descriptions thereof will be omitted as appropriate.

[0182] The urea production plant 208 is equipped with a scrubber SCR. The scrubber SCR is a single-stage scrubber that uses an acidic cleaning solution containing an acid. In the urea production plant 208, a first evaporator EVa and a second evaporator EVc are connected in series.

[0183] 10 illustrates lines L2*, L3*, and L16*. A concentrated urea aqueous solution is discharged from the first evaporator EVa to line L2*. By merging line L2* and line L16*, a mixture of the concentrated urea aqueous solution and the post-cleaning recovered liquid is supplied to the second evaporator EVc via line L3*.

[0184] 10 is not a component of the urea production apparatus 208. As an example, the urea production apparatus 208 does not include a line L28* that supplies the post-cleaning recovered liquid obtained from the scrubber SCR to the urea synthesis unit U0. As another example, although a pipe corresponding to the line L28* is connected to the urea production apparatus 208, the urea production apparatus 208 is not configured to supply the post-cleaning recovered liquid from the pipe to the urea synthesis unit U0.

[0185] As another example, the urea production plant 208 does not include a wastewater treatment unit WWT. As another example, in the urea production plant 208, a line through which condensed water is supplied to the wastewater treatment unit WWT is stopped.

[0186] [Correspondence] The correspondence between the matters in this embodiment and the matters described in the "Means for solving the problems" section above is as follows.

[0187] The first evaporator EVa corresponds to the "first evaporator." The second evaporator EVc corresponds to the "second evaporator." <Method for improving the urea production apparatus> The urea production apparatus 208 may be manufactured as a new apparatus, or may be manufactured by improving an existing urea production apparatus. The method for improving the urea production apparatus will be described below.

[0188] [Improvement Method] The method for improving the existing second urea production unit 110 shown in FIG. 9 is as follows.

[0189] 9 and 10, in the method for improving the urea production apparatus, a second evaporator EVc is added between the first evaporator EVa and the granulation apparatus F. The second evaporator EVc corresponds to an additional evaporator that further concentrates the liquid before it is supplied to the granulation apparatus F to obtain a high-concentration urea liquid, which is a liquid having a higher urea concentration than the liquid, and additional exhaust gas, which is a gas containing water and ammonia.

[0190] 9 and 10, in the method for improving a urea production plant, the water wash scrubber SCR* is replaced with a scrubber SCR. Alternatively, an acid supply pipe L14 for supplying acid to the scrubber is added, so that the water wash scrubber SCR* functions as a scrubber SCR.

[0191] 9 and 10, in the method for improving the urea production plant, a line L31 is added to supply the exhaust gas discharged from the second evaporator EVc to the scrubber SCR. The line L31 corresponds to an exhaust pipe that supplies the additional exhaust gas to the scrubber.

[0192] 9 and 10 , in the method for improving the urea production apparatus, the supply of the post-cleaning recovered liquid discharged from the scrubber SCR to the urea synthesis unit U0 is stopped. For example, the line L28* may be removed, or the line L28* may be configured not to discharge the post-cleaning recovered liquid.

[0193] As shown in Figures 9 and 10, in the method for improving the urea production apparatus, a line L16* is added to supply the post-cleaning recovered liquid discharged from the scrubber SCR to the second evaporator EVc. The line L16* corresponds to a recovery pipe that supplies the post-cleaning recovered liquid to the additional evaporator. The line L16* may be obtained by connecting the line L28* provided in the existing second urea production apparatus 110 to the second evaporator EVc. The line L16* may be provided as a pipe separate from the line L28* provided in the existing second urea production apparatus 110.

[0194] In the method for improving the urea production apparatus, the operation of the urea hydrolyzer UH and the stripper tower ST in the wastewater treatment unit WWT is stopped. Also, in the method for improving the urea production apparatus, a line L33' is added to supply condensed water to a scrubber. In the method for improving the urea production apparatus, a line L32' may be added to supply condensed water to the urea synthesis unit U0.

[0195] Ninth Embodiment A urea production apparatus 209 according to the ninth embodiment will be described with reference to Fig. 11. Differences between the urea production apparatus 209 and the urea production apparatus 50 according to the fifth embodiment will be described. In the urea production apparatus 209, components common to the urea production apparatus 50 according to the fifth embodiment will be denoted by the same reference numerals, and descriptions thereof will be omitted as appropriate.

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

[0197] The concentrated urea aqueous solution is discharged from the first evaporator EVa to a line L2*. By joining the line L2* and a line L29′, a mixture of the concentrated urea aqueous solution and the post-wash recovery liquid is supplied to the second evaporator EVc via a line L3*.

[0198] 11 are not components of the urea production apparatus 209. As an example, the urea production apparatus 209 does not include a line L25. As another example, although a pipe corresponding to the line L25 is connected to the urea production apparatus 209, the urea production apparatus 209 is not configured to discharge wastewater from the pipe.

[0199] [Correspondence] The correspondence between the matters in this embodiment and the matters described in the "Means for solving the problems" section above is as follows.

[0200] The first evaporator EVa corresponds to the "first evaporator." The second evaporator EVc corresponds to the "second evaporator." <Method for improving the urea production apparatus> The urea production apparatus 209 may be manufactured as a new apparatus, or may be manufactured by improving an existing urea production apparatus. The method for improving the urea production apparatus will be described below.

[0201] [Example of an Existing Third Urea Production Apparatus] An example of an existing third urea production apparatus will be described. Fig. 12 shows an existing third urea production apparatus 120. Regarding the existing third urea production apparatus 120, the description of the components common to the existing first urea production apparatus 100 shown in Fig. 4 will be omitted as appropriate.

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

[0203] [Improvement Method] The method for improving the existing third urea production apparatus 120 shown in FIG. 12 is as follows.

[0204] 11 and 12, in the method for improving the urea production apparatus, a second evaporator EVc is added between the first evaporator EVa and the granulation apparatus F. The second evaporator EVc corresponds to an additional evaporator that further concentrates the liquid before it is supplied to the granulation apparatus F to obtain a high-concentration urea liquid, which is a liquid having a higher urea concentration than the liquid, and additional exhaust gas, which is a gas containing water and ammonia.

[0205] 11 and 12, in the method for improving the urea production plant, a line L31 is added to supply the exhaust gas discharged from the second evaporator EVc to the first-stage scrubber SCR1. The line L31 corresponds to an exhaust pipe that supplies the additional exhaust gas to the scrubber.

[0206] As shown in FIGS. 11 and 12 , in the method for improving a urea production apparatus, a line L29′ is added to supply the post-second-stage cleaning recovered liquid discharged from the second-stage scrubber SCR2 to the second evaporator EVc. The line L29′ corresponds to a recovery pipe that supplies the post-second-stage cleaning recovered liquid to the additional evaporator. The line L29′ may be the line L29 included in the existing third urea production apparatus 120, connected to the second evaporator EVc. The line L29′ may be provided as a pipe separate from the line L29 included in the existing third urea production apparatus 120. In this case, the line L29 may be removed, or the line L29 may be configured so that the post-second-stage cleaning recovered liquid is not discharged from the line L29.

[0207] In the improvement method of the urea production plant, the operation of the urea hydrolyzer UH in the wastewater treatment unit WWT is stopped. By not operating the urea hydrolyzer UH, treated wastewater after condensed water has been treated in the stripper tower ST is supplied to a scrubber via lines L23 and L24.

[0208] If the existing third urea production apparatus 120 is equipped with a line L25 for discharging treated wastewater obtained in the wastewater treatment unit WWT to the outside of the system of the urea production apparatus 120, the method for improving the urea production apparatus may be configured to not discharge wastewater from the line L25. For example, the method for improving the urea production apparatus may be configured to not discharge wastewater from the line L25 by stopping use of the line L25. Furthermore, for example, the method for improving the urea production apparatus may be configured to remove the line L25.

[0209] [Tenth embodiment] A urea production apparatus 210 according to a tenth embodiment will be described with reference to Fig. 13. Differences between the urea production apparatus 210 and the urea production apparatus 60 according to the sixth embodiment will be described. In the urea production apparatus 210, components common to the urea production apparatus 60 according to the sixth embodiment will be denoted by the same reference numerals, and descriptions thereof will be omitted as appropriate.

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

[0211] The concentrated urea aqueous solution is discharged from the first evaporator EVa to a line L2*. By joining the line L2* and a line L29′, a mixture of the concentrated urea aqueous solution and the post-wash recovery liquid is supplied to the second evaporator EVc via a line L3*.

[0212] 13 are not components of the urea production apparatus 210. As an example, the urea production apparatus 210 does not include a wastewater treatment unit WWT. As another example, in the urea production apparatus 210, the line through which condensed water is supplied to the wastewater treatment unit WWT is stopped.

[0213] [Correspondence] The correspondence between the matters in this embodiment and the matters described in the "Means for solving the problems" section above is as follows.

[0214] The first evaporator EVa corresponds to the "first evaporator." The second evaporator EVc corresponds to the "second evaporator." <Method for improving the urea production apparatus> The urea production apparatus 210 may be manufactured as a new apparatus, or may be manufactured by improving an existing urea production apparatus. The method for improving the urea production apparatus will be described below.

[0215] [Improvement Method] The method for improving the existing third urea production apparatus 120 shown in FIG. 12 is as follows.

[0216] 12 and 13, in the method for improving the urea production apparatus, a second evaporator EVc is added between the first evaporator EVa and the granulation apparatus F. The second evaporator EVc corresponds to an additional evaporator that further concentrates the liquid before it is supplied to the granulation apparatus F to obtain a high-concentration urea liquid, which is a liquid having a higher urea concentration than the liquid, and additional exhaust gas, which is a gas containing water and ammonia.

[0217] 12 and 13, in the method for improving the urea production plant, a line L31 is added to supply the exhaust gas discharged from the second evaporator EVc to the first-stage scrubber SCR1. The line L31 corresponds to an exhaust pipe that supplies the additional exhaust gas to the scrubber.

[0218] As shown in FIGS. 12 and 13 , in the method for improving a urea production apparatus, a line L29′ is added to supply the post-second-stage cleaning recovered liquid discharged from the second-stage scrubber SCR2 to the second evaporator EVc. The line L29′ corresponds to a recovery pipe that supplies the post-second-stage cleaning recovered liquid to the additional evaporator. The line L29′ may be the line L29 included in the existing third urea production apparatus 120, connected to the second evaporator EVc. The line L29′ may be provided as a pipe separate from the line L29 included in the existing third urea production apparatus 120. In this case, the line L29 may be removed, or the line L29 may be configured so that the post-second-stage cleaning recovered liquid is not discharged from the line L29.

[0219] In the method for improving the urea production apparatus, the operation of the urea hydrolyzer UH and the stripper tower ST in the wastewater treatment unit WWT is stopped. Also, in the method for improving the urea production apparatus, a line L33' is added to supply condensed water to a scrubber. In the method for improving the urea production apparatus, a line L32' may be added to supply condensed water to the urea synthesis unit U0.

[0220] [Eleventh embodiment] A urea production apparatus 211 of the eleventh embodiment will be described using Fig. 14. Differences between the urea production apparatus 211 and the urea production apparatus 30 of the third embodiment will be described. In the urea production apparatus 211, components common to the urea production apparatus 30 of the third embodiment will be denoted by the same reference numerals, and descriptions thereof will be omitted as appropriate.

[0221] The urea production plant 211 is equipped with a scrubber SCR. The scrubber SCR equipped in the urea production plant 211 is a single-stage scrubber that uses an acidic cleaning liquid containing an acid.

[0222] The urea production plant 211 includes a line L16* that supplies the post-cleaning recovered liquid discharged from the scrubber SCR to the third-stage evaporator EVc. Note that the components indicated by the two-dot chain line in Fig. 14 are not components of the urea production plant 211.

[0223] As one example, the urea production plant 211 does not have a line L28 for supplying the post-cleaning recovered liquid obtained from the scrubber SCR to the urea synthesis unit U0. As another example, although a pipe corresponding to the line L28 is connected to the urea production plant 211, the urea production plant 211 is not configured to supply the post-cleaning recovered liquid from the pipe to the urea synthesis unit U0.

[0224] Also, as an example, the urea production apparatus 211 does not include the line L25. As another example, although a pipe corresponding to the line L25 is connected to the urea production apparatus 211, the urea production apparatus 211 is not configured to discharge wastewater from the pipe.

[0225] <Method of Improving Urea Production Apparatus> The urea production apparatus 211 may be manufactured as a new apparatus or may be manufactured by improving an existing urea production apparatus. Hereinafter, a method of improving the urea production apparatus will be described.

[0226] [Example of Existing Fourth Urea Production Apparatus] An example of an existing fourth urea production apparatus will be described. Fig. 15 shows an existing fourth urea production apparatus 130. Regarding the existing fourth urea production apparatus 130, the description of the components common to the existing first urea production apparatus 100 shown in Fig. 4 will be omitted as appropriate.

[0227] The existing fourth urea production unit 130 is equipped with a water wash scrubber SCR*. ​​The water wash scrubber SCR* brings the exhaust gas discharged from the granulation unit F into contact with a wash liquid, thereby obtaining a purified gas and a post-wash recovery liquid.

[0228] [Improvement Method] The method for improving the existing fourth urea production apparatus 130 shown in FIG. 15 is as follows.

[0229] 14 and 15 , in the method for improving the urea production apparatus, a third-stage evaporator EVc is added between the first-stage evaporator EVa and the granulation apparatus F. More specifically, the third-stage evaporator EVc is added between the second-stage evaporator EVb and the granulation apparatus F. The third-stage evaporator EVc corresponds to an additional evaporator. The additional evaporator, for example, further concentrates the liquid before it is supplied to the granulation apparatus F, thereby obtaining a high-concentration urea solution, which is a liquid having a higher urea concentration than the liquid, and additional exhaust gas, which is a gas containing water and ammonia.

[0230] As shown in Figures 14 and 15, in the method for improving a urea production plant, the water wash scrubber SCR* is replaced with a scrubber SCR. Alternatively, an acid supply pipe L14 for supplying acid to the scrubber is added, so that the water wash scrubber SCR* functions as a scrubber SCR.

[0231] 14 and 15, in the method for improving a urea production plant, a line L31 is added to supply exhaust gas discharged from the third-stage evaporator EVc to a scrubber SCR. The line L31 corresponds to an exhaust pipe that supplies the additional exhaust gas to the scrubber.

[0232] 14 and 15, in the method for improving the urea production apparatus, the supply of the post-cleaning recovered liquid discharged from the scrubber SCR to the urea synthesis unit U0 is stopped. For example, the line L28* may be removed, or the line L28* may be configured not to discharge the post-cleaning recovered liquid.

[0233] As shown in Figures 14 and 15, in the method for improving the urea production apparatus, a line L16* is added to supply the post-cleaning recovered liquid discharged from the scrubber SCR to the third-stage evaporator EVc. The line L16* corresponds to a recovery pipe that supplies the post-cleaning recovered liquid to the additional evaporator. The line L16* may be obtained by connecting the line L28* provided in the existing fourth urea production apparatus 130 to the third-stage evaporator EVc. The line L16* may be provided as a pipe different from the line L28* provided in the existing fourth urea production apparatus 130.

[0234] In the improvement method of the urea production plant, the operation of the urea hydrolyzer UH in the wastewater treatment unit WWT is stopped. By not operating the urea hydrolyzer UH, the treated wastewater after the condensed water has been treated in the stripper tower ST is supplied to a scrubber via a line L23.

[0235] If the existing fourth urea production apparatus 130 is equipped with a line L25 for discharging treated wastewater obtained in the wastewater treatment unit WWT to the outside of the system of the urea production apparatus 130, the method for improving the urea production apparatus may be configured to not discharge wastewater from the line L25. For example, the method for improving the urea production apparatus may be configured to not discharge wastewater from the line L25 by stopping use of the line L25. For example, the method for improving the urea production apparatus may be configured to remove the line L25.

[0236] [Twelfth Embodiment] A urea production apparatus 212 according to a twelfth embodiment will be described with reference to Fig. 16. Differences between the urea production apparatus 212 and the urea production apparatus 40 according to the fourth embodiment will be described. In the urea production apparatus 212, components common to the urea production apparatus 40 according to the fourth embodiment will be denoted by the same reference numerals, and descriptions thereof will be omitted as appropriate.

[0237] The urea production plant 212 is equipped with a scrubber SCR. The scrubber SCR equipped in the urea production plant 212 is a single-stage scrubber that uses an acidic cleaning liquid containing an acid.

[0238] The urea production plant 212 is provided with a line L16* that supplies the post-cleaning recovered liquid discharged from the scrubber SCR to the third-stage evaporator EVc. Note that the components indicated by the two-dot chain line in Fig. 16 are not components of the urea production plant 212.

[0239] <Method of Improving Urea Production Apparatus> The urea production apparatus 212 may be manufactured as a new apparatus or may be manufactured by improving an existing urea production apparatus. Hereinafter, a method of improving the urea production apparatus will be described.

[0240] [Improvement Method] The method for improving the existing fourth urea production apparatus 130 shown in FIG. 15 is as follows.

[0241] 15 and 16 , in the method for improving the urea production apparatus, a third-stage evaporator EVc is added between the first-stage evaporator EVa and the granulation apparatus F. More specifically, the third-stage evaporator EVc is added between the second-stage evaporator EVb and the granulation apparatus F. The third-stage evaporator EVc corresponds to an additional evaporator. The additional evaporator, for example, further concentrates the liquid before it is supplied to the granulation apparatus F, thereby obtaining a high-concentration urea solution, which is a liquid having a higher urea concentration than the liquid, and additional exhaust gas, which is a gas containing water and ammonia.

[0242] As shown in Figures 15 and 16, in the method for improving a urea production plant, the water wash scrubber SCR* is replaced with a scrubber SCR. Alternatively, an acid supply pipe L14 for supplying acid to the scrubber is added, so that the water wash scrubber SCR* functions as a scrubber SCR.

[0243] 15 and 16, in the method for improving the urea production plant, a line L31 is added to supply the exhaust gas discharged from the second evaporator EVc to the scrubber SCR. The line L31 corresponds to an exhaust pipe that supplies the additional exhaust gas to the scrubber.

[0244] 15 and 16, in the method for improving the urea production apparatus, the supply of the post-cleaning recovered liquid discharged from the scrubber SCR to the urea synthesis unit U0 is stopped. For example, the line L28* may be removed, or the line L28* may be configured not to discharge the post-cleaning recovered liquid.

[0245] As shown in Figures 15 and 16, in the method for improving the urea production apparatus, a line L16* is added to supply the post-cleaning recovered liquid discharged from the scrubber SCR to the third-stage evaporator EVc. The line L16* corresponds to a recovery pipe that supplies the post-cleaning recovered liquid to the additional evaporator. The line L16* may be obtained by connecting the line L28* provided in the existing fourth urea production apparatus 130 to the third-stage evaporator EVc. The line L16* may be provided as a pipe different from the line L28* provided in the existing fourth urea production apparatus 130.

[0246] In the method for improving a urea production apparatus, the operation of the urea hydrolyzer UH and the stripper tower ST in the wastewater treatment unit WWT is stopped. Also, in the method for improving a urea production apparatus, a line L33 is added to supply condensed water to a scrubber. In the method for improving a urea production apparatus, a line L32 may be added to supply condensed water to the urea synthesis unit U0.

[0247] 13th Embodiment A urea production apparatus 213 according to a thirteenth embodiment will be described with reference to Fig. 17. Differences between the urea production apparatus 213 and the urea production apparatus 50 according to the fifth embodiment will be described. In the urea production apparatus 213, components common to the urea production apparatus 50 according to the fifth embodiment will be denoted by the same reference numerals, and descriptions thereof will be omitted as appropriate.

[0248] The urea production plant 213 is equipped with a scrubber SCR. The scrubber SCR equipped in the urea production plant 213 is a single-stage scrubber that uses an acidic cleaning liquid containing an acid.

[0249] The urea production plant 213 is provided with a line L16* that supplies the post-cleaning recovered liquid discharged from the scrubber SCR to the third-stage evaporator EVc. Note that the components indicated by the two-dot chain line in Fig. 17 are not components of the urea production plant 213.

[0250] <Method of Improving Urea Production Apparatus> The urea production apparatus 213 may be manufactured as a new apparatus or may be manufactured by improving an existing urea production apparatus. Hereinafter, a method of improving the urea production apparatus will be described.

[0251] [Improvement Method] The method for improving the existing fourth urea production apparatus 130 shown in FIG. 15 is as follows.

[0252] As shown in Figures 15 and 17, in the method for improving the urea production apparatus, a third-stage evaporator EVc is added between the first-stage evaporator EVa and the granulation apparatus F. More specifically, the second-stage evaporator EVb is replaced with the third-stage evaporator EVc. The second condenser C2 is then removed. The third-stage evaporator EVc corresponds to an additional evaporator that further concentrates the liquid before it is supplied to the granulation apparatus F to obtain a high-concentration urea solution, which is a liquid with a higher urea concentration than the liquid, and additional exhaust gas, which is a gas containing 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 improved as follows: A third-stage evaporator EVc is added between the first-stage evaporator EVa and the granulation apparatus F, and is connected in series with both the first-stage evaporator EVa and the granulation apparatus F. Then, the second-stage evaporator EVb and the second condenser C2 are stopped.

[0254] As shown in Figures 15 and 17, in the method for improving a urea production plant, the water wash scrubber SCR* is replaced with a scrubber SCR. Alternatively, an acid supply pipe L14 for supplying acid to the scrubber is added, so that the water wash scrubber SCR* functions as a scrubber SCR.

[0255] 15 and 17, in the method for improving a urea production plant, a line L31 is added to supply exhaust gas discharged from the third-stage evaporator EVc to a scrubber SCR. The line L31 corresponds to an exhaust pipe that supplies the additional exhaust gas to the scrubber.

[0256] 15 and 17, in the method for improving the urea production apparatus, the supply of the post-cleaning recovered liquid discharged from the scrubber SCR to the urea synthesis unit U0 is stopped. For example, the line L28* may be removed, or the line L28* may be configured not to discharge the post-cleaning recovered liquid.

[0257] As shown in Figures 15 and 17, in the method for improving the urea production apparatus, a line L16* is added to supply the post-cleaning recovered liquid discharged from the scrubber SCR to the third-stage evaporator EVc. The line L16* corresponds to a recovery pipe that supplies the post-cleaning recovered liquid to the additional evaporator. The line L16* may be obtained by connecting the line L28* provided in the existing fourth urea production apparatus 130 to the third-stage evaporator EVc. The line L16* may be provided as a pipe different from the line L28* provided in the existing fourth urea production apparatus 130.

[0258] In the improvement method of the urea production plant, the operation of the urea hydrolyzer UH in the wastewater treatment unit WWT is stopped. By not operating the urea hydrolyzer UH, the treated wastewater after the condensed water has been treated in the stripper tower ST is supplied to a scrubber via a line L23.

[0259] If the existing fourth urea production apparatus 130 is equipped with a line L25 for discharging treated wastewater obtained in the wastewater treatment unit WWT to the outside of the system of the urea production apparatus 130, the method for improving the urea production apparatus may be configured to not discharge wastewater from the line L25. For example, the method for improving the urea production apparatus may be configured to not discharge wastewater from the line L25 by stopping use of the line L25. For example, the method for improving the urea production apparatus may be configured to remove the line L25.

[0260] [Fourteenth embodiment] A urea production apparatus 214 according to a fourteenth embodiment will be described using Figure 18. Differences between the urea production apparatus 214 and the urea production apparatus 60 according to the sixth embodiment will be described. In the urea production apparatus 214, components common to the urea production apparatus 60 according to the sixth embodiment will be denoted by the same reference numerals, and descriptions thereof will be omitted as appropriate.

[0261] The urea production plant 214 is equipped with a scrubber SCR. The scrubber SCR equipped in the urea production plant 214 is a single-stage scrubber that uses an acidic cleaning liquid containing an acid.

[0262] The urea production plant 214 is provided with a line L16* that supplies the post-cleaning recovered liquid discharged from the scrubber SCR to the third-stage evaporator EVc. Note that the components indicated by the two-dot chain line in Fig. 18 are not components of the urea production plant 214.

[0263] <Method of Improving Urea Production Apparatus> The urea production apparatus 214 may be manufactured as a new apparatus or may be manufactured by improving an existing urea production apparatus. Hereinafter, a method of improving the urea production apparatus will be described.

[0264] [Improvement Method] The method for improving the existing fourth urea production apparatus 130 shown in FIG. 15 is as follows.

[0265] As shown in Figures 15 and 18, in the method for improving the urea production apparatus, a third-stage evaporator EVc is added between the first-stage evaporator EVa and the granulation apparatus F. More specifically, the second-stage evaporator EVb is replaced with the third-stage evaporator EVc. The second condenser C2 is then removed. The third-stage evaporator EVc corresponds to an additional evaporator that further concentrates the liquid before being supplied to the granulation apparatus F to obtain a high-concentration urea solution, which is a liquid with a higher urea concentration than the liquid, and additional exhaust gas, which is a gas containing 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 improved as follows: A third-stage evaporator EVc is added between the first-stage evaporator EVa and the granulation apparatus F, and is connected in series with the first-stage evaporator EVa and the granulation apparatus F. Then, the second-stage evaporator EVb and the second condenser C2 are stopped.

[0267] As shown in Figures 15 and 18, in the method for improving a urea production plant, the water wash scrubber SCR* is replaced with a scrubber SCR. Alternatively, an acid supply pipe L14 for supplying acid to the scrubber is added, so that the water wash scrubber SCR* functions as a scrubber SCR.

[0268] 15 and 18, in the method for improving a urea production plant, a line L31 is added to supply exhaust gas discharged from the third-stage evaporator EVc to a scrubber SCR. The line L31 corresponds to an exhaust pipe that supplies the additional exhaust gas to the scrubber.

[0269] 15 and 18, in the method for improving the urea production apparatus, the supply of the post-cleaning recovered liquid discharged from the scrubber SCR to the urea synthesis unit U0 is stopped. For example, the line L28* may be removed, or the line L28* may be configured not to discharge the post-cleaning recovered liquid.

[0270] As shown in Figures 15 and 18, in the method for improving the urea production apparatus, a line L16* is added to supply the post-cleaning recovered liquid discharged from the scrubber SCR to the third-stage evaporator EVc. The line L16* corresponds to a recovery pipe that supplies the post-cleaning recovered liquid to the additional evaporator. The line L16* may be a line L28* provided in the existing fourth urea production apparatus 130, connected to the third-stage evaporator EVc. The line L16* may be provided as a pipe different from the line L28* provided in the existing fourth urea production apparatus 130.

[0271] In the method for improving the urea production apparatus, the operation of the urea hydrolyzer UH and the stripper tower ST in the wastewater treatment unit WWT is stopped. Also, in the method for improving the urea production apparatus, a line L33' is added to supply condensed water to a scrubber. In the method for improving the urea production apparatus, a line L32' may be added to supply condensed water to the urea synthesis unit U0.

[0272] (Modifications) The above-described embodiments can be modified as follows: The embodiments and the following modifications can be combined with each other to the extent that no technical contradiction occurs.

[0273] In the third embodiment, an example was shown in which the operation of the urea hydrolyzer UH in the wastewater treatment unit WWT was stopped. Alternatively, the operation of the urea hydrolyzer UH and the stripper tower ST in the wastewater treatment unit WWT may be stopped. Furthermore, the condensed water from the condensers C1 and C2 may be supplied to the scrubbers SCR1 and SCR2 via the wastewater treatment unit WWT in which the operation of the urea hydrolyzer UH and the stripper tower ST has been stopped. In such a configuration, exhaust gas is not discharged from the wastewater treatment unit WWT to the line L26.

[0274] In the first and second embodiments, a two-stage scrubber, such as that exemplified in the third embodiment, may be employed. This configuration further reduces the amount of urea discharged from the scrubber to the outside of the system. Since the post-first-stage cleaning liquid from the first-stage scrubber can be supplied to the purification device U2, the load on the second evaporator EV2 can be reduced. When the load on the second evaporator EV2 is low, the size of the second evaporator EV2 can be reduced. By reducing the size of the second evaporator EV2, the amount of air supplied to the second evaporator EV2 can be reduced, allowing the scrubber to be made smaller. As with the above modified examples, a two-stage scrubber may also be employed in the seventh, eighth, eleventh, twelfth, thirteenth, and fourteenth embodiments.

[0275] In the third to sixth, ninth and tenth embodiments, a single-stage scrubber as exemplified in the first embodiment may be employed. According to the above configuration, the scrubber equipment can be simplified.

[0276] In the seventh, eighth, eleventh and twelfth embodiments, the improvement method is exemplified in which the water wash scrubber SCR* is replaced with a single-stage scrubber SCR. Alternatively, in the improvement method, the water wash scrubber SCR* may be replaced with a two-stage scrubber.

[0277] In the third to sixth embodiments, the method of improving the existing urea production apparatus is exemplified by the method of improving the existing first urea production apparatus 100. The existing urea production apparatus is not limited to the existing first urea production apparatus 100. For example, the urea production apparatus 40 in the fourth embodiment may be an improved version of the urea production apparatus 30 in the third embodiment. Similarly, in the improvement methods exemplified in the seventh to twelfth embodiments, the existing urea production apparatus to be improved is not limited to the configurations in each embodiment.

[0278] The urea production apparatus 20 in the second embodiment may be manufactured by improving an existing urea production apparatus. Examples of improvement methods for manufacturing the urea production apparatus 20 by improving an existing urea production apparatus include the following improvement methods [1] to [6].

[0279] [1] (A) The existing second urea production apparatus 110 is improved in the same manner as the improvement method for constructing the urea production apparatus 207 illustrated in the seventh embodiment. (B) A line is added to supply condensed water from the condenser C1 to the urea synthesis unit U0. (C) A configuration is made in which the entire amount of post-treatment condensed water discharged from the stripper tower ST is supplied to the scrubber. For example, the line L27 is stopped. Alternatively, the line L27 may be removed.

[0280] [2] (A) The existing third urea production unit 120 is improved in the same manner as the improvement method for constructing the urea production unit 209 illustrated in the ninth embodiment. (B) The same improvement as (B) in [1] above is made. (C) The two-stage scrubber is replaced with a single-stage scrubber SCR. (D) The line L28* is stopped or removed.

[0281] [3] (A) The existing fourth urea production apparatus 130 illustrated in the eleventh embodiment is improved in the same manner as the improvement method for constructing the urea production apparatus 211. (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 improvement as (B) in [1] above is made.

[0282] [4] (A) The existing first urea production apparatus 100 exemplified in the thirteenth embodiment is improved in the same manner as the improvement method for producing the urea production apparatus 213. (B) The same improvement as (B) in the above [1] is made.

[0283] [5] (A) The same improvement as the improvement method for constructing the urea production apparatus 50 is made to the existing first urea production apparatus 100 exemplified in the fifth embodiment. (B) The same improvement as (B) in [1] above is made. (C) The two-stage scrubber is replaced with a single-stage scrubber SCR. (D) The line L28 is stopped or removed.

[0284] [6] (A) The same improvement as the improvement method for constructing the urea production apparatus 30 is made to the existing first urea production apparatus 100 exemplified 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 improvement as (B) in [1] above is made. (D) The two-stage scrubber is replaced with a single-stage scrubber SCR. (E) The line L28 is stopped or removed.

[0285] Here, the wastewater treatment unit WWT, which is composed only of the stripper tower ST, is substantially the same as the stripper tower ST provided in the urea production apparatus 20 in the second embodiment. Therefore, the urea production apparatus improved by any one of the above improvement methods [1] to [6] has a common configuration with the urea production apparatus 20.

[0286] The urea production apparatus 10 in the first embodiment may be produced by improving an existing urea production apparatus. The urea production apparatus 10 in the first embodiment can be produced by further stopping the stripper column ST in addition to any one of the improvement methods [1] to [6] for producing the urea production apparatus 20 by improving an existing urea production apparatus.

[0287] Other improvement methods for producing the urea production apparatus 10 by improving an existing urea production apparatus include, for example, the following improvement methods [7] to

[12] . [7] The existing second urea production apparatus 110 exemplified in the eighth embodiment is improved in the same way as the improvement method for producing the urea production apparatus 208. It can be said that the urea production apparatus 208 has a common configuration with the urea production apparatus 10.

[0288] [8] (A) The existing third urea production plant 120 is improved in the same manner as the improvement method for constructing the urea production plant 210 illustrated in the tenth embodiment. (B) The two-stage scrubber is replaced with a single-stage scrubber SCR. (C) The line L28* is stopped or removed.

[0289] [9] (A) The existing fourth urea production apparatus 130 illustrated in the twelfth embodiment is improved in the same way as the improvement method for constructing the urea production apparatus 212. (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.

[0290]

[10] (A) The existing first urea production apparatus 100 exemplified in the fourteenth embodiment is improved in the same manner as the improvement method for producing the urea production apparatus 214. It can be said that the urea production apparatus 214 has a common configuration with the urea production apparatus 10.

[0291]

[11] (A) The existing first urea production plant 100 is improved in the same manner as the improvement method for constructing the urea production plant 60 exemplified in the sixth embodiment. (B) The two-stage scrubber is replaced with a single-stage scrubber SCR. (C) The line L28 is stopped or removed.

[0292]

[12] (A) The same improvement as the improvement method for constructing the urea production apparatus 40 is made to the existing first urea production apparatus 100 exemplified in the fourth 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 two-stage scrubber is replaced with a single-stage scrubber SCR. (D) The line L28 is stopped or removed.

[0293] In the above-described embodiments and modifications, the ammonium salt generated in the scrubber SCR or the second-stage scrubber SCR2 is recovered as a mixture with the solid urea product without being discharged to the outside of the system. Alternatively, the urea production apparatus may be configured to discharge the ammonium salt to the outside of the system. Similarly, the urea production method may include a step of discharging the ammonium salt to the outside of the system.

[0294] An example of a urea production apparatus in which ammonium salt is discharged to the outside of the system will be described below with reference to Figures 19 and 20. Figure 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 the urea production apparatus 215 is configured to discharge a portion of the post-cleaning recovered liquid discharged from the scrubber SCR to the outside of the system and to supply the remainder of the post-cleaning recovered liquid to the second evaporator EV2. Regarding the urea production apparatus 215, a description of the configuration common to the urea production apparatus 10 of the first embodiment will be omitted.

[0296] Specifically, the urea production apparatus 215 includes a line L16a that supplies the post-cleaning recovered liquid to the second evaporator EV2, and a line L16b that branches off from the line L16a and discharges the post-cleaning recovered liquid outside the system. The post-cleaning recovered liquid discharged via the line L16b is sent to treatment equipment such as a urea mixing equipment or a concentration and solidification equipment for treating the ammonium salt outside the system. The line L16a corresponds to the recovery pipe, and the line L16b corresponds to the recovered liquid discharge pipe.

[0297] Although FIG. 19 illustrates the line L16a and the line L16b branching out outside the scrubber SCR, two lines for discharging the post-cleaning recovered liquid from the scrubber SCR may be provided instead.

[0298] 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 the urea production apparatus 216 is configured to discharge a portion of the second-stage post-cleaning recovered liquid discharged from the second-stage scrubber SCR2 to the outside of the system. Regarding the urea production apparatus 216, a description of the configuration common to the urea production apparatus 30 of the third embodiment will be omitted.

[0299] Specifically, the urea production apparatus 216 includes a line L29a that supplies the post-second-stage cleaning recovered liquid to the third-stage evaporator EVc, and a line L29b that branches off from the line L29a and discharges the post-second-stage cleaning recovered liquid outside the system. The post-second-stage cleaning recovered liquid discharged via line L29b is sent to treatment equipment such as a urea mixing system or a concentration and solidification system for treating the ammonium salt outside the system. The line L29a corresponds to the recovery pipe, and the line L29b corresponds to the recovered liquid discharge pipe.

[0300] Although FIG. 20 illustrates the line L29a and the line L29b branching out from the second-stage scrubber SCR2, two lines may be provided to discharge the post-second-stage cleaning recovered liquid from the second-stage scrubber SCR2 instead.

[0301] According to the configurations of the urea producing apparatus 215 and the urea producing apparatus 216, urea can be efficiently recovered without being hydrolyzed, and ammonium salt can be discharged outside the system.

[0302] In each of the above-described embodiments and modifications including a scrubber SCR, a portion of the post-cleaning recovered liquid discharged from the scrubber SCR may be configured to be discharged outside the system, as in the urea production apparatus 215 illustrated in FIG. 19 .

[0303] In each of the above-described embodiments and modifications including the second-stage scrubber SCR2, a portion of the post-second-stage cleaning recovered liquid discharged from the second-stage scrubber SCR2 may be configured to be discharged outside the system, as in the urea production apparatus 216 illustrated in FIG. 20.

[0304] The urea production apparatus 216 illustrated in FIG. 20 is configured to discharge a portion of the recovered liquid after second-stage cleaning outside the system. Alternatively, the system may be configured to discharge all of the recovered liquid after second-stage cleaning outside the system. That is, the system may be configured to discharge at least a portion of the recovered liquid after second-stage cleaning outside the system. Even in a configuration in which all of the recovered liquid after second-stage cleaning is discharged outside the system, the effect of efficiently recovering urea without hydrolyzing it can be obtained. This is because the recovered liquid after first-stage cleaning is supplied to the urea synthesis unit U0 in a urea production apparatus employing a two-stage scrubber.

[0305] A urea production apparatus configured to discharge the post-cleaning recovered liquid or the post-second-stage cleaning recovered liquid outside the system may be manufactured by improving an existing urea production apparatus. For example, in addition to the method of manufacturing a urea production apparatus shown in each of the above embodiments and modifications, a step of adding a recovered liquid discharge pipe for discharging a portion of the post-cleaning recovered liquid or a portion of the post-second-stage cleaning recovered liquid outside the system may be performed. The recovered liquid discharge pipe may be branched off from a recovery pipe that supplies the post-cleaning recovered liquid to the additional evaporator, or may be added as a pipe separate from the recovery pipe. Note that, when all of the post-second-stage cleaning recovered liquid is discharged outside the system, a step of adding a recovered liquid discharge pipe may be performed instead of the step of adding a recovery pipe.

[0306] In addition to the urea production methods exemplified in the respective embodiments, the urea production method may include a withdrawal step of discharging a part of the post-cleaning recovered liquid obtained in the cleaning step out of the system. In this case, the second evaporation step is a step of obtaining a high-concentration urea solution and a second exhaust gas from a mixed liquid of the concentrated urea aqueous solution and the post-cleaning recovered liquid excluding the liquid to be discharged out of the system. In other words, when a part of the post-cleaning recovered liquid is discharged out of the system, the post-cleaning recovered liquid is divided into a liquid to be discharged out of the system and a liquid to be mixed with the concentrated urea aqueous solution.

[0307] The present invention will be described in more detail based on the following examples. Note that the present invention is not limited to the configurations described in the examples. Example 1 A solid urea product was produced using the urea production apparatus 10 shown in Figure 1. The details are as follows.

[0308] A urea aqueous solution as a raw material was supplied from line L1. Formaldehyde was added as an additive from line L5. Sulfuric acid was added from line L14 as an acid to be supplied to the scrubber SCR. A solid urea product was obtained from line L7. The components contained in each line per hour are shown in Tables 1 to 3.

[0309] The first evaporator EV1 was operated under conditions of 47 kPa and 104° C. The second evaporator EV2 was operated under conditions of atmospheric pressure and 140° C. The granulation apparatus F used was a fluidized bed granulator.

[0310]

[0311]

[0312]

[0313] Example 2 A solid urea product was produced in the same manner as in Example 1, except that it was based on the urea production apparatus 20 shown in Figure 2. The components contained in each line per hour are shown in Tables 4 to 6.

[0314]

[0315]

[0316]

[0317] <Evaluation> In Example 1, the amount of urea in line L16 shown in Table 3 is equal to the amount of urea entrained in the gas from the evaporator and granulator, i.e., the urea contained in lines L8, L11, and L12, minus the amount of urea contained in line L9 recycled to the urea synthesis unit and the amount of urea contained in line L15 for gas released into the atmosphere from the scrubber SCR. That is, the amount of urea in L16 = the amount of urea in (L8 + L11 + L12) - the amount of urea in (L9 + L15). It can be seen that the urea entrained in the gas in the evaporator and granulator was recovered without decomposition. Furthermore, the wastewater treatment unit did not have a hydrolyzer or a stripper tower, and no wastewater was sent outside the system. In Example 2, the amount of urea in line L16 shown in Table 6 also satisfies the relationship: amount of urea in L16 = amount of urea in (L8 + L11 + L12) - amount of urea in (L9 + L15). It can be seen that urea can be recovered without decomposing the urea entrained in the gas in the evaporator and granulator. No water is discharged outside the system, and no urea hydrolyzer is required. In this way, urea can be recovered without decomposition, which reduces the amount of urea that needs to be resynthesized from ammonia and carbon dioxide produced by the decomposition of urea, thereby improving the efficiency of producing solid urea products.

Claims

1. A plant for the production of urea, comprising: a first evaporator configured to selectively separate a first exhaust gas containing water and ammonia from a urea solution to obtain a first exhaust gas and a concentrated urea solution, wherein the concentrated urea solution is a liquid having a higher concentration of urea than said urea solution; a condenser configured to cool the first exhaust gas to produce condensate water; a second evaporator configured to selectively separate a second exhaust gas containing water and ammonia from the concentrated urea solution to obtain a second exhaust gas and a highly concentrated urea solution, wherein the highly concentrated urea solution is a liquid having a higher concentration of urea than the concentrated urea solution; a granulator capable of producing a solid urea product from a highly concentrated urea solution; a scrubber configured to contact the second off-gas, which is supplied as gas from the second evaporator, and the third off-gas discharged from the granulator, with an acid scrubbing solution containing an acid to produce a clean gas and a purified recovered solution containing an ammonium salt and urea; and a recovery pipe configured to feed purified recovered solution into the second evaporator, wherein the second evaporator is configured to produce a highly concentrated urea solution and a second exhaust gas from a mixture of a concentrated urea solution and a purified recovered solution.

2. A plant for the production of urea according to paragraph 1, additionally containing: a feed pipe configured to feed at least a portion of the condensate water into the scrubber, wherein The scrubber uses at least a portion of the condensate water as make-up water fed to the scrubbing solution.

3. A plant for the production of urea, comprising: a first evaporator configured to selectively separate a first exhaust gas containing water and ammonia from a urea solution to obtain a first exhaust gas and a concentrated urea solution, wherein the concentrated urea solution is a liquid having a higher concentration of urea than said urea solution; a condenser configured to cool the first exhaust gas to produce condensate water; a second evaporator configured to selectively separate a second exhaust gas containing water and ammonia from the concentrated urea solution to obtain a second exhaust gas and a highly concentrated urea solution, wherein the highly concentrated urea solution is a liquid having a higher concentration of urea than the concentrated urea solution; a granulator capable of producing a solid urea product from a highly concentrated urea solution; a scrubber configured to contact a second exhaust gas, which is supplied as gas from a second evaporator, and a third exhaust gas discharged from a granulator, with an acidic scrubbing solution containing an acid to obtain a clean gas and a purified recovered solution containing an ammonium salt and urea; a stripping column configured to strip condensate water with steam to produce a gas containing ammonia and treated condensate water; a recovery pipe configured to feed the purified recovered solution into the second evaporator; and a feed pipe configured to feed the treated condensate water into the scrubber, wherein the second evaporator is configured to produce a highly concentrated urea solution and a second exhaust gas from a mixture of a concentrated urea solution and a purified recovered solution, and The scrubber uses treated condensate water as make-up water fed to the cleaning solution.

4. A plant for the production of urea according to any one of paragraphs 1-3, in which the second evaporator includes a falling liquid film evaporator and is configured to bring the concentrated urea solution into counter-current contact with heated air at atmospheric pressure, and The second exhaust gas contains heated air that has passed through the second evaporator.

5. A plant for the production of urea according to paragraph 4, additionally containing: an air supply device configured to supply heated air to the granulator, wherein the granulator is configured to use heated air as fluidizing air to form a fluidized bed, and Heated air, brought into counter-current contact with the concentrated urea solution in the second evaporator, is supplied from the air supply device.

6. A plant for the production of urea according to paragraph 4, additionally containing: an additive pipe configured to feed an additive into a liquid downstream of the first evaporator and upstream of the second evaporator or into a liquid downstream of the second evaporator and upstream of the granulator.

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

8. A plant for the production of urea according to paragraph 4, additionally containing: an additive pipe configured to supply ammonium salt for regulating the concentration of ammonium salt of the product into the liquid downstream of the first evaporator and upstream of the second evaporator or the liquid downstream of the second evaporator and upstream of the granulator.

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

10. A urea production plant according to claim 9, wherein the ammonium salt for regulating the concentration of the ammonium salt of the product is formed from acid and ammonia fed to the scrubber.

11. A plant for the production of urea according to paragraph 4, additionally containing: a recuperated solution discharge pipe configured to discharge a portion of the purified recuperated solution discharged from the scrubber outside the urea production plant system, wherein the recovery pipe is designed to supply purified recovered solution, excluding the part released outside the system, to the second evaporator.

12. A method for modifying an existing urea production plant, wherein the existing urea production plant comprises: a granulator capable of producing a solid urea product from a liquid containing urea; an evaporator configured to selectively separate a first exhaust gas, which is a gas containing water and ammonia, from a urea solution to obtain a first exhaust gas and a liquid having a higher concentration of urea than the urea solution and fed to a granulator; a condenser configured to cool the first exhaust gas to produce condensate water; a wastewater treatment unit comprising a urea hydrolyzer and a stripping column and configured to process condensate water to produce purified wastewater; a scrubber configured to contact the exhaust gas discharged from the granulator with a scrubbing solution to produce clean gas and a purified recovered solution; and a feed pipe configured to feed purified wastewater obtained in the wastewater treatment unit into a scrubber, wherein the method includes: adding an additional evaporator between the evaporator and the granulator; adding an outlet pipe configured to supply additional exhaust gas, including water and ammonia, from the additional evaporator to the scrubber; adding an acid feed pipe configured to feed acid to the scrubber so that the exhaust gas discharged from the granulator and the additional exhaust gas are brought into contact with an acid scrubbing solution containing the acid to produce a clean gas and a clean recovered solution; adding a recovery pipe configured to feed purified recovered solution to an additional evaporator; and configuring the wastewater treatment unit so that the condensate water is treated by a stripping column to obtain purified wastewater, and the purified wastewater is fed to the scrubber through a feed pipe without operating the urea hydrolyzer.

13. A method for modifying an existing urea production plant, wherein the existing urea production plant comprises: a granulator capable of producing a solid urea product from a liquid containing urea; an evaporator configured to selectively separate a first exhaust gas, which is a gas containing water and ammonia, from a urea solution to obtain a first exhaust gas and a liquid having a higher concentration of urea than said urea solution and fed to a granulator; a condenser configured to cool the first exhaust gas to produce condensate water; a wastewater treatment unit comprising a urea hydrolyzer and a stripping column and configured to process condensate water to produce purified wastewater; a scrubber configured to contact the exhaust gas discharged from the granulator with a scrubbing solution to produce clean gas and a purified recovered solution; and a feed pipe configured to feed purified wastewater obtained in the wastewater treatment unit into a scrubber, wherein the method includes: adding an additional evaporator between the evaporator and the granulator, adding an outlet pipe configured to feed additional exhaust gas, including water and ammonia, from the additional evaporator to the scrubber, adding an acid feed pipe configured to feed acid to the scrubber so that the exhaust gas discharged from the granulator and the additional exhaust gas are brought into contact with an acid scrubbing solution containing the acid to produce a clean gas and a clean recovered solution; adding a recovery pipe configured to feed purified recovered solution to an additional evaporator; and Configuring the wastewater treatment unit to feed condensate water to the scrubber without activating the urea hydrolyzer and stripper.

14. A method for modifying an existing urea production plant, wherein the existing urea production plant comprises: a granulator capable of producing a solid urea product from a liquid containing urea; an evaporator configured to selectively separate a first exhaust gas, which is a gas containing water and ammonia, from a urea solution to obtain a first exhaust gas and a liquid having a higher concentration of urea than said urea solution and fed to a granulator; a condenser configured to cool the first exhaust gas to produce condensate water; a wastewater treatment unit comprising a urea hydrolyzer and a stripping column and configured to process condensate water to produce purified wastewater; a scrubber configured to contact the exhaust gas discharged from the granulator with an acid scrubbing solution containing an acid to produce a clean gas and a purified recovered solution; and a feed pipe configured to feed purified wastewater obtained in the wastewater treatment unit into a scrubber, wherein the method includes: adding an additional evaporator between the evaporator and the granulator; adding an outlet pipe configured to supply additional exhaust gas, including water and ammonia, from the additional evaporator to the scrubber; adding a recovery pipe configured to feed purified recovered solution to an additional evaporator; and configuring the wastewater treatment unit so that the condensate water is treated by a stripping column to obtain purified wastewater, and the purified wastewater is fed to the scrubber through a feed pipe without operating the urea hydrolyzer.

15. A method for modifying an existing urea production plant, wherein the existing urea production plant comprises: a granulator capable of producing a solid urea product from a liquid containing urea; an evaporator configured to selectively separate a first exhaust gas, which is a gas containing water and ammonia, from a urea solution to obtain a first exhaust gas and a liquid having a higher concentration of urea than said urea solution and fed to a granulator; a condenser configured to cool the first exhaust gas to produce condensate water; a wastewater treatment unit comprising a urea hydrolyzer and a stripping column and configured to process condensate water to produce purified wastewater; a scrubber configured to contact the exhaust gas discharged from the granulator with an acid scrubbing solution containing an acid to produce a clean gas and a purified recovered solution; and a feed pipe configured to feed purified wastewater obtained in the wastewater treatment unit into a scrubber, wherein the method includes: adding an additional evaporator between the evaporator and the granulator; adding an outlet pipe configured to supply additional exhaust gas, including water and ammonia, from the additional evaporator to the scrubber; adding a recovery pipe configured to feed purified recovered solution to an additional evaporator; and Configuring the wastewater treatment unit to feed condensate water to the scrubber without activating the urea hydrolyzer and stripper.

16. The method according to paragraph 12 or 13, further comprising: adding a recovered solution outlet pipe configured to discharge a portion of the purified recovered solution outside the urea production plant system, wherein the recovery pipe is designed with the possibility of feeding the purified recovered solution, excluding the part released outside the system, into an additional evaporator.

17. The method according to paragraph 14 or 15, in which the existing urea production plant further includes a recovered solution discharge pipe configured to discharge a portion of the purified recovered solution that is discharged from the scrubber outside the urea production plant system, and a recovery pipe designed to supply purified recovered solution, excluding the portion discharged outside the system, to an additional evaporator.

18. The method according to any one of paragraphs 12-15, further comprising: configuring an additional evaporator to further concentrate the liquid before feeding it to the granulator and the purified recovered solution in the additional evaporator to obtain a highly concentrated urea solution, which is a liquid having a higher concentration of urea than the liquid fed to the granulator.

19. The method according to any one of paragraphs 12-15, in which the additional evaporator includes an evaporator with a falling liquid film and is designed with the possibility of bringing the liquid into counter-current contact with heated air at atmospheric pressure before feeding it to the granulator, and The additional exhaust gas contains heated air that has passed through the additional evaporator.

20. The method according to any one of paragraphs 12-15, wherein the existing urea production plant further includes an outlet pipe configured to discharge treated wastewater that is produced in the wastewater treatment unit outside the urea production plant system, and the method further includes: configuring the exhaust pipe so that no discharge occurs from the exhaust pipe.

21. The method according to paragraph 19, in which the evaporator in the existing urea plant is the first stage evaporator, the condenser in the existing urea plant is the first condenser, The existing urea production plant 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 a separated gas and a liquid having a higher concentration of urea than the liquid supplied from the first evaporator and configured to be fed to a granulator; and a second condenser configured to cool the gas discharged from the second stage evaporator to produce condensate water, wherein the method additionally includes: replacing the second stage evaporator with an additional evaporator, thereby adding an additional evaporator; and removing the second capacitor.

22. A method for modifying an existing urea production plant, wherein the existing urea production plant comprises: a granulator capable of producing a solid urea product from a liquid containing urea; a first stage evaporator configured to selectively separate a first exhaust gas containing water and ammonia from a urea solution to obtain a first exhaust gas and a concentrated urea solution, wherein the concentrated urea solution is a liquid having a higher concentration of urea than said urea solution; a first condenser configured to cool the gas discharged from the first stage evaporator to produce 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 a separated gas and a liquid having a higher concentration of urea than the liquid supplied from the first evaporator and configured to be supplied to a granulator; a second condenser configured to cool the gas discharged from the second stage evaporator to produce second condensate water; a wastewater treatment unit comprising a urea hydrolyzer and a stripping column and configured to process the first condensate water and the second condensate water to obtain purified wastewater; a scrubber configured to contact the exhaust gas discharged from the granulator with a scrubbing solution to produce clean gas and a purified recovered solution; and a feed pipe configured to feed purified wastewater obtained in the wastewater treatment unit into a scrubber, wherein the method includes: adding an additional evaporator between the second stage evaporator and the granulator; adding an outlet pipe configured to supply additional exhaust gas, including water and ammonia, from the additional evaporator to the scrubber; adding an acid feed pipe configured to feed acid to the scrubber so that the exhaust gas discharged from the granulator and the additional exhaust gas are brought into contact with an acid scrubbing solution containing the acid to produce a clean gas and a clean recovered solution; adding a recovery pipe configured to feed purified recovered solution to an additional evaporator; and configuring the wastewater treatment unit so that the first condensate water and the second condensate water are treated by a stripping column to obtain purified wastewater, and the purified wastewater is fed to the scrubber through a feed pipe without operating the urea hydrolyzer.

23. A method for modifying an existing urea production plant, wherein the existing urea production plant comprises: a granulator capable of producing a solid urea product from a liquid containing urea; a first stage evaporator configured to selectively separate a first exhaust gas containing water and ammonia from a urea solution to obtain a first exhaust gas and a concentrated urea solution, wherein the concentrated urea solution is a liquid having a higher concentration of urea than the urea solution; a first condenser configured to cool the gas discharged from the first stage evaporator to produce 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 a separated gas and a liquid having a higher concentration of urea than the liquid supplied from the first evaporator and configured to be fed to a granulator; a second condenser configured to cool the gas discharged from the second stage evaporator to produce second condensate water; a wastewater treatment unit comprising a urea hydrolyzer and a stripping column and configured to process the first condensate water and the second condensate water to obtain purified wastewater; a scrubber configured to contact the exhaust gas discharged from the granulator with a scrubbing solution to produce clean gas and a purified recovered solution; and a feed pipe configured to feed purified wastewater obtained in the wastewater treatment unit into a scrubber, wherein the method includes: adding an additional evaporator between the second stage evaporator and the granulator; adding an outlet pipe configured to supply additional exhaust gas, including water and ammonia, from the additional evaporator to the scrubber; adding an acid feed pipe configured to feed acid to the scrubber so that the exhaust gas discharged from the granulator and the additional exhaust gas are brought into contact with an acid scrubbing solution containing the acid to produce a clean gas and a clean recovered solution; adding a recovery pipe configured to feed purified recovered solution to an additional evaporator; and configuring the wastewater treatment unit to feed the first condensate water and the second condensate water to the scrubber without activating the urea hydrolyzer and the stripping column.

24. A method for modifying an existing urea production plant, wherein the existing urea production plant comprises: a granulator capable of producing a solid urea product from a liquid containing urea; a first stage evaporator configured to selectively separate a first exhaust gas containing water and ammonia from a urea solution to obtain a first exhaust gas and a concentrated urea solution, wherein the concentrated urea solution is a liquid having a higher concentration of urea than said urea solution; a first condenser configured to cool the gas discharged from the first stage evaporator to produce 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 a separated gas and a liquid having a higher concentration of urea than the liquid supplied from the first evaporator, and configured to be fed to a granulator; a second condenser configured to cool the gas discharged from the second stage evaporator to produce second condensate water; a wastewater treatment unit comprising a urea hydrolyzer and a stripping column and configured to process the first condensate water and the second condensate water to obtain purified wastewater; a scrubber configured to contact the exhaust gas discharged from the granulator with an acid scrubbing solution containing an acid to produce a clean gas and a purified recovered solution; and a feed pipe configured to feed purified wastewater obtained in the wastewater treatment unit into a scrubber, wherein the method includes: adding an additional evaporator between the second stage evaporator and the granulator; adding an outlet pipe configured to supply additional exhaust gas, including water and ammonia, from the additional evaporator to the scrubber; adding a recovery pipe configured to feed purified recovered solution to an additional evaporator; and configuring the wastewater treatment unit so that the first condensate water and the second condensate water are treated by a stripping column to obtain purified wastewater, and the purified wastewater is fed to the scrubber through a feed pipe without operating the urea hydrolyzer.

25. A method for modifying an existing urea production plant, wherein the existing urea production plant comprises: a granulator capable of producing a solid urea product from a liquid containing urea; a first stage evaporator configured to selectively separate a first exhaust gas containing water and ammonia from a urea solution to obtain a first exhaust gas and a concentrated urea solution, wherein the concentrated urea solution is a liquid having a higher concentration of urea than said urea solution; a first condenser configured to cool the gas discharged from the first stage evaporator to produce 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 a separated gas and a liquid having a higher concentration of urea than the liquid supplied from the first evaporator, and configured to be fed to a granulator; a second condenser configured to cool the gas discharged from the second stage evaporator to produce second condensate water; a wastewater treatment unit comprising a urea hydrolyzer and a stripping column and configured to process the first condensate water and the second condensate water to obtain purified wastewater; a scrubber configured to contact the exhaust gas discharged from the granulator with an acid scrubbing solution containing an acid to produce a clean gas and a purified recovered solution; and a feed pipe configured to feed purified wastewater obtained in the wastewater treatment unit into a scrubber, wherein the method includes: adding an additional evaporator between the second stage evaporator and the granulator; adding an outlet pipe configured to supply additional exhaust gas, including water and ammonia, from the additional evaporator to the scrubber; adding a recovery pipe configured to feed purified recovered solution to an additional evaporator; and configuring the wastewater treatment unit to feed the first condensate water and the second condensate water to the scrubber without activating the urea hydrolyzer and the stripping column.

26. The method according to any one of paragraphs 22-25, further comprising: configuring an additional evaporator to further concentrate the liquid before feeding it to the granulator and the purified recovered solution in the additional evaporator to obtain a highly concentrated urea solution, which is a liquid having a higher concentration of urea than the liquid fed to the granulator.

27. The method according to paragraph 22 or 23, further comprising: adding a recovered solution outlet pipe configured to discharge a portion of the purified recovered solution outside the urea production plant system, wherein The recovery pipe is designed to supply purified recovered solution, excluding the portion released outside the system, to an additional evaporator.

28. The method according to paragraph 24 or 25, in which the existing urea production plant further includes a recovered solution discharge pipe configured to discharge a portion of the purified recovered solution that is discharged from the scrubber outside the urea production plant system, and a recovery pipe designed to supply purified recovered solution, excluding the portion discharged outside the system, to an additional evaporator.

29. The method according to any one of paragraphs 22-25, in which the additional evaporator includes an evaporator with a falling liquid film and is designed with the possibility of bringing the liquid into counter-current contact with heated air at atmospheric pressure before feeding it to the granulator, and The additional exhaust gas contains heated air that has passed through the additional evaporator.

30. The method according to any one of paragraphs 22-25, in which the existing urea production plant further includes an outlet pipe configured to discharge treated wastewater, which is obtained in the wastewater treatment unit, outside the system of the urea production plant, wherein the method additionally includes: configuring the exhaust pipe so that no discharge occurs from the exhaust pipe.

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

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

33. A method for producing urea, the method comprising: a first evaporation step for selectively separating a first off-gas containing water and ammonia from the urea solution to obtain a first off-gas and a concentrated urea solution, wherein the concentrated urea solution is a liquid having a higher concentration of urea than the urea solution; a condensation stage for cooling the first exhaust gas to produce condensate water; a second evaporation step for selectively separating a second exhaust gas containing water and ammonia from the concentrated urea solution to obtain a second exhaust gas and a highly concentrated urea solution, wherein the highly concentrated urea solution is a liquid having a higher concentration of urea than the concentrated urea solution; a granulation stage to obtain a solid urea product from a highly concentrated urea solution; a cleaning step for bringing into contact the second off-gas obtained in the second evaporation step and the third off-gas discharged in the granulation step with an acidic cleaning solution containing an acid to obtain a clean gas and a purified recovered solution containing an ammonium salt and urea; a stripping step for obtaining a gas containing ammonia and treated condensate water by stripping the condensate water with steam, wherein the second evaporation stage comprises obtaining a highly concentrated urea solution and a second off-gas from a mixture of the concentrated urea solution and the purified recovered solution, and The cleaning stage uses treated condensate water as make-up water fed to the cleaning solution.

34. The method according to any one of paragraphs 31-33, in which the second stage of evaporation uses a falling liquid film evaporator designed to bring the concentrated urea solution into counter-current contact with heated air at atmospheric pressure, and The second exhaust gas contains heated air that has passed through the evaporator.

35. The method according to paragraph 34, further comprising: an outlet stage for releasing a portion of the purified recovered solution obtained in the purification stage, wherein the second evaporation stage comprises obtaining a highly concentrated urea solution and a second exhaust gas from a mixture of the concentrated urea solution and the purified recovered solution, excluding the discharge portion.