Production method and production apparatus of urea

Abstract

To provide: a production method of urea having relatively high urea synthesis rate and relatively little energy consumption: and a production apparatus of urea.SOLUTION: Provided is a production method of urea having: a carbon dioxide separation step to obtain a rich liquid 11 by making a carbon dioxide containing gas 10 be absorbed and separated into a lean liquid 20, which is an absorption liquid; a rich liquid stripping step to obtain a gas 21 containing carbon dioxide at a high concentration by stripping the rich liquid 11; a high pressure absorption step to obtain a carbamate liquid 25 by using the gas 21; and a urea synthesis step using the carbamate liquid 25 as a part of a raw material. Also provided is a production apparatus of urea having: carbon dioxide separation equipment (CO2-AB) for performing these steps; rich liquid stripping equipment (RST); high pressure absorption equipment (HA); and urea synthesis equipment (R).SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method and an apparatus for producing urea, which has a relatively high urea synthesis rate and a relatively low energy consumption. [Background technology]

[0002] Conventionally, various techniques for absorbing and separating carbon dioxide from exhaust gas are known. For example, one of these techniques is a method in which the exhaust gas is brought into contact with ammonia water (absorption liquid) to absorb and separate the carbon dioxide contained in the exhaust gas into ammonia water. This absorption and separation produces an absorption liquid (rich liquid) containing high concentrations of carbon dioxide, ammonia, and water.

[0003] On the other hand, in a urea synthesis process, carbon dioxide and ammonia are usually reacted under high temperature and pressure to obtain a urea synthesis liquid. In such a urea synthesis process, a method is known in which the above-mentioned absorption liquid (rich liquid) after absorbing carbon dioxide is used as part of the raw material for urea synthesis. Patent Document 1 describes such a method. Specifically, Patent Document 1 proposes a method in which an ammonia production plant and a urea production plant are integrated, carbon dioxide in exhaust gas generated by combustion in the ammonia production plant is absorbed into ammonia water (absorption liquid), and the rich liquid is used as part of the raw material for urea synthesis.

[0004] For example, in FIG. 1 of Patent Document 1, an exhaust gas stream 5 generated by combustion in an ammonia production plant 1 is supplied to a CAP absorption tower 10. A mixed stream 11 (a mixture of a lean stream 24, which is an absorption liquid having a low concentration of carbon dioxide, and an ammonia stream 9 produced in the ammonia production plant 1) is also supplied to the CAP absorption tower 10. In this CAP absorption tower 10, carbon dioxide in the exhaust gas stream 5 is absorbed and separated into a solution (absorption liquid) of the mixed stream 11. The solution after absorbing carbon dioxide (rich stream 13 containing a high concentration of carbon dioxide) is supplied to a urea synthesis section 15. In addition, an ammonia stream 8 produced in the ammonia production plant 1 and a carbon dioxide stream 7 separated from a synthesis gas generated in the ammonia production plant 1 are also supplied to the urea synthesis section 15. These are used as raw materials for urea synthesis.

[0005] In FIG. 2 of Patent Document 1, the exhaust gas stream 25 is supplied from the steam reformer 14 to the low-pressure absorber 29. The lean liquid stream 42 is also supplied to the low-pressure absorber 29. In this low-pressure absorber 29, carbon dioxide in the exhaust gas stream 25 is absorbed and separated into a solution (absorption liquid) of the lean liquid stream 42. A partial stream 31 of the solution (semi-rich solvent stream 30 containing a relatively high concentration of carbon dioxide) of the lean liquid stream 42 after absorbing carbon dioxide is supplied to the urea production plant 38, and the other partial stream is supplied to the high-pressure absorber 33 together with the partial stream 32 of the lean liquid stream 42. The hydrogen / carbon dioxide stream 28 is also supplied to the high-pressure absorber 33. In this high-pressure absorber 33, carbon dioxide in the hydrogen / carbon dioxide stream 28 is absorbed and separated into a mixed solution (absorption liquid) of the partial stream of the semi-rich solvent stream 30 and the lean liquid stream 42. A rich liquid stream 37, which is a part of the mixed solution after absorbing carbon dioxide, is supplied to the urea production plant 38. The ammonia stream 43 produced in the ammonia production unit 41 is also fed to the urea production plant 38. These are then used as raw materials for urea synthesis.

[0006] In FIG. 3 of Patent Document 1, a regeneration tower 44 is added between the high-pressure absorption tower 33 and the urea production plant 38 shown in FIG. 2. A rich liquid stream 37, which is a part of the mixed solution after absorbing carbon dioxide in the high-pressure absorption tower 33, is supplied to the regeneration tower 44. In the regeneration tower 44, carbon dioxide is separated and purified from the solution of the rich liquid stream 37, and high-purity carbon dioxide is obtained. This high-purity carbon dioxide stream 45 is supplied to the urea production plant 38. An ammonia stream 43 generated in the ammonia production unit 41 is also supplied to the urea production plant 38. These are then used as raw materials for urea synthesis. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] U.S. Patent No. 9,428,449 Summary of the Invention [Problem to be solved by the invention]

[0008] It is generally known that if a large amount of water is present during urea synthesis, the water inhibits urea synthesis in terms of reaction equilibrium, resulting in a significant drop in the urea synthesis rate (see, for example, Ullmann's Encyclopedia of Industrial Chemistry, Fifth, Completely Revised Edition, 1996, Vol. A27, "Urea", p. 333-365). Therefore, in terms of the urea synthesis rate, the amount of water in urea synthesis must be relatively small. The molar ratio H / C of water to carbon dioxide (water / carbon dioxide) is usually used as a guide to the amount of water. Specifically, it is desirable for the H / C in urea synthesis to be a low molar ratio of less than 1.5.

[0009] On the other hand, when ammonia water is used as an absorbing liquid for carbon dioxide, even the rich liquid after absorbing carbon dioxide contains a large amount of water. For example, the ammonia concentration in the rich liquid described in Patent Document 1 is 2 to 12 mol / L (about 3 to 12 mass%), and the carbon dioxide concentration is 1 to 10 mol / L (about 4 to 27 mass%), and from these mass ratios, the water concentration is estimated to be about 56 mol / L (about 61 to 93 mass%).

[0010] In the method described in Fig. 2 of Patent Document 1, the only source of carbon dioxide to the urea production plant 38 is the rich liquid (partial stream 31 of the semi-rich solvent stream 30 and rich liquid stream 37). Therefore, when urea is synthesized in the coexistence of water contained in the rich liquid (rich liquid composition of about 56 mol / L [about 61 to 93 mass %]), the H / C is estimated to be about 4.1, and the urea synthesis rate is estimated to be significantly reduced.

[0011] On the other hand, in the method shown in FIG. 1 of Patent Document 1, not only the carbon dioxide stream 7 but also the rich liquid stream 13 containing about 56 mol / L (about 61 to 93 mass%) of water is used as a carbon dioxide supply source to the urea synthesis section 15. Therefore, the H / C in the urea synthesis in FIG. 1 is calculated as the molar ratio of water to the total amount of carbon dioxide in the carbon dioxide stream 7 and the rich liquid stream 13, and is therefore higher than the H / C in a general urea synthesis in which only the carbon dioxide stream 7 is used as a carbon dioxide source. Therefore, it is estimated that the method shown in FIG. 1 of Patent Document 1 has a lower urea synthesis rate than a general urea synthesis method.

[0012] As described above, it is estimated that the urea synthesis rate is very low in each of the methods described in Figures 1 and 2 of Patent Document 1. When urea is industrially produced at a low synthesis rate like each of these methods, it is necessary to make each device in the urea plant very large in order to decompose and separate unreacted substances, which results in an extreme increase in energy consumption.

[0013] On the other hand, in the method described in FIG. 3 of Patent Document 1, the rich liquid containing a large amount of water is not used as it is as a raw material for urea synthesis, but a high-purity carbon dioxide stream 45 separated and purified from the rich liquid stream 37 in a regenerator 44 is also supplied to a urea production plant 38. Therefore, the H / C in the urea synthesis in FIG. 3 is estimated to be lower than the H / C in the urea synthesis in FIG. 1 and FIG. 2. On the other hand, although the pressure of the carbon dioxide stream 45 is not specified in Patent Document 1, it is usually estimated to be about 2 MPaG (for example, see Energy Procedia, 114, "Chilled Ammonia Process Scale-up and Lessons Learned" 2017 p. 5593-5615. In particular, on page 5615, there is a description that "Regenerator operates at 19.5 bar g". 19.5 bar g is about 2 MPaG.). In order to introduce such a carbon dioxide stream into a general urea synthesis facility operated at a high pressure of about 15 MPaG, a CO2 compressor is required. Therefore, compared with the method shown in FIG. 1 in which carbon dioxide is sent to the urea synthesis section 15 in both a rich liquid state and a gas state, and the method shown in FIG. 2 in which the entire amount of carbon dioxide is sent to the urea production plant 38 in a rich liquid state, the compression power of the CO2 compressor in the method shown in FIG. 3 in which carbon dioxide is sent to the urea production plant 38 only in a gas state is increased.

[0014] That is, an object of the present invention is to provide a method and an apparatus for producing urea which has a relatively high urea synthesis rate and consumes relatively little energy. [Means for solving the problem]

[0015] As a result of intensive research conducted by the inventors in order to achieve the above object, they discovered that it is extremely effective to strip a rich liquid to separate a gas containing a high concentration of carbon dioxide, use this gas to produce a carbamate liquid, and use this carbamate liquid as part of the raw material for urea synthesis, and thus completed the present invention.

[0016] The present invention relates to a carbon dioxide separation process in which a gas containing carbon dioxide is brought into contact with a lean liquid, which is an absorption liquid containing low concentrations of carbon dioxide, ammonia, and water, and the carbon dioxide contained in the gas is absorbed and separated into the lean liquid to obtain a rich liquid, which is an absorption liquid containing high concentrations of carbon dioxide, ammonia, and water; A rich liquid stripping step in which carbon dioxide is preferentially gasified by stripping at least a portion of the rich liquid obtained in the carbon dioxide separation step, and a gas containing a high concentration of carbon dioxide is separated, and the rich liquid is converted into an aqueous solution containing low concentrations of carbon dioxide and ammonia; a high pressure absorption step in which at least a portion of the gas separated in the rich liquid stripping step is contacted with ammonia under high pressure to obtain a carbamate liquid; and A urea synthesis process in which the carbamate liquid obtained in the high-pressure absorption process is used as part of a raw material to obtain a urea synthesis liquid. The present invention relates to a method for producing urea having the formula:

[0017] The present invention further provides a carbon dioxide separation facility (CO2-AB) for obtaining a rich liquid, which is an absorption liquid containing high concentrations of carbon dioxide, ammonia, and water, by bringing a gas containing carbon dioxide into contact with a lean liquid, which is an absorption liquid containing low concentrations of carbon dioxide, ammonia, and water, and absorbing and separating the carbon dioxide contained in the gas into the lean liquid; A rich liquid stripping facility (RST) for preferentially gasifying carbon dioxide by stripping at least a portion of the rich liquid obtained in the carbon dioxide separation facility (CO2-AB) to separate a gas containing a high concentration of carbon dioxide and convert the rich liquid into an aqueous solution containing low concentrations of carbon dioxide and ammonia; A high pressure absorption unit (HA) for obtaining a carbamate liquid by contacting at least a portion of the gas separated in the rich liquid stripping unit (RST) with ammonia under high pressure; and The carbamate liquid obtained in the high-pressure absorption facility (HA) is used as part of the raw material in the urea synthesis facility (R) to obtain urea synthesis liquid. The urea manufacturing apparatus has the following features.

[0018] Further, the present invention relates to A method for improving an existing urea production apparatus, comprising the steps of: For existing urea production equipment, at least: A carbon dioxide separation facility (CO2-AB) is a facility for bringing a gas containing carbon dioxide into contact with a lean liquid, which is an absorption liquid containing low concentrations of carbon dioxide, ammonia, and water, thereby absorbing and separating the carbon dioxide contained in the gas into the lean liquid, thereby obtaining a rich liquid, which is an absorption liquid containing high concentrations of carbon dioxide, ammonia, and water; By stripping the rich liquid, which is an absorption liquid containing high concentrations of carbon dioxide, ammonia, and water, the carbon dioxide is preferentially gasified to separate the gas containing high concentrations of carbon dioxide, and the rich liquid is converted into an aqueous solution containing low concentrations of carbon dioxide and ammonia. This can be achieved by adding a rich liquid stripping facility (RST). a carbon dioxide separation step of contacting a gas containing carbon dioxide with a lean liquid, which is an absorption liquid containing low concentrations of carbon dioxide, ammonia and water, thereby absorbing and separating the carbon dioxide contained in the gas into the lean liquid to obtain a rich liquid, which is an absorption liquid containing high concentrations of carbon dioxide, ammonia and water; A rich liquid stripping step in which carbon dioxide is preferentially gasified by stripping at least a portion of the rich liquid obtained in the carbon dioxide separation step, and a gas containing a high concentration of carbon dioxide is separated, and the rich liquid is converted into an aqueous solution containing low concentrations of carbon dioxide and ammonia; a high pressure absorption step in which at least a portion of the gas separated in the rich liquid stripping step is contacted with ammonia under high pressure to obtain a carbamate liquid; and A urea synthesis process in which the carbamate liquid obtained in the high-pressure absorption process is used as part of a raw material to obtain a urea synthesis liquid. The present invention relates to a method for improving an existing urea production apparatus, which enables a urea production method having the above-mentioned structure. Effect of the Invention

[0019] In the urea production method of the present invention, the rich liquid obtained in the carbon dioxide separation step is not used as it is as a raw material for urea synthesis, but a gas containing a high concentration of carbon dioxide is separated from the rich liquid in a rich liquid stripping step, and the gas is used in a high-pressure absorption step to obtain a carbamate liquid. The carbamate liquid is then used as a part of the raw material for urea synthesis.

[0020] As a result, the urea production method of the present invention has a lower H / C ratio in urea synthesis and a higher urea synthesis rate than when a rich liquid containing a large amount of water is used as it is as a raw material for urea synthesis (FIGS. 1 and 2 of Patent Document 1). In addition, since the urea synthesis rate is high, the amount of unreacted substances is small, and the unreacted substances can be sufficiently decomposed or separated without enlarging the equipment of the urea plant, making industrial production possible and eliminating the need to increase energy consumption.

[0021] Furthermore, in the urea production method of the present invention, a gas containing a high concentration of carbon dioxide is separated from the rich liquid, and the carbamate liquid is obtained using the gas, and the carbamate liquid is used as a part of the raw material for urea synthesis. Therefore, even if high-purity carbon dioxide separated and purified from the rich liquid is used as a raw material together with the carbon dioxide supply source for urea synthesis, the amount of the high-purity carbon dioxide may be relatively small. Therefore, the urea production method of the present invention consumes less energy than the case where only high-purity carbon dioxide separated and purified from the rich liquid is used as the carbon dioxide supply source for urea synthesis (Figure 3 of Patent Document 1). For example, when the amount of high-purity carbon dioxide is relatively small, the compression power of the CO2 compressor required to introduce it into the urea synthesis equipment is reduced. In addition, when this high-purity carbon dioxide is not used, a CO2 compressor is not required.

[0022] Therefore, according to the present invention, a method and an apparatus for producing urea with a relatively high urea synthesis rate and a relatively low energy consumption can be provided. [Brief description of the drawings]

[0023] [Figure 1]FIG. 1 is a process flow diagram illustrating one embodiment of the method of the present invention. [Diagram 2] FIG. 2 is a process flow diagram illustrating another embodiment of the method of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] <Carbon dioxide separation process> In the present invention, the carbon dioxide separation process is a process in which a gas containing carbon dioxide is brought into contact with a lean liquid, which is an absorption liquid containing low concentrations of carbon dioxide, ammonia, and water, and the carbon dioxide contained in the gas is absorbed and separated into the lean liquid to obtain a rich liquid, which is an absorption liquid containing high concentrations of carbon dioxide, ammonia, and water. The carbon dioxide separation equipment (CO2-AB) is equipment for carrying out this carbon dioxide separation process.

[0025] The type of carbon dioxide-containing gas used in the carbon dioxide separation step is not particularly limited. For example, exhaust gas generated in the combustion process of facilities such as factories and power plants usually contains a large amount of carbon dioxide. In the present invention, it is preferable from the viewpoint of environmental protection to use such exhaust gas as the carbon dioxide-containing gas, separate the carbon dioxide in the exhaust gas, and use it as part of the raw material for urea synthesis. However, the present invention is not limited thereto. Other gases may be used as the carbon dioxide-containing gas as necessary.

[0026] In the carbon dioxide separation process, the carbon dioxide contained in the gas is absorbed and separated into the lean liquid to obtain a rich liquid. The lean liquid is an absorption liquid containing low concentration carbon dioxide, ammonia, and water, and is a liquid (ammonia-containing aqueous solution) that can absorb and separate the carbon dioxide contained in the gas under moderate temperature and pressure. This lean liquid absorbs carbon dioxide, resulting in a rich liquid (absorption liquid containing high concentration carbon dioxide, ammonia, and water) with a high carbon dioxide concentration.

[0027] The composition of the lean liquid used in the carbon dioxide separation process is not particularly limited. It may be any composition that can absorb carbon dioxide in the gas. For example, as described in Patent Document 1, the concentration of carbon dioxide in the lean liquid is usually 0.2 to 4.0 mol / L (about 1 to 13 mass%), the concentration of ammonia is usually 2 to 12 mol / L (about 3 to 15 mass%), and the concentration ratio of ammonia to carbon dioxide is usually 1:1 to 3:1.

[0028] The temperature and pressure in the carbon dioxide separation step are not particularly limited. Known temperatures and pressures in a method for absorbing carbon dioxide in a gas using an absorbing liquid containing ammonia may be applied. For example, as described in Patent Document 1, the temperature in the carbon dioxide separation step is usually 0 to 25°C, and the pressure is usually 0 to 2.5 MPaG.

[0029] The composition of the rich liquid obtained in the carbon dioxide separation process is not particularly limited. It may be any composition after absorbing a desired amount of carbon dioxide in the carbon dioxide separation process. For example, as described in Patent Document 1, the carbon dioxide concentration in the rich liquid is usually 1.0 to 10 mol / L (about 4 to 27 mass%), the ammonia concentration is usually 2 to 12 mol / L (about 3 to 12 mass%), and the concentration ratio of ammonia to carbon dioxide is usually 1:1 to 3:1.

[0030] At least a part of the rich liquid obtained in the carbon dioxide separation step is treated in a rich liquid stripping step described below. Another part of this rich liquid may be treated in a regeneration step described below.

[0031] In the carbon dioxide separation process, the gas after the carbon dioxide is absorbed and separated (e.g., clean gas) may be discharged outside the system. In addition, a small amount of ammonia contained in this gas may be recovered and regenerated and reused as a part of the lean liquid or raw material for urea synthesis.

[0032] The type of equipment used as the carbon dioxide separation equipment (CO2-AB) is not particularly limited. For example, a known equipment for absorbing carbon dioxide in a gas using an absorbing liquid (e.g., ammonia water) can be used. The absorption tower described in Patent Document 1 may also be used.

[0033] <Rich liquid stripping process> In the present invention, the rich liquid stripping step includes: This is a process in which carbon dioxide is preferentially gasified by stripping at least a portion of the rich liquid obtained in the carbon dioxide separation process, and a gas containing high concentration of carbon dioxide is separated from the rich liquid, and the rich liquid is converted into an aqueous solution containing low concentration of carbon dioxide and ammonia. The rich liquid stripping equipment (RST) is an equipment for performing this rich liquid stripping process.

[0034] In the rich liquid stripping process, stripping means that the rich liquid is heated to an appropriate temperature under an appropriate pressure to preferentially gasify and separate the carbon dioxide contained in the rich liquid, and the rich liquid is returned to an aqueous solution state with a low concentration of carbon dioxide. The gas obtained by this stripping contains a high concentration of carbon dioxide and relatively low concentrations of ammonia and water (water vapor). Carbon dioxide has a lower boiling point than ammonia and water, and ammonia has a lower boiling point than water. Therefore, the above gas can be obtained by adjusting the conditions such as the temperature in the stripping.

[0035] The temperature and pressure in the rich liquid stripping step are not particularly limited. Known temperatures and pressures in a method for gasifying and separating components such as carbon dioxide in an absorption liquid (e.g., aqueous ammonia) may be applied. However, the temperature in the rich liquid stripping step is preferably 100 to 230°C, more preferably 130 to 210°C, and the pressure is preferably 1 to 16 MPaG, more preferably 1 to 2.5 MPaG. In particular, it is preferable that the pressure is approximately the same as the pressure in the high-pressure absorption step described below in terms of relevance to subsequent steps.

[0036] The composition of the gas obtained in the rich liquid stripping step is not particularly limited. It is sufficient if the composition is such that the carbamate liquid can be obtained by contacting the gas with ammonia under high pressure in the high pressure absorption step described later. However, the concentration of carbon dioxide in the gas is preferably 70 to 100 mass%, more preferably 80 to 100 mass%, and the concentration of ammonia is, for example, 0 to 25 mass%.

[0037] The composition of the aqueous solution obtained in the rich solution stripping step is not particularly limited. This aqueous solution may be used as the lean solution as it is, or a mixture of this aqueous solution and a part of the rich solution may be used as the lean solution. For example, as described in Patent Document 1, the concentration of carbon dioxide in the lean solution is usually 0.2 to 4.0 mol / L (about 1 to 13 mass%), the concentration of ammonia is usually 2 to 12 mol / L (about 3 to 15 mass%), and the concentration ratio of ammonia to carbon dioxide is usually 1:1 to 3:1. This lean solution is preferably reused in the carbon dioxide separation equipment (CO2-AB) described above. In addition, in order to obtain an optimal composition as a lean solution for separating carbon dioxide, it is also preferable to add ammonia and / or water to this aqueous solution to adjust the concentration in advance.

[0038] The type of equipment used as the rich solution stripping equipment (RST) is not particularly limited. For example, known equipment for separating components in a solution by stripping can be used.

[0039] <High pressure absorption process> In the present invention, the high pressure absorption step is a step of obtaining a carbamate liquid by contacting at least a part of the gas separated in the rich liquid stripping step with ammonia under high pressure. The high pressure absorption equipment (HA) is an equipment for carrying out this high pressure absorption step.

[0040] In this way, when ammonia, which is supplied separately, is brought into contact with a gas containing a high concentration of carbon dioxide under high pressure, a carbamate, which is an intermediate in the urea synthesis reaction, is produced. If a solution containing this carbamate (carbamate solution) is used as a part of the raw material for urea synthesis in the urea synthesis step described below, it becomes possible to produce urea with a relatively high urea synthesis rate and relatively low energy consumption compared to the method of Patent Document 1.

[0041] In the high pressure absorption step, the amount of ammonia to be separately supplied is not particularly limited. It is sufficient that the amount is such that carbamate is produced under high pressure. However, it is preferable that the amount of ammonia is supplied so that the molar ratio N / C (ammonia / carbon dioxide) of ammonia to carbon dioxide in the carbamate liquid is 2.0 to 3.0.

[0042] The pressure in the high pressure absorption step may be any pressure higher than the vapor pressure of the produced carbamate liquid. The pressure is preferably 1.0 to 10 MPaG, more preferably 1.4 to 2.0 MPaG. The temperature in the high pressure absorption step is not particularly limited. It may be any temperature at which the carbamate is produced. However, the temperature is preferably 90 to 180°C, more preferably 90 to 120°C.

[0043] The composition of the carbamate liquid obtained in the high pressure absorption step is not particularly limited, but usually contains carbon dioxide, ammonia and water. The concentration of carbon dioxide in the carbamate liquid is preferably 20 to 50 mass%, and the concentration of ammonia is preferably 20 to 60 mass%. The carbon dioxide concentration and the ammonia concentration are the concentrations of the total amounts of carbon dioxide and ammonia constituting the carbamate and unreacted carbon dioxide and ammonia, respectively.

[0044] Furthermore, in this high-pressure absorption step, it is preferable to use, for example, the separated gas separated in the separation and purification step described below, and the aqueous solution separated in the concentration step described below, together as part of the raw materials for obtaining the carbamate liquid.

[0045] The type of equipment used as the high pressure absorption equipment (HA) is not particularly limited. For example, known equipment for producing carbamate liquid under high pressure can be used. In addition, at least a part of the absorption heat generated in the high pressure absorption equipment (HA) can be used in an absorption refrigeration device for generating the low temperature required in the carbon dioxide separation equipment (CO2-AB).

[0046] <Urea synthesis process> In the present invention, the urea synthesis step is a step of obtaining a urea synthesis solution by using the carbamate liquid obtained in the high-pressure absorption step as a part of the raw material. The urea synthesis facility (R) is a facility for carrying out this urea synthesis step.

[0047] In the urea synthesis step, at least the components contained in the carbamate liquid (carbamate, carbon dioxide, ammonia, etc.) are used as raw materials, and carbon dioxide and / or ammonia separately supplied as necessary are also used as raw materials to cause a reaction. In addition to a reactor directly performing such a synthesis reaction, it is preferable to use a known condenser (carbamate condenser) or a known stripper, for example, as described in JP-A-10-182587 or JP-A-2002-145850, in the urea synthesis step.

[0048] In the urea synthesis step, since a rich liquid containing a large amount of water is not used, the molar ratio H / C of water to carbon dioxide in the urea synthesis is low and the urea synthesis rate is high, compared with the case where the rich liquid is used as it is as a raw material for urea synthesis. Specifically, the H / C is preferably 1.5 or less. In addition, the molar ratio N / C of ammonia to carbon dioxide in the urea synthesis (ammonia / carbon dioxide) is preferably 3.0 to 4.0.

[0049] The temperature and pressure of the urea synthesis in the urea synthesis step are not particularly limited. Known temperatures and pressures in urea synthesis reactions may be applied. However, the temperature of the urea synthesis is preferably 170 to 200° C., and the pressure is preferably 13 to 25 MPaG.

[0050] The composition of the urea synthesis liquid obtained in the urea synthesis step (liquid to be treated in the decomposition and purification step described below) is not particularly limited, but usually contains urea, unreacted carbon dioxide, unreacted ammonia, and water. The amount of urea in the urea synthesis liquid is preferably 40 to 60 mass%, the amount of carbon dioxide is preferably 0 to 20 mass%, the amount of ammonia is preferably 10 to 30 mass%, and the amount of water is preferably 20 to 30 mass%.

[0051] The type of equipment used as the urea synthesis equipment (R) is not particularly limited. For example, known equipment for synthesizing urea under high pressure can be used. Furthermore, it is preferable to use a condenser (carbamate condenser) and a stripper as described above in combination.

[0052] In the present invention, the urea synthesis solution obtained in the urea synthesis step is preferably converted into product urea through desired steps such as a decomposition / purification step and a concentration step described below.

[0053] <Decomposition / purification process> The decomposition and purification process is a process in which separated gases containing carbon dioxide and ammonia are separated from the urea synthesis liquid obtained in the urea synthesis process, and a purified urea aqueous solution is obtained after this separation. The decomposition and purification equipment (D) is a facility for carrying out this decomposition and purification process.

[0054] As mentioned above, it is preferable to act a part of the aqueous solution separated in the concentration process described below as an absorption solvent for the separated gas separated in this separation and purification process, and to use it together with the separated gas as part of the raw material for obtaining the carbamate liquid in the high-pressure absorption process.

[0055] The conditions and equipment used in the decomposition and purification step may be, for example, known decomposition and purification conditions and equipment used in the post-step of urea synthesis.

[0056] <Concentration process> The concentration process is a process in which water and trace amounts of ammonia and carbon dioxide are evaporated from the purified urea solution obtained in the separation and purification process to obtain concentrated urea liquid, the evaporated water, ammonia, carbon dioxide, and urea accompanying the evaporated gas are recovered as an aqueous solution, and the excess water is treated to obtain clean treated water. The concentration equipment (EV) is a facility for carrying out this concentration process.

[0057] As mentioned above, it is preferable to use a portion of the aqueous solution (recycled liquid) separated and recovered in this concentration process as an absorption solvent for the separated gas separated in the separation and purification process, and to use it together as part of the raw material for obtaining the carbamate liquid in the high-pressure absorption process.

[0058] The conditions and equipment used in the concentration step may be, for example, those known as conditions and equipment for concentration used in a post-step of urea synthesis.

[0059] In the present invention, for example, in the regeneration step described below, it is preferable to separate high-purity carbon dioxide gas from a part of the rich liquid and use this as a part of the raw material for urea synthesis. In this case, the H / C ratio in the urea synthesis can be further reduced and the urea synthesis rate can be further increased.

[0060] <Regeneration process> The regeneration process is a process for separating high-purity carbon dioxide gas from at least a part of the rich liquid obtained in the carbon dioxide separation process other than the rich liquid to be stripped in the rich liquid stripping process. The regeneration equipment (CO2-D) is an equipment for carrying out this regeneration process.

[0061] In the regeneration step, a gas containing carbon dioxide is separated from the rich liquid by a known separation method such as distillation or stripping. Furthermore, if the carbon dioxide in the gas contains a trace amount of ammonia, the ammonia in the gas is removed by a purification method such as water washing. As a result, high-purity carbon dioxide gas is obtained. Then, in the urea synthesis step, this high-purity carbon dioxide gas is also used as part of the raw material. When high-purity carbon dioxide gas is also used as part of the raw material, compressor troubles due to the generation and adhesion of solid compounds of carbon dioxide and ammonia during gas compression can be prevented. In addition, in the present invention, not only high-purity carbon dioxide gas is used as a carbon dioxide supply source for the urea synthesis step, but also a carbamate liquid obtained from the gas separated from the rich liquid is used. Therefore, since the amount of high-purity carbon dioxide does not matter, the compression power of the CO2 compressor is reduced compared to the conventional method in which only high-purity carbon dioxide gas is used as a carbon dioxide supply source.

[0062] The conditions and equipment for the regeneration step may be, for example, known purification conditions and equipment for separating high-purity carbon dioxide gas from the rich liquid. The regeneration tower described in Patent Document 1 may be used. Also, a part of the heat recovered from the urea plant may be used as the heat source for the regeneration step.

[0063] BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a process flow diagram showing an embodiment of the method of the present invention.

[0064] The apparatus shown in FIG. 1 has a carbon dioxide separation facility 1 and a urea plant 2. The carbon dioxide separation facility 1 has a carbon dioxide absorption tower (CO2 Absorber) as a carbon dioxide separation facility (CO2-AB). The urea plant 2 has a rich solution stripper (Rich Solution Stripper) as a rich liquid stripping facility (RST), a high pressure absorber (High Pressure Absorber) as a high pressure absorption facility (HA), a urea synthesis facility (R) [having at least a reactor, and may also include a condenser and a stripper], a decomposer as a decomposition / purification facility (D), and an evaporator as a concentration facility (EV).

[0065] In FIG. 1, exhaust gas 10 is supplied to a carbon dioxide absorption tower [carbon dioxide separation equipment (CO2-AB)]. Also, lean liquid 20 from a rich liquid stripper [rich liquid stripping equipment (RST)] is supplied. Then, by contacting the exhaust gas 10 with the lean liquid 20 in the carbon dioxide absorption tower [carbon dioxide separation equipment (CO2-AB)], a rich liquid 11 that has absorbed carbon dioxide and a clean gas 12 from which carbon dioxide has been removed are generated. The generated rich liquid 11 is supplied to the rich liquid stripper [rich liquid stripping equipment (RST)]. Meanwhile, the generated clean gas 12 is discharged outside the system, or discharged outside the system after a further cleaning process.

[0066] In the rich liquid stripper [rich liquid stripping equipment (RST)] shown in Fig. 1, the rich liquid 11 is stripped to generate a lean liquid 20 and a gas 21. The lean liquid 20 is returned to the carbon dioxide absorption tower [carbon dioxide separation equipment (CO2-AB)] for reuse. Before returning the lean liquid 20, ammonia 22 and water 13 are added to the lean liquid 20 to adjust the concentration of the lean liquid 20.

[0067] Gas 21, ammonia 22, separated gas 23 containing carbon dioxide, ammonia and water separated in decomposition and purification equipment D, and recycled liquid 24 from concentration equipment EV are supplied to the high-pressure absorption tower [high-pressure absorption tower equipment (HA)] shown in Fig. 1. Then, carbamate liquid 25 is generated under high pressure. The generated carbamate liquid 25 is supplied to a reactor [urea synthesis equipment (R)].

[0068] A carbamate liquid 25 and ammonia 22 are supplied to the reactor [urea synthesis facility (R)] shown in Fig. 1. Urea is synthesized using these as raw materials to generate a urea synthesis liquid 26. This urea synthesis liquid 26 is supplied to a decomposition device [decomposition and purification facility (D)].

[0069] 1, a separated gas 23 containing ammonia, carbon dioxide and water is separated from the urea synthesis liquid 26 to produce a urea aqueous solution 27. This urea aqueous solution 27 is supplied to an evaporator [concentration equipment (EV)], and the separated gas 23 is supplied to a high-pressure absorption tower [high-pressure absorption tower equipment (HA)].

[0070] 1, the urea aqueous solution 27 is concentrated to produce a urea product 28 and treated water 29. The urea product 28 and treated water 29 are supplied to the subsequent process.

[0071] Figure 2 is a process flow diagram showing another embodiment of the method of the present invention. The apparatus shown in Figure 2 is the same as the apparatus shown in Figure 1 except that a regeneration tower (CO2Desorber) is added as a regeneration facility (CO2-D) to the carbon dioxide separation facility 1. The CO2 gas obtained from the regeneration tower is compressed and then sent to the urea synthesis facility (R).

[0072] <Method of improving urea production equipment> In the present invention, an existing urea production apparatus may be improved by adding at least a rich liquid stripping unit (RST) [and a carbon dioxide separation unit (CO2-AB) as necessary] to enable the method of the present invention, i.e., the urea production method having the carbon dioxide separation step, rich liquid stripping step, high pressure absorption step and urea synthesis step described above.

[0073] As an embodiment of this improvement method, for example, there is a method of improving the apparatus shown in FIG. 1 of Patent Document 1 (an apparatus integrating an ammonia production plant and a urea production plant) by adding a rich liquid stripping unit (RST) and also adding or strengthening a high-pressure absorption unit (HA). In addition, there is a method of improving a normal urea production plant by adding a carbon dioxide separation unit (CO2-AB) and a rich liquid stripping unit (RST) [and a high-pressure absorption unit (HA) if there is no high-pressure absorption unit (HA)]. In addition, by further adding a regeneration unit (CO2-D), it is also possible to use high-purity carbon dioxide gas as part of the raw material in the urea synthesis process. EXAMPLES

[0074] The present invention will be described in more detail below with reference to examples, although the present invention is not limited to these examples.

[0075] <Example 1> A process simulation was carried out for synthesizing urea according to the embodiment of the present invention shown in Fig. 1. The conditions used in the process simulation are as follows. (1) Carbon dioxide separation facility (CO2-AB) Lean liquid composition: Carbon dioxide 8% by mass, ammonia 7% by mass, water 85% by mass Temperature: 21℃ Pressure: 0.01MPaG Composition of rich liquid: carbon dioxide 11% by mass, ammonia 7% by mass, water 82% by mass (2) Rich Liquid Stripping Equipment (RST) Temperature: 162℃ Pressure: 1.7MPaG Gas composition: 88% carbon dioxide, 4% ammonia, 8% water (3) High pressure absorption equipment (HA) Temperature: 107℃ Pressure: 1.7MPaG Composition of carbamate solution: 38% carbon dioxide, 42% ammonia, 20% water (4) Urea synthesis equipment (R) Temperature: 188℃ Pressure: 15MPaG

[0076] As a result of simulation under the above conditions, the H / C ratio in urea synthesis was 1.1, and the urea synthesis rate was 58%.

[0077] <Example 2> A process simulation was carried out for synthesizing urea according to the embodiment of the present invention shown in FIG. (1) Carbon dioxide separation facility (CO2-AB) Lean liquid composition: Carbon dioxide 8% by mass, ammonia 7% by mass, water 85% by mass Temperature: 21℃ Pressure: 0.01MPaG Composition of rich liquid: carbon dioxide 11% by mass, ammonia 7% by mass, water 82% by mass (2) Rich Liquid Stripping Equipment (RST) Temperature: 162℃ Pressure: 1.7MPaG Gas composition: 88% carbon dioxide, 4% ammonia, 8% water (3) High pressure absorption equipment (HA) Temperature: 107℃ Pressure: 1.6MPaG Composition of carbamate solution: 42% carbon dioxide, 37% ammonia, 21% water (4) Urea synthesis equipment (R) Temperature: 182℃ Pressure: 15MPaG (5) Regeneration equipment (CO2-D) Temperature: 146℃ Pressure: 2.4MPaG CO2 compressor suction pressure: 2.5MPaG CO2 compressor power: 29kWh / t-urea

[0078] As a result of simulation under the above conditions, the H / C ratio in urea synthesis was 0.6, and the urea synthesis rate was 64%.

[0079] <Comparative Example 1> A process simulation was carried out for synthesizing urea using only the rich solution as a carbon supply source as in the method shown in Fig. 2 of Patent Document 1. The conditions used in the process simulation are as follows. Composition of rich liquid: carbon dioxide 11% by mass, ammonia 7% by mass, water 82% by mass Composition of carbamate solution: carbon dioxide 13% by mass, ammonia 14% by mass, water 73% by mass Urea synthesis temperature: 188℃ Urea synthesis pressure: 15MPaG

[0080] As a result of the simulation under the above conditions, the H / C ratio in urea synthesis was estimated to be 4.1.

[0081] The H / C ratios obtained by the simulations of the above Examples 1 and 2 and Comparative Example 1 are shown in the following Table 1. For reference, Table 1 also shows generally estimated values ​​of the CO2 compressor suction pressure and the CO2 compressor power of a known example (a typical urea plant).

[0082] [Table 1]

[0083] As shown in Table 1, the H / C ratios in Examples 1 and 2 are low. On the other hand, the H / C ratio in Comparative Example 1 is high, and it is estimated that the urea synthesis rate is low. In Example 2, the required supply amount of high-purity carbon dioxide gas is less than that in the known example (an example in which only high-purity carbon dioxide gas is used as a carbon dioxide supply source), so the power (CO2 compressor power) required per ton of urea is less than that in the known example. [Industrial Applicability]

[0084] The present invention is useful as a method and an apparatus for producing urea, which has a relatively high urea synthesis rate and requires relatively low energy consumption. [Explanation of symbols]

[0085] CO2-AB Carbon Dioxide Separation Equipment RST Rich Liquid Stripping Equipment HA High Pressure Absorption Equipment R Urea synthesis equipment D Decomposition / purification equipment EV enrichment equipment CO2-D regeneration equipment 1 Carbon dioxide separation facility 2. Urea plant 10 Exhaust gas 11 Rich Liquid 12 Clean Gas 13 water 20 Lean Liquid 21 Gas 22 Ammonia 23 Separation Gas 24 Recycled Liquid 25 Carbamate Liquid 26 Urea synthesis solution 27 Urea aqueous solution 28 Urea products 29 Treated Water

Claims

1. A carbon dioxide separation step involves contacting a gas containing carbon dioxide with a lean liquid, which is an absorbent solution containing low concentrations of carbon dioxide, ammonia, and water, thereby absorbing and separating the carbon dioxide contained in the gas into the lean liquid to obtain a rich liquid, which is an absorbent solution containing high concentrations of carbon dioxide, ammonia, and water. A rich liquid stripping step is performed at a temperature of 100 to 230°C and a pressure of 1 to 16 MPaG, wherein at least a portion of the rich liquid obtained in the carbon dioxide separation step is stripped to preferentially gasify carbon dioxide and separate a gas containing a high concentration of carbon dioxide, thereby obtaining an aqueous solution of the rich liquid containing a low concentration of carbon dioxide and ammonia. A high-pressure absorption step is performed at a temperature of 90 to 180°C and a pressure of 1.0 to 10 MPaG, wherein at least a portion of the gas separated in the rich liquid stripping step is brought into contact with ammonia under high pressure to obtain a carbamate liquid, and The urea synthesis process, performed at a temperature of 170-200°C and a pressure of 13-25 MPaG, uses the carbamate solution obtained in the high-pressure absorption process as part of the raw materials to obtain a urea synthesis solution. A method for producing urea having the following characteristics.

2. The method for producing urea according to Claim 1, wherein the rich liquid stripping step is performed at a temperature of 130 to 210°C and a pressure of 1 to 2.5 MPaG, and the high-pressure absorption step is performed at a temperature of 90 to 120°C and a pressure of 1.4 to 2.0 MPaG.

3. The method for producing urea according to claim 1 or 2, wherein the carbon dioxide used in the urea synthesis step is supplied after being added to the carbamate liquid obtained in the high-pressure absorption step from the gas separated in the rich liquid stripping step and pressurized.

4. The method further comprises a regeneration step of separating high-purity carbon dioxide gas from at least a portion of the rich liquid obtained in the carbon dioxide separation step, excluding the rich liquid to be stripped in the rich liquid stripping step. A method for producing urea according to claim 1 or 2, wherein in the urea synthesis step, high-purity carbon dioxide gas separated in the regeneration step is also used as part of the raw materials to obtain a urea synthesis solution.

5. The method for producing urea according to Claim 4, wherein the carbon dioxide used in the urea synthesis step includes a gas obtained by compressing high-purity carbon dioxide gas separated in the regeneration step to the synthesis pressure in a compressor, and carbon dioxide supplied by adding it to the carbamate liquid obtained in the high-pressure absorption step from the gas separated in the rich liquid stripping step and compressing it to the synthesis pressure, thereby reducing the power of the compressor that pressurizes the carbon dioxide gas.

6. A method for producing urea according to claim 1 or 2, comprising a separation and purification step of separating a separation gas containing carbon dioxide and ammonia from the urea synthesis solution obtained in the urea synthesis step to obtain a purified urea aqueous solution thereafter, wherein the high-pressure absorption step uses the separation gas from the decomposition and purification step as a raw material for obtaining a carbamate solution.

7. The method for producing urea according to claim 6, comprising a concentration step of obtaining a concentrated urea solution from the purified urea aqueous solution obtained in the decomposition and purification step, wherein the high-pressure absorption step involves acting the recycled liquid recovered in the concentration step as an absorption solvent for the separation gas, and the obtained carbamate solution is used in combination as part of the raw material for urea synthesis.

8. A carbon dioxide separation apparatus (CO2-AB) for obtaining a rich liquid, which is an absorbent liquid containing high concentrations of carbon dioxide, ammonia, and water, by contacting a gas containing carbon dioxide with a lean liquid, which is an absorbent liquid containing low concentrations of carbon dioxide, ammonia, and water, thereby absorbing and separating the carbon dioxide contained in the gas into the lean liquid. A rich liquid stripping apparatus (RST) for performing a rich liquid stripping process, which is carried out at a temperature of 100 to 230°C and a pressure of 1 to 16 MPaG, wherein at least a portion of the rich liquid obtained in the carbon dioxide separation apparatus (CO2-AB) is stripped to preferentially gasify carbon dioxide and separate a gas containing a high concentration of carbon dioxide, thereby obtaining an aqueous solution of the rich liquid containing a low concentration of carbon dioxide and ammonia. A high-pressure absorption apparatus (HA) for performing a high-pressure absorption process, carried out at a temperature of 90 to 180°C and a pressure of 1.0 to 10 MPaG, wherein at least a portion of the gas separated in the rich liquid stripping apparatus (RST) is brought into contact with ammonia under high pressure to obtain a carbamate liquid, and A urea synthesis apparatus (R) for performing a urea synthesis process, which is carried out at a temperature of 170 to 200°C and a pressure of 13 to 25 MPaG, using the carbamate solution obtained in the aforementioned high-pressure absorption apparatus (HA) as part of the raw materials to obtain a urea synthesis solution. A urea manufacturing apparatus having the following features.

9. The urea production apparatus according to claim 8, wherein the rich liquid stripping step is performed at a temperature of 130 to 210°C and a pressure of 1 to 2.5 MPaG, and the high-pressure absorption step is performed at a temperature of 90 to 120°C and a pressure of 1.4 to 2.0 MPaG.

10. The urea production apparatus according to claim 8 or 9, wherein the high-pressure absorption equipment (HA) supplies the entire amount of carbon dioxide used in the urea synthesis equipment (R) in the carbamate liquid obtained in the high-pressure absorption equipment (HA) from the gas separated in the rich liquid stripping equipment (RST).

11. The regeneration equipment (CO2-D) further comprises separating high-purity carbon dioxide gas from at least a portion of the rich liquid obtained in the carbon dioxide separation equipment (CO2-AB), excluding the rich liquid to be stripped in the rich liquid stripping equipment (RST), The apparatus for producing urea according to claim 8 or 9, wherein in the urea synthesis apparatus (R), high-purity carbon dioxide gas separated in the regeneration apparatus (CO2-D) is also used as part of the raw materials to obtain a urea synthesis solution.

12. The urea production apparatus according to claim 11, wherein the carbon dioxide used in the urea synthesis apparatus (R) includes a gas obtained by compressing high-purity carbon dioxide gas separated in the regeneration apparatus (CO2-D) to the synthesis pressure in a compressor, and carbon dioxide supplied by being added to the synthesis pressure in a carbamate liquid obtained in the high-pressure absorption apparatus (HA) from the gas separated in the rich liquid stripping apparatus (RST), thereby reducing the power of the compressor that pressurizes the carbon dioxide gas.

13. The urea production apparatus according to claim 8 or 9, comprising a separation and purification apparatus (D) for separating a separation gas containing carbon dioxide and ammonia from the urea synthesis solution obtained in the urea synthesis apparatus (R) to obtain a purified urea aqueous solution thereafter, wherein the high-pressure absorption apparatus (HA) also uses the separation gas separated in the separation and purification apparatus (D) as a raw material for obtaining a carbamate solution.

14. The urea production apparatus according to claim 13, further comprising a concentration equipment (EV) for obtaining concentrated urea solution from a purified urea aqueous solution obtained in the separation and purification equipment (D), wherein the high-pressure absorption equipment (HA) acts on the recycled liquid recovered in the concentration equipment (EV) as an absorption solvent for the separation gas, and the obtained carbamate solution is used in combination as part of the raw material for urea synthesis.

15. A method for improving an existing urea manufacturing apparatus, Compared to existing urea manufacturing equipment, at least, A carbon dioxide separation apparatus (CO2-AB) is a facility for obtaining a rich liquid, which is an absorbent liquid containing high concentrations of carbon dioxide, ammonia, and water, by contacting a gas containing carbon dioxide with a lean liquid, which is an absorbent liquid containing low concentrations of carbon dioxide, ammonia, and water, thereby absorbing and separating the carbon dioxide contained in the gas into the lean liquid. By stripping a rich liquid, which is an absorbent liquid containing high concentrations of carbon dioxide, ammonia, and water, the carbon dioxide is preferentially gasified to separate the gas containing high concentrations of carbon dioxide, and the rich liquid is converted into an aqueous solution containing low concentrations of carbon dioxide and ammonia. This is achieved by adding a rich liquid stripping facility (RST). A carbon dioxide separation step involves contacting a gas containing carbon dioxide with a lean liquid, which is an absorbent solution containing low concentrations of carbon dioxide, ammonia, and water, thereby absorbing and separating the carbon dioxide contained in the gas into the lean liquid to obtain a rich liquid, which is an absorbent solution containing high concentrations of carbon dioxide, ammonia, and water. A rich liquid stripping step is performed at a temperature of 100 to 230°C and a pressure of 1 to 16 MPaG, wherein at least a portion of the rich liquid obtained in the carbon dioxide separation step is stripped to preferentially gasify carbon dioxide and separate a gas containing a high concentration of carbon dioxide, thereby obtaining an aqueous solution of the rich liquid containing a low concentration of carbon dioxide and ammonia. A high-pressure absorption step is performed at a temperature of 90 to 180°C and a pressure of 1.0 to 10 MPaG, wherein at least a portion of the gas separated in the rich liquid stripping step is brought into contact with ammonia under high pressure to obtain a carbamate liquid, and The urea synthesis process, performed at a temperature of 170-200°C and a pressure of 13-25 MPaG, uses the carbamate solution obtained in the high-pressure absorption process as part of the raw materials to obtain a urea synthesis solution. A method for improving an existing urea manufacturing apparatus that enables a method for producing urea having [a specific characteristic].

16. The method for improving an existing urea production apparatus according to claim 15, wherein the rich liquid stripping step is performed at a temperature of 130 to 210°C and a pressure of 1 to 2.5 MPaG, and the high-pressure absorption step is performed at a temperature of 90 to 120°C and a pressure of 1.4 to 2.0 MPaG.

17. The method for improving an existing urea production apparatus according to claim 15 or 16, wherein the carbon dioxide used in the urea synthesis step is supplied after being added to the carbamate liquid obtained in the high-pressure absorption step from the gas separated in the rich liquid stripping step and pressurized.

18. By further adding a regeneration facility (CO2-D) that separates high-purity carbon dioxide gas from at least a portion of the rich liquid obtained from the carbon dioxide separation facility (CO2-AB), excluding the rich liquid that is stripped in the rich liquid stripping facility (RST), A regeneration process for separating high-purity carbon dioxide gas from at least a portion of the rich liquid obtained in the carbon dioxide separation process, excluding the rich liquid stripped in the rich liquid stripping process, and A method for improving an existing urea production apparatus according to claim 15 or 16, which enables the use of the high-purity carbon dioxide gas as part of the raw materials in the urea synthesis process to obtain a urea synthesis solution.

19. The carbon dioxide used in the urea synthesis step includes gas obtained by pressing high-purity carbon dioxide gas separated in the regeneration step up to the synthesis pressure in a compressor, and carbon dioxide supplied by adding it to the carbamate liquid obtained in the high-pressure absorption step from the gas separated in the rich liquid stripping step and pressing it up to the synthesis pressure, thereby reducing the power of the compressor that pressurizes the carbon dioxide gas, as described in Claim 18.

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