Urea synthesis method

Abstract

To provide an urea synthesis method capable of reducing energy and running cost necessary for synthesis of urea.SOLUTION: In an urea synthesis method characterized by contacting an urea synthesis liquid generated in an urea synthesis tower A with at least a part of raw material carbon dioxide at a stripper C under heating, condensating a separated mixed gas by introducing into a condenser B, and circulating the condensated liquid to the urea synthesis tower A, an oxygen supply concentration with respect to the raw material carbon dioxide is 100 to 2,000 ppm, the urea synthesis pressure in the urea synthesis tower A is 125 to 145 bar, and the urea synthesis temperature is 175°C to 190°C, having an N / C of 3.5 to 4.0 and a H / C of 0.70 or less.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a urea synthesis method capable of reducing the energy and running costs required for urea synthesis in a method for synthesizing urea from ammonia and carbon dioxide while forming a passive film on the metal surface inside the apparatus with oxygen.

Background Art

[0002] Urea is an important raw material for nitrogen-based fertilizers, which are one of the three essential elements for plant growth, and has supported global food production since ancient times. Even now, with the increasing population in emerging countries around the world, an increase in fertilizer production to meet the increased food production is required, and the construction of urea synthesis plants based on a urea synthesis method with excellent reliability and productivity has become an important issue.

[0003] Here, reliability mainly refers to the property of protecting the metal surface inside this apparatus from ammonium carbamate, which is generated as a reaction intermediate when synthesizing urea from ammonia and carbon dioxide, and has strong corrosiveness to metals. On the other hand, productivity refers to the reduction of the total urea production cost, including not only increasing the reaction yield of ammonia and carbon dioxide but also reducing the apparatus construction cost and improving the running cost by improving the operating conditions.

[0004] Technological development to enhance such reliability and productivity has mainly been carried out from three viewpoints. First is the chemical viewpoint of forming a passive film on the metal surface of the apparatus to enhance corrosion resistance. Second is the material viewpoint of improving the corrosion resistance of the apparatus material itself against ammonium carbamate. Third is the process viewpoint of improving corrosion resistance and reaction yield by improving the manufacturing process and apparatus. However, these are in a mutually conflicting relationship, and a balanced urea synthesis method is still required.

[0005] That is, it is a method of mixing oxygen with starting materials such as carbon dioxide and ammonia and forming a passive film composed of hydroxides of Fe or Cr and oxides of Cr about 1 to 3 nm thick under high temperature and high pressure.

[0006] However, if too much oxygen is supplied as an oxidizing agent, the inert gas increases, and the H / C (molar ratio of water to carbon dioxide), which will be discussed later, increases, lowering the reaction yield. On the other hand, a scrubber is also needed to ultimately separate and remove oxygen from the unconverted raw materials and other volatile components, as well as a hydrogen combustion removal device to prevent the risk of trace amounts of hydrogen contained in the urea product mixing with oxygen. Thus, while increasing the supply of oxygen improves corrosion resistance, it also has the problem of negatively impacting running costs.

[0007] Therefore, development has focused on improving the corrosion resistance of stainless steel itself. For example, as disclosed in Patent Document 1, an austenitic-ferrite duplex stainless steel has been proposed that focuses on the effects of Cr, Ni, Mo, and N, and contains a Cr content of 26% by mass or more and less than 28% by mass, a Ni content of 6-10% by mass, a Mo content of 0.2-1.7% by mass, and an N content of more than 0.3% by mass and up to 0.4% by mass.

[0008] However, the high cost of austenitic-ferrite duplex stainless steel, which has excellent corrosion resistance and a high chromium content, leads to high construction costs for urea plants, making it difficult to build a urea synthesis plant that combines reliability and productivity.

[0009] Therefore, as disclosed in Patent Document 2, improvements have been made to the manufacturing process and equipment. In particular, the introduction of a stripper to effectively reuse unreacted products and the addition of urea synthesis functionality to the condenser, similar to that of the reactor, can increase the reaction yield, reduce the size of the manufacturing equipment, and contribute to improved productivity.

[0010] However, these technologies still had room for improvement as a reliable and productive urea synthesis method that balanced the oxygen concentration required for corrosion resistance, the material of the stainless steel, and efficient manufacturing processes.

[0011] Therefore, a urea synthesis method has been proposed that, despite selecting inexpensive general-purpose stainless steel, minimizes the amount of oxygen supplied as an oxidizing agent (hereinafter referred to as corrosion-preventive oxygen), exhibits excellent corrosion resistance, high reaction yield, and low construction costs, enabling the production of urea at low cost.

[0012] However, conventional urea synthesis methods require further improvements in terms of the energy and running costs involved in urea synthesis. [Prior art documents] [Patent Documents]

[0013] [Patent Document 1] Japanese Patent Publication No. 2003-301241 [Patent Document 2] Japanese Patent Application Publication No. 10-182587 [Patent Document 3] International Publication No. 2014 / 192823 [Overview of the project] [Problems that the invention aims to solve]

[0014] The present invention was made to solve the problems of the prior art described above. Specifically, the object of the present invention is to provide a urea synthesis method that can reduce the energy and running costs required for urea synthesis in a method of synthesizing urea from ammonia and carbon dioxide while forming a passivation film on the metal surface inside the apparatus with oxygen. [Means for solving the problem]

[0015] As a result of diligent research to solve the above problems, the inventors of the present invention found that reducing the urea synthesis pressure in the urea synthesis column is very effective in reducing the energy and running costs required for urea synthesis, and thus completed the present invention.

[0016] The present invention relates to a urea synthesis method comprising: reacting ammonia and carbon dioxide in a urea synthesis tower at urea synthesis temperature and pressure; contacting the resulting urea synthesis solution, which contains at least urea, unreacted ammonia, unreacted carbon dioxide, and water, with at least a portion of the raw material carbon dioxide in a stripper under heating at a pressure approximately equal to the urea synthesis pressure to separate the unreacted ammonia and unreacted carbon dioxide as a mixed gas of ammonia, carbon dioxide, and water; further processing the unseparated urea synthesis solution containing the unreacted ammonia and unreacted carbon dioxide to obtain urea; and introducing the mixed gas separated in the stripper to the bottom of a condenser and contacting it with an absorption medium under cooling to condense the mixed gas; and circulating the resulting condensate back into the urea synthesis tower. The oxygen supply concentration for the aforementioned raw material carbon dioxide is 100 to 2,000 ppm. The urea synthesis pressure in the aforementioned urea synthesis tower is 125-145 bar. The urea synthesis temperature in the urea synthesis tower is 175°C to 190°C. The N / C (molar ratio of ammonia to carbon dioxide) in the urea synthesis tower is 3.5 to 4.0. The urea synthesis method is characterized in that the H / C (molar ratio of water to carbon dioxide) in the urea synthesis tower is 0.70 or less.

[0017] Furthermore, the present invention relates to a method for improving an existing urea synthesis apparatus to reduce the urea synthesis pressure in a urea synthesis tower, In the existing urea synthesis apparatus, in the urea synthesis tower, ammonia and carbon dioxide are reacted at the urea synthesis temperature and urea synthesis pressure, and a urea synthesis liquid containing at least the produced urea, unreacted ammonia, unreacted carbon dioxide, and water is heated in a stripper under contact with at least a part of the raw material carbon dioxide at a pressure substantially equal to the urea synthesis pressure, and the unreacted ammonia and the unreacted carbon dioxide are separated as a mixed gas of ammonia, carbon dioxide, and water, and the urea synthesis liquid containing the unseparated unreacted ammonia and unreacted carbon dioxide is further processed to obtain urea. On the other hand, the mixed gas separated in the stripper is introduced into the bottom of the condenser and brought into contact with an absorption medium under cooling, thereby condensing the mixed gas, and the condensate thus obtained is circulated to the urea synthesis tower. The urea synthesis apparatus is The material of the pipe connecting the stripper and the condenser of the existing urea synthesis apparatus is not any of the materials of the following (a) and (b). An improvement method for an existing urea synthesis apparatus that enables reduction of the urea synthesis pressure in the urea synthesis tower by replacing the pipe connecting the stripper and the condenser of the existing urea synthesis apparatus with a pipe containing the material of the following (a) or (b). (a) General-purpose austenite-ferrite dual-phase stainless steel with Cr content: 21 to 26% by mass, Ni content: 4.5 to 7.5% by mass, Mo content: 2.5 to 3.5% by mass, N content: 0.08 to 0.30% by mass, C content: 0.03% by mass or less, Si content: 1.0% by mass or less, Mn content: 2.0% by mass or less, P content: 0.04% by mass or less, S content: 0.03% by mass or less. (b) Austenite-ferrite dual-phase stainless steel with Cr content: 26 to 35% by mass, Ni content: 3 to 10% by mass, Mo content: 1.0 to 4.0% by mass, N content: 0.2 to 0.6% by mass.

[0018] Furthermore, the present invention is an improvement method for an existing urea synthesis apparatus for reducing the urea synthesis pressure in the urea synthesis region, In the existing urea synthesis apparatus, in the urea synthesis region, ammonia and carbon dioxide are reacted at the urea synthesis temperature and urea synthesis pressure, and a urea synthesis liquid containing at least the produced urea, unreacted ammonia, unreacted carbon dioxide, and water is brought into contact with at least a part of the raw material carbon dioxide in a stripper under heating at a pressure substantially equal to the urea synthesis pressure, so that the unreacted ammonia and the unreacted carbon dioxide are separated as a mixed gas of ammonia, carbon dioxide, and water, and the urea synthesis liquid containing the unseparated unreacted ammonia and unreacted carbon dioxide is further processed to obtain urea. On the other hand, the mixed gas separated in the stripper is introduced to the bottom of the condensation region and brought into contact with an absorption medium under cooling, thereby condensing the mixed gas, and the condensate thus obtained is circulated to the urea synthesis region. This is a urea synthesis apparatus. The urea synthesis region and the condensation region are present in the same container or in separate containers. The condensation region is present in a horizontal container. The material of the pipe connecting the stripper and the condensation region of the existing urea synthesis apparatus is not any of the following materials (a) and (b). An improvement method for an existing urea synthesis apparatus, which enables reduction of the urea synthesis pressure in the urea synthesis region by replacing the pipe connecting the stripper and the condensation region of the existing urea synthesis apparatus with a pipe containing the following material (a) or (b). (a) General-purpose austenite-ferrite two-phase stainless steel with a Cr content of 21 to 26% by mass, a Ni content of 4.5 to 7.5% by mass, a Mo content of 2.5 to 3.5% by mass, a N content of 0.08 to 0.30% by mass, a C content of 0.03% by mass or less, a Si content of 1.0% by mass or less, a Mn content of 2.0% by mass or less, a P content of 0.04% by mass or less, and a S content of 0.03% by mass or less. (b) Austenite-ferrite two-phase stainless steel with a Cr content of 26 to 35% by mass, a Ni content of 3 to 10% by mass, a Mo content of 1.0 to 4.0% by mass, and a N content of 0.2 to 0.6% by mass.

Advantages of the Invention

[0019] In this invention, by particularly lowering the oxygen supply concentration relative to the raw material carbon dioxide, it is possible to lower the urea synthesis pressure in the urea synthesis tower, and as a result, the energy required for urea synthesis can be reduced, and running costs can be reduced. In this invention, "oxygen supply concentration relative to raw material carbon dioxide" means the concentration of oxygen supplied to the urea synthesis apparatus as an oxidizing agent (oxygen in the air if air is supplied) relative to the carbon dioxide supplied to the urea synthesis apparatus as a raw material. For example, in Figure 2, it means the concentration of oxygen relative to carbon dioxide in line 2.

[0020] Specifically, lowering the oxygen supply concentration relative to the raw material carbon dioxide means lowering the amount of oxygen (or air, if oxygen-containing air is supplied). This increases the proportion of the raw material gas (raw material carbon dioxide) in the mixed gas, lowering the partial pressure of gases other than the raw material (inert gas) in the urea synthesis process, thus making it possible to lower the urea synthesis pressure. It should be noted that this method of lowering urea synthesis pressure was completely unprecedented. This is because it was previously believed that the optimal urea synthesis pressure differed depending on conditions such as N / C, and it was not known that urea synthesis pressure could be reduced simply by lowering the oxygen supply concentration, nor that urea synthesis could be performed successfully even at such low pressures. [Brief explanation of the drawing]

[0021] [Figure 1] This is a process flow diagram of a urea synthesis apparatus using the carbon dioxide stripping method for urea synthesis according to the present invention. [Figure 2] This is one embodiment of a urea synthesis apparatus using the carbon dioxide stripping method for urea synthesis according to the present invention, and is a process flow of the urea synthesis method used in Example 1 and Reference Example 1. [Modes for carrying out the invention]

[0022] One embodiment of the present invention is the carbon dioxide stripping method for urea synthesis shown in Figure 1, in which liquid ammonia as a raw material is supplied to urea synthesis tower A, and corrosion-preventing oxygen is supplied as air along with gaseous carbon dioxide (carbon dioxide in Figure 1) as a raw material to urea synthesis tower A and stripper C. In particular, a preferred embodiment of the urea synthesis method is the various synthesis processes disclosed in Japanese Patent Application Publication No. 10-182587, one embodiment of which is shown in Figure 2, but the invention is not limited thereto.

[0023] For example, the raw material liquid ammonia may be supplied to urea synthesis tower A and condenser B, while the raw material gaseous carbon dioxide, including corrosion-preventive air supplied from carbon dioxide compressor H, may be supplied only to stripper C and not to urea synthesis tower A. Alternatively, for example, the raw material liquid ammonia may be supplied only to urea synthesis tower A, while the raw material gaseous carbon dioxide, including corrosion-preventive oxygen supplied from carbon dioxide compressor H, may be supplied only to urea synthesis tower A and not to stripper C (ammonia stripping method). However, as shown in the prior art, it should be noted that differences in the urea synthesis method result in differences in the amount of corrosion-preventive oxygen required by the stainless steel used in this method, as the pressure and temperature conditions and the composition of the fluids that accumulate and pass through each region, such as urea synthesis tower A, stripper C, and condenser B, differ.

[0024] Furthermore, in the process flow shown in Figure 1, ejector G is used to pressurize the liquid from condenser B and circulate the synthesis solution, due to its simple structure, durability, and ease of maintenance. However, other pressurizing means such as pumps can also be used for circulation. However, if good circulation of the urea synthesis solution can be achieved through the arrangement of each device, this may be omitted. For example, by devising the arrangement of each device, it is possible to circulate the urea synthesis solution by gravity alone. However, if pressurizing means such as ejector G are provided, the urea synthesis tower A, condenser B, and stripper C can be installed at a lower position, which is preferable in terms of installation work and maintenance. Moreover, each of the above devices may be installed individually, or condenser B may be combined with a scrubber, etc.

[0025] On the other hand, the condenser can be installed horizontally or vertically. For example, if a horizontal condenser is installed on or near the ground, its low overall height makes installation and maintenance of the urea synthesis tower easier. Furthermore, using a vertical condenser not only saves installation space but also allows for countercurrent gas-liquid contact in the condensing section, enabling a sufficiently long residence time.

[0026] Furthermore, the urea production apparatus used in the urea synthesis method of the present invention may be a newly constructed apparatus entirely from scratch, or it may be a modified apparatus (so-called revamping) in which parts of existing equipment are added or replaced.

[0027] In such a urea synthesis apparatus, at least some of the parts of the urea synthesis tower A, stripper C, condenser B, and the piping connecting them that come into contact with a corrosive fluid can be made of austenitic-ferrite duplex stainless steel having the following composition (a).

[0028] [Composition of austenitic-ferrite duplex stainless steel applied to the present invention (a)] Cr:21~26% by mass Ni:4.5~7.5% by mass Mo:2.5~3.5% by mass N:0.08~0.30% by mass C: 0.03% by mass or less Si: 1.0% by mass or less Mn: 2.0% by mass or less P: 0.04% by mass or less S: 0.03% by mass or less The remainder consists of Fe and, for example, impurities, additives, etc.

[0029] In particular, it is preferable to use S31803 or S31260, which are readily available and inexpensive as general-purpose austenitic-ferrite duplex stainless steels.

[0030] Furthermore, it is more preferable to use an austenitic-ferrite duplex stainless steel having the following composition (a1) as the austenitic-ferrite duplex stainless steel.

[0031] [Composition of austenitic-ferrite duplex stainless steel applied to the present invention (a1)] Cr:24~26% by mass Ni:5.5~7.5% by mass Mo:2.5~3.5% by mass N:0.08~0.30% by mass C: 0.03% by mass or less Si: 1.0% by mass or less Mn: 1.5% by mass or less P: 0.04% by mass or less S: 0.03% by mass or less The remainder consists of Fe and, for example, impurities, additives, etc.

[0032] In particular, it is preferable to use S31260, which is readily available and inexpensive as a general-purpose austenitic-ferrite duplex stainless steel.

[0033] Furthermore, for strippers where corrosion is likely to progress, it is preferable to use austenitic-ferrite duplex stainless steel with the following composition (b).

[0034] [Composition of austenitic-ferrite duplex stainless steel applied to the present invention (b)] Cr:26~35% by mass Ni: 3~10% by mass Mo: 1.0~4.0% by mass, N:0.2~0.6% by mass The remainder consists of Fe and, for example, impurities, additives, etc. (Note that the Cr content may exceed 26% by mass but be 35% by mass or less.)

[0035] In particular, it is preferable to use, for example, S32707 and S32808 as such austenitic-ferrite duplex stainless steels.

[0036] Furthermore, depending on the corrosive environment, such as for piping and valves, S31603 series general-purpose stainless steel can be used, making it possible to further reduce equipment costs without increasing the amount of oxygen required for corrosion protection.

[0037] Specific preferred embodiments of the above stainless steels include, for example, an embodiment in which the material of at least one device selected from the group consisting of a urea synthesis tower, a stripper, a condenser, and piping connecting at least two of the urea synthesis tower, stripper, and condenser is a general-purpose austenitic-ferritic duplex stainless steel of composition (a) or an austenitic-ferritic duplex stainless steel of composition (b); an embodiment in which the material of the stripper is a general-purpose austenitic-ferritic duplex stainless steel of composition (a) or an austenitic-ferritic duplex stainless steel of composition (b); and an embodiment in which the material of the piping connecting the stripper and the condenser is a general-purpose austenitic-ferritic duplex stainless steel of composition (a) or an austenitic-ferritic duplex stainless steel of composition (b).

[0038] In a urea synthesis apparatus made from the materials described above, it is preferable to operate the urea synthesis column A under the following conditions, primarily from the viewpoint of reaction yield: pressure: 125-145 bar, N / C: 3.5-4.0, H / C: 1.0 or less, residence time: 10-40 minutes, temperature: 175°C-190°C; the stripper under the following conditions: pressure: 125-145 bar, particularly preferably 130-140 bar, temperature: 170-190°C; and the condenser B under the following conditions: pressure: 125-145 bar, temperature: 130-250°C, particularly preferably 170-190°C, N / C: 2.5-3.5, particularly preferably 2.7-3.3, H / C: 1.0 or less, residence time: 10-30 minutes. Here, N / C is the molar ratio of ammonia (including ammonia converted to ammonium carbamate and urea) to carbon dioxide (including carbon dioxide converted to ammonium carbamate and urea) (hereinafter referred to as "N / C"), and H / C is the molar ratio of water (excluding water produced in the urea synthesis reaction) to carbon dioxide (including carbon dioxide converted to ammonium carbamate and urea) (hereinafter referred to as "H / C").

[0039] These operating conditions are preferable because, as shown in Figure 1, the reactions shown in Equations 1 and 2 below occur in urea synthesis column A, and also in condenser B, the reaction proceeds while condensing ammonia gas and / or carbon dioxide gas. Equation 1 is the reaction in which ammonia (NH3) and carbon dioxide (CO2) react to produce ammonium carbamate (NH2CO2NH4). Equation 2 is the equilibrium reaction in which urea (CO(NH2)2) is produced by the dehydration reaction of ammonium carbamate (NH2CO2NH4).

[0040] [ka]

[0041] In other words, in Figure 1, the raw material liquid ammonia is pressurized to the desired pressure by an ammonia pump (not shown) and supplied to urea synthesis tower A. A portion of it is heated by a heat exchanger and supplied to ejector G, while urea synthesis solution is also supplied to ejector G from condenser B. From ejector G, urea synthesis solution containing ammonia is pressurized and supplied to urea synthesis tower A. The raw material gaseous carbon dioxide is pressurized to the desired pressure by carbon dioxide compressor H, and most of it is supplied to stripper C. A portion of the remaining carbon dioxide is supplied to urea synthesis tower A for the purpose of temperature control and supplying corrosion-preventive oxygen. Here, "most of" the carbon dioxide supplied to stripper C means, specifically, the amount of raw material carbon dioxide supplied to stripper C [X C (t / d) (for example, the amount of raw carbon dioxide supplied in line 2b in Figure 2) and the amount of raw carbon dioxide supplied to urea synthesis tower A [X A The ratio of (t / d) (for example, the amount of raw carbon dioxide supplied by line 2a in Figure 2) [X C / X AThis means that the ratio is approximately 90 / 10 to 70 / 30. Furthermore, as shown in Figures 1 and 2, if the majority of the raw carbon dioxide, which has been pre-mixed with corrosion-preventive air (however, other corrosion-preventive oxygen-containing gases are also acceptable), is supplied to stripper C, then the majority of the corrosion-preventive oxygen is also supplied to stripper C, effectively preventing corrosion of stripper C, which is relatively susceptible to corrosion. Normally, air is supplied as corrosion-preventive oxygen to the first-stage intake or intermediate stage of carbon dioxide compressor H. Urea is synthesized in urea synthesis tower A and condenser B, and the urea-containing effluent exiting urea synthesis tower A is supplied to stripper C. In this effluent, synthesized urea, water, ammonium carbamate, and unreacted ammonia exist as a liquid phase, while some unreacted ammonia and carbon dioxide exist as a gas phase together with inert gas. Here, inert gas refers to the combination of corrosion-preventive air introduced to prevent corrosion in a urea synthesis apparatus, such as a urea synthesis tower A, stripper C, condenser B, scrubber, and the piping connecting them, and impurities such as hydrogen and nitrogen contained in the raw material carbon dioxide. The synthesis liquid from urea synthesis tower A is supplied to stripper C, where unreacted ammonia and unreacted carbon dioxide are treated. The raw material carbon dioxide is used as a stripping agent.

[0042] Therefore, the operating conditions for the urea synthesis column A, stripper C, and condenser B in the manufacturing apparatus shown in Figure 1 are determined for the following reasons, including Le Chatelier's principle.

[0043] The internal pressure of urea synthesis column A (urea synthesis pressure) is 125 to 145 bar, preferably 130 to 140 bar. Setting this synthesis pressure to 125 bar or higher allows for a more generous operating pressure relative to the synthesis equilibrium pressure at a temperature favorable for urea synthesis (e.g., 180°C or higher) by reducing the partial pressure of the inert gas, and also prevents a decrease in reaction yield due to gasification. Furthermore, setting the synthesis pressure to 145 bar or lower significantly reduces the energy required to pressurize the raw material ammonia, raw material carbon dioxide, and unreacted ammonium carbamate solution, thereby reducing running costs. Here, ammonium carbamate solution refers to the liquid obtained by recovering unreacted ammonia and carbon dioxide as an aqueous ammonium carbamate solution in the recovery process downstream from the synthesis process.

[0044] Next, regarding the synthesis temperature inside urea synthesis column A, the higher the temperature, the more the reaction in Equation 1 proceeds to the left side and the reaction in Equation 2 proceeds to the right side. Therefore, there is a range that is suitable for the reaction yield, and the temperature is set to 175°C to 190°C. Setting this synthesis temperature above 175°C prevents the reaction rate of urea production from slowing down. Also, setting the synthesis temperature below 190°C prevents an increase in the corrosion rate and the risk of so-called activated corrosion. This synthesis temperature can be controlled, for example, by the preheating temperature of the ammonia that drives ejector G and / or the amount of carbon dioxide supplied to urea synthesis column A.

[0045] The N / C ratio inside urea synthesis column A is stoichiometrically 2 according to Equation 1, but it is preferable that an excess of unreacted ammonia is present, and in this invention, it is set to 3.5 to 4.0. More preferably, it is 3.6 to 3.8.

[0046] From the viewpoint of reaction yield, a lower H / C ratio is preferable in the urea synthesis column A, as shown in equations 1 and 2. Therefore, it is preferable to minimize the amount of water supplied to this recovery device, which in this invention is set to 0.70 or less. More preferably, it is 0.6 or less. It would be even better if this H / C ratio could be reduced as much as possible, but in reality, some water is present because the H / C ratio is often determined by the amount of water required for absorption by the recovery device (not shown) that recovers unreacted products (ammonia and carbon dioxide) from the urea synthesis device. There is no need to set a lower limit for H / C.

[0047] The residence time of the urea synthesis solution inside urea synthesis column A is preferably 10 to 40 minutes. Increasing this residence time beyond 10 minutes accelerates the urea synthesis reaction. On the other hand, if the residence time exceeds 40 minutes, the reaction yield is already close to the equilibrium yield, and further increases in the reaction yield are unlikely.

[0048] Under the above operating conditions, the reaction yield inside urea synthesis column A will be approximately 60% to 75%. Here, the carbon dioxide-based reaction yield is the ratio of the number of moles of carbon dioxide supplied to the apparatus or region under consideration to the number of moles of supplied carbon dioxide converted to urea, and is usually expressed as a percentage.

[0049] The operating conditions for stripper C are required to promote the reverse reaction of Equation 1. Basically, higher temperatures and lower pressures are preferable, but considering the balance of the entire urea synthesis apparatus, the pressure is set to 125-145 bar, particularly preferably 130-140 bar, and the temperature to 170-190°C. In stripper C, stripping is performed at a pressure approximately equal to the urea synthesis pressure. Here, "approximately equal pressure" means that the difference from the urea synthesis pressure is within ±5 bar.

[0050] The operating conditions for condenser B, which is equipped with synthesis capabilities, can be considered similarly to those for urea synthesis column A. Under almost the same conditions as urea synthesis column A, namely pressure: 125-145 bar, temperature: 130-250°C, particularly preferably 170-190°C, N / C: 2.5-3.5, particularly preferably 2.7-3.3, H / C: 0.7 or less, and residence time: 10-30 minutes, a reaction yield of 20-60% can be achieved.

[0051] Under these synthesis conditions, as shown in Figure 1, corrosion-preventive oxygen is supplied to the apparatus as air simultaneously with carbon dioxide. The required amount of corrosion-preventive oxygen differs for each component: urea synthesis tower A, stripper C, and condenser B. Stripper C is in the most corrosive environment, followed by condenser B and then urea synthesis tower A. However, in the synthesis apparatus shown in Figure 1, the corrosion-preventive oxygen is distributed throughout the entire apparatus, so it is determined by the minimum oxygen supply required at the stripper, preferably 100 to 2,000 ppm, and more preferably 100 to 1,000 ppm. Furthermore, by selecting the material of the stainless steel used for each part of the synthesis apparatus, it is possible to operate with an even more preferable amount of corrosion-preventive oxygen, 100 to 500 ppm. However, if only general-purpose austenitic-ferritic duplex stainless steel is used, it is preferable to set the above lower limits to 150 ppm or higher.

[0052] In particular, when using austenitic-ferrite duplex stainless steel, such as S32707 or S32808, with a Cr content of 26-35 mass%, Ni content of 3-10 mass%, Mo content of 1.0-4.0 mass%, and N content of 0.2-0.6 mass%, the oxygen supply concentration relative to carbon dioxide can be reduced to 100-500 ppm, especially in strippers where corrosion is more likely to progress and in the piping connecting the strippers to the condenser.

[0053] As shown in the process flow of Figure 1, the preferred method for supplying corrosion-preventive oxygen is to mix corrosion-preventive air with the raw material carbon dioxide and supply it, from the viewpoint of not requiring the installation of a new pressurizing device. However, the present invention is not limited to this supply method. For example, each device may be individually equipped with a corrosion-preventive air inlet and a device to pressurize the air to a predetermined pressure, and corrosion-preventive air may be supplied to each device. In this case as well, since the corrosion-preventive oxygen is distributed throughout the entire synthesis apparatus, the amount of corrosion-preventive air supplied, i.e., the corrosion-preventive oxygen concentration, remains unchanged.

[0054] Thus, by reducing the amount of corrosion-preventive oxygen, the gas composition is removed from the explosion range, and a hydrogen combustion removal device can be omitted. However, it may be installed for safety reasons. If installed, it is preferable that the hydrogen removal device be located in the middle or outlet of the carbon dioxide compressor downstream of the corrosion-preventive air, or upstream of the carbon dioxide introduction stage of the stripper, as it is before the explosion range under high-temperature conditions like those of a stripper, and a portion of the corrosion-preventive oxygen introduced can be used to remove hydrogen by catalytic combustion within the hydrogen removal device.

[0055] Although the method of the present invention has been described above, the present invention can also be used as a method for improving existing urea synthesis apparatus to reduce the urea synthesis pressure in a urea synthesis column.

[0056] In other words, the existing urea synthesis apparatus is a urea synthesis apparatus in which, in the urea synthesis tower, ammonia and carbon dioxide are reacted at the urea synthesis temperature and pressure, the resulting urea synthesis solution containing at least urea, unreacted ammonia, unreacted carbon dioxide, and water is brought into contact with at least a portion of the raw material carbon dioxide under heating in a stripper to separate the unreacted ammonia and unreacted carbon dioxide as a mixed gas of ammonia, carbon dioxide, and water, the unseparated urea synthesis solution containing the unreacted ammonia and unreacted carbon dioxide is further processed to obtain urea, the mixed gas separated in the stripper is introduced to the bottom of a condenser and brought into contact with an absorption medium under cooling to condense the mixed gas, and the condensate thus obtained is circulated back to the urea synthesis tower. The material of the piping connecting the stripper and condenser of the existing urea synthesis apparatus is neither of the following materials (a) and (b): This method is also useful as an improvement to an existing urea synthesis apparatus, as it allows for a reduction in the urea synthesis pressure of the urea synthesis column by replacing the piping connecting the stripper and condenser of the existing urea synthesis apparatus with piping containing the material described in (a) or (b) below. (a) Cr content: 21% by mass or more and less than 26% by mass, Ni content: 4.5-7.5% by mass, Mo content: 2.5-3.5% by mass, N content: 0.08-0.30% by mass, C content: 0.03% by mass or more Bottom, general-purpose austenitic-ferritic duplex stainless steel with Si content: 1.0 mass% or less, Mn content: 2.0 mass% or less, P content: 0.04 mass% or less, S content: 0.03 mass% or less, (b) Austenitic-ferritic duplex stainless steel with Cr content: 26-35 mass%, Ni content: 3-10 mass%, Mo content: 1.0-4.0 mass%, and N content: 0.2-0.6 mass%.

[0057] Furthermore, the present invention can also be used as an improved method for reducing the urea synthesis pressure in other types of apparatus (apparatus in which the urea synthesis region and the condensation region are located in the same container or in separate containers, and the condensation region is located in a horizontal container).

[0058] In other words, the existing urea synthesis apparatus is a urea synthesis apparatus in which, in the urea synthesis region, ammonia and carbon dioxide are reacted at the urea synthesis temperature and pressure, the resulting urea synthesis solution containing at least urea, unreacted ammonia, unreacted carbon dioxide, and water is brought into contact with at least a portion of the raw material carbon dioxide under heating in a stripper to separate the unreacted ammonia and unreacted carbon dioxide as a mixed gas of ammonia, carbon dioxide, and water, the unseparated urea synthesis solution containing the unreacted ammonia and unreacted carbon dioxide is further processed to obtain urea, the mixed gas separated in the stripper is introduced to the bottom of the condensation region and brought into contact with an absorption medium under cooling to condense the mixed gas, and the condensate thus obtained is circulated back into the urea synthesis region. The urea synthesis region and the condensation region are located in the same container or in separate containers, and the condensation region is located in a horizontal container. The material of the piping connecting the stripper and the condensing region of the existing urea synthesis apparatus is neither of the materials described in (a) and (b), This method is also useful as an improvement to an existing urea synthesis apparatus, as it makes it possible to reduce the urea synthesis pressure in the urea synthesis region by replacing the piping connecting the stripper and the condensation region of the existing urea synthesis apparatus with piping containing the material described in (a) or (b). [Examples]

[0059] The present invention will be described in more detail below with reference to examples. However, the present invention is not limited by the examples.

[0060] <Example 1> Urea synthesis was carried out using a urea synthesis process with a urea synthesis plant with a daily production capacity of 2,200 tons, as shown in Figure 2.

[0061] General-purpose austenitic-ferritic duplex stainless steel was used for the piping connecting stripper C to condenser B, condenser B to ejector G, and ejector G to urea synthesis tower A, while S31603 series general-purpose stainless steel was used for the other piping. Corrosion-preventing air was pre-mixed with the raw material carbon dioxide so that the oxygen supply concentration relative to the carbon dioxide in the carbon dioxide compression process was approximately 1000 ppm. The pressure in the urea synthesis system was set to 136 bar.

[0062] 1237 t / d of raw ammonia is supplied from line 1, and 1600 t / d of raw carbon dioxide is supplied to the compressor from line 14. The raw ammonia is pressurized to 180 bar by a pump, heated to 147°C with steam condensate in heat exchanger D from line 1, and supplied to the bottom of urea synthesis tower A through ejector G. In addition, 1528 t / d of raw carbon dioxide and approximately 14 t / d of corrosion-preventing air are pressurized to 141 bar by a compressor, and 1233 t / d of raw carbon dioxide and approximately 11 t / d of corrosion-preventing air are supplied to the bottom of stripper C via lines 2 and 2b. 295 t / d of carbon dioxide and approximately 3 t / d of corrosion-preventing air are supplied to the bottom of urea synthesis tower A via line 2a (ratio [X C / X A ]=approximately 81 / 19). In addition, 72 t / d of raw carbon dioxide was branched off from the intermediate stage of the compressor and supplied via line 13 to the low-pressure cracking tower in the cracking process downstream of the stripper. Meanwhile, 1849 t / d of the recovered liquid from line 7, used as an absorption medium, had the following composition, was pressurized to 141 bar at a temperature of 106°C, and supplied to scrubber F. Urea 0.2%, Ammonia 35.0%, Carbon Dioxide 40.6%, Water 24.2%

[0063] Scrubber F was operated at 136 bar and 160°C. The liquid from the outlet of scrubber F was supplied to the bottom of condenser B via receiver 16 and downpipe 11. Meanwhile, inert gas was released from the top of scrubber F via line 15.

[0064] The mixed gas, consisting of ammonia, carbon dioxide, water, and inert gas separated in stripper C, was sent to the bottom of condenser B via line 5. The shell side of condenser B was filled with the recovered liquid and ammonium carbamate produced by condensation, and the mixed gas was absorbed by this liquid. The heat of absorption was removed by coolers indicated by lines 13 and 14. After a residence time of 20 minutes, the condensate that had absorbed the mixed gas flowed down downpipe 3 by gravity, was pressurized by ejector G, and supplied to the bottom of urea synthesis tower A.

[0065] The liquid raw material ammonia and condensate supplied to the bottom of urea synthesis column A continued the urea synthesis reaction as they rose up urea synthesis column A. The residence time during this process was 20 minutes.

[0066] The synthesis liquid supplied from the top of urea synthesis column A through line 4 to stripper C undergoes thermal decomposition of ammonium carbamate and separation as a mixed gas of ammonia and carbon dioxide. Stripper C was operated at a top temperature of 180°C and a top pressure of 136 bar.

[0067] As described above, the gas and liquid were circulated between condenser B, urea synthesis tower A, and stripper C during operation. The differential pressure between the discharge and suction sides of ejector G, which uses liquid raw material ammonia as the driving fluid, was approximately 3 bar.

[0068] Table 1 shows the power of the pumps that pressurize the raw ammonia and recovered liquid, and the power of the compressor that pressurizes the raw carbon dioxide. Tables 2 to 5 show the operating conditions of urea synthesis tower A, condenser B, stripper C, and ejector G. Table 6 shows the composition of the tower bottom (line 12).

[0069] <Reference example 1> In the urea synthesis process using the urea synthesis plant with a daily production capacity of 2,200 tons shown in Figure 2, urea synthesis was carried out using S31603 series general-purpose stainless steel for all piping connecting each device.

[0070] To prevent corrosion of the S31603 series general-purpose stainless steel used in the piping, the raw material carbon dioxide was pre-mixed with corrosion-preventive air during the carbon dioxide compression process so that the oxygen concentration relative to the carbon dioxide was 5000 ppm.

[0071] To suppress the flow rate of process gases (ammonia, carbon dioxide, and water) accompanying the inert gas flowing from the top of urea synthesis column A and condenser B to scrubber F, the excess pressure, which is the difference between the operating pressure and the saturated vapor pressure of the liquid, was increased, and the operating pressure of the synthesis system was set to 152 bar.

[0072] Table 1 shows the power of the pumps that pressurize the raw ammonia and recovered liquid, and the power of the compressor that pressurizes the raw carbon dioxide. Tables 2 to 5 show the operating conditions of urea synthesis tower A, condenser B, stripper C, and ejector G. Table 6 shows the composition of the tower bottom (line 12).

[0073] [Table 1]

[0074] [Table 2]

[0075] [Table 3]

[0076] [Table 4]

[0077] [Table 5]

[0078] [Table 6]

[0079] <Rating> In Example 1, due to the reduction in the operating pressure of the synthesis system, the power of the ammonia and recovered liquid booster pumps was reduced by 135 kW (approximately 10%) and the power of the raw material carbon dioxide compressor was reduced by 169 kW (approximately 2%) compared to Reference Example 1.

[0080] Furthermore, because the low pressure improves the separation efficiency in the stripper, the concentrations of ammonia and carbon dioxide obtained at the bottom of the stripper column are low even when using heated steam at a low pressure of 18.1 bar. Due to the lower concentration, less energy is required for the separation of ammonia and carbon dioxide in the separation process downstream of the stripper (not shown in Figure 2). In addition, the amount of recovered liquid obtained by recovering the separated ammonia and water as an aqueous solution is reduced, less water is introduced into the synthesis system, the H / C ratio in the synthesis system decreases, and a lower H / C ratio contributes to an improved rate of carbon dioxide reduction. [Industrial applicability]

[0081] The present invention is a method for synthesizing urea from ammonia and carbon dioxide while forming a passivation film on the metal surface inside the apparatus using oxygen, and is particularly useful as a urea synthesis method that can reduce the energy and running costs required for urea synthesis. [Explanation of symbols]

[0082] A Urea synthesis tower B Condenser C Stripper D Heat exchanger F Scrubber G Ejector H Carbon dioxide compressor I. Recovery System Lines 1, 2, 2a, 2b, 4, 5, 7, 12, 13, 14, 15 3.11 Downpipe 16 Receiver

Claims

1. A urea synthesis method comprising the steps of: reacting ammonia and carbon dioxide at a urea synthesis temperature and urea synthesis pressure in a urea synthesis tower; contacting a urea synthesis liquid containing at least produced urea, unreacted ammonia, unreacted carbon dioxide, and water with at least a portion of raw carbon dioxide in a stripper under heating at a pressure substantially equal to the urea synthesis pressure, separating the unreacted ammonia and the unreacted carbon dioxide as a mixed gas of ammonia, carbon dioxide, and water, and further treating the urea synthesis liquid containing the unseparated unreacted ammonia and the unreacted carbon dioxide to obtain urea; introducing the mixed gas separated in the stripper into a bottom of a condenser and contacting it with an absorption medium under cooling, thereby condensing the mixed gas, and circulating the condensate thus obtained to the urea synthesis tower, By setting the oxygen supply concentration relative to the raw material carbon dioxide at 100 to 2,000 ppm, the urea synthesis pressure in the urea synthesis column is set at 125 to 145 bar; The urea synthesis temperature in the urea synthesis tower is 175°C to 190°C, The N / C (molar ratio of ammonia to carbon dioxide) in the urea synthesis column is 3.5 to 4.0, the H / C (molar ratio of water to carbon dioxide) in the urea synthesis column is 0.70 or less, The method for synthesizing urea, characterized in that the operating condition of the stripper is a pressure of 130 to 140 bar.

2. 2. The method for synthesizing urea according to claim 1, wherein the N / C (molar ratio of ammonia to carbon dioxide) in the condenser is 2.7 to 3.

3.

3. The method for synthesizing urea according to claim 1 or 2, wherein the residence time in the urea synthesis column is 10 to 40 minutes.

4. 4. The method for synthesizing urea according to any one of claims 1 to 3, wherein a majority of the raw material carbon dioxide having an oxygen concentration of 100 to 2,000 ppm is supplied to a stripper, and the remainder is supplied to a urea synthesis column, and a ratio [XC / XA] of an amount [XC(t / d)] of the raw material carbon dioxide supplied to the stripper to an amount [XA(t / d)] of the raw material carbon dioxide supplied to the urea synthesis column is 90 / 10 to 70 / 30.

5. The material of at least one of the devices selected from the group consisting of a urea synthesis column, a stripper, a condenser, and a pipe connecting at least two of the urea synthesis column, the stripper, and the condenser is (a) Cr content: 21 to 26 mass%, Ni content: 4.5 to 7.5 mass%, Mo content: 2.5 to 3.5 mass%, N content: 0.08 to 0.30 mass%, C content: 0.03 mass% or less, Si A general-purpose austenitic-ferritic duplex stainless steel having a content: 1.0% by mass or less, a Mn content: 2.0% by mass or less, a P content: 0.04% by mass or less, and an S content: 0.03% by mass or less, or (b) The method for synthesizing urea according to any one of claims 1 to 4, wherein the stainless steel is an austenite-ferrite duplex stainless steel having a Cr content of 26 to 35 mass%, a Ni content of 3 to 10 mass%, a Mo content of 1.0 to 4.0 mass%, and a N content of 0.2 to 0.6 mass%.

6. The stripper material is (a) Cr content: 21 to 26 mass%, Ni content: 4.5 to 7.5 mass%, Mo content: 2.5 to 3.5 mass%, N content: 0.08 to 0.30 mass%, C content: 0.03 mass% or less, Si A general-purpose austenitic-ferritic duplex stainless steel having a content: 1.0% by mass or less, a Mn content: 2.0% by mass or less, a P content: 0.04% by mass or less, and an S content: 0.03% by mass or less, or (b) The method for synthesizing urea according to any one of claims 1 to 5, wherein the stainless steel is an austenite-ferrite duplex stainless steel having a Cr content of 26 to 35 mass%, a Ni content of 3 to 10 mass%, a Mo content of 1.0 to 4.0 mass%, and a N content of 0.2 to 0.6 mass%.

7. The material of the piping connecting the stripper and the condenser is (a) Cr content: 21 to 26 mass%, Ni content: 4.5 to 7.5 mass%, Mo content: 2.5 to 3.5 mass%, N content: 0.08 to 0.30 mass%, C content: 0.03 mass% or less, Si A general-purpose austenitic-ferritic duplex stainless steel having a content: 1.0% by mass or less, a Mn content: 2.0% by mass or less, a P content: 0.04% by mass or less, and an S content: 0.03% by mass or less, or (b) The method for synthesizing urea according to any one of claims 1 to 6, wherein the stainless steel is an austenite-ferrite duplex stainless steel having a Cr content of 26 to 35 mass%, a Ni content of 3 to 10 mass%, a Mo content of 1.0 to 4.0 mass%, and a N content of 0.2 to 0.6 mass%.

8. A method for improving an existing urea synthesis unit to reduce a urea synthesis pressure in a urea synthesis column, comprising the steps of: the existing urea synthesis apparatus comprises: in the urea synthesis tower, ammonia is reacted with carbon dioxide containing anticorrosive oxygen supplied from a carbon dioxide compressor at a urea synthesis temperature and urea synthesis pressure; a urea synthesis liquid containing at least the produced urea, unreacted ammonia, unreacted carbon dioxide, and water is contacted with at least a portion of the raw carbon dioxide in a stripper under heating at a pressure substantially equal to the urea synthesis pressure, the unreacted ammonia and the unreacted carbon dioxide are separated as a mixed gas of ammonia, carbon dioxide, and water, and the urea synthesis liquid containing the unseparated unreacted ammonia and the unreacted carbon dioxide is further treated to obtain urea; meanwhile, the mixed gas separated in the stripper is introduced into a bottom of a condenser and contacted with an absorption medium under cooling, thereby condensing the mixed gas, and the condensate thus obtained is circulated to the urea synthesis tower; The material of the piping connecting the stripper and the condenser of the existing urea synthesis apparatus is neither of the following materials (a) and (b): A method for improving an existing urea synthesis apparatus, comprising: replacing a pipe connecting a stripper and a condenser of the existing urea synthesis apparatus with a pipe containing the material (a) or (b) below; lowering the oxygen supply concentration relative to the raw material carbon dioxide in the carbon dioxide gas compressor, thereby reducing the urea synthesis pressure in the urea synthesis tower to 125 to 145 bar; and enabling the urea synthesis temperature in the urea synthesis tower to be 175 to 190°C and the operating conditions of the stripper to be a pressure: 130 to 140 bar. (a) Cr content: 21 to 26 mass%, Ni content: 4.5 to 7.5 mass%, Mo content: 2.5 to 3.5 mass%, N content: 0.08 to 0.30 mass%, C content: 0.03 mass% or less, S General-purpose austenitic-ferritic duplex stainless steel having i content: 1.0 mass% or less, Mn content: 2.0 mass% or less, P content: 0.04 mass% or less, S content: 0.03 mass% or less, (b) Austenite-ferritic duplex stainless steel having a Cr content of 26 to 35 mass%, a Ni content of 3 to 10 mass%, a Mo content of 1.0 to 4.0 mass%, and a N content of 0.2 to 0.6 mass%.

9. A method for improving an existing urea synthesis unit to reduce a urea synthesis pressure in a urea synthesis zone, comprising the steps of: the existing urea synthesis apparatus reacts ammonia with carbon dioxide containing anticorrosive oxygen supplied from a carbon dioxide compressor at a urea synthesis temperature and urea synthesis pressure in the urea synthesis region, contacts a urea synthesis liquid containing at least the produced urea, unreacted ammonia, unreacted carbon dioxide, and water with at least a portion of the raw carbon dioxide in a stripper under heating at a pressure substantially equal to the urea synthesis pressure, separates the unreacted ammonia and the unreacted carbon dioxide as a mixed gas of ammonia, carbon dioxide, and water, and further processes the urea synthesis liquid containing the unseparated unreacted ammonia and the unreacted carbon dioxide to obtain urea; on the other hand, introduces the mixed gas separated in the stripper into a bottom of a condensation region and brings it into contact with an absorption medium under cooling, thereby condensing the mixed gas, and circulates the condensate thus obtained to the urea synthesis region; The urea synthesis zone and the condensation zone are present in the same vessel or in separate vessels; The condensation zone is in a horizontal vessel; The material of the piping connecting the stripper and the condensation region of the existing urea synthesis unit is neither of the following materials (a) and (b): A method for improving an existing urea synthesis apparatus, comprising replacing a pipe connecting a stripper and a condensation region of the existing urea synthesis apparatus with a pipe containing a material of (a) or (b) below, lowering the oxygen supply concentration relative to the raw material carbon dioxide in the carbon dioxide gas compressor, thereby reducing the urea synthesis pressure in the urea synthesis region to 125 to 145 bar, and enabling the urea synthesis temperature in the urea synthesis region to be 175 to 190°C and the operating conditions of the stripper to be a pressure: 130 to 140 bar. (a) Cr content: 21 to 26 mass%, Ni content: 4.5 to 7.5 mass%, Mo content: 2.5 to 3.5 mass%, N content: 0.08 to 0.30 mass%, C content: 0.03 mass% or less, S General-purpose austenitic-ferritic duplex stainless steel having i content: 1.0 mass% or less, Mn content: 2.0 mass% or less, P content: 0.04 mass% or less, S content: 0.03 mass% or less, (b) Austenite-ferritic duplex stainless steel having a Cr content of 26 to 35 mass%, a Ni content of 3 to 10 mass%, a Mo content of 1.0 to 4.0 mass%, and a N content of 0.2 to 0.6 mass%.

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