Process for the production of urea

Abstract

Provided is a method for producing urea that suppresses corrosion of a urea plant and improves reaction yield. The method for producing urea is a method for producing urea from a raw material for production containing NH3 and CO2 in a urea production plant, wherein the urea production plant has a treatment device including a reactor, a stripping column, and a condenser, and a plurality of pipelines, the inner wall surfaces of the plurality of treatment devices and the plurality of pipelines are composed of stainless steel, at least a part of the plurality of pipelines is composed of austenitic stainless steel, in the method for producing urea, a passivation film is formed on the inner wall surfaces of the plurality of treatment devices and the plurality of pipelines by supplying CO2 as the raw material for production after adding oxygen thereto, the wall thickness of the pipeline composed of austenitic stainless steel is continuously measured, and the amount of oxygen supplied is adjusted based on the measured value of the wall thickness, thereby controlling the corrosion rate and the reaction yield of urea.

Description

Technical Field

[0001] This invention relates to a method for manufacturing urea. Background Technology

[0002] In urea manufacturing equipment, highly corrosive ammonium carbamate is generated as an intermediate substance during the synthesis of urea from ammonia and carbon dioxide. Therefore, corrosion resistance is required for various processing devices and pipelines in the equipment.

[0003] Japanese Patent No. 3987607 discloses an invention of a urea synthesis method and a urea synthesis apparatus, which describes the introduction of anti-corrosion air into the condenser, synthesis tower, and stripping tower (see paragraphs 0028, 0046, 0055, and 0070).

[0004] WO2014 / 192823 discloses an invention of a urea synthesis method. It describes that in a urea synthesis apparatus used to implement the urea synthesis method, at least a portion of the urea synthesis tower A, stripping tower B, and condenser C, as well as the parts of the piping connecting them that come into contact with corrosive fluids, can be made of austenitic-ferrite duplex stainless steel of a specific composition; and that S31603 series general-purpose stainless steel can also be used in piping, valves, etc., depending on the corrosive environment. WO2014 / 192823 also discloses that the amount of oxygen supplied for corrosion prevention can be reduced, the amount of inert gas reduced, and the reaction yield increased (effects of the invention). Summary of the Invention

[0005] The present invention addresses the problem of providing a method for manufacturing urea, which improves the reaction yield of urea by suppressing corrosion of the processing devices and pipelines of the urea equipment during the manufacturing of urea.

[0006] This invention provides a method for manufacturing urea, which involves producing urea from raw materials containing ammonia and carbon dioxide in a urea manufacturing facility.

[0007] The aforementioned urea manufacturing equipment includes: multiple processing units comprising a reactor, a stripping tower, and a condenser, and multiple pipelines connecting the aforementioned processing units.

[0008] The inner walls of the aforementioned processing devices and pipelines are made of stainless steel, and at least a portion of the aforementioned pipelines are made of austenitic stainless steel.

[0009] In the above-described urea manufacturing method, oxygen is added to carbon dioxide, which is used as a raw material, and then supplied to form a passivation film on the inner wall surface of the multiple processing devices and the multiple pipelines. At the same time, the wall thickness of the pipelines made of austenitic stainless steel is continuously measured, and the oxygen supply is adjusted according to the measured value of the wall thickness, thereby controlling the corrosion rate and the reaction yield of urea (control method (A)).

[0010] The present invention also provides a method for manufacturing urea, which is a method for manufacturing urea from raw materials containing ammonia and carbon dioxide in a urea manufacturing facility.

[0011] The aforementioned urea manufacturing equipment includes: multiple processing units comprising a reactor, a stripping tower, and a condenser, and multiple pipelines connecting the aforementioned processing units.

[0012] The inner walls of the aforementioned processing devices and pipelines are made of stainless steel, and at least a portion of the aforementioned pipelines are made of austenitic stainless steel.

[0013] In the above-mentioned urea manufacturing method, oxygen is added to carbon dioxide, which is used as a raw material, and then supplied to form a passivation film on the inner wall surface of the multiple processing devices and the multiple pipelines. At the same time, the concentration of iron, chromium or nickel dissolved in urea or ammonia and the operating temperature are measured. The oxygen supply is adjusted according to the measured values ​​of the concentration and the operating temperature, thereby controlling the corrosion rate and the reaction yield of urea (control method (B)).

[0014] The present invention also provides a method for manufacturing urea, which is a method for manufacturing urea from raw materials containing ammonia and carbon dioxide in a urea manufacturing facility.

[0015] The aforementioned urea manufacturing equipment includes:

[0016] A reactor used to produce urea synthesis solution from carbon dioxide and ammonia;

[0017] A stripping tower is used to decompose ammonium carbamate by heating the urea synthesis liquid generated in the reactor above, and to separate a mixed gas containing ammonia and carbon dioxide from the urea synthesis liquid above.

[0018] The system includes multiple processing units, including a condenser that absorbs at least a portion of the mixed gas obtained from the stripping tower into an absorption medium for condensation, and utilizes the heat generated during condensation to produce low-pressure steam; and

[0019] Multiple pipelines connecting the above-mentioned processing devices,

[0020] The inner walls of the aforementioned processing devices and pipelines are made of stainless steel, and at least a portion of the aforementioned pipelines are made of austenitic stainless steel.

[0021] Implement any one, any two, or any three of the control methods (A) to (C) below.

[0022] Control method (A): In the above-mentioned urea manufacturing method, oxygen is added to carbon dioxide, which is used as a manufacturing raw material, and then supplied to form a passivation film on the inner wall surface of the multiple processing devices and the multiple pipelines. At the same time, the wall thickness of the pipelines made of austenitic stainless steel is continuously measured, and the oxygen supply is adjusted according to the measured value of the wall thickness, thereby controlling the corrosion rate and the reaction yield of urea.

[0023] Control method (B): Measure the concentration of iron, chromium or nickel dissolved in urea or ammonia and the operating temperature, and adjust the oxygen supply based on the measured values ​​of the concentration and operating temperature to control the corrosion rate and the reaction yield of urea.

[0024] Control method (C): By measuring the operating pressure and operating temperature of the above-mentioned multiple processing devices, the flow rate of carbon dioxide introduced as the above-mentioned raw material, the oxygen content in the above-mentioned raw material carbon dioxide, and the flow rate of ammonia introduced as the above-mentioned raw material, the corrosion rate of each of the above-mentioned multiple processing devices and the corrosion rate of the multiple pipelines connecting the above-mentioned multiple processing devices are calculated, and the oxygen supply is adjusted to control the corrosion rate and the reaction yield of urea.

[0025] According to the urea manufacturing method of the present invention, corrosion of the processing equipment and pipelines of the urea manufacturing process can be suppressed, thereby maintaining urea production. Attached Figure Description

[0026] [ Figure 1 This image shows a schematic diagram of the urea manufacturing process in a urea manufacturing facility.

[0027] [ Figure 2 [Figure illustrating one embodiment of a urea manufacturing method using urea manufacturing equipment.]

[0028] [ Figure 3 The graph shows the difference in corrosion rates between the conduit with and without a passivation film in Example 1. Detailed Implementation

[0029] use Figure 1 The present invention describes the urea manufacturing method. Figure 1 The urea manufacturing equipment shown is one embodiment for implementing the urea manufacturing method of the present invention, and is not limited thereto.

[0030] in addition, Figure 1 The urea manufacturing process in the urea equipment shown is itself known, for example, substantially similar to that disclosed in Japanese Patent No. 3987607. Figure 3 The manufacturing process shown, WO2014 / 192823 Figure 2 The manufacturing process shown is the same. Figure 1 The reactor 1, stripping tower 2, condenser 3, heat exchanger 5, and ejector 6 shown are respectively similar to those disclosed in Japanese Patent No. 3987607. Figure 3 The urea synthesis tower A, stripping tower C, condenser B (including washing tower F), heat exchanger D, and ejector G shown are the same.

[0031] A feature of the urea manufacturing method of the present invention is that: for example, in Figure 1 In the urea manufacturing equipment shown, the corrosion rate and urea reaction yield are controlled by adjusting the oxygen supply according to specific measured values ​​during urea production. There are no particular limitations on the manufacturing method, including specific manufacturing procedures and reaction conditions.

[0032] In the urea manufacturing method of the present invention, urea can also be manufactured using, for example, the following manufacturing method: Utilizing the method disclosed in Japanese Patent No. 3987607 Figure 3 The urea manufacturing equipment shown is a manufacturing method that uses the same manufacturing procedures or conditions as those described in paragraphs 0052 to 0062 or in Example 3; or a manufacturing method that uses the same manufacturing procedures or conditions as those described in paragraphs 0040 to 0048 or in paragraph 0060 of WO2014 / 192823.

[0033] exist Figure 1 In the manufacturing process example shown, ammonia is supplied from ammonia supply line 10 to the lower part of reactor 1, while carbon dioxide is simultaneously supplied from carbon dioxide supply lines 11 and 11a to the lower part of reactor 1, causing a reaction to occur inside reactor 1 to obtain a gas-liquid mixture containing urea. Reactor 1 is an apparatus used to generate urea synthesis liquid from carbon dioxide and ammonia as raw materials.

[0034] Reactor 1 is, for example, made of carbon steel, with an inner lining of duplex stainless steel formed on the portion corresponding to the inner wall surface. Therefore, the wall thickness of reactor 1 cannot be measured from the outside using an ultrasonic wall thickness gauge.

[0035] In the gas-liquid mixture obtained in reactor 1, urea, ammonium carbamate (as a reaction intermediate), water, and unreacted ammonia exist in the liquid phase, while a portion of the unreacted ammonia, unreacted carbon dioxide, and inert gas exist in the gas phase. The inert gas consists of air (oxygen) supplied for corrosion prevention and impurities such as hydrogen contained in the raw material carbon dioxide.

[0036] The reaction conditions in reactor 1 are as described above, and can be compared with those disclosed in Japanese Patent No. 3987607. Figure 3 The same conditions apply to the urea manufacturing equipment shown, for example, the pressure is preferably 130 to 250 bar (13,000 to 25,000 kPa), the N / C (molar ratio of ammonia to carbon dioxide) is preferably 3.5 to 5.0, the H / C (molar ratio of water to carbon dioxide) is preferably 1.0 or less, the residence time is preferably 10 to 40 minutes, and the temperature is preferably 180 to 200°C.

[0037] When supplying carbon dioxide to reactor 1, a compressor (connected to carbon dioxide supply lines 11, 11a, but not shown) is used to pressurize the gas, while simultaneously mixing in an adjusted amount of oxygen. This oxygen can be pure oxygen or air. When using air, it is preferable to supply air via an air filter or the like.

[0038] Ammonia is preheated to about 70-90°C via heat exchanger 5 during its journey from ammonia supply line 10 to reactor 1, and then supplied to reactor 1 together with ammonia recovered from condenser 3 using ejector 6.

[0039] The gas-liquid mixture obtained in reactor 1 is conveyed from gas-liquid mixture pipeline 12 to the top of stripping tower 2. Stripping tower 2 is an apparatus for separating a mixed gas containing unreacted ammonia and unreacted carbon dioxide from the urea synthesis liquid generated in reactor 1 by heating the urea synthesis liquid.

[0040] The stripping tower 2 is, for example, made of carbon steel, with an inner lining of duplex stainless steel formed on the portion corresponding to the inner wall surface. Therefore, the wall thickness of the stripping tower 2 cannot be measured externally using an ultrasonic wall thickness gauge.

[0041] Carbon dioxide gas, which acts as a stripping agent, is supplied from the lower part of the stripping tower 2 via carbon dioxide supply lines 11 and 11b. The stripping tower 2 is heated by a heating device (not shown) to raise its internal temperature.

[0042] The operating conditions in stripping tower 2 are as described above, and can be compared with those disclosed in Japanese Patent No. 3987607. Figure 3 The same applies to the urea manufacturing equipment shown, for example, with a pressure of 130–250 bar (13,000–25,000 kPa), preferably 140–200 bar (14,000–20,000 kPa), and a temperature preferably 160–200°C.

[0043] In stripping tower 2, by heating and introducing carbon dioxide as a stripping agent, ammonium carbamate in the gas-liquid mixture decomposes into ammonia and carbon dioxide, which are then transported from return gas line 14 to the bottom of condenser 3 in the form of a high-temperature mixture of unreacted ammonia, carbon dioxide, inert gas, and water (water vapor).

[0044] Urea, undecomposed trace amounts of ammonium carbamate, unseparated ammonia, carbon dioxide, etc. in the gas-liquid mixture are recovered from the urea recovery line 13 at the bottom of the stripping tower 2. The urea recovered from the urea recovery line 13 is further purified in a subsequent process (low-pressure decomposition process) to improve its purity. The residual trace amounts of ammonium carbamate are decomposed to become a low-temperature recirculation liquid containing ammonia and carbon dioxide (also containing unreacted ammonia and carbon dioxide), which is transported from the recirculation line 17 to the top of the condenser 3 (scrubbing tower) as an absorption medium.

[0045] The condenser 3 is a device used to absorb at least a portion of the mixed gas obtained in the stripping tower 2 into an absorption medium to condense it, and to generate low-pressure steam using the heat generated during condensation. The ammonia contained in the high-temperature mixed gas supplied to the bottom of the condenser 3 is cooled and condensed, and then transported from the downflow pipe 15 to the raw material ammonia supply pipeline 10 by the suction of the ejector 6 for reuse as a raw material for urea production.

[0046] A portion of the ammonia, carbon dioxide, and water (water vapor) that are supplied along with the high-temperature inert gas to the bottom of the condenser 3 come into contact with the absorption medium as they are cooled and discharged as low-temperature gases from the exhaust pipe 16. The ammonia and carbon dioxide are absorbed and removed, and the inert gas is discharged from the exhaust pipe 16.

[0047] The condenser 3 is made of carbon steel, for example, with a lining layer made of duplex stainless steel formed on the portion corresponding to the inner wall surface. Therefore, the wall thickness of the condenser 3 cannot be measured from the outside using an ultrasonic wall thickness gauge.

[0048] Inside the condenser 3, cooling water is introduced through the cooling water line 21. The water vapor that vaporizes during internal heat exchange is collected through the steam line 22 and reused as high-temperature steam. The operating conditions of the condenser 3 are as described above, and can be compared with those disclosed in Japanese Patent No. 3987607. Figure 3 The same conditions apply to the urea manufacturing equipment shown, for example, a pressure of 140–250 bar (14,000–25,000 kPa), a temperature of 130–250 °C (preferably 170–190 °C), an N / C ratio of 2.5–3.5, an H / C ratio of less than 1.0, and a residence time of 10–30 minutes.

[0049] For the aforementioned pipelines, conduits made of austenitic stainless steel (single-phase) or duplex stainless steel (austenitic-ferrite duplex stainless steel) can be used. Figure 1 In the example of the urea equipment shown, the pipelines of the wall thickness measurement sections 30 to 37, which are performed using an ultrasonic wall thickness gauge, are made of austenitic stainless steel.

[0050] For austenitic stainless steel, S31603 (316L SS) can be used, and for duplex stainless steel, 25Cr duplex stainless steel (S31260) and 28Cr duplex stainless steel (S32808: DP28W) can be used. Since each pipeline is made of a single material, the wall thickness can be measured externally using an ultrasonic wall thickness gauge.

[0051] Known in Figure 1 In the urea manufacturing process shown in the urea manufacturing equipment, when ammonia reacts with carbon dioxide, ammonium carbamate, which is highly corrosive to metals, is produced as a byproduct. This corrodes the inner walls of reactor 1, stripping tower 2, condenser 3, or the inner walls of various pipelines.

[0052] In the manufacturing method of this invention, a passivation film is formed on the surface of stainless steel by mixing oxygen into the raw material carbon dioxide, thereby inhibiting the contact between ammonium carbamate and stainless steel and suppressing the corrosion of stainless steel. It should be noted that, compared to duplex stainless steel, austenitic stainless steel requires more oxygen to form the passivation film. However, if the oxygen concentration in the raw material carbon dioxide is too high, the temperature inside reactor 1 and condenser 3 cannot be sufficiently raised, and the reaction rate cannot be accelerated, thus reducing the urea reaction yield (yield reduction). If the oxygen concentration is too low, excessive corrosion of the stainless steel will occur.

[0053] It should be noted that Figure 5 in WO2014 / 192823 shows the relationship between oxygen concentration in the gas phase (horizontal axis) and corrosion rate (vertical axis). Compared with 25Cr duplex stainless steel (S31260) and 28Cr duplex stainless steel (S32808), austenitic stainless steel (S31603) is difficult to form a passivation film. Therefore, it shows that if the oxygen concentration is low, the corrosion rate increases. If the oxygen concentration is increased, any stainless steel will form a passivation film, and thus the corrosion rate decreases.

[0054] In the manufacturing method of the present invention, it is preferable to implement any one, any two, or three of the control methods (A) to (C) described below.

[0055] Control method (A)

[0056] Control method (A) is as follows: In the urea manufacturing process, by adding oxygen to carbon dioxide, which is used as a raw material, a passivation film is formed on the inner wall surface of multiple processing units (including reactor 1, stripping tower 2, condenser 3) and multiple pipelines. At the same time, the wall thickness of the pipelines made of austenitic stainless steel is continuously measured, and the oxygen supply is adjusted according to the measured value of the wall thickness, thereby controlling the corrosion rate and the reaction yield of urea.

[0057] exist Figure 1 In the example of the urea manufacturing equipment shown, for example, the thickness t2 of each pipeline at the wall thickness measurement points 30 to 37 during operation is continuously measured using an ultrasonic wall thickness measuring device, and the corrosion rate s (mm / year) is calculated by the following formula: s=(t1-t2) / (operation time) (t1 shows the initial thickness of each pipeline at the wall thickness measurement points 30 to 37 before operation).

[0058] The initial thickness t1 of each pipeline is known (measured or standard value). The difference between t1 and the thickness t2 of each pipeline after operation is divided by the operation time, and the resulting value is used as the corrosion rate s. Therefore, by continuously measuring the thickness t2 of each pipeline during operation, the change in corrosion rate s can be continuously confirmed.

[0059] Therefore, as an example of control method (A), if the corrosion rate s becomes too large, the oxygen supply is increased (when using air, this refers to the amount of air converted to oxygen), and if the corrosion rate s is sufficiently small, the oxygen supply is reduced, thereby suppressing the increase or decrease in the reaction yield of urea and minimizing it as much as possible, so that urea can be produced with a stable reaction yield.

[0060] about Figure 1 The corrosion rate s in each pipeline of the urea manufacturing equipment shown during operation is preferably controlled below 0.2 mm / year, and more preferably below 0.15 mm / year, based on its relationship with the urea reaction yield.

[0061] Control Method (B)

[0062] Control method (B) is as follows: the concentration of iron, chromium or nickel dissolved in urea or ammonia and the operating temperature are measured, and the oxygen supply is adjusted according to the measured values ​​of concentration and operating temperature, thereby controlling the corrosion rate and the reaction yield of urea.

[0063] exist Figure 1 In the example of the urea manufacturing equipment shown, sampling can be performed at sampling positions 40-42, for example. At sampling position 40, for example, a gas-liquid mixture containing urea, ammonium carbamate as a reaction intermediate, and unreacted gases (ammonia and carbon dioxide) flowing through the gas-liquid mixture pipeline 12 is collected, and the temperature is measured simultaneously. Then, the concentrations of iron, chromium, or nickel ions in the sample are measured.

[0064] At sampling location 41, for example, urea and trace amounts of ammonium carbamate flowing through urea recovery pipeline 13 are collected, and the temperature is measured at the same time. Then, the concentrations of iron, chromium and nickel ions in the sample are measured.

[0065] At sampling location 42, for example, a liquid containing ammonia flowing through downflow tube 15 is collected, and the temperature is measured simultaneously. Then, the concentrations of iron, chromium, or nickel ions in the sample are measured.

[0066] The results showed that when the concentrations of iron, chromium, and nickel ions in the sample were high, the passivation film formation was insufficient, and corrosion was ongoing. Conversely, when the concentrations of iron, chromium, or nickel ions in the sample were low, the passivation film formation was sufficient, and corrosion did not occur. The iron, chromium, or nickel ions used in the measurement could be any one type, any combination of two types, or all three types. Furthermore, the results indicated that corrosion proceeded faster at higher temperatures at the sampling location and slower at lower temperatures.

[0067] When implementing control method (B), it is preferable to sample at multiple locations within the urea manufacturing equipment and measure the operating temperature at these multiple sampling locations. There are no particular limitations on the sampling locations (temperature measurement locations), and multiple locations (preferably three or more) can be selected. For example, the outlet side pipeline of reactor 1 (gas-liquid mixture pipeline 12), the outlet side pipeline of stripping tower 2 (urea recovery pipeline 13), and the outlet side pipeline of condenser 3 (downstream pipe 15) are preferred.

[0068] It should be noted that, regarding operating temperature, it is preferable to measure the internal temperature of the instruments in reactor 1, stripping tower 2, and condenser 3 near each sampling point. Temperature measurements can be performed using known thermometers such as thermocouples or resistance thermometers.

[0069] Therefore, as a control method (B), by implementing any of the following methods, the increase or decrease in the reaction yield of urea can be suppressed, thus enabling the production of urea with a stable reaction yield:

[0070] Method 1 of control method (B): When the concentrations of iron, chromium and nickel are high and the temperature at the sampling location is high, increase the oxygen supply to form a passivation film;

[0071] Method 2 of control (B): Reduce oxygen supply when the concentrations of iron, chromium, and nickel are low and the temperature at the sampling location is low;

[0072] The third method of control method (B): When the concentrations of iron, chromium and nickel are high and the temperature at the sampling location is low, increase the oxygen supply (but by less than in the first method) to form a passivation film;

[0073] Method 4 of control method (B): When the concentrations of iron, chromium and nickel are low and the temperature at the sampling location is high, reduce the oxygen supply (but by less than in method 2).

[0074] Control Method (C)

[0075] The control method (C) is as follows: by measuring the operating pressure and operating temperature of multiple processing units (reactor, stripping tower, condenser), the flow rate of carbon dioxide introduced as raw material, the oxygen content in the raw material carbon dioxide, and the flow rate of ammonia introduced as raw material, the corrosion rate of each of the multiple processing units and the corrosion rate of multiple pipelines connecting the multiple processing units are calculated, and the oxygen supply is adjusted to control the corrosion rate and the reaction yield of urea.

[0076] The operating temperature of reactor 1 can be measured, for example, at a measuring point (measuring device) 51 at the upper part (preferably near the top) or at a measuring point (measuring device) 54 at the lower part of reactor 1. The operating temperature of stripper 2 can be measured, for example, at a measuring point (measuring device) 52 at the upper part (preferably near the top) or at a measuring point (measuring device) 55 at the lower part of stripper 2. The operating temperature of condenser 3 can be measured, for example, at a measuring point (measuring device) 53 at the upper part (preferably near the top) or at a measuring point (measuring device) 56 at the lower part of condenser 3.

[0077] The pressures of reactor 1, stripping tower 2, and condenser 3 are almost identical. Their pressures can be measured, for example, in line 11b or in an ammonia injection line (not shown) leading to condenser 3.

[0078] The flow rate of carbon dioxide introduced as a feedstock can be measured, for example, in carbon dioxide supply lines 11 and 11a. When supplying carbon dioxide to reactor 1, it is pressurized using a compressor, and an adjusted amount of oxygen is mixed in simultaneously; therefore, the oxygen content in the feedstock carbon dioxide can be calculated, for example, from the amount of air introduced to the compressor. The flow rate of ammonia introduced as a feedstock can be measured, for example, in ammonia supply line 10.

[0079] The corrosion rates of reactor 1, stripper 2, and condenser 3, as well as the corrosion rates of the multiple pipelines connecting reactor 1, stripper 2, and condenser 3 (gas-liquid mixture pipeline 12, return gas pipeline 14, and downflow pipe 15), can be calculated using the operating temperature, operating pressure, carbon dioxide flow rate, oxygen concentration in carbon dioxide, and ammonia flow rate, which are the data obtained from the above measurements. Based on the relationship between the measured data and corrosion rate in control method (A), the following points can be considered to determine that: the higher the operating temperature, the greater the corrosion rate; the higher the ammonium carbamate concentration, the greater the corrosion rate; and the higher the oxygen concentration in carbon dioxide, the lower the corrosion rate.

[0080] use Figure 2A further preferred embodiment of the urea manufacturing method of the present invention will be described. Figure 2 The implementation shown is that control method (A), control method (B), and control method (C) are implemented in this order.

[0081] In stage (1), for example according to Figure 1 The manufacturing process shown begins the production of urea. After the start of urea production, control methods (A) to (C) are implemented to control the corrosion rate and urea reaction yield by adjusting the oxygen supply.

[0082] In stage (2), control method (A) determines whether to increase the supply of air (oxygen) in the raw material carbon dioxide or keep it constant. When the corrosion rate determined by control method (A) is within the allowable value (Yes), the process transitions to stage (3). When the corrosion rate determined by control method (A) exceeds the allowable value (No), the process transitions to stage (5) to improve the corrosion protection effect, and urea production continues with the increased supply of air (oxygen) in the raw material carbon dioxide. When transitioning from stage (2) to stage (5) and increasing the supply of air (oxygen) in the raw material carbon dioxide, the operations after stage (3) are not performed.

[0083] In stage (3), control method (B) determines whether to increase the supply of air (oxygen) in the raw material carbon dioxide or keep it constant. When the corrosion rate determined by control method (B) is within the allowable value (Yes), the process transitions to stage (4). When the corrosion rate determined by control method (B) exceeds the allowable value (No), the process transitions to stage (5) to improve the corrosion protection effect, and urea production continues with the increased supply of air (oxygen) in the raw material carbon dioxide. When transitioning from stage (3) to stage (5) and increasing the supply of air (oxygen) in the raw material carbon dioxide, the operations after stage (4) are not performed.

[0084] In stage (4), control method (C) determines whether to increase the supply of air (oxygen) in the raw material carbon dioxide or keep it constant. When the corrosion rate determined by control method (C) is within the allowable value (Yes), the process transitions to stage (5). When the corrosion rate determined by control method (C) exceeds the allowable value (No), the process transitions to stage (5) to improve the corrosion protection effect, and urea production continues with the increased supply of air (oxygen) in the raw material carbon dioxide. When transitioning from stage (4) to stage (5) and increasing the supply of air (oxygen) in the raw material carbon dioxide, the operations after stage (6) are not performed.

[0085] In stage (6), control methods (A) to (C) are comprehensively evaluated to determine whether to reduce the supply of air (oxygen) in the raw material carbon dioxide or to keep it constant. While the corrosion rates calculated in control methods (A) to (C) are all below the permissible value, when they are close to the permissible value (e.g., exceeding 95% of the permissible corrosion rate), the process transitions to stage (7) to keep the supply of air (oxygen) in the raw material carbon dioxide constant. When the corrosion rates calculated in control methods (A) to (C) are all significantly lower than the permissible value (e.g., below 95% of the permissible corrosion rate), the process transitions to stage (8) to reduce the supply of air (oxygen) in the raw material carbon dioxide.

[0086] In addition to the embodiments described above, the present invention also includes the following embodiments.

[0087] Figure 1 The reactor 1, stripping tower 2, condenser 3, and other processing units in the urea manufacturing equipment shown in the example are constructed of carbon steel, with a duplex stainless steel lining formed on the inner wall portion. Therefore, the wall thickness cannot be measured externally using an ultrasonic wall thickness gauge. Furthermore, the reactor 1, stripping tower 2, condenser 3, and other processing units operate under high temperature and high pressure, making internal observation impossible. Therefore, the corrosion status of these processing units cannot be directly confirmed during the operation of the urea manufacturing equipment. On the other hand, because... Figure 1 The pipelines shown are made of a single material, stainless steel, so the wall thickness can be measured from the outside using an ultrasonic wall thickness gauge, thus confirming the corrosion status.

[0088] Therefore, in Figure 1 During the operation of the urea manufacturing equipment shown, operational data such as temperature, pressure, and operating time of the reactor 1, stripping tower 2, condenser 3, and other processing devices can be acquired. Simultaneously, the wall thickness of each pipeline (wall thickness measurement points 30-37 mm) is measured and stored as relevant data. Furthermore, the system can be periodically stopped. Figure 1 The operation of the urea manufacturing equipment shown was monitored, and the corrosion status of the duplex stainless steel lining inside the reactor 1, stripping tower 2, condenser 3, and other processing devices was observed and stored as data.

[0089] By comparing and evaluating operational data such as temperature, pressure, and operating time of each processing unit, wall thickness data of each pipeline, and observation data on the corrosion status of each processing unit, the internal corrosion status of each processing unit can be inferred from the wall thickness data of each pipeline. Through this operation, the internal corrosion status of each processing unit can be directly inferred from the wall thickness changes of each pipeline while the urea manufacturing equipment is running continuously. Therefore, the replacement or maintenance period of each processing unit can be determined without stopping the operation of the urea manufacturing equipment, thus achieving stable urea manufacturing operation.

[0090] It should be noted that this implementation method does not increase or decrease the oxygen supply (when using air, it refers to the amount of air converted to oxygen) as described in the control methods (A) and (B) above. It is suitable for manufacturing urea when a certain amount of oxygen (when using air, it refers to the amount of air converted to oxygen) is introduced into the raw materials for urea production. However, it can also be implemented in combination with one or both of the control methods (A) and (B) described above. Example

[0091] Example 1

[0092] Test pieces made of stainless steel (28Cr duplex stainless steel; S32808, austenitic stainless steel; S31603) were immersed in the synthesized urea solution in an autoclave. Under these conditions, oxygen was slowly introduced into the autoclave, and the oxygen content was measured when a passivation film formed on the test pieces (passive corrosion). The test was conducted at a temperature of 195°C. The results are shown in […]. Figure 3 .

[0093] Depend on Figure 3 It can be seen that when a passivation film is formed on S31603 (passive corrosion), the corrosion area is extremely small, only 0.1 mm. However, when the passivation film is not fully formed (active corrosion), the corrosion area greatly exceeds 10 mm. It should be noted that the same experiment confirms that S32808 requires less oxygen to form a passivation film compared to S31603.

[0094] This result confirms that in the urea manufacturing method of the present invention, in Figure 1 While a passivation film is formed on the inner walls of the multiple processing units and pipelines constituting the urea plant, the concentrations of iron, chromium, and nickel dissolved in urea or ammonia, as well as the operating temperature, are measured. The oxygen supply is adjusted based on these concentration and operating temperature measurements to control the corrosion rate. Furthermore, it is a well-known fact that a higher oxygen (air) content during urea production reduces the urea reaction yield. Therefore, it is confirmed that by adjusting the oxygen supply to control the corrosion rate, the urea reaction yield can also be controlled.

[0095] Example 2

[0096] In accordance with Figure 1 The manufacturing process of the urea manufacturing equipment shown includes the following control methods (A), (B) and (C) implemented during the urea manufacturing process.

[0097] Control method (A)

[0098] Sixty days after the start of urea production, the wall thickness (initially 23.01 mm) of the return gas line 14 (wall thickness measurement point 35), constructed of S31603 general-purpose stainless steel (austenitic stainless steel), connecting the stripping tower 2 and condenser 3, was measured using an ultrasonic thickness gauge (GE Sensing & Inspection Technologies Co., Ltd., ultrasonic thickness gauge, small / easy-to-operate / high-performance ultrasonic thickness gauge DM5E series). The corrosion rate, calculated from the difference between the measured wall thickness and the initial wall thickness and the elapsed time, was 0.12 mm / year. The oxygen concentration in the carbon dioxide supplied to the raw material from the start of operation to the measurement point was 5500 ppm, and the operating temperature (average) was 183°C.

[0099] The obtained corrosion rate indicates that a passivation film has formed on the inner wall of the return gas line 14. This shows that... Figure 2 In the implementation shown, stage (2) is "Yes", so the transition proceeds to stage (3).

[0100] Control Method (B)

[0101] The iron concentration in the solution at the outlet (sampling point 41) of stripper 2 was 0.8 ppm, at an operating temperature of 171°C. Based on the obtained iron concentration, it was determined that passivation films had formed on the inner walls of reactor 1, gas-liquid mixture pipeline 12, and stripper 2, all located upstream of sampling point 41. This indicates that... Figure 2 In the implementation shown, stage (3) is "Yes", so the transition proceeds to stage (4).

[0102] Control Method (C)

[0103] The operating temperature and operating pressure of the measuring parts 51 to 53 are as follows.

[0104] Measurement point 51: Temperature 186℃, Pressure 151 kg / cm² 2 G;

[0105] Measurement site 52: Temperature 188℃, Pressure 151 kg / cm² 2 G;

[0106] Measurement site 53: Temperature 180℃, Pressure 151 kg / cm² 2 G.

[0107] The flow rate of carbon dioxide (measured in carbon dioxide supply lines 11 and 11a) is 45,000 Nm³. 3 / hour. The oxygen content in the raw material carbon dioxide is 250 Nm³. 3 / hour (calculated from the amount of air introduced into the compressor). The ammonia flow rate (measured in ammonia supply line 10) is 69t / hour. Based on the above measurement results and corrosion rate data from control method (A), the corrosion rates of each unit and pipeline are calculated as follows.

[0108] (i) Condenser 3 (inner wall surface is S31603 series general stainless steel): 0.09mm / year, temperature (180℃);

[0109] (ii) Stripping tower 2 (inner wall surface is duplex stainless steel): 0.10 mm / year, temperature (188℃);

[0110] (iii) Reactor 1 (inner wall is made of S31603 general-purpose stainless steel): 0.14 mm / year, temperature (186℃);

[0111] (iv) Return gas line 14 from stripper tower 2 to condenser 3 (inner wall surface is S31603 series general stainless steel): 0.16 mm / year, temperature (188℃);

[0112] (v) Downflow pipe 15 from condenser 3 to reactor 1 (inner wall surface is S31603 general-purpose stainless steel): 0.09 mm / year, temperature (180℃);

[0113] (vi) Gas-liquid mixture pipeline 12 from reactor 1 to stripper 2 (inner wall surface is S31603 general-purpose stainless steel): 0.14 mm / year, temperature (186℃).

[0114] In (i) to (vi), the oxygen concentration in the carbon dioxide supplied to the raw material is 5525 ppm in any case. The obtained corrosion rate indicates that a passivation film has formed on the inner walls of each instrument and pipeline. This shows that... Figure 2 In the embodiment shown, stage (4) is "Yes", therefore transitioning to stage (6). As a result, it is determined that the corrosion rate is less than the allowable value, and the oxygen content needs to be reduced, thereby reducing the oxygen concentration in the raw material carbon dioxide to 4500 ppm. Figure 2 The stages shown are (6) → (8).

[0115] Industrial practicality

[0116] The urea manufacturing method of the present invention can use known urea manufacturing equipment, extend the life of the aforementioned equipment when manufacturing urea, and can manufacture urea with a high reaction yield. Therefore, it can be used as a manufacturing method that can reduce the operating cost of equipment and the manufacturing cost of urea.

[0117] Symbol Explanation

[0118] 1: Reactor;

[0119] 2: Stripping tower;

[0120] 3: Condenser;

[0121] 5: Heat exchanger;

[0122] 6: Injector;

[0123] 30-37: Location for wall thickness measurement;

[0124] 40-42: Sampling positions;

[0125] 51-56: Temperature measurement sites.

Claims

1. A method for producing urea, which is a method for producing urea from raw materials containing ammonia and carbon dioxide in a urea production plant, wherein The urea production plant has: a reactor for producing a urea synthesis solution from carbon dioxide and ammonia as raw materials; a stripping column for separating a mixed gas containing unreacted ammonia and unreacted carbon dioxide from the urea synthesis solution produced in the reactor by heating the urea synthesis solution; a plurality of processing devices including a condenser that condenses at least a portion of the mixed gas obtained in the stripping column by absorbing the mixed gas into an absorption medium, and generates low-pressure steam using heat generated at the time of condensation; and a plurality of pipelines connecting the plurality of processing devices, the inner wall surfaces of the plurality of processing devices and the plurality of pipelines are made of stainless steel, and at least a portion of the plurality of pipelines is made of austenitic stainless steel, when three of the following control methods (A) to (C) are implemented in the following order of control method (A), control method (B), and control method (C), whether to increase the oxygen supply amount in the raw material carbon dioxide is determined in accordance with the corrosion rate in the following control method (A), when the oxygen supply amount is increased, the following control method (B) and the following control method (C) are not implemented, and when the oxygen supply amount is not increased, the following control method (B) is transitioned to, when the following control method (B) is transitioned to, whether to increase the oxygen supply amount in the raw material carbon dioxide is determined in accordance with the corrosion rate in the following control method (B), when the oxygen supply amount is increased, the following control method (C) is not implemented, and when the oxygen supply amount is not increased, the following control method (C) is transitioned to, when the following control method (C) is transitioned to, whether to increase the oxygen supply amount in the raw material carbon dioxide is determined in accordance with the corrosion rate in the following control method (C), when the oxygen supply amount is increased, the subsequent operation is not implemented, and when the oxygen supply amount is not increased, whether to maintain the current state of the oxygen supply amount in the raw material carbon dioxide or to reduce the oxygen supply amount in the raw material carbon dioxide is determined in accordance with the corrosion rate of each of the control methods (A) to (C), Control method (A): In the urea production method, a passivation film is formed on the inner wall surfaces of the plurality of processing devices and the plurality of pipelines by supplying oxygen after adding oxygen to the carbon dioxide as raw materials, and the wall thickness of the pipeline made of austenitic stainless steel is continuously measured, and the oxygen supply amount is adjusted in accordance with the measured value of the wall thickness, thereby controlling the corrosion rate and the reaction yield of urea; Control method (B): The concentration of iron, chromium, or nickel dissolved in urea or ammonia and the operating temperature are measured, and the oxygen supply amount is adjusted in accordance with the measured values of the concentration and the operating temperature, thereby controlling the corrosion rate and the reaction yield of urea; Control method (C) : By measuring the operating pressure and the respective operating temperature of the plurality of processing devices, the flow rate of carbon dioxide introduced as the raw material, the oxygen amount in the raw material carbon dioxide, the flow rate of ammonia introduced as the raw material, the respective corrosion rates of the plurality of processing devices and the corrosion rates of the plurality of pipelines connecting the plurality of processing devices are calculated, the supply amount of oxygen is adjusted, and thus the corrosion rates and the reaction yield of urea are controlled.

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