Purification system process principle
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In order to save everyone’s wealth, I will type it out~! It’s very troublesome because there are too many words~! Low-temperature methanol washing part 1. Overview---------------------------------------------------------------------------------------------------------------- 1. Source of acid gas------------------------------------------------------------------------------------------------ 2. Necessity of acid gas removal-------------------------------------------------------------------------------- 3. Method of acid gas removal-------------------------------------------------------------------------------- 4. Physical and chemical properties of methanol-------------------------------------------------------------------------------- 5. Advantages and disadvantages of low-temperature methanol washing -------------------------------------------------------------------------------- 2. Theoretical basis---------------------------------------------------------------------------------------------------------------- 1. Raoult's law and Henry's law-------------------------------------------------------------------------------- 2. Solubility of various gases in methanol-------------------------------------------------------------------------------- 3. Process and principle------------------------------------------------------------------------------------------------ 1. Dehydration---------------------------------------------------------------------------------------------------------------- 2. Absorption of CO2 and H2S-------------------------------------------------------------------------------- 3. Regeneration of methanol-rich liquid and recovery of H2, CO2 and H2S (1) One-stage vacuum flash evaporation---------------------------------------------------------------------------------- (2) Two-stage vacuum flash evaporation---------------------------------------------------------------------------------- (3) Concentration of H2S component----------------------------------------------------------------------------------- (4) Heating regeneration---------------------------------------------------------------------------------- 4. Methanol dehydration distillation---------------------------------------------------------------------------------- 4. Accidents that have occurred in our plant and similar equipment -------------------------------------------------------------------------------- 5. Technical transformation and functions------------------------------------------------------------------------------------------------ Liquid nitrogen washing part 1. Overview------------------------------------------------------------------------------------------------ 2. Theoretical basis------------------------------------------------------------------------------------------------ 1. Basic principle 2. Gas-liquid phase equilibrium of H2-N2-CO system-------------------------------------------------------------------------------- 3. Process and principle------------------------------------------------------------------------------------------------ 1. Pretreatment of raw gas -------------------------------------------------------------------------------- 2. Cooling of raw gas -------------------------------------------------------------------------------- 3. Liquefaction of nitrogen ------------------------------------------------------------------------------------------------ 4. Liquid nitrogen washing------------------------------------------------------------------------------------------------ 5. Nitrogen cooling production-------------------------------------------------------------------------------- 6. Recovery of carbon monoxide fraction cooling capacity -------------------------------------------------------------------------------- 4. Accidents that have occurred in our factory and similar devices -------------------------------------------------------------------------------- 1. Plate fin heat exchanger clogging -------------------------------------------------------------------------------- 2. Programmed control chaos, collapsed torch -------------------------------------------------------------------------------- 5. Technical transformation and function ------------------------------------------------------------------------------------------------ 1. 16-V1 adsorber pre-cooling modification-------------------------------------------------------------------------------- 2. Nitrogen distribution pipeline modification------------------------------------------------------------------------------------------------ 3. Low raw gas flow interlock modification-------------------------------------------------------------------------------- Methanation part 1. Overview------------------------------------------------------------------------------------------------ 2. Basic principles------------------------------------------------------------------------------------------------ 1. Chemical reaction-------------------------------------------------------------------------------- 2. Chemical balance------------------------------------------------------------------------------------------------ 3. Side reactions------------------------------------------------------------------------------------------------- 3. Methanation catalyst------------------------------------------------------------------------------------------------ 4. Process ------------------------------------------------------------------------------------------------ 1. Process conditions ------------------------------------------------------------------------------------------------ 2. Process flow ------------------------------------------------------------------------------------------------ Low-temperature methanol washing part 1. Overview 1. Source of acidic gas In the modern synthetic ammonia production process, no matter what kind of raw material (natural gas, light oil, heavy oil, etc.) or gasification method is used, the following reactions will occur.: 2CO＋O2＝2CO2 CO＋H2O(g)＝CO2＋H2 Therefore, a large amount of CO2 gas is generated. At the same time, depending on the sulfur content of the raw materials used, a certain amount of sulfides are also generated (including inorganic sulfur and organic sulfur). For example, our factory uses residual oil as raw material and uses partial oxidation method to produce raw gas. After the raw gas is transformed, about 46035Nm3/h of CO2 is generated, accounting for 34.10% of the total transformed gas. ； Approximately 328Nm3/h of H2S and COS are generated. Since the above gases all exhibit acidic reactions, they are collectively called acid gases. 2. The necessity of removing acidic gases. During the ammonia production process, so many acidic gases will not only cause ammonia synthesis catalyst poisoning, equipment corrosion, pipe blockage at low temperatures and other accidents, leading to production suspension, but will also occupy expensive high-pressure and high-pressure container space and increase unnecessary power consumption. CO2 is an important raw material for manufacturing chemical products such as urea, soda ash, and dry ice. Sulfur itself is also an important chemical raw material. Therefore, in the modern synthetic ammonia production process, an acid gas removal process must be installed to remove acid gases such as CO2, H2S, and COS that are harmful to ammonia production, and at the same time fully recover the raw materials of other chemical products. 3. Methods for removing acidic gases: To remove impurities such as H2S and CO2 from gases, absorption methods are mainly used. There are many absorption methods, and their basic characteristics are based on the different solubilities of various components in the gas in a certain solvent. The H2S and CO2 are selectively absorbed by the solvent simultaneously or step by step, and then, by increasing the temperature and lowering the pressure, the H2S and CO2 are released from the solution step by step. According to different principles, absorption operations can be divided into two categories: chemical absorption method and physical absorption method. The chemical absorption method uses acid gas to react with alkaline compounds or alkaline solvents dissolved in water to remove them. From an economic point of view, a solution that can be regenerated should be selected. That is, the acid gas can be released from the solution after absorbing the acid gas under reduced pressure and heating conditions, and the regenerated solution can be reused for absorption after cooling. At present, the ethanolamine method and the hot potassium alkali method are widely used. The physical absorption method is carried out by utilizing the different solubilities of various gas components in a certain solvent. Generally, the most suitable solvents for removing acidic gases are various polar liquids, because polar liquids can dissolve acidic gases, but rarely dissolve non-polar components such as H2 and N2. Commonly used physical absorption methods include high-pressure water washing method (Juhua Plant), N-methylpyrrolidine method (Purisol method) and low-temperature methanol method (Reactisol method). In fact, it is very difficult to strictly distinguish between physical absorption and chemical absorption. From the perspective of modern physical chemistry, when a solute is dissolved in a solvent, solvation occurs, that is, a chemical change occurs. Our factory uses physical absorption method to remove acidic gases in the raw gas, that is, using low-temperature methanol to absorb H2S and CO2 step by step in a tower with a pressure of 7.8MPa, so that CO2 in the raw gas is less than 20ppm and H2S is less than 1ppm. 4. Physical properties of methanol (1) Boiling point: 64.5～64.7℃. (2) Melting point: -97.8℃. (3) Autoignition point: 437℃ in air and 461℃ in oxygen. (4) Proportion: 0.7913. (5) Viscosity: It is 0.86cP at 0℃ and 0.59cP at 20℃. (6) Specific heat: The specific heat between 20℃ and 25℃ is 2.508～2.533 J/g·℃. (7) Vapor pressure: Temperature (℃) Vapor pressure (mmHg) Temperature (℃) Vapor pressure (mmHg) -67.4 0.102 30 160 -54.5 0.378 40 260.5 -44.0 1 50 406 -40.0 2.0 60 425 -30 4.0 64.7 760 -20 8.0 70 927 -10 15.5 80 1341 0 29.6 90 1897 10 54.7 100 2621 20 96.0 110 3561 5. Advantages and disadvantages of low-temperature methanol washing (1) Using methanol as the solvent has strong absorption capacity for CO2, H2S, COS, etc. This requires less solution circulation and thus reduces power consumption. (2) Using methanol as the solvent has a high selectivity between the CO2, H2S, and COS components to be removed and the H2, N2 and other components that are not to be removed. On the one hand, it can be understood that methanol has high solubility for CO2 and H2S, but low solubility for H2, N2, etc. This also has the significance of reducing the loss of effective H2. On the other hand, the high selectivity is reflected in the fact that the absorption of H2S by methanol is several times faster than the absorption of CO2. The solubility of the former is also greater than that of the latter, so the stepwise absorption of H2S and CO2 can be achieved in the same tower. (3) The vapor pressure of methanol is low, which reduces the loss of steam taken away from the top gas of the absorption tower and desorption tower and reduces the loss of solution. (4) Methanol has good chemical and thermal stability, will not be decomposed and deteriorated by impurities such as organic sulfur and chloride, will not foam, and is less corrosive (when the water content in CH3OH is <1.0%). All equipment and pipelines can be made of carbon steel or low-temperature resistant alloys. (5) The low viscosity of methanol not only reduces the power consumption when transporting solutions, but also improves heat transfer and mass transfer efficiency. (6) Methanol has a low boiling point, so cheap low-pressure steam can be used in the reboiler of the desorption tower. (7) Methanol has a low melting point, so it can be absorbed at -80°C without the risk of freezing and blocking pipelines. (8) Methanol is cheap and easy to obtain and is not prone to spontaneous combustion. Although methanol has many advantages as a solvent, it also has disadvantages: (1) The regeneration process is long and complex. (2) Methanol is a toxic substance. When a person takes 10 mL, he or she will go blind, and when 30 mL is taken, it can cause death. The allowable concentration in the air is 50 mg/Nm3. Therefore, equipment manufacturing and pipeline installation are required to be tight and leak-free. Do not take it lightly during operation to prevent accidents. (3) Methanol washing is generally performed at low temperature, so its special advantages can only be shown when matched with air separation and liquid nitrogen washing. 2. Theoretical basis 1. Raoult's law and Henry's law (1) Several concepts solution - two or more components are mixed together, and the uniform phase formed by each component being dispersed into other components in the form of molecules or ions is called a solution. Liquid solutions are most commonly used in engineering. Whether two liquids can form a solution when mixed depends on their properties. The closer the polarity of the two liquids is, the greater the mutual solubility. On the contrary, the further the polarity difference between the two liquids is, the smaller the mutual solubility will be, or even almost completely insoluble in each other. For example, water and methanol are mutually soluble in any proportion, but water and kerosene are not mutually soluble. Certain solids or gases can also be dissolved in appropriate liquids to form solutions of limited concentration. Solvent, solute - When a gas or solid is dissolved in a liquid, the liquid is usually regarded as the solvent, usually represented by footnote 1, and the dissolved gas or solid is regarded as the solute, usually represented by footnote 2. (2) Raoult’s law states that the vapor pressure P1 of a solvent in a solution is equal to the product of the vapor pressure of the pure solvent and its mole fraction X1. This relationship is called Raoult's law, expressed mathematically as: = • (1) We can also analyze it theoretically. In a very dilute solution, there are very few solute molecules, and the solvent molecules are almost surrounded by their own molecules. The situation is almost the same as that of a pure substance. That is to say, the force exerted on the solvent molecules has not changed due to the presence of a small number of solute molecules, so its ability to escape from the solution has hardly changed. However, the presence of solute molecules has reduced the number of solvent molecules, so the vapor pressure P1 of the solvent in the solution is discounted by a mole fraction based on the saturated vapor pressure of the pure solvent. (3) Henry's Law Henry's Law is that under constant temperature and equilibrium conditions, the solubility of a gas in a liquid is directly proportional to the equilibrium pressure of the gas. Expressed mathematically as: = • (2) where: X2 is the mole fraction of the gas in the solution at equilibrium, P2 is the partial pressure of the gas on the liquid surface, and K is the Henry coefficient. Its value depends on the temperature and the properties of the solute and solvent and is measured experimentally. However, when the total pressure is high, it will also have an impact on the K value. Generally speaking, Henry's coefficient K is not equal to the vapor pressure of the solute at that temperature. Because when Depending on the concentration and pressure units used, Henry's law can also be written in the following forms:: = = = = = Km, KC, KS, and H in each formula are all called Henry coefficients, and m2, C2, S2, and X2 can be converted to each other. (4) The relationship between Raoult’s law and Henry’s law and its application in production can be proved by experiments and the laws of thermodynamics: In a dilute solution, if the solute obeys Henry's law, the solvent must obey Raoult's law; conversely, if the solvent obeys Raoult's law, the solute must obey Henry's law. Raoult's law and Henry's law are the basis for absorption operations in the chemical industry. Absorption separation is to use the different solubilities of solvents to various components in a gas mixture to selectively absorb gases with high solubility to achieve the purpose of recovering or removing this gas from the gas mixture. Since Raoult's law and Henry's law do not consider the interaction between solvent and solute molecules, in industrial production due to the influence of temperature, pressure, etc., the application of these two laws will produce deviations and must be corrected. 2. Solubility of various gases in methanol (1) Solubility of H2S, COS, CS2 in methanol Hydrogen sulfide and methanol are polar substances. According to the law of polar substances dissolving polar substances, methanol is a good solvent for H2S. The experimental data of the solubility of H2S in low-temperature methanol are as shown in Table 1 and Table 2. Table 1 shows the dissolution of H2S when the partial pressure is lower than 0.1MPa, and Table 2 shows the dissolution of H2S when the partial pressure is lower than 0.1MPa. When the partial pressure of H2S is low, the solubility of H2S in methanol can be estimated using the following empirical formula: lg = where S2——solubility of H2S in pure methanol (Nm3/t) ； T——given temperature, K ； D——is the pressure coefficient, its value is shown in Table 3. Table 1 H2S equilibrium partial pressure mmHg 0℃ -25.6℃ -50℃ -78.5℃ S2 KS S2 KS S2 KS S2 KS 50 2.4 20.833 5.7 8.772 16.8 2.976 76.4 0.65 100 4.8 20.833 11.2 8.929 32.8 3.048 155.0 0.65 150 7.2 20.833 16.5 9.091 48.0 3.126 249.2 0.60 200 9.7 20.619 21.8 9.174 65.8 3.049 300 14.8 20.270 33.0 9.091 99.6 3.012 400 20.0. 20.000 45.8 8.247 135.20 2.959 Table 2 Temperature ℃ Pressure (atmospheric pressure) Temperature of H2S molecule fraction in liquid phase ℃ Pressure (atmospheric pressure) Temperature of H2S molecule fraction in liquid phase ℃ Pressure (atmospheric pressure) Fraction of H2S molecules in the liquid phase 0 2 0.092 -15 2.0 0.165 -25 2.0 0.203 6 0.329 3.4 0.298 3.0 0.327 8 0.484 4.4 0.403 4.0 0.582 10 0.840 5.4 0.585 4.3 0.733 10.2 1.000 6.4 1.000 4.5 1.000 Table 3 H2S pressure mmHg 50 100 150 200 300 400 D 3.34 3.06 2.28 2.75 2.58 2.46 The total pressure of the gas entering the low-temperature methanol wash in our factory is 7.8Mpa, and the H2S concentration is 0.24%, so the H2S partial pressure is 0.019Mpa, that is: 144.4mmHg, so the above data applies. The solubility of carbon sulfide and carbon disulfide in methanol is shown in the following two figures. (2) Solubility of CO2 in methanol The solubility of CO2 in pure methanol is shown in Table 4 based on published data. Table 4: Solubility of CO2 in methanol under pressure (S is Nm3/t·CH3OH ； X is the mole fraction) Pressure (gauge pressure) 0℃ 77.4 117.0 321.4 4.0 0.072 77.0 0.107 113.0 174.0 4.2 960.7 5.0 0.059 106 150 250 6.0 127 0.172 201 362 7.0 155 262 570 8.0 0.145 192 0.240 8.2 355.0 9.0 223 444 10.0 0.114 268 0.327 610 11.5 343 12.0 0.219 385 0.420 13.0 468 0.550 13.7 0.789 14.0 617 15.0 0.174 1142 16.0 0.333 20.0 0.249 0.471 25.0 0.320 30.0 0.462 31.3 0.563 When S2 is less than 175Nm3/t, the solubility of CO2 in methanol can also be calculated using the following empirical formula: = where S2——solubility of CO2 in methanol, Nm3/t·CH3OH ； P——equilibrium partial pressure of CO2 on the solution, mmHg ； P0——The saturated vapor pressure of liquid CO2 at the given temperature. Its value can be determined by the following formula: lg = 17.5011-1999.1/ T - Temperature, K (3) The solubility of H2, N2, CH4 and other gases in methanol is very low. The solubility data of H2, N2, and CH4 in methanol are very low. The solubility data are listed in Table 5, Table 6, and Table 7 respectively. As can be seen from the table, the solubility of H2 in methanol decreases with the decrease of temperature, which is very beneficial to reduce the loss of H2. Table 5: Solubility of H2 in pure methanol 1 10 50 80 0 0.0001515 0.001515 0.0075757 0.0121212 -20 0.0001218 0.001218 0.00609 0.0097442 -45 0.0001062 0.001062 0.0053123 0.0084997 Table 6: Solubility of N2 in pure methanol 1 10 50 80 3.8 0.0002628 0.002628 0.0137406 0.0210249 -25 0.0002646 0.002640 0.0132345 0.0211752 -45 0.000261 0.002617 0.0135889 0.0217391 -60 0.0002873 0.002873 0.00143678 0.0229885 Table 7: Solubility of CH4 in methanol 1 10 50 80 -25 0.00125 0.0125 0.00625 0.1 -30 0.0013227 0.013227 0.0661375 0.1058201 -50 0.0015105 0.015105 0.0755287 0.1208459 -60 0.0016393 0.016393 0.0819672 0.1311475 3. Principle of the process 1. Dehydration The gas entering the low-temperature methanol washing device in our plant is a wet-shift gas from a carbon monoxide shift station with a temperature of 40°C and containing about 102kg/h of saturated water vapor (in addition to some entrained water). In order to enable methanol to better absorb CO2 and H2S , the temperature of the gas before entering the methanol scrubber is required to be below -9°C. To reach this temperature, it must be pre-cooled. During the pre-cooling process, the water in the gas will freeze when it reaches the freezing point, blocking the pipeline equipment and causing difficulties in production. Therefore, the moisture of the converted gas must be removed before entering the low-temperature part. There are many methods of dehydration, such as adsorption with desiccant (silica gel, molecular sieve), which is one of them. Our factory uses the water absorption of methanol to spray methanol into the shift gas to form a methanol-water solution to lower the freezing point. The freezing point of methanol aqueous solution decreases as the methanol content increases, and the relationship is as shown in the figure: Therefore, the methanol spray volume is related to the shift gas temperature and the temperature requirements of the methanol scrubber, which is determined by the following formula: = × ； PME——Saturation vapor pressure of methanol at 283K, MPa ； PS——raw gas pressure, MPa ； E——Correction coefficient, take 2.6~2.7 ； AME——Affluence coefficient, ranging from 1.1 to 1.3. For our factory, V total=6080kmol/h ； PS＝7.77MPa ； PME=0.00686MPa. Therefore, WME=6080×0.00686/7.77×2.6×1.1=15.352kmol/h, which is about 491kg/h methanol. 2. Removal of CO2 and H2S The principle of CO2 and H2S removal has been described before and will not be repeated here. However, it is necessary to introduce the basis for determining the amount of methanol at the top of the 4115-C1 tower. design: The saturation degree of CO2 absorbed by methanol at the bottom outlet of C1 upper tower is 71%. The amount of CO2 absorbed by the lower tower is only 2.5% of the total CO2. The pressure entering the upper tower is 7.718MPa. When the gas phase CO2 concentration is 30% at -11°C, λ is 14.72Nm3/t·bar. Then the amount of methanol required for the upper tower of 15-C1 is: = =189.1t/h In fact, the concentration of gas phase CO2 entering the tower is 33.36%, so the λ value is larger than 14.72, about 16, so: = =156.5t/h, which is about 174m3/h methanol. This is the methanol circulation volume provided by Linde. in the formula: 46940 is the amount of CO2 in the raw gas entering the C1 tower, Nm3/h. 3. Regeneration of methanol-rich liquid and recovery of H2, CO2, and H2S (1) One-stage vacuum flash evaporation 1) Purpose Methanol inevitably dissolves some useful hydrogen during the process of dissolving acidic gases. For example, the amount of dissolved hydrogen in the methanol liquid at the outlet of the 4115-C1 tower in our factory is about 1326Nm3/h, which is equivalent to the amount of hydrogen required to produce 650kg/h ammonia. This part of hydrogen is desorbed with CO2, which is not only a waste, but also affects the purity of the CO2 product. Therefore, before releasing CO2, the solubility of H2 in methanol is much smaller than that of CO2 and H2S, and a first-level flash evaporation is performed to release and recover H2, creating conditions to ensure the purity of the CO2 product. 2) It has been stated before precooling that the solubility of H2 in methanol decreases as the solution temperature decreases as the temperature of the solution decreases. In order to maximize the recovery of H2 and save compression work under a certain pressure, and increase CO2 production, we also hope to desorb CO2, H2S and H2 together as little as possible, so the methanol liquid to be regenerated must be precooled from -11°C and -7°C to -31.6°C and -28°C respectively before the first-stage decompression flash evaporation. Another purpose of pre-cooling is to utilize the cold energy at a lower energy level through one-stage flash evaporation. For example, in order to make the methanol liquid reach a low temperature of -61°C at the outlet of the C3 tower, it is first cooled at E3, E7, E19, etc. to fully utilize the cold energy at the low energy level between the tubes. 3) Determination of pressure The determination of primary flash evaporation pressure mainly depends on the requirements of product CO2. Because the first-stage flash evaporation pressure is high, the amount of H2 released is relatively small. When the pressure is reduced and flashed again, more H2 will go out with the CO2, and the purity of the product CO2 will be low. On the contrary, if the pressure is low, the amount of H2 released will be large, and the CO2 concentration will be high when the pressure is released again. However, when H2 is released, a lot of CO2 is also released, which affects the output of the product CO2. Secondly, when selecting the first-stage flash evaporation pressure, the rationality of the compression ratio of the circulating gas compressor should also be considered to minimize energy consumption. The primary flash pressure of this device is 2.25MPa, the flash temperature is –28~–32℃, and the flash gas volume is 2188Nm3/h. Among them, H2 accounts for 48.77%, and the product CO2 concentration is above 98.5%. (2) Two-stage vacuum flash evaporation 1) Purpose The purpose of two-stage vacuum flash evaporation is to recover more high-quality CO2 to meet the needs of urea production. In the two-stage vacuum flash evaporation, the product CO2 amount (including part of the CO2 released by the methanol solution due to the temperature increase) is 30729Nm3/h, the concentration is 98.74%, (H2+CO) <1.11%, and the total sulfur is <1.4ppm. 2) Determination of pressure During two-stage flash evaporation, the flash evaporation temperature is determined by the first-stage flash evaporation temperature and the saturation of CO2 and H2S in methanol. On the one hand, the flash evaporation pressure must consider the amount of product CO2 that satisfies it. If the pressure is low, the amount of product CO2 will be greater, and vice versa. On the other hand, we must also consider overcoming the resistance of sending the methanol liquid to the hydrogen sulfide concentration tower and eliminating the need for several methanol transfer pumps. The resistance of sending methanol liquid from the C2 tower to C3 is about 0.1MPa, so the flash pressure of C2 must not be lower than 0.2MPa. 3) How to ensure the purity of the product CO2 and methanol-rich liquid to prevent H2S gas from being released together with CO2 during the secondary decompression flash evaporation. In order to ensure the concentration and quality of the product CO2, the absorption of CO2 by methanol does not affect the absorption of H2S and the solubility and dissolution rate of H2S in methanol are greater than that of CO2 under the same conditions. On the 59th tray at the top of the C2 tower, the H2S-free methanol liquid from the C1 tower is used to absorb the H2S gas released from the sulfur-containing methanol liquid. (3) Concentration of H2S components 1) The significance of recovering H2S. In the process of gasifying sulfur-containing residual oil to produce ammonia, a certain amount of sulfide must be generated. The harm of sulfide to ammonia production has been described before, so it is necessary to separate the sulfide. The separated sulfide gas cannot be vented, otherwise it will pollute the environment and cause great waste, because sulfur itself is an important chemical raw material and must be recycled. 2) Requirements of Claus sulfur recovery unit The sulfur-containing gas recovered from the acid gas removal section is first oxidized and then reduced to sulfur in the Claus sulfur recovery unit. The reaction formula is as follows: 2H2S + 3O2 = 2SO2 + 2H2O SO2 + 2H2S = 3S + 2H2O When the H2S concentration is too low, the above reaction temperature is difficult to maintain and excess oxygen will poison the catalyst, so the Claus sulfur recovery unit requires the H2S content in the recycled gas to be >15%. 3) Pressure reduction gas stripping In order to meet the requirements of the Claus device for H2S content, a C3 hydrogen sulfide concentration tower is installed in this device. The nitrogen gas stripping method is used in the tower to further depressurize the methanol liquid and release a large amount of CO2 and other gases into the atmosphere to achieve the purpose of concentrating H2S. The so-called gas stripping is to pass a certain amount of nitrogen into the C3 tower to destroy its original gas-liquid balance, that is, to reduce the gas phase partial pressure of CO2, H2S and other gases in the tower, so that CO2 and H2S dissolved in the liquid phase can be further desorbed, but we do not want H here 2S desorption, because our purpose is to concentrate H2S, and too much H2S is discharged into the atmosphere together with CO2, which will pollute the environment. Therefore, sulfur-free methanol liquid is used at the top of the C3 tower to absorb the H2S gas released from the solution in the tower. (4) Heating regeneration 1) Purpose: Completely desorb CO2, H2S and other gases dissolved in methanol through heating, so that methanol can be completely regenerated and create conditions for recycling. The desorbed gas is cooled, methanol is recovered and sent to the Claus sulfur recovery unit. 2) Determination of the pressure of the thermal regeneration tower. The top pressure of the thermal regeneration tower C4 is determined by the sum of the inlet pressure required by the Claus device (job number 4119) and the system resistance from C4 to job number 4119. The inlet pressure requirement for job number 4119 is 0.145MPa, and the system resistance of C4→4119 job number is 0.1MPa, so the pressure at the top of the C4 tower is determined to be 0.245MPa, and the pressure at the bottom of the tower is 0.275MPa. 3) Determination of the temperature of the thermal regeneration tower. The temperatures at the top and bottom of the C4 tower are also determined after the pressure is determined. They are all considered based on the saturated vapor pressure. The pressure at the top of the tower is 0.245MPa, and the corresponding temperature is 93°C. The pressure at the bottom of the tower is 0.275MPa, and the corresponding temperature is 103°C. The heat source comes from the S5 pipe network and is delivered through E11. 4. Methanol dehydration distillation (1) Tasks of methanol distillation tower 1) Recover spray methanol used for shift gas dehydration ； 2) Waste methanol discharged from various parts of the recovery unit ； 3) Remove impurities. Debris generated by the previous work numbers and brought into the 4115 device, such as: Heavy metal compounds such as Fe and Ni, HCN, HCOOH and moisture are all transferred into the methanol liquid at this time. In addition, when the shift gas enters the C1 tower, some water is still brought into the C1 tower, which continuously increases the water volume in the system. Therefore, 796kg/h methanol liquid (containing 0.2% water) is pumped from the bottom of the C4 tower to the top of the C5 tower. It not only serves as the reflux liquid of the C5 tower, but also removes the above impurities and balances the water volume in the system. (2) Determination of the pressure and temperature of the methanol distillation tower The methanol distillation tower uses the different boiling points of methanol and water to separate methanol and water through distillation. Since the methanol vapor from the C5 tower and the gas from the top of the C4 tower are condensed and separated together, it is determined that the pressure at the top of the C5 tower must be greater than the top pressure of the C4 tower (C5 → C4 pipeline resistance 0.04MPa), that is, 0.284MPa, and the bottom pressure is 0.314MPa. After the pressure is determined, the temperatures at the top and bottom of the C5 tower also decrease, that is, the top is 104°C and the bottom is 145°C. The heat source comes from the S10 pipe network and is delivered through E15. (3) Removal of impurities and control of acidity in wastewater. The impurities contained in the methanol liquid are distilled, and the heavy components remain at the bottom of the tower. They are discharged from the bottom of the tower to No. 0107 along with the waste water. Since the methanol liquid may contain formic acid and hydrocyanic acid, making the sewage acidic, the PH value in the sewage needs to be checked regularly. 4. Accidents that have occurred in our factory and similar devices 1. The freezing process and treatment at the check valve at the pump outlet when circulating cooling after methanol filling: At 11:35 on December 6, 1983, the E4 and E5 ammonia coolers were opened, and the system was cooled. At 0:15 on the 7th, it was found that the inlet pressure of the P3-1 pump was 0MPa, and the pump was ready to be reversed to clean the filter, but P3-1 was turned on. No liquid could be pumped out from pump 2. It was misjudged that the outlet valve plate of the P3-2 pump was faulty. At 2:40, the system was stopped because the liquid level at the bottom of the C3 tower was too high. After multiple tests, it was confirmed that the outlet valve was normal. After disassembling the pump and inspecting it, it was found that the check valve was blocked by ice. Cause analysis: Before methanol enters the system, P3-2 and other similar pumps disengage the flange to drain the water. However, because the outlet check valve of this type of pump has a good seal, the water accumulated on the sealing surface of the check valve cannot be drained. When the system cools down, the accumulated water freezes and blocks the check valve. Improvement suggestions: If the system has been circulated with water, or water may enter during pump maintenance, the pump should be reversed for half an hour to fully mix the water and methanol before cooling the system with ammonia. 2. E4 ammonia cooler tube freeze cracking process and treatment: At 5:05 on November 25, 1983, the system stopped water circulation and drainage. At 16:00, the pressure relief and drainage of tower C1 were completed. From 18:00, C2 and C4 were successively pressurized and replaced. From 0:55 on the 26th, C1 was also filled with N80 for replacement. At 2:00, a strong ammonia smell was found around E4. At 3:50, the 15-LCV-6 pilot shower sample analyzed the ammonia content of 25.3%. At 7:00, the E4 shell discharged liquid and the ammonia content was analyzed to be 87. 33%, water content 12.67%, confirmed to be a pipe leak. On the 28th, the sealing head was lifted to check for leaks. A total of 20 U-shaped pipes were cracked, all in the lower part, accounting for 3.1% of the total number of pipes. They were all welded with stainless steel plugs. After installation, the 8.7MPa pressure test was normal. Cause analysis: 1) The accumulated water in E4 cannot be drained away. Although it is purged with nitrogen, the gas may be short-circuited and cannot be purged. 2) The piping of the gas ammonia main pipe is unreasonable. It stands to reason that the E4 liquid ammonia valve has never been opened, and it is impossible for ammonia to be fed. In fact, ammonia has not been fed. However, the gas phase pipeline of the ammonia cooler is introduced from the bottom of the ammonia gas main pipe. Therefore, as long as there is gas ammonia on the gas ammonia main pipe of the whole system, the condensed liquid ammonia must first be collected into E4. Improvement suggestions: 1) Lead the branch pipe on the gas ammonia main pipe from the top, and add a drain valve at the end of the main pipe. 2) Adding a cut-off valve to the gas ammonia branch pipe of the ammonia cooler can not only prevent the above accidents, but also facilitate the maintenance and cleaning of the ammonia cooler. 3. 15-E13 ammonia cooler blocking process and treatment: On October 22, 1984, the system was in the methanol circulation cooling state. At 17:00, it was found that the pressure of the 15-C4 tower increased from 0.26MPa to 0.38MPa. It was concluded that 15-E13 was blocked. On the 25th, the relevant pipes were disassembled and inspected. It was found that there was a large amount of white crystals (NH4HCO3) in the 15-E13 process gas outlet pipeline. The sampling analysis was as follows: NH3: 33.37% CO2: 48.59%. After heat treatment, it is decomposed, the E13 process gas pipeline is cleared, and a bypass is connected from V8→E14 to maintain production. Cause analysis: During the methanol regeneration process, CO2, H2S, NH3 and other gases in the original circulating methanol are desorbed, but since there is no load, the amount of desorbed gas is small, so the relative content of ammonia is * * When the temperature of the ammonia cooler is low, CO2 and NH3 react to form NH4HCO3 crystals, which gradually accumulate until the pipe is blocked. Improvement suggestions: When starting and stopping methanol regeneration, reduce the load of ammonia cooler 15-E13, or discharge part of the gas through the V8→E14 bypass to reduce the ammonia content in the desorbed gas. 4. Process and treatment of high total sulfur content in CO2 products: November 29, 1984 8: 00～16: 00, the total sulfur in the product CO2 increased from 3.77 mg/Nm3 to an abnormal amount (0.016%), and the urea device was forced to be stopped. The reason was not found during the day shift. After the mid-shift shift, the operator found that the valve opening of 15-FCV-6 was much smaller than usual (the flow indication did not change). The instrument engineer checked that it was a leak on the negative pressure side of the flow transmitter, which caused the FR-6 indication to be falsely high. After treatment, it returned to normal. Lessons learned: Operators should pay attention to the accumulation of daily experience and be aware of the opening of some important valves. 5. Reasons and solutions for the trace high level of CO2 in purified gas: On July 28, 1985, during the process of resuming operation of two gasifiers and connecting them to the grid, the addition speed was too fast, causing 15AP-7CO2 to reach a constant value (no attention was paid to it after the analysis data came in). At this time, 4116 was cooling and conducting gas, and CO2 passed through the adsorber and was brought into the cold box. The plate heat exchanger in the cold box was clogged, and it was forced to stop to heat up and defrost. Lessons learned: When loading or unloading the system, the speed should be controlled strictly according to the operation method, and the changes in the analyzed data should be paid close attention to. 5. Technical transformation and function 1. The nitrogen charging systems of C2 and C3 towers are separated. The original design of the nitrogen used for pressurizing C2 and C3 towers when they are opened and stopped comes from 4116-E2. The pressures of the two towers are mutually restrained, which brings inconvenience to the operation. Before the test run in 1983, C2 , C3 tower nitrogen charging system is separated, and the pressurized gas source of C2 is taken directly from the N5 pipe network, making the pressures of the C2 and C3 towers independent of each other. However, because the C2 charging pipeline is not equipped with a check valve, methanol pours into the N5 pipe network when the nitrogen compressor trips, seriously affecting the parking replacement work. A check valve was installed after commissioning in 1983. 2. Add 15-E13 ammonia cooler bypass
Part of the gas is discharged through this bypass to reduce the ammonia content in the sulfur-rich gas and avoid the formation of ammonium bicarbonate crystals that block the heat exchanger. 3. 15-C5 Tower Renovation (1) The original design of the reflux liquid for the C5 tower was methanol with a water content of 0.2% from the bottom of the C4 tower. It was pumped to the top of the C5 tower with a P6 pump. As production progressed, the water content in the methanol in the system gradually increased. At its highest, the water content in the reflux methanol reached 10%, which seriously affected the normal operation of the C5 tower and the system A. For the removal of water from alcohol, in March 1987, a temporary pipeline was set up between the outlet of the P6 pump and the outlet of the P9 pump. This part of the methanol was sent to the middle of the C5 tower together with the methanol from V5. The reflux liquid of the C5 tower was partially refined methanol condensed from the methanol vapor at the top outlet, and was sent to the top of the tower through the P5 pump. When the device was running continuously at that time, this temporary measure played a certain role, but the effect was not ideal. In October 1987, with the help of the Shanghai Research Institute of Chemical Industry, after a series of measurements and calculations, this part of methanol was officially connected to the C5 tower from an appropriate location. This was implemented during the overhaul in January 1988, which solved the above-mentioned problems of C5 and brought the methanol water content at the top of the C5 tower to meet the requirements. (2) Direct steam is added to the C5 tower. After the transformation of the C5 reflux liquid, the load of the C5 tower has increased, and the capacity of the reboiler E15 is limited due to scaling and other reasons. The C5 tower has insufficient heat, which affects the separation of methanol and water. The methanol content in the wastewater at the bottom of the tower seriously exceeds the standard. In order to change the above situation, S10 was directly passed into the C5 tower in May 1988. The methanol and water distillation achieved satisfactory results. However, adding direct steam was a last resort because nearly 1t/h of steam condensate was discharged with the wastewater and wasted. ； In addition, the temperature at the top of the C5 tower must be strictly controlled during operation, otherwise water vapor will escape from the top of the C5 tower and enter the system. 4. E1→V1 adds a drain pipe. The original design of E1→V1 has a drain pipe at the inlet. Since the front system brings in a large amount of water, the separation capacity of V1 is insufficient. Therefore, it is considered to directly connect the original drain pipe to the liquid phase outlet of V1 to reduce the separation load of V1. It is currently not in operation for some reason. Liquid nitrogen washing part 1. Overview The composition of the raw material gas after washing with methanol is:: H2 CO CH4 N2 + Ar (vol)% 97.65 0.85 0.25 1.25 In order to prevent CO from poisoning the ammonia synthesis catalyst, CO must be completely removed before the raw gas is sent to the synthesis system. However, since CO is neither acidic nor alkaline, and its solubility in various liquids is also very small, it is not easy to remove a small amount of CO. Currently, three methods are mostly used in the ammonia synthesis industry, namely cuprammonia liquid absorption method, liquid nitrogen washing and methanation method. Since the partial oxidation method of our plant to produce raw gas requires oxygen, and therefore has the advantage of having an air separation device (which can provide nitrogen), the liquid nitrogen scrubbing method is particularly suitable for removing CO from the raw gas. Compared with the copper ammonium liquid absorption method or the methanation method, this method has a very high degree of purification. CO can be removed below 1 ppm, and CH4 and Ar can also be removed below 10 ppm. Therefore, the synthesis system does not need to emit purge gas, which not only reduces raw material and power consumption, but also improves ammonia synthesis efficiency. However, the liquid nitrogen washing method has particularly strict requirements on impurities such as H2O and CO2, so it is obviously very reasonable to combine this process with low-temperature methanol washing. The physical and chemical constants of the main components in the raw gas entering this device are shown in Table 1: Table 1. Physical and chemical constants of relevant gases Gas boiling point ℃ Heat of vaporization at atmospheric pressure kJ/kg Critical temperature ℃ Critical pressure atm CH4 -161.4 244.52 -82 45.8 Ar -185.8 157.43 -122.1 448.8 CO -191.5 216.05 -140.2 434.53 N2 -195.67 199.72 -147.1 33.6 H2 -252.81 456.38 -239.9 13.2 2. Theoretical basis 1. Basic principle The principle of liquid nitrogen washing of CO is to use partial condensation and mixed gases such as H2 and CO to contact liquid nitrogen based on the different boiling points of the components in the ammonia raw material gas. Similar to the distillation operation, CO is replaced by N2, thereby separating impurities such as CO, CH4, and Ar that are not needed in ammonia production. It can be clearly seen from Table 1 that the boiling point of H2 is very low, the boiling points of CO and N2 are similar but much higher than H2, and the boiling points of Ar and CH4 are higher than CO and N2. Although liquid nitrogen washing is performed at a higher pressure, and their respective boiling points are slightly different from the above table, the general trend is still the same. Therefore, when CO in the ammonia synthesis feed gas component is replaced by N2 condensation, CH4 and Ar have already been condensed in the CO fraction. Therefore, the entire liquid nitrogen washing process can be operated at a critical temperature much higher than H2. 2. The limit (purification degree) that can be achieved by gas-liquid phase equilibrium nitrogen washing of the H2-N2-CO system depends on the gas-liquid balance relationship of the H2-N2-CO ternary system under operating conditions. Therefore, it is necessary to discuss the gas-liquid balance of the above ternary system and the basic concepts of phase balance. (1) Phase law Phase law is the most basic and important law for studying and solving phase balance problems. It explains the relationship between the number of phases, the number of components and the degrees of freedom. Its mathematical expression is:: f＝C－P＋2 where f——the number of degrees of freedom, that is, the number of independent conditions that can be arbitrarily specified when a multiphase system reaches phase equilibrium. ； C——number of components ； P——Phase number. For example: For the H2-N2-CO system, the component number is 3, and when the gas-liquid phase is in equilibrium, the phase number is 2. From the phase law, it can be seen that the degree of freedom of the above ternary system is f=3-2+2. That is to say, in the above ternary system, if The two conditions of pressure and temperature are determined, such as specifying any one of XH2, XN2, XCO, YH2, YN2, and YCO. With the determination of this condition, the gas-liquid phase equilibrium state of the system is also determined. (2) Gas-liquid phase equilibrium data table 2-(1), (2), (3), (4), and (5) are gas-liquid phase equilibrium data tables, that is, several groups of gas-liquid phase equilibrium compositions in the H2-N2-CO ternary system when the temperature and pressure conditions are determined. Temperature 83.15K, pressure 21.43atm Table 2-(1) Liquid phase composition (equivalent molecule %) Gas phase composition (equivalent molecule %) H2 N2 CO H2 N2 CO 3.98 4.04 4.47 4.60 4.69 4.87 0 21.86 74.83 82.70 87.76 95.13 96.02 74.10 20.70 12.70 8.05 0 90.84 88.65 86.71 86.62 86.58 86.55 0 4.16 10.81 11.69 12.34 13.45 9.16 7.19 2.42 1.69 1.08 0 Temperature 83.15K, pressure 34.02atm Table 2-(2) Liquid phase composition (equivalent %) Gas phase composition (equivalent %) H2 N2 CO H2 N2 CO 6.09 6.72 7.06 7.45 7.61 0 21.70 40.30 61.95 81.76 3.01 71.58 52.64 30.60 10.63 93.20 91.68 90.20 90.18 89.64 0 2.85 4.95 7.10 9.32 6.80 5.17 3.85 2.72 1.04 Temperature 83.15K, pressure 95.26atm Table 2-(3) Liquid phase composition (equivalent %) Gas phase composition (equivalent %) H2 N2 CO H2 N2 CO 17.45 18.28 19.50 21.90 23.60 0 11.22 28.60 62.12 77.00 82.55 70.50 51.90 15.98 0 91.43 90.67 89.50 87.65 87.00 0 2.01 4.53 10.09 13.00 8.57 7.32 5.67 1.86 0 Temperature 83.15K, pressure 136.9atm Table 2-(4) Liquid phase composition (equivalent molecule %) Gas phase composition (equivalent molecule %) H2 N2 CO H2 N2 CO 24.90 28.15 30.00 33.17 34.46 0 22.70 35.00 56.44 65.54 75.10 49.15 35.00 10.39 0 87.90 85.08 83.56 80.90 79.77 0 4.4 7.43 15.50 20.23 12.10 10.52 9.01 3.60 0 Temperature 122.04K, pressure 34.02atm Table 2-(5) Liquid phase composition (equivalent %) Gas phase composition (equivalent %) H2 N2 CO H2 N2 CO 4.26 3.95 3.56 3.32 2.88 2.61 14.86 32.79 50.83 60.88 82.42 97.39 80.74 63.26 45.61 35.80 14.70 0 19.74 16.60 13.57 12.04 9.45 8.21 13.95 30.90 47.91 57.38 77.68 91.79 66.31 52.50 38.52 30.58 12.87 0 It can be seen from the above five gas-liquid equilibrium data tables of the H2-N2-CO ternary system: 1) Under the same temperature and pressure, as the N2 content in the liquid phase increases, the CO content in the gas phase decreases significantly until it is trace. 2) At the same pressure, the same liquid phase N2 amount and below the critical temperature of N2 and CO, as the temperature increases, the gas phase CO content also increases. This is due to the increase in temperature and the decrease in the molecular fraction of H2 in the liquid phase. Although the equilibrium molecule fraction of N2 remains unchanged, the equilibrium molecule fraction of CO in the liquid phase increases relatively. For example, at 34.02atm and 83.15K, the equilibrium molecular fraction of N2 in the liquid phase is 81.76% and H2 is 7.61 %, CO is 10.63%, and under the same pressure, at 122.04K, when the N2 rate in the liquid phase is 82.42%, H2 is only 2.88%, so CO rises to 14.70% relatively, so the CO equilibrium molecular fraction in the gas phase also rises from 1.04% to 12.87%. 3) At the same temperature and when the N2 content in the liquid phase is similar, the CO content in the gas phase decreases as the system pressure increases. In other words, in order to achieve the same equilibrium molecular fraction of CO in the gas phase, the amount of nitrogen used can be reduced at high pressures. However, as the pressure increases, the equilibrium molecular fraction of H2 in the liquid phase rises quickly, that is, the loss of H2 increases significantly, so the system pressure should not be selected too high. (3) Phase balance diagram In addition to listing the relationship between state parameters in phase equilibrium in table form, for ease of use, a diagram can also be made based on the phase balance data, which is called a phase balance diagram. For ternary systems, it is often represented by an equilateral triangle ABC. The vertices of the triangle represent the three pure components A, B, and C respectively, and each of its sides corresponds to the binary system. Suppose we take a point in the triangle and draw three straight lines that are balanced with each side of the triangle. Then the sum of the three line segments is a constant and equal to the length of one side of the triangle. Therefore, each point in the triangle corresponds to the composition of the ternary system. As shown in Figure 1-1, P is any point within the triangle. To determine the composition of point P, draw straight lines from this point parallel to each side of the triangle. Then line segments a, b, and c represent the concentrations of components A, B, and C in the system respectively, that is, A = 50%, B = 20%, and C = 30%. In order to express the equilibrium relationship of the ternary system, the pressure P and temperature T are usually fixed. As shown in Figure 1-2, certain equilibrium steam compositions Y1, Y2...Yn correspond to certain liquid phase compositions X1, The following phase diagram is drawn based on the experimentally measured data (see Table 2 above). Figure 2 The equilibrium composition of the vapor-liquid phase of the H2-N2-CO system at 83K 3. To determine the amount of liquid nitrogen, first use the equilibrium phase diagram of the H2-N2-CO system (see Figure 3): Figure 3 Determines whether the amount of liquid nitrogen is sufficient. Curves a and b in the figure represent the liquid phase and saturated vapor lines of the ternary mixture respectively. Suppose the composition of the feed gas entering the liquid nitrogen scrubber is M, then the liquid phase composition in equilibrium with M must be point N on the equilibrium knot line MN. Set point O as the CO fraction composition at the bottom of the tower, and the amount of liquid nitrogen has the following three situations:: (1) Point O coincides with point N, at which point the minimum amount of liquid nitrogen is used ； (2) If it is to the left of point N, it means that the amount of liquid nitrogen is sufficient ； (3) If it is on the right side of point N, the amount of liquid nitrogen is insufficient. The amount of liquid nitrogen can also be obtained through the material balance of the scrubber. The relationship between the material in and out of the liquid nitrogen scrubber is shown in Figure 4. From the gas physical and chemical constants, it can be seen that the boiling points of nitrogen and carbon monoxide are very different, and the heat of evaporation of liquid nitrogen and the heat of condensation of CO are also almost the same. In this way, by condensing one molecule of CO, one molecule of N2 must be vaporized, so the operation of removing a small amount of CO with liquid nitrogen can be regarded as constant pressure and carried out at a constant temperature. Suppose: + = + (1) Because the L of liquid nitrogen entering the tower does not contain CO, so: mA＝mL mG＝mD Substitute into equation (1) to get: = / •( — ) (2) XD Compared with XG, XD is very small and can be ignored. Therefore, equation (2) can be simplified to: = / • or = • / (3) When the operating conditions are constant, formula (3) can be used to calculate the theoretical amount of liquid nitrogen, for example: It is known that the amount of raw gas mG containing CO is The minimum amount of liquid nitrogen is related to temperature, pressure and CO concentration in the raw gas. Figure 5 shows the minimum amount of liquid nitrogen required to process 100Nm3 raw gas at a temperature of 83K and different pressures and CO concentrations. Figure 5 Liquid nitrogen requirements at a temperature of 83K and different pressures and CO concentrations (processing 100Nm3 raw gas). In actual operation, the number of trays is fixed, and the amount of liquid nitrogen is often adjusted to make the CO content in the top product reach the required index. 3. Process and Principle 1. Pretreatment of raw gas This liquid nitrogen washing is operated at a low temperature of -180 ~ -192°C. Such a low temperature will cause CO2, CH3OH, etc. to form solids, reduce the heat transfer efficiency of the heat exchanger, and even cause blockage of the heat exchanger channel and production interruption. Although impurities such as CO2 and water have been basically removed by low-temperature methanol washing, trace amounts of CO2 and CH3OH vapor will still be brought in, so the raw gas must be pretreated. This device uses K-154 molecular sieve to adsorb trace amounts of CO2 and CH3OH vapor, and switches adsorption and regeneration regularly. 2. Cooling of raw gas In order to ensure that the nitrogen scrubbing operation can proceed normally, the -57°C raw gas must be further cooled to below -180°C before entering the nitrogen scrubber. The cooling of the raw gas is achieved through three plate-fin heat exchangers 16-E3-A/B/C in the cold box. During normal production, the recycled CO fraction cooling capacity and cold-matched nitrogen production are used to cool the raw gas. During startup, liquid nitrogen sent by some 4111 workers is needed for cooling. 3. Nitrogen liquefaction. Liquid nitrogen washing, as the name suggests, uses liquid nitrogen as the detergent. The 4111 job number sent gas nitrogen with a pressure of 7.6MPa and a temperature of about 40°C, so it must be liquefied first. To turn gaseous nitrogen into liquid nitrogen, liquefaction conditions must first be reached, that is,: The gas nitrogen temperature drops below the critical temperature -147°C. The actual operating temperature of this device is -188°C and the pressure is 7.6MPa. In this state, the gas nitrogen is completely liquefied. 4. Liquid nitrogen washing Liquid nitrogen washing is carried out in the washing tower (C1). There are 70 trays in the tower. The first tray at the bottom is a Φ80 Linde bubble cap, and the remaining 69 trays are sieve plates. The liquid nitrogen flows from top to bottom and contacts the raw material gas from bottom to top in countercurrent, and transfers mass and heat on the tray. Because the condensation points of gases such as CH4 and Ar are relatively higher than the boiling points of liquid nitrogen, CH4 and Ar are easier to condense after gas-liquid contact. Although the boiling points of CO and N2 are similar, and the heat of vaporization of the two is also similar, the purity of the liquid nitrogen entering from the top of the tower is very high (99.999%), so the washing process is similar to the distillation operation. The higher the top, the lower the CO equilibrium molecular fraction in the liquid phase, so the CO molecular fraction in the gas phase is also lower, so that the CO content in the final gas leaving the scrubber reaches the index required by the process. 5. Allocation of nitrogen to generate cold. Allocation of nitrogen to generate cold is an ingenious method used by Linde Company to balance the cooling capacity by adjusting the H2/N2 ratio. Since nitrogen washing is operated at low temperature, there will inevitably be a loss of cooling capacity. Coupled with the incomplete heat exchange of hot and cold materials, the cooling loss of the two reaches tens of thousands of kilojoules per hour, so the cooling capacity needs to be supplemented. This device uses the Joule-Thomson effect produced by directly adding liquid nitrogen into the synthesis gas in the cold box to compensate for the above-mentioned cooling loss. 6. Recovery of cold capacity of carbon monoxide fraction As mentioned before, Linde uses cold nitrogen distribution to make up for the cold loss, but it is far from enough to just maintain the cold balance of the nitrogen washing device. A large and effective cooling capacity balance is achieved by recovering the cooling capacity of the CO fraction. This device uses the CO fraction discharged from the bottom of the nitrogen scrubber tower to produce the Joule-Thomson effect by throttling and reducing pressure to obtain a lower temperature. In addition, the nitrogen washing pressure of this device is higher than that of the general nitrogen washing process, and the tail gas pressure after decompression is lower than 0.2MPa, which significantly increases the effect of the Joule-Thomson effect, because the Joule-Thomson effect depends on the pressure and precooling temperature. 4. Accidents that have occurred in our factory and similar devices 1. The plate-fin heat exchanger is blocked. Due to the narrow flow channel, the plate-fin heat exchanger can easily cause blockage and increase the pressure drop. In severe cases, it may even cause the device to stop. During production, our factory and brother factories (Xinjiang and Ningxia) have blocked the flow channel of the plate-fin heat exchanger due to the introduction of CO2 and CH3OH, which has affected the normal production to varying degrees. Here is just one example. The incident and its handling: At 2:30 on August 20, 1985, the 4116 system was ready to be pressurized. It was found that the FCV-3 and 4 valves were opened and the amount of high-pressure nitrogen was still very small. It was confirmed that the N80 flow channel in the cold box was blocked. Sampling and analysis showed that methanol was present in N80. The liquid was drained at 5:30 and heated with nitrogen to thaw until the end of the 22nd, affecting the driving time for 48 hours. Accident causes and measures: On August 18, the nitrogen compressor was tripped for maintenance. At that time, the N80 pipeline pressure was 0, while the 4115-C1 tower pressure was 5.0MPa. Due to the leakage of the N80-1025 nitrogen filling pipeline cut-off valve, a large amount of methanol leaked out of the vent valve between the valves. In order to reduce the leakage, the operator moved the valve between the valves. The vent valve is closed very small, so that under a pressure difference of nearly 5MPa, the methanol from the 4115-C1 tower is poured into the N80 pipeline. When the 4116 is started to cool the system, the methanol in the N80 pipeline is brought into the cold box and freezes at low temperatures until it blocks the flow channel of the plate-fin heat exchanger. measure: 4116 Before using N80, discharge it first, and take a sample at AP-31 to analyze the methanol content in N80. When the methanol content here is <1ppm, N80 is allowed to be passed into the cold box system. 2. Program control chaos, the process and handling of the collapsed torch accident: At 12:30 on January 11, 1985, shortly after the adsorber system was switched from preheating to heating, a muffled sound was heard outside the central control room, and then TAZ-38 alarmed, the interlock light came on almost at the same time, a large amount of white smoke emitted from the constant light torch, and the FR-1 and FR-5 indicators plummeted. , the pressure of PRC-28 dropped. After a while, it was found that the program-controlled valves XSCV-6, 8, 10, and 11 that should be opened were in a closed state, and the program-controlled valves XSCV-4 and 16 that should be closed were in an open state. Stop the 4117, 4116, and 4115 systems to process the torch. Causes and measures: The program control system suddenly went into chaos, causing high-pressure gas to escape into the low-pressure system, causing the safety valve of the low-pressure system to trip, and the gas to be discharged into the torch. There was water accumulation in the Changming torch pipe, causing water hammer and damaging the torch. measure: A condensate discharge tank was added to the flare pipeline, and the adsorber program control system was put into automatic operation. 5. Technical transformation and function 1. V1 adsorber pre-cooling transformation The original design of the V1 adsorber must be pre-cooled with process gas for 13 hours before system gas conduction. During the pre-cooling period, the process gas is vented. In order to shorten the cooling time of the adsorber, speed up the start-up process and save start-up costs, the accumulated liquid (liquid nitrogen) during the cooling of the equipment in the cold box is used to mix with normal temperature nitrogen in the V2 discharge tank to form cold nitrogen at about -70°C. Pre-cool the adsorber to the required temperature before conducting gas. This project was implemented during the overhaul in 1986. After years of production practice, the effect is very good. Now the adsorber basically does not need process gas pre-cooling when starting up, shortening the starting time by nearly 12 hours. The flow chart after the transformation is shown in Figure 6. Figure 6 2. Renovation of nitrogen distribution pipeline (old cold box) Since the plate heat exchanger E3-A/B was put into operation, there has been a phenomenon of poor heat exchange effect, which may be caused by uneven mixing of high-pressure nitrogen and synthesis gas at the cold end of E3-B. Therefore, during the overhaul in December 1986, the relevant piping was modified, namely: Reduce the amount of high-pressure nitrogen in FRC-3, and pass the reduced high-pressure nitrogen directly into the syngas pipeline through the drain line for mixing first, and then into the heat exchanger E3-B. After the transformation, the heat exchange performance of E3-A/B has improved, saving about 1000Nm3/h of high-pressure nitrogen. The modified piping is shown in Figure 7. Figure 7 3. Interlocking transformation with low raw gas flow. The original design of the raw gas flow rate entering 4116 is lower than 9000Nm3/h, and the 4116 interlocks to stop. However, on June 21, 1985 (June 30, 1986), an accident occurred in which a gasifier tripped (4117-K1 tripped) and the 4116 feed air flow rate was low and the interlocking action occurred. After analyzing and reviewing the FR-2 record paper, it was concluded that: When a gasifier trips (or 4117-K1 trips), the gas is instantly subject to violent fluctuations, causing the flow rate to return to zero instantly, resulting in a low flow interlock action. For this reason, the instrumentation staff modified the interlocking system so that FZ-2 would only take effect when the feed gas volume of 4116 is really low (that is, when both gasifiers trip), thus avoiding unnecessary economic losses. Figure 8 shows the before and after transformation. Figure 8 Methanation Part 1. Overview CO and CO2 are poisons for ammonia synthesis and hydrogenation catalysts, so they must be removed before entering the above system. There are many ways to remove large amounts of CO2, the main ones are absorption methods, such as high-pressure water washing, hot potassium alkali method, ethanolamine method and low-temperature methanol washing, etc. Our factory uses low-temperature methanol to elute CO2 from the process gas. ； Methanation can also be used to remove trace amounts of CO2. CO removal generally uses methods such as copper ammonia liquid washing, liquid nitrogen washing and methanation. Our hydrogen supply unit to the oil refining is installed after methanol washing. CO is about 0.77% and CO2 is trace. Therefore, methanation method is used to remove CO and CO2. The task of the methanation process is to remove the residual CO2 and CO in the gas after conversion and desulfurization and decarbonization (CO2), and to produce qualified hydrogen. This part of hydrogen can be sent to refineries as a raw material for hydrogenation or as a raw material for ammonia synthesis. The advantages of methanation are: Low equipment investment, simple operation, no pollution, etc. However, there are also disadvantages in applying this method to remove carbon oxides.: The methanation reaction itself consumes a portion of hydrogen and has high energy consumption. ； In addition, if the methanated gas is sent to the downstream system to be used as synthesis gas, the generated methane will be mixed into the synthesis gas and accumulated in the synthesis loop to be discharged, and a part of the hydrogen and nitrogen will be lost. Therefore, methanation is only applicable when there are not many carbon oxides in the process gas. 2. Basic principles 1. Chemical reaction The chemical equation of methanation reaction is: CO + 3H2 = CH4 + H2O (steam) ΔH2980 = -206.16kJ/molCO CO2 + 4H2 = CH4 + 2H2O (steam) ΔH2980 = -165.08kJ/molCO2 In addition, if oxygen is present, the following reaction will also occur: 2H2+O2=2H2O (steam) ΔH2980=-241.99kJ/molO2 These reactions are all strongly exothermic reactions, causing significant temperature rise. The value of the adiabatic temperature rise is roughly: 72°C per 1% CO2 60°C per 1% CO2 165°C per 1% O2 2. Chemical equilibrium of methanation reaction The equilibrium constant of the methanation reaction decreases as the temperature increases, but it is quite large below 500 to 600°C. Therefore, it can be considered that in this temperature range, as long as the reaction is fast enough, trace amounts of CO and CO2 can always be reduced to extremely low levels. The equilibrium constant of the methanation reaction is shown in the table below (1): Table (1) CO＋3H2＝CH4＋H2O CO2＋4H2＝CH4＋2H2O Temperature ℃ KP KP 200 2.15×1011 9.48×108 300 1.52×107 3.87×105 400 1.69×104 1.44×103 500 1.02×102 2.10×10 600 1.92 0.761 The methanation reaction is a volume reduction reaction. At a certain temperature, when the pressure is increased, the equilibrium content of carbon oxides in the reaction mixture decreases, and the equilibrium content of carbon oxides is inversely proportional to the pressure. Because the content of H2 in general production is too much, even under low pressure conditions, the equilibrium content of the two carbon oxides is still very low. 3. Side reactions (1) Carbon evolution reaction 2CO=CO2+C This reaction is extremely slow at 200 to 500°C. Therefore, this reaction will not occur at the normal operating temperature of methanation. However, if the temperature exceeds 500°C, this possibility exists. (2) Reaction to generate nickel carbonyl Ni+4CO=Ni(CO)4 The lower the temperature and the higher the pressure, the more favorable it is for the generation of nickel carbonyl. However, the operating temperature of methanation is generally above 250°C, so nickel carbonyl will not be generated in actual production. However, special attention should be paid when heating up. Only when the bed temperature is higher than 250°C can the process gas be used to continue heating. 3. Methanation Catalysts The methanation reaction is actually the reverse reaction of methane steam conversion, so the catalysts used for methanation also have a catalytic effect on the methanation of carbon oxides. Methanation catalysts for carbon oxides should have the following characteristics: First, the allowable content of carbon oxides leaving the methane reactor is extremely small, so the methanation catalyst must have high activity. ； Second, the reaction between carbon oxides and hydrogen is a strongly exothermic reaction, requiring the methanation catalyst to withstand a large temperature rise. Methanation catalysts that meet the above requirements must have active components that are highly dispersed in order to have good surface area and pore volume, and the carrier can withstand high temperatures. Nowadays, methanation catalysts are composed of nickel oxide supported on a refractory material carrier. The nickel content is higher than that of methane conversion catalysts, generally 15 to 30% (calculated as Ni). The catalyst can be tableted, extruded, or formed into spheres. The particle size is generally around 6mm. Except for the pre-reduced form, the nickel in the methanation catalyst exists in the form of NiO. Before use, reduce it with hydrogen or decarbonized feed gas. The reaction formula is as follows: NiO＋H2＝Ni＋H2O ΔH2980＝-1.26kJ/mol NiO＋CO＝Ni＋CO2 ΔH2980＝-38.5kJ/mol Although the thermal effect of these reduction reactions is not large, the catalyst is active once reduced. When reducing with raw gas, in order to avoid the bed temperature rising too high, the carbon oxide content must be controlled below 1% as much as possible. The reduced nickel catalyst will spontaneously ignite and should be protected from contact with oxidizing gases. When an accident occurs in the previous process and a high concentration of carbon oxides enters the methanation reactor, the bed temperature will rise rapidly. At this time, measures should be taken immediately to cut off the feed gas. The reduced catalyst cannot be heated with gas containing CO to prevent the formation of nickel carbonyl at low temperature. Nickel carbonyl Ni(CO)4 is a volatile substance at normal temperature, with a freezing point of 25°C and a boiling point of 43°C at normal pressure. The saturated vapor pressure is 320mmHg at 20℃. Nickel carbonyl is not only a catalyst poison, but also very harmful to the human body. The toxic dose is 1 ppb. The initial symptoms of poisoning are headache, dizziness, nausea and vomiting, fever and difficulty breathing. In severe cases, it can be fatal. In addition to the poison whose carbonyl group is the methanation catalyst, it can also be poisoned by sulfur, arsenic and halogens, even in trace amounts. * * Reduce catalyst activity and life. Sulfur is much more harmful to methanation catalysts than to methane conversion catalysts. Because of its lower operating temperature, continued operation with sulfur-free gas will not restore catalyst activity, but this is not the case for conversion catalysts. Therefore, the harm of sulfur to methanation catalysts is cumulative. Adsorption of 0.5% sulfur (based on the weight of the catalyst) results in a complete loss of methanation catalyst activity. For arsenic, when the adsorption amount reaches 0.1%, the catalyst activity will be lost. In the purification process using low-temperature transformation, generally no sulfur will enter the methanation reactor. However, when the decarbonization system uses the sulfone amine method and the arsenic-alkali method, care must be taken to avoid bringing in these solutions containing sulfur or arsenic. When using these methods to remove CO2, in order to prevent poisoning, zinc oxide can also be added as a protective agent in front of the catalyst bed. Due to the strict refining of the raw gas in the previous process and the limitation of the carbon oxide content in the imported gas, poisoning and sintering of the methanation catalyst will not occur under normal circumstances, and the methanation catalyst itself has high strength and the life of the catalyst can reach 3 to 5 years. When taken out or prepared for reuse, the catalyst should be passivated by pre-oxidation in a controlled manner. The reaction formula is as follows: 2Ni+O2=2NiO ΔH2980=240.7kJ/mol 4. Process 1. Process conditions Operating pressure is closely related to the preceding and following processes. The main process condition that needs to be considered is temperature. The nickel catalyst used for methanation is already active at 200°C and can withstand high temperatures of 800°C, but the temperature is not determined based on the catalyst performance. In actual production, the lower limit should be higher than the temperature at which nickel carbonyl is generated, and the upper limit should be lower than the design temperature allowed by the reactor material, which is generally in the range of 280 to 420°C. When the allowable adiabatic temperature rise is 140°C, assuming that the CO2 content in the inlet gas is 0.2%, the temperature rise caused by the CO2 methanation reaction can be calculated to be 12°C, and the allowable CO content due to the CO methanation reaction is 128/72 = 1.78%. However, CO methanation consumes three times the H2 and generates useless CH4. Therefore, it is neither economical nor safe to remove such high CO content by methanation. This is why methanation is widely used only after low-temperature transformation operation is adopted. Methanation operation is simple and does not require special attention. If other conditions are normal and the temperature difference between the inlet and outlet of the reactor gradually decreases from large to small, this means that the catalyst activity decreases. If it suddenly becomes smaller or the bed hot spot moves downward, it is a sign of catalyst poisoning. 2. According to the calculation of the process flow, as long as the raw gas contains 0.5 to 0.7% carbon oxides, the heat released by the methanation reaction can be enough to preheat the imported gas to the required temperature. Therefore, the process only requires a methanation furnace, inlet and outlet gas heat exchangers, water coolers, and water separators. However, considering the temperature rise and reduction of the catalyst and the fluctuation of the carbon oxide content in the feed gas, other heat sources are required. Generally, the heat source can be high variable gas or S100 steam.