Q&A on Methanol Production Operations (Synthesis)

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1. What is the main task of the synthesis section? Answer: The synthesis section converts the feed gas containing H2, CO, and CO2 (3.45 Mpa, 40°C, 81252.26 Nm3/h) into crude methanol under certain conditions of pressure (5.9 Mpa), temperature (220–260°C), and in the presence of a catalyst (NC306). The heat generated by this reaction is used to produce medium-pressure steam at 2.1–3.9 Mpa; this steam is then reduced in pressure to 0.7 Mpa before being fed into the steam network. 2. What are the main reaction equations for methanol synthesis and its influencing factors? Answer: (1) CO + 2H2 → CH3OH + Q; (2) CO2 + 3H2 → CH3OH + H2O + Q. Influencing factors: operating temperature, operating pressure, catalyst performance, space velocity, and the hydrogen-to-carbon ratio of the feed gas. 3. Characteristics of synthetic reactions: Answer: (1) Reactions with volume reduction ； (2) Exothermic reaction ； (3) Reversible reactions ； (4) Gas and solid-phase catalytic reactions ； (5) Multiple side effects occur. 4. What are the main control points in the synthesis section? Answer: (1) Inlet and outlet temperatures of the synthesis tower ； (2) Drum level ； (3) Drum pressure ； (4) Separator inlet temperature ； (5) Separator level ； (6) System pressure ； (7) Hydrogen-to-carbon ratio of feed gas ； (8) Expansion tank pressure ； (9) Bleed air pressure. 5. What is the function of the compressor cycle section? Answer: Inside the synthesis tower, the reaction takes place in a reduced volume; coupled with the condensation and separation of methanol as well as system resistance, the pressure after the reaction decreases. To maintain stable system pressure, in addition to supplying fresh gas, it is necessary to use the circulation section to compress the remaining gas from the reaction and send it back to the synthesis tower for reuse, thereby increasing the overall conversion rate of the gas. 6. What is the definition of air velocity and its impact on methanol synthesis? Answer: Space velocity: The flow rate of gas passing through a unit volume of catalyst per unit time. Increasing the space velocity reduces the one-way conversion rate and slows down the catalytic reaction, which helps to protect the catalyst and increase yield. However, increasing the space velocity raises the energy consumption during the circulation phase; if the space velocity is too high, the reaction temperature drops significantly, and sometimes it becomes difficult to maintain this temperature, resulting in a decrease in yield. 7. What is the impact of pressure on methanol production? What are the principles for selecting pressure? Answer: The methanol reaction is a reaction with a decrease in the number of molecules, and increasing pressure is favorable for the forward reaction. If the pressure increases, the partial pressure of the components rises, and as a result, the production intensity of the catalyst also increases. For the operation of the synthesis tower, pressure control is adjusted accordingly based on the different catalytic activities at various stages of catalyst use; when the catalyst is new and has high activity, the operating pressure can be lower ； As the catalyst ages, its activity declines, and higher operating pressures are often used to maintain a certain level of production intensity. In summary, the selection of operating pressure must be determined based on specific factors such as catalyst activity, gas composition, reactor thermal balance, and system energy consumption. 8. What is the impact of temperature on methanol production? What are the principles for selecting temperature? Answer: Among the process parameters used to regulate the methanol synthesis reaction, temperature has a significant impact on both the equilibrium and the rate of the reaction. Both the reactions involving the formation of methanol from H2 and CO, as well as those involving the formation of methanol from H2 and CO2, are reversible exothermic reactions. For reversible exothermic reactions, increasing the temperature increases the reaction rate constant, but reduces the value of the equilibrium constant. When the composition of the reaction mixture remains constant and the temperature is changed, the reaction rate is influenced by these two conflicting factors. Therefore, an optimal operating temperature is required. The so-called optimal temperature is the temperature at which the reaction rate is highest for a given composition of the reaction mixture. Studies show that the optimal temperature depends on the composition; under the same initial composition, it is related to the reaction rate. When the methanol content is low, the effect of equilibrium is relatively small, so the optimal temperature is higher. As the reaction progresses and the methanol content increases, the influence of equilibrium grows, resulting in a lower optimal temperature. That is, high first and then low. To achieve the optimal temperature, it is also necessary to take into account the properties and lifespan of the catalyst. At the beginning of its use, the catalyst has high activity, so the reaction temperature can be lower; as the catalyst ages, the temperature needs to be increased appropriately. For copper-based catalysts, the operating temperature is around 220–240°C at the initial stage, about 250°C during the intermediate stage, and can be raised to 260–270°C in the later stages. 9. What are the components of the inert gases in the circulating gas? What are the impacts on synthetic methanol? Answer: The inert gases are CH4, N2, Ar. Inert gas components do not participate in the synthetic reaction, but they affect the reaction rate. Too high an inert gas content reduces the reaction rate, increasing the energy consumption required to produce a given amount of output ； To maintain a low inert gas content, the vent volume increases, resulting in greater loss of useful gases. Generally speaking, the appropriate level of inert gas content depends on specific circumstances, and it is also one of the means used to adjust operating conditions; at the beginning of catalyst use, when its activity is high, a higher level of inert gas content can be tolerated ； In the later stages of catalyst use, a low level of inert gas is generally maintained. If high yield is the goal, the inert gas content can be kept low; if low consumption is the goal, a higher inert gas content can be maintained. 10. In what ways does the presence of a small amount of CO2 in the feed gas used for methanol synthesis have a positive effect on methanol production? Answer: (1) From the reaction equation, CO2 can also participate in the reaction to produce methanol. The synthesis of methanol from CO2 requires one more molecule of H2 compared to using CO, and at the same time, one molecule of H2O is produced. Therefore, when the hydrogen content in the feed gas is low, it is necessary to use more H2 and CO to produce methanol. (2) The presence of CO2 inhibits the formation of dimethyl ether to a certain extent. Since dimethyl ether is the product of the dehydration reaction of 2 molecules of methanol, the reaction between CO2 and H2 to produce methanol generates 1 molecule of H2O, and the presence of H2O plays a positive role in suppressing the dehydration reaction of methanol. (3) It prevents CO from being converted into CO2, a reaction that occurs in the presence of H2O. (4) It is more effective at regulating temperature, preventing overheating, protecting the activity of copper-based catalysts, and extending their service life. (5) It can prevent catalyst carbonization. Therefore, the CO2 content in the feed gas is generally controlled at around 3%. 11. What is the adverse effect of CO2 presence on the coupling reaction? Answer: ① Compared to the synthesis of methanol from CO, 0.7 m3 more of H2 is required per kilogram of methanol produced ； ②An increase in the water content in crude methanol and a decrease in the methanol concentration lead to higher steam consumption. 12. What determines the hydrogen-to-carbon ratio composition of the fresh gas? What is the purpose? What is the value of the indicator? Answer: A: It is determined based on the material balance of the entire synthesis system. B: To meet the requirements of chemical reactions and maintain stable production of the system. C: (H2-CO2)/(CO+CO2) = 2.05~2.15. 13 Why should gases be recycled? Answer: Due to balance and rate limitations. The one-pass conversion rate in the synthesis tower is not high, especially when operating at low pressures; therefore, the useful gas needs to be recycled after the products have been separated, in order to increase the gas conversion rate. 14. Why is the purge gas installed after the separator? What is its purpose? Answer: Since the gas exiting the separator is the residual gas remaining after the reaction, and most of the methanol has been separated out, the inert gases have the highest concentration at this point. Therefore, it is cost-effective to set the purge gas at this level, and it also facilitates the stable operation of the system. 15. Why is the liquid level in the methanol separator an important process parameter? What is the value of the indicator? Answer: If the liquid level in the separator is too high, the liquid will be carried into the circulation section along with the gas; in severe cases, this can lead to liquid slugging, which damages the compressor. Moreover, an increased methanol content in the gas entering the tower deteriorates the reactions taking place within the synthesis tower and exacerbates the formation of side reactions ； If the liquid level is too low, air ingress accidents are likely to occur; high-pressure air entering low-pressure equipment systems can lead to explosions. Therefore, the liquid level should be maintained between 30% and 50%. 16. What are the various forms of drum blowdown, and what is their purpose? Answer: There are continuous discharge and intermittent discharge. Function: ① Regulate water quality and improve heat transfer efficiency. ②Fine-tune the drum temperature. 17. How to increase the air velocity? Answer: (1) Reduce the flow rate of the circulation pump ； (2) Increase the fresh air volume ； (3) Reduce the amount of bleed gas (increase system pressure). 18. Which substances in the methanol plant are explosive and prone to causing poisoning? Answer: Flammable and explosive substances: H2, CO, CH4, CH3OH, CH3OCH3. Substances prone to causing poisoning include: CO, CH3OH. 19. What is the function of a methanol expansion tank? Answer: (1) Reduce pressure. (2) Flashing releases the gases dissolved in the methanol solution. 20. Why can the temperature of the synthesis tower be controlled using the drum pressure? Answer: The boiler water in the shell side of the synthesis tower absorbs the heat of reaction generated during methanol synthesis in the tube side, turning into boiling water at a certain temperature. This boiling water rises and enters the drum, where it forms saturated steam at a temperature corresponding to that of the boiling water; the pressure indicated by this steam is the drum pressure. The heat of reaction within the reactor is removed by the steam generated as boiling water rises, thereby keeping the bed temperature stable. Therefore, the bed temperature is directly affected by the temperature of the boiling water, and it changes as the temperature of the boiling water changes. Within a steam drum, for boiling water at a certain temperature, there is a corresponding saturated vapor pressure at that temperature, and the two influence each other in a one-to-one relationship. When the temperature of the boiling water rises (or falls), the temperature of the boiling water will correspondingly rise (or fall) as well, which in turn affects the rise (or fall) of the bed temperature. Therefore, by adjusting the drum pressure, the catalyst bed temperature can be adjusted accordingly. Generally, for every 0.1 Mpa change in the drum pressure, the bed temperature changes by 1.5°C accordingly. 21. What is the model and composition of the catalyst (Cat) used in our factory? Answer: The catalyst model used in our factory is the NC 306 copper-based catalyst. Composition: CuO≥50%, ZnO≥25%, Al2O3≥4%; it also contains small amounts of graphite and water. 22. What are the factors that affect the lifespan of a Cat? Answer: (l) Cat poisoning ； (2) Thermal aging ； (3) Strength of Cat ； (4) Frequency of start-up and shutdown ； (5) The process parameters are not strictly followed, and the operations are improper. 23. What substances can poison cats? How to extend the lifespan of a Cat during operation? Answer: Toxic substances include Cl, S, amines, Fe, and oils (which cause carbon formation and block the active surface). Methods to extend the lifespan of the Cat during operation are: (1) Follow the procedure strictly when loading the Cat to prevent bridging. (2) During the temperature-raising reduction process of Cat. First, ensure that S in the reducing gas is ≤0.1 PPm. Second, follow the heating and reduction procedure strictly; remember that increasing the temperature and adding hydrogen cannot be done simultaneously. To achieve a complete restoration and enable it to exert its catalytic effect fully. (3) Since Cat has high activity in the initial stage of use, its activity gradually decreases over time; therefore, the operating temperature for Cat should be increased over time. (4) During operation, sudden rises or drops in temperature should be avoided, and overheating is strictly prohibited. 24. Name the hazards of sulfur (S), chlorine (Cl), and oil to Cat Answer: Sulfur (S) and chlorine (Cl) react with Cu in Cat to form inactive substances: Cu + S = CuS ； Cu+2 + Cl = CuCl2; when the oil is heated, it decomposes to release carbon, which blocks the active centers of the catalyst and reduces its activity. 25. State the basic procedures for using Cat: Answer: (1) Purge and clean the synthesis circuit ； (2) Loading of Cat ； (3) Ca heating reduction ； (4) In the initial stage of Cat operation, the temperature is 240°C; in the intermediate stage it is 250°C; and in the final stage it is 260°C, with the circulation volume gradually increasing ； (5) Cooling passivation ； (6) Unload and replace. 26. What are the two main characteristics of Cat? Answer: (1) It does not participate in the reaction itself, and its physical and chemical properties remain unchanged before and after the reaction ； (2) It alters the reaction pathway, accelerating the reaction rate. 27. How should the valves in the water system of the synthesis tower be set before heating and reduction in a Cat reactor? Answer: (1) Open the drum vent valve ； (2) All valves of the drum pressure control valve are closed ； (3) The isolation valves before and after the feed water control valve on the drum, as well as the bypass and drain valves, are all closed ； (4) Drain valves on the shell side of the drum and synthesis tower closed ； (5) Add deionized water to the steam drum using a temporary water supply line ； (6) Close the steam valve leading to the injector. 28. Why is cold water added to the drum before heating and reduction in a Cat system? Answer: An important aspect of maintaining Cat activity is to prevent sharp temperature rises during the heating process; otherwise, its mechanical properties will be affected and its service life will be shortened. Adding cold water serves mainly to ensure even temperature rise and prevent damage to Cat. 29. What are the “three” principles of the Cat reduction process? Answer: (1) Three lows: low-temperature effluent, low-temperature reduction, and a period of low-load operation after reduction ； (2) Three stabilities: temperature stabilization, H2 supplementation stabilization, and water output stabilization ； (3) Three prohibitions: It is prohibited to raise the temperature while introducing H2, it is prohibited to allow moisture to enter the tower, and it is prohibited to discharge water at high temperature for an extended period of time ； (4) Three controls: controlling the hydrogen replenishment rate, controlling the CO2 concentration, and controlling the hourly water output. 30. What precautions should be taken regarding the liquid waste generated during heating reduction? Answer: (1) First, prepare the device ； (2) When there is a liquid level in the separator, start discharging it, and carefully weigh and record it ； (3) When releasing it, one should stand upwind to avoid poisoning and suffocation ； (4) Residual liquid must not be discarded randomly; it should be discharged at designated locations to prevent fires ； 31. Judgment of the end point for temperature-driven reduction in cats and precautions: Answer: Judgment of the end point: (1) The percentages of CO + H2 at the outlet of the synthesis tower are basically the same ； (2) The liquid level in the separator (V7002) is no longer rising ； (3) Analyze the water content. Precautions: (1) During the heating process, strictly follow the heating and reduction curve. (2) It is required that the temperature rise be balanced and the water output even, in order to prevent too rapid a temperature increase, which could lead to too fast water output and a decrease in the mechanical strength of the Cat. (3) Throughout the entire process, it is carried out strictly in accordance with the principle of separating hydrogen and raising temperature at different times. (4) Keep proper special records and draw the actual temperature rise curve. 32. What are the purposes and methods of cat passivation? Answer: Purpose: To prevent strong oxidation reactions that occur when Cat comes into contact with the atmosphere after being unloaded, which could lead to fires or equipment damage. Method: A fixed amount of air was introduced into the synthesis system, and a N2 positive-pressure cycle was gradually passed through Cat to oxidize it to the oxide state (CuO). 33. What should the system pressure be maintained at during cat passivation? Answer: The system pressure should be maintained at 0.5 Mpa. 34. During the Cat passivation process, what should be done if a sudden increase in the temperature of the synthesis tower is observed? Answer: Immediately stop oxygen addition, increase the water supply to the drum, raise the drum’s blowdown rate, and increase the drum’s vent volume. 35. Why shouldn’t the temperature be too high at the beginning of using Cat? Answer: Raising the initial temperature during use will accelerate the growth of copper grains, that is, it will speed up the rate of activity degradation. 36. How should high S content in fresh gas be handled? Answer: The supply of fresh gas should be stopped immediately, the purification system should be vented, and corresponding actions should be taken as if it were a short-term shutdown. 37. What is the safety valve setting for the synthetic steam drum? What is the safe valve value for the swelling groove? Answer: The drum valve setting is: 4.8 Mpa at the initial stage (pressure at 260°C), and 5.1 Mpa in the middle and later stages ； The expansion tank value is: 0.6 MPa. 38. What is chemical equilibrium? How do concentration, pressure, and temperature affect chemical equilibrium? Answer: For a reversible reaction, the state in which the rate of the forward reaction is equal to the rate of the reverse reaction (that is, the number of molecules consumed in the forward reaction per unit time is equal to the number of molecules produced in the reverse reaction) is called chemical equilibrium. Concentration: An increase in the concentration of the reactants shifts the chemical equilibrium to the right ； As the concentration of the product increases, the chemical equilibrium shifts to the left. Pressure: Increasing pressure shifts the chemical equilibrium in the direction of reduced volume ； By reducing the pressure, the chemical equilibrium shifts in the direction of increased volume. Temperature: Increasing the temperature shifts the chemical equilibrium in the direction of endothermic reactions ； Lowering the temperature shifts the chemical equilibrium in the direction of exothermic reactions. 39. What is the rate of a chemical reaction? How does a catalyst affect the rate of a chemical reaction? Answer: The rate of a chemical reaction is expressed by the decrease in the concentration of the reactants or the increase in the concentration of the products per unit of time. Unit: mol·L-1·s-1 or mol•L-1•min. Catalysts can alter the reaction pathway, reducing the activation energy of the reaction and increasing the rate of chemical reactions (both the forward and reverse reaction rates are accelerated), while their composition, mass, and chemical properties remain unchanged before and after the reaction. Catalysts also possess selectivity. 40. How is the temperature raised in the synthesis tower? Answer: (1) Use a temporary water supply line to feed deionized water into the steam drum, maintaining the liquid level at 30%. (2) Start the compressor to establish a nitrogen circulation. (3) Open the steam nozzle and inject steam, with a heating rate of 25°C/h. 41. Process flow for methanol synthesis: Answer: Water gas generated by the pressurized gasification of water-coal slurry in a new type of gasifier is purified to produce synthetic gas with a total sulfur content of less than 0.1 ppm, and a hydrogen-to-carbon ratio of (H2-CO2)/(CO+CO2) equal to 2.05–2.15. After being pressurized by the 5-stage impellers in the compression section of the turbine compressor, it mixes in the cylinder at a certain ratio with the recycled gas coming from the methanol separator (40°C, 4.6 Mpa). It is then pressurized to 5.20 Mpa by the 1-stage impeller in the recycling section, and sent to buffer tank V7004, where it becomes gas with a pressure of 5.15 MPa and a temperature of around 60°C, ready to be fed into the tower. The gas entering the tower flows at a rate of 528,903 Nm3 per hour into the shell side of the inlet preheaters E7001A and B. There, it is heated to 225°C by the hot gas exiting the reactors in the synthesis towers (R7001A, B), after which it enters the top of the synthesis towers (R7001A, B). R7001A and B are vertical insulated – shell-and-tube reactors. The tube is filled with an NC306-type low-pressure synthetic methanol catalyst. When the syngas enters the catalyst bed, at 5.10 MPa and 220–260°C, CO, CO2, and H2 react to produce methanol and water, along with trace amounts of other organic impurities. Both reactions for synthesizing methanol are highly exothermic, and most of the heat released by these reactions is carried away by the boiling water on the shell sides of the synthesis towers R7001A and B. The catalyst layer temperature and the outlet temperature of the synthesis tower are controlled by regulating the drum pressure. The hot reaction gases exiting R7001A and B enter the tube side of the inlet tower preheater (E7001A, B), where they exchange heat in counterflow with the syngas entering the tower; as a result, their temperature is reduced to around 90°C, at which point some of the methanol condenses into a liquid. This gas-liquid mixture is further condensed in water coolers (E7002A, B) to a temperature of ≤40°C, before entering the methanol separator (V7002) to separate crude methanol. The gas from which crude methanol has been separated has a pressure of approximately 4.60 MPa and a temperature of around 40°C; it is returned to the circulation section of C7001, where it is pressurized before being reused in the system. To prevent the accumulation of inert gases in the synthesis system, a small amount of recycle gas must be continuously discharged from the system: one portion is discharged directly to the distillation section (V8006), while another portion is washed with water in a wash tower to remove methanol before being sent as vent gas to the gas power generation network. The pressure of the entire synthesis system is controlled by the vent gas discharge control valves (FV7004 and PV7005). The crude methanol separated by V7002, along with the liquid crude methanol discharged from the bottom of the water wash tower (T7001), is reduced in pressure to 0.4 MPa and then sent to the methanol expansion tank V7003 to remove most of the gases dissolved in the crude methanol. Thereafter, it is sent directly to the methanol processing unit or to the crude methanol storage tanks V8001A and B. The pressure of V7003 is controlled at 0.4 MPa by PV7007, and the expanded gas is discharged into the fuel gas system as fuel. The shell sides of the drum vessels (V7001A, B) and the methanol synthesis towers (R7001A, B) are connected by two drain pipes and six vapor-liquid riser pipes to form a natural circulation boiler, which generates 4.0 MPa medium-pressure steam; this steam is then reduced in pressure to 1.3 MPa before being fed into the steam distribution network. The boiler feed water used for the drum comes from the main boiler feed water pipe, with a temperature of 104°C and a pressure of 5.0 MPa. To ensure the quality of the boiler water, continuous and intermittent blowdown systems are installed at the lower parts of V7001A and B, as well as at the lower parts of R7001A and B. The continuous blowdown is discharged into the blowdown expansion tank V7006, while the intermittent blowdown can be discharged directly into the sewer system. The catalyst becomes active only after reduction. Therefore, before use, the synthetic catalyst must be reduced using H2. The reduction reaction is as follows: CuO + H2 = Cu + H2O. The reduction pressure is 0.54 MPa, the maximum reduction temperature is 230°C; the reducing gas is H2, while the diluting gas is N2. It is necessary to follow the specified procedure for heating and reducing the catalyst strictly. The heating of the catalyst in the synthesis tower is achieved by using medium-pressure steam at 3.5 MPa; this steam passes through the steam injectors M7001A and B to drive the circulation of boiler water, thereby gradually raising the temperature of the catalyst layer. Three additional pipelines are connected to the inlet pipeline of the C7001 cycle section as well. An N2 pipeline, used for purging the system before startup and for supplying N2 to carry out temperature elevation and reduction of the catalyst ； A reduction pipeline to supply H2 during the catalyst heating and reduction process ； An air line to supply O2 during catalyst deactivation. To prevent overheating of the catalyst due to the depletion of liquids in V7001A and B, automatic low liquid level interlocks are installed for these units ； When the level of V7002 is too high and excessive liquid is present, liquid slugging can occur, damaging the compressor and affecting the proper operation of the catalyst. Therefore, V7002 is equipped with an automatic high-level interlock that shuts off the feed gas automatically when the level becomes too high. 42. Why is the control temperature of the synthesis tower set at 220~260°C? Answer: Among the process parameters of the methanol synthesis reaction, temperature has a significant impact on both the chemical equilibrium of the reaction mixture and the reaction rate. From the perspective of chemical equilibrium, lower temperatures are favorable for methanol yield, while higher temperatures are beneficial for the reaction rate. To determine an appropriate temperature, it is necessary to take into account the properties of the catalyst as well. The reactions of CO and CO2 with H2 to form methanol are both reversible exothermic reactions. Therefore, although an increase in temperature raises the reaction rate constant, it reduces the value of the equilibrium constant. As a result, when the composition of the reaction mixture remains constant and the temperature is changed, the reaction rate is influenced by these two opposing factors. At lower temperatures, as the value of the equilibrium constant increases, the reaction rate rises with increasing temperature, while the equilibrium constant gradually decreases. If the temperature is raised further, the reaction rate declines again as temperature increases. Thus, within the range of lower temperatures, there is a temperature at which the reaction rate is highest; this is the optimal temperature. The optimal temperature can be defined as the temperature at which the reaction rate is highest for a reaction mixture of a given composition; this temperature is referred to as the optimal temperature for that composition. As the reaction progresses and the gas composition changes, the corresponding optimal temperature also changes. The curve that represents these optimal temperatures is known as the optimal temperature line. The closer the temperature operation line is to this optimal temperature line, the higher the yield and the faster the reaction rate. To achieve the optimal temperature, it is also necessary to take into account the properties and lifespan of the catalyst. At the beginning of its use, the catalyst has high activity, so the reaction temperature can be lower; as the catalyst ages, the temperature needs to be increased appropriately. For copper-based catalysts. The initial operating temperature is 230–240°C, it is around 250°C in the intermediate stage, and the operating temperature can be increased to 260–270°C in the later stage. 43. Why is the pressure controlled at 5.3 MPa in the low-pressure method? Answer: Pressure is also an important process parameter in the methanol synthesis reaction. The methanol synthesis reaction is a reaction with a decrease in the number of molecules; increasing pressure is favorable for the forward reaction. As the pressure rises, the partial pressures of the components increase as well, thereby increasing the catalyst’s production efficiency. Generally, for every 10% increase in reaction pressure, the production capacity of the synthesis tower increases by 10%. Selection of operating pressure. It is related to the catalyst temperature. In the early years, zinc-chromium catalysts were used, with an initial activity temperature of around 320°C, due to limitations imposed by the reaction equilibrium. Only 25~30 Mpa can be selected. At higher pressures and temperatures, carbon monoxide and hydrogen produce by-products such as dimethyl ether, methane, and isobutanol. The heat released in these side reactions is greater than that of the methanol synthesis reaction, which raises the temperature of the bed and accelerates these side reactions. If not controlled in a timely manner, this can lead to a sharp increase in temperature and damage to the catalyst. Copper-based catalysts, which have been widely used in recent years, operate within an activity temperature range of 220–270°C, with the operating pressure maintained at around 5.0 MPa, thereby effectively controlling the occurrence of side reactions. For the operation of the existing synthesis towers in our plant, the pressure is adjusted appropriately according to the varying catalytic activity of the catalyst at different stages. At the beginning of catalyst use, when its activity is high, a lower operating pressure can be used; as the catalyst’s activity declines over time, a higher pressure is typically applied. In summary, the level of synthesis pressure is determined by factors such as catalyst activity, load level, space velocity, the efficiency of condensation and separation, the hydrogen-to-carbon ratio of the gas entering the reactor, and the content of inert gases; it is also possible to adjust the production volume within a limited range in order to facilitate reduced consumption. 44. Why is the hydrogen-to-carbon ratio of the fresh gas controlled between 2.05 and 2.15? Answer: Practice has shown that the composition at the maximum reaction rate is not the stoichiometric composition, but rather the optimal hydrogen-to-carbon ratio determined by the kinetic equation. 45. What is the effect of catalyst particle size on methanol synthesis? Answer: The particle size of the catalyst has a significant impact on the macroscopic rate of methanol synthesis. The particles are small, resulting in a higher utilization rate of the internal surface area and a faster macroscopic reaction rate; this allows for a reduction in the amount of catalyst required. However, the resistance per unit of bed volume increases, leading to higher energy consumption ； The particles are large, resulting in a low macroscopic reaction rate; this reduces the pressure drop per unit of bed volume, and it also provides strong toxicity suppression capabilities. In general, the selection of catalyst particle size should be determined based on the type of tower used, the properties of the gas flow, and other relevant factors. A more reasonable approach is to use small particles in the upper part of the reactor and large particles in the lower part; the optimal particle size that results in the lowest total cost (cost of catalysts and energy costs) can be determined through calculation. The particle size of the catalytic Liu used in our factory is: Φ5×5. 46. Why is it necessary to control the total sulfur content in the fresh gas at ≤0.1PPm during synthesis operations? Answer: Sulfur is the most common toxin and the main substance responsible for the decline in catalyst activity. Sulfur in the feed gas generally exists in the form of hydrogen sulfide (H2S) and carbon disulfide (COS). Hydrogen sulfide reacts with active copper. Under the conditions of methanol synthesis, COS decomposes: COS + H2 = CO + H2S; the H2S produced then reacts with active copper to form inactive copper sulfide. The newly reduced catalyst, at 250°C and a COS concentration of 23–41 PPm, loses its usability after more than 42 hours of poisoning, as the CO conversion rate drops by 40%. The only way to prevent catalyst poisoning is to strictly control the process parameters of the purification and desulfurization steps, ensuring that the total sulfur content in the fresh gas remains at ≤0.1PPm. 47. What is the thermal aging of catalysts? Answer: Copper-based catalysts are very sensitive to heat (temperature). Copper is the active component of the catalyst, and the activity of the catalyst is directly proportional to the surface area of the metallic copper. An increase in operating temperature accelerates the growth of copper grains, thereby speeding up the decline in activity. The empirical relationship between the number of days of use and the peak temperature is given by: Y=5×107(0.959)X, where Y represents the number of days of use and X represents the operating temperature. To prevent the catalyst from losing its activity prematurely due to thermal aging, it is generally necessary, while maintaining production levels and stable operation, to operate in such a way that the increase in the hotspot temperature does not exceed 5°C each time. 48. How is catalyst strength reduced? Answer: The strength of the catalyst can be damaged during operation, especially during reduction processes or emergency shutdowns. Improper handling can lead to a significant decrease in the catalyst’s strength, or even to its disintegration, forcing a shutdown and replacement. Some manufacturers compress the catalyst to enhance its strength, achieving good results. 49. What is the impact of frequent start-up and shutdown on the catalyst? Answer: Shutdowns can be caused by accidents related to the process or equipment. No matter how carefully operations are carried out, it is inevitable that the catalyst’s activity will be impaired during a shutdown; in particular, improper handling and failure to replace the feed gas in the synthesis tower in a timely manner can cause severe damage to the catalyst’s activity. Note: According to some reports, in one of ICI’s methanol production plants, after the circulation pump failed and the plant came to a stop, the feed gas inside the towers was not replaced for 48 hours. The loss of catalyst activity in a stagnant carbonyl atmosphere is equivalent to a 9-month reduction in the catalyst’s service life. And a company in West Germany replaced its catalyst as a result. 50. What are the precautions for catalyst reduction and use? Answer: (1) During the reduction process, it is necessary to closely monitor the temperature at the outlet of the synthesis tower; when the temperature rises sharply, it is essential to immediately stop or reduce the amount of reducing gas supplied, as well as decrease the flow rate of the steam nozzle. (2) Strictly control the effluent rate; the hourly output volume shall not exceed 2 Kg per ton of catalyst. (3) Determination of the reduction endpoint: When the concentration of CO + H2 in the gas exiting the reactor, as determined through multiple analyses, becomes identical to the inlet concentration, it indicates that the catalyst is no longer consuming CO + H2 and that the liquid level in the separator is no longer rising; at this point, it can be concluded that the catalyst reduction process has reached its endpoint. (4) After the reduction is complete, reduce the system pressure to 0.15 Mpa, maintain the temperature of the synthesis tower at no less than 210°C, and replace the N2 in the system with fresh gas until its concentration is below 1%, after which methanol synthesis can be started. (5) When using synthetic pressure elevation, the pressure increase rate must not exceed 0.5 Mpa/h to prevent the catalyst from being damaged due to excessive temperature rise. (6) The operation of the new catalyst involves maintaining a certain production level, followed by gradual pressure increase, increased circulation rate, elevated CO content, and gradual temperature rise. During the first start-up with the new catalyst, the outlet temperature of the synthesis tower gradually increased from 220°C to 230°C. (7) The sulfur content and chloride content in the syngas should both be less than 0.1 ppm; trace amounts of oxygen, heavy metals, water vapor, and carbonyl compounds must not be introduced into the tower. (8) During the methanol synthesis process, the conditions must be strictly controlled; the temperature of the catalyst bed should not drop below 210°C. Sudden changes in catalyst temperature are strictly prohibited, and an optimal space velocity for using the catalyst is 6000–10000 h-1. (9) In the event of a stoppage during operation due to certain reasons, for a short-term stoppage within 24 hours, the fresh gas supply can be cut off to continue the circulation. Until the CO + CO2 reaction in the system is complete, the catalyst bed is maintained at above 210°C. (10) If the parking time exceeds 24 hours, the normal procedure can be followed: after parking as described in (9), reduce the pressure and temperature, replace the air with N2, and maintain the system pressure at 0.5 Mpa. 51. What are the control processes and indicators for catalyst deactivation? Answer: Lower the tower temperature to below 50°C and the tower pressure to below 0.3 Mpa. Perform nitrogen purging until it is satisfactory, then pressurize to 0.5 Mpa. Circulate N2 along with compressed air to gradually increase the oxygen content, while monitoring changes in tower temperature. The passivation is complete once the oxygen content at the outlet reaches 19–21%. Passivation procedure: Tower exit temperature (°C) ≤50 ≤60 ≤60 ≤60; Tower inlet oxygen content (%): 0.1, 1, 4, 21; Time required (hours): 2–3, 7–9, 5–6, 5–6. 52. How is the drum liquid level controlled? Answer: To ensure that the heat energy generated by the synthesis reaction is removed promptly and smoothly, the drum must maintain a certain liquid level. At the same time, in order to ensure timely discharge of steam from the drum and prevent water from entering the steam outlet pipe, the liquid level in the drum cannot exceed a certain upper limit. During normal operation, the liquid level in the drum is generally kept between 1/3 and 1/2 of the drum’s volume. Both the pressure of the deoxygenated soft water supplied to the drum and the opening degree of the water supply valve can affect the drum’s liquid level. The amount of wastewater discharged can also be used to fine-tune the drum’s pressure and liquid level; if necessary, increasing the amount of wastewater discharged can help to rapidly reduce the drum’s liquid level and pressure. 53. How is system pressure controlled? Answer: The level of system pressure reflects, on the one hand, the volume of gas flow, and on the other hand, the activity of the catalyst. At the beginning of catalyst use, its activity is high and when the gas flow is appropriate, the system pressure is low. When the gas flow suddenly increases or the catalyst’s activity is significantly reduced, the system pressure rises, potentially leading to overpressure. Other abnormal operating conditions can also cause overpressure. The system pressure can be adjusted appropriately by changing the volume of gas supplied by the compressor and the amount of gas removed from the tower. 54. Why must the temperature of the material entering the methanol separator be kept below 40°C? Answer: The gas exiting the tower after being cooled by the water cooler has most of its methanol and water condensed into liquids; this gas then enters a separator where crude methanol is separated out. The effective gas that emerges from this separation process contains approximately 0.611% methanol vapor. If the temperature of the gas entering the separator increases, the concentration of methanol vapor rises, and this will affect the quality of the methanol produced when the gas is sent to the synthesis tower under pressure. The temperature entering the separator is affected by the water supply volume, water supply temperature, and circulation rate. As the temperature rises, it is necessary to increase the flow rate of cooling water appropriately, lower the temperature of this water, or reduce the circulation volume, in order to keep the temperature of the gas entering the separator below 40°C and thereby reduce the amount of methanol vapor carried away. 55. How to control the formation of impurities in crude methanol? Answer: (1) When the hydrogen-to-carbon ratio in the gas entering the tower decreases, it facilitates the formation of dimethyl ether; an appropriately high hydrogen level is beneficial for suppressing the formation of dimethyl ether. (2) The presence of alkali metals such as potassium and sodium in the catalyst promotes the production of higher alcohols; therefore, it is necessary to ensure the quality of the catalyst. (3) When the CO content in syngas is too high, it tends to form iron pentacarbonyl on the equipment, resulting in an increase in the content of higher hydrocarbons. (4) When the moisture content in syngas is high, the following reaction occurs easily, resulting in the formation of formic acid. CO + H2O = HCOOH (5) Due to poor separation, the presence of methanol in the recycle gas can lead to the formation of impurities such as dimethyl ether, methyl isobutyl ether, diisobutylene, propanol, and isobutanol within the synthesis tower. (6) When sulfur is introduced into the gas, thiols are formed, which have a foul odor. (7) Amines are easily formed after mixing with ammonia. 56. Necessary conditions and preparatory work for driving the system for the first time: Answer: (1) The catalyst must be fully reduced, the water output should be close to the value calculated under the heating and reduction process; the synthesis tower should be maintained at a constant temperature of 210–220°C, and the system should be pressurized with N2 to 0.5 Mpa. (2) All the crude methanol storage tanks are in standby mode, and all problems that arose during the catalyst heating and reduction process across the entire system have been resolved. (3) Instrument air, medium-pressure steam, boiler feedwater, cooling water, and deionized water have all been introduced into the boundary area for standby use. (4) The waste oil drums for storing the waste methanol from startup are ready. (5) The purification operation is normal, the quality of fresh air meets the requirements, and the total load is ≥30%. (6) The compressor is operating normally, and fresh gas can be introduced into the system at any time. (7) All instruments in this system have been recalibrated, and they are operating normally after debugging. (8) The distillation section is already ready to receive crude methanol. (9) The control room is well-lit; operating tools, safety equipment, shift handover records, production reports, operating procedures, and process parameters are all available. Gas masks and fire-fighting equipment are provided as required. (10) The microcomputer is operating properly, and all parameters have been adjusted. 57. Steps to start up the synthesis system: Answer: (1) Inform the compressor to make all necessary preparations. (2) Coolant is introduced into the water cooler, and it is checked that the water temperature, pressure, and flow rate are normal. (3) Set the drum to operate automatically at 50% to maintain a stable liquid level and analyze water quality. (4) Allow slight continuous discharge of wastewater, at a rate of about 5% of the water intake volume; adjustments can be made based on the analysis of water quality. (5) When the drum pressure is set at 3.9 Mpa, automatic mode is activated and the steam is fed into the steam pipeline network. (6) Start the compressor and circulate N2 in the synthesis system to maintain a pressure of 0.5 MPa; add more N2 if the pressure is insufficient. (7) Slightly open the fresh gas valve, adding to the synthesis system in amounts of about 5% of the total load each time; there must be a 30-minute interval between two additions. (8) During the gas guiding process, pay close attention to the temperature of the synthesis tower; adjust the amount of steam supplied by the ejector as well as the circulation rate in order to maintain the outlet temperature of the synthesis tower at 215°C ± 5°C. (9) As fresh gas is added, the pressure continues to rise. As the synthesis reaction speed increases and more heat is released, the amount of steam supplied to the injector can be gradually reduced. (10) When the injector nozzle is fully closed, steam should be gradually introduced into the pipeline network depending on the drum pressure and the conditions of the steam pipeline system. (11) When the system pressure approaches 4.0 MPa, slightly open the vent to prevent too rapid pressure increase; throughout the entire pressure-raising process, the rate of pressure increase should be controlled at 0.5 MPa/h. (12) When the gas volume load is increased to 50%, production shall be maintained for one week to ensure complete reduction. (13) Sample analysis is conducted at the outlet of the synthesis tower; the gas composition is analyzed at the inlet of the synthesis tower, and the H/C ratio is calculated to determine the appropriate vent volume. (14) When the system pressure reaches 4.9 MPa, adjust the blow-off gas volume to maintain system stability. (15) After operating at half load for one week, the load can be gradually increased until it is at full capacity; at the same time, attention should be paid to adjusting the cooling water volume and increasing the circulation rate. (16) At the beginning of production, crude methanol contains many impurities, which are discharged into waste oil tanks through temporary pipes. Samples must be taken regularly for analysis during this discharge process; discharging should be stopped once the product becomes clear and transparent with no abnormal odors, after which the backflow valve should be closed and the shut-off valve opened, with the liquid level set at 50% to enable automatic operation. (17) When the liquid level in the methanol expansion tank reaches 30%, notify the distillation section to get ready to receive it; open the shut-off valve on site, and set the liquid level at 50% to enable automatic operation. 58. Under what circumstances is it a general accident? Answer: (1) The composition of the gas entering the tower is not up to standard; attempting to contact the previous process unit half an hour in advance is ineffective. (2) The outlet temperature of the synthesis tower is below 210°C for various reasons. (3) The cooling water was shut off, causing the inlet temperature of the separator to exceed 55°C for half an hour. (4) Due to a failure in the feed water pump, the water supply pressure to the steam drum became unstable and the liquid level fluctuated significantly; attempts to resolve the issue within half an hour were unsuccessful. (5) The temperature of the fresh gas is greater than 50°C, or liquid contact treatment proves ineffective after half an hour. (6) This section is currently experiencing process fluctuations, with multiple instruments malfunctioning and unable to operate. (7) Other work IDs require parking. 59. How should one stop the vehicle in the event of a general accident? Answer: (1) Report scheduling. (2) The personnel on site should get ready to park the vehicle. (3) Notify the compressor team to stop supplying fresh gas. (4) When the separator level drops to the low limit, close the shut-off valve. (5) Decide whether to vent and replace with N2 depending on the duration of resuming operation. (6) Monitor the drum pressure and the outlet temperature of the synthesis tower; activate the injector in a timely manner to maintain the outlet temperature at ≥210°C. 60. Steps for a normal short-term shutdown: Answer: Due to the need for emergency repairs on the equipment or requirements from adjacent production units, the expected shutdown time is no more than 12 hours for a short-term shutdown: After receiving instructions from the dispatch team and coordinating with adjacent units, (1) the compressor stops supplying fresh gas. . (2) After releasing the gas, cut off the supply and maintain pressure in the system. (3) Gradually reduce the circulation volume until the reaction ceases, reducing it until finally shutting down by closing the inlet isolation valve. (4) After the separator level has been drained, close the shut-off valve. (5) Close the drum continuous blowdown valve. (6) Maintain pressure in the system while starting the ejector to keep the tower temperature at ≥210°C. 61. Steps for starting the vehicle after a normal short-term stop and a routine accident stop: Answer: The stop time is ≤12 hours, and the synthesis loop is kept insulated and under pressure. (1) Open the fresh gas isolation valve on-site and purge the gas isolation valve. (2) Prepare the compressor for startup; reduce the pressure behind the tower to 2 Mpa and then shut off the purge gas. (3) Start the compressor and operate it with a low circulation rate; adjust the steam flow from the steam nozzle as needed to keep the temperature at the outlet of the synthesis tower at ≥210°C. At the same time, open the blowdown valve on the steam drum by 1–2 turns. (4) The circulation rate can be increased when the tower outlet temperature is between 210–230°C. (5) Supply fresh gas to the synthesis system; the gas supply process should be steady and not too rapid. (6) During the addition of fresh gas, monitor the outlet temperature of the synthesis tower at all times; by adjusting the nozzles of the steam ejector and the blowdown from the drum, maintain the outlet temperature between 210–230°C and keep the drum liquid level at 50%. (7) Once it is detected that the outlet temperature is below 210°C, the supply of fresh gas should be stopped; instead, the fresh gas should be vented, the circulation volume reduced, and the nozzle adjusted to raise the temperature before resuming the supply of fresh gas. (8) When the system pressure reaches 4.0 MPa, gas blowing is initiated to maintain stability in the composition of the gas entering the tower; as the system pressure continues to rise gradually, when it approaches 4.8 MPa, the amount of gas blown is adjusted to prevent further increases in pressure. (9) Gradually increase the circulation rate; when a cooling trend appears, stop increasing it. The amount of fresh air should be increased gradually, not too quickly, until the outlet temperature rises again. (10) Increase the amount alternately, maintain stability during intervals, and finally increase it to full capacity. (11) The separator level is set at 30% and controlled automatically. (12) Once the steam pressure reaches the specified value, it is integrated into the pipeline network. (13) Gradually adjust various indicators to reach a normal operating state. 62. Under what circumstances should an emergency stop be initiated? What are the emergency shutdown steps? Answer: (1) An emergency stop shall be initiated in any of the following situations: (1) Pipeline rupture, cracks in equipment welds, or leaks at manholes. (2) The root pipe of the instrument is broken, causing the drain backflow to rupture. (3) Compressor trip. (4) Instrument air supply is interrupted, power failure occurs, water supply to the steam drum stops suddenly, and the liquid level drops to the minimum level. (5) It was confirmed that the total sulfur level is excessively high. (II) Parking steps: (1) Notify the dispatcher by phone. (2) Cut off the fresh air supply. (3) Stop the compressor. (4) Separator: Close the on-site shut-off valve when the liquid level in the expansion tank reaches the low limit. (5) At the same time, relieve the pressure by releasing the purge gas; when the system pressure is ≤ 0.3 Mpa, open the N2 shut-off valve and introduce qualified N2 to purge the system. (6) Analyze CO + H2