Basic knowledge of methanol synthesis

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I. Chemical reactions for the synthesis of methanol:
Main reactions:
CO + 2H2 = CH3OH + 102.5 kJ/mol
CO2 + 3H2 = CH3OH + H2O + Q kJ/mol

Side reactions:
2CO + 4H2 = CH3OCH3 + H2O + 200.2 kJ/mol
CO + 3H2 = CH4 + H2O + 115.6 kJ/mol
4CO + 8H2 = C4H9OH + 3H2O + 49.62 kJ/mol
CO + H2 = CO2 + H2O – 42.9 kJ/mol
nCO + 2nH2 = (CH2)n + nH2O + Q kJ/mol

II. Calculation of the heat of reaction for the synthesis of methanol from carbon monoxide and hydrogen:
The synthesis of methanol from carbon monoxide and hydrogen is an exothermic reaction; at 25°C, the heat of reaction is 90.8 kJ/mol. The relationship between the reaction heat QT (kJ/mol) and temperature is given by: QT = –74893.6 – 64.77T + 47.78×10⁻³T² – 112.926×10⁻³T³, where T is the absolute temperature in K. The reaction heat for methanol synthesis at different temperatures is shown in the table below: Reaction temperature (°C) | Reaction heat (kJ/mol); Reaction temperature (°C) | Reaction heat (kJ/mol): 100 | 200; 250 | 93303.2, 97068.8, 97926.52. For temperatures of 300 °C and 350 °C, the values are 99370.0 and 102298.8 respectively.

III. Equilibrium constant for methanol synthesis: The synthesis of methanol from carbon monoxide and hydrogen is a reversible gas-phase reaction, and pressure plays an important role in this reaction. The equilibrium constant, expressed in terms of partial pressures, can be calculated using the following formula: kp = p_CH3OH / (p_CO × p_H2). Here, kp represents the equilibrium constant for methanol, while p_CH3OH, p_CO, and p_H2 represent the equilibrium partial pressures of methanol, carbon monoxide, and hydrogen respectively. The reaction temperature is also an important factor affecting equilibrium. The following equation expresses the equilibrium constant for methanol synthesis in terms of temperature: lgKa = 3921/T – 7.9711lg T + 0.002499T – 2.953×10⁻⁷T² + 10.20, where Ka is the equilibrium constant expressed in terms of temperature ； T — reaction temperature, K. The reaction equilibrium constants calculated using formulas are shown in the table below. Reaction temperature (°C) Equilibrium constant (Ka) Reaction temperature (°C) Equilibrium constant (Ka): 0, 100; 200, 667.30, 12.92, 1.909×10⁻²; 300, 400, 2.42×10⁻⁴, 1.079×10⁻⁵. As can be seen from the table, the equilibrium constant decreases rapidly as the temperature rises. IV. Effect of temperature on the methanol synthesis reaction: The methanol synthesis reaction is a reversible exothermic reaction. From the perspective of chemical equilibrium, as the temperature increases, the value of the methanol equilibrium constant will decrease. However, from the perspective of reaction rate, increasing the reaction temperature speeds up the reaction. Therefore, there is an optimal temperature range. For different catalysts, the operating temperature range varies. The operating temperature for the C307 synthetic methanol catalyst is 190–300 °C, while the optimal temperature is 210–260 °C. In actual production, to ensure a long service life for the catalyst and minimize side reactions, it is necessary to operate at as low a temperature as possible, depending on the catalyst’s performance, while still maintaining an adequate methanol yield. (At the beginning of catalyst use, the reaction temperature should be kept low; as the catalyst is used over time, the reaction temperature can be gradually increased.) Furthermore, the higher the temperature of the methanol synthesis reaction, the more side reactions occur, and the content of organic impurities and other components in the resulting crude methanol increases, posing difficulties for the subsequent distillation process of the crude methanol. V. Effect of pressure on the methanol synthesis reaction: The methanol synthesis reaction is a volume-reducing reaction; increasing the pressure shifts the reaction in the direction of methanol formation ； From a kinetic perspective, increasing the pressure raises the partial pressures of the reactants, thereby accelerating the reaction ； Furthermore, increasing pressure is also beneficial for suppressing side reactions and improving the quality of methanol. Therefore, increasing pressure is beneficial for the reaction. However, the pressure should not be too high; otherwise, it not only increases power consumption but also raises the requirements for equipment and materials, leading to higher investment costs. The operating pressure for the C307 synthetic methanol catalyst is 3–15 MPa. VI. Effect of space velocity on the methanol synthesis reaction: The duration of contact between the gas and the catalyst is typically expressed in terms of space velocity, which refers to the amount of gas that passes through each unit volume of catalyst per unit of time. Its unit is m3 standard/(m3 catalyst·h), abbreviated as h-1. Air velocity is an important means of regulating the temperature of the methanol synthesis tower and the methanol production volume. In methanol production, only 3% to 6% of methanol can be obtained when the gas passes through the synthesis tower once; the methanol conversion rate of the fresh gas is not high, therefore the fresh gas must be recycled. Under certain conditions, as the space velocity increases, the contact time between the gas and the catalyst decreases, resulting in a lower methanol content in the gas leaving the tower. However, due to the increase in air velocity, the amount of gas passing through the catalyst per unit time increases, so the actual methanol production rises. When the air velocity increases to a certain level, the increase in methanol production becomes less significant. At the same time, as the air velocity increases, the energy consumed also rises, and the amount of heat carried away by the gas increases as well. When the heat carried away by the gas exceeds the reaction heat, it becomes difficult to maintain the bed temperature. The space velocity for methanol synthesis is influenced by various factors such as system pressure, gas flow rate, gas composition, and catalyst performance. Operating space velocity of the C307 synthetic methanol catalyst: 4000–20000 h-1. VII. The impact of hydrocarbon ratio control on the methanol synthesis reaction: Methanol is produced by the reaction of carbon monoxide, carbon dioxide, and hydrogen, as shown in the following equations: CO + 2H2 ≒ CH3OH; CO2 + 3H2 ≒ CH3OH + H2O. From these equations, it can be seen that the molar ratio of hydrogen to carbon monoxide required for methanol synthesis is 2, while the molar ratio for synthesis using carbon dioxide is 3. When both carbon monoxide and carbon dioxide are present, there are two ways to express the hydrocarbon ratio (f or M) in the feed gas: f = (H2 – CO2) / (CO + CO2) = 2.05–2.15, or M = H2 / (CO + 1.5 CO2) = 2.0–2.05. The composition of the feed gas obtained using different processes with various raw materials often deviates from these values of f or M. In production, the optimal carbon-to-hydrogen ratio should be slightly higher than the stoichiometric ratio; according to the stoichiometric ratio, the f-value or M-value is approximately 2, and in practice the value is kept slightly above 2, meaning that a slightly higher hydrogen content is maintained. Excess hydrogen plays a beneficial role in reducing the formation of iron carbonyl and higher alcohols, as well as in extending the catalyst’s lifespan. VIII. Effect of inert gas content on the methanol synthesis reaction: The inert gases in the methanol system refer to nitrogen, methane, argon, and other non-condensable organic compounds. The high content of inert gas in the system correspondingly reduces the effective partial pressures of CO, CO2, and H2, which is unfavorable for the methanol synthesis reaction and also increases power consumption. Inert gases originate from the feed gas and side reactions in the methanol synthesis process. For methanol manufacturers, the content of inert gases in the recycle gas continues to increase, so it is necessary to regularly discharge a portion of the gas in order to maintain a certain level of inert gases. General control principle: At the beginning of catalyst use, when its activity is high, or when the load on the synthesis tower is low and the operating pressure is low, the content of inert gases in the recycle gas can be maintained at 20%~25% ； Conversely, it should be kept at around 15% to 20%. The main method for controlling the inert gas content in the recycle gas is to vent the gas from after the crude methanol separator. The formula for calculating the volume of exhaust gas is as follows: V_exhaust ≈ (V_fresh × I_fresh) ÷ I_exhaust. Here, V_exhaust represents the volume of exhaust gas, in m3/h ； Vfresh – Volume of fresh gas, m3(Norm)/h ； I: Air release – percentage of inert gases in the released air ； I Freshness – Percentage of inert gases in fresh gas, % ； IX. The effect of carbon dioxide content on the methanol synthesis reaction: Carbon dioxide can also participate in the reaction for synthesizing methanol. For copper-based catalysts, the role of carbon dioxide is complex; it has both kinetic effects and may also act as a chemical additive. In summary, its favorable aspects include: ① A certain amount of CO2 can help increase the yield of methanol ； ②Improving the selectivity of the catalyst can reduce the occurrence of side reactions such as those involving ethers ； ③ It can more effectively regulate temperature, prevent overheating, and extend the lifespan of the catalyst ； ④Prevents catalyst carbon buildup. Its disadvantages are: compared to the synthesis of methanol from CO, 0.7 m3 more of H2 is consumed per 1 kg of methanol produced ； It increases the water content in the crude alcohol and reduces the methanol concentration. In short, when selecting operating conditions, the advantages and disadvantages of CO2 should be weighed. Generally, at the beginning of use, when the catalyst activity is high, it is appropriate to increase the concentration of CO2 in the feed gas, so as to prevent the methanol synthesis reaction from becoming too intense and to facilitate the control of the bed temperature ； During later stages of use, the CO2 concentration in the feed gas can be appropriately reduced to facilitate the methanol synthesis reaction and to control and stabilize the bed temperature. When a copper-based catalyst is used, the CO2 content in the feed gas is usually around 6% (by volume), with the maximum allowable CO2 content being 12%–15%. Generally, it is controlled at 4–6% in the initial stage, and at 2–4% in the middle and later stages. X. Effect of the methanol content entering the tower on the methanol synthesis reaction: The lower the methanol content entering the tower, the more favorable it is for the methanol synthesis reaction to proceed, and it also helps to reduce the formation of by-products such as higher alcohols. To this end, the temperature of the water cooler should be reduced as much as possible, and efforts should be made to improve the efficiency of the methanol separator, in order to minimize the methanol content in both the recycle gas and the gas fed into the methanol column. When synthesizing methanol under low pressure, it is required that the methanol content in the gas after cooling and separation be around 0.6%. Generally, the gas temperature after the water cooler is controlled between 20 and 40 °C. XI. Effect of water-to-carbon ratio on the methanol synthesis reaction: From the perspective of chemical equilibrium, increasing the water-to-carbon ratio is beneficial for methane conversion and also helps to prevent carbon deposition. However, an increase in the water-to-carbon ratio leads to higher energy consumption for vapor generation, a greater thermal load on the furnace tubes, and increased airflow resistance inside the furnace tubes. Therefore, while meeting the process requirements, the water-to-carbon ratio should be reduced as much as possible. In actual production of methanol from natural gas as a raw material, the water-to-carbon ratio is approximately 3.5. XII. Hazards of ammonia entering the methanol synthesis system: When ammonia enters the methanol synthesis tower, it affects the catalyst’s activity and lifespan as well as the quality of crude methanol. Relevant tests show that when the raw gas contains ammonia at a level of (50~100)×10‑6, its activity decreases by 10%~20% compared to when there is no ammonia present (assuming it is not greater than 1×10‑6). According to relevant technical data, even when methanol synthesis gas contains trace amounts of ammonia at a concentration of 20×10‑6, the chemical reactions that occur during methanol synthesis result in the formation of monomethylamine, dimethylamine, and trimethylamine. The corresponding chemical equations are as follows: CH3OH+NH3 → NH2CH3+H2O+20.75 kJ/mol; CH3OH+NH3 → NH(CH3)2+2H2O+60.88 kJ/mol; CH3OH+NH3 → NH(CH3)3+3H2O+407.55 kJ/mol. As a result of these reactions, the crude methanol produced has an fishy odor ； On the other hand, an increase in mixed amines, along with a high alkalinity and low ammonolysis value, facilitates the formation of fusel by-products, thereby increasing the difficulty of the crude alcohol distillation process. This not only affects the quality of the refined methanol but also increases the consumption of various materials. 13. Formation and hazards of paraffinic alkanes: The reasons for the formation of paraffinic alkanes during methanol production are: ① Excessively high reaction temperatures accelerate side reactions, leading to an increase in the amount of alkanes produced ； ② The high CO level in the fresh gas, combined with a high synthesis space velocity, results in too short a contact time for the gases with the catalyst. At a certain level, the production of alkanes increases significantly ； ③ Improper start-up and shutdown procedures, along with the catalyst coming into contact with the feed gas at temperatures below 210°C, lead to a significant increase in the formation of paraffinic alkanes ； ④ During production, a small amount of organic acids causes corrosion of the equipment; this leads to the formation of iron carbonyl compounds that accumulate on the surface of the copper catalysts, reducing their activity and facilitating the formation of alkanes, which results in noticeable waxing ； ⑤ During the manufacturing, storage, transportation, and filling of catalysts, elements such as iron, cobalt, and nickel can end up mixed into the catalysts; the production process can cause CO to undergo dissociative adsorption, thereby promoting the formation of alkanes ； ⑥ When SiO2 or other acidic oxides are present in the catalyst, it promotes the formation of paraffin ； ⑦ When water vapor is present in the feed gas, it can react with carbon monoxide on iron-containing catalysts as follows: CO + H2O → (–CH2–) + CO2. If the ethylene content in the gas entering the tower is high, or if lubricating oil from compressors mixes in, paraffin will be formed. The main hazard of paraffinic alkanes is that they cause blockages in equipment and pipelines such as the water coolers and methanol separators in the methanol synthesis system, resulting in an increase in system pressure differences; in severe cases, production must be halted for wax removal. Furthermore, alkanes with 16 carbon atoms or more are insoluble in methanol and water at room temperature; they will precipitate as crystals in the liquid or cause the solution to become turbid, thereby reducing the quality of methanol and leading to increased consumption of pure methanol and lower yields. 14. What are the requirements of methanol synthesis catalysts for raw gas purification? To extend the service life of methanol synthesis catalysts and improve the quality of crude methanol, it is necessary to purify the feed gas. The task of purification is to remove oils, water, dust particles, iron carbonyl, chlorides, sulfides, and so on, among which the removal of sulfides is particularly important. Sulfides in the feed gas can poison the catalyst; when a copper-based catalyst is used, sulfides react with copper to form copper sulfide, thereby rendering the catalyst inactive. Copper-based catalysts have high requirements regarding sulfur; the sulfur content in the feed gas should be less than 0.1 mL/m3 ； Oil contamination carried in the feed gas into the methanol synthesis tower has a significant impact on the catalyst. At high temperatures, the oil decomposes to form carbon and high-carbon polymers, which deposit on the surface of the catalyst, blocking its pores and reducing its surface activity; this leads to a decrease in the catalyst’s efficiency. Moreover, the presence of sulfur, phosphorus, arsenic, and other elements in the oil can cause chemical poisoning of the catalyst.