Ways to extend the service life of methanol catalysts!!!

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Ways to extend the service life of methanol catalysts. A significant issue affecting the long-term safe operation of methanol plants is the short service life of methanol catalysts, low production intensity, and high shrinkage rates, all of which result in a low methanol conversion rate. Industrially, methanol is generally synthesized using the pressurized catalytic method with CO, CO2, and hydrogen. A typical process includes steps such as raw gas production, raw gas purification, compression, methanol synthesis, and distillation of crude methanol. The methanol synthesis catalyst is a copper-zinc-based catalyst; it exhibits good activity and high selectivity, but it is extremely sensitive to poisons and can easily become deactivated as a result. The factors affecting the service life of methanol catalysts and the corresponding measures include: 1. Temperature-induced reduction. Under normal reduction conditions, only copper oxide in the methanol catalyst is reduced, while the oxides of zinc and aluminum are not reduced. The reduction occurs in layers; the catalyst bed is reduced layer by layer from top to bottom, and each catalyst particle is reduced gradually from the surface inward. The reduction of methanol catalysts is an exothermic reaction; the reduction rate should not be too fast, and it is necessary to strictly control the hydrogen concentration in the loop as well as the rate of temperature increase during reduction. In addition to controlling the hydrogen concentration, it is also necessary to control the CO/CO2 concentration in the synthesis loop; if needed, the amount of nitrogen added can be increased, along with the amount of gas blown out, in order to regulate the CO/CO2 concentration in the loop and ensure effective reduction of the catalyst. In addition, temperature-raised reduction must also adhere to the principles of \"three lows, three stabilities, and three no’s\", namely: low-temperature effluent, low-temperature reduction, and a period of low-load operation after reduction ； Stable temperature rise, stable hydrogen supplementation, stable water output ； Hydrogen extraction and temperature elevation should not occur simultaneously, moisture must not be introduced into the tower, and the time during which water is at high temperature should not be too long. 2. Operating pressure: From the perspectives of chemical equilibrium and kinetics, increasing the pressure accelerates the progress of the reaction. However, increasing the pressure also facilitates the occurrence of side reactions; when these reactions take place, the degree of volume contraction of the gases before and after the reaction is more pronounced than in the methanol synthesis reaction. Side reactions increase, and paraffins and ethers may be formed, covering the surface of the catalyst and blocking its micropores and voids, thereby reducing the catalyst’s activity. Take measures: maintain stable pressure and minimize fluctuations in operating pressure as much as possible. 3. Operating temperature: Significant fluctuations in the temperature of the catalyst bed can cause changes in the structure of the catalyst’s inner surface, leading to rapid aging of the catalyst. An increase in the bed temperature can also lead to an increase in side reactions, which clog the micropores and voids of the catalyst and reduce its activity. Take measures: At the beginning of catalyst use, the reaction temperature should be kept at a low level; as the catalyst is used for a longer period of time, the reaction temperature can be increased gradually. 4. Gas composition: An excess of hydrogen in the gas fed into the methanol synthesis tower helps to reduce side reactions, alleviate hydrogen sulfide poisoning, and decrease the formation of iron carbonyl ； The presence of a certain amount of CO2 can prevent carbon deposition on the catalyst, thereby helping to maintain its high activity ； A certain amount of CO2 can reduce the reaction heat, which helps to maintain the temperature of the catalyst bed. Take measures: maintain (H2+CO2)/(CO+CO2) in the fresh gas at 2.1–2.2, and (H2+CO2)/(CO+CO2) in the recycled gas at 4–5. At the beginning of catalyst use, its activity is high, allowing the content of inert gases in the recycle gas to be kept at a relatively high level, typically around 30%. As the catalyst is used over time, the content of inert gases can be gradually reduced in order to maintain methanol production. 5. Start-up and shutdown: Frequent start-ups and shutdowns, along with repeated pressure changes, can damage the catalyst’s strength, leading to partial pulverization and blockage of the catalyst’s micropores ； In some plants, the methanol synthesis tower is not depressurized during short-term shutdowns, and the catalyst synthesizes carbonyl metals in a stagnant carbonyl atmosphere, resulting in catalyst poisoning. Take measures: strive to avoid unnecessary stops and maintain long-term operation. For short-term parking, after the compressor stops supplying gas, the circulator continues to operate, allowing the carbon oxides in the circulating gas to continue reacting and thereby reducing the pressure in the synthesis loop. If necessary, the loop can be purged with nitrogen, or an electric furnace can be used to maintain the temperature until the carbon oxides in the circulating gas are reduced to zero; thereafter, the circulator is stopped and the system is held at pressure for maintenance purposes ； For long-term parking, pressure must be released and the atmosphere replaced with nitrogen to reduce hydrogen content