Exploration of methods to extend the service life of methanol synthesis catalysts

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Issues such as controlling the temperature of hot spots, controlling gas composition, avoiding frequent start-up and shutdown, paying attention to catalyst loading as well as catalyst heating and reduction, and operating under light load. Extending the service life of methanol synthesis catalysts can increase the production of methanol, reduce production costs, and improve the economic efficiency of the plant. There are many factors that affect the service life of methanol synthesis catalysts, and strict control at every stage is necessary to extend their lifespan. This article focuses on methods to extend the service life of methanol synthesis catalysts in terms of process operations and catalyst reduction procedures. I. Controlling the catalyst hotspot temperature: In the methanol production process, the key to operational control lies in managing the temperature of the catalyst bed, which in turn involves controlling the methanol synthesis reaction. It is necessary to maintain a stable temperature of the catalyst bed while ensuring proper transfer of reaction heat, in order to maximize methanol production. The stability of bed temperature control is closely related to catalyst activity, gas flow rates in and out of the tower, tower load, and the composition of the gas entering the tower. There are many process parameters that affect the temperature of the synthesis tower, which poses significant challenges to temperature control. It should be noted that during operation, it is strictly prohibited to exceed the recommended temperature in an attempt to increase production, as this will **reduce the lifespan of the catalyst**. Preventing the catalyst from overheating during operation is an important measure to extend its service life. Therefore, reducing the catalyst hotspot temperature is a good way to slow down the thermal aging of the catalyst and extend its service life. The main measures to prevent thermal aging of catalysts are: 1. During reduction and start-up/shutdown processes, operate in accordance with predetermined parameters to prevent overheating. 2. While ensuring production volume, operate stably and minimize the temperature at hot spots in the bed; any increase in the temperature of these hot spots should be done carefully, with the increase amount not being too large, usually around 5°C. 3. Appropriately increase the CO2 content in the syngas. The CO2 content in the gas entering the tower must not be less than 1%; at least 2% is required, with 3–5% being ideal. II. Controlling gas composition: To control the gas composition, it is first necessary to regulate the ratio of CO to CO2, making adjustments according to the different usage periods of the catalyst. Next, it is important to control the amount of inert gases; monitoring and analyzing the volume of gas released serves as a basis for optimization. Thirdly, it is essential to control the alcohol content in the circulating gas. The lower the alcohol content in the gas entering the reactor, the more favorable the conditions for the methanol synthesis reaction, and it also helps to prevent the formation of by-products such as higher alcohols. Therefore, it is necessary to reduce the temperature of the gas leaving the methanol water cooler as much as possible, and to separate the condensed methanol promptly. III. Avoid frequent start-up and shutdowns. Many manufacturers inevitably experience multiple start-ups and shutdowns due to equipment or system issues; if these shutdowns are not handled properly, it can damage the catalyst’s activity. Tests have shown that after a short-term shutdown, if the catalyst is sealed within the feed gas without any other processing, its activity suffers a significant decline upon restart. Therefore, after a short-term or emergency shutdown, the following actions should be taken: 1. The area should be immediately purged with nitrogen. If replacement is not possible, the circulation machine can be kept running to ensure complete reaction of the hydrocarbon mixture in the circulating gas, until only inert gases and hydrogen remain in the system, or until the volume fraction of CO + CO2 is less than 0.5%. 2. When the bed temperature drops, the steam supply should be increased appropriately and the circulation rate reduced, so as to maintain the bed temperature at 210°C, while gradually reducing the system pressure to 0.2 Mpa. 3. In the event of a long-term shutdown, nitrogen purging should be carried out; once the purging is complete, the system should be maintained at a slight positive pressure to prevent air from entering during maintenance. IV. Catalyst Loading When loading the catalyst, the following points should be taken into consideration: 1. Copper-based catalysts have poor strength; it is strictly forbidden to drop or collide with them during transportation. 2. Before loading, the catalyst should be gently sieved to remove powder and fragments. 3. The spreading method should be used for loading, in order to minimize the falling height of the catalyst and prevent bridging. Random checks should be conducted on the pressure difference across the catalyst bed, which should remain within the allowable range. 4. Better weather should be chosen for loading to prevent the catalyst from absorbing moisture and losing its activity; once loading begins, it should be carried out continuously without interruptions. After loading, it should be sealed immediately, filled with nitrogen, or subjected to heating and reduction. V. Temperature-induced reduction of the catalyst: Copper-based catalysts for methanol synthesis must be reduced before they become active. The reduction operation is a very important step in the process. The level of catalyst activity per batch depends not only on the production quality and filling quality of the catalyst itself, but also to a large extent on the quality of its reduction process; this has a significant impact on the catalyst’s service life. Therefore, the reduction process must be carried out strictly, meticulously, and carefully. The quality of catalyst reduction upon heating plays a decisive role in the catalyst’s service life in the future. A catalyst that has been reduced well features small grains, numerous internal voids, and a large active surface area; such a catalyst, when used in normal production, offers advantages such as high reactivity, a uniform temperature distribution within the catalyst bed, and a long service life. The temperature-induced reduction of the catalyst plays a significant role in determining its activity and directly affects its service life. Therefore, when reducing the catalyst, the following aspects require special attention: 1. Hydrogen content control. The reduction reaction is highly exothermic; when the hydrogen content is low, the temperature rise in the catalyst bed is proportional to the hydrogen concentration. Generally, an increase of 1% in hydrogen content results in a 28°C rise in the bed temperature. Thus, controlling the rate of hydrogen addition is crucial for the reduction process. During reduction, it is necessary to follow the principle of increasing temperature without increasing hydrogen pressure, and increasing hydrogen pressure without increasing temperature, in order to prevent excessive reduction that could cause a sharp rise in bed temperature and affect the catalyst’s activity. Therefore, in the reduction process, low hydrogen levels and high space velocity are generally used to control the reduction rate. 2. Water output control: The determination of the reduction endpoint has a significant impact on catalyst activity; during reduction, it is necessary to prevent both incomplete reduction and excessive reduction. Many manufacturers strive to maintain as uniform a water output as possible when using syngas reduction. During the reduction process, the theoretical water output should be fairly close to the actual water output, and a stable hydrogen content in the gas entering and leaving the synthesis tower indicates that the reduction is essentially complete. 3. Control of inert gas release volume: Inert gases are generally used as the carrier gas for reducing gases, with nitrogen being the common diluent gas. During reduction processes, inert gases help to control the rate of reduction, make it easier to regulate the bed temperature, and thus contribute to improving catalyst activity while protecting its strength. Furthermore, due to the use of syngas reduction, the CO2 content in the inert gas also affects the assessment of the reduction progress; by examining the CO2 content in the exhaust gas, it is possible to determine the extent to which CO participates in the reduction reaction. As a result, the amount of water produced may be lower than the theoretical value. VI. Light-load operation: The main reaction equations for methanol production are: CO + 2H₂ = CH₃OH + Q; CO₂ + 3H₂ = CH₃OH + H₂O + Q. These reactions are reversible, exothermic, involve a reduction in volume, and take place through gas-solid phase catalysis. At light-load conditions, the catalyst exhibits high activity. In order to help it adapt to high-load operation as quickly as possible and to improve both the efficiency of the catalyst and its service life, it is necessary to maintain stability in flow rate, temperature, pressure, and gas composition during this stage, thereby ensuring the reaction proceeds spontaneously. Special attention must be paid to maintaining equilibrium in the reaction heat to prevent overheating and loss of temperature control. The purpose of operating at light load is not only to stabilize the catalyst’s structure and extend its service life, but also to adjust various operational parameters so as to reach normal production levels. It enables us to understand the operational characteristics and grasp the patterns of operation, thereby preparing for full-load normal production. At the beginning of production, the catalyst has high activity; therefore, it should operate at a low load under conditions of low CO content and high CO2 content for a period of time before it can be used for normal-load production. VII. The sulfur content in the syngas should be kept below 0.06×10‑6, and the chloride content below 0.01×10‑6. The syngas must be free from harmful impurities such as heavy metals and unsaturated hydrocarbons, as well as oil mist. Copper-based catalysts are highly sensitive to sulfur; this is because H₂S in the syngas combines with Cu in the catalyst to form Cu and Cu₂S, which reduces the catalyst’s reaction activity and shortens its service life. Primary deionized water is used as process water to improve its quality and reduce the amount of CI that enters the methanol synthesis tower; the harm that even trace amounts of chlorine can cause to the methanol synthesis catalyst cannot be ignored. Furthermore, the ammonia content and oil present in the gas entering the tower have a significant impact on the catalyst’s activity and service life. Oil decomposes at high temperatures to form carbon and high-carbon polymers, which deposit on the surface of the catalyst and block its pores. Moreover, elements such as sulfur, arsenic, and phosphorus in the oil can cause permanent chemical poisoning of the catalyst. Ammonia reduces the catalyst’s activity; when its concentration is reduced or eliminated, the catalyst’s activity increases, but it cannot return to its original level. VIII. Conclusion In summary, the service life of methanol synthesis catalysts is influenced by the operating conditions during process operation, as well as by the catalyst heating and reduction processes. Only by strictly controlling every aspect can the service life of methanol synthesis catalysts be extended.