Methanol Weekly Topic: What should be the temperature control for methanol catalysts? 2015.08.24----08.31

Generally, the synthesis reactors for methanol are mainly of the Ruhr process and ICI type. It is usually a tubular reactor; the catalyst is located inside the reaction tubes, where syngas is used to produce methanol. Outside the tubes, there is a water bath, creating a condition of coexistence between gas and liquid. Steam is generated to absorb the heat released during the reaction, and the reaction temperature is typically controlled by regulating the pressure of the steam system. It should be noted that the temperature controlled in industry is the temperature at the outlet of the synthesis tower, and it generally has a very good one-to-one relationship with the pressure in the steam system. -----Control methods: If done simply, it can be assumed that the temperature outside the reaction tubes in the synthesis tower remains constant. With this temperature, along with the pressure inside the tubes and the concentrations of the reactants, it is possible to carry out kinetic calculations. Similarly, once the temperature and pressure at the exit of the synthesis tower are known, thermodynamic calculations can be performed based on equilibrium principles. Calculation methods: From a kinetic perspective, increasing the steam pressure and thus the reaction temperature is beneficial for accelerating the reaction rate; it also helps to increase the overall conversion rate. However, from a thermodynamic standpoint, raising the temperature is unfavorable for the equilibrium state that the reaction ultimately reaches. In other words, the effect of temperature on kinetics and thermodynamics is a paradox. In actual production, the temperature of new catalysts is generally kept low at first and increased later; this is because operating at high temperatures too early can cause the catalyst to age, thereby reducing its useful life. The temperature must also not exceed 290 degrees; if it does, aluminum oxide in the catalyst will cause dehydration of methanol. ----Qualitative analysis of temperature on kinetics and thermodynamics. However, if you are designing or simulating the methanol synthesis reaction in a laboratory, you must take into account the actual temperature gradient distribution within the synthesis reactor when performing kinetic and thermodynamic calculations.

The bed temperature in the synthesis reactor is maintained between 200 and 280°C; temperatures below 200°C facilitate wax formation, which leads to frequent filter changes. It may also result in some of the wax remaining permanently on the catalyst, blocking the active areas. The catalyst aging rate increases above 280°C ； Additionally, the adiabatic temperature of the catalyst is <315°C. An excessively high catalyst peak temperature can cause syngas methanation, thereby generating heat and resulting in unacceptably high temperatures in the synthesis tower.

There are various methanol synthesis processes, including high-pressure, medium-pressure, and low-pressure methods, and the composition of the feed gas used varies significantly; therefore, the optimal temperature differs for each specific case. For copper-based catalysts, under high pressure and high reactant concentrations, the optimal temperature usually exceeds the heat tolerance limit; whereas it is only practical to operate at the optimal temperature under low pressure and low reactant concentrations. The situation for zinc-chromium catalysts is similar to that of copper-based catalysts; however, the heat resistance of zinc-chromium catalysts is higher than that of copper-based catalysts, at around 400°C, while their reaction rate is lower than that of copper-based catalysts. Therefore, on a larger scale, operation at the optimal temperature can be utilized to enhance production. The optimal temperature value is also related to the particle size of the catalyst used. For guiding production, it is necessary to take into account both the early and later stages of catalyst use, as well as changes in composition and pressure. The reaction for synthesizing methanol is: 2H2 + CO = CH3OH, with △rGm = –114.17 + 0.244 kJ/mol. Therefore, from a thermodynamic perspective, increasing the temperature is unfavorable for the synthesis of methanol. However, increasing the temperature accelerates the reaction rate; that is, raising the temperature is beneficial from a kinetic perspective. Therefore, there must be an appropriate reaction temperature for methanol synthesis. During the experimental procedure, it was found that the catalyst activity first increased as the reaction temperature rose, and then decreased again as the reaction temperature increased. Excessively high temperatures can cause thermal sintering of the catalyst, reducing its activity or even causing it to lose its activity. However, when the reaction temperature is below 280°C, the thermal sintering effect of copper-based methanol catalysts can be almost ignored. Because whether we were testing the activity of the catalyst before heat resistance testing or after it, we conducted numerous tests within the temperature range of 120°C to 280°C by repeating heating and cooling cycles, and the reproducibility of the activity results was very good. In other words, the variation trends in the activity of both catalysts (whether before or after the heat resistance test) within the reaction temperature range of 210°C to 280°C are primarily a reflection of the combined effects of reaction thermodynamics and kinetics. Thermal deactivation is caused by the growth of copper oxide grains within the catalyst during use; it is related to the structural stability of the catalyst, and the activity stability of the catalyst can be evaluated through accelerated aging tests. For metal catalysts, thermal sintering-induced deactivation is relatively common. Most of the copper-based catalysts in use currently contain one or more oxides as additives, such as Cr2O3, MgO, and Al2O3; the inclusion of these additives can effectively slow down the thermal sintering of the catalyst. To extend the catalyst’s service life and improve methanol production efficiency, it is necessary to strictly control the operating temperature, ensure stable operation, prevent sudden rises or drops in reaction temperature, and make any changes in load gradually and smoothly. Methanol catalysts have poor heat transfer properties; if the rate of the reduction reaction cannot be controlled and the heat generated by the reaction is not removed in a timely manner, it is easy for the catalyst to overheat or get damaged, leading to the serious consequence of the entire catalyst being destroyed.

This is likely due to the duration of use. At the beginning, the operating temperature of the catalyst is low, around 220°C; as time passes, the catalyst’s activity decreases and the temperature gradually rises, increasing by 5°C at a time, until it reaches 280°C. This is mainly done for economic reasons, in order to make the most efficient use of the methanol catalyst