What causes a sudden increase in pressure in the synthesis system?
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Our company has a methanol synthesis plant with an annual production capacity of 200,000 tons; it came online in September last year and was operated at 30% load for over two months before the system had to be shut down due to issues with the feed gas. After parking, the entire system was purged with nitrogen, and then the pressure in the system was maintained. Oxygen content and flammable gas analyses were conducted weekly, with results remaining below 0.5%. It was put back into operation in May this year; 10 days after operation, the system load was increased from 30% to 80%. During this pressure increase, the system pressure rose from 48 kilograms to 52 kilograms within 10 minutes, with a gas supply rate of 40,000 cubic meters per hour at that time. Then, the resistance in the synthesis tower suddenly increased, rising from 0.2 MPa to 0.8 MPa. The gas composition at that time featured a high content of inert gases in the system. Experts, please analyze what could be the reasons for this sudden increase in system pressure? Thank you! ! !
A high content of inert gases results in a relatively low concentration of the active gas components, which affects methanol synthesis and leads to less methanol being produced; in such cases, it is appropriate to increase the vent volume after the tower. But this sudden increase is so significant; it doesn’t seem to be caused solely by inert gases. Check if there is any change in the circulating air volume – has it decreased or been interrupted?
The increase in system pressure and an inappropriate hydrogen-to-carbon ratio are related to the high content of inert gases and the outlet temperature of the water cooler (the methanol gas content in the circulating gas). However, an increase in the resistance of the synthesis tower is abnormal; it is unclear whether this is related to the previous system, what purification process is used, and whether impurities may end up covering the surface of the catalyst. Also, consider whether waxing has occurred.
At that time, the circulation rate remained almost unchanged, at around 200,000. The purification process involved both rough desulfurization and fine desulfurization. At the inlet of the synthesis unit, no total sulfur was detected. Additionally, on the day before that, there was wax accumulation in the inlet filter of the methanol distillation feed pump, and the problem was quite severe; the pump had to be replaced every half hour or so； What I would like to know now is that we are a company that has just started operations; the catalysts have been newly installed. The operating temperature at the inlet of the synthesis tower is kept around 200 degrees, while the peak temperature is maintained at around 230 degrees. Additionally, our coolers use air cooling, and the outlet temperature of these coolers remains around 37 degrees. Could you please advise me on the reasons for this?
It becomes easy to understand if we consider the process of introducing air to increase pressure. The amount of air at the compressor inlet gradually increases, and this increase in flow rate is controlled by the amount of air entering the compressor and the amount of air that is released. During the pressure-increasing process, if more air is introduced while the amount of air released does not increase, the pressure will rise more rapidly. When the flow rate increases, the resistance also increases. If the resistance suddenly rises, it is also due to an increase in accumulation, such as an increase in carbon dioxide levels in the loop. It’s also possible that as methanol production increases, the resistance will grow.
The high resistance in the synthesis tower is likely related to the pulverization and fragmentation of the catalyst, as well as its sintering and caking. Did any accidents occur during production?
It shouldn’t be wax formation. Because of the high reactor temperature, if there was wax, it would have been removed long ago. First, the pressure difference gauge should be checked to see if there is any issue. Next, determine whether there is a pressure increase; if the rate of pressure change is too fast, it may cause the catalyst to break down.
First, check whether there is any change in the amount of fresh gas. Next, see if the amount of vented gas is high. If there are no problems in these two aspects, then check whether the temperature of the synthesis tower is still easy to control. If it isn’t easy to control, it means the catalyst has aged. Also, I don’t know what purity level the nitrogen used for replacement has, as oxidation can occur as long as oxygen is present. If that is the case, you can try raising the temperature of the synthesis tower slightly
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I really don’t know how your catalysts are protected. Generally, high-purity nitrogen at pressures of 4 and 9 is used to check whether there are any issues with the catalysts. Additionally, the amount of off-gas might be too low, and the waxing phenomenon also deserves attention