What are the causes of paraffin formation in methanol synthesis?

Reasons related to temperature: 1. The temperature at the inlet of the synthesis tower is too low, at 170–180°C. 2. The reaction temperature in the synthesis tower is too high, above 300°C.

Reasons related to gas composition: 1. Too low H/C ratio; 2. Excessively high CH4 level

1. Fe(CO)5, Ni(CO)4, and a small amount of Mo(CO)4 are catalysts for the production of CH4. Their presence causes the reaction to release heat rapidly, which is harmful to the catalyst, and also results in the formation of small amounts of higher alkane waxes. When the system contains excessive rust impurities, Fe(CO)5 can be formed at 130–150°C. Fe(CO)5 is a volatile substance that enters the catalyst along with the process gas; at 250°C, it decomposes into elemental iron, thereby blocking the catalyst’s micropores and causing premature deactivation. 2. Methanol synthesis gas contains a small amount of H2S, which can corrode pipelines and equipment and cause rust; the presence of rust facilitates the occurrence of the carbonylation reaction. If the system is not thoroughly purged before operation, the residual rust remaining in the equipment and pipes will accelerate the carbonylation reaction, resulting in the formation of Fe(CO)5, Ni(CO)4, and a small amount of Mo(CO)4, thereby promoting the waxing reaction. 3. Wax formation is likely to occur when the methanol content in the circulating gas exceeds the specified limit. When the methanol separator does not perform well and an excessive amount of unseparated methanol is carried along and enters the catalyst bed again, this can exacerbate side reactions and lead to the formation of higher alcohols.

Excessively high CH4 level (cycle gas content above 30%, even 50%)

What is the mechanism behind wax formation when CH4 levels are too high (with a recycle gas content of over 30%, or even 50%)?