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This post was last edited by MTO001 on 2021-7-18 at 18:13. The main factors affecting the ethylene yield in the demethanization tower are the methane/hydrogen ratio, operating temperature, and pressure. The higher the methane/hydrogen ratio, the less ethylene is lost in the overhead gas, and the higher the yield of ethylene. The higher the content of hydrogen and inert gases in the pyrolysis gas, the lower the methane/hydrogen ratio becomes; as a result, more ethylene and ethane must be lost to meet the requirements of the tower top dew point, which leads to a decrease in the yield of ethylene. Effect of different pyrolysis feedstocks on methane hydrogen ratio. Feedstock methane/hydrogen (molar ratio); temperature at the outlet of the demethanizer top condenser when all ethylene is removed (°C); average mol% of ethylene in the methane hydrogen fraction at -100°C: 3.1 MPa, 4.5 MPa, 3.1 MPa, 4.6 MPa. Kerosene pyrolysis: 3.5:1 – 101, -901–; Propane pyrolysis: 1:1 – 114, -10342; Ethane pyrolysis: 1:4 – 134, -1261911. 1) An increase in pressure and a decrease in temperature help to reduce the loss of ethylene in the exhaust gases and improve the yield of ethylene. As pressure increases, the relative volatility of methane and ethylene decreases, which hinders the separation of the components and affects the purity of ethylene. 2) Reducing pressure and lowering the operating temperature can increase the relative volatility of methane and ethylene, which is beneficial for component separation. Pre-dehydrogenation process: By using pre-condensation, most of the hydrogen and a portion of the methane in the pyrolysis gas are separated, reducing the amount of methane that enters the demethanization unit. This leads to a significant decrease in the methane/hydrogen ratio, improving the operation of the demethanization tower and increasing the yield of ethylene. Post-dehydrogenation process: The pyrolysis gas enters the demethanization tower directly after drying and pre-cooling, and the methane and hydrogen at the top of the tower are then separated through condensation. Comparison of pre-dehydrogenation and post-dehydrogenation: The advantage of pre-cooling is that it utilizes stepwise condensation and multiple feed streams, which allows for the saving of low-temperature refrigerants and reduces the load on the demethanization tower ； The ethylene recovery rate and hydrogen recovery rate are high, allowing for the production of hydrogen at high concentrations. Separating hydrogen before feeding increases the methane/hydrogen ratio, thereby improving the separation efficiency of the demethanization tower. Foaming behavior of the material in the demethanization tower: The material in the demethanization tower exhibits a certain degree of foaming under low-temperature operating conditions, and the foaming tendency is even greater in the low-pressure demethanization tower. This factor must be given due consideration in tray design, in order to control the flow velocity within the tower and the tray spacing. Ignoring the foaming tendency of the material in the demethanization tower will severely affect its processing capacity. The low-pressure demethanizer is only suitable for processes with a high methane/ethylene ratio in the pyrolysis gas. The low-pressure method requires sufficient methane and ethylene to enter the system in order to provide a certain amount of reflux. In ethane cracking, the methane/ethylene ratio is very low, so not enough methane is available for reflux. The pyrolysis gas produced from heavy pyrolysis feedstocks has a higher methane/ethylene ratio and a higher methane yield. There is sufficient methane available for reflux to ensure a minimum reflux flow rate for demethanization.