Can the pressure in the pre-depropanization process be reduced?
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The pyrolysis gas is compressed to 3.6 MPa in order to ensure the attainment of a certain purity level for methane and hydrogen. Since the pre-depropanization process employs a pre-hydrogenation step and does not require high-purity hydrogen, could the final compression pressure of the pyrolysis gas be reduced? It’s just a temporary idea; everyone is welcome to discuss it.
The original poster’s idea won’t work; the device design has already been finalized. Can the final pressure of the compressed pyrolysis gas be reduced? If the pressure at the outlet of the pyrolysis gas is reduced, the suction pressure will increase. This leads to a longer residence time of the feedstock in the furnace tubes of the radiation section, resulting in increased secondary reactions, a lower yield of ethylene, and more coking in the furnace tubes. At the same time, when the pressure in the cold box is low, its cooling capacity remains constant, which reduces the separation efficiency and results in losses of ethylene product.
One of the main functions of the pyrolysis gas compressor is to provide the pressure required for separation; if the pressure is reduced, the boiling point of the pyrolysis gas decreases (for example, methane has a boiling point of -161.5°C at standard pressure). The critical temperature of methane is -82.6°C, and its critical pressure is 4.59 MPa; therefore, a lower temperature of the refrigerant is required for condensation. This results in reduced power consumption at the pyrolysis gas compressor, but increased power consumption is needed at the refrigerant compressor. As far as I know, Lummus has currently developed a low-pressure quenching process: this technology, developed by Lummus, is suitable for pre-hydrogenation processes. The pressure of the material before entering the pre-cooler is only 2.0 MPa. After step-by-step cooling and condensation, as well as separation in a fractional distillation column, the gas stream at the top of the column contains no ethylene. After being compressed by the booster, it undergoes staged cooling and condensation; the material that condenses first is used as reflux for the demethanization tower ； The post-condensed streams are separated as high-pressure methane and low-pressure methane streams ； The non-condensable part is the hydrogen product with a purity of 95%. For the low-pressure demethanization column system, in order to reduce the energy loss caused by pressure reduction through throttling valves, low-pressure quenching is proposed to be used in pre-cooling in order to save energy consumption.
I don’t quite understand; let’s learn it together*.