Low-Temperature Distillation Process for CO2/CH4 Separation: A Study for Avoiding CO2 Freeze-Out

Abstract

In published literature, only very limited studies were carried out for low-temperature biogas upgrading for CO2/CH4 mixture separation due to the freeze-out of CO2 under low temperature, which causes several operational problems. Therefore, the present study aims to provide in-depth analysis for a low-temperature distillation process of a typical model of biogas mixture (CH4 + CO2) to tackle the problem of freezing. The process has been optimized by means of varying distillation column feed pressure, temperature and CO2 concentration, reflux ratio, feed stage number, and produced methane purity to lower the risk of CO2 freezing in the column. The modeling results reveal a substantial feature of the low-temperature process that it can capture CO2 in liquid phase with a purity of 99.5 mol % as a valuable byproduct for transport. Additionally, it is found that increasing the column reflux ratio mitigates the risk of CO2 freeze-out allowing the column to reach higher CH4 purities (up to 97 mol %) without CO2 solidification. Moreover, the occurrence of CO2 freeze-out in the column is not affected within a relatively wide range of feed CO2 concentrations. The low-temperature technique can serve as a new promising approach for biogas upgrading overcoming the risk of CO2 frosting.