Low-Temperature Distillation Process for CO_2/CH_4 Separation: A Study for Avoiding CO_2 Freeze-Out

摘要

In published literature, only very limited studies were carried out for low-temperature biogas upgrading for CO_2/CH_4 mixture separation due to the freeze-out of CO_2 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 (CH_4 + CO_2) to tackle the problem of freezing. The process has been optimized by means of varying distillation column feed pressure, temperature and CO_2 concentration, reflux ratio, feed stage number, and produced methane purity to lower the risk of CO_2 freezing in the column. The modeling results reveal a substantial feature of the low-temperature process that it can capture CO_2 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 CO_2 freeze-out allowing the column to reach higher CH_4 purities (up to 97 mol %) without CO_2 solidification. Moreover, the occurrence of CO_2 freeze-out in the column is not affected within a relatively wide range of feed CO_2 concentrations. The low-temperature technique can serve as a new promising approach for biogas upgrading overcoming the risk of CO_2 frosting.