Upgrading biogas to biomethane and liquid CO2: A novel cryogenic process

Abstract

In this paper, ideas for improving the cryogenic separation of CO2/CH4 mixture are drawn in order to upgrade biogas to biomethane, matching natural gas standards. On this basis, a novel technique is developed to mitigate CO2 freeze-out, which was a crucial obstacle upon implementing cryogenic approaches. In the proposed process, four-stage compression, one distillation column combined with flash separator, and sufficient heat recovery are adopted. The process is simulated in ASPEN HYSYS. The thermodynamic framework is validated against experimental data. Additionally, to reveal the efficacy of the added modifications, different configurations are modeled, starting from the simple system to the proposed one. The operating conditions – distillation pressure, reflux ratio, feed tray location, and inlet composition – are optimized towards minimizing energy demand and diminishing frosting danger. With avoiding frosting presence, the suggested process can boost CH4 purity from 60% up to 97.2% (mol), which has not been reached before in any previous studies using only one column, suitable for commercial uses. Also, a valuable by-product of high-purity liquid CO2 (above 99%) can be generated rather than being emitted into air. Compared to former cryogenic systems, the present process achieves not only the highest CH4 purity (97.2%) – even higher than previous study utilized two columns – but also the least energy penalty (0.38 kWh/Nm3cleaned_gas). Another comparison held against conventional approaches, the proposed process stands among the highest in CH4 and CO2 purities generated, witnesses the lowest in methane loss, is among the least in energy consumption, and shows high competitiveness in investment cost.