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

• Cryogenic distillation process of CO 2 /CH 4 separation is adopted to upgrade biogas. • New technique is developed to mitigate CO 2 freeze-out in the distillation column. • With avoiding freezing, CH 4 purity is enhanced from 60 to 97.2% by only one column. • High-purity liquid CO 2 (>99%) is also generated on merit as a valuable by-product. • The system showed promising energy and cost efficiency against traditional methods. In this paper, ideas for improving the cryogenic separation of CO 2 /CH 4 mixture are drawn in order to upgrade biogas to biomethane, matching natural gas standards. On this basis, a novel technique is developed to mitigate CO 2 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 CH 4 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 CO 2 (above 99%) can be generated rather than being emitted into air. Compared to former cryogenic systems, the present process achieves not only the highest CH 4 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 CH 4 and CO 2 purities generated, witnesses the lowest in methane loss, is among the least in energy consumption, and shows high competitiveness in investment cost. [ABSTRACT FROM AUTHOR]