Enrichment of low-grade methane by dual reflux vacuum swing adsorption

Abstract

Methane (CH4) has been recognized as a clean energy resource and crucial chemical feedstock. However, low-grade CH4 gases (<30%) such as coal mining gas are often released directly to the atmosphere due to limited economic values and technical challenges in enrichment, transportation and utilization, resulting in significant greenhouse gas (GHG) emissions and energy waste. In this work, a non-isothermal model of the dual reflux vacuum swing adsorption (DR-VSA) was developed on the Aspen Adsorption platform to numerically study the enrichment of low-grade CH4 from N2 gas by activated carbon (AC) and ionic liquidic zeolites (ILZ). Since the low-grade methane source was often at atmospheric pressure, feed gas was introduced to a high-pressure column (1 bar) and evacuation was operated in a low-pressure column (0.2 bar) by a vacuum pump, avoiding the utilization of compressors. Two cycle configurations with pressure reversal at the heavy end (type A) and light end (type B) were demonstrated and investigated. The parametric study of DR-VSA cycles was achieved by varying the operating variables such as the light reflux flow rate, theoretical heavy product purity and step duration. A new optimization approach was proposed and demonstrated using a dual convergence algorithm to iteratively vary operating conditions and a multiplicative score function to evaluate separation performances. Results indicated that the A-type cycle achieved better separation performance than the B-type cycle, and ILZ showed a significant superiority to the AC. In the case of 20% CH4 feed gas, the A-type process using ILZ adsorbents achieved a CH4 purity, recovery, and energy consumption of 80.2 mol.%, 95.5%, and 180.8 kJ/(mol CH4 captured), respectively. The enrichment of low-grade CH4 via the DR-VSA cycle is energy-sustainable and can generate high-quality fuel gas and GHG mitigation benefits.