Membrane-Assisted Removal of Hydrogen and Nitrogen from Synthetic Natural Gas for Energy-Efficient Liquefaction

Abstract

Synthetic natural gas (SNG) production from coal is one of the well-matured options to make clean utilization of coal a reality. For the ease of transportation and supply, liquefaction of SNG is highly desirable. In the liquefaction of SNG, efficient removal of low boiling point impurities such as hydrogen (H2) and nitrogen (N2) is highly desirable to lower the power of the liquefaction process. Among several separation processes, membrane-based separation exhibits the potential for the separation of low boiling point impurities at low power consumption as compared to the existing separation processes. In this study, the membrane unit was used to simulate the membrane module by using Aspen HYSYS V10 (Version 10, AspenTech, Bedford, MA, United States). The two-stage and two-step system designs of the N2-selective membrane are utilized for SNG separation. The two-stage membrane process feasibly recovers methane (CH4) at more than 95% (by mol) recovery with a H2 composition of ≤0.05% by mol, but requires a larger membrane area than a two-stage system. While maintaining the minimum internal temperature approach value of 3 °C inside a cryogenic heat exchanger, the optimization of the SNG liquefaction process shows a large reduction in power consumption. Membrane-assisted removal of H2 and N2 for the liquefaction process exhibits the beneficial removal of H2 before liquefaction by achieving low net specific power at 0.4010 kW·h/kg·CH4.