Nitrogen replacement strategy to efficiently enhance methane recovery of the methane purification process using nitrogen equilibrium selective sorbent

MIL-100(Cr) is a very promising sorbent based on an equilibrium effect for rejecting nitrogen from substandard quality natural gas. In this work, methane purification from a 25% N2/75% CH4 mixture has been studied using MIL-100(Cr) as a sorbent, on the basis of which an N2 displacement-assisted vacuum-pressure swing adsorption (N2DIS-VPSA) process is proposed. Adsorption equilibrium isotherms of N2 and CH4 have been measured over MIL-100(Cr) powder and pellets, and N2/CH4 selectivity has also been calculated based on ideal adsorbed solution theory (IAST). A rigorous mathematical model has been used to investigate gas- and solid-phase concentration evolution histories to deeply understand the internal mechanism of the N2 replacement process. Breakthrough experiments have revealed that a returned N2 flow could replace almost all of the co-adsorbed CH4, greatly improving the CH4 recovery. At the same time, both experimental and simulation results showed that the N2 replacement process had no effect on the purity of CH4. However, it should be noted that an excessively long N2 replacement time led to incomplete subsequent regeneration of the sorbent, thereby reducing the purity of CH4 in the N2DIS-VPSA process. By optimizing the N2 replacement duration, a high purity CH4 (97%) product gas could be obtained with 92% recovery, much higher than that of MIL-100(Cr) applied into common PSA process (58%).