Simulation on Vacuum Pressure Swing Adsorption Processes for CH₄ Separation from Biogas

In order to meet vehicle gas requirements, a vacuum pressure swing adsorption (VPSA) method was proposed to acquire high purity CH₄ (over 95%) from biogas composed of 60%(V) CH₄, 30%(V) CO₂ and 30%(V) N₂. The adsorption of CH₄, CO₂, and N₂ using 13X and CMS was measured, and the vacuum pressure swing adsorption (VPSA) process was divided in two stages based on the adsorption results. In the first stage, the 13X molecule sieve was used as the adsorbent for CO₂ removal by a six-bed seven-step process. In the second stage, carbon molecule sieve (CMS) was employed as the adsorbent for N₂ capture by a three-bed six-step process. The breakthrough curves of CH₄, CO₂ and N₂ mixtures on 13X and CMS were calculated by mathematical models and compared with experimental values. The results demonstrate that the calculated values are in good agreement with that of experiments, which validates the accuracy and reliability of the mathematical models. Simulation results show that CH₄ purity of 99% with a total recovery of 65% is achieved. The effects of operating pressure in the first stage and adsorption dynamic parameters of CH₄ and N₂ in the second stage on purity and recovery of CH₄ were discussed. The results show that higher adsorption pressure and lower desorption pressure lead to the improvement of purity but decrease of recovery. When CMS shows fast adsorption of N₂ and slow adsorption of CH₄, the purity of CH₄ can achieve 99%, and this can be used as guidance for the development of CMS.