Study Of Vacuum Pressure Swing Adsorption Process For Biogas Upgrading And Carbon Dioxide Capture

The biogas resulted from biological degradation of organic compound mainly contains methane and carbon dioxide,has been regarded as a kind of clean and renewable energy with the potential to replace fossil natural gas.Upgrading biogas with the removal of CO₂ is an efficient way to improve the quality and heating value of biogas,and the resulted bio-methane is an alternative of fossil natural gas,which could be used as vehicle fuel and injected into natural gas grid.Currently,the upgrading technique for biogas mainly includes absorption,adsorption,cryogenic and membranes,et al.Among of these methods,vacuum pressure swing adsorption process has drawn more attention for biogas upgrading because of its low energy consumption and low capital investment,as well as its high degree of automation and stronger applicability.Hence,in this work,a systematic study of vacuum pressure swing adsorption process for biogas upgrading and CO₂ capture has been performed by experiment and simulation,employed silica gel and carbon molecular sieve as adsorbent.Firstly,the equilibrium adsorption isotherms of components and dynamic-transient breakthrough curves of CO₂-CH₄ mixture on silica gel adsorbent have been investigated by static volumetric apparatus and fixed bed breakthrough experiments,respectively.The selectivity of 45%CO₂/55%CH₄ on silica gel adsorbent can be up to10.6,calculated by IAST-Langmuir model under operating conditions of 25?and1bar.The LDF mass transfer coefficient of CO₂ and CH₄ on silica gel adsorbent were determined as 1.026s^-1 and 1.696s^-1,respectively,by numerical fitted from two-dimensional mathematical model of adsorption bed.All that has demonstrated that silica gel adsorbent was a good alternative for biogas upgrading,meanwhile the equilibrium isotherms and kinetics parameters were necessary for model and design of PSA process.Meanwhile,a four-bed nine-steps VPSA experimental setup with silica gel adsorbent has been constructed for biogas upgrading,and a rigorous mathematical model of VPSA process has also been developed for modeling of VPSA process to reveal transient variation of concentration and temperature profiles in adsorption bed.The experimental and simulation results have indicated that biogas containing55vol%CH₄ balanced by CO₂ could be enriched to a methane product gas with 98%CH₄purity with a CH₄ recovery up to 80%.The well agreement between experimental data and simulation results has demonstrated that one-dimensional mathematical model with LDF mass transfer model for adsorption bed could predicate concentration and temperature profiles accurately.A two stage VPSA process has been further proposed for biogas upgrading and CO₂ capture through numerical simulation.The bio-methane product gas with 98.01%CH₄ purity and 97.31%CH₄ recovery could be achieved with a carbon dioxide product gas with 96.74%CO₂ purity and 97.58%CO₂ recovery simultaneously.Furthermore,adsorption equilibrium and kinetics of CO₂ and CH₄ on carbon molecular sieve adsorbent have been determined volumetrically.CO₂/CH₄ kinetic selectivity on CMS adsorbent was approximately 163 but its equilibrium selectivity on CMS adsorbent was lower to 2,which indicated CMS adsorbent is potential adsorbent for biogas upgrading with kinetic selection mechanism.Experimental results have also demonstrated that a 98%CH₄ purity product gas with CH₄ recovery higher than 83%could be available from biogas gas consisting of 55%CH₄ and 45%CO₂by using a three bed six steps VPSA process with CMS adsorbent under desorption pressure of 0.1bar and product purge ratio of 0.05.The proposed mathematical model for CMS adsorption bed with dual-LDF mass transfer and extended Sips isotherm model could predicate mass and heat transfer behaviors accurately during breakthrough experiments and VPSA experiments.In addition,a systematic comparison and analysis of four bed nine step VPSA process with silica gel adsorbent and CMS adsorbent for biogas upgrading has been performed by using numerical simulation.Simulation results indicated that energy consumption and adsorbent requirement for CMS process were 20.45%and27.25%higher than that of silica gel process,for processing the same biogas throughput.Silica gel VPSA process was superior to CMS VPSA process.Finally,Duplex PSA process with silica gel adsorbent has been proposed and explored for methane enrichment and carbon dioxide capture simultaneously from biogas.Numerical results demonstrated that not only the CH₄ purity and recovery in methane product gas could be up to 98%and 96%,but also the CO₂ purity and recovery in carbon dioxide product gas could achieve to 98%and 96%,respectively,when adsorption pressure was set to be 2bar and desorption pressure was assigned to be 0.2bar in duplex PSA process.However,the Duplex PSA process possessed higher energy consumption and lower unit productivity in comparison with dual stage VPSA process.