Mass transfer approaches for CO2 separation in non-isothermal and non-adiabatic pressure swing adsorption system for biomethane upgrading

The mass transfer of CO2 over solid materials can be quite complex. The complexity arises as a result of the adsorbent's complex surface morphology and dynamic process operating conditions. In this study, a new model was proposed to predict the mass transfer behavior of CO2 over silica gel in biogas upgrading process in which three mass transfer systems (lumped, micro-/macropore, and particle) were introduced. An overview for such mathematical models for CO2 mass transfer was conducted and then a new non-adiabatic and non-isothermal model for pressure swing adsorption (PSA) that is different from the existing models in the literature, i.e., adiabatic and isothermal models, was built and assessed. The new model was executed using Aspen Adsorption and validated using experimental data reported previously over silica gel bed. The validation was performed by comparing molar fractions of CH4 and CO2, temperature profile of silica gel bed, and bio-CH4 purity and recovery with previously reported experimental data. The particle mass transfer system, which estimated the adsorption rate and gas concentration profile in the adsorbent's particle, recorded the highest bio-CH4 purity of 99.12%. Lumped approach, which comprised mass transfer resistances in one overall resistance and ignored the dispersion effect, recorded bio-CH4 purity of 97.63% followed by micro-/macropore, which accounted micro and macropore resistance individually, with purity of 84.65%. Particle mass transfer approach yielded the highest bio-CH4 recovery of 96.745%, followed by lumped (95.41077%), and micro/macropore (82.433%) systems. Particle mass transfer was one of the most realistic systems in predicting the CO2 diffusion over silica gel due to its comprehensive heat/mass transfer consideration along with its assumption of uniform pore structure that fitted well with the true nature of the microporous silica character.