Modelling of membrane contactor for biogas purification

Abstract

Biogas upgrading demands removal of selective carbon dioxide (CO2) while sustaining recovery of methane (CH4) and process complexity. This thesis developed an aqueous potassium hydroxide (KOH) with a hollow-fiber membrane contactor (HFMC) for CO2 absorption by using biogas feed of 40% CO2 and 60% CH4, along with one-dimensional, steady state models. The membrane contactor was divided into 20 axial mass balance segments, with a resistance-in-series formulation of gas-film, membrane and liquid-film transport. Four configurations were analyzed, co-current physical absorption (Model 1), counter-current physical absorption (Model 2), counter-current with constant enhancement factor (Efac) (Model 3), and counter-current with an Efac based on finite-reactant kinetics of CO2-OH− type reactions (Model 4). Models 1−3 were independently implemented in Microsoft Excel and Aspen Custom Modeler (ACM), while Model 4 was solved directly with ACM with Excel based recalculation and post processing. CO2 removal of Model 1 under base conditions was 23.9%, increased to 26.0%, 30.9% and 33.0% in Models 2−4, respectively. While the average CO2 flux of Model 4 is reported highest as 2.28 × 10−4 mol m−2 s−1. The contribution of liquid-side resistance decreased to 12.3−12.8% with the reaction enhancement, while the dominant membrane resistance contributed 64.53−64.87% to total. Results from Excel and ACM agreed on Models 1−3, while Model 4 met mass balance, monotonic profile and enhancement convergence criteria. Literature benchmarking of physical absorption model predicted 27.0% CO2 removal. Sensitivity analysis showed that fractional CO2 removal decreases with increasing gas flowrate (Qg) and vice versa, while it increases with increasing liquid flowrate (Ql), fixed Efac, apparent membrane coefficient (km) and effective membrane area (Aeff) within relevant ranges of models. While the framework is appropriate for comparative model evaluation and preliminary design analysis, experimental validation along with better treatment of membrane wetting, hydroxide depletion and electrolyte speciation are needed prior to predictive industrial application.