Modeling and Simulation: Investigation of Vapor-Liquid Equilibrium of CO2 Absorption in Aqueous Piperazine Solvent for Application in Biogas Upgradation Using ENRTL Model

Abstract

One of the primary objectives of biogas refinement is carbon dioxide (CO₂) removal. Several studies investigated the effect of piperazine (PZ) in a mixture with other amines; however, its capability has rarely been investigated as a stand-alone absorbent. In this study, CO₂ solubility equilibrium simulations are performed using the Aspen Plus (v7.3.2, AspenTech) simulation tool for the CO₂/H₂O and CO₂/H₂O/aqueous PZ component systems to predict the CO₂ absorption rate and absorption capacity of the solvent. CO₂ equilibrium partial pressures (up to120 kPa) were calculated using electrolyte non-random two liquid (ENRTL) model with a full explanation of crucial dissociation representing the complicated electrolyte system based on Henry’s Law. The ENRTL model was used to calculate the activity coefficients in the electrolyte system, while the dissociation and equilibrium constants were implemented as a function of temperature and or obtained from reference state free energy. Aspen Plus calculations from the ENRTL model showed excellent results having less than 4% deviation from the experimental results reported in the published literature. A selected range of 0.1–1.2 m PZ concentration and temperature between 298–343 K was used to estimate CO₂ loadings using the simulations from the validated model. Various concentrations of aqueous PZ were then compared to benchmark solvent, that is, pressurized water for CO₂ absorption loading. The observed result showed that CO₂ absorption at PZ concentration of 0.5 m was 120% higher than the pressurized water solvent at the same temperature and partial pressure conditions. Additionally, the efficiency of  CO₂ removal obtained by using PZ was also observed to be higher than pressurized water scrubbing. In short, the obtained encouraging results portrays that PZ may be used as model solvent for CO₂ absorption from biogas and to limit its effect toward global warming.