Biogas-Dioxolane-Mixed Hydrates: Evaluation of Kinetic Performance Coupled with Separation Study

Abstract

Biogas separation can provide quality CH₄ gas which can be used as a fuel alongside CO₂ capture and sequestration. Hydrate-based gas separation can be a potential technology for effective separation of biogas owing to its compact storage, environmentally benign nature, safe and a simplified process with no chemical reactions involved. This study investigates the effect of 1,3 dioxolane (DIOX) on the kinetics and separation of 50–50 mol % CO₂–CH₄ (biogas) mixture using DIOX concentrations of 1, 3, and 5.56 mol % and presents morphological observations during hydrate formation/dissociation. Addition of bioadditives (l-methionine and l-arginine at 0.5 wt % concentration) was also examined particularly with the stoichiometric concentration (5.56 mol %) of DIOX. Experimental results showed the kinetic promotion effect in terms of rate of hydrate formation, t₉₀, and final gas uptake was enhanced with the increasing concentration of DIOX. The highest gas uptake of 49.88 ± 0.75 mmol/mol was obtained using 5.56 mol % DIOX + 0.5 wt % l-methionine with the shortest t₉₀ of 45.4 ± 1.83 min. Kinetic promotion ability of DIOX was found to increase with the increasing concentration of DIOX for the 50–50 mol % CO₂–CH₄ biogas mixture. A dissociation study showed a decrease in rate of normalized moles of gas release on increasing concentration of DIOX from 1 to 5.56 mol %. On adding additives, dissociation rate increased for 0.5 wt % l-methionine and it was observed to be the highest for 0.5 wt % l-arginine; thus, the kinetic additives have a strong influence on hydrate dissociation despite their effect in enhancing the formation kinetics, gas recovery, and the separation factor. Based on the observations, it was inferred that for the separation of equimolar biogas mixture, nonstoichiometric concentrations (<5.56 mol %) are preferred rather than stoichiometric concentrations of DIOX.