Identification of thermodynamic coupling phenomena and its relevance on flux coupling in pervaporation for bio-refining butanol from multicomponent Acetone-Butanol-Ethanol (ABE) mixtures

A comprehensive investigation of sorption characteristics and the influence of thermodynamic coupling on targeted organophilic pervaporation of bio-butanol from multicomponent Acetone-Butanol-Ethanol (ABE) fermentation solutions is presented, emphasizing the effect of penetrant-penetrant and penetrant-membrane interactions on sorption and permeation. In addition to initial screening of thermodynamic models and simulation of activity coefficients and volume fractions, pervaporation experiments were also analogously conducted for multicomponent ABE feed mixtures. Statistically validated second order models were then derived to forecast the influence of solvent concentrations, feed temperature and mutual interaction terms on butanol volume fractions and partial fluxes. Negative first order coupling effects due to ethanol and acetone are observed to be more significant for both activity coefficients and volume fractions, with sorption being most inhibited when acetone and ethanol are at their highest concentrations in feed. In contrast, contribution of non-linear and second order coupling effects is dominant in flux coupling, with positive "drag effects" due to additional plasticization being less pronounced at higher solvent concentrations. For instance, at 313.15 K, increasing ethanol and acetone concentrations by 3 wt% and 6 wt%, respectively, predicts reduction in volume fractions by approximately 29 % and 43 % compared to 3 wt% butanol-water binary systems. Conversely, the corresponding flux values exhibit initial increase of approximately 17 %, followed by subsequent decrease of around 14 %. However, sorption isotherms indicate that the penetrant-polymer interactions have a negligible influence on butanol volume fractions in the PDMS membrane irrespective of feed composition. Similarly, analysis of flux isotherms also suggests that flux coupling effects may ultimately be traced to thermodynamic coupling phenomena. Consequently, prudent membrane selection for pervaporation becomes crucial to overcome the inherently detrimental thermodynamic coupling effects and improve butanol recovery.