The impact of pH on VLE, pervaporation, and adsorption of butyric acid in dilute solutions

Abstract

Butyric acid (BA) is an intermediate product and a precursor to the production of butanol in ABE fermentation. Ideally, it would be beneficial to retain as much BA in the fermenter as possible to increase butanol productivity. In this study, experiments were performed to assess the impact of the pH of the feed solution on the separation of BA from dilute aqueous solutions using three separation methods: distillation, pervaporation, and adsorption. Results confirm that the pH of the solution, which dictates the level of BA dissociation, controls the degree of separation of BA from dilute aqueous solutions. Indeed, results show that the vapour‐liquid equilibrium (VLE) curve, the membrane selectivity, and the adsorption capacity for BA in dilute aqueous solutions decreased steadily as the pH is increased from below to above its pKa value of 4.82. The separation performance is strongly correlated with the pH of the feed solution, and, as anticipated, a pH increase reduces the level of separation for these three processes. This is advantageous for the ABE fermentation incorporating a solvent recovery process since BA would remain in the fermenter and improve the production of butanol. However, the pH cannot increase excessively as there exists an optimum pH for conducting the fermentation process such that a judicious level of pH must be sought to optimize a fermentation‐separation integrated process.