Equilibrium Study and Analysis of Site Energy Distribution of Butanol Sorption on a Biosorbent

Abstract

A biosorbent derived from oat hulls was employed to dehydrate butanol from water mixture to obtain a high-purity butanol product. As this process involves water and butanol binary sorption, it is important to investigate equilibrium of water and butanol sorption on the biosorbent. Previous studies investigated water sorption from a mixture of butanol and water. This work emphasizes the equilibrium of butanol sorption from pure butanol and butanol–water binary systems. The Dubinin–Polanyi models (applicable to micropore and large pore materials), which are equilibrium models derived from the adsorption potential theory of Polanyi, were applied to simulate the experimental data for butanol sorption on the aforementioned biosorbent. The models agreed with the experimental data. The parameter q0 (limiting mass for sorption) was estimated in the butanol–water binary as well as the pure butanol systems. The comparison of q0 from single and binary systems indicated that the competitive sorption occurred between butanol and water on the biosorbent. In order to analyze the energetics of the butanol sorption, the approximate site energy distribution of butanol sorption was further investigated. The results showed that water sorption had a higher value of weighted mean of site energy distribution than butanol sorption. This biosorbent had higher affinity to water and therefore had higher water sorption capacity. The dipole–dipole attraction, one kind of van der Waals force, could be one of the key mechanisms of butanol sorption by the biosorbent. Additionally, thermodynamic parameters were determined. The results indicated that the butanol sorption process was spontaneous and exothermic.