Preparation, Characterization and Optimization of Silica-PDMS Superhydrophobic Composite Membrane for Pervaporation Concentration of an Aqueous Ethanol Solution

Sweet sorghum bagasse pretreated with 5% (by weight) acetic acid was used in a 96 h fed-batch simultaneous saccharification and fermentation (SSF) at an accumulated solid biomass concentration of 20% (w/v). A composite membrane fabricated by depositing a hierarchy of hexamethyldisilazane modified 8 mum and 100 nm silica and thin layer of PDMS on a stainless steel support of 2 mum pores was used for the pervaporation concentration of ethanol-water solutions. The experimental data from the response surface methodology and central composite rotatable design have been modeled by a multiple regression (MR) quadratic model. The MR models were combined with the desirability function to optimize the pervaporation process via the Design-Expert 8.0.7.1 software. An ASPEN Plus user-defined pervaporation model coupled with a Microsoft Excel spreadsheet was then developed to predict the total permeate flux (Jt) and ethanol selectivity (alpha) from experimental conditions. The effects of the main by-products in a fermentation broth on the pervaporation process were studied.;The fed-batch SSF produced a maximum ethanol concentration of 53.1 g/L and an 88.7% ethanol yield. The SEM surface and cross-sectional morphologies showed a dense PDMS top layer and good interlayer adhesion. The water contact angle of the composite membrane was 151.8° as compared to 100° for the pure PDMS membrane. The optimal Jt and alpha were 589.8 g/m 2·h and 10.3, respectively, increasing the ethanol concentration from 8.5 to 48.9% wt. at 52.3°C (feed temperature) and 30 Pa (permeate pressure). The combined ASPEN Plus and Microsoft Excel model predicted accurately Jt and alpha within the range of experimental conditions and extrapolated with reasonable accuracy. The fermentation by-products increased alpha with the exception of glycerol that decreased alpha and a synergetic effect was observed when the entire components were combined. As expected, the components had reverse effect on Jt.