Fractionation of an acidic hydrolysate from steam-treated wood using pervaporation

Abstract

In this study, commercially available pervaporation membranes were evaluated for the recovery of organic acids and furfural from an acidic hydrolysate stream from steam treated wood. Membranes that did not comply with the imposed feed and process conditions were eliminated during the screening process. It was found that the polydimethylsiloxane (PDMS) and polyether block amine (PEBA) membranes from Pervatech B.V (Netherlands) were best suited for this application since both membranes showed a high affinity towards acetic acid and furfural during the pure component sorption experiments. The influence of the feed temperature on the flux and selectivity through the membranes were investigated by increasing the feed temperature from 40 to 80°C, using intervals of 10°C. Pervaporation experiments were initially carried out with a 5 g/L acetic acid-water solution and a 1 g/L furfural-water solution to determine whether the selected membranes were able to recover acetic acid and furfural from dilute aqueous solutions. Both the PEBA and PDMS membranes were selective towards furfural during the binary mixture experiments, with maximum furfural-water separation factors of 3.78 ± 0.45 and 6.44 ± 0.11, respectively. However, both membranes were more selective towards water than acetic acid, with acetic acid-water separation factors ranging from 0.12 to 0.50. Both membranes were tested for the recovery of selected components from the acidic hydrolysate from steam treated wood. The acidic hydrolysate consisted of water, furfural, organic acids (acetic acid, citric acid, formic acid, and succinic acid), sugars (arabinose, glucose, xylose, cellobiose, and sucrose), sugar alcohols (glycerol, sorbitol and xylitol) and higher molecular weight components (polysaccharides and lignin fractions). It was found that the PEBA membrane only separated water and trace amounts of acetic acid and furfural from the acidic hydrolysate. On the other hand, the PDMS membrane separated water, furfural, organic acids (acetic acid, citric acid, formic acid and succinic acid) and trace amounts of monomeric sugars (glucose and xylose) and sugar alcohols (sorbitol and xylitol) from the acidic hydrolysate. The PDMS membrane performed better than the PEBA membrane in terms of total flux and selectivity towards furfural and organic acids. The PDMS membrane yielded a maximum total flux of 2638 ± 132 g.m-2.h-1 at 80°C, with maximum organic acids and furfural enrichment factors of 0.25 ± 0.02 and 2.12 ± 0.06, respectively. The PEBA membrane primarily removed water from the acidic hydrolysate since the permeate consisted of 99.9 wt% water and the maximum acetic acid and furfural enrichment factors were only 0.022 ± 0.003 and 0.103 ± 0.005, respectively. The highest total flux obtained when separating the acidic hydrolysate with the PEBA membrane was 1203 ± 39 g.m-2.h-1. The PEBA membrane was therefore found to be more suited for the recovery and purification of water. The inferior performance of the PEBA membrane in terms of its selectivity towards organic acids and furfural was ascribed to concentration polarisation. The permeation of components through the membranes was modelled using Fick's law and the solution-diffusion model. Fick's law predicted the partial water flux through both membranes accurately, while both Fick's law and the solution-diffusion model predicted the partial acids flux through the PDMS membrane accurately. Both models were less accurate in predicting the partial fluxes of furfural, sugars and sugar alcohols. The reason being that both models used an Arrhenius relationship to describe the temperature dependence of the flux, while the permeation of furfural, sugars and sugar alcohols did not follow an Arrhenius relationship with increasing temperature