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STABILITY OF MFI ZEOLITE-FILLED PDMS MEMBRANES DURING PERVAPORATIVE ETHANOL RECOVERY FROM AQUEOUS MIXTURES CONTAINING ACETIC ACID
Citation:
BOWEN, T., R. MEIER, F. R. ALVAREZ, AND L. M. VANE. STABILITY OF MFI ZEOLITE-FILLED PDMS MEMBRANES DURING PERVAPORATIVE ETHANOL RECOVERY FROM AQUEOUS MIXTURES CONTAINING ACETIC ACID. Presented at 17th Annual North American Membrane Society Meeting, Chicago, IL, May 14 - 17, 2006.
Impact/Purpose:
To inform the public.
Description:
Pervaporation is potentially a cost-effective means of recovering biofuels, such as ethanol, from biomass fermentation broths for small- to medium-scale applications (~2 - 20 million liters per year). Hydrophobic zeolite-filled polydimethylsiloxane (PDMS) membranes have been shown to effectively remove ethanol (EtOH) from aqueous binary mixtues. The effects of fermentation byproducts on membrane performance, however, are not well understood. One byproduct is acetic acid (HAc), which may be produced during fermentation or generated during hydrolysis of biomass. [1] Uninterrupted pervaporation of EtOH and water feed mixtures through high-silica MFI zeolite-filled PDMS membranes was measured versus time for durations of several weeks. The mixed-matrix membranes were 70 - 90 microns thick and contained 50 wt% of 1 - 3 micron zeolite particles (ZSM-5 with Si/Al = 137) dispersed in the polymer phase. The EtOH/water separation factors were 17 - 20 with 5 wt% EtOH/water binary feeds at 50 deg C; this was 2.1 - 2.5 times higher than the separation factors for pure PDMS membranes using the same feed compositions. When HAc was introduced to the 5 wt% EtOH feed at 1 wt%, the EtOH and water fluxes both decreased to approximately 80% of their original values on the following day. The fluxes returned to their original values when the feed was changed back to an HAc-free 5 wt% EtOH/water feed after two days. These decreases in flux due to short term HAc exposure were attributed to HAc competing with EtOH and water for adsorption sites on the zeolite particles. In contrast, EtOH and water fluxes through a pure PDMS membrane were not affected by the presence of HAc. When the mixed-matrix membranes were exposed to feed solutions containing HAc for periods longer than 2 days, the EtOH and water fluxes decreased gradually with time. Of these, the EtOH flux decreased more rapidly, causing the EtOH/water separation factor to decrease with time. Subsequently swapping the feed back to an HAc-free 5 wt% EtOH/water feed solution resulted in increased fluxes, however the fluxes did not rebound to pre-HAc levels as they had in the short-term HAc exposure tests. This lasting decay in performance might be due to HAc degrading the interface between the polymer and the zeolite particles. Additional measurements investigating the effects of increased feed pH and decreased Al content in the zeolite on the mixed-matrix membrane stability in the presence of HAc will also be discussed. [1] Tengborg, C.; Galbe, M.; Zacchi, G. Enzyme Microb. Technol. 2001, 28, 835. * This is an abstract of a proposed presentation and does not necessarily reflect U.S. EPA policy.