Challenging Butanol Recoveries: Pervaporation and Pervaporation Membranes

Abstract

The membrane-based process pervaporation is one of the most promising separation process for the removal of the ABE fermentation products. Product separation and concentration is crucial for the entire production process performance and economy. A successful pervaporation process needs a high selectivity membrane combined with high fluxes of the fermentation products. With the application of classical PDMS membranes, a selectivity up to 30 can be reached. In mixed matrix membranes, a selectivity of nearly 100 can be realized. The permeate concentration reached with PDMS membranes are 30–40 wt% butanol and for mixed matrix membranes 50–60 wt% butanol. This selectivity increase results in a significant decrease of separation energy needed for the pervaporation step of around 70%. Further process intensification and reduction of the energy demand for the solvent separation can be achieved with a combination of a hydrophobic pervaporation unit with a hydrophilic pervaporation unit resulting in a separation energy reduction of more than 90%. The separation of the nearly water-free organic mixture of butanol, acetone, and ethanol to pure single components can be achieved with a well-known distillation step. Most preferable is the integration of the hydrophobic pervaporation unit direct into the fermentation to ensure butanol concentrations below the toxic level for the whole fermentation period. For a commercial application of pervaporation, the combination of a membrane pretreatment prior to the pervaporation unit, as well as installation of proper cleaning in place (CIP) systems, will be necessary to ensure the long-term performance of the pervaporation membrane.