Solvent Recovery From ABE Solutions Applying Pervaporation Under Realistic Process Conditions

Acetone-Butanol-Ethanol (ABE)-fermentation is well accepted to be a promising route for the sustainable production of green solvents and renewable energy carriers. Significant efforts in contemporary research are undertaken in order to develop and optimize this production route. Nevertheless, a persistent challenge is the self-limitation of ABE fermentation by high product toxicity of butanol for the involved microorganisms resulting in rather dilute product concentrations (up to 20 g/l). Consequently, constant in-situ separation of butanol from the fermentation broth is mandatory to sustain stable production. Furthermore, butanol has to be concentrated from fermentation levels up to 99.9 % (w/w) for further utilization. Conventional distillation is far too energy intensive and therefore no feasible option. Thus, the development and optimization of alternative separation technologies like gas stripping, adsorption, extraction or membrane separation has become the main focus in this research area. The target is to increase solvent recovery and enrichment in the concentrate stream together with a reduction of the energy demand for separation. Among the named technologies, pervaporation has frequently been reported to exhibit excellent performance in the separation of solvents from aqueous ABE-solutions.

The scope of the presented work is the analysis of pervaporation for the recovery of solvents from ABE fermentation broth under realistic process conditions. A multitude of authors published highly promising results for this separation task applying commercially available PDMS membrane material. The more recently developed membrane material POMS also shows great potential for solvent recovery but literature on this topic is still scarce. Current work will show pervaporation performance of POMS material with synthetic ABE solutions as well as with real ABE-fermentation broth. Synthetic aqueous ABE-solutions are analyzed to assess the influence of the various associated components of the fermentation broth like glucose, salts and acids independently. Real broth from fermentation of special model substrate will be used for pervaporation after centrifugation and sterile filtration in long-term experiments to assess tendency of clogging and fouling. As a result, the practical applicability of pervaporation with POMS as well as limitations will be highlighted.

Experimental work is undertaken using a lab-scale pervaporation unit containing a 0.0144 m² flat sheet POMS membrane module. Pervaporation is performed at a feed temperature of 35 °C and an absolute pressure of 10 mbar at the permeate side. Total condensation of permeate is guaranteed using a cold trap cooled with liquid nitrogen. Analysis of compositions of feed and permeate is performed by GC-FID, quantities and flow rates are assessed using scales. Results of this work provide valuable insight to the practical usability of pervaporation process during the recovery of solvents from ABE-fermentation. Membrane performance parameters and their effects on the design of a demonstration plant for the processing of real fermentation broth will be presented. Limitations and constraints of the technology encountered will be discussed.