293808 Membrane-Assisted Downstream Processing for Biobutanol Purification

Wednesday, May 1, 2013: 4:00 PM
Bonham C (Grand Hyatt San Antonio)
Sebastian Heitmann, Daniel Welz, Philip Lutze and Andrzej Górak, Department of Biochemical and Chemical Engineering, Laboratory of Fluid Separations, TU Dortmund University, Dortmund, Germany

One of the main challenges for chemical industry is the change from fossil resources to a sustainable production of chemicals. The change to biobased raw materials and biotechnological processes often leads to product inhibitions of microorganisms and diluted system, resulting in an expensive downstream processing. One bulk chemical that can be produced in a sustainable way is biobutanol. One method of producing biobutanol utilizes the so-called acetone–n-butanol–ethanol (ABE) fermentation that can be carried out on basis of e.g. cellulosic and lignocellulosic biomass or agricultural wastes. However, the productivity of this fermentation is still limited, because n-butanol is toxic towards the microorganisms used in ABE fermentation. That is why the concentration of n-butanol in the fermentation broth is often limited to values lower than 2 wt.-percent [1]. For n-butanol recovery mainly distillation processes are used which - as result of the low concentrations - have a high energy demand. About 80 % of the total energy demand of the downstream process is required for the separation of acetone, n-butanol and ethanol from the fermentation broth. Alternatively to distillation the fermentation can be coupled with a pervaporation unit, thus n-butanol can be separated from the broth continuously to prevent the n-butanol concentration from reaching the toxicity limit. Because of the mild operating conditions, pervaporation enables an energy-efficient n-butanol separation and facilitates the recycling of the broth with lower n-butanol content back to the fermenter.

Therefore, the aim of this work was to evaluate if it is more efficient to separate n-butanol by means of pervaporation or distillation or a combination of both. For this purpose extensive experimental studies were performed to characterize the permeation behavior of a commercial available poly(dimethylsiloxane) (PDMS) membrane (Sulzer PervapTM 4060). These studies provide a wide data basis for the downstream process analysis and are used to identify possible operating windows for the application of a pervaporation. An experimental comparison between the pervaporation of a binary mixture of n-butanol and water, the pervaporation of a synthetic medium and the pervaporation of real fermentation broth was carried out. The synthetic medium contained acetone, n-butanol and ethanol in mass ratios of 3:6:1 together with acetic and butyric acid, which are the solvent precursors. Additionally process parameters like feed composition, pervaporation temperature and permeate pressure were varied. The detailed experimental procedure is described elsewhere [2]. Next to the PDMS membrane, composite membrane materials containing ionic liquids are promising for separation of n-butanol from an aqueous solution. These membrane materials show advantageous permeation properties compared to conventional polymer membranes [2]. Therefore different supported ionic liquid membranes (SILMs) have been tested to evaluate their potential in n-butanol pervaporation.

Based on the experimental results a correlation has been developed describing partial fluxes of acetone, n-butanol, ethanol and water. Fluxes of acetic and butyric acid were neglected. With the help of this correlation modular-based process studies have been carried out using Aspen Custom ModelerTM to find an optimal process configuration for n-butanol separation from the fermentation broth. It was found that pervaporation as stand-alone process as well as distillation is comparatively expensive. Because n-butanol should be almost completely separated from the broth, the concentration at the outlet of the membrane module becomes small, leading to a large required membrane area and a small permeate concentration of n-butanol.

In contrast to the application of distillation and pervaporation as stand-alone processes, a combination of pervaporation and distillation was found to be best suited for efficient n-butanol recovery. A possible membrane-assisted process is shown in Fig. 1.  Pervaporation is used for continuous n-butanol removal in order to extend the fermentation cycles. Because the mass fraction of n-butanol wB,Rec in the recycle stream is only slightly smaller compared to the mass fraction of n-butanol in the fermentation wB,Ferm, the required membrane area is comparatively small and a higher permeate concentration can be obtained. The complete removal of n-butanol from the broth is then carried out in distillation columns. In this case, the extension of the fermentation periods and the reduction of the stream to be processed in the energy-intensive distillation lead to a reduced energy demand and lowered downstream costs for n-butanol. The permeate stream and the solvent phase from the distillation can be processed further in a conventional distillation sequence. As existing biobutanol production plants mainly rely on distillation sequences, retrofit of a pervaporation thus enables an increase in the biobutanol production efficiency, while existing paid-off plants can still be used.


Fig. 1: Solvent removal from fermentation broth by distilliation (a), pervaporation (b) and a membrane-assisted process (c).


The research leading to these results has received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement n° 241718 EuroBioRef. We acknowledge Sulzer Chemtech AG and the Laboratory of Biochemical Engineering, TU Dortmund University for providing PDMS membranes and fermentation broth.

[1] T.C. Ezeji, N. Qureshi, H.P. Blaschek, Bioproduction of butanol from biomass: from genes to bioreactors, Curr. Opin. Biotechnol. 3 (2007) 220–227.

[2] S. Heitmann, J. Krings, P. Kreis, A. Lennert, W.R. Pitner, A. Górak, M.M. Schulte, Recovery of n-butanol using ionic liquid-based pervaporation membranes, Sep. Purif. Technol. 97 (2012), 108-114.

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