Membrane Processes in Biorefineries: From Feedstock Preparation to Downstream Processing

Abstract

1. IntroductionThe third and latest wave of biotechnology is the so called „white biotechnology“ aiming to replace the C2/C3 chemistry based on oil and gas by biological processes. The backbone of biotechnology is the conversion by fermentation which is widely established in the production of antibiotics, enzymes, bioethanol and organic acids. In the past, fermentation processes had to partly compete with chemical conversion but the foreseeable shortage of raw materials for chemical conversion has revived the interest in fermentation in recent years. Since the 1970’s, cross-flow membrane processes have established themselves in the downstream processing of fermentation products for recovery and purification. It is expected that this role will extend into the new concept of biorefineries. The first part of this presentation will provide an overview of the existing potential and future opportunities of membrane processes within the concept of biorefineries in general and the second part provides an application study for a wheat-based biorefinery in details. 2. Membrane processes in biorefineriesThe aim of biorefineries is the full utilisation of feedstock to simultaneously produce e.g. food, biofuels and biochemicals. The integrated production of biomaterials can be based on e.g. sugar, starch and cellulose-based feedstock and as such extend current sugar, starch and pulp factories. Membrane processes have been identified as a key separation technology due to their high selectivity and low energy consumption. The initial step in the concept of biorefineries is conversion of the feedstock, e.g. cellulosic or starch-based materials, into sugars. The sugar stream might be polished by micro-filtration (MF) or ultrafiltration (UF) and concentrated by reverse osmosis (RO) before fermentation. During the fermentation step, the fermentation products can be continuously removed by using either MF/UF or pervaporation (PV) in a side stream or submerged in the fermenter to avoid product inhibitions. In the downstream process after the fermenter, membrane processes can be used for the polishing and concentration of the fermentation products, e.g. biofuels/biochemicals. 3. Application study: Membrane processes in a wheat-based biorefinery The presentation will provide an overview of membrane applications in a wheat-biorefinery focused on four applications with reference to the four key production steps: starch extraction, starch conversion to sweetener, fermentation and downstream processing. The first step in wheat-based biorefineries is extraction of the starch from the wheat. For this the wheat flour is mixed with water and then separated by a 3-phase decanter resulting in an A-starch fraction, a gluten and B-starch fraction, and a fraction consisting of solubles and pentosanes. In order to optimise the water consumption it is possible to apply UF for concentrating the solubles and pentosanes and recovering water for recycling in the process. The overall water balance for the starch extraction can be improved by using this concept. An important step in the subsequent conversion of starch to sweeteners is the removal of the mud fraction after liquification and saccarfication. A combination of UF with a decanter can be used as an alternative to rotary vacuum filters achieving higher purities and a mud fraction which is not contaminated with filter aid kieselguhr and can therefore be added directly in the bioethanol production. Hence, the closed process avoids potentially hazardous filter aids and results in a value-added by-product. Continuous fermentation is an important feature in the concept of biorefineries. MF/UF, either submerged or as side-stream, can be applied to recover the active principles directly from the fermentation broth. In this way, the production can be moved from batch to continuous thus preventing product inhibitions. This concept is valid for bioethanol as such but also other biochemicals such as organic acids or bioplastics. Evaporation is often used in the downstream processing as a stand-alone unit or in combination with RO for concentration of the fermentation products. The resulting evaporator condensate from the concentration can be polished by RO resulting in a permeate stream which can be recycled in the process thus improving the water balance of the process. Pilot and/or production results will be presented for all four focus applications. 4. Conclusions and outlookThis paper demonstrates the great potential of membrane processes in the concept of biorefineries and thus in solving the energy and environmental problems of the future. (Less)