Membrane separation technologies in biorefineries: Current status and future developments

Introduction
The start of the 21st century is combined with the continuous replacement of classic C2/C3 chemistry with biological processes. This conversion process started at the end the 20th century and is combined with the integration of membrane processes as high selective and low energy consuming separation technologies in the production of fermentation based products. In fermentation based production membrane processes are used either as stand-alone units or as process synergies in combining with other separation technologies such as evaporators or high speed separators.
The first part of the presentation will review the current status of membrane processes in biorefineries and fermentation based production in general, while the second part will analyze the opportunities of membrane processes in future biorefineries with regard to new concepts and integration of innovative membrane processes.

Biorefineries an fermentation based production – Current Status
The conventional membrane processes microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) have established themselves in the production of fermentation products such as enzymes, antibiotics and organic acids since the 1970ies. Classical examples are the use of UF combined with diafiltration for the recovery of antibiotics or the combination of reverse osmosis and evaporation for the concentration of yeast. In current biorefineries membrane processes are used from the feedstock preparation to recovery of the fermentation products. The raw materials for current biorefineries are mainly the food commodities starch and sugars and thus in the following application study a wheat-based biorefinery is considered. Analyzing the process from the feedstock preparation to the concentration of the fermentation products, one of the potential first membrane application is the use of UF for the concentration of the soluble and pentosane phase from the 3-phase starch extraction decanter. Applying UF does not only reduce the energy typically required to evaporate this phase but also recovers water which can be recycled back into the process. Further downstream in the production, UF can be used in combination with a decanter to polish the starch-based sweetener after the conversion of starch to sweeteners to achieve higher product qualities. In the subsequent fermentation converting the sweeteners, MF/UF can be used to separate the fermentation products from the fermentation broth and thus converting a batch process to a continuous process. Finally, if evaporation or spray drying are used for the concentration of the fermentation product RO can be either used as pre-concentration step before evaporation/spray drying or to polish the condensate from the evaporator/spray dryer for recycling into the process.


Biorefineries – Future Developments
The future development of biorefineries can be divided into two sections (1) development of new concepts and (2) integration of innovative membrane processes.

Concepts
Main raw materials for current biorefineries are starches and sugars. In order to move to a sustainable concept of biorefineries one trend is to replace these food commodities with lignocellulosic biomass which should be ideally separated into its three key components: hemicelluloses, lignin and cellulose. This can be achieved by combining appropriate pre-treatment methods with MF, UF, NF and RO. In particular, UF can be used to concentrate hemicellulose for the production of e.g. barrier films or to concentrate and classify ligninsulfone from spent sulfite liquor or lignin from Kraft black liquor to use them e.g. as binders.

Technologies
Among the emerging membrane processes in particular pervaporation – both hydrophobic and hydrophilic – has the potential to play a key role in future biorefineries. Hydrophobic pervaporation can be used for the initial concentration of the fermentation products after separation from the fermentation broth with MF, UF or NF. Further downstream in the process hydrophilic pervaporation might be applied to dehydrate the active principles, e.g. in combination with distillation.

Conclusion
Overall, this presentation will show that membrane processes as highly selective and energy-saving separation processes are key units of separation in current and future biorefineries starting from the preparation of the feedstocks to the concentration of the final fermentation products. Furthermore, this trend will be supported by new concepts required to handle lignocellulosic biomass as raw materials and the integration of emerging membrane processes such as pervaporation into biorefinerie processes.