Elsi Koivula

24 Aug 2012

2010 - Master of Science, Lappeenranta University of Technology, Finland

2010 - Doctoral student, Laboratory of Membrane Technology and Technical Polymer Chemistry, Lappeenranta University of Technology, Finland. Topic: Methods for minimization of fouling and improvement of separation efficiency in membrane filtration applications in biorefineries

2011 - Researcher exchange at University of Oviedo, research group of Susana Luque, Oviedo, Spain

 

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Abstract


Pretreatment to manage fouling in ultrafiltration of wood-based solutions

The future forest biorefineries could produce valuable compounds such as hemicelluloses, lignin derivatives and wood extractives for further processing. For example, hemicelluloses or xylooligosaccharides, can be used in production of films, barriers and food-additives etc., and lignin can be used in production of activated carbon, carbon fiber, and activated carbon fiber. The valuable compounds could be extracted from wood at high temperature and at high pressure at slightly acidic or at alkaline conditions, or with enzymes. The stream produced in the extraction process is called wood hydrolysate. It is a mixture of all the compounds possible to extract from wood. Therefore, from the economical point of view, the separation of the valuable compounds from the wood hydrolysate is one of the most significant steps in biorefinery process schemes.

Membrane filtration has been found to be a feasible technique in separation of the valuable compounds from wood hydrolysates. For instance, hemicelluloses can be recovered, fractionated  and purified from wood hydrolysates by ultrafiltration. The wood hydrolysates are, however, challenging to filter due to their great fouling potential and, especially, in recovery of sugar oligomers the fouling problems have to be solved before gaining cost effective membrane processes.

This study is focused on the development of a pretreatment step, which decreases fouling effectively without causing hemicellulose or sugar oligomer losses. It has been shown that lignin derived material and wood extractives foul the membranes. In this study the goal of the developed pretreatment step is on efficient removal of lignin, because the amount of wood extractives is smaller than the amount of (acid soluble) lignin in the hydrolysates. However, both lignin and wood extractives contain phenolic compounds which means that in several pretreatment options the developed pretreatment affects actually both, lignin derived material and wood extractives.

The results achieved so far show that the pretreatment methods, which have decreased most the UV absorbance of the hydrolysate and increased the permeate flux, are pulsed corona discharge treatment (degrading method), polymeric adsorbents (adsorption), and enzyme treatment (degrading method). The pulsed corona discharge treatment improved the filtration capacity six fold compared to the untreated hydrolysate. It has been seen that for instance adsorption cause also hemicellulose losses. This is due to the linkage between some hemicelluloses (mostly xylan) and lignin. Thus, to minimize hemicellulose losses the pretreatment process should consist of two stages. First the hemicellulose-lignin bonds should be cleaved, and the lignin should be removed, for instance by adsorption, only after that. The experiments focused on the two stage pretreatment processes of that kind are at the moment going on.