Evaluation of various ultrafiltration membranes for the treatment of solutions derived from alkali treatment of straw

Biomass is a very interesting source to obtain renewable energy and chemical products based on a renewable substrate. Hard or soft wood as well as forestry and agricultural residues are potential feedstocks for a biomass based biorefinery. Biomass is a complex material and consists of cellulose, hemicellulose and lignin. Minor components are proteins, extractives and minerals. In this work wheat straw has been chosen as substrate. Within wheat straw the polymeric substances can be summarized as C6 sugars comprising mainly cellulose which covers around 40 % and C5 sugars like arabinoxylan with a content of around 23 % of the material. The lignin content is approximately 17 %. The arabinoxylan is acetylated and some of the arabinose residues are esterified at O-5 by phenolic acids like ferulic acid. The rest of the wheat straw consists of proteins, extractives and inorganic
matter. Depending on the pretreatment used different forms of products can be derived from biomass. In cellulosic fibre production sulfit or sulfat treatment at high temperature and pressure is used to dissolve the C5 sugars and the lignin. The C5 sugars can be separated from the solution whereas the lignin usually is used as an energy source for driving the process. If also lignin is targeted as a product an alkaline treatment with sodium hydroxide ore ammonia can be used. Alternatively acid or alkaline organosolv processes are used for this task. For each of these process alternatives the product separation and concentration processes are very important. In our experiments an alkaline sodium hydroxide based process has been used for the decomposition of wheat straw. The derived solution contained mainly lignin and acetate as well as some of the C5 and C6 sugars as monomers or oligomers. After separating the liquid stream from the remaining solids the liquid has been treated using UF membranes from various membrane companies. The aim of the experiments was to separate the lignin from low molar mass compounds and concentrate the lignin for further treatment. For the investigated membranes flux and selectivity will be presented. The target of the investigated membrane process is the production of various chemicals which can be used in chemical industry. To achieve an economic process it is necessary to develop a selective separation process with a high overall
energy efficiency for the whole process.