Membrane Filtration for Fractionation of Biomass Compounds - Process Design for the Separation of Hemicelluloses and Lignin from Alkaline Solutions

Abstract: It is important to replace fossil resources such as oil, coal and natural gas with renewable raw materials for the production of materials and fuels in order to reduce environmental impact. Furthermore, these raw materials should preferably be residues, e.g. agricultural or forest residues, in order not to compete with the production of human food and animal feed. High molecular weight arabinoxylan extracted from wheat bran has shown good potential for the production of barrier films for packaging, while lignin extracted from the kraft pulping process can be used to produce fuels and carbon fibres. However, efficient and sustainable processes are required for the separation and purification of these components for the production of renewable materials and fuels. The use of membrane filtration in combination with other separation processes for the separation and purification of these substances has been studied in the work presented in this thesis. Different pretreatment methods in combination with ultrafiltration were investigated for the separation of arabinoxylan from wheat bran extract. It was shown that the use of dead-end filtration with kieselguhr as a filtration aid was the most promising pretreatment method of those investigated, as it removed some high molecular weight compounds from the solution. The introduction of a preseparation step increased the ultrafiltration flux by 230 %, without significantly decreasing the retention of the product. The final design of the separation process also includes ultrafiltration for the separation and concentration of arabinoxylan, diafiltration for the removal of alkali, and finally evaporation to reduce the energy demand in the subsequent spray-drying step. The introduction of a nanofiltration step to purify the ultrafiltration permeate so that it can be reused in the extraction solution was also shown to be promising. The average flux was above 200 l/m2h and the retention of the impurities was high. It was found that a membrane with a cut-off of 1 kDa was appropriate for the separation of lignin from black liquor, combining high lignin retention (>80 %) with a sufficiently high flux. It was also shown that the introduction of a preseparation step with a 20 kDa membrane before lignin separation resulted in a lignin-rich retentate with a lower content of hemicelluloses and multivalent ions, e.g. calcium, iron and magnesium. Precipitation of the lignin-rich retentate from filtration of the black liquor was performed at 80 °C by adding sulphuric acid to obtain a pH between 4 and 1.5. The resulting lignin slurry showed good filtration properties and had a total dry substance content above 60 %. It was also demonstrated that this process design has a lower acid requirement than the industrial process used today.