Recovery of Hemicelluloses Extracted from Spruce and Wheat Bran : Membrane Filtration Process Development and Cost Estimates

Abstract: Hemicelluloses are a group of abundant polymers found in all plants and constitute about one third of the total plant biomass. These polymers can be used in many different products, such as oxygen barrier films, emulsifiers, foams and probiotic food additives. However, the commercial development and use of new products utilizing hemicelluloses has been limited due to the high production cost. Increased utilization of hemicelluloses would increase the value of biomass, and increase the proportions of products made from sustainable raw materials, and thus reduce our dependence on fossil materials. In the work described in this thesis, hemicelluloses extracted from spruce and wheat bran have been recovered and purified using membrane filtration. As the molecular mass of the hemicelluloses was high, they were retained by the membrane, while water and low-molecular-mass contaminants, such as lignin, salt and sugar mono- and oligomers, passed through the membrane. This allows membrane filtration to be used for simultaneous concentration and purification of hemicelluloses. In order for the membrane processes to be cost-effective, the filtration capacity, i.e., the flux, must be high. One problem associated with treating extracts from biomass is the adhesion of hydrophobic compounds onto the membrane surface, resulting in membrane fouling and thus reduced flux. However, these compounds can be removed by pretreating the extract. Various pretreatment methods have been investigated in attempts to obtain the highest possible flux. Alkali-extracted wheat bran hemicelluloses, mainly arabinoxylan, were purified from contaminants such as suspended material, extraction chemicals and lignin. It was shown that the molecular mass of the hemicelluloses could be reduced by sparging the solution at 80 °C with air. This resulted in an almost fourfold increase in flux. The highest rate of flux increase was obtained when colloidal material had been removed from the solution by dead-end filtration prior to air sparging. Pretreating the solution with air sparging drastically decreased the cost of purifying the hemicelluloses by ultrafiltration and diafiltration. The hemicelluloses recovered from process water after thermomechanical pulping of spruce, galactoglucomannan, were first purified using microfiltration to remove colloidal material. This had a significant positive effect on the filtration capacity of the subsequent hemicellulose recovery step using ultrafiltration. A pilot trial was conducted on-site at a thermomechanical pulp mill to evaluate the performance of the process. The results of this trial, combined with results from laboratory experiments, were used to estimate the cost of the membrane processes used for the recovery of the hemicelluloses from the process water. An additional benefit of recovering dissolved hemicelluloses from the process water is that the cost of waste water treatment at the pulp mill will be reduced, which may compensate for part of the cost of the recovery of hemicelluloses.