Hemicellulose pre-extraction of hardwood.

摘要

Many pulp mills in temperate climates are facing increasing competition from new producers in tropical and subtropical regions. This situation has become critical due to decreasing forest products prices and increased wood costs. A potential solution to this problem is the production of bio-fuels and bio-based chemicals besides pulp. This concept is called the "Integrated Forest Products Biorefinery (IFBR)" (van Heiningen, 2007). The approach is based on separation of the principal wood components and further processing and commercialization of each to create the highest value added.;Autohydrolysis using water is one of the promising methods to extract hemicelluloses from wood prior to pulping in order to make co-products such as ethanol and acetic acid besides pulp. Many studies have been carried out on the kinetics and mechanism of autohydrolysis (Garrote et al., 1999, 2007; Yu et al., 2008). However, most of these studies were performed in batch reactors which are not well suited to determine the intrinsic kinetics of hemicellulose dissolution because of the change in pH during autohydrolysis, and further reaction of the dissolved hemicelluloses in solution. Therefore a fed batch reactor was used in the present study to obtain the intrinsic kinetics of dissolution of hemicelluloses from a mixture of hardwoods.;Another wood pre-extraction method, the green liquor process, is of interest because of its potential to create valuable products such as ethanol and acetic acid without lowering the pulp yield and quality (Yoon S-H and van Heiningen, 2007). The presence of alkali in green liquor neutralizes acetic acid released by cleavage of acetyl groups from xylan. However, reaction of acetic acid by Na2CO3 creates CO2 in the wood chips and the extraction vessel which will cause operational problems. Therefore instead of Na2CO3 we investigated another buffer, sodium acetate (NaAc), which does not produce a gaseous product upon contact with acetic acid.;In the present study the kinetics and mechanism of removing hemicelluloses from a southern hardwood mixture was investigated in a fed batch reactor using aqueous feed solutions containing different concentrations of NaHCO3 , NaAc or acetic acid (HAc). The influences of flow rate, temperature, chemical concentration, pH, and hardwood chip dimension on the kinetics of hemicelluloses extraction were studied. Free acetate in solution and acetyl groups still attached to dissolved xylan were measured since they were considered important for understanding the dissolution of xylan. The content of uronic acids of dissolved xylan was also measured to assess its effect on xylan solubility. Finally the molecular weight distribution of dissolved xylan was determined by GPC. The results were used to explain the kinetics of hemicellulose removal in terms of a chemical mechanism and rate determining step(s).