| Alkaline pulping including kraft pulping and soda pulping is the main pulpingprocesses. Most of the hemicelluloses are dissolved in the cooking liquor together withlignin, and they are removed from the effluent by comubustion at effluent treatmentprocess, which leds to the waste of biomass. Recently, the effective application of allcomponents in the biomass has been gotten more and more attention. Based on theconcept of biorefinery, the fiber materials are pre-hydrolysised prior to pulping, and thepre-hydrolysised raw materials are used for pulp production. The hemicelluloseoligosaccharides(OS) can be obtained from the pre-hydrolysis liquor(PHL) by furtherseparation and purification, and they can be a feedstock for the production ofvalue-added products or a raw material for fuel production, which bring additionaleconomic benefits for the pulp and paper companies.The effects of wood chip size on the dissolution of wood components duringpre-hydrolysis were studied. Combined with the biomass component separationtechniques, different recovery and purification processes were studied.The components in PHL were analyzed. It was found that saccharides(oligosaccharides and monosaccharides) lignin are the main components in PHL, andfurfural, formic acid, acetic acid was also found in PHL. The wood chip sizes affectedthe dissolution of sugar during pre-hydrolysis. The reduction of wood chip size washelpful to the extraction of oligosaccharides. However, compared to the wood chips ofdifferent sizes, more monosaccharides was formed during prehydrolysis of wood meal,which resulted in the reduction of oligosaccharides content in PHL of wood meal.The fraction of lignin with high molecular mass in PHL tended to cause membranefouling. A variety of precipitation strategies were investigated and evaluated. The resultshowed that, for ethanol precipitate and the acidification precipitation of PHL at pH2,it was observed to be nonselective between lignin and the OS, causing equalprecipitation and removal. Poly dimethyl diallyl ammonium chloride(p-DADMAC)precipitation and cryoprecipitation showed a poor lignin removal. Polyaluminiumchloride (PAC) precipitation was highly specific to large molecular lignin compared toother precipitation strategies, leading to25.1%lignin removal with negligible OS lossthrough charge neutralization mechanism. Results of gel permeation chromatography(GPC) for lignin molecular weight distribution before and after precipitation verified that it was mainly the high molecular mass lignin that was precipitated.Lignin and other impurities with small molecule weight could be removed bydialysis. The effects of different molecular weight cut-off dialysis bags on smallmolecular impurities removal and oligosaccharides loss were studied. The resultsshowed that the dialysis was very effective for impurities with small molecule weightremoval, but it also caused significant loss of oligosaccharides. Ultrafiltration(UF) alsocan be used to remove the impurities with small molecular weight, and the purificationefficiency and membrane flux was affected by the kinds and pore size of ultrafiltrationmembrane and osmotic pressure. The precipitation pretreatment could enhance themembrane flux. Anion exchange resin could remove lignin and other impurities withnegative charge from the PHL. Compared to the weak alkaline anion exchange resin,the strongly alkaline anion resin was more effective for the adsorption of impuritiesand lower loss of oligosaccharide. Finally, the resin could be eluted with an organicsolvent, and the desorbed organic substances could be recoveryed.As a summary, the refining processes that produced oligosaccharides with highyield and high purity were obtained, and they were: PAC precipitation followed bydialysis and microporous membrane filtration followed by ultrafiltration and anionexchange resin treatment. |
