Experimental Study For Separation And Concentration Of Model Lignocellulose Hydrolyzates By NF/RO Membranes

As one of the renewable and clean new energies, bioenergy, which has a widerange of resource and promising development, has already been deeply andcomprehensively explored and applied by numerous scientists to subsititute the fossilfuels which have been restricted by the excessive consumption, the rapid increase indemand, the insufficient storage as well as the caused environmental pollution.Bioconversion of lignocellusic biomass for making biofuels is a major way to obtainbioenergy. The hydrolysis of lignocellulosic materials is the most crucial step duringthe entire conversion process. Currently, hydrolysis or pretreatment by dilute acid isthe most common hydrolysis technology for application. However, the lignocellulosichydrolyzates derived from the diluted-acid treatment contain a large number of toxiccompounds obtained from the degradation at high temperature and pressure, whichstrongly inhibit the subsequent microorganism fermentation, and reduce theproductivity and yield of final products (bioethanol). Therefore, in order to ensure theefficient use of resources, these inhibitors need to be removed before the fermentationprocess. By using synthetic model solution, this work comprehensively investigatedthe separation of acetic acid by nanofiltration and reverse osmosis technologies, dueto its higher content and strong inhibition in hydrolyzates. The feasibitily ofsimultaneous acetic acid separation and monosaccharide concentration was examinedas well by using membrane technology. The main results as follows:(1) The relatively optimal experimental parameters were determined byinvestigating the influences of solution pH, transmembrane pressure, operationtemperature and feed concentration. The monosaccharide retention reached85-90%for NF membranes and above99.5%for RO membranes; the acetic acid retentionreached0-5%for NF membranes and around40%for RO membranes. Theperformance of NF for removing acetic was better than that of RO, while theperformance of RO for retaining monosaccharides was better that that of NF.(2) The separation factors of acetic acid over xylose were8.87(NF) and223.2(RO). The separation factors of acetic acid over glucose were56.5(NF) and348.7(RO). The separation factors during RO were extremely higher than that during NF. (3) Previous researches reported that acetic acid with high concentration inhydrolyzates could strongly inhibit the microorgamism fermentation. However, aceticacid with low concentration could conversely improve the growth of microorganismand increase the productivity of ethanol. Thus, keeping acetic acid with lowconcentration in hydrolyzates is a more effective and economic detoxification way.Meanwhile, in order to retain sugar as much as possible, RO membrane is moresuitable for separation acetic acid from hydrolyzates.(4) The optimal separation parameters were obtained by further optimizationexperiment of operation conditions of the best membrane (RO98pHt), that pH2.93(initial pH),40oC and20bar.(5) The result obtained from the real4-times concentrating experiment oflignocellulosic model hydrolyzates is that99.61%recovery of xylose,99.69%recovery of glucose and66.76%removal of total acetic acid content. The resultobtained from the diafiltration experiment of3-times concentrated model solution isthat99.09%recovery of xylose,99.22%recovery of glucose and75.46%decreasingin acetic acid concentration when the separation factor reached4.The above mentioned results demonstrate that separation process of RO98pHtmembrane can effectively and simultaneously separate acetic acid in hydrolyzates aswell as recover and concentrate99%of monosaccharides.