| Summary: | 碩士 === 中原大學 === 化學工程研究所 === 106 === How to efficiently hydrolyze lignocellulose is an important key for commercial production of biomass fuel. Taiwan''s ITRI successfully utilizes zinc chloride solution to efficiently hydrolyze lignocellulose at room temperature. However, the resulting hydrolysate contains high concentration of zinc chloride, hydrolyzed sugar and formic acid. After pretreatment of the hydrolysis solution by vacuum distillation and microfiltration, a small amount of formic acid still remains. It is necessary to further purify and concentrate the hydrolyzed sugar solution to be used as a fermentation liquid. In addition, if the zinc chloride and formic acid in the permeate from membrane filtration of hydrolyzed sugar solution can be separated, the purposes of recovering the zinc chloride solution and obtaining the formic acid by-product can be achieved.
In this study, three commercial NF membranes (NF270, DL and GE) are used to separate the ZnCl2/glucose mixture at different operating pressures and pH to investigate the influence of operating conditions on flux and rejection rate. At the same time, ion concentration in pores, effective pore size and effective thickness of the membrane etc. are estimated theoretically in conjunction with the experimental results to investigate the changes in the electrical and structural properties of the membrane under different pH, and find out why they affect separation efficiency. Selective separation performances of the three membranes are compared to find the most suitable membrane and operating conditions. In addition, a lower MWCO membrane NF40 is used experimentally for studying the feasibility in selective separation of ZnCl2/formic acid mixture.
Filtration results of ZnCl2/glucose mixture show that the flux of NF270 is reduced by 95 % compared with pure water penetration flux. Based on the effective pressure difference after considering the effect of osmotic pressure difference, its water permeability during filtration is still reduced by more than 90%. The rejection of glucose measured is decreased from 80 % to 72 % when pH is raised from 2 to 4, while the rejection of Zn2+ is increased at pH2~3 and fell at pH3~4 with a highest value of 45 % at pH3 where is very near the isoelectric point of NF270(pH 3.1). DL has a higher positive charge among the three membranes at pH2. The significant membrane swelling will lower the rejection of glucose, but the stronger positive electric repulsion makes DL to maintain a higher rejection of Zn2+, and the rejection is not reduced by weakening of sieving and shadowing effect caused by adsorption of counter ion. When pH is increased from 2 to 4, the decrease in positive charge of DL membrane results in a lower rejection rate of Zn2+ while the rejection rate of glucose is gradually increased due to decrease of swelling by low charge repulsion in pores. At pH4, the DL membrane has a low negative charge and higher rejection rate of Zn+2 was observed, which is qualitatively similar to the behavior of NF270 near its isoelectric point at pH3.
Experimental results of zinc chloride/formic acid solution with NF40 membrane show that the filtration rate is obviously affected by the concentration of zinc chloride. The filtration rate is reduced by 75% when the concentration of zinc chloride is increased from 5 to 10 wt%. As the concentration is further increased to 20 wt%, the filtration flux is nearly reduced by 2/3. The rejection rate of Zn+2 also decreases significantly with the increase of zinc chloride concentration. When zinc chloride concentration is increased from 5 to 20 wt%, the zinc rejection rate decreases from 65 to 30%. It may be concluded that filtering zinc chloride/formic acid solution with NF40 membrane has a better selective separation and flux performance under low zinc chloride concentrations.
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