| Xylo-oligosaccharides(XOS), obtained from the enzymatic hydrolyzate by enzymatic hydrolysis of xylan, has some impurities, including enzyme protein, lignin, pigments, inorganic salts etc. The traditional purification method such as ion exchange chromatography, has a long production cycle, complicated operation, and high cost. After decolorization and desalination of enzymatic hydrolyzate, the concentration of XOS solution obtained is too low to sale. The vacuum concentration, which was used often in industry, has high steam consumption, high cost, complicated operation, and more, because of the high operating temperature, it was easy to make the color of product darker.In this paper, the purification and concentration technology of XOS by membrane separation technologies was studied on the limitation above mentioned. Pilot-scale membrane separation equipment was used to research. The associated membrane flux models were built. Membrane cleaning and membrane flux recovery were also studied preliminarily in order to provide theoretical guidance for industrial development and application of good XOS products.Using straw xylanase enzymatic hydrolyzate as raw materials, the effects of feed concentration, operating pressure, temperature, pH on ultrafiltration membrane flux, protein and lignin removal, total sugar transfer rate, light transmittance and decolorization rate of ultrafiltration penetrative fluid were tested. The optimal purification condition was explored:feed concentration of2.00～2.37°Bx, operating pressure of0.18MPa, temperature of25～35℃, pH of7. When feed concentration was2.24°Bx, and temperature was25℃, respectively, the value of membrane flux became steady after21minutes or so, it dropped to137.6L·m-2·h-1from216.8L·m-2·h-1. By ultrafiltration process, the solid content of feed liquid decreased from2.24°Bx to1.75°Bx, total sugar transfer rate was90.1%, protein removal rate was67.9%, lignin removal rate was67.8%, light transmission rate increased from75.7%to93.7%, decolorization rate was80.6%. Decolorization effect of ultrafiltration penetrative fluid was obvious; it was clearer compared with feed liquid. Studying the effect of feed concentration and pressure on ultrafiltration membrane flux, and based on the resistance superposition model, an ultrafiltration membrane flux model was built. The average deviation rate of the model was less than1%, which proved that it could be used to predict membrane flux under different operating conditions.In this work, comparing the effect of pure water permeation flux, desalting and concentration, nanofiltration was proved to be more efficiency and economic than reverse osmosis. By studying the effects of operating parameters of feed concentration, temperature, imported flux, operating pressure on nanofiltration membrane flux, using response surface methodology to optimize parameters, the optimal concentration conditions were ascertained:temperature42℃, imported flux1290L/h, operating pressure0.85MPa. Under this condition, experimental membrane flux was33.20L·m-2·h-1, closing to the predicted value33.79L·m-2·h-1. Quadric polynomial regression model also fitted well. Experiments showed XOS solution could be concentrated from2°Bx to7.64°Bx after23minutes operation, with an increase by nearly4times. Studying the effect of feed concentration and pressure on nanofiltration membrane flux, and based on the classic model of membrane separation, a nanofiltration membrane flux model was built. The correlation coefficient of model predictions and experimental results was0.9937, which proved that the model was good for predicting the experimental results.By comparing the cleaning effect of different cleaning agents, it was concluded: with a compound cleaning agent of0.1%sodium tripolyphosphate and0.025%sodium dodecyl benzene sulfonate, the cleaning effect of ultrafiltration membrane was best, membrane pure water flux recovery rate was99.6%. With0.2%HCl solution, the cleaning effect of nanofiltration membrane was best, membrane pure water flux recovery rate was97.9%. |
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