Study On The Removal Of Xylan From Pulping Black Liquor By Biological Method

Xylanase was used in the treatment of lignin black liquor, involving the use of enzyme to hydrolyze the xylan of high degree of polymerization in the lignin black liquor, the searching of the proper laboratory conditions of enzymatic hydrolysis, the optimization of the process conditions and the enhancement of the sugar-removing ratio. The effect of ultrafiltration, membrane fouling and ultrafiltration on enzymatic hydrolysis was also studied when the lignin black liquor was treated by the membrane with nominal molecular weight cut off 1,000; xylo-oligosaccharide and xylose attained by enzymatic hydrolysis were fermented by Candida utilis and Pichia stipitis so that the purpose of removing xylan and purifying the lignin was gained. The main experimental results were as follows:When Xylanase hydrolyzed the xylan, the suitable pH value was 4.5~5.0 and temperature was 40~50 C. While pH value was 4.8, temperature was 45 C and enzymatic hydrolysis time was 8h, sugar-removing ratio was the highest. But sugar-removing ratio was only 5% lower comparing enzymatic hydrolysis time 4h with 8h. So considering practice, enzymatic hydrolysis time 4h was the best. Substrate concentration has effect on efficiency of enzymatic hydrolysis. The lower the solid concentration of lignin black liquor, the higher the sugar-removing ratio.Enzyme dosage has great impact on sugar-removing ratio. When enzyme dosage was 13.3%(v/v), sugar-removing ratio reaches the highest. Xylanase activity has some effect on enzymatic hydrolysis and there was a limit value of enzyme activity. When enzyme activity was below the value, sugar-removing ratio was lower with lowering enzyme activity; beyond the value, sugar-removing ratio was stable with increasing enzyme activity.The best enzymatic hydrolysis conditions were as follows: pH value 4.8, temperature 45 C, enzymatic hydrolysis time 4h, enzyme dosage 13.3%(v/v). Beyond the limit value of enzyme activity, if solid concentrations were 20g/L, 60g/L and 100g/L, sugar-removing ratio were 65.47%, 50.62%, 40.62% respectively.During the ultrafiltration of lignin black liquor, solvent flux increased with increasing operating pressure. After reaching the limit value of flux, solvent flux was not affected by operating pressure. Feed velocity had no obvious effect on solvent flux. Solvent flux reduced with increasing solid concentration below 80g/L; Solvent flux didn't change much beyond 80g/L. Membrane was apt to be fouled. Batch feed could slow membrane fouling, and at the same time it could explain that membrane fouling was partly reversible.Ultrafiltration had some effect on sugar-removing ratio of lignin black liquor. Batch ultrafiltration had greater effect on sugar-removing ratio than continuous ultrafiltration. Under the same conditions, ultrafiltration had less impact on sugar-removing ratio with higher solid concentration.Both Candida utilis and Pichia stipitis could ferment pentoses. Both yeasts after domestication were adapted to lignin black liquor surroundings. So the utilization of glucose-xylose mixture was very high, reaching 80%. When substrate concentration was high, the utilization of the mixture decreased a little.Using Candida utilis to ferment acid hydrolysis solution of lignin black liquor, the sugar-removing ratio was high, reaching 90% after fermenting 72h. When fermenting xylo-oligosaccharide and xylose through xylanase hydrolysis, the sugar-removing ratio could reach about 85% after fermenting 72h. So Candida utilis could metabolize xylo-oligosaccharide.Using Pichia stipitis to ferment acid hydrolysis solution , the sugar-removing ratio was 88% after fermenting 36h. But after several times of fermentation , the sugar-removing ratio a little. When fermenting the xylanase hydrolysis solution, the sugar-removing ratio was very low.