Kinetic Models And Response Surface Optimization Of Dilute Acid And Salt Hydrolysis Of Corncob For Xylose Production

Lignocellulosic materials represent an abundant and inexpensive source ofsugars which can be microbiologically converted to industrial products. Among thevarious agricultural crop residues, corncob is one of the most abundant agriculturematerials, which can readily be hydrolyzed into xylose due to its high content of xylan.Hydrochloric acid, nitric acid, sodium nitrate and mixed ones were chosen in thepretreatment of corncob because of the hemicelluloses was easier to be hydrolyzedthan cellulose and lignin. The effect was evaluated by the concentration of xylose, theSaeman model and response surface method, which were used to optimize the diluteacid pretreatment conditions, choosing the optimization condition combined withSEM results and activation energy datas.The effects of hydrochloric acid, nitric acid alone on the hydrolysischaracterization of corncob at90℃,120℃and150℃were investigated. The regularpatterns were evaluated by Saeman model, the best reaction condtion was optimizatedby response surface. The results showed that the xylose and glucose concentrationwere increased with the increase of the acid concentration and process time. Theresults of experiments were in accordance with one of the models, the optimalconditions of hydrochloric acid and nitric acid were reaction time of117min, acidconcentration of3%; and reaction time of119min，acid concentration of2.7%,respectively, and the hydrolysis efficiency of hemicelluloses was86%,85%at90℃.Meanwhile, the xylose concentration was increased with the increase of the acidconcentration at the short time at120℃. And the xylose concentration was decreasedas reaction time was extended for hydrochloric acid, however, the nitric acid keptstable. The hydrolysis trends were proved by the model. The best conditions wererespectively reaction time of22min, acid concentration of3.9%; reaction time of51min, and acid concentration of3.9%, corresponding to yields were64%,78%. Thesame result was more obvious at150℃, the highest of the xylose concentration wasreached at10min, and the degradation of the xylose concentration at the present ofhydrochloric acid was obvious with the time prolonging and the acid concentrationincreasing. Meanwhile, the effect of nitric acid was also decreased, but more slowly.The both of the optimial pretreatment condition were reaction time of10min, and acidconcentration of1.0%, corresponding to yield77%,96%. The conclusion showed thatthe effect of nitric acid was better than hydrochloric acid, and the degradation ofhydrochloric acid was much fast and severely. The optimization condtions of temperature, acid concentration and process timefor hydrochloric acid and nitric acid were92℃,3.2%,88.75min;149℃,2.1%,14.84min, corresponding to yields of xylose were78%,92%. The activation energy wasobtained by calculation, was72.5kJ/mol,91.5kJ/mol,which was proved that thecorncob was easy to be hydrolyzed. Because of the cost of nitric acid was much moreexpensive than hydrochloric acid, the effect of pretreatment was better, sodium nitratealone and the mixed system were chosen. The effect of sodium nitrate on pretreatmentwas not obvious, however, the mixed system of hydrochloric acid and nitric acid (1:1)of showed a good choice at the condition of150℃,10min,1.0%, corresponding toyield of95%,93%. In summary, the mixed system of hydrochloric acid/sodiumnitrate and hydrochloric acid/nitric acid can not only reduce the cost, the yield ofxylose was high. The figures of SEM showed that the result was in accordance withthe hydrolysisi effect, the hydrolysis uniformity was improved by adding the salt.