Characteristics Of Mass Transfer And Biochemical Reactions For Simultaneous Enzymolysis And Fermentation Of Rice Straw With Gas Stripping Ethanol

Nowadays, the research on the renewable, environment and friendly energyresources have been extensively investigated because of a series of environmentalproblems caused by the use of traditional fossil fuel. Among these new energy resources,the biofuel has many advantages and has attached wide attention from researchers.Currently, grain was extensively used as raw material to produce fuel ethanol; this willaggravate the world’s food supply shortage. Although cellulosic materials are the mostabundant biomass resources in the world, many of them such as rice straw were burnedin situ or directly abandoned. So the technology converting cellulosic materials intoethanol is very important to promote the sustainable development of society andeconomy, and the protection of the ecological environment.Lignocellulose is mainly composed of cellulose, hemicellulose and lignin, and thecellulose with high crystallinity is covered by lignin and hemicelluloses. Therefore,the lignocellulose needs pretreatment to remove the lignin and hemicelluloses, thus, theabsorption of cellulase on the surface of cellulose is improved and the efficiency ofenzymolysis is increased. To eliminate the inhibitions of reducing sugar to enzymolysisand ethanol to fermentation, simultaneous enzymolysis and fermentation in solid-statewith gas carrier stripping ethanol was proposed. In this system, the mobile phase（carrier gas） and the solid substrate used for simultaneous enzymolysis and fermentation（porous media） constitute a complex multi-component transfer system with gas flowand biochemical reactions, in which the enzymolysis and fermentation simultaneouslyconduct, the reducing sugar produced by enzymolysis is consumed by yeast cells, theethanol produced by fermentation transfers from stationary phase to mobile phase, andis carried out by carrier gas. Thus, the inhibitions of reducing sugar to enzymolysis andethanol to fermentation are eliminated, and the efficiency of conversion of cellulose toethanol is greatly increased. The thesis obtained the following achievements:①Effects of operating conditions on the enzymatic hydrolysis performance wereexperimentally investigated, the optimal conditions for enzymolysis included the watercontent of substrate of1mL/（g substrate）, enzymolysis temperature of50°C, rice strawsize of60mesh, and the cellulase dosage of2.0mg/（g substrate）. Besides,simultaneous thermo-gravimetric analysis was also employed. The results showed thatthe combustion enthalpies of the post-hydrolyzed substrate reduced due to a decrease in cellulose content. According to weight loss of cellulosic substrate after enzymolysis, itcan be concluded that the chemical components of post-hydrolyzed substrates hadvaried.②Effects of different pretreatment methods on the enzymatic hydrolysis ofhigh-solid rice straw were conducted. It was found that the microscopic structure ofstraw obviously changed and the output of glucose was improved after the NaOH,H₂SO₄and ultrasonic pretreatments, and the glucose content achieved to126.3mg/（gsubstrate）after1%NaOH pretreatment. Therefore, suitable pretreatment could destroythe integrity of lignocelluloses, remove the sheath of lignin and hemicellulose, andintensify the absorption of cellulase on cellulose and increase the efficiency ofenzymolysis.③Effects of glucose concentrations on the ethanol production and thefermentation kinetics were performed with gas strip. The experimental results indicatethat with the increase of initial glucose concentration, the logarithmic growth phase ofyeast extended, and the biomass concentration and the output of ethanol in the liquidincreased. With the glucose concentration increased to20%, the output of ethanol hadlittle increment. Besides, the kinetics model on fermentation at the initial glucoseconcentration of15%with a sparging carrier gas was established; the yeastconcentration, the output of ethanol and the consumption of glucose via time weresimulated, respectively. The model results were basically consistent with that ofexperiment. Research results indicate that the optimal performance of ethanolfermentation was obtained at15%initial glucose concentration, that inhibition ofethanol to fermentation was eliminated by gas stripping.④The characteristics of simultaneous enzymolysis and fermentation in solid-statewas invesitigatd in a packed bed with gas stripping. It was found that the optimalenzymolysis and fermentation was obtained at the water content of2mL/（g substrate）.The output of ethanol carried out by gas increased from1.289g to2.298g with the flowrate of gas increased from10mL/（min） to30mL/（min）, and then dropped to2.05gwhile the flow of carrier gas increased to40mL/（min）. The output of ethanol firstlyincreased with an increase of the yeast loading to0.75mL/（g substrate） and thenbecame stable with the further increase of yeast loading.⑤Theoretical model of mass transfer during simultaneous enzymolysis andfermentation in solid-state with gas stripping ethanol in a packed bed was established,and the distribution characteristics of ethanol in the packed bed under the different operating parameters were obtained. While the flow rate of gas carrier increased from10mL/（min） to30mL/（min）, the ethanol concentration at the outlet of packed bedrapidly decreased from69.7g/m³to27.7g/m³, however, it only decreased to21.3g/m³with the carrier gas further increased40mL/（min）. Meanwhile, with the increase ofyeast concentration in the substrate, the ethanol concentration at the outlet of packedbed increased with an increasing of yeast loading. In addition, the ethanol concentrationat the packed bed has little changed with the different substrate porosity.