| Fossil energy shortage in China,and fuel ethanol,as the most likely renewable energy to replace fossil resources,has become a hot research focus.China has a large inventory of aged rice,and the development of fuel ethanol can not only alleviate the energy shortage in China.In fuel ethanol production process,but also consume too much aged rice.In fuel ethanol production process,amylase enzymatically hydrolyzes the raw material during the liquefaction process,and converts starch into substances such as dextrin and maltose for use in subsequent processes.Liquefaction process is one of the key links in fuel ethanol.Moreover,with the breakthrough of concentrated mash fermentation technology,raw materials with high solid content have become the mainstream of fuel ethanol production.While improve the economic benefits and reduce costs,the raw materials with high solid content also cause the increase of liquid concentration.Traditional liquefaction stirring tanks often lack effective mixing and mass transfer,resulting in incomplete mixing in the liquefaction process,which affecting the utilization of amylase for raw materials during the liquefaction process and the yield of the subsequent fermentation process.Aiming at the problems of high viscosity and difficulty in mixing in the liquefaction process,the fluid mechanics model of the liquefaction process was established based on the CFD software,and the flow field flow and power consumption mixing of the three impellers in the liquefaction process were studied,and the dynamic process of the liquefaction reaction under different solid contents was studied.Causes affecting product conversion and mixing.Firstly,using brown rice flour as raw material,the rheological properties of the raw material slurry in the initial and late stages of the liquefaction process under different solid contents were measured.It is found that the raw material slurry exhibits obvious shear-thinning non-Newtonian fluid properties,and its rheological model and parameters are determined.Secondly,the single-phase flow models of inclined large-diameter impeller,multi-blade combined impeller and largest-blade impeller were established by Ansys CFD software,the effects of impeller type and impeller design parameters on mixing in the early and late stages of the liquefaction reaction were studied.The flow field velocities of the three impellers were investigated from the three aspects of velocity cloud,velocity streamline diagram and dimensionless velocity distribution.The results show that the inclined large-diameter impellers have greater radial velocity during the liquefaction process,and the groove bottom and the groove top area have greater axial velocity;the mixing time of the three kinds of impeller in the liquefaction process was investigated by the tracer method,and it was found that the mixing time of the three impellers had a small difference;the liquefaction was also studied.The stirring power of the three types of stirring paddles at different speeds is 15%lower than that of the largest blade type stirring paddle,and 75%lower than that of the multi-blade combined stirring paddle,which can better reduce the production process.cost;The shear rate of the liquefaction process was investigated.The shear rate can reduce the viscosity of the non-Newtonian flow field to a certain extent and reduce the difficulty of mixing.The shear rate is mainly distributed near the impeller.In the early stage of liquefaction,the average shear rate of the inclined large-diameter impeller is 13.26 s-1.In the later stage of liquefaction,the average shear rate of the inclined large-diameter impeller is 7.03 s-1.It is much higher than the other two impellers,indicating that the viscosity gradient of the flow field stirred by the inclined large-diameter impellers is lower and the distribution is more uniform.Through various investigations,the advantages of inclined large-diameter impellers in the liquefaction process are proved.Then,the optimized analysis of the selected impeller structure parameters and agitation tank structure was carried out.It was found that with the increase of the impeller diameter ratio,the stirring power consumption continued to increase,and the proportion of the dead zone distribution gradually decreased.When d/D=0.7 it has lower power and can achieve better flow field distribution;by optimizing the inclination angle of the stirring blade,it is found that when the inclination angle of the stirring blade is 45°,the flow field in the near-wall area is the best,and the power is also have smaller values in order to further reduce the dead zone of stirring,the radian of the bottom of the stirring tank was studied.It was found that the change of the radian of the bottom of the stirring tank would not affect the stirring power,but it could reduce the proportion of dead zone.By optimizing the radian of the stirring tank,the proportion of stirring dead zone was finally reduced to 2.8%.It can be seen from the speed distribution and mixing time that the speed distribution in the optimized mixing tank is more uniform and the mixing time difference is small.Finally,a dynamic model of the liquefaction reaction process was established,and the liquefaction process under three different solid contents was simulated by combining fluid mechanics with simplified reaction kinetics,and the effects of the initial solid content and the enzyme content in the secondary phase on the reactants and products were investigated.The results showed that with the increase of the solid content,the viscosity of the slurry in the stirring tank gradually increased,which increased the difficulty of mixing the enzyme and the raw materials,and the increase of the solid content had an obvious inhibitory effect on the distribution of the product.This phenomenon affects the rate and extent of the conversion of reactants to products,while the content of enzymes has less effect on the flow field.Simulating the liquefaction process by hydrodynamics can explain the conversion between reactants and products,improve the conversion efficiency of reactants,and provide a reference for the design of the reactor. |
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