CFD Investigation Of The Effect Of Mixing Intensity On The Continuous Anaerobic Digestion

The consumption of fossil fuel rapid increasingly has brought enormous environmental pollution problems in recent years.Therefore,the state formulates a series of policies to encourage the development of new energy.As an important part of new energy,biomass can be used by many different ways,this paper mainly studies anaerobic digestion.Anaerobic digestion is a biochemical reaction process that organic compound and animal manure were decomposed by various bacterium to produce the biogas,such as methane and carbon dioxide in the absence of oxygen.This process can realize the recycling of organic wastes,but it is very complex.Many factors have an effect on anaerobic digestion.This technology gained more and more attention in recent years.Therefore,this paper investigated the effect of mixing intensity on biogas production rate,methane content and stability of fermentation by means of the combination of experiment and simulation.The main research content and results of this paper are as follows:1.The biogas production rate,methane content and the process stability of fermentation under different mechanical stirring intensity were studied.The main conclusions were as follows:(1)During the start-up stage,the pH of digester decreased continuously.This trend could be prevented by added alkali liquor into reactor.The pH could be maintained stable by feeding fermentation materials and discharging digester every day.(2)When the Hydraulic Retention Time was 30 days,the biogas production rate without mechanical agitation was 1.56m~3/m~3·d,however,the biogas production rate were respectly 1.5m~3/m~3·d,1.36m~3/m~3·d and 1.33m~3/m~3·d when the rotational speed of impeller were 50rpm,100rpm,150rpm.The average methane content of the four experimental groups was kept at about 70%.It could be seen that the biogas production rate decreased with the increase of stirring intensity.(3)The biogas production rate and methane content were not stable without mixing,but they were more stable in the experiment with mechanical stirring.When the hydraulic retention time was reduced from 30 days to 20 days,the loading of reactor increased from 33 kg/d to 50 kg/d,the pressure in the fermentation tank increased sharply,which resulted in a large number of scum and made the fermentation process unsustainable.2.Five different turbulence models were used to simulate the flow field distribution of CMC aqueous solution in the tank under mechanical stirring.The simulation results were verified by PIV test.The main conclusions are as follows:(1)Compared with standard k-?,RNG k-?,standard k-?and SST k-?turbulence models,according to PIV test,the simulation result of the realizable k-?turbulence model were the best one.Therefore,the realizable k-?turbulence model is recommended to simulate non-Newtonian fluid under mechanical stirring.(2)Fluent was used to simulate the effect of injecting position of tracer particles on mixing time.On the basis of steady flow field,unsteady simulation was carried out to simulate the mixing time.The simulation results showed that the mixing time was affected by the injecting position of tracer particles.(3)When the type of impeller and the mechanical energy consumption were same,the mixing strength of the flow field were affected by the structure of the tank.The average velocity of flow field in spherical bottom tank was 12.8%higher than that in flat bottom tank.3.The realizable k-?turbulence model was used to simulate the flow field in the reactor.The conclusions are as follows:(1)When the rotational speed of two impellers was 50 rpm,100 rpm and 150rpm respectly,there was no dead zone in the reactor.The average velocity decreased,however,the energy consumption of the two impellers increased with the increase of the total solid when the rotational speed was same.When the Total Solid of digester is constant,the velocity gradient increased obviously with the increase of mixing intensity.The type of impeller and the distance between two impellers had an influence on the mixing of digester.Fluent simulated the mixing time required for complete mixing of digester at different Total Solid.The results showed that the mixing time increased sharply with the increase of TS.