Rheology And Heat Transfer Characteristics Of Materials In Vertical Flow Anaerobic Digestion Of Food Waste

Improper disposal of food waste results in pollution of land and water.With the mandatory implementation of garbage classification policy nationwide,the food waste disposal industry will develop fastly.Anaerobic digestion of high solid,also known as dry fermentation,has the advantages of wide utilization of raw materials,large load,low sewage output and energy saving.It is an effective way to dispose of food waste.However,food waste is a non-Newtonian fluid with high viscosity,poor heat and mass transfer performance,and high energy consumption of mixing.The reactor is prone to acidification and local overheating.Therefore,optimizing the heat and mass transfer characteristics of materials by mixing in the reactor is the key problem.Computational Fluid Dynamics(CFD)can simulate the flow field and temperature field of materials in the reactor,which has obvious advantages in economy and accuracy.It is an effective tool for analyzing heat and mass transfer characteristics of materials.In this paper,the laboratory-scale high solid vertical flow anaerobic reactor is taken as the research object.The viscosity and thermal physical property measurement show that food waste is pseudoplastic non-Newtonian fluid,and the higher the solid content,the stronger the non-Newtonian property of the material.Using the gas-liquid two-phase flow model established by CFD,the influence of agitator on the flow and temperature field is analyzed.The results show that the mixing effect is the best with a large amount of liquid discharged by the double helical agitator,and the increase of the impeller diameter ratio,the agitator length and the stirring speed will reduce the dead zone and increase the average speed of the material,which is conducive to mixing,but also increases the stirring power consumption.In terms of the mixing effect and energy consumption,the double helical agitator with a impeller diameter ratio 0.75,a impeller height h/H=0.78,?/H=0.13,and a stirring speed 20 rpm is the best design of the agitator,and the power consumption is 81.71W/m~3.After 2.5h of heat,the temperature field is relatively uniform and the design meets the requirements.Using the gas-liquid two-phase flow model established by CFD,the influence of gas mixing parameters on the flow and temperature field is investigated.The results show that the higher gas injection rate,the higher the average material velocity and the input power,the greater influence on the growth of microorganisms.The nozzle aperture has little influence on the flow field and microbial growth.The more the number of nozzles,the smaller the average velocity of the material,and the more complex the flow pattern.When the nozzle mounting height increases,the average velocity of material tends to decrease gradually.On the condition of optimal design,the stirring power of 75.90W/m~3,nozzle aperture d/D=0.04,1 nozzle and installation height h/H=0.10,the temperature distribution in the reactor was relatively uniform after 2.5h of heating.Compared with the mechanical mixing design,the energy efficiency of mechanical mixing is higher and the mixing effect is better.The anaeroobic reactor with an volume of 18 L was amplified to 84m~3 by using the geometric similarity method with the unit volume power as the amplification factor.The simulated results show that the material flow characteristics in the reactor after amplification were basically the same as before,and the mass transfer of the material is relatively uniform,thus meeting the mixing requirements.Through economic benefit analysis,the reactor can achieve a profit of 18159 yuan per year,thus it has realized the unification of economic benefit,social benefit and environmental benefit.It provides reference for the development of anaerobic digestion reactor of high solid.