Numerical Simulation And Structure Optimization Of Activated Carbon Adsorption Process

Volatile organic compounds（VOCs）are important precursors for the formation of ozone and fine Particles.With the continuous development of economy,the emission of VOCs is increasing,which seriously endangers the ecological environment and human health.Activated carbon adsorption method is widely used in VOCs treatment because of it’s high treatment efficiency,wide applicability,simple operation and low operation investment cost.Based on computational fluid dynamics（CFD）,the reaction process of activated carbon adsorption of VOCs was numerically simulated by FLUENT software to explore the effects of inlet flow,inlet concentration and water content on the adsorption efficiency of activated carbon.According to the established activated carbon adsorption simulation system,four kinds of activated carbon adsorption box structures commonly used in industry are numerically simulated.The structure of the adsorption box and the gas distribution device in the box have an impact on the air flow uniformity,which seriously affects the adsorption efficiency of activated carbon.Therefore,it is of great significance to study the type of adsorption box and the gas distribution device.The main research work is as follows:（1）Numerical simulation and optimization of activated carbon adsorption process,using ethanol and nitrogen to configure different concentrations of mixed gas.Through the analysis of adsorption kinetics and adsorption calorific value based on the actual experimental data,the adsorption performance Parameters such as saturated adsorption capacity,adsorption rate constant and adsorption calorific value are obtained.The three-dimensional model of the actual activated carbon adsorption column is established by using the three-dimensional model establishment software SCDM.The mesh is divided by meshing software and brought into fluent for numerical simulation calculation.The activated carbon adsorption bed is simulated by porous media model,in which the mass transfer and heat transfer between gas and solid phases are programmed by UDF（user-defined）module,and the actual experimental data are used to supplement and optimize the numerical simulation.The numerical results show that when the ethanol concentration at the inlet of the model is 6000ppm and the inlet flow is 4L·min.The pressure drop of activated carbon layer is 305Pa,the penetration velocity of carbon layer is 0.09m·s^-1,and the global temperature of carbon layer is 301k.When the inlet concentration,flow rate and moisture content are changed,the unit adsorption capacity increases significantly with the increase of inlet concentration;The inlet flow has no effect on the slope of the penetration curve,the mass transfer area is the same,and only affects the penetration time;The water vapor content has an obvious inhibitory effect on the adsorption of ethanol by activated carbon.With the increase of water vapor content,the adsorption capacity of activated carbon for ethanol gradually decreases.（2）Numerical simulation evaluation of activated carbon adsorption box structure.Based on the research method of activated carbon adsorption device,an industrial activated carbon adsorption box model is established to study the influence of different box types on gas distribution uniformity.The results show that the air flow velocity on the upper surface of horizontal cylindrical adsorption box is stable between 0.7-3.6m·s^-1,and the velocity of gas penetrating the activated carbon layer is stable at 0.7m·s^-1,which is in line with the provisions of the technical code for industrial organic waste gas treatment engineering by adsorption method.Among the four types of horizontal cylindrical adsorption box,the average velocity value is the smallest,the velocity variation fluctuation is the smallest,and the relative standard deviation of the upper surface velocity of the activated carbon layer is 38%.Therefore,the horizontal cylindrical adsorption box is the best of the four box types.（3）Flow change and optimization of activated carbon adsorption box.In order to meet the actual industrial waste gas pollution,the intake pollutant is xylene nitrogen mixed gas,the intake concentration is 216 mg·m^-3 and the intake flow is 10000 m³·h^-1.In view of the uneven gas distribution in the adsorption box,a baffle gas distributor is proposed and studied by numerical simulation.The results show that when the height of the uniform distribution plate in the horizontal cylindrical adsorption box is set to 0.35 m,the position is set to 1.65 m,the width is set to 0.45 m,and the thickness of the carbon layer is 0.3 m,the air inlet angle is set to 45°,and the calculated relative standard deviation of the velocity is the smallest.Compared with the model without optimization,adding baffle and modifying inlet angle can effectively improve the utilization efficiency of activated carbon by 6.2%,reduce the advance penetration area by0.43 m² and prolong the penetration point time by 12.24 h.Finally,the model structure is optimized to help enterprises improve the adsorption efficiency of activated carbon,save the amount of activated carbon adsorbent and prolong the penetration time of activated carbon adsorption.