CFD Study On Flow?Heat Transfer And Reaction Process In VAc Fixed-bed Reactor

Vinyl acetate is an important organic chemical raw material,mainly used for the production of polyvinyl alcohol.The ethylene vapor-phase synthesis of vinyl acetate has the advantages of high product quality and less environmental hazards,and it occupies a dominant position in the vinyl acetate industry.Ethylene vapor-phase synthesis of vinyl acetate generally occurs in a tubular fixed-bed reactor.These reactors have good heat transfer capabilities and are widely used in highly exothermic reaction processes.However,the pores of the bed of the vinyl acetate synthesis reactor are interlaced and connected to each other,the structure is very complex,the internal flow field characteristics and transmission characteristics are not clear,improper design or operation will cause problems such as reduction of selectivity,deactivation of the catalyst sintering,and so on.Therefore,it is necessary to systematically study the flow,heat transfer and reaction processes in the reactor.Firstly,the discrete element method(DEM)was used to simulate the filling process of catalyst particles in the reaction tube,and a bed structure model was constructed.On this basis,the computational fluid dynamics(CFD)method was used to simulate the flow of reactant gases in the bed.Further,the catalytic reaction kinetic model is set in the particles to simulate the reaction and heat transfer process in the reaction tube.The paper studied the bed structure,flow field characteristics,bed pressure drop,temperature distribution and concentration distribution in the reaction tube,as well as the influence of particle size and operating conditions on the reaction process.The results can be used to guide the development,design and operation of production process of the tubular fixed-bed reaction for vinyl acetate.The simulation results of the bed structure constructed by DEM show that the local porosity of the bed decreases by oscillating from the wall surface to the center of the bed in the radial direction,indicating that it is affected by the wall effect.The flow field analysis results show that the fluid flow direction in the bed includes axial,radial,and circumferential directions,and the flow area is divided into high-speed area,low-speed area,and recirculation area,which are quite different from the traditional fixed-bed model assumed flat-flow.In the simulation of the inlet section of the reactortube,the temperature of the bed gradually increases from the tube wall to the center in the radial direction,and the temperature gradient gradually decreases along the radial direction;the temperature in the center of the bed gradually increases along the axial direction.In the simulation range,the the highest temperature of the particles is in the bottom central area of the bed,and the temperature in the area differs from the wall temperature by 5.54 K.As the reaction tube lengthens,this temperature difference will increase within a certain range,which may result in deactivation of the catalyst.The results show that proper increase of catalyst particle size will decrease the pressure drop,improve the heat transfer effect and improve the selectivity of the catalyst to vinyl acetate.The particle size of the better catalyst is 5mm.Appropriately increasing the inlet velocity helps to promote the removal of the heat of reaction in the tube.Increasing the reaction temperature helps to increase the conversion of ethylene,but causes an increase in the temperature rise in the reaction tube and a decrease in the selectivity to vinyl acetate.By adding nitrogen to the raw material,the explosive concentration range of ethylene can be reduced,and the oxygen content in the feed gas can be increased,which helps to increase the conversion rate of ethylene and selectivity of vinyl acetate.