CFD Simulation Of Catalytic Cracking Reaction In Monolith Reactor

Ethylene and propylene are the basic raw materials of petrochemical industry,mainly used in the production of polyethylene and polypropylene and so on.In recent years,with the increasing demand for its downstream derivatives,the production and consumption of ethylene and propylene have also increased rapidly.In the traditional production process,researchers usually use riser reactor,fixed bed reactor,fluidized bed reactor,and produce ethylene and propylene in the form of catalytic cracking.Traditional reactors,such as fixed bed reactors,have such problems as uneven distribution,excessive pressure drop,no dust blockage,low overall efficiency,high cost of catalyst recovery and catalyst wear.It is particularly important to find a better way to obtain high yield of ethylene and propylene.Monolithic reactor is first applied to the treatment of automobile exhaust,and it is also the most widely used and most mature field.The monolithic reactor has many parallel and regular through-hole channels,and the catalytic active component is made into a very thin coating structure,which is loaded on the inner wall of the channel.Owing to the special cellular channel structure with a monolithic reactor,the structure reactor has the advantages of low pressure drop,uniform bed distribution,large specific surface area,no catalyst wear,simple amplification,flexible operation and easy separation.It can overcome the problems in the conventional granular fixed bed reactor.In this paper,a new method is proposed,which applies structured reactors to catalytic cracking reactions to investigate the feasibility and advantages of structured reactors for catalytic cracking.Using Naphtha as the raw material,the six lumped kinetic model of catalytic cracking was used to establish a mathematical model to describe the catalytic cracking in a monolithic reactor.The numerical simulation of Naphtha catalytic cracking process were carried out by using fluent 14.5.The effects of reactor size and reaction conditions on the yield of ethylene and propylene and Naphtha conversion rate were investigated by changing pore diameter,reactor length,reactor temperature and gas inlet velocity.And the result shows that,the increase of the diameter of the pore leads to the decrease of the yield of the product,at L=20 mm,the reaction is completed.Increasing the reaction temperature and increasing the entrance velocity are beneficial to the production of the target products.The ethylene productivity in monolithic reactor is high up to 19.3%and propylene is 23.1%with a Naphtha conversion of 92%at 680 ℃ and 0.4 m/s by the surface reaction and ethylene productivity high up to 15.4%and propylene to 18.49%with a Naphtha conversion of 68.46%by the volume reaction.Under the fixed bed reaction with the same reaction conditions,the yield of ethylene was 10.34%,the yield of propylene was 13.32%,and the conversion rate of naphtha was 72%.By using 3D simulation to investigate the effects of pore shape,and the results showed that the pore shape of triangle gives better result than the circular,quadrangle,hexagonal,but the pressure drop is much higher.In general,quadrilateral pore shape is more available.In addition,the 1/4 monolithic structured reactor model is selected for 3D simulation.The difference between the 3D reaction result and the two dimension is not much.This shows that it is feasible to investigate structured reactors in a two-dimensional simulation.In addition,the residence time distribution of the monolithic reactor was also investigated,and the results showed that the flow state of the monolithic reactor was close to the flat push flow reactor.Based on the surface reaction,the parameters of the structured reactor model are larger than that based on the volume response model,which explains why the surface reaction has better reaction results than the volume reaction.The conversion of residence time distribution density function,the residence time distribution model and six lumped kinetic model is used to calculate the naphtha conversion rate,the yield of ethylene and propylene,found it close to the simulation results,which further proves the correctness of the catalytic cracking reaction model selection.The residence time distribution density function can be used to solve the reaction conversion rate and product distribution.In addition,the residence time distribution density function is applied to the more complex nine lumped kinetic model of residue catalytic cracking.The conversion rate of residual oil is 82%,and the yield of ethylene and propylene is 5%and 7%respectively.