Design And Optimization Of Feed Mixing Device For Gas-phase Fixed Bed Reactor

Mixing efficiency of gas-phase feeds is key to the high performance for the fixed bed alkylation reactor in the production of ethylbenzene from the fluid catalytic cracking（FCC）off gas.However,due to the lack of knowledge on the swirling mixing mechanism,either the mixer with a very simple structure or the multiphase mixing device was used in the engineering application as an alternative.The former cannot ensure the required mixing standard,and the latter will lead to unnecessary costs.Therefore,it is important to uncover the swirling mixing mechanism for gas-phase and develop a gas mixing device with higher performance and lower operating costs.In this paper,the computational fluid dynamic（CFD）based large eddy simulation（LES）was used to investigate the mechanism of the gas mixing.The expanding swirling flow can be generated from several spraying jets flow with a side-shift distance D₁.The method of enhancing the mixing rate is mainly relied on two aspects:1)the high mixing intensity at the high-velocity gradient from the swirl and 2)the refined residence time distribution due to the backflow region from the expanding behavior.Both the two characteristics have an important relationship with the side-shift distance D₁.The result showed that the mixing rate of the expanded swirling flow can be maximized when D₁=0.10R.Under the condition of the constant total flow rate,the number of spraying jets flow（N）is not the key parameter to the performance of mixing.On the basis of the mechanism study,this paper designed a ring-shaped gas mixing device with the features of side-shifted spraying jets flow.Comparative study showed that the newly developed device has the relatively highest mixing efficiency,better velocity distribution performance,and occupying the lowest reactor space.After that,the injection angle（β）,injection number（N）,injection space（L）,feeding ratio of the cold feed（F_c’/F_c）,and the total flow rate（F_c+F_h）were optimized and evaluated by a fixed bed reactor in diameter of 2.6 m.The optimal structure parameters and operation conditions are:D₁=0.10R,β=0～45°,N=9～18,L>0.6251×10^-4m²,（F_c’/F_c）=0.6～1.5,（F_c+F_h）=0.4～1.6 kg·m^-2·s^-1.