Simulation Of Hydrodynamic And Mixing Characteristics In A New-type Gas-Solid Air Loop Mixer

The characteristics of particles flow and mixing have a considerable influence on mass and heat transfer,as well as reaction efficiency.Compared with a bubbling bed,the gas-solid air loop mixer(GSALM)offers several particle circulation rounds between draft tube and annulus,which leads to relatively strong particle convection,diffusion and shear,thereby improves the particle mixing performance.However,the present research on GSALM mainly focuses on the macroscopic gas-solid flow,mass transfer,heat exchange.The characteristics of local particle flow,mixing,and the influence of mixer configuration are not well understood.Besides,the particles flow along the radial direction in a mixer is weak,affecting the particles mixing efficiency.In this paper,a new type air loop mixer with slots on the draft tube is proposed in order to strengthen particles flow and mixing along the radial direction.The characteristics of particles flow and mixing in the mixer are investigated.Firstly,A 3-D simulation method is established with the Euler-Euler model and the behavior of particles flow in a new-type GSALM is investigated.By comparing the predicted results with the experimental data,the key parameters are determined.The results show that with the energy minimization multi-scale(EMMS)model,a specularity coefficient of 0.5,a restitution coefficient 0f 0.9 and inventory of 58 kg,the predicted results agree reasonably with the experimental data.With the established simulation method,the characteristics of particles flow and mixing in the GSALM under different operating conditions are studied.The results show that after slotting on the draft tube,at the cross section of the slots the pressure in the draft tube>that around the slots>that in the annulus,which strengthens the particles flow and mixing along the radial direction.At the bottom zone,the zone of the central downcomer outlet,the cross section of the slots and the gas-solid separation zone,there exist counter-current or cross-flow contacts of particles,forming a number of particles mixing zones with a high efficiency in the mixer.There exists a bidirectional gas bypassing in the bottom zone:about 5%of the gas from the plate distributor bypassing into the annulus and nearly 83%of the gas from the annular distributor entering the draft tube.With increase in the gas velocity,the circulating particles flow rate increases,but the ratio of the particles flowing into the annulus through the slots decreases.In this paper,the structure of downcomer and the height of the slots on the draft tube are further optimized.The results show that with a diameter of downcomer S_c/S_d=0.112,the height of downcomer outlet 0.558 m and of slots 0.9 h_d,the circulating mass flux of particles is large,which strengthen the particles mixing.In the optimized gas-solid air loop mixer,the particle mixing index reaches more than 0.99and changes little with the gas velocity and the particles feeding flow rate.In order to simplify structure of the new-type gas solid air loop mixer,The characteristics of particles flow and mixing in the GSALM with no slots on the draft tube,the unforced GSALM and the free bed are studied.The results show that with no slots on the draft tube,the particles flow and mixing on the horizontal section are weak.In the unforced GSALM the orderly circulating is weakened,even the circulation cannot be formed.The particles are exchanged and mixed between the center and the zones near the wall in the unforced GSALM,but the effect of the mixing is not significant.Compared with the GSALM,the circulating flow of particles in the unforced GSALM and the free bed are not strong and it is easy to produce deviation flow,and the mixing efficiency of particles decreases.The hydrodynamic characteristics of the industrial GSALM are studied.There are some problems being found,such as particle wear on the wall of the central downcomer,gas bypassing and deviation flow,which seriously affect the particles flow and mixing performance of the mixer.A new type central downcomer outlet with a cone baffle is proposed in this paper.The results show that in the mixer with 1.11 D_c baffle the gas bypassing is eliminated and the circulating mass flow rate is increased by 62.5%.In addition,the mixing index of the industrial GSALM can reach up to 0.996.