| Anhydrous hydrogen fluoride(AHF)is a widely used important chemical raw material,fluorite and sulfuric acid reaction is the main way to produce AHF.But at present,the mainstream process of preparation of AHF-the rotary kiln process which takes fluorite(CaF2)as raw materials has various problems,such as the complex process,large investment,low heat/mass transfer efficiencies and difficulty in dealing with low-grade fluorite ore.Therefore,taking into account the high heat/mass transfer efficiency in fluidized bed,some researchers put forward the process of producing anhydrous hydrogen fluoride in gas-solid fluidized bed.The multistage fluidized bed is adopted in this process to provide the reaction space where fine fluorite particles(average particle diameter of 28.4 ?m)are fluidized by the gas mixture(H2O(g),SO3(g),H2SO4(g)and HF)and react with the gas mixture,countercurrent contact happens between gas phase and solid phase in the bed,which greatly improves the heat/mass transfer efficiencies and is suitable for processing low grade,fine fluorite powder.However,fine fluorite particles are prone to self-aggregation due to strong interparticle forces,which will greatly affect the fluidization quality and thus affect the heat/mass transfer efficiency;at the same time,the exothermic reaction of H2O(g)and SO3(g)needs to match the endothermic reaction of fluorite and H2SO4 in the process idea.The heat balance is strongly coupled with the reaction and the reaction is easily destabilized.Based on these reasons,there are no industrialized successful cases for this process.This work embarks from the development of background of the production of hydrogen fluoride in the multilayer fluidized bed,firstly,the fluidization behavior of the fine fluorite particles in the single-stage fluidized bed is studied,and combined with the characteristics of the process,an effective method for enhancing the fluidization quality was established;furthermore,the fluidization behavior of fine fluorite particles in the two-stage fluidized bed is investigated and the influence of operating conditions on the stable transfer of the fine fluorite particles between the stages is examined,then the stable operation range and particle residence time in the multi-layer fluidized bed was determined;Finally,based on the heat balance and mass balance and combined with the characteristics of the multi-layer fluidized bed reactor,an industrial scale three-phase model in the three-stage fluidized bed is established.The three-phase model of the bed further examines the influence of residence time,H2SO4(1)/SO3(g)in feedstock,and pressure on the reaction and heat transfer.The results of this work confirm the feasibility of the process from the perspectives of fluid dynamics and reaction heat transfer coupling,which provides guidance for the optimal design and stable operation of the process.The main research contents and achievements include:(1)In a Pleixglas fluidized bed with the inner diameter of 240 mm,dry air is used as fluidized gas,the agglomerate fluidization of fine fluorite particles under the condition of room/high temperature is studied through the the initial fluidization curve,bed collapse/expansion curves,and overall nonideal index of the bed,the method to improve the fluidization of fluorite particles by adding calcium sulfate particles is also proposed according to the coordination theory for the first time.The results show that the fluidization of fine fluorite particles is characterized by agglomerate fluidization at both room/high temperature,the fine fluorite particles would de-fluidize at 723 K because the van der Waals force between the particles gradually increases with temperature,but the de-fluidization at high temperature is reversible and the fluidization can be regenerated as the bed gradually cool down.Furthermore,combined with the process characteristics,a coordination theory is proposed,fine fluorite particles and calcium sulfate particles were arranged alternately,then the fluorite particles agglomeration resulted from van der Waals forces was prevented by calcium sulfate particles between them;meanwhile,the calcium particles agglomeration resulted from hydrogen bonds was prevented by fluorite particles.According to the theory,the optimal addition fraction of calcium sulfate is between 16.5%-22.4%.Finally,the correctness of the coordination theory is verified by the fluidization experiment.The experimental results show that the optimal fluidization quality of fine fluorite particles at both room and high temperature is obtained when the addition fraction of calcium sulfate particles is 20 wt%.The optimal value is within the optimal range of theoretical calculations.(2)In a two-stage fluidized bed with the inner diameter of 240 mm,dry air is used as fluidized gas,the pressure drop of the bed and downcomer is analysed to study the effect of the two basic operating parameters of the superficial gas velocity and the solid feeding rate on the stable operating range and the average residence time of the bed of fine fluorite particles in a two-stage fluidized bed,and further contrast the effect of the straight and orifice downcomer on the stable operating range and the average residence time of the bed.The experimental results show that the presence of stable solids transfer between the beds can be judged by combining the pressure drop in the upper bed and the pressure drop in the downcomer;when the pressure drop in the lower bed no longer increases with the increase of solid feeding rate,the lower limit of operation is reached,while when the pressure drop of the downcomer remains constant under certain conditions,the upper limit of operation is reached.According to the determination of the stable operating range of the two-stage fluidized bed,it can be found that both the lower limit of operation(minimum stable operating gas velocity)and upper limit of operation(maximum stable operating gas velocity)increases with the increase of solid feeding rate and the orifice downcomer has wider operating range due to the effective suppression of blow-by and easy to handle the stable transmission between the stages of the cohesive particles.In addition,the average residence time decreases with the increase of the solid flow rate when the gas velocity is constant and the reisdence time decreases with the increase of operation gas velocity when the solid flow rate is certain,.But under the condition of the same operating gas velocity and solid flow rate,due to the outlet diameter of orifice downcomer is less than the straight downcomer,average residence time in the the two-stage fluidized bed with the orifice downcomer(30?100 min)is longer than the bed with the straight downcomer(16?99 min).(3)A mathematical model for the three-stage fluidized bed reactor at an industrial scale(3 wt/a)is established and the material and heat balance in the three-stage fluidized bed is calculated,furthermore,the effect of different operating conditions on the reaction and heat transfer efficiency is investigated.The simulation result shows that in the three-stage fluidized bed,the conversion of fluorite particles on each bed gradually reduce from top to bottom of the fluidized bed and the final conversion of fluorite particles is closely related to the average residence time of the particles in the bed.The final conversion rate of fluorite particles can reach 99%when the bed residence time is 31 min.In order to ensure that the reactions in each bed are conducted under the dew point,the excessive heat in the bed need to be removed.The heat in the upper bed and lower bed that need to be removed is more(5993.51 kJ/h and 3557.62 kJ/h,respectively),while less heat need to be removed from the middle bed,only 346.30 kJ/h.H2SO4(1)/SO3(g)in feedstock has a great influence on the heat balance of the lower bed layer(distribution plate 3)and less effect on the upper and middle bed layers.When H2SO4(1)/SO3(g)=1.70,the surplus heat of the lower bed layer is almost 0,which indicates that the lower bed can achieve self-balancing in this case,but the middle and upper beds still have surplus heat need to remove(351.29 kJ/h in the middle bed and 3593.70 kJ/h in the upper bed).The operating pressure had a significant effect on the size of the reactor.Increasing the operating pressure can effectively reduce the residence time and reactor volume,but micro-negative pressure operation can ensure the safety of the reaction without affecting the reaction and heat transfer efficiency significantly,and is therefore more suitable for the process. |
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