Characterization Of Liquid Spray Zones And Phase Transfer Behaviors In Gas-solid Fluidized Beds With Lateral Liquid Spray

Gas-solid fluidized beds with liquid spray or injection have been widely used in various industrial processes,such as fluid catalytic cracking,Wurster coating,aniline synthesis,granulation for agglomerate production,and super condensation process for olefin polymerization.Due to the strong heat removal capability by liquid temperature increase and evaporation,the hydrodynamics in the liquid spray fluidized beds are quite different from the common ones.Taking the afore-mentioned polymerization process as an example,liquid condensing agent atomized through a nozzle is sprayed into a fluidized bed reactor from the side wall.The sprayed liquid can not only improve the particle-droplet contacting efficiency,and also form multi-zones of different temperature and reactant concentration through evaporation.From the reaction perspective,when particles circulate between the liquid spray zone and liquid-free zone,polymer layers with high molecular weight and branching degree and those with low molecular weight and branching degree are alternatively generated,leading to a molecular-level mixing of different polymer structures and hence an excellent product performance.The liquid spray zone fluctuates periodically with the polyolefin fluidized bed,It affects the gas-solid behaviors,heat and mass transfer between phases and polymerization condition of particles seriously.A comprehensive understanding of the fluctuation features of liquid spray zone is good for designing and operating liquid spray polyolefin fluidized bed.This work mainly focus on the characteristics of liquid spray zone and heat-mass transfer between phases in olefin polymerization fluidized beds with lateral liquid spray based on cold model experiments and computational fluid dynamics.Research work has been conducted in the following aspects:1.The area of liquid spray zone is measured by thermal resistance and conductivity probe in a hot gas inlet gas solid fluidized bed.The distribution of the liquid spray zone has been revealed under different gas velocities,liquid spray velocities and liquid spray heights.And a model has been built to predict the liquid penetration length.Results demonstrate that the liquid spray zone increases due to the decrease of liquid evaporation rate.And the fluctuation frequency of liquid spray zone increase with the increase of liquid spray velocity.The mechanism of liquid effect on particle and bubble motion behaviors is proposed based on temporal-frequency analysis of pressure fluctuation and acoustic emission signals.When the fluid drag force is less than the liquid bridge force between particles,liquid distribute all over the bed.Bubble size increases as the increase of inter-particle force,then decreases owing to the increase of particle weight with increasing liquid flow rate.When the fluid drag force is greater than the liquid bridge force,liquid mainly distribute in the upper part of the bed.And it is difficult for the wet particles to form agglomerates.Besides,the liquid adhesion and evaporation on particles could enhance the particles motion intensity.2.An Eulerian multi-fluid model has been used to simulate the cold model fluidized bed.The main factors that affect the liquid spray zone fluctuation feature are revealed.And the variation process of liquid during the operation of the bed has been clarified.Meanwhile,the effect of liquid spray on heat-mass transfer between phases is acquired.The results demonstrate that the motion of solids determine the fluctuation frequency of liquid spray zone,for the fluctuation frequency of liquid penetration length and liquid spray zone area are smaller than the bed fluctuation frequency.The simulation results show that the liquid mainly distribute on the particles,and most liquid evaporate in the emulsion phase.Then the vapor transport to the bubbles as the bubbles rise to the surface of the bed.Besides,the heat transfer rate between liquid spray zone and other regions can be calculated by analogied to the heat transfer process between bubble and emulsion phasein the bed.3.An Eulerian multi-fluid model coupled with particle heat production has been established on the basis of the validated CFD model.It is found that the position of the nozzle,liquid evaporation rate,heat production and removal rate affect the liquid spray zone and mass-heat transfer between phases remarkably.The liquid penetration length decreases,but the spray zone area and spray zone fluctuation frequency increase with increasing liquid spray height.Besides,the spray zone area and fluctuation frequency increase,but liquid penetration length changes a little with increasing inlet droplet diameter.Moreover,the spray zone area decreases,but the spray zone fluctuation amplitude inceases with increasing gas liquid velocity ratio or particle heat production rate.4.An Eulerian multi-fluid model coupled with ethylene polymerization reaction has been established to simulate the reaction in the bed.The distribution of temperature and reactant concentration is simulated under different liquid spray velocities and nozzles arrangements.And the effect of liquid spray on the product quality has been revealed.The study shows the temperature distribution is narrowed but Cc6H12/Cc2H4 distribution broadens with increasing liquid spray velocity.As a result,the melt flow index and density of polyethylene decrease.When two nozzles spray liquid in the same side of the wall,the temperature and CC6H12/C2H4 distribution are all broader than other nozzle arrangements.Moreover,the melt flow index and density of polyethylene fluctuate more intensively.It's good for obtaining products with quite different structure in different bed regions.Thus multi-nozzles in the same side of the wall is the suggested way to arrange the nozzles.