Dynamic Characteristics And Competition And Coordination Relationships Of Cloudy Zone In Gas-Solid Fluidized Bed With Liquid Spray

Polyethylene is an important basic material for the construction of the national economy and the improvement of people’s lives.The existing polyethylene production capacity in China exceeds 20 million tons per year,but there is a serious structural shortage of high-end special materials.How to achieve high performance of the products has always been a challenging industry problem.The cloudy ethylene polymerization gas phase process is a new idea to break through the bottleneck of producing high-end polyethylene products.Through the spray and evaporation of hydrocarbon liquid,multiple reaction zones with different temperature and concentration(gas-liquid-solid’cloudy zone’ and gas-solid ’non-cloudy zone’)are constructed in a single fluidized bed reactor.The polyethylene product with wide molecular weight distribution and high branching content is produced through the cyclic shuttle and polymerization of catalyst particles between different reaction zones,and the multiple uses of single reactor are realized.In the cloudy ethylene polymerization gas phase process,the sprayed hydrocarbon liquid is sheared by gas and collides with particles.The droplets and wet particles with liquid films are formed.The cloudy zone consists of droplets,wet particles and the gas interacting with liquid and particles.Droplets and liquid films have different effects on mass and heat transfer in the reactor.The liquid film covers the surface of the particles and contacts with gas.The interfaces of liquid-solid and gas-liquid are formed.The temperatures of gas and particle decrease and the monomer and hydrogen concentrations around the catalyst active centers change caused by the presence and evaporation of the liquid films.The presence and evaporation of droplets lead to a decrease in the temperature of the gas around them and does not directly affect the monomer and hydrogen concentration around the catalyst active centers.At the same time,the temperature of the cloudy zone fluctuates around the dew point temperature of the sprayed hydrocarbon liquid and the cloudy zone is synergistically controlled by the liquid spray and gas-solid fluidization,which results in a reversible dynamic distribution of temperature,volume and position.The polymerization environment around the catalyst is affected.Thus,characteristics of the liquid distribution between droplet and liquid film and the dynamic characteristics of the cloudy zone are decisive for the polymerization environment,which have a significant impact on the polymer product.However,it is hard to accurately and effectively control the polymerization environments of cloudy zone and the final product structures without the knowledge of dynamic characteristic of cloudy zone and characteristic of liquid distribution.In this paper,the dynamic characteristics of the cloudy zone and characteristics of liquid distribution in the gas-solid fluidized bed of the cloudy ethylene polymerization are investigated via cold-mode experiments,multiphase flow detection method,mathematical modeling and computational fluid dynamics(CFD)simulations.The main research work and findings are as follows:(1)In the quasi-2D fluidized bed,the coexisting of cloudy zone/non-cloudy zone is constructed.The temperature field and particle velocity field of the cloudy zone are measured simultaneously based on the high-speed camera and infrared camera.The fluctuation characteristics of bed temperature,area and position of cloudy zone,and the particle concentration and velocity fields of cloudy zone and non-cloudy zone are investigated.The results show that when the liquid flowrate is low,the temperature fluctuation in the cloudy zone is intense,and the temperature in the non-cloudy zone is stable.When the liquid flowrate increases,the area of the cloudy zone increases and the temperature fluctuation weakens.Both of the frequencies of the area fluctuation of the cloudy zone and the bubble generation and movement are in the range of 2-3 Hz,indicating that the movement of bubbles is an important factor affecting the fluctuation of the area of cloudy zone.With the increase of the liquid flowrate,the spatial distribution of the cloudy zone centers becomes more dispersed and disordered,and the particle concentration and particle velocity in the cloudy zone decrease whose symmetrical distributions in the bed are broken.The results are significantly important for the construction and control of cloudy zone/non-cloudy zone in industrial reactors.(2)In the quasi-2D fluidized bed,a method of ’infrared thermography and particles with reversible thermochromic material(RTM)’ is established to characterize the characteristic of liquid distribution in the bubbling fluidization.The color of particle with RTM changes according to its temperature,and the information of liquid film is obtained according to the color of the particle.The information of liquid phase is obtained from the temperature field measured by the infrared camera.The difference between those is droplets.This method is verified in the minimum fluidization.(3)Based on the mesoscale theory,two competition and coordination relationships are proposed to dominate the dynamic characteristic of the cloudy zone and the characteristics of liquid phase distribution,and a cloudy zone model is established to describe the hydrodynamic properties and laws of heat and mass transfer in the cloudy zone.It is found that the dynamic characteristic of the cloudy zone is controlled by the suspension energy dissipation of the system,including the suspension energy dissipation of droplets and wet particles in the emulsion phase,the energy dissipation due to the bubble expansion and the suspension energy dissipation of interphases.The characteristic of liquid phase distribution in the cloudy zone is controlled by the energy dissipation owing to liquid phase surface change and suspension energy dissipation of droplets and wet particles in the emulsion phase.Based on the above two competition and coordination relationships,a cloudy zone model is established to obtain the data of hydrodynamic,temperature,and ratio of liquid films and droplets in the steady cloudy zone.The results are verified by the results of void fraction and temperature of the steady cloudy zone.In addition,based on the cloudy zone model,the modified drag coefficient of wet particles is proposed and applied into the CFD simulation to simulate the fluidization of wet particles in the cloudy zone.The wet particle velocity and the fluctuation of the bed pressure are obtained,and the results show that the cloudy zone model predict the macroscopic and local hydrodynamic behaviors and the heat transfer characteristic of the fluidized bed with liquid spray.(4)Based on the cloudy zone model,a model of cloudy zone/non-cloudy zone is established.The stability of cloudy zone and non-cloudy zone is controlled by three competition and coordination relationships.They consist of the two competition and coordination relationships which control the dynamic characteristic and liquid distribution of cloudy zone and one competition and coordination relationship which controls the particle fluidization in the non-cloudy zone.This model describes the hydrodynamics and mass-heat transfer.Combining the energy stability conditions,the characteristic parameters of the steady cloudy zone/non-cloudy zone are calculated.A method of prediction of characteristic parameters of cloudy zone/non-cloudy zone is established and the guidance is provided for the control of product quality and development of new product.