Effects Of Jet On The Flow Field And Reaction Characteristics In Multiphase Reactors

The movement of a fluid through a nozzle into another fluid is called a jet.In the chemical industry,the introduction of jets into multiphase reactors is an important means to achieve process intensification and product property regulation.The complex effects of the jet on the reactor are manifested in several ways.First,the flow structure of the jet itself includes the jet core,the initial section and the boundary layer.The distribution characteristics of each microstructure contained in different types of jets are different,resulting in a complex and variable jet flow field.Introducing a jet into a chemical multiphase reactor,different types of jet flow structures meet and mix with different types of multiphase reactor flow structures,eventually forming a new flow structure.Second,the jet fluid enters the reactor and interacts strongly with the multiphase fluid in the reactor.On the one hand,the momentum carried by the jet enhances the turbulence of the fluid in the reactor,which increases the reaction conversion and spatial and temporal yield by enhancing the mixing and heat transfer of the fluid.On the other hand,if the jet undergoes a phase change,it will change the temperature distribution in the reactor;if the jet contains reactants,it will change the concentration distribution of the reactants in the reactor,which will affect the product properties.Several factors mentioned above lead to complex,variable and unpredictable flow and reaction characteristics of the coupled jet and multiphase reactor system.It is important to systematically investigate the effect of jet on the flow field and reaction characteristics in different types of multiphase reactors,and summarize the common laws of the interaction between jet and different reactors for the design,optimization and application of multiphase reactors with coupled jets.In this work,three multiphase reaction systems from simple to complex,namely gas-liquid reactor with liquid jet,gas-solid reactor with liquid jet,and gas-solid reactor with liquid-solid jet,are studied by mathematical modeling and simulation with the representative objects of jet bubbling reactor,liquid jet gas-solid fluidized bed reactor,and slurry jet fluidized bed reactor,respectively.According to literature research,there is a paucity of studies on the flow field and reaction characteristics in the reactor with jet fluids,and it is difficult to fully characterize the flow field details and reaction processes under the effect of jet fluids only by experimental means.Numerical simulation is a research method to obtain numerical results corresponding to the actual process by solving the computational model.In this paper,we will establish numerical calculation models matching different types of multiphase flow reactors with coupled jet fluids to systematically study the fluid motion evolution,energy conversion,and reaction processes in the reactor.The main research contents and conclusions are as follows:(1)For the gas-liquid two-phase reactor coupled with liquid jet,a PBM model describing the agglomeration and fragmentation behavior of dispersed bubbles was established,and a numerical calculation model for the jet bubbling reactor was developed and its accuracy was verified.The simulation results show that the liquid jet enters the reactor and oscillates periodically from side to side,causing the flow field in the reactor to change periodically.The effect of jet liquid on bubbles includes both transfer and fragmentation:On the one hand,in the jet affected zone near the nozzle,the liquid phase turbulent kinetic energy and turbulent dissipation rate are significantly greater than in other zones of the reactor,and bubbles are more likely to break up.Therefore,the bubbles in the reactor gradually increase in the rising process with the increase of height,the bubble breaking rate gradually increases,the average diameter gradually decreases,and the proportion of small bubbles gradually increases.On the other hand,the liquid phase vortex in the reactor creates,disappears,moves and expands regularly,and bubbles of different diameters are carried and transferred to various zones of the reactor under the periodic motion of the liquid phase,thus allowing the gas and liquid phases to mix thoroughly.(2)For the jet bubbling reactor,the flow field characteristics in the reactor at different liquid jet velocities were simulated numerically to reveal the influence of the input energy of the liquid jet on the evolution period of the flow field,bubble size distribution and reaction rate in the reactor,and to elucidate the energy transfer mechanism in the reactor after the introduction of the jet.The results show that as the input energy of the liquid jet increases,the period of the vibrating fin motion of the main body of the jet gradually decreases,the liquid-phase vortex lifetime also decreases,the depth of jet affected zone gradually increases,the bubble breaking rate increases,and the proportion of small-sized bubbles increases.At different jet input energies,the turbulent vortex structure is mainly present in the zone below the nozzle,where the fluid motion velocity and bubble breakage rate are significantly higher than those in other zones of the reactor.Comparing the spatio-temporal variation of energy with the corresponding flow field structure,it is found that the energy of the liquid jet is transferred through the liquid-phase vortex.The liquid-phase vortex closest to the nozzle outlet contains the largest energy,and when the volume share of the vortex in a zone reaches a maximum value,the total energy in the zone also reaches a maximum value at the same time.Further,for the surface masstransfer controlled methanol carbonylation reaction,the mass transfer coefficients and reaction rates were calculated at different jet input energies,and it was found that the introduction of liquid jets was beneficial to increase the specific surface area and mass transfer efficiency in the reactor,which increased the reaction rate and the spatial-temporal yield of the reactor.(3)For the gas-solid two-phase reactor coupled with liquid jet,the numerical calculation model was obtained by coupling gas-liquid-solid flow,multi-component droplet evaporation,particle dynamics and polymerization reaction dynamics,and the accuracy of the model was verified by comparing the temperature distribution obtained from experiments and the property of the products in industrial production with simulation results.Most of the jet liquid entering the reactor collides with the particles and attaches to the surface of the particles in the form of a liquid film.Under the fluidization of the gas,it moves with the liquid-holding particles to other areas of the reactor.The radial velocities of the solid-liquid phases along the jet axis first drop and then stabilize,but the turning point of the solid velocity is closer to the nozzle than the turning point of the liquid velocity.Increasing the jet liquid velocity will increase the jet affected zone,resulting in an increase in the liquid volume fraction in the reactor,an increase in the time-averaged liquid film mass fraction on the particle surface and the dispersion range of the liquid-holding particles,a decrease in the droplet diameter and an increase in the proportion of liquid evaporated in the form of droplets.The increase of the jet velocity will increase the temperature difference between the jet affected zone and the reactor body,and the evaporation of the jet liquid will lead to the increase of the concentration ratio of hexene to ethylene in the jet affected zone,which will lead to the generation of polyethylene with higher molecular weight.(4)For the gas-solid reactor with liquid-solid jet,the slurry jet fluidized bed reactor is taken as the research object.On the basis of the liquid jet gas-solid fluidized bed,the slurry particle phase is added,and the DPM model is used to simulate it.The interaction force between the slurry particles and continuous phases,the collision mass transfer between the slurry particles and liquid,the evolution of the liquid film on the surface of the slurry particles,and the polymerization reaction of the slurry particles are coupled to establish the calculation model of the slurry jet fluidized bed.The flow,dispersion and reaction characteristics of slurry liquid and slurry particles in the liquid-solid mixed jet in the reactor were studied respectively.The existence form of liquid in the slurry jet is similar to that of the liquid jet.After entering the reactor,most of the liquid quickly collides with the slurry particles or the original solid particles in the reactor and exists in the form of liquid film.The volume fraction of liquid phase in the reactor is very small,and the slurry liquid is mainly concentrated in the zone near the nozzle.The slurry particles are dispersed to the whole bed under the action of fluidized gas,and with the liquid-solid separation in the slurry injection process,the slurry particles gradually enter the high concentration ethylene environment of the reactor.With the passage of time,the particles in the high temperature environment gradually increased,and the proportion gradually increased.In the jet affected zone,the liquid film mass fraction on the surface of the solid particle accounts for the largest proportion,and the liquid film mass fraction on the horizontal section of the nozzle height is axially symmetrical distributed with respect to the jet direction.At the same time,this zone is the main evaporation zone of slurry liquid,the ratio of hexene to ethylene concentration is large,and the polymer with higher molecular weight is formed on the slurry particles.