Experimental Study On Dynamic Behavior And Heat Transfer Characteristics Of Droplet-Polyethylene Particle Collision

The collision of droplets and solid particles is a common phenomenon in nature and industrial applications.Typically,in the "gas-liquid" polyethylene process that uses side walls to spray condensate to create a differentiated polymerization environment to produce high-performance products,the atomized sprayed condensed droplets collide with high-temperature polyethylene particles to form different forms of Liquid membrane and dynamic evolution behavior.For example,when a collision occurs,a part of the droplets spread on the surface of the particles,forming a liquid film that wraps the particles and traverses the particles from the outside,and the remaining liquid film remains on the surface of the particles,providing a basis for the formation of the liquid bridge;or after the liquid film drops on the surface of the particles Rebound,re-assemble to form droplets,and flew away from the particles.A part of the droplets continue to oscillate on the particle surface and stay on the particle.The thickness,temperature,residence time,evolution mode,etc.of the liquid film on the particles significantly affect the diffusion rate of the polymerized monomers and cocatalysts into the pores of the particles,the heat and mass transfer characteristics of the gas-liquid-solid three-phase,and the particle-particle The interaction force will eventually change the aggregation state structure of the polymer product,and may induce the formation of liquid bridges,solid bridges,and even particle agglomerates between the particles.Therefore,exploring the complex dynamic behavior and heat-mass transfer process of droplet-particle collisions at different temperatures has important theoretical value for actively regulating the polymerization microenvironment of fluidized bed reactors,achieving high performance of products,and reducing particle agglomeration and agglomeration.And practical significance.However,the existing research lacks the description of the interaction between droplets and polyethylene particles at industrial production temperatures,which leads to a vague understanding of the internal relationship between liquid film motion-mass and heat transfer-solid bridge formation,and it cannot be targeted It can effectively change the operating conditions and effectively regulate the polymerization environment and product properties.This paper takes the gas-liquid fluidized bed polyethylene process as the main background,using high-speed camera,infrared thermal imaging and image processing methods.First,n-hexane,1-hexene and deionized water droplets and single polyethylene particles are studied The collision behavior of the particles was investigated under different Weber numbers.The thickness of the liquid film at the north pole of the particle,the center angle of the farthest point of the liquid film,the dynamic contact angle,and the arc length with temperature were varied to characterize the formation process of the liquid film.Furthermore,with the help of infrared thermal imaging,the retention behavior of the liquid film on the particles during a long period of time after the collision was investigated,and the climbing area and the climbing distance with a small amount of liquid film remaining were mainly investigated.Finally,by means of infrared thermography and scanning electron microscopy,the morphology of the liquid bridge between the two particles and the effect on the formation of the solid bridge were studied.The effects of particle temperature and particle wetness on the growth of the solid bridge were further studied,and the differences were analyzed and compared.Under the conditions of three fixed bridge growth mode.The research results in this paper illustrate the liquid film evolution behavior after droplet-polyethylene particle collision from two perspectives of fluid dynamics and heat transfer,which provides a basic theory for the process strengthening and optimal design of the gas-liquid fluidized bed polyethylene process.guide.The main research content and results include:(1)Using high-speed imaging and image processing methods,the liquid during the collision of n-hexane droplets,1-hexene droplets and deionized water droplets with a single polyethylene particle at a short time scale(0.15 s)was studied respectively Membrane behavior.It was found that both n-hexane and 1-hexene liquid films have two typical behaviors of direct slipping and recoil.For the n-hexane liquid film,as the Weber number increases,the threshold temperature required for the occurrence of surge increases.The 1-hexene liquid film slips and falls directly when the particle temperature is below 50?,but always rebounds above 70?.The characteristic difference between the two behaviors can be directly extracted from the curve of the dimensionless film thickness of the granular north pole point liquid film with the dimensionless time.When the particle temperature exceeds 70?,both n-hexane and 1-hexene liquid film will evaporate and splash immediately.The deionized water liquid film stays on the particles at low Weber numbers to perform damped oscillation,and at high Weber numbers,weaker regular backlash occurs.Liquid film resurgence increases the contact time between the particles and the liquid phase components.Liquid film evaporation has a local concentration effect on the gas phase components around the particles.Droplet splash increases the gas-liquid mass and heat transfer interfaces.Improve the heat and mass transfer rate.Based on this,the Weber number of droplets and the temperature range of particles in the gas-liquid reactor were determined.(2)The behavior of the liquid film during the collision between a droplet and a single polyethylene particle at a long time scale(6 s)was studied in the field of view of a thermal imager,and a special behavior of liquid film backwash was found.The liquid membranes of n-hexane,1-hexene and deionized water exhibit three different liquid film falling patterns.The shrinkage of the n-hexane liquid membrane is in the upper and lower half and then falls,so that the area near the equator is covered twice;1-hexene The liquid film shrinks in a grid shape and then falls in multiple paths,and the path portion is covered for a long time;the deionized water liquid film directly shrinks into water droplets and attaches to the particles near the south pole.At a certain period of time(0.5-1.5 s)after the liquid film is attached,the particle surface tends to be a stable state where it is continuously heated but there is still a small amount of liquid remaining.During this period,the particle temperature is relatively stable.This is because the liquid film climbed back and forth in a certain area from the particle equator to the southern hemisphere.The climbing area of the n-hexane liquid film is larger,the distance is shorter,the liquid film is thinner,and as the temperature increases,the position of the climbing area moves downwards,close to the South Pole.Although the n-hexane liquid film has a shorter distance when climbing,the coverage area is wider and the liquid film is thinner.Therefore,n-hexane is more suitable as a condensing medium in the gas-liquid reactor,effectively removing heat from particles.The climbing behavior of the 1-hexene liquid film increases the contact time between the particles and the liquid component,increases the heat and mass transfer rate between the liquid film and the particles,and is beneficial to maintaining the concentration of the gas phase components near the particles.(3)Infrared thermography and scanning electron microscopy were used to study the liquid film behavior during the collision between droplets and double polyethylene particles on a long time scale,and three modes of solid bridge growth were found,which were wet double particles at high temperatures.There are no droplet release,continuous release between dry double particles,and continuous release between wet double particles.The two modes of continuous release of droplets are liquid bridge-induced solid bridge formation.When the wet double particles are released at high temperature without droplets forming a solid bridge,the growth of the solid bridge is centered on the indirect contact of the two particles,first up and then down,then first and then slow;when the continuous release of droplets between the dry double particles forms the solid bridge The middle part is long,the upper part precedes the lower part,and then alternates with the lower part;when the droplets are continuously released between the wet double particles to form a solid bridge,the middle part grows rapidly,and the upper and lower parts are dragged.The growth rate of the solid bridge from fast to slow is:continuous release of droplets between wet double particles,no release of droplets of wet double particles,and continuous release of droplets between dry double particles.This shows that for the growth of solid bridges,the effect of the length of the particles in the wet state is greater than the effect of the liquid content of the liquid bridge.The presence of an extremely thin liquid film can promote the growth of the solid bridge,and the influence of the liquid volume of the liquid bridge is in Secondary status.In the gas-liquid reactor with side wall injection of condensate,the formation of solid bridges is an important prerequisite for particle agglomeration.Therefore,this research provides a theoretical basis for the possible path of agglomeration formation.