| Polyethylene is the most widely used and produced synthetic plastic in the world.In recent years,the domestic polyethylene production capacity has been concentrated and rapidly released,however,the homogeneous competition of products is severe.The products are mainly mid/low-end general materials,while high-end polyethylene products are highly dependent on imports.The copolymerization of ethylene and high carbon α-olefins is one of the important development directions for high-performance polyethylene.The increase in the carbon number of α-olefins increases the difficulties of not only the copolymerization reaction,but also the removal of volatiles such as residual comonomers,solvents,and oligomers in the products(i.e.,"devolatilization").Excessive volatile content in polymers can have adverse effects on product quality,production safety,human health,etc.Therefore,it is necessary to devolatilize the polymers.With the deepening of people’s concepts of Green,Health,Safety,and Environmental Protection,as well as the improvement of quality standards for highend polyethylene for special purposes such as medicine and food,traditional polyethylene devolatilization technology can no longer meet the growing demand for devolatilization.Therefore,studying the process intensification methods of polyethylene devolatilization to solve the problem of removing high carbon α-olefins in polyethylene,in order to assist the development of high-end polyethylene products with low volatile content,has important research significance and industrial value.In response to the problems of low heat transfer efficiency and slow diffusion of volatiles within polyethylene particles in traditional gas-solid countercurrent movingbed devolatilizers(degassing bins),a new method for polyethylene particle devolatilization using water as a microwave heating medium is proposed in this work.By introducing water to moisten polyethylene particles,the water molecules on the surface and in the pores of the particles generate heat through friction under microwave irradiation,while vaporizing and condensing to release heat,indirectly heating the particles and significantly enhancing devolatilization.The results of fixed-bed microwave devolatilization experiments show that the temperature rise of polyethylene particles and the devolatilization efficiency increase after the introduction of water,but the temperature distribution and devolatilization effect are uneven.In view of the above problems,the method of improving the temperature distribution and devolatilization uniformity in the microwave devolatilizer by particle fluidization is proposed.A fluidized-bed microwave devolatilization device is built,and the feasibility is demonstrated through experiments.The heating uniformity is significantly improved,and although the devolatilization effect is improved,further improvement is still needed.In order to further enhance the removal of volatiles such as high carbon α-olefins and oligomers,a method of introducing low molecular weight hydrocarbons such as ethylene and propane into the purge gas to assist polyethylene devolatilization is proposed based on the rule of "likes dissolve likes".The effects of low molecular weight hydrocarbons on the devolatilization characteristics of polyethylene particles are revealed through experiments.The main research contents and results of this work include:(1)An intensification process method for microwave devolatilization of polyethylene particles using water as a microwave heating medium is proposed.In a fixed-bed microwave devolatilization experimental device,the effects of conditions such as moisture content and microwave power density on the microwave heating and devolatilization characteristics of polyethylene particles are investigated using optical fiber thermometry,infrared thermal imaging,and headspace gas chromatography.The results show that microwaves can enhance the devolatilization of polyethylene particles.The higher the moisture content of materials or microwave power density is,the greater the bed temperature rise is,and the better the particle devolatilization effect is.However,the radial and axial temperature distribution of the fixed bed is uneven,resulting in different volatile contents of the bed particles at different positions.A diffusion model for the volatiles in polyethylene particles is established,and the relative deviation between the calculated values and experimental values is less than 20%.The results show that the higher the moisture content or microwave power density is,the greater the effective diffusion coefficient of volatiles is.The higher the microwave power density is,the greater the strengthening effect of unit mass moisture on the diffusion of volatiles is,proving that microwaves can enhance the diffusion of volatiles in polyethylene particles.(2)In view of the problems of uneven heating and devolatilization effects of the fixed-bed microwave devolatilizer,a process intensification method of improving the temperature distribution and devolatilization uniformity by particle fluidization is proposed.Firstly,in the fluidized-bed devolatilization experimental device,the effects of fluidization time,temperature,and gas velocity on the devolatilization characteristics of polyethylene particles are investigated.The results show that increasing fluidization time,temperature,and gas velocity are beneficial for enhancing the devolatilization of polyethylene particles.Increasing the gas velocity can change the particle flow regime in the bed,reduce the volume concentration of particles,and thus promote the removal of volatiles.Secondly,the effects of fluidizing gases on microwave heating and devolatilization characteristics of polyethylene particles are investigated in the fluidized-bed microwave devolatilization experimental device.The results show that fluidization can significantly improve the temperature distribution and devolatilization uniformity in the microwave devolatilizer.As the fluidization gas velocity increases,the bed heating rate decreases,and the final volatile content first decreases and then increases,indicating an optimal operating gas velocity.Finally,based on the timetemperature equivalence principle,the fluidization devolatilization model of polyethylene particles is established,and the gas-enhanced devolatilization factor is proposed,which realizes the prediction of devolatilization time to reach the target volatile content at different temperatures or gas velocities.(3)In order to further enhance the removal of volatiles such as high carbon αolefins and oligomers,a method for polyethylene particle devolatilization assisted by low molecular weight hydrocarbon gases such as ethylene and propane is proposed.Firstly,the dissolution and desorption behaviors of propane in polyethylene particles are studied using a magnetic suspension balance.The studies show that increasing temperature and decreasing pressure can reduce the equilibrium adsorption capacity of propane,and the solubility of propane decrease with the increase of polyethylene crystallinity.Propane is easy to adsorb/desorb in polyethylene particles,and is beneficial for the desorption of n-hexane,proving the feasibility of low molecular weight hydrocarbons in assisting polyethylene particle devolatilization.Secondly,in the particle-packed column devolatilization experimental device,the effects of low molecular weight hydrocarbon gas purging on the devolatilization characteristics of polyethylene particles are studied.The results show that ethylene has an anti-solvent effect and is a good purging devolatilization gas medium.Propane has dual devolatilization strengthening effects of anti-solvent and swelling,which promotes the particle devolatilization effect in the early stage.Mixed-purging of ethylene and nitrogen and relay-purging of propane and nitrogen both have good effects on enhancing the devolatilization of polyethylene particles. |
