Investigation Of The Sorption And Diffusion In Polyethylene By Gravimetric And NMR Methods And Its Application

Solubility and diffusion coefficient in polymer are key parameters for many processing procedures in polyethylene (PE) industry, such as polymerization, separation, devolatilization, degassing, extraction and so on. Investigation of sorption and diffusion in polymer can provide theoretical and operational guidance for the process simulation and development of new technology. However, we are still suffering from the insufficiency of solubility and diffusion coefficient for the complicated polyethylene system.Based on the gas-phase polymerization, high pressure polymerization, gas-liquid polymerization and ultra high molecular weight polyethylene (UHMWPE) fiber gel-spinning technology, sorption and diffusion in polyethylene with different structure and morphology were investigated systematically in this thesis. We studied the sorption and diffusion of hydrogen, ethylene, isopentane,1-hexene,1-octene, other long chain a-olefins and their mixtures, together with mineral oil and poly-alpha-olefin (PAO) in nascent PE particles, PE membrane, high pressure low density polyethylene (HP-LDPE) solid, molten LDPE, and ultra high molecular weight polyethylene (UHMWPE) gel by the weighing and nuclear magnetic resonance (NMR) method. A sorption-diffusion model was proposed to correlate the mutual diffusion obtained by weighing method and self-diffusion obtained by NMR. The results were further applied in theoretical guidance for the newly developed gas-liquid ethylene polymerization technology, simulation and optimization of the devolatilization and degassing processes. The operation models were also modified. The main work and results are as follows.(1) Diffusion coefficients of co-monomer such as1-hexene and1-octene, condensing agent such as isopentane in nascent PE particles were measured by the weighing method. The influences of external diffusion, particle size and crystallinity of PE, operation parameters such as temperature and pressure, solvent concentration were investigated. The experimental results were fitted by single particle model and multi-grain model separately to obtain the effective diffusion coefficient Deff and that in semicrystalline PE matrix Ds. It was observed that Deff increases with nascent PE particle size and exhibits linear relationship with square of the radius. The dilute diffusion coefficient Deff-with the initial solvent concentration being1×10-4is one magnitude smaller than Deff, and the log form the two is almost linear.Momentum balance and heat balance were introduced to modify the devolatilization model of the purging bin. The combination of experiment results and the modified model can be successfully used in the simulation and optimization of devolatilization process. The results show that a) the residence time and purging gas flow rate should be above the limited values; b) the difficulty of removing different volatiles follows the sequence:1-octene>>sopentane>1-hexene. Furthermore, a new technology of two purging bins was proposed to remove co-monomers with high boiling point.(2) Sorption and diffusion of ethylene in solid and molten LDPE with different melt indexes were investigated by the intelligent gravimetric analyzer (IGA). It was found that the solubility and diffusion coefficients in the solid PE are smaller than those in the molten PE. The sorption enthalpy and diffusion activation energy in solid PE are bigger than those in molten PE.Silo degassing model was modified based on the radial and axial two-dimensional diffusion model, which is more accurate than the traditional one-dimensional diffusion model in predicting the outlet ethylene concentration. The degassing model together with the measured solubility and diffusion coefficients were applied to simulate the effects of LDPE particle size, melt index, pressure of separator on the minimum purging air flow rate and minimum residence time. The simulation results indicate that the minimum purging air flow rate and minimum residence time increase with the pressure of low pressure separator increasing. LDPE with larger size needs smaller purging air flow rate and longer residence time. Besides, high-melt-index LDPE requires larger air flow rate than the low-melt-index LDPE for safe operation. (3) Sorption of hydrogen, ethylene,1-hexene and their mixtures in PE membranes were studied by our IGA accessorized with a gas distributor. It was found that the solubilities of ethylene and1-hexene decrease with the temperature increasing, while hydrogen exhibits reverse solubility tendency. Ethylene has a desolvent effect on the solubility of1-hexene.The obtained solubilities were applied in the simulation of concentration distribution in the reactor of newly developed gas-liquid ethylene polymerization technology. The results show that the ratio of co-monomer to ethylene is higher while the ratio of hydrogen to ethylene is lower in the lower gas-liquid-solid phase, which promotes to produce PE with higher molecular weight and more branched chains. And it exhibits the reverse tendency in the upper gas-solid phase.(4) Diffusion behaviors of mineral oil and PAO in UHMWPE gel were measured by NMR-MOUSE (Mobile Universal Surface Explorer) during the extraction process of gel-spinning technology. The influences of UHMWPE type, extraction agent type and concentration, ultrasound were investigated. Low viscosity PAO is found to have a lower swelling temperature and can be extracted more easily, compared with traditional mineral oil solvent. The effect of extraction agent can be described by a replacement-dilution dual effect. That is, the extraction agent not only replaces and extracts the mineral oil under the concentration gradient, but also plays a diluting role, enhancing the molecular motion and diffusion of solvent. And the dilution effect can promote the effect of replacement.(5) PFG-NMR (pulsed field gradient) was adapted to measure the diffusion of a-olefins with long chains and their mixtures in semicrystalline PE. The effects of chain length, crystallinity and co-monomer type of PE were investigated. The results indicate that the diffusion of a-olefin decreases exponentially with the carbon number of α-olefin increasing. PE crystallinity has a stronger effect on the diffusion of a-olefins than co-monomer type and another co-existing solvent type.Furthermore, based on the free volume theory, activity coefficient model and Micaels-Hausslein elasticity theory, a multiplicity sorption-diffusion model combining with a morphology factor was presented to correlate the mutual diffusion obtained by weighing method and self-diffusion obtained by NMR. The model was verified in the1-hexene/PE system, and can be used to predict the sorption and diffusion behaviors in different PE samples under a wide range of temperature and solvent concentration.