【作者】 陈美娟；

【导师】 陈建峰； 阳永荣； 蒋斌波； 王靖岱；

【作者基本信息】 浙江大学 ， 化学工程， 2014， 博士

【摘要】 聚合物中的溶解、扩散行为普遍存在于各个聚乙烯工艺的诸多工序之中,例如聚合、分离、脱挥、脱气、萃取等等。研究聚合物中的溶解、扩散行为不仅具有重要的科学价值,而且可为工业过程操作与优化、新工艺的开发等提供重要的理论依据和指导。然而,聚乙烯体系复杂、影响因素众多,溶解和扩散方面的基础数据尚显不足且缺乏系统性。本文选择气相法、高压法、气液法聚乙烯工艺以及超高分子量聚乙烯(UHMWPE)纤维凝胶纺丝工艺所涉及的多种小分子/有机物体系以及不同形态和聚集态结构的聚乙烯为研究对象,基于重量法和核磁共振法(NMR)系统性地研究了氢气、乙烯、异戊烷、1-已烯、1-辛烯、十二烯、十六烯、矿物油和PAO等小分子/有机物及其混合物在聚乙烯原生态粉体、聚乙烯膜、高压低密度聚乙烯(LDPE)切粒体、LDPE熔融体和UHMWPE凝胶等聚乙烯中的溶解和扩散行为。‘并建立了聚合物内溶解-扩散模型,将基于重量法测得的互扩散系数与基于NMR法测得的自扩散系数进行关联。进一步地,将研究结果应用于气液法流化床聚乙烯新工艺和新产品的开发过程、气相法聚乙烯工艺脱气仓的模拟和新脱挥工艺的开发过程、以及高压法聚乙烯工艺脱气料仓的模拟和安全分析,修正了操作模型,并提出了工艺优化的措施。具体研究工作和结果如下：(1)基于重量脱附法,采用智能重量分析仪系统性地研究了共聚单体(1-已烯和1-辛烯)和冷凝剂(异戊烷)等在原生态粉体内的扩散行为,考察了外扩散、粉体粒径、结晶度、操作温度和压力、溶剂种类以及浓度等对扩散行为的影响。采用单粒扩散模型拟合得到有效扩散系数Deff及初始质量分率为1×10-4时的稀释浓度扩散系数Deff-；采用多粒扩散模型拟合得到聚乙烯微观聚集态中的扩散系数Ds。研究结果表明,有效扩散系数Deff与粉体粒径的平方呈线性正相关关系；Deff比Deff小一个数量级,且两者的对数形式近似呈线性关系。通过引入动量传递项和热量传递项,提出了修正的聚合物脱挥模型,并结合溶解-扩散数据,成功应用于脱气仓脱挥过程的操作模拟和优化,考察了挥发组分初始浓度、树脂入口温度、氮气入口温度以及挥发组分种类对脱挥操作过程的影响。模拟结果表明,脱气仓操作所采用的停留时间和吹扫流量应分别大于所计算的极限停留时间和极限吹扫流量。各挥发组分的脱除难易程度排序为：1-辛烯>>异戊烷>1-已烯。进一步地,提出了双脱气仓新工艺,解决了高沸点共聚单体难脱除的问题。(2)基于重量吸附/脱附法,采用智能重量分析仪分别测量了乙烯在不同熔融指数的LDPE的熔融体和切粒体中的扩散系数和溶解度。研究结果表明,乙烯在LDPE切粒体中的扩散系数和切粒体无定形相中的溶解度明显小于其在熔融体中的扩散系数和溶解度；乙烯在LDPE切粒体中的扩散活化能和溶解焓绝对值大于其在LDPE熔融体中的扩散活化能和溶解焓绝对值。建立了基于二维扩散的高压法聚乙烯工艺脱气料仓模型,提高了出口乙烯浓度的预测精度。结合乙烯的溶解-扩散数据,利用该模型考察了LDPE切粒体的粒径、熔融指数、低压分离器压力等对脱气料仓所需最小通风量和最小停留时间的影响。模拟结果表明,低压分离器的压力越大,所需最小通风量越大,最小停留时间越长；粒径越大,所需最小通风量越小,最小停留时间越长；中、高熔融指数的LDPE所需最小通风量比低熔融指数的LDPE的大。(3)基于重量吸附/脱附法,采用智能重量分析仪测量了氢气、乙烯、1-已烯及其混合物在聚乙烯膜中的溶解度。实验结果表明,乙烯和1-已烯的溶解度随着温度的升高而减小,而氢气呈现相反的趋势。乙烯的加入对1-已烯的溶解度存在反溶剂效应,即乙烯存在时,1-已烯的溶解度大大减小将单组分和多组分溶解度实验结果应用于气液法流化床聚乙烯新工艺,模拟研究了反应器内H2/C2H4/C6H12的浓度分布。结果表明,在一定冷凝分率下,反应器下部淤浆区域共聚单体乙烯摩尔比相对较高,而氢气乙烯摩尔比相对较低,有利于生产分子量高且具有较多支链的聚乙烯产品；而反应器上部气固相区域则相反。研究结果有效指导了气液法新工艺高性能聚乙烯产品的生产。(4)基于核磁共振技术,采用NMR-MOUSE (Mobile Universal Surface Explorer)研究了UHMWPE纤维凝胶纺丝工艺的溶剂——矿物油和PAO在UHMWPE凝胶中的扩散过程。结果表明,萃取剂不仅能逐渐替换溶剂,而且可显著提高溶剂在凝胶中的扩散系数。在此基础上,提出了UHMWPE凝胶体系萃取过程的“替换-稀释”双重作用模型,即：在一定浓度梯度下,萃取剂与矿物油溶剂或PAO溶剂相互渗透和扩散,溶剂逐渐被萃取剂替换；另一方面,萃取剂对溶剂具有稀释作用,增大了溶剂分子的运动性,并且萃取剂对溶剂的稀释作用可强化其对溶剂的替换作用。此外,考察了UHMWPE种类、萃取剂种类及浓度、超声等因素对扩散过程的影响,发现较低黏度的PAO的溶胀温度比矿物油低,且更易被萃取。(5)采用脉冲梯度场核磁共振技术(PFG-NMR)研究了C6～C16长链α-烯烃及其混合物在半结晶型聚乙烯内的扩散行为,考察了α-烯烃链长、聚乙烯结晶度和共聚单体类型等因素的影响。结果表明长链α-烯烃在聚乙烯内的扩散系数随链长增加呈现指数衰减趋势；聚乙烯结晶度对扩散的影响大于共聚单体类型和溶剂种类的影响。进一步地,以自由体积理论、活度系数模型和Micaels-Hausslein弹性模型为基础,通过引入形状因子项,建立了适用于宽温度、宽浓度范围的聚合物内溶解-扩散模型,实现了基于重量法的互扩散系数与基于NMR法的自扩散系数的关联,并在已烯-聚乙烯体系中得到了验证。模型能够正确描述有机物及其混合物在聚乙烯不同分子结构及聚集态结构内的溶解-扩散行为。更多还原

【Abstract】 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.更多还原