【作者】 王平；

【导师】 任晓红； 王靖岱； 阳永荣；

【作者基本信息】 浙江大学 ， 化学工程， 2006， 硕士

【摘要】 随着市场对聚烯烃产品多样化需求的不断提高,生产厂家需要频繁地进行牌号切换操作,将产品的牌号切换到市场需求旺盛的牌号。随之而来的是正常生产时间减少,大量过渡废料生成,人力和物力资源浪费等一系列问题。因此,聚烯烃生产过程中牌号切换的研究,不但具有较高的理论价值,而且对于指导工业优化控制具有很强的现实意义。本文以乙烯淤浆聚合工艺为对象,研究了连续聚合过程中的牌号切换问题,利用动态优化的原理和方法,求得切换过程中产品性能参数和各操作变量的优化轨迹,为实际生产提供指导。主要创新性工作包括: 1.通过对淤浆聚合工艺的分析和乙烯二元聚合机理的研究,建立了反应釜中产品的质量模型,提出了造粒机中产品质量预测的改良模型。采用目标函数法,针对具体牌号对模型参数进行分析,为牌号切换中产品质量的预测提供了依据。 2.针对并联操作过程的牌号切换,以切换时间最短为目标,建立了包括后续处理过程在内的全局优化目标函数。用迭代动态规划算法对目标函数进行求解,得到各反应釜中聚合温度（T）、氢烯比（[H₂]/[C₂]）、共聚单体与乙烯浓度比（[C₃]/[C₂]）等操作变量和聚合物产品各性能指标的变化轨迹。结果表明,目标函数中加入瞬时性质项可以加强对产品分子量分布的控制和对瞬时性质过调幅度的约束:加入催化剂进料项以及造粒机中产品的熔融指数项,可以使得模型更加符合实际,对造粒机中产品性能的约束可以对反应釜中产品的性能起到间接制约的作用。研究同时发现,优化操作具有不可逆性,控制过程具有非线性的特征。 3.针对串联操作过程的牌号切换,以“二次混合”规则为基础,建立了同时包括第二釜自身产物的累积性质和两釜产品混合所得聚合物的累积性质的全局优化目标函数。通过对第二釜自身产物的性能指标进行反推,建立了其质量模型。采用全局优化目标函数对切换操作进行优化计算,将优化结果与实际生产过程中的切换操作进行比较,证明用本文提出的优化方法可显著缩短切换所需时间,减少过渡料的生成,经济效益十分明显。针对串联操作,将使用直接求差型目标函数和无因次型目标函数进行优化计算所得结果进行了比较表明,直接求差更多还原

【Abstract】 To satisfy the customer’s demand for product with various qualities, the polymer plants are forced to operate under frequent grade transition policies, which will reduce the time of normal production, produce a huge amount of off-specification product, waste a lot of human work and raw material. So the grade transition research not only valuable in theory but also significative for guiding industry production. In the present study, a model of grade transition has been presented for the industrial continuous slurry high-density polyethylene process. Through the optimization of the objective function, the optimal trajectories of the operating variables and the quality target of polymer product can be gained. The innovative results can be summarized as follow:1. By analyzing the slurry polymerization process and the mechanism of polymerization, a steady-state model of polymer quality in the reactors and an improve model of polymer quality in the pelletizer are set up, which provide a way for the prediction of product quality during the grade transition process.2. On the base of mass conservation calculation, an optimal strategy is presented for the grade transition in parallel process, which has two reactors connected in parallel and one pelletizer in follow. Its object is to minimize the time of grade transition. By using iterative dynamic method to optimize the plant-wide objective function, the trajectories of both operating variables and quality target of product can be achieved, such as polymerization temperature, ratio of hydrogen to ethylene, ratio of co-monomer to ethylene, catalyst feed rates, the quality target of polymer product in the two reactors and the polymer’s melt index in pelletizer. The result shows that, by adding instantaneous properties into the objective functions, the control of polymer molecular weight distribution and the extent of overshoot can be enhanced. By adding the items of catalyst feed rates and quality in pelletizer to optimize in a plant-wide viewpoint, the polymer qualities in reactors can be indirect restricted. After comparing forward transition and converse transition, the result indicates the optimal operation is not reversible, and it reflects the non-linear property of control problems.3. According to the flow character of the second reactor in series process, a twice mixing rule is presented. Base on the rule an optimal strategy for the series process ispresented and the objective function is built up in steps, in which the qualities of the polymer produced only in the second reactor and the mixture between the two reactors are considered at the same time. After the optimization of the plant-wide objective functions, and the comparison between the optimal results and the plant operation data, it shows that following the optimal trajectories can reduce the quantity of off-spec product, cut down the transition time, having considerable economic benefit. By comparing the optimal results using direct-subtract function and non-scaled function, it shows that the direct-subtract function can more effectively restrain the sharp change of manipulated variable and instantaneous properties to ensure the security of grade transition process. So for the slurry technology studied in this paper, the direct-subtract objective function is a better choice.4. According to the transition between different operation modes, parallel and series, a man-intervened control principle is proposed. And base on that, a set of operation method is built up to deal with the grade transition from parallel to series and from series to parallel in order to trail off the serious impact to the system made by the fast change of manipulated variables. Apply it into the optimization and compare the results by using the methods brought forward in the former chapters. The result indicates that the man-intervened grade transition takes a bit longer time, but fairly effective on the restriction of the manipulated variables’ change and on the control of the instantaneous properties’ overshoot. Its advantage is quite predictable, and it is meaningful for guiding industry production.更多还原