Optimization Of Grade Transition For Continuous Slurry High-density Polyethylene Process

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.