Quality Modeling And Optimization Of Grade Transition For Polyolefin Multistage Process

In this thesis, the modeling of polyolefin quality is investigated for on-line measurements and used for grade transition study for multistage process. The polyolefin industry always encounters the problems such as, product quality instability, long time cost and great quantity of off-target product during grade transition. To help control the product quality on line and optimize the grade transition process, the thesis focuses on two typical polyolefin multistage process (Spheripol and Borstar) and developed a theory-based model to predict the product quality including Melt Index, Ethylene content and Density. Based on the quality models, a scheme of optimized grade transition strategy was developed according to the specific transition process of Spheripol and Borstar process.The present study on quality model for polymers has been reviewed at the beginning of the thesis and the advantages and application of those models including mechanistic models, empirical models, wave-net based models and hybrid models has been compared. Several strategies for some typical polyolefin process were introduced in the review, mostly focus on polyethylene(PE) plants. After conclusion of the review, the author presented the target and the importance of this thesis.A scheme is then developed to predict the melt index, ethylene content, and density for both Spheripol Process(PP) and Borstar (Bimodal PE) , which are the typical multistage processes using loop reactors and fluidized reactor. These theoretically-based mathematical models are derived from the available on-line temperatures and reactant compositon. Dynamic data from industrial plants were used for parameter estimation and model validation. The results demonstrate the valid application of these models to industrial plants.The grade transition for multistage process is optimized based on quality models developed so far. The manipulated variable profiles are determined for temperature, hydrogen/monomer, comonomer/monomer in each reactor. Theoptimized transition strategy investigated the function of three reactors in series for Spheripol process. Results showed that because of the three reactors in series weighed differently during the whole plants, the extent of variables large transition is also varied due to the single reactor production ratio. The large transition of Melt Index is mainly determined by the hydrogen/monomer in the first loop reactor. The final grade transition time is determined by optimization of all the quality index, and to minimize grade transition time of Spheripol process, the hydrogen/monomer in the first two loop reactors should undergo a large transition to make Melt Index toward the target as soon as possible, meanwhile, the variables in the third fluidized reactor should mainly focus on Ethylene content target.Due to the special transition process between the membrane material and pipe naterial of Borstar process, a special catalyst transition strategy wu . presented, and with residence time distribution function of CSTR being used, a scheme of optiiix.' 'on model of grade transition is developed. The result reveals the necessity of inclusion of catalyst transition in the grade transition, and if the property if two catalyst deviate considerately from tach other, then the large transition of variables may not be valid during grade transition. The profiles of variable is determined by both the ratio of two catalysts and the difference of the catalysts.