21st European Symposium on Computer Aided Process Engineering – ESCAPE 21 E.N. Pistikopoulos, M.C. Georgiadis and A.C. Kokossis (Editors) © 2011 Elsevier B.V. All rights reserved. Optimal Grade Transitions in an Industrial Slurry- Phase Catalytic Olefin Polymerization Loop- Reactor Series Vassileios Touloupides a,b , Vassileios Kanellopoulos b , Christos Chatzidoukas a,b and Costas Kiparissides a,b a Department of Chemical Engineering, Aristotle University of Thessaloniki b Centre for Research and Technology Hellas, P.O. Box 60 361, Thessaloniki, Greece 570 01 Abstract Present market needs combined with the broad range of polyolefin applications have forced the polyolefin industry to operate under frequent grade transition policies. Consequently, under such market-driven operating schedules, the minimization of off- spec polymer production and grade changeover time is prerequisite to any profitability analysis of the polyolefin production processes. In the present study, the optimal grade transition problem is examined in relation to an industrial Ziegler–Natta catalytic slurry- phase ethylene-1-hexene polymerization loop-reactor series. Keywords: Slurry-phase reactors; Optimal grade transition; Catalytic olefin polymerization; Dynamic simulation; Mathematical modeling. 1. Introduction Polyolefins are the most widely used plastics today due to their low production cost, reduced environmental impact, and wide range of applications (e.g., packaging, building and construction, transportation, etc.). It is believed that the degree of technological and scientific sophistication in relation to the polyolefin manufacturing has no equal among other synthetic polymer production processes. Polyolefins are commonly produced in low-pressure catalytic (e.g., Ziegler-Natta, metallocenes, etc.) bulk, slurry and gas-phase reactors. Presently, the total world polyolefins capacity exceeds 120 million tons per year. Polyethylene (i.e., HDPE, LDPE and LLDPE) and polypropylene cover 60 % and 40 % of the total polyolefins production, respectively. The annual world-wide polyolefins market growth in the coming years is foreseen to be 4-6%, making polyolefin manufacturing a very active research area. Present market needs combined with the broad range of polyolefin applications have forced the polyolefin industry to operate under frequent grade transition policies. This trend has led the polyolefin industry to move away from large continuous production of a single polymer grade to a more flexible production scheme comprising a number of polymer grades of high quality but low volume. In fact, in a polyolefin plant as many as 30–40 polymer grades can be produced. Consequently, under such market-driven operating schedules, the minimization of off-spec polymer production and grade changeover time are prerequisite to any profitability analysis of the process. Commonly, the optimal solution to this problem is based on the minimization of a suitable objective function defined in terms of the grade changeover time, product-quality specifications, process safety constraints and the amount of off-spec polymer. However, optimal operation of a polymerization plant in terms of higher yield and better product quality at