A Grade Transition Strategy for the Prevention of Melting and Agglomeration of Particles in an Ethylene Polymerization Reactor By M.R.Rahimpour*, J.Fathikalajahi, B.Moghtaderi, and A.N.Farahani To satisfy the diverse product quality specifications required by the broad range of polyethylene applications, polymerization plants are forced to operate under frequent grade transition policies. During the grade transition, the reactor temperature must be kept within the narrow range between the gas dew point and the polymer melting point, otherwise the particles melt or agglomerate inside the reactor. In the present study, a dynamic well-mixed reactor model is used to develop a grade transi- tion strategy to prevent melting and agglomeration of particles in an ethylene polymerization reactor. The model predicts the conditions under which the temperature of the reactor is outside the allowable range in continuous grade transition. Manipu- lation of feed flow and cooling water flow rates has shown that the reactor temperature cannot be maintained within the al- lowable range. Hence, a semi-continuous grade transition strategy is used for this case so that the temperature is maintained within the allowable range. In addition, several continuous and semi-continuous grade transition strategies for the production of linear low-density polyethylene (LLDPE), medium density polyethylene (MDPE), and high-density polyethylene (HDPE) are compared. 1 Introduction Polymers have many areas of application, and each area requires different grade specifications. Grade transition op- eration is essential in continuous polymer plants because many grades of polymers are produced from the same pro- cess. For the economical operation of continuous polymer plants it is important to reduce the amount of off-specifica- tion polymer produced during the grade transition opera- tion. Determination of the operating policy during different grade transitions in an ethylene polymerization process is an important consideration that can affect process economic and safety because fluidized bed polyethylene reactors are prone to unstable behavior and temperature oscillations. In addition, the reactor temperature must be kept within the narrow range between the gas dew point and the polymer melting point to prevent particle melting and agglomeration. Therefore, the reactor temperature is an important param- eter that can significantly affect the stability of fluidized bed reactors. In industrial settings, polyethylene grade specifications are generally quoted in terms of Melt Index (MI) and Density 1) (r), rather than molecular weight and co-monomer content. It is essential to produce polyethylene with a given MI and r because different polyethylene applications such as injec- tion, wire coating, film and tubing products, as well as some specialty polymers, require different properties. The fre- quency of grade changes is determined by both prices and market demand. In an industrial fluidized bed polyethylene reactor, the residence time of the polymer in the reactor is generally from 3±5 h [1]. Since grade changeovers can be made every few days, the potential for producing large amounts of off-specification polymer is large. There have been many research works about grade transi- tion operation and control of continuous polymer processes. McAuley and MacGregor [2] implemented the optimal grade transition for the gas phase polyethylene reactor using melt index (MI) and density as the properties for grade spe- cification. Takeda and Ray [3] solved the optimal grade tran- sition problem of polyolefin loop reactors and compared the results with and without the constraints in the state variables. Wang et al. [4] integrated an offline optimizer and a non- linear model predictive controller to perform the optimal grade transition operation for the slurry phase polyethylene reactor. Seki et al. [5] developed a non-linear model predic- tive control for the polypropylene semi-batch reactor and high density polyethylene (HDPE) reactor. Although grade changeovers for ethylene gas phase poly- merization in an industrial fluidized bed reactor are of con- siderable economic importance, very little information on this topic can be found in the literature when a decision must be made on changing the operation mode to avoid melting or agglomeration of particles. In the present work, dynamic simulation is used to compare grade transition performance for the most common polyethylene grades (LLDPE, HDPE, and MDPE); in addition, best policies for accomplishing grade changeovers are determined by dynamic modeling. The temperature profile of the reactor with time is an im- portant parameter that can significantly affect the stability of the reactor in grade transition. Because fluidized bed polyethylene reactors are prone to unstable behavior and temperature oscillations, the reactor temperature must be kept within the narrow range between the gas dew point and the polymer melting point. If continuous grade transition is Chem. Eng. Technol. 2005, 28, No. 7 DOI: 10.1002/ceat.200500055  2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 831 ± [*] Associate Prof. M. R. Rahimpour (rahimpor@shirazu.ac.ir, currently on sabbatical at the University of Newcastle,Australia), Prof. J.Fathikalajahi, A. N. Farahani, Department of Chemical Engineering, Shiraz University, Shiraz 71345, Iran; Associate Prof. B. Moghtaderi, Discipline of Chemical Engineering, Faculty of Engineering & Built Environment, The Uni- versity of Newcastle, Callaghan NSW 2308, Australia. 1) List of symbols at the end of the paper. Full Paper