* Corresponding author. Tel.: #55-11-3818-2237; fax: #55-11-813- 2380. E-mail addresses: jompinto@usp.br (J. M. Pinto), rgiudici@usp.br (R. Giudici). Chemical Engineering Science 56 (2001) 1021}1028 Optimization of a cocktail of initiators for suspension polymerization of vinyl chloride in batch reactors Jose H M. Pinto*, Reinaldo Giudici Department of Chemical Engineering, University of Sa J o Paulo, Sa J o Paulo } SP 05508-900, Brazil Abstract The industrial manufacturing of poly(vinyl chloride) (PVC) by suspension polymerization is carried out in batch reactors. The productivity increase of these processes is directly related to the reduction of the time required to complete each batch. The reactor cooling system is designed such that it is capable to compensate for the maximum rate of heat release by the exothermic polymerization, so that the reactor cooling capacity is underutilized. An attractive mode of operation is to run the industrial process isothermally and to use a mixture (`cocktaila) of di!erent initiators, which is able to spread the polymerization rate over the batch time. In this work, the optimal formulation of an initiator mixture is studied as a dynamic optimization problem, which makes use of a representative mathematical model for the batch suspension vinyl chloride polymerization process. Results show that by choosing the optimal amounts of initiators in the cocktail, a signi"cant reduction of the total processing time for a given polymer speci"cation can be obtained, as compared to the case in which only one initiator is optimally chosen.  2001 Elsevier Science Ltd. All rights reserved. Keywords: Optimization; Polymerization; Vinyl chloride; Initiator mixtures; Reaction engineering 1. Introduction Suspension polymerization is the most important com- mercial process for the manufacture of poly(vinyl chlor- ide) (PVC). Despite the large tonnage involved in the industrial production of this commodity polymer, batch reactors are still used almost exclusively. E!orts to improve the productivity of these processes are directly related to the reduction of the time required to complete each batch, for a given polymer quality (average molecu- lar weight, polydispersity, etc.). Industrial-scale reactors are designed with a maximum reactor cooling capacity corresponding to the maximum rate of heat release by the exothermic polymerization, which normally corresponds to the so-called `gel e!ecta (auto-acceleration of the polymerization rate, or Trom- msdor!}Norrish e!ect). Nevertheless, the average rate of heat release along the batch time is signi"cantly smaller than the maximum cooling capacity of the system, which means that for most part of the polymerization time the cooling system is underutilized. In order to reduce the polymerization time, the reac- tion would be carried out under an almost constant rate of polymerization, consistent with the heat removal capa- city of the cooling system. One mode of implementing this strategy is to apply temperature programming, that is, to change the reactor temperature in such a way that the polymerization rate is kept constant (Feldman & Macoveanu, 1978; Hamielec, Gomez-Vaillard & Mar- ten, 1982). However, the implementation of such optimal temperature policy only by manipulating the temper- ature and #owrate of the cooling #uid in the jacket may be di$cult in large-scale industrial reactors. Hence, more elaborate strategies have been developed, involving not only the manipulation of temperature and #owrate of the cooling #uid but also the initiator concentration and even the heat transfer through evaporation and external condensation (Pinto, 1990). In any event, temperature variations may a!ect negatively the quality of the poly- mer produced, since changes in the polymerization temperature cause an increase in polydispersity (broadening of the molecular weight distribution). 0009-2509/01/$ - see front matter  2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 0 9 - 2 5 0 9 ( 0 0 ) 0 0 3 1 7 - 1