Continuous Emulsion Copolymerization of Acrylonitrile and Butadiene. Computer Simulation Study for Reducing the Off-Spec Rubber Produced between Steady States

MINARI, R.J.; GUGLIOTTA, L.M.; VEGA, J.R.; MEIRA, G.R.

Buenos Aires (Argentina)

Congreso; XXII Interamerican Congress of Chemical Engineering - V Argentinian Congress of Chemical Engineering; 2006

This theoretical work investigates the industrial production of two  acrylonitrile-butadiene rubber (NBR) with the aim of reducing the off-spec product generated in the transitions between steady states (SSs). The emulsion process consists of a train of 8 identical continuous stirred-tank reactors (CSTRs), property of Petrobras Energía S.A. (Pto Gral San Martin, Santa Fe, Argentina), with a reaction volume of 17500 L. To simulate the NBR production in the CSTR train an extension of the mathematical model developed by Vega et al. (1997) [1] for the industrial batch process is employed. The studied NBR grades (BJLT and AJLT) exhibit different average values of chemical copolymer composition (pA), molecular weights (Mn and Mw), and levels of trifuncional branching (Bn3). The following transitions were investigated: i) between Normal SS, with all the reagents fed into the first reactor; and ii) between Improved SS, with intermediate SS additions of acrylonitrile (A), butadiene (B), and chain transfer agent (CTA) along the train [2]. Such transitions involve changes in the employed recipe, without modifying the total feed rate (i.e., with small changes in the polymer production). For the intermediate off-spec rubber, the final product was considered out of specification when any of its average molecular properties (pA, Mn, Mw, Mw/Mn, or Bn3) fell outside predefined specification bands (+-10% of their SS values for Mn, Mw, Mw/Mn, and Bn3; +-2.5% for pA of grade BJLT; and +-2% for pA of grade AJLT). For minimizing the total off-spec generated during transitions between Normal SSs, an open-loop “bang bang” strategy was developed. It involves double step-changes in the feed profiles of A, B, and CTA into the first  reactor, with the following criteria adopted during transients: a) the feed of CTA is reduced for increasing average molecular weights; b) the feed of A (FA) is increased for increasing average copolymer composition; and c) the feed of B (FB) is manipulated to maintain constant the total mass flow of comonomers (FA + FB). The proposed “bang bang” strategy was able to importantly reduce the off-spec rubber with respect to simple step changes in all the reagent feeds (52% for the transition BJLT to AJLT and  35% for the transition AJLT to BJLT). For reducing the total off-spec rubber produced during grade transitions between Improved SSs, the feeds of A, B, and CTA into the intermediate reactors of the train were sequentially manipulated. The following alternative were investigated: i) sequential step changes; ii) sequential “bang bang” profiles; and iii) a set of virtual P+I controllers. In i) and ii) the feed profiles were determined with the aim of minimizing the total off-spec rubber. Alternatively, virtual P+I controllers were sequentially applied along the train with the aim of calculating the required feed profiles of A, B, and CTA, that would be further applied in an open-loop fashion. Compared with simultaneous step changes of all reagent feed, the proposed strategies reduce the off-spec rubber by more than 35%.