Emulsion copolymerization of acrylonitrile and butadiene in a train of CSTRS. Intermediate addition policies for improving the product quality

Abstract

A continuous industrial emulsion copolymerization of acrylonitrile (A) and butadiene (B) carried out in a train of 8 continuous stirredtank reactors (CSTRs) at 10 °C is theoretically investigated, with the aim of producing nitrile rubber (NBR) of a quality similar to that obtained in an equivalent batch process. The rubber quality is specified by the latex characteristics (number particle concentration and number-average particle diameter), and by the molecular parameters (average values of the copolymer composition, the molecular weights, and the degree of branching). A mathematical model of the NBR industrial batch process is here extended for simulating the operation of the CSTR train. Two NBR grades of a different chemical composition are investigated. Compared with the batch product, the continuous Normal Steady State (SS) operation, with the complete recipe fed into the first reactor of the train, produces a deteriorated rubber (higher polydispersity and higher degree of branching). Such undesirable effects can be corrected through intermediate additions of A and chain transfer agent (CTA) along the continuous train. Finally, the reduction of the off-spec product generated during the change of grade between Normal SSs is also investigated. To this effect, “bang-bang” strategies for the comonomers and the CTA feeds in the first reactor of the train allow reducing up to 30 % of the off-spec product generated when a simple step change in all the feeds is applied.