The Loop Process

Abstract

Continuous reactors, specially continuous stirred tank reactors (CSTRs), are used to manufacture high production emulsion polymers [1]. The use of CSTRs provides constant quality products and easiness of on-line control. However, in these reactors the heat removal is not effective due to their low heat transfer area/volume ratio, therefore only small conversion increments can be achieved in a single CSTR, and a series of CSTRs has to be used. Continuous loop reactors, CLRs, are becoming and attractive alternative for the production of emulsion polymers. A CLR, whose employment for the manufacture of emulsion polymers was first time described by Lanthier [2,3], consists of a tubular loop that connects the inlet and the outlet of a recycle pump. Reactants are continuously fed into the reactor and the product is also continuously withdrawn from the reactor. The history of this development can be found in ref. [4]. The CLR presents several advantages for the emulsion polymerization process. Its large heat transfer area/reactor volume ratio allows high conversions (98%) in short residence times (8 min.) to be achieved [5]. This results in a substantial reduction of the reactor volume. Geddes [6] reported that a 5,000 liter batch tank reactor can be replaced in terms of production rate by a 50 liter CLR. Because of the small volume and the short residence time at which this reactor is operated, the CLR can be used with great flexibility and minimum losses in the manufacture of different emulsion polymers. The small volume and the absence of head space make the process intrinsically safe. The loop reactor presents also some disadvantages for the manufacture of emulsion polymers: i) core-shell particles cannot be produced, ii) the particle size distribution is difficult to modify and small particle sizes require substantial amounts of surfactants, iii) recipes with high mechanical stability are required to prevent shear induced coagulation; iv) It may suffer from cyclic behavior due to intermittent nucleations, also found in CSTRs; and v) very little has been published on this process in both open and patent literature. In this paper, a review will be given of the designs and flow pattern of the CLRs, polymerization studies carried out in these reactors, and mathematical modeling of CLRs.