Control of Polymerization Process using an Inhibitor

In a previous paper (Ampelli et al., 2006, J. Loss Prevent. Proc., 19, 419-424) inhibitors were used to prevent runaway reactions during methylmethacrylate (MMA) emulsion polymerization processes. In this work the reaction inhibition was extended to suspension polymerizations. Radical polymerization processes are often characterized by high exothermicity and self-accelerating kinetics, which influence strongly the heat and mass transfer phenomena. The main problem that more frequently may occur in chemical reactors carrying out polymerization reactions is the loss of temperature control. In this situation, if the rate of heat generation by chemical reaction exceeds the rate of heat removal by the cooling system, the temperature of the reaction mass will rise, increasing in turn the heat generation rate and a runaway reaction may occur. The use of aqueous suspensions allows to carry out the polymerization process in presence of water as dissipation means, but leads to several difficulties, such as the homogeneity of particle size, the stability of the suspension and the possible separation of the mixture in two phases with the formation of hot spots within the reactor. All these phenomena are strongly dependent on the operative conditions (reaction temperature and stirring of the reactor). For this reason control of suspension polymerization reactions is of paramount importance and safety considerations are necessary to prevent runaway reactions. Reaction inhibition involves injecting small quantities of a particular substance into the reactor at an early stage of the runaway. The substance used can either stop the reaction completely (inhibitor) or lower the reaction rate (retarder). The suitability of a system for inhibition is dependent on the reaction mechanism occurring in the reactor. Inhibition is ideally suited to processes involving free radicals in the initiation phase, such as suspension polymerization of MMA. In this work two inhibition substances were used: hydroquinone and 1,4-benzoquinone. Preliminary tests were carried out using a Differential Scanning Calorimeter (DSC) to understand the influence of these two substances in bulk polymerization reactions of MMA; afterward other experiments were carried out using a small stainless steel jacketed reactor (200 ml) under isothermal batch conditions. An early warning detection system (EWDS) was used to identify situations that can lead to runaway reactions. EWDS is based on the calculation of the reactor divergence; the detection criterion was introduced by Strozzi et al. (1999, AIChE Journal, 45, 2429-2443). When EWDS warned of alarm, small quantities of an inhibition substance were injected into the reactor in order to control the process. The results have shown the reaction inhibition as being a good method to stop runaway phenomena and control the polymerization process. The use of a retarder, such as 1,4-benzoquinone, should be preferred with respect to an inhibitor, because it does not completely stop the reaction and allows to obtain a high final conversion value.