On-line Monitoring

Increasing demands on product quality and consistency require increasing efforts to control the polymerization reactors. Reaction control strategies rely on both efficient on-line sensors and state estimation and filtering techniques. The lack of robust, rapid and reliable on-line sensors is retarding the implementation of feedback control strategies in polymerization reactors. The development of on-line sensors for emulsion polymerization reactors is a long-standing project in our labs. We developed on-line sampling equipment that was able to automatically inject a latex sample into a gas chromatograph. The system was able to handle 55% solids latexes and has been used to monitor individual monomer conversion in emulsion co- and terpolymerizations. Also, the consumption of chain transfer agent was monitored on-line. More recently, significant efforts have been devoted to use reaction calorimetry as on-line sensor. This technique is easily applicable to large-scale reactors. The use of reaction calorimetry in lab-scale reactors is tricky because of the large heat losses associated with small reactors. For this purpose we use a 2 L RC1 Mettler-Toledo reaction calorimeter. Homemade software has been developed so that an accurate measurement of the polymerization heat is available on-line (the commercial unit only gets this information at the end of the process) allowing on-line control of the process. In addition, state estimators (soft-sensors) have been developed to infer the properties not directly measured by the sensor (e.g., co and terpolymer composition, monomer and CTA concentrations, MWD, ...). Ongoing research  deals with the combination of spectroscopic sensors and reaction calorimetry through advanced chemometrics. A FT-Raman probe installed in the RC1 calorimeter is being used to monitor several emulsion polymerization processes including all acrylic formulations. Soft-sensors for particle morphology will also be considered.

On-line Control of Emulsion Polymerization Reactors.

On-line control is a way to achieve maximum production of polymers of improved and consistent quality under safe conditions. For a number of years we have developed on-line control strategies for emulsion polymerization reactors. Using these strategies, we are able to produce consistently linear co and terpolymers of the desired composition (both homogeneous and variable) and MWD (from minimum polydispersity, PI=2, to broad and bimodal distributions). The control strategies account for heat removal limitations and include safety safeguards. Strategies for on-line control of the microstructure structure of nonlinear polymers are under investigation. Other challenges for the next future will be the on-line control of particle morphology and particle size distribution.

Scale-up

Scale-up of emulsion polymerization formulations is challenging because differences in both heat removal and flow pattern characteristics lead to variations in the properties of the latex produced in the large-scale reactor as compared with the latex obtained in the lab. Heat removal affects mainly productivity and the projects dealing with this topic are described below. The changes in properties observed during scale-up are mostly due to differences in mixing characteristics. Emulsion polymerization in stirred tank reactors depends on mixing (agitation) in several ways. Agitation determines the emulsification of the monomers and affects the transport of the reactants to the polymerization loci (polymer particles). Agitation also affects the blending time, which is critical to distribute homogeneously the entering flow streams in the reactor. Because polymerization is highly exothermic, an efficient agitation is required to ensure a uniform temperature throughout the reactor and to maximize the heat removal rate. Agitation can also have a deleterious effect as it can promote coagulation. Mixing characteristics may be widely varied during scale-up, therefore, it is not surprising that product performance is affected by scale-up. The usual way of dealing with this problem is to fine-tune the formulation through trials in reactors of increasing volume. However, this is expensive and time-consuming. This project is aimed at developing a model-assisted scale-up methodology that minimizes the need of trials in reactors of increasing volume with the corresponding savings in time and costs. A combination of a mesoscopic model for the effect of agitation on latex properties with a macroscopic model for the flow pattern in the large-scale reactor obtained by computational fluid dynamics (CFD) is being used in this project.  An extensive investigation on the effect of agitation on polymerization rate, copolymer composition, molecular weight distribution and particle size distribution for several emulsion polymerization systems has been recently carried out at lab scale. Monomers of different water-solubility (styrene, styrene-butyl acrylate, vinyl acetate-VeoVa10, and vinyl acetate-butyl acrylate) and sparingly water-soluble chain transfer agents were used. A mesoscopic mathematical model is currently being implemented combined with CFD calculations of industrial reactors.