Measurement based modeling and control of bimodal particle size distribution in batch emulsion polymerization

Abstract

In this article, a novel modeling approach is proposed for bimodal Particle Size Distribution (PSD) control in batch emulsion polymerization. The modeling approach is based on a behavioral model structure that captures the dynamics of PSD. The parameters of the resulting model can be easily identified using a limited number of experiments. The resulting model can then be incorporated in a simple learning scheme to produce a desired bimodal PSD while compensating for model mismatch and/or physical parameters variations using very simple updating rules. V V C 2010 American Institute of Chemical Engineers AIChE J, 56: 2122-2136, 2010 Keywords: polymerization in emulsion, reduced model, particle size distribution, nonlinear control, predictive control IntroductionEmulsion polymerization processes are interesting for the production of adhesives and paints. The latex produced in these processes is the dispersion of polymer particles into water. The latex properties (optical properties, film formation) heavily depend on the particle size distribution (PSD) of the latex.1 Particularly, the use of bimodal PSD was found interesting in increasing the solid content (which is of high interest in order to reduce the drying time and transport costs), while keeping a low latex viscosity (see for instance, Refs. 2 and 3).In Refs. 4 and 5 the theoretical controllability of the population balance equations of this system has been analyzed. The authors concluded that the PSD is approximately controllable for unconstrained manipulation of feed surfactant concentration. Then, the authors used online measurements of the monomer conversion obtained by densimetry and delayed measurements of the particle size compensated by an observer to calculate the number of particles and predict the birth rate of particles in a continuous stirred tank reactor. These measurements were then used in a PID controller that manipulates the surfactant feed to control the PSD.Despite the theoretically possible controllability of the system, and because of the lack of online measurements of the PSD, controlling the PSD remained a difficult task. Light scattering technologies mainly concern monodispersed lattices and operate off-line because of the necessity of dilution. However, the analysis time of such techniques can be reduced to about 5 minutes and might therefore be adapted for online use if a dilution system is incorporated in the process. Separative technologies, such as capillary hydrodynamic fractionation (CHDF) are more suitable for multidispersed lattices (see for instance 6 and 7 ) but require a longer analysis time (about 15 minutes) and operate therefore offline. An alternative measurement that can be useful for controlling the PSD is the concentration of the free surfactant in the aqueous phase (since it allows calculating the Correspondence concerning this article should be addressed to M. Alamir at mazen.alamir@gipsa-lab.inpg.fr. 2122AIChE Journal August 2010 Vol. 56, No. 8 concentration of micelles) that can be obtained by conductimetry (...

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