8th International IFAC Symposium on Dynamics and Control of Process Systems MW CONTROL OF CONTINUOUS POLYMER REACTORS Pablo González and Jesús Alvarez Universidad Autónoma Metropolitana-Iztapalapa Depto. de Ingeniería de Procesos e Hidráulica Apdo. 55534, 09340 México D.F, MÉXICO Abstract: In this work, the problem of controlling (possibly open-loop unstable) continuous free-radical solution polymer reactors with continuous measurements of temperature, level and flows, and discrete-delayed measurements of molecular weight (MW) is addressed. The point of departure is a previous control scheme with linear- decentralized PI volume and temperature components, and a material balance conversion component. Here, the problem of designing and incorporating a MW component driven by discrete-delayed MW measurements is considered within a constructive framework. The result is a four-input four-output control scheme that: (i) has linear decentralized PI- type components with reduced model dependency, and (ii) recovers the behavior of a controller driven by continuous measurements. The proposed approach is tested with a representative example through simulations. Copyright © 2007 IFAC Keywords: Polymerization reactor control, decentralized control, discrete measurements, discrete estimator, chemical process control. 1. INTRODUCTION A wide class of materials is produced in continuous free-radical polymer reactors, which are highly nonlinear dynamical systems with complex behavior: strong and asymmetric input-output coupling, multiplicity of steady-states, and parametric sensitivity (Hamer et al. 1981). Industrially, these reactors are controlled with volume and temperature PI loops, and the conversion and molecular weight (MW) are regulated by adjusting the monomer and initiator (and/or transfer agent) dosages via supervisory control schemes. The production rate, stability, safety and quality indicators are met by controlling the temperature, volume, conversion, and MW. In particular, the control of MW is important to met product quality specifications. The MW control problem has been the subject of theoretical, simulation and experimental studies over the past decades. The state-of-the-art can be seen elsewhere (Richards and Congalidis, 2006), and here it suffices to mention that several control approaches have been employed, including linear PI controllers (Ellis et al., 1994) as well as nonlinear geometric (Adebekun and Schork, 1989; Niemiec et al., 2002), model predictive (MPC) (Mutha et al., 1997), and calorimetric (Alvarez et al., 2004) control techniques, including open-loop (Adebekun and Schork, 1989), extended Kalman filter (EKF) (Ellis et al., 1994; Mutha et al., 1997), and Luenberger (L) (Tatiraju et al., 1999) nonlinear observers. Most of the MW control and/or estimation schemes have been driven by size exclusion chromatography (SEC) (Ellis et al., 1994) and gel permeation chromatography (GPC) (Niemiec et al., 2002) measurements, which typically involve low sampling rates and long delays. This feature and the detailed- model dependency of the control schemes affect the functioning, and imply complexity, reliability, and cost drawbacks for industrial applicability. Recently, based on inversion and feedforward (FF) - feedback (FB) control ideas, González and Alvarez (2005) presented a PI-inventory controller that combines industrial-like linear and decentralized PI volume and temperature components, with material balance (MB) monomer and MW controllers. This scheme is driven by continuous-instantaneous measurements of volume, temperature and flows, addresses the complete MIMO control problem (volume, temperature, conversion and MW), and regulates the MW with an offset that depends on the accuracy of the initiator decomposition-chain transfer model, and with a speed that is about twice faster than the one of the open-loop response. The same tasks and results were obtained by a robust control scheme (Alvarez and González, 2006) drawn from Preprints Vol.3, June 6-8, 2007, Cancún, Mexico 61