Continuous polymerization control strategies

Continuous Polymerization Control Strategies hi continuous polymerizations, temperature and pressure are controlled in much the same way as in batch systems. 

In this short initial communication we wish to describe a general purpose continuous-flow stirred-tank reactor (CSTR) system which incorporates a digital computer for supervisory control purposes and which has been constructed for use with radical and other polymerization processes. The performance of the system has been tested by attempting to control the MWD of the product from free-radically initiated solution polymerizations of methyl methacrylate (MMA) using oscillatory feed-forward control strategies for the reagent feeds. This reaction has been selected for study because of the ease of experimentation which it affords and because the theoretical aspects of the control of MWD in radical polymerizations has attracted much attention in the scientific literature. 

Polymer production technology involves a diversity of products produced from even a single monomer. Polymerizations are carried out in a variety of reactor types batch, semi-batch and continuous flow stirred tank or tubular reactors. However, very few commercial or fundamental polymer or latex properties can be measured on-line. Therefore, if one aims to develop and apply control strategies to achieve desired polymer (or latex) property trajectories under such a variety of conditions, it is important to have a valid mechanistic model capable of predicting at least the major effects of the process variables. 

The most common continuous emulsion polymerization systems require isothermal reaction conditions and provide for conversion control through manipulation of initiator feed rates. Typically, as shown in Figure 1, flow rates of monomer, water, and emulsifier solutions into the first reactor of the series are controlled at levels prescribed by the particular recipe being made and reaction temperature is controlled by changing the temperature of the coolant in the reactor jacket. Manipulation of the initiator feed rate to the reactor is then used to control reaction rate and, subsequently, exit conversion. An aspect of this control strategy which has not been considered in the literature is the complication presented by the apparent dead-time which exists between the point of addition of initiator and the point where conversion is measured. In many systems this dead-time is of the order of several hours, presenting a problem which conventional control systems are incapable of solving. This apparent dead-time often encountered in initiation of polymerization. 

Because of its importance, continuous on-line monitoring of emulsion polymerization reactors continues to be an area of considerable investment in terms of research and development. It is evident that to better understand the nucleation mechanisms, and to develop better control strategies,... 

Open-loop control strategies were developed and implemented to allow for reduction of transition times during grade transitions in continuous high-impact styrene polymerizations [61]. Similar strategies were also used to control the MWDs in emulsion homopolymerizations and to control the copolymer composition and the MWDs simultaneously in non-hnear emulsion polymerizations [36,37,182]. 

In continuous polymerizations, temperature and pressure are controlled in much the same way as in batch systems. Obviously temperature trajectories are not employed in continuous reactors. Instead, the various vessels in a series of polymerization reactors may operate at different temperatures. The polymerizing mixture, then, will see different temperatures as it passes from one vessel to the next. Likewise, monomer trajectories are replaced, in continuous systems, with intermediate injection of a more reactive monomer between polymerization vessels. This strategy can be exploited to adjust the copolymer composition distribution. 

Manipulation of PSDs is generally attained through modification of surfactant concentrations (mostly in emulsion polymerizations) [48,49], agitation speeds (mostly in suspension polymerizations) [50], and initial catalyst size distributions and reaction times (residence time distributions in continuous reactors, mostly in coordination polymerizations) [51]. Effects of agitation speeds and surfactant concentrations on the PSD of polymer particles produced in suspension and emulsion polymerizations are discussed in detail in Chapters 5 and 6, respectively. When the catalyst is fed into the reactor as a solid material, as in typical polyolefin reactions, then the residence times and the initial PSD of the catalyst particles are used to manipulate the PSD of the final polymer product. Similar strategies are used in seeded emulsion polymerizations, where an initial load of preformed particles can be used to improve the control over the concentration of polymer particles in the latex and over the PSD of the final polymer product. 

Many membrane separation applications have already benefitted from membrane modification strategies. Chapter 10 describes how bespoke polymeric membranes have been used to improve the crystallization of biomolecules. Membrane crystallization allows through a careful control of the process parameters the production of crystals with controlled shape, size, size distribution, and polymorphism. Further research is required to provide comprehensive understanding of the complex relationships between membrane process parameters and crystal structure. The control of product polymorphism will continue to be important in the pharmaceutical industry, which, as the range of drugs and their specificity increase, will reqnire improved... 

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