Control of semibatch polymerization

The baseline case against which this control scheme will be judged is the 

Polydispersity Initiator added during polymerization 0 (gmoir ) 

Baseline predictive Baseline predictive Baseline predictive 

Another process disturbance of considerable industrial concern is the level of residual inhibitor in the monomer. In order to keep MMA from polymerizing in storage, an inhibitor, such as hydroquinone, is added to the monomer in small quantities. Just before use, the inhibitor is removed from the monomer. Problems in the removal process may result in variations in the level of inhibitor remaining in the monomer charged to the reactor. This will result in variations in the polymerization rate. As before, this disturbance was applied to both the baseline and adaptive predictive control case. When applied to the 

Results indicate the following improvement over isothermal batch polymerization reduced initiator consumption, reduced batch time, good control of average molecular weight, and reduced product polydispersity. The noted 

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Optimal Temperature Control of Semibatch Polymerization Reactors... 

Very little has been reported about the use of spectroscopic methods for monitoring and control of other polymerization systems. Lenzi et al. [191] reported that the NIR spectra collected in a dispersive instrument with a transflectance probe may contain very useful information about the structure of core-shell polystyrene beads produced through simultaneous semibatch emulsion/suspension polymerizations. Lenzi et al. [192] developed a polymerization technique that combines recipes of typical emulsion and suspension polymerizations to produce core-shell polymer beads. More interesting, the appearance of the core-shell structure always led to qualitatively different NIR spectra that could not have been obtained with polymer suspensions, polymer emulsions, or mixtures of polymer suspensions and emulsions. As described by Lenzi et al. [191], different spectral peaks could be detected in the wavelength region constrained between 1700 and 1900nm when the core-shell structure developed. 

An example of a commercial semibatch polymerization process is the early Union Carbide process for Dynel, one of the first flame-retardant modacryhc fibers (23,24). Dynel, a staple fiber that was wet spun from acetone, was introduced in 1951. The polymer is made up of 40% acrylonitrile and 60% vinyl chloride. The reactivity ratios for this monomer pair are 3.7 and 0.074 for acrylonitrile and vinyl chloride in solution at 60°C. Thus acrylonitrile is much more reactive than vinyl chloride in this copolymerization. In addition, vinyl chloride is a strong chain-transfer agent. To make the Dynel composition of 60% vinyl chloride, the monomer composition must be maintained at 82% vinyl chloride. Since acrylonitrile is consumed much more rapidly than vinyl chloride, if no control is exercised over the monomer composition, the acrylonitrile content of the monomer decreases to approximately 1% after only 25% conversion. The low acrylonitrile content of the monomer required for this process introduces yet another problem. That is, with an acrylonitrile weight fraction of only 0.18 in the unreacted monomer mixture, the low concentration of acrylonitrile becomes a rate-limiting reaction step. Therefore, the overall rate of chain growth is low and under normal conditions, with chain transfer and radical recombination, the molecular weight of the polymer is very low. 

Emulsion polymerization reactors are made of stainless steel and are normally equipped with top-entry stirrers and ports for addition of reactants. Control of the reaction exotherm and particle size distribution of the polymer latex is achieved most readily by semibatch (also called semicontinuous) processes, in which some or all of the reactants are fed into the reactor during the course of the polymerization. Examples are given in Chapter 8. In vinyl acetate copolymerizations, a convenient monomer addition rate is such that keeps the vinyl acetate/water azeotrope retluxing. at about 70°C. 

In emulsion polymerizations semibatch operation provides better control of the particle size of the product. The properties of the product polymers can be modified, also, by continuous or intermittent changes in the composition of the monomer feed in emulsion copolymerizations, where a given monomer can be preferentially concentrated in the interior or on the surface of the final particles, as described in Chapter 8. 

In semibatch emulsion polymerizations the polymer particles are kept monomer-starved to obtain higher rates of polymerization and to permit easier control of the rate and particle size distribution. There are two aspects to the control of PSD. The controlled addition of emulsifier during particle growth stabilizes the particles without further particle nucleation. The second aspect is related to the particle sticky stage which often occurs... 

We have presented all these points in order to make the reader realize why relatively few papers in the literature are concerned with the kinetics of aldehyde polymerizations. It is almost impossible to take into consideration all the facts that have been discussed in this introduction in each experiment. Consequently, most authors report simply the time versus conversion curve of the polymerization without a detailed scrutiny of the individual factors. In addition, aldehyde polymerizations are fast, in some cases almost explosive with poor temperature control, and many aldehyde polymerizations are carried out in a semibatch process with continuous addition of monomers, although we know commercial processes are carried out in continuous reaction. 

Liotta, V. Sudol, E.D. El-Aasser, M.S. Georgakis, C. On-line monitoring, modeling, and model validation of semibatch emulsion polymerization in an automated reactor control facility. J. Polym. Sci. Pt. A Polym. Chem. 1998, 36 (10), 1553-1571. 

Crowley, T.J. Meadows, E.S. Kostoulas, E. Doyle, F.J. Control of particle size distribution described by a population balance model of semibatch emulsion polymerization. J. Process. Control 2000, 10 (5), 419-132. 

When operating under semibatch Policy II, common practice is to maintain the reactor contents at low or starved monomer concentrations. This provides for relatively straightforward temperature control and overall reactor operation. However, when such low monomer concentrations are used over the duration of the polymerization, the potential for significant long-chain branching and crosslinking exists. The molecular weight profile would, therefore, be radically different from a batch process. 

Houston, W. E. (1986) Adaptive optimizing control of a semibatch polymerization reactor, MS thesis, Georgia Institute of Technology. 

In this section, the proposed process-control design approach is illustrated with a representative starved emulsion semibatch polymerization and numerical simulations, with a model that emulates and industrial size reactor [11], Moreover, the simulation example corresponds to a scaled-up version of the theoretical-experimental calorimetrie estimation study presented before with a laboratory scale reactor [15]. 

In any case, heat-transfer requirements are largely determined by the operation mode. Batch is the most critical operation because high polymerization rates are achieved due to the high monomer concentration. In semibatch mode, the heat generated can be easily controlled by the monomer feed rate. In these reactors, extra cooling is provided by the cold feed. In continuous mode, the continuous cold feed facilitates the control of the reactor temperature, particularly when the reactor temperature is high. 

FIGURE 13.12 Results of predictive control of for free radical polymerization of Am in semibatch operation. In batch mode, decreases monotonically in time. By computing conditions for isoreactivity (Equation 13.63c) constant during the reaction was achieved. By operating Am flow into the reactor in the flooded regime (Equation 13.63b), a predictable increase in during the semibatch reaction was achieved. Adapted with permission from Kreft T, Reed WF. Predictive control and verification of conversion kinetics and polymer molecular weight in semi-batch free radical homopolymer reactions. Eur Polym J 2009 45 2288-2303. 

The design of a polymerization reactor begins with the selection of the type of reactor (batch, semibatch or continuous) and then proceeds to the sizing and details of the reactor configuration. Only then can the details of operation and control be addressed. To this end, we will begin with a discussion of the basic types of reactors. Ultimately, it will be clear that the choice of reactor type is determined not only by practical considerations such as scale of production and propensity for fouling, but also by the specific polymerization kinetics. More complete discussions of reactor types and their residence time distributions may be found in references [1,2],... 

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