Emulsion conversion control

A Simulation Study on the Use of a Dead-Time Compensation Algorithm for Closed-Loop Conversion Control of Continuous Emulsion Polymerization Reactors... 

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. 

Several control techniques have been developed to compensate for large dead-times in processes and have recently been reviewed by Gopalratnam, et al. (4). Among the most effective of these techniques and the one which appears to be most readily applicable to continuous emulsion polymerization is the analytical predictor method of dead-time compensation (DTC) originally proposed by Moore ( 5). The analytical predictor has been demonstrated by Doss and Moore (6) for a stirred tank heating system and by Meyer, et al. (7) for distillation column control in the only experimental applications presently in the literature. Implementation of the analytical predictor method to monomer conversion control in a train of continuous emulsion polymerization reactors is the subject of this paper. 

The analytical predictor, as well as the other dead-time compensation techniques, requires a mathematical model of the process for implementation. The block diagram of the analytical predictor control strategy, applied to the problem of conversion control in an emulsion polymerization, is illustrated in Figure 2(a). In this application, the current measured values of monomer conversion and initiator feed rate are input into the mathematical model which then calculates the value of conversion T units of time in the future assuming no changes in initiator flow or reactor conditions occur during this time. 

A recent paper by Kiparissides, et al. (8) details a mathematical model for the continuous polymerization of vinyl acetate in a single CSTR. Operating conditions were shown to exist in which either steady-state operation or sustained conversion oscillations would occur for vinyl acetate. Experimental results for both cases were successfully simulated by their model. In addition, regulatory conversion control policies were considered in which both initiator feed rate and emulsifier feed rate were used as manipulated variables (Kiparissides (9)). The problem of conversion control in the operating region in which sustained conversion oscillations occur is one of significant commercial importance. Most commonly, however, a uniform concentration of emulsifier is required in the emulsion recipe and, hence, emulsifier flow rate cannot be used as a manipulated variable. 

Leffew, K. W. and Deshpande, P. B. (1981) A simulation study on the use of a dead-time compensation algorithm for closed-loop conversion control of continuous emulsion polymerization reactors, in Emulsion Polymers and Emulsion Polymerization (eds D.R. Basset and A.E. Hamielec), ACS, Washington, pp. 533-66. 

Emulsion Process. The emulsion polymerization process utilizes water as a continuous phase with the reactants suspended as microscopic particles. This low viscosity system allows facile mixing and heat transfer for control purposes. An emulsifier is generally employed to stabilize the water insoluble monomers and other reactants, and to prevent reactor fouling. With SAN the system is composed of water, monomers, chain-transfer agents for molecular weight control, emulsifiers, and initiators. Both batch and semibatch processes are employed. Copolymerization is normally carried out at 60 to 100°C to conversions of - 97%. Lower temperature polymerization can be achieved with redox-initiator systems (51). 

Continuous emulsion copolymerization processes for vinyl acetate and vinyl acetate—ethylene copolymer have been reported (59—64). CycHc variations in the number of particles, conversion, and particle-size distribution have been studied. Control of these variations based on on-line measurements and the use of preformed latex seed particles has been discussed (61,62). 

Polymers obtained by the bulk technique are usually pure due to the absence of a solvent. The purity of the final polymer depends on the purity of the monomers. Heat and viscosity are not easily controlled, as in other polymerization techniques, due to absence of a solvent, suspension, or emulsion medium. This can be overcome by carrying the reaction to low conversions and strong agitation. Outside cooling can also control the exothermic heat. 

The early attempts at NMP of S in emulsion used TEMPO and related nitroxides and needed to be carried out at high temperatures (100-130 °C) necessitating a pressure reactor. Problems with colloidal stability and molecular weight control and limiting conversions were reported.215 217... 

Morton and Salatiello have deduced the ratio kpp/kp for radical polymerization of butadiene by applying the above described procedure, appropriately modified for the emulsion system they used. The primary molecular weight was controlled by a mercaptan acting as chain transfer agent, as in the experiments of Bardwell and Winkler cited above. Measurement of the mercaptan concentration over the course of the reaction provided the necessary information for calculating % at any stage of the process, and in particular at the critical conversion 6c for the initial appearance of gel. The velocity constant ratios which they obtained from their results through the use of Eq. 

In Figure 23.7, the bubble, cloud, and emulsion regions are represented by b,c + iv, and e, respectively. The control volume is a thin horizontal strip of height dx through tiie vessel. The overall depth of the bed is Lfl, which is related to the holdup of catalyst, Wcat. The performance equation may be used to determine Wcat for a given conversion /A (and production rate), or the converse. 

The polystyrene latexes produced from concentrated emulsions have been used as carriers for the controlled release of herbicides [160], The release of 2-(2,4-dichlorophenoxy) propionic acid (2,4-DP) was found to depend on the water concentration, increasing with increasing dilution of the latex. High conversion to polymer was required to prevent a large initial release of herbicide on dilution however, a significant initial burst was still observed at almost complete conversion. This was reportedly due to dissolution of 2,4-DP at, or near, the surface of the latex particles. 

Industrially, polymerizations are carried out to over 99% conversion and thus there is no need to reduce the unreacted monomer unless very low levels are required to meet regulatory. product, or workplace requirements. Most poly(vinyl acetate) emulsions contain less than 0.5 wt % unreacted vinyl acetate. All of the processes are operated in conventional glass-lined or stainless steel kettles or reactors. Control of the process is important to ensure reproducibility of the product. 

Figure 6. Styrene emulsion polymerization—critical conversion for diffusion-controlled propagation as a function of temperature (---) X cr2 0. 740 + 1.846 X...

The use of a precision digital density meter as supplied by Mettler Instruments (Anton Paar, Ag.) appeared attractive. Few references on using density measurements to follow polymerization or other reactions appear in the literature. Poehlein and Dougherty (2) mentioned, without elaboration, the occasional use of y-ray density meters to measure conversion for control purposes in continuous emulsion polymerization. Braun and Disselhoff (3) utilized an instrument by Anton Paar, Ag. but only in a very limited fashion. More recently Rentsch and Schultz(4) also utilized an instrument by Anton Paar, Ag. for the continuous density measurement of the cationic polymerization of 1,3,6,9-tetraoxacycloundecane. Ray(5) has used a newer model Paar digital density meter to monitor emulsion polymerization in a continuous stirred tank reactor train. Trathnigg(6, 7) quite recently considered the solution polymerization of styrene in tetrahydrofuran and discusses the effect of mixing on the reliability of the conversion data calculated. Two other references by Russian authors(8,9) are known citing kinetic measurements by the density method but their procedures do not fulfill the above stated requirements. 

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