CSTR monomer conversion

Figure 2. Isothermal polymerization of methyl methacrylate in a CSTR (1 5). a. Predicted steady-state monomer conversion vs. reactor residence time for the solution polymerization of MMA in ethyl acetate at 86 °C. h. Steady-state and dynamic experiments for the isothermal solution polymerization of MMA in ethyl acetate (solvent fraction O.k) ( ) steady states,...

Emulsion Polymerization in a CSTR. Emulsion polymerization is usually carried out isothermally in batch or continuous stirred tank reactors. Temperature control is much easier than for bulk or solution polymerization because the small (. 5 Jim) polymer particles, which are the locus of reaction, are suspended in a continuous aqueous medium as shown in Figure 5. This complex, multiphase reactor also shows multiple steady states under isothermal conditions. Gerrens and coworkers at BASF seem to be the first to report these phenomena both computationally and experimentally. Figure 6 (taken from ref. (253)) plots the autocatalytic behavior of the reaction rate for styrene polymerization vs. monomer conversion in the reactor. The intersection... 

The mathematical equations for this imperfectly mixed model consist of 12 differential equations similar to equations (l)-(4), four for each of the three CSTR s. At steady state, they reduce to 12 non-linear algebraic equations which are solved numerically in order to calculate the dependence of initiator consumption on polymerization temperature. An overall balance reveals that the monomer conversion and polymer production rate are still given by equations (5) and (8), while the initiator consumption is affected by the temperature and radicals distribution in the three CSTR s, so that equations (7) and (9) become much more complex. 

The solution of Eq. (8.1) permits to establish the dependence of the copolymer composition X prepared in CSTR on conversion p and xm. This dependence, generally speaking differs from calculated for the batch reactor through Eqs. (5.2) and (5.7). This difference would be largest in the range of middle conversions, since in the vicinity of extreme values p = Oandp = 1, the copolymer compositions (which are equal to m(xin) and xin, respectively) are the same for both reactors. It is of a certain interest that if xin = x, when the monomer feed composition at the reactor input is azeotropic (see Sect. 4.5), the copolymer composition X = x ... 

The theory and the experimental data from this study demonstrates that in a train of CSTRs, essentially all of the particles form in the first reactor. Therefore, it is possible to maximize the monomer conversion in the latex leaving the first reactor by keeping the temperature and the residence time at the first reactor as low as possible in order to produce the maximiun number of polymer particles and so increase the rate of polymerization in the succeeding stages. This is the so-called pre-reactor concept. 

Oscillations in the number of polymer particles, the monomer conversion, and the molecular weight of the polymers produced, which are mainly observed in a CSTR, have attracted considerable interest. Therefore, many experimental and theoretical studies dealing with these oscillations have been published [328]. Recently,Nomura et al. [340] conducted an extensive experimental study on the oscillatory behavior of the continuous emulsion polymerization of VAc in a single CSTR. Several researchers have proposed mathematical models that quantitatively describe complete kinetics, including oscillatory behavior [341-343]. Tauer and Muller [344] proposed a simple mathematical model for the continuous emulsion polymerization of VCl to explain the sustained oscillations observed. Their numerical analysis showed that the oscillations depend on the rates of particle growth and coalescence. However, it still seems to be difficult to quantitatively describe the kinetic behavior (including oscillations) of the continuous emulsion polymerization of monomers, especially those with relatively high solubility in water. This is mainly because the kinetics and mech-... 

Samer [137] studied miniemulsion copolymerization in a single CSTR. Two separate feed streams, miniemulsion (or macroemulsion for comparative studies) and initiator were fed at constant rates into the reactor. SLS was used as the surfactant, HD as the costabilizer, and KPS was the initiator. In the miniemulsion configuration (costabilizer included in recipe), the emulsion stream was continuous. Constant volume was provided by an overflow outlet. Salt tracer experiments were used to validate the ideal mixing model assumed for a CSTR. Total monomer conversion was measured via in-hne densitometry, and copolymer composition via offline NMR. 

Another reason for using different reactor sizes along the CSTR train is the variation of polymerization rate with monomer conversion. This factor is not a major consideration if the final conversion is modest as in the case of styrene-butadiene rubber (SBR) processes. Normal exit conversions are 55 to 65% in such systems, and the residual monomer is recovered and recycled. If a very high conversion is desired one must deal with the problem that the polymerization rate is low at high conversions. The final reactor in the series needs to be very large if the desired conversion approaches 100%. Likewise, batch reaction cycle times become large if high conversions are desired. 

In addition to the above investigations, free-radical high-pressure polymerizations should also be studied in continuously operated devices for three reasons. (1) Because of the wealth of kinetic information contained in the polymer properties, product characterization is mandatory. Sufficient quantities of polymer, produced under well defined conditions of temperature, pressure, and monomer conversion, are best provided by continuous polymerization, preferably in a continuously stirred tank reactor (CSTR). (2) Copolymerization of monomers that have rather dissimilar reactivity ratios, such as in ethene-acry-late systems, will yield chemically inhomogeneous material if the reaction is carried out in a batch-type reactor up to moderate conversion. To obtain larger quantities of copolymer of analytical value, the copolymerization has to be performed in a CSTR. (3) Technical polymerizations are exclusively run as continuous processes. Thus, in order to stay sufficiently close to the application and to investigate aspects of technical polymerizations, such as testing initiators and initiation strategies, fundamental research into these processes should, at least in part, be carried out in continuously operated devices. 

Also, Priddy and Pirc used a chain transfer solvent (ethylbenzene) in a CSTR operating at > 99% monomer conversion, and at high polymer solids (40-50 w/w). Under these conditions, they found that chain transfer to solvent (CTS) was extremely high (2) since the high monomer conversions achieved under steady state (SS) operation resulted in a large ratio (typically, 500 1) of solvent to monomer. Gatske [75] has shown that the CTS increases exponentially with conversion as shown in Fig. 6. 

Cascade of CSTRs In several processes, a cascade of CSTRs may be used to obtain the desired polymer properties and maximize monomer conversion. Conversion of monomer increases from reactor to reactor and thus the solids content and viscosity would increase and heat transfer coefficients decrease for each progressive reactor. For each reactor, an energy balance can be performed using Equation 13.9 by replacing the temperature... 

Figure 8.2 Particle-size histograms for polystyrene particles from a tube-CSTR reactor system d%) and (dp) are average diameters from the tube (seed) and CSTR respectively. Xi and Xi are the monomer conversions from the tube and CSTR. [2]...

In a simulation study, Leffew and Deshpande [62] have evaluated the use of a dead-time compensation algorithm in the control of a train of CSTRs for flie emulsion polymerization of vinyl acetate. In this study, monomer conv ion was controlled by manipulating the initiator flow rate. Experiments indicate that there is a period of no response (dead-time) between the time of increase in the flow of initiator and the response of monomer conversion. Dead-time compensation attempts to correct for this dead-time by using a mathematical model of the polymerization system. Reported results indicate that if the reactor is operated at low surfoctant concentration (where oscillations are observed), the control algorithm is incapable of controlling monomer conversion by the manipulation of either initiator flow rate or reactor temperature. The inability of the controller to eliminate oscillations is most probably due to the choice of manipulated variable (initiator flow rate) rather than to the perfotmance of the control algorithm (deadtime conq)ensation). 

Kipaiissides et al. [36] have applied suboptimal control to the CSTR emulsion polymerization of vinyl acetate. A mathonatical model was used to develop a simulation of the polymerization process. Verification of the model was done by open-loop bench-scale polymerization. Closed-loop control of monomer conversion via manipulation of both monoma and initiator flow rates was... 

The Dowlex process by Dow Chemicals is the dominant process in solution polymerization, but Dow does not license this technology to other companies (Figure 2.39). The Dowlex process uses two CSTRs in series with a high boiling hydrocarbon solvent. Other competing processes include the DSM process and the Sclairtech process by Nova Chemicals. In some configurations, these processes may also have tubular reactors operated in series with the CSTR to complete monomer conversion. 

This phenomenon can be observed in free radical polymerization (nonisothermal) due to the exothermic nature of the polymerization reaction. However, due to the gel effect, it is also observed in some isothermal free radical polymerizations in a CSTR [11-14]. Figure 5.2 shows the rate of polymerization plotted versus monomer conversion for the free radical solution polymerization of methly methacrylate. Unlike a more common reaction in which the rate of reaction falls monotonically with conversion, the rate of reaction rises with conversion due to the onset of the gel effect. Thus the system can be thought of as autocatalytic. At high conversions the polymerization... 

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