Copolymerization in a CSTR

J. Hamer, T. Akramov, and W. Ray. The dynamic behavior of continuous polymerization reactors II, nonisothermal solution homopolymerization and copolymerization in a CSTR. Chem. Eng. Sci., 36 1897-1914, 1981. 

Miniemulsion copolymerization in a CSTR involves some very interesting features. However, in the interest of clarity, these systems will be discussed along with results for batch copolymerization. 

However, this does not preclude mini emulsion copolymerization in a CSTR for extremely water-insoluble comonomers. In spite of the fact that the copolymer composition in the continuous miniemulsion is less than that predicted using the homogeneous copolymerization reactivity ratios, the miniemulsion copolymer might be more uniform than the macroemulsion copolymer, where the possibility of significant droplet nucleation could lead to two separate homopolymers or, at the very best, copolymers of various composition. Therefore, it is very important to use CSTR data to scale up a continuous miniemulsion copolymerization product to take into account the different particle growth kinetics for batch and continuous reactors. 

Jaisinghani and Ray (40) also predicted the existence of three steady states for the free-radical polymerization of methyl methacrylate under autothermal operation. As their analysis could only locate unstable limit cycles, they concluded that stable oscillations for this system were unlikely. However, they speculated that other monomer-initiator combinations could exhibit more interesting dynamic phenomena. Since at that time there had been no evidence of experimental work for this class of problems, their theoretical analysis provided the foundation for future experimental work aimed at validating the predicted phenomena. Later studies include the investigations of Balaraman et al. (43) for the continuous bulk copolymerization of styrene and acrylonitrile, and Kuchanov et al. (44) who demonstrated the existence of sustained oscillations for bulk copolymerization under non-isothermal conditions. Hamer, Akramov and Ray (45) were first to predict stable limit cycles for non-isothermal solution homopolymerization and copolymerization in a CSTR. Parameter space plots and dynamic simulations were presented for methyl methacrylate and vinyl acetate homopolymerization, as well as for their copolymerization. The copolymerization system exhibited a new bifurcation diagram observed for the first time where three Hopf bifurcations were located, leading to stable and unstable periodic branches over a small parameter range. Schmidt, Clinch and Ray (46) provided the first experimental evidence of multiple steady states for non-isothermal solution polymerization. Their... 

These are identical for the limiting values for the batch reactor, except that they require only the assumption of perfect mixing. Thus, while polydispersi-ties of 2.0 and 1.5 for termination by disproportionation and combination respectively represent unattainable minima for batch polymerization, these same values represent feasible operation in a well-mixed CSTR. Thus, the CSTR will give a narrower dead polymer number chain length distribution since it is possible to maintain a constant reaction environment at steady state. The effect of residence time distribution on the polydispersity is negligible since the lifetime of a single radical is far less than the average residence time. Likewise, for a copolymerization in a CSTR at steady state, the constancy of... 

It can be seen that the monomer-polymer composition for terpolymerization in a CSTR will depend on the valnes of reactivity ratios. Consider the expression derived for monomer-polymer composition during copolymerization in a CSTR. Equation (10.12) rewritten in terms of monomer 1 composition only is... 

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. 

The difference in copolymer composition between miniemulsion and macroemulsion copolymerization in a batch reactor was not observed in a CSTR. In this case, the copolymer composition for the extremely water insoluble comonomer in a miniemulsion recipe decreases from a batch reactor to a CSTR. This difference can be attributed to the fact that monomer transport is... 

More recent efforts (primarily at the simulation level) on the optimization of styrene-related systems include Cavalcanti and Pinto [4], suspension reactor for styrene-acrylonitrile, and Hwang et al. [5], thermal copolymerization in a continuously stirred tank reactor (CSTR). 

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. 

Figure 8 Calculated copolymer compositions as a function of (steady-state) conversion for STY-BA copolymerizations carried out in a CSTR using various feed compositions. Reactivity ratios are rsTY=0.95 and rBA=0.18.

Equation (10.8) is called the copolymerization composition equation. In a CSTR, the effluent concentration and the reactor concentration are the same. The reactor concentrations of monomers 1 and 2 are [MJ and [Mj], respectively. The polymer composition of monomer 1 repeat unit in the polymer F, can be seen to be... 

The analysis of copolymerization and terpolymerization resulting in copolymer composition versus monomer composition relations in the prior sections can be generalized to n monomers. Consider n monomer repeat units entering a polymer backbone chain in a CSTR. There are nP- propagation reactions that can be involved in the... 

The copolymerization described in the previous example has to be carried out in a CSTR, in a single phase. Again the ratio p = 20. Tlie desired build-in ratio is p = 1.5. The reactor is designed so that the degree of conversion of ethylene is 0.9. From eq. (13.10) it follows then that c /c = n = 0.517. 

The most significant differences between perfectly mixed and segregated flow in a CSTR occur in copolymerizations. In a batch reaction, the copolymer composition varies with conversion, depending on the reactivity ratios and initial monomer feed composition. In a perfectly mixed CSTR, there will be no composition drifts but the distribution of product compositions will broaden as mixing in the reactor approaches segregated flow. 

Because the MWD polydispersity goes to infinity in a CSTR, step-growth copolymerization is rarely done in a CSTR or CSTR train. Since the reactivity ratios for step-growth monomers vary only slightly, there is little compositional drift in batch step-growth copolymerization, so there is little advantage, in terms of CCD, to step-growth CSTR polymerization. 

TiMe 5. Allan et al. [67] NMR sequence distribution data for SAN copolymerization in ethylbenzene in a CSTR process at 1S0-160°C... 

The influence of changes in these other variables on MWD in a homopolymerization has not yet been tested, but whatever perturbations are introduced to the feed in a radical polymerization in a laboratory-scale CSTR, they are unlikely to introduce dramatic changes in the MWD of the product because of the extremely short life-time of the active propagating chains in relation to the hold-up time of the reactor. This small change in MWD could be advantageous in a radically initiated copolymerization where perturbations in monomer feeds could give control over polymer compositions independent of the MWD. This postulate is being explored currently. 

Figure 5. VCM/VAc emulsion copolymerization (a) conversion vs. time in a batch reactor for extreme cases (b) instantaneous copolymer compostion (c) start-up procedures in an unseeded CSTR.

The use of a single-stage CSTR for HF alkylation of hydrocarbons in a special forced-circulation shell-and-tube arrangement (for heat transfer) is illustrated by Perry et al. (1984, p. 21-6). The emulsion copolymerization of styrene and butadiene to form the synthetic rubber SBR is carried out in a multistage CSTR. 

When such a stirring is absolutely absent in a continuous flow system, as it takes place in the piston reactor (PR), regularities of the batch processes with the same residence time 0 are realized. This implies that in order to describe copolymerization in continuous PR one can apply all theoretical equations known for a common batch process having replaced the current time t for 0. As for the equations presented in Sect. 5.1, which do not involve t al all, they remain unchanged, and one can employ them directly to calculate statistical characteristics of the products of continuous copolymerization in PR. It is worth mentioning that instead of the initial monomer feed composition x° for the batch reactor one should now use the vector of monomer feed composition xin at the input of PR. In those cases where copolymer is being synthesized in CSTR a number of specific peculiarities inherent to the theoretical description of copolymerization processes arises. 

Acrylonitrile/Butadiene/Styrene (ABS) Acry-lonitrile/butadiene/styrene (ABS) polymers are not true terpolymers. As HIPS they are multipolymer composite materials, also called polyblends. Continuous ABS is made by the copolymerization of styrene and acrylonitrile (SAN) in the presence of dissolved PB rubber. It is common to make further physical blends of ABS with different amounts of SAN copolymers to tailor product properties. Similar to the bulk continuous HIPS process, in the ABS process, high di-PB (>50%, >85% 1,4-addition) is dissolved in styrene monomer, or in the process solvent, and fed continuously to a CSTR where streams of AN monomer, recycled S/AN blends from the evaporator and separation stages, peroxide or azo initiators, antioxidants and additives are continuously metered according to the required mass balance to keep the copolymer composition constant over time at steady state. 

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