Polymerization of styrene in a tubular reactor

Continuous-Emulsion Polymerization of Styrene in a Tubular Reactor... 

The work reported here is part of a continuing program on the emulsion polymerization of styrene in a tubular reactor. It is now evident that the reactor construction is of primary importance in avoiding the problem of reactor plugging. The plugging is associated with a wall effect so that both the reactor dimensions and the nature of the wall surface are important. 

Polystyrene can be easily prepared by emulsion or suspension techniques. Harkins (1 ), Smith and Ewart(2) and Garden ( ) have described the mechanisms of emulsTon polymerization in batch reactors, and the results have been extended to a series of continuous stirred tank reactors (CSTR)( o Much information on continuous emulsion reactors Ts documented in the patent literature, with such innovations as use of a seed latex (5), use of pulsatile flow to reduce plugging of the tube ( ), and turbulent flow to reduce plugging (7 ). Feldon (8) discusses the tubular polymerization of SBR rubber wTth laminar flow (at Reynolds numbers of 660). There have been recent studies on continuous stirred tank reactors utilizing Smith-Ewart kinetics in a single CSTR ( ) as well as predictions of particle size distribution (10). Continuous tubular reactors have been examined for non-polymeric reactions (1 1 ) and polymeric reactions (12.1 31 The objective of this study was to develop a model for the continuous emulsion polymerization of styrene in a tubular reactor, and to verify the model with experimental data. 

Figure 1. Sketch of equipment used for the continuous emulsion polymerization of styrene in a tubular reactor...

Rollin AL, Patterson I, Huneault R et al (1977) The effect of flow regime on the continuous emulsion polymerization of styrene in a tubular reactor. Can J Chem Eng 55 565-571... 

Zitlalpopoca-Soriano AG, Vivaldo-Lima E, FlOTes-Hacuahuac A (2010) Grade transition dynamic optimizatimi of the living nitroxide-medialed radical polymerization of styrene in a tubular reactor. Macromol React Eng 4 516-533... 

Rollin et al. have published results for the continuous emulsion polymerization of styrene in a tubular reactor the particular concern is with the transition between laminar flow and turbulent flow in such a reactor. Chiang and Thompson have studied the factors which affect the stability of a continuous emulsion polymerization reactor. Birtwistle, Blackley, and Jeffershave examined a modification of a model proposed by Brooks, in order to ascertain whether it permits the possibility of either periodic fluctuations in rate of polymerization in the vicinity of the steady state, or sustained fluctuations in rate of polymerization in any physically attainable state. The conclusion is reached that neither of these possibilities is realizable in reaction systems which conform to the model considered, nor are sustained oscillations possible in reaction systems which conform to certain variants of the model. 

Continuous Emulsion Polymerization.—A useful discussion of theories of continuous emulsion polymerization and review of experimental data has been published recently by Poehlein and Dougherty. Thompson and Stevens have developed a population-balance approach to the modelling of continuous emulsion polymerization reactions. They base their approach upon the Smith-Ewart recursion formula, and allow for both radical desorption from, and finite rate of termination within, reaction loci. Cauley et aU have also attempted to model a continuous emulsion polymerization by means of a population balance, the assumed reaction system being such that bimolecular termination of radicals occurs instantaneously within reaction loci. The effect of flow regime on the continuous emulsion polymerization of styrene in a tubular reactor is the subject of a paper by Rollin et... 

T. Enright, M. F. Cunningham, B. Keoshkerian, Nitroxide-mediated polymerization of styrene in a continuous tubular reactor, Macromol. Rapid Commun. 2005, 26, 221-225. 

Fig. 10.2. Temperature profiles for the polymerization of undiluted styrene in a tubular reactor with good radial mixing. Tin = 135°C and Tu,on = 20°C. The parameter in the plot is the tube diameter in meters.

Example 13.9 Illustrate temperature and molecular weight changes in a tubular reactor by constructing a simple model of styrene polymerization in a tube. 

The objective was to develop a model for continuous emulsion polymerization of styrene in tubular reactors which predicts the radial and axial profiles of temperature and concentration, and to verify the model using a 240 ft. long, 1/2 in. OD Stainless Steel Tubular reactor. The mathematical model (solved by numerical techniques on a digital computer and based on Smith-Ewart kinetics) accurately predicts the experimental conversion, except at low conversions. Hiqh soap level (1.0%) and low temperature (less than 70°C) permitted the reactor to perform without plugging, giving a uniform latex of 30% solids and up to 90% conversion, with a particle size of about 1000 K and a molecular weight of about 2 X 10 . 

The effects of the discharge power on the distribution of polymer deposition in a tubular reactor (Fig. 20.1) are shown in Figures 20.19-20.22. Figure 20.19 depicts the change in polymer deposition pattern due to the discharge power observed in the plasma polymerization of styrene at a fixed flow rate of 5.6 seem. 

The nonisothermal polymerization process in a tubular reactor at laminar flow is investigated in141 Experimental data on the polymerization of styrene under these conditions are presented in142). 

A complete analytical examination of the role of distribution of the flow velocity over the radius of a tube is obviously impossible. A formulated problem for a complete description of the flow of rheokinetic liquid seems to be quite difficult and it is clear that the first steps in investigating a two-dimensional flow were based on very simple assumptions. In a number of works [43,44], the authors took a fixed parabolic profile which is incorrect in principle for the flow of polymerizing media and leads to important mistakes. This is demonstrated very well in Ref. [45] where the possibility for styrene polymerization in a tubular reactor has been estimated it hse been shown that, if a real distribution of flow velocities and residence times over the radius is taken into account, the answer must be negative, in Ref. [44] however, a positive answer is obtained for an a priori parabolic profile. 

NMP of styrene in a miniemulsion can be also performed in a tubular reactor [213]. In the first step, a macroinitiator is prepared by bulk polymerization in a batch reactor and the subsequent miniemulsion polymerizatiOTi is carried out in a tubular reactor. The polymerization kinetics in the tubular reactor are similar to those in a batch reactor. It is also noteworthy that both preparatimi of a macroinitiator and a miniemulsion polymerization can be achieved in a crmtinuous tubular reactor to obtain polystyrene-h/ock-poly(butyl acrylate) diblock and polystyrene-h/ocA -poly (butyl acrylate)-h/ocA -polystyrene triblock copolymers [214],... 

Solution Polymerization These processes may retain the polymer in solution or precipitate it. Polyethylene is made in a tubular flow reactor at supercritical conditions so the polymer stays in solution. In the Phillips process, however, after about 22 percent conversion when the desirable properties have been attained, the polymer is recovered and the monomer is flashed off and recyled (Fig. 23-23 ). In another process, a solution of ethylene in a saturated hydrocarbon is passed over a chromia-alumina catalyst, then the solvent is separated and recyled. Another example of precipitation polymerization is the copolymerization of styrene and acrylonitrile in methanol. Also, an aqueous solution of acrylonitrile makes a precipitate of polyacrylonitrile on heating to 80°C (176°F). 

Continuous Polymerizations As previously mentioned, fifteen continuous polymerizations in the tubular reactor were performed at different flow rates (i.e. (Nj g) ) with twelve runs using identical formulations and three runs having different emulsifier and initiator concentrations. A summary of the experimental runs is presented in Table IV and the styrene conversion vs reaction time data are presented graphically in Figures 7 to 9. It is important to note that the measurements of pressure and temperature profiles, flow rate and the latex properties indicated that steady state operation was reached after a period corresponding to twice the residence time in the tubular reactor. This agrees with Ghosh s results ). 

The styrene conversion versus reaction time results for runs in the laminar flow regime are plotted in Figure 8. Both the rate of polymerization and the styrene conversion increase with increasing flow rate as noted previously (7). The conversion profile for the batch experimental run (B-3) is presented as a dashed line for comparison. It can be seen that the polymerization rates for runs with (Nj e e 2850 are greater than the corresponding batch polymerization with a conversion plateau being reached after about thirty minutes of reaction. This behavior is similar to the results obtained in a closed loop tubular reactor (7J) and is probably due to an excessively rapid consumption of initiator in a... 

There is an interior optimum. For this particular numerical example, it occurs when 40% of the reactor volume is in the initial CSTR and 60% is in the downstream PFR. The model reaction is chemically unrealistic but illustrates behavior that can arise with real reactions. An excellent process for the bulk polymerization of styrene consists of a CSTR followed by a tubular post-reactor. The model reaction also demonstrates a phenomenon known as washout which is important in continuous cell culture. If kt is too small, a steady-state reaction cannot be sustained even with initial spiking of component B. A continuous fermentation process will have a maximum flow rate beyond which the initial inoculum of cells will be washed out of the system. At lower flow rates, the cells reproduce fast enough to achieve and hold a steady state. 

Nagaki et al. (2008) also demonstrated the use of sec-BuLi 84 in a microflow system for the anionic polymerization of styrene 88, as a means of attaining a high degree of control over the molecular weight distribution of the resulting polymer. Employing a solution of styrene 88 (2.0 M) in THF and sec-BuLi 84 (0.2 M) in hexane and a tubular reactor... 

Figure 1 shows the equipment used. The tubular reactor was 240 ft (73m) long, 0.5 inch (1.27cm) OD, Type 316 stainless steel. The reactor was placed in an agitated, constant temperature water bath. Two gear pumps were used to give metered flow of the two feed streams-an emulsion of styrene in an equal volume of water, and a solution of potassium persulfate in water. Table 1 shows the recipe used for polymerization. 

There is an interior optimum. In this numerical example it occurs when 40% of the reactor volume is in the initial CSTR and 60% is in the downstream PFR. The model reaction is chemically unrealistic but illustrates behavior that can arise with real reactions. An excellent process for the bulk polymerization of styrene consists of a CSTR followed by a tubular postreactor. 

Figure 13.7 Performance of a laminar flow, tubular reactor for the bulk polymerization of styrene with Tm = 135° K and i = Ih (a) stability regions (b) monomer conversion in stable region.

Another type of controlled radical polymerization employs a reversible termination with a nitroxide compound [21]. Rosenfeld et al. [22] reported details of the nitroxide-mediated radical polymerization of styrene and butyl acrylate at 140 °C in a 2.9 m tubular micro-reactor with an inner diameter of 900 gm. Whereas, for the low-heat-producing monomer, styrene, the differences between a batch process and the microtubular reaction were small, in the case of butyl acrylate the difference was high. This situation, which may have been due to the Trommsdorff effect in the batch reaction (Figure 14.11), indicated that the polymerization was no longer under control. By contrast, no such effect was observed in the tubular micro-reactor, and the degree of conversion remained quite low under the applied conditions. 

While vinyl acetate is normally polymerized in batch or continuous stirred tank reactors, continuous reactors offer the possibility of better heat transfer and more uniform quality. Tubular reactors have been used to produce polystyrene by a mass process (1, 2), and to produce emulsion polymers from styrene and styrene-butadiene (3 -6). The use of mixed emulsifiers to produce mono-disperse latexes has been applied to polyvinyl toluene (5). Dunn and Taylor have proposed that nucleation in seeded vinyl acetate emulsion is prevented by entrapment of oligomeric radicals by the seed particles (6j. Because of the solubility of vinyl acetate in water, Smith -Ewart kinetics (case 2) does not seem to apply, but the kinetic models developed by Ugelstad (7J and Friis (8 ) seem to be more appropriate. 

---