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Ind. Eng. Chem. Res. 1997, 36, 1158-1162
Why Are Polymerization Reactors Special? Brian W. Brooks* Department of Chemical Engineering, Loughborough University, Loughborough, Leicestershire LE11 3TU, U.K.
Unlike many other products from the chemical industries, polymer molecules cannot easily be separated from each other. Therefore, the material has to be produced with the required specification at the reactor stage. The correct choice of reaction model is crucial for the prediction of reactor behavior. Allowance must be made for the effects of high viscosities on both heat transfer and reaction kinetics. In suspension polymerization, drop sizes, drop mixing, and sedimentation all require particular attention. With emulsion polymerization, a change in the reactor start-up procedure can lead to changes in dynamic behavior, monomer conversion, and product quality. If we can learn from nature, polymerization reactors may take new forms in the future. Introduction Chemical reactors can take many forms, but in principle, most of them can be assigned to three groups: batch reactors, continuous-flow reactors without back-mixing, and continuous-flow reactors with back-mixing. Simplistic diagrammatic representations of these reactor types are shown in Figure 1. These representations indicate the desired mixing conditions; they do not show the physical manipulations which may be necessary to achieve these conditions. Some multiphase reactors do not fit simply into these groups, especially if mixing is confined to one phase only. Reactors can take a wide variety of shapes and sizes from small pipes to large fluidized beds. In each case, the size, residence time, mixing patterns, and temperature profiles are all important. It should be realized that, although a tube may be a simple piece of equipment, the events occurring inside the tube may be far from simple. Chemical reactions of industrial interest often proceed via complex reaction schemes, and the distribution of products depends on the nature of the mixing pattern in the reactor. Usually, the reaction products must be separated into distinct components. The success of a separation process depends on the differences, in the chemical or physical properties, between the product components. A wide variety of separation processes can be used including distillation, extraction, absorption, adsorption, crystallization, membrane methods, and filtration. Polymeric Products Chemical products such as ammonia, benzene, phenol, or sodium carbonate have a unique and distinct composition. They can be separated from process streams and obtained in pure form. In contrast, the constituent molecules of a polymer product have a variety of sizes. For example, the term poly(ethylene oxide) may be used to describe s[-CH2CH2O]ns but in reality, there is not a single species present but a range of polymers which have different molecular weights. The physical properties which determine the usefulness of the polymer mixture depend on the range of values for n. The chemical differences between the various molecular sizes are not sufficiently wide to permit easy * Telephone: +44(0)1509 222510. Fax: +44(0)1509 223923. E-mail:
[email protected]. S0888-5885(96)00375-2 CCC: $14.00
Figure 1. Basic types of chemical reactors.
fractionation; also, most polymers are involatile and cannot be fractionated by distillation. Therefore, the product mix must be correct at the reaction stage; i.e., the required molecular weight distribution must be obtained in the reactor. There is little scope for changing the size range afterwards. Polymerization reactions also have unusual physical features which pose problems for reactor design. Monomer feeds usually have a relatively low viscosity, but the polymer-containing fluids which are produced in the reactors often have very high viscosities. This imposes severe constraints on the choice of mixing pattern and limits the extent of heat transfer (Carloff et al., 1994). Restricted heat transfer is particularly important when, as often happens, the polymerization is exothermic. Polymerization Reactors Reactors of various types, mentioned above, are used for polymer manufacture, but it can be seen that polymerization reactors have a number of special features which affect design choices. Detailed information about all the constituent chemical and physical steps in a reaction process is often unavailable. Therefore, in order to design a reactor successfully, it is necessary to construct serviceable models which account for important reaction phenomena. A model can take many forms depending on the center of interest. For a chemical reactor, a working model should take into account chemical kinetics, fluid mixing, changes in physical properties, and temperature variations (Soroush and Kravaris, 1993). In this paper, special features of polymerization reactors will be discussed with particular reference to free-radical polymerization. Choosing acceptable simplifications in a model is not always easy. This can be illustrated by considering the dynamic behavior of a continuous-flow reactor which is used for the first stage in a styrene polymerization process. During this process, changes in density and specific heat are relatively small, but if they are © 1997 American Chemical Society
Ind. Eng. Chem. Res., Vol. 36, No. 4, 1997 1159
Figure 2. Styrene polymerization in a stirred-flow reactor. (A) No allowance for changes in properties. (B) Allowance for changes in properties.
Figure 4. Changes in average drop size at low polymer content.
performance of a polymerization reactor (the model also predicts reaction kinetics accurately without further parameter changes (Bogunjoko and Brooks, 1983b)). To test this model, it is necessary to devise a method for distinguishing and characterizing a newly-formed polymer in a reaction fluid which already has a substantial polymer content. Figure 3. Instantaneous molecular weight distribution of poly(methyl methacrylate) at high viscosity. (A) Experimental results. (B) “Ideal model”. (C) New model.
neglected, then the predicted variation of temperature with time takes a somewhat spectacular form as shown by plot A in Figure 2 (Brooks, 1981). A real reactor would become unstable well before the first temperature peak was reached; in effect, our model is telling us that this process is not feasible. However, if the model is modified to allow for the real (but small) changes in density and specific heat, then the prediction is changed dramatically, as shown by plot B in Figure 2. This modified prediction corresponds accurately with reality. Fortunately, in this particular case, workers in industry discovered experimentally that the process is safe long before anyone diverted them with inadequate models. Viscosity Effects The high viscosity which is often encountered in polymerizing fluids not only inhibits mixing and impedes heat transfer but also affects the reaction kinetics. Fast reaction steps can become “diffusion controlled” (Stickler et al., 1984). To allow for this, many workers have devised models for molecular diffusion which can be used to correlate changes in rate coefficients and to predict kinetic behavior (Ray et al., 1995; Vivaldolima et al., 1994; Soh and Sundberg, 1982). These changes in the rate coefficients affect both the overall polymerization rate and also the molecular weight distribution of the polymer product. Therefore, it is important that models account for both of these effects simultaneously. Figure 3 shows the results for the instantaneous polymerization of methyl methacrylate at high viscosity (Bogunjoko and Brooks, 1983a). It can be seen that the molecular weight distribution for the polymer is bimodal., Models which assume a variable, but single, value for the chain-termination rate coefficient all predict an instantaneous molecular weight distribution which is monomodal, as shown by plot B in Figure 3. Changing values for key parameters alters the location of curve B but not the shape. By constructing a model which distinguishes between different sizes of growing polymer molecules, even in a very crude fashion, the essential features of the molecular weight distribution can be reproduced. This is shown by plot C in Figure 3. Thus, a model which is not highly detailed can be sufficient for predicting the
Suspension Polymerization In suspension polymerization, the problems with heat transfer are avoided by dispersing monomer drops in a continuous aqueous phase before polymerization begins. Heat transfer, from the small drops to the aqueous phase and from the aqueous phase to the reactor surroundings, is rapid. Therefore, control of the reactor temperature is better than that which is possible in bulk polymerization. Drop size distribution, which depends on reactor geometry and stirrer speed, is important because it determines the size distribution of the finished polymer particles. The presence of a dropstabilizing agent in the continuous phase is usually necessary. If the product is to be used in particulate form, then the particle size distribution is part of the product specification. It might be expected that, for a given monomer system, the relationships between rate coefficients and physical conditions, which are observed in bulk polymerization, should apply to suspension polymerization (Kalfas and Ray, 1993). Most studies of drop size distribution, in liquid-liquid dispersions, have been concerned with liquids of low viscosity with attention focused on drop size distribution in the steady state. In these cases, the time required to achieve a steady state is often very short and is not considered to be important. However, when the drops are highly viscous or when their physical properties change during the process, some considerable time may be required to achieve a stable drop size distribution. This is the case with suspension polymerization (Lange and Reichert, 1981). Even when the fluid viscosity is low, Figure 4 shows that an appreciable time is required to achieve a stable drop size distribution (Brooks and Richmond, 1994); this could be a significant fraction of the reaction time. Drop sizes which are predicted by “classical” relationships with the Weber number are not always obtained in practice (Zerfa and Brooks, 1996). At higher conversions, viscosity increases and changes in the drop size distribution require even more time. The drop size distribution is obtained by breakage and coalescence of drops. If these processes, which are not well understood (Chatzi and Kiparissides, 1995), become slow, as the viscosity increases, then the mixing of any new ingredient by postaddition to the reactor will not be effective. The presence of drop stabilizers also inhibits drop coalescence and mixing (Zerfa et al., 1995). Also, it is important to consider the possibility of phase inversion (in which the drop phase becomes continuous
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Figure 5. Dangerous consequences of density changes in suspension polymerization as density and viscosity increase.
Figure 6. Schematic representation of dispersed phases in emulsion polymerization.
and the phase which is initially continuous is turned into drops (Brooks and Richmond, 1991)). The drop density often increases during suspension polymerization. This increase may be small (say 10%), but it is important because it can have dangerous consequences. Figure 5 shows an example in which the monomer is less dense than hot water but the polymer is more dense than hot water. If the agitator design is not good, the polymerizing drops sediment and may coalesce. Heat transfer from the sedimented mass is much slower than heat transfer from the small drops, and since the reaction is usually exothermic, the temperature inside the polymerizing fluid will rise. Rapid vaporization of monomer can then occur, leading to disaster. The high potential for thermal runaway is one feature of polymerization reactors which cannot be ignored (Barton and Nolan, 1984; Nemeth and Thyrion, 1995). Emulsion Polymerization In emulsion polymerization, monomer drops are dispersed in water but an assembly of very small polymer particles is generated in the aqueous phase as shown in Figure 6. This is not to scale; drop diameters range between 0.05 and 2.0 mm, whereas the particle diameters are usually less than 0.001 mm. The presence of emulsifiers is required, and the essential chemistry occurs mostly in the polymer particles and in the aqueous phase. In conventional emulsion polymerization, virtually no reaction occurs in the monomer drops which act as reservoirs. However, very small monomer drops can be transformed into polymer particles (Vanderhoff, 1993). In addition to the physical advantages of suspension polymerization, the mechanism of emulsion polymerization often allows the simultaneous occurrence of high polymerization rate and high polymer molecular weight. The final reaction product is a dispersion of very small polymer particles which may be used directly (e.g., for surface coatings or adhesives). In emulsion polymerization, particle nucleation and particle growth are important steps which are not fully understood (Giannetti, 1993). This lack of understanding does not present an insurmountable problem with batch reactors because particle nucleation is usually completed early in the polymerization so that particle
Figure 7. Conversion changes with time for the isothermal emulsion polymerization of styrene in a continuous-flow reactor.
nucleation and particle growth are virtually consecutive rate processes. However, continuous-flow reactors usually require agitation, and as a result of back-mixing, particle nucleation and particle growth become simultaneous processes. The particle size distribution now becomes wide so that the latex which is produced is significantly different from that obtained with a batch reactor (which gives a narrow particle size distribution). Particle nucleation now requires characterization. If a tubular flow reactor is used, then back-mixing can be restricted and a narrow particle size distribution is obtained (Paquet and Ray, 1994). Early work predicted that, even when feed streams to a continuous-flow back-mixed emulsion polymerization reactor were steady and the reactor temperature was constant, both the monomer conversion and the particle concentration in the reactor could oscillate with time during the reaction. Even so, some workers believed that any oscillatory behavior was solely the result of inadequate temperature control and that the conversion would stabilize quickly (after the initial stages when particles were being generated). Early experimental results were deceptive. Figure 7A shows the results obtained from regular sampling of a reactor effluent. It can be seen that, after the start-up peak, the conversion appears to become steady. Figure 7B shows the “obvious” line to draw through the points, but is this correct? If, in addition to the regular samples, data points are acquired at other times, then a different picture emerges (Brooks et al., 1978). Figure 7C shows data in addition
Ind. Eng. Chem. Res., Vol. 36, No. 4, 1997 1161 Table 1. Relative Strengths of Fibersa
a
Figure 8. Effect of start-up conditions for the emulsion polymerization of methyl methacrylate in a continuous-flow reactor. Identical feed streams with small differences in start-up conditions.
material
tenacity (g/denier)
nylon 6,6 polyester poly(acrylonitrile) steel spider silk (drag line)
5.2 5.1 3.2 3.5 7.8
Data from Lucas, 1964.
Often, a number of monomers are polymerized simultaneously to make copolymers. Then, product composition becomes important. In these circumstances, a semibatch reactor may be chosen. Here, there is an inflow but no out-flow so that the volume of the reactor contents changes with time. With reactors of this type, particle nucleation can be manipulated and polymer composition can be controlled. The order in which the reaction ingredients are added is now crucial because it determines the nature of the final product. New ways of describing semibatch polymerization reactors are now being developed (Li and Brooks, 1993; Powell and Brooks, 1995). The Future
Figure 9. Start-up effects in the emulsion polymerization of styrene in a continuous-flow reactor. Identical feed streams with small differences in start-up conditions.
to the results that would have been gained from regular measurements. Clearly, the estimate of the best line through the original points must be revised, as shown in Figure 7D. In this example, the oscillations are relatively small and may eventually die out. In other cases, substantial oscillations are sustained throughout the process even though the reactor is isothermal and the feed rate is steady (Greene and Poehlein, 1975). The long-term behavior of a continuous-flow emulsion polymerization reactor depends noticeably on the way in which the reactor operation is started up (Brooks and Raman, 1987). Figure 8 shows the results of two experiments in which monomer conversion is oscillating around quite different values. In both cases, the feed streams and temperature are identical; the only difference is in the order in which the reactants were first introduced to the reactor. These differences in conversion are accompanied by differences in particle size distribution. In effect, two different products are being made from identical feed streams. Even when conditions are arranged so that conversion does become steady, bifurcation may still be observed; also, the particle sizes may continue to change. Figure 9 shows that, for identical reactor conditions (using identical feed streams), a small change in start-up procedure leads to two different (but almost steady) conversions (Baddar and Brooks, 1984). However, it can be seen that, in one case, the particle size is increasing rapidly (as nucleation almost ceases). In spite of a stable conversion, catastrophic coagulation can be expected to occur very late in this process, when nucleation re-commences (Kiparissides et al., 1980; Brooks and Raman, 1987). Not only is the “wrong” product formed but the physical properties of the emulsion will change so that the emulsion will no longer flow out of the reactor (Brooks and Raman, 1987). Ten tons of immobile gel inside a reactor is most unwelcome. Oscillatory behavior can be avoided by using a prereactor as a continuous seeder (Penlidis et al., 1989).
Products of the polymer industry are becoming more complex with precise quality requirements. Reactor processes are becoming more effective as our understanding improves. However, although substantial advances have been made, the range of our processes is still narrow. Most polymerization reactions require sensitive catalysts, very clean conditions, controlled monomer feed ratios, effective mixing, and precise temperatures. Often, large cumbersome equipment and toxic feedstocks must be used. Perhaps we should ask if anything can be learned from nature. All around us, many green plants are quietly copolymerizing carbon dioxide and water to make polymeric materials (cellulose and carbohydrates). How would we set about copolymerizing carbon dioxide and water (just supposing that we knew how to do it)? Judging by existing technology, extensive apparatus with large compressors, sensitive catalysts, and a variety of control equipment would be required. But green plants work at atmospheric temperature and pressure with a sophisticated (and effective) catalyst which copes with a highly contaminated feedstock (dirty air and water) and functions in all weathers. Trees produce a variety of “grades” of timber with complex polymeric structures (already “laminated”) having very useful physical properties; some trees produce latex rubber. Although latex rubber can be made in polymerization reactors, it does not match the material from the rubber tree. The garden spider (Araneus diadematus) produces webs from very fine silk. The silk is composed of a variety of polyamides (proteins), and the fiber properties are changed to fulfill many functions (Vollrath and Edmonds, 1989). Five types of silk have been identified, each with a controlled composition (Lucas, 1964). From the precursor materials, spiders produce their silk with much-envied ease at atmospheric temperature. In spite of high humidity, they manage to displace reaction equilibrium very rapidly, achieving high conversion to polymer before vaporization of volatiles occurs. In the web, the drag lines are very strong. Table 1 shows that spider silk is stronger than many man-made polymers. Some specialized man-made fibers are as strong as spider silk, but the extension at break is much lower. Although it might be claimed that spiders have been
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“developing their processes” for a few million years, we must ask if our processes will eventually match those of the spider. Literature Cited Baddar, E. E.; Brooks, B. W. Start-up Procedures for ContinuousFlow Emulsion Polymerisation Reactors. Chem. Eng. Sci. 1984, 39, 1499. Barton, J. A.; Nolan, P. F. Runaway Reactions in Batch Reactors. Ind. Chem. Eng. Symp. Ser. 1984, 85, 13. Bogunjoko, J. S. T.; Brooks, B. W. Molecular Weight Distributions of Poly(methyl methacrylate) Produced at High Viscosities. Die Makromol. Chem. 1983a, 184, 1623. Bogunjoko, J. S. T.; Brooks, B. W. Kinetics of Free-Radical Polymerisation at High Viscosities. Die Makromol. Chem. 1983b, 184, 1603. Brooks, B. W. Dynamic Behaviour of a Continuous Flow Polymerisation Reactor. Chem. Eng. Sci. 1981, 36, 589. Brooks, B. W.; Raman, G. Effects of Different Reactor Start-up Procedures on the Continuous-Flow Emulsion Polymerisation of Methylmethacrylate. Chem. Eng. Sci. 1987, 42, 1439. Brooks, B. W.; Richmond, H. N. Dynamics of Liquid-liquid Phase Inversion using Non-ionic Surfactants. Colloids Surf. 1991, 58, 131. Brooks, B. W.; Richmond, H. N. Phase Inversion in Non-ionic Surfactant-oil-water SystemssI. The Effect of Transitional Inversion on Emulsion Drop Sizes. Chem. Eng. Sci. 1994, 49, 1053. Brooks, B. W.; Kropholler, H. W.; Purt, S. N. The Emulsion Polymerisation of Styrene in a Continuous Stirred Reactor. Polymer 1978, 19, 193. Carloff, R.; Pross, A.; Reichert, K. H. Temperature Oscillation Calorimetry in Stirred Tank Reactors with Variable Heat Transfer. Chem. Eng Technol. 1994, 17, 406. Chatzi, E. G.; Kiparissides, C. Steady-state Drop-size Distributions in High Holdup Fraction Dispersion Systems. AIChE J. 1995, 41, 1640. Giannetti, E. Nucleation Mechanisms and Particle-size Distributions of Polymer Colloids. AIChE J. 1993, 39, 1210. Greene, R. K.; Poehlein, G. W. Continuous Emulsion Polymerization of Vinyl Acetate. ACS Symp. Emulsion Polym., Philadelphia 1975, 292. Kalfas, G.; Ray, W. H. Modeling and Experimental Studies of Aqueous Suspension Polymerization Processes. 1. Modeling and Simulations. Ind. Eng. Chem. Res. 1993, 32, 1822. Kiparissides, C.; MacGregor, J. F.; Hamielec, A. E. Continuous Emulsion Polymerization of Vinyl Acetate. Part 1: Experimental Studies. Can. J. Chem. Eng. 1980, 58, 48. Lange, F.; Reichert, K. H. Studies on the Time-dependence of Drop Size in Stirred Liquid-liquid Systems. Chem. Ing. Technol. 1981, 53, 747.
Li, B. G.; Brooks, B. W. Modelling and Simulation of Semi-batch Emulsion Polymerization. J. Appl. Polym. Sci. 1993, 48, 1811. Lucas, F. Spiders and their Silk. Discovery 1964, Jan, 20. Nemeth, S.; Thyrion, F. C. Study of the Runaway Characteristics of Suspension Polymerization of Styrene. Chem. Eng. Technol. 1995, 18, 315. Paquet, D. A.; Ray, W. H. Tubular Reactors for Emulsion Polymerization. AIChE J. 1994, 40, 73. Penlidis, A.; MacGregor, J. F.; Hamielec, A. E. Continuous Emulsion Polymerization: Design and Control of CSTR Trains. Chem. Eng. Sci. 1989, 44, 273. Powell, F. E.; Brooks, B. W. Reactor Performance and Validity of Steady State and Stationary State Assumptions in Semi-batch Free-radical Solution Polymerisation. Chem. Eng. Sci. 1995, 50, 837. Ray, A. B.; Saraf, D. N.; Gupta, S. K. Free Radical Polymerizations Associated with the Trommsdorff Effect under Semibatch Reactor Conditions. 1. Modeling. Polym. Eng. Sci. 1995, 35, 1290. Soh, S. K.; Sundberg, D. C. Diffusion-Controlled Vinyl Polymerization. IV. Comparison of Theory and Experiment. J. Polym. Sci., Polym. Chem. Ed. 1982, 20, 1345. Soroush, M.; Kravaris, C. Optimal Design and Operation of Batch Reactors. 2. A Case Study. Ind. Eng. Chem. Res. 1993, 32, 882. Stickler, M.; Panke, D.; Hamielec, A. E. Polymerization of Methylmethacrylate up to High Degrees of ConversionsExperimental Investigation of the Diffusion Controlled Polymerization. J. Polym. Sci., Polym. Chem. Ed. 1984, 22, 2243. Vanderhoff, J. W. Recent Advances in the Preparation of Latexes. Chem. Eng. Sci. 1993, 48, 203. Vivaldolima, E.; Hamielec, A. E.; Wood, P. E. Auto-acceleration Effect in Free Radical Polymerization. A Comparison of the CCS and MH Modes. Polym. React. Eng. 1994, 2, 17. Vollrath, F.; Edmonds, D. T. Modulation of the Mechanical Properties of Silk by Coating with Water. Nature 1989, 340, 305. Zerfa, M.; Brooks, B. W.; Faraday, A. G. Influence of Experimental Conditions on the Suspension Polymerisation of Vinyl Chloride. 5th International Workshop on Polymer Reaction Engineering, Berlin 1995; DECHEMA Monogr. 1995, 131, 249. Zerfa, M.; Brooks, B. W. Prediction of Vinyl Chloride Drop Sizes in Stabilised Liquid-liquid Agitated Dispersion. Chem. Eng. Sci. 1996, 51, 3223.
Received for review July 2, 1996 Revised manuscript received October 16, 1996 Accepted October 16, 1996X IE960375M X Abstract published in Advance ACS Abstracts, February 15, 1997.
