Tubular reactor batch polymerizations

A summary of the nine batch reactor emulsion polymerizations and fifteen tubular reactor emulsion polymerizations are presented in Tables III IV. Also, many tubular reactor pressure drop measurements were performed at different Reynolds numbers using distilled water to determined the laminar-turbulent transitional flow regime. 

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],... 

Tubular reactor Suspension polymerization to prepare large porous polymer beads in a tubular reactor less polydisperse particle size distribution than in batch stirred-tank reactor 176... 

Various reactor combinations are used. For example, the product from a relatively low solids batch-mass reactor may be transferred to a suspension reactor (for HIPS), press (for PS), or unagitated batch tower (for PS) for finishing. In a similar fashion, the effluent from a continuous stirred tank reactor (CSTR) may be transferred to a tubular reactor or an unagitated or agitated tower for further polymerization before devolatilization. 

The advantages of continuous tubular reactors are well known. They include the elimination of batch to batch variations, a large heat transfer area and minimal handling of chemical products. Despite these advantages there are no reported commercial instances of emulsion polymerizations done in a tubular reactor instead the continuous emulsion process has been realized in series-connected stirred tank reactors (1, . ... 

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 are many interesting reports in the literature where computer simulations have been used to examine not only idealized cases but have also been used in an attempt to explain segregation and viscosity effect in unperturbed polymerization reactors (6). Some experimental work has been reported (7, 8). It is obvious, however, that although there is some change in the MWD with conversion in the batch and tubular reactor cases and that broadening of the MWD occurs as a result of imperfect mixing, there is no effective means available for controlling the MWD of the polymer from unperturbed or steady-state reactors. 

There are many variations on this theme. Fed-batch and continuous emulsion polymerizations are common. Continuous polymerization in a CSTR is dynamically unstable when free emulsifier is present. Oscillations with periods of several hours will result, but these can be avoided by feeding the CSTR with seed particles made in a batch or tubular reactor. 

Polymer production technology involves a diversity of products produced from even a single monomer. Polymerizations are carried out in a variety of reactor types batch, semi-batch and continuous flow stirred tank or tubular reactors. However, very few commercial or fundamental polymer or latex properties can be measured on-line. Therefore, if one aims to develop and apply control strategies to achieve desired polymer (or latex) property trajectories under such a variety of conditions, it is important to have a valid mechanistic model capable of predicting at least the major effects of the process variables. 

Now we shall discuss the method used to calculate the "cup"-averaged MWD-H, in which all portions of a polymerized liquid are mixed and averaged in a "cup" (vessel) positioned after the reactor. In this analysis, recourse was made to the so-called "suspension" model of a tubular reactor. According to this model, the reaction mass is regarded as an assemblage of immiscible microvolume batch reactors. Each of these microreactors moves along its own flow line. The most important point is that the duration of the reaction is different in each microreactor, as the residence time of each microvolume depends on its position at any given time, i.e., on its distance from the reactor axis. 

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. 

Tubular Reactor Studies. The first run in the tubular reactor was with the same recipe as for Seed I in Table I, but the conversion was very low, and there were two distinct phases. The residence time in the tube was equal to the batch reaction time. Apparently the more nearly constant temperature of the tubular reactor prevented rapid polymerization. In the next run, initiator and emulsifier levels were doubled, but still conversion was low, although phase separation was not so severe. With seed latex and still more emulsifier, Run I shown in Table II, monomer conversions of about 60% were obtained at 50 minutes average residence time in the reactor. No phase separation was evident, but later tests indicated that some phase separation was occurring. 

Chain Reaction with Termination. More work has been done on this mechanism, using free radical polymerization as the principle example. As shown in Table IV, batch polymerization has received far more interest within this area than the simpler case of continuous polymerization in a stirred tank, presumably because of commercial laboratory practice. The limited work on tubular reactors is not shown and will be discussed separately later. 

The classical procedures used by the chemist or engineer to obtain polymerization rate data have usually involved dilatometry, sealed ampoules, or samples withdrawn from model reactors—batch, tubular, and CSTR s alone or in various combinations. These rate data, together with data on molecular weight can be used to obtain the chain initiation constant and certain ratios such as kp2/kt and ktr/kp. Some basic relationships are shown in Figure 5. To determine individual rate constants such as kp and kt, other techniques are needed. For example, by periodic photochemical initiation it is possible to obtain kp/kt. If the ratio kp2/kt (discussed above) is also known, kp and kt can each be calculated. Typical techniques are described by Flory (20). 

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. 

The polymerization time in continuous processes depends on the time the reactants spend in the reactor. The contents of a batch reactor will all have the same residence time, since they are introduced and removed from the vessel at the same times. The continuous flow tubular reactor has the next narrowest residence time distribution, if flow in the reactor is truly plug-like (i.e., not laminar). These two reactors are best adapted for achieving high conversions, while a CSTR cannot provide high conversion, by definition of its operation. The residence time distribution of the CSTR contents is broader than those of the former types. A cascade of CSTR s will approach the behavior of a plug flow continuous reactor. 

Also, polymerization reactions are carried out in a variety of reactors including agitated batch reactors, continuous stirred tank reactors (CSTR), multizone autoclaves, loop reactors, tubular reactors, fluidized bed reactors, and a combination of these reactors. 

Free-radical polymerizations are exothermic, and so the heat produced during polymerization must be removed. This is not a significant problem in a laboratory scale however, heat transfer problems constitute a restriction for batch processes in an industrial scale. In the case of semibatch and CSTR, the cold monomer and water feed are beneficial for heat removal so that much higher production rates are feasible than for a batch reactor of the same volume. For tubular reactors, their large heat transfer area is advantageous for the strongly exothermic polymerizations. 

During polymerization with a CSTR, the monomer and the other components of the polymerization recipe are fed continuously into the reactor while the polymerization product mixture is continually withdrawn from the reactor. The application of the CSTR in suitable polymerization processes reduces, to some extent, the heat removal problems encountered in batch and tubular reactors due to the cooling effect from the addition of cold feed and the removal of the heat of reaction with the effluent. Even though the supporting equipment requirements may be relatively substantial, continuous stirred tank reactors are economically attractive for industrial production and consistent product quality. 

Solution In radical chain reactions, the overall rate of polymerization, Rp, and the number-average degree of polymerization, X , are functions of the initiator concentration [I], the monomer concentration [M], and also the temperature via the temperature dependence of the individual rate constants. At constant [M] and [I], the Schulz-Flory MWD is produced. However, if [M] and [I] vary with time, a number of Schulz-Flory distributions overlap and thus a broader MWD is produced. In the ideal CSTR [M] and [I] are constant and the temperature is relatively uniform. Consequently, chain polymerizations in CSTR produce the narrowest possible MWD. In the batch reactor, [M] and [I] vary with time (decrease with conversion) while in the tubular reactor [M] and [I] vary with position in the reactor and the temperature increases with tube radius. These variations cause a shift in X with conversion and consequently a broadening of MWD. 

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