Reactors Emulsion

Two of the most comprehensive discussions of these models were presented by Min and Ray (5) and by Poehlein and Dougherty (6). Min and Ray (5) gave a very general model framework which should be capable of modelling most emulsion polymerization systems. Of course, decisions must be made on the relative importance of the various phenomena occurring in a particular system. Other, more recent efforts on the modelling of emulsion reactors include the ones of Table I. Further details can also be found in (30). 

The other major type of catalytic reactor is a situation where the fluid and the catalyst are stirred instead of having the catalyst fixed in a bed. If the fluid is a liquid, we call this a slurry reactor, in which catalyst pellets or powder is held in a tank through which catalyst flows. The stirring must obviously be fast enough to mix the fluid and particles. To keep the particles from settling out, catalyst particle sizes in a slurry reactor must be sufficiently small. If the catalyst phase is another Hquid that is stirred to maintain high interfacial area for reaction at the interface, we call the reactor an emulsion reactor. These are shown in Figure 74. 

This situation describes an emulsion reactor in which reacting drops (such as oil drops in water or water drops in oil) flow through the CSTR with stirring to make the residence time of each drop obey the CSTR equation. A spray tower (liquid drops in vapor) or bubble column or sparger (vapor bubbles in a continuous liquid phase) are also segregated-flow situations, but these are not always mixed. We wiU consider these and other multiphase reactors in Chapter 12. 

We can generalize this to any two-phase reaction where both phases are mixed. One of the most common examples is the emulsion reactor, where immiscible liquids (usually one aqueous and one nonpolar) are fed into a tank that is stirred sufficiently fast that both phases break into drops or one phase is continuous and the other forms drops. We assumed that... 

The area between phases A is the surface area of the drops. It will clearly be a strong function of the stirring characteristics (we assume that stirring is always fast enough to mix both phases). The presence of surfactants, drop size distributions, stirrer design, and circulation patterns. Interfacial area is frequently an unknown in emulsion reactors, but the above formulation should be applicable. Another complication in emulsion reactors is the fact that mass transfer coefficients depend strongly on drop size and stirring rate. The relevant parameter in an emulsion reactor is A km wilh neither factor known very well. 

In a well-mixed emulsion reactor A in a continuous aqueous phase reacts with B in an organic drop phase in the reaction A + B products. The reaction is very fast and is limited by the diffusion of A into the organic phase. The feeds are 1 liter/min of 9 molar A and 1 liter/min of 2 molar B. The stirring is such that the drops are monodisperse at a diameter of 200/fm. 

The reaction A + B products occurs in a well-mixed emulsion reactor with B entering in the continuous aqueous phase and A in the organic phase. Reaction occurs by A transferring from the organic phase into the aqueous phase, where reaction occurs. The organic phase forms drops 0.1 cm in diameter, and the flow rate of the organic phase is 1 liter/min at Cao ... 

The reaction in a homogeneous solution with a polar organic solvent in which the enzymes and substrates are both soluble, occurs often at the expense of the enzyme stability [4, 5]. Besides immobilised enzymes in organic solvents [6], emulsion reactors, especially enzyme-membrane-reactors coupled with a product separation by membrane based extractive processes [7-9] and two-phase membrane reactors [10-12], are already established on a production scale. 

Figure 8.4 Reactor types used in organic-aqueous biphasic systems (a) Emulsion reactor, (b) Lewis cell, (c) passive membrane reactor, (d) active membrane reactor. E represents enzyme molecules.

The first step in downstream processing is the separation of the product-rich phase from the second phase and the biocatalyst. This may be simplified if the enzyme is immobilized or if a membrane module is included in the experimental set-up. In the case of emulsion reactors, centrifugation for liquid phase separation is a likely separation process [58], although the small size of droplets, the possibility of stable emulsion formation during the reaction, particularly if surface-active... 

A small amount of work has been done with other monomers in continuous emulsion reactor systems. 

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. 

For the calculation of molecular weights in CSTR emulsion reactors, a vsefnl classification comes to mind. This includes those monomer systems whose molecular weight and branching development depends on particle size and those that do not. Styrene falls into the former class and vinyl chloride and vinyl acetate into the latter class. Thus, in vinyl chloride emulsion polymerization where LCB is neglected, the instantaneous molecular weight distribution is given by... 

Fig.3a-d. Interfacial contact membrane reactors (adapted from [110]). B biocatalyst, S substrate, P product. Shaded area organic phase, white area-, aqueous phase. Dead-end reactor is a hybrid system, combining an emulsion reactor and a membrane module in the same imit, through which the organic phase flows, while aqueous phase is rejected... 

The organic phase may also be used as a substrate reservoir, besides their use for product stripping from the aqueous phase. The effectiveness of membrane-assisted organic-aqueous two-phase bioconversions relative to direct-contact two-phase emulsion reactors was demonstrated by Westgate et al. [150]. These authors observed a fivefold increase in the maximum specific activity of hydrolysis of menthyl acetate catalyzed by B. subtilis cells when a 0.2 pm nylon flat membrane reactor was used, as compared to an emulsion reactor. This result was attributed to a continuous interfacial contact, which could only be achieved in an emulsion bioreactor at the cost of high power inputs. Doig and co-workers operated a dense membrane bioreactor for the production of citronellol from geraniol with a product accumulation rate similar to the one obtained in an emulsion reactor [124]. Some examples of membrane-assisted two-liquid phase bio-conversions/fermentations are presented in Table 9. 

The previous discussion leads to the definition of the fourth classical control problem, which is the control of the copolymer composition along the reaction batch (or at the end of the batch). This objective is normally attained through manipulation of monomer feed flow rates [44, 45]. The feed stream usually contains the most reactive monomer species, so that composition control is obtained by keeping the concentration of the most reactive monomer concentration at the desired low levels throughout the batch time. It is important to emphasize that implementation of monomer feed strategies may lead to runaway conditions in the presence of heat transfer limitation [ 46 ], which partially explains why control of copolymer composition in emulsion reactors is normally attained by working under starved conditions. 

The rate of polymerization in a batch or continuous emulsion reactor can be written as follows ... 

Operation in biphasic mixtures using water-immiscible solvents introduces a linked equilibrium in the partition of educt and product and possible transport limitations at the interface, which have to be considered. Besides, enzyme deactivation at the interface and possible effects of the residual solvent solubility in aqueous buffers on enzyme stability have to be checked. Table 3 summarizes some data on stability of ADHs dissolved in aqueous buffers in a biphasic mixture with organic solvents [48]. Two different reactor concepts for continuous operation and enzyme catalysis in homogeneous phase have been studied—a bimembrane reactor [13,14] and an emulsion reactor [49]—which are discussed below with regard to reaction engineering. Using water-inuniscible solvents one can make use of the fact that NAD(P)/NAD(P)H are charged molecules and practically insoluble in apolar solvents. The coenzyme introduced in the reaction is therefore confined and physically immobilized with the enzymes in the aqueous phase. This facilitates efficient use of the coenzyme, especially if the volume fraction of the aqueous phase is kept low [13]. 

The water phase can also be recharged with fresh substrate in an emulsion reactor, where a hydrophilic membrane is used to cleave the emulsion (Fig. lOd) [49]. In this type of reactor as well as in the bimembrane reactor the enzyme is well separated from the organic phase. This is important as interphases, e.g., between two immiscible solvents or between a liquid and a gas differing widely in the dielectric constants, may lead to protein denaturation. A more detailed description of these reactors can be found in the references given or for membrane reactors in general [35,106-108]. 

Scheme 8.26 Suzuki coupling reaction catalyzed by polymer-stabilized Pd emulsion reactors.

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