Suspension phase, bulk

As is shown in Figure 2, in the two-phase model the fluid bed reactor is assumed to be divided into two phases with mass transfer across the phase boundary. The mass transfer between the two phases and the subsequent reaction in the suspension phase are described in analogy to gas/liquid reactors, i.e. as an absorption of the reactants from the bubble phase with pseudo-homogeneous reaction in the suspension phase. Mass transfer from the bubble surface into the bulk of the suspension phase is described by the film theory with 6 being the thickness of the film. D is the diffusion coefficient of the gas and a denotes the mass transfer coefficient based on unit of transfer area between the two phases. 6 is given by 6 = D/a. 

As a first approximation a convective term in the film region has been negleted, u is the superficial gas velocity and u f denotes the gas velocity at minimum fluidization conditions. Tne specific mass transfer area a(h) is based on unit volume of the expanded fluidized bed and e OO is the bubble gas hold-up at a height h above the bottom plate. Mathematical expressions for these two latter quantities may be found in detail in (20). The concentrations of the reactants in the bubble phase and in film and bulk of the suspension phase are denoted by c, c and c, respectively. The rate constant for the first order heterogeneous catalytic reaction of the component i to component j is denoted... 

Phase Behavior and Structure of Colloidal Suspensions in Bulk, Confinement, and External Fields... 

Because the polymerization occurs totally within the monomer droplets without any substantial transfer of materials between individual droplets or between the droplets and the aqueous phase, the course of the polymerization is expected to be similar to bulk polymerization. Accounts of the quantitative aspects of the suspension polymerization of methyl methacrylate generally support this model (95,111,112). Developments in suspension polymerization, including acryUc suspension polymers, have been reviewed (113,114). 

Suspension Polymerization. In this process the organic reaction mass is dispersed in the form of droplets 0.01—1 mm in diameter in a continuous aqueous phase. Each droplet is a tiny bulk reactor. Heat is readily transferred from the droplets to the water, which has a large heat capacity and a low viscosity, faciUtating heat removal through a cooling jacket. 

A key factor in doing a successful suspension polymerization is the composition of the aqueous phase or stabilizer. Too much stabilizer results in emulsion polymerization, which produces small particles (less than 1 /cm). Too little stabilizer results in bulk polymerization. For the production of GPC gels, the ratio of aqueous phase to organic phase should be about 2 1. 

Polymerization reactions can occur in bulk (without solvent), in solution, in emulsion, in suspension, or in a gas-phase process. Interfacial polymerization is also used with reactive monomers, such as acid chlorides. 

The concentration of monomers in the aqueous phase is usually very low. This means that there is a greater chance that the initiator-derived radicals (I ) will undergo side reactions. Processes such as radical-radical reaction involving the initiator-derived and oligomeric species, primary radical termination, and transfer to initiator can be much more significant than in bulk, solution, or suspension polymerization and initiator efficiencies in emulsion polymerization are often very low. Initiation kinetics in emulsion polymerization are defined in terms of the entry coefficient (p) - a pseudo-first order rate coefficient for particle entry. 

When we design commercial polymerization plants we must consider the characteristics of both the monomer and the final product. This allows us to define the optimum configuration to produce a specific polymer. Polymerization reactions can take place in homogeneous solutions or heterogeneous suspensions. For homogeneous processes, the diluted or pure monomer(s) are added directly to one another and the reaction occurs in the media created when mixing the reactants. When the reactants are added directly to one another, the process is referred to as a bulk process. With heterogeneous processes, a phase boundary exists which acts as an interface where the reaction occurs. 

Polymerization of vinyl chloride occurs through a radical chain addition mechanism, which can be achieved through bulk, suspension, or emulsion polymerization processes. Radical initiators used in vinyl chloride polymerization fall into two classes water-soluble or monomer-soluble. The water-soluble initiators, such as hydrogen peroxide and alkali metal persulfates, are used in emulsion polymerization processes where polymerization begins in the aqueous phase. Monomer-soluble initiators include peroxides, such as dilauryl and benzoyl peroxide, and azo species, such as 1,1 -azobisisobutyrate, which are shown in Fig. 22.2. These initiators are used in emulsion and bulk polymerization processes. 

The liquid-phase reaction kinetics of doped molecules in silica nanomatrixes was conducted using the metalation of meso-tetra (4-Ai,Ai,Ai-trimethylanilinium) porphyrin tetrachloride (TTMAPP) with Cu(II) as a model. To demonstrate the effect of the silica nanomatrix on the diffusion, pure silica shells with varied thickness were coated onto the same silica cores, which doped the same amount of TTMAPP molecules. The Cu(II) from the suspension could penetrate into the silica nanomatrixes and bind to the TTMAPP. The reaction rate of TTMAPP metalation with Cu(II) was significantly slower than that in a bulk solution. The increase in the thickness of the silica resulted in a consistent decrease of reaction rates (Fig. 8). 

The production of conventional stationary phases in the form of porous polymer particle is based on suspension polymerization. Namely, the polymerization is allowed to proceed in a solvent under vigorous stirring that assures obtaining particles of the desired diameter. Since the particle size is typically in the range of a few micrometers, no problems with heat transfer are encountered. In contrast, the preparation of monoliths requires a so-called bulk polymerization. A polymer mixture consisting of monomers and porogenic solvent is mixed with an initiator. As the temperature is increased, the initiator decomposes and oligomer nu-... 

The pore size distributions of the molded monoliths are quite different from those observed for classical macroporous beads. An example of pore size distribution curves is shown in Fig. 3. An extensive study of the types of pores obtained during polymerization both in suspension and in an unstirred mold has revealed that, in contrast to common wisdom, there are some important differences between the suspension polymerization used for the preparation of beads and the bulk-like polymerization process utilized for the preparation of molded monoliths. In the case of polymerization in an unstirred mold the most important differences are the lack of interfacial tension between the aqueous and organic phases, and the absence of dynamic forces that are typical of stirred dispersions [60]. 

Polymers may be made by four different experimental techniques bulk, solution, suspension, and emulsion processes. They are somewhat self-explanatory. In bulk polymerization only the monomers and a small amount of catalyst is present. No separation processes are necessary and the only impurity in the final product is monomer. But heat transfer is a problem as the polymer becomes viscous. In solution polymerization the solvent dissipates the heat better, but it must be removed later and care must be used in choosing the proper solvent so it does not act as a chain transfer agent. In suspension polymerization the monomer and catalyst are suspended as droplets in a continuous phase such as water by continuous agitation. Finally, emulsion polymerization uses an emulsifying agent such as soap, which forms micelles where the polymerization takes place. 

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