Monomers mass transfer

Other reactor design considerations may be necessary in special cases. Monomer mass transfer, not normally a problem, can he important if the monomer- aqueous phase interface is small. This is more likely in systems involving gaseous monomers in which the large surface area of the monomer emulsion is not present. In such cases special attention must he paid to gas dispersion and transport. Giher factors that can have a significant effect on reactor design include latex viscosity, heat transfer rates, reaction pressure, and control mechanisms. 

The polymerisation of PO and EO, initiated by polyfunctional starters, to make short chain polyether polyols is a reaction that is strongly dependent on diffusion. The consumption rate of PO or EO is given by two simultaneous factors the rate of the chemical reaction in the liquid phase and the efficiency of the monomer mass transfer from the gaseous phase to liquid phase (see details in section 4.1.5). The PO (or EO) consumption rate, considering the mass transfer, is described by equation 13.27 [45-50] ... 

If the particle resists the stresses and does not break up, the pores fill up with polymer and monomer mass transfer limitations become the rate limiting step. This undesirable situation essentially causes the polymerization to shut down. 

The polymerization of VCM is highly exothermic (i.e., 100 kj mol ). Thus, the efficient removal of the reaction heat is very critical for the operation of large-scale reactors [44]. When all of the free liquid monomer has been consumed, the pressure in the reactor starts to fall as a result of the monomer mass transfer from the vapor phase to the polymer phase due to subsaturation conditions. In industrial PVC production, the reaction is usually stopped when a certain pressure drop has been recorded. Because the polymer is effectively insoluble in its own monomer, once the polymer chains are first generated, they precipitate immediately to form a separate phase in the polymerizing mixture. Thus, from a kinetic point of view, the polymerization of VCM is considered to take place in three stages [45]. 

Miniemulsion polymerization is a technique that, in principle, allows any water-insoluble monomer to undergo polymer reactions (not limited to the conventional free radical polymerization) inside the homogenized monomer droplets dispersed in the continuous aqueous phase. Thus, each miniemulsion droplet can be regarded as an ideal submicron scale reactor that is not controlled by the monomer mass transfer process for the synthesis of a variety of... 

Time constant for monomer mass transfer is much lower than the time constant for polymerisation. So the resistance against mass transfer between the different phases is negligible and the polymerisation obeys intrinsic kinetics. Therefore, the distribution of monomer between the phases is controlled by thermodynamic equilibrium. 

Mass transfer of monomer from the suspended drops through the aqueous phase to the seeded particles continues throughout the polymerization. 

As suggested by Barrett (2), it is assumed that following the particle nucleation stage, the polymerization proceeds in the particle (monomer/polymer) phase with no mass transfer limitation. Therefore, the dispersion polymerization is similar to a mass or suspension polymerization, and kj can not be assumed to be constant even at isothermal conditions, since kp and even kp are dependent on the degree of polymerization because of a gel effect. (2., ,D However, since the application of the model is for a finishing step, with polymer molecular weight and viscosity fairly well established, further changes in kp and kp should be minimal. 

The second major difference found in vapor-liquid extraction of polymeric solutions is related to the low values of the diffusion coefficients and the strong dependence of these coefficients on the concentration of solvent or monomer in a polymeric solution or melt. Figure 2, which illustrates how the diffusion coefficient can vary with concentration for a polymeric solution, shows a variation of more than three orders of magnitude in the diffusion coefficient when the concentration varies from about 10% to less than 1%. From a mathematical viewpoint the dependence of the diffusion coefficient on concentration can introduce complications in solving the diffusion equations to obtain concentration profiles, particularly when this dependence is nonlinear. On a physiced basis, the low diffiisivities result in low mass-transfer rates, which means larger extraction equipment. 

Mass transfer in polymeric solutions by molecular diffusion is a comparatively slow process, and in extraction equipment where mass transfer occurs through a thin wiped film, inordinately large equipment surface areas are often required in order to obtain substantial rates of mass transfer. In commercial practice when this situation occurs, the required surface areas may, instead, be obtained by either one of two methods, both of which involve reducing the pressure in the extraction zone. In one approach the extraction pressure is fixed at a value which is less than the equilibrium partial pressure of the monomer or solvent in the polymeric solution fed to the extraction zone. In these circumstances gas bubbles... 

Since autoacceleration and autodeceleration arise from mass transfer limitations in the polymerizing system, it is important to relate the kinetic constants to diffusivity of the monomer and polymer. Again, these diffusion coefficients for the monomer and polymer can be developed in terms of the free volume in the polymerizing system. Likewise, kp and k, will depend on the diffusion coefficients of the monomer and polymer respectively. 

In principle, silica growth kinetics may be controlled by (1) slow release of monomer via alkoxide hydrolysis in the particle-free reverse micelles, (2) slow surface reaction of monomer addition to the growing particle, and (3) slow transport processes as determined by the dynamics of intermicellar mass transfer. There is strong experimental evidence to support the view that the rate of silica growth in the microemulsion environment is controlled by the rate of hydrolysis of TEOS (23,24,29). Silica growth kinetics can be analyzed in terms of the overall hydrolysis and condensation reactions ... 

Despite the complex interaction between the components of a catalyst recipe, for example consisting of catalyst, co-catalyst, electron donors (internal and external), monomers, chain-transfer agents such as hydrogen, and inert gases and the catalyst support, the local polymer production rate rate (polymerization rate) in a given volume, Rp (kg polymer hr"1), can often be described by a first-order kinetic equation with respect to the local monomer concentration near the active site, cm (kgm"3), and is first order to the mass of active sites ( catalyst ) in that volume, m (kg) ... 

At constant monomer pressure, and without any kind of mass-transfer limitation, Cm equals Cm,o- Furthermore, assuming that kp does not change with time, the polymerization rate can be expressed as... 

Because of the extra mass-transfer resistance through the polymer layer, the monomer concentration may depend on the intra-particle morphology. Micro-porosity, crystallinity, and the thickness of the polymer layer around active sites should influence the transport processes... 

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