Mass ttansfer rate

Since the total mass ttansfer rate of B is zero, there must be a bulk flow of the system towards the liquid surface exactly to counterbalance the diffusional flux away from the surface, as shown in Figure 10.1, where ... 

The reaction can be run under differing mass ttansfer rates by changes in impeller speed and system pressure. These changes can also affect the addition time necessary for completion of gas uptake. If the yield increases with increased mass transfer (higher impeller speed and/or higher system pressure), mixing conditions are clearly demonstrated to be critical. The maximum possible yield may not have been achieved because these factors can all affect overall reaction time and the degree to which by-products can form in the films and the continuous phases in consecutive and parallel reactions. 

Often, the driving force for crystallization in melt systems is given simply by the temperature difference, AT, with the melting point temperature. In general, the lower the temperature of a system, the greater the rate of crystallization until reduced mass ttansfer limits nucleation at very low temperatures. 

Reactions between gases and liquids may involve sohds also, either as reactants or as catalysts. Table 17.9 lists a number of examples. The lime/limestone slurry process is the predominant one for removal of SO, from power plant flue gases. In this case it is known that the rate of the reaction is conttolled by the rate of mass ttansfer through the gas film. 

All reactions involved in polymer chain growth are equilibrium reactions and consequently, their reverse reactions lead to chain degradation. The equilibrium constants are rather small and thus, the low-molecular-weight by-products have to be removed efficiently to shift the reaction to the product side. In industrial reactors, die overall esterification, as well as the polycondensation rate, is con-d olled by mass transport. Limitations of die latter arise mainly from the low solubility of TPA in EG, die diffusion of EG and water in the molten polymer and die mass ttansfer at the phase boundary between molten polymer and the gas phase. The importance of diffusion for the overall reaction rate has been demonshated in experiments with thin polymer films [10]. 

Hoftyzer and van Ki evelen [100] invesdgated die combination of mass ttansfer togedier widi chemical reactions in polycondensation, and deduced die ratedetermining factors from the description of gas absoiption processes. They proposed dii ee possible cases for polycondensation reactions, i.e. (1) die polyconden-sadon takes place in the bulk of die polymer melt and die volatile compound produced has to be removed by a physical desoiption process, (2) die polycondensation takes place exclusively in the vicinity of die interface at a rate determined by bodi reacdon and diffusion, and (3) the reacdon zone is located close to die interface and mass ttansport of the reactants to diis zone is die rate-determining step. 

If the reaction rate is very slow, the concentration difference between Cjg and Cb grows closer. In the limit, Cb is equal to Cg and the maximum reaction rate is obtained at the saturation composition. It almost all cases it is assumed that the continuous or liquid phase is well mixed, so that no gradients exist. This is true in most equipment because the blend time is usually small compared to the mass ttansfer time. This means that Cb is the same at all places in the vessel. 

The published values for the activation energies and pre-exponential factors of transesterification and glycolysis vaiy significantly. Catalysts and stabilizers influence the overall reaction rate markedly, and investigations using different additives cannot be compared directly. Most investigations ate affected by mass transport and witliout knowledge of the respective mass transport parameters, kinetic results cannot be ttansferred to other systems. 

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