Mass transfer zone melting

In industrial PET synthesis, two or three phases are involved in every reaction step and mass transport within and between the phases plays a dominant role. The solubility of TPA in the complex mixture within the esterification reactor is critical. Esterification and melt-phase polycondensation take place in the liquid phase and volatile by-products have to be transferred to the gas phase. The effective removal of the volatile by-products from the reaction zone is essential to ensure high reaction rates and low concentrations of undesirable side products. This process includes diffusion of molecules through the bulk phase, as well as mass transfer through the liquid/gas interface. In solid-state polycondensation (SSP), the volatile by-products diffuse through the solid and traverse the solid/gas interface. The situation is further complicated by the co-existence of amorphous and crystalline phases within the solid particles. 

Hoftyzer and van Krevelen [100] investigated the combination of mass transfer together with chemical reactions in polycondensation, and deduced the ratedetermining factors from the description of gas absorption processes. They proposed three possible cases for poly condensation reactions, i.e. (1) the polycondensation takes place in the bulk of the polymer melt and the volatile compound produced has to be removed by a physical desorption process, (2) the polycondensation takes place exclusively in the vicinity of the interface at a rate determined by both reaction and diffusion, and (3) the reaction zone is located close to the interface and mass transport of the reactants to this zone is the rate-determining step. 

This example follows 9.3, where we used spreadsheet and netpath calculations to deduce the mass transfer reactions along the evolutionary path from snow melt to water in the saturated zone of recharge area, specifically in the Navajo National Monument well. Now, we want to calculate the travel time from Navajo National Monument to well... 

The preceding discussion assumes that no convection exists in the melt, and this is rarely, if ever, the case. Next we shall consider two approaches which account for convection in the melt, a transport mechanism which is especially important in mass transfer because Dl is small and even weak convection markedly alters solute concentration profiles and may cause macrosegregation. First we shall discuss film theory which is a very simple approach that gives qualitative information and often provides considerable physical insight into the mechanisms involved. Second, we shall discuss a simplified model of zone refining. 

On the whole, the mass transfer of dopant at the set-in stage of the zoning crystallization process can be viewed as two autonomous processes that, conditionally, are independent of each other the mass transfer over the chain feeding-melt-crystal and the transit process of dopant component over the route feeding-melt-gaseous phase-furnace cold walls . In the meantime, each process has only its own part of participating dopant component Cg and Cg, the sum of which constitutes the dopant concentration in the feeding material Cf = Cg + Cg. 

The glass melting process always involves a fairly intensive melt flow it must be taken into account when melting in pots or tanks, but is of special significance in the case of continuous tank furnaces where it is a prerequisite of correct furnace function. Melt flow has the positive effect of accelerating the mass and heat transfer. Increased corrosion of refractories and possible carry-over of unmelted batch into the refining and working zone are its undesirable consequences. 

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