Mass transfer with fast homogeneous

These intriguing situations, which are similar to the so-called "diffusion falsification" regime of fluid-porous catalytic solid systems (5), can be successfully handled by the "theory of mass transfer with chemical reaction". Indeed, they can be deployed to obtain kinetics of exceedingly fast reactions in simple apparatuses, which in the normal investigations in homogeneous systems would have required sophisticated and expensive equipment. Further, it is possible, under certain conditions, to obtain values of rate constants without knowing the solubility and diffusivity. In addition, simple experiments yield diffusivity and solubility of reactive species which would otherwise have been - indeed, if possible - extremely difficult. 

Special opportunities for the industrial implementation of structured catalysts are offered by the growing interest in millisecond contact time processes, in view of the associated requirements on pressure drop and flow distribution to be matched with strict size constraints. In this case, a better control of the complex interplay between heat and mass transfer and heterogeneous/homogeneous reactions granted by structured catalysts would provide guidelines for the design of reactors and processes with optimized performances in terms of selectivity, yield, fast transient response and operational flexibility. 

Diffusion rates are high and viscosity is low in a supercritical aqueous mixture. Transport properties and miscibility are important parameters, which influence the rate of chemical reactions. High diffusion rates and low viscosity, together with the complete miscibility with many substances, make supercritical water an excellent medium for homogeneous, fast, and efficient reactions. In addition, SCW is an excellent reaction medium with heterogeneous catalysts, because the high diffusion rate avoids mass transfer limitations and efficient solubility prevents coke formation on, or poisoning of the catalyst. 

Third, the restriction associated with the mass action law was rmtil now used without consideration of kinetics as it was applied only to sufficiently fast reactions. However, in the interaction between water and rock participate reactions of various kinetics. Whereas the relaxation of homogeneous processes in water completes in hours or minutes, it is between water and rock, especially silicate ones, may last for years and decades. Simultaneously, in conditions of lowered temperature hypogene minerals only dissolve, hypergene ones dissolve or form, and the ground water composition continuously maintains thermodynamical equilibrium. In order to account for such kinetic variety of the mass transfer processes, they are tentatively subdivided into two groups reactions of irreversible mass transfer and reactions of instantaneous relaxation. 

Supercritical solvents have generated an increased interest in the last few decades. One reason is that their solvent properties vary considerable with temperature and density. They are tunable solvents [1] and for each purpose - separations or reactions - the optimal properties can be adjusted (see, for example, [1-8]). Usually, supercritical fluids are used as a tool to get homogeneous mixtures. In a homogeneous phase, for example, oxidations are extraordinarily fast and complete. The usually improved heat and mass transfer is a further advantage. Supercritical fluids show their good solvent properties only in the supercritical state. Therefore separation after reaction or extraction is very simply achieved by reducing temperature and pressure. This enables very sustainable processes (for example [1, 9]). Here supercritical carbon dioxide and water are of special interest, because they are cheap, nontoxic or of very low toxicity, in the case of carbon dioxide and nonexplosive. 

In mass-transfer-controlled systems in which extensive complexing or association takes place in the bulk phases, a proper mass transfer model must account for transport of all species. Otherwise, the transport model will not be consistent with a chemical model of phase equilibrium. For example. Fig. 8.4-4 indicates schematically the species concentration profiles established during the extraction of copper from ammonia-ammonium sulfate solution by a chelating agent such as LIX. In most such cases the reversible homogeneous reactions, like copper complexation by ammonia, will be fast and locally equilibrated. The method of Olandei can be applied in this case to compute individual species profiles and concentrations at the interfiice for use in an equilibrium or rate equation. This has been done in the rate analyses of several of the chloride and ammonia systems cited above. ... 

Now, considering the same reactions in microstructured reactors enables one to see the impact of a change in the hierarchy. Indeed, for dimensions below 1 mm, the conduction time is always lower than the reaction time for both slow and fast reactions. That indicates that heat transfer is always so fast that it allows operation without detrimental thermal effects. Moreover, since the heat-transfer time considered here is the conduction time, which is always larger than the convective heat-transfer time, still faster homogeneous reactions can be safely studied in these systems. Similar analysis can be performed by comparison of the reaction times with the mass-transfer time. 

The ratio of timescales for internal diffusion within the catalyst pellet and for external mass transfer is of the order of the mass Biot number given in (3.43e). Since for many practical conditions Bim is reasonably high (10 -10 as estimated in Ref. [72]), it is expected that the external mass transfer dominates over the internal one. If we extend this consideration to the reactor scale, then a very useful pseudo-homogeneous model [51] is obtained. The scaling condition that expresses fast fluid-solid mass transfer compared with other processes (X) in Equation 3.22 is... 

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