Mass transfer with fast homogeneous reaction

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. 

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. 

In this chapter we have shown that the mechanism and kinetics of a chemical reaction scheme can be combined with a rather detailed fine scale picture of the fluid dynamics in a simple reactor to predict yield and selectivity for fast homogeneous reactions. With a lot more information we can predict some of the reaction effects when another phase is present and interphase mass transfer becomes important. Heterogeneous analysis is severely limited because our... 

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. 

In contrast to homogeneous single-phase reactions (Section 4.4), we now have to consider that the effective achievable reaction rate may be influenced by mass transfer steps to and within both phases. In the following, we derive first the equations for mass transport at a gas-liquid interface (Section 4.4.1). Then we discuss the interplay of mass transport both with a slow reaction (Section 4.4.2) and with a fast reaction (Section 4.4.3). 

In these electrode processes, the use of macroelectrodes is recommended when the homogeneous kinetics is slow in order to achieve a commitment between the diffusive and chemical rates. When the chemical kinetics is very fast with respect to the mass transport and macroelectrodes are employed, the electrochemical response is insensitive to the homogeneous kinetics of the chemical reactions—except for first-order catalytic reactions and irreversible chemical reactions follow up the electron transfer—because the reaction layer becomes negligible compared with the diffusion layer. Under the above conditions, the equilibria behave as fully labile and it can be supposed that they are maintained at any point in the solution at any time and at any applied potential pulse. This means an independent of time (stationary) response cannot be obtained at planar electrodes except in the case of a first-order catalytic mechanism. Under these conditions, the use of microelectrodes is recommended to determine large rate constants. However, there is a range of microelectrode radii with which a kinetic-dependent stationary response is obtained beyond the upper limit, a transient response is recorded, whereas beyond the lower limit, the steady-state response is insensitive to the chemical kinetics because the kinetic contribution is masked by the diffusion mass transport. In the case of spherical microelectrodes, the lower limit corresponds to the situation where the reaction layer thickness does not exceed 80 % of the diffusion layer thickness. 

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