Selectivities in chemically enhanced absorption or extraction

Selectivities in chemically enhanced absorption or extraction The situation where reaction and diffusion take place simultaneously in a thin zone close to the interface in a two- phase system, is characterized by very steep concentration gradients. In fact, just as in the case of rapid reactions in mixed viscous media (section 5.2.3), reaction may take place in a zone where the ratio of the concentrations of both reactants varies from 0 to This may have a considerable influence on the selectivity, depending on the type of reactions that are taking place. 

When reactant A can be converted in the reaction phase to an unwanted byproduct via a competitive reaction, an excess of B throughout the liquid phase is desirable, so the situations depicted in figures 5.12a and b, should be avoided. If one should find that one of these situation prevails, one may attempt to increase the selectivity by lowering the A concentration in the supply phase (e.g., by applying a lower gas pressure), and increase the surface area proportionally. 

When an undesired consecutive reaction takes place, e.g., when the reaction product decomposes or reacts with the reactant B or with the solvent, high selectivities may be obtained by minimizing the volume of the liquid phase (e.g., by taking a ga quid packed column instead if a bubble column). This will not affect the main reaction, as long as it takes place entirely in a thin film close to the interface. 

It is obvious fiom figure 5.12 that the situations depicted under b and c would be most unfavourable for competitive-consecutive reactions, since the reactant A, upon entering the reaction phase, would encounter only P, so that a lot of the unwanted product X could be formed there, provided the undesired reaction were sufficiently rapid (which they usually are in the examples given above). This can be avoided by using a sufficiently high ratio of cjc,  

Another interesting selectivity problem arises when there are two different reactants in the supply phase, say A and C, that both react with the liquid phase reactant B, forming P and Q respectively, where the formation of Q is undesired. An example of practical importance is the selective absorption of hydrogen sulfide from an inert gas containing also carbon dioxide, in an alkaline solution (containing, e.g., alkanol amines). Conditions can be such that carbon dioxide (C) reacts rapidly with the alkanol amine, whereas hydrogen sulfide (A) reacts instantaneously. The consequence is that the absorption rate of hydrogen sulfide is practically determined by the gas phase mass transfer rate, and Ae rate of carbon 

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