Reaction, flow and diffusion

Generally, contact catalyses are carried out in flow systems, because these arrangements make the best use of the property of catalysts to act upon successive amounts of the unreacted substances. But in this case, the time (concentration) function of the observed overall reaction 

in principle, the question of mixing by diffusion can be decided. 

But it should be borne in mind that this longitudinal mixing, as can be shown by the kinetic theory of gases, plays no important role at reactor lengths of at least several centimeters and at linear flow rates of at least some centimeters per minute. These conditions will certainly be fulfilled in most of the usual laboratory work, using flow rates of many cubic centimeters per minute and tube cross sections not exceeding a few square centimeters. 

Another question is whether a concentration gradient may occur perpendicular to the direction of the flow of the gases, especially between such layers of the gas near the surface which are deprived of reactant by the surface reaction and the richer free gas volume. However, according to an estimate of Damkohler (1), diffusion is sufficiently fast in a normal laboratory reactor to minimize such differences even in an empty space 1 cm. in width no larger concentration differences are to be expected than about 1 %. 

Regardless of such diffusion effects, the flow rate may act as the limiting factor of the reaction velocity. Although this can easily be detected, it has sometimes led to serious errors. Cases of comparable flow and reaction rates have been treated by Schwab and Drikos (6). 

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Soil reactions are generally classified according to the nature of the main chemical process involved adsorption, ion exchange, dissolution, etc. However, in order to assess the kinetics one should consider the nature and the rate of the transport processes associated with the chemical reaction flow and diffusion in the soil solution, transport across the solid-liquid interface, diffusion in liquid-filled pores and micropores, and surface diffusion penetration into the solid. An expression for the kinetics of soil reactions can be devised by assigning rate equations to transport and chemical processes and combining these equations. The expression finally obtained has to be validated by comparison to experimental results. 

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