In any form of continuous reactor the different reagent molecules spend different residence times in the reacting zone. The degree to which a reaction advances after a lapse of nominal residence time may be somewhat different from that expected from classical (micro) kinetics and the known order of the reaction. If the reaction mixture passes laminarly through a reasonably large cylindrical reactor-one in which intermolecular diffusion is negligible-the effective measure of the advancement of the reaction in terms of the residual concentration of the reagent - measured (i) as an actual distributed system C and (ii) as an idealized system in which all molecules spend an equal tlme of residence Cm. may be expressed by the equation: Equation (29) where n is order of reaction, S=knC0n-1τ0, and τ0 is the minimal time of residence (time for the central core). Equation (29) becomes meaningless when n=1 and is replaced by Equation (30) The ratio C/Cm increases as S increases; the increase, however, is the less pronounced the higher the value of n. If the mixture passes through a mixed tank reactor advancement of the degree of the reaction is measured by a function given by Metzner and Pigford. For a second order of reaction a correct treatment can only be carried out if a distinction is made between the square on the mean concentration and the mean of the square concentration.