EXPERIMENTAL-STUDY OF A BIMOLECULAR REACTION IN POISEUILLE FLOW

A bimolecular chemical reaction (reactant1 + reactant2 --> products) i n laminar Poiseuille flow is experimentally observed using a spectroph otometer. The reaction rate (r(m)) follows the second-order rate law; that is, r(m) = kappa c(1)c(2), where c(m) (m = 1, 2) are the reactant concentrations. The reaction rate constant kappa is independently est imated by monitoring the reaction kinetics in a completely mixed batch reactor using the stopped-flow technique. In the reactive transport e xperiments, the reactants are introduced in a tube and are initially s eparated by a sharp interface. The variation of the fluid velocity ove r the cross section of the tube causes the concentrations of the react ants to vary around their cross-sectional average values ((c) over bar (m)). These spatial variations in the concentrations (c(m)') influence the overall reaction rate. The cross-sectional average reaction rate is given by (r) over bar(m) = kappa<(c(1)c(2))over bar> = kappa((c) ov er bar(1)(c) over bar(2) + <(c(1)c(2))over bar>) = kappa(1 + s)(c) ove r bar(1)(c) over bar(2) where s = <(c(1)'c(2)')over bar>/((c) over bar (1)(c) over bar(2)) is the segregation intensity. The experimentally o bserved breakthrough concentration of the product is in agreement with a numerical model that accounts for the effects of the segregation in tensity. On ignoring the influence of the segregation intensity, the p redicted product concentration substantially exceeds the experimental observations. This shows that for initially non-overlapping reactants the segregation intensity is negative (s < 0) and that the overall che mical transformation rate in flowing systems can be significantly diff erent from that implied by substituting the mean concentrations in the expression for the reaction rate.