THE EFFECT OF MICROMIXING ON PARALLEL REACTIONS

The literature gives little guidance on the factors (e.g. choice of reactor, sequence of adding reagents, imperfect mixing) determining the product distribution of parallel reactors. By considering two irreversible, second-order reactions between three substances, it is shown here how the sequence of adding reagents to a semi-batch reactor will influence the yield when the mixing is perfect as well as when segregation is complete. A mixing model is then applied to predict yields when segregation is partial. Such calculations were made for three mixing sequences when chemically equivalent quantities of the three reagents were employed and when one of the two reactions was instantaneous. The yield then depended upon the mixing sequence, the volume ratio of reagent solutions and the Damköhler number (this is a ratio of characteristic times for micromixing and chemical reaction). Sodium hydroxide, ferric sulphate and ethyl chloroacetate solutions were contacted in the sequences corresponding to cases 1 and 3 of the modelling. The alkali reacted competitively and the extents of the two parallel reactions were measured after it had been fully consumed. The independent variables were stirrer speed, volume ratio of the two reagent solutions and the concentration level at constant stoichiometric ratio. The effects of these operating variables on the product distribution were well predicted. The stoichiometry of the precipitation of ferric ions in alkaline solution is probably more complex than considered in the modelling and the parallel reactions used here need improvement or possibly replacement. A similar comparison was made between measured and predicted yields of nitrobenzene during the competitive nitration of benzene and toluene. Twelve out of 13 yields measured by Tolgyesi (1965) were satisfactorily predicted. © 1990.