A FLUID MECHANICAL APPROACH TO TURBULENT MIXING AND CHEMICAL REACTION PART I INADEQUACIES OF AVAILABLE METHODS

The series treats the mixing on the molecular scale (micromixing) of two miscible streams to bring about reaction between their constituents. The case where mixing and reaction have similar time constants is particularly interesting, although the framework is also valid for slow and instantaneous reactions, The case of fast reactions is important because no generally valid modelling is yet available, although examples of such reactions are now known. Two competitive, consecutive reactions are analysed in detail, where the yield of the intermediate product falls rapidly as inhomogeneity and concentration gradients on the molecular scale increase. Part I shows that no existing formulation of micromixing is sufficiently general. The model of lamellar structures (or double slab model) excludes mass exchange between a flow element and its environment and thus, applied to a continuous reactor, permits only a description of segregated flow i.e. other flow situations are incorrectly described, as is also illustrated with examples. A diffusion-reaction model, originating from this laboratory, artificially confined one of the reagents to a single slab, where all diffusion and reaction steps were assumed to occur. This is too restrictive and can lead to errors, which are again illustrated with examples. Both approaches concentrate on the final, diffusive stage of mixing. They overlook the preceding convective steps and are useless for estimating the mixing time. Again examples are given.