Mass transfer with a chemical reaction of the first order. Effect of axial diffusion

The role of diffusion in the direction of convection transport, which is usually neglected in mass (heat) transfer problems, is systematically investigated. Analytical solutions and asymptotic expressions are derived for cases of mass transfer coupled with a homogeneous and heterogeneous irreversible chemical reaction of the first order in plug flow, in Couette flow and in Couette flow with a moving interface. Irrespective of the flow situation, the effect of axial diffusion is of importance for small transformed distances in the direction of the convective transport. Then, the local mass transfer co-efficient is a square of the coefficient which is evaluated without the axial diffusion term. As expected, with an increase in the transformed distance both mass transfer coefficients tend to the same value. In the case of heterogeneous (surface) chemical reactions of the first order, the diffusion in the direction of convective transport is of minor importance. The same is observed when the case of simultaneous homogeneous and heterogeneous chemical reactions of the first order in uniform flow is considered.