Turbulent mixing and residence time distribution in novel multifunctional heat exchangers–reactors

Abstract

Multifunctional heat exchanger–reactors show significant promise in increasing the energy efficiency of industrial chemical processes. The performance of these systems is conditioned by flow properties and is strongly geometry dependent. Here CFD simulation and laser Doppler anemometry (LDA) measurements are used to investigate the redistributing effects of the longitudinal vorticity generated by rows of inclined trapezoidal tabs on turbulent mixing in static mixers. Studies are carried out on three different configurations: in the first, the tabs are aligned and inclined in the direction of flow (the reference geometry for a high-efficiency vortex (HEV) static mixer), in the second, a periodic 45° tangential rotation is applied to the tab arrays with respect to one another, and in the third the reference geometry is used in the direction opposite to the flow direction (reversed direction). The mixing efficiency, taken as the resultant of the momentum-transfer efficiency of the “mean” flow at different scales, is studied. Macro-mixing entails the dispersive capacity of the flow at the heat exchanger–reactor scale, and is generally measured by the residence time distribution (RTD). At the intermediate scale, meso-mixing is governed by the turbulent fluctuations; this process of turbulent mixing can be characterized by the turbulence kinetic energy (TKE). Micro-mixing is characterized by the local rate of turbulence energy dissipation and is related to the progress of fast chemical reactions and selectivity. It is shown here that the reversed-array arrangement (the third configuration) provides the best performance in micro- (50%) and meso-mixing (25%), but exhibits an approximately 40% increase in power consumption over the classical HEV (reference) geometry and somewhat pronounced bimodal behavior in the RTD.