The development and modelling of high intensity impinging stream jet reactors for enhanced mass transfer in gas-liquid systems

[摘要] ENGLISH SUMMARY: In the majority of gas-liquid contacting systems the kinetics of theheterogeneous chemical reaction is not limited by its intrinsic reaction rate,but by the transport of gas to the liquid phase and hence by the overallinterfacial area available for mass transfer. These rates in turn limitproductivity and are thus a critical design consideration. In view of this, novelhigh intensity impinging stream reactors have been developed at thisinstitution for intensification of these mass transfer processes.The reactors are characterised by small reactor volumes supplied withnozzles, which are directed towards one another. The gas and liquid feedstreams are jetted through the nozzles into the reactor volume, resulting in ahighly turbulent mixture of phases. Under these enhanced mixing conditions,mass transfer rates are increased dramatically.Evaluation of mass transfer parameters exhibited by the three different reactorconfigurations investigated showed that the mass transfer coefficient (kL)could be enhanced substantially by centrifugal acceleration of the fluid andmore efficient promotion of turbulence in the round reactor chambers of the n andó-shaped reactors, compared to that of the kite-shaped reactor. However,it was also found that the jagged/angular reactor chamber of the kite-shapedreactor exhibited higher values of the interfacial area (a) due to more effectivebubble break-up mechanisms and higher relative gas hold-ups. It wastherefore concluded that an optimum reactor design would combine the kL-enhancingeffects of the swirling flow in the a-shaped reactor, with the bubblebreak-up and gas hold-up ability of the kite-shaped reactor.Comparison of experimental results with literature· data for conventionalsystems also revealed that, in terms of both the mass transfer coefficient andthe value of the interfacial area per unit of energy dissipated in the reactor, theproposed reactors provide a significant improvement in mass transfer performance. It is thus suggested that the newly developed impinging streamreactors have the potential to represent superior alternatives to conventionalgas-liquid contacting equipment.The fundamental model for the prediction of interfacial area production in thejet reactors originally proposed by Botes (1995) was also improved andexpanded, resulting in more accurate prediction of trends in interfacial area asa function of various process variables.The model, and its associated bubble breakage mechanism, was verified atthe hand of additional absorption data and alternative bubble breakagemechanisms proposed in the literature. Very good results were obtained, sothat it could be concluded that the model is very flexible and can be appliedover a relatively wide range of hydrodynamic operating conditions, validatingthe potential application thereof in other turbulent gas-liquid systems.Considering the above conclusions, recommendations could finally be madeas to how the performance of the reactors could be improved further: liquidnozzles with larger orifices should be used for optimisation of the energyefficiency of the reactors. The use of gas nozzles with smaller orifices wouldadditionally result in higher linear gas velocities, improving the efficiency ofimpingement of gas into liquid streams.

[发布机构] Stellenbosch University