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

Kleingeld, Anton W. (1999-12)

Thesis (M.Ing.) -- University of Stellenbosch, 1999.

Thesis

ENGLISH SUMMARY: In the majority of gas-liquid contacting systems the kinetics of the heterogeneous chemical reaction is not limited by its intrinsic reaction rate, but by the transport of gas to the liquid phase and hence by the overall interfacial area available for mass transfer. These rates in turn limit productivity and are thus a critical design consideration. In view of this, novel high intensity impinging stream reactors have been developed at this institution for intensification of these mass transfer processes. The reactors are characterised by small reactor volumes supplied with nozzles, which are directed towards one another. The gas and liquid feed streams are jetted through the nozzles into the reactor volume, resulting in a highly 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 reactor configurations investigated showed that the mass transfer coefficient (kL) could be enhanced substantially by centrifugal acceleration of the fluid and more efficient promotion of turbulence in the round reactor chambers of the n and ó-shaped reactors, compared to that of the kite-shaped reactor. However, it was also found that the jagged/angular reactor chamber of the kite-shaped reactor exhibited higher values of the interfacial area (a) due to more effective bubble break-up mechanisms and higher relative gas hold-ups. It was therefore concluded that an optimum reactor design would combine the kL-enhancing effects of the swirling flow in the a-shaped reactor, with the bubble break-up and gas hold-up ability of the kite-shaped reactor. Comparison of experimental results with literature· data for conventional systems also revealed that, in terms of both the mass transfer coefficient and the value of the interfacial area per unit of energy dissipated in the reactor, the proposed reactors provide a significant improvement in mass transfer performance. It is thus suggested that the newly developed impinging stream reactors have the potential to represent superior alternatives to conventional gas-liquid contacting equipment. The fundamental model for the prediction of interfacial area production in the jet reactors originally proposed by Botes (1995) was also improved and expanded, resulting in more accurate prediction of trends in interfacial area as a function of various process variables. The model, and its associated bubble breakage mechanism, was verified at the hand of additional absorption data and alternative bubble breakage mechanisms proposed in the literature. Very good results were obtained, so that it could be concluded that the model is very flexible and can be applied over a relatively wide range of hydrodynamic operating conditions, validating the potential application thereof in other turbulent gas-liquid systems. Considering the above conclusions, recommendations could finally be made as to how the performance of the reactors could be improved further: liquid nozzles with larger orifices should be used for optimisation of the energy efficiency of the reactors. The use of gas nozzles with smaller orifices would additionally result in higher linear gas velocities, improving the efficiency of impingement of gas into liquid streams.

AFRIKAANSE OPSOMMING: In die meerderheid gas-vloeistof kontaksisteme word die kinetika van die heterogene chemiese reaksie nie be perk deur die intrinsieke reaksietempo nie, maar deur die vervoer van gas na die vloeistoffase en derhalwe deur die totale interfase area beskikbaar vir massa-oordrag. Hierdie tempos beinvloed op hul beurt weer produktiwiteit en is dus 'n kritieke ontwerpsoorweging. In die lig hiervan is nuwe hoe intensitiet spuitreaktore by hierdie instituut ontwikkel vir die intensifisering van hierdie massa oordragsprosesse. Die reaktore word gekaraktiseer deur klein reaktorvolumes wat toegerus is met spuitstukke wat opponerend ten opsigte van mekaar georienteer is. Die gas en vloeistof voerstrome word deur die spuitstukke in die reaktorvolume ingespuit, met die gevolg dat daar 'n hoogs turbulente twee-fase mengsel ontstaan. Onder sulke turbulente vermengingskondisies word die tempo van massa oordrag aansienlik verhoog. 'n Evaluasie van die massa-oordragsparameters wat deur die drie verskillende reaktor konfigurasies vertoon is, het getoon dat die massaoordragskoeffisient (kL) aansienlik verbeter kan word deur sentrifugale versnelling van die vloeier en meer effektiewe bevordering van turbulensie in die ronde reaktor volume van die n- en ó vormige reaktore, in vergelyking met die vlieer-vormige reaktor. Daar is egter ook bevind dat die hoekige vorm van die vlieer-vormige reaktor hoer waardes vir die interfase area (a) vertoon deur meer effektiewe borrel-breek meganismes en hoer relatiewe gas retensie volumes. Derhalwe is die gevolgtrekking gemaak dat die optimale reaktorontwerp die kL-verbeteringsvermoe van die malende vloei in die ó vormige reaktor en die borrel-breek en gas-retensie vermoe van die vlieervormige reaktor sal kombineer. Vergelyking van eksperimentele resultate met data uit die literatuur vir konvensionele sisteme het verder getoon dat, in terme van beide die massa-oordragskoeffisient en die interfase area per eenheid energie verbruik, die voorgestelde reaktore en aansienlike verbetering in massa oordragsvermoe bied. Dit word derhalwe voorgestel dat die nuwe spuitreaktore die potensiaal besit om uitstaande alternatiewe vir gas-vloeistof kontak te verteenwoordig. Die fundamentele model vir die voorspelling van die interfase area produksie in die spuitreaktore soos voorgestel deur Bates (1995) is ook verbeter en uitgebou. So is meer akkurate voorspelling van die tendense in interfase area as funksie van verskillende prosesveranderlikes verkry. Die model, en sy geassosieerde borrel-breek meganisme, is ook geverifieer deur middel van addisionele absorpsie data en alternatiewe borrel-breek meganismes soos voorgestel in die literatuur. Baie goeie resultate is verkry, sodat die gevolgtrekking gemaak kan word dat die model baie buigbaar oor en redelike wye gebied van hidrodinamiese bedryfstoestande is. Die potensiele toepassing daarvan in ander turbulente gas-vloeistof kontaksisteme is derhalwe gestaaf. Met inagneming van die bogenoemde gevolgtrekkings, kon voorstelle ook gemaak word aangaande die verbetering van reaktor werkverrigting: vloeistof spuitstukke met groter openinge kan gebruik word vir optimering van die energy-effektiwiteit van die reaktore. Verder kan die gebruik van gas spuitstukke met kleiner openinge gebruik word vir hoer lineere gas snelhede, wat die penetrasie van gas in vloeistof strome sal verbeter.

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