The development and modelling of jet reactors
Date
1995
Authors
Journal Title
Journal ISSN
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Publisher
Stellenbosch : Stellenbosch University
Abstract
ENGLISH ABSTRACT: A novel type of gas-liquid contactor was researched and developed to enhance phase mixing. These high velocity impinging stream reactors are characterised by small reactor volumes supplied with nozzles, which are directed towards each other. The gas and liquid feed streams are jetted through the nozzles into the reactor volume, resulting in a highly turbulent mixture of the phases. Under these enhanced mixing conditions, mass transfer rated are increased dramatically. A literature survey revealed that jet reactors have been developed towards a type of phase contacting device where impingement of type feed streams is increasingly used to enhance interfacial area production and mass transfer. This means that the kinetic energy of the feed streams is used to break up bubbles and droplets, and o create intense turbulence (which promotes convective mass transfer). The recently developed jet reactors differ from previous jet-type reactors in the respect that higher flow rates are used and that the nozzles are situated closer to each other. Consequently, the feed streams impinge on each other at very high velocities and cause more intense mixing than can be achieved in any previous phase con actor. The recently developed jet reactors were investigated by absorbing carbon dioxide gas into sodium hydroxide solutions. A new chemical technique was applied into determine the values of the mass transfer coefficient (kL) and the interfacial area (a), respectively, from the absorption data. The efficiency of the new jet reactors was confirmed by the fact that the value of the combined mass transfer coefficient (k a) was more than an order of magnitude higher for the new reactors than for any other type of mixing device. The investigation showed that the positioning of the nozzles, relative to each other, is the most crucial design aspect of the jet reactors. It was also found that the geometry of the mixing chamber does not have a significant: influence on the absorption efficiency of the reactors, as long as the following two criter1a a1e satisfied: the geometry must allow for the desired nozzle arrangement, and there must be no plug flow regions in the reactor. The literature survey also showed that: a practical, fundamental modelling approach have never been developed for any jet-type reactor. A fundamental mechanism of interfacial area production in the new jet reactors, was proposed in this thesis. The model is based on the fact that bubble breakup in a turbulent environment is governed by the interactions of bubbles with turbulent eddies (existing in the liquid phase). The model was implemented in the form of a Monte Carlo simulation, which proved to be a highly practical and flexible mathematical procedure for this situation. The simulation satisfactorily predicted the trends in the experimental data. Furthermore, a proposal was made on how the current model can be expanded to also predict the value of the mass transfer coefficient. If this can be achieved successfully, it would be the first practical, fundamental method to model the value of the mass transfer coefficient in a turbulent environment.
AFRIKAANSE OPSOMMING: 'n Nuwe tipe fasekotanktor is ontwikkel om beter vermenging tussen gasse en vloeistowwe te bewerkstellig. hierdie hoë-snelheid spuitreaktore word gekarakteriseer deur klein raktor-volumes, wat toegerus is met opponerende spuitstukke. Die gas- en vloeistofstrome 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.
AFRIKAANSE OPSOMMING: 'n Nuwe tipe fasekotanktor is ontwikkel om beter vermenging tussen gasse en vloeistowwe te bewerkstellig. hierdie hoë-snelheid spuitreaktore word gekarakteriseer deur klein raktor-volumes, wat toegerus is met opponerende spuitstukke. Die gas- en vloeistofstrome 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.
Description
Thesis (MIng) -- University of Stellenbosch, 1995.