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Mass Transfer and Chemical Reaction in Gas-Liquid-Liquid Systems

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Mass Transfer and Chemical Reaction in Gas-Liquid-Liquid Systems MASS TRANSFER AND CHEMICAL REACTION IN GAS-LIQUID-LIQUID SYSTEMS PROEFSCHRIFT ter verkrijging van de graad van doctor aan de Universiteit Twente, op gezag van de rector magnificus, Prof.dr. F.A. van Vught, volgens besluit van het College voor Promoties in het openbaar te verdedigen op vrijdag 4 december 1998 te 16.45 uur door Derk Willem Frederik Brilman geboren op 19 februari 1968 te Meppel Dit proefschrift is goedgekeurd door de promotoren Prof. dr. ir. G.F. Versteeg en Prof. dr. ir. W.P.M. van Swaaij aan Sandra, aan mijn ouders Referent: dr.ir. L. Petrus The research in this thesis was financially supported by the Shell Research and Technology Centre in Amsterdam, The Netherlands. Cover © 1998 I.B. Kooijman, S. Nolles, D.W.F. Brilman Copyright © 1998 D.W.F. Brilman, Hengelo, The Netherlands No part of this book may be reproduced in any form by any means, nor transmitted, nor translated into a machine language without written permission from the author Brilman, D.W.F. Mass Transfer and Chemical Reaction in Gas-Liquid-Liquid Systems Thesis University of Twente, The Netherlands ISBN 90-36512212 Printing and binding: Print Partners Ipskamp, Enschede, The Netherlands Summary Gas-liquid-liquid reaction systems may be encountered in several important fields of application as e.g. hydroformylation, alkylation, carboxylation, polymerisation, hydrometallurgy, biochemical processes and fine chemicals manufacturing. However, the reaction engineering aspects of these systems have only been considered occasionally. For systems with very slow reaction kinetics this is not surprising, as the three phases will be at physical equilibrium. In reaction systems with fast parallel and consecutive reactions the effects of mass transfer and mixing on the product yield can be significant. A fascinating example of such a reaction system is the Koch synthesis of Pivalic Acid. In this work this reaction system was chosen as model system to study these effects. In the Koch reaction system the reactants isobutene (or equivalent carbocation precursor), carbon monoxide and water originate each from different phases. They react in the presence of an acidic catalyst in the aqueous sulfuric acid catalyst phase to form Pivalic Acid. Additional to this main reaction isomerisation, oligomerisation and disproportionation can take place, which can lead to a spectacular diversity of byproducts. For analysing the effects of mass transfer and mixing in this complex reaction system the reaction kinetics need to be known and were therefore studied. The first step in the Koch reaction mechanism is the formation of a carbocation. For isobutene (and trans-2-butene) the protonation kinetics were determined by gas absorption experiments using a stirred cell. The sulfuric acid concentration was found to have a pronounced influence on the reaction kinetics (and on the solubility of the hydrocarbons), which could be described with an activity based reaction rate equation using the Hammett Acidity Function for the protonating activity of the catalyst solution. For isobutanol as reactant the Koch synthesis of Pivalic Acid could be followed through the carbon monoxide consumption rate. Using this, the effect of the presence of Pivalic Acid in the catalyst solution has been studied. It is shown that the addition of Pivalic Acid to the sulfuric acid catalyst solution drastically reduces the catalyst solution acidity. This implies for the industrially frequently applied completely backmixed reactors that they operate at a considerable lower acid strength than may be expected by the initial catalyst composition. Further, the apparent reaction kinetics at different temperature, pressure and acid strength were determined. A simple equilibrium model for the four basic reaction steps in the Koch reaction mechanism was able to I Summary describe the sharp increase in the Pivalic Acid solubility in the sulfuric acid catalyst solution without using fit parameters . It has been reported frequently that the addition of an immiscible organic liquid phase to a gas - aqueous liquid phase system may significantly increase the gas absorption rate. The effects reported in literature on the mass transfer parameters, like the gas-liquid interfacial area and gas holdup, are discussed short. No general trends can be derived from the reported results in literature. Additional research in this area is therefore desired. However, these effects are insufficient to account for the observed enhancement of the gas absorption rate. Direct contact of the gas phase and the dispersed organic phase may offer one explanation for the gas absorption enhancement. Scouting experiments have shown the existence of ‘complexes’ of gas bubbles and organic phase drops, irrespective of the initial spreading coefficient for this system. A criterion for the possible formation of these complexes based on a surface energy consideration is proposed. Alternatively, analogous to gas-liquid-solid systems, the presence of small droplets in the mass transfer zone at the gas-liquid interface may increase the gas absorption rate through the ‘grazing’ or ‘shuttle mechanism’. In this work instationary, heterogeneous, multiparticle mass transfer models (1, 2 and 3-Dimensional) have been developed, which were not available in literature. For the instationary heterogeneous multiparticle models developed in this work the effect of different process parameters was studied. Special attention is given to the translation of modeling results into the prediction of gas absorption fluxes, which is required due to the infinite number of particle configurations possible in a multiparticle system. A straightforward strategy is proposed to obtain gas absorption fluxes from single particle simulations. The modeling results obtained by this strategy and mass transfer models are compared to experimental data reported in literature. Turbulence modification by the presence of dispersed phase particles has been reported frequently in literature, especially for gas-solid systems. The extent of these effects in a stirred gas-liquid- liquid multiphase reactor was, however, not clear. Using the well known mixing sensitive diazo- coupling reaction, the influences of the addition of solid particles, gas bubbles and organic liquid drops on the product distribution was investigated. For this reaction system it was found that the experimental results could be described by the Engulfment model, when implementing effective dispersion density and -viscosity. For the liquid-liquid experiments it was found that mass transfer effects caused a de-localisation of the reaction zone. The Engulfment micromixing model II Summary extended with liquid-liquid mass transfer for the bulk liquid phases described reasonably well the experimental results. For tert-butanol and isobutene the Koch synthesis of Pivalic Acid proceeded too fast to follow via the CO consumption rate and a strong influence of the hydrocarbon reactant feed rate on the product yield was observed. This feed rate effect could be attributed to the gas-liquid mass transfer rate. Moreover, saturating the reactant feed (before injection) with carbon monoxide has a beneficial effect on the product yield. Transport of CO to the reaction zone, located near the feed inlet, is therefore the main parameter in determining the product yield. At the same time, it is concluded for the reaction conditions studied (a 96 wt% H2SO4 catalyst solution, 293 K, 40 bar CO pressure) that both the carbonylation reaction and the undesired oligomerisation reactions are fast reactions with respect to gas-liquid mass transfer and mixing. The effect of an immiscible, inert, organic liquid phase on the product distribution was studied for tert-butanol as reactant. A considerable improvement of the product yield was observed, especially at lower stirring rates. This can, at least partially, be explained from an enlarged CO absorption capacity at increasing dispersed phase fraction. Nevertheless, even at conditions in which the initial CO absorption capacity of the liquid-liquid in the semi-batch system is sufficient for complete conversion of the tert-butanol reactant injected dispersion, the conversion to acids remains limited. Insufficient local mixing near the feed inlet and chemical reaction equilibria will play a role in these cases. Additional experimental work and modeling is, however, required to test if the combination of a (micro-)mixing model with the kinetic rate expressions and the equilibrium model obtained in this study successfully describes the reaction products obtained for the Koch synthesis of Pivalic Acid from isobutene at various conditions in an agitated multiphase reactor. III Summary IV Samenvatting Gas-vloeistof-vloeistof meerfasen-systemen worden in diverse toepassingsvelden binnen de procestechnologie aangetroffen, zoals bijvoorbeeld in processen gebaseerd op hydroformylerings-, alkylerings-, carboxylering- en polymerisatie reakties, hydrometallurgische en biochemische processen en in de fabricage van enkele fijnchemicaliën. De specifieke reactorkundige aspecten van dit type reaktiesystemen zijn echter slechts nauwlijks bestudeerd. Wanneer de optredende chemische reacties langzaam verlopen in vergelijking met de stofoverdracht tussen de verschillende fasen, kunnen de fasen op fysisch evenwicht worden verondersteld en vereist de aanwezigheid van drie verschillende fasen geen nadere bestudering. Echter, in het geval van een complex netwerk van snelle, deels irreversibele, chemische reacties zullen aspecten als stofoverdracht
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