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University of Groningen Gas Absorption in an Agitated Gas-Liquid University of Groningen Gas absorption in an agitated gas-liquid-liquid system Cents, A.H.G.; Brilman, D.W.F.; Versteeg, Geert Published in: Chemical Engineering Science DOI: 10.1016/S0009-2509(00)00324-9 IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from it. Please check the document version below. Document Version Publisher's PDF, also known as Version of record Publication date: 2001 Link to publication in University of Groningen/UMCG research database Citation for published version (APA): Cents, A. H. G., Brilman, D. W. F., & Versteeg, G. F. (2001). Gas absorption in an agitated gas-liquid-liquid system. Chemical Engineering Science, 56(3), 1075-1083. DOI: 10.1016/S0009-2509(00)00324-9 Copyright Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons). Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the number of authors shown on this cover page is limited to 10 maximum. Download date: 11-02-2018 Chemical Engineering Science 56 (2001) 1075}1083 Gas absorption in an agitated gas}liquid}liquid system A. H. G. Cents*, D. W. F. Brilman, G. F. Versteeg Department of Chemical Engineering, University of Twente, P.O. Box 217, 7500 AE Enschede, Netherlands Abstract Gas}liquid}liquid systems have gained interest in the past decade and are encountered in several important industrial applications. In these systems an immiscible liquid phase may a!ect the gas absorption rate signi"cantly. This phenomenon, however, is not completely understood and underlying mechanisms need further study. In this work the well-known Danckwerts-plot technique is used to determine the liquid side mass transfer coe$cient k* and the gas}liquid interfacial area a simultaneously in this type of systems. As absorption/reaction system CO absorption in a 0.5 M KCO/0.5 M KHCO bu!er solution catalysed by sodium hypochlorite was chosen. Toluene, n-dodecane, n-heptane and 1-octanol were applied as dispersed liquid phases. The Danckwerts- plot could be well used in gas}liquid}liquid systems and from the results it appeared that two types of systems exist; systems that enhance mass transfer and systems that do not enhance mass transfer. E!ects at low dispersed phase hold-up were observed to be very strong and are thus important, but were not taken into account in further analysis of the e!ect of dispersed phase hold-up on mass transfer. In systems where dodecane and heptane were added to the bu!er solutions no enhancement of mass transfer was observed. However, the addition of toluene and 1-octanol caused an enhancement of mass transfer that could be well described using a homogeneous model of the shuttle mechanism. 2001 Elsevier Science Ltd. All rights reserved. Keywords: Mass transfer; Enhancement; Gas-liquid-liquid; Danckwerts-plot 1. Introduction (Falbe, 1980). Gas}liquid}liquid systems are also encoun- tered in areas in which an additional inert liquid phase is Gas}liquid}liquid systems have gained interest in the added on purpose to a gas}liquid system to increase the past decade due to the introduction of homogeneous mass transfer rate. This latter concept is e.g. applied in biphasic catalysis in various reaction systems, e.g. hydro- a few biochemical applications (Rols, Condoret, Fonade formylation, carbonylation, hydrogenation and & Goma, 1990). However, the addition of a second liquid oligomerization (Cornils, 1999). The main advantage of phase can also retard the gas}liquid mass transfer these systems over catalysis in one phase is the easy (Yoshida, Yamane & Miyamoto, 1970). separation of the catalyst and the reactants or products. In this latter perspective, especially when considering In this way advantages of homogeneous catalysis (no that the gas absorption rate is frequently the capacity internal mass transfer limitations and often higher selec- determining factor, it is very important to understand the tivities) and of heterogeneous catalysis (easy catalyst phenomena occurring in these systems for design and separation) can be combined. Important industrial operation of these systems. applications are, e.g., the SHOP-process and hydro- The gas absorption rate in stirred multiphase reactors formylation of propene to butyraldehyde. is usually characterised by the volumetric mass transfer Gas}liquid}liquid systems are further encountered in coe$cient k*a. In this coe$cient the liquid side mass reaction systems which inherently consist of three phases transfer coe$cient k* as well as the interfacial area a are due to two (or more) immiscible reactants, reaction prod- discounted. ucts or catalyst. For example, in the Koch reaction sys- To study the underlying phenomena of the e!ect of tem all three reactants originate from di!erent phases adding an immiscible liquid phase on the gas absorption rate for a gas}liquid system, it is considered necessary to separate the e!ects on the interfacial area a from those on the liquid side mass transfer coe$cient k*. In this work * Corresponding author. the Danckwerts-plot technique to measure k* and a si- E-mail address: [email protected] (A. H. G. Cents). multaneously by chemical absorption is used to study the 0009-2509/01/$- see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 0 9 - 2 5 0 9 ( 0 0 ) 0 0 3 2 4 - 9 1076 A. H. G. Cents et al. / Chemical Engineering Science 56 (2001) 1075}1083 e!ects of the addition of di!erent immiscible organic Bruining, Joosten, Beenackers and Hofman (1986). They phases. In this work four di!erent gas}liquid}liquid sys- tested the model using oxygen absorption in aqueous tems will be studied. solutions using n-hexadecane as a dispersed phase. Brilman, Goldschmidt, Versteeg and van Swaaij (2000) recently developed 3-D heterogeneous mass transfer 2. Previous work models to study this grazing e!ect in more detail. From their overview of all models describing these e!ects, it can One of the "rst encounters in literature on be concluded that, although these heterogeneous models gas}liquid}liquid systems is the work of Yoshida et al. are to be preferred from a physical point of view, the (1970). In their work the oxygen absorption rate was homogeneous models describe the experimental data measured in aqueous dispersions of a.o. kerosene and equally well for simple cases and require less detailed toluene. The observed e!ects on the addition of these input parameters. dispersed phases were very di!erent. The addition of The second mechanism proposed to describe the mass kerosene resulted in a decrease of the volumetric mass transfer enhancement is the so-called coalescence}redis- transfer coe$cient k*a. On the other hand, toluene persion mechanism, which is based on direct contact caused, after a sharp initial decrease, a strong increase in between gas and the dispersed phase by formation of the mass transfer rate at higher hold-ups. gas}liquid complexes (Rols et al., 1990). The gas absorp- Linek and Benes\(1976) studied gas absorption in tion rate is then enhanced due to the introduction of this liquid}liquid systems with a known value for the inter- second transfer path. facial area to determine the in#uence on the liquid side Concluding from experiments and models reported in mass transfer coe$cient k*. They studied oxygen absorp- the literature, it appears that there is a lot of confusion tion in aqueous emulsions with a mixture of n-alkanes about the gas absorption mechanism and e!ects in and with oleic acid. The value of k* remained practically gas}liquid}liquid systems. By studying experimentally constant in the presence of n-alkanes before phase inver- the e!ects on k* and a separately, more insight on the sion, while the addition of oleic acid caused an initial mechanism may be obtained. decrease of k*. When more oleic acid was added, k* increased again. A few studies have been presented in literature on the 3. Experimental technique in#uence of a second immiscible liquid on the gas}liquid interfacial area Mehta and Sharma (1971) used a fast 3.1. Danckwerts plot reaction, CO}NaOH system, to study the in#uence of 2-ethyl-hexanol on the speci"c gas}liquid interfacial area. For a better analysis of the observed e!ects on the According to the authors the area increased due to a pre- mass transfer rate the simultaneous determination of the vention of bubble coalescence. Das, Bandopadhyay, liquid side mass transfer coe$cient k* and the interfacial Parthasarathy and Kumar (1985) studied the interfacial area a has been selected, using the well-known Danck- area in gas}liquid}liquid systems with both a physical werts-plot technique (Danckwerts, 1970). The major and a chemical method. As dispersed phases 2-ethyl- advantage of this chemical absorption method over a hexanol, toluene and methyl-isobutyl-ketone were used. physical absorption technique is that, when the reaction All systems showed initially an increase in the interfacial is fast enough, the bulk concentration of the dissolved gas area with higher dispersed phase fraction, but above is negligible, which implies that the driving force for mass approximately 10% hold-up the area started decreasing. transfer is well known.
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