AS-LIQUID INTERFACIAL AREA IN OXYEN ABSORPTION INTO OIL-IN-WATER EMULSIONS ómez-díaz, D. a, omes, N. b, Texera, J.A. b, Belo, I. b a Department of Chemcal Engneerng, Unversty of Santago de Compostela, Campus Sur, E-15782 Santago de Compostela, Span. b IBB-Insttute for Botechnology and Boengneerng, Center of Bologcal Engneerng, Unversty of Mnho, Campus de ualtar, 471-57, Braga, Portugal. Abstract: Present work ncludes an exhaustve study about gas-lqud nterfacal area between gas phase and lqud heterogeneous medum generated n an arlft boreactor. The model system studed s composed by water, methyl rcnoleate and Tween 8, snce t s the base of the medum used for the producton of -decalactone through the botransformaton of rcnolec acd, by the yeast Yarrowa lpolytca. Expermental results allow descrbng the hydrodynamc behavor of the gas phase nto the bphasc medum. In the ol-n-water system used, a decrease of the gas-lqud nterfacal area wth the ol concentraton ncrease was found, due to the redstrbuton of surfactant between nterface and bulk phases. Keywords: Ar-lft boreactor, methyl rcnoleate, emulson, gas-lqud nterfacal area 1. INTRODUCTION as-lqud-lqud systems are encountered n many reactons and have ganed specal nterest n chemcal engneerng wth the ntroducton of homogeneous bphasc catalyss. Mass transfer modelng n bphasc meda has been done by several authors usng as a model, systems n whch the second lqud phase (organc) s an addtonal nert and external compound (lke perfluorocarbons and slcone ols, among others) added to the system on the purpose to mprove mass transfer from the gas to the lqud aqueous phase (Dumont and Delmas, 23). These systems are encountered n some bochemcal applcatons where a hydrophobc compound wth hgher oxygen solublty than water s used to mprove the oxygen transfer rate to the culture meda. However, numerous botechnologcal processes are based on the development of mcroorgansms wthn a bphasc medum formed by an ol-n-water emulson where the ol s the substrate to be degraded. In these cases, the organc substrate can affect the gas-lqud mass transfer, thus nfluencng the overall productvty. The system heren presented s an emulson of water and methyl rcnoleate (MR), stablzed by Tween 8, whch consttutes the medum used n the producton of -decalactone (a peach-lke aroma compound of ndustral nterest) through the botransformaton of rcnolec acd (major consttuent of methyl rcnoleate) by the yeast Yarrowa lpolytca (Aguedo et al., 25). The bochemcal pathway leadng to the producton of the aroma has been extensvely studed. However, crucal steps controllng the producton rato of -decalactone aganst derved nonaromatc lactones are not totally understood. The oxygen s beleved to nfluence these sde-reactons, but ths pont remans to elucdate. Snce Y. lpolytca s an oblgate aerobe, oxygen s a crucal operatonal parameter for the control of processes nvolvng ths yeast. Prevous studes (omes et al., 27) have analyzed the nfluence of operatng condtons on oxygen mass transfer and on -decalactone producton n a bubblng strred boreactor. In the present work an arlft boreactor was used and the man am of ths study was to study the nfluence of operatng condtons and emulson composton on oxygen mass transfer to the emulson, especally the gas-lqud nterfacal area, n ths bphasc medum, n an arlft contactor.
2. EXPERIMENTAL DATA 1.1 Lqud velocty The velocty of lqud crculaton was determned wth the neutral buoyancy flow follower technque. A small square pece of plastc (sde length,.5m; specfc gravty, 1.3) was used as a flow follower. The tme taken by the follower to traverse a known vertcal dstance (.2 m) n the downcomer was noted and the flow velocty was calculated from an average of 1 measurements. The densty of the follower was very close to that of the lqud. Fgure 1 shows that the ncrease of the ar flow rate leads to an mprovement n the lqud velocty, although t seems to be a trend to become constant at the hgher aeraton rates tested..2.15 U L / m s -1.1.5 2 4 6 8 Q g / L h -1 Fgure 1. Lqud velocty (U L ) generated nto the ar-lft reactor at dfferent aeraton rates. 1.2 as-lqud nterfacal area To determne the gas-lqud nterfacal area, a photographc method was used. Bubbles dameter was measured usng a photographc method based on mages of the bubbles taken along the heght of the column, from the bottom to the top. A mnmum number of 5 well-defned bubbles along the bubble column were used to evaluate the sze dstrbuton of the bubbles n the lqud phase for the dfferent gas flow rates tested. The geometrcal characterstcs of ar bubbles produced n the contactor enable to calculate the value of the gas-lqud nterfacal area. The bubbles produced n the contactor have ellpsodal shape and for ths reason, major (E) and mnor (e) axes of the projected ellpsod (n two dmensons) must be determned. The dameter of the equvalent sphere was taken as the representatve bubble dmenson. Equaton 1 allows determnng the dameter dstrbuton of a bubble present along the gas-lqud contactor. 3 2 d E e (1) Photographs n Fgure 2 dsplay clearly that when the methyl rcnoleate concentraton ncreases n the lqud phase, an ncrease n bubbles sze s produced.
(a) (b) (c) Fgure 2. Photographs from the bubble column at an ar flow rate of 1.35 L mn -1 for dfferent lqud phase compostons: (a) aqueous soluton wth.93 % (v/v) Tween 8; (b) emulson wth.3 % (v/v) MR and.93 % (v/v) Tween 8; (c) emulson wth.54 % (v/v) MR and.93 % (v/v) Tween 8. The bubble dameter dstrbuton along the bubble contactor has a drect nfluence upon the value correspondng to gas-lqud nterfacal area. The nterfacal area determnaton s developed on the bass of two mportant parameters: the Sauter mean dameter and the gas hold-up. An example of the bubble sze dstrbuton obtaned n the present work s shown n Fgure 3. An ncrease n the surfactant concentraton decreases the bubbles sze n the whole dameter range. Ths behavor has an mportant role n the value of the gas-lqud nterfacal area. The decrease n the value of lqud phase surface tenson caused by the presence of surfactant molecules at gas-lqud nterface produces the decrease n the dameter of the bubbles generated nto the gas-lqud contactor. 1 8 % bubbles 6 4 2 Water [Tween8]=.23% 2 4 6 8 1 d/mm 12 14 16 18 Fgure 3. Influence of Tween 8 concentraton n the lqud phase upon the bubble sze. Q g =.5 L mn -1. Dfferent authors recommend the use of the Sauter mean dameter (d 32 ) (Shah et al., 1982), that can be determned usng the data calculated for the equvalent dameter by equaton (2). n d 3 d32 (2) n d 2
where n s the number of bubbles that have an equvalent dameter (d ). The gas hold-up was determned n a square bubble column contactor, due to the dffculty to measure the varatons on the heght of the lqud. Ths column has geometrcal characterstcs of 4 cm nsde sde-length and heght of 65 cm. Snce the dameter and heght are smlar to those of rser s arlft reactor, results can be extrapolated. The use of ths methodology s possble due to bubbles are not present n the downcomer of the arlft reactor. Ar was used as gas stream n the gas-lqud contactor and t was fed at the bottom of the bubble column usng a fve holes sparger. The gas hold-up,, was calculated usng the dfference between the lqud heght n the absence of gas n the bubble column and under the dfferent expermental condtons of gas flow rate fed to the contactor (equaton 3) (Vasconcelos et al., 23). V V V L (3) where V L s the ungassed lqud volume and V s the volume expanson after gas dsperson, calculated from the lqud level change and the cross sectonal area. The change of volume n the bubble column was calculated based on the change observed n the lqud level and the ncrease of ths value after gassng. The gas hold-up was determned for dfferent lqud phase compostons (pure water, aqueous soluton wth.93 % (v/v) Tween 8, emulsons wth.1 % (v/v) and.54 % (v/v) MR and.93 % (v/v) Tween 8, at dfferent gas flow rates (or surface gas veloctes). The expermental results obtaned for the gas hold-up n the bubble column are depcted n Fgure 4. An ncrease n the gas flow rate (or surface gas velocty) produces a clear ncrease n the value of the gas hold-up wth a lnear trend. Also, t can be observed that no sgnfcant dfferences exst between the dfferent lqud phases compostons tested, although for hgher gas flow rates t seems that the gas hold-up starts to become greater for the aqueous medum. Ths can be explaned by the ncrease of bubbles sze n water and therefore, the decrease of ts resdence tme..5.4.3.2.1.2.4.6.8 1 1.2 1.4 U 1-3 / m s -1 Fgure 4. Influence of surface gas velocty (U ) upon gas hold-up ( ). ( ) Pure water; ( ).93 % (v/v) Tween 8; ( ).1 % (v/v) of MR and.93 % (v/v) Tween 8; ( ).54 % (v/v) of MR and.93 % (v/v) Tween 8. The Sauter mean dameter and the gas hold-up values allow the calculaton of the specfc gas-lqud nterfacal area usng equaton 4 (van t Ret and Tramper, 1991).
a d 32 6 1 (4) Ths expermental methodology allows the determnaton of gas-lqud nterfacal area under the dfferent expermental condtons (lqud phase composton and gas flow rate) assayed n ths work (Fgure 5)..8.6 A / m 2.4.2.4.8 1.2 1.6 Q / L mn -1 Fgure 5. Influence of gas flow rate and methyl rcnoleate concentraton on the nterfacal area n the arlft boreactor. ( ).93 % (v/v) Tween 8; ( ).1 % (v/v) of MR and.93 % (v/v) Tween 8; ( ).3 % (v/v) of MR and.93 % (v/v) Tween 8; ( ).54 % (v/v) of MR and.93 % (v/v) Tween 8. Expermental results shown n fgure 5 ndcate the nfluence of lqud phase composton and gas flow rate upon the specfc nterfacal area generated n the bubble column. When gas flow rate ncreases, the nterfacal area also ncreases due to the prevously observed behavor caused by ths varable upon the gas hold-up. The gas hold-up ncreases wth the gas flow rate because a hgher number of bubbles are generated n the sparger, ncreasng the nterfacal area. When pure water was employed as lqud phase n the bubble contactor at hgh values of gas flow rate, the ncrease n nterfacal area was not observed because under those condtons there s a great number of bubbles that collde producng coalescence and therefore the ncrease n the bubbles dameter (data not shown). The presence of surfactant n the lqud phase produces a clear ncrease n the value of the specfc nterfacal area, comparng to pure water, partcularly notced for hgh values of gas flow rate. Ths behavor s due to dfferent reasons. One of them s the clear decrease n the lqud phase surface tenson when the surfactant concentraton ncreases (ómez-díaz et al., 28) as t causes the formaton of bubbles wth mnor sze ncreasng the nterfacal area (Panmanakul, 25). Also, the presence of surfactant molecules n the lqud phase produces a decrease n the bubbles effectve collsons and then the bubbles small sze remans constant along the bubble column. The expermental results obtaned when emulsons were employed as lqud phases are presented n Fgure 5 that shows a decrease n the specfc nterfacal area value when the ol concentraton ncreases n the lqud phase. Ths effect s neglgble for the lowest gas flow rate analyzed but when t ncreases, the presence of the organc phase has an mportant role on the values of nterfacal area. Ths behavor s assgned to a change n the surfactant concentraton at gas-lqud nterface because when an organc phase s added to aqueous solutons wth Tween 8, a new dstrbuton of surfactant between nterface and bulk phases s orgnated. Ths dstrbuton affects the value of the lqud phase surface tenson and consequently bubbles produced under these condtons have hgher dameters than the ones formed n the absence of organc phase.
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