Ind. Eng. Chem. Fundam. 1982, 27, 269-271
269
Mass Transfer in a Bubble-Agitated Liquid-Liquid System' Sam D. Clinton and Joseph J. Perona' ChemlCel Technology Division, Oak R&e National Laboratory, Oak Rwe, TennesS8e 37830
A polarographic method was used to measure the mass transfer coefficients between a mercury layer and an aqueous layer agitated by gas bubbles. The effects of gas rate, vessel sue, and sparge-tube dmmeter were studied. The Schmidt number of the a ueous hase was varied by the use of sucrose solutions. The data are well represented by NSh= 1.33NR~.70Ngc1P3.
Introduction Bubble agitation of liquids can be used in industry for high-temperature or high-pressure systems where mechanical stirring is difficult and in large shallow vessels or furnaces that do not lend themselves to mechanical stirring. An analogue of the system studied here is the metallurgical slag furnace, in which a layer of slag floats on a molten metal phase and extracts a component from the molten metal solution. Gas lances are commonly used to agitate and promote mass transfer in such systems. Many studies have been made in gas-liquid systems to determine mass transfer between the gas and liquid phases. The present study is concerned with mass transfer between two immiscible, nondispersed liquids agitated by an inert gas sparge. There is a dearth of mass transfer data for bubble agitation, but considerable data are available for the analogous heat transfer situation. Hart (1976) reviewed the literature for heat transfer data between a surface and a bubble-agitated liquid. He observed that the heat transfer coefficient does not depend on vessel diameter, liquid height, or any other characteristic dimension of the system, location or geometry of solid surface, or the type of gas distributor used. By analogy, these observations might also be expected to hold for mass transfer systems. Heat transfer coefficients increase with the superficial gas velocity to about the 0.25 power. Hart successfully correlated his data as well as much of the data in the literature by the equation where
Mass transfer in a liquid-liquid nondispersed system agitated by mechanical stirring was studied by Brown (1979). His experimental method was exactly the same as that used in the present study except for the type of agitation employed. The most significant variables affecting the mass transfer coefficient in Brown's work were agitator speed and diameter and aqueous-phase viscosity. Bulicka and Prochazka (1976) proposed a theory for mass transfer between two turbulent phases. By using the concept of surface renewal of turbulent disturbances and by making certain assumptions about the characteristics of the vortices which control mass transfer, they showed that their model led to a correlation of the form Research sponsored by the Office of Basic Energy Sciences, U.S.Department of Energy, under Contract W-7405-eng-26 with the Union Carbide Corporation. 0196-4313/82/1021-0269$01.25/0
where
The interaction factor, qi,,accounts for the effect of turbulence in the j phase on mass transfer in the i phase. Equations 3 and 4 were shown to fit data from mechanically stirred cells with good agreement for systems that were free of interfacial instabilities. Experimental Section Apparatus. An electrochemical method for mass transfer measurements with diffusion-limited current was introduced in the 1950's (e.g., Eisenburg et al., 1955) and has been used extensively. In this study, a polarographic method (Brown, 1979) measured mass transfer rates in a system containing an aqueous solution floating on a mercury layer (Figure 1). Quinone dissolved in the aqueous phase was reduced at the mercury surface, which acted as a cathode. The initial aqueous solution composition was 0.001 M quinone-0.05 M hydroquinone in a 0.2 M phosphate buffer solution with pH of 7.0. Two contactor vessels were used: one, 10 by 10 cm on the horizontal plane, and the other, 20 by 20 cm. Liquid heights for each layer were 5 cm in the smaller vessel and 10 cm in the larger vessel. The sparge tube, which was 0.635 cm in diameter, was located at the center of the square cross section, and the end of the tube was 0.3 cm from the bottom of the vessel; hence, the bubbles emerged from the tube in the mercury layer and rose through both layers. Orifice sizes of 0.16 cm, 0.079 cm, and 0.04 cm were tested in the bottom of the sparge tube. Photographs of the bubbles rising through the aqueous layer showed they were roughly spherical and approximately 0.5 cm in diameter for all orifice sizes and gas rates. The constancy of the bubble size with different orifice sizes can probably be explained by the vertical downward orientation of the sparge tube. As the bubble is forming, it cannot move upward until it moves horizontally to clear the end of the tube; therefore, the action of the buoyant force in stopping the growth of the bubble is delayed. The anode for each cell was made from 1.6-mm-thick brass sheet sized to slip into the inner perimeter of the cell. The anodes were gold-plated and suspended in the aqueous phase. A saturated calomel electrode suspended in the aqueous phase was used as a reference to control the potential of the mercury surface. 0 1982 American Chemical Soclety
Ind. Eng. Chem. Fundam., Vol. 21, No. 3, 1982
270
Table I. Effect of Sucrose on Aqueous Phase Properties
a
aq sucrose concn, wt %
viscosity, CP
density,
0 45
1.17 10.9
1.02 1.22
g/cm3
Schmidt no.a
6.7
1710 194 000
X
4.6 x
Brown (1979).
1 SdCROsE// 20 x 20 cm VESSEL
n
I
diffusivity, cmz/s
-
.
i
SUCROSE F R E E , IO x IO cm VESSEL
i
i
PHASE
i
~
VCTE" v3-T -
/
