Ind. Eng. Chem. Res. 2000, 39, 4673-4677
4673
Drop Size and Hold-up in Countercurrent Extraction with Supercritical CO2 in a Spray Column B. S. Chun† and G. T. Wilkinson*,‡ Faculty of Food Science and Biotechnology, Pukyong National University, Pusan, Korea, and School of Chemical Technology, University of South Australia, Mawson Lakes, South Australia 5095, Australia
The drop size and hold-up were measured in a continuous countercurrent extraction spray column of 22.7 mm i.d. under near-critical and supercritical conditions. The drop size was measured by analysis of images and hold-up by the shut-off method. The systems studied were 5 and 10 vol % 2-propanol + water and 10 vol % ethanol + water extracted with carbon dioxide, at temperatures from 25 to 45 °C and pressures from 6.9 to 13.8 MPa. Data were correlated with physical properties of the system. The model developed to predict the drop size agreed favorably with experimental data with an average absolute relative deviation (AARD) of 5.6%. In the case of hold-up, the AARD was 12.4%. Introduction Supercritical (and the related near-critical) fluid technology has received intense study in recent decades as an alternative separation process. The major advantage of supercritical fluids (SCF) is that, near the critical state, great variation in solvent properties is achievable with relatively minor changes in pressure or temperature. In addition, their strongly nonideal behavior leads to interesting opportunities with relatively small additions of cosolvents. Carbon dioxide is a popular supercritical extractant particularly in food processing, flavor and aroma isolation, and pharmaceuticals manufacture. A characteristic of one class of separation processes is to recover relatively high concentrations of low-to-medium concentration solutes from a mixture, by direct contact,1-3 such as in the removal of water from alcohols, esters, aldehydes, and aromatics. Wilkinson and Foster4 studied the removal of ethanol and the recovery of volatile flavors from wine, as well as the extraction and refining of essential oils, using supercritical carbon dioxide. A difficulty in scale-up of countercurrent solvent extraction involving SCFs is the paucity of reliable mass-transfer data. SCF densities can vary from gaslike to liquidlike, so spanning the range of hydrodynamic behavior from gas absorption to liquid-liquid extraction. Spray columns have long been used in research because of the insight they offer on interfacial mass transfer in related configurations. Mass transfer of the solute, for example, from the continuous phase into the dispersed, takes place during droplet travel through the column as well as during drop formation at the distributor and coalescence at the interface at the end of the column. The purpose of this work was to investigate hydrodynamic behavior under supercritical conditions and obtain a correlation for estimation of hold-up and drop size needed for the reliable design of countercurrent extraction columns where one phase is a supercritical fluid. * To whom correspondence should be addressed. E-mail:
[email protected]. Phone: +614 1361 4644. Fax: +618 8302 3668. † Pukyong National University. ‡ University of South Australia.
Hydrodynamics of Spray Columns. There have been many studies of conventional liquid-liquid extraction using the spray column.6-11 Despite strong interest in supercritical fluid extraction (SFE), few papers have been published on the hydrodynamic behavior of continuous countercurrent extraction systems. Notable exceptions include the studies of Lahiere and Fair12 and Rathkamp et al.13 on spray columns using supercritical carbon. Aqueous ethanol and 2-propanol were each extracted with dense phase carbon dioxide at selected temperatures and pressures. In agreement with work of Brunner and Kreim,3 extraction efficiencies up to 10 times that achieved in normal liquid extraction were obtained for the supercritical system. More recently, de Haan25 studied hydrocarbon systems in both spray and packed columns and Lim et al.29 investigated the system ethanol/CO2/water. Hold-up. Density, viscosity, solvent flow rate, and interfacial tension influence hold-up. Operational holdup (defined as the fractional volume occupied by the dispersed phase at steady state) may be expressed as a function of the slip velocity and superficial velocities of feed and solvent.16
Vs )
Vd Vc + φ d 1 - φd
(1)
where φd is the dispersed phase hold-up and Vd and Vc are the superficial velocities of the dispersed and continuous phases, respectively. The term Vd/φd gives the dispersed phase velocity relative to the column walls and Vc/(1 - φd) the continuous phase velocity. When the continuous phase flows countercurrently, its superficial velocity will be negative. Idogawa et al.17 studied the effect of fluid properties on the behavior of drops at pressures from atmospheric to 5 MPa using hydrogen, helium, and air. They demonstrated that the drop size was essentially independent of the interfacial tension at pressures above 5 MPa, and that the drop size decreased, as expected, with increasing pressure. They correlated operational holdup at high pressure with system conditions and properties. Several investigators have reported the effect of density on dispersed phase hold-up. Wilkinson and
10.1021/ie000255l CCC: $19.00 © 2000 American Chemical Society Published on Web 11/10/2000
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Ind. Eng. Chem. Res., Vol. 39, No. 12, 2000
Dierendonck18 studied various gases such as H2, N2, Ar, CO2, and SF6 dispersed in water and found that holdup increased significantly with gas density, particularly at high gas velocities. Rush et al.20 investigated the effect of gas density on hold-up in a SCO2 column. They found that density of the dispersed phase was more significant than that in conventional gas-liquid bubble columns and similar to that obtained in liquid-liquid systems. Lim et al.29 showed that hold-up increased with dispersed phase velocity and pressure but decreased with temperature, reflecting changes in the density difference of the phases. Hold-up may be correlated with solvent flow rate, viscosity, density, and interfacial tension. The solvent flow rate greatly influences hold-up, and the sensitivity of density to small system changes near the critical point makes investigation of the effects of density on hold-up challenging. Drop Size. In a spray column, the drop size directly determines the interfacial area for mass transfer. The size depends on distributor dimensions, nozzle exit velocity, nozzle diameter, and fluid physical properties. A distributor perforated with several holes formed the drops in this study. At low nozzle velocities, the drop detaches when the buoyancy force exceeds the interfacial tension. At higher velocities, momentum generates a jet. As the flow is reduced, the jet length reduces to zero; drops are then created at the edge of the nozzle, and their sizes are largely independent of the flow rate. Kumar and Hartland21 provided correlations for the drop size and minimum jetting velocity for 12 liquidliquid systems from 8 different sources, in terms of throughput and fluid properties. Vedaiyan et al.22 studied the drop size and characteristic velocity of droplet swarms of benzene/water, methyl isobutyl ketone/ water, and carbon tetrachloride/water systems using a spray column and gave generalized correlations for the estimation of the (mean) drop size and (mean) velocity of the droplet swarm in countercurrent spray columns. Hughmark24 noted that, in normal operation, nozzle velocities for spray columns are often higher than those covered by earlier drop size correlations. With increasing dispersed phase velocity, the jet length increases and eventually the jet breaks up into drops when the velocity reaches a critical Reynolds value. The Kumar and Hartland21 correlation was suitable for this work, having been obtained from a similar range of process variables: drop diameter ds, density difference ∆F, interfacial tension γ, nozzle diameter Nd, and nozzle velocity Nv. Table 1 summarizes correlations commonly used to predict hold-up and drop size. Experimental Section Figure 1 shows the flow diagram of the countercurrent spray column. The column consisted of a 22.7 mm i.d. tube extraction column equipped with view cells at each end. The column length, from the distributor to the interface, was 867 mm. The dispersed phase distributor consisted of a capped tube perforated with eight, 1.6 mm diameter holes equidistant near the circumference. The column temperature was maintained constant by a controlled-temperature chamber. In-line filters were installed to prevent blockages, particularly upstream of sampling and metering valves. Pressure relief valves guarded against inadvertent overpressure at the pump outlet and of the column.
Figure 1. Schematic diagram of the apparatus. Table 1. Selected Correlations for Predicting the Drop Size and Hold-up in Spray Columns reference
AARD %
correlation Hold-up
Idogawa et
al.17
Hikita et al.19
Rush et al.20
φd γ ) 0.059Vd0.8Fd0.17 1 - φd 72
-0.22 exp(-P)
[ ]
φd ) 0.672
φd ) 1.554
[ ] [ ] () [ ] [ ] () Vdµc γ
0.578
Vdµc γ
0.578
µc4g Fcγ
-0.131
3
µc4g
-0.131
Fcγ3
Fd Fc
0.062
Fd Fc
0.405
28.7
207.3
545.8
Drop Size Perrut and Loutaty23
[ [
0.011
