limiting Flow Phenomena in Packed Liquid-liquid Extraction Columns J O H N H. BALLARD' A N D EDGAR L. P l R E T UNIVERSITY
OF
MINNESOTA, MINNEAPOLIS, MINN.
T h e flow mechanism of countercurrent liquid-liquid extraction in packed columns has been studied, mainly with a 3.75-inch column packed with 0.5-inch porcelain Raschig rings. Water was used as one phase, either continuous or dispersed, and a number of fluid mixtures with widely varying physical properties were used as the other phase. Visual observations supplemented the measurement of maximum column capacities. A "transition point" i s defined for operation with the liquid which preferentially wets the packing as the continuous phase,
beyond which rates the column may flood or else change the flow mechanism t o accommodate higher rates. I t does not appear t o correspond with the flooding points of other investigators. A correlation for the transition point is presented which also agrees with previously published data on flooding of gas-liquid absorption columns. The flooding of columns in which the liquid t h a t preferentially wets the packing is the dispersed phase i s shown t o have a different mechanism and t o require a separate correlation.
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work was more extensive than that used by previous investigators. The column design was made versatile to permit introducing each fluid either as the dispersed or as the continuous phase. Two sizes of packing were employed. On the basis of the observations i t was advantageous to define a new allowable column capacity. These capacities proved capable of general correlation.
ONTINUOUS, countercurrent liquid-liquid extraction in packed columns has long been used without a complete understanding of the fluid flow mechanisms involved. Only recently (6) has a correlation been presented to assist in the prediction of packed column capacity in liquid-liquid extraction. The analogous gas-liquid systems in packed absorption columns, however, have been investigated with some thoroughness, and a satisfactory correlation was obtained by Sherwood (16) and later modified by Lobo (9). There seems to exist a possibility of mutual correlation or a t least similarity of phenomena in these two chemical engineering operations. Available data on the flow capacities of packed liquid-liquid extraction columns are still meager, and were often obtained as incidental to mass transfer measurements. These data must be examined as to whether the column design mras such as to give flooding points that can be considered of general value. Rushton ( l a ) ,Appel and Elgin (I),and Sherwood, Evans, and h n g c o r (16) briefly described the flooding appearance in their columns and gave limited data on flooding rates. Row, Koffolt, and Withrow (11) made tests on an 8.75-inch column using the toluene-benzoic acid-water system, with toluene as the dispersed phase. Flooding velocity curves were presented for operation with 0.5-inch Raschig rings and Bed saddles, as well as other packings. Blanding and Elgin (3) studied both spray and packed column design and operation, and pointed out certain important features of design necessary to proper operation of both types of equipment. Using a 2.89-inch column they measured flooding points for systems of water as one phase and methyl ethyl ketone or industrial xylene as the other, with the water phase either continuous or dispersed. A variety of 0.5inch packings were utilized. More recently, Breckenfeld and Wilke (6) presented the results of their flooding measurements using mainly 0.25-inch carbon Raschig rings and several fluid systems in a 2.6-inch column. They succeeded in obtaining a correlation for their data and those of Blanding and Elgin (3)and Row, Koffolt, and Withrow (21). It was desired, in this work, to contribute further to present knowledge by investigating packed column operation both visually and by measuring the effect of the various solvent properties on the capacity of such columns. In order that the variables of density, density difference, and interfacial tension could be independently varied, a variety of fluid systems were utilized. These were chosen to produce wide and controllable variations in the above properties. As a result, the range investigated in this 1 Preeent address,
University of Southern California, Los Angeles, Calif.
APPARATUS
Two columns were utilized for the experimental work. Both were constructed of standard-wall borosilicate glass tubing, which made it possible to observe the column operation visually. Early data were obtained in a 2.03-inch inside diameter column packed with 0.25-inch outside diameter porcelain Raschig rings; the final correlated data, however, were obtained in a 3.75-inch inside diameter column 4 feet long, packed with 0.5-inch outside diameter porcelain Raschig rings (Figure 1). The rings, which were very slightly porous, were made by Maurice A. Knight, Akron, Ohio. The properties of the packings are summarized in Table I. Table I .
Details of Raschig Ring Packings
Size of packing, inch" Type of material Awarent density, Ib./ou. foot
Siik
0.25
0 5
0.5
139.5
139 5
Unglazed porcelain
139.5
Outside diameter, inch 0.25 0.5 0.6 Inside diameter, inch 0.125 0.250 0,250 0.25 0.50 0.50 Length, inch Size of column packed, inchesb 2.03 3.75 2.03 Surface area term, a. sq. feet/cu. foot 202.0 97.0 85.7 Fractional free volume, F 0.559 0.603 0.650 Factor, ao*aK/F 56.3 32.4 27.7 Factor, a/Fa 1155.0 443.0 312.0 Obtained from Maurice A. Knight, Akron Ohio. b Packed by dropping rings, a few a t E tirne,'into column filled with water. No perceptible settling noticed.
Blanding and Elgin's features of column design were incorporated to ensure that the packed section of the column, and not the end fittings, was the factor causing flooding. The arrangement of fittings for operation with the dispersed phase entering at the bottom is shown in Figure 2 (left). An enlarged conical end on the bottom of the column provided room for flow of the continuous phase around the nozzle head. The packing support was fastened around the nozzle assembly, so that the packing rested directly on top of the nozzles in the center, and on the support around the sides (Figure 3). The packing support was a brass disk with 0.375-inch holes drilled side by side, supported by nuts on a few of the nozzle tips. The dispersed phase entered through twelve '/e, inch inside diameter beveled nozzle tips, which thus projected just above the packing
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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auppurt level. The column proper w m completely filled with thr packing by dumping it, a few rings at a time, through the waterfilled column. The packing was not txmpod or shxken. Abovc~ the column way provided an extra, empty, 11-inch section, similar to the column itself, in whjch wiis held the interface (Figure 4 ) The Pont.iniiiws otmse WBS introduced from the u ~ n e rend of this
tam of the edfurnn, s?. shown in Figure 2 (left). The column was inverted from the position shown in fiiaure 2 (left) when the dispersed phase war more dense and ontend :it, t,hn top. A n enlarged cylindrical section (Figure 5). eonst,ruet
