CORRESPONDENCE Spray Tower Extraction continuous phase inlet, is highly characteristic of the caseTof SIR: Authors Geankoplis and Hixson [IND.ENG.CHEM.,42, thorough mixing in the continuous phase. The most probable 1141 (1950)] and Geankoplis, Wells, and Hawk [IND. ENG. reason, then, for the end effect noted by these workers is vertical CHEM.,43,1848 (1951)] are t o be congratulated on their approach mixing in the aqueous phase. to the problems of the behavior of liquid-liquid extraction spray Since mixing in the continuous phase occurs, then the method of columns. Their development of a method for sampling the taking material balances over short sections of the column used continuous phase a t different levels in the column should help by Geankoplis and coworkers for calculating the solute concentragreatly in elucidating the fundamental characteristics of this tion in the dispersed phase a t different levels is not valid. This type of equipment. method applied to the top section of the column involves the four The writer feels, however, that these authors have failed t o streams and concentrations shown diagrammatically in Figure realize the true significance of the observations they have made. 2A. CT, is the only unknown. In actual fact, there are five The probable explanation of their results, with particular referstreams involved, as shown in Figure 2B, the continuous phase ence to their discovery of the unexpected ('end effect" (Le., the passing the level 2 in both directions. In Figure 2, high proportion of extraction apparently occurring a t the continuous phase inlet), and also an indication of how the use of C = concentration of solute accepted methods for evaluation of column performance is L = flow rate consequently prevented are submitted for consideration. No satisfactory theory has been advanced to explain this end effect, Subscripts and Superscripts although Geankoplis and his coworkers suggest that the mechaT refers to the toluene (dispersed) phase nism of coalescence of the drops of the dispersed phase may be W refers to the aqueous (continuous) phase responsible. 1 refers to the top of the column Verticle mixing in the continuous phase in spray columns has 2 refers to the level in the column where the continuous phase is yampled been noted by Morello and Poffenberger [IND.ENG.CHEM.,42, refers to the continuous Dhase crossing the level 2 in a down1021 (1950)l and by Geankoplis and Hixson who stated that w y d direction "observations of the flow pattern in the column indicated turrefers to the continuous phase crossing the level 2 in an upbulence and hence good mixing of the continuous phase." Such ward direction mixing in the continuous phase would have a pronounced influLw,! - Lw,'f = Lw so ence on the solute concentration in this phase a t any particular point in the column, and this writer ventures t o suggest that this influence would be greater than that of exm 37 traction by (or from) the dispersed phase. Consider, for example, run 5 of the paper by Geankoplis, Wells, and Hawk, where the concentration of solute in the aqueous phase increases from 0 to 1.846 X 10-3 pound moles per cubic foot, and assume the extreme case where mixing in the continuous phase is 100% efficient, i.e., the solute concentration throughout the phase is uniform. By sampling the continuous phase a t a number of levels 20 40 60 %P in the column and then plotting conolsreuce F*- r- /WZW..CI-/!e$ centration versus column height, curve 1 of Figure 1 would result. On the Figure 1. Concentration in Aqueous Phase us. Distance from Top Interface other hand, a case where true counter1. Thorough mixing 2. Countercurrent conditions current conditions exist-i.e., where 3. From Geankoplis, Wells, and Hawk there is no internal mixing in the continuous phase-would, from theoretical considerations, be expected to result in a curve of A material balance over the section yields type 2 (Figure 1). The curve 3 obtained by Geankoplis and coworkers for conditions of "good" but not complete mixing C T ~X LT CW," X LwP" Cw, X L w = in the continuous phase, lies intermediate between the two CT,LT Cwzr X Lwzf (1) extreme cases considered. It must be noted that the sudden or change in slope of their curve, representing the end effect a t the = (Cw,'Lw,' - C W ~ " L W ~ "CTJT - CW,LW)/LT (2) 2457
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2458
INDUSTRIAL AND ENGINEERING CHEMISTRY
If there is a high degree of mixing in the continuous phase, then Lwn’and Lw2“ are both much greater than Lw. Thus,
Lw2 + Lw2’ + L W Z K and (3) The first term on the right-hand side of Equation 3 has considerable significance, and even if Cwzawere only slightly greater than CT,’, would result in a value for C T much ~ lower than that calculated by t h e method which assumes true countercurrent condi-
Vol. 44, No. 10
results t o columns of different height, even with other operating conditions identical, is not feasible. The writer sincerely trusts t h a t these comments will be of interest and value t o all workers in t h e field of liquid-liquid estraction spray column design. M. L. NEWMAS 20 NORMANBY Sr. BRIQHTOX
8.5
VICTORIA. AUSTRALIA
SIR: With regard t o Mr. Newman’s probable explanation of t h e end effects, t h e authors had realized that there must be present some mixing of the continuous phase. It is almost impossible to obtain true countercurrent flow in an apparatus such as a spray tower because the two phases are in close contact and some mixing or LW cocurrent flow must occur. Visual observations Cw, of the continuous phase Aow seemed t o indicate that cocurrent upward flow of the continuous phase in the main part of the toaer was quite small in amount. However, it is most probable that each rising droplet does carry with it an “atmosphere” or surrounding film of the continuous, downcoming phase. Internal samples were obtained a t different points in the cross-section of the tower (1, 2 ) and no appreciable difference was found. This indicates t h a t any large cocurrent flows, if they existed, were not detected. This seems t o negate the possibility of large masses of cocurrent water phase flowing in a direction opposite to the countercurrent water flow. It seems that the important benefit t o be derived A B from internal sampling is a more thorough understanding of the internal operation of the column. Figure 2. Flow Condition in Top Section of Column A concentration gradient does exist which affects A. True Countercurrent conditions the over-all performance of the column. KnowlB. Mixing in continuous phase edge of the over-all type of gradient should aid greatly in the design of new columns. A definite color gradient existed in the continuous phase of tions. As there are five unknowns in Equation 2 (there would the ferric chloride extraction ( 1 ) . At the top inlet the color be nine unknowns for an intermediate section), it is not possible was a deep brown, indicating a high concentration of ferric chlot o calculate a true value for the concentration of solute in the dispersed phase a t different levels in the column, and, conscride. At the bottom the light-straw yellow of a dilute concentraquently, capacity coefficients and H.T.U. for short sections of tion was present. However, at the top inlet great turbulence did seem t o appear the column cannot be determined. and the inlet effect may have been due chiefly to this mixing The occurrence of mixing in the continuous phase in spray or it may have been due t o the coalescence of the bubbles. More columns also prevents the use of accepted methods for evaluaresearch should be done on this question. tion of the performance of this type of equipment. T h e equaThe equations derived by Mr. Xewman might explain theoretitions for determining the extraction coefficient ( K T u ) and cally the behavior of a spray tower, It seems that the behavior H.T.U OT for spray columns involve t h e use of the logarithmic is actually very complex and cannot be represented adequately mean concentration differenceas calculated from the concentration of solute in the two phases at both ends of the column. Now the by a few simple equations. It seems t h a t if a serious channelling did exist, physical concept of logarithmic mean concentration gradient is based on changes in design of the tower should alter the flow pattern and the assumption t h a t the two phases pass each other in a truly the amount of extraction. Radical changes in nozzle design countercurrent manner. This implies that the concentration in and end design (1) did not alter the amount of extraction. the continuous phase a t a given level in the column is the result solely of the transfer of solute to the dispersed phase as the CORtinuous phase moves steadily down the column. Since the turLITERATURE CITED bulent mixing in the continuous phase appears t o have a much (1) Geankoplis, C. J., and Hixson, A. N., IND. ENG.CHEM.,42, 1141 greater effect on the distribution of solute in this phase, the use (1950). of logarithmic mean driving force calculated from the solute ( 2 ) Geankoplis, C. J., Wells, P. L., and Hawk, E. L., Zbid., 43, 1848 (1951). concentrations in the two streams at either end of the column is C. J. GEANKOPLIS not justified. Consequently, the values of capacity coefficient D E P A R T h l E N T O F CHEhfIC.4L E N Q I N E E R I X G and H.T.U. calculated from the logarithmic mean concentration T H EOHIO STATEUNIVERSITY COLIJXBUS 10, OHIO gradient are meaningless, and the application of experimental
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