COMMUNICATION SPRAY TOWER OPERATIONS WITH PACKED DROPLETS Heat and mass transfer in liquid spray towers is discussed. For transferring heat, spray tower Operations with packed droplets are a t least 100 to 200% more efficient than with more conventional free-rising droplets. Although the ”packed state” has been proposed for carrying out mass transfer, limited studies have demonstrated its unstable nature. An explanation, based on Marangoni instability, is presented.
A,TREMENDOUS effort has been expended over the years in evaluating liquid extraction as a means of separation. Researchers such as Varteressian and Fenske (1936a,b), Elgin and Browning (1935, 1936), and their coworkers became most involved in this field in the mid-1930’s. Since this time the simplest of extraction devices, the spray tower, has been extensively investigated by various researchers, as summarized by Treybal (1963). These past studies were mainly concerned with free-rising or falling droplets, while the subject of packed droplets (indicating dispersed-phase holdups of approximately 40 to 80%) was mainly associated with the onset of flooding. Stable operation of spray tower extractors with packed droplets was rarely discussed. The recent interest in sea water desalination and, in particular, development of nonfouling heat exchangers to heat sea water has stimulated the study of spray towers for heat transfer purposes. Work in the early 1950’s, such as that of Garwin and Smith (1953), provided background for the studies directed toward desalination processes around the mid-1950’s. A number of investigations have since been carried out in which water was brought in contact with various types of immiscible liquids in spray towers. As these studies progressed, the value of tower operations with packed droplets became more apparent, when investigators such as Woodward (1961) and Bauerle and Ahlert (1965) presented limited information. More recent publications by Greskovich (1966), Greskovich et al. (1967), Letan and Kehat (1965, 1967), and Kehat and Letan (1968) have considered this type of spray tower operation, and in particular the notion of spray tower operations with packed droplets was extensively assessed. In the study by Greskovich (1966), the increase in heat transfer efficiency when transferring heat from the dispersed kerosine phase into water was in the order of 100 to 2 0 0 5 higher than the efficiency obtained when operating with free-rising droplets. Recently, the idea of packed droplets has been proposed (Kehat and Letan, 1968) for mass transfer systems. I n the ensuing discussion, more extensive results of heat transfer in spray towers containing packed droplets are presented, following which the utility of packed droplets in mass transfer operations is considered. Limited results from the work of Greskovich (1966) are presented in support of this discussion. Heat Transfer with Packed Droplets
Both Greskovich (1966) and Kehat and Letan (1968) have discussed the formation, average drop size, and holdup of packed droplets. Furthermore, the effect of additives on stability was mentioned by the latter. I n
the latter study, kerosine was dispersed into water by 1.5-mm. nozzles and the average drop size was reported to be in the range of 3 to 4 mm. for water and kerosine rates of 0 to 2.85 and approximately 0.5 to 2.0 cm. per second, respectively. These values are in reasonable agreement with the drop size of 0.22 inch reported by Greskovich for % 6-inch orifices for the same system. The holdup of a spray tower completely filled with packed droplets varied from approximately 54 to 62% under varying flow rates. For these same flow rates during free-rise studies, the holdup varied from 2 to 24%. The increase in efficiency of packed over free-rising droplets in these studies is readily apparent upon examining Figure 1. The effect of additives on the stability of pa,cked droplets has been discussed by Letan and Kehat (1965, 1967), and it was generally concluded that additives and/ or surface active agents adequately promote the ready formation of packed droplets. I t was suggested that the additives sufficiently retard coalescence and thus stabilize the dense packing. With this in mind, it was proposed that the “packed state” could be useful in spray tower extractions. Mass Transfer during Free Rise
I n the study by Greskovich (1966), the immiscible kerosine-water system was also used for mass transfer operations. The solute was methyl ethyl ketone (MEK). The effect of the presence of M E K can be noted from
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Figure 1 . Heat transfer efficiency for freerising and packed droplets Distributor. % 6-inch holes 0 Studies with packed droplets AStudies with free-rise droplets
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Effective column height 10 feet
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Figure 2. When compared to the heat transfer studies using the same distributor, in this case %*-inch orifice holes, a decrease in holdup is noted when M E K transfers from the dispersed kerosine phase to water. Equivalent or slightly higher holdups are noted when MEK is transferred from the water phase. The general conclusion is that an increase in the internal coalescence rate occurred when MEK was transferred from the dispersed kerosine drops. A similar conclusion concerning the effect of solute transfer on internal coalescence rates was reached by various investigators, such as Groothuis and Zuiderweg (1964). The inadequacy of drop volume production techniques of Hayworth and Treybal (1950), Null and Johnson ( 1 9 ~ 3 , and Scheele and Meister (1968a,b) for mass transfer systems can be recognized. I n the absence of a solute, drop sizes as calculated from these equations would decrease as interfacial tension decreases. I n the studies with the >? -inch distribution holes reported here, the average drop size for kerosine dispersed in water was measured to be approximately 0.13 inch while the average drop size for kerosine plus MEK dispersed into water through the same distributor was 0.21 inch. The three most prominent drop volume prediction techniques would have yielded lower values. When kerosine was dispersed into water containing MEK, the average drop size was measured to be 0.078 inch. In this case, the drop volume predictions would have been in the right direction. The low rate of coalescence for the case of kerosine drops dispersed in water containing solute can be attributed to the Marangoni effect as rigorously described by Davies and Rideal (1963). The most common conclusion, that the lower interfacial tension caused by the addition of solute to the continuous phase yielded smaller drops, is unreasonable, since the reverse case of solute addition to the dispersed phase yielded larger drops. In each case the dynamic interfacial tension a t the distributor nozzle should be nearly equal. I n previous studies-Le., works by Johnson and Bliss (1946), Fleming and Johnson (1953), and Dunn (1965)-it was generally recognized that .50,
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mass transfer from the continuous phase yielded larger volumetric mass transfer coefficients, ha, than transfer from the dispersed phase. Since higher holdup exists, as previously described for transfer from the continuous phase, the increase in interfacial area, not the mass transfer coefficient, most likely leads to increased values for ha. Mass Transfer with Packed Droplets
For free-rising droplets, dispersed phase holdups are normally small. As the holdups increase with increased flow rate, the slip velocity decreases until the droplets begin to pack. However, if the droplets were forced to pack while transferring mass from the dispersed phase, the layer of packed droplets would build up to a few inches below the coalescer and then collapse within a few seconds. Various flow rates were investigated (Greskovich, 1966); however, it was virtually impossible to attain stable operations with packed droplets. Whenever MEK was transferred from the continuous water phase, much higher holdups were observed in the free-rise region for the same flow rates (Figure 2 ) . For this case, the droplets were highly unstable and collapsed in the order of minutes, before the entire column could be packed. Various factors could be attributed to the initial packing of the droplets, the temporary stability, and subsequent collapse. First, for mass transfer from the continuous phase, smaller drops and subsequently higher free-rise holdups are obtained than for the case of mass transfer from the dispersed phase, indicating that packing of droplets is more probable in the former case. Furthermore, there was greater stability in terms of duration of the layer of packed droplets under the coalescer for the case of mass transfer from the continuous phase. This was attributed to the Marangoni effect, discussed by Groothuis and Zuiderweg (1960). A schematic of the stability of packed droplets for mass transfer in either direction is represented in Figure 3. The subsequent collapse of the packed droplets, even when solute is transferred to the drops, can be explained by noting that a large degree of turbulence and backmixing exists in spray towers (Treyball, 1963). If the liquid film between the packed drops becomes depleted in solute, as is the case for transfer from the film, drop stability is enhanced (Figure 3, a ) . If this film through flow nonuniformities becomes dilute in solute, possible transfer from the droplet could occur, promoting the kind of instability denoted in Figure 3, b. I t was observed that partial collapse leads to extremely large drops which quickly rise through the packed droplets, promoting even more backmixing. Entire collapse then occurred in a short time.
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Figure 2. Effect of presence of solute on holdup
0 Free-rise heat transfer from kerosine AFree-rise mass transfer from kerosine, 3 wt. Yo MEK in feed WFree-rise mass transfer from water, 3 wt. % MEK in feed Effective column height. 10 feet Continuous phase. Water Distributor. % 2-inch holes t b . water/lb. kerosine = 0.49 to 0.51
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Figure 3. Stability of packed droplets when solute is transferred a.
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Movement of droplet Direction of surface flow
Conclusions
Efficient operation of spray tower extractors containing packed droplets is possible if mass transfer occurs from the continuous phase. Furthermore, to ensure stability, the eventual collapse of these droplets might be prevented through the use of a proper surfactant system in a fashion similar to that recently reported by Li (1968). Although it is expected that the interfacial resistance to mass transfer could be sizably increased, the proper system could be chosen such that a modest decrease in the overall mass transfer coefficient, k , could be more than offset by the large surface area per unit volume occurring for packed droplets. Thus, the desired ultimate result would be an increased ha. Acknowledgment
The author expresses his appreciation of the interest and advice of M. R. Fenske and coworkers at the Pennsylvania State University, where the experimental study was performed. literature Cited
Appel, F. J., Elgin, J. C., Znd. Eng. Chem. 29, 451 (1937). Bauerle, G. L., Ahlert, R. C., IND.ENG. CHEM.PROCESS DESIGNDEVELOP. 4, 225 (1965). Davies, J. T., Rideal, E. K., “Interfacial Phenomena,” 2nd ed., Academic, New York and London, 1963. Dunn, I., A.Z.Ch.E. J . 11, No. 1, 159 (1965). Elgin, J. C., Browning, F. M., Trans. A m . Znst. Chem. Eng. 31, 639 (1935). Elgin, J. C., Browning, F. M., Trans. Am. Znst. Chem. Eng. 32, 105 (1936). Fleming, J. F., Johnson, H. F., Chem. Eng. Pmgr. 49, No. 9, 497 (1953). Garwin, L., Smith, B. D., Chem. Eng. Progr. 49, No. 11, 591 (1953). Greskovich, E. J., Ph.D. thesis, Pennsylvania State University, University Park, 1966.
Greskovich, E. J., Barton, P., Hersh, R. E., A.Z.Ch.E. J . 13, No. 6, 1160 (1967). Groothuis, H., Zuidenveg, F. J., Chem. Eng. Sci. 12, 228 (1960). Groothuis, H., Zuiderweg, F. J., Chem. Eng. Sci. 19, 63 (1964). Hayworth, C. B., Treybal, R. E., Znd. Eng. Chem. 52, No. 6, 1174 (1950). Johnson, H. F., Jr., Bliss, H., Trans. Am. Znst. Chem. Eng. 42, No. 2, 331 (1946). Kehat, E., Letan, R., IND.ENG.CHEM.PROCESS DESIGN DEVELOP. 7, 385 (1968). Letan, R . Kehat, E., A.Z.Ch.E. J . 11, 804 (1965). Letan, R., Kehat, E., A.Z.Ch.E. J . 13, 443 (1967). Li, N., IND. ENG. CHEM.PROCESS DESIGNDEVELOP.7, 239 (1968). Null, H. R., Johnson, H . F., A.1.Ch.E. J . 4, No. 3, 273 (1958). Scheele, G. F., Meister, B. J., A.Z.Ch.E. J . 14, No. 1, 9 (1968a). Scheele, G. F., Meister, B. J., A.1.Ch.E. J . 14, No. 1, 15 (1968b). Treybal, R. E., “Liquid Extraction,” 2nd ed., McGrawHill, New York, 1963. Varteressian, K. A., Fenske, M. R., Znd. Eng. Chem. 28, 928 (1936a). Varteressian, K. A., Fenske, M. R., Ind. Eng. Chem. 28, 1353 (1936b). Woodward, T., Chem. Eng. Progr. 57, No. 1, 52 (1961).
E . J . Greskovich Esso Research and Engineering Co., Florham Park, N . J . 07932 RECEIVED for review September 3, 1968 ACCEPTED April 18, 1969
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