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2. Water is as effective as 0.1 il’ hydrogen ion solution in leaching sodium ion from glass (Table I). 3. Glass soaked in 6 N hydrochloric acid for 50 hours shows approximately the same ability to sorb sodium ion from neutral solutions as does glass soaked in water for 50 hours (Figure 2). 4. A change of lo6or more in the hydrogen ion concentration of the immersion solution changes the amount of sodium ion sorbed on glass by only ten- or twentyfold [Figures 3 and 4, and Figure 2 of ( l l ) ] . 5 . Under certain conditions a t least, sodium ion is lost from glass to a neutral aqueous solution of sodium ion faster than it is gained (Figure 1). 6. The rate of sorption of sodium ions by glass is increased by prolonged standing of the sorption solution in soft glass or borosilicate glass prior to the tests (10) but not by contact with platinum. 7. Physical tests show an attack and/or swelling of glass surfaces by prolonged contact with aqueous media ( 3 , d , 6, 8, l g , IS, 17). 8. Glass absorbs phosphate ions, the amount of sorption being dependent on the extent of pretreatment of the glass with caustic (9). ACKNO WLEDGRIENT
This work was supported in part by the United States Atomic Energy Commission, in part by the University Research Committee with money made available by the Wisconsin Alumni Research Foundation, and in part by an AMERICAN CHEMICAL SOCIETYFellowship. The authors are indebted to Philip H. Abelson and the Carnegie Institution of Washington for the Nazz used in this work. The neutron irradiations were made in the heavy water pile of the Ar onne Kational Laboratory, with the helpful cooperation of the faboratory staff.
Vol. 44, No. 4
LITERATURE CITED
(1) Bancelin, M.J., J . chim. phys., 22, 535 (1925). (2) Bauer, C.,Helv. Chim. Acta, 25, 1202 (1942). (3) Berger, E.,J . SOC.Glass Technol., 20, 257 (1936). (4) Biscoe, J., J . Am. Ceram. SOC.,24, 262 (1941). (5) Devaux, H.,and Aubel, Compt. rend., 184,601 (1927). (6) Foerater, F.,2. Imtrumentenk., 13, 457 (1893). (7) Hafner, H.C.,and Jones, F. L., J . Am. Ceram. SOC.,26, 68 (1943). ( 8 ) Ibid., p. 56. (9) Hensley, J. W., Ibid., 34, No. 6, 188 (1951). (10) Hensley, J. W., unpublished work. (11) Hendey, J. W., Long, A. O., and Willard, J. E., IND. ENQ. CHEM.,41, 1415 (1949). (12)Hinson, A. L., Smith, D. C., and Greene, J. F., J . Am. Ceram. SOC.,30, 211 (1947). (13) Hubbard, D., and Hamilton, E. H., J. Research Natl. Bur. Standards, 27, 143 (1941). (14) Kreidl, N. J., Trumm, B. J., and Scott, R. F., J . Am. Ceram. SOC.,24, 225 (1941). (15) Lang, E. P.,IND. ENG.CHEM.,ANAL.ED., 6, 111 (1934). (16) Leutwein, F.,Zentr. Minera2. Geol., 1940A,129. (17)Lyle, A. K.,J . Am. Ce-i-am. SOC.,26, 201 (1943). (18) Marboe, E.C.,and Weyl, W. A.,Ibid., 30, 320 (1947). (19) Scheringa, K.,Pharm. Weekblad, 56, 8 (1919). (20) U. S. Atomic Energy Commission, “Radioisotopes,” Cat. 2 (1947). RECEIVED for review July 28 1951. ACCEPTED October 16, 1961. Presented in part before the Division of Physical and Inorgania Chemistry at 116th Meeting of the AMERICAN CHEXICAL SOCIETY, Atlantic City,, N. J.
Wetted-Wall Tube-Plate Column development
A GAS-LIQUID CONTACTOR JOSEPH J. MARTIN’ UNIVERSITY OF ROCHESTER, ROCHESTER, N. Y.
0
F THE many different types of equipment which have been
proposed for contacting gas and liquid phases, probably the two which figure most prominently in present chemical engineering practice are the plate column and the packed column. These columns are somewhat similar in that in both a rising stream of gas contacts a falling stream of liquid; however, they are quite different in respect to the continuous and discontinuous phases and the manner in which large area of contact between the phases is promoted. I n the packed column both the liquid and gas phases are more or less continuous at all points and contact area is produced by deforming the two streams as they pass over and around packing materials of many different sizes and shapes. I n the plate column the liquid phase is continuous, except at high gas velocity, while the gas phase is made discontinuous by forcing it through small openings into the liquid. The bubbles of gas so produced give large contact area between the phases. The plate column at high gas velocity exhibits a different picture, however, for the gas tends to blow droplets of the liquid into the space above the plate, thus making the liquid phase discontinuous, The area of contact between phases is increased by such spattering or foaming and better mass transfer is noted directly by the greater plate efficiency which results (8). At some point, though, the gas velocity becomes sufficient to carry the liquid 1 Present address, Department of Chemical and iLIetallurgioal Engineering, University of Michigan, Ann Arbor, hlich.
droplets to the plate above and the irreversible mixing of liquids from two plates causes a decrease in plate efficiency. Nevertheless the effect of liquid spattering in promoting mass transfer is so pronounced that in some cases i t has been found that removal of every other plate in the column resulted in better over-all column performance by permitting higher gas velocities and therefore more spray of liquid droplets before entrainment became serious ( 2 ) . One might logically conclude from this description of column operation that if the liquid phase could be dispersed effectively without encountering serious entrainment, it might be possible to obtain high capacity and high efficiency of mass transfer at the same time. With this aim in mind, a new column has been designed and constructed and its performance characteristics as a distillation unit are the subject of this paper. The new column has been designated as a wetted-wall tubeplate column, the details of which are shown in Figures 1 to 4. The column possesses metal plates fitted with tubes (short pieces of commercial pipe) which permit the flow of gas and liquid. The liquid pours over the top of the tubes on each plate and down the inside walls to the pointed ends. The gas enters the archways between the pointed ends which are only slightly immersed in the liquid on the plate below. Passing upward through the centers of the tubes, the gas is forced to make a sharp turn before entering the next set of archway openings, as the tubes are horizontally
April 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY
displaced from plate to plate. This is accomplished by simply rotating the identical plates 60 degrees with respect to each other around the vertical axis of the column. In this investigation three different tube diameters or pipe sizes were used, while the tube length was maintained constant.
921
Raoult’s law for these similar compounds, and because of the ease of determining mixture compositions by measuring refractive index. The test runs were all conducted a t total reflux, permitting continuous recycling of the charge to the reboiler. The overhead vapor was totally condensed under a slight pressure, producing the reflux which was pumped through a rotameter to obtain the flow rate and then conducted to the top plate. The reboiler, each plate, and the condenser were e uipped with liquid sampling the liquid could be deterconnections so that compositions mined. Manometers were installed on the reboiler and the condenser to give the pressure drop across the column.
07
Figure 1. Wetted-Wall Tube-Plate Detail
The column may be considered a sort of combination of a wetted-wall column and the downcomers from the plate column, It differs primarily from the wetted-wall column in that the tubes do not run the full length of the column, and from the plate downcomers in that the bottoms of the t u b a are not submerged below the liquid level. EXPERIMENTAL OPERATION
Figure 3. Column Operating at Low Velocity with Liquid Flowing Smoothly Down Inside of Tubes
The column has so far been used only for distillation, though it mi h t well be utilized for other operations requiring mass transfer%etween gas and liquid phases. As shown in Figure 2, the col-
QUALITATIVE RESULTS
From a purely descriptive point of view two distinct regions of performance were observed through the glass walls of the column. At low gas velocities the column assumed a quiescent appearance (see Figure 3), marked only by a small flow of liquid from the bottom tips of the tubes. As the velocity was increased, a point was reached where drops of liquid were picked off the falling film inside the tubes and swept upward by the gas stream, thus filling the volume between the plates with liquid spray, as shown in Figure 4. Since the gas stream was forced to make a sharp turn, as previously mentioned, before entering the next set of tubes, the droplets of liquid were subjected to a strong centrifugal effect and deposited on the outside of the tubes, extending down from the next plate and on the surface of the liquid on the plate. In this manner the tubes served as entrainment separators. The spray effect started almost simultaneously on all plates and remained to the flooding point. Although stable operation existed in most of the range where spattering occurred, the column became very unstable a t the highest velocities and became flooded in much the same way as any other column. Figure 2. Flow Diagram for Experimental Column
u m n was operated in the usual manner in conjunction with a reboiler and condenser. A mixture of trichloroethylene and tetra-
chloroethylene (“tri” and “tetra”) was employed in the test because of the simple equilibrium curve (see Figure 5 ) obtained from the pure component vapor pressures and the assumption of
t
QUANTITATIVE RESULTS
The quantitative results correspond almost perfectly with the qualitative results, with two distinct regions of operation again noted. The pressure drop across the whole column, as given in Table I and Figure 6, was almost negligible until spattering began, being only about an inch of water or less. It
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will be observed there was c o n s i d e r a b l e variation in pressure drop in this .range; this waa due to the fact that the manometers were not very p r e c i s e a t low pressures. Once s p a t t e r i n g started, which occurred at a vapor velocity in all tubes of about 6 or 7 feet per second, the pressure drop increased fairly rapidly; however, even a t the highest velocities it was only about 6 inches of water for the five-plate column, which is only 1.2 inches per plate and considerably less than in an ordinary bubble-cap column. Moreover, the vapor velocity a t the high pressure drops was much greater than canusually be obtained in a bubble-cap column. The mass transfer results are of the greatest interest. These are given in Table I and Figures 8 and 9 in the form of height of a transfer unit, H.T.U., proposed by Chilton Figure 4. Column Operating at High Velocity with and Colburn (I),as a function Liquid from Inside of of vapor velocity. The data Tubes on Lower Plate might have been equally well Spraying on Outside of analyzed in terms of height of Tubeson Next Higher Plate a theoretical plate or plate efficiency; however, since the calculations were made across only the three center plates, almost always the results would involve one or two theoretical plates and a significant fraction of a plate. Because the fraction of a plate is inconvenient and because the results are to be eompared with other types of c o l u m n s 10 with different plate spacing, analysis by the H.T.U. method I ?OS was preferred. Figure 7 gives the defmition of the transfer unit used in the calc u l a t i o n s , and the $04 height of a transfer E unit was o b t a i n e d 5 simply by dividing the height involving t h e three center Y 8 ae a4 a6 08 lo plates, 19.5 inches, by a, MOLEFRACTION TRlWLOROFMnENE IN the number of transLlOUlO fer units. I n Table I Figure 5. Vapor-Liquid Equilibliquid compositions rium for System Trichloroethylon the first and fourth ene-Tetrachloroethylene plates are given as deBased on Raoult’s law termined from the refractive index measurements. At total reflux these compositions were taken to be the same as those of the vapor on the same plate, thus permitting the calculation of height of a transfer unit based on over-all gas concentration driving force, y * - y, where y = mole fraction of trichloroethylene in the vapor phase and y * = mole fraction of trichloroethylene in vapor which would be in equilibrium with liquid. TaMe I1 gives the properties of trichJoroethylene and tetrachloroethylene as required for this study.
E
io. E”
923 Figure 8 shows that up
to a vapor velocity of about 6 feet per second in the tubes, the H.T.U.iocreases, which means that the column is equivalent to fewer transfer units or fewer theoretical p1at-s. This behavior corresponds closely with that exhibited b y t h e usual packed column, and is exactly what would be expected aa long aa the area of contact between the phases is herely the liquid surface I 2 S 4 5 818SK) I5 inside the tubes and on VAPOR VELOCITY IN NBES,FT/SEO the plates. The outstandFigure 6. Pressure Drop in ing characteristic of t#he Experimental Column column is revealed above a vapor velocity of 6 feet per second when spattering sets in and causes the H.T.U. to decrease sharply with increasing vapor velocity. The decrease is evidently due to the creation of the new contact area of all the droplets of liquid that are formed during spattering, for I t would not be expected that the mass transfer coefficients per unit area would change appreciably. Such an effect is obviously b e n e ficial because it means that a t higher velocities the column is equivalent to more transfer units or more theoretical plates. Therefore, better separation in a given distillation is accomplished at higher column capacities, The decrease of H.T. U. in the spattering range is, of course, the same situation as occurs in the bubblecap column. I n the bubble-cap column, though, the vapor velocity cannot be increased very much because the amount of entrained liquid soon offsets any gain in mass transfer due t o the increased area of droplets created, The principal advantage af the new column is that its unique construction permits high vapor velocities without entrainment. IDEALIZED COLUMN
It seems reasonable that correct interpretation of the h o v e results may lead to the design of a commercial column possessing unusual performance characteristics. I n the experimental column the tubes were not placed nearly so close togefher as they might be. By assuming tubes with wall thickness 0.05 their inside diameter and placed a t the vertices of equilateral triangles whose sides are 1.2 times as long as the inside diameter of t,he tubes, the characteristics of an idealized column have been c a1cu 1a t ed . For this idealized column the crow sectional area of the inside of the tubes en any one plate (Le., on half t h e total number of triangular vertexes, as no tube is directly above or below another) is 31.6% of the total cross-seetional area of the column. Therefore, Figure 7. Number of Transfer Units the curves of Figu r e 9 h a v e been
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Vol. 44, No. 4
3e
28
e4
B g20
a 6 5 16
I2
8 0 SUPERFICIbL VELOCITY BASE0 ON EMPTY COLUMN. FVSEC
Figure 8.
TABLE11.
PROPERTlES
Formula Molecular wt. Spec. gravity 2Oo/4O C. Refraotive inhex a t 25O C. Vapor pressure, mm. Hg. Temp., O F . 131.4 194 203 212 221 230
239
Experimental Column Performance
760 830 970 1120 1290 1480 1680
.,..
249.4
Figure 9.
OF THE DISTILLATION COMPONENTS
Trichloroethylene CzHCli 131.4 1.462 1.4740
VAPOR VELOCITY IN TUBES, FT/SEC.
Tetrachloroethylene CzClr 165.3 1.623 1 ,5030
...
300 353 413 480 560
645 760
Experimental Column Performance
characteristics are approximately the same-Le., same number of transfer units in a column or same number of theoretical platesthe new design would give approximately three times as great a capacity and no more pressure drop. Furthermore, the calruIations for the idealized column still assume the largest tubes to be the 1-inch pipes and the plate spacing to be 6.5 inches, It is entirely probable as indicated by the results that in the commercial column larger tubes and closer plate apacing might be successfully employed. I n that event capacities would be even greater than those indicated in Figure 8 and the H.T.U.’s much lower. For example, if the plate spacing were cut to about 3 inches the H.T.U.’s might conceivably fall to half their value as calculated. There are a number of other factors which would place the new design in a very favorable position for consideration as a gasliquid mass transfer device. The simplicity and low cost of the tubes should not be overlooked. Since these could be made from ordinary pipe or tubing, the cost would probably be but a fraction of bubble cap costs. Also the tubes would have little tendency to plug up since there are no small constrictions, and maintenance costs should be reduced. Uniform distribution of the liquid on a plate would be another advantageous factor. Since every tube serves as its own downcomer as well as the passage for the upflow of gas, there are no liquid gradients to contend with on the plates. I n bubble-cap columns much over 5 feet in diameter severe liquid gradients often cause some caps to be so far immersed in the liquid, they fail to permit the passage of vapor, and only a fraction of all the caps are functioning. I n the proposed column with proper leveling of the plate, all tubes would function alike regardless of column diameter An advantage over the packed column is that intermediate fractions can be withdrawn from different plates just as in the usual bubble plate column
SUPERFICIAL VELOCITY BASED ON EMPTY COLUMN, FT/SEC.
Figure 10.
ACKNOWLEDGMENT
Idealized Column Performance
transferred to Figure 10 by merely multiplying the vapor velocity in the tubes by the factor 0.316. Since in distillation it is desired to obtain maximum capacity with a minimum H.T.U. and since pressure drop is usually not a prime consideration unless it is large, the optimum point of design and operation appears to be to use thel-inch pipe tubesand a velocity of about 3 feet per second based on the empty column. For vapors as dense as those used in the experimental column a bubble-cap column would probably entrain seriously a t a velocity of about 0.9 foot per second, which means that if the mass transfer
The author is indebted to the following men for their assistance in building and testing the new column: Jerome Gillette, Edward Mason, Irving Siller, Robert Stroman, Peter Togailas, and Hendrik Van Ness. LITERATURE CITED
(1) Chilton, T. H., and Colburn, A. P., IND. ENQ.CHCX.,27, 255
(1935).
(2) Perry, J. H., ed., “Chemical Engineers’ Handbook,” 3rd ed., pp. 614-15, New York, McGraw-Hill Book Go., 1950. RECEIVED for review February 15, 1951. ACCEPTED February 7, 1952. Presented at the Sixth Southwest Regional Meeting of the AMERICAN CHEMICAL SOCIETY. San Antonio, Tex.
