Countercurrent Extraction of Benzoic Acid between ... - ACS Publications

F. J. Appel, and J. C. Elgin. Ind. Eng. Chem. , 1937, 29 (4), pp 451–459. DOI: 10.1021/ie50328a022. Publication Date: April 1937. ACS Legacy Archive...
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INDUSTRIAL AND ENGINEERING CHEMISTRY

APRIL, 1937

P P R t t'

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v

v w Y

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B Y

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= partial pressure of solute gas (chlorine) mm. Hg = partial pressure of solute gas (chlorine), atm. = gas conztant C. = temp., = temp., F. = temp., K. = molal volume, liters vol. of tower, cu. ft. = weight solute absorbed, lb. = ratio of ofconcn. of total chlorine in waterto concn. of = chlorine in air = concn. of unhydrolyzed chlorine in water, grams/liter in water, @;rams/1iter Of = = ratio of concn. of unhydrolyzed chlorine in water t o concn. of chlorine in air gas law = p5jRT = coefficient of deviation from the = time, min.

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451

Subscripts: c = critical state G = gas L = liquid M = logarithmic mean of the driving force R = reduced conditions

Literature Cited (1) Arkadiev, V.9 J. Buss. Phw-Chern. Soc.9 50, 205 (1918). (2) Cope, Lewis, and Weber, IND. ENQ.CHEM.,23, 887 (1931). (3) Gilmour, Lockhardt, and Welcyng, Mass. Inst. Tech., School of Chem. Eng. Practice, Bangor Sta. Report, 1928. (4) International Critical Tables, Vol. 111, p. 256, New York, McGraw-Hill Book Co., 1928. ( 5 ) Yakovkin, A., J . Ruse. Phus.-Chem. SOC.,32, 673 (1900). R R E ~ I YNovember ~D 25, 1936.

Countercurrent Extraction of Benzoic Acid between Toluene and Water' PERFORMANCE OF SPRAY AND PACKED COLUMNS The performances of columns of the spray and of the packed type for solvent extraction were studied using the system toluene-benzoic acid-water. Capacities of the columns were evaluated in terms of an extraction coefficient, K W a , based on the water phase. H.E. T. P. and H.T. U. for typical runs were also calculated. Data for the holdup of the discontinuous phase in such columns are included. The capacity of the spray column depended on the rates of feed of both the discontinuous and the continuous solvents, on the flow ratio,

and especially on the size of the drops of the former produced by the entrance nozzle. In the packed column, capacity depended upon the rate of feed of the discontinuous phase and only slightly upon that of the continuous, and varied inappreciably with the drop size produced by the nozzle. In general the dependence of holdup on these variables was similar to the capacity. For present conditions the capacity of the packed column was intermediate between that of the spray for the smallest and largest drop size studied. A spray column may be either more or less effective than a packed, depending upon the drop size produced by the nozzle and the nature of the packing.

F. J. APPEL AND J. C. ELGIN Princeton University, Princeton, N. J.

A

LTHOUGH theoretical aspects of liquid-liquid solvent

extraction operations are becoming well developed, few of the data required for their application t o the design of actual extraction equipment are yet available. This is particularly true of the rate of extraction and the capacity of column contacting equipment. Extracting acetic acid between water and isopropyl ether, Elgin and Browning (8) studied the capacity and operation of a spray-type extraction column as affected by the more important variables determining the performance of such equipment. A theoretical analysis of the spray column was also developed. Qualitatively, this was found to inter-

pret the observed data satisfactorily, and the latter were concluded to be characteristic of the behavior of the spray column. In the present communication additional data on the spray column are reported, using the extraction of benzoic acid between toluene and water, and the work is extended to include the capacity of a packed column with this system. The effect of rates of flow of the two solvents and the drop size of the discontinuous phase at entrance have been more thoroughly investigated, and holdup of the dis1 Absorption and Extraction Symposium. Several of the papers presented at this symposium appear on pages 270-318 of our March issue. Others will be printed in subsequent issuea.

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INDUSTRIAL AND ENGINEERING CHEMISTRY

CONC. OF B E N Z O I C ACID I N BATER, LE. MOLS/CU.

FT.

VOL. 29, NO. 4

(Cg)

FIQURE 1. EQUILIBRIUM DISTRIBUTION OF BENZOIC ACID BETWEEN TOLUENE AND WATER tributed solvent in the column, which is intimately connected with the capacity, determined. Use was made of the toluene-benzoic acid-water system because of suitability as a measuring stick rather than for particular industrial significance. Considerations guiding its selection were suitable physical and chemical properties, availability at low cost, and analytical factors. Negligible mutual solubility of the solvents and approximation to the simple distribution law facilitate calculation of data. The relatively high surface tension of the solvents was also desired because of possible effect on the distribution of the discontinuous phase a t entrance and the possible appearance of drop coalescence within the column as a factor to be considered. Knowledge of the magnitude of extraction rate is supplied by the results, but the principal objective has been the characteristic influence of variables and a comparison of the “efficiency” of spray and packed columns for liquid extraction operations as a general guide to their design and operation.

Equipment While essentially the same in principle as that used by Elgin and Browning (9),the equipment and extraction column were considerably improved in details of construction and operation : The column was constructed of Pyrex glass pipe; the inside diameter was 2.03 inches, and the over-all height 5 feet. Its cross section was 0.0225 square foot, and 6-inch spaces a t each end were allowed as disengaging sections; its effective height and volume were 4 feet and 0.09 cubic foot, respectively. As the s ray ty e this space was unfilled. As a packed column it was ed wit . %0.5 X 0.5 x 0.5 inch commercial stoneware Berl saddle acking su ported on a wire grid immediately above the entrance For the ligEt solvent. Because of the small column diameter, the apparent packing density was roughly 40 per cent less than was specified for this packing under normal conditions. The free space measured by drainage in place was 78 per cent as compared with 68 per cent specified by the manufacturers for ordinary density. The glass construction permitted informative observations of the operation of the column and is recommended for similar studies. Small rotary pumps fed the solvents from storage through control valves and calibrated orifice meters to the column; toluene, the lighter solvent, entered at the bottom and water at the top. The phase made discontinuous was broken up by suitable entrance nozzles (described later). The continuous hase entered at a point just below the interface in the column. b t h packing in place the latter was maintained either immediately above or below the packed level. The interface could be adjusted and set at any desired level in the column, irrespective of throughput,

BP1

TOLUENE FLOW I N CU.FT./SIR./SQ.FT.

FLOW IN THIO FIGURE 2. EFFECTO F VARYING THEI TOLUENE SPRAY COLUMN ON THE COEFFICIENT WITH DIFFERENT ENTRANCE NOZZLES AND CONSTANT WATERFLOWS Letters indicate the nozzle used; number, the water flow.

by the simple hydraulic expedient of arranging the water exit line as one arm of an open U; the column served as the other arm. This provided better, less time-consuming, and less laborious control than either the manual or automatic manipulation of inlet and exit valves used in previous experiments. Toluene was made the discontinuous phase by setting the interface 6 inches from the top; water, by setting it at the bottom of the column. After passing countercurrently in the column, and separating in the disengaging spaces, the solvents were withdrawn from oposite ends and conveyed through large auxiliary settling cham&ersto discard or storage. Sampling cocks were provided in both inlet and exit lines to the column, and differential manometers for the measurement of pressure drop over the entrance nozzles when desired. ENTRANCE NOZZLES.Preliminary experiments with open-pipe entrances for the discontinuous phase, varying from 0.11 to 0.38 inch i. d., and shower-type nozzles with 0.040-inch diameter holes, immediately indicated that these would not produce a sufficiently small drop size of satisfactory range and unifoxmity, and that other means of breaking up the solvent must be sought. From such entrances toluene issued into the water in masses varying in diameter up to 1.5 inches. I n rising, these large masses tended to pancake, thus covering almost the entire cross section and collecting smaller globules in transit. Use of nozzles constructed of porous materials was found to be 8 satisfactory means for breaking up the solvent. Various grades and shapes of such materials as porous carbon and Filtros were tried. Nozzles constructed of either short tubes or disks of grades H and R Filtros were found t o be most satisfactory for present purposes and were adopted together with a Sprayco-5B nozzle for the experiments. The drops produced by grade H Filtros averaged between 0.06 and 0.12 inch, grade R Filtros, 0.2 inch, and Sprayco-5B, 0.4 inch in diameter. These are subsequently designated as nozzles A, B, and C, respectively. Area of the porous nozzles was such that in the experiments, flow through Filtros H ranged approximately from 0.1 to 2.7 and throu h Filtros R from 0.13 to 3.85 cubic feet per hour per square foot of nozzle area. Corresponding pressure drops were approximately from 0.2 t o 6.0 and from 0.6 to 1.6 pounds per square inch for the nozzles, respectively. The Sprayco nozzle was not employed for flows exceeding 50 cubic feet per hour per square foot of column since above this point water tended to emulsify in the issuing toluene drops. By qualitative observation, surprising constancy and uniformity of drop size from a given nozzle a p

APRIL, 1937

INDUSTRIAL AND ENGINEERING CHEMISTRY

'RITRR TLOY' I N CU.FT./HR./SQ.FT.

3. EFFECTO F VARYING THE WATER FLOW I N THE SPRAYCOLUMN ON THE COEFFICIENT WITH DIFFERENTENTRANCE NOZZLES AND CONSTANT TOLUENE FLOWS

FIGURE

Letters indioate the nozzle used; numbers, the toluene flow.

peared to be maintained over the flow range used although, at very high flows with Filtros H, a small percentage of the volume issued as decidedly finer drops. It appears that the size of drops produced by such porous materials is not varied greatly by altering the mechanical construction of the nozzle or by varying the rate of flow per unit of nozzle area. Pressure drop over the nozzle is, however, dependent upon the latter. With porous nozzles in use the entering solvent was filtered through fine screen and a Filtros disk to preclude gradual plugging by entrained sediment.

Experimenta1 Method ComIrrIoNs. Both capacity (rate of extraction) and toluene holdup in the spray and packed column were determined as a function of flow rate of each solvent and drop size of the discontinuous phase, the latter because of its intimate relation to the contact area and flooding. Owing to lack of information, a general procedure involving variation of the several factors individually while the others were maintained constant was necessary. Measurements were made over as wide a flow range as proved feasible with present equipment. I n some cases close approach to equilibrium, in others, characteristics of the entrance nozzle and excessive entrainment in the continuous phase due to entrance design prevented carrying the measurements t o the flooding point as was originally planned. Toluene was made the discontinuous phase except in the few cases noted. Benzoic acid was extracted from toluene by water primarily because control of feed composition was more readily attained with the discard of only small quantities of acid in the used water. Benzoic acid concentration was maintained constant in the toluene feed a t 0.0113 * 2% pound moles per cubic foot in all runs; the water fed contained no acid. Close regulation of the operating temperature of the column by jacketing and preheating the feeds was not deemed worth while. Solvents were stored and the column was operated a t room temperature. This

453

FIQURE 4. TOLUENE HOLDUPIN THE SPRAYCOLUMN WITH VARYINGTOLUENE FLOWFOR DIFFERENTNOZZLESAND CONSTANT WATERFLOWS Letters indioate the nozzle used; numbers the water flow.

varied between 15' and 18' C. (59" and 64.4' F.) during the course of the experiments but was 16" to 17" C. (60.8' to 62.6' F.) during the large majority of runs. Any possible effect of temperature occurred a t random and could not be detected between runs in the present work. Drop size was varied by changing from one to the other entrance nozzle, the range being from 0.06-0.12 to 0.4 inch. With each nozzle toluene flow was varied from 13.3 to 63.3, and then water from 13.3 to 133.3 cubic feet per hour per square foot of column cross section, while the flow of the other solvent was held constant a t each of a series of values. Conditions for the spray and packed columns were the same except that the packed column tended to flood a t lower flow rates, and data were obtained nearer the flooding point. PROCEDURE. Following adjustment of the acid concentration in the toluene feed, water feed from storage was begun and adjusted a t the desired rate. When the column was partly full, toluene feed, which was made the discontinuous phase, was started and adjusted, and the interface set a t the predetermined level. For the water phase discontinuous, the procedure was reversed and the interface was set a t the bottom of the column. Attainment of a steady state as determined by constancy of exit compositions then required 5 to 25 minutes of operation, varying with feed rate and corresponding to total throughput of four to six times the column volume. Another 15-minute period was allowed, and three to four samples of each terminal stream were then withdrawn a t 5-minute intervals and analyzed. Analysis of both solvents was accomplished volumetrically by titration with standardized solutions (of proper concentrations) of sodium hydroxide in ethyl alcohol after first ensuring complete miscibility by dilution of the samples with a known quantity of 95 per cent alcohol. Blanks were run on each solvent. The final composition of each stream was taken as the average of the three check determinations. When operating as a spray column, measurement of the volume of toluene held up in the effective volume of the column under the conditions of an experiment was accom-

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VOL. 29, NO. 4

moles of acid extracted per hour were calculated for each solvent and averaged. Where material balances varied more than 10 per cent, runs were rechecked except for the inclusion of a few a t random where variation did not exceed 15 per cent and rechecking was not feasible. From these the coefficient was calculated by the usual relation:

The usual simplifying assumptions and steady-state diffusion are implied in use of this relation. Hence the theoretical significance of the observed coefficients is subject to the restrictions imposed by these assumptions. It is believed that uncertainties introduced in virtue of these assumptions

b

-

I

I

86.6

9

WATER FLOW IN CU. FTJHR./SQ.FT.

FIGURE 5. TOLUENE HOLDUP IN THE SPRAY COLUMN WITH VARYINQ WATERFLOW FOR DIFFERENT NOZZLES AND CONSTANT TOLUENE FLOWS

\

plished by simultaneously stopping the feeds, permitting the column to drain slowly until the top of the column of liquid was set at the interface level during operation, and measuring the height of the toluene column above the water. Knowing its height during operation, the displacement of water by toluene, and hence the volume of the latter in distributed form in the column during operation, is readily derived. This was checked by draining the contents of the column and correcting the volume of toluene obtained for that held in the disengaging chamber. The latter procedure was that used with the column packed. The results are judged to be accurate to approximately 10 per cent. Merck's U. S. P. grade benzoic acid, Barrett Company's best grade commercial toluene, and Princeton tap water were the materials used. The toluene supply was limited to 50 gallons and was re-used after adjustment of its benzoic acid content. Used water was discarded. EQUILIBRIUM DISTRIBUTION. Data in the literature covered only approximately half the concentration range it was desired to employ, and had been obtained with c. P. materials and at 25" C. Equilibrium distribution of benzoic acid between the toluene and water actually used in the experiments was therefore determined by usual methods. The data obtained a t 14.5', 19', and 24' C. are shown in Figure 1 in terms of the equilibrium concentrations in water, C,, and in toluene, Ct, expressed as pound moles per cubic foot. The simple distribution coefficient, Ct/C,, calculated from these data is closely a linear function of the concentration in water, which fact ,facilitates interpolation and limited extrapolation. The observed data in the lower concentration range were found to agree surprisingly well with those reported in the literature.

Calculation and Presentation of Results The rate data are calculated and reported in terms of a capacity coefficient, Kwa, based on the water phase and expressed as pound moles of benzoic acid extracted per hour per cubic foot of tower volume per unit concentration difference. From terminal compositions and rate of flow, pound

0

20

JO

40

80

TOLUENE FLOl IN CU.Fp./HR./SQ.pT.

FIGURE 6 . EFFECT OF VARYING TOLUENE FLOW IN THE PACKED COLUMN ON THE COEFFICIENT WITH DIFFERENT NOZZLES AND CONSTANTWATER FLOWS

are insignificant compared with the effect of the varied factors and do not vitiate the conclusions reached. Practically, the mutual solubility of the solvents is negligible, the solutions are relatively dilute, and the equilibrium relation is practically linear over the concentration range obtained in the column for most of the runs. For present purposes, choice of the capacity coefficient to express rate of extraction in such equipment rather than other possible methods, such as H. E. T. P. and H. T. U. (I), was made inasmuch as it is the authors' opinion that interpretation and analysis of the results is thereby facilitated. H. E. T. P. is both less fundamental and more complex with relation to rate (4), and H. T. U. is less well known and more difficultly interpretable in such a case. H. E. T. P. and H. T. U. for both spray and packed columns have, however, been calculated and tabulated for typical runs in Table I for comparative purposes. The former is calculated by the common method, the latter by the method described elsewhere (1,3).

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INDUSTRIAL AND ENGINEERING CHEMISTRY

Toluene holdup has been reported as percentage of the effective tower volume, calculated as: VJV

x

100 = f = % holdup

Spray Column RATEOF FLOW.The dependence of the capacity coefficient, Kwa,on the rate of flow of the discontinuous phase, toluene, is shown in Figure 2 for each of the three entrance nozzles and a t different constant rates of flow of water, the continuous phase. Figure 3 shows the effect of varying the rate of flow of water (the continuous phase) for different constant flows of the discontinuous phase. As the figures show, increasing the rate of flow of either the discontinuous or continuous phase continually increases coefficient K,a except for the smallest drop size, nozale A , and a t the highest water flows for nozzle B. K,a varies more rapidly with flow of the discontinuous than the continu-

TOLUENE PLOW

13.3

NOZZLE A

0 NOZZLE B Q nmzIx c

'0

I

I

IS

30

I 4S

TABLE I. TYPICALH. T. U. AND H. E. T. P. VALUESFOR SPRAY AND PACKED COLUMNS ---Flow Rate-Toluene Water H. T. U. H. E. T. P.a Cu.f t . / h r . / s q . ft. Ft. Ft. Smav Column 23 A 21.3 49.6 1.3 2.4 19 A 21.3 73.0 1.6 16 A 63.3 73.0 1.2 i:7 124 B 13.3 44.4 7.2 8.8 24 B 34.7 44.4 3.5 6.2 27 B 34.7 133.3 7.2 29 B 57.7 79.2 1.7 4:2 121 c 13.3 44.4 26.3 27.0 C 34.7 44.4 12.6 48 51 c 34.7 133.3 30.1 53 c 44.4 79.2 13.1 15:0 Packed Column 99 A 13.3 44.4 3.6 5.5 102 A 22.2 26.6 1.7 106 A 34.7 26.6 1.2 4:0 94 B 13.3 44.4 9.4 7.0 72 B 34.7 44.4 2.6 4.9 74 B 34.7 84.5 5.2 76 B 44.4 44.4 1.9 4.3 111 C 13.3 44.4 6.3 7.3 34.7 44.4 2.8 GO C 64 c 44.4 44.4 1.9 4:3 84.5 6.7 8.9 62 c 34.7 a Since the number of theoretioal plates to which the column was equivalent was small, the H. E. T. P. values should be regarded as approximationm. Run No,

The actual toluene volume in cubic feet is readily determined from the results thus expressed by multiplying by 0.0009. Insufficient information and data and the complexity of the relations in such equipment preclude, for the present, the quantitative relation of K ,a to variables and quantitative interpretation of the results. The data are best treated graphically in the form of plots showing the quantities calculated above as functions of the variables studied and are so presented. Rate of flow is expressed throughout as cubic feet per hour per square foot of column.

I

I

J

60

76

90

WATER FLOW IN CU.FT./HR./SQ.Fl'.

FIQURE 7. EFFECTOF VARYING THE WATER FLOW IN THE PACKED COLUMN ON THE COEFFICIUNT WITH DIFFERENT NOZZLES AND CONSTANT TOLUENE FLOWS

ous phase. Further, the dependence of the coefficient on either is influenced by the rate of flow of the other and very markedIy by drop size. It varies less rapidly with larger initial drop size and a t lower rates of flow, the relation becoming almost linear under these conditions.

455

Noz~le

~-

..

..

DROPSIZE. The curves of Figures 2 and 3 fall into three groups which correspond to the different nozzles, A, B, and C, and hence to the drop size obtained. The smaller the drops of the discontinuous phase produced a t the nozzle, the larger is the capacity coefficient in the column, other conditions remaining constant. The marked dependenee of the capacity of a spray column on the subdivision of the discontinuous phase is convincingly demonstrated by these results. This is probably the most important single factor. It is also to be expected on theoretical grounds that a specified fractional reduction in size will be more effective in increasing capacity as the diameter of the drops becomes smaller. It should also be pointed out here that, although a reduction in the size of the discontinuous drops enhances capacity, the maximum throughput for the column is simultaneously decreased. With nozzle A the column was found to flood a t lower rates of flow. This question is being further investigated. COALESCENCE. The behavior of K,JI with increasing rates of flow noted with nozzle A and at the highest rates of flow with nozzle B is attributed to a reduction of the area of contact within the column, resulting from the coalescence of drops under these conditions. Coalescence if progressing in the column would increase the size of drops; increasing drop size, in turn, would function simultaneously to reduce the available surface per unit volume and to accelerate the velocity with which the drops rise through the continuous phase. Visual inspection of the column during operation indicated coalescence to be occurring, which is also supported by data on the toluene holdup presented later. The possibility exists that larger drops were produced by the nozzle itself by reason of the increased rate of flow through it. Inasmuch as the results in question were more pronounced with varying water flow and constant toluene flow (Figure 3) and that appreciably larger drops were not observed to issue from the nozzle as the rate of flow of the discontinuous phase increased, this is believed not to be the case. The constancy of the velocity of rise of the drops subsequently calculated from measurements of the toluene holdup checks the latter observation. Although the effect of variables on K , is unknown, it does not seem feasible on theoretical grounds to account for a decrease in capacity coefficient K,a in terms of K,.

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INDUSTRIAL AND ENGINEERING CHEMISTRY

456

I B

01

0

I 10

I 10

I

I

40

SO

TOLUENE FLON IN CU. FT. /tlR/SQ. FT.

FIGURE 8. TOLUENEHOLDUPIN THE PACKED COLUMN WITH VARYING TOLUENE FLOWFOR DIFFERENT NOZZLES AND CONSTANT WATERFLOWS

,

4 k d-

C l

I

I

dl

BOLUENE PLOW 13.s

4L

do

7r

'VATER FLOiV I N CU.FT./HR./SQ.FT.

m

I

'

FIGURE9. TOLUENEHOLDUP14 THE PACKED COLUMN WITH VARYING WATERFLOWFOR DIFFERENT NOZZLES AND COPSTANT TOLUENE FLOWS

With nozzles A and B the'behavior of the.holdup with It appears, therefore, that drop coalescence in the spray increasing rate of flows corresponds essentially to that noted column may play an important part and exert a practical for K,a. This subdtantiates the conclusion that the effect limit on the extent to which capacity may be enhanced by is due to the coalescence of drops of the discontinuous phase increasing flow and reducing drop dimensions. Small drop in the column. Curves A-O.0, B-O.0, and C (Figure 4) for size, low velocity of rise, high flow rate, and high surface the continuous phase stationary show that in this case the tension, the first three resulting in a large number of drops holdup of the discontinuous phase is practically a linear per unit volume as in t h e present case, are, in general, condifunction of its rate of flow. This is definite evidence that tions conducive to its occurrence. the sizes of drops produced by the nozzle in each case have TOLUENE HOLDUP.The volume of toluene as discontinuous not changed appreciably; if anything, they have decreased. drops held in the column is plotted against toluene flow in DISCUSSION.The experimental curves obtained for Kwaand Figure 4 and against water flow, the continuous phase, holdup are in excellent qualitative agreement with the theoin Figure 5. These data in each case represent holdup retical relations for the spray column advanced by Elgin and under the same conditions for which K d was measured, the Browning ( 2 ) . Qualitatively similar curves for the specific individual curves corresponding to those of Figures 2 and 3, contact area as a function of drop size and rates of flow are respectively. predicted by Equations 7 and 8 of these authors when assumed The striking similarity between K d and holdup in their values of the velocities of rise in a stationary column of the dependence on the rate of flow of both discontinuous and continuous phase of the continuous phases, which order of m a g n i t u d e obmay be seen by compariserved are s u b s t i t u t e d . son of these curves with TABLE 11. CALCULATED VALUES OF THE AVERAGERISE VEThe results are also analoLOCITY OF DISCONTINUOUS TOLUENE^ Figures 2 and 3, confirms gous to those found by these --u cslod. (Equation 2) uo a t the conclusion previously Toluene Water Water flow, Water flow, Water flow, authors for acetic acid exr e a c h e d ( 2 ) that the ca44.4 Flow Flow of 0 79.2 106.3 traction between isopropyl p a c it y coefficient in the Nozzle A ether and water in similar . 21.3 190 145 126 129 spray column is governed 49.7 190 194 210 ... equipment. Although the primarily by the effect of 63.3 180 203 ... ... conditions are not identical, variables on a. In other Av. 187 Nozzle B . .~_._ especially with regard to words, variations in the 22.2 718 653 627 508 drop size, Kwa is roughly contact area mask those of 740 653 608 435 44.4 622 583 443 390 57.7 of the same order of magniK,. As s h o w n ( 2 ) , t h e Av. 693 tude in both systems for volume of the discontinuous Noszle C the S p r a y c o - 5 B nozzle phase in the column is deter1050 1650 22.2 1650 1650 1650 1650 1650 1650 34.7 (nozzle C) e m p l o y e d in mined by drop size and rate 44.4 1650 1650 1650 1650 both cases. It seems safe of flow; the contact area is, Av. 1660 to conclude that these rein turn, a function of holdup a Flows in cu. ft./hr./sq. f t . : u in ft./hr. sults are, in general, charand drop diameter.

-

~~

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INDUSTRIAL AND ENGINEERING CHEMISTRY

-TOLUENE FLOW I N Cy,FT./HR./SQ.FT.

FIGURE 10. COMPARISON OF THE CAPACITY OF SPRAYAND PACKED COLUMNS WITH VARYING TOLUENE FLOW FOR DIFFERENT ENTRANCE NOZZLES Letters indicate the nozzle used; numbers, the water flow.

acteristic of solvent extraction in a spray column and to emphasize that the effect of operating variables on a is the important consideration. Much more work will be required before quantitative correlation of K,a with the variables involved can be expected. It is worth while to point out that the experimentally measured holdup data in conjunction with the theoretical relation shown by Elgin and Browning (2) permit a calculation of the average upward velocity of the discontinuous phase. The relation is Vt

=

(5)

FD

or, in terms of the percentage holdup, f,and F D ~

where u becomes uo when the continuous phase, in the present case water, is stationary; u should be independent of the rate of flow of the discontinuous phase (toluene here) if the drop size at the nozzle is constant. Values of uoand of u at different water flows, calculated for typical runs where evidence of coalescence was not immediately discernible, are recorded in Table I1 for each of the three nozzles. These serve to show the order of magnitude of the velocity of rise and the effect of the countercurrent flow of the continuous phase. The constancy of uo and the negligible effect of the flow of the continuous phase on u for large drop size as compared with the smaller are especially to be noted.

Packed Column OPERATION.The column was operated under the same conditions as the spray or unfilled column. Inasmuch as

457

the column diameter was only four times that of the packing, the wall effect was excessive and the results were not the same quantitatively as those to be expected for a largescale column. Toluene when discontinuous rose in globules through the interstices of the packing, except for a tortuous path through the free space and distortion from the spherical, much the same as in the spray operation. Momentary hanging of the globules a t points of contact between packing and between packing and walls, and coalescence and subdivision of globules were occasionally seen. These were more pronounced a t higher flows of water. After traversing a short length of packing, no further change in globule size or velocity in the packing and no material difference with different nozzles could be observed. The packing appears to function in this case primarily to regulate to approximately constant values the drop size and the velocity of rise. Water when continuous and falling through the toluene, however, appeared to wet preferentially the packing and flow down over its surface in a continuous h.Where such a situation exists there is expectation that changes in the capacity and its relation t o flow may arise from inversion of the phase made discontinuous. The few experiments carried out in the present work with water discontinuous indicate this trend but are too few to be conclusive. RATES OF FLOW.The dependence of coefficient K f i on toluene flow (the discontinuous phase) is shown by the curves of Figure 6. Data for the effect of rate of water flow (the continuous phase) are plotted in Figure 7. K,a increases with the rate of flow of the discontinuous phase, and the rapidity of this increase depends somewhat on the rate of flow of the continuous. Although minor differences in the experimental curves for different nozzles are evident, it is concluded that, in contrast to the spray column, the capacity coefficient of a packed column will be determined almost exclusively by rate of flow and the geometry of the packing rather than the subdivision of the discontinuous phase a t the nozzle. Dependence on water flow, the continuous phase, is small compared in Figure 7 with the continuous and that found in the spray column. The trend is not definite nor is the behavior, although generally similar, entirely consistent between different nozzles. T h a t the flow of the continuous phase has less effect than the discontinuous is, however, apparent. The character of the K,a flow curves with different nozzles as well as the trend of the coefficient with water flow is probably governed to some extent by coalescence, and by the height which drops of different size entering the packing must travel before attaining the size characteristic of it. Nozzle A, which gave drops considerably smaller than appeared .to exist in the packing, gave consistently higher coefficients. If true, this factor would be eliminated by increasing the column height. TOLUENE HOLDUP. The volume of toluene existing in the packed space under the same conditions for which K 5 was measured is plotted against toluene flow in Figure 8, and against the rate of flow of water, the continuous phase, in Figure 9. Based on the packing free space rather than its apparent volume, the percentage holdup would be approximately 28 per cent greater than shown by these curves. The holdup rises with toluene flow and depends only slightly on water flow, except where the former was large. Although the curves differ for different nozzles, the divergences are insufficient to indicate that the size of drops formed by the nozzles is a major factor. It is believed that they may be accounted for principally in terms of random coalescence and channeling and of the height which drops must traverse to assume that size characteristic of the packing. The relation between holdup and K,JLis not so pronounced as in the spray column, but is sufficiently pronounced to in-

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the packed column should not be essentially dissimilar from the spray except that a constant drop size and velocity are maintained in the former. Hence K,a should be a linear function of the rate of flow of the discontinuous phase if K,a is not appreciably varied. That this is approximately realized experimentally can be seen from Figure 6. The weight of the evidence seems to make inescapable the conclusion that we must interpret the characteristics of the packed extraction column primarily in terms of the effect of variables on the contact area.

Comparison of Spray and Packed Column

WATER FLOW IN CU.FT./HR./SQ.FT.

B'IQ~RE 11. COMPARISON OF THE CAPACITY OF SPRAY AND PACKED coLuMNs WATER "Ow 'OR ENT ENTRANCE NOZZLES Letters indicate the nonsle used; numbers, the toluene flow.

dicate that variations in the contact areas are a primary factor. The dependence of K,a on rate of flow would not be expected on theoretical grounds to be identical with that af the holdup in view of the points noted above and inasmuch as the velocity of drops in the column is regulated primarily by the packing. DISCUSSION. If the upward velocity of toluene drops in the packing is relatively constant, Equation 2 demands that holdup of this phase be a linear function of its rate of flow. The curves of Figure 8 show that this condition is approximately realized experimentally. Further, the operation of

Typical data for the capacity coefficient found for the spray and for the packed column under corresponding conditions are reproduced on the same plot to facilitate comparison, Figure 10 for varying toluene flow and Figure 11for varying water flow. Under the present conditions with either nozzle the capacity coefficient for the packed column is approximately the same as (actually somewhat less than) that obtained with nozzle B, the intermediate drop size, in the spray column. Dependence on the flow of the discontinuous phase is essentially the same. For the spray column the capacity is considerably greater than this value for small drops (nozzle A), and less for the larger drops produced by nozzle C. That is, the introduction of packing into the column has destroyed the effect produced by altering the subdivision of the discontinuous phase as it enters the column. The relative order of magnitude of the results would undoubtedly differ with the extraction system employed and with varying design of equipment. However, the data justify certain conclusions regarding spray and packed columns as equipment for solvent extraction. The capacity of a column if operated as the spray type may be either greater or less than if packed, depending upon the size of the drops into which the discontinuous phase is subdivided a t the entrance. Packing will reduce the capacity if it tends to increase this drop size and its velocity, and increase it if it tends to diminish these. I n turn this condition is governed by packing type and dimensions. A choice between spray and packed column for conducting a solvent extraction operation therefore rests primarily upon whether a sufficiently small drop size for the discontinuous phase can be produced a t the entrance and upon the accompanying.cost as against the cost of packing. Flooding of such columns is also a consideration since it limits the throughput possible with a given diameter. A few data obtained in the present study indicate that a packed column will flood a t considerably lower throughputs than a spray when conditions are such that their capacity, K,a, is relatively the same. The packed column with either nozzle B or C allowed only half the throughput possible for the spray column with nozzle B. Further results on this subject will be reported in a subsequent communication.

Water Discontinuous Water was made the discontinuous phase in a few experiments using nozzle C. The data are plotted together with comparable data for toluene discontinuous in Figure 12. The curves show that inverting the solvent made discontinuous alters the coefficient as well as its dependence on flow. The data are too few to be conclusive but indicate that the capacity coefficient will be greater if toluene is discontinuous when the flow ratio of toluene to water is appreciably greater than unity, and will be greater with water discontinuous 8 this ratio is considerably below unity. The FIQURE 12. COMPARISON OF THE CAPACITY OF SPRAYAND PACKED COLUMNS WITH NOZZLE C FOR TOLUENE AND FOR WATER differences obtained for inversion of the phase made discontinuous are probabIy connected to some extent with the DISTRIBUTED

APRIL, 1937

IXDUSTRIAL AND ENGINEERING CHEMISTRY

observation that water, if discontinuous, wets and films over the packing whereas toluene rises in distinct globules.

H.T.U.andH.E.T.P. Values of these quantities calculated for representative runs in the spray and packed column are recorded in Table I. These values are typical of the results obtained. They are presented to show their order of magnitude, and they are varied greatly by the several variables studied. I n general, except for nozzle C, they are somewhat less than were obtained in the spray column operating with extraction in the water-acetic acid-isopropyl ether system (3). The enhanced capacity of the column in the present case is believed to be due largely to smaller drop size in the discontinuous phase.

Acknowledgment The Filtros shapes of various specifications used in constructing the nozzles were kindly supplied by Filtros, Inc., East Rochester, N. Y . , and porous carbon of varying specifications by the National Carbon Company, Inc., Cleveland, Ohio. The Berl saddle packing was supplied by Maurice A. Knight, Akron, Ohio. The cooperation of these concerns is gratefully acknowledged.

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Nomenclature over-all capacity coefficient, Ib. moles/hr./cu. ft./ unit AC Ib. moles extracted/hr. N/e a sp. contact area, sq. ft./cu. ft. of column effective column volume, cu. ft. V concn. gradient based on water phase (C: - Cw), AC lb. moles/cu. ft. (subscripts 1 and 2 refer to opposite ends) holdup as discontinuous phase, cu. ft. v* toluene fraction of effective tower volume occupied by disf continuous phase, ?lo av. upward velocity of discontinuous light phase, ft./hr. av. u ward velocity with continuous phase stationary, ft.)ir. rate of flow of discontinuous phase, cu. ft./hr./sq. ft. of column cross section h effective column height, ft. H. E. T. P. = height equivalent to a theoretical plate, ft. H. T. U. = height of one transfer unit, ft. K,a

Literature Cited (1) Chilton and Colburn, IND.ENQ.CHEM., 27,255 (1935). (2) Elgin and Browning, Trans. Am. Inst. Chem. Engrs., 31, No. 4,

639 (Dec., 1935). (3) Ibid., 32, No. 1, 105 (March, 1936). (4)Sherwood and Gilliland, IND.ENQ.CHEM.,26, 1093 (1934).

RECEIVED January 5, 1937.

Alchemist Lecturing on the Elixir of Life

BY

David Scott

No. 76 in the Berolxheimer Series of Alchemical and Historical Reproductions brings a new artist to our attention. David Scott was born in Edinburgh in 1806, the son of Robert Scott, an engraver. He studied first under his father, then at the Trustees Academyin Edinburgh, and later (1832-1834) in Rome. Scott was an exponent of the romanticist school, with a strong tendency toward

the mystical. Attention is called',to the simplicity of the apparatus used for the demonstration. Many of his paintings are historical in nature. He died in Edinburgh in 1849. We are unable to give the present location of the original painting and suggest that it may be in the possession of George Bernard Shaw, whom the alchemist resembles to a considerable extent.

A detailed list of Reproductions Nos. 1 to 60 appeared in our issue of January 1936, page 129, and the list of Nos. 61 t o 7 2 appeared in January 1937, page 74 wh&e also will be found Reproduction No. 73. Reproduction No. 74 ippears on pagk 166, February issue, and No. 75 is on page 345, March issue.