Sherwood 1939

Extraction

Packed Columns

in Spray and Data are presented on liquid-liquid extraction from single drops which indicate that the interior of the drop is not stagnant but is considerably agitated. Data on extraction in spray and packed columns show an initial increase in extraction coefficient with increase in rate of flow of either continuous or discontinuous phase, presumably due to increase in interfacial surface as the holdup increases. A subsequent decrease in the coefficient at the highest flow rates is ex- plained as the result of drop coalescence, this being especially noticeable at high rates of flow of the continuous phase.

T. IC. SHERWOOD, J. E. EVANS, The coefficient is greater if the discontinuous phase does not AND J. V. A. LONGCOR wet the packing. The coefficients are largest in the packed Massachusetts Institute of Technology, column but the flooding rates are greatest in the spray (un- Cambridge, Mass. packed) column.

ECAUSE of its effectiveness as a complement to distillation in the separation of materials, liquid-liquid extrac- B tion has in recent years assumed considerable im-

portance as one of the unit operations of chemical engineering. Within a relatively few years the solvent refining of petroleum products has become common practice, and much has been done in developing the equipment necessary for large-scale operation.

The development of the more theoretical aspects of extraction as a unit operation has been relatively slow as compared with the widespread adoption of the process on an industrial scale. Hunter and Nash (11-14) have described both the diffusional basis of extraction and the graphical methods of computation which are of value in making the necessary stoichiometric calculations. The latter have also been described with unusual clarity by Evans (8). The analogy between distillation and extraction and the meaning of reflux in extraction are discussed by Saal and Van Dyck (17) and by Thiele (go), and have been presented particularly well by Varteressian and Fenske (22, 23).

Relatively little information is available in the literature with regard to the performance of extraction equipment. Fallah, Hunter, and Nash (9) and Strang, Hunter, and Nash (19) report data on extraction in a wetted-wall column, and on the flow conditions in such a column. Elgin and Browning (7) and Appel and Elgin (2) report investigations of countercurrent extraction in a spray column; acetic acid, isopropyl ether, and water were used in the first case, and benzoic acid, toluene, and water in the second. The latter investigation included a study of the operation of a packed column, but since 0.5-inch Berl saddles were used in a 2.03-inch i. d. column, channeling along the wall was doubtless large. Sherwood (18) reports data of Demo and Ewing (6) on extraction of acetic acid from water by benzene in a 3.55-inch i. d. tower packed with 0.5-inch carbon rings. Varteressian and Fenske (21) report data on extraction in the system benzene-ethyl alcohol- water in a 0.55-inch column packed with small metal chain and nickel wire rings. Rushton (16) describes the results of experiments in which oils were treated by countercurrent extraction with nitrobenzene in a 216/16-in~h tower packed with various rings and saddles, to 1 inch in size.

It is apparent from the limited literature on performance of countercurrent columns that no general correlation of the data for design purposes may be expected until more pub- lished information is available. The present article presents the results of a continuation of the work begun by Demo and Ewing (6) on extraction in a 3.55-inch column packed with 0.5- and 1-inch carbon rings and 0.5-inch Berl saddles, as well as with an unpacked spray column. The investigation is analogous to those of Elgin and Browning and of Appel and Elgin in that definite three-component systems were used, and similar to the work of Rushton in that several packings were investigated. Although the tower was larger than those used by these investigators, the ratio of tower diameter to packing size may have been too small in the case of 1-inch packing. It is sometimes assumed that this ratio should be 8 or larger if the results are to be considered typical of the performance of towers of large cross section.

Simultaneously with the study of the packed tower, an experimental investigation on extraction from single drops was undertaken, This is also described, as it throws light on the mechanism of diffusion into the dispersed phase. In both studies the solute was acetic acid which was extracted from water by benzene and by methyl isobutyl ketone.

, Procedure for Extraction from Single Drops Solvent containing acetic acid was introduced through a glass

nozzle mounted vertically at the bottom end of a glass column, 1.74 inches i. d. and 60 inches tall. The solvent drops formin at the nozzle tip rose through water which filled the unpackes column. The solvent feed was controlled by dropping from an analytical buret into a side tube connected to the glass nozzle. The level in this side tube was maintained constant by close observation and regulation of the buret cock. This provided a uniform feed rate and an accurate measure of the amount of solvent introduced t o the column.

The to of the column was fitted with a cork stopper carrying a 0.55-incR bent glass tube from which the solvent was withdrawn t o a measuring buret. The under side of this stopper was hol- lowed to form a cone-shaped receiver for the solvent drops arriving at the top of the column and so preventing any holdup of drops under this stopper. A small amount of water was introduced at the bottom of the column t o force some water out with the solvent leaving the top and thus to retain in the combined top product all acid present in the solvent. Both phases removed from the top were titrated, and the acid found was assumed to

1144

SEPTEMBER, 1939 INDUSTRIAL AND ENGINEERING CHEMISTRY 1145

have been present in the solvent drops arriving at the top. In using the ketone a correction was made for the small acid concentration of the water phase in the tower, but with benzene this correction was quite negligible. In both cases the water in the tower was changed after each run. Total material balances checked within 2 per cent in all tests.

The acid content of inlet and outlet solvent phases, outlet water phase, and water in the column a t the end of the run were obtained by titration with standard sodium hydroxide using the method described below for the samples from the packed column. The water in the column at the end of the run was sampled at both top and bottom of the column in the runs with the ketone. Rate of solvent and amounts of water fed and of solvent and aqueous products were obtained from the buret readings. The rate of drop formation was obtained by counting the drops formed in 5 minutes. "Time of contact was taken as the average time of drop formation and rise obtained by several measurements with a stop watch on single drops.

Several nozzles were used to obtain different drop sizes; the smallest nozzle was a 0.0010-inch i. d. stainless steel hypodermic needle. The other nozzles were fire-polished glass tubes. The effective height of the column was varied from 2.0 to 57.7 inches by adjusting the position of the lower stopper carrying the nozzle. All runs were made a t 22-28' C. (71.6-82.4' F.).

Packed Tower Solvent and aqueous streams were contacted in a 3.55-inch i. d.

Pyrex glass tower, 66 inches long and mounted vertically. This was fitted with headers and distributing nozzles at both ends and operated empty as a spra tower or acked with one of three packing materials. In a&ition to t%e tower, the necessary auxiliaries included storage vessels, feed and product lines, orifice meters, pumps, and an overflow device to control the interface between the two phases in the tower (Figure 1).

In all runs the direction of diffusion was from aqueous layer to solvent layer-i. e., an aqueous solution, initially 6.0 per cent acetic acid, was extracted by either benzene or methyl isobutyl ketone. The aqueous layer, either feed or raffinate, will be re- ferred to as acid. Since the flow of one phase past the other is necessarily by gravity, the heavier acid phase entered the tower a t the top and was withdrawn a t the bottom, while the solvent passed in the reverse direction.

At each end of the glass tower was a brass header consisting of a cylindrical chamber 3 inches high and 3.5 inches i. d. The in- comin liquid was fed into the side of the chamber whence it entered &e tower through six 0.120-inch i. d. brass tubes extending 2 inches into the tower and 2 inches into the header chamber. These were spaced symmetrically at a radius of 1 inch from the center of the tower. The outgoing liquid was withdrawn through a l /4- inch brass pipe leading from a hole in the center of the header plate. One of these headers is shown in Figure 2. The six small feed tubes extend into the header chamber, with the supply tube feeding the header at the right and the brass pipe through which liquid was withdrawn a t the left. The cover plate (shown removed) was fitted with a petcock and a glass thermometer.

In most of the runs with packing the packed height was 54 inches, with 6-inches free space above and below the packing. The packing rested on a S/~-inch-mesh nickel wire grid. In a few runs only 20 inches of packing were used, with the free space above the packing increased to 40 inches. In order to avoid the occurrence of appreciable extraction in the large free space above the packing in these tests, the six upper feed tubes were extended by 0.24-inch glass tubing to introduce the acid a short distance above the packing. A wood spacer maintained the glass extensions in the same relative position as the short brass tubes previously described.

The aqueous raffinate leaving the column passed from the bottom header through a swivel pi e to an overflow vessel which could {e raised

or lowered as desired. The elevation of this overflow determined the position of the interface between phases in the tower, which could be controlled easily in this way. This overflow device in the raffinate line is indicated in Figure 1.

Both feed liquids were pumped continuously from storage vessels at floor level to head boxes situated on a platform about 14 feet above the floor. The feed to the column was by gravity from these constant-head supply vessels, the overflow in each case being returned to the storage vessels. Calibrated orifices were in- serted in each feed line, as Figure 1 indicates, and fitted with pet- cocks to remove air from the manometer leads. Dyed butyl phthalate was used as a manometer fluid. Glass carboys were used for acid storage, with a copper head box and 0.59-inch glass lines. Steel drums were used for the solvent, with a/,-inch iron pipe lines.

FIGURE 2. BRASS HEADER

The solvent extract was strip ed of acid for re-use by contactin with water or with a dilute sol%ion of sodium hydroxide. Acia raffinate was made up to 6.0 per cent for re-use by adding glacial acetic acid. A layer of solvent was maintained a t all times above the solution in the acid storage carboys, and a layer of water was kept in the solvent drum. Since these vessels were agitated by the continuous circulation to the head boxes, each feed liquid was maintained saturated with the other phase.

Before each run the feed liquids were recirculated through the head boxes for ap roximately one hour in order to saturate each phase with the otRer layer. The continuous phase was admitted until the column was about three quarters full, and the second feed was then started. The position of the interface was ad- justed by means of the overflow control, and the flows were set

and held a t the desired rates. The

1 1 r c @ E

PUMP PUMP

FIGURE 1. DIAGRAM OF APPARATUS

position of the interface was ap roxi mately level with the ends of tge si; small feed tubes either at top or bottom header, depending on which phase was dispersed. After about four com lete changes of the continuous p\ase in the column, judged sufficient to obtain steady state, a set of four samples was obtained. A second set was then obtained from 5 to 15 minutes later, depending on the flow rates, and the test was ended. Since it was possible to adjust the temperature of the room, the t:sts were all made a t 25" * 2" C. (77 * 3.6 ' F.).

The acid samples were titrated with 1 N sodium hydroxide, b means of thymol blue indicator. T%e benzene extract was analyzed by shaking 50-cc. portions with an equal amount of water and titrating the mixture with 0.1 N caustic; thymol blue indicator was used and the mixture was shaken violently until the end point was reached.

A faint blue in the water layer was taken as an end point. As B check it was found that the same end point was obtained when sufficient ethyl alcohol was added to make the two phases completely miscible. The ketone extract was titrated in a similar manner with 1 N caustic. The benzene feed was titrated with 0.01 N caustic; the ketone feed with 0.1 N caustic.

1146 INDUSTRIAL AND ENGINEERING CHEMISTRY VOL. 31, NO. 9

Equilibrium data for the system benzene-acetic acid-water were available in the literature (6). Corresponding data for the system methyl isobutyl ketone-acetic acid-water were obtained experimentally. Whereas the acetic acid concentration in water is roughly thirty times as great as in the benzene phase in equilibrium, it was found that the distribution of acetic acid between methyl isobutyl ketone and water cor- responds to about equal concentrations in both phases. The ketone is obviously a much better solvent than benzene for the removal of acetic acid from water. The mutual solubility and equilibrium data for the ketone system are given in Table I.

TABLE I. EQUILIBRIUM AND MUTUAL SOLUBILITY DATA FOR 25" c. SYSTEM ACETIC ACID-WATER-METHYL ISOBUTYL KETONE AT -Equilibrium Dat-

-Mutual Soly. Data- -Ketone Layer- -Water Layer- Acetic Acetic

Ketone Water Acid acid Density aoid Density % % % % G./cc. % G./cc.

1.55 3 . 7 10.5 17 .4 26 .0 37 .6 5 1 . 6 6 6 . 4 81 .6

98.45 7 6 . 8 57 .5 48 .4 39.6 29 .1 19 .2 1 2 . 0 6.5

0 19 .5 32.0 34 .2 34 .4 33.3 29 .2 2 1 . 6 11.9

97.9 2 . 1 2 0

1 .87 8 . 9

17 .3 24.6 30 .8 33.6

0.798 0.804 0.807 0.809 0.811 0.812

2 .85 11 .7 20 .5 26 .2 3 2 . 8 34 .6

0.995 0 .996 0 .998 0,999 1.000 1,001

Extraction from Single Drops

The quantities varied were drop size, column height, inlet concentration of acid in solvent, and solvent feed rate. Both benzene and methyl isobutyl ketone were used. Six runs were made with an acid concentration of 0.0755 pound mole per cubic foot, and five runs a t a concentration of 0.0474 pound mole per cubic foot in the inlet ketone, all with a column height of 57.7 inches. Thirteen runs were made a t an inlet concentration of 0.0603 pound mole per cubic foot, with the column height varied from 2 to 57.7 inches. Several inlet nozzles were employed with each inlet acid concentration of the ketone. Drop diameters, calculated from the measured feed rate and the drop count on the assumption that the drops were spheres, varied from 0.0745 to 0.137 inch. This range of drop diameters is quite small compared with the range of nozzle diameters used. The smaller drops appeared to be spherical, but the larger drops were noticeably flat, with horizontal axes perhaps twice the vertical axes.

Seven runs were made with benzene containing 0.00576 pound mole of acetic acid per cubic foot a t a column height of 57.7 inches. Twelve runs were made with 0.00374 pound mole of acid per cubic foot in benzene with column heights from 2 to 57.7 inches. The benzene drops were larger than those obtained with the ketone, the calculated diameters varying from 0,109 to 0.221 inch. The drop diameters were calculated on the basis of spherical drops, the drop volume being obtained by dividing the volumetric feed rate by the number of drops formed per unit time. In the runs with benzene the equilibrium concentrations in benzene corresponding to the observed water-phase concentrations were negligible, and the actual benzene concentrations could be taken as equal to the over-all concentration driving forces on the benzene basis.

The calculated transfer coefficients are plotted against drop diameter on Figure 3; all the data shown are for the 57.7-inch column height. The values of K were obtained from the equation :

where L = ketone or benzene rate, cu. ft./hr.

Cp, C1 = acid concentrations of drops at exit and inlet, respec- tively, lb. mole acetic acid/ cu. ft.

A = calculated area term, in sq. f t . , of total drops in column at any time, obtained from measured feed rate, drop count (no. of drops per min.), and measured time of formation and rise of drops from bottom to top.

The calculation of A is based on the assumption that the drops are spherical. The logarithmic mean driving force, AC, m. is based on C1 and Cz and the equilibrium concentrations in ketone or benzene corresponding to the observed concentrations in the aqueous phase a t the bottom and top of the column.

From Figure 3 it is apparent that K increases with drop size for both systems, and that for the same drop size K K is greater than KB. Variations in acid content of the inlet ketone have no effect on KK, but two curves are obtained for K B for the two inlet acid concentrations in benzene. The correlation is essentially the same if K is plotted against Reynolds number for the rising drops. Since the liquid-phase diffusivities are probably almost equal in the two cases, it might be expected that KK and K B would not differ greatly. The interfacial tension for the benzene-water system (10) is between 33 and 35 dynes per cm., whereas that for the ketone (1) varies from 8.8 for ketone-water with no acid to 3.0 for the feed containing 0.075 pound mole of acid per cubic foot. Although the interfacial tension should influence the drop size, it is difficult to see how it might affect K for a given drop size. Except for minor differences in velocity of rise, the conditions outside the drop were essentially the same for both ketone and benzene, and it seems logical to look within the drop for an explanation of the observed differences in K for the two systems.

0 < 0.04 0.08 0.12 0.18 0.20 0.24 L

DROP DIAMETER - IN FIGURE 3. EFFECT OF DROP SIZE ON Ex-

TRACTION FROM SIR'GLE DROPS

If the fluid within the drop is quite stagnant as the drop rises through the column, it should be possible to calculate the amount of acid transferred from the laws of unsteady- state diffusion in a sphere. Xewman (16) gives values calculated from the theoretical equation for a sphere with which the experimental results may be compared. The ratio A of unextracted solute to total solute which it is possible to extract may be taken as approximately equal to the ratio of AC a t the top of the column to AC a t the bottom of the column. This is related by the theoretical equation to the ratio D 8 / R 2 , where D is the diffusivity for the solute in the drop, 8 is the time of contact of drop with water, and R is the drop radius. The correlation of diffusivities given by Arnold (3) indicates that D a t 25" C. (77" F.) should be about 0.3 X sq. cm. per second). For the first run with ketone, A is 0.0072, for which the theoretical value of D 8 / R 2 is 0.45. Since the drop radius was 0.05 inch and the time of formation and rise 14.4 seconds, the actual value was:

square inch per second (1.9 X


De 0.3 X X 14.4 = o.0173 R2 - (0.05) _ -

TABLE 11. COMPARISON OF SINGLE-DROP DATA WITH DIFFUSION THEORY (COLUMN HEIGHT 57.7 INCHES)

Methyl Isobutyl Ketone Benzene Therefore, the extraction proceeds a t a rate corresponding to an effective diffusivity larger than the molecular diffusivity in the ratio of the two values of D9/R2, which is 26. The value of D of 0.3 X I .o

square inch 0 .a per second is an estimate only but is probably good 2 0.4 within 50 per cent. T h e theore t ica l values from New- 3 a 0.2 man assume no L surface resistance 8 to diffusion in the E water phase; if E 0.1 such a resistance is 8 0.08 allowed for the 0.06 ratio of the two, e values of D 0 / R 2 3 0.04 would be even l a rge r . S imi l a r

other typical runs are summarized in Table 11. It may be seen that the 0 50 100 150

to actual diffusivi- FIGURE 4. FRACTION EXTRACTED us.

0.6

ca lcu la t ions for 0.0 2

0.0 I

ratio of effective COLUMN HEIGHT, CM

Ratio of Ratio of ' Drop eflective Drop effective

diameter, to actual diameter, to actual in. diffusivitv In. diffusivitv

0.075 17.7 0.109 11.4 0.10 26.0 0.153 26.0 0.115 23.0 0.185 36.4 0.137 36.6 0.210 43.3

sizes, and the location of the curves would vary with drop size. This would be due not only to the variation in conditions de- termining diffusion into the rising drop but also to the fact that the ratio of time of drop formation to time of rise increases rapidly with drop size. The importance of the latter ratio is also apparent when the feed rate is varied. Two runs with the ketone under quite similar conditions resulted in drops 0.10 inch in diameter, but one feed rate was twice the other. For the higher feed rate, A was 0.088, whereas in the other, A was 0.044. The longer time of drop formation resulted in an appreciable increase in amount of solute extracted.

Continuous Extraction i n Column The experimental data and calculated results are sum-

marized in Tables 111 and IV. The quantities varied were flow rate of dispersed phase, LD, flow rate of continuous phase, Lc, nature of solvent, type of packing, phase dispersed, and number of feed nozzles. In tests with benzene, LD (cubic feet per hour per square foot of tower cross section) was varied from 10 to 40, and LC from 3 to 60. With methyl isobutyl ketone, L D was varied from 10 to 70, and LC from 10 to 100. The packings used were half-inch and one-inch carbon Raschig ties increases with

COLUMN HEIGHT, ILLUSTRATING EX- TRACTION DURING DROP FORMATION drop size, and that

for drops somewhat smaller than those employed, the theoretical diffusion equation might be expected to hold. The most apparent explanation of the results with larger drops is that the interior of the drop is fairly well mixed and not stagnant. As the drop rises there is a frictional drag on the top half inducing convection down the sides and up the central axis. Viscosity damps these currents in very small drops, but in the larger drops they are of sufficient magnitude to transfer solute much more rapidly than would be possible by molecular diffusion.

The variations in K between ketone and benzene shown in Figure 3 are presumably due to variations in convection currents within the drop, caused by differences in density, viscosity, and shape of the liquid drops. Another factor of some importance, however, is the appreciable extraction occurring as the drop is formed before its release from the nozzle. This effect is illustrated by Figure 4, in which A is plotted against column height for both ketone and benzene. It is apparent that 40-45 per cent of the solute is extracted before the drop leaves the nozzle. The data shown in Figure 4 are for approximately constant drop

TABLE I11 EXTRACTION OF ACETIC ACID FROM WATER BY BENZENE Flow Rates

Cu. Ft./(Hr.) (Sq. Ft.)

Benzene Acid

Concn., Lb. Mole Acetic Acid/Cu. Ft. Packed Height

Ft.

KBa, Lb. Mole/(Hr.)

Run No.

Acid Acid Benzene Benzene in out In out Half-Inch Carbon Rings, Benzene Dispersed

1 2 3

10 5.9 10 10 10 20 10 30 10 40 13.2 40 20 3 22 30 30 3 30 10 30 20 30 20 38 3 38 10 38.5 20

0.0611 0.0585 0.0649 0.0621 0.0631 0.0618 0.0604 0.0600 0.0606 0.0604 0.0609 0.0602 0.0605 0.0514 0.0614 0.0602 0.0616 0.0532 0.0646 0,0595 0.0611 0.0590 0.0598 0.0576 0.0615 0.0516 0.0644 0.0577 0.0620 0.0592

0 .000081 0.000128 0.000061 n. no0107

0.00159 0.00192 0.00190 0.00166 0.00170 0.00167 0.00144 0.00169 0.00149 0.00187 0.00151 0.00150 0.00141 0.00180 0.00157

4.42 4.42 4.42 4.42 4.42 4.42 4.42 4.42 4.69 4.42 4.42 4.69 4.69 4.42 4.42

3 . 1 3.2 4 . 9 2 . 0 5 .1 2 . 0 2 . 8 3 . 6 2 .7 3 . 7 5 . 4 2 . 5 6 . 0 3 . 3 7 . 7 2 . 9 6 . 5 4 . 6

11 .3 2 . 6 8 . 3 3 . 6 8 . 7 3 . 4 7 . 9 4 . 8

13 .7 2 . 8 10 .8 3 . 6

4 5Fa 6Fa

0 . ooooss 0.000050 0.000080 0,000064 0.000120 0.000085 0.000048 0.000044 0.000047 0.000070 0.000058 Rings, Aci

SF, 9

10 11 12 13 14 15Fa

IHalf-Inch Carbon d Dispersed 10 30 0.0614 0.0608 0.000053 0.00162 4.69 2 . 9 3 . 4

17 0.0605 0,0595 0,000053 0,00138 4.69 4 . 8 4 .2 18 30 30 0.0614 0.0602 0.000052 0.00131 4.69 5.8 6 . 2

20 30

19 40 30 0.0615 0,0599 0.000046 0,00121 4.69 7 . 4 5 . 4 Spray Column, Benzene Dispersed

20 30 10 0.0619 0.0580 0.000019 0.00160 5.02 7 . 7 3 . 9 21 30 20 0.0626 0.0604 0.000010 0.00164 5 .01 7 . 9 3 . 8 22 30 40 0.0621 0.0609 0.000061 0.00169 5.20 8 . 4 3 . 6 23 30 60 0.0617 0.0609 0.000010 0.00177 5 .21 10 .0 3 . 0

Half-Inch Berl Saddles, Benzene Dispersed 24 30 3 0.0618 0.0488 0.000074 0.00167 4.65 10.7 2 . 8 25 30 6 0.0619 0.0530 0.000040 0.00183 4 .65 13.8 2 .2 26 30 10 0.0617 0,0567 0,000018 0.00189 4 .65 13 .8 2 .2 27 30 20 0.0624 0.0601 0.000048 0.00179 4.65 9 . 9 3 . 0 28 306 10 0.0619 0.0576 0.000197 0.00187 4 .65 12 .1 2 . 5 29F 30b 20 0.0618 0.0592 0.000177 0.00181 4 .65 11 .9 2 . 5

One-Inch Carbon Rings, Benzene Dispersed 30 31 32 33 34 35

30 30 30 30 b 30b 306 30b

10 40 60 10 20

0.0572 0.0610 0.0611 0.0574 0.0893 0.0670 0.0691

0.00172 0.00171 0.00187 0.00176 0.00174 0.00209 0.00173

4 .75 4 .75 4.75 4.75 4 .75 4 .75 4 .75

10 .1 10.6 12.4 1 0 . 5

3 . 0 2 . 8 2 . 4 2 . 8 3 . 0 2 . 8 2 . 8

10.1 10.5 10.5

40 60 36

These runs were approximately at the flooding point. b Using three instead of six inlet nozzles for benzene.

1148 INDUSTRIAL AND-ENGINEERING CHEMISTRY VOL. 31, NO. 9

TABLE IV. EXTRACTION OF ACETIC ACID FROM WATER BY METHYL ISOBUTYL KETONE Flow Rate, Kica,

Concn., Lb. Mole Acetic Acid/Cu. Ft. Lb. h - Packed Mole/(Hr.) Cu. Ft./ (Hr.) (Sq. Ft.) -

Run --*- A,cid Acid Ketone Ketone Hei&t, c& Cu: Ft.) H. T. U., No. Ketone Acid in out in out nit AC) Ft.

Half-Inch Carbon Rings, Ketone Dispersed 1 2 3 4 5 6 7 8 9

10 11 12 13Fa 14 15 l6Fa

10 in

40 40

10 60 10 60 80 10 20 30 40 50 60 70 98 .5 10 30 3 7 . 3

0.0614 0.0618 0.0649 0.0610 0.0611 0.0606 0.0610 0.0611 0.0612 0.0611 0.0694 0.0618 . . . . 0.0582 0.0624 0.0612

. . . . 0.0061 0.0077 0.0153

. . . . 0.0104 0.0243 0.0271

1 .66

1915 53.5 57.1

1.05 0 .55 1.50 0 . 6 2 0 . 6 9 1.87 1.15 0 .87 0 . 7 8 0 .69 0 .60 0 . 5 9

3 :6 1.31 1.22

Half-Inch Berl Saddles. Ketone Disuersed 17 40 10 0.0006 0.0036 0.00077 0.0152 4 .50 19.8 2 . 0 18 40 25 0,0624 0,0144 0,00222 0.0307 4 . 5 0 52 .5 0 . 7 6 19 40 40 0,0618 0.0265 0.00569 0.0344 4 . 5 0 88 .3 0 . 4 5 20 40 70 0,0521 0,0405 0,00638 0,0279 1 .66 87 .3 0 . 4 6

One-Inch Carbon Rings, Ketone Dispersed 21 40 10 0.0820 0.0050 0.00052 0.0151 4 . 6 2 2 1 . 6 1 .85 22 40 40 0,0591 0.0295 0.00069 0.0315 4 . 6 2 40 .5 0 . 9 9

Spray Column, Ketone Dispersed 28 40 40 0.0607 0.0350 0.00113 0.0265 5 . 1 5 16 .1 2 . 5 24 40 70 0,0564 0,0405 0.00108 0.0305 5 . 0 4 31.5 1 .27 25 40 90 0,0607 0.0459 0.00071 0,0344 5 . 2 4 4 6 . 3 0 . 8 6 a These runs were approximately at the flooding point.

0 0 20 40 60 80 100

Lo - FLOW RATE OF CONTINUOUS PHASE, FT. /HR. FIGURE 5. CAPACITY CQEFFICIENTS FOR KETONE, KETONE DISPERSED, IN HALF-INCH

CARBON RINGS

rings and half-inch stoneware Berl saddles. The apparatus was also operated as a spray column with no packing. For the benzene-acetic acid-water system the distribution ratio, CW/CB, is roughly 33, indicating that the acetic acid strongly favors the aqueous phase. For the ketone system the corresponding ratio, CFV/CK, is about 2. For the toluene- benzoic acid-water system studied by Appel and Elgin, CW/CT is in the vicinity of 0.1. Tables I11 and IV give the calculated values of both Ka and H. T. U. although the follow- ing discussion is based on the values of Ka obtained:

H. T. U. = L / K a where Ka is calculated from concentrations in the same phase to which L refers. Both Ka and H. T. U. are calculated from over-all concentration differences based on the solvent phase, since the solubility relations indicated that the major diffu-

Lc- FLOW RATE OF CONTINUOUS PHASE FT/ HR

FIGURE 6. CAPACITY COEFFICI~NTB FOR BENZENE, BENZENE DISPERSED,

IN HALF-INCH CARBON RINGS

sional resistance might be in the solvent rather than in the water phase. Although Ka is expressed in terms of concentrations in the solvent phase, it is an over-all coefficient and may be expected to vary with changes in conditions in either phase. Logarithmic mean driving forces are employed, since the equilibrium curve is essentially straight over the range of concentrations encountered in any one run. The material balances checked within 10 per cent in all but a few cases, and within 5 per cent in all of the ketone runs except four (runs 2, 4, 5, and 20) in which the concentration of acid in the ketone leaving the column was not measured but was calculated by a material balance. In several runs the column was on the point of flooding.

Photographs and visual observation of the operation helped considerably in interpreting these results. In the case of both solvents, drops were formed by the division of the solvent stream issuing from the inlet nozzles. At low acid and benzene rates the drops formed were not spherical but were of roughly uniform size. The drops issuing from the top of the packing were definitely of larger size and were less uniform in both size and shape, Some holdup was evident, and some drop coalescence was obviously occurring in the tower. As the flooding point was approached, a layer of drops ac- cumulated beneath the packing, obscuring the nozzles. As the rate of either phase was increased sufficiently, this ac- cumulation of drops would coalesce to form a slug of solvent which was carried out of the bottom of the tower with the raffinate. That considerable coalescence was taking place, even before this final flooding, was apparent from a comparison of the sizes of the drops a t the bottom and top of the column.

The tendency of the disperse phase to coalesce suggests an explanation of the results of Figures 5 and 6. At a constant LD, Ka increases with increasing Lc, primarily because of the increased holdup and a corresponding increase in interfacial area a. This increase is approximately proportional to Lc as long as drop size remains constant, but coalescence a t the higher flow rates reduces Q and hence Ka. In the ketone system the drops were smaller and holdup less than with benzene. Accordingly, since the tendency to coalesce was less, the maximum in the curve of Ka os. LC is much less apparent


14 - 100 t - 3 U E

3 I2 U . 2 80 3

m d

10 i 5 t d 60 - c L 3 V

Y - - Y - 40

D: I . u? -I v s 20 G 6

2 0 1 4

d 9

9 I m

n rn r

0 IO 20 30 40 50 00 70 80 90 L,- FLOW RATE OF CONTINUOUS PHASE , F T /HR.

2 FIGURE 7. COMPARISON OF CAPACITY COEFFICIENTS FOR 0 10 20 30 40 50 60 VARIOUS PACKINGS AT LD = 40 FEET PER HOUR, KETONE L,- FLOW RATE of CONTINUOUS PHASE, FT / HR.

DISPERSED FIGURE 8. COMPARISON OF CAPACITY COEF- F~CIENTS FOR VARIOUS PACKINGS AT LO = 30 FEET PER HOUR, BENZENE DISPERSED for ketone than for benzene. It seems evident that the

principal effect of increasing the flow rate of either phase is to increase the holdup until flooding occurs. Ka increases with holdup until coalescence causes an actual reduction in a.

Figures 7 and 8 illustrate the results obtained with the three packing materials and with no packing (spray tower). The four curves for ketone are quite similar, but for benzene the two curves for half-inch packings are quite different from those for the one-inch rings and for the spray-tower operation.

smaller packings and only 3.5 for the one-inch rings. The latter served only to increase the holdup somewhat as compared with the operation without packing. It should be noted that although Ka is smallest for the spray tower a t a given flow rate, the flooding rates are much greater than with any of the three packings. The results support the conclusions of Appel and Elgin (2)

Several runs were made on half-inch rings with acid dis- As these authors emphasized, variations persed and benzene the continuous in a appear to mask variations in K , phase. The results are compared in and Ka for any given system is de- Figure 9 with corresponding runs for termined primarily by the interfacial benzene dispersed. Ka is smaller surface obtained. Ka increases roughly with acid dispersed except a t very in proportion to LD, coalescence be-

ing more noticeable with varying LO 5 than with varying LO. The drop size - 10 in the packing, and hence the value of

L 3 Ka, depends more on the packing than U 4 on the size of the drops entering the 2 8 packing. Coalescence a t high values v 5 of LC tends to offset the large holdup, c 6 3 and Ka may go through a maximum U 3 as LC is increased. Flooding rates

are much higher for the spray column than for the half-inch packings.

appearance, whereas with acid dispersed and wetting the packing, there was little visual evidence of motion of any kind.

Comparable data on the various packings for both benzene and ketone are The show the same results as the graphs of Ka- namely, that extraction is rapid with the ketone than

rings give better extraction than the large rings or the spray tower.

on the H. T. u. basis in Figure

The ratio of tower diameter to packing size was 7 for the with the larger benzene drops, and that the saddles and small

Conclusion

on various points.

- t LL 3 c

v - 1 2

- z 4 q 2 m Acknowledgment 9 Thanks are due R. Ewing, J. E. 20

. v) _J

I _J

Demo, and A. W. Barry (4) who con- 0 structed the atmaratus. The Berl BENZENE FLOW RATE, FT /HR

saddles were su&&ed by the Maurice A. Knight Company, and the methyl

Carbon Chemicals Corporation.

0 20 40 60 80 100 FIGURE 9. EFFECT OF INVERTING

HOUR) IN HALF-INCH CARBON RING FIGURE 10. COMPARISON O F DATA ON isobutyl ketone by the Carbide and L C - FLOW RATE OF CONTINUOUS PHASE , FT. / HR.

PHASES (ACID RATE 30 FEET PER

PACKING H. T. U. BASIS

small benzene rates, but flooding did not occur with acid dispersed in the range studied. The acid wet the carbon ring packing, flowing down in narrow rivulets which did not appear to be moving. Under such conditions relatively high rates of flow of the continuous benzene phase did not entrain acid drops to cause flooding. With benzene dispersed, the motion

Nomenclature a = interfacial surface of contact between phases, sq.

A = interfacial surface of contact, sq. ft. CB = concentration of acetic acid in benzene, Ib. mole/cu. CK = concentration of acetic acid in ketone, lb. mole/cu.

ft./cu. f t . packed volume

f t .

f t . of benzene drops through the packing gave the tower a

1150

C T =

cw =

c1 = cz = D =

H. T. U. =

INDUSTRIAL AND ENGINEERING CHEMISTRY VOL. 31, NO. 9

K =

KB = K K =

L =

Le = Lo = R = A =

concentration of benzoic acid in toluene phase, lb.

concentration of benzoic or acetic acid in water, Ib. mole/cu. ft.

mole/cu. f t . acid concentration of feed, lb. mole/cu. ft. acid concentration of drops leaving column, lb.

mole/cu. f t . diffusivity of solute in liquid, sq. in./sec., or sq.

cm./sec. height of packing equivalent to 0netransfer unit =

L/Kn , where K a is based on concentration in the same phase to which L refers

extraction coefficient, Ib. mole/(hr.) (sq. f t . ) (unit AC)

extraction coefficient based on concentrations in benzene phase, lb. mole/(hr.) (sq. ft.) (unit AC)

extraction coefficient based on concentrations in ketone phase, lb. mole/(hr.) (sq. f t . ) (unit AC)

flow rate of benzene, ketone, or water layer, cu. ft./hr.

flow rate of continuous phase, cu. ft./hr. flow rate of dispersed phase, cu. ft./hr. drop radius, in. or cm. ratio of solute in raffinate to solute which would be

extracted if raffinate came to equilibrium with extracting liquid

logarithmic mean over-all driving force, lb. mole/cu. f t .

time of contact, sec.

-

Literature Cited

(4) Barry, A. W., S.M. thesis in chem. eng., M. I. T., 1937. (5) Brown and Bury, J . Chem. SOC., 123,2430 (1923) ; International

Critical Tables, Table 111, p. 404, New York, McGraw-Hill Book Co., 1928.

(6) Demo, J. J., and Ewing, R., S.M. thesis in chem. eng., M. I. T., 1936.

(7) Elgin, J. C., and Browning, F. M., Trans . Am. Znst. Chem. Engrs., 31, 639 (1935); 32, 105 (1936).

(8) Evans, T. W., IND. ENG. CHEM., 26, 439 (1934). (9) Fallah, R., Hunter, T. G., and Nash, A. W., J . SOC. Chem. I n d . ,

(10) Harkins and McLaughlin, J. Am. Chem. SOC., 47, 1610 (1925). (11) Hunter, T. G., and Nash, A. W., IND. ENO. CHEM., 27, 836

(12) Hunter, T. G., and Nash, A. W., J. SOC. Chem. Ind . , 51, 285T

(13) Ibid., 53, 95T (1934). (14) Hunter, T. G., and Nash, A. W., World Petroleum Congr.,

(15) Newman, Trans. Am. Ins t . Chem. Engrs., 27, 203 (1931). (16) Rushton, J. H., IND. ENG. CHEM., 29, 309 (1937). (17) Saal, R. N. J., and Van Dyck, W. J. D., World Petroleum Congr.,

(18) Sherwood, T. K., Absorption and Extraction, New York,

(19) Strang, L, C., Hunter, T. G., and Nash, A. W., IND. ENO. CHEM.,

54, 49T (1935).

(1935).

(1932).

London, 1933, Proc. 2, 340.

London, 1833, Proc. 2, 352.

McGraw-Hill Book Co., 1937.

29, 278 (1937). (20) Thiele, E. W., Ibid. , 27, 392 (1935). (211 Varteressian. K. A., and Fenske. M. R.. Zbid.. 28, 928 (1936) (22) Ibid., 28, 1353 (1936). (23) Ibid. , 29, 270 (1937).

(1) Andreas and Tucker, Sc.D. theses, M. I. T. dept. chem. eng.,

(2) Appel, F. J. , and Elgin, J . C., IND. ENG. CHEM., 29, 451 (1937). (3) Arnold, J . Am. Chem. SOC., 52, 3937 (1930).

PR~SENTED before the meeting of the American Institute of Chemical 1938. Engineers, Akron, Ohio. Abstracted from the doctors thesis of J. E.

Evans and the masters thesis of J. V. A. Longoor, Department of Chemical Engineering, M. I. T., 1938.

Unidirectional Drying of Wood ERNEST BATEMAN,

JOHN P. HOHF, AND

ALFRED J. STAMM Forest Products Laboratory, Madison, Wis.

HE drying of wood is a complicated phenomenon which has thus far defied rigorous theoretical analysis. Most T of the evidence indicates that it is a t least in part a diffu-

sion phenomenon. Even this might be questioned, however, on the basis of the recent findings of Ceaglske and Hougen ( I ) that the drying of granular nonhygroscopic solids is controlled entirely by capillary forces rather than by diffusion. Tuttle (8), Sherwood (6), and Kollmann (4) showed that the moisture gradients obtained in drying wood under definite boun- dary conditions can be theoretically reproduced by Fourier analysis methods (3) with a fair degree of accuracy by as- suming that the phenomenon is one of simple diffusion over the complete moisture-content range. Hawley (2 ) , however, pointed out that diffusion would not be expected to take place above the fiber saturation point on the basis that the fiber saturation point is the moisture content in equilibrium with unit relative vapor pressure. Further, in simple diffusion, the diffusion constant and the diffusivity in the Fourier form of the equation (3) should be independent of the moisture content. This is not the case for transverse drying of wood according to the moisture transfusion measurements of Mart- ley (5) in which the equilibrium moisture gradients set up under steady-state drying conditions were determined.

Measurements were made of the rate of drying from a single face of small cylinders of Sitka spruce at different temperatures and under different relative humidity and atmospheric pressure conditions. Moisture gradients were determined on the speci- mens prior to the complete removal of free water. Drying in all cases gave weight losses that varied directly with the square root of the time. Values for the mean effective diffusion per unit moisture gradient were calculated from the rate of drying and the moisture gradients up to the fiber saturation point. The values increase slightly with an increase in the relative humidity effective in the drying, and increase to a greater extent with an increase in the drying temperature, a decrease in the atmospheric pressure, and a decrease in the spe- cific gravity of the wood.

These complications undoubtedly arise from the complex nature of the capillary structure of wood (2 ) . Water is held with an appreciable reduction in vapor pressure within the cell walls of wood as surface-bound and capillary-held water (7) and within the microscopically visible capillary structure with only a small reduction in vapor pressure. The fiber

Documents

Sherwood 1939