INDUSTRIAL AND ENGINEERING CHEMISTRY
June l9W
Wf
= weight flow rate of continuous fluid phase, pounds per
W,
=
2
5
a
=
&,
=
-
kd
m
=
u/
=
UP
=
UU
=
f
P.
= =
second, Me-' weight rate of flow of solids (discontinuous phase) through tower, pounds er second, M0-I effective height of tower, g e t , L specific contact area of discontinuous phase, sq. feet per cubic foot of tower, L-1 gravitational constant, L B - 8 liquid film mags transfer coefficient for continuous phase, pound moles per hour er sq. foot per unit concentration difference, MO-12)-zAC liquid film mass transfer coefficient within a fluid particle (discontinuous phase), pound moles per hour per sq. foot per unit concentration difference,MO-lL-pAC slope of equilibrium distribution curve average linear velocity of continuous fluid based on empty tower, feet.per second, Le-' average linear velocity of a particle (discontinuous phase) based on empty tower, feet per secohd, L B - 1 slip velocity of particles relative to continuous fluid, ut up u/ - (-up), feet per second, 0 - 1 fraction, or per cent, void, dimensionless densit of solid or discontinuous phase, pounds per cu. g o t , ML-J
+
-
1141
-
density of continuous fluid, pounds per cu. foot, ML-a viscosity of continuous fluid, pounds per foot per second, ML -19 -1 Where used, subscripts c and d refer to continuous and discontinuous phaaes, respectively.
p/ pf
LITERATURE CITED
(1) Allen, H. D., et al., Chem. Eng. Progress, 43, 459 (1947). (2) Blanding, F. H., and Elgin, J. C., Trans. Am. Inst. Chem. Engrs.,
38,305 (1942). (3) Elgin, J. C., and Weiss, F. B., TND. ENO.CHEM.,31,435 (1939). (4) Lewis, W. K., Gilliland, E. R., and Bauer, W. C., Ibid., 41, 1104 (1949). (5) Resnick, W., and White, R. R., Chem. Eng. Progress, 45, 377 (1949). (6) White, A. M., Trans. Am.Inst. Chem. Engrs., 31,390 (1935). (7) Wilhelm, R. H., and Kwauk, M., Chem. Eng. Progress, 44, 201 (1948). ( 8 ) Wilhelm, R. H., and McCune, L. K., IND.ENG.CHEM., 41, 1124 (1949). R E C E I Y ~January D 14, 1950.
Mass Transfer Coefficients in an Extraction Spray Tower CHRISTIE
J. QEANKOPLIS'
AND
A. NORMAN HIXSON
UNIVERSITY OF PENNSYLVANIA, PHILADELPHIA. PA.
A study was made of the mass transfer coefficients in a liquid-liquid extraction spray tower, 1.45 inches in diameter and 2.57 feet long, using t h e system ferric chlorideisopropyl ether-aqueous hydrochloric acid. A movable sampling device was employed t o remove internal samples of t h e continuous phase (aqueous hydrochloric acid) during operation. It was possible t o determine the concentration gradient throughout the column and t o locate any end effects. This internal sampling was performed witho u t disturbing the over-all extraction of the tower. A characteristic of t h e system employed is a very high distribution coefficient In favor of the ether phase. This permitted t h e use of wide ranges of solute concentration. Equilibrium data were obtained for the conditions used in
the experiments. The concentration gradient w i t h i n t h e tower revealed a very large inlet effect a t t h e aqueous hydrochloric acid or continuous phase inlet of t h e tower and none a t t h e discontinuous phase inlet. The only abnormal end effects previously reported were for t h o discontinuous or spray inlet end of t h e column. The effect persisted despite radioal changes in continuous phase i n l e t nozzle and column design. Possible reasons for t h e occurrence of t h e effect are discussed. The movable sampler is a tool which can be used t o determine mass transfer coefficients a t any point in a column. A method of plott i n g spray tower data is suggested whereby t h e inlet effect and the performance In the remainder of t h e column are divorced and correlated separately.
IQUID-liquid extraction is a unit operation of chemical engineering which has occupied a position of increasing importance in recent years. I t is a potentially useful method for aeparating components of solutions which are particularly difficult or expensive to separate by the conventional methods of distillation, absorption, evaporation, or chemical precipitation. Extraction can be used when the substance or substances to be recovered are relatively nonvolatile, close-boiling, heatsensitive, or present in relatively small amounts. Countercurrent extraction can be carried out conveniently in towers when there is a sufficient difference in density between the two liquid phases. Sieve plate, packed, and spray columns are the types encountered most frequently. Of these, the spray tower is attractive for experimentation because of its inherent simplicity, and also because of the greater possible range of flow rates of the two liquid streams. There are many reports on studies of the performance of spray towers in the literature. Using the system acetic acid-isopropyl ether-water, Elgin and Browning ( 4 ) found that the capacity
coefficients were affected by flow rates, direction of solute transfer, and drop size of the dispersed phase. Other investigators (16) studied extraction from single drops and found that an appreciable amount of extraction occurred before the drop left the nozzle. Others (I, 7) studied optimum end design of spray columns and inlet distributors. Studies of the effect of column length (IS) showed that the mass transfer coeffioient increased with decrease in tower height. In all cases the analyses were made on an over-all basis-i.e., by analyzing the inlet and outlet concentrations of spray towers. All extractions were made using dilute solutions. It was felt that much could be learned if the internal operation of a tower could be determined, as well as the column inlet and outlet concentrations of the two phases. Accordingly, a traveling sampling device was constructed to remove internal samples from any point inside the extraction column. With this sampling thief i t was possible to determine the concentration gradient and transfer coefficient a t any point in the column and also the effects of different inlet nozzles and structural design of the tower. In selecting a suitable ternary system for use in this investiga-
I Present address, Department of Chemical Engineering, Ohio State University, Columhus. Ohio.
1142
INDUSTRIAL A N D ENGINEERING GHEMISTRY
Vol. 42, No. 6
Referring to Figure 1, the ether and the water phases were kept in &gallon glass carboys, A , a t an elevation considerably above the column. These solutions were si honed into constanthead tanks, C. The small overflow was coiected, D,for reuse. The ether rate was controlled by the glass valve, E, and the water rate by valve V. The ether, always the discontinuous phase, waa dispersed upon entering the bottom of the tower, F, by a glass spray nozzle 1.125 inches in diameter. This funnel-ty e nozzle contained four holes 0.0985 inch in diameter, equaly spaced on a circle 0.687 inch in diameter, and one similar hole in the center. The holes were flush with the outside surface of the nozzle. The water phase entered the column through a water nozzle, W. The height of the interface level between the water and ether la ers was controlled by an adjustable loop, Q. Water and d e r products from the extraction column were collected in 5. gallon carboys, U. Flow rates were determined by quickly switching the flows from the product carboys to 500-ml. graduates T,by means of two-way switchin6 devices, S. The samplin thief consisted primanly of a 5-mm. (0.12 inch inside diameter7 glass tube, GI which extended into the extraction section and occu ied approximately 1.7% of the column crosssectional area. 8 y means of a hook a t the end of the sampler, a sample of the descending water phase was slowly withdrawn without entraining the rising droplets of ether. The sampler tube touched the wall of the column and was held in lace horizontally b a glass sleeve, H. By means of neoprene tuiin , the top end o f t h e sampler was attached, I , to a short piece glass tubin fastened to a block, J. This block fitted in a guide was raise8 or lowered to move the position of the sampler in the column. A setscrew on the block could be secured in any one of the vertically spaced holes, M , thus fixing the vertical position of the tube. The holes were numbered and calibrated to indicate the exact position of the traveling sampler in the extraction section of the column as in Table I.
CI
tl
c V
07
f E
Table I. Location of Sampler
T
Figure 1. Spray-Type Column
tion, i t seemed advisable to choose a system which afforded the greatest flexibility of operation. If a system were employed which had a very high partition coefficient, a very wide concentration range in the spray tower could be covered while still using a range of flow r a t a normal for such towers. For this reason the system ferric chloride-isopropyl ether-aqueous hydrochloric acid solution was selected. In using this system information would also be obtained concerning the transfer of an inorganic ealt, ferric chloride, between organic and water phases instead of the usual organic solutes such as acetic or benzoic acids. Dodson et al. (3) and Nachtrieb et al. (11, 12) found that the distribution coefficient of ferric chloride between the ether and water phases increased markedly with increase in initial hydrochloric acid concentration in the water phase, but above 8.0 N hydrochloric acid the coefficient dropped abruptly. The highest coefficients (over lo00 in some cases) were in favor of the ether phase and were obtained a t a concentration of 7.75 to 8.0 N hydrochloric acid. Because the performance data in the spray tower were to be obtained while using the highest distribution coefficient possible, a concentration of 7.80 W hydrochloric acid was selected. Inasmuch as no complete data were available for distribution coefficients a t this hydrochloric acid concentration, they were determined experimentally at 25" C. and an initial concentration of 7.80N hydrochloric acid. EXPERIMENTAL METHODS
Spray Column. The diagrammatic sketch of the spray tower equipment is shown in Figure 1.
The materials of construction for the tower and accessory equipment were limited because of the extremely corrosive action of the h drochloric acid-ferric chloride solutions. The inlet nozzles and t i e column itself were made of lass. Saran and neoprene tubing were used for the ether and %ydrochloric acid solutions. Neoprene sto pers were used a t the ends of the extraction column. S eciaHy designed glass needle valves were used for controlling t t e flows of each phase.
Hole or Point No. 1 2 3 4 5
7 11 14
Distance from Top of Extraction Section, Feet 0 0.167 0.333 0.500 0 667 1.OOO 1.667 2.167
Distance from Bottom. of Extraotion Seotion, Feet 2.670 2.403 2.237
2.070 1.903
1.570 0.903 0.403
The glass tube in block J was attached to a 125-ml. suction flask, L,b neoprene tubing. A vacuum of 3 to 4 inchea of mercury was geld on the suction flasks by a water =pirator, 0, in order to draw the water phase samples through the sampling tube into the flasks. The sampling rate was controlled by a screw clamp a t N . Three types of nozzles were used for the entering a ueous hydrochloric acid, which was the continuous phase &igure 2). The first was a plain glass tube, whereas the second had four tubes. In nozzle 3 a Venturi-shaped tube served as the inlet for the water which then impinged upon a shallow glass dish that served as an overflow weir. Two basic types of extraction columns were used (Figure 2). Column I was a plain glass tube of 1.448-inch inside diameter consisting of an extraction section 2.57 feet long with &inch settling chambers a t each end. This column was used for the majority of the runs. A few of the later runs were made using column 11, which has a flared settling section a t the top as shown in Figure 2. Holding the interfacial level a t the top position of the flare gave an interfacial area between the water and ether phases of 102% y t e r than normal; a t the middle point it was 51% greater. n one run column I1 was inverted in order to have the flare a t the ether inlet. The ether funne!-type nozzle was placed at a point 1.18 inches below the beginning of the flared portion to obtain an annular space between the ether nozzle and the walls equal to the cross-sectional area of the column proper. In all runs in columns I or I1 the extraction section was 2.57 feet long. Procedure. To start the column the water solution waa allowed to fill the tower up to the point where the interface waa to be maintained. Then both flows were set with the ether bubbling up through the descending, continuous acid-water phase which contained the ferric chloride to be extracted. The rate of flow of each phase was determined by timing measured volumes of the phase. Because the continuous phase flow normally has a small
June 1950
INDUSTRIAL AND ENGINEERING CHEMISTRY
effect on the extraction rate, it ww held constant throughout all the runs. With both flows correctly set, internal sampling was started a t point 14 (Figure 1) near the bottom of the column. All internal water samples were collected at the rate of 10 ml. per minute, whioh amounted ts 3.3% of the continuous phase flow. The first 2 mnutes of Sam ling were employed to purge the line. This amounted to a twoild turnover of the contents in the total sampling line, The sample was withdrawn for a period of 6 to I1 minutes, depending on the amount re uired for anal sia. Sudden or jerk starting and stopping o? sample witxdrawal were avoidedrto prevent disturbing the flow pattern in the column. The purging and sampling procedure was rep6ated as the sampling device was moved up the column. Smaller samples were collected near the top of the tower, because the solutions were more concentratedin iron salt. The size of sample ranged from 30 to 100 ml. A Sam le of the outlet water phase was taken during the withdrawal of each internal water sample. All outlet water Sam les taken during the run were combined to form a composite w&ch was analyeed. This procedure yielded an outlet water sample taken over 40 to 45 minutes, a period deemed long enough to nullify an minor fluctuations which may have occurred during the run. iamples of the outlet ether were also taken simultaneously with the outlet water samples. Flows of both hases were measured every time an outlet sam le waa taken. d e temperature was held between 22' and 24' The presence of the sampling tube in the column did not a pear to alter the basic flow attern of the ether bubbles t h r o u g t out the tower. Where the [ook a t the end of the samplmg tube protruded into the main water stream, the rising ether bubbles were observed to digress momentarily and then resume their normal upward course. Experiment showed that steady-state conditions in the column were attained after the contents of the column had changed four $ five times. The a ueow hydrochlonc acid streams fed to the column were saturate3 before using with isoprop 1 ether, ,and the isopropyl ether charge steams were equilibrateiwith aqueous hydrochloric acid. The isoprop 1ether used was technical qrade obtained frqm the Carbide and 8arbon Chemicals Corporation. At 25" C. it had a densit of 0.7206 gram per ml. and a refractive index of 1.3657. The hy&ochloric acid and ferric chloride hexahydrate were of reagent grade. Distilled water waa used in the aqueous hydrochloric acid-ferric chloride solutions.
EXTRACTION COLUMNS
n
y *
Measured volumes of washed ether solution and of water solution containing a known concentration of hydrochloric acid and ferric chloride were added to glass-stoppered, 250.ml. Erlenmeyer flasks, which were immersed, in a constant-temperature bath. The flasks were agitated 100 bmes every 0.5 hour for 6 to 8 hours in order to attain equilibrium, and then the contents were analyzed. Analytical Methods. The concentration of ferric chloride in the hydrochloric acid-water solution was determined by the standard dichromate method (9). High and moderate concentrations of ferric chloride were analyzed using 0.1 N potassium dichromate; a solution of 0.01N potassium dichromate was used in titrating dilute solutions. The method for dilute solutions yielded results within 0.6% of the known value. When very dilute solutions of ferric chloride were analyzed a special procedure was followed. Three milliliters of concentrated sulfuric acid were added to between 100 and 400 ml. of the sample to be analyzed. It was evaporated slowly to fumes of sulfur trioxide. Water and hydrochloric acid were then added and the solution was analyzed for ferric chloride with 0.01 N potassium dichromate. This method yielded an average error of 1.0% and a maximum error of 1.7%. This compares favorably with the thiocyanate colorimetric method, which has an accuracy of about 2% (10). The thiocyanate colorimetric method (14) was used to analyze the extremely dilute solutions of ferric chloride present in the equilibrium studies. In a very few cases the volume of sample
TOP POSITION MIDDLE
+BOTTOM W
COLUMN
8
Equilibrium Determination. Equilibrium data for the system ferric chloride-isopropyl ether-hydrochloric acid solutions were obtained a t 25.00' * 0.05' C. The ether and water solutions used for the equilibrium measurements were the same as the feed to the extraction column.
1143
WATER
NO. I Figure 2.
COLUMN II
I
NOZZLES
NO. 2
NO. 3
Extraotion Columns and Water Nozzles
obtained from sampling inside the extraction column was small and, therefore, the colorimetric method of analysis was used. To determine the iron content of the ether solutions, the sample was extracted three times with distilled water. The extract was analyzed for ferric chloride by the dichromate method. This method yielded results which deviated from known values by an average of 0.3%. To analyze the contents of double ether layers, the volumes of the two phases were determined and eamples of each phase were pipetted for analysis. The total chloride concentration of the water phase was determined by the Volhard method (8),and the hydrochloric acid concentration was obtained by subtracting the chloride present in the ferric chloride. In this method the silver chloride precipitate was removed by filtering before titrating with potassium thiocyanate. In determining the hydrochloric acid content of the ether phase, the ether solution was extracted three times with water and the extract analyzed by the Volhard method. This procedure yielded results almost within the accuracy of the Volhard method itself. TREATMENT OF DATA
Equilibrium Data. The equilibrium data made to determine the distribution coefficient of ferric chloride between ether and aqueous solutions of hydrochloric acid are given in Table I1 and are compared with the data of other investigators in Figure 3. The data obtained appear to agree well with the data of Nachtrieb (11, I d ) which were obtained a t lower hydrochloric acid concentrations. At about 0.03 pound mole of ferric chloride per cubic foot of ether and an initial concentration of 0.487 pound mole of hydrochloric acid per cubic foot of water solution, the K reaches a peak value of about 3200. The lowest K i n dilute solutionsis 47. The K values from the curve for an initial hydrochloric acid concentration of 0.487 pound mole per cubic foot were used in the tower
INDUSTRIAL AND ENGINEERING CHEMISTRY
1144
Table I I. Equilibrium Data for Ferric Chloride-Ether-Hydrochloric
(S), who worked in a limited concentration range of ferric chloride in the ether phase and found two ether layers when the concentration of hydrochloric acid in the water was above 0.468 pound mole per cubic foot. Theory. In equipment such as packed and spray columns where the contact area of the two liquid phases is not directly measurable (6),the following equation is employed to calculate the mass transfer coefficient from experimental data: d N = K w a ( C w C w * ) d V (1)
Acid Solution
Original Solutions Equilibrium Solutions *Conon. in Water Solution, Conon. in Water Solution, concn. in Lb. Mole/Cu. Foot Lb. Foot ether, (Ib. mole DistribuF ~ c ~ ~ , Volumes, M1. FeCla, FeCidou. tion ratio, HCI X lo' Water sol. Ethersol. HC1 x 10' ft.) X 10' , K CE/Cw 0.4866 124 45 180 0.1017 32.10 316.0 0.4866 124 150 60 0.1105 342.5 3103 0.4866 124 450 100 0.895 644 720 0.4866 124 160 50 0.1797 435 2420 0.4866 124 158 42 0.4641 0.3030 515 1702 0.4866 124 132 68 0.4572 0.0861 261.5 3040 0.4866 124 110 90 0.4566 0.0836 161.6= 1930 0.4847 124 226 50 0.4660 0.553 595 1072 0.5075 116 242 50 0.4822 0.522 626 1200 0.5075 116 170 42 0.4747 0.2395 536 2240 0.5130 113 145.4 68 0.4672 0.0488 284.0 5820 0.5135 113 121.2 90 0.4654 0.0486 173.5a 3565 0.4983 119 47 215 0.4579 0.0718 27.11 377 0,5240 108 51.7 215 0.4641 0,0581 23.32 402 0.4957 120 149 45 0.4654 0,1572 453 2880 0.4866 12.4 35 175 0.4485 0,0424 3.212 75.8 0,5092 11.4 38 175 0.4597 0.0357 2.63 73.8 0,4866 12.4 50 125 0.4542 0.0539 5.18 96.0 0.5092 11.4 54.3 125 0.4635 0,0403 5.24 130.2 0.4866 12.4 100 100 0.4554 0.0699 13,55 194,2 0.4866 124 144 45 0.4622 0.1778 446 2510 0.4866 12.4 100 50 0.4550 0.0761 29.63 389 0.4866 124 125 50 0.4584 0.1030 346.8 3360 0,5100 12.4 81.4 75 0.4660 0.0668 13.73 205,5 0.4866 1.24 70 140 0.4518 0.0127 0.642 50.5 Ether phase separated into two layers.
-
Q
Table 111.
Over-All Transfer Data for Continuous Extraction Column Over-All FeCla
-
To integrate Equation 1 algebraically certain conditions must be obeyed (4, 6). The distribution law m u s t hold over the range of concentrations encountered; the moles of each liauid solvent must remain constant throughout the extractor-Le., the solvents must be relatively immiscible; K w a must be constant throughout the column; the concentrations must be relatively dilute; and the amount extracted must be small. Integrating Equation 1, Equation 2 results: I
Flow Rate, Transfer Rate (H2rUj(li:/Ft,) (Lb. Hour) &le/ ( NDeviation. E--NW) Run ele" LrP LE 104, Av. N 100/N No. 1 58.9 55.8 23 80.0 2.6 1 56.5 57.5 79.3 3.2 24 I 1 55.7 58.5 79.5 5.9 25 I 2 55.8 56.9 26 80.2 3.2 27 I 1 58.3 38.7 81.5 1.1 I 1 57.3 19.50 72.0 2.4 28 I 1 57.2 10.00 46.1 5.4 29 I 2 58.3 37.8 30 81.5 0.6 I 2 56.3 55.6 31 59.5 0 I 2 56.4 56.0 60.5 0 32 I 2 56.1 38.9 58.4 - 1.0 33 I 2 56.2 57.7 80.0 2.6 34 I 2 56.0 19.44 55.8 1.4 35 I 2 56.5 10.57 38.8 2.1 36 I 2 56.7 9.72 28.8 9.7 37 38 2 57.0 19.60 38.1 - 2.4 II 2 56.5 38.2 39.2 1.0 39 40 I 2 56.8 57.2 39.5 2.3 I 2 56.3 9.67 41 13.76 -15.4 I 2 56.5 20.25 18.17 4.4 42 43 I 2 56.1 9.52 62.0 2,4 2.3 I 2 57.0 19.37 104.5 44 I 2 56.1 38.4 137.5 4.7 45 I 2 56.5 57.5 145.3 46 I 2 56.6 20.10 ' 0.0 47 I 2 56.5 56.6 0.0 48 I 2 57.1 19.60 64.0 2.0 49 I 2 56.8 38.7 79.6 1.8 50 81 2 I 2 56 4 57 5 23 51 65 6 I 2 56 3 19 22 18 52 I 2 56 3 37 8 80 0 23 53 I 2 56 6 56 7 80 8 2 6 54 80 0 14 Raise 2 56 2 37 9 55 3 56 5 37 9 78 3 13 56 57 I 3 56 4 10 00 47 69 6 4 6 0 1 7 I 3 56 0 19 70 58 59 I 2 56 4 19 42 67 0 3.1 I 2 56 3 19 70 68 1 5 4 60 I 2 58 0 19 93 69 8 21 61 I1 Top 2 56 8 38 7 79 4 06 62 I1 Mid 2 56 7 37 8 79 4 0 3 63 I1 Raise 2 56 2 38 0 80 0 19 64 :I Inv. 2 56 4 38 2 78 7 14 65 2 56 1 37 9 77 5 - 0 6 66 I 2 56.2 19 40 68 1 - 2 7 67 0 See Figure 2 for type of column and nozzle. Type of Column' I I
Vol. 42, No. 6
Y2-r
-
--
-
-
N = Kwa VACwlm (2) To calculate the number of transfer units for extraction the Colburn ( 8 ) equation in weight units can be used. 2.3 N .w. = ____ 1 - Rz/rnz log [(m uz/mJ(l w~*/wI) 1 (1 r)wz 1.15 log '2' 1.15 (1 - T ) log (1 T ) w l (3) 1 - WI
-
-
-
- - -
(H.T.U.)o.w. can then be obtained by dividing the height of the column by the number of transfer units. The following simplified expression can be obtained when the solvent volumes are constant throughout the column, the K is constant, and K w a as calculated from Equation 2 does not vary with solute and solvent concentrationsin the tower.
-
-
;
-
--
runs and for calculation of mass transfer coefficients, even though the K is higher for a hydrochloric acid concentration of 0.511 pound mole per cubic foot. The lower concentration was used to conserve hydrochloric acid losses in the ether and to facilitate handling. The equilibrium data show that a t intermediate concentration ranges of ferric chloride in the ether, the ether phase separates into two ether layers. This is also borne out by Dodson
3 -
INIltstluaiOR Le. ORIGINAL MOLEWCU. W GONG, FT. GEAN~KOPLIS .4e7
\
J
1000~.
c'
3
z *
3 u
. -
2
10-
u.
a
:;NAGHIRIEB -
I J
b-*
1 1 1 / / 1
0.1
I
.
I 1 1 1 1 1 1
\ I
I 1 1 1 1 1 1
1
I 1 I L I u .
I IO DO 4mo GONG. OF FECLS IN ETHE8, (LB. MOLES1CU. FT.1 Y IO
Figure 3
INDUSTRIAL AND ENGINEERING CHEMISTRY
June 1950 Table IV. Run No. 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
67
Concentration of Ferric Chiorlde in Water at Various Points in Column
[Concentration of FeCh in water phase, (lb. mole/ou. ft.) X 10'1 Point Point Point Point Point Point 14 11 7 5 4 3 Outlet ... ... 0.408 i:is1 0.443 16:33 0.461 ... 0.880 15:oz 3:355 6:27 28 40 37.1 1.485 16.00 22.07 38.10 50:l 54.60 61.95 13.65 58.6 72.4 56.3 83.1 86.6 93.2 51.3 4.70 12.64 2 665 29.08 40.8 1.295 4.17 2,090 8.55 ... 1.080 0.572 1.722 4.10 .. 1.135 0,642 12.36 ..* 3.092 7.95 17.45 1.693 ... 1.060 1.747 5.62 1.390 ... 16,93 0.695 22.14 12.54 7.00 37.82 5.67 46.6 39.8 30.8 60.3 64.1 32.07 19.60 23.10 28.75 38,35 .. 16.35 3 875 10.54 5.38 20.45 2.620 5.43 .. 1.872 2.863 11.40 0.778 1.310 0.972 2.38 4.91 0.358 8.01 13.82 10.03 18.77 7.35 1.692 3.36 6.10 2.410 1.106 124.1 133.6 152.2 .. 171.4 122.0 62.2 77.3 102.0 135,O 59.3 16.05 52.40 28.10 79.2 9.95 5.16 27.20 9.90 54.27 2.174 ... 0.0 .., 0.0 25:io 30 60 4?:7 .. 62:Q 23.00 15.27 29.3 2.905 5.78 .. 2.005 1.033 2.175 7.30 17.44 0.525 69.9 24.80 33.80 49.4 23.34 4.41 7.85 18.85 *. 39.30 2.560 1.910 3.770 24.70 0.923 10.87 6.52 3.41 *, 2.000 14.80 30.95 4.13 11.47 2.98 26.55 1.800 53.0 67.4 49.10 79 .O 46.3 17.85 21.75 38.00 51.9 14.47 19.89 18.17 ., 15.22 17.15 3.26 5.48 16:55 32105 2.020 1.513 ... 2.062 3.68 13.17 30.20 2.480 3.68 13.03 1.865 28-20 29.90 2.476 3.70 13.29 1.846 .. 38.20 2.840 5.40 20.30 2.400 21.20 28.00 44.1 63.5 17.86 ,..
..
... ...
...
....
...
.. .. .. ..... .
.. .
... :
... ...
I
... ..I
.
.
I
.. .. ... . .. .... .. I .
..
.. .. .. ....
...
... ... ... ... ... ... ...
.. ..
...
..
...
, I
(7)
Point 2
Inlet 120.7 120.7 120.7 124.4 123.1 125.0 124.0 123.7 93.0 44.2 42.5 123.5 93.4 92.8 62.8 61.7 61.6 62.0 30.40 29.84 217.5 217.5 219.0 218.5 0.0 0.0 121.8 123.6 124.7 128.0 125.3 125.9 125.4 123.5 122.0 123.5 123.7 123.7 123.7 123.8 124.2 125.2 123.0 123.5 125.4
..
... ...
..
Table V. Run No. 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37
1145
.. 40:2 20.60 26.7 30.3 32.18 51.1 72.1 47.7 29.30 19.00 10.27 23.m 10.70 189.5 149.8 106.7 78.6
..
7s:5 44.8 35.5 85.5 54.9 43.0 49.10 37.9 87.4 64.4 7413 52:08 41.40 42.2 46.4 55.4 77.8
Because the inlet concentration, Cwl, is a constant for a run,
R~~ No. 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52
Concn of HCl Lb. Moie/Cu. Ft. Outlet Inlet 0.4766 0.4700 0.4656 0.4822 0.4753 0.4834 0.4797 0,4747 0.4694 0.4745 0.4654 0.4808 0.4715 0.4645 0.4813
N w = ALw(Cai
If the distribution coefficient is of such large magnitude that the value of CW* is negligible compared to Cw, then a greatly simplified equation can be derived for (H.T.U.)o.w.. Solving for Kwa in Equation 2 there results Kwa =
-
LW'(CWI ) ,C (CW* Cw**)I (CW, CW,*) 2 . 3 log (Cw* C , * )
hA[(Cw,
-- -
- CW,')
(5)
-
If the CW* terms are negligible compared to the Cw terms, Equation 5 reduces to Kwa =
-
N~ = 81.0
L w 2.3 log (C wl/CW J . h
After Kwa from Equation 6 is substituted in Equation 4 the equation can be solved for h.
x
10-4
81.5
66 66 67
(4)
-
Cwz) = 0.01144 X 58.3 (123.1 X lo-' 1.485 X
Conon of HCI Lb. Moie/Cu.z\L Outlet Inlet
R~~ No. 53 54 55 56 57 58 59 60 61 62 63 64
-k
Hence, if CW* is negligible compared to CK and if the logs of the concentration a t differenb points in the column are plotted against the linear height in the column, the slope of the resulting line a t any point represents the negative reciprocal of (H.T.U.)o.w. a t this particular point. If the rwulting line is a straight line, (H.T.U.)o.v. is constant throughout the entire column. Calculations. The e x p e r i mental data from the 45 runs made in the extraction column are presented in Tables I11 to VI. The over-all m a t e r i a l balance for the runs was calculated as follows: For run 27,
Concentratlon of Hydrochloric Acid in Water Phase in Column
Concn. of HCI Lb. Mole/Cu. lk Outlet Inlet 0.4881 0.4647 0.4881 0.4651 0,4881 0.4653 0,4840 0.4647 0.4869 0,4722 0.4881 0.4797 0.4856 0.4831 0.4861 0.4717 0.4870 0.4669 0.4865 0.4677 0,4859 0.4687 0.4870 0.4670 0.4873 0.4770 0.4866 0.4835 0.4881 0.4835
-(H.T.U.)O.W.log Cw,
h
NE
x
10-4
-N N v x 100 = 1.1%
The experimental data were plotted on semilog paper and smoothed values of the experimental points were determined and used to calculate the individual (H.T.U.)ow. (Table VII) and Kwa for short sections of the columns. This was done by dividing the column into five short sections and calculating the mam transfer coefficienta for each section by means of Equations 2 and 4. The following calculations are for runs 27 and 30, which gave similar results and are treated as one run. For the short 0.403-footsection, outlet to point 14,
NW
1
-
ALw(CW~ C W ~ ) 0,01144 X 58.3 (2.52 X lo-'
- 1.40 X
lo-')
Nw = 0.746 X lo-' To obtain the concentration of the solute in the outlet ether phase at point 14, a material balance was made over the short section of the column.
-
-
0.746 X IO-' = A L E ( C E ~ C E ~=) 0.01144 X 38.7 ( C E ~ 0) cEI
= 1.69
x
10-4
INDUSTRIAL AND ENGINEERING CHEMISTRY
1146
Vol. 42, No. 6
rn.
m
--
OISTANCE FROM TOP OF COLUMN, FT.
Figure 4. Effect of Inlet Concentration of Ferric Chloride in Water Phase
,
0
Effect of Ether Rate
From Figure 3 for C,, = 1.69 X tained.
cw,* =
a K value of 60 was ob-
= 0.0282 X 10-4
6o
Then to calculate the mass transfer coefficient and (H.T.U.)o.w.,
ACwlm =
-
(2.52 - 0.0282) - (1.40 0) (2.52 - 0.0282) 2'3 log (1.40 0)
10-4 = 1.90 x 10-4
-
N hA Cwlm
Kwa
= -=
Kwa
= 84.9
Figure 5. Effect of Inlet Concentration of Ferric Chloride in Water Phase
1
I Btrn, TOP DISTANCE FROM TOP OF COLUMN, FT-
Figure 6.
DISTANCE FROM TOP OF COLUMN, FT.
0.746 X 0.403 X 0.01144 X 1.90 X lo-'
(H.T.U.)o.w. = L = 583 = 0.687 K w a 84.9
1
INLET CONC. OF FECL3 IN ETHER, LB. MOLESICU. FT.
I 2 Blm. DISTANCE FROM TOP OF COLUMN,FT.
-r
Figure 7. Effect of Inlet Concentration of Ferric Chloride in Ether Phase
To determine if the slight curvatureof the operating and equilibrium lines had any appreciable effect on the results, calculations were made using the more exact Equation 3 of Colburn. An (H.T.U.)o.w,value of 0.689 foot was calculated by Colburn's equation, which checks the value of 0.687 obtained from the simplified equation. Calculations on other runs gave similar results; hence, the simplified equations were used throughout. The inlet concentration of the ferric chloride in the water phase with the entrance effect in the continuous phase eliminated was determined by extrapolation of the smooth curves of concentration against height (Figures 4 to 10) to the inlet. The per cent reduction in the inlet concentration because of the inlet effect was calculated m the following manner. For run 27, C W , = 123.1 X lo-' C w extrapolated = 64 X
INDUSTRIAL A N D ENGINEERING CHEMISTRY
June 1960
*
TYPE OF WATER NOZZLE
POSITION OF WATER NOZZLE IN FLARE
0
g
8
I
I I I I I I I I I I
I I I I I I I I I I
I I I I I
TOP I 2 DISTANCE FROM TOP OF COLUMN,
Figure 8.
1147
Btm.
I.
FT.
DISTANCE FROM TOP OF COLUMN,FT.
Effect of Different Water Nozzles
Figure 9.
Effect of Flare in Column
as compared to the concentration when sampling, is approximately 8%. However, the moles of ferric chloride transferred in the watet phase when sampling and when not sampling check to better than 1%. The reason for this is that the outlet concentration is very low and, hence, has a small effect on the moles transferred-for example, in runs 23 and 24 the inlet conaentra-
Concentration of Ferric Chloride i n Ether Phase in Column
Table VI.
DISTANCE FROM TOP OF COLUMN, FT.
Figure 10. Effect of Raising Interface Level above Inlet of Water Nozzle
Extraction a t inlet = (123.1
- 64) X lo-'
= 59.1
x 10-4
-
Per cent reduction = 59*1 X 100 = 48 123.1 DISCUSSION
Justificationof Sampling Technique. To justify the sampling technique it was necessary to show that internal sampling had no effect on the normal over-all extraction of the column. In run 23 the coIumn was operated in the conventional manner with the eampling rod pulled entirely from the extraction column. In runs 24 and 25 the same conditions as in run 23 were used but a sample was removed a t point 14 in run 24 and a sample a t point 4 in run 26. A similar study was made in runs 59 through 61. The data are shown in Table VIII. This shows that the average per cent deviatioh of the outlet ferric chloride concentration in the water when not sampling,
Run No. 23 24 25 26 27 28 29 30 31 32 33 34 35 30 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 03 04 65 66 07
Volume of Tota? Ether Out" TOR Bottom layer layer 42.9 57.1 42.5 57.5 42.9 67.1 43.4 56.6 67.8 32.2
*.
..
32.0 71.2 70.5 53.5 56.5 0.5
68.0 28.8 29.5 46.5 43.5 99.5
a.5 38.9 74.3 88.15 01.0 79.5
96.5 61.1 25.7 11.85 39.0 20.5
....
14.2
..
..
Si..,
,.
..
i0 26.3 ..
99.0 73.7
..
I
..
.
33.'1 34.7
60 9 65.3
..
.... .. ..
36.2 32.b 33.8 36.0 37.1
64.8 07.5 66.2 64.0 62.9
..
..
Concn. of FeCla, (Lb.Mole/Cu. Ft.) x 10' Outlet . OverTOP Bottom Inlet all layer layer 0 123 7 37.2 239 0 122.5 0 36.2 239.5 122.5 0 37.4 236.5 0 125.a 33.1 245.0 0 184.8 32.4 258.0 0 319.4 392.7 0 0 187.7 3i:e 261'.0 0 93 7 28.7 254.5 0 94.4 29.2 251 .O 0 130.6 30.7 245.0 0 123.0 27.8 247.5 0 249.0 .. 249.0 0 317.0 0 246.3 3f:O 255.0 0 168.5 34.1 254.0 0 88.8 39.2 231.8 0 59.7 30.7 231.0 0 114.8 236.0 37.3 0 76.6 32.7 247.0 0 575.0 0 478.0 0 320 .OO 0 228.5 37:7 26i.o 0 0 0 0 02.2 345.0 62.7 244.0 246.2 40:5 03.1 188.0 35.4 242.5 126.0 421.5 126.7 314.0 *. .. 125.4 251.5 186.0 0 3i:i 282.5 0 179.3 30.0 259.0 0 401.0 .. 0 308.0 * * 0 297.0 294.0 0 .. 303.5 0 0 3i:3 180.2 26i.5 0 30.2 183.0 257.0 29.4 0 185.9 206.0 29.0 0 267.0 181.4 27.8 0 267.6 178.6 0 302.5 I
Ether outlet separated into two layers in some rum.
..
.. .. I
.
..
.. I.
..
..
..
.. ....
....
..
..
INDUSTRIAL A N D ENGINEERING CHEMISTRY
1148 Table V I I .
Height of Transfer Unit a t Various Points in Column [(H.II'.U.)o.w. for short sections of column, feet'
Run NO.
Out-14
14-11
11-7
7-4
4-2
0.69 1.63 5.77 0.84 0.68 0.79 1.13 3.70 2.94 1.19 0.75 0.82 2.53 1.53 7.22 2.93 1.03 0.69 2.48 0.80 0.60 2.50 0.78 0.63 0.80 0.84 5.82 1.64 0.73 0.74 0.91 0.83
0.68 1.36 3.51 0 69 0.72 0.67 1.05 3.23 2.73 0.96 0.76 0.81 2.04 1.27 5.94 2.36 1 .OB 0.70 1.78 0.69 0.56 1.79 0.66 0.61 0.78 0.73 3.79 1.58 0.72 0.64 0.63 0.64
0.69 1.28 3.00 0.56 0.70 0.45 1.01 2.25 1.86 0.87 0.74 0 63 1,74 0.90 3.98 2.06 1.05 0.72 1.57 0.73 0.62 1.51 0.76 0.60 0.68 0.68 2.58 1.50 0.69 0.63 0.63 0.60
0.64 1.28 3.05 0.45 0.65 0.44 1.00 1.76 1.50 0.88 0.76 0.52 1.55 0.66 3.32 2.21 1.07 0.79 1.45 0.72 0.48 1.56 0.73 0.57 0.68 0.65 2.56 1.54 0.75 0.60 0.65 0.62
tion is 0.01207pound mole per cubic foot and the outlet in run 23 is 0.0000408 pound mole per cubic foot and in run 24 it is 0.0000443. The average of the deviations in the over-all material balances for the 45 runs performed in the tower is approximately 4y0;therefore, the effect of sampling on the outlet concentration and the moles transferred seems small. In withdrawing samples from the column, the tip or end of the sampling hook was placed at a point two thirds of the distance from the wall to the center of the cross section of the column. To show that the sampling position in the cross section had no effect on the concentration of the ferric chloride obtained, special samples were taken in runs 35, 41, and 42 a t point 4 with the sampling tip almost touching the side of the wall. In run 35 the wall sample was 7% higher than the conventional sample; in run 41, 6% higher; and in run 52, 1% lower. These results indicate that the point from which the sample is withdrawn in the cross-sectional area of the column is immaterial. This was expected, inasmuch as observationsof the flow pattern in the column indicated turbulence and hence good mixing of the continuous phase. The effect of rate of sampling was also investigated, because it appeared that an unduly fast rate might entrain droplets of ether from the countercurrent dispersed ether phase. A special run was made a t point 7,varying the sampling rate over a wide range. The water layer was analyzed for ferric chloride content and the small volume of ether lying on the top of the water phase was measured. The data are given in Table IX. The data show that below about 15 ml. per minute the sampling rate had no effect on the entrainment of ether or on the concentration of iron salt in the internal samples. A further analysis of the entrainment problem was made in runs 66 and 67 in which the entrained ether was measured for all water samples taken. Table X indicates that the water phase contains a small amount of the dispersed phase in the form of entrained ether mist which travels co-currently in the water phase. The data further show that the outlet water, which has passed through a settling chamber where there is no countercurrent ether phase, contains as much entrained ether mist as samples from points 14 and 11. This mist which travels with the continuous phase
Vol. 42, No, 6
would be in approximate equilibrium with the water and, therefore, is not a passing but an equilibrium stream and should not be included in the analysis of the water phase. Inspection of Table X reveals that the amount of ether mist is greater at the top than at the bottom of the column. This may be due to the apparently great turbulence a t the top of the column. Such a phenomenon would yield a larger surface area of the drops and give a larger amount of extraction near the top of the column. As the mist travels down the column it tends to coalesce with the countercurrent ether stream. Inspection of Table V indicates that the concentration of hydrochloric acid in the water phase in the column decreased as much as 10% in passing through the tower. Nachtrieb (11) shows experimentally that the drop in concentration of hydrochloric acid in the water phase after contact with ether is due to two factors: (1) about 1 mole of hydrochloric acid is transferred with 1 mole of ferric chloride to the ether phase; (2)because the water phase has been stripped of the major portion of the iron salt, the solubility of the ether in the water phase increases. This latter effect is the largest and is the reverse of the salting-out effect. Hence, the volume of the aqueous phase increases and the concentration of hydrochloric acid in this phase decreases. This solubility phenomenon helps explain the apparent discrepancies in the material balance in the tower runs. This is especially true in the low ether rate experiment where the amount of ether picked up by the water phase is a noticeable percentage. Another effect on the material balance is the internal sampling of about 3% of the continuous water phase which causes the material balance t o become more positive or show a gain in the ether phase. Operating Variables and Inlet Effect. Inspection of all the curves showing the concentration gradients reveals a break in the smooth curve a t the top of the tower. At the discontinuous phase entrance there is no such inlet effect. Thus, there is an abnormal transfer of solute or end effect a t the continuous phase inlet to the column. Furthermore, this occurred, although varying in amount, regardless of ether rate, concentrations, or physical alterations of tower or nozzle. The semilog plots of concentration of ferric chloride in the water against the distance from the top of the column are shown, but only two of the four constant ether rate runs are shown. All curves in Figures 4 and 5 are similar. The concentration gradient is approximately a straight line except a t the top where the inlet effect occurs. For a constant ether rate, the slope of the straight portion of the lines increases slightly as the inlet concentration of
Table VIII. Run
Effect of Internal Sampling
Point Sampled
No. 23 24 25 59
DevEtion of Outlet Concn.
DeviBon of
Total Moles Extracted
N.l-n I Y " I I 1
14 4 14 2
60
8 6
-0:02 -0.02 -1.0 1.8
-
None
61
Table I X .
Effect of Rate of Sampling
Table X. Volume Per Cent Ether Entrained in Water Run
Outlet
66 67
0 3 0 3
Point 14 0 3 0 3
Point 11 0 4 0.4
Point 7
1 4 0 7
Point 4 1 2 0 9
Point 2 1 8 10
INDUSTRIAL AND ENGINEERING CHEMISTRY
June 1950
5 100 WI"'
66
-
INLET CONC. OF FECL3 IN WATER, LB. MOLESICU. FT.
80-
0
.0219
I RUNS WITH NOZZLES NO. I AND NO. 2
ETHER RATE, CU. FT./(HR.)(SO,fT.)
Figure 11.
Entrance Effect on Column Extraction
aqueous ferric chloride decreases. This indicates a decrease in (H.T. U.)O.V,as the inlet concentration decreases. When the inlet concentration of the water phase is constant, the concentration gradient increases with increasing ether rate, as shown in Figure 8. Another run with an inlet ferric chloride concentration of 0.0062 pound mole per cubic foot gave a similar series of curves. When ferric chloride is added to the inlet, ether phase, the top inlet effect is decreased, but the slope of the concentration gradient in the column remains approximately the same (Figure 7). The correlation (Figure 11)of the per cent reduction of the inlet ferric chloride in the water phase by the entrance effect against the ether rate shows that an increase in rate increases the inlet effect. At a constant ether rate there is a distinct trend of in. crease in entrance effect with decrease in inlet concentration. This shows that the inlet effect is dependent on a t least two and possibly more effects. Column Design and Inlet Effect. If the top inlet effect were due primarily to turbulence caused by poor distribution of the water nozzle, then radical changes in nozzle design should greatly alter this inlet effect. Figure 8 compares $ I-point nozzle, a &point nozzle, and an overflow weir-type nozzle. The curves show that there is no appreciable difference in the results from
f r
"1
c
lo
INLET CONC. OF FECL3 IN WATER. La. MOLESICU. FT. 0 .OZl9 o ,0124 9 .0093 fl ,0062 0
TOP I 2 Btm. DISTANCE FROM TOP OF COLUMN, FT.
Figure 12.
the three widely different nozzles. It appears that there must be other reasons for the top inlet effect. Observations of the flow of ether bubbles up the column showed that in the lower section the bubbles traveled upward in a more or less uniform manner. A t the top inch of the column there appeared to be mixing and tuvbulence near the interface. Drops in layers two or three deep appeared to be lined up a t the interface waiting to coalesce. A t least a portion of the disturbance appeared to be caused by the coalescence of the drops which seemed to shake the interface layer. Hence the inlet extraction might be due to the extra turbulence a t the inlet or to the fact that there is a definite surface contact area a t the interface. To determine whether this inlet effect could be altered by redesign of the upper portion of the column, runs were made in a flared column. One run was made with the nozzle and interface a t the top of the flare and one at the middle position which increased the interfacial area. Figure 9 shows that regardless of the position in the flare, the inlet effect remained the same. Thus it might appear that the inlet effect is not affected much by surface area a t the interface. Further evidence that the interface area did not cause the inlet effect is found in Figure 10. In one run the interfacial level was raised 1 inch above the water nozzle, which had the effect of partially removing the interface from the extraction portion of the column. No difference in inlet extraction was noted. No change in inlet effect was noted when the interface level was raised to the center of the flare in order to increase the interface area while the nozzle was kept a t the bottom of the flare. It seems more than likely that the inlet effect is caused by the inherent turbulence effect of coalescence of bubbles a t the interface. Possibly the rate of formation of the bubbles a t the ether nozzle may exceed the normal rate at which the bubbles can coalesce. Furthermore, the inlet effect may be a function of interfacial tension. When the solute-containing water phase strikes the solvent ether for the first time there may be a large change in interfacial tension and also a large amount of extraction. In studying extraction from single drops, Sherwood (15) found a large inlet effect a t the dispersed phase entrance to the column and not at the contipuous phase entrance. In his experiments the solute was extracted from the dispersed ketone, which is the reverse direction used in the present experiments. It is possible the inlet effect may occur only at the point where the solutecontaining phase enters the column.
I
.0030
Height of Transfer Unit a t Points in Column
1149
3 LL 0
INLET CONC. OF FECLl I N WATER, Le. MOLESICU. FT. .0219 0 .0124 k .0093 p. ,0062
-1
DISTANCE FROM TOP OF COLUMN.FT.
Figure 13.
Height of Transfer Unit a t Points in Column
1150
Vol. 42, No, 6
INDUSTRIAL AND ENGINEERING CHEMISTRY
E
7.
ETHER, Le. MOLESICU. FT.
INLET CONC. OF FECLS IN WATER, Le. MOLES/CU. FT.
,0219 .Ol24 q ,0093 0
-
P' 0 8
-
.0062 .0030
!
i
L
s +u .I 2 Btm, TOP I DISTANCE FROM TOP OF COLUMN, FT.
Figure 14. Effect of Inlet Concentration of Ferric Chloride in Ether Phase on
ETHER RATE, CU. FT./(HRJ(SQ. FT.)
(H.r.u.)o.w.
The dBerent effects noticed may be characteristic of the individual system. In any case, internal sampling of the continuous phase is a positive method of locating any inlet effects. Blending and Elgin ( 1 ) showed that using the &red portion of the column a t the dispersed phase inlet reduced turbulence and permitted greater flow rates at flooding. For this reason, the flared portion of column I1 was placed at the ether inlet in one run. This change had essentially no effect on the concentration gradient. Eeight of Transfer Unit. Inspection of Figures 12 and 13 shows that (H.T.U.)o.w. does not vary appreciably throughout the column except a t the lowest ether rate runs. As the inlet concentration is increased, the average (H.T.U.)o.w. in the column tends to increase. As shown in Figure 14, there seems to be very little effect on (H.T.U.)o.w. in the column when the ferric chloride concentration in the inlet ether phase is increased. The average (H.T.U.)o.w. in the column with the inlet effect eliminated as a function of the ether rate is shown in Figure 15. At first there is a rapid decrease in (H.T.U.)o.w. with increase in rate. At high ether rates, regardleas of the inlet concentration, all (H.T.U.)o.w. values tend to approach a constant value of 0.7 foot. Such an effect would occur if equilibrium conditions were reached a t the bottom of the tower. However, in all cases there was an appreciable driving force throughout the tower. The most probable explanation of this constancy of height of transfer unit is that the area of the ether bubbles does not increase with increase in ether rate a t the higher rates of ether flow. This could be due to coalescence of bubbles or increased size of ether bubbles a t the higher rates. Nan& and Viswanathan (13)showed that the o v e r 4 height of transfer unit increased as column height increased. They did not actually segregate the inlet effects in the column. If an inlet effect were present, increasing the column height would tend to minimize the inlet effect a t either end of the column and give a larger apparent height of transfer unit for the whole column -for example, it was calculated that reducing the column length by 60% in one run would decrease the apparent over-all height of transfer unit from 0.58 to 0.43 foot even though the average height of transfer unit in the column remained constant. Proposed Correlation Procedure. By divorcing the inlet effect and the height of transfer unit in the remainder of the column as shown above, it is possible to correlate the spray tower results.
Figure 15. Average Height of Transfer Unit in Column with Inlet Effect Eliminated
Figures 11 and 15 could be used for predicting the column performance for the ferric chloride-ether-hydrochloric acid system. Further investigations may reveal that the inlet effect can be altered or eliminated by redesign of the column, or that the inlet effect may be negligible in commercial columns. In those runs in which the average concentration of the exit ether was between 0.006 and 0.035pound mole of ferric chloride per cubic foot, the outlet ether separated into two distinct phases. The concentration of ferric chloride in the bottom layer wag approximately 6 times that in the upper layer. This double phase would aid greatly in operating a commercial tower, inagmuch as the top ether phase could be recycled to the inlet of the tower and the bottom ether layer could be withdrawn continuously to a regeneration tower and scrubbed with water. UOMENCLATURE
a
= interfacial area per unit volume of extractor, square
feet
cross-sectional area of column square feet A C E = concentration of solute in phase E, pound moles per
cubic foot CW = concentration of solute in phase W ,pound moles per cubic foot C v * = concentration of solute in phase W which would be in equilibrium with concentration in opposite phase, pound moles er cubic foot ACwlm = log-mean vayues of C W - C W *for the two terminals of column h = effective height of extraction section of tower, feet (H.T.U.)o.w. = over-all height of transfer unit based on phase W , feet Kwa = over-all extraction coefficient based on phase W , pound moles per (hour)(cubic foot) (pound moles per cubic foot) k = constant K = distribution coefficient, concentration in phase E / concentration in phase W La = flow rate of phase E , cubic foot per (hour) (square foot) LW = flow rate of phase W , cubic foot per (hour) (square foot) Lv' = flow rate of phase W cubic foot per hour = dv*/dw, slope of equijibrium line in dilute end = amount of solute transferred, pound moles per hour Ng = amount of solute transferred in phase E, pound moles per hour N w = amount of solute transferred in phase W , pound moles per hour N0.w. = over-all number of transfer units based on phase W
June 1950
= ratio of molecular weight of solute-free stream to that of solvent l& ratio of solvent to solution flow in same units &B m ut = weight fraction of solute in entering solvents V = effective volume of extraction column, cubic feet w = weight fraction of solute in solution tu‘ = weight fraction of solute in solution which would be in equilibrium with solute in solvent Subscripts ends of tower or section of tower = in ether phase W = inwaterphase c
i
INDUSTRIAL AND ENGINEERING CHEMISTRY
-
f2 -
LITERATURE CITED
(1)Blanding, F. H.,and Elgin, J. C., Trans. Am. Inst. Chem. EWE., 38, 305 (1942). (2)Colburn, A. P., IND. ENQ.CHEM.,33,459 (1941). (3) Dodson, R. W., Forney, G. J., and Swift, E.H., J . Am. C h . Soo., 58,2573 (1936). (4)Elgin, J. C., and Browning, F. M,,Trans. Am. Inst. Chem. Engts., 31,639 (1936).
llSl
(5)Ibid., 32,105 (1936). (6) Hunter, T. G., and Nash, A. W., J . SOC.Chem. I d . , 51,285T (1932). (7)Johnson, H.F., and Bliss, H.,Trane. Am. Inst. Chem. Engs., 42,331 (1946). (8) Kolthoff, I. M., and Sandell, E. B., “Textbook of Quantitative Inorganic Analysis,’’p. 544,New York, Macmillan Co., 1936. (9)Ibid., p. 579. (10)Ibid., p. 637. (11)Nachtrieb, N. H.,and Conway, J. G . , J. Am. Chem. SOC..70. 3547 (194s). (12)Nachtrieb, N. H.,and Fryxell, R. E., Ibid., 70,3552 (1948). (13)Nandi, 8. K., and Viswanathan, T. R.,Current Science (India). 15, 162 (June 1946). (14)Sandell, E. B., “Colorimetric Determination of Traces of Metals,” p. 263,Nea York, Interscience Publishers, 1944. (15)Sherwood, T. K., Evans, J. E., and Longcor, J. V. A,, IND. ENQ.CHEM.,31,1144 (1939). RECEIVED November 30, 1949. Based on a dieaertation in ahemioal engineering presented by Christie J. Geankoplis to the faculty of the Graduate School, University of Pennsylvania, in partial fulfillment of the requirements for the degree of doctor o i philosophy, June 1949.
Mechanism of Solute Transfer in Spray Towers WILLIAM
LICHT, JR., AND JOSEPH
E. CONWAY’
UNIVERSITY OF CINCINNATI, CINCINNATI, OHIO
Three separate stages of extraction are postulated for spray tower operation. The existence of thew stages is experimentally verified and some of the faoton affecting each atage are discussed. The extraction of aoetic acid from water using isopropyl ether, methyl isobutyl ketone, and ethyl acetate was studied with the water phase dispersed. Ethyl acetate was found to be the most effective solvent and isopropyl ether the least effective for this separation. Over-all transfer coefficients for two of the stages of extraction are calculated and their signlflcance is discurred. This paper represents the flrst phase of a research program undertaken to supply a more fundamental understanding of spray tower performance.
s
PRAY towers represent one of the iaportant typea of columns in which continuous countercurrent extraction is carried out
industrially. These towers continue t o receive considerable attention in the field of liquid-liquid extraction despite the development of new typea of equipment, such aa that by Scheibel (8, 7), which may exhibit higher extraction efficiency. The advantages of spray towers including ease and simplicity of operation, low maintenance costa, ease of cleaning, etc., are difficult to put aside. Elgin and Browning (8),Sherwood, Evans, and Longcor (8), Blanding and Elgin ( I ) , and Johnson and B l h (6) have all studied spray tower operation extensively. The results of these authors indicate that the capacity of a spray column is dependent upon the rates of flow of both the continuous and the dispersed phases, upon which phase is dispersed, upon the direction of extraction, and in certain caaes upon the inlet solute concentration. Johnson and Blisa (6) have presented some simple rules for predicting the proper phase t o disperse in spray tower operation. However, difficulties still arise in connection with this unit operation and some of the results reported are difficult to explain. It waa felt, therefore, that a more fundamental approach to this 1 Pment addrms, Villanovs College, Villanova, Pa.
topic might be undertaken and the present work is an attempt to elucidate further the mechanism by which solute is transferred in spray towers. It seems evident that a study of the mechanism of solute transfer should recognize three separate stages of extraction in the life of each drop during spray tower operation. The period during which the drop is forming on the tip constitutes stage 1. Stage 2 covers the period of drop fall (or rise) through the continuous phase. This stage begins the instant the drop is detached from the tip and ends when the drop strikes the interface at the opposite end of the tower. Stage 3 begins at the end of stage 2 and consists of the process of the drop crossing the interface a t which it has just arrived. It is probable that different factors affect each stage differently; variation of any operating variable may produce changes in certain of these stages while leaving the other stages relatively unaffected. A proper understanding, therefore, of spray tower operation and performance can be had only from a recognition of these three separate extraction stages and from a complete understanding of the factors which affect these stages. The inclusion of the effect of all three stages in an over-all transfer coefficient for the entire tower may be misleading and tend to conceal the effect of the individual variables upon the tower operation. Experimental work was undertaken to prove the existence of these three separate stages of extraction and to evaluate their relative importance under a certain set of operating conditions. Some of the factors affecting certain of these stages were also investigated. EXPERIMENTAL WORK
The extraction of acetic acid from water was studied using isopropyl ether, methyl isobutyl ketone, and ethylacetate aa solvents. The concentration of the acetic acid solution ww 1.053 N (about 6% by weight); this phase waa dispersed and allowed to f d l through the solvent phase, The materials employed were W follows:
