INDUSTRIAL A N D ENGINEERING CHEMISTRY
1198
(12) Havilicek, Engineming, 129,1 (1930). (13) International Critical Tables, Vol. 111, pp. 291, 293, 310,311, New York, McGraw-Hill Book Co., 1928. (14) Jackson, h.1. L., Ph.D. thesis, University of Minneaota, 1948. (15) Johnstone and Pigford, Trans. Am. Inst. Chem. Engrs., 38, 25 (1942). (16) Kalinske and Pien, IND.END.CHEM.,36,220(1944). (17) Landolt-Bornstein, “Physikalisch-Chernische Tabellen,” Eg IIb, p. 1338, 1931; Suppl. 111, Pt. 1, p. 186, 1937; Ann
Vol. 42, No. 6
(22) Perry, “Chemical Engineer’s Handbook,” 2nd ed., pp. 308, 613,792,New York, McGraw-Hill Book Co., 1941. (23) Richards and Room, Science, 71,290 (1930). (24) Sherwood chapter in “Fluid Mechanics and Statistical Methode
in Engineering,” Bicentennial Conference, Philadelphia, University of Pennsylvania Press, 1941. (25) Sherwood and Woertz, IND.ENQ.CHEM.,31, 1035 (1939). (26) Storrow, J . SOC.Chem. Id., 66,41,73 (1937). (27) Suroweic and Furnas, Trans. Am. Inat. Chem. Engrs., 38, 86
Arbor, Mich., J. W. Edwards.
11842). -_,. I__
(18) Lange, “Handbook of Chemistry,” 6th ed., pp. 933, 1590, Sandusky,Ohio, Handbook Publishers, 1944. (19) Maass et al., Can. J. Research, 2, 388 (1930); 4, 283 (1931); 5, 436,442(1931); 6,428(1932). (20) Othrner, IND.ENC.CHEM.,32,841 (1940). (21) Peck and Wagner, Trans. Am. Inst. Chem. Engrs., 41, 737 (1 945).
(28) Von K&rm&n, Ibid., 61,705 (1939). (29) Walker, Lewis, McAdams, and Gilliland, “Principles of Chemical Engineering,’’ 3rd ed., p. 443, New York, McGraw-Hill Book Co., 1937. (30) Westhaver, IND. ENC.CHEM.,34,126(1942).
RECEIVED November 30, 1949.
Fractional 0.
F. ASSELIN’ UNIVERSITY
AND
T h e effect of returning reflux in solvent extraction processes involving four or five components was lnvestlgated. Two of these were t h e essentially immiscible solvents, water and n-amyl alcohol, and two of the others were the compounds in a mixture to be separated. When a fifth component was used, it served as a separating agent between the other two solutes. The effect on the fractionation achieved caused by varying the relative amounts of the two solvents, t h e total concentration of the two compounds in the feed, and the composition of t h e feed on a solvent-free basis was also observed using a five-stage countercurrent extractor. A stagewise graphical method of calculation i s presented which permits t h e computation of the results of such extraction processes when reflux is employed. The position of a so-called “reflux line” relative t o the equilibrium curve enables one t o estimate very quickly the course which the extraction will follow. Calculations for complete extraction units are presented which indicate t h a t the equilibrium relationships of t h e system must meet certain conditions, if reflux Is t o be very effective. Its use is of greatest advantage with systems employing a flfth component as a separating agent.
A
FRACTIONAL liquid extraction process is one in which an immiscible liquid solvent preferentially extracts one of two or more solutes from the liquid in which they are dissolved. One of the better known commercial examples of this type of process is the Duo-Sol method for refining lubricating oils (6). The purification of certain of the less common metals and the production of iron-free alumina by fractional solvent extraction have also been described (6,IO). Commonly only a few extraction stages are used, but when two highly purified products are desired, a larger number will be required. A large number of stages may be justified in the fine chemicals industry for the separation of valuable materials which are difficult or impossible to separate by other means. A case in point is the separation of thorium from the rare earth elements by fractional solvent extraction (2). The number of stages required in a difficult separation should be kept to a minimum. The most commonly used method of decreasing the number of equilibrium contacts required in distillation consists in returning reflux. Saal and van Dyck (12) were the first to discuss in detail the close analogy betaeen dis1 Preaent
E. W. C O M I N G S
OF ILLINOIS. URBANA. ILL.
address, Humble Oil & ReBning Company, Baytown, Tex.
tillation and extraction and the implications of returning reflux. Their paper was concerned only with threecomponent liquid systems and is not applicable, except in its broadest principles, to the four-component systems encountered in the fractionation of two solutes by means of two immiscible solvents. Scheibel (13) discusses fractionations which employ such four-component systems but makes no mention of the use of reflux. The present investigation shows the effects of returning reflux and presents a method of calculating the course of the extraction when reflux is utilized. EQUIPMENT AND OPERATION
A five-stage extraction system was used in reference to a packed column because it provided a more de&ite number of equilibrium contacts and permitted sampling at definite, intermediate stages. The latter is important when the equipment is used to follow the course of extraction in a system for which complete equilibrium data are not available. Each stage consisted of a ear ump, a mixing column 1 inch in diameter b 2 feet, paeke8 wit{ 0.125-inch glaas beads, and a settling chamger similar in design and construction to that used by Knox et al. (8). The settling chambers were made from 1liter Florence flasks and were provided with a fixed solvent overflow arm and an adjustable arm with rubber tube connections to control the level of the liquid interface. Piping was a/a-inch streamline copper pipe and connections to glms were made with rubber tubing. The provision for reflux consisted of a mechanical reflux splitter, a spray-type extraction column 1 inch in diameter b 5 feet and a glass evaporator similar to Kemmerer’s desi n (7‘f The evaporator was equipped with liquid level control fevices to maintain conatancy of operation, Figure 1 shows an over-all flow diagram of the apparatus. Aqueous feed enters the extractor at one end, proceeds countercurrently to the alcohol through five extraction stagw, and is then split into two streams, one of which is withdrawn as purified product and the other is sent to the evaporator to provide reflux. Thick liquor from the evaporator I stripped of part of its solute content by fresh solvent in the spray column. The aqueous raffinate from the column is recycled to the evaporator and the alcohol extract bearin the reflux is sent to. the extractor. All the solute which enters t f e evaporator IS ultimately transferred to the alcohol phase and returned to the extractor as re flux. The concentration of each solute in the evaporator will adjust itself so that this will be accomplished, irrespective of the efficiency of the spray column or the rate of thick liquor recirculation. Thus, the composition of solutes on a solvent-free basis is the same in the aqueous stream, leaving, and in the alcohol, entering, at one end of the extractor.
INDUSTRIAL AND ENGINEERING CHEMISTRY
June 1950
EXTRACTION STAGES
a jr:PuEOUS FEED
4 IMPURE
ALCOHOL PRODUCT
Figure 1.
Flow Diagram of Experimental Extraction U n i t
-f
a. Aloohol w. Wator
The suction side of the gear pump for each stage waa connected to a short vertical tube open to the atmosphere. The two stream to be mixed discharged into this tube. The speed of the pump was adjusted so that its ca acity slight13 exceeded the flow rate of these two streams. T i e reflux s litter was a cylindrical cup 3 inches in diameter by 3 !riches hi&, fitted with radial partitions extending up about half ita height and dividing the cu into two compartments. Each compartment had an outlet from the bottom, one for aqueous product and the other for reflux. A slow1 rotating inverFd Y was mpunted above the cup, so that liquig fed through it was distributed uniformly around the circumference of the cup and split into two streams in proportion to the size of each compartment and independent of the flow rate, The bafaes were reset when the per cent reflux was changed. The spray column followed design of Blanding and El 'n (5)with spray nozzle for the a ueow hase at the top. %he evaporator consisted of four )-foot Liebig condensers in parallel with the central tubes manifolded together a t the top and also a t the bottom, Steam was introduced into the outer jacket near the bottom and the assembly functioned as a long-tube vertical eva rator. Vapor formed in the tubes passed into the up er m a n i g d and thence through a tangential entrance into an incyined &liter round-bottomed flask which served as a vapor disenga 'ng chamber. The liquid separated from the vapor in this fask was returned b gravity through a connection in the bottom of the flask to the yower manifold of the vertical tubes. Vapor left the flask through the neck and passed to a double pipe condenser. The eva orator operr\ted under a variable vacuum of a proximately 87 inches of mercur The vacuum was regulate! by an air bleed actuated by ef%trical contacts to control the liquid level in the evaporator andthereby maintain the proper rate of evaporation, The feed was introduced to the bottom manifold through a solenoid valve actuated b contacts in a standpipe which maintained a constant liqwd leverupstream from this valve. Operation of the equipment was generally satisfactory. Flow rates were indicated by rotameters, but they were actually measured by collecting the stream leaving the extractor over a timed interval. Operation was continued until consecutive samples of the aqueous product showed the same analysis. A steady state had then been reached, and samples of both phases of each stage were withdrawn directly from the settling chambers. SEPARATION OF OXALIC AND SUCCINIC ACIDS
Twelve preliminary runs with pure oxalic and pure succinic acids under a wide variety of operating conditions established that each stage represented on the average one equilibrium contact. Seventeen runs were then made with mixtures of the two acids. Because only that portion of a complete extraction unit on one side of the feed wm employed, only one purified product could be obtained in this case the oxalic acid in the effluent aqueous streams. The two items of most interest, therefore, were the purity of the aqueous product and the fraction of oxalic acid fed to the extractor which is recovered in a purified form. Experimental runs were arranged in groups to show as clearly as possible the effect on these two quantities of the percentage reflux, solvent-&water ratio, total feed concentration, and feed composition on L solvent-free basis.
1199
In the first series, the concentration of each acid in the feed was constant, and the percentage reflux and solvent-&water ratio were varied. The points in Figure 2 show the variation in purity and recovery of the oxalic acid product and in the recovery of the succinic acid with variation in the solvent-&water ratio a t 0, 54, and 100% reflux. In the second series, the total feed conoentration and the solventdo-water ratio were maintained constant, while the composition of the feed on a solventfree basis and the percentage reflux were varied. The points in Figure 3 show the results of this series of runs, In the last series, all variables were held constant except the total PURE ALCOHOL feed ooncentration. The variation in recovery f EED of both acids and in product purity for thii series is shown in Firmre 4. The results of a11 runs were also predicted by calculations from the equilibrium data, using the graphical meGod described below and taking into account the small effect on the equilibrium distribution of one acid between water and alcohol due to the presence of the other acid. The solid lines in Figures 2, 3, and 4 were obtained from these calculations. No attempt was made to place these lines through the experimental points; hence, the agreement between calculations and experimental values will be seen to be generally good. Table I summarizes the experimentaland calculated results of all the above runs. Purity and recovery vary in opposite directions and increased solvent-to-water ratio and increased percentage reflux have roughly parallel effect8 in improving the product purity and decreasing the recovery of product. The use of reflux thus made it possible to improve the purity with a given number of stages a t a fixed solvent-to-water ratio. Alternatively, the w e of reflux would have made it possible to obtain the same product purity with a reduced number of stages or with a smaller solvent-to-water ratio. The former would be of importance when a large number of stages is required for a
PERCENT
REFLU
I
2F
2; 8> 8& 32
D "% I-
5%
$z
aE 3 a
Figure 2.
ALCOHOL TO WATER R A T I O Extraction of Oxalic-Succinic Acid Mixtures
Fwd oonomtratlon of orrllo end ruwlnlo aoldr, 20 gram8 per
litor of moh
INDUSTRIAL AND ENGINEERING CHEMISTRY
1200
solvent-to-water ratio, which generally differed slightly from the nominal integral value specified on the graphs.
Table I. Summary of Runs with Oxalic and Succinic Acids Feed Concentration,, G& Water R u n Oxalic Succinic Ratio 1
2 3 4 5
. ... .. .. .. .. . I
1Y.I
I.U7
19.5 19.5 21.1 9.75
1.91 2.92 3.76 2.06
%
Reflux
Purity of A
