I
A. H. P. SKELLAND Chemical Engineering Department, Illinois Institute of Technology, Chicago 16, 111.
The Futility of Raffinate Reflux in liquid Extraction Raffinate reflux is valueless in liquid-liquid extraction and involves the pointless use of a solvent mixer and reflux divider
MANY
LEADERS in the profession and the great majority of standard textbooks (7-77) indicate use of-or give design methods for-continuous countercurrent liquid-liquid extraction using raffinate reflux as a beneficial technique Nevertheless it can be shown that raffinate reflux is of no value either when accompanied by extract reflux or when used alone. I t involves the needless use of a solvent mixer, reflux dividing equipment, and possibly an auxiliary pumping unit for the refluxed raffinate. Except for the letter of Wehner (72) this important fact appears to be widely unrecognized. The reasoning can be outlined as follows, with the aid of certain illustrations (9). Raffinate reflux here refers specifically to the procedure in which a portion (I?;) of the primary raffinate product (R,)is returned to the,extraction system after preliminary blending with the extracting solvent in a solvent mixer.
Editor’s Note W e hope that this short article will help prevent the perpetuation of an error regarding a maior design and operating procedure-an error widely disseminated in standard texts (see accompanying literature references). Wehner of Johns Hopkins, reached the same conclusions b y the same algebraic method and called attention to them without the algebra in a brief note in the A.1.Ch.E. Journal (72). The editors, Professor Skelland, and Professor Wehner feel it desirable to re-emphasize this error for those who may not have seen Wehner’s letter or who, even if they did see it, preferred to rely on the authority of these references.
The mass flow rate of the feed and the compositions of the streams F, PR, PE, and S will have respectively constant values throughout. Extract Reflux with and without Raffinate Reflux
Refer to figures 6.47 and 6.61 ( 9 ) : For a given extract reflux ratio, material balances on each ideal stage to the left of the feed stage lead to the same operating or difference point, Q, both with and without raffinate reflux. This is illustrated on the Janecke diagram for a Type I1 system, Figure 1. When pure extracting solvent is used, material balances on each ideal stage to the right of the feed stage lead to a raffinate operating or difference point, W , on the vertical through PR. [PR= R, when using extract reflux alone (Q).]
A solvent-free material balance over the whole plant gives
F” = W”
+ Q”
However, since points F and Q are in the same respective positions both with and without raffinate reflux, this means that point W must also be in the same place in both cases (see Figure 1). Accordingly, the total number of ideal stages required and the location of the feed stage must be identical with and without raffinate reflux. Over-all and component material balances over the whole plant of course show that the product quantities are independent of any form of reflux. In figures 6.47 and 6.61 ( 9 ) a solvent balance on the right-hand end leads in both cases to
S
=
Pzv(Np, - Nw)
N
1‘ Figure 1. For a given extract reflux ratio the locations of points W and Q are independent of any raffinate reflux VOL. 53, NO. 10
0
OCTOBER 1961
799
Nomenclature
The nomenclature is the same as in Chapter 6 (9),except that solvent-free streams are denoted by double-primes instead of boldface type.
\f=E,
literature Cited
/ W
Figure 2. In the absence of extract reflux the locations of points W and independent of any raffinate reflux
where ‘YpR and N w are the ordinates of points Px and W in Figure 1. This shows that the solvent requirements are identical in the two cases. Operation with and without Raffinate Reflux
Refer to figures 6.26 and 6.63 ( 9 ) : Material balances on each ideal stage lead to the same operating or difference point, W , both with and without raffinate reflux. When pure extracting solvent is used W j s on the vertical through PR, as shown on the Janecke diagram, Figure 2. [UT becomes the symbol 0, PR = R,, and Ef is called El when operating without reflux ( 9 ) ] Now F” - E“, = W”, and since points F and Ej (or E l ) are in the same respective positions both with and without raffinate reflux, this means that
800
M are
point 11; must also be in the same place in both cases (Figure 2). Consequently the total number of ideal stages needed must be the same both with and without raffinate reflux. If pure extracting solvent is used, a solvent-free material balance aver the whole plant is fi”
Ef”+ PR’f
1
=
=
M“iV.\r - F“Nr
where N M and iVP are the ordinates of points M and F i n Figure 2. This shows that the solvent requirements are identical in the two cases.
INDUSTRIAL AND ENGINEERNO CHEMISTRY
RECEIVED for review January 12, 1961 ACCEPTED March 30, 1961
M’f
Since points E, (or E l ) ,PR and F have the same respective positions on Figure 2 both with and without raffinate reflux, this means that point M is also in the same place in both cases. A solvent balance yields
S
(l),Brown, G. G., others, “Unit Operations,” pp. 301-2, Wiley, New York, 1950. (2) “Chemical Engineers Handbook” (J. H. Perry, editor), pp. 717-18, 733-9, McGraw-Hill. New York. 1950. ( 3 ) Elgin, J. d,Chem. &’ Met. Eng. 49, NO. 5, 110-16 (1942). (4) Foust, A. S.,others, “Principles of Unit Operations,’’ pp. 51? 54, 55, Wiley, New- York, 1960. (5) Larian, M. G., “Fundamentals of Chemical Engineering Operations,” pp. 452-7, Prentice-Hall, Englewood Cliffs, N. J., 1958. (6) Maloney, J. O., Schubert, A. E., Trans. Am. 2nd. Chem. &grs. 36, 741-57 (1940). (7) McCabe, W. L., Smith, J. CI., ”Unit Operations of Chemical Engineering,” pp. 787-95, McGraw-Hill, New York 1956. (8) Sherivood, T. K., Pigford, K. L., “Absorption and Extraction,” pp. 394-5 McGraw--Hill,New York, 1952. (9) Trevbal, R. E., “Liquid Extraction:” pp. 176-94, 196-8, McGraw-Hill, Kew York, 1351. (10) Treybal, R. E., “Mass Transfer Operations,’’ pp. 412-25, McGraw-Hill, New York. 1955. (11) Varteressian, K. A,, Fenske. hf. R., IND. ENG.CHEM.28, 1353-60 (1936). (12) Wehner, J. F., A.2.Ck.E. Journal 5 , 406 (September 1959).
Correction Removing Carbon Monoxide from Ammonia Synthesis Gas I n this article by Holger C. Andersen and William J. Green [IND.ENG.CHEM. 53, 645 (1961)], there is a n error in the identification of Figures 1 and 2, page 646. The graphs above the captions should be interchanged.
