(340) Suciu, D., Rev. Chim. (Bucharest) 18,245 (1967). (341) Swaddle, T. W., King, E. L., Inorg. Chem. 4,532 (1965). (342) Sykora, V., Dubsky, F., Coll. Czech. Chem. Commun. 32, 3342 (1967). (343) Tera, F., Morrison, G. H., ANAL. CHEM.38,959 (1966). (344) Titze, H., J . Chromatog. 20, 193 (1965). (345) Tomaaic, B., Siekierski, S., Ibid., 21, 98 (1966). (346) Toribara, T. Y., Separation Sci. 2, 283 (1967). (347) Toribara, T. Y., Koval, L., Talanta 14,403 (1967). (348) Torracca, E., Costantino, lT., lfassucci, 31. 8.,J . Chromatog. 30, 584 (1967). (349) Travesi, h., Palomares, J., Dominguez, G., Anal. Chim. Acta 35, 421 (1966). (350) Tsitovich, I. K., Kuz’menko, E. A., Zh. Analit. Khim. 22, 603 (1967).
(351) Tsuk, A. G., Gregor, H. P., J. Am. Chem. SOC.87,5538 (1965). (352) Tustanowski, S., J. Chromatog. 31,269 (1967). (353) T’esugi, K., lIurakami, T., Buns e k i Kagaku 15,482 (1966). (354) Vaughn, J. W., Krainc, B. J., Inorg. Chem. 4, 1077 (1965). (35j) Vinkovetskaya, S. Y., Nazovenko, V. L4., Zavodsk. Lab. 32, 1202 (1966). (356) T’ydra, F., Anal. Chzm. Acta 38, 201 (1967). (357) Wahlborg, E. F., Christensson, L., Gardell, S., Anal. Biochem. 13, 177 I1966 ). (358) Webster, P. V., Wilson, J. N., Franks, M. C., Anal. Chtm. Acta 38, 193 (1067). (359) Werner, G., 2. Chem. 5, 311 (1965). (3601 Werner, G.. J . Chromatoa. 22, 400 (361) J?iersma, J. H., Sandoval, A. A., Ibid., 20, 374 (1965). (362) Wheaton, T. A., Stewart, I., Anal. Biochem. 12,585 (1965).
(363) Wish. L., Foli. S. C.. J . Chromatoa. ’ 20,585 ( l 9 6 i ) . ’ (364) Wolf, F., Koch, H., Z. Chem. 6 , 22 (1966). (365) Yaku, F., llatsushima, Y., Xippon Kagaku Zasshi 87,969 (1966). (366) Yamamoto, D., Fukudome, A., -4nal. Chirn. Acta 34. 240 11066). (367) Yoshino, Y., K’ininoshita, H., Sugiyama, H., Sippon Kagaku Zasshi 86, 405 (1965). (368) Yoshino, Y., Sugiyama, H., Nogaito S.,Kinoshita, H., Sci. Papers of College of General Education, Univ. of Tokyo, 16,57 (1966). (369) Young, T. E., llaggs, R. J., Anal. Chirn. Acta, 38, 105 (1967). (370) Zagrodzki, S., Kurkowska, A., Chem. Anal. (IParsaw) 12, l>!I (1967). (371) Zerfing, R. C., T’eening, H., ANAL. CHEM.38, 1312 (1966). (372) Zilikman, A. N., lleisner, G., Zh. Prikl. Khim. 39, 992 (1966); C.A. 65. 1 4 4 7 6 ~ .
Analytical Distillation Frederick
E. Williams,
Hercules Research Center, Hercules Incorporated, Wilrnington, Del. 7 9899
T
wo
YEARS AGO in the biennial review of analytical distillation, n e predicted the ultimate demise of the art; a t best it could retain no more than vestiges of its former usefulness under the continued buffeting of gas-liquid chromatography with the latter’s vast virtues of speed, sensitivity, potential accuracy, and very nearly infinitesimal sample size. Analytical distillation was never born: it just grew. If it is to be made respectable by being given a date of birth, i t first looked upon the world from the top of a brass tube 0.75 in. in diameter and 52 ft high in an airshaft in Pond Laboratory at Pennsylvania State University in 1930. With this unit Fenske (3) demonstrated that i t 11as possible to separate two isomeric hydrocarbons boiling only 2.0” C apart. Under the impetus of this “impossible” event, analytical dibtillation took on stature. I n less than half a decade, column packings were developed which made it possible to have a n equally powerful separating unit in the confines of a laboratory. Unfortunately, the cult of the instrument maker had not yet been born, or scarcely so, and the clever reqearch and glittering packaging of the ga> chromatography era nere not available to aid the development of analytical distillation. But it grew and prospered through World War 11; it performed notably for that conflict, yet a decade and a half later its day was finished. &Isan art, analytical distillation joins the alembic among the methods and de-
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ANALYTICAL CHEMISTRY
vices men have used to probe into the nature of things. Having thus written the obituary of analytical distillation, we turn now to its useful offspring, small-scale distillation. We are concerned here with small quantities of materials suitable for research or as pure standards for gas chromatography. Such amounts usually involve 10 to 100 grams. Leslie (6) has discussed this aspect of distillation; he considers four types of columns for these uzes and rates them as follom: wetted-wall, 7-15 plates; concentric-tube, 120 platei; spinningband, 50-100 plates; and packed column, 100 plates per meter in separating pon er. The range of sample sizes mentioned above is difficult to handle effectively over the desired pressure range of 1-760 torr. Small packed columns of kno\\n design would be generally suitable for atmospheric pressure or somewhat lower, but for the range of 1-100 torr, pressure drop through the column mould make the packed column generally useless; and at any pressure, the dynamic hold-up of the packing (usually l5-20% of the packing volume) is generally an adverse factor. This situation leaves only the open-tube column or its variant, the spinning-band column, as a fractionating device with the desired characteristics. King and Yates (4) have combined a thorough review of the spinning-band column nith a report on their own extended investigation of this type column
operated with nine band designs. Their xork largely confirmed the results of earlier workers insofar as the latter (a rather unsystematic effort, as a whole) could be interpreted. Values of H E T P (height equivalent of a theoretical plate) of .everal centimeters nere generally found a t all pressures investigated. Considering the boil-up rates used by the workers in this field, the high values of H E T P are not uneqected in the light of Kuhn’s (5) and Resthaver’s (10) theoretical investigations of the xetted-wall column. Heat loss from the walls of a spinningband column might very well be a factor in the poor showing of the columns. Such losses would tend to load the walls in the lower portion of the column. As a result only the upper sections mould have the desirably thin layer of refluu and low vapor velocity to provide good fractionation with the result that overall H E T P values \\auld be unnecessarily high. Neasurement of energy dijsipation on a still-pot with a comparison of a measured boil-up rate would give home indication of column heat losses. I n an overall view, spinning-band columns should be designed and operated with much of the technology of precision calorimetry. Sester and Seater (8) have designed a small column which they call “The hnnular Teflon Spinning h i i d Column.” The rectifying section consists of a tube 5-mm i d. in which there rotates a screw of Teflon. The latter rotates to move Liquid on the wall downward to the
still-pot. The column is claimed to have a separating efficiency of 125 to 150 thcoietical plates at atmospheric pressure. The high pressure drop (3 tori) a t atmospheric pressure suggests that in reduced-pressure operation, very high pressure drops might be eupected. Unfortunately the authors do not disclose data for pressure-drop, reboiler temperatures, or column efficiency a t reduced pressures. This lack of complete data has been common with many other workers in the field of low-pressure fractionation; conceivably, this has done much disservice to the development of low-pressure fractionating columns. Pressure drop, it should be emphasized, is something that develops when a gas or vapor flows through a passage and has nothing to do fundamentally with rectification in a fractionating column. Where laminar flow occurs, as in all properly operated fractionating columns, pressure-drop varies as the 4th power of the diameter in a round, open-tube column. Any obstruction, whether i t be packing or some driven mechanical contrivance, will increase pressure drop if it significantly reduces the cross-sectional area of the vapor passage. Rectification occurs &hen a vapor mixture passes over a liquid of the same qualitative composition (except that of azeotropic composition) to allow the natural tendency toward equilibrium to occur. If the velocity of vapor movement is too great, or the area of contact between the liquid and vapor is too small for equilibrium to take place, rectification is poor. A properly designed column packing provides a large area of vaporliquid contact, while in a bubble plate column, a large area of vapor-liquid contact is generated by the vapor movement through pools of refluxing liquid. Kuhn (5) and Westhaver (10) made their theoretical studies on the simple geometry of the straight open-tube columns. I n this design, vapor-liquid contacting can occur only on the inner surface of the tube wall. T o be effective there, sufficient time is required for necessary diffusional effects to take place; this in turn means that permissible vapor velocity must be kept low. As the operating pressure is reduced, vapor velocities increase for a given evaporation rate. Although coefficients of diffusion increase also about in
inverse proportion to pressure t o provide a compensating effect, the requirement of low boil-up a t any pressure is still a critical limiting factor in the productivity of an open-tube column. Anot,her factor, born of a complex of effects, is the thickness of the film of refluxing liquid and its effect on separating power of the column. Once this film attains a thickness where it is not contacted in depth or poorly contacted by the adjacent vapor, the column starts to behave as a column operating a t a very low reflux ratio, a condition of poor separating power. Packed columns, for the same reason, usually show severe drops in separating power with increase in boil-up rates. Among other types of laborntory columns, the Oldershaiv design working with bubble plates has been of considerable interest. Mead and Rehon ( 7 ) have reviewed the qualities of this column. They concluded the dynamic hold-up in the Oldershaiv design is too great for effective batch distillations. Cooke (9)has developed a new design for a perforated plate column which gives about 15% increase in permissible boil-up rate and 10% increase in efficiency, compared to the original Oldershaw design. This improvement is of most significance to those working with fairly large quantities of materials at atmospheric pressure or as low as 250 torr. An unusual and gratifying addition to distillation literature was made in mid1967 with the publication of Peacock’s (9) paper dealing with the selection of test mixtures for distillation columns. Generally, selection of a test mixture is dictated by what is available and well enough standardized to serve in the desired capacity. Very few such mixtures are known; and most of these are suitable only for atmospheric pressure. The problem of analysis has been a major stumbling block to the development of a wider variety of test mixtures. Gas chromatography should make many very nearly ideal mixtures available, particularly since these binary mixtures whose compositions are synthetically established, act ai standards for the analysis of vapor and liquid samples developed during a test. Peacock’s paper is of great importance in that it shows that there is an optimum relative volatility of a test mixture for a
given separating power a t which analytical errors are least important. A chart is presented showing this optimum, and the effect of departures from the optimum. An error of 10% in measuring the separating power of a distillation column is rarely disastrous. I n fact, our slavish use (largely out of necessity) of hydrocarbons to measure separating power and then imputing the same separating power to the column for mixtures of alcohols, ethers, ketones, amines, esters, etc., may in fact involve us in sizable errors, perhaps more than 10%. I n passing, we suggest that in testing small spinning-band columns for separating power, their very low dynamic hold-up and the minute quantity (microliters) of distillate required for gas chromatographic analysis, make it unnecessary to analyze the pot liquid; the composition of the latter could not be significantly changed from its composition at the time of synthesis. This possibility would be particularly helpful in making tests a t 1-10 Torr where the internal pressure in the system will not be large enough to push out a sample from the still-pot. I n concluding this review, we wish to point out that research in distillation is nearly nonexistent in both university and industrial laboratories. This fact has been noted by Bolles and Fair ( I ) in their review of large-scale distillation. The brevity of this review substantiates the dearth of new work in the field of laboratory distillation. LITERATURE CITED
Bolles, W. L., Fair, J. R., Ind. Eng. Chem., 59,86 (1067). (2) Cooke, G. N., ANAL.CHEM.,39, 286
(1)
( 1 967). , (3) Fenske, M. R., Ind. Eng. Chem., 24, 482 (1932). (4) King, P. J., Yates, B. J., Chem. Process Eng., 47, 214 (1966). ( 5 ) Kuhn, W., Helv. Chim. Acta, 25, 252 (19421. f 6 i Lesfie. R. T., Ann. Y. Y. Acad. Sci., 137, 19’(1966). ( 7 ) Mead, R. W., Itehon, T. R., Chem. Eng. Proc. Sym. Ser. 63 (70), 44 (1967). (8) Nester, R. G., Nester, R. >I., Pittsburgh Conference on Analvtical Chem\ - - -
\
,
-
istry and Applied Spectroscopy,- Fek)ruary 1966. (9) Peacock, D. G., Chem. Eng. Sci., 22, 957 (1967).
(10) Westhaver, J. W., Ind. Eng. Chem. 34, 126 (1942).
VOL. 40, NO. 5 , APRIL 1968
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