FIGURE 1.
BATCH-STRIPPIIVO C O L U h l S
A
FRACTIONAL DISTILLATION COLUMNS With One Hundred Theoretical Plates
tion of the coristituents of petroleum was very limited. As a result of the work done during the last six years on American Petroleum Institute Project 6 a t the Kational Bureau of Standards (6, 6) and also in this laboratory, these three conclusions have been disproved. Previous publications described efficient fractionating columns that were used successfully in fractionating petroleum (4, 7 , 8). However, it was found that, after the fractionation of a barrel of gasoline or naphtha in the largediameter column with seventy-five theoretical plates, one refractionation in columns with approximately thirty or less theoretical plates was practically useless, but one refractionation in a column with about one hundred theoretical plates gave considerable separation and made possible the isolation of many compounds with a purity of 98 per cent or better. In other words, by using columns with seventy-five to one hundred plates, it is possible to separate petroleum either into extremely narrow-boiling fractions or else into practically pure substances. Final purification or further separation can then be effected by the methods developed in connection with American Petroleum Institute Project 6 a t the Sational Bureau of Standards (ti, 6) or in some cases by solvent extraction. Two such hundred-plate columns have been used not only for the refractionation of petroleum fractions but also for the fractionation of water ( 2 ) and for the fractionation of difficult mixtures of alcohols, such as isoamyl and active amyl alcohols, in fusel oil. Since the results on all of this work are being published, this present paper will briefly describe these columns and so prevent future repetition. One column is a conventional type column and the other may be operated either as a conventional enriching column or as an inverted column (batch-stripping column).
Column A This column consists of four sections, totaling 13.1 meters (43 feet) of 33-mm. (16/le-inch) 0. d. and No. 20 Stubs gage wall thickness nickel tubing braaed together. Conical screen distributors are placed in the column every 3.05 meters (10 feet). The packing is 3-mm. (l/a-inch) diameter, single-turn rings made from No. 26 nickel wire. The height of packed section is 12.5 meters (41 feet). The column is wound in four sections with No. 18 Nichrome resistance wire and lagged. A diagrammatic sketch of the column is given in Figure 1 which shows its use as a strippin column; i. e., the highest boiling components are those obtainefi first. By a few minor changes the column could also be operated in the ordinary manner as a batch enriching column. When used as a stripping column, the charge to be fractionated is
M. R. FENSKE, C. 0.TONGBERG, D. QUIGGLE, AND D. S. CRYDER The Pennsylvania S t a t e College, State College, Pa.
Fractional distillation columns with at least one hundred theoretical plates are necessary for the separation of petroleum into its main constituents. The advantages of such columns are realized when it is considered that an increase in the number of perfect plates from thirty to one hundred will increase-the enrichment factor from 19 to 13,780 for substances with a relative volatility (a)of 1.10, or with normal boiling points about 2.8" C. apart. A 13.1-meter, 33-mm. 0 . d. enriching or batch-stripping column and an 11.9meter, 33-mm. 0 . d. enriching column are described. Each column has approximately one hundred theoretical plates.
T
HE constitution of petroleum has been the subject of considerable study ever since petroleum was discovered. Severtheless, despite its scientific and industrial importance, surprisingly meager data on its chemical composition are available. The chief difficulty has been the lack of sufficiently efficient fractional distillation. Early petroleum investigators refractionated the same material as many as one hundred times and twenty to thirty refractionations were regarded as the absolute minimum ( 1 ) . The failure to obtain better separation of the constituents in petroleum by fractionation led to three unfortunate conclusions: (a) Petroleum was hopelessly complex, ( b ) it was composed of many constant-boiling mixtures, and ( c ) the role of fractionation in the isolation and separa,644
JUNE, 1936
INDUSTRIAL AND ENGINEERING CHEMISTRY
contained in A , and a small proportioning pump, M , feeds it as liquid at a constant rate through preheater N to the top of fractionating tower B. The liquid passes down through the column and out through sight glass J and line F into vaporizer C. Here it is completely vaporized by a higher boiling liquid condensing in E. The surface between C and D is large, making it possible to have steady and yet complete evaporation of the liquid in C. The gage indicates the completeness of evaporation. The vapors enter the base of the column through G and, after reachin the top of the column, pass through preheater N and cooler I? and are returned as liquid to container A . As this process is repeated, the highest boiling component in A is gradually concentrated in J and line F . It may be withdrawn continuously or periodical1 through 0. The pressure drop in the column is measured i y manometer H . A fractionation of this type has een used successfully to purify n-heptane (boiling at 98.4" C.) rom small amounts of methylcyclohexane (boiling at 100.8" C.). When used as an enriching column the charge to be fractionated is placed in C. The vapor condensed at the top of the column is fed directly back into the column. The reservoir, pump, ,and preheater are not used when operating the column in this manner.
645
cyclohexane mixtures was 5.5 liters per hour with a pressure drop of 119 mm. of mercury.
It should be pointed out that the degree of separation of two components by fractional distillation under conditions of high or total reflux varies logarithmically, and not linearly, with the number of theoretical plates. The degree of separation may be conveniently expressed as an enrichment factor which is the molal ratio of the two components in the distillate divided by their molal ratio in the still when operating a t total reflux (3). As an example, an increase in the number of perfect plates from thirty to one hundred will increase the enrichment factor from 19 t o 13,780 f o r substances with a relative vohtility (a)of 1.10,or with Column A was tested under total reflux with the binary normal boiling mixture n-heptane and methylcyclohexane; the use of this points about 3.8' test mixture was described previously (7). The results are C. apart. given in Table I. The column was so efficient that, even Therefore equipthough these substances boiled only 2.4" C. apart, the disment such as that tillate and still concentrations of n-heptane were close to 100 d e s c r i b e d here, and 0 per cent, respectively. In this range a slight change in and now in use in concentration of the binary mixture makes a considerable separating petrodifference in the number of theoretical plates. Therefore the leum, is extremely variation in the number of theoretical plates obtained in the valuable and the tests may ewily be due to analytical errors. In tall columns t i m e a n d effort of this type, variations in the number of theoretical plates saved when using are also caused by the procedure used in starting the column, it in future investithe spreading of the reflux over the packing, and the rate of gations of petrodistillation. leum will be considerable. In addition, this equipTABLE I. EFFICIENCYTESTSUSING) TZ-HEPTANE-METHYL-men t sh o u Id CYCLOHEXANE MIXTURE IN COLUMN A enable many comVelocity Total p o u n d s now obPresof Theoreti- Height Equivalent Run cure Liquid n-Heptane ea1 t o a Theoretical t a i n a b l e in low No, Drop a t Top Distillate Still Plates Plate purity to be fracMn. H g L./hr. Mole p e r cent Cm. In. FIGURE2. ENRICHINQ COLUMN B tionated to a high Operating as Batch Enriching Column degree of pur&, and also allow many apparently impossible separations to be made.
P
Operating as Batch Stripping Column 106 12.0 7.6 3.4 99.65 80 12.2 9.6 102 3.1 99.1 78 108 11.7 99.7 6.6 6 79 3.0 108 11.7 97.8 3.2 9 100 3.3 a For runs 6,7 and 8, a conservative assumed value of 99 mole per heptane in the dihillate was used to calculate the number of plates. 7 8
Column
4.7 4.8 4.6 4.6 cent n-
E
This column was similar in general design to the 15.9-meter (%&foot), 19-mm. (a/,-inch) diameter column described previously ( 4 ) . It consists of three sections totaling 11.9 meters (39 feet) of the same nickel tubing used in column A. The packing is 4-mm. (S/aa-inch) diameter, single-turn rings mado from No. 26 stainless steel (Nirosta) wire. Three equally spaced distributors centralize the descending liquid. The height of packed section is 11.6 meters (38 feet). The still consists of a piece of 305-mm. (12-inch) iron pipe with steel plates welded on the top and bottom and is heated by a series of Chromalox ring heaters under the bottom plate. The still has a capacity of 12 liters. A total condenser is used. A diagrammatic sketch of the column is given in Figure 2. This column is used only as an enriching column. When tested with %heptane and methylcyclohexane, the column had the equivalent of one hundred and one theoretical plates when operating at a velocity of liquid at the top of the tower of 3.4 liters per hour. The pressure drop was 45 mm. of mercury. The maximum throughput with n-heptane-methyl-
Acknowledgment The assistance of R . A . Rusk, E. M. Fry, J. E. Xckels, and B. W. Thomas is gratefully acknowledged. Without their help and hearty cofiperation this work would not have been completed.
Literature Cited (1) Coates, C. E., J.Am. Chem. Soc., 28, 384 (1906). (2) Cryder, D. S., e t al., paper presented before Division of Physical and Inorganic Chemistry a t 89th Meeting of A. C. S., New York, N. Y., April 22 to 26, 1935. (3) Fenske, M. R., I K D .ENG.CHEM.,24, 482 (1932). (4) Fenske, M. R., Quiggle, D., and Tongberg, C. O., Ibid., 24, 408 (1932). (5)" Leslie, R. T., and White, J. D., Bur. Standards J. Research, 15, 211 (1935). ( 6 ) * Rossini, F. D., PTOC. Am. Petroleum Inst., 16M (111), 47 (1935). (7) Tongberg, C. O., Quiggle, D.. and Fenske, M. R.. IND.ENG. CHEW.26, 1213 (1934). (8) Ibid., 28, 201 (1936).
* These references list all papers published t o June, 1935, on A. P. I. Project 6 being conducted at the National Bureau of Standards. RECEIVED March 16, 1936.
