have been semiempirically determined to be:
Prediction of Retention Times. Weight of packing: Weight of substrate: Flow rate: Column temperature: Room temperature: Vapor pressure of HzO a t 28” C. Barometric pressure: Column inlet pressure ( p , ): Column outlet pressure ( p o ): Ratio of pressures (pi/po) : factor: (corrected gas flow) :
V? --
- c,
62
Note that c1 is characteristic of the substrate and that v2 and 6 2 hav? been determined from the physical constants of the compounds a and b. For N,N-dimethylformamide, c1 = 2.38 cyclohexane ( a ) :(-2.38) 109.4 =
JC’’
(-)
8.18
- 31.9 cal./cc.
(”99) - 23.8 cal./cc.
benzene ( b ) :(-2.38
-
=
Calculation of Apparent Activity Coefficient. In y2 =
y o = --
615
(9.9 I .
+ 54.0 - 31.9) = 5.71
yo =
Yb =
6x(5.4 + 54.0 - 23.8)
Prediction
of
5.20
181
Yb
Relative
Volatility
(Q , b ) * (I
a,.b =
Yap00 -Y b P’b
at 36” C., P. = 157 mm. and Pob = 155 mm. .’. C‘u,b
1.80
Predicted 1 21 2 06
Predicted 1 . 70
LITERATURE CITED
90.0
1 .
ao,b
2.19 = 1.22
303
Benzene (b): ln
Actual Retention Times Past Air Cyclohexane: 1.22 minutes Benzene: 2.19 minutes Actual Relative Volatility
Cyclohexane (a) : In
From the above operating data and the apparent activity coefficients, the retention times of oyclohexane (1.21 minutes) and benzene (2.06 minutes) are calculated. Comparison with Experimental Data.
(303)( 157) = ___- = 1.70 (181)( 155)
(1) Bayer, E., Angew Chem. 71, 299 (1959). (2) Butler, J. A. V., J . Chem. Soc. 1935, 2~n
(3;%&, 1935,952. (4) Denbigh, K. G., “Principles of Chemical Equilibrium,” pp. .195-201; 23140, Cambridge University Press, Cambridge, 1955. (5) General Discussion, Trans. Faraday Soc. 30 (1934). (6) Hildebrand, J. H., Scott, R. L., “Solubility of Nonelectrolytes,” 3rd ed., Reinhold, New York, 1950. (7) Hill, T. L., J . Am. Chem. Soc. 79, 4885 (1957). (8) Hill, T. L., J . Chem. Phys. 30, 93 (1959).
23 7 grams 30y0 x 23 7 = 7 11 ‘ grams ( W L ) 51 7 cc. He/mi . 36” C. = 3 0 9 ’ s . 28” C. = 301” K 28 mm. 760 mm. 15 # gage = 29 7 # absolute 0 # gage = 14 7 # absolute 2 02 0 638 (51 7) (309/301) (732/760) = 51 1 (9) Hoare, M. R., Purnell, J. H., Trans. Faraday Soc. 52,222 (1956). (10) Keulemaiis, A. I. M., “Gas Chromatogra hy,” pp. 107-39; 161-79, 2nd ed., Xeinhold, New York, 1959. ( I 1 ) Littlewood, A. B., Phillips, C. S. G., Price, D. T., J . Chem. Soc. 1955,1480. (12) McMillan, W. G., Mayer, J. E., J . C h a . Phys. 13,276 (1945). (13) Moelwyn-Hughes, E. A., “Physical Chemistry,” pp. 675-812, Pergamon Press, New York, 1957. (14) Nasanow, L. H., J . Chem. Phys. 30, 1596 (1959). (15) Onsager, L., J . Am. Chem. Soc. 58, 1486 (1936). (16) Pierotti, G. J., Deal, C. H., Jr., Derr, E. L., Porter, P. E., Am. Document.,Doc. No. 5782. (17) Pierotti. G. J.. Deal. C. H.. Jr.. ‘ Derr, E. L.,-Portkr, P. k., Znd.’Eng: Chem. 51,95 (1959). (18) Pierotti, G. J., Deal, C . H., Jr., Derr, E. L., Porter, P. E., J . Am. Chem. Soc. 78,2989 (1956). (19) Porter, P. E., Deal, C. H., Jr., Stross, F. H., Zbad., 78,2999 (1956). (20) Reid, R. C., Sherwood, T. K., “Properties of Gases and Liquids,” McGraw-Hill, New York, 1958. (21) Rushbrooke, G. S., “Introduction to Statistical Mechanics,” pp. 218-34; 287-319, Oxford, London 1949. (22) van Arkel, -4.E., Trans. Faraday Soc. 42B,81 (1946).
RECEIVEDfor review November 14, 1960. Accepted April 21, 1961. Division of Petroleum Chemistry, 139th Meeting, ACS, St. Louis, Mo., March 1961.
Determination of Small Amounts of n-Paraffins by Molecular Sieve-Gas Chromatography F. T. EGGERTSEN and SIGURD GROENNINGS Shell Development Co., Emeryville, Calif.
b Normal paraffins in light petroleum distillates can be determined by a gas chromatographic method based on quantitative elution of n-paraffins, which have been selectively absorbed on Molecular Sieve. A sample of the saturates portion, isolated by liquid is displacement chromatography, passed through a gas-liquid chromatography column, a sieve column, and a combustion tube in series. While the sieve retains the n-paraffins, the nonnormal (iso- and cyclo-) paraffins are
eluted, converted to carbon dioxide, and a chromatogram obtained. By reversing the flow and raising the temperature of the sieve, the n-paraffins are recovered, and their chromatogram similarly obtained. Individual n-paraffins as low as a few hundredths per cent are thus determined.
S
by the Linde Molecular Sieve 5A is the basis for a number of new and promising ELECTIVE ABSORPTION
methods for determining straight chain hydrocarbons (4, 8-18). These methods are useful for evaluating processes in which the n-paraffin content is reduced, for example, by platforming, isomerization, or Molecular Sieve treatment. The gas chromatographic method, introduced by Brenner and Coates (4) and extended by Whitham (If?), is especially attractive because it is relatively simple and yields individual n-paraffins. In this scheme, the sample is chromatographed through a conVOL. 33,
NO. 9, AUGUST 1961
e
1147
ventional gas-liquid chromatography column with and without a n amdiary column of Molecular Sieve 5A, which serves as a subtractor: the peaks missing from the chromatogram when the sieve is employed correspond to the n-paraffins selectively absorbed by the sieve. Although the subtractor scheme is satisfactory when the individual np a r a f f i are present in appreciable concentrations, it is not well suited for determining small amounts, when the n-paraffins appear as small peaks or shoulders in the total chromatogram. It seemed likely that in these cases the determination could be made more accurately if the n-paraffins could be removed quantitatively from the Molecular Sieve subtractor column and chromatographed separately. This would enable their detection a t a higher sensitivity and minimize interference by the nonnormal (iso- and cyclo-) paraffins. The desorption step appeared feasible because it has been reported that n-paraffis can be removed from Molecular Sieve 5A a t elevated temperatures (9,9, 6). A method was developed whereby the n-paraffins can be recovered from the sieve and determined individually in concentrations as low as a few hundredths per cent. EXPERIMENTAL
As in the method of Brenner (4), a Molecular Sieve subtractor is connected in series with a conventional gasliquid chromatography (GLC) column. However, after the n - p a r a h have been sorbed and the chromatogram for the nonnormals obtained, the normals are released by backflushing with carrier gas while raising the temperature of the sieve, and are chromatographed through the same GLC column. An apparatus, recently developed for analysis of wide range distillates (6), was adapted to this application merely by connecting the sieve column downstream from the GLC column. A schematic diagram is given in Figure 1. Apparatus. The apparatus has certain features which are advantageous, b u t perhaps not essential: T h e effluent hydrocarbons are detected as carbon dioxide after oxidation over copper oxide and rising temperature operation of the GLC column is employed. Conversion to carbon dioxide permits all hydrocarbons to be detected and accounted for on a common basis; peak area percentages are equivalent t o weight percentages and calibration factors are not required. It also allows operation of the backflushing or flow-reversing valve a t room temperature. Rising temperature of the GLC column has the advantage of giving optimum separations over a wide boiling range and sharper peaks 1148
ANALYTICAL CHEMISTRY
TLBLI
Figure 1. Schematic diagram of apparatus
for late-emerging components. The principal parts of the apparatus are described briefly below; additional details are in reference (6). GLC COLUMN.This column consists of 5 feet of 1/4-inch copper tubing wound around a grooved aluminum block of 1.75-inch diameter, equipped with four 50-watt cartridge heaters. The packing is silicone oil [General Electric Co. SF-96 (1000)], supported on 30- to &mesh (3-22 insulating brick in a weight ratio of 20 to 100. The unit is enveloped in a copper pipe with closed bottom so that it can be readily heated and cooled again in a liquid bath for the next analysis. The sample v a p o ~ e ris a short piece of stainless steel tubing packed with glass wool and maintained at 200" C. MOLECULAR SIEVECOLUMN. This column is a U-shaped l/&xh stainless steel tubing 18 inches long, 5 inches of which is packed with 1.1 grams of 14 to 30-mesh granules of Linde Molecular Sieve Type 5 8 ; this packing is held in place by glass wool (1- to %inch lengths), the rest of the column being empty. For absorption of %-para&, the sieve column is maintained a t 190" to 200" C. with electrical heating tape, and the whole enclosed in a Dewar jar. For the desorption step the tape heater is removed and the column is brought to 400" C. with a preheated aluminum block furnace constructed in two halves, one of which is grooved to fit the column; the blocks were equipped with cartridge heaters. COMBUSTION UNIT (6). Two combustion tubes are employed so that conversion of hydrocarbons to carbon dioxide takes place by both forward and reverse flow of carrier gas. The tubes, placed in a furnace, which is maintained a t about 675" C., are &inch lengths of '/d-inch stainless steel tubing containing 14- to 48-mesh wire-form copper oxide. Water of combustion is removed by indicating Drierite (calcium sulfate) packed in 6-inch lengths of a/lsinch i.d. Tygon tubing. The connecting lines between GLC column and combustion unit are of 1/4-inch stainless steel tubing, maintained a t 200" C. FLOW-REVERSING VALVE. The Republic Teflon plug valve No. 310-61/8D (Republic Mfg. Co., Cleveland, Ohio) is suitable. The valve is connected across the combustion tubes, and can be maintained a t room temperature because the separated hydrocarbons are
converted to carbon dioxide and water of combustioii is removed. FLOWCONTROLLER.To prevent variations in helium flow which ordi-. narily occur when a GLC column is heated or cooled, a diaphragm-type controller (Moore Products Co., Philadelphia, Pa., No. 63-BUL) is connected across a control needle valve upstream from the columns. I t maintains the flow a t 50 cc. per minute within about 2% when the column is heated from room temperature to 200" C. This type of controller has been described by Guild, Bingham, and Aul (7). HELIUMSLIP-STREAM LINE. This is connected just below the sample inlet to prevent condensation of high-boiling hydrocarbons under the serum cap It during reverse flow operation. sharpens up backflush peaks in the gas oil range, but is probably unnecessary for lower-boiling samples. DETECTION AND RECORDINQ.Thermal conductivity detection was employed, the sensing elements being hot-wire filaments (Gow-Mac Engineering Co., Madison, N. J.), operated with a current of 150 ma. A 1-mv. Brown recording potentiometer was used. Isolation of Saturates. The saturates were isolated from 5 ml. of sample by simple liquid displacement chromatography on silica gel, using the dye mixture as described in the fluorescent indicator adsorption or FIA method (I) as zone indicator. The dimensions of the water-jacketed tapered column are from top to bottom, in millimeters, 15 (i.dJ X 150, 10 X 350, 5 X 350, and 2 X 500, holding about 45 grams of silica gel. The column is designed for a maximum charge of 5 to 6 grams of sample containing not less than 10% of saturates, thus yielding a t least 0.5 gram of saturates; this is considered the smallest amount that can be collected and handled without noticeable loss of light ends. As a safeguard against loss, the charge is injected from a hypodermic syringe into a Lungetype pipet and weighed. The weighing pipet and eluent reservoir are mounted on the packed column, and the sample is drained into the gel, followed by isopropyl alcohol under pressure, not exceeding 2 p.s.i. The column ends in a very fine tip penetrating a serum cap which closes a small ice-cooled vial as receiver; the latter is vented by a capillary tubing connected to a drying tube to protect the saturates from condensed air-moisture. The saturates are collected up to and including the first yellow drop, and weighed. Preparation of the column and isolation of the saturates require about 2 hours. Procedure. About 5 pl. of the saturates portion of the sample is injected into the hot (200' C.) vaporizer with helium flowing in forward direction a t 50 cc. per minute and the slip stream a t 15 cc. per minute. The GLC column, initially a t 10' C., is heated a t 6 ' C. per minute t o 200" C., whereby the hydrocarbons are eluted in boiling point sequence. A chromatogram for the nonnormals is thus obtained in about 30 minutes, the normals being withheld in the Molecular
Sieve column. The latter is maintained a t 190" to 200" C., a t which temperature the Cb and higher normals are retained, while the nonnormals pass through essentially unretarded. After elution of the nonnormals, the sieve is cooled to below 50" C. by removing the tape heater, and the GLC column is cooled to about -30" C. with dry ice. The helium flow is then reversed, the slip stream adjusted to 5 cc. per minute, and the Molecular Sieve column rapidly heated to 400" C. by the aluminum block furnace which had been preheated to that temperature. This operation transfers the n-paraffins to the GLC column where they are retrapped. To obtain the chromatogram for the n-paraffins the cold GLC column is again heated to about 200" C. Thus the normals are eluted in a direction opposite to that for the nonnormals. Attempts t o elute the normals directly during forward-flow operation while heating the Molecular Sieve column to 400" C. were unsatisfactory because they emerged in diffuse and poorly separated peaks. RESULTS AND DISCUSSION
A chromatogram of the saturates portion of a platformate is illustrated in Figure 2. The n-paraffin peaks shown here are a t a concentration level of 0.1% each, based on the total platformate; the area per cent of each nparaffin, Ca and above, is equal to its weight per cent in the saturates portion. If desired, the boiling point distribution of the nonnormals can be determined from their chromatogram by a calibration curve, established with known samples by relating peak positions with boiling points, as described in a n earlier study (6). This chromatogram for the nonnormals also includes much of the n-pentane because it is only partly retained a t the sieve temperature employed, 190" to 200" C. When 2 pl. of n-pentane was charged directly to the sieve column a t 200" C.,very slow elution began in 8 minutes; a similar amount of n-hexane was held 45 minutes. Therefore, in the time required to obtain the chromatogram for the nonnormals-about 30 minutes-all of the n-hexane is retained by the sieve, but some n-pentane is lost. The latter is eluted in the 12- to 30-minute period (Figure 2), but too slowly t o be distinguished as a peak. A lower sieve temperature, for example, 170" C., would hold n-pentane, but is not desirable owing t o increasing interference by nonnormals. Even a t 200" C. the sieve retained traces of unidentified nonnormals which emerged near n-Clo and n-CI1 and interfered slightly with their determination. Under the conditions employed here, therefore, the sieve was completely selective only over about five carbon numbers, Cs to Cl0, inclusive.
GLC COLCMIN i O ' TO 1OO.C. MOL. SIEVE COL 2OO.C. SON-SORMAL SATURATES
GLC COLUMN. -JO* TO ZOO.C,/ MOL. SIEVE cob 4oa.c. n-PARAFFINS'
~
MIXUTES
Figure 2. Chromatogram of the saturates portion of a platformate Blend I, Table
I
Since n-pentane was not obtained by means of the sieve, this component was determined, when desired, by a separate GLC analysis of the saturates portion, or of the whole sample in the absence of pentene which might interfere with n-pentane. This analysis is performed with the GLC column alone, omitting the sieve column. The dependability of the method was tested by analyzing blends of a "normallean" platformate (20.9% w. saturates) with known amounts of added Cs to C11 n-paraffins; the normal-lean material was a platformate which had been subjected to large scale treatment with Molecular Sieve 5A. The results obtained for the saturates of the original normal-lean matrix and the blends are shown in Table I; the known values for the blends include the amounts found before addition of the n-paraffins.
Table 1.
Determination of n-Paraffins by Molecular Sieve-Gas Chromatography
Sample Original Found Blend1 Known Found Blend11
used a t least 20 times before replacement. The procedure is probably longer than necessary, because it was designed to give wide separations, not only of the normals to detect interferences, but also of the nonnormals to indicate their boiling point distribution. The time required to obtain a suitable chromatogram can no doubt be reduced. The GLC column could be heated more rapidly, for example, a t 10" to 15" C. per minute; or, it might even be operated a t a constant high temperature, a t least for the nonnormals, because their detailed separation is not essential to the determination of the normals. The optimum conditions would, of course, be those producing just adequate separation of the n - p a r a h in the minimum time. Although the method was tested only with the saturates from a platformate, it could probably be applied equally well to the saturates from other gasoline-range distillates. With platformates, the method might be expected to apply to the whole sample, thus saving time by omitting the saturates isolation step. Since the olefin content is usually low, 1% or less, n-paraffins are the only straight chain hydrocarbons present in significant quantities. The sieve might be expected to pass gasoline-range aromatics and nonnormal saturates, and retain only n-paraffins. Brenner (4)
Known Found
n-Parafis, % weight (basis whole platformate or blends) CS" cs c, CS co ClO c 1 1 Total 0.98 0.03 0.01 1.07 0.13 0.11 1.03 0.13 0.12 0.110.11 1.65 0.79 0.79 1.59 0.71 0.77 0.72 0.81
