V O L U M E 19, NO. 3, M A R C H 1 9 4 7 points of phenanthrene and anthracene, the analySis shows more than 2.0 rings per molecule, which may indicate the presence of these compounds. Limitations of Method. This method is subject to the same limitations as the per cent ring analysis method. Mixed-type aromatics cause low results for number of rings per molecule nhcre naphthene ring is present, and high results lvhere olefin double bond is present. Table VI1 s h o w that an aromatic nucleus condensed x i t h a naphthene ring or witha cyclc-Jlcfincauses deviations of about -0.4 and -0.2 ring, respectively Table VI11 shows that the corresponding noncondensed aromatic-naphthenes and aromatic-olefins give deviations of about -0.6 and -0.3 ring, respectively. Table I X s h o w that the effect of a double bond in a side chain is +0.1 or 0.2 ring, and Table X s h o w that the effect of two double bonds in a side chain is about f0.3 ring. Again it is apparent that the interfering-type compounds Tvould have to be present in considerable concentration in a petroleum fraction to cause a deviation as large as 0.1 ring, the average deviation for the alkyl aromatics. The method may be checked, as shown in Table V, by determining number of rings per molecule on the completely hydrogenated sample by method of Vlugter, Waterman, and Van Westen (22). The check method is independent of the presence of naphthene ring or olefin double bond in the original aromatic extract. ACKNOWLEDGMENT
The authors are indebted to I . Table VI.
IT. Mills for data used in
189 LITERATURE CITED
Am. Soc. Testing Materials, Tentative Standard Method, Desig-
nation: D875-46T. Calingaert, G., Ind. Eng. Chem.. 33, 103 (1941). Deanesly, R. >I and .,Carleton, L. T., IND. ENG.CHmf., .$SAL. ED.,14, 220 (1942). Doss, M. P., “Physical Constants of the Principal Hydrocarbons”, 4th edition, Kew York, Texas Co., 1943. Eaton, J. L., “Science of Petroleum”, 1701. 2 , London, Oxford University Press, 1938. Griswold, J., and Chem, J. N., Ind. EILQ.Chem., 38, 364 (1946). Hirschler, A. E., J . Inst. Petroleum Tech., 32, 133 (1946). Huggins, M., J . A m . Chem. Soc.. 63, 116 (1941). Kurtz, S. S., Jr., and Lipkin, 11. R . , Ind. Eng. Chem., 33, 779 flCJ41). \ - -
~~
Leendeme, J. J., and Kreulen, D. W., J . Inst. Petrorei~mTech., 25, 801 (1939). Lipkin, M. R., and Kurtz, S.S.,Jr., ISD. ESG. CHEM..Ax*L. ED., 13, 291 (1941). Lipkin, M. R., and Martin, C. C., Ibid., 18, 380 (1946). Ibid., 18, 433 (1946). Lipkin, M. R., Martin, C. C., and Kurtz, S. S.,Jr., Ibid., 18, 376 (1946). &lair,B. J., J . Research, Natl. Bur. Standards, 34, 435 (1945). Mair, B. J., and Foreiati, A. F.,Ibid., 32, 165 (1944). Mair, B. J., and Streiff, H. J.. Ibid., 27, 343 (1941). Mair, B. J., JTillingham, C. B., and Streiff, A . J., Ibid., 21, 581 (1938). Mills, I. W., Hirschler, A . E., and Kurtd, S. S., Jr., Ind. Eng. Chem., 38, 442 (1946). Rossini. F . D., Petroleum Engr., 14, 41 (Feb. 1943). Thorpe, R. E., and Larsen, R. G., Ind. Eng. Chem., 34, 853 (1942). Tlugter, J. C., Waterman, H. I., and Van Westen, H. A., J . Inst. Petroleum Tech., 18, 735 (1932). TVard, A . L., and Kurtz, S.S., Jr., IXD.ESG. CHEW,ASAL. ED.,10, 559 (1938).
Standardization of Chromatographic Adsorbents Rate of Flow of Developing Solvent ARTHUR L. LEROSEN, Coates Laboratory, Louisiana S t a t e L‘nicersity, B a t o n Rouge, La. The usefulness of permeability in characterizing an adsorbent is pointed out and a convenient grouping of terms (kA/760a), for this purpose is suggested; with standard packing it is possible to calculate the rate of flow for different values of the pressure, column length, and soltent viscosity.
T
HE author has introduced several terms for the characterization of chromatographic adsorbents ( 3 ) . Among these is V c ,which is defined as the rate of flow of the solvent through the column after a steady state is reached. I n order to use this term to compare the permeability of adsorbents, the solvent, column length, and applied pressure must be specified. Generally benzene, a full water-pump vacuum, and a 75mm. column furnish sufficiently good data; however, i t is often necessary to estimate the usefulness of an adsorbent in columns of different dimensions viith a variety of solvents and pressures; for this purpose a more general term is needed to describe a n adsorbent. Ideally, a knowledge of the particle size of a substance and the use of a standard degree of packing should permit the direct calculation of V, for any set of conditions. I n most adsorbents, however, there is generally a wide range of particle sizes and shapes, so that, this procedure is not practical. The “permeability” of a porous solid is derived from Darcy’s lan- ( 5 ) and expressed by the equation: k = qQ/A(Pi- P d / L where k is the permeability in darcys, q is the viscosity in centipoises, Q is the flux through the column in milliliters per minute, A is the cross-sectional area in square centimeters, ( P I - Pz) is the pressure difference in atmospheres between the ends of the
column, and L is t h e length in centimeters. When the driving pressure, P , is expressed in millimeters of mercury and Q is replaced by VCa,where (Y is the interstitial volume of the column in milliliters per millimeter, the equation can be arranged as:
V, = If A is calculated and
k ( A / 7 6 0 ( ~P/qL )
is experimentally determined, it is now possible to calculate V,for any values of q , P , and L. The need for the determination of A and (Y can be avoided by groupifig i h e constants:. ( L 4 / 7 6 0 ( ~ ) .When the value of this expression is determined experimentally by measuring V , under one set of conditions for a standard degree of packing, the value of V cfor any other conditions may be calculated from the values of P, L , and q without further measurements. The adsorbent is poured into the column under the full water pump vacuum and settled by tapping the sides of the column, the surface of n-hich is made smooth without applying pressure. In describing an adsorbent, the values for both k and ( k A / 7 6 0 ( ~ ) should be stated. (Y
The linear relation bet&en V,and pressure, and the reciprocal of the column length, suggested by several authors (2, 5),follows from Darcy’s law. Flow in relation to these variables is illustrated in Figures 1 and 2 for the systems benzene-silicic acid and benzene-lime. From these graphs and the experimental values of A and a, it is possible to calculate the values of k and ( k A / 7 6 0 ( ~ for ) the adsorbents. I n additional experiments, given herewith, the independence of diameter, the relation of vis-
ANALYTICAL CHEMISTRY
190 0.08 Table I.
12, Adsorbent Solvent Darcys hlerck silicic 3.1 acid Benzene Mississippilime Benzene 6.7
Table 11.
i..4/76Oa
V c for 75 Mm.-Column and 665-AMm.Pressure Calculated Experimental
6 2 X 10-2 14 X 10-2
9.8 21.9
10 23
vc
EXPERIMENTAL
The chromatographic tubes used in this study were obtained from the Scientific Glass Apparatus Company, Bloomfield, S. J. All diameters refer to the inside diameter of the tube. For the study of the effects of pressure and of length on flow rate, a tube 9 X 130 mm. was used. Suction regulated by the device described by Gilmont and Othmer ( 1 ) was used to draw the developing solvent through the columns. For some determinations a layer of benzene about 10 mm. deep was maintained on the top of the column by addition of the solvent from a buret. In this case no .correction was made for the slight liquid head; for other determinations a buret was attached to the top of the column and a correction was necessary. The average barometric pressure in these determinations Tvas 765 mm. of mercury and the temperature was about 31" C. For the loTver pressures it was necessary to cool the receiver in ice. Adsorbent. Merck reagent silicic acid and hydrated lime, obtained from the Mississippi Lime Company, Alton, Ill., were used. The adsorbents were not prewashed (4). (Evidence just obtained indicates that prewashing may cause a considerable increase in J7c.) Apparatus.
I
1
1
0
700
e
600
! e 5 500 e
I
t
-
0.05
;j
d
/
30.04
,
0.02
0.01
I
I
1
10
20
I 30
1
I
40
50
I
V,, M m . per Minute
Figure 2
Relation between Chromatographic Flow Rate and Column Length Benzene-silicic acid,
0 ; benzene-lime, 8
Data. The data for variation of V, with pressure or column length are given in Figures 1 and 2. For the study of pressure four columns were run for the silicic acid and five for the lime. The flow rate for each column was determined a t successively lower pressures, and these points were plotted. Each point on the V,-length graph represents a separate chromatographic column of length indicated. The V , values obtained for ten columns run under the same conditions, with only the degree of packing as a variable and with special care to try to obtain the same packing, are shown in ~ ten columns of Table 11. Table I11 gives the rate of f l o for equal length but different diameter. I n order to test the validity of the inclusion of viscosity in the formula for flow rate through a column, four columns were run using ligroin (boiling point 60' to 70" C.) in place of benzene. The T', values thus obtained were: 19.6, 18.2, 16.5, and 18.3; the average value was 18.2 as compared with a calculated value of 18.7. Table 111. Variation of V , with Column Diameter
400
(Column length 75 mm.; applied pressure 675 m m . ) Tube Diameter, 11m. vc
2
E 300
10.7 10.5 12.6 9.7 11.4 10.6
9 18 34 44 53 7s
7
-
I
0.06
*2
cosity, and the size of the experimental error were determined., Values of V c (Table 11) were found to show a maximum deviation of 10% from the average value.
800
0.07
=t 1
Variations in I: Due to Column Packing Differences
(Column length 75 mm.; applied pressure 675 m m ) Run vc Run
z
I
Flow Characteristics of Two Adsorbents
.-
c
200 100
LITERATURE CITED
i' ,/
It.. and Othmer, D. F., ISD. ESG. CHEM.,A&AL.ED., 15, 641 (1943). i21 .Jacobs, P. IT.SI.,and Tompkins, F.C . , Trans. Faraday Soc., 41, ( 1 ) Gilnion',
I /
/
395 (1946). & L (3) LeXosen, A . L.. .J. J
10
15
20
25
V,, M m . per J l i n u t e
Figure 1
Relation between Chromatographic Flow Rate and Pressure Benzene-silicic acid, 0 : benzene-lime, 0
(4) I b i d . , 67, 1683
A m . Chon. Soc., 6 4 ,
1905 (1942).
(1915).
(6) Muskat, Morris, "Flow of Homogeneous Fluids through Porous lledia", p. 74. New Tork, McGraw-Hill Book Co., 1937. PRESESTED before the Regional Meeting of the AXERICAS C H E M I C h L SoC I E T T at Baton Rouge, L a . , October 1945.
