Simulated Distillation of Narrow, High Boiling Hydrocarbon Fractions 1.H. Gouw Chevron Research Company, Richmond, Calif. 94802
Simulated distillation by gas chromatography appears to be the most simple, reproducible, and consistent method to describe the boiling range of a hydrocarbon fraction unambiguously. This technique is widely used to obtain distillation curves on a variety of products (1,2). The American Society for Testing and Materials has accepted this technique as a tentative method for the “Determination of Boiling Range Distribution of Petroleum Fractions by Gas Chromatography” (3). The technique works best for samples boiling in a range of a few hundred degrees. Two sources of error are associated with the analysis of very narrow boiling fractions. Because of the narrow boiling range of the sample, the solute will travel in a narrow, highly concentrated band throughout the whole length of the column. The overloading at the point of maximum solute concentration results in a dependency of the retention time on the sample size. Errors in the derived boiling points will result if these temperatures are taken from the temperature-retention time correlation derived from the n-hydrocarbon standard mixture. The size of this error will obviously decrease with decreasing sample size, the use of larger diameter columns, and/or increasing boiling range of the sample. By using very small samples and/or larger diameter columns, overloading can be avoided. In many cases, however, this would entail additional effort, is more time consuming, and is not amenable for routine operations. Sophisticated mathematical correction techniques, such as those used in gel permeation chromatography, can also be employed to correct for this phenomenon. In actual practice, this mathematical correction is difficult to carry out. The second error is related to the finite bandspread generated during passage of the solute through the column. This problem can be illustrated by the following extreme example. For a single pure compound, the actual boiling curve would be a straight, horizontal line; the curve derived by simulated distillation will be a line with a finite slope because the band broadening of the peak results in a finite spread of the retention times. The ASTM Method D 2887-T indicates, therefore, that this test is only applicable to samples with a boiling range higher than 100 OF.
GRAPHICAL TECHNIQUE Knowledge of the average boiling point and some indication of the boiling range of narrow boiling hydrocarbon fractions is generally sufficient for characterization purposes. In many cases, only these two parameters are taken from the full distillation curve and used in further calculations. To obtain these data, we make use of the following simple graphical technique. The procedure is carried out in conjunction with a reasonably good gas chromatograph and a device for integrating peak areas. (1) L. E. Green, L. J. Schmauch, and J. C . Worman, Anal. Chem., 36, 1512 (1964). (2) L. E. Green, Riv. Combust.. 24 (12). 536 (1970). (3) ASTM D 2887-70T, “1971 Annual Book of ASTM Standards,” Part 17. p 1072, American Society for Testing and Materials, Philadelphia, Pa. 19103.
A “boiling point lines grid” is first prepared by taking cross-hatched paper and drawing in a temperature scale on the vertical axis. The boiling points of the n-hydrocarbons are marked off on this scale. For each n-hydrocarbon in the range of interest, a horizontal line is drawn a t the level of its boiling point. There is no scale on the horizontal axis. A chromatogram is now obtained of the sample on a column packed with a nonpolar liquid substrate. The total area of the Chromatogram is determined by one of the many chromatographic integrating devices available. The retention time is determined a t which exactly half the sample is eluted, This point is defined as the average boiling point of the sample, TB. For symmetrical curves, the peak maximum can be used directly to establish this retention time. To describe the range, we have to use an arbitrary scale, such as the 5 9 5 % eluted range. A suggestion with a more. fundamental basis is to take those temperatures between which 68.26% of the middle portion of the material is eluted. This corresponds to an area between -1 u and +1 u (a = standard deviation) if the peak shape corresponds to a normal Gaussian distribution. If A is the total area, then the three points are determined on the chromatogram at which A/2 - 0.34134, A/2, and A/2 + 0.3413A are eluted. From these data and if the peak is indeed Gaussian, the normal distribution tables can be used to calculate the temperatures at any amount of material eluted. The temperatures corresponding to + l u and -1 u show the range and the skewness of the distribution. Figure 1 shows a chromatogram of a distillation cut of a Montebello crude with these three retention times drawn in. A second chromatogram is obtained on a mixture of the sample and 10-20% of a synthetic blend of known n-hydrocarbons boiling in approximately the same range as the sample. Both chromatograms should be obtained with the same temperature program on the chromatograph with t h e other variables being held constant between these two runs. One notes from Figures 1 and 2 that the addition of small amounts of internal standards does not significantly alter the shape or the slope of the sample curve. The three retention times from the first chromatogram are than transferred to the second chromatogram by overlaying the second chromatogram over the first. The process of transferring retention times by overlaying the chromatograms obviates the necessity of having to use a precise gas chromatograph with which very reproducible absolute retention times can be obtained. In the procedure described here, a reasonably good gas chromatograph is sufficient, since small changes in retention times can be corrected for by shifting the chromatograms in relation to each other. After the three retention times are transferred, the second chromatogram is now laid over the “boiling point lines grid” described earlier. Figure 2 shows the results. The temperature scale is shown on the vertical axis. For the sake of clarity, we have drawn only shortened, dashed horizontal lines from the “boiling point lines grid” for Czo to CZ4 and from Cs0 to C34. The intersections are noted ANALYTICAL CHEMISTRY, VOL. 4 5 , NO. 6, M A Y 1973
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Table I. Simulated Distillation of Southern California Crude Fractions Identifi-
cation
TB
4641 C11
4624 C14
790 792 795 836 834 832 863 861 865 890 889 888 851
4624 C16
853 872
4624 C17
871 880
4641 C14
4641 C17
4641 C20
+lu TB -1u
Distillation cut of a Montebello crude with ing range indicated Figure 1.
TB
Av
792
2.5
834
2.0
863
2.0
889
1.0
852
1.4
762 767 772 811 812 81 0 838 841 842 867 861 867 821
Av
s
767 5.0
811
1.0
840 2.1
865
3.5
s=
Av
s
819
5.0
856 4.0
885
6.2
912 4.5
884 4.2
840 3.5
2.8
896 8.5 901 901
851
8.5
845 1.6
820 814 824 860 852 856 883 880 892 912 916 907 881 887 889
842 857 882
+1 u
824 4.2
0.71
884
24
-1 u
827 837 871.5
and boil-
s
902 2.1 904
3.3
4.2
structed calibration line, and drawing horizontal lines from these intersections to the vertical axis to obtain the corresponding temperatures. In Figure 2, Ts is 793 "F, f l u corresponds to 829 OF, and -1 u is equivalent to 763 "F. Since this method makes use of internal standards, the results will be inherently more consistent than in the ASTM D 2887-T method. A special situation occurs when the fraction by itself already contains significant amounts of n-hydrocarbons. This is the case observed with distillation fractions from many petroleum crudes. In these cases, it will not be necessary to run a second chromatogram if the identity of the n-hydrocarbons in the chromatogram of the actual sample is known.
REPRODUCIBILITY OF THE METHOD
r
650L
---\rczl
Figure 2. Graphical construction of erlay on "boiling point lines grid"
--4- -c,,
TB
and boiling range by ov-
for the retention times of the n-hydrocarbons which are drawn as vertical lines through the tip of the n-hydrocarbon peaks and the corresponding horizontally drawn "boiling point lines." The retention time-boiling point correlation is the best line drawn through these intersections. A straight line would be expected, but the result is generally a slightly curved line because of the earlier mentioned highly localized concentration of the solute during passage throughout the entire length of the column. T h e average boiling point, T B , and the boiling range which is bracketed between -1 u and +1 u are now obtained by drawing perpendicular lines from the three corresponding retention tines, noting the intersections with the con988
Two Southern California crude oils were carefully distilled through a %foot by %-inch spinning band distillation column at high reflux ratios. Seven small cuts were chosen a t random from these two distillations, Simulated distillation by the described method was carried out on a Perkin-Elmer 226, a Perkin-Elmer 900, and a Varian Aerograph 1520 gas chromatograph. Different columns were used in each of these units. Packed columns 36 inches long by %-inch 0.d. were used in the Perkin-Elmer 226 and in the Varian Aerograph 1520 gas chromatograph. The column packing used was 3% OV-101 on 60-80 mesh Chromosorb G-HP. A 32-foot by 0.02-inch capillary column coated with OV-101 methyl silicone (4) was used in the Perkin-Elmer 900 gas chromatograph. The Perkin-Elmer units were equipped with flame ionization detectors; the Aerograph was equipped with a thermal conductivity detector. Temperature programming was carried out at a rate between 10 "C and 20 "C/min, depending on the unit. Initial temperatures were varied depending on the boiling range of the sample. Area integration was carried out with an optical integrator 15) or with a Disc integrator. The three retention times are computed and noted on the chromatogram as shown in Figures 1and 2.
ANALYTICAL CHEMISTRY, VOL. 45, NO. 6, MAY 1973
( 4 ) T. H. Gouw, I. M.
Whittemore, and R . E. Jentoft, Anal.
Chem.. 42,
1394 (1970). ( 5 ) J. F. Johnson, R. F. Klaver, F. Baumann, and J. Y . Beach, "Separation Immediate et Chromatographie G.A.M.S.," Paris, June 1315. 1961. p 235.
The results of these runs are shown in Table I. The three (or two) data sets obtained for each sample have been obtained on different chromatographic units. T B is the observed average boiling point; s shows the standard deviation of each set of measurements which is computed from
where n is the number of data points. The data show that consistent TB'Scan be obtained under different operating conditions with different units. The spread in values of the temperatures corresponding to +1 u and -1 u is much larger than that observed for T g . This is understandable since the spread is a function of a number of factors, such as the efficiency of the chromatographic separation and the degree of overloading. The average values for the standard deviation of these three points are given a t the bottom of Table I. The average standard deviations for all three temperatures are considerably smaller than the deviations obtained by the ASTM simulated distillation method. Although very little has been published on interlaboratory reproducibility, we estimate from our experience that the 50% point variability in the ASTM method is a t least 10 "F or more. This graphical approach is, therefore, a more consistent and reproducible method to obtain boiling range
information on narrow, high boiling petroleum hydrocarbon fractions.
CONCLUSION The described method does not require equipment as sophisticated as what is necessary for regular simulated distillation. It appears to yield quite precise values for the average boiling point of narrow boiling, high molecular weight hydrocarbon fractions. The boiling range is subject to somewhat larger deviations because it is dependent on the operating parameters. The method is obviously also applicable to lower molecular weight fractions; in this case, it would probably be more appropriate to carry out a complete component analysis by gas chromatography. The described approach should not be regarded as a replacement or alternative technique to the ASTM D 2887-T method; it should be considered as a complementary method which is very useful in specific applications. Received for review August 14, 1972. Accepted December 27, 1972.
Ionization Sequences in the Ground and Lowest Electronically Excited Singlet States of 3-Hydroxy-2-Naphthoic Acid Peter J. Kovi and Stephen G. Schulman College of Pharmacy, University of Florida, Gainesville, Fla. 32601
Since the pioneering study of Weller ( 1 ) which demonstrated intramolecular proton transfer in the lowest electronically excited singlet state of salicylic acid, the spectroscopy and photochemistry of 0-arylhydroxycarboxylic acids have been subjects of interest. In a study of the pH and Hammett acidity dependences of salicylic acid and methyl salicylate, it was shown that in aqueous solutions the phototautomerism of methyl salicylate in the lowest excited singlet state is bimolecular rather than intramolecular ( 2 ) . Hirota ( 3 ) in a solvent dependence study of the fluorescence of 3-hydroxy-2-naphthoic acid, found that an anomalous low frequency fluorescence appeared in basic solvents and in aprotic solvents containing small amounts of basic solvents. From this, he concluded that intramolecular proton transfer occurred during the lifetime of the lowest excited singlet state of 3-hydroxy-2-naphthoic acid. Recently, the hypothesis of true intramolecular proton transfer in the lowest excited singlet state of 3-hydroxy-2naphthoic acid has been refuted by Ware et al. ( 4 ) who employed fluorescence quenching and singlet state lifetime measurements, in a variety of solvents to show that the phototautomerism of 3-hydroxy-2-naphthoic acid involves intermolecular hydrogen bonding with the solvent in the ground state. Although the studies of the fluorescence of 3-hydroxy-2-naphthoic acid seem to be preoccu(1) A. Weller, 2. Elektrochem., 60, 1144 (1956). (2) P. J. Kovi, C. L. Miller, and S.G. Schulman. Anal. Chim. Acta., 61,
7 (1972). (3) K.'Hirota, 2. Physik. Chem. N.F., 35, 222 (1962). (4) w. R. Ware, P. I%.Shukla, P. J. Sullivan, and R. V. Bremphis, J. Chem. Phys., 55,4048 (1971).
pied with the singlet state phototautomerism of the neutral molecule, several prototropic forms can be derived from this compound (as shown in Scheme I) and not only can the neutral molecule ( N ) phototautomerize to the zwitterion (Z), but also the singly charged anion (A) can phototautomerize, subsequent to excitation, to the singly charged anion (AI). Moreover, it is known that in the ground states of salicylic acid and of 3-hydroxy-2-naphthoic acid, the ionization sequence (C) (N) (A) (D) is followed while in the lowest excited singlet state of (Z) (AI) (D) is folsalicylic acid the sequence (C) lowed. The ionization sequence in the lowest excited singlet state of 3-hydroxy-2-naphthoic acid has never (to our knowledge) been studied. In order to elucidate the relative importances of protolytic dissociation, tautomerization, and hydrogen bonding phenomena in ground and lowestOexcitedsinglet states of 3-hydroxy-2-naphthoic acid, the present absorption and fluorescence spectrophotometric study of the acid-base titrimetry of this compound was undertaken.
- - -
- - -
EXPERIMENTAL 3-Hydroxy-2-naphthoic acid was purchased from Aldrich Chemical Co., Milwaukee, Wis., and purified by multiple recrystallization from ethanol. 3-Methoxy-2-naphthoic acid was prepared by the method of Werner and Seybold ( 5 ) .The commercial apparatus and solvents employed in this study have been previously described ( 2 ) .
(5) A. Werner and W. Seybold, Ber. Deutsch. Chem. Ges., 37, 3661
(1904).
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