It was assumed that the amount of sulfur remaining with the carbon at 790 "C was about 30% of the total sulfur in the case of the VGO fractions as shown in Figure 5. Therefore, the values for BNT and SNR were combined and reduced by 30 to yield a value for a category of thiophenes with four or more aromatic rings (BNT+). After making this correction, the values were normalized to 100%. Corrected results were plotted for the VGO fractions (see Figure 10) and corresponding product fractions (see Figure 11) as a function of the cut point of the fractions. The decrease in the relative percentage of benzothiophenes and the increase in the rela-
tive percentage of dibenzothiophenes can be seen by comparison of Figures 10 and 11. ACKNOWLEDGMENT
The author thanks J. S. Ellerbe for assistance in performing the experimental work and J. R. Wagner for constructing the all-glass pyrolyzer, G C column, transfer line, and combustion tube. RECEIVED for review June 13, 1968. Accepted January 17, 1969.
Composition of Asphalt Based on Generic Fractionation, Using Solvent Deasphaltening, Elution-Adsorption Chromatography, and Densimetric Characterization L. W. Corbett Esso Research and Engineering Co., P.O. Box 51, Linden, N.J. 07036 This paper describes a convenient method for determining asphalt composition based on fractionation into four generic com onents. The method involves solvent deasphaltening g r recovery of asphaltenes, followed by elution-adsorption chromatography to yield saturates, naphthene-aromatics, and polar-aromatics. The densimetric method was then applied to define the average chemical structures present. Examples are presented to show the effect of distillation and crude source, noting that molecular weight as well as chemical structure varies appreciably. This suggests that these two characteristics should be considered along with the proportion of each component when making asphalt composition analyses.
(6) was applied in order to chemically characterize each of the components and to support the logic of the generic classification. EXPERIMENTAL
THE COMPOSITION OF ASPHALT has been the subject of much study in the past because such data can provide help in the handling of problems related to the usage of asphalt. As evidence of this, there are over eighty publications dealing with the separation and/or characterization of asphalt components. A few of the more commonly cited literature references are indicated here (1-5). Considering the variables of both molecular size and the types of hydrocarbons present in an asphalt, one is immediately impressed with the complexity of composition. This consideration also suggests that no one method can handle all of these variables without involving a lengthy procedure. Taking a compromise approach, a simplified method was worked out whereby the separation into hydrocarbon types was emphasized. As a result it was found that a separation could be made which yielded four classes or types of components, and that each of these represented a reasonably distinct generic hydrocarbon class. This paper describes the technique used in making such a separation, together with examples showing the effect of vacuum distillation and crude source on the proportioning of the four components. In addition, the densimetric method
Apparatus. All work reported was performed with standard laboratory equipment except for the chromatographic column. The column was prepared from a piece of 3.1 X 100-cm borosilicate tubing to which was attached a 2-mm stop-cock made of Teflon (Du Pont) with vernier adjustment and with 35/25 ball joint fittings at both top and bottom. Procedure. The procedure used is based upon solvent precipitation of asphaltenes followed by elution-adsorption chromatography of the petrolene fraction, illustrated by the scheme shown in Figure 1. As common to most methods, asphaltenes are precipitated first with a paraffin solvent at a high dilution ratio, by the principle of disparity of molecular size and type between solvent and asphaltenes. Normal heptane is preferred for this step because of its purity and other properties that permit filtration steps without precipitation of wax components. It is also used in one standardized test (7) for asphaltene content of bitumen. The chromatographic step which follows involves the use of active alumina and elution solvents of increasing polarity. This step permits the separation of components of different polarity as illustrated in Figure 2 which shows the relationship between the eluate and the per cent of petrolene fraction recovered. It will be noted that three distinct concentrations can be separated depending upon the eluant and the volume of the eluate. In previous publications (8, 9), it was shown that nonpolar solvents of the type used here will elute saturated hydrocarbons under these conditions with less than 0.5% carry through of aromatics (10) or other polar compounds. A fraction of intermediate polarity can then be removed with benzene as eluant without release of the highly polar components. This intermediate fraction has been designated as naphthene-aromatics because of the predominance of these two hydrocarbon types.
(1) J. Marcusson, 2.Agncw Chem., 29, 21 (1916). (2) F. R. Grant and A. J. Hoiberg, Proc. AAPT, 12, 87 (1940). (3) F. S. Rostler and H. W. Sternberg, Ind. EnR. Chen?.,41, 598 (1 949). (4) R. N. Traxler and H. E. Schweyer, Oil Gas J., 52 (19) 158 (1953). (5) L. R. Kleinschmidt, Jr. Res. NBS 54, 163 (1955).
(6) L. W. Corbett, ANALCHEM., 36, 1967 (1964). (7) Inst. of Pet. Standards for Petroleum and its Products 143/67, Part 1, Sec. 2, 25'" Ed. (1966). (8) L. R. Snyder, ANAL.CHEM., 33, 1538 (1961). (9) L. R. Snyder and W. F. Roth, ibid., 36, 128 (1964). (10) L. W. Corbett, ACS, Petroleum Division, April 1967.
576
ANALYTICAL CHEMISTRY
DISPERSE & PRECIPITATE IN in-H EPTAN E
[ PETROENES
1
I
FILTER
SoLURLES
0 V
INSoLUBLES~4 ASPHALTENES (A)
a w W
I
U
ELUTION -ADSORPTION CHROMATOGRAPHY (,-HEPTANE ELUATE
I
( W r
lI.
0
s
SATURATES (5)
1
I
BENZENE ELUATE
METHANOL-BENZENE + TRICHLOROETHYLENE ELUATE
5
NAPHTHENEAROMATICS
100
POLARAROMATICS (PA'
-S-
The justification of this becomes apparent later as the result of characterization analyses. In order to release and partially solubilize the most polar components, a highly polar solvent such as a 50z mixture of methanol and benzene (ZI) was found to work well at this point. Trichloroethylene was then used to more completely solubilize and chase out the highly polar components. As shown in Figure 2, the eluant solvent is added about 100-200 ml in advance of the fraction cut point in order to provide for the lag between addition of the solvent at the top and the separation made at the bottom of the column. Thus three distinct hydrocarbon types can be separated and recovered by the chromatographic scheme described. Table I lists the stepwise details involved and Table 11, an example of repeat analyses on the same asphalt. Component Characterization. It has previously been reported that asphalt or heavy petroleum fractions may be characterized by use of the densimetric method (6). This involves a calculation based upon the relationship between molar volume and atomic H/C ratio. It requires only the measurement of per cent hydrogen, per cent carbon, and molecular weight followed by a calculation step. For this molecular weights were determined in benzene using a Mechrolab Vapor Pressure Osmometer at 37 "C, standardized with benzil. The analysis is then summarized in terms of molecular weight, fraction of carbon atoms in aromatic rings, average number of carbon atoms per molecule, the number of aromatic and naphthene rings per molecule. The number average molecular weight and the number of carbon atoms pertain to the relative molecular size. The fraction of carbon in aromatic rings and the number of rings per molecule are indicative of the degree of aromaticity, while the number of (1 1) H. E. Schweyer and E. L. Chipley, Proc. HRB Record, No. 178, 30 (1967). APPROX.
3500
BASED ON W T . DIST.
B A S E D ON Mi1 O I S T .
-
& v
A S P HAL TEN E s
500
700
-.i;j
Mn = number average molecular weight.
Figure 3. Typical distribution of generic components in asphaltic residue (89 Pen)
-
NA-
1100
900
V O L U M E OF E L U A T E ,
Figure 1. Scheme for separation into four generic components
-Mn-
300
1111.
-
PA-
Figure 2. Elution-adsorption chromatogram of petrolenes naphthene rings is a relative measure of naphthenicity. This permits a cross comparison of chemical characteristics, bringing in the concept of molecular size and the relative amount of aromaticity and naphthenicity. Table I11 exemplifies the Table I. Procedure for Asphalt Separation 1 . Place 10-15 gram [100/(100-% Asphaltenes)] in 2-1 Erhlenmeyer and add 100-ml of n-heptane (99+ mol %) per gram of
asphalt 2. Heat on steam bath while stirring, then let stand overnight 3. Filter slowly through Whatman No. 1 paper in 12-cm Buchner and wash thoroughly with warm n-heptane until clear 4. Transfer insolubles to fresh heptane, warm to 100 "F, filter, and wash again. Under a nitrogen atmosphere, dry insolubles and
weigh as asphaltenes 5 . Concentrate petrolene-heptane solution on steam bath to a
volume of 50 ml 6. Prepare F-20 alumina by drying at 750 "F for 16 hours and store in tight glass containers 7 . Add about 450 grams of alumina to 3- x 100-cm chromatographic column with gentle tapping and immediately prewet with 50 ml of n-heptane 8. Follow prewet solvent in step 7 with the 50 ml of petrolene solution and n-heptane eluant. Add more eluant and recover eluate according to following schedule: Eluate Fraction Eluant feed vol., ml. recovered n-Heptane 200 Saturates Benzene 100 Saturates Benzene 300 Naphthene-aromatics Methanol-benzene 300a Naphthene-aromatics Trichloroethylene 300 Polar-aroma tics 300 Polar-aromatics Trichloroethylene Trichloroethylene Hold-up Polar-aromatics 9. Combine eluates into tared 400-ml beakers and evaporate to dryness and constant weight using a steam bath and nitrogen sparge 10. Weigh fractions to the nearest 0.01 g and calculate weight per cent a This fraction may be cut short or enlarged depending upon the descent of the black ring (polar-aromatics) during elution. Table 11. Repeat Composition Analyses of a Venezuelan 90 Penetration Asphalt Binder Generic fraction Saturates 13.9% 14.1% 34.2 34.8 Naphthene-aromatics 36.7 35.9 Polar-aromatics 14.0 14.3 Asphaltenes ___ ___ 98.8
99.1
VOL. 41, NO. 4, APRIL 1969
577
FLUX -
BINDER
VEN
PITCH -
.
3 7%
SATURATES-
USA
MEX
MID-EAST
92 114
88 116
115
SATURATES
NAPHTHENEAROMATICS
NAPHTHENEAROMATICS
POLARAROMATICS
POLARAROMATICS
ASPHALTFNES-
ASPHALTENES SOFT PT. 'F PEN 0 77'F.
83 300 *
114
184
89
5
Figure 4. Effect of vacuum reduction on composition
PEN @ 77OF. S O F T P T . OF.
90
114
85
Figure 5. Effect of crude source on composition repeatability of these characteristics when applied to the three chromatographic fractions reported in Table 11. The characterization of asphaltenes was not made because of inherent difficulties in obtaining reliable tests for molecular weight and per cent carbon and hydrogen. The divergent results found in molecular weight measurement of asphaltenes is explained (12) as caused by compositions of individual sheets which associate into larger micelles. RESULTS AND DISCUSSION
In order to study the distribution of generic components in an asphalt residuum, a separation by vacuum distillation was made first in order to yield a series of contiguous boiling point cuts. After deasphaltening of the vacuum distillation residuum, solvent separation was applied to obtain fractions in the higher boiling regions which could not be handled by conventional distillation. Analysis of each of these fractions gave the composite picture on the left in Figure 3, with number average molecular weights shown to indicate the approximate range of molecular sizes involved. Thus it became apparent that the distribution of the generic types is quite dependent upon the degree to which a residuum is reduced. The same analysis, shown in the block diagram on the right of this figure, is based strictly on weight proportion as measured by the analysis. This figure does not tell as much as the previous one, but it is easier to arrive at and satisfies the practical needs for composition analysis. The erect of vacuum reduction on composition is shown by -. ____ -.-___.__ (12) J. D. Dickie and T. F. Yen, ANAL.CHEM., 14, 1847 (1967).
the block illustrations in Figure 4. It will be noted here that both saturates and naphthene-aromatics decrease as distillation progresses, whereas the polar-aromatics and asphaltenes tend to increase by concentration. This points out the adviscompos~tion compar~sonson the Same level ability of Table IV. Comparison of Fraction Characteristics of Asphalt Residua Prepared by Vacuum Distillation Designation: Flux Binder Pitch 83 114 184 Soft pt., "F Pen at 77 ''F 300 A 89 5 Generic fraction
Saturates Molecular weight Density, 2014 No. C atoms/mole No. naphthene rings/mole Naph thene-aromatics Molecular weight Density, 20/4 Fraction carbon, aromatic No. C atoms/mole No. aromatic ringsjmole No. naphthene rings/mole Polar-aromatics Molecular weight Density, 20/4 Fraction carbon, aromatic No. C atomsjmole No. aromatic rings/mole No. naphthene ringsjmole O
O
520 0.864 37 2.9
626 0.870 45 3.1
883 0.871 63 3.7
603 0.984 0.25 42 2.1 3.1
720 0.989 0.26 51 2.8 3.6
918 0.992 0.22 65 3.1 4.1
1004 1.062 0.38 68 4.7 3.3
1045 1.069 0.39 73 6.6 3.7
1309 1.081 0.42 89 8.9 3.9
Table V. Comparison of Fraction Characteristics of Asphalt from Different Crude Sources Table 111. Repeat Characterization Analyses of a Venezuelan 90 Pen Asphalt Binder Generic fractions Saturates Molecular weight 570 543 Density, 2014 ' 0.873 0.889 No. C atomsimole 40 39 No. naphthene ringsimole 3.0 2.9 Naphthene-aromatics Molecular weight 660 671 Density, 20/4 ' 0.993 0.991 Fraction carbon, aromatic 0.26 0.26 No. C atoms/mole 47 48 No. aromatic ringsimole 2.6 2.6 No. naphthene ringsjmole 3.3 3.4 Polar--aromatics Molecular weight 1158 1081 Density. 20/4 ' 1.076 1.072 Fraction carbon, aromatic 0.44 0.44 No. C atomsjmole 81 76 No. aromatic rings'mole 8.4 7.9 No. naphthene ringcimole 35 3.4
__ .-__
578
-. .-
-..
ANALYTICAL CHEMISTRY
___.
Source Pen. at 77 'F Generic fractions Saturates Molecular weight Density, 20/4 No. C atomsjmole No. naphthene ringsimole Naphthene-aromatics Molecular weight Density, 20/4 Fraction carbon, aromatic No. C atoms/mole No. aromatic ringsimole No. naphthene rings/mole Polar-aromatics Molecular weight Density, 20/4 Fraction carbon, aromatic No. C atomsimole No. aromatic rings/mole No. naphthene rings/mole
Ven .
USA 92
Mex.
90 557 0.877 40 3.0
630 0.869 45 3.1
416 0.866 34 2.2
666 0.992 0.26 48 2.6 3.4
800 0.991 0.24 56 2.9 3.7
578 1.001 0.27 41 2.3 3.O
1120 1.074 0.44 79 8.2 3.5
I330 1.084 0.43 90 9.2 4.0
1115 1.067 0.38 75 6.6 3.4
88
Table VI. Summary of Composition and Characterization as Found in Typical SSjlOO Straight Reduced Asphalt Density Mole Wt, Fraction Av No. rings/mole Wt % Physical 2014' (Av) Arom (fa) Nap Arom Chemical structure Component range nature (A4 Pure paraffins pure naphthenes 650 0.00 3.0 0.0 5-15 Colorless 0.87 Saturates mixed paraffin-naphthenes liquid 3.5 2.6 Mixed paraffin-naphthene725 0.25 Naphthene30-45 Yellow to 0.99 aromatics + sulfur-containing red liquid aromatics compounds 1150 0.42 3.6 7.4 Mixed paraffin-naphthene30-45 Black 1.07 Polararomatics in multi-ring structures solid aromatics -!- sulfur, oxygen, nitrogen-containing compounds. 3500 0.50 Mixed paraffin-naphthene5-20 Brown to 1.15 Asphaltenes aromatics in polycyclic structures black solid + sulfur, oxygen nitrogen-containing compounds.
+
of consistency or on asphalts submitted to some common level of treatment. Table IV lists the fraction characteristics of these same asphalts. One notable effect of distillation is the increase in molecular weight from flux to the highly reduced pitch, also the gradual increase in number of rings per molecule. Thus, both the proportion of generic fractions and their chemical nature are dependent upon the amount of reduction by distillation. Crude source is also a factor in composition analysis as illustrated in Figure 5 . In this case all four asphalts have about the same penetration and softening point, while each inherently contains different amounts of the four generic fractions. Table V illustrates how both the molecular size and relative amount of aromatic and naphthene rings can vary according to crude source. Based on a limited number of analyses, it can safely be stated that greater variations than this will be found among asphalts used in paving. Table VI is included in order to summarize the general range of composition found in most straight run asphalts. Every asphalt contains some of each of these components, and it is believed (13) that each contributes different qualities
(13) J. Knotnerus, b i d Eng. Chem., Prod. Res. Develop., 6,43 (1967).
+
to the total asphalt. A brief description of the physical nature is also added in which it will be noted that saturates and naphthene-aromatics are liquid in nature whereas the polararomatics and asphaltenes are solids. This has prompted the added description that saturates and naphthene-aromatics are soft or plasticizing type components whereas the polararomatics and asphaltenes are hard components. Shown also in this table is a general description of the chemical structures involved in each component as judged by the densimetric method of characterization. In general we have found that differences in molecular size and ring characteristics, as dependent upon distillation and crude source, is quite pronounced. This suggests that both the molecular size and the chemical characteristics of each component should be considered along with the compositional proportion of each component. The separation technique used here will provide a basis for determining these parameters, and this will aid in a better understanding of the fundamentals involved and their relationship to the overall asphalt.
RECEIVED for review October 30, 1968. Accepted January 7,1969. This paper was presented in part at the 156th National Meeting of the ACS, Division of Petroleum Chemistry, Atlantic City, N.J., September 1968.
VOL. 41, NO. 4, APRIL 1969
579
