Sulfur Compound Type Distributions in Petroleum Using an In-Line Reactor or Pyrolysis Combined with Gas Chromatography and a Microcoulometric Sulfur Detector Harry V. Drushel Esso Research Laboratories, Humble Oil and Rejining Company, P.O. Box 2226, Baton Rouge, La. 70821
A rapid method for determination of sulfur compound types in petroleum was developed which involves dealkylation of the condensed thiophenes using an in-line catalytic reactor or noncatalytic pyrolysis. The resulting reaction products were separated by gas chromatography and the comaounds containing sulfur were quantitatively detected with a Dohrmann microcoulometric sulfur detector. Nonthiophenic compounds, primarily aliphatic sulfides, were converted to H and detected in the same manner. Precision was 2 to 3% and analysis time was 1 hour. Data agreed very well with high resolution mass spectrometric analysis. Results from use of the method on hydrodesulfurized vacuum gas oils showed that the ease of desulfurization followed the order: benzothiophenes>>dibenzothiophenes > benzonaphthothiophenes. INCREASED EMPHASIS on clean air will require a reduction in the sulfur content of residual fuels for power plants and of fuel oil for home heating use. The petroleum industry has been active in developing desulfurization processes to meet these needs. A knowledge of the types of sulfur-containing compounds present and their fate during hydrodesulfurization would aid in understanding mechanisms and kinetics during process development. The analysis of residual fuels and heavy vacuum gas oils is difficult and time-consuming. Drushel and Sommers ( I ) isolated and characterized sulfur compounds in a vacuum gas oil. The separation procedure, which required several steps involving chemical reaction and liquid-solid chromatography, resulted in the recovery of only about 6 0 z of the sulfur compounds present. Such an approach would be unsuccessful on residua because of decreased reactivity, increased side reactions, and poor selectivity during chromatographic separation. Martin and Grant (2) developed an excellent rapid method for sulfur compound types based on combination of a microcoulometric sulfur detector with gas chromatography. They circumvented the problems of high boiling point and complexity associated with gas oils and residua by subjecting the samples to catalytic dealkylation in a small laboratory-scale reactor. This step provided a quantitative measure of aliphatic sulfides which decompose to H2S and made the remaining condensed thiophenes amenable to separation by gas chromatography. Drushel and Sommers ( I ) compared data from this technique with data from isolation and characterization of sulfur compounds and found the results to be comparable for a vacuum gas oil. This paper describes the use of pyrolysis and in-line reactors combined with gas chromatography and microcoulometry to obtain essentially complete sulfur compound type analysis. The method is rapid, requiring only the time necessary to complete the temperature program cycle on the gas chromatograph of about 1 hour. Precision for each of the sulfur types (I) H. V. Drushel and A. L. Sommers, ANAL.CHEM.,39, 1819 (1967). (2) R. L. Martin and J. A. Grant, ibid., 37, 649 (1965).
was within 2 or 3 % based on the total sulfur content. The method has been applied to vacuum gas oils and their hydrodesulfurized products. Results from the use of this method have provided insight into the mechanism of hydrodesulfurization. EXPERIMENTAL
A simple block diagram of the combined apparatus is shown in Figure 1 . A more detailed description of each component of the system is given below. Gas Chromatograph. A Micro-Tek model 2500-R gas chromatograph (GC) was used throughout this study. For the earlier portion of the study, involving use of the in-line, A1203-filled,dealkylation reactor, %-inch aluminum columns were used. A metal transfer line carried the effluent from the G C to the combustion unit of the Dohrmann microcoulometer. For this study the combustion tube and special G C inlet system as supplied by Dohrmann Instruments was used. The carrier gas was helium; the flow rate was 50 cc/min. For the latter portion of this study, in which a pyrolyzer and glass columns were used, a special modification was made in order to use the temperature programming feature and oven of the GC. The ordinary inlet and outlet blocks were bypassed. All of the glass components of the system were mounted in a transite housing which was inserted between the main housing of the G C and the removable oven cover. This permitted operation of the all-glass system and use of the temperature programming feature of the G C without having to dismantle any portion of the inlet or outlet block section of the chromatograph. The carrier gas was nitrogen; the flow rate was 50 cc/min. In-Line Alumina Dealkylation Reactor. The reactor consisted of a 6-inch length of stainless steel tube of about %'-inch i.d. which was packed with a 3-inch length of F-20 chromatographic grade alumina. The temperature was maintained at 600 "C by means of electrical resistance wire wound around the reactor insulated with a layer of asbestos. The temperature necessary to carry out dealkylation using AL03 was previously established by Martin and Grant (2). The inlet end of the reactor was machined to retain a Silicone rubber septum through which samples were injected. Samples were introduced directly onto the alumina packing. The helium carrier gas, which entered the reactor near the septum, swept the cracked products through a heated %-inch stainless steel transfer line to the G C column. A %$-inch x 10-foot column packed with 10% Carbowax 20M on Chromosorb W was used to obtain the necessary separation between the aromatic sulfur compound types. The temperature was programmed from room temperature to a maximum of 275 or 300 "C in order to achieve elution of the condensed thiophenes. Such high temperatures accelerated deterioration of the column requiring its replacement after three or four weeks. Pyrolyzer. The pyrolyzer was constructed from Vycor glass tubing using several loops representing an overall length of about 12 inches (see Figure 2). At carrier gas flow rates near 50 cc per minute, this length of tubing provided several seconds residence time for pyrolysis. A Silicone rubber septum, through which the sample was injected, was mounted where the carrier gas entered the pyrolyzer. The exit end of the pyrolyzer made VOL. 41, NO. 4, APRIL 1969
569
Transfer Line
bG,
Chrsnatograpti
L.?licrocoulometer
imbsstion
TJ~?
!aj73r 1
-
Recsrder
curve)
Figure 1, Schematic diagram of combined apparatus for sulfur compound type determination
Thernocoup:e
\Well
and
:C Co!unn
Silicone Rubber
GC Column Oven Area
4
1
I
I
"
N2 or He
j
I
I
I
///
r /
I
/
/
/
I
,* / / *,
S 2 or He %eep Gas
', '/I
"'
I - " '
S i l i c o n e Rubber Septum
Carrier Gas
Figure 2. Schematic diagram of all-glass pyrolysis--GC-transfer line system
-
570
ANALYTICAL CHEMISTRY
4
_ _
--
connection with the glass G C column by means of a ball joint. The joint was well greased with Silicone grease and held in place by means of a screw-tightened clamp. Glass Column and Transfer Line. The U-shaped G C columns were made from %'-inch glass tubing with ball joints at each end. Components separated by the column were transferred to the combustion tube by means of a %-inch i.d. glass transfer line. The transfer line was heated to a temperature of about 350 "C using electrical resistance wire insulated with asbestos. The capillary tip of the transfer line made connection to the inlet of the combustion tube by means of a Silicone rubber septum as shown in Figure 2. Combustion Tube. The combustion tube was constructed from Vycor glass as shown in Figure 3. Essentially, it was an open, unpacked combustion tube of large internal diameter into which the sample was swept by means of an inert carrier gas (Nz or He). The combustion zone, through which oxygen flowed, was maintained at a temperature of 850 "C. The inlet area was maintained at 550 "C. This tube permitted the direct injection of up to 5 p1 of sample without carbon formation or incomplete combustion. The SOz produced by the combustion of sulfur compounds was titrated by means of the Dohrmann T-300-P titration cell. The combustion tube shown in Figure 3 was used successfully for the determination of total sulfur by combustion using the Dohrmann microcoulometer. Samples containing as little as 1 ppm sulfur were analyzed in less than 2 minutes by direct injection with a syringe. Precision (2a) was of the order of 3 ppm at the 100-ppm sulfur level. The use of this combustion tube for determination of total sulfur will be the subject of a future publication. Microcoulometer, The Dohrmann microcoulometer was used throughout this study. In most cases, the bias voltage was 100-120 mV, the gain was set at 20, and the range (resistance in ohms) varied according to the amount of sample and sulfur content for each analysis. In addition, the signal from the microcoulometer to the recorder was filtered by means of a simple resistance-capacitance network to reduce short term noise. The current was recorded on a 1-mV recorder with Disc integrater. Reference Compounds. All reference compounds used for calibration purposes were Eastman, white-label, organic chemicals. Solutions of the pure compounds were made up in spectro-grade toluene. Petroleum samples containing more than 0.2 to 0 . 4 z sulfur were diluted with toluene. DISCUSSION OF RESULTS
Method Using an In-Line Alumina Dealkylation Reactor. A typical sulfur chromatogram of a vacuum gas oil (VGO) fraction (425-455 "C) using the alumina in-line reactor at 600 "C
-
LIA
L k
L
-,w
-1-
Dibeniothiophenes
Be"Z0-
Benzothiop5enes
naphthothiophenes
41-
Thiopheres
I
10
20
0
Time {ninutes)
Figure 4. Sulfur chromatogram of a VGO fraction (425-455 "C) using the in-line alumina dealkylation reactor at 600 "C (10-ft Carbowax 2OM Column) is shown in Figure 4. The alumina reactor serves to decompose the aliphatic sulfides (designated as nonthiophenic sulfur, NTS) to HzS and thiols and at the same time dealkylates the condensed thiophenes sufficiently so that they may be separated on the G C column. Figure 4 shows that sufficient dealkylation has taken place for adequate separation of the sulfur compounds by type. A serious difficulty was encountered, however, because of the fact that the system was constructed of metal components. The H2S produced during the reaction was completely lost as a result of reaction with the exposed metal surfaces of the reactor, inlet block, column, outlet block, or transfer line. Even though the H2S was lost, it was possible to calculate the distribution of the various condensed thiophenes. The distribution was calculated on the basis of the relative area for each of the groups of peaks representing the various thiophene types. The area, being proportional to the amount of sulfur detected, provided data in terms of the relative weight per cent of the total thiophenic sulfur present. Also, the results were normalized to 100% rather than calculated on the basis of the amount of sulfur injected into the reactor-GC system. Table I lists some of the data obtained on a typica1425-455 "C vacuum gas oil fraction as well as a corresponding hydrodesulfurized product. These data are also compared with results obtained using the small laboratory alumina cracking reactor as proposed by Martin and Grant (2). The results from the in-line reactor agree quite well with those using the small laboratory reactor. There is also agreement, as shown in Table I, with our earlier study ( I ) in which we isolated the ~~
~
Table I. Sulfur Compound Type Distribution Using the In-Line Alumina Reactor Combined with Gas Chromatography and a Microcoulometric Sulfur Detector (Comparison with Other Methods) Total S
ThioDhene (TI
VGOafraction (425-455 "C)
Relative wt % of thiophene sulfur Benzothioohene Dibenzothionhene (DBT)'
BenzonaDhthothiophene(BP
w
2.76
In-line alumina reactor
3.6 2.5 1.6
30.3 30.7 33.8
52.7 55.3 52.1
13.4 11.6 12.6
Separate laboratory alumina dealkylation reactor
1.5 2.0
30.4 33.6
52.4 51.2
15.6 13.1
2
37
46
15
4.1 4.6
6.4 10.7
68.3 63.0
21.2 21.7
High resolution MS analysis of isolated sulfur compounds Hydrodesulfurized product of above In-line alumina reactor Separate laboratory reactor a Vacuum gas oil.
0.29
VOL. 41, NO. 4, APRIL 1969
571
sulfur compounds and characterized them by means of high resolution mass spectrometry. Method Using Pyrolysis. BEHAVIOR OF SOMEREFERENCE Cohwoums. The distribution of sulfur-containing products from the pyrolysis of reference compounds is tabulated in Table 11. These data show that thiophene, benzothiophene (BT), and dibenzothiophene (DBT) survive pyrolysis at 790 "C with little change. Only a very small amount of H2S is produced from these compounds. However, hexyl sulfide undergoes decomposition to produce essentially H2S. If the pyrolysis temperature is raised to 900 "C even the thiophenic compounds decompose to produce measurable amounts of H r S . In addition, at the higher temperature a significant amount of sulfur is not recovered as compared to pyrolysis at 790 "C. The loss of sulfur at the higher temperature is attributed to incorporation of sulfur into carbon which tends to form more readily at the higher temperature. Figure 5 shows the effect which EFFECTOF TEMPERATURE. pyrolysis temperature has on the per cent of total sulfur which is not recovered during the analysis. As the pyrolysis temperature is increased, more of the sulfur becomes fixed as an integral part of the carbon deposited on the surface of the pyrolyzer. Concurrently, the higher temperature probably promotes the formation of more highly condensed aromatic sulfur compounds (not necessarily present in the sample originally) which cannot be eluted from the gas chromatograph because of their high boiling point. The distribution of sulfur compound types as a function of pyrolysis temperature is presented in Table 111. Near 650 "C the temperature is too low t o achieve sufficient dealkylation of the thiophenic nuclei and they are not resolved during the ~~~
~~
~
~
~~~
300
I
I
I
I
I
400
500
600
-00
SOL1
'3co
P y r o l y s i s Temperaturc 1°C.)
Figure 5. Effect of pyrolysis temperature on sulfur recovery (VGO, 425-455 "C, 2.910,/, S)
gas chromatographic separation. Increasing the pyrolysis temperature increases the relative percentage of H2S, but, conversely, decreases the relative percentage of the condensed thiophenes as well as the overall recovery of sulfur. Figure 6 depicts the conversion of normal hexyl sulfide to H2S (or thiols) as a function of pyrolysis temperature. From this figure it is seen that the decomposition of the normal hexyl sulfide begins at 475-525 "C. The conversion to H2S is essen-
~
Table 11. Pyrolysis Behavior of Some Reference Sulfur Compounds
Pyrolysis at 790 "C rz-Hexyl sulfide Thiophene Benzothiophene Dibenzothiophene Pyrolysis at 900 "C n-Hexyl sulfide Thiophene
% of Total sulfur injected Original Postpeaksb compound
HISa 98.5 0 0.2 2.3
Prepeak s 0 0 0 0
0 95.6 88.7 86.6
0 3.5 11.1 11.4
72.6 6.9 6.1 7.9 5.1 1.3 58.8
12.5 0 0 0 3.1 0.8 0
0 82.2 81.2 84.2 67.0 70.0 0
0 2.9 1.3 1.5 18.7 27.9 21.2
Benzothiophene Dibenzothiophene Phenyl sulfide Includes unresolved thiol. Only slightly greater retention time and would be included in a typical analysis.
Not recovered
. 1.5 0.9 0 0
14.9 8.0 11.4 6.4 6.1 0 20.0
Table 111. Effect of Pyrolysis Temperature on the Distribution of Sulfur Compound Types and the Sulfur Not Recovered (2 pl VGO, 425-455 "C, 2.91% S, 1/10 dilution in toluene)
% of Total sulfur injected Pyrolysis temperature ("C) 565 63 5 690 746 755 774 815 900
572
Aliphatic sulfides, AS (HS)
BT
2.0 6.5 15.4 17.4 16.8 17.3 22.8 28.0
21 .o 17.0 17.4 17.5 12.5 9.5
ANALYTICAL CHEMISTRY
Sulfur not recovered DBT BNT SNR (Sulfur types not dealkylated and resolved) (Sulfur types not dealkylated and resolved) 27.1 22.1 22.0 22.2 16.0 15.8
10.7 12.5 11.9 12.1 10.3 6.7
25.8 31.0 31.9 30.9 38.4 40.0
BNT
+ SNR 36.5 43.5 43.8 43.0 48.7 46.7
,
'tl
I
I
I
A
- -A
/'
/
-HEXYL
400
450
500
330
6ro
700
650
750
S50
800
TEIIPERATURE OF P Y R O L Y T I S ZnYE ("C.)
Figure 6. Effect of temperature on decomposition of aliphatic sulfides to HzS tially complete once temperatures near 650 "C are reached. In this same figure, a plot of the relative percentage of H2S produced from a vacuum gas oil fraction (425-455 "C) is shown as a function of pyrolysis temperature. In this case, the temperature required t o decompose the aliphatic sulfides in the sample appears t o be about 50" or so above that of the pure reference compound. In fact, there seems to be a plateau in the curve around 700-800 "C. Above 790 "C the relative percentage of H?S increases still more. It is felt that this increase above 790 "C can be attributed to the decomposition of sulfur-containing molecules which are more complex than simple aliphatic sulfides. The presence of diary1 or alkyl aryl sulfides is indicated. Thiaindanes may be responsible for such behavior. Some data obtained with pure compounds indicate that about half of the thiaindane yields H2S while the remainder dehydrogenates to form henzothiophenes. INFLUENCE OF SAMPLE BOILINGRANGEON THE RECOVERY OF SULFUR. Figure 7 shows that the sulfur in thiophenic structures with 4 aromatic rings or more plus the sulfur which is not recovered increases as a function of the boiling range of the sample being analyzed. Such a trend is due undoubtedly t o a combination of a real increase in the percentage of highly condensed thiophenes as well as an increase in the amount of sulfur which remains bound t o the carbon produced during pyrolysis. As the boiling point increases, the aromatic structures which are usually present tend t o form carbon more readily. CHOICE OF GLC COLC'MN. An SE-30 column was tried as a lypical nonpolar column for separation according to boiling point. The column functioned well but because of the high temperatures needed the substrate tended to bleed excessively. This caused the formation of deposits of finely divided silica in the combustion tube. A column prepared from de-ashed (to remove metal catalyst residues) ethylene-propylene copolymer on Chromasorb W provided good resolution with separation by boiling point (see Figure 8). A similar column prepared from amorphous poly-
60
I
I
300
'loo
I
I
I
20
200
/
500
6SO
END POINT OF CUT ('C.1
Figure 7. Effect of boiling point of fraction on sulfur in benzonaphthothiophenes plus SNR) sulfur not recovered (BNT
+
propylene supported on Chromasorb W treated with excess potassium carbonate behaved in a similar manner except that the H2S peak was missing (see Figure 8). This permits the more accurate measurement of thiophene which is not always resolved from the H2S. If the HzS peak is large, the thiophene peak may be completely engulfed. Fortunately, the thiophene content of the VGO fractions is rather low and the simple thiophenic species are easy t o remove by hydrodesulfurization. A polar column should separate the condensed thiophene types better than a nonpolar column, as pointed out by Martin and Grant (2). For this reason we selected a Carbowax 20M column. Typical sulfur chromatograms are shown in Figure 9. Note that the resolution between contiguous members of an homologous series is less but the separation by type is better than with the nonpolar columns. One disadvantage of the VOL. 41. NO. 4, APRIL 1969
573
0
_ _
:
,-
d
T b i cnhenes
F;S
Pcn:otFliophenes
-L-
Piheniothiophenes
I Ren2onaphthothicphenes
~
1,'1 1n. x 5 ft. g l a s s column of 10% ethylene-propyiene :o?olyner on Chromasorb I\'
1 so
?@O Colunn Temperature f ° C c . )
100
250
300
1/4 in. x 3 ft. glass column o f 10% amorphous polypropylene on Chromasorb h' treated w i t h excess K2C0j
/-A
L - 1 - I
0
I
10
30
20
4c
Time (minutes)
Figure 8. Behavior of nonpolar columns in separating sulfur compounds from pyrolysis of VGO
-
1H2S
- 1
Thiophenes
100
--
I
-
Beniothiophenes
200
150
Dibenzothiophenes
250
4
Benzonaphthcthiophenes
300
Temperature ("C.)
T i l e !minutes'
Figure 9. Comparison of sulfur chromatograms of pyrolyzed VGO and hydrodesulfurized product (399425 "C fraction) Carbowax 20M column is its temperature limit which permits elution of the condensed thiophenes with up to only four aromatic rings, the benzonaphthothiophenes (BNT). ANALYSIS OF VGO AND HYDRODESULFURIZED PRODUCT FRACTIONS. Figure 9 compares the sulfur chromatograms of a typical VGO fraction with a hydrodesulfurized product fraction of the same boiling range. Note that a very striking decrease in the relative percentage of benzothiophenes (BT) has occurred as a result of hydrodesulfurization. On the other hand, dibenzothiophenes (DBT) and benzonaphthothiophenes (BNT) appear to be more difficult to remove by hydrodesulfurization. The relative area of the H2S peak is about the same after hydrodesulfurization as it is before. It is possible that hydrogenated intermediate products are formed which are 574
ANALYTICAL CHEMISTRY
not catalytically cracked (carbon-sulfur bond scission). For example, benzothiophenes may become hydrogenated to form thiaindanes, which, if not catalytically cracked, would fall in the category of nonthiophenic sulfur (NTS) by this method. The concentration of intermediate thiaindanes might even be higher than the NTS value because pyrolysis of such compounds would yield some H S (or NTS) and some benzothiophenes (BT). This explanation is logical because aliphatic sulfides (the major compound type in the NTS category) originally present in the VGO should have been essentially removed by hydrodesulfurization. Table IV lists results obtained on the distillate fractions of the VGO and its product. The data include the weight percentage of each fraction, the total sulfur content, the ali-
Table IV. Sulfur Compound Type Distributions Using the Pyrolysis-GC-Microcoulometric Method (Pyrolysis Temperature 790 "C)
VGOb fraction Total VGO Init-371 371-399 399425 425455 455482 482-510 510-538 538-552
Aliphatic sulfide sulfura % of Wt :4 total S
Fraction wt %
Total sulfur (wt %)
1.5 3.8 19.0 22.6 16.3 18.6 13.4 3.6 1.2
2.86 2.94 2.93 2.91 2.81 2.79 2.83 3.03 3.24 3.43
0.62 0.62 0.57 0.54 0.53 0.54 0.56 0.57 0.62 0.60
21.7 21.1 19.5 18.6 18.9 19.4 19.8 18.8 19.1 17.5
0.406 0.242 0.338 0.388 0.393 0.425 0.453 0.477 0.508 0.565
0.045
-
__
Bottoms
Hydrodesulfurized product fraction Total product Init-371 371-399 399-425 425-455 455-482 482-5 10 510-538 538-552
7.9 6.5 23.6 15.6 20.8 18.6 2.8 3.6 0.8
-
-~
-
-
.-
._
.-
-
-.
-
-
-
-. _.
-
Bottoms a By direct determination using the iodine complex method (3). Vacuum gas oil..
phatic sulfide sulfur (3),and the sulfur compound type distribution. It is interesting that the aliphatic sulfides and NTS agree in fractions near the middle of the VGO boiling range. The iodine complex method indicates a rather constant relative percentage of aliphatic sulfide sulfur throughout the VGO boiling range. The NTS, however, shows a gradual increase with boiling point. This may imply an increase with boiling point of alkyl aryl sulfides or thiaindanes which are only partially detected by the iodine complex method.
Non-
thiophenic sulfur (NTS) 14.1 15.7 16.4 15.9 18.8 21.1 22.2 22.8 22.6 -
14.1 13.5 14.1 14.0 18.8 20.4 26.6 28.1 29.1
Relative wt % of total sulfur BenzoBenzoDibenzonaphthothiophene thiophene thiophene (DBT) (BNT) CST, 25.4 22.0 20.0 19.0 18.1 16.1 14.4 12.8 10.8
_~-
4.7 5.7 4.4 4.2 3.7 3.9 5.2 4.4 10.2
27.2 27.5 23.7 22.2 16.6 13.2 11.7 10.1 7.7
11.5 9.4 9.7 12.1 13.4 11.1 9.7 8.4 4.9
39.2 42.9 32.3 26.0 16.9 15.1 13.0 11.4 17.9
8.5 13.2 12.5 15.6 12.9 10.6 9.7 8.6 10.5
--_
Sulfur not recovered (SNW
-.
21.8 25.4 30.2 30.9 33.0 38.5 42.0 45.9 54.0
__
~-
33.5 24.7 36.7 40.2 47.7 49.9 45.6 47.5 32.3
A very striking decrease in benzothiophenes is produced by hydrodesulfurization. As a result, DBT and BNT tend to be the major thiophenic species in the product. The ease of sulfur removal, in general, follows the order: BT > > DBT > BNT+.
(3) H. V. Drushel and J. F. Miller, ANAL.CHEM., 27, 495 (1955).
jo
5 1
E NTS
5\T*
r/
bT
400 403
:a3 L~~
FGI\T
501 CF
m i x m ('c
sju
)
Figure 10. Sulfur compound type distribution in VGO fractions PITS -nonthiophenic sulfur BT -benzothiophenes DBT dibenzothiophenes BNT T- --condensed thiophenes with four (benzonaphthothiophenes) or more aromatic rings
450 IT
i L I v r OF F W - I
\
-
:C"
AS
Figure 11. Sulfur compound type distribution in hydrodesulfurized product fractions NTS
-nonthiophenic sulfur -benzothiophenes DBT -4ibenzothiophenes BNTi-condensed thiophenes with four (benzonaphthothiophenes) or more aromatic rings
BT
VOL. 41,NO. 4, APRIL 1969
575
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, GC 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.
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