Anal. Chem. 1985, 57,303-308
303
Simulated Distillation of Coal-Derived Liquids Using Combined Gas Chromatography-Vacuum Thermogravimetry Timothy G. Southern, Angelo Iacchelli, David Cuthiell, and Milan L. Selucky* Coal Research Department, Alberta Research Council, P.O. Bag 1310, Devon, Alberta, Canada
Novel base-llne correction permits vlrtually complete ellmlnation of base-llne nonllnearlty In simulated dlstlllatlon by gas chromatography. A general equatlon of the type no. of counts = A o ( l exp(Bo B , T + B,T*)) can be solved H a thlrd polnt and Influence of program parameters are established. Problems wlth standard selection for Internal standardization of GC runs of coal-derlved llqulds can be avoided by uslng vacuum thermogravlmetry for the determlnatlon of nondlstillable reslduum in the sample. The Influence of sample size and quality and detector response are briefly discussed.
+
+
Ever since the advent of linearly programmed ovens in gas chromatography in the middle 19609, attempts have been made to use the gas chromatograph for the determination of the boiling point distribution of various hydrocarbon mixtures (1-3) in order to obtain a method that is faster than conventional distillation and also applicable to the analysis of very small samples. The classical distillation procedures, such as ASTM D86 (4),Dl160 (5),and D2892 (6), are lengthy, whereas the results of simulated distillation of low-boiling materials can be obtained in less than 1h (D3710 (7)).Heavier samples pose considerably more problems (D2887 (8))whose effects have to be considered, e.g., septum and column bleed, validity of conversion of retention time into boiling point scale, and, in the presence of nondistillables, sample homogeneity and determination of nondistillables. An ASTM method had been proposed (9) for higher boiling materials, consisting of dual runs, one direct and another one with addition of a known amount of internal standard containing a t least four different hydrocarbons and necessary for the calculation of the nondistillable residuum. However, this method so far has not materialized as a standard, mostly because of a t least some of the above mentioned difficulties. In the search for an alternate complement of GC for the determination of nondistillable residuum, we have found that vacuum thermogravimetric analysis satisfies most of the demands imposed on such analysis and can be used for this purpose. Two other questions have to be considered in simulated distillation by GC: homogeneity of unit detector response across the chromatogram, and the fact that FID response is in weight percent, while true distillation is quoted in volume percent (10). A review of previous work on simulated distillation by GC has been published recently (11). This paper describes a novel treatment of the base-line correction using a mathematical approach for constructing an S-shaped base line rather than a straight line and the use of vacuum thermogravimetry for the determination of nondistillable residuum. Further, retention time scale conversion to boiling point scale for highly aromatic materials and detector response measurements are briefly discussed.
EXPERIMENTAL SECTION Gas chromatographic measurements were done on a Hewlett-Packard Model 5880 instrument with automatic injector, Model 7672, linked to a HP 85 desk-top computer via a RS 232 serial interface. The column was 16-ft, l/s-in. diameter stainless
steel tubing packed with 3% OV-101 on Chromosorb W (80/100 mesh). The carrier gm (nitrogen)flow rate was 20 mL/min. Other GC conditions were attenuation chart speed 0.2 cm/min, and slice width 0.1 min. The program started at 0 "C, a rate 10 "C/min, final temperature of 330 "C, and final analysis isothermal period of 5-40 min, depending on sample type. Normally, a 0.5-g sample was dissolved in 2 mL of carbon disulfide and 1 y L injected. TGA measurements were done by using Du Pont thermogravimetric analyzer, Model 951, and Du Pont 1090 data station. The beam of the balance was modified using a hot quartz rod (OmnithermCorp., No. 25036). System evacuationwas done with an Edward Model E2M8 mechanical pump and measured with an Edward Pirani head, Model PRCT10. The thermobalancewith a modified Pyrex bell jar was connectedto an oxygen-freenitrogen tank via a fine control valve, Edward LV5, and an oxygen trap. An on/off switch and a control valve were placed in the vacuum line for fast or controlled system evacuation. Finally, an Edward vacuum indicator and controller, Model 16, was connected to the 1090 data station for recording the vacuum profile (12). Sample (20-25 mg) was placed and evenly spread in a flat circular platinum container (9 mm X 1 mm) of approximately 65-mm2surface area (Omnitherm Corp., No. 25036-1) located at the pan holder at the end of the quartz rod. For further details see the Results and Discussion section. Normally, the vacuum used was of the order of 0.3-0.6 torr. Determination of atmospheric equivalent temperature (AET) = 538 "C (lo00 "F) was done by using the Maxwell-Bonnel equation (13). Percent residuum were determined as the percent remaining on the TGA pan at AET = 538 "C.
RESULTS AND DISCUSSION Linearization of GC Base Line. During the last decade, three major procedures have been developed to overcome problems associated with the nonlinearity of the base line (Figure 1). The first attempts used two identical columns and differential detector signal (11). In programmed temperature runs, it is extremely difficult to construct two columns with identical resistance profiles over the whole programmed temperature range, and since in the actual run the sample is brought onto one of the two columns only, septum bleed cannot be completely eliminated. The second approach is based on the idea that the base line during an experimental run will be very similar to or identical with that of an actual run (14, 15). Thus, storing the data for the base line from the blank run point by point and subtracting it from the actual run should eliminate base-line contribution to the results of the run. However, detailed analysis of a number of base lines reveals that they are neither parallel nor related to one another by any simple transformations about either axis (16). The third approach consists of constructing a linear transition from a point of minimum response a t the beginning of the chromatogram to a point of minimum response at the end of the chromatogram and relatine ., the area to this base line (16-19) (Figure i). Thus, as in the two-column approach, neither of the other base-line treatments provide a solution toward complete elimination of the drift of a "floating" base line in the area-slice mode of data treatment but only alter the magnitude of this
0003-2700/85/0357-0303$01.50/00 1984 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 1, JANUARY 1985
m
400-
-
5
Point o f Maximium Gradient
m
I
c 200-
0"
-
0
- - - - - - - - _ - - - - _ _ __ I
I
_
I I
I
Flgure 1. Schematic representation of "slice" integration and influence of baseline nonlinearity; EBL, electronlc base line; BL, chromatogram base line: B, true base line at end of chromatogram: AD (AC), usual base-line correction.
effect. Therefore, Abbot (16), in his effort to improve the precision of simulated distillation, used only 2% coating and recommended discarding columns where the base-line drift was more than 1% . The novel approach described in this paper is based on the direct use of the base-line information from the actual experimental run. In general, there are only two sections of the base line where information concerning its course can be obtained: the immediate beginning of the chromatogram and its very end. Mathematically, a mere two points uniquely define a straight line only, whereas an infinite number of curves can be accommodated between the two points. The actual base line is, however, nonlinear, and in order to obtain sufficient information, a numerical analysis was undertaken, which shows that the equation no. of counts = Ao(l
+ exp(Bo + BIT + B2T2))
is a satisfactory model, where Ao,Bo,B1,and B2 are run specific constants and T i s the actual column temperature in "C. The column temperature can be related to the run time (retention time) via the oven temperature program rate, the initial oven temperature, and a series of constants which are column and carrier gas flow dependent. However, since the carrier gas flow rate is determined only once for a given column and remains fixed thereafter, the dependence on carrier gas flow rate can be ignored. The various constants required to relate oven temperature to column temperature need to be determined only once. Since the above equation contains four constants (Ao,Bo, B1,and B,) which are run specific, while the experimental base line is unambiguously defined at only two points, either additional information has to be acquired about the base line or the number of constants has to be reduced or both. If the linear term BIT is removed from the previous equation, little additional error is introduced, while the equation acquires the form no. of counts = Ao(l
+ exp(Bo + B2P))
The point of maximum gradient of the base line (Figure 2) always occurs at the instant the oven temperature changes from linearly programmed to isothermal, and the ratio of the height of the curve at this point to the total height as measured from the initial base line remains virtually constant for a given set of operating conditions. Thus, this ratio can be used to define the third point of the base line and uniquely define the constants Ao, B1,and Bz. If the influence of the program rate is to be considered,the time variable ( t )is best converted to To,the oven temperature
at that time. Since at the beginning and in the final isothermal period the column temperature equals that of the oven, column temperature T, can be used instead. In order to establish the relationship between Toand T,, the base-line profiles were fitted by using an iterative procedure to no. of counts = Ao(l
+ exp(Bo + B2(T- AT)2))
w h e r e T = O , A T = O , T # O , A T # O , a n d T , = T-ATsuch that the fiial counts for the run corresponded to the final oven temperature. In order to obtain this fit, three points on the curve were arbitrarily chosen, corresponding to 0,300 (point of maximum gradient), and 330 O C . In the first interative cycle, AT was set equal 0, while counts at the end of the run corresponded to an oven temperature of 330 OC. The results are summarized for various program rates in Table I and show the existence of a linear relationship between program rate and column temperature lag. When the constants Ao,Bo,and B2calculated above were used the difference between the oven and column temperatures could be calculated as a function of A t , Le., the time difference between run time and the run time corresponding to the moment when the oven became isothermal. The apparent differences between the oven and column temperature are listed in Table 11, along with the corresponding At values for the final isothermal section of the chromatogram which must be long enough for the actual base line to get established and which, therefore, depends on the sample type and size. Figure 3 shows that these two data sets belong to the same curve and that one is simply displaced in the At direction with respect to the other. Numerical analysis of the family of curves generated under identical conditions, except for temperature program rate (varied from 5 to 25 OC/min), revealed that a reasonable fit can be obtained by using
T = exp(Al
+ A , ( A t ) + A3(At)')
where A T equals the apparent column and oven temperature difference, At equals the time after the point of maximum temperature gradient, and A,, A2, and A3 are constants for the family of curves. It was mentioned above that three sets of points are needed for the computation of the base line from an experimentalrun,
ANALYTICAL CHEMISTRY, VOL. 57, NO. 1, JANUARY 1985
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400T
Table 11. "Time Lag" between Oven and Column Temperatures at Two Different Program Rates 10 O C min-'
25 "C m i d
At, min
AT,
AT,
OC
At, min
0.01 0.11 0.21 0.31 0.41 0.51 0.61 0.71 0.81 0.91 1.01 1.51 2.01 2.51 3.01 3.51 4.01 4.51
14.96 12.34 9.99 8.09 6.65 5.55 4.73 4.11 3.64 3.26 2.80 2.04 1.42 0.97 0.64 0.40 0.24 0.08
0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50
6.02 4.95 4.03 3.30 2.72 2.28 1.93 1.66 1.46 1.29 1.17 0.77 0.53 0.35 0.24 0.15 0.08 0.02
"C
200-L
C 0
0
,0--
-
Corrected I
I
I
I
I
J
I
Table 111. Ratio of the Number of Counts Corresponding to the Point of Maximum Gradient the Number of Counts at Maximum Base-Line Value at Different Program Rates program rate
y m g / ymaa
esd
equiv, AT
2.5 5.0 7.5 10.0 12.5 15.0 17.5 20.0 22.5 25.0
0.955 0.912 0.871 0.832 0.795 0.760 0.726 0.694 0.664 0.635
0.003 0.005 0.007 0.008 0.010 0.011 0.012 0.014 0.015 0.016
1.51 3.01 4.52 6.02 7.53 9.03 10.54 12.04 13.55 15.05
Program Rate 2 5 ' C min-
G
1
Program Rate I O ' C min-' A t = At + . 5
"
o
, 0
'
l
'
*C.$
l
At
;
(min.)
iT'.
m .
m,
5
Flgure 3. Influence of temperature program rate on the time dependence of the column temperature lag.
so that the values of all three constants can be calculated. The first two values are defined by the front and rear sections of the chromatogram. The third value can be calculated using the fact that the ratio of Y(max gradient)/Y(max) is virtually constant over the range of program rates up to 10 OC/min; see Table 111. Fer this program rate change, the ratio changed only by about 1%while at higher rates the change in the ratio is more pronounced.
Figure 4 shows the base line corrected using values of constants A,, A2, and A3 calculated as AI = 0.211151, A2 = -1.71178, and A3 = 0.7997027, by using the above equation. Finally, the influence of base line on the last 10% off and final boiling point temperature (FBP) is shown in Table IV, summarizing averages of results from three runs. The table clearly shows that base-line correction before the run leads to too low FBP, while the use of base-line correction a t the end of the run gives a very high value for FBP. The method described in this paper (ARC method in Table IV) gives the closest correspondence between the actual and measured value, and also the precision at this point has been considerably improved. Vacuum Thermogravimetry. Thermogravimetry is a method ideally suited for the measurement of sample weight changes in dependence on temperature changes. However, in order to determine distillation residuum by this method, a number of precautions have to be observed in order to obtain reliable results. The most obvious one is that in thermogravimetry at atmospheric pressure, true sample distillation cannot be distinguished in a straightforward manner from sample decomposition, and fossil fuels are known to undergo considerable decomposition sometimes a t temperatures as low as 250 "C, nearly always above 300 OC. Therefore, conversion of the TGA instrument to a configuration allowing vacuum runs has been undertaken, as described in the Experimental Section. There are, however, other, less obvious, problems connected with vacuum TGA which will be discussed to some detail. One of the major problems can be caused by inappropriate sample size and its distribution on the balance pan. With samples exceeding about 30 mg and a t pan areas of 65 mm2,the material tends to sputter out of the pan, once vapor pressure had built up in the still relatively viscous material. Introduction of an isothermal step in the heating program in the region of most likely fast evolution of vapors did not prove either practical or satisfactory. Keeping the sample size within 20-25 mg at the selected size of the pan and evenly distributing the sample in the pan and keeping the temperature program rate at relatively low values (typically 10 OC/min) helped overcome the problem. Also, low heating rates allow a satisfactory heat
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 1, JANUARY 1985
Table V. Repeatability of Vacuum TGA Residuum Determinationn % residue, (at AET = 538 “C)
% gas oil (at AET of, macrodistillation)b
1 2 3 4 5 6 7 8 9
56.1 54.4 55.4 52.7 53.5 55.9 54.1 54.9 53.9
35.3 31.3 31.7 31.5 37.0 32.5 28.8 32.1 30.8
av
54.5
32.3
1.1
2.3
run
std 0
0.2
0.4
0.6
0.8
1.0
Vacuum Level (mmHG)
Flgure 5. Influence of vacuum level on accuracy of AET determina-
tion. transfer and eliminate sample cooling due to the endothermic nature of sample vaporization. The separation of the thermocouple from the sample with concomitant measurement lag also requires a slow-temperature program in order that the measured temperature approaches sample temperature as close as practicable. Similar limitations also influence the practically usable vacuum range. We have found experimentally that if the vacuum applied is under 0.3 mmHg, the temperature conversion to AET tends to be prone to larger error (Figure 5). Another complicating factor in vacuum measurements with electrobalance is base-line drift caused by changes in buoyancy of the balance beam. The apparent increase in weight under these conditions can amount to 0.050-0.200 mg. Since container volume and shape, heating rate, and vacuum level contribute to base-line drift, the drift has to be determined for each new set of experimental conditions. The apparent weight gain can then be corrected for by using the “file modification” program available with the 1090 Du Pont data station. It was also observed that the small gas bleed on the right-hand side of the balance influences the weight drift. Practically, the increase is negligible at a vacuum level lower than 0.1 mmHg and amounts to about 0.050 mg at 0.3-0.6 mmHg. This drift can be corrected with the fine suppression knob at the beginning of the run. In order to eliminate errors in vacuum measurement, controlled gas bleed, strong vacuum pump (180 L/min), and location of the Pirani vacuum head as close to the sample container a possible were the corrective measures introduced. Another practical problem is the possibility of condensation of volatile material on the cooler parts of the electrobalance, which is a gradual process taking place in spite of the small gas bleed used. The repeatability of TGA measurements under the above outlined conditions is demonstrated by Tables V and VI using atmospheric distillation residua from coal liquefaction. The results clearly show the suitability of this method of residuum determination which does not need any additional corrections, e.g., for hang-up, and also the conversion of temperature to AET should be more accurate, owing to a more accurate vacuum measurement close to the sample location not easily performed in conventional distillation. Figure 6 shows a thermogram of vacuum determination of AET = 538 O C residuum, in a sample of atmosphericresiduum from a coal liquefaction experiment followed by the deter-
“Conditions: sample type, ADR of run 54D; sample size, 5-8 mg; container type, flat AL pan (28 mm2, 0.5 mm); and vacuum level, 0.3-0.9 mmHg. % gas oil by macrodistillation = 39.2%.
Table VI. Repeatability of Vacuum TGA Residuum Determination” % residue (at 525 OC = AET)
% gas oil (at AET of
2 3 4
49.0 49.5 49.8 49.6
25.6c 35.0 33.2 30
av
49.5
31
std
0.3
run 1
macro distillation)b
1.5
nsample type, ADR run 52D; sample size, 5-8 mg; container type, flat AL pan (28 mm2, 0.5 mm); and vacuum level, 0.3-0.4 mmHg. % gas oil by macrodistillation = 35.6%. cLarge sample (about 12 mg).
*
THERMOGRAVIMETRY OF COAL LIOUEFACTION PRODUCTS TOTAL DISTILLABLES R E S I D U E CHARACTERIZATION BYVACUUMTGA BY ATM PRESSURE TGA 120 1000
800 600 400
$
H
k E F
200
20
0 0
10
20
30
40
50
60
70
80
90
100 110
Time ( min. 1
Figure 6. Determination of nondistillable reslduum using vacuum TGA ( % residuum measured at 48 min) (then vacuum released, nondistillable, thermally strippable material (50-60 min), nonreactive C (step
at 63 min), and ash (65 min) determined).
mination of residual, thermally strippable matter, unstrippable carbon, and ash. The first part of the thermogram was run under conditions described in the Experimental Section. This was followed by an isothermal period of 3 min, during which the vacuum was released. The residual material was then heated rapidly to about 940 O C , and once the loss of material subsided, the “fixed carbon” was burned off in a stream of oxygen to determine ash. The thermogram shows that additional material would have been counted as “distillables” in atmospheric TGA, while 6.9% of the material represented “fixed carbon”. Finally, ash content determined by TGA was
ANALYTICAL CHEMISTRY, VOL. 57, NO. 1, JANUARY 1985
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~
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CURVE 1 - 36 MG
34
~
- 284
I
I
Table VII. Comparison of Literature Boiling Points and Boiling Points Calculated from a Run Using Straight-Chain Hydrocarbon Standards compound
30
32
34
36
38
40
42
44
46
48
50
256
Time ( min. )
218 360 294 340 375 393 410 448 480 495
naphthalene biphenyl fluorene anthracene fluoranthene pyrene methylpyrene chrysene benzofluoranthene benzopyrene
22
28
lit. bp,
a
Average of 10 runs.
O C
calcd bp,”
O C
207.0 (0.6)* 360.8 (0.2) 278.1 (0.2) 310.6 (0.3) 351.2 (0.6) 356.8 (0.3) 367.9 (0.3) 394.3 (0.3) 425.7 (0.3) 432.5 (0.2)
Estimated standard deviation.
Figure 7. Influence of sample size on the determination of resMuum by vacuum TGA.
7.9%, while ASTM ash analysis gave 8.0% ash. The isothermal part of the program between 30 and 50 min was kept at a temperature corresponding to AET = 538 “C at the vacuum level of the experiment. We have observed that measuring the actual vacuum and setting the final run temperature to a value which converts to AET = 538 “C is the simplest way to determine the “vacuum residuum” by this method. Unlike in true distillation, the vapors of distillable material are being removed from the sample much more effectively (large surface area, gas bleed, no apparatus holdup) and the result is only very slightly dependent on sample size (which always has to be considered as a variable in TGA) as shown in Figure 7, representing the isothermal part of the thermogram of Figure 6, and also very little dependent on the time at which the reading is taken. In the two runs shown in Figure 7 about a 50% increase in sample size resulted in only about a 0.25% change in the estimated weight of the residuum. In general, the introduction of the isothermal period a t the end of the vacuum run minimizes the effect of sample size in relation to the overall method repeatability. Boiling Point Scale and Unit Detector Response. For nonpolar compounds, e.g., hydrocarbons, and on a nonpolar stationary phase, gas chromatographic retention times are proportional to the boiling points. Thus, a series of nonpolar compounds of known boiling points, mostly straight chain paraffins, has been repeatedly used as standards for the conversion of retention time scale to a boiling point scale. In a programmed run, the retention time intervals between the members of a homologous series are approximatelyequal, and, in addition to that, straight chain paraffins have the longest retention time of all hydrocarbons of the same carbon number. For mixtures containing large proportions of polycondensed aromatic hydrocarbons, e.g., anthracene oil, the boiling points are underestimated if straight chain paraffins are used as boiling point standards. The boiling point deviation from actual boiling points on the OV-101 phase is shown for a few polycyclic aromatic hydrocarbons (PAH) in Table VI1 (see also ref 8, ASTM D-2887Table A2). In spite of that, an excellent agreement of D86 distillation and simulated distillation by GC using a straight-chain hydrocarbon scale has been reported for coal liquids (20). On the other hand, Hickerson (17)has shown that for a series of methylbenzenes ranging from toluene to durene, the maximum difference of boiling points found was 3 “C. Thus, in the characterization of liquids derived from liquefaction experiments using such solvents as anthracene oil, an “aromatic”series of boiling point standards should be used for all liquids whose boiling points extend beyond about 200 “C. However, if the reaction products under study have been hydrogenated to a larger extent, even this scale will deviate from the actual boiling point distribution. Therefore, another
Table VIII. Relative Response Factors Measured in the Present System compound naphthalene biphenyl fluorene phenanthrene fluoranthene pyrene chrysene benzopyrene
bp,
O C
218 260 294 339 375 393 448 495
re1 response factof
esd
1.0 0.96 0.97 0.97 1.04 0.87b 0.95 0.87c
0.02 0.01 0.01 0.02 0.02 0.02 0.03
Average of 12 runs. Unresolved peak. Relative response factor on unresolved peak 1.11(2). ‘Relative response factor concentration deoendent.
approach is under study, based on the fact that the “aromatic” and the “paraffin” scales are the two extremes. A mixture of polycyclic aromatic hydrocarbons without side chains has aromaticity f(A) = 1,while the paraffin series has f(A) = 0. We know that in such products the compound distribution is essentially a statistical one. Thus, if the aromaticity of the liquid is determined by an independent method, e.g., 13C NMR, and the actual boiling point distribution is measured by distillation, we should be able to generate a family of calibration curves between the two extremes and use the one arrived at from aromaticity measurements. However, for the time being, we have been using the mixtures based on anthracene oil a simple conversion formula, TBP = CBP X 1.26 - 55.15, calculated from the retention times of PAH’s and paraffins as measured in our system. In principle and for long term studies, the boiling point scale CM be calibrated by the solvent used in the liquefaction experiments, by performing good distillation of the solvent and using the fractions obtained for chromatogram calibration. It has been repeatedly shown (21-26) for various classes of hydrocarbons that the flame ionization detector response across a homologous series is not uniform, but the relative response varies by up to several percent. This is particularly true for distillable components containingheteroatoms. Since insufficient information is available as to how the detector unit response depends on detector configuration, i.e., from one manufacturer to the next, we have measured the relative response of a series of aromatic hydrocarbons in our system. The results are summarized in Table VIII (compare with the data of Witier (21)),where the response for naphthalene was set equal to 1. The table suggests that the percentage of componentsat the high boiling point end of the chromatogram might be underestimated. On the other hand, the relative response will be somewhat influenced by the presence of PAH with a paraffinic side chain. Thus, for our purposes, a homogeneous relative response was assumed to exist across the
308
ANALYTICAL CHEMISTRY, VOL. 57, NO. 1, JANUARY 1985 ANTHRACENE OIL
SAMPLE 528-83
%Off
loor 80 70 c
-
B
8 60-
5040-
30 20 *
200
1
I
I
I
300
400
500
600
0.0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95
Temp OC
erd O C
208.2 251.2 266.5 287.2 297.7 299.8 301.6 304.1 315.3 328.1 338.3 241.1 345 349 359.8 376.4 385 398.8 419.1 446
3.4 6 2 4 3 6 6 6 1 1 4 7 7 5 4 3 4 5 3 1.9
Temperature ( OC J
Flgure 8. Boiling point distribution by GC, of anthracene oil.
whole boiling point range. An approach to the recalculation of weight percent data to volume percent data was demonstrated previously (11). Influence of Sample. Sample quality may obviously play a major role in the analysis. First, samples from liquefaction experiments may contain solids and varying proportions of nondistillable components. If this is the case, utmost attention has to be paid to sample homogeneity. Second, another possible problem may arise with samples which partially solidify a t room temperature. Such samples have to be dissolved in a small volume of suitable solvent. Carbon disulfide was the solvent of choice due to its negligible FID response (24). In addition to that carbon disulfide is the solvent of choice because it completely dissolves asphaltenes and nearly completely pyridine preasphaltenes of such materials as pyridine extracts. Also, sample size plays a role in reproducibility of the experiment (16),mainly due to two causes. First, only the linear FID response range should be used which is relatively easy with bitumen-derived materials, since they are rich in the number of components and the chromatogram acquires the shape of an overall envelope more or less statistically distributed across the chromatogram. With coal-derived liquids, especially those stemming from experiments using anthracene oil, the chromatogram not only consists of a relatively small number of individual peaks, but also there is a preponderance of material in the phenanthrene/anthracene range concentrated into only four or five major peaks. The maximum amount of an individual component should not exceed 120 pg in the Hewlett-Packard instrument (27). Already Hickerson (17) pointed out possible detector overloading for very narrow distillation cuts. McTaggart et al. (28) have demonstrated that in the internal standard method, the amount of standard should not exceed 5% per peak. Therefore, addition of a four-component standard was used in order to increase the relative proportion of standard to sample to about 15% total and thereby reduce the calculation error. Worman and Green (29)extended the method for samples boiling up to 538 "C and were using n-octane in one run for samples with IBP greater than about 150 OC and two runs for full-range samples. The use of vacuum TGA for the determination of residuum obviates the problem of pogsible column overload by the internal standard in cases where internal standardization was imperative. Thus, for each new type of material, care must be taken that signal intensities remain within the linear response range of the detector. In the system described here, 1 MLof 20% or more dilute solution gave satisfactory results. An example of actual analysis is shown in Figure 8.
Second, with samples containing nondistillable material, large sample quantities affect signal return to the actual base line at the high-temperatureend of the chromatogram, thereby introducing an uncertainty in base-line determination (16). The sample size and concentration suggested above seem to be a suitable compromise. Column protection using a short precolumn packed with the same packing material as the main column but with a higher loading of stationary phase is advisable to protect the column from unwanted material and also to improve base-line stability from run to run. CONCLUSIONS It has been shown that a model can be developed approximating the base line even through its curved section and that the base line can be determined directly from the experiment and gives results which are not overestimated at the high boiling point end of the chromatogram. The introduction of vacuum TGA as an alternative method to internal standardization aids to method flexibility for liquids containing nondistillable components provided that precautions are taken to maintain sample homogeneity and eliminated problems with the choice of standards which, for coal-derived liquids (unlike for bitumens), may prove difficult and will vary from case to case. The new method has been successfully used for the simulated distillation of a number of liquids produced in liquefaction experiments carried out in our department. LITERATURE CITED (1) Eggertsen, F. T.; Groennings, S.; Holst, J. J. Anal. Chem. 1980, 32, 904. (2) Green, L. E.; Schmauch, L. J.; Worman, J. C. Anal. Chem. 1984, 3 6 , 1512. (3) Petrocelli, J. A,; Puznlak, T. J.; Clark, R. 0.Anal. Chem. 1984, 3 6 , 1008. (4) "Annual Book of ASTM Standards"; American Society for Testing and Materials: Philadelphia, PA, 1983; Voi. 05.01, pp 8-27. (5) Reference 4, pp 603-615. (8) Reference 4, Vol. 05.02, pp 813-850. (7) Reference 4, Vol. 05.03, pp 442-455. (8) Reference 4, Voi. 05.02, pp 791-799. (9) Reference 4, 1976, Vol. 25, pp 716-722. (10) Gouw, T. H.; Hinkins, R. L.; Jentoft, R. E. J . Chromatogr. 1967, 28, 209. (1 1) Butler, R. D. In "Chromatography In Petroleum Analysis"; Altgelt, K. H., Gouw, T. H., Eds.; Marcel Dekker: New York, 1979. (12) Du Pont "Thermal Analysls Applications", Memorandum 27, May 1983. (13) Maxwell, J. B.; Bonnell, L. S. Ind. Eng. Chem. 1957, 49, 1187. (14) Hewlett-Packard, Simulated Dlstillation 189OOC, Option 840, April 1981. (15) Perkin-Elmer, DHSN-1028, D-2887 Sigma Basic Program for Simulated Distillation, April 1980. (16) Abbott, D. J. J . Chromatogr. Sci. 1983, 21, 425. (17) Hickerson, J. F. In Calculation of Physical Properties of Petroleum Products from Gas Chromatographic Analysls"; American Society for Testing and Materials: STP 577, pp 71-80. (18) Petroff, N.; Collin, J. M.; Feillens, N.; Foliain, C. Rev. Inst. Pet. (France) 1961, 36(4), 467. (19) Mikkelsen, L.; Green, L. E. J . Chromatogr. Scl. 1976, 14, 190. (20) Panneil, R. B.; Sood, A. J . Chromatogr. Sci. 1982, 2 0 , 433. (21) Witier, P. R o c . Congr. FATIPEC-CISTI, 16th 1982, 1 , 301-320. (22) Dletz, W. A. J . Gas Chromatgr. 1987, 5 , 68. (23) Drushel, H. J . Chromatogr. sci. 1983, 2 1 , 376. (24) Ceballo, C.; Murgia, E. Rev. Tec. Intevep. 1983, 3 , 35-45. (25) Dewar, R. A. J . Chromatogr. 1981, 6 , 312. (26) Halnova, 0.;Bocek, P.; Novak, J.; Janak, J. J . Gas Chromafogr. 1967, 5 , 401. (27) Green, L. Hewlett-Packard, Application Note GC 2-73, p 3. (28) McTaggart, N. G.; Glaysher, P.; Harding, A. F. "Correlation of Slmuiated True Boiling Point Curves by Gas-Llquid Chromatography Distillation Data on Crude Oils"; Amerlcan Society for Testlng and Materials: Philadelphla, PA, 1975; ASTM Method D2892-73 pp 81-94. (29) Worman, J. C.; Green, L. E. Anal. Chem. 1985, 3 7 , 1620.
RECEIVED for review June 4,1984. Accepted October 1,1984. This work was supported by the Alberta Research Council and the AlbertaICanada Energy Resources Research Fund, administered by Alberta Energy and Natural Resources.
