Gradient Elution Chromatography Using Ultraviolet Monitors in the Analytical Fractionation of Heavy Petroleums William R. Middleton Research Department, Mobil Research and Development Corporation, Paulsboro, N . J. An analytical procedure using liquid-solid gradient elution chromatography over alumina is described for the separation of any petroleum having an initial boiling point above 250° C. The novel introduction of the 0.1- to 0.2-gram sample by preadsorption on part of the column packing permits the analysis of difficultly-soluble samples such as asphalts and gilsonites. Six molecular type fractions from saturates to asphaltenes are isolated. A layering technique is used to form the four-component gradient and to simplify the gradient elution procedure. The eluate is continuously monitored by ultraviolet absorptiometers which trace strip chart chromatograms showing optimum cut points of fractions. The alumina of precise activity and moisture content as required for reproducible separations is characterized as to physical properties and relative retention of pure compounds. A single separation requires about 6 manhours and shows an average repeatability of +Oms% by weight for the weighed fractions.
ALTHOUGH MANY PUBLICATIONS relate to the subject, an easily applied, meaningful analytical separation broadly applicable to all heavy petroleum stocks has yet to be reported. Several investigators (I-3), notably O’Donnel (4, have described separation schemes which have used combinations of several separation methods. Such combinations of methods are too expensive and time-consuming for general use. Liquid-solid adsorption chromatography has become recognized as the most useful individual method of separating heavy petroleums (3, 5-7). The fundamentals of the method have been thoroughly studied (8-10). A recent paper describes a detailed procedure for the separation of heavy petroleum distillates (5). The Chromatographic separations were followed by periodically removing a drop of eluate for analysis in a conventional ultraviolet spectrophotometer. The five chromatographic groups obtained were further analyzed by chemical and spectrographic methods. Efforts made to detail the cornposition of the fractions obtained in the present work by chemical and spectrographic methods were largely thwarted by their increased complexity, the high concentration of sulfur compounds, and the lack of adequate reference compounds in the 400 to 1000 molecular weight range. In the past, difficulties due to the asphaltenes present in asphaltic stocks have necessitated their removal prior to the
(1) L. R. Kleinschmidt,J.Res. Nurl. Bur. Std., 54,163 (1955). (2) Neal W. Furby, ANAL.CHEM., 22,876 (1950). (3) R. D. Schwartz and D. J. Brasseaux, Ibid.,30, 1999 (1958). (4) Gordon O’Donnell, Ibid.,23,894 (1951). (5) L. R. Snyder, Ibid.,37, 713 (1965). (6) Clarence Karr, Jr., W. D. Weatherford, Jr., and R. G. Capell, Ibid.,26,252 (1954). (7) B. J. Mair, W. J. Marculaitis, and F. D. Rossini, Ibid., 29,
92 (1957). (8) L. R. Snyder, J. Chromatog., 6, 22 (1961).
(9) Ibid.,16,55 (1964). (10) Ibid.,20,463 (1965).
chromatographic fractionation ( I , 11). Asphaltene removal, such as by precipitation with normal pentane, is time-consuming and can lead to changes in the composition of the petroleum samples in the presence of light and air (12). Efforts to eliminate this step were complicated by the reported interference of asphaltenes with the chromatography of the oily components (13). This was disputed later when interference was not found in a study of the determination of saturated hydrocarbons in heavy petroleum (11). Apparently the question of the possible association of aromatics components with asphaltenes was not resolved. Other events have aided the development of a broadly applicable analytical separation method, notably, the availability of ultraviolet absorptiometers ideal for detecting the appearance of various aromatic fractions and the development of improved techniques of gradient elution (14, 15). The use of monitoring instruments, gradient elution, and automatic fraction collectors adapts readily to automation. This paper describes a readily automated, relatively rapid, simple Chromatographic method of analysis which is easily applied to any heavy stock without prior asphaltene removal. The accuracy of the separation is promoted by the use of small samples. The experimental data show no detectable interferences from the asphaltic components. The method which separates up to seven major groups of components has been applied to several hundred stocks without difficulty. EXPERIMENTAL The essential components of the chromatographic apparatus are shown in Figures 1 and 2. (Column and fittings from Polytechnical Products eo., Box 151, Haddonfield, N. J. 08033). The mixing vessel with magnetic stirrer serves to smooth the gradation in composition between adjacent 100-ml increments of the eluent gradient. The flow from the chromatographic column passes in series through single quartz cells in each of two recording ultraviolet absorptiometers and thence to a conventional automatic fraction collector. The first absorptiometer, the LKB Uvicord (LKB Instruments, Inc., 12221 Parklawn Drive, Rockville, Md. 20852) in use prior to the second instrument, operates at the fixed wavelength of 253.7 mp. The second, the Vanguard Model 1056A Ultraviolet Analyzer (Vanguard Instruments Division, Technical Measurements Corp., 441 Washington Ave., North Haven, Conn. 06472) has the advantage of being readily dialed to operate at a selected wavelength in the range 200 to 420 mp. Merck reagent alumina (No. 71695 Aluminum oxide-acid washed, -100 to 300 mesh) was the adsorbent used. The original moisture content of 13.7% (loss when heated in a (11) L. R. Snyder and W. F. Roth, ANAL.CHEM., 36, 128 (1964). (12) W. E. Haines, Proc. Am. Petrol. Inst.,42 (VIII), 51 (1962). (13) P. Harnway, M. Cefola, and B. Nagy, ANAL.CHEM.,34, 43 (1962). (14) E. A. Peterson and H. A. Sober, Ibid.,31, 857 (1959). (15) Theodore Rosett, J . Chromatog., 18,498 (1965). VOL. 39, NO. 14, DECEMBER 1967
1839
12/3 O-RING
7)
ITION FUNNE,L
c
MAGNETIC
STIRRER
BUBBLE
TRAP
OMATOGRAPHIC COLUMN
ADAPTER (HELEX)
12/3 0 - R I
T E F L O N T U B I N G TO ABSORPTIOMETERS a FRACTION COLLECTOR
Figure 1. Gradient elution wth layering column muffle furnace at 750" C for 10 minutes) was adjusted to 10.9 =k 0.1 by heating in an oven at 115" C for 12 to 15 hours. Although not used routinely, Alcoa F-20 alumina was indicated to be equivalent to the Merck alumina when the moisture content was adjusted to -4.5 %. With either alumina the retention characteristics of new lots must be confirmed before use (see Discussion). The standardized alumina was prepared in large batches and stored in tightly sealed bottles. The eluent gradient components consisted of five reagent grade solvents which had no appreciable absorption in the UV between 250 and 410 mp. Normal pentane (99f mole %) and normal hexane (95 mole %) (Phillips Petroleum Co.), reagent grade dichloromethane (DCM), and tetrahydrofuran (THF) were percolated through silica gel (Davison Grade 12) and distilled prior to use. The THF was collected and stored under nitrogen in sealed brown reagent bottles to avoid peroxide formation. The eluting solvent system, increasing continuously in polarity and density, is simply called the eluent gradient or gradient. It was wholly prepared immediately before use by the layering technique of Rosett (15). Following an initial 125 rnl of unaltered normal hexane, nine different 100-ml increments were introduced consecutively at the bottom of the layering column. The composition of each of the nine increments is defined by the midpoints of the concentration curves of Figure 3. The filling of the layering column was completed by a final increment of dense displacing solvent (e.g., 20z DCM in methanol). The layering column consisted simply of a vertical cylinder of 1200-cc capacity lightly packed with stainless steel vacuum distillation packing to avoid convection mixing. The limited mixing at adjacent layer boundaries is smoothed into a continuous gradient by the subsequent flow through the 160-ml eluent mixer (Figure I). When in use, the gradient was displaced from the layering column by further displacing solvent supplied from the addition funnel. Sample Preparation by Preadsorption. Column plugging, channeling, and the failure to adsorb asphaltenes during the introduction of highly asphaltic samples was overcome by preadsorption of the small sample on a portion of the column packing. This method made it possible to consistently place the sample in a uniform band at the top of the column. The 0.1- to 0.2-gram sample was weighed and dissolved in 2:l DCM:cyclohexane. Samples weighing up to 2.0 grams were used in the case of waxes or other nonaromatic stocks
+
1840
e
ANALYTICAL CHEMISTRY
GLASS FRIT
MALE LUER - LOK (PYREX) Figure 2. Detail of chromatographic column in an effort to include 25 to 50 mg of aromatics. In these cases larger amounts of solvent and adsorbent were used. Normally, 3 k 0.2 grams of the column packing was added to the solution. This mixture was blown with nitrogen while swirling gently over a low-heat hot-plate. Solvent evaporation was completed by applying a moderate vacuum with continued swirling. Packing the Chromatographic Column. The bottom closure with Luer fitting was firmly attached and the column clamped in position. Merck alumina was steadily poured into the column during 15 to 20 seconds while energetically tapping
POLAR SOLVENTS ADDED TO NORMAL H E X A N E 0 A S E :
THF TETRAHYDROFURAN MeOH = ANHYDROUS M E T H A N O L
1
2
3
4
5
6
7
8
9
C O N S E C U T I V E 100 M L I N C R E M E N T S
Figure 3. Composition of eluent gradient
I
70
I 50
I
60
I
I
40
I
- TUBE
I
I
0
10
20
30
FRACTION KOLLECTOR
INDEX
SOFT R E S I N PEAK ASPH. PEPKS
300 m y
262 m p
I
ASPHALTENES
I
HARD 'RESIN RESIN
I SOFT RESIN
1
;i\L PI N A405
iI
my
F R A C T I O N CUT
M N A & DNA MNA
I
SATURATES
--I l,. oj START
POINTS
Figure 4. Chromatogram for kuwait residuum the column. A packed height of 15 cm was attained. Uniform but not maximum compaction was sought. The surface was levelled and packed with the large end of a cork conveniently mounted on a steel tube. The cork was sized to just fit the column and was vented through a small center hole. The previously prepared and weighed sample increment was then introduced. The surface of the sample increment was smoothed and uniformly compressed. This was protected by similarly placing a 1- to 2-cm-layer of the Merck alumina above it. The space above this final layer was then filled with clean, inert boiling chips (Alcoa tubular alumina, 12 to 14 mesh), leaving enough space for the Teflon adapter of the upper screwed closure. Before use, the column was flushed with clean dry nitrogen for a period up to one hour to remove oxygen from the packing. Chromatographic Fractionation. With the eluent mixing chamber in position and filled with 160 ml of normal pentane, the two absorptiometers were zeroed with reference to pentane. Nitrogen pressure to the addition funnel was adjusted to -20 mm of mercury and the chromatographic fractionation started by opening the stopcocks controlling the eluent flow. (The magnetic stirrer was not started until -100 ml of the pentane had entered the column.) During the wetting of the column packing, the displaced nitrogen was vented. Firm attachment of the Luer fitting was made with the appearance of the eluate. The fraction collector was started and 15-ml fractions were collected in the numbered test tubes. Correlation of the test tube numbers with event marking on the strip chart chromatograms fixed the location of the fraction cut points. Fractions were collected at the normal eluate flow of 25 to 30 tubes per hour at 20 to 25 mm of nitrogen pressure. Excepting for the occasional observation of this nitrogen pressure and the performance of the fraction collector, the chromatographic fractionation was automatic. Fraction Cut Points and Characteristic Properties. The six standardized fractions described and listed with abbreviations in Tables I, 11, and 111, were obtained by the discrete combining of individual 15-ml fractions. The grouping of the
70 odd fractions into six categories was based upon the appearance of the cut points described in Table I. These cut points are graphically illustrated by the chromatogram reproduced in Figure 4. With similar samples and a constant lot of alumina, the reproduction of the cut points are quite exact. Where there was some variation in molecular weight range and nature of the sample, there were slight variations in tube number at the cut points. Recovery of the Six Fractions. In most instances the residual samples chromatographed had initial boiling points above 250" C. This permitted the simple recovery of the fractions by evaporating the low boiling solvents on a warm plate with the aid of a jet of nitrogen. The noneluted asphaltenes, determined by difference from 100.0%, constituted a seventh, unrecovered fraction.
Table I. Final Cut Points for Six Standard Fractions Absorptiometer wave lengths, mfi Saturates NMA & DNAQ
254 and 262 254 and 300
PNA6
254 and 405
Soft resin Hard resin
405
Eluted asphaltc
254 and
254 and 405
405 a
Readout, optical density Sharp rise (Ped) Beginning of broad peak Sharp rise (peak) Inflection point Inflection point & peak color Return to minimum
Final tube number 10 to 12 24 to 26
41 to 43 45 to 47 59 to 61 68 to 72
Abbreviation of monoaromatics and noncondensed diaromatic
oil.
* Abbreviation of polynuclear aromatic oil. c
Abbreviation of eluted asphaltenes.
Table 11. Typical Physical Properties of Chromatographic Fractions of Petroleum Residua Kinematic viscosity Color Sp gr & 77" F & 210" F (cs) Moleculara weight Saturates Water white 0.87-0.90 650-950 20-50 MNA & DNA Near colorless 0.90-0.94 40-100 650-950 PNA Pale amber 0.94-0.99 150-250 600-900 Soft resin 600-900 Amber plus red 1.00-1$04 3000-8000 Hard resin Deep brown 1000-1500 1.02-1.07 Solid Eluted asphalt Nearly black 1200-2500 .. Solid a By vapor pressure osmometry using benzene solvent.
VOL. 39, NO. 14, DECEMBER 1967
0
1841
Table 111. Characteristic Elemental Composition of the MNA &DNAoil PNA oil Heavy Heavy Asphalts fuels Asphalts fuels
Saturates fraction Heavy Asphalts fuels -670 -600
Mol. Vb’t.Zb ~7.50 -650 -750 -700 Atomic ratio hydrogenlcarbon 1.888 1.907 1.709 1.690 1.553 1,526 lemental analysis, wt Hydrogen 13.58 13.73 12.23 12.24 10.97 10. 83 Carbon 85.58 85.77 85.28 86.29 84.12 84.53 Oxygend ... *.. 0.46 0.33 0.53 0.59 Nitrogen ... ... 0.06 0.05 0.11 0.05 Sulfur 0.21 0.07 1.73 1.15 4.04 3.64 a The individual chromatographic fractions (e.g., saturates) from comparable stocks were composited before analysis to provide adequate samples. By vapor pressure osmometry using benzene solvent. c Sample not fully soluble in benzene. d Direct oxygen by Unterzaucher method.
Table IV. Eluent Gradient Quality 1 2 Hexane, Hexane, ethyl ether, cyclohexane, cc4, HCCls, benzene, benzene, Gradient methanol methanol Separation Saltslaroniatics Good GOOd MNA & DNA/PNA Poor Good PNA/soft resin Poor Good Soft resin/hard resin ... Fair Hard resin/asphaltenes . .. Poor
3
Pentane, hexane, DCM, THF, methanol Good f Good Good moa Fair
RESULTS AND DISCUSSION
Gradient Elution. The advantages of the use of gradient elution in liquid-solid chromatography are analogous to those of temperature programming in gas chromatography. A distinct advantage of each of these two chromatographic techniques is that the later component bands have widths comparable to the earlier ones. An example of the capability of the gradient elution procedure reported here is shown in Figure 5 . Note the width, symmetry, and absence of tailing in the later peaks of the chromatogram (Peak A was dashed in to show the position and width of the cycloparaffin peak). The concept, theory and advantages of gradient elution as
applied to column chromatography has been amply documented by the literature (16-18). The extensive applications reported have rarely touched on the separation of petroleum residues (19). Recent papers (17, 20) point to an increased interest on the part of the petroleum industry. The exceptional capability of gradient elution to resolve the peak concentrations of varied components makes it ideal for the chromatographic separation of the exceeding complex residual petroleums. Eluent gradients empioyed with single beam UV absorptiometers should not absorb at the wavelength employed. Although a dozen or so common eluting solvents do not absorb strongly in the 253.4- to 410-mp range, useful eluent gradients are not immediately devised. Only the last of those gradients described in Table IV was reasonably satisfactory for the desired use. Generally speaking, each major fraction should be eluted by a similar volume of eluent to realize good separation. The elution of the sharply defined soft resin peak by the (16) E. Lederer and M. Lederer, “Chromatography” (2nd ed.), Elsevier, Amsterdam, 1951, p. 41. (11) L. R. Snyder, J . Chromatog., 13,415 (1964). (18) T. K. Lakshmanan and S . Lieberman, Arch. Biochem. Biphys., 53, 258 (1954). (19) W. K. Middleton, Div. Petrol. Chem. Preprints, 3, No. 2, A 4 5 (April 1958). (20) L. R. Snyder and M. D. Warren, J. Chromatog., 15, 344 (1964). I
I 20
I
FRACTION COLLECTOR -TUBE
IO INDEX
E
D
END
Figure 5. Chromatogram of six-component broad range mixture A . 1,3-Dicyclopentyl-2-dodecylcyc~opentane D . 1,1,2-Triphenylethane B. ~1-Pheny~~~ntadecene E. 9,10-Dimethyl-l,2-benzanthracene e. 2,6-Dioctylnaphthalene F. Benz(~u)anthracene-7,12-dione
16142
e
ANALYTICAL CHEMWRY
Chromatographic Fractions of Some Heavy Petroleums" Soft resin Thermal Heavy Asphalts asphalts fuels Asphalts -775 -1130 -500 -765 1.294 9.07 83.50 0.83
0.25 5.71
1.005 7.42 87.98 0.52 0.12 3.21
1.311 9.26 84.18 0.91 0.34 4.99
Hard resin Thermal asphalts -1400
1.174 8.20 83.18 1.31 0.73 5.13
preferred gradient may involve a selective displacing action by the THF-component of the gradient. Characteristics of Alumina. The characteristics of suitable chromatographic aluminas were sought when recently supplied Merck alumina proved to be of low activity. (Three early lots were selected as having standard activity for the separations reported here.) To further characterize the highactivity Merck alumina while the supply lasted, the retention data for varied pure compounds were determined as shown in Table V. Aluminas suitable for reproducing the separations here reported should yield retention data equivalent to that of Table V. (Retention volumes were measured to the peak centers.) Alcoa F-20 alumina has been widely reported in separations with rather long columns (5-7). In the present procedure, earlier tests of F-20 (moisture loss 7 . 6 z ) showed excessive flow rates and inferior saturate/monaromatic separation. Recent publications have provided detailed absorptivity data for F-20 at several activity levels (8, 21). The relative characteristics of F-20 and the Merck alumina are shown in Table VI. At 4 . 7 z moisture loss, the F-20 alumina shows retention volumes similar to those desired. (Because of the coarse mesh, the eluent flow through the F-20 should be controlled.) The surface areas of the F-20 and the preferred Merck alumina were equivalent at 250 i 15 sq M/gram. The value for the recent Merck alumina was lower at 200 i 30 sq M/gram. These surface areas were calculated by the BET theory (22) from the weight of nitrogen adsorbed at -196' C by the McBain-Bakr technique (23). Separation of a Known Blend. The standard cut points described in Table I were established partly by determining the peak locations for API 42 hydrocarbons and similar aromatic hydrocarbons. Mass spectrometry of incremental fractions helped pinpoint the optimum cut point between the first and second aromatic fractions. Finally a known blend was prepared including API 42 hydrocarbons as the waxy saturate and oil aromatic fractions. The soft and hard resin components were typical narrow fractions isolated from heavy petroleum by thermal diffusion and two-stage
(21) L. R. Snyder and B. E. Buell, J. Chem. Eng. Data, 11, 545 (1966). (22) S. Brunauer. P. H. Emmett. and E. Teller. J. Am. Chem. SOC.. . 60, 309 (1938): (23) J. W. McBain and A. M. Bakr, Ibid.,48,690 (1926).
Eluted asphaltenes Thermal Heavy Asphalts asphalts fuels -1710 . . .E -1500
Heavy fuels -loo0
0.9092
1.207
1.259
8.35 82.42 2.81 1.05 4.80
8.79 83.17 1.46 1.06 4.30
6.70 87.80 1.51 0.64 2.53
0.9696
1.292 8.93 82.33 3.01 1.27 4.49
6.98 85.77 2.33 0.83 3.21
Table V. Retention Data for Standard Activity Merck Alumina Retention, ml I. Saturates fraction
n-Tricosane Isoparaffins (-450 MW) 1,3-Dicyclopentyl-2-dodecyl cyclopentane Perhydropyrene
+
0 to 150 =k 15 8
~ 1 0 8 8
DNA oil fraction 151 to 1,2,3,4,5,6,7,8-0ctahydroanthracene 2-n-Butyl-1-hexylindane 1-Phenylpentadecane 2-(Ar)Decyltetralin 1,CDimethyl-2-(3,bdimethylocty1)-benzene Amylbiphenyl (tech) 1,l-Diphenyltetradecane
11. MNA
111. PNA oil fraction
n-Tetradecylnaphthalene 2,6-Dioctylnnphthalene Di-t-butylphenanthrene 1,1,2-Triphenylethane 1,1,4,4-Tetraphenyl-1,3butadiene IV. Soft resin fraction
9-Dodecylphenanthrene Dibenzothiophene Benzyl- 1-napht hyl ether 9,10-Dimethyl-1,2-benzanthracene
Relative retention
... 0.015 0.019 0.015;
0.015
...
360 =k 15 165 195 210 215
0.32
215 255
0.40 0.49 0.68
355
0.38
0.39 0.40
361 to 630 =k 15
...
520 & 540
0.82 0.83 0.96 1.00
575
1.11
631 to 690 I- 15 650 645 660
1.20 1.24 1.22
660
1.22
440
445 500
V. Hard resin fraction
691 to 870 =k 15 CAcetyl-o-terphenyl 730 Benz(c~)anthracene-7,12-dione 735
...
... 1.35 1.36
column chromatography. Table VII shows the results af analyzing the known blend. In addition to showing ace curacy of separation, the data were interpreted as indicating no interference from the asphaltic hard resin components. Separation in Presence of A~phaltenes, The distribution of component types during propane deasphalting was studied by the chromatographic analysis of the charge stocks, asphaltic tars (PD tars) and refined oils (PD rafFmates). This series of analyses provided the opportunity to check 661VOL. 39, NO. 14, DECEMBER 1967
I)
1
~-
~
Table VI.
-.-.-
Characteristics of Aluminas
2500
Merck No. 71695 Old preAlcoa F-20 ferred Current Preferred Lower stock stock activitv activity
10.9
11.0
5.5
5.8 200 i 30
250 f 15
Retention volume, ml 1-Phenylpentadecane 1,1,2-Triphenylethane Benzyl-1-naphthylether Benz(or)anthracene-7,12dione
4.70 0.67 +250
f
15-t
[z
102 455 635
87 409 597
720-735
731
713
39.1
KUWAIT 22.7% RESID.
W -I
0
=
1000
10.6 % RESID.
500
37.7 18.2 14.7
0 20
35.1
40
32.9
25.8
s
80
Figure 6. Moleculiar weight distribution in residua 26.5 2.0
0.0"
60
% WEIGHT
9.8 16.7
11.0 14.8
I
I
A'
A M A L 2 4 . 5 % RESID.
LBREGA
15.5
/
4
200-215 38 520-540 270 645-660 487
19.6
;
-I
Table VII. Separation of Known Blend Chromatographic Calculateda analysis, wt composition, 2 wt Saturates MNA & DNA PNA Total aromatic oil Soft resin Hard resin Total resins Oxygenated resins
/
1500
3 V
697
/
2000
6.24 2.50 =k
.-
-.-.-
Mesh Moisture content Loss, 750"/10 min, Loss, 400°/8 hr, Surface area, sq M/g
SATURATES M N A -b DNA O I L ______._-.. PMA O I L SOFT RESIN WARD R E S I N ASPHALT EME S
evidence of interference by asphaltenes. Such interference if present should lead to errors in the analysis of the asphaltic charge and product tar but not to similar errors in the nonasphaltic raffinates. Such a situation would lead to an un-
Making no allowance for small, likely amounts of oxidation products in the blend. a
Table VIII. Chromatographic Fraction Material Balance for Propane Deasphalting Charge us. Products West Texas sour residuum,. % wt Product Charge Raff ' Tar Saturates MNA & DNA PNA Soft resin Hard resin El. asph. Non-el. asph. a
b
14.6 7.7 8.0 9.5 1.7 1.o 0.8
16.6 10.3 12.2 31.9 12.8 10.5 5.7
1.9 2.0 4.7 21.0 11.o 11.8 4.3
z
Charge
16.5 9.7 12.7
31 .O 12.9 9.1 16.6 10.2 7.0 13.2
30.5
12.7 12.8 5.1
Amal residuum,b % w l Product Raff Tar 24.9 8.0 3.9 3.3 0.9 0.7 0.4
5.9 5.2 4.3 11.5 9.6 6.7 14.8
z 30.8 13.2 8.2 14.8 10.5 7.4 15.2
Sp. gr. @ 20" C = 1.009. Sp. gr. @ 20" C = 0.961. Table IX. Precision in Chromatographic Analysis of Heavy Petroleums Safaniya residuum Kuwait hvy vac gas oil 1.001
0.956 ... .. Sp gr, @ 20" C
Analysis, % weight Deviation Mean from mean ~
Saturates MNA & DNA PNA Soft resin Hard resin El. asph. Non eluted asphalt
44
e
1
2
30.9 18.0 22.7 22.9 2.7 2.6 0.2
30.9 17.6 22.0 23.2 2.9 2.6 0.8
ANALYTICAL CHEMISTRY
1
2
Mean
19.4 11.2 17.6 27.2 8.5 6.9 9.2
19.4 11.4 16.6 27.4 8.6 6.8 9.9
30.9 17.8 22.4 23.1 2.8 2.6
0.00
19.4
0.20 0.35 0.15 0.10
0.5
0.30
11.6 15.6 27.5 8.6 6.7 10.6
0.00
Deviation from mean 0.00 0.20 1.o 0.1.5 0.05
0.10 0.70
Saturates MNA & DNA PNA Soft resin Hard resin El. asph.c Non-el. asph.d a
Table X. Typical Analyses of Various Heavy Petroleum Stocks MidHydrogen Convencontinent No. 2 fuel Brega resid No. 6 fuel Asphaltic processed tional P. D. lube stocka lube stockb raffinate (32.9" API) (13.2" API) (7.9" API) pipe coating 28.8 61.0 24.8 21.0 96.1 86.0 52.4 13.1 5.9 15.2 4.4 3.0 9.7 16.9 11.8 12.9 18.1 8.9 0.2 3.0 12.0 24.6 14.3 25.5 4.0 0.4 0.9 12.5 0.8 12.2 11.8 10.6 0.2 0.1 2.8 10.3 4.3 13.7 0.9 0.1 0.4 2.3 12.0 23.0 6.1 ... ... ... 1.2
Aramco bitumen 6.5" API 9.4 9.9 10.9 27.2 10.6 10.4 21.6
Barber gilsonite 1.4 )1.5
2.3 6.1 45.1 43.6
API gravity 35.0 and VI 125 (nominal). API gravity 32.1 and VI 105 (nominal). Eluted asphaltenes and/or polar resins. Asphaltenes (or loss) by difference from 100.0%. Table XI. Chromatographic Analyses and Properties of Residua and Related PD Tars Mid-continent sweet West Texas sour {Kuwait(Arabia) Amal. (North Africa) Resid PD tar Resid PD tar Resid PD tar Resid PD tar 13.5 7.1 2.7 17.9 11.0 22.7 17.4 24.5 0.979 1. 0076250 1.0490250 1.017 1.072626" 1.020 1.0466260 0.959 ... 985. ... 1337. ... 1093. ... 414.1
Properties Vol % on crude Sp. gr. @, 20" C KV at 210" F Carbon res (CCR), % wt Pour point, F Soft pt., R & B, F Asphaltenes, wta Sulfur, % wt
z
O
Chromatographic analysis, wt Saturates MNA & DNA PNA Soft resin Hard resin El. asph. Non-el. asph. a
12.7 85.
...
7.4 1.19
23.5 19.9 8.9 25.6 10.6 6.1 5.4
24.9
167. 10.7 1.25
17.1 115. 111. 11.3 3.47
3.6 4.4 7.6 34.8 20.5 15.2 13.9
14.0 9.7 12.7 32.7 12 9 11.9 6.1
...
27.3
...
17.0
22.7
...
15.6 >115.
176. 20.8 3.84
105. 102. 10. 4.06
135. 15.1 4.24
0.34
1.9 3.1 7.9 40.5 20.7 17.4 8.5
9.2 8.7 15.0 37.9 12.5 9.3 7.4
3.1 4.7 12.0 41.5 15.6 13.4 9.7
31.0 12.9 9.1 16 6 10.2 7.0 13.2
... 8.0
15.7
...
199. 12.0
0.31
10.1
9.0 7.4 19.9 16.6 11.5 25.5
Precipitated by normal pentane.
balance in major raffinate components in the overall charge and recovery. The data of Table VIII show no evidence of unbalance, indicating no asphaltene interference, thus supporting the conclusion of Synder and Roth (11). The data also demonstrate the quantitative precision of the method. Precision and Repeatability. In addition to the data of Table VIII, the precision of the chromatographic analysis is demonstrated by Table IX. Accuracy and precision of the chromatographic procedure is enhanced by the use of the small samples (0.1 t o 0.2 gram), an approach to the linear elution discussed by Synder (8). It must be emphasized that, in addition to the quality of the separations, precision is directly related to the technique employed in recovering and weighing the eluted fractions. Analysis of Varied Stocks and Asphalts. The analysis of varied heavy petroleums and a gilsonite are shown in Table X. Evident is the usefulness of the method for stocks varying from 96% saturates to 89% asphaltenes. Additional data are presented in Tables XI and XII. The analysis of four residua and their propane deasphalting tars is given in Table XI. These commercial residua were produced according to the requirements of the refineries that made them. They bear no consistent relationship with one another.
Table XII. Composition of Typical Asphalts Paving Thermal Pipe Roofing coating asphalt grade Type s SP 119" F SP 162" F SP 219" F SP 235" F 2.0 Saturates 17.0 21.8 21.3 8.6 MNA &DNA 11.6 4.2 5.3 5.7 12.0 15.6 PNA 12.7 14.1 53.7 Soft resin 21.0 11.1 18.0 14.1 11.8 Hard resin 12.2 8.4 11.2 El. asph. 8.0 12.3 Non eluted asphalts 14.6 24.3 23.8 11.6 Total hard resins & 38.0 asph. 34.8 47.9 49.6 Although a variety of methods have been described ( I , 19, 24) for the separation of asphalts into fractions, none has gained broad acceptance. The present method has been found to easily separate asphalts into six reproducible fractions. The analyses of four typical asphalts are given in Table XII. The soft points of the asphalts are ordered according to their content of hard resins plus asphaltenes. (24) R. N. Traxler and H. E. Schweyer, Oil Gas J . , 52, 158 (1953). VOL. 39, NO. 14, DECEMBER 1967
0
1845
Table XIII. Thermal" SP 259" F
Asphaltene Content of Asphalts and Residua
Roofing grade SP 235" F
Pipe coating SP 219" F
Asphaltenes, % wt Precipitation from pentane+ 86.3; 90.0 38.0 35.1 Chromatographic: Eluted 13.3 11.2 12.3 Non eluted 8.4 24.3 23.8 a Contained 52.1 soft resins and 25.3 hard resins by chromatography. Modification of ASTM D893-60T.
Asphaltene Content of Residua and Asphalts. The conventional method of determining asphaltenes as pentane insolubles does not reliably measure the true asphaltenes. Properly, asphaltenes are those molecules, as described by Ferris, Black, and Glelland ( E ) which , consist of two or more chemically bonded units, each unit equivalent to a typical resin structure. Qccasionally a portion of the pentane insolubles may be from the lower molecular weight range characteristic of the resin fractions. This is particularly true of residua which have undergone severe thermal treatment. With such stocks, the chromatographic separation gives a better measure of asphaltene content as illustrated in the first column of Table XIEI. In some asphalts, such as the pipe coating and roofing grade (both air-blown), there is good agreement between the pentane precipitated and chromatographic asphaltenes. The data in the last three columns of Table XI11 show that n-pentane does not precipitate all of the asphaltenes from some straight run asphalts and residua. The molecular weight data for the chromatographic asphaltenes consistently showed values at least twice the average molecular weight of the resin components. Molecular Weight Distribution between Fractions. The availability of the small chromatographic fractions provided the opportunity to develop further information about the (25) S. W. Ferris, E. P. Black, and J. B. Clelland, Din. Petrol. Clzem. Preprints, 11, No. 2, 0-130 (March 1966).
e
ANALYTKAL CHEMISTRY
Paving asphalt SP 119O F
West Texas sour residuum 7.6 API gr
Midcontinent residuum 17.8 API gr
16.4
11.3
1.1
8.0 14.6
11.9 6.1
7.0 2.9
fractions and the original petroleums. Molecular weights were readily determined by use of the Mecrolab Osmometer. An informative molecular weight distribution curve resulted when the values were plotted against the midpoint of the accumulative weight eluted from the column. Thus the molecular weights of the individual fractions were distributed from left to right in the order eluted. Such distribution curves for three residua are shown in Figure 6. Note the unexpected maxima and minima in the Amal and Brega distributions. The curves, extrapolated toward the 100 point, indicate asphaltene molecular weight distributions similar to those based on gel permeation chromatography as reported by Altgelt (26). ACKNOWLEDGMENT
The author is grateful to Michael Grennert for technical assistance and to F. P. Hochgesang for reviewing the manuscript. The use of numerous pure hydrocarbons supplied by American Petroleum Institute Research Project 42 is acknowledged.
RECEIVED for review June 8, 1967. Accepted August 22, 1967. Division of Petroleum Chemistry, 153rd Meeting, ACS, Miami Beach, Fla., April 1967. (26) K. N.Altgelt, J. Appl. Polymer Sci., 9, 3389 (1965).
