ANALYTICAL CHEMISTRY, VOL 50, NO 9, AUGUST 1978
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fusion under condition I did not appear to prevent reduction. These results illustrate the importance of maintaining proper oxidizing flames for accurate sample preparation by fusion. It has been suggested that a nickel crucible of the same size as the platinum crucible be used in an identical position in the flame t o determine the oxidizing or reducing conditions of t h e flame. We found that the use of a nickel crucible for determining these conditions was not always satisfactory. This work suggests t h a t observation of the color of a few fused samples of mixtures containing calcium and iron may serve as a more sensitive test of flame conditions.
F
LITERATURE CITED 02
04
06
08
10
12
14
16
18
20
22
X IC 8n dtrk
Figure 2. Effect of burner conditions on iron in standard disks. (A) Condition I. ( 0 )Condition I 1
and opaque. At lower iron concentrations, the disks exhibited a greenish blue coloration in contrast to the yellowish brown obtained with disks having t h e same iron content prepared prior to under condition 11. T h e addition of more “,NO3
(1) P A Pella. R. H Mvklebust. M M Darr. and K F J Heinrich. N B S I R ’ 77-1211, June 197:. (2) R. L. Myklebust. C. E. Fiori. D. N. Breiter, and K. F. J. Heinrich, FACSS Abstracts (3rd Annual Meeting) N o . 241, Philadelphia, Pa., 1976. (3) C. 0. Ingamells, Anal. Chim. Acta, 52, 323 (1970).
P.A. Pella National Bureau of Standards Washington, D.C. 20234
RECEIVED for review February 12, 1978. Accepted April 13, 1978.
Solvent Extraction of Coal-Derived Products Sir: T h e recent interest in separating and characterizing coal-derived oils has revealed an intriguing problem. There is, a t present, no standard method or technique that is widely used to separate and characterize liquefied or solvent refined coal products. Most investigators in the field, and those just getting started, are however using a host of similar terms to define dissimilar fractions from coal products. For example, the word asphaltene(s), as it pertains to coal-derived products, can mean a solvent separated material t h a t is benzene or toluene soluble and pentane insoluble ( I ) , or hexane insoluble (21, or cyclohexane insoluble (3) or even derived by specific mixtures of toluene and pentane ( 4 ) . Beside all of these differences, there are a host of techniques being used t o characterize coal-derived products by solubility distribution analysis (5,6),chromatographic analysis (7-9), and by the use of distillation (10). Finally, there is also a redefinition of asphaltenes as being specific chromatographic fractions with one functional group present per molecule (8). The situation for t h e analyst is one of mass confusion. When a chemist wishes to separate coal-derived oils for the first time, he can find two, three, and in t h e case of “asphaltenes” as many as ten different methods, all of which arrive at supposedly the same product. A common separation method should be used on all coal-derived products t o serve as a guide post for comparison by process engineers, chemists, and management. T h e method we present here should be used only as a guide t o t h e development of a standard separation analysis of coal-derived material. We feel it is important that the finally accepted method be rapid (hours), suitable to analytical (1-10 g) a n d preparative (10-100 g) batch size, and not require sophisticated instrumentation. Our procedure can provide an analytical separation of three or four major solvent-defined solubility fractions from the multicomponent coal-derived oil. Separation is carried out a t just below room temperature with solvents of increasing orientation solubility (polarity) parameter as defined by Hildebrand (11). Each fraction is also defined with respect t o the volume of solvent used. We have found that 1 liter of solvent per fraction per 3 grams of starting material is suf-
ficient to define the separation. It has also been our experience that Soxhlet extraction a t elevated temperatures requires days to complete and the “soluble” final product of each extraction contains some insoluble material. Finally we wish to introduce a pre-treatment step of the coal-derived oil prior t o solvent extraction. By freezing the oil in liquid nitrogen, one can grind the solid pieces into a fine powder of great surface area. This contributes to rapid and efficient extraction of the pentane soluble material a t the onset of the procedure.
EXPERIMENTAL A coal-derived product is chosen and mixed well if it has been standing for more than 1 h after processing. Most liquefaction products may be warmed to 60 “C and stirred to obtain a uniform mixture. Solids may be chipped or broken. Three to four grams of the product are weighed into a tared 250-mL Pyrex heavy wall or a 315-mL stainless steel (DuPont No. 00522) centrifuge tube to =k5 mg or less. Add liquid nitrogen to the centrifuge tube slowly until 100 mL can remain as a quiet solution-no rapid evaporation. The now frozen product is ground with a thick glass rod into a fine powder. As the liquid nitrogen nears complete evaporation, place the centrifuge tube in a 50-watt ultrasonic bath and sonicate while adding 240 mL pesticide grade n-pentane and stirring rapidly with a glass or Teflon rod. Continue to sonicate and stir until no large particles are present-only a fine powder-approximately 5 min. When sonication is complete, centrifuge for 10 min at 2500 rpm (up to 10000 rpm have been used), 25 “C. The resulting supernatant is decanted into a tared 250-mL round bottom flask and pentane removed under dry nitrogen flush on a Rotovap from a warm (50 “C) water bath until 5 min after the last drop of pentane is observed to condense. The centrifuge tube is washed with 200 mL n-pentane, sonicated, stirred, and centrifuged again, decanting the supernatant into the same distilling flask. The solvent recovered from the Rotovap should be used as part of the next wash to reduce loss. Washing is continued until 1 L of n-pentane is used. After the last wash, pesticide grade benzene is added to the residue and the sonication and washing process repeated for a total of 1 L of solvent decanting into a second distilling flask. The water bath temperature is raised to 80 “C. The final benzene wash may be followed by a 50-mL n-pentane
This article not subject to U S . Copyright. Published 1978 by the American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 50, NO. 9, AUGUST 1978
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Table 11. Solvent Separated Fractions weight percent N-aEhTCUt
coal liquid
CENTRIFLGE
A
i 1 INSOLUE-E
N *ENTY'.tE
50..UE~f
II
G
1 2 3 4 5
pentane/-benzenejpentane -/benzene THFibenzene --jTHF
asphaltenes
benzene insolubles
74.2 73.3 72.0 74.5 72.0 62.7 62.4 62.6 61.9 57.5 57.3 57.6 55.8 55.7 58.9 57.9 57.5
18.4 22.0 23.2 18.2 19.3 20.8 20.9 19.4 20.8 29.8 30.4 34.0 34.5 35.8 32.9 34.5 34.8
7.4 4.7 4.8 7.3 8.7 16.5 16.7 18.0 17.3 12.7 12.3 8.4 9.7 8.5 8.2 7.6 7.7
" By difference. Direct measurement gave recoveries of >95%.
Table I. Suggested Nomenclature for Coal- Derived Materials frac- solvent solubility, tion solublejinsoluble
oils"
name oils
asphaltenes benzene insolu bles preasphaltenes THF insolubles-residue
wash, whereupon this last pentane supernatant is discarded and the centrifuge tube residue (benzene insolubles) dried at 50 "C in a vacuum oven for 15 min. The vacuum oven is backfilled with dry nitrogen. As an option to drying the benzene insolubles from the step above, tetrahydrofuran (THF) or methylene chloridemethanol (90-10) may be used as the final solvent extractor. If T H F is used, it must be freshly distilled to remove the BHT and any water. Repeat the washing, sonication, and centrifugation steps for five 200-mL washing cycles. After the final THF wash, rinse the residue with n-pentane, centrifuge, decant, and dry in a vacuum oven at 50 OC for 15 min. A t no other time should the centrifuge residue be allowed to dry or come in direct contact with the open atmosphere. The benzene washings should not be taken to dryness but brought to about 20 mL, and this solution is freeze-dried at -lo-' Torr for 1 to 2 h. To sublime the benzene and leave a solvent-free asphaltene a 20-30 mL solution is swirled in a 250-mL round bottom flask while submerging it in a 2-L Dewar of liquid nitrogen. Care should be taken to achieve a smooth distribution of the solution inside the flask walls. When the solution appears to be solid, allow the flask to remain in the liquid nitrogen 3 min longer for complete thermal equilibrium. Remove the flask from the liquid nitrogen and attach with greaseless fittings to a vacuum pump. Allow the flask to remain undisturbed until all the benzene has been sublimed. This will be noticed by light tan flakes gathering at the bottom of flask and adhering to the walls. Heating in vacuo to remove the residual benzene may change the chemical nature of the fraction and reduced benzene solubility will ensue. Toluene may be substituted for benzene, but solvent removal is more difficult. A scheme of the procedure is given in Figure 1. DISCUSSION We suggest the common and operationally defined names in Table I be applied to the solvent separated fractions. The term preasphaltenes is used here t o reflect a class of solvent defined substances. This procedure has been effective in our research efforts to provide reliable results of solvent separated fractions from
coal-derived oils in less than 61/2 hours on 3-4 grams of material. Table I1 lists solvent separation results of a number of coal liquefaction products. These products have a wide range of viscosities from 100 to greater than 3000 centistokes (70 "C). Samples of this kind are also known to age (12) under certain conditions of temperature and excess oxygen, causing changes in the relative amounts of oi1s:asphaltenes:benzene insolubles. With these considerations, a measure of precision from a limited series of samples cannot be stated. This summary should be used only as a guide for future development of a common method of separation and characterization of coal liquids. ACKNOWLEDGMENT The authors acknowledge the cooperation of Sayeed Akhtar and Nestor Mazzocco for providing the coal liquefaction products and associated process data. LITERATURE CITED M. J. Mima, H. Schultz., and W. E. McKinstry, ERDA/PERC/RI-76/6, 1976. S. Weller, M. G. Pelipetz, and S.Friedman, Ind. Eng. Chem., 43, 1575 (1951). T. Aczei, EXXON Research, Baytown, Texas, April 1978, private communication. F. Steffgen, K. Schroeder, and B. Bockrath, manuscript in preparation. I.Schwager and T. F. Yen, Fuel, 57, 100 (1978). H. Sternberg, R. Raymond, and F. K. Schweighardt, Div. Pet. Cbem., Prepr., 21 ( I ) , 198 (1976). J. G. Bendoraitis, A. V. Cabal, R. B. Calen, T. R. Stein, and S. E. Voltz, EPRI R e p . , RP-361, January 1976. M. Farcasiu, Fuel, 56, 9 (1977). R. G. Ruberto and D. M. Jewell. Dresented at the National Science Foundation Workshop "Analytical Needs of the Future as Applied to Coal Liquefaction", Greenup, Ky., August 21-23, 1974. J. E. Dooiey and C. J. Thompson, A m . Cbem. SOC., Div. Fuel Cbem., Prepr., 21 (6), 243 (1976). R. A. Keller, B. L. Karger, and L. R. Snyder, in "Gas Chromatography 1970". Stock and S.G.Perry, Ed., Institute of Petroleum, London, 1971, p 125. F. S. Karn and F. R. Brown, ERDA/PERC/TPR-76/2, 1976.
F. K. Schweighardt* B. M. Thames Department of Energy Pittsburgh Energy Research Center 4800 Forbes Avenue, Pittsburgh, Pennsylvania 15213
RECEIVEDfor review October 17,1977. Accepted May 18,1978.
