Integration of Chromatographic and Spectroscopic Techniques for the Characterization of Residual Oils Douglas M. Jewell,* Edgar W. Albaugh, Burl E. Davis, and Raffaele G. Ruberto Gulf Research & Development Company, Pittsburgh, Pennsylvania 15230
An integrated method is proposed to separate petroleum residuals into discrete fractions suitable for qualitative and semiquantitative characterization by chemical, physical, and spectral means. This characterization includes types of functional groups, quantity and types of chain and aromatic ring structures, heteroatom distribution, and molecular size-weight relationships.
Introduction One of the areas of interest today in the petroleum industry relates to the handling of residuals and the need to understand the compositional changes taking place during their processing. The chemical composition dictates the many properties that they exhibit. This paper will emphasize the development and application of techniques for rapidly studying typical residual products in order to provide reasonable compositional data. Typical refinery operations commonly provide two types of residuals: “atmospheric” and “vacuum.” The former implies an initial overhead temperature of 680°F while the latter extends the overhead temperature (corrected) to 1100°F. Petroleum technologists have commonly defined four classes of components in residuals based on solubility relationships: asphaltenes, soluble in benzene, insoluble in n-pentane; maltenes, soluble in benzene, soluble in n-pentane; resins, soluble in n-pentane, insoluble in propane; oils, soluble in n-pentane, soluble in propane. Characterization of any petroleum residual is relative and never complete, but its degree is directly related to the extent that discrete classes of compounds can be isolated and analyzed by chemical and spectroscopic methods. An integral and comprehensive sequence of operations to arrive at such discrete residual fractions is illustrated in Figure 1. Recent studies by the American Petroleum Institute (Jewell, et al., 1972b) have established methods for removing the polar nonhydrocarbons from heavy distillates and residuals. Recent experimentation in our laboratory has shown that in the application to residuals, polar nonhydrocarbons as defined by these methods are equivalent to “resins,” making it possible to describe this class chemically. “Oils” are, therefore, the remaining saturate and aromatic hydrocarbons plus nonpolar nonhydrocarbons (e.g., ethers, thioethers, etc.). N
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Experimental Section The numerous steps previously developed for isolating different sub-types of “resins,” saturates, and aromatics can be appreciably shortened and/or integrated to provide analytical values and quantities of each. The combination of these steps is called a SARA method (saturate-aromatic-resin-asphaltene) . Figure 2 shows an integrated apparatus for performing these experiments starting with a sample of maltenes derived from n-pentane extraction of a residual. A closed system arrangement of two columns and three on-line monitors is demonstrated. Column 12 (SARA column) consists of four discretely packed zones in the ratios shown: (A) Amberlyst 15, H+ cation-exchange resin; (B) Amberlyst A-29, OH- ion-exchange resin; (C) ferric chloride on clay; and (D) Amberlyst A-29, OH- ion278
Ind. Eng. Chern., Fundarn., Vol. 13,No. 3, 1974
exchange resin. Column 14 consists of activated silica gel (Davison Grade 923). Each column is packed dry, saturated, pressurized, and degassed with n-hexane (-5-9 ml/ min). Both column 12 and column 14 effluents are monitored a t 254 nm using a dual-channel ultraviolet analyzer (Isco Model UA-2). Column 14 effluent is also monitored with a differential refractometer (LDC Refractomonitor). The sample of maltenes, dissolved in n-hexane, is injected to column 12 by means of an appropriate calibrated valve, 10. “Resins” are irreversibly retained on column 12 and effluent response is due to total aromatics in the “oils.” Effluent from column 14 will show a base line separation of saturates from aromatics. When all the aromatics have entered column 14, column 12 is bypassed and total aromatics are desorbed with a chloroform-methanol gradient. The “resins” are calculated by difference after the quantitative gravimetric recovery of isolated saturates and aromatic fractions. Figure 3 shows a typical chromatogram from an integrated SARA experiment. The bottom curve is a typical response of the ultraviolet absorption detector (UAD) when monitoring the SARA column. The top curves are indicative of the response of both the UAD and refractive index detector (RID) when monitoring the silica gel column. Tailing of the SARA column occurs to a slight extent depending on the amount of sample charged. This “tail” is completely aromatic as evidenced by the sharp and symmetrical elution of the saturates from the silica gel column. Once the cut-point is simultaneously observed by both the RID and UAD, the detectors are bypassed. These chromatograms demonstrate that the UAD alone is usually sufficient for determining cut-points between saturates and aromatics. This integrated procedure will analytically handle 50500 mg of maltenes. If larger quantities of material are to be separated and/or “resins” are desired, the same SARA column packing sequence can be employed in a recycle chromatographic column shown in Figure 4. This apparatus also permits the isolation of asphaltenes if the residual is first placed in a cellulose thimble which is set in the top reservoir containing n-pentane. After exhaustive elution of the thimble and SARA column with n-pentane, “oils” are recovered in the bottom flask, “asphaltenes” are left in the thimble, and “resins” are retained by the column packing. After removing the thimble, asphaltenes can be independently recovered with benzene and the “resins” can be desorbed in >90% recovery with an exhaustive chloroform elution. The dimensions of the recycle column can be varied to handle 0.5-1.0-g samples (as shown in Figure 4) up to 100 g. The “oils” can be further separated into saturates and aromatics by either silica gel or alumina techniques. The four major classes of compounds obtained from the
SPARATION O V
SARA
COLUMN P I C K I N G SEOUENCE
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111 n PENTINE EXTRACTION
MALTEMU
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121 SARACOLUMN
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RESIW
76
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76
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25
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IS1 IR. W. NM. ESR. W SPECTROIICOCY 141 SPECIFIC
onamic REACTIONS
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STOPCOCK
Figure 1. Sequence for studying residuals.
A RECYCLF. COLmN PJR
SEPARATING RESINS ANU OILS
B PACK1,NVC SEQJMCE R)R ECYCLE C O L W
Figure 4. Preparative SARA column.
od of Albaugh and Talarico (1972) using a uniform concentration of 5 mg/ml benzene for all the samples. Molecular weights were determined by vapor pressure osmometry (VPO) in benzene.
I. SOLVENT RESERVOIRS, \I. 2. TEFLON TUBINQ. I/8'O.D. 3. TEFLON TUBING, 1116" 0.0. 4. ADAPTER WITH TWO 118" HOLES AND T 2 4 / 4 0 5. 1 0 0 m l ROUND BOTTOM FLASK 6. 1/2" MAGNETIC BAR 1. M A G N E T t C STIRRER a M I L T O N ROY MINIPUMP 9. FLOW-THROUOH PRESSURE GAUQE IO. INJECTION VALVE-SAMPLE LOOP II. BYPASS V A L V E
0 . SARA COCUMN, I 8 . X 112. A. H*lON-EXCHANQE RESIN, 2' E. OH- ION-EXCHAYQE RESIN, 2. C. FICI, -KAOLIN 10' D. OH- ION-EXCHANQE RESIN, I' 13. U.V. DETECTORS, 2 5 4 n n 14. SILICA-QEL COLUMN, 25.X 112' IS. DIFFERENTIAL R . I . DETECTOR 16. FRACTION COLLECTOR 17, UV. W A L CHANNEL RECORDER la R . I . RECORDER
Figure 2. Apparatus for SARA experiments.
1
AROMATIC
[SATURATES
-
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I
ELUTION
'Figure 3. Application of multiple detectors to integrated SARA method.
above SARA method can be handled individually for further analysis and subdivision as shown in Figure l. Normal paraffins are quantitatively removed from saturates by a modification of 5-A molecular sieve techniques (0'Connor, et al.,1962). Gel permeation chromatography "fingerprints" of isolated fractions are obtained by the meth-
Discussion of Results The type of information generated by this sequence of operations is illustrated in Table I. Sulfur-rich Kuwait residuals are highly aromatic and contain large quantities of resins and asphaltenes; saturates always decrease and resins and asphaltenes always increase as the vacuum distillates are removed. Cabinda and South Louisiana residuals are low in sulfur and asphaltenes but rich in saturates. These conclusions are corroborated by GPC which shows the Kuwait vacuum residuals to be heavier than the Kuwait and South Louisiana atmospheric residuals, to be richest in condensed aromatic systems even though the aromatic compounds are evenly distributed throughout the residuals, and to be lowest in the saturates content. The Venezuela (Mara) residuals contain appreciable amounts of sulfur and are very similar to Kuwait residuals with respect to compound class distribution. In all cases the resins and asphaltenes contain one or more distinct functional groups per molecule, all the porphyrin and nonporphyrin metallo compounds, and differ from each other primarily by molecular weight and degree of intermolecular association. This is further shown by the GPC of these two fractions and the whole residual (Figure 5 ) . The asphaltenes and resins show a broader distribution than the whole residual and a shift to lower elution volumes, indicating the presence of molecules of larger size. These are due to increased intermolecular association which occurs upon isolation and concentration. The RID and UAD response curves indicate that asphaltenes and resins are distributed throughout the residual and that the distributions are differen€. More detailed qualitative and semiquantitative information on resins and asphaltenes can be obtained by applying numerous functional group separation and spectral techniques (Jewell, et al., 1972b; McKay, 1972) toInd. Eng. Chem., Fundam., Vol. 13, No. 3, 1974
279
6 7
RESIDUE
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8
10
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20
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26
28
30
32
34
RETENTION TIME
Figure 6. Application of GLC to saturates from: (A) vacuum residual; (B)atmospheric residual; and (C) n-paraffins from (B).
Table I. Comparison of Residuals bv SARA Method SATURATES
AROMATICS
REBINS
ASWALTENES
KUWAIT 1VAC.I
gXlRCE
11.6%
31.m
28.m
1I.W
KUWAIT IATMl
18.0
1.6
16.7
0.8
CABINDA 1ATM.I
56.0
27.4
12.5
S LOUISIANA lATM.1
50.1
56.6
14.2
2s
