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Energy & Fuels 1999, 13, 336-339
Analytical Considerations Related to Asphaltenes and Waxes in the Same Crudes B. J. Fuhr* and L. R. Holloway Alberta Research Council, Edmonton, Alberta, Canada T6N 1E4
A. Hammami D.B. Robinson Research Limited, Edmonton, Alberta, Canada T6N 1E5 Received September 30, 1998. Revised Manuscript Received January 11, 1999
Increasingly waxy crudes containing significant quantities of asphaltenes require characterization, and established methods for separation of the waxes and asphaltenes are often not applicable. An analytical scheme is described employing only one solvent, methylethyl ketone, to separate asphaltenes and waxes, rather than an n-alkane and a ketone. The method consists of an asphaltenes fractionation by filtration at 50 °C, followed by wax recovery by cooling to a specified temperature (dependent upon the user’s needs) and filtration. The carbon number distribution of the wax fraction may be determined by high-temperature gas chromatography, and the soluble oil (minus asphaltenes and wax) can be further characterized by liquid chromatography and mass spectrometry.
Introduction Asphaltene and wax deposition can cause problems in crude oil production, transportation, and processing. To develop remedial action to these problems and to formulate predictive tools for simulating the deposition, analytical protocols are required to supply the data. Analytical methods for separating asphaltenes from heavy crudes1 and waxes from waxy crudes2 have been available for some time. When asphaltenes and waxes coexist in and/or coprecipitate from the same crude in relatively large amounts, analytical separation problems can result. This often leads to waxy asphaltenes or black waxes. The waxy asphaltenes problem was noted by Alex et al.3 in attempting a detailed characterization of wax deposits. The solution in that work was to employ a modification of the UOP method to first remove the asphaltenes and polars as a group and then to separate the wax from the remaining oil by precipitation from acetone at -17 °C. The carbon number distribution of the wax was determined using supercritical fluid chromatography, and the remaining oil was analyzed by column chromatography and gas chromatography/mass spectrometry (GC/MS). Even though the asphaltenes and polars could not be separately quantitated by this technique, the remaining data from this protocol was successfully employed in a wax deposition model for North Sea waxy crudes. (1) Syncrude Analytical Methods for Oil Sand and Bitumen Processing; Bulmer, J. T.; Starr, J., Eds.; Syncrude Canada Ltd.: 1979; pp 121-124. (2) Paraffin Wax Content of Petroleum Oils and Asphalts; UOP Method 46-64; Universal Oil Products Co.: Des Plaines, IL, 1964. (3) Alex, R. F.; Fuhr, B. J.; Rawluk, M.; Kalra, H. Prepr. Am. Chem. Soc., Div. Pet. Chem. 1991, 237-246.
In the course of understanding the pour-point depression mechanism of crudes, Irani et al.4 and Schuster and Irani5 developed a method for isolating waxes. This procedure was a modification of ASTM D-721,6 whereby the samples were diluted with methylethyl ketone (MEK), heated to 65 °C and filtered to remove insolubles (asphaltenes), then cooled to -18 °C and filtered to recover the wax. Analyses of the insolubles, waxes, and oils by high-performance liquid chromatography (HPLC) were also carried out during the course of this study, which determined that asphaltenes could play a role in the depression of the pour point. The objective of the current work was to tailor the analytical separation of asphaltenes, wax, and soluble oil fractions from crude residue cuts, such that the required characterizations of the waxes and soluble oils could be easily carried out. Wax analysis usually required a n-paraffin carbon distribution to the highest attainable carbon numbers. Experimental Section The asphaltene/wax separation employed was a modification of that described by Irani et al.4 and shown in Figure 1. It consists of adding MEK to the sample, in a 10:1 (w/v) ratio, heating to 50 °C and filtering to remove the hot MEK insolubles (or asphaltenes), cooling to an appropriate temperature, and filtering isothermally to recover the wax. Analysis of the wax was carried out by high-temperature gas chromatography (HTGC). A Hewlett-Packard 5890 equipped with flame ionization detector was employed. The column was (4) Irani, C. A.; Schuster, D. S. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1985, 30 (1), 158-168. (5) Schuster, D. S.; Irani, C. A. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1985, 30 (1), 169-177. (6) Oil Content of Petroleum Waxes; ASTM D721; American Society for Testing and Materials: Philadelphia, PA, 1996.
10.1021/ef980205h CCC: $18.00 © 1999 American Chemical Society Published on Web 02/18/1999
Asphaltenes and Waxes
Figure 1. Separation scheme for asphaltenes and wax from crude residue. a 15 m × 530 µm DB-1 of 0.1 µm film thickness. On-column injection was used with a helium flow rate of 10 mL/min and a temperature program of 35-450 °C at 10°/min. A component-type analysis (saturates, aromatics, polars, or SAP) of the soluble oil was carried out using alumina column chromatography. This method is a modification of that described in Alex et al.3 The column was 65 cm × 0.9 cm i.d. packed with alumina activated at 400 °C for 20 h. The respective eluents consisted of 100 mL of n-pentane for saturates, 150 mL of toluene/pentane (70/30 v/v ratio) for aromatics, and 100 mL of methylene chloride/methanol (50/ 50 v/v ratio) for polars.
Results and Discussion Waxy Crudes without Asphaltenes. Waxy crudes without asphaltenes were first investigated in order to simplify method development. The MEK wax separation method from Figure 1 was modified for application to residue cuts from crudes containing no asphaltenes. Basically, the filtering step after heating to 50 °C, to
Energy & Fuels, Vol. 13, No. 2, 1999 337
produce the hot solvent insolubles, was omitted. In this case, a >350 °C residue cut from such a crude dissolved completely at 50 °C, which was deemed a suitable temperature to begin the separation. Lowering the temperature of this very waxy crude to 25 °C resulted in significant wax precipitation. HTGC analyses are shown in Figure 2 for the original residue as well as for the wax. The whole residue begins to elute at about C21, which is expected for a >350 °C cut, and contains a very broad hump, presumably consisting of branched and cyclic paraffins as well as aromatics. Superimposed on this hump is the pattern of n-paraffins, making it difficult to obtain a quantifiable carbon number distribution. The 25 °C wax, on the other hand, as seen from Figure 2, produced a noticeably different chromatogram. This trace is almost devoid of the hump, making it relatively easy to quantify the corresponding n-paraffins up to C70. Of course, because of the relatively high wax precipitation temperature, the paraffins between C21 and C30 are missing. The same separation scheme was applied to the >340 °C residue cut from another crude which also contained no asphaltenes. This time waxes were obtained at several different temperatures from 20 to -15 °C; the amount of wax obtained in each case is indicated in Table 1, along with the maximum carbon number and the distribution widths. The latter quantities are also shown for reference for the whole >340 °C residue. Not surprisingly, the amount of wax increases substantially and the distribution shifts to lower carbon numbers as the temperature decreases; this was also indicated by Schuster and Irani.5 The HTGC chromatograms of the highest and lowest temperature waxes are shown in Figure 3. Several features are worth noting. First of all, the 20 °C wax, even though it produces the least amount because it does not contain the C21-C30 peaks, is the clean-
Figure 2. HTGC chromatograms of >350 °C crude residue and its 25 °C wax fraction.
338 Energy & Fuels, Vol. 13, No. 2, 1999
Fuhr et al.
Figure 3. HTGC chromatograms of 20 and -15 °C wax fractions of >340 °C crude residue. Table 1. Amount of Wax Precipitated as a Function of Temperature temp (°C) whole >340 °C residue 20 10 0 -15
wt % wax
carbon no. max
carbon no. range
3.4 12.4 14.6 36.3
23 31 26 25 24
20-55 23-70 21-64 21-58 21-46
est chromatogram with virtually no hump and showing quantifiable carbon numbers up to C70. Some resolution of i-paraffin peaks is also seen in the C35 area. Second, the -15 °C wax has a relatively larger hump and shows much less detail in the high carbon number region, even though it does contain the C21-C30 paraffins. In summary, this portion of the study demonstrated that cooling of MEK mixtures of waxy crude residues from 50 °C will produce a filterable and recoverable wax. It is assumed that this temperature will be sufficient to dissolve all the wax; if it is not, it should be increased. The temperature to which the mixture is cooled should be determined by what kind of wax characterization is required by the user of the data. In other words cooling to a low temperature, -18 °C, will produce a wax value which will correspond to the UOP standard wax value and contain significant i-paraffins, naphthenes, and aromatics, as well as the C21-C30 n-paraffins. If, however, characterization of the highest carbon number carbons in the wax is desired, the precipitation temperature should be much higher, ambient temperature or slightly less. In this way, a much smaller amount of wax is recovered but naphthenes, aromatics, and the lower n-paraffins (C21-C30) are excluded, which allows a detailed characterization of the n-paraffins to the highest values, that portion of the wax often considered the more troublesome. Some i-paraffins are usually precipi-
Table 2. Comparison of SAP Analysis (as wt % of soluble oil) after Different Asphaltene/Wax Separation Schemes
saturates aromatics polars
omission of hot MEK insolubles step
as in Figure 1
61 27 12
60 26 14
tated along with the n-paraffins and could be quantitated by HTGC as well. Waxy Crudes with Asphaltenes. As stated previously, the scheme used for separation of asphaltenes and wax is shown in Figure 1. Dissolution of the wax is usually ensured at 50 °C, yet allows the hot MEK insolubles (or asphaltenes) to be filtered. Two experiments were carried out with a >270 °C residue cut from a waxy crude containing asphaltenes. The first was as already outlined in Figure 1; the second omitted the filtration and determination of the hot solvent insolubles. In both cases, the wax or wax/asphaltene coprecipitate was obtained at -10 °C. The purpose was to demonstrate that the omission of that step would actually result in coprecipitation of asphaltenes and waxes. In the first case, the separately determined asphaltenes and wax values were 11% and 41%, respectively, for a combined total of 52% (of the original residue). In the second instance, the coprecipitate amount was 48%, close to the separately determined values. Table 2 provides data on the component-type (or SAP) analysis of the soluble oils of the >270 °C residue obtained by the two experiments described previously. The results show that the soluble oils are basically the same, indicating that the asphaltenes which are filtered out in the normal separation will end up in the waxy fraction if not removed first. Although not shown here, detailed characterization of the saturates and aromatics fractions of the soluble oil by GC/MS may be carried
Asphaltenes and Waxes
Energy & Fuels, Vol. 13, No. 2, 1999 339
Figure 4. HTGC chromatograms of -10 °C wax and hot solvent‚insoluble fractions of >270 °C crude residue.
out in order to provide data on asphaltene and wax stability and deposition potential. The HTGC chromatogram of the -10 °C wax discussed previously is shown in Figure 4. A reasonably clean chromatogram with a small hump and some indication of i-paraffins around C30 is seen; a good n-paraffin distribution may easily be determined from such a chromatogram. The corresponding chromatogram of the hot solvent insolubles or asphaltenes shown in Figure 4 suggests a small amount of wax coprecipitated with the asphaltenes, even after heating to 50 °C. This trace wax was estimated based on the GC area counts to be 5% of the asphaltenes fraction. This behavior is similar to that found by Schuster and Irani,5 who stated that saturates and asphaltenes associate with one another and do not totally separate during bulk deasphaltening. Comparison of Deasphaltening Solvents. The definition of asphaltenes is an operational one, and as such it depends on the conditions (solvent and temperature) employed in their precipitation.7 In the current work, the definition is MEK solvent and 50 °C; in Irani et al.4 it was MEK and 65.5 °C. A more common definition of asphaltenes is n-pentane and ambient temperature;1 hot n-hexane has also been employed, particularly for determining small amounts of asphaltenes in waxy crudes. Therefore, a comparison was made among solvents and conditions using a residue sample originating from a heavy oil, which contains no waxy components; the results are shown in Table 3. It can be seen that the hot MEK definition gives a significantly lower value than the n-pentane insolubles; however, it comes close to the hot n-hexane definition. While the latter solvent is also useful for asphaltenes determina(7) Speight, J. G. The Chemistry and Technology of Petroleum; Marcel Dekker: New York, 1980; pp 118-127.
Table 3. Comparison of Solvent Insolubles solvent
temp (°C)
wt %
n-pentane n-hexane MEK
ambient 50 50
20.9 14.4 13.4
tion, its wax solubilization potential is too high for wax determination. It is not the authors’ intention to suggest here that correlations should be made between asphaltene definitions. It should be stressed, however, that anytime an asphaltene value is given, its operational definition should be stated. Conclusions A method has been adapted for the analytical separation of asphaltenes and waxes when they coexist in the same crudes. The method employs one solvent, MEK, rather than an n-alkane for asphaltenes and a ketone for wax. The MEK, when added to the crude and heated to 50 °C, solubilizes the wax but not the asphaltenes (or hot MEK insolubles). The amount of these asphaltenes, as determined by filtration at 50 °C, is similar to that obtained using hot n-hexane. By merely lowering the temperature of the same mixture to an appropriate value, precipitated wax may be recovered by filtration and subjected to HTGC analysis for n-paraffin distribution. The wax precipitation temperature will depend on the type of data required. For example, a temperature of 25 °C will provide good information about the highest carbon number range; conversely, -18 °C will provide a %wax value closer to the UOP value but yield low resolution at the high carbon number end. The soluble oil is available for analysis of component types (SAP), with subsequent detailed characterization by GC/MS if desired. EF980205H
