Preparative asphaltenes

Its purpose is to partially convert heavy fractions highly contaminated by natural compounds such as sulfur, nitrogen, metals Ni, V, and asphaltenes and to prepare feedstocks for deeper conversion or to produce low-sulfur fuel-oil. 

Dispersants for Asphalts. Asphalt and asphaltene components can produce difficulties in various processes in recovering crude petroleum oils and preparing them for transportation through pipelines or in refining separation. 

The details of the XANES experimental setup and data analyses have been described previously 3b,8). All model compounds used in this study were obtained from Aldrich Chemical Company and were used without further purification. The asphaltene samples were prepared from their respective petroleum residua by precipitation from n-heptane following the procedure of Corbett (9). A sample of Rasa coal was generously provided by Dr. Curt White of the Pittsburgh Energy Technology Center. 

XANES of Petroleum Residua. On the left side of Figure 1 the sulfur K edge spectra for three different petroleum residua and the asphaltene samples prepared from them are shown. While the absorption spectra all appear to be similar, differences are revealed by examining the third derivatives of the spectra, which are shown on the right side of the figure. All the residua samples appear to contain sulfur bound in sulfidic and thiophenic forms, the amount of sulfidic sulfur increasing from sample 1 to sample 3. The asphaltene samples prepared from residua 2 and 3 also appear to contain both forms. Assuming that the composition of the sulfur... 

Both XANES and XPS results give the same relative ranking for the thiophenic content of any sample, recognizing the currently established accuracy of 10%. For example, either method confirms that petroleum residuum sample 1 contains the most thiophenic sulfur of those studied, and sample 3 the least. Both methods show that thiophenic sulfur concentrates in the asphaltenes prepared from residua 1 and 3, but not in the asphaltenes from residuum 2. However, the quantitative values are different. 

The extracts were fractionated by a Preparative Liquid Chromatography method - PLC-8 [2], in eight distinct chemical classes FI-saturated hydrocarbons (HC), F2-monoaromatics, F3-diaromatics, F4-triaromatics, F5-polynuclear aromatics, F6-resins, F7-asphaltenes and F8-asphaltols. This method, proposed by Karam et al. as an extension of SARA method [4], was especially developed for coal-derived liquids. It combines solubility and chromatographic fractionation, affording discrete, well-defined classes of compounds which are readable for direct chromatographic and spectroscopic analysis. 

While asphalt itself consists of a complex colloidal dispersion of resins and asphaltenes in oils, introduction of liquid elemental sulfur, which on cooling congeals into finely dispersed crystalline sulfur particles and in part reacts with the asphalt, necessarily complicates the rheology of such a SA binder. Differences and changes with SA binder preparation, curing time, temperature etc. must be expected and may be demonstrated by viscosity characteristics. 

Bonnet [3] prepared aqueous dispersions of bitumen and asphaltenes using the urethane reaction product of 4,4 -diphenylmethane diisocyanate and PolyBd diol. 

McKay s solvent sequence completely eluted the Wilmington asphaltenes but did not elute all the Athabasca asphaltene samples and had to be extended by additional solvent mixtures to obtain good sample recoveries (cf. Figure 2). For large scale preparative separations of asphaltenes, the asphaltenes were dissolved in benzene and eluted with the same solvent, omitting the cyclohexane step. This accelerated the operation, but at the same time, as expected, the percentage of the neutral fraction now increased from 20%-21% to approximately 28%-30%, in reasonable agreement with the bulk results from the cyclohexane experiments (see Table III). Table III also shows the additional solvent systems used. 

Table III. Preparative Scale Separation of Athabasca Asphaltenes on Ion Exchangers IRA-904 and A-15...

Figure 4. Calibration of preparative Styragel 1000-A column ( ) - polystyrene standards (m) - asphaltene fractions MWs of asphaltene fractions determined by VPO from benzene...

The molecular size distributions and the size-distribution profiles for the nickel-, vanadium-, and sulfur-containing molecules in the asphaltenes and maltenes from six petroleum residua were determined using analytical and preparative scale gel permeation chromatography (GPC). The size distribution data were useful in understanding several aspects of residuum processing. A comparison of the molecular size distributions to the pore-size distribution of a small-pore desulfurization catalyst showed the importance of the catalyst pore size in efficient residuum desulfurization. In addition, differences between size distributions of the sulfur- and metal-containing molecules for the residua examined helped to explain reported variations in demetallation and desulfurization selectivities. Finally, the GPC technique also was used to monitor effects of both thermal and catalytic processing on the asphaltene size distributions. 

In this study, six petroleum residua were characterized by a combination of preparative- and analytical-scale gel permeation chromatography (GPC). Each residuum was separated initially by pentane deasphalting into an asphaltene and maltene pair, both of which were separated further by... 

Sample Preparation. The residua samples were separated into asphaltenes and maltenes by deasphalting the resid with a 25 1 (v/v) amount of n-pentane. After stirring, the mixture was allowed to sit overnight, then filtered through a 0.45-p porous glass filter. The asphaltenes were washed with several portions of pentane and dried under vacuum at 90°C. Pentane was evaporated from the filtrate to yield the maltenes. 

Preparative GPC. The preparative GPC work was performed on the experimental setup shown in Figure 1. Four 1-in. i.d. glass columns were packed with Styragel (Waters Associates) with 1 ft 104 A porosity, 2 ft 500 A porosity, and 1 ft 100 A porosity. The Styragel porosites were chosen to give good resolution for the entire range of molecular sizes found in residua. Two separate column systems were used—one for maltenes, the other for asphaltenes. 

Elemental Analyses. Sulfur measurements for preparative cuts and raw and control samples were made on 15-50-mg samples using ASTM Method D1552. The precision was 10% relative for maltenes and 5% relative for asphaltenes as determined by multiple measurements on several asphaltenes and maltenes. 

Table III lists the material balances for the preparative separations. These are the percent weight recoveries for either asphaltene or maltene defined, using the sulfur balance for an example, as the sum of the amount of sulfur in each cut times the cut weight percent divided by the total sulfur. In general, the balances are in the 80-120% range, which is reasonable considering the amount of sample handling involved. The recoveries are out of line only in a few cases, most notably the Prudhoe Bay maltene nickel balance. In addition, a comparison of the calculated elemental values for the total residua differ somewhat from the raw total values for several residua. These discrepancies are probably attributable to the small samples, multiple sample manipulations, and compounding of individual errors when the asphaltene and maltene data are summed. The data-fitting routine described in the next section was used to obtain a set of best fit data, which were used in the subsequent size calculations.

Materials. Athabasca asphaltene was prepared from an oil sand sample from the GCOS (now Suncor Inc.) quarry according to a standard procedure used in this laboratory (9). Solvents were refluxed over CaH2 and redistilled. Nitromethane was distilled and stored over molecular sieves. AlCl3 was sublimed and ground under nitrogen ZnCl2 was dried at 110° C in a vacuum oven. 

HEPTANE Asphaltenes (C7). Asphaltenes were prepared according to References 20, 21 in boiling heptane with a ratio of one part by weight of residue to thirty parts by volume of solvent (for example 1 g/30 mL). In the United States there is a proposed method for asphalt composition analysis (22) using n-heptane and a solvent/ charge of 100 (v) l (w). 

PENTANE Asphaltenes (C5). No established method is available for the preparation of pentane asphaltenes. ASTM D-2006 was discontinued in 1976. Another ASTM method (for rubber extender and processing oils) (23) uses a 10-g sample and only 100 mL of pentane, which is insufficient for a correct dispersion of the sample into the solvent. A very precise method for these asphaltenes from coal-derived liquids has been described (24), but some might object to the use of benzene because of the solubilizing properties of this solvent towards asphaltenes. 

Preparation of Resins. Maltenes (1 g) obtained after removal of asphaltenes (obtained either by C5 or C7) and solvent were dissolved in cyclohexane and the solution was run on a dual column (Davison Silicagel grade 62 and Alcoa F 20 alumina, according to USBM-API method)(25). After eluting the saturate and aromatic fractions ( deasphalted oil ) with appropriate solvent, resins on the column were extracted once with a 50/50 (v/v) mixture of ether and methanol, then with a 75/25 (v/v) mixture of chloroform and methanol, last with a 75/25 (v/v) mixture of carbon tetrachloride and methanol (26, 27). 

We would like to thank Kendall Smith, Alan Witt, and Mike Briggs for their contribution to the reactor tests, Mary Little for the Flasma analysis, and Sue Grayson for her assistance with the asphaltene preparation and the SEC ICP analysis. 

---