COMPARISON OF PRECIPITATION AND EXTROGRAPHY IN THE FRACTIONATION OF CRUDE-OIL RESIDUA

Four different crude oils-Arab Heavy (AH), Arab Beni (AB), Alaskan Nor th Slope (ANS), and San Joaquin Valley (SJV)--were characterized by se parating the crudes into compound classes by two different fractionati on methods--extrography (ext) and asphaltene precipitation followed by extrographic isolation of the resins (ppt + ext). The fractions deriv ed from these two methods of separation were characterized and compare d in order to determine the most, complete and discriminating composit ional analysis for subsequent use in studies concerning the stability of emulsions or sludges which are produced by these crudes in refinery processing. The resin and asphaltene fractions are of primary interes t due to their polar, surface-active nature, the central role these fr actions play in stabilizing emulsions and sludges, and their resulting influence on crude processability. Preparatory-scale separations were developed and utilized to provide sufficient amounts of the polar fra ctions of these crudes for further studies. The H/C ratio and concentr ations of S, N, and O as well as V and Ni contents were determined for each of the crudes and their respective fractions. The results of thi s investigation also provided useful information regarding the H-bondi ng capacity (as gauged by the functional group types and content) and aromaticity of the polar fractions of these particular crude oils. The polar functional groups and carbon types were identified and quantita ted by FTIR and C-13 NMR spectroscopy. FTIR analysis also provided an indirect measure of the polarity of substituent groups attached to the aromatic core of these molecules when comparing the results of the in tegration of the aromatic C=C stretch with the aromaticity as measured by C-13 NMR. analysis. Both the H-bonding capacity and the aromaticit y are considered to play a central role in determining the solubility of asphaltenes in the crude medium which in turn has a significant eff ect on the emulsifying potential of the crude oil. In the comparison o f the two fractionation methods, the resin and total asphaltene yields were very similar for most of the crude types; however, the character istics of these polar fractions varied considerably in all the crudes depending upon the method used to isolate them. There was strong evide nce to support the prevailing sentiment that the fraction which is coi ned ''asphaltenes'' is greatly influenced and defined by the very meth od utilized to isolate this fraction. When considering and analyzing t he characteristics of the polar materials in their totality (i.e., res ins + asphaltenes), these materials were very similar in nature regard less of the fractionation method used to isolate them. However, irreve rsible adsorption of asphaltenic components (up to similar to 0.8 wt % of whole crude) on the silica gel does occur when employing the extro graphic technique. Thus, the ppt + ext fractionation method appears to be the simplest and most robust method to provide further understandi ng of the mechanisms which govern the stability of emulsions produced from different crude types.