K. K. (Adry) Bissada1,2, Ewa B. Szymczyk1, David G. Nolte2 and L. M. Darnell1
1 Petroleum Systems & Geochemistry Institute, University of Houston, Houston, USA; 2Ontario-Texas Technology, Houston, USA.
Analysis of crude oils and bitumen provides important geochemical information for understanding petroleum phase behavior and forecasting asphaltene and paraffin deposition (“arterial blockage”) in production, transportation, refining, and heavy-oil upgrading operations and for averting crude-oil incompatibility problems in blending of opportunity crudes in refinery feed-stock selection. Essential steps in this analysis are group-type separation of the components of the petroleum into Saturated hydrocarbons, Aromatic hydrocarbons, Resins, and Asphaltenes (SARA fractions), and the further separation of the Saturates into n-Paraffins, Iso-paraffins and Naphthenes (PIN fractions). These separations are necessary to determine the relative concentration of each chemical group and to obtain high-purity fractions for subsequent gas chromatographic and spectrometric analyses.
Conventional SARA analysis of crude oils entails preliminary “deasphaltening” and subsequent chromatographic separation of the soluble portion into Saturates, Aromatics and Resins with inherent cross-contamination, and inconsistencies among reporting laboratories. Gravimetric quantification of yields is grossly inaccurate, requiring solvent evaporation before weighing and causes loss of light components from recovered fractions and ends in innate inaccuracies and imprecision in the results. Further P I N separation of the Saturates is extremely complex, requiring mol-sieve complexing or urea aduction of the n-Paraffins and subsequent decomposition of the complexes to recover the Paraffins. Adaptation of multidimensional HPLC, together with utilization of instrumental quantification means that preclude the need for solvent evaporation and weighing of residues, provided two enhanced processes. The SARA HPLC process fractionates an oil sample into four pure fractions without prior de-asphalteneing, and the PIN process further separates the Saturates into three pure PIN fractions with excellent resolution. Extensive testing of chromatographic performance, precision and mass-balance showed radically improved fraction-purity, significantly enhanced accuracy, and essentially full recovery.