A New Experimental Method for a Fast and Reliable Determination of Saturates, Aromatics, Resins and Asphaltenes, SARA, in Crude Oils

Due to the complex nature of the crude oil, its compositional characterization is a very challenging task. One of the most conventional ways to characterize crude oil is SARA (Saturates, Aromatics, Resins, and Asphaltenes) analysis. This fractionation is based on the difference between the solubility and polarity of the four fractions: Saturates, Aromatics, Resins, and Asphaltenes¹.

Currently, three main methods of SARA analysis are used by the industry: Thin Layer Chromatography with Flame Ionization Detection (TLC-FID) or Iatroscan for SARA analysis, ASTM D2007 and High Pressure Liquid Chromatography (HPLC) for maltene (deasphalted crude oil) analysis. Reproducibility of the results has always been a challenge, in addition to the different limitations of each of these methods².

In this study, we propose a new methodology for maltene analysis that aims to overcome some of the limitations of the previous methods. After removing the asphaltene fractions, the remaining mixture, called maltenes, is fractionated in two steps; in the first step, maltenes are separated by selective adsorption into the clay and the alumina columns, and in the second step these components are desorbed and recovered.

In the adsorption step, two chromatographic columns, which are filled with clay and silica gel, are connected in series with the ability to recirculate the effluent if needed. Based on the polarity, the resins are selectively adsorbed onto the clay in the first column while the aromatics are adsorbed onto the silica gel of the second column. The effluent of the silica gel column contains the saturate fraction. The efficiency of the process is verified by UV spectroscopy.

Once the adsorption process is completed, the next step is desorption of resins and aromatics in two separate processes. The columns are washed with a relatively polar solvent. The solvent is recovered in a distillation column, and the residues contain the resins and aromatics, respectively. In the end, the solvent is evaporated by a hot plate, and all fractions are weighted.

In comparison to the ASTM method, our new technique requires between 90 to 95% less solvent and is about three to four times faster. The collected data are expected to be more reproducible because the volatile components are not separated and are measured as a part of saturates or aromatic fractions, and the process automation significantly reduces human error. Another advantage is that the equipment is fairly inexpensive and could be easily assembled in any laboratory.

References:

1. Fan, T., Wang, J. & Buckley, J. S. Evaluating Crude Oils by SARA Analysis. in (Society of Petroleum Engineers, 2002).

2. Fan, T. & Buckley, J. S. Rapid and Accurate SARA Analysis of Medium Gravity Crude Oils. Energy Fuels 16, 1571–1575 (2002).