Optical Measurement of Saturates, Aromatics, Resins, And Asphaltenes in Crude Oil

We describe a novel apparatus and method for rapidly separating and measuring four subfractions of crude oil: saturates, aromatics, resins, and asphaltenes (SARA). This work is an extension of our previous work on the microfluidic measurement of asphaltene content, where a microfluidic technique was used to rapidly separate asphaltenes from crude oil for indirect optical measurement. Here, we extend the measurement by adding column chromatography to fractionate the deasphalted oil into saturate, aromatic, and resin fractions. Saturates are measured by refractive index, whereas the aromatics, resins, and asphaltenes are measured by ultraviolet-visible (UV-vis) absorbance. We evaluate 15 samples from various geographical origins to determine appropriate optical-to-gravimetric response factors. The response factors are then used to enable a seamlessly automated SARA measurement technique. When the 15 samples are run through the automated procedure, the optical-to-gravimetric root-mean-square error (RMSE) values are +/- 3.8 wt % for saturates, +/- 2.7 wt % for aromatics, +/- 2.3 wt % for resins, and +/- 1.2 wt % for asphaltenes-absolute errors. The final microfluidic SARA technique exhibited excellent reproducibility; the measurements were within +/- 0.8 wt % for saturates, aromatics, and resins and within +/- 0.2 wt % for asphaltenes. Further, the technique reduced SARA experimental times from 2 days to 4 h for topped samples while greatly reducing the need for manual labor.