Application of AMD to petrochemical analysis: improved separation and expanded Hydrocarbon Group Type Analysis of heavy petroleum products
A complete molecular separation of heavy petroleum products is not feasible for their quality control. They are instead characterized in terms of chemical family composition, using classical Hydrocarbon Group Type Analysis such as SARA (Saturates, Aromatics, Resins, Asphaltenes). However, SARA does not provide information enough to correlate chemical differences of products with their conversion parameters in the case of heavy products.
AMD appears as an interesting alternative to obtain different separations with increasing level of complexity for these products. In general, and for a given sample, AMD can use the same mobile phase over increasing migration distances (refocusing effect) or use a combination of gradient development and refocusing in the search for an improved separation. Conditions can be fine tuned to obtain at will either an increased expansion of the asphaltenic-resinic chromatographic zone, or an increased expansion of the apolar zone (saturates-naphthenoaromatics) of products such as vacuu m gas oil and residues, bitumes, asphaltenes or base oils.
In the simpler approach, SARA can also be obtained by AMD using a sequence of several elution solvents in order of growing or decreasing polarity, as when using a conventional developing chamber.
Detection has been carried out by UV and fluorescence densitometry. Saturates can be detected by Fluorescence Detection by Intensity Changes (FDIC) as a positive peak using berberine-impregnated plates. We have observed that the presence of Asphaltenes provides negative peaks in the chromatographic zones corresponding to Resins ans Asphaltenes because the most polar compounds produce a decrease in fluorescence with regard to the baseline provided by berberine emission. Asphaltene-free products provide FDIC-berberine chromatograms with exclusively positive peaks, which enables chromatogram exploitation using only fluorescence densitometry.