Trace metals such as vanadium, nickel, and iron in fluid catalytic cracking feedstocks deposit on the catalyst and adversely affect catalyst activity and product yields. Vanadium compounds can also cause damage to furnace linings and corrosion of boiler tubes or gas turbine blades. A rapid and sensitive analytical method is needed to detect these metals in feedstocks, products, and residua. Such a method using energy dispersive X-ray fluorescence (EDXRF) spectrometry for determination of vanadium, nickel, and iron in petroleum and petroleum residua is described.
In order to eliminate matrix effects, samples containing an internal standard (cobalt) were decomposed by heating with concentrated sulfuric acid and the resulting carbonaceous residua were ashed. X-ray fluorescent spectral intensities of V, Ni, Fe, and Co were measured using established instrumental parameters. Since the Fe Kβ and Co Kα lines overlapped, the slope and the intercept of the ratio of Fe Kα to Fe Kβ intensities from the analysis of ash of several iron standards were used for correction of Co spectral intensities. Sample metal concentrations were determined by comparison of spectral ratios to those of ashed calibration standards. Analytical data obtained by this procedure for National Institute of Standards and Technology (NIST, formerly NBS) Standard Reference Materials (SRMs) 1618, 1634b, and 8505 were in very good agreement with the NIST certified values. The metal contents of petroleum and petroleum residua determined by this EDXRF method also agreed with those obtained by a flame atomic absorption method. This analytical approach could be developed into an ASTM standard method and the scope extended to include determination of other metals.