Direct determination of nickel in xylene solutions of raw material for catalytic cracking with application of graphite furnace atomic absorption spectrometry

Abstract

Nickel is a severe poison to cracking catalysts, therefore its content in raw material for cracking (which is a mixture of heavier oils boiling in the range 330–550 °C) has to be controlled at levels below mg kg⁻¹. A reliable, simple and fast method of Ni determination in this kind of material is proposed with a detection limit of 1.2 μg kg⁻¹ in the initial sample at 1 : 5 dilution m/v. The method relies just on sample dilution in xylene and Ni determination using graphite furnace atomic absorption spectrometry, with palladium acetylacetonate and air ashing as a double modifier. The method was successfully validated using mineralization as an alternative sample preparation procedure. The following commonly used methods failed as tools for verification of analysis accuracy: Ni in the investigated cracking feed behaved completely differently than in the available certified reference materials; recovery of standard added to samples was close to 100%, while results were up to 10 times too low; pyrolysis curves did not reveal a decrease of integrated absorbance appearing in the entire investigated temperature range. Decreased evaporation of Ni porphyrins due to interaction with graphite, and increased Ni binding in the solid phase in the presence of asphaltenic matrix, should be taken into account in interpretation of technologically important information on speciation of Ni (volatile and non-volatile Ni forms).