Development of a rapid and accurate method for the determination of sodium in vacuum gas oils (VGOs) by ICP-OES

Sodium in vacuum gas oils (VGOs), even at trace levels, produces corrosion by-products in the refinery pipelines and it is a significant catalyst poison, especially for those from atmospheric or vacuum distillation units, thus its concentration in middle-distillate petroleum products needs to be controlled. In addition, sodium contamination was an issue in this study, as sodium might be present even in the dust floating in the air. The use of an ultrapure sodium-free water and the disposal of a clean and dust-free room were the key to be successful on the development of this method. Different sample preparation methods were studied as sample preparation optimisation was an important step in this study. Dry ashing by different processes, wet acid digestion with different acid mixtures, wet acid microwave-assisted digestion, and dilution with a proper solvent were tried to find the appropriate sample preparation method. An accurate and precise method for the determination of sodium in vacuum gas oils (VGOs) by ICP-OES at trace levels has been developed by ashing the sample with a new piece of equipment designed and created by the Instituto de Tecnología Cerámica (ITC), that permits to calcine the sample in one hour and avoids analyte losses or analyte contamination. The quantification limit achieved by the whole sodium determination method is lower than 1 mg kg^-1, which allows it to be used as control method in the petrochemical industry.