New methodology for the determination of sodium in light and heavy petroleum products

Abstract

Sodium represents an important issue to a refinery plant, as it causes catalyst deactivation and by-products corrosion. A method to determine sodium in vacuum gas oils (VGOs) was developed in a previous work, but this method is not suitable for other type of petrochemical materials, such as crude oils (CO) or fuel oils (FO), that also need a control of the sodium concentration.The great difference between the physical and chemical properties of the petrochemical products make them challenging to analyze and to develop one method suitable for all of them. Thus, samples were classified in light, medium, or heavy according to their API gravity number and the sample preparation methodology was optimized depending on the type of sample. The aim of this work is to develop a methodology with different sample treatment conditions which make it versatile and appropriate for any type of light and heavy petroleum products in a relative short time to be used as a control method and with a quantification limit lower than 1 mg·kg-1.The different sample preparation treatments were set up by incinerating the sample in a new piece of equipment designed and assembled by the Instituto de Tecnología Cerámica (ITC), which was modified to adjust the thermal treatment conditions to the requirements of the sample matrix.Different experiments were carried out modifying not only the heating rate ramp and time of dwelling at maximum temperature but also the atmosphere conditions, working in reducing or oxidizing conditions depending on the matrix sample in order to achieve the complete incineration of the sample without analyte loss. Following this working procedure, a methodology for the determination of sodium was achieved, being able to select a different sample preparation treatment depending on the classification of the petroleum products according to their API gravity number. In this regard, two different sample preparation treatments were optimized, one for light petroleum products, incinerating the sample in 90 min and working in a reductive atmosphere; and another for the heavy ones, which principle modifications included the introduction of an oxygen stream in the last ramp of the heating cycle and a diminution in the heating rate in the first stage of incineration.The developed method for the determination of sodium in petroleum products is a fast and accurate method than can be used as a control one, as the total analysis time is much lower than that of the standardized methods and can be applied to any type of petroleum product. Moreover, the quantification limit reached is lower than 1 mg·kg-1, which is in accordance with the refining industry requirements.