Heavy oil hydrotreating catalyst rejuvenation by leaching of foulant metals with ferric nitrate-organic acid mixed reagents

Abstract

Much effort has been devoted in recent years to develop processes for the rejuvenation of metal fouled spent catalysts from residue hydroprocessing units. In this paper we report the influence of adding ferric salts to some organic acids in selective extraction of foulant metals from the spent catalyst prior to decoking. Leaching experiments were carried out using two different organic acids, namely, oxalic acid and tartaric acid with and without ferric nitrate addition. The spent and the treated catalysts were characterized and the improvements in surface area, pore volume and HDS activity as a result of leaching with different reagents compared. The studies revealed that in the absence of ferric salts, the acids showed very poor activity for leaching foulant metals from coked spent catalyst. Addition of ferric nitrate enhanced the leaching efficiency of each acid to a different degree. The selectivity for the removal of the major metal foulant (vanadium) was different for different leaching reagents. The enhanced leaching by the ferric nitrate-organic acid system has been explained in terms of a synergistic mechanism involving oxidizing and complexing reactions. The improvements in surface area and pore volume recovery was found to be related to the extent of vanadium removal from the catalyst. The HDS activity of the catalyst also increased significantly by leaching of the deposited metals. The study showed that oxalic acid-ferric nitrate reagent was superior to other reagent systems in terms of selectivity for vanadium leaching as well as for surface area, pore volume and activity recovery.