Vanadium poisoning of cracking catalysts: Mechanism of poisoning and design of vanadium tolerant catalyst system

Abstract

The mechanism of vanadium poisoning of cracking catalysts is described. Experimental results identify the poison precursor as volatile vanadic acid, H 3VO 4 which is formed under FCC regenerator conditions by the reaction V 2O 5(s) + 3H 2O(v) 2H 3VO 4(v). The concentration of H 3VO 4 in a typical regenerator (730 °C, 20% H 2O, 2 atm total pressure) is 1–10 ppm. Since H 3VO 4 is a strong acid analogous to H 3PO 4, it can destroy the zeolite by hydrolysis of the zeolite SiO 2 Al 2O 3 framework. A basic solid with reasonable pore structure should be an effective scavenger. Basic alkaline earth oxides such as MgO or CaO are shown to be effective for vanadium scavenging. Microactivity testing shows excellent activity retention when 20% MgO is blended with cracking catalyst at vanadium loadings of 0.67% and 1.34% V by weight on catalyst. However, the SO x , in the regenerator flue gas can form a sulfate that competes with the formation of the vanadate. The degree of competition will be thermodynamically controlled. Since the formation of the vanadate from the oxide expands the lattice, pore structure effects exist similar to those observed for the reaction of calcium oxide with sulfur oxides.