Mild Hydrocracking: Optimization of Multiple Catalyst Systems For Increased Vacuum Gas Oil Conversion

Abstract

Abstract The mild hydrocracking (MHC) process adds VGO conversion capacity (hydro-cracking (HC)) to vacuum gas oil (VGO) hydrodesulfurization (HDS) units. In addition to alumina-based HC catalysts, use is being made these days of multiple catalyst systems, consisting of alumina-based HC catalysts on top of dedicated (M)HC catalysts, which boosts the HC activities of the MHC units. In this paper we present the results of a detailed investigation of the MHC performance of zeolite-based HC catalysts in such systems. As compared with alumina-based single-bed catalyst systems, the multiple systems exhibit much higher apparent activation energies and much lower apparent reaction orders. This can be ascribed to the higher sensitivity of the zeolitic HC catalysts to poisoning by organic-nitrogen-containing molecules, as compared with alumina-based catalysts. On the basis of this phenomenon a computer model has been developed, which gives an excellent fit with the experimental data, and can also be used to investigate and optimize further the multiple catalyst systems. With the multiple systems the VGO conversion levels can be boosted to about 70 %, which is far beyond the normal MHC range and far beyond the capacities of alumina-based catalysts. This can be done with retention of selectivity patterns and product qualities.