Reaction order and role of hydrogen sulfide in deep hydrodesulfurization of gas oils: consequences for industrial reactor configuration

Deep desulfurization of gas oils to sulfur levels below the currently legis lated 0.05 wt.% is rendered difficult by the presence of refractory species as part of a spectrum of sulfur compounds that have widely differing react ivities. This distribution of reactivities is reflected in a high apparent order (approximately 2) in total sulfur for a plug-flow reactor. The conseq uences of this apparent kinetic behaviour for required catalyst volumes and axial profiles of hydrogen sulfide in a reactor for deep desulfurization a re examined. Hydrogen sulfide proved to strongly suppress the conversion ra te of sulfur compounds, including that of the refractory alkyl substituted dibenzothiophenes crucial for deep desulfurization. A number of process con figurations are discussed that avoids or reduces the problem of hydrogen su lfide inhibition. These include several variants of a two-step process in w hich the bulk of sulfur is removed in a first step, followed by further des ulfurization down to low sulfur levels in a second step with fresh, H2S-fre e hydrogen gas. The theoretically most ideal option, that of operation with countercurrent flow of oil and gas, is not possible in packed beds of the usual catalyst particles because of the occurrence of flooding at industria lly relevant fluid velocities. Some novel reactor concepts based on special structured packings or monoliths that allow such countercurrent operation are presented. (C) 1999 Elsevier Science B.V. All rights reserved.