Combination residual oil hydrodesulfurization and thermal cracking

Abstract

Residual oils which are thermally cracked with entrained hot solids in a low residence time riser must be hydrodesulfurized so that sulfur oxide emissions in the flue gas of the solids regenerator are maintained within environmentally acceptable limits. Of a full range residual oil thermal cracking feedstock, the lower boiling distillate feed components are capable of providing a higher ethylene yield with a lower dispersant steam requirement as compared to the high boiling residual feed components. In accordance with the present invention, the high boiling residual components of a thermal cracker feed residual oil are selectively removed during hydrodesulfurization to provide a non-aliquot distillate-residual oil hydrodesulfurization product in which the ratio of lower boiling distillate oil to high boiling residual oil is enhanced. Modifying the aliquot distribution of distillate and residual components in this manner provides an interdependent effect in the combination process since the distillate-enriched cracking feedstock not only provides both an improved ethylene yield and a greater steam economy in the thermal cracking operation but it also reduces hydrogen consumption in the hydrodesulfurization operation. Further interdependence between the hydrodesulfurization and thermal cracking operations is obtained by utilizing the residual oil which is selectively removed during the hydrodesulfurization step as fuel in the solids regenerator of the cracking unit, thereby making the more valuable, highly aromatic black oil produced in the thermal cracking process available for conversion to needle coke and carbon black.