The Atmospheric and Vacuum Residue (AR/VR) hydrotreater is a key refinery unit which improves Residue Feeds for Fluid Catalytic Cracking (RFCC) units and which produces additional diesel as well as low sulfur fuel oil. Residue hydroDeSulfurization units (RDS) improve downstream conversion operations by eliminating difficult feed components thus enabling the refinery engineers to squeezing the most out of the bottom-of-the-barrel, and maximize profitability.