This is the bottom product from the atmospheric distillation of the crude oil. Most crude oils are distilled in the atmospheric crude oil tower to cut the atmospheric residue at a +650?F up to a +680?F cut point. Cutting the residue heavier than +680?F risks the possibility of cracking with heavier coke lay down and discoloring of the distillate products. Those atmospheric crude towers that do operate at higher cut points minimize the cracking by the recycle of cold quench into the bottom of the tower (below the bottom stripping tray) and minimizing the residue hold up time in the tower. The atmospheric residue may be routed to the fuel oil pool as the precursor to several grades of finished fuel oil products. The other options for this stream in a modern refinery are as follows:
- feed to a vacuum distillation unit. (This is the most common option.)
- feed to a thermal cracker (Visbreaker, or coking unit).
- feed to a deep oil fluid catalytic cracker.
- feed to a hydro-cracker or hydro-treater.