101Composition and Treatment
catalyst change-out. Some re-reneries use guard beds to function as sacricial reac-
tors to remove deleterious constituents before the feedstock passes into the main
catalyst reactor (Speight 2014). These guard beds typically tend to last 1 to 3 months
at a maximum (depending on the contaminants in the used oil feedstock) before
switching over to another guard bed reactor. The rst guard bed is then cleaned and
fresh guard-catalyst is added before switching back onstream and repeating the pro-
cess with the second guard bed reactor.
Hydrotreating (hydroprocessing) is a common process for removing sulfur and
nitrogen impurities from oils as well as hydrogenating unsaturated moieties in the oil
(Speight and Ozum 20 ...