A fixed bed of solid particles can be used to remove acid gases either through chemical reactions or ionic bonding. Typically, in solid bed absorption processes the gas stream must flow through a fixed bed of solid particles that remove the acid gases and hold them in the bed. When the bed is saturated with acid gases, the vessel must be removed from service and the bed regenerated or replaced. Since the bed must be removed from service to be regenerated, some spare capacity must be provided. There are three commonly used processes under this category: the iron oxide process, the zinc oxide process, and the molecular sieve process.