Many hydrocarbon streams have sulfur-containing compounds as components. These sulfur-containing compounds may make the hydrocarbon stream “sour,” often viewed as undesirable in the industry. One class of the most common form of sulfur-containing compounds present in the hydrocarbon streams is mercaptans, designated R—S—H. R is commonly a light hydrocarbon radical such as a methyl or ethyl group. Mercaptans generally concentrate in hydrocarbon liquid streams during separation in a processing facility.
Various processes have been used to remove sulfur-containing compounds such as mercaptans. Two of the most common processes are the UOP MEROX™ extraction process and the MERICHEM THIOLEXSM REGENSM process. In the MEROX™ extraction process, the mercaptans are removed in a multistage extraction contactor using high efficiency trays. A caustic regeneration section then converts the extracted mercaptans to disulfide oils, via an air/catalyst oxidation reaction, which are then separated and removed in a disulfide separator vessel. The THIOLEXSM REGENSM process is similar, except that in the place of the multistage contactor, the THIOLEXSM REGEN™ process uses a fiber bundle to facilitate contact between the caustic stream and the hydrocarbon stream.
Air is often used for oxidizing the mercaptans to disulfide oils. The unreacted dissolved components of the air stream, i.e., primarily nitrogen and other inert gases and oxygen, are normally separated from the caustic and disulfide oils. The disulfide separator vessel allows the unreacted air components to exit in a vent gas stream. This vent gas stream contains primarily air, and small amounts of water, hydrocarbons, and disulfide oils. Because of the presence of disulfide oils, this vent gas is often treated as a waste, which often triggers various state and national regulations for treatment and handling. When vented to the atmosphere directly or incinerated, the amount of sulfur-containing compounds emitted from these processes are normally enough to require permitting and/or various other state and national agency oversight actions. Some operators of these plants have attempted to avoid this result by routing this vent gas through an activated carbon bed. This activated carbon must be replaced or regenerated after having absorbed its limit of sulfur-containing compounds. It is then often necessary to dispose of the spent activated carbon, typically as a hazardous waste. In addition, to avoid unit shutdown while these carbon beds are either regenerated or replaced, an operator will most often have a backup carbon bed to remove the sulfur-containing compounds when the primary carbon bed is regenerated or replaced.