The removal and recovery of the sulfur compounds from a high-pressure gas stream is typically a two-step process: an amine or physical solvent sulfur removal step followed by a Claus-type process to convert the sulfur species to elemental sulfur. This normally requires cooling the gas to ambient temperature or lower before treating. In addition the sulfur removal step will usually remove some of the carbon dioxide and generate a medium pressure carbon dioxide stream and/or the carbon dioxide may be with the hydrogen sulfide as a low-pressure gas stream for feeding to a Claus sulfur recovery unit.
The Bechtel Pressure Swing Claus is a single-step sulfur removal and recovery technology that operates at full gas pressure and at medium temperatures. The technology can reduce plant capital cost and improve operating efficiency by eliminating amine treatment and, if required, provide a high purity carbon dioxide stream for use in enhanced oil recovery.
The advantages of removing the sulfur separately from the carbon dioxide containing gas are twofold:
- The process reacts and removes the hydrogen sulfide leaving the gasification gas at pressure and with all of the carbon dioxide. This enhances the mass for the turbine combustion in an integrated gas combustion cycle power generation system and thereby increases the power recovery.
- If removal of most of the carbon dioxide is desirable, then a simple physical solvent with flash regeneration can be used since the sulfur was removed upstream and the solvent does not need to be regenerated severely to meet sulfur specifications.
Bench scale testing has been completed to prove out the basic chemistry and reaction kinetics. Overall sulfur recoveries have been determined and various catalysts evaluated. As a result of this work a patent application was submitted and a U.S. Patent, No. 7,374,742, was issued.
The removal and recovery of the sulfur compounds from a high-pressure gas stream is normally a two-step process: an amine or physical solvent sulfur removal step followed by a Claus-type process to convert the sulfur species to elemental sulfur. This normally requires cooling the gas to ambient temperature or lower before treating. In addition the sulfur removal step will usually remove some of the carbon dioxide and generate a medium pressure carbon dioxide stream and/or the carbon dioxide may be with the hydrogen sulfide as a low-pressure gas stream for feeding to a Claus sulfur recovery unit.
The process involves: gas conditioning; H2S and COS reaction over a conventional Claus catalyst at sub-dewpoint conditions; and regeneration by pressure swing and or heating. This allows sulfur formation, collection and separation in a single step. Multiple reaction stages can be used to achieve very low treated gas sulfur content.
The first step for sulfur recovery in this process is an optional thermal oxidation unit. In the thermal oxidation section, air, enriched air, oxygen, SO2, or a combination thereof, is introduced and a portion of the sulfur species contained in the sour gas stream is oxidized through combustion, which results in the formation of SO2. Afterward, the gas stream is further processed in the catalytic unit(s) of the sulfur recovery process, which is a sub-dewpoint Claus process in this example. In the sub-dewpoint Claus process, the sulfur species are converted to liquid elemental sulfur and are collected on the catalyst. In this process, the liquid sulfur is removed during catalyst regeneration. Sub-dewpoint Claus process reactors can be connected in series of between 2 and 5 in order to subject the sour gas stream to several iterations of sulfur removal. This is in addition to the one to three reactors (one in regeneration step, one in the optional standby reactor, and one in the optional guard reactor) shown in the reactor regeneration cycle offset box.
When the catalyst in one of the sub-dewpoint Claus process reactors loses its effectiveness, the reactor is taken off-line for regeneration. The sour gas is routed to the next reactor online and all the reactors move one step closer to regeneration. Also, the last reactor or the optional guard reactor is replaced with a standby Claus reactor. In this way, the gas flows from the most used or loaded reactor to the least used or recently regenerated reactor, and the reactors cycle counter-clockwise through the system. The loaded ineffective reactor is regenerated by a sequence of steps that remove the adsorbed sulfur and restore the bed catalytic activity. This sequence involves nitrogen purging of the reactor, reactor depressurization, heating the reactor with hot recycled nitrogen to remove sulfur, cooling the sweep gas by low pressure steam generation to condense and separate sulfur, repressurizing the sweep gas in a blower and reheating sweep gas, recycling the nitrogen gas without heating in order to cool the reactor after regeneration, and repressurizing the reactor with sweet gas.