Hydrocarbon Compression

Archive for the ‘Direct Conversion of H2S to Sulfur’ Category

Sulfa-Check Process

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Sulfa-Check process uses sodium nitrite (NaNO2) in aqueous solution to oxidize H2S to sulfur. This process was developed and patented by NL Treating Chemicals and is now a product of Exxon Energy Chemicals. It will generate NOX in presence of CO2 and O2. Therefore, local air quality emission standard should be consulted. This process is most suited for small gas streams, generally 0.1 to 10 MMscfd and containing 100ppmto< 1%H2S.

Sulfa-Check gas sweetening process is generally carried out in a contact tower. The sour gas flows into the bottom of the tower and through a sparging system to disperse the gas throughout the chemical solution. The maximum linear gas velocity should be < 0.12 ft/sec. The sweetened gas exits the contact tower at the top and goes to a gas/liquid separator to catch any liquids that may be carried over. An inverted U with a syphon breaker on top should be designed into the gas inlet line to prevent the liquid from being siphoned back. When the chemical is spent, the system is shut down to remove the spent chemical and recharged with a fresh solution to resume the operation.

Written by Jack

September 19th, 2009 at 3:44 am

IFP Process Diagram

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The Institute Francais du Petrole has developed a process for reacting H2S with SO2 to produce water and sulfur. The overall reaction is 2H2S + 862 -» 2H2O + 3S. Figure 7-10 is a simplified diagram of the process. This process involves mixing the H2S and SO2 gases and then contacting them with a liquid catalyst in a packed tower. Elemental sulfur is recovered in the bottom of the tower. A portion of this must be burned to produce the SO2 required to remove the H2S. The most important variable is the ratio of H2S to SO2 in the feed. This is controlled by analyzer equipment to maintain the system performance.

 IFP Process Diagram

Written by Jack

September 19th, 2009 at 3:41 am

Stretford Process Diagram

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An example of a process using O2 to oxidize H2S is the Stretford® process, which is licensed by the British Gas Corporation. In this process the gas stream is washed with an aqueous solution of sodium carbonate, sodium vanadate, and anthraquinone disulfonic acid. Figure 7-9 shows a simplified process diagram of the process.

 Stretford Process Diagram

Oxidized solution is delivered from the pumping tank to the top of the absorber tower, where it contacts the gas stream in a counter-current flow. The reduced solution flows from the contactor to the solution flash drum. Hydrocarbon gases that have been dissolved in the solution are flashed and the solution flows to the base of the oxidizer vessel. Air is blown into the oxidizer, and the solution, now re-oxidized, flows to the pumping tank.

The sulfur is carried to the top of the oxidizer by a froth created by the aeration of the solution and passes into the thickener. The function of the thickener is to increase the weight percent of sulfur, which is pumped to one of the alternate sulfur recovery methods.

Written by Jack

September 19th, 2009 at 3:39 am

LOCAT Oxidation Process

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The LOCAT® process is a liquid phase oxidation process based on a dilute solution of a proprietary, organically chelated iron in water that converts the hydrogen sulfide to water and elemental sulfur. The process is not reactive to CO2. A small portion of the chelating agent degrades in some side reactions and is lost with the precipitated sulfur. Normally, sulfur is separated by gravity, centrifuging, or melting.

Figure 7-8 represents a process flow diagram of the LOCAT® process. The H2S is contacted with the reagent in an absorber; it reacts with the dissolved iron to form elemental sulfur. The reactions involved are the following:

 LOCAT Oxidation Process

 LOCAT Oxidation Process

The iron, now in a reduced ferrous form, is not consumed; instead, it is continuously regenerated by bubbling air through the solution. The sulfur precipitates out of the solution and is removed from the reactor with a portion of the reagent. The sulfur slurry is pumped to a melter requiring a small amount of heat and then to a sulfur separator where the reagent in the vapor phase is recovered, condensed, and recycled back to the reactor.

LOCAT® units can be used for tail-gas clean-up from chemical or physical solvent processes. They can also be used directly as a gas sweetening unit by separating the absorber/oxidizer into two vessels. The regenerated solution is pumped to a high-pressure absorber to contact the gas. A light slurry of rich solution comes off the bottom of the absorber and flows to an atmospheric oxidizer tank where it is regenerated. A dense slurry is pumped off the base of the oxidizer to the melter and sulfur separator.

Written by Jack

September 19th, 2009 at 3:34 am

Gas Streams Claus Process

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This process is used to treat gas streams containing high concentrations  of H2S. The chemistry of the units involves partial oxidation of  hydrogen sulfide to sulfur dioxide and the catalyticaily promoted reaction  of H2S and SO2 to produce elemental sulfur. The reactions are staged  and are as follows:

 Gas Streams Claus Process

Figure 7-7 shows a simplified process flow diagram of the Claus® process. The first stage of the process converts H2S to sulfur dioxide and

 Gas Streams Claus Process

to sulfur by burning the acid-gas stream with air in the reaction furnace,  This stage provides SO2 for the next catalytic phase of the reaction. Multiple  catalytic stages are provided to achieve a more complete conversion  of the H2S. Condensors are provided after each stage to condense the sulfur vapor and separate it from the main stream. Conversion efficiencies  of 94-95% can be attained with two catalytic stages, while up to 97%  conversion can be attained with three catalytic stages. The effluent gas is  incinerated or sent to another treating unit for “tail-gas treating” before it  is exhausted to atmosphere.

There are many processes used in tail-gas treating. The Sulfreen® and  the Cold Bed Absorption® (CBA) processes use two parallel reactors in a  cycle, where one reactor operates below the sulfur dew point to absorb  the sulfur while the second is regenerated with heat to recover molten  sulfur. Even though sulfur recoveries with the additional reactors are normally 99-99.5% of the inlet stream to the Claus unit, incineration of the  outlet gas may still be required.

The SCOTT® process uses an arnine to remove the H2S. The acid gas  off the amine still is recycled back to the Claus plant. Other types of  processes oxidize the sulfur compounds to SO2 and then convert the SO2  to a secondary product such as ammonium thiosulfate, a fertilizer. These  plants can remove more than 99.5% of the sulfur in the inlet stream to  the Claus plant and may eliminate the need for incineration. Costs of achieving this removal are high.


Written by Jack

September 19th, 2009 at 3:30 am

Direct Conversion of H2S to Sulfur

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The chemical and solvent processes previously discussed remove acid gases from the gas stream but result in a release of H2S and CO2 when the solvent is regenerated. The release of H2S to the atmosphere may be limited by environmental regulations. The acid gases could be routed to an incinerator or flare, which would convert the H2S to SO2. The allowable rate of SO2 release to the atmosphere may also be limited by environmental regulations. For example, currently the Texas Air Control Board generally limits H2S emissions to 4 Ib/hr (17.5 tons/year) and SO2 emissions to 25 tons/year. There are many specific restrictions on these limits, and the allowable limits are revised periodically. In any case, environmental regulations severely restrict the amount of H2S that can be vented or flared in the regeneration cycle.

Direct conversion processes use chemical reactions to oxidize H2S and produce elemental sulfur. These processes are generally based either on the reaction of H2S and O2 or H2S and SO2. Both reactions yield water and elemental sulfur. These processes are licensed and involve specialized catalysts and/or solvents. A direct conversion process can be used directly on the produced gas stream. Where large flow rates are encountered, it is more common to contact the produced gas stream with a chemical or physical solvent and use a direct conversion process on the acid gas liberated in the regeneration step.

Written by Jack

September 19th, 2009 at 3:27 am