Claus plant hydrogen sulfide processing

One of the principal aspects of refinery gas cleanup is the removal of acid gas constituents, ie, carbon dioxide, CO2, and hydrogen sulfide, H2S. Treatment of natural gas to remove the acid gas constituents is most often accompHshed by contacting the natural gas with an alkaline solution. The most commonly used treating solutions are aqueous solutions of the ethanolamines or alkah carbonates. There are several hydrogen sulfide removal processes (29), most of which are followed by a Claus plant that produces elemental sulfur from the hydrogen sulfide. 

Certain of the above reactions are of practical importance. The oxidation of hydrogen sulfide in a flame is one means for producing the sulfur dioxide required for a sulfuric acid plant. Oxidation of hydrogen sulfide by sulfur dioxide is the basis of the Claus process for sulfur recovery. The Claus reaction can also take place under mil der conditions in the presence of water, which catalyzes the reaction. However, the oxidation of hydrogen sulfide by sulfur dioxide in water is a complex process leading to the formation of sulfur and polythionic acids, the mixture known as Wackenroeder s Hquid (105). 

A derivative of the Claus process is the Recycle Selectox process, developed by Parsons and Unocal and Hcensed through UOP. Once-Thm Selectox is suitable for very lean acid gas streams (1—5 mol % hydrogen sulfide), which cannot be effectively processed in a Claus unit. As shown in Figure 9, the process is similar to a standard Claus plant, except that the thermal combustor and waste heat boiler have been replaced with a catalytic reactor. The Selectox catalyst promotes the selective oxidation of hydrogen sulfide to sulfur dioxide, ie, hydrocarbons in the feed are not oxidized. These plants typically employ two Claus catalytic stages downstream of the Selectox reactor, to achieve an overall sulfur recovery of 90—95%. 

COMBISULF A process for removing sulfur compounds from the gases from combined-cycle power plants with integrated coal gasification. Carbonyl sulfide is removed by catalytic hydrolysis hydrogen sulfide is removed by selective absorbtion in aqueous MEDA and the sulfur is recovered from both processes by a modified Claus unit. Developed by Krupp Koppers and first operated in Spain in 1996. See PRENFLO. 

SuRe [Sulphur recovery] A version of the Claus process in which the capacity of the plant is increased by using air enriched in oxygen in the production of the sulfur dioxide. There are two versions SURE SSB [Side Stream Burner], and SURE DC [Double Combustion], In the first, a small portion of the feed stream containing hydrogen sulfide is burnt sub-stoichiometrically in a second burner in the second, the hydrogen sulfide is oxidized in two stages, with cooling and sulfur separation between them. Both of these... 

In the SCOT process, the sulfur compounds in the Claus tail gas are converted to hydrogen sulfide by heating and passing it through a cobalt-molybdenum catalyst with the addition of a reducing gas. The gas is then cooled and contacted with a solution of diisopropanolamine (DIPA) that removes all but trace amounts of hydrogen sulfide. The sulfide-rich diisopropanolamine is sent to a stripper, where hydrogen sulfide gas is removed and sent to the Claus plant. The diisopropanolamine is returned to the absorption column. 

Many processes have been developed for the removal of hydrogen sulfide from gas streams. They can be classified as liquid absorption, liquid oxidation, dry oxidation, and adsorption. One of these processes is usually included in a coal gasification or liquefaction flowsheet since the coal sulfur is converted to H2S and finally elemental sulfur. The Stretford and Townsend direct HpS to S processes and the Recti sol process followed by a Claus plant are frequently included on coal conversion flowsheets (1 ). Kohl and Riesenfeld (2) present pertinent details for many commercial processes. 

Hydrogen sulfide is recovered from natural gas or refinery gases by absorption in a solvent or by regenerative chemical absorption.1819 In either case a concentrated hydrogen sulfide stream is produced that is treated further by the Claus process. A typical Claus plant has a feed stream of at least 45 percent H2S, but with modifications can handle streams containing as little as 5 percent H2S. For gas streams with low concentrations of hydrogen sulfide, direct conversion of the hydrogen sulfide to sulfur is accomplished in the solvent system, for example, the Stretford process or CrystaSulf process. 

Beavon Also called BSR [Beavon Sulfur Removal], A process for removing residual sulfur compounds from the effluent gases from the Claus process. Usually used in conjunction with other processes. Catalytic hydrogenation over a cobalt-molybdena catalyst converts carbonyl sulfide, carbon disulfide, and other organic sulfur compounds to hydrogen sulfide, which is then removed by the Stretford process. A variation (BSR/MDEA), intended for small plants, uses preliminary scrubbing with methyl diethanolamine. Developed by the Ralph M. Parsons Company and Union Oil Company of California in 1971. More than 100 plants were operating in 2000. See also SCOT. 

Once removed from the raw gas, the question arises as to what should be done with the acid gas. If there is a large amount of acid gas, it may be economical to build a Claus-type sulfur plant to convert the hydrogen sulfide into the more benign elemental sulfur. Once the H2S has been converted to sulfur, the leftover carbon dioxide is emitted to the atmosphere. Claus plants can be quite efficient, but even so, they also emit significant amounts of sulfur compounds. For example, a Claus plant processing 10 MMSCFD of H2S and converting 99.9% of the H2S into elemental sulfur (which is only possible with the addition of a tail gas clean up unit) emits the equivalent of 0.01 MMSCFD or approximately 0.4 ton/day of sulfur into the atmosphere. Note that there is more discussion of standard volumes and sulfur equivalents later in this chapter. 

Oil refineries and natural gas plants often produce large amounts of hydrogen sulfide, The most applied process for converting this hydrogen sulfide imo elemental sulfur is the modified Claus process. However, 3 to 5% of the H2S feed is not converted into sulfur and has to be treated alternatively. At the University of Utrecht a new catalytic process has been developed to oxidire hydrogen sulfide in Claus tail gas selectively to sulfur [ I,2 ... 

In the molten carbonate process a molten eutectic mixture of lithium, sodium, and potassium carbonates removes sulfur oxides from power plant stack gases. The resulting molten solution of alkali metal sulfites, sulfates, and unreacted car bonate is regenerated in a two-step process to the alkali carbonate for recycling. Hydrogen sulfide, which is evolved in the regeneration step, is converted to sulfur in a conventional Claus plant. A 10 MW pilot plant of the process has been constructed at the Consolidated Edison Arthur KiU Station on Staten Island, and startup is underway. 

Carbon disulfide is purified by distillation. The hydrogen sulfide byproduct is processed to sulfur in Claus plants. 

Extensive dust removal facilities are required to clean up the sulfur dioxide stream from these sources, which adds substantially to the capital cost of the plant and offsets the raw material cost advantage obtained from these sulfur sources. Sometimes, hydrogen sulfide is simply burned to produce sulfur dioxide when the source of the hydrogen sulfide is near a producing sulfuric acid plant. If this method is used then the need for elemental sulfur production by the Claus process is bypassed. 

The hydrogen sulfide is now easily separated from the liquid hydrocarbon stream by distillation, and is then converted to elemental sulfur, another product of tar sands operations, via the Claus process. The stabilized liquid hydrocarbon stream is the synthetic crude oil product of tar sands extraction plants. 

Hydrogen sulfide accumulated from desulfurization processes and any onsite sulfur dioxide are converted to elemental sulfur using two or three stages of Claus reactors [61] (Chap. 9). However, Claus plant operation is seldom a profitable operation for a refinery since the recovered sulfur amounts to only a few tonnes per day. However, the improved ambient air quality achieved in the vicinity of the refinery makes this measure worthwhile. 

The next four papers concentrate on another emission source, hydrogen sulfide from Claus units. The Claus process has been used for several years to remove hydrogen sulfide from petroleum refinery waste gases. However, current environmental concerns require a more efficient recovery of the sulfur values. Some of the new technology to improve Claus plant efficiencies is discussed in these chapters. 

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