Claus unit

Practically all of the H2S and NH3 is stripped from the water, and, with a small amount of phenols remaining, the gas effluent is sent to the Claus unit. 

Purification of refinery gases by elimination of hydrogen sulfide as well as Claus units for sulfur recovery began to make their appearance. 

In 1991, there were approximately 418 sulfur production plants associated with oil and gas production in operation throughout the world. Approximately 86% of these plants were based on the Claus process, and there were 118 Claus units operating in natural gas processing faciHties (11). 

Alternative means for removal of carbonyl sulfide for gas streams iavolve hydrogenation. For example, the Beavon process for removal of sulfur compounds remaining ia Claus unit tail gases iavolves hydrolysis and hydrogenation over cobalt molybdate catalyst resulting ia the conversion of carbonyl sulfide, carbon disulfide, and other sulfur compounds to hydrogen sulfide (25). 

The amount of combustion ait is tightly controlled to maximize sulfur recovery, ie, maintaining the appropriate reaction stoichiometry of 2 1 hydrogen sulfide to sulfur dioxide throughout downstream reactors. Typically, sulfur recoveries of up to 97% can be achieved (7). The recovery is heavily dependent on the concentration of hydrogen sulfide and contaminants, especially ammonia and heavy hydrocarbons, ia the feed to the Claus unit. 

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%. 

Liade AG offers the Clintox process for sulfur dioxide removal. This process uses a physical solvent to absorb the sulfur dioxide. A concentrated sulfur dioxide stream is produced by regeneration. The Clintox process can be iategrated with the Claus process by recovering sulfur dioxide from the iaciaerated tail gases and recycling the sulfur dioxide to the front of the Claus unit. 

Disposal of the spent caustic solution can be a troublesome environmental problem. Depending on the plant location, acid gases are either sent to a fired heater or treated in a Claus unit for conversion of hydrogen sulfide to elemental sulfur. 

MS As are recirculated. The stripped acid gases are fed to a Claus unit" where elemental sulfur is recovered from hydrogen sulfide. In view of air pollution control regulations, the tail gases leaving the Claus unit, R, should be treated for partial removal of the unconverted hydrogen sulfide. Table 3.10 summarizes the stream data. 

There are many processes used in tail-gas treating. The Sulfreen and the Cold Bed Absorption (CBA) processes use two psirallel 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, tiven 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. 

ATS [Ammonium thiosulfate] A process for removing residual sulfur dioxide from Claus tail gas by absorption in aqueous ammonia to produce ammonium sulfite and bisulfite. Addition of hydrogen sulfide from the Claus unit produces saleable ammonium thiosulfate. Developed by the Pritchard Corporation and first operated by the Colorado Interstate Gas Company at Table Rock, WY. 

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. 

In a coal gasification plant, the "Claus unit is nearly the last in a chain of process steps, and the foregoing steps can have a major effect on the Claus in a number of ways. Most of this paper will be examining the kind of Claus feed, but at the outset re should note that a Claus plant is sensitive to variations in feed rate, and that relatively minor ups and downs in upstream units will tend to amplify into wild fluctuations in the Claus plant. Accordingly our Claus-type plant should be inherently stable, if possible, able to ride through large changes in feed rate as well as composition. 

When the gasified coal is to be used for synthesis of methane methanol, or hydrogen, part or all of it is subjected to the water-gas shift reaction, converting CO and water to CO2 and H2. Sulfur must be removed completely. The acid gases H2S and CO2 are first extracted from the gas before or after the shift conversion these acid gases may be processed in a second step in a Claus unit. The acid gas composition depends on each part of the sequence preceding the Claus unit. 

COS usually constitutes about 5 percent of total sulfur in the gasified coal. Passing over the complex problem of extractiig COS from the gas, its conversion to sulfur in the Claus unit requires some attention. It is best hydrolyzed to H2S, then converted to sulfur. Alumina Claus catalyst is effective for hydrolysis at above 650°F, and promoted catalysts are said to be effective at still lower temperatures. 

The acid gas stream from the purification process is sent to a sulfur recovery unit which is usually a Stretford or a Claus unit. The product from these units is elemental sulfur. 

The elemental sulfur is removed by conventional technology. The gases are purified by the Lurgi Rectisol process which uses a low temperature methanol wash to remove H2S, COS and CO2. The acid gas stream is then passed to a Stretford unit which is preferred to the Claus unit because of the high percentage of carbon dioxide in the stream. Sulfur in the stack gas would be removed by conventional flue gas desulfurization techniques and the sulfur would then remain as sulphite sludge and not be recovered as elemental sulfur. 

After the absorber/stripper unit, in conventional operations the pure H2S is fed to a Claus unit where the H2S is converted to elemental sulphur and H2O. The Claus unit can be equipped with an after-treatment to enhance conversions. Another method to decompose H2S to less harmful compounds is the thermal dehydrogenation of H2S to hydrogen and sulphur. Both processes will be treated in detail in the remainder of this chapter. 

When a partial oxidation synthesis gas is treated for H2S and CO2 removal, the resultant acid gas from the treating system is usually quite lean in H2S concentration due to the relatively large amount of CO2. For example, see the acid gas composition in Table IIA where H2S is only 16.07% vol of the Sulfinol acid gas (dry basis). This H2S concentration is near the minimum level which would permit processing the acid-gas in a Claus unit. Moreover, a special splitflow type Claus unit or other unusual procedures would be required to handle this low concentration. 

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