Tail-gas processing, Claus

Since the Selectox process is capable of well over 90 percent sulfur recovery, the authors recommend a careful cost-benefit study to see if Claus tail gas processing should be required with such plants. It might well be found that—say—95 percent capture of sulfur in the coal conversion plant is acceptable. 

G. Bandel and W. Willing, The Doxosulifreen claus tail gas processes - meeting enhanced sulphur emission standards . Sulphur 96, loose paper. 

Another variation of the Selectox process can be used with the Beavon process in tail gas treating. The hydrogenated Claus tail gas stream is sent to a Selectox reactor. Overall recoveries of up to 98.5% are possible. Use of Beavon/Selectox, however, typically costs more than use of Superclaus. 

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. 

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. 

Environmental regulations may force the "Claus" sulfur recovery to exceed 99%, requiring tail gas treatment. High CO2 in the original Claus feed, hence in the Claus tail gas, works strongly against the tail gas processes which use amine solution to extract H2S, and subsequently require incineration of residual H2S in a large volume of CO2. 

The H2S concentration in the tail gas of a conventional Claus plant is still some 5%. This H2S is normally incinerated to S02 and released to the atmosphere. Due to stricter environmental regulations a large number of new technologies based on Claus tail gas treatment have been developed to minimise the S02 exhaust from sulphur recovery units. The Superclaus process and the Shell Claus Off-Gas Treating (SCOT) process are treated below. For descriptions of other tail-gas processes, the reader is referred to [2],... 

The acid gas from the Sul find regenerator must be disposed of in an environmentally acceptable manner. The Claus process offers an effective means for converting nearly all of the sulfur in the acid gas to saleable elemental sulfur. The tail gas from the Claus plant still contains some sulfur compounds. To minimize sulfur emissions from the plant, the Claus tail gas can be fed to a Shell Claus ff-gas Jreating (SCOT) unit where most of this sulfur is recovered and recycled to the Claus plant. With use of the SCOT Process, additional marketable sulfur is recovered within the Claus plant while tail gas sulfur emissions are substantially reduced, to typically less than 250 ppmv. 

The SCOT Process has been accepted by industry as an excellent means of reducing Claus tail gas sulfur emissions. 

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 ... 

A number of commercial plants are now using the Beavon sulfur removal process to convert the sulfur content of Claus tail gas first to hydrogen sulfide and finally to elemental sulfur. These plants reduce the sulfur content of the tail gas from about 1—3% to less than 100 ppm of which less than 1 ppm is present as hydrogen sulfide. The foregoing con-... 

Goar, B.G., Current Claus Tail Gas Clean-up Processes, Proceedings of the 57th Annual GPA Convention, New Orleans, LA (March 20-22, 1978). 

The process has been tested in a mobile pilot plant capable of treating 200 m /h of gas over a wide range of SO2 concentrations. This unit has been operated on a coal-fired boiler in Eastern Europe. A 3,400 Nm% commercial unit is planned for a Claus tail gas application at an Esso (Exxon) refinery in Norway (Ulset and Erga, 1991 Peterson, 1992). 

The ATS process is claimed to be competitive with existing Claus tail gas technology for removing and recovering sulfur compounds. However, the economics must be calculated for each specific location since the cost of ammonia and the selling price for the ammonium thiosulfate solution vary wideiy. 

In the BSRySelectox version, the process is used for hydrogen sulfide removal from Claus tail gas after hydrogenation in a BSR process hydrogenation section. (The BSR process is described in a subsequent section.) About 99.5% overall sulfur recovery, including the Claus unit, is attainable. Even higher recovery can be achieved if the effluent from the Selectox reactor is treated in a fmal Claus stage. 

The Beavon Sulfur Removal (BSR) process was developed jointly by the Parsons Coip. of Pasadena, California, and the Union Oil Company of Califoinia (now Unocal Corporation of Los Angeles, California). The term Beavon process refers to a group of processes utilized for the removal of residual sulfur compounds from Claus plant tail gases. This family of processes has in common an initial hydrogenation and hydrolysis unit to convert all residual sulfur compounds to H2S. Individual processes within this family differ from each other in the technology used to remove the H2S from the Claus tail gas stream. Process improvements and operating experience have been reported by Andrews and Kouzel (1974). Fenton et al. (1975), Beavon and Brocoff (1976), and Kouzel et al. (1977). 

Processes using sulfur dioxide as oxidant instead of molecular oxygen, several of which were developed in the 1960s, have not become a factor in either natural gas or geothermal applications. With the exception of the IFP Clauspol process, which was tailored to Claus tail gas applications, none of these methods has been commercially successful. 

The Unisulf process, a homogeneous liquid redox catalytic process for oxidizing H2S to sulfur, was developed by Unocal in 1981 and was licensed jointly by Unocal and The Parsons Corporation. It was intended for clean-up treatment of Claus tail gas, Rectisol or Selex-ol off-gas, and oil shale retort off gas. Because of problems encountered in commercial operations, the process is no longer offered by Unocal (Bingham, 1992). 

The first sulfur dioxide-based process was the Townsend process, developed in 1958. This process never advanced beyond the pilot-plant stage due to mechanical and corrosion problems. It was followed by the IFF Clauspol 1500 and the Wiewiorowski processes in 1969. The IFF process is closely related to the Townsend process, but is restricted in application to the treatment of Claus tail gas. The low cost and simplicity of the IFF process has attracted some commercial interest however, the Wiewiorowski process was never commercialized. 

The ready acceptance of the Clauspol process in Europe was not duplicated in the U.S. Only four plants were built in the U.S. and only one was still in operation in 1992 (at the Phillips refinery, in Borger, TX). The IFP Clauspol 1500 process is relatively inexpensive and easy to operate however, the SO2 emissions (1,000 ppm SO2) are high in comparison to other competing Claus plant tail gas processes. 

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