SCOT process

Current methods for removing sulfur from the hydrogen sulfide gas streams are typically a combination of two processes the Claus Process followed by the Beaven Process, SCOT Process, or the Wellman-Land Process. 

SUPER-SCOT An improved version of the SCOT process which emits smaller quantities of sulfur compounds. Engineered by Stork Engineers and Contractors. 

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. 

The catalyst is necessary to ensure that the components react with reasonable speed, bnt unfortnnately, the reaction does not always proceed to completion. For this reason, two or three stages are nsed, with sulfur being removed between the stages. For analysts it is valuable to know that carbon disulfide (CS2) is a by-product from the reaction in the high-temperature furnace. The carbon disulfide can be destroyed catalytically before it enters the catalytic section proper. Generally, the Clans process may remove only about 90% of the hydrogen sulfide in the gas stream, and as already noted, other processes, such as the Beaven process, SCOT process, or Wellman-Lord process are often used to recover additional sulfur. 

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 SCOT process provides an efficient way of removing sulphur-containing compounds from the tail gas of a conventional Claus reactor. The tail gas is heated to about 300°C and fed to a hydrogenation reactor, where all sulphur compounds in the gas are converted to H2S. Almost all H2S is removed in an absorber/stripper combination and fed back to the Claus plant. The off-gas from the absorber contains virtually no sulphur compounds (values as low as 500 ppm are reported [2]) and is incinerated in the Claus incinerator. A schematic diagram of the SCOT process is provided in Figure 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. 

Shell also offers for license a -selective version of the Shell ADIP Process. The ADIP process, which has a flow scheme very similar to Sulfinol, can be used to treat the Sulfinol acid gas to raise the H2S concentration by selectively rejecting the CO2. Some integration of the SCOT process with the ADIP process is often possible thus, reducing overall equipment and operating costs. Costs for the Claus plant are substantially reduced when "selective" ADIP is applied. Two selective ADIP plants are scheduled to come on stream in the first half of 1979. 

Figure 25.4 shows a typical sulfur recovery plant based on the Claus process. The tail gas from the Claus reactors may be further processed to remove any remaining sulfur compounds. Combined H2S removal efficiencies of 99.5-99.99 percent are achievable.20 This may be done by low-temperature Claus-type solid-bed processes (e.g., the Sulfreen process), wet-Claus absorption/oxidation processes (e.g., the Clauspol 1500 process), or hydrogenation of the off-gas to form H2S for recycle (e.g., the SCOT process). Residual sulfur compounds in the tail gas are then incinerated to S02. The residual S02 in the oxidized tail gas may be scrubbed by any of several processes (e.g., the Wellman-Lord process) before being vented to the environment. It is feasible to bring the H2S content of... 

SCGP. See gasification, Shell coal SCO. See oxidation, selective catalytic SCOT process, 471 scrubber technology, 330-331 SCT. See reactor, short contact time selective catalytic reduction (SCR), 464 Selectra PROX I, 345, 352 Selexol, 212-213, 283-285, 471, 472,... 

The Shell Claus Offgas Treating (SCOT) Process... 

The SCOT Process is competitive with other processes for reducing sulfur dioxide levels to the 200-500 ppm range. Table I shows capital and operating costs for a typical unit to serve a 100 long ton/day (LT/D) SRU operating at 94% recovery efficiency per pass. 

The SCOT process was first made public in September 1972, at a technical meeting in Japan by Shell Internationale Research Maatschappij (SIRM). Shell had proved the effectiveness and life of the catalyst in the reduction step in bench-scale work at their Amsterdam laboratory and a semi-commercial demonstration on Claus SRU tail gas at Shell s Gor-dorf, Germany refinery. Confidence in the effectiveness and selectivity of the amine absorption step was based on Shell s extensive use of the ADIP process in worldwide applications bolstered by laboratory bench-scale testing. 

At an early date, Shell Canada Ltd. decided to install the SCOT process at their Waterton, Alberta gas processing plant. This plant, designed in the Netherlands, will treat the tail gas from a SRU capacity of 2,100 LT/D. Because the total sulfur emission allowable from a single... 

In Japan, virtually all SRlTs are in high population density areas. Consequently, the SCOT process has been quickly adopted there. Seventeen SCOT units are slated to be built in Japan at this time. 

The Scot Process (SHELL Claus Off-gas Treating) developed by SHELL Internationale Maatschappij already in 1971 uses two steps ... 

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