If a three-stage or four-stage Sulfur Recovery Unit cannot meet the minimum EPA recovery levels, further processing is required. This involves a Tail Gas Cleanup Unit (TGCU), which can either be integrated with the SRU at the design stage or added to an existing SRU. Tail Gas Cleanup Units are divided into two general types: dry bed processes and wet scrubbing processes
When very high overall sulfur recoveries are required, even the extended capability of the sub-dew point Claus process may not be adequate. For these types of facilities, the amine-based SCOT tailgas cleanup process from Shell is often the best choice. The SCOT process is widely recognized as the most economical process for achieving overall sulfur recoveries of 99.7% or higher, and has more industrial applications than any other process of this type
The SCOT process consists of a catalytic reduction section, a feed conditioning section, and a solvent contacting/regenerating section. In the reduction section, reducing gas (hydrogen and/or carbon monoxide) is mixed with the SRU tailgas and passed through a catalyst bed so that all of the sulfur compounds are reduced to H2S.
In the feed conditioning section, the hot gas leaving the catalyst bed is cooled, first in a waste heat boiler (typically), followed by direct contact with quench water. The cooled gas can then be contacted with a selective gas treating solvent in the solvent section, so that nearly all of the H2S is absorbed while most of the carbon dioxide remains in the gas. The treated gas flows to the incinerator for oxidation of the few remaining sulfur compounds. The rich solvent is regenerated to strip out the H2S, which is then recycled back to the sulfur plant feed gas.
With the SCOT Tail Gas Treating Unit (TGCU) to capture nearly all of the unrecovered sulfur leaving the Claus SRU, nearly complete sulfur recovery is possible even though the SRU only recovers 94-97% of the sulfur in its feed gas. If required, the SCOT process can achieve overall sulfur recoveries in excess of 99.9%, resulting in SO2 concentrations in the incinerator effluent of 250 ppm or lower.
The hydrogenating catalysts used in SCOT contain nickel or tungsten on alumina support and the reaction takes place at 480-570°. GAHARCERAM provide high performance hydrogenating catalyst for this purpose