Tail Gas Treatment
Environmental regulations require a high degree of sulfur removal in Claus units. Typical are the German regulations (ТА Luft), which require 97% sulfur recovery for plants up to 20t/dS, 98% for plants between 20 and 50t/d, and 99.5% for larger plants. The thermodynamics of the Claus reactions do not allow one to reach these yields. It is therefore necessary to incorporate some form of tail gas treatment to achieve these yields.
One approach developed by Comprimo is the inclusion of a selective catalytic oxidation step after the second Claus stage, which goes by the name of Superclaus.
There is an important class of tail gas treatment processes typified by Shell’s SCOT (Shell Claus Off Gas Treating) process, although there are now a number of similar processes on the market. The principle arrangement of all these processes is similar and is portrayed in Figure 8-15. The tail gas is hydrogenated in a catalytic reactor, so that all sulfur species are converted to H2S. The gas is then quenched with water and fed to an amine scrubbing unit to remove the H2S. Since the proportion of H2S in the gas at this point of the plant is low, it is important for the economics to choose a selective washing system, typically MDEA. The sulfur-rich gas from the regenerator is recycled to the Claus furnace. Alternatively, the regeneration can be integrated with that of a main H2S amine wash.
Other systems using hydrogenation and subsequent H2S removal are Beavon, which used a Stretford wash for H2S removal (but now also uses MDEA), and others which use, for example, Flexsorb.
An alternative to hydrogenation and absorption is to continue making use of the Claus reaction, but to do this at lower temperatures, at which the sulfur condenses
out in the catalyst bed. Such processes include Amoco’s cold bed adsorption (CBA) process and the SNEA-Lurgi Sulfreen process.
The Sulfreen process depicted in Figure 8-16 serves as a typical example of this class of tail gas treatment. The process is a cyclic one, which uses one reactor in the adsorption mode while the second is being regenerated. For large plants it can be advantageous to include a third reactor. During the adsorption step, tail gas is passed over the catalyst at a temperature of 120-140°C. The low temperature enables the Claus reaction to proceed further towards the production of sulfur, which is adsorbed on the catalyst. In the regeneration mode, hot tail gas is used to desorb the sulfur, which is then recovered in a steam raising sulfur condenser. The heat for the regeneration loop can be supplied by a separate fired heater (as shown) or by heat recovery from the thermal incinerator. Overall sulfur recoveries of 99.0 to 99.5% can be achieved with a standard Sulfreen unit, depending on the Claus feed gas quality. Additional enhancements can be incorporated to achieve higher recoveries, such as incorporating an upstream hydrolysis (Hydrosulfreen, 99.5-99.7%) or a downstream direct oxidation (Carbosulfreen>99.8%) (Fell 1993).