A schematic
process flow diagram of a basic 2+1-reactor (converter) SuperClaus unit is shown below: The Claus technology can be divided into two process steps, thermal and
catalytic.
Thermal step In the thermal step, hydrogen sulfide-laden gas burns in a substoichiometric
combustion at temperatures above 850 °C. The process requires careful control of the fuel-air ratio. To ensure a stoichiometric Claus reaction in the catalytic step, only of the hydrogen sulfide (H2S) content should convert to SO2. A Claus facility usually maintains several separate fires in lances surrounding a central
muffle to handle different gas sources. The concentration of H2S and other combustible components (
hydrocarbons or
ammonia) then determine how the feed gas is burned. Claus gases with no further combustible contents besides H2S (acid gas) burn in the lances by the following chemical reaction: :2 H2S + 3 O2 → 2 SO2 + 2 H2O (Δ
H = −518 kJ mol−1) This is a strongly
exothermic, free-flame
oxidation of hydrogen sulfide, generating
sulfur dioxide. The central muffle itself burns gas mixtures containing ammonia (from a refinery's
sour water stripper) or hydrocarbons. Sufficient air is injected into the muffle for the complete combustion of all hydrocarbons and ammonia, and the temperature is often maintained above 1050 °C. The high temperature destroys
BTEX (Benzene, Toluene, Ethylbenzene and Xylene) mixtures, which otherwise would
poison the downstream Claus catalyst. To reduce the process gas volume or obtain higher combustion temperatures, the air requirement can also be covered by injecting oxygen. Several technologies utilizing oxygen enrichment are available in industry, but require a special burner in the reaction furnace. The Claus reaction continues downstream, as more hydrogen sulfide (
H2S) reacts with the
SO2, to produce gaseous, elemental sulfur: :2 H2S + SO2 → 3 S + 2 H2O (Δ
H = −1165.6 kJ mol−1) The sulfur forms in the thermal phase as highly reactive S2 diradicals which combine exclusively to the S8
allotrope: : 4 S2 → S8 Usually, 60 to 70% of the total amount of
elemental sulfur produced in the process is already present at the conclusion of the thermal process step.
Side reactions Other chemical processes taking place in the thermal step of the Claus reaction are: Catalytic treatment is normally repeated a maximum of three times. Where an incineration or tail-gas treatment unit (TGTU) is added downstream of the Claus plant, only two catalytic stages are usually installed. The catalytic recovery of sulfur consists of three substeps: heating, catalytic reaction and cooling plus condensation. Reheating the gas prevents sulfur condensation in the catalyst bed, which fouls the catalyst. Several industrial methods achieve the required bed
operating temperature: • Hot-gas bypass: mixing in bypass gas direct from the waste-heat boiler. • Indirect steam reheaters: a
heat exchanger consuming high-pressure steam. • Gas/gas exchangers: a heat exchanger cooling the hot gas from an upstream catalytic reactor. • Direct-fired heaters: fired reheaters burning acid gas or fuel gas substoichiometrically to prevent
oxygen breakthrough The typically recommended operating temperature of the first catalyst stage is 315 °C to 330 °C (bottom bed temperature). The high temperature hydrolyzes
COS and
CS2, combustion byproducts that are otherwise inert during the modified Claus process. For subsequent stages, the catalytic conversion is maximized at lower temperatures, but care must be taken to remain above sulfur's
dew point. The operating temperatures of the subsequent catalytic stages are typically 240 °C for the second stage and 200 °C for the third stage (bottom bed temperatures). After each catalytic pass, the process gas cools in the sulfur condenser to between 150 and 130 °C, whereupon the sulfur formed
condenses. The waste heat and the
condensation heat are captured as medium or low-pressure
steam. The condensed sulfur is removed through a liquid outlet. Before storage, liquid sulfur streams pass a degassing unit, which removes gases (primarily H2S) dissolved in the sulfur. The tail gas from the Claus process still contains combustible components and sulfur compounds (H2S, H2 and CO). It either burns in an incineration unit or is further desulfurized in a downstream tail gas treatment unit.
Sub dew point Claus process The conventional Claus process described above is limited in its conversion due to the reaction equilibrium being reached. Like all exothermic reactions, greater conversion can be achieved at lower temperatures, however as mentioned the Claus reactor must be operated above the sulfur dew point (120–150 °C) to avoid liquid sulfur physically deactivating the catalyst. To overcome this problem, the sub dew point Clauss reactors are oriented in parallel, with one operating and one spare. When one reactor has become saturated with adsorbed sulfur, the process flow is diverted to the standby reactor. The reactor is then regenerated by sending process gas that has been heated to 300–350 °C to vaporize the sulfur. This stream is sent to a condenser to recover the sulfur. ==Process performance==