TheClaus process is adesulfurizing process, recovering elementalsulfur from gaseous mixtures containinghydrogen sulfide, (H2S). First patented in 1883 by the chemistCarl Friedrich Claus, the Claus process remains the most important desulfurization process in thepetrochemicals industry. It is standard atoil refineries,natural gas processing plants, andgasification orsynthesis gas plants. In 2005, byproduct sulfur from hydrocarbon-processing facilities constituted the vast majority of the 64teragrams of sulfur produced worldwide.[1][2][3]
The overall Claus process reaction is described by the following equation:[3]
However, the process occurs in two steps:
Moreover, the inputfeedstock is usually a mixture of gases, containinghydrogen cyanide,hydrocarbons,sulfur dioxide orammonia. The mixture may begin as rawnatural gas, or output from physical and chemical gas treatment units (Selexol,Rectisol,Purisol andamine scrubbers) when e.g. refiningcrude oil.[4]
Gases containing over 25% H2S are suitable for the recovery of sulfur in straight-through Claus plants. Gases with less than 25% H2S can be processed through alternate configurations such as a split flow, or feed and air preheating.[5]
The process was invented byCarl Friedrich Claus, a German chemist working in England. A British patent was issued to him in 1883. The process was later significantly modified byIG Farben.[6]
A schematicprocess flow diagram of a basic 2+1-reactor (converter) SuperClaus[definition needed] unit is shown below:
The Claus technology can be divided into two process steps, thermal andcatalytic.
In the thermal step, hydrogen sulfide-laden gas burns in a substoichiometriccombustion at temperatures above 850 °C.[7] The process requires careful control of the fuel-air ratio. To ensure a stoichiometric Claus reaction in the catalytic step, only1⁄3 of the hydrogen sulfide (H2S) content should convert to SO2.
A Claus facility usually maintains several separate fires in lances surrounding a centralmuffle to handle different gas sources. The concentration of H2S and other combustible components (hydrocarbons orammonia) 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:
This is a stronglyexothermic, free-flameoxidation of hydrogen sulfide, generatingsulfur dioxide.
The central muffle itself burns gas mixtures containing ammonia (from a refinery'ssour 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.[8][9] The high temperature destroysBTEX (Benzene, Toluene, Ethylbenzene and Xylene) mixtures, which otherwise wouldpoison the downstream Claus catalyst.[10]
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 theSO2, to produce gaseous, elemental sulfur:
The sulfur forms in the thermal phase as highly reactive S2 diradicals which combine exclusively to the S8allotrope:
Usually, 60 to 70% of the total amount ofelemental sulfur produced in the process is already present at the conclusion of the thermal process step.
Other chemical processes taking place in the thermal step of the Claus reaction are:[3]
The waste stream from the thermal step is initially cooled to precipitate the sulfur, similar to the post-catalytic cooling discussed below. Further treatment with an activatedaluminum(III) ortitanium(IV) oxidecatalyst then boosts the sulfur yield. One suggested mechanism is that S6 and S8 desorb from the catalyst's active sites with simultaneous formation of stable cyclic elemental sulfur.[11]
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 bedoperating temperature:
The typically recommended operating temperature of the first catalyst stage is 315 °C to 330 °C (bottom bed temperature). The high temperature hydrolyzesCOS andCS2, 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'sdew 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 formedcondenses. The waste heat and thecondensation heat are captured as medium or low-pressuresteam. 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.
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.
Over 2.6 tons of steam will be generated for each ton of sulfur yield.
Thephysical properties of elemental sulfur obtained in the Claus process can differ from that obtained by other processes.[3] In the Claus process, sulfur is usually transported as a liquid (melting point 115 °C). In elemental sulfur,viscosity increases rapidly at temperatures in excess of 160 °C due to the formation of polymeric sulfur chains.
Another anomaly is thesolubility of residual H2S in liquid sulfur as a function of temperature. Ordinarily, the solubility of a gas decreases with increasing temperature but H2S behaves inversely.[12] This means that toxic and explosive H2S gas can build up in the headspace of any cooling liquid sulfur reservoir. The explanation for this anomaly is the endothermic reaction of sulfur with H2S topolysulfanes H2Sx.
Millions of tons of elemental sulfur are produced worldwide by the Claus process each year.
Owing to the high sulfur content of theAthabasca Oil Sands, stockpiles of elemental sulfur from this process now exist throughoutAlberta, Canada.[13]
Another way of storing sulfur, while reusing it as a valuable material, is as abinder for concrete, the resulting product having many desirable properties (seesulfur concrete).[14]