Claus process reactions

The Claus process converts the H2S to sulfur by controlled combustion of the acid gas and Claus reaction on a catalyst. 

Certain of the above reactions are of practical importance. The oxidation of hydrogen sulfide in a flame is one means for producing the sulfur dioxide required for a sulfuric acid plant. Oxidation of hydrogen sulfide by sulfur dioxide is the basis of the Claus process for sulfur recovery. The Claus reaction can also take place under mil der conditions in the presence of water, which catalyzes the reaction. However, the oxidation of hydrogen sulfide by sulfur dioxide in water is a complex process leading to the formation of sulfur and polythionic acids, the mixture known as Wackenroeder s Hquid (105). 

The Claus process, which involves the reaction of sulfur dioxide with hydrogen sulfide to produce sulfur in a furnace, is important in the production of sulfur from sour natural gas or by-product sulfur-containing gases (see Sulfurremoval and recovery). 

When the Claus reaction is carried out in aqueous solution, the chemistry is complex and involves polythionic acid intermediates (105,211). A modification of the Claus process (by Shell) uses hydrogen or a mixture of hydrogen and carbon monoxide to reduce sulfur dioxide, carbonyl sulfide, carbon disulfide, and sulfur mixtures that occur in Claus process off-gases to hydrogen sulfide over a cobalt molybdate catalyst at ca 300°C (230). 

The Claus process converts hydrogen sulfide to elemental sulfur via a two-step reaction. The first step involves controUed combustion of the feed gas to convert approximately one-third of the hydrogen sulfide to sulfur dioxide (eq. 9) and noncatalytic reaction of unbumed hydrogen sulfide with sulfur dioxide (eq. 10). In the second step, the Claus reaction, the hydrogen sulfide and sulfur dioxide react over a catalyst to produce sulfur and water (eq. 10). The principal reactions are as foUow ... 

In the second section, unconverted hydrogen sulfide reacts with the produced sulfur dioxide over a bauxite catalyst in the Claus reactor. Normally more than one reactor is available. In the Super-Claus process (Figure 4-3), three reactors are used. The last reactor contains a selective oxidation catalyst of high efficiency. The reaction is slightly exothermic ... 

Aquaclaus A modification of the Claus process in which hydrogen sulfide is removed from water by reaction with sulfur dioxide. Developed by Stauffer Chemical Company and operated by the Heflin Oil Company, in Queen City, TX. 

OxyClaus A variation of the Claus process, using combustion with oxygen to convert a fraction of the sulfur compounds to sulfur dioxide before reaction. Developed by Luigi Oel Gas Chemie and Pritchard Corporation. 

Extended Claus processes maintain the 2 1 ratio of HS to SO2 in the tail gas. The reaction is extended at low temperatures, below the dewpoint of sulfur. Some processes of this type are the Sulfreen, IFP Clauspol, and CBA processes (9). 

The hydrogen sulfide is then oxidized with air at 1000°C over a bauxite or alumina catalyst. The reactions taking place are given below. The Claus process is increasing in popularity and accounted for 24% of sulfur in 1973, 46% in 1980, 74% in 1991, and 87% in 1999. 

Claus Sulfur Recovery Process The Claus process is a controlled combustion process commonly used for the recovery of sulfur from H2S. Temperatures >2,000°F (1,093.3°C) are achieved during combustion and yields of about 95% are typical. The basic reaction involves the following ... 

The AI2O3 in reaction 6.14 probably functions as a Lewis acid,11 since alcohols usually dehydrate via formation of a carbocation, R—CH2—CH2+. The role of AI2O3 as a catalyst for the Claus process (Section 10.1) may be similarly viewed. 

The efficiency of the Claus Process, long the means of conversion of H2S to sulfur, has been increased through improvements in reaction furnace, catalyst bed and computerized feed composition control leading to recovery efficiencies in excess of 98%. Recent development of a Claus Process under pressure may yield further important improvements. Add on tail gas clean-up processes have further reduced plant effluent in response to environmental protection requirements. 

The production of COS in the front end reaction furnace presents special problems since sulfur in this form may be difficult to remove in the downstream catalytic beds under conditions that are optimal for the Claus redox reaction between H2S and SO COS (and CS2) were known to be generated from hydrocarbon impurities carried over in the acid gas feed thus the efficiency of the up-stream sweetening process became an important factor. The reaction of CO2, a common constituent of the acid gas feed, with H2S and/or sulfur under furnace temperature conditions has also been shown to be an important source of COS. 

These developments and similar tail gas desulfurization processes based on the Claus such as the Bumines Citrate (43) IFP (44) and Clean Air (45) processes are simply methods for forcing the Claus redox reaction of the upstream converter units further to completion. Thus they increase sulfur recovery from H2S and... 

More recently the BSR process has been modified by coupling to the Selectox step (50). This replaces the complex Stretford by directly oxidizing some of the produced H2S to SO2 at low temperature over the new proprietry Selectox-32 catalyst. The SO2 and H2S then react to form sulfur in the traditional Claus redox reaction. While overall conversions by the BSR/Selectox are somewhat less than that possible with the BSR (Stretford), values of 99.8% are claimed. 

Figure 1 illustrates the practical combinations of known processes. With low CO2 acid gas, and only methane as impurity, the conventional Claus process may be used if the reaction furnace is designed to assure complete conversion of hydrocarbons to simple compounds such as COS and H2S. 

The gas-phase sulfur chemistry occurring in the front-end furnace of the Claus process is presumably similar to reactions occurring under fuel-rich conditions in combustion. However, in both systems the chemistry is quite complex and involves a number of unresolved issues. 

The high-temperature chemistry of H2S is important both in combustion and in the Claus process. After initiation H2S is converted to SH by reactions with the radical pool,... 

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