Efficiency, Claus reactions

The hydrolysis reaction however, requires higher temperatures (650°F) than the optimum for the Claus redox. Nonetheless it has been clearly demonstrated in plant studies during the 1970 s that running the first converter bed at this higher COS hydrolysis temperature and sacrificing some Claus redox efficiency that can be recovered in later beds does result in an improved overall sulfur recovery efficiency especially in four stage Claus converter systems. 

Using essentially the same sub-dewpoint Claus reaction principle the Cold Bed Absorption (CBA) process of Amoco (47) achieves the same level of tail gas desulfurization. The low temperature high efficiency swing converters can be in line rather than as a tail gas clean up add on unit. 

As stated earlier, carbonyl sulfide and carbon disulfide hydrolyze fairly readily at temperatures in the range of 600° to 7(X)°F in the presence of water vapor and an active Claus catalyst. It is therefore advantageous to design the first catalytic converter for operation at this temperature level if high conversion efficiency is required and if substantial quantities of caibonyl sulfide and carbon disulfide are present. However, this results in inefficient operation of the first converter with respect to the Claus reaction, and installation of a third converter may be desirable to compensate for this loss in efficiency. If air pollution conirol regulations require high conversion efficiency, it is usually economical to use only two catalytic converters, and then remove residual sulfur compounds in a tail gas treating unit. 

Most of the Claus plant tail gas treating processes that have achieved commercial status can be categorized into three basic types (I) sub-dewpoint Claus processes in which a higher conversion efficiency is obtained for the basic Claus reaction by operating the final catalyst bed(s) of the system at a very low temperature, i.e., below the dew point of sulfur in the gas stream (2) direct oxidation processes in which the Claus process section of the plant is oper-... 

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 ... 

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. 

The Claus Catalytic Cbnverters. These units represent the heart of any sulfur recovery plant. While the bulk of the sulfur yield may be obtained in the front end reaction furnace the high overall recovery levels demanded by environmental regulations are largely dependent on the efficiency of the Claus catalytic converters. 

Among the most effective of the modifications to Claus operating procedure is accurate temperature control of the catalyst beds. Gamson and Elkins (27) in the early 1950 s showed that equilibrium sulfur conversion efficiencies in the catalytic redox reaction rise dramatically as operating temperatures are lowered toward the dewpoint of sulfur. While some highly efficient subdewpoint Claus type processes are now in use the bulk of sulfur production from H2S still requires that the converters be operated above the dewpoint. Careful control of converter bed temperature has, however, contributed to improved efficiencies. This has in large part resulted from better instrumentation of the Claus train and effective information feed back systems. 

Oxidation catalysts were among the first to be described and then developed industrially. Because of the energy evolved, oxidation processes were originally known as catalytically induced combustion. Some of the earliest catalytic oxidation reactions used commercially are shown in Table 4.1. This list could also include the Deacon and the Claus processes, which were described in Chapter 2. Subsequently, nitric acid and formaldehyde were produced on a large scale by catalytic oxidation processes. In most early processes, once a reasonable eatalyst had been developed, production was limited only by demand and the availability of efficient equipment. 

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