Claus reactor system

The elemental sulfur that is formed in the primary reactor system is condensed in a horizontal shell-and-tube steaming condenser (17). This represents over 40% of the total recovered sulfur. The process gas stream then enters the first stage (18) of a two-stage Claus reactor system where the following exothermic reaction occurs ... 

The H2S comes out with the reactor products, goes through the product-recovery system of the FCCU, and eventually goes to a Claus plant for sulfur recovery. The metal oxide adsorbent recirculates with the spent cracking catalyst back to the regenerator for the next SO adsorption cycle. 

Closer inspection reveals that this somewhat superficial and largely self-evident evaluation is by no means exhaustive, and concrete experimental studies on adsorptive reactors expose both additional pitfalls and benefits that are often specific for a particular reaction system and decisive for the success or otherwise of adsorptive reactor concepts. Before illustrating this point with the help of four examples with which the author is personally acquainted - the Claus reaction, the direct hydrogen cyanide synthesis from ammonia and carbon monoxide and, to a lesser extent, the water-gas shift reaction and the Deacon process - it is worthwhile briefly reviewing other reaction systems for which the potential of adsorptive reactors has been examined (Tab. 7.2). 

The study of adsorptive reactors for the Claus process represents a departure from most previous studies in that the equilibrium position is already well on the product side, with a conversion of 93 % being achievable for isothermal operation of gas with 10 mol% H2S without additional measures [30]. The need to attain conversions in excess of 99.5 % to ensure that the residual sulfur emissions meet environmental specifications [31] nevertheless makes the reaction system an interesting candidate for adsorptive equilibrium displacement. 

The first class of systems is illustrated by the Sulfreen (80, 81, 82) and IFP (83, 84) processes. In the Sulfreen process the Claus reaction takes place on the carbon or alumina catalyst in a fixed-bed reactor. At the reduced temperature, the conversion equilibrium is improved, but the sulfur is retained on the catalyst as a liquid and must be removed by... 

The pressure drop through a CBA unit depends to a large extent on the system design. As an add-on to an existing two reactor Claus unit, the addition of two CBA converters, a condenser, and the switching valves can add I to 3 psi to the overall plant pressure drop. However, converting an existing three-converter Claus unit to a three-converter CBA unit will have minimal impact on plant pressure drop. 

In conventional Claus process catalytic reactors, the gas stream approaches thermodynamic equilibrium with regard to reaction 8-3. The concentrations of HjS and SO2 progressively decrease as the process gases proceed through the Claus system while simultaneously the concentration of water vapor increases. The concentration of sulfur vapor increases in each catalyst chamber, but is periodically reduced by condensation. Unfortunately, there is no simple technique for reducing the concentration of water vapor. As a result of both the effect of reacting gas composition and the requirement to maintain the temperature above the sulfur dew point, the conversion of H2S to elemental sulfur in a conventional Claus system is limited to about 97%. 

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