Reactor Claus-type

Figure 25.4 shows a typical sulfur recovery plant based on the Claus process. The tail gas from the Claus reactors may be further processed to remove any remaining sulfur compounds. Combined H2S removal efficiencies of 99.5-99.99 percent are achievable.20 This may be done by low-temperature Claus-type solid-bed processes (e.g., the Sulfreen process), wet-Claus absorption/oxidation processes (e.g., the Clauspol 1500 process), or hydrogenation of the off-gas to form H2S for recycle (e.g., the SCOT process). Residual sulfur compounds in the tail gas are then incinerated to S02. The residual S02 in the oxidized tail gas may be scrubbed by any of several processes (e.g., the Wellman-Lord process) before being vented to the environment. It is feasible to bring the H2S content of... 

While this type of "indirect reheat" exchanger is a fine way to expedite sulfur plant start-ups, it does very little to improve conversion of H2S/SO2 to liquid sulfur. For the 50-T/D Claus unit, a bypass to direct reheat gas to the first fixed bed reactor was installed. Figure 5-5 illustrates the location of the bypass. 

Bulk Recovery-Clous Process. The classic Claus process is the most common method of producing sulfur. Figure 2-8 is a simplified flow diagram of a typical Claus plant for a feed acid gas with greater than 50% H2S. The first thermal oxidation reaction is fast and takes place in a high-temperature (2,300-2,500°F) furnace-type reactor. The second reaction is relatively slow and requires several stages of catalytic reactors (400-500°F). The gas is cooled to condense and remove sulfur and is then reheated between reactors. The heat liberated by the reactions is used to make medium-pressure steam in the boiler and low-pressure steam in the sulfur condensers. 

The Propylur reactor is an adiabatic fixed bed type, similar to that employed in a Claus unit. The operating temperature is approximately 500 "C and pressure is slightly above atmospheric. The hydrocarbon partial pressure is reduced by diluting the feedstock with steam, in order to shift the equilibrium towards the desired product (propylene). This also minimizes coking and gum formation. The reaction is endothermic and requires additional heat. 

The formation of carbon oxysitlfide and carbon disitlfide in the furnace leads to problems when high overall conversion is reqitired. Alitmina catalysts are not sufficiently active to convert carbon disitlfide and oxysulfide in the first reactor unless a high temperature is reached at the bottom of the bed. When this is not possible, the bottom third of the bed can be loaded with either an iron-promoted alumina or a newer titania catalyst. Cobalt/molybdate/alumina catalysts were also tested in early attempts to hydrogenate the impurities, but it was found that conditions in the first reactor favored sulfur dioxide hydrogenation instead. All of the catalyst types used in Claus sulfur recovery plants are described in Tables 2.10 and 2.11. 

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