Sulfur reheating processes

The Claus process is the most widely used to convert hydrogen sulfide to sulfur. The process, developed by C. F. Claus in 1883, was significantly modified in the late 1930s by I. G. Farbenindustrie AG, but did not become widely used until the 1950s. Figure 5 illustrates the basic process scheme. A Claus sulfur recovery unit consists of a combustion furnace, waste heat boiler, sulfur condenser, and a series of catalytic stages each of which employs reheat, catalyst bed, and sulfur condenser. Typically, two or three catalytic stages are employed. 

If a warm sulfur removal process is implemented in the plant, sulfur intolerant membranes can be adopted without significant changes in either plant layout or performance. However, if a conventional cold gas cleanup process is employed, the required syngas cooling for purification and its subsequent reheating for the WGS membrane reactor engenders increased plant complexity, capital cost and thermodynamic losses. 

The main processing steps in the Claus process are thermal conversion, sulfur condensation, reheat, and catalytic conversion. In the thermal conversion step, one-third of the feed H2S is oxidized by air ... 

The SO2 thus formed can then be used to make H2SO4, can be treated with limestone, or can be converted to elemental sulfur. The conversion to elemental sulfur can be achieved either by the Claus process or by treating SO2 with a reductant such as carbon (Trail/Resox process). An attractive way of recovering sulfur is by reacting the SO2 with iron sulfide itself to form iron oxide and elemental sulfur. Thus, if the SO2 formed by the oxidation of iron sulfide is recirculated, elemental sulfur can be produced in a single step. The problem with this scheme is that the elemental sulfur needs to be separated from the exit stream by cooling, and the SO2 needs to be reheated. Karr, et al. ( ) and Schrodt and Best (7) carried out experiments to test the feasibility of this scheme. The latter authors, using coal ash as the sorbent material, concluded that sulfur recovery by this method is both technically and economically unattractive. The same may not be true for the iron oxide-silica sorbent. More studies are necessary before a definite conclusion can be drawn. 

In the process (Fig. 1), sulfur and oxygen are converted to sulfur dioxide at 1000°C and then cooled to 420°C. The sulfur dioxide and oxygen enter the converter, which contains a catalyst such as vanadium pentoxide (V205). About 60 to 65% of the sulfur dioxide is converted by an exothermic reaction to sulfur trioxide in the first layer with a 2 to 4-second contact time. The gas leaves the converter at 600°C and is cooled to 400°C before it enters the second layer of catalyst. After the third layer, about 95% of the sulfur dioxide is converted into sulfur trioxide. The mixture is then fed to the initial absorption tower, where the sulfur trioxide is hydrated to sulfuric acid after which the gas mixture is reheated to 420°C and enters the fourth layer of catalyst that gives overall a 99.7% conversion of sulfur dioxide to sulfur trioxide. It is cooled and then fed to the final absorption tower and hydrated to sulfuric acid. The final sulfuric acid concentration is 98 to 99% (1 to 2% water). A small amount of this acid is recycled by adding some water and recirculating into the towers to pick up more sulfur trioxide. 

Carbonation. In this process the mixed juices from the mills are heated, clarified with lime, and evaporated to about 35 degrees Brix. The syrup is relimed and treated with carbon dioxide, filtered, recarbonated, reheated, and refiltered. After carbonation, the syrup is given a double sulfur treatment and filtered. The resulting syrup is subjected to a three- or four-boiling system with the A and B sugars used as the white sugar product. 

The Air Quality Control Systems (AQCS) using lime/limestone wet scrubbing have three basic types of chemical process equipment (1) scrubbers, (2) reaction tanks, and (3) solid-liquid separators, in addition to several auxiliary pieces of equipment such as pumps, demisters, and reheaters. The SO2 in the flue gas is transferred into the liquid in the scrubber, the sulfur in the liquid is converted to solid calcium sulfite, and calcium sulfate in the reaction tanks and solid calcium sulfite and sulfate are separated from the liquid and disposed from the solid-liquid separators such as clarifiers, vacuum filters, and ponds. 

In Denmark the new SNOX process has been developed by Agerholm et al. [153]. Flue gases with a temperature of about 653 K are cleaned from NO by means of selective catalytic reduction. The gas is reheated up to 673 K and introduced to an SO2 converter which is located downstream from the SCR reactor. SO2 is oxidized into SO3 which is converted to sulfuric acid. The ammonia slip from the SCR reactor is oxidized simultaneously. Advantages of the SNOX process are ... 

An improved magnesium oxide (MgO) flue gas desulfurization process and its comparative economics are described. Innovations made include the use of a spray dryer, a cyclic hotwater reheater, and a coal-fired fluidized-bed reactor for regeneration of the MgO absorbent. Several technical concerns with the proposed design are addressed, including fly ash and chloride buildup. The economic evaluation shows the process to have a capital investment of about seven percent less than that of a conventional MgO scrubbing process and a 40 percent smaller annual revenue requirement. Finally, a sensitivity analysis is shown relating annual revenue requirements to the byproduct sulfuric acid price credit. 

Figures 1 and 2, respectively, show the old and new processes. The major innovations are use of (1) a spray dryer absorber in place of the wet venturi, absorber, centrifuge, rotary dryer combination (2) a cyclic hot-water reheat system interconnecting thermally the calciner product solids and the effluent gas from the spray dryer absorber and (3) a coal-fired, fluidized-bed reactor for conversion of magnesium sulfite (MgSO ) and sulfate (MgSO ) to MgO and SO gas. Otherwise, the two systems are very similar, utilizing a regenerable absorbent to recover the sulfur material as a usable commercial grade of concentrated sulfuric acid.

Some of the advanced techniques used in postcombustion cleaning—such as the use of granular calcium oxide or sodium sulfite solutions—have already been described above. In the SNOX process, cooled flue gases are mixed with ammonia gas to remove the nitric oxide by catalytically reducing it to molecular nitrogen. The resulting gas is reheated and sulfur dioxide is oxidized catalytically to sulfur trioxide, which is subsequently hydrated by water to sulfuric acid, condensed, and removed. 

In a conventional type plant with a single absorber it is practical to design the plant to be autothermal with a gas containing as little as 3% sulfur dioxide because heat is only lost when the gas goes to the absorber plus the normal heat lost to atmosphere. In the double-catalysis process, heat is lost from the gas stream going to each of the two absorbers. In the normal design of absorber, if the total gas stream leaves the interstage absorber at 170-180°F to be reheated to converter temperature it would be impossible for the plant to be autothermal with 4% sulfur dioxide gas. 

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