Sulfur heat-removing media

For a reactor operating with constant output, the criterion for optimal performance is for the cooling medium to have the highest possible temperature in the heat removal system. For a working example of the nonadiabatic reactor, there are 4631 cylindrical tubes with inner diameters of 7 mm packed with a catalyst and surrounded by a constantly boiling liquid at 703 K. Sulfur dioxide and air are fed into the reactor at a total pressure PT, in volume fractions of > s,, 2 =0.11 and >v,2 =0.10. The empirical expression oftakes into account diffusion and reaction kinetics, and we have... 

Od condensed from the released volatdes from the second stage is filtered and catalyticady hydrotreated at high pressure to produce a synthetic cmde od. Medium heat-content gas produced after the removal of H2S and CO2 is suitable as clean fuel. The pyrolysis gas produced, however, is insufficient to provide the fuel requirement for the total plant. Residual char, 50—60% of the feed coal, has a heating value and sulfur content about the same as feed coal, and its utilisation may thus largely dictate process utdity. 

Figure 1 shows how acid-gas-bearing process gases can be generally treated in industrial processes. The sulfur compounds and CO2 may be absorbed in a liquid medium, such as amines, alkali salts (NaOH, K2CO3), physical solvents (methanol, propylene carbonate), or water (3). The absorbed acid gases are released by reduction of pressure and/or by application of heat. Alternatively, the H2S and CO2 may chemically combine with the absorbent (as in NaOH scrubbing) to form salts which are removed in a liquid treatment unit. This requires continual and expensive makeup of sodium to the system. 

The CNG process removes sulfurous compounds, trace contaminants, and carbon dioxide from medium to high pressure gas streams containing substantial amounts of carbon dioxide. Process features include 1) absorption of sulfurous compounds and trace contaminants with pure liquid carbon dioxide, 2) regeneration of pure carbon dioxide with simultaneous concentration of hydrogen sulfide and trace contaminants by triple-point crystallization, and 3) absorption of carbon dioxide with a slurry of organic liquid containing solid carbon dioxide. These process features utilize unique properties of carbon dioxide, and enable small driving forces for heat and mass transfer, small absorbent flows, and relatively small process equipment. 

The most commonly used nitrating medium is mixed acid, i.e., nitric and sulfuric acid mixtures, although mixtures of nitric with phosphoric acid, with acetic anhydride, and 100 per cent nitric acid are also used. The reaction usually proceeds readily at room temperature, and external cooling is frequently necessary to remove the heat of the reaction. 

Depending on subsequent processing and final use, various products and by-products must be removed from the low- and medium-heat-content products that come from a gasifier (Table 20.6). In all cases, hydrogen sulfide and other sulfur compounds must be removed because (in addition to the environmental aspects of gas use) they can poison catalysts in subsequent processing. This may be essentially all of the cleanup that is necessary for low-heat-content gas destined for combustion, whereas gas that is to be methanated requires virtually complete removal of essentially all components except hydrogen and carbon monoxide. 

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. 

---