Treated gases

VOC-bearing offgases from ovens

Treated gases

FIG. 5.20.20 Swingtherm process. (Reprinted, with permission, from Chemical Engineering, 1993 [October]:153.)

H2S is initially removed from a source, such as sour nat- Dry Oxidation ural gas, by dissolution in an alkanolamine solvent. However, for low concentrations of H2S in air, this method is not sufficiently effective for total removal of H2S (Kohl and Riesenfeld 1985). Instead, the H2S is either oxidized completely to SO2, and then the system removes the SO2 by scrubbing or capture, or it is captured by reaction with solid or liquid-phase adsorbents.

For stationary units, any liquid or solid base reacts with H2S. Magnetite (Fe3O4), limestone (CaCO3), lime (CaO), zinc carbonate (ZnCO3), and zinc oxide (ZnO) are each effective, and Fe3O4, CaO, and ZnO are used commercially.

Table 5.20.10 lists the major adsorbents for H2S. The main advantage of using these solutions is to reduce high H2S concentrations in gas streams. Table 5.20.11 lists the advantages and disadvantages of these adsorbents. Using solid adsorbents or dry conversion technology is frequently necessary to remove H2S at the ppm level. Table 5.20.10 also shows examples of dry oxidation technologies. The use of solid adsorbents is expensive; therefore, for air streams containing large amounts of H2S, both scrubbing and adsorption are used in sequence.

The market for H2S is small compared to that for sulfur or sulfuric acid. Thus, regeneration of H2S removed by adsorption from air streams is performed only occasionally. Several proven technologies are available for the dry oxidation of H2S to sulfur.

The dry oxidation process using iron oxide is inexpensive but not pleasant or simple to operate. The following equation empirically gives the suitable box sizes (Steere Engineering Co.):

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