Wet gas Sulfuric Acid process

Other wet gas sulfuric acid processes include SULFOX and Sulfacid . SULFOX is comparable to WSA. Sulfacid only treats low, < 1 volume% SO2 gas using activated carbon. 

Figure X.l Schematic of wet gas sulfuric acid process catalyst bed.

WSA [Wet gas sulphuric acid] A process for recovering sulfur from flue-gases and other gaseous effluents in the form of concentrated sulfuric acid. It can be used in conjunction with the SCR process if oxides of nitrogen are present too. The sulfur dioxide is catalytically oxidized to sulfur trioxide, and any ammonia, carbon monoxide, and carbonaceous combustibles are also oxidized. The sulfur trioxide is then hydrolyzed to sulfuric acid under conditions which produce commercial quality 95 percent acid. Developed by Haldor Topsoe 15 units were commissioned between 1980 and 1995. See also SNOX. 

This is the essence of the wet sulfuric acid process, i.e., its use of condensation to produce high-strength sulfuric acid from H20(g)-bearing, low SO2 strength acid plant feed gas. 

Sulfur oxides (SO,) are compounds of sulfur and oxygen molecules. Sulfur dioxide (SO2) is the predominant form found in the lower atmosphere. It is a colorless gas that can be detected by taste and smell in the range of 1, (X)0 to 3,000 uglm. At concentrations of 10,000 uglm , it has a pungent, unpleasant odor. Sulfur dioxide dissolves readily in water present in the atmosphere to form sulfurous acid (H SOj). About 30% of the sulfur dioxide in the atmosphere is converted to sulfate aerosol (acid aerosol), which is removed through wet or dry deposition processes. Sulfur trioxide (SO3), another oxide of sulfur, is either emitted directly into the atmosphere or produced from sulfur dioxide and is readily converted to sulfuric acid (H2SO4). 

Figure 4-13 shows an example from a three-dimensional model simulation of the global atmospheric sulfur balance (Feichter et al, 1996). The model had a grid resolution of about 500 km in the horizontal and on average 1 km in the vertical. The chemical scheme of the model included emissions of dimethyl sulfide (DMS) from the oceans and SO2 from industrial processes and volcanoes. Atmospheric DMS is oxidized by the hydroxyl radical to form SO2, which, in turn, is further oxidized to sulfuric acid and sulfates by reaction with either hydroxyl radical in the gas phase or with hydrogen peroxide or ozone in cloud droplets. Both SO2 and aerosol sulfate are removed from the atmosphere by dry and wet deposition processes. The reasonable agreement between the simulated and observed wet deposition of sulfate indicates that the most important processes affecting the atmospheric sulfur balance have been adequately treated in the model. 

The gas stream containing sulfur dioxide is either dried before passing to the catalytic oxidation step, or is oxidized in the presence of water vapor with subsequent acid condensation and removal. When acid is produced from elemental sulfur, the air used for sulfur burning is predried. In all cases, typical plant designs use sulfuric acid from the process as a drying agent. Wet catalytic oxidation is relatively uncommon. Some applications of Haldor Topsme s WSA-2 wet gas catalysis process are described in the literature (97). 

A variant of the contact process is operated in coking plants, the moist gas catalysis process, in which wet sulfur dioxide from the combustion of hydrogen sulfide is converted into sulfuric acid. In this process hydrogen sulfide, from coking plants, is converted to sulfur dioxide and water with an excess of air ... 

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.

---