Sulfacid process

With the double contact process it is unnecessary to purify the tail gases to reduce their sulfur dioxide content still further, whereas tail gases from single contact plants have to be purified. This can be realized either by scrubbing with ammonia or with an aqueous solution of sodium sulfite and sodium hydrogen sulfite (Wellman-Lord process), absorption on activated charcoal (sulfacid process from Lurgi) or by oxidative gas purification such as in the peracidox process (oxidation of sulfur dioxide with hydrogen peroxide or peroxomonosulfuric acid). 

Existing single-contact acid plants can also be converted to doublecontact plants (63). In such cases, however, using add-on scrubber systems is an alternative, and several such systems have been used commercially. The Cominco ammonia absorption process has been used for many years (22, 64). The Lurgi Sulfacid process (65) and Wellman-Lord process (66) have had more recent and limited use. The Mitsu-bishi-JECCO process has also been applied to acid plant tail gases (27, 67, 68), but the gypsum by-product would be essentially a waste in the United States. 

A number of inoiganic pollutants are removable by TSA processes. One of the major pollutants requiring removal is S02 from flue gases and from sulfuric acid plant tail gases. The Sulfacid and Hitachi fixed-bed processes, the Sumitomo and BF moving-bed processes, and the Westvaco fluidized-bed process all use activated carbon adsorbents for proven S02 removal (58). Zeolites with high acid resistance, such as mordenite and clinoptilolite, have proven to be effective adsorbents for dry S02 removal from sulfuric acid tail gas (59), and special zeolite adsorbents have been incorporated into the UOP PURASIV S process for this application (54). 

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. 

Tail gas emissions are controlled by improving the SO2 conversion efficiency and by scrubbing the tail gas. In a double-absorption process plant, a five-bed converter has 0.3 percent unconverted SO2, as compared to 0.5 percent for a four-bed converter. A Lurgi Peracidox scrubber may be used to remove up to 90 percent of the residual SO2 in the tail gas from a double-absorption plant. Hydrogen peroxide or electrolytically produced peroxymonosulfuric acid is used to convert the SO2 to H2SO4 in the Lurgi scrubber. Tail gas from single-absorption plants may be absorbed on activated carbon (Sulfacid) or scrubbed with ammonia or sodium sulfite (Wellman-Lord). 

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