Acido sulfurico

POLLUTION COLTROL

Sulfuric Acid Mist and Sulfur Dioxide are the two main pollutants in the DCDA Process for the manufacture of Sulfuric Acid. The Emission Standards given in the literature are: COUNTRY 1.UNITED STATES 2.UNITED KINGDOM 3.WEST GERMANY 4.SWEDEN & DENMARK 5.JAPAN 6.INDIA (200 TPD CAPACITY) SO2 LEVEL Kg/Ton PRODUCT ACID 2.0 3.3 2.9 5.0 0.5 to 1.0 12.0 ACID MIST Kg/Ton PRODUCT ACID0.075 0.4 0.5 5.0

The waste gases from a contact sulfuric acid plant leaving the absorbing tower commonly called the tail gas, consist principally of nitrogen, oxygen and small amounts of sulfur dioxide, the amount of the latter being dependent upon the efficiency of the total plant operation. In addition the tail gases contain sulfur trioxide and sulfuric acid as vapor and mist. The amount ofthese components are normally present in small concentrations in a well-designed and well-operated contact plant. During the start-ups and upset plant conditions, however the concentrations can increase to an amount that constitutes a nuisance. Since sulfur dioxide originating from many sources, is by far the most common noxious gas which is discharged into the atmosphere, it is the firstcontaminant to be singled out for attention in communities that are concerned with air pollution. If present in sufficient concentration, it can be irritating to the throat and nasal passages as well as injurious to vegetation. Sulfur trioxide is also an irritant gas but it immediately forms a sulfuric acid mist with the water vapor in the atmosphere. Sulfuric acid vapor will also immediately form asulfuric acid mist. A very small amount of sulfuric acid will produce an appreciable, visible plume from an acid plant stack. On long exposures, sulfur dioxide concentrations above 0.25 ppm cause injury to growing plants. Elimination of damage to plants from sulfur dioxide emissions by the industry averts any hazard to humans or animals. Permissible limits for prolonged exposure of humans areconsidered to be 10 ppm of sulfur dioxide, 1 ppm of sulfur trioxide or 2-10 ppm of sulfuric acid. Nitrogen oxides are quite harmful and hence their emissions must be kept low. Various Processes for the treatments of tail gases are discussed below: DISPERSAL OF WASTE GASES WITH TALL STOCKS: Tall stacks represent by far the most common method of disposing of the tail gases of sulfuric acid plants. The heightof the stack necessary to get proper dispersion of gases

into the atmosphere in order to avoid ground air pollution will depend on the meteorological conditions which exist at the plant location and the topography of the terrain surrounding the plant, as well as building and structures in the immediate vicinity of the stack. Stacks 150-200ft in height have proved satisfactory for plants whereno usual terrain or meteorological conditions exist. Stacks of more than 400ft height have been used to deal with the problems in special instances. SCRUBBING OF SULFUR DIOXIDE FROM WASTE GASES: SCRUBBING WITH WATER: Several Plants scrub the tail gas with water and report a reduction of 50-75% of sulfur dioxide content. These plants are located where disposal of acidic waste water doesnot presenta problem. SCRUBBING WITH SODA ASH SOLUTIONS: Some plants scrub the tail gases with soda ash solution, producing a marketable sodium bisulfite. SCRUBBING WITH LIME SLURRIES: Neutralization with lime has been employed on flue gases of powerhouses. The lime is supplied as water suspension, and the reaction products are calcium sulfite and calcium sulfate. The slurry of calcium sulfite and calciumsulfate crystals circulates over a grid packed tower. Part of the circulating acid is continuously withdrawn and after passing through an oxidizing tank, the sludge is settled out, with the clarified liquor being returned to the circulation system. SCRUBBING WITH SALT SOLUTIONS: The stack gases from sulfuric acid plant are treated with a slightly alkaline, heated and concentrated saline solution...