3.3.15 Sulfuric Acid
Sulfuric acid (H2SO4) (the historical name oil of vitriol) is an inorganic chemical that is a highly corrosive strong mineral acid that is a pungent-ethereal, colorless to slightly yellow viscous liquid that is soluble in water at all concentrations. Sometimes, the acid may be sold as a dark brown liquid (dye added during production) to alert purchases the hazards of handling this acid.
Sulfuric acid is manufactured in large quantities on a world scale with the production of the chemical often being linked to the stage of development of a country, owing to the large number of transformation processes in which it is used. Sulfuric acid (H2SO4) is a basic raw material used in a wide range of industrial processes and manufacturing operations. A high proportion of the manufactured sulfuric acid is used in the production of phosphate fertilizers and other uses include copper leaching, inorganic pigment production, petroleum refining, paper production, and industrial organic chemical production.
Sulfuric acid is manufactured from elemental sulfur in a three-stage process:
Since the reaction of sulfur with dry air is exothermic, the sulfur dioxide must be cooled to remove excess heat and avoid reversal of the reaction.
The combustion of elemental sulfur is the predominant source of sulfur dioxide used to manufacture sulfuric acid. The combustion of hydrogen sulfide from waste gases, the thermal decomposition of spent sulfuric acid or other sulfur-containing materials, and the roasting of pyrites are also used as sources of sulfur dioxide. Sulfuric acid may be manufactured commercially by either the lead chamber process or the contact process with a modern leaning toward the contact process.
In the contact process, the process plants are generally characterized according to the raw materials charged to them: (1) combustion of elemental sulfur, (2) combustion of spent sulfuric acid and hydrogen sulfide, and (3) combustion of metal sulfide ores and smelter gas burning. More specifically, the contact process incorporates three basic operations, each of which corresponds to a distinct chemical reaction. First, the sulfur in the feedstock is oxidized (burned) to sulfur dioxide:
The resulting sulfur dioxide is fed to a process unit (often referred to as the converter) where it is catalytically oxidized to sulfur trioxide:
Finally, the sulfur trioxide is absorbed in a strong sulfuric acid (98%) solution:
In the Frasch process, elemental sulfur is melted, filtered to remove ash, and sprayed under pressure into a combustion chamber where the sulfur is burned in clean air that has been dried by scrubbing with 93%–99% (v/v) sulfuric acid. The gases from the combustion chamber are cool by passing through a waste heat boiler and then enter the catalyst (vanadium pentoxide, V2O5) converter. Typically, 95%–98% (v/v) of the sulfur dioxide from the combustion chamber is converted to sulfur trioxide, with an accompanying large evolution of heat. After being cooled, again by generating steam, the converter exit gas enters an absorption tower. The absorption tower is a packed column where acid is sprayed in the top and the sulfur trioxide enters from the bottom. The sulfur trioxide is absorbed in the 98%–99% (v/v) sulfuric acid where the sulfur trioxide combines with the water in the acid and forms more sulfuric acid. If oleum (a solution of uncombined sulfur trioxide dissolved in sulfuric acid) is produced, sulfur trioxide from the converter is first passed to an oleum tower that is fed with 98% (v/v) acid from the absorption system. The gases from the oleum tower are then pumped to the absorption column where the residual sulfur trioxide is removed. The single absorption process uses only one absorber as the name implies, but many plants have installed a dual absorption step.
In the dual absorption step, the sulfur trioxide gas formed in the primary converter stages is sent to an interpass absorber where most of the sulfur trioxide is removed to form sulfuric acid. The remaining unconverted sulfur dioxide is forwarded to the final stages in the converter to remove much of the remaining sulfur dioxide by oxidation to sulfur trioxide, from whence it is sent to the final absorber for removal of the remaining sulfur trioxide.
If oleum (fuming sulfuric acid, simply represented as H2SO4·SO3) is produced (a mixture of excess sulfur trioxide and sulfuric acid), sulfur trioxide from the converter is passed to an oleum tower that is fed with 98% (v/v) acid from the absorbers. The gases from this tower are then pumped to the absorption column where sulfur trioxide is removed. Various concentrations of oleum can be produced. Common ones include 20% oleum (20%, v/v sulfur trioxide in 80%, v/v sulfuric acid, with no water), 40% oleum, and 60% oleum.
Sulfur dioxide is the primary emission from sulfuric acid manufacture and is found primarily in the exit stack gases. Conversion of sulfur dioxide to sulfur trioxide is also incomplete during the process, which gives rise to emissions. Dual absorption is considered the best available control technology (BACT) for meeting NSPS for sulfur dioxide. In addition to stack gases, small amounts of sulfur dioxide are emitted from storage and tank-truck vents during loading, from sulfuric acid concentrators, and from leaking process equipment.
Acid mists may also be emitted from absorber stack gases during sulfuric acid manufacture. The very stable acid mist is formed when sulfur trioxide reacts with water vapor below the dew point of sulfur trioxide. Typical control devices include vertical tube, vertical panel, and horizontal dual pad mist eliminators.
During the production of sulfuric acid, a sludge is produced in the carbon dioxide removal unit used to absorb solvent gas. A hydrocarbon solvent is used in the unit, which breaks down into a hydrocarbon sludge during the process. This sludge is usually combusted in another part of the process. Sulfuric acid manufacture also produces a solid waste containing the heavy metal vanadium, when the convertor catalyst is regenerated or screened. This waste is sent to an off-site vendor for reprocessing. Additional solid wastes from sulfuric acid production may contain both vanadium and arsenic, depending on the raw materials used, and care must be taken to dispose of them properly in landfills.