The wet sulfuric acid process (WSA process) is one of the key gas desulfurization processes on the market today. Since the Danish catalyst company Haldor Topsoe introduced and patented this technology in the late 1980s, it has been recognised as an efficient process for recovering sulfur from various process gasses in the form of commercial quality sulfuric acid (H₂SO₄), with simultaneous production of high pressure steam. The WSA process is applied in all industries where removal of sulfur is an issue.

The wet catalysis process is especially suited for processing one or more sulfur containing streams such as.^[1]:

• H₂S gas from e.g. amine gas treating unit
• Off-gas from Sour Water Stripper (SWS gas)
• Off-gas from Rectisol
• Spent acid from e.g. Alkylation
• Claus process tail gas
• Heavy residue or petcoke-fired utility boiler off-gas
• Boiler flue gases from various processes
• Metallurgical process gas

Contents 1 Spent acid regeneration and production of sulfuric acid 2 The Process 3 Industrial applications 4 References

[edit] Spent acid regeneration and production of sulfuric acid

The WSA process can also be used for production of sulfuric acid from sulfur burning or for regeneration of the spent acid from e.g. alkylation plants. Wet catalysis processes differ from other contact sulfuric acid processes in that the feed gas contains excess moisture when it comes into contact with the catalyst. The sulfur trioxide formed by catalytic oxidation of the sulfur dioxide reacts instantly with the moisture to produce sulfuric acid in the vapour phase to an extent determined by the temperature. Liquid acid is subsequently formed by condensation of the sulfuric acid vapour and not by absorption of the sulfur trioxide in concentrated sulfuric acid, as is the case in contact processes based on dry gases.

The concentration of the product acid depends on the H₂O/SO₃ ratio in the catalytically converted gases and on the condensation temperature.^{[2] [3]}

The combustion gases are cooled to the converter inlet temperature of about 420-440 °C. To process these wet gases in a conventional cold-gas contact process (DCDA) plant would necessitate cooling and drying of the gas to remove all moisture. Therefore the WSA process is in many cases a more cost-efficient way of producing sulfuric acid.

[edit] The Process

The main reactions in the WSA process

• Combustion: H₂S + 1.5 O₂ = H₂O + SO₂ + 518 kJ/mole
• Oxidation: SO₂ + ½O₂ = SO₃ + 99 kJ/mole (in the presence of a vanadium (V) oxide catalyst)
• Hydration: SO₃ + H₂O = H₂SO₄ (g) + 101 kJ/mole
• Condensation: H₂SO₄ (g) = H₂SO₄ (l) + 90 kJ/mole

The energy produced by the above mentioned reactions is used for steam production. Approximately 2-3 ton high pressure steam / ton acid produced.

[edit] Industrial applications

Industries where WSA process plants are installed:

• Refinery and petrochemical industry
• Metallurgy industry
• Coal-based industry (coking and gasification)
• Power industry
• Viscose industry
• Sulphuric acid industry

About 80% to 85% of the world’s sulfur production is used to manufacture sulfuric acid. 50% of the world’s sulfuric acid production is used in fertilizer production, mainly to convert phosphates to water-soluble forms, according to the Fertilizer Manual, published jointly by the United Nations Industrial Development Organization (UNIDO) and IFDC. ^[4]

[edit] References