Contact Process: Sulfuric Acid and Oleum

Sulfuric acid and oleum concentration control

Sulfuric acid and oleum are used for different purposes, e.g. for the production of fertilizers, explosives, dyes, and petroleum products. Sulfuric acid can be sold in the form of various solutions of H₂SO₄ in water or sulfur trioxide (SO₃) in H₂SO₄ which is known as fuming sulfuric acid or oleum.

Sulfuric acid and oleum are produced industrially by contact process from sulfur containing gases by burning sulfur, acid regeneration, or metallurgical operations. The process consists of the catalytic oxidation of sulfur dioxide (SO₂) to SO₃, and the hydration of SO₃ to H₂SO₄ by absorption in concentrated acid. There are single or double contact processes at contact plants. 

In the Double Contact Process (also known as DCDA, double contact double absorption), the dry SO₂ containing gas is fed into the converter. Then, the gases are removed from the third catalytic bed, cooled and passed through the primary absorber where SO₃ is absorbed in concentrated sulfuric acid (about 98%). The remaining gases (mostly SO₂) are then returned to the fourth bed of the converter. 

After the converter the gases continue to the final absorber where once again SO₃ is absorbed in concentrated acid. The final product is oleum that is stored in oleum dilution tank and can be diluted with water to receive the desired concentration. The concentration of the liquid is usually kept at 98% to avoid acid scaping from the liquid.

Vaisala Polaris™ Process Refractometer is used at the following stages of the sulfuric acid contact process:

• to monitor and control the concentration of acid during drying, absorption and dilution steps to keep the concentration of H₂SO₄ constant at 93, 98 or 104% by weight.
• to control acid concentration after it has been blended of concentrated
• to control acid circulation to the towers to ensure operation within the optimal concentration range, and maximize the absorption.

In oleum production, the final product is viscous and contains small air bubbles. However, the bubbles, changes in flow or color have no impact on the measurement by the in-line refractometer. The refractometer can withstand the harsh and corrosive environments due to its special wetted parts. It also has intrinsically safe and hazardous area certification.

Lean more details about the contact process for sulfuric acid and oleum from the application note.

Download the application note in PDF by filling the form.

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