Acid number

Also known as: TAN, Total acid number

The total acid number (or TAN) of a crude oil is a measure of the corrosiveness of the crude due to the presence of acids, particularly naphthenic acids.

TAN is measured based on the mg of potassium hydroxide required to neutralize one gram of crude. Generally, an acid number of anything over 1 is considered high.

Refiners have four options for dealing with high acid crude:

  • Dilution - Often high-acid crudes are mixed with low-acid grades to reduce the average TAN to a level that does not pose a corrosion risk
  • Neutralization - High-acid crude can also be neutralized with caustic as it is fed into the distillation tower by injecting the caustic directly into the crude stream
  • Corrosion inhibitors - Phosphorus-based corrosion inhibitors can be injected into the crude and allowed to coat metal surfaces, reducing the likelihood of corrosion. Inhibitors are also often injected into the parts of the process units where the acidic components can create the highest degree of corrosion
  • Metallurgy - The metals used for key pieces of equipment can by upgraded to more corrosion-resistant stainless steels containing molybdenum and other corrosion-resistant alloys

Author: Tim Fitzgibbon, Refining Industry Sr. Expert

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