HF Alkylation Unit One Step Cleaning & Neutralization

When inhibited hydrochloric acid is used to remove iron fluoride scale, the equilibrium in the solution could be described as:

2H⁺ + 2Cl^– + FeF₂ (H₂O) -> 2H⁺ + 2F^– + Fe^-2 + 2Cl^– (H₂O)

The actual amount of HF (H⁺ + F^–) available in the circulating cleaning solution depends on the amount of iron fluoride scale dissolved, and on the quantity of HF remaining in the system after it is evacuated for the cleaning. The problems with this mixture as a cleaning solution for HF Alkylation units concern both corrosion rates and cleaning performance:

• Corrosion inhibitors are inhibitors not preventers. Inhibitors slow down the corrosion rate of hydrochloric acid on carbon steel, but high temperatures or high liquid velocities compromise this performance. Any cleaning solution must be heated to at least 180⁰F for effective polymer removal, but even hydrochloric acid alone (without the fluoride in the solution) is effectively inhibited to only 140⁰F.
• The HF is not inhibited nearly as effectively as the HCl. In utility boiler cleanings, where HCL + HF mixtures are often used to remove silica scale, EPRI (Electric Power Research Institute) guidelines dictate that no more than 0.25% HF should be present in cleaning solutions due to excessive corrosion rates encountered where more HF is present.
• The addition of Boric acid or other organic acids to these HCL mixtures makes them more corrosive and less susceptible to inhibition.
• When HCl solutions are re-used on multiple circulations in an HF alkylation unit (or in any equipment) the solution becomes corrosive due to the total concentration of dissolved iron. This becomes a severe problem at about 1% total iron (ferric iron, which would rarely be seen in an alkylation unit cleaning, and can be reduced to ferrous iron by simple means, is a problem at much lower concentrations). Accelerated corrosion resulting from high iron concentrations is due to a mechanism that is unaffected by conventional inhibitors.

In addition to high corrosion rates and polymer removal problems, the fact that acid cleanings require both rinsing and neutralization creates two distinct disadvantages to this method. The first is added downtime.  EnvTech’s method saves valuable time by including the neutralization in the same step as the cleaning. The second disadvantage is the increased volume of waste liquids created by the rinsing and neutralization. This extra volume is very significant when a large system like an HF Alkylation unit is cleaned.

Furthermore, there is always the risk of leaving something full of acid in a large, complex system like an entire HF Alkylation unit. The failure of even a small piece of piping is a big problem when the unit is started up.

Finally, acid cleanings leave carbon steel surfaces in a very “active” condition.  In other words, the surfaces are very susceptible to additional corrosion.