Hydrogen Fluoride Scrubber

Features

HF scrubbers are custom-designed packed bed scrubbers to provide removal efficiencies from 95% to greater than 99.9% with a low pressure drop.  Drizgas first cools the gas stream to saturation using either a horizontal or vertical quencher depending on the size of the inlet gas flow.

Properties

  • Absorption of Gaseous Hydrofluoric Acid.
  • Solids formation at different fluoride-to-aluminium fatios.
34-HydrogenFluoride_02_(5)

Mechanism

The quencher is constructed using exotic metals for protection against HF corrosion. The HF scrubber uses random dumped packing, a mist eliminator, and a liquid distribution system to remove HF through absorption and neutralization. It is ideally suited for hot combustion exhaust streams including thermal oxidizers, kilns, industrial boilers, or incinerators.

Hydrogen fluoride is very soluble in water and this fact can be used to control fluoride levels in the process gas. Fluorine scrubbers use this fact to scrub the fluoride containing gas in an packed tower with a re-circulated solution of weak HF. The gas enters the bottom of the fluorine scrubber and flows upwards against a flow of scrubbing solution flowing down through the packed section. Hydrogen fluoride will be absorbed into the scrubbing solution and exit the packed section in equilibrium with the incoming scrubbing solution. The scrubbing efficiency is thus dependent on the equilibrium conditions at the top of the scrubber.

The vapour pressure of hydrogen fluoride over aqueous solutions is shown in the following graph.

A solution containing 5 percent HF at 25oC has a equilibrium vapour pressure above the solution of 0.1 mmHg which corresponds to about 130 ppm HF in the gas.   If the concentration of HF in the scrubbing solution is maintained at a lower concentration by purging and the addition of fresh water, equilibrium concentrations of HF approaching zero in the gas can be achieved. The problem with this approach is the large purge and make-up rates that may be required to achieve the desired concentration of HF in the gas exit the scrubber.

The effectiveness of a fluorine scrubber can be increased by lowering the equilibrium vapour pressure at the top of the tower. This can be achieved by allowing the hydrogen fluoride to react to form a compound whose equilibrium vapour pressure is less than that of pure hydrogen fluoride in solution. A substance that achieves the desired affect is sodium silicate or “water glass”.

The reaction of hydrogen fluoride with the silicon oxide in sodium silicate forms fluosilicic acid.

6 HF + SiO2 D H2SiF6 + 2 H2O

The vapour pressure of HF above a solution of fluosilicic acid is significantly less than above a solution of hydrofluoric acid. This allows HF to be reduced to the required levels at the outlet of the fluorine scrubber.

The addition of sodium silicate should be 1 to 1.2 times the stiochiometric amount. The recommended detection methods should be utilized to increase or decrease the addition of sodium silicate as required to ensure fluorides are being removed.

Applications

  • Chemicals industry
  • Waste incineration installations
  • Pharmaceutical industry
  • Storage and transfer of chemicals
  • Surface treatment
  • Gas cleaning system