Remove that Bleach Smell

Q. In our scrubber system for chlorination, we use caustic soda as the oxidation agent. This produces sodium hypochlorite, and unfortunately, sodium hypochlorite has an unpleasant smell. Do you have any ideas on how to remove the smell? J.S.

 

A. The characteristic “bleach” odor from the scrubber can be either the sodium hypochlorite (if adequate amounts of caustic soda have been added) or chlorine gas (if the caustic soda is not being added in the correct proportion).

The first thing to check is the pH of the scrubber water. If it is alkaline (pH >9, preferably >10), the most likely source of odor is the hypochlorite. In these solutions, chlorine gas (Cl2) is in equilibrium with hypochlorous acid and sodium hypochlorite. The chlorine gas, in conjunction with the hypochlorous acid, is favored in acidic solutions, and the sodium hypochlorite is favored in alkaline solutions.

Because you are using caustic soda to oxidize, you should be forcing the solution alkaline. If the scrubber water is not alkaline, you need to increase the addition of caustic soda.

Assuming the addition of caustic soda is adequate and the sodium hypochlorite is the source of the odor, you need to investigate several things:
1) Does the scrubber have an adequate mist eliminator? A faulty or missing mist eliminator can allow water droplets to escape into the air. These droplets will carry sodium hypochlorite into the air, producing the odor.

2) Are there leaks in the system? Leaks prior to the scrubber (escaping chlorine gas) would create nearly the same odor as the sodium hypochlorite.  Leaks in the scrubber plumbing would allow sodium hypochlorite into the air, again creating the odor you are experiencing.

3) Is the control system up to the task? Can the control system react quickly enough to handle changing load conditions? A slowly reacting system may allow chlorine gas to escape unreacted.

4) Is the scrubber sized adequately? Have additions to the process or increased production forced the scrubber to handle more load than it was designed to handle?

If you can answer these maintenance and design questions and the odor persists, another chemical addition to the system may be needed. The addition of sodium thiosulfate (also known as sodium hyposulfite) to the scrubber system will neutralize (reduce) the sodium hypochlorite (and the odor) according to the following reaction:

NaClO + NaS₂O₃ + NaOH NaCl + Na₂SO₄ + H₂O

It is important to contact your scrubber system manufacturer before making any additions to the system.  Also, a couple of byproducts and changes will occur:

1) The neutralization of the hypochlorite uses some of the caustic soda, requiring an increase in caustic feed rate.

2) The reaction produces two salts (sodium chloride and sodium sulfate) that will, over time, build up in the scrubber and “salt out” unless periodically purged.

If for some reason you are unable to add sodium thiosulfate to your existing scrubber, you may have to consider installing a second stage that is designed to recirculate a sodium thiosulfate/caustic solution and remove residual odors of the first stage.

Hope these suggestions help.