I have written many articles on “scrubbing NH3” in preparation for line breaks/equipment openings or emergency removal from a compromised container and with NH3 the scrubbing media is simply water (H2O). I learned these “scrubbing” methods while working one summer in an internship and the safety pro who hired me was also an MSU graduate and he called these simple, yet highly effective, set-ups a “KY Scrubber”. With chlorine, we can have some simple set-ups, but our scrubbing media is a bit more tricky – but still fairly simple. Chlorine is readily absorbed in and reacted with an alkaline solution, but know this… When absorbing chlorine in alkaline solutions, the heat of the reaction is substantial, so be SURE your KY Scrubber is not made with a “redneck bucket”!
A simple absorption system consists of a suitably sized container capable of holding the required alkaline solution. The alkali should be stored in a form such that a solution can readily be prepared when needed. After the solution is prepared, the chlorine can be passed from the container into the solution through a connection weighted to hold the outlet of the transfer hose or pipe beneath the level of the solution.
Do NOT immerse the container!
The Chlorine Institutes “Chlorine Manual” or now called Pamphlet 1 provides us with the concentration of sodium hydroxide (NAOH) we need for optimal scrubbing. But this optimal concentration is hard to come by so oftentimes, our KY scrubber will require us to DILUTE the more commonly available 50% NAOH down to 10-20% concentration. For our KY Scrubber, getting down to 20% will be optimal; unless we are going to try and scrub a full 1-ton cylinder or something larger.
I need to remind everyone on this PSM/RMP note:
Using this NAOH within your process, especially since its use is “critical” as it is being used to absorb Cl2, PLEASE be certain to include this chemical in your PSI and HAZCOM/PSM training. Using the NAOH in this KY Scrubber MUST be done via an annually certified SOP as well as a certified PPE Hazards Assessment. I also tell my clients that this KY Scrubber will fall under the 1910.119(d)(2)(i)(B) Process chemistry and 1910.119(d)(3)(i)(G) Material and energy balances in our PSI. We also MUST address what we are to do with the scrubbing media once we have completed the task as it will be a Hazardous Waste and a quite nasty one at that.
Do NOT attempt to neutralize the final solution using an acid – as the reaction will evolve Cl2 gas!
When scrubbing Cl2 with 15% to 20% caustic, the heat generated from the reaction can make the scrubbing solution boil – which is NOT what we want from a scrubber efficiency point as well as safety and health point. The water vapor generated by the boiling solution dilutes the chlorine and reduces the mass transfer efficiency of the
scrubber. Most of these KY Scrubbers will not have a means available to control the heat of reaction so “temperature control” can also be accomplished by reducing the initial sodium hydroxide concentration or scrubbing with a higher concentration of NAOH. All of this MUST be covered in the SOP and we MUST have the means to monitor/measure temps of the scrubbing solution. Also, make darn certain that the container we are scrubbing the Cl2 in is capable of handling these temps – including the MAXIMUM temp that could be achieved! I have seen a KY Scrubber or two get overwhelmed and the personnel evacuates the area, but the scrubbing continued even after we have stopped Cl2 flow(s). Be ready for the heat rise to continue, especially if we are relying on a remote valve to stop flows into the container!!