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Chlorine Precipitation

Chlorine Precipitation

(OP)
Good Afternoon,

I have read in some guidance notes that for a chlorine in water solution on cooling below 9.6 deg C, crystals of Chlorine Hydrate are deposited.

We may be dosing a solution of 3 ppm, 5 ppm or 7 ppm and the water treatment facility is located where the temperatures are from -10 deg C to +50 deg C.

If the pipework lagging / insulation is only good for keeping the solution at minimum 5 deg C for a -10 deg C ambient temp, how much chlorine hydrate is likely to form?

Regards
John

RE: Chlorine Precipitation

One would expect that it would all form chlorine hydrated. Chlorine is not particularly soluble in water. At atmospheric pressure pressure, maximum solubility occurs at 49.3°F (9.6°C), at which point approximately 1% dissolves. Below this temperature chlorine combines with water to form a solid hydrate, Cl2·8H2O, known as chlorine ice. Trapped chlorine gas in the water is freed rapidly when it is frozen and then melted again.

Chlorine is also not an effective disinfectant at low temperature because the contact time is extended.

RE: Chlorine Precipitation

(OP)
Many thanks bimr.
All I was trying to determine was that no chlorine 'ice' can accumulate and block any orifices etc en route to the injection point. From you answer, you say that although it does form ice as it is easily freed, it re-melts again.
The re-melting in this case, I assume you are saying that even at 5 degrees solution temperature, the chlorine ice is only a temporary condition?

Regards
John

RE: Chlorine Precipitation

Chlorine ice will melt quickly and is highly soluble. Probably not an issue if the system is closed. The ice will float to the surface in a tank.

There may be problems with feeding chlorine. If the withdrawal rate is too high from a bottle, the outlet of the bottle may freeze.

It is unlikely that the chlorine diffuser will foul. However, the chlorine injector may foul with ice. The carrying water (service water) should be maintained above 10o C to minimize problems.



As mentioned above, the contact time is also extended with low temperatures.

RE: Chlorine Precipitation

(OP)
Thank you bimr.

We are feeding 130 kg/h from 5 x ton containers taking off as liquid. (Duty/standby from another bank of five containers with auto changeover valve system) then duty standby evaporators, pressure reduction, vacuum regulator, then on to 4 chlorinators. From the chlorinators to an ejector (one per chlorinator) which is where the motive water comes in.
The motive water will be lagged and trace heated and this is where I could suggest that the water temperature is to be controlled to minimum 10 degrees.
Once the chlorine gas has been reduced to vacuum, the pipe work spec changes to PVC right up to the injectors. I will check but I think the motive water is also PVC pipe. Not sure if PVC is ok for trace heating.
This is a nice 600 MLD new water supply project for north Iraq.

Regards
John

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