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[Fabien]

Hi!

I am a (French) junior process Engineer. I used to work on Municipal projects (potable/waste water). I am working now on an industrial project (ammonium Sulphate plant).

I am doing the design of a tank where Nickel Sulphate solution is mixed with Ammonium Sulphate salts and Anhydrous Ammonia. The resulting Nickel Sulphate solution will be used in a Nickel reduction plant to recover Nickel. The product is almost saturated with Ammonium Sulphate.

Coils (fed with Medium Pressure steam) are installed in the tank to raise the temperature to 85°C.

Ammonia is added through a sparger at the bottom of a 7m high tank (244m3).
The tank is agitated with 2 impellers (diameter 1.5m). Under normal operating conditions, about 5 tons of Ammonia will discharge into the tank. We are receiving Ammonia at 25°C, 13bars(g). A control valve reduces the pressure to about 1.1bar(g). A fraction of the Ammonia will vaporize in the process. The rest of the Ammonia will vaporize at the sparger outlet then dissolve into the solution.

The tank is maintained under vacuum (-0.5Pa).

I have got two questions:

1) Due to the high Ammonia flow rate and due to the high probability of blockage in the sparger, the sparger slots are rather wide (10mm wide, those are more “gaps” than slots…). We are not using smaller aperture because solution is almost saturated and up to 40 tons of solid Ammonium Sulphate salts are discharged into the tank. Hence, very large bubbles of Ammonia may be released from the sparger.

I am afraid of large Ammonia bubbles reaching the surface. Ammonia is highly soluble in water. But the solution is rather hot and almost saturated with Ammonium Sulphate.

With a 85°C solution saturated with Ammonium sulphate, with 5m of solution above the sparger, what’s the likelihood of having large bubbles reaching the surface? (agitator runs at 80 rpm, Ammonia will be released 1m below the agitator blades).

2) Heat of solution of Ammonia.

I have to determine the heat input required to raise the solution temperature to 85°C. The dissolution of Ammonia in water is exothermic. The values of the heat of solution are generally available for specific conditions (e.g dissolution in pure water at ambient temperature: 1794 kJ/kg). In my case, solution is hot, and saturated with Ammonium Sulphate. The heat of solution should be significantly lower, but I can’t figure out how to estimate the value with the real process conditions.

Many thanks,

Fabien