Why is Ammonia Stored at -33ºC and P=1bar?

[Chemist20]

Hi there,

Does anyone know why large scale ammonia (60000 tons) storage is done under a temperature of -33ºC and P=1bar? Why not a bit higher P and T?

 

[Chestermiller]

Hi there,

Does anyone know why large scale ammonia (60000 tons) storage is done under a temperature of -33ºC and P=1bar? Why not a bit higher P and T?

The atmospheric boiling point of ammonia is ~ -33C. Having a pressure of 1 atm inside the tank matches the pressure outside, so this keeps the stress low in the containment vessel walls. Also, when heat comes in through the insulation, the vessel can be vented, and the temperature of the ammonia can be maintained by the ammonia boiling and venting. Regulating the pressure and temperature in the vessel is easy, since it just has to be open through a small vent to the outside. There is no control equipment involved to prevent the vessel from overpressuring and exploding.

 

[Enthalpy]

It's for safety, because ammonia is a nasty poison. Just sniff a bottle if you doubt it. A leak at atmospheric pressure is way not as bad as from a pressure vessel.