Anita
If you are blanketing a large cryogenic tank, you have to put the N2 to the vapor space. You won't go to -44°.
There are usually N2 lines going to the bottom of the tank to be used when you are taking it out of service or inerting.
If you use these lines N2 can bubble tru the ammonia and cause the partial pressure to fall below 0.6 bar. But you need to add a huge amount of N2 and evaporate about 3 to 4 % of the tank content to do that. You will have other problems and limitations before temperature is a concern.
Also, N2 for blanketing is added to the top of the tank. If you make an equilibrium calculation with more than 40% N2 volume at atmospheric pressure, then yes, Aspen will tell you that the temperature is below -44°C. However, that is not how blanketing works.
In fact, N2 it is normally installed as an emergency measure but is usually not used or needed for the causes I mention below, not minimum temperature concerns:
- If you add N2, then your refrigeration compressors will stop working, unless it is designed to handle a large amount of inerts.
- If the controller fails, or the N2 valve fails open it can cause the relief valves to open. Depending on the tank location this could mean anywhere from an annoyance to an authority in your site investigating the release and your company in the news. Consequences of a tank relief it is something that needs to be addressed.
- Boil off generates a large amount of vapor. Typically, a large cryogenic tank have 0.05% per day of boil off with new insulation. In the ammonia case, you have to multiply the volume by about 750 when evaporating at atmospheric pressure. This means that the boil off alone generates 35-40% per day of the tank volume.
- If you are withdrawing liquid to load a truck, then the buffer in your vapor space is more than enough to compensate for the withdrawn volume. You may not even notice the pressure fluctuation.
- If you are sending to a cryogenic cargo vessel, you also have to add the vapor return line as a source of vapor and overpressure. The net result is that you need a refrigeration compressor extracting vapor. Not addition of gas to the tank.
- If the pumps are partially recirculating you have even more vapor generation.
- If you start with 200 mmH2O g pressure, The reduction of pressure to atmospheric will cause a reduction of about 0.4°C in the liquid and enough evaporation to compensate for the liquid volume withdrawn.
- You can cause vacuum only If you are sending to a cargo vessel at a very large rate, without vapor recirculation, and starting from a very low pressure in the tank.