Compensating pressure in Cryogenic Storage Tank

[fadel_n]

Hallo everyone,
I am new here and I am working in CO2 liquifaction.
Our issue in designing the loading station lies in the rate of unloading the liquid CO2 from the Storage Tank. As my understanding and after trying with estimated calculation, for a 300 Ton Tank and with an unloading flowrate of 20 Ton/hr, I recognized that the pressure drop significantly given the initial pressure in the Tank is 14 bar (a) and temperature -31 celsius. The pressure as per calculation become 4.87 bar (I think not logical ).
In this case we are obliged to install a pressure building coil to maintain the pressure inside the Tank and prevent dry ice formation.
Can you please clarify this phenomenon and whether my calcullations were correct?
Another point need to highlight is that when we unload 20 Ton of liquid CO2 from the Tank, we need to replace this volume of liquid by CO2 gas. How can we relate the removed liquid to the gas volume to be replaced. Then we need to change it to liquid to be vaporized and maintain the pressure.
for information: I used the real gas equation (Van der Waal) and mass equilibrium equation.
Thank you for your clarification

 

[GD2]

Hallo everyone,
I am new here and I am working in CO2 liquifaction.
Our issue in designing the loading station lies in the rate of unloading the liquid CO2 from the Storage Tank. As my understanding and after trying with estimated calculation, for a 300 Ton Tank and with an unloading flowrate of 20 Ton/hr, I recognized that the pressure drop significantly given the initial pressure in the Tank is 14 bar (a) and temperature -31 celsius. The pressure as per calculation become 4.87 bar (I think not logical ).
In this case we are obliged to install a pressure building coil to maintain the pressure inside the Tank and prevent dry ice formation.
Can you please clarify this phenomenon and whether my calcullations were correct?
Another point need to highlight is that when we unload 20 Ton of liquid CO2 from the Tank, we need to replace this volume of liquid by CO2 gas. How can we relate the removed liquid to the gas volume to be replaced. Then we need to change it to liquid to be vaporized and maintain the pressure.
for information: I used the real gas equation (Van der Waal) and mass equilibrium equation.
Thank you for your clarification

OP,
If your tank operating conditions are correct (14 bara at -31C), this condition is very near to the liquid-vapour transition. A small drop in pressure will most likely change the liquid CO2 to gaseous state, NOT solid (dry-ice) (refer to any carbon-dioxide pressure-temperature diagram).
It is common that as you offload the liquid CO2, the tank pressure will start dropping as the liquid CO2 is withdrawn. The pressure drop will be faster when the tank is nearly empty.
To increase the tank pressure, a pressure building system like adding a heater is one option but you should look more closely into the filling option as other experts have suggested.