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

 

[LittleInch]

The pressure drop is relative to the volumes of the liquid and the gas phase. You don't say what that is so no one can work this out.

You really don't want to get to the triple point as it can go solid on you.

You can't just shove in gas and expect tit to liquify it without cooling it.

You need to take a step back and describe your system better with a few diagrams, flow rates, pressure and temperature.

By "pressure building coil" I assume you mean some form of heating to maintain a set pressure?

 

[fadel_n]

The pressure drop is relative to the volumes of the liquid and the gas phase. You don't say what that is so no one can work this out.

You really don't want to get to the triple point as it can go solid on you.

You can't just shove in gas and expect tit to liquify it without cooling it.

You need to take a step back and describe your system better with a few diagrams, flow rates, pressure and temperature.

By "pressure building coil" I assume you mean some form of heating to maintain a set pressure?

Thank you LittleInch for your response
pressure building coil can be with heater or it is simply a finned heat exchanger that can change liquid to gas by exchanging temperature with the atmosphere.

 

[GBTorpenhow]

Your calculation sounds like it could be correct, if you take liquid out of an otherwise sealed container the vapor space pressure will drop.

To maintain constant tank pressure you logically need to introduce vapor on a 1:1 volumetric basis with respect to the liquid volume removed.

You can get the mass rate of liquid product required to be vaporized in the pressure build system from the required withdrawal rate along with the withdrawal liquid and vapor return densities, with some assumptions about the conditions of the liquid withdrawal.

 

[LittleInch]

If you have a 300 ton tank and remove 20 tons from it and the pressure drops from 15 bara to 6 bara, then the initial volume for the vapour must be minute -

You have filled your tank too high.

 

[GBTorpenhow]

pressure building coil can be with heater or it is simply a finned heat exchanger that can change liquid to gas by exchanging temperature with the atmosphere.

Any manner of heater or heat exchanger can work but typically a simple finned ambient heat exchanger is used for simplicity, plus the heating requirements for pressure build for a typical industrial gas application are not extreme. Control is usually simple as well, with a cryogenic self registering pressure regulator.

Large withdrawal rates may require a dedicated heat exchanger, parallel ambient finned units with a timed three way valve to allow for defrosting, etc., etc.

 

[LittleInch]

Your finned HX is going to build an ice block on it pretty fast.

I've said we can't check your calculation because you haven't given us enough information, specifically the volume of the vessel and what percent full it is at 300 tons.

Liquid CO2 follows the saturated vapour line so as you do pressure, the liquid will drop temperature.

I'm not sure how much you actually know about CO2, so perhaps you could tell us?

 

[Snickster]

I cannot comment on your calculation for pressure drop in tank during unloading without more details like volume of tank and initial vapor space, etc. as others have indicated. However I did a calculation to determine the required flowrate of liquid CO2 to a vaporizer/heater for vaporizing into gas for input into the vapor space to maintain constant 14 bara pressure in the vessel. Basically you have to replace the same volume per time of liquid pumped out to the tank truck with volume of gas so that the mass of make up gas at pressure and temperature of vapor space, equals volume of liquid pumped. This assumes that any vaporized liquid at any exit temperature of the vaporizer will equalize with the temperature of the vapor space at -31 deg. C. Attached is my calculation:

 

[georgeverghese]

Adding on to Snickster's calcs, makeup vaporisation rate at external vaporiser for = 11.3kg/min. Heat of vaporisation for sat CO2 at Tsat = 242degK is = 300kJ/kg, which leads to a heating rate of 57kW. Add 10% design margin = 62kW. See if you have LP steam for this thermosyphon type heating operation.