5 replies to this topic

[uxahid]

Hello Everyone

 

I am trying to simulate the LCO2 unloading from a tank and then inject it to an EOR site.

LCO2 is available at -20 C and 20 bara which is subsequently heated and pumped to final conditions of 5 C and 80 bara. When the LCO2 is pumped out of the storage tank, the pressure in the tank drops and we need to give a vapor input stream to the tank in order to maintain the tank pressure and avoid any dry ice formation. 

 

My question is how do we calculate the vapor input to the tank in order to maintain tank pressure? Is there any rule of thumb? I believe, heat leaks to the tank, tank size and density of LCO2 may be the important factors in determining the vapor quantity input to tank. 

 

Will appreciate if you can share your thoughts/experience on this.

 

Thank you.

 

uxahid

[IGC]

Off-topic, where is the installation?  

 

You would need to do a mass balance (in terms of volumetric flowrate to make it easier in this instance), around the tank.  For this you would need to know the liquid withdrawal rate from the tank.  As you remove liquid, the liquid level drops increasing the vapour space resulting in a reduction in pressure.  So to maintain your operating pressure, you are going to vaporise some additional liquid, in a separate line, and pass this back into the vapour space, resulting in a pressure increase in the tank.  

 

http://www.cheresour...d-up-vaporizer/

 

See that topic for a method.  Density of saturated liquid and saturated vapour of CO2 at your operating pressure is required.  Generally you can assume the tank is insulated, so heat ingress would not be an issue.  If you are heating up the tank, you'd be building pressure up further anyway?

 

Pressure needs to be kept above 60.5 psig to avoid solid CO2 forming - once it's there, it's difficult to get out!

 

Depending on where you are, you'll probably be looking at an electrical pressure raiser than an ambient pressure builder which is typical for cryogenic storage.  

 

edit:  typo

Edited by IGC, 24 February 2016 - 09:15 AM.

[Art Montemayor]

IGC is wrong.  There is no reduction in pressure inside the tank as Liquid CO₂ is withdrawn.

 

You are dealing with a SATURATED STATE.  As long as the liquid inside the tank is saturated (and it is), the vapor pressure remains the same.

 

Study and learn what is on the CO₂ database or on the Mollier Diagram.

[IGC]

Art Montemayor:

 

I've assumed that it is a rather large liquid withdrawal from the tank for the EOR injection.  I mean, if it were a trickle out of the tank, then yes, the vapour pressure would remain relatively constant.  I can't see that being the case with a large, rapid, liquid withdrawal though?  Most bulk storage CO2 tanks come with electric pressure builders for high liquid withdrawal, e.g.:

 

http://files.chartin...MARCH12_web.pdf

[uxahid]

Dear IGC

 

Thank you. The link to the post was useful and helped me.

The project is still at conceptual design stage and if it turned feasible, it will be located in the East sea (Between Korea and Japan). Liquid withdrawal rate is 580 tons/hr and while I was doing the dynamic simulation for CO2 unloading from the tank, I witnessed the drop in vessel pressure. 

 

Dear Art Montemayor

 

Thank you for the response. I have taken guidance from many of your posts and they are wonderful. 

[Art Montemayor]

IGC:

 

As I've stated, the liquid is saturated, not subcooled, in a conventional liquid CO₂ tank at 20 bara and -20 ^oC.  If IGC is pumping out 580 tons/hr, the appropriate tank for such a withdrawal would be as large as required for the draw-out rate.  I still maintain that the LCO₂ vapor pressure would instantaneously evaporate as the vapor space gets larger.  The larger the tank, the smaller the drop in vapor space proportionately.   All the LCO₂ tanks I've been associated with - and there have been hundreds of them - have no "electric pressure builders for high liquid withdrawal".  The ones that do have an electric vaporizer have it to maintain the 20 bara pressure in the tank since the normal use of bulk LCO₂ tanks are used to extraction of the vapor CO₂, not the LCO₂.  This is required because, as I've stated before, the LCO₂ is saturated and any lowering of it's vapor pressure also reduces it saturated temperature - which cannot be allowed to go below -20 ^oF due to the usual carbon steel material of construction used and the ASME code.

 

These tanks are normally designed for approximately 350 psig and I've operated them between 215 to 250 psig.  So it depends on what pressure you want the tank to operate at.  These tanks are not normally designed to store LCO₂ without a refrigeration unit as well as a vaporizer.  This is done to ensure that the liquid remained stored at the design condition regardless of the usual heat leaks and vapor extractions.

 

Therefore, what will normally happen in IGC's case is that the usual vaporizer furnished with bulk LCO₂ storage tanks will kick in and maintain the vapor space pressure at 20 bara - or whatever pressure he/she want to maintain it at (as long as it remains below the MAWP and above the minimum material of construction allowable temperature).