Co2 Storage Tank

[olmedo]

Design pressures of co2 storage tanks are usually set at 350 psig.But in tropical countries ambient temperature goes as high as 115 deg F. w/resulting vapor pressure will be more than 900 psi. Can we rely on refrigeration to prevent over pressure? How do we determine capacity of refrigeration system.

 

[butelja]

Refrigeration can be relied upon to minimize product loss. However, you MUST have a pressure safety relief valve installed on the vessel that will prevent pressure from rising above the MAWP and have the capacity for relieving the boil-off in the event of refrigeration failure.

 

[olmedo]

Thank you Mr. Butelja, is there no possibility that the pressure relief valve will also fail due to mechanical or operator negligence?

 

[butelja]

Anything is possible. However, it is the owner's responsibility to perform preventative maintenance and have engineering controls in place to ensure proper function of safety critical systems. You may want to consider having two PSV's with a selector valve so that one can be serviced while the other remains in place.

 

[dcasto]

Calculations for heat loss from insulated tanks are in Perry's and other sources. Once the heat loss is known, then the continious refrigeration load is known. To calculate the refrigeration load during filling, this can be calculated from the P-H diagram. Do not forget, you'll need a system to pump out of storage and raise the CO2 temperature back to operating temperatures. There are 100's of solutions of for storing CO2, from 0 psig to 1070 psig (criticle point) each will have it's own capital and operating expense point. There will be different points depending on storage volume which pressure is optimal. See the GPSA data book on storage for a chart on storing propane. Generally, the larger the storage amount, the lower the pressure the storage will be and the colder the refrigeration will be..

 

[tjsharpe]

Are these huge tanks,or a collection of small tanks? Building a small shed with insulation drives refridgeration cost down quickly. Are these tanks mounted on a truck
for delivery of Co2? Each case is a new optimization problem. Are high flow rates involved? This changes thermal exchange requirements drastically. More information is needed to bracket an answer. I agree with butelja above on safety. Loss of the contents of a tank is generally cheaper than loss of a tank because of its contents. Relief valves can be doubled up and inspected periodically and are a cheap form of insurance.

 

[olmedo]

Thank you, my specific requirement is for a 50 metric ton capacity storage tank, horizontal and mounted on concrete saddles.

 

[RSC]

The refrigeration load must handle both the expected ambient heat leak as well as any load from injection of warm CO2 into the tank for storage. Sunlight should considered on top of heat input from the atmosphere. Also, relief valves for this system must be piped to a safe location, meaning high enough that a release will disperse in the atmosphere. Since CO2 is heavier than air it will tend to puddle on the ground if released in a confined or depressed area, creating an asphyxiation hazard. Relief system design is further complicated by the fact that CO2 will form a solid if rapidly expanded as through a relief valve. This dry ice could potentially plug the relief valve or the downstream piping. Loss of refrigeration should cause a relatively low relief rate so these may not be big problems, but they should be considered.

 

[dcasto]

The density of CO2 liquid at 250 psia and -1 deg F is close to 62.5 pounds/ft3. This would mean you are looking for approximately 12,000 gallons of storage which is a reasonable size and the pressure and temperatures are reasonable too. The amount of refrigeration needed to keep this stream cool is about 350 horsepower per 1.0 MMBTU/hr of load. (you'll have to get you heat loss load based on economic insulation cost plus the load to take the produced CO2 to these conditions). A prepackaged freon system could be obtained very easily. The tank would be a standard propane or ammonium tank (assuming no H2O that would make system corrosive which all the CO2 pipelines are carbon steel. The calculated relief temperature from this tank (from 250 to 15 psia) is -50 deg F and there is no solid formation all of this is as predicted by the Peng-Robinson equation of state.

Looks like an easy task.

 

[olmedo]

Thanks. I will consider your inputs.