CO2 Vaporizer Control Valve 1

[dtse86]

Hi,

I'm a mechanical engineer, but I figured that I'd post this in the Chemical Engineering forum given that a lot of you guys seem to know about CO2 liquid storage and their components.

I was inspecting the installation of a new CO2 liquid storage tank and noticed that the drawings indicate the control valve for the vaporizers are drawn on the liquid side of the vaporizers on the P&ID's but in the field they are installed on the vapor side of the vaporizers. They appear to be a strict open close operation and to me I expect if it was meant to handle cold CO2 liquid it could handle the CO2 vapor.

I also figured that even though the tank is set up as a double shell with a vacuum, I still need to be concerned with icing on all of the piping coming out of the tank. Correct?

 

[cloa]

What sort of pressure and temperature is the normal operating envelop- high pressure obviously? Pressurise CO2 enough then you can operate at room temperature.

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[cloa]

Or did you mean supercritical fluid not liquid CO2?

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[dtse86]

Sorry that I did not specify. It is a bulk CO2 storage system for a research facility. It appears that the fill is liquid CO2 and it is actually stored under pressure (between 250psig and 265psig) where I believe it remains in a mixed saturated state.

There are 2 vaporizers on the system. One pressure build vaporizer (PBVap) and one direct to process or distribution vaporizer (DPVap).

From the distribution vaporizer there is a regulator to reduce the pressure to ~60psig to the rest of the building.

 

[Montemayor]

I've designed and built a lot of liquid CO[sub]2[/sub] vaporizers in my youth.

The typical design - whether you use steam or electrical heaters to add the latent heat of vaporization - always is done with the CO[sub]2[/sub] liquid in the saturated state while it is in the storage tank. This means that the saturated liquid CO[sub]2[/sub] is normally at approximately 250-275 psig. The saturated liquid is connected to a vaporizer vessel and allowed to seek the same level as in the tank, using a vapor equalizer line on top of the vaporizer.

When you add heat to the vaporizer, the CO[sub]2[/sub] vapor rises to the top of the tank and resides there as a saturated vapor. This vapor is drawn off by a vapor line from the top of the tank and is regulated by a Valve feeding the process downstream - usually a carbonation plant. As more CO[sub]2[/sub] vapor is drawn off, the pressure in the tank starts to lower and triggers the vaporizer to heat up again, or more rapidly. This process ensures that the vapor generated is in accordance to the pressure required to keep the contents in the liquid state.

Of course, since the vapor drawn off is saturated, it is cold - approximately -8 [sup]o[/sup]F. All pure vaporizers produce saturated vapor and in the case of liquid CO[sub]2[/sub] this happens to be in the minus Fahrenheit range. If your downstream process requires warm or ambient temperature gas, you have to furnish a superheater downstream of the regulator.

The process is very simple and gives no problems or hangups. I have generated a lot of flow diagrams for this system and if needed I can upload one for you.

 

[dtse86]

Montemayer,

You are correct in the way you describe the system and how it was designed.

The temperature of the distribution loop needs to be ambient given that all of the piping is uninsulated.

My concern is with the control valve's ability to seal vapor if it was designed for liquid or vice versa. (Based on the design drawings the control valve is located on the liquid side of the vaporizers and in the field the valve is on the vapor side of the vaporizers).

I am also assuming that the PB vaporizer (meant to maintain pressure during rapid draw from the system and protect the inner tank from extremely low temps) will be drawing cold CO2 liquid from the bottom of the tank and sending back warm vapor. As a result, the liquid lines will need to be insulated to prevent icing on the exterior of the pipe correct?

I'm just trying to understand what a typical installation concerns are from the maintenance side of things. I have seen icing around bulk gas storage tanks, but I doubt that is ideal and probably a pain for the maintenance crew.

 

[Montemayor]

The quickest and most accurate way to describe an engineering process system is with a sketch. Please refer to the attached sketch and see what I mean by a liquid CO[sub]2[/sub] vaporizer as it is used in most industrial applications.

The system shown maintains the design operating pressure of the Liquid CO[sub]2[/sub] tank. This is serves two purposes:

1) It ensures that the pressure and the temperature of the tank will not decrease below the allowable temperature of the tank’s steel;
2) It maintains a vapor inventory that allows for quick, rapid response for a downstream gaseous CO[sub]2[/sub] need.

Another way to vaporize liquid CO[sub]2[/sub] is to put a control valve on the liquid feed line to the vaporizer and control the liquid feed rate to the vaporizer such that the required gaseous CO[sub]2[/sub] is generated and subsequently regulated by another control valve on the gas line feeding the downstream process need. This system is more complicated and further requires additional instrumentation to also vaporize liquid CO[sub]2[/sub] for tank pressure maintenance in the event it is needed. Another negative feature of this system is that its response time to sudden downstream process needs is much slower than the system I show in the sketch.

You will always require maintenance where cold, sub-zero temperatures are involved. The secret to minimum insulation (and piping) maintenance is to ensure that you have a proper vapor sealing on all insulation joints of well designed and installed insulation and continuous inspection and maintenance of the vapor sealing integrity. If you do that, there will be no visual “icing” on the insulation or the equipment.