N-Butane Refrigerated Tank

[ARCH01]

In order to maintain the pressure of an atmospheric refrigerated tank within 1 to 5" H2O for nC4 storage, it has been proposed to circulate a portion of the liquid product through a propane chiller. The amount of refrigerant will be regulated to maintain desired tank pressure. Although from the process point of view it makes sense to remove heat absorbed by the insulated tank wall via the use of a chiller, a very slow response is expected since the tank is 5 MM gallon capacity. I am familiar with refrigerated tanks using re-compressors and condensers to handle tank vapors, but I do not have any previous experience with a system like that described above. I would appreciate any feedback from people who has designed a system using propane chiller.

 

[Montemayor]

Archie:

You are absolutely correct in pointing to the use of simple and direct re-compression of the tank vapors as being an alternative solution. Direct recompression is simpler, cheaper, faster in response, and easier & less expensive to operate (more energy efficient). Liquid Ammonia is commonly stored in recompressed atmospheric tanks at approximately -27 oF.

However, from a safety view point, I am a partisan of the indirect method you are proposing to employ. In my opinion, this is a classical case where diehard engineers have to bite the bullet and go against what is apparently the most economically attractive option – all because of local safety hazard concerns. Allow me to explain the details of this tough decision:

If you employ direct recompression, you will be pulling 32 oF Butane vapors from the tank at only 1 inch of WC (water column) as pressure. You would subsequently condense the Butane with 80-85 oF cooling water and flash the resultant liquid back into the tank vapor space as a direct refrigerant. This is a very efficient and quick way to obtain effective refrigeration. But, regardless of how close you locate a recompressor, you are certain to generate a partial vacuum in the suction line of the system. The further away the recompressor is located (and this is what you really want), the more the suction vacuum generated. This is a very risky and hazardous scenario to contemplate. Should you suck in atmospheric air through a leaky gasket or seal into the recompressor, you could (over time) conceivably build up an extremely dangerous explosive mixture of Oxygen and Butane within the system and your recompressor.

By using a Propane refrigeration cycle you are generating an indirect, cold fluid source as your refrigerating medium and one that, if a leak should appear in your evaporator, would not present an explosive hazard (although you will have to protect your atmospheric tank from overpressure). The propane cycle major equipment will look something like this:

1) evaporator: this is a TEMA shell & tube unit operating around -20 oF & 10.5 psig; (note the nice, positive pressure);
2) compressor: evaporator vapor and compresses it to approximately 150 psig & 90 oF;
3) condenser: this is a TEMA shell & tube unit that uses 80 - 85 oF cooling water to condense the Propane;
4) recirculation pump: this pumps liquid Butane through the evaporator and sprays the subcooled product into the tank vapor space to effect rapid cooling.

Note the extra amount of equipment and instrumentation required for the indirect method (besides the extra energy that is expended through inefficiency and entropy). Also, note that the Propane indirect method can cool down your tank ONLY when the tank has sufficient liquid Butane inventory to recirculate. That means that upon startup with Butane vapor (should that be the case) there will be no refrigeration available to condense the vapors. I presume you know of this frailty and have process procedures to circumvent that situation.

From an instrumentation and process point of view, the direct recompression method is more reliable and quicker in response to a cooling requirement. In contrast, the Propane cycle is slower to respond. Because of the conventional, narrow operating deadband existing in an atmospheric tank between the MAWP (Maximum Allowable Working Pressure) and the normal operating pressure, the cooling response time might be very critical in your case. I also presume you are taking this into consideration and have the services of experienced and capable instrumentation engineers available for this application. I would not undertake to design or implement this type of project without recognized, proven, experienced instrumentation engineer(s) directly involved.

One more item to mention is the fact that all cooling equipment on such a critical piece of equipment is going to require standby, emergency cooling capacity. That means that the Propane system requires 8 major pieces of equipment, while the recompression method only requires 4. You can visualize the difference in maintenance requirements and costs between both systems. As I stated, this is a tough, hard decision. I don’t envy your position.

I wish you all the luck in the world in this serious, important application.


Art Montemayor
Spring, TX

 

[jalvarez]

A comprehensive answer, Art. Let me add my two cents.
ARCH01:
If you have a storage tank for nC4, you probably have another for C3.
If so, the decision is easy, you may use the C3 refrigeration system to reliquify the nC4 vapors.
If you don't have C3's, I think that the investment costs for a refrigeration system with C3 will be very high. It's preferable to use the simplest recompression system that Art described and work with a very good instrumentation protection to avoid the mentioned vacuum.
You must design the line from the tank to the suction of the compressor with a DP low enough to assure a positive pressure close to the suction. The protections must stop the compressor if this pressure is close to zero. In one case I remember, the set pressure for this cutout was 5 mbarg.
Cooling nC4 gases is not so difficult. One of my neighbors claims that they shut the refrigeration system down for a good part of the winter season.
Have a safe day
J.Alvarez