recapturing CO2 from small gas streams 1

[offonoff]

In a medium scale process using liquid co2 as a solvent, we wish to recycle nearly pure co2 gas to 800 psi liquid. co2 gas is above atmospheric pressure and below 20 psia. about 400 lbs (200 kg) of co2 to capture per day.

Because co2 stream to be captured is from a tank in the first place, i assume the gas is relatively pure. The system can be flushed with co2 after being opened to keep atmospheric contaminants to a minimum. Thats only my speculation, I don't have any samples to analyze.

The gas stream is nearly a trickle, and co2 will probably have to be accumulated inorder to compress it. Accumulating it over water is a likely solution. So the gas stream to be compressed will be 2-4% h20, 95% CO2, 2% N2/O2

50-200 gallons (200-800 L) can be easily accumulated.

I have no experience with this process. Where might i source compressors for this? I expect multiple stages with activated alumina scrubber after the first stage. some sort of intercooling.

 

[Montemayor]

OffonOff:

You are trying to recover 0.306 lb/min of CO2 (at 440 lb/day) or 2.67 Scfm (calculated at 18 psia & 70 oF). You didn’t tell us the temperature of the CO2, so I have to guess it's at 70 oF. At that rate, I doubt (at first guess) if anyone can come up with a 3-stage compressor that you could economically justify – and I am not even considering the interstage knock-out drums, and adsorption dryers that you would need for this process.

I am also not considering that you will have to vent off a fair percentage of the CO2 together with the specified non-condensable (air) that you cite. That’s the only way to get rid of the air; otherwise, you fail to liquefy the stream at the predicted thermodynamic conditions.

However, I have to be fair and state that you haven’t revealed the value of the CO2 you are recovering, so it might be so expensive that I am wrong in my first guess. I know this process well because I did it for many years – under the same initial pressure conditions and converting it all to high pressure liquid at 75-80 oF. The smallest compressor I ever used was a Rix, and it was for 50 lb/hr (3X your flow rate). At present Rix advertises that they have “mini-compressors that handle less than 50 Scfm. Go to:

http://www.rixindustries.com/general.html

and study what they offer. I can vouch for their quality and the performance of their compressors. However, be mindful that the American compressor will have much higher unit labor costs in the smaller sizes because of the less efficient unit of scale factor when you employ American labor. Rix is located in Benicia, California so their labor rate is not going to be cheap.

In my opinion, your CO2 replacement cost would have to be astronomical in order to justify the Capital Expenditure involved with what you are describing but I’m willing to be surprised. It could be that you have a research or laboratory setup that has to be done at this scale and there is no other option.

Rix predicts that you can use their Model 2V that has 3 stages, is non-lube (a BIG, BIG feature), is air-cooled, with a 3” stroke and a 1 kW electric motor drive – all on a steel skid. Bear in mind that if you contaminate the suction CO2 with water, you are going to incur the subsequent additional costs of interstage knock-out drums for the condensed water in between the first two stages and the adsorption dryers that would be located between the 2nd and the 3rd stage. Additionally, also make note that if you employ air cooling, you will be unable to liquefy the CO2 at 1100 psia and 85 oF. You may require colder cooling water after the 3rd stage. Here I'm assuming you're storing the liquid CO2 above its critical point.

Make sure that you specifically state what you intend to do with the compressor to Rix. They may be able to include the knock-out drums, but you may have to go elsewhere for the adsorption dryer unit. You can obtain a formal quote from Rix if you specify exactly what you need from them. I’m sure they’ll attend to your tender in a quick and cordial way. They used to be very good folks to deal with.

I don’t understand what you describe in your second paragraph. What system, exactly, is to be “flushed” with CO2 after being opened? Can you describe your system in a PFD or in better detail?

Good luck in your quest.

 

[offonoff]

thank you for the detailed response. This is a pilot/laboratory setup and a larger scale is beyond the use we currently have for the apparatus.

We haven't looked into discounts on co2 when buying at high volumes. At retail prices, electricity is cheaper than new co2. I assumed a compression system would be comparable in cost to a scuba air compressor, which is not astronomical ($1500).

here is some clarification on the application:

The CO2 is being used a solvent for extracting oils from plants, fungi, shop rags, soil, etc (its a laboratory setup). A reactor vessel is opened and filled with the media to extract from, then sealed and filled with liquid co2. On the initial filling, the system will need to be "flushed" (run through quickly with co2 gas) because it was just open to room air. Then the co2 liquid filters slowly through the reactor vessel.

The solvent/solute then passes into a separation vessel where the co2 gassifies, leaving the oil behind. We were originally going to have the separation vessel at 400 psi and then re-liquify the co2 with a refrigeration system, not a pump. But separating at just above atmospheric pressure is safer and requires less expensive containers. If compressing really is astronomically expensive, we may reconsider the refrigeration option.

I assume there are serious and crippling issues with piecing together multiple stages from off the shelf air compressors and r744 (co2) refrigeration pumps...

 

[Montemayor]

Well, the part of your being in a lab doing research and development is at least established.

But we don’t know what your level of development is. It sounds to me that you have barely started if you haven’t established a firm design for the CO2 recovery portion. I believe we both know that you can’t justify any processing if you can’t recover the CO2 – and do it economically. That probably is THE under-pinnings of the proposal, if not one of them.

This response is not intended to tell you how to do your job; I merely want to help you with some experience, if I can. The basis of any CO2 recovery process is generally based on its thermodynamic properties: the critical temperature and pressure. These define the threshold into the liquid phase and then onto the supercritical. Bulk CO2 is normally stored and distributed as a liquid at 250 -300 psig and approximately -8 oF. Small quantities are still stored and distributed in individual cylinders at ambient temperature (70 – 80 oF) and approximately 860 – 975 psig. These are saturated conditions. I don’t know the quantities that you are handling or storing, so I can only speculate as to how you are set up.

It is essential, in my opinion, to establish concretely how you intend to receive, store, and process the CO2. Without this basic design in place, you are going to suffer economically, time-wise, and your subsequent processing decisions. You can’t logically equate the cost of electricity to that of CO2. What is a definite fact is that Coca-Cola and Pepsi-Cola bottling plants in your own neighborhood or surroundings daily receive and consume literally tons of the stuff in their routine bottling operations. They receive the CO2 at 250 psig and -8 oF. You could possibly link your requirements with those of a near-by bottling facility. These are commercial operations, not industrial in size. The costs are not as high as you might imagine – although, again, I don’t know anything about your operation. All you have to do is call Praxair up and get their CO2 price. They may cut you a special price if you can prove that future business may be theirs. We did that wnen I was in that business.

There are two basic handling and storing processes for CO2: the Low-Pressure (LP) Process and the High-Pressure (HP) Process. The LP method involves compression up to 250 psig (2 stages), drying, and subsequent liquefaction with a mechanical refrigeration package (usually ammonia). The HP method is to compress the gas to a maximum of 1,200 psig (in 3 stages, with drying after the 2nd stage) and condense with cooling water at 70 -75 oF. The HP liquid is then expanded down to 250 psig storage conditions and the flash vapor produced is recycled back to the 3rd stage of compression. The HP compressor, then, must have an over-sized cylinder to handle the flash + process gas. Note that the compressors are of the reciprocating type and usually are oil-lubricated in the cylinders. This is a contaminant that must be removed (usually with adsorption) unless you pay more for the compressors up front and have them designed as non-lube construction.

My experience tells me that the LP method can be more Capital-expensive than the HP method – depending on the capacity and controls you require.

Why are you not employing a vacuum process to evacuate the air contamination prior to introducing the liquid CO2 in your supercritical vessel? Surely, this vessel is designed for supercritical conditions and adding on full vacuum design should not be that much more. You are going to have to get rid of the air. It is far more easier (& cheaper) to do it at the lower pressures.

I believe you are wasting your time considering that you have a SCUBA compressor application here. You will find that it will wind up costing you far more in adapting it and engineering it to your needs. The same logic applies to your consideration of “off-the-shelf” mechanical refrigeration units. I have been there and done that industrially. I consider it a waste of valuable time and effort. However, as I don’t know anything of your research plans and budget, this is only my professional opinion based on what I know. I would place my hopes on Rix being able to help you out with your compression needs.

Good Luck.

 

[dcasto]

I have a better solvent for you than CO2, it's used to remove oils from plants, rocks, anything. It's FDA approved as eatable. it's used to dewax lube oils.

google these facts......

 

[offonoff]

dxasto: are you referring to hexane? While approved as an oil substitute in food in low quantities, its msds safety sheet looks hairy at best. Working with Propane looks much safer for the operators.

select hexane risk phrases:
Highly flammable
Harmful by inhalation
Danger of serious damage to health by prolonged exposure
Risk of impaired fertility
Harmful: may cause lung damage if swallowed
hazardous to aquatic ecosystems

I was looking for a little food grade hexane to swish around in my mouth, see what i think of it. but on second thought...


monte:

yes, the need to recycle to gas economically is really underpinning the economic analysis of this process. for 50 lb/hr Rix suggested a specialty pump that puts the system in the range of economically unrealistic. The pump below that (lower flow rate) is more common and i suspect significantly less expensive, but i am awaiting a specific quote.

evacuating the air rather than flushing is an excellent idea. Some processes require that the air be evacuated anyway in order to make the media more permeable.

Also, co2 handling/storage infrastructure is a key perspective that i have glossed over so far while determining the economic reasonableness of different pumps. I am awaiting responses from Rix and Milton Roy.

I did not expect to use a SCUBA compressor, But i was expecting that a different pump with the a similar capacity would be similarly priced. I was also under the mistaken impression that SCUBA compressors were liquefying oxygen, which would make them more similar to my needs. This is obviously not the case.

A danfross type CO2 refrigeration pump would require a strong primary compressor (atleast 10 bar, modified or designed for co2 compatibility) with a dessicant, filters and an accumulator, as well as more elaborate sensing and controls. yes, hardly worth messing with.

montemayor, thank you very much for your time and experience so far. It has been very helpful. Were you involved with CO2 solvent applications specifically? Something similar?

 

[dcasto]

Propane is the correct answer. In liquid form it will disolve the oils. Then add heat, boil the propane away and recompress, send back to system.

 

[dcasto]

oh, I've seen the pilot plant doing this process, really nice.