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CO2 Compressor System - "Expert" Second Guessing Design

CO2 Compressor System - "Expert" Second Guessing Design

CO2 Compressor System - "Expert" Second Guessing Design

Just need some help from some experts on the board.  We have a CO2 compressor system that was designed for us by an outside engineering firm (choice from on high) that is now being questioned by an outside Engineer that was brought on as a consultant (again, decision from on high).  His concern is that the compressor itself will be destroyed by the water (and carbonic acid created) from the wet CO2.  He is also concerned since this Sullair compressor is designed for Air and not CO2.  

Upon looking at the system and components, he advised our management that it was incorrectly designed and will, at most, last a year...likely less.  I don't have a personal stake in this, but a team member does.  I'm hoping y'all can give me some feeling or information to either back up our consultant or confirm the system is solid.  I'll try to be liberal with the information, but if I am missing a parameter that could help, please let me know.

CO2 is provided to us from an industrial partner, they can provide 100X our required capacity.  The industrial CO2 is very wet, I was told 100%.  The CO2 leaves the industrial connection at .5 bar (regulated by a valve system, will not drop below .5 bar) and 150*F.  It then travels through 2" steel pipe for approx. 75ft uncovered, then another 75ft under 3ft of earth (underground pipe is wrapped to prevent corrosion).  It then is stubbed up above ground with a double valve, after the valve it runs 140ft above ground through 2" ABS pipe.  From there, it enters an ABS water trap, then into the system.  System is as below.

System design:
Saturated CO2  will first be cooled in an aluminum after-cooler which has a design rating of 200 CFM @ 100 PSI.  
Supersaturated  CO2 will pass through a filter to remove gross condensate, then enter intake of compressor.  
Saturated CO2 will be compressed to 100 PSI  in oil flooded rotary screw compressor which uses 100% silicone lubricant.  
Compressed CO2  passes through a coalescing particulate filter then enters a refrigerated air dryer.  The dryer reduces the temperature of the compressed CO2  and removes water to a pressure dew point of 35-37 º F.  
Compressed, dry  CO2 passes through a fine coalescer filter and a carbon filter to remove any remaining traces of lubricant.  
Clean, dry CO2  is stored in the 80 gallon receiver @ 100 PSI,  and regulated to 50 PSI as it is discharged for use.

1ea.  200 CFM @100 PSI Air cooled aftercooler, 115 V. 1 HP motor
1ea   SCF 235N   water trap/primary filter w/ auto drain.                        
1ea.  Air compressor, ES-6-10H-24KT, 10 HP, 208V, 24KT Silicone fluid, 36 CFM @ 125 PSI
1ea.  Refrigerated air dryer, SRS 50, 115V, rated at 50 CFM @ 100 PSI
1ea.  SCF 65N Dryer after  filter                                     
1ea.  SCH 65N Final filter                                        
1ea.  SCC Carbon polishing filter                                     
1ea.  80 gallon Vertical ASME Air receiver (tank)    

RE: CO2 Compressor System - "Expert" Second Guessing Design

I have designed, operated, and installed so many CO2 recovery systems in my youth, I lost count.  I know exactly what I would do if it were my project – and it wouldn't look like what you describe.  That doesn't mean you have a lousy system.  It just means I wouldn't do it that way because I know better.  I can't point to any specific piece of equipment as bad because I haven't seen it nor do I have the complete specifications.  But from what you describe I see a lot of amateurish mistakes and attempts to save pennies at the risk of equipment life.  However, since I don't know the background, scope of work, or economic incentive, I can't say that it is a lousy installation.  What I do know from many years of experience doing this very thing is the following:

First of all, you don't furnish even a simple flow diagram.  Therefore, I have to interpret what you have written and imagine the process flow as I read it.

Recovering and transporting water-saturated, gaseous CO2 at 150 oF and 7 psig through overhead carbon steel pipe and then through underground carbon steel pipe is asking for trouble.  The low pressure CO2 will cool and deposit water condensate all along the supply trajectory and the produced condensate will accumulate in the underground carbon steel pipe.  This section of carbon steel pipe will suffer carbonic acid corrosion, act as a low point pocket of condensate and eventually slug over and into the compressor system.  For lack of a better word, this is a stupid way to design a suction line to a compressor.

Sullair makes a good air screw compressor.  I only hope that what was specified and purchased is not an air compressor – but an air compressor body designed for CO2 compression.  Cast Iron is no problem if that is the body material of construction.  However, the seals, rotors, and shafts have to be specified and designed for wet CO2 service.  Otherwise, maintenance problems will be numerous.  A screw compressor designed for CO2 service is more expensive than one for air service.

The way to separate a liquid phase from a gaseous phase is NOT BY USING A "FILTER".  Filters remove solids from fluids – they do not "filter" out liquids.  For that, you design and employ a vapor-liquid separator.

The choice of a refrigerated dryer is – again – the cheap, low capital cost way to select process equipment.  That's OK, if that is what you decide and you are willing to accept a short life span on the the equipment.  A cheap investment is not meant for the long haul.

I could make the described equipment operate for more than a couple of years – maybe as much as 10 years.  But I would need experienced and trained operators and maintenance personnel as well as a liberal maintenance budget.  Usually this isn't what is available for these type of low-capital cost processes.  A screw compressor will tolerate some degree of condensate with the CO2 – but this kind of operation will take its toll sooner than later.  Bearings and seals will fail; clearances will increase as will maintenance.  Eventually, you may have to replace the compressor and start all over again.


RE: CO2 Compressor System - "Expert" Second Guessing Design

    Sure you aren't our consultant? ;)  It may very well be that the outside engineering firm the specified this equipment screwed up big time, something that will cost them - not us - in the long run. I will work on putting together a flow diagram, until then let me try to fill in some of the blanks you brought up in your post.

The steel pipe is not overhead in any section, it is run on-the-ground (on saddles) until it reaches a road, which is where it transitions to underground.  It is installed with a slope towards the supply, where it has a drain installed to remove any condensate.  There is about a 2ft drop over the 150ft span.  The ABS line is level and is installed on-the-ground (on supports) as well.  The supplier that fabricated the system installed a gross water trap before the after-cooler (really a pre-cooler).  The trap is simply a 4in pipe installed vertically, approx 48in long, with a 2in inlet a few inches from the bottom and a 2in outlet at the top and an auto-drain at the very bottom of the 4in pipe.  I think this would be effective at catching any slugs of water pushed through the system, but little else.

The compressor is a bog standard air compressor, as it comes from the factory.  In fact, here is the spec sheet on that line (http://www.sullair.com/Files/Sullair/Global/US-en/industrial/LIT_ES6_LS01E_20110104.pdf) - Sullair doesn't specify material of construction.  I can get in touch with them to find that out.  The supplier also went with their fully-synthetic '24KT' oil - as Sullair specs this as non-absorbing of moisture.  Of course, that is a manufacturers claim.  We are planning on taking samples and sending them out for analysis every 30 days initially.  The system will not run continuously, it will kick on and off throughout the day and will sit idle during the night.

Regarding the filters, my only assumption is that they were put into the system for gross condensate removal.  It's similar to the type of filters you would see on a compressed air system in a shop.

This system needs to last for a minimum of 1 year, with hopefully little maintenance or down-time.  Total down-time has to be kept under one day in a month long period, ideally.   

RE: CO2 Compressor System - "Expert" Second Guessing Design

Here is a flow diagram for the compressor system, this does not include the supply line.  The assumption made on this diagram is that the inlet temperature would be in the 130*F to 175*F range, however after traveling through 150ft of steel pipe and 140ft of ABS pipe, I believe it will be much lower (though, I haven't calculated exactly how much lower yet).  

Also, here is a relevant comment from the engineering firm's proposal:

"A major design consideration is that wet CO2 (particularly carbonic acid) is
corrosive to steel and some other metals.  The selected after-cooler is
constructed with aluminum internals to avoid corrosion issues.  The oil flooded
rotary screw compressor will use 100% silicone lubricant which is formulated for
use in corrosive environments.  It is not expected to assimilate acids that pass
through the compressor while protecting the steel internals of the compressor.  A
dehumidification system upstream of the compressor is not available as the
required minimum inlet pressure for available desiccant or refrigerated air dryers
is 50 psi with most dryers designed to operate at 100 psi.  This is much higher
than that provided from the CO2 vent stream..."

RE: CO2 Compressor System - "Expert" Second Guessing Design


question for your "outside consultants"  - how many wet CO2 systems have they designed/installed?  references are valuable and inquire with current operators to determine if they are satisfied with the operation.

the link provided is for an air compressor, not wet CO2 system compressor.  secondly, the link is for a brochure, not a spec sheet.  this leads me to believe the lack of knowledge by certain individuals.  never trust a brochure (salesperson)!

your concerns appear to be valid and warrant further investigation by skilled and knowledgeable people with wet CO2 design experience.  like the Fram oil filter commercial says, you can pay me now or pay me later.

wet CO2 systems are not to be taken lightly as Mr. Montemayor has written.  Highly recommend following his advise and recommendations!!

hope this helps and good luck!

RE: CO2 Compressor System - "Expert" Second Guessing Design


According to him, he has configured many wet CO2 systems.  I wasn't the guy that brought him in, but this CO2 system impacts my systems that I've designed.  I don't know anything about CO2 compressor systems (don't think I ever said that I did) and was trying to sort the wheat from the chaff, so to speak.  

We have one outside engineering firm that spec'd this system (AFAIK - they have no experience with these kinds of systems). Now, after it has been purchased configured and installed, our CEO decided to bring in another outside engineer who has raised these concerns - but uses phrases like "I'm not sure, it may last a month or it may last a year" or "I'm not a compressor salesman, but I have a buddy that is lead engineer over at Sullair" and the one that made me nervous "you might need to add a refrigerated dryer before the inlet to the compressor system, should cost around $50,000".

Yes, the link provided is a sell sheet - my mistake in saying spec sheet (amazing, I do know the difference).  I don't have a spec sheet that I can link to, Sullair has a terrible website.  Also yes, it is an air compressor - as stated in my original post.

The company that fabricated the system has 25 years of compressor experience (but not wet CO2 experience) - I was wondering if one point he brought up was valid.  He was saying that the compressor is designed to run hot enough to not let water condense in the air end or air-fluid receiver.  Further, he is saying that Sullair's 24KT synthetic oil will not retain any water.  His position being that any moisture that makes it to the compressor would, worst case, end up in the tank (which obviously has a drain on it) and wouldn't be in the compressor to corrode it.

Our industrial partner's engineers have no concern with the steel pipe that they have run, they don't feel that there will be any condensation in their line and if there is the auto-drain at the low point in the system will handle it.  From supply to the double valve is their responsibility.  It's everything after that, most important being the $15K compressor system, that is our domain.

So, long post short (too late!) - being one of two people on our internal engineering team - I'm just trying to learn more about what goes into a CO2 compressor system and what (if anything) makes this system "bad" or "wrong" and maybe possible ways to limit damage to the compressor.  Mostly, I just want to be able to know who is blowing smoke; doesn't know what they are talking about or trying to get themselves a bigger paycheck when I have to listen to all the parties involved point fingers in the time-dragging meetings I must be part of.

RE: CO2 Compressor System - "Expert" Second Guessing Design

water that settles out in your discharge separator will corrode the bottom of the separator  

RE: CO2 Compressor System - "Expert" Second Guessing Design

Looks like we might be adding a 500# activated alumina dryer system versus replacing the entire compressor system.

RE: CO2 Compressor System - "Expert" Second Guessing Design

why not use Calcium Chloride to dry the CO2?

RE: CO2 Compressor System - "Expert" Second Guessing Design

What's the benefit to that over activated alumina?

RE: CO2 Compressor System - "Expert" Second Guessing Design

Just digging around myself, what about using a deliquescent drier system to remove the water from our wet CO2 supply before it gets to the compressor?  

RE: CO2 Compressor System - "Expert" Second Guessing Design

The material in a deliquescent drier is ....... Calcium Chloride.  It can lower dew point about 20F and the brine needs draining pretty often, I designed a modification to the standard deliquescent drier that my client has patented that uses a Ranke Hilsche Vortex Tube in front of the drier drop the temperature 20-40 F before the drier so you get a 40-60F dewpoint depression.  It was designed for fuel gas (the hot side of the tube and most of the water vapor goes back to the compressor suction) so it probably won't help much here, but it is the only way I know of to get significant dew point depression with a salt drier.


RE: CO2 Compressor System - "Expert" Second Guessing Design

How would I go about calculating the amount of water a deliquescent drier would remove if used in my set-up?  We're talking pretty low flow here, the MAX the system could push is 30SCFM but we will really only be using 2-3SCFM during the day (none at night).

Unless of course you're saying a deliquescent drier is not an option at all to prevent water/carbonic-acid from destroying the compressor in out set-up.  I mean, if burning through 100lbs (random number) of calcium chloride a month would keep our compressor in good form, that would be worth it in my mind.

Just trying to see all angles to fixing this system so I can be informed and help us from only having one option...thanks everyone for the help thus far.

RE: CO2 Compressor System - "Expert" Second Guessing Design

Different manufacturers have different claims, but the average is that each lbm of salt can absorb 3 lbm of water vapor.  At 0.5 bar(g) and 150°F, CO2 can hold about 9000 lbm/MMCF.  Your daytime flow rate is 0.05 MMCF/day so your water load is around 450 lbm (54 gal/day).  Based on that, if the salt were 100% effecient you would need to plan on adding 150 lbm of salt every day.  Problem is that it is far from 100% effecient.  You are doing good with a 20°F dewpoint depression.  So a salt drier on the suction would lower your dew point from 150°F to around 130°F--from 9000 lbm/MMCF to 4900 lbm/MMCF and you would still need nearly 70 lbm/day of salt.

A fin-fan cooler on the suction to get the temperature to 100°F would drop the water load to 3700 lbm/MMCF and drop the salt load to around 55 lbm/day (and the residual water content to 1000 lbm/MMCF).  If you had a water source (for cooling) and dropped the inlet temp to 50°F then the residual water would drop to around 180 lbm/MMCF and the salt load would be down to 4 lbm/day.

I'd look at putting the refrigerator on the suction and a salt dryer after it.


RE: CO2 Compressor System - "Expert" Second Guessing Design

I tripped over this forum by accident while doing some recent research. I am the unnamed who designed this system. I respect most of the posts to this string and the experience they represent. In case this string is going to continue, I would like to throw in a few points.

1. The original request included the caveats that this was a research project with approximately a one year life.

2. Cost was a major consideration.

3. Equipment was to be on site within +-two months. This precluded specially designed or manufactured equipment.

4. Design was based on 180F inlet temp, thus the pre-cooler.
In actuality, the gas temp at the inlet of the compressor will be closer to 100F due to distance from source and heat sink effect of buried pipe.

5. System was originally proposed with a statement that it would require more than normal maintenance.

6. Carbonic acid is very stable in a pure state. In the presence of water, it degrades quickly to H2O and CO2.

7. The silicone compressor fluid is hydrophobic, chemically inert and has very high film tenacity to metal surfaces.

8. The ABS water separator between the supply pipe and the system is designed to reduce flow velocity (1 1/2" pipe into a 4" vertical pipe) without reducing flow. It would eliminate any possibility of slugging.

9. Additional separation is provided between pre-cooler and compressor.

10. Refrigerated dryers will not work on inlet line. They do not work at 7 PSI. Deliquescent will work at low pressure. Disposal of effluent can be an issue.

11. These compressors are successfully operated in H2S and salt air environments with a very acceptable service life with proper maintenance.

12. Recommended fluid sampling every 30 days would give early indication of internal corrosion or other impending doom.

13. Sullair did not endorse the design.

I am very interested in any suggestions of a more practical solution based on anticipated longevity, timeline and budget constraints.

RE: CO2 Compressor System - "Expert" Second Guessing Design

Nice list, but why do you say


10.  Refrigerated dryers will not work on inlet line. They do not work at 7 PSI.
At 7 psig and 100°F the gas holds a certain amount of water vapor.  At 7 psig and 50°F it holds less water vapor.  Sounds like a chiller would help.


RE: CO2 Compressor System - "Expert" Second Guessing Design

 I stand corrected... or guided.  More accurately stated, a conventional refrigerated air dryer will not work at 7 PSI  They are normally designed to operate at 100 PSI. A specially designed chiller would be possible.  Pipe  or cv would need to be in the 2" range, typically used on a 500 CFM dryer.  Probably a coil of 50' of 2" copper tubing stuffed into a household refrigerator would serve the purpose, but that would be a bit high tech for me. at that temperature If you consider that the heat of compression any moisture takes the air under compression up to +- 180 degrees at 100 PSI,the moisture in the intake air will not reside in the compressor.  That is why there are no drains on the internal receiver or compressing unit of a rotary compressor prior to the air discharge, The first drain is usually after the built in aftercooler.Your comment is appreciated.       

RE: CO2 Compressor System - "Expert" Second Guessing Design

Ah, gotta love the not-really-anonymous internet.

Airsmybag, thanks for adding some more detail to this topic.  I just want to make it clear to you that I posted this - not to call you out or put blame on anyone (though I can see how my wording could have been better in earlier posts) - just to try and get more information on this problem.  I have zero experience in CO2 compression and this board is a wealth of information.  Hopefully that's the spirit you've taken it in.  Also, welcome to Eng-Tips!

Back OT - As it stands now, the outside Engineering consultant that was brought in is recommending one of two options. Option one would be to replace the entire system with one that is specifically made for CO2 compression, at considerable cost to us, but it would be warrantied by the Mfg. for this use.  The second option is a regenerative dryer system, which oddly enough, is at a similar cost level to replacing the whole system.

RE: CO2 Compressor System - "Expert" Second Guessing Design


In addition to David's correct observation and comments, I would add the following suggestions / comments as per your request:

1. You state, "Carbonic acid is very stable in a pure state.  In the presence of water, it degrades quickly to H2O and CO2".  However, this is not exactly true in this application – unless you meant to describe the gas, Carbon Dioxide and not the water solution of the same gas (which is Carbonic Acid).  

2. Carbonic Acid is described as a stable compound that easily breaks down in the presence of water.  That is a classical description – but one that is based on laboratory observations that call for the exposure of the Carbonic Acid to atmospheric conditions.   That is NOT the case in this application.  In this application we have an atmosphere of carbon dioxide in intimate contact with condensed water 100% of the time.  This scenario – or condition – does not allow for the easy degradation of the Carbonic Acid.  It can't degrade to H2O and CO2 because it is surrounded by a CO2 atmosphere that is constantly contributing to the formation of Carbonic Acid.

3. If ABS plastic piping was used to combat possible corrosion on the suction side, why wasn't it used in the underground portion of the low pressure, suction gas flow?  That position of the piping is the one that is most prone to carbonic acid attack.  Using wrapped carbon steel piping is asking for internal corrosion problems.  Additionally, this doesn't make for logical sense with cost being a major consideration since the carbon steel pipe installation costs more than the ABS.

4. I have never found the disposal of spent Calcium Chloride to be an issue.  This is a common inorganic compound that is simply returned back to the earth where it originally came from in the form of calcium and chlorides.  These are not contaminants to nature.

5. The fact that screw compressors are successfully operated in H2S and salt air environments with a very acceptable service life with proper maintenance does not address the fact that it is the INTERNAL GAS (saturated Carbon Dioxide in this case) that is of concern – not the environment around the compressor.  If you want to compress H2S, you are going to have to go through NACE guidelines and special alloys.

I hope this helps to shed light on the potential problems foreseen.

RE: CO2 Compressor System - "Expert" Second Guessing Design

Montemayor, I just want to address one of your points:

"3. If ABS plastic piping was used to combat possible corrosion on the suction side, why wasn't it used in the underground portion of the low pressure, suction gas flow?  That position of the piping is the one that is most prone to carbonic acid attack.  Using wrapped carbon steel piping is asking for internal corrosion problems.  Additionally, this doesn't make for logical sense with cost being a major consideration since the carbon steel pipe installation costs more than the ABS."

The run that is wrapped carbon steel pipe was done by our industrial partner (large oil/gas company).  It is their responsibility alone and their engineering team signed off on it as it will be a temporary (1 year) situation.  Everything after the valve on the end of the supply line is our responsibility and is ABS.

RE: CO2 Compressor System - "Expert" Second Guessing Design

wish i would have bumped into this thread earlier.  as the thread has gone silent since september, I am believing that all has been solved.  

in any case, listening to those that have experience with CO2 and water compression systems would be wise.  the carbonic acid will develop and it will try to eat and disolve all of your carbon steel internals in the compressor.  while an air compressor will compressor wet CO2 gas, it will do it effectively for very long service.  

if the compressor is for a test purpose and for only limited time, then take your chances and see how long the system lasts.  if however, you are not wanting to take chances and need better risk mitigation, then follow the advice of the experts and change out some of the components so that they will stand up to the carbonic acid formed in the gas stream.


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