11 replies to this topic

[thaqrhys]

Dear all,

 

Does anyone know the recommended or best practices to convert gaseous CO2 at atmospheric pressure inlet to liquid CO2 that can be stored and pumped for efficient distribution ?

 

The CO2 gas inlet is saturated with water moisture, and does not contain any other chemicals. The goal is to produce pure CO2 (99.9% vol) liquefied in a storage tank and use the product for further downstream processing. 

 

Thank you in anticipation.

 

Regards,

Thomas.

[thorium90]

First, you have to remove the moisture. Then you need refrigeration to liquefy it.

[Art Montemayor]

Tomasso:

 

Thorium got part of it right.  The actual process is composed of 3 parts, as shown in the attached workbook which also explains in much more detail.  The process consists of:

1. compression
2. drying
3. refrigeration

 

I hope this addresses your specific query and helps you out.

 

 Liquid CO2 Production - Storage.xlsx   144.93KB   132 downloads

 

[thaqrhys]

Dear Art Montemayor and Thorium,

 

Thank you for the prompt reply.

 

I am planning to conduct the process as follows :

 

Firstly, the wet CO2 gas enters a Refrigerated Air Dryer, hopefully this will highly reduce the moisture content.

Then the half dried CO2 gas the compressor at amospheric pressure and exits at 18 barg.

 

The compressed CO2 gas enters a Desiccant Dryer to further reduce the moisture content. 

 

Compressed dry CO2 gas enters into a BPHE (cooled with Water-Glycol system) to -5 deg.C then further liquefied in a Shell & Tube HE (cooled with R-507) to -40 deg.C.

 

Since this whole process is still on trial phase, the liquefied CO2 will be stored in cylinders. The cylinders is cooled until approx. -2 deg.C, so that the liquid CO2 automatically enters the cylinders.

 

Please the experts in this forum to be free to comment on the process. 

 

Thank you.

[shan]

As indicated in GPSA Fig. 20-4, the lowest water content of CO2 is at about 800 psia.  You should have your dehydration at this pressure level.

[Art Montemayor]

Shan:

 

What you are stating is not correct - nor practical in this application.  The OP is going to store the CO₂ product as a liquid  at 250 psig (approx.) and doesn't have a reason to compress the gas any higher - like 800 psia.

 

What has been done in the compressed gas industry (and continues to be done to date) is that the CO₂ is compressed up to the 250 psig and sent through a fixed bed adsorption dryer that employs activated alumina as the adsorbent.  This type of drying operation results in a gaseous product with only 1-2 ppmv of water (equivalent to about -90 ^oF atmospheric dewpoint).  This  quality of dryness is more than sufficient to liquefy the CO₂ and store it successfully.

 

There is absolutely no need to invest in and employ excessive compression energy just to dry the CO₂.  It is a very simple and direct Unit Operation and has worked successfully for the last 60 years.  If you take a look at the simplified flow diagrams I have submitted you will see how this adsorption unit is employed in the industry.

[Art Montemayor]

Tomasso:

 

My comments on your previous post are as follows:

 

You should not attempt to dry the CO₂ prior to compressing it to 250 psig.  This is a foolish and wrong method that wastes a lot of money and energy without contributing anything to the ultimate water content of the CO₂.  Stop and examine what you are proposing – but as a chemical engineer using common sense.  You know you have to compress the CO₂ in order to ultimately liquefy it for storage.  This, you must do.  You also know, as an engineer, that as you compress the gas the water content the gas contains is less – especially because you have to cool the compressed gas after each compression.  Therefore, by common sense, it is far more practical, cheaper, simpler, and effective to compress the gas through the designated 2 stages of compression, intercooling between each stage and separating the resulting liquid water in a vapor-liquid separator (“a trap”) and draining it.  This removes the greatest amount of water (approximately 90%) without you doing nothing more than compressing and cooling the gas – which is exactly what you have to do anyway!

 

The above is the reason for selecting a proper, proven compressor type and arrangement before you do anything else.  The compressor of choice for this type of operation is a reciprocating, 2-stage, no-lubricated machine with built in intercoolers and separators (with automatic drainers).  The intercoolers are preferentially cooled with water because this is more efficient.  After compression, you send the gas to the adsorption dryer unit where the last remains of the water content are removed.  You can employ other types of compressors but you will have more problems and the process will be more complex.  What I show in my workbook is what has evolved in the industry through many years of experiments and trials as the optimized method of producing liquefied CO₂ for storage and transportation.

 

There is no sound or engineering reason to use a Water-Glycol system prior to sending the CO₂ gas to a refrigeration system as I show in my flow diagrams.  It makes no sense economically or practically to use a pre-cooler prior to the refrigeration system – unless there is something else that you are not telling us about or you simply want to buy excess equipment and make the process more complex and costlier to maintain and operate.  But then, that is not engineering.

 

The conventional and correct method used to produce liquid CO₂ is clearly shown in detail in my workbook.  Please study it and ask any further questions you might have.  It is quite simple and common sense.  It is the way that CO₂ is being produced today and the way it has always been done since I was born (76 years ago!).

 

Additionally, please be specific about what you mean by the term “cylinders”.  Are these conventional, 25 kg capacity CO₂ high pressure cylinders?  Or are you talking about a cylindrical storage vessel?  Please be clear and use diagrams, photos, sketches, or whatever else you need to clearly define what you intend to do.  Conventional CO₂ cylinders have a rather small (1”) valve and you simply can’t “pour” or drain Liquid CO₂ into such an arrangement.  You would have to literally pump the liquid CO₂ at -8 oF into the cylinder and let it warm up to ambient temperature while the contents would rise in pressure up to 1,000 psig.  The point here is that if that is what you intend to do, it isn’t practical to think that you can fill the cylinders without a special high pressure pump.  Please be clear and give ALL THE SPECIFIC DETAILS.

[shan]

Art,

The question is what the CO2 used for? If it is for CO2 injection, you may have to compress the CO2 over 1000 psig anyway. We should knock off free water content in the stream as much as possible before introducing it into the fixed bed adsorption dryer.
CO2 bubble point at 250 psig is -7F and at 1000 psig is 84F. It will save you a lot of refrigeration energy by storing CO2 at 1000 psig than storing at 250 psig.

[Art Montemayor]

Shan:

 

I’m trying to stick to the topic at hand.  The OP clearly stated that the CO₂ is to be produced at 17 barg, as a liquid.

 

There is no mention that this liquid CO₂ is to be produced at high pressure or for EOR application in an oil reservoir.  Which is what the GPSA diagram is intended to be used for.

 

You have a good point about free water, but have you looked at my diagrams?  If so, you will note that a separator is clearly shown before the adsorption dryers so that no free water is introduced into the adsorbers.

 

You are wrong about your economics regarding the most economic storage conditions for CO₂.  Check with all the CO₂ producers and users in industry and you will discover that all bulk CO₂ is produced, stored, and transported to users at 250 psig and -8 ^oF.  That has been the norm established through many years of application.  When I state this, I’m relying on all the years I accumulated in the compressed gas business where I first started my engineering profession.

[shan]

Art,

I am unable to defend 250 psig is the most optimal pressure for storage or transportation of CO2. Why? Vessel material or user's demand?

OK, even if the export pressure has to be 250 psig, we will still have option to compress the wet CO2 to 800 psig dehydration and letdown to 250 psig through a J-T valve. If 100 F, 800 psig CO2 letdown to 250 psig, the outlet temperature is 14.6 F. I believe the cost of CO2 compression power will be about the saving of compression power of refrigeration loop.

88F CO2 water content is 150 lb/MMSCF at 250 psia and is 70 lb/MMSCF at 800 psia. If gas velocity is identical for 250 psig and 800 psig bed adsorption bed, the section area of the 800 psig bed will be 15% of 250 psig bed (264.7/814.7*70/150 = 15%).

[Art Montemayor]

Shan:

 

I don’t know what your experience level is in dealing with compressed gases or cryogenic operations, but I will try to explain some details of the CO₂ industry which are well-documented and known to all that are involved in the business of compressing, refrigerating, and transporting the product at an acceptable profit margin.  I spent over 15 years in the industry, not only generating and liquefying the stuff but also transporting it and converting it into Dry Ice.  I was issued US patent #5,385,023 for the automatic production of dry ice pellets with 100% revert gas recovery.

 

You ask why is 250 psig the optimal pressure for storage and transportation of liquid CO₂ and the answers are:

1. When you are compressing CO₂ from basically atmospheric pressure to the critical pressure, you discover that compressor manufacturers (especially reciprocating compressor manufacturers – on which the process and the industry was founded over 100 years ago) have already set the norms of compression ratios employed and the result is that you normally go from approx. 1 psig to 65 psig in the first stage, 60 psig to 250 psig in the second stage, and 245 psig to 1,000 psig in the final third stage.  That, for all practical purposes is written in stone.
2. Long before you and I were born, steel vessel fabricators in the USA started to fabricate thousands of LPG storage tanks that are conveniently designed for 300 to 350 psig.  This pressure range, together with the fact that at 250 psig the saturated temperature of liquid CO₂ is -8 ^oF made it a no-brainer solution to use that design as the storage pressure for Liquid CO₂ because the steel was rated for minimum temperature of -20 ^oF – well within the service condition.  Additionally, all LPG storage as well as transport tanks were one and the same – which meant that you could also employ the nominal 250 psig transport design for Liquid CO₂.
3. Additionally, the -8 ^oF temperature fits in very well with the then-existing ammonia refrigeration available to liquefy the CO₂ at 250 psig.  There was no other available refrigerant at that time.  So that settled that.
4. Furthermore, when engineers started to generate detailed and specific calculations for optimizing the ideal operating pressure / temperature for a CO₂ liquefaction system, it was discovered that of all other methods for liquefying CO₂, the best, optimum, and most efficient conditions were: 250 psig and -8 ^oF.  I can personally confirm this to be the truth because I participated and made some of those calculations many years ago.

So, you see, from a practical and formal engineering point of view, the industry has already identified the optimum storage and transport pressure and temperature of liquid CO₂ – regardless of what you may suspect or believe.

 

In fact, I have personally operated other CO₂ liquefaction systems:

• The utilization of a closed, CO₂ refrigeration loop to liquefy the product CO₂ at 250 psig;
• The utilization of Freons to liquefy CO₂;
• The utilization of a 3-stage reciprocating compressor to produce high pressure (over the critical pressure and under the critical temperature) liquid CO₂ and adiabatically flash it into a storage vessel maintained at 250 psig, while recycling the flash vapors back to the 3^rd stage of compression.

The simple, 2-stage compression to 250 psig followed by adsorption drying and refrigeration with an ammonia system continues to be the optimum solution from both a practical and economic point of view.  And this is so, regardless of what I say or state and is based on hard-earned engineering experience and optimization done by many engineers in the past.  If you still don’t believe me, use your telephone and call Air Liquide, Praxair, or any world-wide manufacturer and supplier of liquid CO₂.  They will confirm what I already know from personal experience.

[thaqrhys]

Dear Art,

 

I could see clearly from your detail explanation that the key of CO2 Liquefaction process is on the choosing the right compressor.

 

Regarding the Refrigerated Dryer, I will remove it from the system and let the remaining moisture content is absorbed in Desiccant Dryer.

 

Yes, you are right! The 25 kg high pressure cylinder. We actually already have liquid pump.

 

So, we change plan and follow your suggestion : after the liquefied CO2 exits the refrigeration system, it enters liquid pump and finally fill to 25 kg cylinders.