6 replies to this topic

[rsk]

Carbondioxide stored in cylinders at 150 kgsc is being used for pressurisation of a process tank to maintain inert atmosphere. The Cylinders are connected through a two stage PRV to supply cover gas pressure at 4kgsc. During intial pressurisation of tank we requre around 100 cylinders & time required is 24hrs. While using cylinders as pressure reduces frost formation is occuring on external suface which is contorlling the rate of vapourisation of co2 from liquid to vapour which in turn controls the discharge rate from cylinder. hence to reduce the time cylinders are being used only up to 60kgsc & discarded for use in some other application at lower pressure. But this increases the no of cylinders to be replaced & lot of efforts are required.
Instead If carbondioxide is stored at above 31deg celcius [b]CRITICAL TEMPERATURE, it will remain in vapour phase & during withdrawl no flashing & no frost fromation will occur & cylinders can be utilised fully. Also the discharge rate will be governed by cylider pressure only. Is this acceptable i am not fully aware of CO2 cylinders storage guidelines. Please share your great experiences.
regards

[Art Montemayor]

"Carbon dioxide stored in cylinders at 150 kg/cm² (2,134 psig) is being used for pressurization of a process tank to maintain an inert atmosphere. The cylinders are connected through a two stage pressure regulating valve (PRV) to supply blanket gas pressure at 4 kg/cm² (57 psig). During the initial tank pressurization we require around 100 cylinders & the time required for blanketing is 24 hrs. While using the cylinders, the pressure reduction causes frost formation to occur on the cylinders' external surface - which controls the rate of vaporization of CO₂ from liquid to vapor and which, in turn, controls the discharge gas rate from the cylinder. Hence, to reduce the time, cylinders are being used only up to a pressure of 60 kg/cm² (853 psig) & subsequently discarded for use in some other application at lower pressure. However, this procedure increases the number of cylinders to be replaced & a lot of effort is required.

Instead, if carbon dioxide is stored at above 31 ^oC (its critical temperature), it will remain in the vapor phase & during withdrawal no flashing & no frost formation will occur. Thus, the cylinders can be utilized fully. Also, the discharge rate will be governed by the cylinder pressure only. Is this acceptable? I am not fully aware of CO₂ cylinders storage guidelines. Please share your great experiences.
Regards."

RSK:

I have edited and corrected your original posting because it was difficult to interpret. I fear your English proficiency is not too good. But that is OK. As long as you can put your idea across, I can piece together what I think you mean. Please read my redacted version and confirm that the meaning in my version is the same as what you meant to say. Now, allow me to make the following observations and comments based on what I interpreted:

1) Carbon Dioxide is normally stored in cylinders as a saturated liquid, at its critical pressure. Oftentimes it reverts to a sub-cooled liquid due to a drop in the surrounding, ambient temperature. If the external ambient temperature increases above its critical temperature, it will convert into a Supercritical Fluid (SCF). This cylinder method is used extensively in both Europe and the USA. Why you are using 150 kg/cm² (2,134 psig) in India is something I don't understand. Although you get more liquid CO2 storage at the higher density, you have to fabricate (or obtain) off-standard, higher pressure, higher weight, and higher cost cylinders. If you are using these type of cylinders in India as standard, that's OK.

2) The important things to bear in mind and have a complete understanding about are the phase conditions and paths that the expanded (regulated) CO2 goes through. Without a complete understanding of the thermodynamic picture, you will not succeed in designing or operating this system correctly. Please refer to a convenient Mollier diagram of CO2 or, better still, a T-S (temperature – entropy) diagram. You can also resort to the complete, FREE, & official CO2 Thermophysical properties at:

http://webbook.nist.gov/chemistry/fluid/

Select CO2 from the drop-down menu on the compounds offered and then select the units you prefer. If you are dealing with saturated fluids, designate appropriately. If you are dealing with superheated or subcooled fluids, select either the isothermal or isobaric conditions - whichever is more convenient to you. This valuable information is given you absolutely free thanks to the benevolent USA government that administers all the exorbitant taxes that I (& other US taxpayers) pay it for this and other purposes.

I am also attaching a convenient Excel workbook dealing with Carbon Dioxide that includes a lot of the above-mentioned thermo data in spreadsheet format. Note that you can download the NIST data directly into a tabular format which you then transfer to a spreadsheet.

3) I recommend that you do away with the use of high pressure cylinders as a source for your blanketing process gas if you are continuously using the blanketing system. You don't give any details on your process or your CO2 consumption rate or quantity, so I can't go into too many details. However, if you install a Low Pressure liquid CO2 storage tank (complete with a little refrigeration system and vaporizer) you will be much better off and your process time and operations will be 100% simpler and safer. What I am recommending is a normal, conventional CO2 vapor supply as is used by all major bottling companies throughout the world of such beverages as Coca-Cola and Pepsi Cola. I installed and supplied literally hundreds of these systems in my early years as a young engineering grad. I am sure that India has carbonated bottling plants that use this same CO2 supply and storage system and I can also safely assume that they have no problems doing this with very low-tech personnel.

4) You don't state your CO2 source, but I am sure that India has bulk, liquid CO2 supply sources of liquefied CO2 at a nominal saturated pressure of 250 psig. Handling and operating high pressure CO2 cylinders is an economic and efficiency pain. I was part of a task force that set out to make this system obsolete over 48 years ago. I thought we had succeeded on a world-wide basis, but apparently we didn't.

5) You are not correct in stating "Instead, if carbon dioxide is stored at above 31 ^oC (its critical temperature), it will remain in the vapor phase & during withdrawal no flashing & no frost formation will occur". This is simply not true and reflects your lack of understanding of the different thermodynamic phases and path that the CO2 undergoes. Please refer to the T-S diagram or the NIST tables and you will find that when you have CO2 in the Supercritical phase, it still has to undergo an isenthalpic path as it expands adiabatically (irreversibly) and, therefore, has to enter into the saturated "dome" where a two phase, lower pressure system is established. At first, this is a liquid-gas system. As you proceed beyond the TRIPLE POINT (approximately 75 psia), you will arrive at the SOLID-gas system where you have created DRY ICE. Your vaporization and blanketing problems are a result of not employing the thermodynamic properties correctly and you are lacking vaporization/sublimation heat application to the expanding liquid CO2. Please read and study my Excel Workbook – especially the worksheet titled "LCO2 Types". If you expand ("regulate") liquid CO2 down to a pressure below its Triple Point (such as 4 kg/cm²) you WILL create both a SOLID and a gaseous CO2 phase. The solid (dry ice) phase will either plug up your system or slow down the gas flow. You must sublimate the solid formed or the process plugs up or slows down.

If you are a student, then please give us your academic background and preparation level. We don't want to waste time talking down to you when you haven't accumulated sufficient Thermodynamic background. Some of what you write is testimony for this.

What you are now proposing is not acceptable because it is illogical according to the thermodynamic properties of CO2 and the process requirements you are confronting. Please detail out ALL your process details and basic data if you want us to be detailed and specific in our comments and recommendations. This is just like love and marriage: you can't have one without the other!

I await your reply – if you have one.
 CarbonDioxide.xls   386KB   121 downloads

[rsk]

Thanks Mr Art for your nice technical reply, I would like u to visit the link
http://www.catalinac...om/co2heat.html
First i would like to make some changes in my query statement, CO2 is stored in cylinders at 65kgsc not at 150kgsc.
Here it is mentioned that for temperatures above 87deg F, CO2 will remain in gas phase no matter how much pressure. Then while using cylinders why frost formation is occurring when there is no question of phase change. Is it because of Joule Thomson effect?
regards

[Art Montemayor]

RSK:

I believe you are wrong once more. I am attaching a copy of the Catalina Cylinder webpage on CO2 cylinder filling and it does not "mention that for temperatures above 87deg F, CO2 will remain in gas phase no matter how much pressure". Any temperature higher than the critical and at a pressure over the critical will create the supercritical phase - which is undefined as to liquid or gas.

Please reveal to us if you are a practicing engineer or a student. You have posted in both Forums and you haven't told us whether your case is a real one or an academic one. I don't want to go into Thermodynamics or Phase Equilibria through this Forum without knowing that what I write is well understood. Otherwise, I am wasting your time as well as mine. I am afraid that our Forums are not equipped or able to give you tutorials or lessons on Thermodynamics. That, I presume, has already been studied, researched, and learned by those who come to the Professional Forum. Since this is the Student Forum, I remain perplexed as to your experience. Please help us help you.

There is no Joule-Thomson Effect taking place when you vaporize liquid CO2. This is still another misconception. The Joule-Thomson Effect deals specifically with REAL GASES - not liquid.

You have not told us what your project (or assignment is) or the nature and background of your queries. You also do not address my comments or questions. To continue like this is like going down a one-way street: there can be no communication if there are no mutual questions and responses on both sides. If we have a Thermodynamic-poor situation, then please let us know. We can try to tailor our response to your level of learning in order to allow you to at least get an idea of how deep the water is. This will enable you to learn how to swim.

Await your reply.
 Catalina_CO2_Cylinders.doc   50.5KB   73 downloads

[rsk]

Thanks Mr Art
i am practising engineer, I do have sufficient knowledge of thermodynamics acquired from my academics, same i am applying to my day to day jobs to strengthen my learning.
Please revisit ur recent reply to this post & see the highlighted sentence in ur attached file.
regards

[Art Montemayor]

RSK:

I will take you up on your being a practicing, graduated engineer and fully aware of Thermodynamics and therefore being able to understand what I will further detail out. I will also move this thread to the Industrial Professional Forum where I believe it belongs - especially since we will discuss serious Thermo and a Thermo cycle.

I have followed your instructions and revisited my recent reply to this post & looked for the highlighted sentence in my attached file. Now, what do you want me to gather or deduce from what I highlighted? What is your point? What is it that you are alluding to? I still maintain that the webpage does not "mention that for temperatures above 87deg F, CO2 will remain in gas phase no matter how much pressure". And you have failed to tell us where, specifically, you have found that above the critical point CO2 will remain a gas. Do you want to still pursue that assertion or do you want to drop it?

In one of the CO2 Pressure-Enthalpy diagrams that I have, the enthalpy of CO2 at 100 oF and 3,000 psia (well into the Supercritical zone) is -3850 Btu/lb. If you expand this CO2 adiabatically down to 100 psia, the temperature of the resulting mixture of gas + liquid will be approximately -55 oF. If you continue the expansion down to what you said you want (72 psia), then the result will be a mixture of gas + dry ice snow at -110 oF. This is an isenthalpic process.

The Pressure-Enthalpy chart is from the GPSA Engineering Databook which took it from:

"Thermoproperties of Non-Hydrocarbons"; L.N. Canjar, E.K. Pollack, T.W. Cadmarn, W.E. Lee, & F.S. Manning; Hydrocarbon Processing & Petroleum Refiner, January 1966.

I don't know what point you are trying to make. The point(s) I make here is that your statements and assertions are not correct. I repeat my recommendation about not using the high pressure liquid CO2 as an expansion source - unless you install a heater/vaporizer.

[Qalander (Chem)]

Thanks Mr Art
i am practising engineer, I do have sufficient knowledge of thermodynamics acquired from my academics, same i am applying to my day to day jobs to strengthen my learning.
Please revisit ur recent reply to this post & see the highlighted sentence in ur attached file.
regards

Dear RSK Hello/Good Afternoon,Leaving aside many assertions from Art;
A very pertinent query is that; any thoughts for
possible failure of cylinders/Their witholding strength/ their caps securing capabilty?
What actions in such failure physical occurance to protect.
Qalander