8 replies to this topic

[Ravi Nookala]

Hi,

 

I need to design an exchanger for melting dry ice to liquid CO2. How the heat transfer area calculation can be performed ?

 

Can we perform such calculation in HTRI? Does HTRI support solid phase to liquid phase transition and does it have option to provide heat of melting? 

Edited by Ravi Nookala, 08 December 2014 - 06:20 AM.

[Art Montemayor]

Ravi:

 

Are you a graduate engineer?  If not, then this thread belongs in the Student Forum.

 

Dry Ice (solid Carbon Dioxide) DOES NOT MELT.  It is a classical example of SUBLIMATION.  You cannot simply apply external heat to it and expect it to behave as a liquid at the same pressure.  It will SUBLIME (convert itself from the solid phase to the gaseous phase without going through the liquid phase).  You should have - and study - a Mollier Diagram for CO₂ if you are going to work with it.

 

It is impossible for liquid CO₂ to exist at atmospheric pressure.  It can exist as a liquid only at temperatures and pressures above its Triple Point.  Again, refer to the Mollier Diagram - or refer to a thermodynamic database of properties.  If you have Dry Ice at atmospheric pressure (which is the usual case), and you want to convert it to a liquid or a gas, you must do it in a pressure vessel as a batch process.  If you have no experience handling Dry Ice, I caution you to consult with an expert engineer.  You can easily generate pressures above 1,000 psig (69 Bar g).

 

Needless to say, if you are handling the SOLID phase of CO₂, how do you expect to put it through a heat exchanger?  Obviously you can't, so you can forget about HTRI (or any other heat transfer program) having any interest in it.

[Ravi Nookala]

Dear Art,

 

Thanks for your response. I am not looking in region of sublimation. I understand that dry ice is more prone to sublimation but my interest is to operate with the temperature pressure higher than critical point to convert from solid to liquid CO2 (pressure around 20 bar and above). The idea is how to use phase transfer in a continuous basis using heating media not in a batch sequence.

 

Please suggest.

 

Regards,

Ravi

[Clalala]

The critical point of CO2 is 73.8 bara, so I assume you mean triple point of 5.18 bara.

 

How much dry ice are you looking to melt?  Seems bit of a strange way to get liquid CO2 to be honest... is it a by-product from something?

[Ravi Nookala]

Dear Clalala,

 

Yes it is a part of processing.

[Art Montemayor]

Ravi:

 

In your initial post you fail to fully reveal and explain to our members what it is that you are trying to do.  This failure to communicate correctly leads one to the subject of sublimation - which has nothing whatsoever to do with what you now describe.

 

I understand that you intend to CONVERT solid Dry Ice to liquid CO₂.  Presumably, this is for experimental purposes or to generate gaseous CO₂ by vaporizing the liquid.  Am I correct?  If not, then please tell us the correct scope of work.

 

The process of converting Dry Ice to the liquefied state is very old - older than I am.  In the early 1930’s Dry Ice was transported as a solid and “converted” into the gaseous state in pressure vessels called “Converters”.  This method was used in avoiding the transportation of liquid CO₂ in high pressure steel cylinders.

 

Through the years there have been some un-trained and ignorant people experimenting with simple thermodynamic processes - and consequently getting hurt in the process.  For the sake of personal safety, it is very important to fully understand thermodynamic phase equilibria.  You are revealing ignorance of the subject when you state:

 

“….. my interest is to operate with the temperature pressure higher than critical point to convert from solid to liquid CO₂ (pressure around 20 bar and above).”

 

You are obviously confusing the Triple Point with the Critical Point - a mistake often made by neophytes.  This can get you (and others) into a lot of trouble if you don’t know all that is behind what you are trying to do.  The point where solid CO₂ turns into liquid is the Triple point.  The point where saturated CO₂ turns into Super Critical Fluid is the Critical Point.  Many people ignore or don't understand the thermodynamic principles involved because they haven't studied the Thermo or they simply don't understand it.  For example, refer to the two youtube “exhibitions” by persons who are doing things on the edge of serious accidents. 

 

 

Both persons are literally “trusting” the allowable stress of glass will be sufficient to withstand the vapor pressure generated by the conversion of a mass of Dry Ice into liquid CO₂.  This is not engineering; this is pure lunacy and careless operation of what could easily turn into a serious and dangerous event.  Please consider that I am not trying to insult you or your attempt to do what you describe.  I strongly want to prevent you - and others - from getting seriously hurt.  You have not convinced me that you have sufficient experienced knowledge of handling Dry Ice or liquid CO₂ and this concerns me deeply because I have seen the results of others who also tried to experiment with liquid CO₂ without having the necessary engineering training and experience.

 

If you seek, want, or need experienced consultation on the subject of CO₂ processes, then let me know.  I have more engineering experience than most and have specialized in the CO₂ industry for many years.  I have a patent in the production of Dry Ice pellets and have managed the operation of many CO₂ plants.  I can help you.  But you have to furnish all the scope of work and accurate information.

[Ravi Nookala]

Dear Art,

 

Thanks for your thoughtful explanation; indeed I missed in writing triple point and confused every one with critical point, sorry for this.

 

For experimental reasons I want to convert solid CO2 at around 25 bar to liquid CO2 with no vapor formation. Issue is I want to do it a continues fashion so planning to use heating medium for this instead of playing with pressure as a measure. I know challenge is how do you get solid CO2 or CO2 crystals in a heat transfer equipment at first place and secondly how do we do engineering calculations around this equipment.

 

I would highly appreciate if you can suggest way of achieving this using heat transfer.

 

TIA

 

Regards,

Ravi

[Art Montemayor]

Ravi:

 

Please bear in mind that I am - and have always been - a proponent of engineering research and development, especially in fields as SCF (Super Critical Fluids) and phase equilibria.  These, in my opinion, are fields that still lack a lot of pioneering and development work in their application to society’s needs.  I admire your interest and work effort in what is a challenging environment.

 

However, you have not answered my basic question and request in finding out the specific details of your project.  I also am ignorant of your specific background and training in the subject topic.  For example, any attempt to convert solid CO₂ into a saturated liquid (presumably this is what you intend to do) requires expert knowledge in the thermodynamic properties of the fluid, the mechanical design of the apparatus used, thorough knowledge and identification of the potential hazards created or possible, and all safe operating procedures to mitigate any process upsets and hazards, before, during, and after the operation.  I respect your desire for knowledge and to add to further knowledge on the subject, but I lack any information that meets my requirements on the subject of proper, safe engineering design.

 

You state that you “want to convert solid CO₂ at around 25 bar (363 psi) to liquid CO₂ with no vapor formation”… and in a continuous fashion.  My comment is that you can't do this without saturated vapor formation, and the process is a BATCH one with the result being saturated liquid CO₂ at ambient temperature (usually) unless you require it at another temperature state - but then you don’t give a detailed scope nor explanation.  The solid CO₂ will not initially exist at around 25 bar; it must be taken to that vapor pressure using an external gas source or applying heat and raising the system's vapor pressure.  I can’t offer specifics when specific basic data and scope of work are not furnished.  The moral responsibility of ensuring that no negative effects will result as a consequence of engineering advice is too great.

 

I am particularly sensitive - and I would welcome all our Forum members to comment on this - to specifying engineering details to others regarding what I perceive as a potentially dangerous or hazardous operation.  I believe that all of us engineers owe it to ourselves to always be mindful of how our recommendations, comments, ideas, or suggestions can be taken or used by others who may not have the same degree of knowledge or experience in the subject matter.  When we do so - and something negative develops due to our contributions - we are morally liable at the very least.

[Chris Haslego]

Everything Art has stated is correct.  It's difficult at times for our community members to help others without knowing more about their specific background.  In this case, Art, myself, and I'm sure others would urge you to seek out someone with specific experience in CO2 processing and check their credentials as well.  This is absolutely NOT something that you should be trying to do without specific knowledge and experience learned from someone trained in this work.

 

We cannot emphasize the safety hazards enough and our forums are not appropriate for this level of education to ensure safety.  Rather, our community is better suited to folks who have a firm understanding of the process / work involved and just need to a boost in the right direction. 

 

Please, please, seek the assistance of a qualified individual and follow their advice.  

 

I'm going to lock this topic so that anyone else who may be interested in this topic in the future can benefit from the advice given here.