7 replies to this topic

[mirzanr]

Dear colleagues,

I am in the process of simulating various CO2 capture processes in a company's in-house software. In the capture process, CO2 is separated cryogenically, using a combination of flash tanks and distillation column. In one of my distillation column, I have CO2, N2, Ar and O2 as feed. I know the top and bottom product concentrations; and pressure of column. Can somebody guide me how to determine the number of theoretical plates for such a distillation column? If Fenske's equation is recommended then, where could I get the volatility data for said components?

Also, please suggest me some literature where I could read how to determine the CAPex and OPex for my flowsheets.

Please guide.

Nauman.

[Art Montemayor]

Nauman:

Before wasting any time on a simulator, please ask yourself what is going to happen when you try to liquefy the mixture of gases (presumably air and CO₂) you have listed as feed. Are you aware that CO₂ is going to reach its “triple point” (where CO₂ exists as a gas, liquid, and SOLID) before the remaining gases liquefy? If you check an equilibrium diagram of the mixture, you will see that the solidification of CO₂ will commence before or during the liquefaction of the remaining gases. And once solid CO₂ (“dry ice”) is formed, look out! Any orifice or internal restriction inside a distillation column will get totally plugged up with the dry ice. The distillation unit operation will come to a quick and sudden halt.

You probably don’t know this, so I feel I have to advise you of what you will run into if you persist in going through with the idea. I started my engineering career liquefying air for the production of Oxygen and nitrogen, so I know far too much about the headaches and problems that CO₂ can give such a process. That is why the first thing anyone in an air separation plant has to do is to ensure that all gaseous CO₂ (as well as water moisture too) is totally removed from the feed air before attempting to liquefy it.

Your proposal won’t work. The low concentrations of CO₂ can’t be separated cryogenically, using a combination of flash tanks and distillation column. However, the reverse, a gaseous CO₂ stream contaminated with air (99% CO₂ vol.) is normally distilled to produce pure CO₂. But you can’t justify doing that unless your CO₂ mol fraction in the feed stream is approx. 90%+. I don’t think that is your case at all.

[mirzanr]

Dear Admin,

thank you for your detailed response. Actually, I am working with an 80+ mol% CO2 stream. I don't want to hit the CO2 triple point, therefore I would be running the column at high pressure and low temperatures. Perhaps, it won't purely be a distillation column since I don't use any reflux stream. It would be sort of a stripper. I plan to inject a high pressure, low temperature partially liquefied feed (80+ mol% Co2) to the column. Reboil is injected from the bottom (which is 99+ mol% CO2). I collect liquid CO2 (EOR grade) at the bottom, and all the lights are vented at the top of column. I hope the problem is clearer now.

Any help in calculations of theoretical stages and volatility data will be highly appreciated.

[Art Montemayor]

Nauman:

As most students who come to our Forum, you have offered meager basic data and definition of your problem. You haven’t, for example, told us from the beginning what it is exactly that you are confronting. Now, it seems you have a typical CO₂ capturing project. But you haven’t submitted a flow diagram or P&ID either, so it has been difficult to simply understand what it is that you are trying to do.

If it is of interest, I have done the design, installation and operation of the very process that you are facing. I did this when I worked as a young engineer in the compressed gas industry. In fact, I may still have the original hand calculations up in the attic of my house. But it has been many years (1965 -1968) since that time, so I may not find the information.

But the important point here is that I can still draw the basic flow diagram on Excel, and I know how to design it. I remember using the crude CO₂ feed as my reboil heat source and the tower was nothing more than a basic, dissolved gases stripper. You can also employ hot refrigerant interstage gas as reboil heat. There is no rectification, as such, involved as I recall. What you call “high pressure” CO₂ never goes higher than 350 psig – which is not considered “high pressure” in the compressed gas industry. There exists what was called a high pressure CO₂ process (1,200 psig liquid) but that is not used anymore because it is uneconomical as compared with the lower pressure cycle.

This is NOT a cryogenic process as you describe it. It operates in the vicinity of -25 ^oF – which is far above the cryogenic zone (-238 ^oF). If you would like a process description and a photo of a typical liquid CO₂ Stripper, let me know.

Dear Mr Montemayor,

As I am also working on CO2 distillation, I'd be interested to see the process description and photo mentioned in your 26 June message.

Very many thanks,

Inxl6

[Art Montemayor]

Inxl6:

I have waited and waited for Nauman in order to help him/her with whatever information was required, but not a response or even a token "thank you". I wish him/her a lot of luck in the future, but with that type of gratitude for help received I doubt it will go very good.

Attached is just some of the information I prepared, which is an analysis of an existing commercial process plant that the Wittemann company markets. I have installed and operated Wittemann equipment - as far back as 1966, in Trinidad. At that time, they didn't have a liquid CO₂ stripping process; however, at Liquid Carbonic Corp where I worked we had such a process. It was used a lot in recovering CO₂ from by-product sources contaminated with lights such as hydrogen and nitrogen.

I hope this helps you to form a good idea of how to purify the product CO₂.

Attached Files

•  Liquid CO2 Purification.xlsx   112.1KB   84 downloads

Very many thanks for this very helpful document.

I hope the lack of response will not deter you from contributing in the future, since not many people have your degree of experience.

Best wishes,

Inxl6

[arslan.ahmad]

To find the volatalities of your feed concentrations, you have to study the book.......Data book on hydrocorbons by J.B Maxwell