[NZben]

I'm looking at manufacturing relativly small amounts of CO2 from burning natural gas, I'm wanting to purify it by cryogenic seperation - at around 6 bar.  I was hoping I could have several small distillation towers at different temperatures to pull off other gasses (H2O, SO2, NO2, N2O, CO) and progressivly cool the CO2, however I've read on the net that NOx and SOx must be removed before entering the cryogenic cooling stages.  Can anyone enlighten me as to why?

The data I have is NO2 will liquefy below 21 degC, SO2 below - 10 deg, CO won't liquefy in my process cos it does so at -192. (these temperatures are all at 1bar - so will be a bit higher at pressure) At 6 bar I'm looking at about -50 degrees for CO2 to liquify.  Anything else I should be aware of? - I would like the CO2 I manufacture to be at least food grade.

I'm not really that concerned about the energy cost of the cryo seperation as compared with amine as my quantities are quite low, I also want to end up with refrigerated liquid CO2 so am hoping I can do it with minimal plant.
Thanks for any input

Ben

[eugenedakin]

Hello NZben,

I have not had the opportunity to create food-grade CO2.

What you say sounds logical and makes sense.   Unfortunately, I am not sure of problems that you may find.

I wish you the best.

Cheers,

Eugene

[billnotgatez]

There are companies that routinely make dry ice. It seems to me, you could get some ideas from them.

[NZben]

Thanks eugene - will let you know how I get on.

I'm in New Zealand so the only companies that produce dry ice over here either get it in bulk from the natural gas fields down country (that's about 75 cents/kg - in minimum 10 tonne lots), the oil refinery up north or I know of one comapny that imports their gas from singapore.  I know of CO2 plants that asco gas make - they seem to be quite a complex arrangement of systems and use what seems to me as a large amount of electricity, not to mention expense - their smallest unit produces 70kg/hr and costs around 3/4mill!!!

I know I could build a much smaller system for around $20,000 but don't want to get to the end of the project and find out why I'm the only one doing it this way and have to start from scratch again.  My main concern is the NOx and what seems to be a huge effort that many companies seem to go to get rid of it.

I've found one issue - mixing NO and NO2 (NOx)at temperatures below -21C give N2O3 which is apparently highly toxic blue liquid -  however I'm still of the thinking i should be able to distill it out? I know I can significantly limit the NOx formed by combustion if I inject a water spray into the combustion to limit combustion temperature. Will keep studying Cheers for your input so far

[Donaldson Tan]

they typically use a SNOX plant (scrubber) to remove NOx

[NZben]

What's inside the tower?  Do they spray a caustic soultion down while forcing the gas up through it?

[NZben]

Did some research and am now an expert on scrubber towers - thanks for pointing me in the right direction - it ain't as difficult as I thought it might be.

[technologist]

The simple method would be to use

• Water wash column
• 3A Mol sieves for drying
• Chilling for CO2 Liquifaction
• Mol Sieves (Special type for Nox removal

Consult website of Wittemann USA for info