CO2 purification

[NZBen]

I'm wanting to produce relativly small amounts of CO2 with a purity of at least food grade. I was hoping to do this by burning natural gas, compressing the exhaust and cooling through different temperature cooling columns. I figured in my simple brain this should work piece of cake - but reading on the net I've found that NOx and SOx has to be removed before the gas reaches the chilling stages of seperation.

The data I have at 1atm NO2 should liquify below 21 degc, SO2 should liquify below -10deg N2O below -88 and CO below -192 they should all be a bit hotter than that cos I'm wanting to do this at 6 bar - where CO2 should liquefy around -50 and I should be able to let the rest of the gas go? that hasn't condensed by that stage?

Any advice anyone can give me about this would be greatly appreciated. Thanks
Ben

 

[Montemayor]

Ben:

Like I said, I've done what you describe several times and I have a US patent on an automatic, PLC controlled, Dry Ice Pellet machine to boot. I have designed, installed, retro-fitted and operated various Dry Ice systems and plants. The last one I started up and operated was in Decatur, Ill. It used LCO2 supplied by the Archer-Daniels fermentation alcohol plant next door. I operated 3 "conventional" block dry ice presses (Larry, Curley, & Moe) and seven pellet machines from New Zealand (I named them after the 7 dwarfs). The pellet machines were such a pain and a loss that I decided to develop my own design. My patented design was built and worked immediately. My concept is based on bulk production of the pellets to satisfy the chicken and sausage meat producers.

So, I identify with what you are stating. And that is why I'm telling you that you can't produce economically competitive CO[sub]2[/sub] doing it the way that you propose. It just doesn't work for a variety of reasons that would take several threads in this forum to explain to you. Trust me on this one. A lot of what you are assuming you can do is not going to work - not because I say so, but because many others have tried and it has been proven out in the field that it won't work. I can tell you, for example, that you will be unable to control your chiller with the CO[sub]2[/sub] at 2 [sup]o[/sup]C. You're going to freeze the system up trying to pull that off. dcasto put his finger on that in his post and he is right because I have shown that to be a fact in the field. That's not the optimum manner to remove water moisture from CO[sub]2[/sub]. The way to do it is with adsorbers and Activated Alumina. Witteman will tell you the same thing as well as any other experienced CO[sub]2[/sub] process designer/fabricator (there's very few left after Witteman).

My credentials in the CO[sub]2[/sub] and dry ice business started in 1960 and I've designed, consulted, installed, modified, and operated these types of plants and produced countless tons of dry ice in such spots as Kingston, Jamaica; Port-of-Spain, Trinidad; Barbados; Caracas, Venezuela; Lima, Peru, La Paz, Bolivia; Decatur, Ill; Houston,TX; Madrid, Spain; and many more that I've since forgotten. I have probably forgotten more CO[sub]2[/sub] stories than you have read. That's why I thought you might be interested in knowing what I, dcasto, and 25362 are trying to alert you to: do not think that you can do what you propose because you've read about it somewhere. It isn't that simple without the experience.

I hope this experience helps you out.

 

[NZBen]

Wow - yr definatly as qualified a person as I could hope to talk to about this, after all when you started I wasn't even a twinkle in my fathers eye!

What was wrong with the pellet machines you were using - I assume they were asco machines?

Everynow and then people come up with lemon ideas - I guess this was one of mine, at least I haven't committed much time and coin into it, thanks for your input -

but my question is, where to from here? I have a pellet machine that I'd like to start producing pellets from, CO2 here is quite expensive so ideally I'd like to make my own - asco build CO2 plants - do you think I should look at and scale down a design similar to theirs? Do you think I should maybe build my lemon idea on a very small scale (using fridge compressors) and old heat exchangers I have laying around to satisfy myself that the idea is flawed from the start?

Thx

 

[unclesyd]

I'll have to agree with and second the comments by Montmayor in that the mechanics of scale have got you.
I haven't seen anything at your required scale using any method of CO2 purification.

My career too started in an area where we produced very pure N2 form combustion of very clean natural gas. We used Kemp Gas Generators to produce a gas stream which we dried with alumina, removed residual O2 with copper shot, and scrubbed the CO2 with an amine system. This system though state of the art required constant operator attention along with on stream lab analysis.