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

 

[NZBen]

The search engine was down when I wrote this - have found something pretty similar to what I was trying to do but with a guy wanting to run an engine on gasified wood.

But apparently it looks really hard from comments - my idea was to run an engine on CH4 and use the power of the engine to compress and chill it's exhaust - maybe with some electrical input to help if needs be. What is so hard about pulling the NOx (I presume the baddy here is NO2) and SOx out when it gets chilled? The scale I'm looking at is around 30kg/hr. Stainless fittings and heat exchangers are not a problem to me as I do alot of stainless fabrication currently.

Thanks

 

[zdas04]

The easiest way to get what you want is to remove the nitrogen pre-combustion. Then the stack gas is CO2 and water. This oxy-fuel technique is being proposed for several coal-fired power plants around the world.

David

 

[NZBen]

Yeah - I've read about this, but thought it sounded like it would be a bit expensive to do. What is wrong with the NOx getting into the chilling process - what happens? Can you condense the CO2 down to liquid, then distil it again? WOuldn't that remove traces of the NOx?

 

[bonzoboy]

If you are only looking at small quantities of CO2, you could filter the combustion exhaust air through a high surface area filter (after you condense the water). Something like silica gel should work, maybe activated charcoal. You still have to separate the N2 (and O2) from the batch. Maybe use liquid N2 bath?

I think you are better starting with O2 as the oxidant; or just purchase a bottle of CO2....

 

[NZBen]

Wouldn't the O2 and N2 just be lost as gas once I condense the CO2 under the cold temperature?

 

[zdas04]

I think what everyone is not saying, is that it isn't quite as easy as you might hope. First, one of the combustion products you didn't mention is water vapor. If you don't get ALL of that out prior to your chiller, you'll have a frozen vessel in minutes. Second, some of the water will condense as you approach temperatures suitable for dehydration and you quickly end up with some of the nitrogen and some of the sulphur morphing into strong acids.

It is really a tough problem.

David

 

[NZBen]

Sorry, I did mean to mention the water but forgot - first I'd cool it to atmospheric temp and a water seperator. I would have thought that if I had a mini cooling tower that chilled the vapor to say 2-3 deg C and at 6 bar that would remove most all of my water? Then the gas moves on 20 a 2nd mini tower which chills down to say - 40 to get most the rest of everything out, and finally moving to a tower at -55 to get the CO2 - vent everything else. As for the acids forming - wouldn't they be likely to collect in the -40 degree tower? I was also reading on the net somewhere about closed loop injection of CH4 can produce basically nil NOx at fixed speed - is this true or rubbish?

The other thing I wondered is what if I bubbled the exhaust through some cool water? Wouldn't that absorb and cool the acid gasses? When the recirculated water gets to acid I could bin it and put fresh water - maybe even make it slightly caustic?

Will the acids end up in my liquid CO2?

 

[zdas04]

I think that you have all the answers you need, but you seemed to have had them at the start. I say go for it.

David

 

[NZBen]

Thanks for the vote of confidence I just wanted to make sure before I committed $20,000 to building it and then find out - hey that's why they don't do it like this...

 

[mbeychok]

NZBen:

I don't know what compositional specifications that "food grade carbon dixide" must meet and it might be helpful if you told us what those specifications are.

Almost all modern ammonia sytnhesis plants today produce large amounts of by-product carbon dioxide with a purity in the range of 98-99% or higher. Why not contact some of them and find out the exact composition of their by-product carbon dioxide ... and see if it would meet your needs?

There is a list of dozens of the USA ammonia plants at

http://www.energystar.gov/ia/business/industry/industrial_LBNL-44314.pdf

(scroll down to page 39 of the 40 PDF pages).

Milton Beychok
(Visit me at www.air-dispersion.com)
.

 

[NZBen]

I believe these are the requirements of food grade:

Purity 99.9% v/v
Moisture 20ppm v/v
O2 30ppm v/v
CO 10ppm v/v
NH4 2.5ppm v/v
NO/NO2 2.5ppm v/v
Non volatile residue 10ppm w/w
Non Vol organic res 5ppm w/w
Phosphine 0.3ppm w/w
HC (CH4) 50ppm v/v
Acetaldehyde 0.2ppm v/v
Aromatic HC 20ppb v/v
sulfur 0.1ppm v/v
SO2 1ppm v/v
No foreign odor or taste.

 

[Montemayor]

Ben:

You say you want to produce food grade CO₂ by burning natural gas. I've done that more than a couple of times - and in tonnage quantities, 24hr/day & 7 days a week.

In trying to follow this thread, I see that you are putting a lot of twists on the process you want to work with and are still making potential changes - you've even omitted the critical dehydration stage.

The process of food grade CO₂ from the combustion of hydrocarbons is older than even myself. I know that personally because my first job out of college in 1960 was to master this process in my first engineering assignment. You say you are committed to spending $20,000 to building something that you haven't researched or made preliminary calculations on? Is this an experiment or an attempt to make a profit? In either case, from what you are describing, it won't work the way you want it to. If you want to learn how to produce food grade CO₂, I can tell you how it has been done successfully for decades. It is very simple and has nothing to do with NOx or a lot of what you describe. However, I can't tell you how to do it the way you want it done - it doesn't work that way.

You have to clearly state what raw materials you plan to use and how you want to produce the product. You've stated the former, but you haven't said anything about the latter. You also haven't furnished any other basic data or scope of work - other than you are pondering on doing it and don't have a process nailed yet. Before you spend $20,000 I can firmly tell you from personal experience that more detailed information is needed in order to furnish help on what you want to do and allow you to possibly save your money rather than go forward towards a dead loss of time and money.

 

[25362]

NZBen, what are the quantities of CO₂ are you planning to produce ?

 

[dcasto]

I don't see how you are going to cool a water wet gas stream to below 0 C witout making ice.

 

[NZBen]

With regards to the last 2 posts, I'd like to be able to make around 30kg/hr. I was hoping that I can remove the majority of the water as a liquid at around 2 deg C - hoping that most of it should condense out - then remove the water from the tower at that point, the next stage might have to have a cyclic defrost for any vapor that gets through.

Montemayor, Thanks for your input I'll answer your question in depth sometime later tonight (when I have a bit more time)woud be very keen to draw from your experience and knowledge.

Quote: "You say you are committed to spending $20,000 to building something that you haven't researched or made preliminary calculations on?"

I've done some calcs and I'm doing the research part right now!

 

[25362]

Another table of impurities could be found in this link:

http://www.jmcatalysts.com/pct/marketshome.asp?marketid=397&id=400&toolbar

 

[Montemayor]

Ben:

Please be specific and give engineering references to your retrieved data - such as 25362 does when he cites his source of data for CO2 grades' definitions. Your description of food grade actually fits the cited Johnson Matthey Catalysts description of "Beverage Grade" - not "Food Processing (i.e., Grade). There is a difference, and any engineering project has to have a definite and fixed specification of product quality. Otherwise, confusion and chaos will be the result. For example, your specification for NH4 is non-existant. You probably mean NH3 (Ammonia); isn't that correct? What specific grade do you want to produce and for which market or use. Again, is this an academic experiment, a profit-oriented project, or a school assignment? If you are a mechanical engineer, you already know that you have to be specific and accurate when dealing with engineers and seeking consultation - otherwise you're only going to generate more questions than you can handle and few (if any) answers.

Await response.

 

[BenThayer]

if you only have $20,000 to deal with, you will be lucky to install the equipment to receive and handle cylinders delivered to you for that.

do a quick life cycle cost comparison of buying it vs. designing, specifying, procuring, building, commissioning, operating and maintaining something and i am sure that you will see that you are wasting your time.

don't screw around with making it, buy it.

 

[NZBen]

Hey 25362, great website lots of useful info on it. Thanks for that.

BenThayer I do appreciate your concern about the scarce amount of money I'm looking at spending and I know the just go out and buy it psyche works for some people - but it doesn't for me, I just built a dry ice machine entirely stainless with a PLC controller and a production rate of 150kg/hr for $3000 ($2000USD) now for me to buy such a device would cost me somewhere in the region of $80-100,000, I also have a double vacuum insulated stainless vessel in which to store it. Some years ago I helped a friend build a machine that cut and seamed plastic, the bank valued it at $120,000, it cost $800 to build.

Sorry Montemayor - I did mean NH3 - as you can tell I'm not much of a chemist! The CO2 is for a profit making project - to run my dry ice machine. The food grade specification I quoted was from wittemann co2. And titled "Typical food grade specifications" Have just finished my dry Ice machine and I'm looking at manufacturing it for a living, Dry ice where i live sells for quite a premium, and I'm wanting to quit work and try to go out on my own.

Below is my description in my head - no need to pick hairs or be too fussy with intricate detail - I'm just wanting an idea on whether the concept is sound. I know this is not how it’s done in industry. (I’ve read quite a bit about the amine absorption process).

I was hoping to use a 4litre automotive engine to burn CH4 passing the exhaust through the catalyst possibly with additional O2 from a smog pump. I was then hoping to cool the exhaust gasses through a heat exchanger close to ambient (remove any condensate). From there I am hoping to compress the exhaust gas with a large compressor (powered by the engine) to around 6 – 7 bar. Cool the gas after compression and drain any moisture, then run the gas into the first chilling tower, this chilling tower will cool the gases to around 2 degrees C, a moisture trap at the base will expel any condensate. Then will pass to a 2nd tower which will chill the gasses to around -40C any condensate will be removed here also, and on to the third tower where it will be chilled to around -55C, at this point most CO2 should liquefy and be pulled out through the condensate drain to refrigerated holding tank. Any gasses that have not at this point liquefied will be expelled after passing through the tower. The 2nd tower may have to have a cyclic defrost cycle, but I’m hoping most the water will be extracted on the 1st tower and before.

CH4 costs around 5c/kWHr where I live, I’ve calculated this through to yield me CO2 at a price of around 35c/kg. In my initial description I left out the water just by error as at the time I was more concerned about the toxins – I figure the amount of water I’ll get will nearly be just 20% less than the CO2 I produce.

I believe cryogenic separation is used in industry for CO2 production tho rarely due to the energy required and 99% of CO2 production uses the absorption method. I have read that the cryo separation method delivers a more pure product with less requirement for post filtering. So my question is not how is it normally done in industry, but will it work by this method?

thanks