4 replies to this topic

[whoooosh]

Hello

I'm designing an Air Separation Unit to separate Oxygen from air for a project and from what I've found the ASU that uses cryogenic air separation typically has two columns of different pressure. However, I can't find any literature on why this is so. From what I know, the relative volatility of oxygen is higher at lower pressures. Since this is so, why can't we just use one single column at a low pressure for the separation instead of two? I understand that, to separate Argon, the high pressure column is required because Argon and Oxygen have similar boiling points. Since I don't need to take Argon out, would one column be more efficient? Thanks!

[Zauberberg]

Can I spend a weekend in Las Vegas for about 120 USD? Probably I could. It isn't going to be that much funny than if I have 10,000 USD with me, but no one can tell me how good or bad it will be if I don't say what my expectations are.

To come to the point - how can you expect to receive a meaningful, accurate, and comprehensive reply on your query by making such a general question? What is the process configuration, and basic parameters around the unit? How many products you suppose to have from the ASU? What are the product(s) quality specifications? Do you already have products that satisfy your separation/purity criteria, flowing from a single column, or you are making a wild guess whether it can be done in a single column, or not? How does the PFD look like?

Can you design your unit with a single column? Maybe you could. Maybe you cannot.

[Padmakar Katre]

Dear,

Once you are thru with Zauberberg's comments and finalize the objective and basis of the design, just Google with the keywords "Air Separation Unit", I am sure you will find plenty of material available online to clear your doubts.

@ Zauberberg
nicely explained.

[whoooosh]

I'm designing the ASU for an IGCC plant. My plant requirements are 900 tonnes of Oxygen per day at 95% purity.
What I'm asking is why HP/LP columns are used in general for ASUs. I did google the term 'Air Separation Unit' and other related terms, too, but couldn't find any explanation of why ASUs typically have two columns instead of one, which is why I'm asking the question here.

[Art Montemayor]

Whoooosh:

You say:

• “I'm designing an Air Separation Unit to separate Oxygen from air”
• “why can't we just use one single column at a low pressure for the separation instead of two?”
• “would one column be more efficient?”

I don’t know what you mean by “design”; I also don’t know what country you are in. However, the answers to the last two questions are:

• You certainly can use a single column to do a basic air separation and produce oxygen as pure as 99.5% (mol).
• The single rectification column would NOT be more “efficient”. (here you fail to specifically state what you mean by “efficient”. Since you don’t identify what kind of efficiency you mean, you can’t expect any further explanation)

Refer to the attached detailed drawing of a Linde Double-Column rectifier for details of what I refer to. I have installed and operated these types of columns as well as single rectifier models in the past. I even have color pictures of them and of their internal parts. I modified several of them to produce a variety of products. The single rectifier columns produced up to 99.5% (mol) purity oxygen. For 95% purity demand, the operation would be simpler.

As Zauberberg has specifically pointed out and Padmakar has inferred, you haven’t only lacked in being specific, but you haven’t done all the research you can do on the subject. If you had done it, you would know the answers. The Internet isn’t the ONLY source of engineering information. It is one of many available to you. If you resort to a conventional engineering library and similar sources, you will find:

• Bliss and Dodge, “Oxygen Manufacture – Thermodynamci Analyses of Processes Depending on Low Temperature Distillation of Air, Chem. Engineering Progress, 45 (51), 129 (1949)
• Ruheman, “The Separation of Gases”, Oxford University Press, NY, 1945.
• R. Norris Shreve, “The Chemical Process Industries”, 2nd Edition, McGraw-Hill Book Co., 1956

You also ask: “What I'm asking is why HP/LP columns are used in general for ASUs”. If you are truly “designing” an air separation unit, then you should be not only aware of what Zauberberg has specifically pointed out, but you should also have already arrived at the answer by trying to produce what you have as your scope of work. For example, have you already generated the P&ID, the heat and mass balance, and the ultimate P&ID? I seriously doubt it. I believe all you have done is resorted to Google, hoping it would all be done for you there. And that kind of research will never produce a degreed and experienced Chemical Engineer. Further, no serious engineering manager – myself included – would consider hiring a chemical engineering graduate that depends solely on making Google hits for finding engineering “answers” to engineering problems.

I hope I have succeeded in showing you the “light” at the end of the tunnel. You have to ask specific, detailed, and correct questions to generate the same quality of answers. You also are expected to do your homework prior to asking the questions. It’s all about hard work.

Attached Files

•  Air Separation Rev1.xls   930.5KB   124 downloads