3 replies to this topic

[bob789]

In one of my previous posts i told you that i was doing a design project on oxygen removal from air. You were very helpful before and i would be greatful if you could help me again as this is an area some of you might be familiar with.

I am currently at the stage where i am carrying process calculations like: energy balance, mass balance. I was now needing your help in relation with the distillation columns within the process. I have two columns. The first one is at air pressure which recieves the air and separates the argon from it. This air stream (excluding the argon) then flows to a second distillation column which separates the nitrogen and oxygen. I was needing information on the common size of these distillation columns used in industry today; also needing information on the heat loads associated with these. Also the volume 1 hours production would take up in column and find a cylinder that would allow that volume to be held.

thanks again,

Bob

[Art Montemayor]

Bob:

Your questions are of a general nature and can only be answered in a general way: The physical size of the distillation columns depend on the size of the capacity they are designed to handle. This, you should either know or have calculated by now. Additionally, no air separation fabricator is going to tell us (or the public) what size the columns should be for a certain capacity. Get real. No one is going to foster or pass on proprietary information to their competitors. This is not a charity organization we are talking about.

If you required to report the size (diameter) of the distillation columns to your instructor(s), then you should calculate these in the normal engineering manner - by using the Brown-Souders relationship. You should be very familiar with this relationship if you are attempting to process design a distillation train.

You ask for heat loads for the same columns. Well, you calculate these as well in the conventional chemical engineering manner. You state you have made the heat and material balances, so you should have already generated the heat load for the reboilers and condensers - right? I don't understand if this is a problem or if you are asking for confirmation.

I don't understand what you mean by: "the volume 1 hours production would take up and find a cylinder that would allow that volume to be held". Please be clear and expain just exactly what it is that you need. How can we even guess at a vessel size when we haven't been given a capacity - just a hint of 1 hour's production?

Await clear and concise basic data.

[bob789]

Hey

I am also doing this project with Bob (still awaiting registration to come through). I realise he has not made himself very clear but to be honest we don't know how to even begin designing a distillation column. We only need relative sizes and we have been given a week to do it. I was just hoping you might be able to point is the right direction of how to go about it. I have a had a look round some books but have drawn a blank. Do we need incoming pipe sizes before beginning or just knowing the volume that would go through in 1 hour enough? The only thing we know at the moment is flowrate through the column but we would be able to calculate other values if needed. I've uploaded the mass balance and flow diagram so you can see where we are coming from, we are trying to get sizes for D1 and D2. If it's the case where we've done something majorly wrong, it would be great if you could just tell me.

Thanks for you help

Bob:

Your questions are of a general nature and can only be answered in a general way: The physical size of the distillation columns depend on the size of the capacity they are designed to handle. This, you should either know or have calculated by now. Additionally, no air separation fabricator is going to tell us (or the public) what size the columns should be for a certain capacity. Get real. No one is going to foster or pass on proprietary information to their competitors. This is not a charity organization we are talking about.

If you required to report the size (diameter) of the distillation columns to your instructor(s), then you should calculate these in the normal engineering manner - by using the Brown-Souders relationship. You should be very familiar with this relationship if you are attempting to process design a distillation train.

You ask for heat loads for the same columns. Well, you calculate these as well in the conventional chemical engineering manner. You state you have made the heat and material balances, so you should have already generated the heat load for the reboilers and condensers - right? I don't understand if this is a problem or if you are asking for confirmation.

I don't understand what you mean by: "the volume 1 hours production would take up and find a cylinder that would allow that volume to be held". Please be clear and expain just exactly what it is that you need. How can we even guess at a vessel size when we haven't been given a capacity - just a hint of 1 hour's production?

Await clear and concise basic data.

Attached Files

•  Project_Mass_Balance_2.xls   80KB   42 downloads
•  Revised_Flow_Diagram_word.doc   241KB   41 downloads

[astro]

Bob 1 & Bob 2

Well done for posting details. We need to see more of this, rather than general pleas for help that show the person asking has not taken the time or thought to help themselves first.

Regarding column design, I contributed to this post:
Column Flooding
No one out there commented further, so I'd take away from that that the silence indicates my advice was appropriate. The upshot of it will hit the areas that you're looking to obtain guidance. Hopefully this response isn't too late for you to sort yourselves out.

I'm surprised that you 2 are so adrift with this. You shouldn't be being asked to perform this work without the necessary grounding in the theory.


Regarding your flowsheet design and mass balance, I've got a couple of comments for you.
1. There's no quantification of the water in your air intake. The humidity of this stream will set these conditions and if it's not specified then find some weather data to get sensible, typical numbers and look for a basis that will provide a design case (read max H2O load).

2. The heat exchange network is problematic (although I can see that you're trying to extract maximum heat exchange from your process stream, so you get marks for trying).
In practical terms, you will not be able to source inlet air that is bone dry (see point 1 above). Consequently, H1 will suffer from freezing given the wall temperatures that it will see at its cold end (stream 14 is running at -195C). I've seen an ammonia cooled air condenser used where the evaporating ammonia liquid was pressure controlled to achieve a constant 5C temp and avoid the problem of freezing.

You will probably only have air or cooling water as your cooling utility and the moisture removal load will have to be taken up by the mole sieve. I'd think you'd be better off placing the mole sieve after C2, with compressor stage after coolers and liquid knock out as standard items.

The other heat exchangers don't appear to be driven by another stream. Is this because you've simplified the modeling of a plate fin heat exchanger block?

3. The nitrogen storage arrangement doesn't make sense to me. Stream phases aren't stated on the mass balance but with the flows you're dealing with, I'd expect that practical storage will be achieved via liquefaction. I doubt whether thermodynamics will let you compress a gas and cool it to a liquefied state. If that's not the plan then I doubt the practicality of storing the volume of gas that the process will produce.

You can sense check the other product streams yourself.

4. Expansion turbines? Your design uses JT valves on the gas streams. You'll achieve more effective cooling using expansion turbines rather than isenthalpic JTs. This provides the opportunity to harness the associated work to drive your compression and reduce the power bill.

5. Waste Argon? Argon is a commercially viable product that is used extensively for welding. On the surface, I would not consider it "waste".

6. LOX Purge for Hydrocarbons
Read this:
Safe operation of reboilers & condensers in ASUs

Think about the accumulation of hydrocarbon contamination and showing a purge stream. The paper gives design guidance that will help.


Well Bobs, I hope that helps.