4 replies to this topic

[snb15]

I am undertaking a design project and am new to using Chemcad. I am designing an air separation unit using cyrogenic distillation to produce the gases oxygen and nitrogen. I have researched many papers and it appears the air passes through a filter and a water separator before the molecular sieve. I have identified a unit in Chemcad for the molecular sieve, but are there ones for a filter and a water separator or is this purely modelled as a molecular sieve in Chemcad?

Also from researching air separation units using cyrogenic distillation sometimes an expander is used in the process but for different purposes. I am currently looking for clarification as to the instances where and when an expander is used and for what purposes.

Many thanks

[Art Montemayor]

snb 15:

As I understand your queries:

• You are trying to “design” an air sepatation unit to produce gaseous oxygen and nitrogen;
• You state you have researched many papers and don’t know why there should be a filter and water separator before the molecular sieve. You ask where in ChemCad there is a similar model and does ChemCad model only reflect a molecular sieve unit.
• You have found that an air expander is sometimes used and don’t know when an expander is used or for what purpose.

My simple response is: If you don’t understand and dominate the Unit processes taking place in the air separation plant – and for what reason, then you have no business trying to simulate the air separation process. The answers to your queries are so basic, common sense, and simple that it reveals that you literally “don’t know what you are doing”. Allow me to explain:

• If you are dealing with the liquefaction of air, then you should clearly know and understand that you are dealing with thermodynamics. You should have full grasp of an adiabatic, non-reversible process (Isenthalpic) and an adiabatic reversible (Isentropic). The difference in efficiency is what determines why and where you employ an expander.
• Before you can liquefy atmospheric air, you must compress it. Upon after-cooling the compressed product it is obvious and common sense that you will condense some of the water vapor in the atmospheric air. The compressed air is still moist (actually, saturated), but you cannot – by common sense – feed water into a cryogenic process (-150 ^oC). Your lines will block up immediately with water ice. You must separate the liquid water and drain it out of the process and then you must subsequently remove the remaining water vapor in the saturated state by passing it through an adsorption unit. That is when you use the molecular sieve unit. The dried product then can go to a Joule-Thomson expanson valve and a turbo expander.

You have to know EVERY thing about your process first before attempting to simulate. You are inputting garbage and, consequently, your product will be garbage.

[alpen]

Dear Art Montemayor,


I am project engineer in air seperation design and mfg company,
I am trying to enhance my knowledge in process side, I am familer with all componets and sizing, compressors, chiller, seperator, adsorber, thaw and regen heater, recyle compressor, TBX, columns, instrumets PT,TT,FT etc.. I do all that from provided PFD's , in other word if I want to start fresh checm cad simulation for 200 tons /day LOX/LIN plant, I size compressor based on my LOX molecluse. next step can you please help me with column sizing procedure for air seperation plant? and TBX arrangement? ( double column/ LIN/LOX) any document rule of thumbs?

Thank you in advance.

Edited by alpen, 16 January 2013 - 12:25 PM.

[Bodhisatya]

Dear Alpen

I am a Commissioning Engineer for Cryogenic Air Separation Units. As Art Sir pointed out thermodynamics is the basis of ASU and U need to have a very clear insight of the Thermodynamic phenomenon occurring. I am not quite sure about the Filter u referred to in the first post.

For the moisture separator Art sir has clearly pointed out the reason and it's application.

Now, coming to Cold Box where Main Rectification takes place: The dry Air then enters the Main Heat Exchanger where it exchanges heat with all the outgoing Cold Streams to reach it's liquefaction Temp.

Now Coming back to your question about the arrangement : There are quite a few Cycle such as Claude , Lachmann, Waste Air Expansion cycle. Each cycle is unique. Determination of Cycle depends on number of Factors such as Liquid Make of the Plant, Power Consumption, as well as Purity Specification of the Final Product. It has to be Linde Double Column when it comes to O2-N2 separation. U r right about the Compressor Sizing Procedure. When the Final Product spec such as Flow and Pressure already determined (Generally Comes with the Bidding Spec.) u can start sizing your Main Air Compressor.

As I hail from Operation Side, Column designing is something about which am not fully aware. Will Like to learn the honorable delegates associated with this forum. Would be a great boon if U share the documents u possesses related to Air Separation Unit.

Regards
Bodhisatya.

[thorium90]

wow you two just hijacked snb15's thread.

Anyway to answer the simplest parts of the TS's questions;

the air passes through a filter and a water separator before the molecular sieve.

The filter removes dust. The water separator removes water. As you might have guessed, a coldbox is 'cold' and not removing water means it would become ice and ice is solid and chokes coldboxes. Molecular sieve brings all the water and CO2 (which also becomes solid when cold) down to ppm levels.

an expander is used in the process but for different purposes.

The expander provides the 'cold' for the coldbox by reducing the pressure, aka Joule Thompson effect.

As for Chemcad, I'm sorry, but I'm familiar with Hysys models of ASU plants.