Hello,
I am currently in the process of designing an air separation plant for the production of argon, oxygen and nitrogen by cryogenic distillation. I have up to this point managed to design the plant using approximate methods and this has allowed me to create an Aspen simulation using the equilibrium model. The process is as follows:
Air Feed = 100kg/s
Filter
Compressor
Coolers
Adsorption Columns (molecular sieves) for removal of impurities
Air Booster Compressor
Heat Exchanger Network to achieve cryogenic state
HP Column
LP Column
Crude Argon Column
Pure Argon Column
Purpose of Plant: Obtain Argon with 99.9996% purity, Oxygen with 99.995% purity, Nitrogen with 99.999% purity from a 100kg/s air feed.
Then, I did some optimization on Aspen by performing sensitivity analysis tests and manual optimization and obtained some results. I have attached an Excel sheet with the specs of the columns and streams up to this point.
Currently, all the columns are designed to be trayed columns, the HP and LP Column have not yet been joined to form a double column as I wanted to start basic and work towards a more complex design. As you can see, the reflux ratios are considerably large as are the tray numbers which in turn makes the column heights unreasonable. The purities have not been reached, due to the approximate methods used at this point but are getting there.
My next steps are to make the LP and crude column a packed column, introduce the reflux stream into the LP Column and begin a Rate-Based Aspen model. The scale at which this process can be improved is huge and the purpose of my project is to design a viable plant, to a third-year student level.
My questions:
1) Which impurities should the design of the molecular sieves focus on and how do you account for competitive adsorption when calculating the adsorbent required?
2) What is the purpose of this recycle stream from the Crude Column into the LP Column? (better conversion?)
3) How else can I reduce the reflux ratios for the HP, LP, Crude and Pure Argon Column? What should I be looking at to vary? What is a sensible reflux ratio for cryogenic distillation columns (I know standard is under 2, is this applicable here as well)?
4) What shall I be looking at to improve the purities of the products, towards their pure grade equivalent?
5) I know the crude argon column and LP column should use packing, what type of packing is typically used, and is there any sources for the design procedure associated with these?
6) For the HP and Pure column, what are the typical tray types being used? single pass trays, double pass trays, sieve trays etc.
7) I read that there is a way to improve argon purity by running the crude argon through a catalytic combustion reactor and reacting it with hydrogen to remove the oxygen but is the flow rate that this process operating at meet the criteria? How is the packed crude argon column capable of avoiding this?
8) I might attempt this, although seems a little complicated but how do you go about combining the HP and LP column?
Thank you to anyone for your time and the answer to any of these questions would be of great help. Any tips on how to build on the basic design would be greatly appreciated!