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# Air separation

## Air separation

(OP)
Given that it takes 10 distillation trays to purify crude (or 40% purity)
oxygen into 95% oxygen at an air separation unit serving a chemical
complex.  The customer (owner of chemical complex), however, requires
99.9999% purity oxygen to feed a new oxidizer reactor in a pilot R&D
facility.

A new purification column is considered for purifying a side-stream of the
crude oxygen, to meet the oxygen purity demand in pilot facility. How many
trays should be designed into the new purification column ?  Assume that
the mass transfer efficiency per tray is constant (i.e. does not change
with composition and there is no pinch) and that tray gas-liquid flows will
be perfect.

### RE: Air separation

Based upon the information you have given, if you assume that the tray efficiency is unchanged, the concentration of the impurities is multiplied by a factor of 0.78 across each tray.  At high purity levels, I have no idea if the assumption is valid.  If it is, you would need a total of 54 trays, or a second column with 44 trays in addition to your existing 10 tray column.

The math for the impurities is:

(0.6)*(0.78)^54 = 8.94*10^-7

Which gives a purity of:

1-8.94*10^-7 = 0.9999991

### RE: Air separation

I want to ask about material of tray and thermosyphon type heat exchanger in bottom. 99.999% oxygen is likely to combust with aluminium metal. NASA has experienced that and lost lots of space pilots by implosion with oxygen and cloths made of aluminium.
In addition 99.999% oxygen means 1 ppm impurity. In air 50-100 ppm of hydrocarbons is normal. So it means that hydrocarbons accumulates in liquids in bottom when thermosyphon heat exchanger evaporates gas from liquids.
I am not sure of your process flow diagram. But you must change a lot and estimate cost & persuade customers.

Regards ;

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