Dave,
The only thing that I can think of is that the boiling points of oxygen and argon are very close together.
The composition profile in the low-pressure oxygen column is interesting. In this column, waste nitrogen comes off the top and your product oxygen comes off the bottom. Argon...
MikalT,
Piping for cyrogenic liquids is usually vacuum-jacketed piping. A manufacturer of VJ piping, which the company I work for has used in the past, is "Quality Cryogenics."
Quality Cryogneics
425 Gennet Drive
Jasper, GA 30143
(706) 692-6167
http://www.vjpipe.com