Have you looked at a molecular sieve solution? Some of the new materials are custom designable for higher specificity. NASA has worked in this area on a small scale that may be upgradeable.
Years past, I used to surprise friends by doing a curve fit on a Hewlett-Packard calculator, a Vax VMS library function, and a Lotus 123 spreadsheet and get three obnoxiously different answers. ( The Hewlett-Packard was the most correct). The Mathworks Matlab folks, the Numerical Recipies in C...
Are these huge tanks,or a collection of small tanks? Building a small shed with insulation drives refridgeration cost down quickly. Are these tanks mounted on a truck
for delivery of Co2? Each case is a new optimization problem. Are high flow rates involved? This changes thermal exchange...