Fun games with membranes and cold traps. I will assume: The total volume is 4000 l. The circulation speed X l/h = Y l/h = 200 l/h pump speed. The volume is divided into N steps where N=V/X=20 with no diffusion between the steps. The total volume which is rejected at the exhaust is replaced by pure C 4 F 10. The total volume,  V i, which is pumped out at stage i for one step, is transferred to stage i+1 as one volume and then treated at this membrane in 100 mini steps assuming that the partial pressures do not change significantly during a mini step. (This statement is perhaps overly optimistic, but I checked that there is no real difference between 100 and 500.) 1 (I stopped at n=4)

Neomechs CO l/h/mbarp1-p26000 mbar Ar l/h/mbarp3-p41000 mbar O l/h/mbarp5-p6300 mbar N l/h/mbarp7-p8120 mbar CF l/h/mbar C 4 F l/h/mbar 5% air in the C 4 F 10 volume. Numbers from an old logbook 2

Small losses of C 4 F 10 and reasonable purity. (1 % N 2 ) but not fast. 3

Switch off Stage I and use p3-p46000mbar p5-p61000mbar p7-p8200mbar Still tolerable losses for the same purity. 4

Switch off Stage I and II and use p5-p66000mbar p7-p8500mbar Not too surprisingly, the losses are now rather high. 5

CO 2 is easy. For a 5% admixture use p1-p22000mbar p3-p41000mbar p5-p6500mbar p7-p850mbar 6

What about a steady state air leak? Try with 0.5 l/h. That is 0.5 l/h C 4 F 10 []  and 0.5 l/h air []  use p1-p26000mbar p3-p41000mbar p5-p6500mbar p7-p8120mbar and start the membranes when there is 2% air in the volume. Not perhaps the most optimal device for such a state. Still, it will stabilise around 98.5%. The rest being N 2. 7

The fiddling with the pressures is very depending on the actual (measured) throughput of the membranes. The pressures should probably also be adjusted as a function of the admixture in the gas. Look for another type in my old logbook: Generon CO l/h/mbar Ar O N CF C 4 F and use p1-p22000mbar p3-p4500mbar p5-p6100mbar p7-p820mbar Start out with 5 % air 8

9 Will now have a look at gas scrubbing at low temperature. Have used: Tables of Physical and Chemical Constants Physical Properties and gas Solubilities, Journal of Chemical and Engineering Data Vol. 18 No Solubility of gases in fluorocarbons, 3M pub M.F. Costa Gomes et al., Journal of Fluorine Chemistry 125(2004)1325 Ostwald solubility coefficient. Volume of gas dissolved in unit volume at ambient temperature and pressure. In anesthetic practice, these are quoted in tables, assuming a body temperature of 37°C. Note that the volume of gas dissolved is only dependent on temperature, and not pressure (though the number of molecules and the activity of these is pressure-dependent). This differs from Bunsen's solubility coefficient (a) in that the amount of dissolved gas is expressed in terms of its volume at the temperature of the experiment, instead of STPD. Friedrich Wilhelm Ostwald Nobel Prize in Chemistry in 1909 The mole Kingdom:Animalia Phylum:Chordata Class:Mammalia Infraclass:Eutheria Order:Soricomorpha Family:Talpidae Which has taken me through a tour of mole units and Ostwald's coefficient. Just to get (cc gas) in (cc liquid) at a given temperature and partial pressure. It does indeed require some rather large extrapolations (and faith).

10 Water is well known. Compare 3M numbers at one temperature with measurements at many different temperatures. Looks consistent.

11 Use the same set-up as for the membranes. That is Total volume: 4000 l Circulation: 200 l/h → 20 steps No diffusion and no compressibility. Total volume of the scrubber: 2 l 3 bar < scrubber pressure <3.5 bar Pressure stabilizing gas : He at 5 l/h Each step split in 500 mini steps. Checked that there is no real difference between 500 and 1000 steps (apart from time used on my PC).

12 Time (h)C 4 F 10 N 2 O 2 He Composition (%) Start with 5 % air and the cold trap at - 60 degC

13 N 2 will also work fine, but will end up with some 2.5 %. This is less than what I would have expected from some measurements at COMPASS and could indicate that the solubility constants that I am using are too low by about a factor of two. C 4 F 10 N 2 O

14 C 4 F 10 N 2 O 2 He C 4 F 10 N 2 O 2 He No real change in purity, but a rather dramatic increase in losses. CO 2 will not work in a cold trap.

15 Some sort of a Conclusion. Gas scrubbing, cold trap at (at least) -60 degC, is the best choice if the aim of the game is to get air content in the range of ppm. The triple membrane contraption will give oxygen content of about 0.1 % nitrogen of <1 % without any excessive loss of C 4 F 10 A steady state leak, would be better handled with a cold trap. The principle of Gas Scrubbing can easily be understood from one clear sketch found in freepatents.com