Can we enrich xenon by distillation?

1. Can we enrich xenon by distillation? David Sinclair, Xenon enrichment forum, 22 March 2018

2. Acknowledgements • My ideas on xenon distillation have been shaped by many helpful discussions Including: • Ethan Brown, Christian Weinheimer (XENON still) • Nakahata-san (XMASS Still) • Harry Kooijam (Prof of ChemEng at Potsdam and Shell Global Solutions, Amsterdam) and Ross Taylor, Kodak Distinguished Prof. at Potsdam – ChemSep • Henning Back • I have also been influenced strongly by the literature on this subject and do not include an extensive ref. list here.

3. Outline • Motivation and enrichment criteria • A few words about distillation • Critical parameters • Feasibility? • Existing data and Need for measurements • Proposed plan • Schedule

4. Motivation • Costs of xenon double beta decay experiments dominated by cost of isotope • Currently there is a single supplier. No evidence that their price is unreasonable but competition has a way of sharpening the pencils when time comes to bid. • nEXO wants 5T of isotope. As the inverted hierarchy is looking increasingly less likely we should be thinking about how to reach the meV mass scale. This probably needs 50T and Xe is probably the only game in town

5. How much enrichment? • The case for any enrichment is modest. With no enrichment the xenon detector would be scaled up by a factor of 2. This increases electron attachment but also self-shielding. Big driver seems to be the economic win. The re-sale value of the light isotopes looks ~ cost of separation • Physics does not benefit from very high enrichment. In SNO we had 99.92% purity because protons would kill the NC signal. In nEXO I cannot see much difference between 80% and 90% from the perspective of physics

6. Distillation and isotope enrichment • Distillation is a standard technique for enrichment of light systems • EG SNO’s heavy water was prepared by distillation to 99.92% • It works because the vapour pressure of light water is slightly higher than that of heavy water, at a given temperature. • The vapour pressure differences get smaller with increasing mass so technique is not normally used for heavy nuclei

7. Distillation 101 • In a classical still, a fluid is boiled at the bottom of a column and condensed at the top. The condensed fluid fall down over a series of trays while the boiled vapour rises. There is a driving force that causes the higher vapour pressure components to diffuse into the vapour phase while the lower vapour phase components move to the liquid phase • Fenske’s Formula (binary system) • Separation factor • a is the ratio of vapour pressures •  is the number of trays

8. Packed beds • For packed beds the trays are replaced by a packing (either a regular structured material or a randomly oriented ‘dumped’ packing. The objective is to have a very high surface area for liquid-vapour interaction while keeping an open structure with minimal pressure drops • In either case one can consider the height over which the concentrations change by the same amount as a single tray in a conventional still. This is called the Height of Equivalent Theoretical Plate (hetp). • If we know the hetp for our packing and the height of the still, and we measure the asymptotic separation in a still the we can deduce the vapour pressure differences. Formulae are a little more complex for multicomponent fluids

9. Will distillation of xenon be feasible? • We do not know because the vapour pressures are not known • We believe it is possible to separate 39Ar from 40Ar with a still 200m high with a production capacity of 150 kg/week • The theoretical vapour pressure differences for Ar are 22 times larger than for Xe but the mass difference for Xe (ie 136 – residual) is 5 times larger. We can have a still 4 times higher. Production requirements are much lower (driven by feed stock availability). Separation factor sought in Ar is factor of 10. This would be nice but a factor of 5 would work fine for xenon. • We can assume the separation of UF6 by distillation is not competitive • It is critical to measure the actual distillation parameters to address this quantitatively

10. What do we know? • There have been two reports on measurement of vapour pressure differences in xenon by distillation • Clusisus found the vapour pressure difference to be too small to measure • Grigor’ev found a value of 1 x 10-4 for dp/p • Neither of these are considered credible (see Tew or Henning’s paper) • Theoretical value from multiple estimates is 3 x 10-4 at triple point Falls as 1/T2 • Clearly a credible experimental result is required!

11. Proposed plan • Make a tall still – operate at 100% reflux – measure separation factors as function of time • Do this at various gas/liquid flows and at various pressures • Start with argon to determine the hetp for a noble fluid, and the still parameters • Then see if we can predict the properties of Kr • If this works go to Xe. • Start with 6’ column then go to 36’

18. Mass analysis by Quadrupole mass spec Extrel 19mm bore Mass resolution 1/3000

20. Simulations • Initial simulations carried out using PROSIM • Commercial program with semi-infinite capability for chemical processes • Not easy to determine exactly what it is doing

21. Simulated performance of 36’ still - % change at top Isotope Initial 1 hr 2 hr 3 hr 300 hr 129 0.326 1.44 1.53 1.66 2.76 131 0.212 0.00 0.24 0.24 0.00 132 0.269 ‐0.37 ‐0.41 ‐0.97 ‐0.74 134 0.104 ‐0.96 ‐1.73 ‐3.75 ‐2.88 136 0.089 ‐2.25 ‐2.99 ‐3.18 ‐6.56 Asymptotic values give the change in vapour pressure times number of equivalent plates Time constant tells us about the driving force largely dependent on diffusion coefficients and still properties Note the mass 129 abundance actually includes 128, 126 and 124

22. Simulation with Chemsep • Currently working with ChemSep • Made freely available by principals • Very good local support • No need to input hetp – works this out from still properties • First results agreed with Prosim but this was a simplified problem • Work ongoing

23. Plans • Almost all of the still parts are at Carleton • Expect to be in operation within a couple of months • Grad students finish their courses in April so effort available • Expect results by fall • Then move to SNOLAB and install 36’ version • This needs only the local column support • Expect ~ 10% precision with single column, perhaps close to 1% in 36’ column for vapour pressure differences