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Purification of nitrogen (argon)

Purification of nitrogen (argon). Grzegorz Zuzel MPI-K Heidelberg. Outline. Motivation of this research Radioactive noble gases in the atmosphere Methods of analysis Purification of N 2 /Ar Measurements of 222 Rn content in argon gas Conclusions and planned activity. Motivation.

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Purification of nitrogen (argon)

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  1. Purification of nitrogen (argon) Grzegorz Zuzel MPI-K Heidelberg IDEA Meeting, LAL-Orsay, 14-15 April 2005

  2. Outline • Motivation of this research • Radioactive noble gases in the atmosphere • Methods of analysis • Purification of N2/Ar • Measurements of 222Rn content in argon gas • Conclusions and planned activity IDEA Meeting, LAL-Orsay

  3. Motivation • Ultra-pure LN2/LAr will be used in the GERDA experiment - Cooling medium for Ge-crystals - Passive shield against external radiation - Active shield (LAr) • Developed techniques could be applied in other low-level projects IDEA Meeting, LAL-Orsay

  4. Outline • Motivation of the research • Radioactive noble gases in the atmosphere • Methods of analysis • Purification of N2/Ar • Measurements of 222Rn content in nitrogen and argon gas • Conclusions and planned activity IDEA Meeting, LAL-Orsay

  5. Radioactive noble gases in the atmosphere IDEA Meeting, LAL-Orsay

  6. Outline • Motivation of the research • Radioactive noble gases in the atmosphere • Methods of analysis • Purification of N2/Ar • Measurements of 222Rn content in nitrogen and argon gas • Conclusions and planned activity IDEA Meeting, LAL-Orsay

  7. Low-level proportional counters IDEA Meeting, LAL-Orsay

  8. Counter filling line • Sample purification • Mixing with counting • gas (P10) • Counter filling IDEA Meeting, LAL-Orsay

  9. Sensitivities • 222Rn: -only α-decays detected - 50 keV threshold (bkg: 0.2 – 2 cpd) - total detection efficiency ~1.5  abs. detection limit ~30 µBq (15 atoms) • 39Ar and 85Kr: - β-decays detected - 0.6 keV threshold (bkg: 1 – 5 cpd) - total det. efficiency ~0.5  abs. det. limit ~100 µBq (5x10485Kr atoms) IDEA Meeting, LAL-Orsay

  10. Measurements of 222Rn in gases – MoREx (Mobile Radon Extraction Unit) 222Rn detection limit: ~0.3 μBq/m3 IDEA Meeting, LAL-Orsay

  11. Ar and Kr: mass spectrometry Ar: 10-9 cm3 (1 ppb; ~1.4 nBq/m3 for 39Ar in N2) Kr: 10-13 cm3 (0.1 ppt; ~0.1 µBq/m3 for 85Kr in N2) IDEA Meeting, LAL-Orsay

  12. Outline • Motivation of the research • Radioactive noble gases in the atmosphere • Methods of analysis • Purification of N2/Ar • Measurements of 222Rn content in nitrogen and argon gas • Conclusions and planned activity IDEA Meeting, LAL-Orsay

  13. Different possibilities • Distillation - high costs and energy consumption • Sparging (e.g. with He) - boiling point for contaminants must be lower than for the gas to be purified - high purity carier gas needed • Adsorption - successfully used for 222Rn removal from nitrogen - a lot of experience at MPI-K IDEA Meeting, LAL-Orsay

  14. Henrys law • n = number of moles adsorbed [mol/kg] • p = partial pressure of adsorptive [Pa] • H = Henry constant [mol/(kg·Pa)] • H determines the retention volume: n = H  p VRet = HRTmAds IDEA Meeting, LAL-Orsay

  15. Purification in the column n = Hp IDEA Meeting, LAL-Orsay

  16. Adsorption models IDEA Meeting, LAL-Orsay

  17. Purification of N2/LN2 from 222Rn • Strong binding to almost all adsorbers • Easy trapping with activated carbon @ 77 K • Problem: 222Rn emanation due to 226Ra - Requires careful material selection - Activated carbon „CarboAct“: 222Rn emanation rate (0.3  0.1) mBq/kg, ~100 times lower than other carbons IDEA Meeting, LAL-Orsay

  18. Purification of N2/LN2 from Kr • Advanced models needed to describe adsorption of multi-component system N2/Kr – e.g. pore size of the adsorber and its internal polarity has to be taken into account • Henry coefficient is expected to be higher for pure gas phase adsorption (@ T > 77 (87) K for N2 (Ar)) - Cooling: LAr (for N2), pressurized liquid gases (for LAr) or gas phase cooling • Pores, low polarity and adsorption from gas phase should lead to H ~1 mol/kg/Pa IDEA Meeting, LAL-Orsay

  19. Henry constant and pore size IDEA Meeting, LAL-Orsay

  20. Considered adsorbers • Molecular sieves, polar (Si/Al ~ 1) 5/10 Å • Hydrophobic zeolite MFI-type: low internal polarity (Si/Al ~ 75), pores ~6.6 Å • Hydrophobic zeolite BEA-type: Si/Al ~ 200, pores ~5.3 Å • “Carbo Act” F3/F4: low 222Rn emanation rate, wide pore size distribution • Charcoal Cloth FM 1-250, fabric • Activated Carbon C38/2, optimized for solvent recovery IDEA Meeting, LAL-Orsay

  21. Experimental setup IDEA Meeting, LAL-Orsay

  22. Results Synthetic carbon, CarboAct ZEOCAT, 5.3Å, Si/Al  200 IDEA Meeting, LAL-Orsay

  23. Results IDEA Meeting, LAL-Orsay

  24. Purification of N2 – Summary • 222Rn removal rather easy, even from LN2 • Low 222Rn emanation rate of the adsorber required • Ar removal impossible (no Ar reduction observed) • Kr removal requires: • Low temperature gas phase adsorption • Activated carbons seem to be more suitable than zeolits • Pore size-tuning required IDEA Meeting, LAL-Orsay

  25. Purification of Ar • (Almost) no difference between Ar and N2 for adsorption on activated carbon • However higher temperatures have to be considered (~100 K for pure gas phase adsorption) • 222Rn removal is not a problem IDEA Meeting, LAL-Orsay

  26. Outline • Motivation of the research • Production of N2 and Ar • Radioactive noble gases in the atmosphere • Methods of analysis • Purification of N2/Ar • Measurements of 222Rn content in nitrogen and argon gas • Conclusions and planned activity IDEA Meeting, LAL-Orsay

  27. 222Rn in nitrogen and argon LN2 class 4.0, usually ~ 50 m3 (STP) of gas taken CRn ~ 50 µBq/m3 Two measurements performed for Ar, class 4.6 IDEA Meeting, LAL-Orsay

  28. Outline • Motivation of the research • Production of N2 and Ar • Radioactive noble gases in the atmosphere • Methods of analysis • Purification of N2/Ar • Measurements of 222Rn content in nitrogen and argon gas • Conclusions and planned activity IDEA Meeting, LAL-Orsay

  29. Conclusions and planned activity • Nitrogen purification intensively studied - Adsorbers selection based on the adsorption theory - Experimental tests are ongoing (zeolites already tested) - For Kr removal from nitrogen better suitable seem to be activated carbons - Adsorption from the pure gas phase has to be studied • First results of purity and purification tests for Ar available - 222Rn removal from Ar possible - 222Rn concentration in argon seems to be significantly higher than in nitrogen of similar quality • Purity tests for different supply chains are planned (N2/Ar) • Although the program was slightly extended it is progressing as scheduled IDEA Meeting, LAL-Orsay

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