Purification of N2 and ArGas purification by adsorption N 2 purification Ar purification Techniques...

29
1 Purification of N 2 and Ar Hardy Simgen, MPIK Heidelberg

Transcript of Purification of N2 and ArGas purification by adsorption N 2 purification Ar purification Techniques...

Page 1: Purification of N2 and ArGas purification by adsorption N 2 purification Ar purification Techniques for analysis Summary Hardy Simgen, ββ-meeting, 26. + 27.1.04 Page 3 Production

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Purification of N2 and Ar

Hardy Simgen, MPIK Heidelberg

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Outline

RequirementsGas purification by adsorptionN2 purificationAr purificationTechniques for analysisSummary

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Production

N2, Ar are produced fromair by rectification

Traces of atmosphericnoble gases remain in final product

Final purity depends on individual plant

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Radioactive noble gas nuclides in the air

0.5 µBq/m3 air50 µBq/m3 Ar

cosmogenic42Ar

10 to >100 Bq/m3 airPrimordial 238U222Rn

1.4 Bq/m3 air1.2 MBq/m3 Kr

235U fission (nuclear fuelreprocessing plants)

85Kr

17 mBq/m3 air1.8 Bq/m3 Arcosmogenic39Ar

Activity concentration (STP)Sources

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Requirements222Rn:

MC simulations [Baudis 2002]:0.5 µBq/m3 N2 (STP) = 6⋅10-5 events/(kg⋅y⋅keV)

BOREXINO has achieved this goal

42Ar:MC simulations [Schönert]:50 µBq/m3 N2 (STP) = 4⋅10-5 events/(kg⋅y⋅keV)

42Ar naturally low enough

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Requirements

Q-value of 39Ar and 85Kr below 700 keV:

But dead-time problem when Ar scintillation isused (slow decay time: 1 µs)

Assume 30 m3 active volume39Ar rate: 50 kHz OK!85Kr rate not higher ⇒ ≤1 ppm krypton required

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Gas purification byadsorption

Applied when highest purities are required

Based on different binding energies

Pore size distribution is crucial

Activated carbon suited for most applications

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Henrys Law

n = H · p

n = number of moles adsorbed [mol/kg]p = partial pressure of Kr [Pa]H = Henry coefficient [mol/(kg·Pa)]

valid for low partial pressures

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Single componentadsorption model for H

S. Maurer, Ph.D. thesis, TU München (2000)Adsorption on activated carbonbased on van-der-Waals equation

simple empirical formula derived:

[ ][ ][ ]

−⋅

+−=

5.17barK

K8105.0exp

Pakgmol

C

C

PT

TH

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Single componentadsorption model for H

1E+1447.663377Rn

2E+528.255209Kr

2E+221.634126N2

2E+221.649151Ar

H [mol/(kg·Pa)]@ 77 Kelvin

TC·PC-0.5

[K·bar-0.5]PC

[bar]TC

[K]Gas

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N2 purification

Lots of know-how at MPI-K (BOREXINO)

Removal of Ar not possible by adsorption

Rn purification possible but problem with222Rn emanation

Highly pure synthetic carbon „Carbo Act“222Rn emanation <0.3 mBq/kg

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N2 purification

222Rn <0.3 µBq/m3 N2

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Purification of N2 from Kr

Single component adsorption model fails forbinary system N2/Kr

„Competition“ between N2 and Kr foravailable adsorption sites

Henry coefficient for Kr adsorption drops from~102 to ~10-4 when N2 is present

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H and pore sizes

0.01

0.1

1

10

100

1000

10000

100000

1e+06

1e+07

1e+08

1e+09

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Hen

ry c

oeffi

zien

t H [

mol

/(kg*

Pa) ]

Pore size b [ Angstroem ]

Nitrogen

Krypton

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Pore size distributions

Activated CarbonSkeleton

pore size [Å]

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Purification of N2 from Kr

Further improvment by gas phase adsorption at low temperatures

Pores and gas phase lead to H ~1 mol/kg/Pa

More than 500 m3 of N2 (STP) can be purifiedwith ~1 kg of adsorber (V=H⋅R⋅T⋅m)

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Purification of N2- Summary -

Rn removal no problem

Ar removal impossible

Kr removal technically feasible but requireslow temperature gas phase adsorptionPore size-tuned carbon adsorbers

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Single componentadsorption model for H

1E+1447.663377Rn

2E+528.255209Kr

2E+221.634126N2

2E+221.649151Ar

H [mol/(kg·Pa)]@ 77 Kelvin

TC·PC-0.5

[K·bar-0.5]PC

[bar]TC

[K]Gas

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Purification of Ar

(Almost) no difference between Ar and N2for adsorption on activated carbon

However T(LAr) = T(LN2) + 10K

Rn removal no problem

Kr removal for Ar even more challengingthan for N2 (especially for large amounts)

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Techniques for analysis- Nitrogen -

Rn, Ar, Kr analysis in N2 performed at MPI-K within BOREXINO

Rn ↔ low background proportional countersDetection limit: 30 µBq

Ar, Kr ↔ noble gas mass spectrometerDetection limits: 1 ppb for Ar

0.1 ppt for Kr

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Noble gas purification line

getter-pump

connections for differentgases (Kr, Ar, Xe, N , CH , ...)2 4

main line

highly pureold xenon

Toepler pump mercurymanometerto main pump

flow meter

carbon trap forhelium-purification

sample container(dismountable)

NaOH-trapagainst CO2

silicageltrap

smallcarbon

trapbig

carbontrap

chromosorbtrap

gasexit

exchangeablechromatography column

helium

port forproportionalcounter

topump

gasexit

thermistor

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Noble gas purification

line

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Mobile RadonExtraction

Unit (MoREx)

222Rn <0.3 µBq/m3 N2

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Mass spectrometer

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Techniques for analysis- Argon -

Rn analysis in the same way as for N2Same sensitivity can be achieved

Kr analysis difficult (contamination of massspectrometer with Ar)

Requires separation of Ar, Kr in advance (gas chromatography)Still some work to do…

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Which N2 is on the market?

0.06

0.04

0.3

40

Kr [ppt]

?

?

?

~50

222Rn[µBq/m3]

10Air Liquide (4.0)

0.0005Westfalen AG (6.0)

0.005SOL (6.0)

0.06Linde (7.0)

Ar [ppm]Company

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Which Ar is on the market?

Ar 6.0 means ∑(Contaminants) < 1 ppmBut noble gases are not includedN2, Ar produced in a similar way (rectification)

Reasonable to assume that Kr < 1ppm in Ar can be found on the market

Has to be proven ⇒ Technique for analysis

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Summary222Rn requirements similar to BOREXINOKr requirement in Ar ~1ppm

Adsorption techniques good for highest purities

Rn in N2 well under controlpurification as well as analysis

Will also work for Rn in Ar

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Summary

Purification of Ar from Kr not trivialRecently shown at MPI-K:

Purification of N2 from Kr by adsorption is possibleRequires gas phase and special adsorbersAr purification in the same way, but higher T (10 K)

Easiest approach: Buy pure enough N2, ArTo do: Development of technique for

measuring 1 ppm Kr in Ar