Transcript Radioactive Noble Gases in BOREXINO Techniques for

Adsorption
techniques for gas
purification
Hardy Simgen
Max-Planck-Institute for Nuclear Physics
Heidelberg / GERMANY
Outline
 Principles
of adsorption
– Column purification
 Adsorption
on activated carbon
– Single component adsorption
– Multi component adsorption
 Individual
systems
– N2/Rn, Ar/Rn, He/Ar
 The
N2/Kr-system
 Summary
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H. Simgen, MPI for Nuclear Physics / Heidelberg
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Radioactive noble gases
in the atmosphere
Source
42Ar
cosmogenic
39Ar
85Kr
cosmogenic
235U
222Rn
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fission (nuclear fuel
reprocessing plants)
Primordial
238U
Activity concentration
(STP)
0.5 µBq/m3 air
50 µBq/m3 Ar
17 mBq/m3 air
1.8 Bq/m3 Ar
1.4 Bq/m3 air
1.2 MBq/m3 Kr
10 to >100 Bq/m3 air
H. Simgen, MPI for Nuclear Physics / Heidelberg
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Adsorption in pores
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H. Simgen, MPI for Nuclear Physics / Heidelberg
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Henrys Law
valid for low partial pressures
n = H •p
n
= number of moles adsorbed [mol/kg]
 p = partial pressure of adsorptive [Pa]
 H = Henry coefficient [mol/(kg·Pa)]
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H. Simgen, MPI for Nuclear Physics / Heidelberg
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Column purification
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Column purification
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Column purification
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Column purification
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Column purification
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Column purification
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quantitatively …

cV   c0    r
rN
NV
VRe t
r
x
with   e 
r!
x
r
x
for large N (many stages):


1
cV   c 0  1 
 e
t

2


y2

2

dy  with


 V

t  N  
 1
 VRe t 
VRet = HRTmAds
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H. Simgen, MPI for Nuclear Physics / Heidelberg
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Pore size distribution
Activated Carbon
pore size [Å]
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H. Simgen, MPI for Nuclear Physics / Heidelberg
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Single component adsorption model
S. Maurer, Ph.D. thesis, TU Munich (2000)
 Prediction
of Henrys constant for
adsorption on activated carbon


 mol 
81  TC K 


H
 exp  0.05 
 17.5
 

T K   PC bar 


 kg  Pa 


 Only
one gas parameter: TC•pC-0.5
 allows to compare adsorption of
different components
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Maurer formula for H
for single component adsorption in gas phase !
Gas
TC
PC
TC·PC-0.5
H [mol/(kg·Pa)]
[K] [bar] [K·bar-0.5] T=-100°C T=-196°C
He
5.2
2.3
3.4
1E-7
8E-7
Ar
151
49
21.6
2E-4
6E+1
N2
CH4
Kr
Xe
Rn
126
191
209
290
377
34
46
55
59
63
21.6
28.0
28.2
37.8
47.6
2E-4
3E-3
3E-3
2E-1
1E+1
6E+1
4E+4
4E+4
7E+8
1E+13
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Realization in low-level physics
 behaviour
at ultralow concentrations?
 problem of re-contamination
 practical limitations:
– column density (troughput)
– temperature stability
–…
 at
least 2 components
– multi-component adsorption
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N2 / Rn - system
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N2 / Rn - system
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MoREx
(Mobile Radon Extraction Unit)
-196
-100
-196
-100
-196
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MoREx
(Mobile Radon Extraction Unit)
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HD II
proportional
counter
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Noble gas
purification
line
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N2 purification plant
222Rn
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<0.3 μBq/m3 N2
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222Rn
in argon
Ar
222Rn -100
3 Ar
-186
-186
-100
<0.3 μBq/m
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-186
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He / Ar - system
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He / Ar - system
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Homestake experiment
Raymond Davis jr.
Nobel
prize
2002
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N2 / Kr - system
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N2 / Kr - system
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Noble gas mass spectrometer
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Experimental setup
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Kr breakthrough (LN2 cooling)
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Gas phase adsorption
 Problem
of N2 condensation at 77 K
 If pure gas phase, there is higher
– mobility of gases
– surface diffusion
→ Particles reach appropriate pores faster
 But
low temperature is required
 Liquid argon cooling!
 alternatively: pressurized liquid nitrogen
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Kr breakthrough (LAr cooling)
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Adsorption potential
Atoms of adsorber
Atom of adsorbat
z
d
d
d
Phi(z)
g=3
g=2
g=1
g=0
Layers of adsorber atoms
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Potential for Kr adsorption on
activated carbon
0.1
5,9
Potential of adsorption Phi(z) [eV]
0.05
0
single wall
15
6,5
-0.05
-0.1
10
-0.15
8
-0.2
7,5
-0.25
7
-0.3
0
1
2
3
4
5
6
7
8
9
10
Distance z between center of Krypton atom and carbon wall [Angstroem]
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H and pore sizes
1e+09
1e+08
Henry coeffizient H [ mol/(kg*Pa) ]
1e+07
1e+06
Krypton
100000
10000
1000
100
10
1
Nitrogen
0.1
0.01
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Pore size b [ Angstroem ]
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Pore size distribution
?
Carbon molecular
sieve
Activated Carbon
pore size [Å]
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Kr breakthrough (LAr cooling)
Carbon
molecular
sieve
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Three other systems
Xe
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N2 / Ar
Impossible
Ar / Kr
Very
challenging !
Rn
Should work
 Xmass
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Summary
 Adsorption
is powerful tool to produce
ultrapure gases.
– simple operation / cheap
 Activated
carbon is well suited for many
applications.
 Rn purification is very efficient, but
problem of re-contamination.
 Kr removal from N2/Ar can be done with
– optimized adsorbers
– gas phase adsorption
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and
H. Simgen, MPI for Nuclear Physics / Heidelberg
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