PROCEEDINGS of the FIFTH INTERNATIONAL CONFERENCE "CRYOGENICS '98", Praha,
Czech Republic, 1998.
AP4
THE STUDY OF GASES CRYOADSORPTION ON THE ACTIVATED CARBON
SCN-2K FOR HYDROGEN ISOTOPES HIGH PURIFICATION
Ivan A. Alekseev, Sergey D. Bondarenko, Veniamin D. Trenin
Petersburg Nuclear Physics Institute
188350 Gatchina Leningrad district Russia
ABSTRACT
Nitrogen, argon, neon, helium, protium and deuterium adsorption isotherms were measured at
the temperature of 78 K on the synthetic activated carbon type of SCN-2K. Separation factor of H2-HD,
H2-D2, and H2-HT molecules on SCN-2K at 78 K and pressure of 100 Pa was determined. Activated
carbon SCN-2K has more high adsorption capacity for N 2 and Ar in comparison to other adsorbents
(SKT-4, PAU). At the same time its capacity to hydrogen isotopes is substantially less that makes its
more advantageous for the purification of slightly adsorbable gases from N 2 and O2 in comparison with
commonly used charcoal type of SKT-4.
Dynamic characteristics of H2 (D2) cleaning from nitrogen impurity was studied by method of
the breakthrough curve under various conditions. Dynamic characteristics of SCN-2K charcoal are much
better than those of these class adsorbents and better than those are of silica gels and zeolites, the other
conditions being equal.
Due to their good physical properties and their high selectivity towards N 2 and O2 SCN-2K is
potential candidate for adsorbent for cryoadsorption purification of hydrogen isotopes.
I. INTRODUCTION
A cryoadsorption method using a porous adsorbent at the temperature of liquid
nitrogen is one of the most suitable processes for hydrogen purification from traces of
oxygen and nitrogen. The adsorbent for the high purification of hydrogen isotopes
(deuterium and tritium) should have the following properties, such as high durability
and an abrasion resistance, considerable adsorption capacity for oxygen and nitrogen
and low adsorption capacity for hydrogen isotopes. Well-known commercial adsorbents
do not quite match these requirements.
Activated carbon type SCN (spherical carbonite) synthesized for medical
application is attractive because of its high mechanical durability and greatly developed
porosity. This adsorbent is manufactured by carbonization of different nitrogenated
polymer resins followed by activation in flow of carbonic acid gas or water steam.
Activated carbon SCN has been using in medicine as hemosorbent and enterosorbent1.
It has been found an application for adsorption of volative hydrides and organic
compounds in radio-electronic technology2, 3.
In this paper we report result of the studies, which have been carried out to
analyze of an activated carbon type SCN-2K possible application for high purification
of hydrogen isotopes.
II. EXPERIMENTAL
The investigated adsorbent was activated carbon type SCN-2K in the form of
granule 0.5-1.0 mm in diameter and bulk density about 315 g/l. The samples were
washed in distilled water and dried.
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The equilibrium adsorption isotherms for cryogenic gases on SCN-2K were
measured volumetrically at temperature of 78 K in the pressure range from 0.01 Pa to 2
MPa. Prior to each set of measurement, the adsorbent was reactivated for 3 hours at the
temperature of 160 – 180 C and at the pressure of 2 Pa. The adsorption cell was cooled
by immersing in liquid nitrogen. Pressure measurements were made using a
thermocouple gauge and diaphragm pressure gauge.
The separation factor of H2-HD, H2-D2, and H2-HT molecules on SCN-2K
activated carbon at 78 K and pressure of 0.1 MPa was determined with equilibration of
hydrogen isotope mixtures. The mixture was circulated over cooled adsorbent with
membrane pump. The hydrogen isotope composition of the gaseous phase and the
adsorbent phase was determined using gas chromatograph and ionization chamber.
Then the separation factor was calculated.
Dynamic characteristics of SCN-2K for H2 (D2) cleaning from nitrogen impurity
was studied by method of the breakthrough curve. The main part of bench-scale
installation was stainless steel adsorber with inner diameter of 13 mm and height of 200
mm. The total weight of SCN-2K was about 8.7 g. Inlet and outlet concentrations of N2
and O2 in hydrogen were measured by gas chromatograph. The detail of the
experimental procedure is presented in Reference 4.
III. RESULTS AND DISCUSSION
Figure 1 and figure 2 shows the experimental result of the isotherm of gases
obtained in our work. Adsorption on microporous
solids can be described with the Dubinin –
Astakhov equation
a=a0 exp(- (A/E)n),
where a is the total adsorbed gas, a0 is the
limiting value of sorption, A=RTln(PS/P), Ps is
the saturation pressure of the gas at the
temperature T, P is the pressure, R is the ideal gas
constant, E is the characteristic energy of
adsorption, n is the rank of equation. Dubinin
isotherm parameters for gases on SCN-2K was
calculated and presented in Tab.1.
Fig.1 Adsorption isotherms of
Ar and N2 on SCN-2K at 78 K.
Table 1.Parameters of Dubinin– Astakhov equation for gases on SCN-2K
Gas
Ps, kPa
a0, cm3/g E, J/mol
n
Ar
301.39
84030
160090 1.3
N2
1013
72010
294070 1.3
Ne
*
76030
148010 1.4
D2
*
65020
267040 1.5
H2
*
44010
320030 2.1
He
*
58030
198010 2.2
* Ps, calculated as Ps,=Pc(T/Tc)2, where Tc and Pc are critical constant for adsorbed gas.
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We compared our data with the values obtained in work5 for others activated
carbons applied in Russia. SCN-2K has more high adsorption capacity for N2 and Ar in
comparison to other adsorbents (SKT-4,
PAU, BAU). At the same time its
capacity to hydrogen isotopes is
substantially less that makes its more
advantageous for the purification of
hydrogen and helium from N2 and O2 in
comparison with commonly used
activated carbons.
Selective adsorption on a porous
adsorbent can be used to hydrogen isotope
separation6-9. The degree of the separation
is indicated by the value of the separation
factor. The higher value of separation
factors the more degree of separation. The
separation factors of H2-HD, H2-D2, and
Fig.2 Adsorption isotherms of He, Ne, H2,
H2-HT molecules on SCN-2K measured
D2 on SKN-2K at 78 K.
at 78 K and pressure of 100 kPa and also
separation factors for hydrogen isotopes on other adsorbents are presented in Table 2.
Table 2. Separation factor for hydrogen isotopes at 78 K and pressure of 100 kPa
Adsorbent
H2 - HD HD – D2
H2 – HT
H2 - D2
D2 - DT
Activated carbon SCN- 1.130.08 1.100.06
1.240.02 1.240.06
2K
Charcoal Columbia G 1.230.0136
Charcoal Fisher
1.280.0136
coconut
Zeolite 13X
1.320.038 1.510.068 1.950.057 2.100.057
1.120.037
8
8
9
9
Zeolite 4A
1.430.02 1.700.05
1.840.06 2.550.06
1.290.039
Fig.3 The breakthrough curves at gas
velocity of 1.9, 4.0, 6.2 cm/sec.
Values determined by us for H2 - D2
mixture are nearly the same that reported by
Basmadjian6 for charcoals.
The results
obtained show little variation in the
separation factor measured on chemically
identical adsorbent of different origin. The
selectivity of SCN-2K relation to hydrogen
isotope is lower than one of zeolites.
Therefore zeolites is more appropriate for
hydrogen isotope separation.
Figure 3 shows the examples of the
breakthrough curves obtained in the process
of purification of hydrogen isotope mixtures
at the bench-scale installation packed SCN2K at pressure 0.8 MPa. For example inlet
concentrations of N2 and O2 were 0.026% and
210
0.0073%, outlet concentrations of N2 and O2 were 0.000039% and 0.0000059%. From
the breakthrough curves the height of main mass-exchange zone was calculated.
Dynamic characteristics of activated carbon SCN-2K are much better than those of
these class adsorbents are and better than those are of silica gels and zeolites, condition
being equal. Thus, the zone of main mass-exchange (L0) in the process of hydrogen
purification from nitrogen impurity under the next conditions: entry N2 concentration –
0.12% vol., gas flow rate – 1.9 cm/s, amount10 for charcoal type of SCT L0=27cm, for
zeolite type of NaX L0=13 cm, while for SCN-2K its value is 2.4 cm. Dynamic capacity
of adsobents for nitrogen at 78 K is presented in Table 3.
Table 3. Dynamic capacity of adsobents for nitrogen at 78 K.
Pressure, MPa
0.5
0.8
1.5
Inlet C0, vol.%
0.12
0.12
0.12
0.8
0.06
activated carbon SKT10
13X zeolite10
activated carbon SCN-2K
91.1
91.3
86.2
-
112.2
93.4
86.6
-
Due to their good physical properties and their high selectivity towards N2
activated carbon SCN-2K is potential candidate for adsorbent for cryoadsorption
purification of hydrogen. This adsorbent is recommend for cryoadsorption purification
of hydrogen isotopes at detritiation plant of the nuclear research PIK reactor. Activated
carbon SCN-2K is used successfully at the system of compression, purification and
storage of the hydrogen isotope mixtures of Petersburg Nuclear Physics Institute11.
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volative hydrides. Ibid. P.396. In Russian.
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SCN-2K and it’s application for hydrogen isotope high purification from microimpurities. Preprint
PNPI
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adsorption process. Leningrad. 1989. P. 46. In Russian.
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1998. This conference.
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