CARBON MOLECULAR SIEVE MEMBRANES:
CHARACTERISATION AND APPLICATION TO XENON
RECYCLING
S. LAGORSSE PONTES, F. D. MAGALHÃES, A. MENDES
LEPAE, Departamento de Engenharia Química
Faculdade de Engenharia da Universidade do Porto
1. Introduction
Carbon Molecular Sieve Membranes (CMS membranes) are a very recent and
promising material. This paper deals with the characterisation of the
performance of these membranes, in terms of sorption equilibrium and
permeation properties, for Xe, He, CO2, O2 and N2. CMS membranes tested
were supplied by Carbon Membranes Ltd. (Israel).
The motivation for this work has to do with studying the suitability of
these membranes for Xenon recycling within an anaesthetic closed loop. Xenon
is an extremely expensive gas (about €10 to €13 per normal litre) that is known
as a suitable inhalation anaesthetic agent, offering many advantages over the
nowadays-used nitrous oxide. Its economical implementation on surgical
theatres has implied research on automated, fully closed delivery systems that
reuse xenon but remove the carbon dioxide and nitrogen exhaled by the
patients.
2. Results and discussion
The CMS membranes used in this work resulted from the pyrolysis of a
cellulosic precursor under controlled conditions followed by a carbon chemical
vapour deposition (CVD) process and further controlled activation procedure.
These treatments, CVD and activation, influences decisively the membrane’s
sieving properties and can be used to tune its performance towards a desired
separation. The main purpose of the present analysis is to find the most suitable
membrane for Xenon recycling in an anaesthetic closed loop. Table 1 shows
single gas permeabilities at 20ºC in three CMS membranes with different
CVD/activation treatments. Large differences are found for the permeability
values of different gases, because of the molecular sieving character of these
membranes (pores sizes very close to the dimensions of the diffusing gases). A
small change in the effective micropore size can significantly modify the
permeation and selectivity properties of the membranes. Ideal selectivities are
shown in Table 2. For manufactured modules MS1b and MS2, CO2/Xe and
N2/Xe selectivities were found to be quite promising.
TABLE 1 – Single gas permeabilities at 20ºC for a feed pressure of 2 bar and a
permeate pressure of 1 bar
N2
O2
CO2
Xe
Mod. MS1
10
100
270
1
Mod. MS1b
9
80
301
<0.05
Mod. MS2
2
10
50
<0.05
TABLE 2 – Ideal selectivities at 20ºC for a feed pressure of 2 bar and a
permeate pressure of 1 bar
N2/Xe
CO2/Xe
O2/N2
CO2/N2
Mod. MS1
10
270
10
27
Mod. MS1b
>180
>6000
9
33
Mod. MS2
>40
>1000
5
25
Results for bore side feed permeation through the MS2 membrane, measured at
30ºC and 50ºC and up to 4 bar for He, N2, O2, CO2, SF6 and Xe are shown in
figure 1. Gas molecules with a diameter larger than 3.9Å do not exhibit
measurable permeation. He, N2 and O2 permeabilities were found to be weakly
dependent on the feed pressure, however, for strongly adsorbed gases like CO2,
the permeability decreases with the feed pressure. This is partly related to the
adsorption isotherm, that is of type I, which implies a strong concentration
dependence of the diffusion coefficient. However, the Darken relation alone,
designated by model 1 in the figure, fails to describe the CMS permeability
towards CO2.
Assuming a more general form for the diffusivity concentration dependence,
given by equation (1), and further incorporating it into the steady state flux
equation (2), one obtains an alternative model (model 2 in figure 1) that gives
better results [1].
Dc0
Dc 
(1   ) n
z2

q2
Jdz    Dc (q)
z1
q1
(1)
 ln p
dq
 ln q
(2)
All permeation rates through the membrane increased with temperature; the gas
transport occurs according to an activated mechanism.
80
Sample MS2
Permeate pressure= 60 mbar
Experimental data at 50ºC
Model 1 at 50ºC
70
Model 2 at 50ºC
Model 1 at 30ºC
-2
-1
Permeability / LN.m .bar .hr
-1
Experimental data at 30ºC
60
Model 2 at 30ºC
CO 2
50
40
He
30
20
O2
10
N2
0
0
0.5
1
1.5
2
2.5
3
3.5
4
Feed Pressure / bara
Figure 1. Single gas permeability as a function of pressure at 30ºC and 50ºC
The N2, O2, CO2, Xe and SF6 equilibrium adsorption isotherms at three
temperatures and up to 7 bar were measured using the volumetric and
gravimetric methods for two CMS fibre samples MS1 and MS2 (figures 2 and
3). The data were fitted to the Langmuir (dashed-curve) and Sips-type (solidcurve) correlations.
8
10ºC
Sample MS1
7
20ºC
Carbon Dioxide
Xenon at 20ºC
6
40ºC
Oxygen
Nitrogen
q / mol.L -1
5
4
3
10ºC
2
40ºC
1
0
0
1
2
3
4
5
6
Pressure / atm
Figure 2. Equilibrium adsorption isotherms in MS1
6
CMS samples: MS1, MS2
MS1, Carbon Dioxide
MS1, Nitrogen
5
MS2, Carbon Dioxide
MS2, Nitrogen
MS2, Sulphur
hexafluoride
q / mol.L -1
4
3
2
1
T= 30ºC
0
0
1
2
3
Pressure / atm
4
5
6
Figure 3. Equilibrium adsorption isotherms at 30ºC in two samples: MS1 and
MS2
MS2 has lower adsorption capacity, resulting from higher carbon deposition
(CVD) and lower activation. Xe uptake was very slow in this membrane, so that
it was difficult to establish when equilibrium had been achieved. Its uptake is ~
50 times lower than in MS1 [1].
From equilibrium isotherms it is possible to qualitatively predict
competitive adsorption effects in multicomponent permeation. CO2 real
selectivity will be enhanced by its strong affinity to CMS. In contrast, although
Xe permeates very little through the tested membranes, it is preferentially
adsorbed on the micropores. This behaviour will cause a blockage effect, having
a negative influence on the multicomponent permeabilities, especially for the
less adsorbables gases unable to displace the adsorbed xenon.
Adsorption isotherm analysis is also a useful method for gathering
information on the membrane’s ultramicroporosity. MS2 membrane pores are
narrower than 5.02 Å (SF6 molecular diameter) and a fraction is larger than 3.94
Å (Xe molecular diameter) [2]. Pores in the MS1 membrane are larger than in
MS2
The study of the structure of the CMS membranes was complemented
with other methods. From electron microscopy it was found that these
membranes have a uniform membrane thickness of 9 m and an external
diameter of 170 m [3]. SEM pictures also showed a dense, apparently
symmetric, crack-free structure. Wide Angle X-ray diffraction revealed a highly
amorphous structure with an average micropore size of about 4 Å. A density of
1.7g.cm-3 (against the limiting value of 2.3g/cm3 for graphite) was measured by
helium picnometry [1].
3. Conclusions
Recycling of xenon from an anaesthetic closed loop using CMS membrane
modules is a promising application for this new material.
Even though some ideal perm-selectivities seem quite good, these values may
become useless when dealing with multi-component separation of absorbable
gases in CMS; multi-component measurements are therefore essential and are
currently under way.
References
1. Pontes, S., Magalhães, F. and Mendes, A. (2003) “Carbon Molecular Sieve
Membranes. Part 1: Gas Transport Characterization”, J. Membr. Sci.,
(submitted).
2. Koresh J.; Soffer A.(1980) Study of Molecular Sieve Carbons J.C.S.Faraday
I., 76, 2472.
3. Pontes S., Magalhães F., M. Adélio, (2002) “Carbon Molecular Sieve:
Characterisation and Application”, International Congress of Membranes –
ICOM, Toulouse, France.