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
Mod. MS1 / Mod. MS1b / Mod. MS2N2 / 10 / 9 / 2
O2 / 100 / 80 / 10
CO2 / 270 / 301 / 50
Xe / 1 / <0.05 / <0.05
TABLE 2 – Ideal selectivities at 20ºC for a feed pressure of 2 bar and a permeate pressure of 1 bar
Mod. MS1 / Mod. MS1b / Mod. MS2N2/Xe / 10 / >180 / >40
CO2/Xe / 270 / >6000 / >1000
O2/N2 / 10 / 9 / 5
CO2/N2 / 27 / 33 / 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].
All permeation rates through the membrane increased with temperature; the gas transport occurs according to an activated mechanism.
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 (solid-curve) correlations.
Figure 2. Equilibrium adsorption isotherms in MS1
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 mm and an external diameter of 170 mm [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
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.