Development of high quality membranes for industrial applications will lead to cost reductions over traditional separations processes. Silica membranes are a new technology for hydrogen separation that needs R&D specifically to apply them to industrial scales. Past work has shown a carbonised template silica membrane which offered hydrostability. This resulted in better stability under steam and high temperature conditions without compromising the permselectivity for small molecules. In this paper a hydrostable silica membrane was developed for hydrogen separation having a pore cut-off around 3 Angstron units. The carbon templates did not compromise the membrane's ability to permeate hydrogen selectively rather than other major gases in a synthesised coal gasifier mixture of CO, CO2 and N2. The selectivity of H2 to N2 was 26, whilst the hydrostable property of the carbonised template membrane was maintained. Computational fluid dynamics (CFD) can be used to develop membrane systems in tandem with these intrinsic improvements. CFD simulation studies were also conducted to gain better insight into the macroscopic flow parameters

Abdel-Jawad, M. M.

Diniz da Costa, J. C.

Duke, M. C.

Gopalakrishnan, S.

Macrossan, M. N.

University of Queensland eSpace

Purifying hydrogen with inorganic silica membranes at high temperatures 
 
M. C. Duke, S. Gopalakrishnan, M. Abdel-jawad, M. Macrossan, J. C. Diniz da Costa* 
 
ARC Centre for Functional Nanomaterials, School of Engineering, The University of 
Queensland, Brisbane Qld 4072, Australia. 
*also Project Leader, Centre for Low Emission Technology (cLET), PO Box 883, Kenmore, Qld 
4069, Australia 
 
Abstract:  Development of high quality membranes for industrial applications will lead to cost 
reductions over traditional separations processes. Silica membranes are a new technology for 
hydrogen separation that needs R&D specifically to applying them to industrial scales. Past 
work has shown a carbonised template silica membrane which offered hydrostability. This 
resulted in better stability under steam and high temperature conditions without compromising 
the permselectivity for small molecules. In this paper a hydrostable silica membrane was 
developed for hydrogen separation having a pore cut-off around 3Å. The carbon templates did 
not compromise the membrane’s ability to selectively permeate hydrogen over other major 
gases coming from a synthesised coal gasifier gas mixture of CO, CO2 and N2. The selectivity 
of H2 to N2 was 26, whilst the hydrostable property of the carbonised template membrane was 
maintained. Computational fluid dynamics (CFD) can be used to develop membrane systems 
in tandem with these intrinsic improvements. CFD simulation studies were also conducted to 
gain better insight into the macroscopic flow parameters. 
 
Introduction: 
 Membrane separation of gas mixtures has always considered as a more energy 
conservative separation method than the conventional separation processes. Hydrogen 
permselective silica membranes have attracted preference in the membrane gas separation 
applications due to the importance of hydrogen as a fuel and an industrial feedstock for the 
production of various chemicals [1]; especially to derive high purity hydrogen for fuel cell 
applications [2]. Membrane reactors, which combine separations and reactions in one system, 
benefit from such membranes which shift the reaction conversion limited by thermodynamic 
equilibrium to the product side, such as processing syngas in the water gas shift reaction [3]: 
 
 CO + H2O ↔ CO2 + H2      (Eq. 1) 
 
By removing H2 from the reaction system shifts the reaction to the product side deriving 
increased CO conversion than a conventional catalytic reactor, producing more H2 while also 
lowering the operating temperature. This reaction is significant in coal gasification processes 
which produce syngas, offering significant advantages to cost and size reduction of the 
operation. 
 Silica membranes are proven to be thermally and chemically stable offering high 
permeability and selectivity for hydrogen [4] with much work concentrating on their hydrogen 
separation property [5]. In silica membranes, the separation is mainly based on a sieving effect 
through small pores (~3Å) producing reasonably high combined values for flux and selectivity. 
Based on their mode of formation, silica membranes are mainly divided into sol-gel and 
chemical vapour deposition (CVD) membranes. Sol-gel membranes are prepared by coating 
an alkoxide precursor sol atop a porous substrate where the membrane quality is mainly 
determined by the sol chemistry. CVD membranes are prepared by the decomposition of gas 
phase precursors inside the micropores of the substrate, thereby blocking the pore diameter to 
molecular dimensions. The CVD method results in membranes with lower permeance and 
higher selectivity. However, the main concern regarding the silica membranes is that they are 
unstable under hydrothermal conditions. A 50% reduction in H2 permeance under lean 
hydrothermal conditions (50% H2O + 50% N2; atmospheric pressure) was displayed by silica-
sol membranes at 500°C [6]. This paper reports a novel concept in which carbon molecules 
were introduced into the system which position themselves between the silica molecules and 
act as supporting pillars, thus inhibiting the reorganization of silica molecules under 
hydrothermal conditions. Another key issue for membrane development is the engineering of 
separator modules. It is of interest to understand the resulting macroscopic flow parameters 
such as velocity and pressure distributions for tubes composed from this membrane. 
Computational fluid dynamics (CFD) simulation studies were conducted to gain insight into the 
macroscopic flow parameters.  
 
Experimental 
 Asymmetric supported carbonised template molecular sieve silica (CTMSS) 
membranes were prepared over porous α-alumina substrates (99.8% purity; porosity = 30%; 
0.5 -1.0 m) by sol-gel dip coating method as described elsewhere [7]. A γ-alumina layer forms 
the primary layer that brings down the pore size to 4nm, followed by an intermediate layer 
prepared by dipping a diluted acid catalysed sol-gel solution having methyl triethoxysilane 
(MTES, Aldrich) precursor leading to a silica with pore sizes in the region 0.35 – 0.45 nm [8]. 
The final silica layer was prepared by adding a short chained cationic surfactant hexyl triethyl 
ammonium bromide (Aldrich) to 0.125M of silica sol solution followed by the calcination under 
vacuum at 500°C, to preserve the hydrophobic organic pyrolysis products. Samples were 
characterized using adsorption studies and SEM and further tested for gas permeation 
between 50–200°C (2 atm drop). Permeation was measured using dead-end method to 
evaluate the performance of single gases involved in Reaction 1 (H2, CO2, CO and N2). 
 CFD calculations were performed using the commercial code CFD-ACE (ESI 
Software). The code is a compressible finite volume flow solver. Full Navier Stokes solutions 
were implemented. Calorically perfect gas was used and viscosity was modelled using kinetic 
theory. The simulations were axisymmetric. Fixed pressure inflow and outlet conditions were 
prescribed. Isothermal non-catalytic walls were prescribed at temperatures of 100°C. No 
chemical reactions were modelled for this set of simulations. Structured grids were used for 
this simple axisymmetric geometry and these were created using the commercial package 
CFD-GEOM (ESI Software) which is typically used in conjunction with CFD-ACE. 
Computational grids for the tubes were generated with clustering near the walls. Grid numbers 
were minimized as computational resources were limited. The solution was run on a single 
Pentium IV 3.4 GHz processor. Simulations typically required 16 hours each. The inner tube 
diameter was 6mm.   
 
Results and discussion  
 CTMSS membrane permeation through a planar substrate is shown in Figure 1. In 
general, permeation increases with temperature for all gases except for CO2. This trend 
exhibits the classical activated transport mechanism whereby diffusion occurs through 
micropores, which is in accordance with other high quality silica membranes as reported by 
other research groups[9, 10]. The CO2 permeation was essentially unrelated to temperature 
and decreased with temperature as observed elsewhere [8, 11]. CO2 has a high heat of 
adsorption, allowing for a more adsorption based transport. However as the pores become 
smaller in high quality membranes, CO2 transport is mostly restricted by the pore size and will 
also be activated [12]. In this result, no activation with temperature was observed due to two 
competing mechanisms. The separation mechanism in these tight pore spaces is by molecular 
sieving.  
Temperature (°C)
Pe
rm
e
a
n
c
e
 
(m
o
l.m
-
2 .
s-
1 .
Pa
-
1 )
40 60 80 100 120 140 160 180 200 220
1E-10
1E-9
1E-8
1E-7
H2
CO2
CO
N2
 
Figure 1: Permeation trend with temperature for planar CTMSS substrate at  
2 bar pressure drop. 
 
dk (Å)
Pe
rm
e
a
n
c
e
 
(m
o
l.m
-
2 .
s-
1 .
Pa
-
1 )
2.7 2.8 2.9 3 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8
1E-10
1E-9
1E-8
1E-7
H2: 2.89Å
CO2: 3.3Å
N2: 3.64Å
CO: 3.76Å
 
Figure 2: Permeation-molecule size cutoff curve for CTMSS membrane  
at 200°C, 2 bar pressure drop. 
 
The kinetic diameter of the molecule pertinent to molecular scale diffusion can be plotted 
against the experimentally measured permeation as shown in Figure 2. The smaller, more 
mobile H2 molecule was more permeable than the larger CO2, CO and N2 molecules. As most 
of the permeation dropped between H2 (2.89Å) and CO2 (3.3Å), the average pore size is 
around that of H2 (~3Å). The permeation of N2 was lower than CO contrary to the size 
relationship. Since the permeation of both these gases occurs in the tail of the silica pore size 
distribution, there is little size selective advantage available. Since CO is a more adsorbing gas 
than N2, its adsorptive property therefore favoured its transport through the membrane. The 
H2/N2 permselectivity of the membrane was 26, well above the ideal Knudsen value of 3.74, is 
another indicator of the molecular sieving behaviour. CVD membranes has however been 
reported to have far greater selectivity values >1000[13,14].  
 Figure 3 shows the CFD simulation of the non-dimensionalised velocity field and the 
centre line non-dimensional pressure and velocity curves (bounded by the 2 crosses in the 
figure). In this simulation the feed side at the top of the figure has a prescribed velocity. Flow 
passes from the feed side to the permeate side and exits the permeate side at the far right of 
the permeate region. The figure shows that the boundary layer size is significant and extends 
the full radius of the tube and that the velocity profile in the tube axially is linear along the 
centre line of the flow. This is an indication that the model is scalable in length provided that 
the flow remains subsonic, but that scaling radially would be more effective. Associated with 
this is the parabolic pressure distribution dropping from the maximum stagnation pressure at 
the wall to the left of the figure.  
 
Figure 3 Nondimensional velocity field with centre line nondimensional  
pressure and velocity plots for flow inside a porous tube. 
 
Conclusion 
 CTMSS membranes were prepared over asymmetric α-alumina substrates coated 
with γ-alumina layer. Except CO2, all other gases studied displayed activated transport through 
this membrane, indicating molecular sieving effect through this membrane. H2/N2 
permselectivity of this membrane was 26 and has in past work been found to be quite stable 
under hydrothermal conditions. CFD studies revealed that the boundary layer size is quite 
significant and extends the full radius of the tube, and that the velocity profile in the tube axially 
is linear along the centre line of the flow. 
 
Acknowledgement 
 The authors acknowledge financial support by the Centre for Low Emission 
Technology (www.clet.net) in Australia. 
 
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Purifying Hydrogen with Inorganic Silica Membranes at High Temperatures

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Purifying Hydrogen with Inorganic Silica Membranes at High Temperatures

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