Gas permeability through graphite foil: The influence of physical density, membrane orientation and temperature

doi:10.1016/j.seppur.2017.02.023

Separation and Purification Technology

Volume 179, 31 May 2017, Pages 467-474

https://doi.org/10.1016/j.seppur.2017.02.023 Get rights and content

Highlights

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New results of graphite foil permeance for H₂, N₂, CH₄, CO₂ and n-C₄H₁₀ are obtained.
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Noticeable surface flow contribution of condensable gases is found.
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Significant anisotropy of graphite foil matrix permeance and selectivity is shown.

Abstract

Gas permeability parameters of microporous foils based on exfoliated graphite prepared by controlled pressing as membrane material for gas separation are reported. Permeability of H₂, N₂, CH₄, CO₂ and C₁-C₄ lower hydrocarbons for pressed graphite foil (GF) samples with various densities (240–1500 kg/m³) within the temperature range 20–90 °C was studied. It was found that GF samples with density around 1000 kg/m³ demonstrate high H₂/CO₂ ideal selectivity (α = 14–22) which significantly exceeds Knudsen selectivity. The influence of anisotropy structure features of GF membranes, physical foil density, transport pore distribution and gas flow direction on gas transport and separation properties was investigated as well. The evaluation of the surface flow contribution to the overall gas flow was carried out by approximation method using temperature dependences of gas permeance. It is concluded that the presence of noticeable surface flow can lead as to a decrease of gas permeance in comparison with Knudsen flow (for example, for CO₂) as to an increase of gas permeance (for example, for lower hydrocarbons). Development of separation process for particular gas mixture compositions needs to take into account obtained tendency. Future potential applications of GF membranes are discussed as well.

Introduction

One of the promising directions of membrane materials development for gas separation applications is design of microporous inorganic materials, for example, zeolites [1], [2], [3], [4], carbon based molecular sieves [1], [5], [6], [7], [8], glassy membranes [9], [10], [11] and others. Such materials can be effectively used as for separation processes where preferable permeability of light molecules, such as hydrogen, helium, nitrogen is required and can be provided due to sieving effect [1], [2], [3], [4], [5], [6] as for processes where preferable permeability of the strongly adsorptive molecules (e.g., lower hydrocarbons) is required and can be provided due to strong surface flow contribution as in adsorption selective membranes [11], [12]. Besides high selectivity level and high permeance, such materials demonstrate good chemical and thermal stability. Combination of these properties makes the microporous inorganic materials more desirable comparing to traditional industrially produced membranes based on nonporous polymeric materials. However, main disadvantages of such porous materials are brittleness and difficulty in scaling up for large area membranes production. From this point of view, industrially produced inorganic matrices, such as foils based on exfoliated graphite, could extend the area of application of micro porous inorganic materials, particularly for gas separation. Graphite foils (GF) are mainly known as sealing materials. Celzard and others [13], [14], [15], [16] have shown that exfoliated graphite-based materials demonstrate wide range of gas diffusion mechanisms depending on such pore structure parameters as pore diameter, open and closed porosity relation, and anisotropy of structure [13], [16]. Foil based on exfoliated graphite is one of such materials that currently produced at industrial scale as sealing material and potentially could be applied as microporous inorganic membranes for gas separation. Authors of the present paper have previously investigated membranes based on exfoliated graphite with optimal density produced by INUMIT Ltd. (Russian Federation) and showed that the GF can provide high ideal H₂/CO₂ and (lower hydrocarbons)/CO₂ selectivity [17], [18].

This paper is aimed to understanding of GF gas selective properties and presents the set of new permeation data for exfoliated GF in relation to H₂, N₂, CH₄, CO₂ and n-C₄H₁₀. This paper presents the set of new permeation data for exfoliated GF in relation to H₂, N₂, CH₄, CO₂ and n-C₄H₁₀. The estimation of the surface flow contribution to the overall gas flow in GF is carried out for the first time by using of temperature dependences of gas permeance. The gas permeability anisotropy of GF matrix is shown as well. The influence of foil density, transport pore distribution and anisotropy of gas flow direction on transport properties and diffusivity mechanism of gases in the graphite foils studied are considered. Future potential applications of GF membranes are discussed as well.

Section snippets

Materials

The samples of GF studied were prepared from exfoliated graphite particles based on graphite nitrate produced by the previously reported method [19], [20]: natural graphite was first treated by 98% nitric acid at mass ratio 1:0.8, and then intercalated graphite was treated by hydrolysis followed by thermo-expansion at 900 °C. Obtained exfoliated graphite was pressed to foil with required specified thickness and density. The thickness of GF samples was varied within the range of 0.2–1.0 mm,

Microstructure of pore matrix

The foil material investigated has three-layer structure: two higher density surface layers formed due to pressing procedure and lower density bulk layer. SEM image in Fig. 2 shows that GF surface looks like a flake-packed structure with jelly-like macro pores with length of about 2 µm and width about 0.4 µm for all samples within the density range of 300–1500 kg/m³. No influence of the density on surface structure was found. The estimated density of surface layers is equal approximately to 2000

Conclusion

The significant influence of physical density on the GF gas selectivity has been observed in the present study. It was found that GF with density of 1000 kg/m³ demonstrates the highest H₂/CO₂ ideal selectivity for the gas flow across graphite layers. This effect is based on interaction of CO₂ molecules with the surface of a pore and tortuosity factor of GF matrix. The increase of density up to 1500 kg/m³ results in an increase of small/large molecules separation, for example, for a H₂/(lower

Acknowledgements

The authors thank Ph.D. D.V. Dmitriev and «ASPECT» Company (Moscow, Russian Federation) for permoporometry analysis, Group ENSIC (Nancy, France) for help with SEM analysis, Prof. V.V. Avdeev, Ph.D. S.G. Ionov, Ph.D. O.N. Shornikova and Dr. N.E. Sorokina (M.V. Lomonosov Moscow State University, Russian Federation) for the GF samples.

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Gas permeability through graphite foil: The influence of physical density, membrane orientation and temperature

Highlights

Abstract

Introduction

Section snippets

Materials

Microstructure of pore matrix

Conclusion

Acknowledgements

Desalination

Microporous Mesoporous Mater.

Carbon

Carbon

Carbon

J. Membr. Sci.

J. Membr. Sci.

J. Membr. Sci.

Progr. Polym. Sci.

Carbon

Carbon

Carbon

J. Membr. Sci.