INFLUENCE OF WATER VAPOR ON SILICA MEMBRANES

Postgraduate

Abstract

      Silica-based membranes are considered to be promising means of hydrogen separation at elevated temperatures due to their high H2 permeance, very good selectivity and relative ease to scale up. This is especially relevant for their applications in coal gasification and steam reforming where water vapor is present. The membranes studied here are composed of three layers: macroporous α-Al2O3 as the support, mesoporous γ-Al2O 3 as the intermediate layer and microporous amorphous silica as the separation layer.

      In this report, the influence of water vapor on He transport through silica membrane has been investigated in terms of adsorption and percolation effect at relatively low temperatures (i.e. 50 and 90°C). The selected temperatures maximize the difference of mobility between He and H2O molecules and avoid the structural change. He is considered to be a mobile component, while H2O is an immobile one when these two gases transported under the specified conditions.

      Two main methods of characterization on the actual as-deposited membrane layers employed in this study are spectroscopic ellipsometry for water vapor adsorption and the gas permeation for He transport in presence of water vapor. The former is a versatile technique to record the water vapor adsorption in situ, while the later provides the information on the percolation effect.

The isotherms of water vapor adsorption in the silica layer obtained from the ellipsometry are of Type I according to the IUPAC classification, complying with the typical adsorption behavior in the microporous materials. This result is in accordance with the range of pore size of the studied silica membrane (~4 Å). The adsorption isotherms generally comply with the first-order Langmuir isotherm with a slight deviation probably caused by the heterogeneous adsorption of H2O molecules at the different active sites on the silica surface.

      He flux through the silica membrane decreases dramatically in presence of water vapor, even in the low pH2O range due to the blocking effect by the strongly absorbed H2O molecules. The transport of gas molecules through such small silica pores can be assumed not to be continuous any more, with the gas molecules hopping from one occupied site to another unoccupied one under the potential gradient. When the coverage of water vapor in the silica layer increases, the He permeance is affected by the percolation effect. The irregular lattice, heterogeneous sites and gas molecules hopping to the sites on the opposite wall of narrow pore may cause the transition of He flux to happen at the high coverage of immobile component. He flux does not vanish even when the coverage of immobile H2O molecules is close to 100% likely due to the presence of big pores in the silica layer.


INTRODUCTION

 

       Hydrogen is a high-value gas with versatile applications as a chemical feedstock or as an alternative fuel (e.g. for fuel cells). Currently, hydrogen is produced mainly from the processes in terms of coal gasification and steam reforming of methane. Both methods generate a mixture of H2, CO2, H2O and other gases.

 

Microporous silica membrane is one of the feasible means for separating H2 from this industrial gaseous mixture in an energy-efficient and cost-saving way, compared to the conventional separation methods (e.g. pressure swing adsorption and cryogenic separation).

 

 At high temperatures and in the presence of water vapor, silica membrane can undergo the rehydration or viscous sintering, resulting in the change of pore structure. However, in this assignment, we have examined how the presence of water vapor can decrease significantly the permeance of an inert gas through the silica membranes at low temperatures, where no structural change is expected. 

 

The objective of this assignment is to illustrate that the adsorption and blocking by the immobile component (e.g. H2O at low temperature) present in the silica micropores may decrease the permeance of the mobile one (e.g. H2, He) in a binary mixture. In the case of the silica membrane exposed to the binary gas mixture (e.g. H2 and H2O) at low temperatures, H2O is considered to be an immobile component, having a strong interaction with the silica surface. As a result, the permeance of mobile component is lowered due to the adsorption and blocking effect of water vapor being the most pronounced at the concentration near or above the percolation threshold, i.e. the point of no flux of the mobile component due to the absence of a connected path for the mobile component through the membrane. 

 

In a word, this report focuses on the effect of water vapor on silica membranes in terms of sorption and percolation. 


Theory

 

Membranes for hydrogen separation

 

The need for hydrogen will increase greatly in the future as a raw material for the chemical industry and as clean fuels in cars and electric industry (e.g. fuel cells). Currently, hydrogen is produced mainly by the reforming of fossil fuels and coal gasification. However, hydrogen is there mixed with large quantities of non-desired components such as light hydrocarbons, CO and CO2 from fossil fuels [1-2]. The purification or separation of hydrogen from these industrial gases by means of membrane has several advantages, including low energy consumption and cost saving. 

In general, membranes can be classified as organic and inorganic based on their material composition, as porous and dense or as symmetric and asymmetric based on their structure etc. Flux, selectivity, chemical stability and mechanical strength are the important parameters for the membrane performance. Although organic membranes have an advantageously low price and good scalability, they cannot be used at high temperatures or in chemically aggressive environments containing e.g. HCl, SOx, and their poor mechanical strength hinders their highpressure application. Dense metal membranes, usually made of palladium or its alloys, have very high selectivity for hydrogen (~100%) based on the solution-diffusion mechanism, but a deadly sensitivity to CO and H2S, in terms of coal gas application [3]. Proton conductors, such as doped BaCeO3, have a very high selectivity in the water vapor atmosphere, because only protons can migrate through these materials. However, H2 flux through the protonconducting membranes is relatively low (~10-8 mol/cm2·s) [4], and their chemical stability in the presence of certain species (e.g. CO2, H2S) is another major concern. Furthermore, energy consumption is disadvantageous because they must be operated at high temperatures (e.g. 800-1000°C) in order to obtain high flux.

 Inorganic porous membranes can be used in many industrial applications at high temperatures (>200°C), and they have high flux and very good selectivity. Two of the most promising porous materials for membrane are zeolite and silica: the pores in the zeolite membrane are a part of the crystal structure, and hence have uniform dimensions. Many zeolites are thermally stable above 500°C. Zeolite membranes are generally formed on porous supports by hydrothermal synthesis, and hence the membranes have a lot of defects, lowering the selectivity. The most critical barrier for zeolite applications is the difficulty in producing in a large scale. Microporous silica membranes have high hydrogen permeance and high selectivity and excellent capacity to scale up. Hereby, silica-based membranes are promising candidates for hydrogen separation at elevated temperatures, although the steam/water stability of these membranes may be an issue