Membranes hydrothermal stability

The separation factors are relatively low and consequently the MR is not able to approach full conversion. With a molecular sieve silica (MSS) or a supported palladium film membrane, an (almost) absolute separation can be obtained (Table 10.1). The MSS membranes however, suffer from a flux/selectivity trade-off meaning that a high separation factor is combined with a relative low flux. Pd membranes do not suffer from this trade-off and can combine an absolute separation factor with very high fluxes. A favorable aspect for zeoHte membranes is their thermal and chemical stability. Pd membranes can become unstable due to impurities like CO, H2S, and carbonaceous deposits, and for the MSS membrane, hydrothermal stability is a major concern [62]. But the performance of the currently used zeolite membranes is insufficient to compete with other inorganic membranes, as was also concluded by Caro et al. [63] for the use of zeolite membranes for hydrogen purification. 

MicrocrystalUne zeolites such as beta zeolite suffer from calcination. The crystallinity is decreased and the framework can be notably dealuminated by the steam generated [175]. Potential Br0nsted catalytic sites are lost and heteroatoms migrate to extra-framework positions, leading to a decrease in catalytic performance. Nanocrystals and ultrafine zeolite particles display aggregation issues, difficulties in regeneration, and low thermal and hydrothermal stabilities. Therefore, calcination is sometimes not the optimal protocol to activate such systems. Application of zeolites for coatings, patterned thin-films, and membranes usually is associated with defects and cracks upon template removal. 

As the pore size is reduced to 1 nm or less, gas permeation may exhibit a thermally activated diffusion phenomena. For example, in studies at Oak Ridge National Laboratory, for a certain proprietary membrane material and configuration, permeation of helium appeared to increase much faster than other gases resulting in an increase in Helium to C02 selectivity from 5 at 25°C to about 48.3 at 250°C (Bischoff and Judkins, 2006). Hydrothermal stability of this membrane in the presence of steam, however, was not reported. 

Because carbon has a natural affinity for adsorption of heavy hydrocarbon species and polar molecules, CMS membranes need to be used at a sufficiently high temperature to eliminate contribution/interference of the adsorption. In contrast, strong adsorption of heavier molecules may be used to separate those species by adsorption as discussed earlier by the SSF mechanism (Rao and Sircar, 1993b). The SSF carbon membranes typically have pore dimensions much greater than those needed for CMS membranes since the separation is based on the adsorbed species effectively blocking permeation of other components (Fuertes, 2000). Carbon membranes are resistant to contaminants such as H2S and are thermally stable and can be used at higher temperatures compared to the polymeric membranes. For the synthesis gas environment, the hydrothermal stability of carbon in the presence of steam will be a concern limiting its operation temperature. 

The permeation of hydrogen and methane through the membrane was investigated, revealing increased permeability for hydrogen on raising the temperature, while the methane permeability remained at a low level. Between 100 and 525 °C, the separation factor (see Section 2.6.3) increased from 7.5 to 31. The hydrothermal stability of the membrane was verified at 525 °C for 8 h for a feed composed of 18% hydrogen, 18% methane and 74% steam. It revealed a decrease of the separation factor from 31 to 26 [51]. 

In summary, the main goal of the present work is the development of a hydrothermally stable microporous silica membrane with prescribed transport properties. Preferably, these steam stable membranes should have very high permselectivities. Because the permselectivity of a molecular sieving silica membrane will drop to the Knudsen value of the y-alumina supporting membrane when the silica membrane deteriorates under steam reforming conditions, a selectivity of the silica layer higher than the Knudsen selectivity is sufficient. In this way the measurement of the permselectivity is a powerful tool to assess the hydrothermal stability of a supported microporous membrane. 

In most of the above-mentioned processes, high operation temperatures are necessary while the reaction atmospheres usually contain considerable amounts of steam because water is one of the reactants, or because water is added to reduce coke formation. Also in food processing and medical applications, steam is often used for sterilisation. Thus, in many applications the membranes must be sufficiently stable in environments of both increased temperature and containing steam. In this work, this is called hydrothermal stability. 

Somewhat surprisingly, however, only a very limited amount of literature is available on hydrothermal stability of even the most commonly applied mesoporous membrane type, namely y-alumina membranes on OC-AI2O3 supports. These mesoporous y-alumina membranes are the common supports for the microporous silica membranes to be used in membrane steam reformers. In the investigations that finally led to the present study, delamination of the y-alumina membrane from the OC-AI2O3 supports in hot steam was found to be a major compli-... 

The excellent separation properties of silica membranes prepared at temperatures as high as 825°C enables their use for high temperature applications, such as the dehydrogenation of H2S (chapter 8). Unfortunately no hydrothermal stability of the prepared layers could be tested because the mesoporous intermediate layer was not hydrothermally stable, but an indication of the hydrothermal stability of the unsupported material could be obtained from the specific surface area and XRD measurements. These measurements did not show any structural change in the material during SASRA treatment, which is a very hopeful result for the operation of real, supported, membranes at high temperatures and high pressures. 

It is generally accepted that thermal and especially hydrothermal treatment of aluminas and other catalytic materials results in deterioration of porous structure, i.e. increase in average pore radius and diminishing in specific surface area [1-4]. It is very important that such alumina materials as some catalyst washcoats and membranes have to be exploited at higher temperatures and at atmosphere of large humidity. Therefore it is necessary to improve their thermal and hydrothermal stabilization by application of new binder materials or additives. Such additives as silica, ceria or zirconia are known as thermal stabilizers. The aim of this work was to determine the influence of addition of the selected stabilizers on hydrothermal stability of alumina material in the temperature range 150 - 225 °C and time up to 72 hours. 

Alumina membranes. It has been established that several phases of alumina exist and a particular phase of alumina is determined not only by the temperature it has experienced but also by the chemical path it has taken. For commercial membrane applications, the alpha- and gamma-phases of alumina are the most common. Alpha-alumina membranes are well known for their thermal and hydrothermal stabilities beyond 1,000 C. In fact, other transitional forms of alumina will undergo transformation towards the thermodynamically stable alpha-alumina at elevated tcmjxratures beyond 900 C. On the contrary, commercial gamma-alumina membranes are typically calcined at 400-600 C during production and are, therefore, subject to potential structural changes beyond 600°C. Moreover, alumina chemistry reveals that phase transition also occurs beyond that temperature [Wefers and Misra, 1987]. 

When using commercial inorganic membranes for separation or membrane reactor applications at relatively high temperatures, say, greater than 400°C or so, care should be taken to assess their thermal or hydrothermal stability under the application conditions. This is particularly relevant for small pore membranes because they are often made at a temperature not far from 400 C. Even if such an exposure does not yield any phase changes, there may be particle coarsening (and consequently pore growth) involved. 

Hydrothermal stability of the membrane in a membrane reactor under an environment like this is a critical factor in its service life as well as performance. Water is known to promote phase transition of metal oxides at a lower temperature. 

Permeate recovery. To achieve high conversions, it is often desirable to maintain a very low permeate partial pressure which leads to an increase in the permeation rate. Vacuum or a sweep gas is usually employed to attain a low permeate pressure. Vacuum adds some energy cost while the use of a sweep gas may require further downstream processing for the recovery of the permeate (if it contains the desired species) or the separation of the permeate from the sweep gas. The use of a condensable gas or vapor as the sweep gas will facilitate the recovery of the permeate. For example, steam can be condensed at a relatively lower temperature and easily separated from many other gases. However, the issue of hydrothermal stability of the membrane, discussed in Chapter 9, can be critical. Air, on the other hand, is a convenient sweep or carrier gas to use because... 

C.H. Chang, R. Gopalan and Y.S. Lin, A comparative study on thermal and hydrothermal stability of alumina, titania and zirconia membranes. /. Membr. Sci., 91 (1994) 27-45. 

The hydrothermal stability of the materials is more important as far as the use of membranes in methane reforming reactor is concerned. In general, improving the hydrothermal stability of membranes is difficult owing to the metastable nature of porous, particularly microporous structures and their tendency to change in the way of surface area reduction. Yet, recent reports [33, 34] show that improvements have been made in the hydrothermal stability of membranes based on silica, a material... 

The cause and effect relation between hydrothermal stability and water permeation is not apparent. This is a point where more attention is necessary in the future studies. It is possible that the larger H2/H2O separation factor or in other words the low permeation of H2O through the membrane is the reason for the better hydrothermal stability of the membranes. Kinetic limitations have indeed shown to enhance the stability of protective coatings and coated materials. On the other hand, it should also be reasoned that the inherent hydrophobic structure of the material is the source of the improvement in hydrothermal stability and for the reduction in H2O permeation by limiting the number of sites where H2O could be adsorbed (as in Fig. 16.7). The results discussed so far clearly indicate that the improvement in hydrothermal stability of the membranes is linked to the improvement in porous structure (structure and quantity of surface exposed to H2O) of the membranes itself. 

Yoshida K, Hirano Y, Fujii H, Tsuru T, Asaeda M. Hydrothermal stability and performance of siLica-zirconia membranes for hydrogen separation in hydrothermal conditions. J Chem Eng Jpn. 2001 34(4) 523-30. 

Microporous silica hydrogen permselective membranes have been extensively studied as a potentially more practical alternative to Pd membranes. Very recently, a comprehensive review was published, tackling various aspects of silica membrane synthesis, application and economics [63]. It was made evident that state-of-the-art silica membranes have good hydrogen flux and separation, as well as respectable thermal stability. However, the hydrothermal stability of a silica hydrogen permselective membrane is a key factor in determining its suitability for a commercial apphcation of membrane-assisted processes. 

Battersby S, Smart S, Ladewig B, Liu S, Duke MC, Rudolph V, Diniz da Costa JC (2009) Hydrothermal stability of cobalt silica membranes in a water gas shift membrane reactor. Sep Purif Technol 66 299-305... 

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