Membrane fabrication methods

Concerning the preparation of thin membranes directly on porous supports, a lower thickness limit seemingly exists for which a dense metal layer can be obtained. This thickness limit increases with increasing surfaee roughness and pore size in the support s top layer. Clearly, this relation puts strong demands on the support quality in terms of narrow pore size distribution, and the amount of surface defects. Therefore both pore size and roughness of the support surface are often reduced by the application of meso-porous intermediate layers prior to deposition of the permselective metal layer. This procedure facilitates the preparation of thin defect-free membranes beeause it is relatively easier to cover small pores by filling them with metal. It is therefore conceivable that for a certain low Pd-alloy thickness and support pore size, the H2 flux becomes limited by the support resistance.  

3 Palladium-alloys and their Implications for Membrane Stability 

In pure Pd, an a-to-(3 hydride phase transition may occur in hydrogen below about 290 °C, ° and only a few cycles through this transition makes the material brittle and must be avoided. By alloying Pd with different elements, the phase 

The majority of the work related to Pd-based membranes is on the highly permeable Pd-Ag alloy. The drawback is, however, that these alloys are prone to poisoning by CO and sulfur-containing gases leading to reduced permeability, or even to complete deterioration of the membrane. The extent to which this affects stability and flux depends on many parameters, and the reported literature appears only partly consistent in this respect. The latter is probably mainly due to the insufficient understanding of the reactions mechanisms and how the surface properties, which is usually not well described on the atomic level, varies with synthesis methods, thermal history, surface composition, morphology, etc. 

A similar effect has been observed for many other hydrocarbons, i.e. adsorption and subsequent hydrogen flux reduction. Post-inspection of the surface often reveals carbonaceous surface contamination, like carbon, CH4 or propylene. Secondary effects of CO adsorption is the catalytic decomposition and formation of carbon, carbide (Pdi xC ) or carbonate phases. While reversible CO adsorption decreases with increasing temperature, catalytic decomposition appears to increase with temperature. The tendency of deposit forming due to catalytic decomposition is reported to be counteracted by the presence of steam mitigating the flux reduction. 

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MEMBRANE FABRICATION METHODS AND MEMBRANE MODULES 5.3.1 Introduction... 

Kimura, J., Kawana, Y., Kuriyama, T., An Immobilized Enzyme Membrane Fabrication Method Using an Ink Jet Nozzle , Biosensors 4 (1988) 41-52. 

The most common methods for manufacturing thin metal membranes include rolled foil, drawn tubes, and films deposited onto porous substrates (ceramic or sintered metal). Usually, electroless plating or electrolytic plating are the methods used to deposit the permselective metal onto the porous substrates although vapor deposition methods have been the subject of much research effort However, to date, vapor deposition methods have not proven to be a superior membrane fabrication method. There are pros and cons to each of these methods, but commercial membrane modules have only succeeded using rolled foil and drawn tubular membranes. 

In order to be used on an industrial scale, membrane reactors need to be produced at a scale much larger than those currently employed in laboratory set-ups. This implies that membrane fabrication methods for large scale systems need to be defined, as well as the engineering related to module preparation. 

Broadhead and Tresco studied the effects of fabrication conditions on the structures and performances of membranes formed from poly(acrylonitrile-vinylchloride) (PAN-PVC) by using the phase inversion process [85]. They reported the relationship of the fine-surface structure of PAN-PVC membranes to the membrane performance and membrane fabrication method. The fine-surface structure of nodular elements and the size of these elements could be altered by changing the precipitation conditions. Membranes were prepared at 22 on 55 mm diameter polished silicon wafers by spinning at 1500 rpm for 20 s with a spin coater [86]. The film was immediately precipitated in one of the four different precipitation media. The first three media consisted of deionized water at 4,22, and 54 °C. These membranes were referred to as Type 1 , Type 2 , and Type 3 , respectively. The fourth medium was a 50/50 mixture of deionized water and N,iV-dimethylformamide (DMF) at 54 °C and coded as Type 4 . Figure 4.53 shows the histograms of the nodule size distributions observed at the skinned surface of the membranes made under four different precipitation conditions. The sizes of these nodular elements became smaller and more uniform with milder precipitation conditions, which supports the theory that nodules are formed through spinodal decomposition under these conditions. In addition, the size of these nodules could be related to water permeability. Hence, water transport occurred through the interstitial spaces where the pores could be situated. 

Table 13.1 Pd-based selective membrane fabrication methods...

This chapter has reported the basic features of a membrane reactor the properties of selective membranes, fabrication methods, actual markets and a cost analysis are described and assessed. 

To control costs of membranes containing precious metals, such as Pd, and to improve their performance in a commercially viable configuration, considerable development effort has been focused on membrane fabrication methods. The most important factors to consider when selecting a fabrication method are H2 transport rates, selectivity, stability, and life. Three common approaches are discussed here free-standing thin foils, thin films on porous substrates, and composite membranes. 

Gas separation, desalination, ultra- and microfiltrarion, ion transport, electrodialysis, and pervaporation processes rely on membrane fabrication methods that can deliver robust and selective membranes. These processes require fabrication methods that can produce membranes that are defect-free so that hydrodynamic flow does not occur over that expected for the specific pore diameter of the membrane owing to... 

This book brings together experts from a number of disciplines working within membrane technology all with the same goal of the development and fabrication of new membranes for increased and efficient application of membrane separation processes. The authors of each chapter share their experience and insights in a specific membrane fabrication area. In many cases, this information extends into application of the membrane system as the research uses membrane performance data to improve subsequent membrane fabrication methods. 

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