Formation of Carbon Membranes

As mentioned previously, the gas separation performance of a carbon membrane is controlled by the pore size distribution in the material. Several factors have been identified that affect the pore size distribution  

There are other minor factors that also play a role in the separation performance of carbon membranes, but the factors listed here have been identified as the most significant. In the next sections, these factors will be analyzed in more detail. The syntheses of selective surface flow and CMS membranes follow very similar procedures. First, a review of SSF membranes will be given and then the remainder of the review will focus on CMS membranes. 

The desired product of effluent streams is often the small, nonadsorbable component such as hydrogen. SSF membranes hinder diffusion of this component and therefore produce streams enriched in the lighter components at higher pressure, reducing 

The driving force for transport of a component in a gas mixture through SSF membranes is determined by the specific adsorbate loading gradient across the membrane. A large absorbate loading of the selectively adsorbed component can be achieved at a relatively low feed pressure. 

The energy barrier for siuface diffusion is relatively low compared to transport through polymeric membranes, and therefore high flux can be achieved, eliminating the need for very thin membranes. 

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Give an insight into the mechanism of carbonization process and formation of carbon membranes that are unavailable by experimental techniques based on the molecular simulation... 

One of the possible problems in a steam reforming membrane reactor is the formation of carbon, either by cracking of methane (Reaction 4) or the Bou-douard reaction (Reaction 5). 

Although hydrogen is believed to suppress the formation of carbon deposits, coking may still occur in hydrogenation reactions as well. During ethylene hydrogenation, extensive coke deposits are noticed on the feed side of Pd-Y membranes and are believed to eventually lead to the embrittlement and rupture of the dense membranes due to carbon diffusion inside the membrane [Al-Shammary et al., 1991]. A modest deposit of carbon could actually increase the selective formation of ethane which may be indicative of some reaction taking place on the carbon deposit. 

Defect-free zeolite membranes have so far only been produced for membranes of the MFI (silicalite type) with thicknesses of about 50 im on stainless steel supports and 3-10 pm on alumina and carbon supports. They are produced by in situ methods of zeolite crystals grown directly on the support system. There are some reports of formation of defective membranes with, e.g., zeolite A. Much more research is needed to widen the range of available zeolite membrane types especially small and wide pore systems. The permeance values of the defect-free membranes is lower than that of the amorphous membranes (see Chapter 6) and to improve this the layer thickness must be decreased together with improving the crystal quality (no impurities, no surface layers, high crystallinity, crystal orientation) and microstructure (grain boundary engineering). 

Acetazolamide and dorzolamide inhibit CA on both the luminal membrane and in the PCT cell. Inhibition of C02 formation in the lumen decreases its intracellular availability. This, together with inhibition of formation of carbonic acid (CA is reversible), decreases intracellular bicarbonate and H+ levels. 

On the other hand, the oxidative coupling reaction of CH4 in the presence of O2, even when performed in membrane type reactors,188 is mainly catalysed by metal oxides catalysts.185 Also, oligomerisation, aromatisa-tion, and the partial oxidation apply non-metallic heterogeneous catalysts (such as zeolites). The reader is therefore directed to some excellent reviews on these subjects.189,190 At this point, it is perhaps relevant to introduce the formation of carbon nanofibres or nanotubes from methane, these being catalysed by metal nanoparticles, but at this moment this is not considered as a Cl chemistry reaction. Again we direct the attention of the reader to some reviews on this type of process.191 192... 

Carbon can block permeation and create porosity at higher temperatures, which is detrimental to both membrane stability and permselectivity [76-79], espedaUy in combination with oxygen [71]. Exposure to unsaturated hydrocarbons at elevated temperatures is particularly detrimental [77, 80]. The formation of carbon on both the membrane and catalyst is promoted in palladium membrane reactors because of the selective removal of hydrogen [81], which necessitates the study of membrane, reactant/product gas mixture, and spedahzed catalyst in concert [51, 82]. For example, a Pd75-Cu25 (aU compositions in this chapter are given in... 

A material for which the chemical stability in the presence of CO2 and SO2 has been extensively studied is BSCF, as it is one of the membrane materials with the highest flux. The presence of CO2 causes a reversible decline in oxygen flux. Even at concentrations of 500 ppm CO2 (ambient air), a slight decrease in the oxygen permeation rate is observed [34]. This decrease is caused by the formation of carbonates of alkaline earth metals on the exposed surface of the membrane. The carbonate formation of perovskite materials AB03 g in the presence of CO2 can be expressed as follows [72] ... 

Temperature has an effect on the formation of carbonates too. For example, in Figure 4.8, a low oxygen flux can be observed for a BSCF membrane at 900 °C. The full recovery of the membrane material by the reversed reaction is faster at higher operating temperatures. 

Since metallic nickel is a catalyst for the formation of carbon due to the decomposition of methane and the disproportionation of CO, these processes can result in catalyst deactivation and the clogging of the proton-exchange membrane of a fuel element by elementary carbon ... 

As discussed above, one of the main issues with AFC is that of electrolyte and electrode degradation caused by the formation of carbonates with carbon dioxide contamination in the oxidant stream. In order to avoid this problem, alkaline anion exchange solid membranes similar to the PEFC membrane are proposed (Varcoe and Slade, 2005). Recently, development of stable solid-state alkaline polymer electrolyte has been carried out. Most of these membranes contain either trimethylammonium or N-methyl pyridium groups. The trimethylarranonium groups have been foimd to be most stable in hot alkaline solutions. The membranes such as quaternary aimnonium polysulphone are found to be stable up to 120°C (Lu et al., 2008). 

Formation of nanooomposite membrane polymer-carbon nanotubes. 

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