Configurations of Carbon Membranes

Table 3.1 summarizes the configurations of carbon membranes found in the ht-erature. It is noticed that most of the carbon membranes produced from the 1980s to early the 1990s are flat disk or flat sheet membranes. Only in the middle of the 1990s, carbon membranes supported on tubes were fabricated, followed by carbon capillary membranes and carbon hollow fiber membranes. Flat sheet carbon membranes are more suitable for laboratory or research applications while carbon membranes supported on tube, carbon capillary membranes and carbon hollow fiber membranes are more practical and suitable to apply in industry. 

Many membrane modules have been proposed by researchers. The most challenging task that must be considered during the fabrication of such modules is the poor mechanical stability of carbon membranes. This is more cracial in hollow fiber carbon membranes since they are self-supporting. More effort is now needed to develop a suitable module configuration and module design to accommodate carbon fibers. 

Abstract Membrane reactor research has been focused on new membrane materials to be integrated in a compact configuration. Carbon membranes have scarcely been explored in the past because of mechanical drawbacks. For this reason, it is recommended that carbon membranes are supported. However, this can cause the formation of defects which are disadvantageous in membrane reactor (MR) applications. This chapter explores the main variables to be considered in the development of carbon membranes, mainly focusing on when the carbon material has to be supported. Some applications are revised for macro and micro reactors. 

An alternative classification of carbon membranes is based on their configuration, which is described in Fig. 10.3. It is clear that carbon membranes can... 

Mediatorless amperometric measurement of gluconate, alcohol and fructose was demonstrated in the simple configuration of a carbon paste electrode when gluconate dehydrogenase, alcohol dehydrogenase and fructose dehydrogenase, respectively, were immobilized behind a membrane [6]. 

Therraodjmamics of phospholipids depend on their molecular structure the conformation and the geometry of the phospholipids polar head, the orientation of the molecular backbone (glycerol in the case of glycerophospholipids) and the acyl-chaiiis orientation. NMR spectroscopy has been used extensively in order to reveal the lipid orientation in membrane structures [20]. NMR experiments have shown that except for phospatidic acid, in all phospholipid crystal structures a part of the sn-2 fatty acyl chain is parallel to the bilaycr surface. In egg phosphatidylcholines for example the averege size of the sn-2 acyl chain is 18 carbons while the sn- fatty acid is 16 carbons long. The final configuration of the two fatty acyl chains extends to the same depth from the bilayer surface and a thermodynamicaly stable conformation of the acyl lipid chains is formed [21]. 

A cis configuration means that adjacent carbon atoms are on the same side of the double bond. The rigidity of the double bond freezes its conformation and, in the case of the cis isomer, causes the chain to bend and restricts the conformational freedom of the fatty acid. The more double bonds the chain has in the cis configuration, the less flexibility it has. When a chain has many cis bonds, it becomes quite curved in its most accessible conformations. For example, oleic acid has one double bond, and linoleic acid with two double bonds has a more pronounced bend. a-Linolenic acid, with three double bonds, favors a hooked shape. The effect of this is that in restricted environments, such as when fatty acids are part of a phospholipid in a lipid bilayer, or triglycerides in lipid droplets, cis bonds limit the ability of fatty acids to be closely packed, and therefore could affect the melting temperature of the membrane or of the fat. ... 

The increasing interest in inorganic membranes for gas applications is undoubtedly due to their excellent high temperature resistance. Inorganic membrane reactors (including carbon membranes) may thus have a very nice potential for industrial applications. The various configurations of membrane reactors will however not be discussed in the current chapter. Their separation properties may be understood on the basis of the materials used, kinetics, and process conditions. 

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