Flat, carbon membranes

The flat carbon membranes were produced for gas separation from coal tar pitch by Liang et al. [15]. The result showed that the separation power of carbon membranes prepared from coal tar pitch was generally higher by at least three orders of magnitude compared with polymeric membranes. 

As an alternative to unsupported carbon membranes, the carbon separation layer can be supported on flat or tubular substrates to fabricate supported carbon membranes. In the case of supported membrane configuration, we have to consider the influence of the same experimental variables reported in symmetric unsupported flat carbon membranes (pyrolysis temperature. 

Later, the CMRs were also used in an attempt to carry out homogeneous catalytic reactions for example, hydration of propene. Lapkin et al prepared a carbon membrane from a macroporous phenohc resin and constructed a CMR for the hydration reaction. In this gas phase continuous catalytic membrane reactor, the flat carbon membrane was used as a contactor for carrying out reactions at high temperature and pressure. In particular, the hydration of propene, catalyzed by an aqueous solution of phosphoric acid, was selected as a suitable model reaction. Olefin and water were fed separately in order to have the additional benefit of an increased alcohol concentration in the product stream because of the absence of steam in the propene feed. 

The flat carbon membranes (diameter 31 mm thickness 2-4 mm), used for carrying out experiments at 403 K and 2 MPa, were allocated in an SS reactor. The experimental results of both CMR and a conventional SLP reactor were compared using the apparent rate of production (Rp in kgaicohoi /rn membrane pore voiume h) of propan-2-ol. Rp dcpcnds oni the rate of reaction in the liquid phase the rate of gas-liquid mass transfer the mass transfer within the membrane and the convective transport in the gas space above the membrane. Therefore, Rp does not directly correspond to the intrinsic reaction rate. The best experimental measure of "Rp" was referred to equilibrium. In particular, Rp was estimated as the percentage of alcohol concentration in the vapor phase divided by the concentration of alcohol at equilibrium. 

Singh-Ghosal, A. and W.J. Koros, Air separation properties of flat sheet homogeneous pyrolytic carbon membranes, /. Membr. Sci., 174,177,2000. 

With respect to carbon membranes, the molecular sieving carbon membranes, produced as unsupported flat, capillary tubes, or hollow fibers membranes, and supported membranes on a macropo-rous material are good in terms of separation properties as well as reasonable flux and stabilities, but are not yet commercially available at a sufficiently large scale, because of brittleness and cost among other drawbacks [3,6],... 

Until now, flat silica membranes were only tested at relatively low temperatures (up to 300°C) because of limitations in thermal stability of the polymer sealing rings [2], However, with the use of dense alumina rings together with carbon sealing it is possible now to measure membrane properties (e.g. permeance) at much higher temperatures (up to 600°C), which will be described below. 

While several niche applications for OD have been identified, the commercial acceptance of the technology has been hampered by the nonavailability of a suitable membrane-membrane module combination. Fluoropolymer membranes, such as PTFE and PVDF, have been shown to provide superior flux performance, but are still unavailable in hollow fiber form with a suitable thickness for use in OD applications. The inherently low flux of OD requires fhaf membranepacking density be maximized for effective operation, and hence the available flat-sheet form of perfluoro-carbon membranes is unsuitable for commercial use. Four-port hollow fiber modules that provide excellent fluid dynamics are currently available, but only low-flux polypropylene membranes are utilized. 

Fig. 11. Comparison between experimental and simulated (dashed lines) dm t)/dt growth rate of a CdSe film deposited at 45 °C from an amraonia-TEA-selenosulfite bath, without STA (curve a), in the presence of 10" M STA (curve b). The inset presents a TEM image of a CdSe deposit on a carbon membrane before coalescence showing the formation of flat platelets (from [80]). With permission of Elsevier...

Hollow fibre and flat sheet membranes with an interfacially polymerised coating of PA have a permeance for water vapour of about 0.16 m/s [13]. These membranes can serve as a basis for building ventilation, which provide fresh air while recovering about 70% of the specific heat and 60% of the latent heat. Because these membranes are selective for water vapour, the air is exhausted with internal pollutants such as carbon monoxide, formaldehyde and radon. The expense of the ventilator should take about three years to be recovered by reduced heating costs. 

The preparation method of flat supported carbon molecular sieve membranes has been investigated by using different polymeric materials by Fuertes and Centeno. They used 3,3 4,4 -biphenyltetracaiboxyhc dianhydride (BPDA)—4,4 -pheitylene diamine (pPDA) [1, 16], phenolic resin [17] as precursor to make flat CMSMs supported on a macroporous carbon substrate. In a later study, they ehose poly-etherimide (PEI) as a precursor to prepare flat supported CMSMs [18]. PEI was chosen because it was one of PI based materials which can be used economically. On the other hand, these PEI carbon membranes showed performance similar to the CMSMs prepared by Hayashi et al. [19], which was obtained from a laboratory-synthesized PI (BPDA-ODA). 

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. 

As a conclusion, during the preparation of precursor membranes, the process parameters involved must be optimized to form a good precursor for pyrolysis. Moreover, the choices of supported and unsupported carbon membranes depend on the application for which the carbon membranes will be used. Normally, most of today s gas separation membranes are formed into hollow fiber modules [73]. Hollow fiber membranes offer a greater area per specific module volume than flat film membranes, by a factor of 1-40 [31]. 

Chapter 3 is for the carbon membrane configuration. Cunentiy, all flat sheet, tubular, capillary and hollow fiber carbon membranes are available at least for laboratory scale experiments. Their merits and demerits are discussed in this chapter. 

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