Carbon molecular sieving membranes

The most serious disadvantages that have to be overcome or controlled are brittleness and the vulnerability of membranes to oxidizing agents and water vapor resulting in performance loss over time, hence regeneration is needed at intervals. When these effects are detected in the membrane performance, it is important to know how to address the problem (or preferably how to avoid it). It is important to know how to open pores which are blocked, how to regenerate the membrane, or how to optimize 

The CMS membranes may be prepared in two different ways, and in both cases the pore tailoring is the focus for the final membrane  

By careful control of carbonization conditions this is done by controlling heating rate, heating temperature, and choice of inert gas or vacuum during the process [76]. 

As explained in the patent of Soffer et al. [ 10], where microporous cellulose fibers are treated with CVD, and pores are tailored by postoxidation. 

CMS membranes may also easily be functionalized, i.e., metals (like AgNOs, MgO, Fe203 or others) are imbedded in the structure of the precursor, and will enhance the separation for certain gas pairs. 

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Advanced Materials Experimental membranes have shown remarkable separations between gas pairs such as O9/N9 whose kinetic dian ieters (see Table 22-23) are quite close. Most prominent is the carbon molecular sieve membrane, which operates by ultran iicro-porous molecular sieving (see Fig. 22-48c). Preparation of large-scale permeators based on ultran iicroporous membranes has proven to be a major challenge. 

Carbon molecular sieve membranes Resistant to contaminants Intermediate hydrogen flux and selectivity Intermediate hydrogen flux and selectivity High water permeability Pilot-scale testing in low temperature WGS membrane reactor application Need demonstration of long-term stability and durability in practical applications... 

Liu, P.K.T., Carbon Molecular Sieve Membrane as Reactor for Water Gas Shift Reaction, Proceedings of 2006 U.S. DOE Hydrogen Annual Merit Review Meeting, Arlington, VA, May 2006. 

Suda, H. and Haraya, Alkene/Alkane permselectivities of a carbon molecular sieve membrane, /. Chem. Soc. Chem. Commun., 93, 1997. 

Koresh, J. E. and A. Sofler. 1987. The carbon molecular sieve membranes. General properties and the permeability of CHJHj mixtures. Separation Scietice and Technology 22(2 3) 972-82. 

Molecular sieves are porous aluminosilicates (zeolites) or carbon solids that contain pores of molecular dimensions which can exhibit seleaivity according to the size of the gas molecule. The most extensive study on carbon molecular sieve membranes is the one by Koresh and Soffer (1980,1987). Bird and Trimm (1983) also described the performance of carbon molecular sieve membranes, but they were unable to prepare a continuous membrane. Koresh and Soffer (1980) prepared hollow-fiber carbon molecular sieves, with pores dimensions between 0.3 and 2.0 run radius (see Chapter 2). 

Table 6.2. Permeability Data of a Carbon Molecular Sieve Membrane at 950 C for Several Modes of Activation (Pore Opening with Oxygen)...

H. Kita, H. Maeda, K. Tanaka and K. Okamoto, Carbon Molecular Sieve Membranes Prepared from Phenolic Resin, Chem. Lett. 179 (1997). 

Centeno, T.A., Fuertes, A.B. (1999) Supported carbon molecular sieve membranes based on phenolic resin. J. Membr. Sci. 160(2), 201-211. 

Carbon molecular sieve membranes. Molecular sieve carbons can be produced by controlled pyrolysis of selected polymers as mentioned in 3.2.7 Pyrolysis. Carbon molecular sieves with a mean pore diameter from 025 to 1 nm are known to have high separation selectivities for molecules differing by as little as 0.02 nm in critical dimensions. Besides the separation properties, these amorphous materials with more or less regular pore structures may also provide catalytic properties. Carbon molecular sieve membranes in sheet and hollow fiber (with a fiber outer diameter of 5 pm to 1 mm) forms can be derived from cellulose and its derivatives, certain acrylics, peach-tar mesophase or certain thermosetting polymers such as phenolic resins and oxidized polyacrylonitrile by pyrolysis in an inert atmosphere [Koresh and Soffer, 1983 Soffer et al., 1987 Murphy, 1988]. 

Carbon molecular sieve membranes have been prepared on porous supports by controlled pyrolysis. For example, Chen and Yang [1994] prepar carbon molecular sieve membranes on porous graphite supports by coating a layer of polyfurfuryl alcohol followed by conu-olled pyrolysis with a Hnal temperature of 50O C. The procedure can be repeated to deposit a desired thickness of the carbon membrane. The choice of a graphite support is partially based on the consideration of the compatibility in thermal expansion between the carbon and the support. 

Permeability and permselectivity of oxygen, nitrogen and sulfur hexafluoride through modified carbon molecular sieve membranes... 

Y.D. Chen and R.T. Yang, Preparation of carbon molecular sieve membrane and diffusion of binary mixtures in the membrane, Ind. Eng. Chem. Res. 55 3146 (1994). 

The hollow fiber membranes are the optimum choice for gas separation modules due to their very high packing density (up to 30,000 m /m may be attained [1]). Figure 4.21 shows alternative configurations for such modules [108]. Modifications of this configuration exist, where possibility for introduction of sweep gas on permeate side is included, or fibers may be arranged transversal to the flow in order to minimize concentration polarization [109,110]. The hollow fiber membranes are usually asymmetric polymers, but composites also exist. Carbon molecular sieve membranes may easily be prepared as hollow fibers by pyrolysis. 

Hagg MB, Lie JA, and Lindbrathen A. Carbon molecular sieve membranes—a promising alternative for selected industrial appUcations. In Li NN, Drioli E, Ho WSW, and Lipscomb GG, eds. Annals of the New York Academy of Sciences vol. 984 Advanced Membrane Technology. New York The New York Academy of Sciences, 2003, pp. 329-345. 

J.E. Koresh and A. Soffer, The carbon molecular-sieve membranes, general properties and the permeability of CH4/H2 mixture. Sep. Sci. Technol, 22 (1987) 972. 

H. Suda, K. Haraya. Gas transport properties of carbon molecular sieve membranes doped with nano particles in Proceedings of the 8th International Conference on Inorganic Membranes, Jul. 18-22, 2004, Gincinnati, OH, Adams Press, Chicago, IL. 

Carbon molecular sieve membranes are used in gas separation technology, for example, to recover CO2 and H2O from natural gas, and other purification steps. A variety of polymeric precursors for carbon molecular sieve membranes are available, such as poly(imide), poly(acrylonitrile) phenolic resins, and poly(fiuduryl alcohol). PPE can be modified in various ways, which procedure is advantageous for tailoring the selectivity. ... 

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