Molecular sieves carbon membranes

Fuertes, A.B., Centeno, T.A. (1999) Preparation of supported carbon molecular sieve membranes, Carbon 37(4), 679-684. 

A.B. Fuertes, T.A. Centeno, Preparation of supported carbon molecular sieves membranes, Carbon, 1999,37,679-684. 

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. 

Molecular sieve dryers, 10 613 Molecular-sieve effects, 16 821 Molecular sieve membranes, 15 813t Molecular sieve products commercial, 16 838-839t manufacturing processes for, 16 831 Molecular sieves, 16 811-853. See also Carbon molecular sieves Zeolite entries... 

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). 

Activated charcoal was originally regarded as a relatively inexpensive adsorbent with an assortment of pores of ill-defined size and shape. However, in recent years considerable progress has been made in the development of tailor-made porous carbons such as molecular sieves, activated carbon fibres and carbon composites (Marsh et al., 1997). Superactive carbons are now made on a commercial scale with BET areas of around 3000 m2g-1. Activated carbons can be manufactured as fine particles or granules or in the form of a cloth, felt or consolidated membrane. The properties of some of these special types of activated carbon are discussed in Chapter 12. 

To obtain carbon membranes with molecular sieving properties, pore diameters in the range of a few angstroms are required. The associated pyrolysis procedures and post treatments are more involved. This subject will be treated later under section 3.2.10 Molecular Sieving Membranes. 

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. 

Partially pvrolvzed poivsilastyrene membranes. Molecular sieve membranes can also be made from porous solids other than carbon and zeolitic materials. One such potential candidate is the family of precursors of silicon carbide. Among the possible precursors, polysilastyrene (phenylmeihylsilane-dimethylsilane copolymer) is commercially available and soluble in many common solvents such as toluene and tetrahydrofuran and can be crosslinked by UV radiation. Polysilastyrene is comprised of long chains of silicon atoms ... 

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

Other molecular sieve membranes are prime candidates as well. Molecular sieve carbon membranes exhibits very high separation factors in the laboratory. Their microstnictuies can be tailored by adjusting the synthesis and calcination conditions however, the issues of their mechanical, chemical and thermal stabilities under various potential application environments have not been addressed. 

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. 

---