Sieve membranes

Pores Even porous membranes can give very high selectivity. Molecular sieve membranes exist that give excellent separation factors for gases. Their commercial scale preparation is a formidable obstacle. At the other extreme, UF,3 separations use Knudsen flow barriers, with aveiy low separation factor. Microfiltration (MF) and iiltrafiltra-tion (UF) membranes are clearly porous, their pores ranging in size from 3 nm to 3 [Lm. Nanofiltration (NF) meiTibranes have smaller pores. 

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. 

Molecular sieves, membrane permeation Molecular distillation Gaseous diffusion, thermal diffusion Filtration, sieves... 

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... 

If the soluble protein that specifically adsorbs to the fiber can be extrinsi-cally labeled, the background problem can be avoided. Of course, in vivo proteins cannot be labeled. However, it is conceivable that a protein labeled with a bulky extrinsic group (e.g., fluorescent dextrans) could be confined by a molecular sieve membrane (e.g., a dialysis membrane) within a closed volume surrounding the specifically derivatized optical fiber. When exposed to the (unlabeled) protein in the biological fluid under investigation, the membrane-clad fiber would allow some unlabeled protein to permeate in and... 

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)...

Decomposition of RuO, to RuOj and Oj Porous Fe203/molecular sieve membranes Peng, Wang and Zhou (1983)... 

Some small-pore zeolite and molecular sieve membranes, such as zeolite T (0.41 nm pore diameter), DDR (0.36 x 0.44nm) and SAPO-34 (0.38nm), have been prepared recenhy [15-21]. These membranes possess pores that are similar in size to CH4 but larger than CO2 and have high CO2/CH4 selechvihes due to a molecular sieving mechanism. For example, a DDR-type zeolite membrane shows much higher CO2 permeability and CO2/CH4 selechvity compared to polymer membranes [15-17]. SAPO-34 molecular sieve membranes show improved selechvity for separation of certain gas mixtures, including mixtures of CO2 and CH4 [18-21]. 

Weh, K., Noack, M., Sieber, L, and Caro, J. (2002) Permeation of single gases and gas mixtures through faujasite-type molecular sieve membranes. Micropor. Mesopor. Mater., 54, 27-36. 

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

The subscript 0 indicates that this flux is for solvent A without the presence of rejected solute B. This model is appropriate for membranes that have straight pores however, such membranes are not typical of most industrial membranes. Not only are the rate limiting pores in the actual working skin layer tortuous, but a complex porous support layer is generally present that supports the working skin layer. In well-made sieving membranes, the porous support is ideally invisible to the transport process, as is illustrated in Fig. 2. The support, therefore, simply serves as a scaffold for the ideal selective layer. Formation of such structures requires some care, but technology exists to achieve this requirement as is described elsewhere (Koros and Pinnau, 1994). 

The improvements that have been made in the preparation of molecular sieving silica membranes started with the development of high quality membrane supports, because quality of the supporting system is of crucial importance for the quality of the final molecular sieving membrane. To this end, the synthesis of the supports was performed by means of colloidal proc-... 

The remaining gas mixture now has the composition nitrogen 3.7 mol%, oxygen 1.0 mol%, methane 47.9 mol%, C02 47.4 mol%. The third heuristic in Table 3.1 applies try to match the products. The appropriate selector is sharp split . Table 3.8 present lists of characteristic properties. Potential methods are absorption, cryogenic distillation, molecular sieving, membranes and equilibrium adsorption. 

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

For the unique properties of PBs to be exploited, PBs must be deposited properly onto a solid support. It is highly desirable to prepare mechanically robust PBs films with controlled thickness, chemical composition and crystallinity, having ion-sieving membranes and electrochromic devices in mind [6], or to create regular patterns of PB-based single molecule magnets [13],... 

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. 

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