Hollow fiber carbon molecular sieve

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). 

D.Q. Vu, W.J. Koros and S.J. Miller, High Pressure CO2/CH4 Separations Using Carbon Molecular Sieve Hollow Fiber Membranes, Ind. Eng. Chem. Res. 41, 367 (2002). 

In addition to the polymer and facilitated transport membranes, novel materials are being proposed and investigated to achieve membranes with economically attractive properties. Carbon molecular sieve (CMS) membranes prepared by pyrolysis of polyimides displayed much better performance for olefin/paraffin separation than the precursor membranes [39, 46, 47]. Results obtained with CMS membranes indicated properties well beyond the upper-bond trade-off curve, as shown in Figure 7.8. Nonetheless, this class of materials is very expensive to fabricate at the present time. An easy, reliable, and more economical way to form asymmetric CMS hollow fibers needs to be addressed from a practical viewpoint. 

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]. 

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. 

Ethylene has been separated from ethane by a silver nitrate solution passing countercurrent in a hollow fiber poly-sulfone.165 This separation has also been performed with the silver nitrate solution between two sheets of a polysilox-ane.166 A hydrated silver ion-exchanged Nafion film separated 1,5-hexadiene from 1-hexene with separation factors of 50-80.167 Polyethylene, graft-polymerized with acrylic acid, then converted to its silver salt, favored isobutylene over isobutane by a factor of 10. Olefins, such as ethylene, can be separated from paraffins by electroinduced facilitated transport using a Nafion membrane containing copper ions and platinum.168 A carbon molecular sieve made by pyrolysis of a polyimide, followed by enlargement of the pores with water at 400 C selected propylene over propane with an a-valve greater than 100 at 35°C.169... 

Xu LR, Rungta M, Koros WJ. Matrimid derived carbon molecular sieve hollow fiber membranes for ethylene/ethane separation. J Membr Sci 2011 380 138-147. 

Vu, D.Q., Koros, W.J. and Miller, S.J. (2002) High pressure CO2/CH4 separation using carbon molecular sieve hollow fiber membranes. Industrial Engineering Chemistry Research, 41 (3), 367-380. 

Xu, L., Rungta, M., Brayden, M.K. et al. (2012) Olefins-selective asymmetric carbon molecular sieve hollow fiber membranes for hybrid membrane-distillation processes for olefin/paraffin separations. Journal of Membrane Science, 423 24, 314—323. 

A suitable polymer material for preparation of carbon membranes should not cause pore holes or any defects after the carbonization. Up to now, various precursor materials such as polyimide, polyacrylonitrile (PAN), poly(phthalazinone ether sulfone ketone) and poly(phenylene oxide) have been used for the fabrication of carbon molecular sieve membranes. Likewise, aromatic polyimide and its derivatives have been extensively used as precursor for carbon membranes due to their rigid structure and high carbon yields. The membrane morphology of polyimide could be well maintained during the high temperature carbonization process. A commercially available and cheap polymeric material is cellulose acetate (CA, MW 100 000, DS = 2.45) this was also used as the precursor material for preparation of carbon membranes by He et al They reported that cellulose acetate can be easily dissolved in many solvents to form the dope solution for spinning the hollow fibers, and the hollow fiber carbon membranes prepared showed good separation performances. 

D. Q. Vu, W. J. Koros, S. J. Miller, Fouling of carbon molecular sieve hollow-fiber membranes by condensable impurities in carbon dioxide-methane separation, Ind. Eng. Chem., 42, 1064 (2003). 

Polyimide derived from a reaction of 2,4,6-trimethyl-l,3-phenylene diamine, 5,5-[2,2,2-trifluoro-l-(trifluoromethyl)ethylidene]-l,3-isobertzofitrandione and 3,3, 4,4 -biphenyl tetra carboxylic acid dianhydride was used by Jorres and Ko-ros to prepare carbon molecular sieve asymmetric hollow fiber membranes [37]. These membranes were developed and optimized for air separation apphcations. However, they were also effective for the separation of other gas mixtirres such as CO2/N2, CO2/CH4 and H2/CH4. The selectivities obtained were much higher than those foimd for corrverrtional polymeric materials without sacrificing productivity. 

Lagorsse S Magalhaes FD, Mendes A (2007) Xenon recycling in an anaesthetic closed-system using carbon molecular sieve membranes. J Membr Sci 301 (1-2) 29-38 Ismail AF, Li K (2008) From polymeric precursors to hollow fiber carbon and ceramic membranes. In Inorganic Membranes Synthesis, Characterization and Applications, Mal-lada R, Menendez M (Eds.) Membrane Science Technology Ser, Elsevier, Amsterdam, The Netherlands, Vol 13, Ch3, 81-119... 

Suda H, Haraya K (1997) Gas permeation through micropores of carbon molecular sieve membranes derived from kapton polyimide. J PhysChem B 101 (20) 3988-3994 TaniharaN, Shimazaki H, Hirayama Y, Nakanishi S, Yoshinaga T, Kusuki Y (1999) Gas permeation properties of asymmetric carbon hollow fiber membranes prepared from asymmetric hollow fiber. J Membr Sci 160 (2) 179-186... 

Soffer A, Azariah A, Amar A, Cohen H, Golub D, Saguee S (1997) Method of improving the selectivity of carbon membranes by chemical vapor deposition. US patent 5695618 Vu DQ, Koros WJ, Miller SJ (2002) High pressure CO /CH separation using carbon molecular sieve hollow fiber membranes. Ind Eng Chem Res 41 (3) 367-380 Ogawa M, Nakano Y (1999) Gas permeation through carbonized hollow fiber membranes prepared by gel modification of polyamic acid. J Membr Sci 162 (1-2) 189-198 Yamamoto M, Kusakabe K, Hayashi J, Morooka S (1997) Carbon molecular sieve membrane formed by oxidative carbonization of a copolyimide film coated on a porous support tube. J Membr Sci 133 (2) 195-205... 

Membranes with pores having pore diameters in the nanometer range ean be obtained by pyrolysis. Molecular sieves can be prepared by controlled pyrolysis of thermoset polymers [poly(vinylidene chloride), poly(furfuryl alcohol), cellulose, cellulose triacetate, polyacrylonitrile (PAN), and phenol formaldehyde] to obtain carbon membranes, or of silicone rubbers to obtain silica filters. For example, carbon molecular sieves can be obtained by pyrolysis of PAN hollow fibers in an inert atmosphere, which leads to dense membranes whose pores are opened by oxidation, initially at 400°-500°C and finished at 700°C [15]. These membranes are used to separate O2 /N2 mixtures. Le Carbone-Lorraine deposits a resin into a tubular macroporous substrate and then by pyrolysis creates a thin (< 1 pm) carbon active layer. Silicon rubber tubes can be pyrolyzed in an inert atmosphere at temperatures around 700°C followed by oxidation in air at temperatures from 500° to 900°C [16]. The membranes are composed almost completely of Si02 with pores having a maximum porosity of 50% and diameters fi om 5 to 10 nm. The permeabilities for He, H2, O2, and Ar range from 0.5 to 5 x 10 m s Pa. 

In 2000, Itoh and Haraya constructed the first CMR and experimentally examined the performance of a dehydrogenation reaction. Asymmetric polyimide hollow fibers were pyrolyzed in a vacuum oven at 1023 K in order to obtain hollow fiber carbon membranes. Their CMR consisted of SS in which 20 carbonized hollow fibers (0.295 mm diameter and 128 mm long) and catalyst pellets (0.5 wt% Pt/Al203) were allocated. The reactor, used for cyclohexane dehydrogenation to benzene at 468 K, showed a fair improvement over equilibrium conversions. In detail, the temperature dependency of the permeation rates showed that the carbon membrane had micropores with an average diameter close to those of the gas molecules and therefore the permeation process was molecular-sieving controlled. The ideal H2/Ar... 

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],... 

Molecular sieve membranes An ultrafine microporous membrane is formed from a dense, hollow-fiber polymeric membrane by carbonizing or from a glass hollow fiber by chemical leaching. Pores in the range 0.5-2 nm are claimed 45-48... 

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