Carbon molecular sieve membranes polymeric membrane

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. ... 

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. 

The carbon-based membranes show superior permeability-selectivity combination to polymeric ones and are categorized in three classes carbon membranes, carbon molecular sieve membranes and carbon nanotubes. ... 

Chen YD, Yang RT (1994) Preparation of carbon molecular sieve membrane and diffusion of binary mixtures in the membrane. Ind Eng Chem Res 33 (12) 3146-3153 Moaddeb M, Koros WJ (1997) Gas transport properties of thin polymeric membranes in the presence of silicon dioxide particles. J Membr Sci 125 (1) 143-163 Ash R, Barrer RM, Lowson RT (1973) Transport of single gases and of binary gas mixtures in a microporous carbon membrane. J Chem Soc Faraday Trans I 69 (12) 2166-2178 Bird AJ, Trimm DL (1983) Carbon molecular sieves used in gas separation membranes. Carbon 21 (3) 177-163... 

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). 

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... 

Table 2.8. Permeability, Selectivity and Separation Characteristics of Various Polymeric and Molecular Sieve Carbon (MSC) Membranes (Koresh and Soffer 1983) ...

The nanoporous carbon membrane consists of a thin layer (<10pm) of a nanoporous (3-7 A) carbon film supported on a meso-macroporous solid such as alumina or a carbonized polymeric structure. They are produced by judicious pyrolysis of polymeric films. Two types of membranes can be produced. A molecular sieve carbon (MSC) membrane contains pores (3-5 A diameters), which permits the smaller molecules of a gas mixture to enter the pores at the high-pressure side. These molecules adsorb on the pore walls and then they diffuse to the low-pressure side of the membrane where they desorb to the gas phase. Thus, separation is primarily based on differences in the size of the feed gas molecules. Table 7 gives a few examples of separation performance of MSC membranes. ° Component 1 is the smaller component of the feed gas mixture. 

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... 

In contrast to ordinary membranes, mixed matrix membranes are composed of an organic polymer and therein embedded inorganic particles such as zeolites, carbon molecular sieves, or nanoparticles. Mixed matrix membranes are believed to achieve higher performance than conventional polymeric membranes. In addition, the poor mechanical properties of inorganic membranes can be improved by embedding them in a flexible polymeric matrix. ... 

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... 

The bulk phase (phase A) is typically a polymer the dispersed phase (phase B) represents the inorganic particles, which maybe zeolite, carbon molecular sieves, or nanosized particles. Thus, MMMs have the potential to achieve higher selectivity, permeability, or both, relative to the existing polymeric membranes, resulting from the addition of the inorganic particles with their superior inherent separation characteristics. 

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. 

It has been observed that membranes of carbon molecular sieves can exceed the upper bound of conventional polymeric membranes. The carbon molecular sieve membranes are produced by carbonization of aromatic polymers (e.g., polyimides), yielding pore dimensions in the range of O2 and N2 molecular dimensions. Polyimide/poly(vinyl pyrrolidone) blends subjected to carbonization conditions also yielded carbon molecular sieve membranes that exceeded the upper bound limit for conventional polymeric membranes [195, 196]. Specific values were O2 permeability of 560-810 barrers with a O2/N2 separation factors of 10-7 well above the upper bound. 

Adsorption systems employing molecular sieves are available for feed gases having low acid gas concentrations. Another option is based on the use of polymeric, semipermeable membranes which rely on the higher solubiHties and diffusion rates of carbon dioxide and hydrogen sulfide in the polymeric material relative to methane for membrane selectivity and separation of the various constituents. Membrane units have been designed that are effective at small and medium flow rates for the bulk removal of carbon dioxide. 

Membranes with extremely small pores ( < 2.5 nm diameter) can be made by pyrolysis of polymeric precursors or by modification methods listed above. Molecular sieve carbon or silica membranes with pore diameters of 1 nm have been made by controlled pyrolysis of certain thermoset polymers (e.g. Koresh, Jacob and Soffer 1983) or silicone rubbers (Lee and Khang 1986), respectively. There is, however, very little information in the published literature. Molecular sieve dimensions can also be obtained by modifying the pore system of an already formed membrane structure. It has been claimed that zeolitic membranes can be prepared by reaction of alumina membranes with silica and alkali followed by hydrothermal treatment (Suzuki 1987). Very small pores are also obtained by hydrolysis of organometallic silicium compounds in alumina membranes followed by heat treatment (Uhlhom, Keizer and Burggraaf 1989). Finally, oxides or metals can be precipitated or adsorbed from solutions or by gas phase deposition within the pores of an already formed membrane to modify the chemical nature of the membrane or to decrease the effective pore size. In the last case a high concentration of the precipitated material in the pore system is necessary. The above-mentioned methods have been reported very recently (1987-1989) and the results are not yet substantiated very well. 

Purification with PSA and Polymeric Membranes. The PSA process is based on the selective adsorption of gaseous compounds on a fixed bed of solid adsorbent in a series of identical adsorption beds. The adsorbent is an active carbon or a carbon-molecular sieve. Each bed undergoes a... 

Robeson LM. Correlation of separation factor versus permeability for polymeric membranes. J. Membr. Sci. 1991 62 165-185. Koresh JE and Soffer A. The carbon molecular sieve membranes. General properties and the permeability of CH4/H2 mixture. Sep. Sci. Tech. 1987 22 973-982. 

This cost differential can be tolerated only in applications in which polymeric membranes completely fail in the separation [78]. Demanding separation applications, where zeolite membranes could be justified, due to the high temperatures involved or the added value of the components, and have been tested at laboratory scale, are the following separation of isomers (i.e., butane isomers, xylene isomers), organic vapor separations, carbon dioxide from methane, LNG (liquefied natural gas) removal, olefines/paraffins and H2 from mixtures. In most cases, the separation is based on selective diffusion, selective adsorption, pore-blocking effects, molecular sieving, or combinations thereof. The performance or efficiency of a membrane in a mixture is determined by two parameters the separation selectivity and the permeation flux through the membrane. 

RO membranes including polymeric (PA-TFC membrane) and molecular sieve zeolite membranes were investigated for ion removal from the water produced at oil field and coal bed methane sites by a cross-flow RO process [78]. Pretreatments including NF and adsorption by active carbon were implemented. The study revealed that (1) most of permeation tests lasted only 3 months due to severe fouling, (2) multistage pretreatment is crucial to extend membrane life, and (3) only NF treatment could extend the membrane life to 6 months. 

To surpass Robeson s upper bound, materials are emerging that rely on transport mechanisms other than solution-diffusion through glassy or rubbery polymeric materials. In particular, a number of materials have been developed that possess fixed microporosity (2 nm or less) in contrast to the activated, transient molecular gaps that give rise to diffusion in most polymers. These materials include amorphous and crystalline (zeolite) ceramics [68-69], molecular sieve carbons [70], polymers that possess intrinsic microporosity [71-72], and carbon nanotube membranes [73-76]. Transport in such materials is determined primarily by the average size and size distribution of the microporosity - the porosity can be tuned to allow discrimination between species that differ by less than one Angstrom in size. However, surface... 

Molecular sieves such as zeolites or carbon molecular sieves show a much higher selectivity for many gas mixtures than polymeric membranes due to their very defined pore sizes. For example it can be calculated from reported sorption and diffusivity data that zeolite 4A has an oxygen permeability of 0.77 Barter and an O2/N2 selectivity of approximately 37 at 35 °C [308]. 

---