Facilitated transport membranes polymer

Goering RM, Bowman CN, Koval CA, and Noble RD. Mechanisms of olefin transport through facilitated transport membranes. Polym Mater Sci Eng 1997 77 260-261. 

Yet another approach to stabilizing facilitated transport membranes is to form multilayer structures in which the supported liquid-selective membrane is encapsulated between thin layers of very permeable but nonselective dense polymer layers. The coating layers must be very permeable to avoid reducing the gas flux through the membrane materials such as silicone rubber or poly(trimethylsilox-ane) are usually used [26],... 

The prospects for facilitated transport membranes for gas separation are better because these membranes offer clear potential economic and technical advantages for a number of important separation problems. Nevertheless, the technical problems that must be solved to develop these membranes to an industrial scale are daunting. Industrial processes require high-performance membranes able to operate reliably without replacement for at least one and preferably several years. No current facilitated transport membrane approaches this target, although some of the solid polymer electrolyte and bound-carrier membranes show promise. 

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. 

Facilitated transport membrane offers an attractive alternative to achieve high selectivity and high flux simultaneously.10,11 This type of membrane is based on the reversible reaction of the targeted gas with the reactive carrier contained in the membrane. There are two main types of reactive carriers the mobile carrier, which can move freely across the membrane, and the fixed carrier, which is covalently bonded on the polymer backbone and only has limited mobility. In mobile-carrier membranes, the carrier reacts with a targeted component on the feed side of a membrane, and the reaction product moves across the membrane and releases this component on the sweep side. As a result, the component being facilitated permeates through the membrane preferably, and the other components, which are not affected by facilitated transport, are retained on the retentate side. In fixed-carrier... 

Theory. The relationship of the chemical aspects of complexatlon reactions to the performance of facilitated transport membranes Is discussed by Koval and Reyes (108). They describe a procedure which can be used to predict and optimize the facilitated transport of gases, Including measurement of the appropriate equilibrium, transport, and kinetic parameters and structural modification of the carrier to Improve the performance of the membrane. Examples of this procedure and carrier modification are given for derivatives of Fe(II) tetralmlne complexes which reversibly bind CO In nitrile solvents (118). Experimental challenges In the measurement of the appropriate properties for other membrane configurations such as reactive Ion exchange membranes and reactive polymer membranes are also discussed. 

The maximum HaS facilitated flux value corresponds to an HaS permeability of 332 x 10" cm STP)cm/ cm s kPa). Matson (8) reported a HaS permeability range of 2250-3000 x 10 cm cm/(cm s kPa) for an ILM containing aqueous solution of Ka(X), for a temperature range of 363-HO3 K. The feed gas HaS partial pressure In Matson s studies was approximately 20 kPa with a total feed pressure of 2.17 x 10 kPa. Robb (22) reported an ambient temperature HaS permeability of 638 x 10 cm cm/(cm s kPa) for a silicone rubber membrane. However, polymer membranes such as silicone rubber have much lower selectlvltles than facilitated transport membranes. 

In facilitated transport membranes, the carriers can be dissolved in a liquid solvent or fixed to a solid polymer matrix. When the carriers are dissolved in a liquid medium, the carriers at the upstream interface react with a specific small molecule to form an adduct, which then moves across the membrane and releases the small molecule from the adduct due to a releasing reaction, as shown in Fig. 9-1 la. As such, the separation property depends on the binding and releasing reaction rates as well as the mobility of the adduct in the liquid medium. 

Facilitated transport membranes have been attracting attention since they have veiy high selectivity, compared with conventional polymer membranes (7). This high selectivity is attributable to carriers which can react reversibly with permeant specif There are two types of facilitated (carrier) transport membranes. One is the mobile carrier membrane in which the carrier can diffuse in the membrane, and the other is the fixed carrier membrane in ich the carrier cannot move. 

The chapters in this book by Langsam, Xu et aL, Hirayama et aL, Fritsch, and Maier et al focus on polymer structure modification to improve the performance of gas separation membranes relative to the upper bound tradeoff relations. Mahajan et al, describe characteristics of hybrid inorganic/organic membranes as a route to break the simple rules that result in equations 8 and 9, possibly resulting in materials with properties which are above and beyond the upper bound lines. Koval et al and Eriksen et al, describe facilitated transport membranes. They seek to strongly enhance solubility selectivity for penetrant pairs i,e, olefin/paraffin) where... 

To date, many kinds of CO2 separation membranes have been reported, including polymeric membranes, composite membranes, and facilitated transport membranes. Further improvements in membrane performance depend on effective CO2 separation materials, and one candidate is ILs. It has been reported that ILs have good CO2 selectivity, suggesting that they may be a possibility for the development of new CO2 separation materials. Since ILs are liquid at room temperature, it is necessary to affix ILs to appropriate support materials. Supported IL membranes have been prepared by impregnation of commercial porous polymer films with 1-n-hexyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([hmim][Tf2N]) and have obtained good C02/He separation properties [40]. Recently, electrospun Nafion/polyethylene oxide (PEO)-supported IL membranes were fabricated for CO2 separation [41]. In this composite membrane, the electrospun Nafion/PEO material acted as a gutter layer for ILs and PEO was added to form clean nanofibrous... 

Some of the available membrane separation processes can already be applied on an industrial scale. Hence, inorganic ceramic membranes (zeolites and their derivatives, e.g. silico aluminophosphates), organic polymer membranes and facilitated transport membranes, which rely on a carrier molecule with high CO2 affinity to achieve selective CO2 transport (such as metallic ions or liquid amines), have been used in separating CO2 from flue gas in post-combustion. As single-stage separation with these membranes is still difficult, new membrane materials are being developed [1]. Typically, the initial separation of carbon dioxide accounfs for 60-80% of the total cost of CO2 sequestration [24,25]. 

The discussion so far implies that membrane materials are organic polymers, and in fact most membranes used commercially are polymer-based. However, in recent years, interest in membranes made of less conventional materials has increased. Ceramic membranes, a special class of microporous membranes, are being used in ultrafiltration and microfiltration applications for which solvent resistance and thermal stability are required. Dense, metal membranes, particularly palladium membranes, are being considered for the separation of hydrogen from gas mixtures, and supported liquid films are being developed for carrier-facilitated transport processes. 

One promising approach to facilitated transport pioneered by Nishide and coworkers at Wasada University is to chemically bind the oxygen carrier to the polymer backbone, which is then used to form a dense polymer film containing no solvent [28], In some examples, the carrier species is covalently bonded to the polymer matrix as shown in Figure 11.29(a). In other cases, the polymer matrix contains base liquids which complex with the carrier molecule through the base group as shown in Figure 11.29(b). Because these films contain no liquid solvent, they are inherently more stable than liquid membranes and also could be formed into thin films of the selective material in composite membrane form. So far the selectivities and fluxes of these membranes have been moderate. 

H. Nishide, H. Kawakami, S.Y. Sasame, K. Ishiwata and E. Tsuchida, Facilitated Transport of Molecular Oxygen in Cobaltporphyrin/Poly(l-trimethylsilyl-l-propyne) Membrane, J. Polym. Sci., Part A Polym. Chem. 30, 77 (1992). 

Ho, W.S. and Dalrymple, D.C. (1994) Facilitated transport of olefins in Ag + containing polymer membranes./oumal of Membrane Science, 91, 13. 

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