Selective polymeric membrane carbon dioxide

This chapter reviews the recent developments of two types of facilitated transport membranes (1) supported liquid membranes (SLMs) with strip dispersion and (2) carbon-dioxide-selective polymeric membranes, for environmental, energy, and biochemical applications. 

Gas separation membrane technologies are extensively used in industry. Typical applications include carbon dioxide separation from various gas streams, production of oxygen enriched air, hydrogen recovery from a variety of refinery and petrochemical streams, olefin separation such as ethylene-ethane or propylene-propane mixtures. However, membrane separation methods often do not allow reaching needed levels of performance and selectivity. Polymeric membrane materials with relatively high selectivities used so far show generally low permeabilities, which is referred to as trade-off or upper bound relationship for specific gas pairs [1]. 

In Chapter 18, Matsuyama and Teramoto report the preparation of new types of cation-exchange membranes by grafting acrylic acid and methacrylic acid to substrates, such as microporous polyethylene, polytetrafluoroethylene, and poly[l-(trimethylsilyl)-l-propyne], by use of a plasma graft polymerization technique. Various monoprotonated amines are immobilized by electrostatic forces in the ion-exchange membranes and used as carriers for carbon dioxide. With these membrane systems carbon dioxide/nitrogen selectivities of greater than 4700 are obtained with high carbon dioxide flux. 

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. 

The only ceramic membranes of which results are published, are tubular microporous silica membranes provided by ECN (Petten, The Netherlands).[10] The membrane consists of several support layers of a- and y-alumina, and the selective top layer at the outer wall of the tube is made of amorphous silica (Figure 4.10).[24] The pore size lies between 0.5 and 0.8 nm. The membranes were used in homogeneous catalysis in supercritical carbon dioxide (see paragraph 4.6.1). No details about solvent and temperature influences are given but it is expected that these are less important than in the case of polymeric membranes. 

Mixed matrix membranes have been prepared from ABS and activated carbons. The membranes are intended for gas separation. A random agglomeration of the carbon particles was observed. A close interfacial contact between the polymeric and filler phases was observed. This morphology between inorganic and organic phases is believed to arise from the partial compatibility of the styrene/butadi-ene chains of the ABS copolymer and the activated carbon structure. A good permeability and selectivity for mixtures of carbon dioxide and methane has been reported (91,92). 

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. 

Glassy polymeric materials are often plasticized when used in gas membranes due to sorption. This can be overcome by annealing or crosshnking, however, this method does not influence the selectivity of the membrane, instead the permeability is decreased. Another method to stabilize the plasticization is to use polymer blends, as demonstrated with Matrimid 5218 and a copoly(imide) P84. The material is stabilized against carbon dioxide plasticization and the selectivity for a mixture of carbon dioxide and methane is improved. Hollow fiber membranes composed of blends of Pis with enhanced resistance towards hydrocarbons have been developed. ... 

Matsuyama, H., Hirai, K. and Teramoto, M. 1994. Selective permeation of carbon dioxide through plasma polymerized membrane from diisopropylamine. J Memh a. 3 257-265. 

Figure 11.2 SO2 permeability within polymeric membranes and selectivity relative to CO2. Reprinted with permission from Separation Purification Reviews, Effects of minor components in carbon dioxide capture using polymeric gas separation membranes, by Scholes, C. A., S. E. Kentish, and C. W. Stevens, 38 -44, Copyright (2009) Taylor and Erancis...

These stiff polymeric materials, used mainly for commercial carbon dioxide removal applications (polyimides, cellulose acetate, polysulfones) have their selectivity mainly based on molecular sieving effects. Despite their affinity for polar compounds (those membranes are also recommended by membrane... 

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