Gas separation materials

Mahajan, R. and Koros, W.j. (2000) Factors controlling successful formation of mixed-matrix gas separation materials. Ind. Eng. Chem. Res., 39, 2692-2696. 

Materials with selective binding or transport properties will have a major impact on sensor design and fabrication. Selectivity in either binding or transport can be exploited for a variety of measurement needs. This selectivity can be either intrinsic, that is, built into the chemical properties of the material, or coupled with selective carriers that allow a non-selective material to be converted into a selective one (see the section on recognition chemistry). An example of the latter is the use of valinomycin as a selective carrier in a polyvinyl chloride membrane to form a potentiometric potassium ion sensor. Advances in the fields of gas separation materials for air purification and membrane development for desalinization are contemporary examples illustrating the importance of selective materials. As these materials are identified, they can be exploited for the design of selective measurement schemes. 

M. Jagtoyen, F. Derbyshire, N. Brubaker. Y. Fei, G. Kimber, M. Matheny, and T. Burchell, Carbon fiber composite molecular sieves for gas separation. Materials Research Society Symposium Proceedings, 1994, pp. 344. 77-81. 

Gas chromatography (GC) is the most common and successful method of soil-gas analysis. The detection limits are about 1-10 ppb by volume. The basic components of a gas chromatographic system are a carrier gas and a flow control system, a column packed with a gas-separating material, an oven for temperature control of the column, a sample introduction device, a detector and a recording system (Fig. 8-8). 

Type III. Hyperbranched polymers have numerous branch units. They have low viscosity, good solubility and are capable of being chemically modified in terminal functional groups. Hyperbranched polymers have a potential to be good gas separation materials because their molecular-sized spaces between branched polymers can be controlled. 

Sears, K., Dumee, L., Schutz, J., She, M., Huynh, C., Hawkins, S., Duke, M., Gray, S., 2010, Recent developments in carbon nanotube membranes for water purification and gas separation. Materials 3 127-149. 

The advantages of PPO as a gas separation material has therefore been well appreciated. The search for new and improved membranes with highly efficient performance characteristics has however led many researchers to chemically modify PPO. Most chemical modifications of PPO reviewed in this chapter are for improvement of CO2/CH4 or O2/N2 permselectivities. Modifications have been made to improve the solubility selectivity of PPO without sacrificing its high intrinsic permeability. Many researchers have also tried to improve the solubility of PPO in polar solvents by structure tailoring the polymer. It may be noted that PPO can be easily modified via chemical reactions. The following section will describe the advances that have been made in the past few decades in the use of modified PPO as membrane materials for the separation of gases. 

Nonporous Dense Membranes. Nonporous, dense membranes consist of a dense film through which permeants are transported by diffusion under the driving force of a pressure, concentration, or electrical potential gradient. The separation of various components of a solution is related directiy to their relative transport rate within the membrane, which is determined by their diffusivity and solubiUty ia the membrane material. An important property of nonporous, dense membranes is that even permeants of similar size may be separated when their concentration ia the membrane material (ie, their solubiUty) differs significantly. Most gas separation, pervaporation, and reverse osmosis membranes use dense membranes to perform the separation. However, these membranes usually have an asymmetric stmcture to improve the flux. 

Pervaporation operates under constraints similar to low pressure gas-separation. Pressure drops on the permeate side of the membrane must be small, and many prevaporation membrane materials are mbbery. For this reason, spiral-wound modules and plate-and-frame systems ate both in use. 

Distillation appHcations can be characterized by the type of materials separated, such as petroleum appHcations, gas separations, electrolyte separations, etc. These appHcations have specific characteristics in terms of the way or the correlations by which the physical properties are deterrnined or estimated the special configurations of the process equipment such as having side strippers, multiple product withdrawals, and internal pump arounds the presence of reactions or two Hquid phases etc. Various distillation programs can model these special characteristics of the appHcations to varying degrees and with more or less accuracy and efficiency. 

Drying is an operation in which volatile Hquids are separated by vaporization from soHds, slurries, and solutions to yield soHd products. In dehydration, vegetable and animal materials are dried to less than their natural moisture contents, or water of crystallization is removed from hydrates. In freeze drying (lyophilization), wet material is cooled to freeze the Hquid vaporization occurs by sublimation. Gas drying is the separation of condensable vapors from noncondensable gases by cooling, adsorption (qv), or absorption (qv) (see also Adsorption, gas separation). Evaporation (qv) differs from drying in that feed and product are both pumpable fluids. 

Jagtoyen, M. and Derbyshire, F., Carbon fiber composite molecular sieves for gas separation. In Proc. Tenth Annual Con/, on Fossil Energy Materials, CONF-9605167, ORNL/FMP-96/I. Oak Ridge National Laboratory, 1996, pp. 291 300. 

Ionic liquids have already been demonstrated to be effective membrane materials for gas separation when supported within a porous polymer support. However, supported ionic liquid membranes offer another versatile approach by which to perform two-phase catalysis. This technology combines some of the advantages of the ionic liquid as a catalyst solvent with the ruggedness of the ionic liquid-polymer gels. Transition metal complexes based on palladium or rhodium have been incorporated into gas-permeable polymer gels composed of [BMIM][PFg] and poly(vinyli-dene fluoride)-hexafluoropropylene copolymer and have been used to investigate the hydrogenation of propene [21]. 

Decomposition following thermal treatment of the separated material yielding solid oxides and gaseous components (solid-gas interaction). 

There are a number of industrial gas separation systems that use the selective permeability of plastics to separate the constituents. In design problems relating to such applications, the designer must consider the environmental conditions to determine whether the materials having the desired properties will withstand the temperatures and physical and chemical stresses of the application. Frequently the application will call for elevated temperatures and pressures. In the case of uranium separation, the extreme corrosivity of the fluorine compounds precluded the use of any material but PTFE. The PTFE... 

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