SUPPORTS FOR FACILITATED TRANSPORT

Facilitated transport has been briefly described in Chapter 1. In facilitated transport, the selective transport medium is a liquid or molten salt contained or immobilized in a porous support. The liquid membrane is held tightly in the support pores by capillary forces. The liquid or molten salt selectively reacts with a gas or vapor species and the reacting species diffuses across the liquid or salt and desorbed on the other side of the facilitated transport membrane. The major advantage of the facilitated transpoa is that diffusion is generally several orders of magnitude faster than diffusion through solid membranes. The support is, therefore, not a membrane by definition. Comprehensive 

Other than isotopes separation for uranium enrichment described in Chapter 2, inorganic membranes are commercially used for particulate filtration of air or other gases in clean room applications, airborne contaminant analysis and high-purity hydrogen production. In addition, some inorganic membranes are us in pH and ion selective electrodes. 

Yet greater potentials of Inorganic membranes are in gas separation especially at high temperatures. This Held of promising applications relies on the inherent thermal and chemical stabilities of many inorganic membranes and the possibilities of designing the desirable pore sizes and pore surfaces. Material and application development efforts in this area have escalated in recent years with promising results. 

While dense inorganic membranes such as palladium-based or zirconia membranes provide extremely high-purity gases, their permeabilities are usually low, thus making the process economics unfavorable. Therefore, most of the recent investigations focus on porous inorganic membranes. 

In many studies the separation factor, which is indicative of the membrane s ability to separate two gases in a mixture, is predominantly governed by Knudsen diffusion. Knudsen diffusion is useful in gas separation mostly when two gases are significantly different in their molecular weights. In other cases, more effective uansport mechanisms are required. The pore size of the membrane needs to be smaller so that molecular sieving effects become operative. Some new membrane materials such as zeolites and other molecular sieve materials and membrane modifications by the sol-gel and chemical vapor deposition techniques are all in the horizon. Alternatively, it is desirable to tailor the gas-membrane interaction for promoting such transport mechanisms as surface diffusion or capillary condensation. 

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In designing the membranes, these points must be considered. However, there have been very few studies which focus on the development of new ion-exchange membranes as the supports for facilitated transport membranes in gas separations. In most of pervious studies, commercial ion-exchange membranes have been used. 

This study is the extension of our previous study. Here, a variety of microporous ion-exchange membranes was prepared by plasma grafting various ionic monomers, such as acrylic acid (AA), methacrylic acid (MAA) and 2-(N,N-dimethyl)aminoethyl methacrylate (DAMA) on several lands of substrate membranes such as microporous polyethylene (PE) membranes and polytetrafluoroethylene (PTFE) membranes. These membranes were evaluated as supports for facilitated transport of CO2 using several kinds of monoprotonated amines as carriers. N,N-dimethylaminoethyl methacrylate (DAMA) grafted membranes prepared by a similar technique were also evaluated as a fixed carrier membrane for the facilitated transport of CO2. 

Finally, some unconventional uses of inorganic membranes as gas sensors and as the immobilizing support matrices for facilitated transport are discussed. 

More recently, Teramoto et al. [62] performed experiments for facilitated transport of SO2 through a poly(vinilidene difluoride) (PVDF), used as supported membrane, containing pure water as a carrier. The permeance of SO2 at a partial pressure of 0.003 Pa was high as 1.04 X 10 barrer/cm and the best selectivity of SO2 over N2 was estimated to be more than 10000. 

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. 

The function of the electrolyte membrane is to facilitate transport of protons from anode to cathode and to serve as an effective barrier to reactant crossover. The electrodes host the electrochemical reactions within the catalyst layer and provide electronic conductivity, and pathways for reactant supply to the catalyst and removal of products from the catalyst [96], The GDL is a carbon paper of 0.2 0.5 mm thickness that provides rigidity and support to the membrane electrode assembly (MEA). It incorporates hydrophobic material that facilitates the product water drainage and prevents... 

Essentially, the eCTD is a transport format for facilitating electronic submissions. The eCTD serves as an interface for industry-to-agency transfer of regulatory information while at the same time, taking into consideration the facilitation of the creation, review, life cycle management, and archival of the electronic submission. The eCTD specification lists the criteria that will make an electronic submission technically valid. The eCTD represents a major advance in the submission of information to support an NDA. In the future, companies may be able to send their submissions to several regulatory authorities simultaneously with a single stroke of a computer key. 

In order to develop the liquid membrane techniques, i.e., emulsion Hquid membrane (ELM), supported liquid membrane (SLM), non-dispersive extraction in hollow fiber membrane (HFM), etc., for practical processes, it is necessary to generate data on equilibrium and kinetics of reactive extraction. Furthermore, a prior demonstration of the phenomena of facilitated transport in a simple liquid membrane system, the so-called bulk liquid membrane (BLM), is thought to be effective. Since discovery by Li [28], the liquid membrane technique has been extensively studied for the separation of metal ion, amino acid, and carboxyHc acid, etc., from dilute aqueous solutions [29]. 

The support has an internal pore structure (i.e., pore volume and pore size distribution) that facilitates transport of reactants (products) into (out of) the particle. Low pore volume and small pores limit the accessibility of the internal surface because of increased diffusion resistance. Diffusion of products outward also is decreased, and this may cause product degradation or catalyst fouling within the catalyst particle. As discussed in Sec. 7, the effectiveness factor Tj is the ratio of the actual reaction rate to the rate in the absence of any diffusion limitations. When the rate of reaction greatly exceeds the rate of diffusion, the effectiveness factor is low and the internal volume of the catalyst pellet is not utilized for catalysis. In such cases, expensive catalytic metals are best placed as a shell around the pellet. The rate of diffusion may be increased by optimizing the pore structure to provide larger pores (or macropores) that transport the reactants (products) into (out of) the pellet and smaller pores (micropores) that provide the internal surface area needed for effective catalyst dispersion. Micropores typically have volume-averaged diameters of 50 to... 

A novel polymeric bicontinuous microemulsion (PBM) membrane, consisting of an interconnecting network of nanometer pore size water channels, was employed as liquid membrane support [13] for the immobilization of new porphyrin carrier [14] for facilitated oxygen transport. Although the membrane resulted to be stable due to the nanoporous structure, a modest (2.3-2.4) O2/N2 selectivity was achieved. 

One of the first applications of TSOSs was reported in 2002. Davis and his team have shown that an amine-derived imidazolium salt can capture carbon dioxide by forming a ammonium carbamate [28], Primary amine functionalized imidazolium salts have also been used for facilitating C02 transport through a supported liquid membrane showing high selectivity and high stability for CH4/C02 separation [50] (Fig. 18). 

Some researchers have also used nitrocellulose filters as liquid membrane supports. Mochizuki and Forster (5) used this material as a support for hemoglobin solutions to study the facilitated transport of O2 and CO. Enns (6) used the same support for studying the facilitated transport of CO2 using aqueous solutions of carbonic anhydrase. Donaldson and Quinn (7) utilized both nitrocellulose filters and cross-linked protein membranes as supports to investigate CO2 facilitated transport using enzymatically active liquid membranes. 

N/m was applied across the membrane. The membrane was used at a temperature from 363 to 403 K. To prevent evaporative loss of the liquid from the membrane, the relative humidity of the gases adjacent to the membrane was controlled in the range of 60 to 90%. Kimura, Matson, and Ward (16) discussed the immobilization of bicarbonate solutions for CO2 and H2S facilitated transport. They indicated that a major problem was maintaining the integrity of the supported liquid membrane when large pressure differences were imposed across the membrane. 

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