Asymmetric membranes permeation properties

N. Tanihara, H. Shimazaki, Y. Hirayama, N. Nakanishi, T. Yoshinaga and Y. Kusuki, Gas Permeation Properties of Asymmetric Carbon Hollow Fiber Membranes Prepared from Asymmetric Polymer Hollow Fibers, 7. Membr. Sci. 160, 179 (1999). 

We report here on the structure and gas transport properties of asymmetric membranes created by the Langmuir-Blodgett deposition of ultra-thin polymeric lipid films on porous supports. Transmission and grazing angle FTIR spectroscopy provide a measure of the level of molecular order in the n-alkyl side-chains of the polymeric lipid. The level of orientational order was monitored as a function of the temperature. Gas permeation studies as a function of membrane temperature are correlated to the FTIR results. 

We report here on the structure and gas transport properties of asymmetric membranes produced by the LB deposition of a polymeric lipid on porous supports. The effects of temperature on the structure and gas transport is described. The selectivity of CO2 over N2 permeation through the LB polymer films is determined. The polymerized lipid used in this study contains tertiary amines which may influence the CO2 selectivity over N2. The long term objective of our work is to understand how structure and chemistry of ultrathin films influence the gas permeation. 

If a membrane has a graded pore structure but is made in one processing step, frequently from the same material across its thickness, it is called an asymmetric membrane. If, on the other hand, the membrane has two or more distinctively different layers made at different steps, the resulting structure is called a composite membrane. Almost invariably in the case of a composite membrane, a predominantly thick layer provides the necessary mechanical strength to other layers and the flow paths for the permeate and is called the support layer or bulk support. Composite membranes have the advantage that the separating layer and the support layer(s) can be tailored made with different materials. Permselective and permeation properties of the membrane material are critically important while the material for the support layer(s) is chosen for mechanical strength and other consideration such as chemical inertness. The composite membranes can have... 

The flux of 0.03 gfd for the homogeneous polyamide membrane was more than two orders of magnitude too low for commercial desalination. The flux was increased 175 fold with no decrease in salt rejection by casting the membrane with asynmetric morphology. Even higher fluxes, up to 3.5 times that observed for the asymmetric MPD-l/T (100-70/30) polyamide membrane, were obtained with asymmetric membranes cast from polyhydrazides and polyamide-hydrazides. Permeation properties for the three types of aromatic polyamides are shown in Table IX. The RO properties of this group of membranes illustrate the combined effects of Structure Levels I, II and III on membrane performance. 

Table IX. Permeation Properties of Asymmetric Polyamide Membranes...

Cellulosic Membranes. The first asymmetric membrane for gas separation appeared in 1970 (Table II), and It was not surprising that this membrane was a modified CA membrane of the Loeb-Sourirajan type (17). Gelled CA membranes for water desalination must be stored wet In order to maintain their permeation performance. However, In gas permeation, wet, plasticized membranes tend to lose their properties with time due to plastic creep of the soft material under pressure and due to slow drying during which the microporous sublayer may collapse and thus increase the thickness of the dense skin-layer. Gantzel and Merten (17) dried CA membranes with an acetyl-content of 39.4% by quick-freezing and vacuum sublimation at... 

Tanihara, N.H., Shimazaki, Y., Hirayama, S., et al. (1999). Gas permeation properties of asymmetric carbon hollow fiber membranes prepared from asymmetric polyamide hollow fiber. J. Membrane Sci., 160(2), 179—86. 

Most membranes used in industries have an asymmetric structure. Figure 2.1 shows schematically a typical cross-sectional view of an asymmetric membrane [3]. It consists of two layers the top one is a very thin dense layer (also called the top skin layer), and the bottom one is a porous sublayer. The top dense layer governs the performance (permeation properties) of the membrane the porous sublayer only provides mechanical strength to the membrane. The membranes of symmetric structures do not possess a top dense layer. In the asymmetric membrane, when the material of the top... 

Another complication is that thin dense membrane films have been shown to decline in permeation properties over extended periods of time (10000hours) even without external stresses. The rate of ageing effect becomes greater the thinner the film [37]. This loss over extended time is presumably due to cast membrane films slowly reorienting polymer chains to achieve equilibrium properties with reduced free volume. The thin dense films used in these studies (0.4-25 pm) approach the same order of thickness as the active layer of cast asymmetric films at less than 2000 A (0.2 pm). The property appears common in that it was reported in this paper with polysulfone, polyphenylene oxide and polyimide... 

DMMDA), to synthesize the polyimides (Scheme 3.23). Asymmetric polyimide membranes were prepared by phase inversion and the inner structure was analyzed by scanning electron microscopy (SEM). As shown in Fig. 3.14, the cross-sectional SEM image of the 6FDA-MDA membrane consisted of an ultrathin skin layer and a porous finger-like structure. The pervaporation properties of the prepared polyimides asymmetric membranes for n-heptane/thiophene mixtures were investigated at 40-77 °C. The permeation flux and sulfur enrichment factor of the polyimide membranes... 

Wang, Z Yang, N., Tam, X and Li, K. (2008) Preparation and oxygen permeation properties of highly asymmetric Lao,6Sro,4Coo jFeo.gOs-s perovskite hollow-fiber membranes. Ind. Eng. Chem. Res., 48, 510-516,... 

Cheng, S., Gupta, V.K., and Lin, Y.S. (2005) Synthesis and hydrogen permeation properties of asymmetric proton-conducting ceramic membranes. Solid State Ionics, 176, 2653-2662. 

Asymmetric carbon membranes were made by carbonization of asymmetric PI hollow fiber membranes by Tanihara et al. [43], In their study, it was found that the permeation properties of caibon membranes were hardly affected by feed pressure and exposure to toluene vapor. Furthermore, there was only little change in the permeation properties of the caibon membrane with the passage of time. 

Kusuki Y, Shimazaki H, Tanihara N, Nakanishi S, Yoshinaga T (1997) Gas permeation properties and characterization of asymmetric carbon membranes prepared by pyrolyzing asymmetric polyimide hollow fiber membrane. J Membr Sci 134 (2) 245-253... 

The permeation flux expressions (3.4.76) and (3.4.81a) are valid for membranes whose properties do not vary across the thickness. Most practical gets separation membranes have an asymmetric or composite structure, in which the properties vary across the thickness in particular ways. Asymmetric membranes are made from a given material therefore the properties varying across Sm are pore sizes, porosity and pore tortuosity. Composite membranes are made from at least two different materials, each present in a separate layer. Not only does the intrinsic Qim of the material vary from layer to layer, but also the pore sizes, porosity and pore tortuosity vary across Sm- At least one layer (in composite membranes) or one section of the membrane (in asymmetric membranes) must be nonporous for efficient gas separation by gas permeation. The flux expressions for such structures can be developed only when the transport through porous membranes has been studied. 

The permeability and selectivity of a membrane are determined by the material and the structure of the membrane, which essentially determine the separation mechanism and application. Asymmetric membranes are mostly applied in practical applications because they have a thin selective layer to obtain high permeation fluxes and a thick porous support to provide high mechanical strength. For a certain mass separation, the type of membrane and the driving force required depend on the specific properties of the chemical species in the mixture. 

The state of water and mechanism of transport in different polymers have been studied by a number of researchers [43-48]. A number of different water-polymer interactions arise from the surface properties of the polymer and the structure of the dense homogeneous film, selective layer of a asymmetric membrane or a thin film composite membrane and the hyrdophilicity-hydrophobicity of the polymer chains and their attached substituents which result in different permeation characteristics of the polymer membranes [46]. 

Cross-section structure. An anisotropic membrane (also called asymmetric ) has a thin porous or nonporous selective barrier, supported mechanically by a much thicker porous substructure. This type of morphology reduces the effective thickness of the selective barrier, and the permeate flux can be enhanced without changes in selectivity. Isotropic ( symmetric ) membrane cross-sections can be found for self-supported nonporous membranes (mainly ion-exchange) and macroporous microfiltration (MF) membranes (also often used in membrane contactors [1]). The only example for an established isotropic porous membrane for molecular separations is the case of track-etched polymer films with pore diameters down to about 10 run. All the above-mentioned membranes can in principle be made from one material. In contrast to such an integrally anisotropic membrane (homogeneous with respect to composition), a thin-film composite (TFC) membrane consists of different materials for the thin selective barrier layer and the support structure. In composite membranes in general, a combination of two (or more) materials with different characteristics is used with the aim to achieve synergetic properties. Other examples besides thin-film are pore-filled or pore surface-coated composite membranes or mixed-matrix membranes [3]. 

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