Liquid transport properties permeability

General advantages of facilitated transport membranes are improved selectivity, increased flux and, especially if compared with membrane contactors, the possibility to use expensive carriers. The specific prerequisites, advantages and disadvantages connected the mobile carriers, are reported in Table 7.1. So far, mainly conventional liquid membranes have been loaded with different mobile carrier systems to obtain facilitated transport properties [3]. Problems encountered are (evaporative) loss of solvent and carrier, temperature limitations, a too large membrane thickness and therefore too low permeabilities as weU as a limited solubility of the carrier in the liquid medium. The low fluxes achieved have, untU now, limited their application... 

In the next sections, the main gas separation applications using facilitated liquid membranes are reported. The gas permeability and selectivity in various membrane systems with facilitated transport properties are summarized in Table 7.5. 

From that point of view it seems natural that special consideration should be given to the composite membranes in which one component consists of liquid crystal. The transitions from crystalline to mesophase state are connected with mobility increase thus influencing the transport properties of these composite systems. However, it was only recently that a discontinuous jump of permeability to liquids, gases and vapours in the vicinity of transition temperature of the liquid crystal phase has been discovered. Membranes of this type form a new class of composites which deserve special consideration due to their particular properties. 

The porosity and pores structure will be of special importance for liquids transport in concrete. These both properties relate directly to w/c ratio and the degree of hydration, because porosity of concrete is affected mainly by cement paste the porosity of aggregate is generally very low. Continuous pores volume is increasing with w/c ratio and decreases with degree of cement hydration. There ate the permeability controlling factor, because the transport of liqitids in concrete composite occurs principally in continuous pores. 

The transport properties, flux and selectivity, can be correlated with two thennodynamic parameters, i) sorption and ii) preferential sorption (see also chapter V). The sorption value reflects the overall interaction of the liquid mixture towards the membrane material. Figure VI - 24 shows the sorption value (left) and the flux (right) of a mixture of toluene-ethanol in a membrane consisting of a blend of polyvinyl alcohol (PVA)-poly acrylic acid (PAA) [41]. It can be seen that with increasing alcohol concentration in the liquid mixture the overall sorption value increases. The transport resistance in this swollen network will decrease and consequently the flux (or better the permeability coefficient) will increase. In fact both the diffusivity (due to increased swelling) and the solubility (due to increased interaction) increase. 

A similar biosensor was made by Nikolelis and Krull [68] however, they used a Severinghaus-type carbon-dioxide-based sensing element. They tried to find the gas-permeable membrane with optimal carbon dioxide transport properties. In their paper, a kinetic reaction-rate method for the determination of aspartame in dietary foodstuffs was proposed as a rapid and inexpensive alternative to a classical high-performance liquid chromatographic method. 

The ratios of the physical properties, such as tensile strength, elongation, liquid wicking distance, liquid transport rate, dielectric constant, opacity, and permeability between MD and CD in nonwoven fabrics are used to characterise the anisotropy of nonwoven stractures in the fabric plane. However, these anisotropy terms are from indirect experimental methods to characterise the nonwoven stmcture, and they are just ratios in two specific directions in the fabric plane, and could misrepresent the anisotropy of the nonwoven structure. The direct method to study the anisotropy of nonwoven structure is to investigate the architectore and texture of the component... 

Fluid handling properties permeability, liquid absorption (liquid absorbency, penetration time, wicking rate and wicking height, rewet, bacteiia/particle collection, repellency and barrier properties, run-off, strike time), water vapour transport, and breathability. 

Abstract Sulfonated polyimides have been designed to be used as proton conducting membranes in fuel cells. These materials present most of the required properties for this application, including a high level of ionic conductivity, a low gas and methanol permeability, and good mechanical properties. However, they exhibit a low stability when immersed in liquid water and in hydrogen peroxide solutions at elevated temperature due to a high sensitivity of the imide functions to hydrolysis. The aim of this article is to review the different routes of synthesis, the membrane-specific properties, the structural and transport property characteristics, and finally their behavior in fuel cells in terms of performance and stability. 

The challenge for modeling the water balance in CCL is to link the composite, porous morphology properly with liquid water accumulation, transport phenomena, electrochemical kinetics, and performance. At the materials level, this task requires relations between composihon, porous structure, liquid water accumulation, and effective properhes. Relevant properties include proton conductivity, gas diffusivihes, liquid permeability, electrochemical source term, and vaporizahon source term. Discussions of functional relationships between effective properties and structure can be found in fhe liferafure. Because fhe liquid wafer saturation, 5,(2)/ is a spatially varying function at/o > 0, these effective properties also vary spatially in an operating cell, warranting a self-consistent solution for effective properties and performance. 

The final colligative property, osmotic pressure,24-29 is different from the others and is illustrated in Figure 2.2. In the case of vapor-pressure lowering and boiling-point elevation, a natural boundary separates the liquid and gas phases that are in equilibrium. A similar boundary exists between the solid and liquid phases in equilibrium with each other in melting-point-depression measurements. However, to establish a similar equilibrium between a solution and the pure solvent requires their separation by a semi-permeable membrane, as illustrated in the figure. Such membranes, typically cellulosic, permit transport of solvent but not solute. Furthermore, the flow of solvent is from the solvent compartment into the solution compartment. The simplest explanation of this is the increased entropy or disorder that accompanies the mixing of the transported solvent molecules with the polymer on the solution side of the membrane. Flow of liquid up the capillary on the left causes the solution to be at a hydrostatic pressure... 

The mechanisms by which various components in a liquid or gaseous feed stream to the membrane system are transported through the membrane structure determine the sq>aiation properties of the membrane. These transport mechanisms are quite different in liquid and in gas or vapor phases. So are their effects on permeate flux (or permeability) and retention (or rejection) coefficient or separation factor in the case of gas separation. 

The problem of the interactions between membrane and absorbent solution interests, for instance, the removal of CO2. Reactive absorption liquids, such as amines, that are used for this type of removal, usually wet polyolefin membranes. Wettability depends on the surface tension of the liquid, membrane material, contact angle, and pore properties of the membrane. Possible solutions to this problem are to employ more resistant membrane materials, to use different absorbent liquids, and to deposit a nonporous layer on the membrane surface that prevents any passage of the liquid through pores. In order to do not increase too much the resistance to the mass transport, the layer has to be thin and highly permeable to the gaseous species. The dense skin can be useful also for avoiding any possible contamination of the feed gas by the absorbent (Figure 38.4). 

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