Water Vapor Transport Properties

Mondal et al. investigated the influence of functionalized MWNTs on microstructure and water vapor transport properties of SPU membranes (95). The presence of MWNT was expected to decrease the permeability due to the more tortuous path for the diffusing molecules that must bypass through impermeable nanoparticles. Experimental results revealed that there were about 20% reductions 

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Figure 3. Selectivity and water vapor transport property of selected PAMPS-PETE composites and reference barrier materials. Numbers above bars correspond to water VTR (g m day ).

Mondal, S. and Hu J.L. (2006), Segmented shape memory polyurethane and its water vapor transport properties. Designed Monomers and Polymers, 9(6) ... 

The effect of polyelectrolyte domain size and voliune fraction on vapor transport properties is more clearly shown in Table 2. Due to a porous stracture, PETE membranes with selected pore sizes and porosities exhibit high permeability to both water and DMMP vapor. No obvious effect of the pore stractures was observed on transport properties. Filling the pores with linear PAMPS did not change the transport properties of PETE membrane with 500 nm pore sizes. A distinct reduction on DMMP vapor permeability was observed when the domain size of the linear PAMPS decreased to 50 nm, which increased the selectivity by nearly 5 times compared to the original membrane. Table 2 also shows that by increasing the volume fraction of linear PAMPS (in 100 nm pores), the composite was less permeable to DMMP vapor with a higher selectivity. 

The vapor transport properties of nanofiber encapsulated composite membranes are listed in Table 4. Nucrel membrane, which is a random copolymer of polyethylene and 10 % polymethacrylic acid, has a low water vapor transmission rate and poor selectivity. Higher selectivity and high water... 

Hydrogen bonds were also responsible for blending of chitosan and cellulose [147]. The mechanical and dynamic thermomechanical properties of the material appear to be dominated by cellulose. The reduced rate of water vapor transport through the blend appears suitable for wound dressings, providing an optimal moisture environment of the wound by preventing excessive dehydration. The blend demonstrated effective antimicrobial activity against E. coli and S. aureus. 

The water content is the state variable of PEMs. Water uptake from a vapor or liquid water reservoir results in a characteristic vapor sorption isotherm. This isotherm can be described theoretically under a premise that the mechanism of water uptake is sufficiently understood. The main assumption is a distinction between surface water and bulk water. The former is chemisorbed at pore walls and it strongly interacts with sulfonate anions. Weakly bound bulk-like water equilibrates with the nanoporous PEM through the interplay of capillary, osmotic, and elastic forces, as discussed in the section Water Sorption and Swelling of PEMs in Chapter 2. Given the amounts and random distribution of water, effective transport properties of the PEM can be calculated. Applicable approaches in theory and simulation are rooted in the theory of random heterogeneous media. They involve, for instance, effective medium theory, percolation theory, or random network simulations. 

Ideal gas properties and other useful thermal properties of propylene are reported iu Table 2. Experimental solubiUty data may be found iu References 18 and 19. Extensive data on propylene solubiUty iu water are available (20). Vapor—Hquid—equiUbrium (VLE) data for propylene are given iu References 21—35 and correlations of VLE data are discussed iu References 36—42. Henry s law constants are given iu References 43—46. Equations for the transport properties of propylene are given iu Table 3. 

Many factors affect the mechanisms and kinetics of sorption and transport processes. For instance, differences in the chemical stmcture and properties, ie, ionizahility, solubiUty in water, vapor pressure, and polarity, between pesticides affect their behavior in the environment through effects on sorption and transport processes. Differences in soil properties, ie, pH and percentage of organic carbon and clay contents, and soil conditions, ie, moisture content and landscape position climatic conditions, ie, temperature, precipitation, and radiation and cultural practices, ie, crop and tillage, can all modify the behavior of the pesticide in soils. Persistence of a pesticide in soil is a consequence of a complex interaction of processes. Because the persistence of a pesticide can govern its availabiUty and efficacy for pest control, as weU as its potential for adverse environmental impacts, knowledge of the basic processes is necessary if the benefits of the pesticide ate to be maximized. 

Poulsen, T. G. et al., 1999, Predicting Soil-Water and Soil-Air Transport Properties and Their Effects on Soil-Vapor Extraction Efficiency Ground Water Monitoring and Remediation, Vol. 119, No. 3, pp. 61-70. 

Continuity of fhe wafer flux fhrough the membrane and across the external membrane interfaces determines gradients in water activity or concentration these depend on rates of water transport through the membrane by diffusion, hydraulic permeation, and electro-osmofic drag, as well as on the rates of interfacial kinetic processes (i.e., vaporization and condensafion). This applies to membrane operation in a working fuel cell as well as to ex situ membrane measuremenfs wifh controlled water fluxes fhat are conducted in order to study transport properties of membranes. 

Different models determine A in different ways. Nation exhibits a water-uptake isotherm as shown in Figure 7. The dashed line in the figure shows the effects of Schroeder s paradox, where there is a discontinuous jump in the value of A. Furthermore, the transport properties have different values and functional forms at that point. Most models used correlate A with the water-vapor activity, since it is an easily calculated quantity. An exception to this is the model of Siegel et al., ° which assumes a simple mass-transfer relationship. There are also models that model the isotherm either by Flory—Huggins theory" or equilibrium between water and hydrated protons in the membrane and water vapor... 

Unlike the cases of the single-phase models above, the transport properties are constant because the water content does not vary, and thus, one can expect a linear gradient in pressure. However, due to Schroeder s paradox, different functional forms might be expected for the vapor- and liquid-equilibrated membranes. 

Haar L., Gallager J. G, and Kell G. S. (1984). NBSINRC Steam Tables Thermodynamic and transport properties and computer programs for vapor and liquid states of water in SI units. New York Hemisphere Pub. Co., McGraw-Hill. 

T. Kataoka, T. Tsuro, S.-I. Nakao and S. Kimura, Membrane Transport Properties of Pervaporation and Vapor Permeation in an Ethanol-Water System Using Polyacrylonitrile and Cellulose Acetate Membranes, J. Chem. Eng. Jpn 24, 326 (1991). 

Lester Haar, John S. Gallagher, and George S. Kell, NBS/NRC Steam Tables Thermodynamic and Transport Properties and Computer Programs for Vapor and Liquid States of Water in SI Units, Hemisphere Publishing, Washington, DC, 1984. 

The operating pressure is obtained from the vapor pressure and the partial pressure of the gaseous educts and products. In this process, the temperatures applied are between 150 and 500 °C. In recent times, supercritical fluids have attracted a great deal of attention as potential extraction agents and reaction media in chemical reactions. This has resulted from an unusual combination of thermodynamic properties and transport properties. As a rule supercritical reactions like hydrolysis or oxidation are carried out in water. Above the critical point of water, its properties are very different to those of normal liquid water or atmospheric steam. 

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