Water sorption liquid pressure

Figure 14. Adsorption isotherms at 32° C for boards with or without liquid ammonia treatment before water sorption. Severe and mild refer to cooking steam pressures.

The flux also depends on the physical make-up of the membrane and the pore size. The pure water flux vs. hydrauhc pressure difference curve is linear. Liquid flux for membrane processes is shown in F ure 1.5. The data show how the permeation rate varies with the size of the species and pore size (implicit in type of membrane). The ordinate represents the flux of water per unit pressure gradient. Since the pore radius of an RO membrane is 0.6 nm, water molecules whose radius is about 0.1 nm can pass through the membrane freely while dissolved ions and organic solutes (e.g., sucrose) cannot. These solutes are either rejected at the membrane surface, or are more strongly attracted to the solvent water phase than to the membrane surface. The preferential sorption of water molecules at the solvent—membrane interface, which is caused by the interaction force working between the membrane, solvent, and solute responsible for the separation [8]. As the pore size decreases and tends toward a non-porous skin structure, the transport mecharusm changes from convective flow through pores to SD in the membrane polymer. The latter is the transport mechanism in GS and PV. 

However, the increase in resistance with decreasing water vapor pressure in the present case appears to be a new clue and has been useful in at least arguing against some possible mechanisms. It appears feasible that the surface properties of the film are changing in response to the external environment, specifically the relative humidity. There is evidence that a surface of a methacrylate hydrogel is more hydrophobic in air saturated with water vapor than in liquid water [28]. If we assume that ionomers show a similar behavior, and that the surface becomes still more hydrophobic at lower humidities, then greater resistance to water sorption could result, thus explaining the observed results. 

Under equilibration in a saturated vapor atmosphere, a determines the maximal values of radius, swelling parameter and liquid pressure of swollen pores. A typical set of parameters gives= 21 nm, = 105 and P —67 atm. For this case, water sorption from vapor would level off at 0.95. The leveling-off of inte-... 

Membrane structure and external conditions determine water sorption and swelling. The resulting water distribution determines transport properties and operation. Water sorption and swelling are central in rationalizing physical properties and electrochemical performance of the PEM. The key variable that determines the thermodynamic state of the membrane is the water content k. The equilibrium water content depends on the balance of capillary, osmotic, and electrostatic forces. Relevant external conditions include the temperature, relative humidity, and pressure in adjacent reservoirs of liquid water or vapor. The theoretical challenge is to establish the equation of state of the PEM that relates these conditions to A.. A consistent treatment of water sorption phenomena, presented in the section A Model of Water Sorption, revokes many of the contentious issues in understanding PEM structure and function. 

FIGURE 5.1 Experimental setup for water flux measurements on PEMs devised by Romero and Merida (2009). A PEM is mounted as a separator between a chamber with a stagnant fluid (liquid water to saturated vapor) and a flow chamber. Gas flows through the flow chamber at a steady volumetric flow rate, V. The partial pressure of water at the inlet, is held constant and the exhaust partial pressure, pout, is recorded over time. (Reprinted from 7. Memhr. ScL, 324, Monroe, C. et al. A vaporization-exchange model for water sorption and flux in Nafion, 1-6, Figures 1,2,3, Copyright (2008) Elsevier. With permission.)... 

Vapor sorption onto porous solids differs from vapor uptake onto the surfaces of flat materials in that a vapor (in the case of interest, water) will condense to a liquid in a pore structure at a vapor pressure, Pt, below the vapor pressure, P°, where condensation occurs on flat surfaces. This is generally attributed to the increased attractive forces between adsorbate molecules that occur as surfaces become highly curved, such as in a pore or capillary. This phenomenon is referred to as capillary condensation and is described by the Kelvin equation [19] ... 

The vapor pressure of a chemical is the pressure its vapor exerts in equilibrium with its liquid or solid phase. The vapor pressure s importance in environmental work results from its effects on the transport and partitioning of chemicals among the environmental compartments (air, water, and soil). The vapor pressure expresses and controls the chemical s volatility. The volatilization of a chemical from the water surface is determined by its Henry s law constant (see Chapter 4), which can be estimated from the ratio of a chemical s vapor pressure to its water solubility. The volatilization of a chemical from the soil surface is determined largely by its vapor pressure, although this is tempered by its sorption to soil solids and its Henry s law constant between soil, water, and air. A substance s vapor pres-... 

Effect of Water. Wood is usually treated with ammonia in the presence of some amount of water. The effect of water depends not only on the amount of water but also somewhat on the history of the wood sample and the method of treatment. Thus, when oven-dried veneer strips were treated with cold liquid ammonia-water mixtures at ambient pressure, the flexibility of the treated wood was substantially decreased when the moisture content of the ammonia was much above 10% (26). Other protonic solvents act similarly (26). In apparent contrast, the rate of sorption of ammonia from the gas phase by wood is markedly enhanced by moisture in the wood (19). Bone dry wood absorbs ammonia quite slowly at ambient temperatures but if the wood has ten to twenty percent moisture content, sorption and plasticization occur much more rapidly. Presumably the moisture opens the pore structure of the wood and also dissolves ammonia much more readily than bone-dry wood. On continued treatment, the water is presumably displaced from the wood by the ammonia... 

A water pump can reach pressures of 1 Torr. An oil vacuum pump can reach 20 mTorr. A turbomolecular pump can reach pressures of 10 10 Torr (10-8 Pa). A sorption pump can reach pressures of 10-2 Torr by exposing the system to a porous zeolite cooled to liquid nitrogen temperature with a Dewar flask placed on the outside. 

It is customary to construct sorption isotherms or vapor pressure isotherms of foods, where water content (either as mass fraction or as mass of water per unit mass of solids) is plotted against aw. Figure 8.3a gives an example of such an isotherm here, the material is a powder below 10 and a liquid above 30% water. 

Analyte stability in treated stainless steel canisters is related to the polarity of the molecule and the number of water molecules in the matrix air. At a relative humidity of about 10%, sufficient water molecules are available to form a monolayer on the inner surface of a 6 1 spherical canister. Water molecules are thought to coat the active sites and minimize sorption of surface-active VOCs. However, if samples of air at high relative humidity are pressurized, water can condense in the container, and polar molecules can partition into the liquid water. As the air sample is withdrawn for analysis, the water will evaporate, liberating polar molecules dissolved in the water and increasing analyte concentrations in subsequent samplings of the container. 

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