Driving water vapor pressure gradient

Figure 13.13 A schematic illustration of the membrane distillation process showing temperature and water vapor pressure gradients that drive the process...

Feed-side and strip-side concentration polarization result in a reduction in the driving force for mass transfer. There is a decrease in water activity at the feed-membrane interface and an increase at the strip-membrane interface. This results in a reduction in the water vapor pressure gradient across the membrane. The feed side and strip side mass transfer co-efficients, Kf and K, respectively, can be expressed in terms of the solute diffusion co-efficient in the boundary layer, D, ... 

Equation (3) establishes a relationship between controlled water vapor pressure P, capillary pressure Pc and pore radius r. Equation (2) relates Pc to the external gas pressure Pg and the internal liquid pressure P1, whose gradient is the driving force of water flux. The presented formalism, thus, provides a closed set of equations that relate the stationary water profiles in the membrane with its porous structure. 

When essentially all of the resistance is on the gas side of the interface, kx ky, and ky or K x kyKi. A well-known example of pure gas-phase resistance is the humidification of dry air with pure water. Because there is no concentration gradient in the liquid, the interface concentration equals the vapor pressure of water at the temperature under consideration. Thus the only driving force is where = partial pressure of water in the air, equivalent to the humidity. Many... 

Oxygen makes up 21% of air, with a partial pressure of 21 kPa (158 mm Hg) at sea level. The partial pressure drives the diffusion of oxygen thus, ascent to elevated altitude reduces the uptake and delivery of oxygen to the tissues. air is delivered to the distal airways and alveoli, the PO2 decreases by dilution with carbon dioxide and water vapor and by uptake into the blood. Under ideal conditions, when ventilation and perfusion are well matched the alveolar PO2 will be -14.6 kPa (110 mm Hg). The corresponding alveolar partial pressures of water and CO2 are 6.2 kPa (47 mm Hg) and 5.3 kPa (40 mm Hg), respectively. Under normal conditions, there is complete equilibration ( alveolar gas and capillary blood. In some diseases, the diffusion barrier for gas transport may be increased during exercise, when high cardiac output reduces capillary transit time, full equilibration may not occur, and the alveolar-end-capillary Po gradient may be increased. 

Membrane Distillation (MD) is an emerging hybrid thermal-membrane desalination process that uses a vapor pressure differenee, ereated by a temperature gradient aeross a hydrophobie membrane, as the driving foree to produce high quality distilled water (Figure 1) [3], A temperature difference as low as 10°C between the warm and eold streams is suffieient to produce distilled water under the right conditions. 

Direct experimental proof that the true driving force for diffusive transport is the gradient of chemical potential rather than the concentration gradient is provided by the experiments of Haase and Siry who studied diffusion in binary liquid mixtures near the consolute point. At the consolute point the chemical potential (and the vapor pressure) are independent of composition so, according to Eq. (5.6), the diffusivity should be zero. The consolute point for the system n-hexane-nitrobenzene occurs at 20 C at a mole fraction 0.422 of nitrobenzene. The system shows complete miscibility above this temperature and forms two separate phases at lower temperatures. Opposite behavior is shown by the system water-triethylamine, for which the consolute tempera-... 

Membrane distillation (MD) has been successfully applied and demonstrated for effective removal of ethanol from the fermentation broth. In MD, volatile feed components are evaporated through air-fiUed pores of a hydrophobic membrane. In the ethanol production, the aqueous solution containing ethanol is heated and vapors are formed, which go through the porous hydrophobic membrane that favors the transport of ethanol as compared with water vapor. Thus, the membranes in MD separate two aqueous solutions differing in temperature and composition. The driving force for the mass transfer through the membrane is the gradient of partial pressure caused by the temperature difference across the membrane. 

To reduce the drying time without decreasing the quality of the dried product, the drying eonditions must be such that the temperature of the produet is above the boiling point of water. Sueh eonditions ensure that an overpressure exists within the material, which implies that a pressure gradient drives the moisture (liquid or vapor) toward the exehange surfaces (Lowery, 1979 Kamke and Casey, 1988). 

The work of Adachi et al. (2009) represented a first attempt to correlate and validate ex situ and in situ water permeation phenomena in PEMs. Water permeabilities of Nafion PEMs and water transport in operating PEFCs were investigated under comparable ex situ and in situ values of temperature and RH. The examined parameters included the type of driving forces (RH, pressure), the phases of water at PEM interfaces, PEM thickness, and the effect of catalyst layers at the membrane interfaces. Several experimental setups and schemes were designed and explored. Water permeability at 70°C was determined for Nafion membranes exposed to either liquid or vapor phases of water. Chemical potential gradients of water across the membrane are controlled through the use of differences in RH (38-100%), in the case of contact with water vapor, and hydraulic pressure (0-1.2 atm), in the case of contact with liquid water. Three types of water permeation experiments were performed, labeled as vapor-vapor permeation (VVP), liquid-vapor permeation (LVP), and liquid-liquid permeation (LLP). Ex situ measurements revealed that the flux of water is largest... 

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