Water transfer coefficient

Toei, R., H. Imakoma, H. Tamon and M. Okazaki. 1983. Water transfer coefficient in adsorptive porous body. J. Chem Eng. Japan 16(5) 364-69. 

There are many transport conditions where experiments are needed to determine coefficients to be used in the solution. Examples are an air-water transfer coefficient, a sediment-water transfer coefficient, and an eddy diffusion coefficient. These coefficients are usually specific to the type of boundary conditions and are determined from empirical characterization relations. These relations, in turn, are based on experimental data. 

The water transfer coefficient of a membrane depends directly on anolyte and catholyte concentrations. The amount of water transferred across the membrane per mole of sodium transferred, called the water transfer coefficient, decreases with increasing... 

Consider the cathode channel of a PEFC and suppose that the water transfer coefficient through the membrane is zero. Then, in the channel,... 

Thus, by measuring v and p we can determine the local water transfer coefficient aw along the cathode channel. 

Figure 4.1 The shape of the flow velocity along the air channel for the indicated values of the water transfer coefficient a, .

In the polymer electrolyte fuel cell (PEFQ, whieh will be useful power source for automotive and on-site power generation, water management is essential because a thin polymer membrane used as an eleetrolyte in the eell shows high ionic conductivity only in hydration. A fuel eell was specially designed for MRI experiment and the hydration process of the membrane in the fuel cell was examined by time-lapse MRI to determine the water transfer coefficient. ... 

This suggests that the initial curvature is driven by the pressure gradient caused in the ion gel by the electroosmosis flow. Hence, the initial curvature is a function of the ionic charge, Q, and the water transference coefficient, Xla, which describes the number of water molecules transferred per counter cation transferred. The experimentally determined initial curvature of an IPMC actuator (Naflon 117/Au) having various ionic forms could be reproduced with fidelity using the theoretical curves described by Eqs. 17, 18, and 20 (Yamaue et al. 2005). 

Determine the ideal net water transfer coefficient for a hydrogen fuel cell so that external humidification is not needed. How would you design the electrode and diffusion media structure to achieve this goal ... 

Example 16.4 The stream data for a process are given in Table 16.5. Steam is available condensing between 180 and 179°C and cooling water between 20 and SO C. All film transfer coefficients are 200Wm C For lO C, the... 

Lujuid-Pha.se Transfer. It is difficult to measure transfer coefficients separately from the effective interfacial area thus data is usually correlated in a lumped form, eg, as k a or as These parameters are measured for the Hquid film by absorption or desorption of sparingly soluble gases such as O2 or CO2 in water. The Hquid film resistance is completely controlling in such cases, and kjji may be estimated as since x (Fig. 4). This... 

The value of the saturation concentration,, is the spatial average of the value determined from a clean water performance test and is not corrected for gas-side oxygen depletion therefore K ji is an apparent value because it is determined on the basis of an uncorrected. A tme volumetric mass transfer coefficient can be evaluated by correcting for the gas-side oxygen depletion. However, for design purposes, can be estimated from the surface saturation concentration and effective saturation depth by... 

Bed-to-Surface Heat Transfer. Bed-to-surface heat-transfer coefficients in fluidized beds are high. In a fast-fluidized bed combustor containing mostly Group B limestone particles, the dense bed-to-boiling water heat-transfer coefficient is on the order of 250 W/(m -K). For an FCC catalyst cooler (Group A particles), this heat-transfer coefficient is around 600 W/(600 -K). 

The heat-transfer coefficient of most interest is that between the bed and a wall or tube. This heat-transfer coefficient, is made up of three components. To obtain the overall dense bed-to-boiling water heat-transfer coefficient, the additional resistances of the tube wall and inside-tube-waH-to-boiling-water must be added. Generally, the conductive heat transfer from particles to the surface, the convective heat transfer... 

Fig. 17. Heat-transfer coefficient comparisons for the same volumetric flow rates for (A) water, 6.29 kW, and a phase-change-material slurry (O), 10% mixture, 12.30 kW and ( ), 10% mixture, 6.21 kW. The Reynolds number was 13,225 to 17,493 for the case of water.

The use of molal humidity as the mass-transfer driving force is conventional and convenient because of the development of humidity data for, especially, the air—water system. The mass-transfer coefficient must be expressed in consistent units. 

The thermal design of cooling towers follows the same general procedures already presented. Integration of equation 35 is usually done numerically using the appropriate software, mass-transfer coefficients, saturation enthalpies, etc. In mechanical-draft towers the air and water dows are both suppHed by machines, and hence dow rates are fixed. Under these conditions the design procedure is straightforward. 

Under equiUbrium or near-equiUbrium conditions, the distribution of volatile species between gas and water phases can be described in terms of Henry s law. The rate of transfer of a compound across the water-gas phase boundary can be characterized by a mass-transfer coefficient and the activity gradient at the air—water interface. In addition, these substance-specific coefficients depend on the turbulence, interfacial area, and other conditions of the aquatic systems. They may be related to the exchange constant of oxygen as a reference substance for a system-independent parameter reaeration coefficients are often known for individual rivers and lakes. 

The drying rate is represented by differential equation (eq. 6) where h is mass transfer coefficient 1/(hcm ) , specific surface area of desiccant beads, cm /g mass of desiccant, g C, concentration by weight of water in the fluid being dried (7, concentration of water at the surface of the desiccant, ie, concentration of water in the fluid that would be in equihbrium with the instantaneous loading on the desiccant, wt-ppm and t — time, h. 

Steam. The steam system serves as the integrating energy system in most chemical process plants. Steam holds this unique position because it is an exceUent heat-transfer medium over a wide range of temperatures. Water gives high heat-transfer coefficients whether in Hquid phase, boiling, or in condensation. In addition, water is safe, nonpolluting, and if proper water treatment is maintained, noncorrosive to carbon steel. 

Sindlady, heating surface area needs are not direcdy proportional to the number of effects used. For some types of evaporator, heat-transfer coefficients decline with temperature difference as effects are added the surface needed in each effect increases. On the other hand, heat-transfer coefficients increase with temperature level. In a single effect, all evaporation takes place at a temperature near that of the heat sink, whereas in a double effect half the evaporation takes place at this temperature and the other half at a higher temperature, thereby improving the mean evaporating temperature. Other factors to be considered are the BPR, which is additive in a multiple-effect evaporator and therefore reduces the net AT available for heat transfer as the number of effects is increased, and the reduced demand for steam and cooling water and hence the capital costs of these auxiUaries as the number of effects is increased. 

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