Moisture transport coefficients

Table I. Some Moisture Transport Coefficients Used for Wood, Their Assumed Potentials, and Their Relationships to the Diffusion Coefficient D...

Meng, B. (1994) Calculation of moisture transport coefficients on the basis of relevant pore structure parameters, Materials and Structures RILEM, 27 125-34. 

The driving potential assumed for moisture movement based on Equation 39 is the moisture concentration Other driving potentials may also be assumed. Table I lists the potentials that have been proposed, the resulting transport coefficients, and their relationships to D in each case (59). Although one or more of these other potentials may be more descriptive of the driving force for moisture movement, the discussion that follows will be restricted to the diffusion coefficient because it is so well established in the literature, and can be related to any of the others. Furthermore, it appears unchanged in the unsteady-state diffusion equation (Pick s second law), unlike any of the other coefficients. Thus Pick s second law may be written, for one dimension, as... 

Besides any capillary transport of liquid moisture and vapor diffusion, there is sorptive or bound moisture diffusion within the fibers when they are less than fully saturated [50]. At relative humidities below 0.8, the sorptive transport coefficient diminishes exponentially with decreasing values of relative humidity. 

The function existing in the moisture transport parameter/(, 6) (Equation 62.20) and function qA 6) existing in the coefficient of phase transitions [Pg.1249]

Moisture resistance. The adhesive should be formulated to minimise moisture transport through the adhesive itself. The equilibrium water content (A/ ) should not exceed 3% by weight after immersion in distilled water at 20 °C. The permeability, obtained from the product of the coefficient of diffusion (D) and My, should not exceed 5 x 10 " m-/s at 20 °C (see Fig. A.l). A film of adhesive, approximately 1 mm thick cast in polytetrafluoroethylene-lined moulds and weighing at least 3 g, is suggested for this test(3). This requirement is to enhance the potential for a durable adhesive/adherend interface, even if moisture uptake is not deleterious to the adhesive itself. 

This study proposes a simple approach to modelling the dynamics of solids transport within a flighted rotary dryer. The approach taken was to model the system in a series-parallel formulation of well-mixed tanks. The concept of active and passive solids is important, since it will lend itself well to the addition of mass and energy balance relations. This model formulation predicts the RTD of the system. Industrial RTD data was obtained from a 100 tonne per hour dryer and compared with the model predictions. gPROMS parameter estimation has delivered overall transport coefficients for this system. The transport coefficients are not independent, nor completely physically meaningful. However, they produce a very simple model formulation, which forms the basis for more detailed rotary dryer models incorporating mass and energy balances. Future work will see the development of a full dryer model based on the proposed solids transport model. Refinements will be made to the model to incorporate the effects of solids moisture and interaction with the counter current air stream. 

To summarize, in the PCD process convective heat and vapor transfer between the material surface and the drying gas is strongly affected by the flow oscillations of the gas stream. Additionally, sound waves produced by the combustor can penetrate the solid materials, affecting the internal moisture transport rate. The local pressure difference induced by the acoustic field may cause an enhanced removal of water vapor from capillaries, and also promote the movement of liquid water towards the material surface. Scarboroug et al. (2006) observed a significant increase in the transfer coefficients when the ratio of the acoustic velocity amplitude to the mean velocity was greater than 1.8, which means that the acoustic oscillations of an appropriate amplitude could increase the drying rate of the pulse combustion process. 

The surface snow temperature of this system is targeted at —2°C. The dew temperature is defined as the temperature below which water in air condenses. If the temperature of the air that is in contact with the snow falls below the dew temperature of the air, moisture transport from the air to the snow surfacer is in the form of condensation or frosting. If the temperature of the air is above the dew temperature, the moisture transport from snow to air in the form of sublimation takes plaee. For snow with polymer, the fiiction coefficient becomes large if sublimation proceeds as a consequence of the increased polymer content on the snow particle surface. Thus, it has an adverse effect on the sliding phenomenon. The dynamic friction coefficient will be discussed later. Conversely, if too much frost develops, the snow surface will harden. The surface also will melt due to the thermal insulation of the frost. When it freezes again, it turns into ice. The energy necessary to freeze the moisture in air into frost increases the load on the cooling equipment. 

Diverse appHcations for the fabric sometimes demand specialized tests such as for moisture vapor, Hquid transport barrier to fluids, coefficient of friction, seam strength, resistance to sunlight, oxidation and burning, and/or comparative aesthetic properties. Most properties can be deterrnined using standardized test procedures which have been pubHshed as nonwoven standards by INDA (9). A comparison of typical physical properties for selected spunbonded products is shown in Table 2. 

Friction Materials. PhenoHc friction materials are made from mol ding compounds developed to meet the extraordinary demands required by friction elements in the transportation industries. Friction materials are used for brake linings, clutch facings, and transmission bands. A moderately high coefficient of friction, which is temperature-independent, is needed. In addition, the material must be high in strength, low in wear and abrasion, and resistant to moisture and hydrauHc fluids. 

To summarize, the hydration status of the drug molecule and other components of a pharmaceutical formulation can affect mass transport. Solubility of drug crystals in an aqueous or nonaqueous solvent may depend on the presence or absence of moisture associated with the drug. Hydration may also determine the hydrodynamic radii of molecules. This may affect the frictional resistance and therefore the diffusion coefficient of the drug molecules. Diffusion of drugs in polymeric systems may also be influenced by the percent hydration of the polymers. This is especially tme for hydrogel polymers. Finally, hydration of... 

Applications of ultrasonic techniques to solid-gas systems rely on the fact that velocity and attenuation of US-waves in porous materials is closely related to pore size, porosity, tortuosity, permeability and flux resistivity. Thus, the flux resistivity of acoustic absorbents oan be related to US attenuation [118,119], while the velocity of slow longitudinal US is related to pore tortuosity and diffusion, and transport properties, of other porous materials [120]. Ultrasound attenuation is very sensitive to the presence of an external agent suoh as moisture in the pore space [121] and has been used to monitor wetting and drying prooesses [122] on the other hand, US velocity has been used to measure the elastic coefficients of different types of paper and correlate them with properties such as tensile breaking strength, compressive strength, etc. [123]. 

The treatment is divided into four sections. Section II deals with estimation of coefficients of heat transfer and of mass transfer. Because most, or all, of the latent heat of evaporation of the moisture is normally derived from the sensible heat of the carrier gas, our knowledge of the pertinent coefficients of heat transfer from the gas to the surface of the drying solid is summarized. A summary of the analogous mass-transfer coefficients records in condensed form gives our current knowledge of the means of estimating the rate of transport from the solid to the gas of the vapor evolved. 

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