Drying, transport mechanisms

Similar conclusions regarding how high PTFE content decreases the gas permeability and porosity of CFPs (TGP-H-060 and TGP-H-090) were presented by Park et al. [102]. It was also observed and concluded that the main transport mechanism of the water through the DL was shear force or evaporation, instead of capillary forces (main force in CLs). They also showed that a CFP (TGP-H-060) with 15 wt% PTFE had the best performance with relatively dry conditions compared to a thicker paper with the same hydro-phobic content. 

After the tests, Djilali s group used mathematical assumptions and equations to correlate the intensity of the dye in the image with the depth in the gas diffusion layer. With this method they were able to study the effect of compression on diffusion layers and how fhaf affects water transport. Water removal in a flow charmel has also been probed with this technique and it was observed that, with a dry DL slug, formation and flooding in the FF channels followed the appearance and detachment of water droplets from the DL. Even though this is an ex situ technique, it provides important insight into water transport mechanisms with different DLs and locations. 

Airborne particles may be delivered to surfaces by wet and dry deposition. Several transport mechanisms, such as turbulent diffusion, precipitation, sedimentation, Brownian diffusion, interception, and inertial migration, influence the dry deposition process of airborne particles. Large particles (dNIOAm) are transported mainly by sedimentation hence, large particulate PAHs tend to be deposited nearer the sources of emission Small particles (dblAm), which behave like gases, are often transported and deposited far from where they originated (Baek et al., 1991 Wu et al., 2005). 

Solution The full numerical model needs to include shrinkage since the material is 50 percent water initially and the thickness will decrease from 100 to 46.5 lm during drying. Assuming the layer is viscous enough to resist convection in the liquid, diffusion is the dominant liquid-phase transport mechanism. 

Some time ago Kamke and Vanek (1994) compared the performance of a number of within-the-timber drying models, representing mainly diffusion-like and multiple-transport mechanism approaches, for predicting average moisture contents and moisture-content profiles. Four data sets were used, with three sets representing idealised problems. The fourth data set was the experimental results of drying 40 mm boards of Norway spruce, Picea abies, from initial moisture contents of 29-66% at a dry-bulb temperature of 60°C, wet-bulb depressions of 8-25°C, and an air velocity of 6 m s. The required inputs for the models, including physical properties... 

Prediction results from drying models are only as good as the input data supplied. The more sophisticated models do not perform any better than the simple models if the physical property data is inadequate...A simple model will work quite well if good physical property data is available for the species and the drying conditions are within the range for which the model was developed. For research purposes where detailed heat and mass transfer information is required (such as predicting stress and strain behaviour) the models that separate the transport mechanisms may prove more useful. 

Information about the porous support layer rather than the skin layer. The techniques used by these authors, as well as those reviewed by Pusch and Welch (21), provide valuable Insight Into the mechanism of membrane formation and thus may assist membrane scientists In developing better membranes. However, many of these techniques do not characterize the membrane under the conditions of application for example, the ultrafiltration membranes (23,24) are dried prior to gas sorption studies and microscopy. Therefore, caution must be exercised In Interpreting the results of these characterization methods and relating them to membrane performance and transport mechanisms. 

PROBABLE FATE photolysis photooxidation m atmosphere, photooxidation half-life in air 3.4-33.7 hrs, reacts with photochemically produced hydroxyl radicals with a half-life of 0.001 hr oxidation occurs slowly hydrolysis not an important process volatilization principle transport mechanism, expected to volatilize quickly from dry soil, volatilization half-life from a model river 10 days sorption not an important process biological processes biotrans-formation occurs, biodegradation is slow at low concentrations reversible hydration to beta-hydroxypropionaldehyde, half-life 21 days... 

PROBABLE FATE photolysis, no direct photolysis, indirect photolysis is too slow to be important, the vapor is expected to react with photochemically produced hydroxyl radicals, with an estimated half-life of 22.2 hrs oxidation not an important process, photooxidation half-life in water 2.4-12.2 yrs, photooxidation half-life in air 21 hrs-8.8 days hydrolysis expected to be too slow to be important under natural conditions, first-order hydrolytic half-life 8.8 yrs volatilization not considered as important as sorption, however, there is very little data, volatilizes from dry soil surfaces, volatilization may be important in shallow rivers sorption adsorption onto solids and particles and complexation with humic material (flilvic acid) are the principal transport mechanisms biological processes bioaccumulation, biodegradation, and biotransformation by many organisms (including humans) are very significant fates... 

Like carbonation, the rate of chloride ingress is often approximated to Pick s law of diffusion. There are further complications here. The initial mechanism appears to be suction, especially when the surface is dry, that is, capillary action. Salt water is rapidly absorbed by dry concrete. There is then some capillary movement of the salt laden water throngh the pores followed by true diffusion. There are other opposing mechanisms that slow the chlorides down. These include chemical reaction to form chloroaluminates and absorption onto the pore surfaces. The detailed transport mechanisms of chloride ions into concrete are discussed in Kropp and Hilsdorf (1995). 

It is important to recognize that diffusion is not the only transport mechanism for chlorides in concrete, particularly in the first few millimetres of cover. There may be several mechanisms moving the chlorides including capillary action and absorption as well as diffusion. Rapid initial absorption occurs when chloride laden water hits very dry concrete. In many circumstances these will only affect the first few millimetres of concrete. If so then the expedient of ignoring the first few millimetres of drillings and then calculating diffusion profiles will work. If the cover is low, the concrete cycles between very dry and wet or the concrete quality is low then the alternative transport mechanisms may overwhelm diffusion, at least to rebar depth. 

For the determination of factors required for the description of the moisture transport mechanism of wet materials, the distribution of the moisture content of the dried material and even its change during the drying process must be known. These measurements are usually carried out under laboratory conditions. Two main varieties of these measurements are widespread ... 

FIGURE 16.4 (a) Transport mechanisms during drying of supported catalysts (b) schematic of metal adsorption on an oxide support. 

---