Drying transport phenomena

This is an area of strong continental climate with dry and hot summer and severe winter with strong winds that transport dust at short and long distance, for instance yellow sand phenomenon in northwest China. During air transport these soil particles absorb numerous pollutants-carcinogens, like benzo(a)pyrene and heavy metals (Ni, Cd, Co, Zn, Pb, As) both from industrial emissions into the atmosphere and waste landfill sites. 

The level of emissions of SO2 and N0X, and their effects in the United States and in other industrialized countries have been the target of considerable debate. When SO2 and N0X are emitted into the atmosphere, a large fraction of these pollutants can be oxidized to sulfate and nitrate during atmospheric transport, and then deposited as acidic compounds (4-8). Acid rain is the popular term to describe this complex phenomenon. Acidic compounds can be deposited in both wet and dry forms, and this process is more properly referred to as acid deposition or acid precipitation. The wet forms of acid deposition include rain, snow, fog, and dew. Dry deposition occurs via absorption of SO2 and N0X on surfaces and via... 

This is presented schematically in Fig. 6.3, which also shows that the kinetics of these processes is described by the transport rate of A from the wall to the adjacent media. Using Fig. 6.3, we can establish that two elementary processes are presented in this system. The first is the flow induced by the concentration gradient and the second is the mass transfer sustained by the processes on the surface (a chemical reaction in the case of the metal placket immersed in a specifically formulated liquid and the transport through the porosity in the case of the drying wall). The case presented here corresponds to the situation when, in respect of the bulk density, the fluid density begins to decrease near the wall. This generates the displacement of the media and the specific ascension force, which is equivalent to the density difference. This phenomenon depends on the concentration difference in fluid A Aca=(cap - c ). From Fig. 6.3 we can write a list of process variables ... 

Salinity in dryland environment is a natural phenomenon derived from a long-term accumulation of salts on the ground and a lack of adequate flushing in the unsaturated zone. Salt accumulation and efflorescent crusts have been documented in the upper unsaturated zone (e.g., Gee and Hillel, 1988 Nativ et al., 1997 Leaney et al., 2003) and in fracture surfaces (Weisbrod et al., 2000) in many arid areas. The salt formation has been attributed to surface evaporation (Allison and Barnes, 1985), wetting and drying cycles (Drever and Smith, 1978), soil capillarity, and capillarity transport of water and salts from the bulk rock matrix towards fracture surfaces (Weisbrod et al., 2000). 

McCallan and Wilcoxon (1936) were the first to report that fungal spores can solubilise copper. In the bathing medium of Neurospora sitophilus spores they detected malic acid and certain amine acids which were able to dissolve copper even from dry deposits of Bordeaux mixture. In this case, complex compounds of the copper(II) ions are formed and, as shown already by the investigations of Bodnar and Terenyi (1930), the fungicidal action of copper(II) complexes proved to be superior to that of copper(II) ions. Horsfall et al. (1937) made similar observations. This phenomenon can be explained by the much higher lipoid solubility of copper(II) complexes (Horsfall, 1957 Durkee, 1958) which allows them to penetrate the cell more easily. They dissociate in the cell, and copper(II) ions are liberated. Thus, complex-forming compounds seem to aid the transport of the... 

Wet deposition encompasses the removal of gases and particles from the atmosphere by precipitation events, through incorporation into rain, snow, cloud, and fog water, followed by precipitation (Hales, 1986). As in the case of dry deposition, wet deposition is a complex phenomenon which in this particular case involves transport to the surface of a droplet, absorption, and possible aqueous-phase chemical conversion. Wet removal of gases is frequently approximated by assuming that the species is in equilibrium between the gas and aqueous phases. The equilibrium partitioning is represented in terms of a washout ratio, Wg = [C]drop/[C]air, where [C]drop and [C]ajr are the concentrations of the chemical in the aqueous and gas phases (Mackay, 1991). 

It has been found that the diffusion of water through Immobiline gels does not follow a simple Pick s law of passive transport from high (the water phase) to zero (the dried gel phase) concentration regions, but it is an active phenomenon even under isoionic conditions, acidic ranges cause swelling 4—5 times faster than alkaline ones. 

Perhaps the single word that best characterizes the acid deposition phenomenon is competition. The nature of acid deposition depends on competition between gas-phase and liquid-phase chemistry, competition between airborne transport and removal, and competition between dry and wet deposition. The key questions in acid deposition are related to identifying the essential processes involved and then understanding their interactions and quantifying their contributions. In this section we summarize our current understanding of the answers to these questions. 

This phenomenon is a paradoxical situation, inducing a two-way motion of vapor. Some part of vapor is transported toward the surrounding medium and another part is transferred toward the material core. The latter is in fact opposite to what is required for a drying operation (A1 Haddad 2007,2008). Then, in the final period of standard hot air drying, the operation is achieved followingprogression kinetics. 

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