Impermeable surfaces evaporation

Impermeable surfaces (film, foil)—evaporation drying, gravure or flexographic. 

FIGURE 9.25 Evaporation from a horizontal impermeable surface coated with a porous layer with 8 < 8,. + 6g(. The speculated intermittent drying of the layer and the associated temperature and saturation distributions are shown. 

TerraTherm Environmental Services, Inc., a subsidiary of Shell Technology Ventures, Inc., has developed the in situ thermal desorption (ISTD) thermal blanket technology to treat or remove volatile and semivolatile contaminants from near-surface soils and pavements. The contaminant removal is accomplished by heating the soil in sim (without excavation) to desorb and treat contaminants. In addition to evaporation and volatilization, contaminants are removed by several mechanisms, including steam distillation, pyrolysis, oxidation, and other chemical reactions. Vaporized contaminants are drawn to the surface by vacuum, collected beneath an impermeable sheet, and routed to a vapor treatment system where contaminants are thermally oxidized or adsorbed. 

Desert rodents lead the most water-independent life of all vertebrates. Kangaroo rats can so reduce their evaporation that they are able to maintain water balance on only metabolic water. Other species survive on only meiabolic water plus free water in air-dry seeds. Respiratory water loss is reduced by cool nasal mucosal surfaces, which condense water from warm air coming from the lungs, before it can be expired. Skin impermeability involves a physical vapor barrier in the epidermis, pins unknown physiological factors. 

With a slow-drying impermeable species a fan speed as low as 1.5-2.0 m s may be sufficient as the moisture eontent at the surface drops quickly below the fibre saturation point - no mass flow - and there is no point in installing overly powerful fans just to strip off surface moisture for the first few minutes of a long kiln schedule (>14 days) thereafter the slow rate of transfer of moisture from the centres of the boards to their surfaces becomes more important than the rate of evaporation. 

In summary the temperature of the wood surface is determined by the cooling effect of evaporation, rising from the initial wet-bulb temperature when the surface is wet to the dry-bulb temperature as the wood approaches the equilibrium moisture content. With permeable woods the surface temperature remains at the wet-bulb for a considerable proportion of the schedule. With impermeable woods the surface temperature soon begins to rise toward the dry-bulb temperature, as there is no mass flow of water from the interior to keep the surface moist and above fibre saturation. 

With impermeable woods - and heartwood - the supply of moisture from the interior eaimot keep paee with evaporation of water vapour from the surface, because mass flow of water is not possible and diffusion is a much slower process. Thus the surfaee moisture eontent quiekly falls below fibre saturation and the evaporative front starts reeeding into the wood. Figure 8.6 shows the parabolic moisture content profile for a slowly air-dried impermeable hardwood. Similarly for permeable softwoods that have been dried below the irreducible moisture content, Stamm (1964, 1967b) reported parabolic moisture profiles that are consistent with diffusion of both water vapour and bound water. 

A One-Dimensional Analysis for Bo 1. Figure 9.22 depicts the one-dimensional model for evaporation in porous media with heat addition q from the impermeable lower bounding surface maintained at T0> Ts, where T, is the saturation temperature. The vapor-film region has a thickness 8g, and the two-phase region has a length 8g(. 

Preservative film materials intended for packing large-size metal ware with sharp edges should of course show elevated resistance to rupture and puncture. With this aim, polymer films are reinforced by glass fibers and kapron by alternating air impermeable and inhibited polymer layers with non-woven fabric [37,38[. Fibrous materials with developed specific surface and porosity can serve as a container for the evaporating Cl [39[. 

A schematic of the drying process is represented in Figure 7.1.18. The material to be dried is placed on an impermeable substrate. The material consists of solvent and semicrystalline polymer which contains a certain initial fraction of amorphous and crystalline domains. The presence of crystalline domains complicates the process of drying because of die reduction in diffusion rate of the solvent. Evaporation of solvent causes an inward movement of material at the surface and the drying process may change the relative proportions of amorphous and crystalline domains. Equations for the change in thickness of the material and kinetic equations which relate composition of amorphous and crystalline domains to solvent concentration are needed to quantify the rate of drying. 

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