Unsaturated-surface drying

Fig. 5-7. Saturated and unsaturated surface drying, temperature and humidity concentration curves.

Equation for falling-rate period. For the situation where unsaturated surface drying occurs, //(V is constant for adiabatic drying, the rate of drying is directly dependent upon X as in Eq. (9.7-9), and Eq. (9.10-32) applies. 

Unsaturated-surface drying During such a period the rate of drying N will usually vary linearly with moisture content X. Since the mechanism of evaporation during this period is the same as that in the constant-rate period, the effects of such variables as temperature, humidity, and velocity of the gas and thickness of the solid are the same as for constant-rate drying. 

Zone ///, X mathematical treatment. For the case where unsaturated-surface drying occurs and the drying rate is dependent strictly upon the conditions prevailing at any instant, independent of the immediate past history of the drying, Eqs. (12.8) and (12.17) apply. These provide... 

Cobalt leads to more rapid drying at the surface of the film than in the lower layers. If the surface dries first, it will buckle or shrivel when the rest of the film contracts as it hardens. For this reason, secondary driers are usually added with cobalt to accelerate the drying of the bulk of the film. The presence of calcium accelerates the loss of unsaturation in the film. When the levels of the two types of drier are balanced , shrivelling does not occur. Manganese is less pronounced in its surface bias, but is also used with secondary driers. 

In zone II, unsaturated surface and bound moisture are evaporated and the solid is dried to its final value Xj. The humidity of the entering gas entering zone II isH and it rises to The material-balance equation (9.10-23) may be used to calculate f/c as... 

Unsaturation confers drying characteristics on oils— when a thin layer of an oil with a high degree of unsaturation is spread on a smooth surface and exposed to air and light, it takes up oxygen at the double bond sites, and gets oxidised and polymerised... 

These oxazolines have cationic surface-active properties and are emulsifying agents of the water-in-oil type. They ate acid acceptors and, in some cases, corrosion inhibitors (see Corrosion). Reaction to oxazoline also is useful as a tool for determination of double-bond location in fatty acids (2), or for use as a protective group in synthesis (3). The oxazolines from AEPD and TRIS AMINO contain hydroxyl groups that can be esterified easily, giving waxes (qv) with saturated acids and drying oils (qv) with unsaturated acids. 

Paint Driers and Polymer Additives. Paints based on alkyd resins (qv) dry by the oxidation and cross-linking of unsaturated side chains. Metal catalysts are included in paint formulations to promote this drying. Cerium carboxylates, eg, the naphthenate, are used as through driers, ie, to promote drying in the body of the paint film rather than at the film s surface (44). 

As drying proceeds, the point is reached where the evaporating surface is unsaturated. The point of evaporation moves into tne sohd, and the dry process enters the second faJhng-rate period. The drying rate is now governed by the rate of internal moisture movement the influence of external variables diminishes. This period usually predominates in determining the overall diying time to lower moisture content. 

Unsaturated (drying) oils, like linseed oil, etc., will rapidly heat and ignite when distributed on active carbon, owing to the enormous increase in surface area of the oil exposed to air, and in the rate of oxidation, probably catalysed by metallic impurities [1]. A similar, but slower, effect occurs on fibrous materials such as cotton waste [2],... 

Unlike the activated carbons, the surface of silica gel is hydrophilic and it is commonly used for drying gases and also finds applications where there is a requirement to remove unsaturated hydrocarbons. Silica gels are brittle and may be shattered by the rapid release of the heat of adsorption that accompanies contact with liquid water. For such applications, a tougher variety is available with a slightly lower surface area. 

Soils, typically, are not fully saturated by water the soil layer and the region reaching to the water table contain water contents below full saturation. These regions usually are referred to as the vadose zone and said to be unsaturated, but they are more correctly considered partially saturated. The degree of saturation is the ratio of the volume of water to the pore volume within the porous medium. Saturation levels usually are a few percent at land surface (or even zero in perpemaUy dry arid zones) and increase slowly with depth until the region of the capillary fringe (water table), where it increases rapidly to 100%. 

---