Drying unbound moisture

Free moisture content is the Hquid content that is removable at a specific temperature and humidity. Eree moisture may include bound and unbound moisture, and is equal to the total average moisture content minus the equiHbrium moisture content for the specific drying conditions. 

The liquid or moisture in a spray droplet is present in two basic forms bound and unbound moisture. The nature of the solid and the liquid matter determines the drying characteristics of the product. 

The presence of unbound moisture in the droplet means that the drying proceeds at a constant high rate as long as the moisture diffusion within the droplet is able to maintain saturated surface conditions. When the diffusional and capillary flows can no longer maintain these conditions, a critical point is reached and the drying rate will decline until equilibrium moisture content is reached. The evaporation of bound moisture is strongly dependent on the nature of the solid matter in the spray droplet. 

Chang et al. (1991) have studied the dehydration process in relation to IM meats. They stated that if the unbound moisture in meats is defined as that which exerts water vapor pressure like that of pure water, all unbound moisture must evaporate before equilibrium can be achieved with air that is less than saturated. In other words, the water will evaporate until the water vapor pressure of the meat is equal to the partial water vapor pressure in the air. Data on the equilibrium moisture content-relative humidity (i.e., isotherms) of meat or meat mixtures are needed over a wide range of temperatures for dehydration applications. For example, isotherms for meat/meat mixtures below ambient temperature are needed for salami and raw ham, and isotherms above steam temperatures are needed for high-temperature finished dried IM meats (Chang et al, 1991). 

Drying rate curves for cooked muscle bundles implies that the classical fundamental studies on drying properties and mechanisms under constant external drying conditions are empirical in nature. This is shown in the specific study of controlled muscle bundle dehydration, which indicated that for intact tissue the embedded capillaries may become more open and thus create internal free vaporization surfaces as demonstrated in studies by Chang et al. (1991). Internal free surface vaporization operating during unbound moisture removal resulted in identification of the mechanism and is reflected in the reappearance of the constant drying rate periods (Fig. 1) and the multiple critical points and by the porous texture of the product. 

Drying commonly describes the process of thermally removing volatile substances (moisture) to yield a solid product. Moisture held in loose chemical combination, present as a liquid solution within the solid or even trapped in the microstructure of the solid, which exerts a vapor pressure less than that of pure liquid, is called bound moisture. Moisture in excess of bound moisture is called unbound moisture. 

This case exists when drying of the product entirely takes place in the constant drying rate period. It is almost always possible when the solid contains unbound moisture. Textiles, minerals, and inorganic chemicals are examples of such solids. 

Capillary movement in porous solids. When granular and porous solids such as clays, sand, soil, paint pigments, and minerals are being dried, unbound or free moisture moves through the capillaries and voids of the solids by capillary action, not by diffusion. This mechanism, involving surface tension, is similar to the movement of oil in a lamp wick. 

Derivation of equations. To derive the equations for this case, no heat losses will be assumed, so the system is adiabatic. The drying will be for unbound moisture in the wet granular solids. We shall consider a bed of uniform cross-sectional area A m, where a gas flow of G kg dry gas/h m cross section enters with a humidity of//i. By a material balance on the gas at a given time, the gas leaves the bed with a humidity H. The amount of water removed from the bed by the gas is equal to the rate of drying at this time. 

Drying is similar in many ways to other mass transfer operations, particularly to humidification. If the surface of a solid is completely covered with liquid, the rate of its evaporation is controlled by the same mechanism as humidification. In order for equilibrium to exist under this condition, the gas must be 100 percent saturated with moisture, because the solid is. If the surface is free of this unbound moisture, however, the moisture content will vary with the relative humidity of the surrounding gas, at equilibrium. Figure 12.13 shows an equilibrium curve for a typical solid. 

If a solid is initially very wet, the surface will be covered with a thin film of liquid, which we shall assume is entirely unbound moisture. When it is exposed to relatively dry air, evaporation will take place from the surface. The rate at which moisture evaporates can be described in terms of a gas mass-transfer coefficient ky and the difference in humidity of the gas at the liquid surface 7, and in the main stream Y. Thus, for cross-circulation drying... 

The constant-rate period In this period, where surface evaporation of unbound moisture occurs, it has been shown that the rate of drying is established by a balance of the heat requirements for evaporation and the rate at which heat reaches the surface. Consider the section of a material drying in a stream of gas as shown in Fig. 12,11. The solid of thickness is placed on a tray of thickness Zuf. The whole is immersed in a stream of drying gas at temperature Tq and humidity Y mass moisture/mass dry gas, flowing at a mass velocity G mass/ (time) (area). The evaporation of moisture takes place from the upper surface, area which is at a temperature T, The drying surface receives heat from several sources (1) by convection from the gas stream (2) by conduction through the solid (3) by direct radiation from a hot surface at temperature T/j, as shown, all expressed as a flux, energy/(area of solid for heat transfer)... 

The rate of diying of unbound moisture [1], Consider a bed of uniform cross section as in Fig. 12.17, fed with a gas of humidity 7j at the rate of mass dry gas/(area bed cross section) (time). The maximum rate of drying will occur if the gas leaving the bed is saturated at the adiabatic-saturation temperature, with humidity... 

Particles large (3.2 to 20 mm diameter) in shallow beds (10 to 64 mm thick) drying of unbound moisture from porous or noi orous particles. During the constant-rate period the gas leaves the bed unsaturated, and the constant rate of drying is given by Eq. (12.31). For this purpose, ky is given by... 

The moisture content of samples, in equilibrium, in the ESEM is varied at constant temperature (10°C), while lowering the pressure in the ESEM chamber from 9 torr to 2 torr. At the same time, relative humidity varies from 100% to 20%. The created chamber climate induces evaporation of the unbound (free) water in the cement paste (CP) samples without or with an embedded aggregate (to model simple concrete). It has been observed that curing conditions, sample age, water/cement (w/c) ratio, the presence of an aggregate, as well as the value of the RH, gives rise to different drying behaviour of the... 

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