Moisture unbound

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. 

Unbound moisture ia a hygroscopic material is moisture that exerts the same vapor pressure as the pure Hquid at the same temperature. Uabouad moisture behaves as if the material were aot preseat. AH moisture ia a nonhygroscopic material is uabouad. 

Free-moisture content is that liquid which is removable at a given temperature and humidity. It may include bound and unbound moisture. 

Unbound moisture in a hygroscopic material is that moisture in excess of the equihbrium moisture content corresponding to saturation humidity. All water in a nonbygroscopic material is unbound water. 

Unaccomplished moisture change, 9 97 Unagitated columns, in commercial extractors, 10 769-774 Unbound moisture, 9 97-98 Unbridged catalysts, oscillating, 16 109 Unbridged metallocenes... 

Bound moisture is water (or other solvents in nonaqueous systems) held by a material in such a manner that it exerts a lower vapor pressure than that of the pure liquid at the same temperature. Water may be chemically or physically bound. Unbound moisture is therefore moisture in association with a solid that exerts the same vapor pressure as the pure liquid. In a discussion of bound versus unbound water, it should be pointed out that are not only different equilibria to be considered, but that the binding energies and kinetics are different. 

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 category of unbound moisture can be described as the moisture in excess of the bound moisture. A hygroscopic material may contain bound as well as unbound moisture. A nonhygroscopic material contains unbound moisture only. The equilibrium vapor pressure of unbound water is equal to that of pure water at the same temperature. 

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. 

Moisture may be bound to the solids so that full vapor pressure is no longer exerted according to Watson and Harper (1987). They pointed out that the terms unbound (free) and bound are commonly used to distinguish moisture in the relatively large spaces from that held more tightly by other forces. They further stated that there is a continuous transition from unbound to bound moisture, and it is not possible to make a precise dividing line. Unbound moisture is frequently defined as that which exerts the... 

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. 

Water may become bound in a solid by retention in capillaries, solution in cellular structures, solution with the solid, or chemical or physical adsorption on the surface of the solid. Unbound moisture in a hygroscopic material is the moisture in excess of the equilibrium moisture content corresponding to saturation humidity. All the moisture content of a nonhygroscopic material is unbound moisture. Free moisture content is the moisture content removable at a given temperature and may include both bound and unbound moisture. 

As mentioned above, moisture in a solid may be either unbound or bound. There are two methods of removing unbound moisture evaporation and vaporization. Evaporation occurs when the vapor pressure of the moisture on the solid surface is equal to the atmospheric pressure. This is done by raising the temperature of the moisture to the boiling point. This kind of phenomenon occurs in roller dryers. 

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. 

Removal of unbound moisture, narrow particle size... 

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. 

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