Equilibrium Moisture Content of Materials

As in other transfer processes, such as mass transfer, the process of drying of materials must be approached from the viewpoint of the equilibrium relationships and also the rate relationships. In most of the drying apparatus discussed in Section 9.2, material is dried in contact with an air-water vapor mixture. The equilibrium relationships between the air-water vapor and the solid material will be discussed in this section. 

For some solids the value of the equilibrium moisture content depends on the direction from which equilibrium is approached. A different value of the equilibrium moisture content is obtained according to whether a wet sample is allowed to dry by desorption or whether a dry sample adsorbs moisture by adsorption. For drying calculations it is the desorption equilibrium that is the larger value and is of particular interest. 

4B Experimental Data of Equilibrium Moisture Content for Inorganic and Biological Materials 

Typical data for various materials. If the material contains more moisture than its equilibrium value in contact with a gas of a given humidity and temperature, it will dry until it reaches its equilibrium value. If the material contains less moisture than its equilibrium value, it will adsorb water until it reaches its equilibrium value. For air having 0% humidity, the equilibrium moisture value of all materials is zero. 

Typical food materials. In Fig. 9.4-2 the equilibrium moisture contents of some typical food materials are plotted versus percent relative humidity. These biological materials also show large values of equilibrium moisture contents. Data in this figure and in Fig. 9.4-1 for biological materials show that at high percent relative humidities of about 60 to 80%, the equilibrium moisture content increases very rapidly with increases of relative humidity. 

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In diying solids it is important to distinguish between hygroscopic and nonhygroscopic materials. If a hygroscopic material is maintained in contact with air at constant temperature and humidity until equilibrium is reached, the material will attain a definite moisture content. This moisture is termed the equilibrium moisture content for the specified conditions. Equilibrium moisture may be adsorbed as a surface film or condensed in the fine capillaries of the solid at reduced pressure, and its concentration will vaiy with the temperature and humidity of the surrounding air. However, at low temperatures, e.g., 15 to 50°C, a plot of equilibrium moisture content versus percent relative humidity is essentially independent of temperature. At zero humidity the equilibrium moisture content of all materials is zero. 

Equilibrium moisture content of a solid is particularly important in drying because it represents the limiting moisture content for given conditions of humidity and temperature. If the material is dried to a moisture content less than it normally possesses in equilibrium with atmospheric air, it will return to its equilibrium value on storage unless special precautions are taken. 

Equilibrium moisture content of a hygroscopic material may be determined in a number of ways, the only requirement being a source of constant-temperature and constant-humidity air. Determination may be made under static or dynamic conditions, although the latter case is preferred. A simple static procedure is to place a number of samples in ordinaiy laboratoiy desiccators containing sulfuric acid solutions of known concentrations which produce atmospheres of known relative humidity. The sample in each desiccator is weighed periodically until a constant weight is obtained. Moisture content at this final weight represents the equilibrium moisture content for the particular conditions. 

All nylons absorb some moisture from environmental humidity. Moisture absorption characteristics must be considered in designing and joining these materials. They absorb from 0.5 to 2 percent by weight of moisture after 24-h water immersion. Freshly molded objects contain less than 0.3 percent moisture since only dry molding powder can be successfully molded. Once molded, these objects absorb moisture when they are exposed to humid air or water. The amount of absorbed moisture increases until an equilibrium condition is reached based on the relative humidity of the environment. Equilibrium moisture contents of two commercial nylon resins for two humidity levels are as follows ... 

Equilibrium moisture content curves vary greatly with the type of material examined. Insoluble, non-porous materials, such as talc or zinc oxide, have equilibrium moisture contents of almost zero over a wide humidity range. A moisture content between 10 and 15% may be expected for cotton fabrics under normal atmospheric conditions. Drying below the equilibrium moisture content for room conditions may be deliberately undertaken, particularly if the material is unstable in the presence of moisture subsequent storage becomes important. 

A variant of the dynamic method may be, for example, an air conditioner of high accuracy that maintains the constant state of a sample placed in the airflow, which is kept at given T and y/ values [13]. Development of the equilibrium moisture content of the material is attained when the weight of the sample becomes constant. This may be checked by gravimetric measurements. 

Effective moisture diffusivity and effective thermal conductivity are in general functions of material moisture content and tanperature, as well as of the material structure. Air boundary coeffiamts are functions of the conditions of the drying air, that is humidity, tonperature, and velocity, as well as system geometry. Equilibrium moisture content of a given material is a function of air humidity and temperature. The drying constant is a function of material moisture content, temperature, and thickness, as weU as air humidity, tonperature, and velocity. 

A bibliography on sorption isotherms of food materials is presented in Ref. [161]. The collection comprises 2200 references, including about 900 papers with information on equilibrium moisture content of foods in defined environments. The papers are listed alphabetically according to the names of the first author, but they are also grouped according to product. 

A reference material, or a matrix, is a filter paper with constant absorptivity for the whole range of relative air humidities. In calculations and the diagram presented in Figure 45.22, the authors took the equilibrium moisture content Xy p = 0.01 Xp yy y, where Xp yy y Is thc maxlmum equilibrium moisture content of the matrix at 25°C. From the diagram in Figure 45.22, materials of mean sorptive activity (K = 1-1.5, e.g., maize meal, potato flour, and peat) with an absorptivity K that does not depend on air humidity, q>, and materials of high sorptive activity K > 2, e.g., zeolite, silica gel, and activated carbon) with absorptivity that depends to a large extent on the relative air humidity, cp, can be selected. 

Fig. 5-12. Equilibrium moisture content of a typical hygroscopic material. Representation according to Poersch, Babcock-BSH AG, Krefeld.

At present, theoretical understanding of the structure of solids and surface phenomena does not enable us to predict the variation of equilibrium moisture content of various materials from first principles. However, by using models such as those used for adsorption isotherms of multilayers of molecules and others, attempts have been made to correlate experimental data. Henderson (H2) gives an empirical relationship between equilibrium moisture content and percent relative humidity for some agricultural materials. In general, empirical relationships are not available for most materials, and equilibrium moisture contents must be determined experimentally. Also, equilibrium moisture relationships often vary from sample to sample of the same kind of material. 

In Fig. 9.4-1, if the equilibrium moisture content of a given material is continued to its intersection with the 100% humidity line, the moisture is called bound water. This water... 

Typical equilibrium moisture contents of some food materials at approximately 298 K (2j C) (/) macaroni, (2) flow, (J) bread, (4) crackers, (5) egg albundn. [Curoe (5) from ref (El). Curves (/) to (4) from National Research Council, International Critical Tables, Vol. ll. New York McGraw-Hill Book Company, 1929. Reproduced with permission of the N ational Academy of Sciences. ... 

Free moisture content in a sample is the moisture above the equilibrium moisture content. This free moisture is the moisture that can be removed by drying under the given percent relative humidity. For example, in Fig. 9.4-1 silk has an equilibrium moisture content of 8.5 kg H2O/IOO kg dry material in contact with air of 50% relative humidity and 25°C. If a sample contains 10 kgH2O/100 kgdry material, only 10.0 — 8.5, or 1.5, kg H2O/IOO kg dry material is removable by drying, and this is the free moisture of the sample under these drying conditions. 

I. Introduction. In the drying of various types of process materials from one moisture content to another, it is usually desired to estimate the size of dryer needed, the various operating conditions of humidity and temperature for the air used, and the time needed to perform the amount of drying required. As discussed in Section 9.4, equilibrium moisture contents of various materials cannot be predicted and must be determined experimentally. Similarly, since our knowledge of the basic mechanisms of rates of drying is quite incomplete, it is necessary in most cases to obtain some experimental measurements of drying rates. 

In this chapter, we review some of our recent studies on moisture equilibrium and transport in paper materials. Equilibrium sorption is first studied with particular reference to sorption hysteresis. We found that the evolution of the equilibrium moisture content of a paper sheet subjected to arbitrary humidity changes can be predicted by applying the theory of independent domain... 

As implied by the proposal of Schuchmann et al. [7], many adsorption isotherms may be normalized by plotting the equilibrium moisture content against the free energy change of sorption (-i rin [10,11]. It is believed that desorption isotherms can be similarly correlated, particularly at moderate relative humidities when multimolecular adsorption is the dominant mechanism of attachment as the moisture content is directly related to the thickness of the adsorbate layer. Tests with particulate materials have shown that there is a linear relationship between the free energy change and In over a sixfold range in the latter [12]. This correlation implies an explicit relationship for the equilibrium moisture content of the kind... 

The equilibrium moisture content of many materials of interest, such as various food items, wood, etc., are described as a function of the percent relative humidity, as illustrated in Figure 6.3.20(b) for a hypothetical system. The level of bound water in this system at 100% relative humidity is 18 kg H2O/ 100 kg dry material. If the total level of water in this sample is 26 kg H2O/IOO kg dry material, then the amount of free water at 100% relative humidity is 8 kg H2O/100kg dry material. Similarly, one can develop estimates of bound water and free water at other levels of relative humidity. 

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