Thermodynamics of Drying

Water content within solids in the two forms, unbound or free water and bound water. Unbound water is in equilibrium with water in phase, i.e., unbound water has the same vapor pressure as bulk water. Unbound water is mainly held in voids of solid. On the other hand, bound water can exist in several conditions (1) water in fine capillaries that exerts an abnormally low vapor pressure because of the highly 

Latent heat of vaporization is the energy required to evaporate per kg of liquid (in this case water) and latent heat of sublimation is the energy required for 1 kg of solid to evaporate. These values can be obtained from the steam tables. In carrying out heat calculations for dryer, the enthalpy of drying can be calculated using the following equation 

The study of the relationship between air and water is called psychrometry. Psychrometric charts show properties of mixture of air and water as shown in Figiu-e 11.4. Any point on this chart represents specific mixture of water and air. The curved line represents saturation curve for 100 % humidity of air saturated with water as a function of dry bulb temperature. 

The dry bulb temperature (DBT) is the temperature of air measured by a thermometer freely exposed to the air but shielded from radiation and moisture. The wet-bulb temperature (0w) is the temperature a parcel of air would have if it were cooled to saturation (100% relative humidity) by the evaporation of water into it, with the latent heat being supplied by the parcel. In other words, wet bulb temperature is the temperature reached by water surface if the air is passed over it. Wet bulb temperature is a function of dry bulb temperature and humidity. The chart shows dry bulb temperature on the x-axis and moisture content on the y-axis. Any point below the saturation line represents air that is unsaturated, therefore, the chart has relative humidity cxirves going up to 100% relative humidity. Wet bulb temperature lines are constant enthalpy or adiabatic cooling lines. The change in composition of 

Using mass transfer rate, the heat transfer at equlibrium can be written as  

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Lampinen, M.J. 1979. Mechanics and thermodynamics of drying, Acta Polytech. Scand. Mech. Eng. Series, 77. 

Recent studies have resulted in significant advances in the understanding of the thermodynamics of drying hygroscopic materials, kinetics of drying, evaporation... 

Psychrometrics. Psychrometrics is the branch of thermodynamics that deals specifically with moist air, a biaary mixture of dry air and water vapor. The properties of moist air are frequentiy presented on psychrometric charts such as that shown ia Figure 2 for the normal air conditioning range at atmospheric pressure. Similar charts exist for temperatures below 0°C and above 50°C as well as for other barometric pressures. AH mass properties ate related to the mass of the dry air. 

Comparing Examples 2a and 2b we notice that the total air pressure has effects on the humidity x, partial density of dry air p total pressure or pressure of humid air, and enthalpy h. Knowing the total pressure is therefore essential in calculations of the thermodynamic properties of humid air. 

Equation (4.137) is almost exactly the same as the approximation equation (4.123) derived for wet bulb temperature. When the partial pressure of water vapor is low compared with the total pressure—in other words when the humidity x is low—the specific heat of humid air per kilogram of humid air, Cp, and the specific heat of humid air per kilogram of dry air, Cp, are al most the same Cp = Cp. Therefore, in a situation where the humidity is low and Le s 1, the thermodynamic wet bulb temperature is very nearly the same as the technical wet bulb temperature dy... 

There is little data available to quantify these factors. The loss of catalyst surface area with high temperatures is well-known (136). One hundred hours of dry heat at 900°C are usually sufficient to reduce alumina surface area from 120 to 40 m2/g. Platinum crystallites can grow from 30 A to 600 A in diameter, and metal surface area declines from 20 m2/g to 1 m2/g. Crystal growth and microstructure changes are thermodynamically favored (137). Alumina can react with copper oxide and nickel oxide to form aluminates, with great loss of surface area and catalytic activity. The loss of metals by carbonyl formation and the loss of ruthenium by oxide formation have been mentioned before. 

The calcium sulfate in a desiccator is effective at removing water vapor only as long as some anhydrous salt remains. When all the anhydrous salt has been converted to the dihydrate, the desiccator can no longer maintain a dry atmosphere. Fortunately, the thermodynamics of this reaction makes it possible to regenerate the diying agent. At 100 °C (373 K), A S ° contributes more to A G ° than does A H ° ... 

In order to obtain a thermodynamically stable micro emulsion, the analysis of the phase behaviour is indispensable. With bovine serum albumin instead of an enzyme (because of the cost of the bio-catalyst) phase behaviour studies are shown in Fig. 2. A strong shift of the phase boundary is observed, yielding a system that solubilises much less water in the presence of the protein. In case of hydrophobic enzymes, the addition of dry lyophilised protein to an already prepared reverse micellar solution can also work well [53]. 

Kowalski, SJ. (2000) Towards a thermodynamics and mechanics of drying processes, Chemical Engineering Science 55, 1289-1304... 

There is one difficulty which always exists in the use of dry hydroxylic solvents SOH, and that concerns the equilibrium 2SOII SOS + HOH. It can be shown from available thermodynamic data that for the simple alcohols and acids the equilibrium constant at 25°C is considerably greater than unity. Thus these systems are metastable with regard to such an... 

NOTE Compiled by John A. Goff and S. Gratch. See also Keenan and Kaye. Thermodynamic Properties of Air, Wiley, New York, 1945. Enthalpy of dry air taken as zero at 0°E Enthalpy of liquid water taken as zero at 32 E... 

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