HEAT OF DESORPTION OF WATER FROM CELLULOSIC MATERIALS AT HIGH-TEMPERATURE

During the falling rate period of drying the heat requirements for dry ing hygroscopic materials may be increased substantially above the nor mal latent heat by the heat of desorption of water. In various new hig h intensity drying processes which are of increasing interest industri ally, the drying material may reach quite high temperatures for period s of time sufficiently short that there is no degradation of the prope rties of the dried material. However the limited stability of many mat erials at high temperatures precludes determination of heats of desorp tion by direct measurements because of the long experimental times req uired for thermodynamic techniques. The present study develops a therm odynamically valid method whereby heats of desorption determined at lo w temperatures may be extrapolated reliably to the high temperatures w here such information is needed but cannot be measured. A relation bet ween isosteric heat of desorption and latent heat of water is derived in order to compare the new results with literature data. As the resul ts for three temperature sensitive cellulosic materials show that inte gral heats of desorption can be up to 60% higher at 200 degrees C than at 100 degrees C, this temperature dependence is too large to be igno red. The method has a sound theoretical basis and is easy to use for i ndustrial drying processes. This method is demonstrated for three such materials, viscose yarn, cotton and kraft wood pulp.