An investigation of the evaporation of stearic acid using a simultaneous TG-DTA unit

The heat treatment of stearic acid shows a zero-order process which is in a ccord with an evaporation process. In this study, the melting points of thr ee samples of stearic acid (A, B, and C) from the rising temperature experi ments at the heating rate (beta) of 2-12 degrees C min(-1) are found to be in the range of 57.4-73.5 degrees C and that of stearic acid B in N-2 flow rate ranging from 50 to 1000 ml min(-1) is 69.0-70.0 degrees C. From the DT A signals, the beta of 6-8 degrees C min(-1) and an N-2 how rate of 100 ml min(-1) were found to be the optimum conditions for the evaporation process of these materials. The enthalpy of vaporization (Delta H-vap) of stearic acid is calculated by the use of the Antoine [1] and Clausius-Clapeyron equ ations [2]. These Delta H-vap values are equal to 115.7 and 81.6 kJ mol(-1) for the temperature ranges of 349-415 and 457-649 K, respectively. These v alues can be compared to the activation energy (E-act) which is in the rang e of 87.0 to 101.1 kJ mol(-1) determined from the rising temperature experi ments of three types of stearic acid subjected to various experimental cond itions. Generally, E(act)greater than or equal to Delta H-vap. It is predic ted, and verified experimentally, that the equilibrium vapor pressure and t he coefficient of vaporization have a logarithmic relationship. (C) 1998 El sevier Science B.V. All rights reserved.