The natural logarithm of Deff as a function of the reciprocal of absolute temperature was plotted in Fig. 7.
The result shows a linear relationship derived from the Arrhenius-type equation.
From the line slope the values of activation energy can be found to be 24.512 kJ/mol, 23.951 kJ/mol and 25.492 kJ/mol for the whole falling rate period, the first and the second falling rate periods, respectively (Fig. 7). The
value 24.512 should be the average (24.725) of 23.951 and
25.492; the relative error is only 0.87%, which indicated that
24.512 kJ/mol was the average activation energy of the
whole falling rate period. The value of activation energy
in the second falling rate period is higher than that in the
first falling rate period, because the non-bond water has
been removed in the first period, the bond water in the second
period needs more heat to be evaporated due to its closer
connection to the materials.