Lynne Wong's PhD thesis

abscissa. For each moisture level (18 in all), a straight line was obtained, the slope of
which equals -q st /R, from which the net isosteric heat of sorption q st could be calculated at
that particular moisture level (Wang and Brennan, 1991). This procedure was repeated for
each of the nine cane components of R 570 aged 52 and 36 weeks.
This approach assumes that q st is independent of temperature; although this is not always
true, it has been accepted as such (Iglesias et al., 1989). The application of this method
requires the measurement of sorption isotherms, at no less than three temperatures.
The total isosteric heat of sorption Q st , is a measure of the interaction (binding energy)
between water vapour and the adsorbent material; in this work, sugar cane fibre. It is the
sum of the net isosteric heat of sorption q st and the latent heat of vaporisation of pure water,
H L :
Q st = q st + H L
The latent heat of vaporisation of pure water H L at various temperatures can be interpolated
from Table 5.3. Thus, H L values at 30, 45, 55 and 60 °C are 43.47, 44.53, 46.03 and 47.01
kJ mol -1 respectively, but the average value of H L for the temperature range may be used
(Sanchez et al., 1997), and can be taken as constant at 43 kJ mol -1 .
The relationship between the total isosteric heat of sorption Q st and the entropy of sorption
S d is given by (Aguerre et al.,1986):
- ln a w =
Qst S − d
(4)
RT R
where a w is the water activity, Q st is the total isosteric heat of sorption (kJ mol -1 ), R is the
universal gas constant (J mol -1 K -1 ), T is the Kelvin temperature (K) and S d is the entropy of
sorption (J mol -1 K -1 ).
The entropy of sorption S d is proportional to the number of available sorption sites at a
specific energy level (Madamba et al., 1996).
If ln a w is plotted against 1/T, for a constant moisture content, the intercept yields S d /R.
An example of a Clausius Clapeyron plot for stalk fibre aged 52 weeks is shown in Fig 6.5.
The calculated net isosteric heat of sorption q st is plotted against moisture level for all nine
cane components of R 570 aged 52 and 36 weeks in Fig 6.6, and the corresponding entropy
of sorption is shown in Fig 6.7.
250