Lynne Wong's PhD thesis

50
40
30
20
% adsorbed water/dry fibre
10
8
6
5
4
3
2
5 6 8 10 20 40 60 80 100
Water vapour pressure/mm Hg
Figure 5.2. Adsorption isotherms of cane fibre (Kelly, 1957).
According to Kelly (1957), below the transition point, a primary monolayer adsorption
process represented by the first part of the curve took place, and above the transition point,
a secondary process involving the condensation of moisture on top of the primary adsorbed
layer (multilayer adsorption) represented by the second part of the curve, occurred. For the
isotherms at 27.2 °C and 51 °C, he found a similar slope (1.14) for the isotherms below the
transition point and a similar, but much steeper, slope (5.55) above the transition point.
Based on these similar slopes, he calculated that the heat of adsorption involved in the
primary process to be 43 cal g -1 (i.e. 180 J g -1 ) or 775 cal mol -1 (i.e. 3240 J mol -1 ), and in the
secondary process to be 19 cal g -1 (i.e. 80 J g -1 ) or 342 cal mol -1 (i.e. 1430 J mol -1 ).
Kelly concluded that the adsorbed water is firmly attached to the sugar cane fibre, and part
of this water represents the so-called ‘hygroscopic water’ of Behne (1937), which is also
known as Brix-free water.
Foster (1962) conducted vapour sorption experiments on pre-dried cane fibre of variety
Q 50 at 20 °C for 35 – 95% relative humidity. He subsequently extrapolated his results to
100% relative humidity and quoted a value of 25% for Brix-free water.
181