Lynne Wong's PhD thesis

5.6.4.2 Kelly’s type of two-equilibria isotherms
When the ln of EMC results obtained in Tables 5.8 – 5.16 were plotted against ln of vapour
pressure (Table 5.3), as previously described for Kelly’s work (1957) in Section 5.4.1, twoequilibria
isotherms were obtained for all nine cane components of R 570 aged 52 weeks
(Fig 5.6) and 36 weeks (Fig 5.7). The transition point of the primary and secondary
equilibria occurred at ln (EMC/% db) = 2.5, i.e. at EMC/% db = 12.5, compared to the
value of 14.5 found by Kelly. The regression coefficient R 2 and the Freundlich constants s
and k are compiled for 52 weeks samples (Table 5.17) and for 36 weeks samples (Table
5.18). All the slopes s of the primary equilibria for 30, 45, 55 and 60 °C isotherms are
different for all cane components, the same applied to the slopes of the secondary
equilibria for 30, 45, 55 and 60 °C isotherms. Therefore, it was not possible to calculate
the heat of adsorption involved in the primary and secondary equilibria as did Kelly, since
his calculations were based on the identical slope of 1.14 for the primary equilibria of both
27.2 °C and 51 °C isotherms, and 5.55 for their secondary equilibria.
When Kelly’s EMC data (1957) were plotted similarly, the gradients of the primary
equilibria at 27.2 °C and 51 °C were found respectively: 0.854 and 1.137, and of the
secondary equilibria at 27.2 °C and 51 °C were respectively 3.305 and 4.551.
5.6.4.3 Adsorption isotherms
The adsorption isotherms for the nine sugar cane component parts aged 52 weeks are
shown in Fig 5.8. Typical S-shape curves referred to as type II isotherms were found.
According to Van den Berg and Bruin (1981), type II isotherms can be divided into three
different regions: in the first region at low water activity, there is monolayer adsorption of
water held by strong hydrophilic bonds on polar sites by Van der Waal forces. In the
second region, called the multilayer region, water is more loosely held by hydrogen bonds
and is under transition to the natural properties of free water. The least firmly bound water
occurs when a w is above 0.6. In the third region, the isotherm rises steeply as practically
free water becomes mechanically entrapped in the void spaces of the material, mainly as a
result of capillary condensation. Water uptake in the first region is normally rapid, slows
down in the second region and is accelerated in the third region. The adsorbed water can
be classified as monolayer, multilayer or condensed capillary water. The enthalpy of
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