Lynne Wong's PhD thesis
Lynne Wong's PhD thesis 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 203
vaporisation generally decreases from the first to the third region. The nine sugar cane components aged 36 weeks also exhibit type II isotherms (Fig 5.9). Stalk fibre Stalk pith Rind fibre 4 4 4 ln(EMC/% db) 3 2 1 3 2 1 3 2 1 0 0 2 4 6 0 0 2 4 6 0 0 2 4 6 Rind fines Top fibre Dry leaf fibre 4 4 4 ln(EMC/% db) 3 2 1 3 2 1 3 2 1 0 0 2 4 6 0 0 2 4 6 0 0 2 4 6 Dry leaf fines Green leaf fibre Green leaf fines 4 4 4 ln(EMC/% db) 3 2 1 3 2 1 3 2 1 0 0 2 4 6 0 0 2 4 6 0 0 2 4 6 ln (water vapour pressure/mm Hg) 30°C 45°C 55°C 60°C Figure 5.6. Adsorption isotherms of nine cane components aged 52 weeks (as per Kelly’s method, 1957). Note the similar behaviour of all the nine cane components. 204
- Page 205 and 206: 4.7.3 Statistical analysis It is es
- Page 207 and 208: Table 4.23. Analysis of variance (B
- Page 209 and 210: pointing out that at 52 weeks old,
- Page 211 and 212: The crop of R 570 sampled in 2001 w
- Page 213 and 214: 4.7.4. Estimated Brix-free water co
- Page 215 and 216: The main difference in the two sets
- Page 217 and 218: Table 4.27. Predicted Brix-free wat
- Page 219 and 220: 4.8 SUMMARY AND CONCLUSIONS An anal
- Page 221 and 222: component parts, and verify the Bri
- Page 223 and 224: 3) Thermodynamic, water in equilibr
- Page 225 and 226: Langmuir (1916, 1917, 1918) propose
- Page 227 and 228: to determine the moisture sorption
- Page 229 and 230: Table 5.1. Some commonly used isoth
- Page 231 and 232: Lomauro et al. (1985) found that wi
- Page 233 and 234: and on agricultural products such a
- Page 235 and 236: Bruijn (1963) studied the mass incr
- Page 237 and 238: After measuring the EMC of dry corn
- Page 239 and 240: approached, that is, either by adso
- Page 241 and 242: Table 5.4. Water activity (a w ) of
- Page 243 and 244: 5.6.3 Procedure to determine equili
- Page 245 and 246: 5.6.4 Results and discussion An exa
- Page 247 and 248: Table 5.8. Equilibrium moisture con
- Page 249 and 250: Table 5.10. Equilibrium moisture co
- Page 251 and 252: Table 5.12. Equilibrium moisture co
- Page 253 and 254: 30 o C 45 o C 55 o C 60 o C Water w
- Page 255: m/m of 96% activity, a w (g/100g dr
- Page 259 and 260: 30 o C isotherm 45 o C isotherm 55
- Page 261 and 262: 4 0 Stalk fibre 5 0 Stalk pith 5 0
- Page 263 and 264: 5.6.4.4 Fitting of sorption models
- Page 265 and 266: Table 5.19. Parameters of the sorpt
- Page 267 and 268: Table 5.21. Parameters of the sorpt
- Page 269 and 270: Table 5.23. Parameters of the sorpt
- Page 271 and 272: Table 5.25. Parameters of the sorpt
- Page 273 and 274: Table 5.27. Parameters of the sorpt
- Page 275 and 276: Modified GAB Kuhn Iglesias - Chirif
- Page 277 and 278: Table 5.28. Classification of resid
- Page 279 and 280: Stalk fibre Stalk pith Rind fibre 4
- Page 281 and 282: 5.6.4.5 Calculated EMC values of re
- Page 283 and 284: Table 5.30. Calculated equilibrium
- Page 285 and 286: m/m of 96% Table 5.32. Calculated e
- Page 287 and 288: Table 5.33. Parameters of the Hailw
- Page 289 and 290: CHAPTER 6. PROPERTIES OF THE SORBED
- Page 291 and 292: where m is the equilibrium moisture
- Page 293 and 294: Stalk fibre Stalk pith Rind fibre 8
- Page 295 and 296: Stalk fibre Stalk pith Rind fibre 4
- Page 297 and 298: 6.2 THE NUMBER OF ADSORBED MONOLAYE
- Page 299 and 300: 6.3 TOTAL SOLID SURFACE AREA AVAILA
- Page 301 and 302: Thus, for each cane component of ea
- Page 303 and 304: abscissa. For each moisture level (
- Page 305 and 306: Stalk fibre Stalk pith Rind fibre 1
vaporisation generally decreases from the first to the third region. The nine sugar cane<br />
components aged 36 weeks also exhibit type II isotherms (Fig 5.9).<br />
Stalk fibre Stalk pith Rind fibre<br />
4<br />
4<br />
4<br />
ln(EMC/% db)<br />
3<br />
2<br />
1<br />
3<br />
2<br />
1<br />
3<br />
2<br />
1<br />
0<br />
0 2 4 6<br />
0<br />
0 2 4 6<br />
0<br />
0 2 4 6<br />
Rind fines Top fibre Dry leaf fibre<br />
4<br />
4<br />
4<br />
ln(EMC/% db)<br />
3<br />
2<br />
1<br />
3<br />
2<br />
1<br />
3<br />
2<br />
1<br />
0<br />
0 2 4 6<br />
0<br />
0 2 4 6<br />
0<br />
0 2 4 6<br />
Dry leaf fines Green leaf fibre Green leaf fines<br />
4<br />
4<br />
4<br />
ln(EMC/% db)<br />
3<br />
2<br />
1<br />
3<br />
2<br />
1<br />
3<br />
2<br />
1<br />
0<br />
0 2 4 6<br />
0<br />
0 2 4 6<br />
0<br />
0 2 4 6<br />
ln (water vapour pressure/mm Hg)<br />
30°C<br />
45°C<br />
55°C<br />
60°C<br />
Figure 5.6. Adsorption isotherms of nine cane components aged 52 weeks (as per Kelly’s<br />
method, 1957).<br />
Note the similar behaviour of all the nine cane components.<br />
204
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