Lynne Wong's PhD thesis

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6.5 THE NET ISOSTERIC HEAT OF SORPTION q st , THE TOTAL ISOSTERIC HEAT OF SORPTION Q st AND THE ENTROPY OF SORPTION S d The net isosteric heat of sorption q st is defined as the total heat of sorption Q st in the food or fibre minus the latent heat of vaporisation of water H L , at the system temperature (Tsami et al., 1990). Conventionally, q st is a positive quantity when heat is evolved during adsorption and negative when heat is absorbed during desorption. It is indicative of the intermolecular attractive forces between the sorption sites and water vapour (Wang and Brennan, 1991). The values of the net isosteric heat of sorption q st are obtained from the slopes of ln a w versus 1/T plots by linear regression analysis with the assumption that they are constant over the temperature range studied. According to the differential form of the Clausius Clapeyron equation (Labuza, 1984): ⎡ ⎢ ⎣ d d ( ln aw ) ⎤ qst = − ( 1/ T) ⎥ R ⎦ m (3) where a w is the water activity, T is the Kelvin temperature (K), m is the equilibrium moisture content, q st is the net isosteric heat of sorption (kJ mol -1 ), and R is the universal gas constant (8.314 J mol -1 K -1 ). Since equation (3) holds only for constant moisture content, values of the water activity a w at specified moisture contents need to be evaluated from the best-fit isotherm equation (Hailwood-Horrobin model in this case). In this study the water activity at 18 fixed moisture values (namely 0.01, 0.05, 0.1, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30 and 35 g/g db) were calculated. In order to do this, the Hailwood-Horrobin equation: aw m = b + c a w + d( a w ) 2 was written in the form of a quadratic equation: d a 2 w 1 + ( c − ) aw + b = m 0 and solved for a w at the chosen values of m. Only the positive values of a w are taken. This was effected at four temperatures (i.e. 30, 45, 55 and 60 °C) for the same chosen values of the moisture content. For each moisture content, ln a w calculated for the four temperatures are plotted as ordinate against 1/T as 249
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
Page 1 and 2:
THE BRIX-FREE WATER CAPACITY AND SO
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dissolved and hydrated waters, and
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ACKNOWLEDGEMENTS I am particularly
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Page 1.5 THE DELETERIOUS EFFECTS OF
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Page 2.2 THE PHENOMENON OF BRIX-FRE
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Page 3.4.3.3 Cane tops 83 3.4.4 Cha
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4.3.3 Temperature at which Brix-fre
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4.6.1 Materials 143 4.6.1.1 Samples
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CHAPTER 6. PROPERTIES OF THE SORBED
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APPENDIX 3. CALCULATIONS LEADING TO
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LIST OF FIGURES Page Figure 1.1. Fi
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Figure 3.1. Glucose and fructose an
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Figure 5.11. Residual plots for the
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total adsorbed water (m) and the pr
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Table 2.18. Moisture content in sug
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Page Table 4.4. Results of the dete
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Page Table 4.24. Analysis of varian
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Page Table 5.13. Table 5.14. Equili
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Table 6.3. Heat of sorption of the
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GLOSSARY OF TERMS Absorption is the
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Filterability of a raw sugar is mea
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Sorption is the generic term used w
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LIST OF MAIN SYMBOLS Symbol Descrip
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s c s Slope of Caurie I isotherm pl
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number of 255, and cane land covere
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Nouvelle Mon In Trésor ustrie and
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Figure 1.3. Cane sampling by core s
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In Mauritius, most of the sugar fac
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are: cane tops, dry and green leave
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1.4 TRENDS IN CANE QUALITY RECEIVED
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campaign was launched to encourage
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The level of extraneous matter in c
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In Australia (Cargill, 1976), cane
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The effect of soil on factory perfo
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leaves increased the level of impur
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• From 1976 to 1980, when the pro
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Clerget purity of molasses 40 Clerg
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CHAPTER 2. IMPACT OF EXTRANEOUS MAT
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Since the extrapolated purity of mo
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Figure 2.1. Jeffco cutter grinder.
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2.1.4 Results The analytical result
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Table 2.3. Analytical results of re
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Table 2.5. Composition of dry trash
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Table 2.7. Predicted factory perfor
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Boiling house recovery 91.0 89.8 89
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0 5 10 15 20 % EM in cane y = 0.572
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% EM in cane 0 5 10 15 20 0 -2 -4 -
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1 y = 0.020 (% D) R 2 = 1.00 = 0.03
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% EM in cane 0 5 10 15 20 0 -2 y =
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esulting in 0.015 unit sucrose loss
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2.2.1 Experimental procedure Cane m
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filter paper, rejecting the first f
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Table 2.9. Effect of increased addi
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Table 2.11. Effect of increased add
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Table 2.13. Effect of increased add
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various components such as stalk fi
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in the presence of dry leaves, if c
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Table 2.17. Moisture content in sug
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CHAPTER 3. SEPARATION OF THE SUGAR
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Table 3.1. It can be seen that the
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Table 3.2. Fibrous physical composi
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R 579 R 570 M 1557/70 M 1400/86 74
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loosen the fibre. The woody core is
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agitate the mixture in the pot, and
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Figure 3.7. Custom-built fibre-pith
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The extraction of fibres starting f
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pre-treatment in a Jeffco cutter-gr
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Figure 3.19. Stalk cake washed free
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not have many dry leaves attached t
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Table 3.5. Masses of cane samples a
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Table 3.7. Masses of cane samples a
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Table 3.9. Masses of cane component
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3 57.6 126.3 23.7 97.2 31.1 53.8 11
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Table 3.12. Material loss (%) from
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3.5.3 Fibre/pith ratios in cane com
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Table 3.15. Effect of extraneous ma
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Snow (1974) investigated the season
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from Figure 2.9 that the change in
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Table 3.18. Fibre % cane results by
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110
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(a). Dry leaf (b). Green leaf (c).
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dry fibre, or a factor, is used in
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Steuerwald (1912) applied sucrose s
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solution/fibre ratio was lowered fr
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leave some residual moisture on the
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instead of 150 g of 10° Brix sucro
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Table 4.2. Determination of Brix-fr
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Table 4.3. Comparison of Brix-free
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Table 4.4. Results of the determina
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In order to test for homogeneity of
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Table 4.7. Results of the repeat de
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The experiment was repeated with th
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e any residual moisture in the samp
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By means of the same technique, Won
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was still hot. Since the filter was
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value determined could be corrected
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Qin and White’s finding was confi
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A sample size of 3.5 g with 75 g co
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Figure 4.4. Fibre samples drying in
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- One large fibre sample (rind) of
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Table 4.18. Brix-free water values/
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Table 4.20. Brix-free water values/
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4.7.3 Statistical analysis It is es
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Table 4.23. Analysis of variance (B
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pointing out that at 52 weeks old,
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The crop of R 570 sampled in 2001 w
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4.7.4. Estimated Brix-free water co
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The main difference in the two sets
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Table 4.27. Predicted Brix-free wat
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4.8 SUMMARY AND CONCLUSIONS An anal
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component parts, and verify the Bri
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3) Thermodynamic, water in equilibr
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Langmuir (1916, 1917, 1918) propose
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to determine the moisture sorption
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Table 5.1. Some commonly used isoth
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Lomauro et al. (1985) found that wi
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and on agricultural products such a
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Bruijn (1963) studied the mass incr
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After measuring the EMC of dry corn
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approached, that is, either by adso
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Table 5.4. Water activity (a w ) of
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5.6.3 Procedure to determine equili
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5.6.4 Results and discussion An exa
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Table 5.8. Equilibrium moisture con
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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 and 256: m/m of 96% activity, a w (g/100g dr
Page 257 and 258: vaporisation generally decreases fr
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 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 297 and 298: 6.2 THE NUMBER OF ADSORBED MONOLAYE
Page 299 and 300: 6.3 TOTAL SOLID SURFACE AREA AVAILA
Page 301: Thus, for each cane component of ea
Page 307 and 308: A similar procedure was followed to
Page 309 and 310: 10 0 Stalk fibre Stalk pith Rind fi
Page 311 and 312: Moreover, if T β > T hm the proces
Page 313 and 314: Table 6.5. Characteristic parameter
Page 315 and 316: Binding energy/kJ (kg mol) -1 2 0 0
Page 317 and 318: 6.8 CALCULATION OF BOUND WATER AND
Page 319 and 320: The values of K 1 , K 2 and W were
Page 321 and 322: Table 6.7. Separation of the total
Page 323 and 324: Table 6.7. (Contd.) Sample 30 o C 4
Page 325 and 326: 3 0 S talk fibre 4 0 Stalk pith 3 0
Page 327 and 328: 3 0 Reconstituted cane at 30 o C 3
Page 329 and 330: when water is added to dry wood, wh
Page 331 and 332: It is evident that in some cases ma
Page 333 and 334: The number of adsorbed monolayers,
Page 335 and 336: Data in Tables 2.9 and 2.11 show th
Page 337 and 338: particular fibre is systematically
Page 339 and 340: Anon. (1985b). Laboratory manual fo
Page 341 and 342: Blanchi R.H. and A.G. Keller (1952)
Page 343 and 344: Day D.L. and G.L. Nelson (1965). De
Page 345 and 346: Heyrovsky J. (1970). Determination
Page 347 and 348: Kuhn I.J. (1964). A new theoretical
Page 349 and 350: Madamba P.S., R.H. Driscoll and K.A
Page 351 and 352: Prinsen Geerligs, H.C. (1897). Stud
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Sing K.S.W., D.H. Everett, R.A.W. H
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Van der Pol C., C.M. Young and K. D
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Lynne Wong's PhD thesis

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