Lynne Wong's PhD thesis
Lynne Wong's PhD thesis Lynne Wong's PhD thesis
the adsorption isotherms of a number of gases, measured at temperatures at, or near, their respective boiling points were all S-shaped with certain distinctive features. This prompted the recognition that adsorption was not always restricted to monolayer coverage and to the emergence of the Brunauer-Emmett-Teller (BET) theory (1938) of multilayer adsorption. This type of isotherm shows the amount adsorbed increases with increasing pressure up to a point B, then levels off and starts to increase again at higher pressures; it was later referred to as a type II isotherm. Point B is usually considered to represent the completion of the monolayer and the beginning of the formation of the multilayer. In addition to the type I and II isotherms described above, there are type III isotherms, where initially there is very little adsorption, then once a small droplet of adsorbate nucleates on the surface, additional adsorption occurs more easily because of strong adsorbate-adsorbate interactions. Type IV and type V isotherms occur when multilayers of gas adsorb onto the surface of the pores in a porous solid. Initially, the adsorption resembles that of type II or type III adsorption, then the adsorbed layer gets so thick that it fills up the pores, no more gas can adsorb, and the isotherm saturates. Both type IV and V isotherms exhibit a hyteresis loop, the lower curve of which represents measurements obtained by progressive addition of gas to the adsorbent, and the upper curve by progressive removal. These five classifications of isotherms were proposed by S. Brunauer, L.S. Deming, W.S. Deming and E. Teller and are termed the BDDT classification (1940), and are sometimes also referred to as the Brunauer classification (1945). The classification proposed by the International Union of Pure and Applied Chemistry (IUPAC) in 1985 (Sing et al., 1985), shown in Fig 5.1, includes a type VI isotherm which has been observed more recently. It appears as a stepped isotherm, associated with layerby-layer adsorption on a highly uniform surface. This type of isotherm is relatively rare (Rouquerol et al., 1999). 5.3 ADSORPTION OF MOISTURE When a hygroscopic material is maintained in contact with air at constant temperature and humidity until equilibrium is reached, the material will attain a definite moisture content. This moisture is termed the equilibrium moisture content (EMC) under the specified conditions. In order to characterise this water sorption mechanism, it is common practice 173
to determine the moisture sorption isotherm, which describes the EMC of the material and the water activity or relative humidity at a certain temperature. The water in a material is generally measured in terms of water activity, a w , which by definition is given by : water vapour pressure at the solid-gas interface vapour pressure of liquid at the same temperature Thus, when a moist material is in equilibrium with its surroundings, the water vapour pressure of the material is equal to the partial pressure of water vapour in the atmosphere, and so, the water activity in the material is equal to the equilibrium relative humidity of the air. Knowledge of the water adsorption characteristics is needed for shelf life predictions of products that deteriorate mainly by moisture gain and is important in drying, packaging and storage. Thermodynamic properties of the material relate the concentration of water in the material to its partial pressure, which is crucial in the analyses of heat and mass transport phenomena during drying. The EMC determines the end-point to which the material must be dehydrated in order to achieve a stable product with optimal moisture content, and yield a theoretical minimum amount of energy required to remove a given amount of water from the material. The properties also provide an insight into the microstructure associated with the material as well as the theoretical interpretation of physical phenomena occurring at the material-water interface. The control of moisture content is particularly important in foods as well as in materials, such as woody straw fibres, during processing and storage because water has many roles in substrate reactions and keeping quality. In this respect the moisture sorption isotherm is an extremely important tool as it can be used to predict changes in substrate stability and to select appropriate packaging material and ingredients. There are many works on moisture sorption isotherms of substrates over the last two decades; some deal with the determination of moisture sorption isotherms, others, with the development of mathematical models to represent the moisture sorption isotherms. 5.3.1 Moisture sorption isotherm models The models available in the literature to describe water sorption isotherms can be divided into several categories: kinetic models based on a multilayer adsorption mechanism (BET model), kinetic models based on a multilayer and condensed film mechanism 174
- Page 175 and 176: Table 4.3. Comparison of Brix-free
- Page 177 and 178: Table 4.4. Results of the determina
- Page 179 and 180: In order to test for homogeneity of
- Page 181 and 182: Table 4.7. Results of the repeat de
- Page 183 and 184: The experiment was repeated with th
- Page 185 and 186: e any residual moisture in the samp
- Page 187 and 188: By means of the same technique, Won
- Page 189 and 190: was still hot. Since the filter was
- Page 191 and 192: value determined could be corrected
- Page 193 and 194: Qin and White’s finding was confi
- Page 195 and 196: A sample size of 3.5 g with 75 g co
- Page 197 and 198: Figure 4.4. Fibre samples drying in
- Page 199 and 200: - One large fibre sample (rind) of
- Page 201 and 202: Table 4.18. Brix-free water values/
- Page 203 and 204: Table 4.20. Brix-free water values/
- 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: Langmuir (1916, 1917, 1918) propose
- 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 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 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
to determine the moisture sorption isotherm, which describes the EMC of the material and<br />
the water activity or relative humidity at a certain temperature.<br />
The water in a material is generally measured in terms of water activity, a w , which by<br />
definition is given by :<br />
water vapour pressure at the solid-gas interface<br />
vapour pressure of liquid at the same temperature<br />
Thus, when a moist material is in equilibrium with its surroundings, the water vapour<br />
pressure of the material is equal to the partial pressure of water vapour in the atmosphere,<br />
and so, the water activity in the material is equal to the equilibrium relative humidity of the<br />
air. Knowledge of the water adsorption characteristics is needed for shelf life predictions<br />
of products that deteriorate mainly by moisture gain and is important in drying, packaging<br />
and storage.<br />
Thermodynamic properties of the material relate the concentration of water in the material<br />
to its partial pressure, which is crucial in the analyses of heat and mass transport<br />
phenomena during drying. The EMC determines the end-point to which the material must<br />
be dehydrated in order to achieve a stable product with optimal moisture content, and yield<br />
a theoretical minimum amount of energy required to remove a given amount of water from<br />
the material. The properties also provide an insight into the microstructure associated with<br />
the material as well as the theoretical interpretation of physical phenomena occurring at the<br />
material-water interface.<br />
The control of moisture content is particularly important in foods as well as in materials,<br />
such as woody straw fibres, during processing and storage because water has many roles in<br />
substrate reactions and keeping quality. In this respect the moisture sorption isotherm is an<br />
extremely important tool as it can be used to predict changes in substrate stability and to<br />
select appropriate packaging material and ingredients. There are many works on moisture<br />
sorption isotherms of substrates over the last two decades; some deal with the<br />
determination of moisture sorption isotherms, others, with the development of<br />
mathematical models to represent the moisture sorption isotherms.<br />
5.3.1 Moisture sorption isotherm models<br />
The models available in the literature to describe water sorption isotherms can be divided<br />
into several categories: kinetic models based on a multilayer adsorption mechanism (BET<br />
model), kinetic models based on a multilayer and condensed film mechanism<br />
174
Change language