Sorption isotherms temperature effects

Temperature Effects. Equilibrium studies have been performed on the HC1, 10 2M NaCl system at temperatures of 2°, 25°, and 50°C. No significant differences were noted in the resulting isotherms, so that heats of sorption have not been calculated. The results of rate studies on the same systems at the three different temperatures are shown in Figures 13... 

Figure A2.3.4 Effect of temperature on water sorption isotherm. (A) Shift without intersection. (B) Shift showing the point of intersection. Reprinted from Rahman and Labuza (1999), courtesy of Marcel Dekker.

Labuza, T.P., Kaanane, A., and Chen, J.Y. 1985. Effect of temperature on the moisture sorption isotherms and water activity shift of two dehydrated foods../. Food Sci. 50 385. 

L. V. C. Rees (Imperial College, London) When zeolites have increasing amounts of polar molecules sorbed in them, the cations show a great increase in mobility when certain low fillings are obtained. Is it not possible that the decrease in the heat of sorption of NH at low isotherm temperatures represents the point where the cations loosen their attachment to the framework This would be an endothermic movement of a positive charge from the negative framework. At 300 °C, the cations would have sufficient thermal energy to mask this effect. 

Conversely, the correct approach to formulate the diffusion of a single component in a zeolite membrane is to use the MaxweU-Stefan (M-S) framework for diffusion in a nonideal binary fluid mixture made up of species 1 and 2 where 1 and 2 stands for the gas and the zeohtic material, respectively. In the M-S theory it is recognized that to effect relative motions between the species 1 and 2 in a fluid mixture, a force must be exerted on each species. This driving force is the chemical potential gradient, determined at constant temperature and pressure conditions [68]. The M-S diffiisivity depends on coverage and fugacity, and, therefore, is referred to as the corrected diffiisivity because the coefficient is corrected by a thermodynamic correction factor, which can be determined from the sorption isotherm. 

Effect of Relative Humidity and Sorption History. An indirect method for estimating wood moisture content is to measure its equilibrium relative vapor pressure h. This is related to wood moisture content by a sorption isotherm. The percent relative humidity (H) or relative vapor pressure (h) (H = 100 h) is the most important factor in determining the EMC for wood. A curve showing EMC as a function of percent relative humidity or relative vapor pressure at constant temperature is called a moisture sorption isotherm. 

Effect of Temperature. The sorption isotherms for wood generally decrease with increasing temperature (Figure 9) above 0 °C. 

Velazquez de la Cruz G, Torres J, Martin-Polo M. Temperature effects on the moisture sorption isotherms for methylcellulose and ethylcellulose films. / Food Engin 2001 48 91—94. 

These results indicate that the shape of sorption isotherms of pure fluids on MCM-48 silicas, i.e. the occurence of pore condensation and sorption hysteresis as well as details of the hysteresis loop depend on the pore size and temperature. The observed hysteresis loops are of type HI, indicating that networking effects are not dominant for sorption hysteresis in the MCM-48 silica materials studied here, despite the fact that MCM-48 consists of a unique three dimensional pore network. 

Water acts as a plasticizer for soy flour (Yildiz and Kokini, 2001). Therefore, increase in water content will plasticize the matrix causing an increase in available free volume for molecular transport. Moisture diffusion as a result will be effected from the water activity of the system. The relationship between moisture content and a can be established using the moisture sorption isotherm (MSI) of soy flour. The glass transition temperature is a very important concept in the diffusion process. At the vicinity of the glass-transition temperature the diffusion process increases at a higher rate. Figure 46.1 shows the plasticization effect of moisture on soy flour and Gordon-Taylor prediction of Tg vs. moisture content (Yildiz and Kokini, 2001). 

Two selected food materials are presented as an example in Figure 4.13. Potatoes exhibit a typical behavior. Equilibrium material moisture content is increased [173]. Raisins, on the other hand, exhibit an inverse temperature effect at large water activities [174], As shown in Figure 4.13, potatoes and raisins exhibit sorption isotherms of types II and III, respectively. 

Chen, C.S. and Clayton, J.T., The effect of temperature on sorption isotherms of biological materials, Trans. ASAE, 14(5) 927-929, 1971. 

Iglesias, H.A. and Chirife, J., Prediction of effect of temperature on water sorption isotherms of food materials, J. Pood Technol., 11 109-116 (1976). 

The effect of temperature on the sorption isotherm should be accounted for. 

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