Moisture, diffusion effects

The extent of hydration or solvation of a molecule also has a profound effect on the transport of the substance. The apparent solubility of the drug in both aqueous and nonaqueous media may be influenced by the absence or presence of moisture. Diffusion of drugs in polymeric systems may also be influenced by the hydration of the polymers and hydration of the membrane through which transport is occurring for example, skin hydration may enhance the diffusion of drug molecules significantly. 

Although effective moisture diffusivity decreased normally in osmotically treated fruits, there are some quality characteristics that are always better in osmodehydrated-dried than in fresh-dried ones. 

Another factor augmectting the heat conductivuty of plastic foams under conditions is the absorbed moisture. For example, for CCljF-foamed polyurethane at 25 °C and a relative humidity of 65%, the ambient moisture diffusion rate is 10-20 g/m for 24 h. Especially strong is the effect of moisture on heat conductivity if the temperature differential across the sample is considerable. For example, in plastic foams used in cryogenic technology, the inner layers are exposed to low temperatures the water vapor first condenses and is then convected into ice. Since the thermoconductivity of water and ice are 0.5 and 1.5 kcal/m x h °C, respectively, even minor tunounts have a considerable detrimental effect of the heat insulating capacity of a foam material... 

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). 

The effect of glass-transition temperature on moisture diffusivity. Diamonds represent moisture diffusivity data while squares represent Tg as a function of a. Once the matrix undergoes glass transition, diffusion rates increases dramatically. 

Gabas, A.L., Telis-Romero, J., and Menegalli, EC. Determination of concentration dependent effective moisture diffusivity of plums based on shrinkage kinetics. Transport Phenomena in Food Processing, J. Welti-Chanes, J.F. Velez-Ruiz and G.V. Barbosa-Canovas, eds., CRC Press, Boca Raton, FL, pp. 153, 2002. 

Approximate Ranges of Effective Moisture Diffusivity in Some Materials... 

As a result, the thermal strain in a laminate composite will relax (i.e. reduced) on moisture diffusion. The extent of moisture swelling by resins used for composites can vary significantly, so the effect on the residual stress state differs widely. However, the major problem is not the benefits associated with thermal stress relaxation, but the fact that the plasticisation of the matrix may enhance its expansion coefficient. Therefore, on cooling, a wet resin matrix either from its cure temperature or after a thermal cycle can, according to Eqn (12.9), lead to higher values of e and Figure 12.14... 

Of particular importance is the timescale over which diffusion occurs under various conditions of relative humidity (RH) and temperature. The RH determines the equilibrium moisture concentration, whereas higher temperatures will accelerate the moisture sorption process. In order to predict the moisture profile in a particular structure, it is assumed that Fickian diffusion kinetics operate. It will be seen later that many matrix resins exhibit non-Fickian effects, and other diffusion models have been examined. However, most resin systems in current use in the aerospace industry appear to exhibit Fickian behaviour over much of their service temperatures and times. Since the rate of moisture diffusion is low, it is usually necessary to use elevated temperatures to accelerate test programmes and studies intended to characterize the phenomenon. Elevated temperatures must be used with care though, because many resins only exhibit Fickian diffusion within certain temperature limits. If these temperatures are exceeded, the steady state equilibrium position may not be achieved and the Fickian predictions can then be inaccurate. This can lead to an overestimate of the moisture absorbed under real service conditions. 

Effective thermal conductivity and effective moisture diffusivity are related to internal heat and mass transfer, respectively, while air boundary heat and mass transfer coefficients are related to external heat and mass transfer, respectively. The above transport properties are usually coefficients in the corresponding flow rate and driving force relationship. The equilibrium material moisture content, on the other hand, is usually related to the mass transfer driving force. 

Effective moisture diffusivity and effective thermal conductivity are in general functions of material moisture content and tanperature, as well as of the material structure. Air boundary coeffiamts are functions of the conditions of the drying air, that is humidity, tonperature, and velocity, as well as system geometry. Equilibrium moisture content of a given material is a function of air humidity and temperature. The drying constant is a function of material moisture content, temperature, and thickness, as weU as air humidity, tonperature, and velocity. 

Equations T3.5 and T3.6 in Table 4.3 are obtained by considering the activation energy for diffusion as a function of material moisture content. Equations T3.7 through T3.10 are not based on the Arrhenius form. They are empirical and they use complicated functions concerning the discrimination of the moisture and temperature effects (except, of course. Equation T3.7). Equation T3.11 is more sophisticated as it considers different diffusivities of bound and free water and introduces the functional dependence of material moisture content on the binding energy of desorption. Equation T3.12 introduces the effect of porosity on moisture diffusivity. 

FIGURE 4.5 Effect of material moisture content and temperature on moisture diffusivity. (Data for potato are from Kiranoudis, C.T. et al.. Drying TechnoL, 10(4), 1097, 1992 and data for clay brick are from Haertling, M., in Drying 80, Vol. 1, Mujumdar, A.S. (ed.). Hemisphere Publishing, New York, 1980, pp. 88-98.)... 

The effect of pore structure and distribution on moisture diffusion can be examined by considering the material as a two-(or multi-) phase (dry material, water, air in voids, etc.) systan and by considering some structural models to express... 

Despite the limited data of effective moisture diffusivity, a lot of data are reported in the literature for thermal conductivity. Data for mainly homogeneous materials are available in handbooks such as the Handbook of Chemistry... 

Effective moisture diffusivity Air boundary mass transfer coefficient Effective thermal conductivity Air boundary heat transfer coefficient... 

The transport properties of foods received much attention in the literature [184-188]. The main results presented by Saravacos and Maroulis [188] are summarized in this section. The results refer to moisture diffusivity and thermal condnc-tivity. Recently published values of moisture diffusivity and thermal conductivity in various foods were retrieved from the literature and were classified and analyzed statistically to reveal the influence of material moisture content and tempera-tnre. Empirical models relating moisture diffusivity and thermal conductivity to material moisture content and temperature were fitted to all examined data for each material. The data were screened carefully using residual analysis techniques. A promising model was proposed based on an Arrhenius-type effect of temperature, which uses a parallel structural model to take into account the effect of material moisture content. 

Tong, C.H. and Lund, D.B., Effective moisture diffusivity in porous materials as a function of temperature and moisture content, Biotechnol. Prog., 6(l) 67-75,1990. 

Bed temperature is increased by high external heat fluxes. This in turn leads to higher moisture diffusivities and hence higher drying rate. This effect is complex and depends on the relative significance of external and internal resistances to moisture transfer. 

Several researches have been carried out to understand the mechanism of moisture movement in clay during drying. Newitt et al. [12] and Wakabayashi [13] investigated the moisture movanent in clay by liquid diffusion and vapor diffusion, which affect the drying characteristics particularly the falling rate. They concluded that the liquid diffusion dominates the movement until about 20%-dry basis in moisture content for stoneware clay and 30% for the mixture of 80% Kibushi clay and 20% feldspar. Wakabayashi [14] also evaluated the effective moisture diffusion coefficient of some sorts of clay such as Kibushi, Gairome, stoneware, feldspar, and their mixtures. The effective diffusion coefficient is available for the brief description of the moisture movement behavior. The effective diffusion coefficient D can be defined by... 

---