Thermal diffiision process

McKay G. and McConvey I.F., The external mass transfer of basic and acidic dyes by wood. J Chem Technol Biotechnol 31 (1981) pp. 401-408 McKay G., Blair H.S. and Gardner J., The adsorption of dyes in diitin II. intraparticle diffiision processes. JAppl Polymer Sci 28 (1983) pp. 1767-1778 Staszczuk P., Stefaniak E. and Dobrowolski R., Characterisation of thermally treated dolomite. Powder Technol 92 (1997) 257... 

Separation of mixtures based on differences in thermal diffiisivity at present are feasible only for analytical purposes or for production on a very small scale of substances not otherwise recovered easily. Nevertheless, the topic is of some interest to the process engineer as a technique of last resort. 

Numerical results reported (2) on a typical TGDDM-DDS matrix laminate, assuming that the prepregs are suddenly expose to die cure temperature, are diown in Fig. 24 (a,b,c) as me variation of die tenqierature, decree of reaction and viscosity as a function of the processing time, bodi on the dam and on the core of the laminate. Input data of die full model are givmi in Table 9 (2). Due to the contribution of die thermal conductivity of the fibm the tenqierature at the center of die laminate nqiidly reaches the external inqiosed temperature and increases as a con uence of die imbalance between the rate of heat generation and the thermal diffiisivity of the composite (Fig. 24a). When these two quantities are comparable, the temperature profile reaches a maximum. 

Polymer processing is influenced by the thermal characteristics of polymer. They are melt temperature, glass transition temperature, thermal conductivity, thermal diffiisivity, heat capacity, coefficient of linear thermal expansion, and decomposition temperature. The effect of processing (shear and heat) is expected to have an effect on the polymer at the molecular level, reducing molar mass and altering its distribution [3]. 

Whereas heat capacity is a measure of energy, thermal diffiisivity is a measure of the rate at which energy is transmitted through a given plastic. It relates directly to process-ability. In contrast, metals have values hundreds of times larger than those of plastics. 

TPs properties (and processes) are influenced by their thermal characteristics such as melt temperature (T ), glass-transition temperature (Tg), dimensional stability, thermal conductivity, thermal diffiisivity, heat capacity, coefficient of thermal expansion, and decomposition (Tj). Table 6.1 provides some of this data on different plastics (also applicable data for aluminum and steel). All these thermal properties relate to how to determine the best useful processing conditions to meet product performance requirements. There is a maximum temperature or, to be more precise, a maximum time-to-temperature relationship for all materials preceding loss of performance or decomposition. 

The Diffiision Limit Model. In this model heat and mass in the droplet are transported by diffusion alone. Since hquid-phase mass diffu-sivities are typically of the order of 10 cm /sec, which, when compared with typical droplet surface regression rates of 10 cm /sec for combustion and lO -lO cm /sec for pure vaporization, shows that except for very slow rates of vaporization or very small droplet sizes (89), hquid-phase mass diffusion is the rate-limiting process. Hence most of the more volatile compounds will be trapped within the droplet interior, unable to reach the droplet surface to become preferentially vaporized. Further reahzing that the hquid-phase thermal diffusivities are of the order of 10" cm /sec, the droplet interior will be heated substantially during... 

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