Drying process constant rate period

In tests on the drying of sand, Wenzel and White 53 1 found that the use of steam rather than air did not alter the general characteristics of the drying process, and that the drying rate during the constant rate period was determined by the heat transfer rate. In these tests, the heat transferred by radiation from the steam and surrounding surfaces was 7.5-31... 

Figure 10. Illustration of drying process (24). Capillary tension develops in liquid as it stretches to prevent exposure of the solid phase by evaporation, and the network is drawn back into liquid (a). The network is initially so compliant that little stress is needed to keep it submerged, so the tension in the liquid is low, and the radius of the meniscus (rc) is large (b). As the network stiffens, the tension rises as rc decreases. At the critical point, the radius of the meniscus becomes equal to the pore radius the constant rate period ends and the liquid recedes into the gel (c). (Reproduced with permission from reference...

When the drying process covers both a constant-rate period and a falling-rate period, the X2 of Eq, (24.21) equals and the R of Eq, (24.25) equals R. The total time of drying tj- is then... 

Many researches adopted one of the aforanentioned approaches and modified it to include various aspects of the pneumatic drying process. Andrieu and Bressat [16] presented a simple model for pneumatic drying of polyvinyl chloride (PVC), particles. Their model was based on elementary momentum, heat and mass transfer between the fluid and the particles. In order to simplify their model, they assumed that the flow is unidirectional, the relative velocity is a function of the buoyancy and drag forces, solid temperature is uniform and equal to the evaporation temperature, and that evaporation of free water occurs in a constant rate period. Based on their simplifying assumptions, six balance equations were written for six unknowns, namely, relative velocity, air humidity, solid moisture content, equilibrium humidity, and both solid and fluid temperatures. The model was then solved numerically, and satisfactory agreanent with their experimental results was obtained. A similar model was presented by Tanthapanichakoon and Srivotanai [24]. Their model was solved numerically and compared with their experimental data. Their comparison between the experimental data and their model predictions showed large scattering for the gas temperature and absolute humidity. However, their comparisons for the solid temperature and the water content were failed. 

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