Simultaneous heat and mass transfer in packed bed brying of seeds having a mucilage coating

Abstract

-The simultaneous heat and mass transfer between fluid phase and seeds having a mucilaginous coating was studied during packed bed drying. To describe the process, a two-phase model approach was employed, in which the effects of bed shrinkage and nonconstant physical properties were considered. The model took into account bed contraction by employing moving coordinates. Equations relating shrinkage and structural parameters of the packed bed with moisture content, required in the drying model, were developed from experimental results in thick-layer bed drying. The model verification was based on a comparison between experimental and predicted data on moisture content and temperature along the bed. Parametric studies showed that the application of correlations capable of incorporating changes in bed properties gives better data simulation. By experimental-theoretical analysis, the importance of shrinkage for a more accurate interpretation of heat and mass transfer phenomena in the drying of porous media composed of mucilaginous seeds is corroborated.

Significance of Bed Shrinkage on Heat and Mass Transfer During the Transport Phenomenon of Humid Air

“…Due to the contraction of the bed's volume, the variations among the predictions become clearer as the rate of heat transfer is increases. At this stage, we need to suggest perfectly assumptions to control the variation in temperature and the moisture content of the product during drying in order to have a good product quality and to consume the minimum amount of energy [89].…”

Heat and Mass Transfer in Packed Bed Drying of Shrinking Particles

“…Image analysis has become one of the most common techniques for shrinkage evaluation. It is a non-destructive method that allows continuous measurement of important parameters during drying such as area, perimeter, major and minor axis length, Feret diameter, elongation and roundness (Prado & Sartori, 2008;Yadollahinia & Jahangiri, 2009;Ramos et al, 2010). The shrinkage of granular beds has been determined from vertical bed displacement, which can be measured with a linear potentiometer (Lang et al, 1993).…”

“…Therefore, simplified models still remain popular in obtaining values for D ef . Thus, as thin-layer drying behavior of mucilaginous seeds was characterized by occurring in constant and falling rate periods, a two-stage mathematical model (constant rate period and falling rate period) for thin layer drying was developed to be incorporated in the deep bed model, in order to obtain a better prediction of drying rate at each period Prado & Sartori (2008). The drying rate equation for the constant rate period was described as: …”

“…Deep-bed drying experiments were carried out in a typical packed bed dryer (Prado & Sartori, 2008). The experiments were conducted at air temperatures ranging from 30 to 50 o C and air velocities from 0.5 to 1.5 m/s, defined by a 2 3 factorial design.…”

“…However, it should be pointed out that the drying of deformable particles is a problem of significant difference from other drying processes involving rigid particulate material, for which the aforementioned assumptions are valid. As a result, efforts have been directed to characterize physical changes in different materials during drying (Koç et al, 2008;Mayor & Sereno, 2004) as well as to adequate models of packed bed dryers to simulate heat and mass transfer in a deformable porous medium (Prado & Sartori, 2008;Bialobrzewski et al, 2008;Mihoubi & Bellagi, 2008;Kowalski et al, 2007), so that particlefluid interactions can be understood and drying optimized. Such investigations are important for the development of more efficient dryers, once they could lead to reductions in energy consumption when it comes to air flow requirements for materials that present higher bulk porosity upon shrinkage.…”