The model describes the drying process as a penetration process. It is assumed that there is a drying front that moves from the equipment wall into the bulk solid, parallel to the wall. Particles between the drying front and the wall are assumed to be completely dry, and particles beyond the drying front are assumed to be completely wet. This is shown schematically in Figure 2.
The heated surface is shown at the bottom. Below the drying front there is a layer of dry solids with zero moisture content. Above the drying front a layer of wet solids is shown in black. This layer of wet solids is assumed to be at boiling temperature at the prevailing pressure. The vapour phase consists of the pure liquid component.So as far as I can tell, this means that, in a tray dryer with no movement, the material you want to dry will always be furthest away from the heat source. Grrrrrrrrreat.
I've worked at places with tray dryers in the past; we had to routinely break the vacuum, go in there with a scoop and turn the stuff over by hand. It's remarkable how fast a 20L rotovap can work by comparison to a medium-sized tray dryer; one can only imagine how fast a double cone dryer works in comparison.
1. Hoekstra, L.; Vonk, P.; Hulshof, L.A. "Modeling the Scale-Up of Contact Drying Processes." Org. Process Res. Dev., 2006, 10 (3), pp 409–416.