When using Aspen Plus to model any piece of equipment, there are certain assumptions that the software automatically makes to ensure that it will have the ability to converge to a solution.
For modeling a Convective Dryer, the assumptions that are made are as follows:
– The Dryer is adiabatic (no heat losses).
– The Dryer gas and its particles pass the dryer in plug flow.
– The particles are ideally mixed in the lateral direction.
– The moisture content and temperature are spatially constant in every particle.
– Intraparticle drying resistances are taken into account by applying the concept of normalized, single-particle drying curve of van Meel.

The other assumption that we are making is that the air is in a counter current and cross-flow configuration. Figure 1 depicts what counter current flow looks like in practice. Counter current flow is used to put the hottest air to the driest part of the product. Figure 2 depicts cross-flow in practice, and it is used because it provides the most efficient heating of the material. Combining the two diagrams would give you the air flow path that we are trying to model.

References:
[1] Aspen Plus V11 Help Menu. AspenTech, 2019.