Recondensation

Recondensation refers to the undesirable recondensation of liquids that have already been evaporated within a drying apparatus. This process is particularly relevant for powder drying and contact drying, as the solid bed is in direct contact with heated surfaces or hot gas. In principle, drying processes are based on a phase transition from the liquid to the gaseous phase: the liquid evaporates and the resulting vapour is removed from the product zone.

The aim is for the vapour to leave the dryer and only be deliberately liquefied in a condenser outside the apparatus, typically after vapour and dust separation. Thermodynamically, the temperature T_T throughout the drying chamber should be above the dew point temperature T_(T,τ) of the vapour mixture, so that no condensation occurs on component surfaces. If a colder zone with T_W < T_τ is formed locally, this acts as a condensation trap: the steam encounters the cool surface, falls below its saturation state there and condenses.

The condition for condensation can be formulated using the saturated vapour pressure function p_sat(T) (e.g. via the Antoine equation). If the partial vapour pressure p_D is above the vapour pressure at saturation at the wall temperature, i.e.

p_D ​> p_sat​(T_Wall​),

condensation on this surface is thermodynamically favoured. In practice, this means: temperature gradients, dead zones with poor convection or inadequately temperature-controlled equipment components lead to recondensation.

The condensate formed can drip back into the powder bed or run off as a film along the equipment walls. This leads to local re-moistening of the product, triggering mechanistic secondary effects: increased agglomeration due to liquid bridges, caking on heating surfaces, and inhomogeneities in the residual moisture profile, right up to sticky zones in the bulk material. Such zones disrupt particle movement, alter the flow behaviour of the powder, and impair discharge and mixability.

Recondensation reduces the effective drying capacity, extends the process time and degrades product quality, as the additional moisture must be removed again. Particularly in the region of decreasing drying rates, where the product is already heated and diffusion resistance dominates, recondensation events can significantly disrupt the drying process.

For this reason, maintaining as homogeneous a temperature profile as possible within the drying apparatus is of central importance. In amixon® apparatus, this is achieved through the design by means of uniform heating of all surfaces in contact with the product, suitable insulation and aerodynamic routing of the thermal fluid. On the process side, a sufficiently high and correctly controlled gas flow rate, sufficiently large vapour filters, defined pressure and dew point conditions of the drying gas, and an operating mode adapted to the drying section all help to prevent recondensation.