Evaporation effects on residence time in wiped film evaporators with roller wipers

The influence of evaporation on the residence time distribution in a wiped film evaporator (WFE) with movable roller wipers and different solvents was investigated. Up to moderate evaporation ratios the measured residence time distributions showed self-similar behavior. The fluid properties, namely the viscosity, and the density, influence the fluid dynamics in WFE, as a gravity-driven, falling film occurs. Although the peripheral load is the most important parameter for the residence time distribution, the wiper speed and the evaporation ratio also affect the residence time noticeably. Especially for high evaporation ratios associated with high temperature differences the liquid film tends to become instable, leading to significantly increased mean residence time. The associated film rupture was observed visually taking high-speed recordings of the inner evaporator surface during evaporation.