Experimental investigation of the residence time behavior of a wiped film evaporator

Abstract

Wiped film evaporators (WFEs) are characterized by small hold-up and short residence times, making them well suited for the separation of thermally sensitive substances and for fast chemical reactions. Since fluid dynamics and heat transfer in WFEs are highly interdependent, the effects of different wiper geometries, operating parameters and physical properties on the residence time behavior are manifold and poorly understood. In this work, the influence of liquid load, wiper speed and liquid viscosity on the residence time distribution (RTD) of a wiped liquid film was investigated. Tracer experiments were conducted without evaporation in a stainless steel WFE alternatively equipped with a spring-loaded comb wiper and a roller wiper. A detailed comparison of both wipers demonstrated that at a high liquid load and an increased viscosity, shorter residence times were achieved with the roller wiper, which could be explained by a strongly pronounced bow wave with an increased vertical flow velocity. Using a comb wiper, an increase in dispersion with rising liquid load at higher viscosity was demonstrated.