A new approach to simulate the fluid dynamics in a wiped film evaporator using Modelica

摘要

Wiped film evaporators (WFE) or agitated thin film evaporators are efficient equipment for separating highly thermosensitive and viscous mixtures. There are different approaches to tailor residence time distribution (RTD) for specific applications and to maximise heat and mass transfer by equipment design or process parameter adjustment, like liquid load or wiper speed. However, the fundamental principles of fluid dynamics are only known to a limited extent. Although flow patterns and mass and heat transfer are highly interdependent, the effects of different wipers on liquid flow and, finally, on residence time behaviour are poorly understood. In this work, a new model describing fluid dynamics and its impact on RTD is presented. The model is implemented in the equation-based modelling language Modelica. Several system variables - e.g. film thickness, hold-up or velocity profiles within the liquid film - can be estimated. The effect of varying process parameters, like liquid load and fluid temperature, can also be evaluated. The model has been calibrated with results from RTD experiments in a WFE using a roller wiper. It is capable of describing the residence time behaviour of the roller wiper system with good accuracy. Mean residence times can be reproduced within approx. +20%. Further results, such as film thickness or hold-up, provide additional insight into the fluid dynamics of characteristic regions - like bow wave, wiping and thin film zone - and the effects of several process parameters. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.