Heat and mass transfer characteristics of a wiped film evaporator
Wiped film evaporators (WFEs) are often used in the petrochemical, chemical and food industries to remove a volatile component from a very nonvolatile mixture. Wiped film evaporators provide a short residence time and are often operated under vacuum for temperature sensitive mixtures. The wiped film evaporator utilizes a set of wiper blades or rollers to spread and thin the liquid along the surface of the heated wall. The wipers or rollers also induced convection within the liquid film which promotes evaporation of the volatile compound and enhances mass transfer within the film. Unfortunately, very little has been published on wiped film evaporators and users must rely on equipment vendors even for preliminary evaluations.
In this work, a rigorous heat and mass transfer model was developed for a vertical wiped film evaporator. The model includes the capability to perform a single stage flash calculation and for predicting physical properties from available DIPPR constants and from group contribution methods. The actual physical properties may also be added. The study includes evaluating the model with published and data obtained in this work. The effects of wiper type, number of wipers, rotational speed, Reynold's number and system properties were studied and compared with the rigorous model. Three test systems were evaluated: sucrose/water, glycerol/water and ethylene glycol/water.
A new mass transfer model in vertical WFEs is proposed. The well-studied falling film evaporator (FFE) is taken as the base case for the vertical wiped film evaporator. A heat transfer enhancement factor, defined as the ratio of the heat transfer coefficient for a wiped film evaporator to the heat transfer coefficient of a falling film evaporator, was applied to a falling film mass transfer model. The results of the rigorous model compared very favorably with experimental data obtained in this work and the published results.
The rigorous model also allows the capability of a flash calculation. In general, the flash calculation agreed favorably with the experimental data and was shown to represent the wipe film evaporator in a process simulation. However, it should be noted the flash calculation does not provide mechanical and engineering details such as the required diameter and length, heat transfer area, number of blades or rollers and rotational speed.
The goal of the study is to provide a computational tool which could be used to evaluate process changes to an existing wiped film evaporator or to assist in a preliminary design. The final design and evaluation of a process change should include the wiped film evaporation vendor.