BT-10 Falling Film Evaporation of Wide-Boiling-Range Mixtures
In anticipation of increasing use of falling film evaporators in the process industries, a single tube study was carried out to determine some of the most important parameters and most fruitful methods of analysis for improvement of design methods. This study involved experimental measurements and analysis of heat and mass transfer for a binary mixture evaporating in downflow in a heated vertical tube. Heat flux to the falling film, local wall temperatures, and bulk fluid temperatures in the test section were measured, as was the flow rate of liquid to the test section and composition.
The primary mixture was ethylene glycol and water. A second mixture, propylene glycol and water, was also used to a more limited extent. The tests were run at atmospheric pressure. Heat fluxes ranged from about 3000 to 25000 W/m2, emphasizing low temperature difference operation, with no (or a small amount of) nucleate boiling. Film Reynolds numbers ranged from about 300 to 1500 for mixtures and up to 3000 for water.
These data were used to evaluate design methods from the literature, which have not been tested against wide boiling range mixtures. It was found that these methods greatly overpredicted the heat transfer coefficients obtained with the mixtures tested. This overprediction could be corrected by accounting for mass transfer resistance using the film theory approach. A correlation is proposed for the mass transfer coefficient which gave excellent results in predicting the heat transfer data when used with a film theory approach for calculating the mixture correction factor.
The data also indicated that for a given flow rate, the film was continuous only up to a certain heat flux, above which it apparently began to break down. The film breakdown effect was shown to be a strong function of mixture composition and was proposed to be due to surface tension gradients in the film. A tentative correlation was proposed, but more work is needed in this area.