BT-13 Falling Film Evaporation of Aqueous Multicomponent Mixtures

J. W. Palen

The main purpose of this report is to summarize the experimental work and analysis of Dr. Qi Wang at Lehigh University involving heat transfer and film breakdown for ternary mixtures of water, ethylene glycol, and isopropanol vaporizing in a falling film evaporator. This work was supervised by Professor John C. Chen, supported by a joint HTRI/HTFS contract, and was reported to HTRI in the form of Lehigh University's Institute of Thermo Fluid Engineering and Science Report #TS-912.

The Lehigh University Report was complete with respect to describing the results of the research including a sophisticated analysis of the data in the form of a new method for the calculation of multicomponent mass transfer. This present report relates the findings to previous HTRI information and presents a new, simpler method for multicomponent evaporation similar to that previously proposed by HTRI for multicomponent nucleate boiling. This approximation approach is applicable to any number of components and gave good results for all the HTRI falling film data now available. More data, especially data under vacuum and hydrocarbon data, are needed for confirmation under conditions closer to industrial practice.

This report also describes recent experiments at the Bandung Institute of Technology on the breakdown of evaporating mixture falling films as a function of surface tension gradients. These were visual results using the same fluids used at Lehigh, and the evidence obtained supported and extended the Lehigh results. It is proposed that falling films can be classified as ""unstable"" or ""stable"" depending on whether mixture surface tension decreases or increases as evaporation takes place. This finding should prove very important in improving design reliability for industrial falling film evaporators. The work should be further confirmed on hydrocarbon mixtures.

The present status of the work is such that an initial computerized design program could be developed based on existing data. However, the research program should continue if the computer program is eventually to reach the standard of reliability associated with other HTRI Programs.