DISTILLATION WITH SECONDARY REFLUX AND VAPORIZATION: SIMULTANEOUS HEAT AND MASS TRANSFER IN A WETTED-WALL FRACTIONATING EVAPORATOR

A plate-fin heat exchanger shows considerable promise as a viable means of implementing the SRV distillation concept based on the large amount of area available for both heat and mass transfer and on the mechanical simplicity of the device. In order to carry out a detailed evaluation of an SRV distillation column a design procedure and performance data for plate-fin equipment must be made available. To address these aspects, a long-range experimental program has been established.Comparisons of calculated and measured data showed that the model adequately described the composition data, but did not satisfactorily describe the temperature data. In an effort to pinpoint the reasons for the unsatisfactory predictions, two constant parameters, associated with the vapor and liquid phase mass transfer coefficients, respectively, were determined to optimally fit the vapor temperature data for each experimental case. For experiments which were run at high concentration levels of the more volatile component, quite good predictions for both the composition and temperature data were obtained with the adjusted parameters, but for low concentration levels the predictions of the vapor temperature data remained unsatisfactory. The behavior of these parameters with an increase in heat flux into the column indicated a decrease in the mass and heat transfer resistances not accounted for in the model.Because design procedures, which have been reported in the literature for similar processes, cannot be used with confidence for SRV distillation, phase one of this experimental program focuses on the development and verification of a suitable model. In accordance with this plan, a model for describing simultaneous heat and mass transfer with high mass flux effects has been proposed. This model takes into account heat and mass transfer in both phases and utilizes film theory to describe the high mass flux effects. To test the model for its ability to fundamentally describe the heat and mass transfer under conditions of evaporation, experiments were carried out in a wetted-wall fractionating evaporator. Vapor temperature and vapor composition measurements were used as indicators of the ability of the model to describe heat transfer and mass transfer, respectively.