Heat and mass transfer in bubble column dehumidifiers for HDH desalination

Abstract

Heat and mass transfer processes governing the performance of bubble dehumidifier trays are studied in order to develop a predictive model and design rules for efficient and economical design of bubble column dehumidifiers for humidification-dehumidification (HDH) systems. As a result of their high heat transfer coefficients and large interfacial areas, bubble columns are an inexpensive and compact solution for dehumidification in HDH, which has promising applications in small-scale desalination and industrial water remediation. Performance parameters for dehumidifier design for HDH, including a device-specific parallel-flow effectiveness, are explained. A new model for the performance of single bubble trays is developed based on the rapid mixing in the column and the approximation of negligible gas-side resistance. An experiment is performed to measure the heat transfer coefficients outside cooling coils in shallow bubble columns, in which geometric parameters including liquid height and cylinder diameter, height, and horizontal position relative to the sparger orifices are varied. The highest heat transfer coefficients are recorded on cylinders placed in the coalescing region and directly above the sparger orifices. Heat flux and parallel-flow effectiveness of a bubble column dehumidifier are investigated experimentally to validate the model, which predicts the heat transfer rate well with an average absolute error of <3%. The independence of heat flux and effectiveness from liquid depth supports the assumption of negligible gas-side resistance to heat and mass transfer. Despite the mass exchange, the bubble column dehumidifier performs like a typical heat exchanger: the heat flux decreases and effectiveness increases with increasing coil area. The results of this study enable modeling and design of bubble column dehumidifiers for high heat recovery and low capital cost.