| Different from conventional thermal or reverse osmosis desalination process, the dewvaporation desalination is a novel seawater and brackish water desalting process, in which air is used as a carrier gas to evaporate water from saline feed and to form fresh water by later condensation. To increase the thermal efficiency, the evaporation and condensation processes are thermally coupled by heat transfer, so that the condensation latent heat can be transferred to the evaporation side to provide evaporative latent heat for the saline water. Two novel experimental dewvaporation columns with shell and tube structure, one is unbaffled and the other is baffled, were designed and built in this work. The effective heat transfer areas of the built towers are 9.6 m~2 and 2.75 m~2, respectively. The corresponding experimental installation and computer data acquisition system were developed. The process characteristic experiments and heat/mass transfer enhancement experiments are successfully carried out with the built columns, respectively. The experimental results show that the productivity of the unit first increases and then decreases with the increase of the humidifying water flow rate and carrier gas flow rate, while continuously increases with the elevation of humidifying water inlet temperature and external steam flow rate. The productivity of the unbaffled column was usually 0.05~0.2 kg/m~2·h and the suitable operating ranges for film flow rate, carrier gas mass velocity and flow rate ratio of external steam to carrier gas were 0.0007~0.0020 kg/m·s, 0.20~0.35 kg/m~2·s and 0.015~0.075, respectively. It was desirable for the humidifying water inlet temperature to be in the range of 70~90℃. The comparative experimental study shows that the heat transfer process can be effectively enhanced by setting baffle plates at the shell side of the column. The productivity of the baffled unit was usually in the range of 0.2~0.8 kg/m~2·h, which is 4 to 5 times that of the unbaffled one. The suitable operating ranges of film flow rate, carrier gas mass velocity and flow rate ratio of external steam to carrier gas for the baffled column were 0.0015~0.0025 kg/m·s, 1.5~2.0 kg/m~2·s and 0.015~0.04, respectively. The salinity of the produced water was determined to be in the range of 20~30 mg/L, which is comparable with that of the traditional thermal desalting processes. From heat and mass balances as well as heat and mass transfer dynamics, a comprehensive heat-mass transfer mathematical model was developed for the dewvaporation process. The mass transfer coefficient for the humidification process and heat transfer coefficient for the evaporation/condensation processes were extensively analyzed and their corresponding calculation methods were developed. Based on the developed model, the height direction profiles of the stream temperatures, evaporation/condensation heat transfer coefficients in the desalination columns were investigated through mathematical simulation. The complicated relations and effects between the process parameters were also systematically investigated, while their optimal or suitable operating ranges were determined. The productivity of the desalination unit will increase with the tube length and decrease with the tube diameter under certain conditions. When polymer was used as tube material, the productivity reduction of the unit due to the increase of wall thermal conductivity will be within 10% if the wall thickness could be controlled within 0.2 mm. The gained output ratio of the system could be over 6 with due heat recovery measures applied. With an over 1.0 m3/d installation as an example, the design process for the desalination system by dewvaporation process was developed.
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