Heat and mass transfer for the diffusion driven desalination process

This research concerns a diffusion driven desalination (DDD) process in which warm water is evaporated into a low humidity air stream, and the vapor is condensed out to produce distilled water. Although the process has a low fresh water to feed water conversion efficiency, it has been demonstrated that this process can potentially produce inexpensive distilled water when driven by low-grade energy such as waste heat. A dynamic analysis of heat and mass transfer demonstrates that the DDD process can yield a fresh water production of 1.14 million gal/day by utilizing waste heat from a 100 MW steam generating power plant based on a condensing steam pressure of only 10.159 kPa in the main condenser. The optimal operating condition for the DDD process with a high temperature of 50°C and sink temperature of 25°C has an air mass flux of 1.5 kg/m2-s, air to feed water mass flow ratio of 1 in the diffusion tower, and a fresh water to air mass flow ratio of 2 in the direct contact condenser. Operating at these conditions yields a fresh water production efficiency (mf/m L) of 0.035 and electric energy consumption rate of 0.0022 kW-hr/kgfw. This dissertation describes the research progress made in the development and analysis of the DDD process. Throughout the past three years, the main focus of the desalination process has been on the heat and mass transport phenomena in the diffusion tower and direct contact condenser within the packed bed. Detailed analyses required to size and analyze these heat and mass transfer devices have been developed. A laboratory scale experimental DDD facility has been fabricated. Temperature and humidity data have been collected over a range of flow and thermal conditions for the diffusion tower and direct contact condenser. The analyses agree quite well with the current data. The condensation effectiveness of the direct contact condenser with and without packed bed has been compared. It has been experimentally observed that the fresh water production rate is significantly enhanced when packing is added to the condenser. It has also been observed that the condensation effectiveness increases considerably when air and water flow configuration is countercurrent. Recently, it has been recognized that the heat and mass transfer within the packed bed can be significantly diminished with water blockages. High-speed cinematography has been used to observe the liquid formation on the packing material. The cause of this phenomenon is addressed. Further experimental and analytical analyses are required to evaluate its influence on the heat and mass transfer coefficients for liquid and air flow within the packed bed.