Theoretical Study On The Chemical Desorption Of CO₂in Multi-effect Distillation

With the lack of world’s water resource, especially in the coastal areas, it becomes of great importance to get fresh water from the ocean in many countries., and now. many of them have attached great importance to the desalination as a high technology industry. Because of many obvious advantages, such as high heat transfer efficiency, reliability, minima] requirements for intake and pretreatment systems, low corrosion rates, minimal scaling rates and so on, the low temperature multi-effect distillation with the horizontal tube falling film evaporation has attracted much attention. There will be non-condensible gas (NCG) releases when the seawater evaporate outside the horizontal tubes, and even a little NCG will have a significant affect on the heat transfer when the heating steam condensed in the tubes. So it is significant to study the mechanism of the desorption of NCG in the desalination devices.Many researchers in the past are mainly concerned on the effects of NCG on heat transfer, energy consumption and distiller operation, however less is on the NCG desorption process, particularly of CO2release. But the chemical desorption of CO2is a significant source of NCG. In this paper, firstly, the chemical reactions relevant to the CO2release process were studied, and the carbonate system in seawater were described according to the thermodynamics and the reaction kinetics of the carbonate system. Then compared the average reaction time with the residence time, the appropriate volume elements were divided, and a model of the CO2release in the multiple-effect distillers(MED) was presented, coupled mass transfer with chemical reaction. At last, by giving a computer program in MATLAB. the CO2release rate and the amount of the CO2released in a TVC-MED were calculated. The TVC-MED system has6effect evaporators with a capacity of12000t fresh water per day, and the factors that influence the chemical desorption of CO2were analysedThe results indicate that the CO2desorption decreases from the first stage to the last one mainly due to the reduction of the mass transfer coefficient with a reduction in evaporating temperature. The chemical desorption of CO2produces almost equivalent amount of NCG as the N2and O2dissolved in seawater, and the NCG concentration increases along the length direction of the tubes with the vapor condensed, which will significantly affect the heat transfer rate. The carbonate system mainly comprises of the HCO3-and CO32-and the concentration of the CO:is small. The concentration of the carbonate system components increases with the water evaporating, which will contribute the CO2desorption. Temperature is the key fact influencing the chemical reaction rate, the rate of the reaction increases with temperature, and the salinity and the pH value of seawater has important influence before the seawater temperature approaches saturated. Moreover, the physical parameters of the seawater at the inlet of the final condenser have a great influence on the CO2release:the release of CO2increases with increasing salinity, but it decreases with increasing seawater temperature and pH value.