CO2 release in vertical tube falling film evaporators

The release of non-condensable (NC) gases, essentially carbon dioxide, oxygen and nitrogen, from the evaporating brine in desalination distillers notably affects the heat transfer for condensation, the energy consumption, the performance and the material life span of the distillers. A better knowledge of CO2 release and of the interaction with the carbonate system in desalination distillers is very important for the design of the venting system and a stable distiller operation. Since CO2 release and scale formation are closely related to the carbonate system of the brine, a better understanding of CO2 release may contribute to the knowledge of scale formation in desalination distillers and improve scale prediction and prevention methods. A model has been developed for the prediction of the CO2 release rates in falling film evaporators with vertical tubes. The theory of mass transfer coupled with chemical reaction kinetics (chemical desorption) was applied to the problem of CO2 release. The mass transfer and the chemical reaction kinetics were studied for the conditions prevailing in falling film seawater evaporators. In addition to the CO2 release rates the model allows for calculation − 2 of the HCO3 , CO3 −, CO2, H+, and OH− concentrations in the carbonate system of the brine. The model was applied to a falling film evaporator at various operating conditions. The release rates of CO2 along the flow path of the falling film were determined. The paper presents the main principles of the model and discusses the simulation results. The effects of process parameters and the seawater properties such as evaporation temperature, seawater salinity, and pH value on the CO2 release rates are shown. Moreover, the paper discusses in which desorption regime (e.g. slow, fast, instantaneous) the CO2 release process takes place and whether the mass transfer is enhanced by the chemical reactions.

A model has been developed for the prediction of CO2 release rates in vertical tube falling film evaporators. In addition to the release rates − the model allows for calculation of the HCO3 , 2− + − CO3 , CO2, H , and OH concentrations in the carbonate system of the brine. The desorption of CO2 was described as a problem of mass transfer coupled with chemical reaction. The phase interface area, the mass transfer coefficient, and Henry’s law coefficient of CO2 in seawater were presented in detail. Given the main dimensions of the evaporator, the operating data and the measurable pH and total alkalinity of seawater, the CO2 release rates − 2− as well as the HCO3 , CO3 , CO2, H+, and OH− concentrations in the brine can be calculated along the flow path of the falling film. In order to show the capabilities of the model, it was applied to a falling film evaporator with a single vertical tube. The release rates of CO2 were simulated at various evaporation temperatures, seawater salinities, and pH values. The CO2 release rates along the flow path of the falling film were shown. CO2 release increases with increasing evaporation temperature and seawater salinity and decreasing pH of the seawater. Currently, the CO2 release is being measured in a vertical tube falling film evaporator test rig in order to verify and improve the model.