CO 2 release in vertical tube falling film evaporators

Abstract

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 CO 2 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 CO 2 release and scale formation are closely related to the carbonate system of the brine, a better understanding of CO 2 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 CO 2 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 CO 2 release. The mass transfer and the chemical reaction kinetics were studied for the conditions prevailing in falling film seawater evaporators. In addition to the CO 2 release rates the model allows for calculation of the HCO 3 −, CO 3 2−, CO 2, 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 CO 2 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 CO 2 release rates are shown. Moreover, the paper discusses in which desorption regime (e.g. slow, fast, instantaneous) the CO 2 release process takes place and whether the mass transfer is enhanced by the chemical reactions.