CO2 release in multiple-effect distillers controlled by mass transfer with chemical reaction

For predicting the CO2 release rates in multiple-effect distillers, the theory of desorption with chemical reaction was applied. The mass transfer processes and the reaction kinetics controlling the CO2 release were described. In the pH and temperature range found in the evaporator stages of multiple-effect distillers, the alkaline reaction mechanism with the steps CO2 + OH- <----> HCO3- and HCO3- + OH- <----> CO32- + H2O predominates. The reaction CO2 + OH- <---->HCO3- is relatively slow, and therefore it is the rate-determining step in the reaction mechanism. Information for predicting mass transfer coefficients in water films flowing over horizontal tubes is still limited. Different approaches were shown. The phase interface area for CO2 release was described as the sum of the surface area of the liquid film on the tubes plus the surface area of the liquid between the tubes, assuming that liquid columns can be expected to occur between the tubes. Depending on the magnitude of he mass transfer coefficient, it was found that CO2 desorption from the evaporating brine either takes place in the transition regime from a slow to a fast reaction or in the fast reaction regime. Mass transfer and chemical reactions simultaneously take place in the boundary layer at the phase interface. Mass transfer is slightly enhanced by the reaction. The chemical rate constant becomes more important and the mass transfer coefficient less important in the correlation for the CO2 release rate.