Degassed Conductivity - Comments on an Interesting and Reasonable Plant Cycle Chemistry Monitoring Technique Part 2: Degassing of Carbon Dioxide in Technical Degassed Cation Conductivity Monitors and Temperature Conversion of the Cation Conductivity

摘要

The efficiency of degassing carbon dioxide in vented reboiling systems is discussed on the basis of thermo-dynamic equilibrium calculations.Even though the vapor containing carbon dioxide is continuously removed instead of remaining in contact with the liquid,it is believed that the thermodynamic model provides a realistic approximation of the actual process.The essential parameter of the model,the fraction of the solution evaporated,corresponds to the actual rate of evaporation before any possible reflux.The analysis of a number of examples indicates that the efficiency of carbon dioxide removal depends strongly on the rate of evaporation,and therefore,taking into account the construction and operation of typical degassed conductivity monitors,is expected to often be far from complete.The presence of impurity anions lowers the pH of the solution leaving the cation exchange column,suppressing the dissociation of carbon dioxide,and thus increasing its volatility.However,this effect is small at the levels of impurities that meet the usual requirements for high-pressure power plant cycles,and,additionally,the resultant decrease in conductivity can be at least partially masked by the increased concentration of non-volatile impurities.The problem of conversion of the conductivities of mixtures of unknown composition measured near the normal boiling point of water to ambient temperature (25 deg C)cannot be solved due to non-trivial variations in ionic association and equivalent conductances between different electrolytes.The conversion equation proposed here is based on an empirical approach minimizing the probable error.The necessary ionic mobility data are based on a semi-empirical predictive method for the estimation of ionic mobilities as a function of temperature.