A MODEL FOR STAGEWISE CALCULATION OF NON-CONDENSABLE GASES IN MULTISTAGE EVAPORATORS

An analytical model was developed to determine the amount of non-condensable gases - essentially oxygen O2, nitrogen N2 and carbon dioxide CO2 - released during adiabatic evaporation in the chambers of MSF-desalination distillers. The chemical reactions of CO2-production by bicarbonate-decomposition and the mass transport processes are analytically described for a differential volume of brine on it's path from entrance to exit orifice of one stage. The total desorption rates of the different gases are obtained by integration over the average residence time of the brine in the stage. The calculated rates of liberated O2 and N2 are reasonably reliable since chemical reactions can be neglected and the required physical data are available. Only qualitative results can be expected for CO2 since reliable transport coefficients and equilibrium constants, particularly at higher temperatures and in combination with scale control chemicals are not available. Supplementary experimental data are required. Given the concentrations of the different gases and the bicarbonates at brine heater outlet, a stagewise calculation leads to a non-condensable gas desorption profile over all stages of a distiller unit. For a once-through type plant without deaerator and for a recycle type plant with vacuum deaerator the degasification schemes are illustrated and the resulting gas-concentrations in the vapor are compared for different venting rates and cascade configurations.