Modeling thermal and geometrical effects on non-condensable gas desorption in horizontal-tube bundles of falling film evaporation

摘要

Non-condensable gas (NCG) released in the multi-effect distillation associated with a thermal vapor compression (MED-TVC) seawater desalination seriously deteriorates the heat transfer efficiency. Due to the complicated coupled nature between NCG desorption and heat transfer process, little attention has been paid to local NCG desorption conditions which contribute to improving thermal efficiency of MED-TVC. Chemical desorption continuously releases more CO2 than O-2 and N-2 released by physical desorption. A numerical model, considering heat and mass transfer, flow dynamics and chemical reactions, predicts chemical reaction time, evaporation rate and CO2 desorption rate for varying thermal and geometrical parameters. The predicted CO2 desorption rate is in a good agreement with the running data of a venting system in a reference MED-TVC desalination plant. An increasing evaporation temperature, which results in a decrease in chemical reaction time and element volume, contributes to decreasing a specific CO2 desorption rate. With an increase in inlet heating steam velocity, the specific CO2 desorption rate decreases due to the increasing element volume and decreasing specific evaporation rate. The specific CO2 desorption rate increases with an increase in tube diameter and a decrease in dimensionless tube pitch due to the opposite effects of heat transfer and element volume.