Modelling and experiments on vaporization of saline water at low temperatures and reduced pressures

A vapor diffusion model, which takes into account the reduction of droplet temperature during the evaporation process, was used to determine the achievable targets for desalination of seawater at temperatures between 26 °C and 32 °C when the saline water was injected as fine droplets in a low-pressure vaporizer. The temperatures between 26 °C and 32 °C correspond to the warm temperatures of the ocean surface in the tropics. The predictions from the model were verified by a large number of experiments at vacuum pressures between 10 mm and 18 mm mercury. The upper bound of the rate of flow of the saline water in the experiments was 1000 l/h. Typical evaporation time of the droplets was a few hundred milliseconds and this was less than the residence time of the spray provided for in the vaporizer. The yield of fresh water measured in the experiments was between 3% and 4% and matched well with the predictions. Small values of water injection pressures of about 0.1 MPa were found to be adequate when a swirl nozzle, used for garden sprays, was employed. Changes in the height of water injection in the vaporizer did not significantly influence the yield of fresh water.