Water utilities and developers

Multiple effect desalination

All early thermal desalination plants were of the Multiple Effect Distillation (MED) type where sea water is heated by steam circulated in submerged tubes.

Operational efficiency was increased significantly by improving the heat transfer characteristics between the liquidvapour phase using plates or horizontal tubes, and by spraying the saline water onto these in order to enable evaporation to take place from a thin film over a larger surface area.

The first ‘effect’ exchanger tubes are heated by steam and the saline water is sprayed onto these where only a proportion evaporates. After passing through a demister, the vapour is introduced to the heating tubes of the second ‘effect’ onto which the remaining saline water from the first effect is sprayed. A proportion of this vapour condenses into product water and in doing so, gives up its latent heat of vaporisation.

This heat is used to evaporate a proportion of the second ‘effect’ saline feed. This process is repeated over several ‘effects’. Vapour from the final effect is condensed in the heat rejection stage,where the residual energy is transferred to the first effect feedwater. A significant feature of this process is that the top brine temperature of the first effect can be limited to values below 70°C, thereby reducing the potential for scale formation.

Performance can be improved significantly by compressing the vapour phase either mechanically, or thermally by means of steam ejectors. Although size limitations resulted in an early trend towards the use of Multi Stage Flash (MSF) Distillation, the higher thermal efficiency of the MED process and its capability of operation at low steam pressures have attracted more recent interest.