Thermal
Thermal desalination exploits the fact that water molecules vaporise at a lower temperature than dissolved salts and other molecules. Therefore, feed water can be heated and then evaporated to separate out the dissolved salts from pure water.
Three different thermal desalination technologies have emerged: multistage flash (MSF), multi effect distillation (MED) and mechanical vapour compression/ vapour compression (MVC/VC).
Multistage Flash
Multi-stage flash desalination occurs through a series of stages:
1. The feed water, usually seawater, passes through a heating stage.
2. The feed water is heated in successive stages.
3. Then the feed water is heated in a brine heater using externally supplied steam.
4. After that, the feed water is passed through various stages and is subjected to several ‘flashes’ of heat. The temperature and pressure of each successive stage declines.
5. The water boils, evaporates, condenses and the pure water produced is collected.
Some energy recovery occurs when the boiled products in each stage cool releasing heat and through the recycling of brine water. Brine water recycling reduces the costs of desalination by reducing the vaporisation temperature of the water present, although there’s an increased risk of corrosion (6, diagram above). The number of stages relates to the efficiency and energy use and reuse of the system.
Medium, large and very large capacity MSF plants can be produced – the largest plant has a capacity of and thus the potential to produce 800,000 m3 of fresh water per day. Capital costs for the medium and large plants are considerably lower than smaller plants per volume of water produced, and they are very energy efficient. The average life-time of the plants is 20 to 30 years due to recent advances in minimising the corrosion of tubes inside the plant.
This is the most robust desalination technology.
Multi Effect Distillation
Another thermal technology is MED which uses steam to heat the feed water rather than heat ‘flashes’. In a MED plant, water is condensed on one side of the stage wall and a saline mixture on the other wall after a series of condensation-evaporation processes called ‘effects’.
Most MED plants are medium-sized – produce up to 300,000 m3 of water per day. The capital costs for these plants have declined in recent years due to the use of low-cost materials which make the plants easier to operate and more economic; however, they are less rugged.
Increasingly MED is being favoured over MSF because MED has a lower energy and pumping requirement.
MED is used a lot in large cruise ships because it can utilise waste heat from the ship’s propulsion engines and uses less electrical energy than reverse osmosis. Both MED and MSF are used to make water for liquid natural gas and methanol plants using waste heat from the natural gas and methanol facilities. Although, MED is the favoured technology for industrial low-grade, heat-driven desalination.
