In these processes, low-temperature steam is taken from the power plant turbine of the supplying plant to heat the saline solution. In commercial distillation, there are a number of heat recovery stages in series, because of the high heat of evaporation of water. These stages are at progressively lower pressures, resulting in flashing and mechanical vapour compression to occur.

In general, the more stages in place, the more efficient is the process. The number of stages is limited by both economic and technical reasons, e. g. the overall temperature

difference between the heat source and the cooling water sink. The typical temperature reduction per stage for a commercial plant is 2-5°C. In terms of thermodynamic efficiency, expressed as kg of water produced against kg of steam used, the figure is 6-10 for MSF applications and up to 20 for MSD. These processes are described below.

14.4.1.1 MSF Distillation. In this process, seawater is passed through a number of stages where it is progressively heated (see below) until it reaches the main heating section supplied by the process heat source, see for example (IAEA-TECDOC-1056, 1998). The brine is then returned through these stages and freshwater is eventually obtained through a series of flashing and condensation processes. In particular, as the heated brine returning from the heat source passes into the first stage heat recovery section, flashing occurs due to pressure reduction. Vapour is produced which condenses on the entry pipe-work to the heating section within the first stage (providing the progressive heating referred to above). The condensate is collected in trays. This condensate together with the remaining brine (that has not flashed) is passed on the second stage. The process is then repeated for a number of stages and the separation process completed. Non-condensable gases are removed by a steam-jet ejector system. The seawater is also chemically treated to remove scale.

14.4.1.2 MED. This process also consists of a number of heat-exchange sections. At the first stage steam from the heating boiler passes through a tube bundle which is cooled by evaporating the entry seawater on the other side of the tube bundle. The resulting steam is then passed to a second stage heat exchanger. Any seawater not evaporated at the first stage is passed on to the second stage. The process is then repeated to complete the separation process. MED plants require similar scale removing processes as do MSF plants.

Several designs have been used. The main difference is in the design of the heat exchangers. The low-temperature horizontal tube multi-effect process (LT-HTME) has horizontal tubes and the brine is sprayed over the outside of the tubes. In the vertical-tube evaporation process (VTE), the evaporation is inside vertical tubes. The LT-HTME is the more dominant process used.

In general, MED plants are more efficient than MSF plants because their heat transfer processes are more efficient for given heat transfer area and similar temperature difference between the heat source and cooling water.

14.4.1.3 RO. RO is also used as a separation process (IAEA-TECDOC-1056, 1998). This process has been applied commercially and can produce freshwater down to between 100 and 200 ppm of total dissolved solids. The electricity consumption is in the range 4-7 kWe h m_3.

In this process, seawater (brine) and water are held in a vessel in two-solution compartments separated by a semi-permeable membrane. Pressure is applied to the compartment containing the brine, sufficient to overcome the natural osmotic pressure of the solution and the permeate pressure (NB this is negligible compared with the natural osmotic pressure). In these circumstances, water flows from the brine compartment to the water compartment, the brine become more concentrated and purified water is obtained in the water compartment.

As the seawater is fed into the brine compartment, it is compressed up to 70-80 bars, sufficient to overcome a natural osmosis pressure of the saline solution of about 60 bars. In practice, only a portion of this water flows through the membrane, the remainder is discharged. The flow through the membrane is proportional to the pressure gradient of the applied pressure less the solution osmotic pressure. The proportionality factor depends on a range of factors including the geometry (shape, area, thickness) and the chemical properties of the membrane, the pressure, concentration, pH and temperature. Membranes have been used of varying design, spiral-wound, hollow fibre, also tubular, plate and frame type, the former two designs being the most commonly used.

14.4.1.4 Hybrid Desalination. Hybrid desalination systems can be used to combine power generation, with MSF or MED, and RO processes. This combined capability can be utilised to advantage in different ways, depending on the size and type of energy source available and the water quality product requirements. There are economic and technical advantages of hybrid as compared to single process technology.

These include the utilisation of a common seawater intake, optimised feedwater temperature for the RO plant, taking cooling water from MSF or MED plant, blending of product waters, common water treatments and various other optimisations that can be made through common process requirements. Some of the different hybrid desalination systems are reviewed in (Awerbuch, 1997).

Some of the reactor concepts that are under consideration for desalination applications are shown in Table 14.5 and discussed below.