RO has been in use commercially around the world for more than 50 years. It can treat a wide variety of water
types, from recycled sewage to seawater. RO pushes feed water (for instance, sea water) through a salt-removing filter. After this occurs, two liquids remain. One is drinkable, the other heavily saline. Reverse osmosis is used widely for municipal desalination (for instance, in Perth and, soon, Sydney). It is also commonly used for tourist resorts and cruise ships. RO is also used to create industrial feed water supplies of various quality, and to create water that can be used for various cooling and boiler feed needs in power generation. The benefits of RO are its rapidly-falling costs.
A typical RO plant Multi
Effect Desalination (MED)
MED
puts feed water through a series of evaporation stages to separate drinking water from salt. Each stage is generally called an "effect" In each effect, water is evaporated through applying heat and pressure. The vapor water is then recondensed into water and re-evaporated until the required water quality is reached. The benefits of MED include its flexibility, its relatively low maintenance needs and its low energy consumption.
A multi-effects plant in India
Mechanical
Vapor Compression(MVC)
Mechanical vapor compression is the most efficient of the distillation processes because it continuously recycles thermal energy and avoids the large number
of stages or effects that adds complexity to traditional MED designs. With MVC, heat initially evaporates the feed water and a specialised compressor regains much of the heat during the recondensation process and recycles it back into the initial evaporation process. The benefits of MVC are its very low energy needs.
An MVC plant in
Israel
The
Process Application of the "heat pump"
principle continuously recycles and
keeps the latent heat exchanged in the
evaporation condensation process within
the system. In this scheme, the heat
required to evaporate part of the processed
feed, which flows on one side of a heat
transfer surface, is supplied through
the simultaneous condensation of the
distillate-producing vapor on the other
side of the surface.A compressor, acting as the "heat
pump", is the driving force for
this heat transfer, and provides the
energy required to separate the solution
and overcome dynamic pressure drop losses
and other irreversibility’s. The
vapor generated from the solution is
pumped to the higher pressure-enthalpy
level required on the condensing side. This work, plus the fraction required
for liquid pumping, is the only energy
consumed by the process. No additional
heat is required.The
low energy requirement for the MVC is
further reduced to a minimum by adopting
a highly efficient horizontal tube,
falling film evaporative condenser.
Large heat transfer areas are incorporated,
and parasitic pressure drop losses are
reduced by integrating the compressor
within the evaporator. Due to this evaporator
design, the pumping head of the compressor
is low - only 2.50C (50F).
