Question

what is MEB and who it woks

( Thermail desalination )

multi effect boiling

Answer 1

Multi-stage evaporation comes from the chemical industry where water or solvent must be removed in order to concentrate a product in solution. The evaporated liquid in the chemical industry is usually not the product, except for cases where the solvent is recovered from a certain reaction. The evaporation process consumes a great deal of energy. The need to save energy was the basis for the development of this multi-stage process, whereby more equipment (investment) is required in order to reduce the overall amount and cost of energy consumed. In most cases, the process involves 2-4 stages, sometimes called effects, and has been used for more than a century for solution concentration, crystallization, solution purification, etc. Since 1950, it has been used for seawater desalination, yet in the water industry it requires between 2-16 stages. Multi-Effect Distillation (MED) is more energy efficient than other evaporation techniques, including the Multi-Stage Flash system. It is also considered to be more sophisticated. A low-temperature source of energy is used in most cases to feed the process. In most industrial cases, this is spent steam at a slightly elevated pressure exiting from a steam-operated power station, a source of heat that is available in refineries, or any low-level steam or hot fluid from other sources.

The schematic of a horizontal tube Multi-Effect MED unit is presented below:

The steam enters the plant and is used to evaporate heated seawater. The secondary vapor produced is used to generate tertiary steam at a lower pressure. This operation is repeated along the plant from stage to stage. The primary steam condensate is returned to the boiler of the power station since it is of extremely high quality that is needed for turbine steam production. The MED technique is based on double-film heat transfer. Latent steam heat is transferred at each stage by steam condensation through the heat transfer surfaces to the evaporated falling film of seawater. The process is repeated up to 16 times or more in existing plants between the upper possible temperature and the lower possible cooling water, which depends onseawater temperature used for cooling the water. The product water is the condensate that accumulates from stage to stage. A vacuum pump/compressor is used to maintain the gradual pressure gradient inside the vessel by removing the accumulated noncondensable gases together with the remaining water vapor after the final condensation stage. The pressure gradient along the MED effects is dictated by the saturation pressure of the feed stream and the saturation pressure of the condensing steam exiting the last stage and is condensed by cooling with seawater. Typical pressure gradients of 5-50 kPa across the system (less than 5 kPa/stage) are typical.

Steam condensation inside horizontal tubes and seawater evaporation on the outer side is the heart of one of the most common MED processes. Seawater is allowed to fall down a tube bundle. Heat transfer on both sides of the heat transfer area is considered highly efficient due to the low resistance of the thin falling films, which allows efficient operation with a low temperature difference across the tube walls. The low temperature difference is limited by the increasing boiling point elevation due to the increase in salt concentration while evaporating part of the water. It is also limited since at too high fluxes, the film starts to boil, nucleating bubbles, causing dry spots that may lead to salt precipitation. This, of course, should be avoided. The low temperature difference across the heat transfer surfaces allows designing a large number of effects between the steam temperature at the first stage and the temperature of the cooling seawater at the other side.