Desalination, MED/MVC


Low temperature multi effect distillation is a simple, reliable and efficient process utilizing low cost / low grade heat or waste steam. Vacuum enables operation at unparalleled low distillation temperatures (below 70°C) minimizing scale and corrosion, and hence maintenance, to an absolute minimum. A large number of effects consequently results in very high economy ratios (product to steam) and the use of aluminum tubes improving the heat transfer results in lower investment cost compared to conventional high temperature processes.

Evaporation and condensation are achieved by direct transmission of latent heat, avoiding sensible heat pickup, which would decrease the effective temperature differential. Using the falling film concept, static head is eliminated from the flashing process and high evaporation heat transfer coefficients result.

These factors, together with generously designed heat transfer surfaces, provide low thermal driving forces and reduce energy requirements. Furthermore, as the brine circulation rate is an order of magnitude lower than that required in forced circulation systems, there are considerable savings in pumping power.

The design of W-WWT and it’s co-operated partners evaporators is extremely safe as there is no pressurized brine flow, and brine pressure is always lower than steam or product pressure. Thus product contamination is consequently impossible even if there should be tube perforation. Also, due to the laminar film type flow, there is no erosion of the tubes and pitting corrosion rates are minimized.

All standard items of equipment in the plant, such as pumps, motors and controls are of top quality and reliable design, purchased only from reputable international manufacturers.

W-WWT and it’s co-operated partners MED plants are typically installed in power plants, refineries or other industries, and demonstrate very high availability and reliable operation with ease of maintenance.

MEDs are:

  • suitable to treat brackish and seawater
  • able to produce boiler feed water with less than 10 ppm TDS guaranteed
  • superior in availability and reliability
  • flexible with load-following capabilities
  • highly efficient with optional utilization of low cost waste heat energy sources


Mechanical Vapor Compression (MVC) distillation is the most thermodynamically efficient process of single-purpose thermal desalination. The thermodynamic efficiency is derived from the application of the “heat pump” principle by a large volumetric flow compressor acting as the “heat pump”, which continuously recycles and keeps the latent heat exchanged in the evaporation and condensation steps within the system. 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, on the other side of the surface. This cycle of evaporation and condensation operates continuously. The principle is similar to our MED.

The “heat pump” work, plus the fraction required for liquid pumping, is the only energy consumed by the process. No additional heat is required, and no cooling water is needed for the heat rejection, as in other distillation processes.

MVC units are standard, factory-assembled and skid mounted units, proven by strict and thorough prototype testing at an R&D centre and by years of satisfactory experience with commercial plants operating throughout the world. The plants are simple, efficient and automatic.

The mechanical compressor creates a vacuum in the vessel and then compresses the vapor taken from the vessel and condenses it inside a tube bundle. Seawater is sprayed outside of the heated tube bundle where it boils and partially evaporates, producing more water.

A single MVC unit can be erected at site within 10-12 weeks.

The quality of the product water from the MVC unit exceeds public health standards by a wide margin. Typical applications are for downstream oil companies, oil facilities, refineries, petrochemical plants, industries like cement, tourism, factories, chemical plants etc.

MVCs are:

  • suitable to treat brackish and seawater
  • able to produce boiler feed water with less than 10 ppm TDS guaranteed
  • superior in availability and reliability
  • flexible with load-following capabilities
  • fit for smaller plants in the range 500 to 5,000 m3/day per single unit