What is Multi Effect Distillation MED
In Multi Effect Distillation MED systems, saline source water is typically not heated; cold source water is sprayed via nozzles or perforated plates over bundles of heat exchange tubes. This feed water sprayed on the tube bundles boils, and the generated vapor passes through mist eliminators. Which collect brine droplets from the vapor. The feed water that turned into vapor in the first stage (effect) is introduced into the heat exchange tubes of the next effect. Because the next effect is maintained at slightly lower pressure, although the vapor is slightly cooler, it still condenses into freshwater at this lower temperature.
This process of reducing the ambient pressure in each successive stage allows the feed water to undergo multiple successive boilings without the introduction of new heat. Steam flowing through the exchange tubes is condensed into pure water and collected from each effect. Heating steam (or vapor) introduced in the heat exchange tubes of the first effect is provided from an outside source by a steam ejector. The Multi Effect Distillation MED system shown in the figure below is also equipped with a brine recycle system. It allows the introduction of warmer-than-ambient water in the first effects of the system. Thereby reducing both the volume of feed water that must be collected by the plant intake system and the overall energy needs of the system.
The main difference between the Multi Effect Distillation MED and Multi Stage Flash Distillation MSF processes is that while vapor is created in an MSF system through flashing, evaporation of feed water in MED is achieved through heat transfer from the steam in the condenser tubes into the source water sprayed onto these tubes.
schematic multiple effect distillation process system
This heat transfer at the same time results in condensation of the vapor to freshwater. Multi Effect Distillation MED desalination systems typically operate at lower temperatures than MSF plants. Maximum brine concentrate temperature of 62 to 75°C versus 115°C) and yield higher GORs.
The newest Multi Effect Distillation MED technologies include vertically positioned effects (vertical tube evaporators). This may yield a GOR of up to 24 kg of potable water per kilogram of steam. Pumping power required for operation of Multi Effect Distillation MED systems is lower than that typically needed for MSF plants. It is equal to 0.8 to 1.4 kWh/m3/3.0 to 5.3 kWh/1000 gallons of product water. Therefore, Multi Effect Distillation MED is now increasingly gaining ground over MSF desalination. Especially in the Middle East, where thermal desalination is still the predominant method for producing potable water from seawater.
Reference: “Desalination Engineering” by Nikolay Voutchkov
