What is multiple-effect distillation?
Multiple-effect distillation, often known as multi-effect distillation (MED), is a distillation method used to desalinate seawater. It is made up of several stages or "effects." The feed water is heated in each stage by steam through tubes, which are frequently sprayed with saline water. Several of the water evaporates, and the steam goes into the following stage's tubes, heating and evaporating more water.
Each step effectively recycles the energy from the one before it, with progressively lower temperatures and pressures. There are various configurations available, such as forward-feed, backward-feed, and so on. In addition, this steam utilizes some heat to warm the incoming saline water between phases.
Operation of multiple effect distillation
The plant can be thought of as a series of closed chambers divided by tube walls, each with a heat source on one end and a heat sink on the other. Every space is made up of two communicating subspaces: the outside of stage N tubes and the interior of stage n+1 tubes.
Each compartment has a lower temperature and pressure than the one before it, and the tube walls have temperatures that are halfway between both the fluid temperatures on each side. The temperature of the walls of both subspaces cannot be in balance with the pressure in a space. It has a pressure that is somewhere in the middle.
The water evaporates because the temperature is too low or the temperature is too high in the first subspace. The vapour condenses in the second subspace because the pressure is greater or the temperature is too low. Evaporation energy is carried from the warming first subspace to the colder second subspace through this. The energy travels to the colder next subspace via conduction via the tube walls at the second subspace.
Multi Effect Distillation process
Low-temperature multi-effect distillation is a straightforward, dependable, and cost-effective method that makes use of low-cost, low-grade heat or waste steam. Vacuum allows for ultra-low distillation temperatures (below 70 °C), reducing scale and corrosion, as well as maintenance, to a bare minimum.
As a result of the huge number of effects, very high economy ratios (product to steam) are achieved, and the use of aluminium tubes to improve heat transfer results in cheaper investment costs than traditional high temperature processes. Direct transmission of latent heat is used to achieve evaporation and condensation, eliminating sensible heat pickup, which would reduce the effective temperature differential.
The static head is removed from the flashing process using the falling film principle, resulting in high evaporation heat transfer coefficients.Low thermal driving forces and lower energy requirements are provided by these variables, which are combined with generously constructed heat transmission surfaces. There are also significant pumping power savings because the brine circulation rate is an order of magnitude lower than that required in forced circulation systems.
Multi Effect Distillation (MED) plants are commonly used in power plants, refineries, and other industries, and they offer high availability, reliability, and maintenance ease.
Advantages of using Multi Effect Distillation (MED):
• Capable of producing boiler feed water with less than 10 ppm TDS
• Superior in availability and dependability
• Adaptable with load-following capabilities
• Extremely efficient with the option of using low-cost waste heat energy sources
Other advantages of multiple effect distillation:
• Low energy consumption when compared to other thermal processes
• Avoids corrosion and scaling by operating at a low temperature (70 °C) and a low concentration (1.5).
• Does not require pre-treatment of seawater and can withstand a wide range of seawater conditions.
• Extremely dependable and simple to use
• Low-cost maintenance
• Continuous operation with minimal monitoring 24 hours a day, 7 days a week
• Can be used with any type of heat source, such as hot water, waste heat from power plants, industrial processes, or solar heating.
• Consistently produce high-purity distillate