How Desalination by Multi-stage Flash Distillation Works

One of the common method for desalination is using Multi Stage Flash distillation. Most of the existing desalination plants use this method. This is a proven method and has been in service for the last three decades.

In an MSF distillation sea water vaporisation takes place at low temperatures in vacuum. The vapours condense to form fresh water. At vacuum pressures the boiling point of water is low requiring less energy.

A brine heater heats the sea water to around 90 to 110 deg C.

Before reaching the brine heaters the cold sea water passes through condensing coils in the vacuum flash chambers. This has the dual function of

• Preheating the cold sea water before entering the brine heater and
• Condensing the flashed steam in the chambers to produce fresh water.

The hot brine then enters the flash chamber which is at a vacuum. Since the entering water temperature is higher than the boiling temperature at that vacuum pressure, a part of the water flashes to steam. The steam rises to the upper part of the chamber and on contact with condensing coils condenses to form pure water. The salt and other impurities still remain with balance of the brine at the bottom of the chamber. Steam ejectors produce the necessary vacuum in the flash chambers.

The balance brine goes to the next chamber where the process repeats. Multiple chambers increases the quantity of the water product. The balance brine returns to the sea. A 20 % yield of fresh water is possible in MSF systems.

The energy requirement is in two stages.

• Electrical energy for pumping the water.
• Steam energy for heating the brine.

The total energy requirement is in the order of 17 kwhr / m³ of water.

Flashing of the steam forms scales and deposits on the tubes. Periodic cleaning and removal is required.

Feasibility of Desalination plants have to be by comparing costs of desalination with costs of pumping or sourcing fresh water from long distances. But in locations like the deserts of Saudi Arabia there is no other choice.

Adversaries of desalination plants have many reasons like destruction of marine life, water quality, high cost of water, energy consumption etc. But millions of people who do not have access to fresh water or in the deserts where water is life tend to think otherwise.