What is Thermal Desalination Technology

All thermal desalination technologies apply distillation. This means heating the source water to produce water vapor, then condense into a low-salinity water. Since the energy for water evaporation is practically not dependent on the salinity concentration of the feed water source, thermal evaporation is very suitable for desalination of high salinity waters and brine. This is one of the reasons that thermal desalination has been widely adopted by Middle Eastern countries. Such countries are Saudi Arabia, Oman, Qatar, the United Arab Emirates, Bahrain and Kuwait. These countries use some of the most saline water bodies on the planet for water supply: Red Sea, Persian Gulf, Gulf of Oman, and Indian Ocean. At present, approximately 75% of the world’s thermal desalination plants are located in the Arabian Peninsula—half of those are in Saudi Arabia.

All thermal desalination plants have five key streams:
1- Source water (seawater, brackish water, or brine) used for desalination.
2- Steam needed for evaporation of the source water.
3- Cooling water to condense the freshwater vapor generated from the source water’s evaporation.
4- Low salinity distilled water (distillate).
5- And concentrate (brine), which contains the salts and other impurities separated from the source water.

The three most commonly used types of thermal desalination technologies are multi stage flash distillation (MSF), multi effect distillation (MED), and vapor compression (VC). Each of these classes of technology has evolved over the past 40-60 years toward improvements in efficiency and productivity. For example, MSF-BR is the abbreviation for a multi stage flash distillation process with brine recycle. MSF-BR reduces the source water volume and the steam needed for evaporation. Similarly, MED-TC stands for multi effect distillation with thermal compression. It is the state-of-the art MED technology. MVC is an acronym for mechanical vapor compression, a VC technology that can run without the need for an outside source of steam.

thermal desalination and evaporation distillation technologies

The three types of thermal technologies mainly differ by temperature and pressure at which source water is boiled to generate freshwater vapor. The oldest thermal evaporation process—MSF—boils water at near atmospheric pressure and a temperature close to 100°C (212°F). This type of process requires a large quantity of high-temperature steam. Multi Effect Distillation MED and Vapor Compression VC are newer thermal desalination technologies. They improved efficiency stems from the fact that water can be boiled at a lower temperature if the boiling process occurs at a pressure lower than the atmospheric pressure. Boiling water at a lower temperature allows the use of less and lower-quality steam for the production of the same volume of water.

In Multi Effect Distillation MED vessels, the boiling process typically occurs at lower temperatures and pressures than in MSF systems. Vapor Compression VC thermal desalination systems operate at lower pressures than either Multi Stage Flash Distillation MSF or MED. which allows these systems to evaporate water at even lower temperatures and to generate their own steam rather than depend on outside steam sources. The ratio of the mass of low-salinity water (distillate) produced to the mass of heating steam used to produce this water is commonly referred to as the gained output ratio (GOR) or performance ratio.

Depending on the thermal desalination technology used, the site-specific conditions, and the source water quality, GOR typically varies between 4 and 40. Thermal desalination technologies produce 4 to 40 kg of freshwater using 1 kg of steam. The higher the GOR, the more efficient the technology, because it produces more freshwater from the same amount of steam. As seen in the figure, all thermal desalination technologies generate very low-salinity water (TDS in a range of 5 to 25 mg/L). This freshwater also has a very low content of pathogens and other contaminants of concern. Such as boron, bromides, and organics. Thermal desalination is most popular in the Middle East, where seawater desalination is typically combined with power generation. Therefore provides low-cost steam for the distillation process. Thermal desalination requires large quantities of steam. Most power plants outside the Middle East are not designed to yield significant amounts of waste steam as a side product of power generation. This is one of the key reasons why thermal desalination has not found wider application outside of the region.

Today, Seawater Reverse Osmosis SWRO dominates the global desalination market in term of capacities. They exceed both traditional thermal technologies (MSF and MED) which are common in the Middle East and North Africa regions. Seawater Reverse Osmosis SWRO is dominating worldwide over thermal technologies in terms of annual installed capacities. Yet the evolution of seawater reverse osmosis into the Gulf and MENA region, particularly in the Gulf of Arabia, has been slow. The reasons are higher feed water salinity affected by hydrocarbons and Red Tide events. Red tide is a phenomenon caused by algal blooms during which algae become so numerous that they discolor coastal waters. The algal bloom may also deplete oxygen in the waters and/or release toxins that may cause illness in humans and other animals.

Reference: “Desalination Engineering” by Nikolay Voutchkov