Thermal desalination in meeting global water demands

Using thermal desalination to produce potable water has been a vital process in the Middle East since the late 1960s. Multi-stage flash (MSF) desalination plants have been the predominant method used in Gulf Cooperation Council (GCC) countries, producing about 94% of the total water production. 

With the global shift towards less energy-intensive technologies in recent decades, thermal desalination still sees much relevance in the Middle East. The geographical restraints of the region hindered the complete transition to membrane desalination, with challenges faced in membrane compatibility with the harsh salinity of seawater from the Gulf and the Red Sea. High temperatures in the region also restrict the operational capacity of membrane desalination plants. 

Outside of the Middle East, the Asia-Pacific region expects the highest growth rate in the market for desalination systems. With rapid population growth rates and urbanization, there is an increasing demand for water from the agricultural and industrial sectors. In China, membrane desalination accounts for about 65% of its total operating capacity, with multi-effect distillation (MED) desalination accounting for the remaining 35%. 

Multi-Stage Flash (MSF) Desalination 

There are two main process configurations used in MSF, once-through (MSF-OT) and brine recirculation (MSF-BR). Most commercial and large-scale thermal desalination plants use brine recirculation systems consisting of 15-40 stages.

Seawater feed passes throughout MSF-OT systems once through at a time, while a small amount of seawater feed mixes with the rejected brine of the last stage for MSF-BR systems before recirculation. 

The mixed feed enters a solar collector array or a conventional brine heater and raises to near saturation temperature under maximum operating pressure. The heated feed then enters the first flash stage through an orifice with pressure reduced. In the first stage, the water superheats and flashes into steam.

The vapor rises and passes through a demister to remove any entrained brine droplets before passing through the heat exchanger where it condenses. The distillate collects into a tray, with the process repeated throughout the plant to ultimately yield freshwater.

Multi-Effect Distillation (MED) Desalination

In the past, extensive scaling problems rendered MED systems less competitive than MSF thermal desalination plants. The old MED designs submerged the evaporator surface tubes, and high heating of the concentrated brine resulted in scaling. Newer MED system designs reduce scaling by preheating the feedwater using vapor from the effects, and the system feeds forward. 

Due to the lower energy demand in operating MED plants, there is a shift in preference over MSF systems for seawater desalination. Apart from improved energy costing, MED systems do not require pretreatment of seawater feed and can tolerate a wide range of varying water conditions. 

A MED plant consists of airtight elements known as effects, which host consecutive condensation and evaporation processes in decreasing temperatures and pressures. The feedwater is first preheated to saturation temperature and is direct to each effect in equal proportions. The heated feedwater sprays onto the evaporator tubes heated by steam and vaporizes. 

The vapor in the previous effect feeds forward and heats the evaporator in the sequential effect. The incoming feedwater cools the vapor in the last effect of the thermal desalination plant, and the vapor condenses in the final condenser to produce freshwater. 

Addressing scaling and foaming in thermal desalination plants

a) Scale formation 

Scale formation in thermal desalination systems comprises alkaline and non-alkaline scale species. 

Alkaline scale species consist primarily of calcium carbonate and magnesium hydroxide. Alkaline scale species are typically the common problem in temperatures below 120°C, with calcium carbonate as the predominant form in processes operating at 50°C to 90°C. The tendency for magnesium hydroxide formation increases at elevated temperatures of over 80°C. 

Non-alkaline scale species consist of calcium sulfate in the predominant form of hemihydrates. The solubility limits determine the extent and rate of precipitation and are affected by temperature and seawater ionic strength. In desalination units, the repeated concentration of brine can easily cross the threshold limits and propagate scale formation.

To address the various type of scale formation and species encountered in thermal desalination units, the application of CrestoPro™ T Series scale inhibitors has proven to be effective. The CrestoPro™ T series provides extensive inhibitive properties against alkaline and non-alkaline scale species, with added dispersancy benefits. 

The mitigation of scale formation using the CrestoPro™ T Series scale inhibitors reduces the cost of cleaning and replacing equipment parts of the heat exchangers in thermal desalination plants.

b) Foaming 

Excessive foaming is a problem encountered in MSF desalination units and is affected by variations in plant operating conditions. Variations in pressure, temperature, flow, alkalinity, and seawater feed compositions can influence the extent of foaming. 

Foam formation is less than ideal as it can lead to contamination of the distillate and a reduction in operating efficiency. The CrestoPro™ T635 is a high-performance antifoam designed for use in thermal desalination units and has proven to be effective in foam knockdown and reduction of distillate contamination.