Desalination Energy Recovery Tech

In this article Anthony Bennett looks at recent advances in desalination processes and the increasing use of energy recovery technologies

The desalination of seawater (see Figure 1) to produce fresh water for human consumption can be achieved by using thermal or membrane processes, or a hybrid combination of these two process types.

We shall firstly briefly consider thermal technologies. In the late 19th Century, the first major technical advance in desalination technology was the development of the multiple effect distillation (MED) process. Here, preheated feed water flowing over tubes in the first distillation ‘effect’ (or tank) is heated by prime steam, resulting in evaporation of a fraction of the water content of the feed. A process of evaporation-plus-condensation is repeated many times to produce the product water from effect to effect, hence the term ‘multiple effect.’ In the mid-1960s multistage flash (MSF) distillation became popular. Here, a flashing-cooling process is repeated from one stage to the next stage to produce the product water.

The first reverse osmosis membrane was produced at about the same time as MSF was being developed. Reverse osmosis membranes are based around the principle of utilising osmosis, a natural process involving water flow across a semi-permeable membrane barrier. Osmosis is selective in the sense that the water passes through the membrane at a faster rate than the dissolved solids. The difference of passage rate results in the separation of water and solids. The direction of water flow is determined by its chemical potential, which is a function of pressure, temperature and concentration of dissolved solids.

Pure water in contact with both sides of an ideal semi-permeable membrane at equal pressure and temperature has no net flow across the membrane because the chemical potential is equal on both sides. If a soluble salt is added on one side, the chemical potential of this salt solution is reduced. Osmotic flow from the pure water side across the membrane to the salt solution side will occur until the equilibrium of chemical potential is restored. Equilibrium occurs when the hydrostatic pressure differential resulting from the volume changes on both sides of the membrane is equal to the osmotic pressure. This is a solution property that is independent of the membrane.

Application of an external pressure to the salt solution side equal to the osmotic pressure will also cause equilibrium. Additional pressure above the equilibrium point will raise the chemical potential of the water in the salt solution and cause a solvent flow to the pure water side, because it now has a lower chemical potential. This phenomenon is called reverse osmosis (RO) (Figure 1). Applying this RO principle to treating seawater is challenging as high pressures (60-70 bar) are required to overcome the osmotic pressure.

In the early years of RO desalination, positive displacement and centrifugal pumps provided 100% of the energy to power a seawater RO (SWRO) plant, but innovations in the field of energy recovery have improved energy efficiency significantly over the last couple of decades. Waste energy from RO systems can currently be recovered from the concentrate flow, and can account for 25-30% of the energy required to overcome the osmotic pressure of seawater (Figure 2). This lowers the total energy requirement of desalination plants dramatically. Energy is recovered by utilising isobaric energy recovery device (ERD) technology.

Projects

The amount of new desalination capacity that came on line during 2013 was 50% more than previous year’s total, according to data from the International Desalination Association (IDA) and Global Water Intelligence. Desalination plants with a total capacity of 6 Million m3/d became operational during 2013, compared with 4 Million m3/d in 2012. Data has yet to be confirmed for the last twelve months, but preliminary information suggests that the total capacity of plants that are online or under construction exceeds 82 Million m3/d.

Whilst the 2013 growth rate was somewhat lower than 2010, when 6.5 Million m3/d of new capacity was installed globally, clearly the demand for desalination continues to grow. From 2010 - 2013, 45% of new desalination plants were ordered by industrial users such as power stations and refineries, while in the previous four years, only 27% of new capacity was ordered by these industrial water users.

Industrial applications for desalination grew to 7.6 Million m3/d for 2010-2013 compared with 5.9 Million m3/d for 2006-2009. Seawater desalination continues to represent the largest percentage of online global capacity at 59%, followed by brackish water applications at 22%, river water projects at 9%, and wastewater recovery at 5% and pure water systems at 5%.

The world’s largest SWRO seawater desalination plant at the time of writing (January 2015), the Ras Al-Khair plant in Saudi Arabia, produces 1,036,000 m3/d, sufficient to meet the daily drinking water requirements of around 3.5 million people. The plant produced its first freshwater in early 2014, although the project is actually scheduled for completion in December 2015. As the world’s largest hybrid plant, the project uses both membrane technology (RO at 309,360 m3/d) and thermal technology (MSF with a capacity of 727,130 m3/d). This plant also features the largest single MSF trains composed of 8 units with capacity of over 91,000 m3/d each. The RO plant has 17 trains.

The Ras Al-Khair plant is dual purpose in that it produces fresh water for drinking but also power in the form of electricity. It has an export production capacity of 1.025 million m3/d desalinated water and an electricity production capacity of 2,400 MW, providing 1350 MW for the nearby Maaden Aluminium Complex, 1050 MW to the Saudi Electricity Company, and about 200MW for internal consumption on site.

The combined cycle power plant at Ras Al-Khair is one of the more efficient power plants in the world. The total length of the double transmission lines from Ras Al-Khair plant to Riyadh and Hafr Al-Batin region will be 1,290 km. The cost of Ras Al-Khair desalination and power plant project and the transmission lines from the plant has so far reached in excess of a massive US$ 6 Billion, according to the IDA.

The largest MED thermal desalination plant in the world is currently the Jubail Water and Power Plant in the United Arab Emirates, a Marafiq plant, built by SIDEM with an 800,000 m3/d production capacity from 27 MED units. The cost for this plant was US$ 1 Billion. This is also a dual purpose plant generating 2744 MW electricity in addition to desalinated water.

The largest hybrid MED-RO plant is the Fujairah II project, also in the United Arab Emirates, constructed by SIDEM and Veolia as a green field development producing 2000 MW of power and 591,000 m3/d of drinking water. The hybrid system includes five high-efficiency gas turbines operated in combined cycle mode. The Fujairah I project, owned by Emirates Sembcorp Water and Power Company and commissioned in 2004, comprises a hybrid MSF-RO system again combined with power production with an electricity generation capacity of 893 MW and a seawater desalination capacity of 455,000 m3/d.

The largest membrane-only SWRO plant so far has been built by IDE Technologies: the 624,000 m3/d Soreq SWRO plant near Tel Aviv, Israel. It came on line in October 2013. This plant has the unique feature of 16” membrane elements installed in vertical pressure vessels. This compares with the widely accepted, traditional design of 8” diameter membranes installed horizontally.

The Ras Al-Khair, Fujairah II and Soreq SWRO plants all incorporate isobaric ERD units. The first major contract to incorporate the isobaric ERD was awarded for the Ashkelon SWRO project in Israel in 2003. The Ashkelon plant was the largest in the world at the time and was developed as a BOT (Build-Operate-Transfer) project by a consortium of three international companies: Veolia water, IDE Technologies and Elran.

In March 2006, the Ashkelon project was voted "Desalination Plant of the Year" in the Global Water Awards. Since that time, many more ERD projects have been awarded, the technology becoming standard on SWRO projects.

However, Andrews (2010) comments that the developers of earlier SWRO plants were nervous about the reliability and performance of isobaric ERDs. They favoured a safer conservative approach with lower capital expenditure on ERD units in exchange for higher energy losses and higher maintenance costs.

Source: World Pumps

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