Current challenges in energy recovery for desalination

Anthony Bennett looks at the various desalination processes, thermal, membrane and hybrid, and how they have evolved, what the current challenges are for increasing energy efficiency, and what the future might hold for the desalination industry.

Fujairah I MSF-RO Hybrid Desalination Plant. Courtesy of William Chang, Emirates Sembcorp Water and Power Company.
Fujairah I MSF-RO Hybrid Desalination Plant. Courtesy of William Chang, Emirates Sembcorp Water and Power Company.
Soreq RO Desalination Plant. Courtesy of Boris Liberman. IDE.
Soreq RO Desalination Plant. Courtesy of Boris Liberman. IDE.
16 in RO Membrane vertical arrangement at Soreq. Courtesy of Boris Liberman. IDE.
16 in RO Membrane vertical arrangement at Soreq. Courtesy of Boris Liberman. IDE.
Origins of distillation

Today, desalination can be achieved by using thermal or membrane processes, or a hybrid combination.

Desalination using thermal processes was first mentioned by Aristotle, who wrote about seawater distillation around 300-400 BC. Different techniques have been used through the centuries, with the explorers Jean De Lery and James Cook recording that they regularly distilled seawater in the 18th Century.

In the 19th Century, distillation had been commercialized by companies such as Caird & Rayner (a brand which still exists today), with firms located in various countries such as the UK, France, Germany and the US.

As steam engines were developed and high pressure steam became readily available, pure water for boiler feed became necessary to reduce the effects of corrosion and eliminate the likelihood of boiler damage.

On ships such as the Titanic, single effect distillation processes were employed that utilized ‘waste’ heat to produce fresh water for boiler feed, but also for drinking and cooking.

Multiple effect distillation
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 effect is heated by prime steam, resulting in evaporation of a fraction of the water content of the feed.

The water vapor generated by brine evaporation in each effect flows to the next effect, where heat is supplied for additional evaporation at a lower temperature. There the vapor condenses, giving up its latent heat to evaporate an additional fraction of water from the brine.

The process of evaporation-plus-condensation is repeated from effect to effect, each at successively lower pressure and temperature.

The combined condensed vapor constitutes the product: distillate water. The first large-scale MED system for maritime use was installed on the Queen Mary in 1934.

In the 1950s scientists began looking at alternatives to thermal desalination by studying membrane processes. Electrodialysis (ED) was the first of these processes to be developed commercially.
This process was significantly more efficient in brackish water applications (especially those with a higher-temperature feed supply) than distillation, and hence the use of ED became commonplace in lower salinity applications.

The introduction of electrodialysis reversal (EDR) greatly reduced the early scaling problems associated with ED. In EDR, an electric current migrates dissolved salt ions through an electrodialysis stack consisting of alternating layers of cationic and anionic ion exchange membranes.

Periodically, the direction of ion flow is reversed by reversing the polarity of the applied electric current.

Multistage flash distillation

In the mid-1960s virtually all the world's seawater desalination capacity (about 1,000 m3/day) was in the Middle East and was produced by multistage flash (MSF) distillation, a technology that was independently, and at around the same time, invented in the UK and the USA by J & G Weir Ltd and in the USA by Bethlehem Steel and Westinghouse, followed later by Agua Chem and Baldwin Lima Hamilton.

In MSF, seawater (after mixing with the recycle stream) is pressurized and heated to the maximum top brine temperature (TBT). When the heated brine flows into a stage maintained at slightly below the saturation vapor pressure of the water, a fraction of its water content flashes into steam.

The flashed vapor passes through a mist eliminator and condenses on the exterior surface of heat transfer tubing.

The condensed liquid drips into trays as a product water. The unflashed brine enters a second stage, where it flashes again to vapor at a lower temperature, producing a further quantity of product water.

The flashing-cooling process is repeated from stage to stage until both the cooled brine and the cooled distillate are finally discharged from the plant as blow-down brine and product water.

Reverse osmosis

In 1963, Loeb and Sourirajan at the University of California, in Los Angeles, developed the first synthetic reverse osmosis (RO) membrane. In RO, permeate passes from the feed to the product side of the membrane when a pressure exceeding the osmotic pressure is applied.

This ‘reverses’ the natural osmotic effect and concentrates salt ions into a waste concentrate stream. However, high pressure energy-intensive pumps (up to 60-70 bar) were required to drive the process.

By the late 1960s, commercial desalination systems producing up to 8,000 m3/day were beginning to be installed in various parts of the world. Most of these installations used thermal processes but by the 1970s larger scale commercial RO and ED/EDR systems began to be used more extensively.

Initially, in brackish applications, RO had to compete against the now established ED and EDR technologies and early RO was complicated and not always reliable.

The growth of RO was due to market standardization on the spiral wound membrane module, and the introduction of thin film composite (TFC) membranes to replace earlier cellulose acetate materials.

In the late 1970s and 1980s the development by Dow of the FilmTec TFC polyamide membrane brand resulted in process improvements including lower operating pressures, higher fluxes and higher salt rejection, which helped to reduce energy consumption and pumping pressures.

In the 1970s, the introduction of isobaric energy recovery technology significantly reduced the operating costs of seawater RO. By the 1980s, desalination technology had become a fully commercial enterprise and by the 1990s, the use of RO desalination technologies for municipal water supplies had become commonplace.

Emerging technology

Newer emerging desalination technologies take advantage of various processes including forward osmosis, osmotic power, membrane distillation, membrane nanotechnology and more futuristic ideas of desalination like bio-engineered bacteria able to consume specific ions and eat salt. Thoughts on future developments in desalination are included later.

Current desalination status

We asked Leon Awerbuch, Dean of the International Desalination Association's Desalination Academy, about the current global status of the desalination industry.

As of September 2013, the amount of new desalination capacity expected to come on line during 2013 was 50% more than previous year's total, according to new data from the International Desalination Association and GWI DesalData.

Desalination plants with a total capacity of 6 million m3/d were expected to come on line during 2013, compared with 4 million m3/dayin 2012. Data has yet to be confirmed for the last 12 months with the next global plant inventory due later this year. 

Current data takes the total capacity of all 17,277 commissioned desalination plants in the world to at least 80.9 million m3/day. Awerbuch explained, “This is equivalent to nearly 32 years of rain for London or just over 21 years of rain in New York.”

While the 2013 growth rate is somewhat lower than 2010, when 6.5 million m3/day of new capacity was completed, Awerbuch told us that the data shows demand for desalination continues to grow. “An increasing proportion of that growth is coming from the industrial sector,” he explained.

Since 2010, 45% of new desalination plants have been ordered by industrial users such as power stations and refineries, while in the previous four years, only 27% of new capacity was ordered by industrial water users.

Industrial applications for desalination grew to 7.6 million m3/day  for 2010-2013 compared with 5.9 million m3/day  for 2006-2009. Of the 7.6 million m3/day, the power industry accounted for 16%; oil and gas, 12% (up from 7% from 2006-2009); mining and metals, 11%; refining and chemicals, 11%; electronics, 5%; and food and beverage, 3%. Other numerous industrial applications accounted for the remaining 40%.

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 and pure water systems at 5% each.

The largest operational desalination plant in the world had been the 880,000 m3/day  Shoaiba 3 thermal desalination plant in Saudi Arabia. This was displaced in April 2014 by the Ras Al-Khair plant.

As the world's largest hybrid seawater desalination plant, for which Doosan won the construction order in September 2010 from the Saline Water Conversion Corporation, the Ras Al-Khair plant produces 1,036,000m3/day desalinated water (RO at 309,360m3/day  and MSF at 727,130 m3/day ).

The plant is dual purpose with an electricity production capacity of 2,400 MW. The combined cycle power plant is one of the more efficient power plants in the world. The total length of the double transmission lines from the plant to Riyadh and Hafr Al-Batin 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 a massive US$ 6.13 billion, according to the IDA.

The largest MED plant in the world is currently the Jubail Water and Power plant (JWAP) a Marafiq plant built by SIDEM with 800,000 m3/day  production capacity from 27 MED units. The cost 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, by SIDEM/Veolia as a green field development producing 2000 MW of power and 591,000 m3/day of 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 a capacity of 893 MW and a seawater desalination capacity of 455,000 m3/day  (see Figure 1).

The largest seawater RO plant so far has been built by IDE Technologies. The 624,000 m3/day  Soreq SWRO plant (see Figure 2) near Tel Aviv, Israel, came on line in October 2013.

This plant has the unique feature of 16” membrane elements installed in vertical pressure vessels (see Figure 3). This compares to the previous widely accepted traditional design of 8” diameter membranes installed horizontally.

Energy recovery systems

We asked Corrado Sommariva, the immediate past president of the International Desalination Association, about the types of pumps typically used in the different types of desalination systems.

“The major process pump types vary according to the technology – thermal or membrane – and to the application that the particular pump has in the process,” Sommariva said.

“Thermal desalination plants,” Sommariva explained, “normally include large size centrifugal pumps. The major process pumps are characterized by very low available NPSH.

Typically for MSF desalination the available NPSH is obtained for large pumps, such as brine recirculation pumps, by installing the pump in a pit below. For this reason MSF has widely incorporated vertical centrifugal mixed flow machines.”

“Vertical pumps for seawater supply are very similar to the circulating water pumps that are used in power systems. Distillate extraction and blow-down pumps are very similar for MSF and MED systems and are generally horizontal single or double suction with a horizontally split case,” Sommariva added.

In the SWRO process the main power consumption is from the high pressure pump, where seawater is pressurized to 60-70 bar. The pumps are generally multistage ring section type.

Variable frequency control, an important energy saving measure, is generally adopted for all the seawater booster and energy recovery booster pumps.

Positive displacement pumps are generally only used for chemical dosing on larger systems although they are used on small scale RO systems.

“For all desalination process pumps,” Sommariva said, “material selection is one of the most challenging aspects due to a combination of high chlorine, high total dissolved solids and challenging oxygen levels.”

In the past, isobaric energy recovery devices (ERDs), such as those incorporating pelton wheels, were installed on a dedicated skid and often there was a single supplier for pump, motor and energy recovery device.

“With the gradual takeover of state-of-the-art isobaric devices this is no longer necessary, and the pump motor set is generally supplied separately from the ERD,” Sommariva added.

There are various energy recovery devices available today, which have brought substantial whole-life cost savings to RO systems.

“The increasing of pump efficiency is also a great potential for energy improvement,” Sommariva explained. “Not only for new projects but also for retrofit applications.”

For example, a replacement impeller with a new hydraulic design could be installed into an old desalination plant. Several thermal plants installed 20-30 years ago are still in operation with pump efficiencies down at 60-70%.

More efficient retrofit designs and the implementation of variable frequency control can bring further cost savings.

“Improving the pump efficiency by including state-of-the-art technology can decrease pump power consumption,” Sommariva added.

"High pressure pumps are one of the main process utilities in SWRO applications and offer a substantial potential for energy savings. For seawater RO, the use of pressure centres with larger pumps serving more than one RO train in parallel offers the possibility of having higher efficiencies and a great potential for efficiency improvement and energy reduction.”

Depending on seawater salinity, the energy footprint of a modern SWRO system is 3.5-6 kWh/m3. The more water we recover per unit of seawater the more energy we need, but we can reduce down to 0.7 kWh/m3 if the water recovered is only 20% of the seawater mass.

“This energy may be supplied as electricity only or electricity and heat and clearly the possibility of supplying energy in the form of low grade heat would have a tremendous impact on optimizing energy footprint,” Sommariva added.

Ultra-filtration and micro-filtration for SWRO pre-treatment is taking over from traditional sand and media pre-filtration and this offers the possibility of in-line UF or MF system configurations where seawater is pressurized from upstream of the membrane system with the filtered water passing directly to the inlet of the high pressure pumps prior to RO treatment.

“In this case the membrane pre-treatment feed pumps become an important utility as these pumps need to be variable speed and both the size and duty necessary to feed the RO system,” Sommariva said.

Future trends

We went on to discuss the IDA's Global Energy Task Force, with its challenge to the industry to reduce by 20% the energy consumption in all major seawater desalination processes by 2015.

“The objectives of the task force,” Sommariva explained, “stem from the development and implementation of new desalination technologies characterized by lower energy footprints that could be easily combined with renewable energy sources, and the implementation of more energy efficient solutions, retrofitted into existing desalination plants.”

“Technological development in each component of a desalination plant is crucial to obtain the energy reduction target and hence improvements in pump efficiency are required, but also the higher flexibility to operate against different flow rates and pressure conditions.”

In large scale SWRO plants, defined as those producing over 250,000 m3/day of fresh water, it is clear that we will continue to see a further reduction in energy costs per unit desalted water, due to more efficient energy recovery devices, the introduction of new membrane types that are more resistant to fouling, larger membrane housings and faster flows in RO plants.

We will also see the increased use of ‘waste’ heat from power plants for use in desalination, a reduction in material costs (and hence CAPEX) of thermal desalination plants by the use of less expensive coated materials, and further recovery of brine.

As evidenced by the recent desalination plant examples given above, Awerbuch said, “Many of the plants in the Middle East will continue to be built as integrated hybrid solutions where the best properties of thermal process are combined with the best features of membrane technology.”

This would provide optimum solutions for power production and the production of fresh water from desalination systems.

Awerbuch added: “These hybrid projects will combine power generation with MED, and nanofiltration and RO systems will be added to raise efficiency and recovery.”

There has also been a strong drive towards the development of solar powered desalination in recent years. This has been moved forward by a generally more environmental and energy conscious approach to the power and desalination market.

“The successful application of renewable energy in desalination applications will depend on several factors,” Sommariva explained, “and primarily the capacity to create strong interest that involves both researchers and investors, as well as the governments, and includes a set of policies that seriously promote this application further.”

“We will expect to see large scale solar desalination with solar towers, concentrated solar power and solar photovoltaic coupling to seawater desalination. At the end of the decade, I think that coupling of nu clear energy and desalination will be possible,” Awerbuch added.

It is likely that we will see the efficiency of all desalination plants significantly increase. For example, Awerbuch predicts that we will see large MED plants with gain-output ratios (GOR) of 15-20.

This means that they will produce 15-20 tons of water per each ton of steam used, compared to a typical GOR of 10 today. MED capacity is also likely to grow.

In RO we will most likely see established use of the larger Soreq project 16 in diameter modules, their use bringing a reduction in cost and space requirements. Despite improvements in thin-film composite membranes over recent years Awerbuch commented that “there are still shortcomings which will be improved.

There will be high temperature membranes which can increase fluxes and recovery at 50 degrees Celsius and above.” This will bring more challenges for pumping systems in terms of material selection at the higher temperatures.

Larger RO trains and improved fouling resistance will have the effect of increasing energy efficiency, reliability, and the environmental impact of these much larger SWRO systems.

“New technologies for pre-treatment and post-treatment,” Awerbuch explained, “as well as new designs for intake and outfall structures, will increase recovery and minimize environmental impact.”

“Pumps are a crucial component for a desalination plant regardless of the technology,” Sommariva concluded. “In the past, desalination plants, both RO and thermal, have often suffered from shortcomings in pump selection due to poor material specification and often poor design of the system without considering cavitation, vibration, noise and performance issues.”

“Often price and CAPEX reduction has been the driver for the selection of a poor pumping system. The development of a platform where there is a better awareness of pump manufacturers, desalination plant process requirements, and pumping system requirements, will greatly help overcome reliability problems,” Sommariva added.

In addition to the IDA's Global Energy Task force, the organization has also set up the Desalination Academy, run by Awerbuch. The academy is training designers and engineers in all aspects of desalination, with a specific focus on the selection and specification of the correct pumping solutions for individual site-specific installations.

Anthony Bennett is a process technologist at Clarity Authoring.