Desalination of Water and Sea-Water

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Desalination Processes

The following table outlines the commercially viable desalination processes in use worldwide today. The links from the table headings lead to descriptions of the processes. These are only general guidelines. There is no perfect process that covers all situations.

Brackish Water Reverse Osmosis (BWRO), Electrodialysis Reversal (EDR), and Seawater Reverse Osmosis (SWRO) are all membrane separation process. The product water resulting from these processes is referred to as permeate. As a rule of thumb 1 - 2 % of salt in the source, or feedwater, will pass through the membrane into the permeate. For example SWRO produces around 350 PPM permeate from 35000 PPM seawater.

Multiple Effect Distillation (MED), Multiple Stage Flash (MSF) and Mechanical Vapour Compression (MVC) are all thermal processes which produce distilled water. Typically this distillate is very pure with a total dissolved solids (TDS) of 1 - 50 PPM.

Energy Consumption

Membrane processes normally utilize electric driven pumps as the prime source of energy but in remote locations diesel engines have been used. The thermal processes on the other hand use steam in the process. MVC is a notable exception as this process frequently incorporates electric or diesel driven steam compressors as the primary source of energy. MED and MSF are classical thermal processes as they utilize steam from dedicated boilers, waste heat boilers or the extraction or back pressure steam from the turbines in power stations.

Since all desalination processes can use some combination of energy sources and can be designed for different levels of energy efficiency, simple comparisons are difficult to make.

The units for steam consumption are shown as kilograms of steam per kilogram of distillate produced. The industry norm for presenting this information is:

Gained Output Ratio (GOR) = lb of distillate/ lb of steam

which is the same as:

Gained Output Ratio (GOR) = kg of distillate/ kg of steam

Some smart soul in the annals of history noticed that one pound (lb) of steam has an energy content of approximately 1000 British Thermal Units (BTU). This gave rise to the popular performance ratio, also known as economy, which is almost equal to GOR.

Economy = Performance Ratio = lb of distillate/1000BTU

The SI equivalent to this is the less elegant, but still frequently used

Performance Ratio = kg of distillate/2326 kJ

Energy consumption, operational costs and capital investment must all be compared simultaneously to provide the lowest evaluated cost process for any given project. While some processes may be excluded from any particular project simple guidelines are difficult to provide.

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Desalination Materials

This table extends to the right and needs to be scrolled to be seen completely. The left hand column repeats to simplify the scrolling process.

The table merely summarizes which materials can withstand the corrosive and erosive environments listed. Some materials may be incompatible with each other due to galvanic corrosion or other reasons. Some materials may not be available for all duties; examples are;

• PVC, FRP/RTRP and other non metallic pipes are not suitable for the high pressure services of SWRO
• Aluminium Bronze is only suitable for non welded applications such as bolted tube sheets
• 254SMO, 654SMO and other exotic stainless steels may only be available in plate and sheet form. Pipe fittings, instrumentation and other components may not be available.

Service

Carbon Steel

Aluminium

PVC

Polypropylene

FRP/RTRP

Service

304L SST

316L SST

317L SST

317LMN SST

904L SST

Service

254SMO SST

AL6XN SST

654SMO SST

Service

Al-Brass

Al-Bronze

CuNi 9010

CuNi 7030

CuNi 6630

Titanium

Distillate

X

X

Y

Y

Y

Distillate

Y

Y

Y

Y

Y

Distillate

Y

Y

Y

Distillate

Y

Y

Y

Y

Y

Y

Potable

Y

X

Y

Y

Y

Potable

Y

Y

Y

Y

Y

Potable

Y

Y

Y

Potable

Y

Y

Y

Y

Y

Y

Brackish

X

X

Y

Y

Y

Brackish

Y

Y

Y

Y

Y

Brackish

Y

Y

Y

Brackish

Y

Y

Y

Y

Y

Y

Brackish/H2S

X

X

Y

Y

Y

Brackish/H2S

Y

Y

Y

Y

Y

Brackish/H2S

Y

Y

Y

Brackish/H2S

X

X

X

X

X

Y

Seawater (Cold)

X

X

Y

Y

Y

Seawater (Cold)

X

Y

Y

Y

Y

Seawater (Cold)

Y

Y

Y

Seawater (Cold)

Y

Y

Y

Y

Y

Y

Seawater (Hot)

X

X

X

X

X

Seawater (Hot)

X

X

X

X

X

Seawater (Hot)

X

X

Y

Seawater (Hot)

Y

Y

Y

Y

Y

Y

Seawater (Hot d/a)

X

X

X

X

X

Seawater (Hot d/a)

X

X

X

X

X

Seawater (Hot d/a)

Y

Y

Y

Seawater (Hot d/a)

Y

Y

Y

Y

Y

Y

Seawater/H2S

X

X

Y

Y

Y

Seawater/H2S

X

Y

Y

Y

Y

Seawater/H2S

Y

Y

Y

Seawater/H2S

X

X

X

X

X

Y

Brine

X

X

Y

Y

Y

Brine

X

Y

Y

Y

Y

Brine

Y

Y

Y

Brine

Y

Y

Y

Y

Y

Y

Brine/H2S

X

X

Y

Y

Y

Brine/H2S

X

Y

Y

Y

Y

Brine/H2S

Y

Y

Y

Brine/H2S

X

X

X

X

X

Y

Brine (Hot)

X

X

X

X

X

Brine (Hot)

X

X

X

X

X

Brine (Hot)

Y

Y

Y

Brine (Hot)

Y

Y

Y

Y

Y

Y

Uses

Uses

Uses

Uses

Support steel

Pipe

Pipe

Pipe

Pipe

Pipe

Pipe

Pipe

Pipe

Pipe

Pipe

Pipe

Tubes

Tubesheets

Tubes

Tubes

Tubes

Tubes

Flash vessels

Flash Vessels

Flash Vessels

Flash Vessels

Flash Vessels

Flash Vessels

Flash Vessels

Flash Vessels

Tubesheets

Tubesheets

Tubesheets

MED Vessels

MED Vessels

MED Vessels

MED Vessels

MED Vessels

MED Vessels

MED Vessels

Pipe

Pipe

Flash Vessels

Flash Vessels

MED Vessels

MED Vessels

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Process Descriptions

Thermal Desalting

The most common ways to desalt the seas involve some form of boiling or evaporation. In a simple still seawater can be boiled releasing steam which, when condensed, forms pure water. Many stills can be connected together making the process more efficient. To achieve this however each still, or effect, must be at different pressures. At sea level pure water boils at 100 degrees Celsius (212 deg F). In a vacuum it can boil at much lower temperatures. Multiple Effect Distillation (MED) makes use of this phenomenon.

What happens if water is heated to 100 degrees Celsius but held under pressure until it is released into a vacuum chamber? The answer is the water flashes into steam. The difference between flashing and boiling is;

• Boiling is accompanied by a constant input of heat, just like when cooking on a stove.
• Flashing involves heating, holding under pressure, then introducing a vacuum to allow flashing.

Connecting multiple stages at successively lower pressures in the principle behind Multi-Stage Flash desalination (MSF).

Other thermal processes include a variation of the simple still known as vapour compression (VC). This can be done using mechanical (MVC) or thermal (TVC) compressors. MVC uses centrifugal fans or blowers to compress and thereby heat steam making it suitable for driving a desalination process. TVC uses moderately pressured steam to drive a steam jet thermocompressor.

Membrane Separation

Semi permeable and ion specific membranes can also be used to desalt seawater. Membrane processes are based on separation rather than distillation (although membrane distillation has been performed).

Reverse osmosis membranes basically let water pass through them but reject the passage of salt ions. In reality a small percentage, say 1%, of sea salts passes through the membranes, or leaks around seals. For potable water this leakage is acceptable but for industrial purposes it may require further treatment.

The operational pressure of reverse osmosis systems is a function of the salinity of the feedwater. The salinity results in a colligative property known as Osmotic Pressure. The osmotic pressure of brackish waters is very much lower than that of seawaters or brines. Typical 500 PPM potable water has an osmotic pressure of 2 bar while normal seawater is closer to 25 bar.

Since this directly relates to working pressure and energy consumption reverse osmosis has an advantage over thermal processes (where the latent heat of evaporation is very constant irrespective of TDS {well for waters at least}).

Electro-dialysis reversal (EDR) makes use of ion specific membranes which are arrayed between anodes and cathodes to drive salt ions in controlled migrations to the electrodes. While not as widespread as RO it is still in common use. RO is by far the most widely used separation process and has tremendous energy advantages when 1% salt passage can be tolerated, when steam is not available, and when good quality seawater is available.

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