Laredo Advanced Vapour - Compression Evaporation Desalination Plant, United States of America
An advanced vapour-compression evaporation desalination pilot plant has been developed at Laredo in Texas. The plant opened in August 2010. This plant has been built to test the advanced vapour-compression evaporation (AdVE) desalination technology developed by the Texas A&M University (TAMU) and commercialised by Terrabon. The desalination technology is expected to reduce the capital and operating costs of purifying water.
In March 2009, the Laredo City Council approved the joint proposal of Terrabon and the Texas Engineering Experiment Station (TEES) to design, construct and operate the pilot plant. The council spent $1.6m towards the development of the plant. The total production capacity of the plant is 50,000 gallon/day (190,000 l/day) of fresh water.
The key objective of the plant is to treat the underground brackish and salty water and provide fresh water to about 200,000 people of Laredo.
Laredo City Council member Gene Belmares has said that apart from supplying water to Loredo, the plant was built to cater to the needs of the entire state of Texas.
Laredo, a semi-arid area on the border of Texas and Mexico, has limited water sources from the Rio-Grande River. The ground water in the area is brackish and salty. This water is planned to be desalinated through AdVE.
Advanced vapour-compression technology
A by-product of Torrebon’s MixAlco fermentation broth dewatering technology, AdVE desalination technology, uses low-cost and highly efficient StarRotor compressors and non-fouling heat exchangers with higher capacities to treat the salty water.
Mark Holtzapple, a chemical engineering professor, along with his team at TAMU, has developed this technology. The technology will help Loredo produce water at a lower cost compared to the conventional reverse osmosis process. AdVE technology operates at higher pressures than reverse osmosis. The compressing systems used by this technology operate at 172ºC as against the conventional systems that operate at 80ºC. TAMU is the patent holder and Terrabon the licensee.
Mark Holtzapple said: "We desalinate using vapour compression, a method first employed on ships in World War II."
The feed water is desalinated using compressors, engines and heat exchangers. Under the vapour-compression method, the vapour is removed from water in a chamber and compressed. This results in the formation of high-pressure steam. The steam condenses and forms distilled water.
The heat compressor extracts vapours from the low-pressure side, compresses them and sends them to the high-pressure side, where the latent heat required to evaporate more water from the low-pressure side is condensed. This is a cost-effective heat generation process.
Unlike the conventional centrifugal compressor, the gerotor positive-displacement compressor operates at lower speeds and has components that can sustain water injection. This compressor is efficient for compression in wide operating conditions.
The compression requires that the non-condensable gases be removed before they get accumulated in the heat exchanger.
AdVE desalination technology is also required to de-gasify the feed water before the water is sent through further processing.
The AdVE desalination technology helps in the reduction of capital and operating expenditure incurred on purifying water. Water is treated at an average capital cost of $2.85/gallon, as compared to $5.10/gallon through reverse osmosis. Water costs an average $1.65/1,000 gallons, as against $4.95/ 1,000 gallons through reverse osmosis.
Use of the vapour-compression process enables desalination of water at lower energy levels. The latent heat exchanger uses lower temperatures, thus saving on energy.
Ion exchange is used to clear the water of sulphate and carbonate ions. This reduces foul in the heat exchangers.
TAMU has developed the technology and it has been commercialised by Terrabon. The integrators and the analysts of the project were TEES and The Centre for Applied Technology (TCAT), a centre within TEES. The American Water's subsidiary Applied Water Management is operating the plant, as the subcontractor of Terrabon.