Yet, this is no fantasy. It has recently been achieved at unit 9 of Sharjah’s Layyah power and desalination complex, where a joint venture of Belgium’s Besix and UK-based LET Integrated Technologies has raised the distillate water production to more than 7.2 million gallons a day (g/d) from 5 million g/d through a hybrid system combining the NF membrane technology with the existing thermal process.
‘What we have done is all explained by engineering: there is no black box,’ says LET president and the hybrid technology inventor Leon Awerbuch. ‘Everything is the result of smart engineering. It is a breakthrough, not black magic.’
The principle behind the hybrid technology is relatively straightforward. In MSF technology, the amount of water produced from a unit depends on the difference between the bottom and top brine temperature (TBT). The main constraint has been increasing the TBT above 105°C – beyond that temperature, scaling rises dramatically.
Enter NF technology. Under the LET integrated hybrid MSF desalination process, membranes supplied by the US’ Dow Chemical Company are used to soften part of the make-up water, which allows the TBT to be raised to 120°C without scaling. At the same time, modifications are made to the internal design of the existing MSF unit to accommodate the increased capacity, and steps are taken to release carbon dioxide from the distillation process to improve plant operations and the water pH levels.
NF has long been applied in the oil industry: it was first developed by the US’ Marathon Oil Corporation for treating seawater to be injected into oil fields. However, it had never been used in a hybrid combination with MSF. Three years ago, Awerbuch approached Sharjah Electricity & Water Authority (SEWA) with the idea. ‘We did a programme of simulation with SEWA, which then decided to go ahead with the project,’ says Awerbuch. ‘They took some risk. I refused to do a pilot plant, as I was so convinced that it would work: the one thing I didn’t know was by how much.’
Even though the $7.5 million project has been commissioned and has been working in combined operation since January, the question remains as to how much additional production can be squeezed out of the 5 million-g/d unit built in the late 1990s by Italy’s Ansaldo Energia. ‘We have been operating the unit at 7.2 million g/d, which is more than 40 per cent above design. With some further adjustments, we are confident that it can go to 45 per cent, and in the near future we hope to push the envelope beyond 50-60 per cent. On newer plants, we won’t be able to achieve this increase – probably in the range of 20-25 per cent. But by any standards that is incredible: the big engineering companies tend to look at 2-3 per cent improvements,’ he says.
The Sharjah plant provides a strong economic case for the hybrid technology. SEWA spent $7.5 million on the project and has so far achieved additional capacity of 2.2 million g/d. That works out at $3.40 a gallon of capacity installed, compared with more than $5 a gallon for average new-build costs. ‘The thing about this technology is that you don’t need to build a new intake structure: you take the water from the outfall flow. You also don’t have to install additional power capacity as there is enough steam in the system,’ Awerbuch says.
Operating costs are also competitive, he maintains. ‘If you take all the costs and operations of the new elements and base it on a 20-year life and a 6 per cent interest rate, the leverage cost of water comes out at $0.40-0.49 a cubic m