A Massachusetts Institute of Technology (MIT) spinout, Gradiant Corporation, is working toward making hydraulic fracturing (fracking) a water-neutral process, by making water reuse more economical. Fracking produces millions of litres of wastewater and much of it is discarded into deep injection wells, and clean water has to be purchased.
Gradiant’s founders, Anurag Bajpayee and Prakash Govindan have developed cost- effective systems to treat briny oilfield water for reuse, saving millions of litres of water annually.
Gradiant has erected two, 12,000-barrel-per-day tracking plants in the Permian Basin of Texas, partnering with two drilling clients who treat about 10,000 barrels daily there. The plants each use separate technologies that treat varying infeed water, which can be adjusted to customer specifications.
Gradient’s system - carrier gas extraction (CGE) - is a humidification and dehumidification (HDH) technique developed by the Gradiant co-founders at MIT. It heats produced water into vapour, and recondenses it, to remove contaminants. This yields freshwater and saturated brine, commonly used in drilling and completion processes.
(SCE) is a cost-effective version of standard chemical-precipitation techniques — where chemical reactions remove specific contaminants to produce clean brine. CGE and SCE employ custom control algorithms that minimize operator intervention and chemical consumption, while continuously adjusting the process to account for varying feed water quality.
The systems can treat water with high levels of contamination using less energy and at lower costs than competing treatment methods, according to Gradiant.
Reverse osmosis, for example, treats water with a maximum contamination level of around 7%, while legacy thermal desalination reaches about 20 to 22%. But Gradiant’s technology uses still less energy to treat water beyond 25% broadening the range of water that can be treated, Bajpayee says. “Our technology is unique in its capability of going through true saturation limits … to the point where you can actually start seeing crystals in the water,” he says.
But Gradiant’s system — designed by Govindan and colleagues in the laboratories of world water and food security professor at MIT and Gradiant co-founder John H Lienhard, – scaled up well to commercial use. It uses a readily available carrier gas (dry air) that vaporizes water below boiling temperatures, and incorporating a column with microbubbles that optimizes condensing surfaces.
In the Gradiant system’s humidifier chamber, briny water drops through packing material and mixes with dry air to produce a hot and humid vapour stripped of contaminants - such as salts – that forms at the top of the chamber. “We creatively mimic nature’s rain cycle — we create the cloud and then we condense that water back out to create rain,” Bajpayee says.
This “raining” happens in a bubble column, which has several levels of perforated trays, each containing a shallow pool of freshwater. As vapour rises through the bubble column, it passes through the plates’ holes, causing an extremely rapid mixing process that cools and condenses the water within the pools. As levels rise, the water overflows and is captured in a tray as fresh, near-distilled water.
The temperature difference between the warm and cool water is much less than in a conventional dehumidifying system, using less energy, and the surface area provided by the microbubbles in the trays offers a more efficient heat-transfer ratio than a flat, metallic condenser surface. Not using expensive materials, such as titanium, in the heat exchanger also reduces the capital costs.
Heated water is also reused to preheat incoming feed water. Instead of fully heating the incoming water to the desired temperature, Bajpayee says, “you only have to make up the little bit that you couldn’t recover,” which saves energy.
Although Gradiant’s first market is the oil industry, it plans to introduce its technologies to different industries across the globe – wherever there is an incentive to recycle highly contaminated water, according to the company.