USING a temperature-dependent solvent to desalinate water with high concentrations of dissolved salts may prove more efficient than conventional methods.
So-called “hypersaline” brines are produced during a range of different processes, such as oil and gas production, and can pollute surface and groundwater if improperly treated. These brines are challenging and costly to treat. A typical method to remove the high levels of salt is reverse osmosis (RO). However as salt concentration is proportional to the amount of pressure needed to desalinate, hypersalines require prohibitively high pressures using RO. Another method is distillation, which is energy-intensive as it requires a phase change of water.
A team at Columbia University, US, has developed a new approach to desalinate these brines by using a new separation process known as temperature swing solvent extraction (TSSE). A temperature-dependent amine solvent is added to a hypersaline brine. As the low-polarity solvent is immiscible with the brine, the lighter amine solvent floats on top of the brine. Then, due to the hydrophilic properties of the solvent, it extracts water from the saline feedwater, leaving behind a dewatered concentrate. The concentrate is decanted, and the water-in-solvent extract is heated from room temperature to around 60oC – the temperature swing. The solubility of water in the solvent decreases with the increasing temperature, causing the water to separate from the solvent. This desalinated product water is then decanted from the solvent. TSSE can desalinate brines with salinities up to 234,000 ppm total dissolved solids, removing around 98.4% of the salt and recovering 50% of the water. The solvent can also be reused continuously with occasional replenishment.
The heat input can be supplied via sustainable sources such as industrial waste heat, shallow-well geothermal, and low-concentration solar collectors. The desalinated water is unsuitable for potable use due to the amine residues but could still be used for other purposes such as reusing hydraulic fracturing water from shale gas production. The water can also undergo post-treatment using RO to obtain a higher quality end-product. As the salt concentration in the product water is low, the required pressure for RO for post-treatment is low, resulting in a low-energy cost.
"I thought solvent extraction could be a good alternative desalination approach that is radically different from conventional methods because it is membrane-less and not based on evaporative phase-change," said Ngai Yin Yip, Assistant Professor of Earth and Environmental Engineering at Columbia. "Our results show that TSSE could be a disruptive technology – it's effective, efficient, scalable, and can be sustainably powered."
TSSE can desalinate brines that have up to seven times the salt concentration of seawater. The process can be easily scaled up and could be operated continuously. The research team is currently working on improving the process to test it on real-world samples.
“We can eliminate the pollution problems from these brines and create cleaner, more useable water for our planet,” said Yip.
Environmental Science & Technology Letters http://doi.org/c644
Catch up on the latest news, views and jobs from The Chemical Engineer. Below are the four latest issues. View a wider selection of the archive from within the Magazine section of this site.