A new technique uses magnetic nanoparticles to remove oil from produced water
Offshore oil spills may make the headlines, but the industry faces a less obvious and systemic problem with produced water. Dumping it in evaporation pools or re-injecting it may not be permitted by regulators for ever, or desired by operators keen to establish their environmental credibility. If produced water could be made sufficiently clean it could be used for irrigation, but pollution standards for that end-use are presently out of reach, even with membrane filtration or the use of nut-shell.
Researchers from the University of Texas at Austin have developed the use of magnetic nanoparticles to remove very stable micron-scale oil droplets from produced water. The technique uses ferrous oxide magnetite and maghemite nanoparticles, as these are among the strongest naturally occurring magnetic substances. A thin layer of silica is precipitated onto the nanoparticles and then polyacrylic acid is covalently bound to that, before an amine group is added. This process gives the nanoparticles a positively charged surface coating.
The impurities in oil droplets give them a negative charge. When mixed, the nanoparticles and oil droplets are drawn together by electrostatic force. The nanoparticles can then be filtered with a magnet, bringing the oil with them.
The nanoparticles and oil are then separated by altering the pH of the oil, which changes the charge of the nanoparticles’ surface coating. Once detached, the nanoparticles can be removed and reused. The Austin team is currently working on a chemical-free process to reuse the nanoparticles to achieve the same result.
The Austin team has shown that its technique can reduce an initial oil content of 0.25% by as much as 99.9% in separated water, down to 2.5 ppm.
However, the process does require thorough mixing of nanoparticles and contaminated water. One test showed that two hours of mixing resulted in a five-minute reaction time, and work on reducing mixing and reaction times is ongoing. Another variable is of course the concentration of nanoparticles. Mix ratios being tested are in the 1-2 grams per litre range.
The last drop
“This new technique is really aimed at removing that little bit of oil in water that needs to be removed before you can consider it treated,” explained lead author Dr Saebom Ko.
The Austin technique looks most promising for onshore production, where use of evaporation pools is commonplace. In offshore production time and space look like being critical limiting factors, unless the reaction time could be radically accelerated.
With raw material prices around US$2 per kilo, and chemical processing costs at a similar level, the solution looks like costing in the region of US$0.01 per kg of produced water, which scales up to around US$10 per barrel. That puts the technology clearly in the “too expensive” zone, unless the team can develop a recycling system that works in volume and at speed. The process can be tailored to target other dissolved chemicals, including polymers and lead.
While the technology is versatile, and bears some similarities to sequential filtration synthesis being developed at Argonne National Laboratory for similar purposes, a working model of the continuous flow system will be needed to prove its viability in cleaning offshore produced water. This solution therefore falls squarely into the “theoretical” box at present.