Pulling oil from water

Jeff Fitlow, Rice University

ENGINEERS at Rice University, US, in conjunction with Shell Global Solutions, have developed magnetic nanoparticles capable of removing the last drops of crude oil from produced water.

Produced water is a by-product of oil and gas production. The water can be treated and then reused in oil recovery processes or released into water streams. A variety of conventional treatment methods are available, such as chemical demulsification, air flotation, and membrane separation. However, these methods fail to remove all of the remaining oil.

Shell told the team at Rice that 5% of oil remains in produced water. However, Sibani Biswal, associate professor of materials science and nanoengineering, said that this amount “can vary depending on a variety of variables such as composition and separation process used”.

The nanoparticles developed by Rice – iron nanoparticles, surface functionalised with amines – were able to remove up to 99.7% of oil from oil-in-water emulsions.

Nanoparticle function

The nanoparticles disrupt oil-in-water emulsions by interacting with naturally present surface-active components, or surfactants.

Typically, oil and water do not mix well, but in crude oil production, emulsions are stabilised by stresses experienced during fluid transport and the presence of surfactants. The new magnetic nanoparticles  act as “sponges” to remove surfactants from the oil-water interface, thereby destabilising emulsions, Biswal said.

She elaborated: “The oil droplets are negatively charged, and they are stabilised by carboxylate-based surfactants, such as naphthenic acids, which are found naturally in crude oils and are negatively charged. Our nanoparticles have a high absorption capacity for these natural surfactants that stabilise the oil droplets. Once the surfactant is removed from the interface, the oil droplets are no longer stable.”

In trials, nanoparticles were mixed with emulsions made in the lab using model crude oil, or produced water obtained from an enhanced oil recovery pilot. Oil-water bonds were broken in just minutes. Some of the oil rose to the top of the mixture and could then be skimmed off. Nanoparticle-oil droplet complexes – negatively-charged oil droplets bound to positively-charged magnetic nanoparticles – could be pulled from solution by applying a magnetic field.

The group tested the nanoparticles for up to ten cycles, after which they became “fouled irreversibly,” said Biswal. Though she said there is the possibility of refunctionalisation.

Notably, these nanoparticles did not aggregate under the high salinity of the produced water as nanoparticles tend to. They do this due to reduced electrostatic repulsion. “Ours have both an electrostatic (cationic) functionality and a steric repulsive interaction that allows the NPs to be stable,” said Biswal.

Bulk processing and scaling up

A flow-through reactor is now being designed for bulk processing. Biswal said the reactor would enable produced water to be mixed with the magnetic nanoparticles for phase separation to occur. The oil and brine stream could then be separated in a continuous manner.

Biswal hopes that an interested industrial partner will join forces with the lab to design a scale up for industry.

Environmental Science: Water Research & Technology: http://doi.org/cs7d