Fresh Nanotech Solutions to a Freshwater Problem

With fresh water in limited supply, ensuring water security is a global challenge demanding urgent attention. Amanda Jasi spoke to researchers exploring novel nanotechnology to help meet that challenge

SALTWATER provides 97% of the Earth’s water, so exploiting this vast resource through desalination appears to be a no-brainer. Reverse osmosis (RO) is the most used desalination method, but RO plants remain highly electrical energy intensive, while you also need to regularly replace the membranes.

A more sustainable alternative is multistage flash distillation which exploits solar energy to generate clean water from seawater via steam. Lowering electric power consumption, the process also offers the ability to co-generate power using the steam.

This process can still negatively impact land use, however, which is why Tamal Banerjee, chemical engineering professor at the Indian Institute of Technology Guwahati (IIT Guwahati), is exploring nanofluids as a more efficient heat transfer method. “When I talk about nanofluids, it means you have a fluid…and then add some nanoparticles,” he clarifies. Nanoparticles are typically defined as being between 1–100 nm in size.

The process first relies on a solar collector to concentrate solar rays, and then transfers the heat to the nanofluid. The nanofluid is then moved to a heat exchanger where the captured energy is used to heat another system containing a series of distillation columns.

“Once you heat this [system]…water is evaporated, and the remaining water is then transported to another column. Whatever is evaporated is then condensed…and you get clean drinking water,” Banerjee says. In the next column, at a lower pressure, the same happens again, generating more clean water and further concentrating the seawater.

The water goes through multiple stages to output clean drinking water, while the remaining concentrated brine is purged from the system. Thereafter, fresh seawater is again fed into the system and the cycle continues.

Banerjee and his group have successfully created a novel nanofluid for heat transfer based on graphene oxide. “Graphene has tremendous thermal conductivity and electrical conductivity,” he says. Graphene oxide’s structure comprises strongly bound layers, however, making the entire structure hydrophobic, meaning it will not disperse in liquid media.

“To separate these layers, you add a chemical functional group such as amine into this graphene oxide,” says Banerjee. The separation then allows for easy dispersion in a base fluid, which in the group’s case is a deep eutectic solvent (DES), comprising two components which when mixed become a fluid. “Otherwise, they remain a solid,” he says.

The group’s DES is a mixture of diphenyl ether and DL-menthol. Banerjee says that the modified graphene oxide has “excellent” dispersion in the DES.

“We did a simulation from a commercial simulation package to show that this process actually works, and we are able to produce clean water. And we have measured this clean water by a particular parameter called a gained output ratio (GOR). It tells us how much water we are able to produce per amount of heat added to the system.”

The GOR was 10, while the range for multistage flash distillation is 1–10. Nipu Kumar Das, research scholar at IIT Guwahati and first author on the paper about the work, admits that higher figures in the range correspond to a greater number of stages. “Typically, GOR systems with a larger size incur higher costs,” he says. “But they exhibit greater energy efficiency, leading to reduced operational expenses.”

Testing has proven the nanofluid’s promise, including its stability (for at least 27 days) and resistance to agglomeration (when nanoparticles stick together instead of flowing). However, further development work is needed.

While the group has created a process flow sheet incorporating three distillation columns, they have not studied fluid dynamics. “We need to do computational fluid dynamics to know more about the heat transfer characteristics within the pipelines,” Banerjee says.

Scaleup studies will also be dependent on the flow rate of different fluids in the system, such as the nanofluid and distillate, as well as the number of columns that will be used. A process and instrumentation diagram (P&ID) based on the group’s data is being developed.

Pilot studies will also benefit the group, with Banerjee saying they could “simulate the solar energy and see how much of this nanofluid is actually being used up to transfer it to the seawater”. They are in talks with manufacturers that could take on pilot plant development.