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Nanoparticle Heating Distills Ethanol From Water

Separations: How light-driven heating could make distillation cheaper and more energy efficient

by Prachi Patel
November 19, 2015

CORRECTION: This story was updated on Dec. 16, 2015, to correct the description of distillation and to clarify the separation difficulty posed by azeotropes.

A new method for heating liquid mixtures could reduce the energy required for distillation. Researchers have found that shining laser light on gold nanoparticles in an ethanol-water mixture vaporizes the ethanol without bringing the whole mixture to a boil (Nano Lett. 2015, DOI: 10.1021/acs.nanolett.5b02804). The approach also works with sunlight and would be especially suitable for use in remote areas, the researchers say.

Credit: Nano Lett.
In a new distillation process, a near-infrared laser (black, upper left) shines on an ethanol-water mixture containing silica-gold nanoparticles (red). The light heats the nanoparticles and vaporizes the ethanol. A water condenser (blue) cools the vapors, and the ethanol fractions are collected in a flask (yellow).
Credit: Nano Lett.
In a new distillation process, a near-infrared laser (black, upper left) shines on an ethanol-water mixture containing silica-gold nanoparticles (red). The light heats the nanoparticles and vaporizes the ethanol. A water condenser (blue) cools the vapors, and the ethanol fractions are collected in a flask (yellow).

In conventional distillation, a liquid mixture is heated, and its components vaporize preferentially according to their vapor pressures. The vapor, which is enriched in the lower boiling compounds, is then condensed. The process is “extraordinarily energy intensive,” says Naomi J. Halas, professor of electrical and computer engineering at Rice University. In bioethanol production, for example, distillation accounts for 75–80% of energy use.

To reduce the energy needed, Halas and her colleagues turned to nanoparticles. A decade ago, Halas pioneered a process that relies on nanoparticles with a metal shell that absorb certain wavelengths of light and generate intense heat in a small region around them. In 2012, the Rice team showed that focusing sunlight on aqueous solutions of 100-nm-diameter particles with silica cores and gold shells could generate steam (ACS Nano, DOI: 10.1021/nn304948h).

They have now applied the same trick to mixtures of alcohol and water. They use an optical fiber to shine 15-W near-infrared laser light on vials of ethanol-water mixtures with the same silica-gold nanoparticles mixed in. Within a few minutes, the nanoparticles heat up, vaporizing an ethanol-enriched mixture from the surrounding liquid. The researchers separate the vapor and condense it. The relatively large nanoparticles are not carried into the ethanol vapors, Halas says.

The technique also prevents the formation of an azeotrope, a problem with ethanol and water mixtures. Ethanol-water mixtures are impossible to separate completely via conventional distillation without using special drying agents. The researchers tested the nanoparticle technique with mixtures of ethanol and water in different ratios. Because they did not form azeotropes, the resulting ethanol fractions had higher purity than those from a traditional distillation.

The researchers have not calculated how much less energy the nanoparticle technique needs compared with traditional distillation. But, Halas says, the technique could work using sunlight, bypassing grid power altogether. And carbon nanoparticles, which also heat up in response to light, would provide a cheaper option than gold shell nanoparticles.

Halas does not envision the method being used for massive distillation columns in the chemical industry any time soon. Rather, she says, it could be useful for quickly getting rich alcohol fractions on a smaller scale as part of a more complex separation process.

Nanoparticle-based heating has been mainly explored for medical applications until now, says Laura I. Clarke, a professor of physics at North Carolina State University. The new work demonstrates that the method can enhance a commercially important process like ethanol distillation. But more than being a novel application, “this means that not all heating is equivalent,” she says.



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Auntie Markovnikov (November 20, 2015 9:56 AM)
After quickly looking through the Nano Lett article and supporting information I have a question or two: Did the authors try directing the laser at the bottom of the flask and not, as shown in the drawings and photographs, first through the vapor above the liquid and then the vapor/liquid interface? If yes, does this make a difference in the results? Apologies if this is already discussed in the article...
Dr. Josep Font (November 25, 2015 12:44 PM)
Dear Sir,

After reading the paragraph

"In conventional distillation, liquid mixtures are heated, and the component liquids vaporize at different boiling points. The lower boiling compounds vaporize first and are then condensed to obtain the pure liquids."

I see that the authors do not understand a single word about vapor-liquid equilibrium. What means that by distillation one can get the pure liquid? This is against the second principle of Thermodynamics!!!

And the azeotrop disappears... How can an azeotrop disappear if it is intrinsic to the specific VLE behavior of the mixture? The azeotrop can be eliminated if the VLE is modified by the nanoparticles but not by simply heating in a different way...

This seems more closely to an advertisement than a scientific article.
Richard Pires (November 30, 2015 11:28 AM)
You summed up my response to the article perfectly
Joseph Gentilucci (November 25, 2015 3:18 PM)
The experiment looked like a batch distillation. What percent of the feed came off overhead as product and what was the content of the remaining heal? I see great difficulty of converting this to a continuous process since it appears that you need a stack of these assemblies with the bottoms being sent to the next lower group through a trap arrangement. Were you able to get 100% removal of the ethanol from the water/ethanol solutions? If yes, I can see a use for making 100% ethanol from the normal azeotrope without the use of the hazardous benzene currently used,but it would be quite a large unit since all the ethanol goes overhead while the current benzene process takes the benzene/water azeotrope overhead. Interesting concept.
Vinod Sinha (November 26, 2015 2:40 AM)
This is a strange report. In normal distillation, the vapor contains all the components of the liquid phase, in proportion to the concentration of the component in the liquid phase, multiplied by its vapor pressure at the temperature of the liquid (Raoult's law). I don't see how this method of heating prevents water from getting hot when energy is absorbed by the nanoparticle, or when the alcohol vapors travel through the liquid mixture.
Bob Kroshefsky (November 26, 2015 12:24 PM)
I agree with your observation, Vinod. The heat generated within the mixture should be readily transferred throughout the bulk liquid and therefore lead to the vapor mixture you cite above the liquid. The formation of an azeotropic distillate is then simply a natural consequence of such a condition. If the authors have a way to defeat this process, then THAT is the big news in this article.
Ed Chlapowski (November 26, 2015 5:46 AM)
This is another perpetual motion machine event of bad science. Too many unmeasured quantities to draw this conclusion.
A. Anderson (November 27, 2015 2:12 PM)
Primitive cultures heated liquids by dropping hot rocks into the liquid.
jc (November 27, 2015 4:45 PM)
Regarding the part about distillation:
And C&EN published this???
I can't find any more socially acceptable words.
Not tryin to be mean to Prachi, but c'mon.... whoever proofed it should be looking for another job.
Y'all should know a technical faux pas at a basic level isn't gonna fly here.
Corinna Wu (December 17, 2015 11:15 AM)
Hi, I'm one of the editors who worked on the story. Your comment and others made us realize that we needed to correct the explanation of conventional distillation. Please see the corrected story above. Thanks!
Auntie Markovnikov (December 1, 2015 10:15 AM)
I agree with all of the other commenters here. The only reasonable way the authors could have observed such unusual results is, I think, if the vapor phase or surface of the liquid was selectively heated with the laser and not the bulk liquid (from the bottom), as I inquired in my first comment. Otherwise this whole thing is preposterous - if the heated material exchanges heat with the bulk then it can only give the expected result.

Note that this is the SECOND time this work by these authors has been cited for special attention in C&ENews, the first was in 2012: Again, in that piece the authors shine sunlight through the vapor phase and directed at the liquid surface. How were these two papers selected for such attention? Lobbying on the part of the authors?
Schuyler Corry (December 2, 2015 6:57 PM)
"I see that the authors do not understand a single word about vapor-liquid equilibrium. What means that by distillation one can get the pure liquid? This is against the second principle of Thermodynamics!!!"

It seems the commenters on this article did not read the manuscript in question. The simple answer is that this is NOT an equilibrium process, so vapor-liquid-equilibrium does not apply. At equilibrium, yes, there would be an azeotrope; but in fact, the rate of vaporization happens faster than the system can equilibrate, so there are some strange behaviors like "breaking" the azeotrope. Heating in the vicinity of the nanoparticles is so fast that the alcohol is selectively vaporized at a greater rate than water molecules. The vaporized liquid forms an insulating bubble around the particle, making it even more difficult to dissipate heat, so the bubble grows and carries the nanoparticle to the liquid surface, releasing a burst of super-heated vapor (enriched in ethanol).

Microwave heating is another non-equilibrium process, since the rate at which radiative energy is absorbed is greater than the rate that the energy can be released by boiling, hence you see things like superheating (boiling above the EQUILIBRIUM boiling temperature at a given pressure).

So no, not magic, not perpetual motion machine, not a product-placement—just a non-equilibrium process.
Dr. Josep Font (December 3, 2015 5:02 AM)
My comment was about the description of what the "conventional" distillation is or is able to do, not about the article. And be sure that, if my bachelor students describe distillation in this way, they will not pass the course.
Auntie Markovnikov (December 3, 2015 7:57 AM)
Schuyler Corry - I read the articles, hence my questions. In both articles (this most recent one and the one from 2012) the light source is directed at the vapor phase and liquid surface, and not at the bulk liquid, as shown in the diagram accompanying this article. If the light is shined only through the bulk liquid is the reported effect observed? - A heated nanoparticle in an insulated bubble of vaporized, non-equilibrium vaporized liquid will diffuse all the way from the bottom of the vessel to the liquid surface without heat exchange and equilibration? I'll bet a case of beer that it is not.
Schuyler Corry (December 3, 2015 1:57 PM)
Dear Auntie,

1st question: the phenomenon is caused by nanoparticles absorbing light, not the bulk liquid and not the vapor/liquid interface. Consider the liquid to be transparent to the light, and only the particles absorb a significant amount of photo energy.

2nd question: I doubt the transport of the heated particles is through diffusion, but rather through a buoyancy of the bubble that carries the particle to the liquid/vapor interface. It would be energetically unfavorable to re-wet the particle once a bubble was formed, but if that did happen the vapor bubble would still continue to rise. Small bubbles will probably merge with other bubbles, accelerating their ascent to the surface. And although I'm sure some heat exchange occurs, there should not be thermal equilibration if the light intensity is above some threshold.

Radiative heat transport was always glossed over compared to convective and conductive processes in school. But radiative heat transfer has some very weird results. I remember calculating that water can freeze well above 0ºC on a clear night, due to radiative heat losses to outer space. Very weird. And superheating (as in a microwave, or with these nanoparticles) is similarly weird. Even though the nanoparticles transfer heat to the solution, the energy source is ultimately solar / photo radiation.

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