Separation and purification of alcohols in distilleries is usually by distillation. Here, the homogeneous mixture is heated to boiling and the separation is based on the difference in the boiling points of the components. The conventional thermal distillation is the most energy-consuming step of the production, making it the main determinant in the cost of the product. Another limitation of this process is the formation of azeotrope which hinders complete purification of the chemicals, resulting in extensive purification procedures. Azeotrope when further boiled produces vapour mixture with the same proportions of the constituents as that in the unboiled liquid mixture, affecting the purity of the distillate.
Researchers from Rice University, have reported an efficient way of distillation using plasmonic nanoparticles and resonant laser illumination. Halas and his co-workers used plasmonic Au-SiO2 nanoparticles that are capable of both absorbing and scattering the light energy. Upon dispersion of the nanoparticles in liquid mixtures and irradiation at their resonant wavelength, the nanoparticles absorb the optical energy and heat a localized fluid volume, resulting in vaporization of the more volatile component. Combination of optical absorption and multiple scattering can concentrate the light energy into mesoscale volumes near the illuminated surface of the liquid. Thus, in this top illumination method, the bulk volume of the mixture is not heated up resulting in less energy consumption and high-efficiency distillation.
For ethanol-water mixtures, a 99% pure distillate was formed by optical distillation when compared to the 95% distillate from thermal heating. This was achieved because no azeotrope was formed in the optical distillation method. However with 1-propanol – water and 2-propanol – water mixtures, light-induced liquid−liquid phase separation was observed, but the distillate properties were similar by both optical and thermal distillation along with azeotrope formation.
Comparing these results, the group has concluded that the hydrogen bonding network is affected by the optical distillation. Localised heating of nanoparticles causes the disruption of strong hydrogen bonding network within the ethanol–water mixture, allowing greater separation of ethanol from water molecules at higher concentrations of ethanol. Whereas with propanol, a larger alcohol with weaker hydrogen bonding network in the mixture, optical distillation produced the same result as thermal distillation.
“While, optical distillation will not replace the very mature conventional distillation, it could find niche applications for new and different separation methods”, says Dr. Halas. In the meantime, utilization of solar energy, plasmonic nanoparticles and resonant illumination can be considered for bio-ethanol production, paving way for energy-efficient fuel production which is the need of the hour. Alongside, deeper understanding of optical distillation in terms of properties of the liquid mixtures and that nanoparticles, will open up surprising applications for photothermal applications and optical distillation in a multitude of industrial applications.