Nanoparticles

Nanoparticles are extremely small particles with dimensions typically measured in nanometers (nm), where one nanometer is equal to one billionth of a meter (10^-9 meters).

Nanoparticles have unique properties and behaviors compared to their bulk counterparts due to their small size and high surface area-to-volume ratio which allows them to interact with other molecules more easily than larger particles. This makes them useful for a variety of applications, such as drug delivery, cosmetics, and electronics.
 
Some common types of nanoparticles include:

• Semiconductor nanoparticles, including quantum dots: They have unique optical properties that make them useful for applications such as solar cells, drug delivery, electronics and biological imaging. Examples: zync oxide (ZnO NPs) or silicon (SiNPs) nanoparticles.
• Metal nanoparticles: These nanoparticles are made of metals, such as gold, silver, and copper. They are used in a variety of applications, including electronics, catalysis, and medicine.
• Carbon nanotubes: These nanoparticles are made of carbon atoms arranged in a cylindrical shape. They are very strong and lightweight, and they are used in a variety of applications, such as electronics, composites, and drug delivery.
• Polymeric nanoparticles: These nanoparticles are made of polymers, such as polystyrene and polylactic acid. They are used in a variety of applications, such as drug delivery, cosmetics, and food packaging. 

There are many different methods for producing nanoparticles where gas / solid separation is necessary. Some of the most common methods include:

• Flame spray pyrolysis when a metal or metal salt is sprayed into a flame, where it vaporizes and then condenses to form NPs,  
• Chemical vapor deposition (CVD), where material vapor (e.g. ZnO) is generated and then condensed to form NPs.
• Arc discharge: where an electric arc is created between two electrodes, which vaporizes the electrodes and forms nanoparticles.
• Laser ablation: when a laser is used to ablate a metal or metal salt, which vaporizes and then condenses to form nanoparticles.
• Sonochemical synthesis: when ultrasound waves are used to break down a metal or metal salt into nanoparticles. 

 
Nanoparticles pose several challenges for gas/solid separation. Nanoparticles are very small, which makes them difficult to capture with regular cyclones and other mechanical separators. NPs can easily pass through the pores of bag filters or clog the filter elements. They can also change its characteristics and degradate when holding in the filters - a problem which is common to electrostatic precipitators. Finally, wet scrubbers pose even more problems if the nanoparticles are soluble or degrade in a liquid phase.

ACS solutions include cyclones with electrostatic recirculation (ReCyclone EH) which allow for a direct particle capture, avoiding particle degradation and aggregation, using electrostatic separation as shown here. The ReCyclone EH has shown efficiencies of more than 95% to capture ZnO NPs with a median size of ~1 μm (Case Study) and more than 90% to capture silicon nanoparticles of 20-30nm (NASA Article- Pages 32, 33 and 34).