Nanoparticle aerosol generator

A nanoparticle aerosol generator, comprised of a vibrating fluidized bed with a baffle, a vibrating Venturi disperser and a cyclone separator. To generate nanoparticle aerosols, the nanoparticle aerosol generator uses the multiple impaction, vibrating air flow and vibrating high speed air flow to break up larger agglomerates, and multiple dilution to minimize the re-agglomeration of the particles.

Not Applicable

DETAILED DESCRIPTION OF THE INVENTION

The study of the health effects of nano-sized aerosol particles requires the particles delivered to the laboratory animals in the exposure chamber to have: (1) a consistent concentration maintained at a desired level for hours, (2) a homogenous composition free from contaminants, and (3) a size distribution with a geometric mean diameter<200 nm and a geometric standard deviation σg<2.5 (Schmoll et al, 2009). Generation of nanoparticle aerosols at concentrations sufficient to perform toxicology studies is highly challenging because nanoparticles tend to agglomerate due to very strong inter-particle forces and form large fractal structures in tens or hundreds of microns in size (To et al, 2009), which are difficult to be broken up, especially for sticky or cohesive powders such as nano-TiO2dry powder.

In order to perform inhalation studies, the test aerosols can be created by aerosolizing a particle bulk powder. Some common aerosol generators used for this purpose include nebulizers, fluidized beds, Venturi aspirators, and the Wright dust feed (Willeke 1980). However, past test aerosol production typically focused on producing aerosols with a size distribution with median diameter greater than 1 μm (Schmoll et al, 2009). Recently, five different generation methods for producing a nanoparticle aerosol from the bulk powder were evaluated with the goal of producing an acceptable nanoparticle aerosol, however, none of the devices were able to satisfy all criteria for an acceptable aerosol (Schmoll et al, 2009). Existing aerosol generators can not create aerosols in the size range, concentration and duration which are necessary for nanoparticle inhalation toxicology studies. This is particularly true for inhalation exposure chambers such as with a volume of 0.5 m3or more.

The embodied nanoparticle aerosol generator, consisting of a vibrating fluidized bed with a baffle and a vibrating Venturi disperser as well as a cyclone separator, utilizes vibrating high speed shear flow and multiple impaction to disperse larger agglomerates of nanoparticles. Additionally it can use multiple dilution to minimize re-agglomeration of the particles. The particle size and mass concentration produced by the nanoparticle aerosol generator can be controlled by adjusting flow rate of air through dry powder layer, and vibration frequency and amplitude.

A first embodiment, as shown inFIG. 1, is a nanoparticle aerosol generator consists of a vibrating fluidized bed with a baffle, a vibrating Venturi disperser and a cyclone separator. The vibrating fluidized bed with a baffle is comprised of a cylinder101with a proximal and distal ends that can have at least two air feed ports, one port on the proximal end102and the other on the distal end103, and an exit port104on the cylinder top which can be connected to a vibrating Venturi disperser105which can be connected to a cyclone separator106. A vibrator107attached to the cylinder can produce mechanical vibrations. Sample/nanoparticle dry powder108to be aerosolized can rest on a filter109supported by a stainless steel air distributor110on the proximal end in the cylinder. A stainless steel screen111can be placed in the sample/dry powder to break up larger agglomerates. A further embodiment of the nanoparticle aerosol generator can include a baffle112which is connected around the exit port. The vibrating cylinder further can be used as a vibrating fluidized bed with a baffle.

Another embodiment of the nanoparticle aerosol generator can include a cylinder101made of metal or other materials. A vibrator is attached to the cylinder. The cylinder can have a proximal and a distal end. Both ends can have air feed ports102,103. These air feed ports can have their control valves and flow meters113,114, and air feed tubes115attached in which clean and dry air can be pulled into the cylinder through an activated carbon and HEPA filter116. The cylinder can also have an exit port104on the top. A stainless steel air distributor110can be placed above the air feed port102on the proximal end in the cylinder. A filter109can sit on the stainless steel air distributor110. Sample/nanoparticle dry powder108to be aerosolized can be rest on the filter109. A stainless steel screen111with a diameter just smaller than the inner diameter of the cylinder can be placed in the sample/nanoparticle dry powder. The stainless steel screen111can break up larger agglomerates through the impaction between sample and the stainless steel screen. Without the stainless steel screen the output aerosol concentration can be much lower. The vibrating cylinder can induce pressure wave/fluctuation in the air which carries the particles, especially in the air nearby the inner surface of the cylinder. The air pressure wave/fluctuation can help break up the larger agglomerates. Mechanical vibrations can be produced by a vibrator attached to the proximal end of the cylinder. The vibration can be agitated by any known agitation mechanism known to one skilled in the art. Mechanical vibration can be applied parallel with the axis of the cylinder.

The baffle112can be a stainless steel tubing or other material tubing with one dead-end and one open-end. The open-end can be of an effective diameter to be attached in the center of the vibrating cylinder around its exit port104. A hole117near the open-end of the baffle can be used as the aerosol exit in the baffle. The dead-end of the tubing112can extend to just above the top of the sample/dry powder108. The baffle vibrating with the cylinder can transfer the mechanical vibrating energy to the center region of the air that carries the particles in the cylinder. The air flow pressure wave/fluctuations induced by the vibration can help destroy cohesions between particles. When the aerosol flows upward, some of the larger agglomerates can be removed from aerosol streams by the baffle112. Without the baffle, the size of the output particles is much larger.

The mechanical vibration not only generates pressure wave/fluctuations in the air flow to destroy cohesions between particles, but also reduces deposition of the particles on the inner surface of the cylinder and the outer surface of baffle. In the embodied nanoparticle aerosol generator, the extra mechanical vibration energy is transferred to the air flow that carries the particles, and eventually, the energy is used to help destroy cohesions between particles efficiently.

The Venturi disperser105can be connected to the exit port104of the vibrating cylinder. The Venturi disperser vibrates with the vibrating cylinder. The Venturi disperser has a constriction118in a pipe. A high-velocity air jet blowing across the constriction118in the Venturi disperser, can create a vacuum in the cylinder101and clean and dry air can be drawn into the cylinder from the air feed ports on the both proximal and distal ends through an activated carbon and HEPA filter116. A portion of the air drawn in from the proximal end air feed port102will flow through the dry powder layer108in the cylinder to carry some small particles upward to form an aerosol stream, and will move towards the exit port104, while the rest of air enters the cylinder through the air feed port103on distal end and mixes with the upward flowing aerosol stream. The air flow from the distal end air feed port will hit the crossflow aerosol stream resulting in some larger particles being removed from the aerosol stream. The aerosol can also be diluted by the clean and dry air, which helps reduce the probability of re-agglomeration of the particles. When the aerosol stream flows nearby the hole117in the baffle112, some larger particles will be removed because they not able to follow the air to make a 90° turn to enter the hole117. Smaller particles can follow the air flow to leave the cylinder. Once the aerosol enters the Venturi disperser, particles will impact on a tube119in the Venturi disperser first. The large particles will be dispersed by this impaction, while smaller particles will follow the air flow to mix with vibrating high speed shear flow in the Venturi disperser. The vibrating high speed shear flow will continuously disperse the agglomerates, dilute the aerosol, and deliver the aerosol to the cyclone separator106. In the cyclone separator the larger particle can be separated. After passing the cyclone separator106, the aerosol mixes with clean and dry air in a mixing device (which is not shown inFIG. 1) to achieve desired concentrations. The aerosol can be diluted in the mixing device. The dilution can reduce the probability of the particle re-agglomeration before the aerosol enters the inhalation exposure chamber. The use of the Venturi disperser allows for the pressure in the cylinder to be slightly negative. Negative pressure in the cylinder can prevent the particles or other toxin material from escaping from the cylinder. Unlike most of the conventional Venturi dispersers use high speed shear flow to break up large agglomerates, this embodiment uses a vibrating high speed shear flow and multiple impaction to disperse larger agglomerates. The particle size and mass concentration can be controlled via adjusting: (1) flowrate of air passing through the dry powder layer with valve113; and (2) the vibration frequency and amplitude. The higher air flowrate through the dry powder layer, the higher aerosol concentration. For example, the nano-sized TiO2aerosol mass concentration can be increased to 12 mg/m3from 6.2 mg/m3in the inhalation exposure chamber when the air flowrate through the dry powder is increased to 3 LPM from 1.5 LPM.

Characterization of Aerosols Generated by Nanoparticle Aerosol Generator.

In order to test the dust/aerosol generators, nano-sized TiO2dry powder (Aeroxide P25, Evonik, Germany) with primary diameter of 21 nm and density of 3.7 g/cm3and nano-sized CeO2dry powder with primary diameter of 3 nm and density of 7.1 g/cm3were used. Three dust/aerosol generators were tested for the ability to generate nanoparticle aerosols having: (1) a consistent concentration maintained at a desired level for hours, (2) a homogenous composition free contaminants, and (3) a size distribution with a geometric mean diameter<200 nm and geometric standard deviation σg<2.5. Before being aerosolized, TiO2or CeO2dry powders were conditioned in a dry desiccator for 24 hours. The aerosols were delivered to a 0.5 m3stainless steel inhalation exposure chamber for the measurements.

An electric low pressure impactor (ELPI) (Dekati Inc., Finland), a scanning mobility particle sizer (SMPS) (TSI Inc., Shoreview, Minn., USA) were used to measure real time particle size distributions and relative mass concentration of the particles in the inhalation exposure chamber. A TSE dust concentration monitor (TSE Systems GmbH, German) was used to monitor relative concentration of the aerosols generated by TSE dust generators (TSE Systems GmbH, German). The real mass concentrations of the aerosols were determined gravimetrically.

Two embodied nanoparticle aerosol generators were operated simultaneously to deliver TiO2aerosols to a 0.5 m3inhalation exposure chamber at flowrate of 90 LPM. The air flow rate through the dry powder layer in each nanoparticle aerosol generator was 1.5 LPM. A pneumatic vibrator driven by 60 psi compressed air was used to produce mechanical vibration.FIGS. 2A-2Care the particle size distribution, particle mass concentration and particle mean geometric diameter evolution measured with the SMPS. As can be seen fromFIGS. 2A-2C, when the particle concentration reaches the desired concentration in the inhalation exposure chamber, the aerosol generated by the nanoparticle aerosol generators has: (1) a geometric mean diameter=157 nm with a standard deviation σg=2.3 (FIG. 2); (2) a relatively stable particle mass concentration of 6.2 mg/m3during a 5-hour-study (FIG. 3); (3) a stable particle size with a relative deviation less than 7% during 5-hour-study (FIG. 4).FIGS. 5-7are the aerodynamic diameter distribution, relative mass concentration, and particle size evolution measured with the ELPI. The count-median aerodynamic diameter of the particles is 152 nm. The relative mass concentration of the particles is stable during a 5-hour-study.FIGS. 8 and 9are the particle concentrations of TiO2aerosols generated by TSE Bundschuh dust generator and TSE Wright dust generator. The particle concentration of the aerosols generated by TSE dust generators could not be maintained at a constant level. The outcome of the studies is summarized in Table 1.

TABLE 1TestDust GeneratorlocationOutcome1TSE BundschuhOur labA nozzle Venturi disperser was blockeddust generatorby the nano TiO2particles after severalminutes.2TSE WrightOur labNo stable generation and output hole ofdust generatorthe particles was blocked nano by TiO2particles after 30-40 minutes.3NanoparticleOur labNanoparticle aerosol concentrationaerosol generatorremained stable for hours
CO2Aerosols:
One embodied nanoparticle aerosol generator was operated to generate CeO2aerosols from nano-sized CeO2dry powder. The air flow rate through the CeO2dry powder layer in the nanoparticle aerosol generator was 1.5 LPM. The aerosol was delivered to a 0.5 m3inhalation exposure chamber at flowrate of 90 LPM and measured with the ELPI.FIGS. 10-12are the particle size distribution, particle relative mass concentration and particle count median aerodynamic diameter evolution measured with the ELPI. As can be seen fromFIGS. 10-12, when the particle concentration reaches the desired concentration in the inhalation exposure chamber, the CeO2aerosol generated by the nanoparticle aerosol generators has: (1) a count median aerodynamic diameter=145.4 nm (FIG. 10); (2) a relatively stable particle mass concentration during a 3-hour-study (FIG. 11); (3) a relative stable particle size during a 3-hour-study (FIG. 12).

Among the tested aerosol/dust generators, the embodied nano-particle aerosol generator is the only one that can generate nanoparticle aerosols from nano-TiO2or CeO2dry powder directly.

These terms and specifications, including the examples, serve to describe the invention by example and not to limit the invention. It is expected that others will perceive differences, which, while differing from the forgoing, do not depart from the scope of the invention herein described and claimed. In particular, any of the function elements described herein may be replaced by any other known element having an equivalent function.

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