MEASUREMENT AND FILTRATION OF AIRBONRE NANOPARTICLES

Transcription

1 MEASUREMENT AND FILTRATION OF AIRBONRE NANOPARTICLES Zhongchao (Chao) Tan, PhD, PEng 谭中超 Associate Professor University of Waterloo Presentation to DLMU, May 7, 2012; Dalian, China

2 : Assistant Professor of Environmental Engineering 2009-Present: Associate Professor of Environmental Engineering 2006-Present: Doctoral Graduate Student Supervisor Department of Mechanical Engineering 2010 : Associate Professor of Green Energy, Department of Mechanical Engineering : PhD Bioenvironmental Engineering, Department of Agricultural & Biological Engineering : PhD Student, Mechanical Engineering

3 Air Pollution Airborne Nanoparticle Bio-refinery

4 Airborne Nanoparticles (Ultrafine Particles) Relative magnitude PM2.5 Relative # Relative Mass Particle Diameter (um) Whitby, AE (1978) A mass based standard: by mass the majority of particles are > 100 nm. By number: majority are ultrafine particles (<100 nm). Larger particles settle down and can be effectively controlled Standards are developed based on the availability of technologies 1. Measurement of nanoparticles 2. Filtration of nanoparticles

5 Lungs Heart Nemmar et al. (2002) Circulation

6 Scanning Mobility Particle Sizer ~ 60,000 USD

7 Aerodynamic Particle Focusing (APF) Particle beam formed when aerosol passes through an orifice into an evacuated chamber Particle beam has a diameter due to Brownian motion Optimum focused particle d p* is a function of Stk f =1-2, by variable orifice geometry: impractical variable upstream pressure ( particle mean free path) (3 mm orifice is to keep gas flow continuum at very low pressure) Various focusing devices to enhance transmission efficiency for smaller particles (<10 nm) 2 m * d p d p d m p 18Stk v p Aerosol in f f P 1 P f Skimmers p f 0 p Focusing orifice 0 f To pump To pump Source region T T f 0

8 Online Size-resolved Chemical Content Analysis Ionization and TOF MS Various systems developed for chemical analysis, challenging limits of particle size, transmit rate, hit rate, ablation rate, portability. Focusing orifice To Vacuum Pumps Pressure Gage Source region Drying tube Aerosol In Inlet Pressure Control Rotary Valve w/ Orifices Could analyze single particles down to 10 nm TOF MS High frequency ablation laser to increase ablation rate by collinear orientation with counter propagation w.r.t. particle beam Turbo pump Excimer Laser (193nm) Laser trigger pulse DAQ/PC Micro Channel Plate (MCP) MCP Signal Source: Rhoads et al. AAAR 19th Conference (2000)

9 Diffusive Charging of Nanoparticles For nanoparticles, diffusive charging is dominant over field charging and independent on dielectric constant (e) The maximum amount of ions carried by a particle is related to diameter, d p Ions d pkt n 2 2e Particle d ln1 p cπ e 2 2kT N ion t

10 Particle Sizing by APF + Diffusive Charging By scanning mode, determine the size distribution of the particles To Vacuum pump Charged aerosol in v s v f Faraday cup Pressure reducing orifice, d 0 Particle focusing orifice, d f To electrometer (x10-17 A), I n d * p I C 2 d 4 0

11 Experimental Setup for Feasibility Study Polydisperse particles CPC Atomizer Neutralizer Monosized particles Power supply to gold needle To vacuum pump Shielded and grounded to reduce noise Electrometer Faraday cup Focusing orifice Orifice bank Current number of d p Orifices select focused d p

12 Saturated Charging Tested alternative charging devices to achieve saturated charging Near 100% detection efficiency for Faraday cup All particles focused are captured by the Faraday cup Particle beam diameter < Faraday cup aperture (3.175 mm) Measured current (ma) Particle beam diameter (mm) mm Power supply output voltage (kv) 4 mm 12 mm downstream of the focusing orifice Particle diameter (nm) Source : Tan & Wexler (2007) Atm. Env.

13 Selectiveness of the new focusing orifice Selectiveness is important for sensitivity Results: d p =50 nm Function of orifice size and upstream pressure Thickness of the focusing orifice also matters mm thick - charging 0.5 mm thick + charging Current (x10-17 A) mm thick - charging Charts from Tan & Wexler (2007) AE Particle diameter (nm)

14 Prototype 2000 SMPS Good matching!? Area of Investigation!

15 Nanoparticle Generation in Corona Charger Alena Saprykina, MSc, 2009

16 Nanoparticle Generation in Corona Charger High voltage power supply Air HEPA Filter Corona charger Scanning Mobility Particle Sizer (SMPS) Transmission Electron Microscope Analysis Scanning Electron Microscope Analysis & Energy Dispersive X-ray Analysis (chemical composition analysis)

17 Corona Generated Nanoparticles A potential nanoparticle generator for aerosol research Total particle concentration, #/cm E=18818 kv/m E=18065 kv/m E=17312 kv/m E=16559 kv/m E= kv/m E=15054 kv/m E=13549 kv/m E= kv/m dp, nm

18 Corona Generated Nanoparticles Source: Alena MSc thesis (2009) From gold needle Agglomeration occurred Agglomeration

19 SMPS Prototype dp, nm Total concentration of particles, #/cm3

20 Laleh Golshahi, MSc, 2007 Granular Filtration of Airborne Nanoparticles YAN Chao

21 Air Filters Extensively studied filters Fibrous filters Fabric filters Membrane filters Granular filters, less studied for air Capture particles and gases Handle high temperature/pressure Reusable filter materials Mobile filters Packed bed Moving bed Fluidized bed Polluted Air Moving bed Granules Inlet Clean Air Granules Outlet

22 Air Filtration Mechanisms Filtration = Transport and Adhesion Particle transport to a collector, quantified by collision efficiency, Attach to a collector, quantified by adhesion efficiency, Total efficiency = (collision eff.) x (adhesion eff.) Collector surface

23 Particle Transport Mechanisms Gravity settling Interception Impaction Diffusion Single Collector Electrostatic precipitation Streamline Particle trajectory 23 Single 1 D I G P E (1 )(1 )(1 )(1 )(1 )

24 Classical Filtration Theory 100% Relative importance Total Diffusion of the mechanisms For nanoparticles diffusion dominates and efficiency Interception Settling increases inversely Impact with particle size for lower end Assumed adhesion Particle diameter (nm) efficiency = 1 Efficiency

25 Adhesion Efficiency 1 Air molecules are not (supposed to be) captured by filters Filtration efficiency is nearly zero when particle size is approaching molecule size The curve should drop from maximum efficiency to near zero (Red dashed line)? Diffusion Inertia 100% Efficiency Critical size Size of molecules Particle diameter (log scale)

26 Thermal rebound: Analytical When the impact speed is greater than a critical speed, a particle will rebound Assumptions: Elastic impact between nanoparticles and the surface Particle impact speed follows Maxwell-Boltzmann distribution JKR adhesion energy model Sample calculation for fibrous/membrane filters showed that thermal rebound would start at 1-10 nm for (K p +K s )=5x10-11 m 3 /N and σ ps = J/m 2 Mechanical constants and specific surface energy between the particle and the surface are limited for many materials, especially for nanoparticles Specific adhesion energy, σ ps = J/m 2 ½mv 2 Fdr Probability ½mv 2 Adhesion energy Temperature increase Particle thermal speed Wang & Kasper s Model (1991)

27 Experimental 27 Filters: fibrous, fabric, membrane filters Different nanoparticles Almost all failed, even at elevated temperatures. Nobody used granular filters Heim M, Aerosol Sci. & Tech. Japuntich et al J. of Nanoparticle Res. Golanski et al Human & Experimental Toxicology van Gulijk et al J. Aerosol Sci. Shin et al J. Aerosol Sci. Kim et al , J. of Nanoparticle Res. Kim et al J. of Nanoparticle Res. Rengasamy et al J. of Occup. & Env. Hygiene Wang et al J. of Nanoparticle Res. Huang et al J. Aerosol Sci. Reference Particle Filter Air Speed (cm/s) Heim et al Kim et al Kim et al Japuntich et al Wang et al Huang et al Shin et al Rengasamy et al.2008 Golanski et al.2009 van Gulijk et al NaCl, >2.5 nm NaCl >2nm Silver >2 nm NaCl, DOP > 10 nm Steel filter, nickel mesh, polyproylene mesh Glass fiber, 9.1/11.8 μm Commercial fibrous filters H&V filter μm 14.4, 6.58 TR No 2.5 Once 15, 10, 5.3 No 5.3 No Silver> 3 nm Fibreglass No NaCl>4.5nm N95 2.8, 8 No Silver > 3nm NaCl, Silver > 4 nm Graphite >10 nm NaCl, CaCl2 > 6, 10 nm stainless steel grid, 90um N95 P100 Filters 4.17, No Fibre glass 5.3, 9.6 No Ground stainless steel grid, 40um 3.1 No No <500K

28 One cannot measure adhesion efficiency or collision efficiency alone Total filtration efficiency with thermal rebound: Wang & Kasper s model The conclusion of (1-10 nm) was only based on many assumptions no validation. Larger filter collectors may help reveals thermal rebound Can only measure total efficiency Es Thermal rebound effect is shadowed by high collision efficiency when the fibre diameter is small (usually in micrometers) Some filters many not allow elastic impact. Most equipment (e.g. SMPS) could accurately detect particle smaller than 5 nm

29 Our Experimental Setup: Large granules, low face speed The granular filter : Monosized glass beads (2, 4 6 mm) and poly-dispersed sulfur granules (6 mm) Face velocity: (1-4 cm/s) highlights diffusion Two types of aerosol particles Salt (NaCl) particles nm, and Standard test micron particles ( um) Isokinetic measurement, probes connected to SMPS (for nano-) and APS (for micron-) Aerosol Granules Clean air Efficiency derived from numbers of particles before and after passing through the filter SMPS size converted to aerodynamic diameter Average of 5 replications with small errors

30 Glass granules, different sizes, same flow rate Higher bed and finer granules, higher efficiency Valley at μm Thicker bed, higher the collision efficiency Smaller critical size Rebounded particles might be re-captured downstream Less difference in efficiency Implication: Properly designed filters can reduce the negative effect of thermal rebound Source of charts: Golshahi (2007) MSc Thesis H=2.5cm H=7.6cm 2mm Glass Beads 4mm Glass Beads 6mm Glass Beads 2mm Glass Beads 4mm Glass Beads 6mm Glass Beads 2mm Glass Beads 4mm Glass Beads 6mm Glass Beads 60 H=12.7cm Aerodynamic Diameter (m)

31 00 90 Different granules, same flow rate Sulfur Granules 6mm Glass Beads Higher efficiency for micron particles, but lower for certain nanoparticles Thermal rebound started early for sulphur (same salt nanoparticles) Implication: Higher mass efficiency doesn't guarantee the same for nanoparticles Different adhesion energy and/or mechanical constants? Others? H=2.5cm H=7.6cm Sulfur Granules 6mm Glass Beads Sulfur Granules 6mm Glass Beads 60 H=12.7cm Aerodynamic Diameter (m)

32 Different air flow rates (face velocities) It was thermal rebound, rather than re-entrainment or re-suspension Same thermal rebound at two different face velocities Diffusion and thermal speeds of the nanoparticles do not change with the external flow speed d G =4mm H=2.5cm LPM 50 LPM d G =4mm H=12.7cm Source of charts: Golshahi (2007) MSc Thesis d G =6mm H=2.5cm d G =6mm H=12.7cm X-axis: Particle diameter, micrometers Y-axis: Filtration efficiency, %.

33 Analysis following Wang & Kasper model Mechanical constants for NaCl particles and glass surface: K p,s =(1-ν 2 )/(πy) NaCl: ν=0.252, Y=39.98 GPa; Glass ν=0.24, Y=80 Gpa Thermal rebound could occur at near 100 nm on a single collector but multilayer of granules increased the collision efficiency Evidenced in our experiments with the thinnest bed Measured total filtration efficiency and model fit well if σ ps = J/m 2 for the salt-nanoparticle-glass-bead interface adhesion efficiency,% Adhesion efficiency vs. d p s= particle diameter(nm) 100 Filtration efficiency vs. d p Calculated and measured total filter efficiency (d G =2 mm, H=2.5 cm, Q=23 lpm) Source: Yan C. (2010) MSc Thesis in preparation

34 Comparison between experiments and model Overall agreed well between model and experiments for conditions tested More obvious thermal rebound by using harder materials than other researchers Closer to elastic impact between NaCl particles and glass surface Total filtration efficiency, % mm 7.6cm 23lpm mm 7.6cm 23lpm mm 2.5cm 23lpm Particle diameter (nm)

35 Challenges to Existing Model Only one widely cited for fabric/fibrous filters The assumptions seem to be questionable JKR adhesion energy model was for two smooth surfaces; Validated by experiments for large particles (millimeters) Thermal speed of particles follow Maxwell-Boltzmann distribution, which is for molecules. Adhesion energy and mechanical constants are not available for many materials, not to mention nanomaterials, new or existing. The specific surface energy, in general, being J/m 2 Factors missed Collector surface roughness Particle-particle interaction Relative humidity Still rely on experiments to determine for each system. Adhesion Adhesion Adhesion

36 For More Information Tan, Zhongchao ( 谭中超 ) Phone: (519) ext tanz@uwaterloo.ca Web: Natural Sciences and Engineering Research Council (Canada )