National Aeronautics andSpace Administration
IS02
George C. Marshall Space Flight Center
Huntsville, Alabama 35812

NASA/TM—2016–218224

Submicron and Nanoparticulate Matter
Removal by HEPA-Rated Media Filters
and Packed Beds of Granular Materials
J.L. Perry
Marshall Space Flight Center, Huntsville, Alabama
J.H. Agui
Glenn Research Center, Cleveland, Ohio
R. Vijayakumar
Aerfil LLC, Liverpool, New YorkMay 2016

The NASA STI Program…in Profile
Since its founding, NASA has been dedicated to theadvancement of aeronautics and space science. The
NASA Scientific and Technical Information (STI)
Program Office plays a key part in helping NASA
maintain this important role.
The NASA STI Program Office is operated by
Langley Research Center, the lead center for
NASA’s scientific and technical information. The
NASA STI Program Office provides access to
the NASA STI Database, the largest collection of
aeronautical and space science STI in the world.
The Program Office is also NASA’s institutional
mechanism for disseminating the results of its
research and development activities. These results
are published by NASA in the NASA STI Report
Series, which includes the following report types:• TECHNICAL PUBLICATION. Reports ofcompleted research or a major significant
phase of research that present the results ofNASA programs and include extensive dataor theoretical analysis. Includes compilationsof significant scientific and technical dataand information deemed to be of continuingreference value. NASA’s counterpart of peerreviewed formal professional papers but has lessstringent limitations on manuscript length andextent of graphic presentations.
• TECHNICAL MEMORANDUM. Scientificand technical findings that are preliminary or of
specialized interest, e.g., quick release reports,
working papers, and bibliographies that containminimal annotation. Does not contain extensive
analysis.
• CONTRACTOR REPORT. Scientific andtechnical findings by NASA-sponsoredcontractors and grantees.

• CONFERENCE PUBLICATION. Collected
papers from scientific and technical conferences,
symposia, seminars, or other meetings sponsoredor cosponsored by NASA.
• SPECIAL PUBLICATION. Scientific, technical,
or historical information from NASA programs,
projects, and mission, often concerned withsubjects having substantial public interest.
• TECHNICAL TRANSLATION.
English-language translations of foreignscientific and technical material pertinent toNASA’s mission.
Specialized services that complement the STI
Program Office’s diverse offerings include creating
custom thesauri, building customized databases,
organizing and publishing research results…evenproviding videos.
For more information about the NASA STI Program
Office, see the following:• Access the NASA STI program home page at<http://www.sti.nasa.gov>• E-mail your question via the Internet to<help@sti.nasa.gov>• Phone the NASA STI Help Desk at
757 –864–9658• Write to:NASA STI Information Desk
Mail Stop 148
NASA Langley Research Center
Hampton, VA 23681–2199, USANASA/TM—2016–218224

Submicron and Nanoparticulate Matter
Removal by HEPA-Rated Media Filters
and Packed Beds of Granular Materials
J.L. Perry
Marshall Space Flight Center, Huntsville, Alabama
J.H. Agui
Glenn Research Center, Cleveland, Ohio
R. Vijayakumar
Aerfil LLC, Liverpool, New YorkNational Aeronautics andSpace Administration
Marshall Space Flight Center • Huntsville, Alabama 35812

May 2016i

Available from:NASA STI Information Desk
Mail Stop 148
NASA Langley Research Center
Hampton, VA 23681–2199, USA
757–864–9658

This report is also available in electronic form at<http://www.sti.nasa.gov>

ii

TABLE OF CONTENTS
1. BACKGROUND .................................................................................................................

1

2. INTRODUCTION TO HIGH-EFFICIENCY PARTICULATE FILTRATION ...............

2

2.1 HEPA Filtration Media Overview .................................................................................
2.2 Capturing Mechanisms in HEPA-Rated Media .............................................................
2.3 Filter Performance .........................................................................................................
2.4 HEPA Filter Classification and Standards .....................................................................

2
2
5
9

3. PARTICULATE REMOVAL BY PACKED BEDS ............................................................

10

4. CONCLUSION ...................................................................................................................

13

REFERENCES ........................................................................................................................

14

iii

LIST OF FIGURES
1.

Particle-capturing mechanisms on (a) a single fiber and (b) fiber network ....................

3

2.

A SEM micrograph of ISS BFE media ........................................................................

4

3.

Filter efficiency as a function of particle diameter ........................................................

7

4.

Filter efficiency dependence on velocity; lower velocity increases efficiency .................

8

5.

Influence of flow velocity on filter pressure drop ..........................................................

9iv

LIST OF ACRONYMS

BFE

bacteria filter element

EPA

efficient particulate air

ePTFE

expanded polytetrafluoroethylene

HEPA

high-efficiency particulate air

HVAC

heating, ventilating, and air conditioning

ISO

International Organization for Standardization

ISS

International Space Station

MPPS

most penetrating particle size

SEM

scanning electron microscopy

ULPA

ultralow particulate airv

NOMENCLATUREasuperficial surface of a granule per unit volume of bed

Cc

Cunningham slip correction factor

DSE

particle coefficient of diffusion/particle diffusion constant in air

df

diameter of the fiber

dp

diameter of particle/packed bed granule

dpG / ηdimensionless group—granule Reynolds numberED

diffusion component of overall single fiber efficiency

EDR

combined diffusion and interception component of overall single fiber efficiency

EGgravitation capturing efficiency

EI

inertial impaction component of overall single fiber efficiency

ERinterception component of overall single fiber efficiency

EToverall single fiber efficiency

Ggravity/mass velocity in weight per unit time per unit cross section of bed

Htmass transfer zone length

Kn

Knudsen numberkBoltzmann constantP

particle percent penetration

Pe

Peclet numberR

ratio of particle to fiber diameter

Re

Reynolds number

St

Stokes numbervi

NOMENCLATURE (Continued)
Ttemperature (K)Uflow velocityhgas phase viscosity /dynamic viscosity of carrier gas (kg/m-s)lgas molecular mean free path (m)h / rDSE

dimensionless group—particle Schmidt numberr

gas phase density

rpdensity of particletrelaxation time of particle

vii

viii

TECHNICAL MEMORANDUMSUBMICRON AND NANOPARTICULATE MATTER REMOVAL BY HEPA-RATEDMEDIA FILTERS AND PACKED BEDS OF GRANULAR MATERIALS
1. BACKGROUND
Contaminants generated aboard crewed spacecraft consist of both gaseous chemical contaminants and particulate matter. Sources responsible for these contaminants are varied and can
include materials used to construct the spacecraft as well as crewmembers’ activities. In a crewed
spacecraft cabin, suspended particulate matter in the cabin atmosphere can present hazards to crew
health and vehicle system performance. The nature of the particulate matter pollutants such as
their physical, chemical, and biological properties; the concentration level; and the size spectrum
are factors that influence the nature of the hazard which can range from a nuisance to an acutehealth or equipment impact.
Particulate matter of various types have been isolated aboard crewed spacecraft.1–3 The size
range isolated typically falls above the definition of course particles (>2.5 µm); however, fine particles (<2.5 µm) have also been isolated in smaller quantities. It is also highly likely that equipment
and crew activities may also produce submicron particulate matter (<1 µm) and ultrafine particulate matter (<0.1 µm). Material in the latter size range is also classified as nanoparticulate matter,
which is formally defined as the size range between 1 and 100 nm (0.1 µm).
Spacecraft cabin atmosphere purification techniques include filtration and physical adsorption processes to remove particulate matter, carbon dioxide, and chemical contaminants fromthe cabin environment. The efficacy for these separation processes for removing submicron andnanoparticulate matter from the cabin environment are considered by the following discussion.

12. INTRODUCTION TO HIGH-EFFICIENCY PARTICULATE FILTRATIONIn the general community, air filtration is intuitively assumed to be similar to the commonly
known process of sieving or straining. Here, one intuitively relates to the capture of particles larger
than the opening or pore in a filter. Straining mostly occurs at the surface of the filter material.
This is more common in liquid filtration where the cake buildup aids in the filtration. Increased
buildup leads to higher capture efficiency. At one extreme, a fully blocked or loaded filter is 100%efficient.
High-efficiency air filtration is unlike any other straining-dependent filtration process.
The filter media is usually made up of many layers of submicron diameter fibers. In this structure,
particles much smaller than the presumed opening in the filter material are readily captured.
2.1 HEPA Filtration Media Overview
High-efficiency filter media is usually made with microfiber glass on wet laid web-forming
machines similar to those used for manufacturing paper. It has a homogenous white appearance
much like some high rag content ‘bond’ paper. Hence, it is also common to call filter media filter‘paper.’ Fibrous structures needed for the filter media can also be produced by expanded membranes, such as expanded polytetrafluoroethylene (ePTFE). The expansion results in a fibrous
structure with uniform submicron dendrites. ePTFE media are more common in higher efficiency
ultralow particulate air (ULPA) (>99.99% efficiency) media and where resistance to harsh chemicals such as hydrogen fluoride are required.
High-efficiency particulate air (HEPA) filter media is usually made with borosilicate micro­
fibers with diameters from 2 to 500 nm. The fiber lengths and the proportions of the fibers used areusually proprietary. A typical HEPA media is usually under 0.508 mm (0.020 in) thick. Hence, forthe fiber diameters used in its manufacture, the media is comprised of several hundred layersof fibers. Thus, even if there are open spaces in a layer, the layers behind it prevent the furthertransport of particles.
2.2 Capturing Mechanisms in HEPA-Rated MediaThe process of capturing particles on the fibers of the filter media is dependent on the properties and conditions of the incoming flow, and of the flow through the internal microstructure of
the filter, and the particle’s transport (aerosol) properties. In addition, the tortious pathways created by the interconnected interstices between the fibers significantly enhances the residence timeof particles inside the media. These characteristics of the filtration process lead to three basic
flow-related capturing mechanisms known as the inertial impaction, interception, and diffusion
mechanisms. Two additional mechanisms, straining and electrostatic attraction between the particle
and fiber, which in some cases affects the performance of lower performing and HVAC filters, only
plays a secondary or minor role in HEPA filtration. Electrostatic attraction is not discussed here.
2The illustration in figure 1 provides a visual depiction of the particle dynamics and particle/
fiber interactions involved in the different capture mechanisms. In this scenario, the particles are
assumed to generally follow the flow streamlines as they approach the filter fiber. For small fiber
diameters and relatively low velocities, i.e., at low Reynolds numbers (Re < 1), which are typical in
filter flows, the flow field around the fiber can be considered laminar. However, the flow curvature
produced as the streamlines wrap around the fiber imparts a significant amount of flow acceleration that affects the motion of the particles relative to the streamline.

Inertial Impaction
Interception
Flow Streamlines!

Diffusion(a)Diffusion

Interception

Straining

Inertial Impaction(b)

Figure 1. Particle-capturing mechanisms on (a) a single fiber and (b) fiber network.
(© R. Vijayakumar. Used with permission.)

3

The fate of different diameter particles along the isolated streamlines are illustrated in figure 1. Inertial impaction is the dominant capturing mechanism for the large diameter particle, as
shown by the particle following the top streamline in figure 1(a). The increased inertia causes theparticle to deviate from the streamline path when it encounters sufficient flow curvature. If sufficiently massive, the particle directly impacts onto the fiber surface and adheres to it. Adhesion
forces (Van der Waals, electrostatics, and capillary) between small particles (<10 µm) and the fiber
surfaces ensures collected particles stay on the fibers. The interception mechanism is illustrated in
the middle streamline as the particle comes within one radius of the surface of the fiber. In thiscase, the particle has sufficient contact time with the fiber surface to also get trapped. Lastly, verysmall diameter particles, as shown in the bottom streamline, are more susceptible to Brownian
motion and are intermittently and incrementally knocked off the streamline path by molecular collisions. Due to the randomness of the diffusional collisions, the particle has a chance to approach
the fiber surface and also get trapped.
Figure 1(b) shows the mechanisms involved in the straining capturing mode, found more
typically in surface filtration. It is recognized that this mechanism only plays a role for very large
particles that are typically collected on the front face of the filter media. For reference, a scanningelectron microscopy (SEM) micrograph of a filter media coupon cut from an International Space
Station (ISS) bacteria filter element (BFE) returned to the ground for analysis after in-flight service
is shown in figure 2. This micrograph reveals the intricate fibrous structure of the HEPA media.
The micrograph shows the particles captured on the surface of the media during its approximately
2 years of operation aboard the ISS. Any micron and submicron particles that were captured areembedded in the interior of the media and not easily visible in the micrograph.

Figure 2. A SEM micrograph of ISS BFE media.

42.3 Filter Performance
2.3.1 Filter Efficiency
The efficiency of the filter media takes into account the various competing particle-capturing
mechanisms discussed above. One approach to establishing a basic foundation for filter efficiency isthrough the Single Fiber Theory. This considers an idealized analysis which assumes uniform fiber
size and flow conditions around the fibers, and therefore its utility is in showing the dependenceof different particle and flow properties on filter efficiency. According to the theory, the overall single
fiber efficiency (ET) is found from the product of the compliment of all the individual single fiber
efficiencies due to inertial impaction (EI), diffusion (ED), interception (ER), diffusion and interception combined (EDR), and gravity (EG) as shown in equation (1):

(

)(

)(

)(

)(

)

ET = 1− 1− EI 1− ED 1− ER 1− EDR 1− EG .

(1)

Equation (1) can be approximated byET ≈ EI + ED + ER + EDR + EG ,(2)assuming that one capturing mechanism dominates over the others in order to minimize competing
effects of more than one mechanism on the same particle. This is generally the case, as a particle
of a specified size will mostly be dominated by just one of the capturing mechanisms. The approximation in equation (2) simplifies the analysis of overall single fiber efficiency as an additive effectof the individual efficiencies of the different capturing modes.
The Stokes and Peclet numbers are the two key physics-based flow parameters, along withseveral empirically derived constants and coefficients that influence filtration efficiency and performance. Usually only one of these flow parameters tends to have a dominant effect on a specificcapturing mechanism. The influence of Stokes drag on a particle near a fiber is characterized by
the particle’s Stokes number (St):
St =

Uτ
,
df

(3)

where U is the flow velocity, t is the so called relaxation time of the particle, and df is the diameterof the fiber. The relaxation time is found from the classical analysis of laminar flow over a sphereand expressed asτ=ρp d 2pCc
18η

,

(4)

where rp and dp are the density and diameter of the particle, respectively, Cc is the Cunningham
slip correction factor (discussed later in relation to packed beds), and η is the viscosity of the air
5medium. The slip factor accounts for gaseous slip at the surface of the particle, which reduces thedrag, essentially causing the particle to behave as if it were a larger, more inertial particle. The slip
effect becomes a factor with very small particles and at low operating pressures. Under these pressure conditions, enhanced filter efficiency has been reported due to the slip effect.4 The relaxation
time relates to the time it takes for the particle to adjust to new flow conditions, similar to the time
it takes a particle to reach settling velocity in a still fluid medium. When St >> 1, the particle isunaffected by flow curvature and continues in a straight line, as is experienced in inertial impaction. At a very low Stokes number (St <<  1), particles follow the flow perfectly, although diffusional
forces start to come into effect with very small particles. The second parameter is the Peclet number(Pe), which accounts for the relative importance of diffusional effects:
Pe =

Udf
DSE

,

(5)

where DSE is the particle coefficient of diffusion (also discussed in relation to packed beds). Diffusion effects dominate when the Peclet number is very small.
The trend for each of the component single fiber efficiencies can be expressed in terms of
the flow parameters given in equations (3) and (5). Impaction single fiber efficiency is directly proportional to the Stokes number, i.e., EI ~ St. This relation shows that the efficiency increases withparticle diameter and is dominant at the largest particle diameters. In the diffusional regime, theeffect is on small particles where Brownian motion, imparted by the thermal energy of the molecules on the particle, causes a net velocity perturbation on the particle. The single fiber efficiency
due to diffusion is inversely proportional to the Peclet number to the –2/3 power, i.e., ED ~ Pe–2/3.5
Therefore, the efficiency is significantly enhanced at very small particle diameters below the mostpenetrating particle size (MPPS), typically 120 nm for HEPA and ULPA media filters. Current datafrom the University of Minnesota indicate that this theory of filtration holds true for particulatematter down to 5 nm in diameter.6–8 Single fiber efficiency by particle interception is dependent on
the ratio of particle to fiber diameter, R = dp  /df, as ER ~ R2. In the range of the MPPS, diffusion
and interception effects mutually interact and produce a combined fiber efficiency as EDR ~ Pe–1/2.
Lastly, gravitation capturing efficiency (EG) is dependent on the ratio of the particle settling velocity to the mean ventilation or cabin velocity, and typically varies as the square of the particle diameter. This last mechanism is not expected to play a role in reduced or microgravity environments.
In addition, the overall filter efficiency has to also include factors for the filter thickness,fiber diameter, and solidity factor of the filter medium. These factors are built into some of thecoefficients that define the individual single fiber efficiencies above. For a more indepth analysisof the application of single fiber efficiency to filter efficiency, the reader is referred to reference 5.
The overall (total) filter efficiency of a HEPA filter has been empirically obtained as a function of particle diameter and represented in figure 3. Superimposed are plots showing the trends
found from single fiber theory, and showing the relative importance of the different particle capture
mechanisms in contributing to the shape of the efficiency curve. The diffusion regime is dominant
in the range of the smallest particle diameters, submicron and below. A very high proportion of theparticles in this size range, up to 100% at the very smallest particle sizes, get captured by the fibers
6Inertial Impaction
and Interception
RegimeDiffusion andInterception
RegimeDiffusion
Regime100

Capture Efficiency (%)

99.99

Diffusion
Regime

99.98

Total

Impaction

99.97

Interception

99.96

Settling

99.95
99.94

0.01

0.1

1

10

Diameter (μm)

Figure 3. Filter efficiency as a function of particle diameter.
throughout the media. In the upper particle size range, the supermicron range, inertial impaction,and interception are the key capture mechanisms. This regime is also characterized by a very highpercentage of particle capture, particularly at the largest particle sizes. In the intersecting regime
between these two particle size ranges, the effects of the particle diffusion are shown to taper off
while the effects of inertial impaction and interception start to dominate. This is made clear by
the plotted efficiency curves of the individual mechanisms showing the trends derived from single
fiber efficiency theory. The net effect is a small reduction in filter efficiency that is characterized bya well-defined ‘valley’ and minimum point. The concept of the MPPS is applicable in this range
and is typically taken to be the near minimum efficiency point. The MPPS for typical HEPA filters
varies from 200 to 300 nm, depending on flow rate. The key implication to emphasize is that HEPA
filters are nearly 100% efficient at capturing the spectrum of particles down to the very smallest
airborne particles.
Small changes in flow velocities through the media result in large changes in particle penetration through the filter. Particle penetration (P) is defined simply as the number of particles
crossing the filter divided by the number of particles incident on the filter and is related to overall
efficiency (ET) as P = 1 – ET.9 As a rule of thumb, reducing the velocity by half not only reduces
the pressure drop by half but also decreases particle penetration through the filter media by nearly
an order of magnitude. This is illustrated in figure 4 where decreasing the velocity by three-fourths
reduces the particle penetration over three orders of magnitude with a corresponding efficiency
increase from 99.998% to 99.99998%. In other words, a simple HEPA-rated filter will perform as an
ULPA-rated or better filter by simply lowering the flow velocity through the media.
70.01
Velocity
0.001
Penetration (%)

99.998+%5.3 cm/s
4 cm/s0.0001

2.5 cm/s

0.00001

1.3 cm/s

0.15 μm

99.99998+%

0.00001
0.01

0.1
0.15 μm

1

Particle Size (μm)

Figure 4. Filter efficiency dependence on velocity;
lower velocity increases efficiency.
(© R. Vijayakumar. Used with permission.)
2.3.2 Hydrodynamic Performance
The fibrous network structure, unlike that of granular filters, is a fairly open, highly porous
structure even when densely packed as in high-efficiency media. This characteristic structure translates into relatively low pressure drops across the media. Typical pressure drops across clean HEPA
media is usually on the order of 249 Pa (a column inch of water) or less. Typical HEPA filters
operate at ~2.55 m3/0.0929 m2 (90 ft3/min/ft2) or ~27.43 m/min (90 ft/min) velocity. This velocity
corresponds to the air flow into the filter cartridge. However, the velocity of the air flow directly
impinging on the surface of the media in the filter cartridge—in a well-designed HEPA filter—isusually under 1.5 m/min (5 ft/min) and often half that. At these low velocities, the resistance to air
flow is in the D’Arcy regime for flow through porous beds and varies linearly with air flow rate. The
nominal media velocity is determined as the total flow over the face of the filter divided by the areaof media in the filter. The performance of a filter depends on the media velocity.
The relationship between pressure drop and media velocity of a sample filter media is shownin figure 5. Two very distinct characteristic flow regimes are apparent in the performance curve.
First, in the low velocity regime, the resistance to air flow varies linearly with velocity. This performance trend is advantageous since it results in smaller growth in resistance with velocity. A second
advantage is that lower velocities afford greater residence times for particles to be collected due toBrownian motion while at the same time reducing the collection due to inertia. Thus, lower velocities increase collection efficiencies of small particles. The opposite effects result when higher velocities are used. As seen in figure 5, the higher flow regime is characterized by a nonlinear and muchlarger rate of pressure drop increase with media velocity, while particle capturing is dominated lessby diffusional capturing and to a greater extent by inertial impaction of larger particles.
8

Pressure Drop

HVAC

HEPA/ULPA

Linear Pressure Drop
(Darcy Law) Regime

Inertial Regime

Velocity

Figure 5. Influence of flow velocity on filter pressure drop.
(© R. Vijayakumar. Used with permission.)
2.4 HEPA Filter Classification and Standards
HEPA filters are usually tested twice, and often a third time. First they are tested to determine efficiency followed by testing for leaks or integrity to ensure that there are no local areasof the filter that exceeds acceptable limits even if the overall efficiency is as claimed. In addition,
in regulated industries, the filters are tested for integrity in their installed configuration.
Filter testing is governed by United States or International Organization for Standardization (ISO) standards. In all standards, the filter efficiency is determined at the rated flow and at ornear the MPPS. Since particles in the MPPS diameter range are the most penetrating or ‘leaky’ particles, efficiencies equal to or greater than the rated efficiency can be expected for all particle sizes.
Leak testing is usually conducted with particles larger than MPPS simply to avoid false results.
That is, at larger sizes, the filter is more efficient than the rated value and hence any leak is more
readily detected. Since both these tests are nondestructive, 100% of HEPA filters are tested andindividually certified. In fact, most test standards require this individual certification to be classified
as a HEPA filter.
In most standards, the filters are classified according to their efficiency. Whereas some indicate the performance value in the classification in some fashion (ISO 29463), others use a letter designation (U.S. Institute of Environmental Sciences and Technology). Although the standards startwith the same theoretical basis of testing at MPPS, no two are alike; they specify different aerosolsand detectors. For example, in the United States, a photometer is still the main particle detector usedfor HEPA filter testing while particle counters are the common devices used in Europe. Further,both the ISO and European standards have introduced a new filter classification—efficient particulate air (EPA) filters—with efficiencies of 99.95% and lower.

9

3. PARTICULATE REMOVAL BY PACKED BEDS
Packed beds of granular or pelletized porous adsorbent media are typically used for theremoval of gaseous and molecular contaminants. Beds of substantial depths may also provide anassist to media filters for removing particulate matter <0.1 µm. However, using absorbent beds forparticle removal usually reduces the effectiveness and life of the bed for gaseous and molecular contaminants due to loading of the pores in the adsorbent media with particulate matter. The media ina packed bed can function similarly to a depth filter and remove fine and ultrafine particulate matter via impaction, interception, and diffusion mechanisms. Studies on particulate matter removal by
packed beds have found that, similarly to HEPA media filters, the 0.3-µm size is the MPPS.10 Thesestudies, however, found that packed bed efficiencies for removing 0.3-µm-size particles, the HEPA
media filtration MPPS, are an order of magnitude lower than for particulate matter <0.1 µm and>1 µm in size.11 Therefore, while deep packed beds have been shown experimentally to remove ultrafine and course particulate matter, they are not suitable by themselves for providing HEPA-rated
performance for the most penetrating particle size of 0.3 µm. The efficiencies afforded by shallow
packed beds are negligible and, in practice, such beds are unsuitable for providing particulate filtration and must be combined with media filtration to effectively remove particulates and volatilecomponents from a gas stream.
Experimental studies have found that granular adsorbent media such as activated carbon
removes 0.3-µm-diameter particulate matter and gaseous contaminants by a similar mechanism
over a wide range of diffusion coefficient magnitude.12 The removal occurs primarily in the interstitial space between packed bed granules and, for porous granules, to a very small degree in the granules’ macropores. Besides the particulate matter size, the primary factors influencing the removal
are the carrier gas velocity and the diffusion constant of the aerosol. An effective bed penetration
depth represented as the height of a bed mass transfer zone can be estimated by equation (6) which
is a modification of the Mecklenburg approach used for estimating adsorbent bed breakthrough:12t⎛ d pG ⎞ 0.41 ⎛ η
⎞0.67
,
⎜
⎟
⎜
⎟
a ⎝
η ⎠
⎝ ρ DSE ⎠

( )

H = 1

(6)

where Ht is the mass transfer zone length, a is the superficial surface of a granule per unit volume
of bed, dp is the diameter of a packed bed granule, G is the mass velocity in weight per unit time
per unit cross section of bed, η is the gas phase viscosity, ρ is the gas phase density, and DSE is theparticle diffusion constant in air. The dimensionless group, dpG /η, is the granule Reynolds number
while the dimensionless group, η / ρDSE, is the particle Schmidt number. Units of measure used inequation (6) should be consistent for determining the Reynolds and Schmidt numbers and SI unitsof kilograms, meters, and seconds are recommended.

10

The particle diffusion constant, DSE , is defined by equation (7) which is the Stokes-Einsteinequation modified for the slip regime:13
DSE =

kTCc

(7)

3πη dp ,where k is the Boltzmann constant (1.31 × 10–23 J/K), T is temperature in Kelvin, Cc is the Cunningham slip correction factor, η is the dynamic viscosity of the carrier gas in kg/m-s, and dp is theparticulate diameter in meters. The Knudsen number (Kn) relates the gas molecular mean free path(λ) in meters to the particle size (dp) in meters as shown by equation (8):14
K n = 2λ d .

(8)pContinuum flow exists when Kn is <<1 while molecular free flow exists when Kn is >>1, while
the slip regime exists for 0.4 < Kn < 20. The Knudsen number is a parameter used to calculate theCunningham slip correlation factor according to equation (9) for solid particles:15
Cc = 1+ K n ⎡1.142 + 0.558e⎣−0.999/K n ⎤

⎦

.

(9)

Research has observed 75% to 90% removal of 0.004-µm particles by a 1.9-cm-deep activated carbon bed containing 6 × 16 mesh (<3.327 mm and >0.991 mm) carbon granules.15 Deep
packed beds may be expected to approach HEPA-like removal efficiency for ultrafine and nanoparticulate contaminants. This approach to high removal efficiency was observed during a test ofa  38-cm-deep activated carbon bed challenged with 200.5 mg/m3 of 0.6-µm particles. The beginning
efficiency of 88.4% rose to 97% after 135 minutes.13 This reported performance equates to ~2.3%
to ~2.5% per centimeter of bed depth if a linear dependence on depth is assumed. In reality, thisrelationship may be nonlinear. A 2-cm-deep bed, therefore, would at best provide ~4.6% to ~5.1%
removal efficiency for 0.6-µm particles based on these experimental results. To provide reasonable effectiveness, the bed would need to be >40 cm deep, or more specifically, several times deeper
than a comparably performing HEPA media filter to even approach the performance provided bya  HEPA-rated media filter.
Although most adsorbent beds used aboard crewed spacecraft for atmosphere revitalization purposes typically are >30 cm deep, the adsorbent beds used in payloads are typically <5 cm
deep, and high flow rate applications in a cabin ventilation system are typically <10 cm deep. Basedon the results reported in the literature, shallow activated carbon beds used in payload and spacecraft cabin ventilation system applications may be capable of providing some degree of removal
efficiency for ultrafine particles (≤100 nm). However, the expected performance for removing larger
submicron (fine) particles is poor, particularly in the MPPS range. Therefore, granular adsorbent
beds alone are not suitable for providing highly effective removal performance for particles in thefine and ultrafine size ranges. For applications where both particulate removal and gaseous contaminant removal must be provided, both high-efficiency media filtration and adsorbent bed
components are necessary.
11

By this evidence, filtration media effectiveness for removing ultrafine and nanoparticulatematerials can be enhanced to a small degree when used in combination with a packed bed of granular media located downstream of the filtration media. The opposite is also true in that the fine particulate removal effectiveness of a shallow adsorbent bed can be significantly enhanced by including
a media filtration component upstream of the adsorbent bed.
Many gas purification components associated with containing experimental payload andsystem process vents typically include both adsorbent and media filtration components; therefore,in combination, they can provide an effective particulate removal function when used in an appropriate configuration consisting of a media filtration component upstream of a packed adsorbent bed
component.

124. CONCLUSION
The phenomena associated with particulate matter removal by HEPA media filters andpacked beds of granular material have been reviewed relative to their efficacy for removing fine(<2.5 µm) and ultrafine (<0.01 µm) sized particulate matter. The ultrafine particulate range constitutes the range defined as nanoparticulates. Generation sources may be diverse and the means toremove nanoparticulates via conventional methods is a valuable capability aboard crewed spacecraft. Both HEPA media filters and packed beds of granular material, such as activated carbon,
which are both commonly employed for cabin atmosphere purification purposes, are found to haveefficacy for removing nanoparticulate contaminants from the cabin atmosphere. When used alone,
HEPA-rated media provides superior performance for removing virtually 100% of particulates.
However, using these methods in an appropriately combined configuration for removing particulates and gaseous contaminants, consisting of a media filtration component upstream of a packed
adsorbent bed component, provides the most effective performance for a broad range of particle
sizes including nanoparticulates.

13

REFERENCES
1. Perry, J.L.; and Coston, J.E.: “Analysis of Particulate and Fiber Debris Samples Returned
from the International Space Station,” ICES-2014-166, 44th International Conference
on Environmental Systems, Tucson, AZ, July 13–17, 2014.
2. Liu, B.Y.H.; Rubow, K.L.; McMurry, P.H.; et al.: “Airborne Particulate Matter and SpacecraftInternal Environments,” SAE 911476, 21st International Conference on Environmental
Systems, San Francisco, CA, July 15–18, 1991.
3. Meyer, M.: “ISS Ambient Air Quality: Updated Inventory of Known Aerosol Sources,”
ICES-2014-199, 44th International Conference on Environmental Systems, Tucson, AZ,
July 13–17, 2014.
4. Agui, J.H.; Mackey, J.; Vijayakumar, R.; et al.: “Investigation of the Filtration of Lunar Dust
Simulants at Low Pressures,” AIAA 2010-6074, AIAA 40th International
Conference on Environmental Systems, Barcelona, Spain, July 11–15, 2010.
5. Hinds, W.C.: Aerosol Technology, 2nd Ed., Chapter 9, Wiley-Interscience, 1999.
6. Kim, S.C.; Harrington, M.; and Pui, D.Y.H.: “Experimental Study of Nanoparticle Penetration through Commercial Filter Media,” J. Nanopart. Res., Vol. 9, pp. 117–125, 2007.
7. Wang, J.; Chen, D.R.; and Pui, D.Y.H.: “Modeling of Filtration Efficiency of Nanoparticles
in Standard Filter Media,” J. Nanopart. Res., Vol. 9, pp. 109–115, 2007.
8. Japuntich, D.; Franklin, L.; Pui, D.; et al.: “A Comparison of two nano-sized particle air
filtration tests in the diameter range of 10 to 400 nanometers,” J. Nanopart. Res., Vol. 9,
pp. 93–107, 2007.
9. Payet, S.; Boulaud, D.; Madelaine, G.; and Renoux, A.: “Penetration and Pressure Dropof a HEPA Filter During Loading with Submicron Liquid Particles,” J. Aerosol Sci., Vol. 23,
No. 7, p. 724, 1992.
10. Thomas, J.W.; and Yoder, R.E.: “Aerosol Penetration Through a Lead-Shot Column,”
Am. Med. Assoc. Arch. Ind. Hlth., Vol. 13, p. 550, 1956.
11. Kovach, J.L.: “Aerosol and Particulate Removal by Activated Carbon,” in Carbon Adsorption
Handbook, P. N. Cheremisinoff and F. Ellerbush (eds.), pp. 359–369, 1978.
12. Katz, S.H.; and MacRae, D.: “Filtration of Aerosols by Granular Charcoal,” J. Phys. Coll.
Chem,. Vol. 51, pp. 695–597, 1948.
14

13. Sorensen, C.M.; and Wang, G.M.: “Note on the Correction for Diffusion and Drag in the Slip
Regime,” Aerosol Sci. Technol., Vol. 33, pp. 353–356, 2000.
14. Morawska, L.: “Environmental Aerosol Physics,” Queensland University of Technology,
Brisbane, Australia, p. 11, 2012.
15. Browning, Jr., W.E.; and Ackley, R.D.: “Characterization of millimicron radioactive aerosols
and their removal from gases,” U.S. Atomic Energy Commission, TID-7641, 1962.

15

Form Approved
OMB No. 0704-0188

REPORT DOCUMENTATION PAGE

The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining
the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing
this burden, to Department of Defense, Washington Headquarters Services, Directorate for Information Operation and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302.
Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid
OMB control number.
PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS.

1. REPORT DATE (DD-MM-YYYY)

2. REPORT TYPE

01– 05 –2016

Technical Memorandum

4. TITLE AND SUBTITLE

3. DATES COVERED (From - To)
5a. CONTRACT NUMBER

Submicron and Nanoparticulate Matter Removal by HEPA-Rated
Media Filters and Packed Beds of Granular Materials

5b. GRANT NUMBER
5c. PROGRAM ELEMENT NUMBER

6. AUTHOR(S)5d. PROJECT NUMBER
5e. TASK NUMBER

J.L. Perry, J.H. Agui,* and R. Vijayakumar**5f. WORK UNIT NUMBER
7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)8. PERFORMING ORGANIZATIONREPORT NUMBER

George C. Marshall Space Flight Center
Huntsville, AL 35812

M–1414

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)10. SPONSORING/MONITOR’S ACRONYM(S)NASA

National Aeronautics and Space Administration
Washington, DC 20546–0001

11. SPONSORING/MONITORING REPORT NUMBER

NASA/TM—2016–218224

12. DISTRIBUTION/AVAILABILITY STATEMENT

Unclassified-Unlimited
Subject Category 54
Availability: NASA STI Information Desk (757–864–9658)
13. SUPPLEMENTARY NOTES

Prepared by the Space Systems Department, Engineering Directorate*NASA Glenn Research Center, **Aerfil LLC
14. ABSTRACT

Contaminants generated aboard crewed spacecraft by diverse sources consist of both gaseous chemical
contaminants and particulate matter. Both HEPA media filters and packed beds of granular material,
such as activated carbon, which are both commonly employed for cabin atmosphere purification purposes
have efficacy for removing nanoparticulate contaminants from the cabin atmosphere. The phenomena
associated with particulate matter removal by HEPA media filters and packed beds of granular material
are reviewed relative to their efficacy for removing fine (<2.5 µm) and ultrafine (<0.01 µm) sized particulate matter. Considerations are discussed for using these methods in an appropriate configuration to provide the most effective performance for a broad range of particle sizes including nanoparticulates.
15. SUBJECT TERMS

particulate matter, filtration, nanoparticulates, HEPA filter, packed bed, particle capturing mechanisms
16. SECURITY CLASSIFICATION OF:
a. REPORTU

b. ABSTRACT

c. THIS PAGE

U

U

17. LIMITATION OF ABSTRACT

18. NUMBER OF
PAGES

UU

28

19a. NAME OF RESPONSIBLE PERSONSTI Help Desk at email: help@sti.nasa.gov

19b. TELEPHONE NUMBER (Include area code)

STI Help Desk at: 757–864–9658
Standard Form 298 (Rev. 8-98)
Prescribed by ANSI Std. Z39-18

16

The NASA STI Program…in Profile
Since its founding, NASA has been dedicated to theadvancement of aeronautics and space science. The
NASA Scientific and Technical Information (STI)
Program Office plays a key part in helping NASA
maintain this important role.
The NASA STI Program Office is operated by
Langley Research Center, the lead center for
NASA’s scientific and technical information. The
NASA STI Program Office provides access to
the NASA STI Database, the largest collection of
aeronautical and space science STI in the world.
The Program Office is also NASA’s institutional
mechanism for disseminating the results of its
research and development activities. These results
are published by NASA in the NASA STI Report
Series, which includes the following report types:• TECHNICAL PUBLICATION. Reports ofcompleted research or a major significant
phase of research that present the results ofNASA programs and include extensive dataor theoretical analysis. Includes compilationsof significant scientific and technical dataand information deemed to be of continuingreference value. NASA’s counterpart of peerreviewed formal professional papers but has lessstringent limitations on manuscript length andextent of graphic presentations.
• TECHNICAL MEMORANDUM. Scientificand technical findings that are preliminary or of
specialized interest, e.g., quick release reports,
working papers, and bibliographies that containminimal annotation. Does not contain extensive
analysis.
• CONTRACTOR REPORT. Scientific andtechnical findings by NASA-sponsoredcontractors and grantees.

• CONFERENCE PUBLICATION. Collected
papers from scientific and technical conferences,
symposia, seminars, or other meetings sponsoredor cosponsored by NASA.
• SPECIAL PUBLICATION. Scientific, technical,
or historical information from NASA programs,
projects, and mission, often concerned withsubjects having substantial public interest.
• TECHNICAL TRANSLATION.
English-language translations of foreignscientific and technical material pertinent toNASA’s mission.
Specialized services that complement the STI
Program Office’s diverse offerings include creating
custom thesauri, building customized databases,
organizing and publishing research results…evenproviding videos.
For more information about the NASA STI Program
Office, see the following:• Access the NASA STI program home page at<http://www.sti.nasa.gov>• E-mail your question via the Internet to<help@sti.nasa.gov>• Phone the NASA STI Help Desk at
757 –864–9658• Write to:NASA STI Information Desk
Mail Stop 148
NASA Langley Research Center
Hampton, VA 23681–2199, USA

National Aeronautics andSpace Administration
IS02
George C. Marshall Space Flight Center
Huntsville, Alabama 35812

NASA/TM—2016–218224

Submicron and Nanoparticulate Matter
Removal by HEPA-Rated Media Filters
and Packed Beds of Granular Materials
J.L. Perry
Marshall Space Flight Center, Huntsville, Alabama
J.H. Agui
Glenn Research Center, Cleveland, Ohio
R. Vijayakumar
Aerfil LLC, Liverpool, New YorkMay 2016