xvEPA
United States
Environmental Protection
Agency
Environmental Sciences Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S2-81-071 June 1981
Project Summary
Filter Media for Collecting
Diesel Particulate Matter
Frank Black and Lisa Doberstem
Certification of paniculate emissions
from diesel motor vehicles involves
filtration of measured aliquots of the
total air diluted exhaust. Seven com-
mercially available filter media were
examined for this purpose. The media
included a variety of PTFE membrane
filters, glass fiber filters, and PTFE
coated glass fiber filters. Relative flow
resistance (pressure drop), collection
efficiency, and gas phase adsorption
were examined. Filter structural dif-
ferences, which influence particulate
collection mechanisms, sample flow
rates and pressure drops, were studied
microscopically. Two media, a mem-
brane and a fiber filter, were also
examined microscopically with varying
levels of particulate load to determine
the role of collected particles in the
filtration of subsequent particles.
The results obtained indicate that
under the defined test conditions the
membrane filters yield low gram per
mile emissions rates due to difficulties
with collection of Federal Test Proce-
dure phase 1 emissions, and the
Gelman A-E glass fiber filter high rates
due to adsorption of gas phase emis-
sions. The mechanics of particle col-
lection are similar for both membrane
and fiber filters as applied. Diffusion
deposition is important with fiber
filters for about 5 percent of the col-
lected particulate matter, the remain-
ing 95 percent collected primarily by
direct interception. With the membrane
filters direct interception is the domi-
nate process.
This Project Summary was devel-
oped by EPA's Environmental Sciences
Research Laboratory, Research Tri-
angle Park, NC, to announce key
findings of the research project that is
fully documented in a separate report
of the same title (see Project Report
ordering information at back}.
Introduction
On March 5, 1980, the U.S. Environ-
mental Protection Agency published
rules and regulations concerning the
control of particulate emissions from
light duty passenger cars and trucks.
Included were procedures for sampling
and analyzing emissions to determine
the gram per mile (g/mi) emission rate
of total particulate matter from this
category of motor vehicles. The recom-
mended sampling apparatus is illustrated
in Figure 1. Particulate emission rates
are determined by filtration of a mea-
sured aliquot of air-diluted exhaust.
Samples are collected during each of
three phases of the Federal Test Proce-
dure (FTP). Primary and back-up filters
are used to establish acceptable levels
of collection efficiency. If the back-up
filter weight gain during testing is less
than 5 percent of the totalweight gain of
the primary and back-up filters, only the
primary weight need be used in the
emission rate calculation, otherwise,
the total weight gain of both the primary
and back-up filters is used in the calcu-
lation.
During this investigation a variety of
commercially available filter media
were examined for the purpose of
collecting particulate matter fromdiesels
in the light-duty certification practice
Two primary filter types, membrane and
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fiber, are commonly used for this pur-
pose. The mechanisms of particle col-
lection are similar for both and have
been studied for many years. Three
major factors are important to the study
of aerosol filtration: the dispersed
particles, the dispersion medium (air for
ihe case of interest), and the porous fil-
tration medium. Important characteristics
of the particles include their size or size
distribution, shape, mass or density,
electrical charge, chemical composition,
and concentration. The gas flow is char-
acterized by velocity, density, tempera-
ture, viscosity, and humidity; the porous
filtration medium is characterized by
surface area, thickness, size of structural
units (e.g. thickness of fibers in fibrous
filters), porosity, and electrical charge
Two major considerations in using
filters for particulate collection are
filtration efficiency and flow resistance
or pressure drop. Both depend on the
particle, gas flow, and filter characteris-
tics previously mentioned.
Seven filter media, two PTFE mem-
brane filters, two PTFE coated glass
fiber filters, and three standard glass
fiber filters were tested for collecting
diesel exhaust particulate matter using
the procedures required for emissions
certification. Particle composition, size
distribution, concentration, and velocity
at the filter face were held constant as
were the dispersion medium composi-
tion, temperature and humidity, while
the filter medium composition, thickness
and porosity were varied. Pressure
drops and relative collection efficiencies
were determined for each of the media
Relative adsorption characteristics were
also examined. Structural differences
and collection mechanics were studied
microscopically. With all.of the examined
media, greater than 95 percent of the
collected particulate matter was located
on the primary filter of a standard 2 filter
set (see Figure 1). However, with the
membrane filters the g/mi emission
rates were somewhat low because of
difficulties associated with collection of
emissions during test phase 1 of the FTP
and with one of the fiber filters high
rates were obtained because of adsorp-
tion of gaseous components of the
exhaust.
Conclusions
Particulate emission rates were de-
termined for a light-duty diesel passen-
\Dilution Air Filter
Sampling
Train
Optional for
Particulate
I Background Reading
Zero Air
Integrator
Counters
To Background Sample Bag
'acKground Bag
Ambient Air Inlet
Dilution Tunnel
Heated Probe
Particulate Probe
\Mixing Orifice
Vehicle Exhaust Inlet
Back-Up Filter (Phase 1 and 3)
Primary Filter (Phase 1 and 3)
Note: Three filter holders
(one for each phase)
are also acceptable
HC Span Gas
To Outside vent
To Exhaust Sample Bag
Sample Line ("7"! \
".IT
Heat Exchanger
Positive
Displacement
Pump
Primary Filter (Phase 2)
Back-Up Filter
(Phase 2)
To Pump. Rotometer_
'and Gas Meter
as Diagramed^
mmediatel
Below
Manometer
Revolution
Counter
Pick Up
^, Manometer
* \j Discharge
Figure 1. Federal emissions certification sampling system.
2
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ger car under FTP cyclic driving condi-
tions using seven commercially available
filter media. Rates obtained with 1//
PTFE membrane filters were low owing
to difficulties experienced with collec-
tion of FTP test phase 1 emissions. The
rate obtained with the Gelman A-E
glass fiber filter was high owing to
apparent adsorption of gaseous emis-
sion components. All of the examined
media collected greater than 95 percent
of the sample aerosol on the primary
filter of a standard two filter collection
set as determined by comparison of
primary to primary plus secondary
weights (Federal Register procedure).
Microscopic study of the media re-
vealed significant structural differences.
The glass fiber filters had different
densities and fiber sizes, resulting in
different pressure drops during sampling
and slightly different collection efficien-
cies. Two of the examined glass fiber
filters, Pallflex T60A20 and TX40HI,
were PTFE coated. The TX40HI had
significantly larger amounts of PTFE in
the fiber system and also was backed
with a PTFE coated glass fiber "cloth"
improving the filter strength. The PTFE
membrane filters had average pore
sizes about IAť, nearly an order-of-
magnitude smaller than the glass fiber
filters. The membrane had a "quasi-
fiber" structure with PTFE filaments
about 0.2/y in diameter. The membrane
filters had coarse backing materials so
that the membrane itself could be as
thin as possible to minimize sampling
pressure drops, which still exceeded all
of examined glass fiber filters. The
Fluoropore filter used a polyethylene
"ribbon" matrix for backing, while the
Zefluor filter had a coarse PTFE "web-
bing." Microscopic examination also
revealed the particle loads at which the
gas flow passages become clogged, a
condition important to the mechanics of
deposition and collection efficiency.
Under the FTP test conditions used,
more than 95 percent of particulate
collection occurred under "clogged"
filter conditions.
Recommendations
The two filtration problems identified
in the reported program were collection
of FTP test phase 1/y (transient cold
start) emissions with membrane filters
and adsorption of gaseous exhaust
components with Gelman A-E glass
fiber filters. Available resources would
not permit comprehensive examination
of either phenomenon. Further study is
indicated, particularly of the collection
of test phase 1 emissions. Comparison
of test phase 1 data from the various
filters showed scatter exceeding that of
the other test phases. Further, several
parameters important to deposition of
particles on filters, for example particle
velocity at the filter face, temperature
and concentration, were not studied
with the membrane or other media.
Were the observations sensitive to
particulate composition? Would similar
results have been obtained with other
motor vehicles and sampling conditions?
This study emphasized the accuracy
with which total particulate mass emis-
sion rates can be determined with
various filtration media. However, pro-
cedures defined for determining mass
rates are commonly applied to collect
samples of exhaust particulate matter
for chemical analysis and assessment
of potential health effects using various
bioassay techniques. There are many
possible physical and chemical phe-
nomena associated with the laboratory
sampling practice that could seriously
bias results and estimates of the potential
environmental impact of the emissions.
Obviously, exhaust cannot be diluted in
the laboratory exactly as it is on the
roadway. The importance of dilution to
mass has been examined in many
studies and the laboratory procedures
give reasonably accurate results. How-
ever, chemical changes occurring at low
dilution are not well understood. Chem-
ical kinetics would, suggest that any
process resulting in abnormally high
concentrations of reactant compounds
can affect composition. Further, it is
possible that sampling system compo-
nents, including the filter medium, can
catalyze reactions. Substantial further
study is indicated to determine the
adequacy of certification procedures for
collecting diesel particulate matter for
determinations other than total mass.
The authors Frank Black, who is also the EPA Project Officer (see below), and
Lisa Doberstein are with the Environmental Sciences Research Laboratory,
Research Triangle Park, NC 27711.
The complete report, entitled "Filtef Media for Collecting Diesel Particulate
Matter," (Order No. PB 81-197 774; Cost: $6.50, subject to change) will be
available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
i US GOVERNMENT PRINTING OFFICE 1981-757-012/7142
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