Field of the Invention
This invention relates generally to removal of particles
from an aerosol, and, more particularly, to an apparatus and method for removing
particles without appreciably affecting the thermodynamic properties or chemical
composition of the gas phase of the aerosol.
Particles distributed in gas have various effects in the
environment, technical applications, and measurement devices. To, for example, enable
research investigations on particle and gas measurements, particles have to be removed
from the gas phase of an aerosol. So far, mainly fabric filters and in some cases,
electrical filters have been employed. However, these known approaches suffer from
serious drawbacks in certain applications.
Recently, a differential particulate mass monitor which
intrinsically corrects for volatilization losses has been introduced. As described
in U.S. Patent 6,205,842 B1, this mass monitor employs alternately activatable particle
removers for selectively removing substantially all particulate matter from a gas
stream, without appreciably affecting gas stream temperature, pressure and flow
rate. This patent teaches that "Such particle removal can be advantageously implemented
using an electrostatic precipitator of the same general type as is commonly used
in air cleaning equipment. In order to reduce ozone production, an electrostatic
precipitator operating with a positive corona and very low current, e.g. on the
order of tenshundreds nanoamps, is preferred. The current should be sufficient to
cause the precipitator to remove substantially all particulate matter from the gas
stream." (Column 6, lines 48-56)
Ideally, a particle remover for use in such a differential
particulate mass monitor should fulfill the particle separation function without
affecting the gas phase thermodynamic conditions or chemical composition.
Fabric filters are available in different sizes, shapes
and materials. They are used for a broad variety of applications. Small filters
are used for air cleaning to protect measuring instruments and for manual sampling
of ambient particles for mass concentration determinations. Large fabric filters
are used to clean flue gases from industrial and power plants.
Fabric filters remove particles from a sample gas stream
with high efficiency, but the pressure drop across the filter is high and increases
with increasing filter loading. Hence, the gas pressure downstream of the filter
is lower than the actual ambient gas pressure. Further, the gas phase of the sample
is altered due to evaporation of particles at the filter surface. Also, handling
of fabric filters in alternating operation is complicated. The filters have to be
removed from the gas stream, when ambient particle concentrations are required behind
the filter and moved back in-line when particles need to be removed. Frequent maintenance
and filter changing are necessary.
In common electrostatic precipitators (ESP's), particles
are charged by a corona discharge. The charged particles are deflected towards a
precipitation electrode due to electrostatic forces. The size and geometrical arrangement
of ESP's differ according to application requirements. Common arrangements include
(multi) wire-plate (mainly for industrial use, e.g. flue gas treatment and indoor
air cleaners), and pin-plate and wire-tube (both mainly for scientific, laboratory
Common ESP's separate gas and particles with a high efficiency.
The pressure drop across the ESP is generally low and alternating operation is easy
by simply switching the power supply on and off. On the other hand, the gas phase
of the sample is changed significantly, mainly due to formation of ozone and nitrogen
oxides by the corona discharge. Another process leading to an alteration of the
gas composition is evaporation of particles precipitated on the collecting electrode.
Wet ESP's are usually employed in industrial applications,
such as flue gas treatment of industrial and power plants. The operate like common
ESP's, but particles precipitated on the collecting electrode are flushed away by
a thin water layer. This treatment prevents particles from agglomerating on the
precipitation electrode surface that may form tips. These tips may cause opposite
corona discharges leading to particle re-entrainment. Further, the treatment prevents
particles on the collecting electrode from evaporating; although the gas phase of
the aerosol is still significantly altered due to the formation of ozone and nitrogen
oxides from the corona discharge. Additionally, the gas gets humidified by the water.
In the differential particulate mass monitor application, for example, humidification
of the aerosol could cause several severe problems, including change of the particle
phase due to condensation of water on the particle surface and alteration of the
particles size, mass, inertia and aerodynamic behavior; potential electrical spark-overs;
and changes to the transmission of light which could lower sensitivity and hence
lower reliability when used with gas sensors.
Document US-A-4 205 969 discloses an apparatus for removing
particles from an aerosol wherein the corona discharger (ionizing meshes and permeable
electrodes forming the ionizing section) is positioned in the aerosol flow. As a
consequence, the gas phase of the aerosol is changed.
A need thus persists for a highly efficient particle remover
which does not appreciably alter the thermodynamic conditions or chemical composition
of the gas phase of the aerosol, the function of which is not influenced by the
removed particles, and which facilitates quick and easy alternating operation.
Summary of the invention
The present invention provides apparatus and a method which
overcome the deficiencies described above and provide additional significant benefits.
Pursuant to the teachings of this invention, particles can be readily and efficiently
removed from an aerosol with no attendant pressure drop or temperature change, and
no or minimal change to the aerosol's gas composition.
In a first aspect, a method for removing particles from
an aerosol is provided. A charge is imparted to particles in the aerosol with a
corona discharger located outside said flow; alteration of the chemical composition
of the gas phase of the aerosol is prevented. The charged particles are deflected
to produce a particle free portion which is separated from the aerosol.
In another aspect, a gas particle partitioner is provided.
The partitioner includes a selectively activatable particle charger for producing
charged particles in an aerosol with no appreciable change to the chemical composition
of the gas phase of the aerosol. A fractionator operates on said charged particles
to fractionate the aerosol into a particle laden gas stream and a particle free
gas stream. A flow splitter separates said particle free gas stream from the particle
laden gas stream.
The particle charger may comprise a corona discharger and
a permeable electrode. Ions from the corona discharger are transported through the
permeable electrode to interact with and electrically charge particles in the aerosol.
The permeable electrode may separate a corona discharge area on one side of the
electrode from an aerosol charging zone on another side of the electrode. A particle
free fluid may wash the corona discharge area to minimize any transport of gas components
produced by corona discharge from said corona discharger to the aerosol. The particle
free fluid may comprise an air flow, and means may be provided for regulating the
air flow and flow of the aerosol to isokinetic conditions to disallow gas exchange
between the air flow and the aerosol.
The corona discharger may comprise a corona discharge wire,
made, e.g. of electrically conducting material, preferably silver or gold, switchably
connectable to a corona voltage source. A permeable grid electrode may surround
the corona discharge wire such that when an additional voltage is applied to the
grid electrode, an electric field is produced in the space between the grid electrode
and an outer wall, and ions are transported through openings in the electrode due
to this electric field.
Further, means may be provided for controlling ion production
by the corona discharger in response to a measurement of ionic current produced
by the corona discharge. A shielded connector is advantageously employed in the
measurement of ionic current.
The gas particle partitioner may also include an aerosol
inlet for producing a laminar flow of the aerosol to the particle charger. The fractionator
of the gas particle partitioner may include a first electrode, a second electrode
spaced from the first electrode, and means for selectively applying an electric
field between these electrodes, such that, when an aerosol flows between the first
and second electrodes, the charged particles in the aerosol are deflected towards
the second electrode by the applied electric field. The fractionator produces a
particle free gas stream adjacent the first electrode and a particle laden gas stream
adjacent the second electrode when the electric field is applied. The first electrode
may comprise an inner cylindrical wall and the second electrode may comprise an
outer cylindrical wall. The means for selectively applying an electric field between
the first and second electrodes may comprise a voltage supply switchably connectable
to at least one of these electrodes, and a shunt resistor for minimizing switching
The flow splitter of the gas particle partitioner may comprise
a conductive ring located near an outlet of the fractionator, and means for applying
a voltage to this ring.
The present invention provides numerous significant benefits
and advantages. Foremost among these is the ability to separate and remove particles
from an aerosol with high efficiency and without altering the thermodynamic conditions
and chemical composition of the gas phase of the aerosol. Unlike fabric filters,
there is no pressure drop with the present invention which permits the use of smaller
pumps and provides lower acquisition and maintenance costs. Since there is no change
to the thermodynamic conditions of the aerosol, measures to stabilize such conditions
can be avoided. The prevention of changes to the gas composition of the aerosol
enables use of the gas particle partitioner (GPP) in gas measuring devices, and
reduction of unfavorable gas reactions, corrosion, etc.
Further, in the present invention, the removed particles
have no influence on the functionality of the GPP resulting in longer lifetime and
cost reduction. The apparatus of the present invention is also easy to switch on
and off, enabling studies of particle and gas effects and interactions. An integrated
isokinetic flow split avoids changes to the original particle size distribution
and concentration for defined conditions. The gas particle partitioner of the present
invention also exhibits low energy consumption, good chemical resistance, minimal
soiling inside and easy handling. Further, the design is extremely versatile and
can be used in a wide variety of applications.
Brief Description of the Drawings
These and other aspects, features and advantages of the
present invention will be more readily understood from the following detailed description
of preferred embodiments when read in conjunction with the accompanying drawing
figures in which:
FIG. 1 is a schematic illustration of a gas particle partitioner
of the present invention;
FIG. 2 is a schematic illustration of the particle charging
and fractionation sections of the GPP;
FIG. 3 illustrates the operation of the GPP when the particle
charger and fractionator are activated;
FIG. 4 illustrates operation of the GPP when the particle
charger and fractionator are inactive; and
FIG. 5 depicts an experimental setup of a prototype GPP.
In accordance with the principles of the present invention,
apparatus (hereinafter sometimes referred to as the gas particle partitioner or
GPP) 10 for removing particles from an aerosol without appreciably affecting the
thermodynamic conditions or chemical composition of the gas phase of the aerosol,
is illustrated in FIG. 1. GPP 10 generally includes an aerosol inlet 12, a particle
charger 14, a fractionator 16, and a flow splitter 18. In the illustrated embodiment,
an outer cylindrical wall 20 serves as a housing for the GPP and, as more fully
described hereinafter, as one of a pair of electrodes of the fractionator 16. An
inner cylindrical wall 22 serves as the other electrode of fractionator 16, and
also supports a cylindrically shaped, permeable grid electrode 24 of particle charger
14. Inner wall 22 and outer wall 20 define an annular space 26 through which the
aerosol flows within the GPP 10.
Aerosol 28 is led into the GPP through aerosol inlet 12.
The aerosol inlet is advantageously designed to achieve a laminar flow and even
distribution of the aerosol within GPP 10, with minimum particle losses due to impaction,
interception and diffusion. The aerosol inlet may take different forms, e.g. an
upside down funnel on the outside with an ellipsoidal or conical stream line routing
on the inside.
From inlet 12, aerosol 28 enters an aerosol charging zone
30 in the annular space between permeable grid electrode 24 and outer wall 20. An
axially extending corona wire 32 within cylindrically shaped permeable grid electrode
24 produces a corona discharge area 34 about wire 32, when a voltage UCor
is applied to the wire. Corona wire 32, made of electrically conducting material,
advantageously silver or gold, serves as a controlled corona discharger for unipolar
charging of particles in aerosol 28. The corona discharger produces high concentrations
of ions which are transported through openings in permeable grid electrode 24 to
interact with and electrically charge aerosol particles in aerosol charging zone
A voltage U1 is applied from a voltage supply
to permeable grid electrode 24 to produce an electric field. Ions produced by the
corona discharge from wire 32 are transported through openings in electrode 24 due
to this electric field. The ion production is, preferably, monitored and can be
controlled by measuring the ionic current with a measuring electrode 36 (e.g. of
aluminum foil), a shielded connector 38 and a current meter 40. Computer or other
control means, responsive the measurements of ionic current by meter 40, can be
advantageously employed to control ion production by the corona discharger.
Corona discharge area 34 is separated from aerosol charging
zone 30 by permeable grid electrode 24. The corona discharge area is washed or flushed
with a particle free airflow 42 to minimize any transport of gas components produced
by the corona discharge process to the aerosol 28. Mixing of the wash flow 42 with
the aerosol flow is minimized by the separating grid electrode 24, and isokinetic
conditions inside and outside the corona discharge area 34. These measures eliminate
or substantially minimize changes to the chemical composition of the aerosol.
Preferably, corona wire 32 and permeable grid electrode
24 are switchably connectable to their respective power supplies. Thus, particle
charger 14 is selectively activatable. When activated, the particle charger imparts
unipolar (e.g. positive) charges to particles in aerosol charging zone 30 without
appreciably affecting the thermodynamic properties or chemical composition of the
gas phase of the aerosol 28. No ions are produced and no changes to the aerosol
occur in the charging zone when the corona discharger is switched off.
After passing through charging zone 30, aerosol 28 enters
the annular space 26 of fractionator 20. Inner wall 22 serves as a first electrode.
An outer wall 20 serves as a second electrode of fractionator 16. Outer wall 20
may be grounded while a voltage U1 is applied to inner wall 22, producing
an electric field F in a generally radially outward direction, as illustrated in
FIG. 2. If the particle charger and fractionator are active, (i.e. UCor
and U1 voltages applied), charged particles 44 in aerosol 28 are deflected
by electric field F, and transported in the direction of outer wall (second electrode)
20. Accordingly, electrical charged particles 44 in the aerosol are transported
by the electric field F (coulomb force) according to their charge and size when
the gas particle partitioner is switched on. This produces a particle free portion
or gas stream 46 adjacent inner electrode 22. Charged particles 44 may be deposited
on outer wall 20 or transported out of the GPP in a particle laden gas stream 48
adjacent outer electrode 20. In the latter case, the gas particle partitioner can
also serve as a particle concentrator. The different modes can be achieved by changing
the strength of electric field F or the length LF of fractionator 16.
Flow splitter 18 physically separates the particle free
gas stream 46 from particle laden gas stream 48. The particle free gas stream 46
can be used as a sample flow for a differential particulate mass monitor of the
type described in U.S. Patent 6,205,842 B1, while particle laden gas stream 48 is
treated as excess flow, as illustrated in FIG. 3. By removing the particles with
the excess flow and due to the fact that the excess flow passes the deposited particles,
evaporation of material from the walls of the fractionator will only influence the
excess flow and not the sample flow. One or more outlet tubes may be provided for
the excess flow. A pair of outlet tubes is, for example, advantageous in obtaining
homogenous flow conditions and avoiding feedback on the flow in the fractionator.
As depicted in FIG. 3, the sample flow is particle free
if the particle charger and fractionator are active. As shown in FIG. 4, the sample
flow will be unaltered (physically and chemically) compared to the inlet flow if
the GPP is switched off (i.e. no voltages applied). The GPP is thus, ideally suited
to serve as a particle remover in a differential particulate mass monitor, as well
as in a wide variety of other applications.
If flow splitter 18 is a conductive ring, this ring may
not be grounded. Otherwise, the grounded ring will influence the electric field
F near the outlet of the fractionator 16. This would lead to a higher longitudinal
velocity and may cause particles to get into the sample flow. Accordingly, if the
flow splitter 18 is manufactured from electrically conductive material, a partial
voltage U2 should be applied to flow splitter 18, as illustrated in FIG.
2, to leave the electric field in the vicinity of the outlet unaltered.
Exemplary values for the geometric, electrical and flow
rate parameters shown in FIG. 2, are now presented.
Radius of the inner
Radius of the outer
Radius of the flow
Voltage of the inner
Voltage at flow splitter
Length of the charging
Length of fractionator
Flow rate of the aerosol
Sample air flow rate
Excess air flow rate
Wash air flow rate
FIG. 5 is a simplified view of an experimental prototype
of the GPP, and associated equipment. GPP 10 includes aerosol inlet 12 (of the upside
down funnel-conical stream routing type), particle charger 14 (including corona
wire 32 and surrounding permeable grid electrode 24), fractionator 16, electrically
conductive flow splitter 18 and sample outlet 19. The corona discharge area interior
of electrode 24 is washed with a particle free air stream 42.
Pumps 43, 45 and 47, along with filters and mass flow controllers
(not shown) establish the desired flow rates.
An adjustable high voltage power supply 48 provides corona
voltage UCor. to corona wire 32. The corona voltage may be adjusted by
computer or manually, in a fashion well known in the art. The supply of voltage
U1 to inner electrode 22 and of voltage U2 to conductive flow
splitter 18 is realized by one high voltage supply 50. The two different voltages
U1 and U2 are obtained through high resistive voltage divider
52. A relay 54 allows simultaneous switching of high voltage power supplies 48 and
To measure particle concentration in the sample flow, a
condensation particle counter (CPC) 56 was used. Since the inlet flow of CPC 56
was either 0.3 l/min or 1.5 l/min and the sample flow from GPP 10 was 3 l/min, in
the experiments, a flow split downstream of the GPP was employed. A three way valve
58 between the flow split and CPC 56 allowed measurement of the total particle concentration
in ambient air VBy. Computer software resident in personal computer 60
was used to read the concentrations from CPC 56 and to adjust the corona voltage
Measurements have been performed using the experimental
setup of FIG. 5, with ambient laboratory air. Standard values that were used for
the measurements are:
1 = 1000V
2 = 446 V
The flow rate of the washing air was chosen to achieve
the same average velocity of the aerosol flow. The corona voltage was varied to
obtain the dependency of the separation on the corona discharge voltage. Prior to
the separation behavior measurements with applied voltages, the particle losses
inside the GPP were studied. Particle losses with no applied voltages, have shown
to be low (about 1%), if the standard flow rates are maintained.
For the first measurements of the separation behavior,
the standard voltages and flow rates were adjusted and the separation efficiency
was calculated from the measured ambient and sample concentrations. The corona potential
was varied from 0 V to 11 kV. The corona potential is the voltage of the corona
wire 32 against ground potential. The actual corona voltage is the difference between
the corona wire potential and the grid electrode potential Ui, i.e. in
this case, the corona voltage varied from -1 kV to +10 kV. The disruptive discharge
voltage is around 5 kV corona potential, i.e. at around 4 kV corona voltage.
Next, a series of measurements were performed to determine
a possible influence of the washing air on the separation efficiency. No significant
change in separation behavior was observed due to the use of washing air.
Next, it was investigated whether the polarity of the corona
potential has a significant influence on the separation. Generally, a positive corona
potential was chosen to be used with the GPP because it is expected to produce less
amount of ozone and nitrogen oxides. No significant differences were observed up
to a corona potential of approximately 8 kV. For potentials higher than 8 kV, the
separation is higher for positive than for negative polarity.
Gold wire is commonly used in conventional ESP's. Silver
was chosen as the corona wire material to keep the formation of gases like ozone
and nitrogen oxide low. Separation efficiency was found to be higher, when a silver
wire, rather than a gold wire, was used. This result was continuously found for
several measurements. On the other hand, recent investigations have shown that the
life time of gold wire for the corona is higher than the life time of silver wire.
The voltage U1 applied to inner electrode 22
was increased to 1500 V, and the voltage of the flow splitter 18 was increased by
the same factor to 669 V. A comparison of the separation behavior for 1000 V and
1500 V was then undertaken. For a voltage of 1500 V, the results show a significantly
increased efficiency. The maximum separation was about 96.5%. The rest up to 100%
may be due to uncharged nanoparticles. Nanoparticles may be insufficiently charged
by a corona discharge, but, on the other hand have a negligible mass compared to
the larger particles that are assumed to be separated from the sample flow in the
It took approximately 8 seconds after switching the corona
voltage on, before the concentration in the sample stream started to decrease (dead
time of the GPP). To determine the dynamic response of the GPP, the particle concentration
in the sample stream after switching on or off the corona voltage was measured in
short time steps. The dynamic response of the GPP should be as fast as possible.
Taking a dead time of 8 seconds into account, the total t90 time (i.e.
the time it takes to reach 90% of the final separation level) for corona voltages
above 8 kV were determined to be higher than 16 seconds.
In order to keep the dead time low, the velocity inside
the GPP can be increased and hence the total volume inside the GPP will be decreased.
A slimmer or shorter design of the GPP will also cause it to become lighter.
Investigations have shown that the corona wire in the GPP
may be used for a long time with no significant deterioration of the separation
efficiency. A changing interval for the corona wire 32 is expected to be at least
in the range of months.
Finally, frequent cleaning of the GPP is not required since
a large fraction of the particles does not get deposited on the electrodes 20, 22,
but is carried out of the GPP with the excess air flow. Since the sample air flow
is geometrically separated from the outer electrode 20, particulate matter deposited
on the outer electrode, may not reach the sample air flow. Accordingly, maintenance
intervals for the GPP are expected to be much longer than those of conventional
The gas particle partitioner of the present invention can
be used in different areas of technical applications and in measurement devices,
including, but not limited to:
- 1. Measurement devices to determine particle mass concentrations can be influenced
by gas components. The GPP can be used to determine and quantify these influences.
It may also be used for the de-correlation of gas and particle effects.
- 2. Since the GPP removes particles from the gas phase with no or little change
to the gas phase, it can also be employed in gas monitors for e.g. CO2,
CO, H2O, NO2, NH3, H2, HS, CH4,
- 3. It can be used as a pre-filter before mass-flow-controllers, flow measurement
devices, pressure gauges, temperature sensors and other sensors as well as a general
filter in low flow systems.
- 4. It can be employed as a filter in clean boxes.
The gas particle partitioner removes particles from an
aerosol with high efficiency and no or minimal changes to the chemical composition
and thermodynamic conditions of the gas phase. It is versatile in design and adaptable
to various areas of applications. Other major advantages of the device are that
it can easily be switched on and off and externally controlled. No interference
of the aerosol will occur when the GPP is switched off. Further, the GPP is energy
efficient, compact and mechanically robust.