Note: Descriptions are shown in the official language in which they were submitted.
1~46Z45 `--
METHOD ~ND APPARATUS FOR ANALYZING
PARTICLE-CONTAINING GASEOUS SUSPENSIONS
BACKGROUND OF THE INVENTION
,, , . . , , _ _
Many processes require, or would at least benefit
from, on-line monitoring of the chemical composition
and/or other parameters of gaseous suspensions involved.
Such in-situ analysis entails a number of significant
advantages over other techniques (e.g., the analysis of
conversion products), particularly in that all of the
problems associated with sampling and sample handling
are inherently eliminated; it also permits dynamic
monitoring of chemical and/or~physical changes that
occur during the course of combustion, pyrolysis, and
other types of reactions.
As far as is known, very few (if any) of the forms
of instrumentation heretofore available are useful or
satisfactory for the on~line analysis of particle
streams (as used herein, reference to "particles" is to
be understood to include liquids and solids, as well as
mixed phases). In particular, it is not believed that
any such instrumentation is capable of resolving size,
temperature, number density and/or ~uantitative chemical
composition for particle-containing gaseous streams,
especially in a reactive environment.
It is of course well known to utilize
electromagnetic radiation for a variety of analytical
purposes, as evidenced by the body of prior art patents
issued in the United States. For example, in Bertrand
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17 ~5: ~5 2: 2133 2'~ ~4~4 ~O~;TRS HTFD. CT. E~
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WO861~140 P~U~/O~ll
2--
Patent ~o. 2,333,76Z an analyti~al technique is
di~closed ~n which the in~enslty of radiation is ~d to
determine ~he solld content of a gaseou~ medl~m~ A
temper~t~re measu~ement system, ope~atiny upon ~or~ed
and emi~ted radiation, ~s describ~d in Tandler et ~1 No.
2,844,0~2, and in P~tent No. 2,8~ Be~g~on dls~los~s
system ~or analyzing gases b~ me~suring the abs~rption
o~ radian~ energy.
Seelbin~r No. 3,724,9.51 and ~ s ~o. 3,743,43Q
both involve techniques or ~aking aero~ol opacit~r
dete~miJIation~, based upon t~nsmitted radiati~n, and
~nowman No. 3,588,49~ ~eache~ radiAtiOn absorp~i~n
analysis ~ppar~us for identifylng s~mples o~ gase~,
aer~sols and liqu~ds, ~ach of ~he f~llowing p~tent~
uses ir~diation ~cattering a~ a basi~ ~or detect~ng
and/or ~nal~rzing aero~ols or smo~es: ~Llsum N~. ;
3,317,730, Charl~on et al No. 3,700,333, I~pper, ~r. No.
3,7~7,122, and Mueller No. 3,882,477. In Pa~nt No.
4,017,193, Loi~erman describe~ ~pp~ra~u~ o~ measuring
t~e t~ansmi~ance o~ a ~a~eous medium ~rr~ng
p~rticulate matter throug~ a aonduit, and Suga No.
4,021,713 di3~10~e~ ~ppa~atu~ ~or the ~equen~ial
mea6urement of radiat~on tr~n~mitted ~hrough ~moke.
Neu~os~h~l No. ~,70~,~37 disclo~e~ an anal~ical
proce~sor c~p~ble of handling a~ least tw~
characteri~ics o the ~pec~men, slmultane~usly
mea~u~iny and conv~rting th~m ~or digltal prlnt ou~.
V~ness@ No. 4,0~5,~9~ concerns ~ ~echnique ~or
~2~62~5
performing Fourier spectroscopy. In Cashdollar et al
No. 4,142,417, an infrared pyrometer is used to
determine radiation emitted from a gas and/or particle,
with temperatures being determined by correlation of the
radiation data to black-body radiation curves.
Kraushaar et al No. 4,304,491 discloses the use of a
spectrometer to detect both dispersed and undispersed
irradiation for IR imaging.
Cells and associated devices, used for
spectroscopic analysis of samples, are described in
Gaglione No. 3,478,206, Sole et al No. 3,631,237 and
Witte No. 3,730,630. Surface temperature measuring
apparatus is taught by ~randli et al in No. 3,924,469,
and a photometer/detector/amplifier arrangement, for use
in automatic analysis apparatus, is shown in Atwood et
al No. 4,014,612.
In Stein No. 4,440,510, a system is disclosed for
pyrometric gas temperature measurement, carried out by
adjusting and comparing the physical temperature of a
black-body with the radiation temperature thereof
measured through the gas. A spectrometric method for
determining the size of metal particles in oils is
taught in Rauffman et al No. 4,448,887, and a method and
apparatus for determining size distribution of
particles, by fitting a selected parameter distribution
function to scaler representations of data obtained, is
disclosed in Hobbs et al No. 4,453,226.
~ v 11 2~L6Z45 --~
Finally, in an article entitled "Fire Flame
Radiation" (Combustion and Flame 52: 127-135, 1983),
; . ~ . . . .
, ~ Vervisch and Coppalle discuss the use of normalized
emission measurements for determining the temperature of
flames containing soot.
D~spite the foregoing, a need remains for means by
which analyses of the sort described above can be
carried out conveniently and effectively.
Accordingly, it is a primary object of the present
invention to provide a novel method and apparatus by
which gaseous suspensions of liquid and/or solid
particles can readily be analyzed for any of a variety
of physical and chemical properties.
; More specific objects are to provide such a method
and apparatus by which such a suspension can be analyzed
either in-situ, in a reactive environment, or as a
supplied sample, for determinations of particle size,
temperature, number density, spectral emittance, and/or
composition, in a manner that is very fast, convenient,
and effective.
SUMMARY OF THE INVENTION
.
It has now been found that certain of the foregoing
and related objects of the invention are readily
attained by the provision of apparatus comprising, in
combination, interferometer means, radiation collecting
means, radiation source means, and electronic data
processing means for analyzing collected radiation. The
interferometer means is operatively positionable, with
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respect to the suspension to be analyzed, for encoding
radiation projected thereinto and emanating therefrom,
and the collecting means is similarly positionable, with
respect to the suspension and the interferometer means,
for collecting coded radiation from the suspension; the
collecting means is also adapted to discriminate, in
cooperation with the data processing means, radiation
transmi~ted through the suspension from radiation
emanating therefrom. An electromagnetic radiation beam
is provided by the source means, so as to be projected
through the interferometer means for coding and
thereafter for transmission thr~ough the suspension.
It should be noted that, as used herein, reference
to radiation "emanating" from the suspension or
containment means is intended to be exclusive o~
radiation which is transmitted by or through the
suspension or the particles thereof, but inclusive of
any radiation that ls emitted by the particle and/or
scattered by interaction therewith. Also, "transmitted"
radiation is that which passes directly through the
substance, without being diverted (such as by
refraction, diffraction or scattering by another
mechanism) from its original rectilinear path.
In preferred embodiments of the apparatus, the
collecting means will comprise a first collector
operatively positionable for collecting radiation
transmitted through the suspension, and a second
collector separate from the first, operatively
k~ .
v i2,~6245
positionable for collecting the radiation emanating from
the suspension. Generally, the apparatus will be
adapted for use with containment means having a sidewall
defining a chamber for the gaseous suspension. The
sidewall of the containment means will in turn have at
least one port providing optical access into the
chamber, with the "second" collector, and the source
means and/or the "first" collector, being disposed on
the apparatus for positioning so as to function through
the port.
In most instances, however, the apparatus will be
adapted for use with containment means having a pair of
optical access ports aligned transversely on opposite
sides of its sidewall. The source means and "first"
collector of such apparatus will be in effective optical
alignment, and spaced from one another to accommodate
the containment means therebetween, thereby permitting
projection of the beam from the source means through the
aligned access ports to the "first" collector. The
apparatus will desirably include means defining an
aperture of variable size, from which passes the
transmitted radiation for collection by the "first"
collector; this will enhance the usefulness of the
apparatus for making particle size determinations.
For some applications, the apparatus will
additionally include a cell, cooperatively providing the
above~described containment means as an integral
component of the apparatus, together with associated
' " ~'' i~.~t '-
24~i
means for injection of the gaseous suspension. In a
specific embodiment, the cell has a generally
cylindrical sidewall and end walls cooperatively
defining the chamber thereof. The sidewall has a pair
of optical access ports positioned diametrically
thereon, and the end walls have means defining inlet and
outlet channels therethrough, which channels are aligned
substantially on the longitudinal axis of the cell for
the injection and removal of particles, respectively.
Such a cell will also have means by which the
temperature of the inside surface of the sidewall, and
the temperature of the inlet and outlet channel-defining
means, can be independently controlled.
In other preferred embodiments of the apparatus,
the "second" collector will be effectively disposed
along the path of radiation between the source means and
the interferometer, and the apparatus will additionally
include diverter means for establishing a radiation path
between the gaseous suspension and either the source
means, the "second" collector, or both. The diverter
means may be operative to either permit passage of
radiation from the source means to the suspension, or to
block such passage of radiation while simultaneously
directing radiation from the suspension to the "second"
collecting means. As a result, measurements of
radiation transmitted through and emanating from the
suspensionl respectively, can be selectively made.
~2~6~5
_ . .. .. .. _ _
Most desirably, the diverter means will be adapted
to simultaneously permit passage of radiation from the
source means to the suspension while also directing
radiation therefrom to the "second" collecting means.
To do so, the diverter means may have a first portion
which is transparent to the radiation from the source
means, and a second portion which is opaque thereto and
is reflective of radiation emanating from the
suspension, and is directed theretoward. Thus, the
diverter means will permit the transmitted and emanating
radiation to be simultaneously measured, using the
"first" and "second" collecting means, respectively.
In the particularly preferred embodiments, the
apparatus will comprise a Fourier-transform
spectrometer, adapted to develop a spectrum ~
representative of the intensity of the collected
radiation as a function of wavenumber. For that
purpose, the data processing means of the spectrometer
will be programmed to compare the representative
spectrum to preestablished spectra indicative of a
parameter for which the gaseous suspension is being
analyzed, so as to fit the representative spectrum
thereto and thereby determine the parameter. More
specifically, the spectrometer will employ radiation
source means operating in the infrared wavelengths
regions, and the data processing means will beneficially
be programmed to effect the comparisons involved by
~%~62~
application of at least one of the following generalized
formulas:
,
E =
[kSBB (Ts) +kgBB (Tg) +NA~,BB (Tp) +NAQSBB (Tw) ]
[l-exp(-(ks+kg+NAQext)L)]
.
ks + kg + NAQext
and
~ ) = l-exp[-(ks + kg + NAQeXt) ]
As used therein (and in other expressions throughout
this specification), "E" represents any collected
radiation emanating from the gàseous suspension and not
transmitted therethrough; "~" represents the ratio of
any collected radiation that is transmitted through the
suspension, divided by radiation that would be
transmitted in the absence thereoE (i.e.,
transmittance); "ks" and llkgll are the extinction
coefficients ~or any soot present and the gas phases,
respectively, of the suspension; "BB ~Ts) ", "BB (Tg)
"BB (Tp) ", and "BB(TW)" are the black-body spectra
appropriate to the temperature of any soot present, the
gas, the particles, and the medium surrounding the
suspension, respectively; "N" is the number density of
the particles in the suspension; "A" is the geometric
cross-sectional area of the particles; "L" is the
effective path length through the gaseous suspension;
"~" is the spectral emmittance of the partlcles; "Qs~ is
the ratio of the radiation scattering cross section to
q
.. . .
iZ~i~2~i:5 "--
--10--
the geometric cross section of the particles and "Qext"
is the ratio of the extinction cross section to the
geometric cross section of the particles, and is equal
to Q5 + Qabs. The term "Qabs" is used to represent the
ratio of the absorption cross section to the geometric
cross section of the particles, and it should be
appreciated that each of the foregoing quantities~ other
than N, A and L, is wavenumber dependent.
The foregoing generalized formula for "E" is 2
special case of a more basic equation, in which special
case the sample is homogeneous and all quantities are
therefore independent of posit-ion through the sample
volume. The data processing means of the apparatus may,
however, be programmed to effect comparison of the
representative spectrum using the following basic
equation, by which contributions from theoretical slices
of width 'Idl'l, at positions "1" through the suspension,
are integrated for values of "1" from zero to "L" to
determine the radiation emanating from a non-homogeneous
sample:
E = ¦ [{ksBBtTs) + kgBB(Tg)~+ NA~BB(T ) +
NAQSBB(Tw)} exp(-y)]dl,
wherein "y" is the integral: 5 (kg + Ics + NAQe t)dl
It will be appreciated that e~uations other than the
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62~5
,i
foregoing generalized formula for E may also be derived
from the basic equation, and used in the apparatus and
method of the invention, for other special cases in
which particular conditions may exist or be assumed to
exist as a practical matter, as will be discussed more
fully hereinbelow.
Other objects of the invention are attained by the
provision of apparatus particularly adapted for the
analysis of a gaseous suspension to determine
compositional parameters o~ the particles contained
therein, utilizing refracted components of radiationO
Such apparatus will comprise containment means having a
sidewall defining a chamber for the flow of a gaseous
suspension of particles along a path therethrough, with
at least one port being provided in the sidewall to
afford optical access to the path. Source means used
for providing electromagnetic radiation in the apparatus
will be adapted to direct the radiation inwardly from
substantially all peripheral points about the path, and
the apparatus will have means for collecting radiation
emanating from the containment means. To enable the
compositional analysis to be made, the containment
means, the source means, and the collecting means will
be so adapted that components of radiation from the
source means that have been refracted or otherwise
diverted from their original paths, due to interaction
with the particles of the suspension, can be
substantially discriminated from radiation that has not
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2~ ~;29~
been so diverted. Preferably, such apparatus will
additionally include second source means, for providing
an electromagnetic radiation beam, and second radiation
collecting means, the second source means and second
collecting means being disposed in effective optical
alignment with one another, and being adapted to measure
radiation transmitted by the particles of the suspension
during passage through the containment means.
Generally, the apparatus will also include electronic
data processing means for analyzing the radiation
collected by the "first" and 'Isecond'' collecting means,
and for also controlling operation of the "second"
source means.
In particularly preferred embodiments of the
apparatus, the sidewall of the containment means will
substantially surround the flow path, and will have an
energy radiating surface thereon to provide the
first-mentioned source means, which surface will usually
be heated for that purpose. The configuration of the
wall surface, and the positions thereof and of the
collecting means with respect to the access port(s),
will substantially limit the radiation from the
radiating surface impinging upon the collecting means to
that which has been so diverted, thereby effectively
providing the radiation discrimination capability of the
apparatus. Most desirably, the radiating surface will
be of generally circular cross-sectional configuration
in planes trans~erse to the flow path axis, and the
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~ ~2~62~5
-13-
sidewall defining it will have a second optical access
port therein aligned transversely with the "one" port on
the opposite side of the flow path. The "second" port
will provide a non-radiating area on the surface, and
; thereby cooperate to provide the discrimination
capability of the apparatus. An interferometer or other
coding means, radiation diverter means, and other
specific features described above may also be
incorporated into the apparatus of this embodiment.
Additional objects of the invention are attained by
the provisions of analytical methods, broadly defined to
comprise the steps of: a. causing electromagnetic
radiation from at least one source to impinge upon a
gaseous suspension of liquid and/or solid particles to
be analyzed; b. collecting spectral radiation from the
so-irradiated suspension; c. developing a spectrum
representative of the intensity of the collected
radiation as a function of wavenumber; and d. comparing
the representative spectrum to preestablished spectra
indicative of the parameter for which the suspension is
being analyzed, and fitting the representative spectrum
thereto to determine the parameter, the comparison being
made by application of the equations and formulas set
forth and defined herein.
In some cases, the method will include the step of
passing the gaseous suspension through a chamber at a
flow rate of about 1 to 100 meters per second, with the
suspensi~n being irradiated during such passage, and a
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1~4~2~5i
-14-
stream of gas may be passed into the chamber
simultaneously with, and as a sheath about, the particle
suspension. Normally, the irradiation used will be a
beam of infrared wavelengths, preferably brought to a
focal volume or zone within the chamber with the gaseous
suspension being passed substantially therethrough the
focal volume. For certain purposes, it will be
advantageous for the particles of the suspension to be
in the form of a monodispersed stream, and in the most
preferred embodiments of the method the step of
analyzing the radiation will comprise Fourier-transform
spectroscopic measurement thereof.
In more specific aspects of the method, radiation
transmitted through the irradiated suspension, as
measured by transmittance ~ , is discriminated from
radiation "E" emanating therefrom, with spectra
representative of the intensity of the radiation r and E
so collected and discriminated being developed as
functions of wavenumber. Preferably, the comparison of
the representative and preestablished spectra, for
determining the desired parameter, will be made by
application of the generalized formulas given above, or
by application of the basic equation referred to, or
other equations derived therefrom, depending upon the
nature of the sample.
In the method, the particles of the suspension may
be at a temperature "Tp" which is to be determined, the
representative spectrum used for comparison being that
.''' '~ :'. .
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" ~ ~2~6245
.-15-
of normalized emission "En", wherein En = E/~ . When
irradiation is carried out with the gaseous suspension
contained in a chamber, the surrounding medium will be
the wall surface defining the chamber, and the
comparison of spectra will be made based upon the
following equation, derived on the basis that Qext
equals 1, and ks and kg both equal zero:
En = ~ BB~Tp) ~ ) BB(TW).
In other instances, the temperature of the
particles and the temperature of the medium surrounding
the suspension will be known, and the parameter for
analysis will be emittance "~.~", the representative
spectrum used again being normalized emission. If
particle temperature "Tp" is substantially higher than
the surrounding medium temperature ''Tw'', the comparison
will be made based upon the equation: ~ = En /BB(Tp).
If, on the other hand, the surrounding medium comprises
the surface of a wall, the temperature of which is
substantially higher than that of the particle, the
comparison will be made based upon the equation:
~ [En /BB(TW)]. In the latter instance, the
method may include the further step of estimating the
wavenumber-dependent linear absorption coefficient
characteristic "k~" of the composition. This may be
done by measuring the value of En, determining a value
for the average transmission "T'" for the inside of the
particles of the suspension by applicatLon of the
equation: T = En /BB(TW), characterizing the gross
L246Z~5
16-
geometry of the particles of the suspension, in terms of
a characterizing dimension "~", selecting a suitable
preestablished curve expressing ~-ln T') as a function
of k~D, based upon that characterization, and estimating
the value of k~ from the selected curve.
In an embodiment of the method that is specifically
adapted for ~uantitative compositional analysis,
electromagnetic radiation will be caused to impinge at
off-axis angles upon the particles of the suspension
during passage through a chamber, such angles consisting
essentially of angles that are oblique to the optical
access port thereof. The cQllected radiation is
substantially limited, by virtue of the off-axis
impingement, to rays coming from the source that are
refracted or otherwise diverted by the particles. A
spectrum representative of the path and amplitude of the
collected radiation is developed, as a function of
wavenumber, and is compared and fitted to preestablished
spectra indicative of the compositional parameter for
which the suspension is being analyzed, to determine the
same.
Generally, the cavity used in performing such a
method will be defined by a wall substantially
surrounding the gaseous suspension, the surface of which
will be maintained at a temperature substantially higher
than the temperature of the particles, thereby providing
an off-axis, infrared radiation source. Typically, the
wall surface will ~e at a temperature that is about 500
.''. ~ '.'`'
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~L2~624S
-17-
Centigrade degrees or more above that of the particles,
and the suspension will desirably be maintained, prior
to entry into the cavity, at a temperature suitable to
ensure that they will be substantially at room
temperature therewithin; the flow rate of the suspension
through the chamber should be sufficiently high to avoid
substantial heating of the particles by the radiant
energy. As an alternative to using a hot surrounding
wall surface, a high intensity radiation source (such as
laser beam optics) may be moved to incrementally
displaced circumferential positions about the path of
the suspension, to provide an off-axis beam at a
multiplicity of angular relationships.
In especially preferred embodiments of the method
for compositional analysis, a beam of electromagnetic
radiation from a second source will also be caused to
impinge upon the particles, with the collecting step
being carried out by collecting and discriminating the
diverted rays from the components of the second-source
beam that are transmitted through the particles. The
representative spectrum used for comparison will again
be that of normalized emission, with the comparison
being made by application of the designated formulae or
equations. In such a case, the transmitted radiation
components and the diverted rays may be collected
sequentially, under conditions of constant particle flow
rate and density, or they may preferably be collected
simultaneously. This embodiment of the method may also
}..,
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~ ~LZ~6245
-18-
include the further step of estimating the
wavenumber-dependent linear absorbtion coefficient
characteristic "k~" of the composition, in the manner
described above.
Finally, the method may be employed for the
analysis of the size of particles in a gaseous
suspension, by causing a beam of electromagnetic
radiation to impinge upon the suspension, and
selectively collecting radiation transmitted
therethrough. A spectrum representative of the
intensity of the collected radiation, as a function of
wavenumber, is developed, and is compared and fitted to
preestablished spectra indicative of particle size. The
representative spectrum is that o (l-f), and the
comparison is made based upon the formula:
(1 r) - l-exp[-(ks ~ kg + NAQeXt)
wherein ~r is the transmittance or fraction of radiation
transmitted, and is equal to the (wavenumber-dependent)
ratio of measured intensities, with and without
particles in the impinging beam (i.e., I/Io).
Preferably, the gaseous suspension will be contained in
a chamber, and the aperture size of the optical access
port, beyond the zone of impingement of the beam upon
the particles, will be varied to maximize the dependency
of the intensity of collected radiation upon the
wavenumbers of the radiation of the impinging beam. In
this manner, the curve of the representative spectrum
.... ..... .
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iZ~62~5
will be optimized for fittlng to the curves of pre-establlshed
spectra.
The invention will be further illustrated by way of
the accompanying drawings, in which:-
Figure la is a schematic representation of a spec-
trometric system embodying the present invention;
Figure lb is a schematic representation of a disc
for providing a variable size aperture, suitable for use ln
the system of Figure la;
Figure lc is a schematic representation of a plate
suitable for use as a diverter in the system of Figure la;
Figures 4, 5a and b, 6a-d, 7a-d, 8a-d, 9a-d, lOa-d,
lla-d, 12a-d, 13a-d, 14a-d, 15, 16, 17a and b, 18a and b, 19,
and 20a and b.
Turning now in detall to Figure 1 of the appended
drawings, as lndicated above the measurement of particle prop-
erties is preferably performed, using Fourier-transform
infrared spectrometer (FT-IR) apparatus, generally designated
by the numeral 10 therein, fitted with special optics and
programmed to carry out the unlque analysis methodology of the
invention. More specifically, the FT-IR apparatus shown
: schematically in Figure 1 can be any of several commercially
available lnstruments ~e.g., the NICOLET 7199 system), and
will include an infrared source 12 (e.g., a globar), a Michel-
son interferometer, generally designated by the numeral 14, a
sample compartment 16, a
'~
.
-- 19 --
~29~62~5 `-
-20-
radiation collector or detector 18, and a computer 20,
suitably interconnected (by means not shown) for
instrument control and data processing and analysis; it
will normally also incorporate a laser beam source 22
and detector 23, for timing purposes. Generally, the
spectrometer will be capable of spectral resolution
between 0.5 to 8 wavenumbers and of operating at any
appropriate scan frequency and any fre~uency range,
although 400-10,000 wavenumbers is preferred. In
addition to providing suitable mirrors 24, 26, 28, 30,
32 and 34 at appropriate locations within the system, a
second detector 36, a reflect~;ive diverter 38, and a
sample cell, generally designated by the numeral 40, are
incorporated in the illustrated embodiment.
By way of broad description, assuming a measurement
of transmitted radiation i5 to be made for a gaseous
suspension passing through the chamber 42 of the cell
40, the IR beam from source 12 is reflected by mirror 26
into the interferometer 14 fo~ encoding. From the
inter~erometer, it is reflected by focusing mirror 28
through port 44 in one side of the cell wall 46, and is
brought to a focal volume "f" therewithin. Those
components of the beam that are transmitted by the
suspension pass through the second port 48 (laterally
aligned with the first) in the wall 46, and are
reflected by mirrors 32, 34 into the detector 18, which
will be selectively adapted to collect the encoded
'. . i~?
2 ~ 6Z ~ S
radiation. It should be pointed out that the port 48, which
lies beyond the focal zone f (or the zone of particle/beam
interaction, if an unfocused beam is employed) with respect to
the source 12, may have associated means for varying the size
of its aperture, so as to permit adjustments to be made to
, achieve optimal sensitivity for particle size measurement.
; AS shown in Figure lb, such means may, for example,
take the form of a disc 70 having a series of circular open-
ings 72 of graduated size thereabout, the disc being rotated
to align any of them with the port 48.
Radiation emanating from the cell 40 can be col-
lected by the detector 36, being reflected by mirror 28
through the interferometer 14 and encoded for that purpose.
To do so, the diverter 38 is positioned (as shown) in the path
of the beam reflected from the mirror 26, and will serve to
reflect it to the mirror 24 and to the detector 36 therefrom;
as will be appreciated, ln the embodiment shown the diverter
38 will be displaced (such as by pivoting) from the path of
the beam generated by source 12, to permit the above-described
transmission measurement to be made.
The two measurements (i.e., of transmitted and ema-
nating radiation) can be made sequentially with the arrange-
ment illustrated by rapidly shifting the position of the
diverter 38, as lndicated. Alternatively, the measurements
can be effected simultaneously, and this will normally be the
preferred mode of operation. Simultaneous measurements can be
made by use of a diverter having two zones of different optl-
cal properties disposed in the radlation path, one zone being
constructed to pass the beam from the source 12, and the other
being made to reflect radlation emanating Erom the cell 40,
which is directed thereto by the mlrrors 28 and 26. Such a
diverter 38', is schematically illustrated in Figure lc, and
';
- 21 -
6Z~5
takes the form of a plate having a mirrored, upper section 74
and a transparent lower section 76 (the radiation beam circum-
ference being shown in phantom line).
Other arrangements and apparatus features can, of
course, also be employed. For example, since it is desirable
in most instances to utilize suspensions in which the part-
icles are homogeneously dispersed, suitable means for pro-
viding such suspensions doing so may be included. The appa-
ratus may employ a system of flipping mirrors for pro~ecting
the radiation to a common detector location (which may itself
comprise a single collector, if appropriately constructed and
coupled with suitable analytical data processing logic to
perform the desired functions). It is also possible to use
only a single optical access port with an aligned reflector,
; in which case the beam will enter and exit from the same
aperture and provide a double-length transmittance measurement
; through the sample. Moreover,~although infrared spectrometry
ls described and ls preferred, other radlatlon frequencies may
be substltuted.
It should be appreclated that apparatus such as that
of Figure 1 can be employed, as well, to analyze gaseous sus-
penslons at locatlons external to the system; e.g., for the in
situ monitorlng o~ a chemical reaction
- 22 -
~z4~2~5
-23-
in progress. In those instances the cell 40 would not
be used, its functions instead being performed by the
on-site containment means (e.g., the reaction vessel),
which would of course have suitable ports for optical
access, and the mirrors 28, 32 would be positioned (as
necessary) to accommodate the reaction vessel
therebetween. It is also possible to employ the
apparatus for analysis of unconfined volumes (e.g., of a
gaseous combustion mixture flowing from a smokestack or
over a container), in which case the medium surrounding
the suspension would be the ambient, rather than a cell
or reactor wall.
Generally, the function of the sampllng cell 40
will be to either conduct gas suspended particles
through the beam from the source 12, or to provide a
second source of radiation emanating from locations
about the gaseous stream (e.g., the wall surface 42); as
is evidenced by Figure 1, moreover, the cell may serve
both functions. Because the port 48, which is aligned
with port 44 on the optical path, provides an unheated
area on the surface 42, it will effectively represent a
gap in the radiation source surroundir,g the particle
flow path, and will thereby limit the components
collected from that source to those which are refracted
or otherwise scattered by the particles into the optics
of detector 36. Obviously, the same effect could be
achieved by other means in the absence of a port, such
.
.,.:. ~ .', '
.. :................ ~; .. :
, ;.-
-::. , ,.~
. ..
,:,:
v 3~ 2 ~ 6 2 4 5
-2~-
as by cooling the corresponding, on-axis area of the
wall. If a transmission beam were projected through
port 48 toward port 44, that fraction of its rays which
was not scattered out of the optical path would of
course be directed toward the same detector; however,
the coding effects provided by the apparatus permit them
to be discriminated, so as to not contribute to the
refracted radiation measurement. In the particular
arrangement of Figure l, such coding permits the
detector 18 to discriminate and collect the rays from
the source 12 which are transmitted through the
particles, and permits the det~ector 36 to do the same
with regard to the radiation emanating from the cell 40.
In any event, the geometry of the cell should be
such that no appreciable attentuation of the IR beam
occurs in traversing it from port 44 to port 48 unless
particles are present, generally within ~ volume of
focus thereof. Similarly, it should be so designed that
no appreciable radiation from the second radiation
source (e.g., surface 42) reaches detector 36 in the
absence of particles in such a zone of the optical path.
A preferred embodiment of the sampling cell 40 is
schematically illustrated in Figure 2. It consists of a
body 48 having an internal cavity 42 of circular cross
sectionj defined by the inside surface of wall 46.
Means~ (not cho~n)--- is provided for controlling the
temperature of the wall surface (normally, the heated
... ,~ ,. ..
~'. ~,.' ..
: . . ; ~ .
12~L~Z~i
section will be separate from the remainder of the wall 46,
to minimize heat loss and energy requirements), and ports
44, 48 are aligned diametrically on the opposite sides of
the wall 46 and provide optical access to the cavity 42; the
ports are closed by transparent windows 50. Passages in the
top and bottom walls 52, 54, respectively, of the body 48
are aligned on the longitudinal axis of the wall g6, and are
constructed to provide particle injection and collection
features for the cell.
~ore specifically, the injection feature is provided
by two coaxial tubular conduits, the inner conduit being
temperature controlled (by means of 47~ and providing a
channel 56 for the gaseous suspension of particles to be
injected into the cell, and the outer conduit pxoviding a
channel 58, of annular cross section, for delivery of a gas
which is to form a sheath about the suspension. The
collection feature is provided by an insert 60, which is
also temperature controlled by means 47, having a funnel-
like conduit 62 formed therethrough.
As can be seen, the optical path of the spectrometer
beam traverses the ports 44, 48, and is brought to a focal
zone at "f", within the cavity 42. The particles 64 are
injected khrough the conduit 56 (in monodispersed form, in
the illllstrated.embodiment), into the focal volume of the
. . ~
beam for interaction therewith,
-25-
~29~6;~4~;
-26-
and are thereafter removed from the cell through the
cond~it 62. -
~
The design of the illustrated cell serves tominimize any path for radiation to enter the emission
; detector 36 in the absence of a sample stream. It will
be appreciated that the fluid mechanics will be so
designed so that the sample stream will pass through the
cavity virtually without loss and without appreciable
alteration of its temperature, by internal cavity
radiation, when the temperature controlled walls are hot
(relative to the particles). Moreover, under those
conditions the gas velocity mu~;t be sufficiently high to
avoid particle heating; flow velocities of 1 - lC0
meters/second will generally be employed, and residence
times will typically range from fractions of a
millisecond to about one-tenth second.
The carrier gas used can be of any desired
composition; nitrogen and argon will be beneficial in
providing a non-reactive environment which will not
interfere in the IR spectra. On the other hand, the use
of "tracer" gases which exhibit IR absorption, such as
carbon monoxide, will allow for gas temperature
diagnostics. The sheath gas can be nitrogen, argon, or
other non-absorbing gas. The suspended particles can,
as explained above, be solids, including very finely
divided substances such as soot, or liquid droplets;
optimally, the particles will be less than 300
" ",", ~ ..
~Z~6245 ``---
micrometers in diameter, and they can be either
monodispersed or polydispersed in the gas phase.
The Measurements
Generally, the analysis methodology will consist of
obtaining transmittance spectra, emission spectra, or
both, which preferably (particularly Eor the sake of
speed and accuracy) will be taken simultaneously, and
under conditions ensuring homogeneity of the particle
concentration in the gas phase. These spectra are
obtained and analyzed with an FT-IR spectrometer under
computer control, using special computer software
functionally described herei~; to determine desired
parameters and properties of the particles.
For purposes of calibration, spectra are obtained
in the absence of a sample stream. Normally,
calibration will be required only at infrequent
intervals, depending of course upon the stability of the
optical system and detectors.
Emission Measurements
The emission measurements require that a wavenumber
dependent instrument response function, F~ , be
determined for each resolution used. This is done by
obtaining a spectrum "R~" (using a detector such as 36
in Figure 1), from a reference black-body placed at the
focal point "f". R~ is corrected by subtracting the
background with no source present, and is divided by the
black-body curve "BB(TR)" appropriate to the temperature
: :. . ""
: .. -............... . ~ , j;
~ 462~5 J
-28-
of the reference; th~s, the response f~nction is
determined in accordance with the formula:
F~ = R~ /BB(TR)
It should be appreciated that all of the above
quantities are wavenumber dependent, and that the
measured spectral response curve will be obtained with a
collector such as the so-called "MCT", "InSb" or "TGS"
detectors. Instrument response functions and background
spectra were observed to be stable over several weeks,
provided that cell conditions remained constant.
A sample emission spectrum 'tE" is then obtained by
dividing (using the computer~associated with the
spectrometer) the observed spectruM "0" at the detector,
with particles present in the focus "f" and corrected
for background, by the instrument response function F
(the subscript "~" is omitted for convenience). It has
been found that emission measurements, with appropriate
background and instrument response corrections, were
made with good signal-to-noise ratios, in as little as
200 milliseconds. Examples of such data are shown in
Figures 5-8 (in which radiance is plotted against
wavenumber) for a number of cases where the particles
are of different composition and at different
temperatures with respect to the wall.
Absorption Measurements
Measurements of absorption, or transmittance, are
made in the normal way. The spectrum from a globar
\
,,:. ~ ..................... .
.:, , .~ :.:
~2~6Z~S
-29-
source [see Bohren, C. F. and Huffman, D. R.,
"Absorption and Scattering of Light by Small Particles",
John Wiley and Sons, New York, NY (1983)] passing
through the empty cell is measured to determine
intensity 1l Io" at each wavenumber, and the same
measurement is made with the particles in the cell to
give the intensity 11 I 11. As indicated above, the
transmittance 11~11 is defined as the fraction of the
radiation transmitted (~ = I/Io), which term is also
used herein to refer to the transmitted percentage. The
absorbance "A" is given by A = -log10 ~ , and the
fraction percentage of radiation absorbed and scattered
is given by (1-~).
Examples of the transmission, plotted as (1-~ as a
function of wavenumber, are presented in Figs. 9 and 10.
Figures 9a and 9b are for carbon and copper particles.
Particles can of course block radiation by absorption
and by scattering (i.e., reflection, refraction and
diffraction). For particles of diameter greater than
several micrometers, and for wavelengths of present
interest (e. g., 1.6 to 25 micrometers), it has been
predicted and observed, in accordance herewith, that
almost none of the incident radiation is transmitted
directly through the particle along its original
rectilinear path. This is true even for particles that
are completely or partially transparent in the infrared
range, such as potassium chloride (Figure lOa) and fuel
'r,
': . ' . ' S.~ .
~2~6Z~5 `~
--30--
oil (lOd), as long as the refractive index of the
substance differs from unity. In addition, diffraction
and interference can produce a wavelength-dependent
reduction of the transmitted intensity by a factor that
is as much as twice the projected area of the particle
(see Hottel, H. C. and A. F., "Radiative Transfer",
McGraw-Hill Company, New York, (1976) and van de Hulst,
H. C., "Light Scattering by Small Particles":, Dover
Publications, NY, (1981).] in addition to the Bohren and
Huffman reference note~ above).
With regard to Figures 9a and 9b, it is expected
that particles such as carbon and copper block radiation
over their projected surface area at relatively short
wavelengths (large wavenumbers), with diffraction
effects decreasing the transmission at longer wavelength
values. For large particles, therefore, (1-~) at short
wavelengths is taken as a measure of the fraction of the
viewing area which is blocked by the projected area
thereof. For soot particles, of diameter O.l micrometer
(Figure 9c), the level of absorption is highly dependent
upon wavelength, decreasing at longer values.
An FT-IR spectrometer is ideally suited for making
transmission measurements in a hot cell, since the
detector will only record radiation which has been
modulated by the Michelson interferometer and will
therefore reject radiation originating at the hot cell
walls. Of course, such a spectrometer also offers the
.''' .'',' ~ '.,.' ' j.
.: :''' Tl~
.. ? ~j .'j~. :'.
6;~:45 "-
-31-
advantages of high sensitivity, high resolution, and
rapid scan in all applications, and is thereore the
preferred apparatus herein, and the apparatus of first
choice in the practice of the instant method.
Normalized Emission
In analyzing the data from the measurements made,
the determination of radiation extinction by the
particles, relative to their blocking area, is of
primary interest. This is done by use of "normalized"
emission "En", equal to E/(l- ~ . Examples of normalized
emission for several cases of interest are presented in
Figures ll to 14 (plots of radiance versus wavenumber).
As can be seen, the spectra vary substantially with the
composition of the particles and their temperature
relative to the cell wall.
The Analyses
Analys1s of Size and Densit~
To determine the size and concentration (number
density) of the particles in the suspension analyzed,
transmittance spectra are employed, examples of which,
plotted as ~l-r), are presented in Figures 9 and 10. In
the case of particles which block less than 20 percent
of the transmitted light, the quantity (l-~) is
approximately equal to the quantity Qext NA~, and can
readily be evaluated in accordance herewith; when
blockage is greater than 20 percent, valuable
information can still be obtained, but the analysis is
., , ::
.,.,.. . ,~ ,...
. .,. ~: ,:
. tL. ,,
62~5 '-
considerably more complex. For a spectrometer
acceptance angle of ~ , and particles with perimeter "P"
such that (P/~)(sin~) is equal to or less than 3,
diffraction and interference can produce a
wavelength-dependent reduction of the transmitted
intensity which is as much as twice the fractional
projected area. An example of this phenomenon, which is
well understood, is illustrated in Figure 9b, which
shows enhanced scattering at long wavelengths. For
purposes of the present analysis, the Fr-IR spectrometer
used had an acceptance angle of 0.25 radian.
While the full Mie scattering theory is available
to treat the effect of diffraction, the simpler Rayleigh
expression has been employed herein, which has been
shown to be accurate for the larger P/~ ratios (see
Gumbrecht, R. O. and Sliepcevich, C. N., J. Phys. Chem.
57, 90 (1953).] For Figures 9a and 9b, it is expected
that the particles block radiation over their projected
surface area at relatively short wavelengths (large
wavenumbers) with diffraction effects decreasing the
transmittance further at longer wavelengths. Therefore,
for large particles the expression "(1-7~" at short
wavelengths is a measure of the fraction of the viewing
area which is blocked by the particles, while the shape
of (1-1~ is a measure of the particle size. Figure 15
illustrates the calculated shapes of three different
sizes.
,.. ,., i ~
''' , '.'.
v
.' ~ ?,.'
~2~Z~
By way of specific example, the diameters o~
particles monodispersed in a gas stream, traversing a
cell in a system such as illustrated in Figure 1, is
obtained by a least squares fitting routine, which
compares (1-~) to theoretical curves. Least squares
fitting is a technique which seeks, such as through
successive approximations, to minimize the value of the
square of the difference between the actual and the
computed values for a particular selected parameter.
This operation may be conducted iteratively until an
acceptable minimization occurs, whereupon those
particular values of the parameters are outputted, as
providing the best fit.
Comparing the theoretical predictions to the curves
of Figures 9 and 10 gives the following average particle
sizes for the several substances: carbon spheres (Figure
9a) 80 micrometers; copper (Figure 9b) 32 micrometers;
potassium chloride (Figure lOa) 80 micrometers; lignite
(Figure lOb) 56 micrometers; and fuel droplets (Figure
lOd) greater than 100 micrometers. These values are in
reasonable agreement with the corresponding values of
115, 4~, 9S, 60 and 180, respectively, as determined by
sieving or photomicroscopy. Because the latter
techniques indicate the largest dimensions of the
particles, rather than average values, determination by
~ J plotting would be expected to give smaller
particle size indications, as it does.
:... , t
' r:
.,_" '~
-- ~.2~6245 "
-34-
For mixed sizes, the observed spectrum is least
squares fit to the theoretical prediction for a
log~normal (or other) distribution. Range and accuracy
can be improved by obtaining additional data for smaller
acceptance angles ~ , which can be changed by using a
variable aperture between the focus "f" and the detector
18. For example, decreasing ~ by a factor of 3 will
increase the maximum measurable size to approximately
300 micrometers.
In any event, the fitting routine will provide a
determination of Qext and the average particle diameter
(assuming a spherical shape) from which the particle
area "A" can be calculated. Then the quantity "N x L"
(concentration times path length) is determined from the
known quantitites, according to the derived
approximation equation set forth above.
An important factor in analyzing the size
distribution for particles has been found to be the
value of the extinction of radiation caused by the
particles, relative to their blocking area. Extinction
of radiation is schematically illustrated in Figure 16,
wherein "Qext" (which, in the preferred embodiments of
the invention, depends on the entrance aperture of the
FT-IR optics), is plotted against "X" for particles with
wavelength independent optical constants, X being equal
to P/~. In the Figure, the "blocking" region (Qext = 1)
is on the left. For particles larger than about 100
.. .,','.',: ~ ',;''
:' .'', .'~ ~
~.Z~62~5
micrometersl Q is equal to 1 over the whole wavelength
region, depending on the value of the refractive index.
For smaller particles, Q increases to a maximum value of
2, which effect is observed as an increase in absorption
at long wavelengths (see Figures 9a and 9b). Also
indicated in the diagram is the scattering behavior of
very small particles, for which ~ is less than 1. Small
soot particles (see Figure 9c) and possibly ash
particles (Figure lOc) lie in this range, for the
wavelengths of interest for this technique; for such
small particles the quantity "(1 - ~)" decreases at long
wavelengths. ~;
Analysis of Composition
Quantitative analysis of particulate composition is
made using the normalized emission function; emission
spectra alone can be employed for semi-qualitative
analysis. The representative spectra are obtained when
the particles are at low temperatures relative to the
surrounding medium; ideally, the particle will be near
room temperature or below, with the suspension contained
in a chamber having a wall surface temperature of 500
Centigrade, or above.
In the simplest case, there will be no effect from
gas or soot absorption or radiation; the particle will
be assumed to be at a temperature low enough to neglect
its emission, and to be large enough to neglect
diffraction effects. Under these circumstances, BB(Tp)
:, ~ ,',.
~ ,.. ,:
,.. , :i,~ .
".r..
. , .
~12~6Z45 -,i
-36-
will be approximately zero, Qext will equal the quantity
(Qs + Qabs) and will have a value of about unity, and En
will about equal Qs times the black-body specrum at the
temperature of the wall; thus, the general equation for
En, set forth above, will reduce to: En
(l-Qabs)BB(Tw) In the simple case of potassium
chloride, where Qabs is approximately zero (Figure lla),
En equals BB(TW), which agrees with the observed
spectrum.
For samples where Qabs has a value other than zero,
the compositional information is contained in the
function En. For example, the~absorption bands for coal
and fuel oil can be seen in the emission spectra of
Figures 5a and b, and the normalized emission spectra of
Figures llb and lld. The value of Qabs must be related
to the shape and optical properties of the particles.
The various effects which have been observed can be
quantitatively explained on the basis of refraction of
radiation, as schematically indicated in Figure 3, which~
shows the geometry for the emission and transmission
measurements, looking down the axis of the cell 40 with
a particle 64 at the focus "f" of the FT-IR beam. The
emission spectrum consists of actual emission from the
particles (ray "a"), plus radiation (ray"b") from the
walls which is diffracted (or reflected) from virtually
any angle that is oblique to the port 44 (i.e., off the
axis between it and the particle) into the collection
,~ ~
.'.: ~ ,;
~ . . ii ; .
....
.:.:. .,. j)
'12a~6245
~ 37-
optics; ray "c" is a component of incident radiation
(e.g., a transmission beam) which has been refracted out
of the optical path, and scattered to virtually any
angle relative thereto.
For Figures 5 and 11, the particles are cold, so
only the radiation scattered (diffracted, refracted or
reflected) into the collection optics contributes, and
the magnitude of the signal will depend on the size of
the particle, the index of refraction of the substance,
and its absorptivity. For a sphere, exact calculations
can be performed to determine absorptivity from E
given the diameter of the par;ticles, the degree of
scattering on the surface, and the index of refraction.
In the simple case of non-reflecting spheres of a
substance having an index of refraction greater than
1.5, Qabs is approximately equal to the quantity
(l-e k~ D), where k~ is the wavelength dependent
absorption coefficient ~absorbance) of the sample, and D
is the diameter of the sphere, which may be known or
computed from (1-~) as discussed above. Then, the
following derived equation applies:
k~ = -ln(l-Q b ) = -ln(E /BB(T )
D D
`''?~ b
Figure~ presents a comparison of k~ for a jet fuel
composition, computed from the foregoing equation using
the observed diameter of 1~0 micrometers, and measured
.,............... ~. ~
., ~ . . ~,;,~
: ^,: ,~::
,;,.............. . i,
. . ,',''. ,,
~2~62~5 `~,
--38--
in a liquid cell, respectively; it can be seen that the
agreement is excellent.
As indicated above, semi-quan~itative spectra can
be obtained using emission spectra alone, such as those
shown in Figure 5. For that purpose, BB(TW) may be
scaled to fit the highest regions of the emission
spectra.
Analysis of ~mittance
The spectral emittance of particles can be made
either with them cooler or hotter than the surrounding
medium (the particles can be heated, for example, in a
heated injector, an entrained flow reactor or a heated
tube reactor associated with the analytical apparatus).
The simplest case is for large particles (Qext = l),
where the particle temperature is greater than that of
the envirous (Tp > Tw), and where soot and absorbing
gases are absent (ks = kg = O). Under those conditions,
En = ~ BB(Tp) and, conversely, ~ = En/BB(Tp).
To determine ~ , measurements were made in a cell
with a temperature controlled injector heating the
particles to a known equlibrium temperature Tp.
Examples of En and BB(Tp) for char and lignite particles
of two size cuts, at different temperatures, are
presented in Figure 12. The emittance varies with the
degree of pyrolysis and particle size.
To obtain the emittance of cold particles, the
simplest case is for large particles, where Tw is much
,.,.. ~ X:~
',I;t
~L;~6Z9L5 -~
-39-
higher than Tp, once again in the absence of soot and
absorbing gases, in which case the emittance will be
equal to (l-[En/BB(Tw)]). To determine ~ , measurements
are made in the cell with the wall heated to above 500
Centigrade, and with the particle injector cooled to
room temperature or below. Examples of En and BB(TW)
for a lignite, potassium chloride, jet fuel and graphite
are presented in Figure 11. As can be seen, the
spectral emittance varies with the sample composition:
for potassium chloride it is approximately ~ero; for
graphite, it is almost 80 percent in agreement with
expectations; for the lignite (Figure lld), the
emittance is similar to that determined from the hGt
lignite (Figure 12d).
Analysis of Temperature
Temperatures can be obtained for the components of
the sample stream even when different components (gas,
soot, particles) are not at the same temperature.
a. particle temperature
Considering initially the case in which soot and
gas contributions can be neglected (ks = kg = 0), and
diffraction ef~ects are small (Qext is approximately 1),
particulate temperature can be determined from
black-body curves, through application of the equation:
En = ~ BB(Tp) ~ ) BB (Tw). A particularly simple
case occurs when the surrounding wall is much colder
than the particle, in which case En is approximately
' ! ,i,. . ~
gL6~5
-40-
equal to BB(Tp), and Tp can be determined directly by
comparing En to computed black-body curves, as in
Figures 12 and 14; En falls on the black-body curve in
regions where ~ = 1. Another simple case occurs when
the particle and wall are in equlibrium, in which case
En = BB(Tp), as shown in Figure 13.
For other cases, the equation expressing En in
terms of the black-body curve for the wall, provided
above, must be solved using an iterative fitting
procedure. ~his requires knowing ~ for at least two
wayelengths.
b. particle distributions
.
One difficulty with a shape-based determination of
temperature is that a distribution of particle
temperatures can give an emission spectrum which appears
to be a good black-body shape corresponding to an
intermediate temperature. The amplitude, however, is
always fo~nd to be lower than that of an isothermal
distribution at that intermediate temperature, as
illustrated in Figure 18.
Figure 18a compares the spectrum for a 50/50 mix of
radiators at 1100 and 1700 Kelvin, with an isothermal
case at 1400 K (the "average" temperature); there is
clearly a difference. The shape of the 50/50 mix curve
however can be matched to 0.77 times the black-body
curve at 1575 K, as shown in Figure 18b; therefore, the
amplitude of the 50/50 mix does not match the full 1575
- . i~ i'
., '.,',' ~ i~'~'! '
r ".,' ~;,',' .;:,,
~LZ~G245
-41-
black-body curve, and it has been found that the larger
the temperature spread, the larger the discrepancy.
This illustrates the importance of normalized emission
in the particle case. With a good knowledge of ~ ,
quantitative information about the average temperature
of the particles, and the temperature spread, can be
inferred by comparing normalized emission shape and
amplitude to black-body curves.
Although normalized emission spectra are obtained
only in instances in which transmission can be measured,
a similar measurement can be made for emission from an
optically thick combusting sample. In that case, a
calibration can be made on a sample of known single
temperature which fills the spectrometer aperture, and
amplitudes would again have significance.
c. temperat~res ~f components in mlxed phase systems
In monitoring the properties of reacting mixed
phase systems, it is desirable to obtain the temperature
of individual phases. For example, in coal combustion
the spectra contain continuum contributions from both
soot and particulates, as well as band contributions
from the gases. To determine relative contributions of
each phase to absorbance, and the particle, soot and gas
phase temperatures, the last two values are assumed to
be the same, and the contribution of BB(TW) is ignored
in the interpretation of the spectra (as will be clear
from the context, the subscripts "s", "p", and "g" refer
'',., ~ :.'~
"', ~. '.~'''.
-- ~Z~62~5
-42-
to soot, the particles, and the gas, respectively).
The temperature determinations from the continuum
region are made at the three wavenumber regions, chosen
because they lie outside of gas emission lines. For
less that five micrometers, Qext is expected to be unity
for particles of diameter greater than about 16
micrometers (see Figure 16); Qext is taken to be unity
for this analysis. In addition, it is assumed that the
values of ~ are constant with time, and the appropriate
value of ~ will be substituted in the generalized
equation for "E" set forth above, at the three
wavelength regions of interest. From the measured
emission and transmission spectra, ~ln7~ and En are
calculated.
With the above approximations, -ln ~ (i.e., tk
NA)L) at the three wavenumber regions of interest ls
; made up of a part that is linear as a function of
wavenumber (ks), and a part that is independent (NA).
These two straight line contributions can be separated
from the ln ~ data, giving the relative "amounts" of k5
and NA at each wavenumber region, as illustrated in
~, ~o~ ~b
,:?~ Figure~4.
With the approximations made, the normalized
emission in the regions free of gas contributions is: En
= [ks BB(TS) ~ NA BB(Tp) ]/(kS + NA), the ratio kS/NA
being known from the transmittance, as discussed above.
`~ ilLZ462~S ` J
-43-
By dividing the above equation by ks and simplifying,
the expression becomes:
[En (1 + NA/kS) = BB(TS) + (NA/kS) ~ BB(Tp)] i,
the "i" denoting that the equation is for three (or
more) wavenumber regions. The unknown quantities are
the black-body amplitudes, which can be found by a least
squares minimization using an iterative fitting routine,
aEter postulating trial values of Ts and Tp. The
arnplitudes of the black-body curves for all temperatures
can be calculated from the black-body reference
spectrum.
In the region of the spec`tra containing gas lines-,
the ratio of kg to (ks ~ NA~ can be determined from the
-ln7' curve. With this information, and with known Tp
(from the continuum-only measurement discussed above),
the gas temperature can be determined from the
normalized emission, using the generalized formulas, and
the En relationship to emission and trasmittance
spectra, previously set forth.
Where soot is present, the comparison of Ts and Tg
~rom these two methods will provide an extra check on
the data. A comparison of determined En~ with a
theoretical En made up of separate contributions from
gas, soot and particles, is shown in Figure 20. The
agreement with reasonable values for the temperatures of
the separate phases, determined in Figure 19, is seen to
be excellent.
',,:.' ~ .",
',.; ; ~ ' '
: . . . ,~ ~ .
, ,:
:; :............... ; , ',.
~ Z~;245 ` ~
-44-
d. particle temperatures from emission only
In obtaining particle temperatures, the use of
normalized emission has two advantages: (1) the it of
amplitude, as well as of shape, affords improved
precision and the potential for determination of
particle temperature spreads; (2) it provides the
ability to determine soot temperatures. However, some
monitoring applications may preclude the determination
of transmittance, which generally requires entrance and
exit ports along a line-of-sight, and (extinction-path
length) products sufficiently small so that at least 15
percent transmission is achiev~d. When there is a very
small degree of transmission because of particle
blocking, the spectra will tend to that of ~ . For cases
without fine soot, a temperature measurement may be
obtained from the shape of the ray emission; again,
knowledge of ~ is required to make this determination
more accurate.
For several spectra, a temperature of combusting
coal particles has been derived from an "n-color"
black-body fit of the raw emission. The n- original
colors are the five wavelength regions designated on the
raw emission spectra shown in Figure 8 where
interference from gas emission is minimized.
On applying this method to the spectra of
combusting species with higher gaseous emission,
however, it became clear that only the three higher
, .. .. ,:
. .
`- ~Z~6Z4~; `;
-45-
wavenumber regions could be considered to lie outside of
overlapping gas lines. Char is presented in Figure 8a,
lignite in Figure 8b, and bituminous coal in Figures 8c
and d; the circle~ regions of the spectra correspond to
the five wavelength regions chosen. Only the three
higher wavenumber regions were used for the black-body
fits, since the two lower ones appeared to have
interference at high water levels.
The measurement is seen to agree well with those
~rom the normalized emission technique, illustrated in
Figure 14. Eliminating the requirements for
transmission measurements wil~l make the instant
technique more flexible, and therefore more desirable in
some situations.
Th eory
The scattering, absorption, transmission and
emission of electromagnetic radiation by and from
particles depend both upon material properties, in the
form of optical constants, and on morphology, which can
be represented by scales of inhomogeneity relative to
wavelength. The interaction of particles with a
radiation field can be characterized by
wavenumber-dependent efficiency factors "Q", which
express the effective cross sections for scattering or
absorption, divided by the geometric cross section of
the particles; thus, Qext = Qs ~ Qabs~ where the
subscripts stand for extinction, scattering and
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62 ~ 5
-46-
absorption, respectively. As used herein "Qs" refers to
radiation scattered out of the acceptance angle of the
optics, and the other Q's are similarly specific to the
optical beam path.
; In developing the basic equation and generaliæed
formulas underlying the analytical techniques and
apparatus of this invention, and from which the
simplified equations employed for the several analyses
are derived, a model was developed to quantitatively
account for many features of the observed normalized
emission spectra. One feature of the model relates to
the geometry of the particles ~in the sample cell tas
described above in connection with Figure 3), from which
it is concluded that the efficiency for scattering of
radiation out of the beam path in a transmission
experiment (e.g., ray "c") is equal to the efficiency
for scattering wall radiation into the beam in an
emission experiment (e.g., ray "bl'), for parti~les
within the focus volume.
To describe the emission, transmission, and
scattering behavior of a multi-phase suspension (i.e.,
containing gas, particles and sootJ, the model developed
was based upon the assumption that: (1) gas and fine
particulates (soot) are at one temperature within the
analyzed volume; (2) particles larger than 0.3 micron
are at one temperature, not necessarily that of the gas;
(3) the molecular concentration of each constituent,
i
:r ~ :
",~
',,.' '~ ~.;'
~2~6Z45 `~J
-47-
averaged over a volume containing many particles, is
constant througho~t the analyzed volume; and (4) the
density of large particles is small, so that less than
0.2 of the radiation is blocked by them. Also, assuming
that "L" is the effective path length through a sample
located in a cell or reactor, with walls at temperature
Tw surrounding the sample volume, the following
expressions for the radiation "Ei" emanating from the
cell, and for the radiation (expressed in terms of
transmittance "~") transmitted through the sample,
respectively, were developed:
[kSBB (TS) +kgBB (Tg) ~NAf;BB (Tp) +NAQSBB (TW) ]
[l~exp(-(ks+kg+NAQext)L)]
. . ~
ks + kg + NAQext
and
(1-~) = l-exp[-(ks + kg + NAQeXt) ]~
the terms of which formulas are defined elsewhere
herein. The analysis is readily extended, by use of the
above-defined basic equation, or equations derived
therefrom, to include ash, to include samples that are
non-homogeneous along the path length "L", and to
accommodate other deviations from the assumptions made
and expressed herein; such analyses will of course be
correspondingly more complex.
As noted above, the normalized emission has
previously been used for both gaseous and soot flames,
.. .. ,, ...... :
.... . . .
~2~62~5
-48-
in which cases En is simply the black-body curve
appropriate to the temperature of the flame in both
shape and amplitude. The present invention, however,
involves the discovery of the significance o~ normalized
emission for particle spectra, and the use thereof for
analysis of the several parameters of the particle
suspensions discussed herein. It involves, moreover,
the discovery of techniques for utilizing the components
of spectrum of ln7~= (ks + kg + NA Qext)L, the use of En
to obtain composition data for the particles, and of
to obtain particle size and density. Furthermore, it
has been found that, in those cases in which QeXt=l, the
spectral variation o ~ can be determined using
normalized emission~ together with a measurement of Tp
by an auxilliary technique; no other method is believed
to exist for determining the spectral emittance of
particles.
As discussed above, composition analysis is
perormed under conditions where the particles are cool
and the wall is hot, and in the absence of absorbing gas
or soot; where Qext = 1~ the normalized emission is
approximately equal to the expression~ Qabs~BB(Tw).
The compositional information is contained in the Qabs
term, which must be calculated from the properties of
the particles.
Mie theory predicts the scattering of radiation of
particles as a function of wavelength, particle size,
~Z~6245 -J
-49-
and optical constants. Certain aspects of the present
invention utilize the fact that radiation from hot cell
walls ~or other surrounding medium at a temperature
above that of the particles) passes through particles in
tlle center and is diverted along other paths. By
collecting the portion of such radiation that is
directed towards an emission detector, the detected
spectrum, missing energy at wavelengths in the
absorption bands of the particle it has passed through,
can be used for analysis of composition.
To optimize this determination, however, it is
necessary to predict, based upon the shape and optical
constants of the particles, the relative magnitudes of
Qs and Qabs; this is done, in accordance herewith, by
evaluatiny effective transmission "T'" of radiation
scattered therethrough. The plot of Figure 4 shows
calculations of T' for particles that can be categorized
as having one of four basic configurations, or gross
geometries; in the plot, "kd " is the
wavelength-dependent absorption coefficient, and D is a
characteristic dimension for the geometry.
More particularly, a thin film of thickness D will
transmit radiation in accordance with Beer's Law, and
consequently the expression: -ln (T~) = k~D will apply;
for such particles, k~ will equal [-ln(En/BB(Tw))/D].
For sperical particles having a range of surface
''' ~ ,' '"
L2~62~5 "J
-50-
roughnesses, the effective transmission is given by ~the
equation:
T' ~ P -kdd/
wherein the factor "P(d)" expresses the probability that
any particular ray will travel a distance "d" before
emerging from the particle. For perfectly scattering
sperical particles, the applicable equation is:
-k~D
T' = [l-e /kdD],
wherein ~ is the particle diameter.
To derive an expression for prismatic flakes (such
as coal particles), "D" was taken as the diameter of a
sphere of the same volume as the flake, knowing the mesh
size and the typical geometry of the sample. In Figure
4, data bars are extracted from normalized emission
spectra of cold coal particles within a hot environment
regions within the En spectra for both 170 x 200 mesh
Zap lignite and 400 x 500 mesh Zap ligni,te were selected
for calculation of these data points. The quantity
(-lnT') is calculated to be [-ln (En/BB(TW))]~ corrected
to account for a reflective component of five to ten
percent. For the chosen regions of the spectra, k~ was
determined from potassium bromide pellet spectra. The
calculated values of -lnT' were found to be in good
agreement with the measured values.
~.2462~5
-51-
Th~s it can be seen that the present invention
provides a novel method, and novel apparatus for
carrying it out, by which gaseous suspensions of liquid
and/or solid particles can readily be analyzed for any
of a variety of physical and chemical properties. The
invention provides, more specifically, such a method and
apparatus by which a gaseous suspension can be analyzed
either in-situ, in a reactive environment, or as a
supplied sample, for determinations of particle size,
temperature, number density, spectral emittance.and/or
composition, and which is carried out in a manner that
is relatively accurate and is~very fast and convenient.
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