Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Method of Producing Acetylene from Coal
Technical Field
This invention relates to an energy efficient process
for decomposing solid carbonaceous matter with volatile
content to produce a high yield of acetylene at relatively
low cost.
Definitlons
The term "solid carbonaceous matter" is intended to
define a class of materials where the hydrogen to carbon atom
ratio is 0.3 to 2.0 with a volatile content. The preferred
carbonaceous matter is coal. However, lignite, peat, plastic
and others will also qualify.
The term "volatile" is hereby defined as compositions
which separate from the carbonaceous matter when tested. For
example, coal and coke are analyzed pursuant to ASTM
Designation D271-64. This analysis is widely recognized as
"proximate" analysis.
Background of Prior Art
Historically, the conversion of coal and other carbon-
aceous matter to acetylene by means of electric arc heaters,and in particular other heating devices, are well-known.
Conventionally, carbonaceous matter, together with -~
hydrogen, are heated by an electric arc device or other
suitable source of heat. The mixture is heated so that the
carbonaceous matter decomposes. The composition that is
produced as a result of the decomposition of the carbonaceous
matter will depend on the reaction conditions existing in the
decomposition or reaction zone. It is well-known that the
formation of certain compositions are favored under specified
reaction conditions. For example, the formation of acetylene
as an intermediate product is favored where the temperature
of the reaction zone is above 1300K. The formation of acetylene
relative to the simultaneous decomposition of acetylene is
also favored at or near 1300K.
In U.S. Patent No. 1,757,454, there is a process whereby
water vapor is employed as the suspension agent for coal dust.
The combination is fed to an electric arc where a chemical
reaction produces acetylene.
Another arc coal/acetylene conversion process is described
in U.S. Patent No. 2,916,534, which was reissued as RE 25,218.
The process described in this patent makes use of hot hydrogen
molecules which are partly or totally dissociated into atoms
to decompose hydrocarbons. This patent also specifies a
specific flow rate of hydrogen gas through the reaction zone
and also the mole ratio of gas to carbon in the hydrocarbons.
U.S. Patent No. 3,217,056 is another patent which
utilizes the heat available in atomic hydrogen to decompose
carbonaceous matter. The heat content of the hydrogen gas
and also reaction times are specified.
U.S. Patent No. 4,010,090 makes use of a hydrocarbon
gas stream to increase the length of an arc before coal or
other carbonaceous matter is provided to the arc. The
principal virtue of this patent relative to the invention
is the large number of other patents relative to arc reactors
disclosed in column 2, lines 11-17.
U.S. Patent No. 3,395,194 describes a partial oxiaation
process using an electric arc device to heat a gaseous hydro-
carbon in the presence of oxygen and at very high pressures.
Generally, where an electric arc is used as a heat
source, the electric arc passes through the hydrogen gas
causing the gas to increase in temperature to extremely high
temperatures in a very short time. Arc column temperatures
commonly reach 8,000X to 20,000K. The gas, on leaving the
arc and entering the reaction zone, is commonly within the
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neighbourhood of 2,000-5,000K. Under these conditions, the
hydrogen molecules may dissociate partially into hydrogen
atoms.
Once the hydrogen leaves the arc, there is an extremely
rapid tendency for the hydrogen atoms to recombine into mole-
cules, and in doing so, they give off tremendous amounts of
heat. A portion of this heat, in addition to the sensible
heat of the gas, is absorbed by the coal particle, mostly via
conduction, convection and radiation, thereby causing the
coal particle to decompose and more specifically to give off
its volatile content, i.e., to devolatize~
It is also well-known and established that process steps
and conditions will vary greatly with the type of carbonaceous
matter fed to the arc. Heretofore, gaseous and liquid carbon-
aceous matter were the favored feedstocks, as there was noknown way of producing a high yield of acetylene at reasonable
costs from solid carbonaceous matter. Also gaseous and liquid
feedstocks were easier to handle and produced less wear and
tear on the arc apparatus.
Thus in summary though the basic process steps are
known, it is hypothesized that the mechanics and the kinetics
of the process, for example, were not understood well enough,
heretofore, to teach one how to maximize the yield of
acetylene from solid carbonaceous matter in an energy efficient
manner.
The following description represents an understanding
of the process directed specif;cally to solids having volatile
content. The necessary process parameters are proviaed for
heating the solid particles as fast as possible to decompose
the particle forming volatiles as fast as possible so as to
avoid the char forming secondary reactions of these volatiles
in the solid particles.
With due recognition of the prior art:
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It is an object of the invention to provide a process
for converting solid carbonaceous matter into acetylene by
decomposing the carbonaceous matter under specific process
conditions.
It is yet another object oE the invention to describe a
process wherein the enthalpies of the source of gaseous heat
and heat enthalpies of the carbonaceous matter are adjusted
within a specified range in order to increase the yield of
acetylene.
It is another object of the invention to perform the
process of decomposing solid carbonaceous matter within an
electric arc apparatus.
It is yet another object of the invention to provide
operating conditions in an electric arc reactor for decomposing
solid carbonaceous matter for the production of acetylene.
It is still another object of the invention to provide
a process which removes at least 25 percent of the volatiles
from solid carbonaceous matter before the carbonaceous matter
leaves the reaction zone.
In accordance with the invention, a process is described
whereby acetylene is produced from solid carbonaceous matter
by decomposing the carbonaceous matter in hydrogen in a
reaction where the specific gas enthalpy and the specific
carbonaceous matter enthalpy are tightly controlled. In
addition, the size of the particles of carbonaceous matter is
maintained below a specific minimum in order to cause the
removal of more than 25 percent of the volatiles present in
the carbonaceous matter within the time it takes the coal and
gas to pass through the reaction zone.
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The novel features that are considered characteristic of
the invention are set forth in the appended claims; the
invention itself, however, both as to its organization and
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method of operation, together with additional objects and
advantages thereof, will best be understood from the following
description of a specific embodiment, when read in conjunc-tion
with the accompanying drawings, in which:
srief Description of Drawings
FIG. 1 is a series of curves showing SER (Specific Energy
Requirement, Kwhr/lb C2H2 produced) and acetylene yield as
functions of specific solid enthalpy and specific gas enthalpy
at various "frozen" product temperatures, the "frozen" product
temperature being defined as the temperature of the product
stream just before a primary quench.
FIG. 2 is a schematic representation of an electric arc
reactor;
FIG. 3 is a cruve of the mole percent of acetylene
produced as a function of time at specific reaction tempera-
tures;
FIG. 4 is a curve of the fraction of coal mass remaining
as a function of time as the coal has undergone decomposition
within a reaction zone; and
FIG. 5 is a curve of Specific Energy Requirement ~SER)
as a function of time.
Description of the Invention
The invention is applicable broadly to solid carbonaceous
matter such as coal, lignite, plastics, etc. However, the
following discussion will be limited to coal in order to
simplify matters.
If one were to plot the SER as a function of the specific
gas enthalpy and a function of the coal loading (carbon content
of coal per hour per standard cubic foot per minute of hydrogen),
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it would be noted that the production of acetylene from carbon-
aceous matter or coal is heavily dependent on operating para-
meters. The dependency of the SER and yield also changes as
a function of the reaction temperature. The crux of this
invention, therefore, is to pinpoint the operating parameters
which will result in producing high yields of acetylene a-t
low values of SER.
FIG. 1 is a plot of iso-yield curves and iso-SER curves
for two values of "frozen" product temperature plotted on the
coordinates of specific gas enthalpy vs. specific solid
enthalpy. The latter is defined as power input in Kw per
pound carbon flow per hour. The Figure contains lines of
SERs of three and five and acetylene yields of 10 percent and
60 percent at "frozen" product temperatures of 1400K and 1800K.
The curves show that a low SER can be attained even with a
relatively low acetylene yield at the lower temperature.
Likewise, a low SER can be attained even at the high "frozen"
product temperature if the acetylene yield is high. The
respective enthalpies are adjusted by regulating the electric
power, gas feed rate and coal feed rate.
The "frozen" product temperature may, more or less, be
regulated by controlling the reaction time. They are not
merely an independent process parameter, but depend on the
input condition of other process parameters. They also depend
on physical and chemical characteristics of the feed materials
and the reaction system; for example, coals of different
composition will undergo different time-temperature history
under otherwise identical input conditions because they will
experience different time-product composition history. The
maximum "frozen" temperature can be as high as the system
permits, but in order to maintain low SERs with realistic
yields, the temperature cannot be much higher than 1800K.
The process under these conditions proceeds more
efficiently at relatively low pressures, e.g. under 2 atmos-
pheres.
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It is possible from the above consideration and from
FIG. 1 to define an envelope for coal, power and hydrogen
flow rates in terms of specific gas and solid enthalpies.
Assume the lower left half of the boundary of the envelope
is determined by the SER of five at 1400K and the upper right
half by the acetylene yield of 60 percent at 1800K. The
latter limit is chosen because acetylene yields cannot
realistically be more than 60 percent under any circumstances.
The boundary is also drawn at the specific gas enthalpy of
five and the specific solid enthalpy of 2.5 because they are
too far removed from the minimum SER operating conditions
desired. The net result is an envelope which is defined by
the lines joining the x, y coordinates (1.0, 2.5) (2.0, 2.5)
(5.0, 1.5) (5.0, 0.6) (2.0, 0.6) (1.0, 1.25), where x is the
specific gas enthalpy in Kw/SCFM H2 and y is the specific solid
enthalpy in Kw/lb carbon HR.
It is noted that the specific gas and solid enthalpies
can also be expressed in metric units of K cal/g mole of gas
and K cal/grams of carbon by multiplying 0.544 to Kw/SCFM and
1.896 to Kw/lb carbon per hour, respectively. The conversion
may become useful in understanding the process energy require-
ments when the heat input is in the form of thermal energy
instead of the electric energy.
It is also noted that in a commercial embodiment of this
invention, the hydrogen would be recovered from the product
stream and recycled in the process. Therefore, the feed
hydrogen may contain other gaseous species such as carbon
monoxide, water vapor, and hydrocarbons, which may have been
produced as byproducts in the process. In such a case the
specific gas enthalpy noted above would be based on the total
flow of gas including the contaminants.
The invention defined in FIG. 1, th~refore, establishes
the first set of operating parameters in this disclosure.
An analytical model was formulated. The model incorporates
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the basic thermochemistry and fluid dynamics that are
obviously important in the complex interactions within the
high temperature reactor, and additionally includes certain
constraints and assumptions necessary to be consistent with
observed results. The capability to predict the effect of
changes in independent process variables on the critical
output parameters makes the model valuable in directing process
improvement experiments, as well as in scaling the reactor to
larger capacities.
In the formulation of the model the arc coal reactor
was conceptually divided into four zones, as indicated by
the simplified diagram in FIG. 2. Owing to the complex fluid
dynamic and thermochemical processes occurring in the reactor
during its operation, it is necessary to recognize only
those processes which have a first order effect on the
quantity of acetylene produced, thereby simplifying the model-
ing to a more tractable format. This is done by making the
following plausible assumptions.
In the coal dispersion zone, the most important process
is the mixing of the coal-hydrogen feed with the main stream
hydrogen. In this model, it is assumed that all coal
particles are uniformly dispersed in the hydrogen gas by the
time the mixture enters the arc zone.
In the arc zone a rotating arc sweeps through the hydrogen-
coal mixture. Because the arc follows the path of least
resistance, it is reasonable to assume that this path is
provided by the hydrogen gas rather than by a path linked
through the coal particles. In other words, this assumption
implies that the ionized conductivity of hydrogen is much
higher than the conductivity of coal particles. Therefore,
as the coal-hydrogen mixture passes through the arc zone,
the electrical power input heats up only the hydrogen gas and
not the coal particles. The particles, in turn, get heated
by the hot hydrogen gas by conduction and convectlon. Because
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of the short residence time in the arc zone compared to that
in the reaction zone, and also because of the thermal inertia
of the coal particles, it is very likely that the coal
particles remain essentially at their inlet temperature while
the gas reaches its high temperature. In view of the fore-
going, the most important assumption is that in the arc zone
the hydrogen absorbs all of the input power and reaches a
high temperature, which is determined from equilibrium consider-
ations, while the temperature of the coal particles remains
unaltered.
In the reaction zone the hot hydrogen gas heats the coal
particles by conduction and convection. The particles pyrolyze,
and the volatiles produced by pyrolysis mix and react with
the hydrogen. The volatiles thus produced under this rapid
heating condition are composed of species containing essentially
hydrogen and carbon elements and resemble in structure various
functional groups existing in the parent carbonaceous matter.
Above 1400K the hydrocarbon volatiles tend to further pyrolyze
to acetylene and ultimately to carbon and hydrogen unless the
reaction is quenched. Hydrogen is preferred as the reaction
medium because of its high thermal conductivity compared with
most other gases and its possible participation in the forma-
tion reaction of acetylene as it is a constituent of the mole-
cule. It is also known that hydrogen tends to retard the
decomposition o~ acetylene through inhibitory reaction mechanisms
and by diluting the concentration of the molecule in the product
gas.
Thermodynamic equilibrium computations for this reactive
gas mixture indicate that the final composition comprises of
predominantly condensed carbon and hydrogen. Evidently,
acetylene forms through complex transient kinetic routes before
this mixture reaches complete equilibrium.
Overall production of acetylene in the arc coal reactor
is in essence a continual process of formation and decomposition
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of acetylene; the formation rate being somewhat higher than
the decomposition rate. As a result, there always exists the
optimum residence time at which the overall production of
acetylene is maximum and hence the product stream should be
quenched to maximize the net production of acetylene for the
given input condition. In general the quench medium can be
either purely thermal or both thermal and chemieal (i.e.,
additional chemieals are produeed from the quench medium by
absorbing a portion of the sensible heat of the product stream).
If it is a purely thermal quench, the composition of the
quench is immaterial; only its thermal capaeity to reduee the
temperature is important, provided that the resulting acetylene
eoncentration is not too low for eeonomic separation and
purification of acetylene. If it is both thermal and ehemical,
the composition of the queneh is obviously signifieant. For
the moaeling purposes here, the quench is assumed to be purely
thermal and perfect. That is, it is assumed that the composi-
tion of the hot products just prior to the quench is "frozen"
instantaneously by the quench.
As the coal partieles are heated by hot hydrogen, they
deeompose to produce volatiles through complex kinetic routes.
The volatiles are essentially hydrocarbons which are unstable
at elevated temperatures. New compounds are constantly being
formed and deeomposed as the temperature of the volatiles
within the reaetion zone deereases. If the reaetion were
permitted to go to thermal equilibrium, the coal would eventuaIly
form char, condensed earbon, soot, hydrogen, and small amounts
of other hydroearbon speeies. The purpose of the queneh is
to: prevent thermal equilibrium at elevated temperatures;
and to reduee the reaetion produets leaving the reaetion zone
from a temperature whieh favors the produetion of aeetylene
to a mueh lower temperature at which acetylene is stable.
Thus the formation of aeetylene is, in reality, an inter-
mediate step in the thermal deeomposition proeess. Though
it is formed at favourable temperatures, the model and experi-
ments show that aeetylene yield reaehes a peak for a eertain
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optimum residence time. At longer residence times the yield
decreases. The yield decreases because of the degradation of
acetylene to soot and hydrogen that occurs simul-taneously
with its formation. This degradation process is sometimes
called acetylene pyrolysis.
FIG. 3 depicts a curve of mole percent of acetylene
produced as a function of time. The magnitude of acetylene
will change as the operating parameters of the process change;
but the shape of the curve is a general one. The concentration
yield of acetylene reaches a peak in less than one millisecond
and remains high out to at least ten milliseconds. The
concentration and thus the yield then drops off rapid]y.
Having previously described and discussed the benefits
of operating under specified solid and gas enthalpies, a
second important operating parameter is established. The
decomposition reaction should take place in about 0.5 milli-
seconds and should be completed in less than 10 milliseconds
in order to maximize concantration and yield.
Previously, it was noted that as the coal particles heat
up, they tend to decompose. It was also noted that the yield
of acetylene is directly related to the volatile yield. It
follows, therefore, that it will be necessary to achieve
maximum decomposition in a time period of about 0.5 milli-
seconds, but in any event, less than ten milliseconds.
The arc-heated hydrogen is initially heated to high
temperatures, but then rapidly cools as it heats and reacts
with the coal particles. The coal particles, on the other hand,
are heated until they achieve the gas temperatura. Under
conditions wherein the heat content or enthalpy of the gas and
coal operated within the envelope defined previously and shown
in FIG. 1, it has been determined that particles smaller than
~0 microns will reach the gas temperature in less than one
millisecond, while larger particles heat more slowly producing
less volatiles.
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The relation between volatile yield and particle size
is illustrated in FIG. 4. These curves are based on a
specific coal sample where the volatile content was 40%
based on the ASTM proximate analysis. Coal having 18% proximate
volatile content or more is considered suitable for conversion
to acetylene. A typical particle size distribution by weight
is disclosed in the following examples. The data shows that
particles below 33 microns in diameter yield close to the
maximum of 80% of their original weight leaving 20% of their
weight as remaining char. The larger particles yield pro-
gressively less volatiles for the larger sizes until, as the
Figure shows, particles of about 190 microns only produce
volatiles commensurate with the proximate analysis of the coal.
In particular, it will be noted that there is a perfect
correlation between the time interval needed for maximum
acetylene yield and the time interval available for maximizing
volatile yields if the coal particle size is maintained b~low
190 microns and preferably in the neighborhood of 25 to 50
microns.
Test data developed in test runs are provided in the
following examples. Examples 1 and 2 define conditions outside
of the envelope defined in Figure 1. Examples 3 and 4 represent
conditions within the envelope. The test data in Example 4
includes quenching.
EXAMPLE 1
A direct current electric arc reactor in which the d.c.
arc was made to rotate at 1000 - 3000 revolutions per second
by the action of an external magnetic field provided by the
magnetic coils surrounding the arc discharge zone outside the
reactor was used to produce acetylene from coal. The reactor,
a schematic of which is shown in FIG. 2, is capable of operating
at power inputs ranging from 50Kw to 150Kw. The reactor wall
was constructed of a copper tube which was cooled by water
circulating at high velocity in the annular gap between the
copper tube and its external jacket.
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Finely pulverized coal with a particle size distribution
of 2% by weight in 6 microns range, 18% in 13 microns, 20%
in 22.5 microns, 20% in 33 microns, 20% in 50 microns, 18% in
81 microns and 2% iII 190 microns ranges were injected through
the arc discharge zone with hydrogen as a carrier gas. The
coal was a high volatile bituminous coal containing 38% by
weight of volatile matters by proximate analysis. The ultimate
analyses of the coal are as follows: (dry bases) 77.3% by
weight carbon, 5.1% hydrogen, 8.4% oxygen, 1.4% nitrogen, 1.8%
sulfur and 6.0% ash.
The reactor was operated at a subatmospheric pressure with
the following input condition: power 148.8 Kw, coal 601b/hr,
and hydrogen 27.7 standard cubic feet per minute (SCFM). The
corresponding specific solid and gas enthalpies are, respect-
ively, 2.5 Kw/lb coal per hour and 5.4 Kw/SCFM H2. No active
quench material was injected and the product stream was cooled
through the water cooled reactor wall. The total residence
time in both the reaction and cooling zones was approximately
5 milliseconds.
It is noted that a reactor of this configuration does
not have a clear distinction between the reaction zone and
the cooling zone as the cooling of the reaction stream starts
as soon as the reaction commences. However, it is estimated
by heat transfer calculations and energy balance that the
reactor is about 75% energy efficient; i.e., 75% of the power
input was taken up by the reagents and the balance by the
reactor cooling water. The "net" specific solid and gas
enthalpies were then 2.4 Kw/lb carbon per hour and 4.0 Kw/SCFM,
and thus the input condition was situated just outside the
envelope defined in FIG. 1.
The reactor output stream under this input condition
contained 8.0% C2H2 by volume, and the acetylene yield was
17.5 lb. per 100 lb. of coal fed. The gross SER based on the
power delivered at the electrodes was 14.2 Kwhr/lb C2H2.
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EXAMPLE 2
The same reactor of Example 1 was operated with 60 Kw
power, 63 lb. coal/hr. and 30 SCFM H2. The corresponding gross
specific solid and gas enthalpies were 0.95 Kw/lb. coal per
hour and 2.0 Kw/SCFM H2, and the "net" specific solid and gas
enthalpies assuming 75% reactor energy efficiency were 0.92
Kw/lb carbon per hour and 1.5 Kw/SCFM H2, respectively. Thus,
the input condition lay just outside the lower le~t-hand
boundary line of the envelope defined in FIG. 1.
The reactor output under this input condition was as
follows: 7.8% by volume C2H2, an acetylene yield of 14.7 lb.
C2H2/100 lb. coal fed, and an SER of 6.5 Kwhr/lb. C2H2.
EXAMPLE 3
The same reactor of Example 1 was operated with 73.4 Kw
15 power, 63.0 lb coal/hr. and 27.5 SCFM H2. The corresponding
gross speeifie solid and gas enthalpies were 1.17 Kw/lb eoal
per hour and 2.67 Kw/SCFM H2, and the "net" specific solid
and gas enthalpies assuming a 75% reactor energy effieieney
were 1.13 Kw/lb earbon per hour and 2.0 Kw/SCFM H2. Thus,
the input eondition lay inside the envelope defined in FIG. 1.
The reaetor output under this eondition was as follows:
aeetylene eoneentration of 9.1% by volume, an aeetylene yield
of 23.6 lb C2H2/100 lb. eoal fed, and an SER of 5.0 Kw/lb.
C2H2 .
EXAMPLE 4
The same reaetor of Example 1 was operated with 88.0 Kw
power, 93.1 lb. eoal/hr. and 33.4 SCFM H2. In this example,
fine spray of water was injeeted six inehes below the are
diseharge zone, thus limiting the total reaetion time to two
milliseeonds. The eorresponding gross speeifie solid and gas
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enthalpies were 0.95 Kw/lb. coal per hour and 2.6 Kw/SCEM H2,
and the "net" reactor energy efficiency was 1.22 Kw/lb.
carbon per hour and 1.98 Kw/SCFM H2. Thus, the input condition
lay inside the envelope defined in FIG. 1.
The reactor output under this condition was as follows:
acetylene concentration of 9.2~ by volume, an acetylene yield
of 20.3 lb. C2H2/100 lb. coal fed, and an SER of 4.7 Kwhr/lb.
C 2H2 '
FIG. 5 illustrates the effect of power variation on SER
while hydrogen and coal feed rates are held constant at the
experimentally-found optimum conditions. It is interesting to
note that although the power input was varied from 50 to
150 Kw, the experimentally-detailed power input at 75 Kw gives
the theoretical minimum SERo According to the model calculation
for a hypothetical 100 percent energy-efficient reactor,
calculated minimum SER is slightly below 3 Kw hours per pound
acetylene and occurs in about 0.5 millisecond residence time.
In summary, therefore, the process of decomposing coal
in high temperature hydrogen for producing acetylene turns
out to be highly complex. Three operating parameters have
been described for improving the yield of acetylene to reduce
the specific energy required. These operating parameters,
taken alone or in combination, were found through calculations
and experimental data to be highly effective.
Taking into consideration the conditions shown in the
envelope of FIG. 1, with the minimum SER points on the curves
shown in FIG. 4 and the particle size distribution shown in
FIG. 5, it is clear that the preferred residence time is from
0.5 to 5 milliseconds with a preferred range of particle sizes
of 25 microns to about 50 microns.
The various features and advantages of the invention are
thought to be clear from the foregoing description. Various
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other features and advantages not specifically enumerated will
undoubtedly occur to those ~ersed in the art, as likewise
will many variations and modifications of the preEerred embodi-
ment illustrated, all of which may be achieved without
departing from the spirit and scope of the invention as defined
by the following claims:
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