Note: Descriptions are shown in the official language in which they were submitted.
s~9
BACKGROUND OF THE INVENTION
This invention relates to a process for removing
sulfurous by-product gases effluent from streams such as the
stack gas emissions from elevated temperature chemical
processes.
Sulfurous gases are presently a major atomspheric
pollutant. Many elevated temperature chemical processes
result in effluent gases containing sulfurous materials as
by-products of the reaction. Such sulfur-containing gases
are present a-t the output of many industrial processes,
particularly those which utilize coal, oil or other fossil
fuels as raw materials. Removal of the sulfur content from
the output of such processes, or its reduction to environ-
mentally acceptable levels has thus far proven to be
economically prohibitive.
On the other hand, the presence of sulfurous
gases in the effluents of many important industrial processes
- can be highly deleterious. If vented directly to a stack,
they cause atmospheric polIution which becomes objectionable
when such gases are present in the stack in a concentration
greater than a ~ew tenths of one percent. Often the process
effluent is used to feed another process in which practical
yields require the use of catalysts. Sulfurous gases, even
in small percentages, are notorious for their tendency to
poison catalystsO For such processes the removal of
sulfurous gases prior to contact with the catalyst is
essential.
In order to avoid problems with sulfurous gas
emissions, the current practice is to utilize purified
~eedstocks from which the sulfur content has been partially
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to substantially totally removed, or to utilize a chemi~al
scrubber plant to remove sulfur-containing gases directly
from the effluent. Both solutions, however, involve major
costs and constitute a serious drawback to the use of
sulfur-containing raw materials. Examples of some industrial
processes for which sulfur impurities are difficult or
impossible to avoid either in the process feed or 1n stack
emissions, are presented in Table I.
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- OBJECTS OF THE INVENTION
~ n object of the pi^esent invention is the developm~nt
of an inexpensi-~e method to remove sulfurous by-product
constituents from the gaseous emission of elevated temperature
chemical processes.
Another object of the present invention is the
development of a process for removing sulfurous by-product
gases from high temperature reaction products of chemical
processes consisting essentially in:
tl) passing a simple or complex metal oxide in the form of
a coarsely divided powder entrained in a conveying gas
through a high energy transfer zone whereby said powder
is subjected to temperatures sufficient to vaporize said
powder and said powder is vaporized to form a hot effluent
stream containing metal oxide ~apor,
(2) injecting said hot effluent stream containing metal
oxide vapor into a gaseous reaction product, including
sulfurous by-product gases, from an elevated temperature
chemical process,
(3) allowing said metal oxide vapor to condense to solid
particles with reaction ~ith said sulfurous by-product
gases, therebY fixinq the sulfur content in the form
of solid partlcles, and
(4) separating said solid particles containing substantially
all of said sulfur content from said gaseous reaction
product.
These and other objects of the present invention
will become more apparent as the description thereof proceeds.
THE DRAWINGS
Figure 1 is a flow diagram of one process of the
lnvention.
Figure 2 is a graph o~ the effect of arc vaporized
~6~i~9
ash on the SO2 concentration in the effluen-t stream.
DESCRIPTION OF T~-IE INVENTION
We have now found a method of desulfurizing the
gaseous process effluents from elevated temperature chemical
processes which is both practical and efficient, and which
is far less costly than conventional scrubbers. In essence,
this method consists of injecting into the sulfur-containing
gas stream the effluent of a high energy transfer zone, such
- as an electric arc device, to which is fed a convenient carrier
gas with entrained solids of a particular kind~ These consist
of either simple metal oxides or more complex derivatives
thereof, e.g. metal carbonates or silicates or multiple
oxides representing combinations of two or more simple oxides.
More particularly, the present invention involves
a process for removing sulfurous by-product materials from
the high temperature reaction products of chemical processes
consisting essentially in:
(1) passing a simple or complex metal oxide in the form
of a coarsely divided powder entrained in a convenient
conveying gas through a high energy transfer zone
whereby said powder is subjected to temperatures
sufficiently high to vaporize said powder and said powder
is substantially completely vapori~ed to form a hot
effluent stream containing metal oxide vapor,
(2) injecting said hot effluent stream containing metal
oxide vapor into a gaseous reaction product, including
sulfurous by-product gases, from an elevated temperature
chemical process,
: (3) allowing said metal oxide vapor to condense to ultra-
fine, reactive solid particles which then combine with
said sulfurous by-product gases, thereby fixing the
sulfur content in the form of solid particles, and
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(4) separating said solid particles containing substantially
all of said sulfur content from said gaseous reaction
product.
In order to carry out this process, it is necessary
merely to inject into the process reactor and/or the effluent
duct from the process reactor, a jet of a hot effluent stream
containing metal oxide vapor, the latter being derived by
entraining a relatively coarse powder of either simple or
complex metal oxides, entrained in a suitable working gas,
such as air, CO, and/or H2 etc., and injecting said entrained
coarse powder into and through a high energy heat transfer
zone, e.g., an induction furnace, a solar furnace, a resistance
furnace, arc plasma generator, etc., whereby said entrained
coarse powder, if of a complex molecular composition is
decomposed into simple oxides, and said simple oxides are
vaporized at temperatures usually in excess of 2000K to form
a hot effluent jet. -The oxide vapor is then allowed to
condense into an ultra-fine, highly reactive fume inside the
process reactor or duct therefrom where the ambient temperature
and other conditions favor a reaction between the oxide fume
and the sulfur gases, forming thereby solid particles of metal
sulfides, sulfites or sulfates, as the case may be, which
solid particles are thereafter removed from the gas stream
by conventionally known means, such as bag filters, agglo-
merator and cyclone, electrostatic precipitator, etc.
Owing to the treatment of the sulfur fixing
oxides to very high temperatures, generally in excess of
2000X and preferably higher temperatures, for example,
~2500K, it becomes possible ~o utilize almost any type o
metal oxide including waste mineral material.
One of the most effective ways of obtaining this hot
ef~luent stream containing metal oxide vapor is to employ the
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methods and device described in our prior United State~ .
Patents 3,644,781 and 3,644,782. These patents disclose the
device and process for energizing a fluid medium.. containing
an entrained condensed phase, such as a powder of a simple
or complex metal oxide, by means of an arc discharge between
an anode and a cathode having a conical tip, said arc
discharge forming a contraction of the current-carrying area
in the transition region in the vicinity of said cathode, the
. points of inflection of said contraction of the current-
carrying area forming, when extended, an angle a, which
comprises forcefully projecting a fluid medium along said
conical tip of said cathode into and through said contraction
of the current-carrying area in the transition region in the
vicinity of said cathode at a mass flow density at substantially
constant convection rate which is at least sufficient to
effect a rise in the temperature of said arc column at a
constant current level and below a total fluid medium
convection rate at substantially constant mass flow density
which is sufficient to reduce the angle ~ below 40 at a
constant current level, and adding a finely dispersed
nongaseous medium capable of causing an enlargement in the
angle ~ to said fluid medium forcefully projected along said ` ~:
- conical tip at a total fluid medium convection rate at
substantially constant total mass flow density which is below
that sufficient to reduce said enlarged angle ~ below 40 at a
constant current level~ This process is commonly called a
forced convection cathode,
When conducting the presently claimed process
utilizing the device and methods of United States Patent
3,644,7~1, it is important that the entrained condensed phase
of a powder of a simple or complex metal oxide be made to
penetrate the central core of the arc column as completely as
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possible, so as to be caused thereby to become vaporized
and, if applicable, broken down into two or more simple
oxides, before exiting from the arc discharge. This type
of treatment has been shown (U.S. Patent No. 3,644,781) to
be effective in vaporizing the major portion of the solid feed
when properly injected into the conduction column of the arc.
When metal oxides are so vaporized, they condense
very rapidly into a fume upon leaving the arc dischargeO
It is on the peculiar physical and chemical properties of
this fume that the sulfur fixing capability of the claimed
process dependsO For example, refractory materials, such
as most metal oxides, a~ter being vaporized in an electric
arc, condense into ultra-fine particles with sizes typically
in the range of lO0 to lO00 Angstrom units. Thus a small
quantity of such solids become possessed of an enormous
suxface area, typically in the range of 50 to 300 square
meters per gram. Such large surface area provides for
effective absorption of a gaseous constituent of a gas stream
into which the fume is dispersed. Secondly, when the arc
feed consists of complex compounds, such as clay or shale,
- the process of vaporization causes chemical breakdown of
the complex moLecules. Further, the extremely rapid rate of
refractory vapor condensation in the arc effluent favors
- recombination in the form of chemically reactive simple oxides,
as illustrated for the case of feldspar:
KaO A1203 6SiO2 ~ K20 ~ A1203 + 6Sio2
While the complex silicate molecule is quite stable
and will notreact with sulfur gases at the common processing
temperatures (e.g., 1000F to 3000F)~ the simple oxides, such
as K~O and Al~03, and indeed a large variety of other oxides,
including those of the alkali and alkaline earth metals, react
~rapidly with sulfur yases at these temperatures. Finally the
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rapid condensation of the super-cooled vapors issuing ~rom
the arc column causes the particles to congeal with highly
active surface properties, such that even normally non-reactive
materials, such as sio2 ~ become effective in surface adsorption
and are capable of assisting in removing sulfur gases from a
- process stream.
It should be emphasized that the above-described
properties, namely, ultra-fine particle size, high chemical
reactivity and enhanced surface adsorptivity, are characteristic
of vaporized materials only, and, more particularly, of
materials vaporized in a high energy transfer zone, such as
an electric arc.
It is believed that the desulfurizing action,
which has been observed in the effluents of reactions involving
coal or petroleum feed stocks are principally due to chemical
reactions between metal oxide compounds, in an ultra-fine
reactive state and the sulfurous gases in the effluent. These
are illustrated by the following reactions for both reducing
and oxidizing atmospheres:
(1) Reducing atmosphere (sulfurous gas - H2S)
H2S ~ MeO ~ MeS~ + H20
(2~ Oxidizing atmosphere (sul urous gas - SO2)
S02 ~ MeO + ~02 ~ MeSO4~
where "Me" stands for any metal, assumed here to be bivalent.
In both illustrations a sulfurous gas reacts with
a metal oxide to produce a sulfur compound which is a solid
at the prevailing reaction temperatures. The solid sulfur
compounas are then removed from the gas stream by conventional
means, e.g., a bag house or electrostratic precipitation.
3~ Among the desulfurization processes at present
under investigation is the use of fully or partially calcined
limestone or dolomite to react with H2S or SO2, in accordance
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with reactions (1) or (2) above, where Me stands for C~
for limestone and Ca or Mg or dolomite. These oxides, for
example, in a finely comminuted s~ate, are used in a
fluidized bed combustor together with high sulfur coal for
power generation. Temperatures in the firebox of the steam
boilers are typically in the range of 2000F to 2300F. For
these conditions, with calcined limestone and excess air,
reaction t2) proceeds and the amount of SO2 fixed as CaSn4
is sufficient to reduce the residual SO2 in the stack to
acceptable levels (~1000 PPM). However, the kinetics of
this reaction in the operating range of temperatures is
such that satisfactory desulfurization requires large amounts
of lime to be processed per ton of coal burned, and involves
plant equipment of major proportions and high capital cost.
It is also pointed out that the finely-comminuted
- oxides employed have particle sizes which are 10 2 to 10 3
times greater than the fume condensate of the vaporized
material of the present invention.
On the other hand, arc vaporized lime injected
into the firebox of a conventional pulverized coal burner
is effective in reacting with the SOz at the same temperature
with considerably greater speed, owing to the extremely fine
- particle size and high degree of reactivity of the arc
vaporized material, so that virtually quantitative conversion
of SO2 to CaSO4 will occur during the normal residence time
- of the gases in the firebox. As a consequence, the amount of
lime used to desulfurize is reduced and, mo,re importantly,
the size and cost of the auxiliary desulfurization equipment
is greatly decreased.
30 The simple or complex metal oxides employed in
the process of the invention can be any type as indica~ed
above. When desulfurizing the combustion gases from the
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burning of a high sulfur coal, ~ waste mineral matter in
the form of coal ash can be employed for this purpose.
Coal ash generally consists of clays and shale, which are
inert as sulfur absorbents. In the high temperature zone,
however, the stable complex silicates are decomposed into
reactive simple oxides which then become effective in sulfur
gas removal. Thus a waste material is available at the site
as the scrubber feed, lowering process cost considerably.
- Also the waste disposal problem is not agyravated, as is
'~ 10 the case when large amounts of limestone or dolomite have
to be transported to and from the site.
The following specific embodiment illustrates the
practice of the invention without being limitative in any
; manner.
EXAMPLE
I DESULFURIZATION PROCESS
. ~
The apparatus was assembled as shown in Fig. 1.
~` The fluid convection cathode 1 is a double-shrouded cathode
; as disclosed in our U. S. Patent No. 3,900,762. This is fed
through line 2 by an oxide powder entrained in a carrier
gas. The carrier gas is passed from line 3 through the mixer
valve 4 where oxide powder from the feeder 5 is mixed with the
carrier gas. The positive terminus is three anodes 6 consisting
of fluid transpiration anodes as described in our U. S. Patent
No. 3,209,193, arranged symmetrically about an extension of
the cathode axis as described in our U. S. Patent No.
3,931,542. The hot effluent stream containing metal oxide
vapors is collected in a cowl 7 and passed to a mixing area 8
to~ether with SO2 which is inserted at point 9 after passage
3~ from the SO2 tank 10 through line 11, valve 12 and flowmeter
13. At the upper end of mixing area 8, the temperature-is
measured b~ a thermocouple 14. The effluent is then passed
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,
through a reactor section 15 where at a point 16 therein, a
sample of the effluen~ gas is collected by line 17, filtered
through a solids filter 18 and the SO2 content is measured
in a detector 19. A positive flow is maintained by the
action of the blower 20 and the gases are then delivered to
the stack 21.
The oxide powder used was the fly ash from a
commercial coal-burning power generating station. This
consists largely of complex silicates such as shale and clay,
which contain a large percentage of suitable metal oxides.
The ash was placed in the hopper of a vibratory powder feeder 5
and fed through a control valve 4 to the high energy transfer
zone, which in this case consisted of the column of an
electric arc. The type of arc system preferred for this
application featured a double-shrouded fluid convection cathode
(FCC) 1, as disclosed in U. S. Patent No. 3,900,762. The
positive terminus consisted of three fluid transpiration
anodes (FTA) 6 (see Patent No. 3,209,193) arranged symmetrically
about the extension of the cathode axis, as disclosed in U. S.
Patent No. 3,931,542.
The arc effluent, along with tempering air from
the surrounding atmosphere, was drawn into a mixing sector 8
and reactor section 15, via a cowl 7 si-tuated directly above
the arc, by the action of a blower 20 downstream of the
reactor section. The mixing section 8 consisted of four foot
long by 2" ID stainless steel pipe. Sulfur dioxide gas was
introduced into the mixing section 8 just downstream of the
co~l 7 at point 9, the flow being controlled by valve 12 and
measured by a flowmeter 13. The operation of the blower 20
was adjusted to pull approximately 40 cubic feet per minute of
gas, the latter consisting of a mixture of the arc effluent
and tempering air, plus the S02 introduced at point 9. The
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~.Q~658~
volume flow rate through the rnixing section w,as adjusted to
achieve an exit temperature of about 2200F, as determined
by a thermocouple 14 located at the end of the mi~ing section
8. The flow of SO2 into the pipe at point 9 was adjusted
; relative to the amount of gas drawn into the cowl, so that thegas sampled at point 16 near the downstream end of the reactor
section 15 read 1800 parts per million on the SO2 detection
meter 19.
Once the SO2 flow was es,tablished, the arc was
ignited and adjusted so tha-t 70 grams per minute of argon were
fed to the inner shroud and about 2 standard cubic feet per
minute were fed to the outer shroud of the double-shroud
fluid convection cathode 1. Simultaneously 15 grams per minute
; of argon were fed to each of the three porous FTA anodes 6.
The arc gap was adjusted to 2-1/2'inches and the current and
voltage were 600 amps (total) and 130 volts, respectively.
Next the outer shroud argon was replaced with approximately
2.5 SCFM of air, following which the arc voltage rose to 160
volts.
As soon as the outer shroud argon was replaced by
air, the SO2 meter reading rose to about 2000 parts per million
and then fell within a minute or two to about 1700 ppm, where
it remained steady. The arc was ~aintained in this condition
for 3 or 4 minutes while the temperature of the gas stream at
14 rose to a steady value of 2200F. The SO2 meter 19
continued to read 1700 ppm. Then the ash feeder was turned
on and adjusted to feed 38 grams per minute of ash. This was
entrained in the air stream fed into the FCC outer shroud.
A~ incandescent effluent flame of vaporized ash components
was immediately visible entering the cowl above the arc.
After about a half minute the SO2 meter 19 began to fall.
Readings taken, starting with the instant the ash was fed to
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the arc, are plotted in the curve of Figure 2, Within three
minutes, the concentration of SO2 in the gas sampled at
point 16 dropped to less than 200 ppm. This low reading
remained until the ash feed was stopped and the arc was
tur~ed off, after which the SO2 reading rose again to a value
of 1800 ppm.
This proves conclusiyely that arc vaporized ash
is effective in removing substantiall~ all of the SO2 from a
gas stream containing 1700 ppm of SO2.
The preceding specific embodiment is illustrative
of the practice of the invention. It is to be understood,
however, that other expedients known to those skilled in the --
art or disclosed herein may be employed without departing
from the spirit of the invention or the scope of the appended
claims.
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