Note: Descriptions are shown in the official language in which they were submitted.
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CROSS REFI.RENCE iO_REI.A~ED APPLICArlONS
The present application is related to co-pending
Canadian Application No. 267,304, filed December 7, 1976.
BACKGROUND OF THE INVENTION
1. _Field of_the_Invention
The present invention relates generally to the treatment
of exhaust gases for discharge to the atmosphere, and more
particularly to methods and apparatus for treating and recovering
energy from hot exhaust gases.
Exhaust gases suitable for treatment by the system of the
present invention include combustion exhaust gases produced in
fuel burning furnaces, roasters and the like, exhaust gases such
as those produced in cement kilns and the like, and exhaust gases
containing such components as nitrogen, carbon dioxide, carbon
monoxide, hydrogen chloride, hydrogen sulfide, hydrocarbon gases,
and the like. Preferably, the exhaust gases are essentially inert
but include noxious components and traces of combustible gases.
_ Prior Art
Hot exhaust gases generated during the combustion of fuel
have commonly been disposed of by exhausting them to atmosphere
through tall chimneys or stacks. Disadvantages of this method
of disposal include resulting air pollution and its harmful
effects on the environment, a waste of recoverable heat energy,
and the high cost of constructing and maintaining tall stacks.
Loss of recoverable heat energy is unavoidable because gases
discharged into a stack must be substantially hotter than ambient
air to produce an up-draft in the stack and to avoid condensation
in the chimney. Moreover, the latent heat of steam in flue gases
is not generally recovered in order to avoid condensation and the
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attel~dant corrosion, as a result of which additional, available
heat energy is being wasted.
Where the latent heat of steam is not recovered, the system
designer must work with "low heating values" of the gases rather
than "high heating values". Low and hlgh heating values for gases
are given in such handbooks as the John N. Perry Engineering_Manual,
published in 1959 by McGraw Hill, where the following typical
heating values are g-iven:
High Heating Low Heating
Gas _ _ Value _ _Value__
Hydrogen 69,958 Btu/lb 51,571 Btu/lb
Methane 23,861 Btu/lb 21,502 Btu/lb
Methyl alcohol 10,270 Btu/lb 9,080 Btu/lb
(vapor)
As will be apparent from these heating values, about 18 percent
more Btu/lb can be recovered from hydrogen if its high heating
value can be utilized, about 11 percent more from methane, and
about 13 percent more from methyl alcohol vapor. Prior systems
have not been able to utilize the high heating value of such gases.
As the public concern about air pollution has increased,
stack heights have been increased to affect better dispersion of
pollutants. However, increasing stack height adds to the cost of
constructing and maintaining stacks, yet provides no solution to
the underlying problem, i.e., avoiding emission in the first
instance of harmful substances such as sulfur oxides, chlorine
gases, phosphor oxides, etc.
A significant factor in air pollution is the increasing
level of gaseous airborne pollutants which combine with moisture
in the air to produce acids, e.g. carbon dioxide, sulfur dioxide,
chlorine and fluorine. The carbon dioxide content in some
industrial districts is as high as ten times normal. Acid
30 forming pollutants have been found in some instances to increase
the acidity of rainwater from its normal pH of about 6.9 to
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values of 4Ø Rainwater having a pH of 5.5 or less will destroy
aquatic life and can do substantial harm to buildings, monuments,
and other structures.
One proposal for removing acid forming components from exhaust
gases is to scrub the entire flow of exhaust gases with water
prior to discharging them through a stack. EEowever, scrubbing the
entire exhaust gas flow requires large quantities of water, which
are not always available, and requires costly, large capacity
scrubbing equipment. Indeed, scrubbing the entire flow of exhaust
gases from some incinerators requires at least half the amount of
water, by weight, of the solid wastes burned in the incinerator.
Treating the large volume of scrub water needed in such a process
is very costly and contributes to the impracticality of scrubbing
as a total solution to the acid pollutant problem.
Another different pollutant to deal with effectively is
sulfur in tlle flue gases. One proposal for the desulfurization
of flue gas utilizes a series of heat exchangers to extract heat
energy from the flue gas prior to a scrubbing operation. Heat
extracted from the gas is returned to the gas following desulfur-
ization and the gas is exhausted through a tall stack for diffusion
into the atmosphere. This proposal has the disadvantages of
wasting heat energy recovered from the gases, requiring large
volumes of scrubbing water, requiring the use of a tall stack, and
polluting the air with such noxious components as are not removed
during scrubbing.
The problem of disposing of exhaust gases is now recognized
as a major concern in industrial countries throughout the
world. Dispersing emissions through the use of tall stacks
is no longer regarded as an acceptable solution. Applicant's
U.S. Patent 3,970,524 discloses a system for gasification of
solid waste materials and a method for treating the resulting
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gases to produce commercially useable gases in such a manner
that dispersion through stacks is not necessary. A feature
of one embodiment of this patent is pressurization of a combustion
zone to such pressures as will permit blower and compression units
to be eliminated from the gas treatment system. Another feature
is the use of a multichamber gas treatment unit in which noxious
gas components are sublimed or "frozen out" and thereby separated
from the clean useable gas components. A problem not addressed
by U.S. Patent No. 3,970,524 is that of providing a system for
treating combustion exhaust gases and productively reclaiming
heat energy from the hot gases. This problem is, however,
confronted in applicant's copending Application No. 267,304,
filed December 7, 1976. In this application, in addition to
purifying the gases, the sensible and latent heat of the gases is
transferred to a power fluid which is in indirect heat exchange
relationship therewith, as in a conventiona] heat exchanger.
However, the economics of indirect heat exchange is very poor and
reduces the over-all desirability of such a system.
SUMMARY OF THE INVENTION
2~ It is therefore an object of the present invention to
overcome the foregoing economic and other drawbacks of the prior
art, and to provide unique and improved methods and apparatus for
purifying exhaust gases to remove harmful components and for
recovering heat energy therefrom.
Another object is to provide unique and improved methods
and apparatus for treating exhaust gases to permit their
discharge directly to atmosphere without the need for tall
chimneys or stacks.
In one particular aspect the present invention provides
a method of treating hot exhaust gas, comprising the sequential
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,teps of:
(a) removing particulate matter from the gas;
(b) cooling the gas by direct heat exchange with heat
capacitant solids material to efEect a separation of less
volatile components, while compressing the gas prior to the
completion of cooling;
(c) productively reclaiming a portion of the heat
energy transferred from the gas to said heat capacitant
solids material; and
(d) discharging the more volatile components to atmosphere
and recovering the less volatile components.
In another particular aspect the present invention
provides an apparatus for treating hot exhaust gas comprising,
in combination:
(a) separator means for removing particulate matter
from the hot gas;
(b) cooling means housing heat capacitant solids material
for separating the gas into less volatile and more volatile
components;
(c) compression means for compressing said gas prior
to the-completion of cooling by cooling means (b);
(d) discharge means for discharging the more volatile
components to atmosphere; and
(e) recovery means for recovering the less volatile
components.
One noteworthy advantage of the various systems of the
present invention is that they obviate the need for costly
stacks. ~nother advantage of the
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prcsent invent;on is that the systems consume only a small
fraction of their power output as compared with conventional
systems which utilize up to 10% of their power output. Still
another advantage is that the systems of the present invention
utilize sublimation or "freezing out" processes to separate out
harmful gas components which can then be recovered and treated
or neutralized, as by scrubbing, with far less water than would
be required if the entire flow of exhaust gases were to be scrubbed
as in prior proposals. The small volume of scrub water required
for this operation can be treated at minimal cost with scrubbing
equipment having a much smaller capacity than is required where
the entire flow of exhaust gas is scrubbed. Substantial savings
are achieved over prior processes inasmuch as large capacity
scrubbing equipment is not required. The ability to utilize
smaller capacity equipment is important also from the standpoint
of minimizing the amount of expensive corrosion resistant material
needed. As is well known, all scrubbing systems experience a
severe corrosion problem requiring the provision of expensive
corrosion resistant materials. In the present systems, where
small scale rather than large scale equipment can be used due to
the limited scrubbing volume, the amount of expensive corrosion
resistant material needed is minimized.
Gas treatment methods and apparatus of the type disclosed
in U.S. Patent No. 3,970,524 are used to effect a separation of
harmful, less volatile exhaust gas components by the sublimation
or "freezing out" process. The apparatus includes an arrangement
of valve interconnected, packed, refrigerated towers through
which exhaust gas passes to effect sublimation or "freezing out"
of harmful components. Components which can be removed by this
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process include C02, HCl, H2S, S02, C2112, N0x, 1~C~, S03,
and the like. It is noteworthy that the vast majority of the
gas treatment process is of a physical nature. Chemical treatment
is not utilized until noxious gas components, which comprise
only a small fraction of the total gas flow, are separated out.
A particularly useful aspect of the present invention is that it
permits noxious gases from many sources to be treated concurrently,
thereby obviating the need for several separate gas treatment
apparatus installations. Off gases from refinery equipment and
the like can be collected and transferred through a sewer-like
system of conduits and treated at a single installation with
apparatus embodying the invention.
Another advantage of the system of the present invention
lies in its capacity for recovery and utilization of heat energy.
In certain embodiments, this heat energy is used to operate an
external combustion or Rankine cycle engine. U.S. Patent No.
3,702,534 describes one such engine and discloses suitable power
fluids for use therewith and with the system of the present
invention. In another embodiment of this invention the heat
energy is recovered by direct mixing of either the hot flue gas
or the hot purified gas with a power fluid. The mixture is used
to operate an expansion turbine and generator to furnish power to
the system or for other purposes.
Inasmuch as the system of the present invention provides
a relatively simple and inexpensive method of purifying flue
gases, it also permits the use of cheap fuels having a relatively
high sulfur content. The savings which result from the use of
cheaper fuels, the elimination of tall stacks, the ability to
recover energy from the gases, the elimination of large uses of
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scrub water, and t~le reduction in si~ in requiring scrubbing
equipment make the system economically attractive for installations
of a wide range of sizes. Moreover, where the exhaust gases being
treated contain a relatively high concentration of sulfurous
compounds, elemental sulfur and/or sulfuric acid may be obtained
from the compounds, thereby adding to the economy of operation of
the system.
In a preferred practice oc the present invention, exhaust
gases are generated in the firebox of a combustion system, and
the firebox is operated under sufficient pressure to obviate the
need for blowers and compressors in the exhaust gas treatment
system. By pressurizing the combined combustion and gas treatment
system with a compressor upstream of the combustion system, the
need for compression equipment downstream from the combustion
system is eliminated. ~owever, as a practical matter, where
large gas volumes are generated, the combustion system cannot
maintain much of a positive pressure and at least one downstream
compressor is generally necessary.
BRIEF DESCRIPTION OF TI~E DR~WINGS
_ _ _ _ _ . _ _ _ _ _ _ _
A fuller understanding of the invention may be had by
referring to the following description and claims taken in
conjunetion with the accompanying drawings in which:
FIGURE 1 is a schematic flow diagram of a system for
practicing one embodiment of the present invention;
FIGURE 2 is a schematie flow diagram of a system for
praeticing another embodiment of the present invention;
FIGURE 3 is a schematie flow diagram of a system for
practicing still another embodiment of the present invention; and
FIGURE 4 is a schematic flow diagram of a typical Rankine
cycle engine which may be used to recover the heat energy from
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p~lrified hot gases.
DESCRIPTION OF TIIE PREFERRED F~IBODIME~TS
Referring to FIGURE 1, a combustion or other gas producing
system is indicated generally by the numeral 10. The system 10
can include one or more fuel burning furnaces, roasters, cement
kilns and the like which emit hot exhaust gases as a product of
fuel combustion and/or other chemical process which discharge hot
exhaust gases containing such components as nitrogen, carbon
dioxide, sulfur dioxide, hydrogen chloride, hydrogen sulfide,
carbon monoxide, nitrogen oxide, hydrogen cyanide, and hydrocarbon
components.
Fuel is supplied to the combustion system 10 as indicated by
an arrow 11. In preferred operation, the fuel used in the system
10 is inexpensive solid or liquid fuel having a relatively high
sulfur content. This fuel is preferred due to its low cost and
because the sulfur content is easily separated out of exhaust gases
as will be explained.
Air or oxygen enriched air is supplied to the combustion
system 10 as indicated by an arrow 12. In preferred practice, a
compressor 13 is used to pressurize the air supply 12 such that
the combustion system operates under pressure. Depending on the
magnitude of the pressure maintained in the system 10, one or
more downstream gas compression units may be eliminated from the
exhaust gas treatment system of the present invention, as will be
explained. In a preferred form of the invention, the combustion
system 10 is operated under sufficient pressure to obviate the
need for blowers and compressors in the exhaust gas treatment
system. By pressurizing the system with a compressor upstream of
the combustion system 10, the need for compression equipment,
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such as optional compressor 45, do-.~nstream from the combustion
system is eliminated. An exemplary preferred pressure in
combustion system 10 which would eliminate the need for
downstream compressors is about 40 psig.
Exhaust gases generated by the combustion system 10 are
ducted, as indicated by arrows 15, 16, 17 to and through a series
of particle separation units 20, 21, 22. The separation unit 20
is preferably a cyclone separator, and particulate matter as small
as 50 microns in size is separated out of the gases, as indicated
by an arrow 24. The separation units 21, 22 house filters which
remove smaller particles as indicated by arrows 25, 26. The
units 20, 21, 22 are insulated to avoid heat loss.
Exhaust gases which have been cleaned of particulate matter
are ducted into a conduit indicated by the numeral 17. The conduit
17 ducts the exhaust gases either into optional heat reclamation
unit 27 (if it is desired to cool the gases and recover the heat
energy therein prior to purification) or into gas feeder conduit
53 for discharge into gas treatment and separation unit 58. If
the heat energy of the exhaust gases is not reclaimed at this
point, and heat reclamation is desirable, then the reclamation
step can be practiced following purification in unit 58. FIGURE
1 shows in phantom the alternative locations of the heat
reclamation unit 27. The details of the unit 27 are illustrated
(in phantom) for a unit located at the discharge of unit 58,
although it should be appreciated that the very same unit can be
located between filter units 21, 22 and gas treatment and
separation unit 58. The following description of the system of
FIGURE 1 hypothetically locates unit 27 immediately downstream
of the filter units.
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Concluit 17 directs the exhaust gases to an injection drum
18. The drum 18 comprises a housing 19 through which exhaust
gases pass before being discharged into conduit 30. As the
gases pass through the housing 19, they are mixed with a power
fluid which is sprayed or injected through spray nozzle 31 into
enclosure 19. The resulting exhaust gas-power fluid mixture
is gaseous and passes out of enclosure 19 into conduit 30. The
power fluid is heavier and has a higher molecular weight than
the exhaust gas and its mixture therewith causes the resulting
mixture to have an increased volume. The power fluid is injected
into the exhaust gas at about ambient temperature and at about
the same pressure as the exhaust gas, causing the gaseous
mixture to be cooler than but at substantially the same pressure
as the exhaust gas. It is desirable to avoid condensation in the
injection drum 18. However, should there be any condensation,
the condensate is withdrawn through drain line 32 and recycled
via pump 33 and recycle conduit 34 to the power fluid spray
nozzle 31.
The conduit 30 ducts the exhaust gas-power fluid mixture to
an expansion turbine 35 in which the gaseous mixture expands and
cools. The work done by the gaseous mixture on the turbine is
manifested as turbine shaft energy. Coupled to the output shaft
of the turbine is a power generator 36. The cooled exhaust gas-
power fluid mixture exhausting the turbine 35 is carried via
duct 37 into condenser 38 where a flow of a heat exchange cooling
medium~ such as water, through coils 39 causes the power fluid
to condense and further cools the exhaust gas. The condensed
power fluid is returned to the injection drum 19 through return
conduit 40, which communicates with the suction side of pump 33,
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and return conduit 34. The cooled ex]laust gas is discharged
from the condenser 38 into a gas feeder conduit indicated by
the numeral 53.
As exhaust gases enter the injection drum 18 they typically
have a temperature of from 150 to 180C. After admixture with
the sprayed power fluid, the gases exiting the injection drum 18
typically have a temperature of from 80 to 100C. After the
gases have been expanded through expansion turbine 35 and cooled
in condenser 38 they are typically at a nearly ambient temperature
of about 20 to 40C. Of course, the temperature of the gases
exiting injection drum 18 depends, to a large extent, on the
amount of power fluid admixed therewith as well as on the
temperature of the power fluid. If only very little power fluid
is sprayed into the exhaust gas, not only will there be only
a small change in gas temperature, but there will be little effect
on gas volume as well. On the other hand, addition of a larger
quantity of power fluid will have a greater effect in reducing
the temperature of the exhaust gas and in increasing the
volume thereof. The amount of power fluid to be added to the
exhaust gas will vary depending upon, among other factors, the
expansion turbine to be used, the pressure in the combustion
system and the nature of the primary fluid.
It is important that the pressure of the gaseous mixture
exiting turbine 35 be of sufficient magnitude to drive the gases
through subsequent processing equipment. Accordingly, it is
generally desired that the turbine exit gases have a pressure
not less than about 10 psig. Inasmuch as a turbine expansion
ratio of at least 2.5:1 to 3:1 is desired, and keeping in mind
that there will be pressure drops across the separator 20 and
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filter units 21, 22, a minimum combustion system pressurè of
about 40 psig seems desirable. This value is, of course, not
invariable and will depend on the system design, the pressure
drops across various items of filter equipment, and the pressure
needed to drive the gases through subsequent processing.
However, a good rule of thumb is that a compressor, such as
optional compressor 45, should be used where the turbine exit
gases have a pressure less than about 10 psig unless the gas
is entering hot regenerat;on.
The gas feeder conduit 53 receives the cooled gases exiting
condenser 38 (or exiting filter unit 22 if heat reclamation unit
27 is either not used or is located at the discharge of unit 58
ducts gases to a gas treatment and separation unit indicated
generally by the numeral 58. The unit 58 is preferably of the
same type described in ~.S. Patent No. 3,970,524 and is operable
to separate the gases into condensable and noncondensable
components by subliming or "freezing out" noxious, condensable
components of relatively low volatility and components having
similar vapor pressures, such as C3 and C4 fractions.
The unit 58 includes three similar packed towers or columns
59, 61, 63. Each of the towers 59, 61, 63 is similar to a
regenerator described by Russell B. Scott at pages 29-31 of
Cryogenic Engineering, published in 1959 by D. Van Nostrand Co.,
Princeton, N.J. Each of the towers 59, 61, 63 contains loose
solids, for example, ceramic balls, quartzite pebbles, steel
shot and other solids having large surface areas and capable
of acting as heat capacitants and being resistant to corrosion.
Automatic switch valves 64a, 64b, 64c, and 65a, 65b,
65c are provided at opposite ends of the towers 59, 61, 63.
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Tower connection conduits 67, 68 comrnunicate the towers 59, 61,
63 with the valves 64a, 64b, 64c and 65a, 65b, 65c.
The gas feeder conduit 53 connects with the valves 64a. An
acid gas conduit 70 connects with the valve 64b. A vacuum pump
79 communicates with the acid gas conduit 70. A transfer conduit
80 communicates the pump 79 with a compressor 81. An acid gas
discharge conduit 83 communicates with the compressor 81. A
purified gas discharge conduit 71 connects with the valves 64c.
A pair of transfer conduits 73, 74 connect with the valves
65a, 65c. A cooling means, which could be a heat exchanger, but,
if gas pressure is high enough is preferably an expansion turbine
75, communicates the transfer conduits 73, 74. An expansion
turbine has the advantage that it produces useful shaft work at
the same time that it cools the gas. To convert the shaft work to
a more useful form of energy, a power generator 76 is coupled to
the drive shaft of the turbine 75.
The manner by which gases are treated in the unit 58 may be
visualized as that of subjecting the gases to several like cycles
repeated time after time as long as exhaust gases are being
produced by system 10. During each cycle, a different step is
conducted simultaneously in each of the towers 59, 61, 63. While
one of the towers is being cooled by a flow of cooled purified
gas, separation is taking place in another tower, and condensed or
sublimed components are being removed from the third tower.
A first step of one cycle is carried out by opening the
valves 64a, 65a at each end of tower 59 and valves 64c at each
end of tower 63. Gases will then flow through tower 59, will
drive the turbine 75, and will flow through the tower 63. The
gases expand i~ the turbine 75 and, as the gases expand, they
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are cooled. It is the flow of these cooled gases through the
tower 63 that readies the tower 63 for a subsequent gas separation
step. (It is assumed here that the tower 59 has already been
pre-cooled in this manner in a previous cycle so that less volatile
gas components loaded into the tower 59 will be sublimed or
"froæen out".) The gases are allowed Lo flow in this manner for a
short period of time, for example, for about 6 to 8 minutes.
Energy extracted from these gases by the turbine 75 is used to
drive the generator 76.
Gas cools in tower 59 due to contact with the large surface
area of the cooler solids in the tower. Less volatile components
of the gas are condensed or converted into the solid phase and
remain in tower 59. The more volatile, noncondensed or clean
components of the gas pass out of tower 59 and, via turbine 75,
through tower 63. This clean gas is purified by being freed from
the "frozen out", sublimed or condensed components. The turbine
75 expands the gas, thus further cooling it, and delivers the
gas at a pressure of typically about 5 psig into tower 63. The
pressure at which the gases enter the tower 63 is not critical.
What is required is that the pressure ratio reduction effected
in the turbine 75 is of sufficient magnitude to adequately cool
the gases so the gases can properly chill the tower 63.
A second step (which is carried out simultaneously with
the loading of exhaust gas into the tower 59 and the cooling of
the tower 63) is that of cleaning a loaded tower by revaporizing
the "frozen out", sublimed or condensed components remaining in
that tower from a prior cycle. This step is carried out, for
example in connection with tower 61, by closing the valves 65a,
65b, and 65c at the lower end of tower 61 and by connecting the
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other end of that tower through valve 64b to the vacuum pump
79 and compressor 81. The valves 65b are used only if a
regenerator is limited with heat systems. The pump 79 operates
to reduce the pressure in the tower 61 by a ratio of about 10
to 1. As pressure in the tower is reduced, the "frozen out",
sublimed or condensed components are revaporized to form an acid
gas which is drawn out of the tower 61. The withdrawn acid gas
is compressed by the compressor 81 and is discharged into the
acid gas discharge conduit 82. The acid gas typically consists
mainly of C02 with small amounts of H2S, S02, S03, IICN and other
noxious gases. Noxious gases, containing chlorine, sulfur, and
the like, may be neutralized, as by scrubbing with caustic
solution. Combustible components of the neutralized gases are
preferably separated out and retained for use. Such gases can
be burned in the combustion system 10.
The next cycle is like the one just described and consists
of a first step of passing gases from the conduit 53 through one
of the valves 64a into the cooled tower 63, separating, by
"freezing out" or subliming, components of the gases in that
tower, cooling the separated clean gas leaving tower 63 in the
turbine 75 and passing the cooled, expanded clean gas through
the recently cleaned tower 61 to chill that tower in preparation
for receiving the next charge of exhaust gases from conduit 53.
A second step is that of simultaneously revaporizing the "frozen
out", sublimed or condensed components which remain in the tower
59 from the prior cycle to clean that tower in preparation for
chilling during the next cycle.
The next cycle is like the two foregoing cycles. Its first
step is that of passing gases from the conduit 53 into the tower
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61 to separate out gaseous components and cooling the just
cleaned tower 59 with the separated clean gas fraction from
tower 61 and turbine cooling means 75. A second step is to
clean tower 63 by revaporizing components remaining in the
tower 63 from the previous cycle by withdrawing them through
vacuum pump 79 and compressor 81.
The purified gases (which are cool if their heat energy has
been reclaimed) discharged into the conduit 71 can, if desired,
be exhausted to atmosphere without the use of a flue gas stack.
Alternatively they can be put to use. For example, inasmuch as
these gases are dry, they can be used to advantage in evaporative
cooling towers and the like. If the heat energy of the gases has
not previously been recovered, they may be passed into a heat
reclamation unit 27 (shown in phantom) in which the gases will be
cooled while their heat energy is transferred to a power fluid.
The acid gas discharged into the conduit 82 may be transferred
to a scrubbing unit where it is scrubbed with caustic solution.
Alternatively, the acid gas may be put to any suitable use.
Noxious gases created in chemical processes other than
combustion can be mixed with gases in the feeded conduit 53 and
treated in the unit 58. The optional addition of such gases is
indicated by broken line 90 in FIGURE 1. A sewer-like blow down
system of gas collection conduits 101 can be used to collect
exhaust gases from a plurality of gas producing apparatuses 102.
Suitable compression equipment (not shown) can be included in the
conduit system 101 to transfer the collected gases into the
conduit 53.
Many power fluids can be used in the system of the present
invention, including water, carbon dioxide, ammonia, propane,
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but<tne, pentane, hexane, various halogenated methane compounds,
such as the flurocarbons, and lithium bromide. Most desirable
are power fluids which are immiscible with and insoluble in water
to prevent power fluid separation problems in condenser 38.
Prehalogenated benzenes, for example, as are disclosed in U.S.
Patent No. 3,702,534 issued November 14, 1972 to Max F. Bechtold,
are pre~erred because tiley are immiscible with water and can be
used over a wide range of temperatures without risk of de-
composition and toxicity. Moreover prehalogenated benzenes have
the advantage of high molecular weight, low flamability and low
corrosivity.
Referring to FIGURE 2, another embodiment of the present
invention is illustrated in which the exhaust gases are subjected
to treatment to separate them into condensable and noncondensable
components by subliming or "freezing out". The combustion system
and particulate matter separation units shown in FIGURE 2 may be
just like their counterpart units in FIGURE 1. Thus, combustion
system 110 is fueled from a fuel supply source 111 and air or
oxygen is supplied to the combustion system 110 through air or
oxygen supply line 112. Preferably air supply line 112 includes
a compressor 113 to pressurize the air supply and to maintain
the combustion system 110 operating under a positive pressure.
In this embodiment of the invention, where hot gases are treated
throughout, downstream compressors are inefficient and, hence,
undesirable. Therefore the gases in the gas treatment system are
maintained under sufficient pressure by a compressor in the
combustion system 110, such as compressor 113, or by a compressor
located upstream of the combustion system 110.
Exhaust gases generated by the combustion system 110 are
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ducted, as indicated by arrows 115, 116, 117 to and through a
series of particle separation units 120, 121, 122. The
separation unit 120 is preferably a cyclone separator, and
particulate matter as small as 50 microns in size is separated
out of the gases as indicated by an arrow 124. The separation
units 121, 122 house filters which remove smaller particles as
ind;cated by arrows 125, 126. The units 120, 121, 122 are
insulated to avoid heat loss.
Exhaust gases which have been cleaned of particulate matter
are ducted into a conduit indicated by the numeral 117. The
exhaust gases in this conduit have a slightly reduced pressure
due to pressure losses in the filters, but are at substantially
the same temperature as when they exited the combustion system
110. These gases are led from conduit 117 into gas feeder
conduit 153 which directs the hot gases to gas treatment and
separation unit 158. The unit 158 is preferably similar to the
type described in U.S. Patent No. 3,970,524 and is operable to
separate the hot gases into condensively low volatility and
components having similar vapor pressures, such as C3 and C4
fractions.
The unit 158 includes two similar packed towers or columns
159, 163. Each of the towers 159, 163 is similar in construction,
content and function to the regenerator shown as 59, 61, 63 in
FIGURE 1. Automatic switch valves 164a, 164b, 164c and 165a,
165c are provided at opposite ends of the towers 159, 163.
Tower connection conduits 167, 168 communicate the towers 159,
163 with the valves 164a, 164b, 164c and 165a, 165c. The gas
feeder conduit 153 connects with the valves 164a. An acid gas
discharge blowdown conduit 170 connects with the valves 164b.
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A purified hot gas discharge con~luit 171 connects with the
valves 164c. A pair of transfer conduits 173, 174 connect with
valves 165a, 165c of towers 159, 163 and with coils 175, 176,
respectively, of heat exchanger 177.
As with unit 58 of FIGURE 1, the manner by wllich gases are
treated in the unit 158 may be visualized as that of subjecting
the gases to several like cycles repeated time after time as
long as exhaust gases are being produced by combustion system 110.
During each cycle, a different step is conducted in each of
towers 159, 163. While a first tower is being cooled by a flow
of relatively cool purified gas, separation is taking place in
the second tower. Condensed or sublimed components are removed
from the second tower at the beginning of the next cycle by the
initial flow of purified gas therethrough from the first tower,
as will be explained more fully hereinafter.
A first step of one cycle is carried out by opening the
: valves 164a, 165a at each end of tower 159 and valves 164c, 165c
at each end of tower 163. Gases will then flow from gas feeder
conduit 153 through tower 159, in which the gases cool and
components of the gas are sublimed or "frozen out". The gas,
freed of the less volatile components, is further cooled in coils
175 of heat exchanger 177 and flow through tower 163, cooling
the solid packing in tower 163 as it passes therethrough and
becoming reheated itself as it does so. It is the flow of cooled
gases tllrough tower 163 that readies the tower 163 for the next
cycle during which gas separation by sublimation or "freezing
out" will take place therein. (It is assumed here that the
tower 159 has already been pre-cooled in this manner in a previous
cycle so that less volatile gas components loaded into tower 159
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31~:
will be subl;med or "frozen out"). The gases are allowed to
f]ow in this manner for a short period oE time, for example,
for about 6 to lO minutes. Energy extracted from the gases
in the heat exchanger 177 may be used in any suitable manner.
Moreover, inasmuch as the purpose of heat exchanger 177 is
to function as a means for cooling the purif;ed exhaust
gases to permit them, in turn, to cool the tower into which
they next flow, any suitable heat exchange means may be
substituted for heat exchanger 177. Exemplary of a useful
means is an expansion turbine to which a power generator may
be coupled if desired. When an expansion turbine is used,
the gases expand in the turbine and are cooled as they
expand. The expansion pressure ratio in the turbine need
only be sufficient to accomplish the desired cooling. In
view of this additional pressure drop, a system which
utilizes an expansion turbine will generally operate at a
somewhat higher combustion system pressure as compared to a
system which utilizes a conventional heat exchanger. Where
an expansion turbine is not used, it is generally desirable
to expand the cooled gas exiting heat exchange means 177.
For this purpose, valves 165c at the heat exchanger end of
towers 159, 163 may be throttling valves.
The hot, purified gases leaving tower 163 are directed
through line 167 and valve 164c into hot gas discharge
conduit 171. These hot gases may be used for any suitable
purpose or may be further treated, in a manner to be hereinafter
described, to reclaim the heat energy therein. It will be
appreciated that in the immediately previous cycle, tower
163 had been used for the sublimation or "freezing out" step
382
and the less vo]atile components of the gas had been condensed
or converted into the solid phase and had remained within
tower 163, i.e., the tower was loaded. To clean loaded
tower 163 by revaporizing the "frozen out", sublimed or
condensed components from the prior cycle to form an acid
gas, the initial flow of purified gas from tower 159 which
passes through tower 163 is diverted from hot gas discharge
conduit 171, by closing valve 164c and opening valve 164b. The
mixed flow of purified gas and revaporized components, i.e.,
acid gas, are ducted through valve 164b into blowdown conduit
170. The acid gas typically consists mainly of C02 with small
amounts of H2S, S02, S03, HCN and other noxious gases. Inasmuch
as flue gas discharge restrictions preclude emission of these
gases, most noxious components in the blowdown gases are
neutralized by scrubbing or otherwise separated out to permit
; exhausting the cleansed blowdown gas. Cleaning of the loaded
tower in this manner can be accomplished during each cycle by
switching the initial purified gas flow to the blowdown line
170 for just enough time to purge the tower and then switching
the gas flow back to the purified gas line 171.
If during the first cycle the exhaust gases were loaded
into tower 159 and simultaneously the acid gases were purged
from tower 163 while tower 163 was cooled, during the next
cycle the exhaust gases are loaded into cooled tower 163. This
next cycle is like the one just described and consists of passing
gases from conduit 153 through one of the valves 164a into the
cooled tower 163; separating, by "freezing out" or subliming,
components of the gases in that tower; cooling (or expanding
and cooling if a turbine is used) the separated clean gas leaving
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~Z382
tower 163 in heat exchanger 177; and passing the relatively
cooled, clean gas through tower 159 ~which is loaded from the
previous cycle~ to purge and to cool the tower. The initial
flow through tower 159 revaporizes the loaded components and
the mixed clean gas - revaporized components flow out through
valve 164b into blowdown line 170. After a short time this
purging flow is termlnated and the clean gas flow through
tower 159 is diverted through valve 164c into purified gas
discharge line 171.
One means for reclaiming the heat energy contained in the
hot, purified gases is shown in FIG7u7RE 4 and comprises passing
the gases through a heat exchanger 85 wherein they give up
their heat energy to a power fluid which, in turn, operates as
the fluid in a Rankine cycle engine to do useful work. Heat
exchanger 85 preferably has first and second stages 86, 87
arranged in series. As the gases pass through the stages 86,
87 they sacrifice sensible and latent heat to the power fluid
which circulates in coils 89, 90. If the heat of purified gases
is utilized, stages 86 and 87 may be combined into a single unit.
Inasmuch as the gases are cooled below their dew point in the
heat exchanger 85, the latent heat of steam is recovered and
part of the high heating value of the gases is recovered, unlike
prior systems which recovered only the low heating value of hot
exhaust gases. In the first stage 86, the exhaust gases are
cooled to a temperature near but slightly above their dew point.
In the second stage 87, the temperature of the exhaust gases
is further reduced and moisture in the gases condenses.
Condensate is withdrawn from the second stage 87, as indicated
by an arrow 91, and is treated to reclaim the condensed water,
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Z38Z
which may be used as a scrubbing fluid elsewhere in the process,
such as in neutrali~ing the less volatile components.
The power fluid which is heated by exhaust gases passing
through the two stage heat exchanger 85 is used to perform
useful work. In the preferred embodiment, the heat exchanger
coils 89, 90 form the boiler of an external combustion engine.
Such an engine typically includes an expansion turbine 91, a
condenser 92, and a pump 93, connected in series by conduits
94, 95, 96, 97. Power fluid heated during passage through the
coils 89, 90 is expanded in the turbine 91 and serves to drive
a generator 99. The power fluid is then ducted through the
condenser 92 and the pump 93 for return to the heat exchanger
coils 89, 90.
Another means for reclaiming the heat energy of the exhaust
gases is to remove it directly from the heated tower solids
using a heat transfer fluid other than the purified gases.
According to this method, the purified gases entering the heated
tower 159, 163 for reheating are removed from the tower via tap
line 180 after passing through only a portion of the heated
solids in the tower. Typically, the purified gases are tapped
from the tower at a point where they just about reach ambient
temperature and then discharged to the atmosphere or utilized.
The balance of the heat in the tower solids is recovered by
passing a fluid heat transfer medium, e.g., compressed air, into
the tower through line 181 and processing the heated medium
exiting the tower via line 171 in the same manner as reheated
purified gas exiting via line 171 is processed.
Referring to FIGURE 3 there is shown still another
embodiment of the present invention which differs from the
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3~Z
:mbodiment of FIGURE 1 primarily in the manner in which the
exhaust gases are cooled prior to being processed in the gas
treatment and separation unit. The method and system of this
embodiment is particularly useful in Large power plant
installations where the stack or exhaust gas has a volume of
up to 2,500,000 Nm3/hr. In view of the similarity between
the systems in FIGURE 1 and FIGURE 3 like numerals are used to
designate like components.
As in the system of FIGURE 1, the combustion system 10
is fueled from a fuel supply source 11 and air or oxygen is
supplied to the combustion system 10 through air or oxygen
supply line 12. Preferably, air supply line 12 includes a
compressor 13 to pressurize the air supply and to maintain
the combustion system 10 operating under positive pressure.
By pressurizing the system with a compressor in or upstream
of the combustion system 10, the need for compression equipment
downstream from the combustion system is diminished or eliminated.
As a practical matter, however, where the configurat:ion of
FIGURE 3 is used in connection with very large exhaust gas
volumes, the combustion system can generally not maintain much
of a positive pressure. Therefore, a downstream compressor,
preferably installed upstream of the gas treatment and separation `-
unit 58, is generally necessary.
Exhaust gases generated by the combustion system 10 are
directed as indicated by arrows 15, 16, 17 to and through a
series of particle separation units 20, 21, 22. The separation
unit 20 is preferably a cyclone separator, and particulate
matter as small as 50 microns in size is separated out of the
gases, as indicated by an arrow 24. The separation units 21, 22
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23~Z
house filters which remove smaller part-icles as indlcated by
arrows 25, 26. The units 20, 21, 22 are insulated to avoid
heat loss.
Exhaust gases which have been cleaned of particulate matter
are ducted into conduit 17 from which they pass through feed
conduit 17a into exhaust gas cooling and heating unit 200.
Unit 200 is operable (1) to cool the exhaust gases prior to
ducting them to gas treatment and separation unit 58 for
separation into condensable and noncondensable components by
subliming or "free~ing out", and (2) to receive the purified
gases exiting gas treatment and separation unit 58 and to
reheat the purified gases to within a few degrees of the
temperature of the exhaust gases which entered unit 200 through
feed conduit 17a. Unit 200 includes two similar packed towers
or columns 201, 203. Each of the towers 201, 203 is similar
in construction and content to the regenerators shown as 59,
61, 63 in FIGURE 1. Automatic switch valves 205a, 205b are
provided at the end of towers 201, 203 adjacent feed conduit
17a. Tower connection conduits 207 communicate the towers 201,
203 with the valves 205a, 205b. Tower connection conduits 209,
210 communicate the towers 201, 203, respectively, with feed
conduit 53 of the gas treatment and separation unit 58. Feed
conduit 17a connects with the valves 205a. A purified hot gas
discharge conduit 211 connects with the valves 205b. A
compressor 213 is included in tower connection conduit 209 to
provide the positive pressure in the system which is almost
invariably required when very large exhaust gas volumes are
passed through the FIGURE 3 system. Conduits 209 and 210
are cross connected through conduits 215 and 217 (which contain
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;23~32
appropriate flow control valvcs) upstream of the compressor 213
to allow either tower 20l, 203 to function as the heating or
cooling tower.
Gas treatment and separation unit 58 is substantially the
same unit and operates in substantially the same manner as is
shown and described in connection with FIGURE 1. In the FIGURE
3 embodiment, however, unlike the FIGURE 1 embodiment, the
purified gas is directed back through the gas cooling and
heating unit 200 for reheating prior to utilization or discharge
to the atmosphere, all as will be more fully described hereinafter.
The manner by which large volumes of gas are treated in
t the system of FIGURE 3 may be understood from the following.
Fuel and air are burned in the combustion system 10, which is
preferably operated under a positive pressure, such as may be
provided via air line compressor 13 or from some other upstream
compressor unit. The hot exhaust gases resulting from the
combustion are cleaned of their particulate content in cyclone
separator 20 and filter units 21, 22 and are ducted via conduit
17 into gas heating and cooling feed conduit 17a. Inasmuch as
the combustion system boilers cannot be expected to maintain
a sufficiently high positive pressure, a downstream compression
stage is required and cooling of the gas is desirable prior to
the compression stage. This cooling of the gases is very
effectively performed in regenerators, such as packed towers
201, 203, since the pressure drop across the regenerators is
relatively small (about 1.4 psi).
The manner by which gases are treated in unit 200 may be
visualized as that of subjecting the gases in successive like
cycles to heating and cooling in towers 201, 203. During each
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cycle, a different step is being conducted in each of towers
201, 203. While a first tower is serving as the cooling tower
to cool the hot gases, the other tower is serving to heat the
purified gases leaving unit 58. In the next cycle, the roles
of the respective towers are reversed. Thus in a first cycle
one of the towers 201, 203 is selected as the cooling tower into
which the hot particle free exhaust gases are ducted and the
corresponding valve 205a is opened.
If tower 201 is to serve as the cooling tower, valve 205a
associated therewith and valve 205b associated with tower 203
are opened while valve 205b associated with tower 201 and valve
205a associated with tower 203 remain closed. The hot exhaust
gases flow from feed conduit 17a through valve 205a into tower
201 in which the gases are cooled prior to compression in
compressor 213. At the same time the tower 201 is heated by the
hot gases in preparation for serving as the heating tower in the
next cycle. The compressed gases are then directed through
conduit 209 to feed conduit 53 for processing in gas treatment
and separation unit 58. If desired, other noxious gases may
be mixed with the compressed exhaust gases entering feed conduit
53 (optional addition indicated by broken line 90). Following
processing in unit 58, the purified gases leaving towers 59, 61,
63 through valves 64c are ducted via tower connection conduit
210 into and through tower 203 in which the purified gases are
reheated while the tower is cooled (it is assumed that tower
203 had been pre-heated in a previous cycle by passage of hot
exhaust gases therethrough). The hot purified gases leave tower
203 by way of tower connection conduit 207 through valve 205b and
conduit 211 and may be discharged to the atmosphere or, more
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;"'`~
23~
likely, utiliYed, such as by reclaiming the heat energy contained
therein. One means for reclaiming the heat energy is shown in
FIGURE 4. Operation of the FIGURE 4 means to transfer the heat
energy of the gases to a power fluid which, in turn, operates as
- the fluid in a Rankine cycle engine has already been described
herein in connection with the embodiment of FIGURE 2. In a
typical system the hot gases entering the cooling tower 201 are
at a temperature of about 150-180C and are cooled in the tower
to about 20-40C, at which temperature the gases are compressed
and enter unit 5~. The purified gases leaving unit 58 are
reheated in tower 203 to within 2 to 5C of the temperature of
the gases entering tower 201. As was described in connection
with the embodiment of FIGURE 2, another means for reclaiming
the heat energy of the gases is to remove it directly from the
heated tower solids. For this purpose tap lines 220 and heat
transfer medium injection lines 221 are provided in connection
with towers 201 and 203.
The next cycle is like the one just described except that
tower 203 serves as the cooling tower and tower 201 as the
reheating tower. It will be appreciated that following the
previous cycle, tower 201 was left in a relatively heated state
by the passage of hot exhaust gases therethrough whereas tower
203 was left in a relatively cooled state by virtue of having
given up its heat content to the purified gases passing there-
through. The hot exhaust gases flow from feed conduit 17a
through valve 205a into tower 203 in which the gases are cooled
while the tower is heated. They are then ducted via cross conduit
215 to compressor 213 in which they are compressed. The
compressed gases are ducted through conduit 209 to feed conduit
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53 for processillg in gas treatment and separation unit 58.
Following processing in unit 58, the purified gases leaving
towers 59, 61, 63 through valves 64c are ducted via tower
connection conduit 210 and cross conduit 217 into tower 201 in
which the purified gases are reheated while the tower is cooled.
The hot purified gases leave tower 201 by way of tower connection
conduit 207 through valve 205b and conduit 211 and may then be
discharged or utili~ed, such as by reclaiming the heat energy
therein.
The methods described, and apparatus illustrated in Figs.
1, 2 and 3, are quite effective and yield a gas containing only
traces of undesired components. If however complete removal of
sulfurous and other harmful compounds is required, an adsorption
or absorption system can be linked with the processes and
apparatus of the present invention.
While the invention has been described with reference to
particular embodiments thereof, it will be understood that
numerous modifications may be made by those skilled in the art
without actually departing from the scope of the invention.
Accordingly, all modifications and equivalents may be resorted
to which fall within the scope of the invention as claimed.
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