Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Background of the Invention
The invention is particularly useful in industrial furnaces
or ovens from which hot exhaust gases exit through a ~lue into
the ambient atmosphere. Much energy is lost as the hot flue
gases exit the furnace. This energy, if recaptured, can be
utilized to preheat the combustion air to increase the efficiency
o the fuel. Such energy can ~lso be used in the enrichmen~ of
combustion air with oxygen to reduce heat losses by lowering
the amount of waste or exhaust gases. To enrich combustion
air with oxygen, it is necessary to increase the amount of
oxygen from a normal 21 percent to 25-50 percent. It has been
found that the resulting flue gases have a lower level of ~` -
nitrogen, thereby reducing the amount of heat wasted for heating
nitrogen. It has been found that even a small enrichment of
combustion air to 25 percent oxygen produces substantial savings
in fuel.
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Combustion equipment, designed to use pure oxygen, has
special refractories and water cooled members. At present7 it
is uneconomical to use more than a 50 percent oxygen enrichment
of air because of the special equipment needed. Thus, the term
"conventional burner", as used in the specification and claims~
means any regular burner that can operate with up to 50 percent
oxygen content in the combustion air. The savings in fuel pro-
duced by the oxygen enrichment of combustion air, compares
favorably with the savings experienced from using a recuperator
for preheating the combustion air. For examp].e, most industrial
recuperators produce combustion air at a temperature of 1000F.
or less. The enrichment of combustion air with oxygen to about
50 percent at high temperature is equivalent to preheating the
conbustion air to l/-~00F. Tlle invention is directed to the en~
richrnent of combustion air.
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~ In one particular aspect the present invention provides
a m~thod oE enriching combustion air with oxygen comprising
circulating air, containing normal quantities of oxygen, into
contact with a liquid which absorbs oxygen from the air to
form an oxygen enriched liquid, and then subsequently
liberating the oxygen from the oxygen enriched liquid and
mixing the liberated oxygen with air containing normal
quantities of oxygen, to form oxygen enriched combustion air
for subsequent mixing with a combustile fuel.
In another particular aspect the present invention provides
in combination:
(a) a first tower with a vertically elongated chamber .
sealed from the ambient atmosphere;
(b) a second tower disposed in spaced relation from the
first tower and also having a vertically elongate chamber ::
sealed from the ambient atmosphere;
(c) means for circulating air, containing normal quantities
of oxygen, separately to the chambers of the towers;
(d) means for circulating an oxygen absorbing liquid to
the chamber of the first tower for contact with air circulated
therein, to form an oxygen enriched liquid;
(e) means for circulating the oxygen enriched liquid
from the first tower to the chamber of the second tower;
(f) means for causing liberation of oxygen from the
oxygen enriched liquid in the chamber of the second tower, .
for contact with air circulated in said chamber, to enrich said
air with oxygen; and ~ -
(g) means for removing the oxygen enriched air from the ::
chamber of the second tower. .
Description of the Drawing
The following description of the invention will be better
understood by having reference to the annexed drawing, which
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is schematic and illustrates a furnace and an apparatus which
is made in accordance with the invention for enriching with
oxygen, combustion air circulated to the furnace.
Detailed Description of the Drawing
With reference to the drawing, there is shown an
industrial furnace 5 which is used, for example, in the heat
treatment of metals. The furnace 5 has a heat chamber 6 and
connecting flue 7 through which hot exhaust gases exit the
furnace 5.
A conventional burner 8 is provided for heating metals
disposed within the heat chamber 6 of the furnace 5. The
burner 8 essentially comprises a nozzle 9 for receiving any -
suitable fuel, e.g. gas or oil, under pressure; and a burner
block 11 composed of any suitable refractory material. The ;
burner block 11 comprises ~-
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a mixing chamber 12 in which the combustion air and fuel are
thoroughly mixed and ignited for subsequent passage into a
flame tunnel 13 from which a flame 14 and hot gases, including
the products of combustion, are discharged into the heat chamber
6 of the furnace 5. ~n apparatus, generally indicated at 15, is
provided for enriching the combustion air with oxygen, prior to
the mixture of the air with the fuel in the burner 8.
O~ygen Enrichment A~paratus 15 :
The apparatus 15, used in the enrichment of the combustion ~-
air with oxygen, essentially comprises an absorption tower 16 ; ~
in which oxygen lS removed from air containing normal amounts : :
of oxygen, and a desorption tower 17 in which o~ygen is liberated .~ :
for combination with the combustion air being circulated to the :~.
burner 8. The towers 16, 17 are essentially.the same in that : ~ -
both have a vertically elongated chamber 18, a downwardly ~ :
; directed spray 19 adjacent the top 20 of the chamber 18, and a:
bed 21 of conventional tower packing material, e.g. ceramic or
plastic chips, disposed within the chamber 18 intermediate the
spray 19 and.bottom 22 of the chamber 18. The tower packing ~
material provides a more efficient mass transfer of o~ygen within .-
the towers 16, 17.
Air, containing normal amounts of oxygen, is continuously
circulated, under pressure, to the absorption tower 1~ by any
suitable blower 2~, after passage through a conventional dryer
24. A portion of the dry air is also circulated directly to
the desorption tower 17. The air pressure within the absorption ~ .
tower 16 is mai.ntained from atmospheric to 15 pounds.per square
inch (psi~
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An oxygen absorbing liquid 25 is pumped to the absorption -
and desorption towers 16, 17 from any convenient source of supply
26. Any suitable liquid having an affinity for oxygen may be
utilized. Broadly speaking, the oxygen absorbing liquid 25
should have high oxygen releasing characteristics.
The mass flow rate of liquid through the apparatus 15 is
considerably greater than that of the gas. Therefore, the major
portion of the power required to operate the various fans and
pumps of the apparatus 15, is used to circulate the liquid.
I0 Accordingly, the density and viscosity of any liquid used in
the system or process, should be as low as possible to reduce the
power requirements to a minimum.
The consumption of power in moving a liquid is directly
proportional to the flow rate of the liquid, the specific gravity
of the liquid, and a fractional power of the viscosity of the
liquid. The flow rate of the liquid is inversely proportional
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to the oxygen releasing power of the liquid. Mathematically, it :
can be stated that: ;
p ~ (SG3- nZj~ ,
S
where: P is the liquid pumping power, SG is the specific gravity
of the liquid in lbs./cubic foot, n is the viscosity of the
liquid in centipoise, and S is the oxygen releasing power of the
liquid measured in cubic feet of oxygen released per cubic foot
of liquid.
An oxygen absorbing liquid 25 having a parametric value for
(SG) n~ of 60, was successfully used-in a prototype apparatus.
For economieal reasons, the parametric value of any commercially
used oxygen absorbing llquid 25 should not exceed 30.
The speciEic heat o~ an oxygen absorbing liquid 25 should
preferably be lower than 0.4 B~u/lb./F. for temperature swing
operations, wherein there is a variation in the temperatures of
the liquid (temperature swing) in the absorption and desorption ;;~
towèrs 16, 17.
The maximum oxygen enrichment o~ a liquid is also dependent
on the separation characteristic of the liquid for oxygen and
nitrogen. This is measurable by a so-called separation factor
. which is the ratio of the solubility of oxygen in the liquid to
the solubility o~ nitrogen in the liquid under the same tempera-
ture and partial pressure conditions. The o~ygen absorbing
liquid 25 should have a separation ~actor o~ at least 1.25 which
produces a maximum enrichment of about 25%.- Higher levels o~ :
enrichment are achieved by liquids with larger separation ~actors.
The characteristic temperature (k) for an oxy~en absorbing
Iiquid 25 should be greater than 1000R. in a system employing ~ .
temperature swing only. There is no advantage in using tempera-
ture swing when the characteristic temperature (k) is less than
200R. so a pressure swing is utilized, pressure swing being a
variation in the gas pressures in the absorption and desorption
towers 16, 17. For economical reasons, a combina-tion of pressure
.and temperature s~ing is used when the characteristic temperature
(k) is in the range of from 200R. to 1000 R. Mathematicall~,
the characteristic temperature (k) can be expressed as:
f S P ~ ~ Ta Td~ ~
k ~ ~ ln a - ln ~ )
where: ~n is the natural logarithm, Sa is the volumetric solu-
b;lity of oxyten in the liquid in the absorption tower, Sd lS
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the volumetric solubility of oxygen in the liquid in the de,
sorption tower, Pa and Pd are partial pressures of oxygen in the
absorption and desorption towers, respectively, and Ta and Td ;~
are temperatures (R) in the absorption and desorption towers, "
respectively. The characteristic temperature (k) is const~mt
for a particular liquid, but varies with different liquids.
A dimethyl polysiloxane fluid having a standard vicosity
of 50 centistokes at 25C., e.g. a silicone fluid bearing the
trademark SF-96 of the General Electric Company, was effectively
used as an oxygen absorbing liquid 25. Such fluids are clear,
water-white, oily fluids which are non-toxic, inert, tasteless
and odorless.
The air and liquid 25 are brought together in-the absorption
tower 16, where the liquid 21 absorbs at lea$t a portion of the
oxygen in the air. The liquid 25 is-generally at room temperature,
or at a relatively low temperature of from ~0F; to 100F. The
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oxygen deficient air, or air from which oxygen has been removedJ
is discharged from the top 10 of the chamber 18 of the absorption
tower 16 which is also supplied with a suitable mechanism through
which liquid can be drained from the chamber 1~.
The liquid, now enriched with oxygen, i.e. the oxygen en-
riched liquid 25, is circulated by a pump 27 under a relatively
high pressure of from 25 psi to 100 psi to a heat exchanger 28
for preheating and then to the desorption tower 17, where the
preheated oxygen enriched liquid 25 is brought into heat exchan~-
ing relation with hot exhaust gases exiting the flue 7 of the
furnace 5. The oxygen enriched liquid 25 is forced from the
- spra~ 19 onto the bed 21 o~ tower packing material in the de-
sorption tower 17. A conventionally designed heat exchanger 29
composed, for example, of a series of vertically disposed inter-
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connected tubes, is embedded in the bed 21 of tower packing
material. The oxygen enriched liquid 25 trickles downwardly
through the bed 21 and is heated by the hot exhaust gas passing
through the heat exchanger 29 to liberate oxygen which is com-
bined with normal oxygen containing air that is being drawn up-
wardly through the bed 21 and discharged from the desorption
tower 17 through an exhaust port 30 in the top 20 of the chamber
18 by a vacuum pump or fan 31 which is disposed in the air line
between the desorption tower 17 and burner 8. The vacuum pump 31
can be a steam ejector, in which case, a portion of the hot exhaust
gases, exiting the flue 7, can be utiiized in the generation of
steam necessary to operate the steam ejector.
The air pressure within the desorption tower 17 is not
greater than, and preferably less than the air pressùre within
the absorption tower 16. In most cases, the air pressure within
the desorption tower 17 is less than atmospheric pressure which
is the air pressure within the absorption tower 16. The tempera-
ture of the oxygen enriched liquid 25 is raised to a temperature
- of from 150F. to 400F., depending on the liquid used, as it
comes into heat exchanging relation with the hot exhaust gases
being circulated through the heat exchanger 29 within the de-
sorption tower 17. The oxygen combines with the combustion air
which is enriched from a normal 21 percent to a perferred 25-5
percent. The vacuum pump 31 forces-the enriched combustion air
to the gas burner 8 for mixture with the fuel in the mixing
chamber 12 ; ;
The hot liquld from which oxygen has been liberated, or
the oxygen-lean liquid 32, remalning in the bo-ttom 22 of the
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desorption tower 17, is circulated, under pressure, back to the
absorption tower 16 by any suitable pump 33. It is important
to cool the oxygen-lean liquid 32 before it is passed into the
absorption tower 16 for reuse in the process. Accordingly, the
oxygen-lean liquid 32 is passed through the preheater 28 and
into heat exchanging relation with oxygen enriched liquid 25
being continuously circulated to the desorption tower 17. It
has been found necessary to further cool the oxygen-lean liquid
32, so the liquid is subsequen-tly circulated through another heat
exchanger 3~ through which cold water is separately circula~ed.
The properly cooled oxygen-lean liquid 32 is then recycled back
to the absorption tower 16 for reuse in the process.
It is important to note that the hot exhaust gases also
preheat the oxygen enriched combustion air being forced upwa~dly
through the bed 21 of tower packing material in the desorption
tower 17. If desired, a portion o~ the exhaust gas can be used ~ -
outside the desorption tower 17 to further preheat the oxygen
enriched combustion air being circulated to the burner 8. It is
also important to note that the air flows countercurrent to the
liquid trickling downwardly through the beds 21 of tower packing
material of the towers 16, 17.
The solubility of a gas in a liquid increases with the
increase in the pressure of the gas above the liquid and de-
creases with the increase in temperature of the liquid. The
above described process utilizes a variation in pressures
(pressure swing), or a variation in temperatures (temperature
swing), or a combination of both between the absorption and
desorption towers 16, 17. A ternperature swing between the
absorption arld desorption towers 16, 17 i.s preferred because of
the readily available heat in the systern, i.e. the heat of the
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hot exhaust gases. In this particular instance, the absorption
tower 16 is operated at a lower temperature and higher pressure~
whereas the desorption tower 17 is operated at a higher tempera-
ture and lower pressure. Thus, both a pressure and temperature
swing are utilized. The air pressure within the absorption
tower 16 is controlled by the regulation of the inlet and exhaust
air valves 35, 36 in the air lines leading to and from the
absorption tower 16, whereas a desired vacuum is established in
thedesorption tower 17 by the regulation of the inlet and ex-
haust air valves 37, 38 in the air lines leading to and fromthe desorption tower 17.
As previously indicated, the degree of pressure swing
and temperature swing for a liquid depends on the physical
characteristics and oxygen solubility behavior of the liqu~d as a
function of temperature and partial pressure of oxygen. An
oxygen concentration of 27.3 percent was achieved when pressures
in the absorption and desorption towers 16~ 17 were 2 psi and
-18.6 inches mercury, respectively, and the temperatures in
said towers 16, 17 were 60F. and 170F., respectively. The
enriched air flow was 3.5 standard cubic feet per hour, measured
at 30 degrees mercury and 70F., with a liquid flow rate of 0.5
gallons per mlnute. The flow rate of the liquid through the
system is dependent on the oxygen releasing capacity of the
liquid. The higher the oxygen releasing capacity, the lower will ~
be the flow rate for a specific oxygen generation rate. It is ::
more economical to use a lower flow rate because the power input
is severely reduced.
In operation, a desired amount of oxygen absorbing licluicl
25 is pumped into the bottom 22 of the absorption and desorption
towers 16, 17. The necessary fans and pumps are operated to
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cause circulation of the liquid through the system. The cooled
liquid is forced from the spray head 19 onto the bed 21 of
packing material of the absorption tower 16. Air, containing
normal amounts of oxygen, is forced under pressure, upwardly
through the bed 21 into contact with the oxygen absorbing
liquid 25 trickling downwardly through the bed 21 into the
bottom 22 of the absorption tower 16 from where it is removed and
circulated, under pressure, to the desorption tower 17. The now
oxygen enriched liquid is sprayed from the head 19 onto the bed
21 of tower packing material and trickles downwardly through the
bed into contact with the heat exchanger 29 through which hot
exhaust gases are separately circulated. The oxygen enriched
liquid is heated and the oxygen stripped or liberated ~rom the
liquid for combination with combustion air being forced upwardly
through the bed 21 of tower packing material in the desorption
tower l7. The now oxygen-lean liquid 32 is cooled and circulated
back to the absorption tower 16 for reuse in the process. The
heat from the oxygen-lean liquid 13 is utilized to preheat the
oxygen enriched liquid 25 being circulated to the desorption
tower 17. Thus, oxygen absorbing liquid is continuously being
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circulated between the towers 16, 17. Oxygen deficient air from
the absorption tower 16 is exhausted into the ambient atmosphere
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as is the cooled exhaust gas from the desorption tower 17.
Thus, there has been provided a very simple and economical
method and apparatus for enriching combustion air with oxygen.
Further, normally lost heat from hot exhal1st gases exiting the
flue of a furnace, are utllized in the enrichment of the com-
bustion air with oxygen and in the preheating of the enriched
combustion air, as previously indicated or mentioned.
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