Note: Descriptions are shown in the official language in which they were submitted.
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B/~CK~;RO[.iND OF ri'~lE INVr;N'I'ION
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In the manufacture o~ pu]p, certain orc~anic compoullds
are separated fram the cellulose components by cooking the
raw materials in cooking liquors. The cooking liquors include
inter alia inor~anic salts SUCil as sodium sulEide, sodium
bisulfite, sodium hydroxide, sodium carbonate, calcium
bisulfite, magnesium bisulfite, sodium sulfite and sodium
bicarbonate. The cooking process produces a waste li~uor which
is ~nown as "brown liquor" or "black liquor". The black liqùors
eontain lignin and other organic compounds; and therefore, the
black liquor has calorific value. The black liquor also
contains other chemieals which are not completely consumed in
the cooking operation. The organic compound and unconsumed
chemieals are suspended and dissolved in a solvent and medium
which includes washing water added during the pulp processing.
Often, the suspended and dissolved matter are present in the
black liquor at a solids content of 5-15~ by weight.
The inorganic salts are rather expensive; and there-
fore, from a economic viewpoint, it is advantageous to reclaim
~0 them. In the past the black liquor has been concentrated and
then smelted at temperatures of approximately 1700F to burn
any combustible organic or other carbonatious fraction
leaving the residual inorganics at a temperature above their
fusion points. The inorganic chemicals are then subjected to
reducing conditions to convert them into chemicals which can be
used again in the pulping process. Under these conditions,
hydrogen sulfite and sulfur dioxide are usually released and
must be recovered to avoid air pollution. Since the temper-
atures are above the fusion point of the inorganic chemicals, ,t
a portion of these chemicals are usually vaporized and must be
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separated from the remainder of the exhaust gases in orderto be recvvered.
The processes described above are complex requiring
complicated and expensive equipment. Further, the processes
tend to be hard on equipment to thereby limit the useful life
thereof; and consequently, add to the cost of practicing such
processes. Many oE the problems set forth above have been
overcome by treating the black liquor in a fluidized bed
oxidation process which is shown and described in U.S. Patent No.
3,309,262 to George G. Copeland and John E. Hanway, Jr. The
patent discloses a process in which the black liquor is
sprayed over a fluidized bed which is made up of solid parti-
cles of residual inorganic chemicals derived from the black
liquor. An oxygen containing fluidizing gas is passed up-
ward through the bed to maintain the particles in a fluidized
state. under these conditions a portion of the black liquor
will evaporate and another portion of the black liquor will
depGsit on the particles. At least a portion of the fluidized
bed is maintained at a temperature abovc the combustion
2n temperature of the concentrated black li~uor but below the
fusion temperature of the particles. Such a temperature would
probably be within the range of from approximately 1000F to
1800F and often below approximately 16oooF. Under these
conditions the carbon content of the or~anic material is burned
off and agglomerates of residual inorganic material are formed
which are heavy enough to stay within the fluidized bed and not
become entrained in the fluidizing gas to pass out of the bed.
The agglomerates are yrown to a size where they can be taken
out o~ the fluidized bed and eventually recycled to the pulping
~0 operation.
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Such a process is not without its disadvantages. For
one, the heat energy generated by the fluidized bed reaction is
not used with a high degree of efficiency. Further, it has been
necessary to feed the black liquor into a fluidized bed at a
fairly low concentration in order to have sufficlent water
available to keep the temperatures within the bed within the
desired range.
SUMMARY_OF THE INVENTION
It is an object of the present invention to overcome
drawbacks found in the prior art such as those discussed above.
Accordingly, a steam generator is provided with a fluidized bed
made up of particles comprising inorganic materials recovered
from black liquor, a means for injecting the black liquor into
the fluidized bed, means for flowing a fluidizing gas upward
through the fluidized bed to maintain it in a fluidized state
and water tubes which carry water in heat exchange relationship
with the fluidized bed to generate steam and maintain the
fluidized bed at a temperature below the fusion point of the
inorganic materials.
More particularly, in one aspect the invention
pertains to a steam generator adapted to contain a bed of
fluidized particles of inorganic materials derived from waste
liquor. The generator includes means for introducing waste
liquor containing inorganic materials and having more than 45
percent by weight solids into the steam generator, means for
flowing fluidized gas upward through the steam generator, and
means for withdrawing particles from the steam generator. A
plurality of tubes are provided for flowing water in hea-t
exchange relationship with the fluidized bed to extract heat
therefrom, the tubes providing sufficient exchange surface to
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maintain a portion of the bed at a temperclture of between 1000F
and 1800F and to evaporate the water.
Another aspect of the invention comprehellds a process
for oxidizing black liquor from a paper pulp operation in a
fluidized bed steam generator made up of solid particles, the
particles being substantially entirely of the inorganic
components of the black liquor. The process comprises the steps
of providing black liquor so that it contains solids at a
concentration of more than 45 percent by weight, dlrecting
fluidizing gas upward through the bed, the gas containing
enough oxygen to support oxidation of substantially all of each
of the organic components of the liquor in the bed, directing
the black liquor into the bed, indirectly exchanging heat from
the bed to a cooling medium to maintain the bed at a temperature
of between approximately 1000F to 1800F and below the
temperature at which the inorganic components of the liquor
fuse, whereby the inorganic components of the liquor form a
number of the particles, and deposit on the previously formed of
the particles.
~0 BRIEF DESCRIPTION OF DRAWINGS
Fig. 1 is a diagrammatic view showing an embodiment of
the present invention;
Fig. 2 is a view partly in section showing the steam
generator of Fig. l; and
Fig. 3 is a top view in section taken substantially
along the line 3 - 3 of Fig. 1.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
As shown in Fig. l, the present invention incIudes a
reactor in the form of a fluidized bed steam generator l0 which
receives oxygen containing fluidizing gas from conduit 12
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leadin~ from a pre-heater 14 which receives the fluidizing gas,
which may be air, under pressure frorn blower 16~ The amount and
velocity of the fluidizing gas can be controlled b~ means of a
valve 18 which is placed in a conduit 20 which allows air to
pass to the blower 16 from the atmosphere.
The fluidizing gas passes upward through a fluidized
bed 22 to maintain the particles thereof in a fluidized state
and supply oxygen to support combustion.
Exhaust gases containing the products o~ combus-
tion and possibly water vapor along with some entrained solidfrom the fluidized bed 22 pass out of the steam generator 10
and into a duct 24 which passes the exhaust gases into a
cyclone collector 26. The gases then pass through a conduit
28 to a wet scrubbing system 30 where the entrained particles
are removed from the exhaust gases. The exhaust gases are
then vented to the atmosphere at the yent 32. The particulate
matter removed from the exhaust gases passes down through
conduit 34 to a screw conveyor 36 which conveys the particulate
matter back into the fluidized bed 22 within the steam ;~
generator 10.
As will be explained presently, material is con-
tinuously discharged from the fluidized bed 22. Makeup
material is introduced through a conduit 38 which can be
metered by means of a hopper 40 and a feeder valve 42 which
regulates the passage of the makeup material into the screw
conveyor 36.
Fine particulate matter, which is recovered in the
wet scrubbing system 30, passes through conduit 44 and down
through an entrainment separator 46 and a conduit 48 which
leads to a sump 50. In passing from the wet scrubbing system
30 to the sump 50, the particulate ma~ter is carried in a
medium o~ dilute black liquor which is introduced into the wet
scrubbing system 30 through a conduit 52 under action of a pump
54 which receives the liquor through a supply conduit 56. The
fines can be taken out at the sump or alternately can be moved
by a pump 60 through a conduic 62 and conduit 64 to the conduit
56 to be recycled back to the wet scrubber 30 under control of
a valve 66 in the conduit 64.
The supply conduit 56 contains relatively dilute
liquor. More concentrated liquor is available throu~h the
supply conduit 68. Black liquor which has been pre-concentrated
by any conventional evaporation process flows through the
supply conduit 68 to a reservoir 70 and then flows through a
conduit 72 and through a pump 74 which forces the liquor
through a conduit 76.
The liquor`must be introduced into the fluidized bed
22. It can be introduced above the fluidized bed 22 or at a
level below the top oE the fluidized bed 22. If it is desired
to introduce it above the fluidized bed, a valve 78 in the
20 conduit 76 is closed so that the liquor will flow through a .
conduit 80. A valve 82 in the conduit 80 is opened as is a :
valve 84 in a conduit 86 which conveys air under preSsure. The
compressed air mixes with liquor supplied by the conduit 80 and
pressurizes the liquor so that it can be sprayed through a
sprayer 88 at the end of the conduit 86 within the steam ' .
generator 10.
If it is desired to in]ect the liquor at a level
below the top of the ~luldized bed, the valve 78 is opened
as is a valve 90 in a line 92 which connects the conduit
86 with the conduit 76. The valves 82 and 84 are closed so that
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compressed air and liquor will pass down through the conduit
76 to the liquor inlet 93 in the steam generator 10.
In order to moderate the temperatures within the
-steam generator 10 so that they are maintained at a level
lower than the fusion temperatures of the inorganics making
up the fluidized bed, water can be injected through sprayers
94 and 96. The sprayer 94 is at the end of a conduit 98
which supplies water. Compressed air is supplied through
a conduit 100 which connects with the conduit 98. Similar-
ily, the sprayer 96 receives water through a conduit 102 which
is pressurized by means of compressed air supplied through
a conduit 104 which connects with the conduit 102.
The particles grow in the fluidized bed 22 during
combustion as the inorganic components from the black liquor '
deposit on the particles. Dry particles are continuously
discharged from the fluidized bed steam generator 10 through a
conduit 106 into a classifier 108. The rate o discharge is
controlled by a valve 110 which is positioned in the conduit
106. Particles are classified according to size with the
~0 larger particles being discarded or treated chemically torecover the inorganics. The smaller particles are returned to
the fluidized bed through a conduit 112 which connects with the
conduit 38 so that the smaller partlcles will drop into the
hopper 40 to be conveyed by the screw conveyor 36 back into the
fluidized bed 22.
The system described above functlons ln a manner sub-
~, stantially similar to the system described in U.S. Patent
3,309,262. -
In the past where fluidized beds were used to recover
the inorganic chemicals derived from the black liquor, such as
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in the sys~em shown in U.S. Patent No. 3,309,262, the liquorwas introduced at a concentration of approximately thirty-five
percent (35~) solids, and additional water was made available
to maintain the temperature within the fluidized bed below the
eutectic point of the bed material. Most of the available
water was evaporated in the process in order to maintain the
low temperatures. This had several disadvantages. For one, it
limited the concentration of black liquor which could be
treated; and secondarily, i-t was inefficient from an energy
viewpoint. Available heat energy was used to evaporate water
rather than being made available to perform useful work.
In the present invention the steam generator 10 as
shown in Figs. 2 and 3 includes a series of tubes in which
water is heated and evaporated into steam. The front and rear
walls 114 and 116 are made up of a plurality of vertically
extending tubes 118 which are connected by a plurality of fins
120. The tubes 118 of the front wall 116 extend to a roof 122
and pass at a slight angle with the horizontal to a header 123
which passes the steam or steam and water mixture to a steam
drum 124. The roof 122 is also made up of the tubes 118 with
interconnecting fins 120 as are the walls 114 and 116. This is
not shown in detail for the sake of clarity.
The tubes 118 of the front wall 114~extend to a
header 125 which passes the steam or steam and water mixture
to the steam drum 124. Fins between the tubes extend from
the bottom of the front wall 114 to a duct plate 126. Above
the duct plate 126, the tubes 118 are not joined by fins
so that the flue gases can pass through the tubes 118, above
the duct plate 126 and below the steam drum 124.
Water is made available to the tubes of the front and
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rear walls 114 and 116 and to side walls 128 and 130 by a
header 132 which surrounds the furnace. The header 132
receives water from a feeder tube 134 which is connected with a
downcomer 136 extending downward from a water drum 138. The
water drum 138 receivqs feedwater from a feedwater supply tube
140.
A feeder tube 142 is also connected to the downcomer
136 and supplies water to a header 144 which in turn supplies
the water to a plurality of vertically extending water tubes
146 which make up a partition wall 147 as shown best perhaps
in Fig. 3. Only some of the water tubes 146 are shown in Fig.
2 for clarity, it being understood that in the preferred embodi-
ment they extend over the width of the steam generator 10 be-
tween the side walls 128 and 130. As shown in Fi~. 3 the water
tubes 146 are spaced so that they each touch contiguous tubes.
This arrangement is due to the fact that the water tubes 146
are receiving heat from two sides; and therefore, more heat
is transferred through each tube 146 than each of the tubes
118 which receive heat from only one side.
A collection header 152 receives steam from the tubes
146 and conveys the steam into the steam drum 124. A number of
downcomer tubes 148 extend down from the steam drum 124 to the
water drum 138 and a number of evaporation tubes 150 extend
from the water drum 138 up to the steam drum 124.
In practice, feedwater enters the steam generator 10 through
the feedwater supply tube 140 and enters the steam drum 124
and flows down through the downcomer tubes 148 to the water drum
138. The water then flows downward through the downcomer 136
to enter the headers 132 and 144. The water from the header
132 then flows upward through the water tubes 118 and the water
from the heac1er 144 flows upward through the water tubes 146.
The water in the tubes 118 and 1~6 is converted into steam or a
steam-~later mixture and enters the steam drum 124. Steam is
taken off through a steam pipe 154"and delivered to the point
of use.
Water in the steam-water mixture in the steam
drum 12~ will settle on the bottom and flow downward through
the downcomer tubes 148 to the water drum 138. Water will be
heated in the evaporation tubes 150 and rise as steam or a
steam-water mixture into the steam drum 124. The direction of
flow within the tubes 148 and 150 is due to the direction of
flow of the exhaust gases over the tubes 148 and 150 between
the duct plate 126 and the roof 122.~ Since heat is extracted
by the tubes 150 before thè gases reach the tubes 148, the
water in the tubes 148 will not be heated as much as water
within the tubes 150. Thus, the tubes 148, in effect,
serve as heated downcomers. The exhaust gases leave the steam
~enerator 10 through the duct 24.
The black liquor is sprayed in through the sprayer
~0 88 or flowed in through conduit 76. Water can be supplied
through either or both the sprayers 94 and 96. The water may
be made available at critical times such as when the generator
is started up but the use of the water can be min1mized or
completely eliminated because the fluidized bed 22 will be
maintained at temperatures below the fusion point of the bed
material by reason of the extraction of heat from the fluidized
bed 122 by the tubes 146 and 118. ~
With the present invention, it is possible to recover
the inorganics from the black liquor in a fluidized bed without
continuously spraying water into the bed to limit the
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temperatures. Further, the concentrations of the black liquor
can be higher th~n in the fluidized beds of the prior ar-t.
This is advantageous because the higher concentration makes for
a black liquor of higher heating content and consequently more
available energy.
At certain times during operation it may be necessary
to supply heat to start or continue the combustion within the
steam generator 10. Heat is supplied by adding a fuel into the
fluidized bed 22. For example, natural gas can be injected
with the fluidizing gas and burned. Other fuels such as fuel
oil or coal can be burned within the fluidized bed 22 to supply
heat.
The foregoing describes but one preferred embodiment
of the present invention. Other embodiments being possible
without exceeding the scope of the present invention as defined
in the following claims:
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