Note: Descriptions are shown in the official language in which they were submitted.
1~74014
Field of the Invention
The present invention relates to a recovery
system for kraft pulping process. More particularly, the
present invention relates to an improved recovery system
wherein at least a portion of the black liquor is pyrolyz-
ed to produce a reducing gas, the pyrolyzed solids oxidiz-
ed to provide oxidized inorganic solids and the gas gene-
rated during pyrolyzing used to reduce the oxidized inor-
ganic solids.
Background to the Invention
As taught in U.S. patent 4,011,129 issued March
8, 1977 to Tomlinson, it is known to burn a portion of the
black liquor from the kraft pulping in a fluidized bed
oxidation zone and to direct the pellets formed during
this fluidized bed oxidation onto the surface of the char
bed in a conventional kraft recovery boiler and there to
reduce the sodium sulfate in the pellets to sodium
sulfide. This process has proved quite satisfactory and
is currently being practiced at Domtar's Fine Paper Mill
in Cornwall, Ontario. The reduction efficiency of the
furnace has been found to have been, to some extent,
adversely affected by the additional load applied thereto
through the application of pellets on to the bed.
It is also well known to pyrolyze a residual
pulping liquor and subsequently burn the gas generated
while simultaneiously burning the remainder of the orga-
nics in the liquor, thereby to generate sodium carbonate
pellets and to subsequently contact the sodium carbonate
pellets with SO~ derived from the burning of the remainder
,
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of the gas from the pyrolyzer to form sodium sulfate (see
Canadian patent 960,811, issued January 14, 1975 to
Shick).
It has also been proposed in United States
Patent 4,135,968 issued January 23, 1979, to Dehass, to
dry a portion of the black liquor from a kraft mill,
pyrolyze~same and add the dry pyrolyzed solids to the
remainder of the black liquor for injection into the
recovery furnace whereby the inorganics in the pyrolyzed
solids as well as in the remainder of the liquor are
reduced in the recovery furnace. The gas from the
pyrolyzer will generally be treated for sulfur removal and
then burned in a separate unit.
In another arrangement dry residual liquor
solids are pyrolyzed to produce a combustion of fuel gas
from about 70% of the organics present in the dried liquor
and the remaining organics and the inorganics are treated
in a molten salt bath gasifier to produce further fuel gas
and a smelt. The gasifier may be operated under reducing
conditions so that the smelt when dissolved provides a
green liquor. The fuel gases produced in the pyrolyzer
and the gasifier may be burned to generate steam in a
boiler.
Brief Descri tion of Present Invention
p
It is the ob~ect of the present invention to
provide a process for recovering residual liquor from a
kraft pulping operation.
Broadly the present invention relates to a
process for the recovery of chemical from residual liquor
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resulting from a pulping cellulosic material comprising
pyrolyzing at least a portion of the residual liquor to
provide a reducing gas and residual solids containing
inorganic and organic material, said reducing gas contain-
ing sulfur compounds, oxidizing the organic material in
said solids and recombining said sulfur in said reducing
gas with said oxidized inorganics to produce other type of
sulfur compounds and reducing said other type of sulfur
compounds with said reducing gas. Recombining may be per-
formed either in the oxidation stage or ghe reducing stage
or both.
Brief Description of the Drawings
Further features,-objects and advantages will be
evident with the following detailed description of the
preferred embodiments of the present invention taken in
conjunction with the accompanying drawings in which:
Figure 1 is a schematic illustration of one form
of the invention.
Figure 2 schematically illustrates a modified
pyrolyzer system for incorporation into the present inven-
tion.
Descri tion of the Preferred Embodiements
p
of the Invention
As illustrated, the oxidized black liquor say,
about 15% solids enters the system via line 10 and passes
through a set of multiple effective evaporators 12 (only
one shown) and is concentrated say, to about 50% solids
leaving the system via line 14. Steam enters the evapor-
ator via line 13 and condensate leaves via line 15.
The concentrated liquor in line 14 may be
1~74()~4
delivered into a spray dryer or the like 16 where it is
dried and leaves as a coarse powder via line 18. The
fines leaving the system via line 20 are separated from
the gas stream in the cyclone 22 and pass via line 24 to
line 18. Gases are removed from the cyclone 22 via line
26 and some may be recycled via line 28 back into the
dryer 16.
Alternatively, the black liquor may simply be
concentrated to the desired degree in the evaporators 12
and the liquor in line 14 pyrolyzed under pressure in the
presence of water to provide a reducing gas and residual
liquor solids containing both organic and inorganic com-
ponents as will be described hereinbelow.
The powder liquor solids in line 18 passes into
a pyrolyzer generally indicated at 30. During start up,
fuel such as natural gas or the like enters via line 32
and the exhaust gases pass via line 34 into a fluidized
bed boiler system 36. The air entering the pyrolyzer 30
passes through heat exchanger 38 to absorb heat from the
exhaust gases in line 34 and then via line 40 into the
pyrolyzer to provide oxygen for combustion of a natural
gas or other fuel. In operation the heat necessary to
operate the pyrolyzer will normally be derived from the
organics in the liquor.
The pyrolyzed solids containing some of the
organics, generally about 50% to 80% and normally about
70% of the organic originally in the liquor pass via line
42 to a pelletizer or the like 44 (if the particle size is
so small that it would otherwise simply carry out in the
A ~ ,:,
1:~74~
gas stream from the fluid bed) and are injected via line
46 into the fluid bed 36 where the organic components are
burned and the inorganics accumulated in the ~orm o~
pellets. Some of the pyrolyzed powder may be recirculated
to the pyrolyzer 30 via line 48. These pyrolyzed solids
are relatively hot and are mixed to the powdered li~uor
solids from line 18 to heat the liquor solids preferably
to the temperature at which pyrolyzing may commence.
Reducing gas leaves the pyrolyzer via line 50
and may be treated in a variety of different ways depend-
ing on the specific system to be employed and depending on
the manner in which the pyrolyzer is operated. For
example the gases may pass into a unit generally indicat-
ed at 52 which may either be a sulfur separation unit or a
sulfur oxidation unit or the unit may be completely by-
passed. These alternatives will be discussed herein
below.
The fluidized bed 36 in the illustrated arrange-
ment preferably is provided with a heat exchanger gen-
erally indicated at 78. In the illustrated arrangement thewater or steam enters via line 80, passes through the heat
exchanger 78 which is shown in the bed and through a
second heat exchanger 82 mounted in the freeboard area and
steam leaves the system via 84. Alternatively fluid to be
heated (steam or water) may flow in the opposite direc-
tion, it being important to absorb heat from the combus-
tion or oxidation occuring in the fluidized bed 36 to
ensure the bed is not overheated. The bed temperature may
be controlled by adding water but the use of indirect heat
~d
1: L74()~4
-- 7 --
exchange improves the thermal efficiency of the system.
As will be apparent fluidizing air is introduced into the
bed via the line 86 and oxidizes the organic materials
entering the bed via the line 46 as a part of the pyrolyz-
ed solids. ~aseous fuel may also be injected into theunit via line 70 either into the bed (if sufficient heat
exchange surface is available) or into the freeboard and
burned to generate heat which will be recovered in the
boiler 78 and 82. Flue gases leave the fluidized bed 36
via line 88 and pass into the spray dryer 16 to dry the
liquor entering this dryer in the figure 1 embodiment.
As above indicated the solids entering the fluid-
ized bed via line 46 form pellets of sodium
carbonate and sodium sulfate. These pellets as described
above may be crushed in the crusher 68 to a smaller par-.
ticle size an~ then introduced into-the reducer-56. Crush-
ing reduces the time required for reduction i.e the
smaller the particle size the less time required for
reduction. It has been found that if the particle size is
too large the ceducing action is impaired, therefore, in
some cases the particle size may require reduction,
hcwever, the size range of the pellets normally extracted
from the fluid bed (generally about 200-2000 microns range
and an average of about 800 microns) are quite satisfact-
ory.
While it is preferred to use the dry method i.espray dryer 16 etc. and to pyrolyze dry solids, it is also
possible to take the liquor from the line 14 ~at say S0
solids~ as illustrated in figure 2, to pressurize same
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with a pressurizing pump 29 and ~eed the liquor through a
pressurized heater 31 heated by any suitable means and
carry the heated liquor via the line 33 to a separator 35
where the reducing gas produced is separated and leaves
via line 49 which connects to line sn in figure 1 ~or use
in any of the three embodiments described hereinbelow.
The pyrolyzed liquor solids remaining after
separation of the reducing gas passes via line 37 back to
the heater 31 where heat is extracted and used to heat the
liquor entering frorn line 14. The pyrolyzed solids
together with any liquid (water) accompanying same passes
from the heater 31 via line 45 to line 46 shown in figure
1 for injection into the fluidized bed 36. Any further
heat required for pyrolyzing, may be added in the heater 31
by suitable means as indicated by the arrow 39 and the
waste heat recovered, if desired, from line 41.
Either the figure 1 or figure 2 pyrolyzing con-
figuration may be used with the first, second and third
embodiments of the invention to be described herein below,
but means may be required to treat the flue gases from the
fluidized bed to ensure that sulfur loses are maintained
with acceptable limits if the wet technique of figure 2 is
used since the direct contacting of the liquor with the
flue gas may not be used.
In either of the figure 1 or figure 2 arrange-
ments, the amount of inorganics pyrolyzed will be at least
sufficient to generate sufficient reducing gas to reduce
the material ~inorganic solids from the residual liquor)
in the reducer 56. Generally just enough reducing gas
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1~740~4
g
will be generated to reduce the material unless the
requirement for ~uel gas is significantly higher than the
reducing gas requirements, but the sulfur content of the
gas must be recovered.
First Embodiment
If the unit 52 is bypassed the reducing gasses
will pass directly via lines 54 and 58 into a reducer 56
which may be heated e.g. by fuel gas fed in line 59 or by a
portion o~ the reducing gas line 54 directed to line 59
through a line not shown. The exhaust heating gases from
the reducer pass via line 60 into the fluidized bed 36,
while incoming air passes in exchange with the air in line
60 in the heat exchanger 62 and via line 64 to form
combustion air for the gas entering via line 59.
Pellets formed in the fluidized bed 36 are
removed from the bed, pass via line 66 and, if desired a
crusher 68 into the reducer 56 where the sulfur components
(Na2SO4) are reduced by the gases entering via lines 54 and 58.
Gases from the reducer pass via line ~0 into the fluidiz- -
ed bed 36 for combustion and adso~btion of the SO2 by the
inorganics forming the pellets and the reduced pellets
leave the reducer via line 72.
It is normally preferred to introduce these
~ases from the reducer into the bed i.e directly into the
fluidized bed portion where they are oxidized since these
gases may contain $ulfur compound that react with the
material in the bed thereby to prevent escape of SO2 and
to retain the sulfur in the system. However, it is
imperative that the bed temperature not exceed a certain
-
. . ; .
1~714~.4
maximum temperature level and if there is not adequate
heat exchange available in the bed it may be necessary
to introduce and burn this gas in the freeboard area where
the sulfur compounds will combine with dust and the heat
generated absorbed in the boiler section. Any SO2 so
formed and not reacted with the dust can be scrubbed frorn
the flue gas using, for example, the drier 16 or other
suitable means (not shown) such as means for direct con-
tact absorption by the black liquor from line 14.
Second Embodiment
In an alternate arrangement the gases leaving
the ~rolyzer in line 50 and containing sulfur compounds
may be passed through the unit 52 which is a sulfur
separator adapted to separate the reducing gas from
SO2 and passes it into the reducer 56 via line 58 to
reduce the pellets entering the reducer 56 via line 66.
The resu~tant reducing gases are now free of sulfur and
thus may be either fed directly back to the fluidized bed
as indicated via line 70 or alternatively can be passed
via line 76 to form a fuel for any suitable application
e.g. a calciner, power boiler, etc. The SO2 gases
separated in the unit 52 pass via line 74 into the fluid
bed 36 and are absorbed by the inorganics in the bed 36 or
in the drier 16 or in both.
The unit 52 of the above identified arrangement
may provide for catalytic oxidation and the adsorbtion of
H2S and TRS emissions from the pyrolyzer using the follow-
ing chemical reaction in one reactor.
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1~740~4
TRS or H2S + Fe23 _ ~ Fe2S3 + ~2 (1)
Sulfur is then desorbed from the iron to
regenerate ferric oxide in a second reactor according to
the following eq~ation.
Fe2S3 + O2(air) ~ S2 ~ Fe23 (2)
The sulfur dioxide then passes via line 74 as
above described into the fluidized bed 36 or some other
suitable means where it contacts with the inorganics of
the pulping liquor and is captured. These inorganics will
include sodium sulfate and sodium carbonate. A portion of
the later will react with the SO2 at the operating
temperature in the fluidized bed of about 1250CF to form
sodium sulfate according to the following equation:
S2 + Na2C3 ~ Na2SO4 + C2 (3)
15 Finally the sodium sulfate is reduced in the
reducer 56 in accordance with the following reaction:
Na2SO4 + H2/CO~ ~ Na2 S + H2O+CO2 (4)
Third Embodiment
Alternatively unit 52 may function as a
catalytic oxidation unit converting the H2S or TRS in the
gasses leaving the pyrolizer as follows.
TRS or H2S ~ 2 - ? S2 + heat(5)
-catalyst eg. activated
carbon, bauxite
The reducing gas now containing sulfur dioxide
so generated may then be passed directly to the reducing
zone via the line 58 to react with the pellets from the
fluidized bed in particular the sodium carbonate in the
pellets from the bed 36 as follows.
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S2 + Na2a)3 ~_? Na2S04 ~ C2 ( 6
The sodium sulfate formed by the above reaction
as well as any sodium sul~ate in the pellets from the
fluid bed 36 further react under the reducing conditions
5 in the reducer 56 as follows:
Na2So4 + H2/C ~ Na2S + H20/C2
The gases leaving the reducer pass via line 70
back to the fluidized bed either directly into the bed or
into the freeboard. Burning these gases in the freeboard
10 does not require as stringent SO2 recovery as with the
first embodiment since the amount of SO~ in the gas is
relatively small. Alternatively these gases may be directed
via line 76 used in another suitable combustor since their
sulfur content will be quite low.
15 EXAMPLE
Various test were conducted in the lab to prove
the operability of the steps of the present invention.
1. Hardwood kraft pulping liquor was dried in a
pilot plant scale co-current spray drying system having a
20 cross-sectional area and chamber height of about 1.0 ft2
and 8 ft respectively. Hot air was used as a drying gas
and typical results obtained were as follows:
Drying Air StreamBlack Liquor Intlet R~sidua-l
Liquor
Inlet OutletFlow Temp. Solids Powder
flow temp. Temp. Moisture
(SCFI~I) (C) (F)Imp. Gal./hr. (C) (%) (96)
126-135 218-233 113-127 3.4 ~9-92 48.2 1.1-2.3
101-103 188 101 2.4 90 54.7 4.0
2. The burning of kraft pulping liquor produces
pellets containing approximately 32% Na2SO4 by weight or
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- 13 -
15.1 lb Na2SO4/100 lb black Liquor solids. To obtain a
complete Na2SO4/Na2S conversion, assuming the following
reduction reactions:
Na2So4 + ~H2 > Na2S ~ 4H20 ( 1 )
Na2SO4 + 4CO ~ Na25 + 4CO (2)
A stoichiometric requir2ment of four moles of H2
or CO per mole of Na2SO4, corresponding to 0.42 lb moles
reducing gas per 100 lb black liquor solids, would be
required. Literature data indicated that for each 100 lb
black liquor solids, the pyrolysis of kraft liquor at
about 700C produced a non-condensible gas mixture with
about 1.0 lb moles H2 0.15 lb moles CO, 0.10 lb moles CH4
and 0.40 lb moles of CO2. Obviously if the above gas
mixture is the source of reducing gases, the completion of
reactions (1) and/or (2) still leaves about 0.G8 moles of
H2 and CO per 100 lb black liquor solids present in the
spent reducing gas stream, i.e. more than 100% in excess
of the stoichiometric requirement. Since the reducing
process is not 100% efficient it will normally be necess_
ary to provide at least a slight excess of such gas over
and above the ~toichiometric requirements.
3. Inorganic pellets, formed from oxidation of
spent hardwood kraft pulping liquor in a fluidized bed
type combustion unit, and containing about 67% sodium
carbonate and 33% sodium sulfate were reduced in the solid
state using CH4, CO or H2 which are known to be the
principal vapour products of pyrolysis of spent kraft
pulping liquor. The following sulate reduction results
were obtained:
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1~74(~14
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Reducing Gas Stream(3) Pellet Reducing Sulfate Reduc-
Size Duration tion Effic.(l)
Type Temp.
(C) (,~icrons) (hrs) (%)
CH4 680254 5.0 24.7
CH4 680421 ~.~ 13.4
CO 620254 1.0 42.3
10 CO 620254 3.0 66.4
H2 620254 2.0 74.4
H2 620254 2.5 83.2
H2 620421 2.5 63.8
15 H2 620635 2.5 55.6
H2 6201060 1.0 24.2
H2 6201060 2.5 41.4
H2 6201060 5.0 83.3
20 50% H2 in
Nitrogen 620254 2.0 71.2
25% H2 and
25% CO in
25 Nitrogen 620254 2.0 61.8
(1) (moles Na2S formed/initial moles Na2SO4 in pellet)
(2) Temperature at the wall of the tube furnace
(1.0" ID x 13" length)
(3) Flow 0.3 - 0.5 l/min
4. Based on the above table and literature indica-
tions that the composition of the non-condensable reducing
gas produced by the pyrolysis of a spent krat pulping
35 liquor would contain about 60% H2, 10% CO2 and 6% CH4 and
a typical mean pellet size from the fluid bed of abo~ 800
microns, it was estimated that to obtain about 90% overall
sulfate reduction would require a total reducing time of
about 6-10 hrs at a temperature of 620 to 650C.
Having described the invention, modifications
will be evident to those skilled in the art without
departing from the spirit of the invention as defined in
the appended claims.
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