Note: Descriptions are shown in the official language in which they were submitted.
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BLACK LIQUOR CONCENTRATING A~PARATUS INCORPORATING
BLACK LIQUOR OOMBUSTION EXHAUST GAS ABSORBING MEANS
S60pe of the I~entio~:
The present invention relates to a black liquor
concentrating apparatus for concentrating an alkaline waste
liquor in wood pulp producing system for producing wood pulp
5 by treating wood chips with alkaline chemicals contain-
ing substantially no sulfur compounds, so-called AP black
liquor.
~rief Description of the Drawings:
The present invention is described with reference
10 to the accompanying ~rawings in which:
Fig. 1 shows a flow sheet in the prior art;
Fig. 2 shows the change in the viscosity of the black
liquor with the corresponding concentration thereof;
Fig. 3 shows the changes in the viscosity of the black
15 liquor with the corresponding pH thereof;
Fig. 4 shows the changes in the concentration speed
of the black liquor with the corresponding concentration
thereof;
Fig. 5 shows the changes in the boiling point of the
20 black liquor with the corresponding concentration thereof;
Figs. 6 through 8 are flow sheets showing the wor~ing
examples of the present invention;
Figs. 9 and 10 show a spray tower and a Venturi
scrubber, respectively and they are typical examples of C02
gas absorbing means used in the present invention; and
Fig. 11 shows a disk evaporator which may be used as a
part of the secondary concentrator. ~
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1 277~3~8
In the production process of wood pulp, particularly
the production process of chemical pulp, a kraft pulp cooking
method (hereinafter referred to as RP process) is now the
main production method of chemical pulp because the quality
of the so produced pulp is preferable and the method of
recovering chemicals therefrom has been established. An
example of the flow chart with respect to the recovery of
the cooking chemicals is shown in Fig. 1.
On the other hand, it has been confirmed as a result
lo of the recent research that a soda substantially using sodium
hydroxide as a main cooking chemicals gives almost the same
level of yield and quality of pulp as are obtainable in the
KP process. Moreover, in this process (hereinafter referred
to as AP process), no sulfur compound like sodium sulfide is
used as a cooking chemicals, and therefore the process is
seen as a production process of chemical pulps which gener-
ates no sour smell substance during the cooking process which
is in contrast with the KP process.
As the method of recovering chemicals in the AP
process, it is possible to employ a direct causticization
method wherein the black liquor is blended with iron oxide
and subjected to combustion to produce sodium ferrite, which
is then hydrolyzed to recover sodium hydroxide as a cooking
chemicals. This method, whereas it cannot be used in the RP
process involving a sulfur compound, is known to be advan-
tageous in terms of energy efficiency and its process is
simplified as compared with a causticization method as is
employed in the RP process.
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1~7~
However, the viscosity of the AP black liquor i8 very
much higher than that of the alkaline waste liquor of the KP
process (referred to as KP black liquor hereafter) when both
liquors have the same concentration. The higher the concen-
tration, the greater the difference in the viscosity. Asshown in Fig. 2, the viscosity of the AP black liquor
becomes about ten times as high as that of the RP black
liquor at a concentration of 70%.
High viscosity of the black liquor indicates that its
fluidity has deteriorated and the concentration efficiency of
the black liquor in an evaporator is lowered. It also lowers
the efficiency of transporting the black liquor into a com-
bustion furnace and deteriorates sprayability from a burner
in the furnace, so that the combustibility of the black
liquor will be lowered.
As for conventional viscosity lowering methods for the
black liquor which are well known to the art, the following
methods are preferred:
2~ l. lowering of the concentration of the black liquor;
2. raising the temperature of the black liquor;
3. adding a surfactant to the black liquor, etc.
~ owever, if the AP black liquor is concentrated by
using the same apparatus as the prior concentrating apparatus
of the KP black liquor, it is unavoidably necessary to carry
out combustion in a black liquor concentration of 60% which
is 5 to 10% lower than that in with the KP black liquor (65
to 70~).
~r - 3 -
127~
The decrease which occurs in the black liquor concen-
tration subjected to combustion indicates an increase in the
amount of water to be evaporated in a combustion furnace, so
that enerqy cannot be effectively used as heat energy.
There is a problem that more heat energy is required
for raising the temperature of the black liquor. A method of
adding a surfactant to the black liquor has been disclosed in
Japanese Patent Public Disclosure No. 228094/84, in which the
viscosity is only lowered to a level of 1/2 to 1/3 of the
viscosity of the untreated black liquor and thus it is diffi-
cult to lower the viscosity to the same level of that of the
KP black liquor.
The object of this invention is to dissolve the
problems which stem from the fact that alkaline waste liquor,
in particular, AP black liquor, contains substantially no
sulfur compounds and has a viscosity which is 10 times as
high as that of the RP black liquor, and namely, that there
is a deterioration in the fluidity of the black liquor
including a lowering of the concentration efficiency,
decrease in the transport efficiency, deterioration in the
sprayability, lowering of availability of heat energy for
combustion caused by the decrease of the concentration of the
black liquor in order to maintain a certain level of viscos-
ity and the like.
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1;~77B08
Inventions which are similar and related to the
present invention include those disclosed in Canadian Patent
Application 491,196. It has been confirmed as a result of
exhaustive study of these previously filed inventions that
the present invention is even more advantageous. More
specifically, the present invention differs from Canadian
Patent Application 491,196 in that it is unnecessary,
according to the present invention, to oxidize an AP black
liquor containing substantially no sulfur compounds. Also,
there is no necessity with the present invention to add pure
C2 as a substance capable of improving the ability of the
AP black liquor to be concentrated. Although the previously
filed inventions disclose a method wherein C02 is added to the
black liquor in each concentration step, it has been found
that, when absorbing means usinq AP black liquor combustion
exhaust gas is employed, it is more advantageous to absorb the
exhaust gas into an AP black liquor having a black liquor
concentration of 40 to 65% than into an AP black liquor
having a black liquor concentration higher or lower than said
range when taking into consideration the fact that the black
liquor concentration of 40 to 65~ enables a reduction in the
amount of AP black liquor to be treated, the necessity to
avoid foaming of the black liquor in a case where the concen-
tration of the AP black liquor is low, and the absorption of
the exhaust gas into the black liquor and the difficulty in
absorbing exhaust gas into the black liquor in a case where
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lZ7~0B
the concentration of AP black liquor i8 high. In addition,
the above-described prior arts disclose an apparatus in which
the pH of the black liquor into which C02 gas has been
absorbed is measured in the circulation line to control the
amount of black liquor to be delivered in accordance with the
measured pH value. Although this method enables control of
the pH of the AP black liquor, it has been found that such
prior art method disadvantageously involves variations in
the amount of C02 gas-absorbing black liquor supplied to the
black liquor concentrating means. ~he present invention
therefore overcomes this disadvantage and proposes an improved
method wherein the amount of AP black liquor to be introduced
into the exhaust gas absorbing means is controlled by measur-
ing the pH of a portion of the black liquor in the exhaust
gas-absorbing black liquor storage tank. It has been con-
firmed that this novel method enables the amount of black
liquor supplied to the secondary black liquor concentrator
to be stabilized by such pH control of the storage tank and
consequently permits the operation of the secondary black
liquor concentrator to be even more stabilized.
The present invention, which has been completed as a
result of the research on the method of lowering the viscos-
ity of the black liquor, is based on the discovery that the
viscosity of the AP black liquor lowers to the same level as
or less than that of the RP black liquor, when C02 gas
contained in the AP black liquor combustion exhaust gas
(referred to as the AP exhaust gas hereinafter) is absorbed
into the AP black liquor. Thus, the property of the AP black
liquor to be concentrated is improved.
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1277W8
Summary of the Invention:
The present invention is directed to a black liquor
concentrating apparatus for concentrating an alkaline waste
liquor in wood pulp producing system for producing wood pulp
by treating wood chips with alkaline chemicals containing
substantially no sulfur compounds, said system comprising:
(a) means for cooking wood chips at high temperatures and
under high pressure together with an alkaline chemicals
containing substantially no sulfur compounds;
(b) washing means for separating wood pulp from an alka-
line waste liquor, said wood pulp having been produced in
said cooking means, and for washing as well as dewatering
said wood pulp;
(c) multi-effect black liquor concentrating means for
concentrating the alkaline waste liquor, that is, so-called
AP black liquor, discharged from said cooking means and
washing means;
(d) burning and recovering means for burning the black
liquor concentrated by said black liquor concentrating means
together with iron oxide, and recovering burning reaction
products consisting essentially of sodium ferrite, together
with heat energy; and
(e) causticizing means for hydrolyzing and separating the
burning reaction products recovered by said burning and
recovering means into cooking chemicals and iron oxide,
1m~08
wherein said black liquor concentrating means
comprises (1) a primary concentrator for concentrating the AP
black liquor discharged from said cooking means and washing
means to the range between 40 and 65~ by weight of the solids
content, and (2) a secondary concentrator for further con-
centrating the AP black liquor concentrated in said primary
concentrator to the range between 65 and 85% by weight of the
solids content, and for supplying the concentrated AP black
liquor to said burning and recovering means, wherein exhaust
lo gas absorbing means is disposed between said primary and
secondary concentrators, said exhaust gas absorbing means
having an exhaust gas inlet and outlet, said exhaust gas
inlet being adapted to introduce the AP black liquor combus-
tion exhaust gas discharged from said burning and recovering
means such as to absorb the Co2 gas contained in said exhaust
gas into said AP black liquor by directly contacting said
exhaust gas with the AP black liquor, wherein said exhaust
gas absorbing means has a storage tank disposed before said
secondary concentrator, said storage tank having an inlet for
introducing the black liquor from said exhaust gas absorbing
means thereinto, and a supply pipe for supplying the black
liquor to said secondary concentrator, wherein said exhaust
gas absorbing means has a liquid ievel controlling means and
pH controlling means controlling the pH of the AP black
liquor in the storage tank at a predPtermined range thereby
controlling the amount of AP black liquor and the exhaust gas
to be introduced and wherein a vapor-liquid separator is
disposed on the downstream side of said gas outlet of said
exhaust gas absorbing means, the downstream side of said
vapor-liquid separator being connected to a smokestack.
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1;~7~0B
The AP black liquor may be a black liquor which
contains an alkaline waste liquor and which is discharged
from cooking means and bleaching means for bleaching the
pulp, which has been washed by said washing means, with
an alkaline chemicals containing substantially no sulfur
compounds.
The multi-effect black liquor concentrating means may
treat the alkaline waste liquor discharged from either the
cooking means or the washing means.
Detailed DescriPtion of the Invention:
The viscosity of the AP black liquor can be lowered to
the same level as or less than that of the KP black liquor by
absorbing CO2 gas contained in the AP exhaust gas into the AP
black liquor and its ability to be concentrated is sharply
improved.
The viscosity of the AP black liquor can be lowered to
the same level as or less than that of the KP black liquor
and the concentrating efficiency is largely improved by
absorbing CO2 gas containing in the AP exhaust gas into the
AP black liquor.
The range of pH of the black liquor for which lowering
of the viscosity can be expected is between 12.5 or less and
9.5 or more, in which range preferable effect is obtained,
and more preferably a pH range of between 10.0 or more and
12.0 or less in a black liquor concentration of 40% at 80C
(pH values used hereinafter refer to the pH in a black liquor
concentration of 40% at 80C unless otherwise stated). If
the CO2 gas contained in the AP exhaust gas iS added to the
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f
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~277808
black liquor only in such an amount that the p~ of the black
liquor remains in the range of 12.5 or more, the viscosity is
not lowered sufficiently, as shown in Fig. 3. If the CO2 gas
contained in the AP exhaust gas is added to the black liquor in
S an amount sufficient to cause the pH to be in the range of
less than 9.5, it is presumed that the apparent viscosity of
the black liquor is adversely increased due to the formation
of solid substances in the black liquor.
The variation of the concentrating speed in relation
to the concentration of the black liquor is shown in Fig. 4
which is directe~ to the property of the AP black liquor to
be concentrated ~ue to the absorption of the AP exhaust gas.
As is apparent from this figure, the ratio of the concentrat-
ing speed of the black liquor to which the CO2 gas contained
in AP exhaust ga~ according to this invention has been added
relative to that of the conventional untreated black liquor
becomes larger as the concentration increases. As the con-
centration becomes high, the amount of the AP black liquor to
be treated becomes small, but if the level is too high it
becomes impossible to absorb CO2 gas contained in the AP
exhaust gas due ~o the increase in viscosity. If the con-
centration is too low, the amount of the AP black liquor to
be treated becomes large. The concentration of the black
liquor at which CO2 gas contained in the AP exhaust gas is
added in the range of 40% to 65%.
It has been discovered that the addition of CO2 gas
contained in the AP exhaust gas lowers not only the viscosity
of the black liquor, but also the boiling point thereof.
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1277808
Fig. 5 shows the boiling points of the AP black liquor in
which CO2 gas contained in the AP exhaust gas has been
absorbed as well as of an untreated black liquor. The
boiling point of the untreated AP black liquor elevates with
the increase in concentration of the black liquor and reaches
126C at black liquor concentration of 80%. The boiling
point of the AP black liquor to which CO2 gas contained in
the AP exhaust gas has been added gradually elevates with the
concentration of the black liquor up to 50% but it does not
elevate beyond 105 C even if the concentration of the black
liquor increases further.
A satisfactory explanation for this is the fact that,
assuming that the whole sodium content of the AP black liquor
having a concentration of 80% is sodium hydroxide only, the
boiling point of this black liquor is 118C, while, assuming
that the whole sodium content is sodium carbonate only, its
boiling point is 105C.
Decrease in the boiling point of the black liquor
indicates that the vapor pressure of water at the same point
is adversely high, so that the water in the black liquor is
easily vaporized and the black liquor is easily concentrated.
It is understood that the purpose of the present
invention, i.e. the feature of absorbing CO2 gas into the
AP black liquor, is not to separate lignin, silica and
hemicellulose from the AP black liquor.
The following examples will aid in explaining the
present invention but should not be deemed as limiting the
practice thereof.
1;~7~8
ExamPl e
As shown in Figs. 6 and 7, the black liquor which is
present intermediate between the primary and secondary con-
centrators of the multi-effect AP black liquor concentrating
-5 means, that is, the black liquor having a concentration of 40
to 65~, is temporarily stored in a medium concentration black
liquor tank provided on the downstream side of the primary
concentrator. The black liquor stored in the tank is intro-
duced into the absorbing means from the upper side thereof
with a pump. On the other hand, the AP black liquor combus-
tion exhaust gas is introduced into the absorbing means from
the lower side thereof so as to come into direct contact with
the AP black liquor flowing down from the upper side through
a perforated plate so that the CO2 gas contained in the
exhaust gas is absorbed into the AP blaclc liquor. The AP
black liquor having the CO2 gas absorbed therein is discharged
from the lower side of the absorbing means and introduced into
a storage tank having anagitator for storage therein. A pH
controlling means is disposed in the sto-age tank in such a
manner as to contact the black liquor. 'rhe pH controlling
means is adapted to control the amount o~ black liquor intro-
duced into the absorbing means so that the pH of the black
liquor is between 9.5 and 12.5, more pre~erably between 10.0
and 12Ø In addition, the black l quor in the storage tank
is recirculated to the upper side of the absorbing means with
a circulating pump so as to absorb the exhaust gas again
until the pH of the black liquor in the storage tank reaches
a predetermined range. The black liquor, which has reached a
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1Z77808
predetermined pH range is supplied in the same amount as that
to be introduced in the absorbing means stored in a black
liquor cushioning tank which supplies the black liquor to the
secondary concentrator. This is controlled by liquor level
-5 controlling means disposed on the storage tank. The black
liquor stored in the cushioning tank is supplied to the
secondary concentrator with a pump. On the other hand, the
surplus exhaust gas which is discharged from the absorbing
means is passed through a mist separator in order to remove
mist therefrom, and is then discharged into the atmosphere
from a smokestack. The mist thus separated is introduced
into the primary concentrator as a dilute black liquor and
concentrated therein. In this case, since the fairly highly
concentrated black liquor was produced by concentrating a
black liquor to some extent, the amount of black liquor
treated by the CO2 gas contained in the AP exhaust gas is
small, accordingly, the amount of power consumed by the gas-
liquid contact is small, and the viscosity of the black
liquor is not high enough to cause problems concerning the
gas-liquid contact. The reaction of the CO2 gas contained in
the AP exhaust gas with black liquor ends within a relatively
short time, and the black liquor is hardly foamed at all
since the higher the concentration of the black liquor, the
less its ability to be foamed.
Exam~le 2
Fig. 8 shows a case where the black liquor which is to
be introduced into the black liquor concentrating means is
mixed with an alkaline waste liquor that is discharged from a
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bleaching means which follows a washing means and bleaches
washed pulp with an alkaline chemicals containing substan-
tially no sulfur compounds.
In Example 2 also, treatment is carried out by various
means similar to those in Example 1.
These examples are described by referring to the
method of introducing the AP exhaust gas in the AP black
liquor such as to react the CO2 gas with the black liquor,
but this is not intended to exclude a source of CO2 gas other
than this AP exhaust gas, as it is also possible to use in
place of the AP exhaust gas the combustion gas of boiler or a
tai:l gas from a different process which contains CO2 in an
amount of more than 10% by volume.
It is advantageous to use such a tail gas because it
is very cheap as a source of gas. The CO2 gas contained in
the boiler exhaust gas and the tail gas can be used after
concentrating it through the absorption process, the membrane
separation process or the like. The efficiency of absorbing
gas is improved by these processes.
The absorbing means according to the present invention
is set forth below in detail.
The exhaust gas absorbing means of various types can
be used in the present invention, such as a perforated-plate
tower (Fig. 7), spray tower ~Fig. 9), and Venturi scrubber
(Fig. 10).
In considering the efficiency of gas absorption and
pressure drop, a Venturi scrubber is preferably used, more
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lZ7780B
preferably a spray tower and most preferably a perforated
plate tower in these exhaust gas absorbing means.
As the concentrating means, a falling-film type multi-
effect evaporator is employed in the examples, it is partic-
ularly advantageous to employ a disk evaporator (see Fig. 11)as a part of the secondary concentrator for a high solid
concentration.
As discussed above, the viscosity of the AP black
liquor can be greatly reduced by providing the AP exhaust gas
absorbing means absorbing CO2 into the AP black liquor. As a
result, the performance of the concentrating apparatus are
sharply improved.
The elevation of the boiling point of the black liquor
can be maintained to within an extremely small range, so that
it is possible to greatly enlarge the temperature difference.
This not only realizes an improvement in the property of the
black liquor to be concentrated, the miniaturization of the
concentrating apparatus and a reduction in cost, but it also
makes possible the concentration of the black liquor to a
high consistency, and cause an increase in the quantity of
heat recovered from the combustion of the highly concentrated
AP black liquor.
The provision of a storage tank having the liquid
level controlling means and pH controlling means make it
possible to constantly and continuously supply the black
liquor to the secondary concentrator.
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