CA1277808C - Black liquor concentrating apparatus incorporating black liquor combustion exhaust gas absorbing means - Google Patents

Abstract

BLACK LIQUOR CONCENTRATING APPARATUS INCORPORATING
BLACK LIQUOR COMBUSTION EXHAUST GAS ABSORBING MEANS

Abstract of the Disclosure:
A black liquor concentrating apparatus is disclosed 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, wherein said black liquor concentrating apparatus comprises (1) a primary concentrator for concentra-ting the AP black liquor discharged from said cooking means and washing means, (2) a secondary concentrator for further concentrating the AP black liquor concentrated in said primary concentrator and (3) exhaust gas absorbing means disposed between said primary and secondary concentrators.

Description

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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. ~
~ i A ~' 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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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~).

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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.

; - 4 -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 ; - 5 -" .

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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.

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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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 . ., ."~ '`
f _ g _ ~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.

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 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 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 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.

Claims (12)

1. A black liquor concentrating apparatus for concentrat-ing 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, 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 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 level controlling means and pH controlling means controlling the pH of the AP black liquor in the storage tank at a predetermined 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.

2. A black liquor concentrating apparatus according to Claim 1, wherein said multi-effect black liquor concentrating means is for concentrating the alkaline waste liquor dis-charged from said washing means only.

3. A black liquor concentrating apparatus according to Claim 1, wherein said multi-effect black liquor concentrating means is for concentrating the alkaline waste liquor dis-charged from said cooking means only.

4. A black liquor concentrating apparatus according to Claim 1, wherein said exhaust gas absorbing means is a perforated plate tower.

5. A black liquor concentrating apparatus according to Claim 1, wherein said exhaust gas absorbing means is a spray tower.

6. A black liquor concentrating apparatus according to Claim 1, wherein said exhaust gas absorbing means is a Venturi scrubber.

7. A black liquor concentrating apparatus according to Claim 1, wherein said AP black liquor is a black liquor which contains an alkaline waste liquor which is discharged from bleaching means for bleaching the pulp which has been washed by said washing means with alkaline chemicals containing substantially no sulfur compounds.

8. A black liquor concentrating apparatus according to Claim 1, wherein said vapor-liquid separator is a mist separator.

9. A black liquor concentrating apparatus according to Claim 1, wherein said exhaust gas absorbing means and said storage tank are united in one body.

10. A black liquor concentrating apparatus according to Claim 1, wherein said exhaust gas absorbing means has said storage tank in a separate body.

11. A black liquor concentrating apparatus according to Claim 1, wherein said predetermined range of pH is between 9.5 and 12.5.

12. A black liquor concentrating apparatus according to Claim 1, wherein said storage tank has a return pipe for returning the black liquor to said exhaust gas absorbing means.