CA1036758A

CA1036758A - Method of delignification of cellulosic material in alkaline medium using oxygen

Info

Publication number: CA1036758A
Application number: CA210,399A
Authority: CA
Inventors: Saisei Miyao; Katsuhiko Takeda; Tadashi Nagano
Original assignee: Toyo Pulp Co Ltd
Current assignee: Toyo Pulp Co Ltd
Priority date: 1974-02-18
Filing date: 1974-09-30
Publication date: 1978-08-22
Also published as: SE411464B; JPS50112501A; SE7412272L; DE2450608A1; GB1489222A; FI301374A7; DE2450608B2; NO743539L; FR2261371A1; FI56710B; FR2261371B1; JPS5241361B2; AU7383574A; BR7408260A; FI56710C; IT1022768B

Abstract

ABSTRACT OF THE DISCLOSURE

The present invention provides a method of delignify-ing cellulosic material with oxygen in the presence of an alkaline aqueous medium in a reaction vessel, which comprises previously dissolving said oxygen into said alkaline aqueous medium under a partial oxygen pressure of at least 4 kg/cm2 before charging the medium into the reaction vessel, contacting said alkaline medium containing dissolved oxygen with the cellulosic material in said reaction vessel at a temperature of 80 to 160°C to conduct oxidative delignification of said cellulosic material, said cellulosic material being introduced batch-wise or continuously into the reaction vessel, the con-sistency of said cellulosic material in said aqueous medium being so chosen as to form an agglomeration of the cellulosic material in the alkaline aqueous medium which agglomeration does not have any gaseous space therein, and the pressure in the reaction vessel being maintained more than the partial oxygen pressure employed in the oxygen-dissolving step, and discharging a part of the waste liquor formed during delignification from the vessel, while at the same time charging fresh alkaline aqueous medium containing dissolved oxygen into the vessel, so that the displacement of the waste liquor with the fresh alkaline aqueous medium containing dissolved oxygen is carried out continuously throughout the delignification, and so that the chosen consistency of the cellulosic material is kept substantially constant by controlling the discharging and charging volume of the alkaline aqueous medium.

Description

~036758 AS is generally known, an example of applying oxidative delignification with oxygen in a pulp and paper industry has been oxygen bleaching in which delignification is carried out at a pulp consistency of more than about 18~ by weight based on the mixture of pulp and an alkaline aqueous medium, i.e. in the vapor phase. The reason why so high a pulp consistency is necessary is that the solubility degree and the dissolution rate of oxygen into an alkaline aqu~ous medium are small. For this reason, pulp should be fluffed in a reaction vessel at a high pulp consistency to enlarge the surface area of pulp and to let the alkaline aqueous medium exist around the pulp fiber surface only as a thin layer. By contacting oxygen gas with such fluffed pulp through the thin layer of the alkaline aqueous medium, delignification proceeds effectively.

, 10367S~ .
In s~ch`delignification at the high pulp con-sistency as descri~ed above, it is possible, in the case where residual lignin i~n the pulp is small, to operate the system safely withbut risk of explosion due to the heat generation during lignin degra~ation if careful attention is paid to the control of reaction, since the exothermic reaction of lignin degradation by oxygen is not violent in this case. On the other hand, in case of delignification `
of high yield pulp having high lignin content, there is a big risk of explosion so far as the controlling method in commercial use is applied, due to the great heat generation caused by lignin degradation. It is now essential that some fundamental solutions for this problem be obtained.
On the other hand, many studies have also been done as to oxidative delignification at relatively low pulp consistency, i.e in the liquid phase. In general, oxidative delignification is considered to be the heterogeneoussurface reaction in which lignin degradation proceeds by the contact of cellulosic materials, alkaline aqueous medium and oxygen. Therefore, conditions of their contact and reaction conditions such as reaction temperature, pressure, time and type of alkali used have significant effects upon delignification, especially in case of oxidative delignification in the liquid phase.
For this reason, the choice of optimum conditions for delignification is so difficult that oxidative delignification in the liquid phase has never yet been put to complete practical application.
It is therefore an object of the present-invention to provide a new method of delignifying cellulosic material`s ,.

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in which disadvantages of prior art methods as described above are overcome and high lignin content pulp and further wood chips, as well as chemical pulp, can effectively be delignified.
It is another object of the present invention to provide a new method o delignifying cellulosic materials in which a risk of e~plosion due to the heat generation during lignin degradation is substantially eliminated.
It is further object of the present invention to provide a new method of delignifying cellulosic materials in which the contact of cellulosic materials, alkaline aqueous medium and oxygen is effectively made, to conduct effective delignification and to significantly shorten the reaction time.
It is further object of the present invention to provide a new method of delignification of cellulosic materials wherein high quality pulp can be produced.
In accordance with the invention, there is provided, in brief, a me~hod of delignifying cellulosic materials with oxygen in the presence of an alkaline aqueous medium at an elevated temperature in a pressurized reaction vessel which comprises previously dissolving said oxygen into said alkaline aqueous medium, contacting said alkaline medium containing dissolved oxygen with said cellulosic materials in said reaction vessel to conduct oxidative delignification of said cell~losic materials, the consistency of said cellulosic materials being kept so as to form an i agglomeration of the cellulosic materials and the alkaline aqueous medium not having any gaseous space therein~, dis-charging a part of or substantial part of the waste liquor, _ ~ _ ,! .

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formed during delignification, from the vessel, while supplementarily charging the fresh alkaline aqueous medium containing dissolved oxygen into the vessel, whereby the displacement of the waste liquor with the fresh alkaline aqueous medi`um containing dissolved oxygen is carried out continuously throughou~t the delig~ification.
~ The aforementioned and other objects and features of the invention will be apparent from~the following detailed d~scription, when read in conjunction with the accompanying drawings, in which Fig. 1 is a schematic diagram illustrating equipment used in a preferred embodiment of the invention;
and Fig. 2 is a graph showing a relationship between reaction times and Kappa numb~rs of resulting pulp obtained by the present invention and by a conventional method.
. We~have investigated various methods of oxidative delignification of cellulosic materials with oxygen in liquid phase at comparatively low cellulosic materials consistency of between ab`out 5 to about 20% by weight based on the mi~ture of cellulosic materials and the alkaline ! ~ aqueous medium, in order to provide a new and improved ¦ method wherein pulp of any lignin content can be delignified and the problem in vapor phase treatment as mentioned above can be solved. As the results of the investigation, following information has been obtained: in the case ~, where cellùlosic materials are subject to delignification with oxygen in liquid phase, ` 1) The oxidative degreadation of lignin proceeds I very quickly in the alkaline aquoues medium under certain ` ' ~0367S8 conditio~ of temperature and pressure, while the dissolution and transfer rate of oxygen gas in the alkaline aqueous m~dium are low~ For this reason, the` ambient zone of the reaction is considered to continuously suffer from a `'shortage of dissolved oxygen". Due to "oxygen starvation", the reaction to be eff~ected is~delayed. Therefore, in order to keep the reaction proceeding rapidly, it is necessary to maintain constant contac~ between the alkaline aqueous medium containing dissolved oxygen and the cellulosic materials.

2) On the other hand, soluble salts of organic acids which arè produced by oxidative degradation of lignin tend to accumulate inthe interface between the alkaline aqueous medium and cellulosic materials. This not only decreases the solubility of oxygen into the alkaline medium, but also adversely affects the contactive reaction of oxygen wi~h cellulosic materials due to a type of Donnan's membrane equilibrium. Therefore, it is preferred that such salts of organic acids by-produced be removed from the interface for effective delignification with oxygen.
The present invention is based on the above-mentioned discoveries as to delignification of cellulosic materials with oxygen in the alkaline aqueous medium.
According to the present invention, oxygen is previously dissolved into the alkaline aqueous medium and the waste alkaline medium is continuously displaced with fresh medium containing dissolved oxygen, thereby keeping constant contact of the alkaline agueous medium co-ntaining-dissolved oxygen with cellulosic materials.
. . -Furthermore, in order to remove salts of organic acids from the interface, the oxygen dissolved alkaline medium is passed through the pulp or chips suspènsion, mat, layer or the like to thereby break down the so-called Donnan's membrane'equilibrium.
The consistency of cellulosic materials to be employed in the present invention should be determined within the range in which displacement of alkaline medium can easily occur. That is to say, it is necessary to choose the consistency range in which the agglomeration of cellulosic materials in the alkaline medium retains its form as if it were a mat or layer, ~r the like without any gaseous space therein. ~owever, because pulp fibers have good affinity to water compared with undefibrated cellulosic materials such as wood chips, the consis'tency of pulp like materials to form the agglomeration of cellulosic materials and alkaline medium as a fiber mat, iayer or bed becomes iower than that for wood chips. Thus, the pre-ferable consistency varies according to the cellulosic materials being employed. In general, the consistency of cellulosic materials of between about 5 to about 15% by weight based on the mixture of cellulosic materials and the alkaline aqueous medium is preferred for chemical and semi-chemical grade pulp, and about 10 to about 20% for wood chips.
In the process of the present invention, cellulosic materials are treated at relatively low consistency, so ~that there is much medium which absorbs generated heat of reaction. The absorbed heat is therefore always carried away together with'the medium which is discharged from the . ' . .

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reaction vessel continuously throughout delignification, by medium displacement. ~ccor~ingly, there may be no risk of explosion or overheating in the reaction vessel as in the case of delignification at high pulp consistency.
Alkaline chemicals such as sodium hydroxide, carbonate, and bicarbonate may be used as the alkaline medium in the invention. It is preferred, however, to use sodium carbonate solution, from the recovery point of view, in case of delignification o defibrated materials wherein the rate of alkali penetration into materials is minor factor On the other hand, sodium hydroxide is well employed on wood chips wherein rapid and effective penetration of alkaline medium into chips is needed.
It is advantageous that the reaction time is remarkably reduced by.the process of the present invention, so when the process of` the invention is applied in bleaching of che~ical pulp, bleaching is accomplished within about 10 minutes, whereas applied in delignification of semi-chemical grade pulp or chip materials, delignification is attained within about 20 to about 40 minutes, which is quite short when compared with processes of prior art.
An incidental advantage of theinvéntion is that good quality pulp can be obtained, because the shorter the reaction time required, the less oxidative degradation of cellulosic fiber there is~
As far as reaction temperature is çon~erned, it may be generally about 80 to 160C, preferably about 100 to 150C, depending on thè kind o cellulosic materials and the degree of delignification. Partial oxygen-pressure under which oxygen is dissoived in the alkaline aqueous 10367S~
medium previous to the introduction into the reaction vessel maybe at least about 4 kg/cm2, and preferably more than about 7 kg/cm2 when a high degree of delignification is required.
Therefore, the total reaction pressure at the delignification may be required to be more than the partial oxygen pressure which has been previously employed to dissolve oxygen in the alkaline aqueous medium.
In practising th~ process of the present invention, either continuous or batch system may be employed. Fig. 1 shows a schematic device usable for practising a preferred embodiment of the process of the invention in which delignification is conducted continuously. Defibrated or non-defibrated cellulosic materials are continuously fed into a pressurized react~ion vessel (1) from the top thereof. The alkaline aqueous medium in which oxygen has previously been dissolved in an oxygen dissolving tank ~2) is also continuously fed into the vessel, which pressure is maintained at the same as or more than the partial oxygen pressure in the oxygen dissolving ta~nk, through a center pipe (3) having a large number of holes or slits arranged lengthwisely on the surface thereof. The alkaline aqueous medium containing dissolved oxygen is then radially spread out from the center pipe, across cellulosic materials fed into ~he vessel. Thus, the contact of cellulosic materials with the alkaline aqueous medium containing oxygen is performed and deli-gnification proceeds during such contact. Waste liquor ormed during delignification is continuously discharged or extracted through suction pipes -(4) and (5), and, at the same time, supplemental fresh alkaline aqueous medium ,- 8 -containin~ dissolved oxygen is continuously charged or fed into the vessel from the tank t2) through the center pipe (3).
The consistency of cel~ulosic materials inthe reaction vessel i~ kept constant by controlling the volume of input medium fed through pipe ~3) and output waste liquor ex-tracted thxough pipos ~4), ~5~ The lignin content of cellulosic materials is reduced by degrees, as cellulosic mat~rials moVe downwardly in the reaction vessel, and thus obtained delignified pulp is removed from the bottom of the vessel through a blow valve ~6).
Some part of the waste liquor, which is discharged from the relatively early stage of delignification through theupper suction pipe (4), may be transferred to a chemical recovery system (not`shown). The alkaline chemical obtained by the recovery system may be fed to the tank (2) for reuse.
Another part of the liquor, which is discharged from the lr latter stage of delignification through the lower suction pipe (5), may be re~cycled to the tank (2), where the liquor is mixed with the~ alkaline aqueous solution recovered by the recovery system and, if resuired, make-up solution and oxygen is thoroughly dissolved under pressure into the mixed solution. The regenerated orretreated solution thus obtained is recharged into the reaction vessel and reused as the supplemental fresh alkaline aqueous medium containing dissolved oxygen.
In this embodiment, two stages of delignification are considered, i.e., the early stage and the latter stage.
However, it may be possible in an other embodiment tO consider more multiple stages of dellgnification and to assume more .. ~
j, multiple layers corresponding to different degrees of .j . . .
'I _ g _ ,1 .1 ` .

10367Sff delignification of cellulosic materials in the reaction vessel by separately discharging the waste liquor from each of layers in the reaction vessel. By employing such multi-layered system as described above, it becomes possible to use different k~nds of alkali and different pH values of alkaline aqueous medium at the same time in a single reaction vessel.
For instance, when wood chips are used as cellulosic materials in the process of this invention, it is pre-ferred, in order to obtain good quality of pulp, to treat chips with sodium hydroxide solution of relatively high pH at the relatively early stages or layers in the vessel, andthen with sodium carbonate solution of low pH at the latter stages or layers w`nere delignification has proceeded to some extent Furthermore, it may be possible to carry out both delignification of cellulosic materials and washing of the resultantpulp in one vessel by treating the resultantpulp with hot water instead of the alkaline aqueous solution at the final stage or layer in the vessel.
When the liquor extracted from ~he latter stages of deli-gnification is recirc~ulated, after oxygen is thoroughly dissolved thereinto, in the relatively early stages in the vessel, on that occasion; the process may be carried out in a counter-current system~ By employing such a counter-current system, the total volume of the alkaline aqueous medium in use is mada small and the waste liquor having a higher concentration of organic substances is obtained consequently. This is preferable from the chemical recovery point of view. -The following examples are given by way ofillustration only.

, ~0367S8, ~ EXAMP~E 1 Unbleached k~aft pulp made from Douglas fir of a Kappa number 32.8 was treated for delignification according to the prèsent invention.
100 g of the pulp was placed in the center of a 8Q
autoclave ` by sandwiching the pulp with two wire mesh plates so as to provide a pulp consistency between two wire mesh plates of about 7% by weight when immersed in an alkaline`aqueous medium.
The alkaline solution which contained sodium carbonate and sodium hydroxide (~a2C03/NaOH=4/1) and had the alkaline concentratio,n of 5 g/~ as sodium oxide, was used as the alkaline medium for cooking.
The alkaline medium in which oxygen had previously been dissolved under a partial oxygen pressure of 8 kg/cm2 was quickly fed into and filled up the autoclave at a temperature of l10C. Then, into the thus filled-up vassel, fresh alkaline medium containing dissolved oxygen was additionally and continuously pumped from the bottom thereof at the rate of 1 ~/minute for 5 minutes, while the overflow surplus liquor was taken out from the top of the vessel. By this procedure, a part of the alkaline cooking medium was continuously displaced during delignification by the fresh alkaline aqueous medium containing dissolved oxygen.
The delignified pulp obtained had Kap~a number of 6.8 and b~ightness of 46.7 (GE), From the above results, it is evident t~at the treatment,described above is well applied to the first stage of à conventional bleaching process in bleaching kraft pulp .
.
~ 11 --~036758 EXAMPLE :2 Eucalyptus chips were cooked at a temperature of 180C for one ho`ur in an alkaline solution of sodium carbonate at an amount of, as sodium oxide, 15% by weight based on wood, and then we~e~defibrated by a disk.refiner. The crude pulp obtained had a Kappa number of 132 in a yield of 68~
The crude p~lp was delignified in the same manner andunder the sam~ conditions as in Example 1 except for employing the following: `
Partial Oxygen Pressure = 12kg/cm2 Temperature = 140C
Reactlon time = 10 minutes Magnesium carbonate was additionally mixed in the alkaline cooking medium at the concentration of 0.1 g/Q.
For comparison, the same chips were delignified by a conventional Xraft process under the following conditions:
Alkali charge = 22% as Na2O based on woods Sulphidity = 31.6%
Time to maximum temperature = 70 minutes Time at maximum temperature = 60 minutes Wood to liquor ratio = 1:5 The comparative data obtained by the process of the invention and by the kraft process are given in the following Table.

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Screened Kappa Brightness Breaking* Burst* Tear*
Pulp ~ield ~mber - _ Length,Km Facter Factor Pulp by the !
process of ~ 47.77.6 50.2 6~53 5.46 116 invention Kra~t Pulp ¦ ~2.6~ 10,~ 29~5 5.74 5.57 115 ~* PFI mill 400 cc freeness) As is appaxent from the table, the quality of the pulp made by the process of the invention is superior t~
the kraft pulp in yield and brightness, and equivalent in physical parameters such as breaking length, burst and tear factors.

Crude pulp having a Kappa number of 126 was pre-pared from hardwood chips by cooking the chips with sodium carbonate solution and then defibrating the softened chips at the yield of 69.~%.
The thus obtained crude pulp was delignified in the.same manner and under the samè conditions as in Example 1 except for employing the following:
Partial oxygen pressure = ;2 kg/cm2 Temperature - 140C

The relationship between Kappa number of the thus delignified pulp and reaction time is shown by a curv.e A
in Fig. 2.
As a comparative experiment, delignification was carried out in the same manner and under the same-conditions as above-described except for employing an alkaline aqueous . - 13 -103Y~7S8 medium into which oxygen was not previously dissolved, and blowing oxyaen gas directly~into the vessel under a partial oxygen pressure of 12 kg/cm . The relationship obtained is also shown by a curve B in Fig. 2.
From these two curves, it is evidently understood that more than 80~ delignification was attained within 5 minutes in case "A" wherein oxygen had previously been dissolved. On the other hand,~same delignification as in "A"
was not attained even after 30 minutes in case "B" wherein oxygen gas was put in~to the vessel directly.

~ Shredded Douglas fir chips were cooked at a temperature of 170C for 30 ~inutes in an alkaline solution of sodium ~ydroxide at an amount of, as sodium oxide, 15%
by weight based on wood, to thereby obtain undefibrated materials having a ~appa number of 128 in a yield of 62.8~.
The thus prepared undefibrated materials were then delignified in the same manner and under the same conditions as in Example 1 except for employing following:
Consistency of cellulosic materials = 10%
Partial oxy~en pressure = 12 kg/cm2 Tempèrature - 150C
Alkaline a~ueous medium;
at early period of 10 minutes = sodium hydroxide solution at latter period of 10 minutes = mixture solution of sodium carbonate and hydroxide (Na2C03/NaOH=4/1) The results obtained were as follows:

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Screened pulp yield 44.2%
Rejects O
Kappa number 18.2 Brightness 38.2 ~GE) As can be seen from the above described process and examples, the present invention provides an effective method for rapid delignification of any type of cellulosic materials, such as chemical pulp, semi-chemical grade pulp, and wood chips. It is understood that various changes in the details, arrangements, materials, and process s~eps which are herein dèscribed and illustrated to better explàin the nature of the invention may be made by those skilled in the art without departing from the scope of the invention.

- 15.-.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS;

1. A method of delignifying cellulosic material with oxygen in the presence of an alkaline aqueous medium in a reaction vessel, which comprises, previously dissolving said oxygen into said alkaline aqueous medium under a partial oxygen pressure of at least 4 Kg/cm2 before charging the medium into the reaction vessel, contacting said alkaline medium containing dissolved oxygen with the cellulosic material in said reaction vessel at a temperature of 80 to 160°C to conduct oxidative delignification of said cellulosic material, said cellulosic material being introduced batch-wise or continuously into the reaction vessel, the consistency of said cellulosic material in said aqueous medium being so chosen as to form an agglo-metration of the cellulosic material in the alkaline aqueous medium which agglomeration does not have any gaseous space therein, and the pressure in the reaction vessel being main-tained more than the partial oxygen pressure employed in the oxygen-dissolving step; and discharging a part of the waste liquor formed during delignification from the vessel, while at the same time charging fresh alkaline aqueous medium containing dissolved oxygen into the vessel, so that the displacement of the waste liquor with the fresh alkaline aqueous medium containing dissolved oxygen is carried out continuously through the delignification, and so that the chosen consistency of the cellulosic material is kept substantially constant by controlling the discharging and charging volume of the alkaline aqueous medium.

2. The method according to claim 1, wherein said cellulosic material is unbleached chemical pulp.

3. The method according to claim 1, wherein said cellulosic material is semi-chemical grade pulp.

4. The method according to claim 1, wherein said cellulosic material is wood chips,

5. The method according to claim 1, wherein the consistency of said cellulosic material is about 5 to about 15% by weight based on the mixture of the cellulosic materials and the alkaline aqueous medium, and the cellulosic material is unbleached chemical pulp or semi chemical grade pulp.

6. The method according to claim 1, wherein the consistency of said cellulosic material is about 10 to about 20% by weight based on the mixture of the cellulosic materials and the alkaline aqueous medium, and the cellulosic material is wood chips.

7. The method according to claim 1, wherein the waste liquor discharged from the reaction vessel is treated to recover the alkaline aqueous medium therefrom into which medium oxygen is dissolved and the thus obtained alkaline aqueous medium containing dissolved oxygen is reused for said displacement of waste liquor.

8. The method according to claim 1, wherein said waste liquor is separately discharged from different zones of delignification in the vessel.

9. The method according to claim 1, wherein said waste liquor is separately discharged from different zones of delignification in the vessel, and said fresh alkaline aqueous medium containing dissolved oxygen is also separately charged into different zones of delignification in the vessel.

10. The method according to claim 7, wherein said fresh alkaline aqueous medium separately charged into different zones of delignification is comprised of different types of alkali.

11. The method according to claim 7, wherein said fresh alkaline aqueous medium separately charged into different zones of delignification is of different pH, the pH value of the alkaline aqueous medium being chosen according to the degree of delignification of the cellulosic material.

12. The method according to claim 7, wherein oxygen is dissolved into the waste liquor discharged from latter zone of delignification in the vessel, and the thus obtained waste liquor containing dissolved oxygen is charged at an earlier zone of delignification into the vessel as the fresh alkaline aqueous medium for displacement of the waste liquor, whereby said cellulosic materials and said alkaline aqueous medium are contacted countercurrently in the vessel.

13. The method according to claim 1 wherein a major part of the waste liquor is displaced by fresh alkaline aqueous medium throughout the course of delignification.