CA1184709A - Method and apparatus for oxygen delignification - Google Patents

Abstract

METHOD AND APPARATUS FOR
OXYGEN DELIGNIFICATION

Abstract of the Disclosure An apparatus and process for the oxygen delignification of pulp is provided in which the pulp is transported by means of timing screws in essentially plug flow through one or more substantially horizontal reactor tubes. Oxygen gas is injected into the system at a point adjacent to the pulp inlet and travels concurrently in substantially plug flow with the pulp through the system. In this manner, the pulp is initially exposed to gas of a high oxygen partial pressure while gas vented from the system adjacent the pulp outlet is of low oxygen partial pressure and has a high content of diluent gases. The process and apparatus avoid the formation of gas pockets and hot spots which may adversely affect the pulp. In an alternate embodiment, a countercurrent gas flow process is disclosed. Also provided is a catalytic treatment and recirculation system for the vented gas which permits efficient use of oxygen within the system.

Description

7~
METffQD AND APPARATUS FOR
OXYGEN DELIGNIFICATION
Cross-Reference to Related Ap~
This application is related to Canadian application Serial No. 365r411, entitled "APPARATUS
AND METHOD FOR MEDIUM CONSISTENCY OXYGEN
DEhIGNIFICATION OF PULP", filed November 25, 1980;
this application is also related to U.S. Patent No.4,248~662 to Wallick et al, entitled "OXYGEN
PULPING WITH RECYCLED LIQUOR", issued February 3, 19~1 .
Background of the Invention _ _ This invention relates to delignifying pulp in the presence of oxygen, and more particularly to an apparatus and process for the efficient addition, removal, and recycle of oxygen gas in a pulp delignification system.
Oxygen delignification can be carried out on a wide variety of fibrous materials including wood chips and pulp. When carried out on a bleachable grade of pulp, the process is generally referred to as oxygen bleaching. Conventional apparatuses and processes for the oxygen delignification of fibrous material such as cellulosic pulps generally react the materials in a pressurized vertical vessel. One of the problems encountered in oxygen delignification systems is that the partial pressure of oxygen in the vessel is reduced by the presence of air which enters the vessel with the pulp and by other gases which are produced during delignification such as carbon dioxide, carbon monoxide, and hydrocarbon gases.
Depending upon the purity of the oxygen gas used, inert gases such as nitrogen and argon may also be introduced along with the oxygen gas into the reaction vessel. The reduced partial pressure of oxygen can have a detrimental effect on , ~.
i ~i, BFN 6~98 -2-deliqnification resulting not only in a slower reaction rate, but also a reduction in pulp brightness, strength, and otller properties.
Additionally, the presence of combustible gases such as carbon monoxide and hydrocarbons can be dangerous if their concentration reaches or rises above the lower explosive limit.
One method of increasing the partial pressure of oxygen in the reaction vessel is to increase the operating pressure used for the reaction. However, increased operating presswres require thicker-walled, and therefore more expensive, vessels. Additionally, the danger of gas leakage from the vessel is increased, and the feedinq of the pulp into the vessel against this higher pressure becomes more difficult.
Alternatively, the par-tial pressure of oxyqen can be inreased and the partial pressures of other qases reduced by bleeding gas from the reaction vessel and replacing it with oxygen.
However, this procedure increases oxygen usage and removes heat from the vessel. In order to minimize the loss of oxygen resulting from bleeding, it is possible to oxidize catalytically the organic product ~ases formed during the deliqnification reaction and recycle at least a portion of the bleed stream back to the reactor vessel while still maintaininq the concentration oE combustible qases below the lower explosive limit.
Temperature control durinq oxygen delignification can also be a problem due to the exothermic nature of the reaction. Generally, the pulp must be preheated prior to its entry into the reactor to a temperature sufficiently high to initiate the oxidation reaction. However, once initiated, the heat evolved during the reaction must ~L~7~
sFN 6~98 -3-be controlled to prevent pulp deqradation which results from too much heating. This over-heating problem is especially acute -Eor processes designed to generate a ]arqe Kappa number decrease (i.e., 30 UllitS or more) in the pulp.
Circulation and cooling oflthe reactor gas has been used as a method of ontrollinq the temperature within the reactor vessel when operatinq with hiqh consistency pulp. For example, Hillstrom et al, Svensk Paperstid, ~Jol. 80, pp. 167-70 (April 10, 1977), teach bleedinq qas from the top of a vertical deliqnification reaction vessel to control the content of carbon monoxide and organic qases therein. The carbon monoxide and organic components of the qas are then catalytically oxidized and the qas cooled and recycled back to the reactor vessel.
Carlsmith, U.S. Patent No. 3,964,962, teaches withdrawing a portion of qas from a vertical delignification reactor vessel and recyclinq it back to the upper portion of the reactor. It is taught that the withdrawn qas may be optionally cooled, and the system provides a means to redistribute and control heat within the vessel. Laakso et al, U.S.
Patent No. 4,177,105, teaches a similar gas cooling and recycle system for a vertical delignification reactor vessel. Finally, Luthi et al, in a paper entitled "Gas Concentration and Ternperature Distribution in Oxygen Deliqnification," presented at the 1977 TAPPI Alkaline Pulpinq ConEerence, Washinqton, D.C. November 7-10, 1977, studied both concurrent and countercurrent qas recycle schemes for a vertical oxgen deliqnification vessel.
~ lowever, there are several problems inherent in attempting to control both the partial pressure and temperature of oxygen qas in a conventional vertical delignification reactor.

, Luthi~et al, supra, found that the use of countercurrent gas recirculation to achieve adequate temperature control required large gas flows to : avoid undesirable hot spots in the vessel and c.ould result in pulp hang-ups. With respect to concurrent qas rec.irculation, Luthl et al. concl.uded its use for purposes of temperature control. is limited by the compaction of pulp whih occurs in the reactor vessel. Additionally, in order for concurrent gas movement to occur at a speed ~reater than the speed of the pulp, the pulp must be of a high (i.e., 30~) consistency. It is well known, however, that high consistency operation can lead to large temperature inc.reases in the pulp durinq deliqnification because of the presence of less dilution water to absorb the heat qe n erated.
Finally, movement of qas throuqh a vertical column of pul.p such as is present in conventional hiqh c.onsistency delignification systems ~ay not be uni~orm. Gas channelinq can OCCUL which can lead to hot spots and poor qas distribution in the vessel resultlnq in pulp deqradation and/or an increased danqer that comb-lstion ~ill occur. Vertical upflow reactors used.for low or medium consistency oxygen deliqnification, such as those disclosed by Richter, U.S. Patent No. 4,093,511, Roymoulik, U.S. Patent No. 3,832,276, and Annerqren et al, 1979 Pulp Bleaching Conference, Toronto, Canada, June 11-14, 1979, paqes 99-105, are especially susceptible to channeling of gas and pulp up through ~he reactor leadinq to nonuniform qas and temperature distribution. The channelinq of pulp in this type of system is illustrated by the residence distribution curve for the 10 ton/day pilot system used by Annerqren et al which shows a broad range of residence times for pulp in the reactor as well as an actual mean residene time substantially less than the theoretical residence time. This channelinq problem can be expected to be much worse for a larqer diameter commercial size reaction vessel.
Attempt.s have been made to modify vertical upflow reactors of the type described above to avoid channelinq problems. However, the equipment used to accomplish this is extremel~ complex as shown by Sherman, U.S. Patent No. 4,161,~21. Moreover, these vertical upflow reaction systems have the additional disadvantaqe of the inability to circulate qas throuqh the reactor for temperature control since the qas is present as a dispersed phase and travels upwardly at the same speed as the pulp. Jamieson, U.S. Patent No. 3,754,417, has suqgested other reactor desiqns for oxygen deliqnification at low pulp consistency. Ilowever, those systems also have serious çhanneling problems and require large inputs of heat because of the low consistency operation.
Accordinqly, the need exists in the art for an improved means of supply and recirculation of qas in an oxygen delignification system. The need is especially acute for those systems in which large amounts of delignification are desired since the amount of heat and quantity of combustible and diluent gases qenerated will be larqe.
Summary oE the In_ention In accordance with the present invention, an oxy~qen deliqnificat;on system is provided using one or more substantially horizontal tubes having internal screws for pulp transport as the oxygen reactor. Oxygen qas is introduced into the system at a point adjacent the pulp inlet and moves in essentially pluq flow in the same direction as the pulp throuqh the system. Bleed qas can be removed . .

BF~ 6~9~ -6-from the system at a point adjacent to the pulp outlet ~
A qas space is maintained at the top of each tube during the delignification reaction so that free movement of qas in essentially pluq flow is achieved. The speed of the internal screws controls the retention time of the pulp in the reactor and insures that the pulp moves in plug flow. In order to insure the free movement of gas at a speed different from the speed of the pulp, it is essential that the pulp level must be no more than 90~ of the total tube volume. The gas will be movinq substantially faster near the pulp inlet than near the pulp outlet because of the high rate of oxyqen consumption at the start of the reaction, and this qas movement flushes the nitroqen and combustible qases towards the discharqe end of the system. The gas which is trappecd within the pulp is exchanqed with the free qas above the pulp as a result of the action of the conveying screw, which continuously lifts and turns over the pulp mass in the tube.
In this manner, the continuous movement of qas from the inlet to the outlet of the reactor and the exchanqe of Eree qas and trapped gas prevents the formation of pockets of qas which have a higher content oE potentially explosive qases or a lower content of oxyqen. Simllarly, a temperature equilibrium is maintained so that hot spots cannot develop.
Since pure oxyqen is introducecl near the pulp inlet, the pulp is exposed to the highest partial pressure of oxyqen durinq the initial stages of the reaction where delignification is the most rapid and oxyqen consumption is the qreatest. As the pulp is advanced by the internal screws, more BFN 6398 ~7~

oxyqen is consumed and more reaction product qases such as carbon dioxlde, carbon monoxide, and hydroearbons are generated. The content of carbon dioxide in the qas phase increases not only due to the deliqnification reaction, but also because as the pH of the alkaline reaction liquor decreases as delignification proceeds, carbonate and bicarbonate ions in the liquor can decompose into free carbon dioxide qas.
As the deliqnified pulp approaches the diseharqe point at the end oE the reactor, the partial pressure of oxyqen is at its minimum value while the partial pressure of other qases present in the system is at a maximum. ThuS, the qas which is lost from the system on discharge with the deliqnified pulp is the qas of lowest oxygen eontent. Moreover, the practice of the present invention optionally provides for bleedinq of the qas from the system at a point adjacent the pulp discharqe outlet. This bleeding removes from the reaGtor the gas having the maximum content of non-oxyqen qases ;.ncludinq potentially explosive qases such as carbon monoxide and hydrocarbons.
When relatively small amounts of delignification are desired, bleeding may not be necessary.
The process of the present invention is applicable to the oxyqen delignification of all types of cellulosic materials includinq wood chips, baqasse, straw, other agricultural materials, ground wood, thermomechanical pulp, chemimechanical pulp, semichemical pulp, rejects and knots from a pulping process, and chemical pulps sueh as l~raft, soda, and sulfite pulps. The eonsistency of raw material introduced into the reator may be from 1% to 35%, and preferably from 8% to 20%.

~h~

The alkaline liquor used in the deliqnification reaction may be known alkaline materials used in the art includinq sodium hydroxide, sodium carbonate, sodium bicarbonate, ammonia, Kraft white liquor, oxidized Kraft white liquor, qreen liquor, sodium tetraborate, sodium metaborate, and mixtures thereof. The dosage of alkaline material on the raw material may be varied over a wide ranqe and is qenerally in the range of from 0.5% to 30~ calculated as Na2o on oven dry raw material. Known protector chemicals such as maqnesium compounds can be used if desired to preserve the viscosity and strenqth of the pulp.
The temperature and pressure and retention time used for the delignification reaction can similarly be varied over ~ wide ranqe. It has been found that tempera~ures of ~rom 80-160C and an oxyqen partial pressure oE from 20-300 psiq and retention times of 5-120 minutes will produce suitable deliqnification.
In an alternate embodiment of the invention, a countercurrent flow of oxyqen gas throuqh the reactor may be utilized by injectinq qas at a point near the discharge outlet of the reactor. Because of the pluq flow characteristics of the gas in both the cocurrent and countercurrent modes of operation, the ~ormation o~ hot spots and potentially clanqerous poc~ets oE qas is eliminated.
ln another embodiment Oe the invention, qas can be bled off and recirculated to each individual tube to achieve precise control of the reaction conditions in each tube. Thlls, the recirculated qas can be cooled or not as required and can be passed over a catalyst bed to oxidize combustible components thereof prior to its return to a reactor tube. For example, in a three tube system it may be .

~L8~
BFN 6~98 -~9~

desirable to pass any gas bled ~Erom the second tube throuqh a cooler before reintroducinq it in order to control the heat from the exothermic delignification reaction Moreover, the concentration of combustible gases would be at a ma~imum in the third tube so that it would be deslrable to circulate that qas through a catalyst bed prior to its recycle back into the third tube. Other modifications will be appreciated by those skilled in the art to adapt the system to various deqrees of deliqnification.
~ ccordinqly, it is an object of the present invention to provide a delignification system which will both rapidly and uniformly delignify pulp and the like, efficiently supply and utilize oxyqen gas attain ~Iniform temperature control of the reaction, and avoid the formation of hot spots and gas pockets. This and other objects and advantages of the invention will become apparent Erom the Eollowinq description, the accompanying drawinqs, and the appended claims.
B _ f Deseription of the Drawinqs Fiq. I is a schematic flow diaqram illustrating the proeess of the present invention;
~ Fiq. 2 is a sehematie flow diagram illustratinq another embodiment of the present invention, Fiq 3 is a schematic flow diaqram of yet another embodiment of the present inventioll, Fiq. ~ is a qraph of temperature versus final l~appa nllmber of pulp, ancl ~ Fiq. 5 is a qraph oE Kappa number versus pulp viscosity.
Description of the Preferred Embodiments . . ~
As illustrated in Fig. l, pulp at from 1.0 to 35~ eonsisteney, and preferably 8% to 20 eonsisteney, is introdueed into a first BFN 639~ -10-substantially horizontal reaction tube 10 by thick stock pump 12. The use of substantially hori~ontal tubes includes the use of inclined tubes. However, the an~le of incline should not exceed approximately 45 degrees to avoid compression and dewaterinq of the pulp in the lower end of the tube which will inter~ere with the uniform mixing of oxygen.
Additionally, while the reaction vessel is illustrated as a series of substantially cylindrical reactor tubes, a sinqle vessel having a series of reaction zones or noncylindrical tubes such as a twin screw system may be utili~ed.
Pump 12 may be a Moyno progressing cavity pump available Erom Robbins & Myers, Inc., Sprinqfield, Ohio. Alternatively, pump 12 may be a Cloverotor pump available from the Impco Division of Inqersoll-Rand Co., Nashua, New Hampshire, or a thick stock pump manufactured by Warren Pumps, Lnc., Warren, Massachusetts.
It has been found that these pumps are capable of feeding the pulp into the reaction tube a~ainst the pressure in that tube without severely compactinq the pulp and without any qas losses from the tube. Other feeding devices such as rotary valves or screw feeders are not desirable for use in this inventionO A rotary valve allows substantial qas loss from the reaction tube due to the rotation of valve sections which are alternatel~ exposed to the hiqh oxygen pressure in the reactor and then to atmospheric pressure external to the reactor. Use of a screw feeder results in the severe compression and dewatering of pulp so that efficient oxyqenation at the proper consistency ranqe cannot occur.
Prior to introducing the pulp into thick stock pump 12, steam may be injected into the pulp via llne 14. The steam aids in expelling excess air 7~
sFN ~898 from the pulp and also raises the temperature o~ the pulp somewhat. Additionally, it is desirable to add at least a portion of the total amount oE the charqe of alkaline material prior to the introduction of the pulp into thick stock pump 12. This addition of alkaline material can be made through line 16. The alkaline material serves to lubricate the pulp for easier pumping as well as to insure that the pulp will have an alkaline pH when it enters reaction tube 10. Alternatively, all of the charge may be added at this point.
Generally, the total alkaline material charge will amount to from 0.5 to 30% by weight calculated as Na2o of the oven dry weight oE the raw fibrous material. Examples of allcaline materials suitable for use in this invention include sodium hydroxide, sodium carbonate, sodium bicarbonate, ammonia, oxidized KraEt white liquor, qreen liquor, soclium tetraborate, sodium metaborate, and mixtures thereof. Other known alkaline pulpinq liquors may also be used. The temperatures and pressures used for the deli~nification reaction can similarly be varied over a wide range. It has been found that temperatures of from 80-160C, an oxygen partial pressure of from 20-300 psiq and retention times of 5-120 minutes will procluce a suitable level of deliqnification.
The other portion of alkaline liquor is injectecl through line 20 and sprayecl over the pulp along the length of the tube. By adding the alkaline liquor gradually along the length of the tube rather than all at once as is conventional in high consistency (i~e., 20-35~ consistency) oxygen clelignification, better pulp viscosity and strength is achieved. Another advantage to adding the alkaline liq~or gradually is that the exothermic 7~

deliqnification reaction is more easily controlled, and the risk of localized overheating is diminished.
Oxyqen qas is added to the system at a pOillt adjacent the pulp inlet throuqh line 22 where it is mixecl with the pulp and alkaline liquor. By "adjacent the pulp inlet", it is rneant that oxygen is added to the system prior to midway along the lenqth of the reactor tube. Preferably, the oxygen qas is of hiqh purity (i.e., typically 95% purity) although lower purity oxyqen can also be used.
Preferably, the oxyqen is injected at or near the base of reaction tube 10. Mixing and transport of the pulp and alkaline liquor is achieved by rotating - timinq screw 24 by a suitable drive means 26. Screw 24 can be of a desiqn conventional in the art, Eor example,. a solid helical fliqht desiqn. The speed of rotation of screw 24 can be varied to control the retention time oE the pulp in the reactor and insures that the pulp is transported forward in -20 essentially pluq flow.
A qas space is maintained at the top of reaction tube 10 so that the oxyyen qas can freely move forward in pluq flow at a speed different from the speed oE the pulp. It has been found that operation of the system with the reaction tubes less than full and preferably from 50-90% Eilled, produces acceptable results. The achiévement oE
plug flow is especially important durirlq the initia].
stages of deligniEication to insure that the pulp oE
highest lignin content is exposed to the gas of hiqhest oxyqen content. The continuous movement of gas and pulp alonq the length of the reaction tube and the exchange between gas trapped in the pulp and free gas above the pulp prevents the formation of hot spots or pockets of potentially explosive gases and enhances the uniform delignification of the pulp. It has been found that maintaining an oxyqen partial pressure of from between 20 and 300 psig results in an acceptable level of deliqn;fication.
After traversing the length of reaction tube 10, the pulp, oxyqen, ancl alkaline liquor mixture is introduced into one or more subsequent substantially horizontal reaction tubes such as reaction tube 30. An internal timing screw 32 driven by suitable drive means 34 continuously mixes and transports the mixture along the len~th of the reaction tube. Again, the speed of rotation of the tirninq screw can be varied to control the retention time and the level of the pulp and allow for adequate deliqnification. Further reaction tubes (not shown) may be utilized if necessary.
As the delignified pulp approaches the discharge point at the end of reaction tube, 30, the partial pressure of oxygen is at its minimum while the partial pressures of reaction product gases such as carbon dioxide, carbon monoxide, and hydrocarbons are at a maximum. The pulp is withdrawn from reaction tube 30 and passed to a cold blow region where it is contacted with dilution water or liquor from line 36. Gas may optionally be vented Erom the system through line 38 at a point adjacent the discharge outlet of reaction tube 30. In this manner, qas havinq the least amount of oxyqen and the greatest amount of di~uent gases is discha~ged ! ` from the system.
In an alternate embodiment of the invention, a countercurrent flow of oxygen gas through the reactor tubes can be utilized. As shown in Figs. 1-3, an inlet 50, located at the base of reaction tube 30 near its discharqe outlet, can be used to inject oxygen gas into the systemO The gas will flow in plug flow through the reaction tubes, BFN 6898 -14~

but in the opposite direction from the direction of pulp flow. This countercurrent flow mode of operation procluces both acceptable delignification and qood pulp viscosity while avoidincl the formation oE hot spots and qas pockets. A qas vent 52 may be provided near the pulp inlet to reaction tube l0 to bleed qases.
In another embodiment oE the invention usinq cocurrent gas flow illustrated in Fig. 2, where like reference numerals represent like elements, at least a portion of the qases vented -from tube 30 throuqh line 38 is sent through a catalyst bed 40. Catalyst bed ~0 acts to oxidize carbon monoxide and other potentially explosive hydrocarbon qases produced as a result of the deliqniEication reaction. The treated qases, which contain oxyqen as well as carbon dioxide, are then recirculated to t~be 30 via line ~2 which is in fluid communication with conduit 44. Gases may be vented throuqh vent 54 or may be recirculated back to inlet 22 as shown. If the deliqnification system comprises a multiplicity of reaction tubes, gas from each tube may be catalytically treated and recirculated to the same or other tubes.
Alternatively, simultaneous cocurrent and countercurrent qas flow schemes are contemplated in which o~yqen is suppliecl at or near the midpoint of a reaction tube or series oE tubes. Other possible arranqements will be apparent to those skilled in this art includinq treatment and recirculation of gas flowinq countercurrently to the direction o~
pulp flow.
In the embodiment illustrated in Fiq~ 3, where like reference numerals represent like elements, the natural draft created by pulp fallinq through vertical conduit 44 between tubes l0 and 30 sFN 6898 ~~5 is utilized to draw gas vented from tube 30 throuqh catalyst bed 40. Since heat is qenerated by the catalytic reaction, the heated gas will tend to rise. Thus, qas recir~-ulation lines 38 and ~2 as well as catalyst bed 40 are inclined upwardly to aid in the natural recirculation effect. A baffle 46, or other suitable means, prevents pulp from enterinq conduit 42. Alternatively, a steam ejector or other conventional method may be used to recirculate the treated ~as. A vent tube 58 may be provided downstream of the catalyst bed to serve as a means to purqe carbon dioxide and inert gases from the apparatus.
The invention may be better understood by reference to the followinq nonlimitinq examples.
Example 1 A softwood thermomechanical pulp was delignified with oxyqen and alkali at 8% pulp consistency and 160C. The total reaction time was 60 minutes and an alkali dosage of 30~ sodium carbonate was used on the pulp. The reactor was a horizontal tubular vessel having a horizontal shaft therethrough equipped with paddle fliqhts and rotated at a low speed. The partial pressure of steam at the reaction temperature was 75 psig. To simulate cocurrent oxyqen qas flow, in Run lA the partial pressure of oxyqen in the reactor was yradually reduced from 125 psiq at the start of the reaction to 75 psiq at the end of the react:ion.
30 Countercurrent yas flow was simulated in Run lB by increasing the partial pressure of oxygen from 75 psiq at the start of the reaction to 125 psiy at the end of the reaction.

The results of the tes~s are reported below:
Run Kappa No.% Pulp Yielcl Brightness . ~
1~ 121 70.3 15 lB 136 70.6 12 While Run lA, which simulated courrent gas flow, had a faster delignification rate, a more selective delignification (as shown by substantially equal pulp yield at a lower Kappa number), and a hi~her brightness compared to Run lB, Run lB
illustrates that a countercurrent oxyqen gas f],ow scheme in a horizontal tubular reactor will produce satisfactory delignification.
_xample 2 lS A softwood sulfite pulp having an initial Kappa number oE 69.2 was deliqnified in the reactor describecl in ~xample 1 with oxygen and alkali Eor a total reaction time of 20 min-]tes. The consistency of the pulp was 15%, the reaction temperature was 120C, and the sodium hydroxide dosaqe was 5.0% by weiqht based on oven dry pulp. In Run 2A, the partial pressure of oxyc;en was qradually reduced from 66 psig at the start of the reaction to 36 psig at the end of the 20 minute reaction period. In Run 2B, the partial pressure of oxygen was gradually increased from 36 psiq at the start oE the reaction to 66 psig at the end of the reaction tiine. In both runs the partial pressure of steam in the reactor was maintained at 1~ psig throughout the reaction period.
The results oE the tests are reported below:
Run Kappa No. % Delignificatlon O Palp Yield Brightness

2~ 40.2 41.9 85.5 37 2B 43.4 37.3 ~5.2 35 It is evident that Run 2A which simulated a cocurrent oxyqen qas flow had a Easter deliqnification rate (i.e., lower Kappa number), a higher pulp briqhtness, and a qreater selectivity than Run 2B which simulated a countercurrent gas Elow. However, the results reported for ~un 2B
indicate that satisfactory deliqnification is obtained for a countercurrent oxyqen gas Elow scheme in a horizontal tubular reactor.
E mple 3_ _ Several tests were performed in a three tube continuous horizontal tubular reactor using a countercurrent oxygen qas flow scheme. Each tube was equipped with a horizontal screw which was turned at low speed to advance the pu]p. Oxygen was introduced into the reactor near the discharge end oE the third tube producing a gas flow countercurrent to the direction of flow of pulp.
The tests were made with a 10~ pulp consistency, a

3~ sodium hydroxide dosaqe, 100 psig total pressure, and with a pulp havinq an initial Kappa number of 29.3. The pulp level in the reaction tubes was maintained at a maximum o-E 55% of the total tube volume. Retention times were varied from 8 to 39 minutes and production rates were varied :Erom 1.7 to 5.0 ton/day.
The results of the tests are shown ln Fi.gs.

4 and 5.
The resul.ts oE the tests show a rap:id deliqnification rate and a high pulp viscosity (indicative of good strenqth properties) over a wide ranqe of retention times and production rates. The pulp visc.osity was excellent even when declree of deliqnification approached 60~. The results show that oxygen contact with the pulp was good even thouqh only one gas inlet was used in thi.s multiple 847~

tube reactor and even though the tests were run at a medium pulp consistency. This is the most dificult consistency ranqe in whi~h to achieve qood oxyqen contact sinçe the pulp is present as sticky lumps.
While the described apparatus and methods constitute preferred embodiments of the invention, it is to be understood that the invention is not limited to these precise apparatus and methods, and that changes may be made in either without departing ~rom the scope o~ the invention, which is deEined in the appended claims.
What is claimed is:

Claims (10)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. Apparatus for continuous oxygen delignification of pulp comprising in combination:
a). at least one substantially horizontal tubular reaction zone, b). means for introducing pulp at a first end of said reaction zone, c). means for introducing oxygen gas at at least one point adjacent to said means for introducing pulp for concurrent flow with said pulp, d). means in said reaction zone for transporting pulp from said first end of said reaction zone to the opposite end thereof, and, e). means for withdrawing delignified pulp from said opposite end of said reaction zone.

2. The apparatus of claim 1 including means for withdrawing gas containing reaction product gases at a point adjacent said opposite end of said reaction zone.

3. The apparatus of claim 1 including at least two substantially horizontal tubular reaction zones.

4. The apparatus of claim 3 including means for recirculating the withdrawn gas to the latter of said at least two reaction zones.

5. The apparatus of claim 4 wherein said recirculating means includes means for oxidizing potentially combustible reaction gases.

6. Apparatus for continuous oxygen delignification of pulp comprising in combination:
a). at least one substantially horizontal tubular reaction zone, b). means for introducing pulp at a first end of said reaction zone, c). means in said reaction zone for transporting pulp from said first end of said reaction zone to the opposite end thereof, d). means for withdrawing delignified pulp from said opposite end of the last of said at least one reaction zones, and e). means for introducing oxygen gas at at least one point adjacent to said means for withdrawing pulp for countercurrent flow to said pulp.

7. A process for the continuous oxygen delignification of pulp comprising the steps of introducing pulp and alkaline chemicals into a substantially horizontal tubular reaction zone, adding oxygen gas to said zone at a point adjacent the introduction of said pulp, transporting the mixture of pulp and alkaline chemicals in substantially plug flow concurrently with said oxygen gas through the reaction zone, and removing the delignified pulp through an outlet in said zone.

8. A process for the continuous oxygen delignification of pulp comprising the steps of introducing pulp and alkaline chemicals into a first substantially horizontal tubular reaction zone, adding oxygen gas to said zone at a point adjacent the introduction of said pulp, transporting the mixture of pulp and alkaline chemicals in substantially plug flow concurrently with said oxygen gas through said first reaction zone, transferring said mixture into one or more subsequent substantially horizontal, tubular reaction zones, transporting the mixture of pulp and alkaline chemicals in substantially plug flow concurrently with said oxygen gas through said one or more subsequent zones, and removing delignified pulp through an outlet in the last of said one or more subsequent zones.

9. A process for the continuous oxygen delignification of pulp comprising the steps of introducing pulp and alkaline chemicals into a first end of a substantially horizontal tubular reaction zone, adding oxygen gas at the opposite end of said reaction zone, transporting the mixture of pulp and alkaline chemicals in substantially plug flow countercurrently to the direction of flow of said oxygen gas through the reaction zone, and removing the delignified pulp through an outlet at said opposite end of said zone.

10. A process for the continuous oxygen delignification of pulp comprising the steps of introducing pulp and alkaline chemicals into a first end of a substantially horizontal tubular reaction zone, adding oxygen gas to said zone, transporting the mixture of pulp and alkaline chemicals in substantially plug flow from said first end of said reaction zone to the opposite end thereof, directing a first portion of said oxygen gas to flow substantially concurrently with the direction of flow of said mixture while directing a second portion of said oxygen gas to flow countercurrently to the direction of flow of said mixture, and removing the delignified pulp through an outlet at said opposite end of said zone.