CA1063409A - Oxygen bleaching - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Oxygen bleaching of high consistency pulp takes place in an oxygen reactor. Pulp is mixed with oxygen, and emulsified in a defibrator, and then is transported under pres-sure to the bottom inlet of an oxygen reactor. The pulp moves upwardly in an inner, concentric sub-vessel of the oxygen reac-tor and caustic may be inserted into the pulp while moving up-wardly in the inner sub-vessel. At the top of the sub-vessel the pulp is dispensed by a scraper, which prevents channeling, over the edge of the sub-vessel and into the outer chamber sur-rounding the inner sub-vessel. A top portion of the outer chamber, above the sub-vessel, contains pressurized gas, and the concentration of explosive gases in the top portion is sensed, and the gases are exhausted from the top portion and replaced by compressed air (or a similar non-explosive rela-tively inexpensive gas) when the concentration of explosive gases exceeds a predetermined amount.

Description

1(~63~09 BACKGROUND AND SUMMARY OF THE INVENTION

The invention relates to the treatment of cellulose material with oxygen or an oxygen-containing gas at elevated pressure and temperature in a closed oxygen reactor, and the monitoring of the levels of explosive gases in the oxygen reac-tor to minimize the chances of explosion, and the dispensing of oxygen-treated pulp in the reactor to prevent channeling.
While according to the present invention gas monitoring may take place both in reaction vessels that work completely full - -; 10 of pulp (wherein gas pockets can form), the methods and appar-- atus according to the invention are specially adapted for reac-tion vessels wherein pressurized gas is disposed in a gas chamber formed at the top of the reactor vessel.
In the past it has been known to treat pulp from a continuous digestor with oxygen in a reactor having an inner sub-vessel with open top concentric with an outer chamber sur-rouding the sub-vessel. Such a reactor is shown in Swedish `- patent publication 379,069. In such reactors, the oxygen is introduced before the pulp enters the reactor, and/or while it ' 20 is moving upwardly in the sub-vessel of the reactor, and a quantity of excess oxygen is released at the upper level of the pulp in the reactor, along with substances formed in the treat-, ment and steam. Excess oxygen is introduced during the treat-~, ment to insure that the explosive gases that will be produced '5 25 during treatment are diluted by the oxygen at the top of the `'s reactor. As a result, a much larger amount of oxygen is ~'. .
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1~63~(~9 utilized than is necessary for the treatment of the pulp, and consequently the costs of treatment are relatively high since oxygen is expensive. Also, in such known reactors, when the pulp is dispensed over the top of the sub-vessel into the outer chamber, channeling thereof can occur with resultant non-uniform qualities of the pulp.
- According to the present invention there is provided apparatus for oxygen bleaching of pulp comprising an oxygen reactor vessel including an inner concentric sub-vessel having a larger diameter at the open top thereof than at the bottom ~hereof, and an outer chamber surrounding and completely enclosing said inner sub-vessel, said outer chamber having a pressurized gas-filled top portion thereof above the open top of said inner sub-vessel, means for transporting high-consistency pulp to an inlet in the bottom of said sub-vessel, means for adding oxygen and steam to said pulp in said means for -transporting said pulp before entry into said opening in the bottom of said sub-vessel, means for emulsifying the pulp and the oxygen to thoroughly mix them together before entry of said pulp into the bottom inlet in said sub-vessel, rotatable scraper means associated with said inner sub-vessel at the : top thereof to distribute pulp over the top edge of said inner sub-vessel into said outer chamber, an outlet for oxygen bleached pulp from said outer chamber, located sdjacent the bottom of said outer chamber, means for sensing the concentration of explosive gases in said top portion of said outer chamber, and means for exhausting explosive gases from said top portion of said outer chamber upon the concentration of explosive gases in said top portion exceeding . a predetermined amount and replacing the exhausted gases with compressed air . or the like to maintain the pressure in said top portion of said outer chamber . while reducing the potential for explosion therein.
In another aspect, the invention provides a method of bleaching pulp - with oxygen utilizing an oxygen reactor having an inner, open-top, concentric sub-vessel with a bottom inlet therein, and an outer chamber having a pres-~ 30 surized gas-filled top portion disposed above the open top of said sub-vessel, and a pulp outlet adjacent the bottom of said outer chamber, said method comprising the steps of transporting digested, washed pulp having a consistency , ~ :
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between 3-30% toward the oxygen reactor inlet, adding oxygen and steam to said pulp, emulsifying the pulp and oxygen to thoroughly mix them together, feeding the emulsified oxygen-rise pulp into the bottom inlet of said sub-vessel, the pulp moving upwardly in said sub-vessel to the top thereof, maintaining the pressure in the oxygen reactor at about 1-12 atmospheres and the temperature at about 100-140C, scraping the pulp at the top of the sub-vessel so that it is dispensed into said outer chamber from said sub-vessel, expelling the oxygen treated pulp from the outlet at the bottom of the outer chamber of the oxygen reactor, sensing the concentration of explosive gases in the top portion of said outer chamber, and exhausting the explosive gases from the top portion of said outer chamber when the concentration thereof exceeds a predetermined level, and replacing the exhausted gases with compressed air or the like.
The drawbacks inherent in known devices and methods are eliminated, and a pulp of more uniform quality, and utilizing less oxygen for the treat-ment thereof, is provided. According to the present invention, only the amount of oxygen necessary for treatment of the pulp is provided, the oxygen and pulp being thoroughly mixed with the oxygen in an e~ulsifier (such as a defibrator) and bleaching occurring at a pressure of about 1-12 atmospheres, and a temperature of 100-140C. The level of the explosive gases that collect , in the top portion of the reactor are sensed, and if they exceed a certain predetermined value ~i.e. 0.7 - 0.8% for turpentine, 3.5% for C0), gases are exhausted from the top portion of the reactor, and are replaced with compressed air, or another relatively inexpensive gas that does not have explosive potential. The compressed air can be metered into the top portion of the ,, -`~ reactor in response to a drop in the pressure in the top portion.
Also, according to the present invention, a scraping means is provided that does not allow channeling, the scraper comprising a rotatable shaft, a pair of arms extending outwardly from the shaft and each arm having a plur-~; 30 ality of blade members .

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1(~634Q9 -attached thereto. The blade members extend downwardly from the arms, and the bottom edges thereof are disposed substan-tially at the top edge of the sub-vessel. The blade members are arcuate, and the leading edges thereof are closer to the shaft than the trailing edges thereof. The blade members cio-sest to the shaft have rounded portions thereof disposed around the shaft, and the trailing edge of the blade member fu~therest from the shaft is substantially at the radial extent of the edge of the sub-vessel.
It is the primary object of the present invention to provide for less expensive, more uniform oxygen bleaching of pulp. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGURE 1 is a schematic view of exemplary oxygen-bleaching apparatus according to the present invention;
FIGURE 2 is a detail cross sectional view of the upper part of the oxygen reactor of the apparatus of FIGURE l;
~ 20 FIGURE 3 is a top view of an exemplary scraper accord-- ing to the present invention taken along lines 3-3 of FIGURE 2;
- and FIGURE 4 is a top view of an exemplary caustic liquid , dispensing means taken along lines 4-4 of FIGURE 2.

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~- - 4 -1()63409 DETAILED DESCRIPTION OF THE INVENTION

An exemplary treatment system according to he pre-sent invention is shown diagrammatically at 10 in FIGURE 1.
The reactor vessel 12 provides for oxygen bleaching of pulp as the pulp is fed upwardly through the interior concentric sub-vessel 14, and then is uniformly distributed over the top 16 of the sub-vessel 14, and falls downwardly in the outer cham-ber 17. The sub-vessel 14 is narrow at the bottom, and gra-dually widens to the top edge 16. The rotating scraper 18 according to the present invention, at the top 16 of sub-ves-sel 14 is so desiyned that it prevents channeling of the pulpwhile it is being distributed over the top 16 into chamber 17.

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Also, according to the present invention there is provided a gas monitoring system, shown generally at 19, in the top por-~- tion 20 of chamber 17 to control the atmosphere in portion 20 so that an explosive level of CO, turpentine, and like poten-^ tially explosive gases will not be achieved, and this control ~ is achieved with the minimum possible expenditure.
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According to the present invention, washed pulp is fed from a conventional continuous digestor 22 or the like through line 23 to an emulsifiex 24 (a defibrator or refiner).
The pulp in line 23 has a consistency of about 3-30%, about 25% being preferred. The pulp must be washed in order to re-, move the unoxidizable solids therefrom. Oxygen, steam, and magnesium sulphate are added to the pulp in line 23 before the pulp passes into the emulsifier 24, the emulsifying of the oxygen with the pulp making the oxygen readily available for ., , .. . . .

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10634()9 reaction with the pulp, and allowing high consistency pulp to be treated. O~ may also be added to the pulp at other points before passage thereof over edge 16 of sub-vessel 14. NaO~ or a like caustic may also be added to the pulp in line 23, or alternatively, it may be added by rotating distributor 26 in the sub-vessel 14, being added to the pulp after it is emulsi-fied with oxygen. From emulsifier 24, the pulp passes through line 25 into sub-vessel 14 wherein it flows upwardly until it reaches the top edge 16 of the sub-vessel 14. The pressure in reactor 12 is maintained at about l-lZ atms., and the tempera-ture at 100-140C.
At the top edge 16 of the sub-vessel 14, the rotat-; ing scraper 18 is provided, the scraper 18 preferably being ., .
concentric with the rotating caustic distributor 26. The ro-tating distributor 26 is shown most clearly in FIGURES 2 and 4, and comprises a central channel 29 through which the NaOH
or the like is fed, a pair of hollow arms 30 extending radially from the channel 29, and a plurality of nozzles 31 disposed along the length of the arms 30 through which the caustic flows into the pulp in sub-vessel 14. As the distributor 26 rotates ~; in direction A, the caustic is fed through nozzles 31 and mixed into the pulp by the rotating action of the arms 30, and by the natural action of the caustic when it is inserted into the sub-vessel 14 with a rotational velocity component.
Good, even distribution of the caustic is insured by the dis-tributor 26.
- The scraper 18 has a central rotating shaft 33 with - a pair of arms 34 extending therefrom. Each arm 34 has a plurality of arcuately shaped blade members 35 extending from . . .

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-~063409 the bottoms thereof ~see FIGURBS 2 and 3 in particular), in-cluding a pair of blades 35' disposed most closely to the shaft 33 which have rounded portions 36 terminating the blades 35 on the leading side 37 of the arms 34 when rotating in the direction of rotation A. No-rounded portions 36 are provided for the blades 35' on the trailing side 38 of the arms 34 when rotating in the direction of rotation A. The leading edges 35".~in the direction of rotation) of the blade members are closer to the shaft 33 than the trailing edges thereof.
- 10 The scraper 18 is so designed that the furtherest point radi-ally from the shaft 33, the tip 39 of the outer blades 35, is substantially radially even with the edge 16 of the sub-ves-sel 14. With the construction of the scraper shown, there is no channeling of the pulp as it is distributed from the sub- .
vessel 14 into the chamber 17. Any suitable power means 40 may be provided for rotating the distributer 26 and the scraper 18, the channel 29 extending throughout the length of the .~ shaft 33 to deliver caustic to the distributor 26.
Once the pulp has reached the top edge 16 of the sub-vessel 14, substantially all the oxygen therein has been ~: consumed in the bleaching process. Thus, there is no need to introduce pure oxygen - which is e~pensive - into the upper portions 20 of the chamber 17 to replace explosive components since the oxygen will not significantly contribute to the bleaching action. During the treatment process, carbon monox-. ide, turpentine, and methanol are gaseous materials that may .j ' ` ' .

cause an explosion if in large enough concentration, and ad-ditional unsuitable scum suppressing agents ~e. g. photogene) can produce explosive gases. The lower threshold for explo-sion of CO mixed with oxygen is about 12% by volume for tem-peratures that are normally used in oxygen bleaching (althoughthe threshold is relatively little affected by pressure and temperature). A reasonable level of safety for the CO concen-tration is about 3.5%, however.

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In order to keep the CO concentration at 3.5% or below utilizing the techniques of Swedish Patent Publication .
379,069, an excessive amount of oxygen would have to be ex-hausted from the upper portion 20 of the chamber 17, with con-~, sequent wasteful addition of oxygen to the pulp for treatment.

Assuming that 420 g CO forms per ton of pulp, and that the partial pressure of the 2 is 90% of the total pressure, about ~;- 12 kg 2 per ton of pulp would have to be exhausted from the - gas chamber 20, if the CO concentration is not to exceed 3.5%

' by volume. In turn, this means that a corresponding excess of `' oxygen must be supplied to the pulp earlier in the treatment, , 20 as in line 23. Practical experiments have shown that in con-' s,ideration of the lignin-removing reaction, it is advantageous '; to conduct the oxygen treatment with a certain excess of oxygen.

'' This excess may be relatively little, however, only about 2 kg .'~

2 per ton of pulp. In this example then the extra quantity ' 25 of oxygen that must be supplied to keep the exhaust gas CO

concentr-t~on below 3.5~ must thu~ be about 10 kg 2 per ton - ' . .

lQ634(~9 of pulp. For a normal size plant with production of 600 tons of pulp per day, this extra quantity of oxygen entails an extra outlay hundreds of thousands of dollars a year.
Turpentine is found in coniferous wood, especially in the heart of pine wood. ~he main proportion of the turpen-tine is removed in discontinuous cooking by degassing and in continuous cooking in a Kamyr cooker, in the cooker's "~i-~eat" washing ~see ~.S. Patent No. 4,002,528 for an exemplary cooker and washer). Depending upon the method of operation of the cooker, the amount of turpentine in the blow lines can be between 1.0 and 5.0 kg per ton of pulp. If we assume a temperature of 120C in oxygen treatment and a pressure of 10 atm, with saturation there will be maximally about 3.3%
by volume turpentine in the gas chamber. The explosion thres-hold for turpentine in an air mixture is low, only 0.7 to `- 0.8% by volume. If we assume that the amount of turpentine :~ in the pulp that is to be treated is 2.0 kg per ton of pulp - and the 2 partial pressure in the exhaust gases is 90~, -' about 50 kg 2 per ton of pulp would have to be degassed if the turpentine content is not to exceed about 0.8~ by volume.
.: ' i To get the necessary margin of safety the turpentine concen-tration would have to be kept even lower. The consumption of oxygen therefore will be higher than S0 kg per ton of pulp, ; which would make the process very uneconomical. The above calculations are approximate because many variables enter into ~ it. It must however be accepted as an illustration of the .;~` .
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1~63409 dangerous levels that can be reached, and of the need to de-gass the reaction vessel. The calculations further show that if we replace the removed gas conventionally by addition of oxygen, the costs for the extra oxygen will be so high that the treatment method will become economically infeasible.
According to the present invention, the explosive - levels of CO, turpentine, and the other explosive gases that might be present, can be kept well within the safe limits, while no excess oxygen need be added during treatment to make up for oxygen that will be exhausted from the top 20 of cham-ber 17 to keep the explosive levels within the safety range.
Exemplary apparatus according to the present invention is shown generally at 19 in FIGURES 1 and 2~ The apparatus 19 ~ comprises an exhaust line 41 in which an adjustable remote-- 15 controlled valve 42 is disposed, the valve 42 being controlled by a sensing means 43 operatively connected thereto. A probe for the sensing means may be disposed within the chamber 20 as shown in dotted line at 44 in FIGURE 2, or alternatively gas can be fed through pipe 45 to sensing means 43, and then .,~ x ) exhausted through valve 42 after analysis thereof. The sens-ing means may be any suitable conventional type that can moni--~ tor the concentration of the explosive gases (espqcially CO and turpentine) that are expected to be present in the chamber 20 ~` such as a flame ionization detector. When the sensing means 43 senses an unacceptable level of any explosive gas within x) The exhausted explosive gases are discharged to a further station 42', and they are not returned to the reactor 10, or otherwise returned to the process.
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la6340s the chamber 20, the sensing means 43 opens valve 42 to ex-haust enough gas from the chamber 20 to reduce the explosive gas concentration.
Cooperating with the exhaust line 41 is an intake line 50, the line 50 being disposed exactly on the opposite side of the vessel 12 as the exhaust line 41. The line 50 is connected to a source of compressed air 51, or other suitable inexpensive gas that does not have significant amounts of potentially explosive components therein. Valve 52 is dis-posed in line 50, and controls the intake of air into thechamber 20 from the source 51. The valve 52 is remotely con-trolled by a pressure sensing means 53, a probe 54 extending - from sensing means 53 into the interior of the chamber 20.
When the pressure in the chamber 20 is reduced below a pre-determined value necessary for proper pressurization of the vessel 12 for oxygen bleaching, the sensing means 53 controls valve 52 to ~pen it and allow compressed air or the like to - be introduced into the chamber 20 until the desired pressure is achieved. The exhaustion of the gas through line 41 and the consequent introduction of non-explosive, inexpensive gas through the line 50, thus insures that an explosive level of gases in chamber 20 will not be achieved, while not requiring the utilization of excessive amounts of oxygen to control the explosive levels. As mentioned above, since substantially all of the bleaching has taken place before the pulp is dispersed over the top edge 16 of the sub-vessel 14, there is no reason : ' ' .

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' ~0634(~9 to provide excessive amounts of oxygen in the chamber 20.
When the gas from the line 41 is analyzed, it is found that it normally has an oxygen content of about 20%, slightly less than that in normal air (which indicates that all of the oxy-: 5 gen introduced into line 23, and at any other points during the pulp upflow in sub-vessel 14, has been consumed.) ~. Alternatively, the sensing means 43 can control `~ both the valves 42, and 52, coordinating opening and closing thereof to maintain the proper pressure, the means 53 only ' 10 being used ~if at all) as a back-up device to insure proper pressure in chamber 20.
After the pulp is passed over the top edge 16 of the sub-vessel 14, it falls through chamber 17 until it achieves ' :`, a level 60 within the outer chamber 17 of vessel 12. A
plurality of baffles 61 or the like may be provided within the -chamber 17 to break the fall of the pulp in chamber 17, caus-ing a cascading of the pulp until it reaches the level 60. The pulp is fed from the chamber 17 through the outlet 62 of the .. 'l vessel 12 into an outlet line 63 for transfer to further ~, 20 treatment stations. Suitable stirring or diluting apparatus, .~ shown generally at 65 and 66 respectively in FIGURE 1, may be provlded if desired, as may,a recirculation line and pump 67, , 68 (for start up and for facilitating further stirring or di-.' lution~, the line 67 selectively being placed in or out of com-"', 25 munication with outlet line 63 by controlling valves 69.

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~U634()9 Apparatus accordlng to the present invention may be utilized according to the following example: Sulfate pulp from conifer wood that has been washed in continuous digester 22 having a Kappa number of 55, was fed to emulsifier 24 under the pressure of the digester 22. A pulp quantity of 75 tons per day was provided, and the pulp consistency was 8% tnor-mally a consistency of 25~ is desixable). 35 kg of NaOH was added per ton of pulp, and 22 kg of 2 per ton of pulp was added to line 23. In addition, compressed steam was supplied to line 22 so that the pulp suspension was heated to about 110C. The defibrator or emulsifier 24 comminuted that part of the pulp that was not already freed as individual fibers, and completely mixed the oxygen and the pulp so that the oxygen was readily available to the pulp and a homogenous suspension was : .
provided. The pulp suspension flowed continuously through ` line 25 into the sub-vessel 14, and upwardly in the sub-vessel 14. At the actual production level, the residence time of the - pulp in sub-vessel 14 was about 40 minutes. At the top 16 of the sub-vessel 14, the pulp suspension was scraped off by scraper 18, without channeling, and transferred to outer cham-:
ber-17, falling down to level 60 after passing through baffles 61 or the like. Thus, by this oxygen treatment, the Kappa num-ber of the pulp was lowered by 30 units to 25. The pressure in the chamber portion 20 of the chamber 17 was 5 kp/cm2. The reactor pressure was adjusted by delivery of oil-free compressed air from source 51 through line 50, and through line 41 gas was withdrawn from the reactor so that the turpentine content .

~Q63~Q9 thereof did not exceed .25% by volume. The amount of compressed air added from source 51 to compensate for the gas that was exhausted amounted to about 30 kg per ton of pulp.
While the invention has been herein shown and de-scribed in what is presently conceived to be the most practical ` and preferred embodiment thereof, it will be apparent to those of ordinary skill in the art that many modifications may be made thereof within the scope of the invention, which scope is to be accorded the broadest interpretation of the appended claims so as to encompass all equivalent structures and methods.

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Claims (16)

WHAT IS CLAIMED IS:

1. Apparatus for oxygen bleaching of pulp com-prising an oxygen reactor vessel including an inner concen-tric sub-vessel having a larger diameter at the open top thereof than at the bottom thereof, and an outer chamber surrounding and completely enclosing said inner sub-vessel, said outer chamber having a pressurized gas-filled top portion thereof above the open top of said inner sub-vessel, means for transporting high-consistency pulp to an inlet in the bottom of said sub-vessel, means for adding oxygen and steam to said pulp in said means for transporting said pulp before entry into said opening in the bottom of said sub-vessel, means for emulsifying the pulp and the oxygen to thoroughly mix them together before entry of said pulp into the bottom inlet in said sub-vessel, rotatable scraper means associated with said inner sub-vessel at the top thereof to distribute pulp over the top edge of said inner sub-vessel into said outer chamber, an outlet for oxygen bleached pulp from said outer chamber, located adjacent the bottom of said outer chamber, means for sensing the concentration of explosive gases in said top portion of said outer chamber, and means for exhausting explosive gases from said top portion of said outer chamber upon the concentration of explo-sive gases in said top portion exceeding a predetermined amount and replacing the exhausted gases with compressed air or the like to maintain the pressure in said top portion of said outer chamber while reducing the potential for explosion therein.

2. Apparatus as recited in claim 1 further com-prising means for adding caustic to the pulp flowing upwardly in said inner sub-vessel, said means comprising a rotating dis-tributing means rotatable with said scraper means and disposed in said sub-vessel.

3. Apparatus as recited in claim 2 wherein said rotating distributing means comprises a central hollow rotatable shaft, a plurality of hollow arms extending outwardly from said shaft and in fluid communication therewith, and a plurality of fluid distributing nozzles associated with each arm.

4. Apparatus as recited in claim 1 wherein said means for emulsifying the pulp and oxygen comprises a defi-brator.

5. Apparatus as recited in claim 1 wherein said means for exhausting explosive gases from said top portion of said outer chamber and replacing the exhausted gases with com-pressed air or the like comprises an exhaust valve disposed on one side of said top portion of said outer chamber, said exhaust valve controlled by said means for sensing the concen-tration of explosive gases in said top portion of said outer chamber, means for sensing the pressure in said top portion of said outer chamber, and an inlet valve disposed on the op-posite side of said top portion as said exhaust valve and con-nected to a source of compressed air or the like, said inlet valve controlled by said pressure sensing means to inject com-pressed air or the like into said top portion of said outer chamber in response to a low pressure in said top portion.

6. Apparatus as recited in claim 1 wherein said means for sensing the concentration of explosive gases in said top portion of said outer chamber comprises a flame ionization detector.

7. Apparatus as recited in claim 1 wherein said scraper means includes means for preventing channeling of the pulp as it is distributed over the open top of said inner sub-vessel.

8. Apparatus as recited in claim 1 wherein said scraper means for distributing pulp over the top edge of said inner sub-vessel into said outer chamber so that no channeling occurs comprises a shaft member concentric with said inner sub-vessel, a pair of arms extending outwardly from said shaft member and disposed just above the top edge of said open top of said sub-vessel, and a plurality of arcuate blade members mounted on said arm members and extending downwardly therefrom, each of said blade members having a bottom edge thereof disposed substan-tially at the top of said sub-vessel.

9. Apparatus as recited in claim 8 wherein said plurality of arcuate blade members comprises an outer blade member and an inner blade member, said inner blade member hav-ing a rounded portion thereof disposed on the opposite side of said shaft member as the arm to which said blade member is attached, and said outer blade member having the leading edge, in the direction of rotation thereof, disposed closer to said shaft than the trailing edge thereof, the trailing edge of said outer blade member being disposed substantially at the edge of the open top of said sub-vessel.

10. A method of bleaching pulp with oxygen utiliz-ing an oxygen reactor having an inner, open-top, concentric sub-vessel with a bottom inlet therein, and an outer chamber having a pressurized gas-filled top portion disposed above the open top of said sub-vessel, and a pulp outlet adjacent the bottom of said outer chamber, said method comprising the steps of transporting digested, washed pulp having a consis-tency between 3-30% toward the oxygen reactor inlet, adding oxygen and steam to said pulp, emulsifying the pulp and oxygen to thoroughly mix them together, feeding the emulsified oxygen-rich pulp into the bot-tom inlet of said sub-vessel, the pulp moving upwardly in said.
sub-vessel to the top thereof, maintaining the pressure in the oxygen reactor at about 1-12 atmospheres and the temperature at about 100-140°C, scraping the pulp at the top of the sub-vessel so that it is dispensed into said outer chamber from said sub-vessel, expelling the oxygen treated pulp from the outlet at the bottom of the outer chamber of the oxygen reactor, sensing the concentration of explosive gases in the top portion of said outer chamber, and exhausting the explosive gases from the top portion of said outer chamber when the concentration thereof exceeds a predetermined level, and replacing the exhausted gases with compressed air or the like.

11. A method as recited in claim 10 comprising the further steps of adding caustic to the pulp flowing up-wardly in said oxygen reactor sub-vessel.

12 . A method as recited in claim 10 wherein the consistency of the pulp is about 25%, and wherein said step of emulsifying the pulp and mixing it with oxygen is accomplished by passing the pulp and added oxygen through a defibrator.

13 . A method as recited in claim 10 comprising the further steps of diluting and stirring the oxygen-bleached pulp when it is disposed at the bottom of the outer chamber before it is expelled.

14. A method as recited in claim 10 comprising the further step of recirculating the oxygen bleached pulp back into the oxygen reactor outer chamber after it is ex-pelled from the bottom of the outer chamber.

15. A method as recited in claim 10 wherein said step of exhausting explosive gases from the top portion of the outer chamber and replacing the exhausted gases with compressed air or the like is accomplished by controlling an exhaust valve connected to the top portion of the outer chamber in re-sponse to the concentration of explosive gases exceeding a predetermined amount, sensing the pressure in the top portion, and injecting compressed air or the like into the top portion in response to the pressure falling below a predetermined value.

16. A method as recited in claim 10 wherein the explosive gases that are sensed include turpentine and carbon monoxide, and wherein the concentration of turpentine that initiates exhausting of the gases in the top portion is about 0.7 - 0.8%, and the concentration of carbon monoxide that initiates exhausting of the gases in the top portion is about 3.5%.