WO1996023929A1

WO1996023929A1 - Bleaching pulp with peroxide at superatmospheric pressure and in multiple stages

Info

Publication number: WO1996023929A1
Application number: PCT/FI1995/000138
Authority: WO
Inventors: Kaj Henricson; Brian Greenwood; Björn Svedin; Olavi Pikka
Original assignee: A. Ahlstrom Corporation
Priority date: 1995-01-31
Filing date: 1995-03-14
Publication date: 1996-08-08

Abstract

Cellulose pulp is bleached with peroxide under superatmospheric pressure in two (or more) stages, with removal of oxygen and reaction gases from the pulp between the stages, About 5-20 Kg/admt hydrogen peroxide is intimately mixed with the pulp and then introduced into a first pressurized vessel in which the pressure is between about 5-15 bar and the temperature between about 80-120 °C. Bleaching takes place in the first vessel for about 15-60 minutes while oxygen and reaction gases are formed, and then at least 20 % of (and preferably subtantially all of) the oxygen and reaction gases are removed from the pulp before complete consumption of all of the peroxide. Then the pulp is preferably introduced into a second vessel, additional peroxide, alkali, and steam can be added, and peroxide bleaching continues at a pressure of about 1.1-10 bar and at a temperature of about 90-120 °C for between about 30-200 minutes, during which oxygen and reaction gases are formed anew. The majority of the newly formed oxygen and reaction gases -- in fact subtantially all of them -- are separated out, typically upon discharge of the pulp from the second vessel.

Description

BLEACHING PULP WITH PEROXIDE AT SUPERATMOSPHERIC PRESSURE AND IN MULTIPLE STAGES

BACKGROUND AND SUMMARY OF THE INVENTION

In US patent applications 08/120,838 and 07/721,958, it was recognized that pressurized peroxide bleaching following ozone bleaching or the like was extremely advantageous. It has now been recognized that similar advantages can be maximized by practicing peroxide bleaching in two or several consecutive superatmospheric stages, typically in two different vessels.

In practicing pressurized peroxide bleaching processes practical problems have arisen . In large mills with tower heights of 30-40 meters and a pressure requirement of 2-10 bar at the top of the tower it is difficult to find pumps which are able to produce the desired pressure. Also the pressure at the tower inlet in upflow towers is easily over 10 bar , often 15-20 bar, and when the pulp is heated with steam injection a steam pressure much above 10 bar is necessary . Such high pressure steam is not usually available at a pulp mill bleach plant. What has been found according to the present invention is that by using a bleaching vessel configuration such as shown in FIGURES 1 and 2 of the US application serial no. 08/120,838 or modifications thereof as disclosed herein , the problems associated with the prior art can be eliminated.

Mill tests have demonstrated that when hydrogen peroxide is used in pulp bleaching a great deal of oxygen, carbon monoxide, carbon dioxide, and other reaction gases are produced. The oxygen is formed from the hydrogen peroxide while the rest of the reaction gases are formed when the peroxide reacts with the cellulose fibers of the pulp . The gases produced cause flow problems in the bleaching towers, and when the towers are pressurized the gas volume is reduced and allows the flow problems to be πύnimized . Also the bleaching result is improved . By removing a substantial amount of (typically at least 20% of, and preferably at least the majority of, and most desirably substantially all — i.e. 90% or more of the oxygen and reaction gases from the pulp the bleaching process may be performed at a lower superatmospheric pressure than has otherwise been practiced . That is by utilizing at least one intermediate gas separation it is possible to bleach at lower superatmospheric pressures . In an article entitled "Alternatives for Achieving High Brightness TCF Pulps" by Basta et al, it has been suggested that peroxide bleaching be accomplished first in a pressurized pre-reactor and then in an atmospheric larger reactor. However this too has a number of disadvantages. If the larger reactor is atmospheric then the temperature is low , and the peroxide reaction not as effective . Also without superatmospheric pressure it may be more difficult to discharge the pulp from the tower. Further the oxygen pressure is not present, and without the pressure of oxygen decomposition of the peroxide is accelerated . By maintaining superatmospheric pressure in the second reactor, according to the present invention, an oxygen atmosphere is assured. Expressed chemically, the peroxide reaction begins with:

H₂0₂ -> 00H- + H + Thereafter the bleaching reaction continues as follows: 00H- + fibers -> 02 + bleached fibers + other reaction gases

That is oxygen is formed in the suspension. An oxygen atmosphere prevents a harmful peroxide decomposition reaction which is expressed as follows:

00H- -> 02 + H- When oxygen is released by discharging the pulp to an atmospheric tower as suggested by Basta et al the relative amount of oxygen is very low so that oxygen may readily react to form the decomposition reaction set forth above . It has been found according to the present invention that high superatmospheric pressure is not necessary to prevent the decomposition reaction , rather merely a slight superatmospheric pressure (e. g. about 1.1-5 bar) so that the gas atmosphere can be controlled properly.

Basta et al also teach the use of a hot pre-reaction where the amount of chemical utilized is large, and a cool "post-reaction" section. However according to the present invention it has been found that hot pre-reaction increases the reaction speed and peroxide decomposition, and fibers may readily be deteriorated due to the excessively fast reaction. It is better to have a cooler pre-reaction with a large amount of chemicals and a warm "post-reaction" with a lesser amount of chemical.

According to the present invention an advantageous method and system are provided for effecting peroxide bleaching of cellulose pulp . While the invention is particularly applicable to chemical pulp it also is urilizable with mechanical pulps and recycled fiber pulps . The invention is also applicable to a wide variety of different consistency ranges although medium consistency pulp, within the range of about 5-25% solids, and preferably about 8-18%, is preferred .

The invention is applicable to all types of peroxide bleaching including alkaline, neutral, and acidic conditions. The hydrogen peroxide can be added in the form of peracids and Caro's acid. Oxygen can be added to the peroxide stage. Additives like magnesium, EDTA, DTP A and olybdat may be used.

According to one aspect of the present invention a method of bleaching cellulose pulp with peroxide utilizing at least a first pressurized bleaching vessel is provided comprising the following steps: (a) Mixing peroxide (e. g. 5-30 kg/admt) with cellulose pulp to form an intimate mixture. (b ) Introducing the intimate mixture into the first pressurized bleaching vessel and effecting peroxide bleaching of the pulp in the first vessel while maintaining the pressure superatmospheric throughout, so that oxygen and reaction gases are formed during peroxide bleaching of the pulp . (c) Separating out at least 20% (preferably at least a majority of, and most desirably substantially all — that is 90% or more — of) of the oxygen and reaction gases from the mixture before complete consumption of all of the peroxide . ( d ) After step ( c ) allowing the peroxide bleaching of the pulp to continue under superatmospheric conditions until substantially all of the peroxide is consume , oxygen and reaction gases being formed ane . An , (e) separating out the majority (preferably substantially all) of the newly formed oxygen and reaction gases from the pulp . The method also preferably comprises the further step (f) , substantially between steps (c) and (d) , of introducing the pulp into a second pressurized bleaching vessel, step ( d) being practiced under superatmospheric conditions throughout the second vessel. Step (e) is typically practiced substantially contemporaneously with discharge of the pulp from the second vessel.

The temperature in the first bleaching vessel is typically about 80-110 °C and the pressure between about 5-60 bar and the treatment time between about 15-60 minutes. In the second vessel the temperature is slightly higher (e.g. about 10°C) , and the pressure is about 1.1-10 bar, preferably 5 bar or below, and the treatment time is between about 30-200 minutes, preferably about 60-150 minutes. The method can include the use of three or more vessels. The pulp is preferably pretreated before peroxide bleaching . For example it may be treated to remove the transition metals therefrom, and/or bleached with other alkaline, or acidic, bleaching chemicals.

According to another aspect of the present invention a cellulose pulp bleaching system is provided comprising the following elements: A first superatmospheric bleaching vessel having a top and bottom with a pulp inlet at the bottom and a pulp outlet at the top . A first fluidizing discharger at the top of the first vessel for discharging pulp out the outlet and for removing the majority of gas from the pulp . A second superatmospheric bleaching vessel having a top and bottom with a pulp inlet at the bottom and a pulp outlet at the top . A second fluidizing discharger at the top of the second vessel for discharging pulp out the outlet and for removing the majority of gas from the pulp . Means for mixing peroxide and other chemicals into pulp prior to or at introduction into the pulp inlet of the first vessel. And, a connection between the pulp outlet from the first vessel and the pulp inlet to the second vessel.

The mixing means may comprise a pump for simultaneously pumping pulp and mixing pulp with peroxide and chemicals, and/or may comprise a medium consistency mixer. There may be an additional mixing means for mixing peroxide and other chemicals into the pulp at or after the first fluidizing discharger . If after the first fluidizing discharger the additional mixing means may comprise a medium consistency mixer (s) between the first fluidizing discharger and the second vessel pulp inlet. The height of the first vessel is typically about 15-30 meters and it has a volume of between about 100-500 cubic meters, whereas the second vessel has a volume about the same as or greater than the first vessel. A typical installation may consist of two vessels of the same size dimensioned for the treatment time of 45 to 60 minutes .

It is the primary object of the present invention to provide a simple and effective method, and a system for its use, for bleaching cellulose pulp with peroxide . 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 illustration of a first exemplary embodiment of a system according to the invention for bleaching cellulose pulp with peroxide; FIGURE 2 is a view like that of FIGURE 1 only for a different embodiment; and FIGURES 3, 4 and 5 are views like FIGURE 1 for still different embodiments.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring now to FIGURE 1 in detail, pulp is fed to storage unit 10 from a pretreatment stage or stages 11. The stage or stages 11 may be 02 delignification, ozone bleaching, or other acidic or alkaline bleaching or delignification stages; and/or transition metal removal by acid washing or chelating agents such as EDTA or DTPA [A and/or Q stages] followed by washing or pressing (thickening) , or the like. Pulp is transferred from storage unit 10 by a pump 12 [which is preferably a degassing medium consistency pump] through a line or conduit 16 to a fluidizing mixer 14. Mixer 14 is preferably a high-shear, medium consistency mixer such as an MC mixer commercially available from Kamyr, Inc. of Glens Falls, New York, or an AHLMIX mixer available from A. Ahlstrom Corporation of Finland. The mixer 14 has an inlet for the medium consistency (e. g. 5-20%, preferably about 8-15%) pulp suspension connected to line 16, and an inlet port 18, for the pressurized feed of peroxide bleaching chemical. Except for the addition of peroxide (e. g. typically H₂0₂ in a liquid) the apparatus is the same as FIGURE 1 of the US application 08/120,838. Alkali (e.g. NaOH) , Mg, 02 , and /or steam are preferably added with the peroxide . Normally about 5-20 (e.g. about 10) kg/admt hydrogen peroxide (or equivalent other type of peroxide) is added. The charge can, however, be greater, up to 40 kg/H₂0₂.

The pulp suspension is intimately and uniformly mixed with the peroxide, and discharged through a mixer 14 outlet into a conduit 20 and passed within about 2-3 seconds into the bottom 23 of a first upright pressurized reaction vessel 22. After a residence time of about 15-60 minutes (preferably about 15-45 minutes) the mixture of pulp and bleaching chemical, which has not yet been entirely consumed during the bleaching reaction, carries to the top 25 of the reaction vessel 22.

The vessel 22 preferably has a height of about 15-30 meters. Its cross-sectional area (typically diameter) is dictated by the production desired . The volume of the vessel 22 is typically about 100- 500 cubic meters . For example if the desired production is 1500 adt/D and the treatment time is about 30 minutes, the volume of the reactor 22 should be at least about 300 cubic meters , e . g . 300-400 m3. Thus for a reactor 22 with a height of about 30 meters the diameter would be about 3.5 meters , and for a height of about 15 meters the diameter would be about 5.0 meters .

During the entire time the pulp is in the vessel 22 it is at superatmospheric pressure and elevated temperature. The preferred temperature range is 80-110° C, preferably 80-100° . The pressure in the vessel 22 is not high in order to minimize the problems associated with the prior art as described above, preferably about 5-15 bar. During the bleaching reaction in the vessel 22 oxygen and reaction gases are produced .

According to the present invention at at least one intermediate stage during the bleaching at least a substantial portion of the oxygen and reaction gases that are produced during the bleaching reaction are removed , so that the pressure need not be needlessly high in the second vessel. This is accomplished in the embodiment illustrated in FIGURE 1 by removing preferably at least about 20% of the oxygen and reaction gases (preferably at least 50%, and most desirably substantially all of — that is at least about 90% of — the oxygen and reaction gases) in line 28 utilizing the fluidizing discharger 26. Preferably the fluidizing discharger 26 is of the type having a low resistance to flow , or even being capable of raising the pressure , and because of this when additional bleaching chemical, steam, or the like is added in line 24 the pressure is manageable (e.g. below 12 bar) . Normally about 75% of the bleaching chemical is consumed in the reactor 22 with about 25% remaining . Under some circumstances the addition of further hydrogen peroxide, alkali, steam, or the like in line 24 is not necessary although typically at least steam will be added in line 24 to slightly raise (e. g. about 10° C) or maintain the temperature of the pulp as it is discharged through valve 30 into the second pressurized bleaching vessel 32. Good bleaching results are obtained when the pressure is stable in the first reaction vessel 22. The pressure may be maintained at a constant level by utilizing the pressure regulating valve 30, preferably located closely adjacent the fluidizing discharger 26, and utilizing the conventional control loop 31. The mixture of pulp and peroxide bleaching chemical is discharged from vessel 22 into the enlarged inlet portion 33 of the second vessel 32. While superatmospheric , the pressure in the vessel 32 is typically lower than in the vessel 22. For example the temperature in the vessel 32 is preferably between about 90-120° C , more preferably about 90-110° C, while the superatmospheric pressure is between about 1.1-10 bar, preferably between about 1.1-5 bar . In the second vessel 32 the bleaching reaction continues and oxygen and reaction gases are produced anew.

Separated gas is removed from the second vessel 32 through a gas discharge line 34. The pressure in the second vessel 32 , which typically has at least twice the volume of the vessel 22 , is also maintained steady by a separate pressure regulating valve 36 and a control loop 37 in a known manner. The pulp forms a column in the second vessel 32 at or near a predetermined level, through known level control means 38, line 41 pump 44, and pressure regulating valve 47, and is treated for about 30-200 minutes, preferably about 60-150 minutes (and preferably longer than in vessel 22) , to complete peroxide bleaching. The pulp is thereafter discharged at the bottom 42 of vessel 32 by a pump 44, which is preferably also a degassing medium consistency pump , through a valve 47 into a conduit 46 leading to a washer or other suitable treatment (e.g. bleaching) stage.

In the second vessel 32 the majority of the gas , primarily in the form of oxygen and reaction gases, is separated via the conduit 34, while an "oxygen atmosphere" maintained above the pulp in the vessel 32 as indicated by head 39.

While it is preferred that two different pressurized bleaching vessels 22, 32 be utilized, it is possible that the intermediate gas separation and then subsequent continued peroxide bleaching (with or without additional bleaching chemical addition) can take place in the same vessel; or more than two vessels may be utilized, preferably with gas removal between each.

FIGURE 2 shows another embodiment of exemplary apparatus according to the present invention. In FIGURE 2 components comparable to those in FIGURE 1 are shown by the same reference numeral only preceded by a "1" , and therefore the common structures will not be described.

The elements, structure and operation of the embodiment illustrated in FIGURE 2 are substantially the same as the embodiment describe in connection with FIGURE 1 , except that second vessel 132 is provided at its bottom 142 with an outlet 152 which is dimensioned to permit the bleached paper pulp to be fed, due to the pressure head thereof, into a suitable washer 150, preferably a drum diffusion washer as sold by assignee A. Ahlstrom Corporation , with pressurized inlet or dif f user available from Kamyr , Inc . of Glens Falls, New York.

A level control mechanism 38 cooperates through line 148 with an rpm regulator of washer 150 in known manner to maintain the paper pulp level in a second vessel 132 at a predetermined level. Finally, the washed pulp is discharged from washer 150 by a pump 144 , preferably a degassing medium consistency pump through a conduit 146 for further treatment.

FIGURES 3 and 4 show two other embodiments of systems that may be utilized to practice the method of the present invention. In both FIGURES 3 and 4 structures comparable to those in FIGURES 1 and 2 are shown by the same two digit reference numeral only preceded by a "2" .

In the FIGURE 3 embodiment the peroxide, and alkali, magnesium, steam, and/or oxygen are added directly to the medium consistency pump 212 , the pulp flowing in the upflow reactor 222 to the fluidizing discharge device 226. The gas removal efficiency of the device 226 is typically about 50-90%, the gas being discharged as indicated at 228. Additional hydrogen peroxide and alkali, if utilized, may be added directly at the discharger 226, while steam is added in the line 29 connecting the discharger 226 to the bottom of the second vessel 232. The vessel 232 is an upflow reactor, with the discharge mechanism 244 being a fluidizing discharger such as the mechanism 226. The gas that is removed, again typically at least 50-90% is discharged as indicated at 45 while the pulp is discharged at 246, and passes to blowout, a storage tank, washer, and/or subsequent bleaching stages. The temperatures and pressures in the vessels 222, 232 are preferably as described with respect to the FIGURE 1 embodiment; in particular the pressure in the vessel 222 is typically between about 5-15 bar, while in the vessel 232 it is between about 1.1-10 bar, typically less than 5 bar. It is desirable that the temperature in the reactor 232 is slightly higher than that in the reactor 222, e.g. about 10 °C in a typical situation, and the maximum temperature in vessel 22 relatively low, e.g. 120° C or less.

In one operative example, the pulp in the vessel 222 has a temperature of about 100° C and a pressure of about 10 bar, while in the reactor 232 the temperature is raised (by steam addition in line 29) to about 110° C and the pressure is below 5 bar (although superatmospheric) . In this way high pressure is maintained during gas formation but low pressure during steam injection .

FIGURE 4 shows an embodiment only slightly different than that of

FIGURE 3. In the FIGURE 4 embodiment a medium consistency mixer 214 is utilized for the hydrogen peroxide and other chemical intimate mixing, rather than adding the chemicals directly to the pump 212, or in addition to adding them to the pump 212. Also in this embodiment mixers 49 and 58 are provided to which hydrogen peroxide, and alkali and/or steam may be added. In this embodiment while steam addition is also shown in line 29 typically the steam addition would be only in one or both of the medium consistency mixers 49 ,

58. Also in this embodiment while hydrogen peroxide and alkali addition to the fluidizing discharger 226 are illustrated, that can be eliminated and instead any additional peroxide, etc. , added in one or both of the medium consistency mixers 49, 58.

Example 1

An installation shown in Fig 5 was used for testing the process in mill scale. About 30 kg of H₂0₂ was added in mixer 302 before the first bleaching tower 302. The retention time in both towers 302 and 304 was 45 minutes. After the two bleaching towers 302 and 304 a blow tank 306 installed for separating steam and gases before pumping the pulp further. Gases were not separated by a top separator 308 in the second tower 304 , but the mixture was blown to the blow tank 306. Thus the gas separator 308 at the top of the second tower 304 is only optional. The bleaching reaction can, of course, continue in the blow tank 306 or storage chest but at atmospheric pressure .

In the test runs with this installation it was found out that gases fill about 40 % of the volume of the second tower 304 when the gas separator 310 at the top of the tower 302 is shut off. Thus both flow conditions and bleaching time are disturbed giving a poor bleaching result.

Example 2

A single pressurized peroxide bleaching tower was installed at a mill . The temperature and pressure were about 120°C and 3 bar. There were several problems with this installation including: - the high gas volume caused pulsation in the discharge system,

- the flow pattern in the tower was bad, - the high temperature during the fast initial peroxide reactions led to a low strength of the pulp

Due to these bad experiences the system will be rebuilt and equipped with intermediate gas separation. The added first vessel will be smaller, same size or larger than the existing vessel depending on which total reaction time is needed.

It will thus be seen that according to the present invention a method and system for peroxide bleaching are provided which have the advantages of requiring less pumping head, lower required steam pressure, and a less costly construction of the reactors since lower pressure is used in the second reactor 32, 132, 232. While the invention has been described particularly with respect to medium consistency pulp, typically having a consistency of 5-25%, preferably about 8-18%, and chemical pulp, it is to be understood that it is also useful for different consistencies and also for other types of pulps such as mechanical pulp and recycled fiber pulp. The invention is also improved over a configuration in which a second reactor is at atmospheric pressure, the superatmospheric pressure having numerous advantages.

While the invention has been herein shown and described 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 methods and systems . Such a modification may be, as an example, to install two vessels with a non-pressurized gas separation and a second MC pump feeding the second vessel between the two vessels.

Claims

WHAT IS CLAIMED IS:

1. A method of bleaching cellulose pulp with peroxide using at least a first pressurized bleaching vessel, comprising the steps of: (a) mixing peroxide with cellulose pulp to form an intimate mixture;

(b) introducing the intimate mixture into the first pressurized bleaching vessel and effecting peroxide bleaching of the pulp in the first vessel while maintaining the pressure superatmospheric throughout so that oxygen and reaction gases are formed during peroxide bleaching of the pulp; ( c ) separating out at least 20% of the oxygen and reaction gases from the mixture before complete consumption of all of the peroxide;

(d) after step (c) allowing the peroxide bleaching of the pulp to continue under superatmospheric conditions until substantially all of the peroxide is consumed, oxygen and reaction gases being formed anew; and (e) separating out the majority of the newly formed oxygen and reaction gases from the pulp .

2. A method as recited in claim 1 comprising the further step (f) , substantially between steps (c) and (d) , of introducing the pulp into a second pressurized bleaching vessel, step (d) being practiced under superatmospheric conditions throughout the second vessel.

3. A method as recited in claim 2 wherein step (e) is practiced substantially contemporaneously with discharge of the pulp from the second vessel.

4. A method as recited in claim 2 comprising the further step of introducing additional peroxide into the pulp , and mixing it with the pulp , substantially simultaneously with the practice of step (c) .

5. A method as recited in claim 2 comprising the further step of introducing additional peroxide into the pulp, and mixing it with the pulp, just before introduction of the pulp into the second vessel.

6. A method as recited in claim 2 wherein during the practice of step (b) the temperature of the pulp is maintained at about 80- 110° C and the pressure between about 5-20 bar , and wherein the treatment time in the first vessel is between about 15 - 60 minutes.

7. A method as recited in claim 6 wherein during the practice of step ( d ) the temperature of the pulp is maintained at about 90- 120° C and the pressure between about 1.1-10 bar, and wherein the treatment time in the second vessel is between about 30 - 200 minutes.

8. A method as recited in claim 6 wherein during the practice of step ( d ) the temperature of the pulp is maintained at about 90- 110° C and higher than in the first vessel, and the pressure between about 1.1-5 bar and lower than in the first vessel, and wherein the treatment time in the second vessel is between about 60-150 minutes.

9. A method as recited in claim 2 wherein during the practice of step (b) the temperature of the pulp is maintained at about 80- 100° C and the pressure between about 5-15 bar, and wherein the treatment time in the first vessel is between about 15-45 minutes.

10. A method as recited in claim 7 wherein the amount of peroxide added in step (a) is, or equivalent to, about 5-30 kg/admt hydrogen peroxide.

11. A method as recited in claim 1 wherein the amount of peroxide added in step (a) is, or equivalent to, about 5-30 kg/admt hydrogen peroxide.

12. A method as recited in claim 1 wherein step (a) is further practiced by introducing alkali, oxygen, magnesium, and steam with the peroxide.

13. A method as recited in claim 2 comprising the further step of introducing one or several of peroxide, alkali, and steam into the pulp , and mixing it with the pulp, substantially simultaneously with the practice of step (c) or just before introduction of the pulp into the second vessel.

14. A method as recited in claim 3 wherein step (c) is practiced so as to separate out substantially all of the oxygen and reaction gases from the mixture .

SUBSTITUTE SHEET

15. A method as recited in claim 1 comprising the further step of , prior to step (a) , pretreating the pulp so as to remove transition metals therefrom.

16. A method as recited in claim 15 wherein during pretreatment of the pulp to remove transition metals therefrom it is treated with at least one of acid , EDTA, and DTPA, combined with washing or pressing.

17. A method as recited in claim 1 comprising the further step , prior to step (a) , of bleaching or delignifying the pulp with ozone or oxygen.

18. A method as recited in claim 2 wherein the first vessel is an upflow reactor and wherein step (b) takes place as the gas flows upwardly in the vessel, and wherein the second vessel is a downflow vessel and wherein step (d) takes place as the pulp moves downwardly.

19. A method as recited in claim 2 wherein the first vessel is an upflow reactor and wherein step (b) takes place as the gas flows upwardly in the vessel, and wherein the second vessel is an upflow vessel and wherein step (d) takes place as the pulp moves upwardly.

20. A method as recited in claim 1 wherein the pulp during the practice of steps (a) through (d) has a consistency of between about 8-18%.

21. A method as recited in claim 20 wherein the cellulose pulp treated is chemical pulp.

22. A method as recited in claim 2 wherein the height of the first vessel is about 15-30 meters, and it has a volume of between about 100-500 cubic meters, and wherein the second vessel has a volume same as or greater than the first vessel.

23. A method as recited in claim 2 wherein step (e) is practiced to remove substantially all of the oxygen reaction gases from the pulp .

24. A cellulose pulp bleaching system comprising: a first superatmospheric bleaching vessel having a top and bottom with a pulp inlet at the bottom and a pulp outlet at the top; a first fluidizing discharger at the top of the first vessel for discharging pulp out the outlet and for removing the majority of gas from the pulp; a second superatmospheric bleaching vessel having a top and bottom with a pulp inlet at the bottom and a pulp outlet at the top; a second discharger at the top of the second vessel for discharging pulp out from the vessel; means for mixing peroxide and other chemicals into pulp prior to or at introduction into the pulp inlet of the first vessel; and a connection between the pulp outlet from the first vessel and the pulp inlet to the second vessel.

25. A system as recited in claim 24 wherein said mixing means comprises a pump for simultaneously pumping pulp and mixing pulp with peroxide and chemicals .

26. A system as recited in claim 24 wherein said mixing means comprises a medium consistency mixer.

27. A system as recited in claim 24 further comprising additional mixing means for mixing peroxide and other chemicals into pulp at or after said first fluidizing discharger.

28. A system as recited in claim 27 wherein said additional mixing means comprises a medium consistency mixer between said first fluidizing discharger and said second vessel pulp inlet.

29. A system as recited in claim 24 wherein the height of the first vessel is about 15-30 meters, and it has a volume of between about 100-500 cubic meters , and wherein the second vessel has a volume at least twice as great as the first vessel.