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WO1996023929A1 - Blanchiment de pate a l'aide de peroxyde a pression suratmospherique et a etapes multiples - Google Patents

Blanchiment de pate a l'aide de peroxyde a pression suratmospherique et a etapes multiples Download PDF

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Publication number
WO1996023929A1
WO1996023929A1 PCT/FI1995/000138 FI9500138W WO9623929A1 WO 1996023929 A1 WO1996023929 A1 WO 1996023929A1 FI 9500138 W FI9500138 W FI 9500138W WO 9623929 A1 WO9623929 A1 WO 9623929A1
Authority
WO
WIPO (PCT)
Prior art keywords
pulp
vessel
recited
peroxide
bleaching
Prior art date
Application number
PCT/FI1995/000138
Other languages
English (en)
Inventor
Kaj Henricson
Brian Greenwood
Björn Svedin
Olavi Pikka
Original Assignee
A. Ahlstrom Corporation
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by A. Ahlstrom Corporation filed Critical A. Ahlstrom Corporation
Publication of WO1996023929A1 publication Critical patent/WO1996023929A1/fr

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Classifications

    • D—TEXTILES; PAPER
    • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
    • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
    • D21C9/00—After-treatment of cellulose pulp, e.g. of wood pulp, or cotton linters ; Treatment of dilute or dewatered pulp or process improvement taking place after obtaining the raw cellulosic material and not provided for elsewhere
    • D21C9/10—Bleaching ; Apparatus therefor
    • D21C9/16—Bleaching ; Apparatus therefor with per compounds
    • D21C9/163—Bleaching ; Apparatus therefor with per compounds with peroxides

Definitions

  • 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 ⁇ nimized .
  • the bleaching result is improved .
  • 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 .
  • 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.
  • the peroxide reaction begins with:
  • 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.
  • 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.
  • 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.
  • a method of bleaching cellulose pulp with peroxide utilizing at least a first pressurized bleaching vessel 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 .
  • peroxide e. g. 5-30 kg/admt
  • step (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 .
  • step ( 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.
  • 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.
  • a cellulose pulp bleaching system 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 .
  • 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
  • FIGURES 3, 4 and 5 are views like FIGURE 1 for still different embodiments.
  • 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.
  • 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 H 2 0 2 in a liquid) the apparatus is the same as FIGURE 1 of the US application 08/120,838.
  • Alkali e.g. NaOH
  • Mg, 02 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/H 2 0 2 .
  • 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 .
  • 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.
  • the diameter would be about 3.5 meters
  • the diameter would be about 5.0 meters .
  • 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.
  • oxygen and reaction gases are produced .
  • 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.
  • 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) .
  • the pressure in the vessel 32 is typically lower than in the vessel 22.
  • 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 .
  • 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.
  • a pump 44 which is preferably also a degassing medium consistency pump
  • a valve 47 into a conduit 46 leading to a washer or other suitable treatment (e.g. bleaching) stage.
  • 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.
  • FIGURE 2 shows another embodiment of exemplary apparatus according to the present invention.
  • 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.
  • FIGURE 2 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 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.
  • 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.
  • structures comparable to those in FIGURES 1 and 2 are shown by the same two digit reference numeral only preceded by a "2" .
  • 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.
  • 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 ,
  • FIG 5 An installation shown in Fig 5 was used for testing the process in mill scale. About 30 kg of H 2 0 2 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 .
  • 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 added first vessel will be smaller, same size or larger than the existing vessel depending on which total reaction time is needed.

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  • Life Sciences & Earth Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Wood Science & Technology (AREA)
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Abstract

Une pâte de cellulose est blanchie à l'aide de peroxyde sous pression suratmosphérique en deux étapes (voire plus), l'oxygène et les gaz réactifs étant extraits de la pâte entre les deux étapes. Une quantité comprise entre environ 5 et 20 kg/admt de peroxyde d'hydrogène subit un mélange intime avec la pâte pour être ensuite introduite dans une première cuve mise sous une pression d'environ 5 à 15 bar et à une température comprise entre environ 80 et 120°C. Le blanchiment s'effectue dans la première cuve pendant environ 15 à 60 minutes tandis que se forment l'oxygène et les gaz réactifs, à la suite de quoi un minimum de 20 % (et de préférence la quasi-totalité) de l'oxygène et des gaz réactifs sont extraits de la pâte avant consommation complète de tout le peroxyde. La pâte est alors introduite de préférrence dans une seconde cuve dans laquelle on pourra rajouter du peroxyde, des alcalis et de la vapeur et le blanchiment par peroxyde se poursuit à une pression d'environ 1,1 à 10 bar et à une température comprise entre environ 90 et 120°C pendant environ 30 à 200 minutes, durée au cours de laquelle l'oxygène et les gaz réactifs se forment à nouveau. La majeure partie de cet oxygène et de ces gaz réactifs nouvellement formés -- en réalité, leur quasi-totalité -- sont séparés, en général au moment où la pâte est évacuée de la seconde cuve.
PCT/FI1995/000138 1995-01-31 1995-03-14 Blanchiment de pate a l'aide de peroxyde a pression suratmospherique et a etapes multiples WO1996023929A1 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
US38042695A 1995-01-31 1995-01-31
US08/380,426 1995-01-31

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WO1996023929A1 true WO1996023929A1 (fr) 1996-08-08

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Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO1993000470A1 (fr) * 1991-06-27 1993-01-07 A. Ahlstrom Corporation Procede de blanchiment a l'ozone
WO1994029511A1 (fr) * 1993-06-08 1994-12-22 Kvaerner Pulping Technologies Ab Procede de blanchiment de pate de cellulose
DE19509401A1 (de) * 1994-05-20 1995-11-23 Ahlstroem Oy Verfahren für das Bleichen von chemischem, mechanischem, chemisch-mechanischem und/oder Sekundärfaser-Halbstoff mit Peroxid

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO1993000470A1 (fr) * 1991-06-27 1993-01-07 A. Ahlstrom Corporation Procede de blanchiment a l'ozone
WO1994029511A1 (fr) * 1993-06-08 1994-12-22 Kvaerner Pulping Technologies Ab Procede de blanchiment de pate de cellulose
DE19509401A1 (de) * 1994-05-20 1995-11-23 Ahlstroem Oy Verfahren für das Bleichen von chemischem, mechanischem, chemisch-mechanischem und/oder Sekundärfaser-Halbstoff mit Peroxid

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