SE1551539A1 - Method and system for oxygen delignification of cellulose pulp - Google Patents

Abstract

The Invention relates to an improved oxygen dellgniflcaiion stage located between 2 wash positions (W1, W2) in a fiberline producing bleached qualities of cellulose pulp.The inventive method and system divide the oxygen stage into at least 2 phases, and after the first phase Is the pulp suspension effectively depressurized and vented to atmosphere in order to vent away the bulk volume of inert gases formed during the first phase. The partial pressure of oxygen added to final phase may thus be increased and the amount of oxygen dissolved in the liquid phase is Increased enabling better delignificatlon of the cellulose pulp in the final phase.

Description

1/12 METHOD AND SYSTEM FOR OXYGEN DELIGNIFICATION OF CELLULOSE PULP Background of the lnvention Ttie present invention relates to a rnethod and systern for oxygen delignification.

A number of different processes for oxygen deiignification have beer: disciosed, most vvith a focus on achieving alarger deiignificatior: at less order of viscosity iosses. Some have beer: based upon findings in laboratoiyenvironrneiwt where conditions differ qitite much cornpared to systems. in most iaboratory tests is used a srnaiiexperirnentai reactor charged tivith a srnall arnount of puip. iaboratory reactor equipped vvitti rnixingcapabiiities, such as a quanturn rnixer, that induce a thorough rnixing effect ir: the entire puip sampie vvitti a iii/eightof a coupie of hundred grarns. This idnd of rriixing effect may never be obtained in wmmerciai mili systems,producing thousands of tons of bleached puip per day, as the residence time for the puip miii mixers is quiteshort and often less than one second. t/t/hile high order of deiignification may be obtained iaboratory triais, the same resuits rnay be very difficiiit to reach a cominercial rniii system.

Ttie vvood costs is a substantial part ofa rniiis variabie cost arid measures increase tre pulping yieid is norrnaiiyvery cost efficient. Cooking to higher Kappa no and rriaxirnizing deiignification degree in oxygen deiignificationimproves overall yieid and reduce wood costs. i-iowever. this shift in higher kappa number the puip from the digester, increase the demand or: delignification capabiiities of the oxygen stage in a mill environment, Basic principies oxygen deiignification with rernixing and advantages therefrorrt in rniii systern was discoveredas a resuit frorn production rebuiids, where existing singie stage oxygen deiignificatiori obtainedincreased deiignification degree by adding a second reactor ahead ofthe existing reactorvvith remixiitg in-betvveen. Despite alt kind of inodifications of the oid single stage system, the kappa number couid not be furtherreduced with preserved puip guaiity. Hovrever, after rebuild to tvvo phases deiignification vvith remixiitg inbetween kappa nuinber couid further reduced 'without puip quality deterioration. This experience from iniiisysterns supported the fact that rernixing iri deiignification or bieachirig systems using gaseous agents neededremixing in order to expose the entire puip suspension evenly to the deiignification agents. in a presentation entitled "Tvvo-stage fviC-oxyfgen deiignification process and operating experience", vvhich vvasgiven by Shinichiro Kondo, from the Technicai Div. Technical Ûept. Odi PAPER Co. Ltd., at the 1992 Pan-Pacificiïuip èšt Paper 'technology Conference ('99 lïtAN-PAC PPTC), Sept. 8-19, Sheraton Grande Tokyo Bay Hotel å'i'ovvers, the positive effects of rernixing between stages was disciosed, biit then vvith highest pressure at start and ieast pressure in second reactor.

Advantages tvith heating between phases in oxygen stage delignification in system was discovered yvnentrying to optimize singie stage oxyger: deiignification, using higher temperature and iarger cnarges of alkali, acertain kappa number couid be reached vvithout too penalties in puip strength losses. This end point indeiignification was often found at kappa numbers in the range US 5217575 presented an improved systern for ttie oxyger: bieactiing of pulp of rnediurri consistericy vvhere thistemperature profiiing surprisingly showed that a iovver temperature a first phase may resuit in betterdeiignification. in US 521?575 vvas shown that if an optimized singie stage operated at 105°C was preceded by aphase operated at 2Û-4G°C tower temperature, a better delignification could be obtained at higher seiectivity,LJStäZI-šiêïtiö disciosed an alternative for teinperattire profiiipg where the difference in teinperattire shouid be kept between O°C and iöfíï. The airn was to obtain an improved yield and an improved viscositjy, while retaining the SUN15025 SE prio text.docx 2/12 sarrre dvt/eil time, in connection with industriai impierrientation. in order to improve oxygen deiignification further has aiso been proposed to aiter consistency during the oxygendeiignification stage, and US 425Q15O was proposed to operate a first phase at iovrer consistency foiiowed bythicitening ahead of a foiiovt/ing phase. This system was argued to function on high kappa pulp and by usingrepeated phases abie to reduce kappa niirnher frorn 70 down to SfE-C 505 147 presents an aiterriative vvith instead a high puip cohcuitratiorr, in the range of 25-4990, in a firstphase arid a lower' corrcentration of' 846% in a second phase, at ttre same tirrre as the temperature in the secondphase is to be higher than, or the same as, the temperature ih the first phase. The advantages of the soiution inaccordance with SE-C 505 14? are stated to 'pe the possibiiity of admixing more oxygen in the first high-consistency phase without the risk of channeling. Here was unused quantities of oxygen bied off after the first phase for further admrxture in a secoitd mixer prior to the second phase.

A trigh pressure aiternative has been disciosed in 85526843, where the ihitiai pressure in first reactor isestabiished above 15 bar; and where the pressure was nraintairred at high ievei throughout all phases of treoxygen delignification stage. The idea is to increase the amount of oxygen dissolved in the iiduid phase per literiiquid, and thus being abie to react with the puip. Hon/ever, as been reaiized during the development of thepresent invention the amount of inert gases produced increases throughout the phases, the net amount of oxygendissoived may in fact decrease as the totai pressure in the system is oniy the ofthe partiai pressures fromeach present (CO, C02, 0; etc).

Very few of the prior art soiutions identify the problem vr/ith residuai inert gases in the pulp suspension, and oniyUStSSQi 152 identifies this probiern, but here is degassing proposed in a pressurized pump: “tft/here appropriate, itshou/d also be possibíe to deges exhaust gases (residuai gases) in immediate conjurrction with the second pzimp,preferabiy by means of the pump being provided with irrternai degassinggr, preferabiy a pzimp termeci "deggrassingpurnp". Such type degassirrg, performed in a cehtrifugai part of' a separating sorhe in center, rnay not extract aii gases and especiaiiy not those gases dissoived in the iiduid phase at the prevailing pressure. in US622120? is also degassing done upper part of a dot/tfn fiow tower in the iast phase. This specific systemwas deveioped for temperature profilirtg using low pressure steam between a first up-fiovv' reactor and asubsequent dovrn-flotß/ reactor, where degassing from top of second reactor only kept the oxygen dissolved andoxygen rentainirtg after ventirtg. This systern has never been used, and the risk is very itigh that rnajor part ofall oxygerr charged rnay be vented avvay frorn the secord reactor, ie. ohtainirrg no reaction effect from the vented part of charge.

The greater part of the prior art has consequentiy been airnecl at keeping the puip suspension pressurizedthroughout aii oxygen deiignification stages, most often pressurized to highest pressure in iniet to first stage, andusing this pressure to the driving force for piiip fiow throughout the system, which makes sense as ite instaiied pumping effect rnay be kept to a rhiriirnurn.

Besides the optimization of the deiignification conditions in the stages is aiso fiitrate handling in the oxygendeiignification having an impact on kappa number reduction and puip strength. Usage of dirty fiitrate vrith highCOD content in the oxygen deiignification caiis for high amount of oxygen charge, as a large part of the oxygen charge is consurned reaction tfvitit the COEE in the fiitrate.

SUN15025 SE prio text.docx 3/12 Also, if high kappa puip, With kappa after cook in the range 40-129, vvtiere a kappa nurnber above 29may be considered high for hardwfood, is to be deiignified, large amounts of oxygeh is needed for deiignificationthat may resuit in channeiing in the stages. Vtíith high kappa iturnber in puip is also a iarge part of the oxygencharge consurned during delignification ofthe high kappa puip.

So combination of high kappa puip and fiitrates vvith high ievei of CGI) impose immense probierhs for ctiargirtg and rnixing the necessary arnourit of oxygeri to the puip suspension in follovving parts is the normai terminoiogy for oxygen delignification stages used. A stage, i.e, an oxygendeiignification stage in this case, is the treatment done between two Wash positions. What is done in said stagemay aiso be divided into phases, vrith some changes effectiveiy hrought into next phase in aspects of chemicaicharges, heating and/or venting. Aiso, each phase rnay in turn be divided into zones vrhere a first and a secondzone of one phase rnay differ as to changes of ctiernicai charges, heating arid/or venting. in summary, an oxygen deiignification stage between 2 vvash positions rnay be divided into phases, and eachphase may in turn be divided into zohes. This nomenclature is used throughout the description even if some prior art describe 2~phase deiignification systems as 2 stage systems.

The vrash positions may be performed a number of 'i/vays, for exarnpie; ß Using wash presses, 'where a dlsplacement vi/ashing is performed of the puip foiiOwIed by a dewateringto high consistency: e Using wash fiiters, but vvashing effect is obtained by dot/vn the puip to iovv consistency and asuhseqtient dewatering on the drum; ß Using drum dispiacement wash machines, and a dispiacement wash of puip is performed on the drumand optionaiiy finished by some thickening process using vacuum draining or press roiis: e Using slmpie diiute and devi/ater rnachines, dilute the puip to low consistency and subseguently dewatering the iow consistency puip to medium or high consistency puip using a dewatering screvv. in this description are iow, medium and high consistencyf of puip used to define the amount of liduid ih the puip suspension. Lovv consistency (LC) in puip is typicaiiy in range 2--89/5, rnediuin consistencyf (MC) the range 1895, and higti oorisistericy (i-iC) typically above 30%. LC ptiip so diiuted that it behaves iike vvater arid the ptiipsuspension may be readily pumped by conventional iiduid pumps. iviC is an lntermediate range that is pumpabiebut requires speciai fviC-pumps that fluidize the suspension, high consistency puip is not pumpabie by conventionai pumps and instead require transport screws etc. for feeding and transport. in this description is aiso COD content ofthe filtrate used, COD standing for “Chemical Oxygen tltemand” and is a parameter describirig the totai oohterit of oxidizabie materiai the fiitrate. .

SUMMARY OF THE INVENTION One common aim of the inventioh is to improve oxygen deiignification further, and optimiziiwg the order of oxygendissoived the puip suspension such that the oxygen may react with the ceiiuiose to a far greater extent thanpreviousiy possibie, and rnake the oxygert deligitification better suitabie to high kappa cooking.

Vtihile the shift to high kappa cooking and increased deligitification order in oxygen stages has been knovvh forrnany years to beneficial for' ptiip strength frorn iaboratory triais, rniii impiemeritatiohs has been less frequent due to difficuities adding the necessary amounts of oxygen in a reactive phase after high kappa cooking.

SUN15025 SE prio text.docx 4/12 lt been found during laboratory trials that the inert gases, especiaiigf CO and C02, that are forrried during theoxygen stage acts parasiticaiiy on the soiubility of oxygen in the liquid phase, and more or less substitutes theamount of dissoived oxygeit. For the efficiency ofthe delignification effect on the celluiose puip is of outmostimportance that the oxygen to the greatest extent possible is dissoived in the liquid phase such that the liquid maypenetrate the ceilulose fiber network and get in contact vvith the oxidizabie material, i.e. lignin, in the cellulose fiber matrix. in iaboratory triais 'vvith muiti-phase delignification is often the small test reactorfilled 'vvith an excess amount ofoxygen, i.e. the puip sample is not in an hybraulically filed volume instead only occupies 5-40 of entire testreactor, so these testing”s may never come close to real environment reactors being hydraulically filled vvitlw puip suspension and where oxygen is not in excess.

Another aim is enabie airnost a total vvithdravval ofali inert gases that are formed and dissolved the puipsiispension after a first phase of the oxygen delignification stage, vvhere the oxidation process fastest andprodiices most ofthe inert gases. By' dropping the pressure ofthe entire pulp suspension to a pressure close toatrriospheric pressure, couid both the content of non-dissoived gases be rernoved, aiso a large part of gasesbeing dissoived in the liquid at higher pressure. A fiashing effect is thus obtained. By venting away the inert gasesbefore charging a fresh charge of oxygen to the puip suspension could also the amount of oxygen being dissoived in the iiguid phase be increased considerably as competing gas content has been reduced to a large extent. 'fbe method according the invention related to oxygeri delignifirzation of a medium consistency ceiiulose pulpsuspension having a ltappa nurnber exceedirig 18, arid said puip suspension through a first and a secondoxygen delignification phases located between 2 vvash positions (Vä, Wz) for the puip where said phases areseparated by heating the puip at ieast 5°C using steam after the first phase and ahead of the second phase, andboth phases includes charge of allraii and oxygen to each of said first and second phase. The inventive featuresof the rnethod are that both the first and second phase is pressurized to an initial pressure exceeding 5 par, andafter the first phase the puip depressurized to an excess pressure belovv 0.5 bar, preferably depressurized toatrriospheric pressure, i.e. within a pressure interval betvveen O to 9.5 bar, and residual gases released from thepuip during depressurization are vented avvay before the start ofthe second phase, and tivherein the charge ofoxygen to the second phase is charged to the puip after the residual gases has been vented atvay, and vvhereinthe charge of oxygeit to the second phase is distributed into the puip using a mixer. thus establishing increasedpartial pressure ofthe oxygen in the puip suspension. This concepttial principie enable better delignification at higher viscosity, i.e. improved seieotivity of the oxygen delignification stage.

The method is further distinguished in that more than QO% of the inert gases, such as carbon monoxide andcarbon dioxide, formed during the oxidation process the first phase. are vented away by the depressurizatioit,and tvherein the puip suspension for the depressurization effect fed to an upper part of a standpipe andexposed to pressure iri the range 0-05 in the upper part ofthe standpipe having a tieigtit exceeding 3 meter.This veritirig principle applied here reduce the pressure of the puip suspension to such a levei that the soiubiiity aiigases decreases, i.e. follows Henri/s law, and enabie efficient degasification in the stand pipe voiurne. lt isspecificaiiy the inert gases formed during the first phase of delignification that needs to be removed. Even if someresidual oxygen may be lost in this venting is the parasitic effect on oxygen soiubility by the competing inert gasesmore dominant, and by proper dosing of the oxygen charge to the first phase could most ofthe oxygen charged by consumed in the first phase. ln a preferred ernbodiment ofthe method is the first phase divided into at ieast 2 zones, and both zones includes charge of oxygen to each of said first and second zones of the first phase. This division of the first phases into SUN15025 SE prio text.docx 5/12 rriuitipie zories, i.e. at 2 zories, enable a fuii utilization of the reaction ltirietics of oxygeri deiigriificatiori, asrnost of the reactions an oxygen delignification stage occurs during the first half of the total retention time of the oxygen deligiiifirzation stage, and the huik volume of inert gases are forined in this first half. in yet a preferred ernbodimerit of the iriventive method the retentioh tirne for the puip suspension iri the firstphase shorter than the retentiori tirne in the second phase, and that the retentiori tirne of the first zone in the iirstphase is shorter than the retention time ofthe second zone in the first phase. This sizing of reactors may enablean equal amount of oxygen charged to each phase, near the charge voiume possibie to mix into the puipsuspension 'with the niixers used, said charge being further optiinized such that more than 90% is consunwed in each phase and is not Wasted iri the follovving degassing step. ih another embodiment ofthe inventive method is the medium consistency cellulose puip suspension fed to thefirst phase obtained from a preceding thicltening process producing a high ænsistency celluiose puip at aconsistency above 30% and that the medium consistency celiuiose puip suspension prodiiced by ntixiitg thehigh consisteiicy cellulosa puip from the titicifieiwing process yvith oxidizíed fiitrate. tisiitg oxidized fiitraie for formingthe medium consistericy puip avoids adding oxidizabie matter into the pulp stisperision, which oxidizahie matterproduce hydroxyfi radicais during the oxygen stage and in presence of celluiose puip reduce the viscosity of the puip. in a finai entbodimeiit of the inventive method the pulp suspension after the first zone in the first phasedepressurized to an excess pressure below 0.5 preferably depressurized to atmospheric pressure, iivithiria pressure interval between O to G5 bar, and residuai gases released from the puip during depressurization arevented away before the start of the second zone of the first phase, and whereiit a charge of oxygen to the secondzone in the first phase is charged to the puip after the residual gases has been vented away, and wherein thecharge of oxygeit to the second zone of the first phase is distripiited into pulp using a niixer, thus estahiishingincreased pariiai pressure of the oxygen in the puip. This repeated venting of ineri gases formed iniiiaiiy in theoxygen stage is especially suitahie for oxygen delignification of high ltappa puips and wheri the fiitrate addedcontains high concentration of COD, tlvhich in totai introduce huge amounts of oxidizable material to the oxygen stage, and thus generates extreme amounts of inert gases during the oxygeit deiignification.

Froin a system point of tfiew the invention coniprises a system for oxygen deiignification of a mediuin consistencycellulose puip suspension tiaving a kappa ntirriberexceeding 18, and 'iii/here said system is iocated between 2wash positions, said medium consistency puip suspension first pressurized by a first pump passing the puip to atleast a first oxygen mixer and directiy thereafter to at ieast a first oxygen deiignification reactor in a firstdeiignification phase. After the first delignification phase is the puip suspension heated in a steam rnixer ahead ofa second phase in a second oxygen deiignification reactor such thai said phases are separated by heating thepuip at least 5°C using steam after the first phase and ahead ofthe second phase, and both phases includescharge alltali and oxygen to each of said first and second phase. The is further distinguished in thatboth the first and second phase is pressurized by said pumps to an initial pressure exceeding 5 har each of theoxygen delignification reactors, and after the first phase is the puip depressurized to an excess pressure below0.5 bar over a vaive located in an outiet from the first oxygen delignification reactor, preferabiy depressurized toatinospheric pressure, i.e. vvithin a pressure interval between (i to 0.5 bar, in a vented standpipe. i-'äesidiial gasesreleased from the ptilp during depressurization are vented .eiyvay in a degassing line connected to the standpipebefore the start of the second phase, and whereiri the charge of oxygeri to the second phase is charged to asecond oxygen mixer to the puip after the residual gases has been vented away, and wherein the charge ofoxygen to the second phase is distributed into the pulp the second mixer, thus estabiishing increased partiai pressure of the oxygen in the pulp in a second oxygen deiignificatioii reacior.

SUN15025 SE prio text.docx 6/12 The systern is designed ih such a vvay that rnore than 90% of the ihert gases, such as carbon rnorioxide andcarbon dioxide, formad during the oxidation process the first phase, are vented away by the depressurizationover the vaive located in the outiet from the first oxygen deiignification reactor. The puip suspension for thedepressurization effect is fed from the vaive and to an upper part ofa standpipe and exposed to pressure therange (3--05 bar in the uppar part of tiia standpipe having a haigiit excaeding 3 matar. A stand pipa connected toatrnosphere and vvith this height enabie a sufficient practicai voiuriie for estabiishihg a retention tirne in this staridpipe that may allovv inert gases to separate over time. Even though it is not necessary, additionai equipment rnaybe added the standpipe to improve separation of inert gases, such as stirrers or sonication/uitrasonication, butthe costs are most often not inothfated versus the effect obtained, as most of the separation effect occurs vrhen the pressure is reduced suddenly over the vaive arid ahead the stahdpipe.

Further, the system is designed such that the first phase is divided into at ieast 2 zones, with reactors eachzone and both zones inciudas charge of oxygen to each of said first and second zones ofthe first phase mixers ahead of each zone. As indicated above, this staging of zones enable rnaximai charges of oxygen to eachphasa and zones, vrithotit iosses of oxygen in sitbsaqtieiit degassing and ininimtim risks for gaschanneiing in tha systarn. This kind of staging is also preferabiy designed such that the retention tinia for puipsuspension in the first phase is shorter than the retention in the secorid phase, an' that the retehtion time ofthe first zone in the first phase is shorter than the retention time ofthe second zone the first phase, said retention times established by increasing storage voiume in reactors of each phase or zone. in the finai enibodirneiit of the inventive system is tha ntaditim consistency caiiuiose puip suspension fed to thefirst phase obtained frorn a preceding thicitenirig process producing a high consistency ceiiuiose puip at aconsistency above ê-Otiíi, and that the medium consistancy ceiiuiose puip suspension is produced by rnixing thehigh consistency ceiiuiose puip frorn the thickening process with oxidized fiitrate obtained from a fiitrata tania in avvash arranged after the iast phase and having passed through an oxidizing reactor (RE). This additionai reactoris typicaiiy oniy naedad when tiia CGI) leveis in the fiitrate is above 'iÜO g/i, tvhicli inay be tha case if high or rnediurn kappa puip is fed to the oxygeri stage. "fhe invention is described in mora detaii vvith reference to the figures in accordance vvith the foiiovring figure iist.

SREEF BESCFHPTEON OF THE DRAWiNGS 1 snovvs a first ernbodirnerit of the inventive oxygen deiignificatioh systern suitable for incornirig puip vvitii iovvkappa numbers and iow concentration of COD in the filtrate obtained in the oxygen Wash, and Fig. 2 shows a second embodiment of the inventive oxygen delignification system, suitable for incoming puip Withmedium kappa numbers or medium concentration of COD in the filtrate obtained in the oxygen vvash, and Figß shovrs a third einbodiniant of tha inventive oxygen deiignification systam, vvitn similar sat--up as in figure but with an additionai veritirig aiso after the zone in the first stage, and optional degassing purnps, suitabie forihcorhing puip vvitti rnediurri riurnbers and rneditirn concentration of COD ih the filtrate obtairied in theoxygen wash; and Pig/i shovvs a fourth embodiment of the inventive oxygen delignification system, vvith simiiar set-up as in figure 2but vrith an additionai oxidizing reactorfor the fiitrate returned to the oxygen stage; suitabie for incoming puip vvith high Kappa numbers and high concentration of COD in the filtrate obtained in the oxygeii vvash . in aii figures are reactors indicated to be tiydratiiicsaily fiiled vvitri the puip suspension, and in the sarne nianrier is the retention voiurne of the puip suspension in the stand pipes indicated in the same vvay.

SUN15025 SE prio text.docx 7/12 ÛETAELED DESCREPTEÛNin figure 1 is disclosed a first embodiment ofthe iriventive oxygen deiigltificatioii stage located betvveen 2 washpositions tft/t and i/VQ a fiheriine producing bieached quaiities of ceiiuiose pulp. The puip may be obtained fromany kind of continuous or batch chemical pulping process and even mechanicai buip, but most preferabiy pulpis obtairied from a continuous ltraft puipirig process.

The oxygen delignification system is typically iocated directiy after a puip screening stage orimmediately preceding said puip screening stage.

Normally a number of bieaching stages foilows said oxygen deiignification, inciuding chiorinedioxide deiignification and/or bleaching, aikaii extraction, peroxide bieachingr, ozone bleachihg, hot acid treatment, chelating stages, etc etc.

Puip is to the oxygen deiignification stage from a wash position i/“i/t. in this case a irl/ash press»where the pulp fed to a standpipe 1G has a high consistency and the wash filtrate sent to a fittrate tank Fit. Thepuip is diiuted to medium consistency in the standpipe by adding fiitrate from the fiitrate tank FTg that is obtainedfrom the second 'wash press Vxig, together with the buik charge of alkali necessary for estabiishing the aiitaiineconditions for the oxygen stage. 'the aii<;aii charge is typicaiiy in the order of kgr/Ailïi' pulp and the consistency about 12% Aprotector in form of a rnihor charge itligíåOfii is aiso charged. The temperature of the puip suspension in thisstage is typicaiiy around SS-QEPC.

The pulp suspension is ihitiaiiy pressurized by a first pump P1 to a pressure above 5 barie), andpreferabiy about 10 bar, and this pump aiso bring about a mixing effect of the added iiqtiid charges (aiitaii, in formof sodiuin hydroxide oxidized vvhite iiquor or White iiduor, and fiitrate). The conditions are novv set for starting theoxygen deligiiification 'which starts 'with basslng the medium consistency ceiiuiose pulp through a mixer M2dedicated for mixing oxygen into tre puip suspension. Ünce the oxygen is rnixed into the puip suspension, thepuip suspension is fed to an upfiovv tot/ver, i.e. an oxygen delignification reactor 0213,, whereih the oxygendeiighification process proceeds. in this initial phase of the oxygen deiignification the consumption ofaikaii andoxygen is extremeiy high and a iot of inert gases such as CO and C02 are formed, Due to the exothermic heatrelease during the deiignification is teinperatiire increased some 5°C, i.e in the range 1--'7°C. Hovvever, ihsome miii systems the heat release is qiiite iarge and rapid initiaiiy, ihdicating a high oxidation rate.

At the end of the first phase is the puip depressurized from a residuai pressure of about 6 bar-(e),(if the initial pressure was about 1G bar) in top of reactor 0213 and sent to a standpipe 'i i, typicaiiy held atatmospheric pressure and vented directiy to atrriosphere. The standpipe is sufficientiy high, typically more than 5meter, and enabie aiso inert gases dissoived in the iiquid phase to reiease over time. in the bottom of thestandpipe 'lt 'will most of inert gases formed in first have been reieased and vented to atmosphere.

Next pump P2 pressurize the pulp suspension again to a pressure above 5 barie), and preferablyabout 10 bar, and this pump aiso bririg about a rnixing effect of the added iiquid ctiarges (alkali). The conditionsare now set for starting a second phase of the oxygen delignification which starts tivith passing the mediumconsistency ceiiuiose pulp through a second mixer ivig dedicated for mixing a second fresh charge oxygen into thepuip suspension. 'fhis second mixer ivig, is preferabiy aiso used to mix in steam. amount of steam added istypicaiiy in the order that an increase of ahout 5°C is obtained in tre pulp suspension, reaching a ternperature ofabout 100-105°C. The order of temperature increase is dependeht on the arnount of exothermic tieatirig inprevious stages, Once the second charge of oxygen is mixed into the pulp suspension, the puip suspension is fed to a second or finai upfiow tovi/er, i.e. an oxygen deiignification reactor 0.222, 'wherein the oxygeri delignification SUN15025 SE prio text.docx 8/12 process proceeds iri a second phase. in this second or finai phase of the oxygen deiignificatiori stage theconsumption of aikaii and oxygen is moderate and the amount of inert gases such as CO and C02 formed areproduced at a far iovrer rate and at iess volume.

At the end of the second phase is the puip depressurized from a residuai pressure of about 6har(e), (if the initiai pressure was ahout 10 har) in top of reactor aan sent to a staiidpipe 12, typicaiiy heid atatriiospheric pressure and vented directiy to atrnosphere. The standpipe is sufficieritiy high, typically rnoie than 5meter, and enahie aiso inert gases dissoived in the iiquid phase to reiease over time. in the bottom of thestandpipe “l2 will rnost of the inert gases formed in first phase have been reieased and vented to atmosphere.

After finishing of the oxygen deiignification the vented puip suspension is fed to a finai washpress Vit/g, conventionaiiy cailed oxygen wash position, that ends the oxygen deiignification stage. 'the washfiitrate ohtained from this final wash. press Vtig is collected in a second filtrate f-"i'¿, and used as dilution liquidin the first standpipe 10. This tfvash is aiso norrnaiiy used as vvashing/dispiaoernent iiquid in the first washpress lift/i.

The emhodiment shotrvn in figure is preferahiy used for oxygen deiignification of low i-:appa puip and with fiitratehaving low content of COD, By venting the suspension down to atmospheric conditions after the first phase couidalmost aii ofthe inert gases formed he vehted avvay frorn the pulp siispension, and in the suhsediient secondcharge of oxygeii couid an. optirnai amount of oxygeii 'pe dissoived into the puip suspension 'without oompeting vvith residuai arnourits iriert in figure 2 is disclosed a second emhodiment of the inventive oxygen deiignification stage iocated hettrveen 2wash positions tft/i and i/Vg in a fiherline producing hleached quaiities of celluiose puip. This second emhodimentdiffers front the first einbodiment hy additionai pre--reactor OZZfAforming a first zone in the first phase. 'Thisadditional pre--reactor adds yet ah oxygen deiignification with an additionai inixer lVi~. ahead of this additionai pre-reactor OZiA, and rnay suitahiy he designed as a verticai pipe ioop, or using two standard standpipes togetherwith an upper hend connecting the standpipes. in this emhodiment is no venting after the pre-reaotor 02% used,and the second zone of the first phase may foiiow directiy in reactor 0213.

The emhodiment shovvit in figure 2 is preferahiy used for oxygen deiigrtificatioit of medium kappa puip or vrith fiitrate having ntediijni concehtratioh of COD, vviiere a siightiy iarger order of oxidation. takes piace in first phase. in figure 3 is disclosed a third errihodirnerit of the inventive oxygen deligriificatiori stage iocated between 2 washpositions Wt and Wg a fiheriine producing pieached quaiities of ceiiuiose pulp. This third emhodiment differsfrom the first emhodinfent hy an additionai pre-reactor O21Aforming a first zone in the first phase. and asuhseduent standpipe M. This additionai standpipe M adds yet a position for venting of inert gases from the puipsuspension. Additionaiiyf, iii this einhodiment is aiso degassihg used in all ivifl purnps 91A, Pm, P2 P3, enahiing veritirig of inert gases ahead of aii oxygeri rnixers till; arid ivia as »vett as ahead of the finai tfvash'\Ar¿.The emhodiment shotrvn in figure 3 is preferahiy used for oxygen deiignification of medium kappa puip and with fiitrate having medium concentration of COD. where a siightiy iarger order of oxidation takes piace in first phase.

SUN15025 SE prio text.docx 9/12 Finally, in figure 4 is disclosed a fourth erribodiment ofthe inventive oxygen deiignification stage iocated between2 'wash positions Vilt and i/Vz in a fiberiine producing hieached gualities of ceiiulose puip. This fourth ernhodirnentdiffers frorn the third emhodirnent by an additional oxidation reactor RE for the fiitrate pumped by pump PA fromthe filtrate tank FT; and to standpipe ft).

Both oxygeri and alltali may be added to the oxidation reactor RE, as weii as additioriai oxidation stictias peroxide (i-izüg). in this ultimate version of the inverition rnay most of the oxidant added to the puip suspensionhe used for deiignification ofthe iignin content of the pulp, and almost no oxidation of dissoived iignin or otherorganic content the liquid phase ofthe puip suspension takes piace in presence of puip. avoiding viscosityiosses in the pulp due to formation of hydroxyi radicais 'vvhen oxidizing COD content in the iiquid phase. ernhodirneitt shovm in figure 4 is preferatiiyf used for oxygen deiignification of high kappa pulp and vvith filtrate having high conceritratiori COD, where the iargest order of oxidatior takes piace iri first phase.

The residence tirne in reactors 02%, 0219 and 022 shouid be continuously increasing. Typically the residencetime in a pre-reactor iike 02% may be in the range “i-'iO minutes, the residence time in a second reactor iiite 0218in the second zone of the first phase may be the range 29-60 minutes, and the residence time a reactor lil if only tvvo reactors are used, i.e. 0213 and OZ¿ are used, the residence of the first phase may be shoiterthan 20 minutes, and preferaply iri the range 5-60 minutes in totai, foiiovved by a residence time in the range 4G- 180 minutes for the second phase. in aii ernhodiments shown are the inert gases formed dririitg a first phase of the oxygen. deiignification stage vented away ahead of the final phase in the oxygen deiignification stage, such that the oxygen charge added tothe finai phase rnay obtain highest possibie ievei of oxygen soived the liquid phase of the puip suspension. insome applications where the oxidation produce more inert gases, with higher i-:appa number and/or higher COD content in fiitrate, may additional venting 'se irripiemented aiso after a first zone in the first phase. 'tft/hat embodintent of the intfention is to be chosen depends on rate and arnount of oxidation that oocurs infirst vvhich order of oxidation may be controlied hy measuring the temperature profiie oftiie pulpstisperision. if the temperature increase due to the exotherrriic reactions are modest and sioyv it may 'se ftiiiy sufficient with a 2reactor system, vvith venting hetvveen, and further costs for a third reactor and additionai mixer couid likely nothe motivated by the sinaiier improvement in seiectivity. if the ternperattire increase is very rapid the first phase it is fair to assume that the fiitrate contains iiigh CODcontent, as this CGI) content is much easier to oxidize than the organic rnatter bound to the oeiiuiose, and hence the rnotivation to invest in a fiitrate oxidation reactor' RE rnay be ftiiiy rnotivated 'ey the improvement in seiectivity.

Accordingiyf, is to 'ce understood that the embodirrients disciosed are potential embodirnents ofthe presentinvention, and has been described by tvay of iiiustration and not iimitation. Features from the fourth ernhodiment,such as the oxidation reactor for the filtrate, may for instance also be impiemented in a 2 reactor system, and degassirig ltviC ptinips may aiso he used as a cornpierrient in such 2 reactor system.

SUN15025 SE prio text.docx

Claims (1)

1. A method for oxygen deiignification ofa medium consistency ceiiiiiose puip siispension having a kappanumber exceeding 18, said puip suspension passing through a first and a second oxygeri deiigriiiicatioh phases iocatedbetween 2 wash positions (Wi, i/iíz) for the puip where said phases are separated hy heating the puip atieast 5°C using steam after the first phase and ahead ofthe second phase, and both phases inciudescharge of oxygen to each of said first and seænd phase; characterizeof in that hoth the first and second phase is pressurized to an initiai pressure exceeding 5 har, and after the first phase is the puip depressurized to an excess pressure heioiiv 0.5 bar,pi'ei"ei'abiy depressurized to .eitrriosptteric pressure, i.e. ti/ittiin a pressure iritenfai between Û to 0.5 har and residuai gases reieased frorh the puip during depressurization are vented away heiore thestart of the second phase, and vvhereirw the charge of oxygen to the second phase is charged to the puip after the residuaigases has heen vented away, and wherein the charge of oxygen to the second phase is distributed into the puip using a rnixer, thus estahiishing iricreased partiai pressure of the oxygeh in the puip suspension. A method according to ciairn 1 characterized in that more than 94% of the inert gases. such as carbonrnonoxide and carbon dioxide, formed during the oxidation process in the first phase, are vented avi/ay bythe depressurizatiop, and wherein the puip suspension for the depressurization effect fed to an upperpart of a standpipe and exposed to pressure iri the rahge 0-05 in the upper part ofthe standpipe having a height exceeding 3 rrieter. A method according ciairn 1 characterized in that the first phase is divided into at ieast 2 zones, and both zones inciudes charge of oxygen each of said and second zones ofthe first phase Å method according to ciairn 3 characterized in that the retention tirne for the puip suspension in thefirst phase is shorter than the retention tirne in the second phase, and that the retention time ofthe first zohe in the first phase shorter than the retehtion tirne of the second zorie the first phase, A method according to ciaim t cheracterizeci in that the medium consistency ceiiuiose puip suspensionfed to the first phase is obtained from a preceding thickening process producing a high consistencyceiiuiose piiip at a consistency above 309/2), and tiiat the rneditim consistency ceiiuiose puip suspensionis produced 'oy rnixing the high corisistericy ceiitiiose ptiip frorn the thickeriihg process vi/ith oxidizedfiitrate. A inethod according to ciairp 3 ciiaracterized in tiiat and after the first zone in the first phase is puipdepressurized to an excess pressure heiow 0.5 har, preferabiy depressurized to atrhospheric pressure,Le. within a pressure iritervai between O 0.5 'par and residuai gases reieased frorh the puip during depressurization are vented avvay before thestart of the second zone of the first phase. and vvhereirt a charge of oxygen to the second zone in the first phase is charged to the puip afterthe residuai gases has been vented away, and iivherein the charge of oxygen to the second zone of thefirst phase is distributed ihto the puip using a rnixer, thus estahiishirig increased partiai pressure of the oxygeh ih the puip. SUN15025 SE prio text.docx 10. 11. 11/12 A system for oxygen delignification ofa medium consistency cellulose puip suspension having a trappanumber exceeding 18, and vvhere said system is iocated between 2 'vvash positions (Vi/i, tft/g) said medium consistency puip suspension first pressurized by a first pump (Pr/t) passing the puipto at least a first oxygen mixer (itfiz) and directly thereafter to at least a first oxygen delignification reactor(6215) in a deligriitication phase, and after the first deiignification phase is the puip suspensionheated in a stearn mixer (M3) ahead ofa second phase in a second oxygeri deligniiication reactor (022)such that said phases are separated by heating the puip at ieast 5°C using steam after the first phaseand ahead of the second phase, and poth phases includes charge oxygen to each of said first andsecond phase; and charge of aiitaii to at least first phase characterized in that hoth the first and second phase is pressiirizíed hy said pumps to an initiai pressure exceeding ššhar each the oxygen delignification reactors , and after the is the puip depressurized to an excess pressure below 0,5 har over avaive (V2) iocated in an outiet from the first oxygen delignification reactor, preferably depressurized toatmospheric pressure, i.e. vvithin a pressure intervai between t) to 6.5 har, in a vented standpipe (11), residuai gases reieased from the puip during depressurizatioit are vented away in a degassingEine (flifr) connected to the standpipe before the start of the second phase, and vvhereiri the charge of oxygeri to the secorid phase is charged to a second oxygen mixer (M3)to the puip after ttie residuai gases heen vented away, and vvherein the charge oxygen to ttiesecond phase is distributed into the puip using the second mixer, thus estabiishing increased partial pressure of the oxygen in the puip in a second oxygen deiignification reactor (G22). A system according to clairn ? characterized in that more than 99% ofthe inert gases, such as carhonmonoxide and carbon dioxide, formed during the oxidation process in the first phase, are vented away bythe depressurization over the valve (V2) located in the outiet from the first oxygen deiigitificatioit reactor(0219) and vvherein the puip suspension for the depressurization effect is fed from the vaive and to anupper part of a standpipe and exposed pressure in the range Ü--ílö har in the upper part of the standpipe having a height exceeding 3 meter. A systern accordin, to ciaim Tf' characterized in that the first phase is divided into at least 2 zones, withreactors in each zone and hotit zones includes charge of oxygen to each of said first and second zones of the first phase using mixers ahead of each zone. A systern according to clairn 9 characterized in that the retention time for the puip suspension in the firstphase is shorter than the retention time in the second phase, and that the retention time of the first zonein the first phase shorter than the retention time ofthe second zone in the first phase, said retention times estahiished hy increasing storage volume in reactors of each phase or zone. A systern accordin, to ciaim Tf' characterized in that the medium consistency celluiose puip suspensionfed to the first phase is obtained froin a preceding thickening process (Vt/i) producing a high consistencyceiiuiose puip at a consistericy above 30%, and that the medium consistency ceiitiiose puip stisperisionis produced hy mixing the high consistency celluiose puip from the thickening process oxidizedfiltrate ohtained from a filtrate tank in a vvash arranged after the last phase and having passed through an oxidizing reactor (i-'åiI-E). SUN15025 SE prio text.docx