SE538452C2 - Method for operating a two vessel digester system - Google Patents

Abstract

18/18 Abstract The invention relates to an improved two vessel cooking system, wherein the firstvessel is an atmospheric impregnation vessel and the second vessel is a pressurizeddigester, where hot black Iiquor is finally flashed off inside the impregnation vessel forsteaming the cellulosic material and providing the impregnation Iiquor. According to the inventive method is an additional cleaner wash Iiquor heated andadded to final cook zone in digester, while displacing a corresponding amount of hotblack Iiquor before the final cook zone, and the heat value in the hot black Iiquor isrecovered first in transport circulation before being used as source for flashing offsteam used to steam the comminuted cellulosic material inside the impregnationIiquor and final use as impregnation Iiquor. The invention results in a cooking process with high yield and improveddelignification in final cooking phases and reduced COD content in the produced pulp without increasing steam consumption. Fig. (5) KST 14007SE_Eng_text.docx

Description

1/18 Method for operatinq a two vessel diqester svstem TECHNICAL AREAThe present invention concerns a method for the continuous cooking of celluloseaccording to the introduction to claim 1 with the aim of achieving improved washing and delignification at less Iosses in heat economy.

DESCRIPTION OF DRAWINGS The background prior art and the invention will be described using figures 1 to 4describing the development of continuous cooking up to Compact CookingW' G2,Iaunched in 2003, and finally the invention is described with reference to figure 5.Figure 1 shows the development of continuous cooking processes with start from1957; Figure 2 shows schematically the flow routing in Compact Cooking G2; Figure 3 show the upper part of the atmospheric impregnation vessel used inCompact Cooking G2; Figure 4 shows in further detail the liquor routing in Compact Cooking G2;.

Figure 5 show the improvement in Compact Cooking, using the atmospheric firstvessel for recovering the increased flow of heated liquors to the digester system assteam for chip steaming; Figure 6 show a brown wash keeping filtrate pressurized.

THE PRIOR ART As shown in figure 1 has the development of continuous cooking passed severaldevelopment phases since the first commercial continuous digester installed inFengersfors, Sweden by Valmet AB (then Kamyr AB).

The very first generation (see 1957, CC + Cold Blow) of continuous digesters had asimple one stage cocurrent cooking zone (CC) with a final cooling in bottom for coldblow. ln the later generation (see 1962, CC + Hi Heat) of continuous digesters was also animproved internal washing with a countercurrent Hi.Heat washing zone. The designcapacity was in a retrospective view quite low as the countercurrent wash preventedincrease of production, and only half the digester was used for cooking. Typicallycooking temperatures lie at some 160-170°C. A large part ofdigesters still in operation use only half the digester as a cooking zone.

KST 14007SE_Eng_text3.docx 2/18 Next followed a development (see 1983, CC + MCC + Hi Heat) where a part of thewash zone was converted to a countercurrent cooking zone with an addition of alkaliat end of cooking zone in order to even out the alkali profile and use more of thedigester as a cooking zone. This development proceeded with further extension (see1991, CC + MCC + ITCTN' (Hi Heat) of the cooking zone, adding an lTC cooking zoneat full cooking temperature, also countercurrent, below the MCC cooking zone. Thetotal charge of alkali was thus distributed to several positions during the cook, withthe main charge in top of digester, but with additional charges in bottom of the MCCzone as well as in the bottom of the lTC zone. This distribution of alkali at multiplepoints reduced the very high alkali concentration at cooking start. High alkaliconcentration was found to be detrimental to pulp strength, and was established inold conventional cooking where most, if not all, of the alkali was charged to top ofdigester.

The technique of impregnation with black liquor was developed during the latter partof the 1980s and the 1990s (see 1993, BLITM + MCC + ITCTN' (Hi Heat) ), with theaim of obtaining improved cooking economy and heat economy and of obtainingbetter pulp. ln figure “1993” is the black liquor impregnation, i.e. BLI, established in aseparate impregnation vessel in a two vessel digester system, but several singlevessel systems had also BLI in upper part of the digester. ln these early two vesseldigester systems the highest pressure established was obtained in the impregnationvessel and often at such high temperature that the comminuted cellulosic materialwas not thoroughly impregnated with alkali before the cook. Often was a temperaturein excess of 120°C established in the impregnation vessel, which was consideredbeneficial for heat economy in the process but has later on been found to bedetrimental for alkali diffusion into the core of the comminuted cellulosic material. Atabout this time was also Lo-SolidsTM cooking implemented, where black liquor waswithdrawn and replaced with cleaner cooking liquor with less lignin content. ln later development (see 1997, Compact CookingW' ) was the two vessel digestersystem further improved with the main objectives to increase yield and uniformcooking results. Most dominant features in Compact Cooking was impregnation aswell as cooking at high L/\N ratios, enabling presence of high amount of alkali in kgper kg of wood, while reducing the concentrations of alkali, and less cookingcirculations that often established an uneven alkali distribution over the cross section.This uneven alkali distribution was seen in the larger digesters developed at this time KST 14007SE_Eng_text3.docx 3/18 with capacities over 4000 ton pulp per day and with a digester diameter in excess of6 meter or more. During a short development time during the 90-ies was the capacityof digester increased from the typical 2000 t/day capacity to 5000 t/day or morewhich also called for changes in cooking process. Especially as diameters of thedigesters increased from about 4-5 meter, and very tall digesters, up to over 8 meter,making it difficult to implement effective cooking circulations.

Compact Cooking is more of an opposite to Lo-Solids cooking as the content ofdissolved hemicellulose is kept high during the cook in order to increase yield as thehemicellulose starts to precipitate back on the pulp at final phases of cook.Hemicelluose is a large part of the dissolved organic content, which in Lo-Solidscooking deliberately is kept low throughout the cook by multiple extractions of blackliquor, and often the total organic content is kept below 100 g/l throughout the cook,which of course results in losses in yield.

Up until the late 1990-ies was all commercial digester system equipped with chipsteaming systems ahead of the impregnation vessel, and those steaming systemoften had a first steaming bin, using low pressure steam, and a second pressurizedsteaming vessel, also using low pressure steam for heating and displacement of theair bound in comminuted cellulosic material. The steam used was most often steamflashed off from spent cooking liquors or steam boilers where clean steam wasproduced. The necessity of a thorough steaming was considered essential, butadded a lot of investment costs in the digester system. ln conventional two vesseldigester systems at this time also the first impregnation vessel was pressurized, andin fact the highest pressure in the system was established in the first impregnationvessel, with the high pressure sluice feeder feeding the steamed comminutedcellulosic material to the top of the impregnation vessel. This type of two vesselsystem was the conventional standard in cooking for almost half a century,necessitating both steaming equipment as well as high pressure sluice feedersahead of impregnation vessel.

A solution to the tradeoff between investment costs, yield, production capacity andpulp quality was then developed with next generation of Compact Cooking G2, (see2003, Compact CookingW' G2). ln this two vessel digester system was the entire presteaming equipment excluded by using a modified impregnation vessel sold byValmet under the name ImpBinTM. ln the lmpBin was both steaming andimpregnation implemented, and as the lmpBin was atmospheric this limited the KST 14007SE_Eng_text3.docx 4/18 maximum temperature in upper impregnation Iiquor level to about 100°C, providingoptimal conditions for alkali diffusion into the core of the comminuted cellulosicmaterial which has shown to reduce the reject amounts considerably. The rejectamount has been reduced to such an extent in first installations of Compact CookingG2 using lmpBin that reject storage bins dimensioned for “normal” reject handlingvolumes has been used at only fractions of its design capacity, if used at all. ln a recent further improvement of continuous cooking shown in US7112256 is theconcept with lmpBin developed with CrossCircTM. This combination of anatmospheric impregnation vessel with substantial retention time, and a dedicatedcooking vessel is the optimal design of the digester system. First, the atmosphericimpregnation vessel enable a thorough impregnation of comminuted cellulosicmaterial with alkali all the way to the core of the comminuted cellulosic material,reducing the amount of rejects after cook to a minimum. The comminuted cellulosicmaterial are also steamed in the very same vessel by adding hot black Iiquor to theIiquor level established in the lmpBin, allowing steam to flash off and into anestablished chip volume above Iiquor level. The patent US7112256 is related to theCrossCirc features that the bulk volume of return Iiquor from the top separator isinstead sent to this Iiquor level in lmpBin, and before that is instead hot black Iiquoradded to start of transfer (instead of the top separator return Iiquor that wasconventional practice). This improves heat economy as heating takes place towardscooking temperature in transfer to digester, and the residual heat in the top separatorreturn Iiquor is allowed to flash off for steaming, while yet establishing a lowimpregnation temperature in black Iiquor after flashing.

The concepts with Compact Cooking G2, using an lmpBin is disclosed also inChemical Pulping Part 1, Fibre Chemistry and Technology, Book 6, 2“d edition (ISBN978-952-5216-41-7), on pages 350-356.

A more detailed process flow chart of a Compact Cooking G2, using an lmpBin andCrossCirc is shown in figure 2. The digester is simplified with only 2 screen sections,and in the first upper screen section is hot black Iiquor HBL1 withdrawn at full cookingtemperature. This HBL1 is added to end of lmpBin and at start of transfer of thecomminuted cellulosic material to raise the temperature in feed line TSFEED to topseparator according to CrossCirc practice. lts is to be noted that the main charge of KST 14007SE_Eng_text3.docx 5/18 alkali to the digester is at the very top of the digester, so there will be a considerableresidual alkali level in the HBL1 liquor, as this residual alkali level must be sufficient tomaintain the residual alkali level at end of next cooking zone above 6 g/l (avoidinglignin condensation etc). As the retention time in the feed line TSFEED is short, nopractical consumption of alkali occurs, but the temperature of the comminutedcellulosic material will be increased towards full cooking temperature. The returnliquor TSRET withdrawn in top separator, i.e. the transport fluid in TSFEED, The TSalkali residual and temperature is a mixture of the temperature and alkali in the endof the impregnation plus the addition of HBL1 which main purpose is to increase thetemperature of the TS.

When this return liquor TSRET is released at the liquor level of the lmpBin steam isreleased and a steaming effect of the comminuted cellulosic material is obtained withthe residual heat value in the liquor. The pressure reduced liquor establish theimpregnation liquor lmpLiq for the lmpBin.

As indicated, in many mills is additional heating of the return liquor TSRET necessaryin order to generate sufficient amount of steam for comminuted cellulosic materialsteaming to such an extent that the comminuted cellulosic material loses itsbuoyancy due to bound air. But in some mills in warm climates could the heat valuein the return liquor TSRET be fully sufficient for the requested steaming effect,especially for hardwood cooking and using eucalyptus as wood material having ahigh density. Further, steaming comminuted cellulosic material being fed at ambienttemperature directly to lmpBin requires quite different amount of steam if thecomminuted cellulosic material are fed from an outside chip pile at ambienttemperatures of -30 to -40°C, compared to feeding comminuted cellulosic materialfrom an outside chip pile at ambient temperatures of +30 to +40°C. Hence, in somemills the further heating is only done in cold seasons while no further heating is donein warm seasons.

This is the essential principle in CrossCirc, i.e. sending the return liquor flow from topseparator to top of the impregnation vessel and instead adding hot black liquor tostart of transfer system. The heating effect of the hot black liquor is first used toelevate the temperature in transfer towards cooking temperature without any heatlosses, and the residual heat value in the liquor withdrawn in top separator is used toproduce the necessary amount of steaming of the comminuted cellulosic material in lmpBin. As indicated, spent cooking liquor is withdrawn from end of digester, RECg, KST 14007SE_Eng_text3.docx 6/18 and preferably also from a withdrawal at Iiquor level in lmpBin, i.e. RECi, and sentfurther to evaporation plant ahead of final recovery in a recovery boiler.

An essential feature of the atmospheric lmpBin system is shown in more detail infigure 3. The temperature profile established in the chip volume above Iiquor level, and above release point of the return Iiquor TSRET, develop a hot zone (>100°C) where steaming of comminuted cellulosic material is obtained. After a transition zoneis a cold buffer zone established at ambient temperature, i.e. the temperature whichthe comminuted cellulosic material has on entry to the lmpBin. Thus steam andcondensable gases accumulates in condensate layers inside the chip volume and isprevented from escaping by the colder comminuted cellulosic material laying abovethe transition zone. Only air and some non-condensable gases passes through thecold chip volume and is vented off by a DNCG (Diluted Non Condensable Gas)system (as shown in figure 2).

As colder comminuted cellulosic material are continuously fed into the lmpBin is thistransition zone balanced with the amount of steam released in the chip volume, andthe control of steam release is controlled by a temperature probe penetrating the chipvolume (not shown). lf the transition zone moves downwardly more return IiquorTSRET is added or additional heating thereof, or alternatively using low pressuresteam as a complementary steaming effect.

Figure 4 shows in further detail the Iiquor routing in Compact Cooking G2. Thedigester is operated with only 2 concurrent cooking zones, charging the white IiquorWL to top of digester where it mixes well with the inflow of impregnated comminutedcellulosic material. A L/\N ratio of about 5 is established in first cooking zone andafter this zone is hot partially spent cooking Iiquor, i.e. black Iiquor, withdrawn in aL/W ratio of about 3. This hot black Iiquor is charged to start of transfer as shown inorder to raise the temperature in the transport circulation where a L/W ratio of about10-12 is established. ln the top separator is excess transport Iiquor withdrawn at aL/W ratio of about 6-8, and this transport Iiquor is sent to top of lmpBin to be releasedinto the chip pile volume established above the Iiquor level, where steam is released.Thus, the impregnation Iiquor for the lmpBin is established by the hot black Iiquorwithdrawal which after mixing into the transport circulation for heating, is withdrawn intop separator and sent to top of lmpBin. The black Iiquor is thus given an increased retention time in the cooking system as it is recirculated back to lmpBin and KST 14007SE_Eng_text3.docx 7/18 maintained in first cooking zone. The increased retention time is beneficial forincreasing yield, as the hemicellulose dissolved early in cooking need time and highalkali concentration for cleaving off end groups to such an extent that it precipitatesback on the cellulose. The effect of hemicellulose precipitation is dependent on theH-factor exposure of the dissolved hemicellulose. Hence, as is realized in the Iiquorrouting scheme is the carbohydrates dissolved in cooking Iiquor early in cook, i.e. inthe ImpBin and upper cooking zone, recirculated back to start of ImpBin and giventhe necessary retention time for obtaining the intended increase in yield due toprecipitation. The temperature established in the Iiquor level is about 100-102°C andthe only heating subjected to the comminuted cellulosic material during impregnation is the heat release due to exothermic reactions, reaching about 110°C in bottom. This low temperature results in a minimal H-factor and neglectable alkali consumption dueto delignification reactions. However, some amount of alkali is consumed due toneutralization of the wood acidity released during steaming, but much of the woodacidity is withdraw at the Iiquor level in ImpBin. The high L/\N ratio of about 5 stillenable a sufficient amount of alkali in kg per kg of wood, and less drop in alkali concentration due to alkali consumption.

BRIEF DESCRIPTION OF THE INVENTION Now, the invention is all about a further improvement of the Compact Cooking G2concept, where increased yield is one of the major objectives for the cookingconcept. lt has surprisingly been realized that the ImpBin concept used in CompactCooking G2 is beneficial for recovery of heat value in cooking liquors and thusreducing heating costs, i.e. need for fresh steam. Even though total yield is high in aCompact Cooking G2 process, it has shown that the order of delignification in lastphase of cooking is low, typically is the kappa number reduction in last half ofcooking zone in the order of 15-20, while the kappa number reduction in first half is inthe order of 60-100. _ For cooking system operated according to design the carryover of COD in the digester blowline is in the range of 500-700 kg/adt. However foroverloaded digesters or digesters that has to operate at low dilution factors fordifferent reasons (small sized comminuted cellulosic material, low kappa no. ) thecarry over of COD in the blowline can increase levels above 900 kg/adt (or 1000kg/bdt).

KST 14007SE_Eng_text3.docx 8/18 Thus the invention is an improvement of a method for cooking comminuted cellulosicmaterial in which method is used a two vessel continuous digester systemcomprising a first atmospheric impregnation vessel and a second pressurizeddigester vessel, wherein the cellulosic material has a retention time of at least 40minutes in the first impregnation vessel and at least 90 minutes in the seconddigester vessel, and where a liquor level of at least 15 meter is established in the firstimpregnation vessel, said method comprising following steps in sequence; -feeding comminuted cellulosic material kept at ambient temperature to the firstimpregnation vessel establishing a cellulosic material level at least 4 meter above aliquor level established in the impregnation vessel; -addition of hot impregnation liquor at a temperature above the boiling point at aposition inside the volume of cellulosic material above the liquor level, such thatsteam is released into the volume of cellulosic material for steaming, and where thepressure released hot impregnation liquor assumes a temperature corresponding tothe boiling point at the liquor level established in the impregnation vessel; - the impregnation vessel having a withdrawal screen at the liquor level and whereinthe withdrawal screen withdraws at least 1-2 ton liquor/bdt wood of the impregnationliquor, said liquor containing at least a part of the wood acidity released duringsteaming in the chip volume above liquor level; -the cellulosic material is impregnated with the pressure released impregnation liquorduring the retention time in the impregnation vessel, -during feed out of impregnated comminuted material from the impregnation vessel isadded black liquor at full cooking temperature such that the temperature of thetransport slurry of comminuted material to the digester is raised at least 10°C; -in the top of the digester is at least 4 ton liquor/bdt wood of the transport slurrywithdrawn and used as the hot impregnation liquor added to the impregnation vessel;-and that a full digester temperature in the interval 130-170°C is established in acooking zone in the digester using at least 80% of the total retention time of saiddigester as a cooking zone; -said digester having a digester withdrawal screen in the middle of the cooking zoneat a position in the digester corresponding to 35-60% of the total retention time in thedigester, and wherein the cooking withdrawal screen withdraws at least 2.5 tonliquor/bdt wood establishing a first part of the cooking zone with high L/W ratio and a KST 14007SE_Eng_text3.docx 9/18 second part of the Cooking zone with low L/W ratio, which withdrawn Iiquor ischarged during feed out of impregnated comminuted material from the impregnationvessel as black Iiquor at full cooking temperature. What is defined in preceding part isthe preconditions for a two vessel Compact Cooking system with an lmpBin andusing CrossCirc in transfer circulation.

Now the invention is characterized in that in this context is an additional volume of atleast 0.5 ton Iiquor/bdt wood of wash filtrate from a subsequent brown wash positionafter the digester added to the center of the digester vessel at the digester withdrawalscreen after heating the wash filtrate to full digester temperature, said wash filtratehaving a lignin content less than 50% of the lignin content in the black Iiquorwithdrawn in the withdrawal screen, and finally withdrawing a corresponding amount of Iiquor as additional Iiquor alsofrom the withdrawal screen in the impregnation vessel thus using the heat value ofthe heated wash filtrate first in transfer circulation and subsequently as hot impregnation Iiquor releasing more steam for chip steaming.

By adding this cleaner wash filtrate in heated form ahead of final cooking zone, couldthe heat value be fully recovered first in transfer circulation as well as in steamgeneration in the atmospheric impregnation vessel and the produced pulp could beobtained with less COD content, and the yield could be kept at high level as theimpregnation as well as first cooking zone is operated at high hemicellulose content in the cooking Iiquor, due to the re-circulation of the extracted hot black Iiquor. ln a further embodiment of the inventive method is fresh alkaline Iiquor charged tothe two vessel digester system in a total charge of 160-190 (HW) and 190-230 (SW)kg alkali/bdt wood (EA as NaOH) in at least 3 process positions, wherein a firstcharge is added to the top of the digester in an amount exceeding 60% of the totalcharge of fresh alkaline Iiquor, and a second charge to the hot impregnation Iiquorand a third charge to the wash filtrate added to the center of the digester vessel atthe digester withdrawal screen. lncreased delignification could thus be obtained in the final cooking zone in theCompact Cooking G2 process. ln a preferred embodiment is the amount of fresh alkaline Iiquor charged to the washfiltrate added to the center of the digester vessel at the digester withdrawal screen KST 14007SE_Eng_text3.docx 10/18 corresponding to at least 10% of the total charge of fresh alkaline liquor, wherein thecharge is sufficient to establish an initial alkali level in the cooking zone over 10 g/l(EA as NaOH), and preferably above 15 g/l. ln an optional embodiment of the inventive method is the amount of fresh alkalineliquor charged to the wash filtrate charged to a cooking circulation recirculating a partof the liquor withdrawn from the digester Withdrawal screen to the center of thedigester vessel. This enforced circulation could improve radial distribution of themodified cooking liquor ahead of the final cooking zone. ln a preferred embodiment of the inventive is also the amount of fresh alkaline liquorcharged to the hot impregnation liquor corresponding to at least 10% of the totalcharge of fresh alkaline liquor, wherein the charge is sufficient to establish a finalresidual alkali level in the impregnation vessel over 8 g/l (as NaOH), and preferablyabove 15 g/l. Such a charge control could guarantee that sufficient amount of alkali ispresent during the entire impregnation, safeguarding that the alkali diffusion into thecomminuted cellulosic material continues during the entire residence time in the impregnation vessel. ln yet a preferred embodiment of the inventive method is the wash filtrate from asubsequent brown wash position after the digester obtained from a pressurizedwash, maintaining the pressure of the wash filtrate before addition to digester. lf thewash filtrate is kept pressurized the temperature may be equal to or exceed thecorresponding amount of spent impregnation liquid that is withdrawn from theimpregnation vessel, improving heat economy of the two vessel system. ln apreferred alternative of this method is the wash filtrate from a subsequent brownwash position after the digester obtained from a pressurized wash maintaining thetemperature of the wash filtrate before addition to digester above 100°C, andpreferably in the range 100-110°C. The wash filtrate could preferably be obtainedfrom a subsequent brown wash position immediately after the digester, preferably ina pressure diffuser.

The wash filtrate from a subsequent brown wash position after the digester mayalternatively be obtained from a wash after an oxygen delignification stage,maintaining the temperature of the wash filtrate before heating and addition to KST 14007SE_Eng_text3.docx 11/18 digester in the range 85-95°C or above. Such a high temperature wash filtrate maystill be beneficial for reducing the heat losses, as the temperature loss is only 5-15°Chigher temperature in liquor withdrawn from the impregnation vessel than the washfiltrate added to digester.

DETAILED DESCRIPTION OF THE INVENTION The invention will be described in more detail with reference to figure 5, showing themodifications of the Compact Cooking G2 system shown in figure 4. ln following example is liquor-to-wood / L/\N ratio used, meaning the total amount ofliquid in ton per ton of bone dry wood (bdt). The example use a minimum charge ofan additional volume of wash liquor of at least 0.5 in L/\N, in order to indicate theminimum order of liquor charged. But the invention may be applied with increasingvolumes up to a L/\N ratio up to 2, or even more of the added liquor volume chargedas wash liquor and then passed on through the system via first transport circulationand finally to impregnation vessel.

As a first step is a cleaner wash filtrate added to center of digester at the withdrawalscreen in a L/\N ratio of at least 0.5. This cleaner wash liquor WL is heated in a heatexchanger HE to full cooking temperature before addition. The wash liquor ispreferably a wash liquor from a subsequent brown wash position, either from a washdirectly after the digester or a wash after pre-bleaching or oxygen delignification,where the wash liquor still is alkaline. The temperature of the wash liquor from abrown wash position is typically in the order of 90-1 10°C, and in the lower range if thewashing is done in wash machines collecting the wash filtrate in atmospheric filtratetanks, and in the upper range if the washing is done in wash machines keeping thewash filtrate pressurized. The COD content in such brown wash positions is typicallyin the order of 30-80 g/l filtrate, which could be compared with the COD content ofthe mid screen withdrawal in digester which typically may be about 200 g/l, and thushigher COD content at end of cooking zone and before the bottom wash.

A corresponding additional amount of black liquor, i.e. in a L/\N ratio of at least 0.5, iswithdrawn from the withdrawal screen and sent to start of transfer. This additionalamount of hot black liquor will elevate the temperature in the transport circulation, asthe part of the hot black liquor in the transport liquor increases. After having used theheat value in liquors in the transport circulation for heating the comminuted cellulosic material, is the excess transport liquor withdrawn from top separator, but now at an KST 14007SE_Eng_text3.docx 12/18 increased volume in a L/\N ratio of at least 0.5 of the increase. This extra volume ofheated transport Iiquor is thereafter added to top of ImpBin where as a result of theextra volume of heated transport Iiquor also is flashed off a larger amount of steam, before the finally flashed transport Iiquor assumes a temperature of about 100°C in the Iiquor level of the ImpBin. Finally, in the ImpBin withdrawal screen is also theWithdrawal volume increased at an increased volume in a L/\N ratio of at least 0.5.This modification leaves the cooking process otherwise unaffected as to liqour-to-wood ratios during impregnation or cooking, maintaining the advantages of the Compact Cooking G2 process.

As indicated could also an additional amount of alkali be added to the wash Iiquorbefore addition into the middle of the cooking zone. This could increase the alkaliconcentration in the last cooking zone improving delignification further in this lastzone. For improving the distribution of alkali, as well as cleaner wash Iiquor, couldoptionally also a cooking circulation CC be installed, that promotes an enhancedradial distribution of the new cooking Iiquor. Such a cooking circulation normallyincludes also a pump (not shown). The amount of fresh alkaline Iiquor charged to thewash filtrate is charged to a cooking circulation recirculating a part of the Iiquorwithdrawn from the digester withdrawal screen to the center of the digester vessel atthe height position of the digester withdrawal screen (as shown in figure 5), oralternatively is charged to a cooking circulation recirculating a part of the Iiquorwithdrawn from a wash withdrawal screen (the lowermost screen shown in figure 5)to the center of the digester vessel at the height position of the digester withdrawal SCFeen.

Heat Economy As a result, the extra amount of steam flashed off in the ImpBin will reduce the needfor adding extra steam, i.e. LP-steam, to top of ImpBin. This reduction of steamaddition match the need for heating the wash Iiquor in heater HE, so in aspects ofheat economy the change is not increasing steam consumption. ln the boundary limits of the two vessel system is an additional volume of wash Iiquoradded to the digester and the corresponding amount of recovery withdrawal increaseis done from ImpBin screen. lf these volumes are identical and the temperature in the KST 14007SE_Eng_text3.docx 13/18 ImpBin withdrawal is about 100°C, then no increased cost of steam is caused in thetwo vesse| digester system if the wash Iiquor has the same temperature, i.e. 100°C, before heating. For each degree of temperature increase in wash Iiquor is the steameconomy improved as less fresh steam (LP steam) needs to be added to ImpBin topas a result or if the steam to the ImpBin is enough the hot black Iiquor will be used toincrease the temperature of the transfer circulation and the MP-steam demand on the digester top to reach cooking temperature will reduce.

Improved delignification As indicated in figure is typically the COD content in mid-point withdrawal at some200 g/l. lf no dilution of the cooking Iiquor and addition of alkali is made at thisposition the COD content will increase further during the final cooking zone.

While still some hemicellulose may precipitate in such an undiluted cooking zone,most of the hemicellulose precipitation has already been made due to therecirculation phases during impregnation and first cooking zone. On the other hand,the extra volume of hot black Iiquor withdrawn, with its content of hemicellulose thatmay further increase yield in final cooking zone, is not sent directly to recovery,instead it is sent to the first phases of the cook where it may precipitate.

Hence the first phase of the cook is performed with high level of dissolved organiccontent, primarily hemicellulose and lignin in almost equal proportions, but then thefinal phase is modified slightly, decreasing the concentration of lignin and optionallyincreasing the alkali concentration for the final phase of the cook.

This modification of the cooking Iiquor improves delignification, such the typical loworder of delignification that could be obtained in final delignification, reaching a kappareduction in the order of 15-20 units, may be increased by at least 5-10 units.

Less COD in blown pulp As an additional effect of the modification of the cooking Iiquor at final phase of cookis also the residual COD content reduced considerably. ln the system set up asshown in figure 4 is the COD level in pulp typically about 900 kg per bdt pulp (notwood), while after a modification of the cooking Iiquor the COD level may be reducedto below 650 kg. This improves bleachability, as less bleaching chemicals areneeded to reach the desired brightness.

KST 14007SE_Eng_text3.docx 14/18 Summary By the inventive modification of the Compact Cooking G2 system is the increasedyield objective still obtained to a large extent, but further delignification may beobtained at less COD content in the cooked pulp. These results are obtained withoutor reducing heating costs as all heating of cleaner wash liquors is recovered in thelmpBin resulting in less need for fresh steam for steaming the comminuted cellulosic material.

Optional brown wash ln figure 6 is shown a preferred brown wash in form of a pressure diffuser washingthe pulp coming directly from the blow line of the digester. This system installationfollow the DiConnTM concept sold by Valmet, where the filtrate obtained from thepressured diffuser is kept pressurized making it possible to maintain the temperature of the wash filtrate above 100°C. ln conventional pressure diffuser installations is the wash filtrate from the pressure diffuser collected in an atmospheric filtrate tank that will flash off any heat value above boiling point.

KST 14007SE_Eng_text3.docx

Claims (9)

15/18 PATENT CLÅIMS

1. Method for cooking comminuted cellulosic material in which method is used a twovessel continuous digester system comprising a first atmospheric impregnationvessel and a second pressurized digester vessel, wherein the cellulosic material hasa retention time of at least 40 minutes in the first impregnation vessel and at least 90minutes in the second digester vessel, and where a liquor level of at least 15 meter isestablished in the first impregnation vessel, said method comprising following stepsin sequence; -feeding comminuted cellulosic material kept at ambient temperature to the firstimpregnation vessel establishing a cellulosic material level at least 4 meter above aliquor level established in the impregnation vessel; -addition of hot impregnation liquor at a temperature above the boiling point at aposition inside the volume of cellulosic material above the liquor level, such thatsteam is released into the volume of cellulosic material for steaming, and where thepressure released hot impregnation liquor assumes a temperature corresponding tothe boiling point at the liquor level established in the impregnation vessel; -the impregnation vessel having a withdrawal screen at the liquor level and whereinthe withdrawal screen withdraws at least 1-2 ton liquor/bdt wood of the impregnationliquor, said liquor containing at least a part of the wood acidity released duringsteaming in the chip volume above liquor level; -the cellulosic material is impregnated with the pressure released impregnation liquorduring the retention time in the impregnation vessel, -during feed out of impregnated comminuted material from the impregnation vessel isadded black liquor at full cooking temperature such that the temperature of thetransport slurry of comminuted material to the digester is raised at least 10°C; -in the top of the digester is at least 4 ton liquor/bdt wood of the transport slurrywithdrawn and used as the hot impregnation liquor added to the impregnation vessel;-and that a full digester temperature in the interval 130-170°C is established in acooking zone in the digester using at least 80% of the total retention time of saiddigester as a cooking zone; -said digester having a digester withdrawal screen in the middle of the cooking zoneat a position in the digester corresponding to 35-60% of the total retention time in thedigester, and wherein the cooking withdrawal screen withdraws at least 2.5 ton KST 14007SE_Eng_text2.docx 16/18 liquor/bdt wood establishing a first part of the cooking zone with high L/\N ratio and asecond part of the cooking zone with low L/W ratio, which withdrawn liquor ischarged during feed out of impregnated comminuted material from the impregnationvessel as black liquor at full cooking temperature characterized in that an additionalvolume of at least 0.5 ton Iiquor/bdt wood of wash filtrate from a subsequent brownwash position after the digester is added to the center of the digester vessel at thedigester withdrawal screen after heating the wash filtrate to full digester temperature,said wash filtrate having a lignin content less than 50% of the lignin content in theblack liquor withdrawn in the withdrawal screen, and finally withdrawing a corresponding amount of liquor as additional liquor alsofrom the withdrawal screen in the impregnation vessel thus using the heat value ofthe heated wash filtrate first in transfer circulation and subsequently as hot impregnation liquor releasing more steam for chip steaming.

2. Method for cooking comminuted cellulosic material according to claim 1characterized in that fresh alkaline liquor is charged to the two vessel digestersystem in a total charge of 160-190 (HW) 190-230 (SW) kg alkali/bdt wood (EA asNaOH) in at least 3 process positions, wherein a first charge is added to the top ofthe digester in an amount exceeding 60% of the total charge of fresh alkaline liquor,and a second charge to the hot impregnation liquor and a third charge to the wash filtrate added to the center of the digester vessel at the digester withdrawal screen.

3. Method for cooking comminuted cellulosic material according to claim 2characterized in that the amount of fresh alkaline liquor charged to the wash filtrateadded to the center of the digester vessel at the digester withdrawal screencorresponds to at least 10% of the total charge of fresh alkaline liquor, wherein thecharge is sufficient to establish a alkali level in the cooking zone over 10 g/l (asNaOH), and preferably above 15 g/l.

4. Method for cooking comminuted cellulosic material according to claim 3characterized in that the amount of fresh alkaline liquor charged to the wash filtrateis charged to a cooking circulation recirculating a part of the liquor withdrawn fromthe digester withdrawal screen to the center of the digester vessel at the height position of the digester withdrawal screen, or alternatively is charged to a cooking KST 14007SE_Eng_text2.docx 17/18 circulation recirculating a part of the liquor withdrawn from a wash withdrawal screento the center of the digester vessel at the height position of the digester withdrawal SCfeen .

5. Method for cooking comminuted cellulosic material according to claim 3characterized in that the amount of fresh alkaline liquor charged to the hotimpregnation liquor corresponds to at least 10% of the total charge of fresh alkalineliquor, wherein the charge is sufficient to establish a final residual alkali level in the impregnation vessel over 8 g/l (as NaOH), and preferably above 15 g/l.

6. Method for cooking comminuted cellulosic material according to claim 1characterized in that the wash filtrate from a subsequent brown wash position afterthe digester is obtained from a pressurized wash, maintaining the pressure of thewash filtrate before addition to digester.

7. Method for cooking comminuted cellulosic material according to claim 6characterized in that the wash filtrate from a subsequent brown wash position afterthe digester is obtained from a pressurized wash, maintaining the temperature of thewash filtrate before addition to digester above 100°C, and preferably in the range100-110°C .

8. Method for cooking comminuted cellulosic material according to claim 7characterized in that the wash filtrate is obtained from a subsequent brown washposition immediately after the digester, preferably in a pressure diffuser.

9. Method for cooking comminuted cellulosic material according to claim 1characterized in that the wash filtrate from a subsequent brown wash position afterthe digester is obtained from a wash after an oxygen delignification stage,maintaining the temperature of the wash filtrate before heating and addition to digester in the range 85-95°C or above . KST 14007SE_Eng_text2.docx