SE1451340A1 - Method for operating a two vessel digester system - Google Patents

Description

15 20 25 30 2/18 Next followed a development (see 1983, CC + MCC + Hi Heat) where a part of the wash zone was converted to a countercurrent cooking zone with an addition of alkali at end of cooking zone in order to even out the alkali profile and use more of the digester as a cooking zone. This development proceeded with further extension (see 1991, CC + MCC + ITCW' (Hi Heat) of the cooking zone, adding an ITC cooking zone at full cooking temperature, also countercurrent, below the MCC cooking zone. The total charge of alkali was thus distributed to several positions during the cook, with the main charge in top of digester, but with additional charges in bottom of the MCC zone as well as in the bottom of the ITC zone. This distribution of alkali at multiple points reduced the very high alkali concentration at cooking start. High alkali concentration was found to be detrimental to pulp strength, and was established in old conventional cooking where most, if not all, of the alkali was charged to top of digester.

The technique of impregnation with black liquor was developed during the latter part of the 1980s and the 1990s (see 1993, BLIW' + MCC + ITCTN' (Hi Heat) ), with the aim of obtaining improved cooking economy and heat economy and of obtaining better pulp. ln figure “1993” is the black liquor impregnation, i.e. BLI, established in a separate impregnation vessel in a two vessel digester system, but several single vessel systems had also BLI in upper part of the digester. ln these early two vessel digester systems the highest pressure established was obtained in the impregnation vessel and often at such high temperature that the comminuted cellulosic material was not thoroughly impregnated with alkali before the cook. Often was a temperature in excess of 120°C established in the impregnation vessel, which was considered beneficial for heat economy in the process but has later on been found to be detrimental for alkali diffusion into the core of the comminuted cellulosic material. At about this time was also Lo-SolidsTM cooking implemented, where black liquor was withdrawn and replaced with cleaner cooking liquor with less lignin content. ln later development (see 1997, Compact CookingTM )was the two vessel digester system further improved with the main objectives to increase yield and uniform cooking results. Most dominant features in Compact Cooking was impregnation as well as cooking at high L/\N ratios, enabling presence of high amount of alkali in kg per kg of wood, while reducing the concentrations of alkali, and less cooking circulations 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 140073 E_Eng_text.docx 10 15 20 25 30 3/18 with capacities over 4000 ton pulp per day and with a digester diameter in excess of 6 meter or more. During a short development time during the 90-ies was the capacity of digester increased from the typical 2000 t/day capacity to 5000 t/day or more which also called for changes in cooking process. Especially as diameters of the digesters 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 of dissolved hemicellulose is kept high during the cook in order to increase yield as the hemicellulose 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-Solids cooking deliberately is kept low throughout the cook by multiple extractions of black liquor, 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 chip steaming systems ahead of the impregnation vessel, and those steaming system often had a first steaming bin, using low pressure steam, and a second pressurized steaming vessel, also using low pressure steam for heating and displacement of the air bound in comminuted cellulosic material. The steam used was most often steam flashed off from spent cooking liquors or steam boilers where clean steam was produced. The necessity of a thorough steaming was considered essential, but added a lot of investment costs in the digester system. ln conventional two vessel digester systems at this time also the first impregnation vessel was pressurized, and in fact the highest pressure in the system was established in the first impregnation vessel, with the high pressure sluice feeder feeding the steamed comminuted cellulosic material to the top of the impregnation vessel. This type of two vessel system was the conventional standard in cooking for almost half a century, necessitating both steaming equipment as well as high pressure sluice feeders ahead of impregnation vessel.

A solution to the tradeoff between investment costs, yield, production capacity and pulp quality was then developed with next generation of Compact Cooking G2, (see 2003, Compact Cooking TN' G2). ln this two vessel digester system was the entire pre steaming equipment excluded by using a modified impregnation vessel sold by Valmet under the name ImpBinTM. ln the lmpBin was both steaming and impregnation implemented, and as the lmpBin was atmospheric this limited the KST 140073 E_Eng_text.docx 10 15 20 25 30 4/18 maximum temperature in upper impregnation Iiquor level to about 100°C, providing optimal conditions for alkali diffusion into the core of the comminuted cellulosic material which has shown to reduce the reject amounts considerably. The reject amount has been reduced to such an extent in first installations of Compact Cooking G2 using ImpBin that reject storage bins dimensioned for “norma|” reject handling volumes 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 the concept with ImpBin developed with CrossCircTM. This combination of an atmospheric impregnation vessel with substantial retention time, and a dedicated cooking vessel is the optimal design of the digester system. First, the atmospheric impregnation vessel enable a thorough impregnation of comminuted cellulosic material 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 cellulosic material are also steamed in the very same vessel by adding hot black Iiquor to the Iiquor level established in the ImpBin, allowing steam to flash off and into an established chip volume above Iiquor level. The patent US7112256 is related to the CrossCirc features that the bulk volume of return Iiquor from the top separator is instead sent to this Iiquor level in ImpBin, and before that is instead hot black Iiquor added to start of transfer (instead of the top separator return Iiquor that was conventional practice). This improves heat economy as heating takes place towards cooking temperature in transfer to digester, and the residual heat in the top separator return Iiquor is allowed to ?ash off for steaming, while yet establishing a low impregnation temperature in black Iiquor after flashing.

The concepts with Compact Cooking G2, using an ImpBin is disclosed also in Chemical Pu/ping Part 1, Fibre Chemistry and Technology, Book 6, 2“d edition (ISBN 978-952-5216-41-7), on pages 350-356.

A more detailed process flow chart of a Compact Cooking G2, using an ImpBin and CrossCirc 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 cooking temperature. This HBL1 is added to end of ImpBin and at start of transfer of the comminuted cellulosic material to raise the temperature in feed line TSFEED to top separator according to CrossCirc practice. Its is to be noted that the main charge of KST 140073 E_Eng_text.docx 10 15 20 25 30 5/18 alkali to the digester is at the very top of the digester, so there will be a considerable residual alkali level in the HBL1 liquor, as this residual alkali level must be sufficient to maintain the residual alkali level at end of next cooking zone above 6 g/l (avoiding lignin condensation etc). As the retention time in the feed line TSFEED is short, no practical consumption of alkali occurs, but the temperature of the comminuted cellulosic material will be increased towards full cooking temperature. The return liquor TSRET withdrawn in top separator, i.e. the transport fluid in TSFEED, The TS alkali residual and temperature is a mixture of the temperature and alkali in the end of the impregnation plus the addition of HBL1 which main purpose is to increase the temperature of the TS.

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

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

This is the essential principle in CrossCirc, i.e. sending the return liquorflow from top separator to top of the impregnation vessel and instead adding hot black liquor to start of transfer system. The heating effect of the hot black liquor is first used to elevate the temperature in transfer towards cooking temperature without any heat losses, and the residual heat value in the liquor withdrawn in top separator is used to produce the necessary amount of steaming of the comminuted cellulosic material in lmpBin. As indicated, spent cooking liquor is withdrawn from end of digester, RECz, KST 140073 E_Eng_text.docx 10 15 20 25 30 6/18 and preferably also from a withdrawal at Iiquor level in lmpBin, i.e. RECi, and sent further to evaporation plant ahead of final recovery in a recovery boiler.

An essential feature of the atmospheric lmpBin system is shown in more detail in figure 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 zone is a cold buffer zone established at ambient temperature, i.e. the temperature which the comminuted cellulosic material has on entry to the lmpBin. Thus steam and condensable gases accumulates in condensate layers inside the chip volume and is prevented from escaping by the colder comminuted cellulosic material laying above the transition zone. Only air and some non-condensable gases passes through the cold 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 this transition zone balanced with the amount of steam released in the chip volume, and the control of steam release is controlled by a temperature probe penetrating the chip volume (not shown). lf the transition zone moves downwardly more return Iiquor TSRET is added or additional heating thereof, or alternatively using low pressure steam as a complementary steaming effect.

Figure 4 shows in further detail the Iiquor routing in Compact Cooking G2. The digester is operated with only 2 concurrent cooking zones, charging the white Iiquor WL to top of digester where it mixes well with the inflow of impregnated comminuted cellulosic material. A L/\N ratio of about 5 is established in first cooking zone and after this zone is hot partially spent cooking Iiquor, i.e. black Iiquor, withdrawn in a L/W ratio of about 3. This hot black Iiquor is charged to start of transfer as shown in order to raise the temperature in the transport circulation where a L/W ratio of about 10-12 is established. ln the top separator is excess transport Iiquor withdrawn at a L/W ratio of about 6-8, and this transport Iiquor is sent to top of lmpBin to be released into 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 Iiquor withdrawal which after mixing into the transport circulation for heating, is withdrawn in top 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 140073 E_Eng_text.docx 10 15 20 25 30 7/18 maintained in first cooking zone. The increased retention time is beneficial for increasing yield, as the hemicellulose dissolved early in cooking need time and high alkali concentration for cleaving off end groups to such an extent that it precipitates back on the cellulose. The effect of hemicellulose precipitation is dependent on the H-factor exposure of the dissolved hemicellulose. Hence, as is realized in the Iiquor routing scheme is the carbohydrates dissolved in cooking Iiquor early in cook, i.e. in the |mpBin and upper cooking zone, recirculated back to start of |mpBin and given the necessary retention time for obtaining the intended increase in yield due to precipitation. The temperature established in the Iiquor level is about 100-102°C and the 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 due to delignification reactions. However, some amount of alkali is consumed due to neutralization of the wood acidity released during steaming, but much of the wood acidity is withdraw at the Iiquor level in |mpBin. The high LNV ratio of about 5 still enable 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 G2 concept, where increased yield is one of the major objectives for the cooking concept. lt has surprisingly been realized that the |mpBin concept used in Compact Cooking G2 is beneficial for recovery of heat value in cooking liquors and thus reducing heating costs, i.e. need for fresh steam. Even though total yield is high in a Compact Cooking G2 process, it has shown that the order of delignification in last phase of cooking is low, typically is the kappa number reduction in last half of cooking zone in the order of 15-20, while the kappa number reduction in first half is in the order of 60-100. _ For cooking system operated according to design the carry over of COD in the digester blowline is in the range of 500-700 kg/adt. However for overloaded digesters or digesters that has to operate at low dilution factors for different reasons (small sized comminuted cellulosic material, low kappa no. ) the carry over of COD in the blowline can increase levels above 900 kg/adt (or 1000 kg/bdt).

KST 140073 E_Eng_text.docx 10 15 20 25 30 8/18 Thus the invention is an improvement of a method for cooking comminuted cellulosic material in which method is used a two vessel continuous digester system comprising a first atmospheric impregnation vessel and a second pressurized digester vessel, wherein the cellulosic material has a retention time of at least 40 minutes in the first impregnation vessel and at least 90 minutes in the second digester vessel, and where a liquor level of at least 15 meter is established in the first impregnation vessel, said method comprising following steps in sequence; -feeding comminuted cellulosic material kept at ambient temperature to the first impregnation vessel establishing a cellulosic material level at least 4 meter above a liquor level established in the impregnation vessel; -addition of hot impregnation liquor at a temperature above the boiling point at a position inside the volume of cellulosic material above the liquor level, such that steam is released into the volume of cellulosic material for steaming, and where the pressure released hot impregnation liquor assumes a temperature corresponding to the boiling point at the liquor level established in the impregnation vessel; - the impregnation vessel having a withdrawal screen at the liquor level and wherein the Withdrawal screen withdraws at least 1-2 ton liquor/bdt wood of the impregnation liquor, said liquor containing at least a part of the wood acidity released during steaming in the chip volume above liquor level; -the cellulosic material is impregnated with the pressure released impregnation liquor during the retention time in the impregnation vessel, -during feed out of impregnated comminuted material from the impregnation vessel is added black liquor at full cooking temperature such that the temperature of the transport 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 slurry withdrawn 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 a cooking zone in the digester using at least 80% of the total retention time of said digester as a cooking zone; -said digester having a digester withdrawal screen in the middle of the cooking zone at a position in the digester corresponding to 35-60% of the total retention time in the digester, and wherein the cooking withdrawal screen withdraws at least 2.5 ton liquor/bdt wood establishing a first part of the cooking zone with high L/W ratio and a KST 140073 E_Eng_text.docx 10 15 20 25 30 9/18 second part of the cooking zone with low L/W ratio, which withdrawn Iiquor is charged during feed out of impregnated comminuted material from the impregnation vessel as black Iiquor at full cooking temperature. What is defined in preceding part is the preconditions for a two vessel Compact Cooking system with an lmpBin and using CrossCirc in transfer circulation.

Now the invention is characterized in that in this context is an additional volume of at least 0.5 ton Iiquor/bdt wood of wash filtrate from a subsequent brown wash position after the digester added to the center of the digester vessel at the digester 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 the black Iiquor withdrawn in the withdrawal screen, and finally withdrawing a corresponding amount of Iiquor as additional Iiquor also from the withdrawal screen in the impregnation vessel thus using the heat value of the 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, could the heat value be fully recovered first in transfer circulation as well as in steam generation in the atmospheric impregnation vessel and the produced pulp could be obtained with less COD content, and the yield could be kept at high level as the impregnation 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 to the 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 first charge is added to the top of the digester in an amount exceeding 60% of the total charge of fresh alkaline Iiquor, and a second charge to the hot impregnation Iiquor and a third charge to the wash filtrate added to the center of the digester vessel at the digester withdrawal screen. lncreased delignification could thus be obtained in the final cooking zone in the Compact Cooking G2 process. ln a preferred embodiment is the amount of fresh alkaline Iiquor charged to the wash filtrate added to the center of the digester vessel at the digester withdrawal screen KST 140073 E_Eng_text.docx 10 15 20 25 30 10/18 corresponding to at least 10% of the total charge of fresh alkaline Iiquor, wherein the charge 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 alkaline Iiquor charged to the wash filtrate charged to a cooking circuiation recirculating a part of the Iiquor withdrawn from the digester Withdrawal screen to the center of the digester vessel. This enforced circuiation could improve radial distribution of the modified cooking Iiquor ahead of the final cooking zone. ln a preferred embodiment of the inventive is also the amount of fresh alkaline Iiquor charged to the hot impregnation Iiquor corresponding to at least 10% of the total charge of fresh alkaline Iiquor, 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. Such a charge control could guarantee that sufficient amount of alkali is present during the entire impregnation, safeguarding that the alkali diffusion into the comminuted 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 a subsequent brown wash position after the digester obtained from a pressurized wash, maintaining the pressure of the wash filtrate before addition to digester. lf the wash filtrate is kept pressurized the temperature may be equal to or exceed the corresponding amount of spent impregnation liquid that is withdrawn from the impregnation vessel, improving heat economy of the two vessel system. ln a preferred alternative of this method is the wash filtrate from a subsequent brown wash position after the digester obtained from a pressurized wash maintaining the temperature of the wash filtrate before addition to digester above 100°C, and preferably in the range 100-110°C. The wash filtrate could preferably be obtained from a subsequent brown wash position immediately after the digester, preferably in a pressure diffuser.

The wash filtrate from a subsequent brown wash position after the digester may alternatively be obtained from a wash after an oxygen delignification stage, maintaining the temperature of the wash filtrate before heating and addition to KST 140073 E_Eng_text.docx 10 15 20 25 30 11/18 digester in the range 85-95°C or above. Such a high temperature wash filtrate may still be beneficial for reducing the heat Iosses, as the temperature loss is only 5-15°C higher temperature in Iiquor withdrawn from the impregnation vessel than the wash filtrate added to digester.

DETAILED DESCRIPTION OF THE INVENTION The invention will be described in more detail with reference to figure 5, showing the modifications 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 of liquid in ton per ton of bone dry wood (bdt). The example use a minimum charge of an additional volume of wash Iiquor of at least 0.5 in LAN, in order to indicate the minimum order of Iiquor charged. But the invention may be applied with increasing volumes up to a L/\N ratio up to 2, or even more of the added Iiquor volume charged as wash Iiquor and then passed on through the system via first transport circulation and finally to impregnation vessel.

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

A corresponding additional amount of black Iiquor, i.e. in a L/\N ratio of at least 0.5, is withdrawn from the withdrawal screen and sent to start of transfer. This additional amount of hot black Iiquor will elevate the temperature in the transport circulation, as the part of the hot black Iiquor in the transport Iiquor increases. After having used the heat value in liquors in the transport circulation for heating the comminuted cellulosic material, is the excess transport Iiquor withdrawn from top separator, but now at an KST 140073 E_Eng_text.docx 10 15 20 25 30 12/18 increased volume in a L/\N ratio of at least 0.5 of the increase. This extra volume of heated transport Iiquor is thereafter added to top of ImpBin where as a result of the extra 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 the withdrawal volume increased at an increased volume in a LNV ratio of at least 0.5.

This modification leaves the cooking process othen/vise 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 Iiquor before addition into the middle of the cooking zone. This could increase the alkali concentration in the last cooking zone improving delignification further in this last zone. For improving the distribution of alkali, as well as cleaner wash Iiquor, could optionally also a cooking circulation CC be installed, that promotes an enhanced radial distribution of the new cooking Iiquor. Such a cooking circulation normally includes also a pump (not shown). The amount of fresh alkaline Iiquor charged to the wash filtrate is charged to a cooking circulation recirculating a part of the Iiquor withdrawn from the digester withdrawal screen to the center of the digester vessel at the height position of the digester withdrawal screen (as shown in figure 5), or alternatively is charged to a cooking circulation recirculating a part of the Iiquor withdrawn 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 SCFGGFI.

Heat Economy As a result, the extra amount of steam flashed off in the ImpBin will reduce the need for adding extra steam, i.e. LP-steam, to top of ImpBin. This reduction of steam addition match the need for heating the wash Iiquor in heater HE, so in aspects of heat economy the change is not increasing steam consumption. ln the boundary limits of the two vessel system is an additional volume of wash Iiquor added to the digester and the corresponding amount of recovery withdrawal increase is done from ImpBin screen. lf these volumes are identical and the temperature in the KST 140073 E_Eng_text.docx 10 15 20 25 30 13/18 ImpBin withdrawal is about 100°C, then no increased cost of steam is caused in the two vessel 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 steam economy improved as less fresh steam (LP steam) needs to be added to ImpBin top as a result or if the steam to the ImpBin is enough the hot black Iiquor will be used to increase the temperature of the transfer circulation and the l\/IP-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 some 200 g/l. lf no dilution of the cooking Iiquor and addition of alkali is made at this position 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 the recirculation phases during impregnation and first cooking zone. On the other hand, the extra volume of hot black Iiquor withdrawn, with its content of hemicellulose that may 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 organic content, primarily hemicellulose and lignin in almost equal proportions, but then the final phase is modified slightly, decreasing the concentration of lignin and optionally increasing the alkali concentration for the final phase of the cook.

This modification of the cooking Iiquor improves delignification, such the typical low order of delignification that could be obtained in final delignification, reaching a kappa reduction 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 cook is also the residual COD content reduced considerably. ln the system set up as shown in figure 4 is the COD level in pulp typically about 900 kg per bdt pulp (not wood), while after a modification of the cooking Iiquor the COD level may be reduced to below 650 kg. This improves bleachability, as less bleaching chemicals are needed to reach the desired brightness.

KST 140073 E_Eng_text.docx 10 15 14/18 Summary By the inventive modification of the Compact Cooking G2 system is the increased yield objective still obtained to a large extent, but further delignification may be obtained at less COD content in the cooked pulp. These results are obtained without or reducing heating costs as all heating of cleaner wash liquors is recovered in the lmpBin 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 washing the pulp coming directly from the blow line of the digester. This system installation follow the DiConnW' concept sold by Valmet, where the filtrate obtained from the pressured 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 140073 E_Eng_text.docx

Claims (9)

PATENTKRAV 1. Forfarande fOr kokning av finfordelat cellulosamaterial, i vilket fOrfarande anvands ett kontinuerligt tvakarlskokarsystem, innefattande ett forsta atmosfariskt impregneringskarl och ett andra trycksatt kokarkarl, varvid cellulosamaterialet har en retentionstid pa minst 40 minuter i det fOrsta impregneringskarlet och minst 90 minuter i det andra kokarkarlet, och dar en vatskeniva pa minst 15 meter etableras i det forsta impregneringskarlet, vilket namnt forfarande innefattar foljande pa varandra foljande steg: - matning av finfordelat cellulosamaterial fOrvarat vid omgivningstemperatur till det f6rsta impregneringskarlet och darvid etablera en cellulosamaterialniva minst 4 meter Over en vatskeniva etablerad i impregneringskarlet;

1. tillsattning av varm impregneringsvatska vid en temperatur Over kokpunkten vid en position inuti volymen av cellulosamaterial Over vatskenivan, sa aft anga frig6rs in i volymen av cellulosamaterial for angning, och varvid den fran tryck frigjorda varma impregneringsvatskan antar en temperatur som motsvarar kokpunkten vid vatskenivan etablerad i impregneringskarlet; - dar impregneringskarlet har en avdragssil vid vatskenivan och varvid avdragssilen drar av minst 1-2 ton vatska/bdt ved fran impregneringsvatskan, dar namnda vatska 20 innehaller minst en del av den under angning av flisvolymen Over vatskenivan frigjorda vedsyran; -impregnering av cellulosamaterialet med den kali tryck frigjorda impregneringsvatskan under retentionstiden i impregneringskarlet; 2. under utmatning av impregnerat finfordelat cellulosamaterial fran impregneringskarlet tillsatts svartlut vid full koktemperatur sa aft temperaturen pa transportslurryn av finfordelat material till kokaren hojs med minst 10 °C; 3. i toppen av kokaren avdras minst 4 ton vatska/bdt ved fran transportslurryn och anvands som den till impregneringskarlet tillsatta varma impregneringsvatskan; 4. och etablering av en full kokartemperatur i intervallet 130-170 °C i en kokzon i 30 kokaren med anvandande av minst 80 % av den totala retentionstiden i namnda kokare som en kokzon; 5. dar namnda kokare har en kokaravdragssil i mitten av kokzonen vid en position i kokaren som motsvarar 35-60 % av den totala retentionstiden i kokaren, och varvid kokavdragssilen drar av minst 2,5 ton vatska/bdt ved, vilket etablerar en forsta del av KST 1 4007SE_Sv_krav.docx kokzonen med hog L/VV-kvot och en andra del av kokzonen med lag L/W-kvot, vilken avdragen vatska satsas under utmatning av impregnerat finfordelat material fran impregneringskarlet som svartlut vid full koktemperatur, kannetecknat av att en ytterligare volym av minst 0,5 ton vatska/bdt ved av tvattfiltrat fran en efterfoljande 5 bruntvattposition efter kokaren tillsatts till centrum av kokarkarlet vid kokaravdragssilen efter uppvarmning av tvattfiltratet till full kokartemperatur, varvid namnda tvattfiltrat har ett lignininnehall pa mindre an 50 % av lignininnehallet i den i avdragssilen avdragna svartluten, och aft slutligen dra av en motsvarande mangd vatska som extra vatska ocksa fran avdragssilen i impregneringskarlet, saledes med 10 anvandande av det uppvarmda tvattfiltratets varmevarde forst i overforingscirkulation och darefter som varm impregneringsvatska vilket frig6r mer anga far flisangning.

2. Forfarande far kokning av finfordelat cellulosamaterial enligt patentkrav 1, kannetecknat av att farsk alkalisk vatska satsas till tvakarlskokarsystemet i en total sats om 160-190 (HW) 190-230 (SW) kg alkali/ bdt ved (EA som NaOH) vid minst 3 processpositioner, varvid en fOrsta sats tillsatts till toppen av kokaren i en mangd som overstiger 60 % av den totala satsen av farsk alkalisk vatska, och en andra sats till den varma impregneringsvatskan och en tredje sats till tvatffiltratet tillsatt till centrum av kokarkarlet vid kokaravdragssilen.

3. Forfarande f6r kokning av finfordelat cellulosamaterial enligt patentkrav 2, kannetecknat av att mangden farsk alkalisk vatska satsad till tvatffiltratet tillsatt till centrum av kokarkarlet vid kokaravdragssilen motsvarar minst 10 % av den totala satsen av farsk alkalisk vatska, varvid satsen är tillracklig far aft etablera en alkaliniva i kokzonen pa Over 10 g/I (som NaOH), och foretradesvis Over 15 g/I.

4. Forfarande far kokning av finfordelat cellulosamaterial enligt patentkrav 3, kannetecknat av aft mangden farsk alkalisk vatska satsad till tvattfiltratet satsas till en kokcirkulation som atercirkulerar en del av vatskan avdragen fran kokaravdragssilen till centrum av kokarkarlet vid kokaravdragssilens hojdposition, eller alternativt satsas till en kokcirkulation som atercirkulerar en del av vatskan avdragen from en tvattavdragssil till centrum av kokarkarlet vid kokaravdragssilens NO position. KST 1 4007SE_Sv_kra v.docx