METHOD FOR OPERATING A TWO VESSEL DIGESTER SYSTEM

Description

TECHNICAL AREA

[0001] The present invention concerns a method for the continuous cooking of cellulose according to the introduction to claim 1 with the aim of achieving improved washing and delignification at less losses in heat economy.

DESCRIPTION OF DRAWINGS

[0002] The background prior art and the invention will be described using figures 1 to 6 describing the development of continuous cooking up to Compact Cooking™ G2, launched in 2003, and finally the invention is described with reference to figure 5.

Figure 1 shows the development of continuous cooking processes with start from 1957;

Figure 2 shows schematically the flow routing in Compact Cooking G2;

Figure 3 show the upper part of the atmospheric impregnation vessel used in Compact 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 first vessel for recovering the increased flow of heated liquors to the digester system as steam for chip steaming;

Figure 6 shows a preferred brown wash in form of a pressure diffuser washing the pulp coming directly from the blow line of the digester.

THE PRIOR ART

[0003] As shown in figure 1 has the development of continuous cooking passed several development phases since the first commercial continuous digester installed in Fengersfors, Sweden by Valmet AB (then Kamyr AB).

[0004] The very first generation (see 1957, CC + Cold Blow) of continuous digesters had a simple one stage cocurrent cooking zone (CC) with a final cooling in bottom for cold blow. In the later generation (see 1962, CC + Hi Heat) of continuous digesters was also an improved internal washing with a countercurrent Hi.Heat washing zone. The design capacity was in a retrospective view quite low as the countercurrent wash prevented increase of production, and only half the digester was used for cooking. Typically cooking temperatures lie at some 160-170°C. A large part of digesters still in operation use only half the digester as a cooking zone.

[0005] 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 + ITC™ (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.

[0006] The technique of impregnation with black liquor was developed during the latter part of the 1980s and the 1990s (see 1993, BLI™ + MCC + ITC™ (Hi Heat)), with the aim of obtaining improved cooking economy and heat economy and of obtaining better pulp. In 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. In 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-Solids™ cooking implemented, where black liquor was withdrawn and replaced with cleaner cooking liquor with less lignin content.

[0007] In later development (see 1997, Compact Cooking™) 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/W 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 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.

[0008] 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.

[0009] 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. In 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.

[0010] 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™ G2). In this two vessel digester system was the entire pre steaming equipment excluded by using a modified impregnation vessel sold by Valmet under the name ImpBin™. In the ImpBin was both steaming and impregnation implemented, and as the ImpBin was atmospheric this limited the maximum temperature in upper impregnation liquor 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 "normal" reject handling volumes has been used at only fractions of its design capacity, if used at all.

[0011] In a recent further improvement of continuous cooking shown in US7112256 is the concept with ImpBin developed with CrossCirc™. 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 liquor to the liquor level established in the ImpBin, allowing steam to flash off and into an established chip volume above liquor level. The patent US7112256 is related to the CrossCirc features that the bulk volume of return liquor from the top separator is instead sent to this liquor level in ImpBin, and before that is instead hot black liquor added to start of transfer (instead of the top separator return liquor 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 liquor is allowed to flash off for steaming, while yet establishing a low impregnation temperature in black liquor after flashing.

[0012] The concepts with Compact Cooking G2, using an ImpBin is disclosed also in Chemical Pulping Part 1, Fibre Chemistry and Technology, Book 6, 2nd edition (ISBN 978-952-5216-41-7), on pages 350-356.

[0013] 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 liquor 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 TS_FEED to top separator according to CrossCirc practice. Its is to be noted that the main charge of 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 TS_FEED 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 TS_RET withdrawn in top separator, i.e. the transport fluid in TS_FEED,. 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.

[0014] When this return liquor TS_RET is released at the liquor level of the ImpBin 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 ImpLiq for the ImpBin.

[0015] As indicated, in many mills is additional heating of the return liquor TS_RET 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 TS_RET 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 ImpBin 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.

[0016] This is the essential principle in CrossCirc, i.e. sending the return liquor flow 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 ImpBin. As indicated, spent cooking liquor is withdrawn from end of digester, REC₂, and preferably also from a withdrawal at liquor level in ImpBin, i.e. REC₁, and sent further to evaporation plant ahead of final recovery in a recovery boiler.

[0017] An essential feature of the atmospheric ImpBin system is shown in more detail in figure 3. The temperature profile established in the chip volume above liquor level, and above release point of the return liquor TS_RET, 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 ImpBin. 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).

[0018] As colder comminuted cellulosic material are continuously fed into the ImpBin 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). If the transition zone moves downwardly more return liquor TS_RET is added or additional heating thereof, or alternatively using low pressure steam as a complementary steaming effect.

[0019] Figure 4 shows in further detail the liquor routing in Compact Cooking G2. The digester is operated with only 2 concurrent cooking zones, charging the white liquor WL to top of digester where it mixes well with the inflow of impregnated comminuted cellulosic material. A L/W ratio of about 5 is established in first cooking zone and after this zone is hot partially spent cooking liquor, i.e. black liquor, withdrawn in a L/W ratio of about 3. This hot black liquor 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. In the top separator is excess transport liquor withdrawn at a L/W ratio of about 6-8, and this transport liquor is sent to top of ImpBin to be released into the chip pile volume established above the liquor level, where steam is released. Thus, the impregnation liquor for the ImpBin is established by the hot black liquor withdrawal which after mixing into the transport circulation for heating, is withdrawn in top separator and sent to top of ImpBin. The black liquor is thus given an increased retention time in the cooking system as it is recirculated back to ImpBin and 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 liquor routing scheme is the carbohydrates dissolved in cooking liquor early in cook, i.e. in the ImpBin and upper cooking zone, recirculated back to start of ImpBin and given the necessary retention time for obtaining the intended increase in yield due to precipitation. The temperature established in the liquor 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 liquor level in ImpBin. The high L/W 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

[0020] 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. It has surprisingly been realized that the ImpBin 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).

[0021] 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 second part of the cooking zone with low L/W ratio, which withdrawn liquor is charged during feed out of impregnated comminuted material from the impregnation vessel as black liquor at full cooking temperature. What is defined in preceding part is the preconditions for a two vessel Compact Cooking system with an ImpBin and using CrossCirc in transfer circulation.

[0022] Now the invention is characterized in that in this context is an additional volume of at least 0.5 ton liquor/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 liquor withdrawn in the withdrawal screen,
and finally withdrawing a corresponding amount of liquor as additional liquor 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 liquor releasing more steam for chip steaming.

[0023] 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 liquor, due to the re-circulation of the extracted hot black liquor.

[0024] In a further embodiment of the inventive method is fresh alkaline liquor 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 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.

[0025] Increased delignification could thus be obtained in the final cooking zone in the Compact Cooking G2 process.

[0026] In a preferred embodiment is the amount of fresh alkaline liquor charged to the wash filtrate added to the center of the digester vessel at the digester withdrawal screen corresponding to at least 10% of the total charge of fresh alkaline liquor, 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.

[0027] In an optional embodiment of the inventive method is the amount of fresh alkaline liquor charged to the wash filtrate charged to a cooking circulation recirculating a part of the liquor withdrawn from the digester withdrawal screen to the center of the digester vessel. This enforced circulation could improve radial distribution of the modified cooking liquor ahead of the final cooking zone.

[0028] In a preferred embodiment of the inventive is also the amount of fresh alkaline liquor charged to the hot impregnation liquor corresponding to at least 10% of the total charge of fresh alkaline liquor, 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.

[0029] In 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. If 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. In 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.

[0030] 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 digester in the range 85-95°C or above. Such a high temperature wash filtrate may still be beneficial for reducing the heat losses, as the temperature loss is only 5-15°C higher temperature in liquor withdrawn from the impregnation vessel than the wash filtrate added to digester.

DETAILED DESCRIPTION OF THE INVENTION

[0031] 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.

[0032] In following example is liquor-to-wood / L/W 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 liquor of at least 0.5 in L/W, in order to indicate the minimum order of liquor charged. But the invention may be applied with increasing volumes up to a L/W ratio up to 2, or even more of the added liquor volume charged as wash liquor and then passed on through the system via first transport circulation and finally to impregnation vessel.

[0033] As a first step is a cleaner wash filtrate added to center of digester at the withdrawal screen in a L/W ratio of at least 0.5. This cleaner wash liquor WL is heated in a heat exchanger HE to full cooking temperature before addition. The wash liquor is preferably a wash liquor 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 liquor still is alkaline. The temperature of the wash liquor from a brown wash position is typically in the order of 90-110°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.

[0034] A corresponding additional amount of black liquor, i.e. in a L/W ratio of at least 0.5, is withdrawn from the withdrawal screen and sent to start of transfer. This additional amount of hot black liquor will elevate the temperature in the transport circulation, as the part of the hot black liquor in the transport liquor increases. After having used the heat 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 increased volume in a L/W ratio of at least 0.5 of the increase. This extra volume of heated transport liquor is thereafter added to top of ImpBin where as a result of the extra volume of heated transport liquor also is flashed off a larger amount of steam, before the finally flashed transport liquor assumes a temperature of about 100°C in the liquor level of the ImpBin. Finally, in the ImpBin withdrawal screen is also the withdrawal volume increased at an increased volume in a L/W 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.

[0035] As indicated could also an additional amount of alkali be added to the wash liquor 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 liquor, could optionally also a cooking circulation CC be installed, that promotes an enhanced radial distribution of the new cooking liquor. Such a cooking circulation normally includes also a pump (not shown). The amount of fresh alkaline liquor charged to the wash filtrate is charged to a cooking circulation recirculating a part of the liquor 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 liquor 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 screen.

Heat Economy

[0036] 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 liquor in heater HE, so in aspects of heat economy the change is not increasing steam consumption.

[0037] In the boundary limits of the two vessel system is an additional volume of wash liquor added to the digester and the corresponding amount of recovery withdrawal increase is done from ImpBin screen. If these volumes are identical and the temperature in the ImpBin withdrawal is about 100°C, then no increased cost of steam is caused in the two vessel digester system if the wash liquor has the same temperature, i.e. 100°C, before heating. For each degree of temperature increase in wash liquor 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 liquor will be used to increase the temperature of the transfer circulation and the MP-steam demand on the digester top to reach cooking temperature will reduce.

Improved delignification

[0038] As indicated in figure is typically the COD content in mid-point withdrawal at some 200 g/l. If no dilution of the cooking liquor and addition of alkali is made at this position the COD content will increase further during the final cooking zone.

[0039] 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 liquor 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.

[0040] 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.

[0041] This modification of the cooking liquor 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

[0042] As an additional effect of the modification of the cooking liquor at final phase of cook is also the residual COD content reduced considerably. In 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 liquor the COD level may be reduced to below 650 kg. This improves bleachability, as less bleaching chemicals are needed to reach the desired brightness.

Summary

[0043] 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 ImpBin resulting in less need for fresh steam for steaming the comminuted cellulosic material.

Optional brown wash

[0044] In 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 DiConn™ 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. In 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.

Claims

1. 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 second part of the cooking zone with low L/W ratio, which withdrawn liquor is charged during feed out of impregnated comminuted material from the impregnation vessel as black liquor at full cooking temperature characterized in that an additional volume of at least 0.5 ton liquor/bdt wood of wash filtrate from a subsequent brown wash position after the digester is 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 liquor withdrawn in the withdrawal screen,

and finally withdrawing a corresponding amount of liquor as additional liquor 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 liquor releasing more steam for chip steaming.

2. Method for cooking comminuted cellulosic material according to claim 1 characterized in that fresh alkaline liquor is charged to the two vessel digester system in a total charge of 160-190 (HW) 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 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 2 characterized in that the amount of fresh alkaline liquor charged to the wash filtrate added to the center of the digester vessel at the digester withdrawal screen corresponds to at least 10% of the total charge of fresh alkaline liquor, wherein the charge is sufficient to establish a alkali level in the cooking zone over 10 g/l (as NaOH), and preferably above 15 g/l.

4. Method for cooking comminuted cellulosic material according to claim 3 characterized in that the amount of fresh alkaline liquor charged to the wash filtrate is charged to a cooking circulation recirculating a part of the liquor withdrawn from the 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 circulation recirculating a part of the liquor withdrawn from a wash withdrawal screen to the center of the digester vessel at the height position of the digester withdrawal screen .

5. Method for cooking comminuted cellulosic material according to claim 3 characterized in that the amount of fresh alkaline liquor charged to the hot impregnation liquor corresponds to at least 10% of the total charge of fresh alkaline liquor, 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 1 characterized in that the wash filtrate from a subsequent brown wash position after the digester is obtained from a pressurized wash, maintaining the pressure of the wash filtrate before addition to digester.

7. Method for cooking comminuted cellulosic material according to claim 6 characterized in that the wash filtrate from a subsequent brown wash position after the digester is 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 .

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

9. Method for cooking comminuted cellulosic material according to claim 1 characterized in that the wash filtrate from a subsequent brown wash position after the 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 .

Ansprüche

1. Verfahren zum Kochen von zerkleinertem Zellulosematerial, wobei in dem Verfahren ein kontinuierliches Zweibehälter - Kochersystem verwendet wird, das einen ersten atmosphärischen Imprägnierbehälter und einen zweiten druckbeaufschlagten Kocherbehälter umfasst, wobei das Zellulosematerial eine Verweilzeit von mindestens 40 Minuten im ersten Imprägnierbehälter und von mindestens 90 Minuten im zweiten Kocherbehälter aufweist, und wobei in dem ersten Imprägnierbehälter ein Laugenspiegel von mindestens 15 Meter eingestellt wird, wobei das Verfahren folgende aufeinanderfolgenden Schritte umfasst;
Zuführen von zerkleinertem Zellulosematerial, das bei Umgebungstemperatur gehalten wird, in den ersten Imprägnierbehälter, wodurch ein Zellulosematerialspiegel von wenigstens 4 Metern über einem Laugenspiegel eingestellt wird, der in dem Imprägnierbehälter eingestellt wurde,
Zugabe von heißer Imprägnierlauge bei einer Temperatur oberhalb des Siedepunktes an einer Position innerhalb des Volumens des Zellulosematerials oberhalb des Laugenspiegels, so dass Dampf zum Dämpfen in das Volumen des Zellulosematerials freigesetzt wird, und wobei die druckbefreite heiße Imprägnierlauge eine Temperatur annimmt, die dem Siedepunkt an dem Laugenspiegel entspricht, der in dem Imprägnierbehälter eingestellt wurde;
wobei der Imprägnierbehälter ein Abzugssieb an dem Laugenspiegel aufweist, und wobei das Abzugssieb wenigstens 1-2 Tonnen Lauge / bdt Holz aus der Imprägnierlauge abzieht, wobei die Lauge zumindest einen Teil der beim Dämpfen freigesetzten Holzacidität im Chipvolumen oberhalb des Laugenspiegels enthält;
wobei das Zellulosematerial während der Verweilzeit in dem Imprägnierbehälter mit der druckbefreiten Imprägnierlauge imprägniert wird,
wobei während des Abführens des imprägnierten zerkleinerten Materials von dem Imprägnierbehälter, Schwarzlauge bei voller Kochtemperatur hinzugefügt wird, so dass die Temperatur der Transportaufschlämmung des zerkleinerten Materials zu dem Kocher um wenigstens 10°C erhöht wird,
wobei im oberen Bereich des Kochers wenigstens 4 Tonnen Lauge / bdt Holz der Transportaufschlämmung abgezogen werden und verwendet werden als die heiße Imprägnierlauge, die dem Imprägnierbehälter hinzugefügt wird;
und wobei eine volle Kochertemperatur in dem Intervall von 130-170 °C in einer Kochzone in dem Kocher eingestellt wird, unter Verwendung von wenigstens 80% der Gesamtverweilzeit des Kochers als eine Kochzone;
wobei der Kocher ein Kocherabzugssieb in der Mitte der Kochzone an einer Position in dem Kocher aufweist, die 35-60% der Gesamtverweilzeit in dem Kocher entspricht, und wobei das Kocherabzugssieb mindestens 2,5 Tonnen Lauge / bdt Holz abzieht, wodurch ein erster Teil der Kochzone mit hoher L/W Rate und ein zweiter Teil der Kochzone mit niedriger L/W Rate eingestellt wird, wobei die abgezogene Lauge beim Abführen von imprägniertem zerkleinertem Material aus dem Imprägnierbehälter als Schwarzlauge bei voller Kochtemperatur beschickt wird, dadurch gekennzeichnet, dass ein Zusatzvolumen von mindestens 0,5 Tonnen Lauge / bdt Holz von Waschfiltrat von einer nachfolgenden Braun - Waschposition nach dem Kocher in die Mitte des Kochbehälters an dem Kocherabzugssieb zugeführt wird, nach dem erhitzen des Waschfiltrats auf volle Kochtemperatur, wobei das Waschfiltrat einen Ligningehalt von weniger als 50% des Ligningehalts in der im Abzugssieb abgezogenen Schwarzlauge aufweist,
und schließlich Abziehen einer entsprechenden Menge von Lauge als zusätzliche Lauge auch aus dem Abzugssieb in dem Imprägnierbehälter, wodurch der Wärmewert des erhitzen Waschfiltrats zuerst in der Übertragungszirkluation und anschließend als heiße Imprägnierlauge verwendet wird, wodurch mehr Dampf für das Dämpfen der Chips freigesetzt wird.

2. Verfahren zum Kochen von zerkleinertem Zellulosematerial nach Anspruch 1, dadurch gekennzeichnet, dass frische alkalische Lauge in das Zweibehälter - Kochersystem beschickt wird in einer Gesamtladung von 160 - 190 (HW) 190 - 230 (SW) kg Alkali/Bdt-Holz (EA als NaOH) in mindestens 3 Prozesspositionen, wobei dem oberen Bereich des Kochers eine erste Ladung zugegeben wird in einer Menge von mehr als 60% der Gesamtladung an frischer alkalischen Lauge, und wobei eine zweite Ladung zu der heißen Imprägnierlauge und eine dritte Ladung zu dem Waschfiltrat zugegeben wird, hinzugegeben an der Mitte des Kocherbehälters am Kocherabzugssieb.

3. Verfahren zum Kochen von zerkleinertem Zellulosematerial nach Anspruch 2 dadurch gekennzeichnet, dass die Menge an frischer alkalischer Lauge, mit der das Waschfiltrat beschickt wird, das der Mitte des Kocherbehälters am Kocherabzugssieb zugegeben wird, wenigstens 10% der Gesamtladung an frischer alkalischen Lauge entspricht, wobei die Ladung ausreicht, um ein Alkalispiegel in der Kochzone über 10 g/l (als NaOH) und vorzugsweise über 15 g/l einzustellen.

4. Verfahren zum Kochen von zerkleinertem Zellulosematerial nach Anspruch 3 dadurch gekennzeichnet, dass die Menge an frischer alkalischer Lauge, mit der das Waschfiltrat beschickt wird, zu einer Kochzirkulation beschickt wird, wobei ein Teil der Lauge, die von dem Kocherabzugssieb abgezogen wird, zur Mitte des Kocherbehälters an der Höhenposition des Kocherabzugssiebs rückzirkuliert wird, oder alternativ zu einer Kochzirkulation beschickt wird, wobei einen Teil der aus einem Waschabzugssieb abgezogenen Lauge zur Mitte des Kocherbehälters an der Höhenposition des Kocherabzugssiebs rückzirkuliert wird.

5. Verfahren zum Kochen von zerkleinertem Zellulosematerial nach Anspruch 3, dadurch gekennzeichnet, dass die Menge an frischer alkalischer Lauge, mit der die heiße Imprägnierlauge beschickt wird, wenigstens 10% der Gesamtladung an frischer alkalischen Lauge entspricht, wobei die Ladung ausreichend ist, um im Imprägnierbehälter einen endgültigen Restalkalispiegel von über 8 g/l (als NaOH) und vorzugsweise über 15 g/l einzustellen.

6. Verfahren zum Kochen von zerkleinertem Zellulosematerial nach Anspruch 1, dadurch gekennzeichnet, dass das Waschfiltrat von einer nachfolgenden Braun - Waschposition nach dem Kocher von einer druckbeaufschlagten Wäsche erhalten wird, wodurch der Druck des Waschfiltrats vor Zugabe zum Kocher aufrechterhalten wird.

7. Verfahren zum Kochen von zerkleinertem Zellulosematerial nach Anspruch 6, dadurch gekennzeichnet, dass das Waschfiltrat von einer nachfolgenden Braun - Waschposition nach dem Kocher von einer druckbeaufschlagten Wäsche erhalten wird, wobei die Temperatur des Waschfiltrats vor Zugabe zum Kocher oberhalb 100°C und bevorzugt im Bereich von 100-110°C aufrechterhalten wird.

8. Verfahren zum Kochen von zerkleinertem Zellulosematerial nach Anspruch 7 dadurch gekennzeichnet, dass das Waschfiltrat von einer nachfolgenden Braun - Waschposition direkt nach dem Kocher, bevorzugt in einem Druckdiffuser erhalten wird.

9. Verfahren zum Kochen von zerkleinertem Zellulosematerial nach Anspruch 1 dadurch gekennzeichnet, dass das Waschfiltrat von einer nachfolgenden Braun - Waschposition nach dem Kocher von einer Wäsche nach einer Sauerstoff - Delignifizierungsstufe erhalten wird, wobei die Temperatur des Waschfiltrats vor dem Erhitzen und Zugabe zum Kocher im Bereich von 85-95°C oder darüber aufrechterhalten wird.

Revendications

1. Procédé de cuisson de matière cellulosique triturée, dans ledit procédé on utilise un système de lessiveur continu à deux cuves comprenant une première cuve d'imprégnation atmosphérique et une seconde cuve d'imprégnation atmosphérique, dans lequel la matière cellulosique présente un temps de rétention d'au moins 40 minutes dans la première cuve d'imprégnation et d'au moins 90 minutes dans la seconde cuve d'imprégnation, et dans lequel un niveau de liqueur d'au moins 15 mètres est établit dans la première cuve d'imprégnation, ledit procédé comprenant les étapes suivantes en séquence consistant à ;

- alimenter la matière cellulosique triturée maintenue à température ambiante à la première cuve d'imprégnation établissant un niveau de matière cellulosique au moins 4 mètres au-dessus d'un niveau de liqueur établit dans la cuve d'imprégnation ;

- ajouter une liqueur chaude d'imprégnation à une température au-dessus du point d'ébullition à une position située à l'intérieur du volume de matière cellulosique au-dessus du niveau de liqueur, de telle sorte qu'une vapeur est libérée dans le volume de matière cellulosique destiné à la vaporisation, et dans lequel la liqueur chaude d'imprégnation à pression libérée assume une température correspondant au point d'ébullition au niveau de la liqueur établit dans la cuve d'imprégnation ;

- la cuve d'imprégnation ayant un tamis au niveau de la liqueur et dans lequel le tamis retire au moins 1 à 2 tonnes de liqueur/bois anhydre de la liqueur d'imprégnation, ladite liqueur contenant au moins une partie de l'acidité du bois libérée pendant la vaporisation dans le volume du copeau au-dessus du niveau de la liqueur ;

- la matière cellulosique est imprégnée de la liqueur d'imprégnation à pression libérée pendant le temps de rétention dans la cuve d'imprégnation,

- pendant l'alimentation de matière triturée imprégnée provenant de la cuve d'imprégnation, on ajoute de la liqueur noire à une température de cuisson complète de telle sorte que la température de la suspension de transport de matière triturée au lessiveur est accrue d'au moins 10 °C ;

- dans la partie supérieure du lessiveur on trouve au moins 4 tonnes de liqueur/bois anhydre de la suspension de transport retirée et utilisée comme la liqueur chaude d'imprégnation à la cuve d'imprégnation ;

- et qu'une température de lessiveur complète dans l'intervalle compris entre 130 et 170 °C est établie dans une zone de cuisson dans le lessiveur utilisant au moins 80 % du temps de rétention totale dudit lessiveur en tant que zone de cuisson ;

- ledit lessiveur ayant un tamis de lessiveur au milieu de la zone de cuisson au niveau d'une position dans le lessiveur correspondant de 35 à 60 % du temps de rétention totale dans le lessiveur, et dans lequel le tamis de cuisson retire au moins 2,5 tonnes de liqueur/bois anhydre établissant une première partie de la zone de cuisson avec un rapport L/W élevé et une seconde partie de la zone de cuisson avec un rapport L/W faible, dont la liqueur retirée est chargée pendant l'alimentation de matière triturée imprégnée depuis la cuve d'imprégnation comme liqueur noire est chargée pendant l'alimentation de matière triturée imprégnée provenant de la cuve d'imprégnation en tant que liqueur noire à température de cuisson complète caractérisé en ce qu'un volume supplémentaire d'au moins 0,5 tonne de liqueur/bois anhydre de filtrat de lavage à partir d'une position de lavage marron après que le lessiveur soit ajouté au centre de la cuve du lessiveur au niveau du tamis de lessiveur après le chauffage du filtrat de lavage à une température de lessiveur complète, ledit filtrat de lavage ayant une teneur en lignine inférieure à 50 % de la teneur en lignine dans la liqueur noire retirée dans le tamis,

et finalement retirer une quantité correspondante de liqueur en tant que liqueur supplémentaire également provenant du tamis dans la cuve d'imprégnation utilisant ainsi la valeur calorifique du filtrat de lavage chauffé en premier dans la circulation de transfert et ensuite en tant que liqueur chaude d'imprégnation libérant plus de vapeur de la vaporisation de copeau.

2. Procédé de cuisson de matière cellulosique triturée selon la revendication 1, caractérisé en ce qu'une liqueur alcaline fraiche est chargée sur le système de lessiveur à deux cuves en une charge totale comprise entre 160 et 190 (HW), entre 190 et 230 (SW) kg d'alcali/bois anhydre (EA en tant que NaOH) dans au moins 3 positions de traitement, dans lequel une première charge est ajoutée à la partie supérieure du lessiveur en une quantité dépassant 60 % de la charge totale de liqueur alcaline fraiche, et une deuxième charge sur la liqueur chaude d'imprégnation et une troisième charge sur le filtrat de lavage ajouté au centre de la cuve de lessiveur au niveau du tamis de lessiveur.

3. Procédé de cuisson de matière cellulosique triturée selon la revendication 2, caractérisé en ce que la quantité de liqueur alcaline fraîche chargée sur le filtrat de lavage ajouté au centre de la cuve du lessiveur au niveau du tamis de lessiveur correspond à au moins 10 % de la charge totale de liqueur alcaline fraîche, dans lequel la charge est suffisante pour établir un niveau d'alcali dans la zone de cuisson de plus de 10 g/l, en tant que NaOH), et préférablement supérieur à 15 g/l.

4. Procédé de cuisson de matière cellulosique triturée selon la revendication 3, caractérisé en ce que la quantité de liqueur alcaline fraîche chargée sur le filtrat de lavage est chargée à une circulation de cuisson remettant en circulation une partie de la liqueur retirée du tamis de lessiveur au centre de la cuve de lessiveur au niveau de la position de hauteur du tamis de lessiveur, ou alternativement est chargée sur une circulation de cuisson remettant en circulation une partie de la liqueur retirée depuis un tamis de lavage vers le centre de la cuve de lessiveur au niveau de la position de hauteur du tamis de lessiveur.

5. Procédé de cuisson de matière cellulosique triturée selon la revendication 3, caractérisé en ce que la quantité de liqueur alcaline fraîche chargée sur la liqueur chaude d'imprégnation correspond à au moins 10 % de la charge totale de la liqueur alcaline fraîche, dans lequel la charge suffisante pour établir un niveau final d'alcali résiduel dans la cuve d'imprégnation de plus de 8 g/l (en tant que NaOH), et préférablement supérieur à 15 g/l.

6. Procédé de cuisson de matière cellulosique triturée selon la revendication 1, caractérisé en ce que le filtrat de lavage à partir d'une position ultérieure de lavage marron après que le lessiveur soit obtenu à partir d'un lavage pressurisé, maintenant la pression du filtrat de lavage avant l'addition au lessiveur.

7. Procédé de cuisson de matière cellulosique triturée selon la revendication 6, caractérisé en ce que le filtrat de lavage à partir d'une position ultérieure de lavage marron après que le lessiveur soit obtenu à partir d'un lavage pressurisé, maintenant la température du filtrat de lavage avant l'addition au lessiveur au-dessus de 100 °C, et de préférence dans la plage comprise entre 100 et 110 °C.

8. Procédé de cuisson de matière cellulosique triturée selon la revendication 7, caractérisé en ce que le filtrat de lavage est obtenu à partir d'une position ultérieure de lavage marron immédiatement après le lessiveur, de préférence dans un diffuseur de pression.

9. Procédé de cuisson de matière cellulosique triturée selon la revendication 1, caractérisé en ce que le filtrat de lavage à partir d'une position ultérieure de lavage marron après que le lessiveur soit obtenu à partir d'un lavage après une étape de délignification d'oxygène, maintenant la température du filtrat de lavage avant le chauffage et l'addition au lessiveur dans la plage comprise entre 85 et 95 °C ou plus.

Drawing