WO2012158075A1

WO2012158075A1 - Compact process for producing prehydrolyzed pulp

Info

Publication number: WO2012158075A1
Application number: PCT/SE2011/050610
Authority: WO
Inventors: Daniel Trolin; Stefan ANTONSSON; Jonas SAETHERÅSEN; Mikael Lindström
Original assignee: Metso Paper Sweden Ab
Priority date: 2011-05-13
Filing date: 2011-05-13
Publication date: 2012-11-22
Also published as: CA2837277A1; EP2707539A1; ES2737432T3; PT2707539T; CN103687990B; RU2546497C1; EP2707539A4; EP2707539B1; CA2837277C; CN103687990A

Abstract

The invention is related to an improved compact process for producing dissolving pulp in a prehydrolysis kraft cooking process. In order to avoid pitch problems with blocked withdrawal screens and to obtain a distinct ending of the prehydrolysis stage, as well as a thorough alkaline impregnation ahead of the kraft cook stage, alkali is charged to the mixture of prehydrolyzed material to such an extent that the residual alkali concentration after neutralization of the acidic hydrolysate is above 20 g/l EA as NaOH and the temperature of the resulting alkaline treatment liquor for the prehydrolyzed material is lowered by at least 10% in comparison to the temperature in the prehydrolysis stage. The alkali charge will avoid redeposition of hemicelluloses dissolved in the prehydrolyse stage and will abruptly swing the wood material mixture to alkaline conditions favourable for a alkali impregnation stage at reduced temperature ahead of the final kraft cooking stage, which impregnation stage will extract the major part of the hemicelluloses content of the cellulose material.

Description

Compact process for producing prehydrolyzed pulp

FIELD OF THE INVENTION

The present invention relates to a process for the production of pulp in which hemicellulose is hydrolyzed into hydrolysate, and lignin is dissolved by a kraft cooking method for liberating cellulose fibers. Still more particularly, the present invention relates to a process for the production of a pulp which has a high content of alpha cellulose and can be sold as dissolving pulp.

BACKGROUND OF THE INVENTION

Traditionally, there are basically two processes for the production of special pulps having a high content of alpha cellulose. These include acidic sulfite cooking, and prehydrolysis- sulfate (kraft) cooking. The former was developed at the end of the 19th century, and the latter in the 1930's, see e.g. Rydholm, S. E., Pulping Processes, pp. 649 to 672, Interscience Publishers, New York, 1968. The basic idea in both processes is to remove as much hemicellulose as possible from cellulose fibers in connection with delignification, so as to obtain a high content of alpha cellulose, i.e. polymeric chains having a relative high polymerization degree and not short hemicellulose molecules with a randomly grafted molecular structure. In the traditional sulfite process, the removal of hemicelluloses takes place during the cooking, simultaneous with dissolving of the lignin. The cooking conditions in that case are highly acidic, and the temperature varies from 140 DEG C. to 150 DEG C, whereby hydrolysis is promoted. The result, however, is always a compromise with delignification. A drawback is the decrease in the degree of polymerization of the alpha cellulose and yield losses, which also limit the potential for hydrolysis. Various improvements have thus been suggested, such as modification of the cooking conditions, and even a prehydrolysis step followed by an alkaline sulfite cooking stage. The main obstacle in connection with sulfite pulping processes is the complicated and costly recovery processes of the cooking chemicals.

A separate prehydrolysis step permits the desired adjustment of the hydrolysis of

hemicelluloses by varying the hydrolysis conditions. In the prehydrolysis-kraft cooking process the bulk delignification is not carried out until a separate alkaline cooking step, even though some handbooks indicate that as much as 30 kg of lignin per ton of wood may be dissolved in the prehydrolysis (i.e. a small part of the total lignin content as 30 kg per ton of wood corresponds to some 3% of the wood material). The conditions for prehydrolysis is most often established by heating in a hot steam phase or hot water liquid environment, where the natural wood acidity released will usually lower the pH down to about 3.5, most often referred to as auto hydrolysis. Sometimes could also additional acid and a catalyst be added. The subsequent delignification step has been a conventional kraft cooking method, where white liquor has been added to the digester.

Several prehydrolysis-kraft cooking processes have been disclosed and this technology was very much in focus during the late 60-ies and early 70-ies.

In the early publication "Continuous Pulping Processes" by Sven Rydholm, 1970, is described the experiences from "Continuous Prehydrolysis-kraft Pulping" on pages 105-1 19. On page 106, Fig. 8.1 is disclosed a two vessel continuous cooking system with a first up flow prehydrolysis tower followed by a down flow conventional kraft cooking digester, in which the up flow tower experienced severe pitch deposits on the extraction strainers which clogged after only 3-6 days of operation. Another system is disclosed on page 107, Fig.8.2 with a one vessel hydraulic continuous digester system with a first upper prehydrolysis zone and a lower alkaline kraft cooking zone, both zones being separated by a strainer section. However, also in this one vessel design was the strainers subjected to severe pitch deposits and clogging. The pitch problem also migrated into the chip feeding system resulting in the need for an alkali charge in the high pressure feeder to avoid pitch deposition in the high pressure feeder. This pitch problem has been seen in almost every installation of continuous cooking systems used for Prehydrolysis-kraft Pulping. This causes production disturbances and pulp quality variations. Furthermore, the lack of a well defined prehydrolysis zone in previous continuous installations has caused variations in the degree of hydrolyzation, which in turn led to unacceptable quality variations of the final product.

When Prehydrolysis-kraft Pulping is implemented in batch systems the pitch problem is partially solved by the fact that the strainers in the batch digester is switching from

withdrawing acidic prehydrolysate to alkaline cooking liquor and later on black liquor. The volume of acidic hydrolysate withdrawn after a steam phase prehydrolysis is also relatively small in total volume so the exposure in strainers is limited. The latter alkaline stages will then also dissolve and wash out any pitch deposits such that they do not build up over time. This is not possible to achieve in continuous systems as the strainers are located in a stationary process position where the chemical conditions (as of pH, pitch content etc) do not change.

In US5589033 is disclosed a batch process for prehydrolyzed kraft pulp sold by Metso often in connection with Superbatch™ cooking. Here is a hot 170^QC prehydrolysis step in a gaseous steam phase terminated by a hot neutralization step at 155^QC using heated alkali and for a duration of only 15 minutes (as shown in example 3). This neutralization is followed by a hot black liquor treatment step at 148^QC for a duration of 20 minutes and finally the pulp is cooked in a kraft cooking stage at 160^QC for 54 minutes. Here the degree of hydrolyzation could be controlled in a good manner by controlling the duration of each stage. As the hydrolysis step is conducted in a steam phase could the following neutralization and alkalization of the wood material be obtained rather quickly and thoroughly as the wood material has been steamed at high temperature in a steam phase allowing the alkali to penetrate the wood material by diffusion. However, this type of well defined transition zone between the prehydrolyse in a steam phase and the neutralization is not favourable in a continuous system where the wood material is supposed to flow trough reaction towers in a plug flow, hence the hydrolysis is instead most often implemented in a liquid filled stage at least in final parts.

Nowadays dissolving pulp for such end uses as spinning fibers (rayon/lyocell) is considered to be an optional method for producing textiles having less environmental impact compared with production of cotton textiles. Dissolving pulp is also a base product for different additives and consistency agents and fillers in tyre cord and casings, ether and spongs, nitrocellulose and acetate. Hence dissolving pulps may be an alternative product instead of pulp for regular paper pulp making.

A common implementation in most prehydrolysis-kraft cooking processes is that the prehydrolysis stage has been terminated by withdrawal of the prehydrolysate, either in form of a pure acidic prehydrolysate, or in form of a neutralized prehydrolysate. As indicated before would any strainers in such process position be subjected to pitch deposits, both when the prehydrolysate is kept at its lowest pH level or if the prehydrolysate is withdrawn in a transition position where the chip suspension switch from acidic to alkaline.

SUMMARY OF THE INVENTION

One object of the present invention is to provide an improved prehydrolysis-kraft process for the preparation of pulp from lignin-containing cellulosic material. In accordance with the invention, these and other objectives have now been accomplished by a process comprising prehydrolyzing said cellulosic material in a prehydrolysis stage at a temperature in the range of about 120^Q and 180^Q C and during at least 20 minutes so as to produce a prehydrolyzed cellulosic material and an acidic hydrolysate. Thereafter adding a strong alkali chock charge to the mixture of prehydrolyzed cellulosic material and acidic hydrolysate to such extent that the residual alkali concentration after neutralization of the acidic hydrolysate, preferably directly after this charge and after thorough mixing, is above 20 g/l EA as NaOH forming an alkaline treatment liquor. Thereafter maintaining the prehydrolyzed cellulosic material in said alkaline treatment liquor for a sufficient time in an alkaline pre-extraction stage to reduce the alkali concentration by at least 10 g/l EA as NaOH but not to a concentration below 5 g/l EA as NaOH. In said alkaline pre-extraction stage the dissolved carbohydrates as well as any lignin dissolved in the prehydrolysis stage are maintained dissolved during this alkaline pre- extraction stage and further carbohydrates and lignin are dissolved from the cellulosic material in the alkaline pre-extraction stage. After the alkaline pre-extraction stage the cellulosic material is transferred from the alkaline pre-extraction stage to a kraft cooking stage. The characterizing part of the invention is that the strong alkali chock charge is made using liquor volumes that decrease the temperature of the resulting alkaline treatment liquor for the prehydrolysed material by at least 10% in comparison to the temperature in the prehydrolysis stage, preferably at least 12^Q C if the prehydrolyse temperature is about 120 ^QC and at least 18^Q C if the prehydrolyse temperature is about 180^Q C, in order to reduce the alkali consumption during the cooking chemical diffusion process where the alkali treatment liquor penetrates to the core of the lignin-containing cellulosic material. This transition from the acidic prehydrolysis stage to the relative low temperature alkaline pre-extraction stage establish a more thorough impregnation of the lignin-containing cellulosic material to the core thereof which results in a more homogenous cook with low rejects amounts after the subsequent kraft cook stage. Another positive effect is that the cooking temperature can be reduced which increases the final alpha cellulose content in the pulp after the kraft cook. Lowering of the temperature significantly will also reduce reprecipitation of hemicelluloses onto the fibers, since the reprecipitation process is strongly dependent on temperature. According to one preferred embodiment is the acidification of said prehydrolysis established only by heating and optionally adding water, and without adding any external acidifiers, only using the wood acidity released during heating reaching a pH level below 5 during the prehydrolysis. In this embodiment not using any external acidifiers could the acidification of said prehydrolysis be established in a liquid filled phase and preferably is the temperature at end of the prehydrolysis in the range of 150-180^QC. In such case no external acidifier is used is the temperature of the resulting mixture of alkaline treatment liquor and the prehydrolysed material below 130^QC. Preferably is the resulting mixture of alkaline treatment liquor and the prehydrolysed material below 120^QC which is well below an optimal kraft cooking temperature of about 142^QC. According to another preferred embodiment is the acidification of said prehydrolysis established by heating and addition of external acidifiers, reaching a pH level below 3 during the prehydrolysis. In this embodiment using external acidifiers could the acidification of said prehydrolysis be established in a liquid filled phase and preferably is the temperature at end of the prehydrolysis in the range 120-165 degrees C. In such case external acidifier is used is the temperature of the resulting alkaline treatment liquor for the prehydrolysed material below 125 ^QC. Preferably is the resulting mixture of alkaline treatment liquor and the prehydrolysed material below 120^QC which is well below an optimal kraft cooking temperature of about 142^QC.

In both cases of adding or not adding acidifier to the prehydrolysis and if prehydrolysis is established in a liquid filled phase, several advantages for reaching the objective of high yield alpha cellulose are obtained. As mentioned could as much as 30 kg lignin per ton of wood be dissolved during the prehydrolysis, and if in a liquid filled phase could the lignin easier be kept in the dissolved state and prevented from condensing onto the fibers. Condensated lignin could negatively affect the following diffusion/penetration of cooking chemicals, as it has been found to form a layer on the fiber material that obstruct diffusion, and thus may impair the delignification rate during the subsequent kraft cook. Lignin condensation is a well known effect that occur in depleted alkaline environment, especially in acidic conditions, and results in "black cook", i.e. pulp with condensated lignin that is very difficult to delignify further thereafter. Lignin condensation will lead to an increased cooking temperature in order to reach the target kappa number, which in turn has a negative impact on alpha cellulose yield. If condensation of lignin is avoided during the prehydrolysis would the pulp be much easier to cook to the desired kappa number at end of the subsequent kraft cook, and with higher yield and polymerization degree of cellulose, both favorable for special grades of dissolving pulp.

As could be realized from above embodiments using or not using external acidifiers, is a higher temperature needed for similar order of prehydrolyse effect if no acidifier is used, but for both embodiments it is essential that the transition between the prehydrolyse stage and the alkaline pre-extraction stage is made such that an essential lowering of the temperature is obtained.

According to a further embodiment of the invention is the step of maintaining the prehydrolyzed cellulosic material in said alkaline treatment liquor implemented for a time period of from about 10 to 90 minutes. Besides establishing a thorough and even alkalization of the wood material after the prehydrolyse stage according to the main objective it is also important to optimize the conditions for the alkaline pre-extraction as well as alkali impregnation ahead of cook. It has been seen from laboratory tests that only a minor part of the total hemicellulose content is dissolved and hydrolysed to oligo-, monosaccarides and decomposition products thereof in the prehydrolyse stage, while yet a large part of the total hemicellulose content could be dissolved in a subsequent alkaline pre-extraction stage. The total dissolved organic content (DOC) after the prehydrolyse is about 5-10% and as much as 30% after an extended alkaline pre-extraction stage following the prehydrolyse, By DOC is included all organic content from the wood material, including hemicellulose/carbohydrates as well as lignin.

According to yet another embodiment is at least 1 m³/ton OD (Owen Dried) of wood of the used alkaline treatment liquor removed from said alkaline pre-extraction stage with its content of dissolved carbohydrates and lignin before onset of the kraft cooking stage, and thereafter adding alkali to the kraft cooking stage. By extracting the used alkaline treatment liquor at this late stage, while keeping it alkaline, could the pitch problems be avoided in any extraction screen sections in prehydrolysis stage and the total DOC content be accumulated to this point in the process. This is especially advantageous for continuous cooking systems which hereto had seen severe pitch deposit problems when trying to extract acidic or neutral prehydrolysate from extraction screens in such systems. Moreover, extracting hemicelluloses rich liquid before the kraft cook stage reduces the carry-over into the kraft cooking stage, which in turn reduces the risk for reprecipitation of hemicelluloses to occur later during the kraft cook stage, which reprecipitation is dependent on hemicelluloses concentration and high temperature.

In a most preferred embodiment of the invention the process could be implemented in a continuous digester system using at least one vessel for the prehydrolysis and one vessel for the alkaline pre-extraction stage and the kraft cooking stage. In this embodiment could also the alkaline pre-extraction stage be implemented in a separate vessel and the kraft cooking stage in another separate vessel. As the strong alkali chock charge will swing the pH conditions rapidly to the alkaline side after the prehydrolyse not only are potential pitch deposit problems avoided which hereto has been a concern for sturdy continuous processes, but the risk for redeposition of hemicelluloses triggered by low residual alkali concentration is also eliminated. The following alkaline pre-extraction stage is also given optimum conditions for a thorough impregnation of the acidic wood material by diffusion of said alkaline treatment liquor, which results both in higher degree of alkaline dissolution of DOC as well as even pH level to the core of the wood material ahead of the alkaline cooking stage. Diffusion is a time dependent process more than a displacement process obtained from by-flushing alkaline liquids, and thus the conditions for a continuous process is improved as it may not be necessary to implement internal liquid circulations throughout the stage.

When compared with known prehydrolysis-kraft processes, such as US5589033, the present invention offers the following advantages:

A distinct ending of the prehydrolysis stage by both a sharp pH transition to a sufficient minimum alkali concentration and lowering of temperature, i.e. the two most dominant process conditions for prehydrolysis.

The neutralization of the free liquid occurs rather instantly, but more favorable conditions for neutralization of the bound liquid inside the wood material are obtained. The diffusion is favored by high temperature but as the alkali consumption rate is far higher at high temperature is lower temperature essential for alkali to diffuse into the core of the wood material. Most batch cooking processes are coming from a state of the art where heat economy is very much in focus, starting with the RDH-process and the more modern batch cooking process Superbatch. Heat is recovered by using spent hot and warm liquors from end of cooking stage in pretreatment stages ahead of cooking. Like in US5589033 are high cooking temperatures often used (160^QC). There is thus a general approach in batch cooking to try to maintain the high temperatures during the process in order to improve heat economy. In modern continuous kraft cooking has the optimum cooking temperature been lowered since the late 1950-ies when cooking temperature most often was about 160^QC or even up to 170^QC for hardwood, and now in the late 1990-ies the typical cooking temperature for hardwood is about 142^QC. In this process according to the invention is a relatively cold alkali charge used for interrupting the prehydrolysis. This is referred to as a cold alkali charge not being heated before addition. Suitable alkali charge for use herein contains caustic soda, and the preferred agent is alkaline kraft cooking liquor, i.e., white liquor. The effective alkali concentration, i.e. EA, is most often given in % and corresponds to; 1 /2 Na₂S + NaOH. Such white liquor typically holds a temperature of some 80-90^QC when delivered from the recovery island, and a white liquor of this temperature is included by definition in the used cold alkali charge. Preferably could this white liquor be cooled down even further in a heat exchanger in order to reach the desired final temperature after addition at end of the prehydrolysis. Yet, other alkaline filtrates could be added, preferably those having a high alkali content and a low temperature.

The lignin-containing cellulosic materials to be used in the present process are suitably softwood, hardwood, or annual plants.

According to the present invention, prehydrolysis-kraft pulp can be obtained with a high yield of alpha cellulose with a high polymerization degree. BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of the cooking process according to US5589033;

FIG. 2 is a schematic representation of the cooking process according to the invention; FIG. 3a is a schematic representation of the pH through a chip of wood after the

prehydrolysis stage, End of PR, and when exposed to the alkaline treatment liquor, Initial of Ex;

FIG. 3b shows the alkaline consumption rate as a function of temperature;

FIG. 4 show the different pH level established inside a wood chip after a prehydrolysis when exposed to alkaline treatment liquor during a substantial time using cold or hot alkaline treatment liquor;

FIG. 5 show the kappa number after a kraft cook inside a wood chip after the two different treatments as shown in figure 4;

FIG. 6 shows a principal set up for a continuous cooking system using the inventive process, here using a prehydrolysis tower and two subsequent vessels for alkaline treatment and cook;

FIG. 7 shows a principal set up for a continuous cooking system using the inventive process, here using a prehydrolysis tower and one subsequent vessels for alkaline treatment and cook;

FIG. 8a-8d shows the inventive process implemented in a batch digester when ending the prehydrolysis and starting the subsequent alkaline treatment;

DETAILED DESCRIPTION OF THE INVENTION

As a comparison is in FIG. 1 shown the cooking steps of US5589033. The chips are first treated in the prehydrolysis step Pr where chips are heated by steam to 170^QC for 25 minutes. Thereafter is heated white liquor added in order to establish a neutralization step Ne and the acidic prehydrolysate REC_Ac is withdrawn from the process. The neutralization step is established at 155^QC for 15 minutes. Even though the white liquor is heated is the temperature decreased some 8%. After the neutralization step is the neutralization liquid displaced by adding hot black liquor BL_HOT, and this establish an alkaline black liquor impregnation step BL held at 148^QC for 20 minutes. Thereafter the black liquor is withdrawn and a new charge of white liquor is added ahead of the following cooking step Co which is held at 160^QC for 54 minutes. In commercial batch cooking systems like Superbatch™ sold by Metso is the white liquor used heated both in a heat exchange with hot spent cooking liquor as well as steam in order not keep the temperature at high level, before being used as the neutralizing liquid.

In contrast is in FIG. 2 shown the cooking steps according the inventive process. Here is shown a first steaming step ST for the chips but this step may be avoided of the subsequent prehydrolysis is implemented in a steam phase. The chips are thereafter treated in the prehydrolysis step Pr where chips are heated by steam at a temperature of between about 120^Q and 180^Q C and during at least 20 minutes so as to produce a prehydrolyzed cellulosic material and an acidic hydrolysate. Addition of liquid such as water H₂0 is an option, which may be preferable if a liquid prehydrolysis is sought for, for example in a continuous cooking system. Another option is to add an acidifier Ac if a lower temperature is sought for in the prehydrolysis.

According to the inventive process is a distinct ending of the prehydrolysis implemented by adding a strong and cold alkali chock charge WL_COLD with a volume and at a temperature that will reduce the temperature of the cellulosic material by at least 10% in comparison to the temperature in the prehydrolysis stage, preferably at least 12^Q C if the prehydrolyse temperature is about 120 ^QC and at least 18^Q C if the prehydrolyse temperature is about 180^Q C. This will establish an alkaline treatment liquor which after this charge establishes a residual effective alkali concentration above 20 g/l EA as NaOH. Thereafter the

prehydrolyzed material is maintained in an alkaline pre extraction stage Ex for a sufficient time in the alkaline pre-extraction stage to reduce the alkali concentration by at least 10 g/l EA as NaOH but not to a concentration below 5 g/l EA as NaOH. In this pre-extraction stage the dissolved carbohydrates as well as any lignin dissolved in the prehydrolysis are maintained dissolved during this alkaline pre-extraction stage and further carbohydrates and lignin are dissolved from the cellulosic material. Thereafter the cellulosic material is transferred from the alkaline pre-extraction stage Ex to a kraft cooking stage Co. Before transfer to the kraft cooking stage is preferably a large part of the spent alkaline treatment liquor withdrawn to recovery and a fresh charge of alkali WL is added to start of cook. The kraft cook may be implemented in any kind of known kraft cooking method for batch or continuous cooking such as, Compact Cooking, Lo-Solids cooking, ITC-cooking, MCC cooking, EAPC cooking as examples. The kraft cook is then finished by a wash stage Wa, which may be implemented in any kind of known wash equipment, such as a countercurrent wash zone in bottom of a digester or using a pressure diffuser wash or filter wash after the cook.

In figure 3a is disclosed schematically the pH profile trough a wood chip as exposed to the alkali chock charge after a prehydrolysis. The pH level at the core of the chips is as low as established in the prehydrolysis while the outer surface of the chip is exposed to the alkaline treatment liquor established.

In figure 3b is disclosed the reaction rate, i.e. consumption rate, of alkali during a

delignification process as a function of temperature. Here is disclosed the rule of thumb for kraft cooking where the reaction rate is doubled for each increase in temperature by 8^QC. If one starts with a typical cooking temperature of 140^QC this temperature corresponds to a reaction rate which establishes a base reference at 100%. If the cooking temperature is reduced in steps by 8^QC to 132, 126, 1 18 and 1 10^QC the reaction rate would decrease in steps to 50%, 25%, 12.5% and 6.25% respectively. Hence, it is of outmost importance to reduce the temperature if the core of the chip should be soaked with strong alkali, reducing consumption of alkali during diffusion due to lignin delignification reactions. Too high alkali consumption rate during the diffusion process may lead to alkali shortage in the core of the lignin containing cellulose material resulting in increased amounts of rejects after the subsequent kraft cook stage.

In figure 4 is shown the schematic difference in established pH level inside a wood chip if a diffusion of alkali is made into prehydrolysed wood material using either hot or cold alkaline treatment liquor, i.e. Τ_Ηοτ and T_COLD respectively. As could be seen in the right hand picture is the pH level reached after treatment in the cold alkaline liquor, T_COLD, the dotted line, much higher in the core of the cellulose material than if hot alkaline treatment liquor Τ_Ηοτ was used. The reason is due to the reduced alkali consumption rate during the diffusion process.

An even pH profile is of outmost importance for the subsequent cook which is shown in figure 5, where schematically the kappa number through the cooked wood chips is disclosed with the dotted line for hot respectively cold alkaline treatment. As could be seen in the left hand picture is the kappa number higher in the core, i.e. is undercooked UC, while the surface has much lower kappa number, i.e. is overcooked OC. This result in high reject amount from core parts and alkali degradation of the cellulose at the surface. In the right hand picture is shown a more even delignification with less difference between surface and core of wood material due to leveled out pH profile. Both left and right hand pictures resulting in same average kappa number H_Av but the cook conducted after treating with cold alkaline will result in a pulp containing higher alpha cellulose. In figure 6 is disclosed a three vessel continuous cooking system for prehydrolysis and cooking. The chips are first fed to a chip bin 1 and subsequent steaming vessel 2 during addition of steam ST for purging the chips from bound air. From the steaming vessel the steamed chips falls into a liquid filled chute above a high pressure sluice feeder 3, which pressurize the steamed chips and feed the formed slurry of chips in a feed flow 4 to the prehydrolysis vessel 10. Here the prehydrolysis vessel is in form of a steam-liquid phase digester having an inverted top separator 1 1 withdrawing a part of the transport liquid from line 4 back to start of feeding via A. As indicated is steam ST added to top of vessel 10, and optionally could also acid be added from source Ac. In bottom of the prehydrolysis vessel 10 is the cold and strong alkali charge added from source WL. This could be done by mixing in the alkali to the return flow B. The prehydrolysed wood material in its now alkaline treatment liquor is fed in line 14 to a succeeding pre extraction vessel 20, here in form of a hydraulically filled vessel with a downward feeding top separator 21 . After the necessary treatment time in the pre extraction vessel the alkaline wood material is fed to steam-liquid phase digester 30 via line 24, and excess transport fluid is withdrawn by an inverted top separator 31 and sent to C, which is added to bottom of the pre extraction vessel 20 as part of the transfer circulation. Here could a substantial part of the used alkaline treatment liquor be withdrawn to recovery from this return flow C. The kraft cook is established in the digester 30 and finally the prehydrolysed and cooked pulp is washed in a pressure diffuser 40. In figure 7 is disclosed a two vessel continuous cooking system for prehydrolysis and cooking. The difference here in relation to figure 6 is that the pre extraction vessel 20 is implemented as a first stage in the vessel 30, between the top separator 31 and a screen section from where a substantial part of the used alkaline treatment liquor is withdrawn to recovery REC1 . Fresh white liquor for the subsequent cooking phase is added via a central pipe at the level of this withdrawal screen.

In figures 8a to 8d are shown how the inventive method may be implemented in a batch digester in a 4 step sequence. Here is a steam prehydrolysing phase shown in figure 8a, where the steam phase is filling the vessel at the prehydrolyse temperature T_HYD- At ending of the prehydrolysis phase is cold white liquor added to the bottom and as shown in figure 8b which added white liquor catch the acidic condensate on the wood material as a layer PC in front of the rising cold white liquor level. In figure 8c is shown the later phase of the displacement of the cold white liquor trough the vessel, and here is also a larger volume of a mixture MX of cold white liquor and acidic condensate formed between the acidic

condensate PC and the rising cold white liquor. When the liquid level reaches an upper screen, it could be circulated back to the bottom as shown in figure 8d, and at least a part of the acidic condensate PC could be returned, either in the mixed fraction MX or also as a part of the acidic condensate. However, some or all of the acidic condensate PC may be sent to recovery as the purer fraction PC, while only the acidic condensate as contained in the mixture MX may be circulated. The switching from withdrawal to recirculation could be controlled by a pH sensor and/or a temperature sensor..

Claims

PATENT CLAIMS

1 . A process for the preparation of pulp from lignin-containing cellulosic material comprising prehydrolyzing said cellulosic material in a prehydrolysis stage at a temperature of between about 120^Q and 180^Q C and during at least 20 minutes so as to produce a prehydrolyzed cellulosic material and an acidic hydrolysate, adding a strong alkali chock charge to the mixture of prehydrolyzed cellulosic material and acidic hydrolysate to such extent that the residual alkali concentration after neutralization of the acidic hydrolysate is above 20 g/l EA as NaOH forming an alkaline treatment liquor, maintaining the prehydrolyzed cellulosic material in said alkaline treatment liquor for a sufficient time in an alkaline pre-extraction stage to reduce the alkali concentration by at least 10 g/l EA as NaOH but not to a concentration below 5 g/l EA as NaOH, whereby the dissolved carbohydrates as well as any lignin dissolved in the prehydrolysis stage are maintained dissolved during this alkaline pre-extraction stage and further carbohydrates and lignin are dissolved from the cellulosic material in the alkaline pre-extraction stage, thereafter transferring the cellulosic material from the alkaline pre-extraction stage to a kraft cooking stage, characterized in that the strong alkali chock charge is made using liquor volumes that decrease the temperature of the resulting alkaline treatment liquor for the prehydrolysed material by at least 10% in comparison to the temperature in the prehydrolysis stage, preferably at least 12^Q C if the prehydrolyse temperature is about 120 ^QC and at least 18^Q C if the prehydrolyse temperature is about 180^Q C, in order to reduce the alkali consumption during the cooking chemical diffusion process where the alkali treatment liquor penetrates to the core of the lignin-containing cellulosic material.

2. The process of claim 1 wherein the acidification of said prehydrolysis is established only by heating and optionally adding water, and without adding any external acidifiers, only using the wood acidity released during heating reaching a pH level below 5 during the prehydrolysis.

3. The process of claim 2 wherein the acidification of said prehydrolysis is established in a liquid filled phase.

4. The process of claim 3 wherein the temperature at end of the prehydrolysis is in the range 150-180^Q C.

5. The process of claim 4 wherein the temperature of the resulting alkaline treatment liquor for the prehydrolysed material is below 130^Q C.

6. The process of claim 1 wherein the acidification of said prehydrolysis is established by heating and addition of external acidifiers, reaching a pH level below 3 during the prehydrolysis.

7. The process of claim 6 wherein the acidification of said prehydrolysis is established in a liquid filled phase.

8. The process of claim 7 wherein the temperature at end of the prehydrolysis is in the range 120-165^QC.

9. The process of claim 8 wherein the temperature of the resulting alkaline treatment liquor for the prehydrolysed material is below 125^QC.

10. The process of claim 2 or 5 wherein the step of maintaining the prehydrolyzed cellulosic material in said alkaline treatment liquor is effected for a time period of from about 10 to 90 minutes.

1 1 . The process of claim 2 or 5 wherein at least 1 m³/ton of wood OD of the used alkaline treatment liquor is removed from said alkaline pre-extraction stage with its content of dissolved carbohydrates and lignin before onset of the kraft cooking stage, and thereafter adding alkali to the kraft cooking stage.

12. The process of claim 1 wherein the process is implemented in a continuous digester system using at least one vessel for the prehydrolysis and one vessel for the alkaline pre-extraction stage and the kraft cooking stage.

13. The process of claim 12 wherein the alkaline pre-extraction stage is implemented in a separate vessel and the kraft cooking stage in another separate vessel.