WO2020204793A1

WO2020204793A1 - Method for extracting hemicellulose from lignocellulosic material

Info

Publication number: WO2020204793A1
Application number: PCT/SE2020/050331
Authority: WO
Inventors: Lari Lammi; Jouni Karvonen; Susanna Minnaar; Hunphrey LANDMAN; Johann WAUTS
Original assignee: Valmet Ab
Priority date: 2019-04-01
Filing date: 2020-03-31
Publication date: 2020-10-08
Also published as: ZA202104454B; CN113646480B; BR112021012905A2; CN113646480A; SE1950402A1; EP3947809A1; EP3947809A4

Abstract

The present invention relates to a method for extracting hemicellulose from lignocellulosic material, the method comprising - providing (11) a quantity of lignocellulosic material comprising a first amount of hemicellulose, - determining (12) a desired amount of hemicellulose for extraction, wherein the desired amount of hemicellulose is determined (12) as a portion of the first amount of hemicellulose, - supplying (13) the lignocellulosic material to the treatment vessel, - performing (14) hydrolysis of the lignocellulosic material for extracting hemicellulose, and - supplying (15) a volume of displacement liquor, wherein the volume of displacement liquor is supplied until the desired amount of hemicellulose is displaced from the treatment vessel.

Description

METHOD FOR EXTRACTING HEMICELLULOSE FROM LIGNOCELLULOSIC MATERIAL

TECHNICAL FIELD

The present invention relates to a method for extracting hemicellulose from lignocellulosic material, wherein the lignocellulosic material is supplied to a treatment vessel and an acid hydrolysis is performed.

BACKGROUND

During production of dissolving pulp from lignocellulosic material it is generally desirable to remove hemicellulose in order to obtain a pulp with a high content of alpha-cellulose. This is also done for various other kinds of specialty chemical pulp.

The removal is performed by a prehydrolysis step where a treatment liquor such as steam or water is introduced and hemicellulose is hydrolysed from the wood chips during acid conditions. Afterwards, the slurry thus created is neutralized by an alkaline neutralizing liquor to produce a neutralized hydrolysate. Hemicellulose is broken down by the contact with alkali into smaller components and is removed from the slurry in subsequent washing stages during the pulp production.

In some applications, it is now desirable to preserve the hemicellulose in hydrolysate and remove it from the slurry for further use in other fields of industry such as the food industry, livestock industry, or biofuels. However, only a small part of available hemicellulose is removed using prior art methods. It is also difficult to keep the hemicellulose separate from the alkali added to the hydrolysate and used for neutralization, and to remove and preserve hemicellulose without lowering the efficiency of the pulp process.

Since the economic value of the finished pulp is at the moment significantly higher than that of the removed hemicellulose, any commercially viable process for extracting and removing hemicellulose cannot be allowed to impair the pulping process.

There is therefore a need for a method for extracting hemicellulose from lignocellulosic material that takes the above considerations and drawbacks into account and that enables removal of hemicellulose in a more efficient way while at the same time having a minimal impact on the process for producing pulp.

SUMMARY The object of the present invention is to eliminate or at least to minimize the problems discussed above. This is achieved by a method for extracting hemicellulose according to the appended independent claim.

The inventive method for extracting hemicellulose from lignocellulosic material comprises providing a quantity of lignocellulosic material comprising a first amount of hemicellulose, determining a desired amount of hemicellulose for extraction, wherein the desired amount of hemicellulose is determined as a portion of the first amount of hemicellulose. Further, the method comprises supplying the lignocellulosic material to the treatment vessel, performing hydrolysis of the lignocellulosic material for extracting hemicellulose, and supplying a volume of displacement liquor, wherein the volume of displacement liquor is supplied until the desired amount of hemicellulose is displaced from the treatment vessel.

Thereby, the amount of hemicellulose that is to be extracted can be decided and a suitable amount of displacement liquor used to make sure that that amount is removed after the hydrolysis. This also preserves the hemicellulose since the displacement prevents the exposure to a neutralizing liquor that would otherwise have broken the hemicellulose down into smaller molecules.

Suitably, steam and/or water is supplied as treatment liquor for the hydrolysis. Preferably, a combination is used where steam is first supplied and water added later, since this improves the extraction of hemicellulose from the lignocellulosic material to the slurry formed during hydrolysis and makes the process more economical due to the possibility of heating the water in a separate vessel before supplying it to the treatment vessel.

The volume of displacement liquor may advantageously be determined before being supplied to the treatment vessel by selecting a volume in which that portion of hemicellulose can be captured. Thereby, a desired percentage of the total amount of hemicellulose in the lignocellulosic material may be removed by adding a suitable volume based on known properties of the displacement liquor.

Suitably, the volume of displacement liquor is determined based on said desired amount of hemicellulose. Thereby, the volume of displacement liquor can be predetermined to facilitate the displacement of hemicellulose.

The method may advantageously be performed at least twice for different desired amounts of hemicellulose, and a volume of displacement liquor may be determined for displacing each of the desired amounts of hemicellulose. Thereby, information regarding the volume required for displacing different amounts of hemicellulose may be gathered. Suitably, a relationship between the volume of displacement liquor and the amount of hemicellulose displaced by said volume may also be determined. Thereby, the method can easier be performed since the volume of displacement liquor required to displace any desired amount of hemicellulose can be predicted.

Suitably, the volume of displacement liquor may thus be selected based on said relationship to determine the volume of displacement liquor required for displacing a new selected amount of hemicellulose. Thereby, the established relationship may be used to find out with greater accuracy how much displacement liquor should be used. This also makes it possible to disregard other factors such as pH, temperature, or solubility of hemicellulose in the displacement liquor, among others, so that the established relationship based on the method being performed repeatedly under the same conditions will be sufficient to perform the inventive method efficiently.

Suitably, the displacement liquor is at a temperature of 170 °C or less, preferably 160 °C or less and more preferably 140 °C or less when supplied to the treatment vessel. By supplying the treatment liquor at a lower temperature, the hemicellulose is preserved so that a breaking down of hemicellulose into smaller molecules takes place at a lesser degree or is prevented altogether. A larger quantity of hemicellulose can thus be removed from the treatment vessel for further use as biofuel or in the food industry.

The flow of displacement liquor into the treatment vessel is dependent on the treatment vessel volume but in general during displacement may be 200 1/s or less, preferably 150 1/s or less, and more preferably 100 1/s. A lower flow rate is advantageous in allowing an increased amount of hemicellulose to be dissolved in the displacement liquor so that a smaller quantity of fluid may be used. This has the benefit of being energy and cost efficient since a smaller volume of displacement liquor will require less energy for heating and recovery.

Many additional benefits and advantages of the present invention will be readily understood by the skilled person in view of the detailed description below.

DRAWINGS

The invention will now be described in more detail with reference to the appended drawings, wherein

Fig. 1 discloses schematically a method according to a preferred embodiment of the present invention; Fig. 2 discloses a second embodiment of the invention with the method applied to a treatment vessel for producing dissolving pulp in batches;

Fig. 3 discloses a third embodiment of the invention with the method applied to a treatment vessel for producing pulp through a continuous process; and

Fig. 4 discloses a graph relating an amount of hemicellulose in lignocellulosic material to a volume of displacement liquor in which it can be dissolved.

DETAILED DESCRIPTION

The method according to the present invention will now be described, followed by a description of one embodiment where the invention is applied in a batch cooking process and one embodiment where the invention is applied in a process with continuous cooking of pulp. The method is suitable for use with any process where lignocellulosic material undergoes hydrolysis as part of a pulping process.

The term lignocellulosic material is used herein to mean materials containing lignin, cellulose and hemicellulose. One example of such materials is wood, others include other agricultural or forestry wastes. The lignocellulosic material is commonly divided into small pieces, chips or fragments before the pulping process is initiated.

When producing pulp from lignocellulosic material, process parameters are chosen in order to arrive at a finished pulp with a defined quality in as cost efficient and time efficient a way as possible. Thus, the mill has limited possibility to change any process parameters and the finished pulp cannot be allowed to deviate from quality requirements.

Extracting, or rather removing, other substances such as hemicellulose has traditionally been seen as necessary in order to increase the quality of the pulp, but the possibility of using the hemicellulose for other purposes is a relatively new insight. It is well known that hemicellulose would need to be removed before neutralization takes place after the hydrolysis to avoid the degradation of hemicellulose that would otherwise result from the increase in pH during neutralization and subsequent processing of the pulp.

However, the pulping process is dominated by the need for the cost and time efficient production of pulp and although hemicellulose can now be used for a number of commercially viable purposes it would not be cost efficient to allow features of the pulping to be controlled by the desire to preserve and remove hemicellulose other than to a very limited degree. For this reason, properties of the hydrolysis must be determined from the pulping standpoint.

It is however possible to alter the volume of displacement liquor without affecting properties of the pulp in any negative way. Therefore, even where process parameters in general must be kept constant it is possible to vary the volume of displacement liquor and only incur an additional cost of heating and recovering the displacement liquor itself.

It is a particular advantage of the present invention that a desired amount of hemicellulose can be extracted and removed by using a volume of displacement liquor without having to also adjust process parameters of the pulping process to any great degree. Another significant advantage is that the quality of the finished pulp is also increased due to the removal of a larger portion of the available hemicellulose so that a hemicellulose content of the finished pulp is decreased.

The inventive method will now be described with reference to the appended drawings.

According to a preferred embodiment of the present invention, a quantity of lignocellulosic material is provided 1 1. The lignocellulosic material comprises a first amount of hemicellulose that may be previously known or may alternatively be determined by means of, e.g., chemical composition analysis in a laboratory.

A desired amount of the total amount of hemicellulose is determined 12. This desired amount can be expressed as a percentage of the total amount or can alternatively be expressed as a weight or by any other suitable quantification in order to relate it to the total amount of hemicelulose. In prior art methods, only a low percentage of hemicellulose is generally extracted but the present inventors have realized that the extracted amount can be significantly increased by using a volume of displacement liquor to displace the hemicellulose from the lignocellulosic material.

In the following, it will be described that hemicellulose is captured in treatment liquor and/or displacement liquor. This is to be understood as hemicellulose being present in the treatment liquor and/or displacement liquor in such a way that the hemicellulose may be transported from the treatment vessel along with the treatment liquor and/or displacement liquor during displacement. Captured hemicellulose may e.g. be dissolved in liquor or mixed in liquor, and in the following where terms like “dissolved” or “mixed” are used this is to be understood as the hemicellulose being captured. Thereby, the desired amount of hemicellulose can be selected as desired and the extraction can be ascertained by adapting the volume of displacement liquor so that the desired amount can be displaced by the displacement liquor and thereby be extracted and removed from a treatment vessel where the lignocellulosic material has undergone a hydrolysis.

In the preferred embodiment, at least 30 %, preferably at least 40 %, and more preferably 50 %-60 % of the total amount of hemicellulose in the lignocellulosic material may be selected as the desired amount. In some embodiments, a significantly higher percentage of the total amount of hemicellulose could instead be selected such as 70-75 %. Based on the desired amount, the volume of displacement liquor is selected as shown in Fig. 4 that discloses one example of a relationship between the desired amount of hemicellulose and a volume of displacement liquor required. The displacement liquor is in this embodiment water, but other liquids may also be suitable. To exemplify, for removing 50.7 % of an original hemicellulose content in the lignocellulosic material a volume of 2m³ of water per ton of pulp would be suitable; for removing 57.6 % that volume would instead be 4m³ and for removing 62.6 % a volume of 6m³ would be needed. The relationship between the removed amount of hemicellulose and the displacement liquor required to remove it may be established by performing the method according to the present invention repeatedly, as will be described further below.

Determining the desired amount of hemicellulose and the volume of displacement liquor required can take place at any time before introduction of the displacement liquor into the treatment vessel. In the preferred embodiment, the desired amount of hemicellulose is determined before introduction of the lignocellulosic material into the treatment vessel, but it is to be noted that this step of the method can be performed at any time after the quantity of lignocellulosic material has been decided and before introduction of the displacement liquor into the treatment vessel. Similarly, the total amount of hemicellulose in the lignocellulosic material can also be determined at any time before the step of deciding the desired amount and the corresponding volume of displacement liquor. In some embodiments, the total amount of hemicellulose in the lignocellulosic material is measured but in other embodiments the total amount may instead be determined by knowledge of an average or typical amount of hemicellulose in the type of lignocellulosic material used.

In the preferred embodiment, the volume of displacement liquor is generally selected by adding displacement liquor until the desired amount of hemicellulose has been removed from the treatment vessel. This can be established e.g. by monitoring a hemicellulose content in the displacement liquor after it has passed from the treatment vessel.

The lignocellulosic material is supplied 13 to the treatment vessel and the treatment liquor is added so that hydrolysis of the lignocellulosic material can be performed 14. The treatment liquor may be a gas or a liquid that serves to create acidic conditions in the treatment vessel. In some embodiments, the gas or liquid itself has a low pH, but in the preferred embodiment the gas or liquid is water or condensate in the form of steam or fluid and the acidic conditions are created by the lignocellulosic material itself releasing acidic substances when in contact with the treatment liquor at elevated temperatures. This is well known within the art and will not be described in detail herein.

Advantageously, the treatment liquor is first added in the form of steam and then in the form of water. The steam will extract hemicellulose and other substances from the lignocellulosic material in a more efficient way than a liquid. This facilitates hydrolysis and shortens the time needed to arrive at a desired P-factor.

As used herein, the term P-factor signifies how far a hydrolysis has progressed and is determined using the following formula, where T is a temperature in Kelvin and t is a treatment time in hours.

In this embodiment, the desired P-factor is anywhere in the range of 50- 1500, which may be achieved by performing the first treatment for 60-90 minutes at a temperature of 165- 170 °C (438-443 K). As an example, when using eucalyptus wood as lignocellulosic material, a suitable P-factor is 400- 500.

As described above, the P-factor deemed suitable will be the P-factor that is most suitable for producing pulp from the lignocellulosic material. It is a particular advantage of the present invention that a desired amount of hemicellulose may be removed from the lignocellulosic material or the pulp regardless of the P-factor selected for the hydrolysis of the lignocellulosic material as long as that P-factor is actually suitable for producing pulp.

The water added as treatment liquor after the steam is preferably heated to prevent a lowering of the temperature inside the treatment vessel. During hydrolysis, hemicellulose is hydrolysed from the lignocellulosic material and forms part of a slurry that comprises the lignocellulosic material and treatment liquor. In order to remove the desired amount of hemicellulose, the volume of displacement liquor is supplied to the treatment vessel to displace material in the slurry from the treatment vessel. That material comprises hemicellulose and also other substances that are in liquid form or that are dissolved in liquid of the slurry. Larger particles such as the wood matrix of the hydrolyzed lignocellulosic material will be retained inside the treatment vessel.

After the treatment liquor and the substances dissolved or mixed therein have been removed, displacement continues by the displacement liquor passing through the treatment vessel. The displacement liquor is then able to pass through the larger particles of the lignocellulosic material, e.g. the wood matrix, by diffusion and to extract additional hemicellulose that was not already dissolved or mixed in the treatment liquor. This means that removal of hemicellulose can continue after the treatment liquor has been displaced. It also means that the removal of hemicellulose can be controlled at least partly by selecting properties of the displacement liquor such as temperature, pH value or the flow of the displacement liquor. The most important factor is however the volume of displacement liquor used.

The volume of displacement liquor that is suitable for removing a desired amount of hemicellulose can be selected in a number of different ways. In the most general form of the present invention, a volume of displacement liquor is used for displacement until the desired amount of hemicellulose has been removed, and this can be determined by monitoring the amount of hemicellulose that has been removed from the treatment vessel. For instance, sensor means could be provided in conduits through which the treatment liquor, displacement liquor and all substances that have been dissolved or mixed with them passes. Alternatively, sensor means could be provided in a separate vessel into which those liquors are transported. In other embodiments, the amount of hemicellulose that has been removed from the treatment vessel can be determined in any other suitable way.

In another embodiment, the volume of displacement liquor needed to displace the desired amount of hemicellulose can be determined through knowledge of how large a volume of displacement liquor is required to capture the hemicellulose. Also, properties of the treatment liquor or of the pulping process may be taken into account.

In yet another embodiment, the method according to the invention is performed at least twice but preferably a plurality of times for different amounts of hemicellulose in order to determine the volume of displacement liquor required. This is advantageous in decreasing the need for knowing or controlling other parameters so that knowledge of the volume of displacement liquor can be used to extract and remove the desired amount of hemicellulose in later batches of pulp or in later instances in a continuous cooking of pulp.

It is even more advantageous to use information from such repeated performance of the inventive method in order to establish a relationship between the desired amount of hemicellulose to be removed and the displacement liquor needed for that. Such a relationship is shown in Fig. 4 and means that regardless of the particular parameters present when the relationship was established it can be expected that the relationship remains true as long as no significant process parameters or parameters of the lignocellulosic material are altered.

In practice, this means that the present invention can be used at any site where pulp is produced in order to first establish a relationship between the amount of hemicellulose and the volume of displacement liquor and that this relationship can be used to select the volume of displacement liquor for any desired amount of hemicellulose. The method may then be used to remove hemicellulose in following pulping processes using the same or similar process parameters such as P-factor, temperature, pH, treatment liquor, and type of lignocellulosic material, among others. If any of these parameters or other is changed or if some time has passed since the relationship was established so that it may be suspected that it may have changed due to drift or due to unforeseen circumstances, the method is advantageously repeated in order to establish a different relationship or to verify the existing relationship.

After displacement, an alkaline liquid such as white liquor is supplied to increase the pH value and create alkaline conditions in the treatment vessel. Treatment of the lignocellulosic material is then continued in order to produce pulp as is well known within the art.

In the description above, addition of lignocellulosic material and liquids and removal of liquids after displacement have been discussed in a general form that is not limited to pulp produced through a batch process or through a continuous process. Further below, embodiments describing both will be shown in more detail, but the inventive method is not limited to any given production method and can be used in any process where lignocellulosic material undergoes hydrolysis at acidic conditions so that hemicellulose is extracted.

In order to preserve the hemicellulose removed from the lignocellulosic material and prevent it from being broken down into smaller molecules, it is advantageous to supply the displacement liquor at a temperature of 170 °C or less, preferably 160 °C or less and more preferably 140 °C or less. In some embodiments as low a temperature as 120 °C or less may even be used. Generally, there is a desire within the technical field of pulping to prevent cooling of the pulp during the process since the cost of re-heating the pulp for subsequent process stages is high. However, the present inventors have realized that in order to maintain the hemicellulose in its original form it is actually advantageous to lower the temperature during displacement and also during subsequent storage and treatment of the hemicellulose.

The flow of displacement liquor into the treatment vessel during displacement is preferably also controlled and may be kept at 300 1/s, preferably 150 1/s or more preferably 100 1/s in order to facilitate removal of hemicellulose. By keeping the flow rate low an increased amount of hemicellulose may be dissolved in the displacement liquor so that the total amount of fluid required is minimized.

The second embodiment where the inventive method is used in a pulping process using batch cooking of lignocellulosic material to produce dissolving pulp will now be described with reference to Fig. 2. Although this embodiment specifically describes the production of dissolving pulp, it is to be noted that it could be applied to any process where lignocellulosic material is treated in batches in a pulping process.

Fig. 2 shows a treatment vessel 2 in the form of a batch digester having a top 22, a middle 23 and a bottom 24. Also provided are an upper inlet 21 and a lower inlet 25, along with a plurality of screens for inserting and/or withdrawing liquid to and from the treatment vessel 2. In this embodiment, there are a feed screen FS, a top screen TS and a middle screen MS but alternatively there could be fewer or more screens located at the same or other parts of the digester 20. Pumps may be located in withdrawal lines from any or all of the screens but are not shown in the figure.

In this embodiment, lignocellulosic material is fed into the digester 20 through the upper inlet 21 and steam may be supplied through the feed screen FS during filling of the digester 20. The bottom inlet 25 is used for adding steam and different liquors during operation of the digester and liquors may also be added through the middle screen MS and/or top screen TS. Generally, the top screen TS is used for withdrawing displaced liquors and also for withdrawing gases. The middle screen MS is used for withdrawing and circulating cooking liquor during cooking of the lignocellulosic material.

When producing pulp in batches in the digester 20, lignocellulosic material is fed into the digester 20 and undergoes hydrolysis at acidic conditions, followed by a neutralization and a cooking to arrive at a dissolving pulp that is discharged from the digester 20 and that may then undergo later process stages to further treat the pulp. This general process is already well known to the skilled person and in the following, only the hydrolysis will be described with reference to the method of the present invention.

A batch digester may have a volume of about 100-500m³ in order to treat a volume of 20- 150 ton of dry lignocellulosic material, but the capacity may of course vary depending on technical as well as commercial factors for each batch digester.

Thus, lignocellulosic material is fed into the digester 20 through the upper inlet 21 and generally steam is injected though the feed screen FS during filling in order to add a spreading motion to the lignocellulosic material so that an even upper surface of lignocellulosic material inside the digester 20 is obtained.

A treatment liquor is added, and in this embodiment, steam is used as a first treatment liquor followed by a second treatment liquor in the form of water. Alternatively, only one of these liquids could be used or another suitable treatment liquor added instead as already described above with reference to Fig. 1.

The steam is added through the lower inlet 25 and temperature rises through the digester 20 to contact all parts of the lignocellulosic material and initiate hydrolysis. In this embodiment, the hydrolysis is at a temperature of 165- 170 °C but other temperatures could alternatively be used. The steam temperature is generally higher than the hydrolysis temperature. It is advantageous from an economical perspective to add the steam at first a lower pressure of about 2.5-4 bar and change when the temperature rises to a higher-pressure steam of about 10- 12 bar. Other pressure intervals may also be suitable.

After steam hydrolysis, water or condensate is added through the lower inlet 25 for a water hydrolysis. Preferably, the water is pre-heated and pressurized so that it matches the temperature during steam hydrolysis. The volume of water added as treatment liquor should fill up the digester 20 to cover the lignocellulosic material. Generally, the digester is hydraulically full but it can also be partially full while still submerging all the chips and taking into account a chip bed drop during the steam hydrolysis. Hydrolysis is performed until a desired P-factor has been reached and the lignocellulosic material and treatment liquor form a slurry inside the digester 20.

Either before or during hydrolysis in the digester 20, the volume of displacement liquor that should be used to remove the desired amount of hemicellulose from the digester 20 is determined. This may be determined as described above. Fig. 4 shows a graph that discloses one example of a relationship between the amount of hemicellulose to be removed and the volume of displacement liquor needed to achieve this. In the Figure, the volume of water (in m³ per ton of lignocellulosic material) required to extract a desired amount (in % of the total amount) of hemicellulose in the lignocellulosic material. If the desired amount is 50.7 %, 2 m³ would be needed, whereas extraction of 57.6 % requires 4 m³ and extraction of 62.6 % requires 6 m³.

After hydrolysis is completed or at a predetermined time before the final P- factor is reached, the volume of displacement liquor is inserted through the lower inlet 25 so that fluid in the slurry is displaced through the top screen TS. This can be seen as a wash step that washes away hemicellulose and also the acidic fluid in the slurry.

After displacement, a water pad remains in the slurry and a neutralizing liquor such as white liquor is added through the lower inlet 25 in order to raise the pH in the digester 20 before cooking takes place. After cooking, the pulp is discharged from the digester 20 through the lower inlet 25 that also serves as an outlet.

Fig. 3 discloses a third embodiment where the method according to the present invention is used in a continuous pulping process. A digester 30 that is used for hydrolysis of the lignocellulosic material comprises a top 32, a middle 33 and a bottom 34 and has an upper inlet 31 and a lower outlet 35. Also provided are an upper screen US and a lower screen LS for withdrawing liquor from the digester 30 above and below the middle portion 33.

During operation, lignocellulosic material is continuously fed into the digester 30 through the upper inlet 31 along with the treatment liquor that may be supplied through the upper inlet 31 or through separate fluid inlets (not shown) that may be located at the top 32 or between the top 32 and the middle 33. The lignocellulosic material moves downwards through the digester 30 and at the same time undergoes hydrolysis so that a finished slurry can continuously be discharged through the lower outlet 35 and be transported to a second digester (not shown) where cooking takes place.

As in the first and second embodiment described above, both steam and water are preferably used as treatment liquor. Preferably, the steam is supplied at the top and the water is supplied through at least one fluid inlet placed between the top 32 and the middle 33. As the slurry proceeds downwards, the lignocellulosic material undergoes hydrolysis so that the desired P-factor is reached at a desired part of the digester 30. This can be at the middle 33 in this embodiment, but in other embodiments the hydrolysis can be controlled so that hydrolysis is completed and the desired P-factor is reached at any other part of the digester that is deemed suitable. Beneath the part where hydrolysis is completed, the displacement liquor is supplied and allowed to flow through the slurry before being withdrawn. Preferably, the displacement liquor is supplied through a fluid inlet near the lower outlet 35 and withdrawn through the upper or lower screen US, LS. If desired, the discharge liquor can be added through a fluid inlet placed above the bottom outlet 35 and a separate neutralizing liquor can be added at the bottom for neutralizing the slurry before discharge from the digester 30.

Based on factors such as the flow of lignocellulosic material into the digester, the desired amount of hemicellulose may be determined and also the volume of displacement liquor is determined in the same way as described above. When using the inventive method with a continuous treatment process, instead of determining the volume of displacement liquor for extracting the desired amount from a batch of lignocellulosic material, a volume of displacement liquor that should be supplied per unit of time is determined so that the desired amount of hemicellulose can be extracted from the amount of lignocellulosic material supplied per time unit. Thus, if the flow of lignocellulosic material into the digester 30 is at a particular volume or weight per second, the volume of displacement liquor for removing the desired amount of hemicellulose from that amount of lignocellulosic material should be supplied at a desired flow. A dynamic control of the displacement liquor depending on the flow of lignocellulosic material into the digester 30 is therefore advantageous so that the extraction of hemicellulose can be achieved, and a hemicellulose content of the slurry discharged from the digester 30 can be maintained at a stable level. If the amount of hemicellulose differs greatly from time to time in the discharged slurry, the quality of the finished pulp may be affected, and this is generally undesirable.

Fluid inlets for the supply of treatment liquors and displacement liquor and screens for the withdrawal of liquor from the digester 30 may be placed at different levels of the digester 30 to suit the needs of each particular pulping process where the inventive method is applied. In general, however, it is advantageous to supply the treatment liquor or liquors at or near the top 32 of the digester 30 and possibly to add further treatment liquor further down along the digester 30. It is also advantageous to control the treatment of lignocellulosic material so that hydrolysis is completed at a predetermined level in the digester 30 and to circulate withdrawal liquor in parts of the digester 30 that are lower than the level where hydrolysis is completed. Preferably, the withdrawal liquor is supplied at or near the bottom 34 and it is withdrawn at a higher level that may be at or close to the middle 33. That way, the part of the digester between the top 32 and middle 33 will hold the slurry that is undergoing hydrolysis whereas the part of the digester between the middle 33 and the bottom 34 will be where displacement of hemicellulose is performed and possibly also where neutralization liquors are added before discharge of the slurry through the bottom outlet 35.

For all the embodiments described herein a control unit may be used to control operation of the digesters 20, 30 and to receive information regarding an amount or volume of lignocellulosic material that enter the digesters 20, 30. The control unit may further be configured to determine the volume of displacement liquor required for extracting hemicellulose as described herein and based on the various factors and parameters described above. Furthermore, the control unit may continuously receive signals that give information regarding the hydrolysis taking place in the digesters 20, 30 and the control unit may use such information to determine and perform adjustments to the operation of the digesters 20, 30.

It is to be noted that features from the various embodiments described herein may freely be combined, unless it is explicitly stated that such a combination would be unsuitable.

Claims

1. Method for extracting hemicellulose from lignocellulosic material, the method comprising

providing ( 1 1) a quantity of lignocellulosic material comprising a first amount of hemicellulose,

determining ( 12) a desired amount of hemicellulose for extraction, wherein the desired amount of hemicellulose is determined ( 12) as a portion of the first amount of hemicellulose, supplying ( 13) the lignocellulosic material to the treatment vessel,

performing ( 14) hydrolysis of the lignocellulosic material for extracting hemicellulose, and

supplying ( 15) a volume of displacement liquor, wherein the volume of displacement liquor is supplied until the desired amount of hemicellulose is displaced from the treatment vessel.

2. Method according to claim 1, wherein steam and/or water and/or condensate is supplied as treatment liquor for the hydrolysis.

3. Method according to claim 1 or 2, wherein the volume of displacement liquor is determined before being supplied ( 15) to the treatment vessel by selecting a volume in which that portion of hemicellulose can be captured.

4. Method according to claim 1 or 2, wherein the volume of displacement liquor is determined based on said desired amount of hemicellulose.

5. Method according to claim 1 or 2, further comprising performing the method at least twice for different desired amounts of hemicellulose and determining a volume of displacement liquor for displacing each of the desired amounts of hemicellulose.

6. Method according to claim 5, further comprising determining a relationship between the volume of displacement liquor and the amount of hemicellulose displaced by said volume.

7. Method according to claim 6, further comprising selecting the volume of displacement liquor based on said relationship to determine the volume of displacement liquor required for displacing a new selected amount of hemicellulose.

8. Method according to any previous claim, wherein the displacement liquor is at a temperature of 170 °C or less, preferably 160 °C or less and more preferably 140 °C or less when supplied ( 15) to the treatment vessel.

9. Method according to any previous claim, wherein a flow of displacement liquor into the treatment vessel during displacement is 200 1/s or less, preferably 150 1/s or less, and more preferably 100 1/s.