SE1950402A1 - Method for extracting hemicellulose from lignocellulosic material - Google Patents

Abstract

The present invention relates to a method for extracting hemicellulose from lignocellulosic material, the method comprisingproviding a quantity of lignocellulosic material comprising a first amount of hemicellulose,determining a desired amount of hemicellulose for extraction, and based on said desired amount of hemicellulose determining a volume of displacement liquor for displacing said desired amount from a treatment vessel,- supplying the lignocellulosic material to the treatment vessel, performing hydrolysis of the lignocellulosic material for extracting hemicellulose, andsupplying the volume of displacement liquor for displacing the desired amount of hemicellulose from the treatment vessel.

Description

METHOD FOR EXTRACTING HEMICELLULOSE FROM LIGNOCELLULOSICMATERIAL TECHNICAL FIELD The present invention relates to a method for extracting hemicellulose fromlignocellulosic material, wherein the lignocellulosic material is supplied to atreatment vessel and an acid hydrolysis is performed.

BACKGROUND During production of dissolving pulp from lignocellulosic material it isgenerally desirable to remove hemicellulose in order to obtain a pulp with ahigh content of alphacellulose. This is also done for various other kinds ofspecialty chemical pulp.

The removal is performed by a prehydrolysis step where a treatment liquorsuch as steam or water is introduced and hemicellulose is hydrolysed fromthe wood chips during acid conditions. Afterwards, the slurry thus created isneutralized by an alkaline neutralizing liquor to produce a neutralizedhydrolysate. Hemicellulose is broken down by the contact with alkali intosmaller components and is removed from the slurry in subsequent washingstages during the pulp production.

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

Since the economic value of the finished pulp is at the moment significantlyhigher than that of the removed hemicellulose, any commercially viableprocess for extracting and removing hemicellulose cannot be allowed toimpair the pulping process.

There is therefore a need for a method for extracting hemicellulose fromlignocellulosic material that takes the above considerations and drawbacksinto account and that enables removal of hemicellulose in a more efficientway while at the same time having a minimal impact on the process forproducing pulp.

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

The inventive method for eXtracting hemicellulose from lignocellulosicmaterial comprises providing a quantity of lignocellulosic materialcomprising a first amount of hemicellulose, determining a desired amount ofhemicellulose for extraction, and based on said desired amount ofhemicellulose determining a volume of displacement liquor for displacingsaid desired amount from a treatment vessel. Further, the method comprisessupplying the lignocellulosic material to the treatment vessel, performinghydrolysis of the lignocellulosic material for eXtracting hemicellulose, andsupplying the volume of displacement liquor for displacing the desiredamount of hemicellulose from the treatment vessel. Thereby, the amount ofhemicellulose that is to be eXtracted can be decided and a suitable amountof displacement liquor selected to make sure that that amount is removedafter the hydrolysis. This also preserves the hemicellulose since thedisplacement prevents the exposure to a neutralizing liquor that wouldotherwise have broken the hemicellulose down into smaller molecules.

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

The desired amount of hemicellulose may advantageously be determined asa portion of the first amount of hemicellulose and the volume ofdisplacement liquor may be determined by selecting a volume in which thatportion can be dissolved. Thereby, a desired percentage of the total amountof hemicellulose in the lignocellulosic material may be removed by adding asuitable volume based on known properties of the displacement liquor.

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 tothe treatment vessel. By supplying the treatment liquor at a lowertemperature the hemicellulose is preserved so that a breaking down ofhemicellulose into smaller molecules takes place at a lesser degree or isprevented altogether. A larger quantity of hemicellulose can thus be removedfrom 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 thetreatment vessel volume but in general during displacement may be 200 l/ sor less, preferably 150 l/s or less, and more preferably 100 l/s. A lower flow 2 rate is advantageous in allowing an increased amount of hemicellulose to bedissolved in the displacement liquor so that a smaller quantity of fluid maybe used. This has the benefit of being energy and cost efficient since asmaller volume of displacement liquor will require less energy for heatingand recovery.

Many additional benefits and advantages of the present invention will bereadily understood by the skilled person in view of the detailed descriptionbelow.

DRAWINGS The invention will now be described in more detail with reference to theappended drawings, wherein Fig. 1 discloses schematically a method according to a preferredembodiment of the present invention; Fig. 2 discloses a second embodiment of the invention with the methodapplied to a treatment vessel for producing dissolving pulp in batches; Fig. 3 discloses a third embodiment of the invention with the methodapplied to a treatment vessel for producing pulp through a continuousprocess; and Fig. 4 discloses a graph relating an amount of hemicellulose inlignocellulosic material to a volume of displacement liquor in which it can bedissolved.

DETAILED DESCRIPTION The method according to the present invention will now be described,followed by a description of one embodiment where the invention is appliedin a batch cooking process and one embodiment where the invention isapplied in a process with continuous cooking of pulp. The method is suitablefor use with any process where lignocellulosic material undergoes hydrolysisas part of a pulping process.

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

According to a preferred embodiment of the present invention, a quantity oflignocellulosic material is provided 11. The lignocellulosic material comprisesa first amount of hemicellulose that may be previously known or may alternatively be determined by means of e. g chemical composition analysis ina 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 orcan alternatively be expressed as a weight or by any other suitablequantification. In prior art methods, only a low percentage of hemicelluloseis generally extracted but the present inventors have realized that theextracted amount can be selected based on how hemicellulose is dissolved orcaptured in a displacement liquor. Thereby, the desired amount ofhemicellulose can be selected as desired and the extraction can beascertained by adapting the volume of displacement liquor so that thedesired amount can be dissolved or captured in the displacement liquor andthereby be extracted from a treatment vessel where the lignocellulosicmaterial has undergone a hydrolysis.

In the preferred embodiment, at least 30%, preferably at least 40%, andmore preferably 50%-60% of the total amount of hemicellulose in thelignocellulosic material may be selected as the desired amount. In someembodiments, a signif1cantly higher percentage of the total amount ofhemicellulose could instead be selected such as 70-75%. Based on thedesired amount, the volume of displacement liquor is selected as shown inFig. 4 that discloses one example of a relationship between the desiredamount of hemicellulose and a volume of displacement liquor required. Thedisplacement liquor is in this embodiment water but other liquids may alsobe suitable. To exemplify, for removing 50.7% of an original hemicellulosecontent in the lignocellulosic material a volume of 2m3 of water per ton ofpulp would be suitable; for removing 57.6% that volume would instead be4m3 and for removing 62.6% a volume of 6m3 would be needed.

Determining the desired amount of hemicellulose and the volume ofdisplacement liquor required can take place at any time before introductionof the displacement liquor into the treatment vessel. In the preferredembodiment, the desired amount of hemicellulose is determined beforeintroduction of the lignocellulosic material into the treatment vessel, but it isto be noted that this step of the method can be performed at any time afterthe quantity of lignocellulosic material has been decided and beforeintroduction of the displacement liquor into the treatment vessel. Similarly,the total amount of hemicellulose in the lignocellulosic material can also bedetermined at any time before the step of deciding the desired amount andthe corresponding volume of displacement liquor. In some embodiments, thetotal amount of hemicellulose in the lignocellulosic material is measured butin other embodiments the total amount may instead be determined by knowledge of an average or typical amount of hemicellulose in the type oflignocellulosic material used.

The lignocellulosic material is supplied 13 to the treatment vessel and atreatment liquor is added so that hydrolysis of the lignocellulosic materialcan be performed 14. The treatment liquor may be a gas or a liquid thatserves to create acidic conditions in the treatment vessel. In someembodiments, the gas or liquid itself has a low pH, but in the preferredembodiment the gas or liquid is water or condensate in the form of steam orfluid and the acidic conditions are created by the lignocellulosic materialitself releasing acidic substances when in contact with the treatment liquorat elevated temperatures. This is well known within the art and will not bedescribed in detail herein.

Advantageously, the treatment liquor is first added in the form of steam andthen in the form of water. The steam will eXtract hemicellulose and othersubstances from the lignocellulosic material in a more efficient way than aliquid. This facilitates hydrolysis and shortens the time needed to arrive at adesired P-factor.

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

V _ ._ i' _*I:_ __ i* 4~_,.~ï\._ Q, *_ššü ,iššïšff§ï"fšflfšïg i š lï å? s šÜ-ï 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 60-90minutes at a temperature of 165-170°C (438-443 K). As an example, whenusing eucalyptus wood as lignocellulosic material, a suitable P-factor is 400-500.

The water added as treatment liquor after the steam is preferably heated toprevent a lowering of the temperature inside the treatment vessel.

During hydrolysis, hemicellulose is hydrolysed from the lignocellulosicmaterial and forms part of a slurry that comprises the lignocellulosicmaterial and treatment liquor. In order to remove the desired amount ofhemicellulose, the volume of displacement liquor is supplied to the treatmentvessel to displace material in the slurry from the treatment vessel. Thatmaterial comprises hemicellulose and also other substances that are inliquid form or that are dissolved in liquid of the slurry. Larger particles such as the wood matrix of the hydrolyzed lignocellulosic material Will be retainedinside the treatment vessel.

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

In the description above, addition of lignocellulosic material and liquids andremoval of liquids after displacement have been discussed in a general formthat is not limited to pulp produced through a batch process or through acontinuous process. Further below, embodiments describing both will beshown in more detail but the inventive method is not limited to any givenproduction method and can be used in any process where lignocellulosicmaterial undergoes hydrolysis at acidic conditions so that hemicellulose isextracted.

In order to preserve the hemicellulose removed from the lignocellulosicmaterial and prevent it from being broken down into smaller molecules, it isadvantageous to supply the displacement liquor at a temperature of 170°Cor less, preferably 160°C or less and more preferably 140°C or less. In someembodiments 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 preventcooling of the pulp during the process since the cost of re-heating the pulpfor subsequent process stages is high. However, the present inventors haverealized that in order to maintain the hemicellulose in its original form it isactually advantageous to lower the temperature during displacement andalso during subsequent storage and treatment of the hemicellulose.

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

By increasing the ability of hemicellulose to be captured in the displacementliquor, a higher amount of hemicellulose can be removed from the treatmentvessel without increasing the volume of displacement liquor. This also meansthat a smaller volume of displacement liquor is needed for extracting thedesired amount of hemicellulose, giving the benefit of reducing waterconsumption of the inventive method. Since the volume of liquids used inpulping processes is generally kept as small as possible for energy efficiencyand cost efficiency reasons, both water and energy can be saved byincreasing the dissolving of hemicellulose. 6 The second embodiment where the inventive method is used in a pulpingprocess using batch cooking of lignocellulosic material to produce dissolvingpulp will now be described with reference to Fig. 2. Although thisembodiment specif1cally describes the production of dissolving pulp, it is tobe noted that it could be applied to any process where lignocellulosicmaterial is treated in batches in a pulping process.

Fig. 2 shows a treatment vessel 2 in the form of a batch digester having a top22, a middle 23 and a bottom 24. Also provided are an upper inlet 21 and alower inlet 25, along with a plurality of screens for inserting and/ orwithdrawing 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 butalternatively there could be fewer or more screens located at the same orother parts of the digester 20. Pumps may be located in withdrawal linesfrom any or all of the screens but are not shown in the Figure.

In this embodiment, lignocellulosic material is fed into the digester 20through the upper inlet 21 and steam may be supplied through the feedscreen FS during f1lling of the digester 20. The bottom inlet 25 is used foradding steam and different liquors during operation of the digester andliquors may also be added through the middle screen MS and/ or top screenTS. Generally, the top screen TS is used for withdrawing displaced liquorsand also for withdrawing gases. The middle screen MS is used forwithdrawing and circulating cooking liquor during cooking of thelignocellulosic material.

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

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

Thus, lignocellulosic material is fed into the digester 20 through the upperinlet 21 and generally steam is injected though the feed screen FS duringf1lling in order to add a spreading motion to the lignocellulosic material sothat an even upper surface of lignocellulosic material inside the digester 20is obtained.

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

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

After steam hydrolysis, water or condensate is added through the lower inlet25 for a water hydrolysis. Preferably, the water is pre-heated and pressurizedso that it matches the temperature during steam hydrolysis. The volume ofwater added as treatment liquor should fill up the digester 20 to cover thelignocellulosic material. Generally, the digester is hydraulically full but it canalso be partially full while still submerging all the chips and taking intoaccount a chip bed drop during the steam hydrolysis. Hydrolysis isperformed until a desired P-factor has been reached and the lignocellulosicmaterial and treatment liquor form a slurry inside the digester 20.

Either before or during hydrolysis in the digester 20, the volume ofdisplacement liquor that should be used to remove the desired amount ofhemicellulose from the digester 20 is determined. This may be calculateddepending on factors such as the amount of lignocellulosic material,properties of the wood used for the lignocellulosic material, and factors thatincreases dissolving of hemicellulose such as pH, flow or temperature, andalso depending on the volume of water used as treatment liquor and alreadypresent inside the digester 20. Fig. 4 shows a graph that discloses thevolume of water (in m3 per ton of lignocellulosic material) required to eXtracta desired amount (in % of the total amount) of hemicellulose in thelignocellulosic material. If the desired amount is 50.7%, 2 m3 would beneeded, 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 thelower 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 andalso the acidic fluid in the slurry.

After displacement, a water pad remains in the slurry and a neutralizingliquor such as white liquor is added through the lower inlet 25 in order toraise the pH in the digester 20 before cooking takes place. After cooking, thepulp is discharged from the digester 20 through the lower inlet 25 that alsoserves as an outlet.

Fig. 3 discloses a third embodiment where the method according to thepresent invention is used in a continuous pulping process. A digester 30 thatis used for hydrolysis of the lignocellulosic material comprises a top 32, amiddle 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 withdrawingliquor from the digester 30 above and below the middle portion 33.

During operation, lignocellulosic material is continuously fed into thedigester 30 through the upper inlet 31 along with the treatment liquor thatmay 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 themiddle 33. The lignocellulosic material moves downwards through thedigester 30 and at the same time undergoes hydrolysis so that a finishedslurry can continuously be discharged through the lower outlet 35 and betransported to a second digester (not shown) where cooking takes place.

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

Based on the flow of lignocellulosic material into the digester, the desiredamount of hemicellulose is determined and also the volume of displacementliquor in the same way as described above. When using the inventive method 9 with a continuous treatment process, instead of determining the volume ofdisplacement liquor for extracting the desired amount from a batch oflignocellulosic material, a volume of displacement liquor that should besupplied per unit of time is determined so that the desired amount ofhemicellulose can be eXtracted from the amount of lignocellulosic materialsupplied per time unit. Thus, if the flow of lignocellulosic material into thedigester 30 is at a particular volume or Weight per second, the volume ofdisplacement liquor for removing the desired amount of hemicellulose fromthat amount of lignocellulosic material should be supplied at a desired flow.A dynamic control of the displacement liquor depending on the flow oflignocellulosic material into the digester 30 is therefore advantageous so thatthe extraction of hemicellulose can be achieved and a hemicellulose contentof the slurry discharged from the digester 30 can be maintained at a stablelevel. If the amount of hemicellulose differs greatly from time to time in thedischarged slurry, the quality of the finished pulp may be affected and this isgenerally undesirable.

Fluid inlets for the supply of treatment liquors and displacement liquor andscreens for the Withdrawal of liquor from the digester 30 may be placed atdifferent levels of the digester 30 to suit the needs of each particular pulpingprocess where the inventive method is applied. In general, however, it isadvantageous to supply the treatment liquor or liquors at or near the top 32of the digester 30 and possibly to add further treatment liquor further downalong the digester 30. It is also advantageous to control the treatment oflignocellulosic material so that hydrolysis is completed at a predeterminedlevel in the digester 30 and to circulate withdrawal liquor in parts of thedigester 30 that are lower than the level where hydrolysis is completed.Preferably, the withdrawal liquor is supplied at or near the bottom 34 and itis withdrawn at a higher level that may be at or close to the middle 33. Thatway, the part of the digester between the top 32 and middle 33 will hold theslurry that is undergoing hydrolysis whereas the part of the digester betweenthe middle 33 and the bottom 34 will be where displacement of hemicelluloseis performed and possibly also where neutralization liquors are added beforedischarge of the slurry through the bottom outlet 35.

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

It is to be noted that features from the various embodiments describedherein may freely be combined, unless it is explicitly stated that such acombination Would be unsuitable. 11

Claims (5)

1. Method for extracting hemicellulose from lignocellulosic material,the method comprising - providing (11) a quantity of lignocellulosic material comprising afirst amount of hemicellulose, - determining (12) a desired amount of hemicellulose for extraction,and based on said desired amount of hemicellulose determining a volume ofdisplacement liquor for displacing said desired amount from a treatmentvessel, - supplying (13) the lignocellulosic material to the treatment vessel, - performing (14) hydrolysis of the lignocellulosic material forextracting hemicellulose, and - supplying (15) the volume of displacement liquor for displacingthe desired amount of hemicellulose from the treatment vessel.

2. Method according to claim 1, Wherein steam and/ or Water and/ orcondensate is supplied as treatment liquor for the hydrolysis.

3. Method according to claim 1 or 2, Wherein the desired amount ofhemicellulose is determined (12) as a portion of the first amount ofhemicellulose and Wherein the volume of displacement liquor is preferablydetermined by selecting a volume in Which that portion can be dissolved.

4. Method according to any of claims 1-3, Wherein the displacementliquor is at a temperature of 170°C or less, preferably 160°C or less andmore preferably 140°C or less When supplied (15) to the treatment vessel.

5. Method according to any of claims 1-4, Wherein a floW ofdisplacement liquor into the treatment vessel during displacement is 200 l/ sor less, preferably 150 l/s or less, and more preferably 100 l/s. 12