Method for reducing fouling

Abstract

A method for reducing fouling of a steaming reactor during the production process of a carbohydrate composition are disclosed. The method comprises: subjecting a feedstock of wood chips to an impregnation treatment with an acidic impregnation liquid to form an impregnated feedstock; separating the impregnated feed stock into a pressate fraction and a solid fraction; circulating at least part of the separated pressate fraction to a mixing tank; feeding additional liquid into the mixing tank; subjecting the separated solid fraction to steam treatment in the steaming reactor to form hydrolyzed feedstock; transferring the hydrolyzed feedstock to the mixing tank, wherein the hydrolyzed feedstock is mixed with the at least part of the separated pressate fraction that is circulated to the mixing tank to form a slurry.

Description

[0001] METHOD FOR REDUCING FOULING

[0002] FIELD OF THE INVENTION

[0003] The present disclosure relates to a method for reducing fouling of a steaming reactor during the production process of a carbohydrate composition .

[0004] BACKGROUND OF THE INVENTION

[0005] Different methods are known for converting biobased raw material , such as lignocellulosic biomass , into a liquid stream of various sugars . In a biomass- to-sugar process wood chips may be subj ected to various kinds of pretreatment such as washing, impregnating with water and / or other liquids , and heating, in order to prepare them for the later stages of the process . During acid-catalyzed processing, such as acid hydrolysis or autohydrolysis , unwanted dark-colored solid substances may be formed due to degradation of carbohydrates , extractives and condensation of lignin . Such formation retards the economy, safety, and efficiency of biomass pretreatment processes as these substances have high tendency to precipitate on surfaces and therefore may block e . g . the used reactor, pipes and decrease process performance overall . The inventors have recogni zed the need to avoid or hinder said adverse effects during the production of the carbohydrate composition .

[0006] SUMMARY

[0007] A method for reducing fouling of a steaming reactor during the production process of a carbohydrate composition is disclosed . The method comprises :

[0008] - subj ecting a feedstock of wood chips to an impregnation treatment with an acidic impregnation liquid to form an impregnated feedstock;

[0009] - separating the impregnated feedstock into a pressate fraction and a solid fraction by squeezing out liquid from the impregnated feedstock to provide the solid fraction with a total dry matter content of 35 - 80 % ;

[0010] - circulating at least part of the separated pressate fraction to a mixing tank;

[0011] - feeding additional liquid into the mixing tank;

[0012] - subj ecting the separated solid fraction to steam treatment in the steaming reactor to form hydrolyzed feedstock; and

[0013] - transferring the hydrolyzed feedstock to the mixing tank, wherein the hydrolyzed feedstock is mixed with the at least part of the separated pressate fraction that is circulated to the mixing tank and with the additional liquid, to form a slurry .

[0014] BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The accompanying drawing, which is included to provide a further understanding of the method and constitutes a part of this specification, illustrates an embodiment and together with the description help to explain the principles of the above . In the drawing :

[0016] Fig . 1 illustrates one embodiment of conducting the method for reducing fouling of a steaming reactor during the production process of a carbohydrate composition .

[0017] DETAILED DESCRIPTION

[0018] A method for reducing fouling of a steaming reactor during the production process of a carbohydrate composition is disclosed . The method comprises :

[0019] - subj ecting a feedstock of wood chips to an impregnation treatment with an acidic impregnation liquid to form an impregnated feedstock;

[0020] - separating the impregnated feedstock into a pressate fraction and a solid fraction by squeezing out liquid from the impregnated feedstock to provide the solid fraction with a total dry matter content of 35 - 80 % ;

[0021] - circulating at least part of the separated pressate fraction to a mixing tank;

[0022] - feeding additional liquid into the mixing tank;

[0023] - subj ecting the separated solid fraction to steam treatment in the steaming reactor to form hydrolyzed feedstock; and

[0024] - transferring the hydrolyzed feedstock to the mixing tank, wherein the hydrolyzed feedstock is mixed with the at least part of the separated pressate fraction that is circulated to the mixing tank and with the additional liquid, to form a slurry .

[0025] As a result of degradation of carbohydrates , extractives and condensation of lignin, present in impregnated wood chips , during acid-catalyzed processing of the impregnated wood chips taking place in the steaming reactor, unwanted solid and sticky components with low solubility may easily be formed . The formed solid components are easily fouling on the surfaces of the steaming reactor and the compressing conveyor, such as the screw feeder . One group of such components are so called humins , which are degradation products formed during catalytic conversion of carbohydrates and its derivatives ( including C5 and 06 sugars , furfural , hydroxymethylfurfural (HMF) , etc, ) . Such formation may cause fouling of the steaming reactor .

[0026] The term "fouling" is used in this specification to refer to accumulation of unwanted material on the surfaces of the steaming reactor . The fouling materials can comprise or consist of non-living substance ( inorganic or organic) .

[0027] The inventors surprisingly found out that by separating the impregnated feedstock into a pressate fraction and a solid fraction by squeezing out liquid from the impregnated feedstock to provide the solid fraction with a total dry matter content of 35 - 80 % , and by then subjecting the specified solid fraction to steam treatment instead of directing the impregnated feedstock as such to the steam treatment, fouling of the steaming reactor may be reduced.

[0028] Fouling of the steaming reactor may have the adverse effects of raw materials being consumed during by-products formation leading to lower yield of the desired products and lower selectivity of the conversion. Further, steaming reactor walls, screws, and pipes may become covered or even blocked with the build-up precipitates, which may be difficult to remove.

[0029] A feedstock of wood chips is subjected to an impregnation treatment during the production of a carbohydrate composition. The size of the wood chips of the feedstock may be classified by standard SCAN-CM 40:01. The standard SCAN-CM 40:01 (version revised 2001) describes a manner for the classification of wood chips, wherein a sample of wood chips is placed on the top screen of a stack of five screen trays and a fines tray. The screens have holes or slots of specified dimensions and the stack is kept in a reciprocating motion. After a specified time, the screening is stopped and the six classes obtained are weighed separately. The size of each class is its mass, expressed as a percentage of the total mass of all six classes.

[0030] At most 5 weight-% of the wood chips of the feedstock may be overthick wood chips as specified by SCAN-CM 40:01. The feature that the wood chips in the wood-based feedstock are "overthick wood chips" should in this specification, unless otherwise stated, be understood as wood chips that pass the first screen of the chip classifier but are retained on the second screen comprising 8 mm slots, when chip size classification is performed as specified in SCAN-CM 40:01. In one embodiment, at most 3.5 weight-%, or at most 3 weight-%, or at most 2.5 weight-%, or at most 2 weight-% of the wood chips in the wood-based feedstock are overthick wood chips as specified by SCAN-CM 40:01.

[0031] The feedstock of wood chips may comprise fines. The feature that the wood chips of the feedstock are "fines" should in this specification, unless otherwise stated, be understood as wood chips that are not retained even on the fifth screen comprising 3 mm holes, when chip size classification is performed as specified in SCAN-CM 40:01. In one embodiment, at most 10 weight- % , or at most 6 weight-%, or at most 3 weight-%, or at most 1.5 weight-%, or at most 0.5 weight-% of the wood chips of the feedstock are fines as specified by SCAN- CM 40:01.

[0032] In one embodiment, at most 3 weight-%, or at most 1 weight-%, or at most 0.5 weight-%, or at most 0.1 weight-%, or about 0 weight-%, of the wood chips of the feedstock are oversize wood chips as specified by SCAN- CM 40:01. I.e. oversize wood chips may be removed from the feedstock. The feature that the wood chips are "oversize wood chips" should in this specification, unless otherwise stated, be understood as wood chips that do not pass the first screen comprising 45 mm holes, when chip size classification is performed as specified in SCAN-CM 40:01.

[0033] The inventors surprisingly found out that the impregnation liquid is more homogenously absorbed and impregnated into the feedstock of wood chips of the specified size when especially the amount of oversize wood chips is minimized and / or the amount of overthick wood chips in the feedstock is controlled.

[0034] The bulk density of the wood chips, such as beech chips, may be 175 - 245 kg / m3, or 185 - 230 kg / m3, 185 - 220 kg / m3. The bulk densities may be measured in accordance with standard SCAN-CM 46:92 (1992, Wood chips for pulp production - Bulk density) . The bulk density of the wood chips may affect the production capacity in how much in kilograms of feedstock of wood chips can be processed. By controlling the bulk density of the wood chips, one may achieve a suitable production capacity for industrial scale. In addition, the bulk density of the wood chips may ensure that the size of the feedstock is on a suitable level and can be efficiently processed. A too high bulk density of the wood chips, such as 300 - 400 kg / m3, may, however, adversely affect the impregnation treatment making it challenging to carry out.

[0035] Such a feedstock of wood chips may be provided in different manners. Firstly, a wood-based raw material may be provided originating from hardwood, softwood, or their combination. The hardwood may come from e.g. beech, birch, eucalyptus, ash, oak, maple, chestnut, willow, aspen, or poplar. The wood-based raw material may also be any combination or mixture of these. Providing the feedstock of wood chips may comprise subjecting wood-based raw material to a mechanical treatment selected from debarking, chipping, dividing, cutting, beating, grinding, crushing, splitting, screening, and / or washing the wood-based raw material to form the feedstock of wood chips. During the mechanical treatment e.g. wood logs can be debarked and / or wood chips of the specified size and structure can be formed. The formed wood chips can also be washed, e.g. with water, in order to remove e.g. sand, grit, and stone material therefrom. Further, the structure of the wood chips may be loosened before the pretreatment step. The wood-based feedstock may contain a certain amount of bark from the wood logs.

[0036] Providing the wood-based feedstock may comprise purchasing the wood-based feedstock. The purchased wood-based feedstock may comprise purchased wood chips that originate from wood-based raw material.

[0037] The provided feedstock of wood chips may be subjected to pre-steaming before being subjected to an impregnation treatment. The pre-steaming of the feedstock of wood chips may be carried out with steam having a temperature of 100 - 130 °C, at atmospheric pressure. During the pre-steaming the wood-based feedstock is treated with steam of low pressure. The pre-steaming may be also carried out with steam having a temperature of below 100 °C, or below 98 °C, or below 95 °C. The presteaming has the added utility of reducing or removing air from inside of the wood-based feedstock. The presteaming may take place in at least one pre-steaming reactor .

[0038] The provided feedstock of wood chips is subjected to an impregnation treatment with an acidic impregnation liquid. The impregnation liquid may consist of organic or inorganic acid and water. The impregnation liquid may consist of sulphuric acid and water. The impregnation liquid may comprise acid, such as sulphuric acid, in an amount of at most 20 weight-% based on the total weight of the impregnation liquid.

[0039] The total amount of acid added to the feedstock of wood chips may be 0.3 - 5.0 weight-% 0.5 - 3.0 weight- %, 0.6 - 2.5 weight-%, 0.7 - 1.9 weight-%, or 1.0 - 1.6 % w / w based on the total dry matter content of the feedstock. The impregnation liquid containing acid may act as a catalyst in affecting the hydrolysis of the hemicellulose in the feedstock of wood chips. The sulphuric acid may catalyze the hydrolysis of the hemicellulose in the feedstock of wood chips to monomeric sugars. The impregnation treatment may be conducted in an impregnation tank, e.g. in a reactor or vessel.

[0040] In one embodiment, liquid is squeezed out from the impregnated feedstock to provide the solid fraction with a total dry matter content of 40 - 79 % , or 45 - 78 %, or 50 - 77 %, or 53 - 76 %, or 55 - 75 %, or 60 - 70 %. In one embodiment, liquid is squeezed out from the impregnated feedstock to provide the solid fraction with a total dry matter content of 50 - 80 % , or 55 - 75 % , or 53 - 78 % , or 50 - 70 %. The total dry matter content, or the total solids content as it may also be called, includes the amount of both suspended solids and soluble solids . The total dry matter content may be determined according to standard SCAN-CM 39 : 94 ( 1994 ) . A higher total dry matter content of the separated solid fraction has the added utility of less water being present in the steaming reaction causing the adverse effect of fouling . However, a too high total dry mater content may not be used as the hydrolysis requires some amount of liquid to be present .

[0041] In one embodiment , the total dry matter content of the solid fraction is increased by 20 - 50 weight-% , or 25 - 45 weight-% , or 30 - 35 weight-% , compared to the dry matter content of the impregnated feedstock before the separation of the impregnated feedstock into a pressate fraction and a solid fraction . This value may be calculated based on measurements made on the impregnated feedstock going into compressing conveyor and calculations made after the compressing conveyor, which are then compared . Calculations made after the compressing conveyor may be carried out based on the information of the dry matter content of the impregnated feedstock going into the compressing conveyor and of the total amount of liquid ( forming the pressate fraction) that is pressed out from the compressing conveyor .

[0042] Separating the impregnated feedstock in order to provide a solid fraction with a specified total dry matter content has the added uti lity of soluble carbohydrates and soluble lignin being directed into the pressate fraction instead of being fed into the steaming reactor, where they would degrade forming degradation products that would cause foul ing on the surfaces of the steaming reactor .

[0043] Increasing the total dry matter content of the solid fraction that is to be subj ected to the steam treatment in a steaming reaction also has the added utility of enabling maximi zing the dry sol id matter in the steam treatment by decreasing the amount of liquid going into the steaming reaction.

[0044] The separated solid fraction is subjected to steam treatment in a steaming reactor. The steam treatment may be a steam explosion treatment, which may be carried out by treating the solid fraction with steam having a temperature of 130 - 240 °C, or 180 - 200 °C, or 185 - 195 °C under a pressure of 0.17 - 3.25 MPaG (pressure as measured from the gauge) followed by a sudden, explosive decompression of the feedstock. The solid fraction may be treated with the steam for 1 - 20 minutes, or 1 - 18 minutes, or 2 - 15 minutes, or 4 - 13 minutes, or 3 - 10 minutes, or 3 - 8 minutes, before the sudden, explosive decompression of the steam-treated solid fraction.

[0045] In this specification, the term "steam explosion treatment" may refer to a process of hydrothermal treatment in which the solid fraction is treated in a steaming reactor with steam having a temperature of 130 - 240 °C, or 180 - 200 °C, or 185 - 195 °C under a pressure of 0.17 - 3.25 MPaG followed by a sudden, explosive decompression of the feedstock that results in the rupture of the fiber structure of the feedstock. The steam explosion treatment may be conducted in a pressurized steaming reactor. The residence time of the solid fraction in the pressurized reactor may be 0.5 - 120 minutes. The term "residence time" should in this specification, unless otherwise stated, be understood as the time between the solid fraction being introduced into or entering the steaming reactor and the hydrolyzed feedstock being exited or discharged from the same.

[0046] From the steaming reactor the hydrolyzed feedstock may be transferred to the mixing tank.

[0047] In the mixing tank, the extraction of hydrolysed sugar compounds may be carried out. The amount of the circulated separated pressate fraction in the mixing tank may be 0.1 - 25 %, or 0.3 - 15 %, or 0.5 - 10 %, or 0 . 8 - 5 % , or 1 - 3 % , of the total amount of liquid in the mixing tank . In the method additional liquid is fed into the mixing tank . In one embodiment, the method comprises feeding additional liquid into the mixing tank, wherein the amount of the circulated separated pressate fraction in the mixing tank is 0 . 1 - 25 % , or 0 . 3 - 15 % , or 0 . 5 - 10 % , or 0 . 8 - 5 % , or 1 - 3 % , of the total amount of liquid in the mixing tank . Thus , also other liquids than the circulated separated pressate fraction may be fed into the mixing tank . Additional liquid may be water or recirculated proces s water from process stages downstream the mixing tank . In the mixing tank extraction of sugar compounds may then be carried out . The additional liquid may be used to obtain an appropriate solid matter content for the slurry in the mixing tank .

[0048] From the mixing tank the mixture of the pressate fraction, the solid fraction, and additional liquid, may be subj ected separation, where the solid fraction is separated from liquid and washed with washing water . The washing water may be circulated back to the mixing tank to be used as the additional liquid .

[0049] Being able to circulate at least part of the separated pressate fraction into the mixing tank has the added utility of decreasing the need to direct the same into a waste l ine , as not all of the pressate fraction may be re-used in the impregnation treatment . I f all of the separated pressate fraction would be fed back into the impregnation treatment this would in the end result in all the sugar and soluble lignin being directed to the steaming reactor resulting in fouling on the surfaces thereof .

[0050] After the separation, the solid fraction may be subj ected to enzymatic hydrolysis for the production of further carbohydrates .

[0051] In one embodiment , separating the impregnated feedstock into a pressate fraction and a solid fraction is carried out with a compressing conveyor . The compressing conveyor may be a screw feeder or a pi ston . A screw feeder is an example of a compressing conveyor that applies a squeezing force to the transferred material . This is a useful property when the liquid content of the material needs to be influenced upon . As the material passes through the screw feeder it becomes compressed, removing some previously absorbed liquid and causing some - at least partially elastic - compressing deformation . Releasing the compressed material from the screw feeder makes it readily absorb a new impregnating liquid i f one is available , as the elastic deformation relaxes . The compres sing property of a screw feeder is also useful if the material is to be fed into a process stage where it must be under pressure .

[0052] The method may further comprise circulating at least part of the separated pressate to a pressate tank . The method may further comprise circulating the pressate fraction from the pressate tank to the impregnation tank .

[0053] In one embodiment , solid matter is separated from the at least part of the pressate fraction before feeding the at least part of the pressate fraction into the mixing tank . Solid matter to be separated may comprise e . g . wood . The separation of the solid matter from the pres sate fraction may be carried out by f iltration using for example a run-down screen or a rotating screen .

[0054] The method as disclosed in the current disclosure has the added utility that fouling of the steaming reactor may be efficiently reduced .

[0055] The method as disclosed in the current disclosure has the added utility of one being able to operate efficiently for producing the carbohydrate composition without the need to clean the steaming reactor as often as with processes without separating impregnated feedstock into a pressate fraction and a solid fraction, wherein at least part of the pressate fraction i s then by-passed the steaming reactor .

[0056] EXAMPLES

[0057] Reference will now be made in detail to the described embodiments , an example of which is illustrated in the accompanying drawing .

[0058] The description below discloses some embodiments in such a detail that a person skilled in the art is able to uti li ze the method based on the di sclosure . Not all steps of the embodiments are discussed in detail , as many of the steps will be obvious for the person skilled in the art based on this specification .

[0059] For reasons of simplicity, item numbers will be maintained in the following exemplary embodiments in the case of repeating components .

[0060] Fig . 1 illustrates one embodiment of a method for reducing fouling of a steaming reactor during the production process of a carbohydrate composition is disclosed . In the embodiment of Fig . 1 , a feedstock of wood chips is subj ected to an impregnation treatment to form an impregnated feedstock . The impregnated feedstock is then separated into a pressate fraction and a solid fraction by squeezing out liquid from the impregnated feedstock to provide the sol id fraction with a total dry matter content of 35 - 80 % . At least part of the separated pressate fraction is circulated to a mixing tank . Additional liquid is also fed into the mixing tank .

[0061] In the embodiment of Fig . 1 , another part of the separated pressate fraction is circulated to a pressate tank . From the pressate tank the pressate fraction therein is circulated back into the impregnation treatment and / or to the mixing tank .

[0062] The separated solid fraction is subj ected to hydrolysis and steam treatment in a steaming reactor to form hydrolyzed feedstock . The hydrolyzed feedstock is then transferred to the mixing tank . In the mixing tank the hydrolyzed feedstock is mixed with the pressate fraction to form a slurry . Additional liquid may be fed into the mixing tank when needed . From the mixing tank the formed mixture of pressate fraction, solid fraction, and additional liquid may be further processed .

[0063] Example 1 - Reducing fouling of a steaming reactor during the production process of a carbohydrate composition

[0064] In this example fouling of a steaming reactor during the production process of a carbohydrate composition was tested .

[0065] A feedstock of wood chips was provided having the following properties :

[0066] Wood material : Beech chips Properties of the wood chips :

[0067] ; S i z e class 7 - 3 mm % ; 5 . 4 ;

[0068] The feedstock of wood chips was then subj ected to an impregnation treatment to form an impregnated feedstock . The impregnation treatment was carried out in the following conditions :

[0069] Impregnation liquid : H2SO4 at the acid concentration of 13 g / 1 ( titrated result of impregnation liquid) Temperature of the impregnation liquid : 90 °C

[0070] Residence time in the impregnation liquid : 1 . 5 minutes

[0071] The impregnated feedstock was then separated into a pressate fraction and a sol id fraction by squeezing out liquid from the impregnated feedstock . A screw feeder was used to squeeze the liquid . A solid fraction with a total dry matter content of 62 % was provided .

[0072] Then 1 / 3 of the separated pressate fraction was circulated to a mixing tank . Additional liquid was further fed into the mixing tank . The rest of the separated pressate fraction was circulated to a pressate tank, from which it was later circulated back into the impregnation treatment .

[0073] The separated solid fraction was subj ected to hydrolysis and steam treatment in a steaming reactor to form hydrolyzed feedstock . The conditions used in the steaming reactor were as follows :

[0074] Temperature of the steam : 185 °C Pressure : 1 . 02 MPaG

[0075] Time of steam treatment : 6 . 5 minutes

[0076] The hydrolyzed feedstock was then transferred to the mixing tank . In the mixing tank the hydrolyzed feedstock is mixed with the pressate fraction to form a slurry .

[0077] From the carrying out the example , it was noticed that as the process went along, the steaming reactor was not fouled to such a large extent as when directly subj ecting the impregnated feedstock to the steam treatment without taking into account the total solids content thereof . It was notice that fouling of the steaming reactor was reduced . Example 2 - Reducing fouling of a heated reactor by decreasing amount of free liquid during the production process of a carbohydrate composition

[0078] In this example fouling of a heated reactor ( simulating a steaming reactor) during the production process of a carbohydrate composition was tested .

[0079] A feedstock of wood chips was provided and cut to smaller pieces by cutting mill and using a 6 . 0 mm screen in the mill .

[0080] The feedstock of wood chips was then subj ected to an impregnation treatment to form an impregnated feedstock . The impregnation treatment was carried out in the following conditions :

[0081] Impregnation liquid : H2SO4 at the acid concentration of 0 . 50 - 0 . 55 % w / w ( calculated result in dosing) Temperature of the impregnation liquid : 20 - 25 °C Residence time in the impregnation liquid : >12 hours

[0082] Three different samples of the impregnated feedstock with different total dry matter contents were prepared :

[0083] Sample 1 : Impregnated feedstock was separated into a pressate fraction and a solid fraction by vacuum filtration . A sol id fraction with a total dry matter content of 28 . 4 % was provided .

[0084] In this case , after the vacuum filtration there was stil l some free liquid on the surfaces of the wood chips and between the wood chips .

[0085] Sample 2 : Impregnated feedstock was separated in a similar way as when preparing sample 1 , but additionally some impregnation liquid was added into the heated reactor resulting in a total dry matter content of 12 . 1 In this case there was a lot of free liquid in the beginning of the heating in the heated reactor, but the amount of liquid during the processing was decreased due to evaporation .

[0086] Sample 3 : Impregnated feedstock was separated into a pressate fraction and a solid fraction by centrifugation in 2500 rpm and for 5 minutes with special bottom element in the ves sel to separate the liquid . A solid fraction with a total dry matter content of 36 . 9 % was provided .

[0087] The solid fractions with variating liquid amounts ( i . e . total dry matter contents ) were subj ected to hydrolysis and heat treatment in heated reactors ( simulating the steaming reactor) to form hydrolyzed feedstock . The target temperature in the reactor was 191 °C . During the reaction the liquid is released as steam and replacing N2 atmosphere . The amount of deposit or fouling of the heated reactor was visually analysed . The results are presented in table 1 .

[0088] Table 1 .

[0089] From the carrying out the example , it was noticed that the amount of liquid in the reactor affected the amount of fouled material, deposit on the walls of the reactor.

[0090] It is obvious to a person skilled in the art that with the advancement of technology, the basic idea may be implemented in various ways. The embodiments are thus not limited to the examples described above; instead they may vary within the scope of the claims.

[0091] The embodiments described hereinbefore may be used in any combination with each other. Several of the embodiments may be combined together to form a further embodiment. A method, as disclosed herein, may comprise at least one of the embodiments described hereinbefore. It will be understood that the benefits and advantages described above may relate to one embodiment or may relate to several embodiments. The embodiments are not limited to those that solve any or all of the stated problems or those that have any or all of the stated benefits and advantages. It will further be understood that reference to 'an' item refers to one or more of those items. The term "comprising" is used in this specification to mean including the feature (s) or act(s) followed thereafter, without excluding the presence of one or more additional features or acts.

Claims

CLAIMS1 . A method for reducing fouling of a steaming reactor during the production process of a carbohydrate composition, wherein the method comprises :- subj ecting a feedstock of wood chips to an impregnation treatment with an acidic impregnation liquid to form an impregnated feedstock;- separating the impregnated feedstock into a pressate fraction and a solid fraction by squeezing out liquid from the impregnated feedstock to provide the solid fraction with a total dry matter content of 35 - 80 % ;- circulating at least part of the separated pressate fraction to a mixing tank;- feeding additional liquid into the mixing tank;- subj ecting the separated solid fraction to steam treatment in the steaming reactor to form hydrolyzed feedstock; and- transferring the hydrolyzed feedstock to the mixing tank, wherein the hydrolyzed feedstock is mixed with the at least part of the separated pressate fraction that is circulated to the mixing tank and with the additional liquid, to form a slurry .2 . The method of claim 1 , wherein liquid is squeezed out from the impregnated feedstock to provide the solid fraction with a total dry matter content of 40 - 79 % , or 45 - 78 % , or 50 - 77 % , or 53 - 76 % , or 55 - 75 % , or 60 - 70 % .3 . The method of any one of the preceding claims , wherein the total dry matter content of the sol id fraction i s increased by 20 - 50 weight-% , or 25 - 45 weight-% , or 30 - 35 weight-% , compared to the dry matter content of the impregnated feedstock before the separation of the impregnated feedstock into a pressate fraction and a solid fraction .

4. The method of any one of the preceding claims, wherein the method comprises feeding additional liquid into the mixing tank, wherein the amount of the circulated separated pressate fraction in the mixing tank is 0.1 - 25 %, or 0.3 - 15 %, or 0.5 - 10 %, or 0.8 - 5 %, or 1 - 3 %, of the total amount of liquid in the mixing tank.

5. The method of any one of the preceding claims, wherein separating the impregnated feedstock into a pressate fraction and a solid fraction by squeezing out liquid from the impregnated feedstock to provide the solid fraction is carried out with a compressing conveyor .

6. The method of claim 5, wherein the compres- sion conveyor is a screw feeder or a piston.

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