Method and arrangement for heating lignocellulosic material in a digester

Abstract

A method of heating lignocellulosic material in a digester (20), comprising the steps of: transporting lignocellulosic material (4) as a slurry (2) comprising the lignocellulosic material and a liquid before the lignocellulosic material is added to the contents (14) of the digester, separating a major part of the liquid (6) from the slurry so that only a minor part of the liquid is added to the contents of the digester together with the lignocellulosic material, and heating the lignocellulosic material and the minor part of the liquid by steam before the lignocellulosic material and the minor part of the liquid are added to the contents of the digester, wherein the steam is flash steam generated by flashing black liquor withdrawn from the digester A digester system is also provided.

Description

[0001] METHOD AND ARRANGEMENT FOR HEATING LIGNOCELLULOSIC MATERIAL IN A

[0002] DIGESTER

[0003] TECHNICAL FIELD

[0004] The present invention relates to a method and an arrangement for heating lignocellulosic material in a digester.

[0005] BACKGROUND

[0006] In the production of pulp and paper, wood chips or other lignocellulosic material is typically treated e.g. cooked, with chemicals and heat in a digester. Due to the large volumes of material treated and the elevated temperatures required, energy consumption is significant with the pulp and paper industry accounting for about 6% of total global industrial energy consumption.

[0007] The wood chips in the digester are typically heated by steam introduced into the digester. The steam used may typically be fresh steam, i.e. steam produced in the pulp mill for heating purposes.

[0008] Various techniques to decrease the need for fresh steam and thereby improve the energy efficiency of the heating of the wood chips comprise methods to form steam using the heat in spent liquid, i.e. black liquor, which is extracted from the digester.

[0009] The black liquor may thus be used as a heat source for heating another liquid to produce steam. Alternatively, the black liquor may be flashed to produce flash steam. As another alternative, black liquor may be further heated and evaporated to produce steam therefrom.

[0010] There is accordingly a need for further methods and systems for heating lignocellulosic material in a digester.

[0011] There is further a need for methods and systems for efficiently transporting lignocellulosic material into the digester.

[0012] SUMMARY

[0013] At least one of the abovementioned needs or at least one of the further needs which will become evident from the below description, are according to corresponding first and second aspects of the present invention obtained by a method of heating lignocellulosic material in a continuous digester, comprising the steps of: - transporting lignocellulosic material as a slurry comprising the lignocellulosic material and a liquid before the lignocellulosic material is added to the contents of the digester,

[0014] - separating a major part of the liquid from the slurry so that only a minor part of the liquid is added to the contents of the digester together with the lignocellulosic material, and

[0015] - heating the lignocellulosic material and the minor part of the liquid by steam before the lignocellulosic material and the minor part of the liquid are added to the contents of the digester, wherein the steam is flash steam generated by flashing black liquor withdrawn from the digester, and a digester system comprising:

[0016] - a separator configured to: o receive lignocellulosic material as a slurry comprising the lignocellulosic material and a liquid, and o separate a major part of the liquid from the slurry so that only a minor part of the liquid remains with the lignocellulosic material,

[0017] - a continuous digester for receiving the lignocellulosic material together with the minor part of the liquid, wherein the digester further comprises an inlet for receiving flash steam for heating the lignocellulosic material together with the minor part of the liquid,

[0018] - an arrangement configured to generate flash steam from flashing black liquor withdrawn from the digester, and

[0019] - a conduit arranged to lead flash steam, from the arrangement configured to generate flash steam, to the inlet for receiving flash steam.

[0020] The present invention is accordingly based on the recognition by the present inventors that separating a major part of the liquid used to transport the lignocellulosic material reduces the heating requirements for heating the lignocellulosic material since the major part of the liquid does not have to be heated by the flash steam. Expressed differently, by separating the major part of the liquid from the slurry the slurry is transformed into a solid fraction comprising the lignocellulosic material and the minor part of the liquid, i.e. the solid fraction comprises wet lignocellulosic material, and a liquid fraction comprising the major part of the liquid, whereby the liquid fraction does not have to be heated by the flash steam. This in particular reduces the amount of flash steam that has to be generated from the withdrawn black liquor and thus makes it easier to generate sufficient flash steam for heating from the withdrawn black liquor.

[0021] Additionally, as only the minor part of the liquid, i.e. the liquid remaining in the solid fraction, adds to the contents of the digester, the cooking liquor, i.e. the white liquor added to the digester and the black liquor formed during the cooking process, is less diluted by the liquid carrying the lignocellulosic material in the slurry. This helps maintain a concentration of cooking chemicals that is sufficient for efficient cooking for a longer time and thus decreases the need for supplying fresh white liquor to the digester.

[0022] Further, as only the minor part of the liquid adds to the contents of the digester, the volume of black liquor formed in the digester decreases. This makes it easier to handle the black liquor once it is to be removed from the digester, for example generation of flash steam or heat recovery and ultimately incineration or disposal. Additionally, the black liquor will have a higher dry matter content which further reduces the need for concentrating the black liquor before incineration.

[0023] Yet further, by using flash steam generated from black liquor the need for using fresh steam, e.g. steam made from clean water, decreases.

[0024] The method of heating lignocellulosic material in a digester may be referred to as a method of treating lignocellulosic material in a digester. Accordingly, the digester may further comprise liquor such as white liquor and / or black liquor.

[0025] The term lignocellulosic material is used herein to mean materials containing lignin, cellulose, and hemicellulose. One example of such materials is wood, in particular wood chips. Other examples include other agricultural or forestry wastes such as bagasse and wheat straw. When lignocellulosic material is present in a digester it is often present in liquor, i.e. the white liquor that is added to the digester and the black liquor that forms from the white liquor during the cooking process.

[0026] Typically, the lignocellulosic material is present in the digester at a temperature of 70- 180°C, such as 70-150°C.

[0027] Accordingly, the heating of the lignocellulosic material has the purpose of bringing the lignocellulosic material to a temperature of 70-180°C, such as 70-150°C, i.e. bringing it to the temperature of the contents of the digester.

[0028] The digester system may alternatively be referred to as a digester plant.

[0029] The digester is a continuous digester. The steps of transporting, separating and heating are preferably performed continuously. The digester may preferably be a vapor phase digester. The digester is suitable for, e.g. designed for, receiving the lignocellulosic material together with the minor part of the liquid, e.g. as the solid fraction, or as the slurry. Accordingly, the digester may receive the lignocellulosic material together with the minor part of the liquid, i.e. the solid fraction, as part of the slurry when separation, to transform the slurry into the solid fraction, is performed within the digester. Alternatively, the digester may receive the lignocellulosic material together with the minor part of the liquid, i.e. the solid fraction, without the major part of the liquid, i.e. without also receiving the liquid fraction, when separation is performed outside the digester. The digester may thus comprise an inlet for receiving the lignocellulosic material together with the minor part of the liquid, i.e. the solid fraction, as part of the slurry, or for receiving the lignocellulosic material together with the minor part of the liquid, i.e. the solid fraction, without receiving the major part of the liquid, i.e. without receiving the liquid fraction. In either case, the lignocellulosic material is generally fed or introduced into the digester.

[0030] Preferably the lignocellulosic material, as a slurry, i.e. both solid fraction and liquid fraction, or together with the minor part of the liquid and without the major part of the liquid, i.e. only as the solid fraction, is fed into, or introduced into, the digester at the top of the digester. The top of the digester may be defined as the uppermost section of the digester, this section having a diameter smaller than the maximum diameter of the digester. Expressed differently, the top of the digester may be considered to refer to the uppermost ! to 1 / 8 of the digester.

[0031] The inlet for receiving lignocellulosic material may be connected to a pipe or tube connecting the digester with a source of lignocellulosic material such as a source of the slurry or a separator providing a source of the lignocellulosic material together with the minor part of the liquid.

[0032] The digester further comprises an inlet for receiving steam, i.e. flash steam, for heating the lignocellulosic material together with the minor part of the liquid. The inlet is preferably provided at the top of the digester. The top is as defined above. The inlet is more preferably provided at a top gas space of the digester, i.e. the volume above the surface of the contents of the digester or the volume defined by uppermost % to 1 / 8 of the digester.

[0033] It is specified that the method comprises transporting lignocellulosic material as a slurry comprising the lignocellulosic material and a liquid before the lignocellulosic material is added to the contents of the digester. Expressed differently, the lignocellulosic material is transported as a slurry at least at some point in time before the lignocellulosic material is added to the contents of the digester. In other words, the step of separating a major part of the liquid from the slurry occurs before the lignocellulosic material is added to the contents of the digester. As such, the step of separating a major part of the liquid from the slurry may be performed before or after the lignocellulosic material has entered the digester.

[0034] The slurry may have a dry matter content, i.e. a content of lignocellulosic material, in the range 2 wt% to e.g. 20 wt%, such as 5 wt% to 15 wt%, such as 10 wt%, but may be higher or lower. Generally, a slurry comprises particles or pieces of solid matter suspended in liquid. For example, the ratio of liquid to lignocellulosic material may be 8:1 or less such as 7.5:1 or less, such as 7:1 or less. Accordingly, the dry matter content of the slurry may preferably be 2 wt% to 20 wt%, more preferably 2 wt% to 15 wt%, most preferably 2 wt% to 10wt%. Alternatively, the dry matter content of the slurry is 11 wt% or less, such as 10.5 wt% or less, preferably 10 wt% or less. Generally, the slurry may have a dry matter content of 8-11 wt%.

[0035] Transporting the lignocellulosic material as a slurry may comprise any of pumping or feeding the slurry.

[0036] The liquid in the slurry is preferably an aqueous liquid. More preferably, the liquid comprises white liquor.

[0037] The digester system may comprise a conduit for transporting the slurry and / or a pump for pumping the slurry.

[0038] The lignocellulosic material and the minor part of the liquid are added to the contents of the digester. Expressed differently the lignocellulosic material and the minor part of the liquid is brought into contact with and / or deposited on the contents of the digester.

[0039] The lignocellulosic material and the minor part of the liquid may for example comprise 19 wt% to 50 wt%, such as 20 wt% to 33 wt%, for example 25 wt% lignocellulosic material with the remainder up to 100 wt% being made up by the minor part of the liquid.

[0040] The content of the digester is the mixture of lignocellulosic material, white liquor and black liquor that is present in the digester. Typically, the contents of the digester do not completely fill the digester, thus providing an upper surface of the contents of the digester above which there is a gas space. The content of the digester may for example comprise 25 wt% lignocellulosic material with the remainder being liquid.

[0041] The step of separating the major part of the liquid from the slurry may be performed for example using a grate, screen or sieve allowing passage of the liquid but retaining the lignocellulosic material. In particular a conveyor, such as a screw conveyor, may be used to lift the slurry against gravity, whereby the liquid drains from the lignocellulosic material due to the liquid having the ability of flowing between and around the comparatively solid individual pieces of lignocellulosic material.

[0042] The major part of the liquid corresponds to at least 50 wt%, preferably at least 75 wt%, more preferably at least 80 wt% of the liquid that was added to the lignocellulosic material to form the slurry, or that was present in the slurry before separation and / or before the slurry entered the separator.

[0043] The minor part of the liquid thus corresponds to less than 50 wt%, preferably less than 25 wt%, more preferably less than 20 wt% of the liquid that was added to the lignocellulosic material to form the slurry, or that was present in the slurry before separation and / or before the slurry entered the separator. The separator may be configured to receive lignocellulosic material as a slurry comprising the lignocellulosic material and a liquid by having an inlet for receiving the slurry.

[0044] The separator may be configured to separate a major part of the liquid from the slurry so that only a minor part of the liquid remains with the lignocellulosic material by for example comprising a grate, screen or sieve allowing passage of the liquid but retaining the lignocellulosic material. In particular the separator may comprise a conveyor, such as a screw conveyor, lifting the slurry against gravity whereby an enclosure of the conveyor is provided with a grate, screen or sieve to allow the liquid to drain from the slurry. In particular the separator may be a top separator.

[0045] The separator may alternatively be considered a separator for, or suitable for, or capable of, separating a major part of the liquid from the slurry so that only a minor part of the liquid remains with the lignocellulosic material.

[0046] Only a minor part of the liquid is added to the contents of the digester together with the lignocellulosic material. Expressed differently, only a minor part of the liquid remains absorbed by, adsorbed by, adhered to, and / or entrained by the lignocellulosic material as the lignocellulosic material is added to the contents of the digester.

[0047] The minor part of the liquid may comprise liquid that has been absorbed or adsorbed by the lignocellulosic material. The minor part of the liquid may additionally comprise liquid that has wetted the lignocellulosic material or that adheres to the lignocellulosic material. The minor part of the liquid may additionally comprise liquid which, depending on the speed of the separation, does not have time to be separated from the lignocellulosic material, e.g. liquid that is entrained by the lignocellulosic material as the lignocellulosic material is added to the contents of the digester. Typically, the amount of such liquid, e.g. free liquid, or entrained liquid, makes up less than 50 wt% of the solid fraction, i.e. less than 50 wt% of the lignocellulosic material and the minor part of the liquid is liquid that is not absorbed or adsorbed by the lignocellulosic material.

[0048] Expressed differently, in the slurry, liquid is present as absorbed liquid (i.e. liquid in the lignocellulosic material), adsorbed liquid (i.e. liquid bound to the surface of the lignocellulosic material) and bulk liquid, or free liquid (i.e. the remaining liquid).

[0049] The major part of the liquid (that is removed in the separation) is thus made up of by part of the bulk liquid. The minor part of the liquid (that remains with the lignocellulosic material as it is added to the contents of the digester) thus comprises the absorbed liquid and the adsorbed liquid as well as the remainder of the bulk liquid that was not separated as the major part of the liquid.

[0050] As the amount of absorbed liquid and adsorbed liquid typically does not change significantly during the separation step, the minor part of the liquid may generally be considered to be made up solely by the remainder of the bulk liquid after separation of the major part. Accordingly, separating a major part of the liquid from the slurry may thus generally encompass separating, or removing, a major part of the bulk liquid, or free liquid, present in the slurry. The minor part of the liquid thus becomes the remainder of the bulk liquid, or free liquid, that was present in the slurry.

[0051] Heating the lignocellulosic material and the minor part of the liquid by flash steam may comprise transferring heat from the flash steam to the lignocellulosic material. The heat may be transferred directly, by contacting the lignocellulosic material with the flash steam, or indirectly, by heating the digester or a part of the digester that is in contact with the lignocellulosic material. Preferably, the steam, i.e. the flash steam, is used to heat the lignocellulosic material directly, such as by being led to, and into, the digester.

[0052] Correspondingly the conduit is arranged to lead flash steam, from the arrangement configured to generate flash steam, to the inlet for receiving flash steam. Expressed differently, the conduit is arranged and / or configured to lead the flash steam generated in or by the arrangement, to the digester. The conduit may be a pipe or tube.

[0053] The lignocellulosic material and the minor part of the liquid are heated before the lignocellulosic material and the minor part of the liquid are added to the contents of the digester. Expressed differently, heat is added to the lignocellulosic material and the minor part of the liquid at least at some time point before the lignocellulosic material and the minor part of the liquid are added to the contents of the digester. Typically, the lignocellulosic material and the minor part of the liquid are heated after separation from the major part of the liquid. Flash steam may for example be provided around or in the separator. Flash steam may alternately or additionally be provided between the separator and the contents of the digester, such as between the separator and the surface of the contents of the digester. Additionally, the lignocellulosic material and the minor part of the liquid may be heated also after it has been added to the contents of the digester, such as when the lignocellulosic material and the minor part of the liquid have been deposited onto the surface of the contents of the digester.

[0054] The flash steam is generated by flashing black liquor withdrawn from the digester.

[0055] Correspondingly the arrangement is configured to generate flash steam from flashing black liquor withdrawn from the digester.

[0056] Flash steam refers to steam or vapor obtained as a liquid, e.g. black liquor, is subjected to a, preferably sudden, decrease in pressure. The pressure referred to is the pressure impinging on the liquid. The flash steam thus comprises water vapor. The flash steam may further comprise air and / or water droplets.

[0057] The flash steam is generated from black liquor. Expressed differently, black liquor is subjected to a decrease in pressure causing flash steam to be generated from the black liquor. The black liquor after flashing may be referred to as flashed liquor.

[0058] Black liquor is an aqueous solution comprising lignin residues, hemicellulose, and the inorganic chemicals used in the digester. As such, black liquor is formed from white liquor, which comprises water, and the chemicals used in the digester, e.g. sodium hydroxide (NaOH), and sodium sulfide (Na2S), during the cooking of the lignocellulosic material. The black liquor may preferably have a solids content of 12-17% when led from the digester.

[0059] The black liquor may preferably have a temperature of at least 120°C, such as 120°C- 200°C, preferably 120-180°C, more preferably 130-165°C, when withdrawn or led from the digester. Black liquor may be led from different positions having different temperatures due to the cooking process in the digester, and the black liquor from the different positions may either be mixed into a single flow of black liquor before being flashed.

[0060] Flashing generally refers to reducing the pressure of a liquid to allow at least partial removal of water, as steam, from the liquid. The flashing occurs due to a pressure drop whereby the reduced pressure is lower than the pressure of the liquid prior to flashing.

[0061] The flashing of black liquor may be performed in a flashing vessel.

[0062] The flashing vessel generally comprises a pressure container providing at least partial removal, i.e. flashing, of water, as steam, from liquid led into the flashing vessel.

[0063] The flashing vessel comprises an inlet for receiving black liquor, a first outlet for withdrawing flashed black liquor, and a second outlet for allowing flash steam generated in the flashing vessel to leave the flashing vessel. Conventional flashing vessels may be used provided that they have at least these inlets and outlets. The pressure in the flashing vessel should be lower than the pressure of the black liquor entering the flashing vessel. The pressure in the flashing vessel may be set by connecting at least one of the outlets to a volume having the desired pressure. As an example, connecting the flashing vessel, e.g. via the second outlet, to a gas space at the top of a digester will generally provide the desired pressure difference between the pressure in the flashing vessel and the black liquor led from a position below the gas space in the digester.

[0064] The arrangement is preferably provided adjacent to the digester. The arrangement may alternatively be referred to as an apparatus or a device.

[0065] Preferably the digester does not comprise a withdrawal strainer or withdrawal screen for withdrawing liquid at the top of the digester. Expressed differently, there is preferably no further top separator, nor any other device such as strainer or screen which works as an extra top separator. Further expressed differently the major part of the liquid is preferably sufficiently large so that none of the minor part of the liquid that is added to the contents of the digester together with the lignocellulosic material needs to be removed at the top of the digester, or from close below the liquid level of liquid in the digester, such as from a position at which the lignocellulosic material have had a retention time as being part of the contents of the digester, for 0.1-15 minutes. Further expressed differently, there is preferably no further withdrawal of liquid from minor part of the liquid added to the digester before the lignocellulosic material added to the digester together with the minor part has been present in the digester, i.e. present in the liquid in the digester, for more than 0.1 to 15, minutes, preferably for more than 0.1 to 10 minutes, most preferably for more than 0.1 to 5 minutes.

[0066] Preferably the black liquor withdrawn from the digester and used to generate flash steam is not heated while passing from the digester to the arrangement configured to generate flash steam.

[0067] It is also preferred that the major part of the liquid, outside the digester, is not heated by black liquor withdrawn from the digester.

[0068] Generally, the major part of the liquid is at least 51 wt%, preferably at least 52.5 wt%, more preferably at least 55 wt%, most preferably at least 60 wt% such as at least 65 wt%.

[0069] Typically the major part of the liquid is at least 70 wt%, preferably at least 80 wt% of the liquid.

[0070] Correspondingly the separator is preferably configured to separate at least 70 wt% preferably at least 80 wt% of the liquid. Alternatively, the major part of the liquid is at least 65 wt%, preferably at least 67.5 wt%, or at least 77.5 wt% or at least 77.5 wt%.

[0071] Correspondingly the separator is alternatively configured to separate at least 65 wt%, preferably at least 67.5 wt%, or at least 75 wt% or at least 77.5 wt% of the liquid.

[0072] This further decreases the amount of liquid that is added to the contents of the digester and thereby further reduces the heating requirements and the extent of dilution of the white and black liquor in the digester.

[0073] Generally, the major part of the liquid may be even higher, such as at least 85 wt% or at least 90 wt%, such as at least 95 wt% of the liquid. Preferably the heating requirement of the lignocellulosic material and the dilution of the white and black liquor in the digester, caused by the extent of the minor part of the liquid that is added to the contents of the digester, are balanced against the energy requirement and the residence time of the separating step and the separator and against the production rate. As an example, a low extent of separation, i.e. with the minor part being for example 40 wt% of the liquid requires less energy and residence time in the separator to obtain, yet leads to more liquid being added to the contents of the digester compared to a minor part of the liquid being for example 20%, and thus leads to higher heating requirements for the lignocellulosic material and higher dilution of the white and black liquor. Accordingly, the major part preferably is 70 wt% to 90 wt% as this provides a good balance.

[0074] As an alternative to expressing the major part of the liquid by weight percentages of the liquid present in the slurry, the major part of the liquid can be expressed as weight of liquid per weight of lignocellulosic material in the slurry. Further alternatively, the major part may be expressed as a weight percentage of the of the slurry. This is illustrated in the table below:

[0075] As seen, the major part of the liquid may typically be from more than 2 ton to 34.3 ton, preferably from 2.83 ton to 18.05 ton, more preferably from 4.5 on to 8.55 ton, or such as from 3.68 ton to 15.2 ton, more preferably from 3.97 ton to 17.1 ton, most preferably from 5.85 ton to 8 ton per ton of lignocellulosic material in the slurry. More particularly, when the slurry comprises 5-15 wt% lignocellulosic material a major part of the liquid is preferably more than 2 ton to 34.3 ton, more preferably 3.68 ton to 17.1 ton, most preferably 3.97 ton to 17.1 ton liquid per ton of lignocellulosic material in the slurry

[0076] Most particularly, when the slurry comprises 8-11 wt% lignocellulosic material the major part of the liquid is preferably 4.05 ton to 10.93 ton, more preferably 5.26 ton to 10.35 ton, most preferably 5.66 ton to 9.2 ton liquid per ton of lignocellulosic material in the slurry.

[0077] Similarly, the major part of the liquid as a percentage of the weight of the slurry may be from 66.7 wt% to 97.2 wt%, preferably from 73.9 wt% to 94.8 wt%, more preferably from 81.8 wt% to 89.5 wt%, such as from 78.6 wt% to 93.8 wt%, more preferably from 79.9 wt% to 94.5, most preferably from 85.5 wt% to 88.5 wt% of the slurry.

[0078] More particularly, when the slurry comprises 5-15 wt% lignocellulosic material the major part of the liquid is preferably 66.7 wt% to 97.2 wt%, more preferably 78.6 wt% to 94.5 wt%, most preferably 79.9 wt% to 94.5 wt% of the slurry.

[0079] Most particularly, when the slurry comprises 8-11 wt% lignocellulosic material the major part of the liquid is preferably 80.2wt% to 91.6 wt%, more preferably 84.8 wt% to 92.5 wt%, most preferably 85.0 wt% to 90.2 wt% of the slurry.

[0080] Preferably the step of separating the major part of the liquid from the slurry is performed after the slurry enters the digester.

[0081] Preferably, the step of separating the major part of the liquid from the slurry is performed in a top gas space of the digester.

[0082] Correspondingly the separator is preferably provided in the digester, preferably in a top gas space of the digester.

[0083] This is advantageous in that simplifies the transporting of the lignocellulosic material since the lignocellulosic material and the minor part of the liquid may generally be allowed to add to the contents of the digester spontaneously once separated from the major part of the liquid. It thus simplifies the handling of the lignocellulosic material since it is carried by the slurry until it is to be added to the contents of the digester.

[0084] It further simplifies the separation step and the separator since there generally is no need for providing handling of the lignocellulosic material and the minor part of the liquid once separated from the major part of the liquid.

[0085] Generally, the major part of the liquid is led out of the digester, such as for example by providing a pipe or conduit between the separator and the outside of the digester. Alternatively, the step of separating the major part of the liquid from the slurry is performed before the slurry enters the digester.

[0086] Correspondingly, the separator may alternatively be provided outside the digester.

[0087] This is advantageous in that it provides easier access to the separation and the separator, e.g. for maintenance or repairs. A further advantage is that the major part of the liquid never enters the digester and thus is easier to lead off from the separation and from the separator.

[0088] Preferably the separation takes place, and the separator is provided, adjacent or above the top of the digester so as to reduce the distance the lignocellulosic material and the minor part of the liquid have to travel before entering the digester to be added to the contents of the digester. A tube, pipe or conveyor may be provided for leading the lignocellulosic material and the minor part of the liquid into the digester, preferably into the top of the digester.

[0089] Preferably the step of heating the lignocellulosic material and the minor part of the liquid comprises introducing the flash steam into the digester, preferably into a top gas space of the digester.

[0090] Correspondingly, preferably the conduit is arranged to introduce the flash steam into a top gas space of the digester.

[0091] The top gas space of the digester is the volume of the digester above the surface of the lignocellulosic material being treated in the digester. The gas space of the digester may coincide or form part of the top of the digester defined above. Accordingly, the conduit may be arranged to introduce flash steam into the top of the digester.

[0092] Alternatively, the flash steam is introduced into the contents of the digester.

[0093] Preferably the flash steam is generated by:

[0094] - leading black liquor from the digester into a flashing vessel,

[0095] - accumulating at least part of the liquor in a buffer volume of liquor in the flashing vessel, and

[0096] - withdrawing liquor from the buffer volume, heating the withdrawn liquor, and reintroducing the heated withdrawn liquor into the flashing vessel to generate flash steam.

[0097] Correspondingly, the arrangement configured to generate flash steam from flashing black liquor withdrawn from the digester preferably comprises:

[0098] - a first conduit arranged to receive black liquor from the digester and to lead the black liquor into a flashing vessel, - a flashing vessel arranged to receive black liquor from the first conduit and for accumulating at least part of the black liquor in a buffer volume of black liquor in the flashing vessel,

[0099] - a second conduit arranged to withdraw black liquor from the buffer volume,

[0100] - a heater arranged to heat the withdrawn black liquor, and

[0101] - a third conduit arranged to reintroduce the heated withdrawn black liquor into the flashing vessel.

[0102] This is advantageous in that withdrawing black liquor from a buffer volume in a flashing vessel, heating the black liquor, and reintroducing it into the flashing vessel holding the buffer volume provides for more efficiently producing steam.

[0103] As a first factor leading to increased efficiency, the buffer volume in the flashing vessel provides for accumulating and temporarily storing black liquor received from the digester, thereby providing that variations in the mass flow rate of black liquor from the digester can be accommodated by the buffer volume, whereby the mass flow rate of the black liquor that is withdrawn, heated and reintroduced can be set or varied so as to optimize the heating and reintroduction of the black liquor. The mass flow rate of the latter may for example be set to be constant, or substantially constant so as to minimize disturbances and / or the risk of scaling in the conduits holding the black liquor during the heating and / or reintroduction. The mass flow rate and / or the flow rate, e.g. black liquor velocity, may further be set lower or higher, and the dimensions of the conduits and the heater adjusted accordingly, so as to increase the efficiency of the heating and the heater, thereby providing more efficient generation of steam.

[0104] As a second factor leading to increased efficiency, the heating and the heater do not need to handle all black liquor from the digester since the flashing vessel provides for allowing at least a part of the black liquor led from the digester into the flashing vessel to flash to generate steam upon entering the flashing vessel. This means that less than the full volume of black liquor entering the flashing vessel may be accumulated in the buffer volume, and the volume of black liquor to be withdrawn, heated and reintroduced into the flashing vessel is thereby reduced allowing for using a smaller heater and smaller conduits. A further effect is that the black liquor in the buffer volume, if at least part of it is flashed upon entry into the flashing vessel, is colder than the black liquor led from the digester, thus providing a higher delta T in the heater and thereby a more efficient heat transfer to, and heating of, the black liquor and thereby more efficient generation of flash steam when the heated withdrawn black liquor is reintroduced into the flashing vessel. As a third factor leading to increased efficiency, the buffer volume will contain black liquor with a higher dry matter content than the black liquor led from the digester. This is due to any flashing of black liquor entering the flashing vessel from the digester, as well as due to the flashing when the heated withdrawn black liquor is reintroduced into the flashing vessel. The higher dry matter content leads to less frothing when the heated withdrawn black liquor is reintroduced into the flashing vessel. Lesser frothing provides that the flash steam generated in the flashing vessel contains less of the black liquor, e.g. as drops, and thus is drier, cleaner, and thereby more efficient to transport and use. As a side effect, the black liquor in the buffer volume, if not required for generating flash steam by withdrawing, heating and reintroducing, requires less energy, due to its higher dry matter content, to dry further before combustion or other disposal.

[0105] As a fourth factor leading to increased efficiency, the flashing vessel and buffer volume allows for withdrawing, heating and reintroducing the same black liquor several times to generate increasing amounts of flash steam until the amount of flash steam generated is enough to provide the heating needs of a digester alone, i.e. without requiring supplemental fresh steam.

[0106] Further, the flash steam contains sulfur in the form of dihydrogen sulfide, H2S, which sulfur, when the flash steam is used to heat lignocellulosic material in a digester, fortifies the treatment chemicals, such as sodium sulfide, typically used in the digester.

[0107] In this method and arrangement.

[0108] In contrast to the prior art, the black liquor is not heated, such as in a heat exchanger, while being led from the digester to the flashing vessel. A further difference is that a buffer volume of black liquor is accumulated in the flashing vessel, e.g. instead of flashed black liquor being led directly from the flashing vessel to either heat recovery, mixing with black liquor from the digester, or a further flashing vessel. A further difference is that black liquor is withdrawn from the buffer volume and heated before being reintroduced into the flashing vessel. Yet a further difference is that black liquor from the buffer volume is not mixed with black liquor from the digester before being heated.

[0109] Generating flash steam may alternatively be referred to as obtaining flash steam.

[0110] The step of leading black liquor from the digester into the flashing vessel may alternatively be considered a step of receiving black liquor from a digester in a flashing vessel. The black liquor may be led through a conduit such as a pipe or tube. A pump may be provided to pump the black liquor led from the digester into the flashing vessel.

[0111] The black liquor led from the digester to the flashing vessel is black liquor obtained from the digester in which the lignocellulosic material is present and / or treated in. Correspondingly, the first conduit may be a pipe or tube. The first conduit is arranged to receive black liquor from a digester and to lead the black liquor into a flashing vessel. Expressed differently, the first conduit may be configured and / or arranged to receive black liquor from a digester and to lead the black liquor into the flashing vessel. The first conduit may be provided with a branching point to lead off part of the black liquor received from the digester and only leading the remainder of the received black liquor to the flashing vessel. The first conduit is preferably connected to the digester at one end and to the flashing vessel at the other end.

[0112] The flashing vessel generally comprises a pressure container providing at least partial removal, i.e. flashing, of water, as steam, from liquid led into the flashing vessel. The flashing occurs due to a pressure drop whereby the pressure within the flashing vessel is kept lower than the pressure of the liquid entering the flashing vessel.

[0113] The flashing vessel comprises an inlet for receiving black liquor, a first outlet for allowing withdrawing black liquor from the buffer volume, and a second outlet for allowing flash steam generated in the flashing vessel to leave the flashing vessel. Conventional flashing vessels may be used provided that they have at least these inlets and outlets. The pressure in the flashing vessel should be lower than the pressure of the heated withdrawn black liquor, and preferably also lower than the pressure of the black liquor led from the digester into the flashing vessel. The pressure in the flashing vessel may be set by connecting at least one of the outlets to a volume having the desired pressure. As an example, connecting the flashing vessel, e.g. via the second outlet, to a gas space at the top of the digester will generally provide the desired pressure difference between the pressure in the flashing vessel and the black liquor led from a position below the gas space in the digester. Further, the heating of black liquor withdrawn from the buffer volume further increases the pressure difference between the heated withdrawn black liquor and the flashing vessel. The heating also increases the temperature difference between the reintroduced black liquor and the saturation temperature of the black liquor at the pressure in the flashing vessel which further increases the amount of energy available in the black liquor for generating flash steam.

[0114] Accumulating at least part of the black liquor in a buffer volume of black liquor in the flashing vessel may alternatively be referred to as adding at least a part of the black liquor to the buffer volume.

[0115] Generally, all of the black liquor, excluding the part of the black liquor flashed to steam, led to the flashing vessel is accumulated in the buffer volume. However, a small part of the black liquor may for example be entrained as drops in the flash steam and therefore not be accumulated in the buffer volume. In addition, a part of the black liquor may be led from the buffer volume so as to not be heated and reintroduced if there is no need for that part of the black liquor to be heated and reintroduced and / or if there is a need to adjust the volume of the buffer volume. It is noted that the black liquor that is accumulated may be referred to as flashed black liquor.

[0116] The buffer volume preferably is provided in a lower part of the flashing vessel.

[0117] Withdrawing black liquor from the buffer volume may alternatively be referred to as leading black liquor from the buffer volume. The black liquor may be led through a conduit such as a pipe or tube. A pump may be provided to pump the black liquor from the buffer volume to the flashing vessel.

[0118] Correspondingly, the second conduit may be a pipe or tube.

[0119] The second conduit is arranged to withdraw black liquor from the buffer volume.

[0120] Expressed differently, the second conduit may be configured and / or arranged to withdraw black liquor from the buffer volume. The second conduit may be provided with a branching point to lead off part of the black liquor withdrawn from the buffer volume and only leading the remainder of the withdrawn black liquor to the flashing vessel.

[0121] The second conduit is preferably connected to the flashing vessel at one end and to the heater at the other end.

[0122] Heating the withdrawn black liquor increases the temperature and pressure of the black liquor, thereby increasing the amount of flash steam that is generated when the heated withdrawn black liquor is reintroduced into the flashing vessel. The heating may preferably increase the temperature of the withdrawn black liquor by at least 10°C, preferably at least 20°, more preferably at least 40°C.

[0123] Preferably the withdrawn black liquor passes through the heater.

[0124] The heating is preferably performed by indirect heat exchange with fresh steam. The fresh steam preferably has a pressure of 5-17 bar(g), e.g. intermediate pressure steam.

[0125] The heater is arranged to heat the withdrawn black liquor. Expressed differently the heater is configured to heat the withdrawn black liquor. The heater is preferably a heat exchanger. The heat exchanger preferably transfers heat from fresh steam to the withdrawn black liquor.

[0126] Reintroducing the heated withdrawn black liquor into the flashing vessel may alternatively be referred to as leading the heated withdrawn black liquor to the flashing vessel. The heated withdrawn black liquor may be led by a conduit such as a pipe or tube. The heated withdrawn black liquor may enter into the flashing vessel via the same inlet as the black liquor from the digester enters the flashing vessel. Preferably, however, the heated withdrawn black liquor enters the flashing vessel via a further inlet of the flashing vessel.

[0127] Reintroducing the heated withdrawn black liquor into the flashing vessel causes generation of flash steam due to the pressure in the flashing vessel being lower than the pressure in the heated withdrawn black liquor. The amount of flash steam generated by reintroducing the heated withdrawn black liquor into the flashing vessel is typically larger than the amount of flash steam generated, if any, when the black liquor from the digester is led into the flashing vessel due to the former's larger temperature difference vis-a-vis the temperature in the flashing vessel.

[0128] Correspondingly, the third conduit is arranged to reintroduce the heated withdrawn black liquor into the flashing vessel. Expressed differently the third conduit is arranged and / or configured to lead the heated withdrawn black liquor to the flashing vessel. The third conduit may be a pipe or tube. The third conduit is preferably connected to the heater.

[0129] Preferably the method further comprises generating flash steam from the black liquor led from the digester into the flashing vessel.

[0130] Correspondingly the flashing vessel preferably is configured or arranged, by being connected to volume, such as a gas space at the top of a digester, having a pressure lower than the pressure of the black liquor led from the digester into the flashing vessel.

[0131] This is advantageous in that flashing of this black liquor generates further flash steam and decreases the amount of black liquor that is withdrawn, heated, and reintroduced.

[0132] Generating flash steam from the black liquor led from the digester into the flashing vessel may be obtained by providing a pressure in the flashing vessel that is lower than the pressure of the black liquor led from the digester into the flashing vessel.

[0133] Preferably the method further comprises repeatedly performing the step of withdrawing black liquor from the buffer volume, heating the withdrawn black liquor, and reintroducing the heated withdrawn black liquor into the flashing vessel to generate flash steam.

[0134] Correspondingly the arrangement according to the second aspect of the present invention may preferably comprise a pump arranged to circulate the black liquor through the second conduit and the third conduit.

[0135] This is advantageous in that it generates further flash steam and further increases the solids content of the black liquor in the buffer volume.

[0136] Preferably the method comprises continuously performing the step of withdrawing black liquor from the buffer volume, heating the withdrawn black liquor, and reintroducing the heated withdrawn black liquor into the flashing vessel to generate flash steam. Preferably the method further comprises leading the black liquor from the digester into the flashing vessel at a position above the surface of the buffer volume.

[0137] Correspondingly the first conduit is preferably arranged so that the black liquor led thereby enters the flashing vessel at a position at least 'A, preferably at least 1 / 3, of the height of the flashing vessel above the bottom of the flashing vessel.

[0138] This is advantageous in that it facilitates flashing of the black liquor upon entering the flashing vessel. It further reduces the risk of frothing and formation of droplets in the flashing vessel. This further reduces the content of black liquor, e.g. as droplets, in the flash steam generated in the flashing vessel.

[0139] In order to obtain that the black liquor from the digester enters into the flashing vessel at a position above the surface of the buffer volume, the position of the surface of the buffer volume, and thus the amount of black liquor in the buffer volume, may be adjusted by adjusting one or more of the amount of black liquor entering the flashing vessel, the amount of black liquor withdrawn from the flashing vessel, and the amount of heating of the withdrawn black liquor.

[0140] A reduction of the amount of black liquor in the buffer volume may be obtained by one or more of reducing the amount of black liquor entering the flashing vessel, increasing the flashing of the black liquor led from the digester into the flashing vessel, increasing the amount of black liquor withdrawn from the flashing vessel, and increasing the amount of heating of the withdrawn black liquor.

[0141] Oppositely, an increase in the amount of black liquor in the buffer volume may be obtained by one or more of increasing the amount of black liquor entering the flashing vessel, decreasing the flashing of the black liquor led from the digester into the flashing vessel, decreasing the amount of black liquor withdrawn from the flashing vessel, and decreasing the amount of heating of the withdrawn black liquor.

[0142] Additionally, the amount of black liquor in the buffer may be reduced by leading off black liquor before entering the flashing vessel or after being withdrawn from the flashing vessel.

[0143] A further alternative comprises using a flashing vessel having multiple inlets for black liquor and selecting an inlet positioned above the surface of the buffer volume.

[0144] Preferably the method further comprises reintroducing the heated withdrawn black liquor into the flashing vessel at a position above the surface of the buffer volume. Correspondingly the third conduit is preferably arranged so that the black liquor led thereby enters the flashing vessel at a position at least ! , preferably at least 1 / 3, of the height of the flashing vessel above the bottom of the flashing vessel.

[0145] This is advantageous in that it facilitates flashing of the heated withdrawn black liquor reintroduced into the flashing vessel. It further reduces the risk of frothing and formation of droplets in the flashing vessel. This further reduces the content of black liquor, e.g. as droplets, in the flash steam generated in the flashing vessel.

[0146] As described above the amount of black liquor in the buffer volume may be adjusted in order to obtain that the heated withdrawn black liquor is reintroduced into the flashing vessel at a position above the surface of the buffer volume. As above the amount of black liquor in the buffer may be reduced by leading off black liquor, and multiple inlets for black liquor may also be used as described above.

[0147] Preferably the method further comprises reintroducing the heated withdrawn black liquor into the flashing vessel at a position further above the surface of the buffer volume than the position at which the black liquor from the digester is led into the flashing vessel.

[0148] Correspondingly, the third liquid conduit is preferably arranged to reintroduce the heated withdrawn black liquor at a position in the flashing vessel above the position at which the first liquid conduit leads black liquor into the flashing vessel

[0149] This is advantageous in that the heated withdrawn black liquor, with its higher temperature, generally will generate more flash steam upon entry than the black liquor led from the digester and accordingly the higher position provides less entrainment of black liquor, e.g. as droplets, in the generated flash steam.

[0150] It is further contemplated that the heated withdrawn liquor may be mixed with the liquor led from the digester and reintroduced into the flashing vessel in that way.

[0151] Correspondingly the third conduit may lead the heated withdrawn liquor to the first conduit.

[0152] This may be advantageous in that it provides reduced complexity. As an example, the flashing vessel then does not need to have a separate inlet for the heated withdrawn liquor.

[0153] As an alternative, the flash steam is generated by withdrawing black liquor from the digester, heating the black liquor, and flashing the heated black liquor.

[0154] Correspondingly, the arrangement configured to generate flash steam from flashing black liquor withdrawn from the digester may alternatively comprise: - a first conduit arranged to receive black liquor from the digester and to lead the black liquor to a heater,

[0155] - a heater arranged to heat the withdrawn black liquor,

[0156] - a second conduit arranged to lead the heated withdrawn black liquor to a flashing vessel, and

[0157] - a flashing vessel arranged to receive the heated withdrawn black liquor from the second conduit and to flash the heated withdrawn black liquor to generate flash steam.

[0158] This provides a simpler method and arrangement of generating the flash steam. The heater and flashing vessel may generally be configured as discussed above. The first, second and third conduits may generally be implemented as discussed above.

[0159] BRIEF DESCRIPTION OF THE DRAWINGS

[0160] A more complete understanding of the abovementioned and other features of the present invention will be apparent from the following detailed description of preferred embodiments in conjunction with the appended drawings, wherein:

[0161] Fig. 1 shows an embodiment of the method according to the first aspect of the present invention being carried out by an embodiment of a digester system according to the second aspect of the present invention.

[0162] Fig. 2A shows an embodiment of a separator of the digester system according to the second aspect of the present invention being used to perform an embodiment of the separating step of the method according to the first aspect of the present invention.

[0163] Fig. 2B shows an alternative embodiment of the separator of the digester system according to the second aspect of the present invention being used to perform an alternative embodiment of the separating step of the method according to the first aspect of the present invention.

[0164] Fig. 3A shows an alternative embodiment of the step of generating steam in the method according to the first aspect of the present invention being carried out in an alternative embodiment of the arrangement of the digester system according to the second aspect of the present invention.

[0165] Fig. 3B shows a further alternative embodiment of the step of generating steam in the method according to the first aspect of the present invention being carried out in a further alternative embodiment of the arrangement of the digester system according to the second aspect of the present invention.

[0166] All the figures are schematic, not necessarily to scale, and generally only show parts which are necessary in order to elucidate the respective embodiments, whereas other parts may be omitted or merely suggested. Any reference number appearing in multiple drawings refers to the same object or feature throughout the drawings, unless otherwise indicated.

[0167] One or more superscript ‘ appended to a reference number indicates that the feature so reference is a variant of the feature assigned that reference number.

[0168] DETAILED DESCRIPTION

[0169] Fig. 1 shows an embodiment of the method according to the first aspect of the present invention being carried out by an embodiment of a digester system 1 according to the second aspect of the present invention. A slurry comprising lignocellulosic material in the form of wood chips and a liquid is carried, as indicated by arrow 2, to a top separator 10. In the top separator 10, the wood chips are separated from the major part of the liquid. As shown in Fig. 1 the top separator is positioned within a top gas space 12 of a digester 20, hence the wood chips fall into the digester as indicated by arrow 4 together with the minor part of the liquid while the major part of the liquid exits the top separator 10 and the digester 20 as indicated by arrow 6. In the digester 20, the contents of the digester 14, i.e. the wood chips are treated with a treatment chemical, e.g. white liquor, and black liquor is formed. The black liquor is withdrawn from the digester 20 through one or more screen arrangements 22, 24, and 26 as indicated by conduits and arrows 28, 30, and 32. The black liquor, except for any surplus liquor which may be led off as indicated by arrow 34, is then led into a flashing vessel 40. In the flashing vessel 40 the black liquor typically releases some of its water content as flash steam whereas the major part of the black liquor accumulates in a buffer volume 42 of black liquor. Black liquor is then withdrawn from the buffer volume 42 as indicated by second conduit and arrow 44, except for any black liquor that is led off as indicated by arrow 46, to a pump 48. In the pump 48, the pressure of the black liquor is increased so as to cause the black liquor to be pumped through a heater in the form of indirect heat exchanger 50 in which the black liquor is heated using steam as indicated by arrow 52. The condensate of the steam is then led off to recovery as indicated by arrow 54. Heated black liquor is then led from the heat exchanger 50 back to the flashing vessel 40 as indicated by third conduit and arrow 56. Upon reintroduction into the flashing vessel 40 the heated black liquor flashes to produce flash steam which, together with any flash steam produced as the black liquor entered the flashing vessel as indicated by the first conduit 28, is led off and back to the gas space 12 at the top of the digester 20 as indicated by arrow 58. The arrangement for generating flash steam is designated the reference numeral 100.

[0170] The top of the digester 20 and the top separator 10 are shown enlarged and in more detail in Fig. 2A. As seen the slurry 2 enters into the bottom of a cylindrical chamber 110 having a perforated wall. A screw conveyor 112 is carried by vertical shaft 114 and rotated by a motor 116 to lift the slurry 2 upwards. As the slurry is lifted upwards, a major part of the liquid, trapped between the screw conveyor 112 and the perforated walls of the cylindrical chamber 110 drains off through the perforated walls and is collected in an annular chamber 118 formed around the cylindrical chamber 110, from which is led out of the digester 20 as indicated by arrow 6. At the top of the cylindrical chamber 110 the lignocellulosic material, and the minor part of the liquid present in the slurry, is expelled radially outwards to fall over the upper circumferential edge of the cylindrical chamber 110, as indicated by arrow 4, down towards the contents of the digester, i.e. the lignocellulosic material and black and white liquor already present in the digester 20. The flash steam also enters the top gas space 12 to heat the top gas space 12 and thereby the lignocellulosic material 4 together with the minor part of the liquid.

[0171] The method and system 1 provide several advantages.

[0172] The use of the top separator 10 further provides that less liquid enters the digester 20. With less liquid in the digester, less liquid needs to be heated in order to heat the wood chips, and the overall energy requirement decreases.

[0173] Due to the initial release of water as flash steam from black liquor entering the flashing vessel, the temperature and dry matter content of the black liquor in the buffer volume 42 increases. This improves the temperature difference in the heat exchanger 50 and thus improves the heating efficiency of the heat exchanger 50. Further, by providing recirculation of the black liquor via the pump 48 and heat exchanger 50 the dry matter content of the black liquor in the buffer volume 42 may be further increased, thus decreasing the energy need to finally combust the black liquor led off.

[0174] A further advantage is that the heat exchanger 50 does not need to be dimensioned to handle all black liquor led off from the digester 20, nor does it need to be able to handle the black liquor at the flow velocities prevalent between digester 20 and flashing vessel 40. Instead, the pump 48 may be operated so as to obtain a constant flow of black liquor through the heat exchanger 50 and changes in flow of black liquor to the flashing vessel 40 from the digester 20 being accommodated by the buffer volume 42. Yet a further advantage is that the recirculation and heating of the black liquor in the heat exchanger 50 provides for producing sufficient amounts of steam so as to satisfy the heating requirements in the digester without any, or only limited, supplementation with fresh steam.

[0175] The black liquor from the digester 20 preferably enters the flashing vessel 30 at a position above the surface of the buffer volume 42 as shown by the first conduit and arrow 28 in Fig. 1. Alternatively, the black liquor enters the flashing vessel 40 below the surface of the buffer volume 42. Generally, the former is preferred as it aids in the release of water as steam from the black liquor and decreases the risk of black liquor, e.g. in the form of droplets, being entrained in the flash steam. Likewise, it is preferred that the heated black liquor also enters the flashing vessel 40 above the surface of the buffer volume 44, more preferably at a position closer to the top of the flashing vessel 40 as shown by arrow 56 than to the surface of the buffer volume 44, in order to obtain an efficient flashing of the heated black liquor. However, as an alternative, the heated black liquor may enter the flashing vessel 40 below the surface of the buffer volume 42.

[0176] Fig. 2Bshows an alternative embodiment of the separator of the digester system according to the second aspect of the present invention being used to perform an alternative embodiment of the separating step of the method according to the first aspect of the present invention.

[0177] Here the separator 10’ is positioned outside the digester 20’. The digester 20’ differs from the digester 20 in that it neither is required to accommodate the separator, nor is it required to allow the major part of the liquid passage out of the digester. It however includes a top inlet for receiving the lignocellulosic material and the minor part of the liquid. The separator 10’ may be of similar construction as separator 10 in Figs. 1 and 2A. Slurry 2 enters the separator 10’ and a major part 6 of the liquid is separated and led off. The remaining lignocellulosic material and the minor part of the liquid is led into the top of the digester as indicated by arrow 4. As in Figs 1 and 2A the flash steam enters the top of the digester 20’ as shown by arrow 58.

[0178] Fig. 3A shows an alternative embodiment of the step of generating steam in the method according to the first aspect of the present invention being carried out in an alternative embodiment of the arrangement of the digester system according to the second aspect of the present invention.

[0179] The arrangement 100’ shown in Fig. 3A thus differs from the arrangement 100 in Fig. 1 by a modified third conduit 56’ which delivers the heated withdrawn black liquor into a modified first conduit 28’. The heated withdrawn black liquor is thus reintroduced into the flashing vessel 42’, which differs from the flashing vessel 40 of Fig. 1 in that it does not have a separate inlet for the heated withdrawn black liquor, together with the black liquor led from the digester 20.

[0180] This simplifies the construction of the flashing vessel 40’.

[0181] Fig. 3Bshows a further alternative embodiment of the step of generating steam in the method according to the first aspect of the present invention being carried out in a further alternative embodiment of the arrangement of the digester system according to the second aspect of the present invention.

[0182] The arrangement 100” shown in Fig. 3A thus differs from the arrangement 100 in Fig. 1 by a modified first conduit 28” which leads the black liquor from the digester directly into a modified heat exchanger 50’ in which the black liquor is heated before being led by a second conduit implemented by a modified third conduit 56’ into a modified flashing vessel 40’. Here, the flashed black liquor is drained from the flashing vessel 40’ via second conduit 44 and led to further flashing or heat recovery through conduit 46. Alternatively, the flashed black liquor may be recirculated from second conduit 44 into first conduit 28” to be mixed with black liquor from the digester and heated and flashed again.

[0183] This provides an even simpler arrangement for generating flash steam. The use of the step of separating the major part of the liquid from the slurry, and accordingly the use of the separator 10, is especially advantageous in this embodiment of the arrangement as it reduces the amount of black liquor that passes through the heater 50’, and thereby the required capacity of the heater 50’.

[0184] EXAMPLE

[0185] The method according to the first aspect of the present invention was evaluated in an experiment.

[0186] A slurry comprising 90 wt% liquid and 10 wt% lignocellulosic material in the form of wood chips was led at a total flow rate of 3500 ton / h to a separator positioned in the top of a digester. A major part of the liquid was separated from the slurry, and the remaining lignocellulosic material and minor part of the liquid was allowed to add to the contents of the digester, comprising 75 wt% liquid and 25 wt% lignocellulosic material.

[0187] Black liquor at a temperature of 150°C and a mass flow rate of 200 ton / h was led into a flashing vessel having a pressure of 4 bar(g). Upon entry, approximately 0 wt% of mass flow of the black liquor vaporized to flash steam whereas the remainder was accumulated in a buffer volume. The temperature of the buffer volume was 155°C. Black liquor was withdrawn from the buffer volume at a mass flow rate of 4000 ton / h, heated to a temperature of 165°C using fresh steam in heat exchanger, and reintroduced into the flashing vessel. Upon entry, approximately 2 % of the mass flow of the black liquor vaporized to flash steam whereas the remainder was accumulated in the buffer volume.

[0188] The experiment was continued until the dry solids content of the black liquor in the buffer volume reached above 25 wt%, at which time the mass flow rate of black liquor into the flashing vessel was increased and part of the withdrawn black liquor began to be led to heat recovery so as to prevent the dry solids content from increasing further above 25 wt%. The experiment then continued with the dry solids content of the buffer volume at or near 25 wt%. The experiment thus showed the advantages of the method, i.e.: a) that significant amounts of flash steam could be generated, b) that the black liquor in the buffer volume, which is eventually withdrawn from the flashing vessel and not heated and recirculated, had the high dry solids content of 25 wt%.

[0189] FEASABLE MODIFICATIONS

[0190] The invention is not limited only to the embodiments described above and shown in the drawings, which primarily have an illustrative and exemplifying purpose. This patent application is intended to cover all adjustments and variants of the preferred embodiments described herein, thus the present invention is defined by the wording of the appended claims.

[0191] Throughout this specification and the claims which follows, unless the context requires otherwise, the word “comprise”, and variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated integer or steps or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

Claims

Claims1. A method of heating lignocellulosic material (4) in a continuous digester, comprising the steps of:- transporting lignocellulosic material (4) as a slurry (2) comprising the lignocellulosic material and a liquid (6) before the lignocellulosic material is added to the contents (14) of the digester (20),- separating a major part of the liquid (6) from the slurry so that only a minor part of the liquid is added to the contents of the digester together with the lignocellulosic material, and- heating the lignocellulosic material and the minor part of the liquid by steam (58) before the lignocellulosic material and the minor part of the liquid are added to the contents of the digester, characterized in that the steam is flash steam generated by flashing black liquor (28) withdrawn from the digester.

2. The method according to claim 1 , wherein the major part of the liquid is at least 70 wt%, preferably at least 80 wt% of the liquid, or wherein the major part of the liquid is at least 65 wt%, preferably at least 67.5 wt%, or at least 77.5 wt% or at least 77.5 wt%.

3. The method according to any preceding claim, wherein the slurry comprises 5-15 wt% lignocellulosic material and the major part of the liquid is 66.7 wt% to 97.2 wt%, more preferably 78.6 wt% to 94.5 wt%, most preferably 79.9 wt% to 94.5 wt% of the slurry. or wherein the slurry comprises 8-11 wt% lignocellulosic material and the major part of the liquid is 80.2wt% to 91.6 wt%, more preferably 84.8 wt% to 92.5 wt%, most preferably 85.0 wt% to 90.2 wt% of the slurry.

4. The method according to any preceding claim, wherein the step of separating the major part of the liquid from the slurry is performed after the slurry (2) enters the digester (20).

5. The method according to claim 4, wherein the step of separating the major part of the liquid from the slurry (2) is performed in a top gas space (12) of the digester (20).

6. The method according to any of the claims 1-4, wherein the step of separating the major part of the liquid from the slurry is performed before the slurry (2) enters the digester (20).

7. The method according to any of the preceding claims, wherein the step of heating the lignocellulosic material (4) and the minor part of the liquid comprises introducing the flash steam (58) into the digester (20), preferably into a top gas space (12) of the digester.

8. The method according to any of the claims 1-7, wherein the flash steam is generated by- leading black liquor from the digester into a flashing vessel (40),- accumulating at least part of the black liquor in a buffer volume (40) of black liquor in the flashing vessel, and- withdrawing liquor from the buffer volume, heating the withdrawn liquor, and reintroducing the heated withdrawn liquor into the flashing vessel to generate flash steam.

9. The method according to claim 8, further comprising repeatedly performing the step of withdrawing liquor from the buffer volume, heating the withdrawn liquor, and reintroducing the heated withdrawn liquor into the flashing vessel to generate flash steam.

10. A digester system (1) comprising:- a separator (10) configured to: o receive lignocellulosic material (4) as a slurry (2) comprising the lignocellulosic material and a liquid, and o separate a major part of the liquid (6) from the slurry (2) so that only a minor part of the liquid remains with the lignocellulosic material (4),- a continuous digester (20) for receiving the lignocellulosic material (4) together with the minor part of the liquid, wherein the digester further comprises an inlet for receiving flash steam (58) for heating the lignocellulosic material together with the minor part of the liquid, characterized by- an arrangement (100) configured to generate flash steam from flashing black liquor withdrawn from the digester, and- a conduit (58) arranged to lead flash steam, from the arrangement configured to generate flash steam, to the inlet for receiving flash steam.

11. The digester system (1) according to claim 10, wherein the separator (10) is provided in the digester (20), preferably in a top gas space (12) of the digester12. The digester system according to claim 10, wherein the separator (10) is provided outside the digester (20).

13. The digester system (1) according to any of the claims 10-12, wherein the arrangement (100) configured to generate flash steam from flashing black liquor withdrawn from the digester comprises:- a first conduit (28) arranged to receive black liquor from the digester (20) and to lead the black liquor into a flashing vessel (40),- a flashing vessel (40) arranged to receive black liquor from the first conduit (28) and for accumulating at least part of the black liquor in a buffer volume (42) of black liquor in the flashing vessel,- a second conduit (44) arranged to withdraw black liquor from the buffer volume (42),- a heater (50) arranged to heat the withdrawn black liquor, and- a third conduit (56) arranged to reintroduce the heated withdrawn black liquor into the flashing vessel.

14. The digester system (1) according to claim 13, wherein the third liquid conduit (56) is arranged to reintroduce the heated withdrawn black liquor at a position in the flashing vessel (40) above the position at which the first liquid conduit leads black liquor into the flashing vessel.

15. The digester system (1) according to any of the claims 10-12, wherein the arrangement (100”) configured to generate flash steam from flashing black liquor withdrawn from the digester comprises:- a first conduit (28”) arranged to receive black liquor from the digester (20) and to lead the black liquor to a heater (50),- a heater (50) arranged to heat the withdrawn black liquor,- a second conduit (56’) arranged to lead the heated withdrawn black liquor to a flashing vessel (40), and- a flashing vessel (40) arranged to receive the heated withdrawn black liquor from the second conduit (56’) and to flash the heated withdrawn black liquor to generate flash steam.

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