WO2024005685A1

WO2024005685A1 - Method for processing cellulose pulp obtained from a kraft process

Info

Publication number: WO2024005685A1
Application number: PCT/SE2023/050401
Authority: WO
Inventors: Sofia BYLUND; Maria VENNERSTRÖM; Lari Lammi
Original assignee: Valmet Oy; Valmet AB
Current assignee: Valmet Technologies Oy; Valmet AB
Priority date: 2022-06-27
Filing date: 2023-04-28
Publication date: 2024-01-04
Anticipated expiration: 2024-12-27
Also published as: SE2250793A1; SE546238C2; FI20246482A1; CA3259833A1

Abstract

A method for reducing the hemicellulose content of cellulose pulp obtained from a Kraft process, said method comprising subjecting the cellulose pulp to a cold caustic extraction, CCE, stage (1, 2) followed by a hot caustic extraction, HCE, stage (3, 4). A method for producing pulp from lignocellulose material is also provided, which method comprises subjecting the lignocellulose material to a kraft pulping process to obtain cellulose pulp and subjecting at least part of the cellulose pulp obtained from the kraft pulping process to the method for reducing the hemicellulose content of the cellulose pulp. A corresponding system is also provided.

Description

METHOD FOR PROCESSING CELLULOSE PULP OBTAINED FROM A KRAFT PROCESS

TECHNICAL FIELD

The invention relates to a method for processing cellulose pulp obtained from a Kraft process into pulp having low hemicellulose content such as dissolving pulp. The invention also relates to a method for producing cellulose pulp having low hemicellulose content from lignocellulose material.

BACKGROUND

Different grades of pulp may be produced from lignocellulose material such as wood material. In recent years, the market for low-hemicellulose pulp, or more specifically dissolving pulp, has increased. Dissolving pulp has a high alpha cellulose content, typically over 90 % and a low content of hemicelluloses, typically below 10%. Dissolving pulp is used for example to produce textiles, chemicals, and materials such as cellulose acetate. The Kraft process is a well-known process for producing cellulose pulp from lignocellulose material. In conventional kraft pulping, a pulp with high yield, high strength and high brightness is desired to be able to produce paper from the pulp. Such paper grade pulp has a substantially higher hemicellulose content than dissolving pulp.

In order to produce dissolving pulp using the kraft process, additional stages are required. WO 2015/195030 discloses the use of the so-called pre-hydrolysis kraft process, i.e. a kraft process preceded by an acid pre-hydrolysis stage to break down hemicelluloses prior to cooking. Furthermore, the pre-hydrolysis kraft process is followed by oxygen bleaching reinforced by an additional oxidation agent in order to achieve a dissolving pulp of sufficiently low viscosity. In order to implement such a process in an existing Kraft pulping plant substantial modifications may be required, however.

Another known method to reduce the hemicellulose content is to subject the Kraft pulp to cold caustic extraction (CCE). A problem with this method is that CCE only works well for removing C5 hemicelluloses (pentosans), but not for removing C6 hemicelluloses (glucomannans). Thus, this method does not work satisfactorily with softwoods. Furthermore, a CCE stage cannot be used to control viscosity.

SUMMARY

An object of the invention is to provide a method which solves or at least improves on one or more of the problems discussed in the background section above. These and other objects are achieved by the present invention by means of a method and a system according to the independent claims.

According to a first aspect of the invention, there is provided a method for processing cellulose pulp obtained from a Kraft process, said method comprising subjecting the cellulose pulp to a cold caustic extraction (CCE) stage followed by a hot caustic extraction (HCE) stage. Put differently, the method comprises providing cellulose pulp produced using the Kraft process and subjecting the cellulose pulp to caustic extraction in two stages, a first cold caustic extraction stage, and a second hot caustic extraction stage. The method may result in a reduced hemicellulose content, i.e. the cellulose pulp subjected to the method is “upgraded” to low-hemicellulose cellulose pulp, which, depending on operating conditions and optional additional steps may classify as dissolving pulp. The method may thus be referred to as a method for reducing the hemicellulose content of cellulose pulp obtained from a Kraft process. Alternatively, the method may be referred to as a method for producing dissolving pulp from cellulose pulp obtained from a Kraft process. The cellulose pulp may be provided as so-called “never dried” (softwood) pulp, for example obtained directly from the Kraft process. Alternatively, the cellulose pulp may be provided as “partly dried” (softwood) pulp at a consistency of 30-50 %, for example from a press apparatus following the Kraft process. Alternatively, the cellulose pulp may be provided as “fully dried” (softwood) pulp at a consistency of more than 90%, for example from a dryer following the Kraft process.

The invention is based on the insight that by splitting the caustic extraction into two different stages comprising a first stage run at low temperature (a CCE stage) and a second stage at higher temperature (a HCE stage), not only the C5 hemicellulose content, but also the C6 hemicellulose content may be reduced (C5 in the first stage and C6 in the second stage). The invention is thus particularly advantageous when the cellulose pulp has been produced from lignocellulose material comprising C6 hemicellulose (glucomannan) such as softwood.

The invention is further based on the insight that the HCE stage can be used for viscosity control. The method according to the first aspect of the invention is advantageous since it may be used for producing low-hemicellulose pulp from paper grade pulp, i.e. for “upgrading” paper grade pulp. This allows paper grade pulp from an already existing Kraft pulping plant to be used for producing low-hemicellulose pulp without making substantial modifications to the Kraft pulping plant. Further, production of paper grade pulp and low-hemicellulose pulp can be produced simultaneously from a single Kraft pulping plant by subjecting only a part of the pulp from the Kraft process (being paper grade pulp) to the method according to the first aspect of the invention to produce low-hemicellulose pulp, where the remaining part of the pulp is used for paper production (or for other purposes).

It is understood that the CCE and HCE stages each comprise a reaction step in which the cellulose pulp is subjected to caustic extraction using a caustic solution, and further that at least the HCE stage comprises, or is followed by, a washing step in which the dissolved hemicelluloses are removed as part of the washing filtrate.

The CCE stage may be conducted at a temperature of 30-50 °C. The CCE stage may be carried out for a duration of 10-20 minutes. The HCE stage may be conducted at a temperature of 60-120 °C. The pulp consistency in the CCE-stage and/or the HCE-stage may be 6-10 weight-%.

In embodiments, the alkali charge in the CCE stage and/or in the HCE stage is equivalent to 60-120 g sodium hydroxide /I or equivalent to 5-12 weight-% sodium hydroxide. Alkali may be charged in both the CCE and HCE stages, but preferably alkali is charged solely in the CCE stage. Alkali may be added in the form of a caustic solution being for example a sodium hydroxide solution.

In embodiments, the method further comprises washing the cellulose pulp between the CCE and HCE stages. Put differently, the CCE stage comprises a washing step. In embodiments where alkali is added solely in the CCE stage, the washing between the CCE and HCE stages (or as part of the CCE stage) may advantageously be omitted, however.

In embodiments, the method further comprises acid hydrolysis of the cellulose pulp prior to the CCE stage. This may be advantageous since such an acid hydrolysis affects viscosity and also metal content of the cellulose pulp, which allows the viscosity and metal content to be optimized. The method may further comprise controlling the viscosity of the pulp by controlling the acid charge and/or the temperature and/or the residence time of the acid hydrolysis.

In embodiments, the method further comprises controlling the viscosity of the pulp by controlling the alkali charge and/or the temperature of the HCE stage. Advantageously, this embodiment is combined with the above-described embodiment where viscosity of the pulp by controlling the acid charge and/or the temperature and/or the residence time of the acid hydrolysis. By controlling the operating conditions of both the acid hydrolysis stage and the HCE stage, the viscosity of the pulp can be optimized with regard to other parameters such as metal content, cellulose content and hemicellulose content. In embodiments, the method further comprises heating the cellulose pulp between the CCE and HCE stages and/or as part of the HCE stage. The heating may comprise increasing the temperature from 30-50°C (in the CCE stage) to 60-120 °C (in the HCE stage). Heating of the pulp is advantageously achieved by adding steam to the biomass, for example using at least one steam mixer, i.e. one or more mixers comprising steam injection nozzles to inject steam directly into the mixer(s) for mixing/heating with the pulp. Another option would be to heat the pulp by diluting the pulp with a dilution liquid of high temperature but using steam for heating is preferable since the consistency of the pulp is not substantially decreased. The consistency in both the CCE-stage and the HCE-stage is preferably 6-10 weight-%.

According to a second aspect of the invention, there is provided a method for producing pulp from lignocellulose material, the method comprising subjecting the lignocellulose material to a kraft pulping process to obtain cellulose pulp and subjecting at least part of the cellulose pulp obtained from the kraft pulping process to the method according to the first aspect of the invention or embodiments thereof.

In embodiments of the method according to the second aspect of the invention, only part of the cellulose pulp obtained from the kraft pulping process is subjected to the method according to the first aspect of the invention or embodiments thereof, wherein the remaining cellulose pulp obtained from the kraft pulping process is output for use in a paper production process.

In embodiments of the method according to the second aspect of the invention, the kraft pulping process does not comprise a pre-hydrolysis stage and/or is not preceded by a prehydrolysis stage. In other words, the kraft pulping process is not a PHK process.

In embodiments of the method according to the second aspect of the invention, the lignocellulose material comprises glucomannan. For example, the lignocellulose material comprises softwood.

According to a third aspect of the invention, there is provided a system for producing pulp from lignocellulose material. The method comprises a kraft pulping plant arranged to produce cellulose pulp from the lignocellulose material, a cold caustic extraction, CCE, arrangement connected to the pulping plant to receive cellulose pulp therefrom, said CCE arrangement comprising means for adding a caustic solution to the cellulose pulp for caustic extraction of the cellulose pulp, and a hot caustic extraction, HCE, arrangement connected to the CCE arrangement to receive cellulose pulp therefrom, said HCE arrangement comprising means for adding a caustic solution to the cellulose pulp for caustic extraction of the cellulose pulp; said HCE arrangement being configured to operate at a higher temperature than said CCE arrangement.

In embodiments of the system, the HCE arrangement comprises a reactor in which said caustic solution is added and said caustic extraction takes place, and a washing device arranged to wash the cellulose pulp from the reactor to remove a filtrate comprising hemicellulose dissolved in the CCE and/or HCE reactor.

In embodiments of the system, the system further comprises an acid hydrolysis arrangement upstream of the CCE stage. The system may furthermore comprise a control system configured to control at least one operating parameter of the HCE arrangement (the alkali charge and/or the temperature) and at least one operating parameter of the acid hydrolysis arrangement (acid charge and/or the temperature and/or the residence time of the acid hydrolysis). In this manner, the viscosity of the pulp can be optimized with regard to other parameters such as metal content, cellulose content and hemicellulose content.

The features of the embodiments described above are combinable in any practically realizable way to form embodiments having combinations of these features. Further, all features and advantages of embodiments described above with reference to the first aspect of the invention may be applied in corresponding embodiments of the method according to the second aspect of the invention and in corresponding embodiments of the system according to the third aspect of the invention, and vice versa.

BRIEF DESCRIPTION OF THE DRAWINGS

Above discussed and other aspects of the present invention will now be described in more detail using the appended drawings, which show presently preferred embodiments of the invention, wherein: fig. 1 illustrates an embodiment of the method according to the first aspect of the invention, and fig. 2 illustrates an embodiment of the method according to the second aspect of the invention comprising an embodiment of the method according to the first aspect of the invention.

DETAILED DESCRIPTION

Fig. 1 illustrates an embodiment of the method according to the first aspect of the invention. Cellulose pulp produced from softwood using a Kraft process is provided, which cellulose pulp may be a paper grade pulp, i.e. Kraft pulp having significantly higher hemicellulose content than dissolving pulp. The cellulose pulp is first subjected to CCE extraction step 1 , which typically is carried out in a reactor/vessel in which alkali/caustic solution is added at a charge equivalent to 5-12 weight-% sodium hydroxide. The CCE extraction is carried out at a temperature of 30-50°C. The retention time in the CCE extraction step may be 10-20 minutes.

After the CCE extraction 1 , the pulp is optionally washed 2 such that a filtrate comprising spent caustic solution, spent washing liquid and hemicelluloses (mostly C5 hemicellulose, i.e. pentosan) is removed from the cellulose pulp. The optional washing 2 may be considered a part of the CCE stage, i.e. the CCE stage corresponds to 1 and 2.

The cellulose pulp is thereafter subjected to HCE extraction 3 which is also carried out in a reactor/vessel. In this embodiment no heating step is shown explicitly, but it is understood that the HCE extraction 3 comprises, or is preceded by, heating the pulp to a higher temperature of 60-120 °C. The retention time in the HCE extraction step may be 30-120 minutes. The heating is preferably achieved using at least one steam mixer, i.e. one or more mixers comprising steam injection nozzles to inject steam directly into the mixer(s) for mixing/heating with the pulp. Caustic solution is optionally added also to the HCE extraction step 3 such that the alkali charge is equivalent to 5-12 weight % sodium hydroxide also in the HCE extraction step 3. If the optional washing 2 is omitted, addition of caustic solution in the HCE extraction step may be omitted however since part of the alkali charge from the CCE stage is carried over.

After the HCE extraction step 3, the pulp is washed 4 such that a filtrate comprising spent caustic solution, spent washing liquid and hemicelluloses is removed from the cellulose pulp. The washing 4 may be considered a part of the HCE stage, i.e. the HCE stage corresponds to 3 and 4. The hemicellulose removed in stage 4 is mainly C6 hemicellulose, i.e. glucomannan. If the CCE washing 2 is omitted, the filtrate removed in step 4 also comprises substantial amounts of the C5 hemicellulose extracted in the CCE extraction 1.

Fig. 2 illustrates an embodiment of the method according to the second aspect of the invention. Lignocellulose material in the form of softwood wood chips is provided to the Kraft pulping process 8. The Kraft pulping process is well known in the art and will not be described in further detail here. It is noted however that the Kraft pulping process does not comprise (and is not preceded by) a pre-hydrolysis step, i.e. is not a PHK-process. It is understood the cellulose pulp provided from the step 8 is purified pulp (washed and screened brown stock). The cellulose pulp from step 8 is optionally subjected to an acid hydrolysis stage 9 in which acid is added followed by an optional washing stage 10. Thereafter, the cellulose pulp is subjected to CCE extraction 11 , HCE extraction 13 and HCE washing 14 which steps correspond to steps 1 , 3 and 4 described above with reference to fig. 1. In this embodiment, there is no CCE washing step, however. Furthermore, alkali/caustic solution is added solely in the CCE extraction step 11.

It is noted that steps 9-11 and 13-14 in fig. 2 also illustrate an embodiment of the method according to the first aspect of the invention.

Fig. 2 could also be said to illustrate an embodiment of the system according to the third aspect of the invention, which embodiment comprises a kraft pulping plant 8, a pre-hydrolysis arrangement 9, a washing arrangement 10, a CCE arrangement 11 , a HCE arrangement 13, 14 and an optional bleaching arrangement 15. Such a plant and arrangements are all well- known in the art and will not be described in further detail here.

In the embodiments described above with reference to figs. 1 and 2, the consistency of the pulp fed to the CCE-stage and the HCE-stage is 6-10 weight- % (medium consistency).

The description above and the appended drawings are to be considered as non-limiting examples of the invention. The person skilled in the art realizes that several changes and modifications may be made within the scope of the invention. For example, the methods may comprise additional stages after the HCE stage to further reduce lignin content and/or to control viscosity. As further examples, the embodiment in fig. 1 may comprise an acid hydrolysis step and a washing step corresponding to refs. 10-11 in fig. 2, and the embodiment in fig. 2 may comprise a CCE washing step corresponding to ref. 2 in fig. 1.

Claims

1. Method for reducing the hemicellulose content of cellulose pulp obtained from a Kraft process, said method comprising subjecting the cellulose pulp to a cold caustic extraction, CCE, stage (1 , 2; 11) followed by a hot caustic extraction, HCE, stage (3, 4; 13, 14).

2. Method according to claim 1 , wherein said CCE stage (1 , 2; 11) is conducted at a temperature of 30-50 °C.

3. Method according to claim 2, wherein said CCE stage is carried out for a duration of 10-20 minutes.

4. Method according to any of the preceding claims, wherein said HCE stage (3, 4; 13, 14) is conducted at a temperature of 60-120 °C.

5. Method according to any of the preceding claims, wherein alkali is added solely to said CCE stage (11).

6. Method according to any of claims 1-4, further comprising washing the cellulose pulp between the CCE and HCE stages or washing the cellulose pulp as part of the CCE stage (1 , 2).

7. Method according to any of the preceding claims, further comprising acid hydrolysis (9) of the cellulose pulp prior to the CCE stage (11).

8. Method according to claim 7, further comprising controlling the viscosity of the pulp by controlling the acid charge and/or the temperature and/or the residence time of the acid hydrolysis (9).

9. Method according to any of the preceding claims, further comprising controlling the viscosity of the pulp by controlling the alkali charge and/or the temperature of the HCE stage (3, 4; 13, 14).

10. Method according to any of the preceding claims, further comprising heating the cellulose pulp between the CCE and HCE stages and/or as part of the HCE stage (3, 4; 13, 14).

11. Method according to any of the preceding claims, wherein said HCE stage (3, 4; 13, 14) comprises, or is followed by, washing the cellulose pulp after caustic extraction to remove a filtrate comprising hemicellulose dissolved during said caustic extraction.

12. Method according to any of the preceding claims, wherein said CCE stage is conducted with an alkali charge equivalent to 60-120 g sodium hydroxide /I or equivalent to 5-12 weight-% sodium hydroxide.

13. Method according to any of the preceding claims, wherein said cellulose pulp obtained from the Kraft process is a paper grade cellulose pulp formed from softwood.

14. Method for producing pulp from lignocellulose material, said method comprising: subjecting the lignocellulose material to a kraft pulping process (8) to obtain cellulose pulp, and subjecting at least part of the cellulose pulp obtained from the kraft pulping process to the method according to any of the preceding claims. Method according to claim 12, wherein only part of the cellulose pulp obtained from the kraft pulping process (8) is subjected to the method according to any of the preceding claims, and wherein the remaining cellulose pulp obtained from the kraft pulping process (8) is output for use in a paper production process. Method according to claim 12 or 13, wherein said kraft pulping process (8) does not comprise a pre-hydrolysis stage and/or is not preceded by a pre-hydrolysis stage. Method according to any of claims 12-14, wherein said lignocellulose material comprises glucomannan. System for producing pulp from lignocellulose material comprising softwood, said method comprising: a kraft pulping plant arranged to produce cellulose pulp from the lignocellulose material, a cold caustic extraction, CCE, arrangement connected to the pulping plant to receive cellulose pulp therefrom, said CCE arrangement comprising means for adding a caustic solution to the cellulose pulp for caustic extraction of the cellulose pulp, said CCE arrangement being configured for reducing the hemicellulose content of the cellulose pulp, and a hot caustic extraction, HCE, arrangement connected to the CCE arrangement to receive cellulose pulp therefrom, said HCE arrangement comprising means for adding a caustic solution to the cellulose pulp for caustic extraction of the cellulose pulp; said HCE arrangement being configured to operate at a higher temperature than said CCE arrangement, said HCE arrangement being configured for reducing the hemicellulose content of the cellulose pulp. System according to claim 18, wherein said kraft pulping plant is arranged to produce paper grade cellulose pulp from the lignocellulose material comprising softwood.