CN112262240A

CN112262240A - Method and assembly for optimizing filtrate circulation in kraft pulping

Info

Publication number: CN112262240A
Application number: CN201980039115.2A
Authority: CN
Inventors: F·威尔格森
Original assignee: Valmet AB
Current assignee: Valmet Technologies Oy; Valmet AB
Priority date: 2018-06-15
Filing date: 2019-06-12
Publication date: 2021-01-22
Also published as: EP3810852A4; SE1850733A1; BR112020023628A2; EP3810852A1; WO2019240659A1

Abstract

The invention relates to an assembly for optimizing the circulation of concentrated filtrate (64') in kraft pulping, the assembly comprising: -a cold caustic extraction reactor (30) for separating hemicellulose from the pulp; -a first CCE scrubber (41) arranged to receive the input pulp and the input washing liquor, dilute the pulp with the washing liquor and dewater the pulp to produce output pulp and used washing liquor, and further comprising a dewatering press (44) arranged to receive CCE output pulp from the cold caustic extraction reactor (30) and dewater the pulp to produce dewatered CCE output pulp and a concentrated filtrate (64') separated from the dewatered CCE output pulp, wherein the first CCE scrubber (41) is arranged to receive the dewatered CCE output pulp from the dewatering press (44) as input pulp. The invention also relates to a method for optimizing the circulation of the concentrated filtrate (64').

Description

Method and assembly for optimizing filtrate circulation in kraft pulping

Technical Field

The present invention relates to a method and an assembly for optimizing filtrate circulation in Kraft pulping (Kraft process), the assembly comprising a cold alkali extraction reactor for separating hemicellulose from pulp and at least one washer (washer) arranged to receive incoming pulp and incoming washing liquid, dilute the pulp with washing liquid and dewater the pulp to produce outgoing pulp and used washing liquid.

Background

When producing high grade dissolving pulp or dissolving cellulose, after the cooking step and washing, treatment is performed in a cold caustic extraction reactor to remove hemicellulose prior to washing to produce the desired output pulp and recover chemicals. A large number of washing steps are performed and the water consumption of the process is usually high, requiring the addition of large amounts of fresh washing water, and large amounts of water are extracted from the cooking stage, which must be treated by evaporation or other purification processes and can then be reused in the process.

The benefits of reducing the water consumption of the process are significant and in fact the spent liquor from each process stage is typically used as an input liquor to another process stage until it cannot be used again and the liquor must be removed and purified. However, there is still a need to further minimize water requirements, especially to reduce liquid load in the cooking, to reduce the required extraction capacity and to reduce the steam consumption required to evaporate excess liquid.

There are some prior art solutions such as WO2013/178608, WO2011/138633 and SE 1651739. However, further improvements are needed to achieve a more efficient process for producing dissolving pulp, which is also cost effective.

Disclosure of Invention

It is an object of the present invention to eliminate or at least minimize the above-mentioned problems. This is achieved by a method and an assembly for optimizing filtrate circulation in kraft pulping according to the independent claims. Additional features of the invention are defined in the dependent claims.

Thus, according to the present invention, there is provided an assembly for optimizing filtrate circulation in kraft pulping, the assembly comprising

-a digester for receiving wood chips and digesting the wood chips with a cooking liquor to form a slurry,

-a cold caustic extraction reactor for separating hemicellulose from the pulp;

-a first scrubber arranged to receive incoming pulp and incoming washing liquid, dilute the pulp with washing liquid and dewater the pulp to produce outgoing pulp and used washing liquid,

and further comprising a dewatering press arranged to receive the CCE output pulp from the cold caustic extraction reactor and dewater the pulp to produce a dewatered CCE output pulp and a filtrate separated from the dewatered CCE output pulp, wherein the first scrubber is arranged to receive the dewatered CCE output pulp from the dewatering press as an input pulp. Furthermore, the cooking liquor at least partly comprises filtrate from the dewatering press.

Thereby, the filtrate from the cold caustic reactor can be removed before the washing stage after the reactor, resulting in a more concentrated filtrate and reducing the amount of washing liquid required for the washing stage after treatment in the cold caustic extraction reactor. Furthermore, by using concentrated filtrate removed after the reactor in the cooking process, the liquid load in the cooking stage is reduced due to the addition of alkaline filtrate. This is particularly advantageous as it makes the kraft pulping more efficient and reduces water consumption.

The addition of alkaline filtrate to the process for producing dissolving pulp has been seen as disadvantageous because the filtrate also contains hemicellulose, which usually needs to be removed from the pulp in order to produce a high quality dissolving pulp. Although it is advantageous to add filtrate to the cooking stage in other pulping processes, it has been avoided when producing special pulps such as dissolving pulps, which should have a low content of hemicellulose in the finished product.

According to another aspect of the invention, the assembly further comprises a plurality of washers arranged in series, such that each washer is arranged to receive as input pulp the output pulp from a washer before the series and to receive as at least part of the input wash liquor at least part of the used wash liquor from a washer after the series, wherein the first washer arranged to receive as input pulp the dewatered CCE output pulp from the dewatering press is the first one in the series of washers. Thus, the output pulp from the dewatering press is washed in multiple washing stages to produce a dissolved pulp without residual filtrate from the cold caustic extraction reactor in order to reduce wastewater load and recover chemicals.

According to another aspect of the invention, the assembly further comprises at least one, but preferably a plurality of scrubbers arranged in series before the cold caustic extraction reactor, wherein the scrubber or the last one of the series of scrubbers is arranged to provide the input slurry as output slurry to the cold caustic extraction reactor, and wherein the at least one scrubber arranged before the cold caustic extraction reactor is arranged to receive used scrubbing liquid from the first scrubber after the CCE reactor as input scrubbing liquid. Thereby, the need for additional fresh water is further reduced, as the used wash liquid from the wash stage after the reactor can be reused as input liquid in the last wash stage before the reactor.

The present invention also provides a method for optimizing filtrate circulation in kraft pulping, the method comprising:

-cooking the chips in a digester in a cooking stage to produce a pulp,

-treating the pulp in a cold caustic extraction reactor to produce a CCE output pulp,

-dewatering the CCE output pulp to produce dewatered CCE output pulp and filtrate, an

Washing the dewatered CCE output pulp in a plurality of washing stages to produce washed pulp,

wherein the concentrated filtrate is supplied as cooking liquor in a digester in a cooking stage.

Advantageously, the filtrate is supplied as cooking liquor in the cooking stage of kraft pulping. Also, as mentioned above, the pulp is washed in at least one washing stage before being treated in the cold caustic extraction reactor, and wherein a first scrubber of a plurality of washing stages arranged after the cold caustic extraction reactor produces used washing liquid which is used as input washing liquid in the washing stage before the cold caustic extraction reactor.

Numerous other benefits and advantages of the present invention will become apparent to those skilled in the art upon consideration of the following detailed description.

Drawings

The invention will now be described in more detail with reference to the accompanying drawings, in which

FIG. 1 discloses a flow diagram of conventional kraft pulping according to the prior art;

fig. 2 discloses a flow chart according to the prior art in more detail;

fig. 3 discloses a flow chart of the present invention.

Detailed Description

In the following, kraft pulping will be described briefly, but details are only mentioned where important to the invention, since kraft pulping in general is well known in the art. Generally, when it is stated in the following that the liquid is an input liquid somewhere in the process, it is to be understood that the liquid forms at least a part of the input liquid in that part of the process. Similarly, when it is described that the output liquid is used in a certain way in the process, this is to be understood that at least a part of the output liquid is used in this way. As the skilled person will readily understand, it is still possible and sometimes desirable to combine liquids from different sources as inputs and divide the output into portions that are used in different ways.

The assembly and method according to the invention are intended for the production of specialty pulps such as dissolving pulps. In such a process, high quality pulp is obtained by removing substances such as hemicellulose present in the raw material. For this purpose, the raw material is any kind of lignocellulosic material. The term "lignocellulosic material" is used herein to mean a material comprising lignin, cellulose and hemicellulose. One example of such material is wood, other examples include other agricultural or forestry waste. Before starting the pulping process, the lignocellulosic material is typically divided into chips, sheets or fragments. Hereinafter, the term "wood chips" is used to denote the raw material.

Thus, fig. 1 shows a flow diagram of a conventional kraft pulping process, which includes feeding wood chips or other organic pulp-containing raw material 51 to a digester 10 for delignification, commonly referred to as a cooking stage. White liquor 61 and additive liquor are also supplied to the digester 10, which are provided to achieve the desired alkali concentration during cooking. Such added liquor is typically filtrate 64 from a cold caustic extraction reactor and may be

black liquor

62, 63 from an external source (not shown) or from the washing stage 20.

After cooking, brown solid cellulose pulp 52 is released from the digester and passed to a washing stage 20, in which the pulp 52 is washed in at least one, but usually a plurality of washing steps, the series of washers being arranged to separate cellulose fibres from the spent cooking liquor, usually called black liquor. An input liquid 73, typically fresh water or condensate, is also added to the washing stage. The washing stage 20 also typically includes a screening process in which waste material, such as fines, knots, dirt and debris, is removed from the pulp.

After the washing stage 20, the cleaned brown raw material slurry 53 is fed into a cold caustic extraction reactor 30 or CCE reactor 30 where further separation is performed to separate the dissolved material (mainly hemicellulose) from the longer cellulose fibres. Before introduction into the reactor 30, additional liquid is typically added to the purified brown feedstock slurry 53, such as white liquor 61 and/or spent wash liquor from the washing step immediately after the reactor 30.

After cold caustic extraction, the purified pulp or CCE output pulp 54 is transported to the CCE washing stage 40 and washed in at least one, but typically multiple washing steps to remove spent cold caustic solution and dissolved hemicellulose and produce a purified brown pulp 55, which is transported to a subsequent bleaching stage (not shown) before the process is completed. The spent cold alkaline solution and dissolved hemicellulose are called filtrate 64 and are supplied to their appropriate cooking stage due to their high alkali content. A portion of the filtrate 64 may also be moved to a recovery area 65 to limit the amount of hemicellulose added to the cooking stage. A washing liquid 67, typically in the form of fresh water or condensed water, is also supplied to the CCE washing stage 40.

Fig. 2 discloses the process of fig. 1, wherein the washing stage 20 and the CCE washing stage 40 are shown in more detail. Thus, the brown solid cellulose pulp 52 is transported by means of a pump 91 from the digester 10 to the first washer 21 in the washing stage 20, where it is diluted and dewatered by the washing liquid 71 to form an output pulp, which is inserted (input) as input pulp to the second washer 22. The used washing liquid 72 from the first washer 22 is mainly black liquor 63, which may be inserted into the digester 10 or moved to the recovery zone 65. The incoming wash liquor 71 in the first scrubber is spent wash liquor from the second scrubber 22. The washing stage 20 may comprise a larger number of washers, but in this general flow chart only two

washers

21, 22 are shown. The last scrubber 22 in the washing stage 20 receives fresh or condensed water as input liquid 73.

The clarified brown feedstock pulp 53 may be diluted with filtrate 64 prior to insertion into the CCE reactor 30, and the CCE output pulp 54 may also be diluted with filtrate 64 prior to introduction into the first CCE scrubber 41 of the CCE washing stage 40 provided with a plurality of

scrubbers

41, 42, 43. The number of scrubbers may vary, but typically at least one, and preferably at least three

scrubbers

41, 42, 43 are used. As in the previous washing stage 20, used

washing liquid

76, 77 from one

washer

42, 43 is used as input washing liquid for the washer immediately preceding the series, and it can also be used to dilute input pulp into each washer. The last washer 43 in the CCE washing stage 40 receives fresh water 67 as input liquid.

The present invention will now be described in more detail with reference to the preferred embodiment disclosed in fig. 3.

The main advantage provided by the present invention compared to the prior art solution disclosed in fig. 2 is that the filtrate 64 is not diluted, which reduces the liquid load on the digester 10 and the capacity to extract liquid from the digester 10, and since the washing stage 20 uses the used washing liquor 75 from the CCE washing stage 40 as input washing liquor, the amount of fresh or condensed water required for the washing steps before and after the CCE reactor can be kept to a minimum. This also minimizes the amount of liquid sent to recovery or evaporation, thereby generally reducing the necessary capacity in the process to treat used liquid, which reduces the steam required to evaporate excess water added to the wash line.

Therefore, in fig. 3, reference numerals indicating similar or identical components and processes are denoted by the same reference numerals, and those features that have been described above with reference to the prior art will not be described below.

After the clarified brown feedstock pulp 53 has been processed in the CCE reactor 30, the purified pulp or CCE output pulp 54 is transported to a dewatering stage 44, which may form part of the CCE washing stage 40 or may be a separate stage prior to the CCE washing stage 40. In the dewatering unit, the CCE output pulp 54 is dewatered to reduce the amount of liquid present in the pulp. The result is a clarified brown feedstock slurry 56 having a higher dry solids content, and the dewatered CCE output slurry 56 is used as the input slurry for the first CCE scrubber 41. Prior to insertion into the first CCE washer 41, the pulp 56 may be diluted with used wash liquor 75 from the first CCE washer, if desired. The CCE washing phase 40 is then performed in a similar manner as described above with reference to the prior art solution of figure 2. In this embodiment, three

scrubbers

41, 42, 43 are disclosed in the CCE washing stage 40, but it is noted that the number of scrubbers may vary in a similar manner to the scrubbers in the washing stage 20.

The dewatering press 44 in the dewatering stage 44 provides a concentrated filtrate 64', which concentrated filtrate 64' is supplied to the digester 10 and may also be used to dilute the CCE output pulp 54 prior to the dewatering stage 44. A portion of the concentrated filtrate 64' may also be sent to a recovery section 65, if desired.

In the prior art shown in fig. 2, the washing stage 20 requires an input liquid 73, typically fresh or condensed water, since the filtrate 64 from the CCE washing stage 40 is diluted by the used washing liquid 76 from the washer 42. In the preferred embodiment of fig. 3, the washing stage 20 instead receives the used washing liquor 75 from the first CCE washer 41 in the CCE washing stage 40.

In summary, the method according to the present invention comprises treating the pulp in the CCE reactor 30 in the form of a clarified brown stock pulp 53 to produce a CCE output pulp or purified pulp 54 and dewatering the CCE output pulp 54 in a dewatering stage 44 to produce a dewatered CCE output pulp 56 and a filtrate 64 'which is a concentrated filtrate 64'. The dewatered CCE output slurry 56 is then washed in a plurality of

washers

41, 42, 43 arranged in series in the CCE washing stage 40. The concentrated filtrate 64' is supplied as cooking liquor to the digester 10, in which the cooking stage of kraft pulping is performed. Preferably, the brown solid cellulose pulp 52 forming the output from the digester 10 is washed in the washing stage 20 in at least one

washer

21, 22 to form a purified brown feedstock pulp 53, which is provided as input to the CCE reactor 30. The input wash liquor 75 to the wash stage 20 is at least partially generated by the first CCE scrubber 41 in the CCE wash stage.

For any pulping process, it is desirable to reduce water consumption to also reduce energy consumption. Therefore, it is a common strategy to reuse the liquor in different parts of the process, and in some known processes the filtrate is reused in the cooking stage where it is desired to add alkaline liquor. However, it was not previously known to use the filtrate from the CCE reactor 30 in the digester 10 for digesting dissolving pulp, as it is generally considered disadvantageous to add material that has been removed from the pulp at a later stage. The reason for this is that substances such as hemicellulose should be removed to improve the quality of the final dissolving pulp, and it is believed that adding hemicellulose in cooking will make it more difficult to reduce the hemicellulose content of the pulp in the subsequent washing stage. . However, it has been shown in practice that the addition of alkaline filtrate to the digester for digesting dissolving pulp is actually advantageous due to the reduced liquid load in the digestion stage. This results in an overall reduction in the water consumption of the pulping process and it is still possible to remove hemicellulose during the wash stage 20 and the CCE wash stage 40 without making the process less efficient than conventional pulping processes.

It is noted that features from the various embodiments described herein may be freely combined unless such combination is explicitly indicated to be inappropriate.

Claims

1. Assembly for optimizing the circulation of concentrated filtrate (64') in kraft pulping, said assembly comprising

-a digester (10) for receiving wood chips and digesting the wood chips with a cooking liquor to form a pulp,

-a cold caustic extraction reactor (30) for separating hemicellulose from the pulp;

-a first CCE washer (41) arranged to receive an input pulp and an input washing liquor, dilute the pulp with the washing liquor and dewater the pulp to produce an output pulp and a used washing liquor,

and further comprising a dewatering press (44) arranged to receive the CCE output pulp from the cold caustic extraction reactor (30) and dewater the pulp to produce a dewatered CCE output pulp and a concentrated filtrate (64') separated from the dewatered CCE output pulp,

wherein the first CCE washer (41) is arranged to receive the dewatered CCE output pulp from the dewatering press (44) as input pulp, and

wherein the cooking liquor at least partly comprises a concentrated filtrate (64') from the dewatering press (44).

2. The assembly according to claim 1, further comprising a plurality of scrubbers (41, 42, 43) arranged in series such that each scrubber is arranged to receive as input slurry output slurry from a scrubber before the series and to receive as at least a portion of input wash liquor at least a portion of used wash liquor from a scrubber after the series, wherein the first CCE scrubber (41) arranged to receive as input slurry the dewatered CCE output slurry from the dewatering press (44) is the first in the series of scrubbers.

3. The assembly according to claim 2, further comprising at least one, but preferably a plurality of, washers (21, 22), the series of washers (21, 22) being arranged after the digester (10) and before the cold caustic extraction reactor (30), wherein the last of the washer or series of washers is arranged to provide input pulp as output pulp to the cold caustic extraction reactor (30), wherein at least one washer arranged before the cold caustic extraction reactor (30) is arranged to receive used wash liquor from the first CCE washer (41) as input wash liquor.

4. A method for optimizing concentrated filtrate (64') circulation in kraft pulping, the method comprising:

-cooking the chips in a digester (10) in a cooking stage to produce a pulp,

-treating the pulp in a cold caustic extraction reactor (30) to produce a CCE output pulp,

-dewatering the CCE output pulp to produce dewatered CCE output pulp and a concentrated filtrate (64'), and

-washing the dewatered CCE output pulp in a plurality of washing stages to produce washed pulp,

wherein the concentrated filtrate (64') is supplied as cooking liquor in the digester (10) in the cooking stage.

5. The method according to claim 4, characterized in that the pulp is washed in at least one washing stage before treatment in the cold caustic extraction reactor (30), and wherein the first CCE washer (41) of a plurality of washing stages arranged after the cold caustic extraction reactor (30) produces used washing liquor which is used as input washing liquor in the washing stage before the cold caustic extraction reactor (30).