TECHNICAL FIELD

The present invention relates to a method and assembly for optimizing filtrate circulation in a Kraft process, the assembly comprising a cold caustic extraction reactor for separating hemicellulose from pulp and at least one washer, arranged to receive input pulp and input washing liquid, dilute the pulp with the washing liquid and dewater the pulp to produce output pulp and spent washing liquid.

BACKGROUND

When producing high grade dissolving pulp or dissolving cellulose, a cooking step and washing is followed by a treatment in a cold caustic extraction reactor in order to remove hemicellulose before being washed 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 generally high, requiring large amounts of fresh wash water to be added and a large extraction flow from the cooking stage that must be treated by evaporation or other cleaning processes before being able to be re-used in the process.

The benefits of decreasing the water consumption of the process are significant, and indeed the spent liquid from each process stage is generally used as input liquid in another process stage until re-use is no longer possible, and the liquid must be removed and cleaned. However, there is still a need for further minimizing the water requirements and especially of decreasing the liquor load on the cooking to reduce the extraction capacity needed and to reduce the steam consumption required to evaporate the excess liquid.

Some prior art solutions exist, such as WO2013/178608, WO2011/138633 and SE1651739. However, further improvements are necessary in order to achieve a more efficient process for producing dissolving pulp that is also cost effective.

SUM MARY OF THE INVENTION

The object of the present invention is to eliminate or at least to minimize the problems mentioned above. This is achieved through a method and assembly for optimizing filtrate circulation in a Kraft process according to the independent claims. Additional features of the present invention are defined in the dependent claims.

Thus, according to the present invention there is provided an assembly for optimizing filtrate circulation in a Kraft process, the assembly comprising

a digester for receiving wood chips and cooking the wood chips together with a cooking liquid to form a pulp,

a cold caustic extraction reactor for separating hemicellulose from the pulp, a first washer, arranged to receive input pulp and input washing liquid, dilute the pulp with the washing liquid and dewater the pulp to produce output pulp and spent washing liquid, and further comprising a dewatering press arranged to receive a CCE output pulp from the cold caustic extraction reactor and dewater said pulp to produce a dewatered CCE output pulp and a filtrate separated from the dewatered CCE output pulp, wherein the first washer is arranged to receive as input pulp the dewatered CCE output pulp from the dewatering press. Furthermore, the cooking liquid at least partly comprises filtrate from the dewatering press.

Thereby, the filtrate from the cold caustic reactor can be removed before the washing stages after the reactor, resulting in a more concentrated filtrate and a reduction in the amount of washing liquid required for the washing stages after the treatment in the cold caustic extraction reactor. Also, by using the concentrated filtrate removed after the reactor in the cooking process, the liquor load in the cooking stage is reduced due to the addition of alkaline filtrate. This is particularly advantageous since it renders the Kraft process more efficient and reduces the water consumption.

The addition of alkaline filtrate to a process for producing dissolving pulp has long been seen as disadvantageous since the filtrate also contains hemicellulose that generally needs to be removed from the pulp in order to produce a high-quality dissolving pulp. While it is known to be advantageous to add filtrate to cooking stages in other pulping processes, it has therefore been avoided when producing specialty pulp such as dissolving pulp that 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 so that each washer is arranged to receive as input pulp the output pulp from the washer before in the series and to receive as at least part of the input washing liquid at least part of the spent washing liquid from the washer after in the series, wherein the first washer that is arranged to receive as input pulp the dewatered CCE output pulp from the dewatering press is first in the series of washers. Thereby, the output pulp from the dewatering press is washed in a number of washing stages to produce the dissolved pulp without remaining filtrate from the cold caustic extraction reactor in order to reduce effluent load and recover chemicals.

According to a further aspect of the invention, the assembly also comprises at least one but preferably a plurality of washers arranged in series before the cold caustic extraction reactor, wherein the washer or the last in the series of washers is arranged to give as output pulp an input pulp to the cold caustic extraction reactor, and wherein at least one washer arranged before the cold caustic extraction reactor is arranged to receive as input washing liquid a spent washing liquid from the first washer after the CCE reactor. Thereby, the need for additional fresh water is further reduced, since the spent washing liquid from the washing stages after the reactor can be re-used as input liquid in the last washing stage before the reactor.

The invention also provides a method for optimizing filtrate circulation in a Kraft process, comprising cooking wood chips in a cooking stage in a digester to produce pulp,

treating pulp in a cold caustic extraction reactor to produce a CCE output pulp dewatering the CCE output pulp to produce a dewatered CCE output pulp and a filtrate, and

washing the dewatered CCE output pulp in a plurality of washing stages to produce a washed pulp wherein the concentrated filtrate is supplied as cooking liquid in the cooking stage in the digester. Advantageously, the filtrate is supplied as cooking liquid in a cooking stage of the Kraft process. Also, as also 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 washer in the plurality of washing stages arranged after the cold caustic extraction reactor produces a spent washing liquid that is used as input washing liquid in the washing stage before the cold caustic extraction reactor.

Many additional benefits and advantages of the invention will become readily apparent to the person skilled in the art in view of the detailed description below.

DRAWINGS

The invention will now be described in more detail with reference to the appended drawings, wherein

Fig. 1 discloses a flow diagram of a conventional Kraft process according to the prior art;

Fig. 2 discloses a flow diagram in more detail according to the prior art; and

Fig. 3 discloses a flow diagram of the present invention.

DETAILED DESCRIPTION

In the following, the Kraft process will be briefly described but details will be mentioned only where they are of importance for the present invention, since the Kraft process in general is already well known within the art. Generally, when it is said in the following that a liquid is an input liquid somewhere in the process, this is to be understood as that liquid forming at least part of the input liquid to that part of the process. Similarly, when it is said that an output liquid is used in some specific way in the process, this is also to be understood as at least a part of that output liquid being used in that way. It is still possible and sometimes desirable to combine liquids from different sources as an input and to divide an output into fractions that are used in different ways, as is readily understood by the skilled person.

The assembly and method according to the present invention are aimed at the production of specialty pulp such as dissolving pulp. In such processes, a high quality pulp is achieved through the removal of substances such as hemicellulose that are present in the raw material. Raw material is for this purpose any kind of lignocellulosic material. The term lignocellulosic material is used herein to mean materials containing lignin, cellulose and hemicellulose. One example of such materials is wood, others include other agricultural or forestry wastes. The lignocellulosic material is commonly divided into small pieces, chips or fragments before the pulping process is initiated. In the following, the term wood chips is used to denote the raw material.

Thus, Fig. 1 shows a flow diagram of a conventional Kraft process that involves feeding wood chips or other organic pulp-containing raw materials 51 to a digester 10 for delignification, generally referred to as a cooking stage. Also supplied to the digester 10 are white liquor 61 and additional liquors that are provided in order to achieve a desired alkaline concentration during cooking. Such additional liquors are generally a filtrate 64 from a cold caustic extraction reactor and possibly black liquor 62, 63 from an external source (not shown) or from the washing stage 20. After cooking, a brown solid cellulosic pulp 52 is released from the digester and is transported to a washing stage 20 where the pulp 52 is washed in at least one but generally a plurality of washing steps with washers arranged in series to separate the cellulose fibers from spent cooking liquors, generally referred to as black liquor. To the washing stage an input liquid 73 is also added, typically fresh water or condensate. The washing stage 20 also generally comprises a screening process where reject material such as shives, knots, dirt and debris are removed from the pulp.

After the washing stage 20, a cleaned brown stock pulp 53 is fed into a cold caustic extraction reactor 30 or CCE reactor 30, where a further separation is performed, separating dissolved materials (mainly hemicellulose) from the longer cellulosic fibers. Additional liquid is generally added to the cleaned brown stock pulp 53 before introduction into the reactor 30, such as white liquor 61 and/or spent washing liquid from a washing step immediately after the reactor 30.

After cold caustic extraction, a purified pulp or CCE output pulp 54 is transported to a CCE washing stage 40 and washed in at least one but generally a plurality of washing steps in order to remove spent cold caustic solution and dissolved hemicellulose and produce a purified brown pulp 55 that is transported to a subsequent bleaching stage (not shown) before the process is completed. The spent cold caustic solution and dissolved hemicellulose is referred to as the filtrate 64 and is supplied to the cooking stage where it is suitable due to its high alkaline content. Part of the filtrate 64 may also be removed to a recovery area 65 to limit an amount of hemicellulose added to the cooking stage. Washing liquid 67, generally in the form of fresh water or condensate is also supplied to the CCE washing stage 40.

Fig. 2 discloses the process of Fig. 1 with the washing stage 20 and the CCE washing stage 40 shown in more detail. Thus, from the digester 10 the brown solid cellulosic pulp 52 is transported by a pump 91 to a first washer 21 in the washing stage 20 where it is diluted with a washing liquid 71 and dewatered to form an output pulp that is inserted as input pulp into a second washer 22. A spent washing liquid 72 from the first washer 21 is mainly black liquor 63 that may be inserted into the digester 10 or removed to the recovery area 65. The input washing liquid 71 in the first washer is a spent washing liquid from the second washer 22. The washing stage 20 may include a higher number of washers but in this general flow diagram only two washers 21, 22 are shown. The last of the washers 22 in the washing stage 20 receives fresh water or condensate as input liquid 73.

The cleaned brown stock pulp 53 may be diluted with filtrate 64 before insertion into the CCE reactor 30, and the CCE output pulp 54 may also be diluted with the filtrate 64 before introduction into a first CCE washer 41 in the CCE washing stage 40 where a plurality of washers 41, 42, 43 are provided. The number of washers may differ but generally at least one and preferably at least three washers 41, 42, 43 are used. As in the previous washing stage 20, spent washing liquid 76, 77 from one washer 42, 43 is used as input washing liquid for the washer immediately before in the series and it may also be used to dilute the input pulp to 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 a preferred embodiment disclosed in Fig. 3. The main advantages provided by the invention in comparison to the prior art solution disclosed in Fig. 2 are that the filtrate 64 is undiluted, which reduces the liquor load on the digester 10 and the liquor extraction capacity from the digester 10, and that the amount of fresh water or condensate required for the washing steps before and after the CCE reactor can be kept to a minimum since the washing stage 20 uses spent washing liquid 75 from the CCE washing stage 40 as input washing liquid. This also minimizes the amount of liquid sent to recovery or evaporation, thereby reducing the necessary capacity for handling spent liquid in the process in general which reduces the steam required to evaporate the excess water added to the washing line.

Thus, in Fig. 3 the reference numerals that refer to similar or identical components and flows are denoted by the same reference numerals and those features that are already described above with reference to the prior art will not be described again below.

After the cleaned brown stock pulp 53 has been treated in the CCE reactor 30, the purified pulp or CCE output pulp 54 is transported to a dewatering stage 44 that may form part of the CCE washing stage 40 or be a separate stage before 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 cleaned brown stock pulp 56 with a higher dry solids content and this dewatered CCE output pulp 56 is used as input pulp to the first CCE washer 41. Before insertion into the first CCE washer 41 the pulp 56 may be diluted with spent washing liquid 75 from the first CCE washer 41 if desired. The CCE washing stage 40 is then performed in a similar way as described above with reference to the prior art solution of Fig. 2. In this embodiment, three washers 41, 42, 43 are disclosed in the CCE washing stage 40 but it is to be noted that the number of washers may be varied in a manner similar to the washers in the washing stage 20.

The dewatering press 44 in the dewatering stage 44 provides a concentrated filtrate 64' that is supplied to the digester 10 and that may also be used to dilute the CCE output pulp 54 before the dewatering stage 44. Part of the concentrated filtrate 64' may also be sent to recovery 65 if desired.

In the prior art shown in Fig. 2, the washing stage 20 requires an input liquid 73 that is generally fresh water or condensate, since the filtrate 64 from the CCE washing stage 40 is diluted with the spent washing liquid 76 from the washer 42. In the preferred embodiment of Fig. 3, the washing stage 20 instead receives spent washing liquid 75 from the first CCE washer 41 in the CCE washing stage 40.

To summarize, the method according to the present invention comprises treating pulp in the form of cleaned brown stock pulp 53 in the CCE reactor 30 to produce the CCE output pulp or purified pulp 54 and to dewater the CCE output pulp 54 in the dewatering stage 44 to produce the dewatered CCE output pulp 56 and the filtrate 64' that is a concentrated filtrate 64'. The dewatered CCE output pulp 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 liquid to the digester 10 where a cooking stage of the Kraft process is performed. Preferably, the brown solid cellulosic pulp 52 that forms output from the digester 10 is washed in at least one washer 21, 22 in the washing stage 20 to form the cleaned brown stock pulp 53 that is givens as input to the CCE reactor 30. The input washing liquid 75 to the washing stage 20 is produced at least partly by the first CCE washer 41 in the CCE washing stage. For any pulping process it is desired to reduce the water consumption in order to also reduce energy consumption. The re-use of liquids in different parts of the process is therefore a common strategy and there are some known processes where filtrate is re-used in cooking stages where addition of alkaline liquor is desired. However, it has not previously been known to use filtrate from the CCE reactor 30 in the digester 10 for cooking dissolving pulp since it has generally been thought to be disadvantageous to add substances already removed from the pulp at later stages. The reason for this is that substances such as hemicellulose should be removed in order to improve quality of the finished dissolving pulp and it has been thought that adding hemicellulose to the cooking would make it more difficult to reduce hemicellulose content of the pulp at the subsequent washing stages. However, in practice it has been shown that addition of alkaline filtrate to the digester for the cooking of dissolving pulp is actually advantageous due to the reduced liquor load at the cooking stage. This results in a lower water consumption of the pulping process as a whole and it is still possible to remove hemicellulose during the washing stage 20 and the CCE washing stage 40 without rendering the process less efficient than conventional pulping processes.

It is to be noted that features from the various embodiments described herein may freely be combined, unless it is explicitly stated that such a combination would be unsuitable.