EP2992136A1

EP2992136A1 - Method and system for washing of cellulose pulp

Info

Publication number: EP2992136A1
Application number: EP14792129.0A
Authority: EP
Inventors: Anders HEDBLOM
Original assignee: Valmet Oy; Valmet AB
Current assignee: Valmet Oy; Valmet AB
Priority date: 2013-05-03
Filing date: 2014-04-25
Publication date: 2016-03-09
Also published as: CN105189859A; SE537925C2; SE1300319A1; EP2992136A4; WO2014178780A1

Abstract

A method and system for washing cellulose pulp are provided. The pulp is washed two times in the same washing space of a washing apparatus (10) with a movable permeable processing surface (11), filtrate from the pulp passes through the processing surface as the pulp is transported together with the processing surface. There is an intermediate pulp storage (20) connected to the washing apparatus (10) and arranged to store the pulp between the two washing operations (I, II).

Description

METHOD AND SYSTEM FOR WASHING OF CELLULOSE PULP

TECHNICAL FIELD

The present invention relates to treatment of cellulose pulp and more specifically to a method and a system for washing of cellulose pulp.

BACKGROUND

Pulp washing (also known as fiber washing) is a key operation in the treatment of cellulose pulp. Washing is performed at several positions in the pulping line, such as after the digester, in the screening department, and following oxygen reactors and bleaching towers, in order to remove liquor (with chemicals, dirt, dissolved substances, etc) from the pulp.

There are many different types of washers available, including wash presses, dewatering presses, wire presses, screw presses, diffusers, filters and drum washers. A well-known washing apparatus is a twin-roll press of the general type disclosed in US Patent 3,980,518, for example. A twin-roll press has two counter-rotating rolls with perforated outer surfaces. The incoming pulp is distributed lengthwise onto the respective roll. A web of pulp is formed on the respective roll and transported in the direction of rotation in a vat partially surrounding the rolls, until the so-called press nip between the rolls. The twin-roll press uses the washing principles of displacement, where dirty liquid (liquor) in the pulp is replaced by cleaner wash liquid added to the vat, and pressing, where dirty liquid is pressed/squeezed out from the pulp, in particular at the press nip.

A washing apparatus like the twin-roll press, for example, is associated with a limited performance. Sometimes two or more wash presses are arranged in series one after another in order to achieve a desired washing efficiency. However, washing apparatuses such as twin-roll presses are expensive and space-requiring machines. They also consume a lot of energy. Besides, introducing additional machines in the pulp mill is generally problematic.

SUMMARY

An object of the invention is to provide an improved method for washing of cellulose pulp. Another object is to provide an efficient method for washing cellulose pulp, which is comparatively inexpensive and/or requires little space.

These objects are achieved in accordance with the appended claims.

The present invention is based on the recognition that it is possible to achieve efficient washing by using one and the same washing apparatus for two (or more) non-simultaneous washing operations. The washing operations are separated in time by means of an intermediate pulp storage, such that pulp is washed batch-wise in the washing apparatus, and stored in the intermediate pulp storage between the washing operations.

More specifically, a method is provided for washing of cellulose pulp, said method comprising washing, in a first washing operation, the pulp for a first predetermined period of time using a washing apparatus with a movable permeable processing surface. Filtrate from the pulp passes through the processing surface as the pulp is transported together with the processing surface. The pulp washed in the first washing operation is received by an intermediate pulp storage connected to the washing apparatus and stored in the intermediate pulp storage. The pulp is then washed, in a second washing operation, for a second predetermined period of time using the washing apparatus, the pulp being received from the intermediate pulp storage and applied onto the processing surface of the washing apparatus, such that the pulp is washed again in the same washing space of the washing apparatus.

The washing and storing is preferably repeated such that the pulp is washed batch-wise using the intermediate pulp storage, each batch of pulp being washed in two separate washing operations, which occur in the same washing space of the same washing apparatus but are separate in time.

According to another aspect of the invention, a system for washing cellulose pulp is provided.

A general view within the pulping industry is that one should always strive for a continuous pulp flow and stable conditions in the pulping line. The invention, quite to the contrary, proposes to interrupt the pulp flow and perform loop washing in a batch-wise manner. Surprisingly, the washing performance/result becomes satisfactory despite the discontinuous flow through the washing apparatus.

Two (or more) conventional washing apparatuses can be replaced with a larger washing apparatus and an intermediate pulp storage arranged according to the invention. The washing apparatuses replaced may be arranged next to each other in the pulping line, or be from different parts of the pulping line associated with similar process conditions. The invention also means that demands for increased washing efficiency can be handled without the need for an additional washing apparatus. The proposed solution is comparatively inexpensive and consumes less energy than conventional washing with presses in series. It also requires less space in the pulp mill.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with further objects and advantages thereof, may best be understood by reference to the following description and appended drawings, in which:

Fig. 1 is a schematic block diagram of a system for washing cellulose pulp according to an exemplifying embodiment of the invention;

Fig. 2 is a schematic block diagram of a system for washing cellulose pulp according to another exemplifying embodiment of the invention; Fig. 3 is a schematic block diagram of a system for washing cellulose pulp according to still another exemplifying embodiment of the invention;

Fig. 4 is a schematic cross-sectional view illustrating a twin-roll press which is used in an exemplifying embodiment of the invention; and

Fig. 5 is a flow chart of a method for washing cellulose pulp according to an exemplifying embodiment of the invention.

DETAILED DESCRIPTION

In the drawings, similar or corresponding elements are denoted by the same reference numbers.

Fig. 1 is a schematic block diagram of a system for washing cellulose pulp according to an exemplifying embodiment of the invention. The illustrated system 100 includes a washing apparatus 10, an intermediate pulp storage 20, a first and a second process vessel 30, 40 connected to the washing apparatus 10, and a first and a second filtrate tank 51, 52. When the system is in operation, there is a first washing operation I, where pulp from the first process vessel 30 is washed in the washing apparatus 10 during transport together with a movable permeable processing surface 11 (e.g. a rotatable surface, such as the perforated outer surface of a press roll 15), and then led to the intermediate pulp storage 20. In a subsequent second washing operation II, pulp from the intermediate pulp storage 20 is washed in the same washing space of the same washing apparatus 10 as in the first washing operation I, and then output to the second process vessel 40.

During the first washing operation I, filtrate passing through the permeable processing surface 11 is collected into the first filtrate tank 51. During the second washing operation II, filtrate passing through the permeable processing surface 11 is collected into the second filtrate tank 52. In a preferred embodiment, filtrate from the second operation II is returned to the first washing operation I and used as washing liquid. Filtrate may also be used for dilution of the pulp. When the system 100 is in operation, there is normally a continuous flow of pulp in to the first process vessel 30 and out from the second process vessel 40, but a discontinuous flow of pulp within the system (arrows I and II, respectively). The pulp is preferably washed batch-wise using the intermediate pulp storage 20, such that each batch of pulp is washed in two separate washing operations I, II, which occur in the same washing space of the same washing apparatus 10 but are separate in time.

"Washing space" here refers to a space within the washing apparatus 10 where the pulp is transported on the processing surface 11 and washed or dewatered. In other words, the washing space associated with washing operation I is the space within the washing apparatus 10 where the pulp is transported on the processing surface 11 during washing operation I, and the washing space associated with washing operation II is where the pulp is transported on the processing surface 11 during washing operation II. In accordance with the invention, the washing space of washing operation I is the same as the washing space of washing operation II. Washing space refers to a stationary space within the washing apparatus 10.

The processing surface 11, i.e. the permeable surface onto which pulp is applied, is identical in the first and second washing operation I, II. Preferably, the full axial length of the washing apparatus is used. Thus, in the case with a twin roll press 10, substantially the entire length of the press rolls 15 is used in each washing operation I, II.

The input area of the washing apparatus 10, where the pulp is distributed onto the processing surface 11, is substantially the same in washing operation I and II. Similarly, the output area of the washing apparatus 10, i.e. where the pulp is withdrawn from the processing surface 11, is substantially the same in washing operation I and II. The input and output area, respectively, are stationary areas within the washing apparatus 10 and form the beginning and end of the washing space.

The intermediate pulp storage 20 is typically a tank/vessel with a storage volume exceeding the volume of pulp produced in the respective washing operation I, II. In other words, the intermediate pulp storage 20 is dimensioned for storing the pulp between washing operation I and II. The intermediate pulp storage 20 may be a pure pulp storage/buffer or, alternatively, function also as a processing tank/vessel with chemical addition, temperature control, dilution, or other processing of the pulp. Examples include a medium consistency (MC) tank, a high consistency (HC) tank, an acid stage (A), and a bleaching tower/reactor, e.g. chlorine dioxide (D), oxygen (O), peroxide (P), chelating agent treatment (Q), alkali-oxygen (EO), or alkali- oxygen-peroxide (EOP). For applications with comparatively short washing cycles, it may even be possible to use an enlarged stand pipe or the like.

The intermediate pulp storage 20 is external to the washing apparatus 10, i.e. a separate physical unit. The pulp thus leaves the washing apparatus 10 between the washing of washing operation I and II, respectively. There may be embodiments where the intermediate pulp storage 20 comprises two or more parallel tanks.

The process vessels 30, 40 are vessels/tanks in the pulping process (i.e. the pulping line) outputting or receiving pulp washed in accordance with the invention. Each process vessel 30, 40 may be a processing tower/reactor/tank or, alternatively, a pure pulp storage. Examples include a blow tank (BT), an MC/HC tank, an A stage, and a bleaching tower/reactor, e.g. D, O, P, Q, EO, EOP.

The intermediate pulp storage 20 serves as a pulp buffer alternating between a filling mode, during the first washing operation I, and an emptying mode, during the second washing operation II. The first and second process vessel 30, 40 are also arranged to alternate between an emptying mode and a filling mode, but the emptying mode is associated with the first washing operation I, and the filling mode is associated with the second washing operation II.

It should be noted that "emptying mode" does not necessarily mean that the vessel is completely empty after the washing operation. In particular for the process vessels 30, 40, there is often a minimum level of pulp required in the vessel for the overall process to run smoothly. Furthermore, there is a flow of pulp into the first process vessel 30 also during emptying mode, but the outgoing flow is larger. Correspondingly, there is a flow of pulp out from the second process vessel 40 also during filling mode, but the incoming flow is larger. In Fig. 1, valves 5 indicate that the flows of the first and second washing operation I, II are not continuous. A pump 21 is also shown, arranged to output pulp from the intermediate pulp storage 20 during the second washing operation II. The pulp can be diluted by an appropriate dilution means so that it can be pumped from the intermediate pulp storage 20. The system of the invention may be implemented using conventional equipment, such as piping, valves, pumps, dilution screws etc. Details, such as where valves are needed and not, are routine measures for which no inventive skill is needed.

The first washing operation I takes place during a first predetermined period of time, whereas the second washing operation II takes place during a second predetermined period of time. These predetermined periods of time do not have to be fixed time intervals but could also, for example, be determined as the time it takes for a particular volume of pulp to pass through the washing apparatus 10. A possible approach would be to determine the period of time based on at least one process parameter, e.g. such that the level of pulp in a particular process tank does not go beyond a maximum or minimum value. Or the period of time could be determined based on process parameters indicating the pulp quality, degree of bleaching, system performance etc. Various control strategies are possible.

The process conditions, such as pH and temperature, should preferably be about the same or similar in the first and second washing operation. Hence, in cases where the intermediate pulp storage 20 involves processing of the pulp, and not only storing, the processing should preferably be of a similar kind as the processing of the pulp in the process vessel 30 before the first washing operation I. In particular, it is advantageous to share a washing apparatus between process stages with similar content of corrosive substances, such as chlorides. A high degree of chlorides means that the parts of the washing apparatus that come into contact with the pulp have to be of a resistant, expensive material.

Example combinations of intermediate pulp storages 20 and first and second process vessels 30, 40 in accordance with the invention are provided in Table 1. (Dl and D2 are chlorine dioxide stages, PI and P2 are peroxide stages, etc. ) Depending on the application, tanks/vessels of a conventional pulping line can either be used in all three positions, or at least one new tank/vessel has to be introduced. HC tanks may, depending on the application, be used as storage tanks instead of MC tanks.

In Example 2, the first process vessel 30, the intermediate pulp storage 20, and the second process vessel 40 are pure storage tanks. In the other examples acid or bleaching chemicals are added to at least one of the first process vessel 30, the intermediate pulp storage 20, and the second process vessel 40.

Table 1.

Basically, two conventional washing machines, each with production volume V, may be replaced by a large washing machine with production volume V+V operated according to the invention. The invention requires piping, valves etc. and in some cases an additional tank/vessel with an associated pump. Nevertheless, the proposed system leads to reduced costs, since a larger washing machine is much cheaper per produced volume of pulp than two smaller washing machines. Another important advantage is that less energy is consumed by the system of the invention than by conventional washing systems.

Typically, the first and second washing operation I, II is each run during a period of time of 0.03 to 2 hours. In general, a short washing cycle would be preferred or else the volume of pulp in the buffer(s) may be difficult to handle. The filtrate handling is also facilitated by a short washing cycle. A period of time of about 3-5 min per washing operation may for example be used. For applications using bleaching reactors as pulp buffers, a longer period of time would be feasible due to larger tank size. However, the washing cycle cannot be too long, or else the retention time of the pulp will vary too much within the reactor. A period of time of about 10- 30 min per washing operation may for example be used. Therefore, a preferred period of time for each washing operation is in the range of about 3 to 30 min.

Again, the invention contradicts the prevailing view within the pulping industry, that stable conditions in the pulping line is always aimed at. In line with the traditional approach, the washing cycle would be as long as possible for system stability reasons, but here a short period of time is instead suggested.

To obtain an efficient washing system, means can be provided for changing the settings of the washing apparatus 10 between washing operation I and II. In particular, the amount of washing liquid (not shown in drawings) added to the pulp on the processing surface 11 may be adapted to the respective washing operation I, II. Another parameter, which may be adjusted, is the rotation speed of the movable permeable processing surface 11. Another parameter, which may be adapted to the respective washing operation I, II is the amount of added dilution liquid, diluting the pulp to pumping consistency after the washing in the washing apparatus 10. Adapting these parameters (changing settings or intervals) may e.g. be appropriate if the pulp consistency is different in washing operation I and II.

It should be understood that although the washing apparatus 10 is illustrated as a twin-roll press, this is only an example. Another washing apparatus, such as a diffuser, a filter, a drum washer (e.g. drum displacement washer), or a different type of press (e.g. a wire press), may also be used in accordance with the invention. Accordingly, the movable permeable processing surface 11 may comprise a perforated press roll or drum surface, a wire, or a screen of a diffuser.

Fig. 2 illustrates another exemplifying system of the invention with three washing operations I, II, III in the same washing apparatus. This system 200 includes a second intermediate pulp storage 60 in addition to the intermediate pulp storage 20 of Fig. 1. The second intermediate pulp storage 60 is connected to the washing apparatus 10. During the second washing operation II, the washed pulp is led to the second intermediate pulp storage 60, and in a third washing operation III, the pulp is for a third time washed on the processing surface 11 in the same washing space of the washing apparatus 10. The third washing operation III is associated with a third predetermined period of time. What is said about the first and second predetermined periods of time above may be applied also on the third period of time. The process conditions, such as pH and temperature, should preferably be about the same or similar in all three washing operations I, II, III.

There is preferably one filtrate tank 51, 52, 53 for each washing operation and filtrate may be used as washing liquid in a preceding washing operation.

The washing principles of Fig. 2, may be used to replace three conventional washing apparatuses with one single, larger washing apparatus. Basically, three washing machines with production volume V each, may be replaced by a single washing machine with production volume V+V+V, which means that the overall benefit is even larger than for the case with only two washing operations I, II.

Fig. 3 illustrates another exemplifying system 300 of the invention with two washing operations I, II in the same washing apparatus 10. The washing apparatus 10 is connected to the intermediate pulp storage 20 via additional processing equipment 70, such that there is additional processing, such as bleaching, between the first washing in the washing apparatus 10 and the storing in the intermediate pulp storage 20.

The additional processing equipment 70 may for example include a bleaching tower with an associated washer and other associated equipment such as a pump, a dilution screw, etc. There is in such a case a pulp flow through the washer of the additional processing equipment 70 (i.e. the washer arranged after the bleaching tower) during both washing operations I, II. The bleaching tower of the additional processing equipment 70, serves as an additional pulp buffer, arranged to alternate between a filling mode, during the first washing operation I, and an emptying mode, during the second washing operation II.

Using the principles of Fig. 3, a washing apparatus may be shared between process stages, e.g. bleaching stages, that do not follow directly after each other in the pulping line. The process conditions should preferably be about the same or similar in the first and second washing operation I, II.

Examples of combinations of intermediate pulp storages 20 and first and second process vessels 30, 40 for embodiments with additional processing equipment 70 are provided in Table 2.

Table 2.

Example 7 relates to a pulping line including the bleaching sequence DO EOP Dl D2 (generally with washing after each bleaching tower). The DO and Dl stages share the same washing apparatus, operated according to the principles of the invention. In other words, washing is performed in one and the same washing apparatus 10 between the DO and EOP towers and between the Dl and D2 towers, respectively. Sharing a washing apparatus between two D- stages like in Example 7 has the advantage that the process conditions of the two washing operations I, II in the washing apparatus are very similar (same pH, etc.).

Example 8 relates to a pulping line including the bleaching sequence MC DO EO Dl (generally with washing after each pulp tower). The MC tower may, depending on the application, be replaced by a HC tower. The MC/HC tower and EO share the same washing apparatus, operated according to the principles of the invention. In this example, as in Example 7, existing vessels/tanks of the bleaching stages may be used and there is no need for introducing a new tank/vessel as pulp buffer.

Furthermore, the principles of Fig. 2 and 3 may be combined to a system with three washing operations I, II, III where the first and second washing operation I, II and/or the second and third washing operation II, III need not be performed directly after each other. Basically, three washing machines from different parts of the pulping line and associated with production volume V each, may in this way be replaced by a single washing machine with production volume V+V+V.

For sake of simplicity, no valves, pumps, dilution means etc. are shown in Fig. 2 and 3. A person skilled in the art realizes that the invention may be implemented using conventional equipment arranged according to the described principles.

Fig. 4 is a schematic cross-sectional view of an example washing apparatus, a twin-roll press 10, of a preferred system according to the invention. The twin roll press 10 comprises two cooperating cylindrical press rolls/drums 15. The two press rolls 15 are arranged to rotate in opposite directions Dl during operation and each has a permeable outer surface 11, more specifically a perforated metal sheet. The press rolls 15 are partially enclosed by a vat 16 (also known as trough) formed by guide surfaces, arranged at a distance from the perforated outer surface 11 of the respective press roll 15 so as to partially enclose the press roll in the circumferential direction.

A pulp distribution device 12 is associated with each press roll 15. The pulp distribution device 12 is arranged at the upper portion of the press roll 15 for distribution of pulp onto the perforated roll surface 11. It comprises an elongated housing 18, extending lengthwise along a substantial portion of the press roll 15, and is connected to the vat 16. During operation, pulp enters the pulp distribution device 12 via its inlet, which for example can be arranged at the middle of the twin-roll press 10 as seen in the longitudinal direction.

In the pulp distribution device 12, the pulp is distributed in the longitudinal direction and output through the outlet 13 to form a pulp web on the perforated roll surface 11. The pulp web is transported, guided by the vat 16, in the direction of rotation Dl to be pressed in a nip (also known as pinch) 17 where the distance between the press rolls 15 is smallest. The press rolls 15 may comprise axial filtrate channels (not shown) which receive the filtrate that passes through the perforated roll surface 11, and transport it towards the ends of the press rolls 15, where it is output. Washing liquid may e.g. be supplied to the pulp web in the vat 16. The pulp is output by means of a discharge screw 19.

In a system of the invention that includes a twin-roll press 10, like in Fig. 4, the washing space is defined by the perforated roll surface 11 and the vat 16, and basically encloses the portion of the perforated roll surface 11 where there, during operation, is a pulp web. The washing space is thus stationary, although the perforated roll surface 11 is moving.

In Fig. 4, two press rolls 15 are arranged next to each other, with the rotation centers in the same horizontal plane. The invention is also suitable for washing and/or dewatering apparatuses where, for example, the rolls are differently arranged, or only one perforated roll is used, as well as for another apparatus where pulp is dewatered on a movable permeable surface.

One example system of the invention includes a twin roll press 10 with a non-converging vat 16, i.e. a vat which is parallel or diverging from the position where pulp is applied onto the press roll 15 until the press nip 17. The non-converging vat 16 makes the system of the invention less sensitive to varying process conditions, such as varying pulp consistency and flow.

Another advantageous system of the invention includes a twin roll press 10 with means for adjustment of the distance between the press rolls 15 in response to varying operating conditions of the respective washing operations I, II, III. There may for example be a change of nip (e.g. a change between a first and a second interval) if the pulp consistency is different in washing operation I and II. The press nip 17 is preferably automatically varied during operation, between the washing operations I, II, III and preferably also during each washing operation. By means of nip control, the washing in the twin-roll press 10 becomes more efficient and the press 10 becomes less sensitive to varying load.

Still another advantageous system of the invention includes a twin roll press 10, the press rolls 15 of which have a diameter in the range of 0.8-3m, preferably 1.5-3m and a length in the range of 1.2-1 lm, preferably 4-1 lm. By means of a large twin roll press 10, the production volume can be maintained although the press is arranged to handle two different washing operations. Two conventional presses 10, each with production volume V, may be replaced by a large press with production volume V+V operated according to the invention.

Still another advantageous system of the invention includes a washing apparatus having an open pulp distribution device, such as the pulp distribution device 12 illustrated in Fig. 4. The pulp distribution device 12 comprises a rotatable distribution means 14 arranged to distribute pulp onto the movable permeable processing surface 11 and an outlet 13 which opens towards the pulp web, which, during operation, is formed on the permeable processing surface 11. The outlet 13 has a length that corresponds to the width of the pulp web and the rotatable distribution means 14 is arranged to distribute the pulp along essentially the entire length of the outlet 13. There are no flow-restricting elements between the rotatable distribution means 14 and the permeable processing surface 11 in Fig. 4.

The outlet 13 of an open pulp distribution device 12 is open in the sense that no wall or other essential flow-restricting element is arranged in the outlet opening. The open outlet 13 provides for a free flow of pulp from the pulp distribution device 12 towards the permeable processing surface 11.

The open pulp distribution device 12 can with advantage be of the type specified in Swedish patent SE 532 366, in particular with regard to the position of the rotatable distribution means 14. Such a device provides both distribution and forming of the pulp onto the press roll 15.

The open design of the pulp distribution device 12 makes the system of the invention capable of handling varying process conditions, such as varying pulp consistency and flow.

An open pulp distribution device 12 can be used in a twin roll press 10 but it may also be used to apply pulp onto the movable permeable processing surface 11 of another washing apparatus 10 operated according to the invention.

Fig. 5 is a flow chart of a method for washing cellulose pulp according to an exemplifying embodiment of the invention. In step SI, pulp is washed in a first washing operation I using a washing apparatus 10 with a movable permeable processing surface 11. The input consistency of the pulp is preferably in the range of 2-13%. Thereafter, the pulp is transported to an intermediate pulp storage 20 (step S2). Chemicals may be added to the pulp in the intermediate pulp storage 20. Step S3 asks if the washing operation I is completed, i.e. if a predetermined condition determining the time of the washing operation I is fulfilled. If no, the process returns to step S I . If yes, the settings of the washing apparatus 10, or of its associated equipment such as valves, dilution means, etc., may in step S4 (optional) be adjusted to suit the next washing operation. The amount of washing liquid may for example be changed. In step S5, stored pulp is washed in a second washing operation II using the same washing apparatus 10 with the movable permeable processing surface 11. The input consistency of the pulp is preferably in the range of 2-13%. Step S6 asks if the washing operation II is completed. If no, the process returns to step S5. If yes, the settings of the washing apparatus 10 may in step S7 (optional) be adjusted to suit the next washing operation. The process then returns to step SI .

In accordance with Fig. 5, the washing and storing steps SI, S2, S5 are preferably repeated such that the pulp is washed batch-wise using the intermediate pulp storage 20, each batch of pulp being washed in two separate washing operations I, II, which occur in the same washing space of the same washing apparatus 10 but are separate in time.

The adjustment of step S4 may for example comprise adjusting at least one parameter of: a flow of added washing liquid, a rotation speed, a flow of added dilution liquid, and a distance between press rolls 15.

The invention is not restricted to the illustrated embodiments, but covers also equivalents to the disclosed features and may be varied within the scope of the claims. There may for example be embodiments with more than three washing operations in the same washing apparatus. It would also be possible to have a system where a washing apparatus is arranged so that it can be run both conventionally, with one pulp flow through the washing apparatus, and according to the invention, with two pulp flows, separate in time, through the washing apparatus. This may, for example, be useful when a pulping line is run with two different types of pulp with different requirements as to washing result and/or very different pulp flows.

Claims

1. A method for washing of cellulose pulp, comprising the steps of

washing, in a first washing operation (I), the pulp for a first predetermined period of time using a washing apparatus (10) with a movable permeable processing surface (11), filtrate from the pulp passing through the processing surface as the pulp is transported together with the processing surface,

storing the pulp in an intermediate pulp storage (20) connected to the washing apparatus (10) and arranged to receive the pulp washed in the first washing operation, and

washing, in a second washing operation (II), the pulp for a second predetermined period of time using the washing apparatus (10), the pulp being received from the intermediate pulp storage (20) and applied onto the processing surface (11) of the washing apparatus, such that the pulp is washed again in the same washing space of the washing apparatus,

the washing and storing steps being repeated such that the pulp is washed batch-wise using the intermediate pulp storage (20), each batch of pulp being washed in two separate washing operations (I, II), which occur in the same washing space of the same washing apparatus (10) but are separate in time.

2. The method of claim 1, wherein the input area of the washing apparatus (10), where the pulp is distributed onto the processing surface (11), is the same in the first and second washing operation (I, II), and the output area of the washing apparatus (10), where the pulp is withdrawn from the processing surface (11), is the same in the first and second washing operation (I, II).

3. The method of claim 1 or 2, comprising the further step of adding chemicals to the pulp in the intermediate pulp storage (20).

4. The method of any of previous claims, comprising the further steps of

receiving the pulp to be washed in the first washing operation (I) from a first process vessel (30) connected to the washing apparatus (10), and

outputting the pulp washed in the second washing operation (II) to a second process vessel (40) connected to the washing apparatus (10), the first process vessel (30) and the second process vessel (40) being arranged to alternate between an emptying mode, during the first washing operation (I), and a filling mode, during the second washing operation (II).

5. The method of any of previous claims, wherein the first process vessel (30), the intermediate pulp storage (20), and the second process vessel (40) are pure storage tanks.

6. The method of any of claims 1-4, wherein at least one of the first process vessel (30), the intermediate pulp storage (20), and the second process vessel (40) is a bleaching reactor.

7. The method of any of previous claims, comprising additional processing of the pulp between the washing of the first washing operation (I) and the storing in the intermediate pulp storage (20).

8. The method of any of previous claims, comprising the further steps of

collecting, during the first washing operation (I), filtrate passing through the movable permeable processing surface (11) into a first filtrate tank (51), and

collecting, during the second washing operation (II), filtrate passing through the movable permeable processing surface (11) into a second filtrate tank (52).

9. The method of claim 8, wherein the filtrate collected during the second washing operation (II) is used as washing liquid in the first washing operation (I).

10. The method of any of previous claims, comprising the further steps of

storing the pulp in a second intermediate pulp storage (60) connected to the washing apparatus (10) and arranged to receive the pulp washed in the second washing operation (II), and

washing, in a third washing operation (III), the pulp for a third predetermined period of time using the washing apparatus (10), the pulp being received from the second intermediate pulp storage (60) and applied onto the processing surface (11) of the washing apparatus, such that the pulp is washed again in the same washing space of the washing apparatus.

11. The method of any of previous claims, wherein the washing operations (I, II, III) are associated with similar process conditions.

12. The method of any of previous claims, wherein each washing operation (I, II, III) is run during a period of time in the range of about 3 to 30 min.

13. The method of any of previous claims, comprising the further step of

adjusting a parameter of the washing apparatus (10) or its associated equipment to the respective washing operation (I, II, III), the parameter being selected from the group of: a flow of added washing liquid; a rotation speed; a flow of added dilution liquid; and a distance between press rolls (15).

14. A system (100) for washing of cellulose pulp, comprising

a washing apparatus (10) with a movable permeable processing surface (11), arranged for washing, in a first washing operation (I), the pulp for a first predetermined period of time, filtrate from the pulp passing through the processing surface as the pulp is transported together with the processing surface,

an intermediate pulp storage (20), connected to the washing apparatus (10) and arranged to receive and store the pulp washed in the first washing operation, and wherein

the washing apparatus (10) is further arranged for receiving the pulp from the intermediate pulp storage (20) and washing, in a second washing operation (II), for a second predetermined period of time, the pulp again on the processing surface (11) in the same washing space of the washing apparatus, the system further comprising

means for repetition of the washing operations (I, II) and the storing in the intermediate pulp storage (20), such that the system is arranged for batch-wise operation, each batch of pulp being washed in two separate washing operations (I, II), which occur in the same washing space of the washing apparatus (10) but are separate in time.

15. The system of claim 14, wherein the input area of the washing apparatus (10), where the pulp is distributed onto the processing surface (11), is the same in the first and second washing operation (I, II), and the output area of the washing apparatus (10), where the pulp is withdrawn from the processing surface (11), is the same in the first and second washing operation (I, II).

16. The system of claim 14 or 15, further comprising means for adding chemicals to the pulp in the intermediate pulp storage (20).

17. The system of any of claims 14-16, further comprising

a first process vessel (30) connected to the washing apparatus (10),

means for receiving, at the washing apparatus (10), the pulp to be washed in the first washing operation (I) from the first process vessel (30),

a second process vessel (40) connected to the washing apparatus (10), and

means for outputting, from the washing apparatus (10), the pulp washed in the second washing operation (II) to the second process vessel (40), the first process vessel (30) and the second process vessel (40) being arranged to alternate between an emptying mode, during the first washing operation (I), and a filling mode, during the second washing operation (II).

18. The system of any of claims 14-17, wherein the first process vessel (30), the intermediate pulp storage (20), and the second process vessel (40) are pure storage tanks.

19. The system of any of claims 14-17, wherein at least one of the first process vessel (30), the intermediate pulp storage (20), and the second process vessel (40) is a bleaching reactor.

20. The system of any of claims 14-19, further comprising means (70) for additional processing of the pulp arranged between the first washing in the washing apparatus (10) and the intermediate pulp storage (20).

21. The system of any of claims 14-20, further comprising

a first filtrate tank (51) arranged to collect filtrate passing through the movable permeable processing surface (11) during the first washing operation (I), and

a second filtrate tank (52), arranged to collect filtrate passing through the movable permeable processing surface (11) during the second washing operation (II).

22. The system of claim 21, further comprising means for using the filtrate collected during the second washing operation (II) as washing liquid in the first washing operation (I).

23. The system of any of claims 14-22, further comprising

a second intermediate pulp storage (60) connected to the washing apparatus (10) and arranged to receive and store the pulp washed in the second washing operation (II), and wherein

the washing apparatus (10) is further arranged for receiving the pulp from the intermediate pulp storage (60) and washing, in a third washing operation (III), for a third predetermined period of time, the pulp again on the processing surface (11) in the same washing space of the washing apparatus.

24. The system of any of claims 14-23, adapted for operating each washing operation (I, II, III) during a period of time in the range of about 3 to 30 min.

25. The system of any of claims 14-24, further comprising

adjustment means for adjusting a parameter of the washing apparatus (10) or its associated equipment to the respective washing operation (I, II, III), the parameter being selected from the group of: a flow of added washing liquid; a rotation speed; a flow of added dilution liquid; and a distance between press rolls (15).

26. The system of any of claims 14-25, wherein the washing apparatus (10) comprises an open pulp distribution device (12) with a rotatable distribution means (14) and an outlet (13) which opens towards the movable permeable processing surface (11).

27. The system of any of claims 14-26, wherein the washing apparatus (10) is a roll press with two co-operating press rolls (15) and the movable permeable processing surface (11) is a rotatable perforated outer surface of the press roll, a pulp web in operation being formed on the respective rolls and transported in the direction of rotation in a non-converging vat (16) partially surrounding the press rolls, until the press nip (17) between the rolls.

28. The system of claim 27, wherein the press rolls (15) have a diameter in the range of 1.5-3m and a length in the range of 4-1 lm.

29. The system of any of claims 27 or 28, further comprising

means for automatically varying the distance between the press rolls (15) in response to varying operating conditions between the first and second washing operation (I, II), respectively.