SE539501C2 - Method and arrangement for discharge dilution - Google Patents

Description

METHOD AND ARRANGEMENT FOR DISCHARGE DILUTION TECHNICAL FIELD The present invention relätes to pulp manufacturing processes in general, and particularly to adaptations of discharge dilution to enable improvements in screening in such processes.

BACKGROUND Pulp processing (e.g. Kraft process or sulfite process) typically comprises introducing wood chips or other lignocellulosic material into a digester or cooker together with an impregnation liquid e.g. black liquor from a displacement tank and the chips or other lignocellulosic material is preheated and pre-impregnated. Black liquor is the waste product from the so called Kraft process when pulpwood is digested into paper pulp removing lignin, hemicelluloses and other extractives from the wood to free the cellulose fibers. Thereby, black liquor is an aqueous solution of lignin residues, hemicellulose, and the inorganic chemicals used in the process. Correspondingly, white liquor is a strongly alkaline solution typically of sodium hydroxide and sodium sulfide which is used to break the bonds between lignin and cellulose. Thereby, white liquor is the "pure" impregnation liquid whereas the black liquor is recycled impregnation liquid. Subsequently, hot black and white liquor is introduced into the digester and displaces the initially introduced impregnation liquid back into the displacement tank. Due to the introduction of hot liquor the fiber is heated and cooked during a period of maintaining the temperature at a desired level. The cooking dissolves or softens the lignin in the material and the cooking time and temperature is dependent on the material to be cooked as well as desired fiber properties after cooking. After cooking, black liquor e.g. displacement liquor (from the displacement tank) with a lower temperature is introduced into the digester to lower the temperature and thereby terminate the cooking reactions. Finally the digester is discharged to an atmospheric storage tower e.g. discharge tank from which the cooked pulp is transported via piping to e.g. a screening process in order to deliver a finished pulp with predetermined properties e.g. predetermined fibre fractions.

In this context, wood pulp can be defined as wood chips mechanically ground and/or chemically digested at elevated temperatures to produce fibers for use in manufacturing paper or fiber-based wood composites. [1] In the same manner pulp can be manufactured from other lignocellulosic materials, e.g. bagasse. Lignocellulose or lignocellulosic material refers to plant dry matter (biomass), so called lignocellulosic biomass. So called virgin biomass includes all naturally occurring terrestrial plants such as trees, bushes and grass. So called waste biomass is produced as a low value byproduct of various industrial sectors such as agricultural (corn sto ver, sugarcane bagasse, straw etc), forestry (saw mill and paper mill discards). Energy crops are crops with high yield of lignocellulosic biomass produced to serve as a raw material for production of second generation biofuel examples include switch grass (Panicum virgatum) and Elephant grass.

In present pulp manufacture processes, liquor from the displacement liquor tank is utilized in order to dilute the pulp in the discharge sequence. The dilution is necessary in order to enable emptying or discharging the pulp from the digester and into the discharge tank. In the subsequent screening process some of the process steps are fed with a pulp at low consistency and using liquor for its function. The accept from these process steps, which is a low consistency pulp which resembles pure liquor more than a thicker pulp, is subsequently fed to a reject tank and then fed to the discharge tank. The flow from the reject tank then lowers the consistency in the system significantly. To reduce the consequences of the reject tank flow, typically a so called delta thickener is added, which removes most of the liquor from the pulp fed to the discharge tank Consequently, the pulp fed to the discharge tank from batch cooking does not have high consistency enough to handle all the liquor from the reject tank as dilution in the discharge tank without lowering the screen feed consistency more than desired. In many applications today, the consistency is increased with a thickening device e.g. delta thickener or the like in order to enable the screening being fed at a normal consistency. Depending on the manufacturer of the processing equipment, the thickening agent or device varies.

The thickening agent and/or machinery introduces both unwanted process steps and material into the process, thus increasing both time and costs for the pulping process.

Therefore, there is a need for an improved dilution method and arrangement in the discharge sequence without the above mentioned disadvantages.

SUMMARY The present invention relätes to improved discharge method and arrangement in pulp processing.

In a first aspect the proposed technology discloses a method of providing a diluted discharge pulp, comprising the steps of providing pulp with a first consistency in a digester, which pulp comprises cooked lignocellulosic fibrous material. Subsequently, diluting the provided pulp by feeding a reject liquor from a screening process into the digester during a discharge sequence to provide a diluted pulp in the digester, the reject liquor from the screening process being a pulp with a lower consistency than the pulp with a first consistency, and finally feeding the diluted pulp through a discharge pipe to a discharge tank to provide a diluted discharge pulp.

By means of the proposed technology the pulp can be diluted without addition of any unwanted additives and reject liquor can be reused in the discharge process.

BRIEF DESCRIPTION OF THE DRAWINGS The invention, together with further objects and advantages thereof, may best be understood by referring to the following description taken together with the accompanying drawings, in which: Fig. 1 is a flow chart of an embodiment of a method according to the proposed technology; Fig. 2 is a schematic block chart of a system according to the proposed technology. Fig. 3 is a schematic block chart illustrating prior art; Fig. 4 is a schematic block chart illustrating the benefits of the embodiments of the proposed technology; DETAILED DESCRIPTION The present disclosure will be described in the context of a batch cooking process. However, it is easily adapted for a case of continuous cooking. Further, although the background and the description of the proposed technology primarily describes the so called Kraft process, it is equally applicable to sulfite processes or similar with appropriate adaptation of processing terminology.

The inventors have identified the benefits of reusing liquor or low consistency pulp from the screening process for diluting the pulp during the discharge sequence. In other words, instead of using liquor from the displacement liquor tank during the discharge sequence, the same functionality can be met by using liquor from a screening reject tank. As most of the flow from the reject tank will be used in the cooking discharge sequence, only a small part will be sent to the bottom of the discharge tank to control the consistency. The usage of dilution has then reduced so that it is not necessary to add any thickening device to run screening in normal consistency. Thereby, reusing low consistency reject liquor from the screening process for dilution during the discharge sequence will reduce the need for additives or machinery that increase the consistency of the feed into the screening process.

With reference to Figure 1, embodiments of a method of providing a diluted discharge pulp in a pulp processing plant will be described. In this context the pulp can comprise processed wood fibre [1] or other processed lignocellulosic fibre. According to a first embodiment, the method includes the steps of providing S10 pulp with a first consistency, said pulp comprising digested or cooked wood chips or other lignocellulosic fibrous material. Subsequently, diluting S20 the provided pulp to a predetermined consistency with a low consistency screening reject liquor to provide a diluted pulp, and fmally feeding S30 the diluted pulp to a discharge tank to provide a diluted discharge pulp.

According to a further embodiment, the diluting step S20 comprises feeding the low consistency screening reject liquor as a first fraction from a screening process.

According to yet another embodiment, the method includes the further steps of diluting S40 the discharge pulp in the discharge tank with a second fraction from the screening process to a second predetermined consistency, before (which is common for all embodiments) feeding S50 the (diluted or undiluted) discharge pulp to the screening process for screening S60.

According to a further embodiment, the method includes an intermediate step of feeding the low consistency screening reject liquor to a buffer, and subsequently feeding the first and/or second fraction from the buffer to the digester and optionally also to the discharge tank.

In the above described embodiments, no mention is made to the consistency or concentration of the diluted pulp. However, according to a further embodiment, it is possible to control, in step S25, the dilution in order to achieve a diluted pulp with a predetermined consistency or concentration. This may be performed by determining the concentration of the pulp in the digester and diluting the pulp in order to achieve a wanted concentration. More commonly though is diluting the pulp just enough to enable discharging the pulp from the digester.

With reference to Figure 2 embodiments of a discharge dilution arrangement 100 according to the present disclosure will be described.

According to a first embodiment, the discharge dilution arrangement 100, comprises a digester 1 configured for receiving at least a lignocellulosic fibrous material and a cooking liquor to cook into a pulp. Further, the discharge dilution arrangement 100 includes a discharge tank 2 configured for receiving said pulp from said digester 1 and for outputting said pulp to a screening process. The digester 1 and the discharge tank 2 are connectable via a discharge pipe 3, configured for feeding cooked pulp from the digester 1 to the discharge tank 2. Finally, the discharge dilution arrangement 100 includes a digester dilution pipe 4 configured for feeding a low consistency screening reject liquor from the screening process into at least the digester tank 1 to dilute the pulp. Preferably, the screening reject is fed into the digester 1 at the bottom of the tank. However, it is also technically feasible to add the screening reject to other parts of the tank as well.

According to a further embodiment, the discharge dilution arrangement 100 includes a first discharge dilution pipe 4' configured for additionally feeding the low consistency screening reject liquor from the screening process into the discharge pipe 3 to further dilute the pulp to a predetermined consistency.

According to another embodiment, the discharge dilution arrangement 100 further includes a second discharge dilution pipe 5 configured for additionally feeding the low consistency screening reject liquor from the screening process into the discharge tank 2 as well to further dilute the pulp.

In order to improve the control of the dilution and ensure a sustainable and even access to the reject liquor, it might prove beneficial to collect the reject liquor into a buffer tank from which the reject liquor is then distributed to the parts of the arrangements where dilution is required. According to a further embodiment, the discharge dilution arrangement 100 further comprises a buffer tank 6 arranged to receive the low consistency screening reject liquor from the screening process and to provide a first fraction of the low consistency screening reject liquor to the digester dilution pipe 4 and a second fraction of the low consistency screening reject liquor to the discharge dilution pipe, to provide a diluted discharged pulp.

For those cases where a dilution in the discharge pipe 3 is also required, according to a further embodiment, the buffer tank 6 is further configured to provide a third fraction of the low consistency screening reject liquor via the first discharge dilution pipe 4' to the discharge pipe 3.

The above described arrangements can be implemented for both batch digesters and continuous digesters.

In order to further explain the benefits of the above described embodiments, an example of the impact of various concentrations in the different process steps will be described with reference to Figure 3 and Figure 4. The concentration and concentration intervals are to be viewed as non-limiting examples and are indicated merely to illustrate the benefits of the proposed technology.

In Figure 3, which illustrates prior art without the use of a thickening device, a digester 1 is disclosed with a capacity for processing 20 t/h of wood chips or other lignocellulosic material. Impregnation and/or cooking liquid, e.g. black and/or white liquor or similar depending on the chemical process used, is added to achieve a concentration or first consistency of approximately 10%, corresponding to a total volume of 200 m<3>/h of cooked pulp. In order to enable emptying or discharging the digester 1, liquor is added to dilute the cooked pulp and which to reduce the concentration of the cooked pulp to i.e. 5%. In order to handle the processing capacity of 200 m<3>/h from the digester 1, it is necessary to add 200 m<3>/h of dilution liquid e.g. liquor, thereby resulting in a feed of 400 m<3>/h of diluted cooked pulp from the digester 1 to the discharge tank 2. This results in a doubling of the processed volume.

As discussed previously, it is then potentially necessary to further dilute the cooked pulp in order to enable emptying the discharge tank 2 into the subsequent screening process. Accordingly, the discharge 2 is fed with screening reject to further lower the concentration to about 3%. In this example the low consistency screening reject is fed with a capacity of 2 t/h and a concentration or consistency of 0.6 %, which corresponds to 330 m<3>/h. Thereby, the discharger tank 2 outputs 22 t/h with a concentration of 3 % and a volume of 730 m<3>/h. Through the screening process a screened pulp with a consistency of maximum 3% is provided to press feed and about 10 % of the production is provided to a screening reject wash.

With reference to Figure 4, the dilution in both the digester 1 and optionally the discharge tank 2 is provided by adding the low consistency screening reject liquor. In a corresponding manner as in Figure 3, a digester 1 is disclosed with a capacity for processing 20 t/h of wood chips or other lignocellulosic material. Impregnation and/or cooking liquid, e.g. black and/or white liquor or similar depending on the chemical process used, is added to achieve a concentration or first consistency of approximately 10%, corresponding to a total volume of 200 m<3>/h of cooked pulp. In order to enable emptying or discharging the digester 1, a low consistency reject liquor with a consistency of approximately 0.6% is added to dilute the cooked pulp. In order to handle the processing capacity of 200 m<3>/h from the digester 1, it is necessary to add 1.2 t/h of the low consistency screening reject liquor with a consistency of 0.6%, thereby resulting in a feed of 400 m<3>/h of diluted cooked pulp with a concentration of 5.3% from the digester 1 to the discharge tank 2. This results in a doubling of the processed volume, in a similar manner as in Figure 3.

As discussed previously, with reference to Figure 3 it is then potentially necessary to further dilute the cooked pulp in order to enable emptying the discharge tank 2 into the subsequent screening process. Accordingly, the discharge 2 is also fed with the low consistency screening reject liquor. However, in contrast to the prior art solution in Figure 3 the addition of screening reject is performed with 0.8t/h with a consistency of 0.6% corresponding to 130 m<3>/h. Thereby, the discharger tank 2 outputs 22 t/h with a concentration of 4.1 % and a volume of 530 m<3>/h.

In case of using some other liquid than the screening liquid it is easy to see the benefits of the current embodiments. Using a 0 % diluting liquid corresponds to an output volume of 730 m<3>/h from the discharge tank 2 as illustrated in Figure 3 (without a thickening agent) to provide the necessary screening consistency of 3-4.5%. By utilizing a diluting liquid of 0.6 % the processed volume can be reduced to 530 m<3>/h, which is a significant improvement and saves both time and money.

As mentioned before, the above described concentrations are merely examples. The above mentioned first consistency is typically around 10%, but can also vary within concentration intervals of for example around 9,5-10,5%, 9-11%, 8-12%. The low consistency screening reject liquor is potentially 0.6%, but can also vary within concentration intervals of for example 0.3-1%. The resulting diluted pulp is diluted to such an extent that it is possible to empty the digester with a minimum of dilution. As an example the diluted pulp could be between 4-7%, or 4-6%, 4-5%, 4,5-5,5%. Basically, depending on the material to be processed, the chemicals used, the cooking or digesting parameters applied in the process, the type of screening and other variables the specific concentrations can vary also outside the above given example intervals. With the presently proposed technology it is possible to achieve a desired consistency and at the same time avoiding the need for a thickening agent or machinery to be introduced into the system.

The embodiments described above are merely given as examples, and it should be understood that the proposed technology is not limited thereto. It will be understood by those skilled in the art that various modifications, combinations and changes may be made to the embodiments without departing from the present scope as defined by the appended claims. In particular, different part solutions in the different embodiments can be combined in other configurations, where technically possible.

REFERENCES [1] "Fact Sheet Agriculture and Natural Resources", F-85-13, Forest Products Terminology, E. McConnell, N Irby, (2013) Ohio State University, http: / / ohiowood.osu.edu/images/F_85_l 3_FP_Terminology_Fact_Sheet.pdf [2] Chemical Pulping, 6A and 6B, J Gullichsen, C-J Fogelholm, Paper Making Science and Technology, ISBN 952-5216-06-3, (1999) Jyväskylä, Finland

Claims (10)

1. En metod för att tillhandahålla en utspädd utmatningsmassa,innefattande att tillhandahålla (S10) massa med en första konsistens i en kokare, därmassan innefattar kokat lignocellulosahaltigt f1bröst material,kännetecknad av att späda (S20) den tillhandahållna massan genom att mata in enrejektvätska från en silningsprocess i kokaren under en utmatningssekvensför att tillhandahålla en utspädd massa i kokaren, där rejektvätskan frånsilningsprocessen är en massa med en lägre konsistens än massan med enförsta konsistens, och mata (S30) den utspädda massan genom ett utmatningsrör till en utmatningstank för att tillhandahålla en utspädd utmatningsmassa.

2. Metoden enligt krav 1, vari steget att späda (S20) dessutom innefattaratt mata in en första bråkdel av rejektvätskan från silningsprocessen i utmatningsröret för att ytterligare späda den utspädda massan.

3. Metoden enligt krav 2, innefattande de ytterligare stegen att späda (S40) utmatningsmassan i utmatningstanken med en andra bråkdel av rejektvätskan från silningsprocessen, och mata den utspäddautmatningsmassan till silningsprocessen.

4. Metoden enligt något av kraven 1-3, innefattande steget att mata rejektvätskan från silningsprocessen till en buffert, och därefter matarejektvätskan från bufferten till åtminstone kokaren, och valfritt till utmatningsröret, och valfritt till utmatningstanken.

5. En anordning (100) för utmatningsutspädning, innefattande en kokare (1) konfigurerad för att ta emot åtminstone ett lignocellulosahaltigt f1bröst material och en kokvätska att kokas till en massa; lO 13 en utmatningstank (2) konfigurerad för att ta emot massan från kokaren(1) och för att mata ut massan till en silningsprocess; och ett utmatningsrör (3), konfigurerat för att mata kokad massa frånkokaren (1) till utmatningstanken (2),kännetecknad av ett kokarutspädningsrör (4) konfigurerat för att mata in en rejektvätskafrån silningsprocessen i åtminstone en nedre del av kokartanken (1) för attspäda ut massan i kokaren (1), där rejektvätskan från silningsprocessen är en massa med en lägre konsistens än massan i kokaren.

6. Anordningen för utmatningsutspädning enligt krav 5, innefattande ettförsta utmatningsutspädningsrör (4') konfigurerat för att dessutom mata inrejektvätskan från silningsprocessen i utmatningsröret (3) för att ytterligarespäda ut massan i utmatningsröret (3).

7. Anordningen för utmatningsutspädning enligt krav 5 eller 6,innefattande, ett andra utmatningsutspädningsrör (5) konfigurerat för att dessutommata in rejektvätskan från silningsprocessen till utmatningstanken (2) för att ytterligare späda ut massan.

8. Anordningen för utmatningsutspädning enligt något av kraven 5-7, varianordningen vidare innefattar en bufferttank (6) anordnad att ta emotrejektvätskan från silningsprocessen och att tillhandahålla rejektvätskan frånsilningsprocessen till kokarutspädningsröret (4) och valfritt en första bråkdelav rejektvätskan från silningsprocessen till det förstautmatningsutspädningsröret (4'), för att tillhandahålla en utspädd massa med en förutbestämd konsistens.

9. Anordningen för utmatningsutspädning enligt något av kraven 5-8, vari bufferttanken (6) är vidare konfigurerad för att tillhandahålla en andra 14 bråkdel av rejektvätskan från silningsprocessen via det andra utmatningsutspädningsröret (5) till utmatningstanken (2).

10. Anordningen för utmatningsutspädning enligt något av kraven 5-9, vari kokaren (1) är en satsvis kokare eller en kontinuerlig kokare.