SE539501C2 - Method and arrangement for discharge dilution - Google Patents

Abstract

ABSTRACT In a method of providing a diluted discharge pulp, performing the steps ofproviding (S10) pulp With a first consistency, Which pulp cornprising cookedIignocellulosic fibrous material, diluting (S20) the provided pulp With a lowconsistency screening reject liquor to provide a diluted pulp, and feeding (S30) the diluted pulp to a discharge tank to provide a diluted discharge pulp. (Fíg- 1)

Description

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

BACKGROUND Pulp processing (e.g. Kraft process or sulfite process) typically comprísesintroducing wood chips or other lígnocellulosíc material into a digester orcooker together with an impregnation liquid e.g. black liquor from adisplacement tank and the chips or other lígnocellulosíc material is preheatedand pre-impregnated. Black liquor is the waste product from the so calledKraft process When pulpwood is dígested 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, whiteliquor is a strongly alkaline solution typically of sodium hydroxide and sodiumsulfide which is used to break the bonds between lignin and cellulose.Thereby, white liquor is the “pure” impregnation liquid Whereas the blackliquor is recycled impregnation liquid. Subsequently, hot black and whiteliquor is introduced into the digester and displaces the initially introducedimpregnation liquid back into the displacement tank. Due to the introductionof hot liquor the fiber is heated and cooked during a period of maintaining thetemperature at a desired level. The cooking dissolves or softens the lignin inthe material and the cooking time and temperature is dependent on thematerial to be cooked as well as desired fiber properties after cooking. Aftercooking, black liquor e.g. displacement liquor (from the displacement tank)With a lower temperature is introduced into the digester to lower thetemperature and thereby terminate the cooking reactions. Finally the digester is discharged to an atmospheric storage tower e.g. discharge tank from which lO the cooked pulp is transported via piping to e.g. a screening process in orderto deliver a finished pulp With predetermined properties e.g. predetermined fibre fractions.

In this context, wood pulp can be defined as Wood chips mechanically groundand/ or chemically digested at elevated temperatures to produce fibers for usein manufacturing paper or fiber-based wood composites. [1] In the samemanner 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 includesall naturally occurring terrestrial plants such as trees, bushes and grass. Socalled Waste biomass is produced as a low value byproduct of variousindustrial sectors such as agricultural (corn stover, sugarcane bagasse, strawetc.), forestry (saw mill and paper mill discards). Energy crops are crops withhigh yield of lignocellulosic biomass produced to serve as a raw material forproduction of second generation biofuel examples include switch grass (Panicum virgatum) and Elephant grass.

In present pulp manufacture processes, liquor from the displacement liquortank is utilized in order to dílute the pulp in the díscharge sequence. Thedilution is necessary in order to enable emptying or discharging the pulp fromthe digester and into the díscharge tank. In the subsequent screening processsome of the process steps are fed with a pulp at low consistency and usingliquor for its function. The accept from these process steps, which is a lowconsistency pulp which resembles pure liquor more than a thicker pulp, issubsequently fed to a reject tank and then fed to the díscharge tank. The flowfrom the reject tank then lowers the consistency in the system significantly.To reduce the consequences of the reject tank flow, typically a so called deltathickener is added, which removes most of the liquor from the pulp fed to the díscharge tank Consequently, the pulp fed to the discharge tank from batch Cooking does nothave high consístency enough to handle all the liquor from the reject tank asdilution in the discharge tank Without lowering the screen feed consístencymore than desired. In many applications today, the consístency is increasedWith a thickening device e.g. delta thickener or the like in order to enable thescreening being fed at a normal consístency. Depending on the manufacturerof the processing equipment, the thickening agent or device varies.

The thickening agent and/ or machinery introduces both unwanted processsteps 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 relates to improved discharge method and arrangement in pulp processing.

In a first aspect the proposed technology discloses a method of providing adiluted discharge pulp, comprising the steps of providing pulp With a firstconsístency in a digester, Which pulp comprises cooked lignocellulosic fibrousmaterial. Subsequently, diluting the provided pulp by feeding a reject liquorfrom a screening process into the digester during a discharge sequence toprovide a diluted pulp in the digester, the reject liquor from the screeningprocess being a pulp With a lower consístency than the pulp With a firstconsístency, and finally feeding the diluted pulp through a discharge pipe toa discharge tank to provide a diluted discharge pulp.

By means of the proposed technology the pulp can be diluted without additionof any unwanted additives and reject liquor can be reused in the discharge prOCCSS. lO BRIEF DESCRIPTION OF THE DRAWINGS The invention, together With further objects and advantages thereof, may bestbe understood by referring to the following description taken together With the accompanying drawings, in Which: Fig. l is a floW chart of an embodiment of a method according to theproposed technology; Fig. 2 is a schematíc block chart of a system according to the proposedtechnology.

Fig. 3 is a schematíc block chart illustrating prior art; Fig. 4 is a schematíc block chart illustrating the benefits of the ernbodiments of the proposed technology; DETAILED DESCRIPTION The present disclosure Will be described in the context of a batch cookingprocess. However, it is easily adapted for a case of continuous cooking.Further, although the background and the description of the proposedtechnology primarily describes the so called Kraft process, it is equallyapplicable to sulfite processes or similar with appropriate adaptation of processing terrninology.

The inventors have identified the benefits of reusing liquor or low consistencypulp from the screening process for diluting the pulp during the dischargesequence. In other Words, instead of using liquor from the displacement liquortank during the discharge sequence, the same functionality can be met byusing liquor from a screening reject tank. As most of the floW from the rejecttank Will be used in the cooking discharge sequence, only a small part Will besent to the bottom of the discharge tank to control the consistency. The usageof dilution has then reduced so that it is not necessary to add any thickeningdevice 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 diluteddischarge pulp in a pulp processing plant will be described. In this context thepulp can comprise processed wood fibre [1] or other processed lignocellulosicfibre. According to a first embodiment, the method includes the steps ofproviding S10 pulp with a first consistency, said pulp comprising digested orcooked wood chips or other lignocellulosic fibrous material. Subsequently,diluting S20 the provided pulp to a predetermined consistency With a lowconsistency screening reject liquor to provide a diluted pulp, and finally feeding S30 the diluted pulp to a discharge tank to provide a diluted dischargepulp.

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

According to yet another embodiment, the method includes the further stepsof diluting S40 the discharge pulp in the discharge tank with a second fractionfrom 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 stepof feeding the low consistency screening reject liquor to a buffer, andsubsequently 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 consistencyor concentration of the diluted pulp. However, according to a further embodiment, it is possible to control, in step S25, the dilution in order to 6 achieve a diluted pulp with a predetermined consistency or concentration.This may be performed by determining the concentration of the pulp in thedigester 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 dísclosure will be described.

According to a first embodiment, the discharge dilution arrangement 100,comprises a digester 1 configured for receiving at least a lignocellulosíc fíbrousmaterial and a cooking liquor to cook into a pulp. Further, the dischargedilution arrangement 100 includes a discharge tank 2 configured for receivingsaid pulp from said digester 1 and for outputting said pulp to a screeningprocess. The digester 1 and the discharge tank 2 are connectable via adischarge pipe 3, configured for feeding cooked pulp from the digester 1 to thedischarge tank 2. Finally, the discharge dilution arrangement 100 includes adigester dilution pipe 4 configured for feeding a low consistency screeningreject liquor from the screening process into at least the digester tank 1 todilute the pulp. Preferably, the screening reject is fed into the digester l at thebottom 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 100includes a first discharge dilution pipe 4” configured for additionally feedingthe 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 100further includes a second discharge dilution pipe 5 configured for additionallyfeeding 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 andeven access to the reject liquor, it might prove beneficial to collect the rejectliquor into a buffer tank from which the reject liquor is then distributed to theparts of the arrangements where dilution is required. According to a furtherembodiment, the discharge dilution arrangement 100 further comprises abuffer tank 6 arranged to receive the low consistency screening reject liquorfrom the screening process and to provide a first fraction of the low consistencyscreening reject liquor to the digester dilution pipe 4 and a second fraction ofthe 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 toprovide 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 processsteps will be described with reference to Figure 3 and Figure 4. Theconcentration and concentration intervals are to be viewed as non-limitingexamples and are indicated merely to illustrate the benefits of the proposed technology. ln 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 chipsor 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%, lO 8 corresponding to a total volume of 200 m3 / h of cooked pulp. In order to enableemptying or discharging the digester 1, liquor is added to dilute the cookedpulp and Which to reduce the concentration of the cooked pulp to i.e. 5%. Inorder to handle the processing capacity of 200 m3 /h from the digester 1, it isnecessary to add 200 m3/ h of dilution liquid e. g. liquor, thereby resulting in afeed of 400 m3 / 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 thecooked pulp in order to enable emptying the discharge tank 2 into thesubsequent screening process. Accordingly, the discharge 2 is fed withscreening reject to further lower the concentration to about 3%. In thisexample the low consistency screening reject is fed with a capacity of 2 t/ hand a concentration or consistency of 0.6 %, which corresponds to 330 m3/h.Thereby, the discharger tank 2 outputs 22 t/ h with a concentration of 3 %and a volume of 730 m3 / h. Through the screening process a screened pulpwith 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 optionallythe discharge tank 2 is provided by adding the low consistency screening rejectliquor. In a corresponding manner as in Figure 3, a digester 1 is disclosed witha capacity for processing 20 t/ h of wood chips or other lignocellulosic material.lmpregnation and /or cooking liquid, e. g. black and/ or white liquor or similardepending on the chemical process used, is added to achieve a concentrationor first consistency of approximately 10%, corresponding to a total volume of200 m3 / h of cooked pulp. In order to enable emptying or discharging thedigester 1, a low consistency reject liquor with a consistency of approximately0.6% is added to dilute the cooked pulp. In order to handle the processingcapacity of 200 m3/h from the digester 1, it is necessary to add 1.2 t/h of thelow consistency screening reject liquor with a consistency of 0.6%, thereby resulting in a feed of 400 m3/ h of diluted cooked pulp With a concentration of 53% from the digester l 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 potentiallynecessary to further dilute the cooked pulp in order to enable emptying thedischarge tank 2 into the subsequent screening process. Accordingly, thedischarge 2 is also fed with the low consistency screening reject liquor.However, in contrast to the prior art solution in Figure 3 the addition ofscreening reject is performed with O.8t/ h with a consistency of O.6%corresponding to 130 m3 / h. Thereby, the discharger tank 2 outputs 22 t/ h with a concentration of 4.1 % and a volume of 530 m3 / h.

In case of using some other liquid than the screening liquid it is easy to seethe benefits of the current embodiments. Using a O % diluting liquidcorresponds to an output volume of 730 m3 / h from the discharge tank 2 asillustrated in Figure 3 (Without a thickening agent) to provide the necessaryscreening consistency of 3-4.5°/<>. By utilizing a diluting liquid of 0.6 % theprocessed volume can be reduced to 530 m3 / 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 alsovary Within concentration intervals of for example around 9,5-10,5%, 9-1 1%,8- 12%. The low consistency screening reject liquor is potentially O.6%, but canalso vary Within concentration intervals of for example O.3-1°/o. The resultingdiluted pulp is diluted to such an extent that it is possible to empty thedigester with a minimum of dilution. As an example the diluted pulp could bebetween 4-7%, or 4-6°/o, 4-5%, 4,5-5,5%. Basically, depending on the materialto be processed, the chemicals used, the cooking or digesting parametersapplied in the process, the type of screening and other variables the specific concentrations can vary also outside the above given example intervals. lO With the presently proposed technology it is possible to achieve a desiredconsistency 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 shouldbe understood that the proposed technology is not limited thereto. It Will beunderstood by those skilled in the art that various modifications,combinations and changes may be made to the embodiments Withoutdeparting from the present scope as defined by the appended claims. Inparticular, different part solutions in the different embodiments can be combined in other configurations, Where technically possible. 11 REFERENCES[1] “Fact Sheet Agriculture and Natural Resources", F-85-l3, ForestProducts Terminology, E. McConnell, N Irby, (2013) Ohio State University,http: / / ohiowood.osu.edu / images/ F_8 5_1 3_FP_Terrnino1ogy_Fact_Sheet.pdf [2] Chemical Pulping, 6A and ÖB, J Gullichsen, C-J Fogelholm, PaperMaking Science and Technology, ISBN 952-5216-06-3, (1999) Jyväskylä,Finland

Claims (10)

1. A method of providing a diluted discharge pulp, comprising providing (S10) pulp With a first consistency in a digester, said pulpcomprising cooked lignocellulosic fibrous material,characterizeå hy diluting (S20) said provided pulp by feeding a reject liquor from ascreening process into said digester during a discharge sequence to provide adiluted pulp in said digester, said reject liquor from said screening processbeing a pulp With a lower consistency than said pulp With a first consistency, feeding (S30) said diluted pulp through a discharge pipe to a dischargetank to provide a diluted discharge pulp.

2. The method according to claim 1, Wherein said diluting step (S20)additionally comprises feeding a first fraction of said reject liquor from said screening process into said discharge pipe to further dilute the diluted pulp.

3. The method according to claim 2, comprising the further steps ofdiluting (S40) said discharge pulp in said discharge tank With a secondfraction of said reject liquor from said screening process, and feeding said diluted discharge pulp to said screening process.

4. The method according to any of claims 1-3, comprising the step offeeding said reject liquor from said screening process to a buffer, andsubsequently feeding said reject liquor from said buffer to at least saiddigester, and optionally to said discharge pipe, and optionally to said discharge tank.

5. A discharge dilution arrangement (100), comprisinga digester (1) configured for receiving at least a lignocellulosic fibrous material and a cooking liquor to cook into a pulp; lO 13 a discharge tank (2) configured for receiving said pu1p from said digester(1) and for outputting said pu1p to a screening process; agg a discharge pipe (3), configured for feeding cooked pu1p from saiddigester (1) to said discharge tank (2),characterizeå 'hy a digester di1ution pipe (4) configured for feeding a reject liquor fromsaid screening process into at 1east a bottom section of said digester tank (1)to di1ute the pu1p in the digester (1), said reject liquor from said screening process being a pu1p With a lower consistency than the pu1p in the digester.

6. The discharge di1ution arrangement according to c1aim 5, comprising afirst discharge di1ution pipe (4') configured for additionally feeding said rejectliquor from said screening process into said discharge pipe (3) to further di1ute the pu1p in the discharge pipe (3).

7. The discharge di1ution arrangement according to c1aim 5 or 6,comprising, a second discharge di1ution pipe (5) configured for additionally feedingsaid reject liquor from said screening process into said discharge tank (2) to further di1ute the pu1p.

8. The discharge di1ution arrangement according to any of c1aims 5-7,Wherein said arrangement further comprises a buffer tank (6) arranged toreceive said reject liquor from said screening process and to provide said rejectliquor from said screening process to said digester di1ution pipe (4) andoptionally a first fraction of said reject liquor from said screening process tosaid first discharge di1ution pipe (4'), to provide a di1uted discharged pu1p With a predetermined consistency.

9. The discharge di1ution arrangement according to any of c1aims 5-8, Wherein said buffer tank (6) is further configured to provide a second fraction 14 of said reject liquor from said screening process via said second discharge dilution pipe (5) to said discharge tank (2).

10. The discharflage ríiiutiion arrangement according to any of claims 5-9, Wherein said digester (1) is a batch digester or a continuous digester.