SE539140C2

SE539140C2 - Bottom scraper for a cellulose material treatment vessel

Info

Publication number: SE539140C2
Application number: SE1551552A
Authority: SE
Inventors: WILHELMSSON Magnus; Hägglund Magnus
Original assignee: Valmet Oy
Priority date: 2015-11-27
Filing date: 2015-11-27
Publication date: 2017-04-18
Also published as: EP3380666A4; EP3380666A1; WO2017091138A1; SE1551552A1

Abstract

11 ABSTRACT The invention relates to an improved design of the bottom of a treatment vesselfacilitating feed out of comminuted cellulosic material reducing the torque load onconventional bottom scrapers. The bottom of the treatment vessel has a constant radius (R1 = Rz = Ra), or close tothat, and the scraper arms has reduced radial extension, using the slow flowdiversion of the flow of comminuted cellulosic material towards outlet. The steepertransition of the wall avoids chip column hanging close to treatment vessel wall andthereby do not necessitates that scraper arms are extended all the way to the wall. lna preferred embodiment are no scraper blades (4) arranged radially outside of the mid-section of the scraper arm. Fi9 (4)

Description

1/11 BOTTOM SCRAPER FOR A CELLULOSE MATERIAL TREATMENT VESSEL Technical areaThe present invention relates to arrangement for a downflow treatment vesselfacilitating feed out of comminuted cellulosic material slurried in a treatment liquid in said treatment vessel.

State of the Art Several solutions has been proposed to improve the discharge of comminutedcellulosic material from continuous digesters or impregnation vessels. The largestdigesters have a total height of about 100 meters and with a bottom diameter wellover 8 meter, in some digesters over 12 meter. The digester is typically pressurizedto at least 3-6 bar, and in older digesters to about 8-10 bar. When using theconventional discharge scraper in the bottom of the digester, typically a motor with atleast a 600-800 HP rating is needed. lmpregnation vessels are slightly smaller andmay be atmospheric in top.

Even with these huge motors could the operating conditions be exceeded such thatoverload protection is activated at frequent interval. These huge motors alsocontributes in excessive operating costs as well as a risk for reduced pulp strengthdue to extensive mechanical agitation on the comminuted cellulosic material.

The bottom scraper has been perceived as a necessity to maintain a plug-flow ofchips down the impregnation vessel or digester and equal retention time in thedigester for chips passing the wall of the digester as well as for chips passing downin the center of the chip column inside the treatment vessel. The bottom scraper isalso facilitating emptying of the vessel at shut down, and restart with filled vessel. ln figure 1 is disclosed a conventional design of the bottom part of a continuousdigester, but may also be applicable in impregnation vessels. The vast majority of theinstalled digesters of today have a design similar to figure 1. ln the bottom of thedigester is arranged a bottom scraper comprising a revolving shaft 1 driven by amotor M, and with at least two scraper arms 2a,2b and a central cone diverter 3mounted at the revolving shaft. At each arm are also attached scraper blades 4. Thebottom of the digester shell is a cupped/”dished” gable end and the scraper arms arearranged to sweep over the inside of the gable end and push the cooked pulptowards a central outlet 20. The shape of the transition from a cylindrical wall to the KST 15008SE_Eng|ish text.docx 2/11 outlet is curvilinear, but the radius is changing over the transition from a rather smallradius to a larger radius close to outlet. By this design is the retention volume insidethe vesse| maximized. Typically the arms are slightly angled at an angle oi in relationto the horizontal plane in the range between 5°-30°. The cone diverter is used toreduce risk of channeling of pulp flow, which channeling could result in that the corepulp flows quicker trough the digester than pulp passing down over the digester wall,as the wall cause a friction, and thus ends in uneven pulp quality. As in mostdigesters is also the bottom of the digester designed to implement a final wash zone.Conventionally is cleaner wash liquid, which could be brown stock washing filtrate,introduced into the bottom of the digester through several wash dilution nozzles, herevertical nozzles VN and horizontal nozzles HN. A vertical countercurrent flow, asindicated with grey flow arrows, of this wash liquid is established up to a wash screenWS. As a complement to the axial displacement wash is also a radial washdisplacement established by adding wash liquid trough a central pipe CP, whichoutlet mat be located slightly below, or above, the wash screen WS. The washscreen WS is a slotted screen plate or preferably a stave/bar screen which withdrawsused wash liquid and collects it in a wash extraction chamber WEC, which in turn isemptied to a wash header WH before being withdrawn from the digester. Typically isthe total retention time for the pulp in this wash zone about 10-30 minutes, but saidretention time could be lower as production increases down to 5 minutes, and couldbe increased as production decreases up to 45 minutes. The power requirements fordriving this kind of bottom scraper has shown to be quite large, as scraper arms needto extend to the outer wall, subjecting the arms to more torque. ln US 6,280,575 is an example for redesign of the conventional scraper armdisclosing an idea for improved design of the bottom scraper with the objective toreduce the power requirements. The principle is disclosed in figure 2. Here isproposed to install a false bottom in the digester with a frusto-conical form, said conehaving an angel oi between 40-50 degrees to the horizontal line. The allegedreduction in necessary power for a bottom scraper is 10-20%. However, the torquearm length from center of bottom scraper to outer end of scraper arm is the same,and the total chip column surface to be “shaved” by the arms of the bottom scraper isextended as the cone angle increases. Another problem is that the false bottom wallneeds to be supported structurally in order to withstand the total pressure inside thedigester, or needs to be hydraulically balanced such that the void volume behind the KST 15008SE_English text.docx 3/11 false bottom is filled with liquid at same pressure as in the digester. Another problemwith this design is that the pulp passing the wall of the digester (“sheet pulp”) meetsthe bottom scraper sooner than the pulp passing down in the center (“core pulp”),resulting in different retention time in the digester and thus results in uneven pulpquality. Typically the design chip column speed is about 10 minutes/meter, causing adifference in retention time between sheet and core pulp of about 40 minutes in adigester with a diameter of 8 meter using a bottom scraper with arms inclined 45degrees. ln figure 3 is yet another idea for improved design of the bottom scraper, which iscommercially offered to customers having problem with bottom scraper load. ln thisconcept are the arms of the scraper bent upwardly at the outer end, such that theouter ends sweeps closer to horizontal dilution nozzles HN. The outer end has a larger deﬂection angle q2 compared to the arm angel q close to scraper shaft.

Obiect of the inventionA first objective with the invention is to obtain a further reduction in power consumption in an arrangement for a downflow treatment vessel facilitating feed outof comminuted cellulosic material slurried in a treatment liquid in said treatment vesseL Summarv of the invention The invention is based upon an idea to design the very curvilinear shape of thebottom to a nearly constant radius and using scraper arms that cover less than 100%of the cross section in the bottom thus reducing the total torque load on the arms.Thus the inventive arrangement for a downflow treatment vessel facilitating feed outof comminuted cellulosic material slurried in a treatment liquid in said treatmentvessel, comprises following key features;o a cylindrical vessel wall above the lower end of the treatment vessel with afirst cross section area, ando a central outlet in the bottom of the treatment vessel with a second crosssection area less than 25% of the first cross section; ando a bottom scraper with a least two scraper arms arranged on a drive shaftarranged concentric in the central outlet and connected to a motor forrotating the shaft and associated arms in the bottom of the treatment KST 15008SE_English text.docx 4/11 vessel; ando a curvilinear shaped transition wall part between the cylindrical vessel walland the outlet; whereino said curvilinear shaped transition has a substantially constant radiusbetween the cylindrical vessel wall and the outlet varying less than 10%from an absolutely constant value of said radius; ando said scraper arms has an radial extension from the center of the outlet and towards the cylindrical vessel wall less than 90% of the radial distance from the center of the outlet and towards the cylindrical vessel wall.By using this concept it has been seen in a mill that the total load is reduceddramatically and corresponds to the load as seen during water runs of the treatmentvessel (i.e. the vessel only filled with water during first testing).ln one installation tested in a treatment vessel with a bottom diameter of 5.7 meterand filled with slurried wood chips, was the loading of the hydraulic motor as low as arequired hydraulic pressure of only 20 bar, while it has been experienced withconventional bottom scraper design in similar size of treatment vessels that therequired hydraulic pressure could be as high as 200 bar. The required hydraulicpressure of 20 bar was very close to the required hydraulic pressure as experiencedwhen doing a preceding water test.

According to a preferred embodiment of the invention are also each of the scraperarms straight beam elements, where the shortest distance between the outer end ofthe arm and the curvilinear transition wall part is between 150-250 mm, and thedistance between the mid part of the arm and the curvilinear transition wall part is atleast 90% longer. This simple design provides for a less expensive design usingstandard straight beams, and the distance between arm and transition wall part iskept at short distance only close to outer end, reducing risks ofjamming flow ofmaterial between transition wall part and arms. ln yet a preferred embodiment has each of the scraper arms an angle versus ahorizontal plane lying in the range 25-40 degrees. This provides for a perfectinteraction with a curvilinear transition part with constant radius, and has proven tofind a low scraper load when arranged at about 33 degrees in such constant radiustransition part. ln a further embodiment of the inventive arrangement has each of the scraper arms KST 15008SE_English text.docx 5/11 at least one scraper blade arranged orthogonally to and below the extension of thestraight beam elements at a mid-point position of the scraper arm. This location ofthe scraper blade is then located in a zone with the longest distance between armand transition wall, and contributes to push the material that has been broken up bythe above lying arms towards outlet. ln a specific embodiment of this scraper bladedesign has also the scraper arms no scraper blade elements radially outside of themid-point position of the scraper arm, thus reducing the torque load on the arms bysuch scraper blades arranged at the longest distance from the drive shaft.

The inventive arrangement also includes a cone diverter arranged in the center of thebottom scraper and above the scraper arms, and in size covering the cross section ofthe in the central outlet. This cone diverted press core material flow radially outvvardlyto the straight beam elements of the scraper arms and reduce load on the scraper. ln a best mode of the inventive arrangement has also said scraper arms an radialextension from the center of the outlet and towards the cylindrical vessel wall at 80%, +/-2%, of the radial distance from the center of the outlet and towards thecylindrical vessel wall. ln the water test with the scraper was an radial extension of79% resulting in slightly lower load as during operation with comminuted cellulosematerial filling the treatment vessel. ln the very best mode tested had also said curvilinear shaped transition a constantradius between the cylindrical vessel wall and the outlet, but if the inventive idea isfollowed may also in future modifications with no constant radius instead show avarying radius starting with larger radius from cylindrical vessel wall towards smallerradius close to outlet, but still varying less than 10 of the radius at the most. Thematerial flow along the wall of the cylindrical vessel may then be subjected to thelowest possible deflection of material flow direction, and risks of plugging stationary “wall” volumes be minimized, before the material flow meets the scraper arm action.

List of figuresFigure 1; disclose a bottom scraper for a continuous digester according to the most common conventional design; KST 15008SE_Eng|ish text.docx 6/11 Figure 2; disclose an alternative prior art design; Figure 3, disclose yet another alternative prior art design; Figure 4; disclose an embodiment of the invention; Figure 5; disclose the bottom scraper alone as seen from above in figure 4;Figure 6; disclose a perspective view of the bottom scraper in a design drawing without the cone diverter mounted.

Detailed description of the drawinqs ln figure 4 is a first embodiment of the inventive arrangement disclosed. Thearrangement is for a down flow treatment vessel, i.e. where slurried comminutedcellulosic material is fed in from the top and flows out in the bottom.

The arrangement facilitates feed out of comminuted cellulosic material slurried in atreatment liquid in said treatment vessel, which treatment vessel may be aimpregnation vessel, digester vessel or similar. Comminuted cellulosic materialslurried in liquid is often dense packed in the bottom, and normally dense packedwood chips would fill up about 1/3 of the volume while the rest of the volume istreatment liquid. During treatment the chips is softened and part of the wood chipmaterial (lignin, hemicellulose, turpentine etc) is dissolved in the treatment liquid,wich caused compression of the chip volume and locking between wood chipfragments. ln the basic definitions of the treatment vessel it comprises in line with conventionalprior art; o a cylindrical vessel wall above the lower end of the treatment vessel with afirst cross section area. This cross section area is defined by the diameter Dias disclosed in Fig.4. o a central outlet in the bottom of the treatment vessel with a second crosssection area less than 25% of the first cross section. This cross section area isdefined by the diameter Ds as disclosed in Fig.4. o a bottom scraper with a least two scraper arms 2a, 2b arranged on a driveshaft 1 arranged concentric in the central outlet and connected to a motor Mfor rotating the shaft and associated arms in the bottom of the treatmentvesseL The inventive arrangement is further distinguished in that a curvilinear shapedtransition wall part between the cylindrical vessel wall and the outlet; wherein said KST 15008SE_Eng|ish text.docx 7/11 curvilinear shaped transition has a substantially constant radius (R1=R2=Rs) betweenthe cylindrical vessel wall and the outlet varying less than 10% from an absolutelyconstant value of said radius. Further the scraper arms 2a, 2b has an radialextension (=D2/2) from the center of the outlet and towards the cylindrical vessel wallless than 90% of the radial distance (=D1/2) from the center of the outlet and towardsthe cylindrical vessel wall.

As seen in figure 4 is each of the scraper arms 2a, 2b straight beam elements, wherethe shortest distance X1 between the outer end of the arm and the curvilineartransition wall part is between 150-250 mm, and the distance X2 between the midpart of the arm and the curvilinear transition wall part is at least 90% longer.

Each of the scraper arms has an angle oi versus a horizontal plane lying in the range25-45 degrees.

Further, each of the scraper arms 2a 2b has at least one scraper blade 4 arrangedorthogonally to and below the extension of the straight beam elements at a mid-pointposition of the scraper arm. The scraper arms has no scraper blades radially outsideof the mid-point position of the scraper arm.

A cone diverter 3 is arranged in the center of the bottom scraper and above thescraper arms, and in size covering the cross section of the in the central outlet. ln amost preferred embodiment of the invention has the scraper arms an radial extensionfrom the center of the outlet and towards the cylindrical vessel wall at 80 %, +/-2%, ofthe radial distance from the center of the outlet and towards the cylindrical vesselwall.

Also, in yet another specific and preferred embodiment of the invention thecurvilinear shaped transition has a constant radius, R1=R2=Rs, between thecylindrical vessel wall and the outlet. Alternatively, if not constant varying by startingwith larger radius from cylindrical vessel wall towards smaller radius close to outlet,i.e. R1>R2>R3. ln the latter alternative this could correspond in a treatment vesselwith a diameter of 5.7 meter that R1 (closer to cylindrical vessel wall) may start at 3meter, Rg (at midpoint) at 2.9 meter and Rs (closer to outlet) at 2.8 meter, thusvarying less than 10%. But in consideration of practical manufacturing it is to bepreferred that the radius is as constant as possible, with possibly only manufacturingtolerances included.

As the results from first testing of this design, and results indicating that the torqueload on this kind of bottom scraper with vessel filled with chips is close to testing with KST 15008SE_English text.docx 8/11 only water filled treatment vessel, and that corresponding torque load on bottomscrapers with dished ended gables may be 10 times as high, it has practically beenproven that the design reduce torque load on the bottom scraper. The costs forselecting a hydraulic drive that needs 10 times higher hydraulic pressure induce costincreases that are many times higher than the required pressure increase. Requiringdouble effect in drives may often result in not twice the costs, but often 4-9 times thecosts for half of the required effect. The torque effect required for the bottom scraperis also indicative for the amount of mechanical action on the wood chips that typically reduce pulp quality as well.

KST 15008SE_Eng|ish text.docx

Claims

1. An arrangement for a downflow treatment vessel facilitating feed out of comminuted cellulosic material slurried in a treatment liquid in said treatmentvessel, comprising; a cylindricai vessel wall above the lower end of the treatment vessel witha first cross section area, and a central outlet in the bottom of the treatment vessel with a second crosssection area less than 25% of the first cross section area; and a bottom scraper with a least two scraper arms arranged on a drive shaftarranged concentric in the central outlet and connected to a motor for rotating thedrive shaft and associated scraper arms in the bottom of the treatment vessel;and a curvilinear shaped transition wall part between the cylindricai vesselwall and the central outlet; wherein said curvilinear shaped transition wall part has a substantially constantradius between the cylindricai vessel wall and the central outlet varying less than10% from an absolutely constant value of said radius; and said scraper arms have a radial extension from the center of the centraloutlet and towards the cylindricai vessel wall less than 90% of the radial distance from the center of the central outlet and towards the cylindricai vessel wall. _ An arrangement according to claim 1 characterized in that each of the scraper arms are straight beam elements, where the shortest distance between the outerend of the scraper arm and the curvilinear shaped transition wall part is between150-250 mm, and the distance between the mid part of the scraper arm and the curvilinear shaped transition wall part is at least 90% longer. _ An arrangement according to claim 2 characterized in that each of the scraper arms has an angle versus a horizontal plane lying in the range 25-45 degrees. . An arrangement according to claim 3 characterized in that each of the scraper arms has at least one scraper blade arranged orthogonally to and below theextension of the straight beam elements at a mid-point position of the scraper arm. . An arrangement according to claim 4 characterized in that the scraper arms have no scraper blade elements radially outside of the mid-point position of thescraper arms. . An arrangement according to claim 1 characterized in that a cone diverter is arranged in the center of the bottom scraper and above the scraper arms, and in size covering the cross section of the central outlet. _ An arrangement according to claim 1 characterized in that said scraper arms have a radial extension from the center of the central outlet and towards thecylindrical vessel wall at 80 %, +/-2%, of the radial distance from the center of the central outlet and towards the cylindrical vessel wall. _ An arrangement according to claim 1 characterized in that said curvilinear shaped transition wall part has a constant radius between the cylindrical vesselwall and the central outlet, or if not constant varying by starting with larger radius from cylindrical vessel wall towards smaller radius close to the central outlet.