EP3380666A1

EP3380666A1 - Bottom scraper for a cellulose material treatment vessel

Info

Publication number: EP3380666A1
Application number: EP16868987.5A
Authority: EP
Inventors: Magnus WILHELMSSON; Magnus Hägglund
Original assignee: Valmet Oy; Valmet AB
Current assignee: Valmet Technologies Oy; Valmet AB
Priority date: 2015-11-27
Filing date: 2016-11-25
Publication date: 2018-10-03
Also published as: EP3380666A4; SE539140C2; WO2017091138A1; SE1551552A1

Abstract

The invention relates to an improved design of the bottom of a treatment vessel facilitating feed out of comminuted cellulosic material reducing the torque load on conventional bottom scrapers. The bottom of the treatment vessel has a constant radius (R1 = R2 = R3), or close to that, and the scraper arms has reduced radial extension, using the slow flow diversion of the flow of comminuted cellulosic material towards outlet. The steeper transition of the wall avoids chip column hanging close to treatment vessel wall and thereby do not necessitates that scraper arms are extended all the way to the wall. In a preferred embodiment are no scraper blades arranged radially outside of the mid-section of the scraper arm.

Description

BOTTOM SCRAPER FOR A CELLULOSE MATERIAL TREATMENT VESSEL

Technical area

The present invention relates to an arrangement for a downflow treatment vessel facilitating feed out of comminuted cellulosic material slurried in a treatment liquid in said treatment vessel.

State of the Art

Several solutions have been proposed to improve the discharge of comminuted cellulosic material from continuous digesters or impregnation vessels. The largest digesters have a total height of about 100 meters and a bottom diameter well over 8 meter, in some digesters over 12 meter. The digester is typically pressurized to at least 3-6 bar, and in older digesters to about 8-10 bar. When using the conventional discharge scraper in the bottom of the digester, typically a motor with at least a 600- 800 HP rating is needed. Impregnation vessels are slightly smaller and may be atmospheric in top.

Even with these huge motors could the operating conditions be exceeded such that overload protection is activated at frequent intervals. These huge motors also contribute to excessive operating costs as well as a risk for reduced pulp strength due to extensive mechanical agitation on the comminuted cellulosic material. The bottom scraper has been perceived as a necessity to maintain a plug-flow of chips down the impregnation vessel or digester and equal retention time in the digester for chips passing the wall of the digester as well as for chips passing down in the center of the chip column inside the treatment vessel. The bottom scraper is also facilitating emptying of the vessel at shut down, and restart with filled vessel.

In Fig. 1 is disclosed a conventional design of the bottom part of a continuous digester, but may also be applicable in impregnation vessels. The vast majority of the installed digesters of today have a design similar to Fig. 1 . In the bottom of the digester is arranged a bottom scraper comprising a revolving shaft 1 driven by a motor M, and with at least two scraper arms 2a, 2b and a central cone diverter 3 mounted at the revolving shaft 1 . At each arm are also attached scraper blades 4. The bottom of the digester shell is a cupped/"dished" gable end and the scraper arms are arranged to sweep over the inside of the gable end and push the cooked pulp towards a central outlet 20. The shape of the transition from a cylindrical wall to the outlet is curvilinear, but the radius is changing over the transition from a rather small radius to a larger radius close to outlet. By this design is the retention volume inside the vessel maximized. Typically the arms are slightly angled at an angle a in relation to the horizontal plane in the range between 5°-30°. The cone diverter is used to reduce risk of channeling of pulp flow, which channeling could result in that the core pulp 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 most digesters 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, here vertical nozzles VN and horizontal nozzles HN. A vertical countercurrent flow, as indicated with black flow arrows, of this wash liquid is established up to a wash screen WS. As a complement to the axial displacement wash is also a radial wash displacement established by adding wash liquid trough a central pipe CP, which outlet can be located slightly below, or above, the wash screen WS. The wash screen WS is a slotted screen plate or preferably a stave/bar screen which withdraws used wash liquid and collects it in a wash extraction chamber WEC, which in turn is emptied to a wash header WH before being withdrawn from the digester. Typically is the total retention time for the pulp in this wash zone about 10-30 minutes, but said retention time could be lower as production increases down to 5 minutes, and could be increased as production decreases up to 45 minutes. The power requirements for driving this kind of bottom scraper has shown to be quite large, as scraper arms need to extend to the outer wall, subjecting the arms to more torque.

In US 6,280,575 is an example of redesign of the conventional scraper arm disclosing an idea for improved design of the bottom scraper with the objective to reduce the power requirements. The principle is disclosed in Fig. 2. Here is proposed to install a false bottom in the digester with a frusto-conical form, said cone having an angel a between 40-50 degrees to the horizontal line. The alleged reduction in necessary power for a bottom scraper is 10-20 %. However, the torque arm 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 is extended as the cone angle increases. Another problem is that the false bottom wall needs to be supported structurally in order to withstand the total pressure inside the digester, or needs to be hydraulically balanced such that the void volume behind the false bottom is filled with liquid at same pressure as in the digester. Another problem with this design is that the pulp passing the wall of the digester ("sheet pulp") meets the 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 pulp quality. Typically the design chip column speed is about 10 minutes/meter, causing a difference in retention time between sheet and core pulp of about 40 minutes in a digester with a diameter of 8 meter using a bottom scraper with arms inclined 45 degrees.

In Fig. 3 is yet another idea for improved design of the bottom scraper, which is commercially offered to customers having problem with bottom scraper load. In this concept are the arms of the scraper bent upwardly at the outer end, such that the outer ends sweeps closer to horizontal dilution nozzles HN. The outer end has a larger deflection angle a2 compared to the arm angel a close to scraper shaft.

Object of the invention

A first objective with the invention is to obtain a further reduction in power consumption in an arrangement for a downflow treatment vessel facilitating feed out of comminuted cellulosic material slurried in a treatment liquid in said treatment vessel.

Summary of the invention

The invention is based upon an idea to design the very curvilinear shape of the bottom 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 out of comminuted cellulosic material slurried in a treatment liquid in said treatment vessel, comprises following key features:

• a cylindrical vessel wall above the lower end of the treatment vessel with a first cross section area; and

• a central outlet in the bottom of the treatment vessel with a second cross section area less than 25% of the first cross section; and • a bottom scraper with a least two scraper arms arranged on a drive shaft arranged concentric in the central outlet and connected to a motor for rotating the shaft and associated arms in the bottom of the treatment vessel; and

· a curvilinear shaped transition wall part between the cylindrical vessel wall and the outlet, wherein

• said curvilinear shaped transition has a substantially constant radius between the cylindrical vessel wall and the outlet varying less than 10 % from an absolutely constant value of said radius; and

· 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 reduced dramatically and corresponds to the load as seen during water runs of the treatment vessel (i.e. the vessel only filled with water during first testing).

In one installation tested in a treatment vessel with a bottom diameter of 5.7 meter and filled with slurried wood chips, was the loading of the hydraulic motor as low as a required hydraulic pressure of only 20 bar, while it has been experienced with conventional bottom scraper design in similar size of treatment vessels that the required hydraulic pressure could be as high as 200 bar. The required hydraulic pressure of 20 bar was very close to the required hydraulic pressure as experienced when doing a preceding water test.

According to a preferred embodiment of the invention are also each of the scraper arms straight beam elements, where the shortest distance between the outer end of the arm and the curvilinear transition wall part is between 150-250 mm, and the distance between the mid part of the arm and the curvilinear transition wall part is at least 90 % longer. This simple design provides for a less expensive design using standard straight beams, and the distance between arm and transition wall part is kept at short distance only close to outer end, reducing risks of jamming flow of material between transition wall part and arms. In yet a preferred embodiment has each of the scraper arms an angle versus a horizontal plane lying in the range 25-40 degrees. This provides for a perfect interaction with a curvilinear transition part with constant radius, and has proven to find a low scraper load when arranged at about 33 degrees in such constant radius transition part.

In a further embodiment of the inventive arrangement has each of the scraper arms at least one scraper blade arranged orthogonally to and below the extension of the straight beam elements at a mid-point position of the scraper arm. This location of the scraper blade is then located in a zone with the longest distance between arm and transition wall, and contributes to push the material that has been broken up by the above lying arms towards outlet. In a specific embodiment of this scraper blade design has also the scraper arms no scraper blade elements radially outside of the mid-point position of the scraper arm, thus reducing the torque load on the arms by such scraper blades arranged at the longest distance from the drive shaft.

The inventive arrangement also includes a cone diverter arranged in the center of the bottom scraper and above the scraper arms, and in size covering the cross section of the in the central outlet. This cone diverted press core material flow radially outwardly to the straight beam elements of the scraper arms and reduce load on the scraper.

In a best mode of the inventive arrangement has also said scraper arms a radial extension 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 the cylindrical vessel wall. In the water test with the scraper was a radial extension of 79 % resulting in slightly lower load as during operation with comminuted cellulose material filling the treatment vessel.

In the very best mode tested had also said curvilinear shaped transition a constant radius between the cylindrical vessel wall and the outlet, but if the inventive idea is followed may also in future modifications with no constant radius instead show a varying radius starting with larger radius from cylindrical vessel wall towards smaller radius close to outlet, but still varying less than 10 % of the radius at the most. The material flow along the wall of the cylindrical vessel may then be subjected to the lowest 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 figures

Fig. 1 discloses a bottom scraper for a continuous digester according to the most common conventional design;

Fig. 2 discloses an alternative prior art design;

Fig. 3 discloses yet another alternative prior art design;

Fig. 4 discloses an embodiment of the invention;

Fig. 5 discloses the bottom scraper alone as seen from above in Fig. 4;

Fig. 6 discloses a perspective view of the bottom scraper in a design drawing without the cone diverter mounted.

Detailed description of the drawings

In Fig. 4 is a first embodiment of the inventive arrangement disclosed. The arrangement is for a downflow treatment vessel, i.e. where slurried comminuted cellulosic material is fed in from the top and flows out in the bottom. The arrangement facilitates feed out of comminuted cellulosic material slurried in a treatment liquid in said treatment vessel, which treatment vessel can be a impregnation vessel, digester vessel or similar. Comminuted cellulosic material slurried in liquid is often dense packed in the bottom, and normally dense packed wood chips would fill up about 1/3 of the volume while the rest of the volume is treatment liquid. During treatment the chips are softened and part of the wood chip material (lignin, hemicellulose, turpentine etc) is dissolved in the treatment liquid, which caused compression of the chip volume and locking between wood chip fragments.

In the basic definitions of the treatment vessel it comprises in line with conventional prior art:

• a cylindrical vessel wall above the lower end of the treatment vessel with a first cross section area. This cross section area is defined by the diameter Di as disclosed in Fig. 4;

• a central outlet in the bottom of the treatment vessel with a second cross section area less than 25 % of the first cross section. This cross section area is defined by the diameter D3 as disclosed in Fig. 4; • a bottom scraper with a least two scraper arms 2a, 2b arranged on a drive shaft 1 arranged concentric in the central outlet and connected to a motor M for rotating the shaft and associated arms in the bottom of the treatment vessel. The inventive arrangement is further distinguished in that a curvilinear shaped transition wall part between the cylindrical vessel wall and the outlet; wherein said curvilinear shaped transition has a substantially constant radius between the cylindrical vessel wall and the outlet varying less than 10 % from an absolutely constant value of said radius. Further the scraper arms 2a, 2b have a radial extension (=D2/2) from the center of the outlet and towards the cylindrical vessel wall less than 90 % of the radial distance (=Di/2) from the center of the outlet and towards the cylindrical vessel wall.

As seen in Fig. 4 is each of the scraper arms 2a, 2b straight beam elements, where the shortest distance Xi between the outer end of the arm and the curvilinear transition wall part is between 150-250 mm, and the distance X2 between the mid part of the arm and the curvilinear transition wall part is at least 90 % longer. Each of the scraper arms has an angle a versus a horizontal plane lying in the range 25-45 degrees. Further, each of the scraper arms 2a 2b has at least one scraper blade 4 arranged orthogonally to and below the extension of the straight beam elements at a mid-point position of the scraper arm. The scraper arms have no scraper blades radially outside of the mid-point position of the scraper arm. A cone diverter 3 is arranged in the center of the bottom scraper and above the scraper arms, and in size covering the cross section of the in the central outlet. In a most preferred embodiment of the invention have the scraper arms a radial extension 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 the cylindrical vessel wall. Also, in yet another specific and preferred embodiment of the invention the curvilinear shaped transition has a substantially constant radius, between the cylindrical vessel wall and the outlet. Alternatively, if not constant varying by starting with larger radius from cylindrical vessel wall towards smaller radius close to outlet, i.e. Ri>R2>R3. In the latter alternative this could correspond in a treatment vessel with a diameter of 5.7 meter that Ri (closer to cylindrical vessel wall) may start at 3 meter, R2 (at midpoint) at 2.9 meter and R3 (closer to outlet) at 2.8 meter, thus varying less than 10 %. But in consideration of practical manufacturing it is to be preferred that the radius is as constant as possible, with possibly only manufacturing tolerances included.

As the results from first testing of this design, and results indicating that the torque load on this kind of bottom scraper with vessel filled with chips is close to testing with only water filled treatment vessel, and that corresponding torque load on bottom scrapers with dished ended gables may be 10 times as high, it has practically been proven that the design reduce torque load on the bottom scraper. The costs for selecting a hydraulic drive that needs 10 times higher hydraulic pressure induce cost increases that are many times higher than the required pressure increase. Requiring double effect in drives may often result in not twice the costs, but often 4-9 times the costs for half of the required effect. The torque effect required for the bottom scraper is also indicative for the amount of mechanical action on the wood chips that typically reduce pulp quality as well.

Claims

PATENT CLAIMS

An arrangement for a downflow treatment vessel facilitating feed out of comminuted cellulosic material slurried in a treatment liquid in said treatment vessel, comprising:

a cylindrical vessel wall above the lower end of the treatment vessel with a first cross section area; and

a central outlet in the bottom of the treatment vessel with a second cross section area less than 25 % of the first cross section area; and

a bottom scraper with a least two scraper arms arranged on a drive shaft arranged concentric in the central outlet and connected to a motor for rotating the drive shaft and associated scraper arms in the bottom of the treatment vessel; and a curvilinear shaped transition wall part between the cylindrical vessel wall and the central outlet, wherein

said curvilinear shaped transition wall part has a substantially constant radius between the cylindrical vessel wall and the central outlet varying less than 10 % from an absolutely constant value of said radius; and

said scraper arms have a radial extension from the center of the central outlet and towards the cylindrical vessel wall less than 90 % 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 each of the scraper arms are straight beam elements, where the shortest distance between the outer end of the scraper arm and the curvilinear shaped transition wall part is between 150-250 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 the extension 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 the scraper 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 the cylindrical 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 vessel wall 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.