US20080169071A1 - Screen Device For a Digester For Producing Pulp - Google Patents

Screen Device For a Digester For Producing Pulp Download PDF

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Publication number
US20080169071A1
US20080169071A1 US10/569,093 US56909306A US2008169071A1 US 20080169071 A1 US20080169071 A1 US 20080169071A1 US 56909306 A US56909306 A US 56909306A US 2008169071 A1 US2008169071 A1 US 2008169071A1
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Prior art keywords
screen
digester
screen element
digestion
outlet
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Abandoned
Application number
US10/569,093
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English (en)
Inventor
Antti Isola
Matti Rosenqvist
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Valmet Technologies Oy
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Metso Paper Oy
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Assigned to METSO PAPER, INC. reassignment METSO PAPER, INC. ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: ISOLA, ANTTI, ROSENQVIST, MATTI
Publication of US20080169071A1 publication Critical patent/US20080169071A1/en
Assigned to VALMET TECHNOLOGIES, INC. reassignment VALMET TECHNOLOGIES, INC. CHANGE OF NAME (SEE DOCUMENT FOR DETAILS). Assignors: METSO PAPER, INC.
Abandoned legal-status Critical Current

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    • D—TEXTILES; PAPER
    • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
    • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
    • D21C7/00—Digesters
    • D21C7/14—Means for circulating the lye

Definitions

  • the present invention concerns a screen arrangement of a digester, and more precisely, a screen element being located substantially parallel with the vertical axis of the digester, in the middle of the digester, and process chemical being arranged to flow through said screen simultaneously either into the digester or out of the digester, depending on the determinations of the process and the stage of the process.
  • the field of use comprises especially feeding of process chemicals to the digester and removal of those from the digester when producing pulp or paper stock from wood chips in a batch or continuously operating digester.
  • process chemicals are moved with respect to the material to be processed, for example when digesting pulp from wood chips so, that mounted to the inner surface of the process vessel there is a screen with apertures of the kind that chemicals can flow through the screen surface, but the material to be processed is not able to pass through the screen surface.
  • the chemical flow can in the technique of prior art be directed either inwards to or outwards from the vessel, and the material to be processed, for example wood chip, can be either stable or movable to some direction, depending on the need.
  • the pulp produced chemically must as well have a good strength as a low kappa number, in other words, have a low lignin content after the digestion.
  • the digestion conditions must be carefully optimized. Among these digestion conditions are the correct alkali distribution, suitable temperature profile, adequate amount of liquid, and the amount of unwanted gradients being as small as possible, especially f.ex. in the radial direction of a continuous digester. This requires big circulation and expansion flows.
  • the screens can also be broken, when the support constructions fail. On average, this causes need for renovation of screens.
  • Screens nowadays available on the market for the continuous digestion usually comprise a plurality of vertical bar screens in an arrangement resembling for example a chessboard and attached to the inner surface of the digester in places determined by the process specification.
  • the screen bars of the bar screens have an angular form, for example as a metal bar worked in the form of T, or having a circular form.
  • one problem of the screens used for the digestion in prior art is the fact that they clog easily, when the chip particles cling to the slots.
  • this and the radial component of the pressure resulted from the chip column cause a resultant force pushing the particles to the slots of the screens said particles clogging superpositioned from bottom upwards causing blocking, which in turn disturbs the plug flow and causes channelling of the flow.
  • the packing degree of the digester is normal, the wood chips cling to each other inside the plug flow and also on the edges of the plug flow. Small sticks and chip particles follow the quicker radial liquid flow and cling to the sharp edges of the screen bars.
  • New sticks and chip particles cling more easily to these clung particles resulting in clogging of a wider screen area The clogging increases continuously as the screen area decreases.
  • the flows slow down also on the backside of the screen (between the screen and the digester jacket), resulting in that heavier and heavier particles are carried along to the backside of the screen and block the flow area between the screen and the jacket. Due to the location of the screens and the way of blocking, they are difficult to clean, and it is tried to be avoided as far a possible, for example by changing the construction types of the screens to the same place on the inner surface of the digester, on the casing of the pressure vessel.
  • Patents FI 54509 and FI 105 931 disclose methods for cleaning the screens.
  • One of the biggest problems is to provide an even distribution of the process chemical to the whole chip column without disturbing the flows.
  • wood chips are fed from the upper part of the digester and the processed pulp is removed from the lower part of the digester.
  • Cooking chemicals are fed via a longitudinal centre pipe mounted in the middle of the digester to different areas of the digester, determined according to the process technique.
  • the direction of flow is traditionally from the centre of the digester towards the jacket of the digester, where the screens are located.
  • the additional screen capacity is mounted to the jacket of the digester.
  • U.S. Pat. No. 3,475,271 discloses a digester especially meant for cuttings.
  • a rotating centre screen in the middle of the digester, near the bottom thereof.
  • the screen can have a cylindrical or conical form so that the diameter of the cone grows in the plug flow direction of the chip column.
  • the washing liquor is fed to the digester via screens on the walls of the pressure vessel and it flows towards said central screen, through which it is removed to recycling.
  • This solution aims at providing horizontal displacement wash on the bottom of the digester. Problems are caused, however, by the location of the central screen with respect to the feeding point of the washing liquor. The speed of the flow directed away from the wall of the digester towards the central screen is increased due to the reduced flowing area.
  • the digester is filled with chips and digestion chemical. After that, the content of the digester is heated by circulating the chemical with a pump and by heating it in the circulation pipe with a heat exchanger.
  • the digester is equipped with only one screen (circulation screen) mounted in the lower part of the digester, to the inner surface of the jacket. The location of the screen is unprofitable, as for having quickly homogenous digestion. Thus, among others, digestion time is wasted.
  • the thermal content of the previous digestion batches is recovered for the following digestions. This is achieved by displacing the hot liquor in the digester with cooler and purer liquor.
  • the hot liquor displaced from the digester is partly used as a digestion chemical of the following batches and partly for heating the new digestion chemical in the heat exchangers.
  • another screen is typically needed in addition to the above-mentioned recycling screen in order to perform the required displacements.
  • these so called displacement screens have been mounted to the upper part of the digester on the inner surface of the jacket.
  • the displacement batch digestion method is prominently more energy-efficient than the conventional batch digestion method and provides the possibility to produce pulp with better quality. As a result of this, it has become more popular and there is a need to change the existing conventional batch digestion methods to this kind.
  • the object of the present invention is to eliminate disadvantages stated in connection with the both digestion methods by using a central screen element wherein the flow of the process chemical can be directed to a predetermined point either inwards and/or outwards, and which can be installed into a process vessel easily also afterwards.
  • a tubular screen element having one or a plurality of zones in the longitudinal direction.
  • Each zone comprises a screen segment, through which process chemical can be sucked away from the digester, and at least one outlet, through which chemical is supplied to digester.
  • the zone can be also referred to as screen zone.
  • the central screen element In the transversal direction per each zone, the central screen element must have at least two channels in order to allow the flow of the process chemical in different directions also simultaneously.
  • the screen element is installed inside the process vessel, substantially parallel with the vertical axis, in the middle.
  • This kind of a screen element installed in the middle of the digester increases the screen area of the digester, thus improving the screen capacity.
  • the feed and removal of process chemicals can be optimized so, that the digestion is generally better.
  • the optimizing of the feed and removal means firstly, that the chemical can be fed to the right points in the longitudinal direction of the digester, and also to the middle of the cooking chip column. This is to ensure that there is enough of the process chemical and that it is evenly distributed throughout the whole chip column.
  • the feed and removal flows have influence on the movements of the chip column and on the flows of the whole digester, and in the changing digestion conditions the best possible flowing conditions for each digestion stage are provided by adjusting the places of the feeds and removals. This, for its part, improves the quality and homogeneity of the pulp.
  • FIG. 1 shows one embodiment of the structure of the central screen element in accordance with the present invention
  • FIG. 2 shows one embodiment of the location of the screen zones of the central screen element, in the longitudinal direction of the screen element.
  • FIG. 3 illustrates the operation of the central screen element in the campaign operation of a continuous digester.
  • FIG. 4 shows a displacement batch digester comprising a central screen element in accordance with the invention.
  • the central screen element in accordance with the invention can be implemented with many different constructions.
  • the following descriptions of the apparatus are meant to clarify the construction and operation of the screen element. They are in no way limiting the scope of the invention, which only becomes apparent from the enclosed claims.
  • the screen element has a tubular form having the height bigger than the diameter.
  • the tubular form in this connection means “part of a machine” in which there is arranged space for the flows of liquid.
  • the digesters ready in use and to be produced in the future are the so called tailor made digesters, whereby also this central screen element in accordance with the invention must be designed for each digester separately, so that it corresponds optimally to the dimensioning and operating conditions of the apparatus in question.
  • the size of the digester, already existing screen constructions, the capacity of the digester, pipe units and connections as well as other eventual special features of the operation of the digester must be taken into consideration.
  • the central screen element is divided in the longitudinal direction into one or a plurality of screen zones.
  • the number of the screen zones, their size and location with respect to the whole central screen element is dependent on the digester, to which the screen element will be installed, and on the digestion method to be used, as well as on the technical determinations of the process.
  • the screen zones can locate in the central screen element sequentially in the vicinity of each other or at a distance from each other, on the area of digestion zone and/or other area meant to be inside each digestion vessel.
  • the elements of the screen zone, screen segment and at least one outlet can be located in the zone sequentially in the vicinity of each other or at a distance from each other.
  • the outlet can also be located in the area of the screen segment, behind the screen bars in the flow space.
  • the process chemical can be fed to the digester and removed from the digester at optimal points and thus it is possible to support the operation of the existing screens located on the jacket of the digester.
  • the central screen element provides more screen area for the digester, thus increasing the screen capacity of the digester and, additionally, it can be used for controlling the flows inside the digester and thus for improving the quality of the produced pulp.
  • the central screen element is divided into two or more channels.
  • the meaning of the channels is to provide for the flowing of the liquor containing process chemical in and out to/from the central screen element simultaneously without causing disturbances in the operation.
  • the flows in different directions must have their own channels.
  • the same basic idea of the central screen element can be utilized in both digestion methods, in other words as well in the batch digestion as in the continuous digestion.
  • the wood chips to be digested are continuously moving from the upper part of the digester towards the lower part.
  • this is resulted from the settling of the pulp to be processed and the thickening thereof towards the lower part of the digester.
  • the motion is continuous.
  • the motion is significantly quicker, because in this process, new wood chips are fed from the upper part of the digester into the digester, and the produced pulp is removed from the lower part as a continuous operation during the digestion.
  • the pulp to be processed is thickening towards the lower part of the digester, when the wood chips are digested.
  • FIG. 1 shows as an example a vertical cross sectional drawing of one central screen element of the invention and a section X-X in the transverse direction of the same central screen element.
  • the screen element is divided in the longitudinal direction into two screen zones (A) and (B), and in the transverse direction into five channels (flows 1 , 2 , 3 , 4 and 5 ).
  • the flow to the digester can take place in three points, outlets ( 1 . 1 ), ( 2 . 1 ) and ( 3 . 1 ) with flows ( 1 ), ( 2 ) and ( 3 ), respectively.
  • the flow out from the digester can take place in two separate screen zones (A) and (B) in the screen element through the screen segments ( 5 . 1 ) and ( 4 . 1 ), with process chemical flows ( 5 ) and ( 4 ), respectively.
  • the flows in and out can be conducted through different channels independently from each other as required by the changing digestion conditions.
  • the flow can be conducted to one or more zones at a time.
  • the flow to the lower zone or away from it is directed via a channel through the upper zone, in the axial direction of the central screen element.
  • the chemical can be fed to the digester for example simultaneously at all three points, outlets ( 1 . 1 ), ( 2 .
  • the screen element in accordance with the invention can be installed either into a new digester or as a retrofitting into an existing digester.
  • the screen element is installed into the digester substantially parallel with the vertical axis, in the middle.
  • the screen element is installed into the digester substantially parallel with the vertical axis on the central axis of the digester, in order to have symmetrical and equal long paths for the radial flows in the digesting chip column.
  • the central screen element is installed inside the digester arranging the supporting so that the support disturbs the digestion process as less as possible.
  • the supporting can be implemented at any point of the digester, and if necessary, the supporting can also be made to several different points.
  • the central screen element is attached to the digester at the upper part of the digester and if necessary, the lateral supporting is made also to the lower part of the digester. It is especially important to take care of the supporting of the central screen element in a batch digestion vessel, where in connection with the filling of the chips and filling of the impregnation liquor, the chip column can heavily move upwards and. break the constructions.
  • the connection of the central screen element to the liquid flows is preferably made at the upper part of the screen element. For this reason, when installing the screen element into an existing digester, the existing pipe connections must be taken into consideration when designing and sizing the apparatus.
  • the central screen element can be made of any material suitable for that purpose, like austenitic and ferrite-austenitic steels.
  • the construction of the screen segment of the screen zone can be a round bar screen (as described in Patent WO01/31117), which is flow-technically efficient and would significantly increase the hydraulic capacity of the screen.
  • any other possible constructions implementing the characteristic features in accordance with the invention are covered by the scope of the invention.
  • the screen element can be implemented for example with a perforated plate or a slotted plate. The choice is often based on a plurality of criteria, among others on the price.
  • the transversal cross section X-X of FIG. 1 illustrates one embodiment in accordance with the invention, showing the channel construction of the central screen element.
  • other channel constructions for example pipe constructions for conducting the flows to different zones of the central screen element and away from those are possible and are covered by the scope of the invention.
  • the word channel in this connection refers to a construction providing for the flow of the process chemical inside the tubular central screen element.
  • Multiple channels refer to a channel construction providing for flows of the process chemical in different directions also simultaneously.
  • FIG. 2 illustrates one embodiment in accordance with the invention of the central screen element in a continuously operating digester.
  • the central screen element of FIG. 2 comprises four screen zones, the zones A, B, C and D.
  • the screen zones are located as well sequentially close to each other (A and B) as at a distance from each other (C and D).
  • FIG. 3 illustrate the operation of a central screen element in accordance with the invention in a campaign operation of a continuously operating digester, where for example softwood chips and hardwood chips are processed in turn.
  • the washing liquor displaces the contaminated cooking liquor away from the plug flow.
  • a retrofitted central screen and its lower screen zone (A) can be taken into use for increasing the screen capacity.
  • a pressure difference over the screen of the screen zone suitably adjusted, causes that a part of the flow ( 6 ) can be sucked away from the plug flow as a flow ( 5 ) through the screen segment ( 5 . 1 ).
  • the dilute connections ( 8 ) and ( 9 ) of the bottom act in a traditional way for controlling consistency of the blow.
  • the washing liquor displaces the contaminated cooking liquor away from the plug flow.
  • the traditional radial flow ( 6 ) from the outlet ( 6 . 1 ) is supported for example with two radial flows ( 2 ) from the outlet ( 2 . 1 ), and ( 3 ) from the outlet ( 3 . 1 ), and the upper screen zone (B) as well as the lower zone (A) of the central screen element will be taken into use for increasing the screen capacity simultaneously.
  • a maximized screen capacity allows the use of a smaller pressure difference for displacing the same amounts of liquid away from the plug flow.
  • the strongest of the radial flows is the traditional flow ( 6 ) from the outlet ( 6 . 1 ), and supporting that are the smaller radial flows ( 3 ) from the outlet ( 3 . 1 ) and flow ( 2 ) from the outlet ( 2 . 1 ).
  • Mutual differences between the flows and the mutual pressure differences over the screen surface of different screens like the screen of the jacket ( 7 ) and the screen segments ( 4 . 1 ) and ( 5 . 1 ) are adjusted so that the plug flow is purified as efficiently as possible.
  • the radial flow ( 1 ) from the outlet ( 1 . 1 ) is suitable for adjusting the consistency of the bottom of the digester in addition to the traditional dilute connections ( 8 ) and ( 9 ).
  • the radial flow ( 1 ) from the outlet ( 1 . 1 ) can be utilized also in that, without disturbing the other operations. Only the mutual pressure differences of the screens ( 7 ) and the screen segments ( 4 . 1 ) and ( 5 . 1 ) over the screen shall be adjusted according to the conditions. The radial flow ( 1 ) from the outlet ( 1 . 1 ) must be forced to flow upwards towards the lowering plug flow and towards the screen ( 7 ).
  • FIG. 4 illustrates one embodiment of the invention, wherein the central screen element is used in batch digestion.
  • Wood chips are fed to the digester from the upper part of the digester.
  • the process chemical is fed to the digester through the lower part ( 16 ) of the digester.
  • the air brought along with the wood chips is removed from the digester as a result of the process chemical fill through the pipe connections ( 17 ) located in the upper part of the digester and/or the screen segments ( 4 . 1 ) and ( 5 . 1 ) of the central screen element ( 11 ).
  • the returning of the process chemical can be made in the batch digester in accordance with the invention also at several points of the digester, thus providing even distribution of the process chemical to the whole pulp column, and simultaneously the settling of the pulp into the lower part of the digester during the digestion can be decreased.
  • the hot liquor is replaced in the displacement batch digestion by cooler and cleaner liquor.
  • the displacing liquor can be fed also through it from the outlets ( 1 . 1 ), ( 2 . 1 ) and/or ( 3 . 1 ) as flows ( 1 ), ( 2 ) and/or ( 3 ).
  • the displaced liquor is removed through the displacement screens ( 14 ) installed to the upper part of the digester, and/or through the screen segments ( 4 . 1 ) and/or ( 5 . 1 ) as flows ( 4 ) and/or ( 5 ) of the central screen element.
  • the pulp column For discharging the digested pulp settled on the bottom from the digester, the pulp column must be supplied with dilution liquor.
  • liquor can be fed either through the nozzles ( 15 ) located in the lower part of the digester or from the outlet ( 1 . 1 ) of the central screen element, as flow ( 1 ), or through both of them.
  • the upper outlet ( 3 . 1 ) and the upper screen segment ( 4 . 1 ) of the central screen element ( 11 ) in accordance with the invention can be disengaged, because they would operate in a gas space.
  • One embodiment in accordance with the invention is to change a traditional batch digester to a displacement batch digester by installing a central screen element into the digester. In that case, during the change of liquids, air or liquor is discharged through the screen and during the digestion cycle the liquor of the upper cycle returns to the digester through the same, as described above.
  • the multi-function screen element in accordance with the present invention due to its location in the middle of the digester and to its eventual multiple zones, improves the alkali distribution during the whole digestion process as well in a batch digester as in a continuously operating digester.
  • the control of the flows enabled by the multiple channels in the different zones of the central screen element, independently from each other either into the digester or out of the digester improves the optimization of the flows in the digestion conditions changing continuously caused by the digesting wood chips.

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US10/569,093 2004-05-28 2005-05-25 Screen Device For a Digester For Producing Pulp Abandoned US20080169071A1 (en)

Applications Claiming Priority (3)

Application Number Priority Date Filing Date Title
FI20040730A FI117477B (sv) 2004-05-28 2004-05-28 Silanordning
FI20040730 2004-05-28
PCT/FI2005/050174 WO2005116328A1 (en) 2004-05-28 2005-05-25 Screen device for a digester for producing pulp

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US20080169071A1 true US20080169071A1 (en) 2008-07-17

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US10/569,093 Abandoned US20080169071A1 (en) 2004-05-28 2005-05-25 Screen Device For a Digester For Producing Pulp

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US (1) US20080169071A1 (sv)
EP (1) EP1769118B1 (sv)
AT (1) ATE553247T1 (sv)
CA (1) CA2537087C (sv)
FI (1) FI117477B (sv)
WO (1) WO2005116328A1 (sv)

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20160130753A1 (en) * 2014-11-07 2016-05-12 Valmet Ab Method for recovering hydrolysate

Families Citing this family (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CA2638196C (en) * 2007-08-07 2015-12-15 Andritz Inc. Method and system for semi-chemical pulping
WO2012102650A1 (en) * 2011-01-25 2012-08-02 Metso Paper Sweden Ab Continuous digester with a bottom scraper equipped with draining apertures

Citations (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US1231247A (en) * 1913-08-14 1917-06-26 Walter K Freeman Apparatus for harvesting the constituents of wood and vegetable substances, distilling and purifying them.
US1831748A (en) * 1929-05-14 1931-11-10 Morterud Einar Arrangement for the circulation of digesting liquor in wood pulp digesters
US3215591A (en) * 1963-04-22 1965-11-02 Lummus Co Pressure impregnation apparatus for impregnating wood chips and the like
US3348390A (en) * 1965-03-25 1967-10-24 Kamyr Ab Cellulosic material process vessel liquid distribution device
US3475271A (en) * 1966-02-07 1969-10-28 Kamyr Ab Cellulose digester with washing apparatus
US5470437A (en) * 1992-11-18 1995-11-28 Kvaerner Pulping Technologies Ab Digester for cooking pulp having staggered screens
US6120647A (en) * 1998-01-26 2000-09-19 Ahlstrom Machinery Inc. Simplified liquid removal system for a cellulose pulp digester
US6123808A (en) * 1997-12-09 2000-09-26 Ahlstrom Machinery Inc. Distribution of dilution liquor to the discharge of a cellulose pulp digester

Patent Citations (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US1231247A (en) * 1913-08-14 1917-06-26 Walter K Freeman Apparatus for harvesting the constituents of wood and vegetable substances, distilling and purifying them.
US1831748A (en) * 1929-05-14 1931-11-10 Morterud Einar Arrangement for the circulation of digesting liquor in wood pulp digesters
US3215591A (en) * 1963-04-22 1965-11-02 Lummus Co Pressure impregnation apparatus for impregnating wood chips and the like
US3348390A (en) * 1965-03-25 1967-10-24 Kamyr Ab Cellulosic material process vessel liquid distribution device
US3475271A (en) * 1966-02-07 1969-10-28 Kamyr Ab Cellulose digester with washing apparatus
US5470437A (en) * 1992-11-18 1995-11-28 Kvaerner Pulping Technologies Ab Digester for cooking pulp having staggered screens
US6123808A (en) * 1997-12-09 2000-09-26 Ahlstrom Machinery Inc. Distribution of dilution liquor to the discharge of a cellulose pulp digester
US6120647A (en) * 1998-01-26 2000-09-19 Ahlstrom Machinery Inc. Simplified liquid removal system for a cellulose pulp digester

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20160130753A1 (en) * 2014-11-07 2016-05-12 Valmet Ab Method for recovering hydrolysate
US9663896B2 (en) * 2014-11-07 2017-05-30 Valmet Ab Method for recovering hydrolysate

Also Published As

Publication number Publication date
EP1769118A1 (en) 2007-04-04
FI20040730A0 (sv) 2004-05-28
CA2537087A1 (en) 2005-12-08
FI20040730A (sv) 2005-11-29
FI117477B (sv) 2006-10-31
ATE553247T1 (de) 2012-04-15
EP1769118B1 (en) 2012-04-11
CA2537087C (en) 2013-01-08
WO2005116328A1 (en) 2005-12-08

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