Patent Application
20050274468
The present invention is related to a method of continuous cooking and washing of cellulose-bearing raw material, especially wood chips, by using an improved liquor circulation pattern. More particularly, the present invention relates to a digester provided with the screens and piping necessary for carrying out this method.
A continuous digester for pulp manufacture is generally a vertical, essentially cylindrical vessel, receiving a feed of cellulose-bearing raw material (for example, wood chips) in the top of the digester and discharging cooked chips, i.e. pulp, from the bottom of the digester. The vessel is usually a pressure vessel, although this disclosure is not limited to such vessels. At one or several points along the digester, going downwards from the top, there may be an increase in digester diameter. Adjacent such a diameter increase, and generally below it, there is typically an annular screen structure in the vessel inner wall. Through this screen structure, liquor is removed from the downward moving chip column by means of a pressure difference. This liquor can be returned into the process by pumping it back into the digester in the middle of the descending chip column through a central pipe, either above the screen (whereby co-current circulation is generated) or below the screen (providing counter-current circulation). The circulating liquor is heated, and appropriate chemicals and liquids may be added to it to achieve the desired chemical and thermal conditions for the chip column moving downwards past the screen structure. The liquor flowing out through the screen structure can also be finally withdrawn from the digester to be reprocessed at another location. As such extracted liquor is no longer returned to the digester, it is replaced with e.g. washing filtrate from a downstream washing plant. The filtrate is generally pumped into the bottom of the pressurized digester. This washing liquor is forced to flow counter-currently upwards through the packed column to exit through the above-mentioned extraction screens, and is further conducted to reprocessing.
In a prior art cooking method, part of this washing liquor is added to the circulation flows together with cooking chemicals, and a corresponding amount of liquor is extracted from the circulation screens for reprocessing. The general aim is to replace the liquor affecting the chips passing the circulation screens. In this method, the amount of washing liquor to be added to the digester bottom is correspondingly reduced. This assists the downward movement of the chip column, because the counterforce created by the washing filtrate flowing upward through the chip bed is reduced.
A problem associated with the above-mentioned designs is the radial flow of liquor through the chip bed from the central pipe towards the screen structure at the digester wall. The chip column causes a dynamic pressure loss for radial flow, this loss being dependant on the flow rate. This results in a force vector in the radial direction, which force pushes the chip bed against the screen structure. Because the chip bed is flexibly subject to any force, this dynamic radial force causes thickening, i.e. packing of the chip column against the screen structure at the vessel inner wall, which in turn results in an increased dynamic pressure loss, etc. If the screen structure at the digester wall is overloaded by the liquid flow through the chip column, the screen structure is blocked very quickly, and there will be disturbances in the process; the movement of the chip column may even stop, resulting in production losses.
When a screen as described above is used for extraction only, without a circulation flow to displace the extraction liquor, a zone of zero liquid velocity appears in the middle of the digester. Therefore, in the counter-current washing zone of the digester, the washing efficiency remains lower in the middle of the digester than close to the digester wall.
There have been attempts to solve this problem using a so-called quench circulation. The washing liquor, flowing from the digester bottom upwards to the extraction screens, was removed through dedicated quench circulation screens, situated below the extraction screens. This removed liquid was pumped through the central pipe into the middle of the digester above the extraction screens. From there, the liquor was supposed to flow towards the extraction screens to displace the more concentrated cooking liquor to reprocessing. This quench circulation flow, however, increased the amount of liquor flowing from the center of the digester towards the extraction screens, as well as its radial flow velocity, which increased the non-desired chip column packing against the extraction screens. The additional load due to the quench circulation worsened the operation of the screens in the loaded digesters and the movement of the chip column. Therefore, this design has been abandoned and the vacant quench circulation screens have in general been connected in parallel to the extraction screens to reduce the load of the latter.
Due to the problems described above, circulations or extraction flows in the digester cannot be maintained at sufficiently high rates, which results in non-desired chemical and temperature gradients. This has a negative impact on the quality of the product and the production economy. In the worst case, the operation of the whole digester may stall due to screen blockages.
Solutions for the problem have been presented for example in U.S. Pat. Nos. 2,998,064; 3,385,753; and 6,129,816. The aim of these patents is to reduce the chip column compaction at the screen area by means of the screen construction itself, and simultaneously to prevent the screen from blocking and to thus improve the digester operation. Because the above-mentioned patents cannot repeal the laws of physics, i.e. the increase of chip column compaction against the screen caused by radial flow, the advantage of the methods disclosed in these patents is small and possibly non-existent.
In U.S. Pat. No. 3,475,271, a digester essentially for sawdust pulping is disclosed. In the immediate proximity of its bottom, in the middle of the digester, there is a rotating central screen, which is either cylindrical, or a cone the diameter of which increases in the plug flow direction of the chip column. Washing liquor to be added into the digester is pumped through a screen at the pressure vessel wall and flows horizontally towards the mentioned central screen, through which it is extracted to reprocessing. The aim of this arrangement is to carry out horizontal-displacement washing at the digester bottom. Physics causes problems in this design as well, as does the location of the screen in relation to the supply of wash liquid. The flow from the wall of the vessel towards its center encounters a decreasing flow area, which means increasing flow velocity. Consequently, the radial force vector towards the screen in the middle of the digester increases very quickly, even with small liquor flow rates (compared with flow in the opposite direction, from the middle towards the wall). Because of this, a central screen according to U.S. Pat. No. 3,475,271 blocks very easily. To be able to force the required amount of liquid from the distribution screen at the wall of the digester to the screen in the middle of the digester, both screens have to be very high in proportion to the digester volume, which makes this construction expensive. That is why this design has not been supplied commercially for decades.
In accordance with the present invention, these and other objects have now been accomplished by the development of a method for continuously producing pulp from lignocellulose-containing material in a digester including a top, a bottom, and a central pipe extending from the top to the bottom within the digester, at least one central screen assembly disposed on the central pipe, and a wall screen assembly disposed along the inner wall of the digester, the method comprising feeding a porous column of the lignocellulose-containing material from the top of the digester to the bottom of the digester, pumping liquor through the porous column of the lignocellulose-containing material in a predetermined direction, and simultaneously withdrawing the liquor through the at least one central screen assembly and the wall screen assembly. Preferably, the predetermined direction is concurrent, countercurrent, or a radial direction.
In accordance with one embodiment of the method of the present invention, the wall screen assembly and the central screen assembly are disposed at substantially the same level within the digester.
In accordance with another embodiment of the method of the present invention, the method includes withdrawing a plurality of separate streams of the liquor through the central screen assembly. Preferably, the central screen assembly comprises a plurality of distinct central screens.
In accordance with another embodiment of the method of the present invention, the method includes recirculating at least a portion of the withdrawn liquor to the digester through a circulation loop. Preferably, the method includes heating or supplying chemicals to the recirculating portion of the withdrawn liquor in the circulation loop.
In accordance with another embodiment of the method of the present invention, the digester includes a withdrawal screen disposed along the inner wall of the digester, and the method includes adding washing liquor to the digester adjacent to the withdrawal screen through the central pipe, whereby the washing liquor displaces the liquor through the withdrawal screen, and displacing at least a portion of the washing liquor through the central screen assembly by the washing liquor flowing countercurrent through the lignocellulose-containing material.
In accordance with the present invention, these and other objects have also been accomplished by the development of a digester for continuously producing pulp from lignocellulose-containing material, the digester having a top and a bottom and comprising a top inlet for feeding the lignocellulose-containing material into the top of the digester, a bottom outlet for removing the cooked lignocellulose-containing material from the bottom of the digester, at least one wall screen assembly disposed along the inner wall of the digester for withdrawing liquor from the lignocellulose-containing material, a central pipe extending from the top to the bottom of the digester, and at least one annular central screen disposed on the central pipe for withdrawing liquor therethrough complementary with that of the at least one wall screen assembly. Preferably, the at least one annular central screen comprises a cylindrical central screen.
In another embodiment, the at least one annular central screen comprises a conical central screen.
In accordance with one embodiment of the digester of the present invention, the digester includes back-flushing means associated with the at least one annular central screen.
In accordance with another embodiment of the digester of the present invention, the central pipe includes an inlet for introducing washing liquor into the digester, the inlet being disposed adjacent to the at least one wall screen assembly.
According to the present invention, the excessive packing of chips in a moving column against an individual screen, and the zone of zero velocity in the middle of the digester, are avoided by using a screen assembly located on the central tube or pipe for liquor withdrawal together with a wall screen. The required outflow is thus divided between the wall screen and the central screen. In a preferred embodiment, the central screen is essentially at the same level as the wall screen.
The central withdrawal screen structure in the middle of the pressure vessel may be located at a level different from that of the complementary wall screen assembly. In this case, it is so located that liquid flowing towards this central screen has to travel upwards through the chip column, and against the direction of the plug flow. This counter-flow reduces the chip column compaction against the central screen, especially at the bottom part of the screen, and the screen remains open and operational.
With the screen solutions according to the present invention, the amount of circulation or extraction flows can be increased in proportion to the pulp production rate without increasing the load of an individual screen so much that it would disturb the movement of the chip column and have a negative influence on the quality and quantity of product.
The present invention is applicable for all types of continuous digesters having various different circulation and extraction flow patterns, found on the market under a variety of commercial names.
The present invention can be applied in continuous cooking both for the cooking vessel and the impregnation vessel. These vessels can be pressurized or un-pressurized.
The present invention is described in more detail with reference to the following detailed description, which, in turn, refers to the appended drawings, in which:
Correspondingly, the lower circulation (3) liquor withdrawn from the lower screen (12B) is returned at a position in the screen zone as shown. The circulating liquors in the cooking circulation (2) and (3) are heated to the desired cooking temperature. Equipment required for pumping and heating the circulating liquor and/or addition of different chemicals and/or washing filtrate is not shown in
In the zone between the cooking circulation screens (12) and extraction screens (13) cooking reactions occur, resulting in softening of the chips. The gravitational force caused by the chip column height and the dynamic forces created by liquor flows, together with weakening of the mechanical strength of chips, result in increased compaction of the chip column, i.e. decrease of void space between the chips and of the relative proportion of voids in the digester volume. The maximum compaction of the chip column is typically at the extraction screen (13), where the vertical velocity vectors of the cooking liquor (21) flowing downwards and the washing liquid (22) flowing upwards amount to zero. The maximum region of zero velocity (0) in the middle of the digester is also created at the same level. This region of zero velocity (0) weakens the efficiency of liquor displacements in the digester. Liquid consisting of cooking liquor (21) and displacing washing liquid (22), exits through the extraction screens (13) and is conveyed to reprocessing along line (4).
Counter-current washing of the cooked chips is carried out in the zone between the extraction screens (13) and the washing circulation screens (14). Washing liquid, which has been pumped to the bottom part of the pressure vessel (11) through line (7), is first heated by means of washing circulation (5) to a desired value and re-enters the digester through the lower end of the central pipe. Subsequently, the upwards-flowing washing liquid displaces less pure cooking liquor towards the extraction screens (13). Because the flow occurs by the action of pressure difference, the developing flow direction in the case shown in
The cooking reactions are terminated earlier in the central part of the pressure vessel (11) than at the periphery, as hot cooking liquor (21) is removed from the central part of the digester through the central screen (16), and somewhat cooler cooking liquor (21) closer to the wall zone is withdrawn later through the extraction screen (13). Cooking has then taken place at a slightly higher temperature in the middle of the pressure vessel (11) than at the wall, but during a correspondingly shorter period. This results in a uniform cooking result across the whole cross-section of the vessel (11).
The length of the central screen (16) is determined according to the capacity required. If a washing circulation (5) is used as shown in
It is particularly advantageous to apply the present invention to a digester design having no washing circulation (14) in the bottom part of the pressure vessel (11), because washing liquid (22) in such digester designs has a particular tendency to channel, that is to flow directly from the entry point of the washing liquid (22) towards the extraction screens (13).
Using the disclosed method, pulp can be cooked and washed evenly.
Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.
