|Publication number||USRE39208 E1|
|Application number||US 10/829,477|
|Publication date||Aug 1, 2006|
|Filing date||Apr 22, 2004|
|Priority date||Oct 24, 1997|
|Also published as||CA2246024A1, CA2246024C, US6129816, US6375796|
|Publication number||10829477, 829477, US RE39208 E1, US RE39208E1, US-E1-RE39208, USRE39208 E1, USRE39208E1|
|Inventors||Jay K. Sheerer, Joseph R. Phillips, Jerry R. Johanson, John Pietrangelo|
|Original Assignee||Andritz, Inc.|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (12), Classifications (13), Legal Events (2)|
|External Links: USPTO, USPTO Assignment, Espacenet|
This application is a divisional of application serial No. 08/953,880 filed Oct. 24, 1997, now U.S. Pat. No. 6,129,816.
In the art of chemical pulping of comminuted cellulosic fibrous material, for example wood chips, the cellulose material is typically treated with cooking chemicals under pressure and temperature in one or more cylindrical vessels, known as digesters. This treatment can be performed continuously or in a batch mode. In the connection mode, chips are continuously fed into one end of a continuous digester, treated, and continuously discharged from the other end. In the batch method, one or more batch digesters are filled with chips and cooking chemical, capped and then treatment commences. Once the treatment is finished the contents of the batch digester are discharged. In either batch or continuous digesters, a slurry of comminuted cellulosic fibrous material and cooking chemical is treated in one or more a cylindrical vessels.
In both continuous and batch digesters, in order to uniformly distribute both temperature and cooking chemical, cooking liquor is typically circulated through the slurry of chips and liquor, typically referred to as “the chip column”. This circulation is typically effected by some form of screen, located along the internal surface of the cylindrical vessel, a pump, a heater, and a return conduit. The screen retains the material within the digester as the liquor is removed, augmented with other liquors and/or a portion thereof removed, pressurized, heated, and then returned to the slurry in the vicinity of the screen or elsewhere. The proper operation of the digester and the production of uniform product having the best properties, for example, strength, are highly dependent upon the efficiency of this liquid circulation process.
The radial removal of liquor typically produces radial compression of the chip column in the vicinity of the screen assembly. In addition, the weight of the column of chips above the chips near the screen introduces another source of compression of the chips. Furthermore, the vertical movement of free liquor in the chip column, either upward or downward, can vary the compression load, or compaction, of the chip column. It is known in the art that this radial and vertical compression can interfere with the uniform movement of the chip column, which is so essential for the uniform treatment of the chips. For this reason, conventional digesters and screen assemblies are designed so that the diameter of the flow path increases just below the screen. This increase in diameter or “step-out” relieves the compression in the chip column and permits more uniform movement of the column. This step-out typically consists of a radial increase of about 6 inches to 2 feet. Copending application Ser. No. 08/936,047 filed Sep. 23, 1997 discloses several novel methods of accommodating this “column relief” while maintaining a relatively uniform vessel shell diameter.
The radial compression of the chip column against the surface of the screen, due to the radial flow of liquid, also aids in reducing pluggage of the screen surface. For instance, the normal pressure load on the screen surface in conjunction with the downward movement of the chip column acts to scour or “rub” the surface of the screen. This “rubbing” action helps to keep the apertures of the screen free of obstructions, for example, chips, pulp, or other debris. For instance, for vertically-oriented bar-type screens, the vertical rubbing action helps to dislodge any chips that may accumulate between the screen bars.
However, excessive radial compression can interfere with the uniform removal of liquid through the screen. As the radial flow increases, the compression induced by the flow in the chip column can compact the chip column making it difficult to pass liquid through the column. Therefore, the flow of liquid required to uniformly distribute chemical and temperature is typically limited. Thus, though some radial compression which produces a normal pressure on the surface of the screen is desired, this radial compression cannot exceed the compression that reduces the radial flow of liquid or hinders the axial flow of the chip column.
Typically, prior art screen assemblies comprise right cylindrical screen surfaces of relatively uniform diameter. These screen surfaces may comprise or consist of perforated plate, having slots or holes, or parallel-bar type construction having parallel apertures between the bars. These bars typically have a vertical orientation, but that may have various orientations including horizontal or at some oblique angle, for example, at a 45° angle to the vertical.
By analyzing the distribution of the forces within the chip column that are produced by the flow of liquid within the column it has been found according to the invention that by designing these screen assemblies so that they are not uniformly cylindrical, but slightly divergent, the radial compression loading in the chip column and on the screen surface can be reduced, and the volume and rate of liquid that can flow through the chip column and be removed through the screen can be increased. For example, screen surfaces having a slight increase in diameter in the direction of chip flow can reduce the compression load in the chip column and improve the performance of the screen assembly, and the digester in general.
Conical divergent screening surfaces are not unknown in the art of chemical pulping. For example, continuous hydraulic digesters typically include a conical screen surface at the very top of the digester, in the vicinity of where the slurry of cellulose is introduced, to remove excess liquid from the slurry and return it the digester feed system. In two vessel digester systems those screens, which are typically referred to as “bottom circulation” or “BC” screens, are conical in shape but to do not provide the function of the screens of the present invention. Since BC screens are typically located above the chip pile they do not experience the compressive loading that screens located lower down in the digester do. Also, the BC screens typically do not interfere with the movement of the chip slurry through the digester. Conical BC screens would not be used in the cooking or extraction zones as are the screens of the present invention.
One embodiment of this invention comprises or consists of a cylindrical screen assembly for removing liquid from a slurry of comminuted cellulosic fibrous material in a cylindrical vessel having a diameter that diverges in the direction of the of the movement of the slurry. This screen assembly can have an angle of divergence of between about 0.5 and 45°. However, it is believed that using angles of divergence greater than approximately 15° will diminish the rubbing action of the chip column on the screens that is desirable to prevent screen plugging. Though screens having larger angles of divergence can be used, it is preferred that the angle of divergence of the screen be limited to between about 0.5 and 10°, preferably, about 0.5 to 5°, to ensure that at least some form of normal rubbing force is exerted on the screen surface.
According to another aspect of the invention an assembly, per se, for use in screening liquid is provided. The assembly comprises: An annular screen assembly for separating liquid from solid material, the screen assembly having a screen surface with a top, a bottom, and a first internal diameter and a second internal diameter, and the screen assembly having an external diameter defining an annular volume exteriorly of the screen surface. The screen surface may comprise a substantially continuous cylindrical screen surface, or have a wide variety of other configurations as is conventional for screen surfaces per se, particularly for screens in chemical pulp digesters, and has a substantially constant opening size, and percentage of open area, in the flow direction.
According to another aspect of the present invention a method of treating a liquid slurry of comminuted cellulosic fibrous material under cooking conditions in a substantially vertical continuous digester having at least one substantially annular screen surface, and having a top and a bottom, to produce chemical pulp, is provided. The method comprises the steps of substantially continuously: (a) Introducing the slurry of comminuted cellulosic fibrous material into the digester adjacent the top thereof, to flow downwardly in the digester in a first flow path, having a first diameter. (b) Screening the slurry to remove liquid therefrom using the at least one screen surface, having a substantially constant screen surface opening size, and percentage of open area, in the first flow path. (c) During step (b) causing the slurry of comminuted cellulosic fibrous material to transition from the first flow path to a diverging second flow path [preferably having an initial second diameter, equal to or greater than the first diameter]. And, (d) removing the chemical pulp from adjacent the bottom of the digester.
The method also preferably comprises the further step (e), after (c) and before step (d), of causing the downwardly moving slurry to move in a third flow path having a diameter substantially equal to or larger than the second diameter. There is also preferably the further step of repeating steps (b), (c), and (e), at least once prior to step (d), and there is the further step of heating the liquid removed in the practice of step (c), and reintroducing the heated liquid into the digester adjacent where it was removed. As is conventional, some of the liquid flow may be removed, and/or other liquid added, prior to return to the digester.
The invention also comprises a comminuted cellulosic fibrous material treatment vessel assembly. The vessel includes the following components: A substantially vertical vessel having a top, a bottom, an inlet and an outlet, and through which comminuted cellulosic fibrous material flows in a flow direction, the vessel having a substantially cylindrical wall, and preferably with at least one diameter-changing transition between the inlet and outlet. And, a screen assembly (e.g. provided at or just past the transition), the screen assembly comprising an annular screen surface diverging in the flow direction of the comminuted cellulosic fibrous material, and engaging (contacting) the slurry, so as to reduce the radial compression of material thereon [and also preferably to increase the volume and rate of liquid that can flow through the material and be removed through the screen surface compared to a right cylindrical surface of the same construction], the screen assembly having screen surface openings with a substantially constant screen surface opening size [and preferably percentage of open area] in the flow direction.
Typically the outlet is adjacent the bottom of the vessel and the inlet is adjacent the top so that the screen surface diverges downwardly. The screen surface preferably diverges at a substantially constant angle to the vertical of between about 0.5-10°, most preferably between about 0.5-5°. The screen surface may comprise a first screen surface, and the vessel may further comprise a second annular screen surface substantially immediately downstream of the first screen surface in the direction of flow, the second surface also diverging in the flow direction (at the same angles as indicated above). Preferably the annular screen surfaces are continuous, however they can be “checkerboard” in configuration, or have other known configurations. The screen surfaces may have any conventional construction, such as perforated plate, bar, etc. For example, the screen surface comprises a perforated metal surface with perforations of substantially uniform size and density. As another example the screen surface comprises a plurality of bars spaced from each other in a direction substantially parallel to the flow direction, the spacing between the bars being substantially constant both from bar to bar and along the entire lengths thereof in the flow direction.
The invention also relates to a method of treating a liquid slurry of comminuted cellulosic material in a substantially vertical vessel (and preferably having at least one diameter transition). The method comprises the steps of: (a) Introducing the slurry into the vessel to flow substantially vertically therein in a flow direction. (b) while the slurry is flowing in the flow direction screening (e.g. at or just downstream of the diameter transition) the slurry to remove liquid therefrom while causing the liquid to diverge in the flow direction at an angle of between about 0.5-10° using the at least one screen surface, having a substantially constant screen surface opening size [and preferably percentage of open area] in the first flow path. And, (c) downstream of step (b) in the flow direction, removing the slurry from the vessel. Steps (a) through (c) may be practiced substantially continuously, and so that the flow direction is substantially downward. Also there may be the further step of repeating step (b) at least once prior to the practice of step (c).
It is the primary object of the present invention to provide a simplified screen assembly for a comminuted cellulosic fibrous material treatment vessel which allows for increased liquid removal and improved material movement through the vessel. This and other objects of the invention will become clear from an inspection of the detailed description of the drawings and from the appended claims.
Behind each screen 18, 19 typically is a annular cavity 20, 21, for collecting the liquid withdrawn through each screen 18, 19. Beneath each annular cavity 20, 21, are smaller annular cavities 22, 23, commonly referred to as “internal headers”, for collecting the liquid from cavities 20, 21, and discharging it to liquor removal conduits 24, 25. Though these cavities are shown as being located internal to the shell 13, they may also be located external to the shell, that is, “external headers” may be used. Cavities 20, 22 and cavities 21, 23, typically communicate via apertures having specifically-designed dimensions, that is, orifice holes, in order to promote uniform removal of liquid through each screen, as is conventional. Conduits 24, 25 typically join to form a single conduit 26 which communicates with a re-circulation pump 31. Beneath each screen assembly 16 the diameter of the shell 13 is increased at step out 27. This step-out helps to relieve the compressive forces formed in the chip column due to the vertical compression of the weight of the chips and the radial compression of the liquor removed through the screens. This radial increase may range from about 1 to 36 inches, but is typically between about 6 and 24 inches.
As is conventional,
From the pump 31 the liquid is pumped typically through a heater 34, and the heated liquid is reintroduced into the digester 10 using an internal conduit 35 so that the withdrawn liquor is returned near the area where it was removed (typically just above the screen 18). There are a wide variety of different conventional structures for this purpose.
Shell 113 contains a screen assembly 116. The screen assembly 116 is shown as a double screen 118, 119 but it is to be understood that the screen assembly 116 may comprise or consist of one, two, or more screens. Screens 118, 119 may be of the various types of constructions as described for screens 18, 19 above, but unlike screens 18, 19, screens 118, 119 are tapered such that they diverge in the direction of the slurry flow, shown by the arrows F. This divergence is typically at least by about 0.5 degrees from the vertical and is preferably at most approximately 10 degrees from the vertical, and is preferably substantially uniform and continuous. For example, the upper screen 118 has an upper internal diameter D0 and a lower internal diameter D1 greater than D0, preferably with a substantially constant taper between them. Also, screen 119 has an upper internal diameter essentially equal to D1 and a lower internal diameter D2, greater than D1, preferably with a substantially constant taper between them.
Though the blank plates 40, 41, 42, are shown as right-cylindrical cylinders, these may also be divergent conical sections. As an alternative, only one of the two screens 118, 119 may diverge while the other may be essentially cylindrical. That is, D1 may be essentially equal to D0 and D2 be greater than D1; or D1 may be greater than D0 and D2 be essentially equal to D1.
As described above, the divergent flow path provided by the screens 118, 119 sufficiently reduces the compression in the chip column due to the radial removal of liquid that the potential for hang-up of the chip column is reduced or the volume of liquid that can be removed increased compared to conventional right-cylindrical screens.
Screen assembly 116 typically includes annular cavities 120, 121 and internal headers 122, 123 which discharge to conduits 124, 125 as is conventional. Cavities 120, 121 and cavities 122, 123 typically communicate via multiple orifices (not shown). As in
Though the screens 118, 119 are each shown as having a continuously cylindrical screen surface, 43, it is to be understood that the screen surface 43 may not be continuous or cylindrical. For example, the screen surface 43 may also comprise multiple individual circular screens, or the screen surface 43 may comprise alternating screen surfaces and blank plates, commonly referred to as a “checker board pattern”. More than one such screen assembly 116 can be—and almost always is—used in the same vessel 113. The conduit 126 of the screen assembly 116 can also include a recirculation System like the components 31-35 in FIG. 2.
In a method according to the present invention, utilizing the apparatus of
It is highly desirable that the screen surfaces used in the practice of the invention have substantially constant opening size, and percentage of open area, in the flow direction of the slurry. That is, rather than using bars with an increasing slot spacing in the direction of flow (and therefore necessarily an increase in percentage of open area), such as is provided in the strainer of U.S. Pat. No. 3,385,753, if bars are used according to the invention the slot spacing is kept substantially constant in the flow direction. When perforated screen surfaces are used, the size and spacing of the openings are kept substantially constant, so that, again, the percentage of open area in the screen is kept substantially constant in the flow direction. This feature of the invention can best be seen with respect to the two exemplary embodiments illustrated schematically in
The same results achieved for the embodiments of
The uniform spacings 57 may be provided by using bars having a substantially trapezoidal (rather than substantially rectangular) shape in plan. Again, compaction is relieved solely by the increase in diameter of the screen assembly 243 in the flow direction F. The percentage of screen open area may be maintained the same by providing appropriate perforated screen surface sections between the bar sections illustrated in
In all of the
As another alternative for the screen assembly surface according to the invention, a screen surface with slanted apertures (for example milled slots at a 30-60 degree angle, e.g. about 45 degrees, to the vertical) may be used, such as described in Finnish application 950626 (the disclosure of which is hereby incorporated by reference herein, and a copy of an English language translation of which is provided as an appendix hereto).
It will thus be seen that according to the present invention an advantageous digester, screen assembly, and method of treating a liquid slurry to produce chemical pulp, have been provided. The invention reduces the potential for the liquor removal screens to interfere with the movement of the cellulose material slurry while increasing the volume of liquid can be removed. It is to be understood that though the discussion above generally refers to the vessels in which the present invention can be used as digesters, this invention can be applied to any treatment vessel for treating a slurry of comminuted cellulosic fibrous material that requires that liquid be removed from the slurry. These include impregnation vessels, pretreatment vessels, washing vessels, and bleaching vessels.
While the invention has been herein shown and described in what is presently conceived to be the most practical and preferred embodiment thereof, it will be apparent to those of ordinary skill in the art that many modifications may be made thereof within the scope of the invention, which scope is to be accorded to the broadest interpretation of the appended claims so as to encompass all equivalent structures and methods.
|Cited Patent||Filing date||Publication date||Applicant||Title|
|US2634209 *||May 19, 1948||Apr 7, 1953||Chemipulp Process Inc||Strainer for digesters|
|US2640774 *||Jul 16, 1949||Jun 2, 1953||Pulp And Papen Res Inst Of Can||Production of cellulose pulp|
|US2661095 *||Sep 6, 1950||Dec 1, 1953||Vilter Mfg Co||Fluent material concentrator|
|US2845347 *||May 18, 1953||Jul 29, 1958||Byrne Hugh J||Apparatus and method for digesting|
|US2998064 *||Dec 11, 1957||Aug 29, 1961||Improved Machinery Inc||Strainer construction|
|US3385753 *||May 15, 1967||May 28, 1968||Improved Machinery Inc||Strainer|
|US3397110 *||Mar 8, 1965||Aug 13, 1968||Rosenblad Corp||Cleaning of heat exchangers used with cellulose digesters|
|US3700548 *||Apr 28, 1971||Oct 24, 1972||Improved Machinery Inc||Apparatus and methods of continuous digesting|
|US5234550 *||Jul 10, 1991||Aug 10, 1993||Kamyr Aktiebolag||Wall means having slots between adjacent bars for withdrawing liquid from particle material|
|CA807926A *||Mar 11, 1969||Toivo E Aremaa||Method and a device for precipitation or washing of materials containing cellulose|
|FI950826A *||Title not available|
|SE90530C||Title not available|
|U.S. Classification||162/19, 162/251, 162/248, 162/237, 162/17, 162/243|
|International Classification||D21C7/14, D21C7/00, D21C3/26|
|Cooperative Classification||D21C7/14, D21C7/00|
|European Classification||D21C7/14, D21C7/00|
|Sep 23, 2009||FPAY||Fee payment|
Year of fee payment: 8
|Oct 18, 2013||FPAY||Fee payment|
Year of fee payment: 12