Patent Application
20130284390
1. Technical Field
The invention relates to liquid separating devices for removing a stream of liquid from a black liquor slurry stream of liquid and solids (black liquor). The invention particularly relates to screen slots in an in-line drainer for separating a stream of excess liquor from the stream of black liquor, which comprises liquid typically containing at least some wood chips or fine wood particles in a pulping system.
2. Related Art
In-line drainers for pulping systems, such as disclosed in U.S. Pat. No. 6,451,172, are typically used in the chip feed of a pulping system. A common use for in-line drainers is to return strained liquid from a black liquor slurry stream to a slurry of wood chips and liquor flowing through a high pressure transfer device to a pulping vessel, such as a continuous digester vessel. The in-line drainer removes some of the liquid in the black liquor slurry stream from the low pressure outlet of a high pressure transfer device. The strained liquid removed from the black liquor slurry stream by the in-line drainer may be reused in the pulping system. The recirculated liquid, which is a mixture of wood chips and remaining liquor flowing from the in-line drainer, flows back into the chip flow from a chip bin and moves to the high pressure feeder.
A difficulty with in-line drainers occurs in the removal of the liquid from the black liquor slurry stream by screening. The screening process associated with separating some of the strained liquid from the black liquor slurry stream retains wood chips, wood fines, and wood pins within the liquor flowing through the in-line drainer (which liquid will become recirculated liquid). Screening is typically performed with a cylindrical basket in the in-line drainer. The walls of the basket include slots or apertures so narrow that wood fines and pins cannot pass through them. Conventional screen baskets are fashioned from steel bars oriented in a parallel, horizontal, or inclined fashion at an angle relative to the direction of flow of the black liquor slurry stream so that the liquor passes through the slots while retaining wood particles within the in-line drainer so the wood particles can be removed from the in-line drainer in the recirculated liquid stream. Conventional cylindrical screen baskets are formed of a solid sheet of material with slots through the sheet, as shown in U.S. Pat. No. 6,451,172.
The black liquor slurry stream passing through an in-line drainer typically has a liquid to wood ratio (on a volume basis) of greater than 5 to 1 (meaning at least 5 parts liquid to 1 part solids) resulting in a low concentration of solids in the stream to the screen basket within the in-line drainer. The black liquor slurry stream flowing through the screen basket moves at a high velocity, such as 10 feet per second. As a result of the low solids concentration and high velocity through the screen basket, the solid material, e.g., wood chips, pins, and fines, easily aligns parallel to the slots and passes through the slots or becomes lodged in the slots of the screen basket.
To prevent solids from passing through or becoming lodged in the slots, conventional in-line drainers have helical baffles or helical flights that impart a helical movement to the black liquor slurry stream flowing through the screen basket. The helical movement of the black liquor slurry stream causes the solids to move in a helical path through the screen basket and not to become lodged parallel to the slots.
Another approach to preventing clogging and to encourage the passage of solids through the slots in the screen basket is to align the slots of the screen basket obliquely to the axial direction of elongation of the in-line drainer, and thus obliquely to the direction of flow through the screen basket. This other approach does not suffer a pressure loss in the black liquor slurry stream that is commonly found when using helical baffles and flights. The angle of the screen basket slots relative to the direction of black liquor slurry stream flow through the screen basket ranges from about 0 degrees (perpendicular) to 90 degrees.
Helical baffles and flights, and slots oblique to the flow of the black liquor slurry stream have not completely eliminated the problem of slots becoming clogged with solids, especially with wood fines and pins. Such clogging is a particular problem occurring when the slots are created by cutting into screen baskets formed from metal plates. There remains a long felt need for screen baskets having slots that are less prone to becoming clogged with solids, such as wood chips, fines, and pins. It is to this need and others that the present disclosure is directed.
The terms chips, fines, and pins herein generally refer to comminuted cellulosic fibrous material may such as wood chips, sawdust, grasses such as straw or kenaf, and agricultural waste such as bagasse and recycled paper. The in-line drainers disclosed herein are applicable to liquid separators for feed systems of both continuous and batch digesters, and also applicable to feeding several continuous digesters or one or more discontinuous or batch digesters.
An embodiment of a screen basket has been conceived having a novel screen basket slot design comprising slanted slots with curved inlet edges. The screen basket can be created from a metal plate joined at opposite side edges to form the cylinder. The curved inlet edges reduce the tendency of solid material to be caught by the slot as the material flows through the screen basket. The sharp edges of conventional sharp-edged slots, such as slots having a right-angled corner, have a tendency to catch solid material that ideally should flow past the slots and through the screen basket.
The slots having curved inlet slot edges are adjacent an inside surface of the screen plate. The curved inlet slot edges may be rounded, sloped, chamfered, or inclined. For example, inlets can have a generous radius of curvature equal to one third to two thirds the thickness of the plate. The curved inlets can be only on the lower side surface of a slot or on the upper and lower side surfaces of the slot.
Another embodiment of a screen basket has been conceived for a liquid separating device for use in a feed system of a process to produce pulp out of comminuted cellulosic material (such as wood chips). The screen basket comprises a cylindrical housing including an inlet for a black liquor slurry stream of solids and liquid at or adjacent to a first end of the cylindrical housing. The screen basket also has an outlet for the recirculated liquid at or adjacent to an opposite end, an outlet for strained liquid at or adjacent to the first end for the recirculated liquid, and an inside surface. A cylindrical screen basket assembly is centrally mounted in the cylindrical housing, and includes an outside cylindrical surface. An annular cavity is between the outside surface of the screen basket and the inside surface of the cylindrical housing, and an outlet for separated or strained liquid is in fluid communication with the annular cavity. The cylindrical screen basket is made from a plate formed into a cylinder with a straight joint connecting opposite side edges of the plate, and rows or columns of slots extending through the plate. Each slot has a curved inlet corner edge adjacent to an inside surface of the plate and is oblique to a longitudinal axis of the basket. The curved corner inlet slot edge can be rounded, chamfered, sloped, and/or inclined. The curved inlet corner edges of the slots can have a radius of curvature in a range of one third to two thirds of the thickness of the plate. The curved corner edges of the slots can be at only one of a lower edge or upper edge of each slot, or at both edges of the slots.
The axis of each slot extending through the cylindrical plate of the screen basket can be oblique to a respective radial line from an axis of the screen basket such that the inlet of each slot at the inside surface of the plate of the screen basket is downstream in the direction of black liquor slurry stream flow to the outlet of the slot on the outside surface of the plate. In particular, the axis of each slot may be at an oblique angle between 5 degrees to 45 degrees, or 5 degrees to 30 degrees, or 5 degrees to 15 degrees. The orientation of the length of each slot may be oblique to the axis of the screen basket such as at an angle of 1 degree to 75 degrees, or 30 degrees to 60 degrees, 40 degrees to 50 degrees or 45 degrees. In each row of slots in the screen basket, the slots can be uniform in shape, dimensions, height, and orientation. The shape, dimensions, height, and orientation can vary from row to row.
An embodiment of a method has been conceived for draining liquid from a black liquor slurry stream with a liquid separator, known as an in-line drainer, having a cylindrical screen basket formed from a metallic plate with slots cut into the plate. An embodiment of the method comprises the following steps: feeding a black liquor slurry stream of liquid and solid material, such as wood chips, pins, and fines, into an annular region of a liquid separator between a cylindrical housing of the separator and a cylindrical screen basket; separating a portion of the liquid from the black liquor slurry stream of cellulosic material as the black liquor slurry stream passes through the slots in the screen basket, where substantially all solids in the black liquor slurry stream cannot pass through the slots; the solids flowing over curved inlet corner edges of the slots, where the edges are adjacent to a surface of the screen basket plate adjacent to the annular region and facing a flow of black liquor; the portion of the liquid being discharged as relatively clean liquid, and the black liquor slurry stream without the separated portion of the liquid is discharged from the liquid separator.
An embodiment of a liquid separator has been conceived for draining liquid from the liquid separator of a feed system to produce pulp comminuted cellulosic material. The liquid separator comprises a rolled plate formed into a cylindrical screen basket and columns or rows of slots formed in the plate and oriented horizontally. Each slot has a curved inlet corner edge adjacent an inside surface of the plate, and each slot is oblique to a vertical axis of the basket. A single welded joint extends vertically between abutting edges of the plate, and a conveyor screw located within the interior of the cylindrical screen basket. The curved corner inlet slot edge may be rounded, chamfered, sloped and/or inclined. The curved inlet corner edge has a radius of curvature in a range of one third to two thirds of the thickness of the plate. The curved corner edge may be only at one of a lower edge or upper edge of each slot, or on both edges.
Another embodiment of a liquid separator has been conceived for draining liquid from the liquid separator of a feed system to produce pulp comminuted cellulosic material, which comprises a rolled plate formed into a cylindrical screen basket, and columns or rows of slots formed in the plate and oriented vertically, where each slot has a curved inlet corner edge adjacent to an inside surface of the plate, and each slot is oblique to a vertical axis of the basket. A single welded joint extends vertically between abutting edges of the plate, and a conveyor screw is interior to the cylindrical screen basket. The curved corner inlet slot edge may be rounded, chamfered, sloped and/or inclined. The curved inlet corner edge may have a radius of curvature in a range of one third to two thirds a thickness of the plate. The curved corner edge may be only at one of a lower edge or upper edge of each slot, or on both edges of the slots.
Adding recirculated liquid from the in-line drainer 12 to the chips flowing from the chip bin 26 increases the ratio of liquor to chips in the slurry in the chip tube 28 and entering the high pressure transfer device 16. The in-line drainer 12 is in a circulation loop which continually reuses liquor in the transport of chips from the chip bin 26 to the high pressure transfer device 16. The in-line drainer 12 also moves wood solids that passed through the screens at the low pressure liquor outlet 14 of the high pressure transfer device 16 back into the slurry of wood chips being transferred through the high pressure transfer device 16 to the digester vessel.
The cylindrical screen basket 48 may also include a basket lifting eye 54 for removing the cylindrical screen basket 48 for replacement or servicing. The cylindrical housing 36 typically includes a gusseted mounting flange 56 for installing the in-line drainer 12 in the chip feed system 10. A steam purge inlet 58 allows steam to be injected into the cylindrical housing 36 for periodic steam cleaning of the in-line drainer 12 and, particularly, the cylindrical screen basket 48.
The cylindrical screen basket 48 is positioned in the cylindrical housing 36 so that an annular cavity 60 is created between the outside surface of the cylindrical screen basket 48 and the inside surface of the cylindrical housing 36. The annular cavity 60 receives the black liquor slurry stream passing through slots in the cylindrical screen basket 48 and flowing to the strained liquid outlet 35. A black liquor slurry stream flow path through the in-line drainer 12 extends from the inlet opening 40 where the black liquor slurry stream 32 enters, through the hollow center of the cylindrical screen basket 48 and to an upper chamber 62 in the cylindrical housing 36. The upper chamber 62 is separated from the annular cavity 60 such that the strained liquid 35a in the annular cavity 60 does not mix with the recirculated liquid 34a in the upper chamber 62. From the upper chamber 62, the recirculated liquid 34a flows through the recirculated liquid outlet 34. A helical baffle 65 imparts a helical flow to the black liquor slurry stream 32 moving up into the cylindrical screen basket 48.
Though the centerline of the recirculated liquid outlet 34 is positioned at a right angle to the centerline of the cylindrical housing 36, the recirculated liquid outlet 34 may also positioned in the upper cover plate 42 so that its centerline is essentially collinear with the centerline of the cylindrical housing 36. The recirculated liquid outlet 34 collinear with the centerline of the cylindrical housing 36 and the black liquor slurry stream flow may be used for black liquor slurry streams having relatively low liquor to solids ratios. In such black liquor slurry streams, abrupt changes in flow direction, such as turning ninety degrees to the recirculated liquid outlet 34, may result in undesirable flow restrictions and stagnation of solids within the in-line drainer 12. For black liquor slurry streams having relatively high liquor to solids ratios, such as in a black liquor slurry stream from a low pressure outlet of a high pressure transfer device 16 or high pressure feeder, an abrupt change in flow direction is often acceptable and does not result in stagnations of solids.
Conventional cylindrical screen baskets 48 may be fabricated from a series of evenly-spaced vertical bars 64 so that a straining surface is provided having a series of vertical slots 66 between the bars 64. The cylindrical screen basket 48 also typically includes lower unperforated cylindrical section 68 and upper unperforated cylindrical section 70 at each end of the cylindrical screen basket 48.
A pressurized black liquor slurry stream 32, such as a stream of liquor and wood chips, pins, or fines, enters the in-line drainer 12 through the inlet opening 40 of the in-line drainer 12. The black liquor slurry stream 32 may have a pressure ranging from about 0 to about 5 bar gage, or about 0 to about 30 bar gage. The design of the cylindrical housing 36 and cylindrical screen basket 48 will vary depending, among other things, upon this pressure. The helical baffle 65 imparts a tangential velocity component to the black liquor slurry stream 32 so that the flow through the cylindrical screen basket 48 is somewhat helical and oblique to the orientation of the vertical slots 66 between the vertical bars 64. As the black liquor slurry stream 32 passes through the cylindrical screen basket 48, some liquid from the black liquor slurry stream 32 passes through the vertical slots 66, collects in annular cavity 60, and is discharged from of the strained liquid outlet 35. The chips, fines, pins, and other substantial solids material are too large to pass through the slots 66 and remain within the cylindrical screen basket 48. The black liquor slurry stream 32, without the removed liquor, flows to the upper chamber 62 and is discharged from the recirculated liquid outlet 34.
In addition to being formed from parallel bars or wires, such as shown in
The screen basket 80 may be a metal plate formed into a cylindrical shell with a welded vertical joint between abutting side edges of the plate. The screen basket 80 may be formed of another material suitable for use in a treatment vessel which typically houses an environment having acidic and alkaline chemicals.
The screen basket 80 may have a diameter of 10 to 36 inches (0.25 meter to 1.0 meter) and a vertical length of 48 to 120 inches (1.2 meter to 3 meter). These dimensions are exemplary. The screen basket 80 may have a vertical orientation and be coaxial to a cylindrical housing of the in-line drainer. The screen basket 80 may be included in an otherwise conventional in-line drainer 12 such as shown in
Shaped slots 94 in the screen basket 80 may be uniformly shaped in length and width. The shaped slots 94 may be arranged in rows 96 on the screen basket 80. Each shaped slot 94 may have, for example, a length of 2 to 14 inches (50 mm to 360 mm), such as 110 mm. The vertical height of each row 96 may be 2 to 10 inches (50 mm to 250 mm). The number of shaped slots 94 in each row 96 is dependent on the circumference of the screen basket 80. The number of shaped slots 94 in each row 96 and the dimensions, e.g., vertical height, of each row 96 may be uniform in the screen basket 80 or vary from row to row. Within any row 96, the slot sizes (slot width, relief angle, and diagonal angle relative to the horizontal) may remain constant from slot to slot. The slot size may also vary from row to row. The number of rows 96 may depend on the desired open area in the screen basket 80 to allow for the desired flow of liquor extracted from the black liquor slurry stream (not shown in
The shaped slots 94 extend through the plate of the screen basket 80 and are sized to allow liquid to pass and block solid material such as wood chips, fines, and pins. The shaped slots 94 may be slanted with respect to vertical and horizontal orientations. The shaped slots 94 are arranged in rows 96 that may be horizontal rows extending completely around the cylinder of the screen basket 80.
The inside surface 102 of the screen basket 80 (as shown in
The orientation of the shaped slots 94 with respect to the axis of the screen basket may be parallel, perpendicular, or oblique. The orientation of the exemplary shaped slots 94 shown in
As best shown in
The axis 111 of each shaped slot 94 may be offset from horizontal at an angle, e.g., 45 degrees, such that the opening of each shaped slot 94 on the inside surface 102 of the screen basket is axially offset from the outlet of the shaped slot 94 on the outer surface 100. This axial offset is such that the outlet of the shaped slot 94 is below the inlet to the shaped slot 94 in the direction of the black liquor slurry stream flow 112 through the screen basket. In view of the axial offset, the direction of liquor flow 114 through the shaped slot 94 has a component opposite to the black liquor slurry stream flow 112 direction. The axial offset of the shaped slots 94 is selected to enhance the effect of movement of the liquor through the shaped slots 94 and to block solids from entering or clogging the shaped slots 94. The axial offset may be at an angle with respect to the axis of the screen basket of 45 degrees or in a range of 40 degrees to 50 degrees or zero degrees (vertical) to 75 degrees. The upper sidewall 120 of each shaped slot 94 may be offset from perpendicular to the plate 116 of the screen basket by an angle (ω) of between 5 degrees to 45 degrees, or 5 degrees to 30 degrees, or 5 degrees to 15 degrees.
Avoiding sharp angles on the edges of the slots reduces the tendency of solid particulate (wood chips, fines, other cellulosic material, etc.) caught at the edges of the shaped slots 94. For an in-line drainer having an upper inlet, the shaped slots 94 may have a curved edge 118 at the lower edge of the opening of the shaped slot 94. For an in-line drainer having a lower inlet, the curved edge 118 may be at the upper edge of the openings of the shaped slots 94. The curved edge 118 reduces the tendency of the edges of the shaped slots 94 to catch solid particulates (cellulosic material) in the black liquor slurry stream flowing through the in-line drainer. The curved edge 118 on the shaped slot 94 tends to deflect solid particulates into the flow and away from the shaped slot 94.
While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but to the contrary, is intended to cover various modifications and equivalent arrangements, structures, systems, and methods included within the spirit and scope of the appended claims.
This application claims priority to U.S. Provisional Patent Application Ser. No. 61/638,246 filed Apr. 25, 2012 which is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61638246 | Apr 2012 | US |
