TECHNICAL FIELD

The present invention relates generally to a top separator to be used in, for example, a continuous steam-liquid phase digester for the manufacture of pulp from lignocellulosic material, such as wood chips, and more particularly to a top separator comprising a housing, a casing arranged in the housing, a screen basket arranged in the casing, a screw feeder arranged in the screen basket for conveying a chip slurry from a lower end of the screen basket to an upper end thereof, and an inlet arrangement located in a bottom portion of the casing and in close relation to a lower end of the screw feeder and arranged for supplying a chip slurry into the casing for further transport by the screw flight of the screw feeder, said inlet arrangement being directed generally opposite to the rotational direction of the screw feeder and comprising a ramp-like structure, which extends in a helical path with essentially the same pitch as the screw flight of the screw feeder.

BACKGROUND

A top separator is a device which is mounted at the top of a treatment vessel, such as a continuous digester, a prehydrolysis vessel, or a pre-impregnation vessel, for the manufacture of pulp from lignocellulosic material. The main purpose of the top separator is to dewater a chip slurry, which consists of comminuted lignocellulosic material and liquid and which has been fed to the top separator via one or several supply pipes. The top separator comprises a screw feeder, which is vertically arranged in a screen basket, to transport the chip slurry from a lower end of the screen basket to an upper end thereof. During the upward transport of the chip slurry, liquid is withdrawn outwardly through holes, slits or perforations in the wall of the surrounding screen basket, while the chips are transported to the upper end of the screen basket, where the chips fall over the upper edges of the screen basket and down into the treatment vessel for further processing. The suspension of chips and liquid, also referred to as a chip slurry, is via one or several supply pipes pumped to the top of the treatment vessel, where the chip slurry is introduced into the top separator via an inlet arrangement. Typically the inlet arrangement is located in a bottom portion of a top separator housing, but also other positions for an inlet arrangement have been suggested.

The U.S. Patent No. 6,086,717 to Snekkenes et al. discloses a typical top separator arrangement, which comprises a vertical screen basket, in which a rotatable screw feeder is mounted. A pressurized supply line for pumping and supplying impregnated chips into the top separator is arranged at an off-central position in a bottom portion of the screen basket. A problem with this type of vertical inlet arrangement at an off-central position in a bottom portion of a screen basket, is that when the lowest screw flight of the screw feeder passes over the inlet opening of the pressurized supply line, the inlet opening is temporarily blocked, which leads to a sudden pressure build-up, which, in turn, causes a mechanical load in the form of a pulsation in the screw feeder, thereby inducing excessive wear of the screw feeder and a need to make the screw feeder, including bearings and bushings, as well as the entire top separator bigger and stronger and consequently more costly.

Further, in top separators, which include an inlet opening in a bottom portion, it is therefore known to reduce the pulsations induced by the temporary blocking of an inlet opening as a screw flight of the screw feeder passes over the inlet opening by providing a hole in this screw flight, such that the inlet opening is never completely blocked. Such an arrangement will, however, lead to less efficient transport of material and thereby to impaired overall performance of the top separator.

In the U.S. Patent No. 3,902,962 to Reinhall, a liquid separator is disclosed, which comprises a vertical screw conveyor arranged in a perforated wall casing comprising a tube conduit which opens tangentially into the interior of the wall casing at some distance above the bottom of the wall casing. By this location of the tube conduit, the liquid separator does therefore not make efficient use of the whole screening height of the wall casing. Further, an inlet arrangement comprising a tube conduit that extends through a screening casing is a complex construction, not least with modern screen baskets which typically have very permeable and open walls with little support material that can provide sufficient structural integrity for a reliable and durable inlet arrangement. A general object of the present invention is to provide an improved top separator, which reduces the mechanical load on a screw feeder and thereby reduces the wear on the screw feeder and its associated parts and components. Another object of the invention is to provide an improved top separator, which eliminates, or at least reduces, the pulsations created when parts of a screw feeder pass over an inlet opening in the top separator. A still further object is to provide an improved design for a top separator, which design allows the same or higher chip transport capacity without increasing the overall dimensions or rotational speed of a screw feeder arranged in the top separator.

SUMMARY OF THE INVENTION

The above-mentioned objects are achieved with a top separator comprising a housing, a casing arranged in the housing, a screen basket arranged in the casing, a screw feeder vertically arranged in the screen basket, and an inlet arrangement according to the independent claim. Preferred embodiments are set forth in the dependent claims.

The present invention relates to a top separator, which typically is to be used in a continuous steam-liquid digester, although the top separator can be used in other types of pulp-treatment vessels, such as different types of prehydrolysis vessels and pre- impregnation vessels. The top separator is designed to transport a suspension of lignocellulosic material, also referred to as a chip slurry, in a vertical direction and comprises essentially a housing, in which a casing is arranged, a screen basket arranged in the casing, and a vertical screw feeder, which is driven by a motor unit and which at least partly is arranged inside the screen basket, for transporting the material suspension from a bottom or lower portion of the screen basket to an upper portion thereof. During the upward transport, the material suspension is drained as liquid passes through a perforated wall of the screen basket and is collected in a liquid-collecting space provided between the screen basket and the casing, while the now drier lignocellulosic material is fed to the top of the screen basket where it falls over the upper edges of the screen basket and the casing and into the treatment vessel located therebeneath. Since a top separator in general is designed to transport material upwards, the material suspension is typically introduced into the top separator via at least one inlet located in a lower portion of the top separator or at a lower portion of the screen basket, depending on the specific design of the top separator in question. The material suspension is via one or several supply pipes conveyed into the top separator by means of one or several pumps; and a supply pipe connected to the inlet is consequently pressurized, with a pressure higher than the pressure prevailing inside the housing of the top separator. As was discussed above, in top separators where the inlet is located in a bottom portion of a housing, a casing or a screen basket, such that a screw flight (typically the lowest screw flight portion) of a rotating screw feeder temporarily blocks the inlet as the screw flight passes over the inlet opening, a momentary pressure builds up in the supply pipe connected to the inlet, which in turn creates a pulsation that progresses into components and parts of the screw feeder and causes an excessive wear thereof.

A top separator according to the present invention comprises a housing, an interior casing arranged in the housing, a screen basket arranged in the interior casing, and a screw feeder, which is vertically arranged in the screen basket. The top separator comprises further an inlet arrangement, which can be provided in a lower portion of the housing and below the screen basket, or at the level of a lower end of the screen basket. In a preferred embodiment of the invention, the inlet arrangement is provided in a bottom portion of the interior casing that encloses the screen basket. Further, the inlet arrangement, which is the main subject matter of the present invention, comprises an open and ramp-like structure, which extends a distance in a helical path, from a lower level, where the material suspension enters the ramp-like structure via an inlet opening, to a higher level, where the material suspension leaves the ramp-like structure and is introduced into the interior casing of the top separator and encounters a lower flight of the rotating screw feeder. Since the ramp-like, helical structure extends from a first, lower level to a second, higher level, the ramp-like, helical structure can be said to be characterized by a ramp pitch; and preferably this ramp pitch is substantially equal to the pitch of the screw feeder. The helical ramp structure is further characterized by a ramp width, which can be matched to the height of the screw flight of the screw feeder. The helical ramp structure, or rather the upper part of it, is further preferably an open structure, which implies that there is no well-defined and confined opening directed into the interior casing of the top separator, rather the material suspension is introduced into the casing along the entire length of the helical path of the ramp-like structure, or at least along a substantial portion of the helical length of the ramp structure.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be further explained hereinafter by means of a non-limiting example and with reference to the appended drawings, wherein:

Fig. 1 is a schematic illustration of a top separator according to the present invention.

Fig. 2 is a schematic illustration of an inlet arrangement provided in the top separator illustrated in Fig. 1.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Fig. 1 illustrates schematically a top separator 1, which comprises a housing 2, an interior casing 3 vertically arranged in the housing 2, a permeable vertical screen basket 4 arranged in the interior casing 3, and an inlet arrangement 5. The interior casing 3 and the screen basket 4 are dimensioned such that a circumferential liquid-collecting space 6 is provided between the interior casing 3 and the permeable screen basket 4. The top separator 1, including its main parts, extends essentially in a vertical direction and comprises further a rotatable, vertically arranged screw feeder 7, which comprises a central shaft 8 and which is driven by a motor unit 9, which by a gear box 10 is connected to an upper end of the shaft 8. The screw feeder 7 is provided with a screw blade or screw flight 11, which in a conventional manner are characterized by a screw pitch pi and by a flight height hi, as indicated in Fig. 1. Furthermore, for the use of the present invention, a lowest portion of the screw flight 11 is of particular interest, and in Fig. 2 the lowest screw flight portion is indicated with reference numeral 11a. In use, the top separator 1 is mounted on top of a pulp-treatment vessel, e.g. a continuous liquid-steam phase digester, a pre-impregnation vessel or a prehydrolysis vessel. Since such treatment vessels, including the arrangement of a top separator thereupon, are well-known in the art and not essential for the present invention, a treatment vessel is not depicted in the figures. Further, the specific designs, constructions, positions and dimensions of the housing 2 are not essential for practicing the present invention. As will be thoroughly described and demonstrated below, it is the shape, dimensions and position of the inlet arrangement 5 that is the subject matter of the present invention, and in particular the shape, dimensions and position of the inlet arrangement 5 in relation to the screw feeder 7, the interior casing 3 and/or the screen basket 4.

Now returning to Fig. 1, where it can be seen that the inlet arrangement 5 is located in a bottom part of the housing 2, and more specifically slightly below a lower end of the screen basket 4. In the preferred embodiment shown in Fig. 1 and in Fig. 2, the screen basket 4 has an cylindrical shape without a bottom part, and the inlet arrangement 5 is arranged in a base plate or bottom portion 12 of the interior casing 3 and comprises an inlet opening 13, which is connected to a supply pipe 14 for supplying a suspension of lignocellulosic material and liquid into the top separator 1. As is best seen in Fig. 2, the inlet arrangement 5 comprises a ramp structure 15, with a bottom ramp portion 16, an inner ramp wall 17, a rear ramp wall 18, and an outer ramp wall 19 (not seen in Fig. 2), and extends in a helical path, from a first, lower level, where the inlet opening 13 is located and arranged in the rear ramp wall 18, to a second, higher level, where the bottom ramp portion 16 ends and is in level with the bottom portion 12 of the interior casing 3. In the preferred embodiment shown in Fig. 2, the inlet arrangement 5 can therefore be regarded as an integrated part of the bottom portion 12 of the interior casing 3. It should further be noted that the upper area of the ramp structure 15 is open such that a material suspension enters into the interior casing 3 (and thereby into the screening basket 4) along the helical length of the ramp structure 15.

As was described above, the ramp structure 15, and in particular the bottom ramp portion 16, enters the interior casing 3 at a first, lower level and then goes in a helical path to a second, higher level where the bottom ramp portion 16 ends in level with the bottom portion 12 of the interior casing 3. The ramp structure 15 can therefore be characterized by a ramp pitch p ₂ , as is indicated in Fig. 2. Preferably, the ramp pitch p ₂ is equal to, or essentially equal to, the screw pitch pi of the screw flight 11 of the screw feeder 7, and is in particular equal, or substantially equal, to the screw pitch pi of the lowest portion of the screw feeder 7, i.e. the portion of the screw feeder 7 that includes the lowest screw flight portion 11a. The ramp pitch p ₂ can, for example, differ less than 20 % from the screw pitch pi of the lowest screw flight portion 11a of the screw feeder 7, and more preferably less than 10 %, and even more preferably less than 5 %. Furthermore, the supply pipe 14, or at least the last portion thereof, which ends in the inlet opening 13 of the inlet arrangement 5, is preferably directed with the same, or almost the same, pitch p ₂ into the inlet arrangement 5, to thereby ensure that an incoming pulp flow follows the helical path of the ramp structure 15. The supply pipe 14, or at least the last portion thereof, has preferably a circumferential direction which is equal to, or substantially equal to, a circumferential direction of the ramp structure 15. The ramp structure 15 is further characterized by a ramp width W2, which consequently is the width of the bottom ramp structure 16, as is indicated in Fig. 2. The ramp width w ₂ can preferably be made approximately equal to, or somewhat smaller than (as in Fig. 2), the flight height hi of the screw flight 1 1 of the screw feeder 7, and in particular equal to, or approximately equal to, or somewhat smaller than, the lowest screw flight portion 1 la, to effectively utilize the transport capacity of the screw feeder 7.

From the description above and the figures, it can be appreciated that when in use, the inlet arrangement 5 is arranged in such a way that the lowest screw flight portion 11a of the screw feeder 7, whose rotational direction is indicated by the arrow Ri in Fig 2, sweeps over the ramp structure 15 of the inlet arrangement 5, with only a relatively small clearance between a lower edge of the lowest screw flight portion 11a and the surface of the bottom portion 12 of the interior casing 3, to thereby in an effective manner bring the material suspension onto the lowest screw flight portion 11a of the screw feeder 7 for further transport upwards in a vertical direction. It should further be appreciated that the inlet arrangement 5 is arranged and directed such that the ramp structure 15 enters the bottom portion 12 of the interior casing 3 in a tangential or circumferential direction, which further is directed opposite to the rotational direction Ri of the screw feeder 7. As used herein, the direction of the ramp structure 15 is defined as being the same general direction as the direction of the incoming pulp flow. Furthermore, the inlet opening 13 (and preferably at least the last portion of the supply pipe 14) enters the inlet arrangement 5 and ramp structure 15 with essentially the same tangential or circumferential direction as the ramp structure 15 enters the interior casing 3. By the present inlet arrangement 5, with the inlet opening 13 entering the top separator 1 and in particular entering the interior casing 3 in a circumferential direction, or tangential direction, at a radial position slightly inside the interior casing 3 and also slightly radially inside but also below the screen basket 4, which is positioned within the interior casing 3, the incoming pulp flow will encounter and be forced onto the lowest screw flight portion 1 la of the screw feeder 7, which rotates in a direction generally opposite to the direction of the incoming pulp flow. Further, the ramp structure 15 is preferably arranged with a ramp pitch p ₂ , which is approximately equal to the flight pitch pi of the screw feeder 7, such that a minimal amount of impact forces are created between the incoming pulp and the rotating screw feeder 7. Further, to ensure that the incoming pulp flow follows the helical path of the ramp structure 15, the supply pipe 14 and in particular the inlet opening 13 are directed with approximately the same pitch as the ramp pitch p ₂ , to ensure that the incoming pulp flow follows and is guided by the ramp structure 15. By the present invention, with the inlet arrangement 5 comprising the helical ramp structure 15, which ends in the surface of the bottom portion 12 of the interior casing 3 of the top separator 1, the material suspension flow, which is confined between the bottom ramp portion 16, the inner ramp wall 17 and the outer ramp wall 19 will follow and be guided by the helical ramp structure 15 until it encounters the lowest screw flight portion 11a of the screw feeder 7. By the present arrangement the incoming material flow will already when entering the housing 2 and the interior casing 3 have the same general inclination as the lowest screw flight portion 1 la of the screw feeder 7 in relation to the bottom part 12 of the internal casing 3. This feature, in particular in combination with a large open area provided by an open upper area of the ramp structure 15, which ensures that the incoming material flow is never completely blocked by the lowest screw flight portion 11a as the screw feeder 7 rotates, provides for a material transport and dewatering that is essentially free from pulsations, which, in turn, reduces the wear induced into parts and components of the screw feeder 7 and ensures an effective material transport. Although the present invention has been described with reference to specific embodiments, also shown in the appended drawings, it will be apparent to those skilled in the art that many variations and modifications can be done within the scope of the invention as described in the specification and defined with reference to the claims below.