Abstract:
Apparatus for washing and/or dewatering cellulose pulp is disclosed including a movable permeable surface in a pulp transportation chamber having a chamber gap above the movable permeable surface, a pulp distributor for distributing pulp onto the movable permeable surface, a throttle having a throttle gap width and an adjustable throttle adjuster to remotely adjust the throttle gap width so that a volume of pulp flow into the pulp distributor is equal to or greater than the volume of pulp flow out of the distributor during operation.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is a national phase entry under 35 U.S.C. §371 of International Application No. PCT/SE2013/051570 filed Dec. 19, 2013, published in English, which claims priority from Swedish Application No. 1350030-1 filed Jan. 11, 2013, all of which are hereby incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to apparatus for washing and/or dewatering cellulose pulp. 
       BACKGROUND OF THE INVENTION 
       [0003]    Pulp washing is a key operation in a pulping line. There are many different types of washing apparatuses available, some of which are based on press washing and comprise means for pressing the pulp to remove liquid. After pressing, the pulp can, if suitable, be diluted to a desired consistency. 
         [0004]    A well-known washing apparatus is a twin-roil press of the general type disclosed in U.S. Pat. No. 3,980,518, for example. It has two counter-rotating rolls with perforated outer surfaces. A web of pulp is formed on the respective rolls and is transported in the direction of rotation in a vat partially surrounding the rolls, to the so-called press nip between the rolls. The liquid removed from the pulp, i.e. the filtrate, passes through the perforated roil surface in a radial inwards direction and is led to the ends of the press roll, where it is output. Washing liquid or other treatment liquid may be supplied to the pulp web through inlets in the vat. The twin-roll press uses the washing principles of displacement, where dirty liquid (liquor) in the pulp is replaced by cleaner wash liquid added to the vat, and pressing, where dirty liquid is pressed (squeezed) out from the pulp, in particular at the press nip. 
         [0005]    The incoming pulp can be distributed lengthwise onto the respective press rolls by means of a distribution device, for example by using a rotating screw, such as the device shown in European Patent No. 1,229,164 B1, or the device shown in Swedish Patent No. 532,366 C2. There is a problem with the distribution of pulp along the total length of the press roll, with a danger that the end parts of the press rolls operate without pulp. 
         [0006]    In Swedish Patent No. 516,335 a device is described for feeding cellulose pulp, in the form of a pulp web. In this device the outlet includes restrictions in the form of holes, which are arranged along the generator of the envelope surface of the inlet box. The holes are preferably arranged so that their diameter is smaller than the distance between them. In that way, the pressure is maintained in the inlet box such that the pulp is forced out of the outlet and is uniformly distributed along the width of the pulp web. The holes have, however, a tendency to plug. 
       SUMMARY OF THE INVENTION 
       [0007]    An object of the present invention is to provide an improved pulp distribution device for an apparatus for washing and/or dewatering of cellulose pulp that solves the problem in the prior art. 
         [0008]    Some of the advantages of this invention are that the pulp distribution device will operate more filled than in earlier solutions. This leads to a better forming of the pulp web along the press roll are also in the ends of the press roll, independent of the rotation of speed of the press roll. This gives a higher capacity of the washing apparatus and a better washing. The formation of the pulp web will also be smoother, since the pulp is not torn from the pulp distribution device. 
         [0009]    These and other objects and advantages of the present invention may now be realized by the discovery of apparatus for washing and/or dewatering cellulose pulp comprising a first movable permeable surface traveling in a first direction, a pulp transportation chamber wall adjacent to the first movable permeable surface, sized to provide a pulp transportation chamber having a chamber gap between the pulp transportation chamber wall and the first movable permeable surface, the pulp distribution member comprising an inlet and an outlet arranged for distributing the pulp through the outlet onto the movable permeable surface, a throttle adjacent to the outlet of the pulp distribution member in the first direction, the throttle having a throttle gap width, a portion of the first movable permeable surface adjacent to the outlet of the pulp distribution member comprising a second permeable surface for initially dewatering the pulp moving in the first direction before the throttle, the second permeable surface being sufficiently large so as to begin forming and dewatering the pulp before the throttle, and an adjustable throttle member for remotely adjusting the throttle gap width, the throttle gap width being adapted whereby the pulp volume flow into the pulp distribution member is equal to or greater than the pulp volume flow out of the pulp distribution member during operation. Preferably, the pulp distribution member include web means for forming the pulp into a pulp web on the first movable permeable surface. 
         [0010]    In accordance with one embodiment of the apparatus of the present invention, the throttle gap width is adapted so that the pump distribution member remains substantially full during operation. 
         [0011]    In accordance with another embodiment of the apparatus of the present invention, the throttle gap width is adapted so that an internal pressure is created inside the pulp distribution member during operation. 
         [0012]    In accordance with another embodiment of the apparatus of the present invention, the pulp transportation chamber wall includes the movable throttle device for adjusting the throttle gap width. Preferably, the movable throttle device comprises at least one longitudinal segment. In another embodiment, the movable throttle device comprises a flexible or pivotable plate. In another embodiment, the movable throttle device includes a clearance after the throttle in the first direction. Preferably, the apparatus includes moving means for moving the movable throttle device. In a preferred embodiment, the moving means is at least one rod, at least one plate, and/or at least one eccentric puck disposed on a shaft. 
         [0013]    In accordance with another embodiment of the apparatus of the present invention, the throttle gap width may be fixed during operation, and may be set by selecting an exchangeable member. Preferably, the exchangeable member comprises a predetermined number of shims. 
         [0014]    In accordance with another embodiment of the apparatus of the present invention, the apparatus includes automatic control means for controlling the throttle gap width based upon at least one parameter. Preferably, the at least one parameter is one of the following: the filling level of the pulp distribution member, the pressure in the pulp distribution member, the ratio between the pulp volume flow into the pulp distribution chamber, and the pulp volume flow out of the pulp distribution chamber, the temperature in the pulp distribution member, the pressure in the pulp distribution chamber, and the pulp level in the ends of the first permeable surface. 
         [0015]    In accordance with another embodiment of the apparatus of the present invention, the first movable permeable surface comprises a first rotatable permeable surface of a press roll. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The invention, together with further objects and advantages thereof, may best be understood by reference to the following description and appended drawings, in which: 
           [0017]      FIG. 1  is a schematic cross-sectional view illustrating a twin-roll press with pulp distribution devices according to an exemplifying embodiment of the invention. 
           [0018]      FIG. 2  is a schematic cross-sectional view illustrating a pulp distribution device in a washing apparatus (partially shown) according to a first embodiment of the invention; 
           [0019]      FIG. 3  is a schematic cross-sectional view illustrating a pulp distribution device in a washing apparatus (partially shown) according to a second embodiment of the invention; 
           [0020]      FIG. 4  is a schematic cross-sectional view illustrating a pulp distribution device in a washing apparatus (partially shown) according to a third embodiment of the invention; 
           [0021]      FIG. 5  is a schematic cross-sectional view illustrating a pulp distribution device in a washing apparatus (partially shown) according to a fourth embodiment of the invention; 
           [0022]      FIG. 6  is a schematic cross-sectional view illustrating a pulp distribution device in a washing apparatus (partially shown) according to a fifth embodiment of the invention; 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    In the drawings, similar or corresponding elements are denoted by the same reference numbers. 
         [0024]      FIG. 1  illustrates an apparatus  100  for the washing and/or dewatering of cellulose pulp. The apparatus  100  is in this example in the form of a twin-roll press  100 . The apparatus  100  comprises two co-operating cylindrical press rolls  20 . The two press rolls  20  are arranged to rotate in opposite directions during operation (as indicated by the arrows) and each has a first rotatable permeable surface  21 , more specifically a perforated metal sheet or the like. The press rolls  20  are partially enclosed by a vat  22  (also known as a trough) in the circumferential direction. The vat  22  comprises a vat wall  22  formed by guide surfaces and a vat chamber  25 . 
         [0025]    A pulp distribution device  10  is associated with each press roll  20 . The pulp distribution device  10  is arranged at the upper portion of the press roll  20  for distribution of pulp onto the perforated roll surface  21 . The pulp distribution device  10  comprises an elongated housing  18 , extending lengthwise along the press roll  20 . The pulp distribution device  10  is attached to the vat  22 . During operation, pulp enters the pulp distribution device  10  via its inlet  11 , which for example can be arranged at the middle of the twin-roll press  100  as seen in the longitudinal direction. The input consistency of the pulp is preferably in the range of 2-13%. 
         [0026]    In the pulp distribution device  10 , the pulp is distributed in the longitudinal direction and output through the outlet  12  during formation and dewatering of a pulp web on the first rotatable permeable surface  21  of the press roll  20 . 
         [0027]    A pulp transportation chamber  25  is defined as the chamber in which the pulp is transported in a pulp transportation direction, guided by a pulp transportation chamber wall  22  e.g. in the direction of rotation D 1  to be pressed in a nip (also known as pinch)  23 , where the distance between the press rolls  20  is smallest. In the example in  FIG. 1 , the pulp transportation chamber  25  is the same as the vat chamber  25 , but the pulp transportation chamber may also include e.g. a pre-forming zone between the pulp distribution device  10  and the vat  22 . Also the last part of the pulp distribution device  10  may in certain circumstances be considered to be included in the pulp transportation chamber. The pulp transportation chamber  25  has a chamber gap width Wv between the pulp transportation wall  22  and the first permeable surface  21  of the respective press roll  20 . 
         [0028]    Washing liquid may e.g. be supplied to the pulp web in the vat  22 . The pulp is output by means of a discharge screw  24 . 
         [0029]    In  FIG. 1 , two press rolls  20 , each provided with a pulp distribution device  10 , are arranged next to each other, with the rotation centers in the same horizontal plane. The invention is also suitable for washing and/or dewatering apparatuses where, for example, the rolls are differently arranged, or only one perforated roil is used, as well as for another apparatus where pulp is dewatered on a first movable permeable surface, which is movable in other ways than by being rotatable. 
         [0030]    The vat  22  may be formed by one continuous vat structure as in  FIG. 1  or, alternatively, may comprise a number of vat segments linked together (not shown, a number of variants are known, compare e.g. International Application No. WO 2009/075641). In the latter case, one or more vat segments may be movable to and from the press roll  20 , for example so as to facilitate cleaning of the press roll. There could, for example, be one movable vat segment extending into each pulp distribution device  10 , e.g. pivotally attached at one of its ends. 
         [0031]      FIG. 2  is a schematic cross-sectional view illustrating in more detail a pulp distribution device  10  according to an exemplifying embodiment of the invention. The pulp distribution device  10  is arranged in the apparatus  100 , the upper left side of which is shown, so as to distribute pulp onto the first movable permeable surface  21  of the press roll  20 , via the outlet  12 . The pulp distribution device  10  may be of a type similar to e.g. the one described in Swedish Patent No. 532,366 C2 and preferably comprises an elongated housing  18  with a rotatable screw  13  or other rotatable distribution means  13  inside, rotating in a direction of rotation D 2 . Especially when low concentrations are used, other stirring means are also possible to use instead of a rotatable distribution means. The pulp distribution device  10  extends along the entire length of the press roll  20  and the rotation axis of the rotatable distribution means  13  is parallel to the rotation axis of the press roll  20 . 
         [0032]    According to the present invention it has been realised that the pulp volume flow into the pulp distribution device  10  should be at least equal to, but preferably higher than the pulp volume flow out from the pulp distribution device  10 . Since there has to be an equal mass flow of dry pulp into and out from the pulp distribution device  10 , an initial dewatering should be made through a second permeable surface  14  before the main dewatering is made through the first permeable surface  21 . This means that it is preferred that the pulp distribution device  10  operates completely filled and has an internal pressure in order to better distribute the pulp and form a better pulp web along the total length of the press roll  20 , as well as in the ends of the press roll  20 . The pressure in the pulp distribution device  10  may also give an enhanced initial dewatering. 
         [0033]    One way of making the pulp distribution device  10  operating completely filled would be to decrease the rotational speed of the press roll  20 , so that a desired ratio between the pulp volume flow into the pulp distribution device  10  and the pulp volume flow out from the pulp distribution device  10  is achieved. However, it might not be possible due to other parameters, such as vat pressure. 
         [0034]    A preferred way is to have a local throttle  31  in the pulp transportation chamber  25  near the outlet  12  of the pulp distribution device  10 , i.e. that the chamber gap width Wv is locally smaller in a throttle gap width Wt near the outlet  12  of the pulp distribution device  10 . 
         [0035]    This means that the throttle  31  may be positioned e.g. in the beginning of the vat, in the pre-forming zone, if any, or in the outlet  12  of the pulp distribution device  10 . An example may be that the throttle gap width Wt is 30 mm, while the chamber gap width Wv in other places is 40 mm. Note, that this throttle construction is independent of the appearance of the chamber gap width Wv in the rest of the transportation chamber  22 , which chamber gap width Wv in other places may be constant, diverging or converging from the pulp distribution device  10  to the nip  23 . 
         [0036]    The second permeable surface  14  is provided before the local throttle  31  as seen in the pulp transportation direction. As an example the second permeable surface  14  may be provided by having initial dewatering means already in the pulp distribution device  10  e.g. in the form of a permeable wall  14  in the casing  18  (see  FIG. 1 ) or through a permeable surface (not shown) of the rotatable distribution means  13 , or the like. 
         [0037]    Another solution is to position the throttle  31 , so that there is a dewatering segment  14 ′ of the first permeable surface  21  at the outlet  12  of the pulp distribution device  10 , which dewatering segment  14 ′ works as a second permeable surface  14 ′. The dewatering segment  14 ′ should be sufficiently large for forming and dewatering of pulp to start already before the throttle  31 . 
         [0038]    Since the first permeable surface  21  is moving, what is meant with the dewatering segment  14 ′ is of course the segment of the first permeable surface  21  that at a particular moment is at the outlet  12  of the pulp distribution device  10 —i.e. the actual segment is constantly changing, when the first permeable surface  21  is moving. 
         [0039]    In general, the result is better the closer the throttle  31  is positioned to the outlet  12  of the pulp distribution device  10  due to the risk of plugging before the throttle  31  if the distance between the throttle  31  and the outlet  12  is too big, but the invention will work at least in the distance of 0-0.5 m from the outlet  12  of the pulp distribution device  10 . In the embodiment where the second permeable surface  14 ′ is considered to be a segment of the first permeable surface  21 , there should of course be a balance between having a sufficiently large dewatering segment  14 ′ and having the throttle  31  sufficiently close to the outlet  12  of the pulp distribution device  10 . 
         [0040]    The throttle gap width Wt may be fixed and the throttle  31  simply formed as an edge, wedge, knife or similar  30  in the transportation chamber wall  22 . It is, however, preferable that the throttle gap width Wt is adjustable and even more preferable that the throttle gap width Wt is adjustable during operation. 
         [0041]    A simple solution for the adjustment could be to choose an appropriate number of shims to set the position of said edge, wedge or knife  30 . More advanced embodiments can be seen in  FIGS. 2-6 . 
         [0042]      FIG. 2  discloses an embodiment, where the pulp distribution device  10  is provided with a separate throttle device  32 . The edge  30  may e.g. be formed by a sealing. The position of the throttle device  32  may be adjusted from the outside with at least one rod  33  through a sealing lead-through  34 . For simplicity, the edge  30  is preferably divided into a number of longitudinal segments of e.g. 1 m. 
         [0043]    In  FIG. 3  is shown a longer variant of  FIG. 2 , where the edge  30  instead is formed by at least one plate, preferably divided into a number of longitudinal segments. The throttle device  32  is thus also provided with a clearance  35  after the throttle  31  in order to prevent plugging after the throttle device  32 . The clearance angle is preferably 5-10°. 
         [0044]      FIG. 4  is a variant of  FIG. 3 , but where the throttle device  32  instead comprises two plates attached to each other, preferably also each divided into a number of longitudinal segments. The inner plate(s)  36  is/are instead of the rods  33 . 
         [0045]    In the  FIGS. 2-4  the throttle device  32  was long, but comparatively small. In  FIG. 5  is shown an embodiment where the throttle device  32  is somewhat larger. The part of the pulp transportation chamber wall  22  that is closest to the pulp distribution device  10  comprises at least one flexible plate  37  and a wedge  30 . The position of the plate  37  may be adjusted by means of e.g. eccentric pucks  38  or similar. The pucks  38  are fixed to a shaft which goes through the apparatus  100 . In this way adjustment of the throttle gap width Wt may be done e.g. by means of a not shown turning device on the side of the apparatus  100 . 
         [0046]    In  FIG. 6  is shown another variant of  FIG. 5 . In this embodiment the pulp transportation chamber wall part  22  which is closest to the pulp distribution device  10  is a separate plate  37  pivoted on rods  38 . Sealing may be provided by e.g. a transit plate, a seal between the pivoted plate  37  and the casing  18  and sealed lead-throughs for the rods  38 . 
         [0047]    The adjustment of the throttle gap width Wt may be made manually or automatically. If the adjustment is made automatically, then it is probably easiest to control the throttle gap width Wt on the pressure in the pulp distribution device  10 . Another alternative is to control on the inlet ratio, i.e. the pulp volume flow into the pulp distribution device  10  divided with the pulp volume flow out from the pulp distribution device  10 . The outlet pulp volume flow may be calculated as the rotational speed of the press roll  20  times the length of the press roll  20  times the throttle gap width Wt. 
         [0048]    Yet other alternatives may be to control on pulp level or temperature in the pulp distribution device  10 , on the vat pressure or on the pulp level in the ends of the press roll  20 . Of course it is also possible to control on a combination of different parameters. It is, however, preferable to separate the pulp distribution device pressure control from the vat pressure control, since this enables to have a high pressure in the pulp distribution device  10  without necessitating having a high pressure in the vat  22 . 
         [0049]    The practical implementation of the automatic control may be to have one or more sensors (not shown) for the parameter measurement and/or calculation and to use e.g. one or more actuators, e.g. hydraulic, pneumatic and/or mechanic actuators, (not shown) for the throttle gap width adjustment, which actuator(s) is/are controlled by a controller (not shown). The controller may be stand-alone or integrated in a computer in a known manner, preferably as simple feedback control, but feed-forward control is also conceivable or a combination of both. 
         [0050]    For the purpose of this disclosure, “longitudinal distribution of pulp” refers to distribution of pulp along/to the width of the pulp web formed on the first movable permeable surface. The pulp is thus distributed in a direction substantially transverse to the direction of movement of the movable first permeable surface. This means that the rotatable distribution means is arranged with its rotation axis substantially transverse to the direction of movement of the movable first permeable surface. 
         [0051]    Accordingly, in a roll press application “longitudinal distribution of pulp” refers to distribution of pulp along/to the width of the pulp web formed on the press roll. The pulp is thus distributed in a direction substantially transverse to the rotational direction of the press roll. This means that the rotatable distribution means is arranged with its rotation axis substantially transverse to the rotational direction of the press roll. In a roll press application, longitudinal distribution of pulp consequently means lengthwise distribution of pulp, typically along the length of the press roll and along the length of the pulp distribution device. 
         [0052]    Even though the description has been concentrated on an apparatus in the form of a twin roll press, the invention is by no means restricted to a twin roll press, but may be used in any apparatus where a pulp distribution device is distributing pulp on a first moving permeable surface. The first moving permeable surface needs thus not be rotatable, but may be moving in other ways and the apparatus may thus also be e.g. a twin wire press. 
         [0053]    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.