Note: Descriptions are shown in the official language in which they were submitted.
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Apparatus and method for dewatering cellulose pulp with
improved dewatering efficiency
FIELD OF THE INVENTION
The present invention relates to washing and dewatering of cellulose pulp
and in particular to a wash/dewatering apparatus with one, or preferably
two co-operating, cylindrical press rolls.
BACKGROUND
Pulp washing is a key operation in the chemical pulping line. There are
many different types of washing and dewatering apparatuses available, some
of which are based on washing by pressing the pulp such that fluid is
removed.
A well-known type of wash press has two co-operating cylindrical press rolls,
arranged in the same horizontal plane next to each other with parallel axis of
rotation. The outer surface of each press roll is perforated such that liquid
may be extracted from the cellulose pulp that is conveyed in the restricted
space between the press roll and a surrounding vat, through said perforated
surface. During operation, when the press rolls rotate, a pulp web is formed
on the perforated surface of each roll. A pinch or nip is formed between the
press rolls at the point where the distance between them is the smallest, and
the press rolls are arranged to rotate in opposite directions so as to
transport
the respective pulp webs towards said nip, such that the pulp webs merge
shortly before the nip and are subsequently jointly pressed in the nip.
Thus, the pulp webs are continuously dewatered throughout the whole pulp
passage, wherein a final dewatering is achieved in the nip, where the pulp
web is pressed to a desired degree.
The fluid removed from the pulp, i.e. the filtrate, passes through the
perforated roll surface in a radial inward direction and can for example be
transported to the axial ends of the press rolls by means of axial filtrate
channels. There is normally a filtrate tank arranged in connection to the
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wash press to collect the filtrate that is removed from the washing in the
press. Washing liquid is supplied to the wash press in order to wash the
pulp and displace the fluid in the pulp. The washing principle is thus a
combination of displacement, dewatering and pressing.
Conventionally, the vat has been arranged to converge towards the nip in
order to build up a certain increasing pressure. Nevertheless, a wash press
with a non-converging vat geometry is disclosed in FI 102 977 B. The
objective of this wash press is to convey pulp towards the nip under a
relatively low pressure, in order to avoid plugging and other problems related
to high pressures. This is achieved by the use of a vat that, in contrast to
conventional apparatuses, is arranged at a constant distance from the press
roll towards the nip. In FI 102 977 B, no regard is taken to the pressure
profile that is formed inside the pulp passage.
SUMMARY
An object of the invention is to improve the efficiency of the dewatering in a
washing or dewatering apparatus. This is achieved in a first aspect by an
apparatus for dewatering cellulose pulp according to claim 1, and in a
second aspect by a method of dewatering cellulose pulp.
In recent tests it has been discovered that the increasing pressure towards
the nip is almost independent of the vat geometry, such that a pressure
gradient is built up irrespective of the vat geometry. Thus, especially in a
vat
with a non converging geometry, this pressure gradient produces a counter
flow of filtrate based on the filtrate that is rejected from nip.
The invention is based on this discovery, and the presence of a counter flow
opposite the direction of rotation of each press roll is in this invention
used
to enhance the dewatering efficiency of the apparatus.
According to a first aspect, the invention relates to an apparatus for
dewatering cellulose pulp, which apparatus includes two press rolls,
arranged next to each other, each press roll being provided inside a guide
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surface, such that a pulp passage is formed between each press roll and the
corresponding guide surface, wherein an inlet for transferring pulp or the
like to said pulp passage is arranged at a first point at each press roll, and
wherein the press rolls are arranged to rotate in opposite directions so as to
convey the pulp in a first operational direction towards a nip that is formed
between them at a second point, close to where the guide surfaces face each
other, each press roll having a perforated outer surface for withdrawing
filtrate from said pulp or the like. At the point where they face each other,
the guide surfaces include an outlet for withdrawing filtrate that is rejected
from the nip, said outlet being arranged substantially opposite of the nip and
directed such that the filtrate removed through said outlet is conveyed from
the pulp passage through a clearance that is formed in a space between the
guide surfaces.
An advantage of this apparatus is that it presents a possibility to withdraw
the counter flow of filtrate that generally is formed near the nip in a
direction
opposite the main flow of the pulp in the pulp passage. Thus, the efficiency
of the dewatering is enhanced.
In a first embodiment the outlet comprises outlet openings arranged between
the two guide surfaces. Also, the outlet may comprise a plate with outlet
openings arranged at even intervals in said plate, such that strips are
formed between said openings.
Such a placement of the outlet is advantageous, since only the flow rejected
from the nip is directed to pass through said outlet, wherein the main pulp
flow, flowing towards the nip is not affected.
In another embodiment the outlet comprises outlet openings in the form of
perforations through the guide surfaces on both sides of a tip that is formed
by the guide surfaces at their point of intersection. This is advantageous
since it allows for an enhanced' dewatering with a minimum of alterations of
an existing apparatus.
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Preferably the outlet openings are arranged such through the guide surfaces
that they are directed away from the nip substantially opposite the
operational direction of the pulp. Such an arrangement is advantageous,
since it opens towards the flow rejected from the nip such that the main
pulp flow, flowing towards the nip is not affected.
The apparatus may involve a line for conveying the removed filtrate from the
outlet and re-circulating the removed filtrate to the pulp passage. With such
an arrangement the filtrate may be re-circulated and the loss of fibres is
minimised.
Preferably the line is connected to the inlet of the pulp passage for re-
circulation of the removed filtrate to said inlet.
According to one embodiment of the apparatus the outlet openings are
dimensioned so as to withdraw liquid from the pulp passage, such that a
minimum amount of fibres are withdrawn. Preferably, the outlet openings
have a maximum diameter of 10 mm.
In another embodiment of the apparatus the outlet openings are
dimensioned so as to let through some free pulp fibres with the filtrate that
is being withdrawn from the pulp passage. This may be advantageous as it
diminishes the risk of plugging the outlet openings. Preferably, the outlet
openings have a diameter of between 10 and 200 mm.
According to a second aspect, the invention relates to a method of
dewatering pulp or the like, comprising the steps of feeding pulp or the like
to an inlet of a pulp passage formed between a first press roll and a
surrounding first guide surface; conveying the pulp through the pulp
passage by rotating the press roll towards a nip formed between the press
roll and second similar press roll surrounded by a similar second guide
surface which intersects with the first guide surface close to the nip;
dewatering the pulp through perforations in the outer surface of each press
roll; pressing the pulp at the nip between the two press rolls. Filtrate is
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withdrawn through an outlet arranged through the guide surfaces, close to
their intersection near the nip.
In one embodiment of the method the withdrawn filtrate is returned to the
pulp passage, and possibly the filtrate is inserted at the pulp inlet.
5 In order to wash the pulp in the pulp passage wash liquid may be inserted at
at least one point along the pulp passage.
SHORT DESCRIPTION OF THE DRAWINGS
Below, specific embodiments of the invention will be described with reference
to the accompanying drawings, of which:
Fig. 1 schematically shows a conventional apparatus for dewatering
cellulose pulp;
Fig. 2 shows the flow of pulp and filtrate, according to a discovery on
which the invention is based;
Fig. 3 schematically shows a first embodiment of the apparatus
according to the invention for dewatering cellulose pulp;
Fig. 4 schematically shows the apparatus in fig. 3, seen from above;
Fig. 5 schematically shows a second embodiment of the apparatus
according to the invention; and
Fig. 6 schematically shows a third embodiment of the apparatus
according to the invention.
DETAILED DESCRIPTION OF SHOWN EMBODIMENTS
Fig. 1 schematically shows a conventional apparatus for dewatering cellulose
pulp. The apparatus includes two press rolls 2, arranged next to each other.
Each press roll is provided inside a guide surface 3, such that a pulp
passage 4 is formed between each press roll 2 and the corresponding guide
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surface 3. An inlet 5 for transferring pulp or the like to said pulp passages
4
is arranged at a first point in connection to each press roll 2.
The press rolls 2 are arranged to rotate in opposite directions D1 and D2,
respectively so as to convey the pulp in an operational direction F1 towards
a nip 6 that is formed between them at a second point, close to where the
guide surfaces 3 intersect. The term operational direction is intended to
denote the direction in which the pulp is conveyed during operation, i.e. as
long as there is pulp in the pulp passage and as long as the press rolls are
rotating in their intended directions D 1 and D2. In order to dewater the pulp
in the pulp passage 4, each press roll 2 has a perforated outer surface 7,
through which filtrate is withdrawn from said pulp or the like.
Recently it has been discovered that a counter flow F2 opposite the direction
of rotation DI and D2 of each press roll, and thus opposite the operational
direction F1 of the pulp in the pulp passage, is created by the filtrate that
is
rejected from the nip 6. These flows F1 and F2 are demonstrated in fig. 2.
In figures 3-6, three different embodiments of the invention are shown. In
order to facilitate comparison with prior art, the reference numerals used in
figures 1 and 2, are reused in figures 3-6 to denote same or similar details.
In figure 3, a first embodiment of the apparatus 1 according the invention is
shown, which in addition to the details of the conventional apparatus shown
in figure 1, includes some additional details. Firstly, an outlet 8, for
withdrawing the flow F2 of rejected filtrate from the nip 6 is arranged
substantially opposite of the nip 6. Thus, in the illustrated embodiment,
since the pulp is conveyed upwards towards and through the nip 6, the
outlet 8 is arranged to allow the filtrate to exit downwards.
The outlet 8 is formed through or between the guide surfaces 3, where they
face each other and possibly intersect, and is directed such that filtrate
removed through said outlet 8 is conveyed away from the pulp passage 4 via
a clearance 9 that is formed between the guide surfaces 3. A line 10 is
arranged to lead the filtrate from the outlet 8, through the clearance 9 and
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on to subsequent processing steps. Possibly, the filtrate may be conducted to
a tank or, as will be described below with reference to fig. 5, it may be re-
circulated to the pulp passage 4. Preferably, the line 10 includes at least
one
valve 11 in order to control the exit flow. Also, the line may include a pump
(not shown) for pumping the filtrate to subsequent processing steps.
However, due to the relatively high pressure near the nip 6 in the pulp
passage 4, a pump may be omitted, since the pressure of the rejected filtrate
may in itself very well be enough to conduct the filtrate to the desired
location.
Figure 4 shows a view from above of the outlet shown in figure 3. From this
view it is evident that the outlet 8 in this embodiment comprises outlet
openings 12 in the form of oval holes. These oval holes are uniformly formed
in a plate 13 that connects the two guide surfaces 3, since the tip that is
normally formed at the intersection of such guide surfaces has been cut off.
On the plate 13, the outlet openings 12 are separated from each other by
connecting strips 14 or flanges, which unite the two guide surfaces 3. In
figure 4, the openings are oval, and the spacing between the strips 14 is
about 100 mm, and typically the diameter may be between 10 and 200 mm.
It is however also feasible to arrange the openings as one central slit or
several slits extending through part or the whole width of the guide surfaces.
Such a slit may for mechanical reasons include strips that bridge the guide
surfaces 3 to strengthen the construction.
In figure 5, a second embodiment of the apparatus 1 according to the
invention is shown. In this embodiment, the line 10 is arranged to conduct
the filtrate that has been removed through the outlet 8 back to pulp passage
4, up streams of the nip 6, either to the inlet 5 where it may be mixed with
pulp that it is fed to the pulp passage, or further down the pulp passage.
Preferably though, the filtrate is re-circulated to the first part of the pulp
passage 4, in order not to negatively affect the optional ongoing washing of
the pulp.
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This second embodiment may be combined with the first embodiment shown
in figure 3 and 4, or with the third embodiment, which is described below.
In figure 6, a third embodiment is shown. The same reference numerals are
used to indicate details in figure 6 that are similar to details of previous
embodiments. However, for this third embodiment an apostrophe (') is used
to indicate details that differ slightly in design and/or function from the
details of the first embodiment. For instance, in the third embodiment there
is an outlet, however, this outlet 8' differs from the outlet 8 of the first
embodiment.
The outlet 8' of the third embodiment does not include a horizontal
perforated plate, instead part of the guide surfaces 3 have been perforated.
Thus, the guide surfaces intersect at a point just below the nip 6 to form a
tip 15 that points towards the nip 6. The main difference of the third
embodiment with respect to the first embodiment is that the outlet openings
12' in the third embodiment are dimensioned so as to let out mainly liquid
from the pulp passage, such that a minimum amount of the fibers are
withdrawn.
On the contrary, in the first embodiment shown in figures 3-4, the outlet
openings 12 are dimensioned so as to also allow a certain amount of fibers to
exit from the pulp passage through them. Thus, it is of greater interest to re-
circulate the filtrate to the first part of the pulp passage in the first
embodiment, in order not to lose valuable fibers.
The outlet openings 12', in the third embodiment may preferably be located
opposite the nip as directional openings opposite the upwardly main pulp
flow direction Fl. Thus, the outlet openings 12' may be constituted of
vertical holes or slits, through which at least part of the counter flow F2 of
rejected filtrate may exit the pulp passage downwards. An easier manner to
design the outlet openings 12` is, of course, to arrange radial bore holes
through the guide surfaces 3, orthogonally with respect to the surface of the
respective guide surface 3. However, vertical slits are advantageous since
they (A) allow the flow rejected from the nip 6 to flow straight down through
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the guide surfaces, and (B) since they minimize the risk of letting out the
upwardly going flow F1, which may include a high density of pulp.
Preferably, the outlet openings 12' of this third embodiment have a
maximum diameter of 10 mm, such that a minimum amount of pulp fibres
is allowed to exit the from the pulp passage along with filtrate.
The outlet 8' comprises a clearance 9' between the guide surfaces 3, which
clearance involves a chamber that is connected to a line 10 for removal of
the withdrawn filtrate. In order to increase the withdrawal of filtrate a sub-
pressure may be provided in the line 10.
The aim of the invention is mainly to increase the dewatering of a dewatering
apparatus. However, dewatering is an important step the washing procedure
in a washing apparatus. Thus, the apparatus may very well include one or
several inlets for adding wash liquid to the pulp passage 4, wherein the
apparatus would constitute a washing apparatus with increased dewatering
capacity.
Above, three embodiments of the invention have been described. These may
combined in all feasible combinations, such that e.g. the outlet openings
may be dimensioned either to let through filtrate with some pulp fibers, or to
only let through filtrate with as little pulp fibers as possible, regardless
of the
general design of the outlet. Thus, the first embodiment may be provided
with relatively small holes, and the third embodiment may be provided with
relatively large holes.
The invention is however not limited to these embodiments. The scope of the
invention is defined by the appended claims.
