Note: Descriptions are shown in the official language in which they were submitted.
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A method and a device for removing gas from wood chips
Field of the invention
The invention relates to a method for removing gas from wood chips. The
invention
also relates to a device for implementing the aforementioned method.
Background of the invention
When manufacturing cellulose and paper pulp, the ligno-cellulose-containing
wood
chips used as raw material are cooked in an alkaline solution in order to
separate the
fibres and lignin contained in them from each other. The manufacture comprises
several stages, both before and after the cooking. Before cooking, the wood
chips
are directed to the gas removal phase, where gases, primarily air, are removed
from
both inside the wood chips and the spaces between them by directing hot steam
to
the wood chips. Removing gas from the spaces between the wood chips decreases
the amount of gases disturbing the operation of the digester. In addition,
when the
gases are removed from inside the wood chips, the chips absorb cooking
chemicals
better, which improves the yield of the cooking and the quality of the pulp it
provides.
Vertical bins or vessels or horizontal screws are generally used for removing
gas
from wood chips. The screws being used are generally wood chip conveyor
screws,
which convey wood chips, for example, from chipping to the digester.
Generally, this
type of screws are installed horizontally or only in a small angle with the
horizontal
level. The steam needed for removing gas is generally fed from nozzles
installed on
the lower surface of the shell forming the bottom of the screw. However, the
loading
of the screws varies in relation to time, i.e. in relation to the length of
the screw, the
amount of wood chips moving inside varies, resulting in slow warming of
individual
wood chips to the maximum temperature and varying their delay in the screw.
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Consequently, the processing of the wood chips is not uniform. In
addition, the delay of the wood chips in the screw is not long enough in
order for the gas removal to be complete.
When using vertical bins or vessels in removing gas, the wood chips
are generally fed to the bin from its upper end and the steamed wood
chips are removed from the lower end of the bin. The steam is
generally fed to the lower part of the bin and it flows upwards against
the direction of motion of the wood chip material. This kind of a solution
for steaming wood chips is disclosed, for example, in the US-patent
4,867,845, where wood chips are fed to a bin with an downward-
increasing diameter from the upper end of the bin and the steamed
wood chips are removed from the lower end of the bin, by means of a
rotating discharger installed at the bottom of the bin. Steam is fed
vertically to the centre of the bin, via a steam pipe installed parallel to
the vertical central axis of the bin. The end of the steam pipe extending
close to the bottom of the bin, from where the steam is discharged to
the bin, is formed as an expanding taper. Steam flows upwards from
the steam pipe, through the wood chip column packed in the bin.
The publication US 6,199,299 also discloses a chip bin, where steam is
fed to wood chips flowing downwards in the bin from the middle of the
height of the bin, via steam nozzles installed in the shell of the bin.
US-publication 5,628,873 discloses a downwards-tapered steam bin,
where steam is fed to the wood chip flow flowing downward in the chip
bin from above the conical narrowing part. Steam feeding takes place
both via the shell of the bin and the steam feeding pipes installed within
the bin.
The problem with the above-mentioned bins is that the gas removal in
them is not even throughout the entire wood chip flow. The steam fed
via the bin shell cannot penetrate to the middle of the wood chip flow in
such a manner that all the wood chips would receive an identical steam
processing. The steam fed to the middle of the bin, to its lower part,
does not penetrate into the wood chips next to the walls of the bin or
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into the chips on the wood chip surface of the bin efficiently enough to
attain an
even gas removal result. In addition, in the solutions presented above, the
warming of the wood chips is slow in the bin, in which case a part of the wood
chips do not receive a proper gas removal processing.
Brief description of the invention
The purpose of the present invention is therefore to provide a method for
removing gas from wood chips, which avoids the above-mentioned problems and
in which the gas removal from wood chips takes place in such a manner that all
the wood chips receive an identical steam processing. Thus, the pulp resulting
from the cooking following the gas removal is homogenous.
The invention is based on the idea that gas removal from wood chips is
preformed
in a vertical, elongated gas removal vessel, to which the wood chips are fed
from
the upper part of the vessel and the wood chips are removed from its lower
part.
Steam is fed to the wood chips to the wood chip flow travelling downwards in
the
vessel from the middle and edges of the flow, in cross-direction in relation
to the
wood chip flow. The feed rate of wood chips to the vessel and the discharge
rate
from it are maintained on a level that the wood chip flow is moving as a so-
called
plug flow, in which case the wood chips move evenly in relation to the entire
cross-section area of the vessel. Therefore, steam
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is fed into these wood chips in the plug flow. The level of the surface of
wood chips in the vessel is controlled by means of the feed rate and
discharge rate of the wood chips. The surface of the wood chips is
maintained on such a level that the steam fed to the wood chips does
not escape directly to the steam space in the upper part of the vessel.
The steam feeding means are thus arranged to feed steam to a
distance from the surface of the wood chips, inside the wood chip flow.
The height of the gas removal vessel and the placement of the steam
feeding means in relation to the height of the vessel is arranged in such
a manner that the delay time of wood chips in the vessel is long
enough for the gas removal to be as efficient as possible.
The majority of the steam required in the gas removal from wood chips
is fed via the steam feeding pipe arranged in the middle of the wood
chip flow and the rest of the steam is fed from around the wood chip
flow, via steam feeding means arranged in the shell of the vessel. In
order for the gas removal to be efficient enough, the temperature of the
wood chips in the vessel is raised quickly to approx. 1000 C and
maintained there substantially over the entire time the wood chips
remain in the vessel. The amount of the steam being fed is controlled
by means of the temperature and pressure measurements of the steam
gathering in the upper part of the vessel. The steam forming in the gas
removal is removed from the upper part of the vessel, from above the
surface of the wood chips and the condensate is removed from the
lower part of the vessel.
An advantage of the gas removal method according to the invention is
that as a result of both the design of the gas removal vessel and the
transverse steam flow, which does not prevent the flow of the wood
chips, an even, downward plug flow of wood chips is created, resulting
in each wood chip receiving an equally long and identical steam
processing. Consequently, the gas removal within individual wood
chips is efficient and fast. The transverse. steam flow does not prevent
the downward-flow of the wood chips either. Further, the quick raising
of the temperature of the wood chips to the processing temperature
with a transverse steam flow ensures enough delay time for the wood
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chips in the processing temperature, which improves gas removal
significantly. Also, the dimensions of the gas removal vessel and the
placement of the steam feeding means in relation to the height of the
vessel improve gas removal. The gas removal device used in
5 implementing the method, i.e. the gas removal vessel is simple to
manufacture and easy to install in its place and it takes little room in the
cramped device environments of pulp and paper mills.
The invention can also be applied in renewing equipment in factories,
which manufacture pulp. Thus, acquiring a completely new gas
removal device can be avoided and economical savings can be
created.
Brief description of the drawings
In the following, the invention will be described in more detail with
reference to the appended drawings, in which
Fig. 1 shows a schematic side view of a device according to the
invention,
Fig. 2 shows a schematic side view of another device according to
the invention and
Fig. 3 shows a schematic side view of a steam feeding device used
in a device according to an embodiment of the invention.
In Figs. 1, 2 and 3, the same numerals refer to corresponding parts and
they will not be explained separately later on, unless required by the
illustration of the subject matter.
Detailed description of the invention
Fig. 1 shows a device 1 for removing gas from wood chips. The device
is placed in a pulp and/or paper factory before the digester where the
cooking of pulp takes place. The device comprises is an elongated, by
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its diameter downwards tapered and advantageously conical vessel 2
installed in the vertical direction. The vessel can be any vessel suitable
for the purpose, such as, for example, a bin 2. The wood chips are fed
to the bin 2 by means of a feeding device 3, from the upper and of the
bin. The feeding device 3 is, for example, a rotary vane feeder or a
screw. The wood chips can be fed directly to the bin 2 by means of the
feeding device or, such as in the advantageous embodiment shown in
the figure, the wood chips are fed with the feeding device to a "chip,
packer 4 arranged between the roof 19 closing the upper part of the bin
and the feeding device. The chip packer 4 spreads the wood chips as
an even layer to the gas removal vessel 2 by means of the steam fed
to the chip packer. The steam fed to the packer also increases the
temperature of the wood chips before they fall to the bin 2. In the
winter, if the wood chips are frozen, the steam fed to the chip packer
also melts the wood chips. The amount of steam fed to the chip packer
4 is controlled by the temperature of the wood chips. The operation of
the chip packer is obvious to a man skilled in the art, which is why it is
not described more in detail here.
The wood chips that have gone through the gas removal process are
removed from the bin 2 by the discharge means 5 that are connected
to the bottom of the bin. The discharge means comprises several
pushing means 6, which move the wood chips to a discharge opening
7 that is centralized with the midpoint of the bottom of the bin 2. The
pushing means 6 are placed substantially evenly over the periphery of
the entire shell of the bin 2 defining the bottom of the bin. The power
transmission of the discharge means is implemented, for example,
'hydraulically. The discharge opening is connected to a discharge
means arranged under it, i.e. to a screw 8, by means of which the gas-
free wood chips are transferred forward.
The wood chips fed to the bin fill the bin 2 substantially evenly to the
level of the surface 9 of the wood chips. The surface of the wood chips
settles within a distance from the roof 19 of the bin. A free space, i.e. a
gas space 13 remains between the surface 9 of the wood chips and the
roof of the bin. The level of the surface 9 of the wood chips in the bin is
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controlled by controlling the feed rate of the wood chip feeding device 3
and/or the discharge rate of the wood chip discharge means 5 and the
screw 8 discharging the wood chips from the bin. The discharge rate is
controlled according to the production of the digester. For the purpose
of controlling, one or more sensors 10 measuring the level of the
surface of the wood chips are arranged in the bin 2. On the basis of the
data provided by the surface level sensor 10, the controller 21 controls
the rate of the feeding device 3 and/or the discharge means 5 and the
screw 8 and at the same time the delay time of the wood chips in the
delay zone 25 between the steam feeding means 11 and 12 as well as
the discharge means 5. The surface level of the wood chips is to be the
wood chips must be controlled onto such level that the wood chip
surface extends all the time above the steam feeding openings of the
steam feeding means 11 and 12 feeding steam to the bin. Thus, the
steam is prevented from directly escaping to the steam space 13 above
the surface of the wood chips.
As can be understood from what is described above, the wood chips
move downward in the bin 2 by forming a downward moving wood chip
flow, which is described with the arrow A in the figure. The downward
movement and the effect of the steam fed to the wood chip flow causes
the wood chip flow to become dense, thus forming a so-called plug
flow. Thus, the downward expanding design of the bin promotes the
even movement of the flow in such a manner that no blockages or
other obstacles blocking the even flow form in it. The wood chip flow is
guided by means of the flow controller 14 arranged in the lower part of
the bin, which flow controller evens the chip flow in the bin. In addition,
the flow controller 14 prevents the wood chips from directly discharging
to the discharge opening 7 and thus the blockage of the discharge
opening 7.
Steam is fed to the wood chip flow in cross direction in relation to the
wood chip flow with first steam feeding means 11 and second steam
feeding means 12. The first steam feeding means 11, i.e. the steam
feeding pipe 11 is arranged in the middle of the bin 2. The steam
feeding pipe 11 thus directs steam to the middle of the bin 2 in such a
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manner that at least a part of its length combines with the vertical mid-
axis of the bin. The end of the steam feeding pipe 11 that extends
inside the bin is closed, preventing the steam from flowing into the chip
flow directly, in the same direction with it. In the lower end of the steam
feeding pipe 11 there is an around the steam feeding pipe extending
steam distribution means 15, which can be a perforation extending
around the feeding pipe, or some suitable means attached to the
feeding pipe, such as, for example, a screen, which distributes steam
from the steam feeding pipe 11 evenly to the wood chips horizontally
on every side of the pipe. In the embodiment of Fig. 1, a screen 15 for
dividing steam is arranged in the steam feeding pipe. The steam
feeding pipe 11 thus feeds steam to the middle of the bin in a cross-
direction in relation to the wood chip flow, which steam is marked with
arrows in the figure. In the shell of the bin, substantially on the same
level with the screen 15, there are the second steam feeding means 12
extending around the entire shell of the bin. The second steam feeding
means comprising a distribution chamber 12 and a steam distribution
means 20 direct steam to the bin in a cross-direction in relation to the
wood chip flow. The distribution means 12 is arranged on the outer
surface of the bin and it extends around the entire bin shell. The steam
distribution means 20 is, for example, a screen and it is arranged on
the inner surface of the bin shell, in connection with the distribution
chamber and also extends around the bin shell. The cross-directional
steam flow enables the quick raising of the temperature of the wood
chips to a temperature that is advantageous for gas removal. It has
been experimentally established that gas removal from wood chips
takes place in approx. 1000 C. The wood chips are quickly heated by
means of the steam directed to the bin to this temperature and are
maintained in this temperature by means of continuous steam feeding.
The majority of the steam required in gas removal is fed from the
steam feeding pipe 11. The rest of the steam is fed from the steam
feeding means 12 and/or the steam feeding units 17 arranged on the
bottom of the discharge means 5. It is also possible to feed steam to
the bin only from the steam feeding pipe 11, if the steam amount
coming through it is enough to maintain a high enough temperature
required in the gas removal. Controlling the gas removal temperature is
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done by controlling the amount of steam fed to the bin. The amount of
steam is controlled by measuring the temperature prevailing in the
delay zone 25 below the second steam feeding means 12 by means of
a controller 23, which sends a control message to the steam feeding
means (not shown in the figure). The control message is in the figure
referred to with the letter B and dashed lines. It has been
experimentally established, that the delay time of an individual wood
chip piece in the delay zone 25 should be approximately 10 to 30
minutes, advantageously approximately 15 to 25 minutes in order for
the gas removal from it to be as complete as possible. The steam
feeding means 11 and 12 are placed in relation to the height of the bin
2 in such a manner that below the second steam feeding means 12,
between the steam feeding means 11 and 12 and the wood chip
discharge means is formed a so-called delay zone 25, where an
adequate delay time, which is required for gas removal, is guaranteed
for the downward flowing wood chips. The delay zone is approximately
2/3 of the height of the vessel 2.
The gases that the steam has displaced from the wood chips and the
spaces between them, gathers in the gas space 13 between the
surface 9 of the wood chips and the roof 19 of the bin, from where it is
advantageously directed out via a unit 16. It is also possible to remove
gases from the bin 2 by sucking them via the steam feeding pipe 11 or
the distribution chamber 12. Thus, the steam flow can be effectively
directed in a cross-direction through the wood chip flow. In addition,
this way the channelling of the steam in the wood chip flow can be
prevented. The steam gases can be sucked from only one of the steam
feeding means at a time, because feeding enough steam to the wood
chip flow and maintaining a high enough temperature in it must be
guaranteed throughout the entire gas removal process. In other words,
if the steam gases are sucked via the distribution chamber 12, enough
steam must be fed from the steam feeding pipe 11. Further, if the
sucking of steam gases takes place via the steam feeding pipe 11,
enough steam must be fed via the distribution chamber 12 and the
steam feeding units 17.
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By means of the temperature sensor 22 arranged in the gas space 13
above the wood chip surface 9, the temperature of the gas space 13
above the wood chip surface is monitored. If the temperature rises too
high, it is 'a sign of a malfunction of the bin, in which case an alarm is
given. The temperature prevailing in the gas space 13 above the wood
chip surface is lower than the processing temperature of the wood
chips. This is because a part of the steam fed to the wood.ch.ips flows
upstream in the upper part of the wood chip column towards the
surface 9 of the wood chips and displaces the colder gas in the wood
chips, which flows upwards to the gas space 13.
The condensate formed in connection with the wood chip gas removal
flows to the bottom of the discharge means 5 connected to the lower
part of the bin and are removed from there via the condensate removal
units 18. The condensate can also be removed from the screw 8 (not
shown in the figure).
Fig. 2 shows another embodiment of the gas removal device 1
according to the invention. This embodiment differs from the above-
described embodiment of figure 1 for the part of the wood chip
discharge means. Gas removal from the wood chips takes place in a
corresponding manner as is described in connection with figure 1. In
fig. 2 the downward flowing wood chip flow formed by wood chips is
removed from the bin 2, such as in the embodiment of Fig. 1, by means
of the wood chip discharge means 5 in connection with the bottom of
the bin 2. There are several hydraulically moving pushing means 6 in
the discharge means, which move the wood chips from one side of the
bin to the discharge opening 7 placed on the other edge of the bin 2.
The screw 8 moves the gas-free wood chips forward. In this
embodiment no separate flow controller is required for directing the
wood chip flow to the discharge means 5.
The flow of the wood chips in the bin 2 can, if desired, be directed by
means of the design of the steam feeding means 11. Fig. 3 shows an
in the bin 2 arranged steam feeding pipe 1.1 having a closed,
downward convergent tapered flow guide 24 attached to the lower
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surface of a screen provided in the steam feeding pipe. The effect of
the flow guide is based on that it prevents the sudden expansion of the
free cross-sectional area of the bin after the screen 15 and thus evens
the wood chip flow downwards.
The invention is not intended to be limited to the embodiments
presented as examples above, but the invention is intended to be
applied widely within the scope of the inventive idea as defined in the
appended claims. The gas removal vessel can thus also be of the
same diameter in relation to its entire height, a vessel that is round in
its cross section. Further, as the steam used in gas removal can be
used either fresh steam or expansion steam received from another
process of the pulp or paper mill. Also, the steam gas received from the
gas removal device can be compressed and recycled back as heating
steam to the device.
