Note: Descriptions are shown in the official language in which they were submitted.
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Feeding system in connection with the continuous cooking of cellulose
containing material
TECHNICAL FIELD
The present invention relates to a feeding system for feeding comminuted
cellulose
containing material and liquid to a continuously operating treatment vessel,
preferably a
preimpregnation vessel or a digester which may be of steam/liquor phase type
or of a
hydraulically filled type. The feeding system comprises a chute, operating at
a first
pressure level, with a liquid level and a level of material, a high pressure
feeder which,
by means of rotating pockets, sluices the material, together with atl or a
part of said
liquid, to a second pressure level, which is higher than said first pressure
level, for
further conveyance to said treatment vessel, which high pressure feeder also
receives a
return liquid flow, from said treatment vessel, at said second pressure level
and recir-
culates a recirculation flow to said chute or to said high pressure feeder.
BACKGROUND AND PROBLEMS
Continuously operating digesters for cooking comminuted cellulose containing
material
to paper pulp have been known for a long-time and hence also feeding systems
for such
digesters. The requirements on the feeding system are, among other things,
that the
cellulose material, hereinafter called chips, should be evenly fed from a low
(atmos-
pheric) pressure to a higher pressure, that the chips should be heated at the
same time as
vapour and gases are evacuated from it, to be replaced with water or
condensate, and
that the feeding system should be as cheap as possible in terms of investment
costs and
operating costs.
A conventional feeding system comprises a chip bin, a chip meter, a low-
pressure
feeder, a steaming vessel, a chip chute and a high-pressure feeder. The
function of the
high-pressure feeder is to sluice the cellulose material, including some
liquid, to a
continuous digester or to a preimpregnation vessel, which operates at a
relatively high
pressure. Between the high pressure feeder and the impregnation vessel or
digester,
there is conventionally a top circulation, which comprises a feed line for a
mixture of
chips and impregnation liquid, and a return liquid line for separated
impregnation liquid.
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A top separator is arranged in the top of the impregnation vessel or the
digester for
feeding of the chips into the impregnation vessel or digester at the same time
as a part of
the impregnation liquid is separated off and is pumped with a pump through the
return
liquid line, back to the high pressure feeder. The high pressure feeder is
equipped with a
rotor with pockets, whereby one pocket always is in low pressure position, to
be in open
connection with the chute and one pocket always, at the same time, is in high
pressure
position, to be in open connection with the impregnation vessel or digester
via the feed
line. When a rotor pocket which is, to a degree, filled with chips, arrives in
high pres-
sure position, that is in direct connection with the top circulation, it is
flushed clean by
the liquid from the return liquid line, and the suspension of chips and
impregnation
liquid is fed into the top of the impregnation vessel or digester via the feed
line. Liquid,
in a circulation loop on the low pressure side of the high pressure feeder,
which loop is
conventionally equipped with a pump, is at the same time feeding chips from
the chute
into one of the pockets of the high pressure feeder so that this pocket is, to
a degree,
filled with chips. The circulation loop is also, as will be further described
in connection
with the figure description below, equipped with a sand trap and a tubular
screen, refer-
red to as an in-line drainer. Further, a level tank is, via a line from the in-
line drainer,
connected to the return liquid line of the top circulation.
This conventional system was formed a long time ago, when the production
volume of a
continuous pulp mill was perhaps just a tenth of the volume that a modein pulp
mill
produces today. Accordingly, when the conventional feeding system was
developed, the
machines that were used were much smaller. This, for instance, meant that the
pump in
said circulation loop, for recirculating liquid to the chute on the low
pressure side of the
high pressure feeder, was rather smail and hence needed to be protected from
chips that
might enter the circulation loop together with the liquid which is fed out
from the high
pressure feeder. To achieve this protection, the high-pressure feeder was
equipped with
a screen device, a so-called strainer plate, in its outlet to the circulation
loop. The
intention was that liquid should pass the screen and that the chips should
remain in the
pocket of the high-pressure feeder to give a high filling degree of the pocket
In reality,
however, the screen is partially plugged every time a pocket is filled with
chips, which
leads to that the filling degree in a pocket only reaches about 50-70 vol.-%
(counted on
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chips). The rest of the space is filled with liquid. This should be compared
with the
filling degree in the chute, which theoretically should be able to be reached
in the high
pressure feeder as well and which normally is about 80-85 vol.-%. When the
rotor turns,
the strainer plate is again scraped free from plugging chips, but the problem
re-occurs
when the next pocket comes in the same position. The partial filling degree
results in an
uneconomic operation and in that the high pressure feeder must be operated at
a relati-
vely high rotary speed, which in its turn leads to premature wearing of the
equipment.
In addition, the building volume needs to be relatively large in order to
accommodate
the high pressure feeder which must be placed at a relatively high level due
to the neces-
sary suction pipe for the pump on the low.pressure liquid side of the high
pressure
feeder. Also the building volume needs to be large enough to accommodate said
sand
trap, in-line drainer and level tank and this equipment also leads to
investment and
operating costs.
SOLUTION AND ADVANTAGES
It has now surprisingly been found that a well functioning feeding system can
be provi-
ded, which feeding system operates substantially without a screening device in
the
outlet of the high pressure feeder to the circulation loop and also without an
in-line
drainer and a level tank. The system according to the invention further
enables the
pockets of the high pressure feeder to be filled to a theoretical maximum
degree, that is
to the same degree as the filling degree in the chute.
Basically, the background of the discovery that has led to the invention is
that the
machines in modern pulp mills with high production are much larger than they
were at
the time when the conventional feeding system was developed. Especially, the
pump in
the circulation loop, for recirculating liquid to the chute on the low
pressure side of the
high pressure feeder, is much larger today and is hence capable of handling a
certain
amount of chips in the liquid flow to be pumped, since the size of the chips
is not much
different from what it always has been. Thus, the screening device that
conventionally
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stopped chips from entering this circulation, can be excluded, which leads to
several
important advantages.
Firstly, when a pocket of the high pressure feeder moves towards the low
pressure
position it is full with liquid from the return liquid flow from the digester
or preimp-
regnation vessel, hereinafter called the treatment vessel. When the pocket
reaches the
low-pressure position, the liquid is displaced from above with the mixture of
chips and
liquid that is present in the chute, whereby the same filling degree as in the
chute can be
achieved. The filling degree in the chute is nomnally about 80-85 vol.-% since
some
excess liquid is demanded for the chip column to be able to move down into the
high-
pressure feeder.
Secondly, the building height of the high-pressure feeder can be lowered. This
is a
consequence of the fact that essentially no suction pipe is needed for the
pump in the
recirculation flow of the circulation loop, since there is no pressure drop
across a
screening device. Also, the conventional level tank and its in-line drainer
can be exclu-
ded from the system, which results in decreased investment and operating costs
as well
as in a smaller building volume. The reason for the possibility to exclude the
level tank
and the in-line drainer is that in the system according to the invention,
there is always
liquid communication between the pumps on the liquid side of the high pressure
feeder,
and the chute. This means that a chute liquid level control valve can be
placed in conne-
ction with one of these pumps, for regulation of the liquid level in the
chute. In the con-
ventional system, there is no such liquid communication when the screening
device is
plugged and hence, the chute liquid level control valve have to be placed in
connection
with a level tank, normally between the in-line drainer and the level tank.
Yet another advantage is that the rotary speed of the pockets in the high-
pressure feeder
can be increased to a speed which is at least up to twice as high as in a
conventional
feeding system. Thereby, there is a major increase in the capacity of the high-
pressure
feeder, enabling installation of a smaller size of high-pressure feeder and
thus decrease
of costs.
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An additional advantage of the system according to the invention is that the
pump,
hereinafter called the first pump, in the recirculation flow can be coupled in
series with
a second pump, that pumps liquid from the low pressure recirculation flow to
the high
pressure return liquid flow from the treatment vessel. This means that the
pump head of
5 the first pump can be added to the pump head of the second pump, whereby the
second
pump may be one standard pump instead of, as in the conventional system, two
standard
pumps or one high-pressure pump with several impellers. This results in a
major
decrease in investment and operating costs. Accordingly by means of the
invention all
prior restrictions, e.g. feeder screen, inline drainer, installed on the
suction side of
the second pump can be eliminated, resulting in lower costs and improved
availability.
According to one aspect of the invention, said recirculation flow, that is the
volumetric
flow that exits the high-pressure feeder on its low-pressure side is related
by a factor
0.8-1.5 to the volumetric chip flow which is handled by the high pressure
feeder. The
maaimum theoretical volumetric flow can be calculated as the volume of the
pockets
in the high pressure feeder multiplied with the rotary speed of the high
pressure feeder
and by a factor two (since the pockets are filled twice in each complete
rotation).
Another way of calculating the volumetric flow is by dividing the incoming
chip flow
(as measured in a chip meter) by a factor 0.5-0.9.
According to another aspect of the invention, a sand trap is installed in said
return liquid
flow. This location has the benefit, as compared to the conventional location
in the
circulation loop, that the flow is steady, without any essential fluctuations
in its velocity.
The sand trap consists of a cyclone that has to be optimised for a certain
flow velocity
and will thus operate better in the relatively steady return liquid flow from
the treatment
vessel. As an altemative, the sand trap may be installed in the chute. A
further advan-
tage is that the sand is not circulated in said recirculation flow on the low-
pressure side
of the high-pressure feeder, whereby its wearing effect on the equipment is
avoided
These altelnative placements of the sand trap can of course also be made in
other types
of feeding systems, that is not necessarily related to the present invention.
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According to one aspect of the present invention,
there is provided feeding system for feeding comminuted
cellulose containing material and liquid to a continuously
operating treatment vessel, which feeding system comprises a
chute, operating at a first pressure level, with a liquid
level and a level of material, a high pressure feeder which,
by means of rotating pockets, sluices the material, together
with all or a part of said liquid, to a second pressure
level, which is higher than said first pressure level, for
further conveyance to said treatment vessel, which high
pressure feeder also receives a return liquid flow, from
said treatment vessel, at said second pressure level and
recirculates a recirculation flow to said chute or to said
high pressure feeder, wherein said high pressure feeder is
in open connection, with respect to both said liquid and
said comminuted cellulose containing material, with said
recirculation flow, when any of the pockets of the high
pressure feeder is in a lopation which corresponds to an
outlet for said recirculation flow and that said high
pressure feeder lacks any form of screening device which
would be able to, at any extent, prevent said comminuted
cellulose containing material from entering said
recirculation flow.
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BRIEF FIGURE DESCRIPTION
The invention will in the following be fiuther described with reference to the
drawings,
of which:
Figure 1 represents a feeding system according to a conventional system.
Figure 2 represents a feeding system according to a preferred embodiment of
the
invention.
Figure 3 represents a feeding system according to an alternative embodiment of
the
invention.
Figure 4 shows a tramp material catcher.
Figure 5 shows the tramp material catcher in figure 4 in the section view A-A.
Figure 6 shows a typical high-pressure feeder with its pockets.
DETAILED FIGURE DESCRIPTION
Figure 1 represents a feeding system according to a conventional system.
Detail 1 in
figure 1 denotes a low pressure feeder, which sluices chips, that has been
steamed in a
previous step (not shown), from atmospheric pressure into a slight
overpressure in a
chip chute 2. The low pressure feeder 1 may be excluded from the system or may
be
located at an earlier position in the system. In the chute, there are levels
of liquid and
chips. The chips fall by gravity down into a high pressure feeder 3 through a
first
opening 3a in its housing. The high pressure feeder comprises rotating
pockets, whereby
a first pocket, via the first opening 3a, is in open connection with the chute
and, via a
second opening 3b, which is equipped with a strainer plate, in the housing is
connected
to a recirculation flow 4 at the same time as a second pocket, via a third
opening 3c in
the housing, is in open connection with a return liquid flow 5, which
comprises liquid
that has been separated from the chips in a top separator in the treatment
vessel, and, via
a fourth opening 3d in the housing is in open connection with a sluicing flow
6 for
feeding chips and impregnation liquid to the treatment vessel. Due to the
strainer plate
in the second opening 3b, chips are to a degree prevented from entering the
recirculation
flow 4. The pocket which is in open connection with the return liquid flow 5
is filled
with liquid at a relatively high pressure at the same time as a mixture of
chips and
liquid, which was present in the pocket, is displaced into the sluicing flow
6. When this
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pocket moves into the position of the first opening 3a, the liquid in the
pocket is again
displaced by a mixture of chips and liquid from the chute 2. The filling
degree does
however not reach optimum since the strainer plate becomes partially plugged
by chips.
The liquid which has been displaced enters the recirculation flow 4 and is
pumped, by a
first pump 7, to a sand trap 8, where sand and other particles are removed
from the
liquid flow. Thereafter, the liquid recirculation flow 4 continues through an
in-line
drainer 9 and back to the chute 2. A branch flow 10 is extracted through a
screen in the
in-line drainer 9, to prevent any chips that might be present in the flow 4
from entering
the branch flow 10, and is introduced in a level tank 11, in which a certain
liquid level is
maintained at all times. Liquid is pumped, by a second pump 12 which may
consist of
two or more standard pumps or one high pressure pump, in a conduit 13 from the
level
tank 11 to the return liquid flow 5 from the treatment vessel, in order to
constitute a part
of the liquid that displaces the chips in the high pressure feeder. The liquid
in the level
tank 11 has mainly three sources, that is the liquid which is displaced by
chips in the
high pressure feeder 3, condensate and water from the chute 2 and leakage from
the
high pressure side to the low pressure side of the high pressure feeder 3. To
the second
pump 12 there may also be added cooking chemicals, especially white liquor.
The flow
of high-pressure return liquid 5 is maintained by a third fluid pump 16.
It is important to maintain a certain liquid level in the chute. If it becomes
to high, liquid
will get into the low pressure feeder 1 and the steaming vessel with resulting
problems.
If the level, on the other hand is to low, steam will enter the high pressure
feeder 3.
When steam is allowed to enter the high-pressure side of the high pressure
feeder 3, it
collapses which results in bangs and massive vibrations in the feed line 6 to
the treat-
ment vessel. This may result in severe damages in this line. In the
conventional system
according to figure 1, the liquid level in the chute 2 is controlled by a
chute liquid level
control valve 14 which is arranged in the branch flow 10 between the in-line
drainer 9
and the level tank 11. If the liquid level in the chute 2 becomes to low, the
valve 14 will
throttle down, and vice versa. The liquid level in the level tank is in its
turn controlled
by a valve 15 in the conduit 13 between the level tank 11 and the return
liquid flow S.
The recirculation flow 4 is in reality controlled by the existence of a
screening device in
the high pressure feeder 3. When the screening device becomes plugged, the
first pump
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7 does not get any fluid to pump and thus the flow is intenupted.
Figure 2 represents a feeding system according to a preferred embodiment of
the inven-
tion. The equipment that is the same as in figure 1 has been denoted with the
same
reference numbers. The high pressure feeder 3 is, according to the invention;
in open
connection, with respect to both liquid and chips, with the recirculation flow
4', when
any of the pockets of the high pressure feeder is in a location which
corresponds to an
outlet for said recirculation flow 4', that is at the second opening 3b' in
the housing.
This means that the second opening 3b' lacks any form of screening device
which
would be able to, at any extent, prevent the chips from entering the
recirculation flow 4'.
Since there, due to the lack of a screening device, always is liquid
communication be-
tween the suction side of the first pump 7 and the chute 2, the liquid level
in the chute
can be controlled by a chute liquid level control valve 14' in connection with
the second
pump 12', or by controlling the rotary speed of the second pump 12', whereby
no level
tank is needed. The in-line drainer is not needed either, since its only
function was to
prevent chips from entering the level tank where it would accumulate. This
means that
the recirculation flow 4' can be led directly back to the chute 2. This flow
is regulated,
by a valve 17 or by controlling the rotary speed of the first pump 7', against
the flow of
chips that is entering the feeding system, which chip flow is measured by a
measuring
device, for example a so called chip meter screw 18. Such a regulation of the
recircula-
tion flow 4' and thus the first pump 7 is probably needed according to the
invention,
since the first pump 7 is not controlled by any screening device in the high
pressure
feeder 3. If the recirculation flow 4' is not controlled at all, it will
probably lead to that
an excess amount of chips will enter the recirculation flow 4'.
A tramp material catcher 20 is arranged close to the inlet of the first pump
7. The design
of the tramp material catcher 20 is shown in detail in figures 4 and 5. The
pump 7 is
capable of maintaining a pumping action even if large amount of chips will
enter the
recirculation flow 4'. However, tramp material in form of bolts, nuts, tools
and large
stones needs to be separated in order not to destroy the pump 7. The tramp
material
catcher 20 is therefore designed with a pocket 24 in the lower part of the
pump inlet. In
order to improve separation of tramp material is the central part of the pump
whee122
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extended, forming a boss 23 member. The boss member should preferably extend
to
such an extent that it could create a swirling motion in the flow 4' above the
tramp
material pocket 24. Tramp material is thus separated by centrifugal forces in
the flow 4.
The pocket 24 could be retrieved via an inspection cover 21, enabling manual
extraction
of tramp material collected in said pocket. Black liquor, BL, could also be
used in order
to flush the pocket clean of any chips. The black liquor supply is controlled
via valve
HS. The valve HS is preferably opened automatically upon start of the system.
The
addition of black liquor will improve establishment of a stable flow in the
system. Once
the flow is established after a start-up, then the valve HS is closed. The
valve HS could
also be opened automatically if the rotating pockets of the high-pressure
feeder are
stuck. If such a malfunction occurs would the low pressure feeder I be shut
off, and if
black liquor were introduced in front of the pump 7 would the chip chute 2 and
recirculation flow 4 eventually be drained from chips. This would avoid
further pumps
and valves from become stuck. The gradual decrease of chip content in the chip
chute 2
and the recirculation flow 4, would also increase the likelihood for the high
pressure
feeder to assume its proper function.
A branch flow 13' is, via the pump 12' led directly from the recirculation
flow 4 to the
return liquid flow 5 and is controlled, according to the above mentioned, by
the chute
liquid level control valve 14' or by controlling the rotary speed of the
second pump 12'.
By this atrangement, the first pump 7 and the second pump 12' are coupled in
series,
whereby one standard pump is sufficient as the second pump 12'.
Figure 3 represents a feeding system according to an alternative embodiment of
the
invention. In this case, the recirculation flow 4" (in this embodiment called
a first
recirculation flow) is led to the return liquid flow 5 from the treatment
vessel and there-
by to the high-pressure liquid inlet side of the high pressure feeder 3. By
this arrange-
ment, one pump can be saved, as compared to the embodiment of figure 1, but
instead
the first pump 7" needs to be a high-pressure pump, which of course is not
preferred A
second recirculation flow 19 is led from the return liquid flow 5 to the chute
2 for
adjustment of the liquid/wood ratio in the chute, and is controlled by the
flow control
valve 17" against the flow of chips that is entering the feeding system. The
first recir-
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culation flow 4" is regulated, by a chute liquid level control valve 14" or by
controlling
the rotary speed of the first pump 7", against the liquid level in the chute
2. A similar
tramp material catcher 20 is located in the inlet of the pump 7".
5 By removing the restrictions on the suction side of the first pump 7 a
circulation in a
closed vessel is achieved, i.e. the first pump can no longer build up an
excess pressure
in the chute or steaming vessel. Accordingly the safety pressure switches can
be set at
comfortable levels, e.g. at pressures above 3 bars, preferably above 5 bars,
which allows
for stable operation minimising costly unwanted interruptions of the
production.
In figure 6 is a typical valve member 30 for a high-pressure feeder shown. The
rotating
valve member 30 includes at least one, most often four, diametrically through
going
pockets 31, 32. When the pocket 31 is exposed to the inlet 42a in the
stationary housing
40, then this pocket is filled with chips from the chute 2. During the
subsequent rotation
of the valve member 30 is the pocket 31 closed, and the pocket 32 exposed to
the inlet
42b. When each pocket have rotated some 90 from the filling position, then
the pocket
is flushed with liquor and the chips leaves each pocket in the high pressure
feeder via
outlets 41 a and 41b respectively. In the prior art designs have always a
strainer plated
50 been used in the outlet 3b' from each pocket, preventing chips from passing
through
the high-pressure feeder. The function of a high-pressure feeder is explained
in more
detail for example in US 5,236,285, US 5,236,286 or US 4,372,711.
The invention is not restricted by the description of the above-mentioned
embodiments,
but can be varied within the scope of the clainas. The skilled man will thus,
for example,
realise that various equipment may be included in the recirculation flow
although it is
preferred that a minimum of equipment and especially no level tank is present.
Furthermore it is evident for the skilled man that the flow from the second
purnp may
discharge into many other positions than shown above, e.g. directly into an
impregnation vessel, directly into a digester, the pressure side (5) of the
top circulation
pump (third fluid pump 16) and also on the high pressure side (6) of the HP-
feeder. It is
also evident that speed control of the pumps may be used instead of
conventional drive
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and a throttling valve.
In order to keep an open connection between the high-pressure feeder and the
recirculation flow, with respect to both liquid and comminuted cellulose
containing
material, the relevant opening in the high-pressure feeder should be designed
to prevent
chips from stacking up. Preferably, there are no obstructions at all in the
opening, but
the skilled man will realise that a screening device with slots wider than for
instance 25-
30 mm will be included in the scope of the invention, since such a screening
device will
allow the chips to pass through the opening.
Also, it is conceivable that the relevant opening in the high pressure feeder
can be
equipped with a screening device with slots narrow enough to make the chips
stack up,
if the screening device is movable and withdrawn from the opening for a major
part of
the time. If necessary, that is if the operating conditions otherwise result
in an excess of
chips entering the recirculation flow, such a screening device could be
introduced in the
opening just before a pocket is filled up with chips and be withdrawn
immediately
afterwards, when the pocket proceeds to its high pressure position.
The skilled man will also realise that instead of controlling certain flows by
regulating
them against the flow of chips that is entering the feeding system, as
described above,
the flows can be regulated against the rotary speed of the pockets in the high
pressure
feeder. The device that measures the flow of chips that enters the feeding
system may
also control the number of revolutions of the pump in the flow to be
controlled, instead
of controlling a valve in the flow.
