Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02296289 2000-O1-19
Method and apparatus for separating soap
The present invention relates to a method and apparatus for separating soap
from liquor in a filtrate tank. The method and apparatus according to the
invention are particularly well suited for treating soap-containing filtrates
from digester houses, bleach plants and washer rooms in the chemical wood
processing industry.
Soap appears primarily in the liquor leaving either the digester or the
subsequent washing equipment. Such liquor is called weak black liquor and,
besides cooking chemicals, it also contains various organic and inorganic
materials dissolved in liquor during pulping. It has been estimated that these
materials represent about 12 to 18 percent of the black liquor by weight.
Part of these materials precipitate on the surface of the liquor, in the form
of
soap, the maximum density whereof is only a little lower than that of the
liquor. The soap density ranges widely, from 0.98 kg/l down to 0.005 kg/l.
According to a reference, a constituent is referred to as soap if its density
is
higher than 0.1 kg/l and as foam if its density is lower than that. When the
soap density is at the maximum, there is not much difference between the
soap and liquor densities, whereby the major portion of soap lumps, lumps
being the form of soap when it is at its heaviest, remains below the liquor
surface.
From the digester or the subsequent washing equipment, weak black liquor
is discharged into a filtrate tank. Filtrate tanks are so large, their
diameters
being of the order of 8 to 16 m and volumes of about 400 to 4000 m3, that
the retention time of black liquor in the tank is relatively long. This
provides
the soap with an excellent opportunity to separate from the liquor and to
accumulate on the surface thereof. One purpose of the large tank is to serve
as a buffer in production fluctuations. Another purpose is to separate both
the gas and the foaming soap from the liquor in order to be able to
recirculate a substantially gasfree and soapfree black liquor back to the
process, either through chemical recovery or in some other appropriate
manner. In some cases, soap accumulates on the liquor surface, forming a
layer, which may be even dozens of centimeters thick. Depending on mainly
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the density of soap, but also on the composition (on how much black liquor
is contained in the soap), the form and behavior of soap may vary
considerably. If soap is relatively free of black liquor, the soap material
becomes viscous, sticky, sometimes lumpy and difficult to move. Such
material easily clogs pipes, valves and other soap-treating devices. On the
one hand, a soapy foam containing plenty of gas requires a large storage
volume in order to ensure that foam cannot escape through ventilation ducts
out of the filtrate tank. On the other hand, if soap is stored in the filtrate
tank for too long, the soap density may increase to such an extent, when
liquor separates therefrom, that the soap will sink into the liquor mass and
be carried therewith to the evaporators of the recovery plant. Presence of
soap would substantially disrupt the operation of the evaporation plant. For
these reasons, it is important that soap be discharged from the tank
according as it accumulates on the liquor surface.
As an example of soap separation equipment of prior art, a system based on
overflowing deserves to be mentioned. In that arrangement, the tank was
overflowed, by allowing a soap fraction to flow over the edges of a special
chute or the tank, directly into a sewer. It is clear that this is not
possible
today, for both environmental and economical reasons. On the one hand,
soap must not end up in water systems and, on the other hand, it is
possible to prepare, e.g., tall-oil from the soap fraction separated in the
tank. In slightly more advanced soap separation systems, the wall of the
filtrate tank is arranged with outlets. The idea is to discharge the soap
fraction through these outlets and then pump it for further treatment. The
great variation of the filtrate level in the filtrate tank is, however,
problematic, because it makes it impossible for the operator to know the
soap level in the tank, even though the tank were provided with outlets at
several heights. This is further complicated by the fact that one never
knows, outside the tank, how thick the layer of soap is on the black liquor
surface. In other words, it may happen that practically mere liquor is
pumped out of the tank for further treatment of soap.
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In the worst case, soap sinks into the black liquor, whereby it may end up,
e.g., in the evaporation plant and more or less disrupt its operation,
depending on the type of the evaporation plant. For example, in such
evaporator arrangements in which the material to be evaporated flows along
the inner surface of evaporator tubes, soap lumps may clog the entire
evaporator tube.
Another way of separating soap is to arrange several filtrate tanks in series,
one of which is allowed to overflow so that the soap fraction is separated
by the overflow and the black liquor is pumped further to another tank. This
arrangement is expensive, though, because of both the tanks and the
instrumentation needed. In this arrangement, the main role is played by the
level adjustment of the first, so-called soap tank, which level is to be
optimal in view of the overflow. In other words, the overflow has to be
adjusted in such a way that as little as possible of black liquor is entrained
with the soap, yet so, that the soap is efficiently separated in the soap
tank.
Prior art also involves many "freely" floating soap separating systems, in
which ballast is used in order to cause a soap separation funnel or
equivalent to partially submerge in the liquor so that light soap will flow
into
a recovery pipe from one end thereof. The pipe has to be sufficiently large,
though, in order to prevent soap from clogging it. The pipe itself constitutes
a problem because it also serves as a float, in most cases even more
efficiently than the float itself. As soap then flows into the pipe at a
varying
speed, which is characteristic of it, the weight of the pipe changes and the
pipe starts to sink deeper and deeper into the soap/liquor. To avoid this, the
funnel should either be arranged so high up in the soap layer that heavy
soap could not enter the funnel, which would result in the soap ending up to
the recovery system with the consequences described above, or the
position of the funnel should be monitored and ballast adjusted in
accordance with the position of the funnel, so that the funnel would always
be in a certain depth in relation to the liquor level. Practically, this is
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however impossible because there is no way of accurately defining the
position of the liquor level. The weight of the soap in the funnel or outlet
duct, which directly and, in any case, adversely affects the floating depth of
the soap separating funnel, is not reckoned with in any of these freely
floating soap separating arrangements.
A soap skimmer is disclosed in, e.g., US patent publication 5,137,643. It
separates soap from the surface of the soap tank. The operation is based on
a soap skimmer being arranged to rest on at least two floats on the surface
of liquor in a filtrate tank. One of the floats, i.e., a so-called top float
is
arranged to float at least partially submerged in a layer of soap and the
other, i.e., a so-called bottom float is arranged to float either solely in
soap
or partially submerged in black liquor. The top and bottom floats are
connected with a hanger plate, which maintains a predetermined distance
therebetween. A soap inlet opening is arranged in the bottom float, said
inlet opening leading into a soap outlet pipe. The outlet pipe is provided
with
a vacuum effect to draw soap from the surface of black liquor into the pipe.
In other words, since the bottom float with the soap outlet is submerged in
soap, soap which is drawn by the vacuum effect flows continuously
through the outlet opening into the outlet pipe. Furthermore, the above-
described floating assembly is hinged to a swing arm, one end of the swing
arm being hinged to the bottom of the tank. The swing arm is used for
controlling the movement of the floating assembly as the liquor level varies
in the tank.
A drawback of all the prior art means described above is that in all of them
the outlet funnel, outlet chute or outlet opening stays in place on the
surface of black liquor; in some cases it moves transversely only if the level
of black liquor changes. Consequently, there is no other way for the soap to
flow into the funnel or, more broadly, to the outlet, than from the vicinity
of
the outlet opening. The longer the distance from the outlet opening, the
more certain it is that soap remains for a long time in the tank. This
involves
a risk that the soap will sink and end up in the recovery system.
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Another drawback already discussed above is free buoyancy of the soap
separating means, whereby the weight of soap in the outlet funnel or outlet
pipe has an influence on the floating depth of the funnel as well as on its
capacity to remove soap from the tank.
The method and apparatus according to the present invention eliminates,
e.g., the above-described problem so that the soap outlet opening or
equivalent and the soap which is floating on the surface of black liquor are
brought into a motion relative to each other, whereby it is possible to
reliably remove the soap from the entire cross-sectional area of the tank
without any risk of the soap remaining for too long in the tank. Furthermore,
as no floating soap separation means is used in the present invention,
uneven flow of soap into the apparatus does not cause any problems.
It is also characteristic of a method and apparatus according to the
invention that soap and the soap outlet opening are brought into a motion
relative to each other so that the surface layer of the soap is mixed as
little
as possible. This is achieved by effecting the motion without substantially
touching the surface layer of the soap.
Other features characteristic of the method and apparatus according to the
invention are shown in the appended claims.
The method and apparatus according to the present invention are described
more in detail in the following, with reference to the accompanying
drawings, of which
Fig. 1 is an illustration of a soap separation apparatus of prior art,
Fig. 2 is an illustration of a soap separation assembly of prior art,
Fig. 3 is a schematic illustration of a soap separation assembly in
accordance with a preferred embodiment of the invention,
Fig. 4 is a schematic illustration of a soap separation assembly in
accordance with a second preferred embodiment of the invention,
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Fig. 5 is a schematic illustration of an alternative arrangement for a
detail of a preferred embodiment shown in Fig. 4,
Figs. 6a and 6b illustrate soap skimmers in accordance with a third and
fourth preferred embodiments of the invention, taken as a sectional view A-
A of Fig. 3,
Fig. 7 illustrates a structural arrangement for a lip edge of a soap skimmer
in
accordance with a fifth embodiment of the invention,
Figs. 8a and 8b illustrate a novel type of a filtrate tank in accordance with
a
sixth preferred embodiment of the invention, and
Figs. 9a and 9b illustrate a filtrate tank in accordance with a seventh
preferred embodiment of the invention.
An apparatus for separating soap according to prior art, as shown in Fig. 1
and disclosed in US patent publication 5,137,643 comprises a floating
assembly 100, which comprises a floater chamber 101 with a hollow inner
portion 102. The floating assembly further comprises a soap pipe 104,
which is positioned centrally inside the floater chamber 102 to allow soap to
flow along a flat funnel path 105 into a soap pipe 104 and therethrough
further to the soap outlet 107. The floating assembly further comprises
necessary means (pipe 108) for filling the hollow inner portion of the floater
chamber 101 with water and means (conduit 110) for draining water out of
it. Furthermore, the assembly 100 comprises a ballast container 1 1 1, which
is filled with, e.g., lead pellets or equivalent.
The floating assembly 100 is designed to float on the surface 103 of black
liquor. The floating depth of the assembly 100 is adjustable by increasing or
decreasing the amount of water inside the floater chamber. The soap level
1 14 has to be slightly higher than the edge of the funnel portion 105 so as
to allow soap to flow into the pipe 104. A problem with the assembly
according to Fig. 1 is, however, that soap flow into the soap pipe is allowed
from the top portion of the assembly, only. This means that the soap
floating directly on the surface of black liquor and partially even submerged
therein is not allowed to flow out of the tank at all, in this assembly.
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Another problem is that the soap separating arrangement in accordance with
Fig. 1 is capable of removing soap only from the vicinity of the outer edges
of the funnel 105. This is because the capability of soap of moving on the
surface of black liquor towards the separating arrangement is very limited,
especially, if the heaviest soap fraction is not at all removed.
Fig. 2 illustrates a prior art filtrate tank 10, with at least one so-called
deaeration cylinder 20 arranged therein. The cylinder 20 may be arranged
centrally inside the tank 10, but it is also possible that it is not
positioned on
the axis of the tank 10, but more or less aside thereof. If more than one
cylinder 20 is involved, they are arranged in a scattered manner, apart from
each other in the tank 10. In the arrangement of Fig. 2, the deaeration
cylinder 20 is positioned on the centerline of the tank 10. The cylinder 20
comprises an inner pipe 22, whereinto soap-containing filtrate is passed
through a pipe 25 (also referred to as a "means for introducing liquor into
the filtrate tank). The inner pipe 22 is designed to bring the filtrate into a
circular motion in order that air and other gases would be separated from
the filtrate and be discharged through the top portion of the inner pipe 22 to
the gas treatment. The deaeration cylinder 20 also comprises a so-called
outer pipe 24 arranged outside the inner pipe 22, said outer pipe being in
contact with the inner space of the inner pipe 22 through openings 26
arranged at the bottom end of the inner pipe 22. The filtrate, wherefrom
air/gas has been removed, flows through these openings into the outer pipe
24, over the top edge whereof the filtrate flows to a soap separating space
14 (tank space); i.e., to the space 14 between the outer pipe 24 and the
jacket or wall 12 of the tank 10. The soap separating space 14 is provided
with a soap separation assembly 30. According to prior art, the assembly
may be either completely stationary or rest on floaters. The soap
separation assembly 30 comprises a soap separating chute or funnel 32,
wherefrom a flow path 34 leads to a discharging pump 36, which
30 discharges soap fraction. The filtrate wherefrom soap has been separated is
pumped for further treatment from the space 14 with a pump 16.
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A solution is provided, e.g., to the problems set forth above in this
description, by a soap separating assembly 40 arranged inside a filtrate tank
in accordance with a preferred embodiment of the invention, shown
schematically in Fig. 3. In accordance with this embodiment, said assembly
5 is preferably arranged centrally in the filtrate tank 10. The soap
separating
assembly 40 preferably comprises a substantially vertical pipe 42 fixed to
the bottom of the tank 10, a pipe 44 departing from said pipe 42 and
leading out of the filtrate tank, and a pump 46 in connection with the pipe
44 outside the tank. The soap separation assembly 40 also comprises a
10 vertically movable pipe 48 arranged in a telescopically sliding manner in
connection with said pipe 42. In connection with said pipe 48 is preferably
arranged a two-sided, symmetrical chute 50 (also referred to as a "soap
separating assembly) extending substantially to the wall of the tank 10 or in
the vicinity thereof. The bottom of the chute 50 is inclined, as shown in Fig.
3, leading the soap flowed into the chute 50 to the movable pipe 48, and
further via the stationary pipe 42 and the pipe 44 out of the tank to the
pump 46, which then pumps the soap-containing fraction for further
treatment.
Fig. 3 also illustrates how the upper end of the movable pipe 48 is, in this
embodiment, connected with drive equipment 52, which moves both the
pipe 48 and the chute attached thereto. In accordance with a preferred
embodiment of the invention, the pipe 48 and the chute 50 are moved
vertically, for example, with a pneumatic cylinder. The purpose of this is to
follow the filtrate surface by means of the chute in such a manner that the
level of the chute 50 relative to the filtrate surface FS is always the same.
The filtrate surface FS is monitored in ways known per se, for example, by
either gamma detectors, hydrostatic level detectors or with, e.g., a separate
float arrangement. In practice, all above-mentioned measuring modes give a
measuring value, which is proportional to the filtrate surface FS. However,
when the assembly 40 is taken into use, the control system of the drive
equipment 52 has to be separately calibrated so that the filtrate surface FS
is positioned in an optimal status relative to the chute 50. After
calibration,
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the variations of the filtrate surface indicated by the measuring system can
be directly adjusted to be changes in the elevation of the chute. Another
way of adjusting the chute elevation could be adjustment based on the flow
of pump 46. This is based on the assumption that the flow entering the tank
is constant in the normal operating state of the process. Thereby, by
maintaining the flow of the pump 46 constant at a certain rating, also the
filtrate surface stays constant. If the flow of the pump 46 has to be
changed, the change would control also the actuator changing the chute
elevation.
Fig. 3 also shows how the drive equipment 52 may also rotate the soap
separating assembly 40, for example, by means of a hydraulic electric
motor. Such arrangement is particularly suitable if pipes 42 and 48 are
centrally arranged in the filtrate tank 10. In this case, a rotatable chute
arranged on the diameter of the tank is capable of covering the cross-
sectional area of the entire tank.
It has to be also noted that the present invention is not limited to either of
the above-described arrangements; in other words, the present invention
makes it possible to use both the level adjustment of the chute and rotation
of the chute, only one of these alternatives, or in a case neither of these
alternatives.
Fig. 3 also shows how soap accumulates on the filtrate surface FS in the
tank 10. The top surface SS is essentially higher than the filtrate surface
FS. The soap density changes, i.e., the soap becomes lighter when
approaching the surface SS. Although the introduction of liquor into the
tank is not shown in Fig. 3, it may be effected either by applying the prior
art way according to Fig. 2 or in some other appropriate manner. A novel
mode is disclosed in Figs. 8a and 8b.
Fig. 4 illustrates an arrangement in accordance with another preferred
embodiment. This arrangement is primarily applicable to a situation in which
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it is desirable to provide a deaeration cylinder 20 of an existing tank 10
with
a soap separating assembly 40 in accordance with the invention. In this
embodiment, the soap separating assembly 40 comprises a chute 50, a
soap outlet pipe 54 with a pump 46 and drive equipment 52. The drive
equipment 52 (also referred to as "means for moving (or changing the
elevation of) the soap separating assembly") is arranged at the upper end of
the inner pipe 22 of the deaeration cylinder in such a manner that the pipe
22 is provided with a sliding piece 56 which is driven, for example, by a
pneumatic cylinder (not disclosed) fixed to the cover of the tank 10 or
equivalent structures at the upper end of the tank. The sliding piece 56 has
a protruding support 58, and the chute 50 is suspended thereon. Thus, the
chute 50 is vertically movable by means of the sliding piece. If desired, the
chute may be supported relative to the wall 12 of the tank 10 as well, for
example, by means of guide bars, slide bars, or some suitable roller
arrangements. The drive equipment moving the chute 50 vertically may be
positioned also in the middle of the chute, whereby the chute 50 can be
supported at both ends thereof with appropriate sliding arrangements.
Besides half of the diameter of tank 10, as shown in Fig. 4, the chute may
also extend to the entire diameter thereof, in this embodiment, whereby the
number of outlet pipes 54 needed is correspondingly two. For example,
flexible steel hoses available today are applicable to be used as an outlet
pipe, for compensating the variations of the filtrate level. Therefore, the
pipe
54 is also referred to as a "flexible pipe portion."
Fig. 5 illustrates another alternative for arranging soap removal from the
chute 50. In this embodiment, the outlet pipe 54 is composed of two parts:
a stationary, substantially vertical portion 54" connected, for example, to
the bottom of the tank 10, and a portion 54' which is vertically movable in
relation to the portion 54" and connected to the chute 50. This embodiment
makes it possible to move the chute 50 in the vertical direction in the same
way as the embodiment illustrated in Fig. 4. The pipes or portions 42, 4$,
54' and 54" are also referred to as "pipe portions".
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In the above Figures, the bottom of the chute is descending towards its
middle area because the soap outlet pipe is arranged in the middle area of
the chute. In many cases, this is the most optimal location for the outlet
pipe, but in some cases, for structural or other reasons, the outlet pipe has
to be located nearer to one end of the chute or even at one end thereof.
Fig. 6a is a slightly more detailed illustration of the structure of a chute
50
in accordance with a preferred embodiment. Chute 50 comprises a back
plate 502, which is substantially vertical in the embodiment of Fig. 6a, an
inclined bottom plate 504 and a front plate 506 and a lip or lip portion 508
arranged at the front edge of the front plate 506. Preferred structural
arrangements for the chute 50 are, besides the chute being inclined towards
the outlet pipe 54, also the back plate being 502 sufficiently high so that it
extends above the surface SS of soap foam (shown in Fig. 3) in practically
all running situations of the tank. However, especially when the draw
between the chute and soap is very small, it is even possible to make the
chute symmetrical relative to its longitudinal axis, i.e., soap is allowed to
flow in from either side to the chute. It is also advantageous that the
bottom of the chute 50 is composed of at least two levels (504 and 506) as
shown in Fig. 6a or that the bottom of the chute is curved (Fig. 6b). This is
useful in order that soap could not stick to any point of the chute and
thereby slow down the flow of soap towards the outlet pipe 54. The
subassembly of the pump 46 and the pipes 42, 44, 48 and 54 present an
embodiment of what is also referred to as "means for discharging the soap
fraction from tank."
Both Figs. 6a, 6b and 3 and 4 illustrate a structural arrangement for the lip
508 of the chute 50 in accordance with a preferred embodiment. The end of
the lip 508 is either wavelike or bent into a zigzag shape. The purpose of
this bending is to ensure that the soap lumps on the surface of black liquor
end up into the chute 50. A limited flow area on the level of black liquor
surface at the lip of the chute 50 brings about a faster flow, which more
efficiently draws the soap lumps into the chute 50. At the same time, also
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small directional faults of the chute are compensated. In other words, if the
lip edge of the chute is not completely horizontal, the chute will function
any way. If it is desirable to emphasize this aspect, it is possible to
convert
the serration of the chute lip such that the lip is provided with deep, e.g.,
rectangular openings, which naturally allow for bigger directional faults than
a relatively low-gradient serration.
As for the location of the chute 50 relative to the black liquor and the soap
layer floating thereon, it is advantageous to arrange the chute lip 508 in
such a manner that it is substantially on the same level with the black liquor
surface FS (Fig. 3). If the lip 508 is wavelike or serrated as shown in Fig.
6,
it is advantageous that the black liquor surface FS is substantially on the
mid-level of the waves. The lip portion 508 may also be arranged with a
plurality of adjacent flow paths and flow obstacles therebetween. In other
words, the lip portion is so formed that soap lumps have enough space to
find their way into the chute 50 along with the flow, from the bottom points
of the waves along said flow paths. As mentioned above, at least the
considerable draw between the soap layer and the chute should be taken
into account when dimensioning the back plate of the chute 50, so that the
back plate will extend at least substantially to the level of the soap foam
top
surface SS in order to prevent leaking over the chute 50. Adjustment of the
actual elevation of the chute 50 relative to the filtrate surface FS has been
described above. A characteristic of the invention is that the elevation of
the chute in the tank 10 is adjusted in a forced manner so that potential and
even probable variations in the weight of the outlet system are prevented
from influencing on the elevation of the chute.
Figs. 8a and 8b illustrate a novel type of a filtrate tank which is
particularly
well suited for operation with a soap separation assembly or apparatus 40
illustrated in Fig. 3. In the embodiment of Figs. 8a and 8b the deaeration
cylinder 20 has been transferred completely outside of an essentially
cylindrical filtrate tank 10. The structure and operation of the deaeration
cylinder 20 itself is the same as in prior art arrangements. Only the location
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is new. However, the location described gives an opportunity to an
especially efficient soap separation. This is because now it is possible to
use
a rotatable chute system illustrated in Fig. 3 as one embodiment.
Furthermore, the arrangement shown in Fig. 8 is especially energy-effective
in view of the arrangements shown in subsequent Figs. 9a and 9b for
circulating the surface layer of the filtrate from the tank.
As stated hereinabove, it is characteristic of the present invention that a
draw is developed between the chute and the soap floating on the surface
of black liquor, or more broadly, filtrate. This may be carried out either by
moving the chute relative to the black liquor in the filtrate tank (shown in
Fig. 3) or so that at least the surface layer of the black liquor in the tank
10
is brought into a movement relative to the chute. In both cases, the
direction of movement is such that the lip portion 508 of the chute 50
receives the soap from the surface of the black liquor and tends to skim the
soap inside the lip portion. It is therefore also referred to in general terms
as
"means for skimming the soap fraction."
Figs. 9a and 9b show how the above-described circular motion in the tank is
brought about. In the embodiment of Fig. 9a, the tank is provided with at
least one circulating means of the type of a propeller agitator 70 or
equivalent, by which at least the surface layer of black liquor is made to
rotate in the tank 10. In Fig. 9b, it is shown how the tank is provided with
another circulating means of the type of at least one nozzle 72, which is fed
with filtrate taken from the tank 10 by means of piping 74 and a pump 76.
The circulating means 70 and 72 are also referred to as "means for forming
a reciprocal horizontal draw." The number of agitators and nozzles is
determined by both the size and shape of the tank and by the other
dimensions of the agitators and nozzles. Also the vertical location of both
the agitators and the nozzles is influenced by several factors. Firstly, the
filtrate flow discharged from the circulation device must not mix the soap-
containing surface layer of the filtrate, but it has to stay as stabile as
possible, except for circulation in the tank. Secondly, partly for the above
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reason, agitators or nozzles have to be located clearly below the filtrate
surface in all normal running situations of the tank. In other words, even
when the level in the tank is at its lowest, the flow from the circulation
device must not mix the soap layer. In theory, it would naturally be possible
to arrange agitators or nozzles at various levels in the tank and only use the
circulation equipment located on an appropriate level. Most probably,
however, economical factors put a limit to such a solution. A remedy by
which the discharge flow from the circulating equipment could be prevented
from mixing the soap layer is a guide plate arranged on the tank wall above
the agitators or nozzles, for directing the flow to be more precisely
horizontal.
A way of utilizing a circulating flow brought about by agitators or nozzles is
to arrange a guide plate or partition wall on the surface layer of the liquor,
such guide plate or partition wall being spiral in shape and adjustable also
vertically if desired. The guide plate preferably extends spirally from the
tank
wall as far the soap skimming device. Said plate may also be vertically
adjustable if the liquor level varies greatly in the tank. The purpose of the
plate is to enhance the tendency of light soap circulating in the tank to
accumulate in the middle of the tank by leading the soap flow circulating on
the liquor surface towards the middle of the tank or alternatively to a soap
discharge equipment in general. The soap skimming device may be merely a
substantially vertical pipe having an inlet opening for receiving soap into
the
pipe. Preferably the pipe has two portions so that the lower portion of the
pipe is stationary and the upper portion telescopically sliding relative to
the
lower pipe portion. When the soap inlet opening is arranged in the upper
portion of the pipe, the elevation of the upper portion of the pipe may be so
adjusted that the soap sliding along the spiral guide plate towards the
middle of the tank may be collected in the pipe. In accordance with a
preferred embodiment of the invention, the lower end of the vertical pipe is
provided with a cone, for growing the flow velocity of the soap in order to
ease further pumping.
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A still another way of generating a draw between the soap layer and the
soap separation chute is to arrange the tank with at least one doctor
extending essentially to the soap layer or even therethrough. Such doctor is
rotated in the tank, for example, in the same way as the chute described in
some embodiments above. In some circumstances, this arrangement can be
recommended because, at least in large tanks, rotation of the chute may
prove to be technically difficult, or at least expensive. A way of
dimensioning the doctor is to make the doctor extend more or less through
the soap layer so that the doctors wipe relatively near the chute when
moving over it. Correspondingly, the chute has to be deeper so that the
doctor, when passing by the chute, does not wipe soap from the chute.
Still some structural arrangements deserve to be mentioned. These may be
used in some special applications. For example, if it is desired that a chute,
especially a one-sided chute extending from the centerline of the tank to the
wall thereof, moves horizontally in the filtrate tank, it is possible to
support
the chute against the tank wall, e.g., by a rail. This is done so that an
outer
edge of the chute is provided with a wheel, which rolls on the rail, thereby
making the rotation of the chute in the tank possible. Another alternative is
to suspend the outer edge of the chute on the tank wall, for example, in
connection with a preferably annular rail arranged on rollers. In this case
the
rail itself is moved relative to the tank wall. The drive equipment of the
chute, preferably an electric motor may be located in connection with the
rail, whereby the motor itself stays in place and rotates the rail by means
of,
e.g., cogging of the rail. Alternatively, the drive equipment may be located
at the outer edge of the chute, whereby the motor, by means of, e.g.,
cogging of the rail moves the chute relative to the rail which rail is
stationary in relation to the tank. It is also possible to arrange the
rotation of
the chute through the center pipe of the tank. In that case, the rotating
motor is either arranged to rotate the chute by telescopic pipes or it is
suspended on the upper part of the tank, either on the cover of the tank or
other structures at the upper part of the tank wherefrom the force rotating
the chute is directed down to the chute via a shaft mounted centrally
CA 02296289 2000-O1-19
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relative to the tank (disclosed in Fig. 3). All the arrangements described
above are based on the cylindrical shape of the tank, which is the most
common shape today. Furthermore, if the chute is not supported on floats,
but the chute elevation is regulated electrically, the elevation of the
support
of the chute against the outer wall of the tank has to be adjustable as well.
This is naturally possible, e.g., by arranging the structures supporting the
outer end of the chute against vertical guide bars, whereby, at the same
time when the dimension of the telescopic center pipe is changed, the
elevation of the structures supporting the chute on the tank wall is changed
correspondingly.
Although we have presented a large number of arrangements based on
adjusting the elevation of the soap separating chute relative to the level of
the filtrate in the tank, the present invention is also well applicable to
such
arrangements in which the soap level is kept constant in the tank. In such
arrangements, it would be possible to adjust the level of the filtrate to an
optimum, relative to the chute. Vertical movability of the chute would be in
that case unnecessary. The substantially constant level of the filtrate can be
brought about, e.g., by defining the amount of material entering the tank
and by adjusting the amount of material being discharged from the tank to
be substantially the same. Soap foam makes this adjustment nearly
impossible, though. In any event, the primary basis of the present invention,
i.e., to bring about a draw between the soap and the chute is still essential
to the function of the apparatus.
In the tests performed by us, we have noted some things, which have given
reason to review the forming mechanism of foam again. We have noted,
when observing the flow of foam at different points, that there is a high,
bubbling foam column in the filtrate tank at the deaeration cylinder,
although the flow entering the cylinder seems to be of fairly uniform quality
and contain only a little of air. An explanation for this phenomenon is that
the deaeration cylinder does not separate air from the filtrate but mixes it
therewith. Depending on the dimensions of the cylinder, it is possible that a
CA 02296289 2000-O1-19
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spiral flow forming in the cylinder is either so strong that it draws air
inside
it or so weak that no actual field of centrifugal force is formed, but the
filtrate flows into the inner pipe of the cylinder and forms a heavily
turbulent
flow field in which air mixes with the filtrate. In both cases, the filtrate
contains more air than what it did before entering the deaeration cylinder.
As a solution to this problem, it is suggested that the deaeration cylinder
should be designed more carefully. It calls for re-dimensioning of the
deaeration cylinder, which might lead to the pressurizing of the cylinder so
that filtrate would be fed to the cylinder at a certain pressure and gas would
be drawn therefrom by using an appropriate negative pressure arrangement.
It is also possible to omit the inner deaeration cylinder altogether and bring
the filtrate directly into the outer cylinder. A feeding device of a new type
could be, e.g., a pipe with a relatively large diameter, the pipe running
substantially on the level of the filtrate surface and having the upper
surface
provided with openings for discharging gas into the air space of the tank
and the lower surface with larger openings for discharging filtrate among the
filtrate already existing in the tank. If necessary, guide plates could be
arranged below the filtrate discharge openings for directing the filtrate flow
to the filtrate surface, so that soap would have enough time to separate
from the filtrate and that the "fresh" filtrate wherefrom soap has not yet
separated would remain on the surface of the filtrate.
As can be seen from the above description, an entirely novel structure of
filtrate tanks has been developed, which structure is capable to eliminate all
noted drawbacks of prior art apparatus.
