Note: Descriptions are shown in the official language in which they were submitted.
116137~
,
Degassing o~ liquid mixtures
Technical Field
This invention relates to a method of degassing a
mixture of a liquid first material, e.g. white water, and
at least one second material, e.g. paper-making stock or
stuff, comprising mixing the first and second materials
and pumping the mixture so formed into an aeration tank
via mixture cleaning means, e.g. a vortex cleaner. The
invention also relates to a system for performing such a
method.
Background Art
During stock preparation in paper manufacture certain
paper qualities require that the stock/white water mixture is
degassed in an aeration or degassing tank before it is
supplied to a headbox via a headbox pump. The degassing
takes place by maintaining a negative or sub-atmospheric
pressure in the aeration tank by the connection o~ a vacuum
pump thereto, the negative pressure corresponding to the
boiling point of the stock at the stock temperature in
; question and enabling air and other gases to be removed
from the stock/l~hite water mixture. The mixture level in
the tank is normally controlled by the provision of a spill-
way in the tank or by the provision of a level regulator
which influences a valve in a return conduit. The negative
pressure maintained in the aeration tank is normally accom-
plished by locating the tank at a high level relative toa mixing pump for mixing the stock or paper pulp with white
water from a wire pit. This relatively high level of the
aeration tank requires long ~upply and return conduits
to be connected to the aeration tank which increases the cost
of erecting the paper making plant~
The difference in pressure between that generated across
the mixing pump and the negative pressure in the aeration tank
must be sufficiently great to overcome the static lifting
height between, for example, the levels of liquid in the wire
71
pit and the aeration tank, and the pressure drop in filters,
condui~s and vortex cleaners between the mixing pump and the
aeration tank. The pressure drops across different types
of vortex cleaners normally varies between 10 and 30 meters
of water gauge (mwg). There are mainly two types of vortex
cleaners, namely, one type which works towards a negative
pressure and one type which works towards an overpressure.
Because of the negative pressure in the aeration tank, all
liquid levels to which the aeration tank delivers flows must
be located about 10 meters below the liquid level of the
aeration tank, which is another reason for the high loca-
tion of the tank. If the pressure drop across the vortex
cleaners is low, which is aimed at, or such vortex cleaners
are required which are designed to function towards an
overpressure, this high location of the aeration tank is
required to prevent stock/white water mixture from the mixing
pump and rejects, i.e. dirty material, from the vortex
cleaners, from flowing into the aeration tank, thus jeopar-
dizing or making impossible the intended unctions
The present invention aims to provide a method of,
or system for, degassing mixtures of first and second mate-
rials in an aeration tank in which the above-mentioned
drawbac~s are substantially eliminated.
Disclosure of the Invention
,
According to one aspect of the invention theire is
broadly claimed herein a method of degassing a mixture of
a liquid first material and at least one second material,
the liquid first material being initially contained in a
container means, comprising the steps of withdrawing the
liquid first material from the container means and mixing
it with each second material to form a mixture, regenera-
tively pumping the mixture through a mixture cleaning means
and into an aeration tank, and maintaining the level of the
mixture in the aeration tank substantially even with the
level of the liquid first material in the container means
.
~161371
The provision of regenerative pumping enebles the
aeration tank to be at a lower level than otherwise would
have been possible. Thus, when the method is applied to
degassing a stock/white water mixture in a paper making
process, it is possible to clean and degas the mixture with
the smallest possible energy consumption, with the shortest
possible length of conduits to and from the aeration tank,
and with minimum costs for erection of apparatus for per-
forming the degassing method. Furthermore, the overflow in
the aeration tank may be eliminated by common control of
the regenerative pumping into and pumping out of the aera-
tion tank. This makes it possible to regulate the level as
well as the negative pressure in the aeration tank.
According to another aspect, the invention as
broadly claimed herein lies in a system for degassing a
mixture of a liquid first material and at least one second
material which incluaes a container means for containing
the liquid irst material, an aeration tank, a mixture
cleaning means and pumping and mixing means for mixing the
liquid first material with the second material to form a
mixture and pump the mixture through the mixture cleaning
means into the aeration tank, the improvement wherein the
pumping and mixing means includes a regenerative pump
located between the mixture cleaning means and the aeration
tank, the regenerative pump acting to slow down the mixture
flow rate through the mixture cleaning means while permitting
a pressure drop thereacross, and wherein the aeration tank,
the mixture cleaning means and the pumping and mixing means
are positioned to be at substantially the same level as the
top of the container means, and such that the level of liquid
f irst material in the container means can be maintained sub-
stantially even with the level of the mixture in the aeration
tank.
According to a further aspect, the invention as
broadly claimed herein lies in a system for degassing a mix-
11~;1371
ture of stock and white water which includes a container
means for containing the white water, an aeration tank, a
mixture cleaning means, a pumping and mixing means and a
headbox pump, *he pumping and mixing means helping to mix
the white water and the stock to form a mixture and pump
the mixture through the mixture cleaning means into the
aeration tank and the headbox pump helping to pump the
degassed mixture out of the aeration tank, the improvement
wherein the pumping and mixing means includes a regenerative
pump located between the mixture cleaning means and the
aeration tank, the regenerative pump acting to slow down
the mixture flow rate through the mixture cleaning means
while permitting a pressure drop thereacross, and wherein
the aeration tank, the mixture cleaning means and the pumping
and mixing means are positioned to be at substantially the
same level as the top of the container means, and such that
the level of white water in the container means can be main-
tained ~ubstantially even with the level of the mixture in
the aeration tank
Brief Description of Drawinqs
The invention will now be described, by way of
example, with reference to the accompanying drawings, in
which:-
Figure 1 is a schematic view of a prior art system
for degassing a mixture of stock and white water in a paper
making process;
Figure 2 is a graph comparing the pressures at
different parts of the prior art system shown in Figure 1
with the pressures at corresponding parts of a system accord-
ing to the invention; and
Figure 3 is a schematic view of a system, according
to the invention, for degassing a mixture of stock and white
water in a paper making process.
Descri tion of the Preferred Embodiment
p
Figure 1 shows a prior art system for degassing
-- 4 --
~,
~ ,~
li613'71
a mixture of stock (i.e. paper fiber pulp) and dilution or
whi1:e water in a paper making process. The system includes
a m:Lxing pump l for stock, supplied via a conduit 2, and
dilution or white water supplied from a wire pit 3 and/or
from another source. The pump l pumps up this stock/white
water mixture from level +2.6 (see the levels, given in
meters, above a reference level - i.e. the bottom of the
wire pit supplying the white water - in Figure l) via a
valve 4 and possibly also filters (not shown) to a bank of
vortex cleaners 5 and then to a degassing or aeration tank
6. The concentration of the mixture is about l % at the
input to the vortex cleaners and, in the vortex cleaners,
solid particles, such as sand and the like, are removed.
In those cases where shives and other agglomerated products
(twin~) have to be removed, filters are positioned in the
conduit
- 4a -
~4 '
3'71
A ~
to the aeration tank 6. The inlet pressure of the incoming
stock mixture to the vortex cleaners 5 is typically 26 mwg
(meters of water gauge). Since the vortex cleaners 5 are
directly connected to the aeration tank 6 with a negative
pressure of 8.6 mwg, the pressure drop between the incoming
and outgoing stock/white water mixture in the vortex cleaners
is 34.6 mwg. This pressure difference is necessary for the
vortex cleaners 5 to operate effectively whereas the negative
pressure in the aeration tank facilitates the removal of -
residual gases from the stock/white water mixture therein.The residual gases are removed from the tank 6 via a conduit
7 with the aid of a vacuum pump (not shown). The temperature
in the tank is approximately +50C, and the level 8 of the
mixture is maintained substantially constant by means of a
spillway (not shown) at a level of +21.5 meters above the
reference level (i.e. +12 metres above the level of white
water in the water pit~.
From the tank 6, the degassed stock is pumped through
a conduit 9 to a headbox (not shown) via a headbox pump (not
shown), The headbox pump, the wire pit, the rejects outlet
from the vortex cleaners and the outlet from the return con-
- duit from the aeration tank must be located at such a level
in relation to the aeration tank that the system is able to
operate in view of the negative pressure prevailing in the
tank. For practical reasons, the mixing pump and the headbox
pump are often located at approximately the same level.
Thus the aeration tank is located at a relatively high level
which involves a number of problems with tube lengths,
return flows and a constructive erection of the degassing
or aeration tank. Also from the point of view of energy,
this known arrangement has drawbacks
In Figure 3 there is shown an embodiment Or a system
according to the invention for degassing a mixture of stock
and white water in a paper making process. The system mixes,
in a mixing pump 11, white water from a wire pit 16 with
the stock supplied through conduit 15 from a machine chest.
3~1
The mixture is then passed through a vortex cleaner 12
to an aeration tank 13. The level of white water in the
wire pit 16 i5 substantially the same as the level of the
stock mixture in the aeration tank 1~. This permits the
mixing pump 11, the vortex cleaner 12 and the tank 13 to
be placed at substantially the same level (the least energy-
demanding level). Between the tank 13 and the vortex
cleaner 12 there is placed a regenerative pump 14, the
object of which is to slow down the mixture flow to the
aeration tank 13 and bring-about a suitable pressure across
the vortex cleaner 12 which is mecessary for the effective
functioning of the cleaner 12. The regenerative pump 14
also controls the negative pressure in the tank 13 and enables
the vortex cleaner 12 to operate as intended. In particular
the regenerative pump 14 enables the tank 13 to be positioned
at a lower level than would otherwise be necessary to obtain
the negative pressure in the aeration tank 13. Thus the
aeration tank 13 is not at a high level as in the prior art
system although the headbox pump 17 is positioned at a lower
level (as before) in order to avoid cavitation. Numeral 18
designates a pressure filter and 19 is a headbox.
In the tank 13 a cooled baffle 20 is placed to prevent
fluid from escaping from the tank 13 to a vacuum pump
through a conduit 21. Thus only residual gases can pass
to the vacuum pump 21.
The lengths and numbers of the supply and return con-
duits connected to the aeration tank are less than in the
prior art system and this reduction also leads to a reduced
formation of twin,
The mixture level in the tank 13 can be controlled by
the regenerative pump 14. The baffle 20, as well as the
other arrangements, and the regenerative pump 14 ~with energy
feedback to the network) result in a considerable energy
saving over the prior art system. Energy saving is also
achieved by replacing the regulating valve 4 (Figure 1~ with
- 1~6~37
~ 7
a variable-speed control of the regenerative pump 14 and by
maintaining the negative pressure in the tank 13 through
the regenerative pump 14.
Drive members to the regenerative pump 14 may be a
variable-speed controlled mo~or, which may also operate as
a generator, ror example for feedback to the network, such
as by means of a frequency converter. The motor may also be
a dc motor, driven from the network via a rectifier (not
shown).
. .
Figure 2 shows, in graph form, the pressure distribution,
(see broken line) for the various parts of the system accord-
ing to the invention shown in Figure 3 compared with the
pressure distribution (see unbroken line) for the various
parts of the prior art system shown in Figure 1. The y-axis
of the graph of Figure 2 represents the pressure H in meters
of water column (mwg) and the x-axis of the graph represents,
in sequence, the variou8 sy8tem parts or components.
The reference pressure at the bottom of the wire pit
16 is taken to be zero (seé A) for both systems. The
pressure increase after the pump 1 according to Figure 1 is
about 48 mwgf whereas the pressure at the mixing pump 11
according to the invention is 38 mwg. The pressure drops
across the vortex cleaners 5 and 12 are assumed to be the
same in both cases.
,
A certain pressure drop (see B) takes place across the
conduits connecting the wire pit to the aeration tank in
each system, After this there is, in the system according to
the invention, a pressure drop of approximately 36.3 mwg
across the vortex cleaner 12 and a further drop across the
regenerative pump 14 so that the mixture as it enters the
tank 13 has a negative pressure of -8.5 mwg. After this
there takes place a pressure increase to the headbox pump 17.
With the prior art arrangement (see unbroken line in
1161371
~, ~
Figure 2), the pressure drops across the valve 4. The press-
ure also drops because of the static lifting height require-
ment (see C) and because of the required pressure drop
across the vortex cleaner 5. After this a pressure increase
takes place to the headbox pump 9.
The reduced energy requirement in a system according to
the invention is clearly seen from the plots shown in Figure
2. In another embodiment of a system according to the inven-
tion, the aeration tank or tanks may have such a position
relative to the mixing pump that the latter can simultaneously
serve as a regenerative pump. In this case, the combined
mixing and regenerative pump and the headbox pump may all
be variable-speed controlled.
The invention can be varied in many ways within the
scope of the following claims.
