Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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MIXING APPARATUS
BACKGROUND
This invention relates to apparatus for mixing and
dispersing gas in the form of fine bubbles in a body of
liquid in a tank by rotating an impeller to pull the gas
. and liquid into a mixing zone ~elow a submerged shroud
where the bubbles are formed and dispersed in an upward
radial flow pat~ern.
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1 The invention can be used in various types of aeration
apparatus, such as, to add air to sewa~e, or remove
dissolved oxyg~n from water by mixing an inext gas with the
water to displace the oxygen.
The apparatus also can be used in flotation processes
in which solid particles in a slurryl or immiscible liquid
droplets in an emulsion~are separabedfrom the main body of
the liquid. The small bubbles selectively attach themselves ¦~
to the particles or droplets to be suspended~and provide
buoyancy to raise them to the surface of the liquid. The
material to be separated is taken from the surface of the ~ ¦
tank in the form of a froth. Chemical reagents can be added 1;
to the liquid to enhance film-forming and b~bble adherence
to improve separation efficiency. Reagents that induce a
froth are called "frothers". Those that assist in the
selective separation of one solid from another in a liquid
are called "depressers","deflocculating agents" and
"collectors", depending on the specific function performed
by the reagent.
A good di~cussion of mixiny apparatus on which the
present invention is an improvement is in Chemical Bngineerin~,
June 8, 1964, pp 165-220.
The following U.S. patents also describe flotation -
apparatus on which the present invention is an improvement:
953,746 Theodore J. Hoover April 5, 1910
1,976,956 Gordon MacLean October 16, 1934
2,274,658 Robert B. Booth March 3, 1942
2,494,602 Harold M. Wright January 17, 1950
~ ~2,626,052 Robert Carbonnier January 20, 1953
3 ~2,875,897 Lionel E. ~ooth March 3, 1959
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1 3,393,802 Leland ~. Logue et al July 23, 1968
3,393,803 Arthur C. Daman, Jr.
et al July 23, 1968
3,647,069 Robert S. sailey March 7, 1972
3,775,311 Philip H. Mook e~ al November 27, 1973 .. ~
One prior art flotation apparatus, on which the present
invention is an improvement, includes an upright draft tube
extending in~o a body of liquid contained in a flotation
cell or tank, and an inverted bowl-shaped hood, or shroud t f' ` `' '
below the dra~t tube. The shroud is substantially imper-
forate, except for a series of radially extending ~otches
formed at spaced apart intervals around the inner periphexy
(point of maximum elevation) of the shroud adjacent the
draft tube. An upright rotary shaft extends down through
the draft tube and rotates an impeller located under the
shroud. The space under the shroud forms a mixing zone
where gas and liquid are subjected to turbulence by the ;
impeller blades. The action of the impeller forms small
bubbles which flow outwardly ~rom under the hood through
~ the notches around the top of the hood. The bubbles cir-
culate upwardly in the liquid and attach themselves to
material to be removed by flotation. The configuration of s
the shroud also causes the gas-liquid mixture to be driven
down toward the bottom of the tank. This flow pattern tends
to sweep the bottom clean of solids and elevate them to a
point where they attach themselves to the bubbles and are
~loated away. I `
One disadvantage of this prior art mixing apparatus ;~
is that the impeller cannot be set deep enou~h in large
tanks to create sufficient circulation to sweep the bottom
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1 clean and still produce the necessary surface flow pattern
for the air bubbles to effectively remove material by
flotation. This prior art unit also has an undesirable
tendency to generate foam which flows in a rotary pattern
and stagnates around the draft tube. The rotary flow
pattern tends to collect material to be floated in the
corners of the cell. The stagnation causes a build up or
collection of oam in the center of the cell where the foam
eithex dissipates or is "folded under" by the flow pattern.
Therefore, even though contaminants are floated to the
surface, a good part of them are reentrained in the liquid
and have to be floated again. The present invention avoids
these problems by generating a flow pattern of air bubbles
in a radial direction outwardly from the mixing apparatus
toward the edges o the cell where skimmers can remove the
material floated to the surface.
Flotation processes commonly use several side-by-side
tanks or cells through which the treated liquid flows
serially. It is common to have a liquid level gradient
from cell to cell, with the level of liquid in the cell
nearest the inlet being the highest, and the levels in each
cell thereafter being progressively lower. The level in
each cell is commonly set by adjusting the elevation of
weirs on opposite sides of each cell. The present invention
provides a convenient means for adjusting the gas-to-liquid
ratio in each cell which, i~ turn, provides a good way o~
complementing the use of adjustable weirs to adjust the
liquid level gradient from cell to cell.
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Summary of the Invention
In accordance with the invention there is provided
mixing apparatus comprising a t~nk for holding a ~olume of liquid;
and a mixing unit including an upright shaft disposed within.the
tank for extending below an operating level of the liquid; an
impeller blade secured to the shaft at a location below the
operating level of the liquid; an outwardly and downwardly
inclined stationary shroud located below the operating ie~vel of
the liquid and above the impeller blade~ the shroud extending
circumferentially away from the upright axis of the ~haft, the
shroud also having a top ~urface facing away from the impeller
blade; means for admitting ga~ to the liquid below the shroud to
be mixed with the liquid by rotation of the impeller blade; a
plurality of circumferentially spaced apart upright stationary vanes
secured to ~he top surface of the shroud, the va~es extending
substantially radially outwardly from the upright axis of the
shaft; a plurality of holes extending through the shroud
between the vanes; and means for rotating the shaft and the
impeller blade to entrain gas bubbles in the liquid and form a
circulation pattern of bubbles pa~sing upwardly through the holes
in the shroud and directed ~ub~tantially radially upwardly and
outwardly between the vanes and toward the edge9 o the tankO
Further in accordance ~ith the invention there is pro~
vided mixing apparatus comprising a tank for holding a volume of
liquid; and a mixing unit-including an uprig~t dra~t tube disposed
wi~hin the tank for extending below an operating l'evel of the
liquid; an upright shaft di~posed within the draft tube~ an
impeller blade secured to the ~haft at a looation below the
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operating level of the liquid and below the extent of the draft
tube; an outwardly and downwardly inclined stationary shroud
located at the lower end of the draft tube and extending circum-
ferentially away from the lower end of the draft tube and above
the impeller blade, the shroud having an undersurface facing the
impeller blade, the shroud having an undersurface facing the
impeller blade and being in substantially uninterrupted fluid
communication therewith so that an upwardly circulating pattern
of bubbles entrained in the liquid by rotation of the impeller
blade impinges directly on the undersurface of ~he shroud, the
shroud also having a top surface facing away from the impellex
blade; a plurality of circumferentially spaced apart upright
stationary vanes secured to the top surface of the shroud, the
' vanes extending substantially radially outwardly from the upright
axis of the draft tube; a plurality of holes extending through the
shroud between the vanes; means for admitting gas into the draft
tube to be mixed with the liquid; and means for rotating the
shaft and impeller blade to entrain gas bubbles in the liquid and
create a circulation pattern of the bubbles passing upwardly through
the holes in the shroud and directed substantially radially
outwardly between the vanes and toward the edges of the tank.
Briefly, the mixing apparatus of this invention includes
a tank for holding a volume of liquid, and a mixing unit
comprising an upright draft tube disposed in the tank and extending
below an operating level of the liquid in the tankO An upright
shaft is disposed within the draft tube~ and an impeller secured
to the shaft is located below the draft tube. ~n outwardly and
downwardly extending shroud is secured to the lower end
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of the draft tu~e and extends circumferentially away from the
draft tube and around the upper edges of the impeller blades.
A plurality of upright vanes are secured to the top surface of
the shroud. The vanes extend radially outwardly from the axis
of the draft tube. A plurality of holes extend through the shroud
between the vanes. Gas is admitted into the draft tube to be
mixed with the liquid in the tank, and the shaft and impeller are
rotated to force liquid outwardly from the impeller toward the
underside of the shroud to entrain gas bubbles in the liquid and
create a circulation pattern of the bubbles passing through the
holes in the shroud and directed radially outwardly toward the
edges of the tank.
This combination of the pexforated shroud and the radial
vanes generates a surface flow pattern of foam which continuously
moves in a radial direction from the center of the tank toward
the edges of the tank. When used in floatation apparatus; the
radially moving foam is constantly skimmed over the weirs~
and new foam is constantly pulled in place of ito This action
improves the effectiveness of each skimmer blade in removing
floated material from each cell and greatly eliminates
re-entrainment of the floated material.
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1 In a preferred form of the invention, a propeller
is secured to the rotary shaft below the impeller. The
propeller rotates in the bottom portion of an upright,
tubular still-well. Rotation of the propeller pulls
liquid upwardly from the bottom of the tank toward the
impeller. The still-well and propeller increase the velo-
city of liquid at the bottom of the cell which improves
the ability of the mixing unit to sweep clean the bottom
of the cell. The still-well and propeller also improve the
pumping action o~ the mixing unit which, in turn, improves
the flow rate, or recirculation, of the liquid through the
Lmpeller.
~p pa~ S
~ hen the mixing ~m~t of this invention is used for
flotation, it preferably has an overflow weir, and a skLmmer
which sweeps foal on the surface over the weir. A gas inlet
valve above the tank is adjusted to control the flow rate
of gas into the draft tube to vary the amount of gas mixed
with the liquid in the tank. An increase in the air to
liquid ratio raises the operating level of liquid in the
tank. In a flotation unit containing a number of cells in
series, the draft tube for each cell has its own adjustable
gas inlet valve. In use, the valves can be adjusted
independently of each other to produce a gradient in the
levels of the liquid in each cell. Thus, the gradient need
not be adjusted by the more cumbersome procedure of adjusting
the level of each weir in the cells.
These and other aspects o~ the invention will be more
~ully understood by referring to the following detailed
description and the accompany drawing.
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Brief Description of the Drawing
FIGURE 1 is a schematic, cross-sectional elevation view
showing the mixing apparatus emBodying ~he preferred improvement
of this invention;
FIGURE 2 is a fragmentary perspective view~ partly bro~en
away, showing an enlarged view of t~e apparatus within ~he circle
2 of Figure l; and
FIGURE 3 is a schematic elevation view, partly broken away~
showing several of the mixing units of ~igure l connected in
series to provide an improved flota~ion ~eparator.
Detailed Description of the Preferred Embodiments ~-
Referring to Figure l, a mixing cell lO includes a tank
12 with opposite side walls 14 and a bottom 16. m e tank holds
a body of liquid 18 at an operating level 20 just below a pair o~
weirs 22 extending along the upper edges of the side walls on
opposite sides of the tank. Preferably, the weirs are adjustable
in elevation to adjust the operating level of the li~uid 18. A
separate trough 24 is secured to the outside of the tank under each
weir 22 to catch effluent skimmed from the tank over the weirs.
.
A pair of elongated skimmers 26 on opposite sides of the tank
above the weirs skim e~fluent over the weirs, The skimmers extend
a ma~or portion of the length of the ~ank. A cover 28 enclosed the
top of the tank. The cover 2B may or may not seal the top of the
tank,
A mixing unit includes a vertical draft tube 30 which
extends from the tank cover down into the center of the tankO
terminating below the oparating level 20 of the liquid in the tank~
The top of the draft tube may be secured to the cover by welding
so that the gas space 32 above the operating level of the
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106132'7
1 liquid is sealed from the draft tube. Alternately, the
draft tube may not be welded, or otherwise sealed, to the
tank cover.
A vertically extending rotary shaft 34 is coaxially
disposed wit~in the draft tube and supported at its upper
end by a bearing 36 which supports the shaft within the
draft tube. A driven pulley 38 is secured to the upper end
of the shaft above the bearings 36 and is driven by a belt 40
secured around the driven pulley and a drive pulley 42
turned by an electric motor 44 mounted above the tank cover.
An impeller 46 is secured to an intermediate portion of
the shaft 34 to be rotatable about the same vertical axis as
the longitudinal axis of the shaft. In some instances the
impeller includes eight equidistantly spaced apart, outwardly
lS ex~ending blades 48 tshown best in FIG. 2) secured at their
inner edges to the shaftO The number of blades varies with
the size of the unit. The blades also may be slipped onto the
shaft as a removable unit. Each blade preferably has a flat,
rectangular lower portion, and a triangular upper portion
with an inclined top edge 50 which tapers upward}y toward
the central rotary shaft 34. As shown best in FIG. 1, the
impeller blades are located at an intermediate depth below the
operating level of the liquid 18 in the cell.
A perforated, conlcal shaped shroud 52 is secured, say by
welding~around the lower portion of the draft tube to extend
outwardly and downwardly away from the draft tube around the
upper edges of the impeller blades. When the shroud 52 is
viewed in vertical cross-section, as in FIG. l, the shroud
extends downwardly and outwardly from the draft tube along an
inclinel path wi~lout an~s~stantial curvature. The preferr~l an~e of
inclination of the shroud is 20 ~
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1 relative to the horizontal, although results are good using
an angle of inclination in the range of about 10 to 30.
As shown best in FIG. 2, a series of rectangular shaped
up~ight vanes 54 are secured to the top edge of the shroud,
say by welding. The vanes are circumferentially spaced
apart around the entire top surface of the shroud and extend
radially outwardly from the vertical axis of the draft tube
and rotating shaft. As shown best in FIG. 1, the vanes
extend along an intermedia~e portion o~ ~he shroud when the
shroud is viewed in vertical cross-section.
A plurality of spaced apart holes 56 extend through the
portions of the shroud between the vanes. A separate set
of holes is located between each-pair of vanes, so that the
holes extend circumferentially all the way around the shroud.
The holes are also located on the shroud at the point of
highest pressure created on the shroud by the centrifugal
action of the.impeller.
As shown best in FIG. 1, a propeller 58 is secured,
8ay by welding, to the lower end of the rotary sha~t 34.
Preferably, the propeller 58 is located one-propeller
diameter above the bottom 16 of the cell. An upright,
tubular still-well 60 located below the draft tube 30
surrounds the lower portion of the rotary sha~t 34 and the
propeller 58, terminating in the vicinity of the propeller.
The still-well 60 preferably has a diameter greater than that
of the draft tube 30, and is aligned coaxially with the draft
tube. The still-well 60 is supported by a plurality of
circumferentially spaced apart and xadially extending stator
vanes 62. ~s shown best in FIG. 1, the stator vanes are
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l 1 generally trapezoidal in shape, with the top edge of each
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trapezoid being secured to the bottom k ~ of the shroud,
and the inner edge of the trapezoid being secured to the
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1 top outer ~ of the draft tube. The inner edge of each
51 stator vane has a notched section 63 adjacent the impeller
blades. The outer edge o~ each stator vane extends to
the outer periphery of the shroud.
In use, liquid is admitted to the tank 12 and its
operating level is controlled by any suitable means, such
as those described below with reference to FIG. 3. Xf the
apparatus is used for flotation separation, liquid to be
treated flows into the cell through a submerged inlet
(described below and shown in FIG. 3) and the skimmers 26
sweep foam and separated material, or contaminants, over
lS the weirs and into the tro~ghs. Txeated liquid having a
lower solids content leaves the tank through a submerged
outlet. A typical application ~or flo~ation separation is
to separate crude oil from water. In this instance~ air
bubbles generated by the mixing apparatus float the crude
oil to the surface of the water where the crude oil is
s~immed off and into the troughs 24.
The electric motor 44 rotates the shaft 34 and impeller
46 to force the liquid to flow outwardly away from the axis
of rotation of the impeller and toward the undersurface of
the shroud 52. The ro ating impeller and propeller reduce
the pressure at the lower end of the still-well 60 so that
water is drawn up~ardly through the still-well and into
the eye of the impeller. Gas is pulled down the draft
tube and mixed with the liquid driven by the impeller. The
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- 1 space under the shroud forms a mixing zone where ~as and
liquid are subjected to turbulence by the rotating impeller
blades. As the liquid is driven ~outwardly from the
impeller it flows between the adjacent s'tator vanes 62 and
then flows upwardly through the holes in the shroud and
radially outwardly ~rom the shroud between the vanes 54
above the shroud. Small gas bubbles are formed by the
action of the impeller blades, and these bubhles attach
themselves to the material removed by flotation, or else
0 they saturate the liquid with ~he gas used, and also dis-
place any gas dissolved in the incoming liquid. 'rhe arrows
~hown in FIG. l illustrate the flow pattern of the gas
bubbles and the liquid in the tank.
The angular inclination of the shroud 52 affects the
recirculation pattern of air bubbles discharged from ~he
bottom side of the shroud. As to those air bubbles which
are discharged through the holes in the shroud, the angular
inclination of the shroud affects the desired surface flow
pattern of the bubbles moving toward the skimmers. The
preferred angle of 20~re~erred to aboveJproduces a desirable
radial flow pattern of bubbles generated by the impeller~
and also produces a good recirculation pattern in the lowex
portion of the cell. The preferred angle of the shroud
produces a recirculation pattern which minimizes "upwelling"
in the corners of the cell. "Upwelling" is a turbulent
upward flow of liquid in the corners of the cell which pulls
the foam down into the cell from the surface and upsets the
desired ~uiescent con~ition of the surface foam.
106132~
1 The vanes 54 and holes 56 in the shroud combine to
generate a surface flow pattern of foam whîch continuously
moves in a r,adial direction away from the mixing apparatus
toward the edges of the tank. As the foam continuously
moves radially toward the skimmers 2Ç it is skimmed over
the weirs 22 and new foam is pulled in place of it. This
radially moving foam pattern increases the effectiveness
of the skimmer blades in removing the floated material,
and also greatly reduces the possibility of reentrainment
10 of the floated material. The hole pattern being }ocated at
the point of highestpressure created by the impeller effect-
ively dissipates this high energy to produce a good dis-
persion of fine air bubbles upwardly through the holes 56
and away from the shroud, rather than the air bubbles
stagnating or being dissolved. The hole pattern also allows
the fine air bubbles to move in a direction upwardly and -
outwardly toward the vanes 54 which direct the air bubbles
toward the edges of the tank. Further, the hole pattern
greatly minimizes flow patterns which cause upwelling in
the corners of the cell, thus producing a more quiescent
surface foam condition.
The pre~erred mixing unit contains a set of five
equidistantly spaced apart holes 56 between each pair of
vanes 54~ The number of holes is not critical as long as
the hole pattern produces a good dispersion of the entrained
air bubbles through the shroud and the vanes to produce the
desired radial flow of bubbles with the desired quiescent
surface condition and minimal stagnation and upwelling.
The preferred sizes of the holes are from lJ2 inch to
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1 1-1/4 inc~es in diameter, depending upon the size of the
mixing unit. In large cells, say 5000 gpm, the size of
the holes will be about l-lf4 inches in diameter. For
smaller cells, say those having a ~low rate of less than
S 500 gpm, the holes in the shroud are about 1/2 inch in
diameter.
The vanes on top of the shroud direct the flow of
air bubbles, which pass through the holes int he shroud,
radially ou~wardly away from the mixing unit. The vanes ~ -
thus prevent a rotary pattern of surface foam and resulting
stagnation near the draft tube. The centrifugal action of
the impeller has the tendency to cause bubbles leaving the
lmpeller to flow in a swirling rotary pattern, but the vanes
direct the air bubbles $1Owing through the shroud along a
i5 flow path directed upwardly and radially ~rom the mixing
apparatus to prevent the swirling pattern from taking place.
There is no critical limitation on the height of the vanes,
other than they be high enough to divertthe flow pattern
of bubbles upwardly and outwardly toward the edges of the
tank in a radial ~low pattern.
The propeller 58 and still-well 60 greatly reduce the
possibility of "short-circuiting", i.e. the passage of
solids through the ce}l without being floated. The addition
of the still-well 60 around the propeller 58 increases the ~ -
efficiency of the pumping action of the propeller. In use,
the propeller and still-well combine to pull liquid off the
bottom of the ce~l and direct it toward the impeller at a
flow rate such that the entire volume of the cell is re-
circulated through the mixing apparatus about 15 times per
3 minute. The propeller being located one-propeller cliameter
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1 above the bottom of the cell results in an increased velo-
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city o~ the liquid sweeping the bottom of the cell, result-
ing in a more efficient flotation of any solids that might
otherwise tend to collect on the bottom of the cell. ~ -
The stator vanes 62 help disperse the air-rich liquid
~including air bubbles not passing through the holes in the
shroud) in a recirculating pattern directed radially out-
wardly and downwardly from the mixing apparatus. ~ ;~
FIG. 3 shows a flota~ion separation cell 64 which
0 includes Xour of the flotation cells 10 in series, although
fewer or more of the cells 10 can be used in series wi~hout
departing from the scope of the invention.
In use, liquid with matexial or contaminants to be ;
~eparated by flotation is added to an inlet well 66 adjacent
the first mixing cell in the series. The liquid flows into
the ~irst mixing cell through an opening in the lower portion
of the end wall of the first cell adjacent the well 66. The
liquid is mixed with gas due to the action of the impeller ,~
as described above. Some of the material to be floated is
20 buoyed to the surface and swept away by the skimmers in the "~
first mixing cell. The remaining liguid and contaminants '~
flow from the first mixing cell to the second mixing cell
through an opening in the bottom portion of the common end
wall which separates the first mixing cell ~rom the second.
25 The liquid in the second cell is mixed with gas by a --
separate impeller as described above, and additional matter
is buoyed ~o the surface and removed by the skimmers. The
same flotation action occurs in the third and fourth cells, ~;
and by the time the liquid leaves the fourth cell, the bulk
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¦ of the matter to be separated by flotation has been removed.
¦ Treated liquid flows through an outlet in the bottom of the
¦ wall in the fourth cell and into a discharge well 68 at
¦ the downstream end of the flotation separation apparatus.
5 ¦ Treated liquid flows out a discharge pipe 70 in the bottom
¦ of the discharge well 6B. The material removed by flotation
¦ is skimmed off by the rotating skimmer blades 26 over the
¦ adjustable weirs 22 and discharged from the troughs 24 through
¦ a drain pipe 72.
10 ¦ The advantage of the present invention is that each
¦ flotation cell connected in series as ~ust described can be
¦ independently adjusted to provide the optimum gas-liquid
¦ ratio for mixing to achieve the maximum separation efficiency.
¦ This is accomplished by ~ separa~e adjustable gas inlet
15 ¦ valve 74 sealed through the top portion of the draft tube in
each cell. The air admitted ~o each draft tube from outside
¦ the cell passes through the gas inlet valve. Preferably,
¦ each valve is an adjustable ball valve in which tbe amount
¦ of air flowing into the draft tube can be adjusted independ-
20 ¦ en~ly of the air flowing through the valves in the other
¦ cells.
¦ The amount of air added to each cell also controls the
¦ level of the li~uid in the cell. It is common in such
ilotation cells for the operating ~evel of the liquid in
the first cell to be higher than that in the remaining cells,
and for the level to be on a gradient which decreases
~ progressively from the irst cell to the fourth cell. Since
" the ball valves 74 can be used to adjust the amount of air
admitted to each cell independently of the other cells,
they provide an easily adjustable contro} for setting the
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1 level gradients from cell to cell. This complements the
use of ndjustable weirs in controlling the liquid level
gradient from cell to cell.
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