Note: Descriptions are shown in the official language in which they were submitted.
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BA~K~ROU~D OF T~IE TNV~NTION
Field of the Invention:
The present invention generally relates to improved apparatus
for dispersing gas bubbles throughout a liquid bodv.
State of the Art:
It is well known to distribute gas bubbles in a liquid
body in order to accomplish, for example, solid-liquid or liquid-
liquid separation by flotationO Such flotation techniques
are co~only used for separating and concentrating valuable
minerals and chemicals, for removing particulates from liquid
bodies and for separating various liquids. A typical flotation
process in the mineral beneficiation art, for example, includes
the steps of conditioning an aqueous pulp or slurry of crushed
ore with a chemical flotation aid and then dispersing air bubbles
within the pulp to produce a surface froth relatively rich
in the desired mineral. In the field of oil production, similar
flotation processes are frequently used to separate crude oil
from water prior to the reinjection of the water into a well
or prior to surface disposal of the water. In flotation processes
in general, it is important to maximize contact between the
froth-producing gas bubbles and the materials ~hich are to
be floated and, at the same time, to maintain the surface of
the li~uid body fairly quiescent so that the froth is not agitated
so much as to cause the floated materials to separate from
2~ the gas bubbles to which they have become attached.
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Dispersed gas flotation, as distinguished from dissolved
gas flotation, achieves physical separation of a contaminating
substance from a body of primary liquid be effecting contact
between the contaminating substance> which may be either solid
particles or a second liquid, and gas bubbles without first
dissolving the gas in the primary liquid. l~aving achieved
contact, the contaminating materials attach to the gas bubbles
and rise buoyantly to the surface of the primary liquid as
a froth which can be subsequently removed, as by skimming.
In dispersed gas flotation systems, it is important to achieve
small gas bubbles (i.e. high surface-to-volume ratio), good
mixing to assure high gas-particle contact probability, minimum
short circuiting of the primary liquid, and a highly concentrated
contaminant level in the removal stream.
Conventional dispersed gas flotation systems, which utilize
mechanical impellers in flotation cells to ingest gas into
liquids, have inherent features which preclude their application
to many areas, most notably the treatment of wastewater in
municipal plants and in pulp and paper mills. Attempts to
apply mechanical-type gas flotation devices in such areas have
failed because of the inherently high degree of fluid turbulence
produced by the impellers within the separation zone of the
flotation cells and the necessity for baffles in the mixing
zones of the cells. In pulp and paper applications, for example,
high fluid turbulence will break up the relatively ~eak floc
in the wastewater. In municipal waste treatment or ~7hen treating
wastewater from meat-packing plants, as another example, the
mechanical elements and baffles in conventional gas flotation
systems foul due to the presence of "stringy-type" solids.
1~78081
ORJl~CTS OF THE INVENTION
The general object of the present invention is to
provide an improved machine for dispersing gas into a liquid
body.
Another object is to provide a machine for ejecting
two-phase fluid into a contained liquid body in a manner to
provide a nearly complete dispersion or distribution of gas
bubbles throughout the body together with a quiet but frothy
surface .
A more specific object is to provide a hydraulically
actuated flotation machine and method of operation which
eliminates moving parts and stationary baffles from the mixing
and separation zones in the machine.
The process which is preferably practiced with our
machine is described hereinafter.
SUM~ARY OF THE INVENTION
Broadly speaking, therefore, the present invention
provides a dispersed air flotation process wherein hydraulic
effects are utilized to disperse gas bubbles throughout contained
liquid comprising: (a) introducing liquid into the first cell
of a horizontal series of adjacent flotation cells without
baffles; (b) passing the liquid from cell to cell in the series
for sequential processing; (c) discharging processed liquid from
the last of the cells in the series; (d) in each of the cells,
pumping a gas-liquid mixture under pressure into the liquid
contained in the cell with the density and the kinetic energy
of the pumped mixture per unit of volume of the cell, at the
point of e~ection into the cell, being defined by a point within
the area encompassed by Region I in the graph of Figure 3 to
form a dispersion of gas bubb~es which rise tD the free surface
of the cell as a froth without the aid of baffles and mechanical
gas distribution means; and (e) removing the froth from the free
surface of the liquid in each of the cells.
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1~78081
The pl-esent invention also provides a dispersed air
flotation machine wherein hydraulic effects are utilized to
disperse gas bubbles throughout contained liquid comprising:
(a) a housing and a plurality of flotation cells, without
baffles and mecllanical gas distribution means, mounted adjacent
one another in a horizontal series in the housing and each of
the cells containing a liquid body with a free surface; (b) means
connected to the housing for introducing liquid for processing
thereinto; (c) means for passing liquid for processing from cell
1~ to cell within the housing;(d) means for removing processed liquid
from the housing; (e) means for removing froth from the free
surface of the liquid in each of the flotation cells; (f) a
fluid ejection device fixedly mounted in each of the flotation
cells in a position to expel a mixed two-phase fluid into the
liquid contained in the associated flotation cell, each of the
fluid ejection devices including: (i) a hollow tubular expansion
chamber member which is circular in interior cross-section and
which has an open end through which mixed two-phase fluid is
e~ected into the liquid in the cell and an opposite end; ~ii) a
liquid-carrying pipe sealingly connected through the opposite
end of the expansion chamber member, the pipe being of small
and uniform inside diameter relative to the inside diameter of
the expansion chamber member; (iii) gas introduction means for
introducing gas into the interior of the expansion chamber
member for mixing with the liquid therein; and (g) a pump
connected to pump liquid through the liquid-carrying pipes into
the interiors of the expansion chamber members for aspirating
gas thereinto through the gas introduction means for mixing
with the pumped liquid to form the two-phase fluid which is
e3ected into the liquid body in the vessel to form a dispersion
of gas bubbles which rise to the free surface as a froth.
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BRIEF DESCRIPTION OF THE DRAIIIN~S
Fur~her objects and advantages of the present invention
may be readily ascertained by reference to the following
description and appended drawings which are offered by way
of illustration only and not in limitation of the invention,
whose scope is defined l-y the appended claims and equivalents.
In the drawings:
Figure 1 is a schematic diagram of a flotation machine
according to our invention;
Figure 2 is a sectional view of the machine o Figure
l; and
Figure 3 is a graph illustrating the conditions under
which the machine of Figure l is operated.
DETAI~ED DESCRIPTION OF THE PREF~RRED EMBODI~ENT
The dispersed air flotation machine in Figures l and 2
generally includes sidewalls 4 and 5, endwalls 6 and 7, a floor
8 and an optional roof 9 which together comprise a housing.
Within the housing, flotation cells or compartments are arranged
in a horizontal series for holding the liquid to be treated.
At least one o the sidewalls, say wall 4, terminates short
of the roof 9 and its upper horizontal edge serves as an overflo~
weir to discharge froth from the cells into an elongated launder
box 10 m~unted on ~he sidewall 4. Preferably, a conventional
ratary paddle wheel device 14 is ~ounted adjacent the edge
of the sid~wall 4 to urge froth to discharge over the sidewall
into the collection launder lO, from which the froth i5 carried
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to dischar~e via a conduit 15; the paddle wheel drive means
is well known and is omitted from the drawings for purposes
of clarity. An inlet means, illustrated as a conventional
feed box 16, is mounted on the endwall 6 of the housing to
admit an influent stream of liquid for processing into the
first cell via a conventional underflow weir, not shown, which
is located 3ust above the floor ~. An outlet conduit 17 for
discharging treated liquid from the machine is fitted through
the endwall 7.
1~ The flotation cells are all substantially the same and
only one of them, cell 13, is fully shown and will be described
in detailO Cell 13 generally comprises a compartment wherein
is mounted a two-phase fluid ejection device 20 for introducing
a gas-liquid mixture into the contained liquid to form a froth
on the liquid surface. The illustrated compartment of cell
13 is rectangular in shape, being comprised of the housing
sidewalls 4 and 5, the housing floor 8, and end partition walls
23 and 24 mounted transversely between the housing sidewalls
4 and 5. There is an underflow of liquid into and out of each
cell via opening 25 formed through the partition walls just
above the floor 8; this manner of transferring liquid from
- cell to cell in a flotation machine is well ~nown and the openings
2~ are usually called underflow weirs. It should be noted
that the cell 13 does not contain baffles or other gas distribution
means~
The illustrated machine also includes an integral s~i~ning
compartment 27 which receives treated liquid ~ia an underflow
weir from the last flotation cell of the series. The skimming
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:1078081
can
compartment 27~be understood to be identical to a flotation
cell except that it does not contain a two-phase fluid ejection
device 20. In the illustrated embodiment, the skimming com-
partment 27 includes a separate launder box for receiving
froth from the compartment, which froth is carried to discharge
via a conduit 31. It should also be understood that there is
a skimming device mounted in the skimming compartment to urge
froth into the launder box. Treated liquid is discharged
from the skimming compartment via the aforementioned outlet
conduit 17.
As mentioned previously, a single one of the ejection devices 20
is fixedly mounted centrally at the free liquid surface in each
of the cells to eject a two-phase fluid (e.g~ an air water
mixture) downwardly into the liquid body from below the liquid
surface. Each of the ejection devices 20 is connected, via
liquidcarrying branch pipe 19, to a main manifold pipe 18.
A pump is connected to the ~ain manifold pipe to force liquid
therethrough and then into the ejection devices 20. As will
now be explained, the pumped liquid mixes with gas in the
ejection devices 20 to form the afore~entioned two-phase fluid.
As shown, the liquid pumped to the ejection devices 20 can
be a ~raction of the processed liquid discharged through pipe 17.
~ As best shown in Figure 2, each of the eiection devices
20 preferably includes a hollow straight tubular member 21
which is circular in interior cross-section, has uni.orm inside
diameter and an open, unobstructed endc An annular p-ate having
a central aperture is sealingly fixed concentrically to the
upper end of the tubular member 21 (hereinafter called the
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1078C~81
inlet end). The free end of the associated branch pipe 19
is sealingly fitted thrvugh the annular plate and extends
concentrically into the interior of the tubular mer.lber 21 to
thereby define an annular space between the exterior wall of
the pipe 19 and the interior wall of the tubular member 21.
The radial width of the annular space may range from about
20 to 80% of the interior diameter of the tubular member; the
preferred range is 20 to 25~,'. An aperture is formed through
the sidewall of the tubular member 21 to communicate with the
annular space and a gas-carrying conduit 22 is sealingly fitted
into the aperture to convey gas into the annular space. The
gas-carrying conduit 22 extends from the ejection devices 20
for connection to a source of pressurized gas or to an outlet
which is in gaseous flow communication with the atmosphere above
the liquid surface or outside the housing of the flotation
machine.
The space within the tubular member 21 between the end
of the liquid-carrying branch pipe 19 and the discharge end
of the tubular member 21 defines an expansion chamber. The
discharge end of the expansion chamber is open and unobstructed.
In practice, the ratio of the inside diameter of the expansion
chamber to the inside diameter of the branch pipe 19 ranges
from about 1.5 to about 3.5 and, preferably, the ratio is at
least 2. Further, the length of the expansion chamber is at
least twice its diameter and may be t~Jenty or more times its
diameter in some applications, such as for gas~ uid contacting;
the preferred ratio of the length of the expansion chamber
to its inside diameter ranges from about 2 to about 15.
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1~78081
Although the Figure 1 embodiment shows the ejection devices
20 positioned to expel two-phase fluid downwardly into the
liquid-holding vessel 13, the ejection devices can be positioned
to eject at some oblique angle into the tank.
To operate fluid ejection devices 20, liquid is pumped
at a pressure of, say 3 to 15 psig, through the manifold pipe
18 and then through the branch pipe 19 into the expansion chamber
members 21. Upon entering an expansion chamber 21, the pumped
liquid creates a low-pressure turbulent region and the low pressure
aspirates gas into the expansion chamber from the aforementioned :.
annular space in the expansion chamber. Although natural aspiration
from the atmosphere usually draws enough gas into the ejection
device to satisfy the operating parameters described hereinafter,
a source of pressurized gas can be connected to the gas-carrying
pipe 22.
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1078081
The machine of Figures 1 and 2 is preferably o~erated
such that certain energy-density relationships shown in Figure
3 are maintained at the outlet ends of ejection devices 20
In the graph in Figure 3, the vertical axis (ordinate) represents
the kinetic energy of the two-~hase effluent from an ejection
device 20 in terms of foot-pound force per cubic-foot volume
of the receiving tank 13 per second, and the horizontal axis
(abscissa) represents the density of the two-phase effluent
from that ejection device in terms of pound force (iOe. weight)
per cubic footO The area I generally bounded by the solid
curve ABC in the graph describes the preferred operating region
of the machine. Surrounding that region is a transition Region
II whose outer boundary is defined by the dashed curve DEF.
Outside that boundary is Region III, the so-called undesirable
operating region. When the machine is operated under Region
I conditions, the liquid body in a cell is filled with gas
bubbles and the liquid surface in the cell is relatively quiet
but frothy. However, if the machine is operated under Region
III conditions, either the gas bubbles are not distributed
throughout the liquid body or the liquid sur*ace is excessively
turbulent or choppy.
It should be noted that the abscissa of the graph in
Figure 3 is a linear scale on which density values are shown
ranging from 10 to 62.4 pounds per cubic foot. Those values
are based on tests where the effluent was an air-water mixture.
Since the ~ensity of water is 62.4 pounds per cubic foot, the
density of the two-phase gas-water mixture would necessarily
be less than that. It should also be noted that the ordinate
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1~78081
is a lo~arithmic scale and that the energy rates of the two-
phase effluent range from one-tenth to ten pounds per square
foot per second.
In a sense, the curve AB defines a minimum energy boundary
because a point on that curve defines, with respect to a particular
ef~luent density, the minimum energy that can be expended to
achieve the desired conditions. In actual practice, we prefer
to operate at an energy level above the curve AB in order to
provide a margin of safety. Likewise, the curve BC can be
understood to define a maximum energy boundary because a point
on that curve defines, with res~ect to a particular effluent
density, the maximum energy which can be expended ~Jhile still
maintainin~ the desired conditions~ In practice, we prefer
to operate at energy levels well below the boundary BC in order
to conserve power. For that reason, the exact location of
the curve BC is unimportant except to illustrate that the desired
conditions will cease to exist if the two-phase effluent energy
is too great.
From Figure 3, one could also observe that it would be
preferable to operate at an energy-density point generally
within the shaded area of the nose region of the curve ABC
if energy usage were to be minimized. We have found, however,
that operation there is not desirable from a reliability standpoint
because slight changes in the values of the operating parameters
can readily give rise to undesirable conditions in the cells.
For example, if the machine were set to operate at ~oint b
and the ef1uent density shifted to a point b' (about a 10%
increase), the desired conditions in the cell would deteriorate.
Such shifts in the operating parameters could result from hydraulic
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\ ~ 777
1078081
or air blockages and plu~ging, variations in pump speed, normal
mechanical wear experienced during use, and so forth. Therefore,
we usually operate substantially to the left and above the
shaded area of the nose of Region I, say at point b" in the
unshaded portion of the region.
Operation at a point such as b" in Region I which is sub-
stantially removed from the shaded area is also ~referable
for the reason that efficient flotation requires enough gas
to provide a large number of bubbles to contact the material
which is to be floated. Since the quantity of gas which is
introduced to the liquid in a flotation cell is inversely related
to the lensity of two-phase effluent from an ejection device 20,
and since the number of bubbles is a generally increasing
function of the quantity of gas, operation at point b" (low
density~ is normally preferred to operation at point b (high
density) when the number of gas bubbles is a consideration.
The quantitative relationship of the density of the two-phase
fluid, P2~, to the gas flow QA and the liquid flow QL can be
represented by the following expression:
P2~= ~
It should be noted that we are discussing here the relative
number of bubbles and not the distribution of the bubbles;
the bubbles can, of course, be distributed throughout a cell
whether there are relatively many or relatively few bubbles.
Preferably, the two-phase fluid ejection devices 20 are
positioned with their outlet ends below the surface of the liquid
in the cells such that the gas-liquid mixture fro~ tl~e;ejection
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( 777
1078f)~1 ~
devices im~inges upon or sweeps the floors o~ the cells. The
condition of impingement depends upon the depth of the cells
as well as the energy of the two-phase effluent. From our
observations, we believe that the impingement (or "near" im-
pingement, as that term will be explained hereinafter) on thecell floor is important in achieving good gas bubble distribution
and a quiet liquid surface with minimum power usage. We have
also found that the distance between the outlet end of an
ejection device and the floor of a cell affects the size of
1~0 bubbles in the cell; that is, the hubbles decrease in size
as the ejection devices are positioned closer to the cell
floors. In some gas-liquid mixing applications, for example,
t is desirable to position the ejection device within one
diameter of the cell bottoms, where a "diameter" re~ers to
lS the inside diameter of the expansion chamber 21.
With respect to impingement on the cell floors, we have
observed what we call a hysteresis effect in flotation applications
and believe that effect ~artly explains the transition ~egion
II shown in Figure 3. I~e have observed that, as the ejection
energy is increased while ~aintaining the two-phase fluid density
constant, a critical value is reached where a cell suddenly
ills with bubbles and the free surface becomes quiet. Moreover,
we have found that once the critical ener~y value is surpassed,
we could thereafter reduce the ejection energy while maintaining
a constant nozzle effluent density and that the cell would
remain filled with bubbles until an energy value was reached
below the prior critical value. In other words, the energy
value at which the bubble distribution changes from uniform
to non-uniform depends upon whether one is decreasing the energy
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107808~
from a point within Region I or whether one is increasing
the energy from a point in Region III to reach a point within
Region I. Thus, th~ botmdary AB of Region I is the locus of
energy values at which the preferred conditions will arise
S as the ejection energy is increased from a point in Region
III and the dashed boundary DE of the transition Region II
is the locus of points where the preferred conditions will
cease as the ejection energy is decreased from a point within
Region I. The hysteresis effect, we believe, may be closely
related to the impingement of the ejected two-phase fluid on
the cell floors. By taking advantage of that effect, we are
able to reliably operate at values slightly inside the minimum
energy boundary AB because even if the effluent density should
decrease, say by shifting from point b" in ~egion I to b"'in
Region II, the preferred conditions -n the tank would still persist.
In view of the hysteresis effect, the curve AB can be
understood to define the minimum energy levels at which one
is assured of achieving the preferred conditions within the
cells In still other words, the minimum energy required
for assurance of the preferred conditions is a unction of the
two-phase effluent density, and that function is shown by
curve AB,
The Figure 3 abscissa and ordinate values at which the
flotation cells are operated can be determined by skilled workers
in several ways. For example, the density of the eJected two-
phase fluid can be calculated from the aforementioned expression.
The liquid and gas flow rates into the ejection device 20 (QL
and nA, respectively) are readily ~easurable with a conventional
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107808~
venturi meter, a rotameter, a pitot-static device or the like,
or are determinable from pump operating conditions. Knowing
the tank volume, the gas and the li~uid flow rates, and the
density of the two-phase effluent, one can readily determine
the kinetic energy rate ~mv2 of the two-phase fluid per unit
of tank volume, where "m" is defined as the two-phase fluid
"mass" flow rate (in pounds weight per second) as determined
by the density and pipe-geometry relationship, "v" is the effluent
velocity of the two-phase nixture in feet per second and "g"
is the gravitational constant 32.2 ft/sec2. Here again, we
emphasize that the ordinate values shown in Figure 3 are in
terms of the volume of the liquid held in a cell; thus, for
example, if a cell volume is doubled and the two-phase effluent
density is held constant, the two-phase effluent energy must
also be doubled in order to maintain the preferred flotation
conditions and to establish the same operating point in Figure
3. Normally, the effluent energy of the two-phase fluid is
adjusted by varying the speed or flow of the pump which supplies
the liquid to the ejection devices 20, or by varying the fluid
stagnation pressure at the ejection devices. We have determined
the graph of Figure 3 by tests conducted with tank volumes
ranging from 0.83 to 500 cubic feet and believe the illustrated
range applies to flotation cells over a 1000:1 volume range.
The method of operation of the illustrated machine may
now be contrasted with the method of operation of conventional
impeller-driven flotation machines. In such machines,
impeller rotation aspirates gas into a liquid body, but also
creates substantial agitation and shear within the liquid.
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1C~78081
Such eonditions discourage flotation to the extent that the
gas bubbles may have difficulty in remaining attached to the
substance which is to be floated. ~ith the machine of the
present invention, by way of contrast, a natural hydraulically
actuated effect is utilized to accomplish flotation or, more
specifically, the complete filling and mixing of a contained
liquid body with gas bubbles without violent agitation and
with a minimum of shear turbulenee in the flotation cells.
The complete filling of the cells with gas bubbles and the
eireulation of the bubbles optimizes eontaet between the gas
bubbles and material which is to be floated. It is very important
to note that the hydraulie effeet also allows the process to
be earried Otlt without baffles or other mechanical gas distribution
means.
