Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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The present invention relates to processes and
apparatus for carrying out continuous liquid-liquid
contacting having the capacity to control and contain a fire
occurring within.
One example of industrial application of liquid-
liquid contacting processes is in liquid-liquid ~xtraction.
This entails the mixing of two substantially immiscible
liquids of different densities, typically an organic and
an aqueous solution, wherein a component of interest
transfers from one of the liquid phasas to the other.
Separator vessels are used to contain the liquids and allow
separation into their component phases. One process and
apparatus for liquid-liquid extraction is described in United
States Patent No. 4,338,285 to Eberts issued July 6, 19~2.
Since the organic solutions used in such processes
generally include flammable solvents of density less than
that of the aqueous solution, there is a consequent risk that
the upper surface of the organic solution may be ignited
within a separator vessel.
Emergency settler drainage systems used heretofore
provide for a drainage outlet in the side of each settler at
an elevation which includes the interface between the aqueous
and organic layers. The emergency outlet is opened
automatically in response to fire detectors. The outlet is
connected to a drainage conduit which drains the solutions
to a suitable containment area. The drainage conduit
includes a vertical U-bend to prevent propagation of the fire
through the drainage conduit.
Since the elevation of the interface is not fixed,
considerable aqueous must be allowed to discharge together
with the organic, leading to oversized conduits. Also, since
the head of liquid available to discharge the final amount of
organic from the settler declines to zero, complete drainage
of the organic layer is not to be expected.
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Through continuous operation, a gelatinous mixture
of aqueous, organicl and solid particles (commonly called
crud) often accumulates in the settlers at the organic/
aqueous interface~ Conventional apparatus may allow for
removal of this gelatinous material by providing a second,
normally closed, outlet for organics on each settler, with
a set of conduits leading from these outlets to a collection
tank. To discharge the material, the aqueous and normal
organic outflows are blocked, causing the interface to rise
and, by op~ning the normally closed outlet, allowing the
organic layer and gelatinous material to outflow to a
collection tank.
The present invention provides means for overcoming
the disadvantages of the conventional drainage arrangement
and in a manner which utilizes various elPments of the
apparatus for both emergency drainage and crud removal.
In the present invention, each liquid phase is
pumped at a steady flow rate into a mixer apparatus which
combines the two liquids to form a mixture which is passed
into a settler vessel or other separator vessel. The
separated liquid phases are withdrawn separately from the
vessel, and in order to compensate for the variations in the
flow rate provided by the sources of the phases, a
recirculation is permitted from the outlet for each phase
from the separator vessel to the pump for that phase. The
pump is operated at a pumping rate in excess of the maximum
flow rate provided by the source of the liquid phase, so that
a certain amount of recirculation is maintained, at a rate
which rises and falls with the variations in the rate of
supply from the source. The output from the pump is, with
this arrangement, maintained at a steady rate, and no active
control elements are required to regulate the recirculating
flow.
In one advantageous form of the present invention,
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wherein multiple liquid-liquid contacting stages are
employed, the phase layers in each separator vessel are
connected in direct flow communication with the correspo~ding
phase layer in the other vessels through the outlet conduits
that supply the separated phase to the subsequent stage and
through auxiliary conduits that carry the recirculating flow.
With this arrangement, it is possible to regulate upper level
and interfacial level in all of the separator vessels using
a single level controlling means for each liquid phase. The
level-controlling means may conveniently be in the form of
weir overflows controlling the heads of liquid in the
outflows of processed liquid discharged from the first and
last vessels.
Fire suppression within one separator vessel may be
carried out by flooding the vessel with a third nonflammable
liquid of density greater than that of the first flammable
liquid, for example water. A vessel outlet is provided at an
upper fire suppression level above the operating level of the
organic phase for draining away the flammable organic phase
in the event of fire. The flammable organic liquid is
discharged to a containment vessel though an outlet conduit
having a vertical U-bend trap to prevent fire propayation.
A channel weir is provided within each of the vessels, the
upper edge of which is at the fire suppression level. The
flammable liquid is raised when the vessel is flooded with
nonflammable liquid until substantially all of the flammable
liquid has flowed over the weir and into the connected
discharge conduit.
Examples of the present invention will now be more
fully described, with reference to the accompanying drawings,
wherein:
Fig. 1 is a partly diagrammatic plan view of
multiple-stage liquid-liquid contacting apparatus having fire
suppression capability within each separator vessel in
accordance with the invention;
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Fig. 2 is a partial sectional view showing the weir
and discharge conduit during a fire suppression operation;
and
Fig. 3 is a partial isometric view of a preferred
form of multiple stage apparatus having fire suppression
capability, wherein portions are broken away to reveal
interior structure.
Referring to Fig. 1, wherein like reference
numerals indicate like parts, a liquid-liquid contacting
apparatus having three stages is shown.
Each stage in the said apparatus comprises a
settler vessel 10. In the example of Fig. 1, the settler
vessel 10 are rectangular in plan and at one end there is a
distributor 11 through which is supplied a mixture of a
flammable liquid and a non-flammable liquid. At an opposite
end there are outlet conduits 13 and 14 through which are
withdrawn the upper and lower layers of liquid that separate
out in the vessel. In the first stage the outlet conduit 13
connects with a discharge conduit 16 for the lighter phase,
and in the last stage, outlet conduit 14 connects with a
discharge conduit 17 for the heavier phase. Otherwise, the
conduits 13 and 14 connect with supply conduits 18 and 19
feeding the lighter and heavier phases respectively to
adjacent stages of the apparatus. The two phases pass
countercurrently between the sta~es.
In thP last and the first stages, there are supply
conduits 21 and 22 that are connected to sources of supply
of the lighter and heavier liquid phases, respectively.
In each stage, the supply conduits 18, 19 are
connected to the inlet side of pumps P1 and P2, for the
respective phases, through inlet conduits 23 and 24. The
pumps feed liquid into a mixer tube 26 that feeds liquid into
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the distributor 11 for the settler vessel.
The inlet conduits 23 and 24 of the pumps P1 and P2
are also connected through iauxiliary conduits, 27 and 28, to
the outlet conduits 13 and 14, respectively.
Internally, the mixer tubes 26 are furnished with
a known form of motionless mixer elements 29. These mixer
elements 29 are of the kind that will combine the two liquids
into a substantially uniform emulsion provided the flow rates
of the two liquids are maintained constant, and may be of the
10 kind described in U.S. Pat. No. 3,286,992. As described in
more detail in the said patent, to which further reference
should be made for details, these mixer elements 29 each
comprise a curved sheet-like element which divides the
interior of the mixing tube 26 into two separate channels.
One suitable form of mixer element is the kind available
under the trade mark STATIC MIXER from Kenics Corporation,
Danvers, Mass. Other forms of mixer apparatus may of course
be employed.
` In operation, lighter and heavier liquid phases are
supplied countercurrently through the supply conduits 21 and
22, respectively. These liquids may he, for example, such as
are employed in the continuous countercurrent extraction of
metal values from dilute and/or impure aqueous solutions
containing the metal ions in simple or complex form. In such
case, the heavier phase may be a pregnant aqueous solution
supplied through the supply conduit 22 illustrated at the
extreme left-hand side in each Figure, and the lighter phase,
which may be an organic solution, and may consist of an ion
exchange compound dissolved in an organic carrier, is
supplied through the supply conduit 21 at the right-hand side
in Fig. 1. The organic solution preerentially absorbs the
desired metal values from the aqueous solution.
Each settler tank 10 contains an upper separated
phase layer, often of organic solution, and a lower separated
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phase layer, often aqueous. In one example, proceeding from
left to right the aqueous layers in successive settler
vessels will contain progressively decreasing concentrations
of the ion of interest, the settler vessel at extreme right
containing a barren aqueous raffinate which is discharged
through the discharge conduit 17. Proceeding from right to
left, the organic phase will contain progressively increasing
concentrations of the ion of interest, the vessel at axtreme
left containing a loaded oryanic phase, with a relatively
high concentration of the ion of interest, and the loaded
organic phase is withdrawn from the discharge conduit 16. In
the usual applications, the sources of supply of the liquids
each provide a flow of liquid that varies between maximum and
minimum limits. In order to ohtain a constant output from
each of the pumps P1 and P2, each pump is operated at a volume
pumping rate which is in excess of the maximum flow rate
provided by the source that supplies liquid to the pump.
With this arrangement, the variations that occur in the
supplies are compensated for by increased or decreased
recirculation of liquid along the auxiliary conduits 27 and
28 from the streams of separated liquid that flow out from
the settler tanks through the outlet conduits 13 and 14,
respectively.
Thus, the flows of the liquid phases that are
supplied by the pumps P1 and Pz to the mixer tube 26 in each
stage, are each maintained at uniform and constant rates.
As a result, the motionless mixer elements combine the two
liquids into a substantially uniform emulsion, containing
drops of substantially uniform size of one liquid phase
distributed throughout a continuous phase of the other
liquid, with the attendant advantage that as there is
reduced production of fine drops that are excessively slow to
disengage. This permits settler vessels of smaller area to
be employed, as it is possible to reduce the residence times
therein.
It will be appreciated that in applications where
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the supply of only one of the liquid phases is subject to
variation, it is not necessary to recirculate liquid to the
set of pumps P1 and P2 that handle the other liquid. Thus,
for example, if a constant flow is supplied through right~
hand conduit 21, there is no need for an auxiliary connection
27 b~tween the outlet and supply conduits 13 a~d 18.
It will be noted from Fig. l that the outlets for
the lighter phase from each vessel are connected together
through a continuous lin~ of conduits comprising the outlet
lo conduits 13, the supply conduits 18, and the auxiliary
conduits 27. Similarly, the outlets for the heavier phase
are each linked together through outlet conduits 14, supply
conduits 19, and the auxiliary conduits 28. Thus, in Fig.
l, it is possible to control the upper and interfacial levels
of the liquid phases in each vessel lO by controlling the
respective he~ds of liquid in the discharge conduits 16 and
17. As described in my above-m~ntioned U.S. patent
4,338,285, the outlets 13 from each vessel for the lighter
(usually organic~ first phase may be connected to a common
level controlling weir overflow, the level of which
corresponds approximately to the normal operating level of
the lighter phase. Similarly, each of the outlets 14 for the
heavier (usually aqueous) second phase may be connected to a
common level controlling weir overflow.
An advantage of the arrangement above described is
that it is adapted to operate with by-pass of the liquids
around any selected stage of the apparatus in the case of a
failure in that stage, or to isolation in the event of a
fire.
In order to facilitate by-passing of a failed or
isolated stage, the pumps Pl and P2 are provided with
isolation valves, 42 and 43, respectively, in their inlet
conduits 23 and 24 for isolating the pumps from their supply
and auxiliary conduits. On closure of the isolation valves
42 and 43 in a particular stage, that stage will be by-passed
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and the liquid will flow to the next succeeding stage in the
apparatus.
In the embodiment of Fig. 1 further isolation
valves 44 and 46 are connected in the outlet conduits 13 and
14, respectively. On closure of these valves and the pumps
isolation valves 42 and 43, it is possible to completely
drain the settler vessel 10, thus making it possible to
effect repairs in a settler vessel without disturbing the
operation of the remaining stages in the apparatus.
Throttling valves 47 and 48 may be connected to the output
sides of the pumps P1 and P2, which may be employed for
initial fine adjustment of the volume pumping rates provided
by the pumps P1 and P2. These valves 47 and 48 can be shut
off to prevent back flow if it is desired to carry out repair
work on the pumps.
The pumps to be employed in the present apparatus
pumps should, of course, be of a kind that is capable of
maintaining a consistent volume pumping rate over prolonged
periods. Suitably, these pumps will be conventional
propeller pumps or centrifugal pumps of the kind normally
used in chemical and similar process industries.
In the event of a fire in one vessel upon the upper
surface of the lighter phase the following sequence of
operations is followed. In such a case, the lighter phase
may be a flammable organic solution and the heavier phase
may be a nonflammable aqueous solution.
A third nonflammable liquid such as water is used
to flood the separator vessel 10 containiny the fire. The
water has a characteristic density greater than that of the
lighter phase. A source of water supply, such as a water
main, is connected to a third supply conduit 60. The third
supply conduit 60 is connected to the organic solution supply
circuit consisting of the organic supply conduits 21, and 18
or the auxiliary conduits 27.
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An isolation valve 52 is provided to guard against
backflow of organic solvent into the water supply source and
to control the fire suppression operationO Via the organic
supply circuit, the water supply conduit is connested to the
inlet of the pump ~1 f each stage.
Isolation valves 50 and 51 are provided at the ends
of the organic supply circuit to prevent water from flowing
into the organic solution source and loaded organic solution
receptor. When valve 52 is opened in response to a fire
sensor signal, valves 50 and 51 are closed to isolate the
water flow in the organic circuit. Valves 42, 46 and 48 are
closed on all stages and valves 44 are closad on all stages
except the ~urning stage. In this way, the disruption caused
by the fire is restricted since only the burning stage is
flooded with water. The other stages are shut down but
remain otherwise unchanged. In the alternative, the burning
stage may have valve 44 closed ~nd valve 42 opened. In the
former case, water flows through outlet conduit 13 into the
burning vessel lO whereas in the latter case, water flows
through the pump P1 static mixer and perforated distributor
pipe ll into the burning vessel lO. When the water is to be
drained to resume normal operations, the water may be
conducted through the outlet conduit 14.
Within the burning vessel lO, the inflow of water
causes the level of liquid in the vessel lO to rise a small
distance (typically about 4 inches) to the upper Eire
suppression level 61 above the operating level 62 of the
first phase. The vessel lO is provided with a third outlet
63 at the fire suppression level 61 for draining away the
flammable lighter phase. As will be appreciated, the third
outlet should be disposed above the level of the above-
mentioned weir overflow connected to outlets 13 for the first
phase. A third discharge conduit 64 connects to the third
outlet conduits 65 of each stage and drains the discharged
organic solution into a containment vessel 66. The organic
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solution may be recirculated via conduit 67 to the organic
supply circuit if suitable or may be disposed of if
unsuitable for reuse.
Referring to Fig. 2, a detailed view of the third
vessel outlet is shown. A weir channel 68 is provided within
each of the vessels 10. The upper edge of the weir channel
68 determines the upper fire suppression level 61. When the
water floods the vessel 10, the flammable liquid 69 floats
above the combined nonflammable layer of water and aqueous
solution 70. The water is fed continuously into the vessel
10 to drain the flammable liquid 69 as it flows over the
upper edge of the weir channel 68. The weir channel 68 is
connected upstream of the third outlet conduit 65.
Each weir channel 68 is connected to a discharge
conduit 64 which drains the liquid received from the channel
68 and passes it to a containment vessel 66.
In order to prevent the propagation of fire along
the third outlet conduit 65, a trap 71 is provided within
each of the third outlet conduits 65. The trap 71 has an
upstream end 72, a downstream end 73 and a middle portion
74. The middle portion 74 is below the upper fire
suppression level 61 in order that the trap 71 maintains a
liquid barrier in the middle portion 74 for arresting the
propagation of fire along the third outlet conduit 65.
It will be appreciated that the weir channel 68 may
also be used to remove gelatinous material floating in the
organic or at the aqueous/organic interface. In such an
operation, vessel 10 is flooded with aqueous by closing
aqueous outlet valve 46, causing the interface to rise. When
the interface nears the organic withdrawal level, organic
outlet valve 44 is closed. The interface will continue to
rise to the fire suppression weir level and a mixture of
organic, gelatinous material or crud and aqueous will cascade
over the weir and be drained to the containment vessel or to
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another vessel provided for thls purpose, to be treated when
convenient. Thereafter valve 46 is opened to lower the
interface level. When it i5 sufEiciently low valve 44 is
opened and the operation continued in a regular manner.
Figure 3 shows one preferred form of apparatus in
accordance with the invention. The same reference numerals
indicate parts which are similar to those de~cribed above
with reference to Figs. l and 2 and it is believed the
construction and operation of the apparatus will be readily
understood from the above description, and do not need to be
further descr.ibed in detail. It should be noted, however,
that in Figure 3, the positioning of the mixer-settler stages
is reversed, so that the stage which is referred to as the
"first stage" in Fig. 1, which receives the pregnant aqueous
solution or heavier phase to be extracted, is positioned at
the right hand end in Fig. 3.
Briefly, however, in Fig. 3 each stage has outlets
31a and 32a for the lighter and heavier phases, respectively,
each of which is a submerged collector, for example in the
form of a perforated pipe, within the layer of the liquid
which it collects. Each collector preferably extends
substantially the full width of the settler vessel to promote
substantially laminar flow without turbulence.
A level-controlling device 81 and 82 is connected
to each outlet conduit 16 and 17. Each device 81 and 82
may comprise, for example, outer and inner concentric
cylinders 83 and 84. The discharge conduit 16 or 17 feeds
liquid to the space between the cylinders 83 and 84, and the
liquid overflows the upper edge of the inner cylinder and is
collected for discharge through a conduit 86 connected to the
bottom of the inner cylinder 84. In this manner, the height
of the upper edge of the inner cylinder 84 connected to the
conduit 16 determines the normal operating level of the upper
or lighter phase in each tank 10 while the position of the
upper edge of the inner cylinder 84 connected to the conduit
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17 serves to control the level of the interface between the ;
lighter and heavier phases.
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