Note: Descriptions are shown in the official language in which they were submitted.
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MIXER SETTLER COLUMN
BACKGROUND OF THE INVENTION
[0001] This invention relates to a mixer settler column which is capable of
multistage mixing and settling.
[0002] Mixing and settling processes for fluids are usually aimed at
transferring a
maximum amount of a particular material from one fluid to another. A material
which
is transferred might be dissolved in, absorbed by, or otherwise associated
with, each
of the two fluids. Multiple mixer-settler stages and counter-current flows are
commonly used to achieve effective operation.
[0003] The word "light" is used, for the sake of convenience herein, to
designate a
fluid which is less dense than another fluid which is referred to as a "dense
fluid".
[0004] Factors which determine the effectiveness of a counter-current
multistage
mixer-settler process include the following:
a) in the case of liquid-liquid extraction, small enough droplet sizes should
be
5 obtained, with high interfacial areas and short diffusion paths
within the
droplets, so that mass transfer is suitably fast;
b) the degree of mixing should be such that elements of the fluids contact
each
other sufficiently closely under conditions of high shear;
c) the residence time per stage during or after mixing should be long enough
to
ensure satisfactory mass transfer;
d) the efficiency per stage (stage efficiency is the fractional extent towards
equilibrium that mass transfer occurs in a stage) should be high ¨ this factor
depends on points b) and c) referred to;
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e) the benefits of more stages and higher stage efficiencies, which are
interdependent, should be optimised to give a satisfactory efficiency at a
required throughput; and
f) the velocities of rising or settling particles, droplets or bubbles should
be high,
for low rising or settling rates limit the throughput of a process.
[0005] A process which requires mixing and settling is usually carried out in
separate mixing and settling units or in columns.
Separate Mixing and Settling Units for a Liquid-liquid Contacting Process
[0006] Conventional mixer and settler units can provide independently for very
small droplets and adequate residence times for excellent mass transfer and
adequate time and conditions for good settling. However the use of multiple
stages
can be expensive and requires a large floor area and large fluid inventories.
Separate Mixing and Settling Units for a Solid-liquid Contacting Process
[0007] Counter-current decantation is commonly done using multiple thickeners
5 together with mixer units. This approach is, however, expensive and
requires a large
footprint. Each mixing and settling stage has limited efficiency for mass
transfer and,
in order to achieve efficient overall mass transfer, multiple stages, which
can be
expensive, and excessive dilution, are required.
Mixing and Settling in Columns
[0008] Multi-stage, counter-current mixing and settling can be done in
suitably
designed columns which generally have packing material or include simple
plates
with perforations, or complex, custom-designed plates. Some column-based
processes are fully continuous and have steady counter-current flows, while
other
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column-based processes make use of pulsing techniques which promote desirable
flow and mass transfer characteristics.
[0009] Although a column can provide for many mass transfer stages and have a
small footprint, the operation of a column is subject to the constraints of
conflicting
needs for high mixing, for good mass transfer, for fast relative movement of
fluids
and high throughputs.
[0010] In continuous or pulsed columns counter-current fluent streams
generally
pass each other in each physical stage by the rise or fall of particles,
droplets or
bubbles of one fluid through the other fluid which is continuous. If the
settling rate is
not sufficiently high to sustain the required counter-current flow a
phenomenon such
as flooding can ensue. This problem can be addressed by increasing the cross-
sectional area of the column but not by increasing its length. The rising or
settling of
particles, droplets or bubbles of fluid is generally required in all of the
stages of a
column and no mechanism is provided for selectively bypassing one or more
stages
5 so that groups of stages can be used in parallel. The terminal velocity
of particles,
droplets or bubbles in one fluid places a limit on the relative counter-
current flows
that are possible without the flooding process taking place. This flooding
limit is
sensitive to the smallness of the particles etc. and small sizes should thus
be
avoided. On the other hand, small sizes are needed for fast diffusion over the
high
surface areas which are associated with small particles.
[0011] It is known in a column application to facilitate counter-current flow
by means
of a downcomer, which is a duct, between stages, that allows unidirectional
downward flow of a dense stream. This, however, requires a lighter stream to
flow
upwards by some means other than through the same downcomer, or duct. The
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downcomer always protrudes downwards to below the overflow level of a
downcomer of a lower plate. This causes the bottom of the downcomer to
protrude
into a body of dense fluid below. Consequently, the lighter fluid has to rise
through
holes in the plate, or through bubble caps attached to the plate, and then to
pass
through a layer of dense fluid above the plate. The passage of the light fluid
through
the layer of dense fluid constitutes a mixing process and a bulk transfer of
the light
fluid upwards, and has an inherent potential of flooding.
[0012] Column usage has mainly been confined to processes with liquids, but
sometimes to relatively free-flowing solid particles, like resin beads, and
has not
been extended to solids such as ore or precipitated particles. Columns,
continuous
or pulsed, are subject to unwanted accumulations of solids that possibly would
not
be flushed away adequately during normal column operation. For this reason, at
least, continuous columns and pulsed columns have not normally been used for
thickening, counter-current decantation, counter-current leaching or particle
classification.
[0013] US3108859 describes the operation of a pulsed column in a mixer-settler
mode or in an emulsion mode. In the former mode liquid settles into distinct
layers
during quiescent phases of an operational cycle. In the latter mode a
dispersed
liquid remains dispersed in relatively small droplets, throughout the column,
with no
substantial change in droplet sizes occurring during the operational cycle.
The
patent describes an improvement that provides alternate physical regions in
which
the light liquid is dispersed within the dense liquid, and the dense liquid is
dispersed
within the light liquid, respectively, by means an arrangement in the column
of
various perforated plates, and upward- and downward-pointing nozzles on upper
and
5 lower sides of sets of sieve plates having different wetting
characteristics. The
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disclosure in this patent does not provide for only one flow direction to
cause mixing,
nor the use of one of the flow directions to accomplish a bulk transfer of one
of the
liquids over a selectable number of plates. Counter-current flow is
accomplished by
droplets settling or rising through the continuous liquid at all stages within
the
column, a factor which limits throughput and does not provide for a selected
compromise between throughput and the number of mass transfer stages.
[0014] US3979281 describes a pulsed column in which a light liquid flows
upwards
continuously and in which a dense liquid is introduced periodically in pulses,
so that
it is moved downwards from plate to plate. A downwards pulse is not aimed at
producing mixing, as ducts associated with each plate have upward and downward
extensions from the plate so as make the down-coming dense liquid enter a
lower
body of dense liquid below rather than being caused to mix with light liquid.
The light
liquid is moved upwardly through perforations in each plate and so becomes
dispersed and rises through the dense liquid above the plate.
5 [0015] Another pulsed column process known to the applicant provides
for liquid
contacting operations in which the liquids flow counter-current in a column
and flow
pulses are superimposed so as to create and maintain the dispersion of
droplets of
one of the liquids within the other liquid. Plates in the column are shaped as
discs
and doughnuts so that liquid transfer between plates occurs through central
holes
and circumferential annuli in alternate plates. An operational cycle of the
column
does not allow for a no-flow settling period. As the dispersed liquid must
settle or
rise through the other liquid, between all adjacent pairs of plates, a
throughput limit
exists due to possible flooding.
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[0016] PCT/1602/00501 describes a column with perforated plates, wherein each
plate has a transfer duct that extends both upwards and downwards from the
plate.
The plates and transfer ducts emulate quite closely the common use of a
downcomer in a column, for example in distillation, to facilitate the transfer
of dense
fluids downwards between stages in a column while light fluid is made to pass
upwards through perforations in the plates or through bubble caps mounted on
the
plates. The downcomer is used to transfer dense fluids from one plate, from an
overflow weir formed by the downcomer's upper end, downwards to a body of
dense
fluid above the plate below. The bottom of the downcomer forms a seal with the
heavier fluid on the plate below so as to prevent any flow of light fluid up
the
downcomer at any time. Instead, the light fluid flows upwards through the
perforations of the plate and becomes dispersed in, and rises through, the
dense
fluid located on top of the plate above. As the dispersed liquid must rise
through the
other liquid, between all adjacent pairs of plates, a throughput limit exists
due to
possible flooding. The mixing intensity possible in the column is determined
by a
pressure drop across the stages that can be applied to force the light fluid
upwards
and disperse it by the action of the perforations in the plates. This pressure
drop is
limited in each stage by the head of dense fluid in the downcomer as the upper
level
of this fluid in the downcomer reaches the level of an upper weir of the
downcomer.
[0017] US3719455 describes the simultaneous use of separate mixer and settler
compartments for each stage in a complex arrangement. Use is made of a
mechanical stirrer. The settler section operates continuously at a settling
rate which
is determined by the requirement of settling in each stage at the full rate of
the
counter-current streams. The throughput rate is therefore not controllable
over a
wide range and can be affected by flooding when settling is incomplete and
mixed
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liquids are transferred instead of separated liquids. Liquid transfers take
place
simultaneously through two ducts and at least one of the these modes depends
on
the action of gravity in combination with fluid density differences. There are
two
main exit ports and, depending on gravity, it might not be possible to ensure
that the
exiting liquids report to the correct exit ports.
[0018] US2665196 and US3804594 are, to a substantial extent, similar to the
preceding citation. Other prior art documents are US2937078; US3325255;
US3389969; US3984331; US3989467; US4541724; US5194152 and W0249736.
[0019] US3549332 discloses three distinct timed sub-operations for mixing and
for
the transfers of light and heavy fluids in opposite directions. Inter stage
ducts
connect the bottom of one stage to the top of another stage so as to
facilitate
different transfers forwards and backwards. The ducts are not used for mixing.
There is no suggestion in this patent of the possibility of using multiple
physical
stages per operational stage. Liquid transfers always involve one liquid
entering
another liquid. Thus a transfer of a liquid volume greater than the normal
operating
volume of that liquid, in a stage, requires the liquid to settle or rise
quickly through
the other liquid. This can possibly cause adverse mixing and entrainment. This
is
an important factor in large scale systems for adverse mixing can be
problematic as
flow cross-sectional areas in the stages scale up only in proportion to flow
rates
raised to 2/3, for similar residence times.
In this citation mixing is done by
mechanical stirring and there is no suggestion of eliminating a separate
mixing step.
The technique described in this citation is not applicable to liquid/solid
operations
because the transfer of any fluid entails a transfer from deep within a
settled bed of
solids where blockages can easily occur.
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[0020] US2228434 describes a solvent extraction process in which a tower is
partitioned into a plurality of vertically spaced components by means of a
number of
plates. Each plate includes a first duct in the form of a phase spray and a
second
duct which extends in an opposite direction to the first duct which is in the
form of a
spout. It is not seen that this process is suitable for the handling of
slurries.
[0021] US4247521 describes a liquid-to-liquid contacting system in which zones
are
defined by plates in a column and each plate has a downcomer and an up-duct.
These ducts are of constant cross-section and do not affect the velocity of a
liquid
flowing through a duct.
[0022] US1741519 discloses a fractionating column in which partitioning decks
carry multiple nozzles. each of which exhibits a venturi type action.
[0023] An object of the present invention is to provide a mixer settler column
which
allows for mixing, settling and counter-current transfers of fluid to be done
cyclically
in a plurality of distinct modes of operation. The operations of mixing,
settling and
transfer are carried out in different time periods in the same physical
regions, instead
of using different physical regions for the various operations. The feature
holds
particular benefits including a capability for flexibility and functionality
that is not
possible in other columns known to the applicant.
[0024] A further object of the invention is to provide a mixer settler column
which
can be configured and operated to implement any of a number of processes such
as
liquid-liquid contacting operations, counter-current decantation, counter-
current
leaching, thickening, ion exchange, distillation, gas liquid contacting
operations, and
classification.
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SUMMARY OF INVENTION
[0025] The invention provides apparatus for the processing of dense and light
fluids
that includes an elongate column which, in use, is vertically disposed and
which has
a longitudinally extending axis, a plurality of plates located at spaced
intervals from
one another inside the column, each plate extending transversely to the
longitudinal
axis, wherein each plate forms at least part of a respective fluid-containing
volume
which tends to trap fluid and which prevents the trapped fluid from flowing
towards
an adjacent plate and wherein each plate includes at least one respective
transfer
duct with a first end attached to the plate and a second end, which is spaced
from
the first end, a first aperture with a cross-section of a first area at the
first end and a
second aperture with a cross-section of a second area which is larger than the
first
area at the second end, characterised in that fluid flow from one fluid-
containing
volume to an adjacent fluid-containing volume takes place only through the
said at
least one transfer duct.
[0026] It is to be noted, by way of contrast, that in one conventional
distillation
column for example, a transfer duct extends upwards and downwards from a plate
so as to provide for an overflow of dense fluid that moves down through the
duct to
join a similar dense fluid below, without mixing. Upwards flow through the
duct of a
light fluid is blocked and the light fluid must, instead, go through holes in
the plate for
dispersion into the dense fluid above.
[0027] Depending on the mode of operation of the column of the invention it
may be
orientated so that the respective first ends of the transfer ducts are lower
than the
respective second ends of the transfer ducts. Preferably, the transfer ducts
are
positioned e.g. by suitable placement of the plates so that, with respect to a
first
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plate which is adjacent a second plate inside the column, the transfer duct or
ducts
on the first plate are displaced in a circumferential sense relative to the
transfer duct
or ducts in the second plate i.e. a transfer duct in the first plate is not
aligned nor in
register with a transfer duct in the second plate, viewed in a longitudinal
direction
through the column.
[0028] Preferably each plate forms at least part of a fluid-containing volume
which
tends to trap fluid and which prevents the trapped fluid from flowing towards
an
adjacent plate.
[0029] The fluid-containing volume may be formed, at least, by at least one
duct, at
least part of the respective plate and a portion of a wall from which the
column is
made. This volume may be orientated to face downwardly or upwardly (with
reference to the respective plate) depending on the mode of operation of the
column.
[0030] Preferably each transfer duct is shaped and sized so that fluid flow at
a
suitable velocity through the duct from the second end towards the first end
induces
turbulence inside a fluid-containing volume associated with a plate adjacent
the first
end, and fluid flow, at a suitable fluid velocity through the transfer duct
from the first
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end towards the second end, causes laminar or near-laminar flow of the fluid
over a
fluid-containing volume of the respective plate.
[0031] The invention also extends to a method of using the aforementioned
column
which is orientated so that the longitudinal axis is vertical and so that, in
respect of
5 each plate, the respective first aperture is lowermost and the
respective second
aperture is uppermost, the method including the steps of processing dense and
light
fluids in a cyclical sequence of mixing, settling and counter-current
transfers.
[0032] Mixing may be carried out for a predetermined period by introducing a
fluid
with a suitable flow rate into the column at or near its upper end thereby to
cause a
10 downward flow of fluid in the column and mixing of dense and light
fluids between
each adjacent pair of plates within the column.
[0033] The fluid which is introduced at the upper end may be a light fluid, a
dense
fluid or a mixture thereof.
[0034] Settling may be carried out within the column, for a defined period, by
the
step of imposing a net zero flow of fluid through the column.
[0035] After the settling mode has been completed bulk transfer of the light
fluid
within the column may be effected for a defined period by introducing fluid at
a
suitable flow rate into the column near or at a lower end of the column
thereby to
cause upward flow of only light fluid in the column while maintaining the
dense fluid,
substantially unmoved, within the respective fluid-containing volumes of the
plates.
[0036] The fluid which is introduced to the lower end of the column could be a
light
fluid, a dense fluid or a mixture thereof.
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[0037] The apparatus may be inverted so that the column, still in a vertical
orientation, has the first apertures which are associated with the respective
plates
higher than the second apertures which are associated with the respective
plates. In
this form of the invention a mixing step may be effected by introducing a
fluid into the
column at or near a lower end of the column.
[0038] Settling in this mode of operation may be effected by imposing a net
zero
flow of fluid through the column for a defined period.
[0039] In order to carry out the step of bulk transfer (in the second mode of
operation) a fluid is introduced into the column at or near an upper end of
the
column. This fluid, which may be a light fluid, a dense fluid or a combination
of light
and dense fluids, may be introduced at a flow rate which does not displace, to
any
substantial extent, fluid contained within any of the fluid-containing volumes
associated with the respective plates.
[0040] The aforementioned process can be carried out to achieve counter-
current,
liquid-to-liquid contacting operations such as those employed in solvent
extraction.
In a different application, the light and dense fluids are a liquid, and a
slurry of solid
particles and a liquid, respectively, and the process is carried out to
achieve counter-
current decantation.
[0041] As used herein "fluid" designates a gas, or a liquid, or a slurry of
solid
particles and a liquid,
[0042] In another application the light and dense fluids are liquid, and a
slurry of
solid particles and a liquid respectively. With the column in the first mode
of
orientation the settling step and the bulk transfer step are repeated between
each
pair of consecutive mixing sterilAil;'hHrE,Frcess being aimed at thickening.
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[0043] With the column in the first mode of orientation the light and dense
fluids
may be liquid, and a slurry of solid particles and a liquid, respectively, and
the
process may be aimed at counter-current leaching.
[0044] In another application, again one in which the light and dense fluids
are
liquid, and a slurry of solid particles and a liquid, respectively, the
process is aimed
at counter-current leaching and the column is positioned between adjacent
mixing
tanks,
[0045] In another application, one fluid is a liquid, or a slurry of a liquid
and
particles, and the other fluid is a slurry of ion-exchange beads or fibres and
a liquid,
and the process is aimed at a counter-current ion exchange.
[0046] With the column in the inverted form, in one application, the light and
dense
fluids are vapour or gas, and liquid, respectively, and the process is aimed
at
counter-current distillation or gas-liquid contacting operations. In
another
application, the light and dense fluids are a liquid containing slow settling
particles.
and a liquid containing fast settling particles, respectively, and the process
is aimed
at counter-current classification.
[0047] The invention also extends to a material handling process wherein,
within a
vertically orientated column, mixing of light and dense fluids is achieved by
concurrent flow in a first direction, at suitable velocities, of the fluids
through the
column, settling is carried out by reducing to zero, for a defined period, the
net flow
of fluid through the column, and counter-current bulk transfer is carried out
by
directing the light fluid in a second direction opposite to the first
direction through the
column thereby to leave the dense fluid substantially undisturbed and within a
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plurality of fluid-containing volumes which are longitudinally spaced from one
another
within the column,
[0048] The column provides for mixing, settling and counter-current transfers
of fluid
to be done cyclically, in three or more distinct modes of operation in the
same
physical regions, and not in different physical regions. The column has plates
with
specially shaped transfer ducts that facilitate turbulent mixing of the fluids
when the
flow rate is high and downwards, and that facilitate near-laminar flow of only
the light
settled fluid when the flow rate is relatively low and upwards. The plates and
ducts,
and possibly also the column walls, are arranged in a way that provides a
containing
volume in each physical stage that tends to dam the dense fluid, so that it
normally
does not flow downwards. In an inverted version of the column, mixing would
occur
with upward flow, transfer of the dense fluid would occur with downward flow,
and
the column would have a containing volume in each physical stage that tends to
collect the light fluid so that it normally does not flow upwards. The column
is
operated with controlled flow rates of fluids into and out of the column, so
as to
obtain the desired conditions for mixing, settling and counter-current
transfers of
fluids. The top and the bottom of the column have special structures
(internally or
externally) designed to facilitate the required operation of the column.
[0049] The transfer duct attached to a plate comprises a weir which extends in
one
direction only, namely upwards or downwards from the plate. The transfer duct
is
not a downcomer (as referred to in the prior art) that dips below the level of
the
overflow of a lower plate Thus an inter-stage transfer duct is used to
transport
dense fluid downwards and light fluid upwards under circumstances and during
time
periods when settling rates impose no limit on the extent of the fluid flow
rates. Any
reasonable mixing intensity, for example to obtain small bubbles or droplets,
is
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possible without the associated settling velocities having a direct effect on
the
counter-current flow rates. The flow of dense fluids downwards occurs during
mixing
and the flow simultaneously entrains light fluid that moves downwards too,
without
any need for differential flow rates to be effected by a settling mechanism.
The flow
of light fluid upwards during a transfer period bypasses the heavier fluid and
occurs
when the fluids are not required to mix or to be mixed. The flow of light
fluid upwards
exceeds the downwards flow during mixing so that the net flow of light fluid
is
upwards. It is to be borne in mind that the different cross-sectional areas of
opposed
ends of each transfer duct are required to ensure that the high and low fluid
flow
rates. in opposing directions, are achieved in order to facilitate mixing and
bulk
transfer.
[0050] The column of the invention allows for the use of multiple plates or
stages, in
the column, in a parallel sense. This is achieved by making the mixing and
transfer
fluid movements go over more than one plate at a time. Prior art columns,
known to
the applicant, have counter-current fluids interacting with each other
similarly in each
stage and it is therefore not possible to provide a facility of bypassing
stages
selectively to make the plates operate in parallel and so extend the
throughput
efficiency optimization range.
BRIEF DESCRIPTION OF THE DRAWINGS
[0051] The invention is further described by way of examples with reference to
the
accompanying drawings in which:
Figure 1 is a side view in section of a mixer settler column according to one
form of
the invention;
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Figure 2 is a side view in section and on enlarged scale of part of the column
in
Figure 1, illustrating fluid flow during a mixing phase;
Figure 3 is similar to Figure 2 illustrating fluid flow during a bulk transfer
phase;
Figure 4 shows the column of Figure 1 in an inverted mode for a different
application:
5 Figures 5, 6 and 7 depict different types of transfer ducts which can be
used in the
column of the invention; and
Figures 8, 9. 10 and 11 are side views in cross-section illustrating different
forms of
the column of the invention in different applications.
DESCRIPTION OF PREFERRED EMBODIMENTS
10 [0052] Figure 1 shows in cross-section and from one side a general
structure of a
mixer settler column 10 according to the invention. The column includes an
elongate
tubular body 12 with a surrounding wall 14 which encloses an elongate volume
16.
The body has a longitudinally extending axis 18. An upper end 20 of the body
has a
dense fluid feed inlet connection 22, a light fluid return connection 24, and
a light
15 fluid outflow connection 26. A lower end 28 of the body has a bottom
fluid return
connection 30. a mixed or dense fluid outflow connection 32, and a light fluid
connection 34
[0053] A plurality of plates 36 are provided at spaced intervals from one
another at
respective positions which are displaced along the longitudinal axis. Each
respective
plate has at least one transfer duct 38 which allows for fluid flow between
the plates
which define plate stages within the column.
[0054] The plates 36 are spaced closely enough to give adequately short
settling
distances for small particles or droplets of a dense fluid to settle in a
reasonable
time. Each transfer duct has, at a first end 40 (see Figure 2), a first
aperture 42 of a
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first area and, at a second end 44, a second aperture 46 of a second area
which is
greater than the first area.
[0055] The transfer ducts can have various types of construction and Figures 5
to 7,
referred to hereinafter, illustrate possibilities in this regard. The shaping
of each duct
is such that a reasonably high downward fluid flow rate in the column leads to
high
velocity entry and turbulent mixing in the stage below, as is shown in Figure
2
Additionally, as each duct widens in cross-section towards its upper end (the
aperture 46), a reasonably low upwards fluid flow rate in the column leads to
a low
flow velocity and near laminar flow, of light fluid, above settled beds of
dense fluid
(associated with the various plates), without causing unwanted mixing of the
fluids,
as is shown in Figure 3.
[0056] Each transfer duct may be of any convenient three-dimensional shape
which
allows for the aperture 42 to be smaller in cross-sectional area than the
second
aperture 46.
[0057] Each plate is associated with and helps to define a respective fluid-
containing volume 50 which, with the column vertically orientated as in Figure
1, is
for the collection of dense fluid on the plate. The size of the volume is
determined, at
least, by the distance between the first end 40 and the second end 44 of the
transfer
duct. If the dense fluid level is too high, then dense fluid above the level
of the
second end 44 can overflow the second end and can pass through the transfer
duct
to the plate below. In a simple form of construction each plate has one
transfer duct.
The plates are positioned so that successive ducts are on alternating sides of
the
column, as is shown in Figure 1. Also, the transfer ducts, in adjacent plates,
are not
aligned in a longitudinal sense with one another but rather are displaced
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circumferentially from one another. Depending on factors such as cost and
specific
operating conditions, a plate may have one large transfer duct or multiple
transfer
ducts.
[0058] Figure 5 for example illustrates two plates 36A and 36B which have
opposed
ducts 38A and 38B respectively, each of which extends over a circumferential
arc of
the respective plate. Figure 6 shows a plate 36C which has a plurality of
funnel-
shaped ducts 38C at spaced intervals over its surface. Additionally, the ducts
are
grouped in quadrants by means of transversely extending partitions 54 and 56
respectively. In the arrangement shown in Figure 7 a plate 36D has three
channel-
shaped transfer ducts 38D, 38E and 38F.
[0059] In each case adjacent plates are orientated so that, within the column,
no
duct on one plate is immediately above or below a duct on an adjacent plate.
[0060] In operation of the column of Figure 1 the contents of the main body
within
the column are mixed in all local regions and are moved downwards by the fast
entry
of a required volume of fluids into the top of the column. At this end a dense
fluid
feed and a light fluid return are fed through the respective connections 22
and 24.
Flows can occur consecutively but preferably are simultaneously introduced
into the
column. Mixing within the column is induced by turbulent eddies caused by a
high
fluid flow rate, as illustrated in Figure 2. In each duct the downwards flow
of the
fluids through the first aperture 42, which is of relatively small cross-
sectional area,
causes a stream or jet of fluid to be directed, at an increased speed, into
the
respective underlying fluid-containing volume 50 in which turbulence is
induced.
[0061] The magnitude of the total volume of fluids introduced during the
mixing
phase should be adjusted to fix the required volume in the column per mass
transfer
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stage. This could range from a fraction of a physical stage volume (i.e. the
size of
the volume bounded by two adjacent plates 36, extended though their respective
first
apertures 42, and the column wall 14) or any multiple, not necessarily an
integer, of
physical stage volumes.
[0062] During mixing the dense and light fluids both move between the physical
stages_ The total volumes of the dense fluid feed and the light fluid return
are
introduced in the same ratio as is required for the contents of the main body
of fluid
in the column as a whole. This factor determines the volume fraction occupied
by
the dense fluid within a fluid-containing volume 50, rather than the size of
the fluid-
containing volume. However, the ratio of dense to light fluid should not be so
large
as to cause a higher dense-fluid volume per stage than can be dammed by the
fluid-
containing volume 50 of the stage. Generally, the containing volume 50 would
be
made a reasonably high percentage of the volume of a physical stage, by
increasing
the distance between the first end 40 and the second end 44 of a duct, so that
operation over a wide range of fluid ratios is possible.
[0063] After the mixing phase a settling mode is implemented by stopping all
flows
into and out of the column. Sufficient time is allowed for settling to be
completed or
for segregation to be effected to the required extent e.g. for classification
by particle
size. As the plates 38 are closely spaced fast settling can take place even
when
particles, droplets or bubbles (as the case may be) are very small. The
settled
particles etc. accumulate in the respective fluid-containing volumes 50.
[0064] The settling phase is followed by a bulk transfer phase. During mixing,
a
volume of the light fluid would have been moved into and down the column
together
with the dense fluid. Thus, during the bulk transfer phase a similar volume of
fluid,
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drawn or extracted from the mixed fluid outflow at the connection 32, should
be
returned to the bottom of the column via the return 30 so that the light fluid
is pushed
back up the column to its position before mixing. Fresh light fluid is fed
through the
connection 34 to produce an additional upward movement so that the light fluid
has a
net upward flow over the full operational cycle.
[0065] In the main body of the column the light fluid, above each respective
settled
bed of dense fluid in the respective volumes 50, is moved upwards in near
laminar
flow by the moderate flow rate of a required volume of fluids into the bottom
of the
column. The light fluid feed at the connection 34 and the bottom fluid return
at the
connection 30 are made to enter the column at low to moderate flow rates.
These
flows can occur simultaneously or consecutively, preferably with the light
fluid feed
entering through the connection 34 after the bottom fluid return.
[0066] Figure 3 illustrates near laminar flow 60, for the bulk transfer of the
light
fluids in the column, which is induced by low fluid velocities. Ideally the
near laminar
flow 60 should cause plug flow of all the light fluid up the column without
any dead
volumes. In practice, with moderate flow rates that are not too low, some
beneficial
local mixing takes place which tends to prevent dead volumes of the light
fluid that is
not transferred. In liquid-liquid contacting operations, surface tension
effects help to
prevent any mixing of the light and dense fluids during bulk transfer. In
particulate
processes, especially in classification, the flow rate of the light fluid
during bulk
transfer should be set carefully to regulate or minimize the degree of mixing
between
the light and dense fluids.
[0067] The mixed fluid outflow at the connection 32 should preferably be
settled so
as to produce a light fluid to be returned to the bottom of the column (via
the
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connection 30) and a dense fluid product. The bottom of the column can have
features to facilitate this, or the separation can be done using a suitable
external
processing unit such as a thickener.
[0068] Some adaptations to the column may be required to adapt the column to
be
5 suitable for particular applications.
[0069] For example. the separation of fluids from, or within, the ends of the
column,
and accurate timing and implementation of various forward and reverse flows in
the
column, require the upper and lower ends of the column and appropriate
peripheral
equipment to be customised according to the type of process to be implemented.
10 [0070] In most cases, the separation of the light and dense fluids at
an end of the
column. where a return flow is required, is best done within the end of the
column, so
that no mixed light and dense fluids leave the column and so that there is no
need to
separate the mixed fluids externally. For this, the transfer ducts of one or
more plates
at the end of the column might be made wide and without any narrow aperture,
or
15 made to be just relatively large holes in the plates, so that further
mixing tends not to
occur there and so that those plates only serve to facilitate settling. In
other cases.
for example in counter-current decantation, an external separator such as a
thickener might be better.
[0071] Accurate timing and implementation of the various forward and reverse
flows
20 for the operation of the column require a means for flows to be
started, regulated,
stopped and, sometimes, reversed. This can be done by suitable combinations of
process components such as pipes, valves, pumps, holding vessels, and pulse-
generating actuators; measuring devices such as interface detectors, level
detectors
and flow meters; and electronic or computer control systems. Where flows of
fluid
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occur for relatively long times or need relatively large displacement volumes.
switched pumps or on-off valves might best be used. Alternatively, where flows
of
fluid occur for relatively brief times or need relatively small displacement
volumes, for
example as might be required for the mixing mode of the column, a pneumatic or
mechanical pulse-generating actuator might best be used.
Liquid-liquid contacting operations
[0072] The normal, or the inverted, version of the column (i.e. Figure 1 or
Figure 4)
can be used for liquid-liquid contacting operations. When the ratio of light
to dense
liquid volumes in the column is required to be close to unity, the two
orientations of
the column tend to favour dense, and light continuous-phase liquids,
respectively,
otherwise the liquid of higher volume would tend to become the continuous-
phase
liquid during settling. When the ratio of light to dense liquid volumes in the
column is
required to be very high or very low, the orientation of the column should be
normal
or inverted, respectively, to facilitate good liquid dynamics during the bulk
transfer
mode of operation.
[0073] Figure 8 illustrates an example of customisation of the column for
liquid-
liquid contacting operations. In this case, the bottom of the column can
conveniently
be used to provide for separation of the dense and light liquids so that the
bottom
liquid return is separated dense liquid that pushes light liquid up the
column. At the
top of the column, a buffer volume 61 can be allocated to allow for the
reverse-flow
volume of the light liquid during mixing, and for excess light liquid there to
overflow
and to leave the column.
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Counter-current decantation
[0074] Figure 9 illustrates an example of customisation of the column for
counter-
current decantation. In this case, the average slurry density in the column is
made to
be the same as that of the slurry feed, so there is no light fluid return at
the top of the
column. Generally, solids from the process are required to be thickened to a
density
much higher than that of the slurry feed, and a thickener is used downstream
of the
column to thicken the solids and provide a recycled thickener overflow that
serves as
the bottom fluid return. If necessary, an upstream thickener can be used to
thicken
the feed slurry before it enters the column, in which case a downstream
thickener
might not be needed,
Counter-current leaching
[0075] The use of the column for counter-current leaching is similar to the
application for counter-current decantation, but mass transfer rates,
residence times
and solution addition points need attention. The column functions
satisfactorily for
leaching where the rate limiting step in mass transfer is diffusion within
solid
particles. Where the rate limiting step in mass transfer is diffusion in
liquid films, the
downward mixing flows should be short and frequent for optimum operation. By
moderating the downward mixing flow rates optimally, the transfer of solids
down the
column could be made size-dependent so that larger particles have the longer
residence times which are needed for adequate leaching. Different liquid
streams
can be added at the bottom of the column, and at positions higher up, to
provide
washing fluid that leaves with the exiting solids, and to provide one or more
leaching
solutions that should flow upwards, respectively.
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Connecting unit between adjacent stirred tanks for counter-current leaching
[0076] The column can be used to connect a pair of stirred each reactors so as
to
transfer solids from a first tank to a second tank and liquid from the second
tank to
the first tank. The column needs to have only one pipe connected at each of
the two
ends as shown in Figure 10. Liquid is pumped slowly from the top of the column
to
the first tank 62, with one or more breaks in pumping for settling if
necessary, to
ensure that only clear liquid leaves the top and solids are retained on the
plates.
Slurry from the first tank 62 is pumped at a fast rate into the top of the
column,
causing the settled solids in the column to be mixed with the slurry and to
move
downwards towards the connection to the second tank 64. The mixing flow should
push the contents of the column downwards by less than the full volume of the
column. for example half of the volume, to give approximately two mass-
transfer
stages of counter-current decantation.
Thickening
[0077] The operation of the column for thickening is illustrated in Figure 11.
This
requires a more complex cyclic operation. Between the mixing modes of
successive
operational cycles, several sub-cycles of settling and bulk transfer modes are
implemented to build up the thicknesses of settled solids on the plates of the
column.
Mixing is achieved by introducing slurry or recycled clear liquid at a high
flow rate
(66) at the top of the column. The bulk transfer is done by introducing slurry
at a low
or moderate flow rate (68) at the bottom of the column and by stopping the
transfer
when the slurry has displaced most of the clear liquid above the settled
solids. The
settling and bulk transfer sub-cycles are stopped when enough solids have been
loaded onto the plates of the column.
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[0078] The column can be regarded as an advanced form of pulsed column, but
its
structure and operation have special features that make it more effective and
applicable to more processes than a conventional pulsed column.
[0079] A particular feature of the operation of the column is that sets of
adjacent
plates can effectively be made to operate in parallel, thus allowing a trade-
off
between throughput capacity and the number of mass-transfer stages. This is
done
dynamically simply by changing the volumes of fluids transferred upwards and
downwards during the operating cycles. This versatility allows a tall and thin
column,
with many plates, to be operated quite similarly to a shorter and wider column
with
fewer plates and the same total plate surface area. Because bulk counter-
current
movement of streams is not required during mixing or settling, the column can
operate with small particles, droplets or bubbles. with good mixing and with
efficient
settling, without necessarily limiting throughput.
[0080] Splitting of the operation of the column into distinct time-delimited
modes
and the use of multiple plates per mass-transfer stage provide exceptional
flexibility
and functionality. The average feed rates of fluids, the degree of mixing in
the
column, and the ratio of fluids within the column, can all be set
independently.
[0081] Although the use of conventional pulsed columns is generally confined
to
liquid-liquid contacting operations, the column of the invention can be used
for liquid-
liquid contacting operations as well as several other processes, by
customization of
the time-delimited modes. The column can be used for counter-current
decantation,
with many stages of washing, less dilution and lower costs than with multiple
conventional thickeners. Thickening in the column can be achieved by multiple
sub-
cycles of slurry feeds and settling periods, with the build-up of solids on
the plates
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and the displacement of clear liquid upwards, and then a downwards mixing flow
to
drive the thickened pulp downwards. Ion exchange and leaching operations in
the
column can in some cases be very effective, particularly when the rate-
limiting step
of mass transfer tends to be diffusion, or reactions, within resin or solid
particles.
5 Under well-controlled conditions, the column can be used to do counter-
current,
multi-stage classification of solid particles based on their settling
velocities.
[0082] The column does not have the problem of counter-current fluid streams
needing to pass each other in each physical stage by the rise or fall of
particles.
droplets or bubbles of one fluid through the other continuous fluid. Larger
counter-
10 current flows are effected simply by larger fluid transfers per
operational cycle,
independently of the settling part of the process.
[0083] The versatility of the column provided by the degrees of freedom
available
for its operation allows a single column with standard plates to be used for
any of a
number of processes, and at any of a wide range of operating conditions. Three
15 distinct process operations, namely mixing, settling and transfer, in a
time cycle, are
carried out in the same place in the column in simple stages and with no
internal
moving parts. Of importance in the present invention is the step of making one
or
both of the transfers induce mixing to eliminate the need for a separate
mixing step.
The column also has the advantage of providing for essentially instantaneous
shut-
20 downs and instantaneous restarts after any reasonable length of delay,
with no
process disruption, by freezing the column's operation in the settling phase.
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