Note: Descriptions are shown in the official language in which they were submitted.
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COUNTER CURRENT DIFFUSION EXTRACTOR
The present invention relates to an improved counter current extractor having
improved flow
characteristics.
Counter current extractors (CCE's) are well known in the ,food processing,
industry for the
continuous extraction of liquids, solubles, and fine particulate matter from
associated solids.
Such extractors may comprise a screw conveyor or pair of parallel counter
rotating screw
conveyors mounted within an inclined or vertical elongate housing, which takes
the form of a
trough or enclosed tube. Material to be processed is fed into the lower end of
the housing and
carned upward by screw rotation while an extracting liquid is fed into the top
of the housing via
1o spray jets and flows downwards under gravity.
When the solid phase material reaches the top end of the housing,' it is
removed from the
housing, either by a slat-type conveyor, or by being pushed up a ramp by the
material being
expelled behind it. The solid phase material may be pressed to remove liquid
entrained therein,
and this liquid is returned to an intermediate position of the CCE trough.
Generally, strainer means are provided to filter large particles of the solid
phase from the extract
liquid prior to its removal from the lower end of the housing. Problems have
arisen in the past
with particles of solid matter being trapped against the strainer means by the
pressure of liquid
2o flowing through the strainer means, thus blocking flow. This is
particularly so where the material
to be processed is added cold, so that increased quantities of hot recycle
liquid are required to
heat the material to improve extraction.
Another problem which arises from increased extracting liquid flow is the
tendency of the solid
2s phase to disintegrate under the higher spray pressure of the extracting
liquid as it enters the CCE
extractor. This contributes to the amount of small particulate matter within
the extractor which
causes blockage of the strainer
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means. In addition small particulate matter can create a
blockage or reduce the porosity of the solids phase itself
through the entire length of the CCE. This in turn
reduces the f low rate of the liquid phase down the CCE ~.n
relation to the flow rate of the solid phase moving up the
CCE, resulting in a high liquid retention level throughout
the CCE causing the liquid to by-pass the solid phase
resulting in higher dilution and lower yield.
Whilst it is desirable to ensure that the liquid
phase passes freely through the retaining means, it is
also desirable that a controlled amount of liquid be
retained in the lower regions of the housing to enhance,.
the solid/liquid contact in the vicinity of the solids~~
charging point. In many, if not all applications. when
solids are first added to the CCE, there is a net uptake
of liquid into the solid phase, resulting in an increase
in mass and volume of the solid phase. Whilst not wishing
to be bound to any theory of operation, it is believed
that where fruit. for example, enters the CCE with live,
intact cell walls, osmotic pressure drives water into the
cell, whilst the still organised cell wall ejects soluble
compounds into the extracting liquid. After an initial
uptake of Water, lysing of the cell due to heat occurs,
releasing the cytoplasm and added water. A similar effect
is noted for dry material such as coffee beans, due to the
hygroscopic nature of the material, rather than osmosis.
No subsequent release of water occurs in this case.
The transfer of liquid into and out of the solid
phase is believed to have a significant effect on both
concentration of the extract and the yield of soluble
components transferred from the solid to the liquid
phase. Hy controlling liquid level during the early
processing of the solid phase, improved extraction can be .
achieved.
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The present invention seeks to provide means for alleviating or overcoming
one or more of these problems. According to one aspect of the invention there
is
provided a CCE which includes an elongate housing, a screw conveyor provided
in
the housing, strainer means at one end of the housing for filtering a liquid
phase
passing out of the CCE and clearing means associated with the strainer means
for
preventing the strainer means from remaining clogged with particulate
material. The
clearing means includes at least one liquid jet which ejects liquid so as to
dislodge
particulate material from the strainer means. The strainer means comprises a
wire '
screen having an upstream side and a downstream side and having apertures
therein,
the wire screen being formed of wedge wire so that a substantially flat
profile is
created on the upstream side of the screen and the apertures in the screen are
wider on
the downstream side of the screen than the upstream side of the screen.
In another aspect of the invention, there is provided a CCE which has an
elongate housing with a solid phase inlet at a first end portion and ~a solid
phase outlet
at a second end portion. A screw conveyor is disposed within the housing and
is
rotatable about its longitudinal axis to move solid phase material introduced
into the
housing from the first end portion to the second end portion. Means is
provided for
spilling extracting liquid into the second end portion of the housing so that
extracting
liquid flows in a cascade over and through the solid phase material to the
first end
portion. Drive means is provided for causing the screw conveyor to rotate. The
CCE
also includes strainer means for separating liquid phase leaving the housing
from
solid phase material entering the housing. The strainer means includes a wire
screen
having an upstream side and a downstream side and having apertures therein.
The
screen is formed of wedge wire so that a substantially flat profile is created
on the
upstream side of the screen and the apertures in the screen are wider on the
downstream side of the screen than the upstream side of the screen. The CCE
also
includes clearing means for keeping the strainer means substantially clear of
solid
particles. The clearing means includes at least one liquid yet for spraying
liquid onto
the strainer means at sufFcient velocity to dislodge larger particles from the
stream
into the bulk of the solid phase material, while smaller solids are pushed
through the
strainer means into the outflow of the extracting liquid. The strainer means
is
rotatable so that different portions of the strainer means are presented to
the clearing
means. The CCE further includes control means for maintaining extracting
liquid at
the first end portion of the housing at a predetermined level.
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The CCE may have liquid feed means so that a cascade of hot juice flows over
the material to be processed as it is conveyed through the CCE housing by the
screw.
In a preferred embodiment of the invention, there is
provided a CCE including an elongate housing in the form.
of a trough or tube, and having a solid phase inlet at or
adjacent a first end and a solid phase outlet at or
. adjacent a second end, a screw conveyor disposed within
the housing and rotatable about its longitudinal axis to
move solid phase material introduced into the housing from
the said first end to the said second end, means for
introducing an extracting liquid into the said second end
of the housing and to cause it to flow along the housing
to the said first end, drive means to cause the screw
conveyor to rotate, strainer means to separate a liquid
phase leaving the housing from the solid phase material
entering the housing, and clearing means to keep the
strainer means substantially clear of solid particles
wherein the clearing means comprises one or more liquid
jets, whereby liquid may be sprayed onto said wire screen
at sufficient velocity to dislodge larger solids from the
screen into the bulk of the solid phase material, whilst
smaller solids are pushed through 'the screen into the
' outflow of the extracting liquid, while the strainer means
is adapted to rotate so that different portions of the
strainer means are presented to the clearing means, and
wherein the level of the extracting liquid at the first
end of the housing is maintained at a predetermined level
by control means.
The liquid jets are preferably flat fan jets
positioned such that the spray from the jets forms a
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lateral chord across the strainer means approximately one
third of the way from the top of the strainer means. The
jets are preferably angled at approximately 25-45° from
the plane of the strainer means, and are directed
downwardly onto the screen. It is preferred that the
liquid used is obtained from a balance tank used to store
the extracting liquid which is discharged from the first
end of the counter current extractor.
The wire screen is preferably unobstructed across
its entire screening area and has apertures, the minimum
dimensions of which range from about 2.5 millimetres to 10
millimetres, but are normally about 4 millimetres. It is
preferred that the apertures are wider on the downstream
side of the strainer means and this is achieved by using
wedge wire to form a screen with a flat sided profile on
the top-side or product solids side of the screen so that
the smaller solids will clear easily through the wider
profile on the bottom side or liquid side of the screen.
The screen is made up of several segments so that the
wedge wire forms a radial pattern out from the main screw
shaft or axis. This has been designed in this pattern so
that as the screen rotates, the liquid jets strike the
flat profile of the screen at an angle of approximately
35°, causing the solids, that are pressed against the flat
surface, to peel off the screen in a continuous roll back
into the main body of solids in the CCE. As the screen
rotates further the wedge wire presents longitudinally or
vertically in relation to the liquid jets so that the high
velocity liquid spray will pass through the screen taking
the smaller solid particles along with it, leaving the
screen clear and ready to be re-submerged into the liquid
and solids in the lower segment of the CCE.
In a further preferred embodiment, a driving shaft
for the screw conveyor projects through the wire screen.
Clearing means are attached to the driving shaft so as to
rotate therewith, said clearing means being at least one
wiper member in close juxtaposition with the wire screen
so as to clear solid phase material from the wire screen.
The wiper member may comprise one or more blades extending
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radially from the driving shaft sufficiently close to the
wire screen to dislodge solid phase material therefrom.
The clearing means may comprise a liquid jet and a
wiper member in conjunction.
It will be appreciated that rather than having a
fixed wire screen with rotating clearing means, the wire
screen may be fixed to the driving shaft to rotate
therewith, whilst the clearing means remains stationary to
achieve the same effect.
In another embodiment of the invention, the liquid
feed means for introducing liquid into the housing
comprises a reservoir having a weir arranged laterally
across said elongate housing such that extraction liquid
introduced into the reservoir spills over the weir and
flows in a cascade over and through the solid phase
material as said material is conveyed by the screw
conveyor from the first end toward the second end of the
counter current extractor. This avoids the spray of
extracting liquid onto the solid phase of prior art
methods and thereby reduces breakup of the solid phase.
Means are preferably provided for recycling
extracting liquid from the first end of the housing,
heating it to some desired temperature and returning it
via liquid feed means to the housing at a point between
the first end and the second end of the housing. A
suitable proportion of the extracting liquid discharged
from the lower end of the housing is desirably diverted
through an independent heat exchanger where it is heated
and recirculated to the housing at some point above the
said first end. As the objective is to achieve rapid
heating of material at an early stage of processing, the
heated, recirculated extraction liquid should be
introduced into the lower half of the housing, preferably
at a point from about one twentieth to about one quarter
the length of the housing from the said first end.
Suitable liquid level control means may comprise an
adjustable height outlet within the counter current
extractor trough, requiring liquid to reach the height of
the outlet before exiting the trough. Similar control
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means may comprise an adjustable height inlet into a
balance reservoir tank in which liquid is kept for
recirculation purposes. This allows the outlet from the
counter current extractor trough to be flush with the
bottom of the trough, aiding in cleaning of the trough.
Because the inlet to the balance tank is adjusted to be
higher than the bottom of the counter current extractor
trough, liquid pools in the trough until reaching the
level of the inlet to the balance tank. A further method
for controlling the liquid level is by use of a level
sensing control system. The required liquid level is
programmed into a programmed logic control (PLCj unit.
The PLC unit monitors the liquid level using, for example,
an ultrasonic sensor and opens or closes a motorised
liquid outlet valve so as to maintain a constant level.
The housing of the extractor is preferably provided
with a jacket through which a fluid may be passed. In
this way the temperature of the housing and to some degree
the temperature of the contents thereof may be controlled.
The extracting liquid will frequently be water;
however, other liquids including organic liquids or
aqueous or organic solutions could be used. The process
may be used for the extraction of soluble or dispersible
material from fruit or vegetable matter such as sugarbeet,
sweet sorgum, grapes, grape marc or grape pomace, tea,
citrus fruit, citrus peel, apples, pears, coffee beans,
vanilla beans, and from animal matter such as fish heads
and offal. Residue material such as apple peels and
cores, citrus peel and grape marc or grape pomace may also
be exhaustively extracted to recover valuable soluble
components which are normally discarded with these
residues. Soluble sugars recovered from apple or pear
peels and cores could be used in the preparation of
canning syrups or fermented .to alcohol. Other residues
contain material such as flavouring, colours or specific
useful materials such as pectins which may be recovered.
For satisfactory extraction, it is preferred that the
materials are in a particulate form, the shape of the
particles being such that the diffusion path for the
~~VO 94/28735 Z ~ ~ ~ 5 p 3 PCT/AU94I00301
migration of the soluble material out of the particles is
short in at least one direction.
Tests show that the improvements of the invention
result in a marked increase in extract concentration.
Before introduction of clearing means for the strainer
means, liquid extract was either more diluted than
expected, or recovered in small=~r quantities, or both.
Clearing of blocked strainer means required about 20
minutes downtime, with a corresponding drop in production
capacity for a given CCE.
Introduction of liquid jets to clear the strainer
means of a CCE used to extract solubles from citrus peel
resulted in concentration of solubles in extract liquid
streams increasing from 3.5 to 4.5 Brix. Yield
increased from 20kg soluble solids per tonne of peel to
22kg soluble solids per tonne of peel, and the capacity of
the counter current extractor increased from 4.5 tonnes
per hour to 5.0 tonnes per hour.
Further improvement may be obtained by using a weir
distribution system for recycle and extracting liquids.
Soluble material concentrations obtained using this
further improvement are above 4.5 Brix, usually about
6.5 Brix and even as high as 8.0 Brix.
Hereinafter given by way of example only is a
preferred embodiment of the invention described with
reference to the accompanying drawings, in which:
Figure 1 is a semi-schematic diagram of a counter
current extractor according to the present invention.
Figure 2 is a semi-schematic diagram of part of a
counter current extractor according to a further
embodiment of the invention.
Figure 3 is a cross-section of a counter-current
extractor trough showing one embodiment of the level
control means.
Figure 4 is a cross-section of a counter-current
extractor trough and associated balance juice tank showing
a further embodiment of the liquid control means.
Figure 5 is a diagramatic cross-section of a CCE and
associated balance juice tank showing a further
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embodiment of the liquid control means.
As seen in Figure 1, the counter current extractor
comprises a elongate housing 1 in which is disposed a
screw conveyor comprising a driving shaft 3 having a
helical f light 2 disposed about it and a drive means 4 to
rotate the driving shaft 3 about its longitudinal axis.
The elongate housing has a solid phase inlet 5 and solid
phase outlet 6. Solid material to be extracted is
supplied to the elongate housing through inlet 5 and
conveyed by the helical flight 2 to the outlet 6.
Extracting liquid is supplied to the lateral channel 8
from which it flows over weir 8a and then through the
elongate housing to liquid phase outlet 9. The liquid
phase must pass through screen 13 before reaching outlet
9, screen 13 having small aperatures therein to separate
solid phase material from the extracting liquid. A
portion of the extracting liquid flows from outlet 9 to
balance tank 10 from whence it is pumped via line 11 to
liquid jets 12 which direct the liquid laterally onto
screen 13 at sufficient velocity to dislodge solid phase
particles therefrom. Further heated extracting liquid may
also be provided to the elongate trough 1 by means of a
further lateral channel 15 and weir 15a, to heat the solid
phase upon entry into the trough 1 to improve extraction.
Figure 2 shows an alternative embodiment of the
invention wherein screen 13 is cleared of solid phase
particles by means of blades 14 which are attached to
driving shaft 3 so as to rotate therewith. As blade 14 is
immediately adjacent screen 13, the movement of blade 14
past screen 13 dislodges solid phase particles from screen
13 .
It is also envisaged that embodiments of the
invention having a rotating screen may have both liquid
jets 12 and a blade 14 together.
In Figure 3, a liquid control means is illustrated,
comprising an inner tube 16 slidably mounted in an outer
tube 17. The end 18 of tube 17 may be adjusted so that
depth of the liquid, corresponding to distance d between
the bottom of trough 1 and end 18 may be varied. Tube 16
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is held in place by compression sealing means 19,
comprising a threaded flange 20 at the end of tube 17, a
.
compressible member 21 and a collar member 22 adapted to
be screwed onto the threaded flange 20, thereby
compressing compressible member 21 to seal the end of
outer tube 17 and retain inner tube 16 in position.
Figure 4 shows an alternative liquid level control
means which has a flush mounted liquid exit 23, allowing a
liquid level of zero or greater possible and making
cleaning of the trough 1 easier. Liquid exits the trough
1 via exit 23 and passes through valve means 24 to the
outer tube 25. Valve means 24 may also be set to allow
liquid to pass into a washout drain 26 during cleaning.
The outer tube 25 has an inner tube 27 slidably mounted
therein, tubes 25 and 27 being provided with sealing means
28 to prevent leakage between the tubes. The height of
end 29 of tube 27 may be adjusted by adjustment means 30,
which is held in position by locking screw 31. Because
the end 29 is above the liquid exit 23, the depth of the
liquid d in trough 1 is controlled by adjusting the height
of end 29. Liquid flows from end 29 into balance tank 10,
from which it is removed via pump outlet 32. An overflow
tube 33 is provided to control overfilling of balance tank
10. In some embodiments, several pump outlets may be
provided for the juice, eg. a finished juice outlet, a
recycle juice outlet and a screen spray outlet.
Figure 5 shows an ultrasonic level sensor 34 mounted
above trough 1. Level sensor 34 is attached to a
programmed logic control (PLC) unit 35, into which the
desired liquid level has been programmed. In response to
information provided by the level sensor 34, the PLC unit
opens or closes a motorized valve 36, allowing liquid
to flow into balance tank 10.
