Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates generally to a continuous
process and apparatus for separating the ice crystals con-
tained in a slurry of an aqueous solution and ice crystals
from said aqueous solution and washing said ice crystals with
an aqueous wash liquid. Normally this aqueous wash liquid
will be water, but multi stage processes are possible whereby
the wash liquid in any particular stage is the aqueous
solution withdrawn from a succeeding stage. This kind of
slurry occurs in industrial processes whereby either the
aqueous solution is recovered as a product and the ice
crystals are rejected, or conversely, whereby the ice
crystals are recovered as a product and the aqueous solution
is rejected. A process of -the first kind in the food
industry is the freeze concen-tration of aqueous liquids such
as fruit, juices, beer, wine and coffee extract. A process
of the second kind is the freeze concentration of sea water
to produce fresh water; in this process the brine is rejected
and the ice crystals are recovered and melted.
A continuous packed bed wash column is described in
U.S.P. 3,587,859 (Probstein), specifically for producing
fresh water from sea water. In this wash column a porous bed
of particles ice crystals is formed and the liquid medium
brine is caused to flow outwardly from the column at a
screened opening positioned intermediate its ends. A wash
liquid is introduced at one end of the column to displace the
liquid medium from the interstices between the particles as
the porous bed moves continuously through the column.
Control means are provided to control the relationship among
the liquid pressures at the ends of the porous bed and the
pressure at such screened opening so as to maximize the rate
at which the particles can be removed at the output end of
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the column.
In operating this kind of apparatus the two main
difficulties are freezing of the screen producing breakdown
of the operation and losses occurring by the mixing of the
aqueous solution between the ice crystals and the wash
liquid. These losses are exacerbated by the phenomenon of
channeling. This phenomenon entails the formation of
"channels" in the bed of ice crystals, through which wash
liquid escapes. When e.g. liquld foodstuffs are being
concentrated the concentration is partly undone, and in the
case of sea water desalination fresh water is lost, as it
mingles with the concentrated brine which is being rejected.
Moreover wash liquid reaching the screen has the tendency to
produce clogging. The wash liquid has a higher freezing
point than the concentrated solution. The screen is cooled
by this concentrated solution and the wash liquid coming in
contact with the screen will freeze and, as said, produce
clogging, eventually bringing the process to a halt.
The difficulties associated with freezing up of the
20 screen have been well described in U.S.P. 3,885,399 and
3,992,900 (Campbell, assigned to Avco Corp.). Reference is
made specially to column 1, lines 23-57 (USP 3,992,900). To
avoid this clogging in these patents a two stage process has
been proposed using two wash columns as described in U.S.P.
3,587,859, previously mentioned. U.S.P. 3,885,399 and
3,992,900 are proof to the fact that hitherto there was no
solution to the problem of avoiding wash liquid reaching the
screen, which among other things boils down to the problem of
preventing channeling in the packed bed.
The serious difficulties encountered by channeling
and freezing up of the screen are moreover clearly expostu-
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lated in a recent study of Gers~on Grossmann (AICHE Journal,
Vol. 22, No. 6, November 1976, pg. 1033-1042). In order to
get a clear picture of these problems and their implications,
special reference is made to pg. 1033 and pg. 1034 top.
By practice of the present invention all these
difficulties may be mitigated and in specific embodiments
eliminated completely, which goes to say to provide a method
and apparatus whereby freezing up and channeling may be
efficiently avoided and substantially zero loss operation is
assured.
For a clear understanding of the inventive concept
of the present invention the following discussion is
appropriate about the Pro~stein wash column, mentioned above.
We specifically call attention to Fig. 1 of U.S.P.
3,587,859 and to Fig. 1 and 2 of the Grossman publication.
The counter washer normally consists of a vertical
column with a screen in the middle part of its wall. Accord-
ing to U.S.P. 3,587,859 (column 1, line 41; column 4, lines
17-18) the ice particles form a plug.
The aqueous concentrate is said to flow up through
the lower leg of this plug and out through the screen. The
pressure of the aqueous concentrate itself is utilized to
provide the driving force for moving the ice particles upward
(see U.S.P. 3,587,859 column 1, lines 44-45), which goes to
say that the drag of the concentrate on the plug causes the
latter to move up against the friction on the walls and the
restraining forces at the top. These restraining forces at
the top originate e.g. frorn the drag of the bed against the
wash liquid, and the scraper which is used to disintegrate
the plug and feed out the particles created by that disinte-
gration.
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Ideally in processes according to this prior art
only a small amount of wash liquid should flow down through
the upper leg of the ice plug and out through the screen
(which incidentally would in itself tend to clog the screen),
but the balance is upset as soon as channeling occurs.
The situation as depicted in U.S.P. 3,587,859 and
the publication of G~ossman shows a "brine crown" which is in
fact the ~ashfront or in other words the interface between
the concentrate and the wash liquid - which is strongly
curved and also flow lines of the concentrate which are
strongly curved. This is unavoidable in the set up according
to the prior art at issue. The curvature of the wash front
itself and of the flow lines in its neighbourhood correlate
with differences in the density (porosity) of the packed bed,
(when we look at it in a cross section perpendicular to the
axis).
In this connection attention should be drawn to the
American Reissue 23.810 ~specifically Fig. 1, and column 2
lines 47-50). Packed ice crystals are pushed through a zone
(c) where liquid is being withdrawn, a "transition zone"
(b) and a water-ice zone (a). In this set-up it is in the
transition zone (b) where the curved washfront may be
supposed to be formed (if a proper "front" is formed at all).
Stabilization of a curved washfront has, however, proved to
be difficult. To circumvent these difficulties it has been
proposed in U.S.P. 2,854,494 to force a pulsating reflux
stream of wash liquid through the packed bed of ice crystals.
Supposedly a "shake-up" of the transition zone should help to
counteract difficulties such as channeling, discussed above.
According to the present invention a sharp and
stable washfront is only possible if such measures are taken
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that this washfront ~the interface between the concentrate
and the wash liquid) lies in a flat plane, perpendicular to
the direction in which the packed bed is moving.
Thus according to the present invention there is
provided a process for separating the ice crystals contained
in a slurry of an aqueous solution and ice crystals from said
aqueous solution and washing said ice crystals with an
aqueous wash liquid, whereby a packed bed of ice crystals is
created which is subjected to a continuous translatory
movement, while a washfront is being created within said bed
of ice crystals between said aqueous solution and said
aqueous wash liquid, characterized in that said aqueous
solution and said aqueous wash liquid adjacent to said wash-
front are substantially stagnant, and a stable washfront is
being created lying in a straight plane perpendicular to said
translatory movement of said packed bed of ice crystals.
Generally this will encompass a process for sepa-
rating the ice crystals contained in a slurry of an aqueous
solution and ice crystals from said aqueous solution and
washing said ice crystals with an aqueous wash liquid,
comprising sequentialiy the following steps:
a) continuously introducing said slurry in a first cylindri-
cal zone;
b) in said first cylindrical zone continuously withdrawing
part of said aqueous solution from said slurry, producing
a packed bed of ice crystals, and causing said packed bed
to adopt a homogeneous porosity throughout its cross
section;
c) causing said packed bed to move continuously through a
second cylindrical zone, said second zone containing a
stagnant layer of said aqueous solution in a first
56
section and a substantially stagnant layer of wash liquid
in a second section, contiguous to said first section,
thus creating a washfront between said first and said
second section;
d) continuously disintegrating said continuously advancing
packed bed and feeding out the product of that dis-
integration.
Across the washfront there will be a "jump" in
temperature as the aqueous solution between the ice crystals,
being more concentrated than the wash liquid, will have a
lower temperature than that wash liquid. In the normal case
that the wash liquid is water, the temperature will of course
be 0C, so the ice crystals in the aqueous solution will have
a temperature below 0C. In coming into contact with the
wash liquid, some wash liquid entering between the ice
crystals will freeze, and in doing this generally cement
these crystals together. In this way at the washfront a
solid porous ice plug is formed. Thus first a thickened
slurry has to be created, measures have to be taken to allow
for a homogeneous build up of the thickened slurry to a
packed bed and measures to impart to it a translatory
movement. Most important is that before reaching the wash-
front ice crystals have been caused to distribute themselves
evenly throughout the cross section of the packed bed. In
principle it may be possible to use the drag of the flowing
aqueous solution to move the packed bed and nevertheless
assure a homogeneous build up of this bed arriving at the
interface between the aqueous solution and wash liquid, (the
washfront). Drag may transport the ice crystals in the zone
where the aqueous solution is withdrawn. The ice crystals
accumulating there may push others along. At the washfront
homogeneity may be attained if that washfront is located far
enough from the place where the aqueous solution has been
withdrawn, notwithstanding the fact that at the place of
withdrawal of aqueous solution originally an in homogeneous
build up of the thickened slurry is obtained, e.g. in the
case of a wall filter. This thickened and originally in-
homogeneous slurry must however have opportunity or be caused
to homogenize. It has to be homogeneous taken over its cross
section when arriving at the washfront. This is necessary to
avoid radial differences in resistance against liquid flow.
If these differences exist the washfront will not be flat but
in some way or another curved.
Another possible set up is the following. The
slurry is introduced at the bottom of a vertical cylinder.
The bottom is provided with a screen, through which aqueous
solution is withdrawn. A stirrer-scraper connected with a
shaft through the bottom of the cylinder and actuated by a
motor keeps the screen clean and homogenizes the thickened
slurry. The slurry may be introduced sideway or through the
(hollow) shaft of the stirrer-scraper. Also the screen
through which the aqueous solution is withdrawn need not be
in the bottom but can be e.g. in the wall of the cylinder
near to the bottom. The ice crystals accumulating in the
lower part of the cylinder push others up, forming a packed
bed. The liquid in the lower part of the cylinder is aqueous
solution, the liquid in the upper part wash liquid. The
interface between the aqueous solution and the wash liquid is
the washfront, this washfront being kept at approximately
constant level by adjusting the pressure of the incoming
slurry and the pressure of the wash liquid. This may be done
in the manner more fully described in the under following.
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56
In the top of the cylinder a scraper may be located
to disintegrate the washed out packed bed and feed out the
product of this disintegration. This also will be more fully
described in the underfollowing. The disintegration may also
be accomplished by melting.
In an alternative and preferred arrangement the
zone where the aqueous solution is withdrawn from the slurry
may contain hollow perforated (screened) vanes attached to a -
rotatable shaft, through which the aqueous solution is
withdrawn. ~These vanes may be attached to the shaft in such
a way that they are freely rotatable around small axles
perpendicular to the shaft, which goes to say that they are
free to take on a tilt depending on the processual factors at
hand, such as the rotational speed of the shaft and the drag
of the aqueous solution being withdrawn through these hollow
vanes. In this case the vanes do not provide the energy to
push the crystals ahead but the moving force resides in the
drag of the aqueous solution being withdrawn. The crystals
accumulating by the withdrawal of the aqueous solution push
others along. If the shart, to which the screened hollow
vanes are attached and freely movable, is not rotated, they
will take on a position parallel to the axis of the wash
column, pointing to the feed out end, especially if the vanes
are screened at both sides. It is however advantageous to
rotate the shaft. This will greatly help in counteracting
any tendency to the formation of inhomogeneities by the
passage of the thickened slurry along the screen.
Another preferred measure according to the present
invention is the use of mechanical means to impart a trans-
latory movement to the thickened slurry of ice crystals, andin doing so create a steadily advancing packed bed of these
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crystals. This may e.g. be accomplished by using tilted
vanes, attached to a rotatable shaft, in such a way that they
have a fixed tilt. Again the screen to withdraw the aqueous
solution may be located in these vanes, if these are hollow.
By using mechanical means to move the packed bed, this
movement is to a large extent made independent from the
pressures of the incoming aqueous solution, the outgoing
aqueous solution and the wash liquid and their respective
differences. By rotation of the fixed tilted vanes the
filtering layer adjacent to the filter will be continuously
renewed in such a way that homogeneity is assured.
To facilitate a homogeneous build up of the packed
bed in radial direction, (which is as already said, particu-
larly important when it reaches the washfront), an annular
structure of that bed may be recommendable, especially in the
case of large diameters of the wash column and when the
withdrawal of aqueous solution from the slurry is accomplish-
ed through a screen positioned in the direction of movement
of the packed bed (e.g. a filter in the wall of the cylinder).
We shall then have to use an apparatus consisting of a
cylinder coaxially arranged within another cylinder with
larger diameter. The difference in diameter and the distance
between the screen and the washfront have to be chosen in
such a way that under working conditions the packed bed has a
homogeneous build up at the washfront.
This implies that generally when the aqueous
solution is withdrawn through a screen in the wall of one or
both of the cylinders which together define the annular space
in which the packed bed is created, this annular space has to
be rather narrow, e.g. 2 to 5 cm. It will be clear that
withdrawal of the aqueous solution through a screen in the
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wall of the column will tend to favour the creation of in-
homogeneities in the packed bed. This tendency is the better
counteracted the narrower -~he annular ring is (in connection
with the foregoing we refer to the Canadian Patent Appli-
cation nr. 294.723).
In the case that the aqueous solution is withdrawn
through a screen more or less perpendicular to the trans-
latory movement of the packed bed (as already described) the
annulus may be considerably wider. E.g. when we use tilted
vanes and the inside diameter of the outer cylinder is
100 cm, the annulus may be 20 cm wide. The tendency for an
inhomogeneous build up of the packed bed is much less.
In using an annular structure for the packed bed,
the mechanical means (such as tilted vanes), used to impart a
translatory movement to the packed bed, in the direction of
the axis of the cylindrical wash column, and scraper means
used to disintegrate the packed bed after this has passed
through the washfront, may be attached to the inner cylinder.
In this case the inner cylinder has to rotate.
It is also possible to have the inner cylinder
stand still. In this case the mechanical means to forward
the packed bed, and the scraper means (if applied) have to be
actuated separately. This will be described in more detail
in an under following example. The inner cylinder may
however also be attached to the same shaft as the mechanical
means to forward the packed bed. An optimal possibility of
process control may, as the case may be, be obtained by
providing the inner cylinder, the mechanical forwarding means
and the scraper means with separate actuating mechanisms, so
that their respective rotational speeds can be varied at
will .
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When using the tilted vanes as the mechanical means
to propel the packed bed along it may also be advantageous to
make the tilt of these vanes adjustable. This enhances the
possibility to control the process, as may be dictated by the
particular circumstances at hand.
Disintegration of the packed bed of ice crystals
after passing the washfront, may be accomplished by melting.
Particularly good results have however been obtained by the
use of scrapers. Scrapers for this purpose are in themselves
10 known. Reference is made e.g. to U.S.P. 3,872,009 and in the
under following example a somewhat different construction
will be described. As alread,v mentioned the restraining
force counteracting the translatory movement of the bed of
ice crystals will generally pack the bed so firmly that under
influence of the temperature jump at the washfront the ice
crystals will be cemented together, creating a solid porous
ice plug. It has been found in several instances that this
ice plug will consist for about two thirds- of solid ice and
for about one third of interconnected pores, but this
relation may vary according to conditions. The washfront,
while actually standing still, moves relative to the packed
bed through these pores. In this way the bed is washed
"internally". The position of the washfront within the wash
colurnn may be kept substantially fixed by a sensing device
with feedback on the position of that washfront/ This may be
e.g. accomplished by two closely spaced temperature sensors
at different sides of the washfront. This will be described
more in detail in the underfollowing.
The present invention also encompasses an apparatus
for implementing the process described above. According to
the present invention there is thus provided an apparatus for
separating the ice crystals contained in a slurry of an
aqueous solution and ice crystals, from said solution and
washing said ice crystals at a washfront with an aqueous
wash liquid, comprising in operative cooperation:
a) enclosure means defining a first cylindrical space;
b) means for feeding in said slurry in said cylindrical
space;
c) means for withdrawing solution from said slurry in said
cylindrical space while retaining sald ice crystals,
producing a thickened slurry;
d) enclosure means defining a second cylindrical space;
e) means adapted to transfer said thickened slurry from said
first cylindrical space to a feed-in side of said second
cylindrical space;
f) means for causing said thickened slurry to move continu-
ously through said second cylindrical space to a feed-out
side; -
g) means for maintaining a stagnant layer of said aqueous
solution in said second cylindrical space at its feed-out .
side; .
h) means for causing said thickened slurry to homogenize
throughout in radial direction before reaching the
washfront, producing a homogeneously packed bed of ice
crystals;
i) at the feed-out end of said second cylindrical space
means for continuously disintegrating said packed bed of
ice crystals and feed-out the product of disintegration.
The most preferred embodiments of the invention
comprise those whereby an annular structure of the washcolumn
is combined with rotating hollow vanes with a fixed tilt
through which the aqueous solution is withdrawn from the
slurry and whereby the scraper means at the one side and the
tilted vanes at the other side are provided with separate
actuating means. Generally this will imply that the scraper
means will be given a higher rotational speed than the tilted
vanes.
The present invention will now be further described
by way of the following Example and with reference to the
accompanying drawings, in which:
Fig. 1 represents the various processual fonctions
out of which the process according to the invention is
composed;
Fig. 2 serves to illustrate one of the preferred
embodiments of the invention;
Fig. 3 is a top view of the cross section along
III-III in Fig. 2;
Fig. 4 is a cross section of a hollow vane along
IV-IV in Fig. 3;
Referring to Fig. 1, at (1) the slurry of ice
crystals is supplied, (2) represents the withdrawal of the
aqueous solution, which is fed out at (3) leaving a thickened
slurry. The function of homogenization is represented by
(4), and the function of formation of a packed bed by (5).
Movement of the packed bed through a stagnant layer of
aqueous solution is represented at (6), arriving at the
washfront (7). The bed then moves through the substantially
stagnant layer of wash liquid (8), whereby it normally con-
solidates to a solid porous block of icer whereafter disinte-
gration is taklng place at (9), whereas at (10) the product
of disintegratlon is slushed away and fed out at (11). Some
of the functions (2), (4), (5) and (6) may be performed
simultaneously in particular embodiments of the invention,
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which have been described above. E.g. when using an appa-
ratus with an annular space and hollow vanes with a fixed
tilt through which the aqueous solution is withdrawn, the
functlons (2), (4) and (5) may occur practically at the same
time.
Speaking most generally however, the important
thing is, that when the thickened slurry arrives at the
washfront it is homogeneously packed, so as to form a flat,
not curved washfront perpendicular to the axis of the wash-
column.
Referring now to Fig. 2, the hull of the apparatusis cut out lengthwise to show the inside in side view. At
(17)we see a cylindrical hull, with a bottom plate (18) and
a cover plate (19). An inner cylindrical body is shown at
(20). This body is- attached to a shaft (21), in its turn
attached to the strip (22j, bolted to the cover plate (19).
Another cylindrical body is shown at (23) attached to the
shaft (24), carrying the toothed wheel (25). This wheel (25)
cooperates with the toothed wheel (26), which is actuated by
the motor (27). The body (23) carries the hollow tilted
vanes (28), at their upper surface provided with the screen
(29). The vanes are represented here more or less simpli-
fied, for the rest we refer to Fig. 3 and Fig. 4.
The inside of the hollow vanes (28) communicates
with the duct (29') through which liquid can be withdrawn.
The body (23) also carries pins (30), while the hull (17)
carries vertical vanes (31).
A disc (32) provided with knives (33) and slots
(34) acts as a scraper. The disc (32) is attached to the
cylindrical body (35), attached to the toothed wheel (37),
cooperating with the toothed wheel (38). The motor (39)
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actuates the toothed wheel (38).
The slurry enters at (40) and is kept well stirred
by the pins (30) cooperating with the vanes (31), to prevent
settling of the crystals. Aqueous solution is withdrawn
through the screen (29) and leaves through the hollow vanes
(28) and the duct (29'), leaving a thickened slurry at the
surface of the screen (29). This thickened slurry is packed
and pushed through the annular space (41) by rotation of the
body (23) with the vanes (28). The dotted line (42) repre-
sents the washfront, being the dividing plane between thespace (41), in which the liquid is an aqueous solution and
the space (36), in which the liquid is wash liquid (normally
water). The temperature sensing devices (57) and (58) keep
the movement of the washfront within narrow limits. These
will register a certain temperature difference that exists
between aqueous solution and the wash liquid. If the wash-
front moves up or down this temperature difference will
disappear and an electric impulse from the control unit (59)
will actuate the valve (60) opening or closing it more, as
the case may be.
The space (36) will in most cases be filled by a
porous solid ice plug, which will be disintegrated by
rotation of the disc (32) with the knives (33). The parti-
cles created by the disintegration, passing through the slots
(34), will enter the space (43). Wash liquid will be pumped
around by the pump (44). It will enter the space (43) at
(45), flushing out the ice particles at (46). The ice parti-
cles are melted by the melter (47). When the washfront
remains at a fixed place the quantity of water leaving the
system through the valve (60) will equal the quantity of ice
which is fed out by the scraper (32). If the washfront moves
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down the valve (60) is opened somewhat more, so that somewhat
more water is withdrawn and visa versa. In this way the
balance is restored.
In Fig. 3, we see, at (29), the screen of one of
the hollow tilted vanes, the edge of which is shown at (28).
Between two tilted vanes the space (48) can be detected
through which the slurry passes. The vanes are attached to
the annular cylindric body (23), in which a circular groove
(49) is present. The inside of the hollow tilted vanes is
connected to the circular groove (49) by channels (50). The
annular cylindrical body (23) is attached to the shaft (51)
by the protruding ridge (52). The shaft (51) is provided
with a central channel (29') which is connected to the circu-
lar groove (49) by the channel (53).
From Fig. 4 it can be seen that the bottom (54) of
the vane is provided lengthwise with ridges (55), which serve
to support the screen (29). The ridges (55) are provided
with holes (56) for the passage of the aqueous solution. The
vane is attached to the cyllndrical body (23), provided with
the channel (50) and the groove (49) where the liquid of all
the vanes is collected. The shaft to which the cylindrical
body (23) is attached is shown at (51) with the central
channel (29') to drain away the aqueous solution.
It will be obvious that the apparatus as described
and other related embodiments with hollow vanes can serve as
a continuous slurry thickener when the disintegrating device
is left out.
It will also be obvious that the apparatus accord-
ing to the invention can be used quite generally for systems
in which one component can be crystallized out of a liquid
multicomponent system and may be separated out of this system
in a wash column.
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