Note: Descriptions are shown in the official language in which they were submitted.
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EXTENSIONAL FLOW MIXER
l.Field of the Invention
The present invention relates to the mixing of liquids, particularly viscous
liquids, for example plastic materials such as polymers, and especially the
mixing of such materials having widely different viscosities, and when a minor
phase is highly viscous. However, the invention can also be used for mixing
other liquids, for example milk homogenization and preparation of mayonnaise
in the food industry, preparation of explosive emulsions in the explosive
industry, and homogenisation of molten soaps in the chemical industry.
One form of the present invention is an improvement of the motionless
extensional flow mixer described in our U.S.Patent No.5,451,106, issued
Sept.19, 1995, which gives a detailed review of the prior art in this field.
Briefly, it is known to mix polymers by distributive mixing effected by so-
called "motionless mixers" placed between a screw feeder and a die. In most
cases these mixers have a number of alternating right and left-handed helical
elements placed in a tubular housing equipped with temperature control. The
energy for mixing is provided by the pressure loss across the mixer. The
splitting and recombination of streams results in a predictable number of
striations. The advantage of such mixers is that they are accessories to
standard type of compounding or processing equipment, not their integral part,
and their main disadvantages are lack of easy adjustment, limited
effectiveness
in mixing, and inability to provide dispersive mixing. The basic principle
behind
their design is division and recombination of the flow streams. Since the flow
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division is of the shear type, the dispersive forces are usually weak, limited
to
the cases where the two liquids show similar viscosity.
Theoretical calculations and experiments have shown that dispersive
mixing of two Newtonian liquids is more efficient in extensional than in shear
flow. Extensional flow occurs for example when fluid converges from a
reservoir
to a capillary. In shear flow fields it is impossible to disperse liquids that
have
viscosity higher than that of the matrix fluid by more than a factor of 3.8.
By
contrast, the dispersing capability of the extensional flow field is only
slightly
affected by the viscosity ratio. From the kinematics point of view, the
extensional flow field engenders deformation much more rapidly (note the
absence of vorticity in the elongational flow feld). At a given stress level,
the
generated interphase (that is the accepted measure of adequacy of mixing or
"mixedness") is orders of magnitude greater than that generated in shear.
Similarly, the amount of energy required to generate a given degree of
mixedness in elongation is orders of magnitude smaller than that in shear.
Furthermore, the mechano-chemical degradation of the macromolecules is
much less extensive in the elongational than in the shear field.
In spite of all these advantages present mixing equipment (including the
twin-screw extruders) operates mainly in shear. This is due to the ease of
designing equipment that operates on the shear flow principle. By contrast, it
is
difficult to envisage geometry that will engender very large deformations in
the
extensionai flow field. However, one may by-pass this problem by designing a
mixer in which the elongational flow field is engendered in a series of
convergent-divergent geometries, preferably with semi-quiescent zones in
between.
One prior patent describing an extensionai flow mixer was U.S. Patent
No.4,334,783 of Suzaka, which issued Jun.15,1982. The drawbacks of the
Suzaka mixer are described in our aforesaid '106 patent. The mixer described
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in our '106 patent was intended to overcome these drawbacks, and to provide a
mixer having the following characteristics:
1. The mixture of two fluids is exposed to strong extensional flow fields,
each followed by a semi-quiescent zone;
2. The flow fields are generated by a series of convergences and
divergences of progressively increasing intensity;
3. To reduce the pressure drop, as well as to prevent blockage of the
restrictive openings, a series of holes (e.g. of the Suzaka design) are
replaced
by slits;
4. The slit gaps are made adjustable.
The mixer of our '106 patent has a series of chambers separated by
several convergent/divergent surfaces providing narrow openings between the
chambers. The openings are in the form of slits defined by the inner edges of
protrusions formed on die members which provide the convergent/divergent
surfaces. Also, the die members subject the liquids to gradually increasing
stress, since the protrusions of the die members are concentric and are
arranged so that during mixing the liquids pass radially inwards between the
die
members in passing from the inlet to the outlet of the mixer. At least one of
the
die members is made movable to adjust the slit gap, thereby adjusting the
stress level.
In the design shown in our '106 patent, the movable die member is held
at the lower end of a cylindrical block or mandrel which is slidable in a
cylindrical chamber of a housing. Movement of the block, for adjustment of the
gap width, is effected by rotating a wedge-shaped disc between an end of the
housing and a sloping top end of the block. Passageways for the supply of the
mixed liquids to the edges of the die members are formed around the sides of
the block, and communicate with a side inlet into the housing. This
construction
has been found to have two drawbacks.
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Firstly, when using high pressures in the mixer, for example 3,000 psi or
20 MPa, the liquid pressure at the side of the block adjacent the side inlet
tends
to tilt the block causing asymmetrical flow to the edges of the die members.
Secondly, the wedge-shaped disc used to vary the slit gaps was difficult to
adjust. The present invention overoomes these problems.
Another form of the present invention combines features of the '106
motionless mixer patent with some features of known dispersive mixers that are
used in association with screw extruders, particularly single screw extruders,
to
improve the mixing capability of such extruders. Such mixers generally have a
housing defining a cylindrical cavity with inlet and outlet ends, and a
mandrel of
generally cylindrical form which is rotatable in the cavity. The mandrel has
protrusions which may resemble screw threads, but which are interrupted by
gaps, or separated by other, discrete protrusions or indentations, so that the
material being mixed is not merely progressed along the cavity, as in a screw
extruder, but is also caused to move through slits between the outer edges of
the protrusions and the inside surface of the cavity. The side surfaces of the
protuberances provide convergent entrances into, and divergent exits from, the
slits.
The extensional flow mixer of this invention is similar to that of our '106
patent in having:
a housing providing a cavity having an internal surface, and having an
inlet into the cavity which inlet is connectable to a pressurized source of
the
liquids, the end of the housing remote from the inlet having an outlet for the
mixed liquids;
a mandrel located in the cavity;
the mandrel carrying protnrsions having side surfaces which converge
towards their outer edges, the outer edges cooperating with the internal
surface
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of the cavity to divide the space between the protrusions and the intemaf
surface into a series of chambers separated by slits such that liquid passes
successively through all the chambers and slits in moving from the inlet to
the
outlet, the side surfaces providing convergent entrances to, and divergent
exits
from, the slits, and the slits having cross-sectional areas which decrease in
the
liquid flow direction, from an upstream chamber adjacent the inlet, to the
outlet;
and means for adjusting the slit gaps.
To overcome problems with asymmetrical flow of liquids into the
outem~ost cavity, in accordance with this invention the inlet into the housing
is
at an end of the housing, rather than at the side, and the mandrel has a side
portion provided with helical grooves which cooperate with an interior surface
of
the housing to form helical passageways connecting the inlet to the upstream
chamber for distributing the liquids to this chamber.
In a preferred embodiment, at least one helical groove is provided for
each 25 mm of the mandrel diameter, each groove leading from an inlet end of
the block mandrel to the vicinity of the upstream chamber.
In the mixer of our '106 patent, adjustment of the slit gaps was achieved
by moving the block or mandrel, which carried one series of the protrusions.
!n
accordance with another aspect of the present invention, the block or mandrel
is stationary relative to a fixed part of the housing, and this fixed part of
the
housing is connected by screw threads to a relatively adjustable part of the
housing. The relatively adjustable part may cant' protuberances which
cooperate with those of the mandrel to define the slits.
As indicated, one form of the present invention has features in common
with known dispersive mixers having a housing defining a cavity which is
usually of generally cylindrical form, and having a mandrel rotatable in the
cavity, the mandrel having protrusions. However, the present invention differs
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from this prior art, firstly, in that the cavity is frusto-conical having a
large end at
the inlet and a small end at the outlet, and in that the protrusions are
annular,
and are such that said outer edges divide the space between the protrusions
and the internal surface into a series of annular chambers separated by the
slits
such that liquid passes successively through all the chambers and slits in
moving from the inlet to the ouflet. The chambers have a mean diameter which
decreases from an outermost chamber adjacent the inlet to an innermost
chamber adjacent the ouflet.
This form of the invention may also include screw thread means for
adjusting the axial position of a portion of the housing relative to the
mandrel to
alter the slit gaps. Also again, the mandrel may have, adjacent the inlet, a
side
portion provided with helical grooves, the grooves forming helical passageways
with an interior surface portion of the housing, the passageways communicating
with the upstream chamber. This dynamic form of the invention, termed a
dynamic extensional flow mixer (DEFM), has all four elements which constitute
the fundamental principles of the invention: strong, elongational flow fields,
increasing in intensity in the downstream direction, and the use of slits
which
are adjustable.
In this form of the invention, the fact that the mandrel rotates adds
angular shear to the mixing; this is desirable as it prevents an elongated
droplet
from returning to a spherical shape.
L~rief Descri~ion of the drawings.
Preferred embodiments of the invention will now be described by way of
example with reference to the accompanying drawings, in which;
Fig.1 is a sectional elevation of one form of mixer of the motionless type,
Fig.2 is an enlarged sectional elevation of the die members of the Fig.1
mixer,
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Fig.3 is a partial plan view of the die plates of the same mixer,
Fig.4 is an enlarged view of a die of the same mixer, and
Fig.S is a sectional elevation of a dynamic extensional flow mixer
(DEFM).
As shown in Fig.l, the mixer has a cylindrical housing 10 with a
removable top plate 12 held on by bolts 13 which extend up from the bottom of
the housing through the length of its cylindrical side wall. An adapter plate
14 is
fixed to the top of the plate 12 by bolts 15. The plates 12 and 14 have
aligned
axial bores 12a and 14a which together provide an axial inlet into the end of
the
housing. The upper end portion of bore 14a is threaded to receive an adapter
(not shown) connected to an extruder which delivers viscous liquids at
required
pressure to the inlet. The mixer is supported by a support yoke fixed to the
mixer by support rods inserted into side bores in the housing; the position of
these side bores, which are located between the bolts 13, is indicated at 16.
The housing 10 also carries a pressure sensor (not shown), which is located at
90° to the sectional plane shown in Fig.1
The housing 10 surrounds a cavity having a cylindrical sidewall 11 a and
being closed at the top by the plate 12 and at the bottom by a movable end
closure 18 which has a main disc portion 18a surrounded by side wall 18b
which seal against the cylindrical side wall of the cavity, and having a
downwards extending boss 18c provided with a central, axial outlet bore 19.
This end closure 18 provides a holder for a first, movable die member 20 which
provides an internal surface 11 b for the end of the cavity, and which will be
further described below with reference to Figs.2-4. The die member has a
central outlet bore 20a communicating with outlet 19. The end closure is
adjustably supported in the cavity by a disc-like adjusting plate 22 the outer
edges of which are provided with fine screw threads 23 mating with internal
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threads of a lower end portion of the housing side wall. Side portions of the
plate 22 are provided with four partial bores 24, parallel to the plate axis,
two of
which are shown, suitable for receiving projecting parallel spigots of a tool
(not
shown) which can be used to rotate the plate to adjust the axial position of
the
movable die. The threads 23 are fine enough to allow for fine adjustment of
the
plate position; suitable threads provide 2 mm of movement for each 360°
of
rotation of the plate.
The die member 20 is held in place in the end closure 18 by bolts 25
extending up through the main disc portion 18a of the closure into blind
threaded bores in the die member. In addition, bores 26 are provided in the
disc
portion 18a to allow the die member to be knocked out of the closure by
suitable tools, when replacement is needed.
The upper end portion of the housing cavity is occupied by a block or
mandrel 30, the lower end of which carries the second, fixed, die member 32;
again, details of this will be described in relation to Figs. 2-4. The mandrel
30 is
an integral part of the top plate 12. The die member 32 is fixed to the
underside
of the mandrel 30 by bolts 33 having their heads recessed into the top of the
plate and their lower ends engaged in blind threaded bores in the top of the
die
member 32.
The outer edges of the die member 32 are spaced within the inside
surfaces of the closure wall 18b, allowing liquid to flow between these edges.
The space between these edges communicates with passageways formed on
the outside of the mandrel 30, and which communicate with the inset 12a.
Specifically, the outside surface of the mandrel is formed with several, for
example four, equi-spaced side-by-side spiral or helical grooves 34,
resembling
a multi-start screw thread, each groove being of U-shaped cross section and
having its outer edges close to or touching the cylindrical interior surface
11 a of
the cavity and each forming a passageway with this surface. At their upper
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ends the grooves each communicate with a radial passageway 36, several
equi-spaced such passageways being provided, each of which communicates
with a lower end portion of the inlet bore 12a.
Figs.2, 3, and 4 show details of the die members 20 and 32. These carry
lower and upper symmetrically opposed protrusions 20', 32', these protnrsions
having opposed inner edges E separated by slits. The protrusions have sloping
side surfaces adjacent these inner edges which provide converging entrances
into the slits and diverging exits therefrom, and which define in part an
inlet
chamber C1 and two intermediate chambers C2 and C3. Typically, the sloping
convergentldivergent surfaces lie at 60° to the generally horizontal
plane of the
overall flow of the liquids, i.e., angle a in Fig.4 is 120°, although
angles between
t 15° of this preferred angle may be suitable. It will be seen that the
die
members provide parallel faces 20", 32", which define intermediate portions of
the chambers between the dies, these portions being more than one half and
preferably more than 70% the radial extent of the chambers. These provide
semi-quiescent spaces. The slits are adjustable within a wide range by
rotation
of the adjustment plate 22, to provide convergence ratios (i.e., the ratio of
chamber depth to slit gap, or the ratio of the spacing between the parallel
faces
to the spaces or gaps between the inner edges E of the protrusions) preferably
of between 5:1 and 250:1. Accordingly, the transverse dimension of the
intermediate portions of the chambers, as defined by the spacing between the
parallel faces of the die members, is at least twice the slit gap. The lower
die
member 20 has its outlet bore 20a inwardly of chamber C3 leading to the ou~et
19, while the upper die member 32 has a central boss 32a with a central
projection shaped to divert the liquid towards the outlet.
The nature of the die members so far described is the same as that of
the '106 patent. However, one difference over this previous patent, and which
is
illustrated in Fig.4, is that there is a smooth transition of the slope from
the
sides of the protrusions to their inner edges E, i.e., the edges of the
protrusions
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are rounded instead of having a sharp comer as in the previous patent. This is
intended to eliminate the possibility of dead space or the deposition of
immobile
polymer at these comers.
In use, a blend of molten polymers enters the mixer from any pumping
device, e.g., an extruder or gear pump, through an adapter attached to adapter
plate 14. The melt passes from the bore 12a into the radial passageways 36,
and then into the spiral passageways formed by the grooves 34 and the interior
surface of the housing. The melt is smoothly distributed by these passageways,
and is evenly distributed around the outer edges of the die members, a result
not well achieved with the design shown in our previous '106 patent. The melt
then flows from the rims of the die members towards the central outlet 19,
undergoing convergent and divergent deformation before passing out through
the bore 20a and outlet 19. The gaps between the inner edges of the
protuberances can be adjusted by rotating the adjusting plate 22 using the
tool
having spigots which engage in the four holes 24 in this plate. The slit gaps
can
be controlled precisely within the range of from 0 to 3 mm. The pressure and
temperature of the melt are continuously recorded by the sensor inserted into
the melt through the side wall of the housing. The mixer can readily be
mounted on a laboratory or an industrial extruder with a throughput of up to
1,000 kg/hr.
Apart from the even distribution of liquid to the edges of the die
members given by the axial inlet and spiral passageways, other advantages of
this design over that of the prior '106 patent are:
1. The design is sturdy, with little deflection caused by the high
pressures used. The feed is uniform around the mandrel and does not generate
any pressure gradient that may tend to tilt the mandrel. This is important
since
with a small slit gap, say of 50 microns, a deflection of only 5 microns is
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2. The melt stream is partly homogenized before reaching fhe die
members, the melt temperature being more uniform.
3. Pressure drop in the melt distributer system, upstream of the die
members, is relatively low, compared to the pressure drop across the whole
mixer.
4. The machining of the mandrel is relatively easy, compared to that
needed to produce the special groove in the block of the former design.
5. The screw thread allows easy adjustment of the slit gap.
It may be noted that while it is desirable for both the die members 20
and 32 to have ridges, it is also possible to achieve extensional flow mixing
with
ridges only on one die member, this member facing a flat plate.
!n some cases the motionless mixer as described above may not be
convenient to install on the production line, and/or it may not provide
adequate
distributive mixing. For this reason the dynamic extensional flow mixer shown
in
Fig.5 has been developed. This is shown as attached to the conventional barrel
101 and screw 102 of an extruder. The mixer includes a housing 110 the initial
or upstream portion of which is a barrel extension 114 having a flange 114'
attached to a flange of the extruder barrel 101, the upstream end of the
extension defining an inlet 112a into the mixer. The extension has a retaining
ring 115 which retains an inner flange of a rotatable sleeve 112, and this
sleeve
has internal screw threads 123 holding an upstream cylindrical portion 111 a
of
conical mixing barrel 111 forming a downstream part of the housing 110. Below
the retaining ring 115 a cylindrical portion 114b of the extension 114 engages
an inner surface of the mixing barrel portion 111 a via a sealing bushing 124;
these portions are made non-rotatable relative to each other so that the axial
position of the mixing barrel 111 can be adjusted by rotation of the sleeve
112
relative to extension 114.
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The mixing barrel 111 defines a cavity having a frusto-conical inner
surface 111 b converging from an upper end to an outlet 119 at its small end.
A
mandrel 130 has an upper end adapter 130a connected to the screw 102; an
upper portion 130b located within the cylindrical interior 114a of the
extension
114; a main, lower portion 130c located within the cavity of the mixing barrel
111; and a lower end part 130d located close to the outlet 119 from the
housing.
The upper end adapter 130a has a threaded bore by which it is attached
to the lower end of the screw 102, so that the mandrel is caused to rotate
with
the screw. From its upper end this part decreases in diameter to meet the
upper end portion 130b, which then expands to provide a short cylindrical part
130b' which fits closely within the interior surface 114a of the barrel
extension
114, and then reduces in diameter to connect with a cylindrical connecting
part
at the upper end of the main mandrel portion 130c. The upper end portion 130b
is provided with helical grooves 134 which cooperate with the interior surface
114a to form helical passageways connecting the inlet to a chamber
surrounding the main portion of the mandrel.
The main mandrel portion 130c has several, for example three, three co-
axial annular protrusions 132 which are of decreasing diameter so as each to
have an outer edge close to the conical internal surface 111 b of mixing
barrel
111. The protrusions divide the space between the internal surface 111 b and
the mandrel into co-axial chambers C1, C2, C3 and C4. The spacing between
the outer edges of the protrusions and the internal surface 111 b is
adjustable
by rotation of the sleeve 112 to shift the axial position of the barrel 111.
Since a
small amount of axial movement of the barrel results in much smaller radial
changes in the slit gaps, very fine adjustment is possible. Usually gaps of 0
to 4
mm. will be used. Typically, the barrel sides are inclined at between 10 and
15°
to the barrel axis, and an amount of axial movement of 1 mm changes the slit
gaps by about 170 to 270 micrometers.
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While the drawing shows protuberances with apparently sharp outer
edges, these will preferably be rounded, as shown in Fig.4 for the first
embodiment.
The downstream end portion 130d of the mandrel has an upstream flank
increasing in diameter from chamber C4 to a short cylindrical section 130d',
and
a downstream flank decreasing in diameter with a slope slightly larger than
that
of the housing ban-el 111. The two flanks are joined by grooves 140 which
provide passageways between the last chamber C4 and the outlet 119.
In operation, this mixer provides both dispersive and distributive mixing,
the former caused by the convergent/divergent flow of the liquid through gaps
between the rotating members 132 and the extended barrel surface 111 b, i.e.,
the gaps separating chambers C1, C2, C3, and C4. The distributive mixing is
ascertained by mainly shear flow of the melt through the grooves of members
130b and 130d. The pressure of liquids entering the inlet end of the mixer,
via
the grooves 134, causes the liquids to pass successively through the chambers
C1 to C4, passing though all the slits in moving from the inlet to the outlet.
Accordingly, the mixer works, in this sense, similar to that of the first
embodiment. Also, as in the first embodiment, the length, as well as the
areas,
of the slits decreases as the liquids pass through the mixer, so that they are
subjected to increasing extensional stress. Here, however, the mandrel is
rotated by its connection to the extruder screw, and accordingly there are
also
shear forces between the rotating parts of the mandrel and the intemai surface
of the barrel, especially where the liquids are close to the protuberances.
The
amount of shear is however relatively minor, and not such as to cause
degradation of mixed polymers.
The Fig.5 embodiment is easily scaled up and can be adapted for more
stages. Unlike the first embodiment, the number of slits can be increased
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without undue increase in the diameter. Helical grooves can be provided not
only in inlet member 130b but also in outlet member 130d.
The form of the mandrel shown in Fig.5 also provides semi-quiescent
zones, as in the first embodiment, where the liquid body is neither being
strongly contracted or expanded. The shape and size of the chambers C1 to
C4 can be optimised using the finite element flow modelling for the melt of
typical viscoelastic charactertstics.
The Fig.S embodiment doss not need to be attached to a single-screw
extruder, as shown, but may also be incorporated in a design using twin
screws. Also, the mixer can be used as a stand-alone unit, having, if desired,
its
own independent power source, as well as an internal mixer in blow molding
and injection molding machines. While the mixer will provide extensional flow
mixing when stationary, some rotation is desirable for the angular shear it
provides. The rotational speed however may be low. The mixer is not limited to
polymers, and can be used in mixing foodstuffs, homogenizing milk, and
preparation of emulsions.
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