Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
QM 33771
- AGITATORS
~his invention relates to agitator~ for the
dispersion of a fluid in a liquid.
Disc turbine agitators with a plurality of
axially aligned plane paddle blades are known for the
dispersion of sparged gases as small bubbles in liquids
in tanks and the conco~itant mixing of the tank
contents. In u6e, a vorte~ low pressure zone forms
behind each rotating blade of the turbine, and with the
gas flow rates frequently encountered in industry, the
- 10 gas tends to collect and be held as a cavity in this
~one, this disadvantageously reduces dispersion and
~ixing efficiency and can cause turbine blade erosion.
The same problem would be found with a sparged liquid
less dense than the tank liquid. We have now designed
a turbine agitator in which vortex formation and its
deleterious consequences are minimised, and which
provides eEficient dispersion and mixingO
Accordingly, the present invention provides a
turbine agitator assembly comprising
a reservoir for liquid,
a rotor mounted in the re~er~oir and with a plurality
of radially extending blades, and
~eans for sparging a fluid into li~uid in the
reservoir,
the fluid sparging ~eans and the rotor being so
constructed and arranged that, in use, the rotor blades
(submergea in the liquid~ and/or the liquid flow they
generate disperse the sparged fluid,
- characterised in tha~ each of the blades i~ hollow and
ha6 a discontinuous leading edge, only a 6ingle
196
-2-
trailing edge along an acute angle, no external
concave surface and an open radially outer end.
In conventional disc turbine agitator6, we have
found that vortices are generated where fluid flow i6
not streamline along the blade surface, but becGmes
'separated', for example at projecting edgas ~e.g. ~he
;~ axial edges of conventional a~ially-aligned paddle
blades), where a trailing external surface i~ concave,
or where there iB no acute trailing edge, e.g. with
circular, elliptioal, ~quare or oblong cross-section
blades.
We believe that any blade fulfilling the
foregoing criteria for a blade of this invention will
be suitable. Within this, the blade may have a
15 symmetrical cross-section, having a circular, parabolic
or elliptical section leading face merging smoothly
into a sphenoidal (i.e. wedge 6haped) or sharply
elongate parabolic or elliptical section trailing part.
It will be seen that the term 'trailing edge along an
20 acute angle' thus includes both angular and sharply
radiused edges. Parabolic or elliptical seotion
~ leading ~aces are favoured as improving the streamline
- around the blade, although the leading part may also be
sphenoidal. A preferred blade shape is a symmetrical
25 aerofoil-like cross-seotion.
~he blade is hollow and the leading edge is
discontinuou~, for ~xample in the form of holes, or in
the preferred form of a slot symmetrically disposed in
the leading face of a symmetrical cross-section blade.
30 The radially outer end of the blade is at l~a~t
partially open, 80 that ~uch a blade provides a
scooping action which disperses and mixes by pumping
the ~cooped liquid radially outwardsO
Typical dimensions of a blade in the present
assembly are:
blade length = D/4, projected height = D/5, where D is
the overall rotor diameter.
Typically the blade will be made of conventional
~etals or plastics used for turbine agitator paddles.
In its general $orm the blade has two elongate
axes, one radial and one transverse, de~ining a 'blade
plane'. q~is blade plane will generally coincide with
or lie parallel to any plane oE rotation describea by
the blade in use, that is the blade is usually not set
at an 'attack angle' on or with respect to the rotor
shaft. However, this latter possibility is not
excluded, but the skilled man will readily appreciate
that the angle ~hould not be so great that the trailing
(or any leading~ edge behaves effectively as an axially
projecting edge, and/or any trailing part of the blade
surface behaves effectively as a concave surface, in
tending to produce substantial vortices.
The blades of the turbine rotor ~ay be arranged
in the same rotational plane or in any number of
parallel rotational planes. It i6 preferred that the
blade are arranged regularly within any one plane so
that rotational balance is maxi~ised. Preferably they
are also ~as apt) so arranged along the shaft and with
respect to each blade in any other plane in accordance
with routine engineerin~ practi~e that tor ional
balance is maximised, for example, they are arranged
with equal numbers of blades in each plane~ and with
corresponding blades in different planes axially in
register or with all the pla~es regularly rotationally
skewed with respect to one another.
The blades may also be set at any angle to the
rotor shaft in an axial direction, other than a right
angle in order to provide an axial component of the
discharge flow.
The rotor may have 2 or more blades. The mixing
efficiency of the turbine will generally increa.se with
the number of blades in any one plane until such point
that the blades are BO close with respect to their
transverse dimension that in use the action of any one
blade interferes with the action of the following
blade. Similarly the useful number of planes of blades
i8 ]imited by any mutual interference between the
planes due to proximityO The addition of further
planes of blades increasingly remote from a single
axial sparging eource may also not increase the fluide
disper6ing efficiency of the turbine, but may 6till
assist mixing of the liquid and/or liquid fluid
dispersion in the reservoir.
Subject to the foregoing suitable blade numbers
include 2 to 24 coplanar blades, typically 4 to 12, and
up~to 5 planes of blades, typically 1.
Typically, dimensions of the rotor are
deter~ined by the size of the reservoir, and u~ually
-the diameter will be one third to a half the
corre~ponding reservoir transverse dimension.
The fluid ~parging means may have a single
: - \
--5--
- aperture, or multiple apertures ~uch as a row, grid,
rose or ring. Although the 6parging of liquids, in
particular those les~ dense than the reservoir liquids,
is not excluded, the sparged fluid will often be a gas.
The rotor and fluid sparging means may be placed
in any orientation and mutual position which ensures
that the fluid is delivered either to the volume swept
by the rotor blades or to any dixectly adjacent 30ne on
which any liquid 10w generated by the rotor blades
impinges (in both cases 'the dispersion ~one').
The rotor may be mounted in any orien~ation,
although it will often be convenient to mount it
upright with the sparging means mounted on the
reservoir above or below it, e.g. spaced axially from
it, so that the fluid may be delivered to the
dispersion zone through the liquid essentially under
gravity, either from below for a gas or liquid less
- dense than the reservoir liquid or ~rom above for a
denser liquid. The sparging means may then suitably be
a hole, rose or ring coaxial with the rotor.
As is common with turbine agitators the blades
will not generally extend from the rotor shaft itself
but will each be ~ounted on an ar~ or an equivalent
structure on the shaft. It will be apparent that an
axial hole, rose or ring sparging means small diameter
than the overall rotor aiameter which does not overlap
the blades will ~ot deliver fluid to the dispersion
zone without a deflector. In ~uch a case the blades
may conveniently be mounted extending from the
periphery of a rotor disc, the disc acting as a
~ -6- ~ 6
deflector. With the typical blade dimentions given
hereinbe~ore, the disc will typically be 3D/4 in
diameter, where D is the overall rotor diameter.
The fluid may of course be delivered to a zone
radially outside the volume swept by the blades, since
the liquid pumped into this zone by the blades makes it
a dispersion ~one; a sparging ring may ~e used.
Alternatively, the rotor may be mounted cross-
wise with the sparging means mounted on the reservoir
and spaced radially from it above or below, ayain
conveniently to allow delivexy essentially under
gravity. The sparging means may then suitably be an
axially aligned row, a transverse straiyht or arcuate
row or a planar or curved grid depending on the rotor
structure.
In another aspect the sparging means may be
mounted on the rotor, for example as an aperture or
apertures in front of each blade or spaced axially from
the or a blade planeu
Orientations of the rotor appropriate to or
compatible with the disposition of the sparging means
and blades will be self-~vident to the skilled man.
Although useful in all applications where
dispersion of two fluid phases is required, the present
assembly is particularly use~ul for gas-liquid mass
transfer processes, and for low-shear thorough mixing,
e.g. of sensitive substrates such as living cell
fermentation su~pensions or polymer latices or
dispersions subject to ready degradation or
coagulation.
The pre ent invention will now be de~cribed with
9~
: reference to three specific embodiment~ of the rotor
and sparging means, depicted in Figure~ 1, 2, 3 and 4.
In the Figures, a rotor 4 iB rotata~Iy mounted
vertically within a reservoir 2 (not shown~ capable of
holding a liquid 3 (also not 6hown) to sub~erge the
rotor 4. The rotor 4 consi~t~ o~ a sha~t 5 (driven by
an electric motor 6 - not 6hown) on which a plurality
(four or 8iX) radially extending blades 7 are mounted
regularly about the shaft 5 in a ~ingle plane by means
of a disc or arms 8.
Each blade 7 is of symmetrical uniform aerofoil
cros~-section with a single sphenoidal acute-angle
trailing edge 9 extending the length of the blade 7.
Each blade 7 is hollow and its leading face 10 has a
symmetically disposed slot 11 extending the length of
: the blade 7. The ends 12 of the blade 7 are open.
The blades 7 are mounted such that their central planes
~ of sy~metry are coplanar.
: A means for sparging gas 14 is, in Figures 1 to
~: 20 3, mounted on the reservoir below the level of and
coaxial with the rotor. In Figures 1 and 2 it i~ a
~ingle aperture or a sparging ring of apertures which
do not overlap the blades 7. In Figure 3 it is a
sparging ring lying below a zone 19 radially outside
the volu~e 18 swept by the blades in use. In Figure 4
the sparging means 14 ¢onsists of four apertured tubes
mounted on, projecting from, and communicating with the
; hollow interior of the shaft 5, and regularly spread
betweén and coplanar with the blades 6. ~heir
apertureB 15 are in the trailing face 16 of each tube
14 .
--8--
- In use, the reservoir 2 is filled with liquid 3
to sub~erge the blades 7 of the rotor 4, which is ~hen
rotated in the direction of arrow A. Gas 17 i5 then
supplied via sparging apertures 15 (in Figure 4 through
the hollow interior of the rotor shaft 5 and rod~ 13)
to the volume 18 swept by the blades ~in Figure 1 by
deflection by the disc 8) or the zon~ 19. In all cases
liquid 3 is scooped in by the blades 7 through the ~lot
11 and discharged through the open blade end`12 i~to
the dispersion zone 19. In Figures 1, 2 and 4 the gas
17 flows ove~ the outer surface of the blades 7, and in
all cases the gas is dispersed in the zone 19.
