Fig A. IPM
Vane-type
Fig. B Operation of IPM vane-type
head assembly
batch mixer (section)
The key to IPM performance is the use of an
integrated positive-displacement pumping to drive fluid through defined
flowpaths, including nozzles. This action ensures extensional,
shear and impact stressing of the fluid, leading to high energy transfer
and significant particle or droplet size reduction. Additional
blending effects are provided by cutting and folding actions within the
head so that microscopic-scale fluid blending (distribution) effects are
also obtained. The extent of dispersive and distributive blending
can be tuned by varying parameters such as nozzle diameter, rotation
speed and other factors for each model type and for different models in
the range. This allows a high degree of flexibility to ensure that
most fluid mixing applications are covered. IPM mixers are
available in both batch and inline forms although the internal
construction of the mixing head is very similar in both types.
Construction
With reference to Fig. A above:
The outer and inner elements
are held together and are stationary
The outer element has large
inlet holes around part of its circumference
The inner element has large
outlet holes around all of its circumference
The central element contains
slots for vanes to move and nozzles through which the fluid
can pass under pressure
The central element is mounted
off-centre with respect to the inner and outer elements and is
rotated at speeds typically ranging from 100rpm to 1450rpm,
(anti-clockwise in the picture above)
The vanes are free to slide in
the central element slots and are constrained by the inner and
outer element walls.
Operating Principle
Fluid Entry (Induction)
As the vanes move around with
the central element, the chamber formed by the vanes and central
and outer elements start to expand as they approach the inlet
holes in the outer element. Fluid is drawn from the mixing
vessel or a piped feed into the inlet holes by the low pressure
in these chambers and is sheared by the vanes as it passes
through these holes. When the chamber moves past the last
of the inlet holes, it becomes effectively sealed from the fluid
outside the mixing head. The fluid inside is then pressurised as
the volume reduces during the progression of the chamber around
the high pressure side of the mixer.
Compression and Nozzle Flow
As the volume reduces and fluid
pressure increases in the chambers, the fluid is forced inwards
through small nozzles in the central element to create very high
extensional stressing. Fluid passes through the nozzles at
very high velocities (e.g. at over 400kph in water) and impinges on the
wall of the inner element, providing a high degree of impact
stressing.
Post-Stress Conditioning and Exhaust
Once the fluid has impacted on
the inner element wall, it is pumped under low pressure
into the chamber inside the inner element which is sealed at the
top. The fluid therefore passes out axially through the
bottom of the mixing head (out of the page in the diagram).
During its retention in the low pressure side of the mixer, the
fluid experiences turbulent mixing and post-stress conditioning.
For certain material systems, this is known to be beneficial
e.g. in allowing electrostatic charge balance and membrane formation
around droplets or particles.
Benefits
IPM is intended to bridge the existing gap between rotor-stator
high shear mixers and high pressure nozzle homogenisers for
liquid-liquid mixing. It offers many of the benefits
of conventional high shear technology but provides significantly
higher mixing performance. Key benefits include:
Exceptional dispersion performance -
at least one order of magnitude greater specific powers than those
of conventional rotor-stator "high shear" mixers
Lower cost - similar
performance can be achieved for a fraction of the purchasing
and operational costs
Batch operation is
possible - batch IPM machines enable insertion of a
homogeniser directly into a vessel, a particularly
cost-effective way of upgrading an existing batch mixing
setup.
Rapid, effective blending -
delivered by the cutting and folding and the turbulent mixing actions
Integral
positive displacement pumping - often no need for a separate
inline pump
Wide viscosity range - 0.1cP to 107cP
(inline only above 50,000cP)
Easy scalability - due to the
mathematical models underpinning the technology
Controllable - speed and stressing
can be finely controlled and optimised
Low pressure dosing - direct dosing
into the mixing head at near ambient pressure
Multistage - easy to combine into
complex multi-stage machines with no pressure-drop or performance penalties
