IPM vane pumping machines are ideally suited to emulsification tasks although the dispersion and deagglomeration of soft solids are also common applications. They are intended to bridge the existing gap between rotor-stator high shear mixers and high pressure nozzle homogenisers and offer many of the benefits of conventional high shear technology with 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” mixer
- Lower cost – similar performance to standard nozzle homogenisers can be achieved for a fraction of the purchasing and operational costs
- Batch operation is possible – batch IPM machines enable insertion of an homogeniser directly into a vessel, a particularly cost-effective way of upgrading an existing batch mixing setup.
- Rapid, effective blending – delivered by the cutting, folding and turbulent mixing actions
- Integral positive displacement pumping – often no need for a separate pump if operated inline
- Wide viscosity range – 0.1cP to 10,000,000cP (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
The 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.
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.
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.
Independent trials performed by customers and third parties have confirmed IPM to be a new class of mixer, offering performance between rotor-stator homogenisers and high pressure nozzle homogenisers. Benchmarking trials consistently indicate that if a rotor-stator homogeniser delivers a droplet size (Sauter mean diameter) of around 5 microns in an emulsion and a high pressure nozzle homogeniser gives about 1 micron, then an IPM will be in the 2-3 micron range with a very small standard deviation.