Craig Levins, applications engineer, Chemineer, describes how continuously operating in-line pipeline mixers designed to blend, emulsify and breakdown agglomerates, can handle delicate dilution applications such as those found in washing-up liquid manufacture
Pipeline mixers employ a high speed rotor that runs in close proximity to a fixed stator, producing a thorough wetting of dispersed substances to create a uniform product. The high operating speed and reduced inlet port provide a pumping effect that forces the product to pass through the restricted orifices in the stator. As the fluid passes through the rotor and stator, the hydraulic shear breaks the product down to the primary particle size. The inlet is designed so that it is smaller than the line size, which ensures that the fluid is forced through the mixer and acts as the first shear zone. Although the pipeline mixer acts as a pump, it should not be used as a pumping unit.
Variable input flows
Designed with a fixed gap setting of between 0.2–0.3mm, a pipeline mixer can handle viscosities of up to 100,000 cPs – it can handle products with larger viscosities if a positive displacement pump is used. This also allows variable input flows.
Pipeline mixers have a high throughput-to-kW ratio and operate at a nominal speed of 3000rpm, although some of the smaller belt-driven in-line mixers operate at a nominal speed of 7200rpm.
In the manufacture of washing up liquid, the in-line mixer can first carry out the dilution (with deionised water) of FAES (fatty acid ester sulphonate) with 70% active matter, which is taken directly from the storage tank. To meet the needs of this process, it should be equipped with a variable speed drive that adjusts to the actual production capacity – e.g. 9000 – 32,000l/h.
As soon as contact is made with the water, a gel phase (70 -> 30 %) is formed, requiring the mixer to dilute the base product down to a concentration of less than 30%. At the same time, other surfactants and products needed for the basic formulation are added and mixed into the diluted material. The mixers then process the base materials, which requires a temperature-sensitive perfume to be added, together with components such as a matting agent, pigment and preservatives.
A static mixer can also be used for this process. This mixer is designed with a patented helical mixing element that directs the flow of material radially toward the pipe walls and back to the centre. Additional velocity-reversal and flow division results from combining alternating right- and left-hand elements to increase mixing efficiency. All material is continuously and completely mixed, eliminating radial gradients in temperature, velocity and material composition. A suitable static mixer should maximize mixing efficiency without the wasted energy and material blockage typically found in more restrictive motionless mixers.
With a pipeline mixer, the washing-up liquid product only requires one pass through the mixing head to reach the process downstream. This is because the entire process stream is directed through the mixing head, eliminating any chance of bypassing the high shear zone. Mixers are fitted with a tandem shear mixing head, consisting of two rotors and two stators to give additional product refinement. The primary rotor forces material through the restricted openings in the primary stator, then the secondary rotor and secondary stator. A multi-port secondary stator can be utilised as the secondary stator to further increase product refinement.
A robustly designed pipeline mixer should be capable of blending, emulsifying and breaking down agglomerates – as well as producing a thorough wetting of dispersed substances to create a completely uniform product. This also makes them suitable for use in polymer applications, polystyrene de-lumping, paint manufacture, food product refinements and many more.
Because only one pass of the components through the mixer is required to make them ready for the process downstream, an in-line unit can also be used for the rapid blending in the preparation of coating, the emulsification of mayonnaise and the de-lumping of pharmaceutical slurries.