But why are high shear mixers advantageous when compared to conventional mixing equipment such as agitators and stirrers? Agitators or stirrers come in a vast array of shapes and sizes, including marine propellers, foils, axial and radial flow turbines, gate and anchor stirrer/scrapers, etc. Despite the wide range of capacities and viscosities this type of mixer can handle, the low shear nature of agitators limits their effectiveness to performing simple tasks such as blending liquids of like viscosities, maintaining in-tank uniformity, and producing flow in a vessel to promote heat transfer from jacketed vessel walls. Where the product has to be processed in some way – liquids of differing viscosities are being blended, active ingredients dispersed, emulsions formed, particle or micron size being reduced, etc. an agitator can take a long time to achieve the required results or, more often than not, is simply not capable.
That’s where high shear mixers come in. High shear mixers are ideal for these types of mixing tasks and can cut mixing times by up to 90%, offering improved yield, product quality and greater batch-to-batch consistency. To understand why and how high shear mixers can achieve these results, it is helpful to know how they work.
The high-speed rotation of rotor blades within the mixing workhead exerts a powerful suction, drawing liquids into the centre of the workhead. Centrifugal force then drives materials towards the periphery of the workhead where they are subjected to a milling action in the precision-machined clearance between the ends of the rotor blades and the inner wall of the stator. This is followed by intense hydraulic shear as the materials are forced, at high velocity, out through the perforations in the stator and circulated into the main body of the mix. At the same time, fresh material is continually drawn into the workhead, maintaining the mixing cycle.
But how does this translate into improvements to mixing processes? Shear energy is often required to obtain the best results for the following tasks:
Blending liquids of differing viscosities
When blending easily miscible liquids of similar viscosities, an agitator will produce satisfactory results. But where there is a significant difference in viscosity between the two liquids, an agitator tends to move the two liquids around without actually blending them together, and it can take a long time to achieve a uniform blend.
With a Silverson high shear mixer, the positive mixing action of the rotor/stator assembly draws the liquids into the workhead where they are rapidly combined before being forced out through the stator and projected back into the vessel; the liquids are blended almost instantaneously.
Forming stable emulsions
The key to creating a stable emulsion is obtaining the finest possible droplet size. The more shear energy introduced into the mix, the smaller the suspended droplets will become, creating a uniform, stable emulsion. This is especially important in products such as vaccines which must remain stable, with the phases reduced significantly to maximise shelf life.
Due to their low shear nature conventional agitators simply move the two liquid phases around the vessel, so although it may appear as though the phases have blended, the droplet size hasn’t been significantly reduced, so once the agitation stops the emulsion will soon separate into the two distinct phases once more. Repeatability between batches is difficult to obtain, so in applications which require batch-to-batch consistency, this type of mixing equipment is not suitable.
High shear mixers subject the liquids to intense high energy which reduces the droplet size sufficiently and allows the two phases to combine, resulting in a stable emulsion which will not separate once the shear energy has been removed. The high shear rotor/stator design of the Silverson mixer is ideally suited to this application and can easily achieve emulsions with a droplet size of 2 to 5 microns.
Hygienic applications
High Shear mixers can also be beneficial for applications which require stringent hygiene. Where it can be difficult to maintain hygienic processes with conventional mixing equipment, many high shear mixers are suitable for CIP or SIP operations. Silverson manufacture a range of Ultra-Hygienic models specifically for the pharmaceutical industry, designed to comply with both FDA and cGMP guidelines. Further to this range, a Sealed Unit Laboratory mixer is also available, designed for working on sterile materials where it is essential to exclude airborne contamination.
