Technology Zone
Fluid mixing technology brings liquids, gases and solids to a homogeneous product. This zone covers agitators, static and inline mixers, high-shear and vacuum mixers and the tanks they work in.
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What this zone covers
Four jobs hide behind the same word. Blending brings two miscible liquids to one composition; suspension keeps solids off the bottom; dispersion breaks a gas or a second liquid into droplets; and heat transfer moves the product past a jacket fast enough to keep it uniform. Each one has its own impeller, its own speed and its own definition of finished, and a vessel designed for one of them rarely does another well.
Axial impellers, the pitched blade and the hydrofoil, push flow up or down the vessel and are what suspension and blending want. Radial impellers, the Rushton turbine above all, throw flow outward into the baffles and generate the shear that gas dispersion needs. High shear rotor stator heads work on a small volume at very high shear for emulsions. Flow and shear trade against each other, and the choice is which of the two the product needs.
An unbaffled vessel spins the whole contents as a body and mixes almost nothing while looking busy. Four baffles at a tenth of the diameter break that rotation and turn rotation into flow. The rest of the geometry follows the same logic: impeller diameter roughly a third of the tank, clearance about one impeller diameter from the floor, and a second impeller once the liquid height passes about 1.2 times the diameter.
Not everything needs a vessel. A static mixer divides and recombines the stream in a fixed set of elements and blends two liquids in a few pipe diameters with no moving parts and no power beyond the pressure drop. Inline rotor stator and dynamic mixers put high shear into a continuous stream. Both remove a batch step, and both need the flow rate to stay inside the range they were designed for.
A recipe that works in a 20 litre pilot does not transfer by keeping the speed. Something has to be held constant, and which something depends on the job: power per unit volume for blending and heat transfer, tip speed for shear-sensitive dispersion, or an equal Froude number where the surface behaviour matters. Trying to hold all of them at once is what makes scale-up fail, because they cannot all be kept.
| Parameter | Typical range | Rule of thumb |
|---|---|---|
| Impeller to tank diameter | 0.3 to 0.5 | Large and slow for flow, small and fast for shear |
| Baffles | 4 baffles at 1/10 to 1/12 of tank diameter | Without baffles the contents rotate and mixing time goes to nothing useful |
| Off-bottom clearance | About one impeller diameter | Too low and the impeller starves; too high and solids stay on the floor |
| Power input, blending | 0.1 to 0.5 kW per cubic metre | Scale on power per volume when the job is blending or heat transfer |
| Power input, suspension | 0.5 to 2 kW per cubic metre | Just-suspended speed is the target; more only wastes energy |
| Tip speed, shear-sensitive | Below 3 to 5 m/s | Scale on tip speed when cells, crystals or emulsions are involved |
| Rotor stator tip speed | 10 to 25 m/s | Droplet size falls with tip speed and with passes, not with time in the vessel |
| Static mixer length | 5 to 10 pipe diameters for miscible liquids | Check the viscosity ratio; a large one needs far more elements |
| Symptom | Likely causes | What to do |
|---|---|---|
| Solids settle on the vessel floor | Speed below the just-suspended point, wrong impeller type, or too much clearance from the bottom | Lower the impeller to about one diameter above the floor, switch to an axial type, and raise the speed to the just-suspended point |
| A deep vortex forms and draws air in | No baffles, or speed too high for an unbaffled vessel | Fit four standard baffles, or mount the agitator off-centre or at an angle if baffles are not possible |
| Batch takes far longer than the pilot predicted | Scale-up on the wrong constant, usually speed instead of power per volume | Recalculate on power per unit volume, and check the impeller to tank ratio matches the pilot |
| Product is damaged, cells or crystals broken | Tip speed too high, or a radial turbine where an axial impeller belongs | Move to a large slow hydrofoil, cap the tip speed, and check the pump in the loop is not doing the damage instead |
| Emulsion separates within hours | Droplets too large, insufficient passes through the high shear head, or an emulsifier added at the wrong point | Increase tip speed or the number of passes, and add the emulsifier where the shear is highest |
| Mechanical seal on the agitator leaks | Shaft runout, side load from an unbalanced impeller, or the vessel run at a level that lets the impeller break surface | Check runout and balance, keep a minimum level over the top impeller, and confirm the seal support system |
| Heat transfer to the jacket falls off | Fouling on the wall, or too little flow along the wall from an axial impeller alone | Clean the wall, add an anchor or scraper for viscous product, and check the flow pattern reaches the heat transfer surface |
Go deeper: fluid mixing news and case studies · frequently asked questions
Equipment Guide
Silverson
For more than 75 years Silverson has specialised in the manufacture of quality high shear mixers for processing and manufacturing industries worldwide.
Industrial Tomography Systems (ITS/ITOMS)
Providing comprehensive process data and visualisations in real-time, electrical tomography is able to monitor virtually any academic research or industrial application involving mixing, separation, level detection, or flow rate; to facilitate greater process efficiencies and lower production costs.
Landia a/s
Landia is a worldwide supplier of Chopper Pumps and Mixer Solutions with its main businesses being wastewater anaerobic digestion/biogas agriculture and recycling of waste products.
Admix, Inc.
Established in 1989, Admix is a recognized leader in advanced mixing technologies for the food, beverage, dairy, protein, pharmaceutical, cosmetics and chemical industries.
Ask the Expert
Engineers and product specialists who answer technical questions from readers. The answer comes back to you by e-mail; when we publish it, other readers benefit too.
Martin Hangaard AndersenLandia a/s
Martin Hangaard Andersen, Technical Supervisor with Landia a/s has 20 years of experience with mixing technology, pumps and pumping systems. Martin holds… Ask a question
Joshua BrownIndustrial Tomography Systems (ITS/ITOMS)
Josh is a Technical Engineer and is an integral member of Industrial Tomography Systems’ talented engineering team. Josh is responsible for creating… Ask a question
Matt SmithSilverson
Matt Smith is Sales Director at Silverson Machines, which has over 70 years of experience helping customer in the field of high… Ask a question
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What this zone covers
Axial impellers, pitched blade and hydrofoil, for blending and for keeping solids suspended: they move a lot of liquid for the power. Radial impellers such as the Rushton turbine for gas dispersion, where the shear at the blade tip is the point. Rotor stator heads for emulsions and for breaking agglomerates. If the product is shear-sensitive, choose the largest, slowest impeller that still does the job.
In a vertical cylindrical vessel with a centre-mounted agitator, yes. Without them the contents rotate as a body, a vortex forms, air is drawn in and very little mixing happens while the motor draws full power. Four baffles at a tenth of the diameter is the standard, and the alternative in a vessel that cannot take them is to mount the agitator off-centre or at an angle.
Decide first what must stay constant. Power per unit volume for blending, suspension and heat transfer; tip speed where the product is shear-sensitive; an equal Froude number where surface behaviour and vortexing matter. You cannot hold all of them, and scale-up failures are usually the result of holding rotational speed, which is the one thing that means nothing across sizes.
The lowest speed at which no particle rests on the bottom for more than a second or two. It is the honest target for a suspension duty, because the power to reach it rises steeply and anything beyond it buys almost no extra uniformity while costing energy and shear. It is measured, not calculated, and correlations only give you the starting point.
A static mixer if the streams are already flowing and the job is blending two miscible liquids, because it works in a few pipe diameters, has no moving parts and costs only the pressure drop. A vessel if the process needs residence time, a reaction, heat transfer or solids kept in suspension. Static mixers are also unforgiving about flow rate: outside the design range they stop mixing.
Give up on turbulence and design for movement. Anchors, helical ribbons and scrapers move the whole contents and keep the heat transfer surface clean, at low speed and high torque. Standard turbines simply carve a cavern around themselves while the rest of the vessel sits still, which looks like mixing on the ammeter and is not.
Usually because they are not identical where it counts: impeller clearance, baffle condition, fill level, or a replacement impeller of a different type fitted at some point. Check the geometry against the drawing before adjusting the recipe. A missing baffle or an impeller mounted 200 mm higher changes the flow pattern more than any speed adjustment will fix.
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