Shear in the Ribbon Mixer: Why 400x in a Beaker Becomes 240x in the Plant

The lab result was 410 g/g in deionised water, measured on a magnetic stirrer at 200 rpm. The plant ran the same powder in a 600 litre ribbon mixer at 45 rpm and could not get above 250 g/g, so the dose was raised by 60 percent and the batch cost went up with it. Nobody had changed the product. The mixing energy had changed by a factor of thirty, and the gel was being cut apart as fast as it formed.

Scale-up failure in SAP work rarely comes from the chemistry. It comes from shear, residence time and the ratio of water to powder at the moment of contact. This article sets out what a beaker test leaves out and three fixes that recover most of the lab number in production.

What a Beaker Test Leaves Out

A 400 ml beaker with a 30 mm stir bar at 200 rpm gives a tip speed of about 0.3 m/s and a gentle, laminar flow. A 600 litre ribbon mixer with a 500 mm ribbon at 45 rpm gives a tip speed of 1.2 m/s and a turbulent, folding flow with high local shear at the ribbon edge. The gel that forms in the first two seconds meets that edge and is torn into fragments of 1 to 3 mm.

Fragments are not a cosmetic problem. A gel fragment has fresh surfaces that will absorb more water, so far so good, but it also has a much higher total surface area exposed to the continuous phase, and in a system with dissolved salts the osmotic gradient at those surfaces drives water back out. The net effect after 10 minutes of mixing is a lower retained volume than the same gel left undisturbed.

Impeller Tip Speed and Gel Breakdown

Measure tip speed, not rpm. For a ribbon, tip speed equals pi times diameter times rpm divided by 60. Below 0.8 m/s the gel survives; between 0.8 and 1.5 m/s it fragments slowly; above 2 m/s you are making a slurry rather than a gel, and retained absorption falls 30 to 45 percent within 5 minutes of mixing.

The practical target for a 600 litre batch is 20 to 30 rpm on a 500 mm ribbon, which is 0.5 to 0.8 m/s. Many mixers are geared for 45 rpm because that suits powders, and the fix is a variable speed drive rather than a new mixer. A 15 kW drive with a VFD costs less than a week of over-dosing on a plant running three batches a day.

Scaling From 2 L to a 600 L IBC

Keep three ratios constant and the result usually transfers: water to powder ratio at first contact, energy per kilogram, and time to the point where the gel is fully formed. Energy per kilogram is the one people forget. A beaker delivers about 0.5 watts per kg of batch; a ribbon mixer delivers 8 to 15 watts per kg. To match the beaker, you would have to run the mixer for 40 seconds, not 10 minutes.

The practical answer is to add the powder into a vortex of water at 25 rpm, run for 60 to 90 seconds only, then stop the agitator and let the batch stand for 5 minutes. Standing does the swelling; the mixer only has to disperse. Plants that mix for 15 minutes because it feels thorough are the ones with the 240 g/g result.

Three Fixes That Keep 85 Percent of the Lab Number

First, dose into the vortex rather than onto the surface, using a 100 mm pipe that discharges below the water line. Surface dosing produces lumps that never wet out, and those lumps are counted as capacity in the lab but not in the plant. Second, slow the agitator to 25 rpm during addition and stop it after 90 seconds. Third, pre-dissolve any salt in the recipe before the powder goes in, because powder added to a concentrated brine skins over before it disperses.

With those three changes, the 410 g/g lab figure typically lands at 340 to 360 g/g in a 600 litre batch, which is 85 percent and enough to bring the dose back to the design value. Where it does not, the granule is probably too fine for the mixer, and a coarser cut will survive better than a formulation change.

Ask your supplier for a tip speed recommendation with the grade, not just a capacity figure. The polyacrylate granule grades for mechanical dispersion are specified with this in mind.