Setting granule size and structure reliably in a ring-layer granulator relies on the coordinated interplay of mechanical stress, liquid input and drying capacity, binder properties, residence time and fill level, complemented by robust inline process monitoring.

Operating principle of the ring-layer granulator

In the ring-layer granulator, the material being mixed is pressed against the vessel wall in a concentric ring layer by centrifugal force. Wetting, agglomeration, coating/layering, compaction and drying take place within this layer simultaneously or in zones. The resulting granule properties are set reproducibly by means of defined control variables.

Key control variables for granule size and structure

Circumferential speed / rotor speed

The rotor speed determines the centrifugal and shear forces acting and thus the mechanical energy introduced into the ring layer.

Mode of action:

• Higher circumferential speeds (typically approx. 15–35 m/s) generate strong shear and impact forces.
• Consequence: more compact, denser and more spherical granules with higher breaking strength and lower porosity.
• Lower speeds result in less compaction.
• Consequence: looser, more porous structures with a rougher surface and a tendency to generate more dust.

Liquid input and spray parameters

Granule growth is governed largely by the ratio of liquid input to solid (L/S ratio) and by the spray characteristics.

Liquid-to-solid ratio (L/S ratio):

• Increasing the quantity of liquid strengthens the capillary and adhesive forces between primary particles.
• Consequence: faster particle growth and larger final granules.
• Excessive liquid input can lead to over-wetting, sticking and a broad grain size spectrum; insufficient input limits growth and increases the proportion of fines.

Spray rate:

• Determines the rate of growth and thus the shift in the particle size distribution.
• A constant, controllable spray rate supports a narrow grain size distribution.

Droplet size:

• Fine atomisation favours coating and layering processes (build-up of thin layers).
• Coarser droplets promote agglomeration through the formation of stable liquid bridges.

Nozzle position and spray angle:

• Define the wetting zone within the ring layer.
• A deliberate separation of intensive wetting and subsequent build-up granulation/drying increases process stability, since local over-wetting is avoided.

Process reliability:

• Use of precise metering pumps and controlled spray valves.
• Continuous monitoring of the mass flow and the process gas temperature to ensure a stable liquid-to-solid ratio.

Process gas temperature and drying capacity

The balance between the liquid binder introduced and the drying capacity influences both granule size and structure.

Mode of action:

• A high drying capacity withdraws moisture from the ring layer before large agglomerates can form.
• Consequence: smaller, firmer grains with a narrower grain size distribution and a higher proportion of fines.
• A low drying capacity keeps the liquid bridges effective for longer.
• Consequence: stronger grain growth, larger and more porous granules, but a tendency to stick and to form deposits on the wall if the limit is exceeded.

Process reliability:

• Control of the process gas temperature and the gas volume flow, coupled to the spray rate.
• Monitoring of the exhaust air humidity and the product temperature as an indicator of the actual moisture balance.

Binder properties and quantity of binder

The type, concentration and viscosity of the binder determine the strength of the liquid bridges and thus the stability of the agglomerates being formed.

Mode of action:

• More highly concentrated or more strongly binding systems produce firmer granules with greater resistance to breakage.
• Low-viscosity binders distribute more quickly and more evenly but contribute less to grain strength.
• High-viscosity binders have a stronger adhesive effect but require finer atomisation and higher mechanical energy for distribution.
• Consequence: the quantity of binder shifts the grain size distribution as a whole; the type of binder determines above all strength, porosity and dissolution behaviour.

Process reliability:

• A constant binder temperature, since viscosity is temperature-dependent.
• Check the batch consistency of the binder, because fluctuations feed directly through to grain strength.

Residence time and fill level

Residence time and fill level determine how long and how often a particle passes through the wetting, compaction and drying zones.

Mode of action:

• Longer residence times increase the number of contacts and favour grain growth and rounding.
• Shorter residence times limit growth and maintain a narrower grain size distribution.
• The fill level influences the thickness of the ring layer: too thin a layer leads to uneven wetting, while too thick a layer dampens the shear effect and creates gradients.
• Consequence: residence time and fill level act together; they cannot be set independently of one another.

Process reliability:

• A constant mass flow through gravimetric dosing, since throughput and residence time are directly coupled.
• A defined overflow or controlled discharge devices to stabilise the fill level.

Inline process monitoring

Because wetting, agglomeration and drying proceed simultaneously in the ring-layer granulator, the process state cannot be derived from a single variable. Sound process control relies on several signals recorded in parallel.

• Drive power and torque as a measure of the moisture and cohesion of the ring layer; an increase indicates the onset of over-wetting.
• Product and exhaust air temperature together with exhaust air humidity for continuous assessment of the moisture balance.
• Mass flows of solid and binder to secure a constant liquid-to-solid ratio.
• Optical or acoustic methods for inline determination of the particle size distribution, supplemented by regular sieve analyses as a reference.
• Defined action limits and a documented procedure for exceeding them, so that corrections are made reproducibly.

Only the interplay of these control variables makes granule size and structure reproducible. Their specific target values cannot be laid down in general terms but are determined in trials for each material system and then fixed as a process window with defined tolerances.