How to Get Nearly Spherical Granules with Excellent Flow Properties

luozhu

·December 17, 2025

·8 min read

Manufacturers achieve nearly spherical granules with excellent flow properties. Success depends on selecting the right granulation method. It also requires precise control over key process parameters. The global pharmaceutical pellets market shows significant growth. Projections estimate a value of USD 50.35 billion by 2032.

This market expansion highlights the increasing demand for high-quality, uniform granules in modern manufacturing.

Effective methods include High-Shear Wet Granulation, Fluidized Bed Granulation, and Extrusion-Spheronization. A High Shear Granulator, often a Rapid Mixer Granulator, is a popular choice. Data shows nearly 75% of wet granulation lines use this technology, demonstrating its industry-wide importance.

Using a High Shear Granulator for Sphericity

A High Shear Granulator is an exceptionally effective machine for creating dense, spherical granules. Operators achieve this by precisely controlling three critical process parameters: impeller and chopper speed, binder liquid addition, and total granulation time. Mastering these variables allows manufacturers to transform a fine powder into a uniform, free-flowing product.

Optimize Impeller and Chopper Speed

The impeller and chopper have distinct but complementary functions. The main impeller mixes and densifies the powder, while the high-speed chopper breaks down oversized agglomerates. The interplay between their speeds directly shapes the final granule.

Impeller Speed: A slower impeller speed promotes granule growth, creating larger particles. A faster speed increases densification, resulting in harder, more spherical granules.
Chopper Speed: A high chopper speed is essential for sphericity. It cuts large, irregular lumps into smaller, more uniform pieces, preventing uncontrolled growth and promoting a rounded shape.

Pro Tip: Operators often start with a slow impeller to initiate wetting and then increase the speed for densification. The chopper is typically run at a high, constant speed throughout the wet granulation phase to ensure uniformity.

Control Binder Liquid Addition

The rate and method of binder addition are crucial for granule formation. The goal is to distribute the liquid evenly to wet the powder particles consistently. An improper addition can ruin a batch. If the liquid is added too quickly, it creates overwetted, dough-like lumps. If added too slowly, it produces excessive fines and weak granules.

Modern High Shear Granulator systems allow for precise control over the spray rate. This ensures every particle receives the right amount of binder, promoting the "snowball effect" where small nuclei grow evenly into spherical granules.

Manage Granulation Time

Granulation time, or wet massing time, is the period after all binder liquid has been added. During this phase, the granules continue to tumble and collide inside the High Shear Granulator. This mechanical action further densifies the particles, breaks off sharp edges, and rounds them into a more spherical shape. Extending the wet massing time generally improves sphericity and flow. However, excessive time can lead to overly large or hard granules that may be difficult to mill or dissolve later. Finding the optimal time is a key step in process validation.

Achieving Uniformity with Fluidized Bed Granulation

Fluidized bed granulation (FBG) is a premier method for producing highly uniform and porous granules. In this process, a stream of heated air suspends, or "fluidizes," the powder particles while a binder solution is sprayed onto them. This technique offers exceptional control over granule growth, leading to a consistent final product. Success hinges on mastering spray parameters, airflow, and equipment configuration.

Fine-Tune Spray Parameters

Precise control over binder liquid delivery is fundamental to achieving uniform granules. The goal is consistent droplet atomization and distribution across the fluidized powder bed.

Operators achieve this by selecting the right equipment. For example, top spray granulating nozzles with a multi-orifice 'cluster-head' design produce an exceptionally fine and consistent spray. This technology ensures uniform wetting even in large batches, preventing localized overwetting and promoting even agglomeration.

Proper spray rate and droplet size are critical. A rate that is too fast can create oversized, weak agglomerates, while a rate that is too slow can result in excessive fines. Fine-tuning these parameters ensures each particle receives the optimal amount of binder for controlled, spherical growth.

Control Fluidization and Airflow

Airflow and temperature are the engines of the fluidized bed process. They dictate both particle movement and drying efficiency. The inlet air temperature is a powerful tool for controlling granule properties. An appropriate temperature, often between 170–220°C, promotes rapid, symmetrical shell formation around the wet granule, which is essential for sphericity.

Furthermore, the system can use inlet air temperature as a feedback parameter. It adjusts the temperature based on real-time moisture measurements, allowing operators to achieve specific moisture profiles and minimize particle size variation. The airflow rate itself controls the fluidization pattern, ensuring all particles are continuously exposed to the binder spray and drying air.

Use Wurster Inserts for Controlled Layering

The position of the spray nozzle dramatically changes the granulation process. While top-spray is common for general agglomeration, bottom-spray systems, often called Wurster coaters, provide the highest degree of control for creating uniform layers. This configuration is ideal for pellet coating and building dense, spherical granules. The key differences are clear when comparing the main FBG setups.

Feature	Top-Spray	Bottom-Spray (Wurster)	Tangential-Spray
Nozzle Placement	Top of the chamber	Bottom of the chamber	Side of the container
Primary Application	Agglomeration, Drying	High-quality Coating	Granulation & Coating
Granule Quality	Less dense, more friable	Optimal film quality	Denser, less friable

The bottom-spray method directs particles through a highly concentrated spray zone, ensuring each particle receives a uniform coating. This controlled layering process is unmatched for producing granules with superior sphericity and consistency.

The Extrusion-Spheronization Method

Extrusion-spheronization is a multi-step process renowned for producing uniform, dense spherical pellets. This method offers excellent control over particle size and density. It involves preparing a wet mass, pushing it through a die to form cylindrical extrudates, and then rounding these extrudates in a spheronizer.

Create a Consistent Wet Mass

The quality of the final pellet begins with the initial wet mass. A consistent, plastic-like dough is essential for successful extrusion. Several formulation properties are critical for achieving this consistency.

Binder Liquid Holding Capacity: Excipients must effectively hold the binder liquid under pressure.
Rheological Properties: The flow characteristics of the wet mass influence pellet formation.
Elastic Properties: The material needs an optimal level of elasticity for effective processing.

A Mixer Torque Rheometer (MTR) is a valuable tool for optimization. It measures the torque required to mix the mass, identifying the ideal liquid-to-solid ratio (L/S) at the point of maximum torque. Formulations processed at this peak value consistently produce high-quality pellets.

Optimize the Extrusion Process

Extrusion shapes the wet mass into uniform cylindrical strands. The extruder forces the material through a die or screen with fixed-size holes. Two parameters, screen size and extrusion speed, are vital for controlling the extrudate properties. The diameter of the die holes, which can range from 0.5 mm to 2 mm, directly dictates the diameter of the resulting pellets.

Extrusion speed also has a significant impact. Increasing the screw speed often reduces the bulk density and hardness of the extrudate. This happens because higher speeds increase shear, which can enhance dough elasticity and promote expansion. Operators must balance speed and temperature to achieve the desired density.

Perfect the Spheronization Stage

Spheronization is the final shaping step. The cylindrical extrudates tumble on a high-speed rotating plate, causing them to break and gradually round into spheres. The speed of this plate and the processing time are the most important variables.

Operators often find a two-stage speed approach most effective. They may start at a high speed (e.g., 3000 RPM) to cut the extrudates and then reduce it (e.g., 1000 RPM) to gently form the spheres. A longer spheronization time generally improves roundness and surface smoothness. However, excessive time or speed can create fines or irregular particles. Finding the perfect combination of speed and time is key to producing perfectly spherical granules.

Key Material Properties Influencing Sphericity

Process parameters are only one side of the equation. The inherent properties of the starting materials play an equally critical role in determining the final shape and flowability of the granules. Operators must carefully consider the raw powder and the selected binder to achieve consistent, spherical results.

Starting Material Particle Size

The particle size of the raw powder is a foundational factor in granule formation. Finer starting powders possess a larger total surface area. This characteristic allows the binder liquid to wet the material more uniformly during granulation. The result is more predictable nucleation and even growth, which are essential for forming spherical particles.

Coarser or wider particle size distributions can be challenging. They often lead to non-uniform wetting and binder distribution, resulting in irregular granule shapes and a higher proportion of undesirable fines or oversized lumps.

Binder Selection and Concentration

Binders are essential excipients that provide the cohesiveness needed to form granules from powder. The choice of binder and its concentration directly influence granule sphericity and strength.

Common pharmaceutical binders include polymers like N-vinyl pyrrolidone (PVP) and semisynthetic cellulose products such as hypromellose (HPMC). Formulations containing PVP often demonstrate superior flow characteristics. This advantage comes from PVP's excellent wettability, which helps the liquid distribute evenly and form well-rounded granules.

The concentration of the binder solution is just as important.

Higher Concentration: Increasing binder concentration (e.g., from 5% to 15% w/w) produces granules with greater mechanical strength and better flow. The binder holds particles together more strongly, reducing the amount of fine powder.
Lower Concentration: A lower binder concentration can result in larger, more irregular, or even needle-shaped granules.

Studies show that a binder concentration of 20% (w/w) or higher can produce smaller, more spherical granules. Operators must find the optimal concentration to balance sphericity with other critical quality attributes.

Achieving optimal results requires precise management of critical parameters. Operators must control impeller speed in a High Shear Granulator, spray rate in a fluid bed, or spheronizer speed in extrusion. Consistently producing spherical granules is a direct result of mastering the chosen granulation method. The transformation can be dramatic, improving key physical properties.

For example, crystalline Ibuprofen powder with "Very Poor" flowability becomes a spherical product with "Very Good" flowability after processing.

A final tip for success: operators should characterize their material first. They can then select the method that offers the best control for their specific application.

FAQ

Which method is best for achieving spherical granules?

The best method depends on the material and desired outcome. High-shear granulation creates dense spheres. Fluidized bed granulation offers high uniformity. Extrusion-spheronization provides excellent control over size. Operators select the method that best suits their specific application and material properties for optimal results.

Why is a High Shear Granulator effective for sphericity?

A High Shear Granulator uses a powerful impeller and chopper. The impeller densifies the powder mass. The high-speed chopper cuts large lumps into uniform pieces. This combined mechanical action efficiently rounds the particles, creating dense, spherical granules with excellent flow properties in a short time.

How does binder concentration affect sphericity?

Higher binder concentrations generally produce stronger, more spherical granules. The increased cohesiveness reduces fines and helps particles hold a rounded shape. Operators must find the optimal concentration. Too much binder can create overly hard granules, while too little results in weak, irregular particles.

What causes irregular granule shapes?

Irregular shapes often result from poor process control. Common causes include non-uniform binder distribution, incorrect impeller or chopper speeds, or using a raw material with a wide particle size distribution. Consistent processing and proper material selection are key to avoiding this common manufacturing issue.