Wet Granulator Impeller Speed: Influence on Granule Particle Size Distribution

Wet granulation is a critical process in pharmaceutical manufacturing, used to improve the flowability, compressibility, and uniformity of powders. One of the key equipment in this process is the wet granulator, where the impeller speed plays a pivotal role in determining the characteristics of the granules produced.

This article explores the relationship between the impeller rotational speed in wet granulators and the resulting granule particle size distribution, highlighting the impact on overall product quality and downstream processing.

The Role of Impeller Speed in Wet Granulation

The impeller in a wet granulator provides the mechanical energy required to agglomerate powders and binder into granules. Adjusting the impeller speed influences:

• Shear force applied to the wet mass

• Mixing intensity

• Nucleation and growth of granules

• Breakage and attrition rates

Proper control of impeller speed is essential to balance these factors and achieve optimal granule size and distribution.

Impact of Different Impeller Speeds on Granule Size

• Low impeller speed:
  At lower rotational speeds, the shear force is relatively mild. This results in larger granules due to gentle agglomeration, but it may lead to broad particle size distribution and possible formation of oversized granules or lumps.

• Moderate impeller speed:
  Moderate speeds increase the shear and mixing efficiency, promoting more uniform granule growth. This typically results in narrower particle size distribution and granules of desirable size suitable for tableting or capsule filling.

• High impeller speed:
  Excessively high speeds generate strong shear forces which can cause granule breakage and attrition. This leads to smaller particle sizes and a wider size distribution, potentially affecting flowability and compressibility negatively.

Case Study: Optimizing Impeller Speed for Controlled Granule Size

In a recent production trial, a pharmaceutical wet granulator was operated at three impeller speeds: 30 rpm, 60 rpm, and 90 rpm. The granule size distribution was analyzed using sieve analysis:

• At 30 rpm, larger granules were formed, but the broad span indicated uneven size distribution.

• At 60 rpm, granules were smaller but more uniform, representing an optimal balance.

• At 90 rpm, significant attrition caused smaller granules and increased distribution span, which may impair downstream processing.

Practical Recommendations for Industry

1. Start with moderate impeller speeds to balance granule growth and breakage.

2. Monitor granule size distribution regularly using appropriate particle sizing techniques.

3. Adjust binder addition rate in coordination with impeller speed for consistent wet mass characteristics.

4. Consider scale-up effects — impeller speed influence can vary with granulator size and geometry.

5. Perform process validation to determine optimal parameters for each formulation.

Conclusion

Impeller speed in wet granulation critically influences granule particle size and distribution, which in turn affects the final product quality and manufacturability. Understanding and optimizing this parameter helps pharmaceutical manufacturers ensure consistent granule characteristics, improving downstream processes such as drying, milling, and tablet compression.

Effective control of impeller speed, combined with other process parameters, is essential for achieving a robust and reproducible wet granulation process.