#wurstercoating

The Wurster coating technique is indispensable in multiparticulate formulation development, particularly for controlled-release pellets, taste-masked systems, and targeted delivery technologies. Yet, scaling this process is never a matter of simply multiplying settings. The real challenge lies in maintaining the intricate balance among spray formation, moisture evaporation, and particle circulation. Disturb any one of these elements, and the system becomes prone to agglomeration, premature drying, erratic film growth, or inefficient coating yield. This discussion outlines how to preserve these essential dynamics during scale-up.

Spraying Principles

The spray subsystem defines the conditions in which droplets are generated and delivered to particle surfaces. Misalignment of spray parameters can disrupt the process in several ways.

Over-wetting typically stems from droplets that are too large for the available drying capacity. This often arises from high-viscosity coating dispersions, concentrated solid content, insufficient atomisation air, or spray rates that exceed the system’s ability to evaporate solvent in a timely manner.

Spray drying occurs under opposite conditions when droplets evaporate mid-flight. High atomisation pressures, low-viscosity formulations, or excessive nozzle-to-bed distances produce fine droplets with large surface-area-to-volume ratios. These droplets remain suspended in warm air for extended periods, accelerating evaporative losses and resulting in powdery deposits rather than cohesive films.

Uniformity of coating is shaped by the geometry of the spray pattern. Narrow angles deliver deeper penetration into the Wurster column but risk concentrating wetting within a narrow region. Conversely, wide spray angles provide broader coverage but depend heavily on proper nozzle height to prevent losses from overspray or inadequate surface wetting.

Spraying only establishes the starting conditions for coating. True success depends on the alignment of droplet formation with simultaneous drying and circulating forces.

Drying Dynamics

Drying is the mechanism that converts a wet deposit into a continuous film. However, achieving ideal film formation requires the right balance between temperature, humidity, and airflow.

Over-wetting develops when drying is sluggish. Low inlet air temperature, high humidity (high dew point), or insufficient airflow all slow down solvent removal. Sticky surfaces persist longer, increasing the risk of particle bridging and mass agglomeration.

Premature drying occurs when droplets lose solvent earlier than intended. High inlet or product temperatures, overly high air volume, or extremely dry air conditions (low dew point) harden droplets in mid-air, preventing them from spreading on the particle surface. This produces poor adhesion and weak, fragmented films.

Inconsistent coating emerges from poor temperature control relative to the coating polymer’s requirements. Aqueous coatings are especially sensitive to falling below the polymer’s minimum film-forming temperature, which can create brittle layers. Extremely dry air also elevates electrostatic charge, particularly with non-polar coating systems, increasing bed-wide agglomeration.

Drying control is therefore about orchestrating solvent removal at the precise moment droplets land, ensuring coalescence rather than premature solidification.

Fluidisation and Circulation Control

Particle motion underpins the entire Wurster process. Stable fluidisation ensures repeated movement through the spray and drying zones, creating the cyclical exposure required for uniform coating.

Agglomeration often results from excessive fill levels. A dense bed limits mobility and increases the frequency of collisions between wet particles. If inlet air velocity is insufficient—often due to constricted annular gaps—fluidisation weakens, further increasing the probability of agglomerate formation.

Premature drying related to circulation can occur under high airflow conditions. Excessive velocity accelerates droplet evaporation, leading to dried particles before they reach the pellet surface.

Irregular coating distribution is the most common circulation-related issue. A broad particle size or density distribution generates uneven residence and cycle times. Larger particles may remain longer in the spray region while visiting it less frequently, accumulating more coating per exposure and shielding smaller particles. Underfilled beds introduce unnecessary turbulence, and incorrectly set partition heights disrupt the pressure gradient needed for consistent particle draw-in. Tall partitions reduce draw-in efficiency, while short ones restrict exposure time.

Monitoring the pressure drop across the distributor plate provides a practical diagnostic tool. A sudden drop indicates partial collapse of the fluidised bed, whereas slow declines typically indicate entrainment losses. Stable circulation demands proper airflow scaling, consistent particle size distribution, and careful control of draw-in forces.

Variables to Standardise Early in Development

Certain parameters should be set firmly during development, as modifying them during scale-up complicates the process significantly.

Distribution-plate selection must align with column size and particle characteristics to maintain fluidisation consistency at all scales.

Atomisation pressure, despite interacting with spray rate and nozzle design, should be optimised during early trials. This pressure dictates droplet size, spray penetration, and deposition behaviour.

Environmental factors such as dew point, inlet air temperature, and product temperature should remain constant. They directly influence drying rate, polymer film formation, and coating robustness. Significant changes during scale-up should be avoided unless supported by process data.

These elements function as structural anchors for the coating process. Once established, they should remain unchanged while scaling focuses on variables responsive to equipment capacity.

Strategies for Scaling Up

Scaling requires equipment similar in geometry across development, pilot, and commercial units. When this structural consistency exists, mathematical scaling becomes both logical and effective.

Volumetric airflow should increase in direct proportion to the column’s cross-sectional area to maintain linear fluidisation velocity. Spray rate must scale equally with column area to sustain the intended exposure-to-drying ratio. Partition height should adhere to recommended ranges for each column size and will naturally increase as particle diameter grows during coating.

Variables that should be protected across all scales include:

Spray rate, scaled with airflow and column area

Atomisation pressure, adjusted only to maintain equivalent droplet properties

Air volume, scaled to preserve circulation and drying capacity

Inlet air temperature, held constant to stabilise product temperature

Dew point, maintained to control solvent evaporation and static charge

Fill level, matched as a percentage of equipment capacity

Partition height, maintained to preserve the required pressure gradient

Even with correct mathematical relationships, increased batch mass alters total heat load and moisture retention. Early pilot-scale batches typically require iterative adjustments to restore equilibrium.

Particle-Level Insights from Research

While increasing atomisation pressure is commonly used to maintain droplet size at higher spray rates, this approach carries risks. Higher droplet momentum increases the likelihood of pellet attrition and the generation of fines—particularly in regions near the nozzle where larger particles tend to linger.

Computational and tracer-based studies consistently show that residence-time distribution determines coating uniformity. Larger pellets spend longer within the spray zone during each pass but cycle less frequently, accumulating more coating and shading smaller particles. Recirculation events where particles re-enter the spray zone opposite airflow expand this variability further.

Simulations validated by positron emission particle tracking highlight that inlet airflow rate and column height strongly influence particle movement patterns. Errors in scaling these two variables magnify coating variability dramatically at larger scales.

Additional research reveals that solvent evaporation is extremely sensitive to inlet air temperature, while airflow strongly governs coating yield and material losses. CFD–DEM models demonstrate that coordinated optimisation of airflow, spray rate, and drying temperature can significantly reduce spray-drying losses and shorten processing time.

Conclusion