Continuous Improvement in Tablet Coating and Dry Granulation - Pharmaceutical Technology

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Continuous Improvement in Tablet Coating and Dry Granulation
Recent advances in equipment design and operation in spraying, drying, and mixing can improve the tablet-coating process.


Pharmaceutical Technology
pp. s12-s16

The role of coater design in manufacturing

Once pharmaceutical manufacturers understand the basic elements of tablet coating, they can determine which tablet- coater features best suit their solid-dosage product. For example, relying on a coater whose coating pan length exceeds its diameter creates a shallow tablet bed that promotes tablet exposure to the spray. It also allows the installation of more nozzles over the bed than is possible in conventional pans. This combination exponentially improves coating uniformity. There is a limit to the number of nozzles that should be added, however. Manufacturers must weigh the benefit of adding nozzles against the risk of overwetting the tablet bed. It is best to use enough nozzles to cover the bed while avoiding or minimizing overlap.

Airflow must also be evaluated. Conventional coaters have top-to-side airflow, so high-velocity warm air passes through the spray zone. That implementation of the air flow causes turbulence and the possibility of spray-drying the coating droplets. Pharmaceutical manufacturers can reduce those effects by adjusting the airflow, temperature, and spray rate. That correction, however, may slow the coating operation or increase the amount of wasted coating solution.

Minimum air turbulence and improved coating applications can be achieved by using a pan with an inlet and outlet that span the pan's base. This design improvement also allows an improved control over tablet-bed temperature. Additionally, that control yields faster spray rates and reduces coating times by as much as 45%.

Companies using both coating pan designs have found that pans with top-to-side airflow are up to 85% efficient. This result means approximately 15% or more of the spray is deposited within the coating pan on the guns and spray boom or in the exhaust. Designs using base-to-base airflow, which pull the spray onto the tablet bed like a vacuum, can reach approximately 98% efficiency. They also shorten overall coating time because they enable higher spray rates without overwetting the tablet bed.

In applications requiring thick tablet coatings, the tablet bed can grow by one-third or more. This formulation requires an adjustment of the gun-to-bed distance during the process. When tablet coating involves potent substances, manufacturers can consider automating the movement of the nozzles, spray boom, or both. This feature eliminates the need to open the coating pan, thereby increasing efficiency and safety.

Finally, manufacturers should note how their coating pan mixes and handles the tablets. Homogenous mixing promotes content uniformity. Pans equipped with helical mixing baffles constantly move the tablets back and forth across the pan, thereby eliminating dead spots.


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