Issue link: https://www.e-digitaleditions.com/i/1233462
b a c k p a g e Challenges in design and construction of drum tablet coaters Modern tablet coaters must be able to handle tablets and other dosage forms with a wide range of formulations, shapes, and sizes and using a variety of coating solutions. Solvent- based coating solutions require efficient explosion protection, while highly potent active ingredi- ents require appropriate containment measures. Coaters must also manage widely varying batch sizes, and clean- ing must be automated and effective. To maximize efficiency, each step of the coating process should be opti- mized, and automation is key. Feeding a coater can include manually scooping tablets into a hopper in the coater or connecting a container directly to the coater's feed port. Feeding must be fast but gentle, because mechanical forces can potentially damage the tablets. For tablets with highly potent active ingredients, the feeding system typ- ically must include containment valves and closed lines between the feeding container and the coater to ensure containment. Effective mixing of the tablets is critical for product quality. All tab- lets must be sprayed with the same amount of coating solution and at the same frequency to achieve an even surface quality throughout the batch and ensure the coating layer's func- tionality. Optimized, reduced-height mixing elements ensure that the ele- ments are not exposed to the spray flow, which prevents coating solution from drying on the elements and then breaking off into the tablet bed, lead- ing to defective tablets. To apply coating solution or sus- pension as fast as possible, drum coat- ers with an extended drum, a flatter tablet bed, and more spray nozzles have become popular. Air-atomiz- ing spray nozzles that create a highly homogeneous spray pattern at high pressure levels have become com- mon. Air lines for atomization, con- trol, and pattern air are executed as bore holes in the spray boom to avoid unnecessary piping in the coat- ing drum. As the angle of the tablet bed changes during operation, the spray boom automatically adjusts to maintain a consistent angle and dis- tance between the tablet bed and the spray nozzles and prevent droplets from drying prematurely in the air or excessively moistening the tablets. Today's high-spray-rate coating processes require sufficient volumes of treated drying air. The airflow must be optimized to pass through the coating drum with minimal tur- bulence or disturbance to the coat- ing spray pattern to minimize spray loss and ensure product quality. This means optimizing the inlet air distri- bution to keep the air velocity low as the air enters the coating drum, while minimizing pressure loss from the drum perforation and exhaust air system. Minimizing spray loss also typically allows for longer campaigns between cleaning cycles and maxi- mizes coating efficiency. The drying air should gener- ate an even temperature distribution throughout the tablet bed to achieve a uniform coating with the highest spray rates. For certain products, more complex air handling systems may be required to handle solvents or other explosion concerns, as well as for containment applications. Just as with feeding, coater dis- charge should be fast but gentle on the tablets. In recent years, helix- shaped welded discharge elements, which serve as mixing elements during forward movement, have replaced special discharge inserts. When the drum rotates backwards, these discharge elements guide the coated tablets from the drum to a dis- charge port. For containment applica- tions, containment valves and other measures may be required for dis- charge and can be installed directly on the coater. Coating systems require clean- ing after a product change or a specific number of batches. In gen- eral, all surfaces in contact with the product are automatically cleaned using cleaning nozzles, and all pos- sible hose connections between the tank, pump, and nozzles, as well as the nozzles themselves, are rinsed. A GMP-compliant coating system design is necessary for a success- ful cleaning process. Rotating jet cleaners have proven to be success- ful for cleaning coating drums if the cleaning agent pressure is high. Coating drums can also be cleaned using ultrasonic cleaning, in which the drum is placed in a cleaning solution and an ultrasonic gener- ator creates high-frequency pres- sure waves that help to dislodge material from the drum surfaces. As with the coating process itself, the cleaning process should be as auto- mated as possible. T&C Dominik Sander is head of life science applica- t i o n s a t L o e d i g e , a global supplier of coat- ing syste ms, mixers, granulators, dr yers, a n d r e ac t o r s t o t h e phar m ace utical and other industries (www. loedige.de). Pär Almhem is preside nt of Mod- Wave, exclusive distribution partner for Loedige in the US and Puerto Rico (310 918 6772, www.modwave.com).