Issue link: https://www.e-digitaleditions.com/i/1101841
14 April 2019 Tablets & Capsules For containment applications, the contamination risk decreases over the course of the coating process. A risk analysis should determine whether tablet dis- charge through containment flaps is required or whether no special discharge measures are necessary. In any case, process designers must take suitable mea- sures to ensure that no product dust can emerge from the coater prior to, during, or after the entire coating and cleaning process. The cleaning process Coating systems require cleaning after a product change or a specific number of batches of a single product (a "campaign"). A GMP-compliant coater design is always a prerequisite for a successful cleaning process. Modern coaters generally feature cleaning nozzles that wash down all product-contact surfaces, from the air supply flap upstream from the coater to the exhaust air flap downstream from the coater. Operators must also rinse all hose connections between the coat- ing solution tank, pump, and spray nozzles as well as the nozzles themselves. Three-dimensional cleaning nozzles, as shown in Figure 7, have proven successful for cleaning coaters and reducing cleaning time, as they achieve a great mechani- cal effect if the pressure of the cleaning agent is suffi- ciently high. The cleaning process should require as few manual activities as possible, so all cleaning nozzles should be permanently installed or very simple to install without having to access the contaminated areas. For particularly valuable or highly potent products, the coater may also monitor the liquid pressure in each coat- ing solution line and trigger an alarm, automatic nozzle cleaning, or shutdown if it detects a malfunction such as clogging. Visual control systems with imaging technolo- gies are not currently available but are being developed. Drying. A modern coating process, as shown in Figure 5, uses optimized, high-volume spray rates that require suitable volumes of treated drying air. The air-line pro- files and inlet openings on these systems are designed to keep the velocity of the drying air entering the coating drum low to minimize turbulence and pressure loss across the drum perforation and exhaust air system. Turbulent airflow in the coating drum causes some of the spray droplets to dry before contacting the tablet bed and be carried away in the exhaust airstream. This wastes coating solution and results in longer process times. It also affects tablet quality and limits the number of batches of the same product you can run without cleaning the coater. The high drying capacity of modern coating systems makes it possible to coat even highly hygroscopic prod- ucts, such as phytopharmaceuticals or plant extracts, with aqueous solutions. These systems can generate a lot of waste heat, how- ever, so double-walled, insulated coaters that feature air-handling units equipped with heat recovery systems are in high demand. Such systems are not only more energy efficient, they are also safer because they maintain a tem- perature of less than 60°C on all metal surfaces. Moreover, because the systems mix cold and hot air in the air-supply system, they permit quick and precise temperature control. Some products may require more complex machine lines with added components, such as a molecular sieve for air drying; a humidifying system or supply-air humidity control; or, for organic solvents, even a closed drying sys- tem that circulates inert nitrogen gas. Explosion-protected machine lines such as these can be very complex, particu- larly if multiple control circuits overlap and safety-related factors, such as a product with a very low minimum igni- tion energy, significantly affect the procedure. Tablet discharge After coating, tablet discharge should be as quick as possible but gentle enough to prevent damage to the coated tablets. In recent years, helix-shaped welded dis- charge elements have replaced the special discharge inserts (called "scoops") that were previously temporarily installed in the coater opening for the discharge process. These modified permanent elements serve as mixing ele- ments when the drum rotates forward during coating but help to remove all tablets from the coating drum without any residue when the drum rotates backward. For large production coaters, as shown in Figure 6, the number of discharge elements and their volume are selected so that a production batch of several hundred kilograms of tablets can be discharged completely in approximately 10 minutes without operator interference and while remaining gentle to the product. Figure 6 Production coater during discharge process