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solution and create the spray pattern. In most coating operations, the spray air contributes less than 2 percent of the drying capacity. Therefore, for quick calculations, the heat input can be expressed as shown in Equation 3: Heat/ Process air Specific drying input flow rate heat capacity (mass) (SHC) where T In is the inlet-air temperature and T Bed is the product-bed temperature. Since the ER must equal the heat/drying input, the preceding two equations are set equal to each other (Equation 4): The EC can be further defined for a given set of process conditions as (Equation 5): where 1 denotes a known set of process conditions. For a given coating formulation, the evaporative constant (EC) remains the same. The mass flow rate of air is directly related to the volumetric air- flow for a given temperature, pressure, and humidity. For all practical pur- poses, the specific heat capacity of air remains the same for a given humidity 42 January 2017 Tablets & Capsules Before conducting the next trial, two adjustments were made. First, a sealing baffle was added to the exhaust duct to ensure the process drying air flowed through the tablet bed. Second, the pan's rotational speed was increased to improve product movement. The ensuing tri- als were conducted at the processing parameters listed in Table 2. To attain lower bed temperatures, the airflow and/or inlet process air tem- peratures were reduced. The approach to determining the values used for a trial was based on the following reasoning: The size of the largest portion of energy required in a coating process depends on the amount and rate of moisture removal. While increasing the temperature of the solution (solvent and solids) to equal the temperature within the coat- ing pan is part of the energy require- ment of the coating process, it is on the order of 1/500th the magnitude of the energy needed to evaporate the solution's solvent. Therefore, the evaporation requirement (ER) can be expressed as a function of the solvent mass flow rate and heat of vaporiza- tion (Equation 1). Evaporation Solution % Solvent Latent heat of requirement spray rate vaporization (ER) (mass) For a given coating formulation, that equation can be further simpli- fied (Equation 2): Evaporation Solution Evaporative requirement spray rate constant (ER) (mass) (EC) The ER is met by the heat/drying input. Heat/drying energy is related to airflow, specific heat of the air, and the difference between the inlet process-air temperature and the product-bed tem- perature. Another factor that con- tributes to the drying of spray droplets is the amount of air used to atomize the Table 1 Initial coating parameters Parameter Value Inlet-air temperature (Cº) 65 Inlet air flow (m 3 /h) 68 Spray rate (g/min) 10-11 Atomization air (bar) 1.2 Pattern air (bar) 1.2 Gun-to-bed distance (cm) 10 Pan speed (rpm) 15 Product bed temperature (Cº) 40-45 Table 2 Coating parameters of benchmark and of tests conducted at progressively lower product-bed temperatures Figure 1 Tablets produced from the first benchmarking attempt show picking flaws. Baffle Parameter Benchmark Trial T2-3 Trial T2-4 Trial T2-5 Trial T2-6 Inlet-air temperature (Cº) 57-60 60-62 42-47 37-42 30-35 Inlet air flow (m 3 /h) 100 68 68 68 100 Spray rate (g/min) 10.0 10.4 10.4 10.5 8.2 Atomization air (bar) 1.2 1.2 1.2 1.2 1.2 Pattern air (bar) 1.2 1.2 1.2 1.2 1.2 Gun-to-bed distance (cm) 10 10 10 10 10 Pan speed (rpm) 18 18 18 18 18 Product bed temperature (Cº) 40-43 37-38 24-28 20-25 19-22 × EC = × (T In - T Bed ) × SHC Air Process air flow rate (mass) Solution spray rate (mass) Process air flow rate (mass) Evaporative constant (EC) Solution spray rate (mass) 1 1 × (T In - T Bed ) × SHC Air = = = = × × × × × (4) (3) (1) (2) (5) % Solvent (T In - T Bed ) b. Sealing baffle added Exhaust duct a. Inadequate sealing/ location relative to product bed