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This model, by using a few reasonable approximations, reduces the published equation for EE to a greatly simplified expression. While the equation for EE is arrived at via a coupled heat and mass transfer analysis, the aim of this work is to show that EE is equivalently expressed by relating the internal thermodynamic conditions to the saturation conditions (i.e., where the exhaust is fully saturated or 100% relative humidity (RH)) along a constant enthalpy line on a psychrometric chart. The EE factor is a measure of the predicted drying rate occurring in the wetted tablet bed, and if EE is kept constant, it is proposed that equivalent film coating quality can be obtained upon scale-up or changes in operating parameters. The EE model has been frequently cited in recent papers and texts on aqueous tablet coating ( 1– 11). It has been reported ( 5) that the Environmental Equivalency (EE) model proposed by Ebey ( 6) is the most widely used aqueous coating model and has also been incorporated into the TAAC computer program published by Thomas Engineering ( 7).
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A number of first-principles thermodynamic coating models have been reported in the literature as being useful for understanding and predicting the performance and scale-up of aqueous tablet coating and the key parameters which impact the quality of the film coating ( 1– 5). The coating process involves a large number of parameters convoluted with each other, and without a theoretical framework in which to understand these parameters, a purely empirical approach would tend to be cumbersome, laborious, and ultimately unreliable. Since the coating process involves a large number of parameters not independent of each other, an empirical Design of Experiments (DoE) approach alone is insufficient to achieve the process understanding needed to accomplish robust process design and scale-up. Some potential problems encountered in tablet coating as a result of poor control of the thermodynamic conditions are sticking defects from operating at too wet of a tablet bed, while conditions that are too dry can result in spray-drying and a porous, “orange-peel” film coating. This is especially true in the case of functional coatings that require a contiguous, homogeneous film in order to impart the functional properties of the coating. The ability to predict the performance of a tablet coater, specifically in regards to the parameters that impact the film coating quality, is essential for efficient and effective scale-up where processing conditions may vary greatly. For environmental and economic reasons, aqueous coating has become preferred over solvent-based coating. Tablet film coatings are used for aesthetic reasons (color), taste masking, controlling drug release, or for applying an active pharmaceutical ingredient to a core tablet. The psychrometric model herein is presented as a more physically evocative description of the coating process, enhancing process understanding and potentially playing a key role in a Quality by Design approach to defining an aqueous coating design space.Īqueous tablet film coating has become a ubiquitous part of pharmaceutical manufacturing since Abbott marketed the first film-coated tablet in 1953. The EE factor is expressed by an equation involving ten individual parameters however, if the derivation of EE is extended further under the context of an adiabatic process, a much-simplified yet equivalent expression for EE emerges consisting of only three parameters, each directly measurable or obtainable from a psychrometric chart and which bear direct significance to the gross thermodynamic conditions of the coating. A commonly used first-principles model built upon the coupled heat and mass transfer in evaporative mass transfer derives an “Environmental Equivalency” (EE) factor as an indicator of the relative rate of water evaporation from the tablet bed surface and as a relevant scaling factor for aqueous coating. Several mass or heat conservation models for aqueous tablet coating can be found in the literature, varying in approach and proposed method for controlling the coating process. Since many of these variables are interdependent, the choice of parameters by which to control the process or designate a design space is not necessarily obvious.
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Process control of aqueous tablet coating depends on a number of thermodynamic and psychrometric variables.