A Thermodynamic Model for Organic and Aqueous Tablet Film Coating

Abstract

A tablet film-coating model for aqueous- and/or organic-based systems is shown to predict exhaust stream conditions thereby facilitating process optimization and scale-up. This coating model uses the First Law of Thermodynamics and conservation of mass principles to complete a material-energy balance on the coating unit operation for a closed, non-isolated system. Heat loss from the coating pan is incorporated into the model through a parameter called a heat loss factor (HLF) that is directly related to the heat transfer coefficient and pan surface area. For a mixed organic-aqueous coating formulation, the outlet air temperature and humidity are most notably affected by the coating composition and the inlet drying air temperature, which controls the evaporative cooling rate. The coating solution temperature and inlet air relative humidity do not significantly influence the exhaust air temperature, Tair,out. The HLF was determined to be 24 to 62 cal/min degrees C for the LDCS-20 to HCT-30, 360 cal/min degrees C for the HCT-60, 0 cal/min degrees C for the HC-130L and 945 to 1322 cal/min degrees C for the Accela-Cota-48 to Compulab-36 coating pans. This model successfully predicts Tair,out within 3 degrees C for a given coating pan, and within 6 degrees C scaling up from one to 220 kg pans for both organic- and aqueous-based coatings. The model is also useful for probing process and formulation variable sensitivity critical to establishing process robustness.

Scale Up of Pan Coating Process Using Quality by Design Principles

“…The film coating process can essentially be considered an adiabatic evaporative cooling process. The underlying law, which controls the thermodynamic environment inside the coating pan, is the first law of thermodynamics (conservation of energy) . The inlet airflow volume, the inlet temperature, and the moisture content of the air are the thermodynamic factors that govern the evaporation rate of the coating solution from the tablets surface.…”

“…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 process. These models are used to identify the critical coating process parameters, which impact the CQAs of the coated film . Psychrometric charts are a good way of dealing with calculations pertaining to changes in air conditions ( T and relative humidity) and are presented either as Mollier or Carrier diagrams, both of which have the same principle format (axis directions are different).…”

Influence of Process Parameters on Tablet Bed Microenvironmental Factors During Pan Coating

“…Additionally, the heat losses during the coating process are generally not accounted for, and therefore, the models tend to deviate from experimental results when used across different scales, where heat losses may be quite different. While the more recently revised models offer better predictions (16), only macroenvironmental changes are captured and the tablet bed microenvironment is not addressed.…”

Psychrometric Analysis of the Environmental Equivalency Factor for Aqueous Tablet Coating

“…Subscripts: 1, point 1; 2, point 2; ∞, free stream; a, dry air; a+w, moist air; amb, ambient; B, tablet bed; ex, exhaust conditions; film, film conditions; h, heat transfer; i, species i; in, inlet conditions; m, mass transfer; na, nozzle air; pa, process air; w, water; wb, wet bulb conditions. which impact the quality of the film coating (1)(2)(3)(4)(5). 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).…”

“…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). The EE model has been frequently cited in recent papers and texts on aqueous tablet coating (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11).…”