Peer Reviewed: Numerical Simulation of Tablet Coating

The authors investigate the tablet-coating process using a combination of different simulation techniques.
Sep 01, 2012

Figure 1: In the coating process, each tablet receives numerous partial coatings, which leads to a homogeneous coating layer. The scanning electron microscopy (SEM) picture shows a cross-section of the coating layer on the tablet core. (ALL FIGURES COURTESY OF THE AUTHORS)
Drum coaters are widely used in the pharmaceutical industry to produce film-coated tablets. The coating layer(s) around the tablet core can serve many purposes, such as tastemasking, coloring, control of the release of the API from the core of a tablet, application of an additional API, or protection of the tablet core from environmental influences. In this process, an atomizing nozzle above the tablets sprays the coating solution onto the tablets, which are mixed by the rotating drum. Heated air is forced through the system to enhance drying of the coating. In this way, each tablet receives a series of partial coatings (see Figure 1). A crucial issue is coating uniformity, which includes both inter-tablet uniformity (i.e., variation of coating mass from one tablet to the other) and intra-tablet uniformity (i.e., variation of the coating thickness and quality on the surface of a single tablet) (1–3). Inhomogeneity in coating thickness, for example, could lead to significant variations in API content or delivery rate. In many cases, a single tablet that fails testing can lead to rejection of the whole batch.

Parameters that influence drum-coating performance can be divided into two groups. The first group includes parameters that can be adjusted by the operator (e.g., drum-rotation speed, fill level, spraying rate, or drying-air temperature). The second group are parameters that cannot be adjusted directly, but have a direct influence on the quality of the product (e.g., extension of the spray zone, mixing efficiency, air flow pattern, and quality of the coating spray) (4).

Although drum coating is commonly used in the pharmaceutical industry, there have been relatively few scientific investigations reported in the literature. Moreover, process design has been based mostly on trial-and-error and operator experience. Recently, in addition to experimental work, advances in computational simulations have become an important tool for such investigations (5–8).

Figure 2: A schematic representation shows different process regimes of the coating process with an appropriate simulation approach used for each. For (a) and (c), colors denote velocity from slow (blue) to fast (red). In (b), the red color represents coating thickness; the tablet on the right in the lower box has a thicker coating.
The major objective of this work is to show how a combination of different numerical techniques can help to provide a comprehensive, in-silico (i.e., via computer simulation) analysis of the whole tablet-coating process. The following aspects of the film-coating process aspects were identified (see Figure 2):
  • Simulation of the tablet bed mixing using the discrete element method (DEM)
  • Detailed numerical analysis of the interaction between spray droplets and the tablet surface by means of advanced multiphase computational fluid dynamics (CFD) methods
  • Computational analysis of the coater internal flow in terms of drying air, spray, and wall effects using CFD.

The work aims to provide a deeper understanding of:

  • The tablet-mixing process in an industrial pan coater, which allows optimization of the tablet residence time in the spray zone
  • The local behavior of the spray droplets impacting on a single tablet, which leads to a wider understanding of film formation mechanisms
  • The interaction between the spray and coater drying air, thus allowing a detailed analysis of the deposition efficiency of coating material on the tablet bed and insight into operative problems like overspray, filter plugging, or incomplete tablet-coat curing.