Analysis and optimisation of tablet coating processes with EDEM

Stephen Cole
Stephen Cole
Altair Employee
edited April 15 in Altair HyperWorks

For the pharmaceutical, food processing and agriculture industries, spray coating of tablets and other materials is a commonly used method to introduce a coating to a particulate material.  Coating is typically a batch process where the material of interest is placed in a rotating drum.  A spray is then included, as the material passes through the spray zone it receives a quantity of the spray material on the surface.

This coating could be a protective layer, a different colour to differentiate different material types or an active coating containing flavour or pharmaceutical ingredient. It is important to maintain a uniform coating for all the material in the batch process.  Simulation analysis using EDEM can be used optimise nozzle position and geometry to obtain uniform coating.

This post outlines 3 methods to model tablet coating using Altair® EDEM™, simulation files are available for each method and can be downloaded here:

There are many parameters which influence coating uniformity, drum fill levels, rotation speed, spray rate and spray area all are key components which can be investigated with EDEM.  If optimising EDEM simulation results with Hyperstudy then this blog outlines the process:

To perform Coating analysis the first step is to setup a calibrated EDEM simulation.  To get started with EDEM please see the following:

For further examples of EDEM industrial use in process manufacturing you can see example links which include webinars from Novo Nordisk, Firmenich, Inc., Merck Healthcare KGaA, Edinburgh University and Leeds University:

There are different methodologies for tablet coating in EDEM, these include:

  1. Ray Tracing Method (Intra-particle and Inter particle spray coating) 
  2. Spray Coating Model (Inter-particle spray coating)
  3. Residence time analysis

Each method provides valuable coating analysis, they can all be included in 1 simulation or just 1 method chosen.

The Ray Tracing method has the advantage of running in post-processing only, the simulation does not have to be re-run to investigate different spray nozzle locations.  The Spray Coating Model (inter) method considers the trajectory of the spray particles, including effects of gravity, and as it is calculated during run time this method calculates the spray particle trajectory per simulation time-step. The Spray Coating Model method can also be modified further through the API  if additional affects such as cohesion changes due to coating are to be considered. The residence time method is also calculated during run time and can cover different shaped areas of influence.  This residence time method can also be used for other area based analysis, such as time spent in a high temperature zone or investigating material residence time in specific areas for in a continuous process.

 

  1. Ray Tracing Method: Intra-particle and Inter-particle spray coating

 

  • The files required to perform Intra particle spray coating can be found here - Ray Tracing Method - Simulation Files 
  • Instructions on how to run the spray analysis can be seen in this video:


Intra particle coating refers to the distribution of coating on one tablet.  Enabling the tablet analysis to be done on different faces and edges to determine if any area of the tablet is not covered evenly. 

Ray tracing is a computational method used to represent rays of light, the rays in this instance interact with the surface of a tablet to distribute coating.

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This ray tracing method of coating can also be used for Inter-particle analysis, where a batch of material can be analysed to determine the temporal and special distribution of the coating.

This method is run during post-processing of the simulation only, meaning the ray position, density and properties can be varied post-simulation to investigate the influence of different coating setups.  As this is a post-processing method the analysis can only be run on the saved data points, the user must ensure an appropriate rate of saving is set in order to obtain accurate results.  Typical save rate vary between 10 Hz and 100 Hz.  Slow moving or semi-static cases can have low frequency save rates as the velocities and kinematics in the system would be low, high velocity dynamic systems require higher save frequencies to capture the material motion for Ray Tracing analysis.

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A starting point is determined along with an angle and density coefficient.  The rays are then generated and any particles they hit gain an associated amount of coating, depending on the density coefficient.

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The resultant analysis includes the mass coating distribution across a tablet.  As this requires a mesh it is only suitable for Polyhedral Particles in EDEM.

 

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The resultant spray coating can be analysed per particle to ensure a good coverage of the entire tablet or for the complete simulation.  As shown below the simulation has been run to 50 s and the overall spray coverage is inconsistent with a large number of tablets not entering the spray zone during this time.

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Further analysis of intra particle spray coating can be seen here:

 

  1. Spray Coating Model: Inter-particle spray coating

 

  • The files required to perform inter particle spray coating can be found here - Spray Coating Model - Simulation Files
  • Instructions on how to run the spray analysis can be seen in this video:


The inter particle spray coating method in EDEM is a Particle Contact Model  which tracks individual spray particles, these are represented as spheres. On contact with tablets the Spray Coating Model deletes the spray particle and transfers the volume to the tablet.  100% of the volume is transferred on initial contact.

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The user determines the spray location and direction via the Factory options in EDEM. The ‘spray’ factory option creates a fan like distribution of material however other options such as applying linear velocity from the specified area are available.

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This analysis allows for 1 value of coating per tablet and as a Physics Model requires the simulation to be re-run if the spray nozzle location is to be changed. As it requires the simulation to be run the data save rate doesn’t influence the simulation results, any particle type is supported using this method.

 

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Further analysis can then be done to investigate the overall coating efficiency of the design.

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  1. Tablet Coating – Residence Time Method (EDEM 2023)

 

  • The files required to perform the residence time spray coating method can be found here - Residence Time Method - Simulation Files
  • Instructions on how to run the residence time spray coating method can be seen in this video:


 

The residence time method in EDEM is a physics model which allows the user to define a box, cylinder or sphere as an area of interest.  This method only requires the user to specify the zone position, dimensions and orientation, any particles entering this area during run-time get assigned a Custom Residence time value. 

If the area of interest is placed in the simulation representing the known spray zone then the time spent in the spray zone can be considered a measure of the coating. With the particles residing in the spray zone the longest having the greater amount of coating.

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For information about how to simulate coating of particles using EDEM and AcuSolve CFD

 

If you have any questions about EDEM please visit the Altair EDEM Community:

 

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