Developments in scale-up procedures for industrial dryers

Abstract

The aim of dryer scale-up is to increase batch size, or throughput of a continuous process, while maintaining final moisture content, product quality, end-use properties, safety and reliability. This article reviews and updates previously published material on scale-up processes of dryers in general and individual dryer types, particularly material from a Drying Technology special issue on scale-up in 1994. This is adapted to give a basic generic theory for scale-up, applying this to various dryers and noting specific exceptions. It is demonstrated that simplifying assumptions can often be reliably made for scale-up which reduce the level of complexity for modeling compared to equipment design and performance calculations. For example, prediction of drying rate and drying time in spray and flash dryers needs a kinetic model, but for scale-up a heat balance and scaling calculation usually suffices. Approximations such as a one-third power law for drying time are often effective. It is shown that there is some theoretical basis for empirical scale-up rules that have been used by dryer manufacturers. Scale-down is also important, selecting the most appropriate conditions for small-scale tests that will correspond to the desired performance at commercial scale. The individual concepts are not new, but they have not previously been unified into a simply stated general basis for scale-up.

Keywords:

• Operation
• scale-down
• specific drying rate
• particle history
• pharmaceuticals
• fluidised beds 

Acknowledgments

The author would like to thank past colleagues, particularly in the SPS research consortium at Harwell and in GSK, who contributed in many ways to the body of knowledge described above. The author also thanks the reviewers for their helpful and constructive comments which led to important improvements in the paper.

Disclosure statement

The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.