Design and Efficiency Aspects of Rotary Dryers
by Greg Palmer, B.E. Ph.D. and Tony Howes*, B.E. Ph.D.
Palmer Technologies Pty Ltd, Brisbane, Australia
* Department of Chemical Engineering, University of Queensland, St. Lucia 4072 Australia.

Abstract
The drying of products like sand, aggregates, fertilizers and food products is an important step in industrial processes. With an increasing focus on reducing greenhouse gas emissions and energy demand the design of drying units has become critical. In the past rotary dryers have been in some cases thermally very inefficient primarily due to poor design. Fluid bed dryers on the other hand are thermally very efficient due to the interment contact of the gas stream with individual particles and a better understanding of the design principles. Thus, the science of fluid bed dryer design means these units are relatively easy to design even though thermal energy demand between each type of unit is approximately the same. The problem is that the same level of engineering knowledge has not been available for rotary dryers and as consequence these drying units are generally over designed and thermally inefficient.

This paper discusses the difference between a rotary dryer and a fluid bed dryer used to dry slag1. It is important to understand the difference between the two pieces of equipment and the design aspects required in the rotary dryers. One of the difficulties with the design of rotary dryers is determining the amount of material falling through the gas stream at any moment in time.

Due to the complexity in calculating the percentage hold up in the gas stream and the percentage hold up in the lifter various lifter designs have been tried over the years on a trail and error basis, many with poor results. Because of complexity of estimating some parameters most rotary dryers are over designed and as a consequence the final product can be over dried and heated wasting thermal energy and higher than necessary equipment costs.

Work carried out by Palmer Technologies and The University of Queensland has been aimed at understand the aspects of drying in a rotary dryer. This work enabled computer models to be developed and validated against numerous industrial dryers in the sand and cement industries.

The results from drying slag using a fluid bed dryer and a rotary dryer are first compared followed by the design criteria for rotary dryers are discussed in this paper.