Although rotary kilns are designed to be used for driving a chemical reaction, an issue that often comes up in the processing of a material is when to draw the excess moisture from the feedstock.
Many times, there is an excess of moisture that needs to be removed from the material before it is converted in the rotary kiln, and people are faced with a decision: buy another piece of equipment, or use the rotary kiln to do the work. There are costs and benefits to each approach.
While rotary kilns have the ability to remove moisture from a material, this tends to be a very inefficient process. In a rotary kiln, material is typically not showered like in a rotary dryer, but rather, slides along the interior of the rotary kiln. This causes inefficient heat transfer between the material and the gas. Because of this low heat transfer, drying material in a rotary kiln takes much longer than in a rotary dryer, and subsequently requires substantially more energy.
The alternative to drying material in a rotary kiln is to add a rotary dryer into the process prior to the material going into the rotary kiln. Taking this approach would efficiently dry the material before it enters the rotary kiln, leaving the rotary kiln the sole job of converting the material. With a rotary dryer, flights lift the material and drop it through the stream of hot gas, creating a showering effect. This showering effect allows for a good transfer of heat between the material and the gas, drying the material in an efficient manner.
In this situation, much less energy would be required, there would potentially be less wear on the rotary kiln, and the rotary kiln time would be freed up to focus on processing more product, but the down side would be that an additional piece of equipment would be required.
There are pros and cons in each scenario, but ultimately, it is up to the customer to decide what benefits outweigh the costs for their particular situation.
Feeco International is known around the world for our thermal processing equipment. We build it all from one piece to the whole plant. For further questions, see our rotary dryer and rotary kiln equipment pages or contact us.