Though far less common than the need to dry or thermally treat a bulk solid material, some process settings call for preheating a material in preparation of downstream processing. In achieving this objective and meeting the high tonnages associated with bulk solids processing, rotary dryers offer an optimized solution to producers.
The following covers the settings in which rotary dryers are employed as preheating drums, and how they are designed to meet the specific needs of these unique applications.
Why Preheat Bulk Solids?
The diversity present throughout industrial processes, and even across producers manufacturing the same product means no two processes are the same. As such, a preheating drum may be used in any number of industries across a range of materials.
The most common settings in which preheating drums are employed are when producers are looking to either prepare a material for coating or coloring, or to reduce fuel costs in a process involving a rotary kiln (or other type of furnace).
To Prepare Materials for Coating
Preheating a material in preparation for coating helps to promote optimal coating application. Some coatings must be heated for application onto a granule. If granules were not preheated, a sudden or drastic change in temperature between the two materials could cause the coating to “shatter,” a common worry when working with wax-based coatings.
Preheating is also commonly seen in roofing granule production, where the rock granules are heated prior to the addition of the color coating in a subsequent coating drum. This helps the coating to better adhere to the granules.
To Reduce Fuel Costs
In some thermal processing operations, material is preheated prior to entering a rotary kiln, calciner, or other thermal processing device.
Preheating the material in this type of setting makes the kiln more efficient by leaving it with the sole purpose of facilitating the desired chemical or physical reaction.
The lower rate of heat transfer that occurs in a rotary kiln can make bringing material up to the required temperature a slow and energy-intensive process. Instead, a rotary dryer is used to preheat the material to the required processing temperature; the rotary dryer boasts a higher rate of heat transfer, making it a more efficient way to bring material up to temp.
Many facilities also have a source of waste heat that can be used to feed the preheater. This further drives down fuel costs and promotes more sustainable energy use.
Why Use Rotary Dryers for Preheating
The reasons for selecting a rotary dryer to carry out preheating are the same reasons for which rotary dryers are chosen in other process settings.
Rotary dryers are high-capacity machines built for reliability and longevity. They are incredibly robust, offer significant potential for customization, and have a well-earned reputation for reliability.
Rotary dryers are also tolerant to small fluctuations in feedstock and are simple to operate and maintain, making them a widely accepted technology.
All of these advantages carry through to preheating.
Preheating Drum Design
While a preheating drum is not intended to dry the material, the design is much the same as a typical rotary dryer, with the key design decisions being the same:
Direct or Indirect
Preheating drums can be either of the direct or indirect design. In most cases, the direct-fired preheater offers the best option, as it provides the highest level of heat transfer and lowest capital cost, due to the direct contact between the material and products of combustion.
In some special cases, however, the indirect-fired design is the necessary approach. The indirect design is typical with materials that cannot come into contact with combustion gases. This might be because unacceptable pollutants would form, the product would become contaminated, or because the product requires a specific processing atmosphere.
Co-Current or Counter-Current Air Flow
Depending on the unique characteristics of the material, the preheating drum can be configured for either co-current or counter-current air flow. However, since the objective is to heat material for a process following the preheater, it is typically most efficient to have the material exiting the dryer at its hottest point. For this reason, the counter-current air flow is most common, as this puts the material into contact with the hottest gas just prior to discharge. If a co-current configuration were used, the material would potentially have time to cool slightly as it moves through the drum toward the discharge.
Flights
As with typical rotary dryers, the flight (material lifter) design is a very custom aspect of unit design, specific to the handling characteristics of the material to be processed. Both flight design and pattern are customizable and should be engineered to work with the material in order to create the optimal “curtain” or cascade of material through the air stream as the drum rotates. This maximizes air-to-material contact and ultimately heat transfer.
Combustion Chamber
If the material is sensitive to radiant heat, the preheater can be designed to include a combustion chamber. This prevents direct contact between the material and flame.
The need to preheat a material is characteristic of high-value products and intermediary materials where product integrity is critical. As such, a combustion chamber is almost always used in preheating applications.
Drum Knockers
Drum knockers, whether of the pneumatic or hammer design, can be incorporated into the design of the preheater to prevent the potential for buildup in the drum’s interior and subsequent product caking.
Lower Air Volume
Preheating drums have one exception to typical rotary dryer design: because they are not intended to remove moisture, a lower volume of air can be used.
A lower air volume often means a smaller drum will suffice. Similarly, because not as much air is flowing through the system, a smaller off-gas treatment system can be used.
Preheating Drums vs. Rotary Kilns
While the difference between a preheating drum and a rotary dryer is fairly clear, some may wonder how the preheater is any different from a rotary kiln, since both serve to heat the material to a specified temperature.
Because rotary kilns are intended to cause a chemical reaction or physical change in the material, they operate at much higher temperatures than a preheater or dryer. This in turn leads to other changes in the design of the drum itself, in terms of materials of construction, air velocity, burner power, refractory inclusion, and more.
Developing a Thermal Treatment Process With a Preheating Drum
The need to preheat a material is often identified through process development testing, such as that carried out in the FEECO Innovation Center.
The Innovation Center is equipped with a wide range of equipment for testing and optimizing various drying, thermal, coating, and granulation processes. Preheating can be tested as part of a larger process to assess feasibility and establish the process criteria necessary to produce the desired results, or to determine if the addition of a preheating drum could benefit an existing process.
In any case, testing allows producers to determine how preheating affects their process and product, as well as gather essential data such as:
- Feedstock requirements
- Drum speed
- Drum slope
- Inlet and outlet temperature
- Feed and product flow rates
- Retention time
- And more…
Conclusion
Though not common, preheating drums are ultimately another tool in the process engineer’s toolkit, providing another way to optimize process efficiency and product quality. Rotary dryers are the ideal type of equipment for preheating, already built to withstand the temperatures and throughput required and with significant customization potential, plus all of the advantages that come along with rotary dryers.
In business since 1951, FEECO supplies the world’s most reputable rotary dryers. Whether used for preheating or drying, each dryer is designed around the specific process and product goals, with feasibility testing and process development services available in our Innovation Center. For more information on our rotary dryers or testing services, contact us today!