Whether drying, cooling, or calcining material, the design of a thermal processing system gives rise to many questions. With over 75 years of experience in helping our customers design and maintain custom equipment and production lines, weโre covering some of the most frequently asked questions (FAQs) we seeโfrom the benefits of rotary drum technology to whether a kiln can be used for drying and everything in between.
What are the advantages of a rotary drum in drying, cooling, and thermal treatment?
Rotary drums are selected for drying, cooling, and thermal treatment applications for several reasons, most notably for their reliability and high throughput.
Rotary drums are generally low maintenance compared to other thermal devices and rely on simple, well-established technology, making them easy to learn, operate, and maintain.
In some settings, the rolling action that occurs in the material bed also imparts a polishing effect on granules, further smoothing and rounding them, making rotary drums a popular choice when drying granular materials such as fertilizers and soil amendments.
What fuel can dryers and kilns run on?
Rotary dryers and kilns can run on several different fuel types, with the most common being:
- Fuel oil
- Natural gas/propane
- Waste heat
- Biogas
- Electricity (indirect configurations only)
Will my material require pretreatment?
The need for pretreatment is highly case specific. In general, rotary drums perform best when materials are free-flowing and consistently uniform in both particle size distribution (PSD) and moisture content.
Feedstocks exhibiting wide variation in any parameter may require pretreatment to improve uniformity and flowability. This might include a grinding step, back mixing, or additional moisture reduction before material can be successfully processed in rotary equipment.
Producers using a rotary kiln reliant on maximizing surface area for gas-to-solids contact will also frequently agglomerate their material prior to kiln processing to improve bed permeability.
The need for pretreatment, as well as available options, can be assessed through testing in the FEECO Innovation Center.
How can I de-risk the scale-up process?
De-risking the scale-up process can be done through thermal process development testing in the FEECO Innovation Center.
The Innovation Center is equipped with batch- and pilot-scale dryers and kilns to not only confirm feasibility, but to gather the data necessary for successful scale-up.
Testing allows producers to establish key process data required for full-scale, continuous production, including feed and product flow rates, air volume, and other essential variables.
Direct-fired batch kiln used for testing in the FEECO Innovation Center
What sort of maintenance will the system require?
Rotary drum maintenance is specific to each drumโs design and operating conditions. Maintenance requirements generally focus on maintaining proper load distribution and addressing any signs of wear, both of which rely on comprehensive inspection procedures. Beyond proper lubrication to mechanical components, an average drum may require routine realignment, occasional tire and trunnion grinding, regular training (skewing to maintain float), and basic tire mounting assembly upkeep.
Is a cooling step required?
Whether a cooling step is required is highly dependent on the application and post-thermal handling. Materials that will move immediately to bagging or storage typically require cooling to maintain product integrity and prevent caking and bacterial formation. Some operations use an immediate cooling step after a kiln to stop a reaction in progress.
How much energy will the system use?
The amount of energy a given rotary drum will consume is dependent on several factors, including temperature profiles, moisture content, and residence time. A typical fertilizer dryer, for example, might use 1500 BTU per pound of water evaporated.
Energy consumption can be predicted using a mass and energy balance.
What other equipment is necessary to support the system?
Rotary thermal processing systems require several ancillary pieces of equipment to facilitate material feeding and offtake, as well as off-gas treatment and combustion requirements. This can vary considerably depending on the type of system in use, process goals, regulatory requirements, and the material being processed, but typically entails:
- A burner (direct dryers and kilns only)
- Inlet and outlet conveyors or screw feeders for feeding and offtake
- A baghouse, cyclone or scrubber for removing particulates (direct dryers, coolers, and kilns only)
- Some form of exhaust gas treatment such as a thermal oxidizer (secondary combustion chamber) or a quench tower
- A waste heat recovery boiler, where applicable
- A controls system for managing start-up and shutdown
Does mixing of solids occur?
Yes, the tumbling action that occurs in any type of rotary drum promotes solids mixing. The extent of mixing depends on the type of system in use (dryer or kiln (or cooler), direct or indirect) and the design of internal components.
Lifting flights in direct dryers and coolers pick up material and drop it through the stream of combustion gases to maximize heat transfer, inadvertently also mixing the solids.
Lifting flights (at right) shown in a rotary dryer (advancing flights, at left, also visible)
In kilns and indirect dryers and coolers, in which material slides along the interior wall of the drum, bed disturbers are employed to facilitate the desired amount of mixing.
Bed disturbers in a rotary kiln
The incorporation of a dam in any type of rotary drum, typically added to increase retention time, also promotes solids mixing, albeit minimal, as material piles up behind and then spills over the dam, redistributing what was on top to the bottom of the bed.
What data points are necessary for system design?
Engineers require a variety of data points to design a rotary thermal processing system:
Material data points:
- Material (chemical composition)
- Bulk density
- Particle size distribution (PSD)
- Specific heat
- Maximum allowable temperature
- Inlet and outlet moisture content
- Material Inlet Temperature
Processing Conditions
- Capacity
- Plant elevation
- Fuel source (and corresponding higher heating value)
- Ambient air temperature
- Reaction temperature(s), where applicable
- Residence time
Configuration
- Direct or indirect
- Co-current or counter-current air flow (direct configurations only)
- Preferred material of construction
Whatโs the difference between kilns and dryers?
The terms โdryerโ and โkilnโ are often mistakenly used interchangeably, but are fundamentally different. While both types of equipment employ heat to reach the desired objective, rotary dryers are used to remove moisture and rotary kilns are used to change the chemical characteristics of the material.
Kilns typically operate at higher temperatures than dryers. The large difference in operating temperatures means that dryers and kilns are designed and constructed to handle different thermal loads, requiring different materials of construction, the use of refractory, and stainless steel.
Rotary dryers and kilns can also differ in terms of heat transfer. A direct-fired dryer employs convection when showering material through combustion gases. Material is also heated via conduction through contact with the drum shell.
Rotary kilns typically do not utilize flights due to their much higher temperatures, instead heating material through contact with the shell (conduction) and radiation, though the specific mode(s) in use depends on whether the kiln is direct or indirect.
Can a kiln be used to dry material?
Technically, a kiln is capable of drying material, but using a kiln for moisture reduction is not recommended in practice, because in most settings, it is extremely inefficient.
Because the use of lifting flights in the high-temperature environment of a rotary kiln is typically impractical, heat transfer occurs instead through contact with the drum shell or refractory (conduction), where applicable, and contact with the hot gasses moving through the kiln (radiation).
As a result, heat transfer is comparably lower than in a rotary dryer, leading to a longer retention time to meet the target moisture content. Accommodating this additional retention time in the kiln would require a much longer drum, significantly increasing system cost.
Even though an additional system is required, it is often more efficient and economically practical to have a dedicated dryer in place for moisture removal prior to the kiln.
Conclusion
While the thermal treatment of bulk solids, whether drying, cooling, or otherwise, is a complex endeavor of system design, the questions that arise are easily answered when working with an experienced original equipment manufacturer (OEM).
With over 75 years of expertise in building custom rotary dryers, coolers, and kilns, FEECO offers the technical support and pilot-scale testing needed to take a thermal processing challenge from concept to reliable production. To learn more, contact us today!
