Proper Bulk Solids Drying has long been and continues to be a critical tool in industrial processing, facilitating enhanced handling and flowability, greater storage potential, shelf stability, and improved shipping economics.
While many types of industrial dryers have come on the market, rotary dryers remain a favoured option in most industries, offering a reliable, customisable, efficient option for high-capacity processing operations.
What follows is an introduction to how these industrial dryers work, the applications in which they serve, and how to get the most out of them in any industrial setting.
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Rotary Dryer Operation
Rotary dryers rely on long-proven rotary drum technology, in which material is passed through a rotating drum set on a slight angle to allow gravity to assist in moving material through the unit.
Direct Dryers
Rotary dryers can be either directly or indirectly heated, with direct dryers being the most common configuration, due to their higher efficiency.
In a direct dryer, a stream of combustion gas is fed through the unit, allowing direct contact between the process gas and the material being dried.
Flights, or lifters, pick up material from the bed as the drum rotates, and drop it through the stream of combustion gas, creating a “curtain” of material across the drum’s diameter that maximises heat transfer between the material and process gas. This cascading effect has led to some referring to rotary dryers as rotary cascade dryers.
Co-Current vs. Counter-Current Air Flow
Depending on the characteristics of the material being dried and the finished product goals, the gas can be fed through the dryer either co-currently or counter-currently (in relation to the material flow).
A co-current air flow puts the hottest gas in contact with the material at its wettest point, promoting fast initial drying. While this method is slightly less thermally efficient, it allows for greater control over the solids temperature and helps to prevent overheating, attrition, and discolouration of the material.
This configuration also promotes greater dryer longevity because the shell runs at a cooler temperature due to the rapid initial cooling of the process gas. For these reasons, the co-current air flow design is significantly more common than the counter-current approach.
When the material being dried contains bound moisture, the counter-current configuration offers the best approach; this setup fosters gradual moisture removal with a higher thermal efficiency. The positioning of the material at its driest point against the hottest gas encourages removal of any remaining bound moisture. Because of this, however, the counter-current air flow configuration is not recommended for heat-sensitive materials.
Indirect Dryers
In contrast to direct dryers, indirect dryers prevent contact between the material and process gas, instead heating material indirectly, through contact with the rotating drum shell, which is sealed off and heated externally.
Though inherently less efficient and accompanied by a much higher capital cost, the indirect dryer is ideal for processing fines that could otherwise become entrained in the process gas. It also allows for tighter control over the processing atmosphere, which may be necessary when working with materials that are combustible under certain conditions.
Typical Applications for Bulk Solids Drying
The flexibility of rotary dryers lends them to a broad range of applications. Suitable for material pre-treatment, product finishing, and preparation of intermediary materials, rotary dryers can be found processing raw minerals at mine sites, pre-heating granules at roofing granule manufacturing facilities, and drying fertiliser granules in production plants. They are widely used in processing:
- Fertilisers & soil amendments
- Specialty chemicals
- Pulp and paper products
- Minerals, ores, and aggregates
- Animal feed
- Sand
- Sugar
Supporting Bulk Solids Drying Equipment
Some ancillary equipment is required to support a rotary dryer and integrate it into a continuous process. This consists of feeding and offtake conveyors, an exhaust gas handling system, and a burner, each of which offers its own level of customisation to tailor the design according to the process and material goals.
A combustion chamber, used to house the combustion reaction and direct the flow of gas into the dryer, is also frequently incorporated into the system in order to prevent direct contact between the product and flame, which could otherwise cause material degradation.
Keys to Optimal Dryer Performance & Longevity
Rotary dryers hold the potential to offer incredibly reliable, efficient processing for decades—they are recognised for their robust build, simple operation, and an ability to yield consistent results despite minor fluctuations in feedstock characteristics.
Part of their widespread success, however, is the result of proper design and maintenance.
Rotary Dryer Design
The broad spectrum of applications to which rotary dryers can be applied might seem to imply a universally acceptable design. In reality, however, rotary dryers offer the best performance when their design is tailored to the specific characteristics of the material to be processed, in combination with the process goals.
Material characteristics such as those listed below have a significant impact on how the dryer performs, and as such, how various aspects of the unit should be designed to produce the best result.
- Bulk density
- Angle of repose
- Specific heat
- Feedstock moisture vs. final moisture
- Consistency
- Fragility
- Hygroscopicity
- Chemical composition
These qualities influence how the material behaves in a dryer and how it responds to the drying process, which in turn guide the necessary parameters for things like retention time, air flow velocity (direct dryers only), percent fill, temperature profiles, and feed rate. In designing a new rotary dryer, these variables can be established through testing in a facility such as the FEECO Innovation Centre.
For optimal performance and longevity, these process variables in turn should be used to inform on key design decisions, such as:
- Direct or indirect configuration
- Co-current or counter-current air flow (direct dryers only)
- Drum diameter and length
- Feed inlet design
- Whether or not a combustion chamber will be used (direct dryers only)
- Materials of construction
- Bearing & drive components and drive assembly style
- Flight design and placement pattern (direct dryers only)
When operators are experiencing issues with an existing dryer, many of these aspects can be modified or retrofitted to improve performance, however a process audit is always recommended to properly diagnose any issues.
Rotary Dryer Maintenance
It’s important to note that proper maintenance is also critical to maintaining dryer performance and ensuring longevity; even the best-designed rotary dryer holds potential for trouble if not properly maintained.
Worn components such as seals, flights, trunnions, and more, have the potential to impact the dryer’s performance if left untreated. Some aspects of drum health, particularly alignment, can put undue stress on the mechanical components of the drum if not rectified, decreasing dryer life, and adding unnecessary downtime and maintenance costs.
Operators and maintenance personnel should also be properly trained in operating, inspecting, and maintaining their dryers, knowing where potential for trouble lies and how to perform routine maintenance procedures.
Controls & Automation
Controls and automation are powerful tools in optimising dryer performance and longevity. A programmable logic controller (PLC) with motor control center (MCC) can be incorporated to automate start-up and shutdown procedures, as well as facilitate more streamlined dryer operation and performance.
More advanced data collection systems can be employed to provide real-time monitoring, trending, and reporting, all from a single interface or mobile device. This can be programmed to alert operators when key parameters fall out of spec, allowing them to identify and prevent issues before they have the opportunity to escalate. This data is also useful in troubleshooting issues, as well as future maintenance planning, providing valuable benchmark data.
Concluding Remarks
Rotary dryers offer an efficient, reliable solution to high-capacity bulk solids drying operations. Their reliability, combined with their high level of customisation, make them suited to providing optimal performance in a range of settings.
These industrial dryers perform best and offer the longest potential service life when designed according to their specified duty, as well as when properly maintained, with controls offering added performance and monitoring capabilities.
