Granulation drums, also commonly called granulators or agglomeration drums, are used throughout a variety of industrial settings to process fines into a granular product. They have become the basis of modern fertilizer granulation, and continue to make their way into a growing number of applications.
The process of sizing and designing a granulation drum is complex and relies on a number of carefully balanced criteria; several key factors must be considered in order to yield a granulation drum that performs efficiently and produces on-spec product reliably. These considerations are summarized here.
How Granulation Drums Work
The granulation drum is a wet granulation (aka tumble-growth, agitation, or non-pressure agglomeration) device, following the same principles as other wet granulation equipment, which utilize motion, in combination with a liquid binder (in most cases), to promote granule formation.
The equipment functions as follows: raw material is fed into a rotating drum, and at predetermined locations, the liquid binder is introduced, causing the tumbling material to become tacky and pick up additional fines in the material bed. As the material continues to tumble along the length of the drum picking up additional fines, granulation occurs as a result of the rolling motion of the particles in the bed.
Process Parameters That Influence Granulator Sizing & Design
Several parameters play into the optimal configuration of a granulation drum. While in some cases this data may be historically available, in most cases, it will require a thorough granulation testing program to gather the necessary data and assess how the material will respond to wet granulation.
The most influential aspects of granulation drum sizing and design are:
Feed Rate and Desired Production Rate
The desired production rate, and subsequently, the required feed rate, are the most important considerations in granulator sizing, as these parameters define the capacity the drum will need to handle.
How Feed Rate & Desired Production Rate Influence Sizing & Design
The desired capacity not only represents the amount of material the drum must be capable of handling, but also the load that the drum will be expected to bear, setting the stage not just for drum size, but also for drum component design and sizing as well.
Percent Fill
Percent fill is the amount of product in the material bed during processing. In determining the optimal fill percentage, process engineers aim to reach the maximum amount of product in the bed allowable, while still achieving adequate granulation.
How Percent Fill Influences Sizing and Design
The desired fill percentage also impacts drum design, as the optimal fill level will be controlled through drum speed, dams, slope, material feed rate, and drum size.
Depending on the material, a percent fill that is too high could cause clumping of the material, or leave pockets of material unprocessed; too low and the drum may not achieve the desired end product.
Retention Time
Retention time, also known as residence time, is the length of time for which the material must be processed in the unit to reach the desired size and quality.
A retention time that is too low will produce under-sized and potentially weak granules likely to break down during subsequent handling, while a retention time that is too high will potentially produce over-sized granules.
How Retention Time Influences Sizing and Design
Achieving the desired retention time is largely a factor of drum size. In general, the longer the retention time needed, the larger the drum will be.
While retention time is primarily controlled through drum sizing, it can also be controlled through the use of dams. Dams are a physical barrier placed either at the discharge end of the drum, or at some point along the drum’s interior. They function much the same as a water dam; material builds up behind the dam, and then spills over the dam, either continuing processing or discharging (depending on where the dam is located in the drum).
In settings where the required retention time would demand an unfeasibly large drum, dams can be incorporated so that a smaller drum may accomplish the work, though it’s important to recognize that dams are not always an option.
While not as influential as drum size, the speed and slope at which the drum is set will also affect retention time.
Rolling Action
Achieving the right rolling action in the material bed is what allows adequate granulation to occur. During granulation, particles roll against each other and against the sides of the drum’s interior, picking up fines and rolling in a layering effect similar to rolling a snowball.
How Rolling Action Influences Sizing and Design
Optimal rolling action is obtained largely through drum speed (in combination with percent fill), defined in revolutions per minute (RPMs).
The ideal drum speed is highly dependent on several material characteristics, including moisture content, chemical composition, particle size, and more, as each will influence how the material tumbles in the bed.
In a drum that rotates too slowly, granules will not tumble enough to granulate. If the drum rotates too quickly, material will not tumble, and will simply be carried through the drum before it has a chance to granulate.
Drum speed is typically controlled by an electronic variable frequency device (VFD).
Binder Addition (Spray System)
The right addition of liquid binder to the drum is a complex endeavor influenced by many factors. These factors ultimately come together in the design of the granulator’s spray system. Variables used to determine the ideal spray system configuration typically include:
- How the binder is introduced (atomized spray, steam, droplets, etc.)
- At what point(s) in the drum the binder is introduced (multiple locations are often necessary)
- Binder feed rate (the amount of binder supplied at each point)
Compared to the aforementioned variables, the design of the spray system is a much more intricate endeavor. Testing is especially critical in developing the most favorable spray system configuration.
Preconditioning Requirements
Preconditioning requirements are not always required, but do have a direct effect on drum sizing and design. They are an important consideration in looking at system design as a whole, and in some cases, can influence sizing. Such is the case when a pin mixer or pugmill mixer is added to the system before the granulation drum.
When a mixer is incorporated into the system as a means of preconditioning feedstock, it may be possible to use a smaller drum, as production is increased through the addition of a mixer. Mixers pre-mix the liquid binder with the dry feedstock and begin forming seed pellets, relieving the drum of some of its duties and allowing a smaller drum to accommodate the same capacity.
Material Characteristics
It’s important to recognize that the unique characteristics of the material to be processed play into every aspect of granulator design, including all of the factors discussed here. Material characteristics that influence granulator design include:
- Bulk density
- Moisture content
- Particle size distribution
- Chemical composition
- Temperature
- Abrasive or corrosive quality
- And more…
In addition to the variables listed above, material characteristics also influence materials of construction and liner selection/internals when applicable. The variation exhibited across materials, and even materials of the same kind originating from different sources, causes every material to behave differently in the granulator, making testing crucial in engineering a drum suited to the intended application.
Particle Characteristics Controlled Through Process Parameters
In designing a commercial-scale granulation drum, engineers carefully balance all of the factors listed above in order to produce a granular product with the intended specifications. Through these parameters, several end product specifications are controlled, including:
- Bulk density
- Particle size distribution
- Moisture content
- Crush strength
- Binder distrubition
- Attrition (Dust)
- Surface quality
- Green/Wet strength
- Flowability
Given the many variables that must be balanced in granulator design, coupled with the variation in material characteristics and behavior, thorough testing such as that conducted in the FEECO Innovation Center is an essential aspect of effective granulator design and should not be overlooked.
Conclusion
Granulation drums provide a high-capacity, reliable wet granulation device in a variety of settings. Several key criteria must be carefully balanced in order to design and size a reliable, efficient granulator that produces product of the desired quality.
FEECO has been providing the best granulation drums available since 1951. Our granulators are used to process materials across many industries and are known for their robust build and reliable performance. All FEECO granulation drums are backed by thorough testing in our Innovation Center, which we use to assess and refine granulation drum sizing and design. For more information on our granulators, contact us today!