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Rotary Drying: Developing a Process in a Test Setting

| By Shane Le Capitaine and Carrie Carlson, Feeco International

Testing not only illustrates proof of process and provides other critical assurances, but it also reveals the parameters necessary to reliably yield consistent results

Drying is an integral tool in pretreating and finishing bulk solids, which helps producers to meet quality and performance expectations, as well as lower their shipping costs. Designing and optimizing a drying process for efficiency is essential to minimizing operating costs and maximizing the production of a quality product, particularly when it comes to rotary dryers (Figure 1). As such, testing trials and process development are often a critical aspect of meeting dryer system expectations.

FIGURE 1. A rotary dryer is shown here

This article covers how dryer testing services can be used to optimize bulk-solids dryer design.


When is testing necessary?

Rotary dryer testing isn’t always necessary, but it is almost always beneficial. Testing is often undertaken in an effort to do the following:

  • Design a new commercial-scale drying operation
  • Optimize an existing process
  • Meet new target parameters
  • Change existing process conditions or feedstock
  • Retool a plant for a new purpose or product

Developing an efficient dryer designed around the specific process and product goals sets the stage for the life of the operation. Even if a standardized unit performs reliably, over time, small inefficiencies can add up to major costs, especially considering that rotary dryers are often in operation for decades.

In other cases, and particularly when developing a novel process, testing is a necessary step on the path to a commercial-scale operation. Testing reveals the process and equipment parameters required to achieve product quality and production goals with the specific source of feedstock. It also provides critical assurances that the commercial-scale system will perform as expected. When a thorough testing program is not undertaken, producers may find themselves faced with the following:

  • Inability to reach rated capacity
  • Experiencing a high level of attrition in the product
  • Inability to reach target outlet moisture
  • Struggling to achieve consistent product quality
  • Experiencing frequent process upsets

Testing is particularly essential when working with materials that exhibit variation across sources, as this can make a material’s drying behavior unpredictable. For this reason, testing is also likely to play a vital role as more producers begin to reconsider their industrial wastes and process byproducts as resources, drying them for reuse and recovery.


Process development

Developing or optimizing a drying process through testing can take many forms, depending on what is already known about the process and material, as well as the specific process and product goals. Most often, testing centers around one of the following objectives and progresses as necessary.

Showing proof of process. Since all materials can, in theory, be dried, initial testing focuses not on feasibility, but instead on confirming that the specific source material can be dried to the desired outlet moisture content, as well as identifying the basic parameters required to do so. In other words, initial testing is aimed at showing proof of process.

Testing at this stage identifies basic process criteria such as the following:

  • Inlet and outlet temperatures
  • Feedstock requirements (particle size distribution, moisture content, and so on)
  • Retention time
  • Percent fill

As part of the feedstock requirements, testing at this stage may also reveal the need for some type of pretreatment, such as particle size reduction, or further dewatering prior to drying.

Process and product optimization. Beyond this initial testing, further development focuses on fine-tuning the process parameters to optimize for product quality and overall process efficiency.

During this stage, more advanced process data points and their effect on the overall goals are explored and refined. This includes variables such as the following:

  • Feed and product flowrates
  • Rotational speed
  • Drum slope
  • Air flowrate (velocity)
  • Air flow configuration (co-current or counter-current)

This stage of testing also gathers a variety of operating data that informs on process economics, requirements for off-gas handling and treatment, and other decisions. Data gathered at this stage often include such parameters as the following:

  • Exhaust gas flow
  • Burner fuel usage
  • System pressures
  • Gas sampling and analysis
  • Baghouse, cyclone, scrubber efficiency

Establishing optimal flight design. The flights, or material lifters employed in a rotary drum (Figure 2), create a cascading effect, or “curtain” of material across the drum’s cross section. This maximizes the amount of contact, and ultimately heat transfer, between the material and products of combustion.

FIGURE 2. The flights used in a rotary dryer can be customized to match the requirements of a given application

Flights are available in a range of standard and custom designs. Standard designs include straight, single-bend, and double-bend, among others.

In addition to flight design, the placement of flights throughout the drum, or pattern, is also highly customizable. It is not uncommon for flight design or pattern (or both) to change along the length of the drum, as process and material conditions change during the drying process.

Flight design is based around specific material properties, such as moisture content, angle of repose, potential for sticking and more, making this a very custom aspect of the design process. When flight selection is not readily apparent, a flight simulator can be enlisted in the testing process to identify the most suitable design for the material at given stages throughout the drying process.

A flight simulator is an offline rotating drum with flights that are easily changed out for testing. A clear end cap allows for visual observance of how the selected flights work with the material.

Targeting particle characteristics.Whether drying a finished product or an intermediary material, producers are often looking to target specific parameters in the material exiting the dryer. Testing provides the environment necessary to refine the process variables necessary to achieve those qualities. The most commonly targeted parameters during drying include the following:

  • Outlet moisture content
  • Level of attrition (product degradation)
  • Bulk density
  • Crush strength
  • Particle size distribution (PSD)

By manipulating the design of the dryer, in combination with process parameters during testing, producers can reveal the conditions necessary to reach their target specifications.

The testing process varies depending on the user’s goals, but it may also identify inefficiencies and opportunities for improvement, both in the process and the product.


A case study: paper sludge

The paper industry has been under increasing pressure to find beneficial reuse applications for the sludge left over from the recycling process. In this effort, producers have begun drying their sludge for reuse in products, such as absorbents, cat litter, animal bedding, lawn products and more.

Depending on how the paper pulp was processed and dewatered at the plant, its characteristics, and particularly its moisture and clay content, can vary, making testing a critical step in developing a commercial-scale system. Optimizing for efficiency is especially important when working with wastes and byproducts, as the margin for profit is often slim.

Whatever the intended product, testing helps to reveal the process conditions necessary to consistently produce product to specification.

Under-dried material could present a host of issues depending on the end product; excess moisture would not only inflate shipping costs, but could also promote mold or bacterial growth, making the product unfit for its intended use. Similarly, an over-dried product may be too dusty for acceptance on the market.

The producer may also want to test in order to determine the highest level of moisture they can run through their dryer, since paper waste streams are not always consistent.

In this example, producers are likely to focus on finding the right combination of volumetric fill, retention time, dryer temperature and air volume to consistently reach the target moisture content. Paper producers often find that they must employ a size-reduction step prior to drying in order for the process to be effective.


Concluding remarks

Rotary dryers are essential in processing bulk solids, but maximizing the efficiency of a dryer can be challenging without a thorough testing program. Whether developing a new process, or optimizing an existing one, testing facilities such as the one shown in Figure 3, offer an invaluable opportunity to establish a highly efficient operation.

FIGURE 3. A look inside a testing center, with a pilot dryer visible in the background

Testing not only illustrates proof of process and provides other critical assurances, but it also reveals the parameters necessary to reliably yield consistent results, giving producers a leg up on commercializing and refining their operations.

Edited by Gerald Ondrey


Shane Le Capitaine is a process sales engineer at FEECO International (3913 Algoma Rd., Green Bay, WI 54311; Phone: 1-800-373-9347; Email: [email protected]), where he specializes in bulk solids drying, fertilizer granulation, and rotary drum applications. Le Capitaine received a B.S.Ch.E. from the University of Wisconsin-Madison and has over 20 years of experience in his field.


Carrie Carlson is a technical writer at FEECO International (same address as above; Email: [email protected]) with more than a decade of experience. She works closely with engineers and process experts to turn complex ideas into easy-to-understand literature.