Fluid bed dryers offer an effective approach to reducing moisture in bulk solids, leveraging fluidization to achieve uniform drying and high efficiency. However, achieving optimal performance in these systems requires careful consideration of design and operation. Issues such as poor fluidization, excessive fine particle loss, and challenges with sticky materials can impact product quality and process efficiency, often requiring adjustments in equipment design, material pretreatment, and operational practices. This guide addresses common challenges encountered in fluid bed drying and provides strategies for diagnosing and addressing each issue effectively.
Poor Fluidization
Poor fluidization, or the lack of adequate fluidization can be common when working with fluid bed dryers, especially in settings where the material is less-than-flowable.
Signs of Poor Fluidization
Poor fluidization may or may not be apparent, but most often presents itself as material clumping or channeling in the bed. Similarly, if one section of the bed remains stationary while others are dynamic, this also indicates poor fluidization.
Unevenly dried product can also be indicative of a potential issue with fluidization, as the air is not able to access all potentially available surface area during drying.
What Causes Poor Fluidization?
While poor fluidization could be the result of inadequate design, in most cases, it can be tied directly to the material going into the dryer.
The fluidization process is most effective when material characteristics such as particle size, moisture content, and shape (among other parameters), are uniform. Thus, adequate material pretreatment is often a critical aspect of ensuring uniform fluidization in the dryer.
A non-uniform material will experience fluidization at different rates, causing uneven drying.
How to Address Poor Fluidization
Achieving adequate fluidization starts with proper fluid bed dryer design, particularly when it comes to the distributor plate and plenum, or the area responsible for distributing the air flow evenly over the distributor plate. If these components are not properly designed, success will be impossible.
If poor fluidization arises in an existing process, operators should monitor and adjust air velocity carefully to promote optimal fluidization. However, if a change in feedstock has occurred that has caused the material to become non-uniform in some aspect, pretreatment of the material will likely be required.
The type and extent of pretreatment will depend on what characteristics within the material vary. This might include preconditioning the feed with dried material, screening out larger particles, implementing a size reduction step, or other methods of increasing particle uniformity.
Excessive Fine Particle Loss
Excessive loss of fine particles, due to material becoming entrained in the air flow and carried out with the exhaust gas, results in an inefficient operation and product loss.
Signs of Excessive Fine Particle Loss
Indications of excessive fine particle loss include high dust levels in the exhaust system, visible material loss in the form of fines, frequent blockages or wear in downstream filters, and decreased product yield.
What Causes Excessive Fine Particle Loss?
In any fluid bed dryer, the goal is to achieve as much fluidization as possible, while minimizing the loss of fines. In other words, the dryer should dry the widest particle size range possible while still achieving uniform results.
The excessive loss of what are considered fine particles in the operation is the result of air flow velocity that is too high for the size and weight of the material; the particles become swept up into the air flow.
If the feedstock consists mainly of fine particles, achieving the right balance of max fluidization and minimal fines loss can be challenging. In such cases, it may be desirable to agglomerate the material into larger particles to avoid excessive entrainment and fines loss.
How to Address Fine Particle Loss
Addressing excessive fines loss requires identifying the source of the issue. If the air flow velocity is too high, the operator will likely need to reduce it to the level at which proper fluidization is possible, without blowing fine particles out with the exhaust.
It may also be useful to replace the distributor plate with one that has smaller openings to better temper the air flow.
As mentioned, it may also be necessary to pretreat the material via agglomeration or particle size enlargement. A larger particle size and greater bulk density will be less likely to become entrained in the air flow.
If excessive loss of fines is deemed unavoidable, producers can optimize their system by incorporating a cyclone or baghouse dust collection system to capture the fines exiting the system. These fines may then be agglomerated for reintroduction to the system or recovered in other ways.
Over- or Under-Dried Material
The occurrence of over- or under-dried material is another common and frustrating issue fluid bed dryer operators may experience. This can result in lost product, unhappy customers, or the need to reprocess material, all of which can impact the operation’s bottom line.
Signs of Inadequately Dried Material
Signs of over- or under-drying are fairly easy to spot with off-spec product, with regular product monitoring imperative to recognizing an issue quickly. Knowing what’s causing the issue, however, is less straightforward.
What Causes Over- or Under-Dried Material
The potential causes of over- or under-dried material are many. As we’ve already covered, poor fluidization may be the cause of inadequately dried material. This could be the result of poor design, a non-uniform feedstock, or otherwise.
Other causes of inadequate drying might include variation in feedstock moisture or particle size, or a mismatch in the dryer’s key operating parameters and the requirements of the material.
How to Address Over- or Under-Dried Material
Again, resolving the problem must start with identifying the source. If the problem originates with variance in the feedstock, pretreatment may be necessary. If a change in feedstock has occurred, operators may need to address the issue through trial and error, increasing or decreasing residence time and/or the inlet air temperature to suit the new parameters. Operators may also test changes in air flow velocity.
Temperature Control Issues
Proper temperature control is essential to managing the drying process efficiently. As with over- or under-dried product, issues with temperature control may also be apparent in the material exiting the dryer, raising product quality concerns.
Signs of Temperature Control Issues
Issues with temperature control may present as over-dried or even burned product, signs of heat degradation, variations in product color or quality, or higher operational costs resulting from increased fuel consumption.
When temperature control is inadequate, it is also common to see the temperature of the air leaving the dryer fluctuate wildly and not settle into the designated set point. While some fluctuation (+/- 2 ℉) is acceptable, excessive fluctuation (+/- 20 ℉) indicates a problem.
What Causes Temperature Control Issues?
Temperature control issues may be the result of a poorly designed controls/monitoring system, faulty sensors or wires, or even bundling of wires, which can cause problems. Poor tuning of the control loop can also instigate issues with temperature control.
How to Address Temperature Control Issues
Fluid bed dryers should be equipped with a precise temperature control system that allows multi-zone heating or direct feedback from moisture sensors for real-time heat adjustment.
Temperature should be carefully monitored and adjusted according to product requirements.
Difficulty Handling Sticky or Cohesive Materials
Sticky or cohesive materials are inherently challenging for any drying operation, no matter the type of dryer.
Signs of a Sticky or Cohesive Material
In most cases, operators will be aware that they are working with a challenging material. However, in some cases, a change in vendor or feedstock source may bring unexpected challenges to the production line.
Common signs that material has become sticky include buildup on interior fluid bed surfaces, material clumping, blockages that inhibit air flow, inconsistent product quality, and poor fluidization. Large-size chunks stuck to the air grid also indicate a sticky material.
What Causes Material to be Sticky?
A material may inherently be sticky or cohesive simply because of its chemical composition. Some materials may be sticky when they fall within a specific range of moisture content. Similarly, they may become sticky at certain temperatures or when certain process conditions combine.
How to Address a Sticky Material
In the case of designing a fluid bed dryer for drying a sticky material, materials of construction can help to reduce the potential for issues. Similarly, mechanical aids such as pulsating air jets or a mixing arm can also help to minimize buildup and sticking problems.
However, as with many problems, the best solution is prevention and this is again best carried out with material pretreatment.
In addressing sticky materials, pretreatment typically consists of conditioning, or mixing dried material in with the feedstock, as well as running the material through a delumper or other device to break up material clumps and divide the material into a more uniform consistency. The feedstock may also be combined with a coating agent to reduce tackiness prior to drying.
Because many materials become more tacky as moisture content increases, it is also critical to maintain precise control over inlet air humidity and temperature to discourage sticking.
Uneven Air Distribution
Uneven air distribution can cause a number of issues, including inconsistent drying, poor fluidization, and the like.
Signs of Uneven Air Distribution
Inconsistent moisture content in the material exiting the dryer, poor fluidization, and clumping could indicate a problem with air distribution. The most obvious sign of poor air distribution is the presence of static areas in the bed.
If system pressure is being monitored, operators may also notice a change in this regard.
What Causes Uneven Air Distribution?
Uneven air distribution is most commonly the result of a blocked distributor plate. If material is allowed to clog the openings in the distributor plate, an uneven distribution of air will occur.
Other causes of uneven air distribution include blower problems and vessel geometry.
How to Address Uneven Air Distribution
In the case of bed geometry, baffles can be added to promote a more uniform distribution of air flow.
If clogging is an issue, the fluid bed should be regularly inspected and any blockages cleaned to prevent air flow disruption. If blockages are regular, it may be worth investigating material pretreatment to reduce the potential for clogging.
Operators should also routinely check blower performance and adjust as needed.
Wear and Tear on Components
Wear and tear on components is an inevitability in any production setting. However, there are ways to encourage equipment longevity and minimize wear and tear.
Signs of Worn Components
Worn components may present themselves in a variety of ways, from unexplained inefficiencies or frequent shutdowns to visible wear or air leaks.
What Causes Worn Components
Normal wear and tear are simply a natural part of industrial processing. Excessive wear and tear, however, may be the result of improper materials of construction, poor weld quality, or components unsuited for duty.
Addressing Wear and Tear on Components
Prevention of wear and tear begins with choosing the right materials of construction and components suited to the level of duty required. Abrasion-resistant steel, for example, will promote a longer service life and reduced wear compared to carbon steel when working with an abrasive material.
Operators should regularly inspect the equipment, looking (or listening) for any signs of wear and addressing them as quickly as possible to avoid premature and unexpected failure. The equipment should be maintained according to original equipment manufacturer (OEM) recommendations.
Difficulty with Variable Feed Rates
Variable feed rates are a challenge in any drying operation, but they are especially problematic when it comes to fluid bed dryers. While fluid bed dryers offer an efficient drying solution, they require as much consistency in feed rate and material characteristics as possible. At a minimum, variations in such parameters will cause product quality issues; if severe enough, frequent process upsets and shutdowns are not uncommon.
Signs of Variable Feed Rates
Variable feed rates may present a myriad of issues, including changes in product depth, uneven bed height, surges in dryer load, and ultimately, variation in product moisture. When feed rate varies, product quality varies in tandem.
What Causes Variable Feed Rates?
Variable feed rate may be the result of upstream issues or a poorly designed feed rate control system.
How to Address Variable Feed Rates
Ensuring consistent loading requires addressing the root cause, resolving upstream issues as needed and ensuring the feed rate control system is properly designed and functioning.
Further, operators should be properly trained in monitoring and adjusting feed rates in response to changes in process conditions. The incorporation of an adjustable weir dam can assist operators in managing bed depth in accordance with changing conditions.
Weeping
Weeping is the industry term used to describe when solids fall through the distributor plate into the lower plenum. Allowing weeping to occur can cause a number of issues, not the least of which is the frequent need to shutdown the system and clean out the plenum. Weeping is especially problematic when working with flammable materials, as material could catch fire in the lower plenum.
Signs of Weeping
Signs of material falling through the air grid include unexplained inefficiencies, frequent process upsets, and uneven distribution of air.
What Causes Weeping?
Weeping may be the result of a few different factors, ranging from inadequate distributor plate design to poor fluidization or uneven air distribution. Even in a well-designed machine, weeping can occur during shutdown if the system is shutdown without first emptying the fluid bed. This is because during operation, the high velocity of air flow keeps material suspended. Once shutdown is initiated, the fan slows down and velocity is reduced. This can allow fine particles, if present, to fall through the air grid.
Weeping should otherwise not occur in a well-designed fluid bed. A persistent weeping problem often indicates a problem with the distributor plate design.
How to Address Weeping
A persistent weeping problem is typically addressed by redesigning the air distributor plate to have smaller holes or, if needed, tuyeres or nozzle caps.
Testing Fluid Bed Dryer Design
When the dryer design is in question, it can be challenging to pinpoint the issue, particularly in the face of demanding production schedules. This makes the testing capabilities such as those offered in the FEECO Innovation Center a critical resource to resolving the issue as quickly as possible, without affecting the live production environment.
The FEECO Innovation Center is equipped with a pilot-scale fluid bed dryer with an observation window for evaluating equipment design and potential pretreatment requirements, many of which can also be tested in the facility.
Various parameters can be monitored and adjusted during testing to fine-tune the process for optimal performance, as well as establish key process criteria.
The pilot-scale fluid bed dryer available for testing in the FEECO Innovation Center
Conclusion
Efficient fluid bed drying is essential for maintaining product quality and optimizing throughput in bulk material processing. By understanding and addressing issues such as poor fluidization, fine particle loss, and temperature control, operators can maximize equipment performance and minimize operational disruptions. Regular monitoring, material pretreatment, and precise equipment design are key to preventing these challenges. A proactive approach to maintenance and an adaptive process strategy can help fluid bed dryers achieve their full potential, ensuring consistent, high-quality drying results in any application.
FEECO fluid bed dryers are custom designed to meet the unique needs of your material. In addition to manufacturing fluid bed dryers, we can also assist in process design and optimization through the testing capabilities in our Innovation Center. For more information on our fluid bed dryers or testing, contact us today!