Improving Efficiency in Fluid-Bed Drying
Dryer design changes can help boost thermal, energy and operational efficiencies.
Fluid-bed dryers offer an inherently efficient method of moisture removal that has not changed since the systems were developed in the 1950s. The introduction of ever-improving control technologies — personal computer-based and PLC control systems, smart sensors and other technological advances — in the last 40 years, however, has changed how process engineers operate their fluid-bed drying systems. With access to accurate, real-time data, and the ability to adjust the dryer on the fly with precision, process engineers have been able to optimize the process and improve the system efficiency.
Fluid-bed drying offers highly efficient heat transfer because the particles being dried are suspended in the heated process air for intimate contact with the entire surface area. Adding vibration provides a gentle force that helps liberate and allow the product to fluidize. But, to fully capitalize on these capabilities, the airflow needs to be uniform throughout the entire area of the fluid bed. A uniform airflow optimizes the area required for drying.
Dryer Conveying Surface
To introduce the fluidizing airflow, many dryer manufacturers use the same type of drilled-hole deck conveying surface — a design that has been used for decades. This approach has a repeating pattern of drilled holes in a fabricated metal sheet. The heated air is directed upward through the holes, and the material is conveyed over the deck from the infeed to discharge.
Though inexpensive, the drilled-hole deck conveying surface has drawbacks. For example, the diameter of the holes needs to be small to prevent the particles from falling through or clogging the holes. This type of deck also can create hot spots and dead spaces. The hot spots have excessive heat that can cause product charring while the dead spaces lack heat and cannot adequately dry the product. The dead spaces also can lead to stalled material, product layering and inefficiencies, all resulting in off-specifications product. In addition, dead spaces invite clogging, which further contributes to hotter and cooler areas and requires manual unclogging to resume the airflow. Finally, the drilled-hole deck conveying surfaces can make it difficult to maintain the static pressure needed to provide the high jet velocity that keeps the particles suspended in the air.
In a fluid-bed dryer, a drilled-hole deck has a repeating pattern of drilled holes in a fabricated metal sheet. The heated air is directed upward through the holes, and the material is conveyed over the deck from the infeed to discharge. Dryer decks with drilled holes may have trouble maintaining the static pressure required to keep the particles suspended in the airflow.
Another fluid-bed deck approach that supports process efficiency uses a wedge-wire deck design. Instead of using round holes, this design sets the metal decking in narrow slots with a triangular, tapered profile running the entire dryer length. With these wires spaced 0.1875” apart, this concept provides a high velocity jet of air running the entire length of the dryer.
The high, vertical jet of air flowing at a 90° angle perpendicular to the deck creates even air distribution for the entire drying area. Such an airflow and deck design allows particles as small as one micron to be processed without concern for clogged holes. In terms of air volume, the wedge-wire deck provides more open area for airflow than decks drilled with holes. This improvement in airflow efficiency also allows lower airflow volumes and velocities to be used for cost savings in energy and a reduced burden on the fan system.
Often, the most common source of inefficiency in a process is human error. Fortunately, whether by modifying the dryer design or by changing the process conditions, technological improvements are driving these inefficiencies out of the process.
Modern wireless sensors and transducers relay process air temperature and product temperature data — both entering and leaving the dryer — to the operator. In addition, such controls are used to verify product moisture levels before the product enters the dryer infeed and as it exits the discharge. This level of insight allows process variables such as retention time and process air temperature to be adjusted quickly via a centralized personal computer or PLC. Such control systems ensure accurate, repeatable control over:
Touchscreen controls on drying systems help eliminate the potential for human error. Personal computer or PLC-based control systems help ensure accurate, repeatable process management.
- The air temperature and volume.
- Startup and shutdown sequence as well as other parameters.
Computer or PLC-based controls also:
- Allow precise adjustments to be made via a touchscreen control panel.
- Log a history of any changes made.
- Help eliminate the potential for human error, or for human involvement that may run counter to the dryer’s design.
These systems automatically monitor the process continuously for proper operation, and they are constantly verifying the system is running at peak efficiency. In the event the system detects an unplanned change in the process or the product, an alarm may be sent instantly to quality control, production, manufacturing and other departments to rectify the situation before major product losses or line downtime is incurred.
Uniformity of Feed
The dryer itself may be optimized for efficiency based on a given set of process parameters, but what if the process parameters change without notice?
To fully capitalize on a dryer’s design, the equipment upstream must deliver a uniform product to the dryer infeed at a uniform feed rate. A surge in the feed rate would overload the dryer’s capacity for moisture removal and lead to discharging product that fails to meet the moisture requirements. Conversely, a shortage of product entering the dryer infeed may result in overdrying.
For example, suppose a dryer is designed to efficiently reduce the moisture content of a product from 50 percent at the infeed to 10 percent at discharge. Due to a change in the upstream conditions, suppose the product at the infeed actually contains 75 percent moisture. (As an example, suppose the dewaterer is not operating as intended.) In such a case, the dryer will not be able to meet the moisture requirements.
When faced with fluctuating process conditions, one way to improve drying efficiency is to adjust the time allotted to the material in the drying zone — a variable called retention time. The longer the retention time, the more moisture removal that can occur. For example, the retention time can be adjusted is by adding pneumatically operated radius gates called autoweirs. These automated gates rotate 60° upward and above the decks on a cycle timer to periodically slow, block or accelerate the rate of product advance toward discharge. These adjustments can be made in real time to quickly accommodate changes upstream conditions or equipment without compromising drying efficiency.
Wedge-wire decks use narrow slots with a triangular, tapered profile running the entire length of the dryer. A high velocity jet of air flows perpendicular to the deck for even air distribution.
The primary factor in determining whether the process air is heated by natural gas, propane, steam or electricity often is availability. If the facility already has a steam plant in place, steam offers the lowest installation costs. Natural gas offers the least costly operation followed closely by propane. Electricity, with its electric coils and controls and relatively high ongoing costs, is typically cost-prohibitive for applications requiring more than 1 million BTU/hr; however, electricity is a cost-efficient option in less energy-intensive applications such as drying seeds, cereals, some plastics and other delicate products at temperatures below 150°F (66°C). For slurries and products with a high moisture content such as sand and salt — which require drying temperatures above 300°F (149°C) — heating the air with natural gas may improve cost efficiency. The cost efficiency of steam typically peaks when heating the air up to 300°F.
Once the air has been raised to several hundred degrees, it becomes an asset. Rather than allow it to escape the process as exhaust, clever process engineers employing heat recovery to put it to work elsewhere in the process. One method captures 50 percent of the heated process air, cleans it via a dust collector and returns it to the process. For instance, by raising the 250°F air to 500°F (121°C air to 260°C), rather than constantly raising ambient (65°F (18°C]) air to 500°F, the burden on the air heaters is reduced and energy efficiency is improved. Further, the volume of air exhausted to the atmosphere may be cut in half, along with any particulates, to aid in compliance with the many environmental requirements.
In conclusion, implementing improvements in fluid-bed drying technology can improve efficiency, help reduce or eliminate waste, convert more material to saleable product, and yield direct savings in costs. As the tangible returns become visible and understood, these improvements at the dryer help promote a culture of efficiency that can be applied to the entire processing line — for even greater returns.