Improving Drying Efficiency in Fluid Bed Drying
As featured in Process Heating
By Larry Stoma
Fluid bed dryers offer an inherently efficient method of moisture removal. That has not changed since Witte developed the systems in the 1950’s. But in the last 40 years, the introduction of PC/PLC control systems, smart sensors and other technological advances 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 drying efficiency of their systems.
Fluid bed drying offers highly efficient heat transfer because the particle being dried is 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 air flow needs to be uniform throughout the entire area of the fluid bed. A uniform air flow optimizes the area required for drying.
Dryer Conveying Surface
To introduce this fluidizing air flow, many dryers manufactured today use the same type of drilled hole deck conveying surface that has been used for decades. This approach features a repeating pattern of drilled holes in a fabricated metal sheet. The heated air is directed upwards through the holes and the material is conveyed over this deck from the infeed to discharge. Though inexpensive, it comes with several drawbacks. For example, the diameter of the holes needs to be small to prevent the particles from falling through and/or clogging the holes. But maintaining the static pressure needed to provide the high jet velocity that keeps the particles suspended in the air can be difficult. This type of deck can create hot spots with excessive heat that can cause charring, and dead spaces lacking in heat that cannot adequately dry the product, leading to stalled material, product layering and inefficiencies resulting in off-spec product. They also invite clogging, which further contributes to hotter and cooler areas and requires manual unclogging to resume the airflow.
Another approach that better supports process efficiency goals features 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 length of the dryer. With these wires spaced 3/16” apart, this concept provides a high jet velocity of air running the entire length of the dryer. This high, vertical jet of air flowing at a 90 degree angle perpendicular to the deck creates even air distribution for the entire drying area and allows particles as small as one micron in size 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 can offer. 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.
The most common source of inefficiency in a process is often human error. Whether by manually modifying the dryer or by changing the process conditions, technological improvements are driving these inefficiencies out of the process. Today, wireless sensors and transducers relay process air temperature and product temperature – both in and out – to the operator and verify product moisture levels both before it enters the dryer infeed and as it exits the discharge. This allows process variables such as retention time and process air temperature to be quickly adjusted via a centralized PC/PLC. These systems ensure accurate, repeatable control over the air temperature and volume, start-up and shutdown sequence and other parameters, and allow precise adjustments to be made via a touch-screen control panel. They keep a history of changes and 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 24/7 for proper operation and 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 instantly sent to quality control, production, manufacturing, and other departments to rectify the situation before major product losses or line downtime may be 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 the dryer’s design, the equipment upstream needs to 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 discharge product that fails to meet moisture requirements. Conversely, a shortage of product entering the dryer infeed may result in over-drying. For example, if the dryer is designed to efficiently reduce the moisture content from 50 percent at the infeed to 10 percent at discharge and the product at the infeed actually contains 75 percent moisture because the dewaterer isn’t operating as intended then the dryer will not be able to meet the moisture requirements. One way to improve drying efficiency when faced with fluctuating process conditions is to adjust the time allotted to the material in the drying zone, called retention time. The longer the retention time, the more moisture removal that can occur. This can be done by adding pneumatically operated radius gates called autoweirs. These automated gates rotate 60 degrees upwards and above the decks on a cycle timer to periodically slow, block or accelerate the rate of product advance towards discharge. These adjustments may be made in real-time to quickly accommodate changes upstream without compromising drying efficiency.
The primary factor in determining whether the process air is heated by natural gas, propane, steam or electricity is often availability. Steam offers the least costly installation costs, given the facility already has a steam plant in place. 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 1,000,000 Btu’s/hr. but becomes the cost-efficient option in less energy-intensive applications such as drying seeds, cereals, some plastics and other delicate products at temperatures under 150 degrees F. For slurries and products with a high moisture content that require drying temperatures above 300 degrees F such as sand and salt, heating the air with natural gas may improve cost-efficiency The cost-efficiency of steam typically peaks when heating the air up to 300 degrees 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 are using an environmentally friendly alternative that captures 50 percent of the heated process air, cleans it via a dust collector, and returns it to the process. By raising this 250 degree air, for example, to 500 degrees, rather than constantly raising ambient air of approximately 65 degrees to 500 degrees, the burden on the air heaters is reduced and energy-efficiency improved dramatically. 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.
Culture of Efficiency
Implementing these improvements in fluid bed drying efficiency often reduces or eliminates waste, converts more material to profitable, saleable product, and yields direct savings in costs. As the tangible returns become visible and understood, these improvements at the dryer help promote a culture of efficiency that we’ve seen become applied to the entire processing line for even greater returns.
Larry Stoma is a senior sales and design engineer for vibrating fluid bed dryer/cooler manufacturer The Witte Co., Washington, NJ. For more information, call 908-689-6500 or see www.Witte.com.