5 Considerations When Recycling Air and Material in Fluid-Bed Drying
Does recycling the exhaust air makes sense for your process?
It is no coincidence that the energy-cost-reducing technology used to recover and reuse heated process air in fluid-bed drying systems was developed during the energy crisis of the 1970s. As the cost of energy sharply increased, so did the cost to run and operate a fluid-bed dryer. Capturing and reusing as much heat as possible — to take full advantage of every unit of energy — became a logical goal to offset rising costs.
Saving energy also advances worthy environmental goals. It allows executives to position their companies as sustainable organizations that will be around far into the future. If the environmental impact is the only criteria at your company, then the decision to recycle is easy. If there may be a potential cost savings to realize, then consider the following key points to determine if recycling the exhaust air makes sense for your process.
1. Product Drying Temperature
In a fluid-bed dryer, heated air is directed upward through an air-distribution deck, where it mixes with the material to be dried and suspends it in the airstream. The fluidized particles behave like a liquid, allowing the air to surround each individual particle with the continuous, intimate contact needed for highly efficient energy transfer. Because this energy transfer does not rely on direct contact with heated surfaces, the drying occurs gently and without product degradation. This is why the process often is specified for a wide range of products from delicate, frangible foods to coarse mineral products.
Although a fluid-bed dryer may be right for your product, it may not make sense to invest in recovering the air for your process.
Materials such as salt and sand offer high temperature limits and allow the use of high process air temperatures — upwards of 500°F. Given that the exhaust air generated from such a process may approach 250°F, capturing and returning this heated air into the process reduces the burden on the air heaters. It is far less costly to raise 250°F air to 500°F (121 to 260°C) than it is to raise ambient air all the way up to 500°F.
Those delicate, frangible cereals, seeds and some plastics, by contrast, burn or melt even at low temperatures. Given that these products typically require processing at temperatures from 100 to 200°F (37 to 93°C), the spread between the process air temperature and the exhaust air temperature may not be wide enough to warrant the investment in the heat recovery equipment. Further, the cost of heating the ambient air to the process air temperature — relative to the cost to heat the exhaust air up to the process air temperature — may be considered insignificant, depending on the cost of energy and size of the equipment.
2. Product Drying Characteristics
Because vibrating fluid-bed dryers accommodate a range of products with particle sizes ranging from 1” down to 25 microns, there can be significant differences in the rate at which drying occurs, and in how efficiently it occurs. These differences may be due to particle size or shape, or to the inherent properties of the material.
For most materials, the smaller the particle size, the more surface area available for direct contact with the drying air. Large, heavy particles require high air velocities and have less surface area to come in contact with the air. Subsequently, this reduces the energy transfer efficiency. Some materials such as vegetables have a membrane-like outer surface that prevents internal moisture from escaping as readily — again, reducing the drying efficiency. By contrast, some porous materials such as carbon black release their internal moisture easily. Because materials that dry less efficiently leave more heat in the airstream, their drying systems often make fine candidates for air recirculation.
3. Process Requirements
In many applications, the fluid-bed dryer also is used for a second or third purpose beyond drying — for instance, dedusting, cooling or screening. Sometimes, products need to remain heated for a specific amount of time for heat treatment or crystallization. In these cases, the exhaust air temperature will be significantly higher than the exhaust air temperature of a typical drying application.
With this high heat available for capture, these cases present a perfect fit for air recirculation. In systems where the material is cooled directly after drying, both the heat energy originally generated for the drying zone, and the dry, exhaust air from the cooling zone, may be recycled. These special process requirements need to be evaluated when considering recycling fluid bed process air.
4. Cleaning the Exhaust Air
To return any of the exhaust air to the process, entrained particles first must be removed from the airstream. Recycling dirty air contaminated with entrained particles will eventually cause blockages in the heat transfer coils or gas furnace. It also can cause product to accumulate in the underside of the fluid bed.
Achieving this clean air typically demands the air be passed first through a filtering stage. In some cases, a cyclone dust collector may be used to remove the particulates. Most frequently, a reverse-pulse baghouse filter is used for its greater dust-collection capacity. It is a wise idea to put some type of monitoring device on this filter to alert operators of a possible filtration failure. The cleaned, heated air then is directed back to the supply-side equipment, where it is heated to the final processing temperature.
As a side benefit, recirculating air in this fashion also cuts down on the amount of exhaust air being released to the atmosphere. This can be important when applying for air permits under environmental regulations.
5. How Much Air to Recycle
When designing an air-recycled fluid bed, determining exactly how much of the total supplied air to recycle is critical. To the uninitiated, 100 percent may be desired. But drying a product releases water vapor into the exhaust airstream. It is not wise to recycle this water vapor back to the product, which would defeat the original purpose. Most commonly, 50 percent of the supplied air is recycled. This allows the air to be recycled once before it is exhausted to the atmosphere.
Typically, the dryer discharge-end air will be recycled back to the inlet end before being exhausted to the atmosphere. The scenario that would result in the most heat energy recovered at the highest operating efficiency involves capturing heat energy from the first drying zone exhaust air by employing a heat exchanger before the air is exhausted. This captured energy then can be used to preheat the air going to the second zone of the fluid bed.
In conclusion, the 1970s sounded quite the alarm for manufacturers running equipment as if energy was freely available, in nearly infinite supply, at a consistently low cost. The era of unfettered energy use was over, and in its place rose the beginning of the environmental movement. Energy efficiency, conservation, recycling and saving on energy costs became the new normal.
While a combination of regulation and innovation contributed to the clearer skies of today, the public outcry has continued to intensify. Manufacturers today face a maze of federal, state and local rules and regulations. Failure to comply can carry penalties that cost far more than the cost of the equipment needed to capture and recover process air. Many of today’s business leaders understand the basic sensibilities involved in reusing the process air. A green solution that reduces waste and conserves energy, recycled air also more often than not translates directly into tangible financial returns.
Calculating Returns with Fluid-Bed Dryers
Whether heating the process air using natural gas, electricity or steam, this cost is typically the largest cost in any drying system. And while the warm air flows inside the system, it is an asset. But once exhausted, it is gone forever. Therefore, determining the ideal drying system for the application and whether to recycle requires a significant commitment of time, expertise and an extensive series of calculations.
Educated assumptions regarding energy costs and ambient temperatures are made to arrive at an approximate amount of operating costs on a yearly basis. Often, two or three potential systems are compared by looking at the upfront cost relative to projected cost savings over a span of two to five years of operation. Slightly higher maintenance costs are also addressed in systems that recycle due to the need for dust-collection equipment required for the exhaust air.
Consider this sample process where a fluid-bed dryer requires 10,000 scfm of air heated to 500°F (260°C).
This system would require $33 of natural gas per hour, assuming an ambient temperature of 65°F (18°C) and a natural gas cost of $7 per unit. By recycling 50 percent of the air at a recycled air temperature of 150°F, the natural gas cost may be reduced to $29.80 per hour. This savings equates to a potential savings of $23,200 per year in gas alone. Given this type of drying system would likely require a baghouse dust collector at a cost of an additional $35,000 for the dust collector and ductwork required to recycle the air effectively, the recycling system would pay for itself in energy savings after two years of operation.