Fluid Bed Dryers Made Shorter

November 12, 2014

6 Min Read
Fluid Bed Dryers Made Shorter
Illustration of cross flow between air and product in a typical fluid bed dryer

By Craig Anderson, MBA, and Jarrett Bowling, Almo Process, and Michael Pfeiffer and Dr. – Ing. Mathias Trojosky, Allgaier Process Technology GmbH

With integrated heat exchangers in a fluid bed dryer, the combination of conductive heat transfer from fluidization with additional indirect heat transfer from the heat exchanger coils drastically shortens the required length of the fluid bed dryer. The reduction in the length of the fluid bed dryer can be up to 70% by utilizing this type of system. In addition to reducing the footprint of the fluidized bed dryer, efficiencies are also realized due to thermal conservation and less air necessary to fluidize the particles. Up to 80% of the required drying heat can be supplied by the integrated heat exchangers, which drastically reduces the required air. The air supply and exhaust systems for the fluid bed dryer are also proportionately reduced.

Fluidized Bed Dryers
Fluid bed dryers typically utilize cross flow between drying air and a moist product to accomplish drying. The velocity of the upward air flow must exceed the limiting effect of the product bed in the dryer in order to achieve fluidization (as shown in the illustration). This method of drying is reliant on convection to accomplish drying. The particles are immersed in the drying air by the suspension through fluidization. The height of material in the fluid bed is limited to preserve the effectiveness of the typical fluid bed. Overall, a high drying efficiency is achieved. However, bubbling of the material does occur during fluidization. Bubbling allows air to bypass the material. Air that is able to bypass the material reduces efficiency of fluidized heat transfer.

See illustration of Cross Flow between Air and Product in a Typical Fluid Bed Dryer

    Very fine-grained bulk solids (<200 microns) with high surface moisture can be difficult to fluidize uniformly by only the air flow. Moisture is a considerable cohesive element between individual particles, especially fine particles. When the moisture is able to act as a bond between fine particles, the air has a tendency to establish channels in the product layer. The drying air will pass through only these establish channels without the desired effect of particle fluidization. This occurrence is often indicated by crater formation in the product bed. This condition is typically addressed and mitigated by the addition of vibration to the fluid bed dryer. However, further improvement to the process can be achieved by the addition of a supplemental heat source.
    The addition of heating surfaces to the fluidized product provides increased efficiency to the process. The supplementation of the convective drying air flow from fluidization with indirect heat from the heat exchanger coils provides an additional source for heat transfer. During operation, these heat exchangers will be immersed in the product bed. Up to 80% of the heat requirement can be provided by indirect heat surfaces. There is a reduction in the required air supply necessary to support the process as compared to entirely fluidized drying. The main purpose of the air supply for a typical fluid bed dryer is simply to cause fluidization to optimize heat transfer. Since a significant amount of the heat transfer can be accomplished by the heat exchangers, the necessary air flow can be reduced accordingly. This reduces the minimum air requirement necessary to remove moisture from the product. The air supply and exhaust air equipment for the process will also be reduced proportionately. The result is a smaller installation that is less expensive in terms of capital and more efficient in terms of operation.

See illustration of Integrated Heat Exchangers Supplmenting Heat Transfer

    The combination of convective heat transfer with the heat radiated from the heat exchanger coils improves the moisture capacity of the drying air. This results in high water loading in the exhaust air. The additional heat, through the internal heat exchanger, causes very high levels of water evaporation from a smaller quantity of air. Consequently, the exhaust water load is particularly high with risk of dew point in the exhaust pipe. This risk is mitigated by diverting a small amount of hot air from the dryer inlet to the exhaust duct. The temperature of the exhaust air stream is slightly increased and the dew point is decreased.

See illustration of Fluidized Bed Dryer with Integrated Heat Exchangers

    It is necessary to increase the elevation of an installation to accommodate the heat exchangers inside the fluid bed dryer. The overall height of the fluid bed would be somewhat increased while the overall length of the fluid bed would be significantly reduced. The horizontally stacked exchanger coils offer significantly more opportunity for the transmission of heat. The increased height of the fluid bed is paired by increased depth of the product beds inside the dryer. Product beds can be effectively dried at a depth of 1-2 m. Typically, deep product beds are avoided in fluid bed dryers. This is due to the bubbling effect that reduces drying efficiency in fluid bed dryers and is precipitated by deep product beds. However, the horizontally stacked coils of the heat exchangers immersed in the product bed hinder bubble growth and formation. The presence of the heat exchanger coils throughout the product bed transfer heat uniformly through the material. The maximum possible heat transfer is highly dependent on the grain size of the material. Higher thermal transmission can be accomplished with finer particles in the product bed.
    The advantage of utilizing integrated heat changers can be expressed by the Mollier Diagram. Line 1 on the graph indicates a typical fluid bed dryer where heat transfer is supplied by only air. In this case, only a higher temperature is able to increase the moisture capacity of the air. Increasing temperature would be the only method to reduce the amount of required drying air.

See Mollier Diagram Comparison of Fluidized Bed Dryer and Fluidized Bed Dryer with Integrated Heat Exchangers

    Curve 2 on the graph indicates the effect of the additional heat surfaces provided by integrated heat exchangers. As previously noted, it is necessary to provide some heat to the exhaust air due to the high water loading of the air. The higher exhaust air and product temperature is illustrated in the graph by ΔT. The water loading in the dryer exhaust air, illustrated by ΔX, is significantly increased compared to a typical fluid bed dryer application. This directly decreases the required length of the fluid bed dryer, the amount of necessary drying air, and the size of the air supply/exhaust systems. The economical impact is always very large but most evident for products that require low temperature for drying. Temperature sensitive products have the most substantial advantage for integrated heat exchangers installed into fluid bed dryers.

See Installation of Heat Exchanger Coils into a Fluid Bed Dryer: Allgaier Type WS-HF-T-K-5.00

Application Example
•    40 metric tn/hr of potato granules
•    7 in bed immersed heat exchanger bundles
•    Feed moisture of 18%
•    Residual moisture of <8%
•    Final cooling to <50°C
•    Allgaier Installation of WS-HF-T-K

See image of Allgaier Installation of WS-HF-T-K

    The Allgaier-Group has more than 20 years of experience with this technology. This type of technology has been typically applied to applications that require large quantities of moisture to be removed. In particular, crystaline products (such as sodium chloride) and food products have experienced the greatest benefit from this technology. This technology can also be considered for cooling applications. Instead of steam supplied to the heat exchangers, cool water could be utilized for a cooling application. For more information, contact ALMO Process at 513-453-6990 or visit www.almoprocess.com.

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