How fluid bed technology works for cooling, heating and drying bulk solids

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Fluid bed technology is used for cooling and heating bulk solids materials. Heat transfer within the fluid bed takes place by passing a gas (most commonly air) through a perforated distributor plate which then flows through a layer (bed) of solids.

The air performs two functionsL

  • First, the air flows through the bed of solids at a velocity sufficient to support the weight of the particles, which creates a fluidized state enabling the particles to flow.
  • Second, the air in the fluid bed serves to cool, heat or dry the particles as it comes into direct contact with the solids material within the fluid bed chamber.

 

How do fluid bed compare to plate-based moving bed heat exchange technology?

The moving bed heat exchange technology that Solex specializes in offers a highly efficient alternative to fluid beds. These plate-based heat exchangers work by heating, cooling or drying bulk solids indirectly by pumping the heat transfer fluid (e.g., water) through a vertical bank of hollow stainless steel plates while the bulk solid passes between the plates at a rate sufficient to achieve the required cooling. No air is used in the cooling process.

This offers several significant advantages compared to fluid beds:

1. Energy requirements

The premise behind fluid beds for cooling and heating is that large volumes of air are used to both fluidize the material ⁠— required to enable the product to flow ⁠— and to act as the heat transfer medium in order to add or removing heat from the process.

This ambient air is taken in using large fans. And, in most climates, the air must be chilled or treated before being blown across the product using large horsepower fans. The air leaving the fluid bed is then discharged through an emissions stack. Both the chilling process and the circulating fans have high energy requirements.

The challenge with using air to directly cool bulk solids is the large quantity of air required by fluid beds and the expense involved in processing and cleaning that air. Below is an energy comparison between a fluid bed and the plate-based heat exchangers used by Solex

2. Emissions

Fluid beds produce high amounts of emissions. Using air to cool bulk solids is a “once-through” proposition. Air is taken in, chilled and passed across the product. It then must then be disposed of through a stack. The large quantity of air required for this type of direct cooling results in a large quantity of dust and emissions. Permits for stacks are becoming more difficult to acquire, and, with ever-tighter pollution controls, emissions must be cleaned and scrubbed before being dumped into the atmosphere. Associated costs are high.

With Solex indirect cooling, the water is re-used repeatedly by being recycled in a closed-loop system. The cooling media does not come into direct contact with the product, so no dust or emissions are created. This eliminates the need for pollution control equipment and makes tight emission limits easier to meet.

3. Product quality and energy

In fluid beds, since ambient air comes into direct contact with the product as the cooling medium, the temperature and saturation point of the air must both be considered to avoid process issues. In many climates, the temperature of the ambient air is higher than that required to achieve cooling, which means the air must be cooled prior to use.

Another issue is the saturation point of the cooling air. If this is not addressed, moisture can migrate from the air to the product which results in agglomeration and subsequent spoilage.

To ensure saturated air does not come into contact with the product, air must be cooled below the required temperature to condense the water out and then reheated to the optimum cooling temperature. The chilling and reheating ambient air consumes significant amounts of energy. The process of condensing the water out of the air to enable it to be used in the fluid bed requires three times the energy as using chilled water in a plate-based heat exchanger.

This entry was last updated on 2023-1-19

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