Fluid-bed drying in action

To understand the fundamentals of fluidization, consider a quantity of spherical particles at rest in a container. If a gas is forced upward at low velocity through the particles, and at the same time, distributed across the entire cross section of the container, there is some measurable pressure drop in the gas, but no movement of the particles.

As the gas flow is raised, the pressure drop increases until it overcomes the weight of the particles. At this point, the particles begin to move slightly in the direction of the gas flow, thus expanding the bed. Theoretically, the bed can be stabilized with all of the particles suspended in the gas stream and separated from one another. If the velocity of the gas is further increased, movement or agitation in the bed increases, but the pressure drop across the fluidized bed remains constant [5].

This fluidized state can be maintained over a fairly broad range of velocities. At the higher end of the velocity range, rising gas bubbles project a number of particles out of the bed, even if the superficial velocity above the bed is not sufficient to actually transport the particles. This condition is referred to as "spouting." Smaller particles or particles with densities considerably below the average value may continue to be carried in the gas stream exiting the bed. These particles are said to have "elutriated."

If the velocity through the bed is increased to the point where all of the particles are carried by the gas stream, the pressure drop begins to increase and the system is essentially in pneumatic transport. The entire bed is lost. In the fluidized state, the solids tend to behave very much like a boiling liquid. If solids are added to the fluid bed at one point, they distribute across the entire bed, instead of piling up. Fluidized solids flow through weirs, and pour from any opening below the top layer. As a result, fluid beds do not require any slope or mechanical device to move solids from inlet to outlet.

The main advantages of fluidized-bed drying systems are that they permit relatively long residence times and achieve very high heat-transfer coefficients between the particles and the gas. In addition, very close control of product temperature is possible, allowing the processing of temperature-sensitive solids. Furthermore, fluid-bed dryers often operate with exhaust-gas temperatures very close to the dew point of the gas, giving the highest thermal efficiency of any gas-suspension drying system.

On the down side, the requirement to achieve and maintain fluidization limits the range of materials that can be handled in a fluidized bed. The particles should generally be in the size range of 50 mm to 5 mm and of a size distribution not broader than one order of magnitude, that is, 100 mm to 1 mm. They should be reasonably regular in shape and not particularly sticky.

Needle- or platelet-shaped particles are often difficult to fluidize. In many cases, however, all of the above problems can be overcome by agitating or vibrating the fluidized bed.

Material flow schemes

The drying that takes place in the bed is primarily zero-order drying and the product exiting the bed tends to be of very constant moisture. Such a bed can be very efficient in that the gas at the outlet is usually very close to its saturation point. It should be noted that a nonfluidizable feed must be somehow mixed into the bed. Otherwise, the wet feed may consolidate at the feed location, sink into the bed, plug off the fluidizing gas at that point and eventually shut down the system.

Fluid beds can be either cylindrical or rectangular, with the rectangular shape representing the most efficient use of space (Figure 6). Choice of a cylindrical vessel is most often dictated by structural considerations.

Figure 6: transport of solids through the fluid bed may be achieved either by fluidization alone or a combination of fluidization and vibration

When first-order drying is required to remove bound moisture, plug-flow dryers are required. In plug flow, the material moves through a relatively narrow channel, resulting in essentially identical residence time for each particle. This is accomplished in a long and narrow vessel of rectangular or cylindrical shape divided by baffles to create the plug flow. Plug-flow dryers can consistently achieve a very low moisture content in the final product.

It is quite common to use a back-mixed fluid bed to handle the wet feed and reduce the moisture to a nominal level and then follow with a plug-flow drying arrangement to meet the final-product moisture specification. As deeper fluidized layers are employed, the tendency of the solids to backmix even within a narrow channel increases [6]. Consequently, it may not be possible to come up with a simple design for plug flow. In these cases, a tanks-in-series configuration is used to approximate plug flow.

Process design

Before attempting the design of a fluidized-bed drying system, the feed and product characteristics must be established. The engineer must, by experience or lab tests, determine the fluidizing velocity of the powder being dried.

The design of a fluid bed is complicated because the mass flow of drying gas required to evaporate the desired amount of moisture must pass through the bed at the predetermined fluidizing velocity. In cases where the temperature difference from inlet to outlet is small, and product particle size and bulk density are low, this requirement for both mass flow and fluidizing velocity can lead to oversized fluid beds.

One method commonly employed to overcome this difficulty is to immerse a heat-transfer surface in the fluidized layer. The heat-transfer surface usually takes the form of tube bundles or panels heated by hot water, steam or a heat-transfer fluid. Arrangement, spacing and method of installing such heating banks withing a fluid bed vary, but in all cases, they must be submerged in the layer and must not impede the flow of the material through the bed.

Process control

Control of a fluid bed is accomplished by continuous measurement of the inlet temperature, outlet temperature and temperatures of various zones within the bed. Normally, airflow is held constant to provide adequate fluidization, and the pressure drop across the bed is maintained by adjusting the feed rate. The product discharge rate is a function of the feed rate.

With fairly constant solids throughput and constant airflow, the only way to adjust outlet temperature is by adjusting inlet temperature. Because fluid beds exhibit essentially crossflow contact of solids with gas, even the last particles to leave the dryer are in contact with gas heated to the inlet condition. This results in fairly fast response in spite of a very long residence time (typically ranging from 5 min to 1 h) for the solids.

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