Considerations For the Selection of a Vibratory Fluid Bed DryerConsiderations For the Selection of a Vibratory Fluid Bed Dryer

Selecting drying equipment for your application is a major undertaking. Not only will you be investing a significant amount in the initial installation and operation of the system, but you’ll be also spending a great deal of time researching your options and testing potential methods. A decision you make now will have long-term consequences in terms of maintenance cost, production rates, and product quality. The following discussion of drying principles and vibratory fluid bed processing provides the background information you’ll need to select the most appropriate vibratory fluid bed dryer.

Understanding Drying

An understanding of any drying system is based on the fundamentals of drying technology. These may be divided into three categories:

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Understanding Fluid Bed Processing

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Fluid bed processing has a number of applications. The most common process applications are drying and cooling, agglomeration, and air classification. To understand more about fluid bed systems, you should first be familiar with their operation.

How Fluid Bed Processing Works

Fluid bed processing passes a gas directly through a bed of solid material via a perforated plate, nozzles, or ceramic grid, lifting, and mixing the solids. As the velocity of the air increases, so does the pressure drop across the bed until at a certain flow rate, the frictional drag on the particles equals the material bed’s effective weight. At this fluidizing velocity, the material bed attains fluid-like properties and expands beyond the size of the stationary bed. When the air velocity in the free cross section is further increased, the bed expands until particles are carried over by the air. The velocity at which the particles are carried over is called entrainment velocity. Since the mass airflow rate for thermal requirements is usually less than that required to attain fluidization velocity, operation at minimum velocity is desired.

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Fluidization provides maximum exposure of the particles to the process air, reducing fuel costs and improving operating efficiencies. A fluid bed system also has few moving parts, which increases its efficiency in comparison to other systems. For example, the large drums in rotary dryers and the moving trays in tray dryers require frequent maintenance. A fluid bed system’s continuous operation and low maintenance also reduce the amount of operator attention required.

Fluid Bed Vaporization Stages

In fluid bed drying applications, vaporization takes place in two stages:

Fihure 1: Moisture content (dry basis) vs drying time (Carrier Process Equipment Group)

Figure 1 shows an example of how drying test information is generally presented. The figure illustrates a typical drying operation in which there is both surface and bound (internal) moisture. The drying process begins at Point A or A¹ and proceeds to a dry condition at Point E. Segment A-B represents an adjustment period in which the particles and surface liquid may absorb or give up heat, with the drying rate increasing or decreasing depending on the beginning conditions. Segment B-C is the constant rate portion in which surface moisture is evaporated to the surrounding gas until dry spots begin to form on the material surface. Segment C-D starts the falling rate drying period, where the remaining surface moisture is removed, and internal drying begins. Segment D-E is the final drying stage, when the surface is completely dry and bound moisture is being pulled from the interior of the particles.

The falling rate portion of the curve is very important for sizing drying equipment. Less heat is used for evaporation during this cycle, which tends to increase product temperature because of the addition of sensible heat. Thus, drying is more difficult during the later stages of the process.

Understanding Vibratory Fluid Bed Processing

Adding mechanical vibrations to a fluid bed system allows air to pass through the material bed at rates that are below the fluidization velocity, yet still maintain a bed’s fluid properties. Vibratory systems are ideal for processing materials that are difficult to fluidize because of size, shape, or weight.

Understanding Heat Transfer Basics

The following definitions are important for understanding the heat transfer process:

Temperature Scales

To convert between Centigrade and Fahrenheit:

^tF= 9/5 ^tC + 32 where ^tF = Degrees Fahrenheit

^tC = 5/9 ^tF – 32 where ^tC = Degrees Centigrade

To obtain absolute temperature:

^tR = ^tF + 460 where ^tR = Degrees Rankin

^tK = ^tC + 270 where ^tK = Degrees Kelvin

Heat Units

British thermal unit (BTU) is the quantity of heat required to raise the temperature of 1 lb of water by 1˚F. A gram calorie is the quantity of heat required to raise the temperature of 1 g of water by 1˚C. One BTU equals 252-g calories.

Specific Heat

The number of BTUs required to raise the temperature by 1˚F

C = dq/dt(BTU/pounds °F) where C = Specific heat, and dq/dt = Change in heat content with respect to temperature for 1 lb of material.

Since specific heat varies with temperature, it is sometimes necessary to obtain the equation for the specific heat of the material and integrate it to determine the quantity of heat required for a particular temperature change. However, many materials have specific heat values that remain essentially constant over a wide temperature range.

Thermal Conductivity

The rate of heat transferred through a unit of cross-sectional area, for a unit thickness per degree temperature difference between the points to be measured:

q = -kA       dt/dx  (BTU/hour) where q = Heat Transfer rate, k = Thermal conductivity (BTU inch/hour square feet °F), A = Cross-sectional area in square feet, and dt/dx = Change in temperature with respect to thickness (°F/inch)

Thermal conductivity also varies with temperature. However, many materials have conductivity properties that will remain essentially constant over a wide temperature range

Evaporation

The conversion of liquid to vapor that occurs only at the surface. Evaporation goes on at all temperatures and continues until the liquid disappears or until the space above the liquid surface has become saturated with the vapor. Evaporation requires the transfer of heat from the gas to the liquid particles leaving the surface. This quantity is known as the heat of vaporization, and its value depends on the temperature of the liquid and the pressure exerted on the surface of the liquid by the gas/vapor mixture.

Boiling

A three-dimensional process occurs within the liquid volume to convert liquid to vapor. Boiling occurs only when the vapor pressure exerted within the liquid is greater than or equal to the vapor pressure exerted on the surface of the liquid by the surrounding gas/vapor mixture.

Transition

The quantity of heat required to effect a molecular change within a particle without a change of state. Transitions usually occur at constant particle temperature, that is, the particle remains at a fixed temperature until the transition is complete, after which the particle temperature will continue to rise or fall. The heat of transition may liberate or absorb heat.

Heat Load Calculations

Sensible heat is the heat that results in a change in temperature. Assuming that the specific heat of the material is essentially constant over the temperature range:

h_s  = w_m x c_m (t₁ - t₂) where h_s  = Heat Transfer rate (BTU/h), w_m = Material rate (lb/h), c_m = Material specific heat (BTU/°F), t₁ = Input material temperature (°F), and t₂ = Output material temperature (°F)

Latent heat is the heat transferred to a liquid that is present in the input load. This load includes the heat of vaporization, sensible heat load for the liquid, and any other loads resulting from the presence of the liquid. For a liquid/evaporation process:

h₁ = w₁ [c₁(t₁ - t_fg) + h_fg] where h₁ = Heat transfer rate (BTU/h), w₁ = Liquid input rate (1b/h), c₁ = Liquid specific heat (BTU/1b - °F), t₁ = Input liquid temperature (°F), t_fg = Saturation temperature of the gas (°F), and h_fg = Heat of Vaporization t_fg

Transition heat is the heat load resulting from a molecular change:

h_t = w_m (L) where h₁ = Heat transfer rate (BTU/h), w_m = Material rate (1b/h), and L = Heat of transition (BTU/1b)

Fluidization by air velocity alone tends to stratify material by particle size, which makes the process ineffective as particle size increases. The gentle vibrating action of vibratory fluid bed units not only conveys material along the process route but helps to mix and turn the material bed to ensure the bed’s maximum temperature uniformity. In drying applications, the end product’s quality is consistent because of the lack of hot spots or wet spots in vibratory units. Vibration also makes fluid bed drying and cooling applications more efficient by continuously agitating the material, improving heat transfer coefficients analogous to those obtained with turbulent flow in liquid heat transfer operations. Properly applied, a vibratory fluid bed dryer surrounds each material particle with air, reducing the required amount of heat transfer surface.

A vibratory system’s continuous vibrating action also serves as a self-cleaning mechanism, reducing system downtime. Dust collectors can be added to the system to capture nuisance dust, improving the work environment. Both features increase the system’s efficiency.

Vibratory Fluid Bed Dryers

A vibratory fluid bed dryer consists of a U-shaped trough covered by a hood and mounted on a structural air plenum (see Figure 2). The entire unit is vibrated by a set of rotating weights. The drive, located below the fluidized bed, should have an adjustable amplitude, angle of vibration, and frequency so the residence time can be changed for various products, particularly for those that are difficult to fluidize. A manually adjustable weir, located at the right of the dryer unit, is recommended for controlling the material bed’s depth. The process gas enters the plenum chamber and is forced through the trough, which is usually a drilled plate deck. Material introduced by a feeder to the left of the dryer unit vibrates across the trough’s length as it is fluidized by the gas.

You may choose to use a dust collector if your material is likely to generate dust that will be entrained in the exhaust air. Figure 2 shows a dry dust collector capturing dust from a vibratory fluid bed dryer. Other types of dust collectors, such as cyclones or wet scrubbers, may be more suitable for some materials.

Figure 2: Fluid bed with dust collector (Carrier Process Equipment Group)

The rest of the vibratory fluid bed system includes ductwork, blowers, collectors, a heating and/or cooling source, a charging feeder, and controls that coordinate system operation with the vibrating fluid bed unit.

Applications

Vibratory fluid bed units have dried (and in some cases cooled) many different products, including foundry sand, salt, sugar, powdered milk, fertilizers, graphite, limestone, frac sand, pesticides,

glass cullet, fiberglass, candy, pharmaceuticals, carbon black, crumb rubber, ores, resins, cereals, and more. They can also be used to cool products and convey.

Finding Help in Selecting a Vibratory Fluid Bed Dryer

The parameters used in selecting a vibratory fluid bed dryer are complex. The behavior of particles in a fluid bed depends on their particle size and shape, and fluidizing velocity cannot be estimated

by visually inspecting the material bed.

A knowledgeable equipment manufacturer/supplier will help you select the best vibratory fluid bed dryer for your product. They offer guidance in choosing the best deck pattern that routes air from the plenum chamber through your material and ensures that air flows evenly through the dryer’s length. They can recommend the most suitable materials of construction such as stainless steel or other alloy fabrication for specific applications like sanitary food requirements, chemical, or general industry codes. In addition, they can also help you select auxiliary packages, such as ducts, fans, and blowers, that will meet your air requirements.

During the selection process, the vendor will need you to supply several pieces of design data. For instance, to properly size your dryer, the vendor must know the material’s bulk density, particle size, specific gravity, and temperature limits. The vendor must also know several facts about the drying process, such as the moisture content and temperature of the material at the dryer inlet, the desired moisture content of the material at the dryer outlet, the ambient air condition, and the heat and material feeding sources.

Some equipment manufacturers/suppliers can save you time and money by testing your product samples on a vibratory fluid bed dryer and recording data on moisture content, temperature control, product retention time, and pressure. Some vendors may provide field-rental test packages so design data can be obtained at your plant site. In both pilot-testing cases, technical project engineers will work with you throughout the selection process.

Chuck Mitchell is VP of sales and marketing for Carrier Process Equipment Group (Louisville, KY). For more information call 502-969-3179 or visit cpeg.com.