Fluidized Bed Dryers

8

Fluidized Bed Dryers Chung Lim Law and Arun S. Mujumdar

CONTENTS 8.1 8.2 8.3 8.4

8.5

8.6 8.7

8.8

8.9

Introduction ........................................................................................................................................... Advantages and Limitations of Fluidized Bed Dryers........................................................................... Heat Transfer in Fluidized Beds ............................................................................................................ Mathematical Models of Fluidized Bed Drying .................................................................................... 8.4.1 Diffusion Model ......................................................................................................................... 8.4.2 Empirical Model ......................................................................................................................... 8.4.3 Kinetic Model............................................................................................................................. 8.4.4 Single-Phase Model .................................................................................................................... 8.4.5 Two-Phase Model....................................................................................................................... Effect of Operating Parameters on Fluidized Bed Drying ..................................................................... 8.5.1 Effect of Bed Height ................................................................................................................... 8.5.2 Effect of Particle Size.................................................................................................................. 8.5.3 Effect of Gas Velocity................................................................................................................. 8.5.4 Effect of Bed Temperature ......................................................................................................... Types of Fluidized Bed Dryers: Classification and Selection................................................................. Conventional Fluidized Bed Dryers....................................................................................................... 8.7.1 Batch Fluidized Bed Dryers........................................................................................................ 8.7.2 Semicontinuous Fluidized Bed Dryers........................................................................................ 8.7.3 Well-Mixed, Continuous Fluidized Bed Dryers ......................................................................... 8.7.4 Plug Flow Fluidized Bed Dryers ................................................................................................ Modified Fluidized Bed Dryers.............................................................................................................. 8.8.1 Multistage and Multiprocess Fluidized Bed Dryers ................................................................... 8.8.2 Hybrid Fluidized Bed Dryers ..................................................................................................... 8.8.3 Pulsating Fluidized Bed Dryers .................................................................................................. 8.8.4 Fluidized Bed Dryers with Immersed Heat Exchangers ............................................................. 8.8.5 Mechanically Assisted Fluidized Bed Dryers.............................................................................. 8.8.6 Vibrated Fluidized Bed Dryers ................................................................................................... 8.8.7 Agitated Fluidized Bed Dryers/Swirl Fluidizers ......................................................................... 8.8.8 Fluidized Bed Dryers of Inert Particles ...................................................................................... 8.8.9 Spouted Bed Dryers.................................................................................................................... 8.8.10 Recirculating Fluidized Bed Dryers.......................................................................................... 8.8.11 Jetting Fluidized Bed Dryers .................................................................................................... 8.8.12 Fluidized Bed Dryers with Internal Baffles............................................................................... 8.8.13 Superheated Steam Fluidized Bed Dryers................................................................................. 8.8.14 Fluidized Bed Freeze Dryer ...................................................................................................... 8.8.15 Heat Pump Fluidized Bed Dryer .............................................................................................. Design Procedure ................................................................................................................................... 8.9.1 Design Equations........................................................................................................................ 8.9.1.1 Residence Time ............................................................................................................ 8.9.1.2 Sizing of Bed ................................................................................................................ 8.9.1.3 Gas Flow Rate ............................................................................................................. 8.9.1.4 Mass Balance, Continuous Drying, Well-Mixed Bed...................................................

ß 2006 by Taylor & Francis Group, LLC.

174 177 177 178 178 179 180 181 181 182 182 182 182 182 182 184 184 184 184 185 185 185 185 186 187 187 187 188 188 189 190 190 190 191 191 192 192 192 192 193 193 193

8.9.1.5 Heat Balance, Continuous Drying, Well-Mixed........................................................... 8.9.2 A Sample Design Calculation..................................................................................................... 8.10 Conclusion ........................................................................................................................................... Notation ......................................................................................................................................................... References ......................................................................................................................................................

8.1 INTRODUCTION Fluidized bed dryers (FBD) are used extensively for the drying of wet particulate and granular materials that can be fluidized, and even slurries, pastes, and suspensions that can be fluidized in beds of inert solids. They are commonly used in processing many products such as chemicals, carbohydrates, foodstuff, biomaterials, beverage products, ceramics, pharmaceuticals in powder or agglomerated form, healthcare products, pesticides and agrochemicals, dyestuffs and pigments, detergents and surface-active agents, fertilizers, polymer and resins, tannins, products for calcination, combustion, incineration, waste management processes, and environmental protection processes. Fluidized bed operation gives important advantages such as good solids mixing, high rates of heat and mass transfer, and easy material transport. For drying of powders in the particle size range of 50 to 2000 mm, fluidized beds compete successfully with other more traditional dryer types, e.g., rotary, tunnel, conveyor, continuous tray (see Table 8.1). Conventional fluidized bed is formed by passing a gas stream from the bottom of a bed of particulate solids. At low gas velocities the bed is static (packed).

193 195 198 198 199

The bed of particles rests on a gas distributor plate. The fluidizing gas passes through the distributor and it is uniformly distributed across the bed. Pressure drop across the bed increases as the fluidizing gas velocity is increased. At a certain gas velocity, the bed is fluidized when the gas stream totally supports the weight of the whole bed. This state is known as minimum fluidization and the corresponding gas velocity is called minimum fluidization velocity, umf. Pressure drop across the bed remains nearly the same as pressure drop at minimum fluidization even if the gas velocity is increased further. Figure 8.1 shows various regimes of the particulate bed from packed to bubbling bed when the gas velocity is increased. The graphs show the bed pressure drops and bed voidage under various regimes. A fluidized bed is operated at superficial gas velocities higher than the minimum fluidization velocity, umf, normally at 2–4 umf. The minimum fluidization velocity is typically obtained from experiments. There are several ways to determine the minimum fluidization velocity experimentally. It can also be estimated using various correlations. A list of minimum fluidization velocity can be obtained from Gupta and Sathiyamoorthy [1]. It should be noted that these correlations have limitations such as

TABLE 8.1 Comparison of Fluidized Bed Dryers (Conventional Types and Modified Types) with Other Competing Dryers for Particulate Solids Criterion Particle size Particle size distribution Drying time (approx.) Floor area Turndown ratio Attrition Power consumption Maintenance Energy efficiency Ease of control Capacity

Rotary

Flasha

Conveyor

Conventional FBDs

Modified FBDs

Large range Flexible Up to 60 min Large Large High High High Medium Low High

Fine particles Limited size distribution 10–30 s Large length Small High Low Medium Medium Medium Medium

500 mm–10 mm Flexible Up to 120 min Large Small Low Low Medium High High Medium

100–2000 mm Limited size distribution Up to 60 min Small Small High Medium Medium High High Medium

10 mm–10 mm Wide distribution Up to 60 min Small Small High Medium Medium High High High

a

Flash dryer is used only for removing surface moisture from smaller particles at relatively short drying times typically in the range of 10–30 s.


Fixed bed

Expanded Minimum Bubbling bed fluidization fluidization

Pressure drop

umf

Gas velocity

umf

Gas velocity

Bed voidage

FIGURE 8.1 Various regimes of a bed of particles at different gas velocities.

particle size, column dimensions, operating parameters, etc. Thus, they are valid in a certain range of criteria and operating conditions. The effect of wetness of the particles is, however, not included. Particles with high initial moisture content require a higher minimum fluidization velocity than similar bed of dry particles. Due to dominant cohesive forces exerted by wetted surfaces, only the top layer of the bed of solids is fluidized bed. The bottom layers may remain stationary during the initial stage of drying when the solids are quite wet. For the case of dry (or partially dry, no surface moisture) particles, if the fluidizing gas is further increased, the bed of particles goes through different types of fluidization regimes depending on the types of particles with reference to the Geldart classification of powders [2,3]. Based on fluidization quality, powders can be classified into four groups: group A (aeratable particles, easy-to-fluidize when dry), group B (sandlike particles, easy-to-fluidize when dry), group C (fine and ultrafine particles, difficult-to-fluidize due to dominated cohesive forces between particles), and group D (large and dense particles, poor fluidization


quality due to formation of large bubbles in the bed). Figure 8.2 shows the various fluidization regimes exhibited by a bed of dry particles of different classes with increasing gas velocity. Fluidized bed dryers are normally operated in the regimes of smooth and bubbling fluidization. After passing through the fluidized bed, the gas stream is introduced into gas-cleaning systems to separate fine particles (dusts) from the exit gas stream before discharging it to the atmosphere. Figure 8.3 shows a typical setup of fluidized bed drying system. A typical fluidized bed drying system consists of a gas blower, heater, fluidized bed column, gas-cleaning systems such as cyclone, bag filters, precipitator, and scrubber. To save energy, sometimes the exit gas is partially recycled. The bubbling fluidized bed (Figure 8.3) is divided vertically into two zones, namely a dense phase and a freeboard region (also known as lean phase or dispersed phase). The dense phase is located at the bottom; above the dense phase is the freeboard in which the solids hold-up and density decreases with height (Figure 8.3). Fluidizing gas after passing through the bed of particles enters the freeboard region, and carries with it fine particles which are terminal velocities smaller than the operating gas velocity. This phenomenon is known as elutriation. Solids hold-up in the freeboard region decreases as the freeboard height is increased until a height beyond which the solids holdup remains unchanged. This point is known as the transport disengagement height (TDH). TDH can be estimated from several empirical correlations; these correlations are expressed in terms of one or two operating parameters thus, the predictions are generally poor. However, there is no universally accepted equation for calculating TDH. As a result, it is best to determine the transport disengaging height experimentally. In designing a fluidized bed dryer for solids drying, it is important to take note about the occurrence of entrainment of fine particles, especially if the solids are polydispersed (i.e., have wide particle size distribution). The gas exit should be placed at a height above the TDH to minimize elutriation of fines. On the other hand, by means of fines elutriation, solids in fluidized bed can be classified into fine and coarse products. Particles that are elutriated by the fluidized gas stream are known as fine products whereas particles retained in the bed are known as coarse products. This process is called fluidized bed separation or classification or dedusting. For processes that require a certain degree of dedusting (removal of undesirable fine particles) or classification, operating gas velocity and location of gas exit should

#

Fixed bed

Smooth Bubbling

Turbulent

Fast

Pneumatic conveying

# Bubble maximum size greater than 0.66 column diameter Channeling

Gas velocity

FIGURE 8.2 Various fluidization regimes exhibited by different classes of particles with increasing gas velocity.

be chosen carefully in order to achieve the appropriate product cut size. Cut size refers to the critical size that separates the fine (elutriated) and coarse (remain in bed) particles. To ensure uniform and stable fluidization, the type of distributor has to be chosen carefully. This is to prevent poor fluidization quality of solids in certain regions in the fluidized bed, to prevent plugging of distributor-perforated holes, and to avoid solids from dropping into windbox or gas plenum located beneath the fluidized bed. There are many types of distributors available. Figure 8.3 (lower right image) shows four common types of distributors, namely, ordinary (i), sandwiched (ii), bubble cap tuyere (iii), and sparger (iv). It should be noted that pressure drop

Cyclone

across the distributor must be high enough to ensure good and uniform fluidization. As a rule of thumb, for upwardly and laterally directed flow, pressure drop across the distributor must exceed 30% of the pressure drop across the bed [4]. Whereas for downwardly directed flow, the pressure across the distributor must be greater than 10% of the pressure drop across the bed. Upwardly directed flow is normally found in ordinary perforated plates (Figure 8.3, lower right image-i). Sandwichtype distributor is used if reinforcement of the distributor is needed due to heavy load of bed of particles (Figure 8.3 lower right image-ii). Laterally directed flow is normally obtained with bubble caps and nozzle types of distributors (Figure 8.3, lower

Solids reservoir

(a) Freeboard; (b) dense phase

Feeder

Solids hold up Windbox Gas feed Heater

Distributor plate

Blower

FIGURE 8.3 Typical fluidized bed drying setup. Zones in a fluidized bed with its corresponding solids hold-up are shown in upper right side image. Types of perforated distributor plates that can be used are shown in lower right side image.


right image-iii), whereas the sparger type gives laterally or downwardly directed flow (Figure 8.3, lower right image-iv).

8.2 ADVANTAGES AND LIMITATIONS OF FLUIDIZED BED DRYERS Commonly recognized advantages of fluidized bed drying include: high rate of moisture removal, high thermal efficiency, easy material transport inside dryer, ease of control, and low maintenance cost. Limitations of fluidized bed dryer include: high pressure drop, high electrical power consumption, poor fluidization quality of some particulate products, nonuniform product quality for certain types of fluidized bed dryers, erosion of pipes and vessels, entrainment of fine particles, attrition or pulverization of particles, agglomeration of fine particles, etc. See Mujumdar and Devahastin [5] for detailed discussion. Besides drying, fluidized bed has found wide ranges of industrial applications in various industries for mixing, dedusting, granulation, coating, agglomeration, cooling, chemical reactions, incineration, combustion, gasification, etc. Many of these processes can be incorporated with fluidized bed drying in one unit processor to accomplish two or more processes in the same unit. Processes that can be advantageously incorporated with fluidized bed drying are described briefly in the following paragraphs. The mixing effect in a fluidized bed is generally good for particle sizes between 50 and 2000 mm. For fine particles (particle size less than 50 mm), or for particles that are difficult-to-fluidize when wet, vibration is normally applied to improve the fluidization quality and the mixing effect. For large particles, insertion of internals or use of the spouting mode can help to improve the operation. For fluidized bed drying, good particle mixing is essential. Thus, knowledge on particle fluidization characteristics and their properties is required to ensure good performance of a fluidized bed dryer. In addition, the bed of particles can be fluidized by a pulsating flow or by fluidizing sections of the bed periodically such that the entire bed is fluidized in sequence once over a cycle. Clearly, this operation results in saving of drying air and hence electrical power but it also leads to a longer operating time due to the intermittent mode of heat input. Besides, intermittent fluidization can reduce problem of mechanical damage to the particles due to continuous vigorous particle–particle collision as well as attrition-induced dusting. Spray drying, granulation, coating, and agglomeration share the same basic operating principle. A fine spray of solution–paste–slurry–suspension is


atomized and sprayed in the fluidized bed of the drying material itself or inert particles, which are already loaded in the drying chamber. Formation and growth of solid particles takes place in the chamber as evaporation and drying carry away moisture. In granulation, growth of solid particles is carried out by successive wetting and coating of liquid feed onto the solid particles, and solidification of the coated layer by hot drying air. In coating, a layer of expensive active agent can be coated on a less expensive substrate, or to add a surface agent on solid particles, which is needed for downstream processing. By spraying a suitable binder onto the bed of solid particles, agglomerated or granulated solid particles of large particle size are produced. In most cases, spray drying alone is not energy efficient to remove all moisture content inside the solids. This is because considerable amount of heat and time is needed to remove internal moisture that is trapped inside the solids internal. Fluidized bed drying can be incorporated as the second-stage drying to remove the internal moisture. This can be followed by a third-stage fluidized bed cooling to avoid the condensation problem during packaging in some applications.

8.3 HEAT TRANSFER IN FLUIDIZED BEDS Heat transfer in gas-fluidized bed can occur by conduction, convection, and radiation depending on the operating conditions. The contribution of the respective modes of heat transfer to the coefficient of heat transfer depends on particle classification, flow condition, fluidization regimes, type of distributor, operating temperature, and pressure. Heat transfer between a single particle and gas phase can be defined by the conventional equation of heat transfer: q ¼ hp Ap (Tp Tg )

(8:1)

where q is the rate of heat transfer (W), hp is the heat transfer coefficient (W/(m2K)), Ap is the surface area of a single particle (m2), Tp is the temperature of the particle (K), and Tg is the temperature of gas (K). The value of heat transfer coefficient of a single particle in a fluidized bed system is generally not high. It is in the range of 1 to 700 W/(m2K). However, due to the large interfacial surface area, in the order of 3,000 to 45,000 m2/m3, extremely high rates of heat transfer are achieved in this system. The heat capacity is in the order of 106 J/(m3K). As a result, thermal equilibrium is reached quickly. In designing fluidized bed dryers, an isothermal condition is often assumed. The heat transfer coefficient, hp, is a function of the operating parameters, particulate characteristics,

and dryer geometry. It can be estimated from the following correlations depending on the particle Reynolds number, Rep: hp ¼

kg Nup dp

(8:2)

!0:3 Re0t :3

hr ¼

where kg is the gas thermal conductivity (W/(m K)), dp is the particle diameter (m), and Nup is the particle Nusselt number, and Prg is the gas Prandtl number [6]. 0.33 For 0.1 Rep 50, Nup ¼ 0.0282 Re1.4 p Prg and 4 0.48 Prg0.33 for 50 Rep 1 10 , Nup ¼ 1.01 Rep Tubes, single or multiple, as well as flat channels can be immersed in a fluidized bed to provide additional heat for drying by conduction. These surfaces may be vertically or horizontally oriented. Empirical correlations are available in the literature for various geometries and operating conditions. The surface-to-bed heat transfer coefficient, hw ¼ q/aw (Tb Tw), is based on the surface area of the submerged object. This coefficient consists of two components, convective and radiative if the temperature is high. Here aw is wall surface area (m2) and TW is wall temperature (K), Tb is bed temperature (K). The convective heat transfer coefficient, hc, can be estimated using correlation by Vreedenberg [7] for horizontal immersed objects:

m2g hc dt r ¼ 420 s Prg 2 3 kg rg grs dp

The radiant heat transfer coefficient, hr (W/m2 K) can be estimated using the following equation among others [8]:

if

rs Rep 2550 rg (8:3)

!0:44 hc dt r 0:3 rs (1 «) 44 ¼ 0:66Prg Re0: if s Rep 2050 t kg rg rg «

8.4 MATHEMATICAL MODELS OF FLUIDIZED BED DRYING Many mathematical models of fluidized bed drying have been proposed in the literature and verified with experimental data. These models have been developed based on different assumptions.

8.4.1 DIFFUSION MODEL This model assumes that drying of single particles in a fluidized bed is totally controlled by diffusion of moisture inside the particle. For the analysis of particulate drying, diffusion equation for spheres of an equivalent diameter can be used. Zahed and Epstein [23] developed a diffusion model for spout bed drying and later Martinez-Vera et al. [24] applied the same model for fluidized bed drying. This model assumes .

.

In these equations, dt is the column diameter (m), rs is the particle density (kg/m3), rg is the gravitational acceleration (m/s2), mg is the gas viscosity (Ns/m2), « is the void fraction, and Re is the Reynolds number defined by

.

.

.

dt r g ug mg

(8:5)

(8:7)

where s is the Stefan–Boltzmann constant. Radiative heat transfer is insignificant at temperatures, T, lower than 7008C. Typically bed emissivity, «b, is approximately 0.9 and wall emissivity, «w, is between 0.9 and 1.125 [8]. Since most drying processes are carried out at temperatures lower than 7008C, radiant heat transfer can be neglected. The effect of various operating parameters on the heat transfer coefficient is given in Table 8.2.

(8:4)

Ret ¼

s(Tb4 Tw4 ) eb ew eb þ ew eb ew (Tb Tw )

.

Solids are spherical, isotropic, uniform size, and homogeneous. They are perfectly well mixed in fluidized bed. Physical properties of the dry solids remain constant with time. Solids shrinkage and temperature gradient inside the solid are negligible. Drying kinetics is governed by internal moisture diffusion. Thus, moisture at the solid surface is in equilibrium with the bed air humidity. Air is perfectly mixed. Exhaust air is in thermal equilibrium with bed. The dryer is perfectly insulated.

The diffusivity is assumed constant. The following diffusion equation defines moisture transport:

and Rep ¼


dp r g ug mg

(8:6)

@X ¼D @t

@2X @r2

þ

2 @X r @r

(8:8)

TABLE 8.2 Effect of Operating Parameters on Particle Heat Transfer Coefficient Parameter

Effect on Heat Transfer Coefficient, h

Particle Diameter, dp Shape Specific heat, cp Thermal conductivity, kp Gas Velocity, ug Density, rg Viscosity, mg Specific heat, cg Thermal conductivity, kg Fluidized bed Bed height, Hb Bed diameter, db Bed temperature, Tb Bed pressure, Pb

For fine particles, h is higher; for coarse particles, h is lower Higher for rounded and smooth surface particles h /cpn, where 0.25 < n < 0.8 No influence for small Biot number

9 10,11 12,13 14,15

Increases above umf to a maximum value at an optimum velocity, uopt and decreases thereafter Increases with increasing, rg Increases with decreasing, mg At moderate pressure and velocity, no information available At high pressure, increases with increasing cg h / kgn, where 0.5 < n < 0.66 h Increases as bed temperature increases, due to increasing of kg

9

No influence No information available Gas-convective: increases for small particles; decreases for coarse particles No influence on particle-convective heat transfer Gas-convective heat transfer increases

Heat transfer surface Length, L Tube diameter, dtube

No influence Increases with decreasing dtube

where X is the free moisture content, i.e., that in excess of the equilibrium value, D is the diffusivity (m2/s), and r is radial dimension (m). If diffusivity is variable and dependent on the radial distance of drying boundary from the center of the solids, the following diffusion equation is used instead: @X ¼D @t

2 @ X 2 @X @D @X 2 þ þ @ r2 r @r @X @r

(8:9)

Once the diffusivity is known, numerical analysis is applied to the diffusion equation in order to find moisture content profile inside the solid. Diffusivity of various food products can be obtained from Sablani et al. [25]. Average moisture content, X , can be obtained from the following equation: X¼

4p Vp

ð rp

r2 X dr

(8:10)

0

where Vp is the particle volume (m3). Note that moisture content and temperature-dependent diffusivity values can be used to solve the equation numerically.


Reference

10,11 16,17 14,18

12,19 20 21

22 17

8.4.2 EMPIRICAL MODEL In this model, the drying process is divided into different periods where drying mechanisms in each drying period are different. The general solution of Fick’s diffusion expresses the moisture content in terms of the drying time in exponential function. The solution for spherical solids is given in the folowing the equation [26–28]: Sphere: 1 X Xeq 6 X 1 n2 (p2 Deff t=r2sph ) ¼ 2 e Xo Xeq p n¼1 n2

(8:11)

where rsph is the sphere radius (m), Deff is the effective diffusivity (m2/s) and L is slab half thickness. Subscript ‘‘eq’’ denotes equilibrium and ‘‘o’’ indicates initial state. Since the general solution of the diffusion equation is expressed as a series of exponential functions, experimental data obtained from fluidized bed drying can be correlated as an exponential function. Many empirical exponential equations have been proposed. Equation 8.12 is a simple exponential equation. It assumes that the drying rate is proportional to the

difference between the average moisture content and the equilibrium moisture content [29]: X Xeq ¼ ekt Xind Xeq

(8:12)

where subscript ‘‘ind’’ denotes induction period. Equation 8.13 is a modified version of Equation 8.12 by Henderson and Pabis [30]. This equation is also analogous to the theoretical diffusion equation solution for an infinite slab [26,31]. Comparing Equation 8.13 and Equation 8.11, b ¼ Deffp2/r2sph [32]: X Xeq ¼ aebt Xind Xeq

(8:13)

Equation 8.12 tends to overpredict the early stage and underpredict the later stage of drying. Equation 8.14 is an empirical modification of Equation 8.12 by introducing an exponent y [33]. It has been used most commonly because most experimental data can be fitted very well with the following equation: X Xeq y ¼ ext Xind Xeq

(8:14)

Equation 8.15 uses the first two terms from Fick’s second law of diffusion. This equation has been used regardless of solids geometry [34]: X Xeq ¼ a1 eb1 t þ a2 eb2 t Xind Xeq

(8:15)

It should be noted that drying constants in the models mentioned above are empirical and depend on the type of materials, operating conditions as well as dryer dimensions. If one of these models is used for fluidized bed dryer design, experimental investigation on drying kinetics has to be conducted to obtain the drying constant for the particular material prior to the dryer design.

8.4.3 KINETIC MODEL Chandran et al. [35] developed a kinetic model for fluidized bed drying of solids. For a batch fluidized bed kinetic model, it is assumed that the drying process has both constant and falling rate periods. Drying rate in the falling rate period falls linearly with decreasing moisture content. Feed conditions and total contact area between solids and hot airstream remain the same throughout the whole drying process. In the batch drying operation, there is little interaction between the particles (wet and dry particles) in the system. Thus, data on drying kinetics is


sufficient to estimate the residence time of solids in order to achieve the desirable final moisture content. Moisture content of solids in different drying periods can be estimated from the following equations. In the constant rate period, X ¼ Xo at

(8:16)

In the falling rate period, X ¼ Xeq þ (Xcr1 Xeq )ea(ttcr1 )=(Xcr1 Xeq )

(8:17)

where subscript ‘‘cr1’’ denotes the first critical point that distinguishes constant and falling rate periods. For a single-stage continuous fluidized bed kinetics model, solids exit the fluidized bed system with a distribution of moisture content due to the wide residence time distribution. An average value of the moisture content and residence time is used. The average moisture content of solids in a continuous fluidized bed drying is given by X ¼ Xo

ð

X Xo

E(u) du

(8:18)

b

where (X/Xo)b is the moisture ratio in batch fluidized bed dryer, E(u) is the residence time density for the solids, and E(u) ¼ eu. u ¼ t/tcr1 is dimensionless time. Subscript ‘‘b’’ denotes batch process [36]. In the constant rate period, X at ¼1 Xo Xo

(8:19)

In the falling rate period, X at ¼1 Xo (bt þ 1)Xo

(8:20)

For a continuous fluidized bed that exhibits both constant and falling rate periods, the moisture content is then given by the following equation: X at bte uc 1 ¼1þ Xo Xo bt þ 1

(8:21)

where b ¼ a/(Xcr1 Xeq), uc ¼ t/tcr1,X is the average moisture content, t is the average residence time, and a is the drying coefficient. Once the average moisture content is known, equations obtained from mass and energy balances in the following models can be used to calculate the humidity and temperature of the exhaust air as well as the solids temperature. The simplest model is the

single-phase model that treats the fluidized bed as a continuum. As the number of phases considered in the model goes higher, the fluidized bed drying model becomes more complex and involves more transport properties. Complicated fluidized bed drying models that account for many transport processes that occur within and across the phases are beyond the scope of this chapter.

8.4.4 SINGLE-PHASE MODEL In a single-phase model, the fluidized bed is regarded essentially as a continuum (Figure 8.4). Heat and mass balances are applied over the fluidized bed. It is assumed that particles in the bed are perfectly mixed. Equation 8.22 and Equation 8.23 are the equations of moisture balance and energy balance, respectively [24]. Moisture Balance: Ms

dX ¼ Gg (Yout Yin ) dt

(8:22)

where Ms is the mass hold-up of dry solid in bed (kg), X is the average moisture content (kg/kg), Gg is the mass flow rate of dry air (kg/s), and Y is the air humidity (kg(water vapor)/kg(dry air)). Energy Balance: Ms cps

dT ¼ Gg (cg þ Yin cv )(Tin Tout ) dt Gg (Yout Yin )l

(8:23)

Outlet drying air Yout, Tout

where cp is the heat capacity at constant pressure (kJ/ (kg K)) and l is the latent heat of vaporization (kJ/kg). Subscript ‘‘s’’ denotes wet solid, ‘‘g’’ denotes dry air, and ‘‘v’’ denotes water vapor. Equation 8.23 neglects sensible heat of the water in solids.

8.4.5 TWO-PHASE MODEL A simple two-phase model of fluidized bed drying treats the fluidized bed to be composed of a bubble phase (dilute phase) and an emulsion phase (dense phase). The bubble phase contains no particles or the particles are widely dispersed. This model assumes that all gas in excess of minimum fluidization velocity, umf, flows through the bed as bubbles whereas the emulsion phase stays stagnant at the minimum fluidization conditions [37]. Figure 8.5 shows a schematic diagram of the simple two-phase model. Zahed et al. [38] have presented mass and energy balance equations for the dense phase and the bubble phase for fluidized bed drying. Mass balance of liquid in the bubble phase gives the following equation: Vgbb dYbb þ rg (Ybb Yin ) dt Vb t 6Kc rg «bb (Yd Ybb ) ¼ dbb

rg «bb

(8:24)

where subscript ‘‘bb’’ denotes bubble phase and ‘‘d’’ denotes dense phase. The rate of change of mass in the bubble phase can be assumed to be negligible [38] and Equation 8.24 can be rearranged to express humidity in the bubble phase, Ybb in terms of humidity in the dense phase, Yd. In the equation, Vgbb/Vb is the gas flow rate in bubble phase per unit volume of bed.

Solid particles ms, Ts, cs Dense phase Gas crossflow particulate solids

Inlet drying air Yin, Tin, Gg

FIGURE 8.4 Schematic diagram of the single-phase model of fluidized bed dryer.


Dilute phase bubbles

Gas flow

FIGURE 8.5 Schematic diagram of a two-phase model for fluidized bed drying.

Kc is the mass transfer coefficient across the bubble boundary. Mass balance of liquid in the interstitial gas in the dense phase gives the following equation: 6Kc rg «bb Vgd _ (Yd Yin ) þ m (Ybb Yd ) rg dbb Vb t DYd (8:25) ¼ rg «mf (1 «bb ) dt Likewise, the rate of change of mass in the interstitial gas can be assumed to be negligible. In this equation, _ is the mass rate of evaporation of water per unit m volume of bed, which in turn can be obtained from mass balance on dense phase. Vgd/Vb is the gas flow rate in dense phase per unit volume of bed. Mass balance of liquid in the dense-phase particles yields the following equation: _ ¼ rp (1 «mf )(1 «bb ) m

dX dt

(8:26)

The coupled mass and energy balance in dense phase that consists of particles and interstitial gas phases is given in the following equation: rp (1 «mf )(1 «bb )(cps cplX )

dTp dt

Vgd (cpg þ Yin cpv )(Tgin Tp ) DHevap V t b 6Kc rg «bb Vgd (Yd Yin ) rg (Ybb Yd ) Vb t dbb (8:27)

¼ rg

The above equation expresses the change of particle temperature in the dense phase in terms of average moisture content, X , which can be determined from any one equation from Equation 8.8 through Equation 8.21 depending on the operating conditions and the drying model, humidity of dense and bubble phases, Yd, Ybb, enthalpy of evaporation, DHevap, bubble diameter, dbb, and mass transfer coefficient of bubble boundary. Solving Equation 8.27 yields the solids temperature at different drying times.

8.5 EFFECT OF OPERATING PARAMETERS ON FLUIDIZED BED DRYING 8.5.1 EFFECT

OF

BED HEIGHT

For materials with high mobility of internal moisture such as iron ore, ion-exchange resins, silica gel, most drying takes place close to the distributor plate. Bed height has no effect on its drying rate that increasing


bed height beyond a particular value leads to no differences in drying rates. For materials with main resistance to drying within the material, e.g., grains, drying rate decreases with increasing bed height.

8.5.2 EFFECT

OF

PARTICLE SIZE

For group B particles (sandlike particles, according to Geldart Classification of Powdes), drying time that is required to remove a given amount of moisture increases as the square of the particle diameter provided that all other conditions remain unchanged. However, this effect is much smaller for group A (aertable particles, according to Geldart Classification of Powdes) particles because these particles are finer than group B and it exhibits smooth fluidization before entering bubbling fluidization regime.

8.5.3 EFFECT

OF

GAS VELOCITY

Gas velocity has a dominant effect on removing surface moisture. Increasing the gas velocity increases the drying rate. However, gas velocity has no effect at all for particles with high internal resistance to moisture transfer. High internal moisture resistance dominates at the end of the falling rate period.

8.5.4 EFFECT

OF

BED TEMPERATURE

Bed temperature is increased by high external heat fluxes. This in turn leads to higher moisture diffusivities and hence higher drying rate. This effect is complex and depends on the relative significance of external and internal resistances to moisture transfer.

8.6 TYPES OF FLUIDIZED BED DRYERS: CLASSIFICATION AND SELECTION Various types of fluidized bed dryers have been studied, developed, and operated in many industrial processes according to the respective process, product, operational safety, and environmental requirements. It is important to become familiar with the specific characteristics of different fluidized bed types in order to make a logical and cost-effective selection. It should be noted that in many instances several different types may provide similar performance at the same cost. Some novel fluidized bed dryers, which have not found application in industrial drying, are used to overcome disadvantages and difficulties that may occur in conventional fluidized bed dryers. It should be noted that not all modified fluidized bed dryers are necessarily better than the conventional dryers in terms of product quality, or energy efficiency, or drying performance.

TABLE 8.3 Classification of Fluidized Bed Dryers Criterion Processing mode/feed and discharge

Type of Dryer . . .

Batch FBDs (well-mixed) Semicontinuous FBDs Continuous

Subclassification

. . . . .

Particulate flow regime

. . . .

Well-mixed FBDs Plug flow FBDs Circulating FBDs Hybrid

. .

Operating pressure

.

. .

Fluidization gas flow

. .

Fluidizing gas temperature

. .

Low (for heat-sensitive products, low pressure strategy) Near atmospheric (most common) High (5 bars, superheated steam FBDs) Continuous Pulsed FBDs Constant Time-dependent

. . . .

Heat supply

. .

Well-mixed FBDs Plug flow FBDs or Single stage Multistage FBDs Hybrid/combined FBDs

Multistage FBDs (well-mixed—plug flow) Hybrid/combined FBDs

Step down Step up Periodic (zigzag) Combined

Convective Convective/conduction (immersed FBDs)

or . .

Fluidization action

.

Continuous Intermittent (multiple variable strategy) By gas flow (pneumatic)

. .

.

By jet flow

. . .

. .

With mechanical assistance With external field

. . . . . .

Fluidized material

.

Particulate solid (most common)

. . .

.

Paste/slurry

. . .

Fluidizing medium

. . . .


Heated air/flue gases/direct combustion gas Superheated steam/vapor Dehumidified cool air (heat pump FBDs) Air below freezing point of liquid being removed (fluidized bed freeze dryers)

Ordinary FBDs Circulating FBDs Spouted FBDs Recirculating FBDs Jetting FBDs Vibration (vibrated FBDs) Agitation (agitated FBDs) Rotation (centrifugal FBDs) Microwave–radio frequency field (MW–RF FBDs) Acoustic field Magnetic field Group A and B (most common, conventional FBDs) Group C (vibrated FBDs, agitated FBDs) Group D (vibrated FBDs, baffled FBDs, spouted FBDs) Spray onto a bed of inert particles (inert solids FBDs) Spray onto absorbent particles (silica gel, biomass) Spouted FBDs

Table 8.3 classifies the diverse variants of fluidized bed dryers according to various criteria.

Rotation

Wet solids

Exhaust air

8.7 CONVENTIONAL FLUIDIZED BED DRYERS 8.7.1 BATCH FLUIDIZED BED DRYERS A batch fluidized bed dryer is used when production capacity required is small (normally 50 to 1000 kg/h) or several products are to be produced in the same production line. It is preferable to operate batchwise if upstream and downstream processes are operated in batch mode, or several processes are to be carried out in sequence (e.g., mixing, drying, granulation/ coating, cooling) in the same processing unit. Drying air temperature and flow rate are normally fixed at a constant value. However, by adjusting the airflow rate and its temperature, it is possible to save energy and reduce attrition. Mechanical assistance such as agitation or vibration is normally applied for processing materials that are difficult-to-fluidize. Figure 8.6 shows a typical batch fluidized bed dryer with expanded freeboard and built-in internal bag filters. Expanded freeboard is used to reduce elutriation of fine particles.

8.7.2 SEMICONTINUOUS FLUIDIZED BED DRYERS In semicontinuous fluidized bed drying system, the drying chamber consists of a series of subprocessors. The wet product is accurately dosed and charged into the batches. The product is either transported batchwise from one processor to another processor or the batches (the processors with the batches of product) rotates along the process line [39]. This gives uninterrupted continuous operation over a long period. Figure 8.7 shows a schematic diagram of a semicon-

Heated air

Dry solids

FIGURE 8.7 Semicontinuous fluidized bed dryer.

tinuous fluidized bed dryer where the batches are rotated. In addition, gas temperature and velocity at different batches can be varied.

8.7.3 WELL-MIXED, CONTINUOUS FLUIDIZED BED DRYERS The well-mixed continuous fluidized bed dryer (Figure 8.8) is one of the most common fluidized bed dryers used in the industry. As the bed of particles is perfectly mixed, the bed temperature is uniform and is equal to the product and exhaust gas temperatures. However, particle residence time distribution is necessarily wide, thus resulting in wide range of product moisture content. On the other hand, as the feed material is continuously charged into the fluidized bed of relatively dry particles, this gives the added advantage of enhanced fluidizability and better fluidization quality. In some cases, a series of well-mixed continuous dryers may be used with variable operating parameters. In addition, a well-mixed continuous

Exhaust air

Bag filters

Wet solids

Dry solids

lot air or flue gas

Fluidized bed

FIGURE 8.6 Batch fluidized bed dryer.


FIGURE 8.8 Well-mixed fluidized bed dryers.

fluidized bed dryer can be incorporated with other types of dryers such as plug flow fluidized bed dryers to give better drying performance.

8.7.4 PLUG FLOW FLUIDIZED BED DRYERS In plug flow fluidized bed dryers, vertical baffles are inserted to create a narrow particle flow path, thus giving relatively narrow particle residence time distribution. Particles flow continuously as a plug from the inlet toward the outlet through the path. This ensures nearly equal residence time for all particles irrespective of their size and ensures uniform product moisture content. Various paths can be designed such as straight or spiral paths. Length-to-width ratio is normally in the range of 5:1 to 30:1. Figure 8.9 shows a plug flow fluidized bed dryer of straight and reverse paths. Operational problems might occur at the feed inlet because wet feedstock must be fluidized directly rather than when mixed with drier material as in the case of a well-mixed unit. To overcome the problem of fluidizability at the feed inlet, the inlet region may be agitated with an agitator, or by applying backmixing of solids, or by using a flash dryer to remove the surface moisture prior to plug flow fluidized bed drying.

8.8 MODIFIED FLUIDIZED BED DRYERS Various types of modified fluidized bed dryers have been developed and applied in many industrial processes. Modified fluidized bed dryers are applied to overcome some of the problems and disadvantages encountered in conventional fluidized beds.

8.8.1 MULTISTAGE AND MULTIPROCESS FLUIDIZED BED DRYERS As fluidized beds offer many distinct features and advantages for processing of particulate materials,

two or more processes can be carried out and accomplished in a fluidized bed column. This can be achieved by simply changing the operating conditions of fluidized bed to mix, dry, granulate, or coat [40], or cool in a single unit without discharging the material from the unit. In a fluidized bed spray dryer, spray drying is carried out in the upper part of the chamber followed by fluidized bed drying or agglomeration (Figure 8.10a). The large-scale fluidized bed coal dryer is also a particle classifier (Figure 8.10b). Drying and classification (separation of fines) are carried out in the same fluidized bed. By changing the fluidizing gas velocity, cut size (particle size that separates fine and coarse particles) can be adjusted in the classification process. Another example is upper stage of fluidized bed drying that can be followed by a lower stage of fluidized bed cooling (Figure 8.11a). A fluidized bed dryer or cooler consists of first-stage fluidized bed dryer followed by second-stage fluidized bed cooling (Figure 8.11b). In addition, different types of fluidized bed systems can be incorporated in a processing unit as well. For instance, first-stage well-mixed fluidized bed predrying can be incorporated with second-stage plug flow fluidized bed drying (Figure 8.11a). By incorporating different processes and combining different types of fluidized beds, space requirement, installation costs, and energy consumption can be reduced appreciably.

8.8.2 HYBRID FLUIDIZED BED DRYERS Hybrid fluidized bed dryers are useful for through drying of solids that contain surface and internal moistures. Surface moisture can be removed in the first-stage drying using a flash or cyclone dryers. Second-stage drying is then carried out in fluidized bed dryers in which residence time can be easily controlled. Figure 8.12 shows an example of hybrid Exhaust gas

Exhaust gas

Wet solid

Wet solid

Hot air Hot air Perforated distributor Dry solid

Partition plate/ internal baffle Dry solid

Perforated distributor

(a)

FIGURE 8.9 Plug flow fluidized bed dryers. (a) Straight path; (b) reversing path.


(b)

Exhaust gas Solids inlet Exhaust gas Fluidized bed coal dryer Liquid

Solids outlet

Hot gas Dry solids

Heated air (a)

(b)

FIGURE 8.10 (a) Spray fluidized bed dryer; (b) fluidized bed coal dryer and classifier.

cyclone fluidized bed dryer [39]. Wet solids are first charged into the cyclone dryer by exiting fluidizing gas from fluidized bed dryer. Surface moisture content of solids is quickly removed with the gas in the cyclone dryer. Solids and gas are separated in the cyclone. Partially dried solids are then pneumatically conveyed into the fluidized bed for secondstage drying. Other types of hybrid fluidized bed dryers include flash-fluidized bed dryer, filter-fluidized bed dryer [41]. A multistage spray fluidized bed dryer consists of a spray chamber followed by first-stage fluidized bed drying and second-stage fluidized bed cooling (Figure 8.13). When solid powders are formed in the spray dryer, these powders still contain some internal moisture. It is costly to use a spray dryer to remove all of the internal moisture. Instead, using a second-stage fluidized bed dryer is more cost-effective. Lisboa et al. [42] applied fluidization technique in a conventional rotary. The dryer is known as roto-fluidized dryer.

Exhaust air

It was found that the roto-fluidized dryer performs better than the conventional rotary dryer.

8.8.3 PULSATING FLUIDIZED BED DRYERS Pulsating fluidized bed dryers are used to overcome the problems of restricted particle size and size distribution, as well as aggregative fluidization and channeling that occur in a conventional fluidized bed dryer when processing certain types of powders. By pulsating the fluidizing gas stream, the fluidized bed either the whole bed or part of the bed is subjected to variable fluidizing gas velocity (Figure 8.14) [43–46]. This contributes to effective energy costs saving and enhanced drying performance without affecting the fluidization quality and process performance or added extra capital costs. For larger particles (group D particles), intermittent spouting of the bed with a rotating spouting jet has been shown to reduce energy consumption with only a marginal increase in drying time for batch drying.

Inlet solids

Upper stage

Exhaust air

Exhaust air

Wet solids

Lower stage

Product

Heated air (a)

Hot air

Cooling air

Cooling air

(b)

FIGURE 8.11 Two-stage fluidized bed dryers. (a) Upper stage well-mixed fluidized bed followed by lower stage plug flow fluidized bed; (b) first-stage dryer followed second-stage cooler.


Wet solid

Fluidized bed dryer

increases the contacting efficiency between the bed and the heat transfer surface. However, heat transfer coefficient reaches a maximum value. Beyond this point, increasing superficial gas velocity will hinder heat transfer between the bed and the heating surface. This is because of increasing preponderance of bubble (not particles) at the heating surface, which decreases particle-to-wall heat transfer.

Exhaust gas

Cyclone dryer

Pneumatic conveyor Dry solid

8.8.5 MECHANICALLY ASSISTED FLUIDIZED BED DRYERS

Air

Fluidization quality of fine and large particles can be enhanced by the assistance of external means such as vibration or agitation. Moreover, these particles can be immersed in a bed of fluidizable inert particles to improve their fluidization quality [47].

Hot gas

FIGURE 8.12 Hybrid cyclone fluidized bed dryer.

8.8.4 FLUIDIZED BED DRYERS HEAT EXCHANGERS

WITH IMMERSED

8.8.6 VIBRATED FLUIDIZED BED DRYERS Vibration combined with upward flow of air in an aerated bed enables particles to pseudofluidize smoothly. The gas velocity required for minimum fluidization is considerably lower than the minimum fluidization velocity in conventional fluidized bed dryer. Attrition due to vigorous actions between particle–particle and particle–wall is thus minimized appreciably. Hence, application of fluidized bed can be extended to fragile, abrasive, and heat-sensitive materials. The problem of fine particle entrainment is also avoided. For polydisperse powders, low gas velocity fluidizes the fine particles gently whereas vibration keeps the coarse particles in a mobile state. Vibrating fluidized beds are generally plug flow type (Figure 8.16). Vibrating fluidized beds are relatively shallow as the effect of vibration imparted by the vibrating grid decays with distance from the grid.

Fluidized beds equipped with internal heaters or immersed tubes transfer heat indirectly to the drying material. Horizontal tube bundles (Figure 8.15) are used extensively compared to vertical type. Tube pitch is an important design parameter. Fluidizing gas stream fluidizes the material and carries over the evaporated moisture. As a result, total sensible heat of gas and thus quantity of gas required are reduced. Immersed tubes or internally heated fluidized bed dryers are used to dry smaller size or fine powders. This is because heat transfer coefficient decreases with increasing particle size. Instead of tubes, vertical plates are also used as immersed heaters. Heat transfer is highly dependent on the particle heat capacity and mixing. Vigorous bubble action gives better particle circulation and mixing, and thus

Liquid Feed

Exhaust gas

Exhaust gas Heated air Aglomeration chamber

Cyclone Vibro-fluidizer Particles Sieve Heated air

Cool air

Recycled fine, crushed coarse

FIGURE 8.13 Multistage fluidized bed spray dryer.


Desirable Coarse product

Fine

Exhaust air

Product

Wet feed

Perforated distributor

Inlet gas distributor

Hot air

FIGURE 8.14 Pulsating fluidized bed. Parts of the bed are fluidized periodically.

There are some acoustic noise issues associated with such devices. These units can operate in batch as well as continuous modes.

8.8.7 AGITATED FLUIDIZED BED DRYERS/SWIRL FLUIDIZERS Another way to improve fluidization quality of fine particles is to impart mechanical agitation to the bed (Figure 8.17). By agitation, a homogeneous fluidized

Exhaust gas

Solids inlet

Solids flow

Heating fluid

Gas flow Immersed tube

Solids out Hot gas

FIGURE 8.15 Immersed tubes fluidized bed dryer.


bed is formed without channeling or formation of large bubbles. Moreover, agitated fluidized bed dryers are useful for drying pastes or cakes consisting of fine particles [48]. In this case, agitation helps to disintegrate and disperse the pasty feed. The agitator serves as a mixer in the dryer [49]. Moreover, deeper bed depth is possible if the bed is agitated whereas its fluidization quality is maintained.

8.8.8 FLUIDIZED BED DRYERS

OF INERT

PARTICLES

In recent years, the application of fluidized bed drying has been extended to drying of fine powders, pastes, slurries, suspensions, pulp, and enzymes-containing aqueous medium [50–55]. This is accomplished by using inert particles of high heat capacity (Figure 8.18) [56]. Inert particles must be able to fluidize well in a fluidized bed. By mixing the inert particles whose fluidization quality is generally good with the materials mentioned above, the fluidization quality of the materials is improved appreciably [57,58]. In addition, the inert particles with high heat capacity serve as energy carriers that enhance heat transfer [59,60]. Drying on inert particles can be performed in a variety of fluidized beds namely ordinary fluidized bed, spouted bed, spouted fluidized bed, jetting-spouted bed, as well as vibrated fluidized bed [61]. The liquid to be dried is sprayed into the fluidized bed; it coats the inert particle surfaces. The coated layer dries as a result of combined convective heat transfer from hot air and contact heat transfer due to sensible heat of the particles. When the thin layer is

Exhaust air

Flexible couplings

Wet solids

Dry solids

Hot air

Vibrator

FIGURE 8.16 Vibrating fluidized bed.

dry, it becomes brittle, cracks, and is peeled off due to attrition by particle–particle and particle–wall collisions. As a result, a fine powder is formed and is carried over by the exhaust gas to be collected and separated in suitable gas-cleaning devices such as cyclones or bag filters.

8.8.9 SPOUTED BED DRYERS Spouted bed dryers are useful for drying of large (Geldart’s group D) particles (>5 mm), which exhibit slugging under normal fluidization. In a spouted bed, a high gas velocity jet of gas penetrates through an opening at the bottom of the bed of particles and

Exhaust air

transports the particles to the bed surface. Energetic spouting at the bed surface thrusts the particles into the freeboard region at the center of the bed (Figure 8.19). After losing their momentum, these particles fall back onto the bed surface. Through this fountain-like action, good solid mixing is induced. A cyclical flow of particles is thus created. Details of spouted bed drying are discussed elsewhere in this handbook. The spout bed has been applied to drying, granulation, coating as well as to drying of pastes, solutions, slurries, and suspensions. Mujumdar [62] has classified spouted beds into at least 30 different variants, each with a specific set of advantages and limitations. Periodically spouted beds, multiple spouted

Exhaust gas Solids inlet

Liquid Rotation

Agitator

Heated air

Product

FIGURE 8.17 An agitated fluidized bed dryer.


Fluidizing gas

FIGURE 8.18 Inert solids fluidized bed.

Exhaust gas

Bed surface Solids out

Draft tube

Downcomer

Solids flow Gas flow

Spout

Gas and solid feed Conical base

Spouting gas

FIGURE 8.19 Spouted bed dryers.

FIGURE 8.20 Recirculating fluidized bed dryer.

groups of powders and particles. Its application in drying has been reported in coating of tablets in pharmaceutical industries, and in drying of dilute solutions containing solids. However, it is not a common dryer type now.

8.8.11 JETTING FLUIDIZED BED DRYERS beds, two-dimensional spouted beds, and oscillating spouted beds are some of the ideas introduced by Mujumdar in 1985, which have been examined in the literature in recent years.

8.8.10 RECIRCULATING FLUIDIZED BED DRYERS Insertion of a tubular draft tube into an ordinary spouted fluidized bed changes its operational and design characteristics. This type of fluidized bed is known as recirculating fluidized bed (or internally circulating fluidized bed, see Figure 8.20). Unlike spouted beds, recirculating fluidized beds do not have limitation of maximum spoutable bed height and minimum spouting velocity. As spouting gas stream passes through the draft tube, it is confined within the tube and does not leak out horizontally toward the downcomer. After passing through the draft tube, particles follow a certain flow pattern in the bed and flow downward in downcomer region. Since there is more flexibility in operating recirculating fluidized bed, it is applicable to handle all


In an ordinary fluidized bed, inlet gas is passed through nozzles, which are perforated evenly across the distributor plate. Jetting regions appear above every nozzle. In a spouted bed, inlet gas stream is supplied through a centrally located jet, spout in dilute phase is thus created, and penetrates the center region of the spouted bed (Figure 8.19). However, if a fairly large jet replaces the conical centrally located jet in a spout bed, a jetting fluidized bed is formed. One distinctive feature of jetting fluidized bed is that bubbles are formed instead of dilute phase spout (Figure 8.21). Small-scale jetting fluidized beds have been applied in coating and granulation processes.

8.8.12 FLUIDIZED BED DRYERS INTERNAL BAFFLES

WITH

Internal baffles can be inserted into a fluidized bed to divide the bed into several compartments. Various types of baffles can be used, e.g., wire mesh, perforated plate, turn plate, louver plate, and ring [63]. In

efficient baffled fluidized bed dryer can only be determined by carrying out pilot testing.

Exhaust gas

8.8.13 SUPERHEATED STEAM FLUIDIZED BED DRYERS Superheated steam as the fluidizing medium offers a number of advantages, e.g., no fire or explosion hazards, no oxidative damage, better operation performance (higher drying rate) and product quality, environmental friendliness, high energy consumption efficiency, suitability for drying of products containing toxic or expensive organic liquids, ability to permit pasteurization, sterilization, and deodorization of food products [66,67]. Details are available elsewhere in this handbook. The application of superheated steam fluidized bed dryer has been reported for drying of paper and pulp, wood-based biofuels (Figure 8.23), sugar beet pulp, and paddy [65,66]. Superheated steam fluidized bed drying of foodstuff, coal, bagasse, sludges, spent grains from breweries, lumber, tortilla, vegetables, herbs, and spice is also possible [67,70].

Jet

Solids out

Gas inlet

FIGURE 8.21 Jetting fluidized bed dryer.

8.8.14 FLUIDIZED BED FREEZE DRYER

addition, the internals can be placed horizontally or vertically (Figure 8.22). Horizontal baffles are frequently used. The objective of inserting baffles (horizontal and vertical) is to limit bubble growth and coalescence [64,65]. Hence, the baffled fluidized bed is useful to process group B and D particles because large bubbles are formed with such particles. The effect of baffles on the gas and solids flow is very complex and is dependent on bed diameter, distance between baffles, baffle opening and operating conditions. The optimum conditions for operating an

Freeze-drying is one of the low-temperature drying techniques suitable for drying of highly heat-sensitive materials such as drugs, pharmaceutical, biological, and food products. Freeze-drying removes moisture captured inside the solids by sublimation of moisture from solid state (ice) to vapor state. Ordinary freeze-drying is carried out in vacuum. Over the years, new developments showed that freezedrying can be carried out at atmospheric pressure and as well as in a fluidized bed (e.g., Refs. [71–73]). Here the drying rate is very slow. Wolff and Gibert [74] showed that fluidized bed freeze-drying at

Solids in Bubbles

Horizontal baffles

Gas distributor Solids out

Gas in (a)

Gas in (b)

FIGURE 8.22 (a) Horizontal baffled fluidized bed dryer; (b) vertical baffled fluidized bed dryer.


atmospheric pressure with the use of adsorbents can increase the drying rate appreciably (about sevenfold compared to that without adsorbent). In this case, adsorbent particles play a dual role as transfer agent for both heat and mass transfers. But there is difficulty in separating adsorbent particles and frozen dried products at the end of the process. It is thus suggested to use particles that are edible or compatible with human consumption such as starch. Fluidized bed freeze-drying assisted by adsorbent involves three stages, namely freezing of product, sublimation of free-frozen water, and secondary dehydration by desorption. Wolff and Gibert [75] suggested that fluidized bed freeze-drying should be carried out at higher temperature, but lower than the freezing point. They showed that fluidized bed freeze-drying with absorbent contributes to about 35% saving in heat requirement, respectively, although much longer drying time is needed as compared to vacuum freeze-drying.

compressor. The compressor raises the enthalpy of the working fluid and discharges it as superheated vapor at high pressure. Heat is removed from the working fluid and returned to the process air, which has been dehumidified previously at the condenser. As a result, the process air temperature increases. The working fluid is then throttled using an expansion valve to the low-pressure line and enters the evaporator to complete the cycle, whereas the dehumidified and heated process air is charged into the fluidized bed drying chamber to remove moisture of solids. Details on heat pump drying are available elsewhere in this handbook. Figure 8.24 shows a typical heat pump fluidized bed dryer. The fluidized bed drying chamber receives wet solids and discharges dried product whereas dehumidified and heated air is charged into the chamber from the bottom of the chamber. The drying temperature can be adjusted by monitoring the capacity of condenser, whereas the desired humidity of inlet air can be obtained by controlling the motor frequency of compressor. The advantages offered by heat pump fluidized bed dryer are: low energy consumption due to high specific moisture extraction rate (SMER), high coefficient of performance (COP), wide range of drying temperature (20 to 1108C), environmental friendliness, and high product quality. Thus this type of dryer is suitable for heat-sensitive products such as food and products of bio-origin. As chloroflurocarbons (CFC) and hydrochloroflurocarbons (HCFC) are to be phased out very soon, working fluids such as carbon dioxide, ammonia, R717, and R744 can be used as substitutes [76]. Many products have been tested at the Norwegian Institute of Technology, such as food products, fish, fruits, and vegetables [77,78].

8.8.15 HEAT PUMP FLUIDIZED BED DRYER

8.9 DESIGN PROCEDURE

An ordinary fluidized bed drying system consists of a blower, heater, dehumidifier (optional), fluidized bedchamber, and cyclone, whereas an ordinary heat pump drying system consists of evaporator, compressor, condenser, and an expansion valve. By combining fluidized bed and heat pump drying systems, where the evaporator acts as a dehumidifier and the condenser as a heater, a heat pump fluidized bed dryer is formed. The working fluid (refrigerant) at low pressure is vaporized in the evaporator by heat drawn from the exhaust humid air. At the same time, condensation of moisture occurs as the exhaust air temperature goes below dew point temperature. Thus, the process air is dehumidified. The working fluid then goes to

Design procedures for batch and continuous dryers in constant and falling rate periods vary widely. The discussion here is restricted to particulate solids drying.

Steam from dryer

Cyclone

Steam supply

Heat exchanger Pressurized screws

Fluidized bed

Wet product

Dry product Condensate Screw conveyor

Distributor plate Impeller

FIGURE 8.23 Pressurized superheated steam fluidized bed dryer.


8.9.1 DESIGN EQUATIONS 8.9.1.1

Residence Time

If the particles are small, very porous, and sufficiently wet to contain free moisture, the drying rate remains constant throughout the drying process. On the other hand, if the solid particles initially contain surface moisture, falling rate period will occur after a short period of constant rate period. In this case, the design calculation should include two steps: one for the

Wet solids

External condenser Dry solids

Receiver Compressor

Expansion valve

Evaporator

Condensation

Condenser Three way valve

FIGURE 8.24 Heat pump fluidized bed dryer.

constant rate and the other for the falling rate. Table 8.4 shows the equations for calculating residence time at different operating conditions. 8.9.1.2

A¼

Fs tR r b Hb

(8:28)

Gas Flow Rate

Gg ¼ rg ug A where rg is the density of gas.

(8:30)

In this equation, Fs is the solids flow rate (kg/s), X is the moisture content (kg/kg), Gg is the gas flow rate (kg/s), and Y is the absolute humidity (kg/kg). 8.9.1.5

Heat Balance, Continuous Drying, Well-Mixed

Heat balance for the single-phase model gives the following energy balance: Fs Hsin þ Gg Hgin þ Qh ¼ Fs Hsout þ Gg Hgout þ Qw

Gas flow rate (dry basis) is calculated from the following equation. The operating gas velocity, ug, is specified as a multiple of the minimum fluidization velocity, normally it is 2–3umf for fluidized bed drying. Anyway, the suitable operating gas velocity can be determined from laboratory-scale fluidized bed testing as long as the gas velocity yields good fluidization quality during the operation:


Mass Balance, Continuous Drying, Well-Mixed Bed Fs (Xin Xout ) ¼ Gg (Yout Yin )

Sizing of Bed

Sizing of bed is based on simple hold-up mass balance. Cross-sectional area of the fluidized bed can be determined from the following equation after solids flow rate (dry basis), Fs, bed density, rb, and bed height, Hb, are specified, and particle residence time, tR, is determined:

8.9.1.3

8.9.1.4

(8:31) In this equation, Qh is the rate of heat input from immersed tubes (kJ/s), Qw is the rate of heat loss from wall (kJ/s), and H is the enthalpy (kJ/kg). Enthalpy of solids at the inlet and outlet can be obtained from Equation 8.32 and Equation 8.33, respectively: Hsin ¼ (cps þ Xin cl )Tsin

(8:32)

Hsout ¼ (cps þ Xout cl )Tsout

(8:33)

(8:29)

TABLE 8.4 Equations to Determine Residence Time Required for Drying Remarks Batch Drying 1. Constant rate period

Only surface moisture present

2. Falling rate period

(i) From diffusion model

Drying Time Required (Xo X)Ms l Gg cpg (Tin Tout ) 1 X Xeq 6 X 1 [(np)2 Dt=R2 ] ¼ 2 e Xo Xeq p n¼1 n2 tR is obtained by trial and error Ms cps Tp Tin ln tR ¼ Gg cpg Tpo Tin Xcr1 Xeq tR ¼ k1 ln X Xeq tR ¼

(ii) Simplified equation (iii) Empirical formulation Continuous drying (a) Well-mixed 1. Constant rate period

Ð1 Design curve: X ¼ 0 X (t)E(t)dt [79] Only surface moisture

Xo X k 1 Xin Xeq 1 tR ¼ k Xout Xeq

tR ¼

2. Falling rate period 3. Batch drying curve

[80]

(1) Obtain a record of the changing bed temperature Tb during constant inlet air temperature run (2) Divide the constant inlet air temperature batch drying curve X(t) into increments of length, DX. For each increment note the time DtT1 required to accomplish that amount of drying at constant bed temperature of T1 (3) Calculate the time DtT2 required to accomplish the same increment of drying at constant bed temperature, T2 by the use of the following equation: DtT1 [(psat pin )(X Xe )]T1 ¼ [81,82] DtT2 [(psat pin )(X Xe )]T2 (4) Build up the constant bed temperature batch drying curve by increments (5) Obtain drying equation for each curve (6) Obtain residence time from design curve SA Xin Xout [83,84] tR ¼ f *Gb Xout Xeq where S is the bed loading, A is the bed area, f* is ratio of bed loading (S) and flux of gas flow rate (G/A) at constant be temperature, T1

(b) Plug flow 1. Batch drying curve

" # 1 (1 t=tm )2 Residence time distribution function is E(t) ¼ pffiffiffiffiffiffiffi exp 4B 2 pB where 4 Dtm 3:71 10 (u umf ) [82]; B ¼ 2 and D ¼ 1=3 L u mf

D¼

1:49[0:01(Hb 0:05) þ 0:00165rs (u umf )]u0:23

[83] 2=3 umf Note that validity of Reay’s correlation for particle diffusivity has only been confirmed for bed depths up to 0.10 m. There is some evidence in the literature that D may be an order of magnitude larger in much deeper beds [50]. Shallow bed is recommended if the objective is a close approach to plug flow behavior of solid particles in the bed

For a gas–vapor system, Hgin and Hgout can be obtained from Mollier diagram and for organic vapor– inert gas systems, Hgin and Hgout can be obtained from the following equations: Hgin ¼ (cpg þ Yin c1 )Tgin þ Yin l


(8:34)

Hgout ¼ (cpg þ Yout c1 )Tgout þ Yout l

(8:35)

A summary of steps for fluidized bed dryer design is given in Figure 8.25, whereas a simple guide for selecting suitable fluidized bed dryers (FBD) based on material properties is given in Figure 8.26.

Setup laboratory-scale FBD Determine fluidization characteristics Set ug, z, Tg, perform batch drying Calculate constant bed temperature batch drying curve Change z, T

Determine design curve Estimate residence time, tR Fix bed geometry, A and gas flowrate, Gg Determine outlet gas humidity, Yout and dew point Check RH, satisfactory?

Change A, Gg Change Gg

No

Yes Yes

Check for risk of condensation No

No

Optimize? Yes Check the requirement for: • • • • • •

Cooling Good distributor design Efficient gas cleaning Reduction of entrainment High heat demand Justification due to low drying rate

• Stringent requirement for final moisture content • Recovering of valuable solvent • Explosion hazard • Toxic hazard • Ignition hazard

FIGURE 8.25 Design steps starting from laboratory tests.

8.9.2 A SAMPLE DESIGN CALCULATION Wet particulate solids (6000 kg/h) with an initial moisture content of 20% (db) at 208C are to be dried to final moisture content of 4% (db). Inlet air at 1258C with humidity of 0.005 kg/kg is used. Bed depth is 20 cm. Under these conditions, bed density is 500 kg/m3 and equilibrium moisture content is zero. Specific heat of the dry solids and liquid water are 1.0 and 4.2 kJ/kg8C, respectively. Figure 8.27 gives the summary of all available data on this problem. Heat loss at the wall of the dryer is estimated as 5% of the heat content of inlet air. Batch drying curve was obtained at the conditions mentioned above; the relationship


between drying rate and moisture content is given by the following equation:

dX ¼ 0:005X dt

Calculate (a) Mean particle residence time (b) Bed area (c) Mass flow rate of air (d) Absolute humidity of exhaust air (e) Temperature of exhaust air (f) Check whether condensation will occur in cyclone


Crystalline surface moisture

Fragile

Liquids; pastes

Group C, D poor fluidizability particles

Group A, B good fluidizability particles

Colloidal/porous surface + internal moisture

Group C fine particles

Group D large particles

Liquids

Pastes slurries

• V-FBD Heat-sensitive

Mono sized

• PF-FBD • SBD

WM-FBD PF-FBD V-FBD

Poly disperse

• V-FBD

Heat-resistant

Mono sized

• WM-FBD • SBD

Heat-sensitive

Heat-resistant

Surface moisture

Poly disperse

• M-FBD • V-FBD

• WM-FBD • V-FBD

• V-FBD • A-FBD

• SBD

• SBD

• IT-FBD

• H-FBD (sprayFBD) • SBD • V-FBD

• V-FBD

• P-FBD • B-FBD Well-mixed FBD Plug flow FBD Vibrated FBD

FIGURE 8.26 Dryer selection.

SBD M-FBD A-FBD

Surface + internal moisture

Spouted bed Multistage FBD Agitated FBD

P-FBD IT-FBD B-FBD

Pulsating FBD Immersed tubes FBD Baffled FBD

H-FBD IS-FBD SD

• M-FBD • H-FBD (SD-slow), (FED-slow)

Hybrid FBD Inert Solids FBD Spray drying

• A-FBD • SBD • IS-FBD

Wet solids Fs = 6000 kg/h Ts,in = 208C Xo = 0.20 kg/kg rs = 2000 kg/m3 cps = 0.84 kJ/kgK

Exhaust air Bed Hb = 20 cm rb = 500 kg/m3 tR = ??? A = ??? Tb = ???

Wet solid

Dry solid

Hot air ug = 0.70 m/s Yg,in = 0.005 kg/kg rg = 1 kg/m3 cpg = 1.00 kJ/kgK Tg,in = 125 8C Gg = ???

Hot air or flue gas

Dry solids X = 0.04 kg/kg

FIGURE 8.27 Sample calculation.

Solution For continuous, well-mixed dryer operation in the linear falling period, the residence time is given by 1 Xo Xeq 1 tR ¼ k X Xeq

!

1 0:20 0 1 tR ¼ 0:005 0:04 0 tR ¼ 800 s Dry solid mass flow rate is calculated from wet solid mass flow rate, wet solid has initial moisture content of 20%, F ¼ 6000

kg wet solid 1 kg dry solid 1h h 1:20 kg wet solid 3600 s F ¼ 1:389 kg dry solid=s

Bed area is given by A¼

A¼

Fs tR r b Hb

1:389 800 500 0:20

A ¼ 11:11 m2

Gg ¼ 1:0 0:70 11:11 Gg ¼ 7:777 kg=s Outlet air humidity can be obtained from the equation Fs (Xin Xout ) ¼ Gg (Yout Yin ) Yout ¼

Yout ¼

Fs (Xin Xout ) þ Yin Gg

1:389 (0:20 0:04) þ 0:005 7:777

Yout ¼ 0:0336 kg H2 O=kg dry air Outlet air temperature can be obtained from the equation Fs Hsin þ Gg Hgin þ Qh ¼ Fs Hsout þ Gg Hgout þ Qw Qh ¼ 0 since there is no immersed tube. Qw ¼ 0.05GgHgin assumes that heat loss to wall is taken as 5% of the enthalpy of inlet air: Hsin ¼ (cps þ Xin c1 )Tsin Hsin ¼ (0:84 þ 0:20 4:2) 20 Hsin ¼ 33:60 kJ=kg Hsout ¼ (cps þ Xout c1 )Tsout Hsout ¼ (0:84 þ 0:04 4:2)T

Mass flow rate of air is calculated from the equation

Hsout ¼ 1:008T kJ=kg

Gg ¼ rg ug A

Hgin ¼ (cpg þ Yin c1 )Tgin þ Yin l


Hgin ¼ (1:00 þ 0:005 4:2) 125 þ 0:005 2370 Hgin ¼ 139:5 kJ=kg Hgout ¼ (cpg þ Yout c1 )Tgout þ Yout l Hgout ¼ (1 þ 0:0336 4:2)T þ 0:0336 2370 Hgout ¼ 1:141T þ 79:63 Fs Hsin þ Gg Hgin þ Qh ¼ Fs Hsout þ Gg Hgout þ Qw Fs Hsin þ Gg Hgin þ Qh ¼ Fs Hsout þ Gg Hgout þ 0:05Gg Hgin Fs Hsin þ 0:95Gg Hgin þ Qh ¼ Fs Hsout þ Gg Hgout 1:389 33:6 þ 0:95 7:777 139:5 þ 0 ¼ 1:389 (1:008T) þ 7:777 (1:141T þ 79:63) T ¼ 44:6 C From the psychrometric chart, air at absolute humidity of 0.0336 kg/kg has a dew point of 33.58C and relative humidity is 55%. Since the outlet air leaves the dryer at 44.68C (108C higher than the dew point), there is no risk of condensation.

8.10 CONCLUSION Fluidized bed dryers have replaced some of the conventional dryers, e.g., rotary or conveyor dryers in many instances. Among some of the recent developments in fluidized bed drying is the idea of applying microwave energy field continuously or intermittently in a fluidized or spouted bed. Use of superheated steam will probably become more popular in some applications in the future. The presence of moisture on particle surface can cause major changes to fluidization quality as compared with that of dry particles to which most of the available literature is applicable. Also, there are numerous variants of the fluidized bed that require different design information and design strategies. For example, drying using a time-dependent heat input or drying under low-pressure conditions or using superheated steam as the drying medium, etc. must be handled with some modifications and new data sets and models.

NOTATION a, b, x, y, k drying constant, drying coefficient A area, m2 c heat capacity, J/(kg K)


heat capacity at constant pressure J/(kg K) diameter, m or mm diffusivity, m2/s emissivity residence time density solids mass flow rate, kg/s gravity acceleration ¼ 9.80665 m/s2, m/s2 gas mass flow rate, kg/s height, m heat transfer coefficient, W/(m2K) mass transfer coefficient across bubble boundary, m/s thermal conductivity, W/(m K) length, m mass, kg mass rate of evaporation of water per unit volume of bed, kg/(m3 s) integer Nusselt number partial pressure, Pa pressure, Pa Prandtl number rate of heat transfer, W heat input from immersed tubes, kJ/s heat loss to column wall, kJ/s radius, m Reynolds number time, s temperature, 8C, K velocity, m/s volume, m3 moisture content, kg/kg absolute humidity, kg/kg

cp d D e E(u) F g G H h Kc k L M _ m n Nu p P Pr q Qh Qw r Re t T u V X Y

GREEK SYMBOLS root of Bessel function void fraction latent heat of vaporization, J/kg viscosity, Ns/m2 density, kg/m3 Stefan–Boltzmann constant ¼ 5.67108 W/ (m2 K4), W/(m2 K4) f sphericity b « l m r s

SUBSCRIPT b bb c cr1 cyl d

bed bubble convective component first critical cylinder dense

eff eq evap g in ind l mf o opt out p r R s sat sph t v w

effective equilibrium evaporation gas inlet induction liquid minimum fluidization initial optimum outlet particle radiant component residence solids saturated sphere column vapor wall

REFERENCES 1.

2. 3.

4.

5.

6.

7.

8.

9.

10.

Gupta, C.K. and Sathiyamoorthy, D., Fluid Bed Technology in Material Processing, CRC Press, New York, 1999, chap. 1. Geldart, D., Types of gas fluidization, Powder Technol., 7: 285–292, 1973. Geldart, D., Characterization of fluidized powders, in Gas Fluidization Technology, Geldart, D., Ed., John Wiley & Sons, New York, 1986, chap. 3. Karri, S.B.R. and Werther, J., Gas distributor and plenum design in fluidized beds, in Handbook of Fluidization and Fluid Systems, Yang, W.C., Ed., Marcel Dekker, New York, 2003, chap. 6. Mujumdar, A.S. and Devahastin, S., Applications for fluidized bed drying, in Handbook of Fluidization and Fluid Systems, Yang, W.C., Ed., Marcel Dekker, New York, 2003, chap. 18. Chen, J.C., Heat transfer, in Handbook of Fluidization and Fluid Systems, Yang, W.C., Ed., Marcel Dekker, New York, 2003, chap. 10. Vreedenberg, H.A., Heat transfer between a fluidized bed and a horizontal tube, Chem. Eng. Sci., 9(1): 52–60, 1958. Ozkaynak, T.F., Chen, J.C., and Frankenfield, T.R., An experimental investigation of radiant heat transfer in high temperature fluidized bed, in Fluidization IV, Kunii, D. and Toei, R., Eds., Engineering Foundation, New York, 1983, pp. 371–387. Sarkits, V.B., Heat Transfer from Suspended Beds of Granular Materials to Heat Transfer Surfaces (Russian), Leningrad Technology Institute, Lensoviet, 1959. Baerg, A., Klassen, Y., and Gishler, P.E., Heat transfer in fluidized solid beds, Can. J. Res. Sect. F, F28: 287, 1950.


11.

12.

13.

14. 15.

16.

17.

18.

19.

20.

21.

22.

23.

24.

25.

26. 27.

Mersman, A., Zum Wa¨rmeu¨bergang in Wirbelschichten (heat transfer in fluidized bed), Chem. Ing. Tech., 39: 349–353, 1967. Dow, W.M. and Jakob, M., Heat transfer between a vertical tube and fluidized bed air mixture, Chem. Eng. Prog., 47: 637, 1951. Gaffney, B.J. and Drew, T.B., Mass transfer from packing to organic solvents in single phase flow through a column, Ind. Eng. Chem., 42: 1120–1127, 1950. Miller, C.O. and Logwinuk, A.K., Fluidization studies of particles, Ind. Eng. Chem., 43: 1220–1226, 1951. Wicke, E. and Hedden, F., Stromungsformen and Warmeubertragung in von luft aufgewirbelten schuttgutschichten (current forms and heat transfer in Schu¨ttgutschichten air fluidized bed), Chem. Ing. Tech., 24(2): 82, 1952. Traber, P.G., Pomaventsev, V.M., Mukhenov, I.P., and Sarkis, V.B., Heat transfer from a suspended layer of catalyst to the heat transfer surface, Zh. Prikl. Khim., 35: 2386, 1962. Baskakov, A.P., High speed nonoxidative heating and heat treatment in a fluidized bed, Metallurgia, Moscow, 1968. Vreedenberg, H.A., Heat transfer between fluidized beds and vertically inserted tubes, J. Appl. Chem., 2: 26, 1952. Wender, L. and Cooper, G.T., Heat transfer between fluidized bed and boundary surfaces: correlation data, AIChE J., 4(1): 15–23, 1958. Botterill, J.S.M., Teoman, Y., and Yuregir, K.R., Temperature effect on the heat transfer behaviour of gas fluidized beds, AIChE Symp. Ser., 77(208): 330, 1984. Xavier, A.M. and Davidson, J.F., Convective heat transfer in fluidized bed, in Fluidization, 2nd ed., Davidson, J.F., Clift, R., and Harrison, D., Eds., Academic Press, London, 1985, chap. 13. Gelperin, N.I. and Einstein, V.G., Heat transfer in fluidized beds, in Fluidization, Davidson, J.F. and Harrison, D., Eds., Academic Press, New York, 1971, chap. 10. Zahed, H.A. and Epstein, N., Batch and continuous spouted drying of cereal grains: the thermal equilibrium model, Can. J. Chem. Eng., 70: 945–953, 1992. Martinez-Vera, C., Vizcarra-Mendoza, M., GalanDomingo, O., and Ruiz-Martinez, R., Experimental validation of mathematical model for the batch drying of corn grains, Drying Technol., 13(1–2): 333–350, 1995. Sablani, S., Rahman, S., and Al-Habsi, N., Moisture diffusivity in foods—an overview, in Drying Technology in Agriculture and Food Sciences, Mujumdar, A.S., Ed., Science Publishers, Enfield, 2000, chap. 2. Crank, J., The Mathematics of Diffusion, 2nd ed., Clarendon Press, Oxford, 1975, chap. 6. Rossello, C., Canellas, J., Simal, S., and Berna, A., Simple mathematical model to predict the drying

28.

29. 30.

31. 32.

33.

34.

35. 36.

37.

38.

39.

40.

41.

42.

43.

44.

45.

rates of potatoes, J. Agric. Food Chem., 40(12): 2374– 2378, 1992. Senadeera, W., Bhandari, B.R., Young, G., and Wijesinghe, B., Influence of shapes of selected vegetable materials on drying kinetics during fluidized bed drying, J. Food Eng., 58: 277–283, 2003. Lewis, W.K., The rate of drying of solid materials, J. Ind. Eng., 13(5): 427–432, 1921. Henderson, S.M. and Pabis, S., Grain drying theory I: temperature effect on drying coefficient, J. Agric. Eng. Res., 6: 169–174, 1961. Sherwoods, W.C.H., Drying of solids, J. Food Eng. Chem., 21(1): 12–16, 1929. Lopez, A., Iguaz, A., Esnoz, A., and Virseda, P., Thin layer drying behaviour of vegetable wastes from wholesale market, Drying Technol., 18(4–5): 995–1006, 2000. Page, G., Factors Influencing the Maximum Rate of Air Drying Shelled Corn in Thin Layers, M.Sc. thesis, Purdue University, West Lafayette, 1949. Sharma, A.D., Kunre, O.R., and Tolley, H.D., Rough rice drying as a two compartment model, Trans. ASAE, 27: 195–200, 1982. Chandran, A.N., Rao, S.S., and Varma, Y.B.G., Fluidized bed drying of solids, AIChE J., 36(1): 29–38, 1990. Vanecek, V., Markvart, M., Drbohlav, R., and Hummel, R.L., Fluidized Bed Drying, Leonard Hill, London, 1970. Toomey, R.D. and Johnstone, H.F., Gaseous fluidization of solid particles, Chem. Eng. Prog., 48: 220–226, 1952. Zahed, A.H., Zhu, J.X., and Grace, J.R., Modelling and simulation of batch and continuous fluidized bed dryers, Drying Technol., 13(1–2): 1–28, 1995. Romankov, P.G., Drying, in Fluidization, Davidson, J.F. and Harrison, D., Eds., Academic Press, London, 1971, chap.12. Guignon, B., Duquenoy, A., and Dumoulin, E.D., Fluid bed encapsulation of particles: principles and practice, Drying Technol., 20(2): 419–447, 2002. Mujumdar, A.S., and Law, C.L., Filter dryers in process industries, Chemical Industry Digist, April 2006: 34–42, 2006. Lisboa, M.H., Alves, M.C., Vitorino, D.S., Delaiba, W.B., Finzer, J.R.D., and Barrozo, M.A.S., Study of the performance of the rotary dryer with fluidization, in Proceedings of the 14th International Drying Symposium, Silva, M.A. et al., Eds., Sao Paulo, Brazil, 2004, pp. 1668–1675. Gawrzynski, Z. and Glaser, R., Drying in a pulsed fluidized bed with relocated gas stream, Drying Technol., 14(9): 1121–1172, 1996. Gawrzynski, Z., Glaser, R., and Kudra, T., Drying of powdery materials in a pulsed fluid bed dryer, Drying Technol., 17(7–8), 1523–1532, 1999. Kudra, T., Gawrzynski, Z., Glaser, R., Stanislawski, J., and Poirier, M., Drying of pulp and paper sludge in a pulsed fluid bed dryer, Drying Technol., 20(4–5): 917–933, 2002.


46.

47.

48.

49.

50.

51.

52.

53.

54.

55. 56.

57.

58.

59.

60.

61.

Nitz, M. and Taranto, O.P., Drying of beans in a pulsed-fluid bed dryer—fluid-dynamics and the influence of temperature, airflow rate and frequency of pulsation on the drying rate, in Proceedings of the 14th International Drying Symposium, Silva, M.A. et al., Eds., Sao Paulo, Brazil, 2004, pp. 836–843. Mujumdar, A.S. and Devahastin, S., Fluidized bed drying technology, in Mujumdar’s Practical Guide to Industrial Drying, Devahastin, S., Ed., Exergex Corporation, Montreal, 2000, chap. 5. Reyes, A., Eckholt, M., and Alvarez, P.I., Drying and heat transfer characteristics for a novel fluidized bed dryer, Drying Technol., 22(8): 1869–1895, 2004. Adamiec, J., Drying if waste sludges in a fluidized bed dryer with a mixer, Drying Technol., 20(4–5): 839–853, 2002. Lee, D.H. and Kim, S.D., Drying characteristics of starch in an inert medium fluidized bed, Chem. Eng. Tech., 16: 263–269, 1993. Costa, E.F., Cardoso, M., and Passos, M.L., Simulation of drying suspensions in spout-fluid beds of inert particles, Drying Technol., 19(8): 1975–2001, 2001. Grbavcic, Z.B., Arsenijevic, Z.Lj., and GaricGrulovic, R.V., Drying of slurries in fluidized bed of inert particles, Drying Technol., 22(8): 1793–1812, 2004. Chen, G., Zhao, Y., and Chen, Y., Drying of suspending liquor in fluidized bed with inert particles, J. Chem. Eng. (China), 45(4): 474–480, 1996. Taruna, I. and Jindal, V.K., Drying of soy pulp (okara) in a bed of inert particles, Drying Technol., 20(4–5): 1035–1051, 2002. Kirkwood, M.K. and Olson, K.E., Enzyme Drying Process, U.S. Patent 4,617,212, 1986. Grbavcic, Z.B., Arsenijevic, Z.Lj., and Zdanski, F.K., Drying of suspension in fluidized bed of inert particles, in Proceedings of the 11th International Drying Symposium, Akritidis, C.B., Marinos-Kouris, M., Saravakos, G.D., and Mujumdar, A.S., Eds., Ziti Edition, Thessaloniki, Vol. C, 1998, pp. 2090–2097. Jariwara, S.L. and Hoelscher, H.E., Model for oxidative thermal decomposition of starch in a fluidized reactor, Ind. Eng. Chem., 9: 278–284, 1970. Ramakers, B.J., Ridder, R., and Kerkhof, P.J.A.M., Fluidization behavior of woods/sand mixtures, in Proceedings of the 14th International Drying Symposium, Silva, M.A. et al., Eds., Sao Paulo, Brazil, 2004, pp. 1337–1344. Zhou, S.J., Mowla, D., Wang, F.Y., and Rudolph, V., Experimental investigation of food drying processes in dense phase fluidized bed with energy carrier, CHEMICA 98, Port Douglas, Australia, 1998, Paper No. 368. Hatamipour, M.S. and Mowla, D., Experimental and theoretical investigation of drying of carrots in a fluidized bed with energy carrier, Drying Technol., 21(1): 83–101, 2003. Mujumdar, A.S. and Kudra, T., Drying of slurries with particulate media in various gas-particle contactors, in Proceedings of the Second Asian-Oceania

62.

63.

64.

65.

66.

67. 68.

69.

70.

71.

72.

73.

Drying Conference, Daud, et al., Eds., The Institution of Chemical Engineers, Pulau Pinang, Malaysia, 2001, pp. 1–25. Mujumdar, A.S., Spouted bed technology—a brief review, in Drying’ 85, Mujumdar, A.S., Ed., Hemisphere Publishing, New York, 1985, pp. 151–157. Jin, Y., Wei, F., and Wang, Y., Effect of internal tubes and baffles, in Handbook of Fluidization and FluidParticle Systems, Yang, W.C., Ed., Marcel Dekker, New York, 2003, chap. 7. Mujumdar, A.S., Superheated steam drying, in Handbook of Industrial Drying, 2nd ed., Mujumdar, A.S., Ed., Marcel Dekker, New York, 1995, chap. 35. Law, C.L., Tasirin, S.M., Daud, W.R.W. and Geldart D., Effect of vertical baffles on particle mixing and drying in fluidized beds of group D particles. China Particuology science and Technology of Particles, 1(3): 115–118, 2003. Law, C.L., Tasirin, S.M., Daud. W.R.W. and Ng p.p., The effect of vertical internal baffles on fluidization hydrodynamic and grain drying characteristics, Chinese J. of Chemical Engineering, 12(6): 801–808, 2004. Kudra, T. and Mujumdar, A.S., Advanced Drying Technologies, Marcel Dekker, New York, 2002, chap. 7. Jensen, A.S., Pressurized drying in a fluid bed with steam, in Drying’92, Mujumdar, A.S., Ed., Hemisphere Publishing, New York, 1992, pp. 1593–1601. Taechapairoj, C., Dhuchakallaya, I., Soponronnarit, S., Wetchacama, S., and Prachayawarakorn, S., Fluidized bed paddy drying using superheated steam, in Proceedings of the 13th International Drying Symposium, Cao, C.W., Pan, Y.K., Liu, X.D., and Qu, Y.X., Eds., Beijing, Vol. B, 2002, pp. 1218– 1227. Pronyk, C., Cenkowski, S., and Muir, W.E., Drying foodstuffs with superheated steam, Drying Technol., 22(5): 889–916, 2004. Garcia-Pascual, P., Alves-Filho, O., Strommen, I., and Eikevik, T.M., Heat pump atmospheric freezedrying of green peas, in Proceedings of the Second Nordic Drying Conference, Eikevik, T.M., AlvesFilho, O., and Strommen, I., Eds., Copenhagen, Denmark, 2003. Tomova, P., Behns, W., Haida, H., Ihlow, M., and Morl, L., Experimental analysis of fluidized bed freeze drying, in Proceedings of the 14th International Drying Symposium, Silva, M.A. et al., Eds., Sao Paulo, Brazil, 2004, pp. 526–532. Menshutina, N., Korneeva, A.E., Goncharova, S., and Leuenberger, H., Modeling of freeze drying in fluidized bed, in Proceedings of the 14th International Drying Symposium, Silva, M.A. et al., Eds., Sao Paulo, Brazil, 2004, pp. 680–686.


74.

75.

76.

77.

78.

79.

80.

81.

82.

83.

84.

85.

86.

Wolff, E. and Gibert, H., Atmospheric freeze drying. Part 1. Design, experimental investigation and energysaving advances, Drying Technol., 8(2): 385–404, 1990. Wolff, E. and Gibert, H. Atmospheric freeze drying. Part 2. Modeling drying kinetics using adsorption isotherms, Drying Technol., 8(2), 405–428, 1990. Alves-Filho, O. and Lystad, T., A new carbon dioxide heat pump dryer — an approach for better product quality, energy use and environmentally friendly technology, in Proceedings of the 12th International Drying Symposium, Kerkhof, P.J.A.M., Coumans, W.J., and Mooiweer, G.D., Eds., Elsevier Science, Amsterdam, 2000, Vol. B, Paper No. 51. Alves-Filho, O., Eikevik, T.M., and Strommen, I., Characterization of cold-extruded and heat pump dried instant vegetable powders, in Proceedings of the 13th International Drying Symposium, Cao, C.W., Pan, Y.K., Liu, X.D., and Qu, Y.X., Eds., Beijing, Vol. C, 2002, pp. 1635–1642. Strommen, I., Alves-Filho, O., Eikevik, T.M., and Claussen, I.C., Physical properties in drying of food with combined sublimation and evaporation, in Proceedings of the 13th International Drying Symposium, Cao, C.W., Pan, Y.K., Liu, X.D., and Qu, Y.X., Eds., Beijing, Vol. C, 2002, pp. 1698–1705. Vanecek, V., Picka, J., and Drbohlav, R., Some basic information on the drying of granulated NPK fertilizers, Int. Chem. Eng., 17: 2271–2291, 1964. Reay, D., Fluidized bed drying, in Gas Fluidization Technology, Geldart, D., Ed., John Wiley & Sons, New York, 1985, chap. 10. Reay, D. and Baker, C.G.J., Drying, in Fluidization, 2nd ed., Davidson, J.F., Clift, R., and Harrison, D., Eds., Academic Press, London, 1985, chap. 16. Reay, D. and Allen, R.W.K., Predicting the performance of a continuous well-mixed fluidized bed dryer from batch tests, in Proceedings of the Third International Drying Symposium, Ashworth, J.G., Ed., Birmingham, England, Vol. 2, 1982, pp. 130–140. Baker, C.G.J., Predicting the energy consumption of continuous well mixed fluidized bed dryers from kinetic data, Drying Technol., 17(7–8): 2327–2349, 1999. Baker, C.G.J., The design and performance of continuous well-mixed fluidized bed dryers—an analytical approach, Drying Technol., 18(10): 2327–2349, 2000. Reay, D., Particle residence time distributions in shallow rectangular fluidised beds, in Proceedings of the First International Drying Symposium, Mujumdar, A.S., Ed., 1978, pp. 136–144. Baker, C.G.J. and Khan, A.R., Design and performance modeling of plug flow fluidized bed dryers, in Proceedings of the 13th International Drying Symposium, Cao, C.W., Pan, Y.K., Liu, X.D., and Qu, Y.X., Eds., Beijing, Vol. A, 2002, pp. 327–335.

Fluidized Bed Dryers

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