Caballero B. (ed.) Encyclopaedia of Food Science, Food Technology and Nutrition. Ten-Volume Set

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the drying surface). If sensible heat changes are

neglected, the equilibrium between heat and mass

transfer at the drying surface may be represented by:

¼

, ð8Þ

where L

is the latent heat at #

. Therefore, the rate of

drying may be represented by the expression:

¼

ð#

 #

Þ: ð9Þ

The drying rate may be expressed in terms of the rate

of change of moisture content (dW/dt):

¼

ð#

 #

Þ, ð10Þ

where W is the moisture content (dwb), and A

is the

drying surface associated with the unit mass of dry

solids. If the thickness of the slab is l, and the bulk

density of the solids is r

, the rate of drying may be

expressed as:

¼



ð#

 #

Þ, ð11Þ

If W

is the moisture content (dwb) of the wet mater-

ial at the beginning of the constant rate period, and

is the moisture content (dwb) at the end of that

period, the duration of the constant rate period (t

)

may be expressed as:

ðW

 W

Þ

ð#

 #

: ð12Þ

From the equations above, it can be seen that the

higher the temperature and the lower the humidity

of the air, the higher the rate of drying during the

constant rate period. The velocity of the air also influ-

ences the drying rate during this period by affecting

the heat transfer coefficient thus:

, ð13Þ

where G is the mass velocity of the air, D

is the

characteristic dimension of the system, and a, n,and

m are constants. D

has been represented by the

equivalent diameter of the flow channel (cross-

sectional area 4%perimeter) or the length of the

drying surface parallel to the direction of flow of

the air. For most tray-drying calculations, the equiva-

lent diameter is used. Values of n reported in the

literature range from 0.35 to 0.80. Where no specific

values are available, 0.80 is usually used in calcula-

tions. For situations where the air does not flow

parallel to the drying surface, alternative expressions

for h

are available. When some of the heat is

transferred to the drying surface by conduction or

radiation, an overall heat transfer coefficient taking

these sources of heat into account must be used.

0011Many solid foods do not exhibit a constant rate

period. Others do, but it is usually quite short. Some

recent work suggests that, if changes in dimensions of

the solid pieces during drying are taken into account,

more foods would exhibit a constant rate period.

0012As drying proceeds, a point is reached, C in

Figure 3,when the movement of moisture to the sur-

face is no longer fast enough to maintain the surface

in a saturated condition. Beyond this point, the drying

rate starts to fall and continues to do so throughout

the rest of the drying cycle. Point C is known as the

critical point and the moisture content at that point,

, the critical moisture content. The characteristics

of the material being dried and the drying conditions

influence the value of W

. Period C–D(Figure 1), is

known as the falling rate period. Many publications

have reported the existence of two or more falling

rate periods (see below).

0013During the falling rate period, the external condi-

tions, in particular the velocity of the air, have less

influence on the drying rate. This is determined by the

factors that affect the movement of water from

the interior of the solid to the surface. Numerous

mathematical models have been proposed to repre-

sent the falling rate period of drying. Many are based

on the mechanism whereby the moisture is assumed

to move to the surface. Several mechanisms have been

proposed, but the mechanism that has been most

widely accepted is that in which the moisture moves

by diffusion as a result of the concentration gradient

between the surface and the interior of the wet solid.

This type of movement may be represented by Fick’s

second law:

¼ D

, ð14Þ

where W is the moisture content (dwb), t is the time,

l is the distance, and D is the liquid diffusivity.

One well-known solution to this equation, in an

abbreviated form, for a slab-shaped solid, drying

from one large face only is:

W  W

 W



exp Dt







ð15Þ

t ¼



W  W

 W



 ln





, ð16Þ

where W is the average moisture content (dwb) at

time t, W

is the equilibrium moisture content

(dwb), W

is the moisture content at the start of the

falling rate period (dwb), and l is the depth of the slab.

1916 DRYING/Theory of Air-drying

This expression holds for values of (W W

)/W

 W

)

less than 0.60.

0014 Eqn (16) has been widely used to calculate drying

times during the falling rate period. However, when it

is used, it is assumed that D remains constant

throughout the falling rate period. It is widely

accepted now that the value of D changes with

changes in moisture, i.e., it decreases as drying pro-

ceeds. Many authors have found that the falling rate

period could be divided into two or more stages, with

each stage having its own value of D.

001 5 D is dependent on temperature. The relationship

most widely used to describe this dependence is of the

form:

¼ D

exp 



, ð17 Þ

where D

is the average effective diffusivity, D

is the

temperature-independent constant, Q is the energy of

activation for diffusion, R is the gas constant, and T is

the absolute temperature.

0016 Many empirical models, derived from experimen-

tal data, have been used to represent falling-rate

drying. One of the earliest and most widely used is:

¼K

ðW  W

Þ, ð18 Þ

where dW/dt is the rate of drying (rate of change of

moisture content) at a moisture content W (dwb), and

is the drying constant (or mass transfer coeffi-

cient). K

is related to temperature thus:

¼ a exp

b



, ð19 Þ

where a and b are empirical constants.

0017 Integrating eqn (18) gives:

W  W

 W

¼ exp  K

tðÞð20 Þ

t ¼

W  W

 W



:ð21 Þ

Note the similarity between eqns (12) and (17). Al-

though widely used, eqn (15) often does not apply to

the whole falling-rate curve. The introduction of an

empirical exponent, n, can widen its applicability

thus:

W  W

 W

¼ exp  K

ðÞ:ð22 Þ

Many other empirical models have been applied to

describe falling-rate drying. Most are only applicable

under conditions similar to those under which the

model was developed. Many are specific to a particu-

lar food or related materials.

0018Neither the diffusion model nor the empirical

model, outlined above, takes account of other changes

that may occur during drying. Shrinkage may lead to

changes in dimensions, density, and porosity of the

material undergoing drying. These changes may influ-

ence the pattern of drying. Changes in the thermal

properties of the material may influence heat transfer

and, in turn, the pattern of drying. Many more com-

plex models have been proposed that attempt to take

account of these and other changes. Treatment of such

models is outside the scope of this chapter.

0019Spray drying is widely used to dry liquids and

slurries. The feed is converted into a fine mist or

spray, which is brought into contact with heated air

in a large drying chamber. Very rapid drying takes

place, and a powder is formed, usually within 20 s.

Because of the very large surface area created, evap-

orative cooling maintains the surface of the droplets

close to the wet-bulb temperature of the air through-

out most of the drying. If it is assumed that drying

takes place under constant rate conditions, the time, t,

for a spherical droplet to dry from an initial moisture

content, W

, to a final moisture content, W

(dwb),

may be represented by the expression:

t ¼



 W

ðÞ

 #

ðÞ1 þ W

ðÞ

, ð23Þ

where r is the radius of the droplet, and r

is the

density of the liquid.

0020In practice, not all the moisture is removed under

constant rate conditions. More detailed treatments of

drying of droplets during spray drying are available in

the literature.

See also: Drying: Chemical Changes; Equipment Used in

Drying Foods

Further Reading

Brennan JG (1994) Food Dehydration – A Dictionary and

Guide. London: Butterworth-Heinemann.

Brennan JG, Butters JR, Cowell ND and Lilly AEV (1990)

Food Engineering Operations, 3rd edn. London: Else-

vier Applied Science.

Karel M (1974) Fundamentals of drying processes. In:

Spicer A (ed.) Advances in Pre-concentration and Dehy-

dration of Foods, pp. 45–94. London: Applied Science.

Masters K (1991) Spray Drying Handbook, 5th edn. New

York: Longman Scientific and Technical.

Mujumdar AS (1995) Handbook of Industrial Drying, 2nd

edn. New York: Marcel Dekker.

Wang N and Brennan JG (1995) A mathematical model of

simultaneous heat and moisture transfer during drying

of potato. Journal of Food Engineering 24: 47–60.

DRYING/Theory of Air-drying 1917

Drying Using Natural Radiation

M. Mathlouthi, Universite

de Reims Champagne-

Ardenne, Reims Cedex, France

Background

0001 Drying of certain crops and foodstuffs is a very an-

cient and relatively easy technique for food preserva-

tion. Sun-drying has been used by early civilizations

and is still used in many tropical and subtropical

countries where solar energy remains a low-cost

heat energy. However, the traditional method, which

involves spreading the product in a thin layer on the

ground or on a platform and exposing it directly to

the sun has its drawbacks. The quality of the dried

product is reduced owing to color deterioration by

direct exposure to UV radiation, as well as damage to

unprotected products by insects, rodents, and other

animals, and microbial spoilage, especially as the

relative humidity increases. Improvements in sun-

drying have led to solar drying. This technique uses

sun as a heat source, which increases the drying

capacity of air by heating it and reducing its relative

humidity in indirect solar dryers. Recent advances

have produced cheaper solar dryers, suitable for

the drying requirements of the crop, as well as an

improved quality of the dried product. Whatever the

drying system, using either direct exposure to the sun

or circulation (natural or forced) of heated air in solar

dryers, the driving force for dehydration remains the

gradient of water vapor pressure between the product

and the surrounding air. This explains why natural

ventilation is convenient in dry areas like arid zones

of Africa or the Artic, whereas the hot and humid

air of certain tropical regions is inefficient for drying

crops. Such climatic conditions necessitate the use of

adapted solar dryers.

0002 To take advangae of solar energy, collectors are

used, generally made from low-cost packaging films

readily available in developing countries where solar

dryers have been introduced with the aim of reducing

crop losses. Solar-energy-based dryers are generally

low-temperature drying systems. Equilibrium in rela-

tive humidity between the air and the product is

generally reached at a constant ventilation rate and

an average temperature around 50



C. This kind of

drying concerns crops that are dried in bulk in their

storage devices. Fruits with a high sugar and acid

content, like raisins, figs, apricots, or dates, are

particularly well suited to natural drying outdoors.

Sometimes, a pretreatment is needed if the dehydra-

tion rate and storage stability of the product are to be

improved.

Solar Drying Systems

0003Three types of application of solar energy are used to

dry crops:

0004Direct – absorption of solar radiation where nat-

ural convection of the surrounding air is the origin

of mass and heat transfer;

0005Indirect – application of solar radiation, which is

used to preheat air in a solar collector before blow-

ing it on the crop tray and releasing it through a

chimney;

0006Mixed-type drying systems, which use both direct

solar heat and natural ventilation of the drying

chamber with air heated in a solar air heater.

0007There is another type of classification that con-

siders solar drying systems as passive or active,

depending on whether they rely completely on solar

radiation and natural air flow, or only partly on

solar energy, and also on other heating systems, as

well as on motorized fans. All active solar dryers use

forced convection.

Direct Solar Radiation Drying

0008Traditional open-to-sun drying This involves

spreading the crop, or the food on the ground, or

placing it on horizontal shelves exposed to sun, air,

and natural ventilation. From time to time, the crop is

stirred to reduce the drying time. This type of sun

drying is common in developing countries. Although

this method seems simple and cheap, very often, crop

losses result from fungal and insect infestation, or

unexpected rain. The quality of the dried product is

sometimes poor, such as the loss of riboflavin in cous-

cous dried under UV sun radiation as compared with

industrial hot-air-dried products. Even though large

amounts of fruits and vegetables are still sun-dried,

the development of indirect solar dryers seems the

solution for the future if it is necessary to decrease

the drying time, improve the quality, and mini-

mize crop loss.

0009Direct solar-energy absorption dryer In this type of

dryer, the solar radiation is used directly in the dryer,

which is, in this case, a cabinet dryer equipped with a

fan and sometimes an additional electric heat source

(Figure 1), or a greenhouse tunnel covered with a

glass or a polyethylene plastic cover about 150 mm

in thickness and, in some cases, with UV and far-

infrared protection. In the greenhouse-type dryer

(Figure 2), the solar-heated air is circulated through

the product by a fan. A metallic structure made of

arches connected by longitudinal tubes and supported

on vertical lateral tubes is used. This structure is

covered with a polyethylene film of the same type as

1918 DRYING/Drying Using Natural Radiation

that used in cultivation greenhouses. Recycling of air

in a transparent plastic tube is possible. Fans at the air

inlet and exit allow an increased ventilation rate.

0010 Storage-type dryers have roofs that act as a solar

collector. In other cases, both the roof and the wall act

as collectors. The wall is usually black painted con-

crete covered with glass and is both a collector and a

means of heat storage.

0011 Indirect solar dryer In this type of dryer (also called

a distributed-type air-circulation solar dryer), the crop

is placed on shelves in an opaque drying chamber. The

drying air is heated in a separate solar collector and

distributed by either natural or forced (fan) circula-

tion. In this dryer, the crop is not exposed to sun, thus

preventing vitamin loss and other UV-sensitive

reactions like lipid oxidation. The air temperature

is higher than in direct sun-dryers, which reduces

the drying time. A typical solar dryer is shown in

Figure 3. Clearly, the equipment and maintenance

costs of this type of drier are more than that of the

direct sun dryer. Efficiency is increased if forced-air

convection is installed. The optimization of indirect

solar dryers also depends on the quality of the solar

energy heating the collectors.

0012Mixed solar dryer This type of dryer incorporates

both direct and indirect solar-energy drying. The

dryer consists of a preheated air chamber, a drying

chamber and a chimney. However, the drying cham-

ber in this case is glazed so that the product also

receives direct solar energy as well as a flow of drying

air (Figure 4). Depending on the countries and

product, the heat absorber may be made of low-cost

recovered byproducts, as in rice drying, where a layer

of burnt rice husk covered with a transparent plastic

sheet on an inclined bamboo network is used as the

air heater’s absorber. Direct absorption of solar heat

in the drying chamber takes place through a transpar-

ent plastic film, which also protects the crop against

rain and other degradation agents. In the multi-

stacked dryer, different products can be dried at the

same time on a series of shelves. Also, the drying

chamber has a column exposed on one side to the

sun through glazing and, on the opposite side, a

wooden access door and vents, thus obviating the

need for a chimney.

Solar radiation

Glazing

Drying product

Air inlet

Air outlet

Insulated wall

fig0001 Figure 1 Cabinet solar dryer.

air

fig0002 Figure 2 Greenhouse-type solar dryer.

Humid air

Insulated wallFanFiltration screen

for inlet air

Chimney

Transparent side

Absorbing surface

Air outlet

Solar

radiation

Solar

radiation

Transparent film

Air

fig0003 Figure 3 Indirect solar dryer consisting of a collector, drying chamber, and chimney.

DRYING/Drying Using Natural Radiation 1919

Solar Heat Collectors

0013 Solar collectors take advantage of the thermal energy

of solar radiation. There are two types: the bare flat-

type collector and the covered-plate heat absorber.

The surface of the plate is usually painted black; this

absorbs sun radiation and transmits it in the form of

heat to the drying air. The air flow through the col-

lector is by natural convection or by means of a fan.

The efficiency of a collector can be determined by

measuring the inlet and outlet temperatures, the

surface of the collector and flow rate of the air, and

the solar energy available.

Bare Flat-plate Collectors

0014 These collectors consist of a simple air duct with the

upper side acting as a heat absorber and the lower

surface acting as an insulator (see Figure 5). Import-

ant losses of thermal energy are observed at the upper

surface, which limits the use of such collectors to low-

temperature drying like crop drying in storage silos.

In this type of collector a flat plate of aluminum can

be used as a heat absorber. The heat loss is reduced by

placing the air duct in a box filled with glass wool

insulation.

Covered-plate Heat Collector

0015To minimize heat losses from the upper side of the

collector, glass, plexiglass, or transparent plastic is

used as a cover material, placed parallel to the

metallic absorber plate. Covered-plate solar-energy

air heaters are much more efficient than bare-plate

collectors. The air to be heated can be sucked between

the absorber and the transparent cover, or sometimes

underneath the absorber to prevent dust contamin-

ation at the surface of the absorber. In this case, the

air circulates between the lower face of the absorber

plate and the insulation layer, the upper surface of

which is in contact with a static layer of air under-

neath the glazing. This type of back-pass air heater

(Figure 6) is more efficient than one where air flows

between the absorber and the cover.

Efficiency of Solar-heat Collectors

0016Efficiency is defined as the ratio of thermal energy

used to heat the drying air over the total solar

energy absorbed by the collector in the same time

period. The heat used to warm air in the collector is

expressed by:

¼ m

 TðÞ,

where Q

is the heat transmitted to air in the collector

(J s

1

), m

is the mass flow rate of air through the

collector (kg s

1

), C

is the specific heat capacity of

air (J kg

1

), T

is the collector outlet temperature,

and T is the inlet temperature (generally ambient).

0017If A is the collector area (m

) and I the flux of

incident solar radiation (W m

2

), the total incident

energy of the exposed surface of collector is Q

¼ AI,

and the efficiency ratio (Z) can be expressed as:

 ¼

¼ m

 TðÞ

0018In forced-convection air dryers, m

is easy to meas-

ure, as well as the inlet and outlet temperatures. For

natural convection, the flow rate of air is difficult to

Shelf

Hot air

1.50 m

0.5 m

Access

door

Solar

radiation

Glass wall

Transparent roof

Black

galvanized

iron sheet

Vent

fig0004 Figure 4 Mixed solar-energy dryer.

Airflow Absorber

InsulationBox

fig0005 Figure 5 Flat-plate collector.

InsulationBox

Air inlet

Glazing Absorber

fig0006Figure 6 Back-pass air heater.

1920 DRYING/Drying Using Natural Radiation

determine precisely, hence the efficiency ratio is meas-

ured. Incident solar radiation absorbed by collector

depends on the properties of the material used for

the design of the collector like absorbance, transmit-

tance, and reflectance. It is also a function of the

position of the sun relative to the collector, the

solar altitude angle, and the sunrise and sunset of

the collector orientation. A value of 1 kW per square

meter of exposed surface may be considered as an

optimum.

Application of Solar Drying to Selected

Crops

0019 Solar drying is an effective mode of dehydration

especially for certain crops (cereal grains, grapes,

figs, sweet pepper, thyme) in countries with good

sunshine during the harvest period. Many developing

countries have sufficient insolation for sun or solar

drying, but the dryer needs to be of the right design to

minimize crop losses resulting from contamination by

insects, molds, and poor storage conditions. In de-

veloping countries, greenhouses can be used as solar

dryers during the warm period of the year, thus redu-

cing the cost of investment of setting up a new dryer.

Solar collectors may be made from low-cost poly-

ethylene film traditionally used to cover greenhouses.

In all cases, covered heat collectors and drying cham-

bers are preferable to open-air sun dryers to prevent

crop spoilage.

Solar Drying of Grapes

0020 Traditional drying of grapes is still used in many

countries and involves spreading or hanging the

product on galvanized wires (with a coverage of

about 15–20 kg m

2

) on the racks. A typical dryer

contains six to 10 racks each about 50 m long, 2 m

tall, and 1.5 m wide. Racks are placed on open high

land to allow free air flow. The period of drying varies

from 8 to 20 days, depending on the grape variety,

treatment, and climate conditions. The water content

in the grapes is reduced from about 3 kg of water per

kilogram of dry matter in fresh product to about

0.16 kg of water per kilogram of dry matter in the

dried product. Solar dryers using either direct or

indirect solar energy have higher drying rates than

sun dryers. These rates are improved when grapes

are pretreated. Pretreatment solutions contain 4–5%

and 1–2% dipping oil. Changing the surface

tension of grape by dipping for a few minutes in

dipping solution increases water diffusion through

the waxy cuticle and decreases the drying time.

0021 The quality parameters of solar-dried grapes, espe-

cially color, moisture content, and acidity, are more

uniform than those of the sun-dried product. Storage

stability is improved with SO

gas pretreatment and

conditioning in a modified atmosphere (1% O

; 20%

) packages.

Solar Drying of Other Crops

0022For obvious economic reasons, if a solar dryer is

preferred to an open-air sun dryer, it should work to

full output. Therefore, it has to be a multipurpose

dryer equipped for both low-temperature (below



C) and high-temperature (above 50



C) drying,

eventually using the calories produced by the com-

bustion of agricultural waste as part of the total ther-

mal energy required. Tunnel greenhouse dryers are

very efficient during sunny days and can be used

with an auxillary combustion heating system to

ensure a continuous operation. This type of solar

dryer is used for sweet pepper and garlic in Argentina.

In terms of mold contamination, the quality of the

product is better on sunny days than on cloudy,

humid days. A complementary heating source is

needed to improve drying during humid periods and

to increase the working time. Products like thyme can

be dried at about 50



C using a wire basket solar

dryer. Sun-dried thyme herb is suitable for packaging

or grinding as a powder. In terms of the essential oils

extracted after drying, sun-dried thyme is of a better

quality than that of oven-dried thyme, but the

percentage of oleoresin is the same for both drying

methods.

Quality of Solar-dried Products

0023Sun drying usually takes too long, and even solar

drying lasts several days. Such a long process favors

degradation reactions, especially when the water

activity is relatively high, above 0.60. Moreover,

the storage conditions should be controlled so that

the temperature is maintained below 10



C for grain

and nut storage, and the relative humidity below

40%. This requires cooling and dehydration of air.

However, if precautions (e.g., the use of clean crops)

are taken to minimize spoilage, high-quality sun-

dried products can be produced. Other precautions

include the use of covered plate absorbers and the use

of clean drying air. The first stages of dehydration

(first and second day) should be carried out on

sunny and dry days. The most dangerous contamin-

ation is fungal damage, which can yield mycotoxins.

Among the mycotoxins, such as ochratoxin, citrinin,

and ergosterol found in grains and nuts, aflatoxins

are the most important as regards toxicity, occur-

rence, and economic consequences, especially in

developing countries.

0024Other degradation reactions such as browning,

lipid oxidation or vitamin loss may be minimized by

using adequate protection in the dryer. For example,

DRYING/Drying Using Natural Radiation 1921

it is preferable to use low temperatures as well as the

UV radiation filtration using colored glazing.

Conclusion

0025 Solar drying has proved to be technically and eco-

nomically valuable for several crops. It is, however,

necessary to develop large-scale dryers that may be

used throughout the year for different products to

make them attractive to the farmers.

0026 Protection against UV radiation, dust, insects,

mold, and other sources of contamination, as well

as temperature and relative humidity control, are

needed to improve the quality of the product. Like-

wise, the storage conditions of solar-dried products

should be tailored to the crop and the packaging

optimized if a long shelf-life is required.

See also: Drying: Theory of Air-drying; Physical and

Structural Changes; Equipment Used in Drying Foods

Further Reading

Condori M, Echazu R and Saravia L (2001) Solar drying of

sweet pepper and garlic using the tunnel greenhouse

driers. Renewable Energy 22: 447–460.

Ekechukwu OV and Norton B (1999) Review of solar-

energy drying systems II: an overview of solar drying

technology. Energy Conversion & Management 40:

615–655.

Exell RHB (1980) A basic design theory for simple solar

dryes. Renewable Energy Review Journal 1: 1–14.

Pangavhane DR and Sawhney RL (2002) Review of

research and development work on solar dryers for

grape drying. Energy Conversion & Management 43:

45–61.

Fluidized-bed Drying

J G Brennan, The University of Reading, Reading, UK

Principles of Fluidization

0001 If air is made to flow through a bed of solid particles

supported by a perforated plate or grid, frictional

drag will cause a pressure drop to develop across the

bed. The drag, and hence the pressure drop, will

increase as the air velocity increases. When the fric-

tional force overcomes the weight of the particles they

move apart, and the bed expands and begins to behave

like a fluid. In this state it is known as a ‘fluidized bed.’

The air velocity at which fluidization first occurs is

known as the ‘incipient (or minimum) fluidization

velocity.’ This velocity can be calculated, for spherical

particles, using the relationship:

ðr

 rÞg

180ð1  eÞ

where n

(m s

1

) is the minimum fluidization velocity,

(kg m

3

) is the density of the solid particles,

r (kg m

3

) is the density of the air, g (m s

2

) is accel-

eration due to gravity, m (N s m

2

) is the viscosity of

the air, d (m) is the diameter of the particles, and e is

the voidage of the bed (the fraction of the bed that is

void). As the air velocity increases above the min-

imum value, bubbles of air pass up through the bed.

These bubbles are small when they leave the perfor-

ated plate. As they pass up through the bed they

coalesce to form larger bubbles. The movement of

these bubbles causes mixing of the solid particles in

the bed. As the air velocity increases, mixing becomes

more vigorous. At some stage particles will leave the

surface of the bed and fall back on to it. At a still

higher velocity the particles become entrained in the

air and are carried away in the exhaust air stream.

The velocity at which this first occurs is known as the

‘entrainment velocity.’ This velocity may be calcu-

lated, for spherical particles, by means of the rela-

tionship:

ﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ

4dðr

 rÞ



where n

(m s

1

) is the entrainment velocity and C

is the drag coefficient. C

has a value of 0.44 when

the Reynolds number of the air is in the range

500–200 000. When heated air is used to fluidize a

bed of particles containing moisture, dehydration of

the particles will occur. This is the principle of the

fluidized-bed drier.

Fluidized-bed Driers

0002Food particles suitable for fluidized-bed drying

should be in the size range 20 mm to 10 mm, uniform

in size and shape, and not very sticky. They should

not break up easily when subjected to impacts and

abrasion in the bed. Some food materials, e.g., cereal

grains, fluidize readily over a wide range of moisture

contents. Other foods will only fluidize at low mois-

ture content. In the latter cases the wet product may

be fluidized by dispersing it in a bed of already dried

material.

0003Fluidized-bed drying chambers equipped with a

mechanical agitator to help disperse wet or sticky

materials are available. The agitator is located near

1922 DRYING/Fluidized-bed Drying

the feed inlet. It disintegrates the feed and disperses

the pieces into already dry material, thus facilitating

fluidization. Some semisolid foods may be converted

into a form suitable for fluidized-bed drying by

mixing them with some dried material, and extruding

and cutting the mixture into suitably sized pieces.

0004 The air velocity should be as high as possible to

promote rapid drying. The upper limit is often deter-

mined by the loss of fine product, created in the bed,

with the exhaust air. Some loss of fines may be toler-

ated in order to achieve high drying rates. The fine

particles may be recovered from the outgoing air by

means of cyclones or fabric filters. Typical air veloci-

ties for fludizing particles with densities in the range

1000–2000 kg m

3

are presented in Table 1.

0005 For most food applications the support for the bed

is a simple perforated plate. For very fine particles

more complex designs incorporating jets or bubble

caps, similar to those in distillation plates, may be

used. Fluidized-bed driers usually operate at atmos-

pheric pressure. However, by suitable location of fans

they may be run at pressures above or below atmos-

pheric pressure. Heater plates may be incorporated

into the drying chamber to improve heat transfer.

These are not common in food applications. The

drying conditions can be accurately controlled in

batch-operated fluidized beds. High rates of drying

and uniform moisture content in the product can be

achieved. However, batch units are only used for

small-scale operations.

0006 There are two types of continuous fluidized-bed

drier which differ in the pattern of flow of the par-

ticles and air. The plug-flow type (Figure 1a) has a

relatively long narrow bed. The feed is introduced at

the wet end and moves to the dry end in a plug-flow

pattern where the dry product is discharged over an

adjustable weir. The distribution of the residence time

of the particles is relatively narrow so that uniform

drying is accomplished. However, its application is

limited to feed particles which are readily fluidizable.

In the well-mixed, or backed-mixed, fluidized bed

(Figure 1b) the ratio of the length of the bed to its

width is unity or below. The particles are vigorously

mixed and the bed will have a relatively uniform

temperature. The wet feed is introduced into a bed

of relatively dry particles and is quickly dispersed.

Thus, it can handle feeds that are difficult to fluidize.

On the other hand, there is a broader distribution of

particle residence time, as compared with the plug-

flow design, and the product moisture content may be

less uniform. The well-mixed bed is best at removing

surface moisture, whereas the plug-flow type removes

moisture from within the particles. The latter type is

most widely used for food applications. Figure 2

shows a continuous plug-flow fluidized-bed drier

equipped with a cyclone for the recovery of fines.

Vibrated Fluidized-bed Driers

0007Fluidized-bed driers may be fitted with vibrating

bases, to help move the product horizontally from

tbl0001 Table 1 Typical gas velocities for fludidizing different-sized

particles

Averageparticlesize(mm) Velocity (m s

1

)

100 – 300 0.2 – 0.4

300 – 800 0.4 – 0.8

800 – 2000 0.8 – 1.2

2000 – 5000 1.2 – 3.0

From Hovmand CV (1995) Fluidized bed drying. In: Mujumdar AS (ed.)

Handbook of Industrial Drying, 2nd edn, pp. 195–248. New York: Marcel

Dekker with permission.

(a)

(b)

fig0001Figure 1 Representations of (a) a continuous plug-flow fluid-

ized-bed drier and (b) a continuous well-mixed fluidized-bed

drier. Adapted from Mujumdar AS (ed.) (1995) Handbook of Indus-

trial Drying, 2nd edn, by courtesy of Marcel Dekker: New York.

DRYING/Fluidized-bed Drying 1923

the wet end to the dry end. This motion is gentle

and suitable for handling fragile materials and

agglomerates susceptible to mechanical damage. A

vibrating bed is capable of handling particles less

uniform in size as compared with the stationary-

type bed. A vibrating bed is also better at handling

sticky materials. The air velocity required to keep the

particles moving is usually much less in a vibrated bed

than in a stationary one. Bases vibrate in the fre-

quency range 5–25 Hz and a half amplitude of a few

millimeters. Such beds are also known as ‘vibro-

fluidizers.’

Multistage Fluidized-bed Drying

0008 A number of fluidized-bed driers may be used in

series. This enables better control of the drying con-

ditions at different stages in the cycle and can yield a

product with a more uniform moisture content as

compared with a single-stage drier. The use of a

multistage system may also lead to energy savings.

One possible combination is a well-mixed bed, to

remove surface moisture, followed by a plug-flow

bed to complete the drying. Alternatively, a number

of plug-flow beds may be used in series. The latter

arrangement is mainly used for the food applications.

Up to six stages may be used. Vibrating beds are

usually employed. The particles fall under gravity

from one stage to the next. Such an arrangement is

also known as a ‘cascade system.’

Applications

0009Single-stage and multistage fluidized-bed drying

systems are used for a wide range of food materials.

These include peas, sliced beans, diced carrots,

onions, potato granules, meat cubes, grains, flour,

salt, and sugar. The air temperature is selected to

suit each product. Vibrating fluidized beds are used

as after driers and coolers following spray drying. The

Air heater

Air in

Porous plate

or gird

Feed in

Exhaust air out

Fan

Fines

Weir

Product out

Screw conveyor

fig0002 Figure 2 A continuous plug-flow fluidized-bed drier equipped with a cyclone for the recovery of fines. From Brennan JG, Butters JR,

Cowell ND and Lilly AEV (1990) Food Engineering Operations, 3rd edn. London: Elsevier Applied Science with permission.

1924 DRYING/Fluidized-bed Drying

powder from the spray drier has a moisture content of

5–7%. This is reduced to 3–4% on a vibrofluidizer

using dry air at a relatively low temperature. Such

two-stage drying is said to facilitate better control

over the quality of the product and lead to savings

in energy as compared with straight-through spray

drying. Some agglomeration of the powder particles

may occur in the fluidized bed which can improve the

reconstitution characteristics of the product. Such a

system is shown in Figure 3. The product may be

cooled in the same vibrofluidizer or in a second unit

in series with it. Cooling the product facilitates hand-

ling and packaging. Spray driers are available with

fluidized beds built into the main drying chamber.

These are known as integrated fluidized-bed spray

driers.

Spouted-bed Drier

0010 One particular design of fluidized-bed drier is the

spouted bed. In this type of drier the drying air enters

at high velocity through a nozzle located centrally in

the conical base of the chamber (Figure 4). This

creates a spout of fast-moving particles up through

the center of the bed. These particles return to the

base of the chamber as a slow-moving annular bed

surrounding the spout. A proportion of the inlet air

flares out into this annular bed and flows upwards,

countercurrent to the downward-moving particles, at

a relatively low velocity. In the spout, high rates of

heat and mass transfer occur, resulting in high initial

rates of drying. As a result of evaporative cooling

the temperature of the particles in the spout is kept

relatively low. The later stages of drying take place in

the annular bed under relatively mild conditions. This

type of drier is suitable for drying relatively large

particles, greater than 5 mm in diameter. It can cope

with heat-sensitive materials. During start-up a high

fig0003 Figure 3 A two-stage spray-drying process for milk: 1, air filter; 2, heater; 3, cooler; 4, spray drier; 5, cyclone; 6, exhaust fan;

7, fluidized-bed drier; 8, return line for powder. From Mujumdar AS (ed.) (1995) Handbook of Industrial Drying, 2nd edn, by courtesy

of Marcel Dekker: New York.

Air in

Screen

Spout

fig0004Figure 4 The principle of the spouted-bed drier. From Brennan

JG (1994) Food Dehydration ^ A Dictionary and Guide. Oxford:

Butterworth-Heinemann with permission.

DRYING/Fluidized-bed Drying 1925