CHAPTER ONE1.0 INTRODUCTION

Spray drying is an essential unit operation for the manufacture of many products with specific powder properties. It is characterized by atomization of a solution or suspension into droplets, followed by subsequent drying of these droplets by evaporation of water or other solvents.

The manner in which the spray droplets contact the drying medium determines their subsequent drying behaviour and in turn greatly influences the properties of the final product. The form of spray-air contact is determined by the location of the atomizer relative to the air inlet. Broadly speaking, the flow may be considered either ‘co-current’ in which the spray and gas flow in the same direction or ‘counter-current when the spray and gas flow in opposite directions.

Spray drying is used for the manufacture of many consumer and industrial products such as instant food products, laundry detergents, and pharmaceuticals. It is well suited to continuous production of dry solids in powder, granulate or agglomerate particle form from liquid feed-stocks.

Some advantages of spray drying include the ability to produce a dry powder rapidly and the ability to control the particle size distribution. The limitations of spray drying include problems with efficient particle collection and the potential instability of materials sensitive to high temperatures.

1.1 AIMS AND OBJECTIVES The objective of this study is to (a)Demonstrate spray drying of liquid and dissolved solids by varying parameters such as: Air flow rate, Air temperature, Composition Of the feed solution, Flow rate of the feed, Distributor pressure.

(b)Examine the effects of various spray-drying process parameters on the solid product by varying one process parameter and keeping the others constant.

CHAPTER TWO

2.0 LITERATURE REVIEW

2.1 THE DRYING PROCESS Drying is the process of removing liquid from solids

by evaporation. The drying process has been used

for thousands of years to reduce transport weight

and increase the storage life of numerous products

and materials. For centuries, drying meant spreading

a product out in the open air and letting the sun

provide the energy for water evaporation. With the

dawn of the industrial age, many different drying

processes have been developed to increase drying

speed and improve product quality and uniformity.

Dryers are the devices used for carrying out the drying process.

Dryers can be classified on the basis of heating mode or

on the basis of type of feed compartment.

(a) ON THE BASIS OF HEATING MODE

Convective/Direct dryers/ADIABATIC Conductive /Indirect dryers/NON-ADIABATIC Dryers by radiant energy

(b) ON THE BASIS OF FEED COMPARTMENT

•TRAY DRYERS•SCREEN-CONVEYOR DRYER•ROTARY DRYER•TOWER DRYER•SREW-CONVEYOR DRYER•FLUID BED DRYER•FLASH DRYER•SPRAY DRYER•THIN FILM DRYER•DRUM DRYER

2.2 SPRAY DRYERS

2.2.1 Brief History Of Spray Dryers

The development of spray drying equipment and

techniques evolved over a period of several decades

from the 1870s through the early 1900s. The first

known spray dryers used nozzle atomizers, with

rotary atomizers introduced several decades later.

Different types of spray dryers are used for various purposes in different fields ranging from

laboratory sale to industrial sectors. During the last three decades spray drying has undergone

an intensive research and development, so that modern spray drying equipment can meet the

requirements to produce a powder with tailor-made specifications required by the end-user.

One of the first spray drying patents was applied for in 1901 by the German Mr. Stauf, who

sprayed the milk by nozzles into a chamber with warm air. The first real break-through,

however, was in USA in 1913, when the American Mr. Grey and the Dane Mr. Jensen

developed a nozzle spray dryer and started to produce and sell drying installations on a

commercial scale.

Because of the relatively unsophisticated designs of the early spray dryers and practical difficulties in operating them continuously, very little commercial use of the process was made until the 1920s.

By the second decade of the twentieth century, the

evolution of spray dryer design made commercial

operations practical. Milk drying was the first major commercial application of the technology. During the next 20 years, manufacturers developed designs to

accommodate heat-sensitive products, emulsions

and mixtures. Spray drying came of age during

World War II, with the sudden need to reduce the

transport weight of foods and other materials. This

surge in interest led to developments in the

technology that greatly expanded the range of

products that could be successfully spray dried.

The first rotary atomizer was developed by the German Mr. Kraus in 1912, but not until

1933, when the Danish engineer Mr. Nyrop filed his world patent, which was the real breakthrough

of atomization.

In the world of industrial dryers, there are few types that accept pumpable fluids as the feed material at the inlet end of the process and produce dry particulate at the outlet.

Spray drying is unique in its ability to produce powders with a specific particle size and moisture

content without regard for the capacity of the dryer and the heat sensitivity of the product. This flexibility makes spray drying the process of choice for many industrial drying operations.

2.2.2 BASIC STEPS IN SPRAY DRYING

Spray drying is used to dry liquid products. The product to be dried is sprayed into

a stream of heated air. Water evaporates into the air leaving the dry particles to be

Collected.

There are some basic steps that are usually associated with the spray drying process;

2.2.2.1 CONCENTRATION OF FEEDSTOCK

Feedstock is normally concentrated prior to introduction into the spray dryer. The concentration stage increases the solids content thereby reducing the amount of liquid that must be evaporated in the spray dryer.

The limit on the extent of pre-concentration of the

feed is dictated by the viscosity of the liquid, which must not be so high, that the product

cannot be pumped or atomized. For milk powder manufacture, it is common to preconcentrate

the milk (9% total solids in skim milk; 13% total solids in whole milk) to 45% in

an evaporator. For many protein isolates, such a high concentration cannot be used, because

most protein solutions are very viscous.(20) In this case, spray drying must be done with a

concentrate of about 25% total solids concentration. This practice, however, causes the

powder particles to have a lower density. Therefore, these products are typically very light

and fluffy and the unit cost of operation increases dramatically

2.2.2.2 ATOMIZATION

Producing droplets of specific size and

surface area by ATOMIZATION is a critical

step in the spray drying process. The

degree of atomization, under a set of drying

conditions, controls the drying rate,

and therefore the required particle residence

time, and therefore the dryer size.

All of the atomizing techniques can give

good average particle size control,but there

are major differences in the particle size distribution created. The most commonly

employed atomization techniques are:

2.2.2.2.1 PRESSURE NOZZLE ATOMIZATION

A spray is created by forcing the fluid through an

orifice. The energy required to overcome the

pressure drop is supplied by the feed pump. The

narrowest particle size distribution is possible

with this technique.Must be used when minimization

of "fines" is important to the product. The average

particle size produced for a given feed is primarily

a function of the flow per nozzle, the nozzle

orifice pressure drop (ΔP2),and the spray angle. The

spray angle is varied by ΔP1. The higher ΔP1, the

greater the spray angle. The most energy efficient

of the atomization techniques. Requires routine

changing of the internal pieces, usually made of

tungsten carbide. Changing schedule depends upon the

application. Limited to approximately 0.4 GPM flow

per nozzle with a slurry because of potential

plugging with the small orifice

required.

With multiple nozzle dryers, a problem with one

nozzle does not shut operations down.

Typically requires a piston-type positive

displacement pump.

Control of wall buildup can be achieved through

variations of the spray angle.

It Can reduce the capital cost for a dryer because

of reduced diameter required

Figure 2.1: An example of a pressure nozzle

2.2.2.2.2 TWO-FLUID NOZZLE ATOMIZATION

A spray is created by contacting two fluids, the

feed and a compressed gas. The atomization energy is

provided by the compressed gas, usually air. The

contact can be internal or external to the nozzle. A

broad particle size distribution is generated. The

average particle size produced for a given feed is

primarily a function of the flow per nozzle, and the

compressed gas rate and pressure. The least energy

efficient of the atomization techniques. Useful for

making extremely fine particles (10-30 micron)

because of relatively high wear resistance. Also for

small flow rates typically found in pilot scale

dryers. Requires periodic changing of the air and

liquid caps. Can typically use any type of feed

pump. Control of the spray angle is limited. Capital

cost can be lower due to the absence of the pressure

pump and rotary atomizer.

FIGURE 2.2 An example of a two-fluid nozzle

2.2.2.2.3 CENTRIFUGAL ATOMIZATIONA spray is created by passing the fluid across or

through a rotating wheel or disk. The energy

required for atomization is supplied by the atomizer

motor. A broad particle size distribution is

generated. The average particle size produced for a

given feed is primarily a function of the diameter

of the wheel and the RPM.

Requires relatively high gas inlet velocity to

prevent wall buildup, which can increase the amount

of fines produced. Can generally be run for longer

periods of time without operator interface. Usually

the most resistant to wear.

Requires periodic changing of wheel inserts, usually

made of tungsten carbide.

Control of wall buildup is minimal, due to direction

of spray (horizontal) and broad particle size

distribtion, forcing the dryer to be relatively

large in diameter.

Capital cost of the atomizer is typically high.

Comparatively larger diameter dryer can increase

capital cost. As with any high speed rotating

machine, maintenance costs are high. Design of

dryer roof and atomizer support add to fabrication

cost.

FIGURE 2.3 An example of a centrifugal atomizer

2.2.2.3 DROPLET-AIR CONTACT

The central element of a spray dryer is the spray

dry chamber. In the chamber, atomized liquid is

brought into contact with hot gas (usually air, at a

vacuum), resulting in the evaporation of 95%+ of the

water contained in the droplets in a matter of a few

seconds. The way in which the spray makes contact

with the air in the dryer influences the behavior of

the droplet during the drying phase and has a direct

bearing on the properties of the dried product. The

type of contact between the spray and the air is

determined by the position of the atomizer relative

to the air inlet. Nozzle headers are usually located

at the top of the dryer and spray down.

2.2.2.4 DROPLET DRYING

Moisture evaporation takes place in two stages.

During the first stage, the temperature in the

saturated air at the surface of the droplet is

approximately equal to the wet-bulb temperature of

the drying air. There is sufficient moisture in the

drop to replace the liquid evaporated at the surface

and evaporation takes place at a relatively constant

rate. The second stage begins when there is no

longer enough moisture to maintain saturated

conditions at the droplet surface, causing a dried

shell to form at the surface. Evaporation then

depends on the diffusion of moisture through the

shell, which is increasing in thickness. The rate of

evaporation falls rapidly during the second phase.

Different products have differing evaporation and

particle-forming characteristics. Some expand,

others contract, fracture or disintegrate.

The resulting particles may be relatively uniform

hollow spheres, or porous and irregularly shaped.

2.2.2.5 SEPARATION Following completion of drying, the particles of

product must be separated from the drying air.

Primary separation is accomplished by the particles

simply falling to the bottom of the chamber. A small

fraction of the particles remain entrained with the

air

and must be recovered in separation equipment.

Cyclones, bag filters, and electrostatic

precipitators

may be used for the final separation stage. Wet

scrubbers are then often used to purify and cool the

air so that it can be released to atmosphere.

2.3 BASIC FEATURES AND CONFIGURATIONS OF A SPRAY DRYER A SPRAY DRYER, as the name implies, is a device for

drying, utilizing a spray. Spray drying entails

intimate mixing of a heated gas with an atomized

(sprayed) liquid stream within a vessel (drying

chamber) to accomplish evaporation through a direct

contact, adiabatic process.

The unit operation of a SPRAY DRYER includes the following key components:

_ A method for ATOMIZING a solution or slurry._ An air/gas HEATER, or a source of hot air such as a waste flue gas.

_ A gas/spray MIXING CHAMBER with adequate residence time and droplet trajectory distance for

achieving the heat and mass transfer.

_ A means for RECOVERING the solids from the gas stream.

_ A FAN to induce the required air/gas flow through the system.

2.3.1 SPRAY DRYER CONFIGURATIONS

2.3.1.1 CO-CURRENT OPERATION

In a co-current dryer (Fig. 2.4), the spray is

directed into the hot air entering the dryer and

both pass through the chamber in the same

direction. Co-current dryers are the preferred

design for heat-sensitive products because the

hottest drying air contacts the droplets at their

maximum moisture content. Spray evaporation is

rapid, and the temperature of the drying air is

quickly reduced by the vaporization of water. The

product does not suffer from heat degradation

because the droplet temperature is low during most

of the evaporation time. Once the moisture content

reaches the target level, the temperature of the

particle does not increase greatly because the

surrounding air is now much cooler. Dairy and other

heat-sensitive food products are usually dried in

co-current dryers.

FIGURE 2.4 CO-CURRENT FLOW SPRAY DRYER.

1. feed storage

2. pump

3. drying chamber

4. air heater

5. cyclone

6. gas scrubber

7. separator

2.3.1.2 COUNTER-CURRENT OPERATION

In this dryer design (Fig. 2.5), the spray and the

air are introduced at opposite ends of the dryer,

with the atomizer positioned at the top and the air

entering at the bottom. A counter-current dryer

offers more rapid evaporation and higher energy

efficiency than a concurrent design. Because the

driest particles are in contact with hottest air,

this design is not suitable for heat-sensitive

products. Counter-current dryers normally use

nozzles for atomization because the energy of the

spray can be directed against the air movement.

Soaps and detergents are commonly dried in counter-

current dryers.

FIGURE 2.5 COUNTER-CURRENT FLOW SPRAY DRYER

1. drying air

2. feedstock

3. dried product

4. drying chamber

5. cyclone

6. wet scrubber

7. bag filter

8. electrostatic precipitator

2.3.1.3 MIXED FLOW OPERATION

Dryers of this type (Fig. 2.6) combine both

concurrent and counter current flow. In a mixed

flow dryer, the air enters at the top and the

atomizer is located at the bottom. Like the

counter-current design, a mixed flow dryer exposes

the driest particles to the hottest air, so this

design is not used with heat-sensitive products.

FIGURE 2.6 MIXED FLOW SPRAY DRYER

1. drying air

2. feedstock

3. dried product

4. drying chamber

5. cyclone

6. wet scrubber

7. bag filter

8. electrostatic precipitator

2.3.2 SPRAY DRYER FEEDSTOCK One important advantage of spray drying is that the

feedstock can be in virtually any form that can be

pumped. Solutions, slurries, pastes, gels and

suspensions can be successfully spray dried. The

first step in the spray drying process is to

prepare the feedstock for spraying by optimizing

the temperature, concentration, viscosity or other

characteristic.

2.3.2.1 IMPORTANT FEED STOCK PROPERTIESa) Feed temperature

The temperature of the feedstock affects the

viscosity and the transfer of heat from the drying

air in the chamber to the droplets. Both the

temperature of the feedstock and the temperature of

the chamber air need to be considered when

selecting nozzle seal materials.

b) Melting temperature

Some feedstock is solid at room temperature and

must be melted in order to atomize them. Prilling

or spray cooling involves forming pellets or

crystals by spraying melted feedstock into a

chamber through which cooling air is flowing.

c) Abrasion

The abrasiveness of the feedstock must be

considered when selecting the material for the

internal nozzle components. For most applications,

tungsten carbide is the material of choice for

Twist & Dry swirls and orifice disks. Tungsten

carbide has excellent resistance to abrasion and

good corrosion resistance for most feedstock.

d) Corrosion

For some feedstock, corrosive attack on the nozzle

components is a greater problem than abrasion. For

these cases, 316 stainless steel or nickel alloy

C22 may be good choices.

e) Specific gravity

The specific gravity is the density of the

feedstock relative to water. A specific gravity

greater than 1.0 means that the feedstock is denser

than water.

Increasing the specific gravity reduces the flow

through a nozzle. Converting your density units to

g/mL will yield the specific gravity value, since

water has a density of 1 g/mL.

f) Solids content

The solids content is the percent of the feedstock

that is composed of solids. Most feedstock has

about 50% solids, although the range is from about

20% to 70%. Increasing the solids content reduces

the amount of moisture removed in the spray drying

process. As the solids content increases, the

feedstock becomes more difficult to pump and

atomize.

g) Surface tension

Surface tension is the force acting on the surface

of a liquid that tends to minimize its surface

area.

Reducing surface tension makes a feedstock easier

to atomize.

h) Viscosity

Viscosity is the resistance to flow of fluids. The

most commonly used unit is the centipoise.

Increasing viscosity tends to increase droplet

size. For some nozzle designs, including the BETE

Twist & Dry, increasing the viscosity tends to

increase the flow.

2.3.3 SPRAY DRYER PRODUCT(POWDER)

Spray dryers transform liquid feedstock into

particles of powder. Like liquid feedstock, powders

have important properties that are monitored during

the drying operation.

a) Powder Shape

Many spray drying operations produce

sphericalparticles while others result in non-

spherical particles. Particles may be hollow or

solid. Nonspherical particles are characterized by

their aspect ration, which is the ratio of their

longest dimension to their shortest dimension.

b) Powder Size

It is important to differentiate between droplet

size and particle size because the two are

generally not the same. The relationship between

the mean size of liquid droplets and dried

particles is not consistent and no general

statements can be made on this subject.

The methods used to measure the sizes of dried

particles include sieving, microscopy,

sedimentation and laser techniques. Pressure spray

nozzles can produce particles ranging in size from

20 to 600 microns, depending on the nozzle type,

feedstock properties and operating conditions. Two-

fluid nozzles generally produce particles with

sizes in the range from 10 to 200 microns and

larger.

c) Agglomeration

It is sometimes desirable to have product particles

that are larger than those produced by a single

stage spray drying process. Agglomeration is the

process of enlarging particles by getting them to

stick to each other. Agglomerated particles may

have improved solubility, higher bulk density,

improved flow properties and less dust.

Occasionally, the second stage of a two stage dryer

is used to agglomerate product particles. Since the

particles leaving the first stage are still sticky,

they will bond with other particles during the

second stage drying.

Some installations spray wet particles from the

first stage with additional feed in the second

stage. The fresh feed softens the surface of the

particles and allows them to grow. This process is

called “instantizing” or “re-wetting

agglomeration”.

Most dryer operations include the recycling of fine

particles fines captured by the separation

equipment into the spray zone. Returning fines

promotes an agglomerating effect, leading to the

production of a powder that is coarser, freer

flowing and “dust free”.

d) Bulk density

Bulk density is the weight of dried powder per unit

volume. This is a critical factor for most spray

drying operations since it determines the size (or

fullness) of containers and influences the handling

and shipping costs. Bulk density is constantly

monitored during the spray drying process.

6.5 Factors affecting bulk density

• Increasing feed rate increases bulk density if

the residual moisture increases

• If increasing feed temperature leads to the

production of spherical droplets instead of

‘threads’, bulk density increases.

• For easily atomized feeds, increased

temperature can lower bulk density.

• Bulk densities often increase on powder cooling.

• A coarse homogenous powder has a lower bulk

density than a fine homogenous powder.

• A powder with a wide distribution of particle

sizes will have a higher bulk density than a powder

with a narrower distribution of particle sizes.

• Increasing feed solids generally increases bulk

density.

• Feed aeration decreases bulk density.

• Feed suspensions give higher bulk densities

than feed solutions.

• Increasing residual moisture content increases

bulk density.

• Increasing inlet air temperature decreases bulk

density.

• Reducing the outlet air temperature increases

residual moisture and therefore increases bulk

density.

• Co-current dryers produce powders with lower

bulk densities than counter-current dryers.

2.4 Drying history of a droplet containing solids in a spray dryer

Figure 2.7 schematizes the drying history of a

droplet containing

insoluble or dissolved solids, in a spray dryer [3,

4]. At the beginning of the first

drying period, the droplet temperature changes

until reaching the wet bulb

temperature, determining by the operation

conditions. During this first period,

water (solvent) evaporates as a pure liquid, with

the surrounding saturated

vapor film being the only resistance to heat and

mass transfer between gas

and droplet. Moreover, the droplet shrinks,

decreasing its size and

concentrating solids at its surface due to the

inward movement of its

boundary. Once the droplet moisture content becomes

too low to keep this

saturated condition, dissolved solids from solution

start being deposited at the

droplet surface, forming a partial porous crust

around the liquid droplet. As

drying proceeds, the droplet transforms into a

particle with a wet core

completely surrounded by the porous crust. Note

that any change in the spray

(Passos, Birchal – Physical properties of powder)

dryer operation conditions to those, under which

the droplet shrinks slowly at

a rate that assure a uniform distribution of the

solid concentration, can lead to

development of a full particle without a crust.

FIGURE 2.7 Drying history of a droplet containing

solids in a spray dryer

For the case illustrated in Figure 2, the second

drying period starts when

the porous crust covering the entire droplet

surface. At this point, there is no

more a droplet, but a particle constituted by a wet

core and a porous crust.

This period is characterized by thickening the

crust towards the interior of

particle, as well as, by reducing the drying rate,

since the porous crust

constricts the vapor mass transfer from the inner

core surface to the

surrounding gas. Therefore, the particle

temperature increases, supplants the

wet bulb temperature, and tends to reach the

thermal equilibrium with the gas.

For cases, in which the gas operation temperature

exceeds the solution

boiling temperature, a bubble can be generated

inside the particle when its

temperature equals to this boiling point. Under

this condition, vapor is

intensively generated inside the particle, raising

its internal pressure.

Therefore, the particle inflates, or bursts, or

cracks, depending on the

permeability and mechanical characteristics of the

crust. Consequently, by

varying either the spray dryer operation parameters

or the feed solution

properties, many different particle morphologies

can be obtained

In order to evaluate the crust properties, it is

important to emphasize

that, in some cases mainly in the food process, the

crust composition differs

from the droplet solid composition due to the

segregation of these

components during drying. Based on drying kinetics

and mass transfer

fundamentals, solids with smallest mass-diffusion

coefficients must be

concentrated at the particle surface. This

segregation also explains the high

concentration of free fat on the whole milk powder

crust surface [5].

Simultaneously to drying and particle formation

mechanisms, in many

applications, the powder is subjected to

agglomeration, for the purposes of

improving appearance and/or dispersability.

Particle agglomeration can be

enhanced in spray dryer systems for processes, in

which it is required, as the

instant powder production. Therefore, primary,

secondary and tertiary

agglomerations between droplets and/or particles

should be intensified by

respectively:

(a) Using an atomizer device to generate a wider

droplet size distribution. This

promotes, at the atomizing zone, effective

collisions between smaller and

larger droplets due to their different falling

velocity;

(b) Recycling fines (separated from the exhausted

gas) to the upper wet

region of the drying chamber, where droplets are

descended, as shown in

Figures 1b. These fine particles are incorporated

into droplets producing

secondary agglomerates. Different agglomerate

structures can be

achieved depending on the design and location of

the fine return system

[9];

(c) Inserting a fluid bed into the conical region

of the spray dryer to improve or

create agglomerates by rolling down wet particles.

This brief description of drying and particle

formation corroborates the

concept of manipulating the spray dryer operation

variables to achieve the

desirable powder properties in order to optimize

the product quality.

Additionally, it leads to choose the most general

powder properties that can

be manipulated and controlled by the spray dryer

operation. These properties

can be integrated into four groups, as: (i)

moisture content, including the water

activity and sorption isotherms; (ii) particle size

distribution incorporating the

mean particle size and dispersion index, as well

as, the particle shape and its

superficial area; (iii) density, concerning the

solid, particle and in bulk; (iv)

stickiness, including the degree of particle

cohesion and adhesion. Other

properties, specific of the desirable product,

should be added to these general

ones to complete the powder quality requirements.

2.5 Moisture Content Of A Spray Dried PowderThe residual moisture content of spray dried powder

is of great important to define the product

quality, since this quantifies the amount of water

presented in the material. This parameter is

straightly related to drying

conditions and to droplet formation mechanisms.

Consequently, it must vary

with changes in the spray dryer operation variables

(mainly the inlet air

temperature), as well as, the solution or

suspension feed composition and

concentration. Depending on the methods used for

measuring the moisture

content, the different forms of water, available in

the powder, can be

identified, such as water weakly bounded (free or

retained water), water

strongly bound (hydrate water), water with weaker

bonds and water imbibed

in proteins. The drying oven method at a controlled

temperature until constant

mass of the sample is the most widely method used.

Generally for food or

pharmaceutical powders, this method is normalized

concerning the drying

temperature, sample exposure and oven type [11].

Moreover, for these

powders, both, the residual moisture content and

the water activity, are

required in order to develop a procedure to control

adequately the product

shelf life, taste and texture, agglomeration,

contaminant growth and microbial

proliferation [12]. Water activity, expressed by

the air relative humidity in

Two methods are used to express this moisture

content. These methods are wet

basis (m) and dry basis (M). In addition, the

content may be expressed as a percent or

as a decimal ratio. We will use all four forms (wet

basis, dry basis, percent, and decimal

ratio) in analyzing moisture or food products.

The general governing equations for indicating

moisture content are:

260 Food & Process Engineering Technology

where: m = decimal moisture content wet basis (wb)

M = decimal moisture content dry basis (db)

md = mass of dry matter in the product

mw = mass of water in the product

mt = total mass of the product, water plus dry

matter

The percent moisture content is found by

multiplying the decimal moisture content by

100.

In addition, relationships between wet and dry

moisture content on a decimal basis

can be derived from Equations 10.01 and 10.02.

Those relationships are:

Use of the wet basis measurement is common in the

grain industry where moisture

content is typically expressed as percent wet

basis. However, use of the wet basis has

one clear disadvantage—the total mass changes as

moisture is removed. Since the total

mass is the reference base for the moisture

content, the reference condition is changing

as the moisture content changes. On the other hand,

the amount of dry matter does not

change. Thus, the reference condition for dry basis

measurements does not change as

moisture is removed.

For a given product, the moisture content dry basis

is always higher than the wet

basis moisture content. This is obvious from a

comparison of Equations 10.01 and

10.02. The difference between the two bases is

small at low moisture levels, but it

increases rapidly at higher moisture levels.

A final note regarding moisture content relates to

high moisture materials such as

fruits and vegetables. Many of these products have

moisture contents near 0.90 (or

90%) (wb). On a dry basis this would be 900% if

expressed as a percentage. For products

of this type, moisture is often given as “mass of

water per unit mass of dry product,” the decimal

basis we discussed earlier.

2.5.1 Equilibrium Moisture ContentA material held for a long time at a fixed

temperature and relative humidity will

eventually reach a moisture content that is in

equilibrium with the surrounding air.

This does not mean that the material and the air

have the same moisture content. It

simply means that an equilibrium condition exists

such that there is no net exchange of

moisture between the material and the air. This

equilibrium moisture content (EMC

or Me) is a function of the temperature, the

relative humidity, and the product.

2.6 IMPORTANT SPRAY DRYER PARAMETERS

Air Flow RatesThe rate of air flow must be at a maximum in

all cases. The movement of air is decided the

rate and degree of droplet evaporation by

inducing, the passage of spray through the

drying zone and the concentration of product

in the region of the dryer walls and finally

extent the semi-dried droplets and thus re-

enter the hot areas around the air disperser.

A lower drying air flow rate causes an

increase in the product halting time in drying

chamber and enforces the circulatory effects.

The effect of drying air flow rate on powder

solubility depends on its effect on powder

moisture content and density of powder.

Density variation can arise from temperature

changes and migrating pollution. By using

COMSOL multi physics we can able to get the

exact solution which is used for both

temperature and concentration changes. The

rising of air flow rate was led to the

increased of powder moisture content and

decrease in powder solubility (Papadakis,

1998).

Air TemperaturePowder properties such as moisture content,

bulk density, particle size, hygroscopicity

and morphology were affected by inlet

temperature. Normally, the inlet temperature

uses for spray dry technique for food powder

is 150-220oC. Chegini (2005) studied the

effect of inlet temperature (110-190oC) on the

moisture content of orange juice powder. It

was found that at a constant feed flow rate,

increasing the inlet air temperature reduced

the residual moisture content. In other words,

The air inlet temperature to the spray dryer

controls the production rate of the final

powder. For higher production, you run with a

higher inlet temperature.

The outlet temperature is controlled by

adjusting the feed rate and this also controls

the powder moisture based upon a consistent

temperature of inlet, feed solids content and

humidity of inlet air. At higher inlet air

temperatures, there is a greater temperature

gradient between the atomized feed and drying

air and it results the greatest driving force

for water evaporation. The use of higher inlet

air temperature leads to the production of

larger particles and causes the higher

swelling. If temperature is low, the particle

remains more shrunk and smaller. Nijdam (2006)

were obtained the similar results in the

production of milk powder at 120oC and 200oC.

The higher drying temperature is lower the

moisture content and increase its

hygroscopicity.

Composition/Concentration Of The Feed SolutionThe spray concentration influences the

particle size.

The higher the concentration of the spray

solution, the larger and more porous the dried

particles. On the other hand, spray drying a

feed solution that has low concentration will

result in product having finer powders with

less porosity.

The content of the feed solution also affects

the nature of the final products. It may also

place restriction on other spray dryer

parameters. For example, heat sensitive

product requires extreme care in controlling

the inlet and out temperatures of the spray

dryer.

Feed Flow Rate Higher flow rates imply in a shorter contact

time between the feed and drying air and

making the heat transfer less efficient and

thus caused the lower water evaporation. The

higher feed flow rate showed a negative effect

on process yield and that was resulting the

decreased heat, mass transfer and the lower

process yield. In addition, when higher feed

rates were used, a dripping inside the main

chamber was observed, when the mixture was

passed straight to the chamber and that was

not atomized and finally resulting the lower

process yield.

Distributor PressureThe distributor acts as an atomizer. High

distributor pressure translates to high

atomizer speed, and this will result in the

production of smaller droplets and more

moisture will be evaporated resulting from an

increased contact surface.

2.7 ADVANTAGES OF USING A TALL FORM SPRAY DRYER

1. PARTICLE SIZE CONTROL The dry particle size can be easily controlled

by atomization of the liquid and

the design of the hot gas inlet. The correct

dryer design and atomization technique can

eliminate the need for sizing/classification

equipment when the product average

particle size is less than 500 microns.

"Nondusting"

powders can be made which is

beneficial for hazardous products, animal

feeds, dyes, and other products.

2. EVAPORATIVE COOLING OF THE PRODUCT The heat and mass transfer during drying

occurs in the air and vapor films surrounding

the droplet. This protective envelope of

vapor keeps the particle at the saturation

temperature. As long as the particle does

not become "bone-dry," evaporation is still

taking place and the temperature of the

solids will not approach the dryer outlet

temperature.This is why many heat sensitive

products can be spray dried easily at

relatively

high inlet temperatures.

3. SHORT RESIDENCE TIME REQUIRED The surface area produced by atomization of

the liquid feed enables a short gas residence

time, ranging from 3-40 seconds depending

upon the application, which permits drying

without thermal degradation. This allows

for fast turn-around times and product

changes because there is no product hold

up in the drying equipment.

4. REDUCTION IN CORROSION

Because a spray dryer is a gas suspended

process, the dryer chamber remains dry by

design. Therefore, many corrosive materials

can be processed with carbon steel as

the primary material of construction,

which reduces capital and maintenance

costs.

5. HOMOGENEOUS SOLIDS MIXTURE IS PRODUCED Spray drying produces the most homogeneous

product for multi-component solution/

slurries. Each particle will be of the

same chemical composition as the mixed

feed.

6. THE HIGH INLET TEMPERATURE PERMITTED Because spray drying uses direct contact

heating, materials of construction is the

usual limit to inlet temperature. Exceptions

are extremely heat sensitive products

such as proteins, enzymes, and some highly

explosive products. Higher inlet temperatures

equate to better energy efficiency and

smaller equipment for a given process

heat load.

7. THE CHEMICAL REACTION POSSIBILITIES The inherent advantage of surface area also

makes the spray drying process excellent

for gas/solid reactions. For example, recovery

of HC1 and SO2 from a flue gas can be

achieved by atomization of a hydrated lime

slurry. The combination of absorption and

drying results in a dry solid for disposal

instead of a liquid effluent.