Agricultural Process Engineering

8/6/2019 Agricultural Process Engineering

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Agricultural Process Engineering: Drying of Food Product

By

Mustapha Bello

University Putra Malaysia

Q1

a. Different stages of drying

In order to explain the different stages of dying, let us consider a theoretical drying

curve model below

Fig. 1 theoretical drying curve

The different stages of drying are:

I. The Rising Rate Stage (A-B): In this stage of drying, the rate of drying

increases as water is removed. This occurs as the product and the water withinthe product experience a slight temperature increase. Physically, this is

behavior is attributed to the conditioning of sample, e.g. warming-up,

opening of pores etc. This stage is usually short and not always observed in

drying experiments.

The mechanism of water removal at this stage is by evaporation, moisture is

transferred and moves from the centre towards the surface by capillary action.The surface of the food acts like the surface of a pure water and moisture

evaporates to the drying air. Air temperature and velocity are the controlling

factors.

II The Constant Rate Stage (B-C): This is the stage when the drying rate remains


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Constant as water is removed. This constant rate period occurs when the

surface of the food is saturated with water and acts like a surface of

Water. Significant reductions in moisture content occur at constant occur at aConstant rate and at a constant product temperature. This temperature is the

The wet bulb temperature of air.

The mechanism of water removal in this stage is as in the rising stage.

Moisture moves to the surface by capillary action and then

Evaporates in to the air. Constant rate continues as long as any amount ofWater lost by evaporation is replaced by water transported from the interior

To the surface by diffusion. Air temperature and velocity are also the

controlling factors here. Thus, the evaporation from the surface is the

rate-controlling mechanism.

This period continues until the moisture content is reduced to the critical

Moisture content, a point that indicates the on-set of the falling rate period.

III The Falling Rate Stage (C-D): As the drying continues beyond the critical

moisture content, the rate of moisture removal decreases with time, andthis stage is termed the falling rate period. At this stage, the supply of

water to the surface drops below the rate of evaporation and the moisture

content of the surface of the food is no longer saturated; the temperature of

the food rises during the falling rate period and approaches asymptoticallythe dry bulb temperature of the air.

When the supply of water to the surface drops below the rate of

evaporation, the moisture content of the surface begins to decrease rapidlyand approaches quickly the equilibrium moisture content corresponding to

the relative humidity of the air on the isotherm of the material. From that

moment, internal transport and not evaporation is the rate-controllingfactor. Water moves to the surface by liquid water diffusion, water vapor

diffusion and capillary transport.

b. Definitions of Critical and Equilibrium Moisture Contents

I. Critical Moisture Content: This can be defined as the moisture content above

II. Which the rate of moisture removal decreases with time. In other words, it is

the moisture content at which the constant rate period ends and the fallingrate period begins.

III. Equilibrium Moisture Content: This can be defined as the moisture content of

a material after it has been exposed to a particular environment for aninfinitely long period of time. In other words, it can be defined as the

moisture content at which the internal product vapor pressure is in

equilibrium with the vapor pressure of the environment.


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Significance of Critical and Equilibrium Moisture Content

Equilibrium moisture content signifies the minimum moisture content to whichA product can be dried under a given set of conditions whilst the critical

moisture content is significant in calculation of drying time as it tells whether

drying is in constant or falling rate period.

c. Structural construct and operation of tunnel and spray drier

I. Tunnel drier

A tunnel drier, as the name implies, consists of a long tunnel through which trucks

carrying stacks of trays travel with or against a stream of drying air. It consists of a

blower, a heater and trucks all contained in the tunnel as shown below

Fig 2. Outline of a tunnel drier

Fresh air enters and blown into the heater where its heated before its introduced into themain tunnel. The heated air moves at an established velocity though trays of products

being carried on trucks. The products trucks are moved through the tunnel at a raterequired to maintain the residence time needed for the dehydration.

The product can be moved in the same direction as the air flow to provide concurrentdehydration, or the tunnel can be operated in countercurrent manner, with the product

moving in the direction opposite to air flow. A mixed flow is another alternative.

With the concurrent systems, a high- moisture product is exposed to high-temperature air,and evaporation assists in maintaining lower product temperature. At locations near the

tunnel exit, the lower-moisture product is exposed to lower-temperature air. In counter-current systems, a lower- moisture product is exposed to high- temperature air, and asmaller temperature gradient exists near the product entrance to the tunnel. A mixed flow

tunnel drier combines both concurrent and countercurrent flows.

II. Spray drier

A spray drier consists of the following elements:


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i. An Air heater: this is where the drying is heated before being introduced in to

the drying chamber.

ii. An Atomizer: this is the element that used for producing atomic spray of theliquid to be dried.

iii. A Pump: this is used for feeding the liquid product to the atomizer

iv. A Drying chamber: this is the chamber where the actual drying takes place. Itconsists, most commonly, of a vertical section with a conical bottom.

v. Fan/Blowers: this element is used for moving air through the system

vi. A Cyclone: this separates the solid from liquid

An example of spray drier is shown below

Fig. 3 A Spray drier (courtesy of Berk)

Spray driers are used for drying liquid solutions and suspensions with the objective ofproducing light, porous powders. The liquid is dispersed (atomized) as a spray of fine

droplets into a very hot air inside a large chamber. Because of their small size and the

high temperature of the air, the droplets are dried in a matter of seconds andtransformed into particles of solid powder. At the exit from the chamber, the solid

particles are separated from the humid air by the cyclones.

Spray drying is the accepted method for the production of milk and whey powders,

coffee-creamers, cheese powder, dehydrated yeast extract, instant coffee and tea, egg

powder and many other products in powder form. Spray drying is also one of the

methods used micro-encapsulation.

d.

Given dataDiameter of guava piece=6cm

Thickness of guava piece=3mm

Initial moisture content=80 %( d.b)

Final moisture content=5% (d.b)


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Critical moisture content=200% (d.b)

Equilibrium moisture content=3% (d.b)

Diffusivity for constant drying= 9 25 10 m /s1st diffusivity for the two stage= 8 21 10 m /s2nd diffusivity for the two stage= 8 22 10 m / s

Moisture content at the beginning of the second stage=20% (d.b)

Solution:

i) using the following formula;

( ) ( ) ( ) ( )2 2 2ln m m / m m ln 8/ D / 4e c e L =

( ) ( )( ) ( ) ( )

2 2 9 2ln 0.05 0.03 / 0.8 0.03 ln 8/ 5 10 / 4 0.0015

=

33.65066 0.21002 5.48311 10 =

Solving for the , we have,

627.498 s== 10.5 min

ii) for the first stage,

9 2

D=1 10 m / s

Using the same formula, we have,

( ) ( ) ( ) ( )2 2 8 2ln 0.2 0.03 / 0.8 0.03 ln 8 / 1 10 / 4 0.0015 = 1.51059 0.21002 0.010966 =

Solving for, we have,

119s =

= 2 min

For 9 2D=2 10 m / s


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( ) ( ) ( ) ( ) ( )2 2 9 2

3

3

ln 0.05 0.03 / 0.2 0.03 ln 8 / 2 10 / 4 0.0015

2.14006 0.21002 2.1932 10

1.930046 2.1932 10

= 880.02 s

= 14.67 min

=

=

=

Thus the total time is 999.02 s or 17 min

Q2

a. method for obtaining sorption isotherm of a food

The sorption isotherm of a food can be obtained by placing weighed samples of the foodin jars, over saturated aqueous solution of salt and left to equilibrate at a constant

temperature and relative humidity. The samples are taken and weighted at intervals until

such a time when there is negligible or no change between successive weighing. Whenthis happens, it will be assumed that equilibrium is achieved. The moisture content is then

determined and is taken as the equilibrium moisture content (EMC).

The desired relative humidity is provided by the saturated salt solution which has aknown water activities at different temperatures. Other possible chemicals that may be

used are sulfuric acid, glycerol etc. Water activities of common salt solutions are

provided in the literature. Dividing a water activity by 100 gives us the equilibriumrelative humidity (ERH).

Having obtaining the equilibrium moisture content (EMC) at a particular temperature andthe corresponding equilibrium relative humidity (ERH) from the salt solution, a graph of

EMC versus ERH is then plotted. This graph gives us the sorption isotherm of the food.

b. The BET and GAB equations

The BET and GAB equations are mathematical models developed for prediction of

sorption isotherms of foods at low moisture content.

i. The BET equation: this is a model developed by Brunauer, Emmett and Teller

to address the short comings of the earlier models. It is used to fit and draw

the sorption data obtained for a given food. Applied to water vapor sorption,the BET equation is written as follows:


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( ) ( )w

m w w

m

w

CaX

X 1 a 1 a C-1

where:

X= moisture content,

X parameter of the equation, the BET monolayer

C = constant related to the heat of adsorption

a water activity

= +

=

=

mX and C are called the parameters of the BET equation

Procedure for obtaining the BET parameters

In order to obtain the relevant parameters of the BET equation, sorption data of the

food is first determined experimentally. To find mX and C from the data, the BET

equation is rewritten as follows:

( )w

w

w m m

a 1 C 1a

1 a X C X C

= = +

If the group is plotted against wa , a straight line is would be obtained, mX and C

are calculated from the slope and the intercept.

m m

C 1 1slope , intercept

X X

= =

mC and X are obtained simultaneously from the above

The BET equations is found to fit well sorption isotherms, up to water activity values of

about 0.45

ii. The GAB equation: this is a model developed by Guggenheim Anderson and

DeBoer which has a wider range of applicability than the BET model. Theequation is given as:

( ) ( )w

m w w w

C k aX

X 1 ka 1 ka Cka=

+

The GAB equation is a three-parameter equation with k, C and mX as constant.

mC and X have similar significance as in the BET equation. k is a third parameter

that corrects for the difference in properties of adsorbed water relative to liquid water


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and permits the GAB equation to hold over a wider range of moisture content than the

BET. Both k and C are temperature dependent.

Procedure for obtaining GAB parameters

As in the case of BET, the sorption data is first obtained experimentally. The GAB

equation is then rearranged by taking reciprocal, thus,

[ ]

( )

w w

m w

w

w w

m m m

1 1 1 11 ka Cka

X X Cka C

a k C 2 11 C a a

X X C X Ck X Ck

= +

= + +

Thus, a polynomial nonlinear regression of ( )m wa

against aX

will give values for

, the coefficient of the quadratic term, the coefficient of linear term, the

intercept.

[ ]m m m

C 2 k 1= ; = 1 C ; =

X C X C X Ck

These three equations are used to evaluate C, k and mX which are the parameters of

the GAB equation

c. BET and GAB parameters from the given publication sorption isotherms of

lupine at different temperatures

The following data was estimated from the sorption isotherm given in the publication:

wa EMC

0.2 0.02625

0.4 0.04375

0.6 0.08750

0.8 0.18750


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This data is used to obtain the relevant parameters of the BET and the GAB equations

of lupine at30 Co .

I BET parameters.

We calculate the values( )

w

w

a

1 a m and plot it against wa . Thus,

BET Plot y = 24.486x + 4.5216

R2

= 0.9503

0

5

10

15

20

25

30

0 0.5 1

aw

aw/(1-aw)m Series1

Linear (Series1)

Linear (Series1)

The plot gives us y 24.486x 4.5216= +From this,

Slope = 24.486 and the intercept = 4.5216

But the slope and intercept are given by;

m m

C 1 1slope , intercept

X C X C

= =

Therefore,

m m

C 1 124.486 and 4.5216

X C X C

= =

Solving these equations simultaneously, we have,

C=6.4153 and mX = 0.0345


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The value of C falls within 2-50 which is the normal value for type II isotherm to

which the sorption isotherm falls.

II GAB parameters

Here we plot the valuesw

w

aagainst a

m

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