Continuous tunnel dryer 2014

UNIVERSITI TEKNOLOGI MARA

FACULTY OF CHEMICAL ENGEENERING

CPE591 ( PROCESS ENGINEERING I )

Remarks:

Checked by:

________________

Date:

NAME GROUP MEMBER :

NAME GROUP MEMBER STUDENTS ID1.SUDINI BINTI ABDUL KAHAR2. NOORFARAHANIS BT ILIAS3. SITI NURSHAHIRA BT AZIMAN4. FAIZ SHAFIQ BIN ZAILI5. NURFARAHIN FATIHA BINTI SUHAIMI

GROUP : EH 221

ASSIGNMENT 2 : CONTINUOUS TUNNEL DRYERS

DATE SUBMIT :

SEMESTER : 4

PROGRAMME CODE : EH221 BACHELOR (HONS) OF CHEMICAL AND

PROCESS AND ENGINEERING

SUBMIT TO : MADAM PROF DR NORAZAH BINTI ABD RAHMAN

http://i-learn.uitm.edu.my/CourseModule/default.php?case=normal&funcfor=course&funcmod=information&courseid=CPE591&lv=1&refid=938263&typenamez=folder

TABLE CONTENT

No. TITLES PAGES

1. Introduction 3 - 6

2. Principles 7

3. Mechanisms 7 - 9

4. Advantages and Disadvantages 10 - 11

5. Drying Concept 12

6. Drying Rate 12 - 15

7. Equations of Tunnel Dryers 16 - 20

8. Applications of Tunnel Dryers 20 - 21

9. References 22

[Continuous Tunnel Dryers]

Continuous Tunnel Dryer

1.0 Introduction

Drying is a method to removal of water from process materials and other substances by

removal of relatively small amounts of water from material. Heat may be supplied by convection

(direct dryers), by conduction (contact or indirect dryers). Drying is used to remove water from

foods for two reasons so that to prevent (or inhibit) micro-organisms and hence preserve the food

and to reduce the weight and bulk of food for cheaper transport and storage. For effective drying,

air should be hot, dry and moving. These factors are inter-related and it is important that each

factor is correct (for example, cold moving air or hot, wet moving air is unsatisfactory).

Difference between Drying & Evaporation

Drying Evaporation

Removal of small amounts of water from

material

Removal of large amounts of water from

material

Water is removed as a vapor by air Water is removed as vapor as its boiling point

Drying can be described by three processes operating simultaneously:

1. Energy transfer from an external source to the water or organic solvent

Direct or Indirect Heat Transfer

2. Phase transformation of water/solvent from a liquid-like state to a vapour state

Mass Transfer (solid characteristics)

3. Transfer vapour generated away from the API and out of the drying equipment


Figure 1: The figure shown that types of dryer categorized based on direct heat and indirect

heat.

Purpose of drying is to reduce the moisture level of wet granules. Besides that, to improve or

keep the good properties of a material such as compressibility .Drying also not only to reduce the

cost of transportation of large volume materials(liquids) but to preservative and to make the

material easy or more suitable for handling.


What are the

problems

What are the equipmentReason Need Drying

Over drying (bone

dry)

Excess fines

Possible fire hazard

Continuous Tunnel Drying

Direct Heating Moving Solids

Bed Dryers

Fluid Bed Dryer

Indirect Conduction Dryers

To keep the residual

moisture low enough

(preferably as a

range) to prevent

product deterioration

Ensure free flowing

properties

Firstly, drying method can be classified in several different ways, such as a batch, where

the material is inserted into the drying equipment and drying proceeds for a given time or as

continuous, where the material is continuously added to the dryer and dried material is

continuously removed. There are many type of dryer which is tray dryer, vacuum-shelf indirect

dryers, continuous tunnel dryer and other.

Classified of dryer processing:-

Continuous: large quantities/small residence time

Batch: small quantities/ long residence time

Secondly, dryers can be classified according to heat transferring methods by direct or

indirect.

Direct method: - Fluidised,Tray ,Tunnel ,Spray ,Rotary Dryers, and other

Indirect method:-Cone, Tumble, Pan Dryers and Other.

Most commercial dryers are insulated to reduce heat losses, and they recirculate hot air to

save energy. Many designs have energy-saving devices, which recover heat from the exhaust air

or automatically control the air humidity. Computer control of dryers is increasingly

sophisticated and also results in important savings in energy.

Thus, for the continuous tunnel dryers are often batch truck or tray compartments

operated in series , as shown in Figure1. Tunnel dryer is a direct continuous type of dryer. It is a

largest scale dryer.


Figure 2:-Continuous tunnel dryer:- tunnel dryer trucks with countercurrent air flow.

Based on this simple dryer concept, cabinets, trucks or trolleys containing the

material to be dried are transported at an appropriate speed through a long insulated chamber (or

tunnel) while hot drying gas is made to flow in concurrent, counter current, cross-flow or mixed

flow fashion. In the concurrent mode, the hottest and driest air meets the wetted material and

hence results in high initial drying rates but with relatively low product temperature (wet-bulb

temperature if surface moisture is present). For the product heat sensitive, the inlet drying gas

must be at a lower temperature if we use higher gas temperature in counter-current dryers. If the

material to be dried is not heat-sensitive and low residual moisture content is a requirement, one

may employ higher gas temperatures in the countercurrent arrangement as well. For combination

flow or cross-flow arrangements are used less commonly. The latter offer high drying rates but

the tunnels must be designed to fit the trolleys snugly so the drying gas flows through the

material much like a through-circulation packed bed dryer. Standard for operation total drying

times can be handled range from 30 minutes to 6 hours.

Figure 3:- Continuous tunnel dryer :- through-circulation screen conveyor dryer.


2.0 Principle

Basic Mechanisms of drying:

There are two basic mechanics is:-

Migration of moisture from the interior of an individual grain to the surface.

Evaporation of moisture from the surface to the surrounding air.

In this continuous tunnel dryer, the materials to be dried are sent to the air heated tunnel for

drying purpose. The material is entered at one end and the dried material is collected at the other

end of the tunnel. The solids are placed on trays or on trucks which move continuously through a

tunnel with hit gases passing over the surfaces of each tray. The outgoing material met the

incoming air to ensure maximum drying and the out going air contacted the wettest material so

that the air was as nearly saturated as possible.

3.0 Mechanism of action

One of the doors of the tunnel is opened and the materials to be dried are placed to the

trolleys and trucks are pushed slowly in the tunnel and then door is closed. The hot air flow can

be counter current, cocurrent or combination. Hot air is circulated and passed through the rail

truck and perforated trolleys. Many foods are dried in this way. The hot air then followed are

recirculate with the help of fans and the material becoming dried. The moist air is passed out

through the exhaust after completion of drying. The door is opened and the trolleys are taken out

of the funnel and some new trolleys with the wet materials are introduced into the trucks and the

process is repeated.

Basically, for continuous conveyor dryers are up to 20m long and 3 m wide. After that,

when the food is enter it will undergoes drying on a mesh belt in beds 5-15 cm deep. Then, from

Figure 2 and 3, wet material will supplied to the spreader device at the left-hand end is loaded

evenly and in a relatively deep layer on the surface of a slowly moving conveyor belt. The bed of

food will undergoes through by the air initially directed upwards and then downwards the last 1

or 2 sections so that to prevent light-weight, nearly dry pieces from blowing out of the bed. Thus,


to improve the uniformity of drying of a thick layer of wet material, sometimes the drier is

designed so that to produce up-through and down-through flow in alternate sections.

Sectionalising the drier is function to control air temperature, humidity, and velocity

independently in several stages to give optimum output and quality. At the first section, air is

supplied with high temperature and moderate humidity because rapid evaporation from the wet

material keeps its temperature down. Meanwhile, at the final stage dryer can be operated with

very dry air at a low enough temperature to avoid damage during that time by slow approach to

the desired final level of moisture content. Mostly, the foods moisture content is dried to 10-15

% and then the food will transferred to bin dryers for finishing. For the most important feature,

construction of the dryer as two separate conveyors in series makes it possible to discharge the

partly dry material at the end of the first stage. Not only does the mixing aid in making a uniform

product, but also the repiling makes possible a great saving in the flour space required.

The conveyor dryer use centrifugal fan wheels are ordinarily used. A large proportion of

the air passing through the layer of moist material recirculates into the fan, and then to dampers

into the next section, to lattes into fan, may divert part of it; dampers into the next section, to be

replaced by heated fresh air, may divert part of it. The fact hot air temperature is uniform makes

it necessary to control this temperature at a level which is safe to apply to the moist material

leaving the first section and already partly dry. Conveyor dryer equipment has good control over

drying conditions and high production rates. This is because due to used for large scale drying of

foods such as fruits and vegetables are dried in 2 hour to 3.5 hour at up to 5.5 hour). Thus, if we

use some soft, such as starchy or sugary materials may benefit by being subjected first to very

rapid surface drying in such equipment as the belt through drier so that the working surfaces of

the conveyor drier will remain clear and free of sticky build -up. Besides that, conveyor drier has

independently controlled drying zones and is automatically loaded and unloaded, which reduces

it can reduce labour costs. As a result it has largely replaced the tunnel drier.


4.0 a)Advantages and Disadvantages of Continuous Tunnel Dryer

Advantages Disadvantages

1. 1. Comparing with the compartmental dryers

tunnel dryers has the advantage of continuous

operation.

2. 2.A large amount of materials can be dried.

3. 3. Tunnel dryers are used for drying of paraffin

wax, gelatin, soap, pottery, etc.

1. High labor cost for loading and

unloading.

2. Thermolabile substances can’t be dried.

3. Drying rage is slow, so time

consuming.

4. It is not suitable for small scale

production.

5. It is a non-agitated process.

6. Drying of liquid materials is not

possible.

7. There is a chance of accident when

doors are opened before stopping the

hot air circulation.

4.0 b) Selection of Drying Equipment

◦During selections of drying equipment depend to;

1. Physical or Chemical properties of the product.

2. Production Capacity (Kg/hr)

3. Initial Moisture Content and final moisture required.

4. Particle Size Distribution.

5. Temperature and Drying Characteristics.

6. Explosion and Toxicological Characteristics.


The figure shown that characteristics of dries based on the food for each type of equipment.


5.0) Drying concept

To understand the theoretical foundations of drying, there are some definitions of basic concept:

1. Humidity:

The moisture content of a solid can be expressed on dry or wet basis, but the drying solid

is more appropriate to refer the moisture on dry basis (mass of water that accompanies

the dry solid).

2. Moisture equilibrium (X*):

When a wet solid is in contact with air temperature, humidity and certain constants, the

conditions of equilibrium achieved between the air and the wet solid. The equilibrium

conditions are reached when the partial pressure of water that accompanies the wet solid

is equal to the vapor pressure of water in the air.

3. Free moisture:

Is the difference between the moisture of a solid and moisture equilibrium with the air in

the given conditions. It is the moisture that can be lost after some time of contact with air

given and constant conditions.

4. Critical moisture:

The humidity is critical to a solid point that separates the two periods of drying critical

period ahead and critical period post.

6.0) Drying rate

Drying rate is defined by the loss of moisture from the wet solid per each unit of time, and more

specifically by the differential quotient (-dX/dθ) operating in constant drying conditions, when

are air-conditions (temperature, pressure, humidity and velocity) are constant along time.

Analytically, drying rate refers to the unit area of surface drying, according to the equation:


R=−LSAdXdt

Where:

Ls is weight of dry solid use (kg)

A is exposed area for drying (m2)

dX is change in moisture content (kg H20/ kg dry solid)

dt is change in time (h)

Drying curve

Figure 9.5-1: rate of drying curve vs. free moisture (Geankoplis, Fourth edition, page 577)

Drying occurs in three different periods, or phases, which can be clearly defined.


The first phase, or initial period, is where sensible heat is transferred to the product and

the contained moisture.

The second phase, or constant rate period, is when the free moisture persists on the

surfaces and the rate of evaporation alters very little as the moisture content reduces.

The third phase, or falling rate period, is the phase during which migration of moisture

from the inner interstices of each particle to the outer surface becomes the limiting factor

that reduces the drying rate.

Constant rate period (B to C)

In constant rate period, the surface of the solid is initially very wet and a continuous of film of

water exists on the drying surface. This water is entirely unbound water and it acts as if the solid

were not present.

The rate of evaporation under the given air a condition is independent of the solid and is

essentially the same as the rate from a free liquid surface.

Falling rate period (C to E)

At the critical free moisture content Xc, there is insufficient water on the surface to maintain a

continuous film of water. The entire surface is no longer wetted, and the wetted area continually

decrease in the first falling rate period until the surface is completely dry at point D.

Second falling rate period begin at point D where the surface is completely dry. Heat for the

evaporation is transferred to the solid to the zone of vaporization. Vaporized water moved

through the solid into the air stream.


Tunnel dryer

Tunnel dryer possesses all the advantages of tray dryer. Moreover, they have a semi-

continuous operation. This type of dryer, which may be up to 24m long with square or

rectangular cross section about 2x2m, consist of a cabinet equipped with rails to move the tray

racks along the drying chamber. The wetted materials are loaded in a tray that is stacked on

trolley. The trolleys are introduced periodically into the end of the tunnel. They advance through

the tunnel and are removed at the other end. A typical tunnel dryer can be operated in counter

current or co current flow of air and trolleys.

In the counter current systems, the air and the product move in opposite direction while in

co-current systems, they move in same directions. Drying times are similar to those obtained in

cabinet tray dryers, while the air flow rate is linked to the total number of trays. Dimensions of

the tunnel are calculated based on the required drying output, drying time and the capacity of a

single trolley.


7.0) Equation of continuous tunnel dryer

Humidification involves the transfer of water from the liquid phase into a gaseous

mixture of air and water vapor .Dehumidification involves the reverse transfer, whereby water

vapor is transferred from the vapor state to the liquid state. Humidity is defined as the humidity

H of an air where the water vapor mixture is defined as the kg of water vapor contained in 1 kg

of dry air. Humidity depends on partial pressure pA of water vapor in the air and total pressure P

(assume to be 101.325 kPa, 1.0 atm abs, or 760 mm Hg).

Equation involve in determine the humidity:

Humidity,H

Saturation humidity, Hs

Percentage humidity, Hp :

100 times the actual humidity H of the air divided by the humidity Hs if the air were saturated at

the same temperature and pressure,


Percentage relative humidity, HR:

The amount of saturation of an air-water vapor mixture is also given as percentage relative

humidity HR using partial pressures,

Humid heat of an air-water vapor mixture,Cs

The humid heat cs is the amount of heat in J (or kJ) required to raise the temperature of 1 kg of

dry air plus the water vapor present by 1 K or 1°C.

The heat capacity of air and water vapor can be assumed constant over the temperature ranges

usually encountered at 1.005 kJ/kg dry air•K and 1.88 kJ/kg water vapor•K,respectively.

Humid volume of an air-water vapor mixture,VH

The humid volume vH,. is the total volume in m3 of 1 kg of dry air plus the vapor it contains at

101.325 kPa (1.0 atm) abs pressure and the given gas temperature. Using the ideal gas law,


Constant rate drying

In the constant-rate period moisture movement within the solid is rapid enough to

maintain a saturated condition at the surface, and the rate of drying is controlled by the rate of

heat transferred to the evaporating surface. Drying proceeds by diffusion of vapor from the

saturated surface of the material across a stagnant air film into the environment and as the rate of

mass transfer balances heat transfer, the temperature of saturated surface remains constant. If the

heat supplied for drying is solely by convection, the surface temperature approaches the boiling

point temperature rather than the wet bulb temperature.

Typical drying rate curve for constant drying conditions

(a) Plot data as free moisture versus time

To obtain the rate-of-drying curve from this plot, the slopes of the tangents drawn to the curve in

figure a) can be measured, which give values of dX/dt at given values of t.

The drying rate R is calculated for each point by


where;

R = drying rate in kg H2O/h.m2 (lbm H2O/h.ft2)

LS = kg of dry solid used (lbm)

A = exposed surface area in m2 (ft2)

b)The drying rate curve is then obtained by plotting R versus the moisture content


Method using Experimental Drying Curve

The time required for the constant-rate period can be determined directly from the drying

curve of free moisture content versus time.

Method using rate of drying curve for constant rate period

The drying rate is defined as:

The equation can be rearranged and integrated over the time interval from drying from X1 at t1

= 0 to X2 at t2 = t:

Integrating Eq. (9.6-1) for the rate constant period (where R = Rc = constant),


8.0) Applications of Tunnel Dryers

There is several application of tunnel dryer. Tunnel dryer are widely used in research

laboratories and Quality Control department for drying glass wires and small apparatus. It also

used in drying of packaging materials plastic caps, spoons, injectable vials, glass containers, etc.

Other than that, it used in sterilization of containers. Last but not least, tunnel dryer used for dry

food in food production.

As we all know, the tunnel dryer is widely used in so many industries because of tunnel

dryer has the advantage of continuous operation compare to other compartmental dryers. Besides

that, large amount of materials can be dried using continuous tunnel dryer. Tunnel dryers are also

used for drying of paraffin wax, gelatine, soap, pottery, etc. which can’t be dried using other type

of dryers.

Even though, there are a lot of advantages of the using of continuous tunnel dryer. It also

has disadvantages. The disadvantages of using tunnel dryer are it requires a lot of manpower for

loading and unloading the materials which increase the cost and not everything can be dried

using continuous tunnel dryer such as thermolabile substances can’t be dried using the dryer.

Furthermore, continuous tunnel dryer through slow drying rate which can drag the time for

consuming the product. It is not suitable for small scale production and non-agitated process. It

also can’t perform the drying of liquid materials.


9.0) REFERENCES

1. (http://atozstudyzone.blogspot.com/2012/12/tunnel-dryer.html)

2. (http://books.google.com.my/books?id=JE2FMQbawXoC&pg=SA6-PA22&lpg=SA6-

PA22&dq=tunnel+dryers+trucks+with+countercurrent+air+flow+mechanism&source=bl&ots=jB-

)

3. (Q7j7Us6&sig=TdWqOkkuPWJPBZIfvK4n3nE5Y_M&hl=en&sa=X&ei=o8uZU6mXFcyC8gXxjoDoBA

&redir_esc=y#v=onepage&q=tunnel%20dryers%20trucks%20with%20countercurrent%20air

%20flow%20mechanism&f=false)


Continuous tunnel dryer 2014

Engineering

Transcript of Continuous tunnel dryer 2014