Continuous Tunnel Dryer 2014

UNIVERSITI TEKNOLOGI MARA FACULTY OF CHEMICAL ENGEENERING CPE591 ( PROCESS ENGINEERING I )

NAME GROUP MEMBER

:

NAME GROUP MEMBER 1.SUDINI BINTI ABDUL KAHAR 2. NOORFARAHANIS BT ILIAS 3. SITI NURSHAHIRA BT AZIMAN 4. FAIZ SHAFIQ BIN ZAILI 5. NURFARAHIN FATIHA BINTI SUHAIMI GROUP : EH 221 ASSIGNMENT 2

: CONTINUOUS TUNNEL DRYERS

DATE SUBMIT

:

SEMESTER

:4

PROGRAMME CODE

: EH221 BACHELOR (HONS) OF CHEMICAL AND

STUDENTS ID

PROCESS AND ENGINEERING SUBMIT TO

Remarks: Checked by:

________________ ________________ Date:

: MADAM PROF MADAM PROF DR NORAZAH BINTI ABD RAHMAN

TABLE CONTENT

No.

TITLES

PAGES

1.

Introduction

2.

Principles

3.

Mechanisms

4.

Advantages and Disadvantages

5.

Drying Concept

6.

Drying Rate

12 - 15

7.

Equations of Tunnel Dryers

16 - 20

8.

Applications of Tunnel Dryers

20 - 21

9.

References

[Continuous Tunnel Dryers]

3-6

7

7-9

10 - 11

12

22

Page 2

Continuous Tunnel Dryer 1.0 Introduction

Drying is a method to removal of water from pro cess materials and other substances by removal of relatively small amounts of water from material. Heat may b e 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

Removal of large amounts of water from

material

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 vapou r state 

Mass Transfer (solid characteristics)

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


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F igu re 1: Th e figu re shown shown t hat types types of of dryer dryer categori categori zed base based on di r ect heat heat and i ndi r ect 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.

Reason Need Drying

What are the equipment

What are the problems

To keep the residual moisture low enough (preferably as a

Continuous Tunnel Drying Direct Heating Moving Solids

Over drying (bone dry)

Bed Dryers

Excess fines

product deterioration

Fluid Bed Dryer

Possible fire hazard

Ensure free flowing

Indirect Conduction Dryers

range) to prevent

properties


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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.


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F igur e 2:-Continu ous tunnel dryer: dryer: - tunn el dryer dryer tr ucks with countercur countercur rent air f low.

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.

F igur e 3:3:- Continuous tunn el dryer dryer :- thr ough-circulati on scree screen n conve conveyo yorr dryer. dryer.


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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,


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to improve the uniformity of drying of a thick layer of wet material, sometimes the drier is designed so that to produce up-through 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.


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4.0 a)Advantages and Disadvantages of Continuous Tunnel Dryer

Advantages 1. Comparing with the compartmental dryers

Disadvantages 1. High labor cost for loading and

tunnel dryers has the advantage of continuous

unloading.

operation.

2. Thermolabile substances can’t be dried.

2.A large amount of materials can be dried.

3. Drying rage is slow, so time

3. Tunnel dryers are used for drying of paraffin wax, gelatin, soap, pottery, etc.

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.


Page 10

Th e fi gur e shown shown that char acter acter istics of of dri es bas base ed on the food f or each type of equi pment. pment.


Page 11

5.0)

Drying concept

To understand the theoretical foundations of dr ying, 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) (-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:


Page 12

 

   

Where: Ls is weight of dry solid use (kg) 2

A is exposed area for drying (m ) dX is change in moisture content (kg (k g 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.


Page 13



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 s urface.

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 dr y 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.


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Tunnel dryer

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


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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,


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Percentage relative humidity, H R :

The amount of saturation of an air-water vapo r 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 b y 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,


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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


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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


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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 the time interval from 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),


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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 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.


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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=SA6PA22&dq=tunnel+dryers+trucks+with+countercurrent+air+flow+mechanism&source=bl &ots=jB-) 3. (Q7j7Us6&sig=TdWqOkkuPWJPBZIfvK4n3nE5Y_M&hl=en&sa=X&ei=o8uZU6mXFc yC8gXxjoDoBA&redir_esc=y#v=onepage&q=tunnel%20dryers%20trucks%20with%20 countercurrent%20air%20flow%20mechanism&f=false)


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Continuous Tunnel Dryer 2014

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