14

CHAPTER 2

LITERATURE REVIEW

2.1 INTRODUCTION

Drying is perhaps the oldest and most diverse of engineering

operations. Over four hundred types of dryers have been reported in the

literature and more than one hundred distinct types are commonly available.

Energy consumption in drying ranges from a low value of under five percent

in the chemical process industries to thirty five percent in the papermaking

operations (Syahrul et al., 2002). Drying occurs by effecting vaporization of

the liquid by supplying heat to the wet feedstock. This is one of the most

energy-intensive unit operations due to the high latent heat of vaporization

and the inherent inefficiency of using hot air as the (most common) drying

medium. It is reported that in most industrialized countries, the energy used in

drying accounts for 7-15% of the nation’s industrial energy, often with

relatively low thermal efficiencies (Syahrul et al., 2002). Over 85 percent of

industrial dryers are of the convective type with hot air or direct combustion

gases as the drying medium. Over 99 percent of the applications involve

removal of water.

The sugar industry is a typical industry which uses its own

by-product bagasse, as fuel. As mentioned in the previous Chapter, the mill-

run bagasse has a high moisture content which reduces its gross calorific

value. The moisture level of the fuel can be reduced by drying it, using the

waste heat available in the plant. In this way, it is expected to conserve

15

bagasse, providing energy conservation in the process. Hence, in this study,

drying of bagasse using available waste heat is envisaged. The study

comprises experimental and theoretical investigations in bagasse drying. In

this section a detailed literature review for the study is reported under the

following topics.

Review of drying concepts

Thermo gravimetric Studies

Thin layer experimental studies

Thin layer model studies

Energy and exergy Analysis

Numerical simulation studies

Bagasse drying review

2.2 REVIEW OF DRYING CONCEPTS

Drying is generally used to remove moisture or liquid from a wet

solid by bringing this moisture into a gaseous state. In most drying operations,

water is the liquid evaporated and air is the normally employed purge gas. In

general, the main goal of drying is to decrease the moisture content of solid

materials to below a certain limit, which results in quality enhancement, and

ease of handling and further processing (Sokhansanj and Jayas 1995). A

drying process is fundamentally a simultaneous heat and mass transfer

operation and is widely used in a variety of thermal energy applications

(Hossain and Bala 2002). Generally, the heat supplied is transported by

convection from the surroundings to the particle surfaces and from there by

conduction further into the particle throughout the drying process

(Midilli 2001; Dincer and Hussain 2002). The moisture is removed in the

opposite direction as a liquid or vapor. On the surface, it evaporates and

16

passes on by convection to the surroundings (Midilli and Kucuk 2003;

Syahrul et al 2002).

Thus, one of the most important challenges of the drying industry is

to reduce the cost of energy sources for good quality dried products (Dincer

1998). The heat sources with the greatest potential for drying energy in

process industries are secondary heat flows like flue gases and low pressure

steam varying from 3 to 4 bar. Most investigations of drying have been made

from the external viewpoint, wherein the effects of the external drying

medium such as air velocity, humidity, temperature, and wet material shape

and subdivision are studied with respect to their influence on the drying rate.

The results of such investigations are usually presented as drying rate curves,

and the natures of these curves are used to interpret the drying mechanism.

2.2.1 Dryer Types

Drying equipments may be classified in several ways. They can be

classified according to the method of operation and method of heat supply

(Ryozo Toei et al 1994). The first distinction is that between a batch type

dryer and a continuous one (Devahastin and Mujumdar 1999). A continuous

type dryer is used in cases where drying at the rate of many tons per hour is

required. On the other hand, a batch type dryer is suitable for drying at the

rate of less than 100 kg per day. Dryers are also further divided by the type of

heat supply; the heat needed for drying is supplied to the material by one of

the following methods; Convective drying (using a drying medium, i.e., air),

Contact drying (by conduction heat from a surface), Radiation drying,

Dielectric drying and various combinations of drying (Baker 1997).

Dryers can be further classified in a number of ways, on the basis

of pressure (vacuum or near atmospheric), temperature of the product during

17

drying, mode of operation, and the method of material handling within the

dryer (e.g., stationary, agitated, fluidized, converged and falling under

gravity) (Syahrul et al., 2002). The type of dryer should be selected based on

the shape and size of the wet material, the amount of treatment and the drying

conditions. The drying time required for a given product is also a key factor in

the selection of the dryer.

2.2.2 Drying Mechanism

For the drying process, heat is necessary to evaporate moisture

from the surface and a flow of air is needed to carry away the evaporated

moisture. During drying, moisture from the interior has to migrate towards the

surface, where the evaporation of moisture has to take place. Water vapor

diffuses through a boundary film of air and is carried away by the surrounding

air. This creates a region of lower water vapor pressure at the surface of the

material and a water vapor pressure gradient is established from the moist

interior of the material to the dry air (Keey 1972). This gradient provides the

driving force for the removal of water from the material. Water movement to

the surface of the product may be effected due to the following mechanisms;

i) Liquid movement by capillary forces.

ii) Diffusion of liquid, caused by differences in the

concentrations of solutes in different regions of the materials.

iii) Diffusion of liquid which is adsorbed in layers at the surface

of the solid components of the material.

iv) Water vapor diffusion in air spaces within the material caused

by vapor pressure gradients.

18

Typical drying is divided into constant rate and falling rate periods.

The controlling resistance for a drying study may be of energy or mass

transfer due to internal or external conditions. The drying rate in the constant

rate period is determined by conditions external to the material being dried

including temperature, gas velocity, total pressure and partial vapor pressure.

The controlling resistance may be associated with the transfer of energy to the

solid or the transfer of mass away from the solid. Mass transfer during the

constant rate period involves diffusion of water vapor from the material

surface through a boundary layer into the drying medium. During the falling

rate period, the drying rate decreases with time, and the rate of internal mass

transfer to the material surface typically controls the process. A falling drying

rate may be observed when external mass transfer resistance controls and the

surface vapor pressure of the solid decreases as the moisture content drops.

During drying, the controlling resistance may be associated with

the transfer of energy to the solid or the transfer of mass away from the solid.

The product temperature during drying will help to identify whether the

process is controlled by energy or mass transfer. When the product

temperature equals the wet bulb temperature of the surrounding medium, it is

characterized by energy transfer control and when it reaches the dry bulb

temperature of the drying medium, mass transfer control is suggested. The

rate of drying in many practical situations is controlled by internal mass

transfer (Bruin and Luyben 1980). For materials where drying is controlled

by internal resistance, porosity will influence the process. Dense solids of low

porosity will have relatively low mass transfer rates and high energy transfer

rates. In contrast, solids of high porosity will have relatively high mass

transfer rates and low energy transfer rates. In porous solids, internal mass

transfer may occur within the solid phase, or within the void spaces

(Karel 1975). Several mechanisms of internal mass transfer have been

proposed in the drying literature including liquid and vapor diffusion, surface

19

diffusion, hydrodynamic or bulk flow and capillary flow (Waananen et al.,

1993).

Capillary water movement is likely to be important at relatively

high water contents. Diffusion due to vapor or surface pressure gradients is

expected to have significance at low and intermediate moisture contents. The

importance of vapor phase diffusion will increase as solid porosity increases.

Liquid / adsorbed phase diffusion will be predominant in dense solids of low

porosity of less than 5%. Bulk flow of moisture resulting in flux rates several

times faster than diffusion will be important at drying temperatures above the

boiling point of water and will be influenced by porosity. Overall moisture

transfer during drying may be primarily due to one of the mechanisms

mentioned above, or more likely may be due to a combination of more than

one mechanism. A diffusional internal mass transfer mechanism has been

assumed in many modeling studies.

Table 2.1 summarizes some mass transfer driving forces and

postulated mechanisms used in previous drying studies. Driving forces

include gradients in concentration, total pressure and temperature. It can be

noted that in their final form, driving forces for both capillary and diffusion

mechanisms are often expressed in terms of moisture concentration gradients.

The selection of the moisture content gradient or the vapor pressure gradient

as the driving force for diffusion, has been a matter of some controversy. The

vapor pressure gradient more consistently accounts for experimentally

observed phenomena than does the concentration gradient (Bramhall 1979).

The moisture concentration gradient has been successfully used to describe

the drying characteristics of many materials; the diffusion coefficients often

show strong concentration dependence. When the concentration gradient is

taken as the driving force, the concept of thermal diffusion is often introduced

to account for mass transfer due to temperature gradients.

20

Table 2.1 Driving forces and mechanisms proposed in previous studies

(Waananen et al., 1993)

Source Driving Force

Postulated Mechanisms

Application

Schrader et al., (1992) Diffusion Cellulose gel

Ruan et al., (1991) Diffusion Potato

Lartigue et al., (1988) Unspecified Wood

Perre et al., (1988) Vapor diffusion and bulk flow

Clay brick

Suraez and Viollaz (1988) Diffusion Potato slab

Paunder and Ahrens (1987)

Liquid and vapor bulk flow

Wood

Lehtinen (1986)

Vapor diffusion and bulk flow

Paper

Luu and Benner (1986) Diffusion Glass fiber board

Plumb et al., (1985) Liquid diffusion and capillary

Wood

Viollaz and Suarez (1985) Diffusion Shrinking body

Edwards and Adams (1983) Diffusion Wood

Vaccarezza

et al., (1974) Diffusion

Sugar beet root

Husain et al., (1973) Diffusion Potato slices

Chirife (1971) Diffusion Tapioca root

King (1971)

Vapor diffusion and bulk flow

Porous solid

Jeric and Nottage (1967)

Vapor diffusion Fiber board

Bateman et al., (1939) Diffusion Wood

McCready and McCabe (1933)

Vapor diffusion of bound water

Wood, Asbestos

Tuttle (1925) Liquid diffusion Wood

21

2.2.3 Drying Kinetics

For each and every product, there is a representative curve that

describes the drying characteristics for that product at specific temperature,

velocity and pressure conditions. This curve is referred to as the drying rate

curve for a specific product. The drying rate refers to the rate of evaporation

of water from the material. A plot of the drying rate versus product moisture

content (Mp) is called the drying rate curve. Figure 2.1 shows a typical drying

rate curve displaying an initial constant rate period, and at the critical

moisture content (Mc) the drying rate begins to fall with a further decrease in

moisture content (Syahrul et al., 2002).

Figure 2.1 Schematic representation of a drying rate curve

The mechanism underlying this phenomenon depends both on the

material and the drying conditions. The drying rate in the constant rate period

is governed fully by the rates of external heat and mass transfer, since a film

of free water is always available at the evaporating surface. The drying rate in

this period is essentially independent of the material being dried. The falling

Dry

ing

Rat

e (g

moi

stur

e/g

dry

mat

ter *

s)

Product Moisture Content (g water/g dry matter)

22

rate period begins to drop at the critical moisture content, since water cannot

migrate to the surface at the same rate as in the constant rate drying period

because of internal transport limitations. Under these conditions, the drying

surface becomes first partially unsaturated and then fully unsaturated until it

reaches the equilibrium moisture content. Depending on the nature of the

material and conditions of drying, the existence of a continuous drying rate or

falling rate period or combined drying rate can exhibit for the drying

conditions.

In the most general case, after an initial period of adjustment, the

dry basis moisture content decreases linearly with time, following the start of

the evaporation. This is followed by a non-linear decrease in moisture content

with time until the solid reaches its equilibrium moisture content (Me) where

drying stops. The equilibrium moisture content is the limiting moisture to

which a given material can be dried under specific conditions of drying air,

i.e., air temperature and humidity.

2.2.4 Drying Models

The most important aspect of drying technology is the

mathematical modelling of the drying processes. The objective is to allow

design engineers to choose the most suitable operating conditions and then to

size the drying equipment and drying chamber to meet the desired operating

conditions (Hawlader et al., 1997). The principle of modeling is based on

having a set of mathematical equations that can adequately characterize the

system. In particular, the solution of these equations must allow the prediction

of the process parameters as a function of time at any point in the dryer, based

only on the initial conditions (Gunhan et al., 2005).

23

2.3 MOISTURE REDUCTION METHODS FOR BAGASSE

Bagasse, the fuel used in the sugar industry has a very high

moisture content of 50%. Reducing the moisture content of the product

improves its calorific value, and resulting in the overall improvement of plant

efficiency. Various methods of moisture reduction reported in the literature

are (Upadhiaya 1991),

1. Mechanical methods

2. Use of hot imbibition water

3. Chemical methods

4. Drying of bagasse.

Mechanical methods

Cane juice is extracted from sugarcane by first cutting or chopping

the long sticks, a process known as shredding and then passing it through

compounded rollers, a process called milling. Better juice extraction and

moisture removal is possible by improving milling techniques. However,

various steps taken for the reduction of the moisture content in the bagasse

may bring it down to about 45-46% (Edwards 1981).

Use of imbibition water

Normally all the sugar factories use cold water for spraying on the

blanket of bagasse entering the last set of mills. There are many advantages of

using hot water for the imbibition purpose. In the first instance, hot water

extracts more sugar contained in the bagasse and as a result, the sugar loss in

bagasse can be considerably reduced. At the same time by using hot

imbibition water, the temperature of the bagasse is increased and as a result,

24

by the time the wet bagasse travels from the milling plant to the boilers, a part

of the moisture contained in the bagasse can be driven out by exposure to the

ambient air, thereby achieving partial drying of bagasse. Incidentally, a large

quantity of hot water is available in the sugar factories in the form of hot

condensates obtained from the juice heaters, evaporators and vacuum pans.

Even if the condensates contain traces of sugar, such condensates will not be

useful as boiler feed water, but they can be used as imbibition water (Hugot

1986).

In actual practice, by using hot imbibition water the bagasse

becomes soft and as a result, it refuses to enter the mills and slippages take

place. Moreover, the mill roller surface gets polished which adds to the

slippage. But this problem can be overcome by roughening the mill roller

surface. In sugar factories now-a-days, arching of the roller is done manually

while the milling plant is in motion. Therefore, many factories are now using

imbibition water at about 700°C to 800°C using the hot condensates.

Chemical method

The sugar and water are held in bagasse primarily by surface

forces. The reduction in the surface tension on adding suitable chemicals to

the bagasse matrix is expected to reduce the sugar loss and moisture content

in the bagasse. Bacon and Otrahaler (1954) have used a chemical called

‘Extrapol’ a high polymer of ethylene oxide and polyoxy-propylene, and

reported a reduction of the pol % of bagasse from 4-4.5% to 3.6-3.9%, thus

recovering 0.5% extra sugar. Ramaiah et al., (1979) developed an additive

called ‘sushira’, a composition consisting of different surfactant derivatives of

higher alcohol groups on a physico-chemical phenomenon known as

synergism, and have reported 0.6-0.8% reduction in pol along with a decrease

25

in the moisture content of the bagasse by about 3%. The chemical methods

have the basic limitations that the possible moisture reduction is only 3-4%.

Drying of bagasse

A reduction of moisture content below 45% is possible only by

means of drying (Upadhiaya 1991). Drying of wet bagasse involves the

transfer of heat to evaporate the moisture and removal of water vapor by a gas

or air stream. There are several methods like a tunnel dryer, rotary dryer,

pneumatic dryer and fluidized bed dryer which can be used for bagasse

drying. Considering a minimum modification to the existing plant the tunnel

and rotary dryers using direct/indirect heating or by a combination of both is

discussed.

2.3.1 Direct Bagasse Dryer

A direct dryer is also known as contact type dryer; here, the heat is

transferred by direct contact of the hot gas with the product. This hot gases

transfer sensible heat to provide the heat of vaporization of the moisture

present in the solid. Direct heating is preferred for the reason that higher heat

transfer rates result due to direct contact between the hot gas and the product;

in addition to this, a reduction in residence time and uniform drying can be

experienced. A schematic of direct tunnel type driver is shown in Figure 2.2.

Figure 2.2 Direct tunnel type dryer

26

In the sugar industry, one method of direct drying can be carried

out on the ‘Bagasse Elevator’ as the product moves from the mill to the boiler

(Roy et al., 1980). The bagasse elevator has to be designed as a tunnel, by

providing a top rectangular lid throughout its length. The hot gases are made

to pass through the tunnel above the bagasse surface. As the hot gases flow

through the tunnel they are in contact with the product, giving the heat of

vaporization to the product, which results in moisture removal from the

surface of the product.

The other method of direct drying can be carried out using drum

type rotary dryers (Kinoshita 1991) as shown in Figure 2.3. The hot gases are

made to flow in the counter current direction, while the product is dumped

into the rotary drum at a constant rate. The drum is inclined with a horizontal

slope, so that the solids slowly progress through the dryer under gravity. The

longitudinal lifting baffles collect the material and shower it through the hot

gas stream as the barrel rotates. As the bagasse come out of the drum the

moisture content would have been reduced.

Figure 2.3 Rotary drum direct dryer

27

2.3.2 Indirect Bagasse Dryers

Indirect dryers are the other type of dryers in which heat transfer is

only through conduction and forced convection. Indirect dryers can be

adopted for drying bagasse in either of the two methods discussed.

One type of indirect dryers can be used by transferring the heat by

conduction to the product from the bottom of the conveyor. In this case, the

conveyor plate is heated using flue gases or low pressure steam, from where

the heat is transferred to the product by conduction. This facilitates the

trapped water and water vapor in the product to trickle out towards the surface

of the product from where it is washed by the air.

The other type of indirect dryers is a rotary bin type where the

circumference of the bin is lined with vertical pipes. The pipes are fed at the

top with low pressure steam / flue gas with radial outlets from a common feed

header, reaching the individual pipes. The pipes are again connected together

at the bottom end and the condensate / exhaust is removed from the system.

Bagasse is charged to the dryer at the top from belt conveyors and descends

vertically down to the bottom where it is collected. During its travel down the

container bin, the bagasse gets dried by physical contact with the hot pipes

and the liberated water vapor travels up and out of the container bin.

2.3.3 Combined Heating Bagasse Dryers

By combining the direct and indirect type of dryers for bagasse drying, the

tunnel type dryers or the combined rotary type dryers can be used.

28

2.3.3.1 Tunnel type combined heat dryers

In this system while to bagasse moves over the elevator combined

heat in the form of convection and conduction is given to the material

(Massarani and Valence 1981). The bagasse elevator is connected with a lid

on the upper side, forming a tunnel shape, through which the hot air flows

along the length of the tunnel. The bottom of the elevator is attached with a

duct, parallel and in line with the elevator. The schematic of the model is

represented in Figure 2.4. Through the duct, the heating medium is passed

and this raises the surface temperature of the elevator. Heat is transferred

from the elevator surface to the product through conduction.

Figure 2.4 Tunnel type dryer with combined direct and indirect heating

Convection heat from the hot air above the product is used to

vaporize the moisture available in the product surface. As the evaporation of

surface moisture proceeds, the heat supplied by conduction assists the faster

movement of the trapped moisture in the interstices of the product to reach the

surface, where it is vaporized by the hot air flow. Thus, combining the effect

of direct and indirect type, the rate of moisture removal can be increased.

2.3.3.2 Rotary drum combined heat dryers

A rotary bin of conveniently inclined axis is also used for a

combined direct and indirect system of bagasse drying (Kinoshita 1991) as

29

represented in Figure 2.5. The indirect heat is given by conduction from the

steam/flue gas pipe which is fixed on the surface of the bin parallel to the axis

of the drum. The direct heat to the dryer is given by the hot air which is made

to enter the bin at the bottom through an air header and is distributed evenly

to the entire volume through suitable nozzles attached to the radial air header.

Figure 2.5 Rotary drum combined dryer

The hot air moves upward along the drum axis and vaporizes the

free moisture in the product. As the product is in contact with the hot pipe, the

bounded moisture tries to come to the surface, from where it is evaporated by

the hot air.

2.4 EXPERIMENTAL STUDIES ON DRYING

In the present research, a detailed thin layer drying experiments and

the application of TGA methods for drying studies were proposed. These

studies are reviewed on a general basis in this section.

30

2.4.1 TGA Studies

Thermal Analysis (TA) is one of the innovative approaches which

contribute to the study of the drying phenomena of biomass material

especially at high temperatures. Thermal Analysis is defined as the

measurement of changes in the physical properties of a substance as a

function of temperature, while the substance is subjected to a controlled

temperature program, Brown (1988). Thermal Analysis (TA) includes

different experimental techniques such as thermo-gravimetry analysis (TGA),

derivative thermo-gravimetry (DTG) and differential scanning calorimetry

(DSC). TA has been used to characterize different compounds, minerals,

biological substances and pharmaceutical products (Speyer and Robert 1994;

Paulik 1995). However, TA methods have rarely been used to study the

liquid-solid interfaces (e.g., drying kinetics of wet porous solids). In this

research an attempt is made to use TGA studies to determine the drying

kinetics of bagasse samples.

Thermo-gravimetric analysis (TGA) is one of the techniques used

to investigate thermal events and to study the kinetics of liquid-solid

interfaces during thermal analysis of biomass particles (Hatakeyama and

Quinn 1999; Nassar 1999; Mansaray and Ghaly 1999; Kastanaki et al., 2002).

It provides a measurement of weight loss of the sample as a function of time

and temperature. The kinetics of thermal decomposition reactions of

carbonaceous materials is complex, in that the decomposition of carbonaceous

materials involves a large number of reactions in parallel and in series. TGA

provides general information on the overall reaction kinetics rather than

individual reactions. It could be used as a tool for providing a comparison of

the kinetic data of various reaction parameters such as temperature and

heating rate. Other advantages include only a single sample, and few data are

required for obtaining kinetics over an entire temperature range in a

31

continuous manner. Hence, in this section the relevant papers discussing

biomass thermal degradation kinetics, using Thermo-gravimetric analysis are

reviewed.

Moreno et al (2004) conducted an extensive characterization study

on forest biomass particles of Pinus radiate using the thermo-gravimetric (TG)

approach in order to understand and optimize the process of drying wood

particles. The TG results show the weight loss kinetics in relation to the initial

loaded weight as a function of the sample temperature during the experiment.

The weight loss of the biomass took place in three well-defined stages. The

first stage exhibited approximately between 21 and 110°C with a weight loss.

This weight loss was attributed to the release of moisture or of substances

physically absorbed by the surface. The second stage occurred between 187

and 364°C; the weight loss in this stage is mainly due to the consequence of

the thermal decomposition of the wood (pyrolysis or devolatilisation). The

third stage occurred between 364 and 800°C, which corresponds to the

devolatilisation and it results in weight stabilization at a final value. The

region where the weight does not change is between 114 and 187°C. This

means that the biomass particles when dried only at temperatures lower than

187°C undergo a change in mass as a result of the moisture loss and not as a

consequence of devolatilisation.

Vuthaluru (2004) conducted an investigation on the pyrolytic

behaviour of coal/biomass blends using the thermo-gravimetric analysis.

Thermo-gravimetric analysis (TGA) is one of the most common techniques

used to investigate thermal events and kinetics during the pyrolysis of coal or

coal/biomass mixtures. A NETZSCH Simultaneous Thermal Analyzer 409-C

(TGA-MS) was used for this study. TGA experiments generated mass loss

versus temperature or time graphs, reflecting the thermal behavior and

composition of the initial sample, intermediate species and final residue. From

32

the rate of mass loss, the kinetics of pyrolysis for different coal/biomass

blends was determined.

Aiman and Stubingto (1993) have investigated the pyrolysis

kinetics of bagasse at low heating rates. The kinetic study of bagasse

pyrolysis was carried out using a Du-Pont 95-1 Thermo-gravimetric Analyser.

Pyrolysis of dry and wet bagasse occurred in three different zones for

temperatures up to 800°C, with heating rates from 5 to 50°C/min. The major

degradation occurred in the second zone between 195 and 395°C with a mass

loss of 73.8% of the dry sample mass. For the range of experimental

conditions studied, neither the heating rate nor the initial moisture content had

any significant effect on the results.

2.4.2 Thin Layer Drying Studies

Thin layer drying studies are widely used to predict the drying

characteristics of thin moist products for a given set of drying variables

(Doymaz 2004). The drying variables considered for the study are air

temperature, velocity and humidity. In this study, the effect of these variables

on the drying curve is examined. As it is proposed to study the drying

characteristics of thin layer bagasse, a detailed review on thin layer drying is

presented in this section.

Panchariya et al., (2002) have conducted thin-layer experimental

studies of the drying process of black tea. An experimental dryer was

developed for determining the kinetics of black tea drying. The drying

characteristics of tea were examined under varied conditions of air

temperature and air flow velocity. The effective diffusivities were estimated

for a temperature range. The temperature dependence of the diffusivity

coefficient was described by the Arrhenius-type relationship. The results

33

illustrate that inspite of high initial moisture content, the drying of tea

particles takes place only in the falling rate period. It is concluded that this

single-layer drying equation can be used for the simulation of deep-bed

drying of black tea.

Wang et al., (2007) carried out thin layer drying studies on the hot

air drying of apple pomace. The hot air convective drying characteristics of

thin layer apple pomace were evaluated in a laboratory scale dryer. The

drying experiments were carried out at 75, 85, 95 and 105°C and at the air

velocity of 1.20 ± 0.03 m/s. The drying process took place in two falling rate

periods; the effective diffusivities in the second period were about six times

greater than those in the first period. The general relationship of the moisture

ratio against the drying duration in the thin-layer convective drying of apple

pomace was found and can be used in the design and operation of convective

drying.

Kaleemullah and Kailappan (2006) conducted thin-layer drying

kinetics studies for red chillies. The drying kinetics of red chillies in a

thin-layer dryer was investigated within a temperature range of 50-65°C.

Drying curves obtained from the experimental data were fitted to different

drying models, and it was found that the Page and Kaleemullah models are

suitable to predict the moisture ratio of chillies at different drying air

temperatures of a thin-layer dryer. The effective moisture diffusivity (Deff) of

chillies increased from 3.78 x 10-9 to 7.10 x 10-9 m2/s as the drying air

temperature of a thin-layer dryer increased from 50 to 65°C. The activation

energy of diffusion (Ea) was calculated as 37.76 kJ/mol.

Ibrahim Doymaz (2006) conducted thin-layer drying studies for

mint leaves. The thin-layer drying behavior of mint leaves for a temperature

range of 35-60°C was determined in a cabinet dryer. The increase in air

34

temperature significantly reduced the drying time of the mint leaves. The

drying data of this material was analyzed to obtain diffusivity values from the

falling rate-drying period. In this period, the moisture transfer from mint

leaves was evaluated by applying the Ficks diffusion model. Effective

diffusivity varied from 3.067 x 10-9 to 1.941 x 10-8 m2/s and increased with

the air temperature.

Akpinar et al., (2006) conducted thin layer drying studies for

parsley leaves in a convective dryer under the open sun. In this work, the

drying behaviour of parsley leaves was investigated in a convective dryer

with forced convection at a fixed airflow rate of 1 m/s and drying air

temperatures of 56, 67, 85, and 93°C and under the open sun with natural

convection. The initial weights of parsley leaves were about 30 g (± 5 g). The

drying of parsley leaves started with an initial moisture content of around

84% (wb) and continued until a final moisture content of about 10%.

Moisture loss was recorded at 15 min intervals during drying by a digital

balance in the measurement range of 0-3100 g and an accuracy of ± 0.01 g.

During the experiments, the humidity, velocity, inlet and outlet temperatures

of air in the convective dryer were recorded. The constant-rate period is

absent from the drying curve. The drying process took place in the falling rate

period. The drying data were fitted to the different mathematical models and

the Page model showed good agreement with the experimental data obtained

from forced convection drying.

2.5 ENERGY AND EXERGY ANALYSIS

Thermodynamics plays an important role to perform the energy and

exergy analyses of the drying processes. The first law is widely used in

engineering practice and is the basis of the heat-balance method of analysis

that is commonly used in engineering system’s performance analysis.

35

However, the second law involves the reversibility or irreversibility of

processes and is a very important aspect in the exergy method of energy

system’s analysis. During the past few decades, thermodynamic analyses,

particularly exergy analyses, have appeared to be an essential tool for the

system design, analyses and optimization of thermal systems (Dincer and

Sahin 2004). For evaluating the performance of drying systems, the energy

analysis method has been widely used, while the studies on exergy analysis

are relatively few in number. In this section, a detailed literature review of the

energy exergy studies for the drying system has been given.

An energy and exergy analysis of a timber dryer assisted heat pump

was conducted by Ilhan Ceylan et al., (2007). For this research a prototype

heat pump dryer was designed for drying poplar and pine timber. The timbers

have been dried from the moisture contents of 1.28 kg water/kg dry matter

and 0.60 kg water/kg dry matter to 0.15 kg water/kg dry matter in the heat

pump dryer, functioning on the basis of 24h operation. In this study, the

drying air temperature was kept at a mean of 40°C dry bulb temperature with

no additional heat source and an air velocity at 0.8 m/s. From this research it

has been observed that the moisture diffusion from the timber to the air

decreases during drying. Hence, in this work 60% air recirculation is adopted

during the drying studies. For these conditions of drying they have performed

an energy and exergy analysis which indicates that during drying the energy

utilization decreases while the exergy efficiency increases.

In their work Akpinar et al., (2006) performed a first and second

law analysis for the drying of pumpkin slices in a cylindrical dryer with two

sets of trays where the drying air enters the dryer tangentially. The drying

experiments were conducted by varying parameters like the temperature from

60 to 80ºC, relative humidity from 10 to 20% and the air velocity in the range

of 1.0 to 1.5 m/s. The sample with an initial moisture content of 93.7%

36

experienced a weight reduction from 200 g to 14 g during drying in the

convective cyclone type dryer. From the analysis it is observed that the

product in the first tray consumed more energy than that in the second tray.

The EUR of the first tray was higher than that of the second tray. Therefore, it

is said that the energy taken from the heaters was productively utilized for the

thin layer drying of pumpkin slices in the first tray.

Colak and Hepbasli (2006) evaluated the performance analysis of

the single layer drying process of green olive in a tray dryer, using the energy

and exergy method. In this work, the drying process was realized at four

different drying air temperatures (40, 50, 60 and 70°C) and a constant relative

humidity of 15%. The measurements obtained from the experimental data,

were utilized to conduct a system performance evaluation of energy and

exergy efficiencies and their exergetic improvement potential. Exergy

destructions (representing the losses) in the system were also quantified. The

exergy efficiency values were found to be in the range of 68.65-91.79% for a

temperature range of 40 to 70°C. The effects of temperatures and mass flow

rates were investigated. In terms of exergy efficiency, the drying process

realized at a temperature of 70°C obtained the maximum value.

Holmberg and Ahtila (2005) conducted an energy efficiency study

for biofuel drying in a pulp and paper mill by means of energy and exergy

analyses. The evaluation was based on the determination of heat consumption

and the irreversibility rate for energy and exergy analyses, respectively. In

their research they used two alternative heat sources such as, steam at a

pressure of 3 bar and water at a temperature of 80°C to raise the drying air

temperature.

The results show that heat consumption is only dependent to a

small extent on the heat source type or the drying system. It is also observed

37

from this work, that the heat consumption is almost similar for each

comparative test conducted, regardless of the drying temperature or system. In

order to find the quality of energy used during drying they have performed the

exergy analysis by means of the irreversibility rate. It is observed that the

irreversibility rate depends to a considerable extent on the temperature of the

heat source and also on the drying system. It has been concluded that, to

decrease the value of the irreversibility rate, the air recirculation ratio has to

be improved with a lower drying temperature; the dryer area has to be

increased and the number of stages during drying has to be improved.

Akpinar (2004) has conducted an energy exergy analysis for drying

of red pepper slices in a convective type dryer. This test was performed for

the experiments in which drying air inlet temperatures were varied as 55, 60

and 70ºC with an air velocity of 1.5 m/s. During the drying process the

Energy Utilization Ratio (EUR) varied from 1.11 to 18.85% depending on the

drying air temperature in the drying chamber. The exergy efficiency of the

drying chamber varied from 67.28 to 97.92%. It was observed that the EUR

values decreased while exergetic efficiency values increased with the increase

in drying time.

Midilli and Kucuk (2003) examined the energy and exergy analyses

of the drying process of shelled and unshelled pistachios using a solar drying

cabinet. The shelled and unshelled pistachio samples were sufficiently dried

in the ranges between 40 and 60°C, 200 and 800 W/m2 of solar radiation at

1.23 ms-1 of drying air velocity for a drying time of 6 h. For this condition of

drying, the energy analysis was carried out to estimate the amounts of energy

gained from solar air collectors, and the ratios of energy utilization were

obtained. It is recorded that the energy utilization ratio (EUR) varied during

the experiments to a maximum of 59.7%. The exergy analysis was

accomplished to determine the location, type, and magnitude of exergy losses

38

during the solar drying process by applying the second law of

thermodynamics. The maximum value of exergy inflow to the system during

the experiments was recorded as 3.718 kJ/kg, while the exergy losses varied

in the range of 0.15-3.08 kJ/kg.

2.6 EVALUATION OF THIN LAYER DRYING MODELS

The coupled heat and mass transfer phenomena during the drying

process make it very complicated to analyze. The controlling resistance to

heat and mass transfer will define the required accuracy of the transport

properties, and will identify what property must be incorporated in the

mathematical model. However, researchers are striving for a simple

mathematical model with a small number of parameters (Jayas et al., 1990)

which can predict the drying behavior of the product.

Thin-layer equations are often used to describe the drying kinetics

of various types of porous materials. In general, thin layer mathematical

models used for the description of the drying kinetics, can be classified into

three groups: theoretical, semi-theoretical, and empirical (Jayas et al., 1990).

Theoretical equations are based on the diffusion process (Fick’s second law)

or simultaneous heat and mass transfer equations. Semi-theoretical equations

are analogous to Newton’s law of cooling, with some theoretical background.

Empirical equations are easily applied to drying simulations as they depend

on the experimental data (Afzal and Abe 2000). Empirical models fit all

drying data well, but neglect the fundamentals of the drying process.

Among these models, the theoretical approaches take into account

only the internal resistance to moisture transfer while the semi-theoretical and

empirical approaches consider only the external resistance to moisture

transfer between the product and the air (Henderson 1974). In the past, many

39

theoretical and empirical models have been applied for various agro-based

products to determine the best suited drying model for the product. Hence in

this research, to select the suitable mathematical model for the drying kinetics

of bagasse under varied conditions, the drying models were tested. The

related literatures corresponding to thin layer drying models are reviewed in

this section.

Kashaninejad et al., (2007) studied the thin-layer drying

characteristics and modeling of pistachio nuts. The thin-layer drying

characteristics of pistachio nuts were determined experimentally as a function

of temperature, relative humidity and air velocity. Six mathematical models

(Page model, modified Page model, exponential model, diffusion model, two

term exponential model and Thompson model) for describing the thin-layer

drying behavior of pistachio nuts were investigated. The moisture ratio was

fitted to the different thin-layer drying models and was evaluated based on the

coefficient of determination (r), residual mean square error (MSE) and mean

relative deviation modulus (P). The acceptable r value was obtained for all the

six models fitted to all the drying runs. The Page model presented higher

values than the other drying models. The examination of the MSE also

showed that the Page model gave the superior fit to the experimental data

compared to other models due to its lower values.

Menges and Ertekin (2006) studied the mathematical modeling of

the thin layer drying of golden apples. In their study, a laboratory dryer was

used for the thin layer apple drying process and the moisture ratios were

compared with 14 thin layer models such as; Newton, Page, Modified Page,

Henderson and Pabis, logarithmic, two-term, two-term exponential, Wang and

Singh, Thompson, diffusion approximation, Modified Henderson and Pabis,

Verma et al., and Midilli et al., models. The effect of drying air temperature

and velocity on the coefficients of the best suited moisture ratio model were

40

determined by the multiple regression method. In this work, in addition to the

statistical parameters like, Root mean square error (RMSE) and chi-square

(χ2), modeling efficiency (EF) was also evaluated for the determination of the

best suitable model. According to their results, the Midilli et al. model is

superior to the others for explaining the drying behavior of apples.

Kaleemullah and Kailappan (2006) studied the drying kinetics of

chillies in a thin-layer dryer and evaluated the suitability of some drying

models to represent the thin layer drying of chillies. Three thin-layer drying

models (Lewis, Hustrulid & Flikke and Page) were selected in their work for

fitting the experimental data of thin-layer drying results. The three models

were compared with an empirical model which has a temperature term. The

parameters of all the models were estimated by using the SPSS (Statistical

Package for Social Science) software. The suitability of the models was

evaluated and compared, using the mean relative percentage deviation (Em),

standard error of estimate (Es), randomness of residual (ei) and coefficient of

determination (r). It is concluded that the Page and Kaleemullah models stood

first and second, respectively, in predicting the moisture ratio of chillies

during thin layer drying.

Akpinar and Yasar Bicer (2004) reported the thin-layer drying

behavior of egg-plant slices in a Convective-type cyclone dryer. Thin-layer

experiments for varying air temperatures and air velocities were performed. In

order to estimate and select a suitable form of the drying curve, eight different

semi-theoretical and empirical thin layer models were fitted to the

experimental data and comparisons were made for their coefficients of

determination as predicted by the non-linear regression analysis. For the

experiments, the Page model best described the drying behaviour of the

egg-plant slices with a correlation coefficient, r = 0.9999. Also, it is reported

in their work, that several investigators (Afzal and Abe 1999;

41

Karathanos 1999; Hossain and Bala 2002) have reported that the Page model

adequately predicts the thin-layer drying of a wide variety of crops.

Panchariya et al., (2002) in their study described the thin-layer

drying characteristics of black tea particles. The study was carried out to

develop a suitable model to investigate the effect of temperature and air

velocity on the model coefficients which describe the drying characteristics of

black tea particles. The drying experiments were carried out using a batch-

type dryer. The drying data were then fitted to the different semi-theoretical

models such as Lewis, Page, modified Page, two-term and Henderson and

Pabis models, based on the moisture ratios. Though the coefficient of

determination (r), the MSE and coefficient of determination (r2) values for all

the models were quite reasonable, but the chi-square (χ2) values were greater

than the values obtained by the Lewis model. Hence, in this work, the Lewis

model gave better predictions than others. In this study, the nonlinear

regression method was based on the Levenberg–Marquardt (LM) algorithm.

2.7 NUMERICAL ANALYSIS OF DRYING

The description and prediction of the drying kinetics of a given

material is still a weakness in the modeling of drying processes. In the design

and optimization of drying processes, there is a great need for stable and

reliable models to quantify and predict drying rates with a satisfactory

accuracy. Over the last decades several approaches have been proposed about

how to deal with mass and heat transfer phenomena in materials during a

drying process. Wang et al (2007) made an overview of mathematical

modeling of solid drying by reviewing more than 70 published papers; their

study reveals that there is no general model for solid drying due to lack of

availability of fundamental data.

42

2.7.1 Air as Drying Medium

The drying of moist porous solids is a complicated process

involving simultaneous coupled heat and mass transfer phenomena resulting

in a system of coupled nonlinear partial differential equations (Aleksandra

Sander et al 2003). The drying behavior can be influenced by external

parameters, such as temperature, velocity, and relative humidity of the drying

medium, and the internal parameters like density, permeability, porosity, and

sorption–desorption characteristics and thermo-physical properties of the

material being dried. The predominant mechanisms that control moisture

transfer depend on the hygroscopic nature and properties of the materials, as

well as the heating conditions and the way heat is supplied. In most of the

drying situations the drying medium used was air. Hence, in this section

numerical models using air as the drying medium are reviewed.

Go et al., (2007) developed a mathematical model for temperature

and moisture content distribution inside a triangular spouted wheat bed dryer.

The model is based on the analysis of heat and mass transfer inside the dryer.

The equations representing heat balances in the product and the air, mass

balances, and drying rate within a single kernel is considered. Furthermore, it

is assumed that internal moisture content dominates the moisture transfer

process and the surface moisture content is assumed to be always in

equilibrium with the drying air. The solution for these equations was

developed using an implicit finite difference method. The results of the

experimental and model comparison showed that the model was able to

predict accurately the moisture content of wheat during drying. However, the

temperature prediction inside the TSB dryer was less accurate, especially at

high temperatures due to heat losses in the experimental dryer.

43

Younsi et al., (2007) developed a three-dimensional mathematical

model for high temperature thermal treatment of wood The model equations

in their work were solved numerically by the commercial package Femlab for

the temperature and moisture content histories. It is claimed that the model

predicts the behavior of the wood sample exposed to high temperatures, with

sufficient accuracy within the range of temperatures considered.

Kaya et al., (2006) developed a numerical model to represent the

solution of heat and moisture transfer inside a rectangular moist object during

the drying process. The external flow and temperature fields are first

numerically predicted through the Fluent CFD package, and later, the heat

and moisture transfer inside the object. From the temperature gradients

obtained through the Fluent analysis, the variations of the convective heat

transfer coefficients along the surface of the object were determined. A

numerical procedure to analyze the heat and mass transfer through diffusion

was developed. The heat and mass transfer equations were solved using the

finite difference method. The temperature and moisture distributions at

different periods of time were also examined. The model results obtained

from the analysis were compared with the experimental data, and a good

agreement in the results was claimed.

Salah Ben Mabrouk et al., (2006) presented a numerical model for

heat and mass transfer of granular products in a fixed-bed tunnel dryer. A

simplified heat and mass transfer numerical model using the finite volume

method is developed. In their research, the turbulent airflow and granular bed

convection coefficient, as well as the effective conductivity, are estimated

using the turbulent airflow over flat-plate correlations. It is claimed that the

model results were able to reproduce the experimental drying curve data for

grape sample layers. The temperature and moisture distributions in the

product and in the drying air along the tunnel dryer were predicted.

44

Lan Sun et al., (2005) developed a diffusion model for drying a

heat sensitive solid under multiple heat input modes. A two-dimensional

model for mass and energy transport during drying was developed assuming

the processes to be controlled by liquid water diffusion within the drying

object. This drying model permits simultaneous application of conduction and

radiation heat to supplement convection heat. The coupled partial differential

equations were discretized by the finite volume method. A Matlab computer

code was written to solve the governing equations with appropriate boundary

conditions. Four heat input schemes (pure convection, radiation-coupled

convection, conduction-coupled convection and radiation-conduction–

coupled convection) were compared by experimental and numerical studies. It

is claimed, that both the drying rates and temperatures within the slab during

drying under all these four heat input schemes, showed good accord with the

measurements. The radiation-coupled convection is the recommended heat

transfer scheme from the viewpoint of a high drying rate and low energy

consumption.

Chua et al., (2002) developed a mechanistic model capable of

predicting the moisture and temperature distribution of food materials during

isothermal and non-isothermal drying. The model incorporates coupled heat

and mass transfer equations for liquid water and water vapor movements

through porous food material. The model was solved numerically by the

method of finite volume. Evaluation of the model predictions shows good

agreement, for the mean moisture content with the experimental data. Their

results show the existence of a thermal switch that changes the direction of

the thermal gradient for drying, conducted with a step-down varying of the

inlet air temperature. Step-down air temperature drying was also found to

produce more uniform internal moisture distribution towards the latter stage

of drying. The impact of the moisture and temperature distributions on the

product quality was also discussed in the work.

45

Rumsey et al., (2001) developed a two-dimensional simulation

model for cross-flow of rice drying. The dynamic mathematical model was

solved using an explicit finite difference technique called the Mac-Cormack

method. Simulated values of moisture content during drying obtained by the

model showed satisfactory agreement with the experimental data. The effect

of a step disturbance in the inlet product moisture on the outlet moisture

content is also analyzed for different residence times. It is claimed that the

model has proven useful in examining the effects of manipulating the input air

temperature and grain flow.

Naon et al., (1995) proposed a mathematical model which enables

the description of mass and energy transfers during the convection drying of

granular materials. The model shows the evolution of the water content of the

material in time space, air and grain temperature, relative humidity of the air,

layer thickness and power input. In the proposed model, phenomena such as

the phase change of the fluid and heat exchange between air and grain by

convection, is considered. The operation of the model requires the

experimental determination of the water exchange coefficient between the

material and drying air using the thin layer drying kinetics. In the model, the

equations were solved using the implicit finite difference method. The

tri-diagonal algorithm procedure was used to determine the physical values

sought at all points in the drying layer at each instant.

Adriana Franca et al., (1994) presented a numerical simulation of

intermittent and continuous deep-bed drying of biological materials. In this

model a two-dimensional deep bed drying model, which incorporates

modifications in the existing one-dimensional models has been developed,

with the capability of handling both non-uniform velocity profiles and

material properties dependence on temperature and moisture content. In the

model, both the finite element and control volume methods were used to solve

46

the system of differential equations. Corn-drying was simulated in order to

analyze its temporal and spatial drying behavior. Both intermittent and

continuous drying was simulated. The simulation results using both control

volume and finite element method agreed well with the results obtained using

the finite difference method. A comparison of the results from the finite

element, finite volume, and finite difference analysis shows that results of

these techniques are in very good agreement with each other. The results also

showed that continuous drying is more efficient than intermittent drying with

respect to drying time. However, intermittent drying allows more uniform

temperature and moisture distributions.

2.7.2 Steam as Drying Medium

Steam drying could be a better substitute to conventional air drying,

in which continuous generation of low pressure steam is available as a result

of drying, and this could be used for process applications. Steam drying is a

measure that allows recovery of the energy by making use of the vapor

generated during drying for process applications in industries (Soponronnarit

et al., 2002). This superheated steam drying phenomenon, has recently

received considerable attention for drying wet products, due to its several

advantages. The main advantages are oxygen-free drying (free from the risk

of explosion and burning), higher drying rates, energy savings, and reduced

environmental impact. The use of superheated steam can lead to energy

savings as high as 50-80% over the use of hot air or flue gases (Pronyk et al.,

2004). These savings are achieved due to higher heat transfer coefficients and

increased drying rates during the constant and falling periods. High thermal

efficiency can be achieved, only if the exhaust steam is collected and used

elsewhere in the processing operation. As a steam drying model is proposed

in this research, the literature relevant to steam drying for moisture reduction

is reviewed in this section.

47

Uengkimbuan et al., (2006) made a comparative study of pork

drying using superheated steam and hot air. The objective of this work was to

study the application of hot air and superheated steam for producing dried

pork. The effects of the drying media and drying temperature on the drying

characteristics and physical properties in terms of effective diffusivity, color

changes, microstructure, and rehydration capacity of dried pork, are

investigated in this work. The experimental results have shown that the

decrease of pork moisture content in an early drying time was more rapid in

superheated steam than in hot air and appeared to be lower in a latter time.

The effective diffusivity of pork dried at the temperatures of 130-150°C was

in the corresponding ranges of 8.94x10-10 to 15.69x10-10 m2/s for superheated

steam and 8.61 x10-10 to 14.63 x10-10 m2/s for hot air. According to these

results, it is concluded that the prediction of decreasing moisture in

superheated steam is faster throughout the drying time than that in hot air.

Hiromichi Shibata (2005) made an experimental comparative study

of the drying rate curves for porous solids using superheated steam and air as

drying media. In this work, the drying of porous solids such as baked clay,

firebrick, and cemented glass balloons, over a wide range of pore-size

distributions was experimentally investigated in steam at sub atmospheric

pressure and at atmospheric pressure, as well as in air at atmospheric pressure.

Moreover, the drying rate curves in superheated steam at sub-atmospheric

pressure for the porous materials were compared with those in steam at

atmospheric pressure. These comparisons were conducted to elucidate the

mechanism of their differences in steam-drying and in conventional air-drying

at atmospheric pressure, as well as to reveal the superior aspect of drying rates

in steam-drying. The results revealed the distinct differences between critical

moisture contents and normalized drying rate curves, for steam and air at

similar sample temperatures for the materials considered. Comparatively a

longer constant rate period and lower critical moisture content were recorded

48

for steam than that of air. Higher drying rates were observed for steam than

air during the falling rate period.

Namsanguan et al., (2005), conducted a numerical simulation of a

cabinet-type superheated steam drying system and it was validated

experimentally using shrimp as the test material. The experimental shrimp

drying data that were obtained by Soponronnarit et al., (2002) were used to

verify the present model. The inputs for the model are the initial moisture

content and initial mass of the product, the volume flow rate of the drying

medium, the drying temperature, and the fuel flow rate in the burner. In their

research, the simulated moisture content of the shrimp, the drying medium

temperature at the outlet of the dryer, and the performance of the dryer in

terms of drying time, drying rate, specific energy consumption, and drying

efficiency, were verified using the available experimental data. The results

showed that the simulated results agreed well with the experimental results.

The drying temperature and the initial load of the raw product were found to

affect the specific energy consumption (SEC) and the drying efficiency of the

dryer. Based on the present study, the desirable operating conditions were

also evaluated for the dryer for the shrimp by considering the optimized

performance and good product quality.

Pronyk et al., (2004) examined a thin layer superheated steam dyer

for drying food stuffs. The drying kinetics are best determined from thin-layer

drying experiments and can be expanded to modeling deep-bed drying. This

mathematical model will allow an efficient scale-up of laboratory superheated

steam dehydrating systems and produce effective processing parameters for

their operation. The objective of the research was to conduct systematic

studies on the drying of food products in superheated steam. The drier was

constructed with the objective of determining the drying characteristics,

drying rates, and the effect of superheated steam on product quality in thin-

49

layers. Results from the superheated steam drying experiments state that, all

the samples tested (sugar-beet pulp, potatoes, Asian noodles, brewers’ and

distillers’ grain) gained small amounts of moisture in the first few seconds of

processing in superheated steam due to condensation before the product’s

temperature increased to the saturation temperature (100°C). Drying times

decreased and drying rates increased with increasing steam temperatures. For

sugar-beet pulp the decrease in drying time and increase in drying rates were

greater in superheated steam than in hot-air drying.

Elustondo et al., (2002) developed a mathematical criterion to

estimate the optimum working conditions for drying foodstuffs with

superheated steam. In their research an initial drying rate equation was

derived from mass and energy balances, and a new dimensionless number was

proposed to take into account all the key dryer characteristics. The operating

variables in drying with superheated steam are, pressure, temperature, and

velocity. It is predicted in their research that the drying rate always increases

as either temperature or velocity increase, but for these two variables, there

exists at least one pressure at which the drying rate reaches a maximum value.

In order to test their models, numerical experiments were performed for a

parallel and cross flow tray dryer and also for a rotary dryer. In all the cases,

the isothermal plots of the initial dying rates versus pressure show a

maximum, and the maximum shifts to higher pressures as the inlet

temperature is increased.

Iyota et al., (2001), proposed a model for heat and mass transfer in

superheated steam drying. This model focused on the phenomena which occur

during the initial stage of drying; i.e., condensation of superheated steam on

material surfaces and a subsequent shift from condensation to evaporation

leading to the beginning of the actual drying. In the model, the drying

equations considering the reverse process were formulated for an infinite flat

50

plate to calculate changes in the mass of a material with time. The influence

of the initial thickness of the material and the heat transfer coefficient were

also investigated. From their model the following results were found: the

internal temperature rises quickly due to condensation heat transfer, while the

moisture content near the material surface also rises due to condensation.

Tatemoto et al., (2001) studied the effect of operational conditions

on drying characteristics in closed superheated steam drying. In this study,

superheated steam drying in closed system was proposed. The vapor

generated from a sample was used as the drying gas. The effects of

operational conditions on the drying characteristics in closed superheated

steam drying, in which the vapor generated from a sample was used as the

drying gas, were examined experimentally and theoretically. The results were

compared with those in hot air drying. In the theoretical analysis, the

replacement of air with vapor in the drying chamber and the convective vapor

transfer in the sample were considered. The result of this work revealed, that

during the internal evaporation period, the evaporation occurs in the narrow

zone, which moves from the surface to the bottom of the sample.

2.8 BAGASSE DRYING REVIEW

The flue gases flowing from bagasse fired boilers have

temperatures around 300°C. The first interest shown in bagasse drying with

boiler stack gases dates back to 1910, when Prof. E.W.Kerr (Louisiana

Bulletin 1911) showed that it was impossible in Som Louisiana mill at that

time to cover the sugar mill’s energy demands with bagasse alone, owing to

its high moisture content. He built a dryer which reduced the moisture

content from 54.47 to 44.45 %, raising the steam production from 1.63 to

2.53 kg steam/kg bagasse. His dryer was a square tower with bagasse

descending and stack gas rising in a counter current manner. The tower was

51

equipped with deflectors to promote better gas-solid contact. Between 1910

and 1970, only very few bagasse drying works were reported. The reason for

the lack of interest in bagasse drying during this period was the low cost of

fossil fuel. Bagasse was not very attractive as an alternative even in cases

where it was a residue, due to the usage of cheap oil. Because of the energy

crisis in 1970’s, efforts have been concentrated in further reducing the

bagasse moisture. Since then a number of technical reports on bagasse drying

both theoretical and practical have appeared.

Roy et al., (1980) and Keenliside (1983) have reported the use of

moving bed dryers for bagasse drying. Roy et al., (1980) studied the effect of

the temperature of the outgoing flue gases, the velocity of air, length of dryer

and annual profit with that of the percentage of moisture removed the mass of

air flow and the width of the drier and the air heater. The air used for bagasse

drying was first heated using flue gases and then passed through bagasse.

Keenliside (1983) compared three different boiler configurations viz. i) boiler

with no air preheater or bagasse dryer ii) boiler with air preheater and

iii) boiler with bagasse dryer. He showed that the overall increase in steam

production using a bagasse dryer is not significantly greater than when using

air pre-heaters due to the extra peripheral equipment required to operate the

drying systems. Massarani and Valenca (1981) studied intensively the drying

of bagasse in a moving bed dryer. They investigated from a laboratory scale

to a pilot one. The pilot installation was composed of a dryer of 0.40 x 0.50 x

2 m. These two steps led to satisfactory results.

During the fuel crisis, Furines (1976) prepared a feasibility study of

bagasse pre-drying with waste stack gases. He worked with three rotary-drum

type dryers of maximum capacity to operate with the existing boilers, based

on the flue gases temperature of 218ºC. These three dryers processed all the

bagasse produced and lowered the moisture content from

52

54 to 46% (w.b.), provided the gases had a temperature of 218ºC or more. The

use of rotary dryers for bagasse drying was also reported by Guanzon (1980)

and Sarnobat (1987). Guanzon (1980) plotted the capacity of dryer versus

moisture content of bagasse as a function of inlet flue gas temperature. The

moisture removal rate increases with an increase in the capacity of the

bagasse dryer. Sarnobat (1987) calculated the heat transfer area for a rotary

drum dryer inclined at 30º. He reported a bagasse saving of around 30% and

pay back period of three months for a bagasse dryer.

In pneumatic transport, the velocity at which a gas will begin to

transport a specific particle is called the terminal velocity. The terminal

velocity for different bagasse size fractions were determined by Grobert

(1971). They show that at a terminal velocity higher than 13.9 m/s, all the

bagasse particles will be transported pneumatically. At a terminal velocity

lower than 13.9 m/s the raw bagasse will be separated into two fractions. This

separation could enable the use of more efficient systems of pneumatic

transport, and storage in silos, which would be placed between the mill train

and the boilers.

Arrascaeta and Friedman (1987) designed and constructed a bagasse

dryer in 1983 that elutriates the bagasse, separating the particles in different

sizes. This dryer could work with 7 ton/hr and was in operation until 1985.

Later the design was patented in 1987, which used fluidized and pneumatic

conceptions. Nebra and Macedo (1989), developed an industrial dryer which

was designed and built according to a project developed by the Centro de

Tecnologia Copersucar, Brazil. It was a flash drier that could work with 25

tons of bagasse/hour. That is the biggest flash dryer reported until now.

Alarcon and Justiz (1993) also worked with a pneumatic dryer which reduced

the moisture content from 50 to 30% (w.b.) and separated the particles into

different sizes. The biggest particles were used as raw material for paper and

53

pharmacy industries and the smaller ones were burned. Cardenas et al (1994),

described a pneumatic dryer of an industrial size. They studied the energetic

and exergetic efficiencies of a boiler-dryer system. They concluded that the

use of a dryer will improve the boiler efficiency.

Keller (1980) reported the advantages of a suspension dryer over a

rotary drum dryer. He reported the effect of moisture content on combustion

temperature in bagasse furnaces. It was found that with a decrease in moisture

content, furnace temperature increases. He also reported an increase of heat

transmitted to steam per kg of bagasse with a reduction of bagasse moisture

content. Morales (1982) reported the use of a suspension type bagasse dryer

consisting of two units. Each unit was designed for 17.5 ton/hr, with an initial

moisture content of 56% to a final moisture content of 35%. He has reported

bagasse dryer operating data over a period of one year.

Bose et al., (1984) have carried out studies on a fluidized bed

bagasse dryer. Results show a reduction of moisture content from 49-50% to

41-42%. Steam generation increased by 10% and a saving of 0.6 metric ton/hr

of bagasse is achieved. Choh et al., (1984) investigated bagasse drying with

an impulsive fluidized dryer. They found that the temperature of the flue

gases has a great influence on the bagasse drying. At a flue gas temperature of

200°C, the moisture content of the bagasse could be lowered by 10%, whereas

at about 140°C it was 6-8% only. An impulsive fluidized bed dryer with a

contracted-expanded pipe, had a better drying efficiency than a straight pipe,

because the impulsive action in the contracted-expanded pipe changes the

relative velocity and mixing of bagasse particles with the gas flow, inducing

turbulence and improving the heat and mass transfer. Besides, the velocity of

the flow the power consumption is also reduced. They have obtained a steam

output with wet and dry bagasse as 1.94 and 2.4 kg/kg respectively indicating

54

23% more steam production using dry bagasse. Boiler efficiency was also

shown to increase from 63 to 79%.

Meirelles (1984) studied bagasse drying in a fluidized bed dryer.

He observed the necessity of a mixer to allow fluidization, because of the

bagasse cohesive characteristic. By removing the moisture, the agglomeration

decreased and the dried particles were elutriated. The diameter of the bagasse

particles used in that work was from 0.51 to 1.02 mm. He used a very wet

bagasse (71 % (w.b.)). Salermo and Santana (1986) worked with a dryer

composed of a fluidized bed, a pneumatic duct and a cyclone. It is important

to note that they used the cyclone to separate the phases. This system worked

with 10 ton/hour of 47% moisture content (w.b.) bagasse. The final moisture

content was 35% (w.b.) and inlet gas temperature used was 250ºC.

Morgenroth and Batstone (2005) demonstrated effective drying of

bagasse to less than 10% moisture in a 0.3 ton/hr prototype dryer operating at

atmospheric pressure. They found that the boiler efficiency increased by up to

15% and the total electrical surplus power production by approximately 17%.

To compare flue gas drying and steam drying, the boiler efficiency was 10%

more for steam drying. Also, the steam generated using steam drying is 12%

more. In contrast to the flue gas dryer, it offers the advantage of a closed

system. Earlier Ramani and Kothari (1990) reported work on a dryer which

dries bagasse in the presence of steam. They also compared the flue gas and

steam drier, and found an increase in boiler efficiency by 11% in the latter

case. Table 2.2 shows some of the drying systems reported by various

researchers.

55

Table 2.2 Drying systems reported by various researchers

Reference Type of dryer/

type of contact / capacity

Flue gas flow rate

Flue gas temp.

In Out

Moisture content In Out

Remarks

Guanzon (1980)

Rotary/23 t/h /direct

130,000 ft3/min

400-600ºF

50% to 40% -

Maranhao (1980)

Pneumatic/4500 kg/h /direct

1,73,000

m3/h 100 to220ºC

52% to

40%

Team produced increased by 16%

Keller (1980) Suspension - 100 to 220ºC

50% to 35%

Steam produced increased by 15%

Roy et al., (1980)

Counter current flow Indirect

contact 7400 kg/h

10640- 38182 kg/h 300ºC -

Edwards (1981)

Pneumatic/0.5 t/h and 2 t/h/direct

150 t/h

200-390ºC to 140ºC

50% to 35%

Boiler efficiency increased from

60% to 69%

Morales (1982)

Suspension/17.5 t/h /direct

500,000

m3/h 220ºC 56% to

35% -

Keenliside (1983)

Counter current flow - 225-250ºC

and 120ºC 50% and

40%

Dixon (1987) Pneumatic/direct - 706ºC

to 157 -253ºC

46-47% to 21-30%

High temperature

hot air used

Sarnobat (1987)

Rotary/50t/d/ indirect

50,000 to 55,000 ft3/min

300ºC 50% Dryer inclined at 300

Nebra and Macedo (1989)

Pneumatic /12000 kg/h

300ºC

50% to 35.8%

-

Nanda Kumar and Nagesh Kumar (2001)

Combination of fluized bed and

pneumatic conveying/50

t/h/direct

2X105 kg/h 175-180ºC to 70-75ºC

50% to 42%

Boiler efficiency increased from

66% to 71% Steam produced increased from

33 to 36t/h bagasse

Mittal (2005)

Pneumatic/ direct/52t/h

2.5 m/s - 50% to 30-

32%

Boiler efficiency increased from

54% to 62% Steam produced increased from

2.2 to 2.42kg/kg bagasse

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2.9 CONCLUSION

From the Indian sugar cogeneration scenario it is predicted that less

than 15% of the total available cogeneration potential is exploited in the

country. This has lead to a greater focus on the sugar industries to explore the

generation of power to its full potential. It is widely accepted that bagasse

drying is a technique which can improve the moisture content of the fuel and

can provide the surplus fuel required for cogeneration. Hence this research is

focused on studying bagasse drying. The various studies conducted to meet

the objective of this research were reviewed in this chapter on a general basis.

The literature review leads to the following conclusions:

1. The various methods of moisture reduction for bagasse were

reported in this section. Drying is the only technique that can

be adopted to bring down the moisture below 45%. The tunnel

type and rotary bagasse dryer have been described in this

section. Based on simplicity and least modification in the

present fuel handling system, the tunnel dryer with combined

heating is considered for this study.

2. From the literature it is concluded that in most of the drying

studies the moisture removal rate was evaluated for varied

operating parameters such as air temperature, velocity,

humidity and bed thickness.

3. The TGA analysis can be used to make drying curve

estimation by heating the product to a controlled temperature,

well below 180oC. The weight loss in this zone is only due to

moisture loss.

4. The energy Utilization Ratio and Exergy efficiency, which are

obtained from the energy exergy analysis, are used as the

indicator to evaluate the drying system.

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5. The suitability of a thin layer model to represent the drying

characteristics of the product is evaluated based on the

experimental drying curve. This study involves the usage of

non-linear regression analysis and statistical tools to evaluate

the model.

6. In the majority of the modeling works, a two-dimensional

numerical simulation is sought to represent the drying

phenomenon of the wet product. The temperature and

moisture profiles recorded are used to adjust the drying

variables more effectively.

7. Steam drying allows recovery of the energy by making use of

the vapor generated during drying. As the sugar industry

consumes a lot of low pressure steam for process application,

a steam drying model can be investigated for bagasse drying.

Based on these conclusions, it is proposed to examine bagasse

drying studies using the following studies;

Thin layer drying studies to examine the drying kinetics of

bagasse at different operating conditions.

Proposed to investigate the applicability of TGA for drying

studies.

Energy and exergy studies to evaluate the optimal operating

conditions during drying.

Evaluation of thin layer model to identify the suitable model

which predicts bagasse drying.

Develop a detailed numerical model to explain the drying

process. The model is proposed for both air and steam as

drying medium.