Energy 73 (2014) 809e817
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Energy
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Energy and exergy analyses of native cassava starch drying in a tray
dryer
Ndubisi A. Aviara a, *, Lovelyn N. Onuoha b, Oluwakemi E. Falola a, Joseph C. Igbeka c
a
Department of Agricultural and Environmental Resources Engineering, University of Maiduguri, Maiduguri, Nigeria
Department of Mechanical Engineering, University of Maiduguri, Maiduguri, Nigeria
c
Department of Agricultural and Environmental Engineering, University of Ibadan, Ibadan, Nigeria
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 15 February 2014
Received in revised form
19 May 2014
Accepted 24 June 2014
Available online 23 July 2014
Energy and exergy analyses of native cassava starch drying in a tray dryer were carried out to assess the
performance of the system in terms of energy utilization, energy utilization ratio, energy efficiency,
exergy inflow and outflow, exergy loss and exegetic efficiency. The results indicated that for the starch
with ash content of 0.76%, 0.85% crude protein, 0.16% crude fat, negligible amount of fiber, average
granule size of 14.1 mm, pH of 5.88, amylose content of 23.45% and degree of crystallinity of 22.34%,
energy utilization and energy utilization ratio increased from 1.93 to 5.51 J/s and 0.65 to 0.6 as the drying
temperature increased from 40 to 60 C. Energy efficiency increased from 16.036 to 30.645%, while
exergy inflow, outflow and losses increased from 0.399 to 2.686, 0.055 to 0.555 and 0.344 to 2.131 J/s
respectively in the above temperature range. Exergetic efficiency increased with increase in both drying
air temperature and energy utilization and was lower than energy efficiency. Exergetic improvement
potential also increased with increase in drying air temperature. Model equations that could be used to
express the energy and exergy parameters as a function of drying temperature were established.
© 2014 Elsevier Ltd. All rights reserved.
Keywords:
Cassava starch
Energy and exergy
Tray dryer
Improvement potential
1. Introduction
Nigeria is the World's leading producer of cassava [1e3]. Presently, efforts are geared toward promoting the exportation of the
produce and its by-products from Nigeria to other countries. Owing
to the poor storability characteristics of the cassava tuber in its
fresh or unprocessed state, there is the need to have the product
processed into a more storable form in order to minimize deterioration and transportation losses during export. One of the forms in
which cassava can easily be stored and transported without deterioration and losses is as dried cassava starch.
Starch is the common name applied to a white, granular or
powdery, odorless, and tasteless complex carbohydrate, (C6H10O5)x,
which is abundantly found in the seeds of cereal plants and in
bulbs, roots and tubers. It occurs in commercial quantities in such
roots and tubers as cassava, yam and potato and cereal grains such
as sorghum, millet and maize. It consists of two types of molecules
namely the amylose, which constitutes about 20e30% of ordinary
starch, and the amylopectin, which makes up the remaining
70e80%. Starch finds applications as an important raw material in
* Corresponding author.
E-mail address: nddyaviara@yahoo.com (N.A. Aviara).
http://dx.doi.org/10.1016/j.energy.2014.06.087
0360-5442/© 2014 Elsevier Ltd. All rights reserved.
the food, cosmetic, pharmaceutical, chemical and oil industries. It
functions as thickening agent in food, water binder, emulsion stabilizer, bulking agent, flow aid, fat substitute and gelling agent. Its
other industrial uses include the manufacture of synthetic polymers such as plastics and adhesives. It finds application as molecular sieve and binder, and as surface coating for papers. In drug
tablets, starch is used to bind and carry the active components. It
acts as viscosity modifier in paints and is used much in the textile
industry as stiffener. In the oil industry, it is mixed with pumping
water to assist in the cooling of the superheated drilling bits.
Starch is normally extracted from the source material in aqueous
medium. It is usually packaged and supplied in granular or powdery form and this makes drying a fundamental unit operation in
starch processing.
Drying is a complex process involving heat and mass transfer
between the product surface and its surrounding medium [4] which
results in the reduction of the product moisture content to a safe
storage level or to a level required for the commencement of other
processing operations. The role of the dryer is to supply the product
with more heat than is available under ambient conditions so as to
sufficiently increase the vapor pressure of the moisture held within
the product to enhance moisture migration from within the product,
provide the latent heat of vaporization of the moisture and
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N.A. Aviara et al. / Energy 73 (2014) 809e817
significantly decrease the relative humidity of the drying air to increase its moisture carrying capability and ensure a sufficiently low
equilibrium moisture content [5]. The drying industry utilizes large
quantities of energy, making it one of the most energy-intensive
industrial operations. High energy inputs in drying operations
arise due to the high latent heat of water evaporation and relatively
low energy efficiency of industrial dryers [6,7]. Thus, one of the most
important challenges of the drying industry is to reduce the energy
cost for obtaining good quality dried products [8]. Since energy is a
major cost factor, it is essential to perform the energy and exergy
analyses of a drying process to provide energy savings and optimum
process conditions. According to Singh [9], energy analysis is useful
in quantitative evaluation of energy requirements of energy generating and delivery systems and in the detection of mode and evaluation of energy loss. Information obtained from energy analysis can
be used for quantifying energy conservation practices.
The first law of thermodynamics which stands for the principle
of conservation of energy is commonly used in engineering systems
performance analysis. Energy analysis, however, has some deficiencies. Fundamentally, the energy concept is not sensitive to the
assumed direction of the process, e.g. energy analysis does not
object if heat is considered to be transferred spontaneously in the
direction of the increasing temperature [10]. It gives no information
about the inability of any thermodynamic process to convert heat
into mechanical work with full efficiency [11], nor does it provide
any insight into the reason why mixtures cannot spontaneously
separate or unmix themselves. It also does not distinguish the
quality of the energy, e.g., 1 W of heat equals 1 W of work or
electricity. Energy analyses on their own can incorrectly interpret
some processes, e.g., environmental air, when isothermally compressed, maintains its energy (enthalpy) equal to zero, whereas the
exergy of the compressed air is larger than zero.
Exergy is defined as the amount of work that can be obtained
from a stream of matter, heat or work as it comes to equilibrium
with a reference environment, and is a measure of the potential of a
stream to cause change, as a consequence of not being completely
stable relative to the reference environment [5]. It is the combination of the property of a system and its environment because it
depends on the system and its environment. Unlike energy, exergy
is not subject to a conservation law; rather it is consumed or
destroyed due to irreversibilities in real processes such as drying,
with the exergy consumption being proportional to the entropy
generation produced by the irreversibilities associated with the
process. Exergy analysis is a method that utilizes the conservation
of mass and energy principles together with the second law of
thermodynamics for the analysis, design and improvement of energy and other systems. It is a more useful tool for assessing the
efficient use of energy resources [12] as it provides a more realistic
view of process, sometimes dramatically different in comparison to
standard energy analyses. Dincer [13,14] highlighted the importance of exergy and its essential utilization as follows:
It is a suitable technique for furthering the goal of more efficient
energy resource use for it enables the location, types and true
magnitudes of wastes and losses to be determined.
It is an efficient technique revealing whether or not and by how
much it is possible to design more efficient energy systems by
reducing the inefficiencies in existing systems.
It is a primary tool in best addressing the impact of energy
resource utilization on environment.
It is a key component in obtaining sustainable development.
Dincer and Cengel [11], Dincer [5] and Dincer [15] provided
excellent treatises on energy and exergy analyses of the drying
process.
Several other investigators conducted energy and exergy analyses on the drying of different agricultural and food products using
different drying systems. Hepbasli [16] gave out a comprehensive
review of the exergy analysis of renewable energy resources and
provided two approaches for defining the exergetic efficiency as the
brute force and functional approaches. Panwar et al. [17] performed
a detailed review of energy and exergy analyses of solar drying
systems, Akpinar and Kocyigit [10], Sami et al. [18] and Saidur et al.
[19] carried out energy and exergy analysis of different solar drying
systems, Prommas et al. [20] conducted energy and exergy analysis
of porous media drying using heated air, and Aghbashlo et al. [21]
(2013) conducted a thorough review of the exergy analysis of
drying processes and systems. Studies on the energy and or exergy
analysis of food material drying in different drying systems
included solar drying of pistachio [4], pepper, yam slices, water leaf
and okra slices in mixed mode solar dryer [22], olive mill waste
water [23], mulberry [24], jackfruit leather [25], parsley leaves [26],
shelled corn [27], and red sea weed [28]. Others are on fluidized bed
drying of wheat [6], potato [29] and eggplant plant [30]drying in
cyclone type dryer, drying of red pepper slices [31] and coroba
slices [32] in convective type dryer, mint leaves drying in heat
pump dryer [33], green olive [34], palsey [35] and olive leaves [36]
drying in tray dryer, spray drying of fish oil encapsulation [37],
microwave drying of sour pomegranate arils [38]and pasta drying
in an industrial dryer [39]. These investigations show that energy
efficiency is higher than exergy efficiency. Energy utilization, energy utilization ratio, exergy inflow and outflow, exergy loss, energetic and exergetic efficiencies, all varied with product, drying
conditions and type of drying system. Van Gool [40] noted that
maximum improvement in the exergy efficiency of a process or
system could be achieved when the difference between total
exergy output and total exergy input is minimized. Consequently
he suggested the concept of exergetic IP (improvement potential)
as a useful tool in the analysis of different processes and systems.
The rate form as given by Hammond and Stapleton [41] is
commonly used in computing improvement potential.
Information on the energy and exergy analyses of starch drying
appears to be scanty in the scientific literature. The main objective
of this study was to investigate the energetics and exergetics of
native cassava starch drying in a tray dryer and establish the variation of the efficiencies with the drying conditions of inlet and out
temperatures.
2. Materials and methods
2.1. Starch extraction and characterization
The cassava tubers used for starch extraction were obtained
from a farm at the Amina Way in the University of Ibadan, Ibadan,
Nigeria. Starch extraction from cassava was carried out at the Industrial Chemistry Laboratory, Department of Chemistry, University of Ibadan, Ibadan.
The cassava tubers were peeled and thoroughly washed in clean
water. The peels were discarded and the peeled tubers were
crushed in a rasp bar ‘grating’ machine. The resultant pulp was
mixed with sufficient amount of water to form slurry. The slurry
was sieved with the aid of a muslin cloth and 75 mm mesh size
sieve. The fiber was thoroughly washed and discarded. The starch
milk obtained was allowed to settle and the supernatant was
decanted. The starch was resuspended and washed several times
with distilled water to remove the impurities and protein debris.
The cassava starch obtained was divided into two portions and
utilized as follows:
The first portion was dried in open air and used for proximate
composition and pH determination, scanning electron microscopy
N.A. Aviara et al. / Energy 73 (2014) 809e817
and x-ray diffractometry. The second portion was sealed in polyethylene bag and stored in a freezer for use in carrying out drying
experiments at different temperatures in a tray dryer.
Moisture, ash, crude protein, fat and crude fiber content of the
starch were determined using the AOAC [42] method. Amylose
content and pH of starch were determined using the AACC [43] and
AOAC [42] methods respectively. Starch granule micrograph was
obtained using a JSM 35 Genie Scanning Electron Microscope
according the method of Nwokocha et al. [44]. The x-ray diffraction
pattern of starch was obtained using an MD10 2.04 diffractometer
that produced monochromatic CuKa radiation, and the degree of
crystallinity also known as relative crystallinity of starch was
determined using the method reported by Wang et al. [45].
2.2. Drying experimental setup
2.2.1. Drying equipment description and operation
The equipment used for the drying experiments was a Laboratory model tray dryer. A diagrammatic representation of the dryer
and its part list is shown in Fig. 1. It consists of a drying chamber in
which perforated trays were placed horizontally and stacked
vertically, a plenum chamber where the heating elements were
installed, a 0.374 kW axial flow fan that supplied the drying air at a
rate of 0.238 m3 s1 and an outlet for discharging the used air. The
tool frame and lagged casing enclosing the functional units formed
the body of the equipment. The dryer was fitted with a temperature
control device that used a sensor and thermostatic system to
maintain selected temperature to within ±2 C in the drying
chamber. When the dryer is in operation, the axial fan blows air
through the plenum chamber over the heating elements. The air
gets sensibly heated up to the temperature selected using the
thermostatic control. It then enters the drying chamber where it
picks up moisture from the product being dried and gets it
exhausted through the air outlet. The continual picking and
exhaustion of moisture by the drying air leads to the reduction of
mass and therefore the moisture content of the product in the
811
drying chamber. This continues until the reduction in product mass
becomes negligible and equilibrium is assumed to have been
attained with the environment and the drying process is stopped.
The moisture content at which drying terminates is then taken and
termed the dynamic equilibrium moisture content.
(1). Drying chamber (2). Tray (3). Baffle plate (4). Tray support
platform (5). Insulated drying cabinet wall (6). Perforated plate (7).
Inlet air duct (8). Insulated air duct wall (9). Air inlet perforated
plate (10). Heating elements (11). Air supply fan (12). Fan drive
motor (13). Motor mounting (14). Motor mounting support frame
(15). Tool frame (16). Dryer mounting (17). Dryer door (18). Exhaust
air outlet.
2.2.2. Material preparation and drying experimentation
The starch in freezer was brought out and allowed to equilibrate
to ambient conditions for 6 h prior to use in carrying out drying
tests. The initial moisture content of starch was determined using
the AOAC [43] method with modification to prevent or minimize
gelatinization.
The drying test procedure employed by Syrarief et al. [46] and
Ajibola [47] was adopted with modifications to suit the laboratory
and experimental condition. Air at the ambient conditions of
27e38 C dry bulb temperature with an average of 30 C and
50e68% relative humidity with an average of 55% was heated to the
drying temperatures of 40, 45, 50, 55 and 60 C. This range of drying
temperature was selected to prevent gelatinization. For an experimental run at each of the above drying temperatures, the fan was
turned on and the dryer allowed running empty for 2 h to enable it
to stabilize at the specified air conditions before the test began. The
initial moisture content was noted and triplicate samples of starch
each weighing about 25 g and spread in thin layer on a drying dish,
were placed in drying trays and pushed into the drying chamber
with the fan still running. Change in sample weight was monitored
throughout the experiment by weighing periodically using an
electronic balance. Weighing of samples was carried out at every
10 min for the first 1 h; every 30 min for the next 3 h and every 1 h
Fig. 1. Schematic diagram and part list of the laboratory model tray dryer.
812
N.A. Aviara et al. / Energy 73 (2014) 809e817
until three consecutive readings gave identical weights. The test
was then terminated, the time taken was recorded and equilibrium
with the drying environment was assumed to have been reached.
The percentage dry basis moisture contents of the samples were
determined and the average value was taken as the dynamic
equilibrium moisture content, % (db). The moisture contents obtained with time for the starch were used to plot the drying curves
at different drying temperatures. During the experiment, the
ambient temperature, relative humidity and inlet and outlet temperatures of the drying air were recorded.
EU ¼ M_ a ðhai hao Þ
(8)
where hai is enthalpy in J/kg of air at the dryer inlet temperature, hao
is enthalpy in J/kg of air at the dryer outlet temperature and EU is
energy utilization in J/s.
EUR (Energy utilization ratio) was calculated using Equation (9)
given as follows
EUR ¼
M_ a ðhai hao Þ
M_ a ðh ha∞ Þ
(9)
ai
2.3. Energy analysis
The data obtained from drying experiments were used to
perform the energy and exergy analyses of the starch drying process. Drying process was considered as a steady flow process and
from the first law of thermodynamics, for an open system, the
energy balance [48] can be written as follows.
_ ¼
Q_ W
X
M_ ao ho þ
v2o
2
X
"
M_ ai
v2
hi þ i
2
#
(1)
_ is rate of mechanical work
where Q_ is heat energy inflow in J/s, W
_
output in J/s, M a is mass flow rate of drying air in kg/s, hi is air
enthalpy at the dryer inlet temperature in J/kg, ho is air enthalpy at
the dryer outlet temperature in J/kg and vi and vo are air velocities
at dryer inlet and outlet respectively in m/s. Since there is no mechanical work involved in the process of drying native cassava
starch in a tray dryer, Equation (1) becomes
Q_ ¼
X
#
#
"
"
X
V2
V2
M_ ai hi þ i
M_ ao ho þ o 2
2
(2)
Because there is no resultant motion involved in the tray drying
process, the momentum components v2o =2 and v2i =2 become eliminated so that Equation (2) becomes
Q_ ¼
X
M_ ao ho X
M_ ai hi
(3)
where ha∞ is enthalpy of the ambient dry air J/kg, EUR is energy
utilization ratio and M_ a is mass flow rate of air in kg/s.
Energy efficiency was evaluated as the ratio of the energy
expended to the energy supplied using the following expression
hen
_
Ei Eo M a hai hao
¼
¼
100
Ei
M_ a hai
where henis energy efficiency in %, Ei is energy input in J/s, and Eo is
energy output in J/s.
2.4. Exergy analysis
Exergy analysis of the drying process was carried out on the
basis of the second law of thermodynamics which asserts that
energy has quality as well as quantity, and that actual process occurs in the direction decreasing quality of energy [48]. The second
law notes that part of the exergy entering a thermal system is
destroyed within the system due to irreversibilities. In the light of
the above postulation, the total exergy inflow, outflow and losses of
the drying process were estimated. The basic procedure followed
was to determine the exergy values at steady-state points using the
properties the working medium from the first law energy balance.
For this purpose, the mathematical formulations used to carry out
the exergy balance are as show in Equation (11) for an open system.
For M_ ao ¼ M_ ai ¼ M_ a Equation (3) can be written as
Q_ ¼ M_ a ðho hi Þ
(4)
The mass flow rate of the drying air was calculated using
Equation (5) stated as
M_ a ¼ ra V_ a
(5)
where ra is the density of dry air in kg/m3, V_ a is the volumetric flow
rate of the drying air in m3/s.
The enthalpies of the drying air at the inlet and outlet temperatures, hi and ho were calculated using Equation (6).
h ¼ Cpa Tda þ Whsat
(6)
where Cpa is specific heat of dry air in J/kg, Tdais temperature of
drying air in C, W is humidity ratio of drying air (kgH2O/kgDA) and
DA is dry air, hsat is the enthalpy of the saturated vapor in J/kg.
The specific heat of dry air was determined using Equation (7) as
follows
Cpa ¼ 1.0029 þ 5.4 105Tda
(7)
EU (Energy utilization) was determined by applying the first law
of thermodynamics as expressed by Equation (4) and transformed
in Equation (8).
(10)
EX ¼ ½U U∞ T∞ ½S S∞ þ P∞ ðV V∞ Þ þ
y2
þ ðZ Z∞ Þg
2
þ VðP P∞ Þ
(11)
where Ex is exergy in J/kg, U U∞ is internal energy component in
J/kg, T∞ is ambient temperature in C, S S∞ is entropic component in J/kg, P∞(V V∞) is work component in J/kg, y2/2 is the
momentum component in J/kg and (Z Z∞)g is gravity component
in J/kg, ∞ denotes the reference condition.
During the drying process of native cassava starch in a tray dryer
no lifting or lowering of the product occurred and there was no
agitation or relative motion, therefore the gravity and momentum
components in Equation (11) were neglected and eliminated leaving the exergy equation with internal energy, entropy and PV terms
as follows.
EX ¼ ½U U∞ T∞ ½S S∞ þ
P∞
ðV V∞ Þ þ VðP P∞ Þ
J
(12)
This yielded
EX ¼ ½U þ PV ½U∞ þ P∞ V∞ T∞ ðS S∞ Þ
(13)
By substituting enthalpy h, for the U þ PV terms, Equation (13)
became reduced to:
N.A. Aviara et al. / Energy 73 (2014) 809e817
T
EX ¼ Cp ðT T∞ Þ T∞ ln
T∞
(14)
where Cp is specific heat in J/kg.
Equation (14) was used to calculate the exergy inflow and
outflow at the inlet and outlet temperatures of the tray in the
drying chamber, respectively. Then the exergy loss throughout the
process was determined using the following expression:
Exergy loss ¼ Exergy inflow Exergy outflow
X
EXL ¼
X
EXi X
EXo
(15)
(16)
where EXi and EXo, EXL are the inlet and outlet exergy and exergy
loss respectively in J/s.
Exergy inflow to the drying chamber was calculated using
Equation (13) stated as follows
T
EXi ¼ Cpa ðTai T∞ Þ T∞ ln ai
T∞
(17)
Substituting Equation (7) into Equation (16) yielded the exergy
inflow as
5
EXi ¼ 1:0029 þ 5:4 10
T
Tai ðTai T∞ Þ T∞ ln ai
T∞
(18)
(19)
In the form of Equation (17), the exergy outflow as expressed by
Equation (18) becomes
Tao
EXo ¼ 1:0029 þ 5:4 105 Tao ðTao T∞ Þ T∞ ln
T∞
(20)
Exergetic efficiency has been defined as the ratio of exergy
outflow in the drying of the product to exergy of the drying air
supplied to the system [4,32,49]. This is given by the expression
Exergy efficiency ¼
or
Exergy inflow Exergy loss
Exergy inflow
Exergy efficiency ¼ 1 Exergy loss
Exergy inflow
Table 1
Proximate composition of cassava starch.
S/N
Parameter
Value
1.
2.
3.
4.
5.
6.
7.
8.
Ash content, %
Crude fiber, %
Crude protein, %
Crude fat, %
Granule size range, mm
Average granule size, mm
pH
Amylose content, %
0.76 ± 0.001
NIL
0.85 ± 0.017
0.16 ± 0.0025
6.5 to 19
14.1 ± 3.402
5.88 ± 0.121
23.45 ± 0.03
2.5. Statistical analysis
The drying curves (moisture content versus time) of native
cassava starch were plotted at different drying temperatures. Data
obtained from the energy and exergy analyses of the drying process
at different inlet and outlet temperatures were subjected to
regression analysis using Statistix 9.0. Regression equations were
used to express the relationships existing between the energy and
exergy parameters and process variables.
3. Results and discussion
3.1. Cassava starch proximate composition and characterization
Exergy outflow from the drying chamber was calculated using
Equation (13) stated as follows
Tao
EXo ¼ Cpa ðTao T∞ Þ T∞ ln
T∞
813
The results of cassava starch proximate compositions, scanning
electron microscopy and x-ray diffractometry are presented Table 1,
Figs. 2 and 3 respectively. Table 1 shows that the starch had an ash
content of 0.76%, 0.85% crude protein, 0.16% crude fat, negligible
amount of fiber, average granule size of 14.1 mm, pH of 5.88 and
amylose content of 23.45%. Fig. 2 shows that the starch granules are
mostly spherical in shape with a few having indentations similar to
an egg that has been cut at various positions [50,51]. The starch
exhibited the A type crystalline diffraction pattern with major
peaks at 15, 17, 18 and 23 (Fig. 3) and degree of crystallinity of
22.34%.
3.2. Drying curves of cassava starch in a tray dryer
The average initial moisture content of the starch was 82% (db).
Fig. 4 shows the variation of the starch moisture content with time
during drying at different temperatures in the range of 40e60 C.
(21)
(22)
Equation (22) can be stated as
hEX ¼ 1 EXl
EXi
(23)
where hEX is exergy efficiency, %.
The rate form as given by Hammond and Stapleton [42] was
used to determine the exergetic improvement potential of the
drying process. This is expressed as
,
,
,
IP ¼ ð1 hEX Þ EX i EX o
(24)
,
where IP is exergetic improvement potential in J/s.
Fig. 2. SEM (Scanning electron micrograph) of cassava starch, 2000X.
814
N.A. Aviara et al. / Energy 73 (2014) 809e817
Fig. 5. Variation of energy utilization with drying air temperature.
Fig. 3. X-ray diffraction pattern of NCSA (native cassava starch dried in open air).
From this Figure, it can be seen that the moisture content decreased
with increase in time until the dynamic equilibrium moisture
content at each of the drying temperatures was attained. Drying
time and dynamic equilibrium moisture content decreased from
480 to 270 min and 7.70 to 1.70% (db) respectively as the drying
temperature increased from 40 to 60 C. Similar results were obtained in the drying of eggplant [30], potato [29] and coroba slices
[32].
3.3. Energy utilization
The variation of energy utilization in the drying of native cassava
starch using a tray dryer with air at temperatures in the range of
40e60 C is presented in Fig. 5. The Figure shows that the energy
utilized increased with increase in drying temperature and ranged
from 1.93 to 5.51 J/s. Similar results were reported on olive leaves
drying in a tray dryer [36], eggplant and potato slices drying in a
cyclone dryer [30,29], coroba slices drying in convective type dryer
[32] and sour pomegranate arils drying in a microwave dryer [38].
Energy utilization was found to have linear relationship with drying
air temperature that can be adequately expressed with the
following equation:
Fig. 4. Drying curves of native cassava starch at different temperatures in a tray dryer.
EU ¼ 0:1789T 5:319;
R2 ¼ 0:9978
(25)
where EU is energy utilization in J/s, T is drying air temperature in
C and R2 is coefficient of determination.
3.4. Energy utilization ratio
The variation of EUR (energy utilization ratio) with drying air
temperature during the drying of cassava starch in a tray dryer is
presented Fig. 6. Observation from the Figure shows that the energy
utilization ratio decreased from 0.65 to 0.6 as the temperature of
the drying air increased from 40 C to 60 C. Akpinar et al. [29] in
the energy and exergy analysis of potatoes dried in a cyclone type
dryer reported that EUR decreased with increase in temperature
and air velocity. Similar results were reported by Akpinar [30] on
eggplant drying in a cyclone type dryer, Erbay and Icier [36] on the
drying of olive leaves in a tray dryer and Motevali and Minaei [38]
on the drying of sour pomegranate arils in a microwave dryer. Corzo
et al. [32] however, noted that the EUR in the drying of coroba slices
in a convective type dryer increased with increase in drying temperature up to a point and decreased with further increase in
temperature.
The relationship existing between EUR (energy utilization ratio)
and drying air temperature was found to be polynomial of the
second order and can be represented by the following equation:
Fig. 6. Energy utilization ratio at different drying temperatures.
N.A. Aviara et al. / Energy 73 (2014) 809e817
EUR ¼ 5 106 T 2 0:0006T þ 0:6174;
R2 ¼ 0:9851
815
(26)
where EUR is energy utilization ratio, T is drying air temperature in
C and R2 is coefficient of determination.
3.5. Energy efficiency
Fig. 7 shows that the energy efficiency of cassava starch drying
in a tray dryer increased from 16.036 to 30.645% as the drying air
temperature increased from 40 to 60 C. Similar result was obtained by Syahrul et al. [6] on the fluidized bed drying of wheat and
Chowdhury et al. [25] on the solar drying of jackfruit leather. Sami
et al. [18] however noted that energy efficiency in the drying of
chilli using an indirect solar cabinet dryer decreased with time to a
minimum value and thereafter, increased with further increase in
time, while Aghbashlo et al. [37] reported that of the drying of fish
oil encapsulation using a spray dryer decreased with increase in
drying temperature.
The energy efficiency of cassava starch drying in a tray dryer was
found to have a linear relationship with drying air temperature and
this relationship was expressed with the following equation:
henergy ¼ 0:7491T 14:213;
R2 ¼ 0:9939
(27)
where henergy is energy efficiency in %, T is drying air temperature in
C and R2 is coefficient of determination.
3.6. Exergy inflow, exergy outflow and exergy loss
Fig. 8 shows the variation of exergy inflow, exergy outflow and
exergy loss with drying air temperature in the drying of cassava
starch. Exergy inflow, outflow and losses increased from 0.399 to
2.686, 0.055 to 0.555 and 0.344e2.131 J/s respectively, as the air
temperature increased from 40 to 60 C. Similar result was reported
on the solar drying of pistachio [4], eggplant drying in a cyclone
type dryer [30], potato drying in a cyclone type dryer [29], coroba
slice drying in a convective type dryer [32], olive leaves drying in a
tray dryer [36] and fish oil encapsulation drying using a spray dryer
[37]. Colak et al. [33] noted that exergy loss increased with increase
in temperature in the drying of mint leaves using a heat pump
dryer. Motevali and Minaei [38] reported that exergy loss decreased
with increase in temperature and time in the thin layer drying of
microwave pretreated sour pomegranate arils and Akpinar [26]
observed that exergy inflow, outflow and loss decreased with
time in the solar drying of parlsey leaves. Polynomial relationship of
Fig. 7. Variation of the energy efficiency with drying temperature.
Fig. 8. Effect drying temperature on the exergy inflow, outflow and loss.
the second order was found to exist between exergy inflow and
exergy loss with drying air temperature, while that of exergy
outflow with drying temperature was found to be linear. These
relationships can be represented with the following equations:
Exin ¼ 0:0018T 2 0:065T þ 0:1206;
Exout ¼ 0:0249T 0:9594;
R2 ¼ 1:0000
R2 ¼ 0:9912
Exloss ¼ 0:0014T 2 0:0504T þ 0:1112;
(28)
(29)
R2 ¼ 0:9999
(30)
where Exinis exergy inflow in J/s, Exout is exergy outflow in J/s, Exloss
is exergy loss in J/s, T is drying air temperature in C and R2 is coefficient of determination.
Fig. 9 shows that exergy inflow, outflow and losses varied with
energy utilization in a manner similar to their variation with drying
air temperature. Each of them increased with increase in energy
utilization and had relationship with energy utilization that was
polynomial of the second order for exergy inflow and exergy loss,
and linear for exergy outflow. The relationships were expressed
with the following equations:
Exin ¼ 0:0196EU2 þ 0:4961EU 0:6362;
R2 ¼ 0:9997
(31)
Fig. 9. Variation of exergy inflow, outflow and loss with energy utilization.
816
N.A. Aviara et al. / Energy 73 (2014) 809e817
hexergetic ¼ 0:0236T 2 þ 2:6749T 55:011;
R2 ¼ 0:9600
(34)
hexergetic ¼ 0:7847EU2 þ 7:5194EU þ 2:752;
R2 ¼ 0:9360
(35)
where hexergeticis exergetic efficiency in %, T is drying air temperature in C, EU is energy utilization in J/s and R2 is coefficient of
determination.
3.8. Improvement potential
Fig. 10. Variation of exergetic efficiency with drying temperature.
Exout ¼ 0:1394EU 0:2201;
R2 ¼ 0:9912
Exloss ¼ 0:015EU2 þ 0:3913EU 0:4729;
(32)
R2 ¼ 0:9999
(33)
where Exin is exergy inflow in J/s, Exout is exergy outflow in J/s,
Exloss is exergy loss in J/s, EU is energy utilization J/s and R2 is coefficient of determination.
Midilli and Kucuk [4] and Sami et al. [18] similarly, reported that
exergy loss in the drying of pistachio and chilli respectively,
increased with increase in energy utilization.
3.7. Exergetic efficiency
The variations of exergetic efficiency of the tray dryer with
drying air temperature and energy utilization during the drying of
cassava starch are presented in Figs. 10 and 11 respectively. Exergetic efficiency increased with increase in both drying air temperature and energy utilization. Similar results were reported on the
drying of eggplant slices [30], green olive [34], mint leaves [33],
jackfruit leather [25] and sour pomegranate arils [38]. In the temperature range employed, the exergetic efficiency was lower than
energy efficiency. The relationships existing between exergetic efficiency and drying air temperature and energy utilization were
found to be polynomial of the second order. These relationships
were expressed with the following equations:
Fig. 11. Variation of exergetic efficiency with energy utilization.
The effect of drying air temperature on the improvement potential of cassava starch drying in a tray dryer is shown in Fig. 12.
From this figure, it can be seen that the exergetic improvement
potential increased linearly with increase in drying air temperature. Similar results were reported by Erbay and Icier [36] and
Aghbashlo et al. [37] on the drying of olive leaves and fish oil
encapsulation, respectively. The relationship existing between
improvement potential and drying air temperature was represented by the following equation:
IP ¼ 0:0702T 2:5732;
R2 ¼ 0:9912
(36)
where IP is improvement potential in J/s, T is drying air temperature
in C and R2 is coefficient of determination.
4. Conclusions
The proximate composition of the native cassava starch used for
the study was 0.76% ash, 0.85% crude protein, 0.16% crude fat and
23.45% amylose content with average granule size of 14.1 mm. pH
was 5.88 and the starch demonstrated the A type diffraction
pattern.
Energy and exergy analysis of the starch drying process in a tray
dryer revealed the following:
1. Energy utilization increased linearly with increase in drying air
temperature.
2. Energy utilization ratio decreased with increase in drying air
temperature and had a relationship with temperature that was
found to be polynomial of second order.
3. Energy efficiency increased linearly with increase in drying air
temperature.
Fig. 12. Variation of exergetic improvement potential with drying temperature.
N.A. Aviara et al. / Energy 73 (2014) 809e817
4. Exergy inflow, exergy outflow and exergy loss increased with
increase in both drying air temperature and energy utilization.
5. Exergetic efficiency increased with increase in both drying air
temperature and energy utilization and was lower than energy
efficiency.
6. Exergetic improvement potential increased linearly with increase in drying air temperature.
Acknowledgment
Authors are grateful to Dr. Ibrahim Dincer of the Faculty of Engineering and Applied Science, University of Ontario Institute of
Technology, Ontario, Canada.
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Nomenclature
Q_ : energy inflow, J/s
_ rate of mechanical work output, J/s
W:
_ mass flow rate, kg/s
M:
h: enthalpy, J/kg
y: velocity, m/s
r: density of dry air, kg/m3
_ volumetric flow rate of drying air, m3/s
V:
Cp: specific heat of drying air, J/kg
T: temperature, C
W: humidity ratio of drying air, kgH2O/kgDA
EU: energy utilization, J/s
EUR: energy utilization ratio
hen: energy efficiency, %
E: energy, J/kg
EX: exergy, J/kg
U: internal energy, J/kg
S: entropy, J/kg
P: pressure, N/m2
Z: elevation, m
g: gravitational acceleration, 9.81 m/s2
h,ex: exergetic efficiency, %
IP : exergetic improvement potential, J/s
Subscripts
i: inlet
o: outlet
a: drying air
∞: reference or ambient condition
L: loss