Abstract It contains high level of moisture.

Abstract

This
manuscript is about the energy and exergy analyses of thin layer drying procedure
of mint by means of forced solar dryer. After the first law of thermodynamics was
employed, energy analysis was conducted to predict the rates of energy usage
and the amounts of energy achievement from the solar air collector.
Nevertheless, by applying the second law of thermodynamics, exergy analysis was
performed to find out exergy losses at the time of drying process. The drying
experiments were carried out at three different drying mass flow stages
altering between 0.012 kg/sn and 0.033 kg/sn. The influences of inlet air
velocity and drying duration on not only energy but also exergy was
investigated. As a conclusion, It was found out that both energy usage ratio
and exergy loss dropped causing an increase in drying mass flow rate while the
exergetic efficiency increased. 

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Key words :
Energy analysis, Exergy analysis, Thin layer drying, Forced solar dryer, Mint.

1.     
Introduction

It
is frequently accepted that mint is harvested by laying a sheet on the ground. This
is because of the fact that It contains high level of moisture. Due to short
season and the vulnerability of the plant to storage, drying is often utilized
as a conservation method 1. Dried mint is generally used in food and
beverages; and spread over salads. Drying process is described as removing moisture
from the products and is one of the most important processes for conserving
agrarian products because it has a signigicant influence on the charactersic of
the dried crops. It has been long known that in preservation of food fruit and
vegatable drying is one of the most commonly used mothods. What is intended in
drying agricultural crops is to reduce the moster content in them, which allows
them to be stored in safe for longer 
period. Drying the crops via solar means is most commonly used method
both in Turkey and all over the world to establish convenient preservation conditions
for agricultural crops.

Nevertheles,
the process mentioned above has a few problems such as contamination of the
crops of dust, soil, sand particles and even insects2.

Though solar drying has a lot of
disadvantages, it is still widely made use in many parts of the world. Since it
possesses the properties such as being abundant, inexhaustible, renewable,
cheap and non-pollutant, energy obtained from the sun a very important
alternative energy. A short time ago, a number of independent studies about mathematical
modelling and drying kinetics of vegatables, fruits and agrobased products were
carried out by many researchers such as those concerning with pistachio3,22,
carrots4, potatoes and apples5, red peppers6, figs7, crop8, mulberry
2,9,10, mint leaves11,27, hazel nut12, grapes 13,24,25,26, apricot14, silk cocoon23.

 It was proved that Mathematical models were very
useful in both design and analysis of the mass and heat transfer process during
drying.

Thermodynamic analysis, especially
exergy analysis, turned out to be an necessary tool for design, analysis and
optimization of thermal system15. When it comes to equilibrium with a
reference environment, Exergy is defined as the maximum amount of work
producible by a stream of matter, heat or work 16. The objective in the
process of drying is to use the least amount of energy for the highest level of
moisture removal, which is desired in final conditions of crop. Up until now a
lot of investigations have been carried out about exergy analyses of food
drying. However, an extensive literature review carried out for the present
study shows that there is no subtle evidence on energy and exergy analyses of
thin layer drying process of mint by means of forced solar dryer. For this reason,
this paper, since it is different from other studies, focuses on energy and
exergy analyses of thin layer drying of mint by means of forced solar type
dryer by employing the first and second law of thermodynamics. It is believed
that a study of this kind will have great contribution to mint producers as it
removes their problems about energy and exergy all the way through the drying
process. The initial objective of the study is to display energy and exergy
analyses of thin layer drying of mint under various conditions of drying mass
flow rates in a forced solar dryer. Similar to many other significiant studies
on drying process via energy and exergy analysis, the study below may as well be
submitted. The energy and exergy analyses of the drying process of shelled and
unshelled pistachios were conducted by Midilli and Kucuk 17 by using solar
drying cabinet. Furthermore, a new model for thermodynamic analysis of drying
process was developed by Dincer and Sahin 15. Akpinar18 carried out energy
and exergy analyses of drying of red pepper slices by means of convective type
dryer.

In a tray dryer, an exergy
analysis of thin layer drying of green olive was performed by Colak and Hepbasli 19. Akpinar
et al. 20 applied the first and second law analyses of thermodynamic to
drying process of pumpkin.

Under three different air
temperatures, Corzo et al. 21 carried out energy and  exergy analyses of thin layer drying of
coroba slices.  

2.     
Material and methods

Because of its geographical location in the Mediterranean Region (36º
and 42º North latitudes), Turkey has abundant solar energy potential. The
sunshine period in Turkey is 2624 h/year. The maximum period of 365 h/month is
in July and the minumum of 103 h/month is in December. The average solar
radiation intensity is approximately 3.67 kWh/m2 day.

The solar cabinet dryer was mounted in the garden of Technical Education
Faculty of Firat University, Elaz??, Turkey. The solar drying experiments were
conducted from August to September in 2005. Starting at 09:00 am, each test
continued until 17:00 pm. In this process, mint drying was performed in solar
cabinet dryer.

In Fig. 1 a and b, a schematic diagram and a photograph view of the
solar dryer system are presented, respectively. This system consists of basically
four subsytems; they are (a) drying cabinet, (b) solar air collector, (c) air
fan and AC hertz converter (d) data logger. The following data were recorded at
15 min. intervals in these experiments: weather temperature, inlet and outlet
temperature of solar collector and dryer, temperature of the mint center,
relative humidity just above the mint bed surface and solar radiation. While
measuring the temperatures, T Type copper-constant thermocouples were connected
to a ZA9000FST connector element to 5990-0 Almemo digital data logger, with
reading accuracy of ± 0.1  ºC. A thermo
anemometer (FVA645TH3) was employed to measure air speed, with reading 0.1-15
m/s range. By means of FDA612MR pressure module, pressure drop in the collector
was calculated. Mass loss of the mint was recorded during drying process to
determine the drying curves by FKA0251 strain strengetch within the measurement
range of 0.02-10 kN with an accuracy of 0.01 kN. The solar radiation was
computed with the help of Kipp and Zonen solarimeter during the operation
period of drying system. Fresh mint was bought from a local market in Elaz??,
Turkey. All data were gathered by employing Almemo 5990-0 data logger
interfaced to the personal computer; afterwards, they were recoerded at 15 min.
time intervals. Before the sample was placed in the dryer, in order to obtain calibration,
the drying system was operated for at least 60 min.. In the solar dryer system,
there was a centrifugal  fan used for
blowing air into the solar collector through an 82 mm diameter flexible
aluminum duct. Since AC hertz converter was used, the mass air flow could be
controlled. With 1.2 x 0.74 x 0.74 m dimensions, the inner chamber of the dryer
unit was made of a  0.8 mm thick stainless
steel sheet which was respectively enclosed in an outer chamber of 1.5 x 0.75 x
0.75 m made of stainless steel sheet as well. Polystyrene insulating materials
were filled in the space between the two chambers for a quality insulating.

After the air had left the heating chamber, it passed through a (0.3 x
0.2 x 0.2 m) chimney chamber allowing it to mix and have a uniform temperature
before entering into the drying chamber. In the experiments, fresh mint with an
average first moisture content of about 0.3 kg of water/kg of dry solids was
used; it was put in the dryer. Before the experiment, no treatment was carried out
on fresh mint.

3.Analysis

In first and second law analyses of thermodynamics, it was considered
that the drying process was a steady flow one. The basic point of these
analyses is the cases of thermodynamics of damp air.

3.1. The first law analysis

Within the context  of the first law of thermodynamics, in order
to determine a lot more about energy issues and behaviour of drying air by
means of forced solar dryer, an energy analysis of the thin layer drying
process about mint is conducted. The air conditioning process from the
beginning till the end of the mint drying comprises heating, cooling and
humidification processes. In reality, this process can be defined as steady
flow processes analyzed by using the steady flow conservation of mass and
conservation of energy principles.

For the energy and exergy analyses of the thin layer
drying process, the equations below are generally used to calculate mass conservation of the drying air and moisture, the
energy conservation of the process and relative humidity and enthalpy of the drying
air. 

General equation of mass conservation of drying air:

General equation of mass conservation of moisture:

General equation of energy conservation:

The alterations in kinetic energy of the fan were taken
into consideration, on the other hand potential and kinetic energy in other parts of the process were
neglected:

Where w shows the particular humidity, P
atmospheric  pressure, [email protected]
the saturated vapour pressure of the drying air.

The enthalpy of the drying air can be established as
below:

It is presumed that there is no heat loss from the
beginning until the end of  the
connection pipe from the fan to the solar collector to  determine the outlet conditions of the solar
collector, and hence, the inlet conditions of the solar collector are almost
equal to the outlet conditions of the fan as stated in equations (7):

It is possible that the energy conveyed to the drying
air from the solar collector be computed by using the values of both outlet and
inlet temperatures of the solar collector  via following equation:

It is obvious that some small heat losses occur
between the solar collector outlet and dryer inlet when temperature
measurements are taken. Due to the heat losses in this section of the system,
it should certainly be stressed that solar collector outlet conditions are not
equal to dryer inlet conditions.

Therefore, the amount 
of heat losses across the connection pipe between the solar collector and
dryer can be predicted as in the following equation:

During the dehumidification process in drying chamber,
the heat which is used can be estimated by using the equation below and psychrometric
chart:

Energy utilization ratio of the drying chamber (EUR)
was computed during this drying process by using the equation below:

3.2. The second law analysis

With in the context of the second law analysis, the
whole exergy inflow, outflow and losses of the forced solar dryer were
predicted. The fundamental procedure for exergy analysis of the drying chamber
is to find out the exergy values at steady state points and reason of exergy
variations for the process. Using the characteristics of the working medium
from a first law energy balance, the exergy values are computed. To this end,
the general form of exergy equation applicable for steady flow systems was used17.

where the subscript  ? denotes the reference conditions. There are
various types of this general exergy equation. Some, but not all, of the terms
shown in Eq. (12) are utilized in the analyses of many systems. As exergy is
energy easy to obtain from any source, the terms can be build up by using
electrical current flow, magnetic fields, and diffusional flow of materials. A
general simplification is to replace enthalpy for the internal energy and PV
terms that are feasible to steady flow systems. Eq. (12) is frequently employed
under  the conditions where the
gravitational and momentum terms are not taken into consideration. In  addition to all these, the pressure
alterations in the system are also neglected because of V=V?.

In such a case, Eq.(12) is derived as:

Putting Eq.(13) into action, the inflow and outflow of
exergy can be found out based on the inlet and outlet temperatures of the
drying chamber. Then, by employing Eq.(14), the exergy losses from the
beginning until the end of the drying process are determined.

The equation of exergy inflow for the drying chamber can
be stated as follows:

where cpda is the average spesific heat of the drying air. Nevertheless, the equation of exergy outflow can
also be written as;

Eventually, the amount of exergy loss is computed by
employing Eq.(14). The exergetic efficiency 
can be described as the ratio of the exergy which is used in drying the
product exergy to exergy inflow for the drying chamber. However, this case is made
clear as the ratio of the exergy outflow to the exergy inflow for the drying
chamber. Taking this definition into consideration, the exergetic efficiency of
drying chamber can be predicted. Therefore, the common form of the exergetic
efficiency is scripted down as in the following equation17;

4. Results and discussion

It was summer season, from August to September 2005,
when drying experiments were carried out in Elaz??, Turkey. During the
experiments of thin layer mint drying by means of forced solar dryer, the alteration
of solar radiation was between 125 W/m2 and 750 W/m2. The
maximum temperature was recorded at 11.00 
and 15.00. The maximum solar radiation energy was found to be at midday
on the other hand the minimum value was in the  evening on the day when we executed the
experiment. Using data from the results of the experiments and values obtained
out of these computing that were demonstrated in Figs. 3-4-5-6-7-8, and argued
in detail, the energy and exergy analyses of thin layer drying process of mint
via forced solar dryer were performed.

4.1. Moisture content

As a function of drying duration for mass flow rates
0.012 kg/s, 0,026 kg/s and 0,033 kg/s, the moisture variations content are
shown in  Fig. 2. As the moisture content
of mint in the solar type dryer reduces, the moisture diffusion from the mint
into the air lessens as well. It is possible to observe that the relative humidity
in the drying air decreases parallel to the moisture content in mint.

4.2. Energy analysis

By using data which were obtained from forced solar
dryer experiments, the energy analysis of thin layer drying process of mint was
carried out. Figs. 3-4-5 display the results of the energy analysis of thin layer
drying process of mint by means of forced solar dryer. The values of energy
utilization in the drying chamber were computed by using Eq.(10). EUR, which
was calculated via Eq.(11), was described as the ratio of the energy utilization
to the energy that was given from solar collector. Upmost values of Qcol
and Qdc were obtained as 445.6 W and 344.7 W when mass flow rates
were 0.012 kg/s during 480 minute experiment producure, respectively.

At the same time, Fig. 3 demonstrates the energy
utilization ratio (EUR) variations of drying process for 0.012 kg/s mass flow
rate. It was seen that EUR presented differences between 10.4 % and 87.1%
during experiments. Fig. 4 displays the values of Qcol, Qdc
and EUR for 0.026 kg/s mass flow rate of drying air.  Maximum values of Qcol and Qdc
 were found out as 623.4 W and
416.5 W with mass flow rate 0.026 kg/s during 390 minute for experiments,
respectively.

However, Fig. 4 exhibits the variations of the EUR
ranging  from 12.5% to 54.75%  in drying chamber during experiments.

Fig. 5 shows the results of the energy analysis of
drying process for the mass rates 0.033 kg/s. It was found out that Qcol
and EUR ranging from 326.22 W to 705.82 W, 7.9 % and 33.66%, respectively.
Upmost value of Qdc was obtained as 382,34 W via mass flow rates of
0.033.

The average values of EUR for 0.012, 0.026 and 0.033
mass flow rates of drying air were found out as % 87.1, %54.75, %33.66,
respectively. These values demonstrate that EUR of drying chamber dropped with
the increase of mass flow rate of drying air.  

4.3. Exergy analysis

The exergy analysis of thin layer drying process of mint by means of forced
solar dryer was conducted using data that were obtained from the drying
experiments. It is likely that the values of ExL and ?Ex for each mass flow ratio
of drying air be seen in Figs. 6-7-8. In the drying experiments with three
different mass flow rates which were carried out, exergy loss in the drying chamber
rose during the first 300 min., and after that displayed a decaying behaviour.
Clearly, as a result of the alterations in the solar radiation, such a time
variation of the exergy loss came in existence.

The average values of ExL for 0.012, 0.026
and 0.033 kg/s mass flow rates of drying air were calculated as 16.22 W, 8.2 W,
6.88 W, respectively. Maximum value of exergy loss was found out  when the mass flow rate was 0.012 kg/s. The
lowest value of exergy was calculated with a mass flow rate of 0.033 kg/s.
These values demonstrate that the exergy loss was lessened  with increase of the mass flow rate of the
drying air. Moreover, it is probable to state that the amount of radiation
influenced exergy loss. Furthermore, the exergetic efficiencies of the drying
chamber are displayed in Figs. 6-7-8. Employing Eq.(15) depending on the
inflow, outflow and loss of exergy, the exergetic efficiency was computed for
each mass flow rate of drying air. The exergetic efficiency of the drying
chamber rose with the drop in temperature difference between inlet and outlet
of the dryer chamber.

The exergetic efficiency values with a mass flow rate
of 0.012 kg/s were obtained as  33.88 – %75.6
in the experiments. Nevertheless, for 0.026 kg/s mass flow rate of the drying
air, the exergetic efficiency changed from %46.44 to %71.22. The exergetic efficiencies
displayed a difference between %40.1 and % 68.23 for 0.033 kg/s drying air mass
flow rate. These values demonstrate that the exergetic efficiency of the drying
chamber dropped while the energy gained from the solar collector was efficiently
utilized.

5.Conclusions

The impact of connective solar dryer on drying of mint
at three different mass flow rates was investigated one after the other. The duration
of drying lessened considerably when the mass flow rate rose.

The drying process came in existence in failing rate
period. Energy and exergy analyses of thin layer drying process of mint were
conducted within the framework of this study by means of forced solar type
dryer. When the outcomes from these analyses are taken into consideration, the
following statements may be concluded.

–         
The mint samples were dried adequately till final moisture content of
nearly 0.08 kgwater / kgdry matter  was obtained at the ranges from 0.012 to 0.033
drying air mass flow rates during 300-480 min. and under 125 W/m2-
750 W/m2 solar radiation.

–         
It is possible to state that the energy received from the solar
collector rose with the increase of the mass flow rate of drying air.

–         
The energy received from the solar collector was efficiently utilized
for drying chamber when the energy utilization ratio (EUR) went up. As a
crucial note, it is stated that the energy utilization ratio would be presumed
as a significant parameter to analyze the utilization of energy in thin layer
drying process.

–         
The exergy loss fell down with the rise of the mass flow rate of drying
air. Moreover, it will be possible to be said that the value of radiation
influenced the exergy loss.  Great number
of the exergy losses occurred for the 0.012 kg/s mass flow rate.

–         
So as to lower the energy utilization in drying chamber, an optimization  investigation must be conducted leading for
the betterment of collector productivity using various hinderences in the air
flow duct to increase the heat transfer area.

–         
As a result, it is recommended that the layout, structure and moisture
content of the products in the drying chamber be taken into consideration to
lower the energy utilization and exergy losses.

–         
It is essential to display the alterations of exergy with drying time to
determine when and where the maximum and minumum values of the exergy losses
occurred at the time the drying process.

Drying
mint in natural environment lasted nearly 930 minutes for zero mass change for
natural drying before the process of drying was commenced with the help of
solar air collectors. When these data were examined, it is easy to see that
drying process lasted about two days. The moisture content of mint is 3.55
maximum. Drier air has a major effect on the flow of moisture content
alterations over time: m = 0.012 kg / h for 480 minutes for the moisture
content, m = 0.026 kg / s for 390 minutes, and m = 0.033 kg/s is nearly zero to
300 per minute.