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Energy and exergy analyses of thin layer drying of mint in a forced solar dryer

Abstract

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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. 

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.

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