Abstract-- The present study was concerned with convective drying of kiwifruit slices at different drying air temperatures and flow rates. Dehydration behavior of the samples and thermodynamic performance of the process were investigated. The results showed that the process duration was significantly decreased with the increasing temperature and flow rate. The entire process took place in the falling rate period and no constant period was observed. Using the nonlinear regression procedure, the Midilli model represented the best description. The results of thermodynamic analyses indicated that, following the increment in air temperature, the energy utilization and energy utilization ratio values increased from 2.91 to 15.05 kJ s-1 and from 0.24 to 0.47, respectively. The exergy loss and exergy efficiency were specified to be in the ranges of 50-280 J s-1 and 55.89% to 84.13%, respectively. The exergy improvement potential rate and sustainability index varied from and from 12.28 J s-1 to 36.39 J s-1 and from 12.28 J s-1 to 36.39 J s-1, respectively. Generally, despite of energetic indices, higher temperatures of drying air resulted in better exergetic performance.
Keywords-- Convective drying, Thermodynamic analysis, Energy utilization, Exergy efficiency, Sustainability index.
I. INTRODUCTION
In spite of significant advantages of the current industrial dehydration systems, it is reported that approximately 12% of total used energy in manufacturing processes is consumed by the dryers (Colak et al., 2013). Hence, consumption of high amounts of energy and consequent environmental problems is one of the main challenges should be considered and solved by optimizing the existing systems and/or designing and developing new systems (Aghbashlo et al., 2013).
Thermodynamic analysis has appeared as an effective way to examine performance and optimize different energy conversion systems (Aghbashlo et al., 2008). Energy analysis through the use of the first law of thermodynamics is useful in quantitative evaluation of energy generating and delivery systems as well as in the detection of mode and evaluation of energy loss. In general, information obtained from energy analysis can be used for quantifying energy conservation practices employed for investigating and improving the performance of various thermal systems. However, energy analysis could not deliver beneficial information on the quality of different energy forms. Furthermore, the adequacy of energy concept for sustainable improving in the systems performance is criticized (Aghbashlo, 2015). Therefore, to rectify the inefficiencies of energy analysis and provide a more accurate view of the system, the exergy concept is applied too. Exergy is the energy that is available to be used and after the system and surroundings reach equilibrium, the exergy is zero (Dincer and Sahin, 2004). Exergy depends on the system and its environment and, unlike energy which is never destroyed, is always destroyed when a process involves a temperature change due to irreversibilities. Exergy analysis is used in the field of industrial ecology to more efficient usage of energy (Dincer, 2002). In exergy analysis, the conservation of mass and energy principles together with the second law of thermodynamics is used to analyse, design and improve of thermal systems (Aviara et al., 2014).
Several studies have focused on energy and exergy analysis for drying process of different agricultural and food products such as carrot cubes (Nazghelichi et al., 2010), fish oil encapsulation (Aghbashlo et al., 2012), mulberry (Akbulut and Durmus, 2010), mint leaves (Colak et al., 2008), soybeans (Ranjbaran and Zare, 2013), paddy (Sarker et al., 2015), and red seaweed (Fudholi et al., 2014). However, as far as we know, no study has been published in the open literature on thermodynamic analysis for convective tray drying of kiwifruits. Hence, the main objective of the present study was to conducted detailed energy and exergy analyses for single thin layer drying of kiwifruit slices in a tray hot air dryer at different inlet air temperatures and velocities. Furthermore, influences of the air parameters on drying behavior and mathematical modeling of dehydration curves were studied.
II. METHODS
A. Drying experiments and measurements
The kiwifruits were purchased from a local market at Isfahan, central Iran. The initial moisture content of the fresh samples was determined using vacuum oven method at 65 °C for 24 h (Beigi, 2021), and the average value approximately obtained to be 0.83 (w.b.).
The samples were dried using a laboratory scale convective tray dry (Fig. 1) at inlet air temperatures of 45, 55 and 65 °C and flow rates of 1, 1.5 and 2 m s-1. During the experiments, relative humidity and temperature of the ambient were adjusted and remained constant at level of 35% and 30 °C, respectively. For each experiment, about 3 kg of the kiwifruits was hand peeled and cut into 5±0.5 mm thickness and 500 g of the slices was spread as a monolayer on drying tray.
Figure 1: Schematic view of the lab-scale hot air drying system.
During the dehydration processes, changes in the samples mass was monitored using a digital balance accurate to 0.001 g (ViBRA, model EG 620-3NM, Japan), and instantaneous moisture content of the slices was calculated. The experiments were continued until the samples lot reached to an approximate final moisture content of 0.1 (w.b.). Temperature and relative humidity of drying air at inlet and outlet of the drying chamber were recorded by using MTH02 sensors equipped with a data logger.
For each drying condition, the experiments were replicated three times and the average values were used.
B. Experimental uncertainty analysis
To verify the repeatability and accuracy of the determined parameters, the methodology described by Holman was used to perform uncertainty analysis in this work (Beigi et al., 2017):
, (1)
In Eq. (1), 
is the results uncertainty, 
, 
, …, 
are the uncertainty in the independent variables. Also, 
, 
, …, 
show the independent variables and 
represents the independent variables function.
C. Mathematical modeling of drying curves
To describe the drying
curves of the samples, the four most widely used mathematical thin layer models
(listed in Table 1) were selected. In
these models, 
represents the moisture ratio. The equilibrium moisture
content of the slices was ignorable in comparison with the initial (
) as well as instantaneous moisture contents (
) and therefore, the following equation used to determine
the (Xiao et al., 2010):
, (2)
Curve fitting tool of MATLAB 7.10 (MathWorks,
Inc., Natick, MA) and nonlinear regression technique were applied to fit the models to experimental moisture ratio data. The fit goodness
of the models was evaluated and compared using root mean square deviation (
).
C. Thermodynamic analysis
Considering the first law of thermodynamics for
an open system, typical energy analysis for the drying process was performed
and the heat energy utilization (
, kJ s-1) was estimated as follows (Aviara et
al., 2014):
, (3)
, (4)
, (5)
, (6)
, (7)
, (8)
In the equations, 
is enthalpy (kJ kg-1). 
, 
, 
, 
and 
are mass flow rat (kg s-1), velocity (m s-1),
temperature (°C), heat capacity (kJ kg-1 K-1) and density
(kg m-3), respectively. 
is cross section area of the inlet air duct (m-2).
Also, subscripts of a, i, o, and w denote drying
air, inlet, outlet and water, respectively.
The relative
humidity (
) was converted into humidity ratio (
) using the following equations (Zare et al., 2006):
, (9)
, (10)
where, 
is saturated vapor pressure (kPa).
The energy
utilization ratio (
), which is defined as the ratio of energy utilization to
the provided energy in the dryer chamber, was determined using Eq. (11):
, (11)
By applying the properties of the
working medium, exergy analysis for the drying chamber was performed
in scope of the second law of thermodynamics. Employing the general practicable
exergy equation form for steady flow systems, the exergy inflow (
, kJ s-1), exergy outflow (
, kJ s-1) and the exergy loss
(
, kJ s-1) were determined as
follows, respectively (Prommas et al., 2010):
, (12)
, (13)
, (14)
Exergy efficiency (
) was calculated by using Eq. (15):
, (15)
Sustainability relates to environmental aspects as well as economic and social aspects. To obtain sustainable outputs of a thermodynamic system and set optimum conditions as well as responsible use of resources, sustainability must be determined (Beigi et al., 2017). the exergetic improvement potential rate (EIPR, kJ s-1) and sustainability index (SI) were calculated as follows:
, (16)
. (17)
D. Statistical analysis
The effect of practiced variables including drying temperature and flow rate on the studied parameters was analyzed by comparing the obtained results using one-way ANOVA and Duncan tests at the 5% significance level. The statistical evaluation was performed using software SPSS (V.19).
Figure 2. Variation in moisture content of the kiwifruit slices versus drying time at constant air temperature of 65 °C and different flow rates.
Figure 3. Variation in drying rate of the kiwifruit slices at constant air flow rate of 1.5 m s-1 and different temperatures.
III. RESULTS
A. Uncertainty analysis
Based on the obtained results, the uncertainties of the experimental measurements and the total uncertainties of the calculated parameters were below the acceptable error limit of 5%.
B. Drying curves
The typical curves of dehydration kinetic and moisture removal rate for the kiwifruit slices at some randomly selected practicing conditions are presented in Fig. 2 and Fig. 3, respectively. Furthermore, drying duration of the samples under different applied air temperatures and flow rates are shown in Fig. 4.
From Fig. 3, the process took place entirely in
the falling rate period where the moisture removal rate decreases continuously
with the process time. In one hand, according to Zheng et al. (2011),
dehydration of virtually all biological products occurs only in falling rate
period due to the fact that water movement rate from the product interior to
the exposed evaporating surface is controlled by internal molecular diffusion
and water flux is proportional to the moisture content gradient. In the other
hand, the falling rate period starts when the water film covering the
microscopic surface becomes so thin that additional drying will rupture it,
resulting in small dry areas and lit-
Figure 4. Average time for the kiwifruit slices drying at the practiced conditions.
tle wetted area, and consequently decreasing the moisture removal rate. Hence, dehydration rate is mainly affected by the mechanism of moisture movement from within the material to the surface and the surface condition. At any instant, the rate is a function of the relation of the surface and drying medium. Finally, it can be concluded that the falling rate period is an unsteady state condition related to the nature and geometry of material being dried as well as the air temperature, humidity and velocity.
Numerous studies have been conducted on dehydration duration of different agricultural products under various drying systems. According to the reported works on the open literature, drying time of these materials is generally affected by the process method and conditions as well as substantive characteristics, and initial and final moisture contents of drying product. The influences of drying air temperature and velocity on drying duration of the slices can be seen and discussed from Fig. 4. Based on the obtained results, any increment in the air temperature significantly (p<0.01) decreased the process time. Generally, simultaneous heat and mass transfer phenomena occur throughout convective drying and the moisture removal from the material is fulfilled by the hot air as heat source. During the process, water is transferred from inside of the product to its surface by diffusion phenomenon (internal transmission) and from the surface to the air stream by convection (surface evaporation). At higher temperatures of the inlet air into drying chamber, both of the diffusion coefficient and surface evaporation rate. Positive effect of the temperature on the diffusion is expected wherein water viscosity and resistance of water outflow is decreased and consequently, diffusion of water molecules inside the product capillaries is facilitated (Tohidi et al., 2017). In the other hand, as the interfacial moisture concentration is a function of wet bulb temperature of the air, higher temperature results in more interfacial concentration for the products and subsequently, driving force for mass evaporation is increased (Torki Harchegani et al., 2012).
As the results show, increasing air flow rate significantly (p<0.05) decreased drying duration of the kiwifruit slice. The observation could be described according to boundary layer concept and its effect on heat transfer and drying rate. When an air stream flows on a material, a reduced velocity region arises adjacent to the material surface, in which the velocity is less than the free-stream velocity due to its viscosity. The air velocity at the surface of the material is almost zero and increases to the free-stream velocity as the distance from the surface is increased. This film of air is named boundary layer and fundamentally acts as an insulating layer for water evaporation and heat transfer between the material surface and the free air stream. Basically, there are two types of boundary layer including 1) laminar and 2) turbulent. When a uniform air stream passes over a flat plate, the boundary layer thickness is zero at the leading edge and a film builds up as the air moves along the surface. Within this area, the boundary layer is laminar. At a critical distance from the leading edge, laminar flow changes to turbulent flow (Tohidi et al., 2017). Although the turbulent layer thickness is greater than that for the laminar layer but, heat transfer is essentially increased due to the irregular pattern of flow within the turbulent layer. Therefore, thickness reduction of the laminar film and/or making it as turbulent is necessary during drying process. Increasing the air velocity shortening the distance air travels over the material surface are the two primary methods practicable to reduce thickness of the boundary-layer. Similar results regarding the influence of air flow rate on drying time have been demonstrated for drying of different products such as pomegranate arils (Motevali et al., 2011), cactus/brewer’s grains mixture (Chkir et al., 2015), sweet orange (Zanella and Taranto, 2015), and sweet cherry (Doymaz and Ismail, 2011).
C. Modeling of drying curves
The thin layer mathematical models presented in Table 1 were used to describe the drying process of the kiwifruit slices at different drying conditions. Statistical analyses results obtained through fitting experimental moisture ratio data with the models are shown in Table 2. As shown, among the practiced models, the Midilli model with the lowest average values of root mean square deviation and residual sum of squares was found to be the best model to describe the drying curves.
Furthermore, to evaluate the validity of the Midilli model, the practiced moisture ratio values at any particular drying condition were compared with the experimental data. The results for some randomly selected drying curves are shown in Fig. 5. As shown, in general, the points are located on the 45° line, representing the suitability of the Midilli to describe drying curves of the kiwifruit slices. For the other drying conditions, the same trends were also obtained.
Table 1. Thin layer used for mathematical modeling of the kiwifruit slices drying curves.
Table 2. Statistical results obtained from the applied thin layer drying models for prediction of moisture content of the kiwifruit slices drying curves.
|
Model |
T (°C) |
RMSD |
||||
|
1 (m s-1) |
|
1.5 (m s-1) |
|
2 (m s-1) |
||
|
1 |
45 |
0.03235 |
|
0.03783 |
|
0.01973 |
|
55 |
0.01695 |
|
0.01602 |
|
0.02723 |
|
|
65 |
0.01298 |
|
0.00751 |
|
0.01266 |
|
|
2 |
45 |
0.01737 |
|
0.02215 |
|
0.01863 |
|
55 |
0.01828 |
|
0.01155 |
|
0.01197 |
|
|
65 |
0.00778 |
|
0.00254 |
|
0.01394 |
|
|
3 |
45 |
0.02544 |
|
0.03154 |
|
0.02376 |
|
55 |
0.02153 |
|
0.01559 |
|
0.01346 |
|
|
65 |
0.01287 |
|
0.00525 |
|
0.01671 |
|
|
4 |
45 |
0.00353 |
|
0.00596 |
|
0.00436 |
|
55 |
0.00382 |
|
0.00362 |
|
0.00525 |
|
|
65 |
0.00166 |
|
0.00228 |
|
0.00459 |
|
Figure 5. Comparison between the experimental and predicted moisture ratio values.
D. Energy utilization
The average energy utilization values calculated for kiwifruit slices drying at the practiced air temperatures and velocities are shown in Fig. 6. As the results show, energy utilization increased with increasing drying air temperature and flow rate.
A variance analysis represented
that the air temperature and flow rate significantly (p<0.05) affect
the energy utilization. The same observations in term of the effect of drying
air temperature and velocity on energy utilization
Figure 6. Effect of drying air temperature and velocity on energy utilization during thin layer drying of the kiwifruit slices.
have been reported by many researchers such as Aghbashlo et al. (2008, 2009) for drying of potato and carrot slices, Aviara et al. (2014) for native cassava starch and Motevali and Minaei (2012) for pomegranate arils. However, there are some others insisted on the opposite results as the present study where arguing that the increment in the inlet air velocity reduce the energy utilization (Nazghelichi et al., 2010). According to the equations used to calculate energy utilization, positive effect of drying air temperature and flow rate on energy utilization is expected. The increasing temperature and flow rate results in more inlet enthalpy as well as higher heat and mass transfer (as described before) leading to use the majority of the supplied energy for moisture removal from the product and consequently, energy utilization is increased. In addition, at higher temperatures and flow rates of inlet air, heat loss from the drying chamber boundary to the environment is increased due to probably augmented overall heat transfer coefficient enhancing energy utilization value.
E. Energy utilization ratio
The energy utilization ratio (EUR) was calculated by using Eq. (11) and the values were obtained to be in the range of 28.51-43.27%. The utilization ratios obtained in the present study indicate the relatively efficient utilization of energy in the current drying system. However, the values remain still low indicating the availability of high amounts of energy in the outlet airflow. Recycling the outlet airflow energy could be an effective way for enhancing the energy utilization efficiency of the drying system.
Figure 7 presents the variation of energy utilization ratio with air temperature for the applied velocities for the drying process. As shown, at all of the applied air velocities, increasing temperature from 45 °C to 65 °C led to significant (p<0.05) enhancement in the EUR values. For example, at constant air velocity of 1 m s-1, the ratio for drying air temperatures of 45, 55 and 65 °C were obtained to be 28.51, 39.86 and 47.36%, respectively. The same finding in term of the effect of drying air temperature on energy utilization ratio was reported by some researchers such as Nazghelichi et al. (2010) and Amjad et al. (2016).
Figure 7. Effect of drying air temperature and velocity on energy utilization ratio during thin layer drying of the kiwifruit slices.
Figure 8. Exergy inflow at the different drying air temperatures and velocities.
From the Fig. 7, it can be seen that energy utilization ratio decreased with increasing airflow rate at each practicing temperature. However, a variance analysis indicated that the energy utilization ratio was not significantly (p<0.05) affected by the air velocity.
At higher inlet temperature and flow rates of inlet air, a larger portion of the supplied energy to the chamber is used to evaporate moisture from the product. On the other hand, heat loss from the chamber boundary is intensified. However, in comparison with the effect of enhanced heat and mass transfer, the effect of heat loss is small resulting in more energy utilization ratio values.
F. Exergy inflow and exergy loss
Figure 8 presents the results of the exergy analysis. As shown, the values of exergy inflow to the drying chamber varied from 0.37 kJ s-1.
The effects of drying air temperature and flow rate on exergy loss are represented in Fig. 9.
The average exergy loss varied from 50 kJ s-1
to 280 kJ s-1 declaring that high values of the energy were
accessible at the chamber outlet. The results show that the exergy loss
enhanced with both increasing the drying air temperature and flow rate
indicating more utilized exergy for the samples drying and/or more exergy loss
to sur-
Figure 9. Effect of drying air temperature and velocity on exergy loss during thin layer drying of the kiwifruit slices.
Figure 10. Effect of drying air temperature and flow rate on exergy efficiency during thin layer drying of the kiwifruit slices.
roundings. An analysis of variance indicated that the exergy loss was significantly affected by the air temperature (p<0.01) and velocity (p<0.05). This occurs due to the fact that the overall heat transfer coefficient is enhanced at higher temperatures and flow rates of inlet air to the drying chamber. Loss of exergy through drying chamber boundary to the surrounding is one of the main thermodynamic inefficiencies of dryers and must be considered to improving exciting dryers and/or designing and developing new dehydration systems.
G. Exergy efficiency
The effect of drying air parameters including temperature and flow rate on the exergy efficiency are represented in Fig. 10. The obtained values for exergy efficiency ranged from 55.89% to 84.13%.
From the figure and variance analysis, the exergetic efficiency improved slightly following the enhancement in the drying air flow rate. The finding is due to the fact that the increasing inlet air flow rate results in more enhancement in the outlet exergy in comparison with the inlet exergy, and consequently leads to higher exergy efficiency.
Figure 11. Effect of drying air temperature and flow rate on exergetic improvement potential rate of the drying chamber during thin layer drying of the kiwifruit slices.
The finding is in well agreement with the reported results by Beigi et al. (2021) for rough rice and Aghbashlo et al. (2009) for carrot drying.
Based on the results obtained in this study, increasing drying air temperature caused significant (p<0.05) reduction in the exergy efficiency. The observation originates from the inversely proportional of exergy efficiency to inlet exergy (Aghbashlo et al., 2008) and also more overall heat and mass transfer at the higher temperatures leading to more utilized exergy for moisture removal from the drying product as well as more exergy loss from drying chamber boundary to environments.
H. Exergetic improvement potential rate (EIPR)
Figure 11 illustrates the influence of drying air temperature and velocity on the exergetic improvement potential rate of the drying chamber during the thin layer drying of kiwifruit slices. The values range from 12.28 J s-1 to 36.39 J s-1 over the practicing dehydration conditions representing a greatly capable of ameliorating exergy performance for the drying chamber. According to obtained results, the exergetic improvement potential rate significantly (p<0.01) increased with the augment in the drying air temperature and enhanced by any increment in the air velocity. Aviar et al. (2014) conducted exergy analysis for native cassava starch drying in a tray dryer and observed that exergetic improvement potential rate increased linearly from with increment in drying air temperature from 40 to 60 °C.
It is worth to note that unlike the findings, some researchers indicated that a decrement could be induced by increasing drying air temperature and velocity. Colak and Hepbasli (2007) investigated thermodynamic performance of green olive drying in a tray dryer and reported the values of EIPR to be in the range of 103-142 J s-1, while decreased with increasing drying temperature from 40 to 70 °C.
I. Sustainability index
Sustainability index is an important parameter
for exergetic sustainability of dryers in terms of the second-law of thermodynamics
where the higher sustainability index
Figure 12. Sustainability index of the drying chamber during thin layer drying of the kiwifruit slices.
indicates lower environmental footprints. The influences of the air parameters on the sustainability index of drying chamber as far as the thin layer of kiwi fruit slices is concerned are illustrated in Fig. 12. The sustainability index ranged from 2.27 to 6.30 as the air temperature and flow rate varied in the range of 45‒65 °C and 1‒2 m s-1, respectively. Ndukwu et al. (2020) conducted an experimental investigation on hybrid solar-biomass dryer and found the sustainability index to be in the range of 2.30‒6.11. The index for a PV-driven quadruple-flow dryer was reported to be changed from 1.93‒2.73 (Khanlari et al., 2021).
From Figs. 10 and 12, it is obvious that the influence of drying air parameters on the sustainability index of drying chamber and the relative exergy efficiency is same. Hence, to reduce the environmental impact, the exergy efficiency of drying process as an energy intensive operation must be improved.
IV. CONCLUSIONS
In this work, thin layer of kiwifruit slices was dried in a convective dryer considering different levels of air temperature and flow rate and influence of the air parameters on dehydration behavior of the samples as well as energy and exergy indices were studied. Based on the results, following conclusions can be drawn:
· Increasing air temperature and flow rate reduced the process time while the effect of the temperature was more significant than the effect of flow rate.
· Entire dehydration process occurred in the falling rate period indicating that internal molecular diffusion controlled the water movement from inside the product to its surface.
· Higher drying air temperatures and velocities led to more energy utilization amounts. The energy utilization ratios (ranged from 28.51% to 43.27%) demonstrated relatively efficient use of energy during the process.
· The exergy efficiency enhanced with augment in drying air velocity but significantly decreased following the increase in the air temperature.
· Over the practicing conditions, the potential rate for improvement of exergy and sustainability index varied in the range of 12.28-36.39 J s-1 and 2.27 to 6.30, respectively and increased with increasing temperature and flow ate of inlet air. To decrease the environmental footprints and health concerns, the exergy efficiency should be improved.
· In general, accounting to the main disadvantage of convective hot air drying systems, it was concluded that the exergy of outflow air stream obviously is the main factor for thermodynamic inefficiency of the system where a great part of the supplied thermal exergy is lost by the outlet air. Hence, recycling the exhausting air could be a useful approach to overcome the shortcoming. Exergy lost from the drying chamber was found to be the next most significant contributor in the inefficiency in the drying system. Therefore, avoiding heat transfer across the chamber boundary to the environment (by sealing the chamber, selecting appropriate materials as well as choosing the optimum inlet air conditions etc.,) could be a possible approach to improve the thermodynamic efficiency.
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Received: June 29, 2021
Sent to Subject Editor: August 2, 2021
Accepted: November 27, 2021
Recommended by Subject Editor Gianfranco Caruso