Investigation of mass transfer, thermodynamics, and greenhouse gases properties in pennyroyal drying

Correspondence Mohammad Kaveh and Hamed Karami, Department of Biosystems Engineering, University of Mohaghegh Ardabili, Ardabil, Iran. Email: hamed.karami@wur.nl; hamedkarami@ uma.ac.ir (H. K.) and sirwankaweh@gmail.com; sirwankaweh@uma.ac.ir (M. K) Abstract In this research, kinetic analysis, energy, exergy, and greenhouse gases of a hybrid laboratory dryer (solar-hot air) are presented for pennyroyal. Drying was performed at input temperatures of 50, 60, and 70 C and air velocities of 0.6, 1.2, and 1.8 m/s. The effect of drying variables on moisture ratio, effective moisture diffusivity, specific energy consumption, energy utilization ratio, energy utilization, exergy efficiency, and exergy loss was investigated. The highest amounts of effective moisture diffusivity and specific energy consumption were 2.30 × 10 m/s and 48.60 kWh/kg, respectively. Energy utilization and energy utilization ratio varied from 0.0064 to 0.0826 kJ/s and from 0.056 to 0.957, respectively. Exergy loss and exergy efficiency varied between 0.0037 to 0.0510 kJ/s and 0.2428 to 0.8731, respectively. In addition, by increasing the temperature and intake air velocity, drying rate increased and the emissions of greenhouse gases (CO2, SO2, and NOX) were reduced. Practical Applications Modeling of the drying process is an important aspect of drying technology, especially in drying for industrial purposes. The aim of modeling is to select the most suitable drying method and the best operating conditions for obtaining the product. Some of the key issues in drying have been to reduce the price of the energy resources used, increase the drying efficiency, and improve the quality of the dried products. The concept of exergy is defined by the concept of reversible work. The concept of reversibility depends on energy balance and mass regardless of energy quality (exergy loss).


| INTRODUCTION
Pennyroyal belongs to the family of Lamiaceae plants and is of great importance in cosmetic, food, and medical industries. Nepeta L. genus of the pennyroyals is one of the major genera in the Lamiaceae family (Hassanpouraghdam & Hassani, 2014). To date, 250 species of this genus have been reported in the world and 67 species have been reported in Iran.
Drying is one of the oldest methods for preserving agricultural products. Thus, it is of utmost importance in processing herbs because if essential herbs are not dried immediately or their essence is not extracted on time, they will lose their effective ingredients and their volatile compounds (Abbaspour-Gilandeh, Kaveh, & Jahanbakhshi, 2019;Sun, Zhang, & Mujumdar, 2019). Drying the plant immediately after harvest would help preservation of its color and aroma.
Exergy and energy analysis is applied to determine the amount of energy needed to dry the product and exergy loss at each stage of the process. Therefore, exergy and energy analysis is important (Fudholi, Sopian, Othman, & Ruslan, 2014). The concept of exergy is defined using the concept of reversible work. The concept of reciprocity, regardless of the reduction of energy quality (energy dissipation), depends on the balance of energy and mass (Abbaspour-Gilandeh, Akpinar, Midilli, & Bicer, 2006).
Greenhouse gases are gases that act as a barrier to prevent heat from leaving the atmosphere and thus cause global warming. Over the past 100 years, the accumulation of greenhouse gases in the atmosphere has increased the temperature of the Earth's surface and continues to have unknown effects on the Earth's climate in the future (Khoshnevisan, Rafiee, Omid, & Mousazadeh, 2013). Six important greenhouse gases are carbon dioxide, methane, nitrous oxide, hydrofluorocarbons (HFCs), perhydro-fluorocarbons, and sulfur hexafluoride (Ahmadi, Rozkhosh, & Haghighifard, 2014). The burning of fossil fuels such as diesel and gasoline, the change in land use from forests and pastures to agricultural and residential land as well as a number of post-harvest processes for crops are the main causes of increased greenhouse gas emissions (Samani, Choobin, Ghasemi-Varnamkhasti, & Abedi, 2018). Nazari et al. (2010) reported that the total SO 2 , CO 2 , and NO X emissions of Iranian power plants using different fuels were 125.34, 0.255, and 0.465 Tg. Motevali and Tabatabaei (2017) studied energy consumption and GHG emissions (CO 2 , SO 2 , and NO X ) for the drying of rose flowers in different dryers (hot air, infrared, infrared-hot air, microwave, microwave-hot air, vacuum and solar). According to their findings, by increasing air temperature at inlet, infrared power and microwave power, specific energy consumption, and greenhouse gas emissions were reduced. Also, these two parameters were increased by increasing the air velocity.
However, the data reported on energy, exergy, and greenhouse gas emission analyses in the process of drying herbal plants are very few. Given the importance of the pennyroyal plant and its sensitivity to heat application in the drying process, the aims of this research are to optimize the plant's drying conditions, modeling, evaluation and analysis of kinetics, effective moisture diffusion coefficient, activation energy, energy, exergy, and emission of greenhouse gases using a hybrid dryer (solar-hot air).

| Experiments
In this research, leaves and twigs of pennyroyal plant (Mentha pulegium L.) were harvested. The experiments were carried out at four temperatures of 40, 50, 60, and 70 C and three levels of input air velocity: 0.6, 1.2, and 1.8 m/s with three replications.
The dryer used in this study included a solar collector with a 1,000 watt electric element for heating the input air to the drying chamber and five temperature sensors (LM75) with a range of measurements from −55 to 125 C and the measurement accuracy of ±2 C ( Figure 1) (Karami, Kaveh, Mirzaee-Ghaleh, & Taghinezhad, 2018). The position of the sensors was arranged as three sensors on the collector, a sensor at the air inlet into the chamber, a sensor inside the chamber, and a sensor at the air outlet from the chamber. Two humidity sensors (HS1101) with the precision of ±2% were also used to measure the relative humidity of air displacement. Humidity sensors were located at the inlet and outlet inside the chamber. The dryer's fan had the ability to change the amount of the input air that entered the drying chamber.
To measure the speed of the airflow to the dryer chamber, a speedometer of the model AVM-07 made in Taiwan was used with a measurement accuracy of 0.1 m/s. To implement the drying process, the ATMEGA 8 model of AVR microcontroller made in China was used (Karami, Rasekh, Darvishi, & Khaledi, 2017).
In order to do the experiments, 200 g of pennyroyal leaves and twigs were placed in a single layer on the dryer's mesh tray. During the drying process, the product weight changes were recorded by a digital scale (GF-3000, AND) with a precision of ±0.01 g. This scale was attached directly to the trays so that the samples did not need to be brought out of the chamber for weighing. The final moisture content of the samples was then calculated using the oven dryer at 70 C for 24 hours through the following equation (Zhao et al., 2018): F I G U R E 1 Structure of the dryer: (1) solar collector; (2) electric element; (3) solar cell; (4) batteries; (5) drying chamber; (6) damper; and (7) controller box To achieve system stability, all tests started 25 min after the system was turned on. Then, the tray containing the samples was placed in the drying chamber. The process of drying went on like this: the fan transferred the air heated by an electric heater from the collector to the trays. The flow of air absorbed the sample's moisture as it passed through it and dried the plant. In addition, increase in temperature led to the rapid escape of moisture from the sample's texture and thus dried the product.
As laboratory data used for the process of drying pennyroyal plant two parameters of moisture ratio (MR) and drying rate were taken into account which were calculated through the following equations (Abbaspour-Gilandeh et al., 2020: Due to the low value of M e compared to M t andM o , Equation (2) can be simplified as MR = Mt Mo (Badaoui, Hanini, Djebli, Brahim, & Benhamou, 2019). In order to model pennyroyal's moisture in the process of drying in the hybrid dryer (solar-hot air), known models that are frequently applied in the processes of drying agricultural products were used in this study (Table 1). The three criteria of determination coefficient (R 2 ), root mean square error (RMSE), and chi-square (χ 2 ) were used to evaluate the best model according to the following equations: 2.2 | Effective moisture diffusion coefficient (D eff ) Fick's second law (Equation (7)) is widely used to describe diffusion in the process of drying agricultural products (Mohammadi, Tabatabaekoloor, & Motevali, 2019): After expanding Equation (7) and applying the drying conditions for a long time, Equation (8) was obtained to determine the diffusion coefficient (Zhao et al., 2018): The effective moisture diffusion coefficient was obtained using Equation (9) from the gradient (K) of the Ln (MR) graph over time: T A B L E 1 Mathematical models to fit the experimental data

| Activation energy
Activation energy for different temperatures and velocities was calculated using the following equation (Atalay et al., 2017): To obtain E a linear relationship, Equation (11) was used.
By charting ln(D eff ) against 1 Tabs , a line with slope K 2 was obtained.

| Energy analysis
In this study, energy utilization (EU) during the process of drying pennyroyal was calculated using the following equation (Aghbashlo, Kianmehr, & Arabhosseini, 2008): Relations (14)-(16) were used to obtain the input and output air flows and air density, respectively (Taheri-Garavand et al., 2018): The enthalpy of the input or output air is equal to the dry air enthalpy plus the enthalpy of the water vapor. Having obtained these indices exergy loss was calculated through the following equation (Corzo, Bracho, Vasquez, & Pereira, 2008): The heat transfer rate, which leads to evaporation in the dryer, was obtained through the following equation (Nazghelichi, Kianmehr, & Aghbashlo, 2010): In order to obtain the enthalpy of the fresh and dried product, the specific heat of the input or output product had to be first calculated from Equation (19) and the temperature of the input or output product as well as the ambient temperature was measured using a thermometer (Aghbashlo et al., 2008): However, in the wet state, the amount of the input and output air moisture should also be available. This factor is used to determine the input or output air enthalpy (Equation (20)) (Motevali & Minaee, 2012): The rate of heat dissipated from the output air depends on the flow rates of the input air, the specific heat of the input air, and the temperature of the input and output air. The rate of heat dissipated from the output air can be obtained using the following equation (Akpinar, 2010): The heat loss rate from the dryer's body can be obtained from the following equation (Nazghelichi et al., 2010): Finally, the heat loss coefficient of the dryer body can be obtained by equating Equations (21) and (22). As mentioned before, the heat loss coefficient of the dryer body can be obtained by using the following equation (Aghbashlo et al., 2008;Akpinar, 2010): Eventually, the energy utilization ratio (EUR) was determined by (Aghbashlo et al., 2008):

| Exergy analysis
The total exergy of the input and output air for the fresh and dry product can be calculated by the second law of thermodynamics. The basic method for analyzing the exergy of the drying chamber is to calculate the exergy in stable conditions. For this goal, the general form of the exergy equation for stable conditions can be used. The exergy of the input or output air and the exergy of the input or output product can be obtained through the following equation (Ergün, Ceylan, Acar, & Erkaymaz, 2017): The most important step in the exergy analysis is measuring the amount of exergy rate from the dryer's body, which was calculated using the following equation (Rabha, Muthukumar, & Somayaji, 2017): Relations (27)-(29) were used to obtain the amount of exergy loss from the dryer's body (Aktas et al., 2017;Darvishi et al., 2018): Finally, the exergy efficiency was calculated according to the following equation (Abbaspour-Gilandeh et al., 2020;Akpinar, 2010): When drying does not take place, the exergy efficiency is 100% and gradually decreases with the onset of drying.

| Specific energy consumption
Since one of the main objectives of this research was to study the changes in the amount of specific energy consumed and its environmental effects, we tried to apply hybrid drying (solar-hot air) methods to the samples. The equations for calculating the specific energy con- The total production of a power plant required for drying programs can be obtained using the following equation (Motevali & Tabatabaei, 2017):

| Experimental uncertainty analysis
Uncertainty analysis is a method used to find uncertainty in variables.
During the measurement and calculation of the parameters, the uncertainties were determined using the following equation and presented in Table 3.
T A B L E 2 Emissions of greenhouse gases and pollutants from plants using natural gas and heavy oil to produce 1 KW of energy (Nazari et al., 2010)

| Drying rate
Drying rate constantly decreases as the MR is reduced or drying time increased (Figure 3). This shows that drying rate is a strong function

| Modeling
The results of MR against drying time with 14 models for thin layer drying are shown in

| Determination of D eff
The values of D eff for pennyroyal are reported in

| Activation energy
For different temperatures and velocities of the intake air, the activation energy values for pennyroyal in the hybrid dryer were 22.41-25.18 kJ/ mol. These results are shown in Table 5. Kian-Pour and Karatas (2019) reported the E a of apple slices with different temperatures in the range of 20.41-36.51 kJ/mol. In their work, Quispe-Fuentes et al. (2017) reported the E a of Chilean berry dried in a convective dryer at 40, 50, 60, and 70 C and air velocity 2 m/s was 42 kJ/mol.

| Energy utilization ratio
Various experiments were carried out to calculate the energy utilization ratio(EUR) at different temperatures and velocities of the intake air for drying pennyroyal and the results were shown in Figure  The EUR in the hybrid (solar-hot air) dryer increases as the air temperature rises. Increase in the temperature of the dryer leads to increase in the rate of moisture evaporation from the product. In other words, at high temperatures, heat, and mass transfer are high and the loss of moisture is excessive (Motevali, Jafari, & Hashemi, 2018). These results are similar to those reported for drying mushrooms in the fluid bed dryer (Darvishi et al., 2018), drying carrots in the fluid bed dryer (Nazghelichi et al., 2010), and drying rice in the fluidized bed dryer (Sarker, Ibrahim, Abdul Aziz, & Punan, 2015).

| Effect of air velocity on EUR
Intake air velocities for pennyroyal drying experiments ranged from 0.6 to 1.8 m/s. The experiments were carried out at different intake air temperatures and the results were reported in Figure 5. The results show that energy utilization ratio increases as the air velocity rises and decreases by the passage of time. Increase in air velocity increases energy utilization and the evaporation of the moisture content from the product surface . These results are similar to those reported by researchers for drying potatoes  and Kodo millet grains (Yogendrasasidhar & Setty, 2018).

| Effect of air temperature on EU
The results shown in Figure 6 indicate that by increasing the air temperature, EU increases. It can be noted that the input enthalpy has increased with increasing temperature and the amount of intake air, which leads to increased energy consumption. In addition, due to the  (Aviara, Onuoha, Falola, & Igbeka, 2017), and drying coroba slices in a convective dryer (Corzo et al., 2008), the researchers reported that energy utilization increased as the intake air temperature increased.

| Effect of air velocity on EU
The results illustrated in Figure 6 show that by increasing air velocity, energy utilization increases. According to Equation ( 3.8.1 | Effect of air temperature on exergy loss (kJ/s) Exergy loss is the difference between the dryer's input exergy and its output exergy. Input exergy is based on the dryer's wall temperature (Equation (27)) and output exergy is based upon the output air temperature (Equation (28)). Figure 7 shows that by increasing the intake air temperature, the amount of exergy loss increases. Exergy loss is higher in the initial drying phase. In addition, the loss of exergy decreases steadily over time since the passage of time F I G U R E 6 Effect air temperature and air velocity on energy utilization (kJ/s) F I G U R E 7 Effect of air temperature and air velocity on exergy loss (kJ/s) reduces the moisture content in the pennyroyals. A lower amount of moisture has a lower capacity to absorb exergy, so the exhaust air has a lower exergy than the intake air Darvishi et al., 2018). Higher drying temperatures have higher exergy and this exergy reduces the product's moisture or exergy consumption and thus increases exergy loss (Corzo et al., 2008).

| Effect of air velocity on exergy loss (kJ/s)
As the intake air velocity increases, the exergy loss increases too, resulting in a mass transfer rate that decreases the pennyroyal's moisture content. The drying process time also decreases and less energy is transferred to the outside of the dryer chamber. So, the output exergy increases (Aghbashlo et al., 2008;Akpinar et al., 2006). In similar investigations, the researchers reported that with increasing inlet air velocity, the exergy loss increases (

| Exergy efficiency
The exergy efficiency curves are shown in Figure 8 with regard to the temperature and velocity of the intake air for the dried pennyroyal.
The maximum exergy efficiency was 0.8731 at the temperature of 70 C and the air velocity of 1.8 m/s. The minimum exergy efficiency was 0.2428 at the air temperature of 40 C and the air velocity of 0.6 m/s.
3.9.1 | Effect of air temperature on exergy efficiency Figure 8 shows that the exergy efficiency increased during the drying time. Increasing the dryer's temperature caused an increase in exergy loss, but this amount was lower than the increased exergy value.
According to Equation (29), the exergy efficiency increases by increasing temperature at different velocities of the intake air. Exergy efficiency has a direct relationship with the dryer's energy efficiency. The evaporation rate of moisture and mass transfer rate increase as the intake air temperature rises. This increases the exergy efficiency of the hybrid (solar-hot air) dryer. Similar results have been reported in the drying of various products such as soybean (Ranjbaran & Zare, 2013), carrot cubes (Nazghelichi et al., 2010), and green olive (Colak & Hepbasli, 2007).

| Effect of air velocity on exergy efficiency
Increase in the intake air velocity and the drying time increases the exergy efficiency (Figure 8), because the entropy and enthalpy of the dryer's intake air increase by the rising air velocity, which results in increased exergy efficiency. These results were similar to those of Motevali and Minaee (2012)) and Nikbakht, Motevali, and Min-   3.10 | Specific energy consumption Table 6 represents the SEC of the entire drying process for pennyroyal in a hybrid (solar-hot air) dryer. The maximum amount of energy consumed was 48.60 kWh/kg at the intake air temperature of 40 C and the intake air velocity of 0.6 m/s. The minimum amount of energy consumed was 14.11 kWh/kg at the intake air temperature of 70 C and the air velocity of 1.8 m/s. Moreover, the specific energy consumption increased as the air velocity and temperature decreased.
Increase in the intake air temperature and velocity resulted in the rapid transfer of mass and increase in the speed of moisture transfer from the surface of the product. So, the amount of specific energy consumption decreased   Table 7 shows that the highest CO 2 level (116,516.6 g) occurred at 40 C and 0.6 m/s for the gas turbine-gas oil plant while its lowest level (12,721.5 g) occurred at the temperature of 70 C and the air velocity of 1.8 m/s for the combined cycle-natural gas plant. In addition, SO 2 increased by decrease in the temperature and the air flow rate and reached the highest level at the temperature of 40 C and the air velocity of 0.6 m/s. The analysis of NO X changes showed that its variation is inversely related to temperature and air flow velocity. The highest amount of NO X (643.73 g) was produced at 40 C and the air velocity of 0.6 m/s at the gas turbine-gas oil plant. • Increase in the intake air temperature and velocity resulted in decrease in the drying time.

| Greenhouse gases
• Midilli et al's model was selected as the best model for the prediction of pennyroyal MR.
• The EUR increased by increasing the intake air temperature and air velocity.
• The highest and lowest EU rates were 0.0826 kJ/s and 0.0064 kJ/ s, respectively.
• Increasing the intake air temperature and velocity increased the exergy loss.
• Exergy efficiency increased with increasing intake air temperature and air velocity.
• The highest (48.60 kWh/kg) and lowest (14.11 kWh/kg) amounts of specific energy consumption were obtained.
• In the process of drying pennyroyal, increase in the intake air temperature caused the reduction of GHG. Low levels of GHG occurred at high drying temperatures. Increasing the temperature and airflow rate resulted in a significant reduction in GHG.

NOMENCLATURE
A def surface in contact with the dryer's body (m 2 ) A dc cross section of the drying chamber (m 2 ) C a input or output air specific heat (kJ/kg C) C p specific heat of the input or output product (kJ/kg C) C specific heat of the input or output air and the input or output product (kJ/kg C) C ai specific heat of the input air (kJ/kg C)