Introduction
It is of top priority nowadays in every country to reduce the volume of food waste. Hence, the prevention and recycling of food waste is essential to produce promising results. Dry biowaste is very useful as it can reduce toxic leachate from landfills and also can produce green energy. Encouraging people to separate and dry organic waste is a promising project for the management of organic household waste to significantly reduce its volume. Drying time and energy consumption are two important indicators in the field of drying. When the drying time is reduced the energy consumption of the dryer will be reduced. The researchers constructed a local agricultural dryer. They designed for it, a heater with a heating power of 1,500 W and a centrifugal fan with an air volume of 0.2 hp. Their device had a thermal efficiency of 76.9% [1]. A solar dryer for agricultural products was used to dry restaurant food waste. The results showed that the amount of drying depends on the intensity of sunlight and its duration [2]. A small-scale solar dryer was designed and built to evaluate the drying of waste. The results showed that higher levels of solar radiation accelerate the drying process [3]. The researchers investigated the production efficiency of a factory by showing its energy efficiency in the dairy milk industry. Their study was to analyze the thermal energy and production of a dairy milk factory based on annual production. Despite the costs of the power plant for thermal and electrical energy, the energy efficiency was determined at 45.5%. Their results showed that optimizing product efficiency and energy consumption in the production of milk and dairy products positively increases the energy efficiency of the factory [4]. In one study, researchers examined 49 rice dryers and 14 yellow corn dryers. The results showed that the excessive size of the fan/extractor and dryer motor has high energy consumption [5]. Researchers evaluated the effect of different pretreatment methods on drying kinetics, energy consumption, and product quality characteristics of dried apple slices using electrohydrodynamic (EHD) drying. Studied three pretreatment methods of pulsed electric fields (PEFs), ultrasound, and blanching. The results showed that only PEF pretreatment can reduce the drying time of EHD by 39%. [6]. Several studies on energy consumption were achieved in thin layer drying of vegetables and fruits [7–9]. The goal of the waste drying project is to design and evaluate an innovative, flexible, compact, and suitable household dryer for drying food waste to reduce its volume and prevent its spread in air, water, and soil. Therefore, designed a dryer for drying food waste using the advantages of mass rate and high heat transfer, and were investigated drying time, and energy consumption during drying of food waste.
Materials and Methods
Calculation amount of moisture
The dimensions of the dryer’s tray (0.57 × 0.4 × 0.05) m3 were selected. The amount of moisture in food waste that needs to be dried is given by Equation (2) [10]. Initially, moisture content and density of food waste were considered 82% (w.b) and 892 kg/m3, respectively [11],
v: the volume of the tray (m3),
ρ: the density of food waste,
M: dryer capacity (kg),
X1: initial moisture content of the product to be dried (82%),
X2: the maximum desired final moisture content based on experimental results (10%), and
Mw: the amount of moisture in food waste that needs to be dried (kg).
Quantity of heat for drying
The quantity of heat required for drying is given by Equation (3) [10]. Heat source power is given by Equation (4) [12].
∆T: temperature difference in the dryer cabinet (°C),
m: dryer capacity per batch (kg),
Hfg: specific heat of vaporization (kJ/kg),
Mw: amount of moisture to be removed (kg),
Qheat: the quantity of heat for drying (kJ), and
Cp: the specific heat of the food waste (kJ/kg °C).
Food waste has a great variety of thermal properties. Therefore, Cp was calculated by the following equation [13]:
Cp =0.837 + 3.349xw (5)
xw: moisture content of the food waste.
For the specific heat capacity of food waste with humidity (82%), the value of 3/583 kJ/kg °C was calculated. Therefore, the amount of power of the heater was 1.9 kW. To increase the efficiency of the dryer at temperatures above 80°C, a heat power of 2.7 kW was used as the heat source.
Volume of air for drying
The work done by the heater is equal to the work done by the air [14]. Therefore, the mass flow of air and the amount of air were calculated from Equations (7) and (8), respectively [12],
ρ: density of air (kg/m3),
mair: mass flow rate of air (kg/ s),
CP: specific heat capacity of air (kJ/kg °C), and
v: volume of air for drying (m3/s).
Fan selection
The fan aids in heat distribution by drawing ambient air from the surroundings to the heater housing and discharging heated air to the drying chamber. The total pressure loss (∆Htotal) is equal to the sum of the pressure loss due to the straight direction, the pressure loss due to the change in cross-section, and the pressure loss due to the resistance of food waste. The pressure loss of the first part and the second part were determined by the following equations [12]:
∆Hf: pressure loss in the straight direction (Pa),
U: velocity of air in the chamber determined by the momentum equation (m/s),
h: thickness of the material layer in the tray (m),
L: length of the chamber (m),
λ: coefficient of friction,
∆HC: loss due to sudden enlargement,
A2: cross-section area of the dryer chamber (m2), and
Figure 1.
(a) Schematic of the dryer. (b) Dryer components: a - on and off switch, b - temperature controller, c - selector key, d - air velocity control, e - temperature sensor, f - heaters, g - axial fan, h - tray location, and j - door.
A1: cross-section area of the heater chamber (m2).
The coefficient of friction was determined between the range of 0.033 and 0.035. Static pressure loss due to resistance to airflow by food waste was calculated using Equation (11) [15]. The porosity coefficient was determined by Equation (12) [14].
ρp: the density of food waste and
ρf: the density of dry matter.
∆Htotal =∆ Hf + ∆HC + Δpbed. (13)
The total pressure loss was calculated to be 86/14 (Pa).
Model production
After the necessary calculations, it was simulated in the software in Catia 2019 and dryer components were made. The drying chamber was made of a galvanized sheet double-walled with glass wool insulation. Figure 1 shows the schematic of the dryer. Hot air from the tray is transferred vertically to the upper chamber without passing through the sides (Fig. 1b).
The dryer had an automatic temperature controller with an accuracy of ±0.1°C (G-Sense, Iran). The thermocouple was installed below the tray. The air velocity was adjusted at values 0.1, 1.5, and 2 m/s with an accuracy of ±0.1 m/s using an anemometer (UNIT UT363, China).
Figure 2.
Graph of air volume and static fan pressure.
Drying time
Before each test, the dryer was turned on for 30 minutes to stabilize its temperature. The tested materials were food, fruit, and vegetable waste, and components such as metals, glass, paper, and plastic were separated from the waste. After that, the materials were crushed with a shredder to reduce their size to less than 20 mm. Then the wastes were placed in the environment at the same temperature as the environment and then pressed with manual pressure to remove the free water. Finally, the material is spread on the tray with a thickness of 3 cm. Three cubic containers containing 47 g of food waste were placed in three places on the tray. The samples were weighed every 30 minutes using a digital scale with an accuracy of 0.01 g (AND GF-600, Japan). Then, the moisture value was calculated using the initial and final moisture values at any time from the weight of the sample [16]. Then, the samples are placed in an oven at a temperature of 105°C ± 1°C to dry completely and determine the solid mass [17]. Moisture content (XW) of food waste was calculated by the following equation:
wo: sample weight at any given time (kg) and
wd: dried sample weight (kg).
Experiments were conducted at three temperatures of 50°C, 60°C, and 70°C and, three air velocities of 1, 1.5, and 2 m/s.
Energy consumption
Et=A × υ × ρa × Ca × ∆T × Dt (15)
Et: total energy (kWh),
A: cross sectional area (m2),
ρa: density of air (kg/m),
∆T: temperature differences (°C),
Dt: time for drying sample (h), and
Ca: the specific heat of air (kJ/kg °C).
Results and Discussion
The heating power was calculated as 2.7 kW. An axial fan with an air volume of 310 m3/hour, 2,800 rpm, and 110 Pa was used to supply airflow in the dryer. The graph of air volume and static fan pressure is shown in Figure 2.
Drying time
Figure 3. shows the drying time of dry food waste. Researchers have confirmed that increasing the drying temperature decreases the drying time [18–22]. The minimum drying time occurred at a temperature of 70°C and at an air velocity of 2 m/s. The maximum drying time occurred at a temperature of 50°C and at an air velocity of 1 m/s.
Figure 3.
Drying time.
Figure 4.
Energy consumption.
Energy consumption
Figure 4 shows the energy consumption in different temperatures and air velocities. The results show that the dryer has a significant effect on energy savings. The energy consumption of the dryer is reduced by increasing the drying temperature and air velocity. This is different from the proceeding of drying in the hot air dryer [7,8] and cabinet dryer with trays by central and lateral air passage [23]. In them, hot air circulates by the fan throughout the dryer chamber, and the amount of energy consumption increases with the increase in temperature and air velocity. The lowest energy consumption was obtained at 70°C and 2 m/s. The highest energy consumption was obtained at 50°C and 1 m/s. The values of total energy for drying food waste were between 59.41 and 119.62 kWh.
Conclusion
A cabinet dryer for drying food and agricultural waste was designed and evaluated. Compactness, economy, and low energy consumption are the advantages of this dryer. The results showed that the dryer has a significant effect on energy consumption. The minimum energy consumption was at the air velocity of 2 m/s and temperature of 70°C. The maximum energy consumption was at a temperature of 50°C and air velocity of 1 m/s. The minimum drying time was obtained at a temperature of 70°C and at an air velocity of 2 m/s. The maximum drying time was obtained at a temperature of 50°C and at an air velocity of 1 m/s. The energy consumption was reduced by increasing the drying temperature and air velocity.
Acknowledgment
The author would like to thank the Bandar Abbas Municipality Waste Management Organization for its support.
