Drying kinetics of rice using a flatbed dryer with a hot air source from a biomass waste pyrolysis reactor

This study aims to evaluate the process of rice drying using a flatbed dryer integrated with a biomass waste pyrolysis reactor as a heat supplier. The integration of dryer equipment with pyrolysis reactor aims to replace the use of fossil fuel energy with biomass as an eco-friendly source and abundantly available in nature. The current investigation assessed how tray height and drying temperature affected rice’s moisture ratio, drying rate, and drying characteristics. The drying air temperature of 70°C and the tray height of 40 cm were found to yield the optimum drying outcomes, according to the results. The best drying kinetics model that can describe the rice drying process is the Page model.


Introduction
Rice is the main food of Indonesian people.Rice production in Indonesia during 2019-2021 has increased, as shown in Table 1.With increased rice production, proper handling is needed to obtain durable rice that only deteriorates gradually when stored for a long time until it is distributed to the public [1].Ironically, Indonesia still imports rice from other countries.One of the inhibiting factors for rice production in Indonesia is the ineffective rice drying process.Farmers still use the sun's heat to dry grain [2].Actually, open solar drying has several advantages, such as a large energy supply, easy to carry out, and inexpensive start-up costs.Nevertheless, there are a few disadvantages to this method, like the need for a vast area, a lengthy drying period, a reduction in the quality of dried products, and so on [3].
For high-quality rice, it is possible to dry it with a drier.Dryers still typically run on energy derived from fossil fuels, though.Fossil fuel consumption, however, will result in emissions of CO2, SO2, and NOx, which can contaminate the air and cause climate change.Not only is fossil fuel costly, but its supply is diminishing.Thus, biomass can be used in place of fossil fuels.A few benefits of using biomass as an energy source are its low cost, plentiful availability, renewable and sustainable nature, and reduced emissions [3].
One of the rice drying tools that can be used is a flatbed dryer.Flatbed dryers have been used in the rice drying process on both laboratory and pilot scales.Ghiasi et al. [4] have used an electrical energy as heat source in an inclined bed dryer (IBD) and flatbed dryer (FBD) to assess the rice drying process.The findings of the study demonstrate that rice with a commercial quality index that is acceptable can be produced by using FBD and IBD.In addition, FBD-dried rice has less damage and more uniform.Wincy et al. [5] have dried rice using a flatbed dryer integrated with a biomass gasifier on a pilot scale.Exergy analysis showed that the reversible airflow flatbed drier with a biomass gasifier outperformed the traditional biomass burning dryer and can be used for large-scale rice drying.
In this study, we used the flatbed dryer integrated with biomass waste pyrolysis reactor as a hot air source for rice drying.The variations of tray height and drying air temperature were carried out to see the effect on changes in moisture ratio, drying rate, and rice drying characteristics.Finally, a kinetic study of rice drying was also carried out based on the height of the tray and the best drying air temperature.

Dryer apparatus scheme
This study used a flatbed dryer integrated with a pyrolysis reactor, as depicted in Figure 1.The two primary parts of this drying apparatus are the drying chamber and the pyrolysis reactor, which serves as the drying air supply.The drying chamber is equipped with a tray as a place for the sample, the lower part is for the entry of drying air, and the ventilation at the top is for the exit of drying air and moisture resulting from drying.The pyrolysis reactor is a tank made of stainless-steel material with a diameter of 80 cm and a height of 120 cm, which is used to produce charcoal and liquid smoke.The heat from the biomass pyrolysis process will heat the air around the reactor.This hot air is sent to the flatbed dryer.The drying air temperature setting is done by adjusting the gas stove regulator for 10 m/s airspeed to obtain the desired drying temperature (50, 60, and 70°C), then flow to the flatbed dryer.The drying air transfer process uses a blower (220 V, 650 W, 0-15,000 rpm) equipped with an air speed controller.

Drying procedure
This work was started by calibrating the drying temperature through adjusting the gas stove regulator at a drying air speed of 10 m/s to obtain the desired drying temperature.A total of 150 g of rice was put onto the tray.Next, the gas stove is turned on to heat the pyrolysis tank.The blower was turned on to circulate hot air from the pyrolysis process.The rice mass was weighed every five minutes along the drying process until it reached a constant weight.The drying process was carried out by varying the tray height (20, 40, and 60 cm) and the drying temperature (50, 60, and 70°C).

Study on the moisture ratio and drying rate
The study on the moisture ratio and drying rate was done using equation 1 and 2.

Moisture content analysis
As much as 20 g of fresh rice was weighed with aluminum foil of known mass, then dried using an oven at 105°C (±5 ⁰C) for 24 h.After 24 h, the rice was put in a desiccator, and then weighed.Rice moisture content was calculated using Equation 3.

Drying kinetic study
The drying kinetic model was studied using MATLAB based on the best tray height and drying air temperature.The data used were moisture ratio and drying time.The kinetic model evaluated were Newton, Page, and Handerson-Pabis kinetic model.

Results and discussion
Reducing the moisture level of rice is the aim of rice drying, as it improves rice quality and prolongs rice storage.The drying process determines how post-harvest rice is managed [6].Integration of drying equipment with biomass thermochemical equipment has been done, as demonstrated by Wincy et al. [5] to reduce the fossil energy consumption.In this paper, we present the performance of our flatbed dryer integrated with biomass pyrolysis reactor in rice drying process.

The influence of tray height on the moisture ratio
The influence of tray height on rice moisture ratio at various drying temperatures is presented in Figure 2. The moisture ratio tends to decrease by increasing drying time up to the equilibrium moisture content.At 50°C (Figure 2a), the equilibrium moisture content was reached after 75 minutes, 65 minutes, and 95 minutes for tray heights of 20 cm, 40 cm, and 60 cm, respectively.In Figure 2b, the equilibrium moisture content was reached after 65 minutes, 60 minutes, and 85 minutes for tray heights of 20 cm, 40 cm, and 60 cm, respectively.Last, the equilibrium moisture content was achieved after 55 minutes, 75 minutes, and 75 minutes when 70°C of drying temperature was applied.Figure 3 shows the drying conditions at various tray heights.The decrease in moisture ratio lasts longer at a height of 20 cm when compared to 40 cm.The hot air that hit the rice surface at 20 cm (Figure 3a) is less than at 40 cm.This happens because even though the hot air hitting the rice is wider than the height of the other trays (Figure 3c), the hot air output at a tray height of 60 cm is farthest so that the heat energy absorbed by the rice becomes less.To reach a tray height of 60 cm, hot air will spread into the drying chamber so that the water evaporated becomes less even though the drying process is more even.Thus, the best tray height is 40 cm.

Effect of tray height on drying rate
The effect of tray height on the drying rate at various drying temperatures can be seen in Figure 4. Generally, as the drying process progresses, the drying rate will reach a maximum point and then begin to decline.The longer the drying time, the slower the drying rate will be during the procedure.This is due to the fact that a longer drying period lowers the material's water content, making it more challenging to evaporate the water content [7].This phenomena takes place at all variations in tray height and drying temperature.At a drying temperature of 50°C (Figure 4a), the maximum drying rate achieved were 0.1283 g/cm 2 .min;0.2036 g/cm 2 .min;and 0.2163 g/cm 2 .minfor tray heights of 20 cm, 40 cm, and 60 cm.From Figure 4b, the maximum drying rate achieved were 0.2045 g/cm2.min;0.2682 g/cm 2 .min;and 0.2232 g/cm 2 .min.Last, when the temperature of 70°C was applied (Figure 4c), the maximum drying rate achieved was 0.3770 g/cm 2 .min;0.3234 g/cm 2 .min;and 0.1899 g/cm 2 .min.

Effect of drying air temperature on decreasing moisture ratio
Figure 5 shows the drying temperature's influence on the moisture ratio at a tray height of 40 cm.At the beginning of drying, the moisture ratio will decrease faster because the hot air will evaporate the free water contained in the rice.The decrease in the moisture ratio continues until the equilibrium moisture content is reached.In Figure 5, the decrease in moisture ratio at 70℃ is faster than at 60℃ and 50℃.Actually, the moisture content is significantly impacted by the drying temperature.A higher temperature led to a greater loss of moisture content, which in turn shortened the drying time.These might result from a faster rate of moisture removal from the sample caused by an increase in heat transfer between the sample's temperature and the air [8].

Figure 5.
Effect of drying air temperature on the decrease of rice moisture ratio at a tray height of 40 cm.

Effect of drying temperature on drying rate
The drying temperature's effect on rice's drying rate is illustrated through a graph of decreasing the drying rate of rice vs. drying time, as shown in Figure 6.Of all the drying techniques, hot air drying is the most popular.Numerous benefits include its great drying efficiency, ease of use, affordability, and minimal environmental impact [9].In the initial drying period, more water is carried by hot air from the rice to increase the drying rate.Then, the remaining water content in the rice will be less, so the drying rate will decrease.At 70℃, it takes 50 minutes to dry, while at 60℃ and 50℃, it takes 60 minutes and 65 minutes to dry.The primary factor accelerating the mass transfer of water from rice is the maximum heat transfer, which occurs at a higher drying temperature-in this case, 70 °C [10].

Drying characteristics
The drying characteristics of rice can be described through a graph of the relationship between the drying rate and the moisture ratio.Figure 7 shows the characteristics of rice drying at variations of drying temperature (7a) and tray height (7b).The drying rate continues to increase with the length of drying time.This increase continues until the maximum drying rate is reached, then the drying rate decreases.
The drying rate increases because the material's water content remains relatively high.Then the drying rate decreases until an equilibrium moisture content is reached.This is because the movement of water into the drying air comes from within the material, causing the drying rate to decrease.

Mathematical modelling of rice drying
Drying kinetics models is essential to estimate the performance of drying systems.The thin film drying equation can predict the general drying curve.In this investigation, a thin layer equation was used, where the thin layer drying process refers to the drying of particles or granules flowing with drying air [11].
Knowing the appropriate drying kinetics model for drying the product is necessary.In this study, the moisture ratio (MR) value at 70°C of air temperature and a tray height of 40 cm was used.Table 2 displays the drying kinetics model used, while Table 3 presents the curve-fitting results.

Figure 2 .
Figure 2. The effect of tray height on the decrease of rice moisture ratio at various temperature; (a) 50°C; (b) 60°C; (c) 70°C.

Figure 3 .
Figure 3. Drying scheme at tray height: (a) 20 cm, (b) 40 cm, and (c) 60 cm.The white-part represents the part of the tray that is exposed to the drying air.

Figure 6 .
Figure 6.Effect of drying air temperature on the rate of drying rice at a tray height of 40 cm.

Figure 7 .
Figure 7. Rice drying characteristics; (a) Effect of drying air temperature; (b) Effect of tray height.

Table 3 .
[12]linear regression results of drying kinetic model.The best kinetic model that describes the drying characteristics of rice was analysed by the R 2 (Coefficient of Determination) and RMSE (Root Mean Square Error) values.The best model was determined on the highest R 2 value and the lowest RMSE value[12].The results showed that the Page model shows the best fit with an R 2 value of 0.9912 and an RSME value of 0.0336.