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Mr. Nankwasa Crispuss*, Ms. Ogene Fortunate
Nankwasa Crispuss is a Chemical and Process engineer from Kyambogo University. His research interests focus on climate change mitigation by applying all waste management techniques and energy principles in finding solutions to existing problems. For his undergraduate thesis, he designed and fabricated a prototype pyrolysis batch reactor for recycling plastic waste materials into the oil.
Corresponding Author: firstname.lastname@example.org
Plastics, like other materials, have wide engineering applications. This is because of their lightweight and resistance to corrosion, among others. However, the amount of plastic waste generated every day poses a significant environmental threat. There is, therefore, a need to convert plastic waste into useful products using thermochemical techniques. Pyrolysis is a reliable method. However, to the author’s best knowledge, few pyrolysis batch reactor designs exist. During the present study, attention has been focused on designing and fabricating a prototype pyrolysis batch reactor. This is a laboratory-scale model that will serve as a baseline for developing technology for energy recovery from waste plastics. The reactor tank has been made of mild steel with a holding capacity of 1.2 × 10^(-2) m^3/batch of waste plastic. This was tested for five trials using 1000 g of LDPE plastics per trial. The temperature was controlled at 250, 350, and 450°C at an operating residence time of 40, 50, and 60 minutes. Test results showed that the highest amount of oil produced was 250 mL, while the lowest amount of oil produced was 132 mL at 450°C and 250°C. The lowest char quantity produced was 450 g, and the highest was 600 g at 450°C and 250°C, respectively. The highest (19.28%) conversion efficiency was achieved at 450°C, whereas the lowest (10.18%) was obtained at 250°C. Similarly, the waste reduction efficiency of waste plastics into oil increased as the external heat temperature was increased, with the highest value (55%) obtained at 450°C and the lowest (40%) waste reduction value obtained at 250°C. Based on the results of the study, the pyrolysis reactor was found to be more operational and functional at 450°C and 60 minutes of eternal heat temperature and residence time, respectively.
Keywords: Batch reactor, Feedstock, Pyrolysis, Plastic waste, Residence time
Plastics are synthetic or semi-synthetic materials that use polymers as the main ingredient. The plasticity of plastics makes it possible for them to be molded, extruded, or pressed into solid objects of various shapes. Over the past decade, plastic waste has become a major issue both on land and at sea. In a little over a century, plastic has gone from being hailed as a scientific wonder to being reviled as an environmental scourge. The global cumulative production of plastics since 1950 is forecast to grow from 9.2 billion tons in 2017 to 34 billion tons by 2050. Besides the sheer quantity of plastic being produced, one of the major challenges with plastic is its resistance to degradation. According to estimates, it takes an average of 500 years for a single plastic bag to completely biodegrade, resulting in them ending up in landfills or the natural environment. Not only are landfills an eyesore, but they are also incredibly bad for the environment because they release toxins and greenhouse gases into the earth and air. Therefore, it is urgent to “turn off the tap” regarding the production of virgin plastics, reduce the volumes of uncontrolled or mismanaged waste entering the oceans, and increase the level of plastic waste recycling, currently estimated at less than 10 percent.
Through the process of pyrolysis, the challenge of abhorrent plastic waste accumulation can be overcome by the subsequent conversion into liquid hydrocarbon fuels. This will reduce plastics in landfills, reduce emissions, and provide a reliable alternative to depleting fossil fuels. The general objective of the study was to design and fabricate a prototype pyrolysis batch reactor to produce oil from plastic waste. The specific objectives considered through the study included;
- To determine the effect of temperature on oil yield and char production.
- To determine the effect of residence time on conversion and waste reduction efficiency.
This study was conducted for a period of nine (9) months using Low-Density Polyethylene (LDPE) as the test material. This is because LDPE is easily available. The materials were collected from down Banda (Kasenyi), cleaned, and taken to Nelcon Plastics, Bweyogerere for shredding. The reactor was fabricated from the Nabisunsa stage welding station, in Kampala. The quantities of oil and char were weighed and measured by the Uganda Industrial Research Institute (UIRI).
2.0 LITERATURE REVIEW
Aswan et al.  redesigned a pyrolysis reactor prototype for the conversion of plastic waste into liquid fuel. The authors designed a pyrolysis reactor with a capacity of 5 kg. Tests were carried out at 200-400 °C temperature intervals and a range of 40-90 minutes. It was reported that the best product was 86.40% liquid yield at 350 °C with a processing time of 90 minutes. The authors continued to carry out ASTM distillation studies of the oil produced, which showed that 67% of the fraction was in the range of light naphtha, 12% of the fraction was in the range of heavy naphtha, and 21% of the fraction was in the range of medium naphtha. It was concluded that the physicochemical study of all the fractions reveals that the corresponding fraction has either the properties of gasoline, kerosene, or diesel oil and that the hydrocarbons were mainly paraffinic and olefinic, while some aromatic hydrocarbons were also detected but with no significant concentration.
Similarly, Istoto et al.  produced fuels from HDPE and LDPE plastic waste via pyrolysis methods. The authors carried out the production method of pyrolysis at temperatures of 450-621 °C without a catalyst. The results obtained from the experiments indicated that 5 kg of HDPE plastic waste yielded 3.25 liters of naphtha, 0.85 liters of gasoline, 0.325 liters of diesel fuel, and 18.06 grams of residues. The results further indicated that 5 kg of LDPE yielded 0.5 liters of naphtha, 2.9 liters of gasoline, 0.1 liters of diesel fuel, and 19 grams of residue. The conclusions from the study were that the composition of fuels from polyethylene pyrolysis was naphtha, gasoline, and residues.
Salih et al.  designed and fabricated a pyrolysis unit to process 2 kg of waste plastic. The authors obtained a noticeable amount of liquid hydrocarbons alongside gaseous products. The authors concluded that it is possible to produce fuel from plastic waste through pyrolysis.
Jayswal et al.  executed the design, fabrication, and testing of a waste plastic pyrolysis plant. The authors designed and modeled a plant in 3D CAD software, SolidWorks, and a batch reactor was employed to pyrolyze low-density polyethylene at a reactor base temperature of about 600 °C, and the vapor produced was directed to the horizontal, counter-flow shell, and tube condenser. It was reported that the yields depend on various factors like the plastic type used, the cracking temperature of the plastic, rate of heating, operation pressure of the reactor, type of reactor, residence time, use of catalyst, etc. It was further reported that from 10 kg of plastics, the plant yielded 6.63 liters of pyrolysis oil and 2.236 kg of char, on average, at the cost of 3.169 kg of LPG gas.
This section discusses the design dimensions of a batch pyrolysis reactor and the simulation of the pyrolysis process using Aspen Hysys V11 simulator. The effect of temperature and residence time on oil yield and char production was also determined experimentally.
3.1 Simulation of the pyrolysis process
The simulation of polyethylene was carried out in the Aspen Hysys V11 simulator to represent plastic pyrolysis. The simulation helped in the determination of the dimensions of the reactor as shown in Table 1. The reactor wall thickness was determined from mathematical modeling.
The components of the reactant and product stream were selected from the property environment of the component list section. The components were added to the component list based on the depicted reaction equation below.
The following components namely; Ethylene, Propylene, styrene, Methane, Ethane, Carbon, Hydrogen, n-Butane, Propane, Cyclopropane, n-Pcycpentan, 1-Hexadecene were selected.
The fluid package of the pyrolysis process was used to estimate the physical properties of each component implementing the Peng-Robinson model.
The development of the process flow diagram was accomplished by picking the parts of the conversion reactor from the palette of the simulator and connecting the input and output properly. The pyrolytic reactor which is the conversion reactor was picked from the column section of the palette. After developing the flow sheet as described above, it was simulated using a mass flow rate of 1 kg/hr. of waste plastics that is LDPE at the optimum temperature of 450 0C and a pressure of 1 atm as the feed to the pyrolytic reactor. The vapor-liquid fraction was exited from the top of the pyrolytic reactor, which contained petroleum products whereas; char residue was separated from the bottom of the reactor. This generated the reactor dimensions of diameter, height, and volume.
3.2 Experimental procedure.
Plastic wastes were collected, washed, dried then cut into small pieces for easy pyrolysis. 1000 g of shredded plastics was placed into the reactor and heated up to temperatures of 250, 350, and 450 0C in the reactor for a residence time of 1 hour. On the pyrolysis reactor, a temperature regulatory system consisting of a thermocouple, contactor, and a temperature controller was set up for setting the different desired temperatures.
The vapors coming from the reactor were condensed using air and steel condensing pipe to form a condensate or oil.
The condensed liquid fuel from the condensing tube was collected in the fuel collection bottle and measured.
3.3 Determination of the effect of temperature on the amount of oil yield and char production
1000 g of plastic waste for each of the 3 batches in the reactor was heated at different temperatures of 250, 350, and 450 oC while maintaining the residence time of 1 hour.
The temperature was varied using a contactor and temperature controller. The required temperature was set on the temperature controller and the contactor kept switching on and off in case the temperatures went below or above the set value respectively and the amount of liquid fuel and quantity of char produced at different temperatures were weighed.
3.4 Determination of the effect of residence time on oil yield and char production
1000 g of shredded plastic waste for each of the 3 batches was heated at 450 oC in a batch reactor at different residence times of 40, 50, and 60 minutes. A temperature of 450 oC was set on the temperature controller and the contactor kept switching on and off in case the temperatures went below or above the set value respectively and the amount of liquid fuel and quantity of char produced at different residence times were weighed.
4.0 RESULTS AND DISCUSSION
4.1 Batch pyrolysis reactor design dimensions and assembly
After sizing the reactor with the given conditions, the dimensions were obtained and presented in Table 1. The batch reactor was assembled as indicated in Fig 1.
Table 1: Design dimensions of a batch pyrolysis reactor
|Conversion reactor dimensions|
|Parameters||Cylinder vessel sizing|
|Reactor thickness (mm)|
The rector dimensions obtained from the Aspen Hysys V11 simulator were; diameter (0.2168 m), height (0.3252 m), and volume of 1.20E-02 m3 for the cylinder vessel. The reactor wall thickness was 2 mm, and an ellipsoidal head of 2 mm (Table 1).
Image 1: Complete assembly of the reactor and condensing system from the Nabisunsa stage welding station
4.2 Effect of varying temperature on oil yield and char production
The amount of oil and char produced from the pyrolysis of LDPE waste plastic at varying temperatures of external heat supplies is presented in Table 2.
Table 2: Amount of oil and char produced at different temperatures
|Temperature (oC)||Quantity of oil (ml)||Weight of oil (g)||Quantity of char (g)|
At 250 oC of external heat supplied to the reactor, 132 mL (101.772 g) of oil was produced while the amount of char produced was 600 g. At 350 oC , the amount of oil yield was 180 mL (138.78 g) and 520 g of char were produced (Table 2). The amount of oil produced at 450 0C was 250 mL (192.75 g) and the char quantity produced at this temperature was 450 g (Table 2).
The highest amount of oil produced was 250 mL while the lowest amount of oil produced was 132 mL at 450 oC and 250 oC (Table 2). The lowest char quantity produced was 450 g and the highest was 600 g at 450 and 250 oC (Table 2).
4.3 Effect of Temperature on conversion efficiency and a waste reduction Efficiency
As indicated in Figure 2, the conversion efficiency of oil from waste plastic increased with an increase in the amount of external heat temperature supplied to the reactor. The highest (19.28%) conversion efficiency was achieved at 450oC whereas the lowest (10.18%) was obtained at 250oC. Similarly, the waste reduction efficiency of waste plastics in oil increased as the external heat temperature was increased with the highest value (55%) obtained at 450oC and the lowest (40%) waste reduction value obtained at 250oC.
Figure 1: Effect of temperature on conversion efficiency and a waste reduction efficiency
Conversion efficiency is the ability of the equipment to convert waste plastic into oil in terms of weight. Based on the result of the study conducted by Rapsing , the waste plastic oil converter had a 78.1% conversion efficiency at 500oC which is not in agreement with the current study where our maximum conversion efficiency obtained at 450oC was 19.28% which is quite lower at relatively the same temperature of external heat supply.
Waste reduction efficiency is the measure of how efficient the equipment in reducing waste in terms of weight. As a result of the current study, it doesn’t agree with Rapsing  who reported the equipment to have shown 94.3% waste reduction efficiency at 500oC.
The increase in the conversion efficiency and waste reduction efficiency with an increase in the temperature of external heat supplied shows that these two parameters are dependent on the amount of heat supplied to the reactor during the pyrolysis process.
4.4 The effect of residence time on oil yield and char production
Table 3: Amount of oil and char produced at a varying residence time
The highest oil amount produced was 250 mL (192.75g) while the lowest was 185 mL (142.635g) at a residence time of 60 and 40 minutes respectively as shown in Table 3. The highest char quantity (512g) was produced at 40 minutes of residence time while the lowest was 450g produced at 60 minutes of residence time.
Figure 2: Effect of residence time on conversion efficiency and waste reduction efficiency
The conversion efficiency was lowest at 40 minutes while it was highest at 60 minutes of residence time. The waste reduction efficiency was highest (55%) at 60 minutes and lowest (48.8%) at 40 minutes (Figure 2).
The present study results do agree with Aswan et al.  who found out that the best product was 86.40% liquid yield obtained with a processing time of 90 minutes.
The results indicate that the pyrolysis process of waste plastic conversion into oil is dependent on the time of reaction. The higher the residence time, the more the pyrolysis reaction will take place effectively and efficiently to achieve a complete reaction for more oil yield and high waste reduction efficiency.
Based on the result of the study, the batch pyrolysis reactor was found to be more operational and functional at 450oC and 60 minutes of external heat temperature and residence time respectively. Using LDPE waste plastic as feedstock, the equipment was able to garner considerable waste reduction efficiency and a notable conversion efficiency.
The performance of the pyrolysis batch reactor is considered satisfactory. However, there is still a need to enhance its design to further improve its performance. The following recommendations are suggested:
The volume of the reactor tank should be increased to accommodate more amount of feedstock and a thicker material preferably stainless steel should be used in the fabrication of the reactor to withstand a higher temperature and prolong its usability.
Further studies can be conducted to determine the performance of the reactor using other types of waste plastic.
Studies on the chemical properties of oil recovered from LDPE and other types of plastic can be conducted to determine its properties and establish its potential uses or application.
Lastly after determining the possible uses or applications of oil derived from waste plastics, upgrading and pilot testing of the reactor must be conducted to validate the economic and environmental benefits that can be derived from it.
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