Keywords

1 Introduction

Polyethylene terephthalate (PET) is a versatile synthetic polymer that can be thermoformed into various applications, such as containers, disposables, and meditative applications. In the report conducted by Recycling International, PET exploitation will surmount 20 million tons by 2021 and continuously increase per annum [1]. The mass production of PET waste is growing rapidly, with the upsurge of civilization, brisk industrialization, and deviations in the mode of living. Additionally, despite the versatility of PET, it has a fair share of adverse effect in the environment particularly its slow deterioration, causes severe problems in the locale and human health as a whole [2]. Numerous disposal techniques for PET are taken into account but dump or sanitary landfill discarding is the conventional enforcement for the disposal of plastic waste [3]. The dumping of waste plastics in landfills allows habitation of nonentities, stimulating a differing sort of diseases [4].

Moreover, the expense of transit or freight, manual labour, and repairs can escalate reprocessing expenditure [3]. Besides, as a result of fast-paced expansion, areas allotted for dumpsite or landfills, specifically in urban areas, is downsizing. Thus, a better approach and attested process is necessary to discard these PET waste, and the most feasible remedy is pyrolysis.

Pyrolysis refers to a thermochemical conversion of waste plastics without the presence of oxygen with a temperature ranging from 300 to 900 °C to convert plastic waste into liquid oil [5]. The utilization of catalysts helps to reinforce plastic waste's overall pyrolysis process and maximize process performance. Catalysts have a crucial function in fostering process performance, leveling precise output, and lowering operation’s temperature and residence time [6]. A broad category of catalysts is utilized in the pyrolysis of PET withal the foremost utilized catalysts are zeolite, ZSM-5 and Y-zeolite, FCC, and MCM-41 [7]. The catalytic reactivity during the process with the employment of homogenous catalysts might incorporate cracking, oligomerization, cyclization, aromatization, isomerization and the like [8].

Several articles documented the employment of micro structured and mesostructured catalysts for the energy recovery of PET plastic waste to convert it into liquid oil and activated char. One experiment pyrolyzed polyethylene (PET) plastics with HZSM-5 catalysts [9]. From that set-up it is observed that HZSM-5 improved oil yield formation with the conformation of aromatics and iso-alkanes compound. Gaca also conducted an experiment in which he pyrolyzed PET waste with altered MCM-41 and HZSM-5 and disclosed that with the application of HZSM-5 as catalyst yields lighter organic compounds (C3–C4) with higher aromatic compound [10]. Another study employed assorted types of catalysts and noted that merging HZSM-5 with SiO2–Al2O3 or MCM-41 resulted to greater liquid oil yield with lesser gaseous products [11]. Lastly, Aguado also conducted an experiment in catalytic pyrolysis of PET with HZSM-5 and observed the build-up of aromatics and other compounds Simultaneously, the application of mesostructured MCM-41 lowered the aromatics attained due to its minimal acid catalytic reaction. Indeed, employment of artificial catalysts improved general process outcome and the quality of products being produced [12]. Nevertheless, besides zeolite catalysts, numerous alternative catalysts are cost and process-efficient.

Catalysts such as zeolite, Ca(OH)2, and Fe2O3 have enhanced thermal productivity and modified pyrolytic product conformations [13]. These catalysts have unique catalytic properties that can maximize the generated oil and enrich product quality. However, they have pros and cons to be considered. Some catalyst may not be appropriate at elevated temperatures due to coke development, and some catalyst has high gaseous yield due to the reaction time [14]. Herein, the effect of catalyst on the pyrolysis of PET plastic waste is examined taking into consideration that the type of catalyst employed plays a critical role to establish the viability of the whole process.

In this study, Ca(OH)2, Fe2O3, and zeolite have been employed in the catalytic pyrolysis of PET plastics as to differentiate quantitatively and qualitatively the liquid fuel yield’s improvement concerning the catalyst used. This paper can be a future reference for PET pyrolysis-related studies. This study aims to determine the best suitable catalyst in PET plastics’ pyrolysis based on the overall parameters used in the study. Additionally, the paper focuses mainly on differentiating the employed catalyst’s effects in terms of the liquid, gas, and char yield in the process using a batch reactor. The gathered data can be used as a future reference for forthcoming studies relevant to PET waste's catalytic pyrolysis.

The scope of the study is only limited in studying the physical properties of product yield, namely the percent weight of liquid, gas, and char, as well as the liquid’s benzoic acid content, high calorific value, density, and the fire point and flashpoint of the liquid.

2 Material and Methods

2.1 Conceptual Framework

The study claims that the application of catalyst regarding its type in the PET plastic pyrolysis has exhibited varying results in terms of pyrolysis product properties and yield. The addition of such material provides changes in the operational process and improves the resulting output. The experiment consists of three types of variables (see Fig. 1). Parameters like temperature and pressure fall in the independent variables since they have a fixed value for the pyrolysis. On the other hand, dependent variables are parameters that change or are highly affected by the moderator's addition and the area of concern for this study. Lastly is the moderator variable, which alters the result and mainly influences the correlation between the two variables.

Fig. 1
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Conceptual framework

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2.2 Research Design

The research design employed throughout the study is the experimental method. This method is used to determine the significance and suitability of a particular catalyst in the pyrolysis of PET plastic in terms of the liquid properties (flash point, fire point, density, heating value) and the product yield, and the % of benzoic acid. Moreover, the catalyst’s effectiveness to acquire the desired results is based on the pyrolysis product of the plastic alone.

2.3 Research Material and Equipment

The materials used in experimenting are shredded waste PET plastics, empty glass containers, face masks, and an LPG tank. The catalyst used was purchase through online stores, and the obtained catalyst is 500 g Fe2O3, 500 g Ca(OH)2, and 500 g Zeolite, all manufactured by the Dalkem Corporation. The following are the equipment used in the process: condenser, digital thermometer, and the pyrolysis reactor itself.

Upon completing the experiment, the necessary materials used for the sample's testing are as follows: 0.2 M of NaOH, 0.3 g KHP, and distilled water. In acquiring the analysis of the product, specific equipment is utilized, namely the bomb calorimeter, to determine the high heating value, flash point apparatus for the flash and fire pint, pycnometer for acquiring the density, and analytical balance accurate mass measurement, and the burette.

2.4 Experimental Set-Up

The waste PET plastic bottles are collected on the school premises and in the neighboring environment or establishment. After gathering an adequate amount of PET plastic bottles, it is then washed, dry, and sorted out to separate it from its cup.

Moreover, three catalysts were used for the experiment. These are Ca(OH)2, Fe2O3, and zeolite. The catalyst utilized per batch of the experiment consists of five trials and another five trials for the plastic's thermal cracking alone. Both cracking was conducted in a similar operating condition.

2.5 Experimental Procedure

In this study, the collected waste PET plastic bottles are shredded before feeding it in the reactor. A sample of shredded plastic weighing 100 g was placed inside the reactor together with the weighted catalyst. The ratio of the PET to catalyst is 2:1.

The operating parameters are closely monitored to avoid unnecessary accidents or the occurrence of an unfortunate incident. The reactor was checked and ensured that it is tightly sealed before conducting the test and its other parts. Furthermore, as the reaction progress, the following parameters were measured as well: temperature, pressure, and the reaction. Moreover, the process has a residence time of 1 h.

The acquired gaseous product passes through a condenser and cools down by the water after that. The condensed gases were collected in a 500 ml glass container and analyzed using the bomb calorimeter and the flashpoint apparatus. The determination of the benzoic acid content of each sample was also carried out in the experiment.

Sample Preparation for Heating Value determination. The produced liquid product from pyrolysis, which contains some catalyst and the plastic alone, was placed in a tiny vessel and weighed using the analytical balance. The weighted liquid samples have a varying mass but not exceeding 1 g.

The samples gathered were brought to the University Laboratory and placed in the bomb calorimeter with a 2 L of water. A total of 20 testing runs were conducted for the heating value determination. After some time, the values were obtained.

Sample determination for Flash point and Fire point. The samples were tested in the University Laboratory using the flash point apparatus along with digital thermometer for temperature determination, and a burner. An amount of a liquid product is place in a metal cup, and as the temperature rises its temperature was measured as the burner was passed over its surface until some flash is observed and eventually the temperature in which the flame emerges. A total of 20 runs were conducted, five trials for each catalyst employed to increase the accuracy of the result. The flash point and fire point ranges from 28 to 39 °C and 39 to 57 °C respectively.

Sample Preparation for Benzoic Acid Content. The analysis of benzoic acid content of the samples was obtained by using the 0.2 M NaOH solution that was standardized using the 0.3 g of KHP. The standardization and the preparation of the solution were carefully prepared with a supervision of a professor.

After the solution was prepared, the sample was weigh using the analytical balance. The samples were titrated using the NaOH until a slight pinkish color can be observed. For this analysis a total of 20 tests were done. In the case of liquid with catalyst, the benzoic acid content ranges from 0.5 to 11% while the liquid with no catalyst have a content of 11–15%.

Sample Preparation for Density Measurement. The analysis of the density is done using the apparatus called the pycnometer that offers a high accuracy measurement of the said property of the liquid. The empty pycnometer was first place in an analytical balance to determine its weigh which was used as a reference in calculating the density of the sample. Thereafter, it was filled with water, place in an analytical balance and weigh again then emptied. Repeating the procedure for the sample with unknown density but now it was filled with liquid product. Lastly when the value for the weight of Pycnometer with liquid sample was on hand the density is acquired.

2.6 Statistical Treatment of Data

The data acquired was tabulated and analyzed using the calculated mean value of each sample which are presented in a graphical form. These values were used to determine how each catalyst induces changes to the properties and yield of the product. Subsequently, an interpretation is done to elaborate what the values imply.

3 Results and Discussion

3.1 Thermal and Catalytic Pyrolysis Test Result

The test results for pyrolysis of waste PET plastics having a particular type of catalyst and the one with no catalyst is shown in Table 1. It represents the mean value of the product in regards to its % benzoic acid, product yield, high heating value, fire and flash point, and density which are relevant parameters to display the influence of the catalyst.

Table 1 Overall data of the pyrolysis
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Percent Benzoic Acid Content and Product Yield. The percentage of the acid contain in the oil with and without catalyst (see Fig. 2). A 12.8% benzoic acid contains in the oil indicate that it is not recommended to process the given plastic mainly because it could clog piping and heat exchanger and if ever chosen it must be well-monitored. Another effect of the high amount of the acid is it troublesome for the transition of one process to another since the clog oil must be firstly removed that might cause problem. To address this, the catalyst is employed to compensate to the negative effect of the acid. As shown in the figure the catalyst can improve the process by minimizing the amount of the acid and also the damage it can do to the equipment. From the figure it can be clearly seen that the lesser the acid the better.

Fig. 2
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Weight percentage of the product yield

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The presence of benzoic acid in the liquid results in the oil's unfavorable use directly to the internal combustion engine. It is corrosive and may cause deterioration in the quality of fuel, and need serious attention is to be run on an industrial scale [15]. Nonetheless, the addition of catalysts can improve the pyrolysis oil.

Pyrolysis with Zeolite, Ca(OH)2, Fe2O3 have 10.51%, 10.45%, 1.13% benzoic acid respectively. It can be shown that catalyst has a visible effect on lowering the acid characteristic of the oil, decreasing the amount of acid by 0.2% for zeolite, 0.21% for Ca(OH)2, and 1.68% for Fe2O3.

PET plastic normally has a lower yield of liquid with higher yield of gas compared to other plastic. The experiment yield is only 39.6% which is quite low. However, the addition of catalyst could change the % composition of the product promoting an increase in liquid yield. As shown in the figure, with the presence of zeolite, Ca(OH)2 and Fe2O3 the % liquid increase to 69.4%, 70.6%, 62.2% respectively, showing that catalyst not only lower benzoic acid but also enhance the product yield.

PET plastic yield ranges from 23 to 40 wt% and because the experimental value is within range, it clearly shows that PET plastic is not suitable to be used as a material due to low production of the oil, but if gaseous product became a preference this type of plastic is of good choice. In contrast, the experimental yield especially the liquid with the presence of catalyst increases and having a % value more than the expected range. This indicates the high influence of the catalyst to assist the liquid production and improving the performance of the plastic. Additionally, if the PET is used and high liquid yield is desired, it is advisable to have a catalyst considering how distinguishable the value from the original one-plastic alone.

Density. The figure below (Fig. 3) shows the density of pyrolytic oil with and without the presence of catalyst. The density of none, zeolite, Ca(OH)2, Fe2O3 were 906.49 kg/m3, 887.36 kg/m3, 794.80 kg/m3 and 805.95 kg/m3 accordingly. The decrease in the density of oil means that the usage of catalyst is favorable since the value is approaching the commercial standard of gasoline and diesel which is 780 kg/m3 and 870 kg/m3 [16].

The presence of high density in fuel is advantageous since fuel consumption will be less. As shown in the figure the pyrolytic of oil without catalyst is quite high with a value of 906.49 kg/m3. However, it doesn’t necessarily mean it’s preferable since it is too high with respect to the commercial standard-too high or too low value is undesirable. In comparison to the oil without the catalyst having a value close to either the gasoline or the diesel and thus showing a desirable result if conventional fuel is replaced by the pyrolytic oil.

Fig. 3
figure 3

Density of the liquid yield

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High Heating Value. High Heating value refers to the energy content of the fuel [17]. It is an important property in assessing the quality of the fuel and a significant criterion that must be considered in utilization of energy. Figure 4 shows the sample's experimental calorific value without a catalyst is 7987.6699 while using zeolite, Ca(OH)2, and Fe2O3 the calorific value were 9710.0779 cal/g, 9284.0513 cal/g, and 8983.8340 cal/g respectively. The figure displays the increase in calorific value by applying catalyst, making it almost the same as the commercial-grade, which is 10152.89 cal/g for gasoline and 10,272.33 cal/g for diesel [16].

Fig. 4
figure 4

Experimental high heating value (HHV) of oil yield

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The heating value of pyrolytic oil ranges from 30 to 45 MJ/Kg and normally PET plastic have a low calorific value of less than 30 MJ/kg. The resulting value of oil without catalyst is higher than its normal value which is a good one but much lower to the prescribe range, indicating the low efficiency of the pyrolytic oil and limiting its usage as a fuel in engine. On the other hand, the pyrolytic oil with catalyst have a high value that is very close to the gasoline and diesel indicating its potentiality for application.

Flash point and Fire point. A characteristic that refers to the lowest temperature where fuel can disperse to stimulate the formation of an ignitable mixture in the atmosphere, which can ignite, is known as a flashpoint. Correspondingly, it is a vital factor for fuel characterization in assessing fire hazards. On the other hand, the degree to which a liquid fuel will burn for a particular time or sustain a combustion reaction is defined as the fuel’s fire point. The prior characteristic is accustomed in evaluating the hazard associated with its usage and storage. Typically, the flashpoint is much lower for any liquid oil than the fire point for about (5–10) °C.

The graph in Fig. 5. shows the important property of the oil acquired at constant temperature. For a diesel fuel its flashpoint value is 55 °C and a fire point 62 °C whereas, the kerosene has a value 38 °C and 46 °C. In terms of the pyrolysis oil its flash and fire point ranges of 30–39 °C and 39–55 °C with respect to the catalyst employed. As shown in the above figure, the oil without the presence of catalyst has higher flash point compared to the ones with catalyst. The flash point of pyrolysis oil with zeolite, Ca(OH)2 and Fe2O3 has comparable temperatures ranging 28–30 °C. A low value of the flash point specifies that a fuel is highly inflammable and the transportation and handling will need extra consideration. Withal, kerosene and diesel a fuel under light petroleum distillate have a higher flash point compared to the pyrolysis oil which specifies that these are easy to handle however additional precautions are still required.

Fig. 5
figure 5

Flash and fire point of oil yield

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4 Conclusion and Future Works

This study shows that the employment of catalysts influences the pyrolysis of PET plastics. It was observed in the test results of samples, it varies upon the type of catalyst used as well as with samples without catalyst. Yet, despite the variance, the improvement in the pyrolysis oil has been visible. Properties, such as benzoic acid, has been reduced to a significant value—indicating the effectiveness of the catalysts since the presence of acid greatly deteriorates the quality of oil due to its corrosiveness. Furthermore, PET plastics normally have low liquid yield with high gaseous product however, with the used of catalyst in the process, gaseous product decreases and the oil yield increases. In addition, the density decreases and the high heating value (HHV) increases compared to its original value of 7987.67 cal/g. Accordingly, catalyst shows prominent effect on PET plastics by providing huge impact in improving the overall quality of the oil and the process as well.

For further development of this study, the authors recommend to conduct the study in a different heating rate and in a different type of reactor as to investigate the difference of its effect in the process and in the product yield. Additionally, the authors recommend to examine the properties of gaseous product produce during pyrolysis since it could be a good potential for other energy source. Lastly, further analysis in the properties of the liquid yield can be done such as its determining its alkane content and the like.