ENERGY AND RESOURCE RECOVERY FROM WASTE USING HYDROTHERMAL TREATMENT 1

Title

Energy and Resource Recovery from Tetra Pak Waste using Hydrothermal Treatment

Authors

Baskoro Lokahitaa, Muhammad Azizb, Kunio Yoshikawaa, Fumitake Takahashia

Affiliation aGlobal Engineering Course for Development, Environment, and Society, School of

Environment and Society, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku,

Yokohama-shi. 226-8502, Japan. bInstitute of Innovative Research, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro- ku, Tokyo 152-8550

*Corresponding author:

E-mail: [email protected]; [email protected]

Tel. +81-80-8851-5896

ENERGY AND RESOURCE RECOVERY FROM TETRA PAK WASTE USING HYDROTHERMAL TREATMENT 2 Abstract

Since the early 90s, many researchers have tried to improve the life cycle of Tetra Pak products by improving the process for Tetra Pak waste. In this , a novel method is proposed in which a hydrothermal process is utilized to produce solid fuel and recover the aluminum and composites. The experiment was done at the three different holding times between 0 and 60 min and temperatures between 200 and 240 °C. A total of nine experiments were conducted to understand the effects of holding time and temperature on the quality of solid fuel and composites. The results showed that hydrothermal treatment could effectively produce hydrochar, which is comparable to sub- bituminous coal after the aluminum part is removed. In addition, the aluminum and polyethylene composites were well formed. The holding time and temperature had a positive influence on the results of the analyses. As the carbon content increased, the high heating value (HHV) also increased, whereas the ash content decreased accordingly. The highest calorific value was found at an operating temperature of 240 °C and holding time of 60 min.

A Van Krevelen diagram showed that the main reaction during hydrothermal treatment was dehydration. Hydrothermal treatment can increase the calorific value of biomass from a Tetra

Brik by up to 25.22 MJ/kg, which is comparable to lignite and coal. In addition, the aluminum yield from the process was as much as 37%.

Keywords: Hydrothermal, Tetra Pak, recycling, waste-to-energy

ENERGY AND RESOURCE RECOVERY FROM TETRA PAK WASTE USING HYDROTHERMAL TREATMENT 3 1. Introduction

Packaging material has become an essential in every food and beverage industry. The idea of utilizing paper for liquid started in the early nineteenth century and was developed by applying different to improve its performance. Tetra Pak and Elopak are the major players in the paper packaging industry. In the beginning, only non-aseptic packaging was available on the market. Wax- was applied to store products for distribution. A problem arose because such products could not be stored for a long time, and there was a high risk of waste generation. By adding a polyethylene layer, aseptic packaging was introduced. This packaging could extend the of the product by several months. The development continued by applying an aluminum layer to enhance protection of the stored product [1].

Tetra Brik is one of the products from Tetra Pak that utilizes aluminum to provide a barrier against oxygen, loss of flavor, and light. It is the combination of polyethylene- and aluminum-coated that results in a six-layer composite [2]. paper uses virgin fibers, which are an excellent source of solid fuel. The increase in material complexity and high proportion of non-paper material have created challenges in the recycling of the Tetra

Brick product [3].

The recycling of Tetra Paks comes with the benefit of reducing the need for virgin material and reducing air pollution. Current recycling facilities use a modified paper mill to separate aluminum and low-density polyethylene (LDPE) from paper. The results from this process are a paper fiber and a compound of aluminum and polyethylene with an aluminum content up to 15% [3, 4]. This process is quite costly, and some research suggests that it is hard to make a profit by recycling the waste [5].

The compound of aluminum and polyethylene can be used as a material for rigid board manufacturing using a hot press [6, 7]. Murathan, et al. [8] tried to build low-density ENERGY AND RESOURCE RECOVERY FROM TETRA PAK WASTE USING HYDROTHERMAL TREATMENT 4 board by applying various resins to Tetra Pak waste. Zhang et al. [9] sought to recover the aluminum from the compound by dissolving the polyethylene in an organic solvent.

Thermal processes are one of the alternative and potential treatments for recycling

Tetra Pak waste. By adding cellulose to the polyethylene, both thermal and biological degradation can be improved [10, 11]. Haydary and Susa [12] investigated the thermal degradation kinetics of Tetra Pak waste and showed a similar result to the study done by

Lomakin et al. [11].

Some researchers are using a thermal process such as pyrolysis or combustion to volatilize the paper and polyethylene parts, and recover the aluminum [13–18]. The results yielded two different curves of a thermos gravimetric (TG) analysis, which indicate the degradation of cellulose and polyethylene. The addition of a catalyst could enhance the production of bio-oil during the pyrolysis process [19].

Among the available thermal processes, hydrothermal treatment is able to effectively convert cellulosic biomass, industrial waste, and municipal solid waste into homogeneous carbonaceous solids with higher calorific values [20–25]. Makela and Yoshikawa [25] investigated the dominant effects of hydrothermal operating conditions. The results showed that temperature has a major role in improving the quality of hydrothermal products.

Hydrothermal treatment can alter intrinsic characteristics of municipal solid waste to give better control of the combustion behavior [20]. In addition, burning hydrothermally treated products together with coal can improve the combustion rate, ignition, and devolatilization of coal [26].

This work presents a novel method for recycling Tetra Pak waste by using a hydrothermal process, which is able to break down paper and polyethylene. The process eliminates the effect of the polyethylene, and then separates the aluminum from the paper. The hydrothermal process also degrades the paper into a coal-like material with a high calorific ENERGY AND RESOURCE RECOVERY FROM TETRA PAK WASTE USING HYDROTHERMAL TREATMENT 5 value, which is excellent as a solid fuel. The quality of the solid fuel and aluminum produced during the hydrothermal process is assessed in this paper.

2. Materials and Methods

The process of solid fuel production is performed by removing the aluminum part from the paper part, and at the same time carbonizing the paper part. The detailed steps of the experiment are further explained herein.

2.1. Material

One Tetra Pak product, Tetra Brik was used in this experiment. Paper, polyethylene, and aluminum are the main constituents of Tetra Brik. They are arranged in six ordered layers: polyethylene, paper, polyethylene, aluminum, polyethylene, polyethylene. Paper is dominant as the main structure of Tetra Brik, comprising up to 75% of the total weight.

Polyethylene acts as an adhesive agent between the aluminum film and inner polyethylene layer, between the aluminum and paper layer, and encloses the outer . Polyethylene represents up to 20% of the total weight. Furthermore, aluminum, despite being only 5% of the product, plays a major role as a thermal stabilizer.

Samples were obtained from Tetra Brik refuse, which was collected by Tetra Pak

Japan. Tetra Pak Japan collects Tetra Pak waste from municipalities and brings it to its recycling plant. The samples were received in a clean, flattened condition. They were chipped into pieces approximately 1 × 1 cm using scissors and then dried overnight. After the drying process, the moisture percentage was calculated based on the final weight. Therefore, the process described in the next section used the dry mass of the samples.

ENERGY AND RESOURCE RECOVERY FROM TETRA PAK WASTE USING HYDROTHERMAL TREATMENT 6 2.2. Hydrothermal Treatment

The Tetra Paks were hydrothermally treated using a lab-scale reactor, series MMJ-

500, made by OM Lab-tech Co., Ltd., Japan. The reactor vessel was a with a volume capacity of 500 ml. A stirrer with an electric motor provided a centrifugation effect during the holding time. The temperature was controlled using a proportional–integral– derivative (PID) controller. The reactor was also equipped with a pressure gauge to monitor pressure changes during the reaction.

Nine grams of solid samples and 81 g of distilled water were used in each of the experiments. First, the materials were mixed in the reactor tube. The tube was then inserted into the reactor, and the reactor was tightly sealed. Then, the reactor was purged with argon gas to create an oxygen-free environment during the process. The stirrer, which operated at

400 rpm, was connected to the electric motor by a rubber belt. Next, the reactor was heated to the targeted temperature for the required holding time. Finally, the reactor was cooled down, and the product was discharged from the reactor.

Nine experiments, which consisted of three variations of temperature and time, were conducted in this study. The reaction temperatures were 200, 220, and 240 °C, and the holding times were 0, 30, and 60 min. By using this arrangement, we could understand the influence of the temperature and time parameters on the resulting product and observe the worst and best possible results. The coded experimental parameters are presented in Table 1.

Table 1 Coded experimental parameters

Time (minutes) Temperature (°C) Code 0 200 A-1 0 220 A-2 0 240 A-3 30 200 B-1 30 220 B-2 30 240 B-3 60 200 C-1 ENERGY AND RESOURCE RECOVERY FROM TETRA PAK WASTE USING HYDROTHERMAL TREATMENT 7 60 220 C-2 60 240 C-3

The final product was wet and consisted of hydrochar and a composite of aluminum and polyethylene. The liquid was removed using vacuum filtration. Then, the hydrochar and composites were separated using a 3-mm sieve. Next, the products were dried overnight in an oven at a temperature of 105 °C. Finally, the dried products were stored in a sealed to prevent contamination before further analysis.

2.3. Products Analyses

The hydrochars were analyzed for their thermal properties using ultimate and proximate analyses. The composites were analyzed for their aluminum content and density.

2.3.1. Proximate Analysis

The proximate analysis was performed using a Shimadzu D60 TGA/DTA analyzer.

About 10 mg of hydrochar was packed into the crucible and then fed into the analysis chamber. The chamber was purged with nitrogen for 5 min to create an inert atmosphere during the process. It was rapidly heated to 40 °C before the measurement began. During the measurement, samples were heated at a constant rate of 10 °C/min and held at 105 °C for 20 min to remove any moisture. Thermal decomposition of the volatile matter was observed when the samples were heated at a constant rate of 50 °C/min and held at 950 °C for 7 min.

Combustion of the final residue was done to measure the amount of fixed carbon and ash by holding it for 15 min at 950 °C while purging the analysis chamber with air.

ENERGY AND RESOURCE RECOVERY FROM TETRA PAK WASTE USING HYDROTHERMAL TREATMENT 8 2.3.2. Ultimate Analysis

The ultimate analysis of the hydrochars was performed using a Vario MICRO Cube elemental analyzer (Elementar, ) to obtain the composition of carbon, hydrogen, oxygen, and nitrogen. From those elemental measurements, the high heating value (HHV) and atomic ratio could be calculated. This was done using the formula used by Phichai et. al

[27] on a dry, ash-free basis. Furthermore, the atomic ratios of O/C and H/C in the product were used to build a Van Krevelen diagram to classify the quality of hydrochar based on the type of kerogen in the material. The major reaction during the hydrothermal process could also be seen from the changes in atomic ratio for each operating condition.

2.3.3. Aluminum Content

The aluminum content was measured by combusting the composite using a Shimadzu

D60 TGA/DTA analyzer. A differential thermal analysis (DTA) graph was used to determine the appearance of aluminum in the composite by observing the thermal behavior of the material. Aluminum is known to melt at a temperature of 660 °C [28], whereas polyethylene melts at 115–135 °C [29]. From the DTA graph, the melting point of a substance can be determined as the point of intersection of the leading edge of the melting peak with the extrapolated baseline.

A cross section of the composite was analyzed using scanning electron microscopy

(SEM) combined with energy dispersive spectroscopy (EDS) from JEOL Ltd. From the

EDS/SEM analyses, the mixture of metals and other non-metal substances could be understood from differences in the X-ray intensities reflected by the materials [30].

ENERGY AND RESOURCE RECOVERY FROM TETRA PAK WASTE USING HYDROTHERMAL TREATMENT 9 2.3.4. Density of Aluminum

A pycnometer was used to determine the density of solids by comparing the mass difference between liquids with and without the target product. The volume of the measured object was the difference between the volume of water that filled the empty pycnometer and the volume of water that filled the pycnometer containing the object. The density of the solid object was calculated by dividing its mass by its volume [31].

3. Results and Discussion

3.1. Characteristics of Raw Material

The raw material was tested for moisture content, and proximate analysis and

TGA/DTA analyses were performed. The moisture content was counted as 0.85% after overnight drying at 105 °C. The proximate analysis showed a volatile content of 82%, fixed carbon content of 6% and ash content of 12%. The source of volatiles and fixed carbon was the paper and polyethylene parts, whereas the source of ash was mainly the aluminum parts.

From the thermal gravimetric analysis (TGA) (Fig. 1), two mass degradation humps were detected at temperatures below 900 °C, which indicates the deterioration of paper and polyethylene parts. Paper is known to start degrading at temperatures above 200 °C, which corresponds to the first degradation region [32], whereas polyethylene degradation begins at temperatures above 400 °C, which corresponds to the second degradation region [33, 34].

The TGA curve obtained in our experiment shows results similar to the previous research

[12]. Three melting peaks were detected in the DTA graph: the first two peaks were from polyethylene and paper, whereas another peak, at 660 °C, was from aluminum. ENERGY AND RESOURCE RECOVERY FROM TETRA PAK WASTE USING HYDROTHERMAL TREATMENT 10

Fig. 1 TGA and DTA of Tetra Brik before hydrothermal treatment.

3.2. Hydrochar Characteristics

The composition of the elements in hydrochar was different for every operating condition. Table 2 summarizes the results of the proximate and ultimate analyses. The variation in the time and temperature shows a positive trend toward an altered carbon content and heating value. The percentage of volatile matter is decreasing because the decarbonization of polyethylene and paper has broken those materials down during the hydrothermal treatment.

Table 2 Proximate and ultimate analyses of hydrochar (percentage on dry basis)

Fixed HHV Code Volatile Ash C H N O Carbon (MJ/kg) A-1 94.8% 1.0% 4.2% 43.7% 6.2% 0.1% 42.9% 18.8 A-2 91.4% 3.6% 5.0% 42.9% 5.9% 0.2% 47.8% 18.6 A-3 82.2% 9.4% 8.4% 45.2% 5.7% 0.2% 44.9% 19.2 ENERGY AND RESOURCE RECOVERY FROM TETRA PAK WASTE USING HYDROTHERMAL TREATMENT 11 B-1 94.7% 1.8% 3.5% 43.2% 5.7% 0.1% 49.0% 18.6 B-2 87.4% 8.4% 4.2% 45.0% 5.9% 0.2% 44.8% 19.2 B-3 57.6% 39.5% 2.9% 59.9% 4.3% 0.2% 27.8% 23.2 C-1 92.3% 3.2% 4.5% 43.6% 5.9% 0.2% 44.8% 18.8 C-2 82.1% 14.7% 3.3% 46.0% 5.5% 0.2% 45.1% 19.4 C-3 59.8% 35.4% 4.8% 65.7% 4.3% 0.2% 27.5% 25.2

Similar to the previous research, the highest heating value at the highest operating condition was 25.2 MJ/kg, which is equal to sub-bituminous coal [21].

The major reactions during the hydrothermal treatment of biomass are dehydration and decarboxylation; therefore, higher times and temperatures can produce a coal-like material [35, 36]. The hydrothermal process has the potential to change the atomic ratio of

O/C and H/C, which is shown in the Van Krevelen diagram (Fig. 2). During the experiments, the lowest atomic ratio was observed at 240 °C for 30 min and 60 min, which is near the lignite region. On the other hand, other experimental conditions sit near the biomass region.

The changes in atomic ratio of these hydrochars confirm the reactions of dehydration and decarboxylation, which also happen during the process.

ENERGY AND RESOURCE RECOVERY FROM TETRA PAK WASTE USING HYDROTHERMAL TREATMENT 12

A-1 RDF C-1 Paper A-3 Wood

B-3 C-3

Lignite Atomic Atomic H/C Ratio

Atomic O/C Ratio

Fig. 2 Van Krevelen diagram of hydrochars.

3.3. Aluminum Composite Characteristics

A hard, robust, aluminum-rich material was formed during the hydrothermal treatment. The representative appearance of the aluminum composite can be observed in Fig.

3.

(a) (b)

Fig. 3 Composite appearance: (a) B-2 and (b) C-2.

ENERGY AND RESOURCE RECOVERY FROM TETRA PAK WASTE USING HYDROTHERMAL TREATMENT 13 Hydrothermal treatment is believed to be able to promote more dense materials compared with other recovery methods from previous research, which produced a sheet- or powder-like material [3, 17, 37]. The properties of these materials could be considered similar to those of aluminum dross, because the aluminum content in the composite was between 20% and 25%. The aluminum content for each experiment is presented in Fig. 4. On average, the experiment with the 30 min holding time yielded a higher amount of aluminum compared with other holding times. The highest aluminum content was found in the composite product from an experiment performed at 220 °C for 30 min.

26%

25%

24%

23%

Aluminum Aluminum Content 22%

21%

20% A-2 A-3 B-1 B-2 B-3 C-1 C-2 C-3 Experiment

Fig. 4 Aluminum content for each experiment.

The SEM/EDS and DTA analyses confirmed that aluminum was the only metal found in the composite. The sample products from the experiments conducted for 30 min (B-1, B-2, ENERGY AND RESOURCE RECOVERY FROM TETRA PAK WASTE USING HYDROTHERMAL TREATMENT 14 and B-3) were used in the SEM/EDS analyses because they yielded the highest aluminum content. In Fig. 5, the images from the analyses show that on the surfaces of the material cross sections, only carbon and aluminum were present. Aluminum occupies the smaller area but with a higher intensity, as seen in the high-saturation regions. Carbon, which occupies the wider area, behaves like the inverse of aluminum. However, its intensity is lower, as seen by the brighter hue in the low-saturation regions.

Aluminum Carbon

B-1

B-2

B-3

Fig. 5 SEM/EDS images of aluminum and carbon presence in experiments B-1, B-2, and B-3.

The DTA graph shown in Fig. 6 shows the similarity of thermal properties of the products among all of the experimental conditions. The peaks below 200 °C are the polyethylene melting condition. The downward bend occurring between 400 and 600 °C occurs because of the endothermic reaction from polyethylene decomposition. In addition, ENERGY AND RESOURCE RECOVERY FROM TETRA PAK WASTE USING HYDROTHERMAL TREATMENT 15 the endotherm peak around 660 °C occurs because of aluminum melting. Those spectra confirm the presence of aluminum and polyethylene in the composite.

Fig. 6 DTA spectra of aluminum composite.

Figure 7 shows the density of the composite, measured by pycnometer. The trend indicates that an increasing operating temperature also increases the density of the composite.

The highest density of 0.93 g/cm3 was obtained at a temperature of 220 °C with 0 min of holding time. The composite produced from a Brazilian paper mill also had a similar density of 0.9 g/cm3.

ENERGY AND RESOURCE RECOVERY FROM TETRA PAK WASTE USING HYDROTHERMAL TREATMENT 16

1

0.9

0.8

0.7 Density(g/cm3) Density(g/cm3) 0.6

0.5 A-2 A-3 B-1 B-2 B-3 C-1 C-2 C-3 Experiment

Fig. 7 Density of aluminum composite.

In this study, hydrothermal treatment showed promising results as a unique alternative for Tetra Pak waste recycling. The hydrochar can be used as a co-combustion material with other solid fuels, especially coal. In addition, the composites of aluminum and polyethylene have a high potential to be used as rigid board material or in aluminum refining.

4. Conclusions

This research investigated the possibility of hydrothermal treatment as an alternative for processing post-consumer Tetra Brik. The investigation was comprised of two parts: solid fuel properties and aluminum recovery. The investigation of the product characteristics proved that hydrothermal treatment can increase the carbon content of hydrochar by breaking down the paper parts of the Tetra Brik. Energy dispersive spectroscopy and DTA analyses showed that composites of aluminum and polyethylene were also formed.

As the carbon content increased, the HHV also increased, whereas the ash content decreased accordingly. The highest HHV was found at an operating temperature of 240 °C and holding time of 60 min. The HHV is important because it indicates the amount of energy ENERGY AND RESOURCE RECOVERY FROM TETRA PAK WASTE USING HYDROTHERMAL TREATMENT 17 in the fuel. A Van Krevelen diagram indicated that the main reaction during the hydrothermal treatment was dehydration because the trend line indicated the lowest atomic ratio at the highest operating condition. Hydrothermal treatment also decreased both volatile and ash content while raising the fixed carbon content. Hydrothermal treatment can also increase the

HHV of biomass from Tetra Brik by up to 25.22 MJ/kg, which is comparable to lignite and coal. Furthermore, the aluminum yield from the process was as much as 37%.

As current Tetra Pak recycling treatments cannot generate a positive income without incentives, hydrothermal treatment is a promising technology for recycling Tetra Pak waste.

In addition to its simplicity, hydrothermal treatment requires less capital cost. Currently, several countries also give higher incentives for renewable energy plants, including waste.

However, the practical application of hydrothermal treatment as a Tetra Pak recycling process still needs further investigation. The binding behavior of composites and polyethylene removal should be studied to obtain a better product. The effect of the velocity gradient made by the stirrer in the reactor would also be an interesting subject for further investigation. The hydrochar, which has coal-like properties, could be used as a refuse-derived fuel (RDF) for co-firing with coal. Finally, there is the possibility of recovering metal from the composite using the same method that is used on aluminum dross. ENERGY AND RESOURCE RECOVERY FROM TETRA PAK WASTE USING HYDROTHERMAL TREATMENT 18 Acknowledgment

The authors acknowledge the support from Indonesian Endowment Fund for Education

(LPDP) for the research.

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