(514f) Gas Separation Properties of Poly(Ethylene) Terephthalate Derived Carbon Adsorbents

The method of developing new materials from waste for the purpose of solving environmental concerns, called the “waste-treat-waste” approach, is gaining popularity. In this study, we focus on producing effective carbon adsorbent from waste Poly(Ethylene) Terephthalate (PET) for separation of CO₂ from flue gas for the development of low emission, low cost carbon capture technology. Carbon adsorbent was produced from carbonizing waste PET followed by physical activation of the consequent solid product. The surface structure of PET derived carbon adsorbent was studied through SEM analysis and BET/BJH analysis. The adsorption equilibrium and breakthrough curve of CO₂ was measured using a dynamic adsorption system using multiple gas component to study the adsorption characteristics and adsorption capability of PET derived carbon adsorbent.

1. Introduction

Poly(Ethylene) Terephthalate (PET) bottle recycling in Japan is highly dependent on overseas factories located in Southeast Asia, which has caused extreme environmental and health impacts due to the insufficient recycling capability of these countries. Japan and other exporting countries need to decrease their dependency on exporting PET bottle wastes to Southeast Asian countries and that domestic circulation of PET bottle recycling needs to be reinforced to ensure the sustainability of PET bottles. One of the main reasons for the diminishing local PET recycling business is the treatment of non-recyclable PET bottles. They are bottles which are difficult to purify and separate, and therefore usually disposed as combustible waste. Non-recyclable PET bottles are the main cause of profit loss in the recycling industry. Therefore, incentives need to be taken to improve the material value of waste PET in order to boost up profit in the domestic recycling industry. This will lead to the decrease of dependency on exporting non-recyclable PET bottles to Southeast Asian countries. Our research group proposed a “waste-treat-waste” approach through upgrading non-recyclable PET bottles into activated carbon (AC) for the purpose of separating CO₂ from flue gas for the development of low emission, low cost carbon capture technology.

The aim of this study is to determine the gas separation properties of PET derived carbon adsorbents obtained through carbonization and activation of waste PET. The adsorption capability of CO₂ of carbon adsorbents were evaluated using a dynamic adsorption system and the breakthrough curve of multiple gas components were obtained. Ultimately, we aim to separate CO₂ from flue gas through pressure swing adsorption (PSA) process operated at ambient pressure, aiming at operating pressure less than 0.4 MPa-G using economically and environmentally friendly adsorbent with high regenerative and selective properties for negative carbon effect of greenhouse gas.

2. Materials and experimental procedures

Adsorbent used in this study is PET derived carbon adsorbent (PET-AC) obtained from carbonization and activation experiments. Carbonization was conducted using an autoclave batch reactor operated at 400-480ï‚°C and 120 min while activation was conducted using a furnace heated at 900ï‚°C under CO₂ atmosphere.

The adsorption equilibria of pure CO₂ on the adsorbents were measured using a batch system consisting of a 150 mL pending tank and 20 mL adsorbent tank. The system was equipped with two pressure indicators and a thermocouple to monitor the change in pressure and temperature in the system. The adsorbent was weighed to calculate the bulk density and void volume before supplied into the adsorbent tank. As pretreatment, the system was heated to 150ï‚°C and vacuumed for 1 hour to allow conditioning of adsorbents. The pressure was then increased in a step wise manner up to 0.4 MPa. The equilibrium adsorbed amount was determined when there was no change in pressure. All experiments were conducted at 20ï‚°C (293K).

The adsorption breakthrough experiment was conducted using a dynamic system consisting of a 150 mL pending tank, a 150 mL adsorbent tank, a mass flow indicator, two pressure indicators, a thermocouple and an outlet attached to a gas bag for gas sampling. Sample gas consisting of 20 vol% CO₂, 14 vol% CO, 6% H₂, 4 vol% CH₄ and N₂ (balance) was used in the breakthrough experiment. As pretreatment, the system was heated to 150ï‚°C and vacuumed for 1 hour to allow conditioning of adsorbents. Argon was then supplied to allow adjustment of pressure to 0.1 MPa and gas flow to 20 mL/min. Subsequently, while maintaining the initial pressure and flow rate, sample gas was supplied to initialize the breakthrough experiment. Gas collected in the gas bag was analyzed using a GC-TCD (Shimadzu GC-2014) after each interval. The amount of gas adsorbed, gas adsorption capacity, average gas adsorption amount and gas selectivity were calculated.

3. Results and discussions

Experimental isotherm data was obtained and fitted by Langmuir model. Maximum uptake of CO₂ was 2.76 mol/kg at 0.4 MPa and 293 K. Similar experimental procedure was conducted using commercial activated carbon (Shirasagi grade, Osaka Gas Chemical Co. Ltd., Japan) and commercial zeolite (5A, Fujifilm Wako Co., Japan). Maximum uptake of CO₂ measured for commercial activated carbon and commercial zeolite were 5.55 and 3.89 mol/kg, respectively.

Figure 1 shows the breakthrough curves of multiple gas components for PET-AC under a span of 140 mins. In the case of H₂, it can be seen that all H₂ gas reached breakthrough point in less than 1 min while reaching H₂ saturation point in less than 20 minutes. In the case of CO, CO gas for PET-AC reached breakthrough point in 10 min before reaching CO saturation point in less than 20 minutes. However, concentration ratio of CO overshoots to 120% due to the adsorption of other gas components (CO₂), causing a decrease in overall gas volume, thus increasing the molar fraction of CO gas species in the system (CO concentration at time t Initial CO concentration). With increasing run time, it can be seen that the overshoot of concentration ratio of CO decreases and returns to 100%. In the case of CH₄, we can see that PET-AC reached breakthrough point at 30 mins and reaches CH₄ saturation point at 40 mins. In the case of CO₂, PET-AC showed good CO₂ holding ability, with breakthrough point at 55 mins and CO₂ saturation point at 120 mins.

Overall, it is agreeable that PET-AC showed excellent gas adsorption properties, with relatively high ability to separate gas and high potential to be applied in the PSA system. Although the selectivity of gas in batch system is poor for PET-AC, by implementing the difference in gas species breakthrough point, we can separate gas species easily to obtain purified gas.

4. Conclusion

In this study, we have successfully produced high efficient low cost carbon adsorbent from waste Poly(Ethylene) Terephthalate (PET-AC). Results show that the maximum CO₂ uptake of PET-AC is comparable to the commercial activated carbon and zeolite. Multiple gas component breakthrough study of PET-AC shows that PET-AC showed excellent CO₂ holding ability that is suitable to be utilized in the PSA process for the effective separation and capture of CO₂.