Carbon dioxide capture by carbon adsorbents derived from waste plastics

Abstract

In the present era, one of the major threats to environment is climate change and the well known and accepted basis for this is greenhouse gas (GHG) emissions. CO2 is the key contributor promoting global warming and subsequently affecting the climate as manifested by frequent occurrence of floods and droughts, progressive rise in temperature and sea levels, heat waves and melting of glaciers. To economically sequester CO2, it is imperative to comprise cost-effective capture for which the adsorption-based post-combustion CO2 capture has been identified as a promising alternative due to its lower energy requirements, cost-effectiveness, and simplicity in use at a broad range of temperatures. However, the main challenge in successfully commercializing adsorption technology is the development of effective and low-cost adsorbents. Further, plastic waste generation is another major problem that is causing a deleterious effect on our environment. It is estimated that approximately 15,000 tonnes of plastic wastes are generated every day in India and polyethylene terephthalate (PET) waste takes around 500-700 years to biodegrade, therefore environmentalists are seriously concerned about its disposal. Literature studies indicate that carbons from PET wastes especially chemically activated ones have immense potential to adsorb CO2 due to generation of micro/meso pores post carbonization. Also, it has been observed that majority of the CO2 capture studies on PET-based adsorbents have been performed thermogravimetrically which is a suitable method for preliminary studies and cannot be implemented on large scale at industry level. Moreover, in this method adsorption is measured on weight gain basis which may involve higher component of error due to adsorption of other gases. Therefore, there is a need to perform CO2 adsorption studies in a packed column which can be scaled up. Hence, in current study our aim is to prepare porous carbons from PET, post-consumer plastic waste and evaluate its CO2 adsorption capacity under dynamic conditions by performing breakthrough experiments. O-enriched porous carbonaceous adsorbents have been developed from low cost, abundantly available polyethylene terephthalate (PET) waste with high carbon content by directly carbonizing at different temperatures (500 to 800 °C) and then chemically activating using variable impregnation ratios of KOH to carbon (1 to 4). The prepared carbon adsorbents were characterized for their textural and surface chemical properties using nitrogen sorption, CHN, FTIR, XRD, SEM, HRTEM, TPD, and XPS techniques. Further, to assess their CO2 adsorption-desorption performance under dynamic conditions, breakthrough experiments were conducted in fixed-bed adsorption set up. Porous carbon obtained at 700 °C with KOH to carbon mass ratio of 3 (Act-3-700) exhibited best textural properties with BET surface area of 1690 m2 g-1 and micropore volume of 0.78 cm3 g-1 and showed highest CO2 uptake of 1.31 mmol g-1 at 30 °C and 12.5% CO2 concentration. Five adsorption-desorption cycles establish adsorbent’s remarkable stability and regeneration. The kinetics of adsorption process was studied using Lagergren’s pseudo-first-order model, pseudo-second, and fractional order, Elovich, and Avrami models. Among these five models, fractional-order model best explained the CO2 adsorption kinetics. Further, three isotherm models were used to analyze the equilibrium data and Freundlich isotherm showed a superior fit, thus confirming the heterogeneous nature of adsorbent’s surface. Negative values of Gibbs free energy (ΔG°) and adsorption enthalpy (ΔH°) indicate the spontaneous and exothermic nature of adsorption process. Thermal energy needed for desorbing CO2 from the activated adsorbent was found to be 1.34 MJ per kg CO2, which corresponds to ~11.84% energy penalty. The average value of isosteric heat of adsorption was estimated to be 12.08 kJ mol-1, indicating predominance of physisorption. Adsorption column breakthrough profiles were modeled using Linear Driving Force (LDF) approximation for mass transfer. Model equations were solved using the Method of lines in MATLAB environment. Results show that the model closely approximated the column breakthrough curves for different CO2 concentrations (5%-12.5%) and different adsorption temperatures (30-100°C).