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http://hdl.handle.net/10266/3867
Title: | Development and Characterization of Nanostructured Carbon Adsorbents for Carbon Dioxide Capture |
Authors: | Goel, Chitrakshi |
Supervisor: | Bajpai, Pramod K. Bhunia, Haripada |
Keywords: | CO2 capture;mesoporous carbon;nanocasting;kinetics;isotherm;thermodynamic;CHED |
Issue Date: | 28-Mar-2016 |
Abstract: | The growing concern over global climate change because of increasing anthropogenic carbon dioxide (CO2) emissions has triggered the requirement to develop technologies for reduction of these emissions. Capture of CO2 by solid adsorbents has received attention because of its ability to reduce energy penalty associated with capture process. This calls for the development of adsorbent materials with high adsorption capacity, selectivity and fast kinetics coupled with good thermal and mechanical stability. Carbon based adsorbents have been extensively used for adsorption of CO2. Synthesis of carbon materials, having specific pore structure, from nanocasting technique have been reported by many research groups but their application to CO2 adsorption is very limited. The objectives of the present work are to develop nanostructured carbon adsorbents having specific pore structure, from nanocasting technique, for their application in CO2 capture under simulated flue gas conditions by dynamic method and to investigate adsorption kinetics, binary system adsorption equilibria and thermodynamics. In the present work, two different types of nanostructured carbons have been developed by nanocasting technique by using different starting materials. Two polymeric materials namely melamine-formaldehyde resin and resorcinol-formaldehyde resin have been synthesized and were used as polymeric precursors for nanocasting method with mesoporous silica as hard template. A series of adsorbents were developed by varying the carbonization temperature followed by their thorough characterization. Fixed-bed experiments were carried out, under different adsorption temperature and inlet concentrations, to evaluate their CO2 adsorption potential, selectivity and regenerability. Pure component adsorption isotherms for CO2 and N2 on the prepared carbons were also evaluated at four different temperatures ranging from 30–100 °C and were then correlated with three pure component adsorption isotherm models namely Langmuir, Sips, and dual-site Langmuir (DSL) models. Adsorption equilibria of binary CO2-N2 adsorption was then predicted by extending Sips and DSL equations empirically along with usage of ideal adsorbed solution theory and was compared with experimental data obtained from the breakthrough curves through various phase diagrams. Thermodynamic functions such as molar Gibbs free energy change, entropy change, and enthalpy change were evaluated numerically for pure component system. Finally, energy duty for desorption of adsorbed CO2 was also estimated. Mesoporous carbons having high nitrogen content were obtained from melamine-formaldehyde resin as polymeric precursor. Carbonization temperature controlled the textural parameters of nitrogen doped porous carbon adsorbents in consort with evolution of functional groups associated with oxygen and nitrogen heteroatoms. Carbonization at 700 °C produced porous carbons having maximum specific surface area of 266 m2 g-1 with nitrogen amount of 21 wt% and surface basicity of 4.07 meq g-1. X-ray diffraction analysis and transmission electron micrographs confirmed the development of nanostructured carbons with partial amorphous character. CO2 capture performance of the prepared carbons was investigated in a fixed-bed adsorption study set up at different temperatures (30 °C to 100 °C) under varying CO2 concentrations (5–12.5%). Multiple cycles of adsorption-desorption were also carried out to examine the reusability of the prepared carbons. Adsorbent synthesized at 700 °C exhibited the highest dynamic CO2 adsorption capacity of 0.83 mmol g-1 at 30 °C under 12.5% CO2 feed concentration in N2. Breakthrough time and CO2 equilibrium adsorption capacity were investigated from the breakthrough curves and were found to decline as the adsorption temperature increased. Experimental CO2 uptake data at all adsorption temperatures was fitted to three kinetic models with fractional order model giving the best fit to the experimental data over the entire range of adsorption with maximum error of 5% between data predicted by kinetic models and data obtained from breakthrough experiments. From three isotherms models used to analyze the equilibrium data of pure component system, Sips and dual-site Langmuir isotherm models presented a nearly perfect fit implying the heterogeneous adsorbent surface. But their extended forms along with ideal adsorbed solution theory failed in explaining binary CO2 adsorption equilibria. Thermodynamic parameters confirmed the feasibility of adsorption process and indicated the formation of more ordered configuration of CO2 molecules on adsorbent surface and hence exhibited higher heats of adsorption as compared to N2. Additionally, isosteric heat of adsorption was found to vary with surface coverage implying heterogeneous adsorbent surface with an average value of 17 kJ mol-1. Silica templated nanostructured carbons were developed from resorcinol-formaldehyde polymeric precursor by varying the carbonization temperature from 400 °C to 800 °C. Prepared carbons were characterized thoroughly for their textural, chemical and surface properties followed by dynamic CO2 capture performance at various adsorption temperatures from 30 °C to 100 °C under simulated flue gas conditions. Both the textural properties and surface chemistry had an effect on the CO2 adsorption performance of the prepared carbons. RF-700 exhibited the highest dynamic CO2 uptake of 0.761 mmol g-1 at 30 °C in a binary mixture of 12.5% CO2 in N2 attributing to well-developed porous structure and high surface basicity of 1.93 meq g-1. It also demonstrated high selectivity towards CO2 over N2 and stable adsorption capacity over multiple adsorption-desorption cycles. CO2 adsorption on prepared carbons was well described by fractional order kinetic model. Mixed-gas adsorption equilibria of CO2 and N2 on RF derived carbons could not be explained by various isotherm models and IAST which was due to dissimilar adsorbate molecules, adsorptive strengths and adsorbent heterogeneity. Total and partial adsorbed amounts and selectivity towards CO2 were highly under estimated by these isotherm equations. Thermodynamics of CO2 adsorption on carbon material suggested exothermic and spontaneous nature of the process. Thermal energy required for desorption of CO2 was also estimated to be around 1.9 MJ per kg CO2 with average isosteric heat of adsorption of 15.74 kJ mol-1. |
Description: | PHD, CHED |
URI: | http://hdl.handle.net/10266/3867 |
Appears in Collections: | Doctoral Theses@CHED |
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