Isotherms and thermodynamics of CO2 adsorption on a novel carbon-magnetite composite sorbent

Significance 

Although methane and chlorofluorocarbons have a higher greenhouse effect per mass of gases, carbon dioxide is the main greenhouse gas owing to its abundance and high dumping rate into the atmosphere. Therefore, reduction of CO2 emissions has been a core topic of many international climate-concerned sittings and government sponsored research. Consequently, several promising techniques have been devised. In particular, carbon sequestration has been seen as the most viable and most efficient technique for carbon disposition. This technique involves capturing CO2 and storing it in underground caverns or utilizing it in its inert form.

Industries burning coal or other fossil-based fuels are the highest carbon emitters. As these industries were developed at a time when carbon emission was not a concern, they do not have the necessary carbon trapping systems in place. As of now, three major approaches are applicable, they include: post-combustion capture, pre-combustion capture and oxy-fuel combustion. Post-combustion is the most applicable as it can be applied without implementing radical changes on the existing plant architecture. For it to work, efficient gas separation measures are necessary. Among all the available alternatives, adsorption using solid sorbents is particularly attractive due to its low regeneration energy consumption, great capacity, selectivity, ease of handling over a relatively wide range of operating temperatures. In this framework, the equilibrium study is crucial for the design of any sorption system for separation of CO2 from flue gases and for the identification of the adsorption performance. In particular, in order to apply adsorption equilibrium data to a specific gas-separation application, an accurate mathematical representation of the adsorption equilibrium is required.

Recently, Federica Raganati, Michela Alfe, Valentina Gargiulo, R. Chirone and Paola Ammendola from the Combustion Research Institute – Italian National Research Council investigated the mechanism (isotherm), nature/strength (thermodynamics) of CO2 adsorption on obtained coating of a low-cost carbon black (CB) with magnetite fine (FM) particles composite in a lab-scale fixed bed reactor by performing dynamic breakthrough experiments. They also investigated the effect of both temperature and CO2 partial pressure. Their work is currently published in the research journal, Chemical Engineering Research and Design.

In brief, the research method employed commenced with the CB-FM testing at different adsorption temperatures (18–150 C) and CO2 partial pressures. Next, the researchers applied five adsorption models, namely: Langmuir, Freundlich, Sips, Toth and Dubinin–Radushkevich (D-R), so as to describe the measured CO2 adsorption isotherm in the low-pressure region typical of a combustion flue gas, thus evaluating the feasibility of adsorbate–adsorbent interaction. Lastly, they evaluated the most important thermodynamic properties, i.e. the standard Gibbs free energy, standard enthalpy changes and entropy change and isosteric heat of adsorption as a function of surface coverage.

The research team observed that all the models used were efficient in predicting the results. In particular, from the analysis of the evaluated error estimations, it was inferred that the Freundlich and Toth models were the most suitable ones for describing the CO2 adsorption process among the adopted two- and three-parameter models, respectively. In addition, the D–R model fitting allowed the researchers to obtain some preliminary information on the mechanism of CO2 adsorption on CB-FM, which they found to be neither purely physisorption nor purely chemisorption.

In summary, their study presented an in-depth evaluation of the mechanism and nature/strength of CO2 adsorption on CB-FM composite under fixed bed operating conditions. The obtained results showed that adsorption temperature and pressure had opposite effects on the thermodynamics of the process. Altogether, the CO2 adsorption capacity increases with increasing pressure and decreases with increasing temperature.

Reference

F. Raganati, M. Alfe, V. Gargiulo, R. Chirone, P. Ammendola. Isotherms and thermodynamics of CO2 adsorption on a novel carbon-magnetite composite sorbent. Chemical Engineering Research and Design, volume 134 (2018) page 540–552.

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