(204h) Chemisorption of Carbon Dioxide On Chemically Treated Fly Ash Surface: Experimental and Kinetics Modeling

Reyad A. Shawabkeh

AICHE members

Department of Chemical Engineering

King Fahd University of Petroleum & Minerals

Dhahran, Saudi Arabia

rshawabk@kfupm.edu.sa

Fly ash from power generation plant was chemically treated with 30% ammonium hydroxide and used for adsorption of carbon dioxide from CO₂/N₂ mixture. After treatment the activated ash was tested for its surface area, pore size distribution, morphology and attached functional groups.  

The surface area and pore size were measured for both untreated and treated samples using N₂ adsorption at 77 K and showed an increase from 68 to 318 m²/g and from 0.036 to 0.678 cm³/g, respectively, with average pore diameter of 13.28.  SEM morphology illustrated porous, spherical particles with an average particle size of 50 µm.  FTIR analysis provided the existence of amine functional groups appeared at 3178, 2929 and 2770 cm^-1.

The treated fly ash was tested for adsorption of CO₂ using isothermal packed column and found that both chemical and physical adsorption of CO₂ takes place on the surface of the treated ash. The maximum adsorption capacity was 249 mg/g achieved when the initial concentration was 795 mg/L operated at 40 ^oC, 1 L/min, and 11% relative humidity. The isotherm data were fitted to Langmuir, Freundlich and BET model and found that BET best fit these data with R²= 0.97.

Effect of kinetic parameters on the rate of chemisorption such as initial concentration of CO2 (350 -760 mg/L), flow rate (0.3-1 L/min) and humidity (10% -80% RH) of the gas, temperature (-20-40 ^oC) were studied. Then a theoretical kinetic model was developed for the rate of chemisorption of carbon dioxide on chemically treated fly ash in packed bed column. It is developed based on assumption that surface reaction, adsorption and internal diffusion resistance are involved in the rate of chemisorptions.  The corresponding reaction constant and order, adsorption equilibrium constant and bed and particle diffusion coefficient were evaluated and found to be  (L.min/mol), 1.3, (L/mol.g), cm²/g, respectively.

Effect of temperature and nature removal of CO₂ by the treated ash showed an endothermic behavior with heat of adsorption of 178.5 kJ/mol, which revealed a chemisorption process.