in Wiley Online Library (wileyonlinelibrary.com).Thermogravimetric analysis is used to study the adsorption kinetics, equilibrium, and thermodynamics of CO 2 on immobilized polyethylenimine sorbent impregnated on a mesoporous silica over the range of 300-390 K and 5-100% CO 2 concentration. Adsorption isotherm models were fitted to the experimental data indicating that a change in adsorption mechanism occurred near 70 C. Below this temperature, the adsorption data followed the heterogeneous isotherms, while data taken at higher-temperatures followed isotherms for homogeneous surfaces. Heat of sorption was estimated to be 130 kJ/mole for the low-temperature regime, but this decreased to 48 kJ/mole above 70 C. The rate of CO 2 fractional uptake decreased as temperature increased. A phenomenological kinetic model was derived from the Weibull distribution function using a nucleation growth theory to describe the two-step process. The kinetic model was used to predict the uptake at different operating conditions and resulted in good agreement with experimental data. Published 2012 American Institute of Chemical Engineers AIChE J, 59: 923-935, 2013 Experiment MaterialsMethanol (reagent grade) and PEI (MW N 423) used in the preparation of the immobilized sorbents were purchased from Sigma-Aldrich Chemical and used without purification.

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Thermogravimetric analysis (TGA) of magnetite (Fe 3 O 4 ) oxidation was conducted at temperatures ranging from 750 to 900 °C over 10 oxidation cycles. Oxidation experiments were carried out in a continuous stream of air for period of 30 min. The oxidized magnetite (Fe 3 O 4 ), which resulted in formation of hematite (Fe 2 O 3 ), was then reduced by using continuous stream of CO (5% and 10%) with N 2 balance. The rate of oxidation was determined by the oxygen weight gain. Analysis of the data indicated that the oxidation behavior followed a two-stage process. The initial oxidation, which was very fast, took place in 2 min and was described using nucleation and growth processes with a low activation energy of about 4.21 ± 0.45 kJ/mol. As the reaction developed within the surface, oxygen transport through the product layer become the rate-controlling step with activation energy of 53.58 ± 3.56 kJ/mol.

Kinetics analysis of hematite (Fe 2 O 3 ) reduction by hydrogen was evaluated by the thermogravimetric analyser (TGA) in the temperature range of 700−950°C, using continuous streams of 5, 10, and 20% H 2 concentrations in N 2 . A number of kinetic models have been considered, including the single and multi-step models to describe the experimental reduction data. The details of nucleation and growth during the isothermal reduction are described in terms of the local Johnson−Mehl−Avrami (JMA) exponent and local activation energy. The variations of n values (JMA exponent) and activation energies for the reduction conversion indicate the presence of the multi-step reaction process. The reduction was shown to be one-dimensional (1D) growth with a decrease in the nucleation rate.

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The rate of the reduction reaction of a low cost natural hematite oxygen carrier for chemical looping combustion was investigated in a fixed bed reactor where hematite samples of about 1 kg were exposed to a flowing stream of methane and argon. The investigation aims to develop understanding of the factors that govern the rate of reduction with in larger reactors as compared to mostly TGA investigations in the literature. A comparison of the experimental data with a model indicated that reaction between the methane and the iron oxide shows multi-step reactions. The analysis also shows that the conversion occurs with a process that likely consumes all the oxygen close to the surface of the hematite particles and another process that is likely controlled by the diffusion of oxygen to the surface of the particles. Additional analysis shows that the thickness of the fast layer is on the order of 8 unit crystals. This is only about 0.4% of the hematite; however, it comprises about 20 to 25% of the conversion for the 10 min reduction cycle.

Iron oxide (Fe 2 O 3 ), known in its natural form as hematite, is potentially able to capture CO 2 through the chemical-looping combustion (CLC) process. Magnesium (Mg) is an effective methyl-cleaving catalyst, and as such it was combined with hematite to assess any possible enhancement to the kinetic rate of the reduction of Fe 2 O 3 with methane. Therefore, in order to evaluate the effectiveness of Mg as a hematite promoter, the behaviors of Mg-modified hematite samples (hematite−5% Mg(OH) 2 ) were assessed for any enhancement to the kinetic rate of the CLC process. The Mg-modified hematite was prepared by hydrothermal synthesis. The reactivity experiments were conducted in a thermogravimetric analyzer using a continuous stream of CH 4 (at concentrations of 5, 10, and 20%) at temperatures ranging from 700 to 825 °C over 10 oxidation− reduction cycles. The mass spectroscopic analysis of the product gas indicated the presence of CO 2 , H 2 O, H 2 , and CO in the gaseous product. The kinetic data obtained by isothermal experiments at the reduction step are well fitted by two parallel rate equations. The modified hematite samples showed higher reactivity than the unmodified hematite samples during reduction at all the investigated temperatures.