Intrinsic kinetics of some high temperature gas-solid reactions

Abstract: Many materials processes are based on gas-solid reactions.An understanding of the mechanisms underlying these reactionsis essential for optimization of the process parameters andinvention of new process routes. Modelling of gas-solidreactions is made difficult in view of the complexitiesinvolved, like the chemical reaction including adsorption anddesorption, determination of the diffusion constants and masstransfer coeffrcients as well as the heat conductivity and heattransfer coeffrcients. The present doctoral thesis deals withthe experimental studies of a few selected reactions, which areof both theoretical as well as practical interest, with a viewto develop reaction models from the measured kineticparameters.The reactions studied in the present work are:SrcO3(s) = Sro(s) + CO2(g)BaCO3(s) = BaO(s) + CO2(g)FeCr2O4(s) + H2(g) = Fe(s) + Cr2O2(s) + H2O(g)FeTiO3+ H2(g) = Fe(s) + Tio2(s) + H2O(g)CoTiO,(s) + H,(g) = Co(s) + Tio,(s) + H,O(g)NiTiO3+ H2(g) = Ni(s) + Tio2(s) + H2O(g)The main experimental technique used in the present work isthe thennogravimetric analysis (TGA) coupled with differentialthermal analysis (DTA). The chemical reactions were highlightedin some of the experiments by conducting the reactions inshallow powder beds of fine particles, while the mass and heattransfer aspects were focused on in experiments with densecompacts. The TGA-DTA experiments were complemented by SEM-EDSand X-ray analysis and in some cases with BET measurements ofthe stiace area. In the case of decomposition of SrCO3the laser flash technique was used in order todevelop a model for the calculation of the effective thermalconductivity of porous SrC03and SrO.Decomposition of alkaline-earth carbonates: From the resultsof the measurements on the decomposition of fine synthetic SrC03powder, the activation energy for the reaction wasevaluated as 2 10 kJ'mol-1. The reaction rate was found to be unaffected bythe phase transformation occurring in the carbonate. Thedecomposition of porous pellets of SrC03was studied. The overall reaction rate wascalculated for a decomposing compact of SrC03based on the heat and mass transports and thechemical reaction. The kinetics of decomposition of BaCO, wasfound to be complicated as an eutectic melt was fornred at theinterfac; between BaC03and the BaO product. The activation energy forBaC03decomposition was found to be 305 kJ.mol-1.Reduction of Transition metal oxycompounds: In the case ofthe reduction of transition metal oxycompounds, FeCr2O4was found to be first reduced to iron and chromiumoxide before the latter was further reduced to chromium below1373 K. At higher temperatures, the reduction of Cr2O3appeared to occur before the fi.rll recovery ofiron.The reduction of FeCr,O, was found to proceed in two stepsbelow 1373 K, viz.FeCr2O4(s) + H2(g) = Fe(s) + Cr203(s) + H2O(g) Cr2O3(s) + 3H2(g) = 2Cr(s) + 3H2O(g)The activation energy for the reduction of FeCr2O4to Fe and Cr203was calculated to be 13 1 kJ.mol-1.The reduction studies of FeTiO3showed that, below 1186 K, the solid reductionproducts are Fe and TiO2. The activation energy for this reaction was foundto be 108 kJ.mol-1.At higher temperatums, TiO2was found to be reduced to TiO2Ti2O3.The activation energy for the reduction of CoTiO3by hydrogen to Co and TiO2was evaluated to be 15 1 kJ.mol-1and the activation energy for the reduction ofNiTiO by hydrogen to Ni and Tio, was evaluated to be 153 kJ.mol-1.A better fit of the experimental reaction rate data wasachieved by multiplication of AG0by 1/T which was plotted as the x-axis of theArrhenius plot.Key words:Chemical kinetics, decomposition, reduction,mass transfer, heat transfer, gas-solid reactions, SrC03, BaCO3FeTiO3,CoTiO3, NiTiO3, FeCr2O4, thermogravimetry, thermal diffusivity, Arrhenittsequation.