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On The Kinetics of Hydrogen Reduction of NiO-WO3 Precursors in Fluidized Bed- A Modeling Approach
KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Materialens processvetenskap.
KTH, Skolan för industriell teknik och management (ITM), Materialvetenskap, Materialens processvetenskap.
2011 (engelsk)Inngår i: ISIJ International, ISSN 0915-1559, E-ISSN 1347-5460, Vol. 51, nr 9, s. 1383-1391Artikkel i tidsskrift (Fagfellevurdert) Published
Abstract [en]

In the present work, Fluidized bed reduction of NiO-WO3 precursors was investigated isothermally at temperatures 973-1 273 K. The reaction progress was monitored by analysis of H2O evolved during the reaction process using a gas chromatograph instrument. A theoretical model based on intrinsic chemical reaction rate constants and thermodynamic equilibria was developed to estimate the apparent reaction rate constant for the reduction reaction. In developing the model, the particles are considered to be in a completely mixed condition and gas flow is described as plug flow. The proposed model is also suitable for scale-up calculations. The interfacial chemical reaction model was found to fit the experimental results. The apparent activation energy values of the reduction process at different stages were calculated accordingly. The present investigation proved that the fluidized bed technique can be successfully utilized in bulk production of intermetallics containing W and a transition metal (or a composite material) wherein the process conditions would have a strong impact on the particle size of the end product.

sted, utgiver, år, opplag, sider
2011. Vol. 51, nr 9, s. 1383-1391
Emneord [en]
Fluidized-bed, Gas-solid reactions, H2, Kinetics; Modeling, NiO-WO3
HSV kategori
Identifikatorer
URN: urn:nbn:se:kth:diva-26081OAI: oai:DiVA.org:kth-26081DiVA, id: diva2:369831
Merknad
Updated from submitted to published. QC 20120323Tilgjengelig fra: 2010-11-12 Laget: 2010-11-12 Sist oppdatert: 2017-12-12bibliografisk kontrollert
Inngår i avhandling
1. Investigations of the Kinetics of Reduction and Reduction/Carburization of NiO-WO3 Precursors.
Åpne denne publikasjonen i ny fane eller vindu >>Investigations of the Kinetics of Reduction and Reduction/Carburization of NiO-WO3 Precursors.
2010 (engelsk)Doktoravhandling, med artikler (Annet vitenskapelig)
Abstract [en]

Kinetic studies of reduction of the mixtures of NiO and WO3 having different Ni/(Ni+W) molar ratios in flowing hydrogen gas were investigated by means of Thermo Gravimetric Analysis (TGA), Fluidized Bed (FB) technique as well as Thermal diffusivity measurements under isothermal conditions. In the case of TGA, the reaction progress was monitored by mass loss, while evolved gas analysis by a gas chromatograph was the indicator of the reaction progress in the case of FB. The results indicate that the reduction reaction proceeds through three consecutive steps, viz.

NiO-WO3 Ni-WO3 Ni-WO2 Ni-W

The present results show that the fluidized bed technique can be successfully utilized in bulk production of intermetallics containing W and a transition metal (or a composite material) wherein the process conditions would have a strong impact on the particle size of the end product.

During the investigations, it was found that there was a delay in the reaction during the hydrogen reduction of NiO-WO3 mixed oxides in a fluidized bed reactor. In order to understand the same, a theoretical model was developed to estimate the apparent reaction rate constant for the reduction reaction from the intrinsic chemical reaction rate constant. Appropriate differential mass balance equations based on intrinsic chemical reaction rate constants and thermodynamic equilibria were developed. The proposed model was successfully applied in predicting the overall reaction kinetics of a fluidized bed reactor. This model is also suitable for scale-up calculations.

SEM images showed that the particle size of the final product was dependent on the Ni/(Ni+W) molar ratio; smaller particles were formed at higher nickel contents. X-ray diffractions of the reduced precursors exhibited slight shift of Ni peaks from the standard one indicating the dissolution of W into Ni.

A new method for studying kinetics of the hydrogen reduction of NiO-WO3 precursors was developed in which the reaction progress was monitored by following the change of thermal diffusivity of the precursors. Activation energies of reduction as well as sintering were calculated. This method is considered unique as it provides information regarding the physical changes like sintering, change of porosity and agglomeration along with the chemical changes occurring during the gas/solid reaction.

As a continuation of the kinetic studies, Ni-W-C ternary carbides were synthesized by simultaneous reduction–carburization of Ni-W-O system using H2-CH4 gas mixtures by TGA. The results showed that the reduction of the oxide mixture was complete before the carburization took place. The nascent particles of the metals formed by reduction could react with the gas mixture with well-defined carbon potential to form a uniform product of Ni-W-C. The above-mentioned experiments were conducted in such a way to ensure that the reaction was controlled by the chemical reaction. The activation energies of the reduction as well as carburization processes at different stages were calculated accordingly.

The present dissertation demonstrates the potential of the investigations of gas/solid reactions towards tailoring the process towards materials with optimized properties as for example introduction of interstitials. The present process design is extremely environment-friendly with reduced number of unit processes and the product being H2O.

sted, utgiver, år, opplag, sider
Stockholm: KTH, 2010. s. viii, 54
Emneord
Reduction, Reduction-Carburization, Gas-solid Reactions, Kinetics, Hydrogen, Methane, Thermogravimetric analysis, Fluidized bed, Simulation, Thermal diffusivity, Thermal conductivity, Laser flash
HSV kategori
Identifikatorer
urn:nbn:se:kth:diva-26015 (URN)KTH/MSE--10/51--SE+THMETU/AVH (ISRN)978-91-7415-774-1 (ISBN)
Disputas
2010-11-25, Salongen Lavoisier, Biblioteket, Osquars Backe 31, KTH, Stockholm, 09:00 (engelsk)
Opponent
Veileder
Merknad
QC 20101112Tilgjengelig fra: 2010-11-12 Laget: 2010-11-09 Sist oppdatert: 2012-03-23bibliografisk kontrollert

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