Tue Johannessen

Computational fluid-particle dynamics for the flame synthesis of alumina particles

Abstract A mathematical model for the dynamics of particle growth during synthesis of ultra fine particles in diffusion flames is presented. The model includes the kinetics of particle coalescence and coagulation, and when combined with a calculation of the temperature, velocity and gas composition distribution in the flame, the effluent aerosol characteristics are calculated. The model is validated by comparison with an experimental study of the synthesis of alumina particles by combustion of Al-tri-sec-butoxide. Two parameters of the coalescence kinetics are estimated by regression of the model predictions to the measured specific surface area of the product particles. The estimated kinetics can be used to predict the surface area and shape of the particles for a wide range of synthesis conditions.

Metal ammine complexes for hydrogen storage

The hopes of using hydrogen as an energy carrier are severely dampened by the fact that there is still no safe, high-density method available for storing hydrogen. We investigate the possibility of using metal ammine complexes as a solid form of hydrogen storage. Using Mg(NH3)6Cl2 as the example, we show that it can store 9.1% hydrogen by weight in the form of ammonia. The storage is completely reversible, and by combining it with an ammonia decomposition catalyst, hydrogen can be delivered at temperatures below 620 K.

Computational analysis of coagulation and coalescence in the flame synthesis of titania particles

Abstract A method of combining computational fluid dynamics with a mathematical model for the particle dynamics has been applied to simulate experimental data from the synthesis of TiO 2 -particles in diffusion flames. Parameters of the coalescence kinetics are estimated by fitting the model predictions to the measured specific surface area of the product particles. The estimated kinetics can be used to predict the surface area and aggregate structure of the particles for a wide range of synthesis conditions. The regular equation for the rate of coagulation is modified to take into account the effect of dilution. The accuracy of the results, especially the degree of aggregation, i.e. the aggregate size, is highly dependent on the inclusion of this effect. When the dilution is accounted for, the predicted aggregate sizes (numbers of primary particles per aggregate) compare well with reported data based on small-angle X-ray scattering measurements.

Preparation of ZnO–Al2O3 Particles in a Premixed Flame

Zinc oxide (ZnO) and alumina (Al2O3) particles are synthesized by the combustion of their volatilized acetylacetonate precursors in a premixed air–methane flame reactor. The particles are characterized by XRD, transmission electron microscopy, scanning mobility particle sizing and by measurement of the BET specific surface area. Pure (γ-)alumina particles appear as dendritic aggregates with average mobile diameter 43–93 nm consisting of partly sintered, crystalline primary particles with diameter 7.1–8.8 nm and specific surface area 184–229 m2/g. Pure zinc oxide yields compact, crystalline particles with diameter 25–40 nm and specific surface area 27–43 m2/g. The crystallite size for both oxides, estimated from the XRD line broadening, is comparable to or slightly smaller than the primary particle diameter. The specific surface area increases and the primary particle size decreases with a decreasing flame temperature and a decreasing precursor vapour pressure. The combustion of precursor mixtures leads to composite particles consisting of zinc aluminate ZnAl2O4 intermixed with either ZnO or Al2O3 phases. The zinc aluminate particles are dendritic aggregates, resembling the alumina particles, and are evidently synthesized to the full extent allowed by the overall precursor composition. The addition of even small amounts of alumina to ZnO increases the specific surface area of the composites significantly, for example, zinc aluminate particles increases to approximately 150 m2/g. The gas-to-particle conversion is initiated by the fast nucleation of Al2O3 or ZnAl2O3, succeeded by a more gradual condensation of the excess ZnO with a rate probably controlled by the cooling rate for the flame.

Mixed Phase Pt-Ru Catalyst for Direct Methanol Fuel Cell Anode by Flame Aerosol Synthesis

A spray-flame aerosol catalyzation technique was studied for producing Pt-Ru anode electrodes for the direct methanol fuel cell. Catalysts were produced as aerosol nanoparticles in a spray-flame reactor and deposited directly as a thin layer on the gas diffusion layer. The as-prepared catalyst was found to be a mixture of nanocrystalline, mostly unalloyed Pt and an amorphous phase mostly of Ru and to a lesser extent of Pt oxides on top of the crystalline phase. The flame-produced Pt1Ru1 demonstrated similar onset potential but -60% higher activity compared to commercially available Pt1Ru1/Vulcan carbon. The kinetics of methanol oxidation on the mixed phase catalyst was also explored by electrochemical impedance spectroscopy.

The dynamics of aerosols in condensational scrubbers

A mathematical model for the simulation of the dynamics of aerosol change in condensational scrubbers and scrubbing condensers is proposed. The model is applicable for packed column gas/liquid contact when plug flow can be assumed. The model is compared with experimental data for particle removal in a pilot plant condensational scrubber. The model can satisfactorily predict particle growth and particle deposition by diffusional, convective and inertial mechanisms for a wide range of conditions. The parameters of principal importance for the model precision are identified and a procedure for their estimation is proposed. The behaviour of scrubbers and condensers for some important technical applications is demonstrated by model simulations.

The Formation of Porous Membranes by Filtration of Aerosol Nanoparticles

Flame-generated aerosol particles of Al2O3 were deposited by gas filtration on two types of porous and ceramic tubes of α-Al2O3 with mean pore diameters of 450 and 2700 nm, respectively. The particles were aggregates with average mobility diameters in the range of 30–100 nm and primary particle diameters of 4–8 nm. The particles are characterized by differential mobility analysis, transmission electron microscopy, and by their specific surface area. The deposited membranes are characterized by gas permeability measurements, scanning electron microscopy, and by their pore size distribution from nitrogen capillary condensation. The particles form a distinct, homogeneous membrane layer with a porosity of ∼90% on top of the substrate surface and only penetrate slightly into the substrate structure. The mean pore sizes of the deposited membranes determined by nitrogen condensation agree approximately with those determined by gas permeation and the specific surface area. The mean pore diameter varies in the range of 30–70 nm. The gas permeability of the deposited membranes is related to the specific surface area but influenced by the high porosity. The mean pore size and the permeability of the membranes are almost independent of the substrate structure.The development of a membrane with uniform properties is preceded by a short initial period in which the deposited particles, with an equivalent membrane thickness of roughly 2 μm, have a significantly lower permeability than the ultimately developed uniform membrane layer. This effect is particularly significant for the aerosol particles with the lowest mean size, probably due to particles deposited in the pore mouths of the substrate.The particles and the deposited membranes are X-ray amorphous but retain their specific surface area on heating to even high temperatures. When the membranes are heated to 1473 K for 10 h, X-ray diffraction shows a mixture of θ- and α-alumina, accompanied by a partial disintegration of the membrane and a considerable loss of surface area.

Catalyst dynamics: consequences for classical kinetic descriptions of reactors

Abstract The modelling of catalytic reactions/reactors has undergone great improvements since the introduction of empirical power-law kinetics in chemical reaction engineering and micro-kinetic models based on insight into the nature of elementary steps have appeared for many reactions. However, recent in situ studies and surface science investigations has brought added attention to the fact that catalysts may behave in a dynamic manner and reconstruct depending on the reaction conditions. This feature severely limits traditional kinetic descriptions. In the present paper, we present examples of the dynamical behaviour of some catalytic systems and discuss the corresponding limitations in existing models for catalytic reactions and reactors. Catalytic reactors operated in non-steady-state are becoming more frequent in industry. The additional efforts needed to accurately simulate these types of reactors are discussed. Finally, we discuss the role of computational fluid dynamics (CFD) as a tool for detailed simulation of catalytic reactors.

Synthesis of nano-particles of ZnO/Al2O3 in a premixed flame

Abstract Materials with very high specific surface areas can be synthesized by combustion of volatilized precursors together with an additional fuel like methane, hydrogen etc. The structure and properties of the product powder can be varied depending on the temperature and particle residence time in the high temperature zone and on the precursor concentration. The vapour phase synthesis of zinc and copper oxide composites by combustion in a premixed flame of aluminum acetylacetonate and zinc acetylacetonate is investigated. The laboratory setup consists of a burner where premixed methane, air and precursor vapours are combusted above a flame arrestor and saturation units where the precursors are evaporated into a nitrogen stream. This burner setup provides a very homogenous flame environment and ensures a low precursor concentration in the flame zone. The synthesized particles are collected on polycarbonate filters and are characterized by their specific surface area, aggregate size distribution, their morphology as observed by TEM and by their atomic structure and crystalline dimensions by x-ray diffraction. This talk will focus on the relationships between the particle synthesis and processing and the morphology of the product particles. The composite particles consist of zinc oxide deposited on top of an γ-aluminum structure probably due to the different vapour pressures of the oxides. Thereby the alumima serve as a structural support for a high surface area zinc oxide powder. The specific surface areas are 25 m 2 /g and 200 m 2 /g for pure zinc oxide and pure alumina respectively and the surface area of the composite powders varies within these limits.