Methane decomposition over a Ni/Cu/Al 2 O 3 catalyst is studied in a two-stage fluidized bed reactor. Low temperature is adopted in the lower stage and high temperature in the upper stage. This allows the fluidized catalysts to decompose methane with high activity in the high temperature condition; then the carbon produced will diffuse effectively to form carbon nanotubes (CNTs) in both low and high temperature regions. Thus the catalytic cycle of carbon production and carbon diffusion in micro scale can be tailored by a macroscopic method, which permits the catalyst to have high activity and high thermal stability even at 1123 K for hydrogen production for long times. Such controlled temperature condition also provides an increased thermal driving force for the nucleation of CNTs and hence favors the graphitization of CNTs, characterized by high resolution transmission electron microscopy (HRTEM), Raman spectroscopy and XRD. Multistage operation with different temperatures in a fluidized bed reactor is an effective way to meet the both requirements of hydrogen production and preparation of CNTs with relatively perfect microstructures.

Production of hydrogen and carbon nanotubes from methane decomposition in a two-stage fluidized bed reactor

Publisher Summary Flow properties of the fluidized catalyst bed (FCB) are different from those of other conventional fluidized beds. The different treatment required is very significant for research and development on fluidized catalytic beds. This chapter discusses the factors affecting the flow properties. These properties include particle size distribution, also heat and mass transfer, and mixing properties. In the FCB, the dilute phase plays an important role in advancing the catalytic reaction when reaction rates are high. This factor provides a basis for identifying the appropriate reaction model and clarifying the effect of the dilute phase on selectivity and stability. The simplified theory for a bubble column is applied to understand particle circulation in the FCB. As a result, it is possible to analyze flow patterns of bubbles and “emulsion.” This chapter considers the uses of fluidized beds for catalytic reactions, using a viewpoint quite different from studies directed toward the physical handling of solid particles or toward gas–solid noncatalytic reaction. Particle size distribution has a great effect on most aspects of fluidization. Fluid beds are usually operated in the turbulent-flow region to obtain good contact and sufficient throughput of reactant gas. The uniformity and contact efficiency of a fluid bed can be improved by immersion of a surface within the bed.

Transport Phenomena and Reaction in Fluidized Catalyst Beds

This review is concerned primarily with the Lagrangian approach to mixing. First, the concepts of residence time and residence time distribution are introduced and defined. After a general treatment of this topic in the second chapter, the concept of micromixing is discussed in the third chapter with due emphasis on the application to chemical reactors. The final chapters extend the theory and applications to unsteady stale systems and to systems which are non-isothermal or non-homogeneous.

Residence time distributions and micromixing

Oxygen absorption from air into water and axial dispersion in the aqueous phase have been measured in a 5 cm diameter reciprocating plate bubble column. The volumetric mass transfer coefficients in semi-batch conditions were found to increase with agitation and were correlated with the specific power input and air flow rate. Under countercurrent conditions, it was found that axial mixing had little effect and conditions approached plug flow. The volumetric mass transfer coefficients were correlated with specific power input, air and water flow rates. Mass transfer coefficients were estimated using holdup and bubble diameter results. Comparison of the coefficients with the literature values indicated that the bubble surfaces were partially mobile.



On a mesure l'absorption de l'oxygene de l'air dans l'eau et la dispersion axiale dans la phase aqueuse dans une colonne a bulles a plateaux reciproques de 5 cm de diametre. On a trouve que les coefficients de transfert de matiere volumetriques dans des conditions semi-continues augmentent avec l'agitation; ces coefficients ont ete correles avec l'apport d'energie specifique et le debit d'air. On a montre qu'a contre-courant, le melange axial a peu d'effet et que l'on se rapproche des conditions d'ecoulement piston. Les coefficients de transfert de matiere volumetriques obtenus dans ce cas ont ete correles en fonction de l'apport d'energie specifique, du debit d'eau et du debit d'air. On a estime les coefficients de transfert de matiere a l'aide des diametres des bulles et la retention. La comparaison des coefficients avec les valeurs publiees indiquent que la surface des bulles est partiellement mobile.

Characteristics of a countercurrent reciprocating plate bubble column. II. Axial mixing and mass transfer

The use of residence time distribution analysis to characterize the liquid phase hydrodynamics in bioreactors is reviewed, with special emphasis on packed bed bioreactors. The importance of tracer selection, the method of data analysis and other uses for the data collected in RTD measurements are discussed.

Y.B.G. Varma

Papers

Axial mixing of continuous phase in reciprocating plate columns

A model for residence time distribution in multistage systems with cross-flow between active and dead regions

Residence time distribution of solids in multistage fluidisation

An experimental investigation of the residence time distribution of solids in multistage fluidisation

A stochastic model for residence time and contact time distributions of the gas in multistage fluidised beds