Kaihu Hou

The kinetics of methane steam reforming over a Ni/α-Al2O catalyst

Abstract Experiments have been carried out to study the kinetics of the methane steam reforming, accompanied by the reverse water gas shift reaction over a commercial Ni/α-Al2O3 catalyst in an integral reactor under conditions of no diffusion limitation. The experiments demonstrated that both CO and CO2 are formed as primary products, and the rate of methane disappearance is proportional to the partial pressure of methane at low product concentrations. The effect of total pressure on initial reaction rates indicated that the rate controlling steps of steam reforming are surface reactions between adsorbed species. Six possible reaction mechanisms were considered in detail, and intrinsic rate equations were derived by using the Langmuir–Hinshelwood–Hougen–Watson (LH–HW) approach and Freundlich’s adsorption concept. Applying the method of parameter estimation and model discrimination, a satisfactory model of intrinsic kinetics for methane steam reforming over the catalyst used was determined. Good agreement was obtained between the experimental data and results predicted from the kinetic model.

The effect of external mass transfer, competitive adsorption and coking on hydrogen permeation through thin Pd/Ag membranes

The effect of external mass transfer, competitive adsorption and coking on hydrogen permeation through thin Pd/Ag membranes has been evaluated by means of comparisons between experimental and predicated data at temperatures of 548–723 K and a total pressure of 2 bar. Predicted data were obtained by solving a mathematical model taking into account the effect of external mass transfer on hydrogen permeation only and verified from permeation experiments with a mixture feed of H2–N2. It was found that steam and CO have significant inhibitive effects on hydrogen permeation through the membrane due to their competitive adsorption with hydrogen on the metallic surface at temperatures less than 623 K, whereas, CO2 displayed only slight inhibitive effect at these temperatures. The effect increased with the amount of steam, CO or CO2 in the mixed feed, and decreased with temperature. Only very slight effects from steam and CO were noted at temperatures higher than 673 K. Experiments with a mixed feed of hydrogen and methanol confirmed that methanol in the feed had a significant impact on hydrogen permeation as the membrane surface was contaminated and partly covered by a carbonaceous deposit produced from the decomposition of methanol even at a very low concentration of 1% methanol in the mixed feed. On the other hand, if steam was present in the mixed feed the membrane performance could be maintained by oxidation of the carbonaceous deposit into CO or CO2.

Preparation of thin and highly stable Pd/Ag composite membranes and simulative analysis of transfer resistance for hydrogen separation

Composite Pd/Ag membranes supported on an alpha-alumina substrate have been prepared by sequential electroless plating and the morphology of the membranes has been examined by SEM and XRD analysis. SEM pictures revealed that the Pd/Ag film was of columnar form perpendicular to the substrate. Homogeneous alloy films of Pd/Ag were obtained by annealing the separate Pd and Ag deposited layers at temperatures higher than 600oC in a hydrogen atmosphere.Hydrogen fluxes of up to 0.35mol/(m2s) and hydrogen/nitrogen selectivities of 4500 were obtained and a long-term temperature cycling test showed no deterioration in membrane performance. A simulative analysis was carried out to investigate the effect of boundary layer and the distribution of hydrogen transfer resistance between the boundary layer, the Pd/Ag film and the substrate.

Ethane dehydrogenation in a catalytic membrane reactor coupled with a reactive sweep gas

Abstract An experimental and simulation study has been carried out for the dehydrogenation of ethane to ethylene in a catalytic membrane reactor, with and without reaction on the permeate side. The membrane comprised a palladium-silver alloy deposited as a thin film on a Vycor glass support and palladium catalyst pellets were packed inside the membrane tube. Permeation data for the model were determined separately. The differential equations for transport and reaction within the membrane module were solved using orthogonal collocation to give concentration profiles as a function of contact time, reactor length and radius. The simulation was validated with experimental data and was observed to correctly predict the increase in conversion with contact time for the range of experimental conditions investigated.

Mathematical analysis of ethylbenzene dehydrogenation: Comparison of microporous and dense membrane systems

Abstract A comprehensive mathematical model has been developed and used to simulate the catalytic dehydrogenation of ethylbenzene to styrene using two types of membrane reactor system, i.e. a composite membrane consisting of a thin but continuous metal membrane film of PdAg alloy deposited on the outside surface of a porous ceramic substrate and a microporous membrane tube with the thin permselective layer on the outside of the tube. Various operating parameters have been considered including the separation layer thickness, porosity, reactor length and contact time. Feed stream compositions were also varied. Superior performance in terms of conversion enhancement far above the equilibrium value was observed for the PdAg composite membrane system and was attributed to its exclusive hydrogen permeability characteristics.

Equilibrium-shift in alkane dehydrogenation using a high-temperature catalytic membrane reactor

Abstract This study involves the application of a tubular fixed-bed catalytic membrane reactor to effect equilibrium-shift during the catalytic dehydrogenation of ethane to ethylene and hydrogen. The specific characteristic behaviour of the reactor to shift the equilibrium was analysed using a two-dimensional mathematical model which was solved using the orthogonal collocation method. An important extrinsic variable, the time factor (W/F Ao ) was manipulated to yield results which were then used to explain reactor performance. Experimental results showed that under optimal conditions an eight-fold shift in the equilibrium conversion could be attained. Generally, good agreement was obtained between model and experimental predictions for reactor operation using pure nitrogen as sweep gas. When air was employed as sweep, the agreement was not as expected; possibly due to oxidation of the Pd surface.

A COMPARATIVE EVALUATION OF HIGH-TEMPERATURE MEMBRANE SYSTEMS FOR CATALYTIC PROCESSING

Abstract An investigation has been carried out using mathematical simulation to evaluate the relative performance of various composite membranes incorporated in high-temperature membrane reactors. Two membrane categories (porous and dense) and six types of composite membrane systems (Pd/Ag, polyimide, silica, inert porous and Ru-dispersed porous silicalite zeolites and carbon molecular sieve) have been compared. Also considered is the special case of the Pd/Ag composite system having imperfections (pinholes and cracks). The industrially important reaction of ethylbenzene dehydrogenation over promoted iron oxide catalyst using rate constant values from the literature has been studied as the model reaction. Important characteristic results have been obtained which show that while the dense systems possess higher performance levels at lower membrane thicknesses, the porous composite systems especially those containing highly dispersed active metal particles within their micropores can have advantages due to ...