Bernard P. A. Grandjean

Methane Steam Reforming in Asymmetric Pd- and Pd-Ag/Porous SS Membrane Reactors

Abstract This work is devoted to applying electrolessly deposited Pd- and Pd-Ag/porous stainless steel composite membranes in methane steam reforming. The methane conversion is significantly enhanced by the partial removal of hydrogen from the reaction location as a result of diffusion through the Pd-based membranes. For example, at a total pressure of 136 kPa, a temperature of 500°C, a molar steam-to-methane ratio of 3, and in the presence of a commercial Ni/Al2O3 catalyst together with continuous pumping on the permeation side, a methane conversion twice as high as that in a non-membrane reactor was reached by using a Pd/SS membrane. These effects were examined under a variety of experimental conditions. A computer model of the membrane reactor was also developed to predict the effects of membrane separation on methane conversion.

Structurally stable composite PdAg alloy membranes: Introduction of a diffusion barrier

Abstract This work investigates the improvement of the structural stability of PdAg alloy membranes by introduction of a diffusion barrier. Asymmetric PdAg films were deposited on porous stainless steel (SS) substrate by electroless plating. The formation of these alloys was achieved by annealing the as-deposited membranes at temperatures higher than Tamman temperatures of the alloy components in a hydrogen atmosphere. The composite PdAg membranes were characterized using XRD and Auger electron depth profiling. The atomic interdiffusion of silver and palladium resulted in PdAg alloys in an fcc structure. To improve the structural stability of PdAg alloy/SS membranes, an ultrathin intermediate layer of titanium nitride being 0.1 μm thick was introduced as a diffusion barrier between PdAg and the SS substrate. The Auger electron depth profiling analysis indicated that the improved membranes were thermally stable at temperatures as high as 973 K, and practical for the catalytic membrane reactor use. An estimation of diffusion coefficients revealed that the presence of hydrogen in the annealing atmosphere favoured the PdAg interdiffusion and thus the formation of PdAg alloys.

Pressure drop through structured packings: Breakdown into the contributing mechanisms by CFD modeling

Determination of dry pressure drops is often the preliminary diagnostic tool for characterizing structured packing-containing columns. One conventional approach that ushered in this area evolves around the use of Ergun expressions along with mandatory experimental pressure drops for the fitting of some empirical constants characterizing a given packing. This method is strictly representational, and incapable of predicting the impact on bed pressure drop of changes in packing geometry, e.g., corrugation angle, channel size, or packing topography. In this work, a combined mesoscale—microscale predictive approach was developed to apprehend the aerodynamic macroscale phenomena in structured packings. The proposed method consists in identifying recurrent mesoscale patterns (the representative elementary units, REU) wherein the constitutive microscale dissipation mechanisms occur. The dissipative phenomena that were identified to be important are: the elbow loss and jet splitting at the packed bed entrance, the elbow loss at the column wall, the elbow loss at the jump from one layer to another, and the collisional losses at the criss-crossing junctions. Each mechanism was simulated over a wide Reynoids range spanning the pure creeping flow to the fully developed turbulent flow using three-dimensional computational fluid dynamics (CFD). Postulating additiveness of dissipation, the overall pressure drop was reconstructed. The approach was validated using experimental dry pressure drop data for five packing types (Flexipac, Gempak, Mellapak, Sulzer BX and Montz-Pak) having different channel sizes, corrugation angles, and surface topography. Our goal was to advocate CFD as a quicker and cheaper means for design and optimization, in terms of energy dissipation, of new structured packing shapes.

Dynamic modelling of the activated sludge process: Improving prediction using neural networks

A procedure has been developed to improve the accuracy of an existing mechanistic model of the activated sludge process, previously described by Lessard and Beck [Wat. Res. 27, 963–978 (1993)]. As a first step, optimization of the numerous model parameters has been investigated using the downhill simplex method in order to minimize the sum of the squares of the errors between predicted and experimental values of appropriate variables. Optimization of various sets of parameters has shown that the accuracy of the mechanistic model, especially on the prediction of the dissolved oxygen (DO) in the mixed liquor, can be easily improved by adjusting only the values of the overall oxygen transfer coefficients, KL a. Then, in a second step, neural network models have been used successfully to predict the remaining errors of the optimized mechanistic model. The coupling of the mechanistic model with neural network models resulted in a hybrid model yielding accurate simulations of the five key variables of the activate sludge process.

A neural network methodology for heat transfer data analysis

Abstract Neural networks have been until very recently a topic of academic research. Recent developments of powerful learning algorithms and the increasing number of applications in a great number of disciplines suggest that neural networks can provide useful tools for modelling and correlating practical heat transfer problems. This paper presents an introduction to computing with neural networks. To evaluate the potential of neural networks for correlating heat transfer data, three different examples are solved, using a three-layer feedforward neural network. Two different learning algorithms, including the traditional backpropagation algorithm, are used to teach the neural network. It is shown that neural networks can be used to adequately correlate heat transfer data.

Asymmetric PdAg/stainless steel catalytic membranes for methane steam reforming

Abstract Pd-Ag/porous stainless steel asymmetric membranes were prepared by successive palladium and silver platings in electroless hydrazine baths, followed by a thermal treatment in hydrogen of the as-deposited membranes above the Tamman temperatures for the alloy formation. The prepared membranes were permselective toward hydrogen separation. A membrane reactor made of stainless steel was designed to perform methane steam reforming. At mild reaction conditions, methane conversion is significantly enhanced by partial removal of hydrogen from the reaction location as a result of diffusion through the Pd-based membrane. These effects were examined under a variety of experimental conditions.

Gas}liquid interfacial mass transfer in trickle-bed reactors: state-of-the-art correlations

The state-of-the-art of the gas-liquid mass transfer characteristics in trickle-bed reactors was summarized and its quantification methods were reevaluated based on a wide-ranging data base of some 3200 measurements. A set of three unified whole-flow-regime dimensionless correlations for volumetric liquid- and gas-side mass transfer coefficients, and gas–liquid interfacial area, each of which spanned four-order-of-magnitude intervals, were derived. The correlations involved combination of artificial neural networks and dimensional analysis. The dimensionless interfacial area, ShL and ShG were expressed as a function of the most pertinent dimensionless groups: ReL, ReG, WeL, WeG, ScL, ScG, StL, XG, MoL, FrL, Eom, Sb.

Hydrodynamics and mass transfer in trickle-bed reactors: an overview

Abstract The fluid dynamic and the gas–liquid mass transfer characteristics of trickle-bed reactors were revisited and their quantification methods reevaluated based on extensive experimental historic flow databases (22,000 experiments) set up from the open literature published over the last 50 years. The state-of-the-art of trickle-bed fluid dynamics was summarized and a set of unified and updated estimation methods relying on neural network, dimensional analysis and phenomenological hybrid approaches were discussed.

Surface segregation of PdAg membranes upon hydrogen permeation

Abstract PdAg membranes are permeable to hydrogen. Hydrogen treatment results in a small chemical shift (±0.1–0.2 eV) of Pd 3d core level but no change in the Ag3d level. A new valence band in the binding energy region of 7–9 eV corresponding to the interaction between H 1s and Pd4d appears on a hydrogen permeated membrane surface. Quantitative XPS analysis reveals that Pd segregates at the membrane surface toward the high hydrogen pressure side while Ag segregation occurs at the surface on the low hydrogen pressure side after hydrogen permeation. Both surface segregations are explained based on an MTCIP-1A (modern thermodynamic calculation of interface properties — first approximation) approach. It is concluded that hydrogen chemisorption induces palladium segregation on the PdAg membrane surface.

Tailoring the pressure drop of structured packings through CFD simulations

A computational fluid dynamic methodology is proposed to breakdown into elementary dissipation mechanisms the overall single-phase gas flow bed pressure drop in towers containing corrugated sheet structured packings. The goal behind was to allow piecewise geometry optimization of such packings in terms of capacity enlargement and efficiency enhancement. The dissipations sorted in order of decreasing importance were the collision losses by jet streams at criss-crossing junctions within corrugated channels, elbow loss by form drag at interlayer transition, elbow loss by jets striking wall and subsequent flow redirection to upper channels, and elbow loss in bed entrance. Replacement of sharp bends at the interlayer junctions by progressive direction change was beneficial for the reduction of the dissipations at the wall and the interlayer junction thus stretching capacity of the structured packing. However, this improvement was not spectacular because the most energy-intensive component (criss-crossing) remained unaffected by such modifications. Computational fluid dynamics is foreseen to be a successful, rapid and economic tool to theoretically explore new geometries coping with this limitation.