Ahmet K. Avci

Heterogeneous reactor modeling for simulation of catalytic oxidation and steam reforming of methane

Abstract An autothermal, dual catalyst, fixed-bed reaction system proposed for hydrogen production from methane is mathematically investigated using different catalyst bed configurations and feed ratios. Consecutive placement or physical mixture of the oxidation and reforming catalysts, Pt/ δ –Al 2 O 3 and Ni/MgO–Al 2 O 3 , respectively, are the two configurations of interest. Reactor operation at different feed ratios is analyzed for both catalyst bed configurations on laboratory scale and industrial scale via a series of simulations by using one-dimensional heterogeneous fixed-bed reactor model. The type of heterogeneous components implemented into the model is decided by checking related criteria. Hydrogen production is predicted to be higher when the catalysts are in a physically mixed state as well as at low methane-to-oxygen and high steam-to-methane ratios, which are in agreement with the experimental results reported for a bench scale integral reactor. The optimum operating conditions for obtaining maximum hydrogen production are also investigated.

On-board fuel conversion for hydrogen fuel cells: comparison of different fuels by computer simulations

Abstract The conversions of methane, propane, octane and methanol to hydrogen under conditions pertinent to fuel cell operation have been described quantitatively and examined by a series of computer simulations. Catalytic conversion may be achieved by direct partial oxidation or by a combination of total oxidation and steam reforming. Both systems have been simulated, using conversion data and kinetic equations reported in the literature for various catalyst configurations and hydrocarbons. The results show that, in terms of hydrogen produced per weight of fuel and water carried, direct partial oxidation of propane or oxidation/steam reforming of octane are the best alternatives. The latter possess the ease of operation but coke formation may be more of a problem. Methanol, often suggested as a fuel, is much less efficient. Operation of a vehicle using the catalytic conversion system would require fueling by both hydrocarbon and water.

Ignition Characteristics of Pt, Ni and Pt-Ni Catalysts Used for Autothermal Fuel Processing

Oxidation of propane and n-butane over supported Pt, Ni and Pt-Ni catalysts was studied under fuel-rich conditions. Light-off temperatures followed the order of propane > n-butane and of Ni > Pt-Ni > Pt and were found to have minimum values at optimal fuel:oxygen ratios over Pt-Ni. The bimetallic Pt-Ni catalyst is likely to involve (i) synergistic interactions between the two metals and (ii) pronounced effect of Pt metal during surface ignition. Differences in oxidation activities of Pt and Pt-Ni catalysts seem to be related to the degree of dispersion of Pt metal. For both hydrocarbons, coke formation was not observed under the conditions employed.

Quantitative investigation of catalytic natural gas conversion for hydrogen fuel cell applications

Abstract Hydrogen generation from natural gas for driving proton exchange membrane fuel cells in residential small-scale combined heat and power (CHP) applications is investigated by a series of computer simulations. Natural gas is converted into hydrogen either by the combined oxidation–steam reforming, i.e. indirect partial oxidation mechanism on Pt-Ni catalyst or by the direct, one-step partial oxidation mechanism on Pt monoliths. A water–gas shift converter and a catalytic selective carbon monoxide oxidation unit are used for reducing carbon monoxide levels to a value which the anode of proton exchange membrane fuel cell (PEMFC) can tolerate. Unconverted hydrocarbons and hydrogen rejected from the fuel cell are considered to be oxidized in a Pt catalyst packed afterburner in order to supply energy to the system. Reactor simulations based on available kinetic data together with energy integration calculations indicate direct partial oxidation to give higher hydrogen yields corresponding to increased electrical power outputs and elevated efficiencies. Indirect partial oxidation has the advantage of operating simplicity, since the direct route runs only at millisecond level residence times and high temperatures. In both mechanisms, water injection and energy integration are critical issues in adjusting product yields and in temperature control. The simulation outputs are compared and validated by the results based on the thermodynamics of the pertinent mechanism.

On-board hydrogen generation for fuel cell-powered vehicles: the use of methanol and propane

Although hydrogen has been found to be the most acceptable fuel for vehicle-mounted fuel cells, the storage and transportation of the gas presents difficulties. As a result, attention has been focused on on-board conversion of more readily available fuels such as methanol. Comparisons have been made of hydrogen generation from methanol and propane. Simulations are based both on thermodynamic and kinetic data. The results show that a mixture of oxidation and steam reforming (indirect partial oxidation) produces more hydrogen than direct partial oxidation. Propane is found to produce more hydrogen per weight carried than methanol, but suffers from the disadvantage that reaction does not start at room temperature. The necessity to fuel a vehicle with an organic fuel and with water is demonstrated.

Multiphase Catalytic Reactors: Theory, Design, Manufacturing, and Applications

• Provides a holistic approach to multiphase catalytic reactors from their modeling and design to their applications in industrial manufacturing of chemicals • Covers theoretical aspects and examples of fixed-bed, fluidized-bed, trickle-bed, slurry, monolith and microchannel reactors • Includes chapters covering experimental techniques and practical guidelines for lab-scale testing of multiphase reactors • Includes mathematical content focused on design equations and empirical relationships characterizing different multiphase reactor types together with an assortment of computational tools • Involves detailed coverage of multiphase reactor applications such as Fischer-Tropsch synthesis, fuel processing for fuel cells, hydrotreating of oil fractions and biofuels processing

Reactor Design for Fuel Processing

Publisher Summary This chapter discusses reactor designs for fuel processing. Fuel reforming reactor is the part of the fuel processing system in which catalytic conversion of hydrocarbon fuels (fossil or renewable) to a hydrogen-rich mixture takes place. In general, the type of catalyst and fuel dictate the reaction temperature, molar ratio of steam, and hydrocarbon in the feed and the product distribution. Design of a reformer unit is mainly a function of the fuel conversion mode and the nature of the catalyst used. If the conversion route is endothermic, such as in steam reforming (SR), structures providing improved heat input to the catalyst bed should be implemented. Similarly, exothermic reactions require effective distribution of the generated heat to the catalyst bed. This study provides the details of such requirements for each fuel, as the nature of the hydrocarbon is as important as the conversion route and the catalyst in the reformer design. Following this, it outlines various catalytic reactor types. These include fixed beds, monoliths, microchannels, foams, and wire gauzes, that can be used in the specific steps of fuel processing, i.e., reforming, WGS, carbon monoxide removal, and desulfurization. Finally, it presents model equations that describe the operation of basic reactor types and membrane reactors that can be used in fuel reforming, WGS, CO removal, and desulfurization stages of the fuel processing operation.

Quantitative description of protein adsorption by frontal analysis

Abstract Protein adsorption in packed columns was investigated theoretically and experimentally by frontal analysis and ion exchange chromatography. A dynamic model describing interfacial and intraparticle mass transfer and surface reaction phenomena was used in theoretical analysis. The effect of operating conditions, including temperature, feed concentration, column length and diameter, on the performance of breakthrough curves were determined. Two different techniques are employed in the estimation of the mass transfer parameters and the interaction rate constant. Use of model parameters obtained from finite bath studies results in the prediction of the 70% of the breakthrough profiles whereas simultaneous parameter estimation and solution of the partially differential model equations yield full prediction of the adsorption phenomenon. The former technique can be used in the design and scale-up of chromatography columns only if the later parts of protein adsorption are not of importance.

2.16 Catalysts

This chapter provides a concise and holistic coverage of solid catalysts, starting from the fundamentals to their commercial use in industrial reactors. The first section involves the concepts of adsorption involved in and kinetics of solid-catalyzed reactions. Methods and processes of synthesis, preparation, and commercial production of solid catalysts, porous materials used in heterogeneous catalysis, and mechanisms of catalyst deactivation are discussed in the second section. A detailed classification of reactors, the units that enclose the catalysts, and cases summarizing commercial use of catalysts and catalytic reactors are outlined in the third section of this chapter.

Fatigue Behavior and Damage Assessment of Stainless Steel/Aluminum Composites

Fatigue crack growth and related damage mechanisms were investigated experimentally in a stainless steel/aluminum laminated composite with middle through thickness crack, and two different fracture mechanics approaches applied to the composite to reveal their differences under fatigue loading. The larninated composite material, which has a unidirectional continuous AISI 304 stainless steel as fibers and Al 1060 as matrix, was produced by using diffusion bonding. Fatigue tests were conducted in accordance with ASTM E 647. The relationships between fatigue crack growth rate (da/dN), stress intensity factor (ΔK), and strain energy release rate (ΔG) were determined; and damage behavior was discussed. Both linear elastic fracture mechanics (LEFM) and compliance method were used, and the results were compared with each other. It is found that as the crack propagates, the LEFM overestimates the ΔG values. Interlaminar and fiber/matrix interface damage were evaluated by fractographic examination.