Catalyst Surface Temperature Research Articles

Applying spatially resolved concentration and temperature measurements in a catalytic plate reactor for the kinetic study of CO methanation

In this work, the successful application of spatially resolved measurements in an optically accessible catalytic plate reactor was demonstrated, which allows the detailed investigation of the reaction kinetics of the exothermic CO methanation reaction with high initial CO partial pressure. By means of a movable sampling capillary, the axial gas species concentration profiles over the catalyst plate were measured, and the catalyst surface temperature was determined simultaneously along the reactor through a quartz glass window by means of infrared thermography. A one-dimensional model of the catalytic plate reactor and a Bayesian approach were applied to estimate the kinetic model parameters of the proposed Langmuir–Hinshelwood rate expressions by comparing simulated and measured gas concentration profiles. The validity of using a computationally efficient one-dimensional model was proven by solving a two-dimensional model using the kinetic parameters determined with the one-dimensional model and by comparing the two results.

Catalytic reforming of gasoline to hydrogen: Kinetic investigation of deactivation processes

Abstract Catalytic reforming of gasoline to a hydrogen-rich gas is a possible route to feed a fuel cell for electricity production on-board a vehicle. To properly design a fuel processor system, knowledge about the kinetics of the different reactions involved in the reforming is needed. Kinetic studies are hampered by the fact that sulfur compounds present in commercial gasoline may lead to a progressive deactivation of the catalyst. We have undertaken such a study with an optically accessible catalytic channel flow reactor enabling concentration profiles and catalyst surface temperatures to be measured. The concentration profiles measured at different times on stream revealed a progressive deactivation of the catalyst. Isothermal reaction rate constants, depending on the time on stream, were derived by fitting a Langmuir–Hinshelwood kinetic model to the experimental species concentration profiles. The modeling results indicated that the steam reforming of higher hydrocarbons was more strongly affected by the presence of sulfur in the feed than the water gas shift reaction and the steam reforming of methane. Carbon formation was inferred from changes in surface emissivity during the experiments. It is suggested that the primary reason for the observed deactivation is due to the presence of sulfur compounds in the feed. The deactivated catalyst would then promote the formation of coke at the surface, i.e. coke formation is probably a consequence of the deactivation and not a cause for it. Although the variability in preparing the coated catalytic plates affected the measured kinetic rate parameters, the observed trends were in general consistent for all runs.

Fe 2O 3/β-SiC: A new high efficient catalyst for the selective oxidation of H 2S into elemental sulfur

Over the last decades, sulfur recovery from the H 2S-containing acid gases (issued from oil refineries or natural gas plants) has become more and more important due to the ever increasing standards of efficiency required by environmental protection pressures. The H 2S-tail gas was directly oxidized by oxygen to yield elemental sulfur. A significant improvement of the H 2S conversion and selectivity has been developed, however, the support which is the core of the process still needs to be improved. Recently, β-SiC has been reported to be an efficient and selective catalyst support for the H 2S-to-S reaction. One expected reason for this superior yield should be due to the high thermal conductivity of the support. The high thermal conductivity of the silicon carbide plays an important role in the maintenance of the high selectivity by avoiding the formation of hot spots on the catalyst surface which could favor secondary reactions. On the other hand, insulator supports such as alumina exhibit a poor selectivity due to catalyst surface temperature runaway.

Carbon nanofiber growth in plasma-enhanced chemical vapor deposition

A theoretical model to describe the plasma-assisted growth of carbon nanofibers (CNFs) is proposed. Using the model, the plasma-related effects on the nanofiber growth parameters, such as the growth rate due to surface and bulk diffusion, the effective carbon flux to the catalyst surface, the characteristic residence time and diffusion length of carbon atoms on the catalyst surface, and the surface coverages, have been studied. The dependence of these parameters on the catalyst surface temperature and ion and etching gas fluxes to the catalyst surface is quantified. The optimum conditions under which a low-temperature plasma environment can benefit the CNF growth are formulated. These results are in good agreement with the available experimental data on CNF growth and can be used for optimizing synthesis of related nanoassemblies in low-temperature plasma-assisted nanofabrication.

High thermal conductive β-SiC for selective oxidation of H2S: A new support for exothermal reactions

Iron supported on a medium surface area silicon carbide (β-SiC) is a high efficiency catalyst for the selective oxidation of hydrogen sulphide into elemental sulphur at reaction temperatures above the sulphur dew point. The strong interaction between the topmost layer of the support and the active phase allows a high dispersion of the iron-based catalyst particles which provide a high active phase-reactants contact surface to maintain the high desulphurization activity even at high space velocity. The high thermal conductivity of the silicon carbide plays an important role in the maintenance of the high selectivity by avoiding the formation of hot spots on the catalyst surface which could favor secondary reactions. On the other hand, insulator supports such as alumina exhibits a poor selectivity due to catalyst surface temperature runaway. The thermal profile of the bed temperature calculated by a simple heterogeneous model was in agreement with the previous hypothesis. The intrinsic chemical inertness and the surface property of the material render the silicon carbide usable in very drastic conditions, i.e. in a corrosive atmosphere (2% H2S) and with a low O2/H2S ratio compared to silica and/or alumina based catalysts. In such conditions, the catalyst remains stable over 1500h on stream with high sulphur yield up to 95%. These performances make silicon carbide based catalysts the most promising catalyst support suitable for the high-temperature selective oxidation process.

E223 小型熱源に向けたDME触媒燃焼器の吸着熱を利用した低温始動性向上(固体燃焼・微小重力燃焼・触媒燃焼)

This study investigates low temperature startability of dimethyl-ether (DME) catalytic combustor as a small size heat generator. A procedure of DME pre-supply followed by mixture supply into the reactor shows great improvement of startability. With this procedure catalyst surface temperature increases caused by DME adsorption heat at the period of DME pre-supply and increased temperature promote catalytic reaction. When the period of DME pre-supply is excess, the reactor temperature turns decrease by increased heat loss in comparison with adsorption heat. Consequently, it is suggested that the optimum condition for DME pre-supply exists to give best startability of DME catalytic combustor with this procedure.

Experimental and numerical investigation of the catalytic partial oxidation of [formula omitted] mixtures diluted with [formula omitted] and [formula omitted] in a short contact time reactor

The catalytic partial oxidation (CPO) of fuel-rich CH 4 / O 2 mixtures, heavily diluted with H 2 O and CO 2 (46.3% and 23.1% volumetric feed composition, respectively), was investigated experimentally and numerically at 5 bar. Experiments were carried out in an ∼ 8 ms residence time prototype gas-turbine honeycomb reactor coated with a Rh/ ZrO 2 catalyst and included temperature measurements along the reactor and exhaust gas analysis. Simulations with detailed hetero-/homogeneous chemical reaction schemes were performed using a steady, full elliptic 2D code for both the gas and solid phases. The employed catalytic reaction scheme overpredicted mildly the impact of the total over the partial oxidation route and this effect was more pronounced at lower reactor inlet temperatures ( < 523 K ). The contribution of the various chemical pathways to the synthesis gas yields and selectivities has been elucidated. It was shown that the addition of water provided a source of surface oxygen and hydroxyl radicals, which in turn enhanced the CH 4 conversion and H 2 selectivity and reduced the CO selectivity. On the other hand, the addition of CO 2 had a minor impact on the aforementioned parameters. The increase in the H 2 / CO product ratio with water dilution is highly desirable in new power generation processes with large exhaust gas recycle, which utilize the “catalytic rich combustion” methodology (the partial oxidation products stabilize a post-catalyst flame). At steady operation the catalyst surface temperatures exceeded by 200 K the adiabatic equilibrium temperature and standard (passive) heat transfer mechanisms in the solid were shown to be ineffective in providing proper reactor thermal management. Catalytic ignition was achieved at 670 K, however, the strong ignition/extinction hysterisis allowed for sustained steady CPO at inlet temperatures as low as 473 K.

Characteristics of Methane Conversion under Combined Reactions of Solid Catalyst with Microwave Plasma

This study focuses on the reaction characteristics of active hydrocarbon species mainly composed of methine radicals and active hydrogen species mainly composed of hydrogen radicals, to both of which methane as a raw material is converted by microwave irradiation to create plasma that comes into contact with the Pt/Al 2 O 3 catalysis field. The results indicate that when 4 chemical species, i.e., active hydrocarbon species, active hydrogen species, acetylene molecules (produced by coupling of the active species) and hydrogen molecules are supplied to the solid catalyst, an increase in catalyst surface temperature facilitates apparent hydrogenation on the surface of the catalyst and thereby increases the yields of ethylene and ethane. It also appears that addition of hydrogen molecules to microwave-irradiated methane causes significant hydrogenation at 500 K or lower.

Advanced Catalytic Pilot for Low NOx Industrial Gas Turbines

This paper describes the design and testing of a catalytically stabilized pilot burner for current and advanced Dry Low NOx (DLN) gas turbine combustors. In this paper, application of the catalytic pilot technology to industrial engines is described using Solar Turbines’ Taurus 70 engine. The objective of the work described is to develop the catalytic pilot technology and document the emission benefits of catalytic pilot technology when compared to higher, NOx producing pilots. The catalytic pilot was designed to replace the existing pilot in the existing DLN injector without major modification to the injector. During high-pressure testing, the catalytic pilot showed no incidence of flashback or autoignition while operating over wide range of combustion temperatures. The catalytic reactor lit off at a temperature of approximately 598 K (325°C/617°F) and operated at simulated 100% and 50% load conditions without a preburner. At high pressure, the maximum catalyst surface temperature was similar to that observed during atmospheric pressure testing and considerably lower than the surface temperature expected in lean-burn catalytic devices. In single-injector rig testing, the integrated assembly of the catalytic pilot and Taurus 70 injector demonstrated NOx and CO emission less than 5 ppm @ 15% O2 for 100% and 50% load conditions along with low acoustics. The results demonstrate that a catalytic pilot burner replacing a diffusion flame or partially premixed pilot in an otherwise DLN combustor can enable operation at conditions with substantially reduced NOx emissions.

High-pressure experiments and modeling of methane/air catalytic combustion for power-generation applications

The catalytic combustion of methane/air mixtures is investigated experimentally and numerically at gas turbine relevant conditions (inlet temperatures up to 873 K, pressures up to 15 bar and spatial velocities up to 3×10 6 h −1). Experiments are performed in a sub-scale test rig, consisting of a metallic honeycomb structure with alternately coated (Pd-based catalyst) channels. Simulations are carried out with a two-dimensional elliptic fluid mechanical code that incorporates detailed transport and heat loss mechanisms, and realistic heterogeneous and homogeneous chemistry description. The methodology for extracting heterogeneous kinetic data from the experiments is presented, and the effects of catalytic activity and channel geometry (length and hydraulic diameter) on reactor performance are elucidated. A global catalytic kinetic step provides excellent agreement (at temperatures below 950 K) between the measured and predicted fuel conversion, over a wide range of parameter variation (channel hydraulic diameter and length, pressure, and inlet temperature). It is shown that, under a certain combination of catalytic activity and channel length, the absolute temperature rise across the catalyst becomes essentially independent of pressure, a feature highly desirable for many practical systems. Even though the computed catalyst surface temperatures remain well below the decomposition temperature of PdO, a significant section of the catalyst—amounting up to 30% of the total reactor length—contributes minimally to the total fuel conversion, suggesting catalytic activity design improvements in the reactor entry.

Catalytic activity of CeO2 and Pr2O3 for the liquid-phase benzylation of o-xylene to 3,4 – dimethyldiphenylmethane

The liquid phase benzylation of o-xylene with benzyl chloride over rare earth oxide catalysts like CeO2 and Pr2O3 was studied in a batch reactor at atmospheric pressure and 363 K. Surface area, pore volume, DTA, acid strength distribution on the catalyst surface and optimum temperature of the catalyst are reported.

High-Yield Butane to Maleic Anhydride Direct Oxidation on Vanadyl Pyrophosphate Supported on Heat-Conductive Materials: β-SiC, Si3N4, and BN

Both the preparation and the characterization of vanadyl pyrophosphate (VPO) supported on new heat-conductive ceramic supports (β-SiC, Si3N4, and BN) are described. These catalysts provide a very significant gain in maleic anhydride yield by reacting air and butane in a fixed-bed reactor, because of control of the catalyst surface temperature. The stable high maleic anhydride selectivity at high temperature, ensured by the presence of the support, allows the use of significantly high partial pressures of butane. In addition, the β-SiC support protects the catalyst against high-temperature accidental excursions.

On the controlling factor of catalyst temperature in C3H8-air mixture

Catalytic combustion of propane-air mixture was investigated. Platinum catalysts over a flat stainless steel with γ-alumina washcoat were employed. The employed burner has three catalysts set parallel to the mixture flow, spaced at an interval of 5, 10 and 15 mm. Both experiment and numerical simulation were made at inlet temperature of 553 K, inlet velocity of 3 to 7 m/s and equivalence ratio of 0.3 to 0.5. In the numerical simulation, two-dimensional, steady state model was developed to calculate the temperature and species concentration in gas-phase. In this model, chemical reaction on the catalyst surface and that in the gas phase were assumed to occur in three-steps. The numerical results show good agreement with experimental results. It was found that the properties of the catalyst strongly affect the catalyst surface temperature. Especially, the thermal conductivity of catalyst has a great effect, while the emissivity of catalyst has less effect.

Liquid - Phase Benzoylation of o -Xylene to 3,4-Dimethylbenzophenone Using Rare Earth Oxide Catalysts

The liquid phase benzoylation of o-xylene with benzoyl chloride over rare earth oxide catalysts like CeO2 and Pr2O3 is studied in a batch reactor at atmospheric pressure and 411 K. Surface area, pore volume, acid strength distribution on the catalyst surface and optimum temperature of the catalyst are reported.

Heterogeneously catalyzed reactions in the presence of irreducible transport phenomena

In view of the tendency towards the intensification of chemical processes, the importance of fast reactions is growing. Chemists have developed heterogeneous catalysts which allow industrially very important reactions such as catalytic cracking of gas oil or the removal of NOx from stack gas to proceed very rapidly indeed. It is well known that transport phenomena do limit the obtainable rates, however. When designing or optimizing a reactor, the catalytic reaction engineer takes these transport limitations into account by writing down and solving the corresponding conservation equations of mass and energy, in which both transport and intrinsic chemical kinetics are accounted for in an appropriate way. The textbook approach consists in obtaining the latter separately, i.e. at conditions where the effects of transport phenomena can be neglected. There are more and more cases, however, where such a separation is not possible without loosing essential features of the catalyzed reaction. In his book ‘Kinetics of Chemical Processes’ M. Boudart spends a chapter on this category with as title: ‘Irreducible Transport Phenomena’. The present set of papers provides some examples of irreducible transport phenomena, linked to catalyst deactivation during catalytic cracking, reduction of NO and oxidative dehydrogenation of light alkanes. Some contributions focus on dedicated experimental set-ups. One of the messages is that the involved transport phenomena should be amenable to a quantitative treatment without adjustable parameters, e.g. by the use of structured reactors and by maintaining laminar rather than turbulent flow. Others provide illustrations as to how important reaction variables, like the catalyst surface temperature, can be measured. In order to remind the reader of more ‘exotic’ consequences of irreducible transport phenomena, a contribution on the simulation of autonomous oscillations caused by relatively fast surface reactions has also been added.

Oxidative coupling of methane – the transition from reaction to transport control over La 2O 3/MgO catalyst

A 10% La 2O 3 supported on fused MgO (periclase) catalyst has been studied in the 0.45–3.04 atm range for methane partial oxidation to ethane plus ethylene. X-ray diffraction showed that this catalyst, operated by a mechanism La 2O 3→2La 3+O 2−e −+[O], provides active oxygen and vacancies e − for the surface reaction. Four independent reactions were found that allowed calculation of the rates and approximate kinetics of every species in the reacting system: 2CH 4+1/2O 2→C 2H 6+H 2O; C 2H 6+1/2O 2→C 2H 4+H 2O; 2CH 4+3O 2→2CO+4H 2O; 2CO+2H 2O⇌2CO 2+2H 2.The equilibrium proceeds so fast that it is not possible to distinguish whether CO or CO 2 is the primary product. It was found that oxygen conversion was in a transition regime between surface reaction and mass-transport control in the 700–825°C range. Oxygen conversion is strictly transport controlled at 850–875°C while methane reaction rate remains surface controlled. It is in this oxygen-transport controlled regime at 875°C that our highest yield of 16.2 mol% C 2 was obtained. We have fitted kinetics in both transition- and transport-controlled regimes. Finally, it should be noted that catalyst surface temperatures were calculated to be far above than those of the bulk gas temperatures as a consequence of the transport control; as much as 222°C temperature rise at 875°C gas temperature was observed.

Development of combustion catalysts for monolith reactors: a consideration of transport limitations

In the search for catalysts to be utilized in a new generation of catalytic gas turbine combustors it is not unusual to hear of catalysts that appear to have worked well in a laboratory environment but do not do so when installed in a high pressure pilot-scale rig. The influence that interphase and intraphase transport limitations may have on the rate of catalytic combustion of methane in a monolith reactor (with a 1.1 mm cell size) is investigated at a Reynolds number, Re = 10 3 (with) P = 2bar) and at Re = 10 4 (with P = 15bar), for bulk gas temperatures varying from 623–873 K and at a bulk methane mole fraction of 0.02. It is shown that it is not an easy task to evaluate chemical kinetic expressions at conditions which truly represent even the first stage (e.g., at gas temperatures from 623–973 K) in a catalytic gas turbine combustor, for as reaction rates become very fast, interphase and intraphase transport limitations also become significant, e.g., when comparing reaction rates evaluated at incorrectly assigned conditions, then even at a relatively low bulk fluid temperature= 773K, errors may vary from ca. 35% to 80% for Re = 10 4and10 3, respectively. In a laboratory test environment, where experiments may be performed close to atmospheric conditions, it is also shown that it is easy to over-temperature and hence damage the catalyst, and draw false conclusions about the catalyst being too active, e.g., considering interphase resistances alone, at a bulk fluid temperature of 773 K and Re = 10 3 the catalyst is damaged, whereas at Re = 10 4 this would not have occurred. As catalyst surface temperature exceeds 800 K, intraphase diffusion may become significant and the effectiveness factor soon becomes very much less than one. Therefore, reactor scale-up should be based on calculations where the combined effects of interphase and intraphase transport processes are coupled with reactions; scale-up based on simple multipliers, e.g., those derived from residence time or gas hourly space velocity, are unlikely to work. Modelling is shown to have an important role in catalyst and system design; if applied correctly, system development costs may be reduced.

Flameless combustion of hydrogen on the surface of a hydrophobized catalyst

The flameless combustion of hydrogen on the surface of a hydrophobized catalyst is investigated experimentally under various conditions. A quantitative interrelationship is discerned between the rate of catalytic oxidation of hydrogen and the catalyst surface temperature. It is shown that the presence of helium in the gas—air medium diminishes considerably the hydrogen oxidation rate by lowering the temperature of the catalytic surface by virtue of the high thermal conductivity of helium. An approximate mathematical model is proposed for the flameless combustion of hydrogen on the surface of a catalyst. It is shown that the insertion of catalytic rods in metal tubes of diameter 20 mm significantly enhances the performance of each element by the judicious direction of strong convection flows from the heated surface of the rod.

Photocatalytic reaction of 2-propanol over [formula omitted] using high flux radiation

The decomposition of 2-propanol is used as a model to investigate the photocatalytic properties of tungsten (VI) oxide on silica gel. A high flux, simulated solar radiation source is utilized as the energy source for the reactions. The UV-visible photocatalytic activity decreases with an increase in percent of WO 3 loading on SiO 2 from 1% to 3%. IR radiation, carrier gas, and the lamp power also play an important role in this reaction. The IR and the higher lamp power tend to enhance thermal reaction by heating the surface of the catalyst. In contrast to helium, use of argon carrier gas also enhances thermal reaction. Presumably the catalyst surface temperature is greater in the presence of argon due to the lower gaseous thermal conductivity of argon. UV-visible photocatalytic activity is most obvious when a water filter is used to remove the IR radiation and helium is used as the carrier gas. This study also supports a procedure established earlier to evaluate photochemical reactions, including photocatalysis.

Kinetics and fixed‐bed reactor modeling of butane oxidation to maleic anhydride

AbstractSelective oxidation kinetics of n‐butane to maleic anhydride in air were studied over a commercial, fixed‐bed vanadium‐phosphor oxide catalyst. The temperature range was 573–653 K with butane concentrations up to 3 mol % in the feed, which is within flammability limits but below ignition temperatures.The rate data were modeled using power law kinetics with product inhibition and included total oxidation and decomposition reactions. Kinetic parameters were estimated using a multiresponse, nonlinear regression algorithm showing intercorrelation effects. The kinetics were combined with independent measurements of catalyst diffusivity and reactor heat transfer using a one‐dimensional heterogeneous reactor model. Model predictions and observed temperatures and concentrations from non‐isothermal pilot plants were compared up to 115 days on stream. Agreement was acceptable with inlet butane concentrations up to 2.7 mol %. For example, runaway was predicted at a salt temperature 3 K higher than observed. Effectiveness factors around the hot spot were estimated at 0.6 with the catalyst surface temperature 2–3 K higher than the average gas temperature.