Fischer–Tropsch on Iron with H2/CO and H2/CO2 as Synthesis Gases: The Episodes of Formation of the Fischer–Tropsch Regime and Construction of the Catalyst

Fischer–Tropsch synthesis experiments have been performed with reduced precipitated Fe-Al-Cu-K₂O catalysts, using a H₂/CO and a H₂/CO₂ synthesis gas. Samples of the reaction products and of the catalyst were taken at distinct run lengths. The product samples were analyzed in detail and from their composition the kinetic data of elemental reaction steps (growth, branching, desorption as olefin or paraffin) calculated, applying the extended model of “nontrivial surface polymerization”. The catalyst samples were characterized by BET, XRD, Mössbauer spectroscopy, XPS and TPD in hydrogen and thus specifically phase changes of e.g. alpha iron, iron oxides, and iron carbides observed. It has been measured how the product composition changed with time, up to the steady state of synthesis. Several episodes with their own kinetic regimes were identified. These were then correlated with compositional and structural changes of the catalyst. This is addressed as “construction of the true iron Fischer–Tropsch catalyst”.     

Oxidation of low concentrations of hydrogen sulphide: Process optimization and kinetic studies

Low concentrations (e.g. < 3) of H₂ S in natural gas can be selectively oxidized over an “granular Hydrodarco” activated carbon catalyst to elemental sulphur, water and a small fraction of by-product sulphur dioxide, SO₂. To optimize the H₂ S catalytic oxidation process, the process was conducted in the temperature range 125—200 °C, at pressures 230—3200 kPa, with the O/H₂ S ratio being varied from 1.05 to 1.20 and using different types of sour and acid gases as feed. The optimum temperature was determined to be approximately 175 °C for high H₂ S conversion and low SO₂ production with an O/H₂ S ratio 1.05 times the stoichiometric ratio. The life of the activated carbon catalyst has been extended by removing heavy hydrocarbons from the feed gas. The process has been performed at elevated pressures to increase H₂ S conversion, to maintain it for a longer period and to minimize SO₂ production. The process is not impeded by water vapour up to 10 mol% in the feed gas containing low concentrations of CO₂ (< 1.0). A decrease in H₂ S conversion and an increase in SO₂ production were obtained with an increase in water vapour in the feed gas containing a high percentage of CO₂. The process works well with “sour natural gas” containing approximately 1% H₂ S and with “acid gas” containing both H₂ S and CO₂. It gives somewhat higher H₂ S conversion and low SO₂ production with feed gas containing low concentrations of CO₂. A kinetics study to determine the rate-controlling step for the H₂ S catalytic oxidation reaction over “granular Hydrodarco” activated carbon has been conducted. It was concluded that either adsorption of O₂ or H₂ S from the bulk phase onto the catalyst surface is the rate-controlling step of the H₂ S catalytic oxidation reaction.     

Kinetic modelling of the catalytic conversion of synthesis gas

A kinetic study for the one-step conversion of synthesis gas to gasoline on a ZnO–Cr₂O₃–ZSM-5 catalyst is described. On this catalyst, three reactions are involved in the overall transformation of synthesis gas: the methanol synthesis, the conversion of methanol to hydrocarbons and the water–gas shift reaction. Under the operating conditions selected for the study, it was found that the water–gas shift was at equilibrium and the methanol was completely converted to hydrocarbons. Consequently, it was postulated that the kinetics of the limiting reaction step, the methanol synthesis on the ZnO–Cr₂O₃ component, was the one that controls the overall reaction rate. Three kinetic model equations describing the rate of synthesis gas conversion on the bifunctional catalyst, were considered to fit the data of the experimental runs performed in a Berty well-mixed reactor. Those equations were derived under very special conditions where the methanol decomposition term could be neglected. It was also observed that in the kinetic equations a term involving the fugacity of CO₂ was required to predict the rate properly. The catalyst deactivation was also taken into account in the analysis.     

Photocatalytic degradation of malic acid using a thin coated TiO2‐film: Insights on the mechanism of photocatalysis

Decontamination of opaque fluids using photocatalysts and near Ultraviolet (UV) irradiation involves major technical challenges. This study considers a thin TiO₂ layer placed in a new Chemical Reactor Engineering Centre (CREC)‐photoreactor cell. This new photoreactor cell is used for the photocatalytic degradation of malic and malonic acids, typical apple juice components. Conversion of organic species can only proceed through the “dark side” of the TiO₂ layer, which is in direct contact with the fluid. Under the selected operating conditions both external mass‐transfer limitations and photolysis are found to be negligible. Macroscopic radiation balance shows that 92% of near UV radiation is absorbed by the ‘back side” of the TiO₂‐film. Photocatalytic degradation experiments with 10, 20, 30, and 40 ppm malic acid initial concentrations, show that malonic acid is a main intermediate. Complete malic acid conversion occurs after 5–8 h of irradiation. Kinetic modeling of malic and malonic acid photodegradation with kinetic parameter estimation is performed using both an “in series” and an “in series‐parallel” reaction networks. The “in series‐parallel” reaction network displays better ability for predicting CO₂ formation, showing maximum quantum yields of 14.2%. Given that in the CREC‐photoreactor cell with a thin TiO₂‐film, photocatalysis can only proceed via the transfer of mobile “h⁺” sites from the irradiated side to the “dark side', this study demonstrates the significance of this step on the overall photocatalysis mechanism. © 2014 American Institute of Chemical Engineers AIChE J, 60: 3286–3299, 2014     

Catalytic performance of a Ti added Pd/SiO2 catalyst for acetylene hydrogenation

A series of Pd/SiO₂ and Pd–Ti/SiO₂ catalysts were prepared by the incipient wetness impregnation method. The catalytic performance for selective hydrogenation of acetylene to ethylene was measured under “high concentration acetylene”, “high space velocity” and “no dilution gas” conditions. The crystal structure and particle size of the catalysts were characterized by the X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS), nitrogen physisorption using the BET method and transmission electron microscope (TEM). The results showed that the titanium oxide in Pd–Ti/SiO₂ catalyst was amorphous and the addition of Ti reduced the particle size of Pd significantly. Comparing to the Pd/SiO₂ catalyst, the ethylene yield increased from 64.1% to 88.3% under Pd–Ti/SiO₂ catalytic system.     

Towards understanding the bifunctional hydrodeoxygenation and aqueous phase reforming of glycerol

Kinetically coupled reactions of glycerol in water over bifunctional Pt/Al₂O₃ catalysts are explored as a function of the Pt particle size and the reaction conditions. Detailed analysis of the reaction network shows that “reforming” and hydrodeoxygenation require the presence of a bifunctional catalyst, i.e., the presence of an acid–base and a metal function. The initial reaction steps are identified to be dehydrogenation and dehydration. The dehydrogenation of hydroxyl groups at primary carbon atoms is followed by decarbonylation and subsequent water gas shift or by disproportionation to the acid (and the alcohol) followed by decarboxylation. Hydrogenolysis of the C–O and C–C bonds in the alcohols does not occur under the present reaction conditions. Larger Pt particles favor hydrodeoxygenation over complete deconstruction to hydrogen and CO₂.     

Conversion of natural gas to C2 product,hydrogen and carbon black using a catalytic plasma reaction

In the microwave and RF plasma catalytic reaction at room temperature, the decomposition of natural gas over Pd–NiO/γ-Al₂O₃ was carried out. The decomposition of methane is caused by collision by excitation of unstable electronic state. Measuring the flow rate and plasma power can represent kinetic data and mechanism. The conversion of C₂ hydrocarbons was increased from 47% to 63.7% in the microwave plasma catalytic reaction within electric field. Comparing the activities of catalysts, Pd–NiO/γ-Al₂O₃ bimetallic catalyst was more active than Pt–Sn/γ-Al₂O₃ catalyst because of modifying the surface of catalysts by carbon formation. In RF plasma catalytic reaction, we obtained high C₂ yield of 72%, in which the conversion and selectivity of C₂ hydrocarbons were related to the applied power and feed rate of natural gas.     

Kinetic Reaction Models for the Selective Reduction of NO by Methane over Multifunctional Zeolite‐based Redox Catalysts

Kinetic measurements of the selective catalytic reduction (SCR) of NO by methane were performed over CeO₂/H‐ZSM‐5, In‐ZSM‐5, and CeO₂/In‐ZSM‐5 catalysts. The parameter space covered NO, CH₄, and O₂ concentrations varying from 250 to 1000 ppm, from 500 to 2000 ppm, and from 0.5 to 10 vol.‐%, respectively, space velocities between 5000 and 90000 h^–1 and temperatures between 573 and 873 K depending on the catalyst activities. With CeO₂/In‐ZSM‐5 an additional series of measurements was performed with moistened feed gas (0.5–10 vol.‐% H₂O). On the basis of a pseudo‐homogeneous, one‐dimensional fixed‐bed reactor model, the data were fitted to a kinetic model that includes power rate laws for the reduction of NO and for the unselective total oxidation of methane. From analyses of isothermal data sets, almost all reaction orders were found to vary significantly with changing temperature, which indicates that the simple kinetic model cannot reflect the complex reaction mechanism correctly. Nevertheless, the data measured with In‐ZSM‐5 could be modeled with good accuracy over a wide range of reaction temperatures (150 K) while the accuracy was less satisfactory with the remaining data sets, in particular for data with the moist feed over CeO₂/In‐ZSM‐5. With the latter catalyst it was not possible to represent the data measured in dry and in moist feed in a single model even upon confinement to fixed reaction temperatures. A comparison of the separate models established showed strong changes in the reaction orders in the presence of water, which occur apparently already at a very low water content (≤ 0.5 vol.‐%). The kinetic parameters found are in agreement with earlier conclusions about the reaction mechanisms. With In‐ZSM‐5, both reaction orders and the activation energy show a rate‐limiting influence of NO oxidation on the NO reduction path which is removed by the presence of the CeO₂ promoter. A difference in the reaction mechanism over CeO₂/In‐ZSM‐5 and CeO₂/H‐ZSM‐5 is reflected in different kinetic parameters. The differences of the kinetic parameters between dry‐feed and moist‐feed models for CeO₂/In‐ZSM‐5 reflect adsorption competition between the reactants and water.     

Processing Low‐Oxide ZrB2 Ceramics with High Strength Using Boron Carbide and Spark Plasma Sintering

A phase diagram‐assisted powder processing approach is shown to produce low‐oxygen (0.06 wt%O) ZrB₂ ceramics using minimal B₄C additions (0.25 wt%) and spark plasma sintering. Scanning electron microscopy and scanning transmission electron microscopy with elemental spectroscopy are used to identify “trash collector” oxides. These “trash collector” oxides are composed of manufacturer metal powder impurities that form discreet oxide particles due to the absence of standard Zr–B oxides found in high oxygen samples. A preliminary Zr–B–C–O quaternary thermodynamic database developed as a part of this work was used to calculate the ZrO₂–B₄C pseudobinary phase diagram and ZrB₂–ZrO₂–B₄C pseudoternary phase diagrams. We use the calculated equilibrium phase diagrams to characterize the oxide impurities and show the direct reaction path that allows for the formation of ZrB₂ with an oxygen content of 0.06 wt%, fine grains (3.3 μm) and superior mechanical properties (flexural strength of 660 MPa).     

Notes on the issues of equilibrium in the Fischer–Tropsch synthesis

The product distribution for the Fischer–Tropsch synthesis is normally described using the kinetically derived (Anderson–Schultz–Flory) ASF model. Variations of the kinetic model have been proposed to explain deviations from the ASF distribution. The Fischer–Tropsch system can be equally well described using a pseudo‐element (CH₂, H₂, O) equilibrium approach. A one‐parameter equilibrium model is derived for the product distributions for alkenes, alkanes and alcohols. The Fischer–Tropsch system should be considered as three separate partial equilibria systems: the product homologous series; the water gas shift system, and the redox behaviour of the catalyst with the H₂/O ratio of the gas. This approach correctly predicts the impacts of changes in a variety of parameters (temperature pressure, feed composition) on the ASF product distribution. In addition, the catalyst phase changes with gas composition and pressure, indicative of an equilibrium response. Equilibrium is of much greater importance to the Fischer–Tropsch system than previously thought, and the decision to use a complex kinetics‐based model rather than a simpler equilibrium based model should be taken with care.     

Adsorption separation of N2, O2, CO2 and CH4 gases by β-zeolite

In the present study, adsorption equilibrium and kinetic separation potential of β-zeolite is investigated for N₂, O₂, CO₂ and CH₄ gases by using concentration pulse chromatography. Adsorption equilibrium and kinetic parameters have been studied. Henry’s Law constants, heat of adsorption values, micro-pore diffusion coefficients and adsorption activation energies are determined experimentally. The three different mass transfer mechanisms, that have to take place for adsorption to occur, are discussed. From the equilibrium and kinetic data, the equilibrium and kinetic selectivities are determined for the separation of the gases studied.With β-zeolite, carbon dioxide has the highest adsorption Henry’s Law constant at all the temperatures studied, followed by methane, nitrogen and oxygen. Carbon dioxide separation from oxygen, nitrogen and methane has good equilibrium separation factors. This factor is not very high for methane/nitrogen and methane/oxygen systems and is the lowest for nitrogen/oxygen system. Micro-pore diffusion is the dominant mass transfer mechanism for all the systems studied, except CH₄, with β-zeolite. The kinetic separation factors are very small at high temperatures for all the systems studied. Nitrogen/carbon dioxide and oxygen/carbon dioxide can be separated in kinetic processes with reasonable separation factors at low temperatures. Both equilibrium and kinetic separation factors decrease as column temperature increases. Considering all the observations from this study, it was concluded that β-zeolite is a good candidate for applications in flue gas separations, as well as natural gas and landfill gas purifications.     

Partial oxidation of methane to synthesis gas over Rh/α-Al2O3 at high temperatures

The partial oxidation of methane to synthesis gas has been studied in a continuous flow reactor using a Rh/α-Al₂O₃ catalyst under conditions as close as possible to those industrially relevant: pressures up to 800 kPa and temperatures higher than 1274 K in order to avoid the formation of carbon and to obtain high equilibrium selectivities to CO and H₂. Intrinsic kinetic data were obtained when the feed was diluted with helium. Gas-phase reactions were found to occur at 500 kPa when the feed was not diluted. A reaction network has been derived from experimental results in which oxygen conversions range from 0 to 1. CO₂, C₂H₆ and H₂O are the primary products. C₂H₄ is formed by oxidative dehydrogenation of C₂H₆. CO and H₂ are formed by reforming of CH₄ by CO₂ and H₂O; an additional direct route to CO and H₂ at low oxygen conversions cannot be excluded. The catalyst appears to be present in two states, the transition being at an oxygen conversion of 0.4 under the conditions used. The support probably enhances oxidation reactions by reverse spillover of oxygen or hydroxyl species onto rhodium. The support as such behaves similarly to the catalyst at low oxygen conversions, but shows no reforming activity. This revised version was published online in July 2006 with corrections to the Cover Date.     

Separation of C2–C4 hydrocarbons from methane by zeolite MFI hollow fiber membranes fabricated from 2D nanosheets

Separation of higher hydrocarbons from methane is an important and energy-intensive operation in natural gas processing. We present a detailed investigation of thin and oriented MFI zeolite membranes fabricated from 2D MFI nanosheets on inexpensive α-alumina hollow fiber supports, particularly for separation of n-butane, propane, and ethane (“natural gas liquids”) from methane. These membranes display high permeances and selectivities for C₂–C₄ hydrocarbons over methane, driven primarily by stronger adsorption of C₂–C₄ hydrocarbons. We study the separation characteristics under unary, binary, ternary, and quaternary mixture conditions at 298 K and 100–900 kPa feed pressures. The membranes are highly effective in quaternary mixture separation at elevated feed pressures, for example allowing n-butane/methane separation factors of 170–280 and n-butane permeances of 710–2,700 GPU over the feed pressure range. We parametrize and apply multicomponent Maxwell–Stefan transport equations to predict the main trends in separation behavior over a range of operating conditions.     

Simulation of fluidized-bed reactors for catalytic oxidative coupling of methane to C2-hydrocarbons

The “bubble assemblage model” of Kato and Wen was applied to simulate the catalytic oxidative coupling of methane to C₂-hydrocarbons in a fluidized bed reactor. Simulation results were compared to experimental data obtained in a laboratory-scale fluidized bed reactor. To improve the accuracy of predictions, the influence of fluid bed hydrodynamic and kinetic submodels was investigated by applying a sensitivity analysis. It was shown that the most important element in the model is the applied reaction scheme; the consecutive reactions of C₂ hydrocarbons occurring most probably in the gas phase should be considered.     

Dry reforming of methane over palladium–platinum on carbon nanotube catalyst

A dry reforming (DR) catalyst based on bimetallic Pd–Pt supported on carbon nanotubes is presented. The catalyst was prepared using a microwave-induced synthesis. It showed enhanced DR activity in the 773–923?K temperature range at 3 atm. Observed carbon balances between the reactant and product gases imply minimal carbon deposition. A global three-reaction (reversible) kinetic model—consisting of DR, reverse water gas shift, and CH₄ decomposition (MD)—adequately simulates the observed concentrations, product H₂/CO ratios, and reactant conversions. Analysis shows that, under the conditions of this study, the DR and MD reactions are net forward and far from equilibrium, while the RWGS is near equilibrium.     

Equilibrium calculations for direct synthesis of dimethyl ether from syngas

Thermodynamic analysis of single‐step synthesis of dimethyl ether (DME) from syngas over a bi‐functional catalyst (BFC) in a slurry bed reactor has been investigated as a function of temperature (200–240°C), pressure (20–50 bar), and composition feed ratio (H₂/CO: 1–2). The BFC was prepared by physical mixing of CuO/ZnO/Al₂O₃ as a methanol synthesis catalyst and H‐ZSM‐5 as a methanol dehydration catalyst. The three reactions including methanol synthesis from CO and H₂, methanol dehydration to DME and water–gas shift reaction were chosen as the independent reactions. The equilibrium thermodynamic analysis includes a theoretical model predicting the behaviour and a comparison to experimental results. Theoretical model calculations of thermodynamic equilibrium constants of the reactions and equilibrium composition of all components at different reaction temperature, pressure, and H₂/CO ratio in feed are in good accordance with experimental values.     

Gas hydrate concentration measurements on sucrose solutions using a new pilot test rig

A new 750 cm³ pilot test rig based on the “isochoric pressure method” was designed and commissioned for the hydrate measurements to concentrate sucrose solutions. The reactor included an improved agitation system and enabled sampling of the sucrose solutions. The experimental method was validated be performing dissociation measurements for the CO₂ + water system. Gas hydrate kinetic and sampling data were measured for the CO₂ + sucrose solutions at sucrose concentrations between (12–60) ^oBrix, within the temperature range of (274.65–276.15) K and at pressures up to 3.70 MPa. Results showed that sucrose is a kinetic inhibitor. The data were modeled to obtain hydrate formation rate, storage capacity, gas consumption and apparent rate constant. Stage-wise concentration measurements were performed with reactor conditions at 274.65 K, 3.70 MPa and 130 rpm mixer speed with liquid sample withdrawal. A final sucrose product of approximately 60 ^oBrix was obtained.     

Synthesis gas production using steam hydrogasification and steam reforming

Experimental work has been carried out on the mixed reforming reaction, i.e., simultaneous steam and CO₂ reforming of methane under a wide range of feed compositions and four different reaction temperatures from 700 °C to 850 °C using a commercial steam reforming catalyst. The experiments were conducted for a CO₂/CH₄ ratio from 0 to 2 and a steam to methane ratio from 3 to 5. The effect of CO₂/CH₄ ratio on the exit H₂/CO ratio and the conversions of the reactants indicate that the dry reforming reaction is dominant under increased carbon dioxide in the feed. Steam reforming of typical steam hydrogasification product gas consisting of CO, H₂ and CO₂ in addition to steam and methane has also been investigated. The H₂/CO ratio of the product synthesis gas varies from 4.3 to 3.7 and from 4.8 to 4.1 depending on the feed composition and reaction temperature. The CO/CO₂ ratios of the synthesis gas varied from 1.9 to 2.9 and 2.0 to 3.3. The results are compared with simulation results obtained through the Aspen Plus process simulation tool. The results demonstrate that a coupled steam hydrogasification and reforming process can generate a synthesis gas with a flexible H₂/CO ratio from carbon-containing feedstocks.     

Analysis and Modeling of the Hydroisomerization Process for <Emphasis Type="Italic">n</Emphasis>-Hexane

A stage six-path mechanism for the reaction of n-hexane hydroisomerization on a BEA Pt-zeolite catalyst has been proposed. The mass flow rate of n-hexane in the feed stream varies in the range of 0.7?2.8 h^?1; the mole ratio of hydrogen to the n-hexane range of 2.7–14.6; the reactor temperature falls in the range from 453 to 573 K; and the gas flow pressure varies in the range of 1.0–10.0 atm. Thirty experiments have been carried out. In the product stream, the concentrations of n-hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane, 2,3-dimethylbutane, propane, C₁-alkanes, and C₂-alkanes are analyzed by gas chromatography. A kinetic model has been constructed for the six-path mechanism of the reaction of n-hexane hydroisomerization, which contains 15 kinetic constants. Based on the results of laboratory experiments, the kinetic constants of the model have been estimated by the method of nonlinear least squares. The model has been shown to correspond with the experiment in the selected field of experimentation.     

Theoretical and experimental study on the emission characteristics of waste plastics incineration by modified O2/RFG combustion technology

For mitigating the emission of greenhouse gas CO₂ from general air combustion systems, a clean combustion technology O₂/RFG is in development. The O₂/RFG combustion technology can significantly enhance the CO₂ concentration in the flue gas; however, using almost pure oxygen or pure CO₂ as feed gas is uneconomic and impractical. As a result, this study proposes a modified O₂/RFG combustion technology in which the minimum pure oxygen is mixed with the recycled flue gas and air to serve as the feed gas. The effects of different feed gas compositions and ratios of recycled flue gas on the emission characteristics of CO₂, CO and NO_x during the plastics incineration are investigated by theoretical and experimental approaches.Theoretical calculations were carried out by a thermodynamic equilibrium program and the results indicated that the emissions of CO₂ were increased with the O₂ concentrations in the feed gas and the ratios of recycled flue gas increased. Experimental results did not have the same trends with theoretical calculations. The best feed gas composition of the modified O₂/RFG combustion was 40% O₂ + 60% N₂ and the best ratio of recycled flue gas was 15%. As the O₂ concentration in feed gas and the ratio of recycled flue gas increased, the total flow rates and pressures of feed gas reduced. The mixing of solid waste and feed gas was incomplete and the formation of CO₂ decreased. Moreover, the emission of CO was decreased as the O₂ concentration in feed gas and the ratio of recycled flue gas increased. The emission of NO_x gradually increased with rising the ratio of recycled flue gas at lower O₂ concentration (<40%) but decreased at higher O₂ concentration (>60%).