Kinetic modeling of oxidative dehydrogenation of propane with CO2 over MoOx/La2O3–Al2O3 in a fluidized bed

A 4-step kinetic model of CO₂-assisted oxidative dehydrogenation (ODH) of propane to C₂/C₃ olefins over a novel MoO_x/La₂O₃–γAl₂O₃ catalyst was developed. Kinetic experiments were conducted in a CREC Riser Simulator at various reaction temperatures (525–600 °C) and times (15–30 s). The catalyst was highly selective towards propylene at all combinations of the reaction conditions. Langmuir-Hinshelwood type kinetics were formulated considering propane ODH, uni- and bimolecular cracking of propane to produce a C₁-C₂ species. It was found that the one site type model adequately fitted the experimental data. The activation energy for the formation of propylene (67.8 kJ/mol) is much lower than that of bimolecular conversion of propane to ethane and ethylene (303 kJ/mol) as well as the direct cracking of propane to methane and ethylene (106.7 kJ/mol). The kinetic modeling revealed the positive effects of CO₂ towards enhancing the propylene selectivity over the catalyst.     

Oxidation of ethane on high specific surface SmCoO3 and PrCoO3 perovskites

An adapted sol–gel method allowed synthesizing SmCoO₃ and PrCoO₃ oxides with high specific surface (ca. 28 m² g⁻¹) and a relatively clean perovskite phase at 600 °C, a temperature much lower than the one required in ceramic methods. The perovskites were investigated as catalysts for the oxidation of ethane in the temperature range 300–400 °C. Both catalysts were very active: ethane was activated already at 300 °C, i.e., 100 °C below the temperatures previously reported for perovskites. The main product was CO₂ on both catalysts, but on PrCoO₃ oxidehydrogenation (ODH) to ethylene was observed already at 300 °C, with the low selectivity. Even so, this was quite unusual for simple perovskites, and for such a low temperature. TPR data showed that praseodymium decreases the reducibility of Co³⁺ in the perovskite, what could explain the observed ODH, and suggest it proceeds via a Mars–van Krevelen mechanism. Kinetic study showed a similar apparent activation energy for both catalysts (ca. 80 kJ/mol), but a difference in the nature of the participating oxygen species: while on PrCoO₃ both adsorbed and lattice species contribute to the reaction, on SmCoO₃ contribution of adsorbed species is practically negligible, due to its very high oxygen lability. The results show that these simple perovskites may be promising catalysts for ethane oxidation at relatively low temperatures.     

Analysis of Membrane Reactors for Integrated Coupling of Oxidative and Thermal Dehydrogenation of Propane

This study presents strategies capable to intensify the thermal dehydrogenation of propane (TDH) using integrated reactor concepts. An inert packed bed membrane reactor for distributed dosing of oxygen to realize the oxidative dehydrogenation (ODH) was studied and compared to a reactor with catalytically active membrane. The latter concept allows to combine TDH and ODH in one apparatus to overcome the chemical equilibrium by in situ conversion of the by‐product H₂ using O₂ or in a reverse water‐gas shift reaction by CO₂. If CO₂ is used as active sweep gas the reactor offered better performance regarding yield and selectivity. Strategies for further thermal integration are discussed.     

New Integrated Catalytic Membrane Processes for Enhanced Propylene and Polypropylene Production

Newly reported integrated processes are discussed for aliphatic (paraffin) hydrocarbon dehydrogenation into olefins and subsequent polymerization into polyolefins (e.g., propane to propylene to polypropylene, ethane to ethylene to polyethylene). Catalytic dehydrogenation membrane reactors (permreactors) made by inorganic or metal membranes are employed in conjunction with fluid bed polymerization reactors using coordination catalysts. The catalytic propane dehydrogenation is considered as a sample reaction in order to design an integrated process of enhanced propylene polymerization. Related kinetic experimental data of the propane dehydrogenation in a fixed bed type catalytic reactor is reviewed which indicates the molecular range of the produced C₁-C₃ hydrocarbons. Experimental membrane reactor conversion and yield data are also reviewed. Experimental data were obtained with catalytic membrane reactors using the same catalyst as the non-membrane reactor. Developed models are discussed in terms of the operation of the reactors through computational simulation, by varying key reactor and reaction parameters. The data show that it is effective for catalytic permreactors to provide streams of olefins to successive polymerization reactors for the end production of polyolefins (i.e., polypropylene, polyethylene) in homopolymer or copolymer form. Improved technical, economic, and environmental benefits are discussed from the implementation of these processes.     

Bi4Cu0.2V1.8O11−δ based electrolyte membrane reactor for selective oxidation of propane to acrylic acid

An electrochemical membrane reactor using Bi₄Cu_0.2V_1.8O_11?δ as a solid electrolyte membrane was employed to investigate the selective oxidation of propane to acrylic acid over a MoV_0.3Te_0.17Nb_0.12O catalyst. By applying an external current to the membrane reactor, the ionic oxygen was pumped to the surface of the MoV_0.3Te_0.17Nb_0.12O catalyst, which exhibited higher conversion of propane and selectivity to acrylic acid than that in the fixed-bed reactor. The results indicate that the enhancement of catalytic performance in the membrane reactor is mainly due to the presence of the lattice oxygen, which has been proved to be necessary for the formation of acrylic acid. Thus, the Au/Bi₄Cu_0.2V_1.8O_11?δ/Au/MoVTeNbO membrane reactor achieved a higher conversion of propane (42%) and selectivity to acrylic acid (79.6%) at 380 °C with a 0.6 A current.     

Solid–fluid phase behaviour of linear polyethylene solutions in propane, ethane and ethylene at high pressures

The solid–fluid phase transitions for a series of linear polyethylene fractions (M_w: 800, 7000, 23 625, 52 000, 59 300 g/mol) in propane, ethane and ethylene were measured in the temperature range from 360 to 423 K and at pressures up to 2000 bar. Conditions of precipitation of the solid polyethylene from supercritical solutions seem to have minor influence on dissolution of the solid polymer in supercritical solvents. In general, an increase of pressure results in a shift of the solid–fluid phase transition to higher temperatures, i.e. solubility of polyethylene decreases. The solid–fluid phase boundaries for systems composed of low-molecular weight polyethylene and propane show temperature minima. The effect is not observed in the high-molecular weight polyethylene + propane systems and systems with ethane or ethylene as solvents. It was observed that the temperature of the solid–fluid phase transition measured at a constant pressure and a constant composition is not a monotonic function of molecular weight of the polymer. The order of the dissolution temperatures depends on pressure.     

Selective oxidation of propane and ethane on diluted Mo–V–Nb–Te mixed-oxide catalysts

Te-free and Te-containing Mo–V–Nb mixed oxide catalysts were diluted with several metal oxides (SiO₂, γ-Al₂O₃, α-Al₂O₃, Nb₂O₅, or ZrO₂), characterized, and tested in the oxidation of ethane and propane. Bulk and diluted Mo–V–Nb–Te catalysts exhibited high selectivity to ethylene (up to 96%) at ethane conversions <10%, whereas the corresponding Te-free catalysts exhibited lower selectivity to ethylene. The selectivity to ethylene decreased with the ethane conversion, with this effect depending strongly on the diluter and the catalyst composition. For propane oxidation, the presence of diluter exerted a negative effect on catalytic performance (decreasing the formation of acrylic acid), and α-Al₂O₃ can be considered only a relatively efficient diluter. The higher or lower interaction between diluter and active-phase precursors, promoting or hindering an unfavorable formation of the active and selective crystalline phase [i.e., Te₂M₂₀O₅₇ (M = Mo, V, and Nb)], determines the catalytic performance of these materials.     

Model-based Analysis of Fixed-bed and Membrane Reactors of Various Scale

A packed-bed membrane reactor in a distributor configuration is studied theoretically for the oxidative propane dehydrogenation and compared with a fixed-bed reactor. Based on detailed 2D models considering two different heat and mass transport models the reactor scale-up including various reactor-to-particle diameter ratios (D/d_P) is analyzed with respect to reactor performance, heat transfer and hot spot formation. Higher selectivities at lower hot spot temperatures occur in the packed-bed membrane reactor for the same reaction conditions.     

Theoretical study of the application of porous membrane reactor to oxidative dehydrogenation of n-butane

This paper is the theoretical study of the oxidative dehydrogenation of n-butane in porous membrane reactors. Performance of the membrane reactors was compared with that of conventional fixed-bed reactors. The porous membrane was employed to add oxygen to the reaction side in a controlled manner so that the reaction could take place evenly.Mathematical models for the fixed-bed reactor and the membrane reactor were developed considering non-isothermal condition and both radial heat and mass dispersion. From this study, it was found that the hot spot problem was pronounced particularly near the entrance of the conventional fixed-bed reactor. In addition, the assumption of plug flow condition did not adequately represent the reaction system. The effect of radial dispersion must be taken into account in the modelling.The use of the porous membrane to control the distribution of oxygen feed to the reaction side could significantly reduce the hot spot temperature. The results also showed that there were optimum feed ratios of air/n-butane for both the fixed-bed reactors and the membrane reactors. The membrane reactor outperformed the fixed-bed reactor at high values of the ratio. In addition, there was an optimum membrane reactor size. When the reactor size was smaller than the optimum value, the increased reactor size increased the reaction and heat generation and, consequently, the conversion and the selectivity to C₄ increased. However, when the reactor size was larger than the optimum value, oxygen could not reach the reactant near the stainless steel wall. It was consumed to react with the product, C₄. As a result, the yield dropped. Finally, it was found that the increase of wall temperature increased the yield and that the feed air temperature could help control the temperature profile of the reaction bed along the reactor length.     

Modelling and experimental studies on unsteady-state SO2 converters

This paper deals with experimentation of the unsteady-state fixed-bed reactor with flow reversal for SO₂ conversion. A laboratory reactor with compensatory heating coils has been set up, resulting in 96–99% conversion for SO₂ = 2.62–9.52 vol%, and u = 0.2 m/s. The results were from agreement with simulation of a really adiabatic unsteady-state reactor, but agreed well with modelling by considering the radial heat transfer. A three-stage pilot-scale converter has also been run with the M-typed temperature profile produced by heat loss from the chamber between stages. Good agreement was achieved with modelling by taking into account this feature. The results show that, while the adiabatic requirement is difficult to fulfill in the well-accepted experimental reactors, simulation and modelling are powerful means for compensation.     

Continuous Oxygen Ion Transfer Medium as a Catalyst for High Selective Oxidative Dehydrogenation of Ethane

An oxygen permeable mixed ion and electron conducting membrane (OPMIECM) was used as an oxygen transfer medium as well as a catalyst for the oxidative dehydrogenation of ethane to produce ethylene. O^2- species transported through the membrane reacted with ethane to produce ethylene before it recombined to gaseous O², so that the deep oxidation of the products was greatly suppressed. As a result, 80% selectivity of ethylene at 84% ethane conversion was achieved, whereas 53.7% ethylene selectivity was obtained using a conventional fixed-bed reactor under the same reaction conditions with the same catalyst at 800 °C. A 100 h continuous operation of this process was carried out and the result indicates the feasibility for practical applications.     

A comparative study of combination of Fischer–Tropsch synthesis reactors with hydrogen-permselective membrane in GTL technology

This work proposes a one dimensional heterogeneous model to analyze the performance of combination of Fischer–Tropsch synthesis (FTS) reactors in which a fixed-bed reactor is combined with a membrane assisted fluidized-bed reactor. This model is used to compare the performance of the proposed system with a fixed-bed singlestage reactor. In the new concept, the synthesis gas is converted to FT products in two catalytic reactors. The first reactor is water-cooled fixed-bed type while the second reactor is gas-cooled and fluidized-bed. Due to the decrease of H₂/CO to values far from optimum reactants ratio, the membrane concept is suggested to control hydrogen addition. Moreover, a fluidized-bed system has been proposed to solve some observed drawbacks of industrial fixed-bed reactors such as high pressure drop, heat transfer problem and internal mass transfer limitations. This novel concept which has been named fluidized-bed membrane dual-type reactor is used for production of gasoline from synthesis gas. The reactor model is tested against the pilot plant data of the Research Institute of Petroleum Industry. Results show an enhancement in the gasoline yield, a main decrease in CO₂ formation and a favorable temperature profile along the proposed concept.     

Selective oxidation of ethane: Developing an orthorhombic phase in Mo–V–X (X = Nb, Sb, Te) mixed oxides

Mo–V–X (X = Nb, Sb and/or Te) mixed oxides have been prepared by hydrothermal synthesis and heat-treated in N₂ at 450 °C or 600 °C for 2 h. The calcination temperature and the presence or absence of Nb determines the nature of crystalline phases in the catalyst. Nb-containing catalysts heat-treated at 450 °C are mostly amorphous solids, while Nb-free catalysts heat-treated at 450 °C and samples treated at 600 °C clearly contain crystalline phases. TPR-H₂ experiments show higher H₂-consumption on catalysts with amorphous phases. Catalytic results in the oxidative dehydrogenation of ethane indicate that the selective production of the olefin is strongly related to the development of the orthorhombic Te₂M₂₀O₅₇ or (SbO)₂M₂₀O₅₆ (M = Mo, V, Nb) phase (the so-called M1 phase), which is mainly formed at 600 °C. This active and selective crystalline phase is characterized to show moderate reducibility and active centers enough for the selective oxidative activation of ethane with the minimum quantity possible of active centers for ethylene activation. In this sense, the best yield to ethylene has been achieved on a Mo–V–Te–Nb mixed oxide.     

High-temperature parallel screening of catalysts for the oxidative coupling of methane

The oxidative coupling of methane (OCM) was investigated with a specifically designed multi-channel device operating fixed-bed reactors at high temperature and atmospheric pressure. The device allows precise temperature measurement in each channel selected for analysis and possesses a quench cooling unit right after the reaction zone. Analysis is based on a mass spectrometer allowing a time resolution of only 3 s per analysis and 30 s per reactor channel. Successful screening is demonstrated using a reactant feed of O₂ and CH₄ diluted in Ar at flows between 100 and 166 mL/min per reactor channel. As expected, Li/MgO-based catalysts showed good initial performance, but rapid deactivation at 800 °C excludes their use in high-temperature applications. Good C₂ selectivity up to 80% and high yields up to 20% were observed for La/Sr/CaO catalysts. Even more interesting, no decline in performance was observed for those formulations identifying 10% La/20% Sr/CaO as best catalyst in an initial library. Screening of various La and Sr loadings at different operating conditions identified a optimal content of 5–10% for La and 20% for Sr. The exploration of operating conditions showed increasing C₂ productivities with increasing reactant partial pressure, reaching at 800 °C values up to 1.3 × 10⁻⁵ mol (C₂) s⁻¹ g(cat)⁻¹.     

Performance evaluation of a novel reactor configuration for oxidative dehydrogenation of ethane to ethylene

A one-dimensional non-isothermal steady state model was developed to simulate the performance of three-reactor configurations for the oxidative dehydrogenation of ethane (ODHE) to ethylene. These configurations consist of side feeding reactor (SFR), conventional fixed bed reactor (CFBR) and membrane reactor (MR). The performance of these reactors was compared in the terms of C₂H₆ conversion, C₂H₄ and CO₂ selectivity and temperature profiles. The use of sectional air injections on the wall of SFR with a limited number of injection points showed that the performance of reactor significantly improves and optimum pattern of oxygen consumption is also obtained. Moreover, our SFR with a liquid coolant medium operates in an effectively controlled temperature profile that is comparable with that of the MR, which is cooled by a coolant stream of air. Hence, an enhancement in the level of selectivity is obtained for the SFR configuration. Consequently, the side feeding procedure can decrease the high operating temperature problem and low ethylene selectivity in the ODHE process. According to obtained results, the SFR would be a proper alternative for both the MR and CFBR.     

Concentration and residence time effects in packed bed membrane reactors

A fixed bed reactor (FBR) and a packed bed membrane reactor (PBMR) were compared with respect to their performance in the oxidative dehydrogenation of ethane over VO_x/γ-Al₂O₃ catalyst. The experiments were carried out at high space velocities and under oxygen excess conditions. In the PBMR, the oxidant air was distributed from the shell side of the membrane.

At similar overall feed configurations, the conversion of ethane was found to be higher in the PBMR. This effect was most pronounced at the highest space velocity. Mostly ethylene yield was higher in the PBMR than in the FBR. However, the yield of carbon oxides increased more. Thus, an improvement of olefin selectivity was not observed. There were even sets of experimental conditions, where the ethylene yield in the PBMR fell below the corresponding value for the FBR. In the PBMR under oxygen excess conditions, the consecutive oxidation of ethylene is more favoured than in the FBR.

Two essential reasons for the observed differences in the reactor performances are discussed. At first, there are different local reactant concentrations. Secondly, there are essential differences in the residence time behaviour of the reactants in the FBR and PBMR. In order to exemplify the latter aspect additional experiments have been carried out using a cascade of three identical PBMRs. Varying the specific oxygen flow rates over the individual membrane segment walls different dosing profiles were implemented. The results obtained in this study emphasise the general potential, but also the limits of membrane reactors compared to the FBR.     

Development and Application of Oxygen Permeable Membrane in Selective Oxidation of Light Alkanes

In this paper, oxygen permeable membrane used in membrane reactor for selective oxidation of alkanes will be discussed in detail. The recent developments for the membrane materials will be presented, and the strategy for the selection of the membrane materials will be outlined. The main applications of oxygen permeable membrane in selective oxidation of light alkanes will be summarized, which includes partial oxidation of methane (POM) to syngas and partial oxidation of heptane (POH) to produce H₂, oxidative coupling of methane (OCM) to C₂, oxidative dehydrogenation of ethane (ODE) to ethylene and oxidative dehydrogenation of propane (ODP) to propylene. Achievements for the membrane material developments and selective oxidation of light alkanes in membrane reactor in our group are highlighted.     

Optimization of ODHE membrane reactor based on mixed ionic electronic conductor using soft computing techniques

This work presents the optimization of the operating conditions of a membrane reactor for the oxidative dehydrogenation of ethane. The catalytic membrane reactor is based on a mixed ionic–electronic conducting material, i.e. Ba_0.5Sr_0.5Co_0.8Fe_0.2O_δ−3, which presents high oxygen flux above 750 °C under sufficient chemical potential gradient. Specifically, diluted ethane is fed into the reactor chamber and air (or diluted air) is flushed to the other side of the membrane. A framework based on Soft Computing techniques has been used to maximize the ethylene yield by simultaneously varying five operation variables: nominal reactor temperature (Temp); gas flow in the reaction compartment (QHC); gas flow in the oxygen-rich compartment (QAir); ethane concentration in the reaction compartment (%C₂H₆); and oxygen concentration in oxygen-rich compartment (%O₂). The optimization tool combines a genetic algorithm guided by a neural network model. This shows how the neural network model for this particular problem is obtained and the analysis of its behavior along the optimization process. The optimization process is analyzed in terms of: (1) catalytic figures of merit, i.e., evolution of yield and selectivity towards different products and (2) framework behavior and variable significance. The two experimental areas maximizing the ethylene yield are explored and analyzed. The highest yield reached in the optimization process exceeded 87%.     

Partial Oxidation of Propane over Rubidium-Promoted MoO3/SiO2 Catalyst

Silica-supported Rb₂MoO₄ and rubidium-promoted MoO₃ were used as catalysts for the partial oxidation of propane in a fixed-bed continuous-flow reactor at 770–823 K using N₂O as oxidant. The main hydrocarbon products of the reaction were propylene, ethylene, propanal and methane. Addition of various compounds of rubidium to the MoO₃/SiO₂ greatly enhanced the conversion of propane and promoted the formation of propanal at the expense of ethylene and propylene. The highest yield for the production of this compound was found over Rb₂MoO₄/SiO₂ catalyst.