Oxidative coupling of methane over a La2O3/CaO catalyst. Optimization of reaction conditions in a bubbling fluidized-bed reactor

Oxidative coupling of methane over a La₂O₃/CaO catalyst was investigated in laboratory-scale fluidized-bed reactors (ID = 5 and 7 cm) in the following range of reaction conditions: T = 700 – 880°C, P = 41 – 72 kPa and P = 6 – 29 kPa. The maximum C₂₊ selectivity and yield amounted to 73.8% (T = 800°C, X = 13.1%, Y = 9.7%) and 16.0% (T = 840°C, X = 34.0%, S = 47.2%), respectively. Axial gas concentration profiles revealed that C₂₊ selectivity was not only influenced by oxidative consecutive reactions, but also by steam reforming of ethylene. When diluting the catalytic bed (m_cat = 145 g) with quartz (m = 200 and 400 g), a slight decrease of the selectivity (1–2%) was observed. The dilution of the feed gas with nitrogen only led to only a small increase (< 2%) of the C₂₊ selectivity.     

Reaction engineering studies in a polytropic fixed-bed reactor over a highly active and selective catalyst for oxidative methane coupling

The oxidative coupling of methane (OCM) was carried out in a polytropic fixed-bed reactor applying a Zr/La/Sr catalyst developed by the Neste company. Over this catalyst the OCM reaction follows a complex reaction scheme which includes primary parallel reaction steps to CO, CO₂ and C₂H₆ and consecutive reactions of ethane to ethylene or CO_x. Yield of higher hydrocarbons C₂₊ obtained with this catalyst strongly depended on reaction conditions, i.e. low partial pressures of methane and oxygen obtained by diluting the feed gas with nitrogen and high reaction temperatures promoted C₂₊ selectivity and yield. The maximum yield amounted to 21.4% (20 Vol.-% CH₄, 9 Vol.-% O₂, 71 Vol.-% N₂, T = 860°C; X_CH4 = 41.8%, S = 52.5%). This result belongs to the highest yields reported in the open literature.     

Comparative study of the two-zone fluidized-bed reactor and the fluidized-bed reactor for oxidative coupling of methane over Mn/Na2WO4/SiO2 catalyst

In this work, oxidative coupling of methane over Mn/Na₂WO₄/SiO₂ catalyst is studied in a two-zone fluidized-bed reactor (TZFBR) and its performance is compared with a fluidized-bed reactor (FBR). Diluted oxygen in argon was introduced into the bottom of the TZFBR through a quartz ferrite and methane was entered at higher positions along the fluidized bed. The catalyst circulated between the oxygen-rich and methane-rich zones in the TZFBR reactor. The effects of the main operating variables including bed temperature, the methane/oxygen ratio (R_mo), and the height at which methane was introduced into the reactor (H_m) were investigated. It is found that under some operating conditions the TZFBR gives a higher C₂ selectivity than that obtained in the FBR reactor. Reaction of methane with lattice oxygen of the Mn/Na₂WO₄/SiO₂ redox catalyst in the methane-rich zone may have led to the higher selectivity.     

Experimental Investigations of Catalytic Partial Oxidation of Methane to Synthesis Gas in Various Types of Fluidized‐Bed Reactors

The partial oxidation of methane to synthesis gas over Ni/α‐Al₂O₃ catalysts (1 and 5 wt.‐% Ni loading, 71–160 and 250–355 μm particle diameter) was investigated in different types of fluidized‐bed reactors, i.e., the bubbling fluidized bed (FlB), the spout fluid bed (SFB) and the internally circulating fluidized bed (ICFB). A methane‐to‐oxygen ratio of 2:1 was used in all experiments and the temperature was varied between 700 and 800 °C. Gas velocities and catalyst masses were adjusted to assure a stable and controllable reactor operation. A nearly isothermal operation was established in all reactors. The thermodynamic equilibrium values were achieved in the FlB and SFB reactor whereas in the ICFB reactor slightly lower conversions and selectivities were obtained. Taking the direct scale‐up concept of the ICFB reactor into account, significant higher space‐time yields were obtained in this reactor than in the industrial‐scale bubbling fluidized‐bed reactor. No increase of the space‐time yield in comparison to the FlB was obtained in the SFB reactor.     

Oxidative dehydrogenation of n-butane on V/MgO catalysts. Influence of the type of contactor

A comparative study of the catalytic performance of a selective V-Mg-O catalyst in the oxidative dehydrogenation of n-butane is presented using three different types of reactor: (i) an adiabatic fixed-bed reactor; (ii) a fluidized-bed reactor; and (iii) an in situ redox fluidized-bed reactor. The results obtained indicate that the in situ redox fluidized-bed reactor outperforms the conventional fixed- and fluidized-bed reactors, especially at high n-butane conversions. Thus, a selectivity to C₄ olefins of 54% at n-butane conversions of 60% was achieved at 550°C using an in situ redox fluidized-bed reactor while selectivities to C₄-olefins lower than 43% were obtained on the other reactor types under the same reaction conditions (isoconversion and reaction temperature). This revised version was published online in July 2006 with corrections to the Cover Date.     

Performance studies of various cracking catalysts in the conversion of canola oil to fuels and chemicals in a fluidized-bed reactor

Studies were conducted at atmospheric pressure at temperatures in the range of 400–500°C and fluidizing gas velocities in the range of 0.37–0.58 m/min (at standard temperature and pressure) to evaluate the performance of various cracking catalysts for canola oil conversion in a fluidized-bed reactor. Results show that canola oil conversions were high (in the range of 78–98 wt%) and increased with an increase in both temperature and catalyst acid site density and with a decrease in fluidizing gas velocity. The product distribution mostly consisted of hydrocarbon gases in the C₁–C₅ range, a mixture of aromatic and aliphatic hydrocarbons in the organic liquid product (OLP) and coke. The yields of C₄ hydrocarbons, aromatic hydrocarbons and C₂–C₄ olefins increased with both temperature and catalyst acid site density but decreased with an increase in fluidizing gas velocity. In contrast, the yields of aliphatic and C₅ hydrocarbons followed trends completely opposite to those of C₂–C₄ olefins and aromatic hydrocarbons. A comparison of performance of the catalysts in a fluidized-bed reactor with earlier work in a fixed-bed reactor showed that selectivities for formation of both C₅ and iso-C₄ hydrocarbons in a fluidized-bed reactor were extremely high (maximum of 68.7 and 18 wt% of the gas product) as compared to maximum selectivities of 18 and 16 wt% of the gas product, respectively, in the fixed-bed reactor. Also, selectivity for formation of gas products was higher for runs with the fluidized-bed reactor than for those with the fixed-bed reactor, whereas the selectivity for OLP was higher with the fixed-bed reactor. Furthermore, both temperature and catalyst determined whether the fractions of aromatic hydrocarbons in the OLP were higher in the fluidized-bed or fixed-bed reactor.     

Pyrolysis of methane in the presence of hydrogen

The kinetics of methane pyrolysis were studied in a tubular flow reactor in the temperature range 1200 to 1500°C at atmospheric pressure. To avoid excessive carbon formation the reaction time was short and the methane feed was diluted with hydrogen. Ethene, ethyne, benzene and hydrogen were the main gaseous products. Ethane was observed as a product at very low conversions of methane. More than 90% selectivity was obtained for C₂ products. The ratio of ethyne to ethene increased with increasing temperature. The yield of C₂ products is limited by gas-phase equilibrium at lower temperatures. Formation of carbon was strongly depressed by hydrogen at higher temperatures. The maximum yield of ethyne was found to increase from about 10% to about 50% when the temperature was increased from 1200 to 1500°C, with hydrogen dilution H₂: CH₄ = 2: 1. A mechanistic reaction model was used to simulate the pyrolysis of methane at the actual conditions. A sensitivity analysis was performed to evaluate the elementary reactions which influence the formation and consumption of the species in the model system.     

Reaction engineering simulations of a fluidized-bed reactor for selective oxidation of fluorene to 9-fluorenone

The catalytic oxidation of fluorene to 9-fluorenone in a fluidized-bed reactor was investigated by modeling of the reactor and simulation of its performance. The “Bubble Assemblage Model” of Kato and Wen, the “Bubbling Bed Model” of Kunii and Levenspiel and the “Countercurrent Backmixing Model” of Potter were applied. From a comparison of simulation results obtained by the various fluidized-bed models and a fixed-bed model conclusions were drawn about the influence of interphase mass transfer and gas backmixing on the conversion of fluorene and slectivity of 9-fluorenone formation. Furthermore, the dependence of conversion and selectivity on temperature and hydrodynamic conditions was investigated. In particular, the implications of a change of hydrodynamic conditions for scale-up were analysed. The highest yield of 9-fluorenone predicted for a bench-scale fluidized bed amounted to 88% (X_F = 97%, S_NON = 91%). This yield was lower than in a fixed-bed reactor (Y_NON = 92%, X_F = 99%, S_NON = 93%). A further drop of the yield was predicted when scaling-up from a bench-scale reactor to a commercial size unit (Y_NON = 54%, X_F = 86%, S_NON = 63%).     

Oxidative Coupling of Methane in a Negative DC Corona Reactor at Low Temperature

Oxidative coupling of methane (OCM) in the presence of DC corona is reported in a narrow glass tube reactor at atmospheric pressure and at temperatures below 200°C. The corona is created by applying 2200V between a tip and a plate electrode 1.5 mm apart. The C₂ selectivity as well as the methane conversion are functions of methane‐to‐oxygen ratio, gas residence time, and electric current. At CH₄/O₂ ratio of 5 and the residence time of about 30 ms, a C₂ yield of 23.1% has been achieved. The main products of this process are ethane, ethylene, acetylene as well as CO and CO₂ with CO/CO₂ ratios as high as 25. It is proposed that methane is activated by electrophilic oxygen species to form methyl radicals and C₂ products are produced by a consecutive mechanism, whereas CO_x is formed during parallel reactions.     

Electrochemical methane conversion over SrFeO3−δ perovskite on an yttrium stabilized zirconia membrane

Oxidative methane coupling and the related chemical reactions have been studied in an electrochemical membrane cell of the type: CH₄, (O₂), SrFeO_3– , Au¦8%Y₂O₃:ZrO₂¦Ag, air. The results are compared to a fixed bed study of SrFeO_3– . The C₂₊ selectivity and the alkene/alkane ratio may be higher in the cell reactor than in the fixed bed reactor, but the C₂₊ yield never exceeded fixed bed data. The maximum C₂₊ yield observed in the cell reactor was 3.1%. The electric fields in the cell when electrodes were connected influenced the selectivity to CO₂ in a manner which may be related to the NEMCA effect.     

Oxidative coupling of methane to C2-hydrocarbons over lithium–cerium-promoted MgO and MgO–CaO catalysts

The oxidative coupling of methane to ethylene and ethane was studied over lithium–cerium-promoted MgO and MgO–CaO catalysts in the presence of molecular oxygen at 730°C and at atmospheric pressure in a continuous flow, fixed bed quartz reactor. The catalysts were prepared by an impregnation method and finally calcined at 900°C. The surface area, pore size distribution and pore volume of the catalysts were determined. The feed consisted of only methane and oxygen in the molar ratio of 2:1. The results obtained over the catalyst systems, viz. (i) lithium–cerium-promoted MgO and (ii) lithium–cerium-promoted MgO–CaO, have been compared. A relatively high C₂-selectivity has been obtained with Li–Ce-promoted MgO–CaO catalysts. The optimum yield and selectivity for C₂-hydrocarbons were found to be 21·5% and 76·8% respectively at a methane conversion of 28% over Li (7 wt%)–Ce (2 wt%)-doped MgO–CaO (3:1 wt ratio) catalyst. The various factors governing the activity and the selectivity of the catalyst systems have been discussed.     

Optimal operation of a membrane reactor network

In this contribution, the operation of a membrane reactor network (MRN) for the oxidative coupling of methane is optimized. Therefore, three reactors, a fixed bed reactor (FBR) and two packed bed‐membrane reactors, are modeled. For the (CPBMR), a two‐dimensional (2‐D) model is presented. This model incorporates radial diffusion and thermal conduction. In addition, two 10 cm long cooling segments for the CPBMR are implemented based on the idea of a fixed cooling temperature positioned outside the reactor shell. The model is discretized using a newly developed 2‐D orthogonal collocation on finite elements with a combination of Hermite for the radial and Lagrangian polynomials for the axial coordinate. Membrane thickness, feed compositions, temperatures at the inlet and for the cooling, diameters, and the amount of inert packing in the reactors are considered as decision variables. The optimization results in C₂ yields of up to 40% with a selectivity in C₂ products of more than 60%. The MRN consisting of an additional packed‐bed membrane reactor with an alternative feeding policy and a FBR shows a lower yield than the individual CPBMR. © 2013 American Institute of Chemical Engineers AIChE J, 60: 170–180, 2014     

Numerical simulation of particle/monolithic two-stage catalyst bed reactor with beds-interspace distributed dioxygen feeding for oxidative coupling of methane

A three-dimensional geometry model of the particle/monolithic two-stage reactor with beds-interspace distributed dioxygen feeding for oxidative coupling of methane (OCM) was set up. The improved Stansch kinetic model adapting different operating temperatures was established to calculate the OCM reactor performance using computational fluid dynamics (CFD) and FLUENT software. The results showed that the calculated values matched well with the experimental values of the conversion of CH₄ and the selectivity of products (C₂H₆, C₂H₄, CO₂, CO) in the OCM reactor. The distributed dioxygen feeding with the percentage of 5–20% based oxygen flow rate of top inlet promoted the OCM reaction in monolithic catalyst bed and led to the conversion of CH₄ and the selectivity and yield of C₂ (C₂H₆ and C₂H₄) increase obviously. The distributed dioxygen feeding was 15%, the conversion of CH₄, the selectivity and the yield of C₂ reached 34.1%, 68.2% and 23.3%, respectively.     

Segregation effects on consecutive competing reaction in a CSTR

The steady state behaviour of a continuous stirred tank reactor at a constant temperature of 30°C was studied using the saponification of ethylene glycol diacetate by sodium hydroxyde. The reactor studied was respectively fed by premixed and unmixed reactants. Experimental values of the level of segregation lay within the range of 0.15 to 0.5. The level of segregation has been correlated with the physicochemical dimensionless group k₁C/β and we may conclude that the reactor would be in a state of maximum mixedness when the magnitude of this group is smaller than 10^?4.     

Controlled Oxidative Coupling of Methane by Ionic Conducting Ceramic Membrane

Oxidative coupling of methane was performed on a tubular dense membrane reactor made of catalytically active fluorite-structured Bi_1.5Y_0.3Sm_0.2O_3-. Methane and air were separately fed to the tube and shell side of the membrane. The tubular dense membrane reactor gives 35% one-pass C₂ (C₂H₄+C₂H₆) yield for oxidative coupling of methane with a C₂ selectivity of 54% at 900 °C.     

Conversion of methane to higher hydrocarbons in pulsed DC barrier discharge at atmospheric pressure

Conversion of methane to C₂/C₃ or higher hydrocarbons in a pulsed DC barrier discharge at atmospheric pressure was studied. Non-equilibrium plasma was generated in the barrier discharge reactor. In this plasma, electrons which had sufficient energy collided with the molecules of methane, which were then activated and coupled to C₂/C₃ or higher hydrocarbons. The effect of the change of applied voltage, pulse frequency and methane flow rate on methane conversion, selectivities and yields of products was studied. Methane conversion to higher hydrocarbons was about 25% as the maximum. Ethane, propane and ethylene were produced as primary products, including a small amount of unidentified C₄ hydrocarbons. The selectivity and yield of ethane as a main product came to about 80% and 17% as the highest, respectively. The selectivities of ethane and ethylene were influenced not by the change of pulse frequency but by the change of applied voltage and methane flow rate. However, in case of propane, the selectivity was independent of those condition changes. The effect of the packing materials such as glass and A1₂O₃ bead on methane conversion was also considered, showing that A1₂O₃ played a role in enhancing the selectivity of ethane remarkably as a catalyst.     

Formation of C m H n (m ≥ 3, n ≤ m) hydrocarbons from C1 and C2 molecules by high‐temperature (≥1000°C), short‐contact‐time (1–10 ms) nozzle beam reactions

A nozzle, fabricated from nickel, molybdenum, iron, palladium, and quartz was utilized to produce longer chain hydrocarbons, C _m H _n (m ≥ 3, n≤ m) from C₂ (ethane, acetylene) and C₁ (methane) reactants at nozzle temperature range 1000–1150°C. The conversion of ethane was close to 100% at T _noz = 1000°C, while that of methane reached 20% at T _noz = 1150°C. The contact time in the nozzle is in the 10^-3–10^-2 s range. The reactions are first and higher order in reactant pressure. The reaction mechanism involves the formation of free radicals at the nozzle surface followed by gas‐phase reactions. This revised version was published online in July 2006 with corrections to the Cover Date.     

Modeling the influence of mineral rocks,active in different hot gas conditioning systems and technologies,on the production of light α-olefins

A mechanistic reaction model of the pyrolysis of vaporized n-heptane in the presence of steam (P ≈? 18&–32 kPa) is developed based on overall first-order decomposition kinetics, which satisfactorily describes the experimental data. The mechanistic model predicted that calcined, naturally occurring and readily available mineral rocks, Swedish quarried calcium oxide (quicklime) and Norwegian (Norsk Hydro) magnesium oxide (dolomitic magnesium oxide) applied as the catalytic materials increase the conversion of the vaporized feedstock (P = 2.9–4.7 kPa) without changing the distribution of light α-olefins (i.e., C₂H₄, C₃₊, C₄H₈), compared to pyrolysis in an empty reactor viz. thermal steam-cracking. The simulation calls attention to the fact that the endothermic pyrolysis reaction of the alkane n-heptane is proceeding not only in the homogeneous gas-phase but also on the mineral particles (catalytic cracking) with the same mechanism.     

Oxidative coupling of methane in a bubbling fluidized bed reactor

The oxidative coupling of methane to higher hydrocarbons (C₂₊) was studied in a bubbling fluidized bed reactor between 700°C and 820°C, and with partial pressures of methane from 40 to 70 kPa and of oxygen from 2 to 20 kPa; the total pressure was ca 100 kPa. CaO, Na₂CO₃/CaO and PbO/γ-Al₂O₃ were used as catalytic materials. C₂₊ selectivity depends markedly on temperature and oxygen partial pressure. The optimum temperature for maximizing C₂₊ selectivity varies between 720 and 800°C depending on the catalyst. Maximum C₂₊ selectivities were achieved at low oxygen and high methane partial pressures and amounted to 46% for CaO (T = 780°C; P_CH4 = 70 kPa; P_O2 = 5 kPa), 53% for Na₂CO₃/CaO (T = 760°C; P_CH4 = 60 kPa; P_O2 = 6 kPa) and 70% for PbO/γ-Al₂O₃ (T = 720°C; P_CH4 = 60 kPa; P_O2 = 5 kPa). Maximum yields were obtained at low methane-to-oxygen ratios; they amounted to 4.5% for CaO (T = 800°C; P_CH4 = 70 kPa; P_O2 = 12 kPa), 8.8% for Na₂CO₃/CaO (T = 820°C; P_CH4 = 60 kPa; P_O2 = 20 kPa) and 11.3% for PbO/γ-Al₂O₃ (T 2= 800°C; P_CH4 = 60 kPa; P_O2 = 20 kPa).     

Optimal oxygen concentration strategy through an isothermal oxidative coupling of methane plug flow reactor to obtain a high yield of C2 hydrocarbons

An optimal oxygen concentration trajectory in an isothermal OCM plug flow reactor for maximizing C₂ production was determined by the algorithm of piecewise linear continuous optimal control by iterative dynamic programming (PLCOCIDP). The best performance of the reactor was obtained at 1,085 K with a yield of 53.9%; while, at its maximum value, it only reached 12.7% in case of having no control on the oxygen concentration along the reactor. Also, the effects of different parameters such as reactor temperature, contact time, and dilution ratio (N₂/CH₄) on the yield of C₂ hydrocarbons and corresponding optimal profile of oxygen concentration were studied. The results showed an improvement of C₂ production at higher contact times or lower dilution ratios. Furthermore, in the process of oxidative coupling of methane, controlling oxygen concentration along the reactor was more important than controlling the reactor temperature. In addition, oxygen feeding strategy had almost no effect on the optimum temperature of the reactor. Finally, using the optimal oxygen strategy along the reactor has more effect on ethylene selectivity compared to ethane.