Empty Reactor Research Articles

Inhibition of gas-phase oxidation of ethylene in the oxidative conversion of methane and ethane over CaO, La 2O 3/CaO and SrO–La 2O 3/CaO catalysts

The conversion of methane, ethane and ethylene has been investigated either in an empty reactor or in the presence of a series of calcium-based catalysts including unpromoted calcium oxide (C), calcium oxide promoted with lanthane (LC) and with both lanthana and strontium oxide (SLC). It was found that these solids inhibit the gas-phase oxidation of ethylene, probably by trapping chain-carrier radicals. At T>1000 K, the inhibiting effectiveness varies in the order C<LC<SLC which parallels the C 2+ yield sequence in the oxidative coupling of methane (OCM). This strongly suggests that the inhibiting effect, thus observed in separate experiments, is operating in OCM conditions, preventing ethylene from further degradation. This sequence is also found to be similar to that of the strong basicity of solids as deduced from the temperature of CO 2 desorption, indicating that this inhibiting effect might be related to the basicity of the catalyst. Finally the beneficial role of each components in the optimized catalyst (SLC) is discussed: lanthana might be an inhibitor of methyl radical or methane oxidation into CO, calcium oxide a promoter of ethane dehydrogenation into ethylene and strontium oxide might play the role of an inhibitor of the gas-phase oxidation of ethylene.

The partial oxidation of methane to methanol: An approach to catalyst design

In this study a new approach for the design of catalysts for methane partial oxidation to methanol is outlined and results arising from this approach are discussed. The approach involves identifying metal oxide catalyst components which activate methane and oxygen, but do not readily destroy methanol, the desired product. A series of catalysts based on MoO 3 and WO 3 were prepared by impregnation with solutions containing the metal ions of the second component. The Cu/MoO 3 catalyst showed a selectivity advantage over the homogeneous gas phase oxidation in a reactor bed packed with quartz chips. In general, the WO 3 based catalysts were less effective for the production of methanol. Additionally, catalysts composed from physical mixtures of Ga 2O 3 and ZnO with MoO 3 were prepared. The Ga 2O 3/MoO 3 catalyst showed a maximum methanol yield greater than the homogeneous gas phase reaction over an inert bed of quartz particles. The increased methanol yield was attributed to a synergistic effect combining the beneficial reactivities of the MoO 3 and Ga 2O 3 component oxides. Comparison with data obtained from an empty reactor tube showed that none of the catalysts were as active or selective for methane partial oxidation to methanol.

Selective Catalytic Oxidation of NH3 in Gasification Gas. 1. Effect of Iron Sinter and Dolomite on the Reactions of NH3, NO, and O2 in Gasification Gas

Possibilities to selectively oxidize NH3 in gasification gas to N2 have been investigated. Either O2 or NO or both oxidizers simultaneously were added to the hot synthetic gasification gas, which was led through a catalyst bed positioned in the electrically heated furnace. In the empty reactor and in the nonporous quartz bed, only a slight NH3 reduction was achieved. When dolomite or iron sinter was used as catalyst, the added NO formed more NH3. The reactions of H2 with NO in fact may limit the use of certain catalysts in gasification atmosphere. In the experiments with silicon carbide, the silicon carbide surface was oxidized to porous oxide layer, which catalyzed the selective oxidation of NH3 to N2 at the temperature of 700−800 °C. Even with O2 addition alone large NH3 reduction was achieved. At the same time, significant NO formation was, however, noticed. The reason for the different behavior of the porous silicon dioxide layer, and dolomite and iron sinter, is probably their different capability to...

Qualitative Measurement of Heating Uniformity in a Multimode Microwave Cavity

Qualitative tests of heating uniformity in a domestic multi-mode cavity were made using thermal paper. The power distribution in the empty cavity was non-uniform. A stationary four-bladed mode stirrer modified the distribution; rotation at 5 rpm improved uniformity. A cylindrical quartz reactor and alumina-silica insulating cylinder were placed in the cavity, concentrating the field in the reactor and insulation. The field was stronger at the bottom of the empty reactor. Stainless steel sheathed thermocouples in the reactor perturbed the field slightly. When the reactor was filled with granular activated carbon the field was concentrated at the top of the reactor.

Catalytic upgrading of tarry fuel gases: A kinetic study with model components

As a contribution to the development of a process for catalytic upgrading of tarry fuel gases, e.g. coke-oven gas, the conversion of naphthalene, benzene and methane on a nickel catalyst in the presence of H 2 and H 2O was studied. The experiments were performed in a tubular flow reactor (total pressure: 1.6 bar; residence time with respect to the empty reactor: 0.3 s; temperature: 400–950°C; and particle diameter of catalyst: 1.5 mm). The kinetic data were obtained by systematic variation of the reaction conditions. At temperatures of more than 800°C, each hydrocarbon is cracked and converted with H 2O to CO and H 2. Soot formation does not occur at any temperature. In case of simultaneous conversion of all three hydrocarbons, competitive reactions have to be considered. The rate of chemical reaction on the catalyst is substantially decreased in the presence of H 2S. Nevertheless, in a reactor of industrial scale, H 2S has only slight influence. The catalyst would be applied with a particle diameter of 19 mm (experiments: 1.5 mm), and the overall reaction rate of hydrocarbon conversion is significantly affected by gas film diffusion.

Fuel—Gas injection to reduce N 2O emissions from the combustion of coal in a fluidized bed

A laboratory-scale, fluidized-bed reactor (29 mm i.d.) has been used for experiments in which a stream of simulated combustion gases is passed through a countercurrent flame of either methane or propane. The effects of N 2O concentration and bed temperature on N 2O reduction have been analyzed. Additionally, the effects of NO, SO 2, O 2, and carrier gas (N 2 or He) in the inlet stream have been studied. An attempt to establish whether N 2O decomposition in the flame proceeds via radical or thermal mechanisms was carried out by assuming an ideal reaction model in the flame. Up to 99% N 2O decomposition was achieved at a gas/oxygen equivalence ratio of 0.83 (12 vol.% O 2) and a total flow rate of 1 L/min, for both methane and propane injected into the reactor. The analyses indicate that NO x is formed in the flame mainly via a “prompt NO” mechanism. Metallic surfaces can alter the N 2O chemistry, either enhancing (empty reactor) or inhibiting (flame) N 2O decomposition. Both NO and SO 2 play a minor role in the decomposition of N 2O, and so does the carrier gas, though in this case, N 2 can produce considerable amounts of NO x under particular circumstances. Under the conditions used, thermal decomposition accounts for only around 10% of the high N 2O conversions achieved in the flame, radical mechanisms playing a major role.

Catalytic Contribution of Reactor Wall Materials on Oxidative Coupling of Methane

Gas-phase oxidative coupling of methane (OCM) was investigated with empty tubular-flow reactors made of various materials (quartz, ceramic and stainless steel tubes). When the temperature is higher than 680°C, the CH 4 conversions in three kinds of empty reactors are measurable, and the gas-phase OCM becomes noticeable at 750°C or higher. High temperature is beneficial to the activation of CH 4 and the dehydrogenation of C 2 H 6 to C 2 H 4 . Residence time of reactants in the heated volume plays an important role in the gas-phase OCM. Experiments indicated that the gas-phase OCM can be minimized by increasing reactant gas velocity or filling the free volume of the reactor with inert materials. Reactor surface also has an effect on the reaction and may participate in some surface reactions. Based on the kinetic results and the SEM investigation on the surfaces of reactors, a gas-phase reaction mechanism of OCM is proposed.

Comparison of Catalyzed and Homogeneous Reactions of Hydrocarbons for Selective Catalytic Reduction (SCR) of NOx

When NO2 (0.21% in He) was passed over CoZSM-5 or HZSM-5 at SVH = 45,000 h-1, equlibrium with NO + O2 was approached at temperatures above 673 K. The reactions of NO2 + CH4 NO2 + CH4 + O2, and NO + CH4 + O2 were compared over these catalysts and in the empty reactor. The latter two reactions yielded essentially identical results when catalyzed, as did NO2 + CH4 up to about 22% conversion of CH4, i.e., to the point where the oxygen supply became exhausted. Without added O2, NO appeared as a reduction product of NO2 along with N2. In the empty reactor, no N2 was formed although NO2 could be quantitatively reduced to NO; combustion of CH4 with O2 (or with NO) alone was not observed at temperatures less than 873 K, but light-off with NO2 or NOx + O2 occurred at about 723 K. In the absence of O2, the homogeneous CH4 conversion was limited to about 22% at 873 K where the conversion of NO2 to NO reached 100%. With added O2, conversion of CH4 reached 62% at 873 K and approached 100% under more severe conditions. These data illustrate the key role played by NO2 in the selective catalytic reduction reaction. They also show that a catalyst is necessary for the formation of N2 and emphasize the importance of O2 in maintaining an adequate supply of NO2, particularly at temperatures above 800 K where equilibrium favors NO. When C3H8 or i-C4H10 was substituted for CH4, the order of reactivity was i-C4H10 > C3H8 > CH4 in both the catalyzed and the homogeneous reactions. Moreover, in the empty reactor dehydrogenation to the corresponding olefins was found to be important with the former two; the mass balance did not close with CH4, possibly due to the formation of formaldehyde.

Catalytic combustion of methane over magnesium oxide

Abstract The catalytic activity of magnesium oxide for catalytic combustion of methane was studied. The effects of temperature, space velocity, concentration of the reactants and the influence of catalytically initiated homogeneous gas-phase reactions were examined. At a space velocity of 100 000 h±1, the temperature for 10% conversion was lowered by 270°C compared to an empty reactor. It was also shown that catalytically initiated homogeneous gas-phase reactions are significant for the kinetics.

Preparation, characterisation and activity of an iron/sodalite catalyst for the oxidation of methane to methanol

An iron sodalite catalyst similar to that reported to have good selectivity for the oxidation of methane to methanol by Lyons et al. has been prepared, tested and characterised. In a limited range of temperature at 34 bar total pressure the selectivity of the catalyst to methanol is a little better than that of an empty silica glass reactor. Before reaction, X-ray powder diffraction and Mossbauer spectroscopy confirm the presence of Fe(III) in the sodalite framework. After reaction Mossbauer spectroscopy identifies Fe(II) and also small particles of iron(III) oxide, < 1 μm in size.

Partial oxidation of ethane and ethylene in the presence and absence of carbon-13-labeled methane on reducible and nonreducible oxide catalysts

Ethane and ethylene were partially oxidized in the presence and absence of 13 C-labeled methane and catalyst to determine the relative reactivity of ethane and ethylene compared to methane. The relative reactivities are in the order of ethylene>ethane>>methane for reactions in the gas phase (empty reactor) and on catalysts containing reducible oxides of transition metals such as Ca/Ni/K. However, this order changed to ethane>ethylene>>methane for a nonreducible catalyst, such as sodium-promoted Sm 2 O 3 . The results also showed that in the presence of methane and catalysts ethane and ethylene, depending on their partial pressures, compete for active species in the gas phase and active centers on the catalyst. The effects of pressure, temperature, and contact time on partial oxidation of ethane and ethylene were investigated in an empty alumina reactor. It was found that the temperatures at which 50% of oxygen converted (at steady state) during partial oxidation of ethane and ethylene were significantly less than the temperature at which 50% oxygen converted during partial oxidation of methane. The higher reactivity of products compared to the reactant appeared to be a major factor preventing achievement of the yield required for an economical process to convert natural gas to liquid fuels

Kinetic-transport models of bimodal reaction sequences—I. Homogeneous and heterogeneous pathways in oxidative coupling of methane

A reaction-transport model that combines gas-phase reactions occurring within interstitial and intrapellet voids with surface reactions occurring on catalytic sites was used to describe the oxidative coupling of methane in packed-bed reactors, a typical example of bimodal (homogeneous-heterogeneous) reaction systems. A kinetic model for gas-phase reactions was assembled from available literature data; it describes well experimental results in empty reactors. Simulations suggest that C 2 yields greater than 8–9% are unattainable with CH 4/O 2 mixtures in homogeneous reactors. Staging the introduction of the oxygen reactant along the reactor length minimizes secondary oxidation reactions by lowering the local O 2 pressures, and leads to a slight increase in maximum yield (12% for 200 injection points) but also to much larger required reactor volumes. The introduction of an ideal catalytic function (methyl and ethyl radical formation without full oxidation) also increases maximum C 2 yields by increasing the concentration of methyl radicals involved in bimolecular coupling steps. However, C 2 yields greater than 30% require selective catalysts with very high turnover rates (100 s −1); higher rates become ultimately limited by intrapellet diffusion rates. Again, staging oxygen by multiple injection schemes increases attainable yields but requires larger reactor volume and catalytic sites with low reaction order (⪡ 1) in oxygen. For optimum conditions, staged oxygen injection techniques lead to C 2 yields as high as 50%.

Theoretical and Experimental Investigations of Chlorine RF Glow Discharges: I . Theoretical

A comprehensive model of chlorine plasma etching of polysilicon in a parallel plate reactor was developed. The Boltzmann transport equation and a bulk plasma model were used to calculate the rate coefficients of electron impact reactions. These coefficients were then used in a transport and reaction model to calculate the atomic chlorine concentration distribution. The same methodology may be applied to other plasma systems as well. Theoretical results for an empty reactor (no polysilicon film) are presented in this paper. Unified plots were developed which relate the electron density, self‐sustained electric field, and electron impact coefficients in the bulk plasma to pressure, power, and reactor geometry. The calculated atomic chlorine concentration showed similar dependence on pressure and electrode spacing for either first or second order surface recombination kinetics. Experimental verification of the model predictions is presented in the accompanying paper.

Inhibition of the gas phase oxidation of ethylene by various solids and influence of their addition on the catalytic properties of lanthanum oxide towards the oxidative coupling of methane

Ethylene oxidation has been studied in an empty clean quartz reactor in the absence and in the presence of Li/MgO and Li 2CO 3 catalysts. It is shown that the gas phase oxidation of ethylene is inhibited by these solids, suggesting that one of the role of the catalyst in oxidative coupling of methane is to inhibit ethylene oxidation into CO x by trapping very reactive chain-carriers. Furthermore, a series of flame retardant, including phosphates and halides of alkali and alkaline earth metals are shown to inhibit the gas phase oxidation of ethylene into CO x. Blending these compounds (5 % wt) with La 2O 3 (physical mixtures) improves selectivity towards C 2+ hydrocarbons without change of the ethylene/ethane ratio. This is interpreted assuming that additives interfere with the early stages of the reaction which involve methyl radicals, rather than invoking the inhibition of secondary reactions such as ethylene oxidation.

Thermal decomposition of a wood particle. Temperature profiles on the solid surface

The calculation of the weight loss during the thermal decomposition of lignocellulosic materials involves the solution of the heat and mass balance equations. In order to solve these equations it is necessary to know the temperature on the solid surface, which can vary at different points and can also be very different from the gas temperature. In this work, an experimental study was carried out in a system which allows the use of large particle sizes and the simulation of different operating conditions. In this study, the temperatures on the solid surface were measured and compared with those corresponding to the empty reactor.

Oxidative coupling of methane over praseodymium oxide catalysts

The catalytic properties of PrO x and Li/PrO x for the oxidative coupling of methane have been studied in fixed bed reactors. The results obtained, combined with those of the reaction of oxygen with methane, ethane, ethene and carbon monoxide in an empty reactor, allow us to suggest a reaction scheme for the catalytic process. The experimental results have shown that a fraction of the carbon oxides formed comes from the secondary reactions of C 2, but with PrO x the major part of the carbon oxides results from a reaction parallel to C 2 formation. The two catalysts were studied by thermogravimetric analysis (TGA) and X-ray photoelectron spectroscopy (XPS). TGA experiments in various atmospheres including methane—oxygen mixtures have shown that the oxidation state of praseodymium oxide increases with oxygen partial pressure in methane as it does in an inert gas. The TGA studies indicate carbonate formation on the Li/PrO x surface. These carbonates are associated with lithium since none were formed on PrO x , but the TGA experiments indicate that gaseous oxygen is necessary for their formation. XPS measurements of PrO x and Li/PrO x samples variously oxidized have confirmed the evolution in the oxidation state of praseodymium oxide and the carbonate formation on Li/PrO x . The XPS results suggest that the addition of lithium on praseodymium oxide leads to the formation of a mixed carbonate layer and that these carbonates stabilize the Pr 3+ oxidation state and suppress the Pr 4+ ions which are associated with the labile oxygens responsible for surface total oxidation of methane and C 2.

Nonlinear Control of Free-Radical Polymerization Reactors

Free-radical polymerization reactors exhibit autoacceleration kinetics, exothermicity and limited heat removal that pose a sensitive, nonlinear, interactive, multivariable control problem for batch, semibatch and continuous operation. We address a tracking strategy with a nonlinear feedforward-feedback control scheme. Consideration of measured inputs (which are not controls) yields time-varying nonlinear systems which admit coordinate changes to linear autonomous Brunovsky canonical forms where: realizable tracking reference inputs and states are established, and the feedbacks are constructed and tuned. The coordinate change yields linear, noninteractive control designs. The tracking control strategy is able: to take an empty reactor to a stable continuous operation (which, otherwise is open-loop unstable), and to provide a stable periodic continuous operation.

Etude expérimentale de la pyrolyse de sciure de bois dans un lit fluidisé de sable entre 630 et 940 °C

The authors present the results of a study of the pyrolysis of wood sawdust fed continuously into a bed of sand particles fluidized by nitrogen between 630 and 940 °C. The bed diameter was 50 mm. The influence of the bed temperature, the flow rate of nitrogen, the flow rate and the size of the sawdust particles and the nature of the wood were investigated. Other experiments without sand were also carried out for comparison. The results are better in the reactor filled with the fluidized sand bed than in the empty reactor. Interest in the fluidization technique is justified on chemical grounds as well as on energetic ones. The results show that the residence time of sawdust particles in the reactor and the temperature are the main parameters which control the pyrolysis. The performance of the reaction system is enhanced by increasing the bed temperature and shortening the residence time of the sawdust.

Preparation and evaluation of catalysts for the production of ethylene via steam cracking: Effect of Operating Conditions on the Performance of 12CaO-7Al 2O 3 Catalyst

Various types of catalyst samples selective for the production of ethylene via steam cracking were prepared. Controlled ratios of two or more oxides and high calcination temperatures were used in order to obtain the desired crystal phases. BET surface-area measurement and X-ray diffraction techniques were used for the examination of the samples. The prepared samples were tested in an experimental pyrolysis unit using n-hexane as a feed. It was found that a catalyst with the formula 12CaO-7Al 2O 3 has the highest selectivity ratios. The catalyst activity varied widely depending on the operating conditions. At short residence times the catalyst increases the conversion of the feed and hence the olefin yields compared with “inert” α-alumina and thermal cracking (empty reactor). At relatively long residence times and high hydrocarbon partial pressure the catalyst promotes the formation of carbon oxides. Therefore, with a judicious choice of the operating conditions, the catalyst 12CaO-7Al 2O 3 can act as a steam cracking and/or a steam reforming catalyst. Testing of a sample following treatment with hydrogen at high temperature showed no catalytic activity. It is believed that the increased catalyst activity is due to the excess of oxygen that exists in the crystal phase Ca 12Al 14O 33 of the 12CaO-7Al 2O 3 catalyst.

Pyrolysis of methane and the role of surface area

A range of catalysts was screened for activity and product selectivities in the pyrolysis of methane. Experiments were performed in a flow reactor at 1125 °C and atmospheric pressure. It appears that the pyrolysis of methane is primarily affected by the specific surface area of the catalyst and not by the particular type of catalyst. The highest yields to gaseous and liquid products are obtained in an empty reactor tube, whereas high specific surface areas produce mainly graphitic coke and hydrogen. The results are compared with specific surface area effects in the catalytic oxidative dimerization of methane.