Effective approach to cyclic steady state in the catalytic reverse-flow combustion of methane

Computer-based simulations of a reverse-flow reactor should be carried out till the attainment of the so-called cyclic steady state. Usually this state is achieved by the method of direct dynamic simulations. In the paper of Unger et al. (Comput. Chem. Eng. 21 (1997) 5167.) special approaches, making use of various minimization algorithms based most often on Newton algorithms, are proposed. In the present paper one deals with a comparison and an appraisal of these methods, applied to the reverse-flow catalytic combustion of methane that occurs in coal-mine ventilation air.     

Homogeneous vs. catalytic combustion of lean methane—air mixtures in reverse-flow reactors

To carry out a comparative assessment of a recently proposed idea of using thermal flow-reversal reactors (TFRR) for mine ventilation air, the results for the catalytic flow-reversal reactor (CFRR) investigated within the European Project (2003) are briefly presented. Next, experimental investigations of thermal combustion are presented in this paper. These consisted of the kinetic study of homogeneous combustion in the pelletized bed and in the monolith. Kinetic equations for the two cases are derived and discussed. Experimental autothermal reverse-flow operation in a laboratory setup was performed. Due to the high heat capacity of the wall and insulation of the pelletized bed reactor, with considerable heat losses to the surroundings, autothermal operation was successful only in the monolithic reactor. It is finally concluded that the thermal combustion can be competitive compared with the catalytic oxidation.     

Catalytic combustion of methane in non-permselective membrane reactors with separate reactant feeds

Catalytic combustion of methane over Pd and Pt/SiO₂/-Al₂O₃ membranes was studied in the temperature range 300–650 °C. Fuel and oxygen were fed at opposite membrane sides. In order to improve reactor controllability the -Al₂O₃ membranes were impregnated with SiO₂ sol resulting to smaller pore size. Methane conversions up to 100% for the palladium membrane and up to 42% for the platinum membrane were achieved. The results indicated a transition from kinetic to mass transfer control within the temperature range investigated. This was accompanied by reduction of methane slip from tube to shell side with increasing temperature. CO and H₂ were detected in the product gases of the palladium membrane. Their concentration could be reduced by applying a trans-membrane pressure difference. Low concentrations of CO were observed for the Pt/SiO₂/-Al₂O₃ membrane, while no CO or H₂ were detected for a Pd/-Al₂O₃ membrane operating in dead-end configuration.     

Application of recurrent neural networks in batch reactors: Part I. NARMA modelling of the dynamic behaviour of the heat transfer fluid temperature

This paper is focused on the development of nonlinear models, using artificial neural networks, able to provide appropriate predictions when acting as process simulators. The dynamic behaviour of the heat transfer fluid temperature in a jacketed chemical reactor has been selected as a case study. Different structures of NARMA (Non-linear ARMA) models have been studied. The experimental results have allowed to carry out a comparison between the different neural approaches and a first-principles model. The best neural results are obtained using a parallel model structure based on a recurrent neural network architecture, which guarantees better dynamic approximations than currently employed neural models. The results suggest that parallel models built up with recurrent networks can be seen as an alternative to phenomenological models for simulating the dynamic behaviour of the heating/cooling circuits which change from batch installation to installation.     

Bifurcation analysis of the conversion degrees in systems based on the cascade of tank reactors

The scope of the paper is the analysis of the possibility of increasing conversion in different types of systems based on continuous stirred tank reactors. A numerical model of the cascade with constant direction of material flow is tested, a model of changes of direction flow and a relaxation model. The analysis is conducted by means of parametric continuation method and numerical simulation, with the designation of bifurcation diagrams and time series. An essential impact of the relaxation method on the increase of the conversion degree is indicated.     

Effect of fuel properties on biomass combustion. Part II. Modelling approach—identification of the controlling factors

Biomass fuels come from many varieties of sources resulting in a wide range of physical and chemical properties. In this work, mathematical models of a packed bed system were employed to simulate the effects of four fuel properties on the burning characteristics in terms of burning rate, combustion stoichiometry, flue gas composition and solid-phase temperature. Numerical calculations were carried out and results were compared with measurements wherever possible. It was found that burning rate is mostly influenced by fuel size and smaller fuels result in higher combustion rate due to increased reacting surface area and enhanced gas-phase mixing in the bed; combustion stoichiometry is equally influenced by fuel LCV and size as a consequence of variation in burning rate as well as the mass ratio of combustible elements to the oxygen in the fuel; for the solid-phase temperature, material density has the strongest influence and a denser material has a higher maximum bed temperature as it results in a less fuel-rich combustion condition; while CO concentration in the flue gases is mostly affected by both fuel calorific value and size, CH₄ in the exiting flow is greatly affected by material density due to change in reaction zone thickness.     

Modelling of unsteady-state hydrodynamics in periodically operated trickle-bed reactors:Influence of the liquid-phase physical properties

Process intensification using periodic operation of trickle bed reactors (TBRs) is still a long way from replacing conventional steady-state operation in industrial use, despite the numerous benefits described in the literature. Complex interactions between hydrodynamics, mass transfer and reaction phenomena make the design of periodically operated TBRs an almost insurmountable challenge. The development of hydrodynamic models able to provide reliable quantitative predictions of flow behaviour and possessing a sound physical basis, is an essential prerequisite for obtaining the necessary insights into this complexity. In this work, the two-phase pressure drop and dynamic liquid hold-up during max/min and on/off periodical operation were predicted using a model based on the relative permeability concept. In order to demonstrate the utility of this approach, a systematic investigation of the quantitative influence of the liquid-phase physical properties was carried out. The results obtained show that the modelling of the hydrodynamics in periodically operated TBRs using the relative permeability concept is feasible. By selecting suitable permeability parameters, unsteady-state hydrodynamics for different periodic operating modes can be predicted successfully.     

Study of gas-liquid stirred reactor stability taking into account the temperature influence on gas solubility

The paper is devoted to the dynamic behavior and stability of gas-liquid stirred reactor taking into account the temperature influence on gas solubility. Since the rate of gas-liquid processes is very sensitive to concentration of gas reactant dissolved in liquid, even weak fluctuations of temperature can significantly influence on process pass. There are two cases of temperature influence on gas solubility are possible: (1) the solubility decreases with increasing temperature; (2) the solubility grows with increasing temperature. The first case is typical for majority of gases. The second case occurs more rarely but has a great practical importance. It takes place, for example, for the hydrogenation of many compounds in organic solvents (such as benzene, toluene, isopropyl alcohol and others). A model of gas-liquid process has been developed to demonstrate the stability of gas-liquid reactor. It has been shown that the gas solubility behavior has an influence on the form of heat production curve and therefore on the multiplicity of the steady states. The areas of multiplicity and limit cycles were found and the phenomenon of hysteresis in the reactor was shown. A criterion to determine whether the multiplicity is possible under the given conditions was found. By means of an analysis of a mathematical model the stability of steady states of the reactor was studied.     

Catalytic combustion of natural gas as the role of on-site heat supply in rapid catalytic CO2---H2O reforming of methane

Development in highly active catalysts for the reforming of methane with H₂O, CO₂, and H₂O+CO₂, and partial oxidation of methane was conducted to produce hydrogen with high reaction rates. A Ni-based three-component catalyst such as Ni---La₂O₃---Ru or Ni---Ce₂O₃---Pt supported on alumina wash-coated ceramic fiber in a plate shape was very suitable for both reactions. The catalyst composition was set at 10 wt.-% Ni, 5.6 wt.-% La₂0₃, and 0.57 wt.-% Ru for example, or molar ratios of these components were 1:0.2:0.03. Even with such a low concentration, the precious metal enhanced the reaction rate markedly, and this synergistic effect was ascribed to the hydrogen spillover effect through the part of precious metal and it resulted in a more reduced surface of the main catalyst component. In particular, a marked enhancement in the reaction rate of CO₂-reforming of methane was observed by the modification of a low concentration Rh to the Ni---Ce₂0₃---Pt catalyst. Very high space-time yields of H₂ (i.e., 8300 mol/1 h in partial oxidation of methane at 600°C with a methane conversion of 37.5%, and 3585 mol/1 h in CO₂reforming of methane at 600°C with a methane conversion of 58%) were realized in those reactions. By combining the catalytic combustion reaction, methane conversion to syngas was markedly enhanced, and even with a very short contact time (10 ms) the conversion of methane increased more than that at 50 ms. The space-time yield of hydrogen amounted to 2,780 mol/1 h with a methane conversion of 90% at 700°C. Furthermore, in a reaction of CH₄---CO₂---H₂O---O₂ on the four components catalyst, an extraordinarily high space-time yield of hydrogen, 12 190 mol/1 h, could be realized under the conditions of very high space velocity (5 ms).     

Catalytic combustion of methane in a monolith washcoat: Effect of water inhibition on the effectiveness factor

This paper reports the results of a numerical investigation of the diffusion and reaction of methane in the washcoat of a catalytic monolith reactor. The kinetic rate expression used is an empirical equation determined experimentally for a palladium oxide catalyst. The effect of water inhibition on the reaction rate is included in the model. A multi-species diffusion and reaction model is used to simulate the process. The model is solved in a 2-D space using a finite element method. It is observed that the inhibiting effect of water tends to lower the observed reaction rate and that a higher surface water concentration results in an increase in the observed effectiveness factor. The effectiveness factor depends on three dimensionless parameters. Strong diffusion limitation can lead to high water concentrations at the interior of the catalytic washcoat.     

Modelling of methanol synthesis in a network of forced unsteady-state ring reactors by artificial neural networks for control purposes

A numerical model based on artificial neural networks (ANN) was developed to simulate the dynamic behaviour of a three reactors network (or ring reactor), with periodic change of the feed position, when low-pressure methanol synthesis is carried out. A multilayer, feedforward, fully connected ANN was designed and the history stack adaptation algorithm was implemented and tested with quite good results both in terms of model identification and learning rates. The influence of the ANN parameters was addressed, leading to simple guidelines for the selection of their values. A detailed model was used to generate the patterns adopted for the learning and testing phases. The simplified model was finalised to develop a model predictive control scheme in order to maximise methanol yield and to fulfil process constraints.     

Mathematical modelling of Sec pathway mechanism in Escherichia coli: a case study for periplasmic translocation of maltose binding protein-glucose isomerase fusion protein

Within the framework of reported information on the Sec pathway mechanism, a mathematical model for the periplasmic translocation of fusion proteins in bacteria was developed. The mathematical model includes all stages of the targeting stage and assume that the ATP-driven translocation stage is completed in a single step. The equations for the targeting stage involved cytoplasmic folding rate and SecB binding kinetics. Rate equations for the translocation stage were derived using King-Altman and network reduction techniques. Experimental data for maltose binding protein-glucose isomerase fusion protein (MBP-GI) translocation and reported data for MBP translocation were used to estimate the parameters. The simulation results show that the model fits well to the experimental data of cytoplasmic and periplasmic MBP-GI distributions. When the values of the targeting stage parameters, k₁ and k₃ or the concentration of SecB are changed, the model correctly predicts the expected changes in the MBP-GI distribution to the cytoplasmic and periplasmic spaces. The SecB complex and the preprotein concentrations predicted attain steady state immediately, within seconds, and their amounts are very low when compared to MBP-GI in either compartment. The model can be made applicable to any protein that uses the Sec pathway, and with ATP and Sec pathway protein limitations.     

Effects of fuel devolatilisation on the combustion of wood chips and incineration of simulated municipal solid wastes in a packed bed

Detailed mathematical simulations as well as experiments have been carried out for the combustion of wood chips and the incineration of simulated municipal solid wastes in a bench-top stationary bed and the effects of devolatilisation rate and moisture level in the fuel were assessed in terms of ignition time, burning rate, reaction zone thickness, peak flame temperature, combustion stoichiometry and unburned gas emissions at the bed top. It is found that devolatilisation kinetic rate has a noticeable effects on the ignition time, peak flame temperature, CO and H₂ emissions at the bed top and the proportion of char burned in the final stage (char burning only) of the combustion. However, it has only a minor effect on the other parameters. Reaction zone thickness ranges from 20 to 55 mm depending on the moisture level in fuel and an increase in the moisture level causes a shift of the combustion stoichiometry to more fuel-lean conditions.     

分离型热管蒸发段流动特性和传热特性的试验研究

对分离型热管管内蒸发段流动特性和传热特性进行了试验研究 :在保证热管工作效率及安全性的前提下 ,分离型热管蒸发段工质流动形式除单相液流和泡状流 (低热流密度时为弹状流 )外 ,在蒸发段上部约有 42 %～ 50 %的不稳定飞溅降膜区 ;合理充液率随热流密度的增加而减少 ;随着热流密度的增加 ,核态沸腾区及飞溅降膜区的换热系数均增加 ,蒸发段总换热系数也增加。     

Investigation of the combustion of neat cottonseed oil or its neat bio-diesel in a HSDI diesel engine by experimental heat release and statistical analyses

An experimental study is conducted to evaluate the effects of using neat cottonseed oil or its neat ME (methyl ester) bio-diesel, on the combustion behavior of a standard, high speed, direct injection (HSDI), ‘Hydra’ diesel engine located at the authors’ laboratory. Combustion chamber and fuel injection pressure diagrams are obtained at medium and high load using a developed, high-speed, data acquisition and processing system. A heat release analysis of the experimentally obtained cylinder pressure diagrams is developed and used. Plots of histories in the combustion chamber of the heat release rate and other related parameters reveal some interesting features, which shed light into the combustion mechanism when using these bio-fuels. These results, combined with the differing physical and chemical properties of the bio-fuels between themselves and against those for the diesel fuel, which constitutes the baseline fuel, aid the correct interpretation of the observed engine behavior performance- and emissions-wise. Moreover, the possible existence of cyclic (combustion) variability is examined as reflected in the pressure indicator diagrams, by analyzing for the maximum pressure and its rate, and the dynamic injection timing and ignition delay, by using statistical analysis for averages, standard deviations and probability density functions. The key results are that with the use of these bio-fuels against the neat diesel fuel case, the ignition delay is hardly affected, the fuel injection pressure diagrams are very slightly advanced accompanied with higher injection pressures, maximum cylinder pressures remain the same with the vegetable oil and slightly increased with the bio-diesel, maximum cylinder pressure rates are increased with the bio-diesel and decreased with the vegetable oil, while the cyclic irregularity is not affected with these bio-fuels remaining at the acceptable neat diesel fuel case levels.     

Modelling of a moving bed furnace for the production of uranium tetrafluoride. Part 2: Application of the model

The French nuclear fuel making route uses, prior to enrichment, uranium tetrafluoride UF₄ obtained from the reduction, followed by hydrofluorination of uranium trioxide UO₃. These two steps are carried out in a specific reactor known as a moving bed furnace. We developed a steady-state numerical model of the moving bed furnace, described in Part 1. In the Part 2, calculation results for a reference set of operating parameters of the furnace are presented in term of temperature, reaction rates, solid and gas compositions. Results analysis enlightens the detail of the furnace behaviour in its different zones. Unknown features have been revealed, such as thermodynamic limitation of the hydrofluorination reaction in the hot core of the moving bed. A sensibility study of various operating parameters shows how some can influence the UF₄ quality and underlines the strong coupling between the different zones of the furnace. Finally, the model is applied to define an optimal temperature progression in the furnace and suggests geometrical modifications. Besides, the validity of using the law of additive reaction times for calculating the reaction rates in such a reactor model has been checked for the first time against a numerical grain model.     

Effect of wall temperature modulation on the heat transfer characteristics of droplet-train flow inside a rectangular microchannel

The numerical studies of water–oil two-phase slug flow inside a two-dimensional vertical microchannel subjected to modulated wall temperature boundary conditions have been discussed in the present paper. Many researchers have contributed their efforts in exploring the characteristics of Taylor flows inside microchannel under constant wall heat flux or isothermal wall conditions. However, there is no study available in the literature which discusses the impact of modulated thermal wall boundary conditions on the heat transfer behavior of slug flows inside microchannels. Hence, to bridge this gap, an effort has been made to understand the heat transfer characteristics of the flow under sinusoidal wall temperature conditions. Initially, a single phase flow and heat transfer study was performed in microchannels, and the results of the fully developed velocity profile and heat transfer rate were validated with benchmark analytical results. Then an optimal selection of the combination of sinusoidal thermal wall boundary conditions has been made for the two-phase slug flow study. Later, the effects of amplitude(0 b ε b 0.03) and frequency(0 b ω b 750π rad·s~(-1)) of the sinusoidal wall temperature profile on the heat transfer have been studied using the optimal combination of the wall boundary conditions. The results of the numerical study using modulated temperature conditions on channel walls showed a significant improvement in the heat transfer over liquid-only flow by approximately 50% as well as over two-phase flow without wall temperature modulation. The non-dimensional temperature contours obtained for different cases of temperature modulation clearly explain the root cause of such improvement in the heat transfer. Besides,the results based on the hydrodynamics of the flow have also been reported in terms of variation of droplet shapes and film thickness. The influence of Capillary number on the film thickness as well as heat transfer rates has also been discussed. In addition, the measured film thickness has also been compared with that calculated using standard empirical and analytical models available in the literature. The heat transfer rate obtained from the numerical study for the case of unmodulated wall temperature was found to be in a close match with a phenomenological model to evaluate slug flow heat transfer having a mean absolute deviation of 7.56%.     

Effect of perturbations in the exhaust gas composition on three-way catalyst light off

The influence of perturbations in the exhaust gas composition on the operation of three-way catalytic converters (3WCC) has been the subject of many research works. This paper aims to investigate the effect of such transients on the light-off temperature of a commercial 3WCC, by using a dynamic mathematical model for 3WCC simulation. This modeling approach embodies a comprehensive oxygen storage and release submodel into an existing 3WCC quasi-steady model. The dynamic model developed is validated against previously published experimental data, which imply that the presence of transients in the exhaust gas results in an improved low-temperature catalyst performance. Having validated the model, the improved light-off performance is investigated, and attributed primarily to exhaust gas stoichiometry and secondarily to the heat released in the catalyst by the oxidation reactions (self-acceleration). Finally, a parametric study is performed to assess the influence of different patterns of transient exhaust gas composition. The results obtained show that lean composition of the exhaust gas is more favorable during the light-off phase, while frequency and amplitude of the composition oscillation play only a minimal role. This investigation encourages further application of mathematical modeling in areas like lambda control strategy optimization, which were beyond the scope of earlier 3WCC models.     

Effect of hydrogen combustion reaction on the dehydrogenation of ethane in a fixed-bed catalytic membrane reactor

A two-dimensional non-isothermal mathematical model has been developed for the ethane dehydrogenation reaction in a fixed-bed catalytic membrane reactor. Since ethane dehydrogenation is an equilibrium reaction, removal of produced hydrogen by the membrane shifts the thermodynamic equilibrium to ethylene production. For further displacement of the dehydrogenation reaction, oxidative dehydrogenation method has been used. Since ethane dehydrogenation is an endothermic reaction, the energy produced by the oxidative dehydrogena-tion method is consumed by the dehydrogenation reaction. The results show that the oxidative dehydrogenation method generated a substantial improvement in the reactor performance in terms of high conversions and significant energy saving. It was also established that the sweep gas velocity in the shell side of the reactor is one of the most important factors in the effectiveness of the reactor.     

Development of a multi-scale, multi-phase, multi-zone dynamic model for the prediction of particle segregation in catalytic olefin polymerization FBRs

A comprehensive multi-scale, multi-phase, multi-compartment dynamic model is developed to analyze the extent of particle segregation in catalytic, gas-phase ethylene-propylene copolymerization fluidized bed reactors (FBRs). From the numerical solution of the proposed integrated model, the temporal-spatial evolution of the morphological (i.e., particle size distribution, PSD) and molecular (i.e., molecular weight distribution, MWD) polymer properties in a catalytic polymerization FBR can be predicted. In particular, the polymer molecular properties are determined by employing a generalized multi-site, Ziegler-Natta kinetic scheme. To determine the growth of a single catalyst/polymer particle, the random pore polymeric flow model (RPPFM) is utilized. The RPPFM is solved together with a dynamic discretized particle population balance equation (PBE) to calculate the dynamic evolution of PSD in the various compartments of the FBR. Moreover, overall dynamic mass and energy balances are derived in order to assess the dynamic behavior of catalytic gas-phase FBRs. The effects of various fluidized bed operating conditions (e.g., fluidization gas velocity, temperature and catalyst feed rate) on the morphological and molecular distributed polymer properties are thoroughly analyzed.