Autothermal reforming of diesel fuel in a structured porous metal catalyst: Both kinetically and transport controlled reaction

Autothermal reforming (ATR) of diesel fuel into syngas was studied experimentally and theoretically. The experiments were performed in a reactor consisting of two cylindrically shaped monoliths 50 × 55 mm. Different catalytically active components and supports (Co, Mn, Rh, BaO, La₂O₃/Al₂O₃ and SiO₂) were tested. The reactor parameters were as follows: O₂/C = 0.5, S/C = 1.5–1.7, T_in = 350–400 °C. The regularly structured catalytic monoliths were prepared using various metal porous supports. The most active and coke resistant catalyst was determined. The original modeling approach was based on the assumption that ATR involves two parallel reaction routes: (1) complete hydrocarbon oxidation, (2) steam reforming of hydrocarbon. The experimental data and the results of reactor modeling agreed well and allowed a conclusion that the ATR rate is controlled by inter-phase mass transfer. However, the contribution of the reaction routes (1) and (2), i.e., the distribution of hydrocarbon flux between these reactions is determined by the ratio of the reaction rate constants and oxygen concentration near the surface.     

Asymmetric temperature profiles in a single catalyst pellet

For rapid reactions the differences in local coefficients of heat and mass transfer around the surface of a catalyst particle lead to a distribution of surface temperatures and an asymmetric internal profile. The nature of these distributions and their changes with flow rate were predicted for the hydrogenation of ethylene on a single spherical pellet of nickel on silica alumina. The measured surface and internal temperature were in good agreement with the predictions.     

Optimal catalyst profiles for a two-step reaction sequence in a packed bed

Optimal catalyst profiles have been determined for a two-step reaction sequence, where each step is catalyzed by a separate substance, for two cases: one where the remaining amount of initial reactants is minimized, and the second where the concentration of final product is maximized. The effect of varying levels of intraparticle diffusion, initial reactant concentration, residence time, and catalyst loading was investigated. It was found that increasing conversion led to a greater duration of intermediate control, where both catalysts are present in proportions that varied with reactor length. At no point during intermediate control, however, did the catalyst for the initial reaction increase in concentration with increasing length.     

Concentration and temperature profiles for complex reactions in porous catalysts

A method of computation for stationary concentration and temperature profiles in porous catalysts was developed. These calculations can be used with highly complex reactions and with any type of rate equation or transport model. The differential equations are transformed by the collocation method to non-linear algebraic equations. These are solved by a method which uses the time-derivatives of the variables. The method finds stationary points for all problems which can be formulated as ? = f(y) and is demonstrated on two examples.     

Pt-Re催化剂重整高温F-T合成石脑油的催化性

选用SB粉制得γ-Al₂O₃载体,采用共浸渍法制备Pt-Re催化剂,并对其进行BET、XRD、NH₃-TPD、H₂-TPR和ICP表征。以高温F-T合成石脑油为原料,在反应温度500 ℃、反应压力1.0 MPa、空速2.0 h^-1和氢油体积比1 000条件下,考察Pt-Re催化剂的重整活性及其稳定性。结果表明,Pt-Re催化剂能高效催化重整高温F-T合成石脑油,240 h重整过程中,高温F-T合成石脑油液体收率79.89%,芳烃质量分数61.60%,直链烷烃质量分数降低了28.15%,重整转化率达200.53%,研究法辛烷值提高35个单位,表明Pt-Re催化剂能有效催化重整高温F-T合成石脑油,使之转化为汽油调和组分成为可能。     

Enhancement of reforming efficiency by optimising the porous structure of reforming catalyst: Theoretical considerations

A modified form of the Thiele modulus was proposed for the reaction of n-heptane in the bidispersive grain of the platinum reforming catalyst. To assess the rate of conversion under conditions of diffusion use was made of a modified formula, which incorporates the bidispersivity of the grain. An algorithm was proposed for the assessment of grain efficiency and reaction rate related to the parameters of the porous catalyst grain structure. The contribution of the porous structure was analysed, and its optimisation was carried out by maximising the grain effictiveness factor and reaction rate under conditions of diffusion.     

Simultaneous intraparticle forced convection,diffusion and reaction in a porous catalyst

The effect of intraparticle forced convection on a heterogeneous reaction within a porous catalytic pellet is examined. It is found that an appreciable enhancement of the catalyst performance can be expected for liquid-phase reactions on coarse-grained particles. Quantitative results are obtained for an irreversible isothermal first-order reaction in flat, spherical and cylindrical geometries with uniform internal flow. A maximum increase in the effectiveness factor is observed in the region where the kinetics just becomes diffusion controlled.     

Prediction of concentration and temperature profiles for non-isothermal ethane cracking in a pipe reactor

Thermal crackers are mostly modeled as plug flow systems, disregarding the lateral gradients present. In this paper, a 2-dimensional model has been established for ethane cracking in a thermal cracker in laminar flow, using a molecular mechanistic model for ethane cracking. The model, consisting of 9-coupled partial differential equations, is solved using the backward implicit numerical scheme. The resulting product distribution and temperature profiles are predicted throughout the reactor. The concentrations of acetylene and propylene show a maximum within the reactor. The effect of certain operational parameters — tube radius, wall temperature and mass flow rate — is also studied on these profiles. The parameters are varied in the range of 0.005–0.0125 m for tube radius, 1.25 kg/hr-2.5 kg/hr for mass flow rate and 850–1,050 ‡C for tube wall temperature. It is observed that an increase in wall temperature and an increase in tube radius or decrease in flow rate favours the conversion of ethane.     

Optimal temperature profiles for parallel-consecutive reactions with deactivating catalyst

A cocurrent tubular reactor with temperature profile control and recycle of moving deactivating catalyst has been investigated. For the temperature-dependent catalyst deactivation, the optimization problem has been formulated in which a maximum of a profit flux is achieved by the best choice of temperature profile and residence time of reactants for the set of catalytic reactions A+B→R and R+B→S with desired product R, the rates of reactions have been described separately for every reagent by the expressions containing (temperature dependent) reaction rate constants, concentrations of reagents, catalyst activity, as well as catalyst concentration in the reacting suspension and a measure of the slip between reagents and solid catalyst particles. The algorithms of maximum principle have been used for optimization. The optimal solutions show that a shape of the optimal temperature profile depends on the mutual relations between activation energies of reactions and catalyst deactivation. It has been proved that the optimal temperature profile is a result of the compromise between the overall production rate of desired reagent R (production rate in the first reaction minus disappearance rate in the second one), necessity of saving of reagents residence time (reactor volume) and necessity of saving catalyst; the most important influence on the optimal temperature profile is associated with necessity of saving the catalyst. When catalyst recycle ratio increases (mean number of catalyst particles residing in reactor increases), optimal temperatures save the catalyst, as the optimal profile is shifted in direction of lower temperatures. The same is observed when catalyst slip increases (catalyst residence time in reactor increases). Despite of variation in the catalyst concentration the optimal profile is practically the same because the decay rate is affected only by instantaneous activity of catalyst. When reactor unit volume price decreases, catalyst residence time increases, whereas optimal temperature profile is shifted to lower temperatures. When economic value of unit activity of outlet catalyst increases (catalyst with a residual activity still has an economic value), catalyst saving should be more and more intense. As far as possible, the optimal profile is shifted in direction of lower temperatures, whereas the optimal residence time is still the same. Then the optimal profile is isothermal at the level of minimum allowable temperature, whereas the catalyst is saved as its residence time in reactor decreases.     

Velocity,temperature and concentration profiles in a vertical flow reactor

A vertical, adiabatic, steady-state, flow reactor in which a homogeneous chemical reaction takes place has been studied. Release of heat by the reaction sets up density gradients, which cause free convective flow to be superimposed on the laminar upflow. The coupled differential equations of momentum, energy, and mass transfer have been solved numerically by two different finite difference methods. In the first numerical method, the momentum transfer equation formulated in terms of the stream function required large amounts of computer time and storage for solution. In the second method, a simplified form of the momentum transfer equation was solved with a significant reduction of the computer requirements. Velocity, temperature, and concentration profiles have been obtained from the two numerical methods and the results analyzed in terms of dimensionless groups. The excellent agreement which has been found between the two methods confirms the accuracy and usefulness of the simplified method.     

On the bifurcation points for chemical reaction in porous catalyst particle

The methods for calculation of the bounds of region of multiple solutions in the case of a chemical reaction in the catalyst particle of different shapes are considered. To determinate bifurcation points Aris' method is used in case of a slab while the Weisz-Hicks method is employed for cylindrical and spherical geometry. Rather than attempting a comprehensive calculation of the bifurcation points as functions of parameters, a rotatable design of nine pairs of dimensionless activation energy and heat effect is used and a regression analysis performed on the results from these to give an interpolation formula of sufficient accuracy and great rapidity of calculation.It is shown that use of the same hydraulic radius for the particles of different shapes does not bring together the bifurcation points for different shapes.     

Sensitivity analysis for a chemical reaction in a porous catalyst with external heat and mass transfer resistance

Using variational techniques, a computationally efficient procedure is developed for determining the sensitivity of the observed reaction rate in a porous catalyst with external transport resistances. Derivatives of the observed rate with respect to bulk fluid conditions or any other parameters in the mathematical model of the catalyst may be easily calculated for slab, cylindrical, or spherical catalyst geometry.     

Optimal temperature profiles in tubular reactors for several catalyst decay laws

Theoretical and experimental work on tubular reactors has shown that a high temperature reaction zone can be induced to move counter to the flow of reactants by conduction and/or radiation through the solid phase. This naturally occurring phenomenon represents a transient mode of operation that can be argued to be preferable to conventional modes of operation. In order to test this hypothesis the complete time and space dependent optimal temperature policy was generated for a tubular reactor subject to several catalyst decay laws. The solutions revealed that a critical parameter is the ratio of the activation energy for the main reaction to that for the catalyst decay reaction. For values of this ratio less than one, the optimum temperature profiles represent a low temperature reaction zone that occupies the entire active bed at all times, essentially a conventional mode of operation. However, for this ratio greater than one the profiles represent a short high temperature reaction zone that propagates with time along the length of the reactor. For a catalyst decay law independent of conversion, propagation in either axial direction was optimal, but for a decay law dependent upon conversion the optimization procedure only generated reverse propagating zones.     

The influence of calcination temperature on catalytic activities in a Co based catalyst for CO2 dry reforming

The carbon dioxide dry reforming of methane (CDR) reaction could be thermodynamically favored in the range of 800 to 1,000 °C. However, the catalyst in this reaction should be avoided at the calcination temperature over 800 °C since strong metal support interaction (SMSI) in this temperature range can decrease activity due to loss of active sites. Therefore, we focused on optimizing the temperature of pretreatment and a comparison of surface characterization results for CDR. Results related to metal sintering over support, re-dispersion by changing of particle size of metal-support, and strong metal support interaction were observed and confirmed in this work. In our conclusion, optimum calcination temperature for a preparation of catalyst was proposed that 400 °C showed a higher and more stable catalytic activity without changing of support characteristics.     

从反应温降波动分析重整催化剂活性变化

介绍了所用催化剂以及工艺流程的概况。详细讨论了正在运行的第八周期中重整反应器的有关情况。对重整反应器1在2002年初发生的催化剂失活，从失活的现象、可能的原因以及采取的措施进行了详细的分析。从生产效益考虑，应整体更换催化剂。     

Catalytic reforming of tar during gasification. Part II. Char as a catalyst or as a catalyst support for tar reforming

Char, char-supported catalysts and ilmenite were investigated for the steam reforming of biomass tar derived from the pyrolysis of mallee wood in situ. Special attention was given to the reforming of aromatic ring systems in tar. The results indicated that the char-supported iron/nickel catalysts exhibited much higher activity for the reforming of tar than the char itself. Ilmenite and the char-supported iron catalyst contained similar active phase but showed different tar reforming activities. Kinetic compensation effects demonstrated that the reaction pathways on the char-supported catalysts were similar but were different from those on ilmenite. The proprieties of support could play important roles for the activities of the catalysts and the reaction pathways on the catalysts. Char would not only disperse the catalysts but also interact with the catalysts to enhance their activity for the steam reforming of tar.     

Bio-template fabrication of nanoporous Ni@Al2O3: Durable catalyst for biogas reforming reaction

Following green chemistry principles and the significant role of catalysts in chemical transformations, in this study, for the first time, Ni@Al₂O₃ nanoparticles were prepared via a green, a sustainable, and practical approach using the waste management concept. To achieve this goal, the eggplant skin was employed as an abundant and green bio-template to synthesize desired materials, applying for biogas reforming (CH₄/CO₂ = 1:1) at four temperatures (600–750 °C). Synthesized materials were fully characterized and according to the findings, the bio-template was able to induce its structure on prepared materials and had a considerable effect on the activity of nickel-based alumina catalysts thanks to the high dispersion of Ni nanoparticles on the prepared materials. This catalyst could tolerate the reaction conditions even after 30 h of the catalytic run at 650 °C, along with a remarkable coke resistance (less than 4% carbon deposition).     

FeNi doped porous carbon as an efficient catalyst for oxygen evolution reaction

Polymer-derived porous carbon was used as a support of iron and nickel species with an objective to obtain an efficient oxygen reduction reaction(OER)catalyst.The surface features were extensively characterized using X-ray diffraction,X-ray photoelectron spectroscopy and high-resolution transmission electron microscopy.On FeNi-modified carbon the overpotential for OER was very low(280 mV)and comparable to that on noble metal catalyst IrO₂.The electrochemical properties have been investigated to reveal the difference between the binary alloy-and single metal-doped carbons.This work demonstrates a significant step for the development of low-cost,environmentally-friendly and highly-efficient OER catalysts.     

Understanding of resonance phenomena on a catalyst under forced concentration and temperature oscillations

Resonance is an interesting phenomenon that may be observed for reactions on catalytic surfaces during periodic forcing of operating variables. Forcing of the variables for non-linear systems may result in substantially changed time averaged behaviour. These resonance phenomena have been observed experimentally by coincidence rather than by systematic analysis. It is not clear for what type of reaction kinetics such behaviour may be expected and predictions are therefore impossible. Clearly, this forms a serious obstacle for any practical application. In this work we set out to analyse the nature of resonance behaviour in heterogeneously catalysed reactions. A Langmuir Hinshelwood microkinetic model is analysed. It is demonstrated that for weakly non-linear forcing variables — as inlet concentrations — forcing leads to resonance phenomena in terms of the reaction rate only in case high total surface occupancies exist in the steady state. In contrast, forcing of strongly non-linear variables — like temperature — may give rise to resonance phenomena for both low and high surface occupancies. Necessary conditions for resonance to occur are derived. The analysis of resonance phenomena is greatly simplified by the availability of explicit analytical expressions as can be derived from Carleman linearization. We will demonstrate the merits of Carleman linearization as compared to numerical integration.