Feed Composition Cycling Research Articles

NOx storage and reduction by H2 over highly dispersed Pt on Co1Mg2Al1Ox-LDO for stationary applications: A transient kinetic study

The performance of a highly dispersed Pt/Co1Mg2Al1Ox-LDO catalyst towards NOx-storage and reduction by H2 for NOx control in stationary applications was investigated under forced feed composition cycling (“swing-reactor” system). The influence of 7 vol% CO2 and 5 vol% H2O in the NO-containing and H2-containing feeds on the dynamic evolution of adsorbed NOx-s and their reduction performance (conversion and N2-selectvity) were determined in the 200–400 oC range. The effects of catalyst pretreatment with 20 or 50 ppm SO2/He gas mixture on its NOx-s storage and reduction by hydrogen dynamics were also investigated. Reduction of NOx-s formed on the support involves a hydrogen spillover effect from Pt to the metal-support interface and support NOx-s sites. A very stable NSR performance was obtained at 350 oC for 2 h of successive NOx-adsorption (3 min)/reduction by H2 (1 min) steps. The nature of active and inactive NOx-s in H2 was probed by in-situ DRIFTS.

Unsteady-state operation of reactors with fixed catalyst beds

Abstract The review is dedicated to the research and development work made in USSR and Russia in the area of catalytic processes with artificially created unsteady-state conditions. The paper discusses the reverse-flow operation of catalytic reactor, forced feed composition cycling and sorption enhancement of catalytic reactions. It is demonstrated that under proper choice of process concept and control strategy these approaches may result in creation of new technologies with improved efficiency, lower capital and operation costs, higher process stability under variation of process conditions and increased target product yield in thermodynamically limited reaction and complex reaction systems.

CO Oxidation Kinetics over a Nanostructured Cu0.1Ce0.9O2-yCatalyst: A CO/O2Concentration Cycling Study

Feed composition cycling as a transient kinetic technique provides detailed information about elementary steps in CO oxidation over a nanostructured Cu0.1Ce0.9O2−y catalyst. This catalyst has a great potential as a future PROX reactor catalyst since it has great oxygen storage capacity as well as high reoxidation rate under oxygen rich conditions.

Acetylene and carbon monoxide oxidation over a Pt/Rh/CeO 2/ γ-Al 2O 3 automotive exhaust gas catalyst: kinetic modelling of transient experiments

The transient kinetics of acetylene (C 2H 2) conversion by oxygen over a commercial Pt/Rh/CeO 2/ γ-Al 2O 3 three-way catalyst have been modelled. Experiments to validate the model were carried out in a fixed-bed reactor with two separate inlets, enabling alternate feeding of acetylene and oxygen. Frequencies of feed composition cycling up to 1/6 Hz were applied. The experimental conditions resemble the cold-start period of an Otto engine. Two types of adsorbed acetylene species seem to exist. A selective catalyst deactivation for oxygen adsorption, due to deposition of carbonaceous deposits, was found. Ceria proved to have a significant influence on the acetylene oxidation. A kinetic model was developed for the conversion of acetylene to carbon monoxide, based on elementary reaction steps. This model was combined with the published kinetics for transient carbon monoxide oxidation to carbon dioxide over the same catalyst (Nibbelke, R. H., Chapman, M. A. J., Hoebink, J. H. B. J., & Marin, G. B. (1997). Kinetic study of the CO oxidation over Pt/V-Al 2O 3 and Pt/Rh/ CeO 2/ γ−Al 2O 3 in the presence of H 2O and CO 2. Journal of Catalysis, 171, 358–373.), in order to describe the total oxidation of acetylene quantitatively. The combined model is able to describe the results of the transient experiments on simultaneous acetylene and carbon monoxide oxidation. During a transient, both acetylene and carbon monoxide react mainly in a front moving through the reactor, carbon monoxide hardly influencing the acetylene oxidation.

Simulations of non-stationary reactors for the catalytic conversion of methane to synthesis gas

Mathematical simulation results are compared for several types of reactors for the conversion of methane to synthesis gas via catalytic partial oxidation of methane coupled with the simultaneous steam reforming. The simulations concerned reactors operating under non-stationary conditions, i.e. at a continuous, forced unsteady state. Both reverse-flow reactors with pelletized and monolith catalyst were simulated, and those with the pelletized catalyst, operating under conditions of feed composition cycling. The simulation results were compared with data for similar reactors operating at steady state. The basic process parameters compared include the conversion of methane and oxygen and the selectivities of the products (CO and H 2). To provide a comprehensive assessment of the simulation results, thermodynamic conditions for the formation of permanent carbon deposits were also analysed.

CO oxidation in a fixed bed reactor with high frequency cycling of the feed

An experimental set-up is presented, which allows cycling of the feed for transient operation of fixed bed microreactors at frequencies up to 10 Hz. The set-up was applied for the transient oxidation of CO by O 2 over Pt/ γ-Al 2O 3. The results gave clear evidence that CO may adsorb on oxygen covered sites under simultaneous production of CO 2. O 2 does, however, not adsorb on CO covered sites, which causes a delay in the transient of the CO 2 production. A kinetic model including rate parameters was presented, which gives an adequate description of the experimental data and provides much more detailed information than could be obtained from steady-state experiments. A simulation study is performed, with CO oxidation as an example, to explore how feed composition cycling is affected by diffusion inside catalyst pellets. If the time scale of diffusion inside the catalyst is much smaller than the time scale of the applied oscillation, intrinsic kinetics might be obtained from experiments with cycling of the feed. This statement supposes that diffusion limitation would be absent in the corresponding steady-state without composition cycling, and that the reaction kinetics is in the dynamic regime.

Slow convergence to cycle-invariance during forced oscillations of the NO+CO reaction over a Pt catalyst

The transient behavior of the NO+CO reaction over a Pt catalyst during forced composition cycling has been investigated using an external recycle reactor. The convergence to a cycle-invariant state was very slow requiring as much as 90 hours of feed composition cycling. The effects of the feed composition, flow, temperature and the cycling frequency have been studied. The results can be partially explained by a mathematical model based on a seven-step reaction mechanism. Slow convergence to cycle-invariance only occurred when the catalyst was subjected to an oxidizing pre-treatment.

Increasing of carbon monoxide methanation rate by forced feed composition cycling

The methanation of carbon monoxide on the commercial Ni/SiO 2 catalyst was studied experimentally under transient reaction conditions. Two types of concentration changes of the reactor feed were used: (i) step-changes for the investigation of elementary system dynamics, and (ii) forced feed composition cycling for examination of the effect of the periodic feed modulation on the time-average reaction rate. The experiments were performed in a laboratory flow microreactor in the temperature range between 458 and 538 K. It has been found that forced feed composition cycling significantly increases the reaction rate for any composition of reactor feed including composition leading to the highest steady-state rate. The enhancement of the methanation rate is caused by massive deposition of carbonaceous intermediates on the catalyst surface. Those deposits are unreactive under steady-state conditions due to the blocking of active sites for adsorption of hydrogen by CO but highly reactive during feed concentration cycling for alternating between CO-rich and CO-lean feeds.

CO oxidation on Pt: Variable phasing of inputs during forced composition cycling

AbstractA combined experimental and theoretical investigation of the effect of forced feed composition cycling for CO oxidation on platinum has been performed. A novel approach to forced composition cycling was examined, in which the phase angle between the two input streams was varied. Reaction rate enhancement is shown to occur, and by varying the phasing of the feed streams it is possible to achieve a global maximum in the time‐average reaction rate. This phenomenon can be explained quantitatively by a model based on an adsorbate‐induced phase change of the Pt surface combined with CO adsorption self‐exclusion. This mathematical model can also quantitatively describe the complex steady‐state behavior (uniqueness‐multiplicity transitions) observed for this reaction. The predictions of the model have been validated further through a detailed experimental study of the effects of feed flow rate, temperature, size of catalyst charge, and cycling frequency on the instantaneous and time‐average conversions during forced cycling of the feed composition.

Numerical studies of feed cycling in a CSTR

A numerical study of feed composition cycling (periodic operation) of stoichiometrically simple reactions described by Langmuir-Hinshelwood or Eley-Rideal kinetics reveal that although time-average production rates may be higher than the corresponding steady-state ones at the same time-average feed composition, they do not exceed the maximum steady-state rates at a given space velocity. This behaviour is explained by the dynamics of surface concentrations of adsorbed species during a cycle. Although these findings support other numerical studies, they are at variance with experimental observations. Thus, there is a need to develop more sophisticated kinetic models for the prediction of dynamic behaviour of heterogeneous catalytic systems.

Application of periodic operation to maleic anhydride production

AbstractTwo‐step and multi‐step feed composition cycling of the partial oxidation of butadiene to maleic anhydride have been studied. Multi‐step cycles were created by N2 flushes between steps containing just one reactant. Both production and selectivity to maleic anhydride are decreased over a cycle in this type of periodic operation. Selectivity to furan is increased, however. N2 flushing concentrates production of maleic anhydride in the butadiene step, which may have process advantages.

Kinetics of NO reduction by CO over supported rhodium catalysts: Isotopic cycling experiments

The catalytic activity of Rh Al 2O 3 and Rh CeO 2 catalysts was investigated for the reduction of NO by CO under feed-composition cycling conditions as well as under steady feed conditions using a packed-bed reactor and an isotopic reactant, 13CO. Results indicate that the formation of N 2O is an important intermediate step during the (CO + NO) reaction over Rh Al 2O 3 . The enhanced catalytic activity of Rh CeO 2 compared with Rh Al 2O 3 under cyclic operating conditions is discussed in light of the oxygen storage capacity of ceria support. The effect of asymmetric cycling is examined in view of the nature of the rate-controlling step which changes with temperature. On the basis of our observations an overall reaction scheme is proposed for the (CO + NO) reaction over Rh Al 2O 3 in which N 2O is included as a reaction intermediate.

The fischer—tropsch synthesis over a ruthenium catalyst under composition cycling

AbstractThe influence of feed composition cycling on the activity and selectivity of the Fischer‐Tropsch synthesis over an alumina‐supported ruthenium catalyst was investigated using a fixed‐bed reactor at 211°C and 446 kPa total pressure. Forced composition cycling suppressed the overall synthesis rate but increased the rates of formation of the C1 to C4 paraffins and shifted the paraffin/olefin ratio.

Influence of forced feed composition cycling on catalytic methanol synthesis

AbstractStudies have been carried out between 230 and 270°C and 1.97 and 2.93 MPa total pressure on the synthesis of methanol over an industrial catalyst of the copper‐zinc oxide‐alumina type using a gradientless reactor. Two modes of forced feed composition cycling were explored: periodic variations of the mole fraction of H2 and CO and periodic variation of the CO2 content. The latter mode substantially improved methanol production and suppressed the parasitic formation of methane, while the former mode had just the opposite effect. Measurements of methanol, methane and CO during a cycle as well as separate observations of the response to step‐changes in concentration indicated substantial storage of CO on the catalyst surface. Rapid response of methane to the CO2 content suggests over‐reduced areas of the catalyst surface are responsible for methane formation and that these regions are readily oxidized. CO2 or H2O associated with methane formation appear to relate to the dynamics of methanol formation.

Modelling of non‐stationary ammonia synthesis kinetics over an iron catalyst II. Modelling results of forced feed composition in periodic operation and for step‐changes

AbstractSimulation using the two‐site dissociative adsorption model given in Part 1 of this study fails to predict the improvement observed in the time‐average rate under cyclic operation even though it does recreate the observed steady‐state and transient response features following a step‐change in the feed composition. The improvement in the time‐average cycling rate is probably due to the diffusion of nitrogen into the bulk‐phase of the iron catalyst, which is a metastable nitrogen? Iron system under forced feed composition cycling. A two‐site adsorption model, modified to take account of the inventory of nitrogen in the bulk‐phase of the iron catalyst, is presented. This model predicts the improvements in ammonia production observed under periodic operation.

The Claus reaction: Effect of forced feed composition cycling

The Claus reaction was studied, over bauxite catalyst, under steady-state conditions. The orders of the reaction with respect to the reactants; i.e. hydrogen sulfide and sulfur dioxide were found to be approximately 1 and 0.4, respectively. The activation energy of the Claus reaction was determined in the temperature range of 463–573 °K and was found to be 35.1 kJ/mol. The effect of forced feed composition cycling on the Claus reaction, over bauxite catalyst, has also been investigated. The study of the Claus reaction under cycling conditions was accomplished by using square-wave cycling of the reactants ratio. An improvement of about 50% in the time-average-rate of reaction was obtained at the stoichiometric ratio of the reactants. A maximum in the time-average reaction rate was obtained in the high frequency region of the cycle (2 minutes). The minimum in the time-average rate was observed in the low frequency region of cycling (between 6–12 minutes).

Influence of forced cycling on the fischer‐tropsch synthesis. Part II. Response to feed concentration square‐waves

AbstractThe influence of forced feed cycling was investigated on the product distribution from C1 to C9 of the Fischer‐Tropsch synthesis. A promoted iron catalyst in a small‐scale differential reactor at 246°C and 384 kPa total pressure was employed with square‐wave composition cycles generated at a constant flow velocity.Except for methane, the product distribution during feed composition cycling followed the steady‐state Schulz‐Flory model. For some combinations of period, cycle‐split and mean composition, the time‐average methane rate was higher than the corresponding steady‐state rate. Decreasing the period increased the time‐average rate both for methane and other hydrocarbon products. High methane rates, found only during a partial cycle with pure hydrogen, indicated that methane was probably being produced by a parallel reaction path, the hydrogenation of a surface carbide. The methane reaction rate via this mechanism should be strongly inhibited by CO.Product selectivity may be altered by forced composition cycling.

Influence of forced cycling on the fischer‐tropsch synthesis. Part I. Response to feed concentration step‐changes

AbstractThe response of the Fisher‐Tropsch synthesis to composition step‐change was studied as part of a larger study on the influence of forced feed composition cycling on the product distribution from this synthesis reaction. Using a promoted iron catalyst, studies were conducted at 384 kPa total pressure in a fixed‐bed differential reactor held at 246°C by means of an oil bath. It was determined that the reaction rate was not influenced by mass or heat transfer effects.The maxima in the curves of steady state rate versus feed composition for various products occurred considerably above the stoichiometric ratio for each product olefin. This fact suggested that the rate was probably controlled by hydrogen adsorption in the initiation sequence. Support for this interpretation was also found from the response of the rates of production of light hydrocarbons (C2 to C5) to the step‐changes in feed concentration.

Influence of forced feed composition cycling on the rate of ammonia synthesis over an industrial iron catalyst part I — Effect of cycling parameters and mean composition

AbstractStudies have been carried out at 2.38 MPa total pressure and at 360 and 400°C of the synthesis of ammonia over a triply‐promoted iron catalyst (United Catalysts Inc.). Runs were carried out using a gradientless reactor in steady‐state and periodic modes, the latter employing forced composition cycling. The catalyst had a particle diameter of 0.30 mm; the resulting reaction rates were free from transport intrusions. The range of mean compositions for steady‐state measurements was from 0.1 to 0.9 mole fraction of hydrogen and for periodic operation from 0.5 to 0.9. The cycle‐split ratio (used to describe non‐uniform cycling) varied from 0.2 to 0.85. Effects of cycle period, mean composition, cycle‐split, and temperature were investigated. Instantaneous rate variations within the cycle are reported.Results showed that the cycling mode of operation results in an improvement in the time‐average rate of ammonia formation of up to 30%, depending upon the choice of operating parameters. Some implications for the mechanism of ammonia synthesis under periodic operation are presented.

Modelling of resonant behavior during forced cycling of catalytic reactors

AbstractA model, based on a Langmuir‐Hinshelwood type mechanism, for the oxidation of carbon monoxide on a platinum catalyst is presented. This model is used to explain resonant behavior which has been observed during forced feed composition cycling of catalytic reactors. This model further predicts that catalytic reactions can display pathological behavior under certain conditions of feed cycling in agreement with experimental observations.