R. R. Hudgins

Periodic operation of catalytic reactors—introduction and overview

A review, with 89 refs., is presented on the subject of periodic operation of catalytic reactors by compn. forcing. Possible objectives of this mode of reactor operation are increased conversion, improved selectivity, reduced catalyst deactivation and insight into mechanisms of reactor models. Several forcing strategies may be used: manipulating one or more reactant concns., or interspersing pulses of inerts between pulses of reactants. These strategies are distinct from the variables in periodic operation, i.e., frequency, wave shape, amplitude, and phase lag. Lab.-scale equipment for periodic forcing makes use of single reactors along with the control of reactant and/or diluent flows. On an industrial scale, two catalyst beds are used, each operating with different feeds under different conditions. Catalyst transfers between the beds. A large literature has developed over the 25 yr since periodic operation was first proposed. [on SciFinder (R)]

Investigations of periodically operated trickle-bed reactors

Abstract This study concerns experimental investigations of the forced unsteady-state operation of trickle-bed reactors. The hydrogenation of cyclohexene to cyclohexane and α-methylstyrene to cumene on palladium catalysts were taken as model exothermic reactions. Changes in the control variables (e.g. feed composition, feed rate, temperature) strongly influence the regime and performance of a trickle-bed reactor. The aim of the present study of cyclohexane hydrogenation was to seek conditions of periodic operation that would enable higher average inlet concentration to be used without evaporating the feed mixture. For α-methylstyrene hydrogenation, the aim was to discover periodic operating conditions that would improve the time-average conversion relative to that under steady-state operation.

Forced cycling of the catalytic oxidation of CO over a V2O5 catalyst—I: concentration cycling

Abstract A study was performed on the behaviour of a catalytic reactor in which CO was oxidized over V2O5 under transient conditions using square-wave cycling of the reactant ratio. A maximum in the time-average reaction rate was observed at a period of about 20 min, resembling a resonance phenomenon. A minimum in the time-average reaction rate occurred in the range of periods between 40 and 60 min.

Activity and selectivity control through periodic composition forcing over Fischer-Tropsch catalysts

Abstract Data collected under steady-state and periodic composition forcing of the Fischer-Tropsch synthesis over three commonly used catalysts demonstrate that both activity and selectivity can be changed by the latter operating mode. Synthesis of hydrocarbons up to C 7 are favored at the expense of the higher carbon numbers for the Co catalyst, while for the Ru catalyst, only the C 3 and lower species are favored. Only methane production is stimulated with the Fe catalyst. Fe and Ru catalysts shift production from alkenes to alkanes. Transient data is interpreted in the paper.

Methanol synthesis from H2, CO, and C02 over Cu/ZnO catalysts

Methanol synthesis kinetics at steady state are reported for two catalysts: Cu/ZnO (as Cu : Zn  30 : 70 atomic ratio) and Cu/ZnO/Al2O3 (Cu : Zn : Al  60 : 30 : 10 atomic ratio) for various syngas compositions and temperatures from 200 to 275°C. In addition, catalyst deactivation is discussed along with XPS-Auger measurements of surface composition for fresh and used catalysts. Apparent activation energies for methanol synthesis on the Cu/ZnO catalyst depend on both temperature and fractional C02 in the COCO2 mixture and reflect a change in the importance of the CO and C02 as sources of methanol in the synthesis. The water-gas shift reaction does not proceed to equilibrium for CO mole fractions below 0.05. Experimental results confirm earlier observations that methanol is formed directly from CO and C02. However, in syngas mixtures, methanol production from CO and C02 is not just additive; some interconversion must be involved.

Transient study of oxidative dehydrogenation of propane

Abstract Kinetics and the mechanism of the oxidative dehydrogenation of propane were investigated using various transient techniques. Results support a redox reaction mechanism in which propane and intermediate products react with lattice oxygen, reducing the catalyst surface, which is reoxidized by gas-phase O 2 . Partial reduction of the catalyst occurs during the start-up to a steady state. Successive pulsing with C 3 H 8 reduced V 5+ in the magnesium ortho -vanadate phase to V 2+ . Carbon-containing species were observed upon interruption of the reaction, although only minute amounts were formed. Cycling increases the amount of the carbon deposited, but this carbon is reactive and most of it is oxidized in the succeeding O 2 pulse. Temperature-programmed oxidation (TPO) experiments on the catalyst used in steady-state operation revealed mainly strongly bound carbonaceous matter on the catalyst, but this carbon deposition did not affect catalyst activity. Thus, adsorbed oxygen is an important source of total combustion. Our experiments show, however, that lattice oxygen also produces total oxidation. Propene selectivity of the reaction in the absence of gas-phase O 2 was superior to steady-state selectivity, at the same propane conversion. Propene selectivity could be further improved by increasing the degree of reduction of the catalyst.

Composition cycling of an SO2 oxidation reactor

Experiments are described in which the second stage of a two-bed catalytic reactor is used to study the catalytic oxidation of SO2 over a potassium When the feed to Stage 2 is continuously cycled the rate of oxidation SO2 in Stage 2 is higher than when Stage 2 is operated continuously. Mass spectrometric measurements of the time-resolved relative ratio of SO3 in the output from the Stage 2 reactor, coupled with measurements of the A tentative explanation for the increased rate of SO2 oxidation observed is presented. It is suggested that SO3 in the catalyst phase or potass

Investigation of periodic operation of a trickle-bed reactor

Abstract Gas and liquid loadings of a co-current downflow trickle-bed reactor were separately modulated employing α -methyl styrene (AMS) hydrogenation on Pd/Al 2 O 3 to determine if the hydrogenation rate could be increased. Modulating the liquid flow rate resulted in a large increase in the AMS conversion rate. The explanation of the improvement in rate appears to be that modulation caused a transition from the trickling to the foaming-pulsing hydrodynamic regimes. The foam created in the latter regime persisted in the trickling portion of the cycle. Modulation of the gas loading was not effective probably because a regime transition did not occur.

A cycled trickle-bed reactor for SO2 oxidation

Abstract The effects of cycle period, cycle split, and liquid flow rate on SO 2 removal in a periodically operated trickle bed of activated carbon were investigated using a simulated flue gas containing 2500 ppm SO 2 . The highest measured SO 2 removal was 98%. Temperature waves observed in the bed were the consequence of periodic flushing with liquid coupled with the exothermicities of SO 2 absorption and its oxidation to SO 3 as well as the endothermicity of evaporation. The outlet SO 2 concentration from the trickle bed also varied and appeared to be closely related to the changing bed temperature. The period and split affect the SO 2 removal apparently as a result of limits in the minimum flushing time combined with the rate of oxygen transfer to the catalyst. Increasing the period at a fixed split has almost the same effect as decreasing the split with a fixed period. The liquid flow rate has no significant effect on the catalytic activity within the range of experimental conditions studied.

Effect of ethene addition during the Fischer-Tropsch reaction

An investigation has been conducted on the addition of from 1 to 2% ethene to different feed mixtures of synthesis gas. Experiments were carried out over a reduced commercial cobalt catalyst at a temperature of 473 K and 110 kPa pressure. Hydrocarbons from C3 to C7 revealed rate increases of from 50 to 100%. Interestingly, methane production rates were unchanged, suggesting that fragmentation of ethene into C1 did not occur to any appreciable extent. As a result, the relative selectivities of the higher hydrocarbons were improved. However, the certain feed compositions, the chain growth factor, α, was unaffected, reinforcing the belief that C2 species can participate as a chain initiator. A mechanism for the ethene-dosed Fischer-Tropsch reaction is also proposed.