P. L. Silveston

Factors affecting the reactivity of coal chars during gasification, and indices representing reactivity

Literature data for the rate of gasification of coal chars with oxygen, CO2 and steam have been reviewed to clarify the factors controlling this process. Based on steam gasification data for 95 chars from 68 coals whose carbon contents ranged from 65.0 to 93.7%, the gasification rates of the chars from lower rank coals (C 80%) were controlled by the intrinsic reactivity of the coal, which is related to active sites in the coal matrix associated with carbon atoms bonded to heteroatoms, nascent sites, dangling carbon atoms, edge carbon atoms, etc., The number of active sites can be estimated from the amount of chemisorbed oxygen at around 100 °C. Highly dispersed metals such as Ca, K, Na and Mg act as catalysts for gasification, and the degree of dispersion of the metals can also be estimated from the amount of chemisorbed oxygen or CO2. The selectivity of the steam gasification is related to the catalytic action of the coal minerals. Finally, the problems associated with employing the chemisorbed oxygen as a reactive index are discussed.

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.

Challenges for the periodic operation of trickle-bed catalytic reactors

Since the earliest publications just over a decade ago, the literature on periodic operation of trickle beds has grown rapidly. There are now over 30 published papers. Two applications, flow and feed composition modulation, for control of hot spots in large-scale reactors are sufficiently advanced for full-scale implementation if that has not already taken place. Models are now available that are capable of representing the time-average performance of periodically operated trickle beds, but these are not detailed enough to reproduce all of the transient behavior observed. Much about performance under periodic operation remains to be discovered. Research challenges are discussed under separate types of periodic operation: flow interruption, flow augmentation for hot spot control, feed composition modulation for hot spot control and to improve rate and/or to modify selectivity, and flow variation for enlargement of the pulse flow regime.

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.

Pore development during carbonization of coals

Abstract Changes in pore properties during carbonization at a constant heating rate and no external pressure were followed for bituminous and subbituminous coals by porosimetry, sorption and density measurements. Observations were supplemented by scanning electron microscopy and rate of volatilization measurements. All coals exhibited similar devolatilization behaviour and pore structure development, showing maximum micro and macropore volumes around 600 °C, and a partial collapse of pore structure after 800 °C. The micropore structure did not collepse in the case of non-caking coal. Pore volume maxima occur at least 100 °C above the temperature of the devolatilization rate maxima. Some interpretation is provided.

Automotive exhaust catalysis under periodic operation

Abstract Catalytic converters in automobiles now number in the millions. All of these devices are periodically forced about the stoichi-ometric air-fuel ratio at a frequency of about 1 H2 and a small amplitude. Experiments show that this periodic forcing suppresses rather than enhances conversions under normal operating conditions, even though many publications demonstrate that large rate and conversion improvements do occur for the converter reactions over the individual noble metals used in the three-way catalysts. This review concludes that improvements are not found because enhancement becomes very small in the 400–600°C temperature range and forcing at about 1 Hz is sub-optimal.

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.

Periodic operation of chemical reactors — a review of the experimental literature

The decade 1970–80 has seen researchers focus on the dynamic behaviour of chemical reactors. This has led to active programmes in many countries exploring the use of periodic operation to improve reactor performance. Most of these explorations have centred on the use of periodic changes in reactant composition. In the time domain, these appear usually as concentration square waves; they lead to a time varying product composition, which averaged over a long enough period provides a constant mean. Periodic operation introduces new variables which can be utilized for reactor design or control. Among the variables that have been explored so far, frequency of the periodic changes and symmetry of the successive square waves have proved to be the most important. Research results demonstrate that periodic operation can offer higher activity per unit of catalyst load. There is no general physical explanation for the success of periodic operation, however.