Determination of the kinetics of ethene epoxidation

Abstract

Several problems and pitfalls in the use of laboratory reactors for the determination of the kinetics of ethene epoxidation over industrial silver on E-alumina catalyst are discussed. Also, commonly used methodologies for kinetic studies are dealt with because of the general nature of some problems. Some advice is given in choosing and using the appropriate reactor type. Further, a method is discussed to determine kinetics in a cooled tubular reactor without having to use heat transport relations.The activation and deactivation of the silver catalyst have been studied in a Berty-type reactor, in a novel internal recycle reactor and in a cooled tubular reactor. It was found necessary to activate the silver catalyst for approximately 170 reaction hours under reaction conditions to obtain a stable and reproducible catalyst activity. Thermal sintering was probably of importance in experiments at the maximum temperature of 543 K. Deliberate addition of small amounts of 1,2-dichloroethane resulted in rapid deactivation of the catalyst. The activity could be restored by addition of small amounts of ethane to the feed. Also, fluorine and silica have been shown to poison the catalyst. Differences in the behaviour of the catalyst in the three reactors may be attributed to the sensitivity of the catalyst towards tiny amounts of poisons present in the reactors and feed mixtures used.

Deactivation of a commercial catalyst in the epoxidation of ethylene to ethylene oxide—basis for accelerated testing

“…Despite the wide list of possible causes for deactivation, it is broadly accepted that sintering of silver is the primary reason. In contrast to the commercial significance of the system, results reported on deactivation of Ag-based catalysts have been mostly related to model catalysts with just a few exceptions dealing with commercial catalysts [3,4].…”

“…Assuming sintering as the main deactivation mechanism of an alumina-supported Ag catalyst [3,4,[13][14][15][16], and due to the observation that the rate of catalyst sintering often depends on oxygen concentration [24], both high reaction temperatures and different oxygen concentrations were chosen as parameters supposed to bring accelerated and true deactivation. Given that reduced capacity for oxygen adsorption on Ag-based catalyst and its deactivation seem to be directly related [29], the decrease of the rates of oxygen conversion besides ethylene consumption and EO formation was chosen as a measure of catalyst deactivation during time on stream.…”

“…Some authors claim that poisoning by chlorine is reversible only when certain impurities in the feed are present, like ethane. Its role as a scavenger might explain the reversible nature of Ag-catalyst poisoning by chlorine in commercial applications [4,7].…”

“…Thus, an increase in space velocity, e.g., will eliminate concentrations of a specific compound due to high conversion as the cause for catalyst decay, but might cause major catalyst poisoning if some impurities are present in the feed [20]. Sulfur and fluorine, which are often present as impurities in the feed, were both claimed as being effective poisons for Ag catalysts operated under both commercial [3] and laboratory [4] conditions. Therefore, additional precautions, by means of checking the influence of space velocity on deactivation rate, must be undertaken to exclude interference of side-by-side poisoning mechanisms in the global picture of deactivation of the catalyst.…”

“…Over the years deactivation of Ag-based catalysts has been a subject of extensive research, with the aim of understanding its mechanism and of improving catalyst stability. Several possible causes for Ag-catalysts deactivation, such as catalyst poisoning either by impurities [3,4] or by an excess of chlorine-based promoters in the feed [5][6][7], accumulation of carbon deposits on the catalyst surface [8][9][10][11][12], and increase of both silver particles size [3,4,[13][14][15][16] as well as support crystallite size [16], have been discussed. Despite the wide list of possible causes for deactivation, it is broadly accepted that sintering of silver is the primary reason.…”