(58aa) Design, Development and Demonstration of a Tail-End Acetylene Hydrogenation Catalyst | AIChE

(58aa) Design, Development and Demonstration of a Tail-End Acetylene Hydrogenation Catalyst

Steam cracking of hydrocarbon feeds is the preferred commercial route for the production of ethylene. The yield of by-product acetylene is typically between 0.5 and 2.5 kg per metric tonne of ethylene produced. The modern specification for acetylene in ethylene product is less than 1 ppm, due to downstream process requirements. However conventional distillation cannot separate acetylene from ethylene, therefore selective hydrogenation is practiced in most plants.

Palladium catalysts have been used for the hydrogenation of acetylene for a number of years. In an industrial plant with a tail-end configuration the ethylene purification step is down-stream of the de-ethanizer and usually consists of multiple catalyst beds in order to manage the heat of reaction and maximize ethylene yield. Within a multiple bed system, the lead bed typically removes the majority of the acetylene from the ethylene stream, up to 80%. Downstream reactors are used to lower the acetylene level to the sub ppm levels required to meet the specification. Tail-end systems control selectivity both through the selection of an appropriate catalyst, and control of over-hydrogenation through hydrogen stoichiometry and bed temperature. Efficient removal via selective hydrogenation presents some unique challenges for heterogeneous catalysts. Aside from the high activity and selectivity targets associated with such duties the propensity of catalysts within a tail-end selective hydrogenation system, particularly in the lead bed where the catalyst is in a hydrogen lean environment, to form green-oil is well known. Whilst the formation of green oil, a mixture of C4 to C20 unsaturated components, is well known and has been observed for a number of years, the formation mechanism is poorly understood and little studied. Formation of green-oil lowers the performance of the hydrogenation catalysts and requires frequently regeneration to remove the polymeric carbon deposits. Such regenerations are difficult to manage and require several days of down-time on the manufacturing facility.

Critical factors in maximizing activity, selectivity and controlling the rate of deactivation include formulation, metal/promoter interactions and both physical and chemical attributes of the catalyst/carrier material, coupled with reproducible and cost effective catalyst manufacturing methodologies. To understand the catalyst characteristics, and effectively design and manufacture a catalyst to overcome the challenges faced within industrial operation, innovative approaches must be taken to understand the reaction mechanisms and interactions between the components of the catalyst and the reactants.

Within this paper we will present our approach to the understanding of green-oil formation, and the strategy to translate this into improved catalyst formulations. Through advanced characterization, coupled with state of the art testing facilities, we can apply the fundamental understanding of the process requirements we have developed into industrial catalysis, both through the launch of new products and by working closely with industrial customers to enable the optimization of their plant operations. Within our presentation we will demonstrate the methodologies we’ve applied and how these have advanced our catalyst offerings for selective hydrogenation. We will also highlight the improvements to ethylene yield and cycle length that have been attainable on an industrial plant by applying the knowledge gained through extensive testing of our formulations under different process conditions at laboratory scale.