Kinetics of Direct Olefin Synthesis from Syngas over Mixed Beds of Zn–Zr Oxides and SAPO-34

Abstract

A packed bed containing a physical mixture of both Zn−Zr mixed oxide catalyst and SAPO-34 converts syngas directly into a mixture of C 2 −C 5 olefins and paraffins. Specifically, the mixed oxide catalyst is responsible for intermediate oxygenate synthesis from syngas while the molecular sieve catalyzes olefin synthesis from the oxygenate intermediates. Kinetic measurements with cofed propylene over each catalyst independently confirm olefin hydrogenation activity over both components of the composite bed. The addition of either water or CO to the feed drops the activity of propylene hydrogenation over the Zn−Zr oxide. In sum, under reaction conditions of syngas feed and produced water, olefin hydrogenation predominantly occurs over the SAPO-34 catalyst, rather than over the catalyst responsible for hydrogenating CO into oxygenate intermediates. A developed kinetic model consistent with this conclusion describes measurements at differing feed compositions, temperatures, space velocities, and bed catalyst mixing ratios. Technoeconomic analysis of the process indicates that the olefin-to-paraffin ratio is a key performance metric for commercial scale syngas conversion and highlights the importance of considering olefin hydrogenation rates over the molecular sieve component.

Optimization Modeling for Advanced Syngas to Olefin Reactive Systems

“…Finally, we examined the impact of internal or external mass/heat transfer limitations using the GradientCheck tool. 19 For this range of conditions, we found that there are no significant internal or external mass/heat transfer limitations. However, this does not rule out Knudsen diffusion limitations within the nanopores of SAPO-34, especially for oxygenate species such as methanol.…”

“…DeWilde and co-workers showed, in their technoeconomic study, that the olefins-to-paraffins ratio (O/P) is critical for a feasible operation. 19 The single-shot mixed-catalyst reactor design is only feasible for the STO mechanism when O/P is >3. High O/P values can only be achieved at high space velocities, which compromises the conversion.…”

“…Finally, Zn−Zr is responsible for catalyzing methanol further to methane and hydrogenating olefins to paraffins, which are all irreversible side reactions. This is a high-level summary of STO reactions over the mixed catalysts, and the detailed reaction network can be found in the work by DeWilde et al 19 The SAPO-34 catalyzes methanol conversion to ethylene and propylene. Ethylene can be converted further to propylene, propylene to butene, and butene to pentene over the SAPO catalyst.…”

“…Finally, Zn–Zr is responsible for catalyzing methanol further to methane and hydrogenating olefins to paraffins, which are all irreversible side reactions. This is a high-level summary of STO reactions over the mixed catalysts, and the detailed reaction network can be found in the work by DeWilde et al…”

“…Finally, we examined the impact of internal or external mass/heat transfer limitations using the GradientCheck tool . For this range of conditions, we found that there are no significant internal or external mass/heat transfer limitations.…”