(104c) Reactive CFD Modeling, Experimental Validation and Scale-up of Non-Oxidative Ethane Dehydrogenation to Ethylene

Ethylene is the largest by volume commodity chemical manufactured world-wide and represents the single largest source of carbon emissions in the chemical industry. Alternatives to ethane or naphtha steam cracking (ESC) to ethylene seek reaction pathways that are lower in energy use and consequently have reduced carbon emissions. One potential pathway under investigation by our team is the non-catalytic non-oxidative dehydrogenation (NODH) of ethane to ethylene, C₂H₆ → C₂H₄ + H₂. This endothermic reaction is distinct from the related exothermic oxidative dehydrogenation (ODH) of ethane to ethylene (C₂H₆ +0.5O₂ → C₂H₄ + H₂O) or the exothermic oxidative coupling of methane (OCM) to ethylene (2CH₄ + O₂ → C₂H₄ + 2H₂O) as the reactor is simply an empty tube devoid of packings or internal structures, while OCM and ODH require heterogeneous catalysts to proceed. These three reactions share similar gas-phase reaction chemistries; however surface reactions are absent in NODH while important in OCM and ODH, as the latter two depend on the presence of catalysts for reaction initiation [1].

Recently we have demonstrated the effectiveness of NODH for ethylene production from ethane [2]. This non-catalytic endothermic pathway avoids the need to provide superheated steam and complex reactor geometries needed by ESC. Key features of the reaction include use of inert reactor housings, including quartz, and the ability to provide heat of reaction by conventional (external steam, combustion) or non-conventional low-carbon (solar-thermal, concentrating solar power) means. Use of renewable power coupled with less complex downstream separations, due to the absence of steam, can enable up to at least an estimated 40% reduction in carbon emissions associated with ethylene production. Commercial acceptance of our approach depends not only on the reduce energy and carbon impact potentials, but also on throughput and ethylene yield. To this end we used COMSOL Multiphysics computational fluid dynamics software to model ethane conversion and ethylene yields in single reactor tubes capable of producing kg-levels per day ethylene, with a full-scale plant consisting of multiple tubes scaled to metric-tons per day productivity.

An experimental campaign to understand the impacts of diluents (methane, nitrogen), inlet velocities, and reaction temperatures was completed. These measurements were performed in a thermally calibrated quartz reactor tube held in a tubular furnace and at close to atmospheric pressure. Product compositions, including N₂, H₂, CH₄, C₂H₄, C₂H₆, C₃H₆, C₃H₈ and C4-C8 species were measured with a micro-gas chromatograph configured and calibrated for refinery gas analysis. No CO or CO₂ were measured, verifying that the reaction proceeded in the desired oxygen-free atmosphere. For all tests N2 was used as an internal standard for carbon and hydrogen mass balance closures which were typically from 95-100%. Results were interpreted in terms of ethane conversion, ethylene yield, and selectivities to C3+ species. These results were used to build an irreversible reaction global kinetics model describing the primary features of the results. Experimental first order kinetic parameters (Ea/R and Af) were calculated for the following reactions, written to fit first-order irreversible rate expressions:

• C₂H₆ → C₂H₄ + H₂
• 0.5C₂H₆ + 0.5H₂ → CH₄
• C₃H₈ → C₃H₆ + H₂
• C₂H₄ → 0.5C₄H₈
• 0.5C₂H₄ + 0.5C₄H₈ → 0.5C₆H₁₂

Reactive CFD modeling (RCFD) was then used to optimize ethane conversions and ethylene yield while minimizing C3+ selectivity. Parameters considered including reactor length, diameter, external heat exchange fluid temperature, and total inlet flow rate. Kinetic parameters were refined in an iterative fashion to improve match to experimental conversions and selectivities; good fits between model and data were obtained over a full range of inlet experimental ethane concentrations from 5 to 95% and temperatures from 650 to 850 °C. An example of the RCFD results is presented in the Figure. RCFD identified optimum reactor design parameters in terms of tube diameter, length, and flow rate as well as quartz reactor wall temperature. An optimum 68% single-pass ethylene yield at 95% ethane conversion was obtained, byproducts include 20% selectivity to methane and 8% selectivity to C6’s. These results compare favorably to current commercial ethane steam cracking of ~70% single-pass ethylene yield. Our RCFD model, validated against experimental data, shows that NODH is capable of meeting current levels of ESC productivity in terms of product ethylene yields, while significantly reducing the carbon impact of ethylene manufacturing.

Figure Caption: Ethylene yields (fraction) predicted by RCFD as functions of inlet flow rate from 1000 (top plane) to 10,000 (bottom plane) cm³/min, as functions of reactor length (x-axis) and diameter (y-axis); for a quartz tube reactor operated at nominally 750 °C, 0.7 meter total length, 0.031 meter diameter.