(213f) Catalytic Propane Oxidative De-Hydrogenation with High Propylene Selectivity in a Downer Fluidized Bed Reactor: Kinetics and CPFD Simulation

Propylene is one of the major building block in the chemical industries. The oxidative dehydrogenation (ODH) process is a low-temperature process offering potentially high olefin yields and extended catalyst life given the low coke deposition. But ODH is still hindered by low alkane conversion and poor olefin selectivity. This prevents the ODH process from being applied in large scale processes. ODH kinetics involve a complex reaction network. In the presence of gas phase oxygen, olefin combustion and correspondingly lower olefin selectivity are favored.

To demonstrate the catalytic propane oxidative dehydrogenation (PODH) with high propylene selectivity, runs were performed in a mini-fluidizable chemical reactor engineering center (CREC) Riser Simulator [1] using a novel catalyst 7.5 wt. % vanadium supported on a ZrO₂-γAl₂O₃ (1:1 wt. %) [2]. Reactions were carried out at 500-550°C, 10-20 seconds, atmospheric pressure under oxygen-free gas phase conditions. Given that, the ODH circulating fluidized bed system requires a limited degree of catalyst re-oxidation, the ODH catalyst was evaluated using consecutive propane injections (e.g. 10 cycles). There was no catalyst regeneration in between the 10 cycles. Propylene selectivities up to 94% with a 25% propane conversion were obtained.

These data were employed for establishing a kinetic model. This kinetic was based on a Langmuir-Hinshelwood rate equation and a parallel-series reaction network [3,4]. The 6-independent intrinsic kinetic parameters were calculated via numerical regression. The high propylene selectivity led to a much larger 2.82×10^-5 mol.gcat^-1s^-1 frequency factor for propylene formation versus the 1.65×10^-6 mol.gcat^-1s^-1 frequency factorfor propane combustion. Calculated energies of activation (55.7 kJ/mole for propylene formation and 33.3 kJ/mole for propane combustion) appear however, to moderate this effect, with frequency factors influence prevailing. Furthermore, propylene conversion into carbon oxides (CO_x) oxidation appears as a non-favoured reaction step, given the 98.5 kJ/mole activation energy and 4.80×10^-6 mol.gcat^-1s^-1 frequency factor.

To address the issue of ODH process development, our research group at the CREC-University of Western Ontario, Canada has led the implementation of a PODH circulating process involving a downer unit and a dense phase fluidizable regenerator. This reactor systems overcome the issues of fixed-bed reactors, namely non-isothermal conditions and mass transfer limitations. So, propylene selectivity is highly influenced by a suitable catalyst, reactor configuration, operation parameters and proper kinetics.

As far as we are aware of, this is the first contribution where a large-scale simulation of PODH process in a circulating fluidized bed reactor is reported. Regarding the selected downer, it has 10 cm diameter and 10 m length, with two inlets in the top cyclone, one is for the partially reduced catalyst particles and a second one is for the oxidized catalysts (10/1 ratio). After leaving the cyclone, propane is fed into annular feeding section and the reaction starts in earnest with all chemical changes accounted via the developed kinetic model. Catalyst particles with a 87.13 μm mean diameter, a 3357 kg/m³ particle density and 20 seconds total reaction time are considered.

Computational particle-fluid dynamics (CPFD) is employed to demonstrate the main features of process, based on an energy minimum multi-scale (EMMS) drag model coupled with CPFD. This Hybrid Barracuda CPFD model uses the Eulerian-Lagrangian approach called multi-phase particle-in-cell (MP-PIC). The software’s numerical methodology considers a direct element method wherein solids are modeled as discrete particles with both size and density distributions, and the fluid is modeled as a continuum. Barracuda CPFD results are reported including gas and particles in the downer feeding section, particle-fluid flow in the stabilized region and gas compositions at different axial locations. Results are useful to establish downer performance, including propane ODH conversions in the 20% range with 92-94% propylene selectivity.

It is anticipated that thus research allows: (a) to gain insights on PODH performance (propane conversion and propylene selectivity) at various catalyst particle loadings; (b) to show the anticipated favourable flow dynamics in downer units; (c) to evaluate the mixing of partially reduced and oxidized particles streams at the top cyclone-feeding section.

References:

1. de Lasa, H. Riser Simulator. 1992.
2. Rostom, S.; de Lasa, H. I. Propane Oxidative Dehydrogenation Using Consecutive Feed Injections and Fluidizable VO_x/γAl₂O₃ and VO_x/ZrO₂–γAl₂O₃ Catalysts. Ind. Eng. Chem. Res. 2017.
3. Al-Ghamdi, S.; Moreira, J.; de Lasa, H. Kinetic Modeling of Propane Oxidative Dehydrogenation over VO_x/γ-Al₂O₃ Catalysts in the Chemical Reactor Engineering Center Riser Reactor Simulator. Ind. Eng. Chem. Res. 2014, 53, 15317–15332.
4. Hossain, M. M. Kinetics of Oxidative Dehydrogenation of Propane to Propylene Using Lattice Oxygen of VO_x/CaO/γAl₂O₃Catalysts. Ind. Eng. Chem. Res. 2017, 56, 4309–4318.