Caracterización in-situ de la concentración de 1-hexeno con un láser Helio-Neón en la presencia de catalizador sólido

This study provides evidence that a helium-neon (He-Ne) laser operating in the Mid-infrared (MIR) at a wavelength of 3.39 μm can detect variations in 1-hexene concentration in the presence of a solid catalyst. The in-situ and online characterization of the concentration of 1-hexene, as an example of a hydrocarbon, is relevant to enhance the current understanding of the interaction between hydrodynamics and chemistry in different heterogeneous catalytic processes. We designed and built a laboratory-scale downer unit that enabled us to analyze heterogeneous catalytic reactions and provided optical access. The lab-scale reactor was 180-cm long, had an internal diameter of 1.3 cm, and was made of fused quartz to allow the passage of the laser beam. 1-hexene was carefully measured, vaporized, and fed into the reactor through two inlets located at an angle of 45 degrees from the vertical descendent flow and 70 cm below the input of a solid catalyst and a purge flow entraining N2. A system of five heaters, which can be moved in the vertical direction to allow the passage of the laser beam, guaranteed temperatures up to 823 K. Computational Fluid Dynamics (CFD) simulations of the hydrodynamics of the system indicated that a uniform temperature profile in the reaction section was reached after the catalyst and the feed mixed. The estimated catalyst to oil ratio and time on stream in the experiments were, respectively, 0.4 to 1.3 and 2 s. After a correction for laser power drift, the experimental results showed a linear response of the fractional transmission to the 1-hexene concentration that was independent of temperature in the 373 K–673 K range. Even in the presence of a catalyst, the absorption of 1-hexene at the MIR frequency of the laser was high enough to enable the detection of 1-hexene since the fractional absorption of the absorbing path length in these experiments was close to zero (0.013 m) and the 1-hexene concentrations were higher than 1.254 × 10-5 mol/cm3. This result demonstrated the ability of the laser system to measure the concentration of 1-hexene in the presence of a catalyst and indicates that it can be used to better decouple hydrodynamics from kinetics in heterogeneous catalytic processes.Se presenta evidencia de que un láser de helio-neón (He-Ne), que opera en el infrarrojo medio (MIR) a una longitud de onda de 3.39 μm, puede detectar variaciones de la concentración de 1-hexeno en presencia de catalizador sólido. La caracterización in situ y en línea de la concentración de 1-hexeno, un ejemplo de hidrocarburo, es importante para mejorar el entendimiento de la interacción entre la química y la hidrodinámica en procesos de reacción heterogénea. En esta investigación, se diseñó y construyó una unidad downer a escala de laboratorio. El reactor tiene una longitud de 180 cm, un diámetro interno de 1.3 cm y fue fabricado en cuarzo fundido para permitir el paso del rayo láser. El 1-hexeno se dosificó, se vaporizó y se introdujo en el reactor a través de dos entradas ubicadas en un ángulo de 45 grados desde el flujo descendente vertical y 70 cm por debajo de la entrada de un catalizador (0.5 g / s) y un flujo de 0.55 lpm de purga de N2 de arrastre. Un sistema de cinco calentadores, que se puede desplazar en dirección vertical para permitir el paso del rayo láser, garantiza temperaturas de hasta 823 K. Simulaciones de dinámica de fluidos computacional (CFD) de la hidrodinámica del sistema muestra que se alcanza un perfil de temperatura uniforme en la sección de reacción luego de la mezcla del catalizador con la alimentación. La relación estimada de catalizador a aceite y el tiempo en la corriente en los experimentos fueron de 0.4 a 1.3 y 2 s, respectivamente. Después de la corrección de la variación de potencia del láser, los resultados experimentales mostraron una respuesta lineal de la transmisión fraccional con la concentración de 1-hexeno que era independiente de la temperatura en el rango de 373 K a 673 K. Incluso en presencia de catalizador, la absorción de 1-hexeno en la frecuencia del MIR del láser utilizado en los experimentos es lo suficientemente alta como para permitir la detección de 1-hexeno ya que la absorción fraccional es cercana a cero para la longitud del camino de absorción ( 0.013 m) de estos experimentos y concentraciones de 1-hexeno superiores a 1.254 × 10-5 mol/cm3. La configuración experimental permitió demostrar la capacidad del sistema láser para medir la concentración de 1-hexeno incluso en presencia de un catalizador. Esto indica que es posible su uso para distinguir mejor el efecto de la hidrodinámica de la cinética en procesos de catálisis heterogénea

Propane Oxidative Dehydrogenation Under Oxygen-free Conditions Using Novel Fluidizable Catalysts: Reactivity, Kinetic Modeling and Simulation Study

Propane oxidative dehydrogenation (PODH) was studied using VOx/γAl2O3 and VOx/ZrO2-γAl2O3 (1:1 wt.%) catalysts, as well as consecutive propane injections under oxygen-free conditions. These catalysts were synthesized with 2.5, 5 and 7.5 wt.% vanadium loadings, and prepared using a wet saturation impregnation technique. Different characterization techniques were used to establish catalyst properties including NH3-TPD, pyridine FTIR and NH3-TPD kinetics. As well, PODH runs in the CREC Riser Simulator were developed under oxygen free atmospheres at 500-550°C, close to 1 atm., 10-20 s and 44.0 catalyst/propane weight ratio (g/g). Propylene selectivity obtained were up to 94%, at 25% propane conversion. Using this data, a “parallel-series” model was established based on a Langmuir-Hinshelwood rate equation. Adsorption constants were defined independently, with this leading to a 6-independent intrinsic kinetic parameter model. These parameters were calculated via numerical regression with reduced spans, for the 95% confidence interval and low cross-correlation coefficients. A 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 factor for propane combustion was obtained. The calculated energies of activation (55.7 kJ/mole for propylene formation and 33.3 kJ/mole for propane combustion) appeared to moderate this effect, with the influence of frequency factors prevailing. Furthermore, propylene conversion in COx oxidation appeared as a non-favored reaction step, given the 98.5 kJ/mole activation energy and 4.80×10-6 mol.gcat-1s-1 frequency factor. This kinetic model was considered for the development of a scaled-up twin fluidized bed reactor configuration. For this, a hybrid computational particle-fluid dynamic (CPFD) model featuring either “Particle Clusters” or “Single Particles” was employed. Results obtained in a 20-m length downer unit showing a 28% propane total conversion and a 93% propylene selectivity using the “Single Particle” model. However, and once the more rigorous particle cluster flow was accounted for, propane conversion was limited to 20%, with propylene selectivity staying at 94% level. Thus, the obtained results show that a PODH simulation using CPFD requires one to account for “Particle Clusters”. This type of comprehensive model is needed to establish unambiguously the PODH downer reactor performance, being of critical value for the development of down-flow reactors for other catalytic processes

Cluster Acceleration and Stabilization in Downflow Catalytic Reactors: Experimental and CPFD Simulation Studies

Particle cluster dynamics in downflow reactors are of great importance for the implementation of large scale, environmentally friendly catalytic processes. Studies should address particle cluster velocities, solids holdups, and individual cluster sizes to establish reliable models for the unit scale up. In this PhD dissertation, the individual characteristics of particle clusters, such as cluster size, velocity, and particle volume fraction, were measured in the feeding, intermediate, and fully developed flow sections of a cold-flow model unit using CREC-GS-Optiprobes. The downer unit employed in this research had a 0.051 m ID and a 2 m high acrylic column. The feeding section included a cyclone and a ring gas injector with eight nozzles angled at 45º. A fluid catalytic cracking (FCC) catalyst with a mean diameter of 84.4 μm and a density of 1722 kg/m3 was used. The operating conditions for the experiments were superficial gas velocities of 1.0-1.6 m/s and solids mass fluxes of 30-50 kg/m2s. The results obtained showed close to normal particle cluster size distributions near the feeding region, and skewed distributions with a higher frequency of short clusters in the fully developed flow section. Additionally, significant changes were noticed when clusters evolved from the feeding section to the fully developed flow section: the average cluster size changed from 7-9 particles to 3-4 particles, and 0.5-0.9 m/s cluster slip velocities in the downer entrance increased to 1.1-1.4 m/s in the stabilized region. Regarding the obtained findings, it was observed that the cluster slip velocity is a function of the measured axial cluster length. On the basis of the data obtained, it was also established a quasi-spherical shape for the clusters in the entry downer section and a strand shaped cluster for clusters in the stabilized downer region. Furthermore, by using computational fluid dynamics simulations (Multiphase Particle-in-Cell (MP-PIC) Method) and accounting for the experimentally determined cluster size distribution, a Hybrid Experimental-Numerical Cluster Model was postulated and successfully validated. Finally, and to establish the relevance of the fluid dynamic model, a fluidized catalytic cracking (FCC) pilot-scale downer unit, was simulated using the developed Hybrid MP-PIC Model and kinetics obtained in a CREC Riser Simulator. Radial and axial temperature distributions show the adequacy of the gas-solid feeder employed. This was the case given the very effective gas-solid mixing leading to quick gas-solid radial thermal stabilization. On this basis, it was proven that flow stabilization can be achieved in a 1-2 m downer unit length, and this for typical FCC operated with 5-7 C/O (catalyst/oil) ratios