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 Fluid Dynamics in Down Flow Reactors: Experimental and Modeling Study

Gas–solid concurrent downers possess unique features when compared to other gas–solid systems. Establishing their fluid dynamic properties requires both experimental measurements of gas-solid flow properties and computational modeling. Measuring gas-solid flow properties such as cluster solid concentrations, individual cluster slip velocities, and cluster sizes, involves the use of specialized optical equipment, as well as a rigorous data analysis methodology. In addition, the modeling of the fluid dynamics of gas-solid flows in downer units offers special challenges such as establishing a proper drag model, cluster configuration and sizes, sphericity, boundary conditions, among other issues. In this PhD dissertation, the fluid dynamics of gas-solid flows in downer reactor units are analyzed in the context of a wide range of operating conditions. To accomplish this, local cluster particle characteristics are determined for the first time, using two separate downer units and a significantly enhanced data analysis. This involves individual cluster signals recorded by the CREC-GS-Optiprobes and a method for setting the data baseline using solid mass balances. The proposed methodology allows the calculation of individual cluster slip velocities, agglomerate particle sizes, individual particle cluster size distributions, and cluster drag coefficients. Gas-solid flows in downers are simulated in the present PhD dissertation, using a Computational Particle Fluid Dynamics (CPFD) Numerical Scheme. The CPFD model includes particles represented as clusters. This model is validated with experimental data obtained from the two independent downer units which have different downer-column internal diameters (a 1 inch ID and a 2 inch ID). CPFD simulations are implemented using average particle cluster sizes as obtained experimentally. Experimentally observed time-averaged axial and radial velocities, solid concentration profiles, and cluster particle acceleration regions are successfully simulated by a CFPD model. These findings support: a) a narrow distribution of particle cluster catalyst residences, b) the characteristic particle “forward” mixing, and c) the relatively flat radial solid concentrations and solid cluster velocities. It is found that CPFD simulations agree well with experimentally determined particle cluster velocity and the solid void fraction in the downer core region, with this being the case for all the operating conditions studied

Catalytic Conversion of 1,3,5 TIPB Over Y-Zeolite based Catalysts Catalyst/oil ratio(C/O) Effect and a Kinetic Model

A typical FCC unit involves the transport and rapid catalytic reaction of chemical species using 60-70 micron fluidizable catalyst particles. In FCC, hydrocarbon species evolve in the gas-phase are adsorbed on, and then react with the catalyst particles. In this case, large molecular weight hydrocarbons (vacuum gas oil) are converted into lighter products (gasoline). FCC also yields undesirable products such as light gases and coke. Coke promotes catalyst activity decay and as result, is detrimental to catalyst performance. Given the significance of coke as a catalyst decay agent in FCC, it is the objective of this PhD research to study catalyst deactivation by coke. To accomplish this, three different Y-zeolite FCC catalysts, designated as CAT-A, CAT-B and CAT-C were employed in the present PhD study. Catalyst samples studied were characterized in terms of Crystallinity, Total Acidity, Specific Surface Area (SSA), Temperature Programmed Ammonia Desorption (NH3-TPD) and Pyridine Chemisorption. Catalytic cracking runs were carried out in a CREC Riser Simulator using a model hydrocarbon species (1,3,5-TIPB) as a hydrocarbon feedstock. This bench-scale mini-fluidized batch unit mimics the operating conditions of large-scale FCC units. Temperatures within the 510°C-550°C range and times ranging from 3s-7s were selected for catalyst evaluation. For every experiment, 0.2g of 1,3,5-TIPB was contacted with a catalyst amount ranging from 0.12g to 1g. This was done to achieve a C/O ratio in the range of 0.6 to 5. Results obtained showed a consistent 1,3,5-TIPB conversion pattern for the three catalysts studied: increasing first, stabilizing later, and finally decreasing modestly. In spite of this, coke formation and undesirable benzene selectivity always rose. On this basis, a mechanism involving both single catalyst sites for cracking and two sites for coke formation was considered. In this respect, coke formation was postulated as an additive process involving coke precursor species, which are either adsorbed on two sites located in the same catalyst particle or adsorbed in two close sites in different catalyst particles

Optimal design and operation of an industrial fluidized catalytic cracking reactor

YesFluidized catalytic cracking (FCC) is regarded one of the most significant operations in the oil refining industries to convert feedstock (mainly vacuum gasoil) to valuable products (namely gasoline and diesel). The behavior of the fluidized catalytic cracking process is playing a main part on the overall benefits of refinery units and improving in process or control of fluidized catalytic cracking plants will result in exciting benefits economically. According to these highlights, this study is aimed to develop a new mathematical model for the FCC process taking into account the complex hydrodynamics of the reactor regenerator system with a new six lumps kinetic model for the riser. The mathematical model, simulation and optimization have done utilizing vacuum gas oil (VGO) as a feedstock and zeolite as a catalyst under the following operating conditions: temperature (733K, 783K, and 813K), weight hourly space velocity (5, 20 and 30hr−1) and catalyst to oil ratio (4, 7 and 10). The best kinetic parameters of the relevant reactions are estimated using the optimization technique based on the experimental results taken from literature. The effect of operating condition (mainly, reaction temp (T), catalyst to oil ratio (CTO) and weight hourly space velocity (WHSV) on the product composition has also been discussed. The optimal kinetic parameters obtained from the pilot plant scale have been employed to develop an industrial FCC process, where optimal operating condition based on maximum conversion of VGO with minimum cost in addition to maximizing the octane number of gasoline (GLN), have been studied. Minimum coke content deposition the catalyst within the regenerator is also investigated here. New results (the highest conversion and octane number, and the lowest coke content) have obtained in comparison with those reported in the literature

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

The Calcium-Looping (CaCO3/CaO) Process for Thermochemical Energy Storage in Concentrating Solar Power Plants

Articulo aceptado por la revista. * No publicado aún [28-06-2019]Energy storage based on thermochemical systems is gaining momentum as potential alternative to molten salts in Concentrating Solar Power (CSP) plants. This work is a detailed review about the promising integration of a CaCO3/CaO based system, the so-called Calcium-Looping (CaL) process, in CSP plants with tower technology. The CaL process relies on low cost, widely available and non-toxic natural materials (such as limestone or dolomite), which are necessary conditions for the commercial expansion of any energy storage technology at large scale. A comprehensive analysis of the advantages and challenges to be faced for the process to reach a commercial scale is carried out. The review includes a deep overview of reaction mechanisms and process integration schemes proposed in the recent literature. Enhancing the multicycle CaO conversion is a major challenge of the CaL process. Many lab-scale analyses carried out show that residual effective CaO conversion is highly dependent on the process conditions and CaO precursors used, reaching values as different as 0.07-0.82. The selection of the optimal operating conditions must be based on materials, process integration, technology and economics aspects. Global plant efficiencies over 45% (without considering solar-side losses) show the interest of the technology. Furthermore, the technological maturity and potential of the process is assessed. The direction towards which future works should be headed is discussed.Ministerio de Economia y Competitividad CTQ2014-52763-C2, CTQ2017- 83602-C2 (-1-R and -2-R)Unión Europea Horizon 2020 Grant agreement No 727348, project SOCRATCES

Sulfur Species Selective Adsorption Using A New Offretite Based Additive

Fluid Catalytic Cracking (FCC) unit is one of the most important conversion processes used in petroleum refineries. Nowadays, petroleum refineries, and specifically FCC units, have to be improved. New technologies must be developed to increase the refineries revenues as well to comply with environmental regulations. One area of significant concern is the necessary in-situ FCC gasoline sulfur reduction. In this respect, this PhD dissertation proposes a new Zn-Offretite (Zn-OFF) additive for gasoline sulfur reduction via selective adsorption in FCC units. The PhD research developed covers the preparation of the Zn-OFF zeolites and their physicochemical characterization. This physicochemical characterization leads to the demonstration that zinc species are most likely included in the OFF framework. Furthermore, it is also shown that the Zn in the OFF zeolites, may considerably increase acidity as well as the abundance of Lewis acid sites. As a result, it is found that the Zn in the OFF is tailored for thiophenic species selective adsorption. Regarding the Zn-OFF performance, it is proven in this PhD dissertation, that the Zn-OFF additive displays an excellent performance for 2-methylthiophene (2MTh) selective adsorption. The best sulfur removal was found using the Zn(3.5wt%)-OFF additive and 2MTh at 530 °C and 5 s. On the basis of the results obtained, it is anticipated that the Zn(3.5wt%)-OFF additive can provide a valuable in-situ sulfur selective adsorption for the thiophenic compounds. It is also established that the used Zn(3.5wt%)-OFF additive, when blended with a FCC commercial catalyst reduces both coke production and sulfur in coke. It is thus, demonstrated that under typical FCC unit operating conditions, the Zn(3.5wt%)-OFF additive can selectively adsorb sulfur contained species. This additive can also decrease sulfur in coke with this leading to a mitigation of SOx emissions in the FCC regenerator, where coke is combusted and catalyst reactivated

Experimental Investigations on the Instantaneous Flow Structure in Circulating Fluidized Beds

Knowing the instantaneous flow structure is of great importance for the understanding of gas-particle interaction and for the prediction of reactor performances. In this thesis, a systematic and comprehensive study has been conducted on the instantaneous flow structure in a narrow rectangular riser (19 mm in thickness, 114 mm in width and 7.6 m in height), in a cylindrical riser (76 mm in diameter, 10.2 m in height) and in a cylindrical downer (50 mm in diameter, 4.9 m in height). A wider range of operating conditions has been achieved in risers and downer with superficial gas velocity from 3.0 to 9.0 m/s and solids circulation rate from 50 to 700 . With high-speed imaging and optical fiber sensing, it has been found that there are crest clusters, coalesced particles, trough clusters and dispersed particles in CFBs. Crest clusters are surrounded by a cloud of coalesced particles, while trough clusters are immersed in dispersed particles. Then, instantaneous flow dynamics are computed with the tracking of image blocks. The existence of aggregations influences both the particle velocity and the solids flux. After that, a physically meaningful threshold was proposed to characterize crest cluster and trough clusters in terms of solids holdup, size and shape. As the optical fiber probe can be used in high-density conditions, a discrimination method was also proposed for probe signals using wavelet transform. A thorough characterization of crest clusters, coalesced particles, trough clusters and dispersed particles is conducted in the rectangular CFB and their properties are consistent with those originated from the images. Moreover, how particle properties influence phase characteristics was also investigated by comparing phase information formed by FCC and glass beads. Finally, probe signals captured in the cylindrical riser and cylindrical downer are processed to investigate the phase properties in high-density conditions. Phases, including crest clusters, coalesced particles, trough clusters and dispersed particles, are characterized over the riser and downer in terms of length, frequency and time fraction. With the systematic comparison of flow information between the CFB riser and downer, it has been found that aggregation in the CFB downer is less severe than that in the CFB riser

Reactor Performances and Hydrodynamics of Various Gas-Solids Fluidized Beds

The reactor performances and hydrodynamics were systematically studied in a multifunctional fluidized-bed system which included a bubbling fluidized bed (BFB), a turbulent fluidized bed (TFB) as well as a newly identified circulating turbulent fluidized bed (CTFB) using the same batch of activated FCC particles. Catalytic ozone decomposition was employed as the model reaction to experimentally investigate the reactor performances of BFB, TFB and CTFB. The complete mappings of ozone concentration were obtained in these fluidized beds, showing close relationship with the solids holdup distributions. In the BFBs and TFBs, the study of scale-up effect revealed that the static bed height had almost no influence on the ozone concentration distributions, whereas the bed diameter affected the concentration distributions, especially in the bubbling regime. This work, for the first time, examined the reactor performance of a CTFB: the ozone concentration decreased along the axial direction with a large portion of reaction happening in the entrance section, while the “centre-high” and “wall-low” radial profile of ozone concentration was presented. Comprehensive evaluations on reactor performance were then conducted across the full spectrum of the commonly used fluidized-bed reactors, including BFB, TFB, CTFB, riser and downer, in order to illustrate the superior and inferior features for each. The clear correlations between ozone concentrations and solids holdups confirmed that the reactor performances were essentially controlled by the flow structures including gas/solids behaviour and distributions. Furthermore, the CTFB and downer showed a comparable reactor performance that was very close to that of a plug-flow reactor, resulting from the uniform flow structures with little backmixing. While the TFB demonstrated favourable reactor performance, the CTFB is still superior in reactor efficiency. The further deviation of the BFB and riser from a plug-flow reactor was due to the significant gas bypassing and backmixing. The performances of the various fluidized beds were then quantitatively characterized by gas-solids contact efficiencies. The hydrodynamics of the BFB, TFB and CTFB were also studied in order to help understand their reactor performances. An optical fibre probe was used to obtain the spatial distribution (i.e., axial and radial profiles of time-average data) and the temporal variation (i.e., time-serial data) of solid holdup. By analysing the instantaneous signals of solids holdup, the BFB was found to be dominated by a dense (solid) phase with a discrete dilute (bubble) phase, while the TFB exhibited a dynamic flow structure with the comparable dense (cluster) and dilute (void) phases. The CTFB experienced even more transient behaviour over the TFB, causing more interfacial activities. In addition, the CTFB successfully achieved a gas/solids upflow with solids circulation rates as high as 300 kg/m2s while maintaining a dense bed with solids holdup ranging from 0.25 to 0.35. The CTFB possesses many hydrodynamic advantages, such as uniform axial flow structure, homogeneously inter-diffused dilute/dense phases and no net solids downflow, leading to very favourable mass transfer and highly efficient gas-solids contact