Riser Simulator Research Articles

Fluidized bed oxidative dehydrogenation of ethane to ethylene over VOx/Ce-γAl2O3 catalysts: Reduction kinetics and catalyst activity

Cerium-doped γ-Al2O3 (Ce-γAl2O3) catalyst supports are prepared with various amounts of cerium (0.2–5wt.%). An excess solvent impregnation method is used to saturate the vanadium (5wt%) on the cerium-modified supports. The XRD and FTIR characterization techniques indicate the presence of crystalline CeO2 phase, and amorphous VOx phases on the γAl2O3. Raman analysis shows the coexistence of poly-vanadate and mono-vanadate species. BET analysis reveals a high surface area for the synthesized catalysts with a type-IV adsorption isotherm. TPR/TPO analyses show that VOx/Ce-γAl2O3 catalysts are very active and stable over repeated reduction/oxidation cycles. The stability/purity of the synthesized catalyst is also confirmed by TGA analysis. NH3-TPD characterization shows mild acidity while the TPR kinetics predicts lower activation energy for reduction. Oxidative dehydrogenation (ODH) of ethane experiments are developed over VOx/Ce-γAl2O3 catalysts under gas-phase oxygen-free conditions in a fluidized CREC Riser Simulator. Temperatures and residence times are varied between 450 and 600°C and 10–50s, respectively. A maximum ethylene selectivity of 87.9% with 10.0% ethane conversion is achieved at 550°C using the 1.0wt.% cerium-doped VOx/γ-Al2O3 catalyst. The successive ethane injections and subsequent catalyst regeneration show the stable redox characteristics of the prepared ODH catalysts.

Iron Oxide over Silica-Doped Alumina Catalyst for Catalytic Steam Reforming of Toluene as a Surrogate Tar Biomass Species

An iron oxide over silica-doped alumina catalyst was successfully synthesized using one-pot, solvent-deficient method. The prepared Fe2O3/SiO2–Al2O3 catalyst was characterized using TGA/DTG, XRD, N2 adsorption isotherm, NH3-TPD, and SEM. The TGA/DTG and XRD results show that the presence of Si enhances the stability of γ-Al2O3 support at high temperatures. The prepared Fe2O3/SiO2–Al2O3 catalyst was obtained by calcination at 950 °C, with a high BET specific surface area (49 m2/g). The NH3-TPD showed that Fe addition can significantly increase catalyst acidity. SEM images confirmed the textural properties of the catalysts in term of surface morphology. The catalytic activity of Fe2O3/SiO2−Al2O3 catalysts was examined in a CREC fluidized riser simulator using toluene as model compound for biomass tar. Experiments with this catalyst yielded high toluene conversions. Composition of gases produced (H2, CO, CO2, and CH4) were close to chemical equilibrium at 25 s and 600 °C. These results indicate that the Fe2O...

Propane Oxidative Dehydrogenation Using Consecutive Feed Injections and Fluidizable VOx/γAl2O3 and VOx/ZrO2–γAl2O3 Catalysts

Propane oxidative dehydrogenation (PODH) was studied using VOx/γAl2O3 and VOx/ZrO2–γAl2O3 catalysts and consecutive propane injections. These catalysts were synthesized with 2.5, 5, and 7.5 wt % vanadium (V) loadings. Temperature-programmed reduction by hydrogen displayed one reduction peak for VOx/γAl2O3 and two for VOx/ZrO2–γAl2O3 (5 and 7.5 wt % vanadium). Temperature-programmed desorption of ammonia (NH3-TPD) and pyridine Fourier transform infrared spectroscopy showed that zirconia on γAl2O3 reduces the catalyst acidity. NH3-TPD kinetics gave for VOx/ZrO2–γAl2O3 higher desorption activation energies than those for VOx/γAl2O3. PODH runs in the Chemical Reactor Engineering Center Riser Simulator were developed under an oxygen-free atmosphere at 550 °C, close to 1 atm, 20 s, and a 42.0 catalyst/propane weight ratio (g/g). PODH runs for the 7.5% V/ZrO2–γAl2O3 showed (a) 93% propylene selectivity and 25% propane conversion (based on propane converted into gaseous carbon-containing products), (b) 85% propyl...

Cross-flow vortex-induced vibration of a flexible riser transporting an internal flow from subcritical to supercritical

This study aims to model and explore the IFE (Internal Flow Effect) on the CF (Cross-Flow) VIV (Vortex-Induced Vibration) of a flexible riser transporting an axial single-phase internal flow. The simulation model employs the nonlinear equations describing the coupling of axial and transverse vibrations of a fluid-conveying pipe as the structural model, and adopts a distribution of van der Pol oscillators to create the VIV effect. The governing equations can be solved via a Galerkin-based multi-mode approach combined with the Houbolt's finite difference scheme. The developed code has been validated for VIV effect and IFE from subcritical to supercritical region. The Argand diagram of a flexible riser is plotted at first, and the varying natural frequencies with the increase of internal flow velocity and the critical internal flow velocity can be obtained. Then simulations of the flexible riser at two uniform currents with the increase of internal flow velocity in a transition range from being subcritical to supercritical are conducted. The IFE on CF VIVs are examined by the space-time modifications of riser responses and dominant vibration frequency for which the mode switching and sharing can be identified. It has demonstrated that internal flow influences the vibration amplitude and the dominant vibration frequency. Internal flow can trigger new natural modes and switch the role of the most predominant one. Moreover, a buckling-flutter coupled instability is captured where the riser is experiencing a static divergence and a Hopf bifurcation via the first natural mode.

Intracrystalline mesoporosity over Y zeolites. Processing of VGO and resid-VGO mixtures in FCC

Prototypes of FCC catalysts containing 30%wt. Y zeolite with different intracrystalline mesoporosity generated by alkaline desilication, on an inert silica matrix, were tested in cracking a conventional vacuum gasoil (VGO) feedstock, alone and mixed with an atmospheric tower residue (ATR). The experiments were performed in a fluidized bed CREC Riser Simulator reactor (reaction time: 5–30s, temperature: 550°C, catalyst/feedstock: 4.5). Independently of the catalyst, the conversions of the ATR-VGO mixture were higher than those of the VGO at the same conditions. With both feedstocks, the catalyst including the zeolite with increased intracrystalline mesoporosity and higher acidity showed a higher observed activity than the catalyst with the parent zeolite. In addition, the catalyst with the modified zeolite showed higher selectivity to LCO, which is an intermediate product in FCC and a contributor to the diesel pool, especially with the ATR-VGO mixture. This effect was the consequence of the enhanced diffusion transport of primary product molecules in the zeolite pore system which, in diffusing faster, have fewer chances to be further cracked. The catalyst with higher mesoporosity showed higher coke selectivity, coke being less condensed, due to the higher free volume in the porous system allowing accommodate precursors of coke.

Kinetics of Oxidative Dehydrogenation of Propane to Propylene Using Lattice Oxygen of VOx/CaO/γAl2O3 Catalysts

This article presents the kinetics of oxidative dehydrogenation (ODH) of propane to propylene over VOx/CaO and VOx/CaO-γAl2O3 catalysts in the absence of gas phase oxygen. In addition to their catalytic role, the catalysts also serve as the source of lattice oxygen. XRD and temperature-programmed reduction analysis indicate the availability of different types of VOx species on the prepared catalyst surfaces. The presence of CaO influences the formation of VOx species, the reducibility, and the oxygen carrying capacity of the catalysts. All these have a significant influence on propylene selectivity. The ODH of propane experiments are conducted in a CREC Riser Simulator under circulating fluidized bed conditions. It is observed that VOx/CaO-γAl2O3 displays higher propylene selectivity (94.1%) and lower selectivity of CO2 due to it is moderate active site–support interactions. The reaction network and Langmuir–Hinshelwood type kinetics model is developed using a wide-ranging set of experiments. The availabi...

Internal flow effect on the cross-flow vortex-induced vibration of a cantilevered pipe discharging fluid

Motivated by the application of ocean CCS (Carbon dioxide Capture and Storage) proposal and the fact that a cantilevered pipe can gain or lose energy due to the discharging flow, this study devotes efforts to investigate the IFE (Internal Flow Effect) on the CF (Cross-Flow) VIV (Vortex-Induced Vibration) of a cantilevered pipe discharging fluid. In this study, a simple fluid-structure interaction model is proposed which employs a linear structural equation to describe the transverse vibrations and utilizes a distributed wake oscillator to create the VIV effect. The simulations are justified by comparing with experiments for VIV effect and the non-conservative IFE of flexible risers. Then simulations of a flexible riser are performed by varying incoming currents and internal flow rates. The IFE on VIVs are examined by the space-time modifications of riser responses and dominant vibration frequency for which the mode switching and sharing can be identified. When the internal flow velocity is small, the pipe is losing energy to the inner flow and the VIVs can be depressed significantly. However, the pipe would lose the stability when the internal flow exceeds a critical value. The critical internal flow velocity depends on the current velocity and VIV dominant mode.

Cross-Flow Vortex-Induced Vibration Simulation of Flexible Risers Employing Structural Systems of Different Nonlinearities With a Wake Oscillator

To achieve a reliable structural model for vortex-induced vibration (VIV) the prediction of flexible risers, this paper employs structural systems with different geometrical nonlinearities (including a linear structure, a nonlinear one, a coupled cross-flow, and axial nonlinear one) and a classical oscillator to simulate cross-flow VIV. By comparing the experimental and simulation results, it is found that when the drag coefficient is assumed to be a fixed constant along the cylinder (i.e., the damping model is linear function of current velocity), it can affect the vibration amplitude considerably and may alter the dominant modes. When the excited mode of VIV is bending-stiffness dominant, the cross-flow structural nonlinearities can have a profound stiffening effect on vibration response. Although the introduction of axial deformation can reduce this function, the coupled cross-flow and axial nonlinearities still have the effect of decreasing the VIV amplitude.

Modelling and simulation of an industrial riser in fluid catalytic cracking process

Fluid Catalytic Cracking (FCC) unit is an important unit of modern refineries and any improvement in the unit’s operations and design to increase yield and meet the ever increasing demand for fuel brings about the overall profitability of the FCC. In this work, simulation of an FCC riser of varied diameter was carried out to improve the unit’s operations and design, and the results are compared with risers of different diameters. The riser with varied diameter produces 53.4wt%, a 3.18% increased yield of gasoline at low catalyst to oil (C/O) ratio of 1.27 compared to 51.7wt% from a 1m diameter riser. At increased C/O ratio, more gases and coke are produced in the varied diameter riser. Larger diameter demands more catalyst but yields more gases. Process variables can be directly correlated with yield of gasoline, which can aid process design.

Vortex-induced vibrations of a vertical riser with time-varying tension

Numerical simulations of a vertical tensioned riser in a sheared flow are performed, where the riser top end oscillates sinusoidally in the vertical direction. The oscillating top-end motion causes tension variations and changes in the natural frequencies of the riser. The flow around the structure causes vortex shedding, oscillating lift forces and vortex-induced vibrations (VIV). It is well-known that the vortex shedding is affected by structure motion, and may lock on to the riser’s natural frequencies. However, the vortex shedding frequency must remain close to the Strouhal frequency, and may therefore excite different modes of vibration as the riser tension changes. With this in mind, the overall aim of this paper is to investigate how tension variations affect the VIV response. The riser dynamics are simulated in time domain using a non-linear finite element structural model combined with an empirical hydrodynamic load model. The latter includes a synchronization model which simulates how the vortex shedding reacts to the structure motion to obtain lock-in. Simulations are run using different amplitudes and frequencies for the top-end motion, and the resulting cross-flow displacements and bending strains are studied. The results show that, when the riser top-end oscillates, the VIV response contains several modes, and the dominating mode may vary with time. The number of active modes are found to be strongly dependent on the period of the riser tension.

Computational Fluid Dynamics study of the CREC Riser Simulator: Mixing patterns

The CREC Riser Simulator Reactor is a novel mini-Riser unit. This study considers a laminar-turbulent CFD (Computational Fluid Dynamics) simulation using the COMSOL Multiphysics® module to establish gas-solid mixing patterns. The proposed CFD calculations are verified using both experimental data (<10–15% difference) and mass balance errors (<0.1%). Results show the significant detrimental effect of basket vertical baffles implemented in earlier designs of the CREC Riser Simulator Reactor. On the basis of this, it is demonstrated that a double vortex flow leading to high gas mixing is favored in a new reactor configuration “without” basket vertical baffles. For instance, at 4200rpm, in a basket unit loaded with 0.8g of catalyst, close to ~69cm/s gas axial velocities were observed. This shows turbulent or fast fluidization conditions with smaller than 0.51s mixing times. In addition, this new suggested design with enhanced mixing places the CREC Riser Simulator in a new class of laboratory scale fluidized catalytic units.

Phenomenologically based kinetics of ODH of ethane to ethylene using lattice oxygen of VOx/Al2O3–ZrO2 catalyst

This study addresses the kinetic modeling of ethane oxidative dehydrogenation (ODH) over a 10%wt VOx catalyst supported on Al2O3–ZrO2 (1:1) in the CREC Riser Simulator Reactor. The catalyst was synthesized using impregnation by a soaking method. Following this, the catalyst was characterized using BET, XRF, TPR, NH3-TPD and Raman Spectroscopy. The ODH catalyst was studied in a fluidized CREC Riser Simulator at a temperature range of 525–600°C and with 20–50s contact time. Good ethylene selectivity of up to 82% at 8.5% ethane conversion was attained. A kinetics model was established using a Langmuir–Hinshelwood rate equation. Catalyst activity (availability of lattice oxygen) was first modeled using TPR results. This model was then incorporated with the developed L–H model for the series-parallel reaction network. Nonlinear regression was employed to estimate kinetic parameters and activation energies with their respective confidence intervals. The proposed kinetics for ODH, satisfactorily predicted ethane conversion and selectivity at the selected operating conditions.

Structural changes in FeOx/γ-Al2O3 catalysts during ethylbenzene dehydrogenation

The structural changes that occur in a FeOx/γ-Al2O3 catalyst during the dehydrogenation of ethylbenzene in a fluidized CREC Riser Simulator have been investigated. Chemical and morphological change...

Kinetics of oxidative cracking of n‐hexane to olefins over VOx/Ce‐Al2O3 under gas phase oxygen‐free environment

The kinetics of oxidative cracking of n‐hexane to olefins using lattice oxygen of VOx/Ce‐Al2O3 is investigated. The TPR/TPO analysis shows a consistent reducibility (79%) of VOx/Ce‐Al2O3 in repeated redox cycles. The total acidity of the sample is found to be 0.54 mmol/g with 22% are strong acid sites that favors olefin selectivity. The oxidative cracking of n‐hexane in a fluidized CREC Riser simulator gives approximately 60% olefin selectivity at 30% n‐hexane conversion. A kinetic model is developed considering (1) cracking, (2) oxidative dehydrogenation (ODH), and (3) catalyst deactivations. The proposed cracking mechanism considers adsorption, C–H and C–C bond fission and desorption as elementary steps and implemented by pseudo steady state hypothesis. A Langmuir‐Hinshelwood mechanism is found to represent the ODH reactions. The developed model fits the experimental data with favorable statistical indicators. The estimated specific reaction rate constants are also found to be consistent with the product selectivity data. © 2016 American Institute of Chemical Engineers AIChE J, 63: 130–138, 2017

VOx-Nb/La-γAl2O3 catalysts for oxidative dehydrogenation of ethane to ethylene

The present study deals with oxidative dehydrogenation (ODH) of ethane to ethylene in absence of gas phase oxygen using VOx-Nb/La-γAl2O3 catalysts. In catalyst formulation, La is used to modify γ-Al2O3, stabilizing its bulk phase transformation. Nb is introduced as a promoter of VOx species. The prepared catalysts are characterized using various physicochemical techniques in order to demonstrate reducibility, oxygen carrying capacity, stability, metal–support interaction and acidity of the catalysts. TPR shows that VOx-Nb/La-γAl2O3 is highly active and stable over repeated reduction and oxidation cycles. XRD analysis indicates that VOx appears both as amorphous and crystalline phases. The presence of Nb minimizes the formation of crystalline V2O5 phases, which is undesirable for ODH reaction. The NH3-TPD analysis reveals an intermediate acidity of the VOx-Nb/La-γAl2O3 catalysts. NH3-TPD kinetics analysis shows that the addition of Nb increases the activation energy of ammonia desorption, suggesting increased V–support interaction and minimized V–V interaction. The ODH of ethane activity and stability is evaluated in a CREC fluidized riser simulator. The addition of Nb increased ethylene selectivity (85.7%) at 20.1% conversion comparatively at low reaction temperature (550°C) and short contact time. Thus, Nb enhances VOx isolation and forms a secondary oxide, resulting a superior catalyst activity.

A Zn-Offretite for the adsorption of thiophenic species under fluidized catalytic cracking conditions. Synthesis, characterization and reactivity

A zinc-containing offretite (Zn-OFF) was synthesized using a hydrothermal method. The goal was to prepare a Zn-OFF zeolite with adsorption properties for selective removal of thiophenic chemical species in gasoline. It was expected that this new sorbent would perform favorably under the conditions close to the ones of FCC units. Following the Zn-OFF synthesis, several characterization techniques were used such as: a) temperature programmed desorption (TPD), b) temperature programmed reduction (TPR), c) temperature programmed oxidization (TPO), d) X-ray diffraction, e) N2 isotherm, f) UV–vis spectra, g) Scanning Electron Microscopy (SEM) and h) Fourier Transform Infrared Spectroscopy (FTIR). In addition, chemical characterization was developed using Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES) and X-Ray Fluorescence (XRF). It was proven on this basis, that zinc species are most likely included in the zeolite framework. Regarding the influence of Zn on the OFF zeolites, it was shown that Zn considerably increases (doubles) the weak acidity measured by TPD as well as shifts the TPD peak towards a higher temperature range. Additionally, FTIR pyridine adsorption also showed in the case of the Zn-OFF in comparison with the OFF, a 2.5 times higher abundance of Lewis acid sites versus Brönsted sites. It is speculated on this basis, that the Zn in the OFF structure increases species adsorption, changing the adsorption bonding strengths. Regarding the Zn-OFF performance, it was proven in a CREC Riser Simulator unit that one can achieve 37wt% of 2-methylthiophene removal with less than 1.3wt% being converted into coke.

Catalytic steam gasification of biomass surrogates: Thermodynamics and effect of operating conditions

Thermodynamic chemical equilibrium of biomass steam gasification is considered using both stoichiometric and non-stoichiometric analyses. These thermodynamic analyses include gaseous products, tars and coke, as well as consider a wide range of operating conditions. It is shown that both stoichiometric and non-stoichiometric approaches provide close results. Catalytic steam gasification of biomass surrogate species (glucose and 2-methoxy-4-methylphenol) is developed in a CREC Riser Simulator under the expected conditions of a circulating fluidized bed gasifier. A highly active and stable fluidizable Ni/La2O3-γ-Al2O3 catalyst is employed in this study, to investigate the effects of gasifier operating conditions. This catalyst yields 98% carbon conversion of glucose to permanent gases with no tar formation and negligible coke deposition at 700°C. Catalytic gasification results with the variation of temperature and steam/biomass ratio show limited deviation from equilibrium predictions. The deviation between experimental results and equilibrium predictions can be attributed to the relatively shorter reaction time (20s). As the reaction time is increased, experimental results approach chemical equilibrium. For 2-methoxy-4-methylphenol gasification, 90% carbon conversion is obtained with only 5.7wt% tar formation and 3.3wt% coke deposition. Again chemical equilibrium predictions are close. The only exceptions are: (a) coke and methane being overestimated and (b) tars being underestimated. Regarding synthesis gas from glucose and 2-methoxy-4-methylphenol gasification, the Ni/La2O3-γ-Al2O3 catalyst yields a H2/CO ratio greater than 2.0.

Synergy in the Cracking of a Blend of Bio-oil and Vacuum Gasoil under Fluid Catalytic Cracking Conditions

The catalytic cracking of a blend of raw bio-oil (20 wt %) from black poplar sawdust fast pyrolysis and vacuum gasoil (VGO, 80 wt %) has been studied. The runs have been performed in a riser simulator reactor (RSR) under fluid catalytic cracking (FCC) conditions; that is, 500–560 °C, catalyst to feed mass ratio of 6 gcat (gfeed)–1 on a dry basis, reaction times of 3–10 s and a equilibrated commercial FCC catalyst. The cofeeding effect has been quantified by comparing the yields of product lumps and individual components in the gaseous fraction and gasoline lump in the cracking of VGO, raw bio-oil, and their blend (20% bio-oil/80% VGO), at the same conversion values (60–70 wt %). The cofeeding has a favorable synergy because it promotes the formation of C3–C4 and gasoline (C5–C12) hydrocarbon lumps, attenuating the formation of CO2, CO, and also coke formation. In the cracking of the blend (80% VGO/20% bio-oil) at 70 wt % conversion, a deoxygenation degree of 95 wt % has been obtained, with a total liquid ...

A fluidizable VOx/γ–Al2O3–ZrO2 catalyst for the ODH of ethane to ethylene operating in a gas phase oxygen free environment

A new fluidizable VOx/γ-Al2O3–ZrO2 catalyst for the oxidative dehydrogenation (ODH) of ethane to ethylene is investigated. This catalyst operates in an oxygen free environment. The prepared catalyst is characterized using various physicochemical techniques. Nitrogen adsorption/desorption isotherm analysis shows a significant extent of monolayer coverage suggesting high active sites dispersion. TPR/TPO results demonstrate that the VOx/γ-Al2O3–ZrO2 catalyst is very reactive and displays consistent hydrogen consumption during repeated reduction and oxidation cycles. NH3-TPD analysis indicates that the support acidity decreases after catalyst modification with ZrO2. The estimated energy of ammonia desorption is found to be 75.4, 77.8 and 80.5kJ/g for the Al2O3/ZrO2 ratio of 2:1, 1:1 and 1:2, respectively. This higher energy of desorption with ZrO2 content, it is assigned to the increased ZrO2-support interaction. XRD patterns display the presence of a VOx crystal phase on the support surface. When evaluated in a fluidized CREC (Chemical Reactor Engineering Center) Riser Simulator, the ZrO2 modified catalyst shows at 600°C close to 90% ethylene selectivity while the unmodified catalyst gave 42% ethylene only. Furthermore, ethylene selectivity remained stable with catalyst time-on-stream. It is concluded that ZrO2 enhances metal-support interaction, influencing favorably the VOx reducibility and ethylene selectivity.

Experimental and numerical study of pseudo-2D circulating fluidized beds

We present experimental investigations and numerical simulations of a pseudo-2D riser. Experiments were performed for various airflow rates, particle types/diameters, and particle size distributions. Pressure distributions along the wall of the riser were measured. Additional measurements from a smaller pseudo-2D riser (Kallio et al., 2009; Shah et al., 2012) were used to analyze horizontal solids volume fraction profiles. The experimental data were compared with simulation results carried out using an Euler–Euler approach. A mesh sensitivity study was conducted for numerical simulations and effects associated with simplifying real 3D geometry to a 2D model were examined. In addition, the effect of using an algebraic equation to represent the granular temperature versus a full partial differential equation also was examined for numerical simulations. Results showed small but significant near-wall sensitivity of the flow variables to mesh size. Substantial differences in mean pressure, solids distribution, and solid velocities were obtained, when 2D and 3D simulation results were compared. Finally, applying the simplified granular temperature equation for turbulent fluidization and for dilute-phase transport can lead to incorrect predictions in models.