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Performing an adequate fouling study for the heat exchangers in the convection section of a steam cracker requires reliable data on circumferential tube wall temperature profiles. A hybrid Computat...

Lead oxide (PbO) formation can occur in Lead-Bismuth Eutectic (LBE)-cooled nuclear systems in case of oxygen ingress or temperature decrease of the coolant beyond the normal operation ranges. In the present work the formation of lead oxide in an actively cooled LBE flow is studied. Computational fluid dynamics (CFD) is used to predict the nucleation, growth and dissolution of PbO particles. Solid oxide particles are modeled as a pseudo-continuous phase, using the Kinetic Theory of Granular Flow (KTGF) to account for particle-flow interaction. The particle size distribution (PSD) is accounted for using Population Balance Equations/Models (PBE/PBM). The results obtained from the model are qualitatively in good agreement with experimental results obtained in the MEXICO loop at SCK·CEN. The calculated PSD reveals that the majority of the oxide particles are expected to be in the sub-micron range. Experimental results indicate that in the studied conditions PbO nucleates in the LBE bulk leading to suspended particles in the LBE flow.

Photocatalytic nanomaterials, using only light as the source of excitation, have been developed for the breakdown of volatile organic compounds (VOCs) in air for a long time. It is a tough challenge to immobilize these powder photocatalysts and prevent their entrainment with the gas stream. Conventional methods for making stable films typically require expensive deposition equipment and only allow the deposition of very thin layers with limited photocatalytic performance. The present work presents an alternative approach, using the combination of commercially available photocatalytic nanopowders and a polymer or inorganic sol–gel-based matrix. Analysis of the photocatalytic degradation of ethanol was studied for these layers on metallic substrates, proving a difference in photocatalytic activity for different types of stable layers. The sol–gel-based TiO2 layers showed an improved photocatalytic activity of the nanomaterials compared with the polymer TiO2 layers. In addition, the used preparation methods require only a limited amount of photocatalyst, little equipment, and allow easy upscaling.

The Gas-Solid Vortex Unit is an advancing fluidization technology with the potential to overcome the limitations of conventional fluidized beds. The conditions for stable fluidization are investigated, that are the upper and lower limit, i.e. minimum and maximum capacity (W s,min and W s,max ). Based on dimensional analysis three non-dimensional groups are identified, governing the fluidization phenomena: the superficial radial particle Reynolds number Re p , R , the swirl ratio S and the unit loading λ. Data from different authors is gathered for minimum (42 datasets, 3 geometries) and maximum (251 datasets, 8 geometries) capacity and used in regression analysis. Parameters are estimated for different proposed functional dependencies of the identified dimensionless groups. The model equations describing the minimum and maximum unit loading best, including their 95% confidence intervals, are: λ max = 4.0 ± 0.4 10 − 3 R e p , R 0.443 ± 0.011 S 0.454 ± 0.018 λ min = 1.15 ± 0.05 10 − 4 R e p , R The two equations describe the limits of the operational range of a GSVU for which stable fluidization is possible. The applicability of the model equations is verified against a wide range of data taken from different publications.

The hydrodynamics of secondary flow phenomena in a disc-shaped gas vortex unit (GVU) is investigated using experimentally validated numerical simulations. The simulation using ANSYS FLUENT® v.14a reveals the development of a backflow region along the core of the central gas exhaust, and of a counterflow multivortex region in the bulk of the disc part of the unit. Under the tested conditions, the GVU flow is found to be highly spiraling in nature. Secondary flow phenomena develop as swirl becomes stronger. The backflow region develops first via the swirl-decay mechanism in the exhaust line. Near-wall jet formation in the boundary layers near the GVU end-walls eventually results in flow reversal in the bulk of the unit. When the jets grow stronger the counterflow becomes multivortex. The simulation results are validated with experimental data obtained from Stereoscopic Particle Image Velocimetry and surface oil visualization measurements.

A gas–liquid vortex reactor (GLVR) is considered one of the promising Process-Intensified Equipment for applications that require high mixing and mass transfer efficiency. To gain understanding of the fundamentals of GLVR operation, the gas–liquid hydrodynamics in the GLVR has been investigated by high-speed imaging, digital images analysis and cross correlation. Gas bubbles and liquid ligaments are observed in the GLVR over a wide operating window. The dynamic liquid layer thickness in the GLVR chamber varies in the range of 26–36 mm. The instantaneous and time-average velocity fields of the liquid phase are obtained with a guaranteed high signal-to-noise ratio (cross correlation peak ratio higher than 10), allowing the analysis of liquid turbulent properties including the turbulent kinetic energy and its dissipation rate. Results show that the high turbulent kinetic energy dissipation rate ranging from 23 to 114 m2/s3 contributes to a high mixing and mass transfer efficiency in the GLVR.

This work evaluates the feasibility of biogas air-fuel combustion and natural gas oxy-fuel in steam cracking furnaces. Four cases, namely air-fuel combustion of pure natural gas, 20% CO2, 40% CO2 diluted natural gas, and oxy-fuel combustion of natural gas are investigated both experimentally and numerically. The John Zink Hamworthy Combustion test furnace, representing a section of a steam cracking furnace, is used for experimental studies. A three-dimensional steady-state CFD model is also developed to simulate the test furnace. The simulation results of the air-fuel combustion scenarios are in good agreement with the experimental data, with the maximum and average relative errors of furnace temperature of 3.86% and 1.78%, respectively. The reduction of flame length with increasing CO2 mole fraction in the fuel is observed in both experiments and simulations. It is shown that CO2 dilution has minor effect on the overall heat flux profile, which is beneficial for retrofitting existing furnaces. On the other hand, the oxy-fuel combustion simulation using default EDC model predicts a significant flame lift-off and incident radiative heat flux shift towards the higher elevations which was not observed in the experiments. This can be mainly attributed to the reduced reaction rate in a CO2 and H2O enriched combustion environment. Adjusting the EDC model parameters helps to achieve better agreement between simulation results and experimental data, while additional lab-scale experiments are essential for further validation of the numerical model. Moreover, it is of particular interest to study the optimal mole fraction of O2 in oxy-fuel combustion scenario.

Innovative gas-solid fluidized beds with process intensification capabilities are among the most promising alternatives for the current state of the art in the chemical industry. In the present work the advantages of such a reactor that sustains a rotating fluidized bed with gas-solid slip velocities much higher than those in conventional fluidized beds are illustrated computationally and experimentally. A Gas-Solid Vortex Reactor (GSVR) demonstration unit is designed to operate at typical biomass fast pyrolysis conditions targeting the production of chemicals and fuels from renewable feedstocks. For the demonstration unit preheated N 2 supplies the thermal energy required by the fast pyrolysis process but alternative sources can also be evaluated. A broad range of operation conditions in the 80 mm diameter and 15 mm height GSVR can be evaluated: N 2 mass flow rates of 5–10 g s −1 and biomass feed mass flow rates of 0.14–1.4 g s −1 . Particle-free and particulate flow experiments confirmed that the carrier gas is evenly distributed around the GSVR cylindrical chamber as anticipated by computational fluid dynamic simulations. The latter also supported the inclusion of a profiled bottom end wall and a diverging exhaust. Cold flow experiments with biomass confirmed that the GSVR sustains a rotating fluidized bed with average bed height of 10 mm and solids azimuthal velocities of 6–7 m s −1 .

To study fouling in steam cracker convection section tubes, accurate tube wall temperature profiles are needed. In this work, tube wall temperature profiles are calculated using a hybrid model, combining a one-dimensional (1D) process gas side model and a computational fluid dynamics (CFD) flue gas side model. The CFD flue gas side model assures the flue gas side accuracy, accounting for local temperatures, while the 1D process gas side model limits the computational cost. Flow separation in the flue gas side at the upper circumference of each tube suggests the need for a compartmentalized 1D approach. A considerable effect is observed. The hybrid CFD-1D model provides accurate tube wall temperature profiles in a reasonable simulation time, a first step towards simulation-based design of more efficient steam cracker convection sections.

Biomass fast pyrolysis has been considered one of the best alternatives for the thermal conversion of biomass into bio-oil. This work introduces a new reactor technology for biomass fast pyrolysis, the Gas-Solid Vortex Reactor (GSVR), to obtain high bio-oil yields. The GSVR was designed to decrease the residence time of the pyrolysis vapors; thus, the secondary cracking reactions are reduced, to enhance the segregation of the char and the unreacted biomass and to improve the heat transfer rate. Biomass fast pyrolysis experiments have been carried out for the first time in a Gas-Solid Vortex Reactor (GSVR) at 773 K, using softwood (pine) and hardwood (poplar) as feedstock. Char yields as low as 10 wt. % in the GSVR were comparable to those reported for the same feedstocks processed in conventional fluidized bed reactors. The yields of non-condensable gases in the range of 15–17 wt. % were significantly lower than those reported for other commonly used biomass fast pyrolysis reactors. Two-dimensional gas chromatography (GC × GC) revealed noticeable differences at the molecular level between the bio-oils from the GSVR and bio-oils from other reactors. The aromatics in the pine bio-oil consist almost entirely (85 wt. %) of guaiacols. For poplar bio-oils no predominant group of aromatics was found, but phenolics, syringols, and catechols were the most pronounced. The experimental results highlight the advantages of the GSVR for biomass pyrolysis, reaching stable operation in around 60 s, removing the formed char selectively during operation, and enabling fast entrainment of pyrolysis vapors. Results indicate a great potential for increasing yield and selectivity towards guaiacols in softwood (e.g., pine) bio-oil. Likewise, decreasing pyrolysis temperature could increase the yield of guaiacols and syringols in hardwood (e.g., poplar) bio-oil.

Účinek vysoceemisivních povlaků na radiační přenos tepla v parních krakovacích pecích není zdaleka znám. Chybí experimentální data popisující emisivní vlastnosti materiálů, které se používají v parních krakovacích pecích. Z tohoto důvodu se provádí měření spektrální normálové emisivity na zvýšených teplotách, přičemž se hodnotí emisivní vlastnosti žáruvzdorných šamotových cihel před a po depozici vysoceemisivního povlaku. Po nanesení povlaku s vysokou emisivitou na základní šamotový materiál se výrazně zvýší jeho emisivita. Provedené experimenty s parní krakovací jednotkou ukazují 5% snížení rychlosti spalování topného plynu po nanesení vysoceemisivního povlaku na žáruvzdorný materiál krakovací komory. Parametrické studie zabývající se účinkem emisivních vlastností reaktorové cívky a stěny pece na radiační přenos tepla ve spalovací komoře potvrzují, že pro přesné modelování chování povlaků s vysokou emisivitou je vyžadován jiný model než model šedého plynu. Přestože model šedého plynu postačuje k zachycení chování chladiče reaktorové cívky, je nezbytný jiný, nešedý model, který zobecňuje absorpci a opětovné vyzařování plynu ve spcifických spektrálních pásmech, aby mohl přesně zachytit výhody využití vysoceemisivního povlaku na stěně pece. The effect of high emissivity coatings on the radiative heat transfer in steam cracking furnaces is far from understood. To start, there is a lack of experimental data describing the emissive properties of the materials encountered in steam cracking furnaces. Therefore, spectral normal emissivity measurements are carried out, evaluating the emissive properties of refractory firebricks before and after applying a high emissivity coating at elevated temperatures. The emissive properties are enhanced significantly after applying a high emissivity coating. Pilot unit steam cracking experiments show a 5 % reduction in fuel gas firing rate after applying a high emissivity coating on the refractory of the cracking cells. A parametric study, showing the effect of reactor coil and furnace wall emissive properties on the radiative heat transfer inside a tube-in-box geometry, confirms that a non-gray gas model is required to accurately model the behavior of high emissivity coatings. Even though a gray gas model suffices to capture the heat sink behavior of a reactor coil, a non-gray gas model that is able to account for the absorption and re-emission in specific bands is necessary to accurately model the benefits of applying a high emissivity coating on the furnace wall.

A three-dimensional numerical analysis of the flow in a Gas-Solid Vortex Unit (GSVU) is carried out. The numerical model is first validated by comparing the bed pressure drop and solids velocity with experimental data. Next, the influence of gas flow rate, solids density, and particle diameter on the pressure drop, solids velocity, bed void fraction and slip velocity between the two phases is studied. A stable, solids bed is achieved for the entire range of gas flow rates tested (0.1–0.6 Nm3/s). No particle entrainment is observed when varying the solid density (1800–450 kg/m3) or the particle diameter (2–0.5 mm). A shift to bubbling fluidization regime is observed for small particle diameters (0.5 mm). The observed changes in the GSVU flow patterns are discussed by analyzing the changes in the cumulative centrifugal to drag force ratio over the bed.

To clarify the three-dimensional structure of near-wall jets observed in disc-like gas vortex units, experimental and numerical studies are performed. The experimental results are obtained using Particle Image Velocimetry (PIV), Laser Doppler Anemometry (LDA), pressure probes and surface oil flow visualization techniques. The first three techniques have been used to investigate the bulk flow hydrodynamics of the vortex unit. Surface oil flow visualization is adopted to visualize streamlines near the end-walls of the vortex unit. The surface streamlines help determine the azimuthal and radial velocity components of the radial near-wall jets. Simulations of the vortex unit using FLUENT® v.14a are simultaneously performed, computationally resolving the near-wall jet regions in the axial direction. The simulation results together with the surface oil flow visualization establish the three-dimensional structure of the near-wall jets in gas vortex units for the first time in literature. It is also conjectured that the near-wall jets develop due to combined effects of bulk flow acceleration and swirl. The centrifugal force diminishes in the vicinity of the end-walls. The radially inward pressure gradient in these regions, no longer balanced by the centrifugal force, pushes gas radially inward thus developing the near-wall jets. This article is protected by copyright. All rights reserved.

The radiation and convection section of a steam cracker are thermally coupled. Optimization and design requires a coupled simulation of both sections. In this work a 1D model for the convection section, CONVEC-1D, is developed. Several models for the different heat transfer phenomena are implemented and evaluated. For flow boiling, an empirical and a mechanistic model are developed and compared for both single- and multicomponent hydrocarbon feeds. The latter is performing best over a wide range of operating conditions, taking into account the different two-phase flow regimes. The coupled iterative procedure is demonstrated for an n-pentane steam cracker convection section.

This work discusses fouling in the vapor–steam mixture overheater in the convection section of an industrial steam cracker due to the thermal degradation of heavy hydrocarbon droplets deposited on the tube wall. A spray of heavy hydrocarbon multicomponent droplets is injected in a tube of the vapor–steam mixture overheater and the path of the droplets through the tube is followed by an Eulerian–Lagrangian computational fluid dynamics simulation. To study tube fouling, the droplet impingement behavior on the wall, the evaporation of the deposited liquid, and a coking model describing thermal coke formation due to degradation of heavy hydrocarbons are required. To describe the droplet impingement behavior, a regime map for single component millimeter-sized droplets is taken from the literature. Two simulations are performed to study fouling problems in a vapor-mixture overheater tube. Simulation results are found to be grid sensitive. By analyzing and comparing simulation results it is concluded tha...

Coincident (simultaneous) three‐component particle velocity measurements performed using two laser Doppler anemometry probes at the outlet section of a 9 m high cylindrical riser are for the first time presented for dilute flow conditions. Near the blinded extension of the T‐outlet a solids vortex is formed. Particle downflow along the riser wall opposite the outlet tube is observed, which is restricted to higher riser heights at higher gas flow rates. Increased velocity fluctuations are observed in the solids vortex and downflow region as well as at heights corresponding to the outlet tube. Contrary to the rest of the riser, in the downflow region time and ensemble velocity averages are not equal. Given the local bending of the streamlines, axial momentum transforms to radial and azimuthal momentum giving rise to the corresponding shear stresses. Turbulence intensity values indicate the edges of the downflow region. © 2016 The Authors AIChE Journal published by Wiley Periodicals, Inc. on behalf of American Institute of Chemical Engineers AIChE J, 62: 3575–3584, 2016

Lead-Bismuth Eutectic (LBE) is an interesting candidate as coolant for future Accelerator Driven Systems (ADS) and Generation IV (Gen. IV) nuclear power plants. A well-known issue of liquid metal coolants is the need for coolant chemistry control and continuous purification. This paper presents a purification strategy for control of the dissolved oxygen content in LBE by means of a LBE cold trap. Although cold traps have been explored in the past for Sodium Fast Reactors (SFR) and more recently for lead-lithium (PbLi), advances on cold trapping for LBE are lacking behind. Formation of lead oxide (PbO) is promoted in a non-isothermal purification device (cold trap) to lower the dissolved oxygen concentration present in the liquid metal. Attachment of PbO particles to the stainless steel filter material is observed and continuous growth of captured PbO consumes oxygen from the liquid metal melt. A long-term purification campaign of more than one month is presented to demonstrate the increase in cold trap effectiveness with loading.

Processes requiring intensive interfacial momentum, mass and heat exchange between gases and particulate solids can be greatly enhanced by operating in a centrifugal field. This is realized in the Gas-Solid Vortex Reactor (GSVR) with centrifugal accelerations up to two orders of magnitude higher than the Earth's gravitational acceleration. Here, the flow patterns of two 1g-Geldart B-type particles are experimentally assessed, over the gas inlet velocity range 82–126 m s−1, in an 80 mm diameter and 15 mm height GSVR. The particles are monosized aluminum spheres of 0.5 mm diameter, and walnut shell in the sieve fraction 0.50–0.56 mm and aspect ratio 1.3 ± 0.2. Two dimensional Particle Image Velocimetry combined with Digital Image Analysis and pressure measurements revealed that periodic fluctuations in solids azimuthal and radial velocity between gas inlet slots are strongly related to the average solids azimuthal velocity and bed uniformity. Aluminum particles feature steeper changes in azimuthal velocity and more attenuated changes in radial velocity than walnut shell particles. Within the assessed gas inlet velocity range the solids bed of aluminum exhibits average azimuthal velocities and bed voidages 40–50% and ≈10% lower than those of walnut shell. The aerodynamic response time of the particles, i.e. ρsdp2/18μg, emerged as an important parameter to assess the influence of the carrier gas jet on the radial deflection of the particles and the interaction solids bed-outer wall. Too low aerodynamic response time relates to nonuniformity in bed voidage due to solids radial velocity fluctuations. Excessive aerodynamic response time indicates low solids azimuthal velocities due to solids bed-outer wall friction.

Vortex units are commonly considered for various single and multiphase applications due to their process intensification capabilities. The transition from gas‐only flow to gas–solid flow remains largely unexplored nonetheless. During this transition, primary flow phenomenon, jets, and secondary flow phenomena, counterflow and backflow, are substantially reduced, before a rotating solids bed is established. This transitional flow regime is referred to as the vortex suppression regime. In the present work, this flow transition is identified and validated through experimental and computational studies in two vortex units with a scale differing by a factor of 2, using spherical aluminum and alumina particles. This experimental data supports the proposed theoretical particle monolayer solids loading that allows estimation of vortex suppression regime solids capacity for any vortex unit. It is shown that the vortex suppression regime is established at a solids loading theoretically corresponding to a monolayer being formed in the unit for 1g‐Geldart D‐ and 1g‐Geldart B‐type particles. The model closely agrees with experimental vortex suppression range for both aluminum and alumina particles. The model, as well as the experimental data, shows that the flow suppression regime depends on unit dimensions, particle diameter, and particle density but is independent of gas flow rate. This combined study, based on experimental and computational data and on a theoretical model, reveals the vortex suppression to be one of the basic operational parameters to study flow in a vortex unit and that a simple monolayer model allows to estimate the needed solids loading for any vortex device to induce this flow transition.

An increased interest in the use of liquid metals for novel energy conversion systems is present today. The Accelerator Driven System (ADS) called MYRRHA under design at the Belgian Nuclear Research Centre (SCK-CEN) is an example of such an innovative system. The use of Lead-Bismuth Eutectic (LBE) as a coolant for this reactor implies that an accurate knowledge on the chemical properties of the coolant needs to be available. An important factor is the risk of coolant oxidation due to oxygen ingress in the system. Although the formation of lead oxide (PbO) is well understood, the deposition mechanism and kinetics are not yet studied. In this work the deposition mechanism of PbO on 316L stainless steel is investigated. The evolution of the dissolved oxygen concentration during thermal cycling of LBE indicates that fouling of isothermal surfaces by PbO can only proceed by particle deposition. On the other hand, the fouling of non-isothermal surfaces by PbO is dominated by crystallization fouling. A real-time measurement of the PbO deposition rate shows an asymptotic behavior of PbO crystallization fouling. By predicting the onset of PbO nucleation and subsequent growth, a kinetic model for the crystallization fouling is put forward. Quantitative agreement between deposition rate predictions and validation measurements is obtained around 673 K.

A unique normalized radial pressure profile characterizes the bed of a gas‐solid vortex reactor over a range of particle densities and sizes, solid capacities, and gas flow rates: 950–1240 kg/m3, 1–2 mm, 2 kg to maximum solids capacity, and 0.4–0.8 Nm3/s (corresponding to gas injection velocities of 55–110 m/s), respectively. The combined momentum conservation equations of both gas and solid phases predict this pressure profile when accounting for the corresponding measured particle velocities. The pressure profiles for a given type of particles and a given solids loading but for different gas injection velocities merge into a single curve when normalizing the pressures with the pressure value downstream of the bed. The normalized—with respect to the overall pressure drop—pressure profiles for different gas injection velocities in particle‐free flow merge in a unique profile. © 2015 The Authors AIChE Journal published by Wiley Periodicals, Inc. on behalf of American Institute of Chemical Engineers AIChE J, 61: 4114–4125, 2015

Regime maps that predict the heavy hydrocarbon droplet impingement behavior on a heated wall (Weber number of the impinging droplet v/s wall temperature) are constructed based on CFD simulations using the Volume of Fluid model with the geo-reconstruct scheme. Based on the simulation results, maps are constructed for single-component droplets with a diameter of 50 and of 100 µm. The applied CFD model is validated by comparing these with regime maps available in literature, constructed based on experimental data for model liquids and liquid mixtures. The impingement regimes of Splash, Stick, Rebound and Breakup are well-predicted. Two distinct types of Splash (Splash with ligament formation and Splash with ring detachment) are reported for the first time. Using the validated CFD model, regime maps are constructed for multi-component heavy hydrocarbon droplets with a diameter of 50 and of 100 µm. Significant differences between the single-component and the multi-component droplet impact behavior are observed. Improved and new correlations for regime transitions, droplet stretching on the wall, droplet rebounding velocity and number of splashed droplets are derived based on energy balances. They are found to correlate well with CFD predictions.

Coincident three-component particle velocity measurements are performed in an almost 9 m high cylindrical riser (internal diameter 0.10 m) of a pilot-scale cold-flow Circulating Fluidized Bed set-up, using two Laser Doppler Anemometry (LDA) probes. Experiments are performed with superficial gas velocities of 3.5 m/s and 5.3 m/s near the solids inlet line and with an average solids volume fraction of 0.0002. The particle flow is observed to be highly disturbed due to the asymmetrical position of both the gas and the Y-shaped solids inlet line. A jet-like particle flow and a by-passing streaming gas jet around the particles are formed. Just below the solids inlet line a downward oriented particle velocity is measured. Particles are immediately lifted by the ascending gas and spread over the entire cross-sectional tube area. Near the solids inlet line the axial and radial particle velocity fluctuations are of the same order of magnitude as the corresponding mean particle velocity components, showing that the flow is anisotropic. The radial particle velocity fluctuations decrease fast as the radial inflow of particles is transformed into axial particles flow. The turbulence intensity in the inlet section of the dilute riser flow is found to be extremely high, indicating that the flow is far from being fully developed. The total particle shear stress profiles near the solids inlet line indicate a high momentum transfer from radial to axial particle transport. The particle fluctuation energy values calculated based on experimental data are nearly double than the values corresponding in fully developed flow studied previously.

As large floor-fired furnaces have many applications in refinery and (petro-) chemical units and about 80% of heat transfer in these furnaces is by radiation, the accurate description of radiative heat transfer is of the most importance for accurate design and optimization. However, the impact of using different radiation models in coupled furnace/reactor simulations has never been evaluated before. Therefore, coupled furnace/reactor simulations of an industrial naphtha cracking furnace with a 130 kt/a capacity have been conducted. Computational fluid dynamics simulations were performed for the furnace side, while the one-dimensional reactor model COILSIM1D was used for the reactor simulations. The Adiabatic, P-1, discrete ordinates model (DOM), and discrete transfer radiation model (DTRM) were evaluated for modeling the radiative heat transfer. The results with DOM and DTRM are very similar both on the furnace and the reactor sides. The flue gas temperature using DOM is higher than when using the P-1 radiation model, resulting in higher incident radiation. Comparing the simulated results of all radiation models to the industrial product yields and run lengths shows that DOM and DTRM outperform the others. As DOM has a broader application range than DTRM, and because the current implementation of DTRM in FLUENT/14.0 cannot be run in parallel yet, DOM is the recommended radiation model for run length simulations of steam cracking furnaces.

The process intensification possibilities of a gas–solid vortex reactor have been studied for biomass fast pyrolysis using a combination of experiments (particle image velocimetry) and non-reactive and reactive three-dimensional computational fluid dynamics simulations. High centrifugal forces (greater than 30g) are obtainable, which allows for much higher slip velocities (>5 m s–1) and more intense heat and mass transfer between phases, which could result in higher selectivities of, for example, bio-oil production. Additionally, the dense yet fluid nature of the bed allows for a relatively small pressure drop across the bed (∼104 Pa). For the reactive simulations, bio-oil yields of up to 70 wt % are achieved, which is higher than reported in conventional fluidized beds across the literature. Convective heat transfer coefficients between gas and solid in the range of 600–700 W m–2 K–1 are observed, significantly higher than those obtained in competitive reactor technologies. This is partly explained by reducing undesirable gas–char contact times as a result of preferred segregation of unwanted char particles toward the exhaust. Experimentally, systematic char entrainment under simultaneous biomass–char operation suggested possible process intensification and a so-called “self-cleaning” tendency of vortex reactors.

IMPROOF will develop and demonstrate the steam cracking furnace of the 21st century by drastically improving the energy efficiency of the current state-of-the-art, in a cost effective way, while simultaneously reducing emissions of greenhouse gases and NOX per ton of ethylene produced by at least 25%. Therefore, the latest technological innovations in the field of energy efficiency and fouling minimization are implemented and combined, proving that these technologies work properly at TRL 5 and 6 levels. The first steps to reach the ultimate objective, i.e. to deploy the furnace at the demonstrator at commercial scale with the most effective technologies, will be discussed based on novel pilot scale data and modeling results.

In a Gas–Solid Vortex Reactor (GSVR), also referred to as a Rotating Fluidized Bed in Static Geometry, a fluidized bed is generated in a centrifugal field by introducing the gas via tangential inlet slots to the reactor chamber. Better heat and mass transfer are observed, making this a promising reactor type for Process Intensification. Developing GSVRs on industrial scale requires, amongst other, a good insight and understanding of the hydrodynamics of the granular flow. In the present work experiments are performed over a wide range of operating conditions in a cold flow pilot-scale set-up. The set-up has a diameter of 0.54 m, a length of 0.1 m and 36 tangential inlet slots of 2 mm. Different materials with solids density between 950–1800 kg/m3 and particle diameters of 1–2 mm, at varying gas injection velocities from 55 to 110 m/s are tested between minimum and maximum solids capacities. All these operating conditions are used to follow the change of granular flow by performing PIV. The rotating fluidized bed can change from a smoothly rotating, densely fluidized bed to a highly bubbling rotating fluidized bed depending on the operating conditions. Bubbling diminishes with increasing solids density and particle diameter. Experimental measurements of azimuthal particle velocity fields in a GSVR are for the first time reported. Azimuthal solids velocity is found to decrease with higher solids density and larger particle diameter. The critical minimum fluidization velocity, that is the minimum velocity at which the complete bed is fluidized, is calculated and the centrifugal bed behavior is mapped in terms of a dimensionless radial gas velocity and a dimensionless particle diameter, as conventionally done for gravitational beds.

A drag model for the Gas-Solid Vortex Unit is presented. A large set of experimental data, obtained using static gauge pressure measurements, Particle Image Velocimetry and Digital Image Analysis in different GSVU geometries is used to estimate the model parameters with regression analysis. A large range of radial particle Reynolds numbers, particle densities and particle diameters is investigated. The newly proposed drag coefficient correlation, including 95% confidence intervals, reads: C D , GSVU = 15.00 ± 4.65 e 2 R e p , R − 0.28 ± 0.05 S 0.76 ± 0.03 The drag coefficient correlation is found to depend on the swirl ratio S, a variable determined by the GSVU geometry. The model suggests that, for the operating conditions in the GSVU, the particles mostly behave independently of one another. The performance of the model to predict the pressure drop over fluidized beds in a centrifugal field is high as compared to the standard drag models for fluidized beds in the gravitational field. Additionally, the drag model eventually allows to calculate the azimuthal solids velocity in a GSVU under varying operating conditions using an Archimedes/Reynolds number correlation derived from the radial momentum balances for both phases.

Coke formation in the convection section of a steam cracker occurs when heavy feeds are cracked. This work presents CFD simulations of coke formation in the mixture superheater tubes in the convection section of a steam cracker. The hydrocarbon feed used for the simulations is a gas condensate. Eleven representative chemical species are selected, based on their boiling points, to mimic the entire range of feed components. The liquid–vapor spray flow in the mixture superheater tube is simulated based on an Eulerian–Lagrangian approach using ANSYS FLUENT 13.0. Evaporation of multicomponent droplets suspended in the vapor phase or deposited on a tube wall is considered. The mixture superheater tubes make three horizontal passes (11.3 m long and 0.077 m diameter) through the convection section. The droplet–wall interaction model considers ‘Splash’, ‘Rebound induced breakup’, ‘Rebound’ and ‘Stick’. The amount of liquid deposited on the mixture superheater tube wall is obtained by simulating the spray flow. The amount of coke formed from the liquid deposited on a wall is based on the phase separation model of (Wiehe, 1993). Industrial & Engineering Chemistry Research 32, 2447–2454. Spatial variations of the coke layer formed in the mixture superheater tubes as a function of outer tube wall temperatures and initial droplet diameter are presented. For outer tube wall temperatures lower than the boiling point of the highest-boiling species in the feed a 1 mm thick coke layer is formed over a period of 1 month. For outer tube wall temperatures higher than the boiling point of the highest boiling component in the feed no coke is formed in the mixture superheater tubes. This work provides guidelines to minimize the extent of coke formation in the steam cracker convection section when a heavy feed is cracked. It also provides possible remedies to completely eliminate the coking problem when cracking heavy feeds.

An industrial project was developed to optimize the biodiesel production from crude palm oil. This process was developed for a one ton scale application on the palm oil production facility in equatorial Africa, to be used on the plantation to provide fuel for the fleet of the company. Because of the specific conditions (crude palm oil as starting material, application in technologically difficult conditions), it was essential for the developed procedure to be robust and simple, and to use minimum amounts of chemicals. The process was optimised on lab-scale in 2005 and 2006, scaled up in the following year, and is since then successfully applied as intended on the palm oil plantation. The produced biodiesel is used pure, without mixing with diesel fuel and without additives. After several years of continuous use, no negative effects were noticed on the engines. The process efficiency and durability are therefore confirmed.

Coincident three-component solids velocity measurements are performed in a 8.7-m-high cylindrical riser (internal diameter 0.10 m) of a pilot-scale cold-flow Circulating Fluidized Bed set-up, using two Laser Doppler Anemometry (LDA) probes. Experiments are performed with superficial gas velocities of 3.5 and 5.3 m/s at riser heights from 0.9 to 5.5 m and with an average solids volume fraction of 0.0002. At the lower riser heights, the solids flow is observed to be highly disturbed due to the asymmetrical position of both the air and solids inlet. A fully developed parabolic axial solids velocity field is observed about 2 m above the solids inlet line. The measured radial and azimuthal particle velocity components are significantly smaller than the axial one. The particle velocity fluctuations in the axial flow direction are larger than in the radial and azimuthal directions, showing that the solids flow is anisotropic. However, both the radial and azimuthal velocity fluctuations are higher than the corresponding mean velocities, as observed when studying the turbulence intensity in the three directions. All fluctuating velocities are decreasing significantly with increasing riser height, i.e. as the flow becomes more developed. No average down-flow of solids is recorded, not even close to the wall, due to the highly diluted flow. The total particle shear stress profiles designate a mean transport of fluctuating momentum along the riser radius due to velocity fluctuations. The particle fluctuation energy calculated based on experimental data indicates that the solids flow is dominated by wall-particle rather than intra-particle collisions.

A particle population balance formulation for a circulating fluidized bed, involving aggregation and breakage of particles, is solved using the homotopy perturbation method (HPM). The homotopy method deforms a difficult problem into a simple problem, which then can be easily solved. The HPM solution is compared with the solution obtained using a standard finite difference method. Using homotopy, a good approximation of the finite difference solution is obtained within a few iteration steps. The results reveal that the homotopy method is an effective and simple tool to solve nonlinear partial integro-differential equations and has a wide scope and applicability to solve complex engineering problems. The reliability of the algorithm is tested using three different feed inlet particle size (diameter) distributions to indicate the robustness of this method.

Liquid lead-bismuth eutectic (LBE) is an important candidate to become the primary coolant of future, generation IV, nuclear fast reactors and Accelerator Driven System (ADS) concepts. One of the main challenges with the use of LBE as a coolant is to avoid its oxidation which results in solid lead oxide (PbO) precipitation. The chemical equilibria governing PbO formation are well understood. However, insufficient kinetic information is currently available for the development of LBE-based nuclear technology. Here, we report the results of experiments in which the nucleation, growth and dissolution of PbO in LBE during temperature cycling are measured by monitoring dissolved oxygen using potentiometric oxygen sensors. The metastable region, above which PbO nucleation can occur, has been determined under conditions relevant for the operation of LBE cooled nuclear systems and was found to be independent of setup geometry and thus thought to be widely applicable. A kinetic model to describe formation and dissolution of PbO particles in LBE is proposed, based on Classical Nucleation Theory (CNT) combined with mass transfer limited growth and dissolution. This model can accurately predict the experimentally observed changes in oxygen concentration due to nucleation, growth and dissolution of PbO, using the effective interfacial energy of a PbO nucleus in LBE as a fitting parameter. The results are invaluable to evaluate the consequences of oxygen ingress in LBE cooled nuclear systems under normal operating and accidental conditions and form the basis for the development of cold trap technology to avoid PbO formation in the primary reactor circuit.

The feasibility of performing the SO2/NOx adsorption process in a gas-solid vortex reactor (GSVR) is examined and compared with the more traditional riser technology. The multiphase reacting flow is modeled using the Eulerian-Eulerian two-fluid model. Models of nonreacting flows were validated using data from a bench-scale experimental setup. The GSVR has the potential to significantly improved heat/mass transfer between phases, as compared to more conventional fluidization technologies. Process intensification opportunities are investigated. The model predicts continuous removal efficiencies greater than 99% for SO2 and approximately 80% for NOx. The gas-solid slip velocity and convective mass transfer coefficient for the riser were 0.2-0.5 and 0.06-0.12 m/s, respectively, whereas the values for the GSVR were 6-7 and 1.0-1.1 m/s, respectively. This order of magnitude increase in the external mass transfer coefficient highlights the potential intensification opportunities provided by the GSVR.

The hydrodynamic behavior of dispersed gas–solid flow is simulated for an industrial-scale Fluid Catalytic Cracking (FCC) riser using the multi-fluid approach, with complementary information from the Kinetic Theory of Granular Flow (KTGF) for the transport coefficients of the solid phase. A continuous Particle Diameter Distribution (PDD) is considered for the solid phase catalyst. The three-dimensional gas–solid flow is considered to be non-isothermal and turbulent. The hydrodynamics of the riser reactor is coupled to a 12-lump FCC kinetic model to predict the influence of a polydisperse distribution on gas–solid reactive flow. The kinetic model involves lumped species consisting of paraffins, naphthenes, aromatic rings, and aromatic substituent groups in medium and heavy fuel oil fractions and includes the effect of aromatic ring adsorption and catalyst deactivation due to coke formation. In this study, a Computational Fluid Dynamics (CFD) model using the Eulerian–Eulerian multi-fluid approach and a Population Balance Model (PBM) are coupled. The hydrodynamic equations are solved by means of a finite volume method, while the population balance equations are solved using the Direct Quadrature Method of Moments (DQMOM). The moments of the solids phase velocity are modeled using classical kinetic theory, while the moments of the PDD are described using quadrature weights and abscissas. To account for the catalyst PDD, three different approaches namely simple, surface-averaged and volume-averaged coupling have been attempted. The latter two approaches, assuming surface and volumetric reaction kinetics respectively, predict with a good precision the yields of the different product families obtained in an industrial FCC unit showing a slight improvement from a conventional heterogeneous reactor model with a monodispersed distribution.

The Kiliani reaction, i.e. the elongation of the carbon chain by means of cyanide addition to the carbonyl group of carbohydrate derivatives, is studied on lower C3-, C4- and C6-ketoses under continuous flow conditions. Depending on the process parameters, the corresponding cyanohydrins or alpha-hydroxycarboxylic acids are obtained. A simple on-line purification of the effluent is studied using cation exchange resins. Reactions provide high yields and selectivity within short residence times, emphasizing the assets of the continuous flow process versus the batch process.

Riser outlet effects induced by an L-outlet and by abrupt T-outlets with different extension heights and outlet surface areas are studied experimentally and computationally. Experiments are carried out in a cold flow riser. The cold flow riser has a diameter of 0.1 m and a height of 8.765 m and is operated in the dilute flow regime with a superficial gas velocity of 2.48-7.43 m/s and a solids flux of 3.0 kg/m²/s. Particle velocities are measured using Laser Doppler Anemometry (LDA). Vortex formation in the extension part of the riser is observed. The vortex circulates the solids along the wall opposite to the outlet, thus inducing a solids reflux. The flow pattern upstream the outlet is, however, hardly affected in the small diameter riser. The vortex position and length are affected by the extension height, but hardly by the outlet surface area and the superficial gas velocity. The use of an L-outlet significantly reduces the vortex formation. The experimental measurements are used to validate a 3D Eulerian–Eulerian and Kinetic Theory of Granular Flow (KTGF) based gas–solid flow model. In general, the calculated trends are qualitatively in agreement with the experimentally observed phenomena. The exact shape of the vortex is not always accurately predicted.

A Eulerian–Lagrangian method is implemented to simulate turbulent two-phase gas–solid riser flow, using a mean-field/probability density function (PDF) method. The mean-field method is applied to the gas phase while the PDF method is applied to the solid phase. Using the PDF method for the solid phase is advantageous as there is no need for closure models for the convection term in the momentum equation contrary to the situation where the mean-field method is used. The present method is implemented to investigate the influence of a side inlet on the flow pattern in a dilute gas–solid riser configuration. When applying the Eulerian–Lagrangian method, a perfect correspondence with the experimental observations is obtained. On the contrary, when using the Eulerian–Eulerian method significant differences with the experimental data are observed.

A coupled and a decoupled solution method are applied to the steady-state simulation of one-dimensional, reactive gas flow. The accuracy of the numerical steady-state solution depends to a significant extent on the molar expansion of the gas. The convergence rate of the coupled solution method is almost independent of molar expansion. The convergence rate of the decoupled solution method is at least 50% lower for strong molar expansion than for nonexistent molar expansion. By adding a transport equation for molar mass, the convergence rate of the decoupled solution method becomes almost independent of molar expansion. The results for the decoupled solution method apply to other methods in which composition is determined on a fixed flow field. A tool is presented to quantify the perturbation of the flow field by the chemical reactions.

A hybrid Lagrangian-Eulerian methodology is developed for the numerical simulation of turbulent reactive gas-solid flow. The SO2-NOx Adsorption Process (SNAP) in a riser reactor with dilute gas-solid flow is taken as a test case. A threedimensional time-dependent simulation is performed. By using the transported composition PDF method [1], modeling of the mean chemical source term and mass transfer terms in the gas-solid flow model equations is no longer needed. A notional particle-based Monte-Carlo algorithm is used to solve the transported composition PDF equations. A Finite-Volume technique is used to calculate the hydrodynamic fields from the Reynolds Averaged Navier Stokes (RANS) equations combined with the k-e turbulence model for the gas phase and the Kinetic Theory of Granular Flow (KTGF) for the solid phase [2]. The newly developed hybrid solution technique is tested with the SNAP chemistry that has a total of 13 scalars (i.e., 5 gas phase components and 8 solid phase species) for which the composition fields of the reactive species are calculated. A good agreement between simulated and experimental gas-outlet composition of a demonstration unit is obtained.

s Hydrodynamics of a gas–solid rotating bed reactor (RBR) in static geometry are investigated. Tangential injection of a gas at mass flow rate of 0.4–1 kg/s in a cylindrical vessel with a diameter of 0.54 m generates a rotating gas phase flow field. Introduction of solid particles in this field results into an annular dense gas–solid rotating bed. A stable annular gas–solid rotating bed without solids particles loss is achieved over a wide range of operating conditions. Goal of the presented work is to investigate, by means of experiments, the window for a stable operation of the gas–solid RBR, as a function of the solid particle diameter and density, the geometry of the RBR and the gas flow rate. If the solid particle diameter is comparable to tangential gas injection slot width, the establishment of a stable flow is delayed due to an increased slugging tendency. The upper limit of the solids content is found to decrease with decreasing solid particle diameter. Obtained experimental cold flow results are the initial steps in assessing the potentials of a RBR as an efficient gas–solid processing reactor from a process intensification point of view.

The presence of liquid hydrocarbon droplets in the convection section of a steam cracker may cause serious fouling problems due to coke formation, especially in high temperature zones. In order to investigate these fouling problems, a model has been developed and implemented in a CFD code to accurately simulate the behavior of a hydrocarbon droplet impinging on a hot surface. The impact energy of the droplet and the hot surface temperature are found to determine the impact behavior. On the basis of the newly developed model, the positions where droplets preferentially collide with the convection section tube walls and liquid material is deposited are determined. Furthermore, a kinetic model describing coke formation out of liquid hydrocarbon droplets in the temperature range of 450−700 K has been developed to calculate the rate of coke formation in each zone of the convection section tube. The calculated coke layer thickness on the most vulnerable tube locations and the industrially available values corre...

A method is proposed to simulate reactive flow, fully taking into account the effect of the reactions on the flow. Operator splitting is used to separate the computation of convection and reaction. A fast solver for mildly stiff ordinary differential equations and parallelization of the reaction term evaluation have been implemented to reduce the CPU time. The proposed method is applied to the steady-state, two-phase gas–solid simulation of a Fluid Catalytic Cracking riser reactor. A relatively simple kinetic model with four lumped components is used to demonstrate the feasibility of the method. The results show that the method is able to handle reactive flow with significant feedback of the reactions on the flow.

Computational fluid dynamics (CFD) simulations of the hydrodynamic behavior of rotating fluidized beds in static geometry (RFB-SG) are carried out for gas–solid flows. The rotating motion of the reactor bed is induced by the tangential injection of the gas along the circumference of the fluidization chamber. Steep gradients in the gas velocity fields both in radial and tangential direction generate turbulence. The radial and tangential drag forces fluidize the particle bed in both radial and tangential direction. An Eulerian two-fluid model is used. Gas phase turbulence is accounted for by a k–e model adapted for rotational flows. The RFB-SG simulations provide guidelines for a design and operation with a high efficiency in gas–solid momentum transfer, excellent gas–solid separation and limited solids losses. Hydrodynamic variables like the centrifugal force, the injection pressure, the radial and tangential slip velocities, solids hold-up are calculated for both polymer particles (300 μm, 950 kg/m3, Geldart Group B) and glass beads (70 μm, 2500 kg/m3, Geldart Group A) to allow for a comparison among different fluidization chamber designs. Unstable bed behavior, like slugging and channeling, is also numerically predicted.

A coupled simulation of the flue gas and process gas side of the convection section of a steam cracker is performed, making use of the CFD software package Fluent. A detailed overview of the operating mode of the different heat exchangers suspended in the convection section is obtained. The asymmetric inlet flow field of the flue gas in the convection section, and the radiation from the convection section walls leads to large differences in outlet temperatures of the tubes located in the same row. The flow fields and temperature fields in the tubes of a single heat exchanger differ significantly with e.g., outlet temperatures of the hydrocarbon-steam mixture ranging from 820 K to 852 K. Moreover, the simulations reveal the presence of hot spots on the lowest tube row, possibly causing fouling. © 2009 American Institute of Chemical Engineers AIChE J, 2009

A hybrid Lagrangian-Eulerian methodology is developed for the numerical simulation of turbulent reactive gas-solid flow. The SO2-NOx Adsorption Process (SNAP) in a riser reactor with dilute gas-solid flow is taken as a test case. A three-dimensional time-dependent simulation is performed. By using the transported composition PDF method [1], modeling of the mean chemical source term and mass transfer terms in the gas-solid flow model equations is no longer needed. A notional particle-based Monte-Carlo algorithm is used to solve the transported composition PDF equations. A Finite-Volume technique is used to calculate the hydrodynamic fields from the Reynolds Averaged Navier Stokes (RANS) equations combined with the k-epsilon turbulence model for the gas phase and the Kinetic Theory of Granular Flow (KTGF) for the solid phase [2]. The newly developed hybrid solution technique is tested with the SNAP chemistry that has a total of 13 scalars (i.e., 5 gas phase components and 8 solid phase species) for which the composition fields of the reactive species are calculated. A good agreement between simulated and experimental gas-outlet composition of a demonstration unit is obtained.

A model has been developed allowing to simulate the flow boiling process of a hydrocarbon feedstock. For the calculation of the different horizontal two-phase flow regimes that evolve during flow boiling, use is made of the Volume Of Fluid (VOF) model that uses a Piecewise Linear Interface Calculation (PLIC) method to reconstruct the interface between both phases in each computational cell. In-house developed codes calculate the mass and energy transfer phenomena occurring during this flow boiling process. As such, an existing CFD code is completed with a newly developed complete evaporation model. The developed model is used to simulate the flow boiling process of a hydrocarbon feedstock in the tubes of a convection section heat exchanger of a steam cracker. The simulation results show a succession of horizontal two-phase flow regimes in agreement with the literature.

The efficiency of the application of high-emissivity coatings on the furnace walls in steam cracking technology can only be evaluated on the basis of a description of radiative heat transfer distinguishing between the frequency bands. To this end, a non-grey gas radiation model based on the exponential wide band model (EWBM) has been developed and applied in the context of three-dimensional CFD simulations of an industrial naphtha cracking furnace with side-wall radiation burners. Applying a high-emissivity coating on the furnace wall decreases the net outgoing radiation from the furnace wall in the absorption bands and increases the net outgoing radiation from the furnace wall in the clear windows. Since radiation that is emitted by the furnace wall and travels through the flue gas in the clear windows can reach the reactor tubes without partially being absorbed by the flue gas, contrary to radiation that is emitted by the furnace wall and travels through the flue gas in the absorption bands, the thermal efficiency of the furnace increases. It was found that application of a high-emissivity coating on the furnace walls improves the thermal efficiency of the furnace (∼1%), the naphtha conversion (∼1%) and the ethylene yield (∼0.5%). These differences are small but, considering the industrial importance and scale of the steam cracking process, significant.

The endothermic thermal cracking process of hydrocarbons takes place in tubular reactor coils suspended in large gas-fired pyrolysis furnaces. Heat transfer to the reactor tubes is mostly due to radiation from the furnace refractory walls and the flue gas. A three-dimensional (3-D) simulation of the flow in an industrial scale steam cracking furnace is performed. The renormalization group (RNG) k − ɛ turbulence model is used. The combustion kinetics is modeled by a three-step reaction mechanism, while turbulence–chemistry interaction is taken into account through the Finite Rate/Eddy-Dissipation model. The Discrete Ordinates model (DOM), the P-1 and the Rosseland Radiation model are used for modeling of the radiative heat transfer. The results of using the different radiation models are compared mutually with adiabatic simulation results. The absorption coefficient of the gas mixture is calculated by means of a Weighted-Sum-of-Gray-Gas model (WSGGM). The effect is discussed of the use of different radiation models on the predicted wall, tube skin and flue gas temperature profiles and heat fluxes towards the reactor tubes, as well as on the predicted species concentration profiles and structure of the furnace flames under normal firing conditions.

To account for meso-scale phenomena in coarse grid simulations, correlation terms appearing in the filtered gas-solid flow equations have to be modeled. Possible approaches to describe filtered gas-solid momentum transfer are evaluated via a mixture speed of sound test. In contrast to the effective drag coefficient closure model for the filtered drag force, the generalized added mass closure model for the correlation between the solid volume fraction and the gas phase pressure gradient shows an acceptable behavior. Furthermore, the generalized added mass may become determining in describing filtered gas-solid momentum transfer when sufficiently coarse grids are used. 3D steady-state riser simulations demonstrate the impact of the generalized added mass based description of filtered gas-solid momentum transfer on the calculated gas-solid flow behavior, in particular on the gas-solid slip velocity profile. (c) 2007 Elsevier Ltd. All rights reserved.