Downer Reactor Research Articles

Mixing effects of high-speed jets in gas-solid riser and downer reactors

For the high-temperature and short-contact time gas-solid reaction process, riser and downer are considered appropriate reactors. To realize an intensive and complete mixing of reactants with catalysts, the feed raw is always introduced in the form of high-speed jets. In this study, in order to investigate the mixing effects of different types of high-speed jets in riser and downer, traceable ozone is injected with the high-speed feed jets to react with catalyst particles. By detecting the decomposition of ozone, the gas-solid mixing and reaction in riser and downer under the influence of both co-current and counter-current injections are analyzed. The relative ozone concentration is used to reflect the location reaction extent and its radial nonuniformity index is proposed to compare the results in riser and downer. It is found that the jet influence zone in downer provides a relatively better environment for the mixing of feed jets with catalysts. In the riser, introduction of counter-current injections could improve the uniformity of gas-solid mixing in the initial contact region of feed with catalysts.

Multiscale modeling of an entrained flow solar calciner for thermochemical energy storage via calcium looping

Calcium looping has been proposed as an alternative route addressing the intermittent nature of solar energy in terms of thermochemical energy storage, with the added benefit of CO2 capture. In this work, a multiscale approach aims to predict and optimize the performance of a pilot-scale solar calciner operating at realistic conditions, incorporating a diffusion–reaction model with multiphase flow considerations in a downer reactor. The model has been successfully validated with experimental data from the literature studying the effect of particle size on the observed rate, and with limestone decomposition experiments carried out in a lab -scale downer reactor. It was revealed that particles larger than 100 μm exhibit internal mass transport resistances, due to CO2 accumulation within the particle, whereas heat transport limitations are not expected in the operating conditions of interest. A reference scenario demonstrated 92 % limestone conversion at realistic operating conditions, feasible in concentrated solar power plants. The solar flux density on the reactor wall influences the efficiency of the system critically since heat loss to the environment leads to greater solar input. To tackle the low efficiency, a partially insulated, indirectly irradiated reactor was investigated: the efficiency increased from 8 % to 26 %, while integration in a Calcium-Looping system can further boost the efficiency to 34 %. This approach constitutes a basis for future scale-up considerations.

Mass transfer model based on the spatial superposition assumption for simulating the catalytic ozone decomposition in downer reactor

A mass transfer model based on the spatial superposition assumption is proposed in this work to better reflect the effect of heterogeneous structure on the mass transfer behavior of the downer. Attributing to the special assumption of this model, fewer concentration parameters are required compared with the classic multi-scale method, which largely saves the calculation consumption. The simulation results of this model also could be in good agreement with the experimental data. Additionally, the calculated mass transfer coefficients are in the range from 10−3 to 10−1, which are approximately three orders of magnitude lower than that of the homogeneous structure. It is proved that the existence of clusters would seriously affect the gas-solid mass transfer effectiveness, thereby deteriorating the reaction rate. By simulating the distribution of ozone concentration in the catalytic ozone decomposition, it can be found that the mass transfer near the inlet region plays the dominant role while along the flow direction, the influence of reaction process becomes stronger than that of the mass transfer process and gradually turns to control the whole decomposition process. These results indicate that the present model is useful and could be applied for the simulation of mass transfer process in other gas-solid fluidized systems with heterogeneous structures.

Simulation of the coal pyrolysis process in downer reactor with a mass transfer model based on the spatial superposition assumption

A mass transfer model based on the spatial superposition assumption is constructed to investigate the coal pyrolysis process in the downer. The simulated volatiles yield using this developed model are in better agreement with the experimental data than that based on the conventional Jung-La Nauze equation. It is found that the mass transfer coefficients (Kf̅) calculated by the Jung-La Nauze equation are approximately two orders of magnitude higher than that of this work, indicating that the mass transfer process predicted with the uniform structure is overestimated. Moreover, the concentration difference between the surface of dilute particles and gas bulk is small and close to zero, which means that the reaction is the rate controlling step, while the concentration difference between the surface of the clusters and gas bulk is relatively large, which implies that the mass transfer process also becomes the rate controlling step. All these results obtained in this work are expected to deepen the understanding of the mass transfer and reaction process by considering the influence of heterogeneous structures in gas-solid fluidized system.

Flow distribution analysis of a novel fcc system through experiment study and atomic model

As the largest palm oil producer in the world, Indonesia has a promising potential to produce green fuel through the Fluid Catalytic Cracking (FCC) process. A novel FCC configuration, FCC Proto X 3, which combines a riser reactor and downer reactor in the system, has been developed. However, several valves including in the FCC system remain a black box to the flow distribution in the system. The objective of this paper is to investigate the effect of the valve setting variation on the airflow distribution of the FCC system. The methodology uses experiment and acausal modeling. The effect of valve setting variation on pressure and average velocity of the airflow has been investigated. The experiment is conducted under cold test conditions, while the acausal model of the FCC system is built by using OpenModelica. It is obtained that valve 2 which controls the flow at the channel toward the regenerator is essential due to its role in controlling the air supply combustion process in the regenerator and driving the spent catalyst particles to the regenerator. Valve 3 is responsible for controlling the flow toward the riser reactor directly. Later, it is responsible for supplying the lifting fluid to support the catalytic cracking reaction at the riser sections. Valve 4 contributes to controlling the lifting fluid to the downer reactor. It will also be responsible for supplying thermal energy from the high-temperature particle catalyst to the reactor. When all valves toward the regenerator and reactor are 100 % open, the measured average velocity at the flue gas outlet and the product outlet are 8.04 m/s and 5.775 m/s respectively. The result shows that the airflow at the FCC system tends to flow through the regenerator. The atomic model estimation also shows a similar trend to the experiment result

Particle Residence Time Distribution in a Concurrent Multiphase Flow Reactor: Experiments and Euler-Lagrange Simulations

The present work focuses on investigating the residence time behavior of microparticles in a concurrent downer reactor through experiments and numerical simulations. For the numerical simulations, a three-dimensional multiphase model was developed using the Euler-Lagrange approach. The experiments were performed in a 0.8 m-long steel reactor with gravitational particle injection. The effects of different operating conditions, e.g., the sheath gas velocity on the particle residence time distribution were assessed. An increase in the sheath gas flow rate led to a decrease in the peak residence time, although the maximum residence time increased. Regarding the lowest sheath gas flow rate, the particles’ peak residence time was twice as high compared to the peak residence time within the highest flow rate. The particles’ residence time curves presented a broad distribution coinciding with the size distribution of the powder. The numerical results agreed with the experimental data; thus, this study presents a numerical model for predicting the particle residence time behavior in a concurrent downer reactor. Furthermore, the numerical simulations contributed to a better understanding of the particle residence time behavior inside a concurrent downer reactor which is essential for optimizing thermal rounding processes. Dimensionless correlations for the observed effects are developed.

Fast Pyrolysis Downer Reactor: Effect of Reactor Geometry on the Hydrodynamics

Fast Pyrolysis Downer Reactor: Effect of Reactor Geometry on the Hydrodynamics

Scaling of catalytic cracking fluidized bed downer reactor based on CFD simulations-Part II: effect of reactor scale.

The practical realization of the scaling up of gas–solid multiphase flow reactors with chemical reactions is hindered by chaotic flow behaviors and complex heat and mass transfers in the reactor. In addition, a law to scale up complex reaction mechanisms in multiphase flow systems has been rarely proposed in the existing literature. Thus, this study aims to investigate the scaling up of the catalytic cracking fluidized bed downer reactor based on the similitude method of chemical reaction performance. Three downer reactor scales with a height of 5, 15, and 30 m, were investigated. To anticipate the behavior of reactive flow, a Eulerian–Eulerian CFD model, two-fluid model, was constructed, which was combined with the kinetic theory of granular flow. A four-lump kinetic model was chosen to represent the mechanism of the catalytic cracking reaction of heavy oil from the pyrolysis of waste plastic. The CFD model accurately predicted the species composition distribution. The scaling law based on the geometric similarity, kinematic similarity, and chemical reaction similarity, was proposed. The catalytic cracking performance similarity of the downer reactor was obtained. With variances in the range of 10% and mean relative absolute error less than 5%, the axial and lateral distributions of chemical performance (heavy oil conversion, gasoline mass fraction, and gasoline selectivity) were found to be extremely similar.

Catalytic conversion of gaseous tar using coal char catalyst in the two-stage downer reactor

The effect of catalyst height (0–50 mm) and catalyst temperature (600–900 °C) on the catalytic cracking of gaseous tar is investigated using a two-stage down reactor. Results show that the utilization of catalyst promotes the removal of coal tar. The yield of coal tar at the pyrolysis temperature of 900 °C is 7.14% and tar is composed of aromatics and high-carbon-number aliphatics. The coal char prepared by fast coke-making method at 900 °C is directly used as a catalyst in the catalytic cracking of coal tar. When the catalyst temperature and the catalyst height are 900 °C and 50 mm, the tar produced by coal pyrolysis is connected with high-temperature coal char for a short time, reducing the tar yield from 7.14% to 4.26%. Among them, aliphatic hydrocarbons with carbon number of C10 and C11 and aromatics are the main components of coal tar. Similarly, compared with the pyrolysis, the content of CH4 and H2 obtained with the addition of the catalyst increase from 0.56 to 0.97 to 1.64%g/g coal and 1.02%g/g coal, respectively. Moreover, GC/MS results show that the increment of catalyst height and catalyst temperature increases aromatics content and facilitates the conversion of high-carbon-number component to low-carbon-number ones.

Numerical Experimentation and Validation of Shrinking Particle - Gas Reaction in a Pilot Plant Downer Reactor

ABSTRACT Downer reactors have been the choice for catalytic reactions necessitating control on intermediates, short residence time and high solid loading. The noncatalytic gas-solid reactions exhibit different hydrodynamics due to continuous variation of particle morphology during the reaction. This study addresses the challenges with numerical experimentation of reactive gas-solid system possessing shrinking particle characteristics and verification with the results from pilot scale test facility. Petcoke with ash content about 0.322% by weight is selected to justify the shrinking particle characteristics during combustion. The particle size ranges from 2 µm to 130 µm with an average size of 60 µm. The powder is cohesive with Hausner ratio of 1.43 and occupies cohesive regime in the Geldart’s classification. Ansys Fluent v19.0R1 is employed in simulating the reaction with a shrinking particle model and the kinetic parameters deduced with the help of TGA has been incorporated in the numerical model as well. The optimum multiphase interaction parameters using numerical experiments established with coupled CFD-RSM method also been included. The simulation tests are conducted for the same experimental conditions with the pilot plant test facility. The temperature profile and concentration profile are obtained for different solid/gas loading ratios and compared with the experimental data obtained with the pilot plant test facility. The study establishes the significance of appropriate drag models along with the interacting parameters for reliable and robust simulation. The simulation model presented here can be used for design and analysis of the performance of a downer reactor for reactive systems with shrinking particle characteristics.

Computational study of particle distribution development in a cold-flow laboratory scale downer reactor

The use of downer reactors (gas-solid co-current downward flow) in the Fluid Catalytic cracking (FCC) process for the upgrading of heavy crude oil into more valuable products has gradually become more common in the last decades. This kind of reactor is characterized by having homogeneous axial and radial flow structures, no back mixing, and shorter residence times as compared with the riser reactor type. Although downer reactors were introduced a long time ago, available information in literature about the multiphase hydrodynamic behavior at FCC industrial scale is scarce. Therefore, it is necessary to conduct experimental and computational studies to enhance the understanding of the hydrodynamics of two-phase co-current downward flow. The Computational Fluids Dynamics (CFD) software, Ansys Fluent, is used to study two-dimensional gas (air) and solid (catalyst particle) flow in a downer section of a cold-flow circulation fluidized bed (CFB) system at a laboratory scale. The implemented computational model is validated by comparing numerical results for solid velocity and volume fraction with measurements carried out on a CFB system using a fiber-optic probe laser velocimeter. According to numerical results obtained for different gas velocity and solid flux, flow development cannot only be estimated by considering solid axial velocity changes along the reactor; it is also necessary to take into account solid volume fraction axial variations as radial profiles can change even when velocity profiles are developed.

Hydrodynamic simulations of non-spherical particle dispersions in downer reactor with second-order moment turbulence model

A second-order moment kinetic frictional stress model coupled with a non-spherical drag coefficient between particle and gas phases is proposed to model the hydrodynamics of non-spherical particle flow in downer reactor. The heterogeneous dispersions of particle and the Reynolds stresses between gas and particle are numerically simulated. Results show that predictions are in good agreement with the experimental data. An increase in irregular shape degree leads to the enlargement for the dense ring of particle volume fraction, the particle velocity and its fluctuation, the normal and shear stress, and the granular temperature, as well as the encouragement of granular temperature peaks towards wall region. Strongly heterogeneous characteristics are found at the second acceleration region, which the amplitudes of normal and shear stresses are approximately twice larger than those of fully development flow. Higher heterogeneous indexes correspond to the homogeneous flow, and its larger gradient indicates that hydrodynamics is transiting from homogenous to heterogenous flow. Frequency of article collisions is on the order of 102.

Investigation on ultralight particle dispersions in dense two-phase flow using a kinetic friction stress model

The graphite particle dispersion characteristics with irregular shapes of dense gas-solid turbulent flow in downer reactor is numerically investigated using an improved kinetic friction stress model and a adopted drag coefficient describing momentum transfer between gas and non-spherical particles. Hydrodynamic predictions are in better line with experimental results. Redistributions of Reynolds normal and shear stresses with strongly anisotropic dispersions are captured. Comparisons to glass bead, ultralight particles elongate the second acceleration length, gather the concentration peaks at near wall region, and weak the maximum dense ring. Maximum kinetic energy at fully development region is approximately 1.2 times larger than that of second acceleration region. Lower particle sphericities are contribution to the enhancement of particle velocity, particle concentration, particle shear stress, particle collision frequency, and granular temperature, except for the reduction of turbulent kinetic energy. The negative amplitudes of shear stresses at fully development regions are approximately 4.0 times larger than those of second acceleration regions.

Particle Lagrangian CFD Simulation and Experimental Characterization of the Rounding of Polymer Particles in a Downer Reactor with Direct Heating

Polypropylene (PP) powders are rounded at different conditions in a downer reactor with direct heating. The particles are fed through a single central tube, while the preheated sheath gas is fed coaxially surrounding the central aerosol jet. The influence of the process parameters on the quality of the powder product in terms of particle shape and size is analyzed by correlating the experimental results with the flow pattern, residence time distribution of the particles and temperature distribution predicted by computational fluid dynamics (CFD) simulations. An Eulerian–Lagrangian numerical approach is used to capture the effect of the particle size distribution on the particle dynamics and the degree of rounding. The simulation results reveal that inlet effects lead to inhomogeneous particle radial distributions along the total length of the downer. The configuration of particle/gas injection also leads to fast dispersion of the particles in direction of the wall and to particle segregation by size. Broad particle residence time distributions are obtained due to broad particle size distribution of the powders and the particles dispersion towards the wall. Lower mass flow ratios of aerosol to sheath gas are useful to reduce the particle dispersion and produce more homogenous residence time distributions. The particles’ residence time at temperatures above the polymer’s melting onset is determined from the simulations. This time accounts for the effective treatment (rounding) time of the particles. Clear correlations are observed between the numerically determined effective rounding time distributions and the progress of shape modification on the particles determined experimentally.

From 3D to 1D: Capturing the effect of particle clusters in downers in the fluid catalytic cracking of gasoil

The effect of particle clusters on the chemical performance of a downer reactor during fluid catalytic cracking of gasoil is investigated by performing several 4-way coupled reactive 3D Discrete Element Method - Large Eddy Simulations (DEM-LES) in OpenFOAM. A one-meter-long cylindrical downer was used as a canonical case to analyze the dynamics of particles via 3D DEM-LES. A fluid mesh of 7×106 hexahedral cells sufficed to meet the y+<1 criterion for wall-resolved simulations. A four-lump kinetic model was selected to describe the chemistry of the process in which gasoil, gasoline, light gases, and coke are considered as the lumps. The difference on the validation of the reactive 3D DEM-LES with experimental data found in literature was 10 rel.% at a catalyst-to-oil ratio of 8.1 and less than 5 rel.% at a catalyst-to-oil ratio of 15 and higher. Two effectiveness factors that take into account the frequency, mean lifetime, size and shape of clusters were proposed to capture the effects of clusters on the prevailing chemistry. In this way, it is possible to translate the information obtained with the intensive 3D DEM-LES to a regular 1D model. The first effectiveness factor was based on two empirical correlations that relate the particle Reynolds number and the Stokes number with the frequency and mean lifetime of clusters, determined from 3D DEM-LES. A second effectiveness factor relates to the shape and size of clusters to compute the efficiency of the catalytic reactions. A modified 1D model that takes into account these two effectiveness factors predicted values of conversion within 3 rel.% of deviation from experimental data, similar to those of the 3D DEM-LES with a computational time 4×108 times lower.

Structural optimization of baffle internals for fast particle pyrolysis in a downer reactor using the discrete element method

Abstract The structural optimization of baffle internals for fast pyrolysis of coal with particulate mixing and heat transfer in a downer reactor using the discrete element method (DEM) has been investigated in this research. The pyrolysis terminal temperature at the exit of the downer reactor is not only decided by the volume-feeding-rate ratio of the coal to the sand, but also is affected by the inner structural design of the baffle internals in the downer reactor. As presented in the previous publication of the author, the inhibition from the baffle internals in a downer reactor can improve the particulate-mixing degree and heat carrier, and increase the mean residence time of the coal and heat-carrier particles in the downer reactor. The structure of the baffle internals in the downer reactor mentioned in this research can be optimized by the independently developed 3D soft-sphere model of the DEM programme of a 40-mm baffle length, a 30° baffle-slope angle and at least four baffles designed in the downer reactor, which is beneficial for the process design of coal pyrolysis with a heat carrier in the downer reactor.

Multi-fluid Eulerian simulation of binary particles mixing and gas–solids contacting in high solids-flux downer reactor equipped with a lateral particle feeding nozzle

The performance of binary particles mixing and gas–solids contacting, which is considered qualitatively to have a significant influence on the heat transfer in internal heated circulating fluidized beds, is carefully investigated by means of a numerical approach in the newly developed high solids-flux downer lignite pyrolyzer (φ0.1 m × 6.5 m). Since binary particles are used in this system, a reasonably validated 3D, transient, multi-fluid model, in which three heat transfer modes relating to the convection, conduction and radiation are considered, is adopted to simulate the flow behavior, temperature profiles as well as volatile contents. The simulation results showed that the solids stream impinges the left wall surface initially and turns towards the right wall in the further downward direction and then shrinks during this process resulting in that the solids concentrate a little more at the central region. In the further downward section of the downer, the particle flow disperses near the right wall and develops uniformly. Meanwhile, the coal phase is slowly heated in the downer and it is found that most of the heat absorbed by the coal is from the convection heat transfer mode. To explore the heat transfer mechanism more quantitatively, two indexes (mixing index and contacting index) are proposed, and it is found that the mixing index initially increased fast and later remained at a relatively flat state. For the contact index, it shows a trend with a first rising and then falling, finally rising continuously. Also, it is found that the convection heat transfer is closely correlated to the contacting status of gas–coal which indicates that the improving of the gas–coal contacting efficiency should be an effective way to strengthen the coal particle heating process.

High-temperature fast pyrolysis of coal: An applied basic research using thermal gravimetric analyzer and the downer reactor

The study explored the influence of temperature and particle size on the high-temperature fast pyrolysis behaviors of Shenmu(Yulin) coal through thermal gravimetric analyzer and the downer reactor. Results of TG-FTIR show that the thermal hysteresis causing by high heating rate make TG and DTG curves shift to high-temperature zone. The infrared spectra reveal that the release characteristics of typical products and functional groups are consistent with the weight loss curve. Moreover, experimental results of the downer reactor reveal that the content of coal-char and coal-tar decreased and coal-gas increased with the increment of temperature. Better syngas can be obtained at 900 °C. The yield of coal-char firstly increases and then decreases with the decrease of particle size and the coal-tar is opposite. More importantly, during the pyrolysis process of 35–60 mesh coal, the largest amount of coal-char and the smallest amount of coal-tar are not only obtained, but high-quality syngas is also obtained. Through the characterization of coal-tar, it is found that coal-tar is mainly composed of aromatic compounds, and the content can reach more than 50%. Moreover, the change of infrared absorption intensity of O–H, CO and C–O groups in coal-char varies with the change in pyrolysis parameters.

Parametric effect of biomass partial hydropyrolysis process in a downer reactor to co-produce high-quality tar and syngas

Fast partial hydropyrolysis of biomass was carried out at the level with hydrogen concentration of 0% to 30% and temperatures ranging from 700 to 900 °C by using a downer pyrolyzer. A theoretical parametric effect on yields and properties of the hydropyrolysis products were clarified. It was found that the volatile matter evolved during pyrolysis was substantially increased in the presence of hydrogen. The yields of CH4, C2H6 and light tar increased with increasing elevated H2 ratio, CH4 showed an especially large yield increase. The produced methane mainly comes from the reactions of hydropyrolysis of biomass and secondary hydrogenation of tar. Moreover, the presence of alkali and alkaline earth metals matters was significant catalyzes on both reactions. The CH4 yield increased with elevating the operating temperature and optimal sweeping-gas flow rate is 150 mL/min. These results will be strengthening regulation of feedstocks-operating-products in the practical Biomass Fast Partial Hydropyrolysis technology.

Hybrid Particle Cluster CPFD Simulation in the Acceleration and Stabilized Sections of a Downflow Circulating Fluidized Bed

This study reports a numerical hybrid cluster simulation of a downer reactor unit using CPFD Barracuda VR software. This model includes an experimentally determined particle cluster size distribution to describe the particle cluster morphological changes as well as the cluster acceleration. Gas–solid flows are considered for a fluid catalytic cracking catalyst with an apparent particle density of 1722 kg/m3, flowing in a 5.08 cm-ID column with air at 300 K and with a pressure close to atmospheric. Conditions for the simulations consist of 1.0–1.6 m/s superficial gas velocity and 30–50 kg/m2 s solid mass flux ranges. Calculations developed involve the Hölzer–Sommerfeld and Ganser drag correlations. The proposed numerical model is validated by comparing experimental and simulated radial cluster particle velocities and particle volume fractions. Simulations developed show that the Hölzer–Sommerfeld drag function in conjunction with a proper selection of cluster geometry provides good cluster velocities and particle volume fraction values both in the cluster acceleration and cluster-stabilized flow unit sections.