Downer Reactor Research Articles

Experimental Study of a Novel Fast Gas-Solid Separator for Pyrolysis Reactors

From escalating global concern over the exhaustion of non-renewable energy sources comes an imperative need for the use of renewable resources. Biomass pyrolysis has emerged as a very promising renewable alternative for fuel and chemical production. Previous work has shown that the most effective biomass pyrolysis processes require a careful control of temperature and residence times, and downer reactors appear to be the best technology to achieve such goals. Ongoing research at the Institute for Chemicals and Fuels from Alternative Resources (ICFAR) has led to the development of a new downer reactor design for the pyrolysis of biomass feedstock. However, this process requires a gas-solid separator that achieves a minimum spread of the gas residence time distribution – i.e., as near to plug flow as possible.As a result, a novel integrated gas-solid inertial separator has been designed for implementation in the downer reactor. This new separator combines both primary separation and solids stripping within the same device. This is intended to decrease the product vapor residence time and to reduce the severity of vapor overcracking compared to other fast separation methods. The gas-solid separation section of the new device features a uniflow configuration and a vertical, axial entry with either swirl vanes or a deflector cone.In the current study, various geometry configurations and operating conditions were tested for their effect on separation efficiency and pressure drop. A 70 mm-diameter separator was tested under cold flow modeling conditions using silica sand, glass beads or FCC catalyst particles as solid material. The sand, glass beads and FCC particles had Sauter mean diameters of 200 ?m, 63 ?m and 43 ?m, respectively. Air was used in all cases with a gas inlet velocity varying from 0.95 to 13 m/s and solid loadings ranging between 0.075 and 19 wt/wt. The separation length was adjusted from 0.1 to 2 separator diameters.Initial cold flow experiments using silica sand revealed that the separator performance was influenced greatly by the separator geometry. The measured separation efficiency was greater using flow deflector blades (i.e. swirl vanes) than all tested cone deflectors. Solids recovery in excess of 99.99% was achieved. Particle collection efficiency decreased steadily as the separation length increased, but was in general not as strongly affected by solid loadings or gas inlet velocity. Experimental cold testing of glass beads in air gave typical efficiencies above 98.50%. Recovery of the lighter and smaller FCC catalyst was also very high, with a minimum measured efficiency of 98.80%.

The fluid dynamics of a downer fluidised bed using a cluster-based approach (CBA)

The fluid dynamics of a downer reactor were numerically resolved by adapting a mathematical conservation model. The mathematical model was based on the solid and fluid properties and physical characteristics using a cluster-based approach (CBA). Comparing the numerical results to the experimental data found in the literature indicated that the mathematical model could satisfactorily predict the experimental data. The mathematical simulation determined that there were three fluid dynamic areas in the downer reactor which were characterised by accelerated, slowed -down and fully-developed flow. The fully developed flow area in the downer decreased with increased gas surface speed keeping solid flux constant.

Slip Velocity in Downer Reactors: Drag Coefficient and the Influence of Operational Variables

Cluster formation in downer reactors is considered a most important factor influencing slip velocity between the flowing phases. This study investigates the formation of clusters using CREC-GS-Optiprobes. This probe is a nonintrusive device that records information from a well-defined optical measuring volume. This probe can provide accurate measurements tracking closely gas−solid flows in downer units. This study shows that cluster drag coefficients can be correlated as function of both Reynolds number and number of particles contained in agglomerates. This yields a close to linear relationship between cluster size and average slip velocity.

CFD–DEM simulation of gas–solid reacting flows in fluid catalytic cracking (FCC) process

The CFD–DEM coupled approach was used to simulate the complex gas–solid reacting flows in fluid catalytic cracking (FCC) processes accommodated in riser or downer reactors. Considering the solid catalyzed gas-phase reactions, the model particularly incorporated the descriptions for heat transfer behaviors between particles and between gas and particles, the instantaneous catalyst deactivation, and the lumped kinetics in the gas phase for FCC process, together with the governing equations for the hydrodynamics. The distinct advantage of the present approach is that the catalyst activity can be calculated in time by tracking the history of the particle movement with the occurrence of heat transfer and chemical reactions. The simulation results captured the major features of FCC process very well either in riser or in downer, which had reasonable agreement with the experimental data in the literature. The reduced selectivity to the desired intermediate products in risers, especially under high catalyst-to-oil ratios, can be clearly understood from the simulated backmixing behavior of solid catalysts and the deactivation of catalysts at different locations in the reactor, which caused the non-ideal reaction progress inside the reactor space. It can be concluded that this type of modeling approach forms a solid basis for the cross-scale modeling of general multi-phase catalytic reacting flows.

Inlet effect on the coal pyrolysis to acetylene in a hydrogen plasma downer reactor

AbstractCoal pyrolysis to acetylene in hydrogen plasma is a clean process for the coal utilization. A gas–solid downer reactor was employed to facilitate the high temperature reactions of coal pyrolysis in milliseconds. The effect of the inlet design on the coal injection was studied using CFD simulations, which were qualitatively compared with the cold model experiments in the prototype of a 2 MW hydrogen plasma reactor. The results revealed that the distribution of the coal particles near the inlet nozzles was significantly influenced by the layout of the flat‐shaped nozzles and the operating conditions. Accordingly the heating efficiency of the particles by the hot gas showed strong dependence on the inlet design. The hot model tests demonstrated that the reactor performance characterized by the concentration of acetylene in the product gas increased from ∼7.6 to 9.6% by optimizing the nozzle design, which indicated the critical role of the nozzle design in the coal pyrolysis process.

Mass transfer and reaction performance of the downer and its hydrodynamic explanation

AbstractCatalytic decomposition of ozone was carried out in a 15‐m high, 90‐mm inner diameter downer reactor. Radial distributions of ozone concentration at different axial elevations under different operating conditions were measured. The results were explained from the flow structure experimental data and corresponding signal analysis in time domain and frequency domain. The aggregation status of solid particles, relative movement between the gas and solids, and the dynamic behaviour of clusters were considered to have a joint influence on the mass transfer and reaction process in the downer.

Downer reactor: From fundamental study to industrial application

Downer reactor, in which gas and solids move downward co-currently, has unique features such as the plug-flow reactor performance and relatively uniform flow structure compared to other gas–solids fluidized bed reactors, e.g., bubbling bed, turbulent bed and riser. Downer is therefore acknowledged as a novel multiphase flow reactor with great potential in high-severity operated processes, such as the high temperature, ultra-short contact time reactions with the intermediates as the desired products. Typical process developments in industry have directed to (1) the new-generation refinery process for cracking of heavier feedstock to gasoline and light olefins (e.g., propylene) as by-products; and (2) coal pyrolysis in hydrogen plasma which opens up a direct means for producing acetylene, i.e., a new route to synthesize chemicals from a clean coal utilization process. This paper is to give a comprehensive review on the development of fundamental researches on downer reactors as well as the particular industrial demonstrations for the fluid catalytic cracking (FCC) of heavy oils and coal pyrolysis in thermal plasma.

Evaluation of downer reactor performance by catalytic ozone decomposition

Radial distribution profiles of ozone concentrations were measured along an 8.50 m high and 0.09 m inside diameter gas/solid co-current down-flow circulating fluidized bed (downer) to characterize the reactor performance. Tests were conducted under a series of operating conditions at room temperature and near atmospheric pressure, with FCC particles as the bed material. Results show that the concentration distribution of the ozone tracer gas correlates well with the flow structure of the downer. There is quite a uniform radial distribution of ozone concentrations in the core region of all tested axial sections in the fully developed region of the downer, except for the near-wall region where there is a sharp decrease in ozone concentration. And there exists a relatively significant non-uniform distribution in the entrance acceleration region of the downer.

A CFD–DEM study of the cluster behavior in riser and downer reactors

This paper presents a numerical study of the particle cluster behavior in a riser/downer reactor by means of combined computational fluid dynamics (CFD) and discrete element method (DEM), in which the motion of discrete particles is obtained by solving Newton's equations of motion and the flow of continuum gas by the Navier–Stokes equations. It is shown that the existence of particle clusters, unique to the solid flow behavior in such a reactor, can be predicted from this first principle approach. The results demonstrate that there are two types of clusters in a riser and downer: one is in the near wall region where the velocities of particles are low; the other is in the center region where the velocities of particles are high. While the extent of particle aggregation appears to be similar, the duration time for the first type in a downer is shorter than in a riser. Furthermore, it is demonstrated that the formation of clusters is affected by a range of variables related to operational conditions, particle properties, and bed properties and geometry. The increase of solid volume fraction, sliding and rolling friction between particles or between particles and wall, or damping coefficient can enhance the formation of clusters. The use of multi-sized particles can also promote the formation of clusters. But the increase of gas velocity or use of a wider bed can suppress the formation of clusters. The van der Waals force may enhance the formation of clusters when solid concentration is high but suppress the formation of clusters near the wall region when solid concentration is low.

Flow dynamics in a four-inch downer using solids concentration measurements

Local solids concentration fluctuations were measured in a long downer reactor (0.1 m ID, 9.3 m tall) using an optical fiber probe. Axial flow development and radial flow dynamics were analyzed using both statistical and chaos methods. Core, transition and annulus regions were identified and each region showed different flow behavior. Cross-sectional averaged chaos parameters were correlated to cross-sectional averaged solids holdup to develop relationships between non-linear flow dynamics and operating conditions.

Flowability Modification of Lactose Powder by Plasma Enhanced Chemical Vapor Deposition

The surface of 5.5 µm lactose particles was modified by gas phase reactions in a plasma enhanced chemical vapor deposition process to increase the flowability of the bulk solid. The particles were treated in the radio frequency plasma zone (RF) of a downer reactor, using a process gas mixture of oxygen, argon, and hexamethyldisiloxane (HMDSO). By increasing the flow rate of HMDSO (17–103 sccm) and the RF-power (50–200 W), respectively, the flowability can be increased up to a factor of 2 compared to the untreated substrate material. No significant change in flowability was observed by varying the oxygen-to-HMDSO ratio from 5 to 30.

Effect of Injecting Inert Particles on Coking Prohibition and Particle Velocity Uniformization in Downer Reactors

The coking observation and particle flow behaviour in both thermal plasma and cold plexiglas downers were investigated in a binary particle system formed by injecting coarse inert particles (carrying coke away and scouring wall) and fine coal powders into the downer reactor. The results demonstrate that this scheme is a rational selection to prevent coking on downer walls and improve particle velocity distribution along the radial direction. When injected coarse particles mixed with fine powders in downers, the fluctuation of local particle velocity in the radial direction becomes smaller and two peaks in the radial distribution of local particle velocity occur due to the improved dispersing character and flow structure, which are beneficial to the thermo-plasma coal cracking reaction and coking prevention.

Characteristics of gas–solid mass transfer in a cocurrent downflow circulating fluidized bed reactor

The characteristics of gas–solids mass transfer were studied in a downer reactor with the adsorption of CO2 tracer by activated charcoal particle. Axial distribution profiles of the CO2 tracer concentration were measured under several different operating conditions. Mass transfer coefficient and axial dispersion coefficient were evaluated with a non-linear regressing method (Marquardt method). It was found that the operating conditions such as solids circulation rate and gas velocity have complicated effects on the gas–solids mass transfer coefficient, and the effect of superficial gas velocity seemed to be more significant than that of solids circulation rate. The following empirical correlations were found to best fit the experimental data for axial dispersion coefficient, Ea and overall mass transfer coefficient, KF:Pe1=LugEa=0.226Re0.499m0.302Pe2=ugKFaL=0.000185Re0.618m−0.983with m = Gs/(ugρp) = vs(1 − ɛ)/(ugɛ).

Effect of empirical coefficients on simulation in two-scale second-order moment particle-phase turbulence model

A two-scale second-order moment two-phase turbulence model accounting for inter-particle collision is developed, based on the concept of particle large-scale fluctuation due to turbulence and particle small-scale fluctuation due to collision. The proposed model is used to simulate gas-particle downer reactor flows. The computational results of both particle volume fraction and mean velocity are in agreement with the experimental results. After analyzing effects of empirical coefficient on prediction results, we can come to a conclusion that, inside the limit range of empirical coefficient, the predictions do not reveal a large sensitivity to the empirical coefficient in the downer reactor, but a relatively great change of the constants has important effect on the prediction.

CFD Simulation of fluid catalytic cracking in downer reactors

A mathematical model has been developed for the simulation of gas-particle flow and fluid catalytic cracking in downer reactors. The model takes into account both cracking reaction and flow behavior through a four-lump reaction kinetics coupled with two-phase turbulent flow. The prediction results show that the relatively large change of gas velocity affects directly the axial distribution of solids velocity and void fraction, which significantly interact with the chemical reaction. Furthermore, model simulations are carried out to determine the effects of such parameters on product yields, as bed diameter, reaction temperature and the ratio of catalyst to oil, which are helpful for optimizing the yields of desired products. The model equations are coded and solved on CFX4.4.

Flow stability in a downer mixing chamber

Circulating fluidized-bed reactors operating in either riser or downer modes are used in many applications such as fluid catalytic cracking and fluid coking. In such applications, uniform contact of the liquid feed droplets and entrained particles is essential for minimizing heat and mass transfer resistances and achieving high yields of desirable products. This contact may be intensified in the mixing chamber of downer reactors. In order to achieve the best results, a mixing chamber should provide a stable liquid–solid dispersion. The objective of this study was to measure the stability of a liquid–solid flow in a downer equipped with a mixing chamber at its entrance. A novel technique was developed to determine the stability of the flow in the downer. Tribo-electric probes were used to obtain the local solids flowrates, and their signals were analyzed to determine the stability of the solids flow. A variety of different start-up conditions and injected liquids were tested to determine their effects on the stability of the system. This novel measurement technique has been proven to be a very reliable method of determining the stability. Although the system was non-chaotic, it was found that the system was not very stable locally, and was very sensitive to changes in start-up conditions. There were significant variations between replicate runs in the distribution of the local solids flux over the downer cross-section, which can be attributed to slight changes in start-up conditions. However, globally the system was found to be stable since, although shifted within the downer cross-section, similar structures were observed between replicate runs.

Measurement techniques to characterize the contact between injected liquid and circulating solids in a downer mixing chamber

Gas–solid fluidized-bed reactors operating in either conventional or circulating (riser or downer) modes are used in many applications such as fluid catalytic cracking and fluid coking. In such applications, uniform contact of the liquid feed droplets and entrained particles is essential for minimizing heat and mass transfer resistances and generating high yields of desirable products. This contact may be intensified in the mixing chamber of downer reactors. The objective of this study was to measure the quality of mixing between injected liquid feed and circulating solid particles in a downer mixing chamber. An efficient method was developed to determine the local quality of solid–liquid mixing on a short time scale. The measurement technique used two types of measurements in order to obtain the cross-sectional distribution of liquid-to-solid ratios. First, temperature measurements were used to characterize the solid/liquid distribution and, second, tribo-electric probes were used to obtain the local solids fluxes. The measurement technique has proven to be a very reliable and reproducible method to determine the extent of liquid and solid mixing resulting from a variety of injection configurations. Very good and rapid contact between sprayed droplets and particles can be achieved by using appropriate mixing chamber geometries where four or more liquid spray jets impact each other.

Hydrodynamics of High-Density Downer Reactors Using a Novel Solids Feeder

Downer reactors have many advantages over risers for reactions requiring very short residence time. However, its application for reactions where a high solid/gas ratio is required has been restricted by the low solids holdup in the fully developed region (typically less than one percent). In this paper, we present the performance of a 0.078 m diameter and 3.2 m long high-density downer reactor equipped with a recently developed novel solids feeding system.Experiments were carried out using fluid coke particles of particle density 1600 kg/m3 and mean diameter 133 mm, with solids fluxes up to 1400 kg/m2s and superficial gas velocities ranging from 0 to 6 m/s. The downer was operated under batch mode, with solids fed from a solids feeder installed inside a fluidized bed at the top of the downer and the bottom end of the downer connected to a receiving hopper. The solids flux was controlled by a butterfly valve installed 0.4 m below the downer entrance. Air was injected into the downer through four 45-degree-angled nozzles located right below the butterfly valve. Solids were lifted up from the bottom receiving-hopper to the upper fluidized bed through an external riser after each run.The cross-sectional average solids hold-up calculated from the integration of local measurements obtained from a capacitance probe was found to be a function of both the solids flux and superficial gas velocity. The solids hold-up increased with increasing solids flux at a given superficial gas velocity, but decreased with increasing gas velocity at a fixed solids flux. A cross-sectional average solids holdup of 16.5% was achieved at an axial position of 3.0 m below the air injection point under the operating condition of Gs=1400 kg/m2s and Ug=2.0 m/s. The shape of radial solids distribution varied not only with the superficial gas velocity but also with the solids flux in high-density downers. The radial solids distribution became more uniform under higher solids fluxes and lower gas velocities. The dense ring at the near wall region disappeared under the high-flux operating condition (Gs=1400 kg/m2s, Ug=2.0 m/s). The radial solids holdup profiles became less uniform, denser near the wall and more dilute and non-uniformly distributed in the central region, as the gas velocity increased from 3.0 to 6.0 m/s at a fixed solids flux of about 400 kg/m2s, which is significantly different from flow patterns reported in the literature under lower solids flux conditions.

CFD study of gas–solid behavior in downer reactors: An Eulerian–Eulerian approach

In this paper, a multiphase model applied to a downer reactor is proposed, where the gas–solid modeled with some special characteristics, such as: unsteady and 3D flow for two fluids on an Eulerian–Eulerian approach; dissipation of turbulent kinetic energy only on the gas phase modeled as real fluid; and the assumption that the solid phase behaves like a hypothetical fluid without shear stress from molecular causes. Studies based on first (HIPWIND) and second order (HIGHER UPWIND) interpolation schemes were carried out. The difference in the results using the two schemes was found to be negligible. The Computational Fluid Dynamic Technique (CFD) was adopted as a tool to predict downer reactor dynamic, comparing the proposed model with experimental data available in the literature. Good agreement between experimental data and model predictions was verified. Therefore CFD techniques proved to be a powerful tool to predict the fluid dynamic of inlet changing of downer reactors, and it can be used with success in chemical process optimization.

Short-time plasma surface modification of HDPE powder in a Plasma Downer Reactor – process, wettability improvement and ageing effects

The effectiveness of improving the wettability of HDPE powders within less than 0.1 s by plasma surface modification in a Plasma Downer Reactor is investigated. A correlation is revealed between the XPS results (O/C-ratio) and the wettability (contact angle, polar surface tension by capillary rise method). The O 2-content in the plasma feed gas has been adjusted for best wettability properties. XPS results indicate the formation of C O and COOH functional groups on the powder surface. The O/C-ratio increased from 0.0 (no oxygen on the non-treated powder) up to 0.15 for the plasma treated HDPE powder surface. With pure O 2-plasma treatment, a water contact angle reduction from >90° (no water penetration into the untreated PE powder) down to 65° was achieved. The total surface free energy increased from 31.2 to 45 mN/m. Ageing of treated powders occurs and proceeds mostly within the first 7 days of storage. Contact angle measurements and O1s/O2s intensity ratio data support that ageing is mainly a diffusion-controlled process. Nevertheless, XPS results show the presence of oxygen functional groups even after 40 days, which explains why the powder is still dispersible in water without any addition of surfactants.