Gas-solid Distribution Research Articles

Combing mobile electrical capacitance tomography with Fourier neural operator for 3D fluidized beds measurement

AbstractDespite the practical importance, 3D measurements of gas–solid distribution in fluidized beds calls for further breakthroughs. Here an approach combing a recently developed mobile electrical capacitance tomography (ECT) sensor with Fourier Neural Operator (FNO) is developed, in which the fluidized bed is divided into a series of cross‐sectional slices along axial direction. At any given instant, the gas–solid distribution in one slice is measured by mobile ECT and the others, meantime, are predicted by FNO pre‐trained using experimental data. We verified this approach via computational fluid dynamics (CFD) simulations and experimental measurement of static object (i.e., cone, cylinder, and sphere) in fluidized bed. Following we applied this approach to direct measure 3D gas–solid distribution in a bubbling fluidized bed, and found that satisfactory image correlation coefficients and solid concentration average absolute deviation could be obtained, which indicates the proposed approach is promising for 3D fluidized bed measurements.

Modeling and optimizing gas solid distribution in fluidized beds

In fluidized bed reactors, uniform distribution of gas and solids is critical to achieve the desired hydrodynamic and kinetic performance. For FCC units, Technip Energies uses a proprietary distributor design to terminate both reactor risers and spent catalyst lift lines to distribute the exiting gas-solid stream into a fluidized bed.In Technip Energies' Resid FCC unit, air and spent catalyst are distributed from an upward flowing stream into a catalyst bed for coke combustion on catalyst. Proper distribution is key to achieve a uniform coke burn and an even temperature profile throughout the bed. In PropyleneMax™ Catalytic Cracking (PMcc™) technology, the reactor riser also terminates in a fluidized bed, requiring proper distribution of the hydrocarbon vapor and solid mixture into the reactor bed, which is key to providing adequate contact for further cracking to valuable products. This paper discusses Technip Energies' gas-solid distribution technology and the recent work performed to develop an improved distributor for use as a riser termination device in both the PMcc reactor and the RFCC regenerator to achieve improved performance. The use of CFD modeling to screen and evaluate the initial concepts and optimize the final selected concept are presented here. The performance matrices used to quantify the benefits of the optimized distributor design with respect to the existing one in the reactor application are highlighted, showing the benefits of improved gas solid distribution on hydrodynamic and kinetic performance. The paper presents a systematic approach to develop a novel industrial device using physics-based computational tools.

Desulfurization characteristics of slaked lime and regulation optimization of circulating fluidized bed flue gas desulfurization process—A combined experimental and numerical simulation study

Circulating fluidized bed flue gas desulfurization (CFB-FGD) process has been widely applied in recent years. However, high cost caused by the use of high-quality slaked lime and difficult operation due to the complex flow field are two issues which have received great attention. Accordingly, a laboratory-scale fluidized bed reactor was constructed to investigate the effects of physical properties and external conditions on desulfurization performance of slaked lime, and the conclusions were tried out in an industrial-scale CFB-FGD tower. After that, a numerical model of the tower was established based on computational particle fluid dynamics (CPFD) and two-film theory. After comparison and validation with actual operation data, the effects of operating parameters on gas–solid distribution and desulfurization characteristics were investigated. The results of experiments and industrial trials showed that the use of slaked lime with a calcium hydroxide content of approximately 80% and particle size greater than 40 μm could significantly reduce the cost of desulfurizer. Simulation results showed that the flow field in the desulfurization tower was skewed under the influence of circulating ash. We obtained optimal operating conditions of 7.5 kg·s−1 for the atomized water flow, 70 kg·s−1 for circulating ash flow, and 0.56 kg·s−1 for slaked lime flow, with desulfurization efficiency reaching 98.19% and the exit flue gas meeting the ultraclean emission and safety requirements. All parameters selected in the simulation were based on engineering examples and had certain application reference significance.

The Influence of Shear-Thinning Characteristics on Multiphase Pump Vortex Structure Evolution, Pressure Fluctuation, and Gas-Solid Distribution

The Influence of Shear-Thinning Characteristics on Multiphase Pump Vortex Structure Evolution, Pressure Fluctuation, and Gas-Solid Distribution

Research on Pollutant Emission Characteristics of Typical Biomass-fired Boilers

The combustion of biomass fuel boilers not only produces inorganic pollutants such as carbon oxides, nitrogen oxides, sulfur compounds, but also organic pollutants such as VOCs and PAHs, which directly affect the global biogeochemical cycle and atmospheric changes. Compared with foreign countries, there is relatively less research and related standards on the emission of organic pollutants from biomass fuel boilers in China. Therefore, it is necessary to study the emission characteristics of organic pollutants from biomass boilers, which can provide reference for the establishment of organic pollutant emission standards for biomass fuel boilers in China.The emission characteristics of volatile organic compounds(VOCs) and polycyclic aromatic hydrocarbons(PAHs) organic pollutants in the flue gas of three typical biomass-fired boilers in Chongqing were studied, and the emission concentrations of particulate matter, sulfur dioxides(SO2), and nitrogen oxides(NOX) were measured.The results show that the sulfur dioxide emissions from the three types of biomass-fired boilersrs are lower than the local standards in Chongqing, while nitrogen oxides and particulate matter have a phenomenon of exceeding the emission limits of the local standards in Chongqing. The main components of VOCs emitted from biomass powder and biomass particle combustion are OVOCs and aromatic hydrocarbons, while the main components of VOCs emitted from wood biomass combustion are alkynes. The gas-solid distribution pattern of PAHs during the combustion of three types of biomass is basically consistent. Low ring PAHs are mainly enriched in the gas phase, while medium and high ring PAHs are mainly enriched in particulate matter.

Effect of back pressure on gas-solid distribution characteristics of cyclone distributor applied in powdered coal-fired circulating fluidized bed combustion system

The cyclone distributor is an essential part of the powdered-coal-fired circulating fluidized bed (PC-CFB) combustion system, and its gas-solid distribution characteristics are significantly affected by the back pressure of the lower outlet. A four-way coupling method using a dense discrete phase model (DDPM) was adopted to simulate the cyclone distributor. The results indicate that the effect of the back pressure on the axial gas velocity is more pronounced compared to that on the tangential gas velocity. With the increase in the back pressure, the particle separation efficiency decreases monotonically, while the rate of change gradually increases. Meanwhile, the gas separation efficiency increases as the back pressure becomes higher. Furthermore, increasing back pressure improves the particle concentration at the outlets of the cyclone distributor. To maintain high particle separation efficiency and ensure the safe operation of the boiler, it is recommended to set the back pressure between 1000 Pa and 2000 Pa.

Simulation analysis of gas–solid flow characteristics and water evaporation in flue gas semi‐dry desulphurization process based on CPFD method

AbstractThe gas–solids flow in an industrial‐scale semi‐dry method desulphurization tower is simulated by the computational particle fluid dynamics (CPFD) approach. Compared with previous studies on desulphurization towers, this study focuses on analyzing particle distribution characteristics such as particle volume fraction, temperature distribution, and residence time. The simulation fully considered the particle–fluid, particle–particle, and particle–wall interactions in the desulphurization tower. Based on these considerations, the effects of flue gas inlet velocity and temperature on the gas–solid distribution characteristics of the desulphurization tower are simulated. An optimization scheme for adjusting the gas–solid flow in the desulphurization tower is proposed. The research results show that the error between the CPFD simulation data and experimental data is small and the changing trend is consistent. The particles in the bed of the desulphurization tower show a typical core–annulus flow. The distribution of gas and particles in the bed has a serious deviation with the increase of the flue gas inlet velocity and temperature. As the axial height of the desulphurization tower increases, the flue gas velocity, temperature, particle concentration, and water vapour distribution in the bed become more uniform. The relatively stable operating conditions for the gas–solid flow in the desulphurization tower is that the flue gas inlet velocity and temperature are 15 m/s and 393 K, respectively. Under these operating conditions, the pressure loss caused by the venturi accounted for 73.6% of the total pressure loss of the desulphurization tower. When the particle radius is between 0–150 μm, the particle size and the flue gas inlet velocity have the greatest influence on the particle residence time. Finally, the distribution of gas and particles before and after the adjustment of the desulphurization tower is compared, which showed that adjusting the bottom structure of the desulphurization tower could optimize the gas–solid flow.

MP-PIC simulation on CO2 gasification of biomass in a pilot plant circulating fluidized bed gasifier

CO2 gasification of biomass offers a potential carbon-negative solution which combines biomass-to-energy conversion with carbon capture and utilization. In this study, biomass gasification with CO2 in a pilot plant circulating fluidized bed (CFB) gasifier is numerical investigated via a reactive multiphase particle-in-cell (MP-PIC) model. Gas-solid hydrodynamics and thermochemical characteristics under different percentages of CO2 in gasifying agent (mixed with air from 0 to 100 wt%) are comprehensively analyzed, in terms of the biomass conversion, bed temperature, gas species distributions, chemical reaction rates, product gas yields, lower heating value of product gas (LHVgas) and cold gas efficiency (CGE). Simulation results indicate that the CO2 percentage in gasifying agent has insignificant influence on gas–solid distribution, while it directly affects the char conversion and product gas compositions. CO2 gasification favors the CO production, increases the CO/H2 ratio in product gas and mitigates the CO2 emission. However, increasing the CO2 percentage in gasifying agent results in a decreased char conversion rate and reduced bed temperature, which restricts the use of pure CO2 as gasifying agent. The optimal CO2 percentage occurs to 60 wt% where the gasifier has the best thermochemical performance, i.e., the highest compositions of combustible gases, the largest LHVgas and CGE. This research sheds light on biomass gasification with CO2 in a large-scale CFB facility, and would be helpful for the design and scale-up of this technology.

Catalyst-scale investigation of polydispersity effect on thermophysical properties in a commercial-scale catalytic MTO fluidized bed reactor

The methanol-to-olefin (MTO) process is commercialized worldwide, yet the understanding of in-furnace phenomena is still lacking. Accordingly, this work explores the multiphase flow and thermochemical characteristics in a commercial-scale MTO fluidized bed reactor using a three-dimensional multiphase particle-in-cell model. After model validations, the intrinsic relationship of solid transportation and thermochemical properties is elucidated together with the study of the polydispersity effect on reactor performance. The non-uniform gas-solid distribution in the reactor demonstrates the turbulent fluidization regime. Heat transfer coefficients (HTCs) of about 300 and 200 W/(m2·K) are observed in the medium and upper regions, respectively. The small velocity, particle Reynolds number, temperature, and HTC correspond to those large-size catalysts in the dense region where particles are nearly close packing with a large solid volume fraction. Increasing particle size distribution (PSD) width increases the catalyst HTCs and the mass fraction of C2H4, promoting reactor performance. Besides, increasing the PSD width enlarges gas viscosity, gas specific heat capacity, and gas thermal conductivity but decreases gas density.

Optimization of annular distributor for industrial riser reactor with weakened catalyst attrition

AbstractCatalyst attrition and nonuniform gas–solid distribution were observed in an oil refining and chemical enterprise with straight pipe as C4 mixture gas feed zone, which induced unfavored catalytic reaction and unqualified products. In this work, a fluidized attrition experiment circulating system consisting of a riser reactor at a bench scale was constructed to implement fluidized attrition experiment. Four annular pipe distributors were designed as C4 mixture gas feed zones and compared with traditional straight pipe in order to gain insight into the catalyst attrition condition in the riser reactor. At the same time, computational fluid dynamic technique was applied to investigate the gas–solid mixing situation in the riser reactor with five different C4 mixture gas feed zones. Structure four of annular distributor with nine nozzles distributed as well as an inclination of 30° displayed a catalyst attrition of 5.7% in 12 h of fluidized circulation and possessed the best gas–solid mixing condition as well as the highest catalyst volume fraction distribution in the main catalyst reaction space. Therefore, structure four was selected as C4 mixture gas feed zone for industrial riser reactor. Flow behavior in the industrial riser reactor with the optimized C4 annular distributor was further investigated via numerical simulation and compared with traditional straight pipe as C4 mixture gas feed zone so as to provide reference for the optimization of industrial riser reactor. When annular distributor was applied, propylene yield enhanced to 19.86%, and meanwhile, enhancement of catalyst particle with size smaller than 40 μm obviously decreased in comparison with traditional riser reactor.

A computational fluid dynamics study of gas–solid distribution of Geldart Group B particles in a swirling fluidized bed

The study's main goal, as reported in this research paper, was to investigate the gas–solid flow behaviors in a twin cyclonic fluidized-bed combustor (T-FBC) in swirling mode under a range of operating conditions. Silica sand in three size ranges, 300–500 μm, 500–710 μm, and 710–1000 μm, with 1700 kg/m3 solid density was used as the bed material in the study's experimental tests and its computer simulations. The Eulerian–Eulerian approach for two-phase fluid flows was selected to investigate the cold hydrodynamic characteristics of gas–solid particles, at a constant ambient temperature, in the fluidized-bed combustor. The three different turbulence models; the standard k-ε model, the renormalization group (RNG) k-ε model, and the Reynolds Stress Models (RSM) were studied in order to select the most appropriate model. Finally, the RNG k-ε turbulence model was applied in all simulations for both phases of our work. The computational simulations were performed using the same parameters and operating conditions as in the experimental tests, with the validity of the computational simulations thus experimentally verified. In the simulation results, it was shown that the increasing size of the bed particles, the solid hold-up tended to decrease, the time duration for initializing full-bed fluidization increased, and the bed fluctuation frequency decreased. When increasing excess air (EA), the simulation results showed the opposite trend. The secondary to total air ratio (S/T) had substantial effects on time duration to initialize bed fluidization and the bed fluctuation frequency in the swirling fluidized bed; however, it had slightly effects on the radial solid hold-up and radial solid velocity in the dense bed region.

Influence of burden profile on gas-solid distribution in COREX shaft furnace with center gas supply by CFD-DEM model

This paper presents a numerical study of the influence of burden profile on gas-solid distribution in a COREX shaft furnace through CFD-DEM model. The model takes into account the transient features of gas and particle flow under evaluation of five burden profiles, and the results show that the burden profile has a minor effect on the burden descending velocity. Caused by the segregation, the overall maximum and minimum porosities in the packed bed are observed in the case of ‘V' shape and ‘Flap’ shape profile, respectively. The mean square deviation of gas velocity in furnace under ‘W' shape profile is relatively small. Further considering the uniform descending behavior of solid in furnace, the ‘W' shaped burden profile should be practiced in actual production to achieve the uniform of gas-solid flow. The lowest pressure drop can be found in the case of ‘V' shape burden profile. When considering the pressure drop in practical production, the ‘V' shape burden profile should be given priority in practice.

CFD study of nonuniformity of gas-solid flow through a chemical looping combustion system with symmetrical series loops

Multiple identical paths are usually connected to form circulating loops in many industrial reactors including chemical looping combustion (CLC), however the flow pattern may be highly nonuniform in these identical paths, compromising the stable operation. In this study, by using the Eulerian multifluid model, the nonuniformity in a CLC apparatus is studied. First, the symmetry of computational domain is verified by simulation and the model setting is validated by experimental data. Then the typical nonuniformity phenomenon is discussed and evaluated in terms of bed inventory evolution and gas-solid distribution in symmetrical parts. The effects of key operating parameters including total bed inventory, gas aeration rate, and wall roughness on nonuniformity degree are then studied. The simulating results show that the nonuniformity degree between the two symmetrical parts increases with the increase of total bed inventory and gas aeration rate and this trend will be intensified in the predictable short-term future. And through the analysis of pressure fluctuation, the instability of the system is aggravated. However, wall roughness shows an indefinite influence and tendency. This work provides an insightful understanding of nonuniformity and will help further optimise symmetrical CLC systems.

Gas–Solid Distribution Theory in a Pulsed Fluidized Bed Based on the Intermediate Phase

Fluidized bed technology is widely used in chemical industries. The introduction of pulsating energy into a traditional fluidized bed can effectively reduce the generation of bubbles and improve the fluidization quality. The accuracy of gas–solid distributions has a significant influence on the density control of fluidized beds. Here, a gas–solid distribution model and bed expansion prediction model were established with the wake taken as the intermediate phase to make up for the lack of a gas–solid distribution theory in a pulsed fluidized bed. The correlation coefficient of the theoretical model was validated using data obtained from collapse experiments. The linear fitting correlation between the bed expansion height H(p) and correction coefficient Y is quite good. The error of the modified model of the bed expansion ratio was found to be within 20%, which enables it to provide theoretical guidance for industrial applications of pulsed fluidized beds.

Review of Computational Fluid Dynamics Studies on Chemical Looping Combustion

Abstract Chemical looping combustion (CLC) is an attractive technology to achieve inherent CO2 separation with low energy penalty. In CLC, the conventional one-step combustion process is replaced by two successive reactions in two reactors, a fuel reactor (FR) and an air reactor (AR). In addition to experimental techniques, computational fluid dynamics (CFD) is a powerful tool to simulate the flow and reaction characteristics in a CLC system. This review attempts to analyze and summarize the CFD simulations of CLC process. Various numerical approaches for prediction of CLC flow process are first introduced and compared. The simulations of CLC are presented for different types of reactors and fuels, and some key characteristics including flow regimes, combustion process, and gas-solid distributions are described in detail. The full-loop CLC simulations are then presented to reveal the coupling mechanisms of reactors in the whole system such as the gas leakage, solid circulation, redox reactions of the oxygen carrier, fuel conversion, etc. Examples of partial-loop CLC simulation are finally introduced to give a summary of different ways to simplify a CLC system by using appropriate boundary conditions.

Fluidization of micro-interconnected fluidized beds for chemical looping

The micro-interconnected fluidized bed (MIFB) was designed to improve the evaluation condition of oxygen carrier, which is difficult to perform in a lab-scale interconnected fluidized bed because of the large demand for bed inventory. The reduction of bed inventory in the MIFB was mainly achieved by the appropriate miniaturization of the reactor size, in which the wall effect and operating flexibility should be taken into consideration. With hematite serving as the oxygen carrier, stable and flexible fluidization could be realized with 342.9g of bed inventory. Internal perforated plates were arranged in the middle of the reactor to improve gas–solid distribution, which also could restrain the slugging formation and increase the particle residence time by 28.9%. A different fluidization phenomenon was observed in this two-stage reactor in which the particle fluidization was reconstructed in the upper chamber. Throughout 48h of cold operation, the hematite oxygen carrier attrition rate was evaluated as 0.151wt.%/h corresponding to 660h lifetime, where 12.5% of particle attrition was contributed by internal perforated plates. An excellent fitting performance was found between the pressure difference in the risers and the upward particle flow, but the correction factor should be adjusted according to the fluidization flow.

Effect of baffles on bubble behavior in a bubbling fluidized bed for chemical looping processes

A bubbling fluidized bed is generally used as the fuel reactor in chemical looping processes. However, insufficient gas–solid contact remains a steadfast problem. As such, we propose the use of internal baffles to split bubbles at a high gas velocity and to restrict the movement of large-scale solids in the fluidized bed. We investigated the effect of baffle arrangement on the bubble size and energy, pressure gradient, particle distribution, and particle velocity in a fluidized bed using a computational particle fluid dynamics simulation based on our experimental results. We discuss the main influencing factors, which include presence of caps, number of baffles, and the baffle opening ratio. The baffle structure with caps efficiently broke the large bubbles without creating jet flow. Three baffles were deemed suitable in the bed owing to the small bubble size and uniform gas–solid distribution in each compartment. We selected a baffle opening ratio of 20.5%, which improved the flow pattern without causing a significant increase in the pressure gradient and particle velocity through the baffles. We also discussed the potential for scaling up this baffled fluidized bed for industrial applications.

Investigating bubble dynamics in a semi-cylindrical gas-solid fluidized bed

Bubble dynamics (i.e., bubble shape, bubble size, bubble rise velocity) and bed voidage in a semi-cylindrical gas-solid fluidized bed at various superficial gas velocities and particle sizes were measured by fiber optic probe and image analysis. Measurements in the semi-cylindrical bed were compared with those in the cylindrical bed to investigate the similarity of two fluidized beds and verifying whether a semi-cylindrical bed can be used for measuring hydrodynamic properties of fluidized beds. Experiments were carried out in two Plexiglas semi-cylindrical and cylindrical fluidized beds of 14 cm in diameter at ambient pressure and temperature. Glass beads of various sizes (120, 290 and 450 μm) were used as the bed material. The superficial gas velocity was varied in the range of 0.2–0.8 m/s and both beds were operating in the bubbling fluidization regime. It was shown that increase in the particle size leads to decrease in the emulsion phase fraction while increases the fraction of bubble phase. At low gas velocity, the semi-cylindrical fluidized bed contains normal bubbles while at higher superficial gas velocity, elongated bubbles with normal rising velocity can be found. Increase in the particle size leads to increase in the aspect ratio of bubbles. It was found that the average bubble size and rise velocity increases with increasing particle size and superficial gas velocity. Results indicated that the gas-solid distribution, bubble rise velocity and bubble size in both beds are similar and geometry of the cross-section of column (semi-circular and circular) has no significant effect on these parameters. It was shown that the hydrodynamics of both beds are very similar and the difference is negligible. Since measuring the bubble size and rise velocity in the semi-cylindrical bed can be easily done by visual observation compared to those in a cylindrical bed, using a semi-cylindrical fluidized bed instead of two dimensional fluidized bed in the laboratory scale is the preferred choice which provides bubble dynamics close to that in a three-dimensional bed.

Stability Analysis of Gas–Solid Distribution through Nonidentical Parallel Paths

Gas–solid fluidized beds in parallel can share some common units yet couple different reactions without extra pressure drop. The bottleneck, however, in such a parallel system, especially with noni...

Numerical simulation of a 3D full loop iG-CLC system including a two-stage counter-flow moving bed air reactor

A three-dimensional two-fluid model is established for an in-situ gasification chemical looping combustion system to predict its dynamic characteristics and non-uniformity in gas-solid distribution. In the experiment, the fuel reactor (FR) and air reactor (AR) were designed as a high-velocity riser and a novel low-velocity moving bed, respectively. Simulations results show that the predicted pressure distributions agree reasonably well with the experimental data. AR operation did not show any obvious influence on oscillations in the full-loop particle circulation. Each stage of the moving bed AR is found to operate better at relatively lower gas fluidizing number, and the maximum allowable fluidizing number is estimated to be around 0.7. With increase in fluidizing number in AR, gas concentration, gas velocity and solid velocity uniformity index increase, while solid concentration uniformity index decreases. Compared to the parallel arrangement of gas distributors and exhausts, gas concentration and velocity uniformity indices are smaller except for the near-exhaust section under the vertical arrangement of gas distributors and exhausts. Increase in first-stage or second-stage AR outlet pressure has an opposite influence on gas leakage in both the first-stage AR and second-stage AR.