Hydrodynamic simulations of gas–solid spouted bed with a draft tube

Flow behavior of particles in a two-dimensional spouted bed with a draft tube is studied using a continuous kinetic-friction stresses model. The kinetic stress of particles is predicted from kinetic theory of granular flow, while the friction stress is computed from a model proposed by Johnson et al. (1990). A stitching function is used to smooth from the rapid shearing viscous regime to the slow shearing plastic regime. The distributions of concentration and velocities of particles are predicted in the spouted bed with a draft tube. Simulated results compare with the vertical velocity of particles (Zhao et al., 2008) measured and in the spout bed with draft plates and solid circulation rate (Ishikura et al., 2003) measured in the spouted bed with a draft tube. The impact of the friction stress of particles on the spout, annulus, fountain and entrancement regions is analyzed in gas–solid spouted bed with a draft tube. Numerical results show that the gas flow rate through the annulus increases with the increase of the entrainment zone. The solids circulation rate decreases with the decrease of inlet gas velocity and the height of the entrainment zone. The effect of spouting gas velocity on distributions of concentration, velocity and particle circulation is discussed.     

TG–DSC analysis of straw biomass pyrolysis and release characteristics of noncondensable gas in a fixed-bed reactor

Under the double pressures caused by the energy shortage and environmental damage, to exploit the agricultural wastes and convert into available clean fuels are becoming more and more urgent in modern society. The aim of learning the pyrolysis characteristics of soybean straw and corn straw, the nonisothermal thermogravimetry and differential scanning calorimetry (TG-DSC) method was used in this work. The results showed that both of biomass feedstocks all underwent four different pyrolysis stages, with the increase in heating rate, the peak temperature shifted toward the high-temperature interval, and that the yield of bio-char also increased correspondingly; potassium had an influence on the thermal cracking of biomass, and that the existence form of potassium and impregnation increment of sylvite would result in the yield of bio-char was distinct. In addition, temperature and catalyst had a significant impact on the gaseous products of biomass pyrolysis. Increasing the pyrolysis temperature could enhance the yield of CO and H₂ and CH₄ content reached the maximum at 600°C. For both of the biomass, sylvite had a negative effect on the formation of CH₄, and H₂ content of soybean straw reached a maximum with 5% K₂CO₃ and corn straw with 5% KCl.     

Numerical simulation of micro-mixing in gas–liquid and solid–liquid stirred tanks with the coupled CFD-E-model

The coupled CFD-E-model for multiphase micro-mixing was developed, and used to predict the micro-mixing effects on the parallel competing chemical reactions in semi-batch gas–liquid and solid–liquid stirred tanks. Based on the multiphase macro-flow field, the key parameters of the micro-mixing E-model were obtained with solving the Reynolds-averaged transport equations of mixture fraction and its variance at low computational costs. Compared with experimental data, the multiphase numerical method shows the satisfactory predicting ability. For the gas–liquid system, the segregated reaction zone is mainly near the feed point, and shrinks to the exit of feed-pipe when the feed position is closer to the impeller. Besides, surface feed requires more time to completely exhaust the added H⁺ solution than that of impeller region feed at the same operating condition. For the solid–liquid system, when the solid suspension cloud is formed at high solid holdups, the flow velocity in the clear liquid layer above the cloud is notably reduced and the reactions proceed slowly in this almost stagnant zone. Therefore, the segregation index in this case is larger than that in the dilute solid–liquid system.     

Solid–liquid mass transfer in a gas sparging contactor equipped with a tube of circular fins

The solid–liquid mass transfer rate at a stack of circular fin surfaces in a gas sparging contactor was investigated. A diffusion-controlled dissolution technique of copper in an acidified chromate solution was employed. Variables studied included the number of actively exposed fins ranging from 5 to 20, pertinent physical properties of the solution, and air superficial velocity. Experimental data showed that the rate of the diffusion-controlled mass transfer increases with increasing superficial air velocity and decreases with increasing chromate solution acid concentration. Moreover, at relatively low superficial air velocity, increasing the number of actively exposed fins results into a continuous increase in the mass transfer coefficient. At relatively higher superficial air velocity, however, the mass transfer coefficient decreases in the 5–10 range of actively exposed fins and then reverts to increase in the 15–20 range. An empirical correlation relating the mass transfer j factor to Re_g, Fr, and a dimensionless height defined as the ratio of the height of actively exposed fins to the column equivalent diameter was developed based on the data generated in this study, with ±6.45% average deviation.     

Hydrodynamics of gas–solid flow in the circulating fluidized bed reactor for dry flue gas desulfurization

Process design and scale-up require a fundamental understanding of the hydrodynamics of gas–solid flow in the circulating fluidized bed flue gas desulfurization (CFB-FGD) reactor although the CFB system has been widely used in flue gas desulfurization and flue gas cleaning processes. The hydrodynamics in the CFB-FGD reactor model was investigated by pressure measurements and specially designed sampling probe based on three dimensionless groups for practicable similarity of industrial CFB-FGD process. The results show that the pressure drop in the venturi section is predominant as high as 60% of the total pressure drop and the total pressure drop significantly increases with the increasing external solid circulating rates at the same superficial gas velocity. Moreover, the measurements of radial solid mass fluxes show that the flow pattern in the CFB-FGD reactor is a typical core–annulus flow and this flow structure prevails until the top of the reactor. Reflux ratios are used to quantitatively evaluate the internal solid reflux in the reactor and the values in the low section of the reactor are much higher than those in the upper section.     

Experimental investigation and CFD simulation of liquid–solid–solid dispersion in a stirred reactor

For understanding the monosodium aluminate hydrate crystallization from the supersaturated aluminate solution containing red mud as the leaching liquor of bauxite, the liquid–solid–solid dispersion of a simulant system, i.e. glycerite, red mud and sand, in a stirred reactor has been experimentally investigated as well as simulated using computational fluid dynamics model (CFD) for the first time. The computational model is based on the Eulerian multi-fluid model along with RNG k–ε turbulence model, where Syamlal–obrien-symmetric drag force model (Syamlal, 1987) of the inter-phase momentum transfer between two dispersed solid phases is taken into account. A good agreement is obtained between the experimental data of solid distributions and the simulation results in the flow fields of liquid–solid–solid as well as liquid–solid systems. The solid suspension qualities of both liquid–solid and liquid–solid–solid systems in the stirred reactors with and without draft tube were also studied in detail based on mixing time, the standard deviation of solid concentration proposed by Bohnet and Niesmak (1980), the flow pattern and power number. The influence of the interaction between two dispersed solid phases on the suspension of red mud is found significantly greater than that of sand. The holdup of sand below the impeller is considerably larger than that above the impeller and the red mud dispersion approaches homogeneous in the reactor. The mixing time of liquid–solid–solid suspension is longer than that of liquid–solid suspension under the same conditions, and the mixing times of both systems in the stirred reactor with draft tube are longer than that in the reactor without draft tube. Furthermore, the distributions of sand and red mud in the reactor with draft tube were found less homogeneous than those without draft tube in most cases.     

Numerical simulation of local and global mixing/segregation characteristics in a gas–solid fluidized bed

Researches on solids mixing and segregation are of great significance for the operation and design of fluidized bed reactors. In this paper, the local and global mixing and segregation characteristics of binary mixtures were investigated in a gas–solid fluidized bed by computational fluid dynamics-discrete element method (CFD-DEM) coupled approach. A methodology based on solids mixing entropy was developed to quantitatively calculate the mixing degree and time of the bed. The mixing curves of global mixing entropy were acquired, and the distribution maps of local mixing entropy and mixing time were also obtained. By comparing different operating conditions, the effects of superficial gas velocity, particle density ratio and size ratio on mixing/segregation behavior were discussed. Results showed that for the partial mixing state, the fluidized bed can be divided into three parts along the bed height: complete segregation area, transition area and stable mixing area. These areas showed different mixing/segregation processes. Increasing gas velocity promoted the local and global mixing of binary mixtures. The increase in particle density ratio and size ratio enlarged the complete segregation area, reduced the mixing degree and increased the mixing time in the stable mixing area.     

Dynamic characteristics of the volatile cloudy zone in a gas–solid fluidized bed with hydrocarbon liquid spray

In a gas–solid fluidized bed with continuous hydrocarbon liquid spray, a volatile “cloudy zone” could be formed, defined as a dynamically steady liquid-affected zone, including droplets, wet particles, and the gas which passes through the zone. A new flow pattern with the dynamic coexistence of cloudy zone and non-cloudy zone (gas–solid zone), is accordingly established. The temperature, particle concentration, and particle velocity fields are measured in real-time via infrared thermography and particle imaging velocimetry, respectively. Results show that the area and range of central position of the cloudy zone illustrate a heavier fluctuant trend with the increasing velocity of liquid spray, and the main frequency of area fluctuation is close to that of the bubble rising. Moreover, the particle concentration and particle velocity in the cloudy zone are lower than those in the non-cloudy zone, breaking the conventional symmetrical distributions of hydrodynamic parameters of particles in a gas–solid fluidized bed.     

Profiles of solid fraction and heterogeneous phase structure in a gas–solid airlift loop reactor

The time-averaged and transient local solid fractions in a gas–solid airlift loop reactor (ALR) were investigated systematically by experiments and CFD simulations. To demonstrate the macro-flow pattern, the time-averaged local solid fractions in four regions of the ALR were measured by optical fiber probe under the conditions of different superficial gas velocities and particle circulation fluxes. The experimental results show that the lateral distribution of time-averaged local solid fraction is a core-annulus or heterogeneous structure in the three regions (draft tube, bottom region, particle diffluence region), but a uniform lateral distribution in the annulus. The operating conditions have different effects on the lateral distribution of time-averaged local solid fraction in each region. In the CFD simulation, a modified Gidaspow drag model considering the formation of particle clusters was incorporated into the Eulerian–Eulerian CFD model with particulate phase kinetic theory to simulate and analyze the transient local solid fraction and the two-phase micro-structures in the gas–solid ALR. The predicted values of solid fraction were compared with the experimental results, validating the drag model. The contours of transient flow field indicate that the flow field of the ALR should be divided into five flow regions, i.e., draft tube, annulus, bottom region, particle diffluence region and constrained back-mixing region, which further improves the understanding of the airlift reactor where only four divisions were determined from the experiments. The transient local solid fraction and its probability density function profoundly reveal the two-phase micro-structures (dilute phase and emulsion phase or cluster phase in the constrained back-mixing region) and explain the heterogeneous phenomenon of solid fraction in the ALR. The dilute phase tends to exist in the center of bed, while the emulsion phase mainly appears in the wall region. The results also indicate that the gas–solid ALR has the common characteristic of aggregative fluidization similar to that in normal fluidized beds. The simulated two-phase transient micro-structures provide the appropriate explanations for the experimental core-annulus macro-structures of time-averaged local solid fraction.     

Developments in the understanding of gas–solid contact efficiency in the circulating fluidized bed riser reactor:A review

In the last several decades, circulating fluidized bed reactors have been studied in many aspects including hydrodynamics, heat and mass transfer and gas–solid two phase contacting. However, despite the abundance of review papers on hydrodynamics, there is no summary paper on gas–solid contact efficiency to date, especially on high density circulating fluidized beds(CFBs). This paper gives an introduction to, and a review of the measurement of contact efficiency in circulating fluidized bed riser. Firstly, the popular testing method of contact efficiency including the method of heating transfer experiment and hot model reaction are discussed, then previous published papers are reviewed based on the discussed methods. Some key results of the experimental work are described and discussed. Gas–solid contact efficiency is affected by the operating conditions as well as the particle size distribution. The result of the contact efficiency shows that the CFB riser is far away from an ideal plug flow reactor due to the characteristics of hydrodynamics in the riser. Lacunae in the available literature have been delineated and recommendations have been made for further work.     

Nature and characteristics of gas–liquid flow regimes in a micro-packed bed reactor

Micro-packed bed reactors (μPBRs) have the advantages of high heat and mass transfer efficiency and excellent safety, and they have been successfully applied to hydrogenation and oxidation reactions. However, the study of gas–liquid flow regimes in the μPBR, which is essential for the mass transfer modeling and reactor scale-up, is still insufficient due to the limitation of micro-scale and complexity of capillary force. In this work, the flow regimes in the two-dimensional μPBR were systematically studied by visual method utilizing a high-performance camera. Four typical flow regimes and characteristics were captured, and flow regime transition was revealed. Effects of gas and liquid superficial velocities, liquid physical properties, and particle sizes on liquid spreading areal fraction and pressure drop were investigated. Flow regime transition correlation of churn flow and pseudo-static flow in the μPBR was provided for the first time based on the summary of the current and previous published results.     

The effect of the draft tube geometry on mixing in a reactor with an internal circulation loop – A CFD simulation

The main topic of this paper is to describe the effect of geometrical parameters on mixing time in a reactor with an internal circulation loop. The design of the draft tube inside the reactor is an important geometric parameter and has a big influence on two phase hydrodynamics as well as on mass transfer in the reactor. In the first section, the validation of the selected mathematical model is carried out. The results of experimental measurements (liquid velocity and gas hold-up) obtained on the laboratory scale reactor are compared with the CFD simulations. The CFD simulation (bubbly flow and mass transfer models) was made using COMSOL Multiphysics 3.5a. The results of the numerical simulation are in good agreement with the experimental data. In the second section, the study of mixing in the reactor is described with the new geometry of the draft tube. The standard experimental techniques for testing mixing processes are quite problematic because common tracers (soluble salts) have significant influence on two phase hydrodynamics inside the reactor. Though, an alternative nontrivial method had to be used. The split of the draft tube into two or three section has a significant effect on mixing (mass transfer) in the reactor.     

Prediction of solid–gas equilibria with the Peng–Robinson equation of state

In many applications related to Supercritical-Fluid (SCF) technology, solids are dissolved in SC fluids. Experimental data are now available for many systems but cannot cover all cases of potential practical interest. The prediction of solid solubilities in SC fluids, often in the presence of co-solvents, is useful for rational design of SCF extraction and related processes. Recently, thermodynamics has made considerable steps towards describing complex systems (gases with polar compounds) at high pressures using the so-called Equation of State/Excess Gibbs Free Energy (EoS/G^E) models. The success of these models is so far restricted to Vapor–Liquid Equilibria (VLE) for which they have been primarily developed and tested. In this work we evaluate such a predictive model, the LCVM EoS, for solid–gas equilibria (SGE) including systems with co-solvents. LCVM is chosen due to its success for VLE of asymmetric systems such as CO₂ with heavy alkanes and alcohols. Successful predictions are obtained for several solids as well as for some systems with co-solvents, but the results are less satisfactory for complex, multifunctional solids. A discussion of several factors, which affect modeling of SGE with cubic EoS, is included.     

Modelling and gas–solid mixing characterization in the jiggled bed reactor (JBR)

The jiggled bed reactor (JBR) is a new multiphase laboratory-scale microreactor consisting of a sealed container attached to a piston that is rapidly moved up and down by a pneumatically powered actuator. Particles and fluids in the container are mixed by this up and down motion instead of mechanical agitators or a fluidizing gas. This alternating motion provides intense mixing of all phases (gas, liquid, or solid) and intense contact between phases. Small rods inside the solids bed are heated by induction, allowing for excellent control of bed temperature and heating rate. The JBR is inexpensive and easy to operate, and it has been applied to catalytic gasification of bio-oil, biomass pyrolysis, activated carbon production, high-pressure oil hydrogenation, and hydrocarbons adsorption. Experiments demonstrated that solids mixing depends on the reactor platform maximum accelerations during both up and down strokes. A minimum acceleration, 55 m²/s for the tested JBR, was required to achieve good solids mixing. A physical model was developed to predict the reactor platform motion and its maximum acceleration. It requires a few preliminary experiments (around 10) to obtain its four empirical parameters. The model can then determine how to adjust the actuator compressed air pressure or modify the equipment to eliminate performance bottlenecks.     

Fundamental studies on the hydrodynamics and mixing of gas and solid in a downer reactor

The effect of flow direction on hydrodynamics and mixing in the upflow and downflowcirculating fluidized beds is discussed in details.Similar profiles of gas and solids velocities andsolids concentration are found in both risers and downers.When the flow is in the direction ofgravity(downer),the radial profiles of gas and particle velocity are more uniform than that inthe riser,the solids mixing is very small and the flow pattern approaches plug flow,while theflow is against gravity(riser),the solids backmixing significantly increase and the flow pattern isfar from plug flow.Among many of factors the flow direction has the largest influence onhydrodynamics and axial mixing of gas and solids.     

Numerical simulations and validation of gas–solid flows in a fluidized-bed roaster based on the CFD-DPM model

In view of the complex gas–solid flow characteristics in a fluidized-bed roaster, the discrete phase model (DPM) provided by ANSYS software was used to numerically analyze the model using a coupled algorithm. The asymmetric flow phenomenon in the transition section at the top of the furnace was found to be unfavourable to the gas–solid flow, and an inverted U-shaped furnace structure was proposed to optimize the transition section at the top of the furnace. A large cold experimental setup was built to verify the model. The results showed that the air velocity is mainly in the axial upward direction; under the action of the gas, the solid particles and the air velocity are basically in the same direction. The main furnace and subfurnace connection section changes the movement of the gas–solid mixture, and its unreasonable structure leads to the asymmetric flow phenomenon of the gas–solid fluid at the top of the furnace. Compared with the previous furnace structure, the uniformity of gas–solid flow in the optimized ‘inverted U-shaped’ structure has been significantly improved. The cold experimental results are in good agreement with the numerical simulation results, which verifies the accuracy of the proposed model.     

Simulation of gas–solid two-phase flow in the annulus of drilling well

This paper investigates the hydrodynamic behavior of gas–solid two-phase flow in the annular space of an air drilling well under different arrangements by using three-dimensional approach. Two-fluid model is used to solve the governing equations in the Eulerian–Eulerian framework. Effect of eccentricity and drill pipe rotation on the pressure drop, volume fraction and velocity profile are examined. The results are compared with available data in the literature and good agreement is found. The results show that the presence of solid particles in the annulus change the air velocity profile significantly and create two off-center peaks velocity close to the walls instead of one peak velocity in the middle. Eccentricity of drill pipe makes more accumulation of the cuttings in the smaller space of the annulus. Increasing the eccentricity increases pressure drop due to impact of particles with annulus wall and also particles collision with each other. Rotation of the drill pipe shifts maximum air velocity location toward smaller space of the annulus which results more uniform cutting distributions in the annulus and improvement in their transportations. Pressure drop in the annulus increases as eccentricity and rotation of drill pipe increase.     

An assessment of the ability of computational fluid dynamic models to predict reactive gas–solid flows in a fluidized bed

Fine grid, two dimensional simulations of reactive gas–solid flows occurring in a fluidized bed reactor were carried out using the Eulerian multi-fluid kinetic theory of granular flow (KTGF) approach in the commercial flow solver, ANSYS FLUENT 12.1. The fuel reactor of a pilot scale Chemical Looping Combustion rig, operated in the bubbling fluidization regime at the Vienna University of Technology, was simulated. Grid dependence studies were carried out as well as sensitivity studies to the fuel inlet condition and the inclusion of gas phase turbulence. Simulations could not accurately reproduce the experimental trend for the case when highly reactive nickel oxide was used as the oxygen carrier material, but in general satisfactory quantitative agreement was observed. The failure to correctly capture the experimental trend was primarily attributed to the fine length-scales at the feed gas inlets not being adequately resolved even at the finest grid investigated. The trend quickly worsened when coarser grids were used, indicating that the generality of the model is lost when grid dependence effects are present. A number of possible dimensional effects were also discussed. Subsequently, the model was used to successfully capture another experimental trend obtained with a much less reactive ilmenite oxygen carrier material. The model captured this trend correctly because the reaction was now limited by the reaction rate and not by species transfer to the large scale gas-emulsion interfaces. Results were therefore not as sensitive to the correct hydrodynamic modelling of the interface, especially near the gas inlets, and the model retained its generality over a wide range of operating conditions.     

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 3 D,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.