Increase In Superficial Gas Velocity Research Articles

An Inlet Area for Particle Mixing in a Two-Dimensional Fluidized Bed Using a CFD-DEM Model

Fluidized beds are widely used in many industries since they are effective in mixing process. The distinct element method (DEM) has recently received more attention for investigating the phenomena of multiphase flow because the technique is effective in gathering detailed information on the complex phenomena without physically disturbing the flows. A CFD-DEM model has been developed for calculating the minimum fluidization velocity and particle mixing in a two-dimensional fluidized bed. In this research, the inlet area on the particle mixing was investigated. From the result, it was indicated that the developed CFD-DEM model was performed adequately in predicting the phenomena in a two dimensional fluidized bed. The minimum fluidization velocity predicted by the developed model agreed well with the theory and correlation of Grace. Based on Lacey mixing index, it was found that the mixing index increased with an increase in time and superficial gas velocity. In addition, the inlet area of 20% gave a good mixing.

Simulated second-order moments of clusters and dispersed particles in riser

The second-order moment model for solid phase combining with large eddy simulation for gas phase was used to simulate distribution of moments and volume fraction of particles in the riser. This study shows that the simulated solids volume fractions along height using a two-dimensional model are in agreement with experiments. The axial second-order moments are larger than the lateral second-order moment by 2–3 times. The second-order moments of clusters and dispersed particles are calculated. Simulations show that the second-order moment of dispersed particles is about 3.9 times larger than that of clusters in axial direction. The simulated second-order moments of clusters and dispersed particles increase with the increase of superficial gas velocity and solids mass flux. The particle–particle collisions result in the reduction of anisotropic profiles for the solid phase in risers. Anisotropy of the normal second-order moments increases with the increase of superficial gas velocity. Both axial and lateral second-order moments increase with the increase of diameter of particles.

Effect of Vibrating Sparger on Mass Transfer, Gas Holdup, and Bubble Size in a Bubble Column Reactor

Abstract Bubble size has a key role in gas holdup and mass transfer in bubble column reactors. In order to have small and uniform bubbles, a new structure was designed; the reactor operates in two modes, with vibrating sparger and conventional bubble column in which sparger is fixed. In vibrating mode, the sparger vibrates gently during gas entering. The vibrating sparger performs like a paddle, resulting in a forced recirculation of gas–liquid inside the reactor; moreover, the bubble detachment is accelerated. The superficial gas velocity was between 0.003 and 0.013 ms− 1, and the vibration frequency was changed between 0 and 10.3 Hz. The bubble size was measured at three various positions of the reactor height by photographic method and using MATLAB 7.0.1 software. The mass transfer coefficient was determined by means of the dynamic gassing-out method. The results show that the bubbles were bigger in vibrating mode than those working without vibration. The bubble size decreases with increase in height from sparger. Gas holdup increased with increase in superficial gas velocity and vibration frequency. The effect of vibration increased the gas holdup with an average of 70% for all superficial gas velocities. Volumetric mass transfer coefficient was almost stable as vibration frequency increased.

Kinetic characteristics of pulverized coal combustion in the two-phase flow

The kinetic characteristics of pulverized coal combustion in the two-phase flow were studied using the micro fluidized bed kinetic analyzer (MFBKA) so as to get closer to the actual working condition. On the basis of the performance of CO and CO2 emissions, the effect of various operating parameters, such as reaction temperature, superficial gas velocity, coal particle size, oxygen concentration and coal properties, on the combustion rate was investigated. The yield ratio of CO to CO2 remained 1:1 until the reaction temperature exceeding 850 °C and then decreased gradually, indicating that the combustion mechanism changed at such temperature. The increase of superficial gas velocity and oxygen concentration and the reduction of coal particle size had positive effects on promoting the combustion process, but their contributions became feeble as the superficial gas velocity and the oxygen concentration were respectively greater than 0.10 m/s and 50vol%, and the coal particle size was smaller than 74 μm. The relations of reaction rate constant with various operating parameters were established. The kinetic parameters of pulverized coal combustion determined by MFBKA presented the same changing tendency as the results of thermogravimetric analysis (TGA), but the former apparent activation energy is relatively lower because of its optimized kinetic conditions.

Experimental study on lateral mixing of particles in a quasi-slot-rectangular spouted bed

A novel fluidized bed ash cooler called Bi-spouted-bed fluidized bed ash cooler was developed for circulating fluidized bed (CFB) boilers. It consists of chambers that have some characteristics of slot-rectangular spouted bed (SRSB), and the flow structure is similar to a combination bubbling bed and SRSB, thus we call it quasi-slot-rectangular spouted bed (QSRSB). A cold model of quasi-slot-rectangular spouted bed (QSRSB) with a cross-section of 900 mm × 200 mm was built. And a binary mixture of different sized particles was adopted, and the jetsam (bigger) and flotsam (smaller) component were used as tracers and bed material respectively. Experimental investigation on lateral mixing of tracer particles has been conducted. The lateral mixing process of tracer particles was studied. In addition, the effects of particle size of bed material dp, superficial gas velocity u, static bed height h and air inlet section width δ on the effective lateral dispersion coefficient of tracers were investigated. Meanwhile, a comparison of the effective lateral dispersion coefficient of tracers in QSRSBs and the bubbling fluidized bed with the same upper cross-section was conducted. It is found that the tracers disperse from regions of higher tracer concentration to regions of lower tracer concentration. Furthermore, the effective lateral dispersion coefficient of tracers increase with the increase of superficial gas velocity u, static bed height h and air inlet section width δ, but decrease with the increase in size of bed material dp, respectively. Additionally, the effective lateral dispersion coefficient of bubbling fluidized bed appears to be larger than that of QSRSBs when gas velocity at the upper section of bed, U, is greater than 0.3 m/s. However, the minimum U that ensuring the normal operation of QSRSBs is less than that of bubbling fluidized bed in the present work, which means that QSRSB has a better adaptability to the tracer granularity with the same gas flowrate.

Laboratory study of hydrogen sulfide removal in slug flows in a high pressure crude oil loop

Abstract Hydrogen sulfide (H2S) undergoes decompression during petroleum extraction from oil reservoirs, during which a certain amount of H2S-scavenging liquid compound is added to absorb H2S in the pipelines. All experiments were conducted in a high pressure flow loop with a thousand feet of coiled tubing to simulate the horizontal section of the pipeline that runs along the ocean floor from the reservoir at 20 bar (1 bar=101.3 kpa) and room temperature with a 40 ppm H2S gas concentration in nitrogen (N2). This paper reports experimental results on the role of superficial liquid and gas velocities (0.2–0.5 m/s and 0.4–1.1 m/s) on H2S removal. All experimental tests were conducted in the slug flow regime, thus this paper also reports the effects of pressure and the distance to which this flow regime prolongs, which is correlated to the H2S removal efficiency. Results indicate that an increase in superficial gas velocity at low superficial liquid velocity decreases the scavenger efficiency while the opposite is seen at high superficial liquid velocity. In addition, an increase in pressure shifts the transition zones in the flow regime altering the scavenger efficiency.

Effect of Void Fraction on Pressure Drop in Upward Vertical Two-Phase Gas-Liquid Pipe Flow

Experimental data for the void fraction and two-phase frictional pressure drop from various sources has been compiled and analyzed. The experimental data revealed that at the lower range of superficial gas velocity and void fraction, the variations of the two-phase frictional pressure drop with superficial gas velocity and void fraction are relatively flat. However, as the superficial gas velocity and void fraction increase to higher values, the frictional pressure drop became significantly sensitive to the two parameters. In a situation when the two-phase pressure drop is sensitive to the variation of the void fraction, it is then that the proper and accurate characterization of the void fraction becomes significant. From the experimental data, regions where the pressure drop is sensitive to the variation of the void fraction are identified and evaluated.

Numerical Simulation of Slag Foaming in High Temperature Molten Metal with Population Balance Modeling

Abstract A computational fluid dynamic (CFD) model has been developed for the simulation of slag foaming on bath smelting slag (CaO-SiO-Al 2 O 3 -FeO) by considering foam as a separate phase which is comprised of a mixture of gas and liquid. The model accounts for the formation of foam due to transformation of both gas and liquid into foam and it's destruction due to liquid drainage and bursting of bubble. The bubble break-up and coalescence was considered in gas-liquid dispersion whereas in the foam layer, the bubble coalescence due to film rupture was incorporated. Population balance modelling was used to track the number density of different bubble class. The model predicted the foam height of the slag system (CaO- SiO-Al 2 O 3 -FeO). The foam height was found to increase with the increase of superficial gas velocity. The content of FeO was changed and its effect on the foaming index was observed. The results from the present model show that foaming index decreases with increase of FeO content in slag. The results from the CFD model also show that the foaming index of a slag with Al 2 O 3 is higher than that of slag without Al 2 O 3 . The predicted results from the present study are in reasonable agreement with the experimental data.

Hydrodynamic characteristics and overall volumetric oxygen transfer coefficient of a new multi-environment bioreactor

The hydrodynamic characteristics and the overall volumetric oxygen transfer coefficient of a new multi-environment bioreactor which is an integrated part of a wastewater treatment system, called BioCAST, were studied. This bioreactor contains several zones with different environmental conditions including aerobic, microaerophilic and anoxic, designed to increase the contaminant removal capacity of the treatment system. The multi-environment bioreactor is designed based on the concept of airlift reactors where liquid is circulated through the zones with different environmental conditions. The presence of openings between the aerobic zone and the adjacent oxygen-depleted microaerophilic zone changes the hydrodynamic properties of this bioreactor compared to the conventional airlift designs. The impact of operating and process parameters, notably the hydraulic retention time (HRT) and superficial gas velocity (U(G)), on the hydrodynamics and mass transfer characteristics of the system was examined. The results showed that liquid circulation velocity (V(L)), gas holdup (ε) and overall volumetric oxygen transfer coefficient (k(L)a(L)) increase with the increase of superficial gas velocity (U(G)), while the mean circulation time (t(c)) decreases with the increase of superficial gas velocity. The mean circulation time between the aerobic zone (riser) and microaerophilic zone (downcomer) is a stronger function of the superficial gas velocity for the smaller openings (1/2 in.) between the two zones, while for the larger opening (1 in.) the mean circulation time is almost independent of U(G) for U(G) ≥ 0.023 m/s. The smaller openings between the two zones provide higher mass transfer coefficient and better zone generation which will contribute to improved performance of the system during treatment operations.

The cultivation of Anabaena variabilis in a bubble column operating under bubbly and slug flows

In a bubble column reactor with an inner diameter of 6cm and a height of 63cm for the culture of cyanobacteria two different shapes of bubbles can be generated, resulting in bubbly flow or slug flow. Growth of Anabaena variabilis under slug flow (1.9g/l/day) was 1.73 times higher than that under bubbly flow (1.1g/l/day) when the specific irradiation rate was maintained above 10μmol/s/g dry cell. Although a stepwise increase in superficial gas velocity enhanced the average cell growth rate under bubbly flow by 1.57 times, the average cell growth rate during the deceleration phase under bubbly flow (1.98g/l/day) was 0.61 times smaller than that under slug flow (3.22g/l/day). These results demonstrate that the bubble shape in the slug flow was advantageous in regards to the radial circulation of cells.

Lagrangian–Eulerian simulation of slugging fluidized bed

This work studies gas–solid slugging fluidized beds with Type-D particles, using two-dimensional simulations based on discrete element model (DEM). DEM performance is quantitatively validated by two commonly accepted correlations for determining slugging behavior. The voidage profiles simulated with bed height corresponding to Baeyens and Geldart (1974) correlation for onset of slugging demonstrate a transitional flow pattern from free bubbling to slugging. The present calculated values for the maximum slugging bed height are in good agreement with the correlation from Matsen et al. (1969). Simulations show that fluidized beds with Type-D particles can operate in the round-nosed slugging regime and also shows that wall slugs and square-nosed slugs tend to be formed with increase in superficial gas velocity and in bed height, respectively.

CFD simulation and wavelet transform analysis of vortex and coherent structure in a gas–solid fluidized bed

In a gas–solid fluidized bed, the particle vortexes play an important role in determining the heat transfer capability, while the coherent structures dominate the hydrodynamic characteristics of the turbulent flow field. A two-dimensional Eulerian–Eulerian model integrating the kinetic theory of granular flow (KTGF) is used to simulate the particle vertical fluctuating velocity in a gas–solid fluidized bed, based on which the continuous wavelet transform technique is for the first time introduced to analyze the particle vortex behavior. The results show that the time scale of a particle vortex has a linear relationship with the wavelet scale. Some particle vortexes perform periodic motion and merge with or separate from others until the end of the vortex period. As the superficial gas velocity and bed height increase, particle vortexes are stretched and their boundaries become unclear. Furthermore, to investigate the intermittency of the gas–solid flow field, the probability density function (PDF) of wavelet coefficients is computed. It is found that the wavelet coefficients and the probabilities of intermittent events both improve as the wavelet scale, superficial gas velocity and bed height increase. Since intermittency is caused by coherent structures in the flow field, the effects of coherent structures on the intermittency are studied through a coherent structure extracting method and the extended self-similarity (ESS) scaling law. Before extraction, wavelet coefficients at the same scale increase as the superficial gas velocity and bed height increase, while the scaling law curve at Ug=0.75m/s deviates from those at other superficial gas velocities, but shows no obvious change with the bed height. After extraction, the discriminations of scaling law curves under different operating conditions are eliminated and all curves merge in one. The effects of bubble formation, their size and propagation on particle vortex behavior are also analyzed. Bubble motion becomes more violent as gas velocity and bed height increase, which enhance the particle vortex size and activity and coherent structure generation, leading to stronger intermittency of granular flow field. As gas velocity increases, the amplitude of pressure fluctuation first increases and then decrease, while one sharp peak changes to be uniform lower peaks. As bed height increases, the major frequency decreases slightly and the amplitude increases slightly. This work provides deep insights into the turbulent flow field in a gas–solid fluidized bed by CFD simulation and the continuous wavelet transform technique.

Hydrodynamic Studies on Three-Phase Combined (Internal & External) Loop Airlift Fluidized Bed Reactor Using Newtonian and non-Newtonian Liquids: Minimum Fluidization Velocity and Liquid Holdup

A novel combined airlift loop fluidized bed reactor was proposed in this work. The internal and external loops were combined and the hydrodynamic parameters like minimum fluidization velocity and liquid holdup were studied for Newtonian and non-Newtonian fluids. Studies were conducted using Newtonian fluids of water, n-butanol, 60% and 80% glycerol and non - Newtonian fluids such as 0.25%, 0.6% and 1.0% Carboxy Methyl Cellulose (CMC) aqueous solutions were employed in the liquid phases. Spheres, Bearl saddle and Raschig ring with different sizes were used as solid phase. The experimental results indicated that the increase in particle size and sphericity increased minimum fluidization velocity whereas increase in superficial gas velocity decreased minimum fluidization velocity. In addition, the liquid holdup increased with increase in particle size and superficial liquid velocity. Furthermore, an increase in superficial gas velocity decreased the liquid holdup for Newtonian and non-Newtonian systems. Two separate correlations were developed to predict the minimum fluidization velocity and liquid holdup based on the experimental results for both Newtonian and non-Newtonian liquids for a wide range of operating conditions. The capability of the proposed correlation for minimum fluidization velocity and liquid holdup was examined and reasonable agreement between predicted and experimental results of Newtonian and non-Newtonian liquids suggested the applicability of the proposed correlations.

Studies on Minimum Fluidization Velocity and Liquid Hold in an Internal Loop Airlift Fluidized Bed Reactor

The influence of superficial gas and liquid velocities, particle diameter and sphericity, physical and rheological properties of liquids on minimum fluidization velocity and liquid holdup were studied in an internal loop airlift fluidized bed reactor. Spheres, Bearl saddles and Raschig rings were used as solid phases. Water, n-Butanol, two concentrations of Glycerol (60 and 80%) were used as Newtonian liquids and three concentrations (0.25%, 0.6% and 1.0%) of Carboxy Methyl Cellulose (CMC) solutions were used as non-Newtonian liquids. Superficial gas velocity was varied from 0.142 x 10-3 m/s to 5.662 x 10-3 m/s and superficial liquid velocity was varied from 0.001 to 0.12 m/s. The experimental results showed that increase in particle size and sphericity increased minimum fluidization velocity whereas increase in superficial gas velocity decreased minimum fluidization velocity. The liquid holdup increased with increase in particle size and superficial liquid velocity. An increase in superficial gas velocity decreased the liquid holdup for Newtonian and non-Newtonian systems. Based on the experimental results separate correlations were developed for the prediction of minimum fluidization velocity and liquid holdup for both Newtonian and non-Newtonian liquids for a wide range of operating conditions.

Investigation of Temperature Distribution and Heat Transfer in Fluidized Bed Using a Combined CFD-DEM Model

Fluidized beds are widely used in many industries because they are effective for both mixing and drying. The distinct element method (DEM) has recently received more attention for investigating the phenomena of multiphase flow because the technique is effective in gathering detailed information on complex phenomena without physically disturbing the flows. However, most studies have focused on the aerodynamics of the particles. In this study, a combined computational fluid dynamics (CFD)-DEM model, which allows prediction of gas and particle temperature profiles and heat transfer coefficients in a two-dimensional fluidized bed, was developed. The predicted results were compared with the experimental results at the superficial gas velocities of 2.04, 2.22, and 2.41 m/s and at the controlled inlet temperature of 343 K. Based on the comparison between the predicted and experimental results, it was found that the developed model performed adequately in predicting the gas temperature profiles, and the predicted particle temperature profiles were higher than the experimental data. The predicted heat transfer coefficient was slightly higher than the experimental data. However, the predicted and experimental results had a similar trend in which the heat transfer coefficient increased as a function of an increase in superficial gas velocity. In addition, the minimum fluidization velocity predicted by the developed model agreed well with the experimental data. Such predictions can provide essential information on temperature and heat transfer coefficients inside the fluidized bed for design and scale-up.

Process Simulation Development of Coal Combustion in a Circulating Fluidized Bed Combustor Based on Aspen Plus

A process simulation model has been developed to simulate and predict the condition of three kinds of coal combustion in a 30 kW circulating fluidized bed combustor based on Aspen Plus with considering gas−solid hydrodynamics via Aspen Plus in-line FORTRAN codes and combustion reaction kinetics via some external FORTRAN subroutines simultaneously. The related mathematical models in the process simulation, such as a gas−solid hydrodynamics model in the CFB riser, equivalent pyrolysis model of coal, and combustion kinetics model of coal pyrolysis products, have been presented in detail. The profiles of voidage and gas pressure along the height of riser are discussed and preliminarily predicted by the developed model in this paper. It can be concluded from the results of the process simulation model that increasing gas superficial velocity can lead to a decrease of voidage in the dense region and lower acceleration region, and a slight increase of voidage in the upper acceleration region and completely fluidized region. Furthermore, increasing gas superficial velocity can also effectively increase the total pressure drop of the riser. Therefore, a larger inlet pressure is required when more primary air is introduced into the riser bottom.

Hydrodynamics and Adsorption Mass Transfer in a Novel Gas−Liquid−Solid Circulating Fluidized Bed Adsorber

A novel gas−liquid−solid circulating fluidized bed adsorber for the separation of natural products or traditional Chinese medicines from extracts was successfully developed in this work. The hydrodynamics, axial dispersion, and mixing characteristics of phases and the adsorption and desorption of Ginkgo flavonoids in macroporous resin particles with prepared extract were investigated. The results showed that with the increase of downer superficial gas velocity, the deviation degree of flow pattern from plug flow, and the liquid back-mixing in the downer were enhanced because the liquid Pe number of the downer became smaller while its axial dispersion coefficient got larger. Similar tends were found for the riser as the riser superficial gas velocity climbed. Compared with that of the downer, the flow in the riser was more close to plug flow. Investigations on the adsorption kinetics of macroporous resin particles showed that the adsorption separation of Ginkgo flavonoids from extract was controlled by the...

Prediction of gas holdup in a combined loop air lift fluidized bed reactor using Newtonian and non-Newtonian liquids

Many experiments have been conducted to study the hydrodynamic characteristics of column reactors and loop reactors. In this present work a novel combined loop airlift fluidized bed reactor was developed to study, the effect of superficial gas and liquid velocities, particle diameter, fluid properties on gas holdup by using Newtonian and non-Newtonian liquids. Compressed air was used as gas phase. Water, 5% n-butanol, various concentrations of glycerol (60 % and 80 %) were used as Newtonian liquids, different concentrations of Carboxy Methyl Cellulose (0.25 %, 0.6 % and 1.0 %) aqueous solutions were used as non-Newtonian liquids. Different sizes of Spheres, Bearl saddles and Raschig rings were used as solid phases. From the experimental results it was found that the increase in superficial gas velocity increases the gas holdup, but it decreases with increase in superficial liquid velocity and viscosity of liquids. Based on the experimental results a correlation was developed to predict the gas holdup for Newtonian and non-Newtonian liquids for a wide range of operating conditions at a homogeneous flow regime where the superficial gas velocity is approximately less than 5 cm/s.

Liquid-Phase Back mixing in Bubble Columns

Liquid-phase axial dispersion coefficients have been measured for air-water system in bubble columns of 10, 15 and 30 cm diameter. The experiments are carried out using a transient method (the tracer response method). Dispersion coefficient is obtained by adjusting the experimental profiles of tracer concentration with the predictions of the model. The experimental results show that one-dimensional axial dispersion coefficient, Dax,L, reveal strong scale dependence. Backmixing of liquid phase increases with the increase of reactor diameter and superficial gas velocity. Axial dispersion coefficient for large column reactors can be easily predicted from the developed relation . Comparison of calculated with the experimental data and with the published data of other authors shows good agreement which ensure the reliability and confusability of the adopted correlations to be used in further design and scale-up purposes.

Liquid-Phase Back mixing in Bubble Columns

Liquid-phase axial dispersion coefficients have been measured for air-water system in bubble columns of 10, 15 and 30 cm diameter. The experiments are carried out using a transient method (the tracer response method). Dispersion coefficient is obtained by adjusting the experimental profiles of tracer concentration with the predictions of the model. The experimental results show that one-dimensional axial dispersion coefficient, D ax,L, reveal strong scale dependence. Backmixing of liquid phase increases with the increase of reactor diameter and superficial gas velocity. Axial dispersion coefficient for large column reactors can be easily predicted from the developed relation . Comparison of calculated with the experimental data and with the published data of other authors shows good agreement which ensure the reliability and confusability of the adopted correlations to be used in further design and scale-up purposes.