Low Gas Velocities Research Articles

Influence of impeller diameter on local gas dispersion properties in a sparged multi-impeller stirred tank

Vertical distributions of local void fraction and bubble size in air–water dispersion system were measured with a dual conductivity probe in a fully baffled dished base stirred vessel with the diameter T of 0.48m, holding 0.134m3 liquid. The impeller combination with a six parabolic blade disk turbine below two down-pumping hydrofoil propellers, identified as PDT+2CBY, was used in this study. The effects of the impeller diameter D, ranging from 0.30T to 0.40T (corresponding to D/T from 0.30 to 0.40), on the local void fraction and bubble size were investigated by both experimental and CFD simulation methods. At low superficial gas velocity VS of 0.0077m·s−1, there is no obvious difference in the local void fraction distribution for all systems with different D/T. However, at high superficial gas velocity, the system with a D/T of 0.30 leads to higher local void fraction than systems with other D/T. There is no significant variation in the axial distribution of the Sauter mean bubble size for all the systems with different D/T at the same gas superficial velocity. CFD simulation based on the two-fluid model along with the population balance model (PBM) was used to investigate the effect of the impeller diameter on the gas–liquid flows. The local void fraction predicted by the numerical simulation approach was in reasonable agreement with the experimental data.

Investigating the conveying mechanism of particulate plugs with stationary layers

This paper presents an investigation into pneumatic conveying mechanism of particulate plugs with stationary layers. A plug flow regime may exist in pneumatic conveying systems operating with high-pressure gradients and low gas velocities. The most significant advantages of transporting particulate materials in a plug form is low energy consumption, low particle attrition and low pipe erosion. However, due to inaccurate approximations of the required pressure gradient of the plug, this kind of conveying processes can lead to pipeline blockage. This study presents a model for predicting pressure loss of particulate plugs that include a pick-up mechanism of a stationary layer of particles to be introduced between the plugs. The mechanism takes into account various parameters including the fraction of stationary layer (α), pipe and particulate characteristics, plug velocity and slope of pick-up layer. It was shown that the plug repose angle, θ, plays a very important role in determining the characteristics of plug flow conveying regime. Moreover, it was showed that the bulk density ratio (between the bulk density of the plug and the stationary layer) in some cases may make a significant contribution to pressure loss prediction. The model developed in this study was validated by the experimental measurements of other research teams and good agreement was found. The results of the present work provide information for updating major characteristic variables for pneumatic conveying systems operating as a plug flow. This can be useful both for designers and for future research studies.

A Design of Experiments (DOE) approach to optimise temperature measurement accuracy in Solid Oxide Fuel Cell (SOFC)

In SOFC, accurately measuring the hot-gas temperature is challenging due to low gas velocity, high wall temperature, complex flow geometries and relatively small pipe diameter. Improper use of low cost thermometry system such as standard Type K thermocouples (TC) may introduce large measurement error. The error could have a negative effect on the thermal management of the SOFC systems and consequential reduction in efficiency. In order to study the factors affecting the accuracy of the temperature measurement system, a mathematical model of a TC inside a pipe was defined and numerically solved. The model calculated the difference between the actual and the measured gas temperature inside the pipe. A statistical Design of Experiment (DOE) approach was applied to the modelling data to compute the interaction effect between variables and investigate the significance of each variable on the measurement errors. In this study a full factorial DOE design with six variables (wall temperature, gas temperature, TC length, TC diameter and TC emissivity) at two levels was carried out. Four different scenarios, two sets of TC length (6 – 10.5 mm and 17 – 22 mm) and two different sets of temperature range (550 – 650 °C and 750 – 850 °C), were proposed. DOE analysis was done for each scenario and results were compared to identify key parameters affecting the accuracy of a particular temperature reading.

CO2 Dissolution and Design Aspects of a Multiorifice Oscillatory Baffled Column

Dissolution of CO2 in water was studied for a batch vertical multiorifice baffled column (MOBC) with varying orifice diameters (d0) of 6.4–30 mm and baffle open area (α) of 15–42%. Bubble size distributions (BSDs) and the overall volumetric CO2 mass transfer coefficient (KLa) were experimentally evaluated for very low superficial gas velocities, UG of 0.12–0.81 mm s–1, using 5% v/v CO2 in the inlet gas stream at a range of fluid oscillations (f = 0–10 Hz and x0 = 0–10 mm). Remarkably, baffles presenting large do = 30 mm and α = 36%, therefore in the range typically found for single-orifice oscillatory baffled columns, were outperformed with respect to BSD control and CO2 dissolution by the other baffle designs or the same aerated column operating without baffles or fluid oscillations. Flow visualization and bubble tracking experiments also presented in this study established that a small do of 10.5 mm combined with a small value of α = 15% generates sufficient, strong eddy mixing capable of generating and...

CFD modelling of gas flow through a fixed bed of Raschig rings

In this work, direct numerical simulation of incompressible gas flow through a complex geometrical structure of Raschig rings is performed. The bed structure is obtained in a separate simulation in which Raschig rings are added one by one and fall freely into a cylindrical container until mechanical equilibrium is reached. The gas is injected at the top of the container, flows through a packed layer of randomly oriented rings, then leaves the container at the bottom. The flow equations are solved with the use of classical second order solver and projection method for structured regular grid and the complexity of domain geometry is handled by a variant of immersed boundary technique. The model allows to study the characteristics of the flow within the bed, pressure distribution in particular, and can be applied to development and validation of simplified approaches. The simulations are performed for various grid resolutions and the results show good convergence to grid independent solution, especially for lower gas velocities. The obtained dependence of total pressure drop on the inlet velocity is in reasonable agreement with the literature data but the demand for grid resolution significantly increases with the gas velocity.

CFD–DEM simulation of the pneumatic conveying of fine particles through a horizontal slit

Fine particles play a significant role in many industrial processes. To study the dynamic behavior of fine particle and their deposition in rock fractures, the pneumatic conveying of fine particles (approximately 100μm in diameter) through a small-scale horizontal slit (0.41m×0.025m) was studied, which is useful for the sealing technology of underground gas drainage in coal mining production. The CFD–DEM method was adopted to model the gas-particle two-phase flow; the gas phase was treated as a continuum and modeled using computational fluid dynamics (CFD), particle motion and collisions were simulated using the DEM code. Then, the bulk movement of fine particles through a small-scale horizontal slit was explored numerically, and the flow patterns were further investigated by visual inspection. The simulation results indicated that stratified flow or dune flow can be observed at low gas velocities. For intermediate gas velocities, the flow patterns showed pulsation phenomena, and dune flow reappeared in the tail section. Moreover, periodic flow regimes with alternating thick and sparse stream structures were observed at a high gas velocity. The simulation results of the bulk movement of fine particles were in good agreement with the experimental findings, which were obtained by video-imaging experiments. Furthermore, the calculated pressure drop versus gas velocity profile was investigated and compared with relative experimental findings, and the results showed good agreement. Furthermore, the particle velocity vectors and voidage distribution were numerically simulated. Selected stimulation results are presented and provide a reference for the further study of fine particles.

Experimental analysis and development of correlations for gas holdup in high pressure slurry co-current bubble columns

The effect of liquid and gas velocities, solid concentrations, and operating pressure has been studied experimentally in a 15 cm diameter air-water-glass beads bubble column. The superficial gas and liquid velocities varied from 1.0 to 40.00 cm/s and 0 to 16.04 cm/s, respectively, while the solid loading varied from 1 to 9%. The gas holdup in the column was reduced sharply as we switched from batch to co-current mode of operation. At low gas velocity, the effect of liquid velocity was insignificant; while at high gas velocity, increasing liquid velocity decreased the gas holdup. Drift flux approach was applied to quantify the combined effect of liquid and gas velocities over gas holdup. For co-current three phase flows, the gas holdup decreased with increase in solid loading for all pressures. But for batch operations, when solid loading was 5% or more, settling started leading to higher gas holdup. Increasing pressure from atmospheric conditions increased the gas holdup significantly, flattening asymptotically.

High Ash Fine Coal Dry Cleaning and Stability of Shallow Bed Dense-Phase Gas–Solid Separation Fluidized Bed

Fine coal dry separation technology has been one of the international research hotspots in the field of coal preparation. This article proposes the shallow bed dense-phase gas–solid fluidized bed separation technology to remove gangue from the high ash fine coal. According to the criterion of center pressure drop, the CP value can be finally reduced by reducing the bed height, H, to decrease the bed pressure drop, ΔPB, under the condition of a fixed gas distributor pressure drop, ΔPD. A fluidized bed of uniform and stable density was formed. The medium of 33% fine particles showed a better performance in −6+3 mm, −3+2 mm, and −2+1 mm fine coal separation than the medium of different fine particles content by ash segregation. A 100 mm bed height with a medium of 33% fine particles was selected to separate −6+3 mm fine coal. The ash content of cleaned coal reached 9.66% under the optimized conditions, whereas the clean coal yield reached more than 77.75%. For −3+2 mm fine coal, the cleaned coal yield was 82.59%, the ash content of clean coal was 10.93%, and the deash rate could reach 51.96%. The shallow bed dense-phase gas–solid fluidized bed exhibited poor liquidity and high viscosity under conditions of low gas velocities. The shallow bed dense-phase gas–solid fluidized bed (100 mm bed height) with medium containing 33% fine particles can separate the −6+2 mm fine coal.

Gas mixing study in freely bubbling and turbulent gas–solid fluidized beds with a novel infrared technique coupled with digital image analysis

A novel experimental technique using a high speed Infrared (IR) camera combined with an improved Digital Image Analysis (DIA) method for non-invasive concentration measurement with high spatial and temporal resolution has recently been developed by Dang et al. (2013). This paper reports the extension of the IR technique to freely bubbling and turbulent fluidization regimes to investigate and quantify lateral gas mixing characteristics in gas–solid fluidized beds. The mechanism of lateral gas mixing in the bubbling regime studied with the novel technique is in good agreement with values reported in the literature. The experimental results, interpreted with a plug flow model with superimposed dispersion for a homogeneous flow, show that the lateral gas mixing coefficient first increases with the increase of superficial gas velocities from the bubbling to the turbulent flow regime and then decreases for even higher velocities, which is consistent with earlier literature studies. The dependency of the lateral gas mixing coefficient on the Reynolds number using Amos׳ correlation (Amos and Mineo, 1993) has shown large discrepancies at low gas velocities (where the equation was extrapolated), while a good match was obtained at higher gas velocities. The experimental findings reported in this paper indicate that the novel IR/DIA technique can successfully be applied for mass transfer and gas mixing studies in gas–solids multiphase flows.

Catalytic Ozone Decomposition in a High Density Circulating Fluidized Bed Riser

Reactor performance of a high density circulating fluidized bed (CFB) riser is studied using ozone decomposition with extremely high solids circulation rates up to 800 kg/m2s and superficial gas velocities ranging from 5 to 9 m/s. Results show that the axial and radial distributions of the ozone concentration are consistent with those of the solids holdup. Higher solids flux and/or lower gas velocity lead to higher reaction conversion in the riser. Solids holdup is the dominant factor affecting reaction conversion. Effects of solids holdup on overall ozone conversion are more significant under high superficial gas velocity and/or low solids circulation rate. Overall conversion in the high density riser is not only less than that of plug flow reactor but also less than that of completely stirred tank reactor indicating that, due to the two-phase nature inside the riser, a CFB riser is still far from an ideal plug flow reactor.

Experimental Investigations of Droplet Deposition and Coalescence in Curved Pipes

Even though there have been several studies conducted by the industry on the use of different inlet devices for gas–liquid separation, there have been limited laboratory and field evaluations on the use of external piping configurations as flow conditioning devices upstream of a separator inlet. The results of a systematic study of droplet deposition and coalescence in curved pipe and pipe fittings are reported in this paper. A facility has been designed consisting of two main test sections: a fixed horizontal straight pipe section and an interchangeable 180 deg return pipe section (or curved pipe section) of the same length. Both inlet and outlet to the 180 deg return are horizontal, but the plane of the 180 deg return pipe section can pivot about the axis of the inlet horizontal pipe to an angle as much as 10 deg downwards allowing downward flow in the return section. Various pipe fittings of different radius of curvature can be installed for comparison in the 180 deg return. Fittings evaluated in this study included: 180 deg pipe bend, short elbow bend (with standard radius of curvature of 1.5D), long elbow bend (with custom radius of curvature of 6D), target tee bend, and cushion tee bend. Experiments have been carried out using water and air, and varying gas velocities and liquid loadings. In order to compare the performance of geometries, Droplet Deposition Fractions (DDF) were measured in the horizontal straight pipe section and in the 180 deg return pipe section as a measure of coalescence efficiency. The results demonstrate that higher DDF occurs for curved fittings as compared to the straight pipe section. The short elbow bend has approximately 10% DDF higher, whereas long elbow bend along with 180 deg pipe bend perform better (by 15–20% DDF) than straight pipe. It was found that the cushion tee and target tee bends can coalesce droplets at lower gas velocities but break up droplets at higher gas velocities. Additionally, no significant differences between DDF's in three different inclination angles of a curved pipe were observed. It can be concluded that 180 deg pipe bend or two 6D long radius elbow bend can serve as a droplet coalescer; a pair of cushion tees or target tees can also work as coalescers at low kinetic energy but as atomizers at high kinetic energy.

Atmospheric Pressure Atomic Layer Deposition of Al2O3 Using Trimethyl Aluminum and Ozone

High throughput spatial atomic layer deposition (ALD) often uses higher reactor pressure than typical batch processes, but the specific effects of pressure on species transport and reaction rates are not fully understood. For aluminum oxide (Al2O3) ALD, water or ozone can be used as oxygen sources, but how reaction pressure influences deposition using ozone has not previously been reported. This work describes the effect of deposition pressure, between ∼2 and 760 Torr, on ALD Al2O3 using TMA and ozone. Similar to reports for pressure dependence during TMA/water ALD, surface reaction saturation studies show self-limiting growth at low and high pressure across a reasonable temperature range. Higher pressure tends to increase the growth per cycle, especially at lower gas velocities and temperatures. However, growth saturation at high pressure requires longer O3 dose times per cycle. Results are consistent with a model of ozone decomposition kinetics versus pressure and temperature. Quartz crystal microbalance (QCM) results confirm the trends in growth rate and indicate that the surface reaction mechanisms for Al2O3 growth using ozone are similar under low and high total pressure, including expected trends in the reaction mechanism at different temperatures.

On-line monitoring of primary and agglomerate particle dynamics

The detailed evolution of primary and agglomerate particle growth during flame spray synthesis of 30g/h of zirconia nanoparticles is monitored online by a differential mobility analyzer (DMA), an aerosol particle mass (APM) analyzer and a condensation particle counter (CPC) as well as thermophoretic sampling/microscopy. The differences between measured projected-area equivalent diameter, diameter of gyration and mobility diameter are elucidated. Particles were sampled from distinct positions in the flame at up to 1700K and 1018particles/m3 using a probe allowing continuous aerosol extraction and rapid dilution with quench air that effectively suppressed coagulation. Primary particle growth was complete at ~10cm above the burner at about 1500K as the primary particle size did not change radially and axially downstream. As expected, the average agglomerate size increased with axial distance from the burner. Larger agglomerates, however, were observed at the edges of the aerosol plume attributed to prolonged residence time due to lower gas velocities there. Results were compared to off-line particle size characterization by nitrogen adsorption and thermophoretic sampling/transmission electron microscopy.

An experimental investigation on the onset from bubbling to turbulent fluidization regime in micro-structured fluidized beds

Membrane-assisted fluidized beds have been investigated for a number of industrially important applications. Recently, micro-structured membrane-assisted fluidized beds have been proposed and particular for efficient hydrogen production in particular for maximising the volumetric production rate by maximising the installed specific membrane areas. Another important advantage of micro-structuring became evident from computational studies. CFD calculations have shown that micro-structured fluidized beds can be operated in the turbulent fluidization regime at much lower superficial gas velocities compared with larger fluidized bed [1]. This work focuses on an experimental validation of the transition velocity in micro-structured fluidized beds. The influence of the reactor scale and the particle diameter on the transition gas velocity from the bubbling to the turbulent fluidization regime in micro-structure fluidized beds was measured in pseudo 2D column using different experimental techniques: in particular, the pressure drop fluctuation measured by pressure sensors and the local solid holdup fluctuation measured by digital image analysis on images recorded by high speed camera. It was found that smaller reactors filled with particles of smaller diameter can be operated in the turbulent fluidization at a relatively lower gas velocity compared with larger reactors. Interestingly, the different measurement techniques result in somewhat different transition velocities, i.e. the pressure drop fluctuation method generated higher critical velocities than the local solids' holdup fluctuation method. This difference is discussed in this paper. The experimental results confirmed that micro-structured fluidized beds can indeed be operated in the turbulent fluidization regime with lower fluidization velocities thus anticipating increased energy efficiencies and enhanced mass transfer rates.

Hydrodynamic Performance of a Spout–Fluid Bed with Draft Tube at Different Temperatures

Temperature is one of the key factors exerting a strong effect on the hydrodynamic performance of spout–fluid beds. In this study, experiments were conducted in a 160-mm-diameter spout–fluid bed with a nonporous draft tube at four temperatures: 293, 373, 440, and 500 K. The effect of temperature on the hydrodynamic performance indexes of pressure drop, gas bypassing, minimum spouting velocity, and flow pattern was investigated. The pressure drop in the annular region was higher at 500 K than at 293 K. The minimum spouting velocity (Ums) increased with temperature at a low superficial fluidizing gas velocity (Ums < 0.010 m/s). However, Ums decreased with temperature at a high superficial fluidizing gas velocity (Ums > 0.020 m/s) because the amount of gas flow rate transferred from the fluidizing gas into the draft tube increases with temperature. Six flow patterns identified by pressure drop fluctuations and digital images show that the gas velocity ranges in the two flow regimes of spouting with aeration and spout–fluidization are narrow or limited at 500 K compared with those at 293 K. The effect of temperature on these hydrodynamic performance indexes should be considered in the use and design of spout–fluid beds.

Study of Droplet Spray Behavior of an Atomization-Based Cutting Fluid Spray System for Machining Titanium Alloys

The atomization-based cutting fluid (ACF) spray system has been found to be effective for improving the tool life and overall productivity during the machining of titanium alloys like Ti–6Al–4 V. The aim of this research is to study droplet spray characteristics of an ACF spray system including droplet entrainment zone (e.g., angle and distance) and droplet-gas co-flow development regions with respect to three ACF spray parameters, viz., droplet and gas velocities, and spray distance. ACF spray experiments are performed by varying droplet and gas velocities. Machining experiments are performed in order to understand the effect of the droplet spray behavior on the machining performance, viz., tool life/wear, and surface roughness during turning of a titanium alloy, Ti–6Al–4 V. The flow development behavior with respect to the spray distance is studied by modeling the droplets entrainment mechanism. The model is validated by the ACF spray experiments. Experiments and the modeling of flow development behavior reveal that a higher droplet velocity and a smaller gas velocity result in smaller droplet entrainment angle leading to a gradual and early development of the co-flow, and a better distribution of the droplets across the jet flare. Machining experiments also show that a higher droplet velocity, a lower gas velocity and a longer spray distance significantly improve tool life and surface finish.

Numerical Simulation and Analysis of CO2Removal in a Polypropylene Hollow Fiber Membrane Contactor

This present study shows a comprehensive 2D numerical model for removal of CO2in a polypropylene (PP) hollow fiber membrane contactor (HFMC) using the computational fluid dynamics (CFD) method. Monoethanolamine (MEA) solution was used as the liquid absorbent in a nonwetting mode. The simulation results represented that higher liquid velocity and concentration and lower gas velocity and concentration led to higher percent of CO2removal. The most proper parameters for CO2removal were less than 1 mol m−3gas concentration and 0.2 m s−1gas flow rate, and for MEA the values were above 8 mol m−3concentration and approximately 1 m s−1liquid velocity. Furthermore, the model was validated with the experiment results. Therefore, the modeling results provided references to the selection of absorbents and operation parameters in the experimental study and pilot-scale applications.

Mass Transfer And Hydrodynamics In A Split-Column Airlift Photobioreactor

Overall gas-liquid mass transfer coefficients of oxygen gas (KLa) at different axiallocations of the riser and downcomer of a split-column Airlift Photobioreactor wereinvestigated using FOXY Fiber Optic oxygen sensor supported by NeoFox viewer software.The effect of bubble dynamics on mass transfer was considered. A 4-point optical probe wasused to measure local interfacial area (a) and other bubble dynamics parameters for both theriser and the downcomer of a split-column airlift photobioreactor. Having both overall masstransfer (KLa) and interfacial area (a) measured at different locations, local oxygen masstransfer coefficient (KL) can be determined for each location along the riser and thedowncomer of the airlift bioreactor. Effects of different operating conditions (0.3 – 2.8 cm/ssuperficial gas velocity) on both mass transfer and bubble dynamics of the riser and thedowncomer have been investigated for air-water system. It was found that both local masstransfer and gas-liquid interfacial area are considerably varying from the bottom to the top ofthe riser and the downcomer of the bioreactor. It was found that local mass transfer coefficientvaries by 100% from the bottom to the top of the riser at low superficial gas velocity (1 cm/s)and by 30% at relatively high velocities (2-2.8 cm/s). The Trend of the variation of the KLa inthe downcomer section of the column is very different from that of the riser, as it was noticedthat insignificant mass transfer takes place at the lower part of the downcomer. The results ofmass transfer and bubble dynamics in the riser and the downcomer of the studied conditionswill be presented and discussed.

Non-Newtonian Liquid-Gas Non-Uniform Stratified Flow With Interfacial Level Gradient Through Horizontal Tubes

This paper presents the predictions of the axial distribution of liquid level and interfacial level gradient (ILG) for nonuniform non-Newtonian liquid-gas flow in horizontal tubes. The non-Newtonian liquid is described using power-law model, while the model of Heywood and Charles for uniform non-Newtonian liquid-gas two-phase flow, which was developed based on one dimensional energy equation, is extended to describe nonuniform stratified flow by incorporating the effect of interfacial level gradient. Two different critical liquid levels are found from the energy equation and are adopted as boundary condition to calculate the interfacial level distribution upstream of the channel exit. The results from the model are compared with the published numerical and experimental data. The results show that the model can predict the interfacial level distribution and interfacial level gradient for nonuniform stratified flow. Low liquid velocity, low gas velocity and high liquid viscosity are beneficial for forming a nonuniform flow with interfacial level gradient. The difference between the analytical model and the published data is smaller than 10%.

Moving bed syngas conditioning: Modelling

This paper presents a modelling approach for simulating tars and particulate (dust) removal in a moving bed heat exchange filter (MBHEF) in order to satisfy gas requirements of end-use syngas applications: engines and turbines. The two-dimension, adiabatic, steady-state proposed model accounts for two-phase (gas and solid) and neglects conduction and mass diffusion. Tars condensation is modelled through representative tar class lumps: phenol (class 2), naphthalene (class 4), pyrene (class 5). The model also considers tar concentration influence on the tar dew point. The filtration model is taken from literature. A sensitivity analysis is performed varying the particle size and the superficial gas velocity. Maps of temperature and tars abatement efficiency are presented. The simulation results indicate the feasibility of the use a MBHEF as tars removal equipment benefiting its advantages against others gas-cleaning methods with acceptable pollutant removal efficiencies, ranging 88–94% for ranges studied. Results also point out low gas velocities (0.5-1 m/s) and high particle size (400–700 μm) for reducing operational costs in MBHEFs with compact size.