Holdup Measurements Research Articles

Influence of whip speed ratios on the inclusion of air into a bakery foam produced with a planetary mixer device

In the present study, dimensional analysis has been developed in order to identify the ratios governing the foam development network, with mixers combining dual whip revolution speeds. It is first suggested that, for a given planetary whip mixer and foaming solution, the set of dimensionless ratios characterizing the course of gas hold-up can be reduced to modified Froude and revolution numbers, as in the case of a classical mixing system. This requires the introduction of a characteristic velocity, instead of the two individual rotational speeds of the whip, in the corresponding list of governing parameters. An analytical expression for this characteristic velocity is then proposed, and gas hold-up measurements are made at various whip speed ratios, using a given food model-recipe, containing whole eggs and sugar. Our experimental results were evaluated, in order to validate the proposed dimensional analysis. It is shown that the aeration process requires a very low, modified revolution number to reach steady state values. It is also shown that the asymptotic values of gas hold-up are influenced by the modified Froude number. Depending on the values of the modified Froude number, it is noted that coalescence can occur, which then contributes to a reduction in gas hold-up. It has also been established that the modified Froude number can account for the eccentricity of planetary mixers, and consequently, that this number is a robust indicator for the determination of the optimum gas hold-up value, when air incorporation is produced by various planetary whipping devices. The results described in this paper enable foaming processes using planetary whipping mixers to be optimized, by proposing a framework in which the impeller speed ratio and aeration time can be set in such a way as to control the simultaneous entrainment and disentrainment of gas. Finally, this work can be further extended to other gas/liquid applications in which planetary mixers are used, such as surface aeration in wastewater treatment plants.

Minimum Energy Dissipation under Cocurrent Flow in Packed Beds

Functional analysis of mass balances on gas and liquid, and pressure drop equations describing the concurrent flow in packed beds has yielded two criteria. Positive values of these criteria has been shown sufficient to achieve energy savings by synchronized periodic pulsations of inlet gas and liquid velocities or by liquid velocity only compared to the situation under the same mean steady inlet phase velocities. This, however, is not the case of pulsation by gas inlet velocity alone. The developed criteria have been evaluated from data obtained by experimental measurement of liquid holdup and gas pressure drop in a column 0.05 m in diameter packed to 1 m by 0.003 m glass spheres using water/air and toluene/air system. Our experiments showed these criteria to be fulfilled in the whole domain of gas and liquid velocities tested. Evaluation of the energy savings under pulsations relative to the steady state operation showed that the savings for water/air system may reach up to 4% with the maximum in region of high and intermediate liquid velocities and low gas velocities. For the toluene/air system the savings are somewhat lower and occur in the same velocity domain.

Capacitance sensor for hold-up measurement in high-viscous-oil/conductive-water core-annular flows

Capacitance sensors are widely used in multiphase flows, for example, to estimate the hold-up in a given section of the pipe, taking advantage of the different permittivity values of the two liquids. The estimation is obtained by capacitance measurements between two electrodes, flush mounted on the external surface of the experimental pipe. Usually, capacitance sensors are used to investigate flows with non-conductive fluids, but they have the possibility to work also when, for example, conductive water is used. However, the capacitance technique applied to conductive fluids develops some issues. In this paper, we present a concave electrode sensor system developed for oil/conductive-water flows. A key contribution is to propose a modelization to the problem of capacitive sensing in presence of conductive fluids, based on a new approach to the parasitic couplings outside the measurement section. Thanks to this modelization, we propose a new design method for the working frequency and the electrode measurement head.

Core-annular flow in horizontal and slightly inclined pipes: Existence, pressure drops, and hold-up

An analysis of high viscosity oil/water flow in horizontal and slightly inclined pipe is presented. We propose experimental flow maps – in particular we focus on the core-annular flow pattern boundary – pressure drops, and oil hold-up measurements. Experimental data are used to validate some models present in literature. Since hold-up measurement in liquid–liquid systems is particularly challenging and, therefore, only few data are available in literature, they are analyzed and commented in detail. The slip ratio computed by hold-up data is also provided and analyzed.

CFD simulation and experimental measurement of gas holdup and liquid interstitial velocity in internal loop airlift reactor

This paper documents experiments and CFD simulations of the hydrodynamics of our two-phase (water, air) laboratory internal loop airlift reactor (40 l). The experiments and simulations were aimed at obtaining global flow characteristics (gas holdup and liquid interstitial velocity in the riser and in the downcomer) in our particular airlift configurations. The experiments and simulations were done for three different riser tubes with variable length and diameter. Gas (air) superficial velocities in riser were in range from 1 to 7.5 cm/s. Up to three circulation regimes were experimentally observed (no bubbles in downcomer, bubbles in downcomer but not circulating, and finally the circulating regime). The primary goal was to test our CFD simulation setup using only standard closures for interphase forces and turbulence, and assuming constant bubble size is able to capture global characteristics of the flow for our experimental airlift configurations for the three circulation regimes, and if the simulation setup could be later used for obtaining the global characteristic for modified geometries of our original airlift design or for different fluids. The CFD simulations were done in commercial code Fluent 6.3 using algebraic slip mixture multiphase model. The secondary goal was to test the sensitivity of the simulation results to different closures for the drag coefficient and the resulting bubble slip velocity and also for the turbulence. In addition to the simulations done in Fluent, simulation results using different code (CFX 12.1) and different model (full Euler–Euler) are also presented in this paper. The experimental measurements of liquid interstitial velocity in the riser and in the downcomer were done by evaluating the response to the injection of a sulphuric acid solution measured with pH probes. The gas holdup in the riser and downcomer was measured with the U-tube manometer. The results showed that the simulation setup works quite well when there are no bubbles present in the downcomer, and that the sensitivity to the drag closure is rather low in this case. The agreement was getting worse with the increase of gas holdup in the downcomer. The use of different multiphase model in the different code (CFX) gave almost the same results as the Fluent simulations.

Tomography measurements of gas holdup in rotating foam reactors with Newtonian, non-Newtonian and foaming liquids

Rotating solid foam reactors have already proven to show high mass transfer rates and to be a potential alternative to slurry reactors. The rotation of a foam block stirrer results in a high mass transfer and in the development of different reactor sections showing specific hydrodynamics and gas holdup distributions. In order to optimize the reactor system the hydrodynamics in a lab scale reactor are studied using γ -ray tomography, a powerful method to measure the gas holdup in three-phase reactors. The influence of liquid properties, such as viscosity and surface tension, and the rotational speed on the gas/liquid distribution in the different reactor sections is investigated. Especially the viscosity has a strong effect on the entrapment of gas bubbles in the foam block structure, while the surface tension is the dominant parameter in the outer reactor section. The influence of these parameters on the inset of foaming and the collapse of the gas/liquid dispersion is investigated. Conclusions on the mass transfer performance are drawn and recommendations for further optimizations of the reactor design and the operational conditions depending on the liquid properties are developed.

Effect of operating conditions and design parameters in a continuous powder mixer

An experimental investigation was carried out to study the mixing performance and flow behavior in a continuous powder mixer for a typical pharmaceutical mixture. Blender performance, characterized by the relative standard deviation (RSD) of composition of blend samples taken at the blender discharge and by the variance reduction ratio (VRR) of the blender, was measured as a function of impeller rotation rate, flow rate and blade configuration. The flow behavior in the continuous mixer was characterized using the residence time distribution (RTD) and powder hold-up measurements. To quantify the strain applied to the powder in the blender, the number of blade passes experienced by the powder in the blender was calculated using the residence time measurements. The relationship between different experimental parameters and mean residence time and mean centered variance was examined. The mixing performance was largely dominated by the material properties of the mixture, which had a larger effect than the ingredient flow rate variability contributed by the feeders. Holdup was strongly dependent on impeller rotation rate; as impeller rotation rate increased, holdup (and therefore, residence time) decreased sharply. As a result, intermediate rotation rates showed the best mixing performance. Blade configuration affected performance as well; blade patterns where some of the blades push the powder backwards improved the mixing performance.

Holdup measurements on an SRNL Mossbauer spectroscopy instrument

Gamma-ray holdup measurements of a Mossbauer spectroscopy instrument are described and modeled. In the qualitative acquisitions obtained in a low background area of Savannah River National Laboratory, only Am-241 and Np-237 activity were observed. The Am-241 was known to be the instrumental activation source, while the Np-237 is clearly observed as a source of contamination internal to the instrument. The two sources of activity are modeled separately in two acquisition configurations using two separate modeling tools. The results agree well, demonstrating a content of (1980 ± 150) μCi Am-241 and (110 ± 50) μCi of Np-237.

Characterizing the liquid dynamics in cocurrent gas–liquid flows in porous media using twin-plane electrical capacitance tomography

The hydrodynamics of gas–liquid cocurrent down- and upflow packed beds was studied experimentally using twin-plane electrical capacitance tomography (ECT). The ability of ECT to measure liquid holdups was examined by performing different calibration procedures. Confrontation of liquid holdup measurements from single-/twin-plane ECT, RTD and bed drainage revealed that ECT calibration between pre-wetted and flooded bed captured accurately the free-draining liquid holdups. A new modality of imaging tracer transients, referred to as twin-plane ECT/RTD, was proposed whereby upstream injections and detection of permittivity-contrasting liquid tracer impulsions allowed access, via voxel-affixed RTD and cross-correlation analyses, to interstitial velocity, liquid holdup and volumetric flux maps as well as to their degrees of uniformity. Single-plane ECT was successful at monitoring the liquid drainage dynamics showing its dependence with respect to gas flow rates and particle sizes, as well as to identify regime transitions and to unveil the relationships of pulse frequency and pulse velocity with respect to particle diameter and fluid throughputs in upflow and downflow.

On the measurement of local gas hold-up and interfacial area in gas–liquid contactors via light sheet and image analysis

Particle image velocimetry techniques coupled with advanced image processing tools are receiving an increasing interest for measuring flow quantities and local bubble-size distributions in gas–liquid contactors. Aim of this work is that of providing a numerical procedure able to reconstruct local gas hold-up and specific interfacial area from images obtained by laser sheet illumination. A correction for measured quantities accounting for laser sheet thickness is proposed and tested by means of Monte Carlo simulations. The algorithms proposed are robust and independent of any measured parameters of the actual bubble size distribution.

Single-beam gamma densitometry measurements of oil–water flow in horizontal and slightly inclined pipes

Gamma densitometry is a frequently used non-intrusive method for measuring component volume fractions in multiphase flow systems. The application of a single-beam gamma densitometer to investigate oil–water flow in horizontal and slightly inclined pipes is presented. The experiments are performed in a 15 m long, 56 mm diameter, inclinable stainless steel pipe using Exxsol D60 oil (viscosity 1.64 mPa s, density 790 kg/m 3) and water (viscosity 1.0 mPa s, density 996 kg/m 3) as test fluids. The test pipe inclination is changed in the range from 5° upward to 5° downward. Experimental measurements are reported at three different mixture velocities, 0.25, 0.50 and 1.00 m/s, and the inlet water cut is varied from 0 to 1. The gamma densitometer is composed of radioactive isotope of Am-241 with the emission energy of 59.5 keV, scintillation detector [NaI(Tl)] and signal processing system. The time averaged cross-sectional distributions of oil and water phases are measured by traversing the gamma densitometer along the vertical pipe diameter. Based on water volume fraction measurements, water hold-up and slip ratio are estimated. The total pressure drop over the test section is measured and frictional pressure drop is estimated based on water hold-up measurements. The measurement uncertainties associated with gamma densitometry are also discussed. The measured water hold-up and slip ratio profiles are strongly dependent on pipe inclination. In general, higher water hold-up values are observed in upwardly inclined pipes compared to the horizontal and downwardly inclined pipes. At low mixture velocities, the slip ratio decreases as the water cut increases. The decrease is more significant as the degree of inclination increases. The frictional pressure drop for upward flow is slightly higher than the horizontal flow. In general, there is a marginal difference in frictional pressure drop values for horizontal and downwardly inclined flows.

Non-invasive imaging of shallow bubble columns using electrical capacitance tomography

This paper deals with application of non-invasive electrical capacitance tomography to study the hydrodynamics of shallow bed bubble columns. Two bubble columns with different height to diameter ratio were used. Air–kerosene system that represents dielectric two-phase mixture was investigated. The ECT provided good measurement of the gas holdup at different gas velocities compared to the classical pressure measurements. The ECT was able to provide the gas hold up and the bubble velocities distribution across the column diameter at different gas velocities. The study revealed that spatial gas holdup and bubble velocity distributions are sharp with parabolic shape in the small bubble column (HD/DC=5). However, in the large bubble column (HD/DC=4) the gas holdup and bubble velocity profiles were flatter indicating improvement in the mixing homogeneity and leading to well-mixed reactor. 3D graphical visualization of the flow regimes and transition points were also examined using the ECT. In the small bubble column flow regimes were heterogeneous to slugs flow especially at high flow rate, resulted in downward flow near the walls and imperfect mixing.

Measurement of Dynamic Liquid Distributions in a Fixed Bed Using Electrical Capacitance Tomography and Capacitance Wire-Mesh Sensor

An intricate problem associated with fixed bed operation is liquid maldistribution, which denotes the fact that the liquid does not homogeneously flow through the bed. In a comparative study we evaluated two capacitance imaging methods—capacitance wire-mesh sensor and electrical capacitance tomography (ECT)—with respect to their capability of measuring static and dynamic liquid holdup in a fixed bed. The capacitance wire-mesh sensor as an invasive instrument is able to disclose flow structures at higher spatial resolution and was therefore considered the reference instrument for liquid holdup measurement. We found that both methods predict dynamic liquid holdup in the column in a similar way with only small systematic deviation. The results therefore prove that noninvasive electrical capacitance tomography can reliably measure cross-sectional dynamic liquid holdup in a fixed bed, even with a simple and fast linear back projection reconstruction algorithm.

CFD Modeling and Gas Holdup Measurement in Three-Phase Slurry Bubble Column

CFD Modeling and Gas Holdup Measurement in Three-Phase Slurry Bubble Column

GAS holdup and RTD measurement in an industrial flotation cell

In this work, the measurement of the axial gas holdup profile and gas residence time distribution (RTD), in a 130 m 3 self-aerated flotation cell, is presented. For this purpose, a radioactive tracer gas was activated in the Nuclear Reactor of the Chilean Commission of Nuclear Energy. The gas tracer, Freon 13B1, was injected as an impulse signal at the gas (air) inlet point, located at the top of the cell, from which the gas tracer circulates first through the rotor, where the bubble dispersion occurs, and then the gas becomes well distributed over the whole cross-sectional area before leaving the cell. The axial gas holdup profile was estimated from the transient gas concentration measurement at different depths inside the cell. From these experiments it was found that the air entering the cell was preferentially distributed in the upper half of the cell, while the gas entrainment into tailings was negligible. The mean gas holdup was 8.8%, and consequently the effective pulp volume of collection zone was 91.2%. The gas concentration on top of froth was recorded for RTD measurement of the gas leaving the cell. It was found that the mean gas residence time was around 42 s and the mixing condition for gas and pulp was similar.

An Experimental Study on Holdup Measurement in Fluidized Bed by Light Transmission

An Experimental Study on Holdup Measurement in Fluidized Bed by Light Transmission

The effect of hydrodynamic multiplicity on liquid phase trickle flow axial dispersion

Residence time distributions (RTDs) were measured for trickle flow in a bed of glass beads at different liquid flow rates and hydrodynamic states to obtain liquid holdup and axial dispersion measurements using the piston-dispersion (PD) model. The different hydrodynamic states were achieved by means of different pre-wetting procedures. It was found that all dispersion data show the same relationship to interstitial velocity, independent of the employed pre-wetting procedure. This relationship strongly reminds of Aris–Taylor type dispersion found in laminar flow through tubes. Based on the analogy of laminar liquid flow through a set of parallel tubes, a theoretical relationship between partial wetting, dispersion and liquid flow rate can be determined. Wetting efficiencies calculated from this relationship agree reasonably well with existing correlations.

Hydrodynamic and mass transfer characteristics of asymmetric rotating disc extractors

Asymmetric rotating disc contactor (ARDC) offers potential advantages in having comparable mass transfer coefficient ( k C a _ ) and lower back mixing over the conventional rotating disc contactor (RDC). In the present work, measurements of dispersed phase hold-up (∈ D ), effective interfacial area ( a _ ), continuous phase mass transfer coefficient ( k C a _ ) and axial dispersion coefficient for the dispersed as well as continuous phase have been performed in 80 mm and 150 mm i.d. ARDC over a wide range of operating variables and physical properties. From these measurements, the values of slip velocity and sauter mean drop diameter ( d 3,2) have been estimated. Suitable correlations have been proposed for ∈ D , V S , k C a _ , a _ , k C , d 3,2, Pe C and Pe D . The results on true mass transfer coefficient ( k C ) have been compared with those published in the literature.

Influence of the lift force closures on the numerical simulation of bubble plumes in a rectangular bubble column

Numerical Eulerian–Eulerian simulations of the unsteady gas–liquid flow in a centrally aerated two-dimensional bubble column were carried out in order to understand the effect of different formulations of the lift force coefficient ( C L ) on the computational results. Three different values of the superficial gas velocity ( U G =2.4, 12.0 and 21.3 mm s −1) that ensure the existence of different flow regimes were experimentally and computationally studied. The validation of the simulated results was based on visual observations and measurements of the global gas hold-up ( ε G ) and the plume oscillation period (POP). The results presented reveal that, at U G =12.0 and 21.3 mm s −1, using C L <0 results in under- and over-estimation of the ε G and POP, respectively. On the other hand, taking C L >0 does not affect the POP while it leads to increasingly higher ε G values, which are different from those experimentally reported. At U G =2.4 mm s −1, the effect of the lift force is not so evident, although it slightly improves the prediction of experimental values. Particularly interesting is the case of C L >0.4 at U G =21.3 mm s −1, producing a non-symmetric bubble plume oscillation. Since using Tomiyama's lift coefficient correlation does not improve the results, including the lift force into the simulation of bubble plumes is not recommended.

Monitoring Filtration in Trickle Beds Using Electrical Capacitance Tomography

Experiments were carried out to monitor the evolution of the deposition of fine particles in trickle-bed reactors during the flow of nonpolar hydrocarbon oil-like liquid suspensions using electrical capacitance tomography (ECT) imaging. The accuracy of the ECT rendition was validated in the pristine (i.e., deposit-free) bed state by comparing the liquid holdup measurements from ECT with the liquid holdup from residence time distribution (RTD) measurements. The pulse-flow characteristics (pulse velocity and frequency) estimated from the ECT signals were in agreement with existing literature data. For filtration experiments, the effects of the initial liquid suspension distribution, the gas and liquid superficial velocities, and single-phase flow (i.e., zero gas velocity) on the structure of the deposition in the bed were studied. ECT imaging successfully tracked the unsteady-state progression of bed plugging throughout the trickle bed. It was found that increasing the liquid or gas superficial velocity res...