Froth Regime Research Articles

A new tomography-based approach for the fluid dynamic description of conventional structured packings and sandwich packings

Pressure drop, holdup and flooding points are essential factors for the hydraulic design of columns equipped with structured packings. This is also true for sandwich packings representing a combination of two alternating layers of conventional structured packings with different geometric surface areas. Such a combination results in a heterogeneous flow pattern evolving within certain operating ranges. In order to describe accurately the fluid dynamics over the entire operating range of both common structured packings and sandwich packings, a modelling approach is proposed based on conventional measurements and tomographic investigations. The approach distinguishes film-like flow patterns and froth regimes. For film-like flow patterns, packing-typical descriptions are used, whereas the froth in packings is described using an analogy to trays. Correlations were derived to determine the liquid holdup for film and froth flow, the dry and irrigated pressure drop as well as the gas velocities at the loading limits. The fluid dynamic model was verified with 1595 measurements for pressure drop and 510 measurements for holdup for the test system water/air.

Identification of Phase Inversion on Sieve Trays

AbstractThe fundamental differences in hydrodynamics of the froth and spray regime account for the ongoing interest in search for the point of phase inversion. This short communication presents a new approach for identification of phase inversion on sieve trays in terms of an image‐based measurement technique. Image analysis of entrained droplets reveals a distinct increase in Sauter mean diameter and droplet frequency during phase inversion. Further measurement methods like pressure drop, gravimetric analysis of entrained liquid, froth height assessment and photographic observation of the flow regime serve as a reference value and complement the discussion. A flow map based on the experimental data comprises each regime and shows a good agreement with phase inversion correlations from literature.

Comparative assessment of different image processing methods to determine the gas–liquid interfacial area in froth regimes of sandwich packings from ultrafast X-ray tomography image data

Sandwich packings consist of alternatingly stacked structured packing layers of different specific surface area. In such packings froth two-phase flow appears when the packing is operated between the loading limits of the layers. For this highly agitated flow regime, there is a lack of hydrodynamic data, in particular on gas-liquid interfacial area. Ultrafast X-ray tomography, a cross-sectional imaging technique with a frame rate of more than 1000 cross-sectional images per second, is applied to visualize the gas-liquid flow and to extract the gas–liquid interfacial area data via image post-processing methods. For that, we assessed different segmentation methods, that are, level set and gray level contour techniques.

Experimental Investigation of Fluid Dynamics in Sandwich Packings with Ultrafast X-ray Tomography

Sandwich packings consist of two layers of corrugated sheet structured packings with higher (holdup layer) and lower (de-entrainment layer) specific surface area alternatingly placed in the column. They are preferentially operated between the flooding points of both layers, which results in zones of bubbly flow, froth regime and liquid film flow in each sandwich element. Compared to conventional packed columns, sandwich packings can reach higher capacity and also higher separation efficiency as a result of intensive phase interactions. In the scope of a collaborative project, sandwich packings are experimentally and theoretically investigated. To provide detailed information on the heterogeneous flow patterns to derive reliable process models, ultrafast X-ray tomography is applied as a non-invasive measurement technique with high temporal and spatial resolution. In addition to local liquid holdup in the different layers of the packing, cross-sectional liquid distribution and axial transitions between the flow regimes are estimated. Furthermore, a method for the detection of the gas-liquid interfacial area is proposed.

Modeling and simulation of downward vertical two-phase flow with pipe rotation

One of the major problems in two-phase flow research is prediction of flow pattern at different configurations without requiring expensive experimental tests. Numerical modeling and simulation is a suitable and emerging approach for this purpose which has many benefits including saving in time and cost. In this study, for the first time, the effect of pipe rotation on the flow patterns of air–water two-phase flow in downward direction was numerically studied. For this reason, Eulerian–Eulerian multi-fluid approach was utilized in Ansys-Fluent software. At first, apparent conditions of each regime in various revolution were recorded from experimental tests and then the simulation was carried out to find if the results are compatible. It was shown that pipe rotation has important effects on the flow patterns map and shift transitions boundaries of slug and annular flow toward lower gas superficial velocity. Comparison of numerical results with experimental data show acceptable match for all regimes. Good agreement was observed for falling film, bubbly and slug regimes but the least agreement was observed for froth regime due to high turbulence and perturbation of flow.

Froth image analysis by use of transfer learning and convolutional neural networks

Deep learning constitutes a significant recent advance in machine learning and has been particularly successful in applications related to image processing, where it can already surpass human accuracy in some cases. In this paper, the use of a convolutional neural network, AlexNet, pretrained on a database of images of common objects was used as is to extract features from flotation froth images. These features could subsequently be used to predict the conditions or performance of the flotation systems. Two case studies are considered. In the first, froth regimes in an industrial flotation plant could be identified significantly more reliably with the features generated by AlexNet than with previous state-of-the-art approaches, such as wavelets, grey level co-occurrence matrices or local binary patterns. In the second case study, the arsenic concentration in the batch flotation of realgar-orpiment-quartz mixtures could be predicted more accurately than was possible with features extracted by wavelets, grey level co-occurrence matrices, local binary patterns or by use of colour. These results suggest that feature extraction with convolutional neural networks trained on complex data sets from other domains can serve as more reliable methods than previous state-of-the-art approaches to froth image analysis.

Entrainment Performance and Model of Multidowncomer Sieve Trays

The experiments about entrainment performance, as well as mass-transfer efficiency, of multidowncomer (MD) sieve tray were investigated in a 1200-mm-diameter tower with air/water system. The results indicate that the entrainment rate of MD sieve tray is lower and its behavior with increased liquid rate contradicts that of the conventional sieve tray. According to the nonmonotone variation phenomenon of entrainment at various liquid loads given by Lu and Kister, we developed the Double-Transition Point Theory to interpret the entrainment features under different contact regimes and the entrainment mechanism of MD sieve tray. By comparing the correspondence match of entrainment, pressure drop, and efficiency curves, we discussed the operating upper limit and mass-transfer efficiency. Therefore, based on the formulas of Fell’s and Hunt’s correlations, we developed the model for MD sieve tray where a semitheoretical derivation on entrained droplets in the froth regime was made.

Hydrodynamics and mass-transfer characteristics analysis of vapor–liquid flow of dual-flow tray

In the present study, the behavior and details of vapor–liquid contact on the dual-flow trays (DFTs) were investigated using a 3D CFD model within the two-phase Eulerian framework. The simulation provided good agreement with experimental results, which verified the reliability of the model. Firstly, the operation range was divided into four regimes having different characteristics of flow phenomena, and most attention was paid to the hydrodynamic behavior of froth regime and fluctuating regime due to its importance in operation and structure improvement. Then, the liquid flow characteristics of these two regimes were revealedwith the analysis of velocity profiles and liquid phase distribution contour. Meanwhile, some in-depth picturing of local events of hydrodynamics and mass-transfer performance was also presented through the discussion of cyclohexane liquid volume fraction distribution and vapor-phase mole fraction distribution. The froth regime was proved to have higher and homogeneous point efficiency at bulk zone around the plate center area than the fluctuating regime. Thus, the improvement of DFT should be focused on the restriction of “free-flow” to prolong the froth regime and delay the formation of vortex flow with circulation cells. Finally, an orthogonal wave tray (OWT) was designed and compared with DFT for evaluation of the effect of proposed modifications on the enhancement of tray hydraulics, mass-transfer efficiency and stability.

Rate-based modelling and simulation of distillation columns with sandwich packings

Sandwich packings are innovative column internals consisting of two layers of ordinary packing with different geometrical surface area. The layer with the high surface area, hold-up layer, has a lower flooding point as compared to another, de-entrainment layer. Sandwich packings can be operated even when the hold-up layer is flooded. In operation, different fluid dynamic regimes occur along the column. The flow in the flooded hold-up layer is comparable with that in trays. A froth regime is established above the hold-up layer, whereas a film flow develops in the remaining part of the de-entrainment layer. The proper design of sandwich packings requires a mass transfer model capable of considering all these different regimes. In this work, the rate-based concept is used to develop a mass transfer model of sandwich packings. This model employs two alternate column segments. One of them corresponds to de-entrainment layer, and here, mass transfer correlations for packed columns are applied. Another segment is related to the hold-up layer; it is described with correlations for sieve tray columns. The developed mass transfer model is validated with experimental efficiency data for sandwich packings.

The influence of gas physical properties on entrainment inside a sieve tray column

Existing entrainment prediction correlations, often used in the design of sieve tray distillation columns, have been developed mostly with air/water data. Therefore, the aim of this work was to investigate the influence of gas physical properties on entrainment, by also utilising gases with physical properties notably different from air. Air, CO2 and SF6 were passed through a rectangular sieve tray column (tray hole diameter of 6.3mm and fractional hole area of 0.156). These three gases were chosen in order to cover a large gas density range of 1.2–5.8kg/m3. Water, ethylene glycol and n-butanol were used as liquids in order to achieve the desired wide range of liquid physical properties. Tray spacing and weir height were set at 615mm and 51mm, while the liquid rate was varied from 2.9 to 80m3/(hm) to cover both the spray and froth regimes. Gas flow factors ranged between 1.9 and 4.8m/s(kg/m3)0.5. A database of over 500 experimental data points was generated in this study. The main objective was to use the experimental data to describe the effect of gas physical properties and gas-and-liquid flow rates on entrainment. The data were also used to evaluate the scope and limitations of current entrainment prediction models. Existing correlations fitted the air/water data well, but showed rather large deviation from the non-air/water data. A new approach to describe the influence of gas physical properties on entrainment is proposed. This approach utilises a variation of the Reynolds and Froude numbers along with the ratio of the gas to liquid density to correlate entrainment.

Computational fluid dynamics simulation of multiphase flow in packed sieve tray of distillation column

Computational fluid dynamic models (CFD) have been used for the description of hydraulic characteristics of packed sieve tray (PST). PST is a conventional sieve tray combined with a slice thickness of packing on the tray deck. Eulerian-Eulerian framework has been used to solve the equations of both liquid and gas phases assumed as two interpenetrating phases. A commercial scaled sieve tray has been simulated based on a three-dimensional unsteady state model. Comparison with experimental data proves good agreement for the simulation results under the studied conditions. Effects of the packing on the liquid velocity distribution, clear liquid height and vertical liquid volume fraction distribution have been investigated. The simulation results show that 3.08 cm of packing thickness could increase the clear liquid height up to 17 percent and froth height up to 10 percent as well as promoting froth density by 6 percent with the only drawback of increasing wet pressure drop up to 16 percent in froth regime.

New Fraction Jetting Model for Distillation Sieve Tray Efficiency Prediction

The two phases on distillation trays can function in several different regimes depending on the physical properties of the system and the liquid-to-vapor ratio. Predicting the fraction of the vapor transported as jets, or fraction jetting, on a tray operating in the mixed-froth regime can bridge froth and spray regime models, explain the gradual changes in tray efficiency during the froth–spray transition, and eliminate the need to predict the froth–spray transition point when separate froth and spray regime models are used. Fraction jetting models will also facilitate multiregime models, such as the Syeda et al. (Chem. Eng. Res. Dev.2007, 85, 269–277) sieve tray efficiency model, that are valid for both froth and spray regimes. Current fraction jetting models are empirical and developed from limited data. In this paper, the concept of fraction jetting on a sieve tray is discussed and a new semiempirical model is presented. The fraction jetting model was developed using the existing data of Raper et al. (Chem. Eng. Sci.1982, 37, 501–506) and no new data were generated. Furthermore, the applicability of the fraction jetting model to tray efficiency models that incorporate jetting is demonstrated using the Syeda et al. sieve tray efficiency model.

New froth behaviour observations and comparison of experimental sieve tray entrainment data with existing correlations

A thorough understanding of the hydrodynamics in tray columns is required to optimise column and tray design for specific operating capacities and conditions. Liquid transported by the rising gas to the tray above, defined as entrainment, is one way of measuring the tray column capacity limit. Entrainment correlations available in the literature have been developed with predominantly air/water data, because of the limited availability of non-air/water data. In this work an experimental setup was constructed to measure entrainment, tray pressure drop and weeping for various gas and liquid systems. The experimental entrainment data for three systems, namely air/water, air/ethylene glycol and air/silicone oil, is compared to existing correlations. The effect of liquid physical properties on entrainment under flow factors ranging from 1.6kg0.5/(m0.5s), for a 415mm tray spacing to 4.0kg0.5/(m0.5s) for a 615mm tray spacing within a liquid flow range of 2.9–112m3/(hm) was observed. The experimental results showed a somewhat complex dependency of entrainment on liquid physical properties. At gas flow factors of 2.2kg0.5/(m0.5s) for the 415mm tray spacing, entrainment reached a maximum in the froth regime and then decreased with increasing liquid rates. Notably, the liquid viscosity – not included in previously developed correlations – significantly influences the entrainment behaviour. Existing entrainment correlations agree better with the air/water data than with the air/ethylene glycol or air/silicone oil data.

A Study on Chimney Type Centrifugal Tray Lower Operating Limit

In this work, a computational fluid dynamics (CFD) model is developed to predict the hydrodynamics of industrial-scale Centrifugal tray that is a new kind of tray and can be used in distillation and absorption towers. Three-dimensional two-phase flow of gas and liquid is considered in which the interaction was modeled based on the concept of phasic volume defined in the Volume of Fluid (VOF) multiphase model through k-epsilon Simulation. All governing equations including surface tension and wall adhesion are solved simultaneously using the FLUENT code. Gas-liquid interfaces and the existence of froth regime are clearly visualized via VOF model. Due to the importance of trays, it is vital to predict lower operating range limits for such gas-liquid contacting devices. Weeping phenomenon is observed in bubbling regime and occurs at low vapour flow rates. To approach this aim, continuity and momentum equations are solved simultaneously for the fluid on tray and the results were compared with data of sieve trays. In the Comparison of sieve type trays with centrifugal chimney type, in low loading vapour, no weeping was observed in the centrifugal tray. The simulation results show that this innovated centrifugal tray can be used for solving the weeping problem and increasing the performance of tray towers.

A Fundamental Model for Prediction of Sieve Tray Efficiency

A phenomenological model for froth structure is proposed based on the analysis of froth images of an active sieve tray taken from a 0.153 m distillation column. Froth is defined as a combination of bubbles and continuous jets that break the surface of froth projecting liquid splashes and drops above the surface. To estimate the fraction of small bubbles in froth, a fundamentally sound theoretical expression is derived from turbulent break-up theory. A new model for predicting point efficiency of cross-flow sieve trays has been developed based on the hydrodynamics of an operating sieve tray represented by the proposed froth structure model. This efficiency model is applicable for both froth and spray regime. Fraction of by-passed or uninterrupted gas jet is considered as the determining factor for froth to spray transition. The net efficiency is estimated by adding up the contributions of both bubbles and jets present in the dispersion. The model is tested against the efficiency data of cyclo-hexane/n-heptane and i-butane/n-butane mixtures.

On the froth–spray transition at multiple orifices

A new theoretical model of the transition between the froth and spray regimes on a multi-orifice plate is proposed. The model considers the force balance on a film of liquid adjacent to the gas core at the onset of jetting at a single orifice, and estimates the thickness of the film by drawing the analogy with surface waves in gas–liquid annular flow. The model predictions correlated very well with a wide range of experimental data from published sources.

Mass Transfer in Foam and Froth Regimes in Distillation with Sieve Tray Column.

An experimental study on separation performance of sieve tray column in binary and ternary distillation is made under total reflux conditions with different tray specifications. Two different flow regimes are observed on the tray in different concentration regions for the ethanol-water, ethanol-benzene and ethanol-benzene-water systems. The liquid holdup in the foam regime is well correlated in terms of the weir height, free area and vapor phase Reynolds number. Although higher mass transfer rate is observed in the foam regime than that in the froth regime, the dimensionless volumetric vapor phase diffusion flux in the foam and froth regimes are well correlated in terms of the ratio of Froude to Weber number, free area and vapor phase Reynolds number.

Model for Liquid Phase Flow on Sieve Trays

A continuum based model has been used to predict liquid phase flow on sieve trays. The model uses the time and volume averaged momentum and continuity equations to predict steady state liquid velocities of froth regime flows. Vapour phase flows and the interphase interactions are specified as part of the model. Empirical correlations for distillation hydraulics are used to develop the constitutive relationships needed for closure of the model. The model was evaluated for F-factors (defined as usρG (kg 0.5 /m0.5 s) ) ranging from 0.462 to 1.464 and liquid loadings (volumetric flow per length weir) of 27 m2/h to 68 m2/h. Experimental data from Solari and Bell1 were used to evaluate the model. On average the model is able to predict liquid velocities within 33 percent of measured values. This is a substantial improvement upon the most common assumptions and models for liquid flow on sieve trays. At low vapour loadings and high liquid loadings, the basic assumptions of the model become progressively less relevant and hence the degree of flow maldistribution is under-predicted.

A mechanistic analysis of sieve tray froth height and entrainment

AbstractPhenomenologically based model for forth height was developed using the assumption of both vapor‐and liquid‐continuous zones within the total froth height. The model demonstrates the importance of liquid and vapor rates and determines that the Weber number (and therefore droplet size and surface tension) has little effect on froth height.A composite air/water database including entrainment data for both the spray and froth regimes is used to develop an entrainment correlation as a function of the ratio of tray spacing to froth height (Ts/h2ϕ) and the average froth density (ϕ2ϕ = hL).For conditions resulting in the ratio of liquid inventory to the perforation diameter (hL/dH) <4, the height, equation requires an empirical correction for froth height, and entrainment depends onϕ2ϕ.This results from the predominantly vapor‐continus nature of the froth. For large values of hL/dH, the theoretical value for h2ϕ appears adequate and entrainment is essentially independent of ϕ2ϕ, since most of the liquid is contained in the liquid‐continuous region located far away from the underside of the tray above. In such cases, entrainment is from a light spray on top of the liquid‐continuous region, and therefore entrainment depends much less on ϕ2ϕ.The differen dependency of entertainment on ϕ2ϕ can be used as a method to identify the spray‐froth transition. For hL/dH<2, a spraylikecondition exists; for hL/dH > 2, a frothlikecondition exists.The reported dependency of air/water system entrainment on major geometry and operating conditions is described well by the new correlation. Alternate correlations for the non‐air/water entrainment data, however, are required. New methods for estimating the impact of entrainment on efficiency are given.

An Assessment of Froth Behaviour in Full Scale Coarse Flotation Cells

A description is given of recent developments in flotation control strategy at Peak Downs Mine based on the ASHSCAN’coal gauge. Additional testwork undertaken to enhance this control strategy is detailed. Both short and long term behaviour is outlined, particularly in relation to the concentrate froth. Simple empirical relationships are derived between concentrate ash % and several key operating parameters. Froth mobility is discussed in terms of a sharp transitional phenomenon between two different froth regimes. In the mobile regime, characterised by free flowing watery froth, concentrate ash can be readily controlled by manipulating pulp level. However, no such control is possible in the immobile regime, characterised by a viscous dry froth. On-line data logging has also uncovered underlying long term plant cyclic behaviour.