Dispersion Height Research Articles

Measurement of Gas Hold-Up Profiles by Gamma Ray Tomography: Effect of Sparger Design and Height of Dispersion in Bubble Columns

Radial variation of the gas hold-up was investigated in 0.385m i.d. bubble column using gamma ray tomography. Radial profiles were measured at three axial locations ( H D /D= 0.259, 3 and 5 ). In all the experiments water was liquid phase and air was gas phase. Superficial gas velocity was varied from 0.06 to 0.3ms −1 . Perforated plates were used as spargers. The free area of sparger plates ( FA ) and the hole diameter ( d 0 ) were varied in the range of 0.135 to 5.4% and 0.8 to 87mm respectively. The hold-up profiles were found to depend strongly on the sparger design, at least near the sparger. Further, the height of dispersion was found to influence the hold-up profiles near the sparger. All the results have been correlated on the basis of drift flux model and parabolic holdup profile.

Effect of scale-up on average shear rates for aerated non-Newtonian liquids in external loop airlift reactors.

Average shear rates have been estimated experimentally in a 700-dm3 external loop airlift reactor. Aqueous pseudoplastic carboxymethylcellulose and xanthan gum solutions were used to simulate non-Newtonian behavior of biological media. Average shear rates of non-Newtonian solutions were found by analogy with Newtonian glycerol solutions using downcomer liquid velocity as the measurable parameter. Due to the complexity of local shear rate measurement, an average shear rate was assumed to exist and is proportional to superficial gas velocity. The data from this work and those in the literature were used in producing a new correlation for estimating average shear rates as a function of superficial gas velocity, geometry, and dispersion height. Wall shear rates were found to be significant. The ratio of wall shear rates to bulk shear rates were varied from 5% to 40%. Furthermore, it has been found that shear rates generated in airlift loop reactors are lower than those generated in bubble columns. Copyright 1999 John Wiley & Sons, Inc.

Mass transfer characteristics of a fermentation broth in a reactor: co-current downflow contacting reactor

Gas holdups, dispersion height, volumetric mass transfer coefficients and kLa of water and a yeast fermentation broth were studied in a co-current downflow contacting reactor. kLa and gas holdup increased with increasing superficial gas velocity and there was a parallelism between kLa and gas holdup. The values of kLa gas holdup, and dispersion height measured in the air/fermentation broth system were all lower than those in air/water system.

Effect of Sparger Design and Height to Diameter Ratio on Fractional Gas Hold-up in Bubble Columns

The combined effect of sparger design and dispersion height on fractional gas hold-up (−ɛG) was investigated in a 0.385 m i.d. bubble column. Perforated plates were used as -spargers. Free areas of sparger plates (F. A.) and hole diameters were varied in the range of 0.13% to 5.0% and 0.8 mm to 87 mm, respectively. The height to diameter ratio (HD/D) was varied in the range of 1–8. In all the cases, the superficial gas velocity was covered in the range of 30-300 mm s−1. In order to investigate the combined effects of sparger design and the HD/D ratio together with the coalescing nature of the liquid phase, three liquid systems were considered, namely, water, an aqueous solution of an electrolyte and an aqueous solution of carboxymethyl cellulose (CMC). The comparative behaviour of these systems has been presented. The effects of sparger design, HD/D ratio and the gas- liquid system have also been analysed on the basis of drift flux model. A predictive procedure has been given for the drift flux constants by simulating the flow pattern in bubble columns using Computational Fluid Dynamics (CFD).

Effect of Dispersion Properties on the Separation of Batch Liquid−Liquid Dispersions

The effect of dispersed phase holdup and dispersion height on the separation of batch liquid−liquid dispersions is experimentally investigated in terms of the variation in the heights of the sedimenting and coalescing interfaces with time. A model is presented which predicts these separation profiles, knowing the initial drop diameter and the interfacial coalescence time for a dispersion of given height, dispersed phase holdup, and physical properties. This is verified using the present and published experimental data.

EFFECTS OF GEOMETRY ON HYDRODYNAMICS IN EXTERNAL-LOOP AIRLIFT REACTORS

Experimental investigations were carried out in model external-loop airlift reactors. Two reactors of laboratory scale (riser liquid height ranged between 1.16-1.56 m, riser diameter 0.03 m, AD/AR ratio between 0.111-1,000, total liquid volume VT = (1.189-2.446).10−3m3) and pilot-plant scale (riser liquid height of 4.4 and 4.7 m, respectively, riser diameter 0.200 m, AD/AR ratio of 0.1225 and 0.040 m, total liquid volume, VT = (0.144-0.170) m3) were used. The influences of reactor geometry characterized by some parameter as: AD/AR ratio, liquid height in riser and downcomer and liquid height in gas separator, together with the amount of introduced air, on the basic hydrodynamic design parameters: gas holdup and liquid circulation velocity were analysed. The influence of gas sparger design on gas holdup and liquid velocity was found to be negligible. The experimental liquid circulation velocity was correlated using a simplified form of the energy balance in airlift reactors, valid for external-loop airlift reactors with almost complete phase separation at the top. An original dimensionless correlation for gas holdup prediction involving superficial velocities of gas and liquid, cross sectional areas, dispersion height, riser diameter, as well as Froude number, was obtained.

Interfacial Area and Gas Holdup in a Bubble Column Reactor at Elevated Pressures

The influence of pressure, liquid viscosity, and gas velocity on the gas holdup and specific gas−liquid interfacial area in a bubble column reactor has been studied. The 18.7 L reactor had an inner diameter of 15.6 cm with a dispersion height set equal to 3 times the diameter and was operated at pressures between 0.1 and 6.6 MPa. By means of the chemically enhanced absorption of CO2 in diethanolamine, the gas−liquid interfacial area in the reactor has been determined. The viscosity has been changed in the range from 1 to 9.4 mPa·s by adding ethylene glycol to the mixture. It is determined that pressure has a small effect on the gas holdup in pure water, whereas it shows a pronounced effect for the more viscous liquids. This can be attributed to the influence of the increased pressure on the flow regime transition. For the most viscous liquid all interfacial area data were obtained in the fully heterogeneous regime. Here the interfacial area increased with increasing pressure and was moderately affected by...

Kinetics of phase separation for polyethylene glycol–phosphate two‐phase systems

Phase separation times for polyethylene glycol (PEG)-4000-phosphate aqueous two-phase systems were studied, for small scale (5-g) and large scale (1300-g) systems, as a -function of the stability ratio. Profiles of dispersion height for both large and small scale systems were represented as a fraction of the initial height and were found to be independent of the geometrical dimensions of the separator. Furthermore, by plotting time as a fraction of the initial height the total time of separation can be calculated for a given height of system at a particular stability ratio. This generalization is important for the design of large scale aqueous two-phase separators. Phase separation times were also found to be dependent on which of the phases is continuous. A characteristic change in phase separation time was also observed at the phase inversion point (i.e., where the dispersed phase changes to a continuous phase and vice versa) and this point tends toward higher volume ratios as the tie-line length (TLL) is increased. Furthermore, the phase inversion point at each TLL corresponds to a fixed phosphate concentration.

Disengagement of the gas phase in bubble columns

The technique of gas disengagement is popularly used to assess the bubble size distribution in bubble columns. The technique involves the dynamic measurement of dispersion height when the gas supply is stopped. In this paper a mathematical model has been proposed for the process of dynamic gas disengagement. It has been shown that the initial faster disengagement is due to the presence of internal liquid circulation and not due to the presence of very large bubbles. Further, slower disengagement has been attributed to the transition from heterogeneous dispersion to homogeneous dispersion. The new model also explains the effects of superficial gas velocity, column diameter, column height and liquid phase physical properties on the gas disengagement.

Effect of pulp cleaning zone on gangue control in column flotation

A physical modelling technique has been used to evaluate the role of the zone between the feed inlet and the pulp froth interface (pulp cleaning zone) to control gangue recovery in column flotation. The behavior of gangue was simulated using Ottawa sand having d 80 of 150 ,um and 250 pm. The column used had an internal diameter of 0.06 m and the dispersion height was maintained at 0.84 m. By varying the feed position, the average solid concentration in the pulp cleaning zone was estimated. The results obtained indicate that a 0.20 m separation between the feed inlet and the interface was sufficient to effect the rejection of coarse gangue particles. Under similar operating conditions, the finer material exhibited negligible changes in the solid hold-up in this zone, thus indicating that the transport mechanisms for fine and coarse particles in flotation columns are different. The implications of these findings in column flotation are discussed.

Batch Settling of Liquid-Liquid Dispersion

A liquid-liquid two-phase flow takes place in many industrial processes. Common examples are water separation from crude oil, dispersion separation in different solvent extraction settlers, or dispersion separation at the ends of differential direct contact mass transfer columns. A liquid-liquid dispersion separation process was reviewed to enlighten a few aspects. A physical comprehensive model, followed by a mathematical model, was developed and compared to a set of experiments for the investigation of the liquid-liquid batch settler phase separation. Different parameters were checked experimentally, as initial dispersion heights, settler diameters, dispersion types, etc. Results allow the use of the experimental procedure, with the mathematical model, as a tool in monitoring the dispersion behavior in commercial units.

A unified approach to the scale-up of gas—solid fluidized bed and gas—liquid bubble column reactors

The aim of this work is to develop a unified approach to the scale-up of gas—solid (G—S) fluid bed and gas—liquid (G—L) bubble column reactors. The unified approach relies on analogies in the hydrodynamic behavior of G—S and G—L systems in both the homogeneous and heterogeneous flow regimes. The homogeneous G—S fluidization regime is to be identified with homogeneous G—L bubbly flow while the heterogeneous G—S fluidization regime is to be identified with the churnturbulent regime for G—L bubble columns. In the heterogeneous flow regime of operation, the classic two-phase theory developed for G—S fluidized beds can be applied with profit to describe also the hydrodynamics of G—L bubble columns provided the “dilute” phase is identified with the fast-rising large bubbles, and the “dense” phase is identified with the liquid phase containing entrained “small” bubbles. Quantitative analogies in the hydrodynamic behavior of G—S and G—L systems are demonstrated by the use of extensive experimental data obtained in five columns of three different diameters (2 × 0.1 m, 1 × 0.19 m and 2 × 0.38 m). The total expanded bed height in the experiments varied in the range 0.5–3.5 m. About 4,000 dynamic gas disengagement experiments were carried out with various systems to determine the total gas hold-up and the hold-ups of the “dilute” and “dense” phases. The gas phases used in the experiments were air, helium, argon and sulfur hexafluoride. Fluidized cracking catalyst (FCC) was used as the solid phase in fluid bed experiments. In bubble column operations the liquid phase used was water, paraffin oil or tetradecane. Sintered plates were used for gas distribution in all the columns. The gas hold-up in the “dense” phase, ɛ df was found to be practically independent of the scale of operation. The hold-up of the fast-rising “dilute” phase, ɛ b , on the other hand was found to be a significant function of the column diameter, D T , and of the total dispersion height, H. The “dilute” phase gas hold-up can be modeled for both G—S and G—L systems using a theory allowing for bubble growth in the region above distributor. The bubbles are assumed to grow in diameter up to a distance h *, at which the bubbles reach their equilibrium size. The equilibration height h * was found to increase with the superficial gas velocity through the dilute phase, U — U df ). For air—FCC, the value of h * varies in the range 0.4–1.2 m. For bubble columns the values of h * are significantly smaller and lie in the range 0–0.5 m. Increasing gas density increases the total gas voidage in both G—S and G—L systems but has no significant effect on the hold-up of the dilute phase. In G—L bubble columns, the liquid properties affect the total gas hold-up but have only a minor influence on the dilute phase hold-up. The unified model to describe the bubble hydrodynamics in G—S fluid beds and G—L bubble columns is a useful tool in scaling-up these two reactor types, because of the possibilities of cross-fertilization of design data.

Operating and hydrodynamic characteristics of a cocurrent downflow bubble column reactor

The hydrodynamic stability of the gas—liquid two-phase dispersion in a cocurrent downflow column (CDC) depends critically on the column configuration and such operating parameters as the inlet liquid velocity, column liquid velocity and bubble dispersion height. The hydrodynamic characteristics of the CDC were investigated using a column of 0.076 m (3in) i.d. with liquids of different surface tension, viscosity and coalescence properties. The two-phase flow patterns, bubble formation, stability and entrainment were observed for oxygen with water and aqueous solutions of 1-propanol, glycerol and sodium alginate. The minimum inlet liquid velocity required to maintain a stable gas—liquid dispersion was determined, as was the maximum column liquid velocity which did not give bubble entrainment in the outlet. An empirical correlation defining stable column operating conditions was proposed. Strongly coalescing liquids, such as water and water containing low concentrations of 1-propanol (≪0.15 wt%) or glycerol (≪5wt%), gave large diameter bubbles (3-5mm) and good gas disengagement which allowed the use of relatively high column liquid velocities (up to 0.14 m/s). Coalescence-inhibiting liquids such as aqueous 1-propanol (>0.015 wt%) and aqueous glycerol (>5 wt%) gave rise to very small bubbles (<1 mm) and required liquid outlet velocities in the range of 0.001–0.015 m/s in order to ensure bubble disengagement. The CDC compared with other types of downflow bubble columns was found to be capable of greater operational flexibility. No excess gas input or recycling was required in the CDC due to its ability to achieve 100% gas utilization. The CDC would be particularly suited to applications where high ratios of liquid-to-gas throughputs were required, such as in waste water or effluent treatment.

Flow injection dispersion characteristics with inductively coupled plasma atomic emission spectrometry. Part 1. Methodology

A flow injection (FI) dispersion study was conducted with inductively coupled plasma atomic emission spectrometric detection. Both the dispersion height and width behaviours were characterized. The dispersion time periods measured at 61 and 10% of the transient height (σ0.61, Δt0.1, tf and tr) successfully represented the dispersion width behaviour. The transient front and rear section slopes assumed a logarithmic dependence of the instantaneous emission signal with time. However, the slopes were sometimes non-linear. Deviations from conventional behaviour of the dispersion heights and widths for FI parameters were observed, and differences between normal and reverse FI modes occurred at low flow rates and/or large reactor volumes.

Mixing characteristics of external-loop airlifts: non-Newtonian systems

Mixing time (θ) was measured in a 45 or 60 1 working liquid volume external-circulation-loop airlift (ECL-AL) fermentor, and in a geometrically similar 6 1 prototype, both equipped with perforated-plate spargers (1 mm dia. holes). Carboxymethylcellulose solutions were used to simulate the rheological behaviour non-Newtonian fermentation broths. The specific mixing time (θ/ V) is shown to be highly dependent on the dispersion height and gas superficial velocity in the riser section, as well as on the riser diameter and the effective viscosity of the liquid phase. In the range studied (0.111⩽ A D / A R ⩽0.46), the downcomer-to-riser cross-sectional-area ratio has a relatively minor effect on θ/ V. The superficial velocity of the circulating liquid in the riser ( u LR ) was also measured. The results have been combined with those of a previous investigation, by Popovic and Robinson, to better elucidiate the effect of dispersion height on the rate of liquid circulation. A modified correlation of u LR is presented and discussed.

The continuous separation of liquid/liquid dispersions

A model allowing for binary and interfacial coalesecence of drops is presented which predicts the variation in dispersion height, throughput, droplet velocity and diameter along the length of a horizontal settler operating at the steady-state. Experimental results obtained in a such a box-settler verify the model which is applicable to both wedge-shaped and deep-bed dispersion bands of varying thickness. The analysis is extended to include the variation in hold-up, drop diameter and relative velocity in the sedimentation zone of a vertical settler under both laminar and turbulent flow conditions. Equations are presented which enable the height of the sedimentation zone to be predicted from the throughput, phase ratio, hold-up and binary coalescence time.

Gas-Liquid mass transfer in a bubble column with organic liquids

Water, methanol, toluene and ligroin were aerated in bubble column of 0.15 m I.D. and a height of 4.3 m . Using a gas disengagement technique, the holdups of large were found to be similar in all liquids whereas the holdups of small bubbles differed significally. Oxygen mass transfer studies with a dynamic absorption method were impaired by height-and time-dependent oxygen depletion in the gas phase. Only at low dispersion heights, there was agreement with the results of a steady state method. Photographic determination of the small bubbles' diameters and knowledge of their holdup fraction allowed to estimate their potential contributions to the mass transfer rate at gas flow rates up to 0.2m/s. These contribution were generally high and in ligroin the estimates even exceeded the observed total oxygen transfer rate. It is conclude that the finite rates of coalescence and break-up reduce the driving force for mass transfer from the small bubble fraction.

Hydrodynamic modelling and blank running of a pilot‐scale internal loop airlift reactor for wastewater treatment

Abstract The pilot‐scale airlift internal‐loop reactor for wastewater treatment (AILRW) differs in hydraulic characteristics and parameter values from the other types due to a shrunk top cover and low gas supplement. A blank running performed in AILRW indicated that the gas holdup in the riser had an average value fit the model of α=2.47Vgs 0.97 . The local value was linear with the axis coordinate. The linear distribution coefficient kα was measured as a function of the average gas holdup. The top connecting gap size had a great effect on the circulation. A model based on frictional pressure drop at the bottom was shown to predict successfully the liquid circulation while other models failed due to the limited head space and the effects of high gas holdup in the connecting section at the top of reactor. A model to estimate the height of dispersion in the gas drain was proposed. The overall oxygen transfer coefficient which was measured for a low range of gas superficial velocity of 0.28 to 2.4(E‐2 ms‐1) ...

Analysis of dispersion structure of fermentation media by dynamic gas disengagement method

In this communication, a digital dynamic gas disengagement sensor which is capable of measuring the transient dispersion height continuously in real time, is used to evaluate the dispersion structures of some synthetic media to demonstrate the usefulness of the technique

Effect of dyes and surfactants on the dynamic behaviour of liquid—liquid gravity settlers

The dynamic response of the steady-state dispersion height in gravity settlers due to the pulse injection of aqueous solutions of dyes and surfactants into the mixer or settler inlet is experimentally investigated. Positive changes in dispersion height are observed when an anionic or nonionic surfactant solution is injected at the settler inlet. Ampholytic surfactants produce a small negative change and cationic surfactants have more complicated effects, first producing a positive change followed by a negative change, probably related to the decrease in interfacial tension with concentration combined with the variation in interfacial tension gradients within the dispersion as the surfactant passes through. In all cases an increase in surfactant concentration decreases the interfacial tension but does not significantly affect the pH. When dyes are injected, positive and negative changes in dispersion height are observed depending on whether the pH increased or decreased; the dispersion height returning to its original value after a short time when the injected dye has passed through the system. The magnitude of the effect depends on the concentration and is lower when the dye is injected at the settler inlet rather than in the mixer. By adjusting the concentration of mixtures of dyes producing positive or negative changes it is possible for the dispersion height to be unaffected by the injection. Dyes have a pronounced effect on the pH but only a small effect on the interfacial tension. This is also true for mixtures of dyes and surfactants. Batch experiments indicate that reducing the pH so the solution is acidic increases the rates of both binary and interfacial coalescence whereas increasing the pH, so the solution is alkaline, decreases the interfacial coalescence rate but has no effect on the binary coalescence. These observations substantiate the effect of acidic and alkaline dyes on the dynamic behaviour of continuous dispersions.