Sauter Mean Drop Diameter Research Articles

A study of the drop size distributions and hold-up in short Kühni columns

The hydrodynamic behaviour of a short Kühni column was investigated under no mass transfer conditions using the binary system water (continuous phase) and Exxsol D-80 (dispersed phase). The counter-current flow pattern of the liquid phases was characterised regarding the Sauter mean drop diameter, drop size distribution and hold-up; a photographic method was used to assess drop sizes. The following operating variables were studied: rotor speed, flow rate of both liquid phases and column stage. The log-normal probability density function was found to be adequate to fit the experimental drop size distributions along the column. As expected, smaller drops and more uniform drop size distributions were obtained with the increase of rotor speed and column stage number, thus indicating the predominance of drop breakage phenomena in short columns. The total hold-up was influenced mainly by rotor speed and flow rate of the dispersed phase. Recommended correlations available in the literature were found to be inadequate for predicting experimental drop sizes and hold-up, so alternative expressions, valid only for short Kühni columns, were proposed.

CFD Modelling of Turbulent Drop Breakage in a Kenics Static Mixer and Comparison with Experimental Data

This study has investigated drop breakage in liquid–liquid systems by means of advanced numerical modelling tools based on computational fluid dynamics (CFD). A static mixer with 24 standard Kenics inserts was analysed with the volume fraction of dispersed phase equal to 0.1%. Four superficial velocities were used resulting in Reynolds numbers of 12 000, 15 000, 18 000 and 21 000, respectively. To simulate the evolution of the drop size distribution (DSD), the population balance equation (PBE) was coupled with the multi-phase description and added to the standard equations in the CFD code with proper kinetic expressions for the breakage rate. Instead of solving the original PBE, transport equations for the moments of the DSD were solved and the closure problem was overcome by using the quadrature method of moments (QMOM). As expected, the predicted drop size decreased along the mixer and pressure drop, final size distribution and Sauter mean drop diameters, d 32, all agreed well with data reported in a previous experimental study, the best agreement for d 32 being from −10% to +9%, obtained with the finest mesh, 927 k.

The effect of agitation intensity on alkali-catalyzed methanolysis of sunflower oil

The sunflower oil methanolysis was studied in a stirred reactor at different agitation speeds. The measurements of drop size, drop size distribution and the conversion degree demonstrate the effects of the agitation speed in both non-reaction (methanol/sunflower oil) and reaction (methanol/KOH/sunflower oil) systems. Drop size distributions were found to become narrower and shift to smaller sizes with increasing agitation speed as well as with the progress of the methanolysis reaction at a constant agitation speed. During the methanolysis reaction, the Sauter-mean drop diameter stays constant in the initial slow reaction region, rapidly decreases during the fast reaction period and finally reaches the equilibrium level. Due to the fact that the interfacial area increases, one can conclude that the rate of reaction occurring at the interface will also be enhanced progressively. The “autocatalytic” behavior of the methanolysis reaction is explained by this “self-enhancement” of the interfacial area, due to intensive drop breakage process.

Effect of Impeller Blade Height on the Drop Size Distribution in Agitated Dispersions

AbstractA common method to achieve a contact of two liquid phases – required for many chemical engineering operations – is the dispersion of one into the other by mechanical agitation. The drop size distribution in such an agitated dispersion is a result of the dynamic equilibrium existing between the breaking and coalescing drops. A comparison has been made of drop diameters produced by four disk type impellers differing only in blade height (DW = 1, 2, 4 and 6 cm). Measurements in situ at 200, 250, 300, 350, 400, 450 rpm and at holdup fractions 0.02, 0.05, and 0.07, showed that the Sauter mean drop diameters increased up to 140 % as the impeller blade height decreased from 6 to 1 cm. Plots of ln α32 vs. ln N, lnα32 vs. ln DT and ln α32 vs. ln αmax gave straight lines.

Comparison of mean drop sizes and drop size distributions in agitated liquid–liquid dispersions produced by disk and open type impellers

Abstract A comparison is made of drop diameters produced by a disk and an open type six-blade impeller having the same impeller diameter and width and rotated at the same speed. Drop size measurements in situ at 200, 250, 300, 350, 400 and 450 rpm and at hold-up fractions 0.01, 0.025, 0.05 and 0.10 showed that always the Sauter mean drop diameters produced by the open style impeller were 6–82% larger than the ones produced by the disk impeller. Plots of ln d 32 versus ln N and ln d max versus ln N gave straight lines. Plots of d 32 versus d max gave relationships not quite linear.

Effect of the Number of Impeller Blades on the Drop Sizes in Agitated Dispersions

A comparison is made of drop diameters produced by disk impellers having two, four, six and eight blades, all of the same impeller diameter and rotated at the same speeds. Measurements in situ at rotational speeds between 250 and 560 RPM and at hold-up fractions 0.01, 0.025, 0.07 showed that the Sauter mean drop diameters increase with the decrease of number of impeller blades and this increase may reach up to 52 %. Plots of ln d32 versus ln N, ln d32 versus ln n and d32 versus dmax gave straight lines.

Suspension polymerisation of methyl methacrylate using sodium polymethacrylate as a suspending agent

Aqueous solutions of sodium polymethacrylate (PMA-Na) are used as suspending agents for the suspension polymerisation of methyl methacrylate. These aqueous polyelectrolyte solutions are viscous and they show a non-Newtonian shear thinning behaviour. The Sauter mean drop diameter decreases with increasing continuous-phase viscosity, but increases with increasing stirring speed. The maximum drop sizes follow the same trends. The Sauter mean diameter also decreases, initially, for increasing hold-up, reaches a minimum, and then increases with increasing hold-up. Taylor's theory of viscous break-up seems to be suitable to describe the breakage of the droplets.

Hydrodynamic Behavior of a Rotating Disc Contactor Under Low Agitation Conditions

ABSTRACT The hydrodynamics of a rotating disc contactor (76.2 mm diameter, 19 stages, and operating height of 815 mm) has been investigated at limiting operational conditions of low holdup (φ ≤ 3%) and low agitation (Re R ≤ 22,000) for the water/n-heptane system in the absence of mass transfer. Water was chosen as the continuous phase, and it was kept stagnant inside the column. Local drop size distribution, local mean drop diameter, and total holdup have been measured at different flow rates of the dispersed phase (70 to 640 mL/min) and rotor speeds (0 to 700 rpm). Drop size distributions along the column were suitably described using the log-normal probability density function, and empirical expressions were derived to predict the parameters of the distribution curve as a function of operating conditions. An empirical expression was also proposed to predict the Sauter mean drop diameter along the column with a mean deviation of 3.4%. Statistical analysis has revealed no significant effect of the rotor speed on the total holdup, which changed solely with the dispersed phase flow rate for the operating conditions investigated.

Drop size distribution in highly concentrated liquid–liquid dispersions using a light back scattering method

AbstractNew data are presented on drop size distribution at high dispersed phase fractions of organic‐in‐water mixtures, obtained with a light back scattering technique (3 Dimensional Optical Reflectance Measurement technique, 3D ORM). The 3D ORM technique, which provides fast, in‐situ and on‐line drop distribution measurements even at high concentrations of the dispersed phase, is validated using an endoscope attached to a high‐speed video recorder. The two techniques compared favourably when used in a dispersion of oil (density (ρ) = 828 kg m−3, viscosity (µ) = 5.5 mPa s, interfacial tension (σi) = 44.7 mN m−1) in water for a range of 5–10% dispersed phase fractions. Data obtained with the ORM instrument for dispersed phase fractions up to 60% and impeller speeds 350–550 rpm showed a decrease in the maximum and the Sauter mean drop diameters with increasing impeller speed. Phase fractions did not seem to significantly affect drop size. Both techniques showed that drop size distributions could be fitted by the log‐normal distribution. Copyright © 2005 Society of Chemical Industry

Scale-down studies on the hydrodynamics of two-liquid phase biocatalytic reactors.

The maintenance of constant interfacial area per unit volume is a key parameter for the successful scale-up of two-liquid phase bioconversion processes. To date, however, there is little published information on the hydrodynamics of such systems and a suitable basis for scale-up has yet to be defined and verified. Here we report power input and hydrodynamic data for a whole-cell bioconversion process using resting cells of Rhodococcus R312 to catalyse the hydration of a poorly water-soluble substrate 1,3-dicyanobenzene (1,3-DCB). Experiments were performed in geometrically similar 3-L and 75-L reactors, each fitted with a three-stage Rushton turbine impeller system. The two-phase system used comprised of 20% v/v toluene dispersed in 0.1 M aqueous phosphate buffer containing up to 10 g(ww) x L(-1) of resuspended biocatalyst and 20 g x L(-1) 1,3-DCB. The power input to the 3-L reactor was first determined using an air-bearing technique for both single-phase and two-phase mixing. In both cases, the power number attained a constant value of 11 at Re>10,000, while the measured power inputs were in the range 0.15-3.25 kW x m(-3). Drop size distributions and Sauter mean drop diameters (d(32)) were subsequently measured on-line in both reactors, using an in-situ light-backscattering technique, for scale-up on the basis of either constant power input per unit volume or constant tip speed. At both scales d(32) decreased with increasing agitation rate, while the drop size distributions obtained were log-normal. All the measured d(32) values were in the range of 30-50 microm, with the lowest values being obtained in systems with biocatalyst present. In all cases, constant power input per unit volume was found to be the most suitable basis for scale-up. This gave virtually identical d(32) values and drop size distributions at both scales. A number of correlations were also identified that would allow reasonable prediction of d(32) values for various agitation rates at each scale. While the results obtained are for a particular phase system, the scale-down methodology presented here would allow the rapid evaluation of other bioconversion processes in the 3-L reactor with a 25-fold reduction in scale. In this way, potential problems that might be encountered at the larger scale, such as the carryover of antifoam from the fermentation stage, could be quickly and efficiently identified.

Measurement of interfacial areas with the chemical method for a system with alternating dispersed phases

The interfacial area for a liquid–liquid system has been determined by the chemical reaction method. The saponification of butyl formate ester with 8 M sodium hydroxide has been used to this end. A correlation has been derived to describe the mole flux of ester through the interface and the kinetic rate constants have been calculated. For a continuously operated reactor a correlation has been derived for both reaction in the dispersed phase as well as reaction in the continuous phase. A viscosity factor had to be incorporated to obtain one single correlation for the Sauter mean drop diameter. The validation for this chemically enhanced reaction regime is presented and discussed.

Study of drop and bubble sizes in a simulated mycelial fermentation broth of up to four phases.

The mean sizes and size distributions of air bubbles and viscous castor oil drops were studied in a salt-rich aqueous solution (medium), first separately, and then simultaneously as a three-phase system. The dispersion was created in a 150-mm-diameter stirred tank equipped with a Rushton turbine, and the sizes were measured using an advanced video technique. Trichoderma harzianum biomass was added in some experiments to study the effect of a solid phase under unaerated and aerated conditions to give either three-or four-phase systems. In all cases, the different dispersed phases could be clearly seen. Such photoimages have never been obtained previously. For the three phases, air-oil-medium, aeration caused a drastic increase in Sauter mean drop diameter, which was greater than could be accounted for by the reduction in energy dissipation on aeration. Also, as in the unaerated case, larger drops were observed as the oil content increased. On the other hand, mean bubble sizes were significantly reduced with increasing oil phase up to 15% with bubbles inside many of the viscous drops. With the introduction of fungal biomass of increasing concentration (0.5 to 5 g L(-1)) under unaerated conditions, the Sauter mean drop diameter decreased. Finally, in the four-phase system (oil [10%]-medium-air-biomass) as found in many fermentations, all the phases (plus bubbles in drops) could clearly be seen and, as the biomass increased, a decrease in both the bubble and the drop mean diameters was found. The reduction in size of bubbles (and therefore increase in interfacial area) as the oil and bio- mass concentration increased provides a possible explanation as to why the addition of an oil phase has been reported to enhance oxygen transfer during many fermentations.

Newtonian power curve and drop size distributions for vibromixers

AbstractPower input data are presented for a twin flat disk up‐and‐down moving (vibromixer) impeller operating in a small vessel with a range of Newtonian liquids. Vibromixer power number and Reynolds number are defined and are used to establish the Newtonian power curve for this type of mixer. Drop size distributions are presented for xylene‐in‐water dispersions under turbulent flow conditions in the vibromixer and are shown to vary with the maximum velocity of the disk (2πAf). The Sauter mean drop diameter of the distribution is related to the vibromixer Weber number, (We =ρ(2πAf)2D/σ), by an equation of the type d32/D = C (We)−3/5 with the coefficient C = 0.37.

Prediction of vinyl chloride drop sizes in stabilised liquid-liquid agitated dispersion

Drop sizes and drop size distributions have been determined for stabilised dispersions of vinyl chloride (VCM) in agitated aqueous solutions of hydrolysed poly(vinyl acetate). For this system, where drop sizes depend on breakage (rather than on coalescence), experimental measurements confirm the assumptions that the Sauter mean drop diameter is proportional to the maximum drop diameter. Increases in volume fraction of the dispersed phase lead to widening of the drop size distribution. When the agitator speed is reduced, the size distribution changes from monomodal to multimodal. By using estimates of interfacial tension and dispersion density, a complete relationship between drop size, volume fraction of VCM and Weber number is obtained.

THE EFFECT OF TWO-LIQUID PHASE RHEOLOGY ON DROP BREAKAGE IN MECHANICALLY STIRRED VESSELS

Abstract Experiments were carried out to determine the effect of the dispersed phase concentration,.φ, on the Sauter mean drop diameter of xylene in water in a mechanically agitated vessel. In the course of the experiments, the rheology of the dispersions was also measured. It was found that the viscosity of the dispersion exhibited strong viscous non-Newtonian characteristics for dispersed phase concentrations greater than approximately 50% by volume. The increase in the apparent viscosity of the dispersion with increasing dispersed phase concentration changed the flow condition in the tank from turbulent regime to transitional and laminar flow regime. The Sauter mean drop diameter measured as a function of the dispersed phase was successfully interpreted in terms of a turbulent eddy-drop breakage mechanism for φ < 50% and by a boundary layer drop breakage model for φ > 50% by volume.

Effect of analyte and solvent transport on signal intensities in inductively coupled plasma atomic emission spectrometry

Aerosols generated with several different nebulizers have been characterized by their major aerosol characteristics, namely the Sauter mean drop diameter, D 3,2, the analyte mass transport, W tot, the solvent mass transport, S tot, the analyte transport efficiency, ϵ n, and the solvent transport efficiency, ϵ s. These characteristic aerosol parameters have been compared with the magnitudes of analytical signals, I net, generated in an inductively coupled plasma, in order to examine the possible existence of interrelationships between them. For three of the nebulizers tested, a reasonably linear correlation between W tot and I net was observed with water as solvent. For the fourth, a crossflow-type nebulizer, which produced a tertiary aerosol with a significantly larger mean drop size than the other nebulizers, the relationship between W tot and I net was not linear. For the organic solvents tested, methanol and n-butanol, solvent/plasma interactions were sufficiently strong as to make the relationship between W tot and I net quite complex.

Simulation of mass transfer in a batch agitated liquid-liquid dispersion

A simulation model is developed to compute the transient Sauter-mean drop diameter and the fractional mass transfer in batch agitated liquid-liquid dispersions when the continuous phase, dispersed phase, or both phases offer significant resistance to mass transfer. This model accounts for mass transfer during drop breakup, drop-drop coalescence or rebounding, drop oscillations, and free motion of the drops throughout the vessel

Countercurrent pulsed and reciprocating plate extractors. Prediction of Sauter mean drop diameter

Six published correlations for Sauter diameter d32 in pulsed and reciprocating plate columns are compared with drop size measurements from seven different data sources. The correlation of Kumar and Hartland gives the best fit of data in the whole range of agitation rates. Also the semiempiric equations of Misek and of Boyadzhiev and Spassov are of general validity at higher agitation levels. A new semiempiric equation covering the whole range of agitation rates and describing experimental data with a standard deviation of 10% is presented.

Performance of Spray, Sieve Tray, and Packed Contactors for High Pressure Extraction

Abstract The performance of various contactors in high pressure extraction service was studied. The mass transfer efficiency of a spray column and columns filled with sieve trays, 1.27 cm ceramic Raschig rings, and no. 15 metal Intalox saddles were obtained from a “windowed” high pressure extraction column with an internal diameter of 9.88 cm and a column height of 168 cm. The extraction column has an internal diameter of 9.88 cm and a contacting height of 168 cm. Glass windows in the extractor allowed visual observation and photography. Supercritical and liquid carbon dioxide were used to extract isopropanol from water at pressures of 81 to 150 atmospheres and 297 to 318 [ddot]K. The sieve tray extractor yielded the highest mass transfer efficiency followed closely by Raschig rings and metal Intalox saddles. The spray column was by far the least efficient. The mass transfer efficiency was observed to be independent of the operating conditions studied in this work. Sauter mean drop diameters and dispersed...

Dispersed phase holdup and drop size distribution in pulsed plate columns

AbstractDispersed phase holdup was measured in a pulsed plate column for the kerosene‐water system under binary conditions and under solute transfer from dispersed to continuous and continuous to dispersed phases. The experimental data were satisfactorily modelled through a recirculation regime model.The drop size distribution, measured by a photographic technique, exhibited a multinodal character at low agitation rates and high dispersed phase flow rate. Sauter mean drop diameter was found to depend on the agitation rate, the dispersed phase flow rate, the mass transfer direction and the plate free area. Correlations for d32 and the interfacial area were presented using Kolmogoroff's isotropic turbulence model.