Drop Size Measurements Research Articles

Produced-Water-Treating Systems: Comparison of North Sea and Deepwater Gulf of Mexico

In general, water-treatment systems in the North Sea differ from those in the deepwater Gulf of Mexico (GOM). The two most apparent differences are the extensive use of hydrocyclones in the North Sea, and the use of large, multistage horizontal flotation units in the deepwater GOM. Deepwater-GOM platforms use hydrocyclones, but not nearly to the extent that they are used in typical North Sea platforms. Typically in the North Sea, if flotation is used at all, it is a vertical compact unit. The objective of this paper is to provide an understanding of the reasons for these differences. In this paper, field data and modeling results are presented to explain these differences. The models accurately correlate the measured drop size and oil-in-water concentration observed in the two regions. In addition, the modeling tools are used to answer hypothetical “what if” questions. This allows isolation of individual variables such as fluid temperature, shear, separator residence time, and fluid density. Thus, the modeling provides a detailed understanding of the relative importance of these variables. It also provides a direct comparison of the performance of North Sea vs. GOM process configurations. While the qualitative conclusions are well-known (i.e., deepwater separation systems are designed to minimize weight and space), the detailed understanding provided here provides insight into the design of water-treatment systems in general. It also emphasizes, in a quantitative way, the importance of carrying out effective water treatment early in the process and the necessary use of large end-of-pipe equipment when this is not possible.

Breakup of Volatile Liquid Jet in Hot Cross Flow

Liquid jet in cross flow has been a widely studied subject although most of the experimental work has been carried out at ambient conditions. The present study involves the investigation of the behavior of an ethanol jet injected into a cross flowing air at elevated temperatures up to 428K. Spray parameters such as the trajectory and drop size distributions are measured and compared for cold and hot flow conditions. Experiments are conducted for a range of liquid-to-air momentum ratios (Q) ∼ 4 – 13 and for Aerodynamic Weber numbers (We) ∼ 15. Though most of the existing literature attributes the effect of elevated temperature on the effective change in Q due to density variation, the present study shows interesting observations where the results vary in spite of maintaining similar magnitudes of Q between cold and hot flow conditions. It is observed that a higher trajectory is consistently manifested by the ethanol jet at elevated temperatures as compared to that found at ambient temperatures. It is proposed that the physical parameter,density ratio, needs to be considered as an additional parameter to account for these differences. Therefore, a new trajectory correlation is proposed which is observed to match experimental data at different temperatures. The drop size measurements indicate considerable amount of evaporation of the fine droplets formed at the leeward portion of the resultant spray of ethanol jet in cross flow.

Prediction of emulsion drop size distributions in colloid mills

Colloid mills are the most common emulsification devices used in industry for products with high oil content. Drop breakage occurs when the emulsion is flowed through a small gap between rotor and stator under laminar shear conditions. In this paper, we have developed the first full population balance equation (PBE) model for colloid mills and used the model to better understand the relevant drop breakage mechanisms. The PBE model accounted for both drop breakage and coalescence and generated predictions of the drop size distribution after each pass of the emulsion through the colloid mill. Drops were assumed to break due to capillary instability with the distribution of drop sizes resulting from each breakage event studied in detail. A viscosity model was developed to predict the emulsion viscosity as function of the oil fraction and the high shear rates commonly used. Nonlinear optimization was used to estimate adjustable parameters in the breakage and coalescence functions to minimize the least-squares difference between predicted and measured drop size distributions for high oil-to-surfactant emulsions. We concluded that experimentally observed drop volume distributions could not be predicted with daughter drop distribution functions reported in the literature. Improved predictions were obtained using a new bimodal distribution function which captured drop breakage into multiple, nearly uniform daughter drops with a large number of small satellite drops. We also investigated model extensibility for changes in the oil fraction, emulsion flow rate and rotor speed.

Comparison of Drop Size and Velocity Measurements by a Laser PrecipitationMeter and Low-Speed Photographyor an Agriculture Sprinkler

<abstract> <bold><sc>Abstract. </sc></bold>Kinetic energy of water droplets has a substantial effect on development of a soil surface seal and infiltration rate of bare soil. Methods for measuring sprinkler droplet size and velocity needed to calculate droplet kinetic energy have been developed and tested over the past 50 years, each with advantages, disadvantages, and limitations. Drop size and velocity of an impact sprinkler at three operating pressures and one nozzle size were measured using a laser precipitation meter and compared with published values obtained using a photographic method. Significant differences in cumulative volume drop size distributions derived from the two measurement methods were found, especially at the highest operating pressure. Significant differences in droplet velocities were found between measurement methods as well. Significant differences were attributed to differences in minimum drop sizes measured; 0.5 mm for the photographic method versus 0.2 mm for the laser precipitation meter. The laser precipitation meter provided smaller cumulative volume drop size distributions compared to the photographic measurement method. The laser precipitation meter tended to provide greater drop velocities which were attributed to altitude differences at experimental sites. The difference in calculated droplet kinetic energy per unit drop volume based on drop and size velocity data from the laser precipitation meter and the photographic method ranged from +12.5 to -28%. The laser precipitation meter generally provided a lower estimate of sprinkler kinetic energy due to the measurement of a greater proportion of smaller drop sizes. Either method can be used to obtain drop size and velocity sprinkler drops needed to calculate sprinkler kinetic energy. The laser precipitation meter requires less skill and labor to measure drop size and velocity.

Characterization of hydromechanical stress in aerated stirred tanks up to 40 m(3) scale by measurement of maximum stable drop size.

BackgroundTurbulence intensity, or hydromechanical stress, is a parameter that influences a broad range of processes in the fields of chemical engineering and biotechnology. Fermentation processes are often characterized by high agitation and aeration intensity resulting in high gas void fractions of up to 20% in large scale reactors. Very little experimental data on hydromechanical stress for such operating conditions exists because of the problems associated with measuring hydromechanical stress under aeration and intense agitation.ResultsAn indirect method to quantify hydromechanical stress for aerated operating conditions by the measurement of maximum stable drop size in a break-up controlled dispersion was applied to characterize hydromechanical stress in reactor scales of 50 L, 3 m3 and 40 m3 volume with a broad range of operating conditions and impeller geometries (Rushton turbines). Results for impellers within each scale for the ratio of maximum to specific energy dissipation rate ϕ based on measured values of maximum stable drop size for aerated operating conditions are qualitatively in agreement with results from literature correlations for unaerated operating conditions. Comparison of data in the different scales shows that there is a scale effect that results in higher values for ϕ in larger reactors. This behavior is not covered by the classic theory of turbulent drop dispersion but is in good agreement with the theory of turbulence intermittency. The data for all impeller configurations and all aeration rates for the three scales can be correlated within ±20% when calculated values for ϕ based on the measured values for dmax are used to calculate the maximum local energy dissipation rate. A correlation of the data for all scales and all impeller configurations in the form ϕ = 2.3∙(ϕunaerated)0.34∙(DR)0.543 is suggested that successfully models the influence of scale and impeller geometry on ϕ for aerated operating conditions.ConclusionsThe results show that besides the impeller geometry, also aeration and scale strongly influence hydromechanical stress. Incorporating these effects is beneficial for a successful scale up or scale down of this parameter. This can be done by applying the suggested correlation or by measuring hydromechanical stress with the experimental method used in this study.

Shipborne Polarimetric Weather Radar: Impact of Ship Movement on Polarimetric Variables at C Band

Abstract The effect of ship motion on shipborne polarimetric radar measurements is considered at C band. Calculations are carried out by (i) varying the “effective” mean canting angle and (ii) separately examining the elevation dependence. Scattering from a single oblate hydrometeor is considered at first. Equations are derived (i) to convert the measured differential reflectivity for nonzero mean canting angles to those for zero mean canting angle and (ii) to do the corresponding corrections for nonzero elevation angles. Scattering calculations are also performed using the T-matrix method with measured drop size distributions as input. Dependence on mean volume diameter is examined as well as variations of the four main polarimetric parameters. The results show that as long as the ship movement is limited to a roll of less than about 10°–15°, the effects are tolerable. Furthermore, the results from the scattering simulations have been used to provide equations for correction factors that can be applied to compensate for the “apparent” nonzero canting angles and nonzero elevation angles, so that drop size distribution parameters and rainfall rates can be estimated without any bias.

Effects of melting layer on Ku-band signal depolarization

Propagation effects on Ku-band over an earth-space path is carried out at Kolkata, India, a tropical location, by receiving a Ku-band signal with horizontal plane polarization transmitted from the geostationary satellite NSS-6 (at 95°E). The amplitude of co-polar attenuation has been monitored along with the measurements of rain rate, rain drop size distribution and height profile of rain rate. The cross-polar enhancement of the signal is also monitored by receiving the same signal in orthogonal direction with another identical receiver. The experimental observations are used to study the effect of melting layer on both co-polar attenuation and cross-polar enhancement for the rain events observed during 2012–2013. Melting layer is indicated by the bright band signature in vertical profile of rain rate. The ground based drop size measurements indicate that the stratiform rain has more number of small drops whereas convective rain composed of large rain drops. The results indicate that the depolarization due to melting layer is more dominant compared to that due to the drop deformation mechanism at low rain rates.

Evaluation of the New Version of the Laser-Optical Disdrometer, OTT Parsivel2

Abstract A comparative study of raindrop size distribution measurements has been conducted at NASA’s Goddard Space Flight Center where the focus was to evaluate the performance of the upgraded laser-optical OTT Particle Size Velocity (Parsivel2; P2) disdrometer. The experimental setup included a collocated pair of tipping-bucket rain gauges, OTT Parsivel (P1) and P2 disdrometers, and Joss–Waldvogel (JW) disdrometers. Excellent agreement between the two collocated rain gauges enabled their use as a relative reference for event rain totals. A comparison of event total showed that the P2 had a 6% absolute bias with respect to the reference gauges, considerably lower than the P1 and JW disdrometers. Good agreement was also evident between the JW and P2 in hourly raindrop spectra for drop diameters between 0.5 and 4 mm. The P2 drop concentrations mostly increased toward small sizes, and the peak concentrations were mostly observed in the first three measurable size bins. The P1, on the other hand, underestimated small drops and overestimated the large drops, particularly in heavy rain rates. From the analysis performed, it appears that the P2 is an improvement over the P1 model for both drop size and rainfall measurements. P2 mean fall velocities follow accepted terminal fall speed relationships at drop sizes less than 1 mm. As a caveat, the P2 had approximately 1 m s−1 slower mean fall speed with respect to the terminal fall speed near 1 mm, and the difference between the mean measured and terminal fall speeds reduced with increasing drop size. This caveat was recognized as a software bug by the manufacturer and is currently being investigated.

Airborne Rain-Rate Measurement with a Wide-Swath Radar Altimeter

Abstract The NOAA Wide-Swath Radar Altimeter (WSRA) uses 80 narrow beams spread over ±30° in the cross-track direction to generate raster lines of sea surface topography at a 10-Hz rate from which sea surface directional wave spectra are produced. A ±14° subset of the backscattered power data associated with the topography measurements is used to produce independent measurements of rain rate and sea surface mean square slope at 10-s intervals. Theoretical calculations of rain attenuation at the WSRA 16.15-GHz operating frequency using measured drop size distributions for both mostly convective and mostly stratiform rainfall demonstrate that the WSRA absorption technique for rain determination is relatively insensitive to both ambient temperature and the characteristics of the drop size distribution, in contrast to reflectivity techniques. The variation of the sea surface radar reflectivity in the vicinity of a hurricane is reviewed. Fluctuations in the sea surface scattering characteristics caused by changes in wind speed or the rain impinging on the surface cannot contaminate the rain measurement because they are calibrated out using the WSRA measurement of mean square slope. WSRA rain measurements from a NOAA WP-3D hurricane research aircraft off the North Carolina coast in Hurricane Irene on 26 August 2011 are compared with those from the stepped frequency microwave radiometer (SFMR) on the aircraft and the Next Generation Weather Radar (NEXRAD) National Mosaic and Multi-Sensor Quantitative Precipitation Estimation (QPE) system.

Comparative study of the main flotation frothers using a new HYDROMESS adapted technique

Stability of bubble size in the pulp zone and froth control is an important parameter for a successful flotation process. The present study compares the action of the main frothers in terms of: mean bubble size in the pulp zone, froth stability, surface-tension-lowering ability and reagent participation in flotation of a naturally hydrophobic mineral. Bubble size was determined using a new technique adapted from HYDROMESS, a device designed to measure drop size. The remaining indicators were obtained by determining surface tension and conducting flotation tests with the addition of hydrophobic (graphite) and hydrophilic (calcite) particles. The results indicate that methyl isobutyl carbinol (MIBC) is the most efficient frother in controlling coalescence, with critical coalescence concentration (CCC) reached at 20 ppm. Moreover, this frother has greater potential for selective action, since it increased calcite entrainment by only 2.3% at a concentration of 30 ppm. In flotation tests with a sample of graphite, MIBC produced higher recoveries.

Maximum stable drop size measurements indicate turbulence attenuation by aeration in a 3 m3 aerated stirred tank

In this study, break-up controlled drop dispersion experiments in a 3m3 pilot scale reactor with 1.2m inner diameter are presented that allow a correlation of the maximum local energy dissipation rate in stirred reactors with intense aeration. Experiments with two different setups of 6-bladed Rushton turbine impellers with diameters of d=0.41m (d/DR=0.34) and d=0.51m (d/DR=0.43) in a 3 impeller configuration were conducted. The results from experiments without aeration are well in agreement with the existing literature on drop dispersion. The results from experiments with aeration indicate a strong attenuation of turbulence intensity in stirred tank reactors by the presence of air. The ratio between maximum and volume-averaged energy dissipation rate was reduced by aeration by 64% for the d/DR=0.34 impeller and by 52% for the d/DR=0.43 impeller when compared with unaerated operating conditions on the basis of equal volumetric power input. The value of the aeration rate had no measurable effect in the range of aeration rates applied, which was between 0.1vvm (volume of gas/volume liquid/minute) and 1vvm.

Mass transfer and emulsification by chaotic advection

This study characterizes a new mixing process based on chaotic advection for the production of water/oil (w/o) emulsified engine fuel. At low and intermediate Reynolds numbers, in curved pipes, Dean roll-cells induce radial convective mass transfer, a mechanism exploited in the technology of helical static mixers. The succession of bends of alternating curvature planes produces in addition spatially chaotic flow trajectories that enhance the mixing over and above that in the reference helically coiled geometry made of coplanar bends. This feature is assessed in the present work by comparison of the droplet sizes obtained in helical (regular) and alternating (chaotic) static mixers of the same cross section and the same tube length. The coils are assembled from 90° bends, and the chaotic configuration is obtained by turning each bend by ±90° with respect to the previous one.To cover a large range of Reynolds numbers [30–350] and capillary numbers [0.1–1], a lipophilic solvent (Butanol) is added to the continuous phase (oil) to decrease the viscosity and a surfactant (Tween 20) is used beyond the critical concentration (covering the whole w/o interface) to decrease the interfacial tension. Drop-size measurements at the exit of the mixer in the chaotic advection configuration show substantial drop-size reduction and tightening in drop-size distribution. Moreover, it is shown that mixing intensification by chaotic advection is almost independent of Reynolds number in the studied range, with a mixing efficiency enhancement around 30%.

COMPARISON OF DROP SIZE DATA FROM GROUND AND AERIAL APPLICATION NOZZLES AT THREE TESTING LABORATORIES

Spray drop size is a critical factor in the performance of any agrochemical solution and is a function of spray solution, nozzle selection, and nozzle operation. Applicators generally select a particular nozzle based on the drop size reported by manufacturers and researchers. Like most population sampling methods, the accurate measurement of spray drop sizing is a function of sampling methodology, accuracy of the measurement, and inferences about a total population from a subset. Studies were conducted to determine the repeatability and accuracy of spray drop size from a standardized set of spray nozzles at three different application technology research laboratories (USDA-ARS in College Station, Texas; University of Nebraska-Lincoln in North Platte, Nebraska; and University of Queensland, Gatton in Gatton, Australia). To minimize differences in drop size measurements between laboratories, the same set of nozzles was used at each location. The three laboratory measurements of drop size varied by less than 5% except for the measurement of the very largest drops in a spray plume. Day-to-day differences in drop size measurements within each lab were also found to be around 5%. This work shows that through careful monitoring of spray pressure, air speed, and measurement distance, very close agreement in drop size measurements can be obtained between different facilities.

Review on Measurement Techniques for Drop Size Distribution in a Stirred Vessel

A literature review on measurements of drop size distribution in liquid–liquid dispersion produced in a stirred vessel is presented in this work. The methods of measurement can be classified into in situ and external measurement. Two main groups of measurement techniques, namely, a laser system and image analysis, are reviewed. Several issues regarding the applications of the techniques and possible ways to overcome the problems are discussed. The suitability of different techniques depends on the operating conditions and properties of the drops. Laser systems provide fast in situ measurements which are useful for online monitoring and detecting process changes but unable to deliver reliable drop size and distribution values. In situ image analysis techniques could give accurate measurement of drop size, but a long time is required to analyze drops from a large number of images. However with development of automated image analysis, analysis time can be reduced. Therefore real-time monitoring and process control by image analysis techniques can be possible.

A combined lossy capacitor population balance model (LCPBM) for calculating the influence of frequency on electric field enhanced coalescence in a static-mixer settler setup

In this work a combined lossy capacitor population balance model (LCPBM) is developed to predict the effect of a square wave frequency on electric-field coalescence (drop size) in a static-mixer settler setup for the caprolactam–toluene–water system. The static-mixer is used to mix the organic and aqueous phases. The electric field is applied by placing insulated electrodes at the end of the static-mixer. The electrical-circuit model of the system is based on the lossy capacitor model where the insulation and the dispersion are considered as leaky dielectrics. The charge and the electric field are determined from the electrical-circuit model and coupled in the hydrodynamic equation to calculate the velocities of drops. The velocities are in turn used in the population balance model (PBM) to determine the time evolution of the drop size. Three approaches are used to calculate the electrical force and the charge on the drops from the circuit model: (1) the Bailes′ charge hypothesis where the drops pick their charge from the free dispersion–insulation interfacial charge, and the electrical force calculated based on electrophoretic contribution, (2) the electrical force determined from the Taylor–Melcher leaky dielectric model considering the electrical force due to the free charge and the force due to the difference in dielectric permittivity's of the dispersion and insulation, and (3) by accounting for the free charge convection by the fluid motion which results in equal redistribution of the polarization charge between the dispersion and the insulation. The (LCPBM) model was validated by measuring drop sizes using a square wave of 0.4kV/cm and frequencies between 3 and 100Hz for two flowrates. With all approaches, the mean sauter diameters were calculated within 10% relative error at lower frequencies for both flowrates. At higher frequencies of 50Hz and 100Hz, a 20% relative error was obtained for the first approach. A better prediction within 10% was found for the second and third approaches with the later approach giving the best prediction.

Single-Pass Emulsification Processes in Two Different Inline High Shear Mixers

Single-pass continuous emulsifications were studied in kerosene– and silicone oil–aqueous systems with two inline high shear mixers (HSMs) adopting two main commercial configurations, including the dual-row ultrafine-toothed and the single-row blade–screen units. The effects of the processing parameters on the measured drop sizes and power consumptions were investigated. Bimodal drop size distributions (DSDs) were observed within both inline HSMs due to the inhomogeneity of the turbulence and shear levels accompanied with the recirculation and re-entrainment flow patterns. The drop sizes increase with increases in the dispersed-phase volume fraction and the continuous-phase flow rate and decrease with increases in the rotor speed and the continuous-phase viscosity. Correlations for the Sauter mean diameters were also obtained. The results obtained here are fundamental for the assessment of emulsification capability as well as the design and selection of inline HSMs.

Intensification of metal extraction with high-shear mixing

A laboratory flow reactor (Vr=9.5mL) with a high shear mixer was used to study the intensification of extraction and stripping. The stirring rate was 15,000rpm and the reactor space-time was varied from 1.3 to 13s in extraction experiments. The phases were separated with an in-line centrifuge. Results were compared to those made in batch reactor (Vr=7.7L) equipped with conventional pitched-blade turbine impeller. The residence time and drop size distributions of flow reactor were measured. Copper extraction was made from sulfate solution using a hydroxyoxime reagent (LIX 984). Both extraction and stripping reached equilibrium in a few seconds in the flow reactor, while the same required about 250s in the batch reactor. Residence time distribution was utilized in the model of extraction kinetics. Calculated pseudohomogeneous extraction kinetic constants of the flow reactor and the batch reactor were 5.9 and 0.034Lmol−1s−1, respectively. Difference was over 150-fold. Measured drop size distributions indicate that differences in generated interfacial area can explain only a part of rate increase. Such dramatic intensification of extraction in flow reactor is interpreted here to be due to both increased interfacial area and decreased diffusion path length.

Measurement of maximum stable drop size in aerated dilute liquid–liquid dispersions in stirred tanks

Turbulence intensity, or hydromechanical stress, is a controlling parameter in many industrially relevant processes. Especially fermentation processes are often characterized by intense aeration and agitation, operating conditions for which the measurement of turbulence intensity is extremely difficult. Since the maximum stable drop diameter in a break-up controlled dispersion is directly correlated with turbulence intensity, the measurement of drop sizes can enable an indirect access to the intensity of turbulence under such operating conditions. This work presents the constraints and the development of a method for the measurement of maximum stable drop size in aerated liquid–liquid dispersions in stirred tanks for the purpose of characterizing turbulence intensity. Continuous and dispersed phase properties were selected to achieve a break-up controlled dispersion with negligible coalescence. This was accomplished mainly by applying a dilute dispersion, a low ionic strength and by incorporating a dispersed phase, paraffin oil, with a negative spreading coefficient. The negative spreading coefficient prevents coalescence due to drop–bubble interactions for aerated operating conditions. It was demonstrated that the off-line measured drop size distributions are representative for the conditions in the bioreactor and are not altered by sample handling. The sampling and measurement procedure was found to be highly reproducible with a standard deviation for the maximum stable drop size for independent experiments of approximately 10%. Relevant constraints for the application of the method in large-scale experiments were discussed and accounted for during method development to allow a later application in production scale equipment.

Flapping instability of a liquid jet

We study the flapping instability observed when a liquid jet is incompletely atomized by a fast parallel gas stream: the remaining liquid jet is destabilized over a scale large compared with its radius, and breaks into liquid fragments. We characterize the symmetry of this instability and its frequency. The intact liquid length is measured as a function of gas and liquid velocity, and turns out to be longer than the one predicted by Raynal (1997) for a planar mixing layer. The frequency of the instability is measured with a spectral method, and is in agreement with the frequency observed for the planar shear instability, though slightly smaller. The planar, and not helical, symmetry of the instability makes it akin to a flapping instability, observed when a planar liquid sheet is atomized by two planar gas streams. We next measure drop sizes when the flapping instability is present, with a method based on image processing. Measured size distributions are in agreement with distributions observed in a mixing layer geometry for low gas velocities (long tail distribution). The mean drop diameter depends weakly on liquid velocity, and decreases as d10∼Ug0.9. On the contrary, Sauter diameter depends strongly on liquid velocity.

Predicting the effects of surfactant coverage on drop size distributions of homogenized emulsions

Population balance equation (PBE) models have been extensively used to predict drop size distributions of processed oil-in-water emulsions. Our previous work on high pressure homogenization demonstrated that both drop coalescence and breakage must be included to obtain satisfactory distribution predictions at high oil-to-surfactant ratios. While it was capable of predicting drop size distributions over a range of surfactant concentrations, this PBE model was not extensible to other surfactant types without re-estimation of adjustable model parameters. The objective of the present study was to develop a new PBE model that allows satisfactory prediction for emulsion systems with different surfactant types and concentrations given model parameters estimated from drop size distribution data collected for a single surfactant at a single concentration. This extended PBE model was developed from our previous model by adding a surfactant mass balance, including the effects of the free surfactant concentration on the interfacial tension and the surface coverage of drops and by modeling the coalescence frequency to be a function of the surfactant coverage. To demonstrate the approach, model parameters were obtained by nonlinear optimization using measured drop size distributions collected at 50wt% oil and 1wt% Pluronic F68 surfactant. These parameters were used to predict drop size distributions at 50wt% oil and 0.5–2.0wt% surfactant for Pluronic F68 and three other surfactants from the Pluronic family. We found that the extended PBE model generated substantially improved distribution predictions compared to our previous model, with the degree of improvement dependent of the surfactant used. These results represent an important step towards the use of PBE models for emulsified product design.