Representative drop sizes and drop size distributions in A/O dispersions in continuous flow stirred tank (original) (raw)
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Effect of Impeller Blade Height on the Drop Size Distribution in Agitated Dispersions
Chemical Engineering & Technology, 2006
A 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 (D W = 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 a 32 vs. ln N, lna 32 vs. ln D T and ln a 32 vs. ln a max gave straight lines.
Industrial & Engineering Chemistry Research, 1989
T h e microscopic phenomena occurring in an agitated vessel are extremely complex. The exact mechanisms of coalescence and breakage in bubble and drop systems are generally not very well understood. Several attempts have been made in the literature to improve our understanding of these phenomena in multiparticle systems. In the present work, the most comprehensive models describing the coalescence and breakage processes in a styrene/water dispersion system are incorporated in a numerical algorithm to calculate the steady-state drop size distributions in a batch stirred vessel. A new breakage distribution function is introduced that considers droplet broken into two daughter and several satellite droplets. As a result, bimodal diameter density distributions are obtained. It is shown that the proposed model has the ability t o fit reasonably well a series of experimental data obtained for a low-coalescence system a t different impeller speeds, temperatures, and dispersed phase hold-up fractions.
Population Balance Modeling of Liquid−Liquid Dispersions in Homogeneous Continuous-Flow Stirred Tank
Industrial & Engineering Chemistry Research, 2009
This work presents population balance modeling to predict representative drop sizes and drop size distributions in dispersions of dilute phosphoric acid in a mixture of n-paraffin, tributyl phosphate, and di-2-ethyl hexyl phosphoric acid (D2EHPA) created in a continuous-flow stirred tank agitated with a top-shrouded pump-mix impeller having four trapezoidal blades. The stirred tank has been assumed to be homogeneous. Three population balance models, differing in droplet breakage rate models, have been evaluated. A part of the experimental data has been used to obtain the optimum values of the parameters of all the three models. The remaining part of the experimental data has been used to compare the models for their ability to predict the representative drop diameters and the number probability density distributions. On the basis of this comparison, the best model has been identified.
Drop breakup in turbulent stirred-tank contactors. Part I: Effect of dispersed-phase viscosity
AIChE Journal, 1986
Numerous experiments were conducted in four, baffled cylindrical tanks of standard geometry, equipped with six-blade Rushton turbines, by photographically examining dilute suspensions of silicone oils in water. Five grades of oil, ranging in viscosity from about 0.1 to 10 Pas and exhibiting the same interfacial tension with water ([0.0378 N / m), were employed. The range of variables studied includes 13,000 < Re < 101.000. 44 < We < 1.137, and 0.065 < < 0.50 m2/s3. The objectives of the experimental program were to examine the extent to which dispersed-phase viscosity influences equilibrium mean drop size and drop size distribution at constant interfacial tension, and to determine the relevance of the predicted correlating parameters and the range of applicability of the semiempirical theory.
AIChE Journal
Previous studies on emulsification have used the maximum drop size (d max) or Sauter mean diameter (d 32) to investigate the effect of viscosity on the drop size distribution (DSD), however these parameters fall short for highly polydispersed emulsions. In this investigation (Part I) we studied the steady-state DSD of dilute emulsions using of silicon oils with viscosities varying across 6 orders of magnitude at different stirring speeds. Different emulsification regimes were identified; our modelling and analysis is centred on the intermediate viscosity range where interfacial cohesive stresses can be considered negligible and drop size increases with viscosity. The bimodal frequency distributions by volume were well described using two log-normal density functions. In Part II 1 we expanded this study by using other continuous phases of different viscosity while keeping the Power number constant, thus examining the effects of viscosity ratio.
Industrial & Engineering Chemistry Research, 1991
Surface-active agenta play a very important role in the production of polymers by suspension polymerization. Their function is to adsorb to the monomer/water interface, thus, preventing coalescence of monomer droplets. One commonly used stabilizer is poly(viny1 acetate) partially hydrolyzed to poly(viny1 alcohol) (PVA). The degree of hydrolysis and the molar mass of PVA affect its ability to act as a stabilizer. T o assess the effectiveness of PVA in stabilizing monomer drops, a model system of 1% styrene in water was selected for our experimental studies. The stabilizer had a viscosity average molar mass of 22 OOO and a hydroxyl group content of 97.5-99.5 mol 70. The interfacial tension of the styrene/water system was measured by the Wilhelmy plate method for various concentrations of PVA. A number of experiments were carried out to study the effect of stabilizer concentration on the evolution of drop size distribution. The minimum time required for a system to reach steady state and the corresponding average drop diameters were measured as a function of stabilizer concentration, temperature, and agitation speed. Correlations were developed to relate the minimum transition time, t h , and Sauter mean diameter, dZ, with the Weber number of the main flow. Zwick, M. M. Poly(viny1 alcohol)-iodine complexes. J. Appl. Polym.
Time evolution of the drop size distribution for liquid–liquid dispersion in an agitated tank
Chemical Papers, 2017
Agitating two immiscible liquids or a solid-liquid suspension is an operation frequently performed in the chemical and metallurgical industries, for example, in suspension/emulsion polymerization, heterogeneous/phasetransfer catalytic chemical reactions, and hydrometallurgical solvent extraction. For emulsification, suspension polymerization, solid particle dispersion, and crystallization, it is essential to be able to predict the mean drop/particle size and the drop/particle size distribution. A simple model was proposed for predicting the time evolution of drop size distribution during drop breaking, and was successfully tested on data published by Ruiz and
Droplets size evolution of dispersion in a stirred tank
EPJ Web of Conferences
Dispersion of two immiscible liquids is commonly used in chemical industry as wall as in metallurgical industry e. g. extraction process. The governing property is droplet size distribution. The droplet sizes are given by the physical properties of both liquids and flow properties inside a stirred tank. The first investigation stage is focused on in-situ droplet size measurement using image analysis and optimizing of the evaluation method to achieve maximal result reproducibility. The obtained experimental results are compared with multiphase flow simulation based on Euler-Euler approach combined with PBM (Population Balance Modelling). The population balance model was, in that specific case, simplified with assumption of pure breakage of droplets.