UMD Theses and Dissertations
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Item Liquid-liquid Dispersion in Batch and In-line Rotor-Stator Mixers(2013) Rueger, Paul Edwin; Calabrese, Richard V; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This two-part dissertation investigates the behavior of batch and in-line rotor-stator mixers separately. In the first study, water was dispersed into viscous oil using a batch Silverson L4R rotor-stator mixer. The flow regime was determined by reference to published Power number data and by qualitative differences in drop size data. Drop breakup in laminar flow was analyzed by comparison to published single drop breakup experiments in idealized flow fields. The breakup mechanism in laminar flow was similar to that for simple shear flow and equal to about twice the nominal shear rate in the rotor-stator gap. Drop breakup in turbulent flow followed a mechanistic correlation for mean drop size for drops less than the Kolmogorov microscale, but still large enough that both inertial and viscous effects were manifest. Surfactants decreased drop size with Marangoni effects observed near the CMC for laminar, but not for turbulent flow. Below phase fractions of 0.05, d32 increased in a log-linear fashion with phase fraction for all conditions tested including: laminar and turbulent flow, presence of surfactant, and hydrophobically treated high-shear surfaces. The significant effect of phase fraction was caused by the flow structure being locally laminar near the drops, and was permitted by sufficiently low fluid viscosities which promoted film drainage. Above phase fractions of 0.1, drop sizes plateaued. This was attributed to decreasing coalescence rate and efficiency, along with increasing breakup. In the second study, the power consumption of an IKA 2000/4 in-line pilot scale rotor-stator mixer was measured with a purpose-built torque meter. The power spent by the mixer on pumping was insignificant compared to viscous dissipation. A constant power number was obtained for turbulent flow using constant power per stage with an empirically determined effective diameter for each generator type. For conditions where mean drop size was close to equilibrium, as determined by flowrate independence, previously reported mean drop size data were calculated using the well-known inertial subrange scaling law along with the power draw measurements of the present study. The maximum local energy dissipation rate was found to be nine times the average energy dissipation rateItem Effect of Surfactants on Drop Size Distributions in a Batch, Rotor-Stator Mixer(2004-12-17) Padron, Gustavo A.; Calabrese, Richard V; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Surfactants are often required to stabilize liquid-liquid dispersions produced by rotor-stator mixers. Since drops are deformed rapidly due to the high power input of these devices, the dynamic interfacial properties governed by the surfactant adsorption rate have a significant effect on the resulting drop size. The objective of this work is to develop a fundamental link between surfactant adsorption dynamics, interfacial properties, and turbulent emulsification processes in rotor-stator mixers. The mean drop size and drop size distributions (DSD) of dilute dispersions produced by a batch rotor-stator mixer were studied. Silicone oils of various viscosities were dispersed in aqueous nonionic surfactant and aqueous methanol solutions. The aqueous methanol (clean) systems allowed comparison of surfactant-laden to surfactant-free systems with similar equilibrium interfacial tensions. The DSD were measured via a video microscopy/automated image analysis technique. The equilibrium interfacial tension of clean and surfactant systems was measured, via a pendant drop technique, as a function of methanol and surfactant concentration, respectively. The dynamic surface tension of surfactant solutions was similarly measured. By fitting the data to the Langmuir adsorption isotherm and a long time approximation to the Ward Tordai equation, the adsorption parameters and surfactant diffusivities were obtained. This information, with an estimate of the drop deformation timescale, allowed estimation of the surface dilational modulus (Esd). This is a measure of the Marangoni stresses acting on the drop' surface due to interfacial tension gradients. Trends observed in the mean drop size and DSD experimental results are explained in terms of the interfacial and rheological properties. Below the CMC, Esd peaks and the drop size increases with concentration, despite a decrease in equilibrium interfacial tension. Above the CMC, Marangoni stresses are small but the presence of the surfactant still modifies the rheology of the interface, increasing the effective viscosity of the drops. A comprehensive set of mechanistic models for drop size in turbulent flows was developed and modified to partially account for the effect of surfactants via an appropriately defined effective viscosity. Various model choices were systematically fit to the drop size data to select the most appropriate mechanistic correlation. Normalized experimental DSD data collapsed to a single log-normal volume distribution.