Influence of different silica nanoparticles on drop size distributions in agitated liquid‐liquid systems

The impact of different silica nanoparticles on rheology, interfacial tension and drop size distributions in liquid‐liquid systems is determined experimentally. The particles vary in wettability and specific surface area. In contrast to commonly used high‐energy devices for Pickering emulsion preparation, low energy input by stirring allows to quantify drop breakage and coalescence in steady state and dynamic conditions. The experiments can provide essential information for drop size model development in nanoparticle‐stabilized emulsions.DFG, 56091768, TRR 63: Integrierte chemische Prozesse in flüssigen MehrphasensystemenTU Berlin, Open-Access-Mittel - 201

Generalized breakup and coalescence models for population balance modelling of liquid-liquid flows

Population balance framework is a useful tool that can be used to describe size distribution of droplets in a liquid-liquid dispersion. Breakup and coalescence models provide closures for mathematical formulation of the population balance equation (PBE) and are crucial for accu- rate predictions of the mean droplet size in the ow. Number of closures for both breakup and coalescence can be identi ed in the literature and most of them need an estimation of model parameters that can di er even by several orders of magnitude on a case to case basis. In this paper we review the fundamental assumptions and derivation of breakup and coalescence ker- nels. Subsequently, we rigorously apply two-stage optimization over several independent sets of experiments in order to identify model parameters. Two-stage identi cation allows us to estab- lish new parametric dependencies valid for experiments that vary over large ranges of important non-dimensional groups. This be adopted for optimization of parameters in breakup and co- alescence models over multiple cases and we propose a correlation based on non-dimensional numbers that is applicable to number of di erent ows over wide range of Reynolds numbers

Phase inversion in nonionic surfactant-oil-water systems

This study has been concerned with the inversion of water in oil (W/0) emulsions, to oil in water (O/W) emulsions and vice-versa. It has been shown that there are two types of emulsion phase inversion that can occur in nonionic Surfactant-Oil-Water (nSOW) systems: (i) A "transitional" inversion, which is brought about by changing the nSOW phase behaviour, by altering the surfactant's affinity for the oil and water phases and, (ii) a "catastrophic" inversion, induced by increasing the dispersed phase fraction and occurs at closest packing of unstable dispersed phase drops. The inversion mechanism of the two inversion types has been characterised. The two inversion types can be represented as boundaries on a "map" relating nSOW phase behaviour with water to oil volume ratio. The form of the map depends on the nature of the oil. At the transitional point, the nSOW system can be 3 phase - an oil phase, a water phase and a surfactant phase microemulsion. Ultra-low interfacial tension exists between the phases - this property is of interest for producing extremely fine emulsions with low energy input. Transitional inversions are sometimes reversible. In nSOW systems, true catastrophic inversions can be induced by moving the water to oil ratio in one direction only. Double emulsion drops (W/O/W or O/W/O) are sometimes produced before inversion and inversion points are dependent on dynamic conditions. A thermodynamic relationship between nSOW phase behaviour, oil type, surfactant type, surfactant concentration and temperature has been derived, based on the partitioning of surfactant between oil, water and a surfactant micelle phase. It has been shown how this can be used to classify nonionic surfactants. The effect of agitation conditions, water addition rate and oil phase viscosity, on the drop types and drop sizes of emulsions present before and after inversion (for each inversion type) has been studied extensively. Surfactant type and concentration also affect drop behaviour and drop sizes. Various drop types have been identified and qualitative and quantitative analysis of the factors controlling the drop sizes of emulsions at each stage of a phase inversion has been developed

Scale-up study of suspension polymerisation in an oscillatory baffled reactor

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Breakage of drops in two-liquid phase dispersions in mechanically agitated vessels

Measurements of drop size distributions were made for a range of xylene-water and sunflower oil-water dispersions in a 0.15 m diameter unbaffled cylindrical vessel, mechanically agitated with a range of standard Rushton turbine impellers. The effects of dispersed phase concentration, viscosity of the two-phase liquid mixture, impeller speed and diameter on drop size distribution were investigated. All dispersions contained 0.3%w/w of the surfactant. Sodium Dodecyl Sulphate (SDS) used to minimise the effect of drop coalescence during the experiments. Experimental evidence for xylene-water dispersions showed that as the dispersed phase concentration increased beyond 50% by volume, the physical properties of the dispersion changed, notably the rheological properties of the dispersion deviated from Newtonian behaviour and became shear thinning; the apparent viscosity of the two-phase liquid mixture increased exponentially with dispersed phase concentration. Measurements of the electrical conductivity and visual assessment of the dispersion using a photomicrography technique showed no evidence for the occurrence of global phase inversion during the experiments. The evidence however indicated that the structure of the dispersion changed progressively as the dispersed phase concentration was increased. For xylene phase concentration greater than 50% by volume, the bulk flow in the vessel was non-turbulent (Re < 10000) and the average drop diameter was successfully described using a model developed in this thesis based on shear breakage in the boundary layer flow next to the rotating disk of the impeller. For xylene phase concentration less than 50% by volume, the rheology of the dispersion was primarily determined by the viscosity of the continuous phase (water) and drop size distributions were described well by the established equations based on Kolmogoroff's (1949) theory of isotropic turbulence. The drop size distribution data obtained for sunflower oil-water dispersions were more erratic and less reproducible compared with the xylene-water system, especially for sunflower oil concentrations greater than 50% by volume. However, the general trend in data was similar for both dispersions. The lack of reproducibility and conformity of drop size distribution data for sunflower oil-water dispersions with the model was largely attributed to the flow regime remaining predominantly turbulent and the possible presence of small amounts of contaminants present in the low-grade food oil samples purchased locally for these experiments

Modelling of liquid-liquid dispersions in batch stirred vessels

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Inverse microsuspension polymerisation of aqueous acrylic acid using redox initiators

In an inverse microsuspension polymerisation initiated by a redox pair, at least one component of the redox pair must be segregated from the monomer initially to restrict the extent of polymerisation before the dispersion is established. Normally, the monomer and the oxidant in aqueous solution are dispersed in an oil phase, and the aqueous reductant is separately added to start polymerisation. Thus, the reaction system initially consists of two types of aqueous drop and the distribution of reactants is heterogeneous in nature. In the literature, no report to date has been found concerning the mechanistic aspects of this polymerisation process. The present work concerns polymerisation of aqueous acrylic acid in drops that are dispersed in a paraffinic oil and stabilised with a non-ionic surfactant. A sodium metabisulphitel potassium bromate redox initiation system is used. Drop mixing in agitated dispersions has been investigated by examining the aqueous drop behaviour in simulation systems using various developed techniques. The evolution of aqueous drop size and size distribution has been characterised throughout the course of polymerisation using a freeze-fracture technique with electron microscope. The variations of polymerisation rate, limiting conversion, final particle size and size distribution have been investigated by altering several key parameters, such as agitation intensity, volume and composition of the aqueous reductant and ways of adding the aqueous reductant. Aqueous polymerisation of acrylic acid in a singlephase, initiated by the redox pair, has been also studied and a reaction scheme has been proposed. Kinetic relationships for polymerisations in both an aqueous homogeneous medium and a heterogeneous medium have been obtained and rationalised in terms of mechanism. A new hypothesis for the polymerisation process has been developed which differs from both conventional suspension and emulsion polymerisations. It is believed that the polymerisation takes place in the aqueous drops as a result of continuous simultaneous coalescence and break-up of the different types of aqueous drops. The surfactant has important effects on the course of polymerisation. A preliminary model has been developed for describing this specific polymerisation process. The publications arising from the present project are listed in Appendix VIII

Three-way coupling simulation of a gas-liquid stirred tank using a multi-compartment population balance model

© 2016 by De Gruyter.Modelling of gas-liquid stirred tanks is very challenging due to the presence of strong bubble-liquid interactions. Depending upon the needs and desired accuracy, the simulation may be performed by considering one-way, two-way, three-way or four-way coupling between the primary and secondary phase. Accuracy of the prediction on the two-phase flow generally increases as the details of phase interactions increase but at the expense of higher computational cost. This study deals with two-way and three-way coupling of gas-liquid flow in stirred tanks which were then compared with results via four-way coupling. Population balance model (PBM) based on quadrature method of moments (QMOM) was implemented in a multi-compartment model of an aerated stirred tank to predict local bubble size. The multi-compartment model is regarded as three-way coupling because the local turbulent dissipation rates and flow rates were obtained from a two-way computational fluid dynamics (CFD) simulation. The predicted two-phase flows and local bubble size showed good agreement with experimental data

Modelling of mass transfer in gas-liquid stirred tanks agitated by Rushton turbine and CD-6 impeller: a scale-up study

A combined computational fluid dynamics (CFD) and population balance model (PBM) approach has been applied to the simulation of gas-liquid stirred tanks agitated by (i) a Rushton turbine or (ii) a CD-6 impeller, operating at aeration numbers from 0.017 to 0.038. The multiphase simulations were realised via an Eulerian-Eulerian two-fluid model and the drag coefficient of spherical and distorted bubbles was modelled using the Ishii-Zuber equations. The effect of the void fraction on the drag coefficient was modelled using the correlation by Behzadi et al. (2004). The local bubble size distribution was obtained by solving the PBM using the quadrature method of moments (QMOM). The local kLa was estimated using both the Higbie penetration theory and the surface renewal model. The predicted gas-liquid hydrodynamics, local bubble sizes and dissolved oxygen concentration were in good agreement with experimental measurements reported in the literature. A slight improvement in the prediction of the aerated power number was obtained using the non-uniform bubble size distribution resulting from the coupled CFD-PBM simulation. Evaluation of the prospective scale-up approaches indicates a higher probability of maintaining a similar level of mass transfer in a larger tanks by keeping the Pg/V and VVM constant. Considering its predictive capability, the method outlined in this work can provide a useful scale-up evaluation of gas-liquid stirred tanks