D Dirk Thoenes

Aggregation kinetics of small particles in agitated vessels

Rapid coagulation by turbulence in stirred tanks was studied for particles and aggregates smaller than the Kolmogorov microscale. The coagulation kinetics are determined by the floc structure and by the hydrodynamic and colloidal interactions between the colliding particles. The collision efficiency for doublet formation in the heterogeneous shear field of a stirred tank follows from particle trajectory analysis of solid particles in simple shear flow, provided the simple shear rate is made to correspond to the residence time weighted turbulent shear rate. Experimentally, the resulting aggregates proved to be fractal-like with their porosity increasing with aggregate size. Porosity within the aggregates results in penetration of the floc surface by the fluid flow, giving rise to enhanced collision efficiencies compared to solid particles. The collision efficiencies between porous flocs may be estimated by a model that pictures a porous floc as consisting of an impermeable core and a completely permeable shell. With the collision efficiencies from this shell-core model the aggregate growth could be described adequately.

Surfactant-free emulsion polymerization of styrene using crosslinked seed particles

The aim of this research was to prepare a monodisperse polystyrene latex without surfactants adsorbed at the particle surface. Conventional polymerization formulations usually lead to large amounts of oligomers. Furthermore, they are characterized by a low reproducibility with respect to particle size. This was overcome by using a seed latex that was crosslinked in order to overcome dissolution in the monomer phase. By adjusting the seed concentration, any desired particle size in the range 0.5–1.2 μm could be obtained. The monodispersity was very good.

Dynamic modeling of limited particle coagulation in emulsion polymerization

In emulsion polymerization, a limited degree of particle coagulation may occur. Particle coagulation is caused by a loss of colloidal stabilization when the surface coverage of emulsifier on the particles drops below a critical value. It has been demonstrated experimentally in a previous article that the time scale for particle coagulation is small compared to the time scale for particle growth by polymerization and absorption of monomer. This indicates that the coagulation process can probably be described by von Smoluchowski kinetics. Based on this result, a comprehensive dynamic model for the simulation of limited particle coagulation in emulsion polymerization has been developed. It has been shown that there is a reasonable agreement between simulations with the dynamic model and experimental data (e.g., conversion time history, particle number, and particle size distribution).

A novel reactor for continuous emulsion polymerisation

Abstract It has been demonstrated earlier that a Pulsed Packed Column is a suitable reactor for continuous emulsion polymerization of styrene. The emulsion polymerization of vinyl acetate follows a different mechanism, which renders the process more liable to instabilities when carried out in a continuous flow reactor. It was demonstrated that emulsion polymerization of vinyl acetate in a Pulsed Packed Column can be carried out in a stable manner, and with a sufficient reaction rate, provided the rate of backmixing is limited.

Particle sizing by laser diffraction spectrometry in the anomalous regime

The application of laser diffraction spectrometry to determine the size distributions of particles in the anomalous diffraction regime, i.e., particles with a refractive-index ratio close to one, has been examined. From a computer simulation, using the Mie theory and the geometrical optics approximation, it could be concluded that for suspensions with a refractive-index ratio near 1, the corresponding scattering matrix is required for calculation of the correct particle size distribution, even in the case of particles that are much larger than the wavelength of the incident light. In a system with the refractive-index ratio almost at unity, a suspension of ice crystals in a sucrose solution, the ice particles were sized by means of optical microscopy and laser diffraction spectrometry, and the results were compared.

Application of the plug flow with axial dispersion model for continuous emulsion polymerization in a pulsed packed column

The performance of a pulsed packed column (PPC) has been investigated for the continuous emulsion polymerization of styrene. In a PPC, good local agitation is combined with little backmixing. Good local agitation provides proper emulsification, intensive radial mixing and high rates of heat transfer to the reactor wall. As a result of the low backmixing in the PPC, sustained oscillations in conversion and particle number, which are often observed in a CSTR, can be avoided completely. This indicates that the PPC is a promising reactor for continuous emulsion polymerization and that the performance of the PPC can be predicted by a steady-state model. In this study a comprehensive steady-state model has been derived for the continuous production of seed latexes in a PPC. The model is based on micellar nucleation and plug flow with axial dispersion. A rate equation for particle nucleation has been derived from batch experiments. It has been shown for the emulsion polymerization of styrene that there is a good agreement between simulations with the developed model and experiments in the PPC.

Emulsion copolymerisation in a flexible continuously operated reactor

The performance of a pulsed packed column (PPC) has been investigated for the continuous seeded emulsion copolymerisation of styrene and methyl acrylate. In the PPC, intensive radial mixing is combined with little axial mixing. Intensive local mixing provides proper emulsification and high rates of heat transfer to the reactor wall. It has been demonstrated that the performance of the PPC in terms of conversion and intermolecular chemical composition distribution approximates that of a batch process even for high pulsation velocities. Copolymers with a narrow composition distribution can be produced in the PPC by feeding the more reactive monomer styrene to the reactor contents at some axial positions in the column. Feed streams to adjust the monomer composition in the reacting particles have been calculated with a comprehensive multimonomer mechanistic emulsion polymerisation model. The results of this work indicate that a wide variety of latex products can be produced in the PPC with stable operation and a high conversion.

Numerical Particle Tracking in a Turbine Agitated Vessel

Modelling orthokinetic turbulent coagulation in a stirred tank requires knowledge of parameters describing size dependent growth and breakup rates. The residence time weighted strain rate is used for estimating the initial coagulation rate. As the aggregates grow larger, breakup occurs due to the shear stresses exerted upon the aggregates in a limited region near the impeller. In this region further growth is excluded resulting in a reduced coagulation rate. The reduced coagulation rate as well as the breakup frequency as a function of the aggregate size can be obtained from numerical particle tracking. The particle tracking program predicts the movement of the aggregates in the turbulently agitated vessel. The program requires knowledge of the flow pattern and the shear field in the turbine agitated vessel, which were determined from laser doppler velocimeter measurements.

High conversion emulsion polymerization in large scale reactors

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The polymerization rate of freshly nucleated particles during emulsion polymerization with micellar nucleation

A simple procedure was developed to account for the contribution of freshly nucleated particles to the total polymerization rate during micellar nucleation. It has been shown that the polymerization rate of the freshly nucleated particles cannot be described by a steady-state solution for a radical population balance over the particle size distribution, i.e., the classical Smith-Ewart recursion relation. Once nucleated, the particles grow for a significant period of time with one radical before either radical desorption or radical absorption, followed by instantaneous bimolecular termination, occur. For most emulsion polymerizations, radical desorption is the dominant process for radical loss of the freshly nucleated particles. A relation for the mean time that the freshly nucleated particles grow with one radical was derived.