Jg Johan Wijers

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.

Analysis of Fouling in Refuse Waste Incinerators

Gas-side fouling of waste-heat-recovery boilers, caused mainly by the deposition of particulate matter, reduces the heat transfer in the boiler. The fouling as observed on the tube bundles in the boiler of a Dutch refuse waste incinerator varied from thin and powdery for the economizer to thick and sintered for the superheater. Analysis of process data showed that both types of layers resulted in a 27% decrease of the heat transfer coefficient of the bundles. To determine the important mechanisms in the deposition of particles, layers taken from the different bundles are analyzed using electron microscopy. The analysis revealed the existence of a melt in the thick deposit. The melt, giving rise to a liquid phase, increases the sticking efficiency of the deposit and leads to larger deposition rates. For the economizer and the superheater the actual deposition rate is calculated from the change in heat transfer. On the basis of a comparison between the calculated deposition rates and deposition rates to be expected in the case of a pure diffusion and thermophoresis process, it is shown that for both types of deposits inertia-controlled transport is the dominant transport mechanism of particles.

Hydrodynamics in a ceramic pervaporation membrane reactor for resin production

The hydrodynamics in a pervaporation membrane reactor for resin production has been investigated. In this type of reactors it is important to reduce concentration and temperature polarization to obtain high water fluxes during operation. The influence of secondary flow on polarization, as induced by small density differences, is studied using Computational Fluid Dynamics in a model system. This model is operated in three parallel flow situations: horizontal, vertical opposed and vertical adding flow. Density-induced convection is found to be most effective in the horizontal situation increasing water fluxes up to 50%. Water fluxes were also determined experimentally using the horizontal setup. Influence of density-induced convection was observed experimentally.

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.

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.

Measurement of 2D-temperature distributions in a pervaporation membrane module using ultrasonic computer tomography and comparison with computational fluid dynamics calculations

Temperature polarization effects can considerably limit the overall performance of pervaporation modules. This study shows the measurement of temperatures in a pervaporation membrane module using ultrasonic computer tomography (U-CT). With this technique complete 2D-temperature distributions that occur in the module can be measured within seconds. Temperature distributions were also determined using computational fluid dynamics (CFD). Both temperature distributions are comparable and show that in the tubular pervaporation system studied, temperature segregation occurs. This segregation is a result of the occurring hydrodynamics in the system that is determined by mixed convection. Also, the measured water fluxes through the membrane show to be dependent of forced and natural convection.

Mixing with a pfaudler type impeller; the effect of micromixing on reaction selectivity in the production of fine chemicals

Publisher Summary Stirred tank reactors with a Pfaudler type impeller are frequently used in the production of fine chemicals, but information on the mixing performance of this type of impeller is limited. This information is required to predict selectivities of mixing sensitive processes. For low feed rates, which are often used in the production of fine chemicals, the mixing process is controlled by micromixing. To investigate the micromixing in stirred tank reactors with a Pfaudler type impeller, the product distributions of mixing sensitive reaction sets are determined. These experiments show that in a large part of the reactor the product distribution is not a function of feed point position. With a micromixing model (E-model) using the average energy dissipation rate, the product distribution is calculated. The calculated product distributions are in reasonable agreement with the measured product distributions for partially baffled reactors with a Pfaudler type impeller for a broad range of process conditions.

DETERMINATION OF THE MIXING RATE OF A HIGH VELOCITY FEED STREAM IN AGITATED VESSELS

Abstract In semi-batch or continuously stirred reactors, often a feed containing one or more reactants has to be mixed with the contents of the vessel. For fast competitive or consecutive reactions the mixing rate of the feed stream with the vessel contents has a large influence on the product quality. The mixing rate is often controlled by the turbulent dispersion process. Therefore, it has been suggested in the literature to keep the turbulent dispersion time constant upon scale-up to obtain a constant product quality. In this study, based on a combination of a theoretical model, Planar Laser Induced Fluorescence experiments and Laser Doppler Velocimetry experiments, the turbulent dispersion coefficient is determined. This has been done for the case that a feed stream is mixed in a stirred vessel by a combination of feed stream and stirrer generated turbulence. The turbulent dispersion coefficient is used to derive an equation for the turbulent dispersion time as function of several design and process variables.

Scaling-Up of Reactive Crystallizers

In precipitation reactors the formation of solids is a fast process. When stirred vessels are used nucleation occurs almost entirely near the feed stream inlet. In that case the mixing rate of the feed stream with the bulk contents, on a scale larger than the Kolmogorov scale, determines the product quality such as particle size or particle size distribution. It is recommended to keep the time needed for mixing at this larger scale, the mesomixing time, constant on scaling-up.

Mass transfer to and copper deposition on a round bar in a new type of electrolytic cell: the helix cell

High-rate electrodeposition of copper from CuSO4-H2SO4 baths can be achieved by using crossflow of solution. To obtain copper layers of uniform thickness and quality, a new type of electrolytic cell, the helix cell, has been proposed. An experimental dimensionless relation has been given to describe the mass transfer to a round bar, in crossflow, in a helix cell. Moreover, the current efficiency of copper deposition has been obtained as a function of current density, flow rate of solution, temperature and weight per cent CuSO4 in the CuSO4-H2SO4 solution.