Pjam Piet Kerkhof

Electric field driven separations : phenomena and applications

Abstract The efficiency of separation processes can usually be assured either by an energy input or by external forces. The majority of conventional sep- arations applies mechanical or thermal energy in combination with pressure or gravity forces. The ability of superimposed electric fields to improve separation processes has been well known for many years. The familiar industrial applications range from solid-solid separation in the beneficiation of ores in the mining industry, coalescence of “water-in-oil” emulsion in the petroleum industry, to cleaning of exhaust gases from solid particles in various technologies.

Unsteady-state flux behaviour in relation to the presence of a gel layer*

Abstract The unsteady-state flux behaviour has been studied for silica and dextran in a stirred ultrafiltration cell. Under the experimental conditions dextran and silica show a clearly different flux behaviour. During the filtration of dextran only a polarization layer is build up. For silica also a gel layer formation occurs. As a result the time to reach steady-state flux is less than a minute for dextran, whereas the formation of the silica gel layer takes more than one hour. The osmotic pressure model provides a good description of the flux for the experiments with dextran. If mass transfer coefficients are used which are higher than those electrochemically measured the transient flux for silica can be rather well predicted by the gel-polarization model. The use of flux measurements the presence of a gel layer is discussed.

Measurement and Modelling of the Air Flow Pattern in a Pilot-Plant Spray Dryer

The air flow pattern in a co-current pilot plant spray dryer (diameter 2.2 metres) was modelled and measured for the case without spray. The swirl angle was zero and the modelling was done with a computational fluid dynamics package (FLOW3D by CFDS). The boundary conditions for the CFD-model (velocity and turbulence quantities at the inlet) were derived from measurements with a hot-wire probe. To validate the CFD model, air velocity magnitudes were measured at numerous locations in the spray drying chamber. To interpret the data, a novel approach was developed based on the interpretation of velocity distributions rather than time-averaging the signals. This was necessary because of flow reversals and large fluctuations in the air velocity. The measurements were compared with the CFD model results and the agreement between model and measurements was reasonable.

AIR FLOW, TEMPERATURE AND HUMIDITY PATTERNS IN A CO-CURRENT SPRAY DRYER: MODELLING AND MEASUREMENTS

ABSTRACT In a co-current pilot plant spray dryer measurements were done of the airflow pattern (no spray) and the temperature and humidity pattern (water spray). These patterns were simulated with a computational fluid dynamics package (FLOW3D) The measured air velocities showed large fluctuations. The measured and predicted flow pattern showed good agreement qualitatively, but the measured profiles showed less variation than the predicted ones The measured temperatures and humidities showed good agreement in large areas of the dryer, but the agreement in the zone near the central axis leaves room for improvement.

Measurements of particle residence time distributions in a co-current spray dryer

ABSTRACT Measurements of particle residence time distributions by means of tracer analysis were done in a co-current pilot plant spray dryer operated with a pressure–nozzle. A system is described for injecting tracer into the feedstream just before the nozzle. Tracer concentrations were measured in two product streams: the tower product stream (mean particle size 134 microns) and the cyclone product stream (mean particle size 67 microns). The measurements show a very wide range of residence times: some particles have residence times shorter than 3 seconds, others have residence times longer than 10 minutes. The median of the distribution is 58·5 s for the tower product stream and 42.2 s for the cyclone product stream.

On the isothermal binary mass transport in a single pore

For the transport in an inert pore the local species momentum balance is reconsidered. This leads to a Maxwell–Stefan type equation for a component α in which the gradient in chemical potential, the interspecies friction and the α–α shear stress form the momentum balance. From the set of equations the component axial velocity profiles can be derived, and so we call this model the velocity profile model (VPM-1), in which 1 stands for the fact that we only consider here the flow in one direction. For binary systems the set of equations is solved, and pore-integrated velocities are derived. This is done both for liquids with a no-slip boundary condition and for gases with Maxwell-slip boundary condition. The pore-averaged velocities can be expressed in the same form as the binary friction model. The use of the difference in pore-averaged velocities instead of the pore-averaged differences requires a correction function, which is derived for both fluid types. For liquids the component-wall friction factors are equal to those in the binary friction model, for gases a slightly different form is obtained. Comparison of predictions for liquid ultrafiltration and gas transport through porous plugs shows in general very small differences between the present model and the BFM, and good agreement with experimental data. The VPM-1 predicts a second flow reversal point of (near-)equal mass isobaric diffusion of gases at different pressures, and a reversal with temperature. From the model follows a new expression for the velocity difference. Velocity profiles for various situations are explored such as liquid ultrafiltration and diffusion, counterdiffusion of gases and for the Stefan-tube. In the latter we find that for a zero average flux of inert gas there is a core of inert gas moving in the direction of the water vapor, and a reverse flow in the area near the wall. The model can be extended to more-dimensional flow problems such as in adsorption and heterogeneous catalysis.

Solute rejection in the presence of a deposited layer during ultrafiltration

During ultrafiltration deposited layers are often formed on the membrane surface. These layers not only reduce the volumetric flux through the membrane, but also may influence the rejection of other solutes in the feed. In the present paper we will show that besides an increase in the rejection, a decrease in rejection may also occur, which can completely alter the aimed selectivity of the separation process. The influence of deposited layers has been studied experimentally by two types of depositing components: silica sol and the protein BSA. In the presence of a relatively open silica deposit a strong drop in the rejection of PEG and dextran was found compared to the rejection on a clean membrane. For thick deposit layers the rejection even decreased to zero, thus resulting in a total permeation of a normally partially rejected solute. On the other hand an increase in PEG rejection occurred in the presence of a BSA deposit. Due to the compressibility of the protein deposit the highest rejections were measured at the highest pressures. The effects were the most pronounced at the isoelectric point of BSA. A model is presented to describe the underlying phenomena.

Drying: a fascinating unit operation

This special issue contains a selection of papers presented at the 12th International Drying Symposium, held from 28 to 31 August 2000, in Noorwijkerhout, The Netherlands. Other selections are presented in special issues of Drying Technology [1,2]. The proceedings are published by Elsevier Science [3]. The final programme and some background information can be found at website http://www.ids2000.tue.nl. In this journal, it might be worthwhile to make a few statements about drying in the context of chemical engineering. In 1972, Keey [4] wrote in the preface to his first book: “Drying is a commonly practised art, but a neglected science, at least by workers whose mother-tongue is English.” And in Chapter 1 of his book: “The reasons for drying are almost as diverse as the materials that are dried.” And one more statement: “The diversity of purpose is matched by the diversity of methods.” Now, three decades later, the diversity has not decreased; but what about the art and science? For extending drying as a science, the development of a “drying community” was very important. The following initiatives in the past have been invaluable: organising of the International Drying Symposia by Arun Mujumdar, start of the journal Drying Technology, and the publication of a series of handbooks and monographs. As a result of the founding of national working parties on drying in a number of countries, the EFChE has incorporated a Working Party on Drying. All these initiatives have roused great interest, enthusiastic participation and cooperation from workers in the field, all coming from a very diverse world. Meanwhile, the International Drying Symposium has found a firm basis in the International Advisory Panel. In many respects, drying remains still an art, but its character is changing from alchemy and craftsmanship to inspired virtuosity. The science of drying is no longer neglected; it is flowering. However, both in the world of drying itself and in that of chemical engineering, appreciation and application still have considerable room for improvement. For this reason, we will touch upon a few aspects of drying in general,

Isothermal vapour and liquid transport inside clay during drying

ABSTRACT A model is presented to describe the moisture transport inside a partially saturated porous material. The transport is caused by vapour diffusion and liquid diffusion. The evaporation inside the porous material is described with a mass transfer coefficient and a specific evaporating surface. Predictions of the model for moisture profiles are compared to experimentally oblained profiles found in the literature. The model needs further extension in the form of incorporating sorption isotherms.

FLUIDIZATION BEHAVIOR OF WOOD/SAND MIXTURES

In conversion of biomass to secondary energy carriers, several routes are possible, such as gasification, combustion and pyrolysis. In many of these processes it is necessary or advantageous to dry the biomass before further processing. For wooden biomass, fluidized bed drying in superheated steam is a promising option. Given the difficulty to fluidize wood particles alone, it is very common to fluidize these kinds of particles with sand. This also gives better defined fluidization behavior. Especially when the wood particles come in various size and shape (i.e. from sawdust to chopped wood), this gives a more reliable scale-up. Also heat transfer to the wood particles may benefit from the use of sand. However, not much is known about fluidization behavior in pressurized steam of binary mixtures with large particle size ratio and large particle density ratio. Therefore minimum fluidization velocity and bed porosity of wood/sand mixtures in air have been experimentally determined and compared to correlations known from literature. The experimental values show a clear trend, but correlations from literature appear not to be very accurate. So more experiments have to be done to find a correlation that gives more accurate predictions in case of the specific particles used in this work. From segregation experiments could be found that, to keep the wood/sand bed well-mixed, finer sand (0.1-0.5 mm) with maximum 10 weight-% wood should be used, and the superficial gas velocity should be at least 3-4 times the minimum fluidization velocity.