William Resnick

PARTICLE SEPARATION IN A MAGNETICALLY STABILIZED FLUIDIZED BED

Abstract Magnetically stabilized fluidized beds provide the possibility of performing dry particle separation on the basis of density difference. Such separations are generally performed by wet media methods and are expensive and difficult, if not impossible, to perform for the separation of high density materials. An experimental study of this separation potential is described. Encapsulated magnetite comprised the bed material and bed stabilization was provided by a uniform axial magnetic field. Depth of penetration and settling times were measured for objects of different sizes and densities as a function of bed operating conditions. The separation potential for small particles was also investigated. The experimental results show that dry density separation is possible and that it can be monitored by appropriate manipulation of magnetic field intensity, gas velocity and bed stability which provide different levels of resistance to particle penetration and motion in the bed.

Mass transfer from gas bubbles in an agitated vessel with and without simultaneous chemical reaction

Abstract A simplified model is proposed that permits the prediction of total mass transfer rates for a sparingly soluble gas in a gas-liquid contactor. The model is based on the average residence time of the gas bubbles in the continuous phase, and permits the estimation of the rate of diffusion per unit area as well as the total area for diffusion. A knowledge of bubble diameter, reaction rate constant, gas diffusivity and contactor operating conditions is necessary to arrive at a numerical solution of the equations. The equations have been solved for a large range of variables and the results are presented graphically. The model predicts correctly the effect of changes in diffusivity, reaction rate constant, gas and liquid flow rates, bubble diameter and agitation intensity on total mass transfer rates.

A modified electroresistivity probe technique for steady- and unsteady-state measurements in fine dispersions—I: HARDWARE AND PRACTICAL OPERATING ASPECTS

Abstract Significant modifications of the electroresistivity probe technique of Neal and Bankoff were made so as to permit its use in liquid—liquid dispersions with drops in the range of 0·1–0·3 mm dia. Modifications of both hardware and signal processing theory are necessary because sensor length is no longer negligible with respect to bubble or drop size and a multi-level rather than a two-level signal is produced. The principle of operation, the hardware, the mode of operation under different conditions, instrument calibration techniques and final expressions used for signal processing are presented in this paper. The method can be used for monitoring interfacial area in liquid—liquid or gas—liquid extraction or reaction equipment, but not in suspension polymarization reactions. Its main advantages include the smallness of the sensor and the fact that virtually on-line reading of hold-up, average size and interfacial area, can be obtained from a single sensor.

Particle and bubble behaviour and velocities in a large-particle fluidized bed with immersed obstacles

Abstract A stereo-photogrammetric technique supplemented by cine-photography was used to study particle and bubble behaviour in a two-dimensional bed of large spherical particles whose Umf was 79.4 cm/sec. The bed was equipped with immersed objects that simulated horizontal tubes. Single obstacles of several shapes as well as obstacles in array were studied. A detailed and accurate mapping of particle velocities, particularly in the neighbourhood of the obstacles, was done. At fluidizing-air velocities of 80 – 90 cm/sec, bubble velocities of up to 39 cm/sec were measured. Particle caps above the obstacles and stagnant air bubbles attached to the bottom of the obstacles were observed and their size and behaviour for various obstacles and arrays were noted.

Gas-Liquid Dispersions

Publisher Summary Dispersion of gases in a liquid can be achieved by bubbling the gas through the liquid by using sparge pipes at the bottom of the contactor. Mixing impellers of the turbine type are frequently used to increase the rate of mass or heat-transfer over that obtained without the use of the mixer. Consequently, the contacting of gas and liquid in a mixing vessel equipped with a rotating impeller is generally used for the absorption of sparingly soluble gases. Some of the more common applications are in biochemical fermentation, hydrogenation, hydrocarbon oxidation and various other oxidation processes. Efficient contact is produced between the phases in agitated gas–liquid contactors and, therefore, this type of equipment can also be useful for those absorption and stripping operations for which conventional plate or packed towers may not be suited. This chapter refers to mechanically agitated gas–liquid dispersions. However, most of the theoretical and experimental conclusions also apply to any type of gas–liquid dispersion. The total surface area for diffusion is increased because the bubble diameter is smaller than for the free-bubbling case at the same gas flow rate; hence there is a resultant increase in the overall absorption rate. One approach to the analysis of gas–liquid dispersion has been to develop simplified theoretical models that provide an approximation to the real processes and mechanisms operating in the system. Typically, their application to the design problem requires knowledge of factors, such as bubble velocity relative to the agitated liquid and rate of surface renewal or film thickness as functions of mixing conditions.

Displacement and velocity fields in hoppers

Abstract A stereoscopic technique was used to investigate a number of factors that affect flow in a two-dimensional bin with horizontal discharge slot. The technique proved to be a suitable, accurate and efficient method to carry out flow field measurements in flowing bulk materials. Four flow regions were found: a stagnant layer adjacent to the wall; a transition region — a shear zone in which large velocity changes occur at each horizontal section; a central core at the upper section of the center of the bin in which groups of particles exhibit velocity fluctuations about the average velocity; and a discharge zone next to the exit slot in which the particles accelerate rapidly. Direct mass flow measurements were made and compared with calculated results and literature correlations.

Determination of flow patterns for unsteady-state flow of granular materials

Abstract A “freezing” technique was employed to study the solids flow pattern development when a bed, initially at rest, begins to empty. The bed was composed of horizontal layers of differently coloured particles and the flow pattern that had developed could be observed by sectioning the bed after freezing. As flow is initiated, a dilation wave moves up the column resulting in an increase of bed porosity. The porosity at rest was 0.41, whereas the flow porosity was equal to 0.47 which is the value also obtained at minimum fluidization conditions. The height of the stagnant region was equal to half the bed diameter and levels in the outer annulus were observed to remain horizontal. Neither the flow pattern nor the solids flow rate was affected by the bed height.

Hysteresis phenomena in magnetized-fluidized beds

Abstract Hysteresis phenomena in magnetized fluidized beds are described. It is shown that magnetic structural effect is the source for the different hysteresis phenomena observed. Variable bed stability is shown to exist due to effects of gas flow profiles producing a progressive stabilization of flow from the wall inwards. Hysteresis of transitions between different levels of bed stability is established and related to magnetic structural effects. Finally, it is shown that properties of the bed such as degree of particle alignment with the field and its permeability to gas flow are dependent and hence can be monitored by selecting the path leading to its formation.

Dynamic behaviour of an agitated two-phase reactor with dynamic variations in drop diameter-I: Numerical simulations

Abstract A model assuming infinite drop coalescence and break-up rates was used to study the dynamic behaviour of chemically reacting liquid-liquid dispersions with simultaneous interphase heat and mass transfer in continuous reactors. The investigation has shown that, in some cases, it is important to take into account dynamic variations in interfacial area for proper description of reactor behaviour. It was found that in most cases in which there is a dynamic dependence of drop size on phase composition, phase temperature and/or impeller speed, oscillatory behaviour of concentration and temperature results that can bring the reactor into undesirable situations. The oscilliatory character of the transient response is a result of the dynamic lag between drop size and system variables.

Development of a dynamic circulation-interaction model for mechanically agitated, liquid-liquid reactors

Abstract A mathematical model based on the circulation-interaction concept was developed for the dynamic behaviour of continuous, mechanically-agitated, liquid-liquid reactors. The model is described by a set of ordinary, non-linear differential equations and a set of algebraic equations. These equations provide the relationships between the state variables of the system and the inputs to the system. The model permits the study of the dynamic response of the state variables of a liquid-liquid reactor to disturbances and manipulation of the operating conditions. The model can assist in understanding the dynamics of such a system and should be useful for reactor design and design of control systems. Two numerical examples with reasonable and realistic parameters and data values were studied. “Hot” spots can develop during steady-state operations and sensitive and dangerous cases can develop during the dynamic response.