M. H. I. Baird

Mixing Through Oscillations and Pulsations—A Guide to Achieving Process Enhancements in the Chemical and Process Industries

In this article we review the concepts and key developments of mixing enhancement through pulsation and oscillation. We focus on more recent research using oscillations in baffled tubes or columns, which generate discrete vortices as distinct from the more random type of flow produced by the well-established pulsed plate and reciprocating plate column devices. The scope of this review ranges from the basic chemical engineering concepts and key findings from recent research projects, including heat/mass transfer and residence time distribution (RTD); to applications and case studies involving specific reactions, e.g. polymerization and process intensification. We hope that this review will enable the reader to identify additional potential and perhaps an unexpectedly wide range of applications for the oscillatory baffled flow technology.

Drop size and holdup in a reciprocating plate extraction column

This paper reports measurements of droplet sizes and holdup in liquid—liquid systems in a 2 in. dia. reciprocating plate extraction column, in the absence of mass transfer. The Sauter mean droplet diameter was found to depend on the product of plate amplitude and frequency and on the system properties, but it was not in general affected by the throughput. It could be correlated fairly well using a turbulent dispersion model similar to that which has been applied successfully to dispersions in stirred tanks. A simple hydrodynamic model is proposed for the holdup, assuming the droplet diameter to depend on turbulent energy dissipation. This model is consistent with the experimental data if it is assumed that droplets in kerosene undergo hindered circulation while droplets in methyl isobutyl ketone circulate freely. The model also agrees with some previously published data on holdup in a gas—liquid system in a pulsed column, assuming that the gas bubbles circulate freely.

Drop phase mass transfer coefficients for liquid—liquid systems and the influence of packings

Abstract Drop phase mass transfer coefficients have been measured for unhindered motion and for disturbed motion through a vertical regular array of two lines of Raschig rings a short horizontal distance apart. The influence of additions of sodium dodecyl sulphate surfactant has also been investigated. The effect of the Raschig rings is to slow drops down and despite drop distortions and erratic motion the mass transfer coefficient was linearly dependent on drop velocity over a wide range of drop sizes. A new model is proposed to describe mass transfer coefficients for a wide range of drop size, taking contact time and contamination by the surfactant into account.

Spontaneous oscillation of a gas-fluidised bed

Abstract Well defined spontaneous oscillations can occur in a gas-fluidised bed with a low resistance support and a large gas space beneath the support. Measurements of the instantaneous pressure drop as a function of time have been carried out in a 10 cm dia. bed with gas space volumes between 11 and 191. At superficial gas velocities in excess of the minimum fluidisation value, the pressure drop fluctuates regularly but not sinusoidally. The effects of variables such as bed mass, gas velocity and gas space volume upon the oscillation frequency have been measured and it is concluded that the oscillations are a relaxation phenomenon connected with the periodic formation of gas bubbles.

The Use of Immobilised Biomass to Remove and Recover Radium from Elliot Lake Uranium Tailing Streams

Abstract Return sludge from a municipal wastewater treatment plant was killed and subsequently immobilised in the form of particles of approximately 1 mm in diameter. The immobilised biomass particles were then tested for their capacity to adsorb selectively radium-226 from the waste waters of the uranium mining and milling operations in Elliot Lake, Ontario. Both the adsorption equilibrium and the kinetics of radium uptake were examined. The elution of the adsorbed radium from the biomass particles and the subsequent reuse of the particles for additional radium uptake were also looked at. This information has provided an improved understanding of the way the immobilised biomass functions as a radium adsorbent.

Radium in uranium mill tailings ― some observations on retention and removal

Abstract The various mechanisms by which radium can be retained by uranium mill tailings are discussed. These include coprecipitation (solid solutions), adsorption on silica and hydroxides, ion exchange in clay minerals, and retention in pores. Previous work on radium removal from mill tailings and on radioactive disequilibrium in uranium deposits is critically reviewed in relation to these mechanisms. Fresh data are presented, indicating the roles of charge-depressant ions and of organic complexing agents in radium removal. The addition of reducing agent has been found effective in the presence of complexing agents because it can convert some low-solubility metal hydroxides to their more soluble, lower valency form, thereby eliminating radium adsorption sites. A radium level of 30 pCi/g in the tailings is concluded to be environmentally safe for surface disposal of tailings.

A model for axial mixing in reciprocating plate columns

Abstract Axial mixing in reciprocating plate columns with different plate geometry has been investigated for single-phase flow. Data are presented on the steady-state concentration profile of a salt tracer around a single reciprocating plate and between two reciprocating plates in a 5 cm diameter column. These data show the existence of two separate hydrodynamic regions in which there are two mechanisms for axial dispersion. One region is in the volume swept out by the reciprocating plate; here the axial mixing is high. The other region is between the plates, where the amount of axial mixing is significantly lower. A model is proposed based on the observed flow structure which accounts for the effects of flowrate, frequency, amplitude, plate spacing, hole size and fractional free area. There are only two adjustable parameters in the model, which are estimated independently from the concentration profile between the plates. The predictions of this model also compare favourably with published data on other reciprocating plate column geometries.

Coextraction of uranium and radium from ore with ferric chloride

Abstract Ferric chloride leaching, at temperatures around 74°C, is found to remove up to remove up to 97% of the uranium from ores occurring in the Elliot Lake area of Canada. Radium removal is poor due to the formation of sulphates from the sulphides present in the ore. However, if the sulphides are removed initially by flotation, then ferric chloride can extract as much as 92% of the radium, giving tailings which are sulphide-free and with radium levels approaching 24 pCi/g. The main steps in a ferric chloride leaching process have been tested at the bench scale. Radium may be removed by adsorption on manganese dioxide and uranium is removed by liquid extraction with D2EHPA (DAPEX process). The ferric chloride may be recirculated for further leaching and the net usage is expected to be in the order of 5 kg/Mg of solids treated. Because of the recycle, it is possible to keep chloride ion levels in the effluent below the prescribed level of 750 mg/L.

The elution of radium adsorbed by microbial bioman

The present work examines the equilibrium and kinetics of the elution of radium-226 adsorbed by inactive (dead) microbial biomass. Two biomass types, P. chrysogenum and activated sludge, were examined. Values were determined for engineering design parameters such as eluant type, eluant concentration and solid to liquid mtio. Five eluants (EDTA, NTAA, TAC, HCl, HNO,) were selected and studied. The mineral acids and EDTA were shown to be the most efficient radium eluants. The radium readsorption potential of the biomass following elution was also examined and shown to be severely limited, especially when mineral acids were used as eluants. The rate of radium desorption from the biomass, using ‘mineral acids, is shown to be very rapid, with complete elution accomplished within one or

Axial mixing of fine particles in fluidized beds

Abstract The axial dispersion of fine particles in fluidized beds can be predicted by a semitheoretical correlation based on the isotropic turbulence theory. Th particles in fluidized beds has been modified on the basis of the theory which indicates that axial mixing is more pronounced in a bed of fine particle