J. Garside

Industrial crystallization from solution

Abstract This review approaches crystallizer performance from a reaction engineering viewpoint in which emphasis is placed on the interaction of crystallizer fluid mechanics and residence time distributions with the physical chemistry of nucleaton and growth kinetics to produce the crystal size distribution within a crystallizer. The present state of knowledge in these various areas is assessed, particular attention being paid to work carried out over the past decade. Great advances have been made during this period but significant gaps in our understanding still exist, particularly in the areas of crystallizer fluid mechanics (especially the role of micromixing), primary nucleation, habit modification, comparison of predicted crystallizer performance with actual operating characteristics, and the control of full scale crystallizers.

Solute clustering in supersaturated solutions

Abstract Concentration gradients in a vertical column of aqueous supersaturated solution have been measured for a number of solutes. It is assumed that these gradients result from the presence of solute clusters. A theoretical analysis based on the assumption of steady-state quasi-equilibrium allows the size of these clusters to be estimated. Typically they contain of the order of 103 molecules and are between about 4 and 10 nm in size.

Application of CFD Modelling to Precipitation Systems

A CFD package was employed to simulate the steady state precipitation of barium sulphate in jet mixing devices (coaxial flow pipeline mixers and tee-junction mixers). Both 2D and 3D descriptions were employed. Solutions of the Navier-Stokes equations along with the standard two equation k-e turbulence model provide the velocity and energy fields while generation and development of the solid crystal phase were described by the moment equations of the population balance. Spatial distributions of species concentrations, supersaturation, total crystal number and magma density were calculated and the average size and coefficient of variation of the crystal product determined. Visualization of these distributions enables a rapid assessment of the effect of the actual flow pattern within a particular jet precipitator. The influence of the positions and orientation of the two inlet streams, as well as the length of the mixer on the mixing and the precipitation were investigated.

Mixing, reaction and precipitation: Limits of micromixing in an MSMPR crystallizer

The effect of micromixing limits on a process involving elementary chemical reaction and subsequent crystallization is considered. Two limiting cases are analysed; maximum species and age mixedness (Model I) and maximum species but minimum age mixedness (Model II). Although the conversion of the chemical reaction is similar in both situations, the crystal size distributions can be very different. Such differences arise from variations in the supersaturation profiles experienced by fluid elements. The sensitivity of the two models to the reaction rate constant and the nucleation kinetic parameters is explored.

Solute clustering and interfacial tension

Abstract The effect of surface curvature on surface tension has been included in the theory of homogeneous nucleation to show that, under certain conditions, cluster formation results in a decrease in Gibbs free energy. This cluster formation is thus a spontaneous event and a quasi-equilibrium concentration of clusters of narrow size range may then exist in supersaturated solutions. Previous experimental work suggests the existence of solute clusters in a variety of aqueous solutions. The implications for crystal nucleation and growth theory are discussed.

Transformation of calcium oxalate hydrates

Abstract The transformation kinetics of calcium oxalate trihydrate and calcium oxalate dihydrate to the thermodynamically stable monohydrate have been studied in batch precipitation experiments. A combination of size distribution measurements, optical microscopy, X-ray diffraction and thermogravimetric analysis were used to characterise the processes involved. The transformation appears to be a solution mediated process in which dissolution of the unstable phases is followed by growth of the monohydrate; corresponding dissolution and growth rates have been determined. The distribution of hydrates in the initial stages of the precipitation was very sensitive to the mixing process; for example the use of a magnetic stirrer produced a different distribution to that formed with a propeller agitator.

The influence of biuret on the growth kinetics of urea crystals from aqueous solutions

Abstract The growth kinetics of urea in the [001] and [110] directions are presented for both pure solutions and solutions containing up to 6% biuret. This impurity is formed during the synthesis of urea and acts as a powerful habit modifier. In pure solutions urea grows very quickly in the [001] but very slowly in the [110] direction. As a result long needle-like crystals are produced. In the presence of biuret [001] growth is dramatically reduced while [110] growth is virtually unaffected, so producing a crystal with a much lower length: breadth ratio. The influence of biuret is explained by the replacement of two urea molecules in the crystal lattice by a biuret molecule. The resulting disruption of the (001) face leaves insufficient NH 2 -groups to satisfy the hydrogen bonding linkages.

Nucleation and growth kinetics of barium sulphate in batch precipitation

Precipitation kinetics of barium sulphate were determined from a series of batch experiments. The thermodynamic supersaturation ratios were calculated from conductivity measurements while the crystal size distributions were measured by means of an electrical zone sensing particle size analyser. Using the measured crystal size distributions the nucleation and growth rates were determined by the moments method. Observed nucleation rates were strongly affected by the thermodynamic supersaturation ratio as well as by the stirrer speed and magma density and indicated the occurrence of both primary and secondary nucleation. The nucleation kinetics were therefore expressed as a combination of primary and secondary nucleation terms. The dependence of the growth rates on the thermodynamic supersaturation ratio and the stirrer speed suggested that both the diffusion and surface integration processes influenced the kinetics.

Non-isothermal effectiveness factors and the role of heat transfer in crystal growth from solutions and melts

Abstract The effect of heat transfer and the role of the surface integration process in crystal growth are analysed quantitatively in terms of a non-isothermal effectiveness factor, defined as the ratio of actual growth rate to the rate that would be obtained if conditions of the supercooled or supersaturated bulk liquid existed at the crystal surface. The expression for simulataneous transfer of mass and heat in the liquid phase in the vicinity of the growing crystal surface is developed. Surface conditions of growing crystals, freely settling in a supercooled or supersaturated liquid, are determined for seven binary systems including organic melts and aqueous solutions of inorganic salts. Organic melt systems and some aqueous systems exhibit effectivenss factors significantly less than unity, indicating that the liquid phase transport phenomena play an important role in crystal growth. As a measure of the contribution of heat transfer, the inorganic systems this ratio increases at high concentrations of the crystallizing component and high heats of crystallization. An empirical criterior for the importance of thermal processes is that the non-dimensional group βd ≡ (k′ ϱfΔH/h′)(dz*A/dT) should be greater than 0.01.

Crystallization from oil in water emulsions: Particle synthesis and purification of molecular materials

The use of emulsions as a means of studying the kinetics of homogeneous nucleation has been well known for a number of years. In this contribution, however, attention is given to the possible use of emulsions in novel process technology, on the one hand as a means of directing the size and morphology of particulate materials and on the other as the basis for melt purification. m-Chloronitrobenzene serves as a model compound in this work, although the central concepts are valid for any low melting molecular material. Particular attention has been paid to the choice and mode of action of the emulsifier since it is this which ultimately controls the outcome of the crystallization process.