Narayan S. Tavare

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.

BATCH CRYSTALLIZERS: A REVIEW

This review outlines the advances that have been made in experimental data reduction and analysis techniques which can be used to characterise the crystallization kinetics and crystal size distributions in batch crystallizers. A process description based on batch conservation equations describing population, mass and energy balances together with appropriate kinetic events represented by phenomenological models and proper boundary conditions should be used. Several general and useful techniques to extract reliable growth and nucleation kinetics based on solution and solid-side information are reviewed. Good compilations of kinetic studies, design and analysis methods and investigations of operating systems are reported. The review should help as a starting point in the rational design, understanding and better utilization of batch crystallizers.

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.

Crystal Size Distribution

The crystalline solid state is unique in that it exhibits properties related to form (i.e., size and shape) and composition. The processes employed for manufacturing crystalline solid materials in bulk invariably yield a distribution of characteristic shapes and sizes of the resultant solid-phase material. The crystal size distribution is an important property influencing the end-use applications as well as interacting strongly with the crystallization process itself. It is often necessary to describe quantitatively the crystal size distributions as well as to predict it a priori from the process simulation analysis. If all crystals produced have roughly similar shape, it is often adequate to describe the material by a one dimensional crystal size distribution. In this chapter, possible empirical, one-dimensional functions suitable for such use are first discussed. A unified theory is then presented for multidimensional crystal distributions, which can be used to analyze and predict such distributions in certain process configurations.

SEPARATION OF O AND P CHLORONITROBENZENES THROUGH HYDROTROPY

Hydrotropic solutions can be used in separation of two isomers as hydrotropes at high concentrations can enhance solubilities of these isomers in aqueous solution differently. Solubility data are determined for o and p chloronitrobenzenes (CNBs) in aqueous solutions of sodium butyl monoglycol sulphate (NaBMGS). o CNB is highly solubilized as compared to p CNB and can be precipitated out selectively on dilution with water at a reasonable recovery. Precipitation kinetics of p CNB determined from a solvent capacity varying batch crystallizer using the method of s plane analysis show that the relative order with respect to linear growth rate in conventional power law relative nucleation correlation is near unity.

Silica precipitation in a semi-batch crystallizer

Abstract Precipitation of silica achieved by neutralization of dilute sodium silicate solution with dilute sulphuric acid in a 751 pilot-scale agitated vessel with two impellers was used to investigate both crystallization and agglomeration kinetics from the measured transient crystal volume distributions. The overall neutralization process was represented by a homogeneous chemical reaction followed by precipitation and resulting in an agglomerated silica precipitate. It was carried out in three stages involving a discontinuous semi-batch operation, primary neutralization, ageing and secondary neutralization being the three stages involved. The method of moments analysis in crystal volume coordinates was used to deduce the kinetics rates. All the kinetic rates obtained from the measured crystal volume distributions were correlated by empirical kinetic expressions in terms of observables . The calculated time variations of solution silica concentration, total crystal number and population average crystal volume from the process simulation using the derived kinetic correlations compared favourably with experimentally observed values.

On the confluence of dissolution, reaction and precipitation: The case of boric acid production

Abstract Reactions of borax with organic acids were studied in a batch mode. Product boric acid resulting from the reaction of solid borax with liquid propionic acid was found to deposit on the surface of the unreacted borax core to form an ash layer. Sharp interface unreacted core model was used for simulation of this process. The analytical solution to the model involving diffusion through a liquid boundary film, diffusion through the solid reaction products, and surface reaction was obtained. Reaction of borax solution with oxalic acid crystals exhibited a different route, i.e., reactive precipitation process. Crystallization of boric acid took place in the solution phase after complete dissolution of oxalic acid crystals. Nucleation and growth rates were evaluated from a series of experiments by the method of s plane analysis. Relative nucleation kinetics were correlated in terms of the significant process variables.

Mixing, reaction and precipitation: an interplay in continuous crystallizers

The two-environment micromixing model of Ng and Rippin (1965,Third Europ. Symp. Chem. React. Engng, pp. 161–165. Pergamon Press, Oxford) and the IEM (Interaction by Exchange with the Mean) model are extended to a process involving the elementary chemical reaction between two reactant species and subsequent crystallization of product in a continuous crystallizer with premixed feeds at the entry. The sensitivity of these models to Damko¨hler number and dimensionless micromixing parameter is explored. The computationally efficient two-environment model appears to characterize satisfactorily the micromixing effects in a continuous reactive precipitator while the IEM model description in a continuous MSMPR (mixed suspension mixed product removal) crystallizer is better suited to a nearly segregated system.

Ammonium Sulfate Crystallization in a Cooling Batch Crystallizer

Abstract Crystallization kinetics of ammonium sulfate crystals are determined in a 25-L draft tube baffled, agitated crystallizer from a series of batch cooling experiments performed in an integral mode. The method of s-plane analysis for relative nucleation kinetics and the method of initial derivatives for growth rate kinetics are used to establish conventional kinetic expressions. A simulation technique is developed to calculate the product population density functions for the same system in a seeded cooling batch crystallizer configuration.

Separation of o- and p-chlorobenzoic acids : hydrotropy and precipitation

The aqueous solubilities of o- and p-chlorobenzoic acids (CBAs) in different concentrations of sodium butyl monoglycol sulphate (NaBMGS) solutions at 25 and 45°C are determined by the weight disappearance method. The precipitation of o- or p-CBA by dilution with water from their single component NaBMGS solution is carried out in a 2 L agitated jacketed crystallizer. The crystal size distributions (CSDs) of the precipitated solids are determined using a multichannel Coulter counter. The effects of significant process variables, namely, temperature, water addition flow rate and speed of agitation on the CSDs are investigated in a series of batch experiments. The results are analyzed by the method of s-plane analysis and correlations between the nucleation and growth rates are presented.