Jaroslav Nývlt

Kinetics of nucleation in solutions

Abstract Some results of experiments on the kinetics of nucleation from water solution are presented in this paper. The experimental work, carried out in the Research Institute of Inorganic Chemistry inUstinad Labem is based on measuring the width of the metastable zone of concentrations under varying conditions. When the supersaturation P exceeds a certain limit the nucleation rate increases very rapidly; in a system where the supersaturation is achieved by moderate cooling it is practically impossible for the concentration to exceed the limit of metastable supersaturation under given conditions and, therefore, the nucleation rate at the beginning of nucleation will be identical with the supersaturation rate for a limited period of time, i.e. d m z d τ = k P m = k 1 b where k1 represents the weight of the substance precipitated by cooling a unit volume of the saturated solution by 1 °C and b is the cooling rate (°C/hr). From the measurements of the metastable zone width P at various cooling rates b the kinetic constant of nucleation k and the order of nucleation m can be calculated. The measurements were carried out by using many different systems of salt solutions. The measuring and evaluation method is described in the paper, the results of the measurements are discussed and some generalized conclusions are presented. Data obtained according to the described method may be used for calculation of crystal product size in a commercial crystallization unit.

Metastable zone-width of some aqueous solutions

Abstract Measurements of the metastable zone-widths of 25 different systems and their dependence on the cooling rate, the temperature and the presence or absence of crystals are reported and the results are discussed. The nucleation parameters, which may be obtained from metastable zone-width measurements, are shown to be dependent on certain physiochemical properties of the solutes. These data and the method of their prediction are considered to be of utility in the design of industrial crystallizers.

Programmed cooling of batch crystallizers

Abstract An analysis of the programmed cooling of batch crystallizers, starting with the solution of the momentum equations of the population balance, is presented. Based on a general solution, conditions are discussed leading to limiting equations which had been published earlier. The model described in this paper determines the optimum cooling curve for a batch crystallizer with an arbitrary seeding and nucleation ratio, hence a more detailed analysis of the physical conditions of the process and a better comparison of the theory with experiment are possible. Kinetic data of crystallization can also be obtained from batch experiments.

The periodic behaviour of continuous crystallizers

Abstract A mathematical model of a continuous stirred crystallizer is presented which demonstrates that periodic changes of supersaturation, solid phase content, crystal size and production rate can readily occur, and that under certain conditions the steady state may not be established. These theoretical considerations help to explain the results from a pilot-plant study.

Crystallization Studies by Thermometric Methods

The effect of thermal history of a solution (i.e. time and temperature of overheating preceding the measurements) has been analyzed on a theoretical basis and described by semiempirical equations taking into account structural changes in solution but using only measurable quantities. Induction periods so estimated have been employed for predictions of the kinetics of nucleation under given experimental conditions. The spontaneous crystallization of a closed sample of supersaturated solution with defined thermal history has been studied using computer simulation and compared with direct measurements of KC103 crystallization in a Calvet microcalorimeter. The agreement of both these results is very good and stimulating for further studies.

INFLUENCE OF HYDRODYNAMIC CONDITIONS ON THE CONTROLLED DOUBLE-JET PRECIPITATION OF SILVER HALIDES IN MECHANICALLY AGITATED SYSTEMS

Abstract The controlled double jet precipitation of silver halides from a homogeneous reaction in the liquid phase involves a chemical reaction which produces supersaturation, nucleation, and growth of crystals. Mixing influences all three of these processes. The mixing/stirring conditions play an important role in controlling the chemical composition, average grain size, and grain size distribution of formed particles. The purpose of this study was to determine the dependence of the mixing and stirring conditions on the controlled double jet precipitation of silver halides. Experiments were conducted in a flat-bottom crystallizer, a profiled bottom crystallizer, and a profiled bottom crystallizer equipped with a conical draft-tube. The average crystal diameter of the formed silver chloride microcrystals was independent of the crystallizer configuration if the frequency of revolution of the 50 mm diameter impeller was greater than 300 RPM. The variation coefficient decreased if the controlled double jet precipitation was carried out in a crystallizer with a profiled bottom and/or a draft-tube.

Nucleation and growth rate in mass crystallization

Abstract The quality of product crystals in mass crystallization is largely determined by the kinetics of two successive steps: nucleation and crystals growth. Many problems can be treated in a similar way as they are dealt with in studies of growing single crystals. There are several features, however, which make the difference between the growing of single crystals and mass (industrial) crystallization to be more profound. The main difference is that, because of the large number of crystals encountered in mass crystallization, it is impossible to describe the properties of individuals and hence the statistics with definition of certain average properties and of their probable deviations becomes more important. Theoretical description of the process is thus limited to simple laws and simple relationships describing the effect of various technological variables. In addition, mutual interaction of crystals in the slurry has to be taken into consideration. The main theories and methods of measurement of the kinetics of crystallization are briefly reviewed in the paper with the emphasis of those which are characteristic for mass crystallization.

Batch salting-out crystallization

Abstract Based on moment equations and materials balance, the optimum rate of addition of the second solvent in salting-out crystallization has been determined. At the start of the batch operation, a low feed rate of the second solvent should be used; the feed rate can be increased only during the run. A simple model is proposed to express the dependence of the mean crystal size on the rate of stirring and on the concentration of the salting-out solvent.

Enthalpy of mixing in KCl-KBr mixed crystals at 25°C

Abstract The enthalpy of mixing in KCl-KBr mixed crystals (molar ratios of 1:0.33, 1:0.63 and 1:1.79) was calculated from calorimetric measurements of integral dissolution enthalpies at 25 °C. The results are compared with experimental data reported in the literature and with values calculated by using different theories.

Fluidized-bed apparatus for crystal growth rate measurements and its testing with copper sulphate

Apparatus for measurement of crystal growth rate in a fluidized bed has been modified. The improvement consists of a new way of introducing and removing seeds, which substantially speeds up the measurement and, due to better stability of the temperature regime, leads to improved reliability of results. Growth rates of CuSO4 · 5H2O in aqueous solutions, measured in the original and improved apparatus, are compared and they demonstrate the advantages of the new version of the equipment.