Characterizing the product crystals from a mixing tee process

Abstract

The classical theory of nucleation has been acceptably well developed, particularly as it pertains to vapor-liquid transformation in pure systems. When liquid solution-solid crystal transformations are considered the theory is still applied, typically, but with less confidence. Perhaps the best measure of the extent to which one can accept classical theory for these latter systems is provided by a study of the experimental results of Nielsen (1969Nielsen ( , 1971. He carried out homogeneous nucleation studies for a variety of ionic crystal-aqueous solution systems. H e was able to show reasonable agreement between classical theory and experiment for some salts (e.g., 2,2 electrolytes), while success was mixed for others. The major goal of this work is the characterization of the crystals which are produced through homogeneous nucleation by the Nielsen process. In order to do this, a preliminary experimental study of the process is required to identify the mechanisms by which the observed product is formed. This paper describes work serving to standardize the experimental procedure. ExperimentalPrecipitation experiments were carried out using a modified Nielsen apparatus, Figure 1. Nielsen's original apparatus was activated by two syringes operating in parallel, each of capacity 5 mL. These operated simultaneously to force the two reactant streams to intimately mix in a downstream tee. The device provided a sample of the precipitate for subsequent study-primarily the counting of the product crystalline entities; it also permitted a measure of the reaction or induction time t i , i.e., the period elapsed between the time of mixing and the visually determined onset of precipitation. The general idea behind this was to mix reactant solutions rapidly so that the precipitation mechanisms occurred from a well-mixed homogeneous solution. The mixing occurred in a specially designed mixing tee whose characteristic mixing time was estimated to be about one millisecond. Obtaining a particular relationship between the number of particles, Nvs. initial supersaturation, So, ensured that homogeneous nucleation had occurred.The apparatus modification involved in this work included a larger volume of reagents, say a maximum of 525 mL, and a mixing tee of simpler configuration. The motivating idea behind this latter change was to enable easy characterization of the tee in the event that future scale-up and modifications were to be considered. The tubing sizes given in Figure 1 are about the same as those used by Nielsen (1961). The larger quantities of reactants were desirable as the goals of the experiments included providing product crystals in sufficient quantities for more elaborate characterization and further processing. Plexiglass vessels D served to contain the reactants under air pressure as they were fed to the mixing tee. In Figure I , C is the pressure gauge, B, the pressure regulator and A, an in-line filter; vessels D were fed from vessels I whose contents were transported through filters G (0.22 wm) by the diaphragm ...

In situ WAXS synchrotron radiation study on particle formation of precipitated barium sulfate

“…Angerhöfer explains this fact with the occurrence of aggregation in the sampling beaker, with the result that only aggregates are counted instead of primary particles, leading to a strong decrease in the number of counted particles. The nucleation rate data measured by Mohanty et al 2 is much too high to produce any reasonable simulation result. For further validation of the primary crystallization kinetics in situ, timeresolved measurements were performed.…”

“…Nucleation kinetics of barium sulfate have been studied by several authors (Nielsen, 1 Mohanty et al, 2 and Angerhöfer 3 ) under stoichiometric conditions (R 5 1). Although all three authors studied the same crystalline substance, namely barium sulfate, their reported experimental data and the proposed models are not very consistent ( Figure 7).…”

“…The quantity to describe the driving force of the precipitation process, used to present the nucleation data in the original literature-concentrations in the case of Nielsen, 1 concentration-based supersaturation in the case of Mohanty et al 2 and Angerhöfer…”

“…Numerous theoretical and experimental research studies are reported on nucleation and growth kinetics [1][2][3][4] and on the influence of reactor geometries and mixing conditions on the resulting particle size or to test precipitation models. [5][6][7][8] Most of these studies were carried out as batch or semi-batch processes, where mixing of the reactants has a dominant influence due to the very fast nucleation and growth kinetics of barium sulfate.…”

Kinetics of nucleation from aqueous solution

“…Data obtained for several systems by Nielsen (1 967) and others (Mohanty et al, 1988;Bhandarkar et al, 1989;Makhija, 1988) are presented graphically in Figure 2. The wide spread of values for the intercept log P for these systems are listed in Table 1.…”