Hiroshi Takiyama

Morphology of NaCl Crystals in Drowning-Out Precipitation Operation

The drowning-out precipitation of sodium chloride was carried out to observe nucleation phenomena by mixing two saturated aqueous ethanol solutions having different concentrations. The shape of the crystals was found to differ with the operating conditions. Crystals with hollow surfaces that may lead to a decrease in purity were observed only when the initial supersaturation ratio σ 0 was high. From the induction time, primary nucleation was found to be predominant, with homogeneous nucleation becoming significant at σ 0 > 0.7. The number of crystals generated increased slightly with increasing initial supersaturation ratio, but started to decrease after a certain supersaturation, where the crystals were agglomerated from the early stages of crystallization. The ‘hollow’ crystals were generated in such high supersaturations. The crystals generated by homogeneous nucleation agglomerated readily. Since the number of crystals depended on the ethanol concentration in aqueous solution and/or the agitation condition, nucleation was strongly influenced by local mixing.

Determination of solid-liquid phase equilibria by using measured DSC curves

Abstract Solid–liquid phase equilibria (SLE) data are essential in designing apparatus and operation of industrial crystallization because they determine the possibility of separation and predict purification limits. In this study, a SLE determination system by using differential scanning calorimetry (DSC) was improved and applied not only to eutectic systems but also to partial solid solution systems. This method is different from conventional techniques that use onset and peak temperatures to determine SLE. A classification method of phase diagram types is proposed. This determination system was applied to the naphthalene (naph)– p -chloronitrobenzene ( p -CNB) and naph–catechol (cate) systems (eutectic systems) and the indole–naph system (partial solid solution system). From the calculated results, the determined SLE was in good agreement with the literature data.

Purification of organic solid solutions by melt crystallization: comparison between layer and suspension crystallization

Abstract In order to compare the degrees and rates of purification achieved by layer and suspension melt crystallization operations, purification by “sweating” of naphthalene-benzothiophene solid solution crystals was carried out after they were formed either on a cold surface (layer crystallization) or in supercooled melts (suspension crystallization). The purification rate coefficients obtained for the suspension system crystals were found to always be larger than those for the layer system, furthermore, the amount of impurities unable to be removed by “sweating” from the layer was higher than that from the particles. These two facts lead to the conclusion that the overall efficiency for the purification by sweating is higher for the suspension crystallization than the layer one.

Operational design for start-up of chemical processes

Aiming at designing more operable chemical processes, this paper presents an operational design methodology for the case of process start-up which evaluates and improves a process flow diagram (PFD) into an operational flow diagram (OFD) by modifying the process structure, adjusting the process behavior and adding more facilities. An important controlled group unit (CGU) concept is introduced, which can be operated independently from other parts of the process. This CGU concept makes it convenient to decompose the whole process into CGUs so that the complicated design problem can be narrowed down and simplified. A multi-level blackboard architecture is used to implement such a design methodology which supports an interactive environment for engineers. Object orientation is used to implement the knowledge sources which improves the blackboard performance. Finally, the methodology is illustrated by a simple example and then is evaluated by an industrial hydrogen-desulfurization process design case study.

Polymorphism, morphologies and bulk densities of DL-methionine agglomerate crystals

To explain the experimentally determined operation conditions to produce DL-methionine (DL-Met) crystalline particles having high bulk density from reaction crystallization, the morphologies of DL-Met agglomerated crystals in addition to the crystallographic structures were investigated. The existence of the third polymorph, the γ-form, was confirmed. Both the observed and predicted morphologies for the elementary particles were plate-like and were strongly affected by the crystal structures. The effect of polymorphism and morphology of resulting particles from reaction crystallization on the bulk density of the product agglomerates was discussed. The bulk density was strongly affected by the agglomeration and the morphology of the constituent elementary crystals. The morphology change with pH of the solution was also experimentally found. Local pH during the reaction crystallization is concluded essential to determine the morphology of the elementary particle, and hence the bulk density of the agglomerates.

Analysis of nucleation of zeolite A from clear solutions

Abstract It is recently reported that the preparation of zeolite membranes from clear solutions with low concentrations of Si 4+ and Al 3+ could prevent secondary nucleation and hence results in tight structures. Therefore, it is essential to study the phenomena of nucleation and crystal growth of zeolites from low concentration and clear solutions. In this study, the dependence of induction periods for nucleation of zeolite A on solution compositions and temperatures was analyzed. The induction periods of nucleation were found to decrease with the saturation ratios. The dependence of induction periods on saturation ratios was represented by two straight lines and the lines intersected at critical points that corresponded to the points of changing the types of precipitate structures. In the range of the saturation ratio above the critical point, precursors precipitated, while zeolite A precipitated in the rest of the range. This suggests that the induction periods are related to the type of precipitate structures.

Growth rate enhancement of potash alum crystals by microcrystals

Abstract During the steady growth of a single crystal of potash alum fixed in a clear supersaturated solution, secondary nucleation was intentionally induced by adding ground potash alum crystals and the resulting changes in the growth rate and the solution concentration were measured. The growth rates after the nucleation were found to abruptly increase by a factor of up to 3, and to gradually return to the steady growth rates or to some constant values. At the same time, the solution concentration started to decrease at the moment of the nucleation. As a measure of the growth rate increase the enhancement coefficient, ϵ 0 , was introduced which was defined as the ratio of the growth rates in the presence to the absence of microcrystals at the same supersaturation. The values of ϵ 0 were found to be almost independent of the growth rate in the absence of microcrystals, i.e. the solution supersaturation.

Crystallization phenomena in ternary systems: nucleation of KCl during dissolution of NaCl

Abstract Spontaneous nucleation of potassium chloride crystals onto the surfaces of dissolving sodium chloride substrate crystals was observed in the ternary system of NaCl–KCl–H2O. The phenomena were understood being controlled by the local conditions of the mother liquor adjacent to the dissolving surface of the sodium chloride substrate and are specific to ternary systems or systems having more than two components, since the local supersaturations are created by the equilibrium conditions. When sodium chloride crystals are dissolving in either aqueous potassium chloride solutions or NaCl–KCl–H2O solutions, the solution adjacent to the dissolving surface is assumed to be saturated with respect to sodium chloride, and this can cause local high supersaturation with respect to potassium chloride enough to induce its nucleation. The mechanism of nucleation of potassium chloride is discussed on the basis of ternary phase equilibrium and kinetics of dissolution.

Mechanism of growth rate enhancement by micro-crystals for the potash alum–water system

Abstract Phenomenon of growth rate enhancement (GRE) caused by newly born micro-crystals was experimentally measured and quantitatively analyzed for the system of potash alum and water. Micro-crystals were prepared by mixing stoichiometric amounts of aqueous solutions of aluminum and potassium sulphates, and were fed as slurry. The size and number were controlled by the age and volume of the slurry, respectively. The age showed a substantial effect on the growth enhancement in such a way that the younger the age greater the effect. The degree of enhancement was found to be proportional to the square of the slurry volume added to the mother liquor. An empirical correlation was obtained among the degree of GRE, the slurry age and the slurry volume added. The size and number of micro-crystals were then estimated from the analysis based on a simple nucleation – growth model. Although the maximum effective size of micro-crystals could not be to determined due to the lack of reliable values of surface tension for the present system, it was estimated to be several micrometers from the experimental observations. In addition, the growth rate of such micro-crystals was found to be much slower than that of coarse ones.

Growth rate enhancement by microcrystals and the quality of resulting potash alum crystals

Abstract Growth rate enhancement resulting from the addition of ground powder crystals to a growing larger crystal has been examined for the potash alum–water system. The crystal quality of the large crystal after the addition and subsequent growth rate enhancement was evaluated in terms of the formation of inclusions and dislocations. Inclusions were observed with an optical microscope and the dislocations were analysed using X-ray transmission topography. It was found that the development of inclusions occurs at the time of the addition to the solution of the ground powder crystals and their attachment to the growing crystal surface. Simultaneously, dislocation bundles were generated. It is proposed that the inclusions form as the growing crystal surface envelopes the adhering particles and that the dislocations form both as a consequence of the strain that develops and the lattice mismatch required to refacet the surface. Both result in the development of additional growth centres which cause the growth rate enhancement.