Masaaki Yokota

Supersaturation dependence of crystal growth in solutions in the presence of impurity

Abstract It is proposed to describe the rate of crystal growth in solution in the presence of impurity as a function of supersaturation. The theoretical equations are based on the pinning mechanism of Cabrera and Vermilyea for the inhibition of step advancement, assuming that impurity species are adsorbed one-dimensionally on the step lines. The Langumir adsorption isotherm is assumed to hold for impurity adsorption. Experimental growth rate data from the literature are used to assess the model. The supersaturation dependence of crystal growth rate in the presence of impurity is also well described with the model. The critical supersaturation below which a crystal does not grow is correlated with the impurity concentration. For the case where an impurity has no effect at high supersaturation, two possible mechanisms are discussed: one related to a change of rate-determining process and the other related to competition between impurity adsorption and growth unit deposition processes.

Seeding policy in batch cooling crystallization

Abstract The effect of seeding in batch cooling crystallization is discussed on the basis of original experimental data on potassium alum–water system obtained for both controlled and natural cooling modes. Regardless of the cooling mode, unimodal product crystal size distributions of grown seeds were obtained, provided that enough seed loadings are above a critical concentration. Slow cooling was not a necessary condition to avoid secondary nucleation. The critical seed concentration can be determined easily from several batch trials with the help of a seed chart, in which mean mass size of product crystals normalized with the seed size is plotted as a function of seed concentration, with seed size as a parameter. This type of seeding effect could be observed for cooling crystallization of other systems and for crystallization employing other supersaturation-generating methods.

The combined influence of supersaturation and impurity concentration on crystal growth

Supersaturation dependence of crystal growth in the presence of impurity is discussed. The supersaturation dependence was divided conveniently into two types: the case where growth rate is suppressed over a wide range of supersaturation and the case where it is suppressed only in the low range of supersaturation while at the higher supersaturation range the impurity effect disappears completely. The latter case is explained theoretically by assuming slow unsteady state adsorption of impurity species at higher supersaturations. Literature data of the growth rate of an ammonium sulfate crystal are successfully explained. It is pointed out that classification of impurity action into the two types is not essential. The former type is a special case of the second one where the time constant of adsorption process becomes very small even at higher supersaturations.

Effect of Cooling Mode on Product Crystal Size in Seeded Batch Crystallization of Potassium Alum

Potassium alum was crystallized by seeding in a batch crystallizer under controlled and natural cooling modes. Regardless of the cooling mode, the product crystal size distribution (CSD) became bi-modal at low seed concentrations because of enormous secondary nucleation. The mean mass size of the product was smaller for the natural cooling mode compared to that for the controlled cooling mode with more intensive secondary nucleation. On the other hand, at high seed concentrations, the product CSD became uni-modal with the same mean mass size for both cooling modes, where the crystallization was dominated by seed growth. The low supersaturation caused by the growth of enough seeds plays a key role to produce uni-modal size distribution with suppressed nucleation. Adhering of small crystals (secondary nuclei) to growing seed crystals is also considered to be another mechanism for generating uni-modal CSD.

Unsteady-state impurity effect of chromium (III) on the growth rate of potassium sulfate crystal in aqueous solution

Abstract Growth length of a single potassium sulfate crystal was measured at a constant supercooling of 7°C in a flow cell in the presence of a trace of chromium (III) (up to 5 ppm) under natural and controlled pH conditions. The impurity action of chromium (III) exhibited unsteady-state behavior. The growth rate decreased gradually, reaching zero in about 100 min at an impurity concentration of 2 ppm under the condition of pH = 5.8. At higher impurity concentrations (3 and 5 ppm), the growth was completely stopped at an early stage of the run nearly at the same pH level. No impurity effect was observed at higher pHs (= 7 and 10), while, at lower pHs (= 3 and 5), the effect was very strong even at an impurity concentration of 2 ppm. The unsteady-state growth behavior is reasonably explained by a model assuming that the impurity species, which acts as a stopper for the step advancement, adsorbs very slowly on the step lines. The effect of pH on the impurity action is explained by assuming that the first hydrolysis product, [Cr(H2O)5(OH)]2+, of the aqua complex compound of chromium (III) acts as the active species for growth inhibition.

Liquid inclusions in crystals produced in suspension crystallization

Abstract Sodium chloride, potassium chloride, succinic acid and potassium hydrogen phthalate crystals were crystallized batchwise in an agitation vessel and the total volume of liquid inclusions per crystal, V (μm 3 ), was measured as a function of crystal size, d (μm). The found data of the total volume, including literature data from a continuous crystallizer, was correlated with a simple equation, V =4.0×10 −6 d 4 . Most of the inclusions observed in sodium chloride and potassium chloride crystals were layer inclusions, which were aligned two-dimensionally in parallel with crystal faces. This type of inclusion pattern was suggested, with the help of the results of additional in situ experiments under a microscope, to be caused by the adhesions of small crystals to growing crystals and mechanical contacts imposed to these crystals during growth. Although the layer pattern was not found in the other crystals examined, the same mechanism was considered to work for the inclusion formation.

Scale-up effect on the rate of contact nucleation caused by collisions of crystals with an impeller

This paper proposes a rate equation of contact nucleation caused by crystal–impeller collision Jc-i=k′·p·ns·dNp·uN+1·D-4b, where k′ is the coefficient, p the impeller blade number, ns the crystal number, dp the crystal size, u the tip speed of an impeller, and Db the impeller blade size. Contact nucleation rates of potassium alum measured in three stirred tanks of different size (1.57×10-3, 6.28×10-3, 5.02×10-2 m3 of working volume, P=4,6,ns=10–2000,dp=3.585×10-4–1.545×10-3 m, u=0.817–2.136 m/s, Db=9.45×10-3,1.5×10-2,3.0×10-2 m) were successfully explained by this equation when the value of an unknown parameter (N) was equal to 3. The meaning of N=3 is also discussed using the measured frequencies of particle–impeller contact (non-nucleation experiments).

A simple method for evaluating kinetic parameters in non-isothermal batch crystallization

This paper propose a kinetic parameter evaluation technique that can be flexibly applied to various types of batch crystallization. The technique allows simultaneous evaluation of growth and nucleation kinetic parameters.

A note of the purity of crystals produced in batch suspension crystallization

Potassium chloride, potassium hydrogen phthalate and succinic acid were crystallized from aqueous solution by unseeded batch crystallization. The agglomerates produced under high loading conditions were purer than the less agglomerated crystals produced under low loading conditions for all the substances examined. This is because agglomerates are composed of small primary crystals of high purity.

SIMPLE BATCH OPERATION FOR SELECTIVE CRYSTALLIZATION OF METASTABLE CRYSTALLINE PHASE

Abstract This paper describes effectiveness of a “short-time” batch operation for obtaining a metastable crystalline phase (L-glutamic acid, α-form) based on the following experimental results. Seed crystals of α-form are charged initially into a crystallizer to commence crystallization. In the case of slowly cooled batch operation, the crystallization accompanying nucleation of unwanted stable crystalline phase (β-form). However, nucleation of β-form can be reduced significantly by increasing the cooling rate (“short-time” batch operation). Additionally, selection of the α-form seed crystals that do not include any other solid phase is effective for avoiding the formation of unwanted (stable) nuclei.