Stirred Tank Crystallizer Research Articles

New insights into the effect of polyvinylpyrrolidone (PVP) concentration on methane hydrate growth. 1. Growth rate

To further understand the effects of polyvinylpyrrolidone (PVP) on methane hydrate growth, the current study evaluates initial hydrate growth rate over a wide range of PVP concentrations from 0.7 to 20000ppmw (2wt%) in a stirred tank crystallizer. Tests were undertaken at temperatures ranging from 275.1 to 279.1K, pressures from 4645 to 7183kPa and PVP molecular weights of 10000, 40000 and 360000g/mol. Throughout all these conditions, a common sigmoidal trend between growth rate and PVP concentration was observed. Initial growth rates were the same as that of pure water at PVP concentrations under 10ppmw but decreased rapidly to near zero growth at concentrations above 1000ppmw. PVP reduced the initial growth rate by roughly 50% over the concentration range of 10 to 100ppmw. Such low concentrations have not previously been investigated and provide insight into the PVP inhibition mechanism. PVP was shown to be less effective at inhibiting hydrate growth as reactor pressure increased. The various molecular weights investigated showed similar effects on growth rate at the same mass concentrations, showing no improvement in inhibition by varying polymer chain length over the range tested.

Analytical technology aided optimization and scale‐up of impinging jet mixer for reactive crystallization process

Reactive crystallization is widely used in the manufacture of active pharmaceutical ingredients (APIs). As APIs often have low solubility, traditional stirred tank reactors and the route of process operation and control using metastable zone width are not effective. This work investigated the integration of an impinging jet mixer and a stirred tank crystallizer that can take advantage of both the reaction and crystallization characteristics, the focus being on design optimization and scale‐up using process analytical techniques based on the Fourier transform infrared spectroscopy and focused beam reflectance measurement, as well as X‐ray diffraction and particle imaging Morphologi G3. The parameters for process operation and design of the impinging jet mixer were optimized. The research was carried out with reference to the manufacture of an antibiotic, sodium cefuroxime, first in a 1L reactor, then a 10L reactor. The crystals produced showed higher crystallinity, narrower size distribution, higher stability, and purity. © 2014 American Institute of Chemical Engineers AIChE J, 61: 503–517, 2015

Effect of impeller design and power consumption on crystal size distribution

Crystallization processes in a 500 mL stirred tank crystallizer with computational fluid dynamics (CFD) and population balances toward estimating how crystal size distributions (CSDs) are influenced by flow inhomogeneities was explored. The flow pattern and CSD are presented here though extensive phase Doppler particle analyzer measurements and CFD predictions for three different impeller designs (disc turbine, pitched blade turbine, and Propeller) and each rotated at three different speeds (2.5, 5, and10 r/s). As crystallization processes in practice could involve break‐up and aggregation of crystals, some selected break‐up and aggregation kernels are incorporated. Extensive comparison of simulations with experimental data showed consistent trends in the proper quantitative range. An attempt has also been made to develop scaling laws: (a) mean particle size with average power consumption per unit mass and (b) particle‐size distribution with the turbulent energy dissipation distribution. © 2014 American Institute of Chemical Engineers AIChE J, 60: 3596–3613, 2014

An investigation into the effect of mixing on the secondary nucleation of sodium chlorate in a stirred tank and an oscillatory baffled crystallizer

This paper reports the outcome of the examination of the effect of mixing intensity on the secondary nucleation mechanism of seeded crystallization of sodium chlorate in various configurations of stirred tank crystallizer (STC) and oscillatory baffled crystallizer (OBC). The results show that for the STC, an un-scraped system always yielded crystals of the same enantiomorphism as the seed crystal. The introduction of scraping to the STC resulted in product crystals of the opposite enantiomorphism to the seed being formed, however changing the rate of stirring (RPM) had no influence over the percentage similarity to the seed crystal. For the OBC, scraping always gave a product crystal crop with less than 100% similarity to the seed. Similarly, this was unaffected by altering the frequency of oscillation. Removing the scraping effect brought an increase of the similarity to the seed but the product crystals never reached 100% similarity to the seed enantiomorphism. For both scraped and un-scraped OBCs, reduction of the oscillation amplitude did increase the similarity of the products to the seed and 100% similarity to the seed was repeatedly achieved at the lowest operating amplitude for the un-scraped OBC. These findings provide further insight into the nucleation mechanism in the OBC and suggest that an alternative mechanism to that in the STC is observed.

Growth of calcite seeds in a magnetized environment

Using the constant-composition technique, the growth rates of calcite seeds in a simulated cooling-water environment, i.e., at a temperature higher than room temperature and in the presence of impurity, were measured in a stirred-tank crystallizer under the influence of a magnetic field. When the impurity was absent, the magnetic field would gradually reduce the growth rate of calcite seeds and would have varying impact on the time needed for the seeds to stop growing for different levels of supersaturation, temperature, and pH. In the presence of impurities, including Sr2+ and Fe2+ in the concentration range between 0 and 2.0ppm, the growth experiments were conducted at two temperatures, i.e., 25°C and 35°C. In the presence of Sr2+ the calcite seeds grew steadily after a premagnetization time of 20h, which was long enough to completely stop the growth of calcite seeds in the absence of impurities. On the other hand, the effect of Fe2+ on calcite growth rate in the presence of magnetic field was rather complicated; either reduced or enhanced rate has been observed. The magnetic field indeed inhibited or stopped the growth of calcite seed for some instances.

Kinetics of Gypsum Precipitation for Designing Interstage Crystallizers for Concentrate in High Recovery Reverse Osmosis

Treating desalination concentrates by precipitating sparingly soluble salts is a promising method for extending water recovery limits of such processes and thereby reducing concentrate volumes. One of more common technologies for carrying this out is the use of fluidized bed crystallizers, often with sand grains as the crystallizing surface. Most of the works done to date on such crystallizers for water treatment purposes have been on water softening, and little has been done on use of such crystallizers for precipitation of gypsum. This study was on the crystallization kinetics of gypsum at initial supersaturation levels typical of reverse osmosis (RO) concentrates, in the presence of silica sand grains or gypsum seeds, in a batch stirred tank crystallizer. The findings show that silica sand grains were efficient primary heterogeneous nucleators for gypsum. Crystallization kinetics increased with larger grain size or higher grain number concentration. Considering the different nucleation mechanisms, a ce...

Stirred batch crystallization of a therapeutic antibody fragment

Technical-scale crystallization of therapeutic proteins may not only allow for a significant cost-reduction in downstream processing, but also enable new applications, e.g., the use of crystal suspensions for subcutaneous drug delivery. In this work, the crystallization of the antigen-binding fragment FabC225 was studied. First, vapor diffusion crystallization conditions from the literature were transferred to 10μL-scale microbatch experiments. A phase diagram was developed in order to identify the crystallization window. The conditions obtained from the microbatch experiments were subsequently transferred to parallelized 5mL-scale stirred-tank crystallizers. This scalable and reproducible agitated crystallization system allowed for an optimization of the crystallization process based on quantitative measurements. The optimized crystallization process resulted in an excellent yield of 99% in less than 2h by increasing the concentration of the crystallization agent ammonium sulfate during the process. The successful scalability of the Fab fragment crystallization process to 100mL-scale crystallizers based on geometric similarity was demonstrated. A favorable crystal size distribution was obtained. Furthermore, a wash step was introduced in order to remove unfavorable low-molecular substances from the crystals.

Probing into Nucleation Mechanisms of Cooling Crystallization of Sodium Chlorate in a Stirred Tank Crystallizer and an Oscillatory Baffled Crystallizer

In this paper we report our recent studies into why different nucleation behaviors in cooling crystallizations were observed in a stirred tank crystallizer (STC) and an oscillatory baffled crystallizer (OBC). Using sodium chlorate as the model compound and setting up a number of hypotheses, we identified and verified different nucleation mechanisms for the above two systems: while under the same operational conditions, secondary nucleation occurred exclusively in the STC where the product crystals bore the same handedness as the seeds; a combination of primary and secondary nucleation was displayed in the OBC where a mixture of left- and right-handed crystals coexisted when a single-handedness crystal was seeded. Through further hypothesis, we established the vital link between the nucleation mechanisms and the ways mixing is generated in the two systems and discovered that the scraping action between the baffle edge and the wall of the crystallizer could be the main culprit for the unexpected primary nuc...

Growth Behavior of Aragonite under the Influence of Magnetic Field, Temperature, and Impurity

With and without magnetization, the growth rates of aragonite seeds were measured in a stirred-tank crystallizer in the presence of Fe2+ or Sr2+ acting as an impurity at two temperature levels, using the constant-composition technique. The concentration range of the impurity investigated was between 0 and 2 ppm, and the temperature settings were either 25 or 35 °C. The individual factors each had a distinct effect on the growth rate of aragonite, and the combined effects became complicated, especially in the presence of Sr2+, which changed the surface structure of the aragonite seed dramatically.

Synergetic effects of temperature and magnetic field on the aragonite and calcite growth

The effects of temperature and magnetic field on aragonite and calcite growth were investigated in a stirred-tank crystallizer, using the constant-composition technique. At room temperature without magnetization, the calcite seeds grew but the aragonite did not. When the growth rates of the aragonite and calcite seeds were measured in the presence of a magnetic field at temperatures above room temperature, the synergetic effects were significant but with differing effects on the different seeds. The cluster-transformation mechanism was used to explain the growth rate data. Finally, the aragonite growth rate data were compared with data obtained in a fluidized bed.

A comparison of growth behavior between CaCO 3 and CaF 2 crystals in a constant-composition environment

For the comparison of crystal growth behavior of different sparingly soluble salts, a constant-composition technique was used to study the influence of process variables on the growth of calcite and fluorite in a stirred-tank crystallizer. The growth experiments were conducted in the metastable regions determined in this study and the growth rates were evaluated from the solution volume added to the crystallizer for keeping the system at constant composition, using the cured natural calcite and fluorite as seeding. Factors affected the growth rates, such as the pH of the solution, ionic strength, activity ratio and relative supersaturation, were identified and the influence of individual factor on the growth rate was compared between the two systems. Finally, growth rate expressions of calcite and fluorite were established and compared with that reported in the literature.

Influence of mixing and solid concentration on sodium bicarbonate secondary nucleation rate in stirred tank

AbstractThis work aims to investigate the influence of the solid concentration in suspension on the contact secondary nucleation rate of sodium bicarbonate crystallization in a stirred tank crystallizer and to show the necessity of a local description of the mixing for a nucleation rate influence study. Experiments and computational fluid dynamics (CFD) simulations are realized. Crystallization kinetic parameters are extracted from experimental data using a mass distribution fitting approach. CFD and the experimental results allow identifying that a mixing property correlated with the measurements of the secondary nucleation rate in the stirred tank crystallizer appears to be the turbulent dissipation rate on the edge of the impeller. Its influence and the influence of the solid concentration in the suspension on the secondary nucleation rate are estimated by the evaluation of their exponents in a kinetic law. The obtained exponent values are then discussed qualitatively. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Purification of Natural Gases with High CO2Content Using Gas Hydrates

An alternative process for the conditioning of natural gases with high CO2 content using gas hydrates has been designed, simulated, and compared to the well-established process of pressurized gas scrubbing (PGS). Simulation calculations were made with a gas mixture consisting of CO2 (1) and CH4 (2), with y1 = 0.5 being exemplary for biogas (digester gas) and landfill gas. The feed was separated using a cascade of continuously stirred tank crystallizer vessels, which can also be regarded as an ideal crystallizer column resembling a gas-hydrate-based scrubbing process. It was found that the selective enclathration of CO2 molecules does not show potential for competitive CO2 removal from natural gas streams.

Reaction rate constant of methane clathrate formation

Particle size distribution measurements were performed during the growth stage of methane hydrate formation in a semi-batch stirred tank crystallizer. Experiments were carried out at temperatures between 275.1 and 279.2 K and pressures ranging from 3873 to 5593 kPa. The reaction rate constant of methane hydrate formation was determined using the model of Bergeron and Servio (AIChE J 2008;54:2964). The experimental reaction rate constant was found to increase with temperature, following an Arrhenius-type relationship, from 8.3 × 10 −8 m/s to 6.15 × 10 −7 m/s over the 4° range investigated, resulting in an activation energy of 323 kJ/mol. An increase in pressure of approximately 600 kPa did not have any effect on the reaction rate constant. Population balances, based on the measured critical nuclei diameter and that predicted by homogeneous nucleation theory, were also used for comparison purposes. The initial number of hydrate particles was calculated using the mole fraction of methane in the bulk liquid phase and compared to that predicted by an energy balance.

A Study on Nucleation for Protein Crystallization in Mixed Vessels

This study investigated mechanisms of secondary nucleation for the crystallization of the protein hen egg white lysozyme. A parametric study of the secondary nucleation of lysozyme in a stirred tank crystallizer was performed, testing the influence of stirrer speed, the size and volume of added seed crystals, solution temperature, supersaturation, and sodium chloride concentration on measured nucleation rates. The measured rates were highly variable, which made assessment of the influence of experimental parameters difficult. Only stirrer speed exhibited significant influence over experimental variability (10-fold increase in nucleation rates with a 50% increase in stirring speed). These results were interpreted by comparison with parameter influence that is commonly observed with secondary nucleation of nonproteins. A novel tubular crystallizer was used to assess the influence on secondary nucleation of fluid shear forces acting on lysozyme crystals. It was observed that fluid shear forces acting on mounted lysozyme crystals do not notably influence secondary nucleation. Overall, the results suggest that attrition is an important source of secondary nuclei in lysozyme crystallization and that secondary nucleation caused solely by fluid shear forces is not.

Reaction rate constant of propane hydrate formation

Experimental data on the rate of formation of propane hydrates were obtained using a particle size analyzer capable of detecting particles with diameters as small as 0.6 nm, while operating in a closed loop system. Experiments were carried out at temperatures around 274 K and pressures between 315 and 340 kPa in a semi-batch stirred tank crystallizer. The experimental data were analyzed using a newly developed kinetic model based on crystallization theory and as a result, the actual reaction rate constant for propane hydrate formation was successfully determined. A value of 2.15 × 10 −7 m/s is reported at 274.2 K, while it was found that the dissolution rate of propane at the vapor–liquid water interface was enhanced by a factor of five during the growth stage, compared to the value obtained during typical solubility experiments.

Investigation of crystallization kinetics of sodium bicarbonate in a continuous stirred tank crystallizer

In our previous work [J. Crystal Growth 263 (2004) 459], a supersaturation determination method using a high-precision densimeter was designed and developed for monitoring sodium bicarbonate supersaturation in the sodium carbonate–bicarbonate system, a two-component solution in equilibrium. In the present paper, we apply this method to the study of sodium bicarbonate continuous crystallization. It gives us access to the sodium bicarbonate supersaturation evolution. The sodium bicarbonate supersaturation evolution and their crystallization kinetics obtained in a continuous stirred tank crystallizer are investigated under different operational conditions, such as residence time, initial supersaturation and seeding. The induction time determined by density measurements is discussed. Mathematical modeling is used to interpret the supersaturation evolution. It is shown that measuring the density gives satisfying performances for studying the crystallization of a two-component solution in equilibrium, such as the sodium carbonate–bicarbonate system.

A kinetic study on crystallization of calcium gluconate in external loop airlift column and stirred tank for an immobilized glucose oxidase reaction with crystallization

The batch crystallization of calcium gluconate from the reaction solution of the immobilized glucose oxidase catalyzed oxidation of glucose was kinetically studied not only in an external loop airlift column but also in a small scale stirred tank. The solubility of calcium gluconate under a wide range of the glucose oxidation conditions was measured and correlated. The crystallization was started by charging successively a saturated calcium gluconate solution prepared at 30 °C and a known amount of its seed crystals in either crystallizer being thermostated at a constant temperature between 5 and 15 °C. The time courses of the dissolved calcium gluconate concentration during crystallization were observed to be unaffected either by superficial gas velocity in the airlift column or by impeller speed in the stirred tank. The time courses in the column were found to be identical with those in the tank under the same operating conditions of pH, temperature, and glucose and seed crystals concentrations. Almost no decrease in the dissolved calcium gluconate concentration was observed during the crystallization without any seed crystals, while negligible change in the crystal size distribution was microscopically found during the crystallization with seed crystals. These results obtained suggested that the crystallization proceeded under the surface reaction control and no secondary nucleation occurred in both crystallizers. A crystal growth model was proposed to reproduce the time courses of the dissolved calcium gluconate concentration during crystallization under the various operating conditions. The order of the crystal growth rate with respect to the calcium gluconate supersaturation was found to be 3.2. The calculated time courses of dissolved calcium gluconate concentration agreed well with the observed ones in both airlift and stirred tank crystallizers.

Measurement of Calcite Growth Rate in the Seeding Continuous Stirred Tank Crystallizer

Measurement of Calcite Growth Rate in the Seeding Continuous Stirred Tank Crystallizer

Analysis of a continuous crystallizer with agglomeration

The analysis of a continuously stirred tank crystallizer in which the particles may undergo agglomeration is presented. It is shown that a solution presented previously is in error due to an assumption made in the development of the model, and that a solution in the form of a Taylor series expansion can be derived easily. The accuracy of this revised solution is determined by the number of terms used in the series. A rather large number of terms is required to generate accurate results in some cases. It is shown that physically meaningful solutions do not exist for some values of the agglomeration parameter. For acceptable values of the parameter it is shown that the particle-size distribution is monomodal. For unacceptable values of the parameter, the model solution is unbounded at large sizes, and does not predict a bimodal size distribution.