Narrow Crystal Size Distribution Research Articles

Crystallization of alpha-lactose monohydrate in a drop-based microfluidic crystallizer

A microfluidic process for producing crystals of controlled size by confining the crystallization within drops is demonstrated. The process consists of a drop producing stage which segments the mother liquor into monodisperse drops followed by crystallization in a temperature controlled tubular crystallizer. The process is implemented to produce lactose crystals with significantly narrower crystal size distribution (CSD) as compared with crystals produced in a stirred bulk crystallizer. By controlling the drop size and initial supersaturation the mean crystal size can be controlled. The distribution of the number of crystals per drop and the CSD are measured as a function of supersaturation at temperatures between 20 and 40 ∘ C for 150 and 300 μ m drops. In the case where drops contain only one crystal, a very narrow CSD is obtained with a coefficient of variation of crystal size as low as 7%. The crystallization of lactose in a microfluidic tubular crystallizer is modeled by treating nucleation in drops as a Poisson process with a nucleation rate based on classical nucleation theory. Experimental results are in good agreement with predictions from a Poisson process model over the range of temperatures, supersaturations and drop sizes tested.

In2O3 and tin-doped In2O3 nanocrystals prepared by glass crystallization

Glasses in the system Na2O/B2O3/Al2O3/In2O3/(SnO2) were melted and subsequently tempered in the range from 540 to 700°C. This led to the crystallization of In2O3, besides also NaBO2 and Na2Al2B2O7 were formed. The crystallite sizes were in the range from 12 to 34nm, depending on the temperature of thermal treatment, as calculated from XRD-line broadening. Crystal sizes determined by TEM were in the same range. Particle size histograms show a narrow crystal size distribution. After crystallization, the glass matrices and the non indium-containing crystalline phases were dissolved in diluted acetic acid. Spectra of the obtained suspensions containing tin-doped In2O3 nanocrystals exhibited a sharp UV-cut off, high transmission in the visible range and strongly decreasing transmission at wavelengths >1000nm.

Organic-free synthesis of ZSM-5 with narrow crystal size distribution using two-step temperature process

The MFI type of highly silica zeolite, ZSM-5 with well-defined crystal morphology was successfully synthesized in the absence of organic template using two-step temperature process. In this process, the crystallization process was conducted at two different temperatures by separating the nucleation and crystallization stage; that is, the nucleation stage at higher temperature (190 °C) to accelerate the nucleation followed by the crystallization stage at lower temperature (150 or 165 °C) so as to control the crystal size and size distribution. The effect of nucleation rate on the final crystal size and size distribution adopting higher temperature of the first stage for nucleation than that of the second stage for crystallization was investigated. The results show that two-step temperature process could control the crystal size and size distribution of ZSM-5. The result also shows that the time duration at higher temperature for the acceleration of nucleation strongly affects the crystal size and size distribution. It implies that rapid nucleation at higher temperature forms large number of nuclei followed by slow crystal growth probably due to the lack of energy at lower temperature. It is very critical, therefore that the optimum time duration at higher temperature should be chosen to accelerate the nucleation within initial stage.

The synthesis of large crystals of zeolite Y re-visited

The major synthesis routes described in the literature so far for zeolite Y (FAU) with an elevated n Si/ n Al-ratio and larger crystallite sizes were systematically re-investigated. Syntheses in the presence of triethanolamine with or without added bis(2-hydroxyethyl)dimethylammonium chloride yielded large crystals (10–100 μm) of faujasite-type zeolites. However, only products with low n Si/ n Al-ratios below 1.8 were obtained, and zeolite P was an inevitable side product or even the main product. Furthermore, the outcome of these syntheses was poorly reproducible. Following another known literature procedure, phase-pure zeolite Y could be obtained with the cyclic polyether 15-crown-5 as a structure-directing agent. The influence of ageing, homogenization and seeding of the synthesis gel, the crystallization time and temperature, different Si- and Al-sources and the addition of NaF as a mineralizer or of co-templates such as triethanolamine, ethylene glycol or 1,3,5-trioxane on the resulting product was investigated. A carefully optimized procedure led to large crystals of highly phase-pure zeolite Y with well defined octahedral morphology and a narrow crystal size distribution in the range of 4–5 μm. Such crystals can be reproducibly synthesized in high yields with an n Si/ n Al-ratio of 3.4–4.0. Neither crystallization seeds nor an ageing of the synthesis gel before hydrothermal crystallization are required.

Membrane distillation crystallization of concentrated salts—flux and crystal formation

Membrane distillation crystallization (MDC) can be used to process highly concentrated aqueous solutions. In MDC, membrane distillation (MD) is used to recover water and to generate the desired supersaturation in the crystallizer where product crystals can be precipitated. This paper discusses factors influencing flux at close to saturation and the formation of salt crystals. The flux behaviour was investigated using two aqueous salt solutions of sodium sulfate (Na 2SO 4) and sodium chloride (NaCl) which have different solubility–temperature coefficients. For both salts it was observed that MD was operable at high concentrations at feed temperatures of 50 and 60 °C with fluxes up to 20 L m −2 h −1. It was found that both concentration and temperature polarization influenced the performance of MD. When operated in batch concentration mode without the crystallizer the flux gradually declined due to vapour pressure suppression and concentration polarization, up to a critical degree of saturation. The flux data could be predicted by heat and mass transfer modeling or by a simple empirical relationship involving the overall vapour pressure driving force and the degree of saturation. Beyond the critical degree of saturation, rapid flux decline was observed due to crystal deposition and scale formation on the membrane which reduced the membrane permeability. The Na 2SO 4 solution was able to operate at slightly higher degrees of saturation, which may be because the negative solubility–temperature coefficient favours solubility in the polarization layer. In MDC the temperature and saturation level both in the MD and the crystallizer are critical operating parameters. Anhydrous sodium sulfate crystals can be produced by means of MDC with a relatively narrow crystal size distribution and average size of 60–80 μm.

Batch cooling crystallization study based on in-line measurement of supersaturation and crystal size distribution

The in-line information of supersaturation and crystal size distribution is very important for the controlling and optimization of batch cooling crystallizations. Based on the in-line measurements of supersaturation with an attenuated total reflectance-Fourier transform infrared spectrophotometer and chord length distribution with an MTS laser reflection analyzer, the batch cooling crystallization of an organic compound, labeled as C15 is studied in this work. The crystal size distribution of the final product was also analyzed with a laser diffraction analyzer to verify the data obtained from the in-line instrument. Various cooling modes are tested, and the seeding policy is applied to improve the final crystal size distribution. The effects of the cooling modes and seeding are analyzed based on the obtained in-line supersaturation and off-line crystal size distribution data. Improved operation conditions are proposed in terms of large mean crystal size and narrow crystal size distribution. The result of this work shows that the spectrophotometer gave accurate solution concentration results with high resolution; the MTS in-line analyzer can be used to monitor chord length distribution and count rate trends.

Control of calcium carbonate morphology by precipitation in compressed and supercritical carbon dioxide media

This paper reports a new method to produce calcite crystals with scalenohedral and rhombohedral morphologies in the Ca(OH)2–H2O–CO2 system, without the use of any tailor-made additive. Compressed CO2 vapour (25°C, 50bar), liquid CO2 (25°C, 200bar) and supercritical CO2 (45°C, 200bar) were used to precipitate CaCO3. De-aggregated calcite crystals of rhombohedral habit and narrow crystal size distribution were prepared using a novel route that consists in the reaction of an aqueous suspension of Ca(OH)2 with compressed carbon dioxide. In the new method, morphological control, overall production rate and conversion efficiency are enhanced with respect to the established industrial carbonation process.

Production of large crystals with a narrow crystal size distribution by a novel WWDJ batch crystallizer

A novel batch crystallizer has been developed and used for the crystallization of l-aspartic acid, resulting in large product crystals with a narrow crystal size distribution. The batch crystallizer was equipped with a slurry sprinkler fixed on the shaft of an impeller, and a double-deck jacket. The slurry sprinkler, known as Wall Wetter, is specially designed for agitating the slurry and sprinkling it on the wall of the crystallizer headspace which is covered by the upper jacket. It was expected that by setting the upper jacket temperature at an appropriate high temperature, fine crystals could be dissolved during the fall of slurry along the wall, and consequently the crystal size distribution would shift to the large side. l-aspartic acid was chosen as an example to demonstrate this process. l-aspartic acid was crystallized at an initial supersaturation ratio ( C/ C s) of 1.7 with no seeds. The classification exponent n and characteristic size D e of product crystals, which were evaluated by the Rosin Rammler Sperling Bennet (RRSB) distribution function, were 3.9 and 302 μm, respectively, compared with 2.2 and 237 μm, respectively, in a control crystallization experiment not using the Wall Wetter.

Continuous cooling crystallization from solution

In this paper, a model was constructed in order to simulate the behaviour of slow growing crystals in high viscosity systems in a tower crystallizer. This type of crystallizer, ideally, is an application of the plug–flow concept. Plug–flow conditions are an advantage when a narrow crystal size distribution is the goal desired. The model was based on the following assumptions: steady state operation, no nucleation or growth rate dispersion, no breakage, size-dependent growth and settling of crystals. A simulation example for sucrose is presented although the model applies to other similar systems.

Polymers designed to control nucleation and growth of inorganic crystals from aqueous media

Water soluble graft polymers prepared by copolymerization of either methacrylic acid (MAA) or vinylsulfonic acid (VS) with α-methoxy-ω-methacroyl-oligo(oxyethylene)s (PEO n -MA) serve to control nucleation and crystal growth during precipitation of inorganic crystals from aqueous media. Precipitation of zinc oxide crystals ('zincite') is used as example for such mineralization processes. Homogeneous and narrow crystal size distributions are obtained in presence of ppm-amounts of graft copolymers. Copolymer is incorporated into the crystals demonstrated by using latex particles with -CO 2 H-group rich surfaces as controlling additives. Incorporation of these particles leads to single crystals with pores of the size of the particles ('Swiss cheese' morphologies).

The preparation of highly ordered single layer ZSM-5 coating on prefabricated stainless steel microchannels

An elegant way to prepare catalytically active microreactors is by applying a coating of zeolite crystals onto a metal microchannel structure. In this study the hydrothermal formation of ZSM-5 zeolitic coatings on AISI 316 stainless steel plates with a microchannel structure has been investigated at different synthesis mixture compositions. The procedures of coating and thermal treatment have also been optimized. Obtaining a uniform thickness of the coating within 0.5 mm wide microchannels requires a careful control of various synthesis variables. The role of these factors and the problems in the synthesis of these zeolitic coatings are discussed. In general, the synthesis is most sensitive to the H 2O/Si ratio as well as to the orientation of the plates with respect to the gravity vector. Ratios of H 2O/Si=130 and Si/template=13 were found to be optimal for the formation of a zeolitic film with a thickness of one crystal at a temperature of 130°C and a synthesis time of about 35 h. At such conditions, ZSM-5 crystals were formed with a typical size of 1.5 μm×1.5 μm×1.0 μm and a very narrow (within 0.2 μm) crystal size distribution. The prepared samples proved to be active in the selective catalytic reduction (SCR) of NO with ammonia. The activity tests have been carried out in a plate-type microreactor. The microreactor shows no mass transfer limitations and a larger SCR reaction rate is observed in comparison with pelletized Ce-ZSM-5 catalysts.

The nucleation period for TPA-silicalite-1 crystallization determined by a two-stage varying-temperature synthesis

A two-stage-varying-temperature synthesis procedure, which involves a rapid change in temperature at some point during the course of crystallization, was applied to the synthesis of discrete colloidal particles of TPA-silicalite-1. As the duration of the period at the initial synthesis temperature was extended, the crystal concentration and ultimate crystal size varied until they were approximately equal to those obtained for a complete synthesis at the initial temperature. At this point in the crystallization, it was concluded that the nucleation stage was completed. For syntheses performed at 60, 80 and 100°C, the duration of the nucleation period determined by this method was about 100 h, between 4 and 6 h and less than 2 h, respectively. Thus, nucleation, for this system, is a continuous process, and it was found that the rate of nucleation, which is initially high, declines throughout the nucleation period. In all cases, nucleation occurred during an induction period when little or no crystal growth was observed, which explains why the syntheses yielded a product with a rather narrow crystal size distribution. If, for the two-stage syntheses, the temperature change was made after completion of the nucleation period, the second synthesis temperature controlled only the linear growth rate of the crystals and the final yield of silicalite-1 obtained.

An Improved Method for the Purification of Polyether−Polyols Using Phosphoric Acid as Neutralization Agent

The aim of this study was to develop a crystallization process for the removal of the alkaline catalyst (potassium hydroxide) from crude polyether−polyols. The process involves a first step of neutralization with an aqueous solution of phosphoric acid and subsequently the removal of water to reach supersaturation and crystal growth. Finally, the crystals formed are removed by filtration. Different amounts of phosphoric acid and water were investigated in order to establish a suitable recipe for this process. Different depressurization vs time profiles for the water evaporation were used to produce an uniform and narrow crystal size distribution with a large mean crystal size in this kind of experimental device.

Control of ZnO Crystallization by a PEO-b-PMAA Diblock Copolymer

Amphipolar diblock copolymers used as admixtures provide a unique effect on crystallization processes from solution. In an example given here, nucleation of ZnO crystals and growth of specific crystal faces is controlled by minute amounts of PEO-b-PMAA copolymers. A narrow crystal size distribution and retardation of growth parallel to [001] are observed in dependence on the concentration of the block copolymer, which is added in the ppm range.

Zeolite synthesis using a semicontinuous reactor, Part 1: Controlled nucleation and growth of ZSM-5 crystals having well-defined morphologies

ZSM-5 has been synthesized at 90 and 130°C in reactor systems in which reagents were supplied continuously, and homogeneous product slurry was withdrawn semicontinuously. In some cases, precursor crystals were nucleated in situ, whereas other preparations started from dispersions of seed crystals. Guided by a simple mathematical model, conditions were chosen to give a steady, near-linear crystal growth regime. The crystals grew from solution in a medium free of solid gel in which the predominant source of nutrient silica was the monomeric monoanion [(HO) 3SiO] −. The products were well formed, of high crystallinity, and of narrow crystal size distribution. By providing a means for regulating solution composition and effective supersaturation, the experimental method allowed direct control over nucleation and growth processes and of the radial composition of the resulting crystals. Continuous methods of zeolite synthesis provide a versatile preparative tool and a powerful technique for studying crystallization behavior.

Large AlPO 4-5 crystals by microwave heating

Microwave heating is applied to the synthesis of AlPO 4-5. After 60 s of heating, large AlPO 4-5 crystals are obtained. XRD, polarization microscopy, and adsorption measurements prove the regularity of the AFI framework. In an optimized microwave synthesis prismatic AlPO 4-5 crystals up to 130 μm long and 40 μm thick could by synthesized. In a two-step synthesis, however, slightly smaller, but very uniform AlPO 4-5 crystals with a narrow crystal size distribution without any amorphous or crystalline byproducts could be obtained. Several possible mechanisms for the fast crystallization within 60 s by microwave radiation are discussed: an increased dissolution of the gel by lonely water molecules, the almost T-gradient-free and, therefore, convection-free in situ heating, and the existence of organic-inorganic arrays as local microassemblies which could transform directly into the AFI framework.

Controlled precipitation

AbstractIn a process of precipitation, two or more reactants form a product. In many cases, the solubility of this product is very low and high supersaturation is generated which leads to nucleation and crystal growth. The number of the formed nuclei and their growth govern the particle size distribution of the precipitate. As a rule, the precipitate is separated from the mother liquor in a centrifuge or on a filter. This separation and the subsequent drying process of very fine particles can be very expensive with respect to investment and energy costs. Therefore, it is desirable to produce as coarse a precipitate as possible with a narrow crystal size distribution.

THE NARROWING OF CRYSTAL SIZE DISTRIBUTIONS IN A SONICATOR-MSMPR CRYSTALLIZER SYSTEM

Obtaining small crystals of a narrow crystal size distribution is an important problem in many pharmaceutical, biotechnological, and ceramic processing applications. This paper describes the use of a two-stage system to reproducibly produce pilot-scale quantities of a ceramic precursor powder. The population balance is used to determine the crystal size distributions obtained from a staged crystallization system. The staged crystallization system consists of two units, a mininucleator (sonicator) and an MSMPR crystallizer. Experimental and theoretical studies of this system showed that the use of the mininucleator leads to narrowing of the crystal size distribution obtained from the MSMPR crystallizer. The resulting expression for the population density distribution allows direct fitting of experimental log population density data from the second stage to determine the growth and nucleation kinetics for both the mininucleator and the MSMPR crystallizer

Preparation of Silver Doped High Temperature Superconductors by the Controlled Double-Jet Precipitation

Chemical precursors of high temperature superconductors were prepared by the controlled double-jet precipitation of their oxalates to obtain suspensions with submicron average crystal size, narrow crystal size distribution, desired chemical composition and internal structure. To improve mechanical and electrical properties of HTSC layers the chemical precursors of HTSC were doped by silver oxalates using the same method. This technique enables to achieve better mixing of reagents in compare with mixed oxide processing. The suspensions were coagulated, washed and coated on various substrates. During carefully controlled heat treatment the grains readily sintered and formed HTSC layers. The layers prepared by this technique showed some improvements in mechanical and electrical properties. When coated on silver substrate, high density layer and a strong bond between the HTSC layer and substrate were achieved

ChemInform Abstract: The Molten Salt Synthesis of Large Crystal‐Sized SrTiO3.

AbstractA process is developed for the preparation of SrTiO3 crystallites with a relatively narrow crystal size distribution and an average crystal size ranging from 0.3 to 13.0 μm using molten alkali metal carbonates as reaction medium and SrTiO3 "seed" crystals having the desired size characteristics.