Freeze Crystallization Research Articles

Eutectic freeze crystallization in a 2nd generation cooled disk column crystallizer for MgSO 4·H 2O system

Eutectic freeze crystallization is a new separation process to produce pure ice and salt from concentrated salt solutions. For this new process, specialized crystallizers are being developed. A new, 2nd generation cooled disc column crystallizer (CDCC-2) with 150-l capacity and 5.6 m 2/m 3 cooling area was designed and tested for MgSO 4 crystallization. A heat flux of 1720–5750 W/m 2 was achieved at a temperature difference between coolant and crystallizer bulk solution of 3–6.5 K and a residence time of 1–3 h. Crystallization and gravitational separation of ice and salt are discussed.

Impurity gradients in solution-grown ice and [formula omitted] crystals measured by cryo-laser ablation and high-resolution-induced-coupled plasma mass spectrograph

During the eutectic freeze crystallization (EFC) of an industrial aqueous MgSO 4 solution, ice and magnesium sulfate duodecahydrate ( MgSO 4 · 12 H 2 O ( s ) ) were crystallized simultaneously near the eutectic point. It was found that the crystallization was highly selective: although the industrial feed solution contained appreciable levels of inorganic impurities (typically: 320 mg / L Cl - , 410 mg / L Ca 2 , 40 mg / L Mn 2 + , 70 mg / L Na + and 50 mg / L K + ), the formed ice and salt crystals contained lower levels of impurities (typically: 17 mg / L Cl - , 8 mg / L Ca 2 + , 17 mg / L Mn 2 + , and 5 mg / L Na ). Also the ice was pure: only traces (typically: 20 mg / L SO 4 2 - and 5 mg / L Mg 2 + ) of magnesium and sulfate were found in the ice crystals. In this work the spatial distribution of the impurities in the crystals is investigated. Gradients of composition in solids are measured by laser ablation high-resolution inductively coupled plasma mass spectrometry (LA-HR-ICP-MS). A special cryogenic sampling cell ( < - 80 ∘ C ) for laser ablation has been constructed. The focused (5– 10 μ m width) laser shoots at the frozen sample, scanning its surface. The vapor is fed to the mass spectrometer. In this way, the impurity concentration as a function of position in the crystal can be measured. The results of this method with MgSO 4 · 12 H 2 O and ice are presented.

Industrial Crystallization: Developments in Research and Technology

In this review paper developments in industrial crystallization are discussed with emphasis on recent highlights in the field. One of the most important past developments can be found in the introduction of the population balance concept. This will be discussed together with more recent developments and topics currently of great interest such as molecular modelling, polymorphism and polymorph prediction at the fundamental scientific level as well as melt crystallization equipment, eutectic freezing crystallization and crystallization control by means of on-line techniques at the applied, industrial level. Potential future directions for industrial crystallization are discussed with reference to past and present developments. The future of industrial crystallization is envisaged to lie in areas such as nano-particle production by means of crystallization and protein crystallization, in addition to some of those mentioned above.

Evaluation of the Performance of a Newly Developed Eutectic Freeze Crystallizer: Scraped Cooled Wall CrystalIizer

Eutectic freeze crystallization (EFC) separates aqueous inorganic solutions into pure water and pure salt. By operating at the eutectic point, ice and salt can be formed simultaneously as two separate phases. The scraped cooled wall crystallizer (SCWC), specially developed for eutectic operation, is introduced. It features cooling through an outer wall combined with an internal cooling cylinder. Experiments were executed in a 1151 model SCWC using a ternary aqueous system of KN0 3 –HN0 3 . The SCWC is equipped with temperature sensors measuring bulk temperature and the temperatures of the coolant entering and exiting the cooling walls. The total heat flux from coolant to bulk equals 490-585 W m −2 K −1 at a temperature difference between cooling wall and bulk of 5–6K. At this heat flux the production capacity equals 3.8 x 10 −4 kg m −2 K −1 s −1 for ice and 3.1 x 10 −5 kgm −2 K −1 s −1 for salt. The KN0 3 crystals produced are reasonably well faceted with average size of 80–230 μm. Both ice and salt crystals are easily filtered. Impurities in the ice crystals drop below 50ppm K+-ions after three washings. Ice solid content measurements indicated that 0.01 wt% of the ice slurry consisted of entrained salt crystals. Washed salt samples contained less than 5 ppm of ionic impurities while impurity levels in the feed ranged from 50 to 500 ppm.

Eutectic Freeze Crystallization with an Aqueous KNO3−HNO3 Solution in a 100-L Cooled-Disk Column Crystallizer

Eutectic freeze crystallization is a novel technique for processing waste and process streams of aqueous electrolyte solutions, separating them into pure water and pure solid salt. A dedicated apparatus, a 100-L cooled-disk column crystallizer, has been developed and tested, combining crystallization and solid/solid separation of ice and salt. Experiments have been performed with a ternary system of aqueous KNO3−HNO3. Heat-transfer rates were achieved in the range of 1300−4600 W·m-2·K-1, which are related to the square root of the scraping rate. The ice product slurry contained 0.15 wt % salt. The ice impurity content after three washing cycles was as low as 15 ppm K+. Well-faceted KNO3 crystals are produced with high purity.

Development of a Vacuum Crystallizer for the Freeze Concentration of Industrial Waste Water

Freeze concentration has proven to be a viable technology for the concentration of hazardous industrial waste waters before incineration. Owing to the relatively high investment cost of the technology, its applicability has been limited until now. This paper investigates the feasibility of a vacuum crystallizer as potential replacement of the scraped surface heat exchanger, which is one of the most cost- and maintenance-intensive items in conventional freeze concentration technology. Different crystallizer configurations and vapour treatment methods were evaluated for the system design. A thermodynamic analysis was performed to predict the phase transition behaviour in the condenser. A hydrodynamic analysis was performed to establish design rules and to determine the operating window. Finally, the principle of vacuum freeze crystallization was proven experimentally.

Eutectic freeze crystallization in a new apparatus: the cooled disk column crystallizer

Eutectic freeze crystallization (EFC) is a technique for simultaneous crystallization of ice and salt that is energetically more efficient than conventional evaporative crystallization. In this paper, a new type of crystallizer is introduced for simultaneously conducting EFC and separating the ice from the salt crystals, the cooled disk column crystallizer (CDCC). Crystallization is achieved by indirect cooling with wiped disks and separation by means of gravity. The liquid feed stream enters the column at the center, ice leaves the column at the top and salt is discharged at the bottom. The heat flux from bulk fluid to disk in a 14 l CDCC using CuSO 4–water as a model system (eutectic temperature −1.6 °C) was 1.9 kW m −2 for a temperature difference between disk and bulk of 4.5 K. The CuSO 4 · 5H 2O crystals formed were well faceted with an average size of 200 μm. Ice crystals were disk shaped with an average size 150 μm. Both ice and salt crystals were easily filtered. The impurity level in the ice crystals was below 100 ppm Cu.

Scale formation of ice from electrolyte solutions on a scraped surface heat exchanger plate

Freeze crystallization processes often suffer from an ice scale layer which is formed on cooling surfaces if no preventive action is taken. This causes a severe decrease in heat transfer rate, leading to stagnation of the crystallization process. Scraping of the heat transfer surface can prevent scaling of ice. This research investigates the phenomenon of ice scale formation on scraped cooled surfaces. Specific attention is paid to the influence of supersaturation, scraping rates and different electrolyte solutions of various concentrations. Ice formation experiments are performed with KNO 3, HNO 3 and CaCl 2 solutions. An induction time is measured, determined by the minimum scraping rate required to keep the heat transfer surface free of an ice layer. The required scraping rate increased with increasing supersaturation. Comparison of HNO 3 and CaCl 2 experiments shows that besides thermodynamics other parameters play an important role in ice scale formation. Experiments with silicon scraper blades show that ice scaling is rather prevented by mechanical removal of the initial crystals than by refreshment of the thermal boundary layer.

Dynamic modeling and simulation of eutectic freeze crystallization

A computer-based dynamic model of an eutectic freeze crystallizer comprised of coupled kinetics, population, mass, and energy balances has been developed to simulate its transient behaviour. The model assumed an MSMPR crystallizer and was used to simulate the transient responses of crystal size distribution of salt and ice during the start-up of the crystallizer. The temperature and the solute concentration inside the crystallizer achieved constant values after two residence times, whereas the steady state of the crystal size distributions were reached after ten residence times. A coupled effect of the simultaneous crystallization of salt and ice was clearly visible when the high mass production rate of ice induced the occurrence of a second peak of the supersaturation of salt.

Eutectic Freeze Crystallization Using CO2 Clathrates

Abstract: Highly soluble salts can be separated from aqueous solutions by eutectic freeze crystallization (EFC). This technique delivers significant energy savings in comparison with evaporative crystallization. The eutectic temperature can be raised by crystallizing CO2 clathrates instead of normal ice. Application of this process is especially efficient for salt solutions with eutectic points at low temperatures, such as CaCl2 (−55°C). In this work the shift of the eutectic point is predicted by use of a model for NaCl and CaCl2 solutions. Experiments are executed under eutectic conditions, with these substances proving the feasibility of the eutectic clathrate freezing process. Energy calculations show that by using eutectic clathrate crystallization instead of regular EFC the electric energy consumption can be reduced by 32%.

Eutectic freeze crystallization simultaneous formation and separation of two solid phases

Eutectic freeze crystallization (EFC) separates aqueous inorganic solutions into pure water and pure salt. By operating at the eutectic point, ice and salt can be formed simultaneously as two separate phases. Two aqueous systems were investigated in batch and continuous crystallization experiments: copper sulfate (eutectic temperature 271 K) and mono-ammonium phosphate (269 K). Below the eutectic temperature, ice can be formed without any salt formation by seeding with ice seeds and solely salt is formed by seeding with salt crystals. When however, the solution is seeded with both salt and ice crystals, two distinct solid phases are formed: when the stirrer is turned off, ice rises and salt settles. This shows that EFC separates aqueous solutions into ice and salt as two distinct phases. Based on these results, a 15 l cooled disk column crystallizer (CDCC) has been built, designed to crystallize and separate both the solids simultaneously in a single apparatus.

Eutectic freeze crystallization: Application to process streams and waste water purification

Two case studies are presented using eutectic freeze crystallization (EFC) as an alternative for evaporative crystallization: a 7.8 ton day −1 35 w% aqueous sodium nitrate and a 24 ton day −1 12 w% copper sulfate stream. The proposed crystallizer is a cooled disk column crystallizer (CDCC), using indirect cooling for heat transfer. In single stage operation, the formed ice crystals are used to pre-cool the feed stream. A two stage refrigeration unit utilizes the formed ice crystals in the condensation of refrigerant. Expressed as a thermal equivalent energy requirement, EFC requires 1282 kJ kg −1 NaNO 3 and 1037 kJ kg −1 CuSO 4·5H 2O. Compared to conventional multi-step evaporation, the energy reductions are 30% for sodium nitrate and 65% for copper sulfate.

Efficacité des techniques de régénération d'eau pour une usine de fabrication de papier intégrée sans effluent. Analyse exergétique

Water regeneration efficiency for pulp and paper plant. Exergy analysis. The water network of a pulp and paper mill is contaminated by a large number of components which are brought into the system as wood constituants or process additives. Keeping the contaminant concentrations below certain limits, to safeguard paper quality and process runnability, must be part of any systems closure strategy. This can be achieved by purging contaminants using well known separation techniques such as evaporation, freeze crystallization and membrane filtration. The purpose of this study was to compare the thermodynamic efficiency of these techniques by means of an advanced exergy analysis. The contaminated stream was modelled using key components for both organic and inorganic contaminants. The activity coefficients were calculated using the Chen algorithm. The approach developed by Brodyanski, Sorin and Le Goff was used to determine the exergy efficiency. This analysis shows that freeze crystallisation is the most efficient technique when the exergy of the output stream is considered as a useful effect. When only the useful work obtained by separation is considered, membrane separation becomes the most efficient process. The analysis of the process incorporating a separation operation produces the actual efficiency. This example illustrates the advantage of using exergy methods to determine the efficiency of processes based on different physico-chemical phenomena.

Freeze Crystallization of Imipenem

This work is a study of the freeze crystallization process developed for the production of a rapidly soluble and stable crystalline form of imipenem, an antibiotic. The objective is to understand the relationship between process conditions and the product crystallinity during the freeze crystallization. Solutions of imipenem and sodium bicarbonate, a drug stabilizer, were crystallized in acetone-water solvent systems at various ratios and temperatures. The degree of crystallinity of the resulting products was measured using an X-ray powder diffraction analysis technique. The acetone-water S-L phase diagram was used to correlate the resulting degrees of crystallinity of imipenem products and equilibrium properties of the system. It was discovered that the conversion of amorphous imipenem to crystalline products was directly related to the percentage of equilibrium liquid during the freeze crystallization. Solubility data indicated that imipenem is virtually insoluble in these equilibrium liquid phases. This suggests that the phase transition from the amorphous state to the crystalline state is mediated by the presence of a liquid phase.

Mixed Waste Separation Technologies

Abstract The United States Department of Energy policy for management of mixed (radioactive and hazardous) waste is in the development stage. Regardless of the approach to treatment of mixed waste, significant preprocessing will be necessary. The separation requirements for preprocessing of mixed waste will vary greatly depending on the downstream treatment requirements, the contents of the mixed waste stream, and the regulatory requirements at the waste treatment site. During the past year, the Department of Energy's Mixed Waste Integrated Program (MWIP) has begun to identify separation requirements, applicable commercial technologies, and emerging technologies that may meet specific requirements. The current emphasis on developing emerging separation technologies for the MWIP include: freeze crystallization for gross separation of dissolved solids and organics from an aqueous waste stream; a bio-catalytic process for nitrate destruction; the General Electric KI/I2 process for separating mercury from non...

Melting, freezing, and channeling phenomena in ice counterwashers

AbstractThermal phenomena are associated with the operation of ice counter‐washers, a major component for the separation of ice and brine in freeze crystallization processes. Melting, freezing, and channeling in the upper leg of the counterwasher occur because of the temperature difference between the wash water and the ice and have important effects on the operation of the device. In the present study, these phenomena are modeled and analyzed and their scaling laws derived. Ice and wash water temperature distributions as well as variations in porosity and wash water flow rate due to melting and freezing in the ice plug are computed and plotted. The effect of channeling on counterwasher operation is studied and formulated in terms of the channel dimensions and ice plug properties. Growth rates of channels due to melting by the wash water are calculated. Results indicate that warm wash water and slow motion of the ice plug cause rapid channel growth and may be deleterious to counterwasher operation. Best conditions for recovery from channeling are obtained with wash water temperature close to freezing and high plug velocity.

Photoconductivity of Gallium Selenide Crystals

Single crystals of GaSe have been prepared by reaction of the elements, followed by gradient freeze crystallization. Crystals as grown were $p$-type, probably because of Cu acceptors, with hole mobility of 15 ${\mathrm{cm}}^{2}$/volt sec, hole concentration of ${10}^{16}$ ${\mathrm{cm}}^{\ensuremath{-}3}$, and acceptor ionization energy of 0.12 ev. In many ways the properties of GaSe crystals are very similar to those of ZnTe crystals. The absorption edge of GaSe is at 6310 A, corresponding to a band gap of 1.97 ev; the temperature coefficient of band gap is about -4\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$ ev/deg. Insulating and photosensitive GaSe crystals can be prepared by compensating the acceptor impurities by incorporated donors from Groups VII or IV. Rectification tests indicate $p$-type photoconductivity. Thermal quenching of photoconductivity corresponds to an ionization energy of about 0.5 ev, whereas optical quenching corresponds to an ionization energy of about 1.0 ev; thus a large Franck-Condon shift is indicated. Below the temperature at which thermal quenching of photoconductivity occurs, the sensitivity is within the range of sensitive CdS or CdSe crystals.