Sandra Svilović, Marija Ćosić, Renato Stipišić, Davor Rušić, Nenad Kuzmanić

Effect of hydrodynamics on ion exchange kinetics in a batch reactor is still not properly studied despite the fact that proper mixing parameters may considerably improve the process of solid suspension and influence the hydrodynamic layer thickness responsible for the mass rate transfer. For this reason the aim of this work was to analyse the influence of hydrodynamic conditions generated by different impeller diameters on the maximum amount of copper exchange onto zeolite NaX and exchange kinetics as well. Investigations were carried out in a 2.1 dm3 baffled batch reactor at the state of complete suspension. Mixing was conducted using straight blade turbine (4-SBT) which impeller to tank diameter ratio (D/T) were varied in the range from 0.46 to 0.68. Solution containing Cu2+ was prepared by dissolving the appropriate weight of Cu(NO3)2∙3H2O. The initial concentration (7 mmol/L) was checked by Perkin Elmer Lambda 25 UV/VIS spectrophotometer. Zeolite NaX was crushed and sieved to obtain particles in the range from 0.071 – 0.09 mm. In the first part of the examination just suspended impeller speed, NJS, was determine for different impeller diameters and mass of zeolite (10.5, 13.125, 15.75 and 18.345 g) according to the Zwietering criterion. For all examined conditions power consumption was determined as well. In the second part of examination the influence of hydrodynamic on the kinetics of copper exchange on zeolite were analysed. Kinetics experiments were carried out at just suspended impeller speed (NJS) for all impellers and solid/liquid ratios used, while solution temperature was kept constant. Obtained experimental kinetics data were fitted using Elovich and Blanchard model [1-2].

The obtained results showed that just suspended impeller speed decreases as impeller diameter increases. The increase in zeolite mass causing the slight increase of NJS which is more pronounced in the systems with smaller impeller. Power consumption, at the state at complete suspension, generally decreases as impeller diameter increases. Kinetics results indicated that amount of copper ion sorbed increases sharply with the time in the initial stage and then gradually until the equilibrium is reached for all hydrodynamics conditions examined. Considering the relationship between amounts of copper exchanged and power consumption at the state of complete suspension it is possible to suggest more viable (D/T) ratio for the examined process.

Antonija Čelan, Marija Ćosić, Davor Rušić, Nenad Kuzmanić

In the last few decades, an interest in batch crystallization had increased significantly due to the numerous advantages of this separation method. Even though the number of scientific papers dedicated to the subject is continuously increasing, only a few of them deal with the impact of mixing on the process of crystallization. Additionally, the number of those researches in which mixing is performed by dual or even a multi-impeller system is almost negligible. Since mixing ensures favorable conditions for mass and energy transfer, an impact of this operation on every aspect of a crystallization process should be investigated. For this reason, the aim of this work was to find how the off-bottom clearance influences mechanism and crystal growth kinetics. 

Batch crystallization of disodium tetraborate decahydrate (borax) by controlled cooling was conducted in a crystallizer of 15 dm3, which was equipped with four baffles. Saturated solution was prepared at 30°C and was cooled at the rate of 6 °C/h. Since liquid height to vessel diameter ratio (H/dT) equaled 1.3, mixing was performed by using two impellers of a standard diameter (D/dT=0.33). Radial SBT impeller was mounted above an axial PBT impeller on a single shaft (PBT-SBT dual impeller system). Impeller spacing (S/D) was kept constant and equal to S/D=1 while the off-bottom clearance was varied in the range of 0.2 to 1.3. Impeller speed, at which crystallization was conducted, ensured the state of a complete suspension (N/NJS=1). Solution concentration was monitored continuously by Na-ion selective electrode. Linear crystal size over process time was determined by microscopic analysis of the slurry samples with Motic Images Advanced 3.2 software. In order to explain obtained results, it was also necessary to analyze the fluid flow generated by impellers. For this purpose, the overall fluid flow pattern was photographed according to the method developed by S. Ibrahim and A. W. Nienow.

Results of this investigation indicate that the change of off-bottom clearance reflects on the values of linear crystal size over process time and consequently on the crystal growth kinetics. It was found that the overall crystal growth coefficient, kg, increases with an increase of impeller off-bottom clearance. A deviation from this trend is only present when the flow generated by the lower impeller is transformed due to the vicinity of the crystallizer bottom. An analysis of numerical values of the order of overall crystal growth process, g, did not imply a regular dependence of this parameter on the off-bottom clearance. To determine the crystal growth mechanism, a model proposed by Mersmann was used. By this model, in all tested conditions, crystal growth was integration limited.

Zvonimir Jurun, Antonija Kaćunić, Nenad Kuzmanić

Utjecaj operacije miješanja na proces šaržne kristalizacije često se zanemaruje, premda utječe gotovo na svaki segment tog procesa. S obzirom da se u industrijskoj praksi procesi kristalizacije provode u sustavima volumena i do nekoliko desetaka m3, pri čemu se značajno odstupa od standardnih geometrija reaktora s miješanjem, nameće se potreba uvođenja drugog ili više miješala na zajedničku osovinu. Pri odabiru optimalne konfiguracije miješala, nužno je voditi računa o energetskim utrošcima kao i o hidrodinamičkim zbivanjima u sustavu. Vrijeme homogenizacije, brzina nukleacije kao i utrošena snaga miješanja su veličine koje pri tome mogu pružiti veoma korisne informacije.

Svi eksperimenti u ovom radu  provođeni su u kristalizatoru s kontroliranim hlađenjem volumena 15 dm3 te omjera visine stupca matične otopine boraksa i promjera kristalizatora 1.3, što je omogućilo uvođenje drugog miješala na zajedničku osovinu. Cilj je bio sagledati kako različite konfiguracije dvaju turbinskih miješala mogu utjecati na gore navedene parametre. U ispitivanjima su korištene različite konfiguracije dvaju miješala: PBT-PBT, SBT-SBT te PBT-SBT pri čemu je brzina vrtnje miješala osiguravala stanje potpune suspenzije u sustavu.

Vrijeme homogenizacije je određivano potenciometrijskom metodom korištenjem Na-ion selekivne elektrode, dok je brzina nukleacije određivana iz podataka o prezasićenosti matične otopine primjenom Mersmannovog kriterija. Utrošak snage miješanja je kontinuirano praćen tijekom eksperimenta mjerenjem zakretnog momenta.

Dobiveni rezultati ukazuju na to da se geometrijske i hidrodinamičke karakteritike ovakvih sustava značajno odražavaju na vrijeme homogenizacije, širinu metastabilne zone te brzinu nukleacije. S obzirom na navedeno, odgovarajućim odabirom konfiguracije dvaju miješala moguće je direktno utjecati na karakteristike finalnog produkta procesa šaržne kristalizacije.

Antonija Kaćunić, Marija Ćosić, Nenad Kuzmanić

The production process of crystalline products is governed by two kinetic steps - nucleation and crystal growth. Nucleation kinetics is highly correlated to the width of the metastable zone and with regard to product characteristics it usually has a strong influence on crystal growth. Considering the fact that batch crystallization is usually carried out in suspension, mixing can affect the width of the metastability region, nucleation kinetics and, finally, the properties of the product which include crystal size distribution, purity, morphology, etc. Thus, for a complete understanding of the crystallization process, besides a rigorous description of the crystallization phenomena, the hydrodynamics of the system needs to be analyzed in detail [1,2]. The majority of the researches dealing with the influence of mixing on crystallization are conducted in a single impeller crystallizer, which is common in laboratory scale investigations, but on an industrial scale, multiple impeller crystallizers are usually employed. It is well known that hydrodynamic conditions in stirred reactor depend on the impeller and crystallizer geometry. The flow fields in these systems are very complex, with vortical structures that are not always well defined and may significantly change with impeller position. The flow complexity greatly increases if a configuration with two impellers is used, especially when applied impellers are of the different type, each having its own flow characteristics.

The aim of this work was to investigate the influence of impeller position on the nucleation mechanism and nucleation rate of disodium tetraborate decahydrate (borax) in a dual impeller batch cooling crystallizer.

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Antonija Kaćunić, Marija Ćosić, Nenad Kuzmanić

Mnoge operacije u kemijskoj industriji u velikoj mjeri ovise o uspješno provedenom miješanju. Koliko će ono biti uspješno moguće je procijeniti kroz nekoliko parametara, pa tako i kroz poznavanje vrijednosti snage miješanja. Snaga miješanja predstavlja energiju preko miješala dovedenu u sustav. Njezina vrijednost, između ostalog, utječe i na veličine važne pri provedbi procesa kristalizacije kao što su  stupanj izmješanosti te koeficijent prijenosa tvari.

U ovom istraživanju, šaržna kristalizacija boraksa je provedena u sustavu volumena  15 dm3 pri čemu je omjer visine stupca matične otopine i promjera kristalizatora (H/T) iznosio 1.3. Miješanje se provodilo uporabom dvaju radijalnih turbinskih miješala (tzv. 4-SBT tip miješala) pri onoj brzini vrtnje koja je osiguravala stanje potpune suspenzije (N = NJS). U sustavu su kontinuirano praćene koncentracija matične otopine (uz uporabu Na-ion selektivne elektrode) te zakretni moment (korištenjem mjerila zakretnog momenta Himmelstein and Co.). Ispitan je utjecaj veličine miješala – D/dT (promjer miješala/promjer miješalice), utjecaj udaljenosti donjeg miješala od dna – c/D te međusobni razmak dvaju miješala – s/D na vrijednost snage miješanja. Snaga miješanja je izračunata iz izmjerenih vrijdnosti zakretnog momenta te je izražena kao snaga utrošena po jedinici mase suspenzije – P/m.

Rezultati su pokazali kako utrošak snage izrazito ovisi kako o veličini tako i o položaju miješala unutar kristalizatora. Obzirom da je udio troškova vezanih uz utrošak snage miješanja u ukupnim troškovima proizvodnje znatan, nužno je razmotriti alternativne konfiguracije miješala kako bi se proizveo produkt zadovoljavajućih karakteristika uz minimalne troškove proizvodnje.

Antonija Kaćunić, Marija Ćosić, Nenad Kuzmanić

This work gives an insight into the impact of impeller speed in a dual-impeller crystallizer on the process of batch cooling crystallization of disodium tetraborate decahydrate (borax). Experiments were conducted in a 15 L baffled batch cooling reactor with specific geometrical characteristics at linear cooling rate of 6°C/h. Mother liquor mixing was performed using the straight blade turbine (4-SBT) that created a radial flow pattern in the system. Mentioned specific geometrical characteristics primarily relate to the liquid level to vessel diameter ratio (H/dT).

Nenad Kuzmanić

Ovim predavanjem želi se dati što potpuniji uvid utjecaja operacije miješanja na procese koji su okarakterizirani prisustvom čvrste i kapljevite faze, a koji se odvijaju u turbulentnom režimu strujanja. S obzirom da se i kristalizacija provodi u suspenziji cjelovitije će biti sagledan utjecaj geometrijskih i hidrodinamičkih karakteristika reaktorskog sustava na provedbu šaržne kristalizacije uz kontrolirano hlađenje.

Antonija Kaćunić, Marija Ćosić, Davor Rušić, Nenad Kuzmanić

U ovom radu je ispitivan  utjecaj parametara miješanja na širinu metastabilne zone u procesu šaržne kristalizacije dinatrijevog tetraborat dekahidrata (boraksa). Miješanje se provodilo upotrebom dvaju radijalnih turbinskih miješala (tzv. SBT miješala) pri brzini vrtnje koja je osiguravala stanje potpune suspenzije.

Sandra Svilović, Renato Stipišić, Davor Rušić, Nenad Kuzmanić

Katalizatori na osnovi zeolita i bakra kao aktivne komponente imaju izuzetno široku primjenu u procesnom inženjerstvu. Za njihovu uspješnu pripremu nužno je poznavanje utjecaja različitih parametara na brzinu ionske izmjene kao i kinetički model izmjene. Stoga je u ovom radu ispitan utjecaj koncentracije bakra na brzinu ionske izmjene u kinetičkom području te je izvršena usporedba različitih kinetičkih modela s dobivenim eksperimentalnim podacima.