Antonija Kaćunić, Marija Ćosić, Davor Rušić, Nenad Kuzmanić

The aim of this work was to experimentally investigate the influence of impeller off-bottom clearance on crystal growth kinetics of disodium tetraborate decahydrate (borax) in a dual-impeller batch cooling crystallizer. Examinations were carried out in a crystallizer of 15 dm3 with liquid height to crystallizer diameter ratio (H/dT) of 1.3. In this research, two pitched blade turbines (PBT) with diameter D/dT=0.33, were mounted on the same shaft with a constant spacing between impellers (s/D=1.0). Mixing was performed at the impeller speed which ensured the state of complete suspension in the system (N=NJS). The impeller off-bottom clearance (C/D) was varied from 0.2 to 1.3. Crystallization was carried out by a controlled cooling of the solution saturated at 30°C. Concentration of the solution was measured in line by potentiometric method, while the changes of crystal size over time were determined by optical microscope and image analysis software. 

Experimental results indicate that impeller off-bottom clearance influences the change of crystal size over process time. But, regardless of the impeller off-bottom clearance, crystal growth is integration limited. The value of an order of crystal growth rate changes insignificantly with C/D ratio, while the crystal growth rate constant is affected by the change in hydrodynamics. Obtained results are a consequence of the hydrodynamic conditions in the dual-impeller crystallizer. In order to analyse the overall fluid flow pattern, in which an interaction of the flows generated by each of the impellers occurred, photographs of the flows were taken and simulations by VisiMix 2000 Turbulent were made. Taking into account the values of kinetic parameters of crystal growth, characteristic of obtained crystals, power consumption and the overall fluid flow pattern, it is possible to define the impact of impeller off-bottom clearance on borax crystal growth kinetics.

Zvonimir Jurun, Antonija Kaćunić, Nenad Kuzmanić

Utjecaj operacije miješanja na proces šaržne kristalizacije često se zanemaruje, premda ona utječe gotovo na svaki segment tog procesa. Pri odabiru optimalne konfiguracije kristalizatora, veličine koje mogu pri tome mogu pružiti veoma korisne informacije su vrijeme homogenizacije, brzina nukleacije kao i utrošena snaga miješanja. 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 jednog i dvaju turbinskih miješala mogu utjecati na gore navedene parametre. 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.

Ključne riječi: Šaržna kristalizacija, vrijeme homogenizacije, brzina nukleacije, SBT miješalo, PBT miješalo.

Branka Gotovac, Antonija Kaćunić, Davor Rušić

Ovaj rad prilog je sustavnom inženjerskom istraživanju u marikulturi s teorijskog motrišta. Tematikom pripada području matematičkog modeliranja biološkog procesa uzgoja morskih mikroalgi. Za biološko-inženjersku znanost od značaja je poznavanje kinetičkih modela. U radu je prezentirana metodologija njihovog razvoja utemeljena na podacima koji su rezultat simulacije.

Polazište su biološke faze: faza indukcije, eksponencijalna faza, faza usporenja rasta, stacionarna faza i faza odumiranja stanica. Procijenjeni su prijelazi jedne faze u drugu (koncentracija i vrijeme) i korelacijsko-regresijskom analizom definirane su analitičke funkcije za svaku od faza. Kinetički modeli faza razvijeni su deriviranjem funkcija. Navedeni slijed je preduvjet iznalaženja integracijskih konstanti. Naime, koncentracija i vrijeme prijelaza postaju početni uvjeti uz diferencijalne jednadžbe dobivene deriviranjem analitičkih funkcija. Njihova rješenja utemeljena na početnim uvjetima u potpunosti matematički definiraju kinetiku faza. S obzirom da je uzgoj algi vremenski kontinuirani biološki proces i da predstavlja “cjelinu”, primijenjena je matrična algebra kojom su objedinjene sve matematičke formulacije, a predstavljaju svojevrsni “matematički preslik” fenomena biološkog procesa.

Ključne riječi: uzgoj morskih algi, matematičko modeliranje, kinetički modeli bioloških faza, matrični model.

Antonija Kaćunić, Marija Ćosić, Nenad Kuzmanić

Utrošak snage pri provedbi operacije miješanja može značajno ovisiti o geometrijskim karakteristikama sustava, posebno miješala. U ovom radu sagledan je utjecaj veličine i pozicije miješala na utrošak snage miješanja pri provedbi šaržne prekristalizacije boraksa. Ispitivanja su provedena u kristalizatoru volumena 15 dm3 u kojem je omjer visine stupca kapljevine i promjera kristalizatora iznosio 1,3. Zbog navedenog omjera miješanje se provodilo dvama aksijalnim turbinskim miješalima s ravnim lopaticama nagnutim pod kutom od 45° (PBT tip miješala). Utjecaj veličine miješala na utrošak snage ispitivan je primjenom miješala različitih promjera, dok je pozicija miješala sagledavana kroz udaljenost donjeg miješala od dna te kroz razmak između miješala. Iz rezultata je uočeno da utrošak snage značajno ovisi o primijenjenim parametrima miješanja. S obzirom da se utrošak sagledavao pri postizanju stanja potpune suspenzije kristala, dobiveni rezultati mogli su se objasniti tek uvidom u strukturu toka kapljevine. U tu svrhu izvršeno je fotografiranje tokova te je analizom slike određen tok kapljevine kojeg razvija pojedino miješalo. Ovisno o veličini i poziciji miješala na slikama je uočeno preklapanje nastalih tokova. Na osnovu provedenih ispitivanja utvrđeno da je utrošak snage pri suspendiranju kristala boraksa ovisi o strukturi sveukupnog toka kapljevine formiranom uslijed djelovanja tokova dvaju miješala. Za suspendiranje kristala boraksa potom je predložena optimalna konfiguracija dvaju aksijalnih miješala.

Ključne riječi: utrošak snage, konfiguracija dvaju miješala, šaržna kristalizacija, boraks dekahidrat.

Sandra Svilović, Ivan Smajo, Renato Stipišić, Davor Rušić, Nenad Kuzmanić

U  ovom  radu  ispitivan  je  utjecaj  brzine  turbinskog  SBT  miješala  na  kinetiku  ionske  izmjene
bakra  iz  vodene  otopine  bakrova  (II)  nitrata  na  sintetskom  zeolitu  NaX  u  neprotočnom
kotlastom reaktoru. Različite brzine vrtnje miješala uzrokuju i različita stanja suspenzije zeolita
u reaktorskom sustavu što se reflektira na kinetiku izmjene.
Kinetička ispitivanja su provedena praćenjem količine izmijenjenih iona bakra s vremenom pri
različitim  brzinama  vrtnje  SBT  miješala  uz  konstantnu  početnu  koncentraciju,  temperaturu  i
veličinu  čestica.  Analizom  eksperimentalnih  rezultata  utvrđeno  je  da  je  proces  izmjene  iona
najbrži u početnom periodu nakon čega usporava do uspostave ravnoteže te da je brzina izmjene
veća  u  stanju  potpune  i  intermedijalne  suspenzije  zeolita  u  odnosu  na  stanje  nepotpune
suspenzije te stanja pri kojem dolazi do pojave površinske aeracije.
Lagergrenova  jednadžba  pseudo-prvog  reda  i  Blanchardov  model  korišteni  su  za  obradu
eksperimentalnih  kinetičkih  podataka.  Kinetičkom  analizom  je  utvrđeno  najbolje  slaganje  sa
Blanchardovim modelom.
 
Ključne riječi: bakar, zeolit NaX, SBT miješalo 

Antonija Kaćunić, Marija Ćosić, Renato Stipišić, Davor Rušić

The influence of mixing on crystallisation is often ignored despite the fact that it affects almost every aspect of the process what is especially pronounced in multiple impeller systems where flow complexity is increased. The predominant motivation for this work was to investigate the effect of mixing on nucleation kinetics of borax in the process of batch cooling crystallisation. Crystallisation was conducted in 15 dm3 crystalliser with an aspect ratio (H/dT) equal to 1.3. Such aspect ratio required an installation of a second impeller on the same shaft. Mixing was performed by using two down pumping pitched blade turbines (PBT impellers). Mother liquor was saturated at 30 °C and it was cooled down at 6°C/h. The impact of several mixing parameters, such as impeller speed, size and position, was examined. A key parameter for evaluating the blending effectiveness of stirred tanks – mixing time, was determined by potentiometric method whiles the nucleation mechanism and rate was determined using the Mersmann's nucleation criterion. Obtained results indicate that investigated mixing parameters affect the values of mixing time and consequently the nucleation kinetics. Considering the stated, by choosing proper mixing parameters, it is possible to influence the characteristic of the final product of crystallisation.

Keywords: Mixing time, nucleation rate, PBT impellers, borax, crystallisation.

Antonija Kaćunić, Marija Ćosić, Nenad Kuzmanić

Interaction between mixing and crystallization is often ignored despite the fact that it affects almost every aspect of the operation including nucleation, growth and maintenance of the crystal slurry. This is especially pronounced in multiple impeller systems where flow complexity is increased. By choosing proper mixing parameters, what closely depends on the knowledge of the hydrodynamics in a mixing vessel, the process of batch cooling crystallization may considerably be improved. The values that render useful information when making this choice are mixing time and power consumption. The predominant motivation for this work was to investigate the extent to which radial dual impeller configuration influences mixing time, power consumption and consequently the values of metastable zone width and nucleation rate. In this research, crystallization of borax was conducted in a 15 dm3 baffled batch cooling crystallizer with an aspect ratio (H/T) of 1.3. Mixing was performed using two straight blade turbines (4-SBT) mounted on the same shaft that generated radial fluid flow. Experiments were conducted at different values of N/NJS ratio (impeller speed/minimum impeller speed for complete suspension), D/T ratio (impeller diameter/crystallizer diameter), c/D ratio (lower impeller off-bottom clearance/impeller diameter) and s/D ratio (spacing between impellers/impeller diameter). Mother liquor was saturated at 30°C and was cooled at the rate of 6°C/h. Its concentration was monitored in line by Na-ion selective electrode. From the values of supersaturation that was monitored continuously over process time, it was possible to determine the metastable zone width and subsequently the nucleation rate using the Mersmann’s nucleation criterion. For all applied dual impeller configurations, the mixing time was determined by potentiometric method using a pulse technique, while the power consumption was determined using a torque meter produced by Himmelstein & Co. Results obtained in this investigation show that dual impeller configuration significantly influences the values of mixing time, power consumption as well as the metastable zone width and nucleation rate. A special attention should be addressed to the impeller spacing considering the flow interaction that could be more or less pronounced depending on the spacing value.

Keywords: Dual impeller crystallizer, mixing time, power consumption, metastable zone width, nucleation rate.

Antonija Kaćunić, Lea Lokas, Marija Ćosić, Nenad Kuzmanić

The aim of this work was to investigate the influence of fluid flow pattern in the crystallizer created by different impellers on the nucleation mechanism and nucleation rate of disodium tetraborate decahydrate (borax) in the process of batch cooling crystallization. Examinations were performed in the crystallizer of 15 dm3 with liquid level to tank diameter ratio (H/dT) of 1,3 and diameter ratio impeller to tank (D/dT) of 0,33. The fluid flow patterns in the crystallizer were generated by axial and radial turbine impellers and by their different dual combinations. Impeller speed ensured the state of complete suspension in the system for all configurations used (N = NJS).

Mirella Žanetić, Maja Jukić Špika, Renato Stipišić, Antonela Jukić, Sandra Svilović

HETMIX in cooperation with the Institute for Adriatic Crops and Karst Reclamation.

Effect of malaxation time and temperature on Levantinka virgin olive oil (VOO) properties during production process was studied. Mixing of olive pastes was performed at 26, 36, 46 ºC. Also, for every temperature effect, malaxation time was studied during 30, 45, 60 min. In order to estimate the  olive  oil  quality  the  investigation  of  free fatty acid, peroxide value, UV  spectrophotometric indices, total phenols content, sensory evaluation, oxidative stability and antioxidant activity was carried out.