Title Utjecaj miješanja na kinetiku nukleacije i rasta kristala boraksa u šaržnom kristalizatoru s dvama miješalima Title (english) Influence of mixing on nucleation and crystal growth kinetics of borax in dual impeller batch crystallizer Author Antonija Kaćunić Mentor Marija Ćosić (mentor)Committee member Nenad Kuzmanić (predsjednik povjerenstva)Committee member Jasna Prlić Kardum (član povjerenstva)Committee member Marija Ćosić (član povjerenstva)Granter University of Split Defense date and country 2016-11-10, Croatia Scientific / art field, TECHNICAL SCIENCES Universal decimal classificationUDC ) 66 - Chemical technology. Chemical and related industries Abstract Cilj rada je bio ispitati utjecaj hidrodinamičkih uvjeta u kristalizatoru s dvama turbinskim miješalima na kinetiku kristalizacije dinatrijevog tetraborat dekahidrata (boraksa) tijekom njegove šaržne kristalizacije kontroliranim hlađenjem. Proces je provođen u kristalizatoru volumena 15 dm3, u kojem je omjer visine stupca matične otopine i promjera kristalizatora iznosio 1.3 što je zahtijevalo uvođenje drugog miješala u sustav. Miješanje se provodilo korištenjem triju različitih konfiguracija dvaju miješala: PBT-PBT, SBT-SBT te PBT-SBT. Za svaku primjenjenu konfiguraciju miješala detaljno je ispitan utjecaj brzine vrtnje, promjera miješala, udaljenosti donjeg miješala od dna te međusobne udaljenosti dvaju miješala na mehanizam i brzinu nukleacije, kinetiku rasta kristala boraksa te granulometrijske karakteristike finalnog produkta. U radu je također analiziran i utjecaj brzine hlađenja matične otopine na kinetiku ispitivanog procesa. Da bi se dobio potpuniji uvid u hidrodinamička zbivanja u ispitivanom sustavu, pri svim uvjetima miješanja je određeno vrijeme homogenizacije te je vizualizirana struktura toka kapljevine fotografiranjem tokova kao i simulacijom istih uporabom programskog paketa Visimix 2000 Turbulent. Primjenom Mersmannovog nukleacijskog kriterija je utvrđeno da je pri svim ispitivanim parametrima miješanja nukleacija započela primarnim heterogenim mehanizmom. Za definirani mehanizam su određene brzine nukleacije čime je sagledan utjecaj parametara miješanja i brzine hlađenja na ovu veličinu. Priroda sveukupnog toka suspenzije, definirana strukturom toka kapljevine, intenzitetom konvekcijskog toka te smičnim naprezanjima u zoni oko miješala, također se odražava i na promjenu linearne veličine kristala. Na temelju tih podataka su izračunate brzine te određeni parametri rasta kristala. Korištenjem Mersmannovog modela je pokazano da se pri svim ispitivanim uvjetima provedbe procesa rast odvija integracijskim mehanizmom. Hidrodinamički uvjeti se u konačnici odražavaju i na svojstva finalnog produkta procesa kristalizacije. Granulometrijska analiza prosijavanjem ukazuje na pojavu sekundarne nukleacije i aglomeracije pri određenim uvjetima provedbe procesa. Ukoliko je pak u bilo kojem od ispitivanih sustava bilo osigurano stanje intermedijalne suspenzije, aglomeracija je bila manje izražena. Najpravilniji kristali uz najužu raspodjelu veličina sepak dobivaju primjenom SBT-SBT konfiguracije miješala. Primjena različitih parametara miješanja se odrazila i na utrošak snage miješanja. Najveći utjecaj na ovu veličinu, pri svim ispitivanim konfiguracijama, ima brzina vrtnje miješala, a potom promjer korištenih miješala. S obzirom na navedeno, zaključuje se kako je pri odabiru uvjeta provedbe kristalizacije boraksa kontroliranim hlađenjem u šaržnim kristalizatorima s dvama miješalima neophodno detaljno ispitati utjecaj operacije miješanja na sve stupnjeve kristalizacijskog procesa. Kako parametri miješanja bitno utječu na kinetiku nukleacije i rasta kristala sasvim jasno da oni određuju i karakteristike finalnog produkta kristalizacije. Pravilnim odabirom konfiguracije dvaju miješala i podešavanjem uvjeta miješanja moguće je dobiti produkt željenih karakteristika uz minimalne energetske troškove
Abstract (english) The aim of this research was to investigate the influence of mixing in a dual impeller system on crystallization kinetics of borax in a batch cooling crystallizer. Crystallization was conducted in 15 dm3 crystallizer of tank diameter to liquid height ratio equal to 1.3. This aspect ratio required the use of a second impeller which was mounted on the same shaft. Stirring was conducted by using three different impeller configurations: PBTPBT, SBT-SBT and PBT-SBT. For each of the configurations listed before, an influence of impeller speed, size and position in the vessel on nucleation and crystal growth kinetics, as well as on the granulometric properties of the final product was analyzed in detail. In this work an influence of cooling rate was investigated as well. In order to gain a complete insight into the hydrodynamics of the crystallizer, mixing time was measured and the fluid flow was visualized, using the streak photograpy method and by simulation in Visimix 2000 Turbulent. Using the Mersmann's nucleation criterion, it was determined that the primary heterogeneous nucleation was a dominating mechanism in all experiments conducted in this research. Based on the nucleation mechanism, the rate of nucleation was calculated in order to analyze the influence of mixing parameters and cooling rate on this value. The nature of the overall fluid flow, defined by the flow pattern, the intensity of the overall convection fluid flow and shear stress in the zone around the impeller/s, is reflected on the size of the growing crystal as well. From the change of values of linear crystal size in process time, growth rates and growth parameters were determined. By using the Mersmann’s model, it was determined that the growth rate was governed by the integration mechanism at all process conditions examined. Applied mixing parameters ultimately determined on the properties of the final product of crystallization. Granulometric analysis by sieving showed that both secondary nucleation and agglomeration were present at certain process conditions. But, ifintermedial suspension state was achieved, agglomeration was less pronounces regardless of the impeller configuration. By far, the most regularly shaped crystals were obtained by using the SBT-SBT impeller configuration. Hydrodynamic conditions in the vessel, generated by different mixing parameters reflected the power consumption as well. Impeller speed and size have the biggest influence on this value. Considering the stated, it can be concluded that it is of the essence to investigate the influence of dual impeller mixing on the process of batch cooling crystallization in detail. Since mixing parameters determine the nucleation and crystal growth kinetics it is clear that, at the same time, they determine the properties of the final product of crystallization. Therefore, by choosing the suitable dual impeller configuration and by adjusting the mixing conditions, it is possible to obtain a product of wanted characteristics with minimal energy cost
Keywords
Miješanje
sustav s dvama miješalima
šaržna kristalizacija
boraks
tokovi fluida
nukleacija
rast kristala.
Keywords (english)
Mixing
dual impeller system
batch crystallization
borax
fluid flow pattern
nucleation
crystal growth
Language croatian URN:NBN urn:nbn:hr:167:437428 Study programme Title: Chemical engineering Type of resource Text Extent str. 269 File origin Born digital Access conditions Open access Terms of use Created on 2018-01-09 12:09:36
