Comparison of the effect of mechanical stirring and ultrasound irradiaton on crystal growth kinetics of borax in a batch cooling crystallizer : diploma thesis

Cilj ovoga rada je bio ispitati utjecaja miješanja i ultrazvuka u šaržnom kristalizatoru s hlađenjem na kinetiku rasta kristala dinatrijevog tetraborat dekahidrata (boraks). Proces kristalizacije izvodio se u kristalizatoru laboratorijskog mijerila u kojem je volumen matične otopine boraksa iznosio 2,14 dm 3 . Detaljno je analiziran utjecaj brzine vrtnje miješala (100, 200, 300 i 350 o. min -1 ) te amplitude ultrazvuka (20, 30, 40 i 50 %), na promjenu koncentracije odnosno prezasićenosti matične otopine, promjenu linearne dužine kristala tijekom procesa, raspodjelu veličina kristala te privedenu snagu po jediničnoj masi otopine. Pri svim ispitivanjima korištena je otopina zasićena pri temperaturi od 30 °C hlađena je linearnom brzinom od 6°C h -1 . Eksperimentalni rezultati ukazali su da brzina vrtnje miješala kao i amplituda ultrazvuka utječu na kinetiku rasta kristala boraksa. U sustavima s miješanjem red rasta kristala iznosi oko 1,3 dok je u sustavima s ultrazvukom njegova vrijednost oko 2,0. U sustavu s miješanjem konstanta brzine rasta kristala raste s porastom brzine vrtnje miješala dok se u sustavu s amplitudom ultrazvuka nije uočen pravilan trend promjene ove veličine s amplitudom. Djelovanje ultrazvuka uzrokuje dobivanje produkta malog masenog promjera čija se veličina značajnije ne mijenja s promjenom amplitude, dok je u sustavu s miješanjem srednji maseni promjer kristalnog produkta raste s porastom brzine vrtnje. Privedena snaga po masi otpine je značajno veća u sustavu s miješanjem, ali u ovom slučaju je postignuto i stanje potpune suspenzije pri brzinama vrtnje većim od 100 o. min -1 , dok je u sustavu s ultrazvučnim djelovanjem ovo stanje nije postignuto ni pri najvećoj amplitudi. Zbog toga primjena ultrazvučnog djelovanja u sustavu bez miješanja nije preporučljiva, unatoč velikoj brzini rasta kristala.The aim of this research was to investigate the effects of mechanical stirring and ultrasound irradiation on crystal growth kinetics of disodium tetraborate decahydrate (borax) during the batch crystallization by controlled cooling. The crystallization was carried out in a laboratory scale crystallizer in which volume of the mother liquor was 2,14 dm 3 . The influence of the impeller speed (100, 200, 300 and 350 rpm) and the ultrasound amplitude (20, 30, 40 and 50%), on the concentration changes of the mother liquor, the changes of linear crystal size over process time, the crystal size distribution of the product and the power consumed per unit mass of the solution was analyzed in details. Experimental results indicated that the impeller speed as well as ultrasound amplitude influence the crystal growth kinetics of borax. In the system with mechanical stirring the value of the crystal growth order is 1.3, while in the systems with ultrasound it is about 2.0. In the agitated systems, the crystal growth rate constant increases with an increase of the impeller speed while in the system with the ultrasound it is not found a regular trend of this parameter with the amplitude. The ultrasound irradiation in the crystallizer causes the production of fines which size does not change significantly with amplitude, while in the agitated crystallizer crystal size increases with increasing impeller speed. The power consumed per unit mass of solution is significantly higher in the stirred crystallizers, but in those systems the state of complete suspension was achieved at the rotation speeds higher than 100 rpm, which in the systems with ultrasound this state was not attained even at maximum amplitude. Therefore, the sonocrystalization alone, without mixing is not recommended, despite of high crystal growth rates