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U ovom radu je sagledan utjecaj promjera aksijalnog PBT miješala (engl. Pitched blade turbine) te amplitude ultrazvuka, A, na kritiĉnu brzinu vrtnje miješala, N JS, pri suspendiranju kristala natrijevog tetraborat dekahidrata (boraksa) u šaržnom kristalizatoru standardnih dimenzija koji je sadržavao razbijala virova. Ova ispitivanja provedena su u svrhu utvrĊivanja brzine vrtnje korištenih miješala za naknadnu provedbu sonokristalizacije boraksa u miješajućem šaržnom kristalizatoru s hlaĊenjem. U prvom dijelu rada izvršena je šaržna kristalizacija boraksa iz otopine zasićene pri temperaturi od 30 °C, te su odreĊene granulometrijske karakteristike kristala. Potom je izvršeno suspendiranje kristala u zasićenoj matiĉnoj otopini s tri promjera PBT miješala (d m /T = 0,43, 0,53 i 0,63). Pri svakom promjeru je prvo odreĊena minimalna brzina vrtnje miješala potrebna za postizanje stanja potpune suspenzije kristala, N JS , primjenom Zwietringova kriterija 1s. Potom je pri djelovanju ĉetiriju razliĉitih amplituda ultrazvuka takoĊer za svaki promjer miješala odreĊena kritiĉna brzina vrtnje miješala spomenutim kriterijem. Kako bi se sagledali energetski zahtjevi sustava izmjeren je zakretni moment pri provedbi miješanja kao i koliĉina osjetne topline koja se tijekom sonofikacije u sustavu povećavala. Iz ovih veliĉina konaĉno je izraĉunat ukupan utrošak snage privedene sustavu. Rezultati su ukazali da se kritiĉna brzina vrtnje miješala za postizanje stanje potpune suspenzije kristala mijenja s promjenom promjera miješala. Ova pojava posljedica je promjene strukture toka kapljevine koja prati promjenu veliĉine miješala. UtvrĊeno je takoĊer da se pri ultrazvuĉnom djelovanju stanje potpune suspenzije kristala postiže pri manjoj vrijednosti N JS , meĊutim utjecaj ultrazvuka ovdje je znaĉajno manje izražen od utjecaja promjera miješala. U radu je takoĊer utvrĊeno koji od ispitivanih uvjeta istodobnog djelovanja miješanja i ultrazvuka zahtijevanju najniži utrošak snage pri suspendiranju korištenih kristala. In this work the influence of pitched blade turbine (abbrev. PBT) diameter and ultrasonic amplitude, A, on just suspended impeller speed, N JS , of sodium tetraborate decahydrate (borax) crystals in a batch baffled crystallizer was investigated. These tests were carried out in order to determine the PBT impeller speed for the subsequent sonocrystallization of borax in an agitated batch cooling crystallizer. In the first part of the work, crystallization of borax from a solution saturated at 30 ° C was performed, and the properties of the product obtained were determined. Then, the crystals were suspended in saturated mother liquor at three diameters of PBT impeller (d m /T = 0,43, 0,53 i 0,63). At each of the impeller diameter, the minimum impeller speed required to achieve the state of complete crystal suspension, N JS , was determined applying Zwietring's criterion 1s. Then at four different amplitudes of ultrasound, just suspended impeller speed at each of PBT impeller used were determined by the same criterion. In order to estimate the energy requirements of the systems, during the mixing the torque was measured, as well as the amount of sensitive heat that increased during sonication of the system. From these quantities, the power consumption brought to the system was finally calculated. The results indicated that the critical impeller speed to achieve the state of complete suspension of the crystal changes with the change of PBT impeller diameter. This phenomenon is a consequence of the changes in fluid flow pattern caused by the size of impeller. It has also been found that under ultrasonic action, the state of complete suspension of crystals is achieved at a lower N JS value, however, the influence of ultrasound here is significantly less pronounced than this of impeller diameter. In this work is also determined which of the tested conditions of simultaneous mixing and ultrasonic actions required the lowest power consumption when suspending the crystals used. |
