Naslov
Influence of impeller diameter on crystal growth kinetics of borax decahydrate in PBT-SBT dual impeller batch cooling crystallizer

Autori
Čelan, Antonija ; Ćosić, Marija ; Pehnec, Igor ; Kuzmanić, Nenad

Vrsta, podvrsta i kategorija rada
Sažeci sa skupova, prošireni sažetak, znanstveni

Izvornik
23rd International Congress of Chemical and Process Engineering CHISA 2018 Prague / - , 2018, 1-2

Skup
23th International Congress of Chemical and Process Engineering (CHISA 2018)

Mjesto i datum
Prag, Češka Republika, 25.08.2018. - 29.08.2018

Vrsta sudjelovanja
Predavanje

Vrsta recenzije
Međunarodna recenzija

Ključne riječi
impeller diameter ; crystal growth kinetics ; PBT-SBT configuration

Sažetak
Crystallization as a solid-liquid unit operation is usually carried out in an agitated vessel. During this operation, mixing needs not only to ensure the state of complete suspension of growing crystals but it also needs to establish appropriate hydrodynamic conditions for crystal growth. Hydrodynamics in such a vessel depends, in the first place, on the number as well as on the type of impellers used since both affect the characteristics of the generated fluid flow pattern. Therefore, besides the operating conditions in the crystallizer, its geometric configuration, i.e. impeller geometry needs to be chosen in such a way that it ensures the production of crystals of desirable properties, such as crystal size distribution, crystal shape and yield, at minimum operating costs. The aim of this work was to analyse the influence of impeller diameter on crystal growth kinetics of borax dehydrate in dual-impeller batch cooling crystallizer. Crystallizer was a flat bottom vessel equipped with four baffles. Vessel diameter, T, equalled 0.24 m while liquid height to vessel diameter ratio, H/T, equalled 1.3. Since the vessel H/T ratio exceeded 1.2, an installation of two impellers on the same shaft was required. In this work, two impeller types were used: four-blade straight blade turbine (SBT) was mounted above a four-blade pitched blade turbine (PBT) thus forming a PBT-SBT dual impeller configuration. In order to analyse the influence of impeller diameter, D, on the conducted process, experiments were carried out at different impeller D/T ratios in the range from 0.27 to 0.40. During the experiments, the lower impeller off-bottom clearance (C/D=1) as well as the spacing between impellers (S/D=1) were kept constant. At all examined conditions, mixing was performed at just suspended impeller speed (N=NJS). This speed was determined in preliminary examinations by a visual method 0.9H proposed by Mersmann and Einenkel (1977). Unseeded batch cooling crystallization was carried out from borax solution saturated at 30 °C and was cooled down at constant cooling rate of 6 °C/h. After nucleation commenced, samples of crystal suspension were withdrawn from the reactor in a defined process time interval. Samples were then analysed by means of microscopy and sized using the Motic Images Advanced 3.2 software. During the process, supersaturation change was determined from concentration which was measured in line by a sodium ion-selective electrode. In order to analyse energy requirements for mixing, torque was measured by Himmelstein MCRT-48000 torque transducer from which the power consumption per unit mass was calculated. Lastly, the crystal product was analysed by laser diffraction particle size distribution analyser Horiba LA-300 to gain an insight into the influence of impeller diameter on the crystal size distribution. At all examined conditions, obtained results indicated that during the process of crystal growth, consumption of supersaturation increased as the impeller diameter decreased. This decrement of supersaturation is attributed to the change of linear crystal size which was much more emphasized in the system with the smallest D/T ratio investigated. Obtained results reflected later on the values of crystal growth rates which were calculated based on the change of crystal size in process time. Experimental result indicated also that in examined supersaturation ranges, crystal growth rate was the highest at the ratio D/T=0.3. The crystal growth rates were than represented as a function of supersaturation according to the well-known relation: G=k_g Δc^g By applying this equation, the values of crystal growth kinetic parameters ; overall crystal growth rate constant, kg and order of crystal growth, g, at different impeller diameters were estimated. Data showed that the values of parameter g were very similar in all experiments indicating that it is unaffected by hydrodynamic conditions in the crystallizer. On the other hand, the values of kg were the highest at D/T=0.33 while the lowest value was present at D/T=0.4. In order to clarify the relation between kg and the hydrodynamic conditions in the crystallizer, dimensionless mixing time was experimentally determined and CFD simulations of fluid flow by ANSYS FLUENT 17.2 were made. Results showed that mixing time was significantly influenced by PBT-SBT configuration diameter ; it decreased as D/T increased. CFD simulation indicated that as PBT-SBT impeller configuration diameter increased, the overall fluid flow was more discharged in radial direction and the region of high fluid velocity, placed between two impellers, became more pronounced. Obviously, mixing intensity was higher in the system with larger D/T ratio but overall, the conditions for crystal growth were more suitable at lower D/T ratios investigated. To establish if the process of crystal growth was governed by diffusion or integration growth mechanism, for all examined mixing conditions the experimental values of crystal growth rate were presented in a dimensionless growth rate vs. dimensionless supersaturation diagram (Mersmann et al. 1992). At all examined PBT-SBT diameters, crystal growth was governed by integration mechanism. The process of crystal growth at different PBT-SBT diameters eventually resulted in different crystal size distributions. It was interesting to notice that crystals with smallest mean size were obtained at D/T=0.3. At this ratio, a higher value of product yield was also obtained. Even though the values of crystal mean size obtained at other examined PBT-SBT impeller D/T ratios had approximately the same value, crystals obtained in those cases were more agglomerated. Change in impeller diameter reflected eventually on power consumption. It was found that power consumption for the ratios D/T=0.27 and 0.4 had approximately the same value, while a slightly higher values of power were noticed at the ratio D/T=0.33.

Izvorni jezik
Engleski

Znanstvena područja
Kemijsko inženjerstvo

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