www.cet-journal.com Page 1 Chemical Engineering & Technology Received: July 28, 2017; revised: January 22, 2018; accepted: April 11, 2018 This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the final Version of Record (VOR). This work is currently citable by using the Digital Object Identifier (DOI) given below. The final VoR will be published online in Early View as soon as possible and may be different to this Accepted Article as a result of editing. Readers should obtain the final VoR from the journal website shown below when it is published to ensure accuracy of information. The authors are responsible for the content of this Accepted Article. To be cited as: Chem. Eng. Technol. 10.1002/ceat.201700443 Link to final VoR: https://doi.org/10.1002/ceat.201700443 This article is protected by copyright. All rights reserved. Borax crystallization kinetics in PBT-SBT dual impeller crystallizer at different impeller positions Antonija Čelan*, Marija Ćosić, Nenad Kuzmanić 1 University of Split, Faculty of Chemistry and Technology, Department of Chemical Engineering, R. Boškovića 35, 21000 Split, Croatia *Correspondence: Antonija Čelan (E-mail: akacunic@ktf-split.hr), University of Split, Faculty of Chemistry and Technology, Department of Chemical Engineering, R. Boškovića 35, 21000 Split, Croatia Abstract Main focus of this work was to investigate the influence of hydrodynamic conditions on crystallization kinetics and properties of borax crystals obtained in a 15 dm 3 dual impeller batch cooling crystallizer. The two impellers used, pitched and straight blade turbine, were mounted on the same shaft. Hydrodynamics was analyzed through the values of mixing time and specific fluid flow patterns generated. Results indicate that wider metastable zones were generally observed at impeller positions characterized by higher mixing times. In those cases, growth rate constants were lower what resulted in a formation of smaller but more regularly shaped crystals. These findings imply that dual impeller position should be taken into account in order to produce crystals of desired characteristics. Keywords: Crystallization, Impeller off-bottom clearance, Impeller spacing, Mixing, Dual impeller system 1. Introduction Numerous advantages of crystallization have made this separation method popular in many industries in the last few decades. In a general sense, crystallization is defined as a phase change in which a crystalline product is obtained from a solution [1]. The process starts with nucleation, continues with the growth of created nuclei and ultimately results in a formation of the final crystal product [2]. When performed batch-wise in a stirred vessel, crystallization kinetics and, ultimately, the characteristics of final product are bound to be affected by mixing. Mixing ensures beneficial conditions in a surrounding in which crystals form in terms of providing homogeneity in the vessel. At first, before nuclei are formed, mixing is in charge of keeping the supersaturation level uniform. After crystals are formed, its role gets another dimension in ensuring that particles are evenly suspended in the vessel. The degree of solids suspension that is most often employed is the state of complete suspension (N=N JS ), at which the maximum surface area of particles is exposed to the fluid [3]. It has already been proven that mixing influences crystallization kinetics in single impeller systems [4- 6]. But researches concerning dual impeller systems are still quite rare [7-8] even though the use of multiple impeller systems is very common in chemical and other related industries. Compared to