Mehdi Hasan PURIFICATION OF AQUEOUS ELECTROLYTE SOLUTIONS BY AIR-COOLED NATURAL FREEZING Thesis for the degree of Doctor of Science (Technology) to be presented with due permission for public examination and criticism in the Suvorov auditorium of Technopolis at Lappeenranta University of Technology, Lappeenranta, Finland on the 19th of October, 2016, at noon. Acta Universitatis Lappeenrantaensis 717 Supervisors Professor Marjatta Louhi-Kultanen LUT School of Engineering Science Lappeenranta University of Technology Finland Associate Professor Jaakko Partanen LUT School of Engineering Science Lappeenranta University of Technology Finland Reviewers Professor Joachim Ulrich Department of Thermal Process Technology Martin Luther University Halle-Wittenberg Germany Associate Professor, Dr. Elif Genceli Güner Department of Chemical Engineering Istanbul Technical University Turkey Opponents Professor Joachim Ulrich Department of Thermal Process Technology Martin Luther University Halle-Wittenberg Germany Professor Henrik Saxén Department of Chemical Engineering Åbo Akademi University Finland ISBN 978-952-335-004-5 ISBN 978-952-335-005-2 (PDF) ISSN-L 1456-4491 ISSN 1456-4491 Lappeenrannan teknillinen yliopisto Yliopistopaino 2016 Abstract Mehdi Hasan Purification of aqueous electrolyte solutions by air-cooled natural freezing Lappeenranta 2016 78 p. Acta Universitatis Lappeenrantaensis Diss. Lappeenranta University of Technology ISBN 978-952-335-004-5 ISBN 978-952-335-005-2 (PDF) ISSN-L 1456-4491 ISSN 1456-4491 Freeze crystallization is a particular type of a purification method where the solvent freezes out, which constricts the volume of the solution, leaving thus behind a more concentrated solution. In the case of freezing an aqueous solution, water is the solvent which crystallizes and can be separated from the concentrated solution by the virtue of buoyancy. In an ideal situation, freeze crystallization of an aqueous solution produces ice crystals that do not contain any of the impurities present in the original solution. As the process continues, the original solution becomes more concentrated and the freezing temperature declines progressively. Freezing point depression (FPD) is of vital importance in characterising the freezing behaviour of any solution. Due to this necessity, a new calculation method to predict FPD is presented in this work. In this method, designated ion-interaction parameters for the Pitzer model are extracted from reliable FPD data found in the literature, other than calorimetric data. The extracted parameters from FPD data are capable of predicting the freezing point more accurately than those resulted from the calorimetric data. The calculation method is exemplified for numerous 1-1 and 1-2 types of electrolytes. Impurities in excess of the maximum recommended limits must be removed from wastewater prior to discharge because of their persistent bio-accumulative and detrimental nature. Natural freezing is suggested in the present work as a purification technique to treat huge volumes of wastewater in a sustainable and energy-efficient manner. The efficiency of freeze crystallization in the purification of wastewater by imitating natural freezing in a developed winter simulation with the provision of altering winter conditions is scrutinized in this thesis. Hence, natural freezing is simulated experimentally for ice crystallization from unsaturated aqueous Na2SO4 and NiSO4 solutions to assess the feasibility of such a technique to be used to purify wastewaters containing electrolytes. This work presents a series of data in similitude of natural freezing of water from aqueous Na2SO4 and NiSO4 solutions in various concentrations and freezing conditions. The influence of solution concentration and different freezing conditions, such as ambient temperature, freezing time and freezing rate, on the efficiency of the purification process is investigated by analysing the effective distribution coefficient (K) of the solute between ice and the solution. The experimental results demonstrate clearly that high purity ice can be obtained from slow freezing of the solution with the concentration typically found in industrial wastewater. During freeze crystallization, the diffusion of impurities from the solid-liquid interface to the bulk of the solution, along with the growth mechanism of the solid phase play an important role in determining the purity of the ice layer. Therefore, a calculation method is introduced to estimate the concentration of the solution at the advancing ice–solution interface in terms of the limiting distribution coefficient (K*) from experimental K values at different growth conditions. The heat transfer -controlled growth rate of the ice limited by the free convective heat transfer coefficient of air (hair) rather than the thermal conductivity of the ice (kice) and the heat transfer coefficient of the solution (hsol) was found to prevail over the mass transfer of rejected solute molecules from the ice–solution interface to the bulk solution of experimental interest. A simplified and robust model is developed to estimate the thickness and growth rate of the ice layer formed from solutions at different freezing conditions, and the model is validated with experimental results. In addition, inclusion formation within the ice matrix during freezing is investigated for various solution concentrations, both macroscopically and microscopically. Keywords: Freeze crystallization, eutectic point, purification of wastewater, natural freezing, crystal growth kinetics, suspension crystallization, static layer crystallization, electrolyte, Pitzer model, freezing point depression, heat transfer, mass transfer, distribution coefficient, ice growth rate, ice purity. Acknowledgements All praise to THE MOST EXALTED for providing me the wisdom, intellect, patience, perseverance, wiliness, enthusiasm, strength, and health with other innumerable bounties to accomplish the goal of my doctoral study well ahead of the schedule. Even a million thanks would belittle the contribution of Prof. Marjatta Louhi-Kultanen for generating the theme of the dissertation, and sorting out scientific challenges and financial supports. I would like to acknowledge Dr. Jaakko Partanen for his initiatives, expert guidance, and supports with endless manoeuvres. As this study was carried out in the Thermal Unit Operations research group of the School of Engineering Science at Lappeenranta University of Technology (LUT), I am hence grateful to all staffs of this unit. Special thanks to exchange students - Thomas Regal, Mael L’Hostis, Bolormaa Bayarkhuu, Nirina Ramanoarimanana, and Olaya Aranguren, LUT student – Mikko Brotell, and Rasmus Peltola, and LUT graduate MSc. Kaisa Suutari for helping me by performing the experiments and sharing their insight. I would like to remember the assistance of the late Mr. Markku Maijanen during the building of the experimental setups and also Ms. Anne Marttinen for her help in administrative formalities. I am grateful to Prof. Ville Alopaeus, Dr. Rüdiger U. Franz von Bock und Polach of Aalto University, Prof. Alison Lewis’s group in the Crystallization and Precipitation Unit (CPU) of the University of Cape Town (UCT), especially to Mr. Jemitias Chivavava, Mr. Edward Peter, and Mr. Dereck Ndoro for their scientific discretion and the fruitful discussions. Thanks to Ariful Islam Jewel and the staffs of UCT, and countrymen for supporting me with everything during my research visit in South Africa. I would like to convey my gratitude to the Academy of Finland, the Graduate School of Chemical Engineering (GSCE), the LUT foundation, and LUT graduate school for supporting my research work financially. I am thankful to my friends – Hasnat Amin, Ashraf khan, Muhammad Adeel Maan, Hafiz Maan, Fahad Rezwan, Fayaz Ahmad, and Jitu Kumar for all their help and support. Last but not least, my sincerest thanks go to my wife, parents, brother, and relatives for their perpetual encouragement and compassionate understanding, patience and continuous support during my doctoral study. Mehdi Hasan October 2016 Lappeenranta, Finland Contents Contents 7 List of publications 9 Nomenclature 10 1 Introduction 13 1.1 Background ............................................................................................. 13 1.2 Problem statement ................................................................................... 14 1.3 Objectives of the thesis ............................................................................ 16 1.4 Potential exploitations ............................................................................. 16 1.5 Plan of development ................................................................................ 17 2 Freeze crystallization kinetics and ice purity 18 2.1 Introduction ............................................................................................. 18 2.2 Freeze crystallization ............................................................................... 18 2.2.1 Basic concept of freeze crystallization ........................................ 18 2.2.2 Advantages of freeze crystallization ........................................... 19 2.2.3 Classification of freeze crystallization ........................................ 20 2.3 Influence of heat and mass transfer on crystal growth kinetics of freeze crystallization .......................................................................................... 21 2.3.1 Differential mass transfer model ................................................