Iowa State University Capstones, Theses and Retrospective Theses and Dissertations Dissertations 1976 Effects of supersaturation and temperature on nucleation and crystal growth in a MSMPR crystallizer James Everett eltH Iowa State University Follow this and additional works at: https://lib.dr.iastate.edu/rtd Part of the Chemical Engineering Commons Recommended Citation Helt, James Everett, "Effects of supersaturation and temperature on nucleation and crystal growth in a MSMPR crystallizer " (1976). Retrospective Theses and Dissertations. 6213. https://lib.dr.iastate.edu/rtd/6213 This Dissertation is brought to you for free and open access by the Iowa State University Capstones, Theses and Dissertations at Iowa State University Digital Repository. It has been accepted for inclusion in Retrospective Theses and Dissertations by an authorized administrator of Iowa State University Digital Repository. For more information, please contact [email protected]. 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Xerox University Microfilms 300 North Zeeb Road Ann Arbor, Michigan 48106 76-28,248 HELT, James Everett, 1947- EFFECTS OF SUPERSATURATION AND TEMPERATURE ON NUCLEATION AND CRYSTAL GROWTH IN A MSMPR CRYSTALLIZER. Iowa State University, Ph.D., 1976 Engineering, chemical X©rOX UniVOrSity Microfilrns, Ann Arbor, Michigan 48106 Effects of supersaturation and temperature on nucleation and crystal growth in a JBMER crystallizer by James Everett Eelt A Dissertation Submitted to the Graduate Faculty in Partial Fulfillment of The Requirements for the Degree of DOCTOR OF PHILOSOPHY Department: Chemical Engineering and Nuclear Engineering Major: Chemical Engineering Approved: Signature was redacted for privacy. :harge of Major Work Signature was redacted for privacy. For the Major Department Signature was redacted for privacy. Iowa State University Ames, Iowa 1976 il TABLE OF CONTENTS page INTRODUCTION 1 LITERATURE REVIEW 4 Solutions and Solubility 4 Nucleation 7 Primary nucleation 8 Secondary nucleation 11 Crystal Growth 13 (^pes of Ciystallizers l8 Crystal Population Balance 19 Relation Between Crystal Size Distribution and Kinetics 23 Importance of Supersaturation in Understanding the Operation of a MSMER Crystallizer 24 Effects of Secondary Nucleation 27 Measurement of Supersaturation 27 Refractive index 20 Use of refractive index in crystallization measurements 29 THEORETICAL DEVELOPMENT 32 Objectives of Present Work 32 Supersaturation Relationships 33 Effects of Temperature 3^ EXPERIMENTAL 36 Equipment 36 Crystallizer 36 Feed supply system 38 Withdrawal system kO ill Page Coolant syctem kO Sampling and instrumentation 4l Calibration of Refractometer 42 Procedure 46 Feed preparation 46 Coolant system 47 Operation of crystallization system 48 Sampling and filtration of crystals 49 Supersaturation measurement $0 RESULTS 52 Treatment of Data 52 Determination of Growth and Nucleation Rates ^6 Growth and Nucleation Kinetics 59 Supersaturation dependency 59 Temperature dependency 80 Discussion of Inverse Nucleation Rate with Temperature 88 COKCmSIONS 93 RECOMMENDATIONS 9^ NOMENCIATURE 98 BIBLIOGRAPHY 102 ACKNOWLEDOÎENTS 107 APPENDIX A. VOLUMETRIC SHAPE FACTOR 108 APPENDIX B- CALCULATION OF NUMBER OF CRYSTALS FROM WEIGHT Œ KNO3 IN SCREEN ANALYSIS 109 APPENDIX C. EXPONENTIAL LEAST SQUARES ANALYSIS 110 APPENDIX D. RESULTS OF EXPERIMENTAL RUNS 111 1 IHîRODUCriON Crystallization can be defined as the formation of solid particles within a homogeneous phase. The solid particles can be formed from the vapor phase, as in the case of snow. They can be formed from a liquid melt, as in freezing of water or the manufacture of large single crystals. The solid can also be formed from liquid solutions. This last case of crystallization from solution is very important industrially because of the wide variety of materials that is marketed in a crystalline form. The main benefit of the operation is that a crystal formed from an impure solution can itself be pure. It also affords a practical method of obtaining pure chemical substances in a satisfactory condition for packaging and storing. As a result, crystal­ lization is used in a wide range of industries from the production of bulk fertilizers to the preparation of fine cheniicals and drugs. In industrial operations the yield, purity, size, and shape of the crystals produced are important factors. Many times, for example in sugar production, it is especially necessary that the crystals be of uniform size. Unifom size is desirable for appearance, for ease in filtering and washing, and for consistent behavior in use. Uniformity of size also minimizes caking in the package. Attainment of the desired size distribution is one of the main objectives in the design and operation of industrial crystallizers. It is the relationship between the conçeting kinetic rates of nucleation and crystal growth that determines the size distribution of the product. For sssj years achieviiig the correct relationship concerning the kinetics 2 was very much an art rather than a science. In the last ten to twenty- years, however, there has been a great deal of research effort in an attempt to better understand the kinetics of crystallization. To under­ stand the kinetics it is necessary to understand the two mechanisms of crystallization; namely, crystal nucleation and crystal growth. These two mechanisms co^ete for solute in tenas of their respective dependence on supersaturation. The classical theories of nucleation do not predict the observed nucleation rates in continuous crystallization from solution. A considerable amount of theoretical work has been done in the area of continuous, mixed-suspens ion, mixed-product-removal crystal 1 izers. This type of crystallizer is like mary used in industrial processes. The analysis technique of using a crystal population balance provides a convenient way to study the kinetics of nucleation and growth in a mixed-suspens ion crystallizer. Since both nucleation and growth are direct functions of the level of supersaturation in the crystallizer, to fully understand the kinetics of crystallization it is important to know what level of super- saturation exists, normally the magnitude of supersaturation is quite small in a )GMER cooling crystallizer and this makes the measurement of small changes in concentration very difficult. In most research to date, the magnitude of supersaturation has been inferred from the resulting crystal size distribution rather than the more desirable situation of predicting the crystal size distribution from knowledge of the level of supersaturation. Also, relationships between the level of super- saturation and the birth and growth rates have not been available. 3 The objectives of this vork are therefore twofold. First of all to show that refractive index methods offer a way of continually monitoring the level of supersaturation in a MSMER crystallizer. Secondly, to develop the needed relationships relating the birth and growth rates of crystals to the level of supersaturation and to the crystallizer temperature. k LITERATURE REVIEW Solutions and Solubility A solution may be defined as a homogeneous mixture of two or more substances. The substances may be in any of the three states of matter— gas, liquid, or solid. The solution is usually divided into two parts, the solutes and the solvents. It is conventional to call the con^xsnent in excess the solvent. For a solution con^sed of an inorganic salt dissolved in water the water is the solvent and the salt the solute. The solubility of a solute in a given solvent is usually stated as the parts by wei^t per part by weight of solvent. If a hydrated SRlt is dissolved in water, the