Suspension Melt Crystallization In Tubular

Suspension Melt Crystallization in Tubular

and Scraped Surface Heat Exchangers

DISSERTATION

zur Erlangung des akademischen Grades

Doktor-Ingenieur (Dr.-Ing.)

vorgelegt der

Mathematisch-Naturwissenschaftlich-Technischen Fakultät

(Ingenieurwissenschaftlicher Bereich)

der Martin-Luther-Universität Halle-Wittenberg

von Herrn Dipl.-Ing. Tero Tähti

geb. am 18.08.1972 in Hämeenlinna, Finnland

Dekan der Fakultät: Professor Dr.-Ing. Holm Altenbach

Gutachter:

1. Professor Dr.-Ing. Joachim Ulrich

2. Professor Dr. Jörg Kreßler

3. Dr. Marjatta Louhi-Kultanen

Die Arbeit wurde am 19.10.2004 verteidigt.

urn:nbn:de:gbv:3-000007337

[http://nbn-resolving.de/urn/resolver.pl?urn=nbn%3Ade%3Agbv%3A3-000007337]

Acknowledgement

The work presented in this book has been constructed during my work as scientific researcher

at the Institut für Verfahrenstechnik, Martin-Luther-Universität Halle-Wittenberg, Germany.

I would like to express my very special thanks to Professor Dr.-Ing. habil. Joachim

Ulrich, my supervisor at the Martin-Luther-Universität. I am grateful for the opportunity to

work with him in his field of expertise: melt crystallization. In addition to the professional

advising, his encouragement and support have played an essential part in accomplishing the

results leading to completion of this work.

I also thank Professor Dr. rer. nat. habil. Jörg Kreßler for taking the task to be a referee

for this work.

Dr. Marjatta Louhi-Kultanen from Lappeenranta University of Technology, Finland, I

do not only thank for being a referee for my work, but also for being an excellent scientific

colleague and for the support I have received during my whole scientific career.

I would like to appoint my sincere thanks also to Professor Dr.-Ing. habil. Lutz Brendler

and Dr.-Ing. Dieter Möhring for scientific guidance and invaluable and inspiring discussions

over wide range of scientific and technical topics.

I am grateful to the guidance and support I have received during my cooperation with

Niro Process Technology B.V., the Netherlands. The view to the background of industrial

process engineering has provided me with valuable insight, helping me to assess my work in a

different light. My special thanks go to Dr.-Ing. Reinhard Scholz, Mr. Bart Schreurs and Mr.

René-Jeroen Verschuur.

I thank all my colleagues at the Martin-Luther-Universität Halle-Wittenberg for creating

a friendly working climate and for being such a young and dynamic team. I also thank all my

students for the support in the experimental work.

My very special gratitude is addressed to my loved wife Jun Jun, who has supported and

fostered me during the years, and without whom the completing of this work would have been

much much more difficult.

Tero Tähti

Halle (Saale), 20.10.2004

Table of contents

TABLE OF CONTENTS

1. Introduction 1

2. Suspension Melt Crystallization 3

2.1 Effect of Crystallization Kinetics on Suspension Melt Crystallization 4

2.1.1 Nucleation 5

2.1.2 Crystal growth 6

2.1.3 Secondary growth phenomena 8

2.1.4 Population balance 9

2.2 Suspension Melt Crystallization Processes 10

2.3 Solid-Liquid Separation in Suspension Melt Crystallization 14

2.4 Scraper Surface Heat Exchangers 15

2.4.1 Heat transfer properties of scraped surface crystallizers 16

2.5 Freeze Concentration 22

2.6 Summary of Existing Suspension Melt Crystallization Research 23

3. Crystalline Deposits in Heat Exchangers 25

3.1 Effect of Flow Conditions 28

3.2 Effect of Crystalline Suspensions 30

3.3 Effect of Surface Structure of Heat Exchanger 32

3.4 Summary of Existing Research on Crystalline Deposits 33

4. Experimental Work 35

4.1 Introduction to Experimental Work 35

4.2 Suspension Melt Crystallization in a Tubular Heat Exchanger 35

4.2.1 Experimental equipment 35

4.2.2 Used compounds 38

4.2.3 Suspension density measurements 39

4.2.4 Limiting surface temperature difference for incrustation 40

4.2.5 Heat transfer properties of the double-pipe heat exchanger 43

4.2.6 Particle size measurement 44

Table of contents

4.3 Experiments with Pilot Plant Equipment 46

4.3.1 Experimental equipment 46

4.3.2 Suspension density in the crystallizer loop 48

4.3.3 Crystal size and habit 50

4.3.4 Scraper speed of SSHE 51

4.3.5 Reduction in cooling efficiency due to heat production and

losses to environment 52

4.4 Particle Characterisation from Laboratory Scale

Suspension Melt Crystallization 56

4.4.1 Particle characteristics under scraping action 56

4.4.2 Particle characteristics under free growth in a suspension 62

4.4.3 Particle characteristics of ice crystals from stirred tank 67

4.5 Conditions for Formation of Crystalline Layers 69

4.5.1 Growth of pure components 71

4.5.2 Crystallization of fatty acid mixtures 74

5. Discussion 76

5.1 Discussion to Crystallization in the Tubular Heat Exchanger 76

5.2 Discussion to Pilot Plant Equipment 80

5.3 Discussion on Laboratory Scale Suspension Melt Crystallization 86

5.3.1 Particle formation in laboratory scale SSHE 86

5.3.2 Crystal growth in suspension 88

5.3.3 Secondary growth of ice crystals 89

5.4 Discussion to Layer Growth Experiments 90

5.5 Conclusions and Outlook 93

5.5.1 Conclusions 93

5.5.2 Outlook 94

6. Summary 95

7. Zusammenfassung 97

8. List of Symbols 99

9. References 102

Introduction

1

1. Introduction

Suspension crystallization processes offer a highly selective and energy-efficient method for

separation of chemical mixtures. In the crystallization of organic melts heat transfer

phenomena control the rate of crystal formation. The growth rate of crystals depends on the

heat transfer coefficient, the heat of crystallization and the undercooling of the melt. The heat

removal from the crystallization process is usually carried out using indirect heat exchangers,

where heat is removed from the melt by a cooling medium through a separating heat

exchanger wall. In such processes the problem of incrustations on heat exchanger surfaces by

the crystallizing component results in additional resistance to heat transfer. The increased heat

transfer resistance reduces the heat transfer rate, or necessitates a higher temperature driving

force. Thereby, the energy-efficiency of the process is reduced and the costs for energy and

cleaning are increased.

Another difficulty in suspension melt crystallization arises from the fact that the product

from the crystallizer is a suspension consisting of the pure crystals and the impure liquid from

which the crystals were grown. The final product purity depends on how well the solid-liquid

separation can be achieved. In melt crystallization this is often difficult due to high

viscosities, possible formation of soft deformable crystals and the sensitivity to temperature

changes. High demands are set also by the aim of purification in melt crystallization

processes, where the impurity concentration of the final product is often limited to a few ppm.

This requires almost complete separation of crystals and mother liquor.

The difficulties with incrustations and the solid-liquid separation result in high

construction requirements for suspension melt crystallizers. Suspension crystallization from

melts is usually carried out in equipment with continuous mechanical cleaning of the heat

exchange surface. Scraped surface heat exchangers are often used for this purpose. However,

the relative complexity of such processes increases the investment costs and the need for

maintenance.

Therefore, an optimization of the crystallizer construction has to be carried out in order to

obtain a less complex equipment configuration. By reducing the number of moving parts the

operating costs can be reduced. Together with the lower investment costs brought by the

increased simplicity, the total costs of the crystallization process will be decreased. The

characteristics of the crystals produced with such equipment also have to be investigated in

order to estimate the influence of the simplification on the overall process efficiency.

The aim of this work is to investigate the possibility of simplifying the crystallization

process. The crystal characteristics from suspension melt crystallization with and without

scrapers were examined in order to optimize the process conditions. The improved control of

the product crystal characteristics allows a more efficient application of the suspension melt

crystallization processes in the industrial practice. The aim was also to investigate the

incrustation mitigation in suspension melt crystallization with the aid of controlling the

process conditions, e.g. the flow velocity of the suspension. Efficient deposit removal by

adjusting the process conditions would allow simple standardized heat

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