CONTINUOUS EUTECTIC FREEZE CRYSTALLISATION

Loading...

Taking too long?

Reload document

| Open in new tab

CONTINUOUS EUTECTIC FREEZE
CRYSTALLISATION

Report to the
Water Research Commission

by

Jemitias Chivavava, Benita Aspeling, Debbie Jooste, Edward Peters, Dereck
Ndoro, Hilton Heydenrych, Marcos Rodriguez Pascual and Alison Lewis
Crystallisation and Precipitation Research Unit
Department of Chemical Engineering
University of Cape Town

WRC Report No. 2229/1/18
ISBN 978-1-4312-0996-5

July 2018

Obtainable from
Water Research Commission
Private Bag X03
Gezina, 0031

orders@wrc.org.za or download from www.wrc.org.za

Printed in the Republic of South Africa
© Water Research Commission

ii

DISCLAIMER
This report has been reviewed by the Water Research Commission (WRC) and approved for
publication. Approval does not signify that the contents necessarily reflect the views and
policies of the WRC nor does mention of trade names or commercial products constitute
endorsement or recommendation for use.

Executive summary

Eutectic Freeze Crystallisation (EFC) has shown great potential to treat industrial brines with
the benefits of recovering potentially valuable salts and very pure water. So far, many studies
that have been concerned with the development of EFC have all been carried out (1) in batch
mode and (2) have not considered the effect of minor components that may affect the process
efficiency and product quality.

Brines generated in industrial operations often contain antiscalants that are dosed in cooling
water and reverse osmosis feed streams to prevent scaling of heat exchangers and membrane
fouling. These antiscalants may affect the EFC process, especially the formation of salts. The
first aim of this work was to understand the effects of antiscalants on thermodynamics and
crystallisation kinetics in EFC.

A continuous EFC process is appropriate for the treatment of large volumes of brines since it
allows better control of product quality, reduces waste and has less labour costs. The second
aim of this work was to develop a continuous EFC process. To this end, a laboratory size
continuous EFC plant was designed and commissioned. It is essential that continuous
solid-solid-liquid separation is achieved in order to allow a smooth continuous EFC operation.
This requires large ice and salt crystals which easily separate mechanically and since product
properties depend on the operating conditions, it is important to understand the effect of
residence time and degree of undercooling on product characteristics. Further to this, large
production rates are required at industrial process scale. This is sometimes limited by scale
formation and a study focused on understanding dynamics around ice scale formation was
conducted. The third aim was therefore to investigate the interaction between operating
conditions and product quality, as well as operational constraints of a continuous EFC process.

Since industrial brines contain more than one component, there is an opportunity to recover ice
and more than one salt product. Although the treatment of such brines was proven using batch
EFC, the use of continuous EFC would be beneficial for the treatment of large volumes of
brines. Therefore, the fourth aim of this research was to investigate the treatment of real
multicomponent brines using continuous EFC.

Phosphonate-based antiscalants were found to have a marginal effect on the thermodynamics
of a Na2SO4-H2O system but showed significant effects on the crystallisation kinetics of both
ice and salts. This was mainly attributed to the alteration of the surface free energy and surface
coverage. The continuous 2 ℓ EFC plant was successfully commissioned and the crystalliser
cooling capacity was found adequate for feed flow rates of 44 to 100 mℓ/min. Gravitational
separation was achieved in the separation zones of the crystalliser. Increasing residence time
at a constant operating temperature was found to enhance the mean crystal size of ice while
increasing the degree of undercooling, at a constant residence time, increased the mean crystal
size more significantly. The increase in size was attributed to more time for growth and faster
growth rate, respectively. As expected, higher scraper speeds were found to delay the formation

iii

of an ice scale layer and the presence of impurities increased the induction time for scale
formation.

Ice and Na2SO4.10H2O were produced from continuous EFC of ternary Na2SO4-MgSO4-H2O
synthetic solutions using a non-scraped agitated crystalliser. It was found that increasing the
concentration of MgSO4 in the ternary Na2SO4-MgSO4-H2O depressed the eutectic
temperatures for Na2SO4.10H2O and ice crystallisation. A real brine was also treated using a
continuous EFC process that employed a jacketed column crystalliser. Ice and Na2SO4.10H2O
were produced from this hypersaline brine but the production rates of both products varied
widely at constant operating conditions. This was attributed to the coupled or interdependent
crystallisation dynamics of the products in eutectic systems and short residence times in the
column crystalliser, which limited time for salt crystallisation in some runs. The ice product
had a high Na impurity content but this reduced significantly after washing. Only traces of
impurities were detected in the Na2SO4.10H2O product but these could not be washed
effectively.

This report provides a literature review and synthesis, experimental methodology and results
for each of the four aims of this project.

iv

Acknowledgements

The project team wish to express their gratitude to –

the staff of the Crystallisation and Precipitation Research Unit and the Mechanical Workshop,
in the Chemical Engineering Department at the University of Cape Town for their contribution
to the research reported here;

the Water Research Commission (WRC) and University of Cape Town for the funding support;

the members of the Reference Group for their contributions to and guidance of the project,
namely:

Dr JE Burgess, Water Research Commission
Mr A Wurster, Independent member
Ms R Muhlbauer, Anglo American
Mr JS Beukes, Coaltech
Prof LF Petrik, University of the Western Cape
Mr IW van der Merwe, Proxa
Mr P Gunther, Prentec
Mr A Viljoen, Prentec
Mr D Howard, Worley Parsons
Dr G Madzivire, University of the Western Cape
Mr HM du Plessis, Independent member
Dr L Baratta, Sasol
Mr G Gericke, Eskom
Mr T Harck, Solution H

v

Table of Contents

Executive summary …………………………………………………………………………………………………… iii

Acknowledgements …………………………………………………………………………………………………….. v

Table of Contents ………………………………………………………………………………………………………. vi

Table of Figures ………………………………………………………………………………………………………… xi

List of Tables ………………………………………………………………………………………………………….. xiv

Nomenclature …………………………………………………………………………………………………………… xv

Glossary …………………………………………………………………………………………………………………. xvi

1

Introduction ………………………………………………………………………………………………………… 1

1.1 Problem statement ………………………………………………………………………………………….. 2

1.1.1 Effect of antiscalants on the EFC process…………………………………………………….. 2

1.1.2 Continuous EFC ……………………………………………………………………………………….. 2

1.1.3 Operational considerations and limitations in continuous EFC ……………………….. 2

1.1.4 Multi-component EFC ………………………………………………………………………………. 3

1.2 Research aims ………………………………………………………………………………………………… 3

1.2.1 Research objectives …………………………………………………………………………………… 4

1.3 Scope of research ……………………………………………………………………………………………. 4

2 Theory of crystallisation ……………………………………………………………………………………….. 5

2.1 Crystallisation ………………………………………………………………………………………………… 5

2.2 Crystallisation kinetics ……………………………………………………………………………………. 5

2.2.1 Nucleation ……………………………………………………………………………………………….. 5

2.2.2 Crystal growth ………………………………………………………………………………………….. 6

2.3 Population balance ………………………………………………………………………………………….. 7

2.4 Product quality ……………………………………………………………………………………………….. 7

2.4.1 Morphology (Habit) ………………………………………………………………………………….. 7

2.4.2 Crystal size distribution …………………………………………………………………………….. 8

2.5 Continuous crystallisation ……………………………………………………………………………….. 8

2.6 Theory of EFC ……………………………………………………………………………………………….. 8

2.7 Multicomponent solutions ……………………………………………………………………………… 12

2.7.1

Ionic interactions …………………………………………………………………………………….. 12

2.8

Inclusions …………………………………………………………………………………………………….. 13

2.8.1 Liquid inclusions …………………………………………………………………………………….. 13

2.8.2

Isomorphous inclusions …………………………………………………………………………… 13

vi

2.9 Heat transfer in eutectic freeze crystallisers ……………………………………………………… 13

2.9.1 Heat balance …………………………………………………………………………………………… 14

2.9.2 Scaling…………………………………………………………………………………………………… 14

2.10 Operating conditions ………………………………………………………………………………….. 15

2.10.1 Residence time …………………………………………………………………………………….. 16

2.10.2 Undercooling ………………………………………………………………………………………. 16

2.10.3

Solids fraction ……………………………………………………………………………………… 16

2.11 Gravitational separation ……………………………………………………………………………… 16

2.12 Additives and / or impurities ……………………………………………………………………….. 17

2.12.1 Antiscalants ………………………………………………………………………………………… 17

2.13

EFC summary and context of application ……………………………………………………… 18

3 Literature review ………………………………………………………………………………………………… 19

3.1 History of EFC……………………………………………………………………………………………… 19

3.2 Eutectic Freeze Crystallisation ……………………………………………………………………….. 19

3.2.1 Development of Eutectic Freeze Crystallisers …………………………………………….. 19

3.3 Additives ……………………………………………………………………………………………………… 21

3.3.1 Effects of antiscalants on salt crystallisation kinetics …………………………………… 21

3.3.2 Effect of impurities on ice crystallisation …………………………………………………… 24

3.3.3 Continuous EFC process ………………………………………………………………………….. 25

3.3.4 Effect of residence time …………………………………………………………………………… 26

3.3.5 Effect of undercooling …………………………………………………………………………….. 27

3.4

Ice scaling in scraped wall crystallisers ……………………………………………………………. 29

3.5 Continuous EFC of multi-component brines …………………………………………………….. 32

3.5.1 Single salt crystallisation …………………………………………………………………………. 32

3.5.2 Multiple salt simultaneous crystallisation …………………………………………………… 36

3.6 Gap analysis …………………………………………………………………………………………………. 38

3.7 Hypotheses…………………………………………………………………………………………………… 43

3.8 Research questions ……………………………………………………………………………………….. 44

3.8.1 Effect of antiscalant on EFC process …………………………………………………………. 44

3.8.2 Continuous EFC crystalliser …………………………………………………………………….. 44

3.8.3 Effect of operating conditions on ice product quality …………………………………… 44

3.8.4

Ice scaling behaviour ………………………………………………………………………………. 45

3.8.5 Continuous EFC of multi-component brines ………………………………………………. 45

vii

4 Effect of antiscalants on Eutectic Freeze Crystallisation …………………………………………. 46

4.1 Material and methods ……………………………………………………………………………………. 46

4.1.1 Experimental design and reagents …………………………………………………………….. 46

4.1.2 Experimental set-up ………………………………………………………………………………… 46

4.1.3 Experimental procedure …………………………………………………………………………… 47

4.2 Results and Discussion ………………………………………………………………………………….. 48

4.2.1 Effect of antiscalant on phase equilibria …………………………………………………….. 50

4.2.2 Effect of antiscalant on ice crystallisation kinetics ………………………………………. 51

4.2.3 Effect of antiscalant on Na2SO4.10H2O crystallisation kinetics …………………….. 54

4.3 Conclusion …………………………………………………………………………………………………… 58

5 Commissioning of a continuous 2 ℓ EFC plant ………………………………………………………. 59

5.1 Design details of the equipment ……………………………………………………………………… 59

5.2

Installation and assembly……………………………………………………………………………….. 60

5.3 Preliminary tests …………………………………………………………………………………………… 62

5.4 Commissioning …………………………………………………………………………………………….. 63

5.5 Results and Discussion ………………………………………………………………………………….. 63

5.5.1 Heat transfer evaluation …………………………………………………………………………… 64

5.5.2 Gravitational separation performance ………………………………………………………… 66

5.5.3 Product quality ……………………………………………………………………………………….. 67

5.6 Conclusion …………………………………………………………………………………………………… 67

6 Operational considerations and limitations in a continuous EFC process…………………… 68

6.1 Materials and methods …………………………………………………………………………………… 68

6.1.1 Experimental design and reagents …………………………………………………………….. 68

6.1.2 Experimental set-up ………………………………………………………………………………… 68

6.1.3 Analytical methods …………………………………………………………………………………. 68

6.1.4 Experimental procedure …………………………………………………………………………… 69

6.2 Results and Discussion ………………………………………………………………………………….. 71

6.2.1 Effect of operating conditions on ice characteristics ……………………………………. 71

6.2.2 Effect of scraper speed, undercooling and impurities on ice scale formation ….. 74

6.3 Conclusion …………………………………………………………………………………………………… 79

7 Treatment of multi-component brines …………………………………………………………………… 81

7.1 Materials and methods …………………………………………………………………………………… 81

7.1.1 Experimental design and reagents …………………………………………………………….. 81

viii