the quality of sugar crystals and their storage …

Conference on

Technical progress in the sugar industry

organized by

Polish Sugar Technicians Association

on 12 and 13 May 2014 in Zakopane.

THE QUALITY OF SUGAR CRYSTALS

AND THEIR STORAGE STABILITY

Mohamed MATHLOUTHI Association Andrew VanHook

Reims, France

1. Introduction2. Quality of sugar crystals

3. Causes and consequences of sugar caking

4. Moisture control and storage stability

Regular shape of sucrose crystal

Defects :Twins, Conglomerates - agglomerates

Amorphous, dust, … inclusions

Definition of caking

Seeding and Crystal quality

Crystal growth and quality

Washing in centrifugals

Drying and conditioning

Moisture repartition in the crystal

Water vapor sorption isotherm

Prevention of caking

5 . Conclusion

OUTLINE

Introduction

-Breakage of sugar crystals generally originates from

defects such as shape and size, agglomeration,

inclusion and/or adsorption of impurities, amorphous

state, …

-The defects of sugar crystals are formed during the

different steps of process: crystallization, drying,

screening and handling which lead to dust formation

-Water Vapor Sorption Isotherms and Image Analysis

allow understanding of the origins of the lack of stability

of white sugar during storage and to predict its prevention

- White sugar instability especially as regards caking,

generally results from crystal defects and fines <0.2mm

Quality of sugar crystals


Regular shape of sucrose crystal obtained in pure solution: eachface has a characteristic growth . In presence of impurities, somefaces are blocked; other faces grow in needle or triangle shape…

REGULAR SHAPE

TWINS (Type 3)TWINS (Type 2)


Formation of Twins at high supersaturation in

presence of impurities


AGGLOMERATE

CONGLOMERATEABRASION

ScreeningIn receiver

Supersaturation > 1.2

Temperature decrease

In Pan boiler for high supersaturation


BREAKAGE

Bucket elevator, Screw transporter…


AMORPHOUS sugar

Re-crystallization

Sugar crystals (< 100 µm) (sugar dust from factory)

single crystals (< 100 µm) (B.S. patent)

CRYSTAL QUALITY: shape and size :sugar dust

By heating in oven of crystals, inclusion can be revealed

Breakage

of the crystal at the

level of the inclusion

INCLUSION

196h 213h 290h 503h

Cleavage of crystal


DUST FORMATION DURING DRYING

SURFACE BEHAVIOUR OF SUCROSE CRYSTALS

Drying at high temperature (>70°C) may generate a

layer of amorphous sugar surrounding the crystals.

Dust and broken crystals are more active in water

adsorption than well formed separated monocrystals.



HANDLING - CONDITIONING

Forces applied on sugar crystals during handling and conditioning induce fractures or particle breakage.

Causes and consequences

of sugar caking

Definition of caking

Sugar caking is a spontaneous phenomenon of

adhesion of particles which change from free

flowing behaviour to soft lumps in a first stage and

then into agglomerated non flowing solid

The major factors affecting the caking phenomenon

are:

The quality of sugar crystals (grain size distribution,

surface defects, broken crystals, inclusion of impurities, …)

Water content (total moisture, surface moisture, bound

water)

Equilibrium Relative Humidity (R.H. of air at equilibrium)

Temperature of sugar after drying/cooling

Gradient of Temperature and R.H. in the bag or the silo

WATER ADSORPTION AND CAKINGSchematic steps of lumping

Solide

Air

Eau

Solide

Eau

SolideEau

a)

b)

c)

d)

Solide

Air

Eau

A- Pendular step

B- funicular step

C- capillary step

D- drop step

E

R

H

CAKING OF CRYSTALLINE WHITE SUGAR

1 mm

Pendular step

0% < ERH< 44%

Solid

Air

Syrup


500 µm

44% < ERH < 75%

Liquid bridge

Funicular step

Solid

Air

Liquid


75% < ERH < 85%

Solid bridge

Capillary step

Solid

Syrup


500 µm

ERH> 85 %

Syrup surrounding crystals

Solid

Liquid

Drop step

Causes of Caking

The main causes of caking are related to crystal

quality:

The risk of caking is minimized when sugar

crystals have a regular shape and size with CV <

30% and fines (<0.2mm) less than 1%

To obtain these conditions of crystal quality the

work starts in pan boiler

All steps of crystallization in pan boiler are

important to optimize especially for :

SEEDING

CRYSTAL GROWTH

SEEDING AND CRYSTAL QUALITY


Magma seeding : production of magma (Dm 100µm) and seeding of the

different vacuum pans with magma

- Adjustment of magma quantity in function of final crystal size

Automatic control of magma seed cooling

Quality of crystals (low CV) depends on quality of seeding crystals

Theoretically = the number of seed particles (dm 10 µm)

is equal to the number of final crystals with size Dm

Direct seeding by slurry injection:

- Supersaturation around 1.15 (to prevent false grain or dissolution)

- Optimised mechanical stirring of slurry if speed is :

- high enough to prevent melting in overheated zones

- low enough to avoid fine crystals formation

Requirements for seed slurry


Quality of seeding crystals

Seed Defects :

Breakage : Twins and conglomerates


ETHANOL: Abandoned : because Tb=78.5°C ebullition yields vapour which leads to seed loss during evapocrystallisation

ISO-PROPANOL: Tb = 85°C (no loss by evaporation)

- good dispersion (2 kg sugar/4 L iso-propanol) with easy settling

- better grinding yield in Ditmar (2h instead of 3)

POLYETHYLENE GLYCOL: used to replace isopropanol because of

regulation issues. Proves to be a better disperser. Less decantation.

RESULTS obtained with PEG 300:

Homogeneous seed crystals 10 µm (very slow settling)

DISPERSING AGENT

SEEDING AND CRYSTAL QUALITYQuality of seed slurry

PHOTOS OF SLURRY WITHDIFFERENT DISPERSING AGENT

ISO-PROPANOL Polyethylene glycol PEG 300


The seed magma preparation installation

• Single stage seed magma preparation (Braunschweig process):

• Seed magma cooling crystallizer - Seed magma receiver

• External heat exchanger – Pan boilers seeding circuit

Seed magma cooling crystallizer

Magma receiver


temperature and supersaturation during seed magma preparation


Results of optimised seed magma preparation

• Seed magma preparation optimized :

– Crystal content: 20.6 %

– Temperature: 27.5°C

– MA: 76 µm

– CV: 32

No coarse crystals anymore

Aggregate level less than 20%

• Improved final product grain size distribution

– CV: 30 – 35

– % fines (< 0.20 mm) < 1%


Evolution of seed particles in vacuum pan : Formation of conglomerates

CAUSES :

High level of syrup supersaturation at seeding

Insufficient Agitation or too high Agitation (> 286 rpm)

Insufficient Proportion of seed slurry

Low dispersion of seed particles

CONSEQUENCES :

Increase of ash in sugar

Difficulty of sugar drying

SOLUTION :

Adding of non ionic surfactant (~0.1%)

Improvement of slurry quality and Agitation

CRYSTAL GROWTH

Optimal growth at constant supersaturation

Supersaturation trend

1:increase ( = 1.15) for footing;

2: stabilization for seeding;

3: decrease by adding water (thinning and dissolution of fines);

3-4:GROWTH:at constant supersaturation ( = 1.05 – 1.10);

4-5: decrease during massecuite final Brixing.

CRYSTAL GROWTH AND QUALITY

During growth, a rough surface can be transformed into a flat

crystal face with inclusion of mother liquor

inclusions


Explosion of vapour bubbles at crystal

surface

promotes the inclusion of mother liquor


During growth, the presence in mother liquor

of impurities induces modified crystal shapes

Dextran Invert Sugar

WASHING IN CENTRIFUGAL

WASHING IN CENTRIFUGAL AND QUALITY

1. Optimal Loading

Constant Speed

Constant load (position of sensor)

Constant Quality of massecuite

(consistancy)

All optimisation parameters depend

on layer thickness constancy


2. Pre-washing After removal of runoffs from crystal surface

Massecuite layer color changes from « brown

to clear yellow »

Speed ~350 rpm ( if lower : re-appeance of

brown color with repelled wahing water)

Adapted to the type of centrifuge

Duration of pre -

washing

Duration of

washing

Coloration / °Brix

0

0

3

3

9

7

5

7

36

45

30

27


Optimum Washing

(position in cycle - Results)

Cycle duration

Speed rpm

EU Pts

Coloration Ash Total

3.8 5.9 9.7

3.6 5.5 9.1

3.8 6.0 9.8

850 rpm

910 rpm

980 rpm

1030 rpm

1100 rpm

3.9 7.0 10.91150 rpm

4.6 8.6 13.2

4.1 5.8 9.9

Conclusion

Sugar Quality for a given volume of washing water varies

in function of :

Position in time during centrifugal cycle (optimum

depends also on centrifugal type)

Path of water between crystals and efficacy of

washing device

The Volume of water used depends on massecuite

characteristics

Compacity (mother liquor viscosity + % crystals)

Grain size distribution

Excess of wahing water does not improve sugar quality and

contributes to increase processing cost

For example, 1 kg water in excess represents 1 kg of

sugar lost (°Brix = 50)


DRYING AND SCREENING

DRYING SCREENING AND CRYSTAL QUALITY

Moist sugar Exhaust

air

Dry sugar

Dry

air

Nearly fluidised-bed conditions

Rotary Louvre dryer - principle of operation

http://www.elma.co.za/projects/sugardryer1.jpg


What makes the process of sugar drying different

from water evaporation from a film of pure water?

Partial pressure of water above the syrup film depends on syrup composition (variable due to drying!)

Mass transfer resistance of the syrup film (absent for pure water!)

Crystallization of sugar from syrup

Possibility of formation of amorphous sugar.




Formation of amorphous sugar crust

MECHANISM

Rapid evaporation from the outer layers of the syrup film

Insufficient rate of water diffusion through the inner

layers of the syrup film

CONSEQUENCES

• High supersaturation of syrup near the interface

• Formation of an amorphous sugar crust at the interface

• Water trapped within the syrup film

• A dramatic decline of the rate of drying





Syrup film

Amorphous layer

(once formed)

Initially, for

typical industrial

conditions,

3-4 µm

Crystal

Hot air

Crystal of sugar and syrup film in the dryer



DUST FORMATION DURING DRYING

Crystals are covered by sugar dust. These dust particles arecomposed of amorphous sugar : short time high temperaturedrying is at the origin of dust formation. Also abrasion duringscreening


Moisture control and storage stability

MOISTURE IMPLICATIONS IN STORAGE STABILITY

Air MoistureSyrup Water

Crystal sucrose

Syrup sucrose

Included water

WATER IN AND AROUND SUGAR CRYSTAL

WATER IN AND AROUND SUGAR CRYSTAL

0

0 ,005

0 ,01

0 ,015

0 ,02

0 ,025

0 ,03

0 ,035

0 ,04

0 ,045

0 ,05

eau t o t al e 5 ,246 0 ,0397 0 ,0313 0 ,0267 0 ,0236 0 ,0228 0 ,0203

eau de su r f ac e 5 ,0579 0 ,0159 0 ,0126 0 ,0083 0 ,0041 0 ,0035 0 ,0024

eau inc l use 0 ,1881 0 ,0238 0 ,0187 0 ,0184 0 ,0195 0 ,0193 0 ,0180

0 ,9 0 ,85 0 ,82 0 ,75 0 ,58 0 ,44 0 ,33

WATER ACTIVITY

Included water

Surface water

included water content is almost constant and surface

water increases with increased water activity

MOISTURE IMPLICATIONS IN STORAGE STABILITY

WATER SORPTION ISOTHERM OF CRYSTALLINE SUGAR

CRYSTAL QUALITY

Amorphous sugar hygroscopicity

Roth D. (1976)

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

0,3 0,4 0,5 0,6 0,7 0,8 0,9Aw

Wat

er c

onte

nt (g/

Kg

M.S

.)

< 250 µm 400-500 µm 500-800 µm > 800 µm

< 250 m

Fraction 250-400 m

> 800 m

CRYSTAL SIZE DISTRIBUTION

AND WATER VAPOR SORPTION

10°C

35°C

EFFECT OF TEMPERATURE ON WATER SORPTION

ISOTHERMS

(Crystalline sugar)

EFFECT OF TEMPERATURE ON WATER SORPTION

ISOTHERMS

(Crystalline sugar)

dp T = 9°C (DT = 1°C)

dp T =13.5°C (D= 1.5)

Tdp = 18°C(D= 2)

Tdp = 23°C (D=2)

Tdp= 27°C(D=3)

Tdp =31°C(D=4 )

CRYSTAL QUALITY and STABILITY:

hygroscopicity of milled sugar

0

0.05

0.1

0.15

0.2

0.25

0.3

50 60 70 80 90 100HRE %

Te

ne

ur

en

ea

u (

g/1

00

g M

.S.)

sucre Référence microcristaux(< 250 µm) < 250 µm (sucre standard)

0

0.05

0.1

0.15

0.2

0.25

0.3

50 60 70 80 90 100HRE %

Te

ne

ur

en

ea

u (

g/1

00

g M

.S.)

sucre Référence microcristaux(< 250 µm) 21% de broyés

70% de broyés < 250 µm (sucre standard)

21 %Milled

70 %

Single crystals

Manufactory crystals

CONTROL AND PREVENTION

OF CAKING

CONTROL OF FLOWABILITY

•Flow rate •Friability Angle

•Angle of repose

•Cohesion : Jenike cell

•Microscopical observations

FRIABILITY ANGLE DEVICE

0

10

20

30

40

50

60

70

80

90

100

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Aw

An

gle

de

fri

ab

ilité

°

< 0,25mm 0,40mm< < 0,50mm 0,50mm< < 0,80 mm > 0,80 mm

>0.80 mm0.5<<0.80.4<<0.5

<0.25 mm

FRIABILITY ANGLE

METHODS FOR PREVENTION OF CAKING

Storage at appropriate temperature and R.H.

Stabilisation of sugar moisture and temperature by

maturation ( 4 – 6 days)

Drying to low moisture content (Surface moisture < 1/3

total moisture)

Maintaining the lowest proportion of fines possible ( <

1%): Fine particles and amorphous responsible of caking

Avoid gradients in silo (layers with different quality:

moisture, color, size, …)

CONCLUSION

• STABILITY OF WHITE SUGAR DEPENDS ON CRYSTAL QUALITY

•CRYSTALS QUALITY depends on seeding conditions:

- Supersaturation 1.15 (no false grain or dissolution)

- Optimised stirring of slurry and homogeneous dispersion

CRYSTALS QUALITY if growth at constant supersaturation

Minimize defects and control size distribution (CV < 30%)

Optimisation of washing in centrifugal:

- Elimination of ash easier than coloration

- Optimising of washing water quality and quantity

CONCLUSION

CONCLUSION

• DRYING AND SCREENING CONDITIONS OPTIMISED:

• - Drying temperature not too high ( ~ 65°C )

• - Minimize amorphous and dust formation

• - Avoid crystal breakage, abrasion, …

CONTROL AND PREVENTION OF CAKING:

Temperature and R.H. (water vapor sorption curves)

Sugar flowability

Maturation in silo

Homogeneous quality (MA – CV)

the quality of sugar crystals and their storage …

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