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BEHIND SUGAR THE PEOPLE
Presents a Technical Paper to BSST 11/10/2012
Decolourisation Techniques used in Sugar Refining
THE PEOPLE BEHIND SUGAR
What is Refined Sugar?
Early Sugar Refining in London
Bone Char Treatment
Taylor Filters
The Wash Floor
What is Refined Sugar?
Typical Refined Sugar Colours
EEC Grade I 20 IU colour
Bottlers Grade 35 IU colour
EEC Grade II 45 IU colour
EEC Grade III 60 IU colour
White Sugar
What is Refined Sugar?
Other Typical Refined Sugar Requirements
Filtered (as a dissolved solution)
Suspended solids less than 2ppm
Pol 99.9 minimum
Ash 0.015% maximum
Invert 0.020% maximum
Other The full list of requirements for refined sugar may vary from one grade of sugar to another and these are often on a sliding scale as in the case of EEC Sugars. However, the important parameters listed above along with sugar colours are typical specifications for refined sugars. Sugars such as bottlers grade may be subject to individual purchasers specifications, and the likes of the international Cola beverage companies have their own world standards for acceptable sugars for their drinks formulations.
What is Refining?
Affination
Melting
Crystallisation
Clarification
Decolorisation
Filtration
Evaporation
Bagging and
packing
Drying
Conditioning
Phosphotation or
Carbonatation
Ion Exchange Resin ,
Powdered Activated
Carbon or Granular
Activated Carbon
Pressure Filtration or
Deep Bed Filtration
3 , 4 or 5 boiling or
backboiling
A series of steps for removing impurities and Colour
Primary Decolourisation
Phosphatation
Carbonatation
Secondary Decolourisation Ion Exchange Resin
Powder Activated Carbon
Granular Activated Carbon
Types of Colorants in Raw Sugar
Types of Colorants in Raw Sugar
Indicator Value = Colour pH9 / Colour pH4
Origin of Colorants in Raw Sugar
Colour Profile in Raw Sugar
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0
0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0
A
B
C
D
E
High Molecular Weight Low
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0
0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0
Process Percent Removed
A B C D E
Affination 56% 43% 34% 34% 34%
Colours after Affination
A
B
C
D
E
High Molecular Weight Low
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0
0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0
Process Percent Removed
A B C D E
Affination 56% 43% 34% 34% 34%
Carbonatation 80% 50% 50% 50% 20%
Colours after Carbonatation
A
B
C
D
E
High Molecular Weight Low
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0
0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0
Process Percent Removed
A B C D E
Affination 56% 43% 34% 34% 34%
Carbonatation 80% 50% 50% 50% 20%
Acrylic 0% 92% 67% 93% 50%
Colours after Ion Exchange Resin
A
B
C
D
E
High Molecular Weight Low
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0
0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0
Process Percent Removed
A B C D E
Affination 56% 43% 34% 34% 34%
Carbonatation 80% 50% 50% 50% 20%
Acrylic 0% 92% 67% 93% 50%
Carbon 33% 40% 50% 72% 50%
Colours after Carbon
A
B
C
D
E
High Molecular Weight Low
Decolourisation Systems Comparison
0.00
2.00
4.00
6.00
8.00
10.00
12.00
14.00
16.00
18.00
20.00
0 5 10 15 20 25 30 35 40
0.00
2.00
4.00
6.00
8.00
10.00
12.00
14.00
16.00
18.00
20.00
0 5 10 15 20 25 30 35 40
Resin/Granular Carbon Bone Char
Processing Choices
RAW SUGAR
REFINED
SUGAR
IER
CRYSTAL-
LISATION
&
CENTRIFUGAL
SEPARATION
AFFINATION
CARBONATATION
PHOSPHATATION
GAC
PAC
50% 50% 50-80% 90% COLOUR
REMOVAL
ACROSS
PROCESS
Removal Chart
How do they Work?
Affination
Syrup
Raw
Sugar
Bin
Syrup
Overflow to
Recovery
To Melter
Wash
Water
Steam Weigher
44oC
How do they Work?
Affination
A large proportion of the Raw Sugar Colour is on the
surface syrup layer and the rest is included in the Crystal
It is easy in Affination to OVERWASH and dissolve
crystal rather than just remove surface syrup
Only two Variables that can be controlled by the Affination Process:
Magma Temperature
Wash Water Addition
How do they Work?
Affination
Magma Too Hot Too Much Crystal will be dissolved
Magma Too Cold Impurity not removed from Surface of Crystal
Magma Temperature Just Right All Surface Impurity Removed and No Crystal Dissolved
Around 44C is correct Magma Temperature
Magma Temperature needs to be measured not Green Syrup Temperature
How do they Work?
Affination
Syrup is a closed circuit with just an overflow to Recovery
The only way Impurity or Crystal can be dissolved is by the addition of Water at the Centrifuge
Typically aim for 72 brix and 84-88 purity to give 7% of Raw Sugar going into Green Syrup
How do they Work?
Carbonatation
How do they Work?
Carbonatation
CAPTURE of impurities so that they can be
filtered out of the syrup
REMOVAL of impurities by filtration
Note: In carbonatation the CaCO3 that is
formed is the filter aid
A SIMPLE SERIES OF REACTIONS:
CaO + H2O Ca(OH)2
Ca(OH)2 + CO2 CaCO3
Ca 2+ + imps Ca(imps) 2+
Ca(OH)2 + CO2 + Imps CaCO3(Imps)
How do they Work?
Carbonatation
Adding lime into the syrup does three things:
o Raises the pH
o Changes the ionic strength of the solution
o Changes the ionic environment of the impurities
The local environment of any dissolved species is significantly disturbed.
Many species are no longer soluble (or as soluble as they were)
Bubbling CO2 into the mixture causes precipitation of CaCO3 to occur
These crystallites provide nucleation sites for the co-precipitation of some of the impurities
Many of the high Mw species are acids at the higher pHs seen in carb they form anions which can then complex with Ca 2+
Liquor is a concentrated solution
There is barely enough water to dissolve the
sucrose so not much is left over to dissolve
anything else
Some impurities are already out of solution
(turbidity) or close to
precipitation
How do they Work?
Carbonatation
Filterability vs % Lime
0.00E+00
5.00E-08
1.00E-07
1.50E-07
2.00E-07
2.50E-07
3.00E-07
3.50E-07
4.00E-07
4.50E-07
0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2
Amount of lime (% CaO on brix)
F
F (Rock Lime) F (Powd Lime)
Final colour vs % LIme
600
650
700
750
800
850
900
950
0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2
% Lime
Fin
al
co
lou
r
Colour (RL) Colour (PL)
b
Large bubbles
Slower kinetics
Poor mixing
Less crystallites but larger
Lime
Quality Amount added
Method of addition
CO2 Gassing rate
Bubble size
Kinetics of absorption
Small bubbles
Faster kinetics
Propensity to form Foam
Lots of crystallites
How do they Work?
Carbonatation
Mixing
Rate determining step is the transfer of Ca(OH)2 to the surface of the gas bubbles
Transfer of CO2 through the phase boundary layer can become rate limiting
CO2 concentration and pressure of the gas can influence the CO2 absorption efficiency
pH
CO2
GAS
LIMED SYRUP
Phase boundary
layer
Ca 2+ We are titrating a basic solution with an acidic
gas.
Our reactor is continuous so every stage of the titration is represented within the vessel
How do they Work?
Carbonatation
How do they Work?
Phosphatation
How do they Work?
Phosphatation
Mechanism;
Precipitate formation: Phosphoric acid and Lime reacts to produce a large amount of calcium phosphate crystals.
Those crystals adsorb in their surface the colloids in suspension producing a primary floc.
A cationic decolorant (high molecular cationic polymer) can be used to capture negative charged colorants
Flocculant (high molecular anionic polymer) is added to coagulate primary flocs into bigger secondary flocs.
There are two steps in the process: Floc Formation and Floc Separation.
3Ca(OH)2 + 2 H3 PO4 = Ca3(PO)4 + H20
How do they Work?
Phosphatation
FC
FC
Ph
FC
Acid
Lime
Decol.
Flocculant
Air
TC
FC
Raw Liquor ScumsClarified
Liquor
Steam
Liquor
Heating
Chemical
Addition
Reaction
Primary
Floc
Formation
Aeration
Flocculant
Addition
Secondary
Floc
Formation
How do they Work?
Phosphatation
Cavitation:
A disc at the end of a hollow shaft rotates in the liquor producing microscopic air bubbles.
Dissolved air: (Scum Desweetening)
Compressed air is fed into the eye of the aeration pump impeller. The air blends with the liquor and dissolves under the pressure. It requires an aeration chamber or a pressurized tank that provides time for the air to dissolve. The pressure should be 70 to 100 psig. When the pressure is released the air comes out of solution in the form of microscopic bubbles.
The more air in the system the better the
performance of the clarifier.
How do they Work?
Phosphatation
The thickness and consistency of the scums bed is controlled by:
The weir box setting that regulates the level of liquor.
The speed of the scums rake that controls the scums removal.
Self Draining
How do they Work?
Phosphatation
Temperature : 85oC (176oF).
Lower temperatures increases the viscosity of the liquor.
Higher temperatures favours inversion and color formation.
Liquor Concentration: 63 to 65 brix.
Acid dose: 150 to 500 ppm P2O5
Ph on reaction tank: 7.0 to 7.5 Controlled by the addition of lime
Cationic Decolorant: 100 to 300 ppm active ingredient.
Flocculant: Max 10 ppm
How do they Work?
Phosphatation
How do they Work?
Activated Carbon
How do they Work?
Activated Carbon
10 millimetres
Macroscopic
Crack / Crevice
Graphite
plate
1000 angstroms
Graphitic Crystallite
Coal Particulates
Macroscopic Crack
1 millimetre
How do they Work?
Activated Carbon
pH Non-dissociated form is more strongly adsorbed.
Dissociated form behaves as competitor for adsorption space.
Temperature As temperature increases, capacity decreases
This may reduce capacity for the most volatile compounds 10-20%.
Particle Volume Pore size distribution has a dramatic influence on adsorptive performance. Removal of trace
levels of contaminants requires an extensive micropore volume.
Bed Depth
Increasing bed depth means the Mass Transfer Zone is a smaller percentage of the bed.
Flow Rate
Increasing the flow rate does reduce the efficiency.
How do they Work?
Activated Carbon
Rely on external surface area
Cannot normally be reactivated
Requires a precoat filter to be removed
Varying Quality of feed is accommodated
Low Capex, high opex option
Typical dose 0.05 to 0.5% on Sugar Solids
Contact time 20-30 mins
Solid effluent
Rely on internal surface area
Can be thermally or chemically re-activated
Requires on site regeneration
Varying Quality of feed is NOT accommodated
Requires large inventory of GAC
Typical burn rate 0.5 to 0.8% on Sugar Solids
Contact time 2-5 hrs
Liquid and Gaseous effluents
PAC GAC
How do they Work?
Activated Carbon
USCE - Egypt
How do they Work?
Ion Exchange Resins
How do they Work?
Ion Exchange Resins
Resin
+
Colour
-
Colour
- Colour
-
+ Resin
+
Colour
-
Colour
-
Colour
- + SUGAR
SYRUP
Cl-
Cl- Cl-
SUGAR
SYRUP
Colorant molecular weight Charge density Type of charge Degree of hydrophobicity pH Ionic strength of the medium.
Ion exchange: The colorants exhibit mostly an anionic behavior at alkaline pH and thereby they can be exchanged against the mobile chloride ions. However this mechanism is not the only one in color removal.
How do they Work?
Ion Exchange Resins
Steric effect
Porosity of the decolorizing media is a key parameter. This illustrates why the decolorization of sugar juices is carried on at relatively low flow rate.
Hydrophobic effect
Polymeric adsorbents have a polarity. The colorants are basically hydrophobic (not highly soluble in water) and will tend to be adsorbed on the hydrophobic part of the adsorption media.
Van der Waals forces effect
These are attractive forces between chemical groups in contact. They result from a temporary dipole formation.
Hydrogen bonds
It is an electrostatic attraction that occurs between molecules in which hydrogen is in a covalent bond with a highly electronegative element.
How do they Work?
Ion Exchange Resins
Anionic Resins used in sugar have usually a quaternary ammonium functional group and are in Chloride form.
Styrenic
Has aromatic groups in the structure
Selective for colour but low regeneration
efficiency
Acrylic
Is mostly aliphatic
Less selective but easier
regeneration
Acrylic Macroporous Styrenic Macroporous
Typical Decolorization 50-60% 65-75%
Regeneration Efficiency Excellent Good
Regenerant 10% NaCl 10% NaCl + 0.5% NaOH
Matrix Aliphatic Aromatic
Max Feed Colour IU 2500 800
Colour Loading BVIUs 35000 25000
Cost High Medium
How do they Work?
Ion Exchange Resins
LARGE EFFLUENT VOLUME
Step Volume
BV Flow
(BV/hr) Temperature
(C) Material In Material Out
Sweet off 1 1 2-4 60-80 Hot Water Liquor Feed Tank
Sweet off 2 1 2-4 60-80 Hot Water Sweetwater Tank
Backwash Lower Bed 1.25 2-4 60-80 Recovered Water Effluent
Backwash Top Bed 1.25 2-4 60-80 Recovered Water Effluent
Caustic Regen 1 0.64 1-2 60-80 Reclaimed/Fresh Brine Reclaim Water Tank
Caustic Regen 2 0.58 1-2 60-80 Reclaimed/Fresh Brine Effluent
Caustic Regen 3 0.43 1-2 60-80 Reclaimed/Fresh Brine Effluent
Rinse 1 1 2 60-80 Reclaim Water Effluent
Rinse 2 1.5 2 60-80 Fresh Water Effluent
Rinse 3 1.5 2 60-80 Fresh Water Reclaim Water Tank
Sweet On 1 0.6 2-4 70-80 Feed Liquor Reclaim Water Tank
Sweet On 2 1 2-4 70-80 Feed Liquor Sweetwater Tank
Service 2-4 70-80
How do they Work?
Ion Exchange Resins
OPTIM UM REM OVAL PROFILE
-2
0
2
4
6
8
10
12
14
30 40 50 60 70 80 90 100 110 120
DS
NaCl
Other (COLOUR)
How do they Work?
Brine Recovery
What does Nanofiltration do?
It removes colour from the regeneration effluent (brine) in order to
reuse it.
Why is that important?
To save money: reduce Salt, Caustic and water consumption.
Reduces Chloride to effluent
Key NF figures:
Typical: 75 % decolourisation, 85 % recovery
Feed
Permeate
Concentrate
Membrane
How do they Work?
Brine Recovery
How do they Work?
Brine Recovery
Na+ Cl-
Na+ Cl-
Na+ Cl-
Colour
-Charged
Colour
-Charged
Colour
-Charged
Na+
Na+
Na+
Na+ Cl-
Na+ Cl-
Regeneration Effluent Nanofilter
Na+ Cl-
Na+ Cl-
Na+ Cl-
Na+ Cl-
Na+ Cl-
Reclaimed Brine
Feed & Concentrate
mixture
Nanofilter Permeate
High pressure side Trans Membrane Pressure
(TMP)
Low pressure side
How do they Work?
Brine Recovery
How do they Work?
Brine Recovery
How do they Work?
Primary Decolourisation Comparisons
Phosphatation operates at higher Brix (65) compared to Carbonatation, so less steam used, and less combustible fuel
Carbonatation requires double filtration, phosphatation does not Phosphatation uses less power than carbonatation Phosphatation is more flexible than carbonatation, especially on flow
capacities
Carbonatation has a more capital intensive cost Docolourisations similar percentage Operational costs similar
How do they Work?
Secondary Decolourisation Comparisons
Some Key Points
Carbon decolourisers are superior in removing impurities & flavenoids
GAC & Bone Char have air emissions
IER has waste water emissions
GAC has traditionally had the lowest operating cost
GAC & PAC do not have de-ashing capabilities
IER and Bone Char do have de-ashing capabilities
Bone char uses 90% more energy than GAC for the same decolourisation
IER has the option of membrane treatment to recover 90% of the salt and drastically reduce waste emissions
Phosphatation + IER Low Capex & Low Opex
Phosphatation + GAC Low/High Capex & Low Opex
Phosphatation + PAC Low Capex & High Opex
Carbonatation + IER Mid Capex & Low Opex
Carbonatation + GAC High Capex & Low Opex
Carbonatation + PAC High Capex & High Opex
Conclusions
Summary of Decolourisation Process Options
References
Robert Albright, Albright Consulting - Architecture and App.ppt
Cane Sugar Refining
with Ion Exchange Resins - Purolite
Brad Ahlgren, Calgon Carbon Corp. - Carbon 101.ppt
Norit - Introduction to the purification of
liquid
sugar with Norit Activated Carbon
Colour
1) The chemistry of colour removal: a processing perspective SB Davis Proc S Afr Sug Technol Ass (2001) 75
2) Separation and identification of sugar colourant (TM Letcher and PG Whitehead ) Proc S Afr Sug Technol Ass (1996) 70
Comparison of methods
1) A Comparative evaluation of Carbonatation and Phosphatation (AS Vawda) SIT 940
2) Pros and Cons of various decolorisation processes for production of refined sugar SIT Savannah May 2010
3) Removal of colour in sugar cane juice clarification by defecation, sulfiation and carbonation
4) Analysis of Refinery Clarification Processses (TLPT) July 2004
T&L HydraCoRe 70pHT Presentation, Thames Jan 2006.ppt
THE PEOPLE BEHIND SUGAR
THE PEOPLE
BEHIND SUGAR
Thank You!
To ensure the most efficient and effective
refinery, with maximum output and minimized
energy use and environmental impact, or just to
get the best out of your upgrade and refit, talk
to the people behind sugar.
