Cane Sugar Refining - Purolite

Decolorization with Ion Exchange Resins
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Cane Sugar Refining with Ion Exchange Resins
Ion exchange resins used in the sugar industry as decolorizers are of the strong-base
anionic type, with quaternary amine functional groups. They are operated in the
chloride form.
The resin matrix is usually composed of one of two types of polymeric material:
polystyrenic with divinylbenzene crosslinks, resulting in a hydrophobic matrix, or
a polyacrylic with divinylbenzene crosslinks which results in a more hydrophilic matrix.
Some refineries, for the major decolorization, use only one resin type, acrylic or
styrenic. Others employ an acrylic resin followed by a styrenic resin for polishing.
The acrylic resin removes large molecular weight colored compounds which tend to
foul the styrenic resin, providing good protection to the polishing resin.
Acrylic resins have a slightly lower selectivity for sugar colorants but, unlike
polystyrenic resins, they can be completely regenerated with sodium chloride solutions.
This can be explained by the more hydrophilic character of the acrylic resin matrix.
Even dark regenerant effluents from styrenic resins can be used to regenerate acrylic
resins. The use of acrylic resin is justified when the color load is high and a low level of
decolorization may be expected. Also, the use of a two pass system gives a more constant
level of decolorization than when only one resin bed is used.
Styrenic resins are more prone to fouling by organic compounds than acrylic resins
and need occasional acid regeneration. Also, their decolorization capacity is higher than
that of acrylic resins, but the color is not so efficiently removed during regeneration and
capacity can drop rapidly if overloaded.
Styrenic resins have a higher decolorization power because of their ability to fix
colorants both through ionic bonds to the ionic active groups and through hydrophobic
interactions with the resin matrix. Hydrophobic interactions increase with salt
concentration, explaining the difficulty of using salt solutions to remove colorants fixed
to the resin matrix.
Polystyrenic resins with an aromatic matrix should have a higher affinity for
unsaturated colorants than acrylic resins.
Styrenic resin types are favored when the inlet color is on the low end (i.e. up to
800 ICUMSA) and acrylic types when it is higher. Numerous plants today operate
with a combination of both types, which is advisable when the inlet feed color fluctuates
during the season.