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Resins - Decolourization

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By Bento, Luis San Miguel
Posted on 2006-09-20    Last edited on 2009-09-23

The application of ion exchange resins as main decolourizer in sugar industry has began in the seventies of last century with the introduction of macroreticular resins in the market. After then, with the introduction of new resins, with equipments completely automatic and improved, with a better knowledge of colourants nature and inter-action with resins, and with new regeneration processes and effluents treatment, resins application has become one of the best decolourizer technique known.

For sugar decolourization the main resins used are strong base anionic resins with ammonium quaternary poles bond ionically to chloride ions. Resin matrix can be of two types: polystyrenic divinil benzenic and acrylic. This last resin type, although present some advantages regarding poisoning resistance, present a lower decolourization capacity and for food applications they must work at temperatures lower than 76ºC. At higher temperatures there is the risk of acrilates release from resin matrix.

Resina_estirenica

Macroreticular polystyrenic resins are formed by micro spheres in agglomerates of 0,75mm. These resins present a great physical resistance and an excellent decolourizing capacity. Their characteristics are (Rohm + Haas, 1997):


Medium size
650µm – 820 µm

Density 1,050 – 1,080 (in Cl- form)

Ion exchange capacity
1 equivalent/liter (minimal)

The percentage of divinil-bezen in the resin, that is, the percentage of cross linkage, is important for resin functionality. A high cross link degree make the resin with low porosity, decreasing the accessibility of colourants to ion exchange centers. On the contrary, a low cross link degree make the resin mechanically too fragile. The percentage of divinl-benzen normally used, in strong base polyestirenic resins is 4% a 6% (Marhol, 1982).

Resins are normally placed in vertical columns where liquor flows in an up or down flow. The contact time is between 2 and 3 BV / hour. The application of resins at industrial level is described in the publication: “Ion Exchange Resins in the Sugar Industry” (Bento, 1997).

In sugar decolourizatin, resins works in three steps:

1. Decolourization - liquor flows through resin bed.
2. Regeneration – includes: desweetning, washing, decompactation, regenerant flow through resin, final washing.
3. Stoppage.

The aim of regeneration is the removal, as much as possible, of colourants fixed to the resin, in order to put the resin near its initial capacity. This removal is done by exchange the coloured anions by the regenerant chloride ions, and by the removal of colourants fixed hydrophobicaly to the resin matrix.

In order to make maximum utilization of resins, during decolourization and regeneration, it is necessary to understand the mechanisms involving the colourants fixation to resins.

The great majority of cane colourants presents an amphiphilic character, with a part polar and other apolar. The fixation of these compounds to resins can be done through different mechanisms:

- by ionic bond between the ammonium quaternarium pole, fixed to the resin and the anionic colourant;
- by hydrophobic inter-action between the colourant apolar part and the resin matrix;
- by weak bonds as by van der Waals forces.

These mechanisms can act separately or in conjunction within the same colourant molecule (Williams and Bhardwaj, 1988). Another phenomenon that can occurs is the switch between fixation mechanisms, due to the variation on concentration of the solution that surrounds the resin. This fact occurs mainly during resin regeneration when NaCl concentration passes from 100g/l to almost 0g/l (Bento, 1992).

Fixation of colourants by hydrophobic inter-action, depends on resin matrix nature. Styrenic resins present a hydrophobic matrix while acrylic resins have a more polar one. This difference is noted when resins are placed in sugar solutions containing sodium iodite, nitrate or chloride. In these conditions, it is observed that when salt concentration increases, there is a complete decolourization inhibition when acrylic resins are used. When styrenic resins are used, it is observed some decolourization, even at high salt concentrations (Figure). This is due to the fixation of colourants to the resin matrix, by hydrophobic inter-action (Bento, 1992).

Resina_acrilica_estirenica


Bibliography

Bento L.S.M., 1992, Organic and Inorganic compounds influence on the sugar colourant - ion exchange
          resin inter-action, Proc. of S.I.T. Conf., 201-220
Bento L.S.M., 1997, Ion exchange resins for sugar liquors decolourization, Proc. of S.I.T. Conf., 251-271
Marhol M., 1982, Ion exchangers in analytical chemistry. Their properties and use in inorganic chemistry,
          Comprehensive Analytical Chemistry, Ed. Svehla G., Pub. Elsevier Sc. Pub. Co., Vol. XIV
WilliamJ.C., Bhardwaj C.L., 1988, The use of HPLC to investigate the mechanism of resin decolorization,
          Proc. of S.P.R.I. Conf., 37-61


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Decolorization (ion exchange)