Saccharate Regeneration - Decolourization Resins

 It was proved that calcium ions in a alkaline sucrose solution, enhance colourants removal from styrenic anionic resins (Bento, 1996a).
In a test of colourants removal from styrenic resins with various regenerants, the mixture of calcium chloride, calcium hydroxide and sucrose (saccharate solution), is more efficient than an alkaline sodium chloride solution, at same chloride concentration. The presence of sucrose is necessary as it improves the solubility of calcium in alkaline solutions (Silin, 1964).
In order to obtain the same quantity of colourants removal when an alkaline sodium chloride solution at 100 g/l of Cl- is used, it is necessary a concentration 10 g/l of chloride ions, when sacharate solution is used (Figure1). However, in practice a solution of 35 g/l of Cl-  is used, to profit the maximum possibilities of this regeneration process. By this way it is possible to make an efficient regeneration of styrenic resins at low chloride concentrations.

Figure 1: Colourants desorption from styrenic resins

The possible formation of a complex between colourants, calcium and sucrose, can explain the efficiency increase of this regeneration process

With this principle, a new regeneration system was developed, using a solution of calcium chloride (35 g/l), sucrose (130 g/l) and calcium hydroxide (7g/l de CaO) (Bento 1996b). This process was patented in Europe (Patent nº EP 0 840 805 B1 - 19.12.1995) and in U.S.A. (Patent nº 5,932,106 - Aug.3, 1999).
During 200 cycles of liquor treatment, resin decolourization capacity was maintained at 90%, identical to the initial capacity (Bento and Correia, 2000) (Figure 2). If a normal regeneration, with alkaline sodium chloride, was applied, capacity would have decreased to 60%, with the same number of working cycles.

The decrease of resin poisoning, when Saccharate regeneration is used, can be explained by the simultaneous removal of both colourants, fixed ionically and by hydrophobic inter-action.

Resin in a high concentration of salt shrinks, due to water molecules release, from the interior of the resin, due to osmotic pressure. This change of volume can provokes a resin deterioration at each NaCl regeneration. With Saccharate Regeneration this volume change does not occur. This fact can explain the increase of resin life when this regeneration is applied.

    Figure 2: Liquor decolourization by resins with Saccharate
    Regeneration

Summarizing, the advantages of Saccharate Regeneration process, in comparison with classic regeneration, are:
-    resins decolourization capacity is practically maintained through its life;
-    resins life increase;
-    effluents from this regeneration contain a lower concentration of chloride ions.

The sucrose contained in low purity syrups or in low brix solutions, as in sweet water, can be used to prepare the regenerant to this regeneration process. Therefore the chemicals cost will be lower than when white sugar is used.  However, processes for effluents recovery were tried: tangential filtration and carbonatation.

In the process to recover regenertion effluents with tangential filtration, a tubular membrane of 4000D was used. A permeate flow of 20 to 31 l/m2/h was obtained (Bento, 1996b). In the test done, a concentration of 5 was achieved (feed volume / retentate volume). With an effluent with an atteunance of 8360 (at 420nm pH 9.0) it was obtained a permeate with 2020 of attenuance, representing a decolourization of 53%. The permeate, after chemical make-up, was used in the following regenerations. The retentate can be mixed with refinery low purity syrups of Recovery section. 

Another process tried to treat Saccharatev Regeneration effluents was their carbonatation (Bento and Correia, 2000).
In this treatment, regeneration effluent is divided in three parts (Figure 3):
-    EFL1 (1BV) this first part of the effluent presents low salt concentration and is treated biologically before be sent to effluent treatment stations (Guimarães et al., 1999);
-    EFL2 (2BV) this part of th effluent is treated with carbonatation and, after that, it is used in next regenerations;
-    EFL3 (1BV) this is the final part of the effluent, with a low colour, and is used for next regeneration, after chemicals make-up.

Figure 3: Resins effluents treatment

In trials made in a one liter resin column, from cycle 165 to cycle 200, effluents were divided as described and carbonatation treatment was made to EFL2 effluent. This treatment consisted in an addition of calcium hydroxide to the effluent, 4g/l, and bubbling of CO2 till a pH 8,5. The formed precipitate was filtered and filtrate was used in next regeneration. Effluent attenuance decreased from 10948 to 6734, representing an average decolourization of 37,1% .
This treatment to the effluent EFL2 was made in consecutive cycles during seven to ten cycles. After each of these periods new regenerant was prepared and a new group of treatment was made.
With this process, a reduction of about 50% of effluents discharged was obtained, in comparison with classic salt regeneration.

The chemicals consumption with Saccharate Regeneration is presented in Table 1 (Bento, 1999b). As it is observed, this regeneration presents a chemicals cost 28% lower than classic regeneration with NaCl. Besides that, there is a 76% chloride reduction in effluents discharged (from 182kg to 44kg / m3 of resin).

Table 1 – Comparison between NaCl regeneration and Saccharate Regeneration

As Saccharate Regeneration use a lower concentration of chlorides, resulting effluents contain a lower salinity. In some cases these effluents can be returned to sugar processing streams. This procedure was applied in a beet sugar factory of Inner Mongolia, P.R. of China. In this factory C sugar was melted and decolourized with resins regenerated with Saccharate Regeneration process. Effluents from this regeneration were mixed with beet juice before Carbonatation. The majority of colourants in C sugar are melanoidins, formed in high concentrated syrups and massecuites (Paton, 1994). These colourants are easily removed by resins (Guimarães et al., 1999) and easily precipitated in carbonatation (Bento and Correia, 2000).
In this process, as used in China, colourants (mainly melanoidins) are moved from Crystallization (C sugar) to the processes were they are more easily removed (resins and carbonatation).
This process was industrially applied during 62 days. A decolourization of 50% in C sugar was obtained and a colourants precipitation of 70% in carbonatation was achieved. With this application, white sugar quality was improved, circulation of syrups in Crystallization decreased with a reduction in energy consumption.

Recently, Saccharate Regeneration process was simplified by the application of a mixture of calcium chloride and sodium hydroxide . In this process, filtration of regenerant solution is not necessary, making the process simpler.     

In Table 2 a comparison of chemicals consumption for the different regenerations, is made.

Table 2 – Comparison between regeneration systems

I cane sugar mills attached to an ethanol plant, this process can be used with advantages. Effluents with sacharate solution and colurants can be sent to the ethanol plant. Sucrose contained in the efluents will be used to produce ethanol. Doing this, the resin decolourization process will be free of effluents!

Bibliography

 Bento L.S.M. (1996a) “Regeneration of decolorization ion-exchange  resins - a
              new approach”  Proc. of S.I.T. Conf.
Bento L.S.M. (1996b) “Sugar Colourants and ion exchange resins: Influence of
              calcium and sucrose in sugar colourants removal from ion-exchange
              resins”  Proc. of  S.P.R.I. Conf.
Bento L.S.M. (1999) “Utilization of calcium chloride in an alkaline saccharate
              solution to regenerate ion exchange resins for cane sugar liquors
              decolourisation” Proc. of C.I.T.S. Conf.
Bento L.S.M., Correia C. (2000) “Regeneration of a styrenic divinylbenzenic
              resin for sugar decolourisation with calcium chloride in a calcium
              saccharate solution” Ion Exchange in The Millenium, Ed. J.A. Greig,
              Imperial Colledge Press, Cambridge, UK, 298-305
Silin P.M., (1958) “Technology of Beet-Sugar Production and Refining” PPI
   Pishchepromizdat, Moskva 1958, Israel Program for Scientific
Translations, Jerusalem 1964