During milling there is a decrease of chlorogenic acids, neutral phenols and flavonoids indicating the formation of enzymatic browning products. According to Goodacre and Coombs, 1978, this reaction contributes to more than half of colour in cane juices.
The juice from the first mill in the tandem is then more pure, with more sugar and less colourants, than the second expressed juice. Therefore, separation of these juices, to produce sugar from the first extracted juice and ethanol from the second extracted juice, is highly advantageous. Separation of the juices from the last mills to produce ethanol, as is done in Brazil, will:
- decrease colour of juice entering the clarifyer;
- sugar production will be more economical;
- less energy is consumed;
- sugar quality increases.
4. JUICE DECOLOURIZATION WITH GAC
GAC is a powerful product for removal of colorants from sugar solutions. However, its application in the sugar industry is limited by the necessity for thermal regeneration. For sugar cane mills, not running all the year it is not economically viable to install a very expensive regeneration kiln to treat the used carbon. Chemical regeneration may be a solution to overcome this problem.
A chemical regeneration process was developed and tried in a pilot plant scale in a cane sugar mill. This process to regenerate GAC uses ethanol, NaOH and H2O2 ((Patent pending; Bento, 2006). This mixture proved to be more efficient than sodium hydroxide regeneration or even the mixture of sodium hydroxide and ethanol.
In a trial at a Louisiana sugar mill (Rein et al, 2006) this chemical regeneration was applied to treat clarified juice. Chemical regeneration did not achieved the efficiency of thermal treatment but its application will maintain the GAC operation for a period of cycles enough to keep the carbon working during all the sugar season. After the season carbon can be replaced or sent to a kiln to be re-activated.
Figure2: Softening and decolourization with GAC and resins in a cane sugar mill
The application of this chemical regeneration in sugar mills producing sugar and ethanol will be attractive. High pure juice, from first mill, will be clarified and concentrated to 50 – 60 obrix. This syrup will be clarified again and decolourized in GAC columns. Ethanol, from one intermediate production phase from the Ethanol Plant, will be used for carbon regeneration, according to the referred regeneration process. The resulting colored effluent from GAC regeneration, containing ethanol, will be mixed with juice or molasses before fermentation or fed directly to distillation (Figure 2). Colourants in the effluent have the same chemical nature as colorants in molasses and therefore they will not affect the fermentation process.
The advantages of GAC decolourization in a sugar mill with an ethanol plant are:
- possibility of GAC regeneration using chemicals without the necessity of thermal regeneration with a kiln;
- no carbon losses due to handling (in chemical regeneration carbon is not moved from the columns as in kiln regeneration);
- less energy consumption (energy for kiln regeneration is avoided);
- no liquid effluents (regeneration effluents are sent to the Ethanol Plant);
- white sugar quality will improve.
5. SYRUP SOFTENING AND DECOLOURIZATION USING RESINS WITH SACCHARATE REGENERATION PROCESS
Colourants and calcium are harmful impurities in fine liquors, before crystallization. Calcium form complexes with colourants and polysaccharides and will enhance colour inclusion into sugar crystals. Roge et al., 2007, refer that macromolecules complexing calcium are very likely present as inclusions inside the crystals. Therefore, removal of calcium before crystallization is important to obtain low colour sugar.
Figure 3 – Comparison of colourants desorption at increase NaCl concentration in normal (low) and saccharate regeneration (up)
A process to remove calcium and colourants from sugar solutions using ion exchange resins regenerated by the saccharate process was developed (using NaCl, Ca(OH)2 and sucrose; Patent pending; Bento, 2001). Anionic resins used to decolorize sugar liquors, are efficiently regenerated using calcium chloride in an alkaline calcium saccharate solution (Bento, 1996) . An explanation for this high efficiency is the formation of a complex between colourants, calcium and sucrose. The formation of this complex can dislocate the regeneration reaction allowing colourants removal even at low chloride concentration (Figure 3: 10 g/L instead of 100 g/L of NaCl, at same regeneration performance). The saccharate mixture was used to regenerate anionic and cationic resins in the same column.The idea behind the utilization of saccharate regeneration for sugar liquors decolourization and decalcification is the following: if the presence of calcium and sucrose helps to remove efficiently colourants fixed to anionic resins; the presence of colourants and sucrose will help to remove efficiently calcium fixed to cationic resins (Figure 4). This process was applied in a mixed bed column with a mixture of anionic and cationc strong base resin to treat carbonated liquor (Bento, 2001). Results are presented in Table 2.
Figure 4: Regeneration of anionic and cationic resins with the saccharate proces.
This treatment can be applied to syrup after GAC in a sugar mill (Figure 2). Resulting fine syrup will be concentrated and crystallized to produce white sugar. Regeneration effluents, containing sucrose, can be sent to the ethanol plant. As the concentration of salts are reduced the mixture of these effluents with molasses or cane juice do not alter significantly the total amount of inorganic compounds in the input of the ethanol plant. In this way sucrose used in the saccharate regeneration is not lost.
Table 2 – Results of softening and decolourization liquor using resins with the saccharate process
Calcium in ppm of Ca on dry solids
The advantages of applying softening and decolourizer resins using the saccharate regeneration process in a cane sugar mill with an ethanol plant are:
- advantages of calcium removal:
· calcium can form insoluble compounds that can form incrustations in heating surfaces;
· calcium enhance colourants affinity to sugar crystals;
- an extra decolourization after GAC will produce syrup colours and high quality white sugars;
- no sugar losses – sucrose used in the saccharate process will be converted to ethanol in the Ethanol Plant;
- no liquid effluents – regeneration effluents will be sent to the Ethanol Plant.
Ethanol can be used to remove colourants fixed irreversibly to styrenic anionic resins. In a test with one liter resin column after a cycle of 50 BV of carbonated liquor resin was regenerated using 3 BV of NaCl at 100 g/L followed by 1 BV of a mixture of NaCl at 100 g/L with 20 % v/v of ethanol (Bento, 1992). It was observed that in the second part of regeneration, with salt and ethanol, a great amount of colourants was removed. This quantity was more than with the normal regeneration, the first part. This process can also be used in a sugar plant attached with an ethanol plant as the ethanol in the regeneration effluents can be recovered in the Ethanol Plant.
Cane sugar mills with attached ethanol plants can produce high quality white sugar. Sugar process in the mills must be changed to profit the proximity of the ethanol plant and to improve sugar quality. The proposed process changes are: juices separation; syrup clarification, decolourization using GAC with chemical regeneration and softening using resins with the saccharate regeneration process.
With these changes final syrup quality will have a suitable quality to produce a high quality white sugar. Moreover, this process will be more economical, less pollutant and with less energy consumption than the usual process.
These economical advantages must be considered when comparing sugar cane with other crops for ethanol production.
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