Ethanol and Sugar Production - AdvantagesBy Bento, Luis San Miguel
Posted on 2009-10-14 Last edited on 2010-08-04
Advantages in processing white sugar when sugar cane factories are associated
with ethanol plants
(Paper presented in the CORM Carbohydrates as Organic Raw Materials - Building a Sustainable Future 5th meeting, Lisbon, January 2009)
Cane sugar industry utilizes sugar cane to produce raw sugar in mills. Raw sugar is then refined to white sugar, in separate unities. Some sugar mills produce ethanol from molasses or from juice in attached ethanol plants, jointly with white sugar. In this case, the sugar processing can be changed in order to have a more efficient, more economical and less pollutant process. In this paper are presented the changes that ethanol production allows to the sugar processing. Some of these changes, as separating the low purity juices for ethanol production, are already applied. However other changes, as utilization of ethanol as a regeneration chemical in decolourizers systems are not applied nowadays. These changes will open the possibility to produce white sugar directly in cane mill with a much more economical process. Some of the changes presented here can also be applied in beet sugar industry.
White sugar is industrially produced from two sources: sugar cane and sugar beet. Although the final product is very similar, processing techniques are quite different. In sugar beet the process is accomplished in only one step where white sugar is produced directly from beets. On contrary, in sugar cane industry, the process is done in two steps. In sugar mills sugar is extracted from cane and a raw sugar is produced. This raw sugar is afterwards refined to white sugar in independent refineries. In some sugar mills the refinery is installed jointly to the mill. Other mills have a white end section where it is produced plantation white sugar with a lower quality than refined sugar.
The main reason for this difference between cane and beet industry are the non sugars present in respective juices. The main impurities responsible for process difficulties in cane are sugar colourants and polysaccharides.
In this paper we will try to prove that in a cane sugar mill producing sugar and ethanol, it will be possible to produce, directly in the mill, white sugar with high quality. The main processing changes discussed are: juice separation in the mill tandem; decolourization of clarified juice with GAC (granular activated carbon) regenerated with ethanol; fine syrup softening and decolourization with resins regenerated with sccharate process.
2. CANE JUICE COLOURANTS
Sugar colourants are one of the most important impurities in sugar industry. Generally colourants are divided in two categories: colourants produced by degradation of hexoses (melanoidins, HADPhexoses degradation products and caramels) and those presented in sugar cane, as cane pigments and phenolic compounds that can form high molecular weight colourants during cane sugar processing.
Phenolic acids occurring in sugar cane are usually derived from cinamic acids (Farber e Carpenter, 1972). They are the building blocks of flavonoids.
Phenolic acids are normally linked to alcohols, to other phenols or to polysaccharides by ester bonds. An example of this esterification is the one that occurs in CWP Cell Wall Polysaccharides, composed by polysaccharides, as arabinoxylan with glucoronic residues esterified with ferulic acids. These high molecular complexes exits in cells membranes and have a plant protection function, against external aggressions (insects, cuttings, etc.) (Clarke et al., 1988). Being soluble and with a low anionic charge, these compounds are not removed in most factory and refining processes, appearing in the white sugar (Clarke et al., 1988). Due to their high molecular weight CWP cause great difficulties in sugar processing, increasing sugar products viscosity, poisoning resins, causing difficulties in crystallization and massecuite purging in centrifugals. Also, they decrease refining process efficiency and increase energetic consumptions and affect sugar quality (Clarke et al., 1988).
Phenolic acids esterified with polysaccharides, when undergoing an enzimatic browning reaction, may contribute to the formation of cross-links bewteen polysaccharides, resulting in very high molecular weight complexes with high colour. An example of this kind of reaction is the enzimatic oxidation of ferulic acid in CWP (Clarke et al., 1988) (Figure 1). These compounds have a high molecular weight, are difficult to remove and have tendency to enter the crystal during boiling (P. Smith, et al., 1981).
Figure 1: Enzymatic oxidation of CWP and their degradation (Clarke et al., 1988)
This enzymatic browning reaction occurs during the first part of sugar production, in the mills during extraction. After this step enzymes are denaturated due to the high temperatures and pH in clarification.
Other colour formation reaction occurring in sugar mills during extraction involves phenolic compounds and iron. Iron soluble in juices may form complexes with anthocianins, as with most of phenolic compounds forming dark colourants of great stability (P. Smith and Paton, 1985; Riffer, 1988). This reaction may happen whenever sugar solutions are in contact with iron, particularly during sugar extraction in mills.
Most of the browning products from sugar beet, are insoluble in water (Wyse, 1971; Paton, 1992). This can be the main reason why beet white sugar can be produced directly from beet syrups and cane sugar not.
3. JUICE SEPERATION IN THE MILL TANDEM
Colourants in cane juice, before extraction, are mainly phenolic compounds free or bonded to polysaccharides. Cane juice colour is yellowish, varying from pale to strong yellow, with colours between 4,000 IU and 20,000 IU (ICUMSA Unities absobancy at 420 nm; pH 7.0) and an IV between 10 and 33 (Indicator Value quocient of colour at pH 9.0 and colour at pH 4.0, at 420 nm). This colour is influenced by phenolic compounds, as the high IV suggests.
During milling extraction, besides sugar, polysaccharides (originates from cane or formed after harvesting) and cane pigments including flavonoids and other phenolic compounds are also extracted. In the mill tandem colour and flavonoids increases from the first mill to the last one (Paton, 1992). Along the mill, flavonoids concentration increases from 50 ppm - 150 ppm, on solids, to values until 2,000 ppm in the last mill (P. Smith and Paton, 1985).
An example of colourants through the tandem is presented at Table 1. As a result of colour increase through the mill, the mixed juice has more colourants than the first expressed juice. P. Smith et al, 1981, reported a colour increase, from first expressed juice to mixed juice, of 66% on first expressed juice colour
Table 1 Colours (pH 7) along the mill tandem (P. Smith et al, 1981)
1st expressed juice 11,100
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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