Electrodialysis (ED) as a technique is known from the 1950's and it is widely used for example in desalting of water and whey and within the inorganic chemical industry e.g. for recovering organic acids from solutions. Desalting of sugar cane or sugar beet solutions via ED has been established on 1960's to 80's in various patent publications. Electrodialysis separates salts from a sugar solution using alternative cation and anion exchange membranes. This is done by passing a direct current through a membrane stack, causing the anions to move through the anion exchange membrane and the cations through the cation exchange membrane. The cations cannot move through the anion exchange membrane.
U.S. Pat. No. 3,799,806 discloses a process for the purification and clarification of sugar juices, involving ultrafiltration followed by purification with electrodialysis. Sugar is separated by crystallization from the purified juice.
U.S. Pat. No. 3,781,174 discloses a continuous process for producing refined sugar from juice extracted from sugarcane. This process comprises further removing the impurities and colouring matter by using a combination of ion-exchange resin and ion-exchange membrane electrodialysis, concentrating the purified juice and crystallizing the concentrated juice to form refined sugar.
U.S. Pat. No. 4,331,483 discloses a process for purifying beet juice by contacting the juice to be purified with at least two ion exchangers formed of a porous mineral support covered with a film of cross-linked polymer containing or bearing quaternary ammonium salt groups for at least one of the ion exchangers and sulfone groups for at least one of the other ion exchangers. The ion exchange is used for removing proteins, amino acids and betaine. Further, the purified juice might be demineralized by ion exchange or electrodialysis. Sugar is then separated by crystallization from the purified juice.
U.S. Pat. No. 4,083,732 discloses a method of treating fresh sugar juice at about room temperature which includes removing non-sugar impurities, concentrating the resulting cold, water white juice by reverse osmosis to form a syrup which is evaporated to form direct white sugar and edible molasses. Also a method of removing ions from the syrup by electrodialysis to produce edible molasses is disclosed.
Thus, electrodialysis is well known as a method for desalinating sugar cane syrup or molasses of a relatively high concentration. In case of sugar syrup or molasses, however, it has been considered defective in that organic non-sugar contents would adhere to and precipitate on the anion exchange film and make cleaning of films difficult. A method for the reduction of fouling by the precipitation of calcium and silicon before electrodialysis is disclosed in U.S. Pat. No. 4,492,601. It describes a process for clarifying and desalinating sugar cane syrup or molasses, wherein inorganic oxy-acid and organic acid impurities are removed from raw sugar cane or molasses solutions by the steps of (1) admixing with the raw sugar cane syrup or molasses solution a water-soluble chloride of an alkaline earth metal ion which reacts with inorganic oxy-acid anions and radicals and with organic acids to form a water-insoluble precipitate of said oxy-acid anions and radicals and organic acids, (2) separating said precipitate from said solution, (3) diluting the precipitate-free solution, and (4) subjecting said diluted solution to an electrodialysis using cation exchange film and neutral film arranged in an alternating manner.
However, ED has not commonly been used until late 1990's in sugar industry due to its high capital costs and due to fouling problems caused by anion products removed by ED from molasses. Various extensive pre-treatment methods to overcome the fouling problem have been patented, e.g. U.S. Pat. No. 4,711,722 and JP 58-082124.
The development of fouling resistant and high temperature resistant anion exchange membranes and the design of electrodialysis stacks has facilitated the economical use of ED in the sugar industry. Eurodia Industrie S.A. has established commercially viable ED technology for desalting of cane molasses, sugar beet syrup and liquid sugar. Lutin describes electrodialysis as a purification technology in the sugar industry especially to partially replace ion exchange resins for the demineralization and purification of sugar syrups (Zuckerindustrie 125, No 12, pp. 982-984, 2000 by Lutin). It should be noted that ion exchange technology does not provide an identical result to ED and that the regeneration of ion exchange resins necessarily involves the use of strong acids and bases while the ED resins are easily cleaned occasionally by an acid wash followed by an alkali wash with less chemicals than in ion exchange.
Further, alkali metal cations have been suspected of being highly melassigenic by holding sugar in the molasses and preventing it from being recovered as crystalline sugar. Elmidaoui et al. (Elsevier, Desalination 148, 2002, pp. 143-148) describe the removal of melassigenic ions especially Na+, K+ and Ca2+ for beet sugar syrups by electrodialysis using an anion-exchange membrane.
However, none of the above-mentioned prior art discloses a process wherein chromatographic separation is utilized.
Chromatographic separation has been used in the sugar industry e.g. to recover sucrose, betaine and/or raffinose from sugar solutions, such as molasses. U.S. Pat. Nos. 5,795,398 and 6,224,776 describe prior art processes for such recovery.
The article “New technologies in the sugar industry” by Matild Eszterle (Cukoripar liv, vol 54, (2001) No 1, pp 4-10) discloses separation techniques used in sugar industry including chromatography and electrodialysis. These techniques are disclosed as alternatives for the purification of sugar juices. This article does not disclose any specific combination of these techniques and it is only directed to provide a method which would decrease the amount of energy consuming crystallization steps.
U.S. Pat. No. 6,406,547 discloses a process for producing sugar from beets comprising multiple steps including two separate ultrafiltration steps. In this process the second ultrafiltration permeate is nanofiltered. The nanofiltration retentate can be used in evaporation and crystallization operations to produce crystals of white sugar. The process can optionally include ion exchange and/or electrodialysis purification steps, prior to or after the nanofiltration step. Recycle syrups can be treated with a chromatographic separator to remove raffinose from the sugar solution.
It is also known in the art to use electrodialysis to remove salts from corn fiber hydrolyzate before a simulated moving bed (“SMB”) chromatographic separation step (U.S. Pat. No. 6,586,212 or U.S. Pat. No. 6,352,845).
Despite the advances made in the art, there exists a continued need for the development of novel processes for the separation and recovery of sucrose and non-sucrose components from sugar beet and/or sugar cane origin. Specifically, many of the prior art approaches discussed hereinabove involve the use of electrodialysis alone for the purification, and are silent about the use of chromatographic separation. Thus, the prior art does not disclose electrodialysis treatment of a sucrose-containing solution selected from molasses and non-nanofiltered sugar juices and sugar liquors before chromatographic separation. The objective problem to be solved is to improve overall yield of components and to enable recovery of higher purity fractions of sucrose and/or non-sucrose components from said sucrose-containing solutions and/or higher resin capacity and reduced evaporation volumes in the chromatographic separation.
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