Strong base anion resins are often used for decolorization purposes, in which colored impurities are removed from a feed stream. As one example from the cane sugar refining industry, decolorization is used to produce a crystallized sugar having an acceptable white color. Because many of the color bodies in a cane processing stream are hydrophobic and anionic, strong base anion resins are conventionally used for decolorization since the color bodies will bind to this type of resin through ionic and hydrophobic interactions. For this reason, cane refinery decolorization using strong base anion resins has become common.
Using ion exchange resin for decolorization has been less common in the beet sugar industry since the process, when operated conventionally, is capable of producing a white crystallized sugar without additional purification. However, more recent processes which increase the recovery of beet sugar, such as chromatography, have resulted in higher color feed to the crystallization step and have motivated a consideration of decolorization. The use of chemicals and waste disposal problems associated with anion resin decolorization have hindered widespread adoption of this technology. Similar problems exist in the cane sugar industry but are often accepted because decolorization is required to produce a sellable sugar.
The various problems associated with the anion resin decolorization process are primarily due to the method of regenerating the strong base anion resin. The regeneration is a well-known process and typically includes feeding a regenerant solution that includes sodium chloride (NaCl) and sodium hydroxide (NaOH) across the strong base anion resin, as shown in FIG. 1. The regenerant solution includes 10% NaCl and 1.0% NaOH. For additional regeneration, fresh NaCl and NaOH are added to the regenerant solution. The regenerant solution displaces the exchanged and adsorbed color bodies. The used regenerant is referred to as regenerant waste or spent regenerant, and is considered a waste stream. The spent regenerant contains most of the NaCl from the regenerant solution in addition to various organic color compounds previously removed by the strong base anion resin. Since the spent regenerant is a caustic brine, the spent regenerant is an undesirable material because it has a very high chemical oxygen demand (COD), high salt content, and poorly degradable color bodies. In some cases, the spent regenerant disallows the use of the process due to effluent discharge limits.
To overcome some of the waste problems, a reprocessing-recycle technique using nanofiltration to recover a fraction of the NaCl has been in use since the 1990s. In this method, the spent regenerant is passed through a crossflow membrane nanofiltration system, producing a membrane permeate and a membrane retentate, as shown in FIG. 2. A significant amount of the NaCl and NaOH of the spent regenerant passes through the membrane and is recovered in the membrane permeate. Fresh NaCl and NaOH are added to the membrane permeate for subsequent resin regeneration. A significant reduction in salt consumption may be achieved by recycling part of the spent regenerant in this manner. However, the process still requires a fresh NaCl and NaOH makeup and a spent regenerant bleed-off and salt/organic containing concentrate must be dealt with. The membrane retentate is sent to waste.