Ion Exchange Resin Regeneration and Cleaning

Color bodies are desorbed from the strong base anion resins using a 10% NaCl solution (for acrylic strong base anion resin) or a mixture containing 10% NaCl and 0.5% NaOH (for styrenic strong base anion resin). The NaOH also provides additional desorption energy. The NaCl solution dehydrates the resin forcing exchanged and adsorbed color bodies out. The regeneration is concentration dependent and not stoichiometric. It requires the salt concentration inside a resin bead to be at about 3+% NaCl. To achieve this, 10% NaCl solution is required. The regeneration flow direction is down-flow at a rate of 2 BV/h (0.25 gpm/ft³).

Special Cleaning for Sugar Decolorization Resins
It is strongly recommended to use the procedure below for decolorization resins every 30 cycles or when an important drop in throughput is noticed.

It is very important that resin be regenerated prior to beginning this treatment.

Step 1: Prepare a 2 BV solution with 6% HCl or 10% H₃PO₄.

Step 2: Pass half the volume of acid solution at a flow rate of 1 BV/h (0.12 gpm/ft³) for one hour down-flow.

Step 3: Introduce the second half of your acid solution and keep it in contact with the resin for two hours. Injection of air every 20 minutes assists in declumping.

Step 4: Pass condensate or demineralized water at 3 BV/h (0.37 gpm/ft³) down-flow for 1 hour.

Step 5: Rinse the resin bed with condensate or demineralized water at 8 BV/h (1 gpm/ft³) down-flow until the pH equals 7.0.

After completing this procedure, perform a normal regeneration but with twice the standard amount of NaOH in order to neutralize residual acid and the acid adsorbed onto the small amount of weak base sites of the resin.

The acid cleaning should restore decolorization performance for an extended period if the resin is in good shape. As the resin nears the end of its life, the acid cleaning will provide a more temporary improvement in decolorization.

See also, Cleaning of Organically Fouled Anion Resins. 

Alternative Regenerant Solutions
It has been observed that NaCl, alone or alkalized with NaOH is not the best regenerant for this kind of resin.

It has been suggested that during regeneration at high salt concentration, amphiphilic compounds will be fixed to the resin by hydrophobic interaction. During salt washing, as salt concentration decreases, these compounds will be removed from the resin matrix but will be fixed to the ionic groups (switch effect). Washing the resin at low pH, however, can avoid this. Another possibility is to regenerate the column at a low chloride concentration in the presence of calcium in an alkaline medium; a process developed by Dr. Luis Bento. Sucrose is added in order to maintain calcium in solution. During this regeneration, hydrophobic interaction is reduced and colorants are removed from the ionic bond; possibly through the formation of a colorant/calcium/sucrose complex.

As mentioned above, NaCl does not efficiently remove sugar colorants from ion exchange resins because at high concentrations it does not remove compounds fixed by hydrophobic interactions to the resin matrix. An organic solvent, such as ethanol, mixed with salt can be used to improve the removal of hydrophobic colorants. With this mixture, salt will remove anionic colorants and ethanol will decrease the hydrophobic interaction effect. This has been tried using a mixture of NaCl, at 100 g/l (6.24 lb/ft³), and Ethanol, at 20% (v/v), with a great improvement in removal of colorants from the resin. Although efficient, this process is not economical if applied to each regeneration.

Another possibility, as mentioned above, is to use salt at low concentration with a mixture of calcium hydroxide in sucrose solution. This mixture greatly enhances the removal of colorants fixed to ion exchange resins. The formation of a complex sucrose-calcium-colorant, may shift the equilibrium towards regeneration. Thus, chloride ions, even at low concentration, can efficiently dislocate colorants fixed ionically to resins. At low salt concentrations, non-polar colorants are also easily removed. A regeneration process using 3 BV of a mixture of CaCl (0.4 N) and CaO (10 g/l (0.62 lb/ft³)) in sucrose solution (150 g/l (9.36 lb/ft³)) has been tried, for instance.

With this regeneration process, an average decolorization of more than 90% is obtained; more so than results with standard regenerations using alkalized salt solutions. Colorants adsorbed on the resin matrix may form a layer with a degree of anionic character. During regeneration, not all colorants are removed, and doubly positively-charged calcium ions may be bonded to this layer, producing extra positive fixed ions in the resin. This may explain the increase of resin capacity often found when using this regeneration system.

This regeneration process uses lower quantities of chemicals than classic regeneration systems. However, it is necessary to use sucrose to maintain the calcium hydroxide in solution. In order to recover sucrose from the effluents, tangential filtration techniques are used. An ultra-filtration (UF) membrane has been used to concentrate colorants in the retentate, while the permeate was used in the regeneration process. The retentate, with minor proportions of the sucrose, calcium and chloride ions, and the major part of the sugar colorants, can be sent to affination and/or recovery sections of the refinery or to the low grade sections of sugar factories. As this effluent is highly alkaline, it can be used to make pH corrections in such processing units, instead of the more commonly-used lime.

Regeneration using sodium carbonate has also been tried. In this process, carbonate ions bind to the resin and during decolorization, the ions are released to sugar liquor where they precipitate any calcium ions present. An average carbonated liquor decolorization of 89% and decalcification of 70% has been achieved using this technique.