Ion exchange resins are widely used for beet sugar juice processing. The purpose of this article is to review their use and present the latest improvements regarding the softening processes. We will especially focus on strong acid cation resins which present the almost unique feature of salt conversion reactions and thin juice softening. The salt conversion process is based on selective affinities for different ionic species. The decalcification of thin beet juice is a necessity nowadays in order to avoid evaporator scaling by calcium carbonate, oxalate or silicate resulting in high energy consumption. Different techniques have been used to reduce scaling. The most common ones are anti-scaling agents or cationic ion exchange resins.
Originally they were regenerated with a brine solution; however, this has been progressively abandoned due to the waste handling cost. The industry has developed several more environmental friendly alternatives such as the NRS or Gryllus processes.
In fact, the phenomenon is called salt conversion and is based on a higher affinity for divalent species than for monovalent ones with cationic ion exchange resins. This can be illustrated by the following reaction:
2RSO3-K+ + Ca2+ >>> 2R-Ca + 2 K+
It is interesting to note that for weak acid cation resins (WAC), the exchange reactions are the following:
2R-COOH + Ca(HCO3)2 >>> (R-COO-)2 Ca2+ +2H2CO3
R-COOH + CaCl2 >>> no reaction
Since potassium is the main monovalent cation, and calcium and magnesium are the divalent species, the equilibrium equation can be written as follows:
X = Fraction in the resin / X = Fraction in the solution
The left part represents the ionic fraction within the cationic resin and the right part what is left in solution; CR is the resin capacity and CS is the solution salinity. We can note that in a monovalent/divalent exchange, the removal efficiency is function of the resin capacity and of the thin juice salinity. The calcium uptake is higher when CS is low (curve 1) rather than high (curve 2). This translates into the fact that it is easier to remove calcium from dilute solutions (16 Brix thin juice) than concentrated ones (molasses). Moreover, we can see that when the feed solution concentration is high (curve 3) it is possible to regenerate the resin.
Assuming the following feed composition: 150 mg CaO/l of juice -> ~ 5.35 meq/l Ca and a monovalent composition of Na + K ~ 43 meq/l resulting in a total salinity of 50 meq/l.
, the apparent selectivity coefficient defined as Kapp=
We see that the apparent selectivity is very favorable for calcium uptake when the solution concentration is low. During the regeneration step, in a conventional NaCl regeneration process, we would use a 12% solution or 130 g/l (=2.2 eq/l), the Kapp becomes ~ 2.3 it is almost reversed which makes the regeneration possible.
Besides chromatography for molasses desugarization, the softening process is the main one in application. Among the different possible regeneration techniques, the Applexion® NRS (New Regeneration System) is the most successful one.
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