On the issue of regeneration of cationite in Na – form
The article discusses how it is possible to simply determine the specific salt consumption for the regeneration of the working exchange capacity of cationite in Na form.
The specific salt consumption for cationite regeneration is the most important parameter for the high-quality operation of the water softening plant. If this indicator is too high, then part of the salt will be inefficiently spent on cationite regeneration. There may be an overspending of salt and, accordingly, an excess of highly mineralized wastewater. If this indicator is too low, the softening unit will be regenerated imperfectly, which will lead to a decrease in the filter cycle and a deterioration in the quality of softened water.
What is the specific salt consumption required for effective regeneration of cationite?
It is known that the strength of the interaction of cations with cationite depends on the charge of the cation and its hydrated radius. The smaller the radius of the hydrated ion and the larger its charge, the stronger it interacts with the cationite. There is the following sequence:
Li < Na < NH4 < K < Mg2+ < Ca2+
In this sequence, calcium has the greatest “adhesion” with cationite. Then magnesium, etc. Calcium, like magnesium, has a charge of two, but at the same time calcium has a smaller radius of the hydrated ion (Table 1). That is, it is easier for calcium to enter the cationite compared to magnesium. Also for sodium and potassium. Each of these ions is monovalent, but the radius of the hydrated potassium ion is less than sodium. Therefore, calcium will have an advantage when exchanged with cationite in the presence of magnesium and other monovalent ions.
In fact, the less energy an ion spends interacting with water (hydration energy), the easier it is for it to pass to cationite from water compared to another ion. However, this rule only works for comparing ions having the same charge.
Table 1
hydration energy, kJ/mol | Radius of the hydrated ion, 10-1 nm | |
К | 322 | 1,1 |
Na | 406 | 1,83 |
Ca+2 | 1615 | 3,09 |
Mg+2 | 1953 | 3,46 |
For a visual representation of the exchange of ions of the solution with cationite, let’s look at the 3 schemes in Figure 1.
Figure 1
The schemes show the cationite in the form of an insoluble matrix and a diffusion layer in which the exchange between the ions of the cationite and the solution takes place. Assume that there are only two ion-exchange bonds on the cationite matrix. In scheme 1, both cationite bonds hold one divalent calcium cation. At the same time, only two sodium cations are present in the solution. In this case, an equilibrium will be observed in the interaction system. Calcium is held by two bonds for the cationite matrix. At the same time, the same two bonds of the cationite are claimed by two sodium. If both sodium pass to cationite, they will “push” calcium into the solution. But then, calcium, which has an advantage over sodium, having no competition over other sodium cations in solution (which are no longer in solution), will simply exchange back with the sodium of the cationite. Turning to the concentrations from individual cations, we can say that in this case, 1 g-eq of calcium is in equilibrium with 1 g-eq of sodium. Or 1 mole of calcium is in equilibrium with 2 mole of sodium. At the same time, there is calcium on the cationite. Indeed, in accordance with the above sequence, 1 mole of calcium has an advantage in ion exchange over 1 mole of sodium.
It is obvious that in order to knock out calcium from the cationite and replace it with sodium, at least twice as much sodium will be required in the solution as calcium in equivalent amounts. scheme 2 shows that if there are 4 sodium cations in the solution, the ion exchange process will shift towards replacing one calcium cation of the cationite with two sodium cations of the solution. As a result of ion exchange, an equilibrium state will be obtained, shown in scheme 3. Two sodium cations will be on the cationite, but 2 sodium cations will also remain in the solution. These cations will “compete” with the calcium cation providing the resulting equilibrium. One calcium cation, according to the charge, has an advantage over two sodium cations, but since there are already sodium cations on the cationite, the exchange will lead to an increase in the concentration of sodium cations twice in solution (will lead again to scheme 2), and they will be exchanged with calcium again, bringing it into solution. Therefore, with such a concentration of sodium, equilibrium will be reached when the calcium is already in solution, and there will be sodium on the cationite matrix. If there are 3 sodium cations in the solution, then if divalent calcium passes into the solution, it will compete with only one monovalent sodium cation. Accordingly, the calcium will return to the cationite, and there will be 3 sodium cations in the solution. Only 4 sodium cations will “force” calcium out of the diffusion layer of cationite.
We obtain, in order to regenerate the cationite qualitatively (the absorbed hardness ions from the source water are completely replaced with sodium ions of the regeneration solution), the amount of sodium in the regeneration solution should be twice as much as the amount of hardness cations on the cationite taken in g-eq. This is a theoretically necessary minimum of sodium.
Moreover, analyzing this scheme, it becomes clear that with such an amount of sodium, it does not matter whether the regeneration solution is supplied from below the cationite or from above. In any case, all hardness ions will be replaced.
Consider an example. The softening filter is filled with cationite in Na form. The volume of cationite is 0.55 liters. The hardness of the source water is 2.8 mg-eq/l. As a result of the experiment conducted to obtain softened water, it was found that the total exchange capacity of 1 liter of cationite was 1.65 g-eq/l (Fig. 2), and the working capacity was 1.27 g–eq/l. Within this working capacity of the cationite, it was possible to obtain water with the required hardness (less than 0.2 mg-eq/l).
The question is, how much sodium chlorine will be required to regenerate the working exchange capacity of the cationite?
In accordance with the above scheme of exchange of divalent cations for monovalent ones, it can be said that for withdrawal from cationite (regeneration) 1.27 g-eq of calcium and magnesium, a theoretically necessary minimum of 1.27 * 2 = 2.54 g-eq of sodium will be required. Let’s take the regeneration efficiency coefficient (ensuring the completeness of regeneration) equal to 1.1.
The regeneration efficiency coefficient provides a small excess of sodium in the regeneration solution slightly greater than the required theoretical minimum. This allows for complete regeneration.
We get that to regenerate 1 liter of cationite containing 1.27 g-eq of hardness cations, we will need 2.54* 1.1 = 2.8 g-eq of sodium, or 2.8*58.5 = 163.8 grams of table salt. (58.5 g/mol is the molar mass of sodium chlorine).
In this case, the specific salt consumption for the regeneration of 1 liter of cationite is 163.8 g/l.
If it is required to regenerate the total capacity of the cationite, suppose, in the case when the next regeneration of the cationite was skipped, then the specific salt consumption will be equal to 1,65*2*1,1 =3.63 g-eq/l or 3.63 * 58.5 = 212 g/l.
Figure 2
The total exchange capacity of modern strongly acidic cationites averages about 1.9 g-eq/l. In this case, complete regeneration of the resin will require 1,9*2*1,1*58,5 = 244 grams of salt per 1 liter of cationite.
The total exchange capacity of modern weak acid cationites averages about 3.8-4.2 g-eq/l. Then the specific salt consumption for the complete regeneration of this cationites will be 4,2*2*1,1*58,5 = 540 g/l.
It is necessary to identify an important point. Ion exchange on cationite works according to the presented scheme only at concentrations of an aqueous solution of more than 20 g/l. Full compliance with the presented scheme is achieved at a concentration of aqueous solutions from 70 g/l. If a low concentration of ions is observed in water, then divalent cations have a much greater advantage in ion exchange with cationite compared to monovalent ones, and the scheme in Figure 2 works with a greater error. I.e., this scheme can only be used for concentrated regeneration solutions.
In my opinion, this circumstance can be explained by the fact that at low concentrations of ions (softening of fresh water), both charge and mobility are of decisive importance for cations. At low concentrations, it is difficult for more mobile sodium, which has a smaller radius of the hydrated ion, to compete with slightly less mobile, but divalent calcium at this low concentration. In concentrated solutions, ions significantly lose their mobility and the valence of ions comes to the fore, and the influence of mobility begins to decrease, and ion exchange begins to occur according to the scheme in Figure 1.
This can explain why in the process of Na – cation of fresh water, hardness ions can be almost completely removed from it. At the same time, an increase in the total concentration of ions in the initial non-softened water leads to an increase in the concentration of divalent hardness ions in the softened water even at the beginning of the filter cycle on fully regenerated cationite.
It is important to note that in order to obtain deeply softened water, it is necessary that there is always only sodium in the lower part of the cationite. This condition for the source fresh water will ensure the production of softened water with almost zero hardness. In Figure 2, it can be seen that the lower “inert” part of the cationite should contain only sodium. If calcium appears there, let’s assume from an incorrectly installed filter cycle, then this calcium will exchange with the sodium of the softened water in the lowest part of the filter and the softened water will have increased hardness. For the example considered in Figure 2, the necessary “inert” sodium part of the cationite according to the results of the experiment was (1,27/1,65) *100% = 23 % of the total volume of cationite. The greater the total concentration of ions in the source water, the greater the inert part of the cationite should be. Based on this condition, the working exchange capacity is always less than the total exchange capacity of the cationite.
As a result, knowing the amount of water that has passed through the filter, the initial and obtained water hardness, it is quite simple to determine the optimal specific salt consumption for full-fledged cationite regeneration. The specific salt consumption will be equal to the amount of hardness ions absorbed by 1 liter of cationite per filter cycle taken in gram equivalents multiplied by 2 and by the regeneration efficiency coefficient.
For the case when it is required to obtain deep-softened water, the value of the hardness of the softened water can be neglected. Then the value of the specific salt consumption for the regeneration of 1 liter of cationite will be equal to,
С = ((Gsource*F)/V)*2*k*58.5 , грамм соли/1 л катионита,
где,
Gsource – the hardness of the source water, mg-eq/l,
F – volume of the filter cycle, m3,
V – volume of cationite in the filter, l,
k – the regeneration efficiency coefficient, k = 1.1-1.2.
For automated softening systems, it is necessary to take into account seasonal fluctuations in the hardness of the source water of surface water supply sources. For example, for the Volga River in the Saratov region, the water hardness ranges from 2.5 mg-eq/l in August-September to 4.5 mg-eq/l in March – April. In this case, it is possible to set a single value of the specific salt consumption only for the maximum hardness value during the regime adjustment of automated softening plants. Otherwise, during a certain period of the year, the installation will not provide the necessary water softening. But this leads to significant salt overruns in the warm season. This circumstance must be taken into account during the commissioning.