The features of operation of a mixed bed ion exchanger in the process of water desalination
Ivan Tikhonov
This article discusses the influence of carbon dioxide contained in water on the process of ion exchange desalination of water with a mixed bed ion exchange resin. Experimental data are presented. Recommendations are given for conducting the desalination process using a mixed bed ion exchange resin.
The main indicator of the quality of ultrapure water is the specific electrical conductivity of water (electrical conductivity). The minimum possible electrical conductivity of water is 0.055 µS/cm. It is believed that at this value of electrical conductivity, water does not contain substances that can transfer an electric charge. The electric charge in water is transferred by means of ions. There are several ways to remove ions from water. Currently, the most common are 2 ways:
1-ion exchange
2- reverse osmosis
To remove ions by ion exchange, cationic and anionic resins in the form of H+ and OH – are used, respectively. That is, all cations contained in water are replaced on the cationite by hydrogen cations (H+) and anions are replaced on the anionite by hydroxyl anions (OH-). As a result, water molecules are obtained and, accordingly, all ions are removed from the water, provided that the concentration of cations is equal to the concentration of anions in mg-EQ/l.
To achieve the highest efficiency in removing ions, a mixed bed ion exchange resin is used. This resin consists of cation and anion resin which are mixed in certain proportions. Regeneration of such a resin is quite complex and, as a rule, after the depletion of the ion exchange capacity for H-OH, such a resin is simply replaced with a new resin. Obviously, it makes sense to use this resin only to remove small concentrations of ions from the water that remain after the main stage of desalination. As a rule, reverse osmosis installations are used at the main stage of desalination. After reverse osmosis, about 1% of the ions from their concentration in the source water remains in the water. Accordingly, this will allow you to effectively operate the filter with a mixed bed resin.
However, in this situation, there is one serious problem caused by the existence of carbon dioxide in the water. Let’s look at this in more detail.
Any water that comes into contact with atmospheric air contains carbon dioxide, since carbon dioxide reacts with water to form carbonic acid.
Carbon dioxide also enters the water from various geological or biochemical processes. The more carbon dioxide gas is in the water, the lower the degree of its dissociation in the water is. Carbon dioxide dissociates in water to form carbonic acid according to equation (1).
СО2 + Н2О <--> Н2СО3 <--> Н+ + НСО3– (1)
Thus, when the water contains 11 mg/l of carbon dioxide gas, the share of its dissociation is only 4.2 %. Since the average carbon dioxide content in surface waters is between 2.5 and 15 mg/l, most of it is in the gaseous state, and not in the form of carbonic acid or dissociated H+ and HCO3– ions. Accordingly, carbon dioxide gas is not hydrated by water in contrast to ions and during reverse osmotic desalination, carbon dioxide passes through the separating membrane and enters the filtrate. If the filtrate is then sent to a mixed bed ion exchanger, the carbon dioxide gas is removed in the filter according to the following scheme. The bicarbonate ion (HCO3– ), as the dissociation anion of carbonic acid, is exchanged on the anionic part of the mixed bed resin for hydroxyl (OH–). The hydrogen cation H+ of carbonic acid cannot be exchanged for the hydrogen cation contained in the cationic part of the mixed bed resin. After the exchange of bicarbonate for hydroxyl, a water molecule is obtained instead of carbon dioxide and the pH of the water increases, respectively, the dissociation of the remaining carbon dioxide gas continues. As a result, all carbon dioxide gas is removed from the water. At the same time, it is contained in the form of bicarbonate on the anionic part of the mixed bed resin.
It is obvious that part of the ion exchange capacity of the anionite is occupied by bicarbonate of carbonic acid while the ion exchange capacity of the cationite remains the same (original). Accordingly, when passing water containing in addition to cations and anions of salts in equal equivalent concentrations, also carbon dioxide, the exchange capacity of the anionite ends much faster than the exchange capacity of the cationite.
Manufacturers of mixed bed resin try to make such a ratio of cationite and anionite that the exchange capacity for cations is equal to the exchange capacity for anions.
For example, if the ion exchange capacity for cations in the H – form is about 1.8 g-EQ/l, and for anions in the OH -form about 1.1 g-EQ/l, then cationite and anionite are mixed in a ratio of 1:1.5.
As a result, when passing through a filter with 2.5 liters of mixed resin (1 liter of cationite and 1.5 liters of anionite) water with an ion concentration of 10 mg/l (0.119 mmol/l converted to NaHCO3) and a concentration of carbon dioxide of 10 mg/l (0.227 mmol/l), the filter cycle of this filter should be calculated based on the total concentration of 0.119+0.227=0.346 mmol/l, and not from the concentration of salt ions only.
The filter cycle in this case, when using the full exchange capacity, when calculating for anionite, will be
1,65/0,346= 4,768 m3 = 4768 liters.
where,
1.65- ion exchange capacity of anionite (1.1*1.5=1.65 g-EQ/l)
If there was no carbon dioxide in the water, the filter cycle would be
1,65/0,119=13,865 m3=13865 liters.
As you can see, the filter cycle in the absence of carbon dioxide is three times larger for these conditions than in the presence of carbon dioxide.
It is quite expensive to use a mixed bed ion exchanger as a stage for decarbonization of water.
Let’s conduct a small experiment. The source water undergoes reverse osmotic desalination and then passes a mixed bed ion exchanger to produce deionized water. The electrical conductivity of the source water is 400 µS/cm. The concentration of carbon dioxide in the water is 11 mg/l. When the end of the filter cycle of mixed bed ion exchanger was approaching, calculated taking into account the concentration of carbon dioxide, the filter was connected directly to the source water to obtain a more visual result of the experiment. The electrical conductivity of the filtrate after the filter was 2.4 µS / cm, while the pH of the filtrate was 8.4. This pH value indicates that there is almost no carbon dioxide in the filtrate. The residual amount of bicarbonate in the filtrate provides pH = 8.4 (not 7.0) in accordance with the carbon dioxide balance in the absence of carbon dioxide. The pH value of 7.0 will only be observed if the filtrate’s electrical conductivity is less than 0.1 µS / cm (in fact, in the absence of salt ions).
The pH value =8.4 indicates that the mixed bed ion exchange resin effectively removes carbon dioxide from the water. After a certain amount of water passed through the filter, an increase in the electrical conductivity of the filtrate and a decrease in the pH value were observed. This indicates that the exchange capacity of the anionite in the OH – form has run out. At the same time, there is still an exchange capacity of the cationite in the H-form. Accordingly, the filtrate contains only hydrogen cations and monovalent anions begin to appear, which reduces the pH of the water and increases the electrical conductivity. The process of anions appearing in the filtrate is quite interesting. Obviously, chlorides and bicarbonates first appear in the filtrate, after depletion of the ion exchange capacity of the anioniter. At the same time, since there are only hydrogen ions as cations, the pH reduces and bicarbonates, which are in the water in the form of carbonic acid, turns into carbon dioxide gas. Accordingly, there are no bicarbonates in the water passing through the filter, since they pass into carbon dioxide, and in accordance with the law of acting masses, bicarbonates located on the anionite begin to actively pass into water and immediately pass into carbon dioxide gas. This reduces the pH value. This process occurs up to a pH value of 4.5. After this pH value, the anionite is depleted by bicarbonate and chloride begins to pass into the water and, accordingly, hydrochloric acid appears. As you know, anions of strong acids begin to appear in water below the pH value of 4.5. At the same time, the value of electrical conductivity of water begins to increase sharply, since hydrochloric acid does not pass into a gaseous state, unlike carbonic acid.
Figure 1 shows the dependence of the electrical conductivity of the filtrate of mixed bed ion exchanger on the pH under conditions of depletion of the capacity of the anionite in the OH-form for the experiment presented above.
The figure shows that before the pH value of 4.5, the electrical conductivity increased slightly (from 2.4 to 20 µS/cm). With a further decrease in pH, the electrical conductivity began to increase sharply, because strong acids began to appear in the filtrate. At the same time, the concentration of carbon dioxide at pH =4.5 was about 200 mg/l, and at pH =3.5, CO2= 350 mg/l. This indicates that 350/44=7.94 mg-EQ/l of bicarbonate was exchanged into water from anionite.
Figure 1
Table 1 shows calculated data on the dissociation of carbon dioxide in water.
Table 1
| СО2, mmol/l | СО2, mg/l | рН | Н2СО3, mmol/l | Н2СО3, µS/cm | Н2СО3, mg/l | n | the share |
| 0,045454545 | 2 | 5,35 | 0,004497474 | 1,754014875 | 0,27884339 | 0,158 | 0,098944429 |
| 0,090909091 | 4 | 5,2 | 0,006360389 | 2,480551625 | 0,394344104 | 0,158 | 0,069964277 |
| 0,136363636 | 6 | 5,11 | 0,007789854 | 3,038042881 | 0,48297092 | 0,158 | 0,057125593 |
| 0,181818182 | 8 | 5,05 | 0,008994948 | 3,50802975 | 0,557686781 | 0,158 | 0,049472214 |
| 0,25 | 11 | 4,98 | 0,010547512 | 4,113529506 | 0,653945716 | 0,158 | 0,042190046 |
| 0,363636364 | 16 | 4,9 | 0,012720778 | 4,96110325 | 0,788688209 | 0,158 | 0,034982138 |
| 0,568181818 | 25 | 4,8 | 0,015900972 | 6,201379062 | 0,985860261 | 0,158 | 0,027985711 |
| 1,136363636 | 50 | 4,65 | 0,02248737 | 8,770074375 | 1,394216952 | 0,158 | 0,019788886 |
| 1,704545455 | 75 | 4,56 | 0,027541291 | 10,74110361 | 1,707560062 | 0,158 | 0,016157558 |
| 2,272727273 | 100 | 4,5 | 0,031801944 | 12,40275812 | 1,971720522 | 0,158 | 0,013992855 |
| 3,409090909 | 150 | 4,41 | 0,038949268 | 15,1902144 | 2,414854598 | 0,158 | 0,011425119 |
| 4,545454545 | 200 | 4,35 | 0,04497474 | 17,54014875 | 2,788433904 | 0,158 | 0,009894443 |
| 6,818181818 | 300 | 4,26 | 0,055082583 | 21,48220723 | 3,415120123 | 0,158 | 0,008078779 |
| 22,72727273 | 1000 | 4 | 0,100566577 | 39,22096494 | 6,23512776 | 0,158 | 0,004424929 |
| 76,81818182 | 3380 | 3,73 | 0,184889402 | 72,10686671 | 11,46314291 | 0,158 | 0,002406844 |
The table sequentially (left to right) presents data on the concentration of gaseous carbon dioxide in mmol/l and mg/l, then the pH of the water containing this amount of carbon dioxide, then the concentration in mmol/l carbon dioxide that is dissociated in water and the value of electrical conductivity caused by this quantity of dissociated carbonic acid, then the concentration of carbonic acid in water in mg/l, the conversion factor mg/l of carbon dioxide in µS/cm (n) and the share dissociation of carbon dioxide.
As you can see from the table, when the concentration of CO2 in water is 2 mg/l, the share of dissociated CO2 is about 10 % (0.0989). At 100 mg/l, only 1.4% of carbon dioxide is converted to carbonic acid in water. At the maximum concentration of carbon dioxide in water (at a water temperature of 0 0C) equal to 3380 mg/l, the share will be 0.24 %.
Figures 2, 3 show the dependence of the electrical conductivity of water on the concentration of carbon dioxide in it. The figures differ only in the maximum concentration of CO2 in water. As we can see, the electrical conductivity increases quite quickly in the range of small CO2 values (up to 10 mg/l). Then the electrical conductivity changes slightly in relation to the increase in the concentration of CO2.
Accordingly, if you look at figure 1, it is obvious that up to the pH value of 4.5, the electrical conductivity of water is mainly determined by carbon dioxide. But after the pH value of 4.5, strong acids begin to appear in the water and the electrical conductivity of the water increases sharply and is mainly determined by strong acids.
Figure 2
Figure 3
The main conclusion is that before the mixed-action filter, it is necessary to ensure the removal of carbon dioxide from the water. This will significantly (in some cases by several times) increase the filter cycle of the mixed bed ion exchanger and improve the quality of the filtrate.
This can be done by dosing caustic soda into the water before installing reverse osmosis to the pH value of 8.5. This will allow the carbon dioxide to bind into sodium bicarbonate and remove it from the membrane to a concentrate. This can also be done by using separate H-OH ion exchange after reverse osmosis. In the first case, water softening will be required before reverse osmosis. In the second case, you will need to regenerate the H-OH filters.
23.03.2020
