Removing oxygen from the hot water system. Removal of oxygen. Chemical methods for oxygen removal - dosing equipment of the MWT R series
The water treatment process is often accompanied by the removal of gases such as carbon dioxide, oxygen and hydrogen sulfide. These gases are corrosive, as they have the ability to cause or enhance corrosion of metals.In addition, carbon dioxide is aggressive towards concrete, and the presence of hydrogen sulfide gives the water an unpleasant odor. Due to the above, the task of most completely removing these gases from water is urgent.
Degassing of water- this is a set of measures aimed at removing gases dissolved in it from water. There are chemical and physical methods of water degassing. Chemical methods of water degassing involve the use of certain reagents that bind gases dissolved in water. For example, deoxygenation of water is achieved by introducing sodium sulfite, hydrazine or sulfur dioxide into it. When sodium sulfite is introduced into water, it is oxidized to sodium sulfate by oxygen dissolved in water:
2Na 2 SO 3 + O 2→2Na2SO4
Sulfur dioxide introduced into water reacts with it and turns into sulfurous acid:
SO 2 + H 2 O → H 2 SO 3,
Which, in turn, is oxidized by oxygen dissolved in water to sulfuric acid:
2H 2 SO 3 + O 2 → 2H 2 SO 4
At the same time, modified solutions of sodium sulfite are currently used (reagents, etc.), which have a number of advantages in comparison with a pure solution of sodium sulfite.
Hydrazine promotes almost complete deoxygenation of water.
Hydrazine introduced into water binds oxygen and promotes the release of inert nitrogen:
N 2 H 4 + O 2 → 2H 2 O + N 2
Deoxygenation of water using the latter method is the most advanced, but at the same time, the most expensive method (due to the high cost of hydrazine). In this regard, this method is used mainly after physical methods of deoxygenation of water in order to remove residual oxygen concentrations. At the same time, hydrazine belongs to substances of the first hazard category, which also entails restrictions on the possibility of its use.
One of the variants of the chemical method is treating water with chlorine:
a) with the oxidation of hydrogen sulfide to sulfur:
H 2 S+Cl 2 → S+2HCl
b) with the oxidation of hydrogen sulfide to sulfates:
H2S+4WITHl 2 + 4N 2 ABOUT-> H 2 SO 4 + 8HCl
These reactions (as well as intermediate reactions for the formation of thiosulfates and sulfites) occur in parallel; their ratio is determined primarily by the dose of chlorine and the pH of the water.
Disadvantages of chemical gas removal methods:
a) The water treatment process becomes more complicated and expensive due to the need to use reagents. At large hourly flows through degassing with chemical reagents, despite the comparative simplicity of its implementation, it begins to lose significantly to thermal degassing in terms of operating costs.
b) Violation of the dosage of reagents leads to a deterioration in water quality.
These reasons determine the significantly less frequent use of chemical gas removal methods at large facilities than physical ones.
There are two main ways to remove dissolved gases from water by physical methods:
1) aeration - when water purified from gas is actively in contact with air (provided that the partial pressure of the gas being removed in the air is close to zero);
2) creating conditions under which the solubility of gas in water is reduced to almost zero.
Aeration usually removes free carbon dioxide and hydrogen sulfide from water, the partial pressure of which is atmospheric air close to zero. Degassers that carry out aeration, depending on the design, the nature of the movement of water and air and the course of the degassing process, are divided into:
1) Film degassers (decarbonizers) are columns with a nozzle (wooden, Raschig rings, etc.) through which water flows in a thin film. The purpose of the nozzle is to create an extensive contact surface between water and air. The air pumped by the fan moves towards the flow of water;
2) They blow compressed air through a layer of slowly moving water;
The second method is used when removing oxygen from water, since it is clear that the first method will not work here due to the significant partial pressure of oxygen in the atmospheric air. To remove oxygen, water is brought to a boil, and a sharp decrease in the solubility of all gases in water occurs.
Bringing water to a boil is carried out: 
1) by heating it (in atmospheric deaerators);
2) reducing the boiling point of water by lowering the pressure (in vacuum deaerators).
IN In atmospheric deaerators, preliminary deaeration is carried out in special deaeration columns for due to the excess amount of steam entering the deaeration tank through the supply steam line , and the final one - in deaeration tanks due to steam blowing. In vacuum degassers (deaerators), special devices (such as vacuum pumps or water-jet ejectors) create pressure at which water boils at a given temperature.
In water treatment process, the main application is in removal processes carbon dioxide found film degassers to remove hydrogen sulfide (together with a number of other tasks - supplying oxygen as an oxidizing agent in , ) - bubbling, and for deoxygenation of water in the presence of steam sources at the facility - thermal, in the absence - vacuum.
The design of degassers involves determining the area cross section degasser, the height of the water column in it, the required air flow, the type and surface area of the nozzle required to achieve the desired degassing effect.
V.V. Volkov, I.V.Petrova, A.B.Yaroslavtsev, G.F.Tereshchenko
Despite the fact that the content of dissolved oxygen in water is relatively low (under normal conditions about 8 mg/l), in microelectronics, energy and Food Industry Quite stringent requirements are set to reduce its concentration in process waters to the level of several µg/l. For example, in the food industry, oxygen contained in water deteriorates the quality of a number of products, in particular, it causes a decrease in the aging resistance of beer. In the energy sector, to reduce corrosion and scale deposits in order to increase the service life of heating networks and equipment by 10 years or more, the oxygen content in water should be at the level of 5 μg/l.
The most stringent requirements for the quality of ultrapure water are put forward by the semiconductor industry - in some cases the required level should not exceed 1 µg/l. All enterprises in the microelectronics industry today already consume huge amounts of ultrapure water. Ultrapure water is not on the market as a commercial product. In the microelectronics industry, it is produced directly at enterprises and supplied through pipelines to workshops where it is used. Currently, ultrapure water is often used to wash silicon substrates in integrated circuit manufacturing. The presence of dissolved oxygen causes the formation of an oxide layer on the surface of the substrate, the growth rate of which depends on the time of interaction of water with the surface and on the concentration of dissolved oxygen. Oxide layer formation occurs even when ultrapure water with low dissolved oxygen levels of 40-600 μg/L is used.
Removing dissolved oxygen from water can be achieved by both physical and chemical methods. Chemical methods allow for deep reagent purification of water from dissolved oxygen. However, traditional chemical methods (reduction with hydrazine hydrate or sodium sulfite at elevated temperatures) have a significant drawback - the introduction of impurities (reagents) into the water during the purification process.
Traditional physical methods such as thermal degassing, vacuum degassing or nitrogen bubble deaeration are expensive, require large plant sizes and have a small active surface area per unit volume. In addition, it is quite difficult to reduce dissolved oxygen concentrations from a few parts per million to a few parts per billion using these approaches.
The use of membrane contactors makes it possible to achieve deeper degrees of purification and has a number of advantages: a significant increase in the gas-liquid surface area per unit volume, high mass transfer rates, lack of dispersion between phases and the possibility of scaling (modular designs). These advantages make membrane methods an attractive choice among other available physical methods for oxygen removal. For example, recently new water treatment systems consisting of two compact membrane contactor modules with a total area of 260 m 2 were installed at nuclear power plants in South Korea (Kori and Wolsung). This technology makes it possible to reduce the content of dissolved oxygen in process waters of nuclear power plants to 0.39 and 0.18 mg/l, respectively, by physical blowing with a carrier gas and evacuation at 50 o C.
However, such methods have a number of disadvantages, for example, partial evaporation of water during the process, high consumption of inert gas (for example, nitrogen) or steam, and the use of additional equipment to create and maintain technical vacuum. In addition, to achieve high degrees of water purification from dissolved oxygen (less than 1 µg/l), the use of two-stage systems is required: a preliminary stage - reduction to 100 µg/l, and final purification to a level of 1 µg/l and below.
A promising chemical method for removing dissolved oxygen is the process of catalytic reduction of oxygen with hydrogen on a palladium catalyst to form water. A significant disadvantage of such methods is the need to pre-saturate the water with hydrogen. This problem is partially solved today in industry by using special nozzles or membrane contactors. Thus, existing catalytic removal methods require a two-stage process: preliminary dissolution of hydrogen in water and subsequent reduction of dissolved oxygen in water with hydrogen on a palladium catalyst.
Recently, the Institute of Petrochemical Synthesis named after A.V. Topchiev RAS (INHS RAS) together with the Dutch Organization for Applied scientific research(TNO) developed and patented a method for depositing palladium metal on the outer surface of hydrophobic polymer membranes. The developed technology of applying a palladium catalyst to the outer surface of porous membranes in the form of nano-sized particles made it possible to combine in one module the advantages of highly efficient gas-liquid contactors with a high depth of water purification characteristic of chemical reactors (Fig. 1). An important advantage of this combined approach is the implementation of a one-stage process for removing dissolved oxygen from water at room temperature without the stage of hydrogen bubbling in water.
The principle of operation is that water containing dissolved oxygen washes the membrane from the outside, and hydrogen, used as a reducing agent, is supplied inside the porous hollow fiber membrane and diffuses through the pores of the membrane to the outer palladized surface, where the reduction reaction of oxygen with hydrogen takes place formation of water molecules.
Fig.1. The principle of one-stage removal of dissolved oxygen from water in a membrane contactor/reactor.
The developed method of applying palladium to the outer surface of polymer membranes makes it possible to obtain catalytic membranes with an amount of palladium less than 5 wt.%. According to scanning electron microscopy data, it is clear that palladium is located on the outer side of the membrane (Fig. 2), while X-ray diffraction, EDA and EXAFS methods have proven that palladium on the surface of hollow fibers is only in metallic form with a particle size of the order of 10-40 nm .
Fig.2. External surface Pd-containing porous polypropylene hollow fiber membranes: a – optical microscopy (magnification 70 times), b – scanning electron microscopy (magnification 8500 times).
The developed application method was successfully adapted to a non-separable commercial membrane contactor Liqui-Cel Extra Flow (1.4 m2; USA). To study the process of removing dissolved oxygen from water, a gas mode was used, in which physical blowing was completely eliminated and removal was possible only through a catalytic reduction reaction. When hydrogen is supplied, a sharp drop in the oxygen concentration in water at room temperature is observed only due to the catalytic reaction.
Fig.3. Dependence of the concentration of dissolved oxygen in water on the time of the experiment in flow mode: 1 – helium (water flow 25 l/h); 2 – hydrogen (water consumption 25 l/h); 3 – hydrogen (water flow 10 l/h).
During pilot tests of a catalytic membrane contactor/reactor in water recirculation mode in the system (temperature 20 o C), the concentration of dissolved oxygen in water was reduced by more than 4 orders of magnitude to a level of 1 μg/l and lower only due to the catalytic reaction. This implementation eliminates the inevitable high consumption of gas or steam compared to the traditional process of physical blowing. The results obtained meet the most stringent industry requirements for ultrapure water at present.
Long-term (6 months) tests showed high stability of the catalytic activity of membrane contactors. It was found that even in the event of catalyst poisoning or deactivation, it is possible to re-deposit palladium on the surface of the membranes of an operating membrane contactor/reactor.
As a result of the research carried out by the Institute of Chemistry and Chemistry of the Russian Academy of Sciences, together with TNO, a catalytic membrane contactor/reactor was developed, containing a palladium catalyst deposited in a special way on the outer surface of porous polypropylene hollow fiber membranes. Moreover, the technique is adapted in such a way that the application process is carried out without disassembling industrial membrane contactors, ensuring simplicity and scaling of their production to the required level. The cost of the palladium deposition process can be estimated at 5-7 euros per 1 m 2 of membrane.
The developed one-stage method for removing dissolved oxygen is completely ready for commercialization and makes it possible to obtain ultra-pure process water for various fields of microelectronics, energy and the food industry.
The most important factor in the corrosion of iron in water is dissolved oxygen. In condensate return lines of heating systems, free carbon dioxide is also of paramount importance.
The degree of free oxygen removal required to prevent severe corrosion depends on the operating temperature and, to a lesser extent, the amount of water passing through the system. In cold water systems, it is desirable that the oxygen content does not exceed 0.2 ml/l. When it is necessary to achieve a lower oxygen content than is possible with single-stage deaeration, additional chemical treatment of the water leaving the deaerator is used (with sodium sulfide salt or by using multi-stage deaeration). At 70°, as is the case in many hot water systems, it is usually not necessary to reduce the oxygen content below 0.07 ml/l. For steam boilers operating at pressures below 17.5 kg/cm2 - (without economizers), the desired limit should not exceed approximately 0.02 ml/l; for high pressure boilers (or when using economizers), an almost complete absence of oxygen is required, i.e. i.e. below 0.0035 ml/l.
CHEMICAL METHOD FOR REMOVAL OF GASES DISSOLVED IN WATER (DEACTIVATION)
Removal of gases by chemical means is carried out by contacting hot water, at a temperature of about 70 °, with a large surface of perforated iron sheet or scrap iron for half an hour or more - until the oxygen is almost completely consumed by corrosion. For this purpose, special installations for heating systems, equipped with sand filters, were designed; however, such installations are too bulky and require constant maintenance. Therefore, this method has been replaced, to a large extent, by the physical method of removing gases - deaeration. Sodium sulfide salt is used to remove residual dissolved oxygen and is worth its cost only in cases where 95% of the free oxygen is previously removed by deaeration. To remove 1 kg of oxygen dissolved in water, about 8 kg of sodium sulfide salt is required. To ensure complete removal of oxygen in boilers, about 30 mg/l of excess sodium sulfur salt is required. To a lesser extent, iron sulphide salt neutralized with caustic soda is used for deaeration.PHYSICAL METHOD FOR REMOVAL OF GASES DISSOLVED IN WATER (DEAERATION)
By selecting temperature and pressure ratios at which gases become practically insoluble, it is possible to completely remove them from the water. Behind last years the design of gas removal equipment has been significantly improved. There are now several successful types of deaerators available, each adapted for a special purpose. There is also an installation for removing CO2, H2S and MH3 from water.Cold water deaeration
There are installations for deaerating water without heating that produce 15,000 m3 per day and reduce the oxygen content to 0.22 ml/l, which is considered sufficient to prevent corrosion and the formation of tubercles in a long steel pipeline. In such a device, water is sprayed into special trays in a chamber under low pressure. Gases can be removed by steam ejectors with refrigerators or vacuum pumps.
Hot water deaeration
The main condition for deaeration is to maintain water in a finely atomized state (for a sufficient time) at a boiling point corresponding to the pressure at which dissolved gases are freely released. With a simple type of open feedwater heater, the deaerator, when heated to 88 - 93° and freely venting gases into the atmosphere, reduces the oxygen concentration to approximately 0.3 ml/l. This significantly reduces corrosion of steam boilers low pressure. However, in economizers or high-pressure boilers, corrosion increases so strongly with temperature that more complete oxygen removal is necessary.
Deaerators for hot water systems
This type of deaerator is intended primarily for large buildings, for example, hospitals, hotels, etc. Water is heated under vacuum so that its boiling point does not exceed 60-80°. The heating steam passes through the coils and therefore the water does not come into contact with it and does not become polluted. Water is sprayed down onto plates and heated by two rows of steam coils. The temperature of the steam entering the lower coils is higher than the temperature of the water, which consequently evaporates; steam entrains the released gases through a valve cooled by the incoming cold water. The condensate from the valve flows back into the tray chamber, while the gases are expelled by a vacuum pump or steam ejector.
The deaerator is placed in the basement of a building and requires a hot water circulation pump; sometimes it is installed at a high enough level to allow water to be supplied by natural circulation. Under such conditions, an oxygen concentration of 0.04 ml/l is achieved, which protects the system from corrosion at temperatures below 70°.
Deaerators for boiler feed water
In these deaerators, direct contact of water with steam occurs. Most often, plate-type deaerators operating under pressure or vacuum are used. The spray deaerator, operating under low pressure, is widely used in boiler installations. In a tray-type deaerator, cold feed water passes through a refrigerator, then enters a chamber heated by steam, where it is sprayed onto metal trays. The water then flows into a storage tank. Steam fills the entire space, and the direction of its movement is such that it heats the water and removes the released gases. In this way, it is possible to achieve an almost complete absence of oxygen in the water.
In a more modern model of the deaerator, water is sprayed into a steam atmosphere at a pressure of approximately 0.1 kg/cm2. This type of deaerator is designed for marine boilers. It will probably also find application for stationary boilers.
The deaerator consists of a condenser, a steam-heated section, a deaeration section surrounding the steam inlet, and a deaerated water storage section located at the bottom of the apparatus. Cold feed water passes through the refrigerator, then through the spray nozzles, enters the chamber heated by steam, and again through the nozzles into the deaeration chamber, and then into the water collector. The steam enters the deaeration chamber under a pressure of 0.7 kg/cm2 and rises into the refrigerator, where the removed (non-condensable) gases are released, and the heat of the steam is transferred to the water entering the apparatus. Most dissolved oxygen is removed from water when it is initially heated; the last 5% of oxygen is much more difficult to remove. For this purpose, a deaeration chamber is used, which ensures almost complete removal of oxygen from the water.
The most powerful deaerators also remove all free carbon dioxide and partially-semi-bound carbon dioxide and other gases. At the same time, due to the removal of carbon dioxide, the pH of the water increases.
The development of new types of deaerators has practically resolved the issue of eliminating corrosion in water systems and steam boilers. Such a device should be considered an integral part of a modern boiler installation.
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The choice of method for removing impurities from water is determined by the nature and properties of the impurities. Thus, suspended impurities are most easily removed from water by filtration, colloidal impurities by coagulation. If ionic impurities can form a poorly soluble compound, then they can be converted into this compound, oxidizing impurities can be eliminated by reduction, and reducing impurities can be eliminated by oxidation. Adsorption is widely used to remove impurities, with uncharged impurities adsorbed on activated carbon or other
adsorbents, and ions - on ion exchange substances. Charged impurities can also be removed by electrochemical methods. Thus, knowledge of the composition and properties of impurities allows you to choose a method of water purification.
Removing oxygen from water.
Oxygen dissolved in water causes corrosion of the metal of power plant steam generators, station pipelines and heating networks, and therefore must be removed from the water. Oxygen is removed by deaeration and chemical reduction.
Deaeration is based on the use of Henry's law, according to which the solubility of a gas is directly proportional to its pressure above the liquid. By reducing the partial pressure of a gas above a liquid, its solubility in the liquid can be reduced. The partial pressure can be reduced either by reducing the total gas pressure or by displacing a given gas with another gas. In practice, both techniques are used. Typically, water is purged with steam, which reduces the partial pressure of oxygen. However, the deaeration method cannot ensure deep removal of oxygen. The latter is achieved by the interaction of oxygen with chemical reducing agents. Initially, sodium sulfite was used for these purposes, which upon oxidation turns into sodium sulfate:
This method is still used at low-power stations. However, when treating water with sulfite, the salt content increases, which is unacceptable at power plants operating at high steam pressure. At such stations, oxygen is removed using hydrazine, which is a strong reducing agent. When hydrazine interacts with oxygen, nitrogen and water are formed according to the reaction equation
At the same time, the salt content does not change. The disadvantage of hydrazine is its toxicity, therefore, when working with it, appropriate safety regulations must be observed.
Water softening by sedimentation method.
For slightly soluble salts at a constant temperature, the products of ion activities are observed to be constant, called the solubility product. For example, at 20 °C for equilibria
The concentration of an ion in a poorly soluble compound can be reduced by increasing the concentration of an ion of the opposite sign in the same compound. For example, the ion concentration can be lowered by increasing the ion concentration accordingly. This principle
can be used to precipitate unwanted impurities from solution. The method of precipitation of poorly soluble compounds is used to purify water, for example, to soften it (reduce hardness). To reduce carbonate hardness, the liming method is used, in which lime is added to the treated water. As a result of the electrolytic dissociation of lime, the pH of the water increases, which leads to a shift in the carbon dioxide balance towards the formation of carbonate ions:
As a result, the product of the solubility of calcium carbonate is achieved and the latter precipitates:
In addition, with an increase in the concentration of hydroxide ions, the solubility product of magnesium hydroxide is achieved and the latter precipitates
The reactions that occur when lime is added can be written in molecular form by the equations
As can be seen, with the introduction of lime, the concentration of ions decreases (softening), (reducing alkalinity) and
The liming method is not suitable for reducing non-carbonate hardness. For these purposes, it is necessary to introduce a highly soluble salt containing carbonate ions. Usually, soda is used for this, which, when dissociated, produces ions
The carbon dioxide equilibrium can also be shifted to the right when heated:
As a result, the concentration of carbonate ions increases and the solubility product of calcium carbonate, which precipitates, is achieved. This softening method is called thermal. Hardness removed by heating is called temporary hardness. The thermal method is used only when there is no need for deep softening and when the water must be heated according to technology in other devices.
To purify natural and waste waters from impurities, methods of cationization, anionization and chemical desalination are widely used.
Ion exchange.
The ion exchange method is widely used to remove ions from water. Ion exchange occurs on ion exchangers, which are solid polyelectrolytes in which ions of the same charge sign are fixed on a solid matrix, and ions of the opposite charge sign are able to pass into solution and be replaced by other ions of the same charge sign.
Some have the ability to ion exchange natural compounds, for example aluminosilicates. However, synthetic ion exchangers, which are usually polymeric materials, have become more widely used. Copolymers of styrene with divinylbenzene and methacrylic acid with divinylbenzene can be mentioned as polymers that serve as the basis (matrix) for ion exchangers. The ion exchanger consists of a matrix on which there is big number functional groups. The latter are either introduced into the monomer or into the reaction mixture during polymerization, or grafted onto the polymer after polymerization. Functional groups are capable of dissociating in solution, with ions of one charge sign remaining on the ion exchanger, and ions of the other charge sign moving into the solution. Depending on which ions go into the solution, a distinction is made between cation exchangers and anion exchangers.
With cation exchangers, cations pass into solution, which can then be exchanged for cations in the solution. The functional groups of cation exchangers are usually sulfo groups, phosphoric acid groups, carboxyl groups, hydroxyl groups. When the ion exchanger comes into contact with a solution, these groups dissociate, sending ions into the solution. As a result of this, the ion exchanger is charged negatively, and the solution around the ion exchanger is charged positively. Depending on the degree of dissociation of functional groups, strong and weak cation exchangers are distinguished. The cation exchanger after the dissociation of functional groups can be conventionally denoted by the formula, and the ion exchange can be represented by the equation
where are cations participating in ion exchange. In anion exchangers, when functional groups dissociate, they send anions into the solution, and positively charged ions remain on the ion exchanger. The functional groups of anion exchangers are usually amino groups and quaternary ammonium bases. When these groups dissociate, the ion exchanger is charged positively, and the solution near the ion exchanger is charged negatively. The anion exchanger after the dissociation of functional groups can be denoted by the formula and the anion exchange can be represented by the equation
where are the anions participating in ion exchange. Anion exchangers can also be strong or weak.
Water cationization.
Most often, for the treatment of natural water by the cationization method, cation exchangers are used, in which the exchanged ions are Na+ ions (Na-cation exchangers) or H+ (H-cation exchangers). Na-cation exchanger exchanges Na+ ions for ions contained in natural water. Since the main cations in natural water are ions, water softening occurs during -cationization:
As a result of Na-cationization, both carbonate and non-carbonate hardness decreases. However, the salt content practically does not change, since ions pass into the solution. The cationization process consists of passing water through filters loaded with Na-cationite powder. As work progresses, the Na-cation exchanger filter is depleted (the ion exchanger goes into the Ca-Mg form). After the cation exchanger is depleted, it is regenerated. The regeneration process is the same ion exchange reaction, but carried out in the opposite direction. Typically, regeneration is carried out with a solution of table salt:
As a result of regeneration, the ion exchanger again restores its ability to soften water.
During H-cationization, the ions of the ion exchanger are exchanged for cations contained in water:
As a result of this exchange, ions are removed from the water
etc. In water, the concentration of ions increases which are partially bound by carbonate and bicarbonate ions:
As a result of H-cationization, water softens, alkalinity and salt content in water decrease. However, this reduces the pH of the water and makes it corrosive. Therefore, H-cationization is usually carried out in combination with other ion exchange methods. Regeneration of the N-cation exchanger is carried out with an acid solution. As an example, consider one of the reaction equations that occurs during the regeneration of an N-cation exchanger:
Cationization is used to purify not only natural but also waste water. Harmful wastewater cations are exchanged for harmless ion exchanger ions. For example, to remove ions from wastewater, the latter can be subjected to Na-cationization:
Cationization of natural and waste waters is usually carried out as one of the final stages for deep purification, since the cost of ion exchange treatment is quite high. If the concentration of impurities in the water is high, then the bulk of the impurities are first removed by other, cheaper methods.
Anionization of water.
Anionization involves the exchange of anions contained in water with anion resin anions. The ions exchanged are usually ions less commonly and other anions. The process of anionization of natural water can be represented by the following equations:
Anionization is used for purification natural waters, usually in conjunction with other methods. Anionization is also used to purify wastewater from harmful anions, for example ions of radioactive anions, etc.
Chemical desalination of water.
When creating powerful thermal power plants, a serious problem arose in obtaining large quantities high purity water. This problem was solved by developing a method of chemical desalination of water. Chemical desalting of water consists of sequential repeated treatment of water in H-cation exchange and OH-anion filters. As a result of H-cationization, H+ ions pass into water, and as a result of OH-anionization -
OH- ions. They mutually neutralize and, as a result, impurities remain on the ion exchangers. After the ion exchange filters are worn out, they are regenerated with acid and alkali solutions, respectively. Anions are the most difficult to remove from solution weak acids, especially silicic acid anions. For this purpose, strong anion exchangers are used, in which the functional groups are completely dissociated. Ion exchange with hydrosilicate anion proceeds according to the equation
Rice. XIV.3. Electrodialyzer circuit:
A - anode; K - cathode; - anion exchange membrane; M cation exchange membrane
Removing silicic acid anions is a very important operation in thermal power engineering, since this acid easily passes into high-pressure steam and then deposits on turbine blades, which reduces the efficiency of the power plant. Chemical desalting is the final operation for preparing water entering the steam generator. First, the bulk of impurities are removed by coagulation, sedimentation, etc. methods.
Electrodialysis.
Removing ionic impurities from solutions electrochemical method using membranes or diaphragms is called electrodialysis. Consider the removal of sodium sulfate from water in an electrodialyzer with ion exchange membranes. The simplest electrodialyzer (Fig. XIV.3) consists of three compartments separated by two ion-exchange membranes and two electrodes. The membrane consists of an ion exchange material capable of passing through either cations (cation exchange membrane - or anions (anion exchange membrane - Water containing sodium sulfate is supplied to the middle compartment of the electrodialyzer. When voltage is applied, sodium and hydrogen ions move through the catnonite membrane to the cathode and sulfate - ions and hydroxide ions through the anion exchange membrane - to anode A.
In accordance with the value of the electrode potentials (see § VII.3), only the reduction of hydrogen ions can occur at the cathode
Enter department II. (Anions can pass through the anion exchange membrane, but cations cannot. The cation exchange membrane allows cations to pass through and does not allow anions to pass through.) As a result, the ion concentration in compartments decreases, and in compartments II it increases, so purified water is removed from compartments, and from compartments II - a solution in which the salt concentration is increased (brine). The same reactions occur at the cathode and anode as in a three-chamber electrodialyzer.