Research Institute of Nuclear Physics. D.V. Skobeltsyn Moscow State University M.V. Lomonosov Institute of Nuclear Physics (SINP MSU) proposes a new method of oxygen removal based on the initiation of oxidative radical chain reactions in water. At the Institute of Nuclear Physics of Moscow State University, generators of ozone-hydroxyl mixture were developed, which make it possible to initiate radical chain reactions of oxidation of impurities in water. The process of chain oxidation of a solution of phenol and phenolic compounds was experimentally observed. Wastewater*. It is proposed to use two processes leading to deoxygenation of water: blowing water with a gas that does not contain oxygen; radical chain reactions. The setup diagram is shown in fig. one.

The installation consists of a radical generator, an ejector pump (E), a buffer tank and pipelines. The flow of treated water will be 50 m3/h. 10% water, i.e. 5 m3/h, is fed to the ejector, which sucks the gas mixture out of the generator. A flash corona electric discharge burns in the radical generator, the discharge current is 15 mA, and the power consumption is 150 W. All gas cavities of the facility are purged with natural gas before the discharge is switched on. The gas is mixed with the liquid in the ejector. The gas-water mixture flow from the ejector enters the buffer tank, where it mixes with the main water flow and oil. Oil is added as the main substance that will interact with oxygen.

Oil consumption, taking into account its solubility (50 mg/l, or 50 g/m3), with a water flow of 50 m3/h, will be 2.5 l/h. Natural gas circulates inside the unit: it is sucked out of the radical generator by the ejector, mixed with water in the ejector, separated from the water in the buffer tank and returned to the radical generator through the return pipe. The oxygen separated from the water and carried away by the gas from the buffer tank burns part of the natural gas at the electrodes of the radical generator. The gas circulation rate is equal to the water circulation rate through the ejector (5 m3/h), while the gas is little consumed and almost all of it flows from the buffer tank back to the generator. Gas consumption is compensated by natural gas replenishment.

To do this, it is possible to organize gas blowing through the system with flame ignition in the outlet stream after blowing. The volume of the buffer tank should be such that the water retention time is greater than the oxygen removal time. This time can be up to 15 minutes (taking into account the inaccuracies made in the numerical estimates), i.e. tank volume - 10-15 m3. Approximate characteristics of the proposed installation for removing oxygen from water are as follows: water flow - 50 m3/h; the power consumed by the radical generator is 150 W; oil consumption - 2.5 l / h; gas consumption (for oxidation and drainage) - 500-1000 l / h; the volume of the buffer tank is 10-15 m3. The exact characteristics of the installation depend on the needs of the customers.

The constants required for the calculation of installations should be obtained as a result of research and development. SINP MGU manufactures radical generators with a power of 50 to 150 W, designed to oxidize impurities in water. They can be modified to generate organic radicals. Ejector pumps are also designed and manufactured at SINP.

* It should be noted that the simplest and cheap way obtaining water that does not contain oxygen is the use of water from underground sources where there is no oxygen. Traditional methods of removing oxygen from water, as well as the process of chain oxidation of a solution of phenol and phenolic wastewater, are discussed in the article "Removal of oxygen from water" on the website http://depni.sinp.msu.ru/~piskarev/ in the section "Projects requiring investment."

sometimes binding of oxygen and carbon dioxide is required. Deaeration can be done by various methods. Even in the presence of deaeration equipment (deaerator), it may be necessary to additionally reduce the concentration of dissolved oxygen and carbon dioxide using special .

Methods for deaeration of feed water in boiler rooms

. Use of reagents

To bind oxygen in feed and network water, you can use complex ones, which allow not only to reduce the concentration of oxygen and carbon dioxide to standard values, but also to stabilize the pH of the water and prevent the formation of deposits. Thus, the required quality of network water can be achieved without the use of special deaeration equipment.

. Chemical deaeration

The essence of chemical deaeration is the addition of reagents to the feed water, which make it possible to bind the dissolved corrosive gases contained in the water. For hot water boilers we recommend the use of a complex rust and scale inhibitor. To remove dissolved oxygen from water during water treatment for steam boilers - , which often allows you to work without deaeration. If the existing deaerator does not work correctly, then we recommend using a reagent to correct the water chemistry regime. For food production the use of Advantage Reagent 456 is also recommended

. Atmospheric deaerators with steam supply

For deaeration of water in boiler rooms with steam boilers mainly thermal two-stage atmospheric deaerators (DSA) are used, operating at a pressure of 0.12 MPa and a temperature of 104 °C. Such a deaerator consists of a deaeration head with two or more perforated plates, or other special devices, thanks to which the source water, breaking into drops and jets, falls into the storage tank, encountering countercurrent steam on its way. In the column, water is heated and the first stage of its deaeration takes place. Such deaerators require the installation of steam boilers, which complicate the thermal scheme of a hot water boiler and the scheme of chemical water treatment.

. Vacuum deaeration

In boiler rooms with hot water boilers, as a rule, vacuum deaerators are used, which operate at water temperatures from 40 to 90 ° C.
Vacuum deaerators have many significant disadvantages: high metal consumption, a large number of additional auxiliary equipment (vacuum pumps or ejectors, tanks, pumps), the need for location at a considerable height to ensure the performance of make-up pumps. The main disadvantage is the presence of a significant amount of equipment and pipelines under vacuum. As a result, through the seals of the shafts of pumps and fittings, leaks during flange connections and welded joints, air enters the water. In this case, the effect of deaeration completely disappears, and even an increase in the oxygen concentration in the make-up water is possible compared to the initial one.

. Thermal deaeration

Water always contains dissolved aggressive gases, primarily oxygen and carbon dioxide, which cause corrosion of equipment and pipelines. Corrosive gases enter the source water as a result of contact with the atmosphere and other processes, such as ion exchange. The main corrosive effect on the metal is oxygen. Carbon dioxide accelerates the action of oxygen, and also has independent corrosion properties.

Deaeration (degassing) of water is used to protect against gas corrosion. Thermal deaeration has found the greatest distribution. When water is heated at constant pressure, the gases dissolved in it are gradually released. When the temperature rises to the saturation (boiling) temperature, the concentration of gases decreases to zero. Water is freed from gases.

Underheating of water to a saturation temperature corresponding to a given pressure increases the residual content of gases in it. The influence of this parameter is very significant. Underheating of water even by 1 °C will not allow to achieve the requirements of "PUBE" for the feed water of steam and hot water boilers.

The concentration of gases dissolved in water is very low (of the order of mg/kg), so it is not enough to separate them from the water, but it is also important to remove them from the deaerator. To do this, it is necessary to supply excess steam or evaporation to the deaerator, in excess of the amount necessary to heat the water to a boil. With a total steam consumption of 15-20 kg / t of treated water, the evaporation is 2-3 kg / t. Reducing flash steam can significantly degrade the quality of deaerated water. In addition, the deaerator tank must have a significant volume, ensuring that water stays in it for at least 20 ... 30 minutes. A long time is necessary not only for the removal of gases, but also for the decomposition of carbonates.

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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 rather stringent requirements are set to reduce its concentration in process waters to a level of several µg/l. So, for example, in the food industry, oxygen contained in water degrades the quality of a number of products, in particular, it causes a decrease in the aging resistance of beer. In the energy sector, in order 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 of the microelectronics industry are already using a huge amount 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 fed through pipelines to workshops at the places of its use. Currently, ultrapure water is often used to wash silicon substrates in the production of integrated circuits. The presence of dissolved oxygen causes the formation of an oxide layer on the substrate surface, the growth rate of which depends on the time of interaction of water with the surface and on the concentration of dissolved oxygen. The formation of an oxide layer occurs even when ultrapure water with a low level of dissolved oxygen of 40-600 µg/L is used.

The removal of dissolved oxygen from water can be achieved by both physical and chemical methods. Chemical methods make it possible to carry out 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.

Conventional 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 the concentration of dissolved oxygen from a few parts per million to a level of several 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, no dispersion between phases, and the possibility of scaling (modularity of structures). These advantages make membrane methods an attractive choice among other available physical methods for oxygen removal. For example, recently at nuclear power plants in South Korea (Kori and Wolsung) new water treatment systems were installed, consisting of two compact membrane contactor modules with a total area of ​​260 m 2 . This technology makes it possible to reduce the content of dissolved oxygen in NPP process waters to 0.39 and 0.18 mg/l, respectively, by physical purge with a carrier gas and vacuum at 50°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, the use of additional equipment to create and maintain a 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 the removal of dissolved oxygen is the process of catalytic reduction of oxygen by hydrogen on a palladium catalyst with the formation of water. A significant disadvantage of such methods is the need to pre-saturate the water with hydrogen. This problem is now partially solved in industry by using special nozzles or membrane contactors. Thus, the existing catalytic removal methods require the process to be carried out in two stages: preliminary dissolution of hydrogen in water and subsequent reduction of dissolved oxygen in water with hydrogen on a palladium catalyst.

Recently, the A.V. Topchiev Institute of Petrochemical Synthesis RAS (INHS RAS), together with the Dutch Organization for Applied Scientific Research (TNO), developed and patented a method for applying metallic palladium to the outer surface of hydrophobic polymer membranes. The developed technology for applying a palladium catalyst to the outer surface of porous membranes in the form of nanosized 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 of 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 fed into the porous hollow fiber membrane and diffuses through the pores of the membrane to the outer palladium surface, where the reaction of oxygen reduction with hydrogen proceeds with the 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 polymeric membranes makes it possible to obtain catalytic membranes with a palladium content of less than 5 wt.%. According to scanning electron microscopy data, it can be seen that palladium is located on the outer side of the membrane (Fig. 2), while X-ray diffraction, EDA and EXAFS methods proved that palladium on the surface of hollow fibers is found only in a metallic form with a particle size of about 10-40 nm .

Fig.2. External surface Pd-containing porous polypropylene hollow fiber membranes: a – optical microscopy (70 times magnification), b – scanning electron microscopy (8500 times magnification).

The developed application method was successfully adapted to the non-separable commercial membrane contactor Liqui-Cel Extra Flow (1.4 m 2 ; USA). To study the process of removing dissolved oxygen from water, the gas mode was used, in which physical purge was completely excluded and removal was possible only due to the 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 the flow mode: 1 – helium (water flow rate 25 l/h); 2 – hydrogen (water consumption 25 l/h); 3 – hydrogen (water flow rate 10 l/h).

During pilot tests of a catalytic membrane contactor/reactor in the water recirculation mode in the system (temperature 20 ° 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 gas or steam consumption compared to the traditional physical purge process. The results obtained meet the most stringent industry requirements for ultrapure water at the present time.

Long-term (6 months) tests showed high stability of the catalytic activity of membrane contactors. It has been found that even in the event of catalyst poisoning or deactivation, it is possible to reapply palladium to the membrane surface of an operating membrane contactor/reactor.

As a result of the studies carried out by the Institute of 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 disassembly of industrial membrane contactors, ensuring the simplicity and scaling of their production to the required level. The cost of the process of applying palladium can be estimated at the level of 5-7 euros per 1 m 2 of the membrane.

The developed one-stage method for the removal of dissolved oxygen is completely ready for commercialization and makes it possible to obtain ultrapure process water for various areas of microelectronics, energy and food industries.

The removal of oxygen from water is carried out not only by desorption (physical), but also by chemical methods. The chemical binding of oxygen into corrosion-inert substances is carried out in several ways, each of which is based on redox processes. Since these processes are also typical for a number of typical water treatment methods, for example, for cleaning from biological contaminants, and are important in assessing the corrosion of structural materials of the main and auxiliary equipment, we will analyze their main provisions.

Redox reactions consist of the processes of oxidation (donating electrons to substances) and reduction (receiving electrons to substances). A substance that donates its electrons during a reaction is called a reducing agent, and a substance that accepts electrons is called an oxidizing agent. Some substances can exist in oxidizing and reducing forms and are able to change from one form to another by gaining or losing electrons. With the exception of oxygen and hydrogen, which are oxidizing and reducing agents, respectively, the remaining substances, depending on the conditions, can be either oxidizing or reducing agents, which is characterized by the redox potential of the reaction system or the redox potential. The redox potential depends on the activity of the redox form according to the Nornst equation:

where n is the number of electrons involved in the redox reaction; k is a temperature dependent parameter; E 0 is the standard potential, which determines the equality of the activities of the oxidizing and reducing forms.

The redox potential is a measure of the redox and redox capabilities of a system. The strongest oxidizing agents are ions and used to determine permanganate or dichromate oxidizability, as well as fluorine, ozone and chlorine.

Chemical methods for removing dissolved gases from water consist in binding them into new chemical compounds. Strict regulation of oxygen content when using reducing water regimes in the circuits of thermal power plants with drum boilers, in heating networks determines the need to use not only physical methods of degassing, but also chemical methods of additional deoxygenation based on redox reactions.

The reducing agents used include reagents such as sodium sulfite, hydrazine, and redox groups created on high molecular weight, water-insoluble polymers.

Water treatment with sodium sulfite is based on the oxidation reaction of sulfite with oxygen dissolved in water:

2Na 2 SO 3 + O 2 2Na 2 SO 4.

The reaction proceeds quite quickly at a water temperature of at least 80 0 С and pH ≤ 8. This deoxygenation method is used only for medium-pressure boilers (3–6 MPa) and for make-up water of the heating network, since at temperatures above 275 0 С and pressure more 6 MPa sulfite undergoes hydrolysis and the process of self-oxidation - self-healing:

Na 2 SO 3 + H 2 O 2NaOH + SO 2; 4Na 2 SO 3 Na 2 S + 3Na 2 SO 4 .

For once-through boilers and drum boilers of high and ultra-high parameters, deoxygenation of water with hydrazine in the form of hydrazine hydrate (N 2 H 4 ∙ H 2 O) is used, which does not increase the salinity of water.

N 2 H 4 ∙ H 2 O O 2 3H 2 O + N 2.

The main factors determining the rate of this reaction are temperature, pH, excess hydrazine, and the presence of catalysts. Thus, at a temperature of 105 0 C, pH = 9 ÷ 9.5 and an excess of 0.02 mg/kg hydrazine, the time for complete oxygen binding is 2–3 seconds. At pH< 7 гидразин практически не связывает кислород. При рН = 9 ÷ 11 достигается максимум скорости реакции. Органические катализаторы интенсифицируют реакцию, повышая скорость взаимодействия в 25 – 100 раз. Каталитически влияют на скорость реакции также соединения меди и некоторых других металлов.

In boiler water and in superheaters, excess hydrazine decomposes with the formation of ammonia:

3N 2 H 4 4NH 3 + N 2.

In the presence of metal oxides, the decomposition of hydrazine with the release of H 2 is also possible:

3N 2 H 4 2NH 3 + 3H 2 + 2N 2 .

Redox reactions can be carried out by filtering water through water-insoluble macromolecular substances containing redox groups capable of reversible oxidation and reduction. An example of such substances are electric ion exchangers (EI) used in deoxygenation schemes for additional water of heating networks that has passed the preliminary stage of thermal deaeration. EI is obtained by introducing into the structure of the ion exchanger during the synthesis of the material. On such resins, simultaneous and independent occurrence of ion-exchange and redox processes is possible. EI can be obtained on the basis of copper and bismuth.

The determining factor in choosing the type of ion exchanger for packing redox substances on it is the ability of the matrix to firmly hold the deposited compounds. This ability depends on the sign of the ionite surface charge.

Lecture #10

Organization of chemical deoxygenation.

Sodium sulfite solution for medium pressure boiler feed water treatment is prepared in a tank protected from contact with the atmosphere. A solution with a concentration of 3 - 6% is introduced into the supply pipeline in front of the pumps using washer and plunger dispensers. The dose of sodium sulfite for processing 1 m 3 of feed water after thermal deaeration is calculated by the formula:

where g is the consumption of technical sulfite, g/m3;

Oxygen concentration in treated water, g/m 3 ;

k - excess reagent (2 - 3 g / m 3);

When organizing hydrazine treatment, it is necessary to take into account the properties of hydrazine hydrate. Hydrazine hydrate N 2 H 4 · H 2 O is a colorless liquid that readily absorbs oxygen, carbon dioxide and water vapor from the air, and is readily soluble in water. Hydrazine is toxic at concentrations greater than 40%, flammable, delivered and stored as a 64% solution in sealed stainless steel containers. Hydrazine vapors cause irritation of the respiratory tract, organs of vision, hydrazine solutions act on the skin, therefore, when handling hydrazine, the relevant safety regulations must be strictly observed.

The calculated dose of hydrazine should take into account not only its consumption for oxygen binding, but also for interaction with metal oxides. Its dosage is calculated by the formula:

g g \u003d 3C 1 + 0.3C 2 - 0.15C 3,

where g g is the calculated dose of hydrazine hydrate, mg/kg;

C 1 - C 3 - the concentration in the feed water, respectively, of oxygen, iron and copper compounds, mg / kg.

Dosing of hydrazine is carried out at one of two points: at the suction of feed pumps or into the turbine condensate before the heater low pressure(PND). The estimated amount of 100% hydrazine φ, mg/kg, required for loading into the pre-dilution tank, is determined from the ratio:

where D - feed water consumption, m 3 / h;

τ is the time between tank recharges, h.

The tank capacity of 10 m 3 for 20% concentration hydrazine provides approximately two months of reagent supply for a hydroelectric power station (GRES) with a capacity of 3600 MW.

At a given feed water flow rate, the hourly reagent consumption d, kg/h, is calculated by the formula:

Typically, an excess concentration of hydrazine is maintained in the feed water during normal operation, 0.03 - 0.06 mg/kg.

We will consider the technology of using chemical deoxygenation using the example of using an iron oxide electric ion exchanger (EI). EI of this type is capable of deoxygenation and at the same time softening of water in schemes with preliminary vacuum deaeration. Preliminary deaeration of water ensures its heating to 60 - 80 0 C and partial removal of dissolved oxygen, which positively affects the efficiency of the method under consideration. Under the noted temperature conditions, the process can be based on typical designs of ion-exchange filters. With the initial oxygen content of the treated water up to 1 mg/kg, the electric ion exchanger provides a decrease in the oxygen content to 5–20 µg/kg.

The presence of iron hydroxide on the surface of the electric ion exchanger also contributes to iron removal.

The given technological characteristics provide high efficiency of using this material for deoxygenation of make-up water of a closed-type heating system.

Water purification by distillation methods.

distillation method.

Purification (desalination) of waters with a high salinity, including sea waters, as well as the processing of highly mineralized waste solutions in order to protect the environment is the most important scientific and technical task.

Treatment of highly mineralized waters and solutions can be carried out, firstly, by removing dissolved impurities from water, which is realized, as a rule, without phase transitions of the solvent (water) into a vapor or solid state; secondly, by the method of extracting hydrogen molecules from a solution, based on a change in their state of aggregation (by distillation).

The first way of extracting salts from a solution is theoretically more appropriate, since the mole fraction of dissolved even highly mineralized impurities is about 100 or more times less than the number of water molecules themselves. However, technical difficulties in implementing such a path do not allow in all cases to economically realize this advantage.

When heated aqueous solutions water molecules acquire energy exceeding the forces of molecular attraction and are carried out into the vapor space. When the saturation vapor pressure in water becomes equal to the external pressure, the water begins to boil. Ions and molecules of dissolved substances contained in water and in a hydrated state do not have such an energy reserve and pass into steam at low pressures in a very small amount. Thus, by organizing the process of boiling aqueous solutions, it is possible to separate the solvent (water) and the impurities contained in it. Distillation (thermal desalination) is carried out in evaporation plants (Figure 1), in which water, due to the production of heat from the primary steam supplied to the heating system, is converted into secondary steam, which is then condensed.

Figure 1 - Scheme of the evaporation plant:

1 – primary steam supply line; 2 - heating section; 3 – evaporator housing; 4 - line for the removal of the formed (secondary) steam; 5 - capacitor; 6 – primary steam condensate discharge line; 7 - feed water supply line; 8 – purge line; 9 - emptying line; 10 – distillate withdrawal line.

Primary steam is usually taken from a steam turbine. Substances that pollute the water remain in the volume of evaporated water and are removed from the evaporator with the discharged (purge) water. The distillate - secondary steam condensate - contains only a small amount of non-volatile impurities that enter it due to the droplet entrainment of evaporated water (concentrate).

Assuming in the first approximation that the transition of impurities into the secondary steam is equal to zero, we will estimate, on the basis of the material balance in the evaporator, the concentration of impurities in the evaporator water C w. The material balance equation has the form:

R p.v C p.v \u003d R p C p + R pr C v.i,

where R p.v - feed water consumption (P p.v \u003d R p + R pr);

P p - steam capacity.

Considering that С n = 0, (Р n + Р pr)С a.c. = Р pr С v.i, whence .

The larger the blowdown, the lower the concentration of impurities in the evaporator water (in the blowdown). The negative temperature coefficient of solubility of hardness salts during the evaporation of water concentrations of Ca 2+, Mg 2+,,, OH ions - up to limits exceeding the product of the solubility of CaCO 3, CaSO 4 and Mg (OH) 2, is the reason for the formation of scale on heat transfer surfaces in evaporators . Scale formation reduces the performance of evaporators and worsens their technical and economic performance.

Evaporation plants are single- and multi-stage. If the secondary vapor is condensed directly in the evaporator condenser, then such an evaporative plant is a single-stage one. In multi-stage installations (Figure 2), the secondary steam of each stage, except for the last one, is used as heating steam for the next stage and condenses there.

Figure 2 - Scheme of a multi-stage evaporative plant:

1 - heating steam supply line; 2 - 4 - evaporator, respectively, 1 - 3 steps; 5 – secondary steam outlet line; 6 - capacitor; 7 – condensate drain line; 8 - feed water supply line; 9 – feed water heater; 10 - purge line.

With an increase in the number of stages, the amount of condensate (distillate) obtained in the evaporation plant from one ton of primary steam also increases. However, with an increase in the number of stages, the temperature difference between the heating and secondary steam decreases, which necessitates an increase in the specific heat exchange surfaces, which ultimately leads to an increase in overall dimensions, specific metal costs, and an increase in the cost of the installation.

The power supply of a multi-stage installation can be carried out in parallel with the supply of each evaporator from a common collector, but more often - in series, as shown in Figure 2. In this case, all the feed water is supplied to the first stage of the installation, and then, after its partial evaporation, the water flows into the next stage, and from the latter it is discharged into the drain. Multistage evaporation plants are used in combined heat and power plants with large total and external losses of steam and condensate. Single-stage evaporation plants are used at condensing power plants (CPPs) with small losses (1-3%) and are included in wastewater treatment schemes of water treatment plants with prohibited discharges.

At present, distillate is mainly produced from water previously softened on ion-exchange filters, but in some cases water that has undergone simplified processing is used. The steam supplied to the evaporator is called primary, and the steam formed from the water entering the evaporator is called secondary.

In flash evaporators, steam is formed not by boiling, but by boiling water, preheated to a temperature several degrees higher than the saturation temperature of water, in the chamber in which vaporization occurs. They do not require high quality feed water, since the process of evaporation of water during boiling is carried out without heat transfer through the surface. Flash units are also called adiabatic or "flash". Since the saturation temperature depends on the saturation pressure, when boiling at a pressure below atmospheric, it is possible to organize the operation of the evaporators of this type at a temperature below 100 0 C, which reduces the likelihood of scale formation.

A single-stage forced circulation flash evaporator works as follows (Figure 3).

Figure 3 - Single-stage forced circulation flash evaporator.

The source water enters the condenser 1, after which part of it is sent to the evaporation chamber 3. The circulation pump 5 takes water from the evaporation chamber and pumps it through the heater 6, returning the water through the nozzle 2 to the evaporator body. When non-condensable gases are sucked out by steam ejector 8, the pressure in the chamber decreases below the vapor saturation pressure, as a result of which evaporation occurs from the surface of the droplets and the mirror. Separation of moisture droplets is carried out in device 7. The distillate is pumped out of the evaporator by pump 4, its amount in single-stage plants is approximately equal to the amount of condensing steam.

Flash evaporators can be built according to a multi-stage scheme, which provides a lower specific heat consumption. In seawater desalination plants, the number of stages can reach up to 30 - 40. When such a plant is included in the scheme of regenerative heating of boiler feed water, it is performed according to the heat balance conditions as a single stage or has three or four stages.

Prevention of scale formation in evaporative plants.

The experience of operating evaporators with salt water supply indicates serious difficulties arising from the rapid formation of scale on heat transfer surfaces, a decrease in the heat transfer coefficient α and a decrease in the efficiency of the evaporators.

The growth of a dense layer of crystalline deposits occurs from a supersaturated solution as a result of the growth of crystals existing on the surface (primary scale formation), as well as due to the adhesion and adsorption of fine particles already formed in the evaporated water (secondary scale formation).

As a rule, scale formation of both types proceeds simultaneously. The formation of scale on the surface can be represented as follows: the formation of embryonic crystals in the recesses of metal microroughnesses; the appearance of formations such as a coral bush; filling the gaps between the branches of the "bush" with small particles of the solid phase formed in the solution and transported to the heat transfer surface.

Methods for carrying out calculations related to the assessment of scale formation intensity have not yet been developed, since all the factors influencing this process are far from being studied, in particular, it is necessary to know the exact values ​​of the scale-forming ion activity coefficient for the actual parameters of the evaporator operation.

Methods for dealing with scale formation in evaporators can be divided into physical, chemical and physico-chemical; in addition, it is possible to use special designs and materials for evaporators to reduce scale formation.

Reagentless methods.

The contact stabilization method was proposed by Langelier and so named because of the absence of solid phase precipitation on the heat transfer surface during its use. It is based on the fact that the energy of crystal formation on undissolved impurity particles is less than the energy of spontaneous appearance of crystallization centers. Crystallization on a stabilizer substance proceeds at a lower supersaturation of the solution. Due to the many centers of crystallization is the deposition of excess over solubility of the number of scale formers. Crushed materials are used as a stabilizer: limestone, marble, sand, through the filter layer of which evaporated water circulates.

The height of the filter should be 1.8 - 2 meters. The rate of rise of the brine in order to avoid the entrainment of the stabilizing material should not exceed 35 m/h. The use of contact stabilization makes it possible to reduce the amount of scale in the evaporator by 80 - 90%, but it is structurally complex.

Magnetic treatment of water consists in pumping it through an apparatus in which a magnetic field is created. It is known that installations are equipped with magnetic devices, when water is not stable, that is, it is supersaturated in CaCO 3, they work efficiently. The theory of magnetic treatment has not yet been formed, but the following studies have established the following. Contained in water that is transported through steel pipes, ferromagnetic corrosion products and colloidal particles with electric charge and magnetic moment accumulate in the magnetic field created by the magnetic apparatus. An increase in the concentration of the solid microphase in the gap of the magnetic apparatus promotes the crystallization of calcium carbonate from unstable water in its volume, as a result of which the rate of scale formation decreases, but the concentration of sludge increases with further heating and evaporation of water subjected to magnetic treatment. Since the chemical and disperse composition of natural water impurities change over seasons and regions, and the degree of CaCO 3 supersaturation of water also depends on temperature, the efficiency of magnetic treatment can vary over a wide range up to zero values.

Ultrasonic processing during the evaporation of water can create due to elastic mechanical vibrations environments of significant energies conditions leading to a violation of the kinetics of crystallization in the near-wall layer. The action of ultrasonic waves on the heating surface can excite alternating bending forces at the boundary of crystalline bonds with the surface, which ultimately cause scale exfoliation. The mechanism of the impact of ultrasound on scale formation is not fully understood.

E.F. Tebenikhin, Reagent-free methods of water treatment in power plants. M.: Energoatomizdat, 1985.

Lecture #11

Prevention of scale formation in evaporators

installations by chemical and other methods.

Chemical methods. Stabilization by acidification is used to prevent the formation of calcium carbonate and magnesium hydroxide scale on the heat transfer surfaces.

Natural water containing Ca 2+ , , , CO 2 , depending on the state of carbon dioxide equilibrium of the system, can be aggressive, stable or unstable. The main criterion for the stability of such a system, used in practice, is the “stability index” proposed by Langelier.

For natural waters, the ratios pH equal ≥ pH fact are fulfilled. The difference between the actual and equilibrium values ​​is denoted by Y and is called the stability index or the Langelier index:

pH fact - pH equal = Y.

At Y = 0, water is stable, at Y< 0 она агрессивна, при Y >0 water is unstable and prone to deposit formation. In stabilized water treatment by acidification, the stability index is close to zero. Knowing the nature of the change in pH fact \u003d f 1 (A) and pH equal \u003d f 2 (A) with a decrease in the alkalinity of water as a result of acidification, these equations can be solved in relation to ΔA (decrease in alkalinity to a stable state).

The required dose, mg / kg, technical sulfuric or of hydrochloric acid can be determined by the formula:

where e is the equivalent mass of acid, mg-eq/kg;

The dose of acid depends on the alkalinity of the feed water, the temperature of the distillation process and the evaporation rate and is usually 70 - 90% of the alkalinity of the source water. An overdose of acid can cause corrosion of the structural materials of the evaporation plant, and therefore careful control of the dosing process is necessary. The use of sodium bisulfate is similar to acidification, since as a result of the dissociation of NaHSO 4, hydrogen ions are formed.

For acidification, ferric chloride can be used, while along with the hydrogen ion during hydrolysis, a suspension of iron hydroxide is formed, the particles of which serve as centers of crystallization of scale formers.

Physical and chemical methods. They are based on the use of chemical reagents-additives-surfactants introduced into the evaporated water in such a small amount (1-20 mg/kg) that their reaction with water impurities does not play a significant role. The effectiveness of such additives is due to the fact that, due to their high surface activity, the crystallization of scale formers on the heating surface is sharply reduced. Surfactants are adsorbed in the form of a monomolecular film on the surface of seed crystals, or hinder their adhesion on the surface.

Strong stabilizing-peptizing properties, capable of preventing coagulation of particles in a wide range of solids content, are characterized by some antiscale substances present in water, usually in the form of micelles and micromolecules.

In addition to the listed reagents, some complexing agents are also used, for example, sodium hexametaphosphate Na (NaPO 3) 6 and some other polyphosphates.

At high temperature(up to 120 0 C) and high water hardness, a good effect was the use of anti-scale reagents containing polyacrylic acid, EDTA salts (trilon B), sulfonic acid and others.

In addition to the above, scale is removed (cleaned) from the surfaces of the apparatus by a chemical method using reagents - sulfuric, hydrochloric, citric, acetic and others.

Technological methods for limiting scale formation. They are primarily used in evaporative systems with vertical tube heating sections. Examples of technological methods for limiting scale formation can be the use of an organized gas removal (gas purge) of evaporators to saturate the feed water with carbon dioxide. During the thermal decomposition of bicarbonates into the gas phase, as is known, carbon dioxide is released. By mixing it with water in an amount that exceeds the equilibrium value, the water is given aggressive properties with respect to calcium carbonate, which prevents its release in feedwater heaters. It should be taken into account that with an excess content of carbon dioxide in water, which lowers pH, corrosion processes of structural materials are intensified.

Methods for obtaining pure steam in evaporation plants.

Pollution of saturated steam with inorganic compounds is associated, firstly, with the entrainment of moisture (mechanical entrainment) and, secondly, with the solubility of certain substances in water vapor. The main contribution to steam pollution is made by mechanical (droplet) entrainment. Usually, the evaporated water is carried out in the form of droplets with a size of 0.5 to 3 microns, which are formed during the destruction of steam bubbles that go beyond the limits of the water volume.

The removal of salts with steam is intensified when the evaporator water is foamed, and the structure of the foam depends on the load and pressure in the evaporator. It should be emphasized that the patterns of entrainment of moisture droplets by steam operate in the same way both for evaporative installations and for other units that produce steam. To ensure high purity of steam in evaporators, the following is used: volumetric separation in the steam space, for which the height of the steam space is chosen to be at least 1.5 meters, and for highly foaming solutions - 2.5 - 3 meters; perforated sheets in front of steam pipes to equalize steam velocities in this zone; louvered separators for trapping drops of moisture.

An effective remedy Steam cleanliness is ensured by flushing the steam with feed water. Washing is usually carried out by bubbling small steam bubbles through a layer of washing water, the salinity of which is much less than the salt content of the evaporated water, which ensures a washing efficiency of at least 90%. At high requirements to the quality of the distillate, steam washing is carried out with external or own condensate, in some cases a two-stage steam washing is organized. The considered measures make it possible to obtain distillate when feeding evaporators with softened water that meets the requirements of the PTE of power plants and networks, used for feeding without additional purification as make-up water (feed water) for drum boilers. At power units with once-through boilers, additional distillate purification at the BOU is required.

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§ 132. Removal of dissolved gases from water

Most often, the process of water treatment requires the removal of carbon dioxide, oxygen and hydrogen sulfide. All three gases are corrosive gases that cause or enhance the processes of corrosion of metals. Carbon dioxide is also aggressive towards concrete. The property of these gases to cause and enhance corrosion processes, as well as the unpleasant odor that hydrogen sulfide imparts to water, in many cases makes it necessary to remove them from the water as completely as possible.

A set of measures related to the removal of gases dissolved in water from water is called water degassing.

Chemical and physical methods of water degassing are used.

The essence of the former is the use of certain reagents that bind gases dissolved in water. For example, deoxygenation of water can be achieved by introducing sodium sulfite, sulfur dioxide or hydrazine into it. Sodium sulfite, when introduced into water, is oxidized by oxygen dissolved in water to sodium sulfate:

2Na2SO3 + O2 -> 2Na2SO4.

In the case of the use of sulfur dioxide, sulfurous acid is formed:

SO2 -f H2O - "- H2SO3,

which is oxidized by oxygen dissolved in water to sulfuric acid:

2H2SO3-fO2-*-2H2SO4.

A chemical reagent with which it is possible to achieve

almost complete deoxygenation of water is hydrazine.

When it is introduced into water, oxygen is bound and inert nitrogen is released:

N2H4 + O2->-2H2O-f-N2.

Last chemical method deoxygenation of water is the most perfect, but at the same time the most expensive due to the high cost of hydrazine. Therefore, this method of application is mainly for the final removal of oxygen from water after physical methods of its deoxygenation.

An example of a chemical method for removing hydrogen sulfide from water is the treatment of water with chlorine:

a) with oxidation to sulfur:

HJS+C12-»-S+2HC1;

b) with oxidation to sulfates:

H2S + 4C12 + 4H2O -> H2SO4 + 8HC1

These reactions (as well as the intermediate reactions of the formation of thiosulfates and sulfites) proceed in parallel in certain ratios, which depend primarily on the dose of chlorine and the pH of the water. Chemical methods of gas removal are characterized by the following disadvantages: a) the need to use reagents that complicate and increase the cost of water treatment; b) the possibility of deterioration of water quality in case of violation of the dosage of reagents. As a result, chemical gas removal methods are used much less frequently than physical ones.

Physical methods for removing dissolved gases from water can be carried out in two ways: 1) water containing the removed gas is brought into contact with air if the partial pressure of the removed gas in air is close to zero; 2) conditions are created in which the solubility of the gas in water becomes close to zero.

With the help of the first method, i.e., with the help of aeration of water, free carbon dioxide and hydrogen sulfide are usually removed, since the partial pressure of these gases in atmospheric air is close to zero.

The second method usually has to be resorted to when deoxygenating water, since at a significant partial pressure of oxygen in atmospheric air, oxygen cannot be removed from it by aeration of water. To remove oxygen from water, it is brought to a boil, at which the solubility of all gases in water drops to zero. Water is brought to a boil either by heating it (thermal deaerators) or by lowering the pressure to such a value that the water boils at its given temperature (vacuum degassers).

Removal of dissolved gases from water in the process of water treatment is carried out on degassers various types, which, according to their design, the nature of the movement of water and air, and the environment in which the degassing process is carried out, can be classified as follows:

1) film degassers, which are columns loaded

with one or another nozzle (wooden, Raschig rings, etc.),

through which water flows in a thin film. The nozzle is used to create

developed surface of contact between water and air injected

fan towards the flow of water;

2) bubbling degassers, in which I slowly move through the bed

compressed water is blown through with compressed air;

3) vacuum degassers, where with the help of special devices

(vacuum pumps or water jet ejectors), this pressure is generated

the temperature at which water boils at a given temperature.

In water treatment technology, film degassers are mainly used, and vacuum (or thermal) ones are used for deoxygenation of water. Bubble degassers are used as an exception due to the relatively high operating cost (electricity consumption for air compression).

When designing degasifiers, the following quantities must be determined: the cross-sectional area of ​​the degasser, the required air flow, the surface area of ​​the nozzle required to achieve a given degassing effect.

The cross-sectional area of ​​the degassers should be determined by the admissible spray density of the nozzle, i.e., by water consumption per 1 m2 of the cross-sectional area of ​​the degasser. With deep removal of carbon dioxide from water (up to 2-3 mg / l) on degassers loaded with Raschig rings (25X25X3 mm), the allowable irrigation density of the nozzle is 60 m3 / (m2 "h), the specific air consumption is 15 m3 / m3; on degassers loaded with a wooden nozzle from boards, respectively 40 m3 / (m2 "h) and 20 m3 / m3; when deoxygenating water on vacuum degassers, the admissible density of nozzle irrigation is 5 m3 / (m2 "h).

The required surface area of ​​the nozzles loaded into the degasser is determined by the formula given in § 131. The methods for determining the remaining quantities included in this formula are also indicated there. The K values ​​are found for each type of degasser according to the respective graphs1.