Implementation of devices for removing oxygen from water. Removal of gases from water. Deaeration of cold water
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 properties to cause or enhance the corrosion of metals.In addition, carbon dioxide is aggressive towards concrete, and the presence of hydrogen sulfide gives the water an unpleasant odor. In view of the above, the task of the most complete removal of these gases from water is urgent.
Water degassingis 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
The sulfur dioxide introduced into the 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 contributes to the 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
Water deoxygenation by the latter method is the most perfect, but at the same time, the most expensive method (due to the high cost of hydrazine). In this regard, this method is mainly used after physical methods of water deoxygenation in order to remove residual oxygen concentrations. At the same time, hydrazine belongs to the 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 the treatment of 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:
H 2 S + 4FROMl 2 + 4H 2 O-> H 2 SO 4 + 8HCl
The course of these reactions (as well as the intermediate reactions of the formation of thiosulfates and sulfites) occurs 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 process of water treatment is complicated and expensive by the need to use reagents. At high hourly flows through degassing with chemical reagents, with the relative simplicity of its implementation, it begins to lose much to thermal degassing in terms of operating costs.
b) Violation of the dosage of reagents leads to a deterioration in water quality.
These reasons cause a much 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 the water being purified from gas is actively in contact with air (provided that the partial pressure of the removed gas in the air is close to zero);
2) creation of conditions under which the solubility of gas in water decreases almost to zero.
Aeration usually removes free carbon dioxide and hydrogen sulfide from water, the partial pressure of which is atmospheric air close to zero. Degasifiers that carry out aeration, depending on the constructive device, the nature of the movement of water and air and the course of the degassing process, are divided into:
1) Film degassers (calciners) are columns with a packing (wooden, Raschig rings, etc.), through which water flows in a thin film. The purpose of the packing is to create an extensive water-air contact surface. The air blown by the fan moves towards the flow of water;
2) .They are blowing 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 atmospheric air. To remove oxygen, water is brought to a boil, and the solubility of all gases in water decreases sharply.
Bringing water to a boil is carried out:
1) its heating (in atmospheric deaerators);
2) lowering the boiling point of water by lowering the pressure (in vacuum deaerators).
AT 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 purging. In vacuum degassers (deaerators), special devices (such as vacuum pumps or water-jet ejectors) create a pressure at which water boils at a given temperature.
In the water treatment process, the main application is in the removal processes carbon dioxide found film degasifiers to remove hydrogen sulfide (together with a number of other tasks - supplying oxygen as an oxidizing agent to , ) - 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 cross-sectional area of the degasser, the height of the water column in it, the required air flow, the type and surface area of the nozzle required to achieve a given degassing effect.
The presence of oxygen in the heating steam system leads to corrosion of boilers, heating networks, and reduces the efficiency of heat transfer with steam.
There are chemical and physical methods to remove oxygen from the feed water. Physical methods of deaeration are carried out by vacuum, thermal method, nitrogen bubble deaeration.
Chemical Oxygen Removal Methods - MWT R Series Dosing Equipment
- On low pressure boilers up to 7.0 MPa, using sodium sulfite, sodium metabisulphite;
- On boilers of high, ultrahigh, supercritical pressure, using hydrazine hydrate (nitrogen and water are formed during oxidation), diethylhydroxylamine, isoascorbic acid, carbohydrazine, hydroquinone, film-forming amine - chelamine.
The degree of extraction of free oxygen to prevent boiler corrosion, corrosion of networks, depends on the temperature of the coolant, the volume of water. The oxygen content in feed water systems with single-stage aeration reaches a value of not more than 0.2 ml/l, and if the oxygen content is less than 0.07 ml/l, additional water treatment is applied by dosing chemicals.
Catalytic method for deep oxygen removal on a palladium catalyst, pressure filtration - equipment of the MWT Pl series
Deep removal of dissolved oxygen from water from 20 µg/l, calculated filtration rate from 5 – 80 m/h. The extraction of dissolved oxygen in the incoming water is based on the principle of the interaction of palladium ion exchange material with the reduction of oxygen by hydrogen. Catalytic filter material chemically resistant to acids, alkalis - insoluble in organic solvents, water, non-toxic, non-flammable, non-explosive. The filter is washed by reverse current in the presence of undissolved compounds, or without washing under conditions pure water up to 10 microns.
Specifications of the filter material:
Indicators |
Description |
Conformity |
Composition granulometric: |
0,45 – 1,05 |
resp. |
Mass fraction of water, % |
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Filtrate oxidizability in terms of oxygen, mg/g, not more than |
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Osmotic stability, %, not less |
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Bulk weight, kg/m3 |
Membrane Degassing for Deep Oxygen Removal - MWT MD Series Equipment
The use of deep oxygen removal technology for steam and hot water systems, using hydrophobic membranes in membrane contactors, makes it possible to achieve a deep degree of water purification up to 1 µg/l, and if necessary, oxygen removal less than 1 µg/l by two-stage degassing, with physical blowing off with gas and vacuum , with a preliminary decrease to 100 mcg / l.
Benefits of using MWT MD membrane degassing:
- Block scaling for increased productivity;
- Regulation of the degree of extraction of dissolved oxygen;
- Stable indicators High Quality degassing;
- Insignificant operating costs;
- Reagentless degassing.
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 consumed little 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 easiest and cheapest way to obtain water that does not contain oxygen is to use 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 applications, Advantage 456 is also recommended.
. Atmospheric deaerators with steam supply
For water deaeration 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, due 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 thermal scheme hot water boiler and chemical water treatment scheme.
. 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 operation of make-up pumps. The main disadvantage is the presence of a significant amount of equipment and pipelines under vacuum. As a result, air enters the water through the seals of pump shafts and fittings, leaks in flanged joints and welded joints. 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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