RU2686146C1 - Water degassing method - Google Patents

Abstract

FIELD: chemistry.SUBSTANCE: invention can be used to produce deaerated and decarbonised water and its use in heat engineering. Method of water degassing includes preliminary clarification of initial water, supply to Na-cation filters, wherein hardness of softened water is maintained in range of 0.02–0.1 mg-eq/l. Solution of caustic soda is dosed into the softened water; the amount of sodium hydroxide is not more than 10–15 % of the amount of carbon dioxide in the water. Solution of sodium sulphite is then dosed into the water, wherein the amount of sodium sulphite is selected based on the concentration of dissolved oxygen in the treated water. Then, water is directed to reverse-osmosis water desalination unit, at that ratio of permeate to initial flow is set within 75–90 %. Obtained partially desalted and degassed permeate is supplied to consumer, concentrate is drained into sewerage.EFFECT: method provides higher efficiency of chemical bonding of oxygen dissolved in water, excludes increase of salt content of treated water, and also considerably reduces total salt content of treated water.3 cl, 1 dwg, 4 tbl, 1 ex

Description

Technical field

The invention relates to technological processes for the removal of dissolved corrosive gases CO ₂ and O ₂ from water, which cause corrosion of equipment and pipelines, and can be used to obtain deaerated and decarbonated water for its use in various technological processes.

Prior art

A known method of operation of the installation for the preparation of desalinated water containing sequentially connected heat exchanger for preheating the source water, pre-clarification unit, ultrafiltration unit, ultraviolet disinfection unit, filter unit and two-stage reverse osmosis unit. The source water, passing through a plate-type heat exchanger, is heated. A coagulant solution (for example, aluminum oxychloride grade “Aqua-Aurat 30”) and an alkali solution, for example, sodium hydroxide, are consistently dosed into the source water line. Then the heated water is mixed with the washing water of the ultrafiltration unit and sent to the pre-clarification unit. In the pre-clarification unit the formation and precipitation of flakes (sludge) occurs. The clarified water is collected in the receiving chute in the upper part of the clarifier and is fed by gravity into the receiving tank of the ultrafiltration unit, where purification from suspended substances, colloidal and organic impurities takes place. Further, the clarified water is pumped (one working and one reserve) for treatment into the UV sterilizer for disinfection. Then the water disinfected and free from suspended substances enters the stage of two-stage osmosis. For the correction of pH and for the removal of carbon dioxide after the first stage of osmosis and before entering the second stage of the osmosis unit, sodium hydroxide solution is dispensed. After cleaning on the reverse osmosis membranes, desalinated water is collected in a tank, from where pumps are supplied for production (see RF patent for invention No. 2506233, IPC C02F 9/04, C02F 103/04, publ. 10.02.2014).

The disadvantage of this method of operation of the installation for the preparation of desalinated water is that the scheme of water preparation does not take into account the preliminary removal of calcium ion from the source water or the dosing of salt formation inhibitor. Therefore, the first stage of reverse osmosis desalination will be subject to significant deposition of solid calcium carbonate. This process will proceed extremely quickly, since the method provides for the return of the concentrate after the second stage of reverse osmosis to the input of the first stage. This concentrate has a pH value of water more than 8.5, which will contribute to the deposition of calcium carbonate in the first stage of reverse osmosis. Another disadvantage is that not all non-condensable gases are removed from the water, but only carbon dioxide by dispensing sodium hydroxide solution before the second stage of the osmosis installation, while the dissolved oxygen is not removed from the water.

A method of obtaining desalinated water and high-purity water for nuclear power plants research centers, including the supply of treated water for pre-treatment on a bulk coal filter and microfilter, further desalination of water on two consecutive reverse osmosis filters by directing the filtrate through the first intermediate tank to the second input, and the second filtrate is used for the additional treatment of the ion exchange filter, the accumulation of purified water in the tank of purified water, the return of the concentrate of the second reverse osmosis filter in the original capacity and direction of the concentrate of the first reverse osmosis filter for discharge in the absence of radioactive or chemically toxic contaminants in it, and, if they are present, for disposal, while the filtrate of the first reverse osmosis filter with stiffness above 0.5 mg • eq / l is returned from intermediate tank in the source water tank for re-treatment in the first reverse osmosis filter, and with hardness up to 0.5 mg • eq / l the filtrate of the first reverse osmosis filter before the direction to the second return osmotic filter is corrected in the intermediate container by basification to pH 8,3-9,0 (see. RF patent for invention №2448057, IPC C02F 9/08, C02F 1/28, C02F 1/42, C02F 1/44, publ. 04/20/2012).

The main disadvantage of this method is the lack of removal of dissolved oxygen from water, as well as the lack of prior removal of calcium ion from the source water before the first stage of reverse osmosis.

The closest to the technical nature of the proposed invention is a known method of purification of natural waters, including two stages of mechanical processing, desalination with reverse osmosis, bactericidal treatment, while bactericidal processing is carried out by chlorination before mechanical treatment of water, then after two stages of mechanical treatment, sodium sulphite is dechlorinated, then the water is purified by microfiltration and the inhibitor is added, desalination by reverse osmosis is carried out in two stages, after the first stage, the end waste is dumped, and an inhibitor and caustic soda are added to the permeate, raising the pH to 10.4, then the second stage of desalination is carried out with reverse osmosis, and the concentrate after the second stage of reverse osmosis is mixed into the stream at the inlet of the first stage of desalination, and acid is added to the permeate and passed it through air-conditioning filters with calcium-magnesium loading (see RF patent for invention № 2225369, IPC C02F 9/08, C02F 1/50, C02F 1/44, C02F 103/04, publ. 10.03.2004).

The main disadvantage of this method is the use of sodium sulfite only for dechlorination of water. At the same time, dissolved oxygen remains in the water, which does not allow this water to be used directly for heat and power facilities. Also to the disadvantage can be attributed that the known method involves the use of only two-stage reverse osmosis desalination, which is economically unjustified for fresh water.

The present invention is to increase the efficiency of the process of chemical deaeration and decarbonization of water.

The technical result achieved when solving this problem is to increase the efficiency of the reaction of chemical binding of dissolved oxygen in water, the lack of increase in the salinity of the treated water, as well as a significant decrease in the total salinity of the treated water during chemical deaeration and decarbonization of water.

This technical result is achieved by the fact that the method of water degassing is that the water is pre-clarified, sent to Na-cationite filters, while the hardness of the softened water is maintained within 0.02-0.1 mg-eq / l, then to the softened water is metered with a solution of caustic soda, while the amount of caustic soda is chosen not more than 10-15% of the amount of carbon dioxide in water, then sodium sulfite solution is metered into the water, while the amount of sodium sulfite is selected based on the concentration of dissolved oxygen in the prepared then, the water is directed to the installation of reverse osmosis water desalination, the ratio of permeate to the initial stream is set within 75-90%, then the obtained partially desalinated and degassed permeate is sent to the consumer, and the concentrate is drained to the sewage system.

The amount of sodium sulfite is chosen either equivalent to the amount of oxygen dissolved in water or at least 10-30% more than the amount of oxygen dissolved in water.

The hardness of the softened water is maintained within 0.02-0.1 mEq / l, in order to prevent the deposition of hardness salts on the membrane of the reverse osmosis unit, in the absence of carbon dioxide in water, i.e. when the pH of the water is more than 8.37, otherwise the water hardness of more than 0.1 mEq / l will be enough to quickly clog the membranes with calcium carbonate,

The amount of caustic soda is chosen not more than 10-15% of the amount of carbon dioxide in water, while the condition that the deeper the initial water softens, the more caustic soda can be dosed, but not more than 15% of the amount of carbon dioxide in water to avoid excessive alkalinity of water , and, accordingly, in order to prevent the reduction of the selective properties of the membrane of the reverse osmosis unit.

The amount of sodium sulfite is determined by the fact that in water, after a reverse osmosis water desalination unit, the amount of dissolved oxygen must be contained, which is allowed when using prepared water (degassed permeate) in a particular process, based on the condition that per 1 mg of oxygen dissolved in water 8-10 mg of sodium sulfite is required. The amount of sodium sulfite is chosen either equivalent to the amount of oxygen dissolved in water, or at least 10-30% more than the amount of oxygen dissolved in water. If the amount of sodium sulfite is equivalent to the amount of oxygen dissolved in water, then there is a probability that oxygen will not be fully bound and removed from the water. If the amount of sodium sulphite is no less than 10-30% more than the amount of oxygen dissolved in water, this will provide an excess amount of sodium sulphite more than the equivalent in water, to ensure the binding of dissolved oxygen.

In this case, an excess of sodium sulfite is necessary for the guaranteed binding of all the dissolved oxygen in water. Excessive dosing of sodium sulfite will result in a large amount of sulphites being discharged into the sewage system. Dosing of sodium sulphite without excess may not ensure the process of complete binding of oxygen dissolved in water.

A solution of caustic soda is dosed before the sodium sulfite solution, since caustic soda raises the pH of the water and the reaction of oxygen binding by sodium sulfite to sodium sulfate proceeds faster.

The ratio of permeate to the source stream is set within 75-90%, with a decrease in the permeate to source water ratio below 75%, an unreasonably high flow of water discharged into the sewer system will be obtained, and with an increase in this ratio over 90%, a solid precipitate may be deposited osmosis.

Brief Description of the Drawings

The invention is illustrated by the image, which shows a flowchart of a water treatment system with membrane degassing.

Positions in the drawing indicate the following:

1 - installation of continuous water clarification;

2 - installation of a system of continuous Na-cationic water softening;

3 - installation of dosing solution of caustic soda;

4 - installation of dosing solution of sodium sulfite;

5 - fine filter;

6 - installation of reverse osmosis water desalination;

7 - membrane accumulator tank;

8 - boiler feed pump or filtrate pressure booster pump;

9 - accumulative high-temperature tank.

Detailed Description of the Invention

The proposed method of degassing water is as follows.

Method of degassing water, contains the following process steps. The water passes the clarification stage at the unit 1 for continuous clarification of water and enters the installation 2 of the system of continuous Na-cation-exchange water softening. It is advisable that at least two filters be installed that allow the system to operate in continuous mode. The hardness of softened water should be in the range of 0.02-0.1 mEq / l. The value of the hardness of the softened water will be determined based on the amount of caustic soda solution metered into the softened water after the installation of water softening 2. The higher the hardness of softened water and the greater the consumption of caustic soda, the higher the probability of formation of a solid precipitate of calcium carbonate on the membrane.

After installing 2 softening in water using the installation of 3 dosing is dosed with a solution of caustic soda. The amount of caustic soda is chosen not more than 10-15% of the amount of free carbon dioxide in water. Free carbon dioxide is bound to bicarbonate ion (equation 1)

NaOH + H ₂ CO ₃ = NaHCO ₃ + H ₂ O (1)

The pH value of the water increases to 8.2-8.5. Then, a solution of sodium sulphite is dosed into the water using the installation 4. The amount of sodium sulfite is chosen either equivalent to the amount of oxygen dissolved in water, or at least 10-30% more than the amount of oxygen dissolved in water.

Then the water, passing through the fine filter 5, enters the installation 6 reverse osmosis desalination of water. At installation 6 of reverse osmosis desalination, the source water is divided into two streams: permeate (desalinated water) and concentrate (water saturated with salts and discharged into the sewer system). The operation of this installation is organized so that most of the concentrate is returned to the input of the installation 6 reverse osmosis desalination. Thus, a recirculation of a part of the concentrate stream (recycling) is obtained.

The sodium sulphite entering the water through the dosing unit 4 reacts with dissolved oxygen. The result is sodium sulfate (Equation 2).

2Na ₂ SO ₃ + O ₂ -> 2Na ₂ SO ₄ (2)

This reaction proceeds rather quickly in hot water or in water with a pH value of more than 8.5. The water supplied to the unit 6 reverse osmosis desalination, has a temperature of from 2 to 40 ° C. Nevertheless, the reaction is not fast enough (equation 2) is compensated by the effective mixing of sodium sulfite in water in the fine filter 5 and in the reverse osmosis membrane element itself. Thus, most of the sodium sulfite, which did not bind oxygen in front of and inside the reverse osmosis element, returns to the input of the reverse osmosis element with a flow of recycling. This ensures a sufficiently complete reaction (equation 2) to and within the reverse osmosis element.

Reverse osmosis membrane element transmits gases dissolved in water, but practically does not pass salts of ions dissolved in water. Thus, carbon dioxide bound to bicarbonate with caustic soda does not pass through the membrane, but is discharged in the form of bicarbonate ion to the sewage system. The same principle works when binding oxygen dissolved in water. As a result of the reaction (equation 2), the oxygen dissolved in water is bound by sodium sulfite to sodium sulfate and then discharged with the concentrate stream to the sewer.

Thus, the reverse osmosis unit 6 undergoes a process of simultaneous desalting and degassing of water, which is a fundamentally new approach to the operation of such devices.

Desalinated and degassed water is sent to the consumer. It is important to prevent secondary water pollution by oxygen and carbon dioxide in atmospheric air. For this purpose, it is recommended to use a diaphragm accumulator tank 7 in front of the pump with booster 8. A booster pump 8 is needed if permeate pressure is required above 1.0-2.0 bar.

When operating the system as a water treatment system for steam and hot water boilers, prepared water should be sent either directly to the boiler or to a high-temperature storage tank 9 in which the water temperature is maintained at least 100 ° C.

Simultaneous desalting and degassing of water at the reverse osmosis unit can significantly reduce heat loss associated with the boiler blowdown, as well as the operation of the thermal deaerator. This significantly reduces the corrosivity of the returned condensate, simplifies the technology of water degassing and, accordingly, the number and composition of equipment, and also significantly reduces the cost of the entire water treatment system. The system is easily automated and does not require constant monitoring.

The proposed scheme is quite variable. If it is required to remove only oxygen from the water, then it is possible to refuse the use of a softening plant and exclude from the scheme the dosing of caustic soda solution before the reverse osmosis installation.

The invention is illustrated by the following example.

Example. The use of water degassing system according to the proposed method of water treatment for fresh water from a drinking water supply. Water is prepared for use in a low pressure steam boiler. The composition of the source water is presented in table 1.

Table 1 Indicator unit of measurement Value Total hardness mg-eq / l 3.7 Total alkalinity mg-eq / l 2.7 Chlorides mg / l 28 Sulfates mg / l 15 Sodium + Potassium mg / l five Salt content mg / l 269 Silicon (in the form of H ₄ SiO ₄ ) mg / l ots Dissolved iron mg / l 0.3 Oxidation mg O ₂ / l 4.8 Dissolved oxygen mg / l 10.2 Free carbon dioxide mg / l 11.5 PH value Unit pH 6.8-7.0

Technological scheme of the water treatment system is shown in the drawing.

The source water of this composition after the installation of 1 water clarification is supplied to the installation 2 of the system Na-cationic water softening. Unit 2 consists of two filter columns that operate in series. One column passes (softens) water, the second column is in regeneration or in standby mode. The columns are filled with ion exchange resin at 70% of their volume. The ion exchange resin is initially in the Na + form. The calcium and magnesium ions contained in the source water are exchanged in the column in the volume of cationite for sodium ions. The residual hardness of softened water should be from 0.02 to 0.1 mg-eq / l. After installing 2 softening, water is dosed using dosing stations 3 and 4 successively sodium hydroxide solution and then sodium sulfite solution. Both dosing stations 3 and 4 consist of a dosing pump with water lifting and pressure pipes with non-return valves and two polymeric tanks. Caustic soda reacts with free dissolved carbon dioxide in water. The result is sodium bicarbonate. The amount of metered caustic soda is 12.6 mg / l, which is equivalent to the amount of caustic soda to bind all free carbon dioxide. Thus, almost all of the original free carbon dioxide is bound to the bicarbonate ion. Sodium sulfite reacts with oxygen dissolved in water. The result is sodium sulfate. Sodium sulfite is dosed with a slight excess to ensure that all dissolved oxygen is bound. The amount of sodium sulfite being dosed is 93 mg / l, which is 10% more than the required equivalent amount of sodium sulfite for binding all dissolved oxygen. The composition of softened water after dosing of caustic soda and sodium sulfite are presented in table 2.

table 2 Indicator unit of measurement Value Total hardness mg-eq / l 0.05 Total alkalinity mg-eq / l 2.96 Chlorides mg / l 28 Sulfates mg / l 62.4 Sodium + Potassium mg / l 112.2 Salt content mg / l 378.2 Silicon (in the form of H ₄ SiO ₄ ) mg / l ots Dissolved iron mg / l less than 0.1 Oxidation mg O ₂ / l 4.8 Dissolved oxygen mg / l 0.05 Free carbon dioxide mg / l 0.1 PH value Unit pH 8.4

As can be seen from table 2, the salinity of water after dispensing caustic soda and sodium sulfite increases significantly. The pH value increases to 8.4 and the oxygen value decreases to 0.05 mg / l.

Then the water, passing the fine filter 5, enters the installation 6 of reverse osmosis water desalination. Water pressure rises before reverse osmosis desalination using a high-pressure centrifugal multistage pump. The pressure at which the process of reverse osmosis water separation takes place is 7-20 bar. At this pressure, a part of softened water seeps through the surface of the reverse osmosis membrane. The result is two streams: permeate and concentrate. The ratio between the original stream and the permeate is set at 85%. Those. To obtain 1 m ³ permeate, 1.15 m ^{3 of} source water is fed to the reverse osmosis unit. At the same time 0.15 m ^{3 of} concentrate is discharged into the sewer. Permeate - desalinated water supplied directly to the consumer or for correction. Concentrate - water flow saturated with salts, discharged into the sewer. The parameters of the reverse osmosis desalination process are set by adjusting the concentrate flow rate and the level of water pressure at the inlet to the membrane unit. The proportion of permeate in relation to the source water is 0.85. For example: from 1 m ^{3 of} source water, 0.85 m ^{3 of} permeate is obtained and, accordingly, 0.15 m ^{3 of} concentrate.

 The composition of the permeate is presented in table 3.

Table 3 Indicator unit of measurement Value Total hardness mg-eq / l less than 0.02 Total alkalinity mg-eq / l 0.25 Chlorides mg / l one Sulfates mg / l 2 Sodium + Potassium mg / l one Salt content mg / l 9 Silicon (in the form of H ₄ SiO ₄ ) mg / l ots Dissolved iron mg / l 0.0 Oxidation mg O ₂ / l 0.0 Dissolved oxygen mg / l 0.05 Free carbon dioxide mg / l 0.25 PH value Unit pH 7.5

As can be seen from table 3, the composition of the permeate meets the requirements for the feedwater of low-pressure steam boilers, in addition to the pH value. This parameter is easily adjusted.

Table 4 presents the composition of the concentrate and evaluated its ability to form solid deposits on the membrane.

Table 4 Indicator unit of measurement Value Total hardness mg-eq / l 0.28 Total alkalinity mg-eq / l 16.6 Chlorides mg / l 204 Sulfates mg / l 397 Sodium + Potassium mg / l 721.18 Salt content mg / l 2372.8 Silicon (in the form of H ₄ SiO ₄ ) mg / l ots Dissolved iron mg / l 0.3 Oxidation mg O ₂ / l --- Dissolved oxygen mg / l 0.05 Free carbon dioxide mg / l 4.0 PH value Unit pH 8.5 Langelier index (at 15 ^° С) 0.42

As can be seen from table 4, the Langelier index of the concentrate is in the range of 0-0.5, which indicates a slight ability of the concentrate to form solid calcium carbonate deposits.

The present invention is not limited to the above example, given only as an illustration of a specific variant of its implementation.

The proposed simple technology of membrane degassing of water allows to obtain a significant reduction in the cost of water treatment equipment, as well as a significant simplification of the technological scheme in comparison with thermal degassing. The proposed scheme also avoids the drawbacks of traditional chemical de-aeration of water, since in the proposed scheme there is no increase in the salinity of the chemically deaerated water. The proposed scheme allows to remove free carbonic acid from water, while the usual alkalization of water only binds carbonic acid without removing it from the water.

Claims (3)

1. The method of water degassing, which consists in the fact that the water is pre-clarified, sent to Na-cationite filters, while the hardness of the softened water is maintained within 0.02-0.1 mg-eq / l, then the caustic solution is metered into the softened water soda, while the amount of caustic soda is chosen not more than 10-15% of the amount of carbon dioxide in the water, then sodium sulfite solution is metered into the water, while the amount of sodium sulfite is chosen based on the concentration of dissolved oxygen in the prepared water, then the water is sent to the reverse system otic desalination of water, while the ratio of permeate to the original stream is set in the range of 75-90%, then the resulting partially desalinated and degassed permeate is sent to the consumer, and the concentrate is drained to the sewer. 2. The method according to p. 1, characterized in that the amount of sodium sulfite is chosen equivalent to the amount of dissolved oxygen in water. 3. The method according to p. 1, characterized in that the amount of sodium sulfite is chosen not less than 10-30% more than the amount of dissolved oxygen in water.

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