Water degassing using reverse osmosis membranes

Tikhonov I.A.

The presence of dissolved corrosive gases CO2 and O2 in water causes corrosion of equipment and pipelines. As the water temperature increases, the mobility of oxygen molecules increases, and the corrosive aggressiveness of water increases.

The problem of removing oxygen and carbon dioxide from water is solved mainly in two ways. It is thermal and chemical degassing (deaeration).

Thermal degassing removes dissolved gases from the water in the deaeration column. The water in the state of saturation spreads on the plates of the deaeration column like a thin film. In this case, part of the water is evaporated, carrying with it dissolved gases that are released from the surface of the water when it boils. The larger the surface of water evaporation and the higher the saturation temperature, the more efficient the water degassing is.

 Gases are not removed by chemical degassing. They are only turned into inorganic compounds.

When using sodium sulfate

2Na2SO3+O2–>2Na2SO4           (1)

When using hydrazine hydrate

N2H4*H2O+O2–>3H2O+N2        (2)

The turning of carbon dioxide into bicarbonate ion (alkalization) occurs by the reaction:

NaOH+H2СO3=NaHCO3+H2O       (3)

Chemical deaeration and alkalinization has a number of significant disadvantages:

1.          The salinity of the feed water significantly (for surface water up to 50 percent or more) increases when chemical deaeration has been carried out and, respectively, the continuous blowdown of steam boiler increases. To bind 1 mg of oxygen, 10 mg of sodium sulfite is spent. It should be noted that when using hydrazine hydrate, the salt content of water does not increase, but the reagent itself is extremely toxic (belongs to the first class of danger), fire hazardous and requires specific storage conditions, which excludes its use for steam boilers, especially those working in food production.

2.          Sulfites remain in the water after the chemical deaeration, this is due to their excessive dosing for the reliable bonding of oxygen. As a rule, boiler manufacturers strictly regulate the content of sulfites in boiler water (5-10 mg/l), which presents a significant difficulty in the organization of the process of dosing of sodium sulfite into feed water. Sulfite ion (SO3) is a strong reducing agent and significantly enhances the corrosion processes occurring in the boiler and the steam condensate path by destroying the passivating layer on the metal surface. Interaction of sulfite ion with the product is not allowed. Sodium sulfite belongs to the substances of the 3rd hazard class. Sodium sulfite is most suitable for binding a small amount of residual oxygen in the feed water after thermal deaerator.

3.          Another point that is not always taken into account is that when sodium sulfite is dosed into water, sodium sulfate Na2SO4 is formed, which, in fact, increases the content of ion sulfate in the feed water and when hardness salts enter the boiler insoluble calcium sulfate CaSO4 (gypsum) may be formed in the boiler, it is possible to form insoluble calcium sulfate CaSO4 (gypsum). Calcium sulfate forms dense deposits (contamination) on the evaporative surfaces, which significantly increase the thermal resistance, and leads to overheating of the pipe metal and significant waste of flue gas. Moreover, gypsum is almost impossible to remove from the surface of the pipes by chemical washing of the boiler with inhibited hydrochloric acid.

4.          Increasing the alkalinity of the feed water with caustic soda only turns carbon dioxide into sodium bicarbonate (equation 3), which will again pass into carbonic acid in the boiler, it will be released as carbon dioxide into the steam when the water boils and subsequently passes into the condensate, causing a decrease in the pH of the condensate and significantly increasing its corrosion properties. Thus, the alkalinization of feed water helps to avoid only carbon dioxide corrosion of the feed and boiler path of the steam boiler, but increases the corrosion aggressiveness of the condensate.

Thermal degassing for steam boilers is currently the most acceptable option. The deaerator is also a storage tank of feed water, where feed water and condensate are supplied. Due to a small overpressure, there is no repeated contamination of water with aggressive gases from the atmosphere.

Nevertheless, thermal degassing requires a number of complex technical solutions in the design and has a significant cost of the main and auxiliary equipment. So it is necessary to provide heating of make-up water before the deaerator at least 80 ° C, which is a significant technical difficulty especially with variable flow of make-up water. With a sharp decrease in the flow of make-up water to the deaerator, due to the inertia of the steam regulator to the make-up water exchanger, the temperature of the make-up water after the heat exchanger increases sharply and the water boils in the pipeline from the heat exchanger to the deaerator. In this case, the release of oxygen from the water begins inside the pipeline and oxygen corrosion becomes intense. To avoid damage to this pipeline, it is advisable to make it from stainless steel.

The design guidelines prescribe the proportion of vapour in the deaerator, which equals to 2 kg per 1 ton of deaerated water. In practice, to obtain oxygen in deaerated water less than 50 µg/l, the evaporation rate can be increased by more than 10 times. In addition, automation of the deaerator often causes difficulties. Since it is necessary to simultaneously maintain the set pressure in the deaerator, the water temperature in the deaerator and the water level in the deaeration tank. The water level in the deaerator tank decreases with a sharp increase of a flow steam, and to maintain it the consumption of make-up water increases in the deaerator above the passport value. The quality of deaeration is sharply reduced.

Thus, for small steam and especially water-heating boilers the organization of thermal deaeration is extremely expensive, both in terms of capital and operating costs. Moreover, vacuum deaerators are used for water heating boilers, the design of which is unreliable and does not provide the necessary water quality.

As a rule, in practice, for boilers with a capacity of less than 3.0 – 6.0 t/h for steam, even if the thermal deaerator is installed, it does not provide the required degassing of feed water and the deaerator in fact works as a storage tank of feed water.

It is advisable to use membrane degassing of water for more effective degassing of feed water in boilers

It is known that membrane degassing of water can be carried out with the help of hydrophobic membranes, or so-called membrane contactors.

Currently, hydrophobic membrane contactors are used for water degassing in many industries. These are hollow fiber structures with a large branched surface area. Through this surface the mass transfer of gas from liquid to inert gas flow or vacuum is carried out. The inert gas is inside the fibers. Water flows outside the fiber. The fibers are made of hydrophobic material. The fiber does not absorb (does not pass water), but uncharged gas molecules can freely pass through the microporous structure of the fiber in the presence of the difference in gas concentrations inside and outside the fibers.

Degassing when using membrane contactors is effective enough to remove carbon dioxide from water, because as an inert gas it can be used atmospheric air. But it is necessary to use nitrogen of high degree of purification with vacuum to remove oxygen from the water. This circumstance demands the application of additional expensive and power-consuming equipment in a boiler room. In this case, the standard value for oxygen will not be achieved and there is a need for dosing sodium sulfite to bind residual oxygen. It should be noted that the pH value of water is quite difficult to get above 8.5 immediately after the membrane contactor. This circumstance makes it necessary to dose caustic soda into feed water, which subsequently leads to a high content of carbon dioxide in the condensate.

However, if the company has a system of centralized nitrogen production, this scheme of membrane degassing can be quite competitive in comparison with thermal deaeration.

The authors propose to use traditional polymer reverse osmosis membranes for water degassing, which are used everywhere for water desalination. These membranes are hydrophilic and cannot prevent the passage of water through them. At the same time, ions of metal salts dissolved in water practically do not pass through hydrophilic membranes. The selectivity of modern reverse osmosis membrane elements for water purification is from 99.0 to 99.7 %. Almost all salts are delayed.

The gases dissolved in water pass through polymer reverse osmosis membranes. Accordingly, in order to remove gases from the water, these gases must be transferred to inorganic compounds dissolved in water before the membrane.

The caustic soda solution NaOH should be dosed into the water before installation of reverse osmosis water desalination for removal of dissolved carbon dioxide

NaOH+H2O+ CO2<->NaHCO3+H2O

As a result, carbon dioxide turns into sodium bicarbonate, which is removed from the membrane with the concentrate stream.

It is necessary to dose a solution of sodium sulfite (equation 1) to bind oxygen. The resulting reaction (1) sodium sulfate will also be removed with the concentrate.

As a result, demineralized water coming out of the reverse osmosis desalination unit does not contain aggressive dissolved gases.

This method of degassing is fundamentally different from chemical degassing. In this method, gases are bound and removed from the water. During chemical deaeration gases are only bound in inorganic compounds. Thus, the salt content of water does not increase and the amount of bicarbonate and sulfate ion in the feed water does not increase.

The main advantages of membrane degassing by hydrophilic membranes:

1. Degassing of water takes place with simultaneous desalination of the feed water.

2. Dissolved gases are not bound, but removed from the water. Together with a small amount of alkalinity of make-up water after osmosis it allows obtaining a minimum value of carbon dioxide in the steam condensate, thereby ensuring reliable and efficient operation of the equipment of the steam condensate path.

3. In comparison with chemical degassing there is practically no sulfite ion SO3 in osmotic (feed) water. There is a limit on the content of sulfite ion in boiler water.

4. A much larger amount of sodium sulfite can be dosed into the water before the reverse osmosis unit for guaranteed oxygen binding.

5. Dosing of sodium sulfite also allow to bind free chlorine in the water entering the membrane. Thus, it is possible to use chlorine to conduct and increase the efficiency of the processes of coagulation, iron removal, disinfection, etc, in the installation of pre-treatment of water before osmosis.

6. Blowdown of steam boilers is reduced by the simultaneous use of desalination and degassing of make-up water. This leads to a decrease in water and heat losses and an increase in the efficiency of the boiler, as well as to a significant reduction in contaminated effluents from the boilers and, accordingly, the entire boiler house.

This method has some disadvantages:

1.    If it is necessary to remove carbon dioxide simultaneously with deaeration, it is necessary to dose caustic soda before the membrane, which requires preliminary softening of water before reverse osmosis. This will prevent precipitation of calcium carbonate salts on the membrane. Accordingly, there will be costs for salt, and it will require activities for the disposal of highly mineralized wastewater, which is generated during regeneration of the softening unit.

2.    There is a high risk of secondary water pollution with oxygen in this technology of degassing with reverse osmosis membranes. i.e., the water must be sent either directly to the feeding of the boiler without storage tank or a collecting tank with a steam sparging and a temperature of water of at least 100 C.

3.    There are costs of sodium sulfite and caustic soda compared to thermal degassing.

The proposed method of water degassing is carried out as follows (figure 1).

The method of water degassing contains the following technological stages. Water goes through the installation of continuous clarification of water (1) and fed to the installation of system of continuous Na- cation exchange water softening (2). It is advisable to have at least 2 filters installed that allow the system to operate in continuous mode. The hardness of the softened water should be within 0.02-0,1 mmol/ l. The hardness of the softened water will be determined on the basis of the amount of sodium hydroxide solution dosed into the softened water after the softening installation. The higher the hardness of the softened water and the greater the consumption of caustic soda is the higher the probability of formation of a solid precipitate of calcium carbonate on the membrane will be.

Solution of caustic soda is dosed into the water using a dispenser (3) after installing the water softening (2). The amount of caustic soda is chosen not more than 10-15% of the amount of free carbon dioxide in the water. Free carbon dioxide is bound into the bicarbonate ion (equation 3).

The pH value of water increases to 8.2-8.5. Then, the sodium sulfite solution is dosed into the water by means of the installation 4. In this case, the amount of sodium sulfite is chosen either as the equivalent to the amount of oxygen dissolved in water, or not less than 10-30% more than the amount of oxygen dissolved in water.

Then water, passing through the microfilter (5), enters the installation of reverse osmotic desalination of water (6). At the installation of reverse osmosis desalination (6), the initial water is divided into two streams: permeate (demineralized water) and concentrate (water saturated with salts and discharged into the sewer). The operation of this unit is organized so that most of the concentrate is returned to the input of the desalting unit (6).

Sodium sulfite entering the water by means of the dosing unit (4) reacts with dissolved oxygen. The result is sodium sulfate (equation 1).

This reaction takes place fairly quickly in hot water or in water with a pH value of more than 8.5. The water supplied to the desalting unit (6) has a temperature of 2 to 40°C. Nevertheless, the reaction (equation 1) proceeds sufficiently fully due to the effective mixing of sodium sulfite in water in the microfilter (5) and inside the reverse osmosis membrane element. Moreover, most of the sodium sulfite, which has not reacted with oxygen before and inside the reverse osmosis element, is returned to the input of the reverse osmosis element with the flow of the recycle. This ensures a sufficiently complete reaction (equation 1) before and inside the reverse osmosis element.

  Reverse osmosis membrane element passes the gases dissolved in water, but practically it does not pass the ions dissolved in water. Thus, carbon dioxide converted into bicarbonate by caustic soda does not pass through the membrane, but it is discharged in the form of bicarbonate ion into the sewer. The same principle works when oxygen is bound by sodium sulfite. As a result of the reaction (equation 1), dissolved oxygen in water is bound by sodium sulfite into sodium sulfate and then is discharged into the sewer with the concentrate flow.

Thus, the process of simultaneous desalting and degassing of water takes place at the reverse osmosis unit (6) that is a fundamentally new approach in the operation of such devices.

Demineralized and degassed water is sent to the consumer. It is important to prevent secondary contamination of water with oxygen and carbon dioxide from the air. It is recommended to use the membrane accumulator tank (7) in front of the pressure booster pump (8).  Pressure booster Pump (8) is needed if you want a higher permeate pressure of 1.0-2.0 bar.

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

Simultaneous desalting and degassing of water at the reverse osmosis plant can significantly reduce heat losses associated with  blow down of the boiler, as well as the operation of the thermal deaerator. This significantly reduces the corrosive aggressiveness of the condensate, simplifies the water degassing technology and therefore reduces the amount and composition of the equipment, as well as significantly reduces the cost of the entire treatment system. The system is easily automated and does not require constant monitoring.

The proposed scheme is quite variable. If you want to remove only oxygen from the water, you can refuse to use the softening unit and exclude from the scheme the dosing of caustic soda solution before the reverse osmosis unit.

In conclusion, it can be said that the reverse osmosis degassing of make-up water of steam and hot water boilers may well be applied for automated boilers without maintenance personnel with a thermal capacity of up to 200 MW.

For this technology of water treatment a patent application was submitted and registered. Registration number 2018138802 (date of registration – 05.11.2018).

 

1 – the installation of continuous water clarification;

2 – the installation of a system of continuous Na- cation exchange water softening;

3 –  dosing system for sodium hydroxide;

4 – sodium sulfite solution dosage system;

5 – microfilter;

6 – reverse osmosis water desalination system;

7 – membrane accumulator tank;

8 – boiler feed pump or filtrate pressure booster pump;

9 – high temperature storage tank.

Figure 1 Scheme of water treatment system with water degassing on reverse osmosis unit