RU2477538C1 - Method of cleaning liquid radioactive wastes and apparatus for realising said method - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: method is disclosed, which involves cleaning liquid wastes by preheating and evaporation to form vapour and brine while keeping pressure in the evaporation chamber below atmospheric pressure. The method involves recycling brine on a closed loop, compressing the formed vapour and condensation. Condensation is carried out by passing compressed vapour through a supersonic ejector while simultaneously feeding a portion of the obtained condensate into the mixing chamber of the ejector. Pressure below atmospheric pressure is maintained by circulating the condensate on a closed loop which includes the supersonic ejector, a pipe for removing condensate and a heat exchanger. Also disclosed is an apparatus for realising said method, in which units and components are connected to form two closed loops: for recycling brine and for circulating condensate.

EFFECT: low power consumption with high degree of purification of the medium, simple apparatus and reliability thereof.

12 cl, 1 dwg, 2 ex

Description

The invention relates to the field of processing and purification of solutions with high salinity, using evaporation and condensation. In particular, it can be used for complex treatment of contaminated and saline liquid radioactive wastes (LRW) at various nuclear facilities.

In the nuclear industry and, in particular, in all modern nuclear power plants, LRW treatment methods based on the distillation method are widely used (B.E. Ryabchikov. “Liquid Radioactive Waste Treatment”, M., DeLi Print, 2008, p.134-148 ) The most common evaporation method at nuclear power plants to date is evaporation using high-pressure steam as a coolant supplied to the upper part of the external heating chamber, where it heats the initial LRW going in countercurrent from the bottom up. Then LRW, heated to high (about 200 ° C) temperatures, from the upper part of the heating chamber enter the middle part of the separator, where their instantaneous volume boiling occurs. The resulting secondary steam through a droplet collector enters the condenser, and then into the condensate after-treatment system, where it is further treated, mainly from cesium radionuclides, which enter the condensate due to droplet entrainment. The resulting LRW concentrate (still bottom) with a salt content of 150-350 g / l is usually sent for cementing.

The disadvantage of this method is the high energy consumption. In addition, the evaporation plants used in the method are distinguished by large dimensions, metal consumption and high operating costs. In the presence of hardness salts in LRW, the process of evaporation is even more complicated due to the formation of insoluble sediments on the walls of the apparatus.

Therefore, the development of new evaporation systems with greater compactness and simplicity is relevant.

Closest to the proposed group of inventions are a method and installation for processing liquid radioactive waste (see Prospectus of the company NUKEM, "Evaporation of radioactive liquids", RWE NUKEM GmbH, Januar, 2002).

The known method includes pre-heating LRW, their supply to evaporation with the formation of steam and brine while maintaining the pressure in the evaporation chamber below atmospheric pressure, recirculating the resulting brine, compressing the vapor, condensing it, removing condensate and concentrated brine.

The method implements the process of "flash" evaporation. Pre-cleaned from mechanical impurities and petroleum products, LRW is fed to an evaporator equipped with an electric heater. The initial LRW heated with the help of an electric heater is fed into a tubular heat exchanger, where they are heated by the heat of hot condensate passing on the other side in countercurrent heat. After that, the heated LRW is piped to the upper part of the evaporator, in which a reduced pressure is created, which ensures the evaporation of water at 78-91 ° C. Part of the water from the LRW evaporates, and the rest in the form of brine is discharged by gravity back into the tank, which is a cyclone separator. Steam is supplied through a steam purification system from water (droplet eliminator, reflux condenser) through pipes to the compressor in order to increase its pressure to a value exceeding the pressure on the water saturation line. This technique allows the condensation of steam to return the heat of condensation to the recirculating brine and thereby refuse to use an external source of steam cooling. Compressed steam, having a temperature of about 105-110 ° C, is fed to the outside of the vertically mounted heat transfer pipes, where its heat is transferred to the cold brine passing through the internal cavity of these pipes, due to which the steam condenses. Hot condensate is re-passed through a heat exchanger, where it transfers heat to the incoming source LRW. After this, the condensate, which is essentially distilled water, is sent to a discharge or post-treatment.

A known installation for cleaning LRW as described above. It contains a brine tank equipped with supply lines for the initial LRW and brine discharge, a brine circulation line equipped with a pump, an evaporation chamber, a steam pipe, a compressor, a vertical tubular heat exchanger-condenser, a recovery heat exchanger operating in a liquid-liquid system, a condensate tank with an outlet pipe , shut-off and control valves and instrumentation. At the entrance to the evaporation chamber, a deaerator is installed as a mandatory device in this installation. The brine tank is made in the form of a cyclone separator equipped with a multi-stage steam purification system.

Thus, in the known technical solution, steam condensation takes place in a heat exchanger in a steam-liquid system, while another heat exchanger operating in a liquid-liquid system is used to heat the initial LRW.

The disadvantages of the known technical solutions adopted for the prototype are as follows. The method is characterized by high energy consumption. The implementation of steam condensation in a vertically tubular heat exchanger requires a complex control system, precision execution of the tubes and their installation. The number of tubes in the evaporator reaches several hundred. Misalignment of the tubes should not exceed the height of tenths of a millimeter, which complicates the manufacture of the installation. In addition, the installation has an increased sensitivity to changes in process parameters.

The objective of the invention is to provide a reliable and economical method of processing liquid radioactive waste.

The problem is solved by the described method for cleaning liquid radioactive waste, which includes pre-heating the waste, evaporating it with the formation of steam and brine while maintaining the pressure in the evaporation chamber below atmospheric, recirculating the brine, compressing the steam, condensing it, draining the condensate and concentrated brine, while condensing compressed steam is carried out by passing it through a supersonic ejector with the simultaneous supply to the mixing chamber of the ejector of the part obtained in the condensate process, in the pressure in the evaporation chamber is maintained below atmospheric pressure due to the circulation of the said condensate in a closed circuit, including a supersonic ejector, a condensate drain line and a heat exchange device that provides indirect heat exchange between the condensate and the recirculating brine.

According to the method, from the diffuser of a supersonic ejector, the gases released at the stage of condensation of the compressed vapor are periodically removed.

If necessary, the brine recirculating during the process can be subjected to additional purification from mechanical impurities.

The problem is also solved by the claimed installation for implementing the method. The claimed installation includes a brine tank made in the form of a cyclone and equipped with a tubular heat exchanger inside, a brine supply line for evaporation, equipped with a circulation pump, an evaporation chamber connected to the brine outlet line with a brine tank, and a steam line to a compressor, further connected to a supersonic ejector consisting of a nozzle block, a mixing chamber and a diffuser, a condensate drain line equipped with a condensate tank and a circulation pump, while the tank for the condensate is connected to the said tubular heat exchanger located inside the brine tank, and the tubular heat exchanger, on the other hand, is connected to the mixing chamber of the ejector with the formation of a closed loop for condensate circulation.

The closed loop for brine recirculation is advantageously formed by a series-connected brine tank, a pump, a brine supply line, an evaporation chamber and a brine discharge line.

The brine tank is preferably provided with an inlet for supplying liquid radioactive waste.

The evaporation brine supply line is preferably provided with a concentrated brine outlet.

The condensate discharge line is preferably provided with a condensate outlet pipe.

The claimed installation can be additionally equipped with a mechanical filter mounted on the supply line of brine for evaporation in front of the pump,

Preferably, a vacuum pump is installed in the upper part of the supersonic ejector diffuser.

Preferably, the evaporation chamber is equipped with a reflux condenser.

The claimed installation is equipped with connecting pipelines, shut-off and control valves and instrumentation.

The invention is illustrated with the help of figure 1, which schematically depicts an installation containing the following components and details:

1 - pump for brine;

2 - capacity for brine;

3 - brine discharge pipe;

4 - brine supply line;

5 - a line of removal of brine;

6 - evaporation chamber;

7 - reflux condenser;

8 - compressor;

9 - tapering steam line;

10 - nozzle block of the ejector;

11 - mixing chamber of the ejector;

12 - ejector diffuser;

13 - a vacuum pump;

14 - condensate drain line;

15 - capacity for condensate (desalinated water);

16 - pipe drain condensate (desalinated water);

17 - circulation pump for condensate (desalinated water);

18 - heat exchanger;

19 - pipe for supplying liquid radioactive waste.

20 - mechanical filter (if necessary)

The claimed method is as follows.

The initially heated brine (LRW) from the brine tank - 2 with a pump - 1 through the brine supply line - 4 is fed into the evaporation chamber - 6, where the pressure is maintained below atmospheric and volumetric boiling of the brine is ensured at a low temperature (78-92 ° C). The steam formed during boiling through the reflux condenser - 7 enters the compressor - 8, and the remaining brine is recycled through the brine discharge line - 5 to the brine tank - 2. Thus, the circulation principle is implemented in the evaporation chamber of the unit, therefore, when the pump - 1 is constantly running lines 4 and 5, brine is recycled. In the compressor - 8, steam is compressed, after which it is supplied through the steam pipe - 9 to the ejector mixing chamber - 11. At the steam supply point, the nozzle block of the ejector - 10 is installed, from which into the mixing chamber of the ejector - 11 from a container with condensate (desalinated water) - 15 with a pump - 17 pre-prepared desalinated water is injected at the start of the installation or condensate obtained during the operation of the installation. As a result of mixing the compressed steam with the flow of fresh water in the mixing chamber of the ejector, almost complete condensation of the steam occurs, and the resulting gas-liquid mixture, consisting of water, steam residues and dissolved gases released from the brine, enters the ejector-12 diffuser, in which braking occurs supersonic gas-liquid flow. Condensate with an elevated temperature leaves the diffuser - 12 due to the condensation that occurred in the mixing chamber of the ejector - 11 with the transfer of excess heat of vaporization to the water. The gases released from the brine appear in the upper part of the diffuser - 12, from where they are periodically removed using a vacuum pump - 13. The pressure of the water injected into the ejector is provided by the pump - 17, and the water temperature by the heat exchanger - 18, in which heat exchange between the condensate (desalinated water) takes place. leaving the diffuser - 12, and brine in the tank - 2. In the process of installation on the condensate drain line - 14 using a pump - 17, the condensate circulates between the tank - 15 and the heat exchanger - 18, passing along a closed circuit along the way, with containing a supersonic ejector equipped with a nozzle block, a mixing chamber and a diffuser. Excess condensate is discharged through the branch pipe - 16. Compensation for the loss of brine due to evaporation is carried out through the pipe for supplying the initial LRW - 19. The brine content is monitored and the excess concentrated brine is drained through the brine pipe - 3. This drain is carried out when the salinity reaches brine at the level of 300-600 g / dm ³ . If the installation is additionally equipped with a mechanical filter - 20, then it is used to filter salts deposited during the concentration of LRW.

The following are examples of the method, as described above, containing specific parameters of the cleaning process.

Example 1

A unit with a capacity of 0.16 kg / s (approximately 580 l / h) of condensate was used. The method is carried out under the following conditions: the temperature of the initial LRW (evaporated brine) supplied by the pump - 1 along the supply line of brine - 4 to the evaporation chamber - 6, is t = 91 ° C; the temperature of the brine discharged after evaporation of the calculated amount of steam along the brine removal line - 5 to the tank - 2 is t = 81 ° C; the temperature of desalinated water (condensate) supplied to the nozzle block - 10 for the condensation of compressed steam is - t = 81 ° C.

To obtain a given condensate capacity, a continuous supply of 8.78 kg / s of brine to the evaporation chamber - 6 is provided. This amount of brine, cooling during evaporation from t = 91 ° C to t = 81 ° C, will give 0.16 kg every second a pair of 87.84 kcal.

The resulting low pressure primary steam is compressed by 1.3 times in the compressor, while its pressure from the initial 0.05 MPa increases to a pressure of 0.065 MPa. Due to the compression of the vapor, its temperature at the outlet of the compressor is = 110.14 ° C or 383.14 K. Moreover, the steam velocity at the exit of the narrowing steam line - 9 reaches 300 m / s. The adiabatic work of vapor compression at such a flow rate and pressure drop is 4.4 kJ. The energy consumption for compressing steam in the compressor is 10.2 kW · h / m ³ .

To condense the amount of steam received, desalinated water is supplied to the nozzle block of the ejector at a temperature of t = 81 ° C under a pressure of 0.1 MPa and in an amount of 8 kg / s. As a result, a water stream is formed at the outlet of the ejector diffuser with a temperature t = 91.2 ° C = 364.2 K and a pressure of 0.075 MPa.

To heat the initial LRW to t = 91 ° C, taking into account the recovery heat, approximately 20 kW · h / m ³ is required. The total energy consumption for the drive of both pumps is 1 kW · h / m ³ . Taking into account the energy consumption for the vacuum pump - 13, pumping the gases contained in the original LRW (not more than 1.3 kW · h / m ³ ), the total energy consumption in this specific example is 20 + 10.2 + 1 + 1.3 = 32.5 kWh / m ³ . The coefficient of LRW purification from radionuclides (determined by the content of cesium radionuclides in the condensate) was 4 × 10 ⁵ .

Example 2

The installation according to example 1 was used. The method is carried out under the following conditions: the temperature of the initial LRW (evaporated brine) supplied by the pump - 1 along the brine supply line - 4 to the evaporation chamber - 6, is t = 78 ° C; the temperature of the brine discharged after evaporation of the calculated amount of steam along the brine removal line - 5 to the tank - 2 is t = 70 ° C; the temperature of desalinated water (condensate) supplied to the nozzle block - 10 for the condensation of compressed steam is - t = 70 ° C.

To obtain a given condensate capacity, they provide a continuous supply of 8.78 kg / s of brine into the evaporation chamber - 6.

The resulting low pressure primary steam is compressed in a compressor 1.32 times, while its pressure from the initial 0.04 MPa increases to a pressure of 0.053 MPa. Due to the compression of the vapor, its temperature at the outlet of the compressor is about 105 ° C. At the same time, the steam velocity at the exit from the steam line - 9 reaches a value of 280 m / s. The energy consumption for compressing steam in the compressor is 11.2 kW · h / m ³ .

To condense the obtained amount of steam, desalinated water is supplied to the nozzle block of the ejector with a temperature t = 70 ° C under a pressure of 0.12 MPa and in an amount of 7.8 kg / s. As a result, a water stream is formed at the outlet of the ejector diffuser with a temperature t = 78 ° C and a pressure of 0.062 MPa.

To heat the initial LRW to t = 78 ° C, taking into account the recovery heat, approximately 15 kW · h / m ³ is required. The total energy consumption for the drive of both pumps is 1.2 kW · h / m ³ . Taking into account the energy consumption for the vacuum pump - 13, pumping the gases contained in the initial LRW (not more than 1.3 kW · h / m ³ ), the total energy consumption in this specific example is 15 + 11.2 + 1.2 + 1, 3 = 28.7 kWh / m ³ . The coefficient of LRW purification from radionuclides (determined by the content of cesium radionuclides in the condensate) was 8 × 10 ⁵ .

In the prototype method, when working on the installation described in the prototype, with a similar predetermined condensate capacity, it is required to use about 70 kW * h / m ³ .

As can be seen from the description, the technical result of the claimed group of inventions is to ensure the simplicity of the technology, which consists in the absence of strict requirements for installation elements and the absence of complex automation. The proposed LRW processing technology does not have strict requirements for the quality of the cleaned medium (LRW) and the content of associated gases in it. The method can be implemented when containing in LRW oil products and other organic substances, usually interfering with the implementation of the method based on the known evaporation technology.

Claims (12)

1. The method of purification of liquid radioactive waste, including pre-heating the waste, its evaporation with the formation of steam and brine while maintaining the pressure in the evaporation chamber below atmospheric pressure, recirculation of the brine, compression of the vapor, its condensation, removal of condensate and concentrated brine, characterized in that the condensation is compressed the steam is carried out by passing it through a supersonic ejector with the simultaneous supply to the mixing chamber of the ejector of the part obtained in the condensate process, in the evaporation chamber the pressure is lower atmospheric, maintained by the circulation of said condensate in a closed circuit comprising a supersonic ejector condensate manifold and heat exchanger providing indirect heat exchange between the recirculated exhaust condensate and brine. 2. The method according to claim 1, characterized in that periodically from the diffuser of a supersonic ejector remove gases released at the stage of condensation of compressed steam. 3. The method according to claim 1, characterized in that the brine recirculating during the process is subjected to additional purification from mechanical impurities. 4. Installation for implementing the method described in claim 1, comprising a brine tank made in the form of a cyclone and provided with a tubular heat exchanger inside, a brine supply line for evaporation equipped with a circulation pump, an evaporation chamber connected to the brine outlet line with a brine tank, and a steam line - with a compressor further connected to a supersonic ejector, consisting of a nozzle block, a mixing chamber and a diffuser, a condensate drain pipe equipped with a condensate tank and a circulation pump, wherein the condensate tank is connected to the said tubular heat exchanger located inside the brine tank, and the tubular heat exchanger, on the other hand, is connected to the ejector mixing chamber to form a closed loop for condensate circulation. 5. Installation according to claim 4, characterized in that the closed loop for brine recirculation is formed by serially connected brine tank, pump, brine supply line, evaporation chamber and brine discharge line. 6. Installation according to claim 4, characterized in that the brine tank is equipped with an inlet pipe for supplying liquid radioactive waste. 7. Installation according to claim 4, characterized in that the supply line for brine for evaporation is equipped with a branch pipe for discharge of concentrated brine. 8. Installation according to claim 4, characterized in that the condensate drain pipe is provided with a condensate outlet pipe. 9. The installation according to claim 4, characterized in that it further comprises a mechanical filter mounted on the supply line of brine for evaporation in front of the pump. 10. Installation according to claim 4, characterized in that a vacuum pump is installed in the upper part of the supersonic ejector diffuser. 11. Installation according to claim 4, characterized in that the evaporation chamber is equipped with a reflux condenser. 12. The installation according to claim 4, characterized in that it is equipped with connecting pipelines, shut-off and control valves and instrumentation.

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