SE1451118A1

SE1451118A1 - A method for cleaning contaminated gas

Info

Publication number: SE1451118A1
Application number: SE1451118A
Authority: SE
Inventors: Sabrina Tietze; Christian Hardy Ekberg; Mark Russell St-John Foreman
Original assignee: Westinghouse Electric Sweden
Priority date: 2014-09-22
Filing date: 2014-09-22
Publication date: 2016-03-23
Also published as: TW201615261A; WO2016045980A1

Abstract

A method for cleaning contaminated gas from a containment (1) of a nuclear site is disclosed. The method comprises providing a wet scrubber (3), comprising a vessel (5) containing a scrubber solution, activating the wet scrubber by introducing said contaminated gas in the scrubber solution in the vessel at a position beneath a surface of said scrubber solution, bringing said contaminated gas into contact with said scrubber solution, so that the scrubber solution absorbs or dissolves contaminants of said contaminated gas, thereby producing a cleaned gas, and releasing said cleaned gas from the vessel. A phosphine additive comprising a phosphine or a mixture of phosphines is supplied to the wet scrubber in order to be mixed with the aqueous scrubber solution at least when the wet scrubber is activated.(Fig 1)

Description

A method for cleaning contaminated gas TECHNICAL FIELD OF THE INVENTION The present invention refers to a method for cleaning contaminated gas from a containment of a nuclear site according to the preamble of claim 1, see WO 2014/008977.

BACKGROUND AND PRIOR ART It is known to provide a wet scrubber for cleaning the gases enclosed in a reactor containment of a nuclear plant in case of an event, such as an accident, or any other event requiring the gases in the reactor containment to be decontaminated. In case of such an event, the wet scrubber is activated and the gases to be vented are conveyed to a vessel of the wet scrubber. The gases are then brought into contact with a scrubber solution in the vessel. Various contaminants in the gases are then absorbed by the liquid. The gases cleaned from these contaminants may be discharged to the atmosphere via a stack.

Various such wet scrubbers for nuclear sites have been proposed in the art.

WO 2014/008977 discloses a wet scrubber comprising a vessel, a gas inlet for feeding contaminated gas to the vessel containing a scrubber solution, a manifold connected to the gas inlet and having a plurality of gas outlets, a set of metal fiber filters, each being connected to a respective one of the outlets, and a gas outlet. WO 2014/008977 suggests to add sodium thiosulphate to the scrubber solution in order to enable capture of gaseous iodine.

US 4,936,878 discloses a wet scrubber comprising a vessel, a gas inlet for feeding contaminated gas to the vessel containing a scrubber solution, a manifold connected to the gas inlet and 2 having a plurality of gas outlets, a set of venturi nozzles, each being connected to a respective one of the outlets, and a gas outlet. Also US 4,936,878 suggests to add sodium thiosulphate to the scrubber solution in order to enable absorption of gaseous iodine.

SUMMARY OF THE INVENTION A problem of the cleaning of the contaminated gas from the containment of a nuclear site is the difficulty to remove all contaminants from the gas, in particularly iodine, including, but not limited to, elemental iodine, inorganic iodine compounds, and organic iodine compounds, from the gas. The capability of the scrubber solution of absorbing contaminants depends inter alia on the chemical composition of the scrubber solution, which comprises an aqueous medium, consisting of water and possible additives, and how the scrubber solution reacts with or dissolves the contaminants of the gas.

The object of the present invention is to provide an improved method of cleaning contaminated gas from a containment of a nuclear site. More specifically, the present invention aims at a method enabling removal of iodine, including, but not limited to, elemental iodine, inorganic iodine compounds, and organic iodine compounds, from the gas.

This object is achieved by the method initially defined, which is characterized in that a phosphine additive comprising a phosphine or a mixture of phosphines is supplied to the wet scrubber in order to be contained in said scrubber solution at least when the wet scrubber is activated.

Such an addition of a phosphine additive to the wet scrubber results in an improved scrubber solution being capable of 35 removing iodine substances and compounds from the gases. The phosphine will react with elemental iodine, inorganic iodine 3 and organic iodine in different ways. The phosphine additive breaks the chemical bond between the organic chain and the iodine atom in the molecule. Elemental iodine is transferred into the aqueous medium while the organic part remains in the organic phosphine additive.

Phosphines are non-water miscible organic compounds. If the phosphine additive is added to the scrubber solution before the wet scrubber is activated, it will float as a separate layer on top of the aqueous medium. Once the wet scrubber is activated through the introduction of the contaminated gas and the entire liquid phase, i.e. the aqueous medium and the phosphine additive, is bubbling or otherwise intimately mixed, droplets/bubbles as a fine emulsion will be formed, which means that each droplet/bubble is covered with a thin phosphine layer.

When the introduction of gas is interrupted and the mixing in the vessel is stopped, the separate layer of phosphine or phosphines on top of the aqueous medium will be reformed 20 again.

According to an aspect of the invention, the phosphine or phosphines comprise, or consist of, alkyl phosphines, in particular trialkyl phosphines. Alkyl phosphines may react with both elemental iodine, 12, and organic iodine, e.g. methyl iodide.

According to a further aspect of the invention, the phosphine or phosphines comprise, or consist of, trioctyl phosphine.

According to a further aspect of the invention, the phosphine or phosphines comprise, or consist of, trihexyl phosphine.

According to a further aspect of the invention, the phosphine or phosphines comprise, or consist of, tributyl phosphine. 4 According to a further aspect of the invention, the phosphine or phosphines comprise, or consist of, phosphines with branched side chains or phosphines mixed with acryl and/or alkyl side chains.

According to a further aspect of the invention, the phosphine additive is supplied to the scrubber solution in the vessel before the wet scrubber is activated. The phosphine or phosphines will then form a separate layer on top of the aqueous medium, as mentioned above.

According to a further aspect of the invention, the phosphine additive is mixed with the scrubber solution upon activation of the wet scrubber. The activation of the wet scrubber, i.e. the introduction of the contaminated gas into the scrubber solution, will cause bubbling and mixing of the scrubber solution, i.e. the aqueous medium and the phosphine or phosphines.

According to a further aspect of the invention, the contaminated gas is introduced into the vessel via a gas inlet pipe connected to a gas distribution device, which is located in the vessel and comprises a plurality of outlet openings for the gas beneath said surface.

According to a further aspect of the invention, the phosphine additive is supplied to the scrubber solution at a position below or upstream the outlets, and are mixed with the aqueous medium upon activation of the wet scrubber, i.e. when the contaminated gas is introduced into the scrubber solution and brought into contact therewith.

The phosphine additive may according to this aspect remain beneath or below the outlets until the wet scrubber is activated, i.e. as long as the wet scrubber is in passive state.

According to a further aspect of the invention, the phosphine additive is supplied to the scrubber solution via the gas inlet pipe.

The phosphine additive is according to this aspect supplied to the scrubber solution via the outlet openings and the gas distribution device when the wet scrubber is activated. The phosphine or phosphines will thus be mixed with the scrubber solution upon entry into the scrubber solution.

According to a further aspect of the invention, the scrubber solution is circulated through the vessel by means of a liquid inlet pipe and a liquid outlet pipe.

According to a further aspect of the invention, the phosphine additive is supplied to the scrubber solution via the liquid inlet pipe.

Also according to this aspect of the invention, the phosphine or phosphines may be supplied to the scrubber solution when the wet scrubber is activated. The phosphine or phosphines are thus supplied when the contaminated gas is introduced and the mixing of the scrubber solution is initiated.

According to a further aspect of the invention, said contaminated gas is brought into contact with said scrubber solution by means of mixing means associated with the outlet openings to promote mixing of the gas with the scrubber solution. Such mixing means promotes the mixing and bubbling of the scrubber solution, i.e. the aqueous medium, and the phosphine or phosphines, and ensures that the contaminated gas is properly brought into contact with the scrubber solution and the phosphine or phosphines, and thus to ensure the above mentioned formation of a fine emulsion. 6 According to a further aspect of the invention, the mixing means comprises one or more metal filters associated with the outlet openings. Advantageously, a plurality of metal filters are provided, wherein one metal filter is provided above or downstream with respect to said outlet openings.

According to a further aspect of the invention, the mixing means comprises a plurality venturi nozzles, each being associated with or provided above or downstream a respective one of said outlets openings.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention is now to be explained more closely 15 through a description of various embodiments and with reference to the drawings attached hereto.

Fig 1discloses schematically a nuclear plant comprising a wet scrubber.

Fig 2discloses a first variant of a gas distribution device of the wet scrubber.

Fig 3discloses a second variant of the gas distribution device of the wet scrubber.

Fig 4illustrates the reaction of tributyl phosphine with elemental iodine.

Fig illustrates the reaction of organic iodide with trialkyl phosphines DEATAILED DESCRIPTION OF VARIOUS EMBODIMENTS Fig 1 discloses a nuclear site comprising a containment 1 housing a nuclear reactor 2, such as a Boiling Water Reactor, a Pressure Water Reactor, an Advanced gas cooled reactor (AGR), CANDU, Magnox, RBMK, VVER, fast and pebble bed reactors, etc. The invention is applicable also to other nuclear sites, such as fuel reprocessing plants, storage and final 7 repository installations, radionuclide (radioisotope) production facilities, etc.

The nuclear site also comprises a wet scrubber 3 comprising a gas inlet pipe 4, a vessel 5 and a gas outlet pipe 6 connected to a stack 7. The gas inlet pipe 4 is connected to the inner space of the containment 1. The gas inlet pipe 4 may comprise a valve and potentially a rupture disc arrangement 4a configured for manually or automatically open the gas inlet pipe 4, in case of an event requiring gas from the containment 1 to be released into the atmosphere via the vessel 5 and the stack 7.

In the embodiments disclosed, the vessel 5 of the wet scrubber 3 is provided outside the containment 1. It should be noted, however, that it is possible to provide the vessel 5 inside the containment 1.

Furthermore, the wet scrubber 3 comprises a liquid inlet pipe 8 and a liquid outlet pipe 9.

The vessel 5 may be pressurized or operated at atmospheric pressure. The pressure in the vessel 5 may be regulated by means of a valve arrangement 6a provided on the gas outlet pipe 6. The valve arrangement 6a may be combined with or replaced by a passive flow restriction.

The vessel 5 contains a scrubber solution comprising or consisting of an aqueous medium comprising water and possible additives, e.g. sodium thiosulphate, additives for regulating the pH of the scrubber solution at an appropriate level, etc.

The scrubber solution is circulated through the vessel 5 by means of a liquid inlet pipe 8 and a liquid outlet pipe 9. Water may be added to the scrubber solution via the liquid inlet pipe 8. 35 The additives may be added to the scrubber solution via a 8 dosing equipment 10 and the liquid inlet pipe 8. The liquid outlet pipe 9 may be connected to a drain 11.

The vessel 5 may thus be filled with the scrubber solution cleansing liquid to a certain level forming a surface S of the aqueous medium beneath a gas filled upper space of the vessel 5.

The wet scrubber 3 also comprises a gas distribution device 12 located in the vessel 5. The gas distribution device 12 is connected to the containment 1 via the gas inlet pipe 4. The gas distribution device 12 comprises a plurality of outlet openings 13 for the contaminated gas to be cleaned.

The outlet openings 13 are located in the vessel 5 below the surface S of the aqueous medium. The outlet openings 13 are thus located at a lower level in the vessel 5 as indicated in Fig 1. Contaminated gas in the containment 1 may thus be introduced into the scrubber solution via the gas inlet pipe 4, the gas distribution device 12, and the outlet openings 13.

In order to promote further the mixing of the contaminated gas with the scrubber solution, the gas distribution device 12 comprises mixing means 14 associated with the outlet openings 13. The mixing means 14 is provided directly above or downstream the outlet openings 13. Also the mixing means 14 is provided below the surface S of the scrubber solution.

A first variant of mixing means 14 comprises one or more metal filters 15, see Fig 2. As schematically indicated in Fig 2, a plurality of metal filters 15 are provided so that each metal filter 15 is associated with a respective one of the outlet openings 13. The contaminated gas is conveyed through the metal filters 15 thereby being thoroughly mixed with the scrubber solution. 9 It is to be noted that the plurality of metal filters 15 may be replaced by a fewer number of metal filters 15 covering a number of the outlet openings 13, or one single metal filter 15 covering all outlet openings 13.

A second variant of mixing means 14 comprises a plurality venturi nozzles 16, schematically indicated in Fig 3. Each venturi nozzle 16 is provided above or downstream with respect to the outlet openings 13. When the contaminated gas is led through the outlet opening, the scrubber solution will be sucked into the venturi nozzle 16 via an inlet 17 of the venturi nozzle 16.

It should be noted that also other variants of mixing means 14 could be employed, for instance mixing means comprising both nozzles, such as venturi nozzles, and filters or mixer elements.

According to the invention, a phosphine additive comprising a phosphine or a mixture of phosphines is supplied to the wet scrubber 3 in order to be mixed with the aqueous scrubber solution. The phosphine or phosphines may be added to the scrubber solution in the vessel 5 before the wet scrubber 3 is activated. Since the phosphine or phosphines will not be mixed with the water of the aqueous medium and have a lower density than water, they will float as a separate layer 18 on the surface S of the aqueous medium, see Fig 1.

The quantity of phosphine additive may be, for instance, approximately 1-10 percent by volume of the scrubber solution.

The phosphine additive may be supplied to the vessel 5 from a suitable source 19 via a separate pipe 20 or via the liquid inlet pipe 8.

Both the separate pipe 20 and the liquid inlet pipe 8 may enter the vessel 5 at a position below the surface 17 and above the mixing means 14. The phosphine or phosphines will then rise in the scrubber solution and form the separate layer 17 on the surface S of the aqueous medium of the scrubber solution as long as the wet scrubber 3 is in the passive state.

Alternatively, the phosphine additive may be supplied via a separate pipe, not disclosed, entering the vessel 5 at a position below the gas distribution device 12. Also in this case the phosphine or phosphines will rise, but may due to the presence of the mixing means 14 at least partly be retained below the mixing means 14 in the passive state until the wet scrubber 3 is activated.

It is also possible to supply the phosphine additive when the wet cleaning apparatus 3 is activated. Such a supply of the phosphine additive from the source 19 may take place via the gas inlet pipe 4 or the gas distribution device 12, as is indicated by dashed lines in Fig 1, or as described above.

Various derivatives of phosphine may be used. Especially, alkyl phosphines, such as trialkyl phosphines, have proved to be suitable.

Alkyl and aryl phosphines react with both elemental iodine, 12, and organic iodine, e.g. methyl iodide.

Examples of trialkyl phosphines that may be used are trioctyl phosphine, trihexyl phosphine, and tributyl phosphine. These phosphines are advantageous with respect to handling properties and toxicity.

Tributyl phosphine, trihexyl phosphine and trioctyl phosphine are all suitable due to their chemical properties. The chain length of the trialkyl phosphine does not affect the capture of the iodine 35 species. 11 Trihexyl phosphine and trioctyl phosphine are more easy and convenient to handle, and are advantageous for safety and environmental reasons. A long chain length makes the phosphine less volatile, less oxygen sensitive, less flammable and less malodorous.

Tributyl phosphine, trihexyl phosphine and trioctyl phosphine are all available as bulk chemicals. They could easily be used as a replacement for each other one in case of an unlikely shortage of one of the chemicals.

When the wet scrubber 3 is activated, contaminated gas is introduced via the gas distribution device 12 into the scrubber solution causing the aqueous medium and the phosphine or phosphines to mix. Fine bubbles/droplets of the gas will form a fine emulsion. Each such bubble/droplet will be covered with a thin layer of phosphine. The phosphine will then react with the iodine in the droplets/bubbles emulsion.

Elemental iodine Elemental iodine will react with the phosphines as shown in Fig 4. Fig 4 schematically indicates the reaction of tributyl phosphine with elemental iodine (12). The product of the reaction is an iodinated alkyl phosphine salt, which will hydrolyse in the aqueous medium and form a trialkyl phosphine oxide. Iodine will be washed out in form of iodide ions into the aqueous medium of the vessel 5. The trialkyl phosphate salt can partially dissolve in the aqueous medium but the reaction will not have any pH or ionic strength effect.

Organic iodides In the reaction of organic iodides, the alkyl chain of the alkyl iodides will react with phosphine to form directly a phosphonium salt, see Fig 5 schematically indicating the reaction of organic 12 iodides (RI) with trialkyl phosphines, such as the reaction of tributyl phosphine/trihexyl phosphine with organic iodides R = CxHy.

The iodine will be released and washed into the aqueous medium and will also be in form of I- ions under the aimed conditions in the scrubber solution, comprising pH = 8-13.

At very low pH (pH < 2) iodide ions could be reoxidised to elemental iodine which would again react with the phosphines in the organic layer to be reduced to iodide ions (see Fig 4) or which reacts with the thiosulphate ions in the solution.

The present invention is not limited to the aspects described above, but may be varied and modified within the scope of the following claims. 13

Claims

1. A method for cleaning contaminated gas from a containment (1) of a nuclear site, the method comprising providing a wet scrubber (3) comprising a vessel (5) containing a scrubber solution, activating the wet scrubber (3) by introducing said contaminated gas in the scrubber solution in the vessel (5) at a position beneath a surface of said scrubber solution, bringing said contaminated gas into contact with said scrubber solution, so that the scrubber solution absorbs or dissolves contaminants of said contaminated gas, thereby producing a cleaned gas, releasing said cleaned gas from the vessel (5), characterized in that a phosphine additive comprising a phosphine or a mixture of phosphines is supplied to the wet scrubber (3) in order to be contained in said scrubber solution at least when the wet scrubber (3) is activated.

2. A method according to claim 1, wherein the phosphine or phosphines comprise alkyl phosphines.

3. A method according to claim 2, wherein the phosphine or phosphines comprise trioctyl phosphine.

4. A method according to any one of claims 2 and 3, wherein the phosphine or phosphines comprise trihexyl phosphine.

5. A method according to any one of the preceding claims, wherein the phosphine or phosphines comprise, or consist of, tributyl phosphine. 6. A method according to any one of the preceding claims, the phosphine or phosphines comprise, or consist of, phosphines with branched side chains or phosphines mixed with acryl and/or alkyl side chains. 14

6. A method according to any one of the preceding claims, wherein the phosphine additive is added to the scrubber solution in the vessel (5) before the wet scrubber (3) is activated.

7. A method according to claim 6, wherein the phosphine additive is mixed with the scrubber solution upon activation of the wet scrubber (3).

8. A method according to any one of the preceding claims, wherein the activation step comprises introduction of the contaminated gas into the vessel (5) via a gas inlet pipe (4) connected to a gas distribution device (12), which is located in the vessel (5) and comprises a plurality of outlet openings (13) for the gas beneath said surface.

9. A method according to claim 8, wherein the phosphine additive is supplied to the scrubber solution at a position or 20 upstream the outlet openings (13) and are mixed with the aqueous medium upon activation of the wet scrubber (3).

10. A method according to claim 9, wherein the phosphine additive is supplied to the scrubber solution via the gas inlet pipe (4).

11. A method according to any one of claims 8 to 10, wherein the scrubber solution is circulated through the vessel (5) by means of a liquid inlet pipe (8) and a liquid outlet pipe (9).

12. A method according to claim 11, wherein the phosphine additive is supplied to the scrubber solution via the liquid inlet pipe (8).

13. A method according to any one of claims 8 to 12, wherein said contaminated gas is brought into contact with said scrubber solution by means of mixing means (14) associated with the outlet openings (13) to promote mixing of the gas with the scrubber solution.

14. A method according to claim 13, wherein the mixing means (14) comprises a plurality of metal filters (15), each being associated with a respective one of said outlet openings (13).

15. A method according to claim 13, wherein the mixing means (14) comprises a plurality of venturi nozzles (16), each being associated with a respective one of said outlet openings (13).