NO346293B1

NO346293B1 - AN ARRANGEMENT AND A METHOD FOR REDUCING A CONTENT OF DISSOLVED GAS FROM A GAS-CONTAINING LIQUID

Info

Publication number: NO346293B1
Application number: NO20200388A
Authority: NO
Inventors: Per Lothe
Original assignee: Knutsen Tech As
Priority date: 2019-04-03
Filing date: 2020-03-31
Publication date: 2022-05-23
Also published as: WO2020204726A1; EP3947290A1; EP3947290A4; NO20200388A1

Description

AN ARRANGEMENT AND A METHOD FOR REDUCING A CONTENT OF DISSOLVED GAS FROM A GAS-CONTAINING LIQUID

The present invention is related to an arrangement and a method for reducing a content of dissolved gas from a gas-containing liquid. The gas may be one of or a combination of two or more of for example oxygen, carbon dioxide, nitrogen, hydrocarbons, and chemicals such as for example Volatile Organic Compounds, VOC. Volatile Organic Compounds are organic chemicals with molecules comprising carbon, hydrogen, oxygen, and nitrogen. The liquid may for example be water, oil, or chemicals, for example glycol.

The invention is applicable in a variety of industries wherein it may be desirable or necessary to at least remove a substantial portion of dissolved gases from a liquid. An example of such industries is the oil and gas industry where for example oxygen is removed from so-called injection water that should not contain oxygen. Another example is the fishfarming industry where gases such as nitrogen and carbon dioxide are removed from water being circulated in a closed loop system. A further example of an industry wherein the invention can be applied, is the shipping industry where ballast water treatment may include removal of oxygen as part of disinfecting ballast water.

Within the industry, several methods exist for reducing a dissolved gas from a liquid. The solubility of gas obeys Henry's law, that is, the amount of a dissolved gas in a liquid is proportional to its partial pressure. Therefore, placing a solution under reduced pressure makes the dissolved gas less soluble. Sonication and stirring under reduced pressure may enhance the efficiency of reducing a content of dissolved gas from a gas-containing liquid. This technique is often referred to as vacuum degasification wherein vacuum chambers, called vacuum degassers, are used to degas liquids through pressure reduction. A vacuum degasification may also be combined with nitrogen purging and chemical injections to enhance the process. The conventional vacuum degasification requires several sub-processes and requires large plants in order to provide a continuous process for large volumes.

Another method known in the industry is based on the fact that an aqueous solvent dissolves less gas at higher temperature, and vice versa for organic solvents (provided the solute and solvent do not react). Consequently, heating an aqueous solution may expel dissolved gas, whereas cooling an organic solution has the same effect. Ultrasonication and stirring during thermal regulation are also effective. These methods need no special apparatus and are easy to conduct. In some cases, however, the solvent and the solute decompose, react with each other, or evaporate at high temperature, and the rate of removal is less reproducible. For this method also, it is difficult to create a continuous process since heating a large amount of fluid will be energy demanding.

Gas-liquid separation membranes allow gas but not liquid to pass through. Flowing a solution inside a gas-liquid separation membrane and providing an underpressure on an outside of the membrane makes the dissolved gas go out through the membrane. This method has the advantage of being able to prevent redissolution of the gas, so it is used to produce very pure solvents. However, a gas-liquid separation membrane has a limited flow capacity. The above techniques are normally also combined with use of chemicals to meet the most stringent gas removal requirements such as removal of dissolved oxygen in water.

When oxygen is to be removed from a liquid, adding reductants to the liquid may sometimes be effective. For example, especially in the field of electrochemistry, ammonium sulfite is frequently used as a reductant because it reacts with oxygen to form sulfate ions. By such a method, the dissolved oxygen is almost eliminated. However, this method can be applied only to oxygen and involves the risk of reduction of the solute.

The invention has for its object to remedy or to reduce at least one of the drawbacks of the prior art, or at least provide a useful alternative to prior art.

The inventor has previously developed technologies suitable for treatment of water.

One such technology is disclosed in publication WO 2011/105911 which regards a method and an apparatus for the treatment of ballast water, wherein the ballast water is carried into a ballast tank via a down pipe. A down pipe may also be denoted a drop pipe. An underpressure which is formed at the upper portion of the down pipe induces formation of gas bubbles in the ballast water. Before the ballast water enters the down pipe, the ballast water is carried via a throttling device into a closed container at the upper portion of the down pipe. The closed container disclosed in WO 2011/105911 has a larger flow area than the flow area of the down pipe. The closed container and the down pipe are arranged in series, so that the water flows through the closed container and then through the down pipe.

Publication WO 2009/022913 discloses a method for treating ballast water wherein ozone is added to the ballast water by using a down pipe.

Another such technology is disclosed in publication WO2005/009907 which regards an apparatus for reducing an oxygen content of ballast water on a ship where ballast water is introduced into an upper part of a downcomer, also known as a down pipe or “drop line”, to produce a pressure drop, especially in the upper part of the downcomer. This pressure drop facilitates the release of gases from the seawater. The separated gas is allowed to leave the seawater after flowing through the downcomer together with the seawater. The upper part of the downcomer, which is situated well above a deck of the ship, is directly connected to a water supply pipe, and the lower part of the downcomer is communicating with the ballast compartment of the ship.

Publication JP3593398 B2 discloses a condenser drain recovery system attached to a cooler for cooling an exhaust gas of an ejector in a steam power plant. The drain recovery system comprises an air-water separation pipe configured for recovering condensed gas. The air-water separation pipe is open to the atmosphere for releasing non-condensed gas and for providing a pressure difference between the air-water separation pipe and a condenser.

The inventor has previously developed technologies for preventing gases to be separated from liquids.

One such technology is disclosed in publication WO 03/048028 which regards a method and an apparatus for reducing evaporation of volatile organic compounds (VOC) or other gases during the filling of a liquid petroleum product on a storage and/or transport tank. A feed pipe discharges into a loading column, where the cross section of the loading column is significantly larger than that of the feed pipe.

The apparatuses and methods disclosed in the publications referred to above, are capable of handling large volumes and have limited moving parts. The applicant has surprisingly found that combining at least some of the teaching of said publications, i.e. facilitating release of gases from a liquid and reducing evaporation of gases from a liquid, may provide a system or arrangement suitable for continuous treatment of large volumes of liquid, and remedy or reduce at least one of the drawbacks of the prior art.

At least some of the above-mentioned publications refer to a Froude number. The Froude number F, which is known from open-channel hydraulic theory, is dimensionless and is defined as a ratio between the inertial force and the gravitational force acting on a fluid:

in which V = the fluid velocity in metres per second, g = the earth gravitation in metres per second<2 >and hm = the hydraulic mean depth.

By replacing the hydraulic depth hm in the formula with the diameter D of a pipe, an expression is found that has turned out to be appropriate for the selection of suitable diameters of a separation column and a drop line for use in the arrangement according to the present invention.

A Froude number of a fluid depends on the property of the fluid, inter alia its viscosity, and can be found from literature or from experiments. As an example, water has a Froude number of about 0.33. When knowing the Froude number (found from literature or from experiments), the earth gravitation in metres per second<2>, and the liquid supply and discharge rates of each of the separation column and the drop line, the fluid velocity V in metres per second can be calculated. Thus, the diameter D of each of the separation column and the drop line can be found from the expression

The necessary height of the separation column depends on the vapour pressure of the gas-containing liquid as will be discussed below.

The inventor has surprisingly found that at least one of the drawbacks of the introductorily mentioned prior art and use of a vacuum pump for extracting any released gas may be remedied or reduced by combining at least some of the teaching of his above referenced publications.

The object of the invention to remedy or to reduce at least one of the drawbacks of the prior art, or at least provide a useful alternative to prior art is achieved through features, which are specified in the description below and in the claims that follow.

The invention is defined by the independent patent claims. The dependent claims define advantageous embodiments of the invention.

In a first aspect of the invention there is provided an arrangement for reducing a content of dissolved gas from a gas-containing liquid, the arrangement comprising:

- a closed separation column having an inlet at a top portion for receiving the gascontaining liquid, a liquid outlet, and a gas outlet for gas being released from the gas containing liquid; and

- a drop line having an inlet at a top portion for receiving a drive liquid, and an outlet in a bottom portion, wherein the gas outlet of the separation column is in fluid communication with the inlet of the drop line, the separation column is designed with a Froude number allowing dissolved gas of the gas-containing liquid to raise within the separation column, and the drop line is designed with a Froude number allowing gas to follow the liquid flow through the drop line, so that a gas released from the gas-containing liquid within the separation column is transferred to the drop line where the gas flows together with the drive liquid through the outlet of the drop line, and liquid with a reduced content of dissolved gas flows through the outlet of the separation column.

In one embodiment are the separation column and the drop line designed with lengths which allow the operating static head of the separation column being equal to or lower than the operating static head of the drop line.

By providing a drop line that in operation contains a liquid providing a static head being equal to or higher than the static head of the separation column, a pressure at a top portion of the drop line will be equal to or lower than a pressure at a top portion of the separation column. This has the effect of facilitating or enhancing the transference of gas released from the liquid within the separation column into the drop line by means of suction. The drop line may be designed to provide a pressure within its top portion that is equal to or preferably less than a vapour pressure of the liquid flowing through the separation column.

From the first aspect of the invention it should be clear that the liquid flowing out through the outlet of the separation column is separate from the drive liquid flowing out through the outlet of the drop line. Thus, the flow through the separation column and the drop line may be denoted parallel flows, contrary to for example the teaching from WO 2005/009907 wherein a liquid in one embodiment flows in series through a downcomer (drop line) and a separating pipe (separation column).

By means of providing a fluid communication between the top portion of the separation column and a top portion of the drop line, the gas released from the gas-containing liquid supplied to the separation column is transferred into the drive liquid flowing through the drop line. Thus, the liquid flowing out of the lower portion of the separation column may be denoted a “gas-clean liquid”, while the liquid flowing out of the lower portion of the drop line may be denoted a “gas-contaminated liquid”. By the term “gas-clean liquid” is meant a liquid with at least a reduced content of dissolved gas as compared with the gascontaining liquid prior to flowing through the separation column.

The gas-containing liquid and the drive liquid may be supplied from the same source. Such an embodiment may be of particular interest when the source of the gas-containing liquid is the sea, and the water shall be used for example as injection water in the oil and gas industry, as ballast water in the shipping industry, and in some cases also in the fishfarming industry.

A gas-containing liquid and a drive liquid supplied from the same source may be supplied to the inlet of the separation column and the inlet of drop line by means of individual pumps, i.e. a pump for supply of liquid to the separation column, and a pump for supply of water to the drop line. Alternatively, a gas-containing liquid supplied from the same source may be supplied by means of a common pump, wherein the gas-containing liquid and the drive liquid is diverted or split into separate conduits or lines prior to entering the inlet of the separation column and the inlet of the drop line, i.e. upstream of the inlets.

When the liquids to the separation column and the drop line are supplied from the same source, the Froude number of the separation column should be less than the Froude number of the drop line. This is necessary for achieving the above-mentioned transfusion of the gas released from the gas-containing liquid supplied to the separation column into the drive liquid flowing through the drop line.

For an arrangement configured for reducing a content of dissolved gas from a gascontaining liquid in the form of water, a Froude number being less than 0.33 will result in gas bubbles raising, while a Froude number larger than 0.33 will result in gas bubbles following the flowing liquid. Thus, for a gas-containing liquid in the form of water and a drive liquid in the form of water, the separation column forming part of the arrangement according to the present invention, should be configured for having a Froude number being less than 0.33, while the Froude number of the drop line forming part of the arrangement according to the present invention should be larger than 0.33. Preferably, the Froude number of the separation column configured for a gas containing water may be less than 0.25, and a Froude number of the drop line may be larger than 0.4.

As an alternative to supplying the gas-containing liquid and the drive liquid from the same source as discussed above, the gas-containing liquid and the drive liquid may be supplied from separate sources. One effect of this is that the drive liquid may be selected from a low-cost liquid source. For example, if the dissolved gas is to be reduced or substantially removed from a gas-containing liquid such as for example an oil or a chemical such as glycol, a drive liquid would normally not be from the same source. Instead, a drive liquid would typically be water. Although being supplied from separate sources, the drive liquid and the gas-containing liquid may in one embodiment be the same type of liquid.

In one embodiment, the drive liquid may be circulated in a loop. Such a loop may, but does not have to, comprise a vessel arranged downstream of the outlet of the drop line for receiving drive liquid with gas sucked out of the top portion of the separation column. The vessel may be configured for allowing a retention time sufficient for at least a portion of the gas to be separated from the drive liquid being circulated. Obviously, a volume of a drive liquid circulating in a loop, is limited. Therefore, a drive liquid circulating in a loop is of particular interest for example when water is a limited resource, or when there is a need for a drive fluid being other than the gas-containing liquid. Examples of when there may be a need for a drive fluid being other than the gas-containing liquid are when hydrocarbons are used to remove hydrocarbons in water, or when sea water is used for removal of dissolved gases such as volatile organic compounds from fresh water.

Preferably, the length (effective hydraulic height) of the drop line is longer than a length (effective hydraulic height) of the liquid column within the separation column. This has the effect that the pressure in a top portion of the drop line is lower than a pressure in a top portion of the separation column, at least when the gas-containing liquid and the drive liquid are supplied from the same source, or the liquids have substantially the same specific gravity and temperature. Preferably, the pressure in a top portion of the drop line is equal to or less than a vapour pressure of the gas-containing liquid within the separation column. The vapour pressure is the pressure exerted by a vapour in thermodynamic equilibrium with its condensed phases at a given temperature in a closed system. The vapour pressure is also known as bubble point. For example, for a water having a temperature of 4 ºC, the vapour pressure will be about 0.81 KPa. For a water having a temperature of 20 ºC, the vapour pressure will be about 2.33 KPa. Vapour pressure for relevant liquids at various temperatures can be found from literature.

In one embodiment, the inlet at the top portion of the separation column has an extension to enlarge a horizontal cross-sectional area at the inlet to increase a surface area of a liquid level when the arrangement is in operation. Tests have surprisingly shown that an increased surface area of the liquid level at the inlet of the separation column increases the amount of dissolved gases released from the liquid entering the separation column.

In one embodiment the extension may be provided by means of a tube arranged downstream of a separation column supply pipe, i.e. between a downstream end portion of the separation column supply pipe and an upstream end portion of an inlet of the separation column. To effect the increased surface area of the liquid level at the inlet of the separation column, the tube has preferably a larger cross-sectional flow area than the separation column supply pipe. Alternatively, or additionally to arranging a tube downstream of the separation column supply pipe, the extension may be provided by means of an enlarged cross-sectional area of the separation column at the top portion connected to the separation column supply pipe. Such an enlarged cross-sectional area may for example be provided by means of a barrel-shaped portion of the separation column.

In one embodiment, the separation column supply pipe may be provided with a heating device to increase a temperature of the gas-containing liquid flowing therethrough so that the vapour pressure of the gas-containing liquid is increased. An increased vapour pressure of the gas-containing liquid entering the separation column will increase the release of dissolved gases from the liquid flowing through the separation column. A heating device for the purpose of heating the liquid flowing through the separation column supply pipe, may for example be a heat exchanger type equipment known per se.

In one embodiment, a drop line supply pipe is provided with a cooling device for reducing a temperature of the drive liquid supplied to the drop line so that the vapour pressure of the drive fluid is reduced. For the arrangement disclosed herein, the effect of reducing the temperature of the drive liquid will be similar to that of increasing the temperature of the gas-containing liquid entering the separation column since a reduced temperature of the drive liquid will increase the transfer of released gas or gases from the separation column to the drop line.

In one embodiment of the arrangement, the separation column supply pipe is provided with the heating device, and the drop line supply pipe is provided with the cooling device to further increase or “double” the effect. By “double” is meant adding the effect provided by heating the gas-containing liquid, to the effect of reducing the temperature of the drive liquid.

The gas outlet in the top portion of the separation column communicates with the top portion of the drop line typically by means of a pipe. Preferably, a connection point between such a pipe and the drop line comprises an ejector device. In order to reduce a risk of liquid from the separation column from entering into the drop line, it is an advantage if the ejector device and/or at least a portion of the pipe between the separation column and the drop line is arranged at a higher elevation than a liquid level within the separation column.

The inlet at the top portion of the separation column may be provided with a pressure reduction unit to enhance separation from the gas-containing liquid. Preferably, the pressure reduction unit is configured for spreading the gas-containing liquid that flows through the inlet at the top portion of the separation column.

In one embodiment of the invention, two or more separation columns may be arranged in series so that the outlet of one of the separation columns arranged upstream of a second one of the separation columns is in fluid communication with the inlet of said second one of the separation columns. In such an embodiment, the top portion of each of the at least two separation columns are in fluid communication with the drop line. This has the effect of removing dissolved gases from the gas-containing liquid in at least two steps. Such an arrangement is of particular interest when there is a desire for providing a liquid wherein substantially all dissolved gas or gases has been removed. It can also be an advantage in cases where the water comprises for example living organisms that during cell destruction will release gases and where several steps may be required to ensure sufficient gas removal. Some liquids may also have different viscosities that will require the dissolved gas to be removed in several steps.

In a second aspect of the invention there is provided a method for reducing a content of dissolved gas from a gas-containing liquid. The method comprises:

- - providing an arrangement according to claim 1;

- flowing the gas-containing liquid through the inlet at a top portion of the separation column;

- flowing a drive fluid from a drive fluid source into the top portion of the drop line and out through the outlet at a bottom portion;

- providing fluid communication between the gas outlet in the top portion of the separation column and the top portion of the drop line so that gas released from the gas-containing liquid in the separation column is transferred into and flows together with the drive fluid out through the outlet of the drop line.

In one embodiment, the method comprises supplying the gas-containing liquid and the drive liquid from the same source. In an alternative embodiment, the method comprises supplying the gas-containing liquid and the drive liquid from the separate sources.

In the following is described examples of preferred embodiments illustrated in the accompanying drawings, wherein:

Fig. 1 shows an arrangement according to an embodiment of the invention comprising a separation column and a drop line supplied with liquid via two separate supply lines;

Fig. 2 shows an arrangement according to an embodiment of the invention comprising a separation column and a drop line supplied with liquid via a common supply line;

Fig. 3 shows an arrangement according to an embodiment of the invention comprising three separation columns arranged in series, and a drop line, wherein the series of separation columns and the drop line are supplied with liquid via two separate supply lines; and

Fig. 4 shows an arrangement according to an embodiment of the invention wherein liquid is supplied to a separation column and a drop line by means of gravity.

Fig. 5 shows an arrangement similar to that of fig.1, but with the top portion of the separation column provided with an extension to enlarge a surface area of the liquid.

Positional specifications such as above, under, left, right, upstream, and downstream etc., refer to the positions shown in the figures.

In the figures, same or corresponding elements are indicated by same reference numerals and letters.

A person skilled in the art will understand that the figures are just principle drawings. The relative proportions of individual elements may also be strongly distorted.

In the figures reference numeral 1 denotes an arrangement according to the present invention configured for at least reducing a content of dissolved gas from a gas-containing liquid.

The arrangement comprises a separation column 3 having an inlet 5 at a top portion 7 of the separation column 3. The inlet 5 is for receiving a gas-containing liquid from a liquid source (not shown). The column 3 is provided with an outlet 9 in a bottom portion 11 thereof.

To provide an underpressure chamber within the separation column 3, the inlet 5 is arranged at some distance, for example 0.5m, from a top end of the column 3, i.e. at a top portion 7.

In the embodiments shown, an end portion of the inlet 5 at the top portion 7 of the separation column 3 is provided with a pressure reduction unit 4 to enhance separation of gas from the gas-containing liquid. The pressure reduction unit 4 is configured for spreading the gas-containing liquid that flows through the inlet 5. In one embodiment, the pressure reduction unit 4 may for example be a throttling device in the form of a disc valve configured for spreading the gas-containing liquid when entering the separation column 3. An example of a suitable throttling device is disclosed in WO 2011/105911.

It should be noted that a pressure reduction unit of a type known per se may alternatively or additionally (not shown) be arranged upstream of the inlet 5 of the separation column 3.

The top portion 7 of the separation column 3 is provided with a gas outlet 13 connected to a gas pipe 15. The gas pipe 15 is in fluid communication with a top portion 22 of a drop line 20 via an ejector device 23 known per se.

The drop line 20 has an outlet 24 in a lower end portion thereof. The drop line 20 itself is of a type known per se, for example from WO 2005/009907 A1 wherein the drop line is denoted “downcomer”.

In operation, a supply of gas-containing liquid through the inlet 5 of the separation column 3 is balanced with the discharge of liquid through the outlet 9 in the bottom portion of the separation column 3 and the gas outlet 13 in the top portion 7 of the separation column 3. Thus, in operation, a liquid level L3 in the column 3 is at a constant elevation or height balanced by the vapor pressure of the gas containing liquid. Similarly, a supply of drive liquid through the inlet 22 of the drop line 20 is balanced with the discharge of liquid plus the transferred gas from the top portion 7 of the separation column 3, through the outlet 24 in the bottom portion of the drop line 20. However, a flow rate through the inlet 5 of the separation column 3 can be different from a flow rate through the inlet 22 of the drop line 20 since the flow of liquid through the separation column 3 is parallel with the flow of liquid through the drop line 20. The gas released from the gas-containing liquid in the separation column 3, is however in serial flow with the flow through the drop line 20.

When a liquid flows (indicated by dotted arrow) through the column 3 from the inlet 5 through the outlet 9, an underpressure will be generated in the top portion 7 of the column 3 between a liquid level L3 and the gas outlet 13. The underpressure is generated i.a. due to the weight of liquid in the column 3 and, in the embodiment shown, due to a suction provided by means of the ejector device 23 in the top portion 22 of the drop line 20. Thus, the underpressure is dependent on i.a. the length (height) of the column 3. The underpressure should be sufficient to generate release of gas from the gas-containing liquid. Therefore, the pressure in the top portion of the separation column is equal to or preferably less than a vapour pressure of the gas-containing liquid so that at least a portion of the dissolved gas or gases releases from the gas-containing liquid. Gas or gases released from the liquid will be transferred or sucked out through the gas outlet 13 and into the drive liquid flowing down the drop line 20.

In a prototype of an arrangement according to the invention for reducing a content of for example dissolved oxygen from water, a height of the water column within the separation column was 10 meters. The height of the separation column was about 10.5 – 11 meters to provide the underpressure chamber between the liquid level L3 and the outlet 13.

In the figures, the pressure P1 and pressure P2 in the lines 30, 28 upstream of the pressure reduction unit 4 and the inlet 22 of the drop line, respectively, are larger than the vapour pressure(s) of the relevant gas-containing liquid and drive liquid. Pressure P3 and P4 should be less than or equal to the vapour pressure of the gas-containing liquid. The pressures P1, P2, P3 and P4 are shown on relevant positions of their relevant lines.

As discussed above, to facilitate reduction of the content of dissolved gas or gases from a gas-containing liquid, the separation column is configured to achieve a Froude number allowing gas of the gas-containing liquid to raise within the separation column 3, and the drop line 20 is configured to achieve a Froude number allowing gas to follow the liquid flow out through the outlet 24 of the drop line 20.

In fig.1, the inlet 5 of the separation column 3 is in fluid communication with a source of gas-containing liquid via a separation column supply pipe 30. In the embodiment shown, the separation column supply pipe 30 is provided with a pump 31 and a valve arranged downstream of the pump 31.

The drop line 20 is in fluid communication with a source of drive liquid via a drop line supply pipe 28. In the embodiment shown, the drop line supply pipe 28 is provided with a pump 29 and a valve arranged downstream of the pump 29.

In the embodiment shown in fig.1, the gas-containing liquid and the drive liquid are supplied to the separation column 3 and the drop line 5, respectively, independently of each other. Such an independent supply allows for different sources of gas-containing liquid and drive liquid. This has the effect that for example water, which in many cases may be taken from an inexhaustible source such as a sea or a lake, may be used for reducing a content of dissolved gas, for example CO2, from for example glycol. Further, an independent supply of drive liquid and gas-containing liquid allows for regulating the flow of drive liquid through the drop line 20. Regulating the flow of drive liquid through the drop line 20 will influence the suction provided by the ejector 23 providing fluid communication between the gas pipe 15 and the top portion 22 of the drop line 20.

Although the gas-containing liquid and the drive liquid in the embodiment shown in fig.1 are supplied to the separation column 3 and the drop line 5 independently of each other, it should be clear that such independent supply does not prevent the gas-containing liquid and the drive liquid being collected from the same source.

In fig.2, the gas-containing liquid and the drive liquid is supplied from a common source by means of a common pump 31. Upstream of the inlet 5 of the separation column 3 and the top portion 22 of the drop line 20, a common fluid pipe 32 is split into a separation column supply pipe 30 and a drop line supply pipe 28 by means of a flow diverter 34. In one embodiment, the flow diverter 34 comprises a valve arrangement configured for regulating the flow rate through the separation column supply pipe 30 and a drop line supply pipe 28.

The embodiments shown in figures 1 and 2 disclose an arrangement configured for a “one-step” reduction of gas from a gas-containing liquid. Such a one-step reduction of a gas from a gas-containing liquid will in many cases be sufficient for obtaining a desired reduced level of dissolved gases in a liquid.

However, in some cases it is necessary to provide a liquid being substantially free from dissolved gases. An example of such a case is in the oil and gas industry wherein socalled injection water must be substantially free of oxygen. Oxygen in concentrations of 0.5 ppm in hydrogen-sulphide-free water and 0.01 ppm in water containing hydrogen sulphide, is generally considered to be sufficient to cause corrosion problems in the facilities and bacteria-plugging problems in an injection reservoir. Thus, an injection water must have a concentration of dissolved oxygen being less than the above-mentioned concentrations.

Fig. 3 shows an arrangement according to the present invention providing a “multi-step” reduction of gas from a gas-containing liquid. In the embodiment shown, three separation columns 3, 3’, 3’’ are arranged in series. The columns 3, 3’, 3’’ are connected to a drop line 20 via gas line 15 in the same way as discussed above. Gas-containing liquid from a liquid source (not shown) is pumped into a first separation column 3 (the left column in fig.

3) and at least a portion of dissolved gas is removed therefrom while the liquid flows through the first separation column 3. By means of a pump 37 liquid flowing out of the outlet 9 in the bottom portion 11 of the first separation column 3, is pumped into the inlet 5 of a second separation column 3’ (the intermediate column in fig.3) wherein the content of dissolved gas is further reduced. This process is continued by pumping by means of pump 39 the liquid from the outlet 9 of the second separation column 3’ to the inlet 5 of the third separation column 3’’ (the right column in fig.3). Liquid flowing out of the outlet 9 of the third separation column 3’’ has thus been subject to a three-step reduction of dissolved gas.

In alternative embodiments (not shown), the multi-step reduction of dissolved gas from a liquid may be provided by two separation columns arranged in series, or more than three separation columns arranged in series.

The arrangements shown in figures 1-3 and 5 require one or more pumps for supplying the gas-containing liquid and the drive liquid to the separation column 3 and the drop line 20, respectively. However, if a liquid source for providing one or both of a gas-containing liquid and a drive liquid has a hydraulic head sufficient to supply said liquid(s) into the separation column 3 and/or drop line 20, then the respective pumps are superfluous.

Fig. 4 shows a liquid source 40 having a hydraulic head being higher than the inlet 5 of the separation column 3 and the top portion 22 of the drop line. The hydraulic source 40 may for example, but not limited to, be a lake or an artificial source. A liquid, for example water, flows due to gravity into the separation column 3 via inlet 5, and into the top portion 22 of the drop line 20. The flow is spilt into a separation column supply pipe 30 and a drop line supply pipe 28 by means of a flow diverter 34. The flow diverter 34 may comprise a valve arrangement configured for regulating the flow rate through the separation column supply pipe 30 and a drop line supply pipe 28. The gas released from the gas-containing liquid is sucked into the drop line 20 as discussed above.

In the embodiment shown in fig.4, the arrangement for reducing a content of dissolved gas from a gas-containing liquid is provided without any pumps that require power for operating. The only movable parts required for the principle arrangement in fig.4, are the valves for controlling the supply of liquid from the source 40, and any valve arrangement provided with respect to the flow diverter.

In fig.5, the inlet 5 at the top portion 7 of the separation column 3 has an extension 6, here in the form of a tube 6, to enlarge a horizontal cross-sectional area at the inlet to increase a surface area of a liquid level L3 when the arrangement 1 is in operation. In the embodiment shown, the tube 6 is arranged downstream of the separation column supply pipe 30, and the tube 6 forms part of the separation column 3. The tube 6 has a larger cross-sectional flow area than the separation column supply pipe 30. In the embodiment shown in fig.5, a longitudinal cross-sectional area of the tube 6 is about half that of the separation column 3.

In fig.5, the separation column supply pipe 30 is provided with a heating device 36 to increase the temperature of the gas-containing liquid flowing therethrough to so that the vapour pressure of the gas-containing liquid is higher than the vapour pressure of the drive fluid. A vapour pressure of the gas-containing liquid flowing into the separation column 3 being higher than the vapour pressure of the drive liquid will increase the release of dissolved gases from the liquid flowing through the separation column.

Alternatively to, or additionally to providing the separation column supply pipe 30 with a heating device 36, the drop line supply pipe 28 may be provided with a cooling device 38 as shown in fig.5. The purpose of such a cooling device 38 is to reduce a temperature of the drive fluid supplied to the drop line 20 to reduce the vapour pressure of the drive fluid.

The gas released from the gas-containing liquid can evacuate only through the gas pipe 15 providing fluid communication between the gas outlet 13 and the inlet of the drop line 22. Further, due to the drop line 20 being designed having a Froude number allowing gas to follow the liquid flow, the gas released from the gas-containing liquid flowing into the separation column 3 will be transferred to the drive fluid flowing through the drop line 20.

The heating device 36 and/or the cooling device 38 discussed above, may be included in the embodiments shown in figures 1 - 4. From the disclosure herein, the skilled person will appreciate that the arrangement and method for reducing a content of dissolved gas from a gas-containing liquid may be used for example within the oil and gas industry, fish farming industry, chemical industry, process industry, and shipping industry.

A typical application in the shipping industry is for ballast water treatment, wherein there is a need for substantially removing the oxygen from the ballast water to kill at least some of the living organisms in the ballast water. By combining the arrangement and method according to the invention with for example chemical treatment and/or ultraviolet radiation known per se, a highly disinfected water may be achieved.

In an embodiment wherein sea water is pumped from deep water, for example 60-100m below sea level, some of the organisms therein may die due to a rapid change in pressure, followed by removal of oxygen therein. Thus, the invention may be suitable for example for producing injection water in the oil and gas industry.

From the arrangement and method disclosed herein, it will be understood that dissolved gas may be reduced continuously from large volumes of liquid flowing through the arrangement. The arrangement is very simple and reliable with few movable parts, if any, in operation.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements.

Claims

C l a i m s

1. An arrangement (1) for reducing a content of dissolved gas from a gas-containing liquid, c h a r a c t e r i s e d i n that the arrangement comprising: - a closed separation column (3) having an inlet (5) at a top portion(7) for receiving the gas-containing liquid, a liquid outlet (9), and a gas outlet (13) for gas being released from the gas-containing liquid; and

- a drop line (20) having an inlet at a top portion (22) for receiving a drive liquid, and an outlet (24) in a bottom portion,

wherein the gas outlet (13) of the separation column (3) is in fluid communication with the inlet of the drop line (20), the separation column (3) is designed with a Froude number allowing dissolved gas of the gas-containing liquid to raise within the separation column (3), and the drop line (20) is designed with a Froude number allowing gas to follow the liquid flow through the drop line (20), so that a gas released from the gas-containing liquid within the separation column (3) is transferred to the drop line (20) and flows together with the drive liquid through the outlet (24) of the drop line (20), and liquid with a reduced content of dissolved gas flows through the outlet (9) of the separation column (3).

2. The arrangement (1) according to claim 1, wherein the separation column (3) and the drop line (20) are designed with lengths which allow an operating static head of the separation column (3) being equal to or lower than an operating static head of the drop line (20).

3. The arrangement (1) according to claim 1 or 2, wherein the gas-containing liquid and the drive liquid are supplied from the same source.

4. The arrangement (1) according to claim 1, 2 or 3, wherein the gas-containing liquid and the drive liquid are supplied by a common pump (31) and split into separate conduits (28, 30) prior to entering the inlet (5) of the separation column (3) and the inlet of the drop line (20).

5. The arrangement (1) according to any one of claim 1 - 4, wherein a Froude number of the separation column (3) is less than the Froude number of the drop line (20).

6. The arrangement (1) according to any one of the preceding claims, wherein the gas-containing liquid is water, and the drive liquid is water, the Froude number of the separation column (3) is less than 0.33, preferably less than 0.25, and the Froude number of the drop line (20) is larger than 0.33, preferably larger than 0.4.

7. The arrangement (1) according to claim 1, wherein the gas-containing liquid and the drive liquid are supplied from separate sources.

8. The arrangement (1) according to claim 7, wherein the drive liquid is circulated in a closed loop (28, 20).

9. The arrangement (1) according to any one of the preceding claims, wherein a length of the drop line (20) is longer than a length of the separation column (3).

10. The arrangement (1) according to any one of the preceding claims, wherein the inlet (5) at the top portion (7) of the separation column (3) is provided with a pressure reduction unit (4) to enhance separation of gas from the gas-containing liquid.

11. The arrangement (1) according to any one of the preceding claims, wherein the inlet (5) at the top portion (7) of the separation column (3) has an extension (6) to enlarge a horizontal cross-sectional area at the inlet (5) to increase a surface area of a liquid level (L3) when the arrangement (1) is in operation.

12. The arrangement (1) according to claim 11, wherein the extension is provided by means of a tube (6) arranged downstream of a separation column supply pipe (30), the tube (6) having a larger cross-sectional flow area than the separation column supply pipe (30).

13. The arrangement (1) according to claim 12, wherein the separation column supply pipe (30) is provided with a heating device (36) to increase a temperature of the gas-containing liquid flowing therethrough so that the vapour pressure of the gas-containing liquid is increased.

14. The arrangement (1) according to any one of the preceding claims, wherein a drop line supply pipe (28) is provided with a cooling device (38) to reduce a temperature of the drive fluid supplied to the drop line (20) so that the vapour pressure of the drive fluid is reduced.

15. The arrangement (1) according to any one of the preceding claims, wherein two or more separation columns (3, 3’, 3’’) are arranged in series so that the outlet (9) of one of the separation columns (3) arranged upstream of a second one of the separation columns (3’) is in fluid communication with the inlet (5) of said second one of the separation column (3’), the top portion of each of the at least two separation columns (3, 3’, 3’’) being in fluid communication with the drop line (20).

16. A method for reducing a content of dissolved gas from a gas-containing liquid, c h a r a c t e r i s e d i n that the method comprises the following steps:

- providing an arrangement according to claim 1;

- flowing the gas-containing liquid through the inlet (5) at a top portion (7) of the separation column (3);

- flowing a drive fluid from a drive fluid source into the top portion (22) of the drop line (20) and out through the outlet (24) at the bottom portion;

- providing fluid communication between the gas outlet (13) in the top portion (7) of the separation column (3) and the top portion (22) of the drop line (20) so that gas released from the gas-containing liquid in the separation column (3) is transferred into and flows together with the drive fluid out through the outlet (24) of the drop line (20).

17. The method according to claim 16, wherein the method comprises supplying the gas-containing liquid and the drive liquid from the same source.

18. The method according to claim 16, wherein the method comprises supplying the gas-containing liquid and the drive liquid from separate sources.