Technical Field

Present invention relates to a system and a method for mixing of gas and fluids. More specifically, the invention relates to a system and method for mixing nitrogen into a stream of water for producing an oxygen depleted flow of water. Even more specifically, the invention relates to a system and method for mixing an oxygen depleted and nitrogen rich gas into a stream of seawater for use in re-injection in oil and gas reservoirs.

Background Art

In a process for deoxygenation of water for reinjection of the water into a gas or oil reservoir, nitrogen gas from the process is injected into the water to be treated by means of compressors. However, in some instances, when sea water is used for the water to be treated, the use of a compressor can cause some complications, since the gas contains some entrained water. The compression of the gas leads to a rise of the temperature in the gas and the entrained water containing salt, will evaporate and lead to salt deposits in the compressor. The compressor is the most vulnerable component in the deoxygenation system and is very sensitive to salt and external stress. Since the compressor is a complicated device with a very high rotational speed, frequent shutdowns of the compressor are necessary for maintenance and removal of salt deposits. This will again lead to reduced running income and increased maintenance costs.

In order to avoid the problems described above, it has been surprisingly found that use of the compressor can be avoided by replacing the compressor with the system according to the invention. In the system according to present invention, the flow of water to be treated, is divided in two separate streams. The stream passes through a gas ejector where gas is mixed with the water, and a second stream, bypassing the first stream, via a control valve.

However, use of gas ejectors for mixing of gas and fluids are well known.

NZ260581 A relates to portable apparatus for the introduction of gaseous substances into a fluid supply, for instance the introduction of chlorine or other bactericidal substances into a water supply. The incoming fluid flow is divided into two paths, where one of the paths, the injection path, includes an injector for introducing a gaseous substance, and the other paths is a bypass path. The injection path and/or the bypass path includes valve means to maintain a substantially constant proportional distribution of flow through said bypass and injection lines.

US 6.517.727 B2 describes a method to dissolving a solid chemical material, such as calcium hypochlorite, with a solvating liquid, e.g. water, in a chemical feeder operating under positive pressure wherein an inert gas, e.g. air, is injected into the solvating liquid, e.g. by use of an injector nozzle, and the resultant mixture of inert gas and solvating liquid is forwarded to the chemical feeder. Sufficient air is charged to the chemical feeder to limit the level of solvating liquid in the feeder and thereby limit the amount of solid chemical material contacted by the solvating liquid.

US 6.488.853 describes a method for treating sewage water in a sewage treatment plant. Sewage water from a reservoir is split into two flows, where the first flow is forwarded to a gas injector introducing gas, e.g. ozone, into the water and this ozone containing water is led back to the reservoir. The second flow is led to a filter and a UV-radiation system and further to a discharge tank. The first and second flows are not combined after the introduction of gas.

US 7.022.225 relates to treatment of water from a swimming pool by ozonation. A water stream to be cleansed is divided into two streams. A first stream is led via a gas injector (e.g. venturi valve) to a water/gas mixing apparatus. The mixing apparatus also comprises a degasser for the removal of surplus ozone. The so treated first water stream is combined with the second stream, which has been led through a ball valve, back to the pool. This solution does not comprise any regulation of the second (bypass) water stream, except for the ball valve, providing a very limited possibility to regulate the flow rate in the bypass stream

EP2899165A2 relates to an ozone injection method and system, preferably a method and system for treatment of ballast water in ships, where a portion of water from a flow of water in a conduit to be treated is diverted, ozone is injected into this portion, e.g. by use of a venturi, to provide an ozonated portion, the ozonated portion is recombined with the flow of water in the conduit, and the diverted portion is preferably controlled/regulated to provide a minimum diverted portion flow rate according to flow in the conduit and proportion of ozone in the injected gas. By this method, it is the flow rate of the diverted portion, which is to be ozonated, that is regulated/controlled.

US 5006133A1 describes a process and an apparatus for deoxidation of sea water for injection into an oil and/or gas well, where a nitrogen rich gas

(stripping gas) from a separator is mixed with a partial stream of sea water by means of an ejector. The mixed stream injected with gas is led to a separator for deoxygenation and is mixed with the main water flow fed to the separator.

CN108397929A describes gas-liquid separator where a gas stream from a gas- liquid separator is led to the water downstream of a gas-liquid separator. The feeding of gas is made by an ejector to mix water and gas. The mixed stream injected with gas is led back to the gas-liquid separator for degassing.

SE51 1555C2 describes a device for purification of sea water, where the incoming water is divided into two streams, the first stream is mixed with a gas (ozone) by means of an ejector, the second stream is mixed with the first stream which has been injected with gas, in a mixer.

One object with present invention is to provide a method and device for deoxygenation of water which eliminate the use of one or more compressors. Summary of invention

The above objects are achieved with a method for injecting gas into a stream of water, said water is fed to a process for the deoxygenation of water, said method comprising the following steps:

- separating a stream of water to be treated into a first stream and a second stream;

- leading the first stream to a gas ejector;

- regulating the second stream so that the flow of water to the gas ejector is maintained at an approximately constant flow rate and pressure,

- mixing the first stream with an oxygen depleted, nitrogen rich gas stream from a later step in the deoxygenation process;

- combining the first stream and the second stream after having passed the gas ejector and the regulator valve, respectively;

- leading the combined stream to the deoxygenation process.

In another preferred embodiment, the pressure of the first stream is increased by means of a pump if the pressure of the incoming water is too low.

In a preferred embodiment, the water to be treated is preferably sea water.

In yet another preferred embodiment, the deoxygenated water is injected into a gas and/or oil reservoir.

The invention also relates to a device for injecting gas into a stream of water to be deoxygenated for the production of oxygen-depleted water, comprising:

- a supply of water to be treated, said supply is divided into a first stream to be injected with gas, and a second stream,

- an ejector in the first stream injecting oxygen depleted, nitrogen rich gas into the first stream,

- a regulator valve in the second stream, and

- a first static mixer receiving the combined first and second streams.

Preferably, the first stream includes a pump upstream of the ejector. In a preferred embodiment, the water to be treated is sea water.

In yet another preferred embodiment, the treated water is deoxygenated water for injection into a gas and/or oil reservoir.

Brief description of drawings

Figure 1 shows a deoxygenation system according to prior art.

Figure 2 shows a deoxygenation system according to present invention.

Fig. 3 shows an exemplary gas ejector for use in the system according to the invention.

Detailed description of the invention

The invention will now be described in detail with reference to the enclosed drawings, where Fig. 1 shows a prior art deoxygenation system and Fig. 2 shows a deoxygenation system according to present invention. In Figs. 1 and 2 identical reference numerals have been used for identical or similar parts.

According to Fig. 1 , in a prior art system for deoxygenation of water for use as injection water in gas and/or oil reservoirs, oxygen depleted, nitrogen rich gas (60) from a subsequent process step, is injected upstream of a first stage static mixer (100), to remove the free oxygen in the water (10) to be treated. The mass transfer of oxygen takes place in the first static mixer (100). After the mixing of water and gas, this mixture (25) is led to a first stage separator (1 10) to separate water and gas. The water (30) from the first stage separator (1 10) is led to a second stage static mixer (120). Upstream of the second stage static mixer (120), oxygen depleted, nitrogen rich gas (40) is injected to reduce the oxygen concentration in the water down to the current water specification. After the second stage static mixer (120) water and gas is led to second stage separator (130) where water and gas is separated again into a gas stream (60) and water stream (50). The deoxygenized water stream (50) can now be injected into a gas and/or oil reservoir. The gas (70) from the first stage separator is led to a heat exchanger (140) or an electric preheater to ensure that the gas temperature is optimal for subsequent regeneration of nitrogen in a reactor (150). After the heat exchanger/preheater (140), fuel (80) is mixed with the heat exchanged gas. This mixture of gas and fuel is led to the reactor (150) where the fuel reacts with the oxygen over a volume of catalyst in an

exothermic reaction. From reactor (150) the oxygen depleted and nitrogen rich gas (40) with a very low content of oxygen is mixed with the water flow upstream of the second stage static mixer (120), preferably after having passed the heat exchanger (140). This gas and water mixture is then led to the second stage separator (130) in order to further reduce the oxygen concentration in the water to the lowest possible value. Since relatively little oxygen is removed in the second stage separator (130), the gas (60) discharged from the second stage separator (130) can be re-used in the first stage separator (110). The gas (60) from the second stage separator (130) must therefore be led upstream of the first stage static mixer (100). Due to pressure reduction through the system, the pressure of the gas (60) from the second stage separator (130) will be lower than the pressure of the seawater (10) to be treated. According to the prior art system, as depicted in Fig. 1 , a compressor (160) is used to increase the pressure of the gas (60) from the second stage separator (130). Flowever, the use of a compressor (160) can cause some complications, since the gas (60) from the second stage separator (130) contains some entrained water. The compression of the gas (60) from the second stage separator (130) leads to a rise of the temperature in the gas and the entrained water containing salt, will evaporate and lead to salt deposits in the compressor (160), especially if the water to be deoxygenated is sea water,. The compressor (160) is the most vulnerable component in the deoxygenation system and is very sensitive to salt and external stress. Since the compressor (160) is a complicated device with a very high rotational speed, frequent shutdowns of the compressor (160) are necessary for maintenance and removal of salt deposits. This will again lead to reduced running income and increased maintenance costs. A solution to these problems is shown in Fig. 2, depicting the deoxygenation system according to present invention. The deoxygenation system according to Fig. 2 is identical with the prior art system according to Fig. 1 , except that the compressor (160) in Fig. 1 has been replaced by a gas injection system according to the invention and the incoming water (10) to be treated is split into two streams. The gas injection system according to present invention comprises a gas ejector (200), a regulator valve (180) in a bypass line (190), bypassing the gas ejector (200). In one

embodiment, the system also comprises a pump (170) upstream of the gas ejector (200). An exemplary embodiment of the gas ejector (200) is shown in Fig. 3. The gas ejector (200) comprises an inlet (300) for water to be treated, a gas inlet (310), a nozzle (330), a diffuser (340) and an outlet (320) for the gas/water mixture. The gas ejector (200) upstream of the first static mixer (100) will replace the prior art compressor (160). The ejector (200) will use the inlet pressure of the water (10) to be treated to create an underpressure in the gas inlet (310) of the ejector (200). The result of creating this underpressure in the gas inlet (310) is a pressure drop over the nozzle (330) to a lower discharge pressure in the gas/water mixture from the ejector (200). The diffuser (340) in the ejector (200) will also provide turbulent mixing of gas and water and some mass transfer of oxygen from the water to the gas. If the inlet pressure to the nozzle (330) is not sufficient (i.e. the pressure of the water (10) to be treated), this inlet pressure can be increased, in one embodiment, by use of a water pump (170). Flowever, by replacing the compressor (160) with a water pump (170), the energy requirement of the water pump (170) is similar to the energy requirement of the compressor (160), but the water pump (170) is a more robust and working at much lower rotational speed than a compressor, which means that the running costs and maintenance cost are significantly lower compared to a compressor (160). The temperature rise due to the compression is also negligible so there is no risk of salt deposits forming.

Leading the total water flow (10) through the ejector (200) can cause

operational problems of the system and reduce the deoxygenation effect of the deoxygenation system. As soon as the flow of water (10) to the ejector (200) is reduced, the necessary underpressure created by the nozzle (330) will immediately be affected. This results in a very limited operational window. By further reduction of incoming water flow, the underpressure will quickly be reduced to a point where the gas flow (60) ceases completely and the

deoxygenation of the water stops.

In order to solve this operational problem, the flow of incoming water (10) to be treated, are divided in two separate streams. The first stream (210) is led through the ejector (200), via the pump (170) and the second stream (bypass stream) (190) is led through a regulating valve (180). This first and second streams (210, 190) are combined upstream of the first static mixer (100). The pump (170) will increase the water pressure sufficiently to create the necessary underpressure in the nozzle (330) to be able to deoxygenate all the water flow with only a part of the incoming water (10). The outlet pressure from the ejector (200) will be somewhat lower than the inlet pressure (10) due to pressure drop over the regulating calve (180). The remaining water flow will pass through the bypass line (190) and through the regulating valve (180). The regulating valve (180) can reduce the flow in the bypass line (190) to zero. When the regulator valve (180) is completely closed, all the water will flow through the ejector (200). The pressure and flow rate of water to the ejector (200) will be held

approximately constant and within the operational range of the ejector by regulating the regulating valve (180). The pressure of the gas (60) into the ejector (200) will change somewhat since less volume of oxygen gas will be in the gas flow (60) to the ejector due to the lower flow rate of incoming water (10). This also results in that the circulating volume of gas through the ejector (200) will change correspondingly. After the ejector (200), the first and second flows (210, 190) are combined and are led to the first static mixer (100) as a combined stream. This first static mixer (100) is necessary to mix the water containing oxygen depleted, nitrogen rich gas from the ejector (200) with the untreated water (190) from the bypass line to ensure adequate mass transfer of free oxygen from the total water flow to the stripping gas.

In an exemplary embodiment, by leading all the water (10) through the ejector (200), the volumetric ratio of gas/water is about 1 :1. By splitting the water 50/50 in a first (210) and second stream (190), only 50% of the total water (10) will pass the ejector (200). This will result in that the volumetric ratio of gas/water in the ejector (200) is about 2:1. This increased volumetric ratio is obtained by increasing the pressure drop over the nozzle (330) in the ejector (200). The water will bring the gas on and this two-phase stream from the ejector (200) is combined with the untreated water from the bypass line (190). This results in a total volumetric gas/water ratio of about 1 :1 at full water flow after combining the first and second stream.

In a preferred embodiment, the process and system is used for deoxygenation of water, especially sea water, for injection of the so-treated water into a gas and/or oil reservoir.

According to another preferred embodiment of the present invention, a separate water stream at an elevated pressure is available. In this embodiment, the split of the two water streams can be omitted. The available water stream at the elevated pressure can either be wastewater form a downstream or separate process. It can also be taken from the injection water stream downstream the booster pumps or injection pumps. In this event, the available water stream can be used as the required motive pressure to entrain the gas through the ejector. The advantage by this embodiment, is that use is made of water with an elevated pressure that else will be wasted.

LIST OF REFERENCE NUMERALS

10 water to be treated

20 compressed gas from compressor 160 25 gas/water mixture to 1 ^st stage separator 30 water from 1 ^st stage separator

40 nitrogen gas from reactor

50 water from second stage separator

60 gas from second stage separator 70 gas from 1 ^st stage separator

100 first static mixer

1 10 first stage separator

120 second static mixer

130 second stage separator

140 heat exchanger or heater

150 reactor

160 compressor

170 water pump

180 regulator valve

190 second water stream

200 gas ejector

210 first water stream

300 water inlet ejector

310 gas inlet ejector

320 gas/water outlet

330 nozzle

340 diffuser