Technical Field

The present invention relates to a system and method for deoxidizing water, in particular for deoxidizing seawater to be injected under high pressure into an oil and/or gas reservoir for enhanced oil and/or gas recovery.

Background Art

Removal of oxygen from seawater has been an important issue for decades. Examples of such systems and methods are known from inter alia the following references:

WO 2012/065243 describes a process and apparatus for reduction of dissolved oxygen in seawater by using a separator in horizontal alignment to provide high gas-liquid contacting area for separation and de-entrainment within the separator, and thereby provide a higher output. The first bed typically comprises multiple layers of high efficiency structures packing or random packing with high surface area per unit packing volume. The second bed preferably comprises a high void fraction structured packing or grid with lower surface area per unit packing volume. The seawater is heated to 30 - 60 °C to enhance removal of oxygen from the seawater and fuel gas is used as stripping gas. The stripping gas and entrained oxygen is combusted for heating the seawater. The stripping gas is used once in order to avoid operating problems arising from fouling of downstream equipment due to entrained salt.

US 5.006.133 describe a method and apparatus in which a fuel including natural gas is used as both a stripping gas to remove oxygen from seawater and a fuel to be oxidized by said stripped oxygen in a catalytic process.

US 4.612.021 describe a process for reduction of unwanted gas in a liquid by contacting with another gas. The second gas serves as a stripping gas, for example for reduction of oxygen in seawater by supplying nitrogen as said seawater is injected into a main gas ejector. In an auxiliary gas injector, positioned above the main gas injector, dissolution of entrained nitrogen displaces the oxygen dissolved in the seawater, which is then reacted in a catalytic burner. Make up nitrogen is then provided.

US 4.752.306 describes a system in which an inert gas stripping gas is mixed in turbulent co-current flow with seawater to remove dissolved oxygen. The resulting gas mixture and liquid are separated, the gas mixture is then treated to remove oxygen, and the stripping gas is then returned to treat more seawater. A distinguishing feature of this process is that nitrogen is purified after use by removal of oxygen in a separate catalytic reaction chamber before recycling through the process.

N0158283 with the same applicant as present invention relates to a

deoxygenation system comprising two gravitational separators and a catalytic reactor for reducing the oxygen content of seawater. The apparatus is compact and can be used at hazardous and non-hazardous locations through use of suitable enclosures. This system comprises a series of stages. Methanol is added to the effluent of the first stage stripping gas stream and the mixture of gases is heated over a catalyst so that the methanol reacts with oxygen to form carbon dioxide and water. The deoxygenated stripping gas is then returned to a second stage for further use. A problem that can occur with the recycle system of this type is that the catalyst and other equipment in the gas loop are prone to fouling from entrained saltwater.

US 20150283481 describes a type of inline separator where a fluid is passed through a tube element and the fluid is separated into light and heavy fluids.

The heavier fluids are discharged to a vessel. A connection tube is coupled between the upper part of the vessel and the tube element to return the lighter fluid. The purpose of the inline separator is to remove e.g. water or separate the different constituents in a hydrocarbon mixture before further transport. This publication fails to describe use of in-line separators for separating stripping gas from seawater for subsequent injection of oxygen-depleted seawater into and oil or gas well.

NO340761 describes a method and apparatus for removal of oxygen from seawater. The stripping gas is continuously circulated, and the oxygen is removed by oxidation of a fuel that is added into the stripping gas circuit. Salt amount in seawater entrained with the stripping gas circuit is diluted by injection of fresh water.

NO20130670, US4.937.004, GB 2202167, GB 212771 1 , EP 458737, EP 391839, EP 327491 , US 4.612.021 and US 4.5656.34 relates similar methods and apparatus, but fail to describe use of inline separators in seawater deoxygenation systems, and injection of fresh water into the stripping gas to dilute amount of salt in the entrained seawater, to reduce salt deposits in the gas loop.

WO 2006/086046 A2 discloses a process where catalytic combustion is used to regenerate the second reactant (nitrogen) which has been used in a process where oxygen has been removed from an oxygen containing liquid.

US 2010/230366 A1 and US 2016/096745 A1 describe use of membrane extractors for deoxygenation of seawater.

Membranes are known generally in the art for extracting or separating gasses. In a standard membrane system, a Pressure Swing Adsorber (PSA) N2 generator would be used. This is a fairly complex system comprising a large air compressor, a large air tank, two Pressure Swing Adsorber (PSA) vessels and an N2 holding tank. To produce lean N2 from air in a PSA system is needed transportation and storage of a large amount of air. This results in a large air compressor and air reservoir with corresponding large footprint and energy consumption. Experience shows that a PSA system is not very stable and has problems producing N2 with the required purity >99.99%. A PSA system has no regeneration of N2. The vacuum pump discharges the N2 to the atmosphere.

Summary of invention

The present invention has as its primary object to facilitate the use of a membrane extractor, also called deaerator, and optimize the nitrogen regeneration process. According to the process of the invention an N2 regeneration loop is provided that regenerates the discharged O2 rich N2 in a catalytic process. A reaction is created when a fuel, mixed with the N2 upstream of the deoxidizer vessel, reacts with the O2 over a catalyst bed inside the deoxidizer vessel. This process removes the O2 from the N2.

From the deoxidizer vessel N2 with a purity that can be in excess of 99.99% exits at a somewhat elevated temperature.

It has however been found that the membrane extractor is so efficient that the N2 exiting the membrane extractor has a very high O2 content. As this O2 enriched gas is fed through the deoxidizer vessel, the result is a very high temperature in the catalytic process inside the deoxidizer vessel. The high temperature is challenging with regards to materials in vessels, valves and piping/tubing.

To address this issue the present invention introduces a circulation loop in the N2 regeneration process. An excess of N2 will be circulated in the regeneration loop. A portion of the N2 will be routed to the deaerator membranes, the rest of the N2 will be routed to the vacuum discharge line from the membrane extractor. The high O2 in N2 gas is mixed with virtually pure N2 and hence the O2 content is diluted, resulting in a lower process temperature in the deoxidizer.

It is preferred to use hydrogen as fuel in the deoxidizer. In that case, the only product of the reaction, apart from lean N2, is water. In addition, the hydrogen would further reduce the temperature in the reaction, making hydrogen a preferred fuel for this process.

However, it is also possible to use other types of fuel. In the event of using carbon containing fuels, such as methanol, ethanol or methane, a small amount of CO2 will be produced. Unlike common prior art deoxygenation units, in a membrane deaerator there is no direct contact between gas and water. This means that the CO2 produced by the catalytic process will not escape the gas with the water. This means a build-up of CO2 will occur in the gas loop. To remedy this, the CO2 content has to be monitored and some of the gas needs to be discharged and replaced with top up gas whenever the CO2 exceeds a predetermined limit.

These and other advantages are obtained by a system for removal of oxygen from water, comprising: - a membrane extractor, said membrane extractor having an inlet for oxygen rich water an and inlet for nitrogen; said membrane extractor bringing water and nitrogen into contact with opposite sides of a membrane, causing oxygen to migrate from the water to the nitrogen through the membrane; said membrane extractor having an outlet for oxygen depleted water and an outlet for oxygen and nitrogen mixture;

- a deoxidizer receiving said mixture of oxygen and nitrogen and fuel; said deoxidizer removing oxygen from said nitrogen through a catalytic process; said deoxidizer having an outlet for purified nitrogen, said outlet being coupled to the nitrogen inlet of said membrane extractor; wherein a bypass line extends from said nitrogen outlet of said deoxidizer to downstream of said membrane extractor, where the flow of nitrogen in the bypass line is not constant but regulated so that the temperature in the bypass line is at an appropriate level to prevent damage to the equipment.

Other preferred embodiments of the system according to present invention are described in the dependent claims.

Present invention also relates to a method of removal of oxygen from water, comprising the following steps:

- bringing water and nitrogen into contact with opposite sides of a membrane in a membrane extractor, causing oxygen to migrate from the water to the nitrogen through the membrane,

- removing oxygen from the mixed flow of nitrogen and oxygen exiting from the membrane extractor, wherein the flow of deoxidized nitrogen is split into two flows, a first flow that is fed into the membrane extractor and a second flow that is bypassed the membrane extractor and mixed with said mixed flow of oxygen and nitrogen downstream of the membrane extractor.

Brief description of drawings

The invention will now be explained in further detail, referring to the exemplary embodiment of the enclosed single figure:

Figure 1 shows schematically a system according to an embodiment of the invention.

Detailed description of the invention

An embodiment of the invention will now be described referring to figure 1.

Seawater enters the system at 1. The seawater flows through a heat exchanger 2 where it acts as a coolant for N2 gas exiting from a deoxidizer 12. Between the heat exchanger 2 and the membrane extractor 3 is a dewatering unit 19 that removes water, produced in the deoxidizer and condensed in the heat exchanger, from the N2 gas. The extracted water may be handled in a separate process (not shown) and drained to sea either directly or via a purifying process where contaminates are removed, or alternatively be injected into the

deoxidised water downstream the deaeration membrane and hence be injected into the oil and/or gas reservoir.

The seawater and the N2 enters opposite sides of a membrane 3a of a membrane extractor 3. The membrane extractor 3 will actually have several membranes and the membranes are not flat objects but may for example comprise a plurality of narrow tubes to take up less space and increase the exposed area against the media (water and N2). The illustrated extractor is merely showing a simplified principle and the actual design of the membrane extractor 3 will be a choice based on technology known to the person of skill.

In the membrane extractor 3 the oxygen dissolved in the water will escape through the membrane 3a due to osmotic pressure and mix with the N2. This is a well-known process per se and hence does not need any further explanation.

The seawater that has been depleted from oxygen flows out of the membrane extractor 3 at 4 for further use, such as injection into an oil or gas reservoir.

The O2 enriched N2 is sucked out from the membrane extractor 3 through a vacuum line 5 by a vacuum pump 6. From the pump 6 the N2/O2 mixture flows through a vacuum discharge line 7 and a line 8 to a compressor 9, where the N2/O2 mixture is compressed to an elevated pressure. The pressurized N2/O2 mixture is preheated by a heater 10 and fed to the deoxidizer vessel 12. Fuel, such as hydrogen, methanol, ethanol or methane is added to the N2/02 mixture through a line 1 1 before the mixture of N2, O2 and fuel enters the deoxidizer vessel 12.

In the deoxidizer vessel 12 is a catalyst bed 13 of a type that is well known per se to the skilled person in the art. In this bed 13 the O2 reacts with the fuel to produce mainly water vapour and, depending on the fuel, small amounts of CO2. In the event of CO2 build up in the gas loop, the CO2 enriched gas is expelled from the process as exhaust through a line 15. The CO2 enriched gas may be removed in a separate process (not shown), or alternatively be injected into the deoxidised water downstream the deaeration membrane and hence be injected into the oil and/or gas reservoir.

The now virtually pure N2 exits the deoxidizer at 16 and flows through the heat exchanger 2 back again to the membrane extractor. The hot nitrogen exiting the deoxidiser can alternatively be routed through an additional loop to the pre heater 10 for preheating the N2/O2 upstream the deoxidizer. The pre-heater 10 would in this event be a heat exchanger.

If the process had been run as described above, the temperature in the deoxidizer vessel 12 would have increased to an inconveniently high level due to the high proportion of O2 in the N2/O2 mixture. The high proportion of O2 is due to the high efficiency of the membrane extractor, as in a membrane deaerator a large amount of O2 is extracted from the water using a relatively low amount of N2. As explained above, this results in a very high O2 content of the N2 exiting the membrane deaerator. This high O2 in N2 content results in a very high temperature in catalytic process inside the deoxidizer. The high temperature is challenging with regards to materials in vessels, valves and piping/tubing.

To avoid such high temperatures, a bypass loop 17 of N2 has been included, which bypasses some of the virtually pure N2 exiting from the deoxidizer vessel 12 to the line 7 downstream of the vacuum pump 6. An adjustable valve 18 is provided to adjust the ratio between N2 fed to the membrane extractor 3 and bypassed through the bypass line 17.

When the N2/O2 mixture is mixed with pure N2 the oxygen concentration in the mixture is substantially reduced. This results in a reduced working temperature of the deoxidizer vessel. Hence the lifespan of the catalyst bed, vessel, valves and other components can be extended. It also results in an overall cooler process in the system.

The system may conveniently comprise one or more temperature sensors that detect the temperature in the deoxidizer and/or the temperature of the nitrogen at the outlet of the deoxidizer and adjusts the split of flows between the membrane extractor and the bypass. If the temperature is above certain level, the flow of nitrogen in the bypass is increased. If the temperature drops below another level, the flow is decreased.

As there inevitably will be some build-up of CO2 in the N2 over time, it may be necessary to exchange the nitrogen for fresh nitrogen or air. Supply of nitrogen or air may conveniently be done upstream of the compressor 9, as shown by the line 14. If air is used to replenish nitrogen to the process, a gas analyser will be arranged downstream of the pre-heater 10 to measure the O2 concentration so that the right amount of fuel can be injected to obtain a complete combustion of O2.