This invention relates to a method and system for purification of gas, in particular hydrocarbonaceous gas such as natural gas comprising H ₂ S, mercaptans, C0 ₂ and other acidic contaminants. Background

Purification of pressurized hydrocarbonaceous gas, including natural gas, biogas etc, hereinafter named as gas, may be desirable or necessary for a number of reasons. Removal of acidic contaminants such as C0 ₂ and or sulphide contaminants from the gas improves the quality of the gas product. The acidic contaminants include C0 ₂ , H ₂ S, mercaptans, CS ₂ , COS, and S0 ₂ .

Some of the challenges to be achieved with respect to purification are to minimize plant size, energy consumption, operational and investment cost, carry over of absorbent and pressure drop.

Prior art

US 6228145 describes a method for purifying gas using membranes and absorbent, whereby the absorbent passing on one side of the membrane is absorbing components to be purified from the gas on the other side of the membrane. This publication points at some benefits utilizing membranes, such as that the division between the gas and solvent phases makes it possible to employ a high gas rate in the absorber without the solvent being carried over, and that a membrane contactor has a high packing factor (m ² /m ³ ).

W099/13963 discloses a method for removing C0 ₂ from natural gas including turbulent mixing of gas and liquid absorbent.

US2002/014438, US4,997,630 and US4,999,031 discloses different types of multiple step absorptions processes.

WO2010/102877A1 discloses a method and apparatus for washing absorbent.

From US 2002/195251 it is known to cool gas to remove and separate

contaminants/condensates upstream a membrane, and to heat the gas up again by using a cross heat exchanger before entering a membrane separator. However recognising that the solubility of carbon dioxide is a strong inverse function of the temperature of the absorbent, especially a physical absorbent such as seawater, cooling of the gas upstream and removing the contaminants including condensate upstream the first absorption step is preferable.

NO201 10472 illustrates a system and method for utilizing two step absorption including co-current bulk removal and in a second step a membrane contactor. Disclosed are different systems for desorption and recycling of the absorbed in closed loops.

The industry has looked into various ways to utilize membrane technology combined with liquid absorbent to achieve a more compact and efficient purification plant, but still these existing solutions can be extensive as membranes and regenerating absorbent to lean absorbent are expensive. Hence, a further way to minimize membrane and lean absorbent extent would be strongly desirable.

Objective of the invention

An objective of the present invention is to provide a method for purification of gas with reduced overall plant size, energy consumption, operational cost and investment cost.

Another goal is to provide a method with a low pressure drop especially when the gas to be purified is pressurized.

Yet another objective is to provide additional pressure control.

A further goal is to provide a system applicable for subsea installation.

Also a system limiting the use of environmental demanding absorbents is intended. A further objective is to provide a system able to performing the method. Description of the invention

The present invention provides a method for purification of a pressurized gas stream comprising acidic gases, wherein the method comprises

- in a first absorption step bringing the pressurized gas stream in direct contact with a first absorption solution absorbing at least part of the acidic gases, obtaining a gas liquid mixture,

- separating the gas liquid mixture in a partly purified pressurized gas stream and a first rich absorption solution,

- in a second downstream absorption step bringing the partly purified pressurized gas in contact with a second absorption solution through a membrane contactor, obtaining a second rich absorption solution and a purified pressurized gas stream, - wherein the first and second absorption solutions comprise a physical absorbent,

- and condensate is separated and extracted upstream the second absorption step. The term "pressurized" as employed here to describe the gas stream being treated by the method of the present invention refers to a gas stream with a pressure above atmospheric pressure, such as a pressure between 2-130 bars, or 10-120 bars or 20- 1 10 bars. The flow directions of the gas to be treated and the absorbent in the two absorption steps can be either co-currently or counter currently. In one embodiment of the flows are co-current in the first step and counter current in the second step. In another embodiment the flow directions are counter- current in both the first and the second absorption step. The separation and extraction of condensate upstream the second absorption step, may take place upstream the first absorption step and or between the first and the second absorption step. The separated condensate can optionally be transported together with the purified gas to a top side installation or optionally be transferred together with separated acid gases and or produced water and or rich absorbent solution and or sea water to an injection well.

In one aspect of the method the first absorption solution and the second absorption solution are lean absorption solutions.

In another aspect of the present invention the physical absorbent comprises seawater. In a further aspect the physical absorbent consists mainly (more than 90 v/v %) of seawater.

The first and second absorption solutions may further comprise a hydrate inhibitor.

In yet another aspect the method comprises separating first rich absorption solution from the mixture at one or more rich absorbent draining points.

In another aspect of the method according to the present invention the method further comprises injecting the rich absorption solution into a subsea injection well.

In yet another aspect of the method the mixture is separated to the magnitude of the vapour pressure of the absorbent.

Method may according to another aspect further comprise obtaining the partially lean absorption solution by depressurization of the first rich absorption solution, or the second rich absorption stream or a mixture of the first and the second rich absorption solution. Further in another aspect the method comprises separating produced water together with the condensate, and separating the produced water from the condensate in a subsequent stage.

In one aspect the method further comprises cooling the gas stream before the first absorption step. This cooling may in one aspect comprise heat exchanging the pressurized gas stream with the partly purified pressurized gas stream or with the first rich absorption solution and or with a third lean absorption solution to cool the pressurized gas so that liquid condensate is formed. In a further aspect the seawater comprised in the physical absorbent is drawn from different locations and or heights, and optionally combined, to generate a desired mixture. Thereby the temperature and the ability of the seawater to absorb acid gases may be adjusted to the desired level. In one aspect of the present invention the method comprises utilizing the pressure of the seawater available subsea. The seawater pressure at a subsea seabed location can be higher than the reservoir pressure and this pressure of the seawater can be utilized to drive the purification process or parts thereof.

Method also comprises in one aspect thereof controlling the pressure over the membrane to maintain a stable overpressure on the gas feed side. In one aspect the pressure is controlled by regulating a flow between the first rich absorption solution and the second rich absorption solution upstream the

depressurization step obtaining the partially lean absorption.

In one aspect of the method according to the present invention the second absorption solution is a lean absorption solution. In another aspect of the method according to the present invention the second absorption solution is a partially lean absorption solution and wherein the first and the second absorption solutions contain a physical absorbent.

Accordingly the present invention provides a method comprising two absorption steps where the first step is based on direct contact between the gas and the absorbent, where the second step is based on the use of a membrane contactor. The use of a direct, co-current contactor is known to provide a compact absorber but also to result in carry over of absorbent to the gas phase. If a second absorption step is needed downstream a direct contactor, then liquid content in the gas phase due to carry over traditionally has been considered to be limiting for the choice of absorption technique for the second step.

Further, membrane contactors are pressure sensitive and normally require pressure control or equilibration of pressure over the membrane. This has generally been considered as a limitation as to which processes that can be performed upstream a membrane contactor. Now, however the present inventors have found a solution to limit the carry over and control the pressure thereby making it possible to employ a compact membrane contactor in the second step. In one aspect of the present invention the mixture is separated to the magnitude of the vapour pressure of the absorbent.

The method according to the invention may further comprise obtaining the partially lean absorption solution by depressurization of the first rich absorption solution, or the second rich absorption stream or a mixture of the first and the second rich absorption solution.

In one embodiment the method further comprises cooling the gas liquid mixture before it is separated.

The method according to the invention may in the first absorption step comprise feeding the first absorption solution co-currently with the gas stream. Alternatively the method comprises in the first absorption step feeding the first absorption solution counter- currently with the gas stream.

In another aspect of the present invention the method further comprises in the first absorption step spraying the first absorption solution into the gas stream. To limit the carry over of absorbent to the membrane contactor the method may further comprises washing the partly purified gas stream and or passing the partly purified gas stream through a demister.

In yet another aspect the method according to the invention may further comprise equalizing the pressure over the membrane. The term "equalization of pressure" and similar as used here generally refers to the need to prepare a stable and controlled pressure surroundings for the membrane. In a preferred aspect of the method according to the present invention the method comprises controlling the pressure over the membrane to maintain a stable overpressure on the gas feed side.

In a different aspect of the method according to the present invention the pressure is controlled by controlled combination of the first rich absorption solution with the second rich absorption solution downstream the second downstream absorption step. Measurements of the pressure of the partly purified gas and the pressure of the second rich absorption solution can in this aspect be employed to control of the first rich absorbent stream into the second rich absorbent stream upstream or

downstream of the valve controlling the flow of absorbent solution into the depressurization step.

In one embodiment the method further comprises mixing lean absorbent into the partially lean absorbent upstream the first absorption step to increase the absorption capability and the partially lean absorbent stream. In another embodiment the method according to the invention further comprises cooling the partially lean absorbent upstream the first absorption step. The method may further upstream the condensate separation and extraction comprise adjusting the pressure of the pressurized the gas stream to be within an acceptable pressure window to avoid hydrate formation. For high pressure gasses the use of seawater as absorbent could otherwise result in formation of hydrates. Acceptable pressure windows will depend on composition of the stream, content of produced water and temperature. Examples are 2-10 bar, 2-20 bar, 2-25 bar, 2-50 bar. To further control and avoid the formation of hydrates the method may further upstream the pressure adjustment comprise adding a kinetic hydrate inhibitor and or a hydrate inhibitor to the pressurized gas stream. Further the present invention provides a system for purification of a pressurized gas stream comprising acidic gases, wherein the system comprises

- a first direct absorber unit comprising a gas inlet, an absorbent solution inlet and a gas liquid mixture outlet,

- a separation unit comprising a gas outlet, a rich absorbent solution outlet and an inlet connected to the gas liquid mixture outlet,

- a second membrane absorber unit comprising on a first side of the

membrane a gas inlet in fluid communication with the gas outlet of the separation unit, a purified gas outlet, and comprising on a second side of the membrane a lean absorbent solution inlet and a rich absorbent solution outlet, and

- a physical absorber inlet.

In one aspect of the present invention the system may further comprise a conduit for establishing fluid communication between the rich absorbent solution outlet of the second absorber unit and an injection well. This conduit provides the possibility to inject rich absorbent and or acidic gas and or produced water into a reservoir via the injection well.

In a further aspect the system also comprises a conduit establishing fluid

communication between the rich absorbent solution outlet of the first absorber unit and an injection well. In another aspect of the system the first direct absorber is arranged with a declined angle, preferably 3-10 degree, to drain absorbent towards the separation unit, and or draining points (one, two or several) in fluid communication with the separation unit.

In yet a further aspect the system comprises a condensate separator upstream the first direct absorber unit, such that condensate is removed from the pressurized gas stream before it enters the first absorber. A cooler or heat exchanger may be arranged upstream the condensate separator to cool the gas to liquefy the

condensate.

In a further aspect the system according to the present invention further comprises a pressure adjustment valve upstream the first absorption unit for adjusting the pressure of the pressurized gas to be within an acceptable pressure window to avoid formation of hydrates. Preferably the valve is arranged upstream the cooler or heat exchanger. In another aspect the system comprises a kinetic hydrate inhibitor and or hydrate inhibitor inlet upstream the pressure adjustment valve.

In one embodiment of the system according to the present invention the inlet to the separation unit is tangentially and the separator unit is configured as a cyclone.

Further in one aspect of the system according to the invention, the absorbent solution inlet to the first absorber unit is equipped with at least one nozzle for spraying the absorbent solution into the gas. Where more than one nozzle is used the nozzles are separated over a distance forming a length of nozzles. The term "direct absorber" as used here refers to a co-current absorber where the gas containing the compound to be absorbed is brought in direct contact with the fluid comprising the absorbent.

In another aspect of the system according to the present invention the first direct absorber unit is combined with the separation unit forming - a combined first direct absorber separation unit comprising in a lower part the gas inlet, the absorbent solution inlet and the rich absorbent outlet, and in a upper part the gas outlet.

In this aspect a gas liquid mixture is formed in the lower part of the separation unit and the gas liquid mixture inlet and the gas liquid mixture outlet are the inlet and outlet to the lower part of the combined first direct absorber separation unit.

The present invention provides a system for purification of a pressurized gas stream comprising acidic gases, wherein the system comprises

- a combined first direct absorber separation unit comprising in a lower part a gas inlet, an absorbent solution inlet and a rich absorbent outlet, and in a upper part a gas outlet,

- a second membrane absorber unit comprising on a first side of the membrane a gas inlet in fluid communication with the gas outlet of the combined absorber and separation unit, a purified gas outlet, and comprising on a second side of the membrane a lean absorbent solution inlet and a rich absorbent solution outlet. The systems for purification according to the present invention may in one aspect comprise a system, wherein the inlet of the flash desorber is also in fluid

communication with the rich absorbent solution outlet of the second membrane absorber. In another aspect of this system the rich absorbent solution from the separation unit is via a first and second valve in fluid communication with the rich absorption solution outlet from the membrane both upstream and downstream a third valve. The output from pressure measuring units arranged on the gas inlet and absorbent outlet side of the membrane unit are arranged to control the opening and closing of the three valves resulting in control of the pressure over the membrane. In one embodiment the system for purification according to the present invention, the separation unit or the upper part of the combined absorber and separation unit further comprises a chimney tray and a system for introduction and recycling of a wash liquid.

In another embodiment the system, the separation unit or the upper part of the combined absorber and separation unit further comprises a demister.

In yet another aspect the system for purification further comprises one or more coolers arranged upstream the inlet to the separation unit or the inlets to the combined absorber and separation unit and or the absorbent solution inlet to the first direct absorber unit, and or the gas inlet to the first direct absorber unit. The term "membrane" as employed herein shall be interpreted to refer to any material type, shape and form suitable for diffusion of acidic compounds there through in a semi permeable way. One particular suitable membrane type is hollow fibre type, which inner diameters are typical 0.01-0.1 micron, wherein gas is flowing. These small inner diameters are potential sensitive to liquids filling up volume and blocking gas flow.

The term "acidic gases" as used here refers to acidic contaminants such as C0 ₂ and or sulphide contaminants including H ₂ S, mercaptans, CS ₂ , COS, and S0 ₂ . In a preferred aspect of the invention the acidic gas is C0 ₂ .

The absorbent liquid employed in one embodiment of the present invention is filtrated seawater. The seawater can have the natural present content of dissolved salts. Seawater is a physical absorbent where the absorption of C0 ₂ is based on the solubility of C0 ₂ in the physical absorbent without any chemical reaction taking place. Both fresh water and salt water are known to be able to absorb C0 ₂ .

Other physical absorbents may also be used; especially physical absorbents that reduces the risk for hydrate formation such as Mono Ethylene Glycol (MEG) or other glycol based inhibitors and or Kinetic Inhibitors (KHI). The absorbents can be employed alone or in combinations. The term "lean absorption solution" as utilized herein refers to a fresh absorption solution or to absorption solution that has been fully regenerated thereby generally regained its full absorption capacity, or to absorption solution comprising a mixture of fully regenerated absorption solution and partially regenerated absorption solution.

The term "partially lean absorption solution" as utilized herein refers to a rich absorption solution that has been only partly regenerated. The partially lean absorption solution has regained a considerable part of its capability to absorb acidic gases but also still contains significant amounts of acidic gases. The partially regeneration is generally obtained through depressurization and sometimes without heating.

One aspect of the present invention is the reduction of the mass rate of lean absorbent required in a membrane absorbing process, achieved by bulk removal of contaminants upstream of the membrane absorbing process. Since the mass rate of lean absorbent is reduced, the equipment and energy needed to regenerate the absorbent are reduced.

The inventors of the present invention have realised the possibility of utilizing the seawater as an easily available absorbent, and that this absorbent could be injected through an injection well with out removing the absorbed acidic gasses. When injected the absorbent would increase the reservoir pressure in the same way as known water injections.

The absorption capacity of seawater is lower than specially developed absorbents but the availability of seawater makes up for the lower absorption capacity.

The use of membrane contactors as such is well known. Yeon et al. disclose in "Application of pilot-scale membrane contactor hybrid system for removal of carbon dioxide from flue gas", Journal of Membrane Science Volume 257, Issues 1- 2, 15 July 2005, pages 156-160, a study comparing membrane contactors to traditional absorption towers.

One benefit of using membrane absorption is that there are no carry over of absorbent into the gas. In conventional absorption towers the minimum carry over of absorbent is limited to evaporation pressure of absorbent, minus optional effect of extra washing methods, as illustrated in WO2010/102877A1.

For example if at 90 bar one type of absorbent, can approximately absorb 0.6 mole C0 ₂ per unit absorbent. When the absorbent pressure is reduced, e.g. in a flash separator, this absorption capacity is typically reduced to approx. 0.3 mole C0 ₂ per unit absorbent, and a partly lean absorbent is obtained. So the C0 ₂ content of the feed gas can by reduced significantly when brought in contact with the partly lean absorbent, and the absorbent can be regenerate without the need for regeneration by heat generation. The C0 ₂ level in the feed gas can typically be reduced from 10 % to 5 % by such bulk C0 ₂ removal. A challenge is however to avoid absorbent coming into the feed side of the membrane absorbent contactor utilized for further lowering the C0 ₂ content. Absorbent on the feed side may disturb (by clogging etc.) the absorption process. In one embodiment the membrane consists of hollow membrane tubes. On the gas side these have typically very small size diameter 0.1 - 1.0 micron. Furthermore if the absorbent is not separated efficiently before it is feed into the membrane absorber it will end up as a carry over in the gas product. The invention solves this challenge by an efficient absorbent separation between the first step bulk absorption and the second step membrane absorption. Preferably the separation is to the magnitude of the vapour pressure of the absorbent. This can be achieved by using a separation volume large enough to accomplish this. Any known separation vessel suited for efficient separation of liquid and gas could be considered for this purpose, including vessel geometry, entry angles, and a mesh system to avoid mist and droplets to be carried over. In addition cyclones and/or a washing system can be provided for an even more efficient separation, if required.

The vapour pressure is by natural law a function of the temperature of the gas. The gas temperature will increase by the exothermic absorbent reaction, hence the pressure of the absorbent vapour that potentially could enter the membrane absorber on the gas feed side are high. The present invention solves this potential problem by an intermediate gas/ absorbent separation, combined with gas cooling to reduce vapour pressure of absorbent. The gas cooling can be indirect with extra cold absorbent or/and with cooling of the gas in between the first step bulk absorption and the second step membrane absorption. In addition further steps for separation can also be used as for instance water wash and/or mist filter(s).

In one embodiment of the present invention rich absorbent solution is provided from the bottom of the membrane contactor to a flash unit, where the partially lean absorbent is obtained, and then it is pressurized and mixed with the feed gas. The combined stream of feed gas and absorbent solution formed in the first step passes through a cooler (co -currently) this does not only cool the solvent, but also removes the heat of reaction and increases the equilibrium acid gas loading due to lower temperature and higher residence time available.

Due to the high acid gas concentration in the feed gas, high solvent loading can be achieved at the lower temperature that is maintained in the cooler.

In another embodiment feed gas and or the partially lean absorbent are cooled by separate coolers prior to the first absorption step. The cooling can be obtained through heat exchange with the rich absorbent from the first absorption step, or through heat exchange with the partially purified gas.

The removal of bulk acidic gas in the first step prior to the second step membrane contactor will effectively reduce the temperature increase due to the exothermic reaction in the membrane contactor which will help protect the membrane.

The overall solvent circulation is reduced due to the increase in acid gas loading obtained by the lower temperatures achieved in the cooler. This reduces the overall size of the plant and capital cost of the overall system. Substantial energy savings will be realized with the reduction in solvent circulation, as regeneration of absorbent solution is energy demanding. Both size/capital cost and the energy savings are directly proportional to the solvent circulation rate.

Depending on the temperature, the pressure and the composition of the gas stream the cooling may result in the formation of condensate or hydrate formation. The present invention further provides additional steps and equipment adapted to handle condensate and avoid hydrate formation.

The needed degree of separation for the gas/liquid separation prior to entering the membrane contactor must be based on the membrane tolerance of the liquid carry over.

Brief description of the figures

Figure 1 illustrate a subsea absorption process with partly recycling of the absorbent.

Figure 2 illustrates schematically a second embodiment of a subsea system without recycling of absorbent.

Figure 3 illustrates schematically a third embodiment of the present invention. Figure 4 illustrates schematically a fourth embodiment of the present invention.

Figure 5 illustrates schematically a fifth embodiment of the present invention.

Figure 6 illustrates a further embodiment of the present invention arranged top-side.

Figure 7 illustrates schematically an embodiment with increased recycling of absorbent. Figure 8 illustrates schematically an alternative embodiment.

Figure 9 illustrates an embodiment with reuse of cooling energy. Figure 10 illustrates an embodiment with full recycling of absorbent. Figure 1 1 illustrates another embodiment also employing a hydrate inhibitor. Figure 12 illustrates a further embodiment utilizing a hydrate inhibitor. Detailed description of the invention

The present invention will now be described in further detail with reference to the enclosed figures and the illustrated embodiments.

Figure 1 shows the layout of a process according to a first embodiment of the present invention. The system is arranged subsea on the seafloor 92. A gas stream 1 comprising acidic gases is feed into a direct, co current absorber 25 where the gas is brought in contact with an at least partly lean absorption solution 26. The obtained mixture 31 is fed into a separator 33. The rich absorbent solution leaves through the bottom as a first rich absorbent stream 28 whereas the partially purified gas leaves over the top as stream 30. In the upper section of the separator a demister 36 is installed to remove droplets from gas stream to avoid liquid being present in the stream 30 when it enters the gas side of a membrane contactor 3. The demister 36could typically be a mesh pad type, an axial flow demister type or any other suitable demisting type device. Through the membrane the gas is brought in contact with an at least partially lean absorption solution 1 1 which is past counter currently through the membrane contactor 3. The thereby obtained purified gas 2 can be transported to a top-side 90 or to an onshore installation (not shown), or a connecting subsea station (not shown) for further processing or transport. The obtained second rich absorption solution 60 is together with the first rich absorption stream 28 transported to a flash desorber 41. By employing a pressure reduction through valve 80 at least a part of the absorbed acidic gas is released from stream 62 entering the flash desorber 41. The released acidic gas leaves through line 63 and the obtained at least partly lean absorbent solution is via pump 70 recycled as stream 26 to the first absorption step, in the direct absorber 25.

The absorbent solution enters the system as stream 85 and is fed as lean absorbent stream 1 1 into the membrane contactor 3. If necessary, to achieve the desired rate of purification, lean absorbent stream 89 can be mixed into the partially lean absorbent stream 26 to increase the absorption capability. Valves 94 and 93 control the inlet of lean absorbent.

The released gas 63 is sent into a subsea formation through an injection well, or transported to an any other adequate carbon storage facility or process. To keep maintain stable liquid level in the process, a portion of rich absorbent liquid from the separator 33 is bleed of, preferably back into the same stream 63 as released acidic gas via line 66 through valve 67. Optionally the bleed 66 can be used in an ejector (not shown) to pressurize acidic gas being sent back to a subsea formation. If the this even is not sufficient pressure a pump (not shown) can be employed on the stream 87, to further pressurize the liquid, hence creating even higher motion fluid pressure for the said ejector (not shown)

An important aspect of this bleed function is also allowing the system to bleed of any produced water from the gas reservoir, which normally would include salts and particles, which could cause significant problems to the system if these would be allowed to accumulate. Furthermore any appropriate de-sanding system, such as de- sanding cyclones or flushing system etc., could be employed in the system whereas the sand would be deployed via preferably the bleed 66. Accumulated condensate would also in this configuration be bleed of and returned to a subsea formation. As the absorbent 26 is feed into the direct absorber 25, the gas is cooled; hence here some gas may condensate. In this embodiment of the invention, this condensate is extracted out from separator 33 in the bleed 66 at a height level higher than extraction level of the first rich absorbent 28. If sand and particles accumulates in the separator 33 these can be extracted from the bottom region of the separator 33 via line 131.In one preferred embodiment the condensate and or sand and particles are send to a reservoir together with extracted C0 ₂ in line 96 into reservoir 64. The extraction of condensate and or particles and sand from the separator can be done in batches or as a continuous operation.

It is important to remove the condensate to avoid carry over to the demister and or membrane, as this will upset the performance of the demister and or membrane. If condensate would have been allowed to accumulate in the top of the separator 33, this would create foaming carrying over to the demister, which carry over would result in salty produced water coming into the membrane contactor 3, potentially clogging up the membrane(s). The level at extraction of rich absorbent 28 is done at a controlled high level, preferably at a level between 0.2 and 2 meters below the extraction of the condensate, and or 0.2 and 2 meters above the extraction of the particles and or sand of the separator 33.

The direct absorber is in a preferred embodiment is in declined angle (not shown), preferably 3-10 degree, to drain absorbent towards the separator. Optionally the direct absorber has absorbent draining points 132, 133 (one, two or several) in conduit to the separator 33 or in conduit (not shown) to any downstream absorbent process line 28 from this separator 33.

If the reservoir pressure is higher than the available seawater pressure absorbent entering the system 85 can optionally be pressurized with a subsea pump (not shown), and in the same way gas and liquid to be returned into a subsea formation could also be pressurized with a pump (not shown). Figure 1 illustrates a solution for controlling the pressure over the membrane. It is important that the control of the pressure equalisation is rapid and precise, and that the cross sectional diameter of the pressure equalisation line and proximity are designed for sufficient response capability. The pressure equalisation should be regulated with adequate response time and form part of the membrane protection systems.

Membranes should preferably have an overpressure on the gas side, i.e. the tube side of the absorber membrane. This is to avoid a collapse of the tubes. This can be controlled by using a liquid equalisation line 28 between the separator 33 and the absorbent solution on the membrane side. The line 28 also provides the function of transporting the first rich absorption solution to a flash unit 41. The pressure difference can be measured by the pressure sensors 76 and 77. The system comprises three valves 78, 79, and 80 which are employed to control the pressure of the rich absorption solution at the outlet of the membrane unit. At stable pressure conditions valve 79 is open and valve 78 is closed. Such a system will always have enough liquid to adjust pressure as liquid supply is provided by the first absorption step, provided that the separator is not drain out by the flow trough valve 79 into the flash separator 41. The prevention of the separator being drained off can be controlled by adjusting the valve 79 against a sufficient liquid level in the separator 33. If one would not have this sufficient supply of liquid 85 and control of liquid level in separator 33, a gas could instead enter the liquid equalization line. The gas would be compressible, and not as efficient to adjust pressure efficiently enough. The configuration in figure 1 enables a combined efficient membrane protection and pressure control, and also provide equipment savings as the system does not require any other buffer tanks.

As liquid is relatively incompressible compared to gas this will create a rapid pressure adjustment to pressure variation of the gas feed 1.

In the figures, where applicable, equal reference numbers is used for elements equal to elements present on figure 1. Figure 2 illustrates a second embodiment of the present invention. Here the reuse of absorption solution through a flash unit is avoided. Instead lean seawater is employed in both steps of the absorption process. The seawater stream is past through a filter unit 82 and an orifice or valve system 86 forming the lean absorbent solution stream 85. This stream is split via valves 94 and 95 to provide the absorbent solution streams 1 1 and 26. The combined rich absorbent solution 62 is sucked into an ejector unit 88 driven by a lean seawater stream 87. The obtained combined stream 65 is pressurized through pump 43 before it is injected as stream 64. In case the gas stream 10 has a pressure higher than then available seawater pressure, then the seawater will be pressurized in stead of being passed through the orifice 86 (pump not shown for such case). The ejector 88 will then also not be needed, i.e. the stream 62 is passed directly to stream 65. Also illustrated on figure 2 is an initial treatment system. The well stream 10 is past through a pressure control unit 18, cooled by heat exchanger 12 to condensate any condensate present in the well stream 10. Separation takes place in unit 14 where the condensate 15 is removed, either through other systems or mixed into the purified gas stream 2. The gas stream leaving unit 14 is heated in the heat exchanger 12 before being fed as stream 1 to the direct absorber 25. Accordingly the unit 12 is a cross heat exchanger.

Figure 3 illustrates a third embodiment of the present invention where the heat exchanger 12 used for cooling the well stream is installed to heat exchange with the partially purified gas stream 30' instead. The dotted lines and heat exchanger 17 are used for start up and possible other situations where additional cooling is required to facilitate condensation of condensate. Also illustrated on figure 3 is the possibility to omit unit 86 illustrated in figure 2 as pressure control can be obtained through valves 94 and 95. Also the ejector is exchange with an optional mixer 97 where additional seawater 87 optionally can be added to the injection stream.

Figure 4 illustrates a fourth embodiment of the invention, which is an alternative to the embodiment illustrated on figure 2. Here the heat exchanger 12 for cooling the well stream is connected to the rich absorption stream from the separator 33 and heat is transferred to stream 28. Optionally an additional cooler 27 may be employed to cool the lean absorbent solution 26 prior to entering the direct absorber. The seawater has increased ability for absorbing acidic compounds at lower temperatures.

Figure 5 illustrates a fifth embodiment which is an alternative to the embodiment of figure 3. Here the mixer 97 is avoided and the composition of stream 62 is not changed. The pressure measuring unit 76 has been moved upstream the heat exchanger 12 close to the gas outlet from the separator 33.

Figure 6 illustrates a sixth embodiment of the present invention which is arranged onshore or on a topside facility. This requires that the absorption solution seawater 81 is pumped up to the facility via pump 84. Otherwise the system has not changed. However for such a topside installation different maintenance solutions are available.

Subsea installation of filters and other equipment generally requires that

maintenance systems are considered. For filters the possibility for back flushing of the filter would provide a subsea applicable maintenance solution. Alternatively or additionally the separator 33 may further comprise a demister 36 included to limit the carry over of absorbent into the second absorption loop. The partly purified gas is introduced to the first side of a gas liquid membrane contactor unit 3 wherein the purification is completed to a satisfactory level and the purified gas is obtain through conduit 2. The liquid absorbent introduced counter currently to the contactor 3 from conduit 1 1 consists of lean absorbent regenerated in a similar manner as described above in relation to figure 1. The utilization of lean absorbent secures that a satisfactory level of purification is obtained. Rich absorbent comprising the absorbed acidic gases leaves the contactor and is feed to a flash separator 41. Within the second flash separator 41 a part of the absorbed acidic gases are desorbed and removed through conduit 63. In the embodiment illustrated on figure 1 the absorbent recycle loop for the first step is kept fully separated from the absorbent solution of the second absorption step.

The absorbent in the invention is a physical absorbent. As physical absorbents are based on the solubility of the C0 ₂ in solvent, and not a chemical reaction, less or no heat is generated from this C0 ₂ absorption and less cooling would be required upstream absorption. Subsequently less or no heating would be required for desorption, hence saving in equipment and energy. In fact the physical absorbent can function as an open cooling system for the gas upstream the membrane contactor. Regeneration using physical absorbent is often accomplished sufficiently by pressure release only, i.e. no heating.

The invention describes a first and second absorption process, whereby the second absorption process utilizes membranes. The invention also includes that the first absorption process can be configured be a number of first type absorption processes run in parallel and or in series , and that the second absorption process can be configured as a number of the second type absorption processes run in parallel and series.

Figure 7 illustrate a modified version of the system in figure 1 , where a portion 189 of the partly lean absorption solution 26 is recycled back to stream 1 1 1 , hence a much smaller amount of fresh absorbent 85 entering the system would be required. The absorbent stream is this embodiment is a mixture of fresh absorbent and partially lean absorbent. Further illustrated on figure 7 is the splitting off of the bleed stream 166 from the rich absorbent stream 28. Valve 167 provides control of the bleeding.

Figure 8 illustrates a system that comprises a first absorption step 98, either by feeding the absorbent 26 co-currently or counter currently. The mixture 99 of rich absorbent and the gas is separated in a next step using a cyclone 100, preferably an inline separator such a disclosed in US 6752860. The gas exist via line lOland a goes through a demisting step 102, and then the gas enters the membrane 3 in a third step. Any liquid removed in the demister 102 is transferred to the absorbent stream 103. The separated rich absorption liquid 103 is optionally run through a cyclone 104 to remove any remaining or new condensates. Separated condensate is removed through line 106, preferably connected to the condensate removing line 15 from separation unit 14 to a common line 108. This condensate can be transported to an onshore or topside offshore facility, and if it is a long distance this condensate 108 can be transported together with the gas 2 as illustrated.

The gas is cooled via the condenser 12, which can by a static cooler based on the surrounding seawater, or based on forced cooling medium as a normal cooler. The supply of the forced cooling medium is not shown. Anyhow, the cooling for condensation must not be more than the gas from the reservoir is securely outside the hydrate formation window. Optionally hydrate inhibitors (such as kinetic inhibitor (KHI) or a glycol based inhibitor such as mono-ethylene glycol (MEG), di- ethylene glycol (DEG) or tri-ethylene glycol (TEG) etc) can be utilized upstream the cooler 12. These will then follow the condensate return line 108 and or be transferred to the reservoir 64 via line 62. Optionally any produced water separated out in the separator unit 14 is in this embodiment transferred via conduit 107 to pump 43 for reinjection in the reservoir 64.

Figure 9 illustrate a system using same absorption steps as in figure 4, but where most condensate is removed upstream the absorption step via line 15, and any remaining or new condensate from produced water 109 or rich absorbent 28, is removed via a cyclonic separator 1 10, and conducted to the gas stream 2 via line 108. A key feature of this embodiment is the sophisticated supply of forced cooling fluid to the cooler 12, utilizing the rich absorbent from the cyclone 1 10 with low temperature before it is returned via line 1 14 and line 62 to the reservoir 64, but also utilizing forced cooling fluid directly from sea 82 via line 1 13, optionally from seawater from several locations or depths illustrated by 82 and 82', to find an optimal cooling fluid temperature. The feature to utilize the rich absorbent will reduced the overall seawater feed to the system.

The several location or depths of seawater accessing the system 82 and 82', can also be used to target a desired absorbent characteristics, as these characteristics are significantly dependent of the absorbent temperature, such flexibility are specially beneficial in case there are fluctuations in the amount of C0 ₂ being produced from the reservoir, hence more stable flow rates through the system can be achieved. Such stable flow rates enables better control of the pressure difference across the membranes 3, thus better membrane protection is also achieved.

What is special with gases comprising high loads of C0 ₂ , especially at the temperature where condensate can be separated (normally in the range of 0- 15°C), is that C0 ₂ hydrate forms at a much lower pressures than methane hydrate. Typically no formation of methane hydrates takes place at pressures below approx. 24 bar, whereas to be secure against C0 ₂ hydrates one needs to be below 10 bar, hence the gas pressure in such cases needs to be reduced to below the hydrate formation curve, especially if no hydrate inhibitors are used. This can pressure can be controlled with a control valve 18, typically to a pressure below 20 bar with when the gas 10 comprises limited (typically less than 10%) C0 ₂ and typically below 10 bar if the gas 10 comprises much (typically more than 10%) C0 ₂ .

In the embodiment illustrated on figure 9 if the density of the rich absorbent stream 62 is above the density of the surrounding water and the environmental quality is acceptable at least part of the stream can be released to the surrounding sea as stream 68. Fig 10 illustrates a system using hydrate inhibitor (HI) such as MEG, as a physical absorbent. The HI 285 is used first and second absorption step, and optionally also upstream the first step, preferably before cooling 12 and or depressurization 218 as illustrated by adding stream 221. Here the produced water and or condensate is removed upstream the first absorption step, preferably conducted to the gas 2 via line 208. If the produced water and or condensate 215 also contain particles (precipitated salt, sand etc.) these are removed before conducted to the gas 2, using for example cyclone 210, where the separated particles and produced water are being returned to the reservoir 64 via line 207.

Any suitable chemicals supressing scaling, wax deposition etc. could optionally also be included.

Normally the rich absorbent separated in the separator 33 exits at adequate level above the bottom of the separator 33, preferably 0.2- 2 meter above bottom, via the line 228 to process line 227, but in the case of high condensate or particles production or built up at this process step, a portion of the rich absorbent exits via line 220 and is separated with cyclone 204, the separated absorbent returning to line 227 via valve 205, and the condensate and or particles are removed, preferably via valve 206 and line 207 to reservoir 64. A key feature of using HI as a physical absorbent is that one reduces the risk for hydrate formation, even if one which to remove condensate at high pressures, in addition to being an as efficient physical absorbent as water.

The system will only require a make up of HI comparable to what is exiting via line 208, and any loss through line 207. Particles and produced water are removed from the system a first stage, preventing set up later down in the C0 ₂ removal system. As water and HI is miscible, some HI may follow the condensate via line 208 and leave the illustrated system via conduit 2, but this HI may be recovered and returned to line 285. The flash separator 41 at least partly regenerates the HI absorbent 262 from the first and the second absorption step and the at least partly regenerated HI absorbent is redirected to the first step as stream 226 and 236 and to the second step as stream 289.

Fig 1 1 illustrates a system similar to the one illustrated on figure 9, except that a Hydrate Inhibitor of the type Kinetic Inhibitors (KHI ) is feed with the line 321 upstream the cooling 12 and depressurizing 18, and where the KHI is removed with produced water 109 back to reservoir 64. A key issue here is that the

depressurization is done so low that hydrate formation will not occur even though seawater is added as a physical solvent in subsequent absorbent steps, and that the depressurization at 18 is then done safely in the presence of KHI. In figure 12 illustrate the same as fig 1 1 , except that HI such as MEG is used instead of KHI added via line 321 prior to the depressurization over valve 18. This would require a lot more HI consumption than KHI as HI often requires

approximate a 50% mix with produced water flow where as KHI often only requires 0.1-5 % KHI mix with the produced water amount. Due to the amount of MEG required the Hi/produced water mixture 315 needs to be recovered, i.e. transported back to a topside or onshore processing MEG recovery system via line 308. KHI can also be combined with the MEG in the embodiment disclosed in figure 9. The use of a HI or KHI added prior to the depressurization over valve 18 and cooling in cooler 12 of the crude gas inhibits the formation of hydrates of C0 ₂ and if necessary methane in the initial condensate and produced water separation and extraction section. The main part of the HI can be separated out, preferably upstream first absorption step and preferably together with the condensate and transferred to regeneration. In an especially beneficial embodiment the HI separated out is mixed with the purified gas stream 2. This enables hydrate inhibition upstream and downstream the absorption processes, without significant loss of hydrate inhibitors due to any downstream absorption processes. In case KHI is the main HI portion, which is less expensive than MEG or similar, then separation of the HI is in one embodiment not necessary (as shown in figure 1 1), but instead could be transferred out with the produced water (109 in figure 1 1) Now back to figure 12, the pressurized gas stream 1 transferred to the first absorption step is at a pressure and temperature stage that the mixing with the first absorption solution comprising seawater will not result in formation of hydrates.

The invention also includes system configurations having a bypass mode of the membrane, allowing the membrane 3 to be shut down, removed or maintained by utilizing bypass valves, illustrated by a valve 131 in a closed position in figures 1 1 and 12.

The figures included here illustrate some embodiments of the present invention but these should however generally not be interpreted as limiting for the scope of the present invention as defined in the claims as a person skilled in the art will appreciate. A person skilled in the art will further appreciate that different solutions presented in the different illustrated embodiments can be freely combined without departing from the scope of the present invention.