METHOD OF REMOVING CARBON DIOXIDE FROM A FLUID STREAM AND FLUID SEPARATION ASSEMBLY

Abstract

The invention relates to a method of removing carbon dioxide from a fluid stream by a fluid separation assembly. The fluid separation assembly has a cyclonic fluid separator with a tubular throat portion arranged between a converging fluid inlet section and a diverging fluid outlet section and a swirl creating device. The separation vessel has a tubular section positioned on and in connection with a collecting tank. In the method, a fluid stream with carbon dioxide is provided. Subsequently, a swirling motion is imparted to the fluid stream so as to induce outward movement. The swirling fluid stream is then expanded such that components of carbon dioxide in a meta-stable state within the fluid stream are formed. Subsequently, the outward fluid stream with the components of carbon dioxide is extracted from the cyclonic fluid separator and provided as a mixture to the separation vessel. The mixture is then guided through the tubular section towards the collecting tank while providing processing conditions such that solid carbon dioxide is formed. Finally, solidified carbon dioxide is extracted.

Description

FIELD OF THE INVENTION

The invention relates to a method of removing carbon dioxide from a fluid stream. In particular, embodiments of the present invention relate to a method of removing carbon dioxide from a natural gas stream. The invention further relates to a fluid separation assembly.

BACKGROUND OF THE INVENTION

Natural gas from storage or production reservoirs typically contains carbon dioxide (CO₂). Such a natural gas is denoted as a “sour” gas. Another species denoted as “sour” in a fluid stream is hydrogen sulphide (H₂S). A fluid stream without any of aforementioned sour species is denoted as a “sweet” fluid.

CO₂ promotes corrosion within pipelines. Furthermore, in some jurisdictions, legal and commercial requirements with respect to a maximum concentration of CO₂ in a fluid stream may be in force. Therefore, it is desirable to remove CO₂ from a sour fluid stream.

Fluid sweetening processes, i.e. a process to remove a sour species like carbon dioxide from a fluid stream, are known in the art. Such processes typically include at least one of chemical absorption, physical absorption, adsorption, low temperature distillation, also referred to as cryogenic separation, and membrane separation.

The use of such methods for removing carbon dioxide from a fluid stream is complex and expensive.

SUMMARY OF THE INVENTION

It is desirable to have a method of removing carbon dioxide from a fluid stream which operates more efficiently than the methods mentioned above. For this purpose, an embodiment of the invention provides a method of removing carbon dioxide from a fluid stream by a fluid separation assembly comprising:

• • a cyclonic fluid separator comprising a throat portion arranged between a converging fluid inlet section and a diverging fluid outlet section and a swirl creating device configured to create a swirling motion of the carbon dioxide containing fluid within at least part of the cyclonic fluid separator, the converging fluid inlet section comprising a first inlet for fluid components and the diverging fluid outlet section comprising a first outlet for carbon dioxide depleted fluid and a second outlet for carbon dioxide enriched fluid;
• a separation vessel having a first section in connection with a collecting tank, the first section being provided with a second inlet connected to the second outlet of the cyclonic fluid separator, and the collecting tank being provided with a third outlet for solidified carbon dioxide, wherein said separation vessel is operated at a pressure and temperature combination that is at or in the vicinity of the phase boundary between a vapour/liquid/solid coexistence region (IVb) and the vapour/solid coexistence region (IVa);
  the method comprising:
  • providing a fluid stream at the first inlet, the fluid stream comprising carbon dioxide;
  • imparting a swirling motion to the fluid stream so as to induce outward movement of at least one of condensed components and solidified components within the fluid stream downstream the swirl creating device and to form an outward fluid stream;
  • expanding the swirling fluid stream, so as to form components of liquefied carbon dioxide in a meta-stable state within the fluid stream, and induce outward movement of the components of liquefied carbon dioxide in the metastable state under the influence of the swirling motion;
  • extracting the outward fluid stream comprising the components of liquefied carbon dioxide in the meta-stable state from said cyclonic fluid separator through the second outlet;
  • providing the extracted outward fluid stream as a mixture to the separation vessel through the second inlet;
  • guiding the mixture through the first section of the separation vessel towards the collecting tank, while providing processing conditions in the first section such that solidified carbon dioxide is formed out of the components of liquefied carbon dioxide in the meta-stable state;
  • extracting the solidified carbon dioxide through the third outlet.

In an embodiment, the invention further relates to a fluid separation assembly for removing carbon dioxide from a fluid stream, the fluid separation assembly comprising:

• • a cyclonic fluid separator comprising a throat portion arranged between a converging fluid inlet section and a diverging fluid outlet section and a swirl creating device configured to create a swirling motion of the carbon dioxide containing fluid within at least part of the separator, the converging fluid inlet section comprising a first inlet for fluid components and the diverging fluid outlet section comprising a first outlet for carbon dioxide depleted fluid and a second outlet for carbon dioxide enriched fluid;
  • a separation vessel having a first section in connection with a collecting tank, the section being provided with a second inlet connected to the second outlet of the cyclonic fluid separator, and the collecting tank being provided with a third outlet for solidified carbon dioxide, wherein said separation vessel is operated at a pressure and temperature combination that is at or in the vicinity of the phase boundary between a vapour/liquid/solid coexistence region (IVb) and the vapour/solid coexistence region (IVa);
    wherein the fluid separation assembly is arranged to:
  • receive a fluid stream comprising carbon dioxide at the first inlet;
  • impart a swirling motion to the fluid stream so as to induce outward movement of at least one of condensed components and solidified components within the fluid stream downstream the swirl creating device and to form an outward fluid stream;
  • expand the swirling fluid stream, so as to form components of liquefied carbon dioxide in a meta-stable state within the fluid stream, and induce outward movement of the components of liquefied carbon dioxide in the meta-stable state under the influence of the swirling motion;
  • extract the outward fluid stream comprising said components of liquefied carbon dioxide in the meta-stable state from the cyclonic fluid separator through the second outlet;
  • provide the extracted outward fluid stream as a mixture to the separation vessel through the second inlet;
  • guide the mixture through the first section of the separation vessel towards the collecting tank, while providing processing conditions in the first section such that solidified carbon dioxide is formed out of the components of liquefied carbon dioxide in the meta-stable state;
  • enable extraction of the solidified carbon dioxide through the third outlet.

Throughout the description, the term “fluid” is used. This term is used to refer to liquid and/or gas.

DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts and in which:

FIG. 1 schematically depicts a longitudinal sectional view of a cyclonic fluid separator that may be used in embodiments;

FIG. 2 schematically depicts a cross-sectional view of a separation vessel that may be used in embodiments;

FIGS. 3a, 3b depict an exemplary phase diagram of a natural gas containing carbon dioxide in which schematically different embodiments of the method are visualised,

FIGS. 4, 5, 6, 7, 8a and 8b schematically depict further embodiments.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a longitudinal sectional view of a cyclonic fluid separator 1 that may be used in embodiments of the invention. Such a cyclonic fluid separator is described in more detail in international patent application WO03/029739. It must be understood that, in embodiments of the invention, also cyclonic fluid separators of a different type may be used, e.g. a cyclonic fluid separator as described in WO99/01194, WO2006/070019 and WO00/23757.

The cyclonic fluid separator 1 comprises a converging fluid inlet section 3, a diverging fluid outlet section 5 and a tubular throat portion 4 arranged in between the converging fluid inlet section 3 and diverging fluid outlet section 5. The cyclonic fluid separator 1 further comprises a swirl creating device, e.g. a number of swirl imparting vanes 2, configured to create a swirling motion of the fluid within at least part of the cyclonic fluid separator 1.

The cyclonic fluid separator 1 comprises a pear-shaped central body 11 on which the swirl imparting vanes 2 are mounted and which is arranged coaxial to a central axis I of the cyclonic separator 1 and inside the cyclonic separator such that an annular flow path is created between the central body 1 and separator housing 20.

The width of the annulus is designed such that the cross-sectional area of the annulus gradually decreases downstream of the swirl imparting vanes 2 such that in use the fluid velocity in the annulus gradually increases and reaches a supersonic speed at a location downstream of the swirl imparting vanes 2.

The cyclonic separator 1 further comprises a tubular throat portion 4 from which, in use, the swirling fluid stream is discharged into a diverging fluid separation chamber 5 which is equipped with a central primary outlet conduit 6 for gaseous components and with an outer secondary outlet conduit 7 for condensables enriched fluid components. The central body 1 has a substantially cylindrical elongated tail section 8 on which an assembly of flow straightening blades 19 is mounted. The central body 11 has a largest outer width or diameter 2R_{o max} which is larger than the smallest inner width or diameter 2R_{n min} of the tubular throat portion 4.

The tubular throat portion 4 comprises the part of the annulus 3 having the smallest cross-sectional area. The maximum diameter of the central body 1 is larger than the minimum diameter of the tubular throat portion 4.

The converging fluid inlet section 3 comprises a first inlet 10. The diverging fluid outlet section 5 comprises a first outlet 6 and a second outlet 7.

The function of the various components of the cyclonic fluid separator 1 will now be explained with respect to a case in which the cyclonic fluid separator 1 is used to separate carbon dioxide from a fluid stream comprising carbon dioxide in accordance with an embodiment of the invention.

The fluid stream comprising carbon dioxide is fed through the first inlet 10 in the converging fluid inlet section 3. In an embodiment of the invention, the fluid stream comprises a mole percentage carbon dioxide larger than 10%. The swirl imparting vanes 2 create a circulation in the fluid stream and are oriented at an angle α relative to the central axis of the cyclonic fluid separator 1, i.e. the axis around which the cyclonic fluid separator 1 is about rotationally symmetric. The swirling fluid stream is then expanded to high velocities. In embodiments of the invention, the number of swirl imparting vanes 2 is positioned in the throat portion 4. In other embodiments, of the invention, the number of swirl imparting vanes 2 is positioned in the converging fluid inlet section 3. Again, the central body 11 has a largest outer width or diameter 2R_{o max} which is larger than the smallest inner width or diameter 2R_{n min} of the tubular throat portion 4.

In embodiments of the invention, the swirling fluid stream has a transonic velocity. In other embodiments of the invention, the swirling fluid stream may reach a supersonic velocity. The expansion is performed rapidly. With respect to an expansion two time scales may be defined.

The first time scale is related to a mass transfer time t_eq, i.e. a time associated with return to equilibrium conditions. The t_eq depends on the interfacial area density in a two-phase system, the diffusion coefficient between the two phases and the magnitude of the departure from equilibrium. The t_eq for a liquid-to-solid transition is typically two orders of magnitude larger than for a vapour-to-liquid transition.

The second time scale is related to an expansion residence time t_res of the fluid in the device. The t_res relates to the average speed of the fluid in the device and the axial length of the device along which the fluid travels. An expansion is denoted as ‘rapid’ when

$\frac{t_{\mathrm{eq}}}{t_{\mathrm{res}}}>1.$

Due to the rapid expansion which causes a high velocity of the fluid stream, the swirling fluid stream may reach a temperature below 200 K and a pressure below 50% of a pressure at the first inlet 10 of the converging inlet section 3. As a result of aforementioned expansion, carbon dioxide components are formed in a meta-stable state within the fluid stream. In case the fluid stream at the inlet section 3 is a gas stream, the carbon dioxide components will be formed as liquefied carbon dioxide components. In case the fluid stream at the inlet section 3 is a liquid stream, hydrocarbon vapours will be formed whilst the majority of carbon dioxide components remain in liquid form. In the tubular throat portion 4, the fluid stream may be induced to further expand to higher velocity or be kept at a substantially constant speed.

In the first case, i.e. expansion of the fluid stream to higher velocity, aforementioned formation of carbon dioxide components is ongoing and particles will gain mass. Preferably the expansion is extended to a solid coexistence region (region IVa or IVb in FIGS. 3a, 3b). However solidification will be delayed with respect to equilibrium, since the phase transition from liquid to solid is associated with a barrier of the free energy of formation. As will be further discussed with respect to FIGS. 3a, 3b, a portion of the carbon dioxide may solidify.

In case the fluid stream is kept at substantially constant speed, carbon dioxide component formation is about to stop after a defined relaxation time. In both cases, i.e. expansion of the fluid stream to higher velocity and keeping the fluid stream at a substantially constant speed, the centrifugal action causes the carbon dioxide particles to drift to the outer circumference of the flow area adjacent to the inner wall of the housing of the cyclonic fluid separator 1 so as to form an outward fluid stream. In this case the outward fluid stream is a stream of a carbon dioxide enriched fluid, the carbon dioxide components therein being liquefied and/or partly solidified.

Downstream of the tubular throat portion 4, the outward fluid stream comprising the components of carbon dioxide in aforementioned meta-stable state is extracted from the cyclonic fluid separator 1 through the second outlet 7 of the cyclonic fluid separator 1. Other components within the fluid stream not being part of aforementioned outward fluid stream are extracted from the cyclonic fluid separator 1 through first outlet 6 of the cyclonic fluid separator 1.

FIG. 2 schematically depicts a cross-sectional view of a separation vessel 21 that may be used in embodiments of the invention. The separation vessel 21 has a first section, further referred to as tubular section 22, with, in use, a substantially vertical orientation positioned on and in connection with a collecting tank 23. The collecting tank 23 is provided with a third outlet 28 and a fourth outlet 26. The tubular section 22 is provided with a second inlet 25 and a fifth outlet 29. The second inlet 25 is connected to the second outlet 7 of the cyclonic fluid separator 1. In an embodiment, the second inlet 25 is arranged to provide a tangential fluid stream into the separation vessel 21, e.g. the second inlet 25 is arranged tangent to the circumference of the separation vessel 21. The separation vessel 21 further comprises a cooling arrangement, in FIG. 2 schematically represented by reference number 31, and a separation arrangement, in FIG. 2 schematically represented by reference number 33.

The function of the various components of the separation vessel 21 will now be explained with respect to a case in which the separation vessel 21 is used in a method of removing carbon dioxide from a fluid stream in accordance with an embodiment of the invention.

The cooling arrangement 31 is configured to provide a predetermined temperature condition in the separation vessel 21. The temperature condition is such that it enables solidification of the carbon dioxide enriched fluid, which enters the separation vessel 21 through the second inlet 25 as a mixture. In other words, the temperature within the separation vessel 21 should remain below the solidification temperature of carbon dioxide, the latter being dependent on the pressure conditions in the separation vessel 21.

Within the separation vessel 21, a mixture comprising carbon dioxide originating from the second outlet 7 of the cyclonic fluid separator 1 is split in at least three fractions. These fractions are a first fraction of gaseous components, a second fraction of hydrocarbon, predominantly in a liquid state, and a third fraction of carbon dioxide, predominantly in a solid state.

The first fraction is formed by gaseous components which are dragged along with the liquids exiting the second outlet 7. The cooling arrangement 31 is configured to keep the temperature within the separation vessel 21 below the solidification temperature of the fluid. The gaseous components do not contain much carbon dioxide as most carbon dioxide will be dissolved in the mixture liquid, as will be explained in more detail with reference to FIG. 3. The carbon dioxide depleted gaseous components may leave the separation vessel 21 through the fifth outlet 29.

The vessel 21 may be equipped with one or more inlets 25 which are positioned tangent to the perimeter of the vertical section 22, such that a rotational flow in section 22 results. Furthermore the top gas outlet 29 may extent as a vertical pipe in said vertical section 22 as to form a so-called vortex finder. The edge of said vortex finder is at a vertical lower position compared to the vertical position of the inlet(s) 25. This is explained in more detail below with reference to FIG. 7.

The edge of the vortex finder (i.e. lowest part of the gas outlet 29), is below the inlet 25 to allow the components that enter through the inlet 25 to separate before reaching the edge of the vortex finder. So this distance is provided to prevent liquids and solids from entering the vortex finder. The liquids and solids will be forced to the outer perimeter due to the rotational forces and will not enter the gas outlet 29.

The sections 22 and 23 of vessel 21 may be physically separated by a conical shaped vortex breaker of which the outer perimeter has a clearance C with respect to the inner perimeter of the vertical section 22. This clearance C can range typically from 0.05 to 0.3 times the inner diameter of section 22. This is explained in more detail below with reference to FIG. 7.

As a result of solidification of carbon dioxide out of the liquid within the mixture, a phenomenon which will be explained in more detail with respect to FIG. 3, the mixture, which no longer holds gaseous components, may be split in a liquid component containing hydrocarbon and a solid component of carbon dioxide by means of a separation arrangement 33. Possible separation arrangements 33 include a gravity separator, a centrifuge and a hydro cyclone. In case a gravity separator is used, it preferably comprises a number of stacked plates. In case a centrifuge is used, it preferably comprises a stacked disc bowl. The separation arrangement 33 in the separation vessel 21 is configured to enable carbon dioxide enriched hydrocarbon liquid components to leave the separation vessel 21 through the fourth outlet 26, and to enable solidified carbon dioxide to leave the separation vessel 21 through the third outlet 28.

In an embodiment, the fluid separation assembly further comprises a screw conveyor or scroll type discharger 35 in connection with the third outlet 28. The scroll type discharger 35 is configured to extract the solidified carbon dioxide from the separation vessel 21.

In yet another embodiment, interior surfaces of elements of the fluid separation assembly being exposed to the fluid, i.e. cyclonic fluid separator 1, separation vessel 21 and the one or more tubes or the like connecting the second outlet 7 of the cyclonic fluid separator 1 and the second inlet 25 of the separation vessel 21, are provided with a non-adhesive coating. The non-adhesive coating prevents adhesion of solidified fluid components, i.e. carbon dioxide, on aforementioned interior surfaces. Such adhesion would decrease the efficiency of the fluid separation assembly.

FIGS. 3a, 3b show an exemplary phase diagram of a natural gas containing carbon dioxide in which schematically different embodiments of the method according to the invention are visualised. The phases are represented as a function of pressure in bar and temperature in degrees Celsius. In this particular case, the natural gas contains 71 mol % CO₂. Additionally, the natural gas contains 0.5 mol % nitrogen (N₂), 0.5 mol % hydrogen sulphide (H₂S), 27 mol % C1, i.e. hydrocarbons with a single carbon atom therein, and 1 mol % C2, i.e. hydrocarbons with two carbon atoms therein. The phases are labelled as follows: V=vapour, L=liquid, C=solid CO₂. Areas of different coexisting phases are separated by calculated phase boundaries.

In FIG. 3a, the condition of the fluid stream at the first inlet 10 of the cyclonic fluid separator 1 schematically depicted in FIG. 1 corresponds to the coordinate of 80 bar and −40° C., denoted by [START] in the diagram of FIG. 3a. The isentropic trajectory along arrow A is in the liquid region (II), whereas the isentropic trajectory along arrow B is in the vapour/liquid coexistence region (III). As a result of the expansion in the coexistence region (III), a meta-stable state in the liquid/vapour regime may be reached while following arrow B, until phase transition occurs at a certain super saturated condition. The resulting evaporation process will then restore equilibrium conditions. Further expansion of the fluid stream along the arrow C may result in the fluid to reach a meta-stable state in the vapour/liquid/solid coexistence region (IVb) or in the vapour/solid coexistence region (IVa). Even though along the expansion trajectory denoted with arrow C, a phase transition to form solid carbon dioxide will not occur instantaneously, the carbon dioxide fraction in the vapour will deplete, while more carbon dioxide dissolves in the liquid. In embodiments of the invention, the fluid stream may be separated by a cyclonic fluid separator, e.g. a cyclonic fluid separator as described in International patent application WO2006/070019, in a carbon dioxide enriched fluid stream and a carbon dioxide depleted fluid stream at the end of the expansion trajectory denoted by arrow C. The separated, carbon dioxide enriched fluid is in a state of non-equilibrium, which will only last for a limited period of time, in the order of 10 milliseconds. Therefore the carbon dioxide enriched fluid is recompressed in the second outlet 7 of the diverging outlet section 5 of the cyclonic fluid separator 1 and discharged via the second outlet 7 to the separation vessel 21, preferably within said time period that the meta-stable state exists. A breakdown of said meta-stable state results in solid formation which in practice means that dissolved carbon dioxide in the liquid solidifies. As a result of the solidification of carbon dioxide, latent heat is released causing the temperature of the fluid to rise. Therefore the separated, carbon dioxide enriched fluid entering the separation vessel 21, may be cooled in order to ensure that the fluid remains in the vapour/solid or vapour/liquid/solid coexistence region. Said process of cooling and recompressing the carbon dioxide enriched fluid is denoted by arrow D. In embodiments of the invention, the process of further solidification takes place in the separation vessel 21. The state of the fluid at a newly developed equilibrium within the separation vessel 21 is denoted as [END]. Solidified carbon dioxide is removed through the third outlet 28 as described above.

In FIG. 3b, the condition of the fluid stream at the first inlet 10 of the cyclonic fluid separator 1 schematically depicted in FIG. 1 corresponds to the coordinate of about 85 bar and about 18° C., denoted by [START] in the diagram of FIG. 3b. The isentropic trajectory along arrow A′ is in the vapour region (I), whereas the isentropic trajectory along arrow B′ is in the vapour/liquid coexistence region (III). As a result of the expansion in the coexistence region (III), a meta-stable state in the liquid/vapour regime may be reached while following arrow B′, until phase transition occurs at a certain super-cooled condition. The resulting condensation process will then restore equilibrium conditions. Further expansion of the fluid stream along the arrow C′ may result in the fluid to reach a meta-stable state in the vapour/liquid/solid coexistence region (IVb) or in the vapour/solid coexistence region (IVa). Even though along the expansion trajectory denoted with arrow C′, a phase transition to form solid carbon dioxide will not occur instantaneously. In embodiments of the invention, the fluid stream is separated by the cyclonic fluid separator 1 in a carbon dioxide enriched fluid stream and a carbon dioxide depleted fluid stream at the end of the expansion trajectory denoted by arrow C′, a process described above with reference to FIG. 1. Additionally, further details with respect to such a process may be found in international application WO03/029739. The separated, carbon dioxide enriched fluid is in a state of non-equilibrium, which will only last for a limited period of time, in the order of 10 milliseconds. Therefore the carbon dioxide enriched fluid is recompressed in the diverging outlet section 5 of the cyclonic fluid separator 1 and discharged via the second outlet 7 to the separation vessel 21, preferably within said time period that the meta-stable state exists. A breakdown of said meta-stable state results in solid carbon dioxide formation from the liquefied part of the fluid stream. As a result of the solidification of carbon dioxide, latent heat is released causing the temperature of the fluid to rise. Therefore the separated, carbon dioxide enriched fluid entering the separation vessel 21, may be cooled in order to ensure that the fluid remains in the vapour/solid or vapour/liquid/solid coexistence region. Said process of cooling and recompressing the carbon dioxide enriched fluid is denoted by arrow D′.

In embodiments of the invention, the process of solidification takes place in the separation vessel 21. The state of the fluid at a newly developed equilibrium within the separation vessel 21 is denoted as [END]. Again, solidified carbon dioxide is removed through the third outlet 28 as described above.

For the examples provided above with reference to FIGS. 3a and 3b, the maximum carbon dioxide solid fraction for a given temperature T is obtained at a pressure P intersecting the phase boundary between regions LVC (IVb) and VC (Iva).

As explained above, the function of the separation vessel 21 is to remove a maximum amount of carbon dioxide in the solid phase. Therefore, according to an embodiment, the separation vessel 21 is operated at a pressure P and a temperature T at or close to the phase boundary between regions LVC (IVb) and VC (IVa). This phase boundary is shown in FIGS. 3a and 3b.

In the example provided in FIGS. 3a and 3b, this phase boundary is crossed by arrow D, which represents the process of cooling and recompressing the carbon dioxide enriched fluid as it takes place in the separation vessel 21. As shown, the state of the fluid at a newly developed equilibrium within the separation vessel 21 is denoted as [END]. According to the embodiment described here, [END] is chosen at or close to the phase boundary between regions LVC (IVb) and VC (IVa). This is done as the amount of solid carbon dioxide is at its maximum at this phase boundary.

In this embodiment, the term “close to the phase boundary” is used to indicate a margin in the temperature of ±5° C. with respect to the indicated phase boundary and a margin in the pressure of ±2 or ±5 bar or a margin of 10% or 20% with respect to the indicated phase boundary.

Thus according to an embodiment, the separation vessel 21 is operated at a pressure P within 5 bar and at a temperature T within 5° C. within the phase boundary between regions LVC (IVb) and VC (IVa).

This conditions may be controlled by controlling the pressure and temperature within the separation vessel 21. The temperature in the separation vessel 21 may be controlled by using cooling arrangement 31. The pressure in the separation vessel 21 may be controlled by a pressure regulating valve which is located in the gas outlet stream 29.

According to an embodiment, the separation vessel 21 is operated at a pressure and temperature combination that is at or in the vicinity of the phase boundary between the vapour/liquid/solid coexistence region (IVb) and the vapour/solid coexistence region (IVa).

According to the examples provided with reference to FIGS. 3a and 3b, the separation vessel 21 may be operated at a pressure in the range of 5-25 bar. The proposed temperate range for these examples is in the range of −70° C. to −90° C.

FIGS. 4, 5 and 6 schematically depict a further embodiment, in which the screw conveyor or scroll type discharger 35 is replaced with a perforated screen 40. FIG. 4 shows a side-view of such a perforated screen 40, where FIG. 5 shows a top view of such a perforated screen according to a possible embodiment. FIG. 6 schematically depicts such a perforated screen 40 in combination with separation vessel 21.

According to this embodiment the solidified carbon dioxide is removed from the separation vessel 21 by means of a perforated screen 40 comprising tapered openings/slots or conical holes. The perforated screen 41 may be heated and a pressure difference may be maintained between a feed side 42 and a collection side 43, such that the pressure at the feed side is always higher than or equal to the pressure at the collection side.

The perforated screen 40 may be provided with a plurality of perforations or openings 41. The openings 41 may be rectangular openings, openings formed as slots, or may be circular openings as shown in FIG. 5.

The solidified carbon dioxide particles that leave the separation vessel 21 through the third outlet 28 are transported to the feed side 42 of the perforated screen 40, as shown in FIG. 4. The solidified carbon dioxide particles are transported through the openings 41 from the feed side 42 to the collection side 43 of the perforated screen 40. The size and shape of the openings 41 are such that, in use, the solidified carbon dioxide particles fill the openings 41 and form a layer of solidified carbon dioxide, thereby preventing transport of gases and liquids from the collection side 43 to the feed side 42.

To create such a layer of solidified carbon dioxide and thereby avoid seepage flow of liquid or gas through the openings 41 from the collection side 43 to the feed side 42, the openings 41 may be provided with a tapered shape or conical shape, i.e. the openings 41 are provided with a cross section at the feed side 41 that is larger than a cross section of the opening 41 at the collection side 43. This is shown in FIG. 4.

An angle of convergence α of these openings 41 can be in the range of 5° and 30° with respect to a longitudinal axis 44 of the opening 41. According to a further embodiment, the angle of convergence α of the openings 41 is in the range of 10° and 20°.

The typical inlet size D42 of the opening 41 (e.g. the diameter for circular openings 41) at the feed side 42 of the perforated screen 40 may be at least 2 times the typical grain size of the solidified carbon dioxide.

The typical outlet size D43 of the opening 41 (e.g. the diameter for circular openings 41) at the collection side 43 may be approximately equal to the mean grain size of the solidified carbon dioxide. However, according to a further embodiment, the typical outlet size D43 of the opening 41 at the collection side 43 is substantially smaller than the mean grain size of the solidified carbon dioxide. The diameter D43 of a circular opening 41 at the outlet side can range from 0.5 to 5 mm though is preferably between 1 and 3 mm.

The depth D41 of the openings 41 measured in the direction of longitudinal axis 44 may typically be two times the inlet size D42 of the opening 41. However, the depth D41 of the openings 41 may also be more than two times the inlet size D42 of the opening 41. Preferably the depth D41 is less than 5 times the inlet size D42.

The tapered shape and dimensions of the openings 41 allow a dense packing of solidified carbon dioxide particles to form in and possibly above the openings 41. In use, the solidified carbon dioxide particles will be present in the openings 41 and on top of the perforated screen 40. The dense packing of solidified carbon dioxide particles have a relatively low porosity and ensure that no leak paths are present for gases or liquids to seep through from the feed side 42 towards the collection side 43.

Furthermore blocking said leak paths in order to obtain an impermeable layer of solidified carbon dioxide at the perforated screen 40 may be established by providing means to apply static head to the solidified carbon dioxide grains. The term head is used to refer to a column or layer of liquid and solids which result in pressure on the dsolids on the perforated screen 40.

This increases the mutual contact pressure between the carbon dioxide grains and between the carbon dioxide grains and the side walls of the openings 41. By increasing the cohesion and adhesion forces, the layer of carbon dioxide is made more dense.

In order to allow the solidified carbon dioxide particles to travel through the openings 41 towards the collection side 43 the solidified carbon dioxide particles are melted from the collection side 43. This may be accomplished by maintaining a suitable temperature T43 at the collection side 43 and/or maintaining a suitable pressure P43 at the collection side 43.

The collection pressure P43 at the collection side 43 is controlled at a pressure which is typically 2 bar lower than a pressure P42 at the feed side 42 and in the separation vessel 21. So, in case the separation vessel 21 is operated at a pressure of 20 bar, the pressure P42 at the feed side is approximately equal to 20 bar and the pressure P43 at the collection side may be controlled to be approximately 10-18 bar.

The temperature T43 at the collection side 43 of the perforated screen 40 may be chosen such that given the relevant pressure, the carbon dioxide is in a liquid phase. For instance for a pressure of typically 10-18 bar, a temperature may be chosen between approximately −55° C. and 0° C.

The temperature at the collection side may be controlled by a temperature arrangement (not shown) or by an arrangement that heats the perforated screen to a desired temperature within the liquid phase of carbon dioxide to melt off liquid carbon dioxide from the perforated screen 40.

As a result of the temperature and pressure T43, P43 at the collection side 43 the underside of the layer of carbon dioxide that is formed will melt and carbon dioxide will drip and may be collected in a suitable vessel or the like.

The above described embodiment provides an efficient way of separating carbon dioxide. By having carbon dioxide present in the solid state within the separation vessel 21 the carbon dioxide is separated from for instance methane (that would otherwise mix with carbon dioxide in liquid phase). At the same time, at the collection side 43 of the perforated screen 40 the carbon dioxide is available in liquid phase, allowing easy further transportation and processing.

By providing the perforated screen 40 a solid carbon dioxide barrier is provided between the feed side 42 and the collection side 43 allowing controlling the collection side and the separation side at different conditions (pressure/temperature).

FIG. 7 shows a further embodiment.

The vessel 21 may be equipped with one or more inlets 25 which are positioned tangent to the perimeter of the vertical section 22, such that a rotational flow in section 22 results. Furthermore the top gas outlet 29 may extent as a vertical pipe in said vertical section 22 as to form a so-called vortex finder. The edge of said vortex finder is at a vertical lower position compared to the vertical position of the inlet(s) 25.

The sections 22 and 23 of vessel 21 may be physically separated by a conical shaped deflector plate or vortex breaker 30 of which the outer perimeter has a clearance C with respect to the inner perimeter of the vertical section 22. This clearance C can range typically from 0.05 to 0.3 times the inner diameter of section 22.

The vortex breaker 30 breaks the rotational motion of the flow from the first section 22 to the collection tank 23, to prevent eddies to be formed in the collection tank 23.

Also, the vortex breaker may prevent gaseous components to travel from the vertical section 22 into the collection tank 23 and deflects these gaseous components towards the top gas outlet 29.

The perforated screen 40 is now provided as part of the collection tank 23. In use, a layer of CO2 will form on top of the perforated screen 40. An overflow wall 34 is formed to provide an overflow connection. The overflow connection allows liquids that will typically form on top of the layer of CO2 to pass the overflow wall 34 and leave the collection tank 23 via fourth outlet 26.

FIG. 8a schematically depicts a further embodiment. FIG. 8a depicts a vessel 21 and two cyclonic fluid separators 1 as described above. However, it will be understood that instead of two, any suitable number of cyclonic fluid separators 1 may be provided.

According to this embodiment the fluid separation assembly further comprises a feedback conduit 81 that is on one side connected to the fourth outlet 26 and on the other side connected to a feedback inlet of the cyclonic fluid separator 1. The feedback conduit 81 further comprises a pump PU.

The carbon dioxide enriched hydrocarbon liquid components that flow via the fourth outlet 26 are pumped by means of the pump PU through the feedback conduit 81 to the feedback inlet of the one or more cyclonic fluid separators 1. According to FIG. 8a, the feedback inlet is upstream of the pear-shaped central body 11 and coincides with the ‘normal’ inlet 82 of the cyclonic fluid separators 1. However, the feedback inlet may also be provided at another position, for instance halfway the cyclonic fluid separator 1.

By providing such a feedback conduit 81, it is possible to achieve partial or even complete solidification of the CO2, without the need of additional cooling in the vessel 21 where the temperature reaches its lowest value. Instead the carbon dioxide enriched hydrocarbon liquid stream is first pumped to the feed pressure and combined with the stream of conduit 82 to form a new feed stream transport indicated as the conduit 81+82, where after said combined feed stream may be cooled to a new temperature which is lower than the temperature in conduit 82 and higher than the temperature level present in the vessel 21. Typically the difference between the feed stream temperature in conduit 81+82 and the temperature in vessel 21, is 25 degrees C. In order to achieve the cooling, a cooling unit 85 may be provided in conduit 81+82, as shown in FIG. 8b.

The first outlets 6 of the cyclonic fluid separators 1 may be combined together with the fifth outlet 29 of the tubular section 22 to form an outlet 83. The fluid through the inlet 81 of the cyclonic fluid separator 1 may comprise approximately 70% CO₂ and 30% C_xH_y, while the outlet 83 may comprise approximately 15% CO₂ and 85% C_x H_y.

Further Remarks

According to an embodiment there is provided a method of removing carbon dioxide from a fluid stream by a fluid separation assembly comprising:

• • a cyclonic fluid separator comprising a throat portion arranged between a converging fluid inlet section and a diverging fluid outlet section and a swirl creating device configured to create a swirling motion of the carbon dioxide containing fluid within at least part of the cyclonic fluid separator, the converging fluid inlet section comprising a first inlet for fluid components and the diverging fluid outlet section comprising a first outlet for carbon dioxide depleted fluid and a second outlet for carbon dioxide enriched fluid;
  • a separation vessel having a first section in connection with a collecting tank, said first section being provided with a second inlet connected to said second outlet of said cyclonic fluid separator, and said collecting tank being provided with a third outlet for solidified carbon dioxide;
    the method comprising:
  • providing a fluid stream at said first inlet, said fluid stream comprising carbon dioxide;
  • imparting a swirling motion to the fluid stream so as to induce outward movement of at least one of condensed components and solidified components within the fluid stream downstream the swirl creating device and to form an outward fluid stream;
  • expanding the swirling fluid stream, so as to form components of liquefied carbon dioxide in a meta-stable state within said fluid stream, and induce outward movement of said components of liquefied carbon dioxide in said meta-stable state under the influence of said swirling motion;
  • extracting the outward fluid stream comprising said components of liquefied carbon dioxide in said meta-stable state from said cyclonic fluid separator through said second outlet;
  • providing said extracted outward fluid stream as a mixture to said separation vessel through said second inlet;
  • guiding said mixture through said first section of said separation vessel towards said collecting tank, while providing processing conditions in said first section such that solidified carbon dioxide is formed out of said components of liquefied carbon dioxide in said meta-stable state;
  • extracting the solidified carbon dioxide through said third outlet, wherein the method further comprises:
    • forming a layer of solidified carbon dioxide extracted from the third outlet 28 on a feed side 42 of a perforated screen 40 comprising openings 41 towards a collection side 43,
    • applying temperature and pressure conditions on the collection side 43 of the perforated screen 40 to melt of carbon dioxide from the layer and collect the melted carbon dioxide through the openings 41 at the collection side 43.

The collection side 43 may be operated at a temperature and pressure combination for which carbon dioxide is liquid. The feed side 42 may be operated at a first pressure and the collection side 43 may be operated at a second pressure, the second pressure being equal or lower than the first pressure. The temperature at the collection side 43 may be in the range of minus 55° C.-0° C., and higher than at feed side 42. The openings 41 have an inlet size D42 at the feed side 42 that is greater than an outlet size D43 at the collection side 43. The outlet size D43 may be approximately equal to or substantially smaller than the grain size of solidified carbon dioxide.

While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced in another way than described. The description above is intended to be illustrative, not limiting. Thus, it will be apparent to a person skilled in the art that modifications may be made to embodiments of the invention as described without departing from the scope of the claims set out below.

Claims

1. Method of removing carbon dioxide from a fluid stream by a fluid separation assembly comprising: the method comprising:

a cyclonic fluid separator comprising a throat portion arranged between a converging fluid inlet section and a diverging fluid outlet section and a swirl creating device configured to create a swirling motion of the carbon dioxide containing fluid within at least part of the cyclonic fluid separator, the converging fluid inlet section comprising a first inlet for fluid components and the diverging fluid outlet section comprising a first outlet for carbon dioxide depleted fluid and a second outlet for carbon dioxide enriched fluid;

a separation vessel having a first section in connection with a collecting tank, said first section being provided with a second inlet connected to said second outlet of said cyclonic fluid separator, and said collecting tank being provided with a third outlet for solidified carbon dioxide, wherein said separation vessel is operated at a pressure and temperature combination that is at or in the vicinity of the phase boundary between a vapour/liquid/solid coexistence region (IVb) and the vapour/solid coexistence region (IVa);

providing a fluid stream at said first inlet, said fluid stream comprising carbon dioxide;

imparting a swirling motion to the fluid stream so as to induce outward movement of at least one of condensed components and solidified components within the fluid stream downstream the swirl creating device and to form an outward fluid stream;

expanding the swirling fluid stream, so as to form components of liquefied carbon dioxide in a meta-stable state within said fluid stream, and induce outward movement of said components of liquefied carbon dioxide in said meta-stable state under the influence of said swirling motion;

extracting the outward fluid stream comprising said components of liquefied carbon dioxide in said meta-stable state from said cyclonic fluid separator through said second outlet;

providing said extracted outward fluid stream as a mixture to said separation vessel through said second inlet;

guiding said mixture through said first section of said separation vessel towards said collecting tank, while providing processing conditions in said first section such that solidified carbon dioxide is formed out of said components of liquefied carbon dioxide in said meta-stable state;

extracting the solidified carbon dioxide through said third outlet (28).

2. Method according to claim 1, wherein the method further comprises:

forming a layer of solidified carbon dioxide extracted from the third outlet (28) on a feed side (42) of a perforated screen (40) comprising openings (41) towards a collection side (43),

applying temperature and pressure conditions on the collection side (43) of the perforated screen (40) to melt of carbon dioxide from the layer and collect the melted carbon dioxide through the openings (41) at the collection side (43).

3. Method according to any one of the claims 1-2, wherein the collection side (43) is operated at a temperature and pressure combination for which carbon dioxide is liquid.

4. Method according to any one of the claims 2-3, wherein the feed side (42) is operated at a first pressure and the collection side (43) is operated at a second pressure, the second pressure being equal or lower than the first pressure.

5. Method according to any one of the claims 2-4 wherein the temperature at the collection side (43) is in the range of minus 55° C.-0° C., and higher than a temperature at the feed side (42)

6. Method according to any one of the claims 2-5, wherein the openings (41) have an inlet size (D42) at the feed side (42) that is greater than an outlet size (D43) at the collection side (43).

7. Method according to claim 6, wherein the outlet size (D43) is approximately equal to or substantially smaller than the grain size of solidified carbon dioxide.

8. Method according to any one of the preceding claims, wherein said extracted outward fluid stream is provided to said separation vessel tangential to a perimeter of the first section, such that a rotational flow in the first section (22) is generated.

9. Method according to any one of the preceding claims, wherein said first section of the separation vessel is further provided with a fifth outlet, and said method further comprises extracting carbon dioxide depleted gaseous components through said fifth outlet.

10. Method according to any one of the claims 8-9, wherein the fifth outlet is formed by a vortex finder, comprising a substantially vertical pipe extending into the first section in a through an upper part of the first section in a downward direction, wherein the lower end of the pipe is at a vertical lower position than the second inlet.

11. Method according to any one of the preceding claims, wherein there is provided a vortex breaker (30) in between the first section and the collection tank.

12. Method according to any one of the preceding claims, wherein said collecting tank is further provided with a fourth outlet (26), and said method further comprises extracting hydrocarbon liquid components through said fourth outlet (26).

13. Method according to claim 12, wherein the hydrocarbon liquid components through the fourth outlet (26) are fed back to the cyclonic fluid separator.

14. Method according to any one of the preceding claims, wherein said separation vessel further comprises a cooling arrangement configured to provide a predetermined temperature condition therein, said temperature condition enabling solidification of the carbon dioxide enriched fluid.

15. Method according to any one of the preceding claims, wherein said fluid separation assembly further comprises a scroll type discharger in connection with said third outlet (28), and said extracting the solidified carbon dioxide is performed by conveying by means of said scroll type discharger.

16. Method according to any one of the preceding claims, wherein said fluid stream comprises a mole percentage carbon dioxide larger than 10%.

17. Method according to any one of the preceding claims, wherein said expanding of the swirling fluid stream is such that the swirling fluid stream reaches supersonic velocity.

18. Method according to claim 17, wherein said expanding is further such that a temperature below 200 K is reached.

19. Method according to claim 17 or 18, wherein said expanding is further such that a pressure is reached below 50% of a pressure at the first inlet of the cyclonic fluid separator.

20. Method according to any one of the preceding claims, wherein said providing said outward fluid stream as a mixture to said separation vessel through said second inlet is arranged such that a tangential fluid stream is provided.

21. Fluid separation assembly for removing carbon dioxide from a fluid stream, the fluid separation assembly comprising: wherein said fluid separation assembly is arranged to:

a cyclonic fluid separator comprising a throat portion arranged between a converging fluid inlet section and a diverging fluid outlet section and a swirl creating device configured to create a swirling motion of the carbon dioxide containing fluid within at least part of the separator, the converging fluid inlet section comprising a first inlet for fluid components and the diverging fluid outlet section comprising a first outlet for carbon dioxide depleted fluid and a second outlet for carbon dioxide enriched fluid;

a separation vessel having a first section in connection with a collecting tank, said section being provided with a second inlet connected to said second outlet of said cyclonic fluid separator, and said collecting tank being provided with a third outlet (28) for solidified carbon dioxide, wherein said separation vessel is operated at a pressure and temperature combination that is at or in the vicinity of the phase boundary between a vapour/liquid/solid coexistence region (IVb) and the vapour/solid coexistence region (IVa);

receive a fluid stream comprising carbon dioxide at said first inlet;

impart a swirling motion to the fluid stream so as to induce outward movement of at least one of condensed components and solidified components within the fluid stream downstream the swirl creating device and to form an outward fluid stream;

expand the swirling fluid stream, so as to form components of liquefied carbon dioxide in a meta-stable state within said fluid stream, and induce outward movement of said components of liquefied carbon dioxide in said meta-stable state under the influence of said swirling motion;

extract the outward fluid stream comprising said components of liquefied carbon dioxide in said meta-stable state from said cyclonic fluid separator through said second outlet;

provide said extracted outward fluid stream as a mixture to said separation vessel through said second inlet;

guide said mixture through said first section of said separation vessel towards said collecting tank, while providing processing conditions in said first section such that solidified carbon dioxide is formed out of said components of liquefied carbon dioxide in said meta-stable state;

enable extraction of the solidified carbon dioxide through said third outlet (28).

22. Fluid separator assembly according to claim 21, wherein the fluid separator assembly further comprising: wherein said fluid separation assembly is further arranged to:

a perforated screen (40) comprising a feed side (42) and a collection side (43), the feed side (42) positioned to collect solidified carbon dioxide from the third outlet (28), the perforated screen further comprising openings (41) towards the collection side (42),

form a layer of solidified carbon dioxide extracted from the third outlet (28) on the feed side (42) of the perforated screen (40),

applying temperature and pressure conditions on the collection side (43) of the perforated screen (40) to melt of carbon dioxide from the layer and collect the melted carbon dioxide through the openings (41) at the collection side (43).

23. Fluid separator assembly according to claim 22, wherein the collection side (43) is arranged to be operated at a temperature and pressure combination for which carbon dioxide is liquid.

24. Fluid separator assembly according to claims 22-23 wherein the feed side (42) is arranged to be operated at a first pressure and the collection side (43) is operated at a second pressure, the second pressure being equal or lower than the first pressure.

25. Fluid separator assembly according to any one of the claims 22-24, wherein the fluid separator assembly is arranged to be at a temperature at the collection side (43) that is in the range of minus 55° C.-0° C., but higher than a temperature at the feed side (42).

26. Fluid separator assembly according to any one of the claims 22-25, wherein the openings (41) have an inlet size (D42) at the feed side (42) that is greater than an outlet size (D43) at the collection side (43).

27. Fluid separator assembly according to claim 26, wherein the outlet size (D43) is approximately equal to or substantially smaller than the grain size of solidified carbon dioxide.

28. Fluid separator assembly according to anyone of the claims 21-27, wherein said second inlet is a tangential inlet to a perimeter of the first section, such that a rotational flow in the first section (22) is generated.

29. Fluid separation assembly according to any one of the claims 21-28, wherein said first section is further provided with a fifth outlet, said fifth outlet being configured to enable extraction of carbon dioxide depleted gaseous components.

30. Fluid separation assembly according to any one of the claims 28-29, wherein the fifth outlet is formed by a vortex finder, comprising a substantially vertical pipe extending into the first section through an upper part of the first section in a downward direction, wherein the lower end of the pipe is at a vertical lower position than the second inlet.

31. Fluid separation assembly according to any one of the claims 21-30, wherein there is provided a vortex breaker (30) in between the first section and the collection tank.

32. Fluid separation assembly according to any one of the claims 21-31, wherein said collecting tank is further provided with a fourth outlet (26), said fourth outlet (26) being configured to enable extraction of hydrocarbon liquid components.

33. Fluid separation assembly according to claim 32, wherein the fluid separation assembly comprises a feedback conduit (81), the feedback conduit (81) being arranged to feedback the hydrocarbon liquid components from the fourth outlet (26) to the cyclonic fluid separator.

34. Fluid separation assembly according to any one of claims 21-33, wherein said separation vessel further comprises a cooling arrangement configured to provide a predetermined temperature condition therein, said temperature condition enabling solidification of a carbon dioxide enriched fluid.

35. Fluid separation assembly according to any one of claims 21-34, wherein said fluid separation assembly further comprises a scroll type discharger in connection with said third outlet (28), said scroll type discharger being configured to enable extraction of said solidified carbon dioxide through said third outlet (28) by conveying.

36. Fluid separation assembly according to any one of claims 21-35, wherein said second inlet of said separation vessel is arranged tangent to the circumference of the separation vessel.