Note: Descriptions are shown in the official language in which they were submitted.
CA 2,748,128
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i Method of Removing Carbon Dioxide from a Fluid Stream and Fluid
Separation Assembly
2
3 Description
4
Field of the invention
6 The invention relates to a method of removing carbon dioxide from a fluid
stream. In particular,
7 embodiments of the present invention relate to a method of removing
carbon dioxide from a
8 natural gas stream. The invention further relates to a fluid separation
assembly.
9
Background of the invention
11 Natural gas from storage or production reservoirs typically contains
carbon dioxide (CO2). Such
12 a natural gas is denoted as a "sour" gas. Another species denoted as
"sour" in a fluid stream is
13 hydrogen sulphide (H2S). A fluid stream without any of aforementioned
sour species is denoted
14 as a "sweet" fluid.
CO2 promotes corrosion within pipelines. Furthermore, in some jurisdictions,
legal and
16 commercial requirements with respect to a maximum concentration of CO2
in a fluid stream may
17 be in force. Therefore, it is desirable to .remove CO2 from a sour fluid
stream.
18 Fluid sweetening processes, i.e. a process to remove a sour species like
carbon dioxide
19 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
21 to as cryogenic separatiOn, and membrane separation.
22 The use of such methods for removing carbon dioxide from a fluid stream
is complex and
23 expensive.
24
Summary of the invention
26 It is desirable to have a method of removing carbon dioxide from a fluid
stream which operates
27 more efficiently than the methods mentioned above. For this purpose, an
embodiment of the
28 invention provides a method of removing carbon dioxide from a fluid
stream by a fluid
29 separation assembly comprising:
- a cyclonic fluid separator comprising a throat portion arranged between a
converging
31 fluid inlet section and a diverging fluid outlet section and a swirl
creating device
32 configured to create a swirling motion of the carbon dioxide containing
fluid within at
33 least part of the cyclonic fluid separator, the converging fluid inlet
section comprising a
34 first inlet for fluid components and the diverging fluid outlet section
comprising a first
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outlet for carbon dioxide depleted fluid and a second outlet for carbon
dioxide enriched
2 fluid;
3 a separation vessel having a first section in connection with a
collecting tank, the first section
4 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
6 dioxide, wherein said separation vessel is operated at a pressure and
temperature
7 combination that is at or in the vicinity of the phase boundary between a
8 vapour/liquid/solid coexistence region (IVb) and the vapour/solid
coexistence region (IVa);
9 the method comprising:
- providing a fluid stream at the first inlet, the fluid stream comprising
carbon dioxide;
11 - imparting a swirling motion to the fluid stream so as to induce
outward movement of at
12 least one of condensed components and solidified components within the
fluid stream
13 downstream the swirl creating device and to form an outward fluid
stream;
14 - 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
16 the components of liquefied carbon dioxide in the meta-stable state
under the influence
17 of the swirling motion;
18 - extracting the outward fluid stream comprising the components of
liquefied carbon
19 dioxide in the meta-stable state from said cyclonic fluid separator
through the second
outlet;
21 - providing the extracted outward fluid stream as a mixture to the
separation vessel
22 through the second inlet;
23 - guiding the mixture through the first section of the separation vessel
towards the
24 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
26 the meta-stable state;
27 - extracting the solidified carbon dioxide through the third outlet.
28 In an embodiment, the invention further relates to a fluid separation
assembly for removing
29 carbon dioxide from a fluid stream, the fluid separation assembly
comprising:
- a cyclonic fluid separator comprising a throat portion arranged between a
converging fluid
31 inlet section and a diverging fluid outlet section and a swirl creating
device configured to
32 create a swirling motion of the carbon dioxide containing fluid within
at least part of the
33 separator, the converging fluid inlet section comprising a first inlet
for fluid components
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1 and the diverging fluid outlet section comprising a first outlet for
carbon dioxide depleted
2 fluid and a second outlet for carbon dioxide enriched fluid;
3 - a separation vessel having a first section in connection with a
collecting tank, the section
4 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
6 dioxide, wherein said separation vessel is operated at a pressure and
temperature
7 combination that is at or in the vicinity of the phase boundary between a
8 vapour/liquid/solid coexistence region (IVb) and the vapour/solid
coexistence region (IVa);
9 wherein the fluid separation assembly is arranged to:
- receive a fluid stream comprising carbon dioxide at the first inlet;
11 - impart a swirling motion to the fluid stream so as to induce outward
movement of at least
12 one of condensed components and solidified components within the fluid
stream
13 downstream the swirl creating device and to form an outward fluid
stream;
14 - 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
16 components of liquefied carbon dioxide in the meta-stable state under
the influence of
17 the swirling motion;
18 - extract the outward fluid stream comprising said components of
liquefied carbon dioxide
19 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
21 the second inlet;
22 - guide the mixture through the first section of the separation vessel
towards the collecting
23 tank, while providing processing conditions in the first section such
that solidified carbon
24 dioxide is formed out of the components of liquefied carbon dioxide in
the meta-stable
state;
26 - enable extraction of the solidified carbon dioxide through the third
outlet.
27 Throughout the description, the term "fluid" is used. This term is used
to refer to liquid
28 and/or gas.
29
Description of the drawings
31 Embodiments of the invention will now be described, by way of example
only, with reference to
32 the accompanying schematic drawings in which corresponding reference
symbols indicate
33 corresponding parts and in which:
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1 - Figure 1 schematically depicts a longitudinal sectional view of a
cyclonic fluid separator
2 that may be used in embodiments;
3 - Figure 2 schematically depicts a cross-sectional view of a
separation vessel that may be
4 used in embodiments;
- Figures 3a, 3b depict an exemplary phase diagram of a natural gas
containing carbon
6 dioxide in which schematically different embodiments of the method are
visualised,
7 - Figure 4, 5,6, 7, 8a and 8b schematically depict further
embodiments.
8
9 Detailed description
to Figure 1 schematically depicts a longitudinal sectional view of a
cyclonic fluid separator 1 that
I may be used in embodiments of the invention. Such a cyclonic fluid
separator is described in
12 more detail in international patent application W003/029739. It must be
understood that, in
13 embodiments of the invention, also cyclonic fluid separators of a
different type may be used,
14 e.g. a cyclonic fluid separator as described in W099/01194,
W02006/070019 and
W000/23757.
16 The cyclonic fluid separator 1 comprises a converging fluid inlet
section 3, a diverging fluid
17 outlet section 5 and a tubular throat portion 4 arranged in between the
converging fluid inlet
18 section 3 and diverging fluid outlet section 5. The cyclonic fluid
separator 1 further comprises a
19 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.
21 The cyclonic fluid separator 1 comprises a pear-shaped central body 11
on which the swirl
22 imparting vanes 2 are mounted and which is arranged coaxial to a central
axis I of the cyclonic
23 separator 1 and inside the cyclonic separator such that an annular flow
path is created between
24 the central body 11 and separator housing 20.
The width of the annulus is designed such that the cross-sectional area of the
annulus
26 gradually decreases downstream of the swirl imparting vanes 2 such that
in use the fluid
27 velocity in the annulus gradually increases and reaches a supersonic
speed at a location
28 downstream of the swirl imparting vanes 2.
29 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
31 equipped with a central primary outlet conduit 6 for gaseous components
and with an outer
32 secondary outlet conduit 7 for condensables enriched fluid components.
The central body 11
33 has a substantially cylindrical elongated tail section 8 on which an
assembly of flow
34 straightening blades 19 is mounted. The central body 11 has a largest
outer width or diameter
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1 2R0 max which is larger than the smallest inner width or diameter 2R. min
of the tubular throat
2 portion 4.
3 The tubular throat portion 4 comprises the part of the annulus 3 having
the smallest cross-
4 sectional area. The maximum diameter of the central body 11 is larger
than the minimum
diameter of the tubular throat portion 4.
6 The converging fluid inlet section.3 comprises a first inlet 10. The
diverging fluid outlet
7 section 5 comprises a first outlet 6 and a second outlet 7.
8 The function of the various components of the cyclonic fluid separator 1
will now be
9 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
11 embodiment of the invention.
12 The fluid stream comprising carbon dioxide is fed through the first
inlet 10 in the
13 converging fluid inlet section 3. In an embodiment of the invention, the
fluid stream comprises a
14 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 a relative to the
central axis of the
16 cyclonic fluid separator 1,i.e. the axis around which the cyclonic fluid
separator 1 is about
17 rotationally symmetric. The swirling fluid stream is then expanded to
high velocities. In
18 embodiments of the invention, the number of swirl imparting vanes 2 is
positioned in the throat
19 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
21 width or diameter 2R0 maxwhich is larger than the smallest inner width
or diameter 2Rn min of the
22 tubular throat portion 4.
23 In embodiments of the invention, the swirling fluid stream has a
transonic velocity. In other
24 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.
26 The first time scale is related to a mass transfer time teq, i.e. a time
associated with return
27 to equilibrium conditions. The teq depends on the interfacial area
density in a two-phase system,
28 the diffusion coefficient between the two phases and the magnitude of
the departure from
29 equilibrium. The tõ for a liquid-to-solid transition is typically two
orders of magnitude larger than
for a vapour-to-liquid transition.
31 The second time scale is related to an expansion residence time trõ of
the fluid in the
32 device. The tres relates to the average speed of the fluid in the device
and the axial length of the
33 device along which the fluid travels. An expansion is denoted as 'rapid'
when teq
___________________________________________________________ >1.
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1 Due to the rapid expansion which causes a high velocity of the fluid
stream, the swirling
2 fluid stream may reach a temperature below 200 K and a pressure below 50%
of a pressure at
3 the first inlet 10 of the converging inlet section 3. As a result of
aforementioned expansion,
4 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
6 formed as liquefied carbon dioxide components. In case the fluid stream
at the inlet section 3 is
7 a liquid stream, hydrocarbon vapours will be formed whilst the majority
of carbon dioxide
8 components remain in liquid form. In the tubular throat portion 4, the
fluid stream may be
9 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
11 formation of carbon dioxide components is ongoing and particles will
gain mass. Preferably the
12 expansion is extended to a solid coexistence region (region IVa or IVb
in figures 3a, 3b).
13 However solidification will be delayed with respect to equilibrium,
since the phase transition from
14 liquid to solid is associated with a barrier of the free energy of
formation. As will be further
discussed with respect to Figures 3a, 3h, a portion of the carbon dioxide may
solidify.
16 In case the fluid stream is kept at substantially constant speed, carbon
dioxide
17 component formation is about to stop after a defined relaxation time. In
both cases, i.e.
18 expansion of the fluid stream to higher velocity and keeping the fluid
stream at a substantially
19 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
21 separator 1 so as to form an outward fluid stream. In this case the
outward fluid stream is a
22 stream of a carbon dioxide enriched fluid, the carbon dioxide components
therein being liquefied
23 and/or partly solidified.
24 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
26 cyclonic fluid separator 1 through the second outlet 7 of the cyclonic
fluid separator 1. Other
27 components within the fluid stream not being part of aforementioned
outward fluid stream are
28 extracted from the cyclonic fluid separator 1 through first outlet 6 of
the cyclonic fluid separator
29 1.
31 Figure 2 schematically depicts a cross-sectional view of a separation
vessel 21 that may
32 be used in embodiments of the invention. The separation vessel 21 has a
first section, further
33 referred to as tubular section 22, with, in use, a substantially
vertical orientation positioned on
34 and in connection with a collecting tank 23. The collecting tank 23 is
provided with a third outlet
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1 28 and a fourth outlet 26. The tubular section 22 is provided with a
second inlet 25 and a fifth
2 outlet 29. The second inlet 25 is connected to the second outlet 7 of the
cyclonic fluid separator
3 1. In an embodiment, the second inlet 25 is arranged to provide a
tangential fluid stream into the
4 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
6 Figure 2 schematically represented by reference number 31, and a
separation arrangement, in
7 Figure 2 schematically represented by reference number 33.
8 The function of the various components of the separation vessel 21 will
now be explained
9 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.
11 The cooling arrangement 31 is configured to provide a predetermined
temperature
12 condition in the separation vessel 21. The temperature condition is such
that it enables
13 solidification of the carbon dioxide enriched fluid, which enters the
separation vessel 21 through
14 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
16 dependent on the pressure conditions in the separation vessel 21.
17 Within the separation vessel 21, a mixture comprising carbon dioxide
originating from the
18 second outlet 7 of the cyclonic fluid separator 1 is split in at least
three fractions. These fractions
19 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.
21 The first fraction is formed by gaseous components which are dragged
along with the
22 liquids exiting the second outlet 7. The cooling arrangement 31 is
configured to keep the
23 temperature within the separation vessel 21 below the solidification
temperature of the fluid. The
24 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 Figure 3. The
26 carbon dioxide depleted gaseous components may leave the separation
vessel 21 through the
27 fifth outlet 29.
28 The vessel 21 may be equipped with one or more inlets 25 which are
positioned tangent
29 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
31 form a so-called vortex finder. The edge of said vortex finder is at a
vertical lower position
32 compared to the vertical position of the inlet(s) 25. This is explained
in more detail below with
33 reference to Fig. 7.
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The edge of the vortex finder (i.e. lowest part of the gas outlet 29), is
below the inlet 25 to
2 allow the components that enter through the inlet 25 to separate before
reaching the edge of the
3 vortex finder. So this distance is provided to prevent liquids and solids
from entering the vortex
4 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.
6 The sections 22 and 23 of vessel 21 may be physically separated by a
conical shaped
7 vortex breaker of which the outer perimeter has a clearance C with
respect to the inner
8 perimeter of the vertical section 22. This clearance C can range
typically from 0.05 to 0.3 times
9 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
11 phenomenon which will be explained in more detail with respect to Figure
3, the mixture, which
12 no longer holds gaseous components, may be split in a liquid component
containing
13 hydrocarbon and a solid component of carbon dioxide by means of a
separation arrangement
14 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.
16 In case a centrifuge is used, it preferably comprises a stacked disc
bowl. The separation
17 arrangement 33 in the separation vessel 21 is configured to enable
carbon dioxide enriched
18 hydrocarbon liquid components to leave the separation vessel 21 through
the fourth outlet 26,
19 and to enable solidified carbon dioxide to leave the separation vessel
21 through the third outlet
28.
21 In an embodiment, the fluid separation assembly further comprises a
screw conveyor or
22 scroll type discharger 35 in connection with the third outlet 28. The
scroll type discharger 35 is
23 configured to extract the solidified carbon dioxide from the separation
vessel 21.
24 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
26 more tubes or the like connecting the second outlet 7 of the cyclonic
fluid separator 1 and the
27 second inlet 25 of the separation vessel 21, are provided with a non-
adhesive coating. The non-
28 adhesive coating prevents adhesion of solidified fluid components, i.e.
carbon dioxide, on
29 aforementioned interior surfaces. Such adhesion would decrease the
efficiency of the fluid
separation assembly.
31
32 Figures 3a, 3b show an exemplary phase diagram of a natural gas
containing carbon dioxide in
33 which schematically different embodiments of the method according to the
invention are
34 visualised. The phases are represented as a function of pressure in bar
and temperature in
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1 degrees Celsius. In this particular case, the natural gas contains 71
mol% CO2. Additionally, the
2 natural gas contains 0.5 mol% nitrogen (N2), 0.5 mol% hydrogen sulphide
(H2S), 27 mol%
3 i.e. hydrocarbons with a single carbon atom therein, and 1 mol% C2, i.e.
hydrocarbons with two
4 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.
6 In Figure 3a, the condition of the fluid stream at the first inlet 10 of
the cyclonic fluid
7 separator 1 schematically depicted in Figure 1 corresponds to the
coordinate of 80 bar and -40
8 00 , denoted by [START] in the diagram of figure 3a. The isentropic
trajectory along arrow A is
9 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
12 transition occurs at a certain super saturated condition. The resulting
evaporation process will
13 then restore equilibrium conditions. Further expansion of the fluid
stream along the arrow C may
14 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
16 denoted with arrow C, a phase transition to form solid carbon dioxide
will not occur
17 instantaneously, the carbon dioxide fraction in the vapour will deplete,
while more carbon
18 dioxide dissolves in the liquid. In embodiments of the invention, the
fluid stream may be
19 separated by a cyclonic fluid separator, e.g. a cyclonic fluid separator
as described in
International patent application W02006/070019, in a carbon dioxide enriched
fluid stream and
21 a carbon dioxide depleted fluid stream at the end of the expansion
trajectory denoted by arrow
22 C. The separated, carbon dioxide enriched fluid is in a state of non-
equilibrium, which will only
23 last for a limited period of time, in the order of 10 milliseconds.
Therefore the carbon dioxide
24 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,
26 preferably within said time period that the meta-stable state exists. A
breakdown of said meta-
27 stable state results in solid formation which in practice means that
dissolved carbon dioxide in
28 the liquid solidifies. As a result of the solidification of carbon
dioxide, latent heat is released
29 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
31 in the vapour/solid or vapour/liquid/solid coexistence region. Said
process of cooling and
32 recompressing the carbon dioxide enriched fluid is denoted by arrow D.
In embodiments of the
33 invention, the process of further solidification takes place in the
separation vessel 21. The state
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1 of the fluid at a newly developed equilibrium within the separation
vessel 21 is denoted as
2 [END]. Solidified carbon dioxide is removed through the third outlet 28
as described above.
3 In Figure 3b, the condition of the fluid stream at the first inlet 10 of
the cyclonic fluid
4 separator 1 schematically depicted in Figure 1 corresponds to the
coordinate of about 85 bar
and about 18 C , denoted by [START] in the diagram of figure 3b. The
isentropic trajectory
6 along arrow A' is in the vapour region (I), whereas the isentropic
trajectory along arrow B' is in
7 the vapour/liquid coexistence region (III). As a result of the expansion
in the coexistence region
8 (III), a meta-stable state in the liquid/vapour regime may be reached
while following arrow B',
9 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
11 arrow C' may result in the fluid to reach a meta-stable state in the
vapour/liquid/solid
12 coexistence region (IVb) or in the vapour/solid coexistence region
(IVa). Even though along the
13 expansion trajectory denoted with arrow C', a phase transition to form
solid carbon dioxide will
14 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
16 depleted fluid stream at the end of the expansion trajectory denoted by
arrow C', a process
17 described above with reference to Figure 1. Additionally, further
details with respect to such a
18 process may be found in international application W003/029739. The
separated, carbon dioxide
19 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
21 diverging outlet section 5 of the cyclonic fluid separator 1 and
discharged via the second outlet 7
22 to the separation vessel 21, preferably within said time period that the
meta-stable state exists.
23 A breakdown of said meta-stable state results in solid carbon dioxide
formation from the
24 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
26 enriched fluid entering the separation vessel 21, may be cooled in order
to ensure that the fluid
27 remains in the vapour/solid or vapour/liquid/solid coexistence region.
Said process of cooling
28 and recompressing the carbon dioxide enriched fluid is denoted by arrow
D'.
29 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
31 21 is denoted as [END]. Again, solidified carbon dioxide is removed
through the third outlet 28
32 as described above.
33
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1 For the examples provided above with reference to Fig.'s 3a and 3b, the
maximum carbon
2 dioxide solid fraction for a given temperature T is obtained at a
pressure P intersecting the
3 phase boundary between regions LVC .(IVb) and VC (Iva).
4 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
6 separation vessel 21 is operated at a pressure P and a temperature T at
or close to the phase
7 boundary between regions LVC (IVb) and VC (IVa). This phase boundary is
shown in Fig.'s 3a
8 and 3b.
9 In the example provided in Fig.'s 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
11 it takes place in the separation vessel 21. As shown, the state of the
fluid at a newly developed
12 equilibrium within the separation vessel 21 is denoted as [END].
According to the embodiment
13 described here, [END] is chosen at or close to the phase boundary
between regions LVC (IVb)
14 and VC (IVa). This is done as the amount of solid carbon dioxide is at
its maximum at this phase
boundary.
16 In this embodiment, the term "close to the phase boundan/ is used to
indicate a margin in
17 the temperature of 5 C with respect to the indicated phase boundary and
a margin in the
18 pressure of 2 or 5bar or a margin of 10% or 20% with respect to the
indicated phase
19 boundary.
Thus according to an embodiment, the separation vessel 21 is operated at a
pressure P
21 within 5 bar and at a temperature T within 5 C within the phase boundary
between regions LVC
22 (IVb) and VC (IVa).
23 This conditions may be controlled by controlling the pressure and
temperature within the
24 separation vessel 21. The temperaturein the separation vessel 21 may be
controlled by using
cooling arrangement 31. The pressure in the separation vessel 21 may be
controlled by a
26 pressure regulating valve which is located in the gas outlet stream 29.
27 According to an embodiment, the separation vessel 21 is operated at a
pressure and
28 temperature combination that is at or in the vicinity of the phase
boundary between the
29 vapour/liquid/solid coexistence region (IVb) and the vapour/solid
coexistence region (IVa).
According to the examples provided with reference to Fig. 3a and 3b, the
separation
31 vessel 21 may be operated at a pressure in the range of 5 ¨ 25 bar. The
proposed temperate
32 range for these examples is in the range of -70 C to -90 C.
33
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1 Fig.'s 4, 5 and 6 schematically depict a further embodiment, in which the
screw conveyor
2 or scroll type discharger 35 is replaced with a perforated screen 40.
Fig. 4 shows a side-view of
3 such a perforated screen 40, where Fig. 5 shows a top view of such a
perforated screen
4 according to a possible embodiment. Fig. 6 schematically depicts such a
perforated screen 40
in combination with separation vessel 21.
6 According to this embodiment the solidified carbon dioxide is removed
from the separation
7 vessel 21 by means of a perforated screen 40 comprising tapered
openings/slots or conical
8 holes. The perforated screen 40 may be heated and a pressure difference
may be maintained
9 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.
11 The perforated screen 40 may be provided with a plurality of
perforations or openings 41.
12 The openings 41 may be rectangular openings, openings formed as slots,
or may be circular
13 openings as shown in Fig. 5.
14 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
16 Fig. 4. The solidified carbon dioxide particles are transported through
the openings 41 from the
17 feed side 42 to the collection side 43 of the perforated screen 40. The
size and shape of the
18 openings 41 are such that, in use, the solidified carbon dioxide
particles fill the openings 41 and
19 form a layer of solidified carbon dioxide, thereby preventing transport
of gases and liquids from
the collection side 43 to the feed side 42.
21 To create such a layer of solidified carbon dioxide and thereby avoid
seepage flow of
22 liquid or gas through the openings 41 from the collection side 43 to the
feed side 42, the
23 openings 41 may be provided with a tapered shape or conical shape, i.e.
the openings 41 are
24 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.
26 An angle of convergence a of these openings 41 can be in the range of 5
and 30 with
27 respect to a longitudinal axis 44 of the opening 41. According to a
further embodiment, the
28 angle of convergence a of the openings 41 is in the range of 10 and 20
.
29 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
31 solidified carbon dioxide.
32 The typical outlet size D43 of the opening 41 (e.g. the diameter for
circular openings 41) at
33 the collection side 43 may be approximately equal to the mean grain size
of the solidified carbon
34 dioxide. However, according to a further embodiment, the typical outlet
size D43 of the opening
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1 41 at the collection side 43 is substantially smaller than the mean grain
size of the solidified
2 carbon dioxide. The diameter D43 of a circular opening 41 at the outlet
side can range from 0.5
3 to 5 mm though is preferably between 1 and 3 mm.
4 =
The depth D41 of the openings 41 measured in the direction of longitudinal
axis 44 may
6 typically be two times the inlet size D42 of the opening 41. However, the
depth D41 of the
7 openings 41 may also be more than two times the inlet size D42 of the
opening 41. Preferably
8 the depth D41 is less than 5 times the inlet size D42.
9 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
11 carbon dioxide particles will be present.in the openings 41 and on top
of the perforated screen
12 40. The dense packing of solidified carbon dioxide particles have a
relatively low porosity and
13 ensure that no leak paths are present for gases or liquids to seep
through from the feed side 42
14 towards the collection side 43.
16 Furthermore blocking said leak paths in order to obtain an impermeable
layer of solidified
17 carbon dioxide at the perforated screen 40 may be established by
providing means to apply
18 static head to the solidified carbon dioxide grains. The term head is
used to refer to a column or
19 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
21 between the carbon dioxide grains and the side walls of the openings 41.
By increasing the
22 cohesion and adhesion forces, the layer of carbon dioxide is made more
dense.
23
24 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
26 collection side 43. This may be accomplished by maintaining a suitable
temperature T43 at the
27 collection side 43 and/or maintaining a suitable pressure P43 at the
collection side 43.
28 The collection pressure P43 at the collection side 43 is controlled at a
pressure which is
29 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
31 the feed side is approximately equal to 20 bar and the pressure P43 at
the collection side may
32 be controlled to be approximately 10 - 18 bar.
33 The temperature T43 at the collection side 43 of the perforated screen
40 may be chosen
34 such that given the relevant pressure, the carbon dioxide is in a liquid
phase. For instance for a
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1 pressure of typically 10 ¨ 18 bar, a temperature may be chosen between
approximately -55 C
2 and 0 C.
3 The temperature at the collection side may be controlled by a temperature
arrangement
4 (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
6 screen 40.
7 As a result of the temperature and pressure T43, P43 at the collection
side 43 the
8 underside of the layer of carbon dioxide that is formed will melt and
carbon dioxide will drip and
9 may be collected in a suitable vessel or the like.
The above described embodiment provides an efficient way of separating carbon
dioxide.
11 By having carbon dioxide present in the solid state within the
separation vessel 21 the carbon
12 dioxide is separated from for instance methane (that would otherwise mix
with carbon dioxide in
13 liquid phase). At the same time, at the collection side 43 of the
perforated screen 40 the carbon
14 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
16 the feed side 42 and the collection side 43 allowing controlling the
collection side and the
17 separation side at different conditions (pressure/temperature).
18
19 Figure 7 shows a further embodiment.
The vessel 21 may be equipped with one or more inlets 25 which are positioned
tangent
21 to the perimeter of the vertical section 22, such that a rotational flow
in section 22 results.
22 Furthermore the top gas outlet 29 may extent as a vertical pipe in said
vertical section 22 as to
23 form a so-called vortex finder. The edge of said vortex finder is at a
vertical lower position
24 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
26 deflector plate or vortex breaker 30 of which the outer perimeter has a
clearance C with respect
27 to the inner perimeter of the vertical section 22. This clearance C can
range typically from 0.05
28 to 0.3 times the inner diameter of section 22.
29 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.
31 Also, the vortex breaker may prevent gaseous components to travel from
the vertical
32 section 22 into the collection tank 23 and deflects these gaseous
components towards the top
33 gas outlet 29.
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1 The perforated screen 40 is now provided as part of the collection tank
23. In use, a layer
2 of CO2 will form on top of the perforated screen 40. An overflow wall 34
is formed to provide an
3 overflow connection. The overflow connection allows liquids that will
typically form on top of the
4 layer of CO2 to pass the overflow wall 34 and leave the collection tank
23 via fourth outlet 26.
6 Fig. 8a schematically depicts a further embodiment. Fig. 8a depicts a
vessel 21 and two
7 cyclonic fluid separators 1 as described above. However, it will be
understood that instead of
8 two, any suitable number of cyclonic fluid separators 1 may be provided.
9 According to this embodiment the fluid separation assembly further
comprises a feedback
io conduit 81 that is on one side connected to the fourth outlet 26 and on
the other side connected
11 to a feedback inlet of the cyclonic fluid separator 1. The feedback
conduit 81 further comprises a
12 pump PU.
13 The carbon dioxide enriched hydrocarbon liquid components that flow via
the fourth outlet
14 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
16 upstream of the pear-shaped central body 11 and coincides with the
'normal' inlet 82 of the
17 cyclonic fluid separators 1. However, the feedback inlet may also be
provided at another
18 position, for instance halfway the cyclonic fluid separator 1.
19 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
21 temperature reaches its lowest value. Instead the carbon dioxide
enriched hydrocarbon liquid
22 stream is first pumped to the feed pressure and combined with the stream
of conduit 82 to form
23 a new feed stream transport indicated as the conduit 81+82, where after
said combined feed
24 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
26 between the feed stream temperature in conduit 81+82 and the temperature
in vessel 21, is 25
27 degrees C. In order to achieve the cooling, a cooling unit 85 may be
provided in conduit 81+82,
28 as shown in Fig. 8b.
29 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
31 cylonic fluid separator 1 may comprise approximately 70% CO2 and 30%C,1-
1y, while the outlet
32 83 may comprise approximately 15% CO2 and 85%CxHy.
33
34
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1 Further remarks
2 According to an embodiment there is provided a method of removing carbon
dioxide from
3 a fluid stream by a fluid separation assembly comprising:
4 - 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
6 create a swirling motion of the carbon dioxide containing fluid within at
least part of the
7 cyclonic fluid separator, the converging fluid inlet section comprising a
first inlet for fluid
8 components and the diverging fluid outlet section comprising a first
outlet for carbon
9 dioxide depleted fluid and a second outlet for carbon dioxide enriched
fluid;
- a separation vessel having a firstsection in connection with a collecting
tank, said first
11 section being provided with a second inlet connected to said second
outlet of said cyclonic
12 fluid separator, and said collecting tank being provided with a third
outlet for solidified
13 carbon dioxide;
14 the method comprising:
- providing a fluid stream at said first inlet, said fluid stream
comprising carbon dioxide;
16 - imparting a swirling motion to the fluid stream so as to induce
outward movement of at
17 least one of condensed components and solidified components within the
fluid stream
18 downstream the swirl creating device and to form an outward fluid
stream;
19 - 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
21 components of liquefied carbon dioxide in said meta-stable state under
the influence of
22 said swirling motion;
23 - extracting the outward fluid stream comprising said components of
liquefied carbon
24 dioxide in said meta-stable state from said cyclonic fluid separator
through said second
outlet;
26 - providing said extracted outward fluid stream as a mixture to said
separation vessel
27 through said second inlet;
28 - guiding said mixture through said first section of said separation
vessel towards said
29 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
31 said meta-stable state;
32 - extracting the solidified carbon dioxide through said third outlet,
33 wherein the method further comprises:
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1 - forming a layer of solidified carbon dioxide extracted from the
third outlet 28 on a feed
2 side 42 of a perforated screen 40 comprising openings 41 towards a
collection side 43,
3 - applying temperature and pressure conditions on the collection side
43 of the perforated
4 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.
6 The collection side 43 may be operated at a temperature and pressure
combination for
7 which carbon dioxide is liquid. The feed side 42 may be operated at a
first pressure and the
8 collection side 43 may be operated at a second pressure, the second
pressure being equal or
9 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
11 the feed side 42 that is greater than an outlet size D43 at the
collection side 43. The outlet size
12 D43 may be approximately equal to or substantially smaller than the
grain size of solidified
13 carbon dioxide.
14
While specific embodiments of the invention have been described above, it will
be
16 appreciated that the invention may be practiced in another way than
described. The description
17 above is intended to be illustrative, not limiting. Thus, it will be
apparent to a person skilled in
18 the art that modifications may be made to embodiments of the invention
as described without
19 departing from the scope of the claims set out below.
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