Note: Descriptions are shown in the official language in which they were submitted.
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ME T HOD FOR REMOVING A CONTAMINANT FROM A CONTAMINATED
STREAM
The present invention relates to a method for
removing a contaminant from a contaminated stream, in
particular a contaminated gas stream.
Various methods are known to remove contaminating
components, such as water, liquid hydrocarbons (also
called "condensate"), hydrates, carbon dioxide (CO2)
and/or hydrogen sulphide (H25), SO2, COS, mercaptans
(RSH) and other organic sulphur species, from a gas
stream such as a natural gas stream. The methods may be
based on physical and/or chemical separation techniques.
Physical separation techniques use differences in e.g.
boiling, condensation and/or freezing points of the
various contaminating components to selectively remove
one or more of these components in a fractionating
column, or differences in density to separate components
with different densities by gravity (e.g. gravity
settler), by a swirling flow (e.g. in a cyclonic
separator) or by spinning flow (e.g. in a centrifugal
separator). Chemical separation techniques may employ
selective absorption or catalytic reactions to convert a
contaminating component into a composition that can be
easily separated.
When the contaminants are removed using an absorbent
component, at least a contaminant-depleted stream and a
contaminant-enriched absorbent stream are obtained. The
contaminant-depleted stream may be further subjected to
further processing steps, if desired, before being sent
to its intended end-use. The contaminant-enriched
absorbent stream is typically regenerated in order to be
able to reuse the absorbent.
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A suitable process for removing 002 and/or H2S from a
gas comprising 002 and/or H2S is disclosed in EP 2 283
911 Al.
A problem that may occur in the known absorbent
regeneration processes is that after removing the
contaminant from the contaminant-enriched absorbent
stream the resulting contaminant-depleted absorbent
stream may form two liquid phases under the prevailing
conditions. In case the contaminant-depleted absorbent
stream is subsequently drawn off and reused in the
contaminant absorption process, the absorbent stream may
not have the right ratio of components as one phase is
predominantly removed. This problem of liquid phase
separation under regeneration conditions has been
acknowledged in e.g. paragraph [0014] of WO 2007/021532.
In Figures 1 and 6 of US 2010/0132551 an emulsion 8
is withdrawn from the regeneration column and liquid
phase separation is performed in an extra separation
device Bl. Streams 9 and 10 are withdrawn from device B1
using two separate outlets (and pumps) for the two
separate liquid phases. Additionally, stream 6 is
withdrawn from the regeneration column. Streams 9 and 6
are combined and recycled as stream 4 to the absorber. In
Figure 5 of US 2010/0132551 liquid phase separation is
performed in a downcomer in collection tray P in the
regeneration column and streams 9 and 10 are withdrawn
using two separate outlets (and pumps) for the two
separate liquid phases.
In Figure 1 of U54251494 an extra reboiler 30 is used
for phase separation. Two separate streams 16 and 18,
with different compositions, are withdrawn from reboiler
30 and recycled at different heights of the absorber.
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It is an object of the present invention to solve or
at least minimize the above issue of phase separation, in
particular during the regeneration of a contaminant-
enriched absorbent stream.
Another object is to provide a method which requires
less phase separation devices, outlets and pumps as
compared to the prior art.
It is a further object of the present invention to
provide an alternative method of removing a contaminant,
in particular 002 and/or H2S, etc., from a contaminated
stream.
One or more of the above or other objects are
achieved according to the present invention by providing
a method of removing a contaminant, in particular 002
and/or H2S, from a contaminated stream, the method at
least comprising the steps of:
(a) providing a contaminated stream (10), preferably a
gaseous contaminated stream;
(b) contacting the contaminated stream (10) in an
absorber (2) with an absorbent solution (80), thereby
obtaining a contaminant-depleted stream (20) and a
contaminant-enriched absorbent stream (30);
(c) separating the contaminant-enriched absorbent stream
(30) in a regenerator (3), thereby obtaining
- a contaminant-depleted absorbent stream (60) at a
bottom of the regenerator (3) and
- a contaminant-enriched stream (50),
the contaminant-depleted absorbent stream (60) forming a
lower layer of a first liquid phase (A) and a higher
layer of a second liquid phase (B) at the bottom of the
regenerator (3);
(d) simultaneously removing the first liquid phase (A)
and the second liquid phase (B) at the interface (X) of
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the lower layer of the first liquid phase (A) and the
higher layer of the second liquid phase (B) using a
liquid outlet (33); and
(e) reusing the contaminant-depleted absorbent stream
(60) as removed in step (d).
It has been surprisingly found according to the
present invention that by simultaneously removing the
lower layer of the first liquid phase and the higher
layer of the second liquid phase at the interface (X) of
these liquid phases using the same outlet, the two liquid
phases are removed in the same ratio as they are formed,
ensuring that the absorbent content of the removed stream
remains substantially constant over time.
An advantage of the present invention is that this
controlled removal of liquid phases can be done without
the need for two or more separate outlets (with
associated pumps) or additional process control. Also, no
mechanical agitation in order to create a finely
dispersed emulsion of the phases is required (with
optional use of emulsion stabilizers).
In step (a) of the method of the present invention, a
contaminated stream, preferably a gaseous contaminated
stream, is provided.
The contaminated stream is not limited in any way (in
terms of composition, phase, etc.) and may for example be
a natural gas stream, a combustion gas, synthesis gas, an
air stream, etc.; the contaminated stream may also be a
contaminated liquid hydrocarbon stream such as a
contaminated LPG stream. Preferably, the contaminated
stream is a methane-rich stream such as natural gas,
containing at least 30 wt.% methane, preferably at least
50 wt.% methane. The person skilled in the art will
readily understand that the contaminant is not limited to
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certain compounds and may include a broad variety of
compounds such as carbon dioxide (CO2) and/or hydrogen
sulphide (H2S), SO2, COS, mercaptans (RSH) and other
organic sulphur species. However, the present invention
is in particular suitable for the removal of 002 and/or
H2S from a contaminated gas stream such as natural gas or
a combustion gas.
In step (b), the contaminated stream is contacted in
an absorber with an absorbent solution, thereby obtaining
a contaminant-depleted stream and a contaminant-enriched
absorbent stream. Typically, the contaminant-depleted
stream is obtained at the top of the absorber and
subsequently removed. The contaminant-depleted stream may
be further processed if needed before it is sent to its
end use. Usually, the contaminant-enriched absorbent
stream is obtained at the bottom of the absorber. As the
person skilled in the art is familiar with the design and
functioning of an absorber, this is not further discussed
here in detail. Preferably, the absorber is operated at a
temperature in the range from 10 to 100 C, more
preferably from 20 to 80 C, even more preferably from 20
to 70 C. Typically, the absorber is operated at a
pressure in the range from 1.0 to 110 bar, more
preferably from 20 to 90 bar.
Preferably, the absorbent solution in step (b) is an
aqueous absorbent solution comprising water and an
absorbent component. In step (b), the absorbent solution
is preferably a single phase. The absorbent solution may
comprise two or more absorbent components. The one or
more absorbent components are not limited in any way.
Usually, the one or more absorbent components are amine
compounds. Suitable absorbent solutions have been
extensively described in the prior art, see for example
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A.L. Kohl and F.C. Riesenfeld, 1974, Gas Purification,
2'd edition, Gulf Publishing co. Houston and R.N. Maddox,
1974, Gas and Liquid sweetening, Campbell Petroleum
Series. Hence, the absorbent components are not further
described here in detail.
In step (c), the contaminant-enriched absorbent
stream is separated in a regenerator, thereby obtaining
- a contaminant-depleted absorbent stream at a bottom of
the regenerator and
- a contaminant-enriched stream,
the contaminant-depleted absorbent stream forming a lower
layer of a first liquid phase (A) and a higher layer of a
second liquid phase (B) at the bottom of the regenerator
(3).
The first liquid phase (A) has a higher density than
the second liquid phase (B). This is the case when the
contaminated stream (10) is a gaseous contaminated
stream, and also when the contaminated stream (10) is a
liquid contaminated stream, e.g. LPG or another light
density stream comprising hydrocarbons with 3 or 4 carbon
atoms.
The regenerator serves to separate the absorbent
solution and the (one or more) contaminant(s), for
example using one or more flash vessels, a column or a
combination thereof. As the person skilled in the art is
familiar with the design and functioning of a
regenerator, this is not further discussed here in
detail. Typically, the regenerator is operated at a
temperature sufficiently high to ensure that a
substantial amount of contaminant is liberated from the
contaminant-enriched absorbent stream. Preferably, the
regenerator is operated at a temperature in the range
from 60 to 170 C, more preferably from 70 to 160 C, even
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more preferably from 100 to 140 C. Typically, the
regenerator is operated at a pressure in the range from
0.001 to 50 bar, more preferably from 1.0 to 30 bar.
Typically the contaminant-enriched stream is obtained
at the top of the regenerator and is subsequently removed
from the regenerator for further processing, if needed.
The contaminant-depleted absorbent stream is obtained at
a bottom of the regenerator, as the regenerator may
comprise several vessels or draw-off trays. Hence, the
person skilled in the art will understand that the term
"bottom" as meant according to the present invention
refers to a place in the regenerator vessel where liquid
accumulates; the bottom is (although preferably it is)
not necessarily the absolute bottom of the regenerator
vessel, but may also be a local bottom such as a draw-off
tray. If the regenerator consist of one vessel or column
(which it preferably does from the viewpoint of
simplicity), the contaminant-depleted absorbent stream is
typically obtained at the (absolute) bottom thereof.
The lower layer of a first liquid phase (A) and the
higher layer of a second liquid phase (B) are formed
under the prevailing conditions in the regenerator.
Preferably, the first liquid phase as formed in step (c)
is a water-enriched phase and the second liquid phase is
a water-depleted phase. The second water-deplete phase
may comprise various absorbent components (and is
typically amine rich).
In step (d), the first liquid phase (A) and the
second liquid phase (B) of the contaminant-depleted
absorbent stream are simultaneously (but as separate
phases) removed (typically from the bottom of the
regenerator). This removal is performed using the same
single liquid outlet. Of course, more than one liquid
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outlet may be present for simultaneously removing the
first and second liquid phases.
A short time after the beginning with the removal of
the first and second liquid phases, the two liquid phases
are removed in the same ratio as they are formed,
ensuring that the absorbent content of the removed stream
remains substantially constant over time. Also, as a
result, the interface of the first and second liquid
phases will be situated at the height level of the
outlet.
Hence, a short time after the beginning with the
removal of the first and second liquid phases, the two
liquid phases are removed at the interface (X) of the
lower layer of the first liquid phase (A) and the higher
layer of the second liquid phase (B) using the same
single liquid outlet.
Preferably, the liquid outlet in step (d) is located
above at least 1/20 of the liquid height in the bottom of
the regenerator (or relevant vessel thereof), preferably
above at least 1/10, more preferably above at least 1/4.
This, to allow that the two separate liquid phases are
simultaneously removed via the liquid outlet in a
controlled manner; if a minimum volume of liquid is not
present above the liquid outlet, a substantial amount of
gas may be removed with the liquid phases thereby
disturbing the ratio of the liquid phases in the stream
removed through the liquid outlet in step (d). Also, if a
minimum volume of liquid is not present, this may
potentially cause sub-optimal or even hazardous operating
conditions in downstream equipment, such as pumps and
heat exchangers. A minimum amount of liquid above the
liquid outlet avoids the occurrence of large scale slug
flow of both liquid phases, wherein both liquid phases
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are withdrawn in alternating 'slugs' of lighter and
heavier phase, by creating a stable interface.
Preferably the liquid outlet in step (d) is located
above at least 1/20 of the liquid height in the bottom of
the regenerator, preferably above at least 1/10, more
preferably above at least 1/4. Additionally or
alternatively, the liquid outlet in step (d) preferably
is located below at most 19/20 of the liquid height in
the bottom of the regenerator, preferably below at most
9/10, more preferably below at most 3/4.
According to a preferred embodiment of the method
according to the present invention, a part of the first
liquid phase is removed from the regenerator, heated and
reintroduced into the regenerator at a point above the
interface of the first liquid phase and the second liquid
phase; preferably the removed part of the first liquid
phase is reintroduced at a point above the second liquid
phase.
Also, it is preferred that the liquid outlet in step
(d) comprises an element for avoiding liquid flow along
the wall and into the liquid outlet. This, to avoid that
the ratio of the liquid phases in the stream as removed
in step (d) is disturbed. Preferably, said element also
ensures that the liquid flow of the first liquid phase
and the second liquid phase through the liquid outlet is
substantially horizontal. Also it is preferred that the
liquid outlet in step (d) comprises a splash protector,
to prevent disturbance of the interface between the first
and second liquid phases. The person skilled in the art
will understand that the element can be shaped in various
ways to avoid liquid flow along the wall and/or splashing
as described above. As an example, the element may
comprise a baffle plate or the like located just above
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the liquid outlet. Also, the liquid outlet may be
sticking into the vessel, as a result of which the liquid
outlet opening is at a selected distance from the wall of
the vessel.
In step (e), the contaminant-depleted absorbent
stream as removed in step (d) is reused. The contaminant-
depleted absorbent stream may be reused in several places
but is preferably at least partly reused in the absorber
of step (b) as the absorber solution.
According to a preferred embodiment, the contaminant-
depleted absorbent stream as removed in step (d) is
cooled before reusing in step (e), thereby obtaining a
cooled contaminant-depleted absorbent stream. By
sufficient cooling of the contaminant-depleted absorbent
stream as removed in step (d) the two separate liquid
phases form a single phase which is of benefit in the
absorber. Hence, typically, the cooled contaminant-
depleted absorbent stream is a single phase. The person
skilled in the art will readily understand that the
amount of cooling needed for forming the single phase
will depend on the composition of the absorbent stream;
also, the person skilled in the art will understand how
to determine the suitable amount of cooling.
Further, it is preferred that the cooled contaminant-
depleted absorbent stream is passed to a collector before
reusing in step (e), in particular if the contaminant is
to be reused in the absorber. Typically, the collector
will be a simple vessel; the
volume of collector vessel
is typically at least the volume of the two liquid phases
as formed in step (c).
Preferably, the optionally cooled contaminant-
depleted absorbent stream is reused in the absorber as
the absorbent solution.
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Hereinafter the invention will be further illustrated
by the following non-limiting drawings. Herein shows:
Fig. 1 a schematic embodiment of a method according
to the present invention;
Fig. 2 a close-up of the bottom of the regenerator as
used in Fig. 1; and
Figs 3-6 close-ups of alternative embodiments of a
bottom of the regenerator as used in Fig. 1.
For the purpose of this description, same reference
numbers refer to same or similar components. Also, a
single reference number will be assigned to a line as
well as to a stream carried in that line.
Fig. 1 shows a simplified embodiment of a method in
accordance with the present invention for removing a
contaminant from a contaminated stream 10. An apparatus 1
is shown comprising an absorber 2, a regenerator 3, a
reboiler 4, a collector 5, pumps 6 and 8 and heat
exchangers 7 and 9. Pump 6 is preferred in case of
operation of the absorber 2 below about 5 bara; at higher
absorber pressures, pump 6 is typically not present.
During use, the (preferably gaseous) contaminated
stream 10 is, after feeding via inlet 21, contacted in
the absorber 2 with an absorbent solution 80 fed via
inlet 24, thereby obtaining a contaminant-depleted stream
20 (removed at overhead outlet 22) and a contaminant-
enriched absorbent stream 30 (removed at bottom outlet
23).
The contaminant-enriched absorbent stream 30 is sent
(using pump 6, if present; otherwise by the positive
pressure difference between absorber 2 and regenerator 3)
to the regenerator 3, via a heat exchanger 7 in which the
contaminant-enriched absorbent stream 30 is heated to
obtain a heated contaminant-enriched absorbent stream 40.
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The person skilled in the art will readily understand
that the regenerator 3 may be a single vessel or column
or may instead comprise several vessels and columns; in
the embodiment of Figure 1, the regenerator comprises a
single column (with packing 37). The heated contaminant-
enriched absorbent stream 40 is fed into regenerator 3 at
inlet 31 and separated in the regenerator 3, thereby
obtaining a contaminant-depleted absorbent stream 60 at
the bottom of the regenerator 3 and a contaminant-
enriched stream 50 which is removed at overhead outlet
32. The contaminant-depleted absorbent stream 60 forms
two separate phases in the regenerator 3 (which in the
embodiment of Fig. 1 are collected at the bottom
thereof), viz, a lower layer of a first water-enriched
liquid phase A and a higher layer of a second water-
depleted (and typically amine rich) liquid phase B (which
is further explained in Fig. 2).
The first liquid phase A and the second liquid phase
B of the contaminant-depleted absorbent stream 60 are
simultaneously removed at the liquid outlet 33 of the
regenerator 3; as a result, a short time after the
beginning with the removal of the first and second liquid
phases, the two liquid phases A,B are removed in the same
ratio as they are formed, ensuring that the absorbent
content of the removed stream 60 remains substantially
constant over time. Also, as a result, the interface X of
the first liquid phase A and the second liquid phases B
will be situated at the height level of the outlet 33.
Subsequently, the removed contaminant-depleted
absorbent stream 60 is reused. In the embodiment of
Figure 1, the contaminant-depleted absorbent stream 60 is
reused in the absorber 2 as the absorbent solution 80 fed
via inlet 24.
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In the embodiment of Figure 1, the contaminant-
depleted absorbent stream 60 is cooled before being
reused in the absorber 2, thereby obtaining a cooled
contaminant-depleted absorbent stream 70; this cooling
allows the two separate liquid phases A,B to form a
single phase. Further, the cooled contaminant-depleted
absorbent stream 70 is passed to a collector 5 before
being reused in the absorber 2.
In the embodiment of Figure 1, a part 90 of the first
liquid phase A is removed (at outlet 34) from the
regenerator 3, heated in a reboiler or heater 4 and
reintroduced as stream 100 into the regenerator 3 at a
point (inlet 35) above the interface X of the first
liquid phase A and the second liquid phase B. Preferably,
the stream 100 is reintroduced at a point above the
second liquid phase B (see e.g. Fig. 3).
Figure 2 shows a close-up of the bottom of the
regenerator 3 as used in Fig. 1. As can be seen, liquid
is drawn off at interface X of the first liquid phase A
and the second liquid phase B of the contaminant-depleted
absorbent stream 60. Typically, the area C above the
second liquid phase B is filled with gas. Preferably, and
as shown, the outlet 33 is positioned as a side draw-off
outlet having an opening placed at a distance from the
wall 36 of the regenerator 3. Alternatively (see also
Fig. 5), the outlet 33 may be a (substantially vertical)
pipe inserted from the bottom of the regenerator 3.
Further, and as shown, the outlet 33 is located above
at least 1/20 of the liquid height in the bottom of the
regenerator 3, preferably above at least 1/10, more
preferably above at least 1/4.
If desired, the liquid outlet 33 comprises an element
(not shown in Fig. 1; cf. plate 38 in Fig. 3) for
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avoiding liquid flow along the wall 36 and into the
liquid outlet 33. Further, the liquid outlet 33 may
comprise a splash protector (not shown in Fig. 1; see
plate 39 in Fig. 4) to prevent disturbance of the
interface X between the first liquid phase A and the
second liquid phase B.
Figures 3-6 show close-ups of alternative embodiments
of a bottom of the regenerator 3 as used in Fig. 1.
In Figure 3, the outlet 33 opens at the wall 36 of
the regenerator 3 (so not at a distance from the wall 36
as in Fig. 2). Further, in the embodiment of Figure 3, a
plate 38 is placed just above the outlet 33 for avoiding
liquid flow along the wall 36 and into the outlet 33.
In Figure 4, the regenerator 3 is provided with a
(anti-)splash plate 39 and a (substantially vertically
arranged) baffle plate 41. The baffle plate 41 is
provided with openings 42. The baffle plate 41 assists in
creating a stabile interface X between the first liquid
phase A and the second liquid phase B, at least near the
outlet 33.
In Figure 5, the outlet 33 is a (substantially
vertically arranged) pipe inserted from the bottom of the
regenerator 3. If desired, further internals such as
plate 38 (see Fig. 3), splash plate 39 and baffle plate
41 (see Fig. 4) may be present in the embodiment of
Figure 5.
In Figure 6, the bottom of the regenerator 3 is in
the form of a draw-off tray 51 (and hence is not the
absolute bottom of the regenerator 3). The draw-off tray
51 is provided with channels 52 allowing gas passage
through the tray 51; hence the area D below the tray 51
and the area C above the second liquid phase C are in gas
communication.
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The person skilled in the art will readily understand
that many modifications may be made without departing
from the scope of the invention. Further, the person
skilled in the art will readily understand that, while
the present invention in some instances may have been
illustrated making reference to a specific combination of
features and measures, many of those features and
measures are functionally independent from other features
and measures given in the respective embodiment(s) such
that they can be equally or similarly applied
independently in other embodiments.
