U.S. patent application number 10/521944 was filed with the patent office on 2006-01-12 for method for liquefying a hydrocarbon-rich flow while simultaneously obtaining a c3/c4-rich fraction.
Invention is credited to Martin Gwinner, Pentti Paurola, Rudolf Stockmann.
Application Number | 20060005573 10/521944 |
Document ID | / |
Family ID | 30128278 |
Filed Date | 2006-01-12 |
United States Patent
Application |
20060005573 |
Kind Code |
A1 |
Stockmann; Rudolf ; et
al. |
January 12, 2006 |
Method for liquefying a hydrocarbon-rich flow while simultaneously
obtaining a c3/c4-rich fraction
Abstract
A process for liquefying a flow of natural gas with simultaneous
recovery of a C.sub.3/C.sub.4-rich fraction is described, whereby
the liquefaction of the flow of natural gas is carried out in heat
exchange for at least one refrigerant and/or mixed refrigerant
flow, and the flow of natural gas that is to be liquefied, after
precooling, is subjected to a rectifying column, in which higher
hydrocarbons are separated from the flow of natural gas, and then
is subjected to further cooling and liquefaction, whereby a
C.sub.2+-rich fraction that is recovered in the subsequent cooling
of the flow of natural gas is fed to the rectifying column as a
reflux liquid. According to the invention, a C.sub.4/C.sub.5-rich
fraction (20, 35) is fed directly and/or indirectly to rectifying
column (T1) as an additional reflux liquid, whereby the feed point
of the C.sub.4/C.sub.5-rich [fraction (20, 35) is located at the
top of rectifying column (T1) or is identical to the feed point of
the C.sub.2+-rich fraction (5).]
Inventors: |
Stockmann; Rudolf; (Buchloe,
DE) ; Gwinner; Martin; (Polling, DE) ;
Paurola; Pentti; (Hafrsfjord, NO) |
Correspondence
Address: |
MILLEN, WHITE, ZELANO & BRANIGAN, P.C.
2200 CLARENDON BLVD.
SUITE 1400
ARLINGTON
VA
22201
US
|
Family ID: |
30128278 |
Appl. No.: |
10/521944 |
Filed: |
July 1, 2003 |
PCT Filed: |
July 1, 2003 |
PCT NO: |
PCT/EP03/07003 |
371 Date: |
July 25, 2005 |
Current U.S.
Class: |
62/620 |
Current CPC
Class: |
F25J 3/0247 20130101;
F25J 2220/60 20130101; F25J 3/0209 20130101; F25J 2270/90 20130101;
F25J 3/0242 20130101; F25J 3/0233 20130101; F25J 2200/30 20130101;
F25J 2205/50 20130101; F25J 2215/64 20130101; F25J 2245/02
20130101; F25J 2260/20 20130101 |
Class at
Publication: |
062/620 |
International
Class: |
F25J 3/00 20060101
F25J003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 23, 2002 |
DE |
102 33 410.2 |
Claims
1. Process for liquefying a hydrocarbon-rich flow, in particular a
flow of natural gas, with simultaneous recovery of a
C.sub.3/C.sub.4-rich fraction, whereby the liquefaction of the
hydrocarbon-rich flow is carried out in heat exchange for at least
one refrigerant and/or mixed refrigerant flow, and the
hydrocarbon-rich flow that is to be liquefied, after precooling, is
subjected to a rectifying column, in which higher hydrocarbons are
separated from the hydrocarbon-rich flow that is to be liquefied,
and then is subjected to further cooling and liquefaction, whereby
a C.sub.2+-rich fraction that is recovered in the subsequent
cooling of the hydrocarbon-rich flow is fed to the rectifying
column as a reflux liquid, characterized in that a C.sub.4
/C.sub.5-rich fraction (20, 35) is fed to rectifying column (T1) as
an additional reflux liquid, whereby the feed point of the
C.sub.4/C.sub.5-rich fraction (20, 35) is located above the feed
point of C.sub.2+-rich fraction (5), and a mass transfer zone (M)
is provided between the feed point of the C.sub.4/C.sub.5 -rich
fraction (20, 35) and the feed point of C.sub.2+-rich fraction
(5).
2. Process according to claim 1, in which the higher hydrocarbons
that are recovered in the rectifying column are separated by
rectification in several steps, whereby one of these steps
comprises the feeding of higher hydrocarbons in a depropanizer
(T2), wherein the C.sub.4/C.sub.5-rich fraction (20, 35) that is
fed to rectifying column (T1) as an additional reflux liquid is
recovered in a depropanizer side column (T3), which is fed to a
C.sub.4+-rich fraction (14) that is drawn off from depropanizer
(T2).
3. Process according to claim 1, in which the higher hydrocarbons
that are recovered in the rectifying column are separated by
rectification in several steps, whereby one of these steps
comprises the feeding of higher hydrocarbons in a debutanizer,
wherein the C.sub.4/C.sub.5-rich fraction (20, 35) that is fed to
rectifying column (T1) as an additional reflux liquid is recovered
in the debutanizer.
4. Process according to claim 1, wherein the C.sub.4/C.sub.5-rich
fraction (20, 35) that is fed to rectifying column (T1) as an
additional reflux liquid is recovered in a side column (T4) of
rectifying column (T1) by a C.sub.4+-rich fraction (30) being fed
to this side column (T4) from rectifying column (T1).
5. Process according to one of the preceding claims 1 to 4, wherein
the C.sub.4/C.sub.5-rich fraction (20, 35) that is fed to
rectifying column (T1) as an additional reflux liquid is cooled
before the feeding thereof (E5, E7).
6. Process according to claim 5, wherein the C.sub.4/C.sub.5-rich
fraction (20, 35) that is fed to rectifying column (T1) as an
additional reflux liquid is at least partially condensed in its
cooling (E5, E7).
7. Process according to one of the preceding claims 1 to 6, wherein
the benzene content of the C.sub.4/C.sub.5-rich fraction (20, 35)
that is fed to rectifying column (T1) as an additional reflux
liquid is less than 500 ppm, preferably less than 300 ppm.
Description
[0001] The invention relates to a process for liquefying a
hydrocarbon-rich flow, in particular a flow of natural gas, with
simultaneous recovery of a C.sub.3/C.sub.4-rich fraction, whereby
the liquefaction of the hydrocarbon-rich flow is carried out in
heat exchange for at least one refrigerant and/or mixed refrigerant
flow, and the hydrocarbon-rich flow that is to be liquefied, after
precooling, is subjected to a rectifying column, in which higher
hydrocarbons are separated from the hydrocarbon-rich flow that is
to be liquefied, and then is subjected to further cooling and
liquefaction, whereby a C.sub.2+-rich fraction that is recovered in
the subsequent cooling of the hydrocarbon-rich flow is fed to the
rectifying column as a reflux liquid.
[0002] Processes of this type that are carried out in particular in
natural-gas liquefaction plants are designed either as so-called
LNG Baseload Plants--i.e., plants for liquefying natural gas for
supply with natural gas as a primary energy--or as so-called Peak
Shaving Plants--i.e., plants for liquefying natural gas to cover
peak demand.
[0003] The above-mentioned Peak Shaving Plants are operated with
turboexpanders or refrigerant mixtures in refrigeration circuits.
The refrigeration circuits often contain only one or a few
components.
[0004] In these plants, the heavy hydrocarbons that are contained
in the natural gas must be separated to produce the inventory of
the mixture circuits, to cover losses during operation and because
of product requirements.
[0005] This usually happens in that the hydrocarbon-rich flow that
is to be liquefied is precooled to a temperature of -10 to
-25.degree. C. and then is fed to a rectifying column. By means of
a suitable column configuration, the higher hydrocarbons--here, the
C.sub.3+-rich hydrocarbons as well as benzene are meant--are then
separated from the flow to be liquefied in the rectifying column.
At the top of the rectifying column a C.sub.2--rich fraction is
drawn off and subjected to further cooling as well as liquefaction.
In this case, the C.sub.3+-hydrocarbon fraction that is recovered
at the bottom of the rectifying column is concentrated with higher
hydrocarbons to the extent that the rectifying column is set to the
desired heating value in the C.sub.2--rich top product.
[0006] The fraction that contains higher hydrocarbons and that is
separated from the flow that is to be liquefied in the rectifying
column is normally then separated into its components by
rectification, whereby the latter are partially used as so-called
Make-up Fractions--for example C.sub.2H.sub.6 or
C.sub.3H.sub.8--for the mixture circuit or circuits or are
recovered as additional product flows and optionally are fed to a
further processing.
[0007] The separation of the above-mentioned C.sub.3+-rich fraction
is performed with, for example, the following rectifying columns:
demethanizer, de-ethanizer, depropanizer and optionally
debutanizer. These columns allow the production of the following
fractions: methane, ethane and ethane make-up, propane and propane
make-up, LPG (liquid gas) as well as a C.sub.5+-fraction.
[0008] It is disadvantageous in the above-described process,
however, that the yield of C.sub.4-hydrocarbons is comparatively
low.
[0009] A generic process, in which in addition to the recycled
C.sub.2+-rich fraction, a C.sub.4 fraction is fed to the rectifying
column as an additional reflux liquid, is known from the technical
article "LPG--Recovery Processes for Baseload LNG Plants Examined"
from OIL AND GAS JOURNAL (November 1997). This process allows a
comparatively high C.sub.3 yield, but the expense as far as
processing is concerned for the production of the C.sub.4 fraction
that is required as a reflux liquid is significant.
[0010] The object of this invention is to indicate a generic
process that makes possible the recovery of a C.sub.3/C.sub.4-rich
fraction--the so-called LPG fraction--with as high a C.sub.3 yield
as possible.
[0011] To achieve this object, it is proposed that a
C.sub.4/C.sub.5-rich fraction be fed to the rectifying column as an
additional reflux liquid, whereby the feed point of the
C.sub.4/C.sub.5-rich fraction is at the top of the feed point of
the C.sub.2+-rich fraction, and a mass transfer zone is provided
between the feed point of the C.sub.4/C.sub.5-rich fraction and the
feed point of the C.sub.2+-rich fraction.
[0012] To produce the C.sub.4/C.sub.5-rich faction that is required
as an additional reflux liquid, two processes that can also be
combined are now conceivable.
[0013] If the higher hydrocarbons that are recovered in the
rectifying column are separated by rectification in several steps
that are downstream from the rectifying column, whereby one of
these steps comprises the feeding of the higher hydrocarbons into a
depropanizer, the C.sub.4/C.sub.5-rich fraction that is fed to the
rectifying column as an additional reflux liquid can be recovered
in a depropanizer side column, to which a C.sub.4+-rich fraction
that is drawn off from the depropanizer is fed. If the separation
of . . . by rectification that is downstream from the rectifying
column comprises . . . corresponding to an alternative,
advantageous embodiment of the process according to the
invention.
[0014] To produce the C.sub.4/C.sub.5-rich faction that is required
as an additional reflux liquid, two processes that can also be
combined are now conceivable.
[0015] If the higher hydrocarbons that are recovered in the
rectifying column are separated by rectification in several steps
downstream from the rectifying column whereby one of these steps
comprises the feeding of the higher hydrocarbons into a
depropanizer, the C.sub.4/C.sub.5-rich fraction that is fed to the
rectifying column as an additional reflux liquid can be recovered
in a depropanizer side column, to which a C.sub.4+-rich fraction
that is drawn off from the depropanizer is fed. If the separation
of higher hydrocarbons by rectification downstream from the
rectifying column comprises a debutanizer, the C.sub.4/C.sub.5-rich
fraction that is required as an additional reflux liquid can also
be produced in the latter.
[0016] The possibility also exists of recovering the
C.sub.4/C.sub.5-rich fraction that is fed to the rectifying column
as an additional reflux liquid in a side column of the rectifying
column by a C.sub.4+-rich fraction being fed to this side column
from the rectifying column.
[0017] The process according to the invention as well as additional
embodiments thereof that represent the subjects of the dependent
claims are explained in more detail below based on the embodiments
that are shown in FIGS. 1 and 2.
[0018] In this connection:
[0019] FIG. 1: shows an embodiment of the process according to the
invention, in which the C.sub.4/C.sub.5-rich fraction is recovered
in a depropanizer side column
[0020] FIG. 2: shows an embodiment of the process according to the
invention, in which the C.sub.4/C.sub.5-rich fraction is recovered
in a side column of the rectifying column.
[0021] According to the process that is shown in FIG. 1, a
precooled and--if necessary--pretreated flow of natural gas, which
has a temperature of between -10 and -25.degree. C., is fed via
line 1 to rectifying column T1. The optionally necessary
pretreatment steps, such as, for example, drying, CO.sub.2 removal,
sulfur removal, etc., are not considered in more detail below; the
standard processes are known to one skilled in the art. The flow of
natural gas that is brought in via line 1 typically has a pressure
of between 30 and 90 bar.
[0022] A C.sub.2--rich fraction is drawn off at the top of
rectifying column T1 via line 2, and it is cooled in heat exchanger
E1 to a temperature of between -25 and -55.degree. C. and in this
case partially condensed. The partially condensed flow is then fed
via line 3 to a separator D.
[0023] According to an advantageous embodiment of the process
according to the invention, the benzene content of the
C.sub.4/C.sub.5-rich fraction that is fed to rectifying column T1
as an additional reflux liquid--on which still more detail is given
below--is less than 500 ppm, preferably even less than 300 ppm.
This has the result that the C.sub.2--rich fraction that is drawn
off at the top of rectifying column T1 via line 2 has a benzene
content of less than 1 ppm. An undesirable freezing-out of the
benzene in the liquefaction part downstream from rectifying column
T1 can thus be effectively avoided.
[0024] A C.sub.1-rich fraction is drawn off via line 4 at the top
of separator D and subjected to further cooling and liquefaction,
not shown in FIGS. 1 and 2.
[0025] A C.sub.2+-rich fraction is drawn off via line 5 from the
bottom of separator D and fed to rectifying column T1 in its upper
area as a reflux liquid.
[0026] A mass transfer zone M is arranged above the feed point of
the C.sub.2+-rich fraction that is fed via line 5 to rectifying
column T1. The latter typically has 3 to 10 additional floors; this
corresponds to about 2 to 7 theoretical floors.
[0027] A C.sub.3+-rich fraction is drawn off via line 6 from the
bottom of rectifying column T1 and optionally subjected to
additional process steps. In this connection, a partial flow of the
C.sub.3+-rich fraction that is drawn off from the bottom of
rectifying column T1 is fed as a reboiler stream to rectifying
column T1 via line 7, in which a heat exchanger E2 is arranged.
[0028] The normally multistage rectification of the C.sub.3+-rich
fraction that is drawn off from the bottom of rectifying column T1
via line 6 is also sufficiently known to one skilled in the art
with its different process variants. In this connection, line area
6' that is shown in dotted lines stood for the most varied
processes, thus, for example, the feeding of the C.sub.3+-rich
fraction, drawn off via line 6, in a demethanizer as well as a
downstream de-ethanizer. A C.sub.3+-hydrocarbon-rich fraction that
is treated as always is fed to depropanizer T2 via line 6''.
[0029] A C.sub.3-hydrocarbon product fraction is drawn off via line
9 at the top of depropanizer T2, and it is cooled in heat exchanger
E3. A partial flow of this fraction is then fed as reflux via line
10 to depropanizer T2. Product flows with a high C.sub.3 purity can
be drawn off in gaseous or liquid form via lines 22 and 23; the
latter are used, for example, as C.sub.3 make-up for the
refrigerant mixture circuits. The residual portion of the
C.sub.3/C.sub.4 product fraction that is drawn off at the top of
depropanizer T2 via line 12--the so-called LPG product fraction--is
mixed in; the thus combined fractions are then released via line 13
and optionally fed to further processing.
[0030] A C.sub.5+-rich hydrocarbon fraction is drawn off via fine 8
from the bottom of depropanizer T2, and if desired, it is also fed
to further processing.
[0031] A C.sub.4+-hydrocarbon fraction is removed in depropanizer
T2 via side hood 14, and it is fed to side column T3. Side column
T3 is used in particular to remove benzene to a large extent from
the fed C.sub.4+-hydrocarbon fraction. For this purpose, a
C.sub.4/C.sub.5-rich fraction is drawn off at the top of column T3
via line 16, cooled in heat exchanger E4 and fed via line 17 to a
branch point, at which a partial flow is fed as reflux via line 18
to column T3, while the residual portion of the
C.sub.4/C.sub.5-rich fraction is fed via lines 19 and 20 to
rectifying column T1 as an additional reflux liquid.
[0032] A C.sub.5+-rich fraction is drawn off via fine 15 from the
bottom of side column T3 and fed to depropanizer T2 in its lower
area.
[0033] The C.sub.4/C.sub.5-rich fraction that is fed to rectifying
column T1 as an additional reflux liquid is sub-cooled in a heat
exchanger E5 preferably to a temperature of between -30 and
+50.degree. C. The optimum starting temperature of the
C.sub.4/C.sub.5-rich fraction that is fed to rectifying column T1
is determined essentially by the conditions within rectifying
column T1.
[0034] Instead of indirectly feeding the C.sub.4/C.sub.5-rich
fraction that is used as additional reflux liquid, the latter can
also--completely or partially--be fed via line 21, shown in dashes
and dots, to separator D. The (partial) flow that is fed to
separator D was then fed from its bottom via line 5 with the
C.sub.2+-rich fraction of rectifying column T1.
[0035] In the embodiment of the process according to the invention
that is depicted in FIG. 2, the C.sub.4/C.sub.5-rich fraction that
is required as an additional reflux liquid now is not recovered in
a side column of depropanizer T2, but rather in a side column 14
that is associated with rectifying column T1.
[0036] To this end, a flow is removed from rectifying column T1 in
its lower area via line 30, and it is fed to side column T4. A
C.sub.5+-rich fraction is drawn off via line 31 from the bottom of
this side column T4, and fed to rectifying column T1.
[0037] A C.sub.4/C.sub.5-rich fraction is drawn off via line 32 at
the top of side column T4 and cooled in heat exchanger E6; a
partial flow of this fraction is fed as reflux via line 33 of side
column T4. The residual flow of the C.sub.4/C.sub.5-rich fraction
that is drawn off via line 32 from the top of side column T4 forms
the additional reflux fraction for rectifying column T1 and is fed
to the latter via lines 34 and 35.
[0038] In turn, a heat exchanger E7, which is used in the
cooling-off or sub-cooling of the C.sub.4/C.sub.5-rich fraction,
can be provided in line 34.
[0039] Also, with this approach, the C.sub.4/C.sub.5-rich fraction
that is used as an additional reflux liquid can be fed to separator
D either partially or completely via line 36 that is shown in
dashes and dots.
[0040] It is to be noted that an approach is selected, in which the
C.sub.3 content of the C.sub.4/C.sub.5-rich fraction that is fed to
rectifying column T1 is as small as possible to be able to carry
out a separation of the C.sub.3+ components with the highest
possible yield in rectifying column T1. Also, the content of
C.sub.4 components in the C.sub.4/C.sub.5-rich fraction that is fed
to rectifying column T1 should be as small as possible not to
impair the high yield of C.sub.4-hydrocarbons already achieved in
rectifying column T1. Consequently, the content of
C.sub.5-hydrocarbons should be selected as large as possible to
produce an efficient separation of C.sub.3+-hydrocarbons.
[0041] The LPG that is recovered by means of the process according
to the invention is therefore of great economic value, since LPG
can be liquefied at ambient temperature, has a high energy content
and is readily transportable. Moreover, LPG can undergo combustion
in a nonpolluting manner. The C.sub.3 yield of the process
according to the invention is more than 60%; that of the LPG's is
more than 90%.
* * * * *