Process for dehydrating and stripping a wet natural gas

Details Process for dehydrating and stripping a wet natural gas Process for dehydrating and stripping a wet natural gas

2002-03-29

2004-09-21

Smith, Duane S. (Department: 1724)

Gas separation: processes

Liquid contacting

And degasification of a liquid

C095S179000, C095S180000, C095S193000, C095S231000, C095S209000

Reexamination Certificate

active

06793714

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to an improved process for dehydrating and fractionating a high-pressure wet natural gas.
BACKGROUND OF THE INVENTION
In French patent FR-B-2,787,870, the applicant has described a process allowing to fractionate a high-pressure natural gas (for example above 5 MPa) containing hydrocarbon constituents referred to as <<heavy>> constituents, consisting of hydrocarbons having at least 3 carbon atoms, and constituents referred to as <<light>> constituents, essentially consisting of methane and ethane.
This process comprised in combination at least the following stages:
1) cooling the gas from T
0
to a temperature T
1
,
2) separating gas phase G
1
from liquid phase L
1
obtained during cooling stage (1),
3) sending at least part of gas phase G
1
from separation stage (2) to an expansion stage (X
1
) so as to obtain a mixed phase M
2
at a temperature T
2
and a pressure P
2
,
4) sending mixed phase M
2
to a stage of fractionation by heat exchange (7) wherein it serves as a cooling agent, after which it is heated,
5) sending liquid phase L
1
to an expansion stage (V),
6) sending heated mixed phase M
2
and expanded liquid phase L
1
to a separation stage so as to obtain a gas phase and a liquid phase, and
7) fractionating the gas phase by distillation carried out by means of continuous heat exchange with mixed phase M
2
and extracting the <<light>> constituents in form of gas and the <<heavy>> constituents in form of condensates, fractionating stage (7) being carried out after expansion stage (3) producing mixed phase M
2
.
Liquid phase L
1
expanded during stage (5) can be sent to a stabilization stage in order to obtain stabilized condensates and a gas phase G
3
to be fractionated, sent to separation stage (6).
It is also possible to use at least part of the scrubbed gas from fractionating stage (7) as an additional cooling agent for this stage.
It is also possible to use at least part of the scrubbed gas to cool the gas during cooling stage (1).
The process according to the prior document is described hereafter in connection with FIG.
1
.
The natural gas to be fractionated is sent at high pressure P
0
and at a temperature T
0
through line
1
into a heat exchanger E
1
. Inside E
1
, it is cooled by heat exchange with cooling water circulating in line
2
or sea water, or air. The cooled gas sent through line
3
is then cooled in a second heat exchanger E
2
to a temperature T
1
. Heat exchange is for example performed by means of at least part of the scrubbed gas from the fractionating and purification process, circulating through line
18
.
The cooled mixed phase comprising a gas phase and condensates from exchanger E
2
is fed through a line
4
into a separation device, a separating drum
5
for example. In this separating drum, the condensates are separated, a gas phase G
1
is extracted at the top of the drum through a line
6
and the separated condensates or L
1
are extracted at the bottom of the drum through a line
7
.
Gas phase G
1
is sent to an expansion device such as an expander X
1
so as to obtain a mainly gaseous mixed phase M
2
cooled by expansion to a temperature T
2
. This cooled mixed phase M
2
is used as a cooling agent during the fractionating and purification stage carried out in exchanger-dephlegmator D
1
described hereafter.
Liquid phase L
1
consisting of the condensed C
3
+
and of part of the C
1
and C
2
is expanded for example through an expansion valve V. Two-phase fluid M
3
resulting from this expansion is for example sent through a line
8
in a stabilization column
9
. A gas phase G
3
is discharged at the top of stabilization column
9
, through a line
10
, and the stabilized condensates L
3
are discharged at the bottom through a line
11
.
The stabilization column is for example reboiled by means of a hot-oil exchanger E
3
. The C
3
+
mixture at the bottom of the column only contains a small amount of light products (C
1
and C
2
).
The fractionating and purification system described in the prior document comprises an assembly consisting of at least one dephlegmator D
1
associated with a separating drum B
1
. Dephlegmator D
1
is for example a plate exchanger known to the man skilled in the art, which comprises passages whose size and geometry are suited for circulation of the liquid and gas phases; these passages are referred to as <<passes>> within the scope of this application. Dephlegmator D
1
comprises at least two passes, one pass P
1
suited for circulation of a fluid, for example mainly gaseous mixed phase M
2
from expander X
1
, serving as a cooling agent, and a pass P
2
or <<reflux pass>> in which the gas to be fractionated circulates upwards. As a result of cooling through thermal exchange with mixed phase M
2
, a condensation occurs in reflux pass P
2
as the condensed liquid causes a distillation effect while flowing down. The exchanger-dephlegmator can also comprise a third pass P
3
and possibly other passes.
Mixed phase M
2
from expander X
1
is sent through a line
12
into first pass P
1
of the dephlegmator where it circulates with a descending flow as shown by the dotted line in FIG.
1
. After acting as a cooling agent, this mixed phase, heated to a temperature T
3
in relation to its inlet temperature T
2
and depleted in liquid, is extracted through a line
13
.
This mixed phase and gas phase G
3
extracted from stabilization column
9
through line
10
are fed and mixed together in separating drum B
1
. The gas phase and the liquid phase separate inside separating drum B
1
.
The condensates (or liquid phase) separated in drum B
1
are extracted through a line
14
and sent by a pump
15
and through a line
16
to be mixed with two-phase mixture M
3
from expansion valve V. These condensates partly consist of the liquid of the mixture separated in B
1
, and of the liquid condensed in the reflux pass.
The gas phase obtained by separation in separating drum B
1
is at the dew point. It circulates in an ascending flow in reflux pass P
2
and cools down as it flows therethrough. In this pass P
2
, the liquid condensed by heat exchange with mixed phase M
2
circulating in a descending flow in pass P
1
circulates with a descending flow and causes a distillation effect. A scrubbed gas is thus obtained, which is discharged through a line
17
at the top of exchanger-dephlegmator D
1
. The scrubbed gas is at a temperature T
4
close to T
2
(temperature of mixed phase M
2
at the expander outlet). This scrubbed gas has in most cases lost between 90 and 99% of the propane present in the feed introduced through line
1
.
Scrubbed gas G
4
extracted through line
17
is for example re-introduced into a third pass P
3
of dephlegmator D
1
to be used as a second cold source. It circulates with a descending flow in P
3
, cocurrent to the circulation of mixed phase M
2
and countercurrent to the direction of circulation of the gas phase separated in the dephlegmator drum. At the outlet of this third pass P
3
, a scrubbed and heated gas flow G
5
(temperature T
3
) is for example recycled through a line
18
to heat exchanger E
2
. The gas, after being used as cooling agent and therefore heated in heat exchanger E
2
, is sent to a compressor C
1
prior to being exported through a line
19
. Compressor C
1
is for example driven by expander X
1
.
Document FR-B-2,787,870 also described a process wherein a wet natural gas (that had not been subjected to a previous drying treatment) was treated, this process using methanol to prevent hydrate formation between the water and the gas during cooling thereof.
During cooling of the gas, a liquid phase containing water and methanol was thus collected besides a hydrocarbon liquid phase consisting of the condensates, i.e. C
3
+
hydrocarbons or NGL.
The process then comprised in combination at least the following stages:
(a) a first part of the gas to be scrubbed is contacted with aqueous liquid phase L′ comprising aqueous liq

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