High-pressure gas fractionating process and system

Details High-pressure gas fractionating process and system High-pressure gas fractionating process and system

1999-12-23

2001-11-20

Bennett, Henry (Department: 3744)

Refrigeration

Cryogenic treatment of gas or gas mixture

Separation of gas mixture

Reexamination Certificate

active

06318119

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a high-pressure gas fractionating process during which at least part of the gas is expanded in order to serve as a cooling agent, expansion being carried out prior to the fractionation and scrubbing operation, the latter being carried out in a device allowing to perform simultaneously distillation and heat exchange. This device is for example a dephlegmator exchanger.
BACKGROUND OF THE INVENTION
The prior art describes various processes and industrial plants for selectively extracting propane and compounds heavier than propane, or ethane and compounds heavier than ethane.
In most cases, the gas to be processed is partly condensed, either by external low-temperature refrigeration, or by means of an expander turbine, prior to being separated in a separating drum. The liquid and vapour phases recovered are thereafter sent to a conventional multilevel distillation column. The wanted heavy compounds are recovered at the bottom of this column in the liquid form, and the scrubbed gas is recovered as vapour distillate. The condenser of the column requires very low-temperature external refrigeration.
More elaborate configurations using a dephlegmator have been described. For example, the Extrapack layout (marketed by NAT and IFP) replaces the distillation column head by a dephlegmator exchanger. An external refrigeration is still used with this configuration.
U.S. Pat. No. 4,921,514 describes a very elaborate layout using both a dephlegmator and a distillation column, the condenser using an external refrigeration cycle.
U.S. Pat. No. 4,519,825 describes a layout with a dephlegmator. The gas coming from the separator is directly sent to the high-pressure reflux pass of the dephlegmator, and the cold in the reflux exchanger is produced by the scrubbed gas that has been expanded through an expander after passage in the exchanger. This layout is suited for processing gases at a pressure below 4 MPa at the inlet.
SUMMARY OF THE INVENTION
The process according to the invention consists in proposing a new process and device configuration where the expansion operation on the gaseous fraction used as cooling agent is performed prior to the fractionating and scrubbing stage. The fractionating and scrubbing stage or separation is carried out in a dephlegmator exchanger for example.
The expression “dephlegmator exchanger” designates, for the present description, all the devices suited to carry out simultaneously a heat exchange and a distillation operation (rectification part).
Similarly, the expression “stripper exchanger” designates a device allowing to perform simultaneously heat exchanges and distillation (stripping part).
Similarly, the expression “high-pressure gas” designates a gas having a pressure at least equal to 5 MPa.
The invention relates to a process allowing to fractionate a gas comprising constituents referred to as heavy constituents and constituents referred to as light constituents, the gas being initially at a temperature T
0
and at a pressure P
0
.
It is characterized in that it comprises in combination at least the following stages:
a) cooling the gas from T
0
to a temperature T
1
,
b) separating the gas phase G
1
from the liquid phase L
1
obtained during cooling stage a),
c) sending at least part of gas phase G
1
from separation stage b) to an expansion stage (X
1
) so as to obtain a mixed phase M
2
at a temperature T
2
and at a pressure P
2
,
d) sending mixed phase M
2
to a heat exchange stage (stage g) where it acts as a cooling agent, and after which it is heated,
e) sending liquid phase L
1
to an expansion stage (V),
f) sending the heated mixed phase and the expanded liquid phase to a separation stage in order to obtain a gas phase and a liquid phase,
g) fractionating the gas phase by distillation performed by means of continuous heat exchange with mixed phase M
2
, and extracting the light constituents as gas and the heavy constituents as condensates, the fractionation stage being carried out after the stage of expansion of mixed phase M
2
.
The expanded liquid phase 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 f).
At least part of the scrubbed gas from fractionation stage g) is for example used as additional cooling agent for this stage.
At least part of the scrubbed gas can be used for cooling the gas during refrigeration stage a).
Stage a) is for example cooled so as to obtain a temperature below −15° C.
Expansion stage c) can be carried out to obtain a gas at a pressure below 2 MPa.
It can comprise for example a dehydration stage prior to refrigeration stage a).
It comprises for example a stage wherein said scrubbed gas is sent to a compression stage, at least one scrubbed and compressed gas fraction G
R1
is diverted, said fraction is sent to a refrigeration and expansion stage after which a mixed phase is obtained, the liquid phase is separated from the gas phase, and the liquid phase is used as reflux supplement in the distillation stage.
The gas fraction is for example cooled and liquefied to a temperature below −100° C.
The invention also relates to the system for carrying out fractionation of a gas comprising constituents referred to as light constituents and constituents referred to as heavy constituents, comprising in combination:
gas refrigeration means and a separation device at the outlet of which a gas phase (G
1
) and a liquid phase (L
1
) are obtained,
expansion means (X
1
) for expanding said gas phase (G
1
), at the outlet of which a mixed phase M
2
at a temperature T
2
is obtained, said expansion means being connected by a line to a device (D
1
, P
1
, P
2
) allowing to perform heat exchange and distillation, said mixed phase circulating through a pass (P
1
),
separation means (B
1
) for separating the mixed phase after passage through the heat exchange and distillation device,
expansion means (V) for expanding liquid phase (L
1
), said expansion means being connected to separation means (B
1
), in order to obtain a gas phase mixed with the gas phase obtained by separation of the mixed phase, all of the gas phases circulating through reflux pass P
2
, circulation of the gas phases in passes P
1
and P
2
being a countercurrent circulation,
a line intended for discharge of the liquid phases obtained by separation in B
1
,
at least one line intended for discharge of the lighter constituents in the gas form, obtained by heat exchange and distillation in device D
1
.
The device is for example suited for heat exchange and distillation, and it comprises a third pass (P
3
) intended for passage of at least part of the gas extracted through the line.
The device that can be suited for heat exchange and distillation is a dephlegmator exchanger (D
1
) comprising at least two passes, including a reflux pass in which fractionation is carried out.
It comprises for example stabilization means situated after the expansion valve.
The stabilization means and the gas refrigeration means are for example included in a single device.
The means for refrigerating and the means for separating the gas to be fractionated are for example a stripper exchanger.
The system comprises for example compression means (C
1
) for compressing the scrubbed gas, a line (
50
) for diverting at least a fraction of the scrubbed gas, means (E
5
) for refrigerating and for expanding (V
2
) said fraction and separation means (B
2
) associated with fractionating device (D
1
) so as to obtain a liquid phase at a sufficiently low temperature used as additional reflux in reflux pass P
2
.
The process and the system according to the invention are notably suited for fractionation of a gas essentially comprising methane and hydrocarbons with one or more carbon atoms.


REFERENCES:
patent: 4519825 (1985-05-01), Berhard et al.
patent: 4525187 (1985-06-01), Woodward et al.
patent: 4714487 (1987-12-01), Rowles
patent: 4720294 (1988-01-01), Lucadamo et al.
patent: 4921514 (1990-05-01), Rowles
patent: 5287703 (1994-02-01), Bernhar

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